Intramolecular Electron Transfer in an Iridium d⁸-d⁸ Donor-Acceptor System Citation Fox, Lucius Seiberling (1989) Intramolecular Electron Transfer in an Iridium d⁸-d⁸ Donor-Acceptor System. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/hapm-b986. https://resolver.caltech.edu/CaltechETD:etd-06072007-081509 Abstract A series of donor-acceptor complexes, [Ir₂(µ-Pz*)₂(CO)₂(Ph₂POCH₂CH₂-Py⁺-R)₂](Ph₄B)₂ (Py⁺-R = 2,4,6-trimethylpyridinium, 4-methylpyridinium, pyridinium, and 4-phenylpyridinium; Pz* = 3,5-dimethylpyrazole), have been synthesized for studying the rate of photoinduced electron transfer from the metal localized (dσ*pσ) excited states of d⁸-d⁸ chromophores. The pyridinium electron acceptors are covalently attached to the iridium metal centers (donor) via a three atom hydrocarbon linker bound to the terminal phosphine ligands. The x-ray crystallographic structure of [Ir₂(µ-Pz*)₂(CO)₂(Ph₂POCH₂CH₂-Py⁺)₂](Ph₄B)₂ reveals a metal-metal distance of 3.219(1) Å and a solid state donor-acceptor separation of 5.34(1) Å. Additional donor-acceptor separations and orientations are available to the compounds in fluid solution through rotations about the Ir-P and linker group C-O, C- N, and C-C bonds. Steady state emission spectra show that the fluoresence and phosphoresence quantum yields in these compounds are substantially reduced relative to a appropriate model complex. To date the excited state reactivity of d⁸-d⁸ metal dimers has been exclusively attributed to their ³(dσ*pσ) states. These findings represent the first evidence for reactivity from a shorter lived ¹(dσ*pσ) state. Picosecond and nanosecond laser flash-photolysis techniques were employed to measure the rates of photoinduced electron transfer and charge recombination in these systems. Values of k ET , obtained from these studies, vary between 10⁶ sec⁻¹ and 10¹² sec⁻¹ as a function of reaction exoergicity (ΔGᵒ' = -0.8 eV to 1.92 eV). Clear evidence for the inverted behavior predicted by classical and semiclassical electron transfer models is seen at high driving force. Rate constants for reactions involving the ¹(dσ*pσ) and ³(dσ*pσ) excited states, as well as the rates of charge recombination are characterized by very similar values of λ and H ab . Our data are adequately modeled by the classical theory of electron transfer proposed by Marcus (λ = 1.0 eV, H ab = 35 cm⁻¹) or by a semiclassical model for k ET where nuclear tunneling involves a low frequency metal-ligand mode (λ = 1.0 eV, H ab = 35 cm⁻¹, ω = 100 cm⁻¹). These findings are explored with regard to utilizing the donor-acceptor complexes as molecular photochemical energy storage systems. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: Chemistry Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Awards: The Herbert Newby McCoy Award, 1989 Thesis Availability: Public (worldwide access) Research Advisor(s): Gray, Harry B. Thesis Committee: Chan, Sunney I. (chair) Bercaw, John E. Gray, Harry B. Marcus, Rudolph A. Defense Date: 31 January 1989 Record Number: CaltechETD:etd-06072007-081509 Persistent URL: https://resolver.caltech.edu/CaltechETD:etd-06072007-081509 DOI: 10.7907/hapm-b986 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 2499 Collection: CaltechTHESIS Deposited By: Imported from ETD-db Deposited On: 14 Jun 2007 Last Modified: 15 Jul 2021 00:16 Thesis Files PDF - Final Version See Usage Policy. 19MB Repository Staff Only: item control page

lntramolecular Electron Transfer in an Iridium d 8 -d 8 Donor-Acceptor System Thesis by Lucius Seiberling Fox In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy California Institute of Technology Pasadena, California 1989 (Submitted January 31, 1989) ii (c) 1989 Lucius S. Fox All Rights Reserved iii Acknowledgment Well, at this point with all of the experiments done and the body of my thesis written, it is time to reflect on my tenure at Caltech and try to thank many of the people who have enriched my graduate career. I owe a special debt of thanks to my research advisor and friend Harry Gray. Apart from being an outstanding scientist, Harry is probably the most generous and supportive person I have ever had the opportunity to work for. I can not count the number of meals at the Acapulco Lounge that I have enjoyed at Harry's expense; each accompanied by numerous rounds of Margaritas! He has cheered me on in lab, thumbs always pointing toward the ceiling, and cheered me up when things weren't going so well. Most importantly, he has provided me with the opportunity to independently seek out my own graduate education. When I joined his group in 1983, I found this "hands off approach" somewhat frustrating. Over the years I believe that the "Gray Way" of graduate education has developed my ability to identify and pursue interesting chemical problems. I value this single skill more than any other and know that I will continue to appreciate it as my career unfolds. One of the exciting aspects of my graduate research was that it involved collaborating with several outstanding scientists at Caltech, SERI, and Brookhaven National Laboratory. Dan DuBois at SERI was responsible for initiating research surrounding the preparation of the redox phosphine ligands and was a valuable source of information during the synthesis phase of this project. Janet Marshall coached me in preparing the iridium dimer complexes and was involved in their inital characterization. Professor Dick Marsh at Caltech solved the crystal structure of [lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py+)2J(Ph48)2 and my interactions with him enhance my appreciation for the science of crystallography and proper data analysis. thank him for the time he took to make sure that our data were sound, our interpretations were correct and to answer all of my questions. During the winter iv quarter of 1987 I had the unique opportunity to travel to Brookhaven National Laboratory to conduct picosecond laser experiments on my compounds with Jay Winkler. Needless to say without these data my thesis would not be complete. I thank Jay and his wife Melissa for their hospitality during my visit. It was certainly nice to spend Sunday afternoons having dinner with the Winklers rather than having the "entree du jour" at the Brookhaven caferteria. Through my interactions with Jay I learned what I know about reaction kinetics and picosecond laser spectroscopy and I am grateful to him for his advice and insights. As I read about the many theories of electron transfer, I found that they are based on readily understandable concepts which are often enshrouded in technical jargon and complicated mathematical formalisms. Luckly for me John Hopfield employed two of electron-transfer's up and coming theoreticians who were willing to answer many of my questions. Jose and Dave, I think I understand your latest papers but I'll give you a call just to be sure. When I arrived at Caltech in 1983, the photogroup was inhabited by three seasoned group members. Mike Hopkins, Tom Zeitlow, and Janet Marshall made key contributions to my education by showing me the ropes in lab, talking to me about potential projects, and teaching me the ins and outs of inorganic photochemistry. Mike and I became good friends and I thank him for the many hours we spent talking about research ideas and graduate school survival techniques. "Jedi Master" Walther Ellis provided me with seemingly endless torture at the hands of his Ligand Field Theory course. Walther is a man who continues to amaze me; master scientist, master drinker, and photocopy machine expert! I certainly wish him and the Xerox machine at Utah all the best. Special thanks go to David Smith, Miriam Zeitlow, and Steve Mayo as my comrades at arms during graduate school and for their friendship. It has been a lot of fun being in the Gray Group thanks to the great people that I have had the oppportunity to V work with and count among my friends. John Brewer and Mike Therien proofread and critiqued my thesis and I thank them for their insights. Outside of the Gray Group there are several people who have had an influence on my development both prior to and during my graduate career. Gary Christoph and Bruce Bursten gave me my first opportunity to conduct research in inorganic chemistry. Both men have helped me realize many of my goals and I thank them for their friendship and support. Sunney Chan has been a valuable friend and sounding board during graduate school and I truly appreciate his kindness. He introduced me to realchinese food and I will miss our regular trips to "Sun Tung Loe" for dim sum. Most Importantly, I want to thank my mother, father, and my lovely wife Jayne for all the love and support they have given me during my education. My parents have stood behind me through out college and graduate school and Jayne has made these last two years at Caltech my happiest. Mom, Dad, I think I am finished with school now. Jayne, I promise that we'll never live near an earthquake fault again! I also want to thank my good friend Chris Thompson for helping me keep things in perspective. Finally, I thank the British Petroleum America Corporation for a fellowship during the spring quarter of 1988. vi To Mom, Dad, and Jayne vii Abstract: A series of donor-acceptor complexes, [lr2(µ-Pz*)2(CO)2(Ph2POCH2CH2Py+-R)2](P h4B)2 (Py+-R= 2,4,6-trimethylpyridinium, 4-methylpyridinium, pyridinium, and 4-phenylpyridinium; Pz*=3,5-dimethylpyrazole), have been synthesized for studying the rate of photoinduced electron transfer from the metal localized (dcr*pcr) excited states of d8-d8 chromophores. The pyridinium electron acceptors are covalently attached to the iridium metal centers (donor) via a three atom hydrocarbon linker bound to the terminal phosphine ligands. The x-ray crystallographic structure of [lr2(µ-Pz*)2(CO)2(Ph2POCH2CH2-Py+)2](Ph48)2 reveals a metalmetal distance of 3.219(1) A and a solid state donor-acceptor separation of 5.84(1) A. Additional donor-acceptor separations and orientations are available to the compounds in fluid solution through rotations about the lr-P and linker group C-O, C-N, and C-C bonds. Steady state emission spectra show that the fluoresence and phosphoresence quantum yields in these compounds are substantially reduced relative to a appropriate model complex. To date the excited state reactivity of d8-d 8 metal dimers has been exclusively attributed to their 3 (dcr*pcr) states. These findings represent the first evidence for reactivity from a shorter lived 1(dcr*pcr) state. Picosecond and nanosecond laser flash-photolysis techniques were employed to measure the rates of photoinduced electron transfer and charge recombination in these systems. Values of kET, obtained from these studies, vary between 106 sec- 1 and 10 12 sec- 1 as a function of reaction exoergicity (~G 0 '= -0.8 eV to 1.92 eV). Clear evidence for the inverted behavior predicted by classical and semiclassical electron transfer models is seen at high driving force. Rate constants for reactions involving the 1(dcr*pcr) and 3 (dcr*pcr) excited states, as well as the rates of charge recombination are characterized by very similar values of A and Hab . Our data are adequately modeled by the classical theory of electron transfer proposed by Marcus (A=1.0 eV, Hab=35 cm- 1 ) or by a semiclassical model for kET where nuclear tunneling involves a low frequency metal-ligand mode (A=1.0 eV, Hab=35 cm- 1, co=400 cm- 1 ). These findings are Vi ii explored with regard to utilizing the donor-acceptor complexes as molecular photochemical energy storage systems. ix Contents Acknowledgment iii Abstract vii List of Figures X List of Tables xiii Chapter 1: Introduction. 1 Chapter 2: Synthesis and Structure. 24 Chapter 3: Electronic Spectrscopy and Electrochemistry. 117 Chapter 4: Time-resolved Experiments. 197 Appendix: Structure Factor Tables. 284 X List of Figures Chapter 1 Figure 1.1: A schematic representation of an electron-transfer hypersurface. 8 Figure 1.2: A simplified MO diagram for d8 -d 8 metal dimers. 11 Figure 1.3: [lr2(Pz)2(COD)2]. 14 Figure 1.4 : The driving force data of Marshall et al. 16 Chapter 2 Figure 2.1: A schematic representation of the donor-acceptor complexes. 26 Figure 2.2: Structures and abbreviations for the phosphinite redox ligands. 45 Figure 2.3: Synthetic scheme for the phosphinite ligands. 47 Figure 2.4: Synthetic scheme for the donor-acceptor complexes. 51 Figure 2.5: Structures for the cis and trans isomers of lr2(Pz*)2(C0)2(Ph2POCH2CH3)2. 59 Figure 2.6: Proton NMR spectra of [lr2(Pz*)2(C0)4] and [I r2( Pz*)2(C0)2( Ph2P OCH 2C H3)2]. 61 Figure 2.7: Chemical shift assignments in lr2(Pz*)2(C0)2(Ph2POCH2CH3)2. 64 Figure 2.8: Diastereotopic protons in lr2(Pz*)2(C0)2(Ph2POCH2CH3)2. 66 Figure 2.9: Proton NMR spectrum of lr2(Pz*)2(C0)2(Ph2POCH2CH2-Me3Py+)2. 69 Figure 2.10: Diastereotopic protons in lr2(Pz*)2(C0)2(Ph2POCH2CH2-Me3Py+)2. 71 Figure 2.11: NMR spectrum of lr2(Pz*)2(C0)2(Ph2POCH2CH2CH2CH3)2. 73 Figure 2.12: Proton NMR spectra of [lr2(Pz**)2(C0)4) and lr2(Pz**)2(C0)2(Ph2POC H2CH2-4 PhPy+)2. 75 Figure 2.13: ORTEP of lr2(Pz*)2(C0)2(Ph2POCH2CH2-Py+)2. 79 xi Figure 2.14: Preparative scheme for high driving force ligands. 87 Figure 2.15: Structure showing phospinite ligands' different degrees of freedom 93 Figure 2.16: Structures showing lr-P rotational conformers. 95 Figure 2.17: Structures showing 0-C and C-C rotational conformers. 1 O1 Figure 2.18: Proton NMR spectrum of lr2{Pz*)2(C0)2{Ph3P)3. 11 O Chapter 3 Figure 3.1: Electrochemical cell used in CV experiments. 12 3 Figure 3.2: Molecular orbital diagram for d8 -d 8 metal dimers. 12 7 Figure 3.3: Absorption spectrum of lr2{Pz*)2{C0)2{Ph2POCH2CH3)2. 133 Figure 3.4: 77° K absorption spectrum of lr2{Pz*)2{C0)2{Ph2POCH2CH3)2. 13 6 Figure 3.5: Emission spectra of lr2(Pz*)2{C0)2(Ph2POCH2CH3)2. 13 9 Figure 3.6: Room T emission spectra of the donor-acceptor complexes. 14 3 Figure 3.7: Excitation spectra of lr2{Pz*)2(C0)2{Ph2POCH2CH3)2. 145 Figure 3.8: Potential energy surfaces showing the origin of rigidochromic effects. 1 5 2 Figure 3.9: CVs of lr2{Pz*)2{C0)2{Ph2POCH2CH3)2. 161 Figure 3.10: CVs of lr2{Pz*)2{C0)2{Ph2POCH2CH3)2 following bulk electrolysis. 163 Figure 3.11: IR spectra of lr2{Pz*)2{C0)2{Ph2POCH2CH3)2 prior to and following bulk electrolysis. 16 6 Figure 3.12: CVs of lr2{Pz*)2{C0)2{Ph2POCH2CH2-Me3Py+)2. 169 Figure 3.13: CVs of lr2{Pz*)2{C0)2{Ph2POCH2CH2-NEt3+)2{PFs)2. 172 Figure 3.14: CVs of lr2(Pz*)2{C0)2{Ph3P)3 prior to and following bulk electrolysis. 177 Figure 3.15: IR spectra of lr2(Pz*)2{C0)2{Ph3P)2 prior to and following bulk electrolysis. 179 Figure 3.16: Electronic spectra of lr2{Pz*)2{C0)2(Ph3P)3 and [lr2{Pz*)2 {C 0) 2{Ph3 P)3]+. 1 81 xii Figure 3.17: State diagram for the donor-acceptor complexes. 188 Chapter 4 Figure 4.1: Block diagram of the picosecond laser used in our kinetics studies. 2 OO Figure 4.2: Inert atmosphere cell used in picosecond absorption studies. 2 O3 Figure 4.3: Difference spectra and kinetics for the model compounds. 2 O6 Figure 4.4: Difference spectra and kinetics for lr2(Pz*)2(C0)2(Ph2POC H2C H2-4Ph Py+)2. 21 3 Figure 4.5: Difference spectra and kinetics for lr2(Pz*)2{C0)2(Ph2POCH2CH2-Py+)2. 21 6 Figure 4.6: Difference spectra and kinetics for lr2(Pz*)2(C0)2(Ph2POCH2CH2-4MePy+)2. 21 9 Figure 4.7: Difference spectra and kinetics for lr2(Pz*)2(C0)2(Ph2POC H2CH2-246Me3Py+)2. 222 & 22 7 Figure 4.8: Kinetic scheme for lr2(Pz*)2(C0)2(Ph2POCH2CH2-Me3Py+)2. 23 2 Figure 4.9: Excitation spectra of lr2{Pz*)2(C0}2(Ph2POCH2CH3)2. 241 Figure 4.10: Potential energy surfaces showing th origin of the inverted region. 24 6 Figure 4.11: Driving force plot for the charge separation and recombination reactions in the donor-acceptor systems. 254 Figure 4.12: Driving force data fit to a classical Marcus model. 257 Figure 4.13: Driving force data fit to a semiclassical model as a function of co. 261 Figure 4.14: Driving force data fit to a semiclassical model as a function of Ao, Ai, and ro 264 Figure 4.15: Photo physical scheme for the photosynthetic reaction center. 272 Figure 4.16: A kinetic scheme for the donor-acceptor complexes. 276 xiii list of Tables Chapter 2 Table 2.1: Atomic Coordinates and Anisotropic Thermal Parameters. 38 Table 2.2: Final Crystallographic Parameters. 41 Table 2.3a: 1H and 31 P NMR Data for the Phosphinite Redox Ligands. 49 Table 2.3b: 1H and 31 P NMR Data for the Iridium Dimer Complexes. 54 Table 2.4: Selected Distances and Angles. 24 Table 2.5: Selected Torsional Angles. 83 Table 2.6: Metal-metal Distances for Selected A-Frame Complexes. 91 Chapter 3 Table 3.1: Spectral Parameters for the Iridium Dimer Complexes. 13 O Table 3.2: Oscillator Strengths for the (dcr* ➔ pcr) 1,3 and Band Ill Electronic Transitions. 1 31 Table 3.3: 77° K Emission Parameters for the Iridium Dimer Complexes. 14 i Table 3.4: Spectral Data for Selected A-Frame Complexes. 142 Table 3.5: Emission Spectral Parameters for lr2(Pz*)2(C0)2(Ph2POCH2CH3)2. 14 9 Table 3.6: Temperature Dependent Emission Data for lr2{Pz*)2{C0)2(Ph2POC H2C H3}2. 15 O Table 3.7: Photophysica! Data for the Iridium Dimer Complexes. 15 6 Table 3.8: Electrochemical Potentials for the Iridium Dimer Complexes. 175 Table 3.9: Final Energetic Parameters for the Donor-Acceptor Complexes. 19 0 xiv Chapter 4 Table 4.1 : Quantum Yield and Lifetime Data for Selected A-Frame Complexes. 211 Table 4.2: Kinetic Parameters for the 2,4,6-Me3Py Complex. 23 8 Table 4.3: Photophysical Parameters for the Iridium Dimer Complexes. 251 Table 4.4: Driving Force and Rate Data. 252 Table 4.5: Predicted Electron-Transfer Rates and Photophysical Parameters. 278 1 Chapter 1 Introduction 2 Electron transfer (ET) is one of Nature's m~st fundamental chemical reactions. Charge transfer processes are of central importance in.a wide number of complex biological and chemical systems that are currently being explored in the scientific community. For example, photosynthesis continues to be an area of interest in chemistry and chemical biology, 1 especially due to the recent crystallographic characterization of the photosynthetic reaction center of Rhodopseudamonas viridis by Deisenhofer and coworkers. 1c Electron-transfer studies concerning model systems 1a and the reaction center itself1 a,3 have greatly enhanced our understanding of this remarkable photoredox system. Efficient long distance electron-transfer events are also important in the energy storage reactions that take place via the membrane-bound proteins in the mitochondria of cells.2 Here, electron-transfer reactions, which are coupled to a proton pumping process, produce a membrane potential that is used to synthesize ATP from ADP and inorganic phosphate. Electron transfer is also a fundamental solid state process and an area of significant interest with regard to the renaissance taking place in solid state chemistry. 4 Many of the same fundamental principles that govern electron transfer between molecules must also play a role in the transport of charges through a semiconducting solid. In fact, some of the theories for molecular electron-transfer reactions have been developed by analogy to the theories of small polarons in solid state systems. One area where a fundamental understanding of electron-transfer processes will certainly have an impact in coming years is Scanning Tunneling Microscopy (STM). 5 STM is of intense interest to the physics and chemistry communities, because this technique offers the unique capability of being able to record real time images of surfaces and surface adsorbates with molecular and, in some instances, atomic resolution. Because the STM technique is based on electron tunneling between the tunneling-tip and surface, an understanding of the molecular properties that govern charge-transfer processes will be important for interpreting images of surface-bound molecular species. In addition to being of academic interest, electron- 3 transfer processes are also important to a number of current and developing commercial technologies. Electron-transfer reactions form the basis of the photographic and xerographic processes. 6 From exposure to development, the transformations required to capture an image on photographic film involve both solid state and molecular redox reactions. Finally, photoinduced and thermal electron-transfer reactions are intimately linked to developing solar energy storage technologies. Recent reports have indicated that solar energy conversion efficiencies as high as 28% can be attained using solid state photoelectrochemical devices. 7 These considerations indicate that detailed studies, concerning the fundamental parameters which control the rates of electron transfer, are certainly warranted. This thesis deals with the mechanistic aspects of excited state intramolecular electron transfer in an iridium d8-d 8 A-frame metal complex. A long range goal of our electron-transfer research is to couple photoinduced charge separation to bond forming chemical reactions. Our studies have focused on d8-d8 complexes, because they are powerful excited state reductants21d,22i and their d8-d7 counterparts are reactive toward C-X and C-H bond activation. 24 The following chapter introduces the concepts and terminology surrounding electron transfer and d8-d8 compounds that are called upon in later discussions. We first review some of the theoretical models associated with electron-transfer reactions followed by some background information on d8-d 8 metal dimers and an outline of the objectives of this research program. Electron-Transfer Theory: Over the past two decades the theoretical models for electron-transfer reactions have played a central role in developing an understanding of these fascinating chemical processes.a These models have provided a central framework for designing and interpreting experiments that explore the roles played by various molecular parameters in determining ket, Because many of the concepts expressed in these theories will be important in later discussions, we will give a brief overview of these concepts and introduce some of the terminology associated with electron-transfer 4 models. A detailed treatment of these models can be obtained by referring to the review articles and original papers cited at the end of this chapter. In principle, a redox system must overcome two types of barriers to electron transfer in going from reactants to products. The first of these is related to the energy required to reorganize the reactant bond distances, angles, and salvation environments to a configuration indicative of the transition state. While bonds are neither formed nor broken in an outer-sphere electron-transfer reaction, the bond distances, angles, and salvation environments in the reactants and products are different. Thus, to form the transition state, these degrees of freedom in the donor and acceptor must rearrange to some configuration intermediate between the reactants and the products. This reorganization process assures that energy is conserved in the transition state by equalizing the energy levels of the transferring electron on the donor and acceptor. Once a transition state configuration has been achieved, the system must overcome an electronic barrier to electron transfer imposed by the intervening medium between the donor and acceptor. If the medium between the redox partners is a good electrontransfer mediator, then the electronic barrier is small and electron transfer occurs with near unit efficiency. Conversely, if the intervening medium is an insulator, then electron transfer will be a less probable event. According to the classical, semiclassical, and quantum mechanical theories of electron transfer, ket for a given reaction can be described by the three kinetic quantities seen in equation 1.1. k et =vx:(FC) eq. 1.1 Here, v is the frequency of the vibrations that destroy the transition state, K is an electronic transmission factor that expresses the probability of charge transfer in the transition state region, and FC is the Frank-Condon factor for electron transfer. These quantities represent the different barriers to electron transfer encountered by a 5 reactive redox system along its reaction coordinate. They have been treated using several different theoretical approaches. The thermal barriers to electron transfer or Frank-Condon factors have been described using both classical and quantum mechanical models. Most notable amongst the classical models is the theory proposed by Marcus for the rate of outer-sphere electron-transfer reactions (equations 1.2 to 1.4).Sf,g,h The total activation free energy in the Marcus model is a function of the reaction free energy (dG 0 ') and the intrinsic thermal barrier A., which is the sum of a molecular (Ai) and solvent related component (A. 0 ). Ai is referred to as the inner-sphere reorganization energy and represents the energy required to rearrange the bond distances and angles of the reactants to their transition state configurations. The later quantity, A. 0 , represents the energy required to reorganize the solvation environment of the reactants. Marcus derived specific expressions for these parameters by assuming that the molecular degrees of freedom could be modeled by classical harmonic oscillators and that the surrounding solvent was a dielectric continuum. One feature of the Marcus model that has stimulated considerable experimental interest concerns its predictions regarding the variation of ket as a function of reaction exoergicity. Taking the natural log of equation 1.2 and plotting RTln(ket) versus dG 0 ' reveals that the rate of electron transfer should increase as a function of driving force in what has been termed the normal region until dG 0 '=A. and then decrease as a function of driving force into what 6 Marcus labeled the inverted region . In addition to the Marcus model, several quantum mechanical and semiclassical models for electron-transfer reactions have also been proposed. One of the more popular semiclassical relationships for ket is has been developed by Jortner and Bixon (equations 1.5 and 1.6).Bi,j,k,I eq. 1.5 In equations 1.5 and 1.6 the outer-sphere reorganization energy (As) is treated classically using a dielectric continuum model. However, the higher frequency molecular vibrations of the donor and acceptor are treated quantum mechanically to allow for nuclear tunneling. This equation reduces to the Marcus relationships at high temperature and has been useful in describing electron-transfer reactions at low temperature and at high driving force where nuclear tunneling effects are most important. While electron transfer in the normal region is for the most part understood, reactions in the inverted region have remained largely unexplored. A lack of information regarding the mechanism of electron transfer at large reaction exoergicities is directly related to the fact that prior to 1984 little experimental evidence for inverted behavior had been observed. 9 Thus, high driving force reactions are of continued interest in electron-transfer research. 1 0 The electronic transmission coefficient (K) controls the rate of electron transfer in the transition state region. When multiplied by the nuclear frequency, v, it represents the activationless reaction rate constant. This factor is determined by the electronic coupling between the two redox partners, the velocity of the reaction 7 coordinate through the transition state region, and the time scale for energy exchange between the reaction coordinate and other degrees of freedom. 11 As stated earlier, K represents the probability that charge transfer will take place once the system has reached the transition state. When K=1 charge transfer takes place with unit efficiency and the reaction is termed adiabatic. A K:<1 is associated with a less efficient nonadiabatic electron-transfer process. The reaction adiabaticity is directly related to the relative magnitudes of the electronic coupling between the redox partners and the kinetic energy of their nuclei near the top of the col (figure 1.1 ). If the kinetic energy of the reaction coordinate (ksT) is less than the electronic interaction (A}, then the system is confined to the lower energy surface and passes ballistically from reactants to products. In contrast, if A<ksT, then the system can make transitions to an excited state surface (i.e. follow the dashed line figure 1.1) and remain in an electronic state corresponding to the reactants. This makes electron transfer a less probable event and requires that the system make multiple crossings through the transition state region before forming products. Recent theories have indicated that the electronic coupling between two redox partners is an exponential function of their through space separation and is related to the structural and electronic properties of the intervening medium.12 Understanding the molecular factors that govern the electronic interactions between donor and acceptor is an area of significant interest in experimental electron-transfer research and is currently being investigated in a number of synthetic 1Oa,g, 13e-h and biological redox systems.1 Ob, 13a-d,24d In addition to being sensitive to electronic coupling effects, K is also a function of the rate of energy exchange between the reaction coordinate and bath modes. 14 Bath modes correspond to degrees of freedom that are not included in the reaction coordinate but are coupled to it. They essentially accept excess energy associated with the transition state and allow the products to relax to an equilibrium configuration. Energy exchange between the RC and bath has been explored in theoretical studies by associating 8 Figure 1.1: A schematic representation of an electron-transfer hypersurface near the transition state region. If electron transfer is adiabatic, the reactants are confined to the lower surface and pass ballistically through the transition state region. If the reaction is nonadiabatic, the system can be excited to higher energy surface in the transtion state region and retain an electronic configuration indicative of the reactants. * marks the position of the transiton state along the reaction coordinate. TS labels the transition state region, which is defined as that portion of phase space within ksT of the transition state. LZ (Landau-Zener region) is that portion of phase space where the electronic coupling between the redox partners is strong. It includes an area within ti of the transition state. 9 products Reactants E ' -----' / '/ /' / * ' ------r----- _ _ _J_ __ TS Reactants --~•-~_J __ products RC 10 a classical friction with motion of the system through the transition state region. Under conditions of low friction (i.e. slow energy exchange between the RC and bath modes) the system can not relax rapidly enough to an equilibrium "products configuration" and must make multiple crossings before being trapped in the products well. Such a situation is referred to as underdamped. Under conditions of high friction (i.e. rapid energy exchange between the RC and bath modes), the system moves through the transition state region by Brownian diffusion. In this overdamped situation the system also makes multiple crossings through the transition state region before forming products. Finally, when the reaction friction is optimal (i.e. critically damped ) energy exchanges between the RC and bath on a time scale commensurate with the formation of products. Under these conditions, the reaction adiabaticity is controlled predominantly by electronic coupling. Behavior characteristic of an overdamped ET system has recently been observed by Kosower, 15 Simon, 16 and Eisenthal17 in organic charge transfer systems dissolved in alcoholic solutions. In these studies the rate of electron transfer was shown to directly correlate with the longitudinal relaxation time of the solvent medium. A greater understanding of the role played by solvent dynamics in electron-transfer reactions will unfold as additional systems with ET rates greater than or equal to 1O13 sec- 1become available. d8-d 8 Metal Dimers:The electronic structure and excited properties of ds-ds transition metal complexes are areas of current interest in inorganic photochemistry .1 8 Recent research has indicated that their low lying excited states display a variety of different chemical reactivities including the excited state oxidative addition of halocarbons, 19 carbon-hydrogen bond activation,20 and outer-sphere electron transfer.21 These diverse photochemical and photophysical properties can be traced to their (dcr*pcr) excited states, which result from the axial metal-metal interactions inherent in these compounds. According to Gray and coworkers, 22 the electronic structure of these intriguing chromophores can be schematically represented by the 11 Figure 1.2: A simplified MO diagram for a d8 -d 8 metal dimer showing the ground state electronic configuration. The (dcr* ➔ pcr) excited states can be accessed by exciting the complex with visible light as indicated by the white arrow. 12 Pz , ,, , ,, ... , ... , ,, ... ,, pcr ... ... ,, .... . . , ,, ... ,, ... , ,, ,, , Pz pcr hv d2 2 , , ,, ... , ,, ... , , ,, ... ,, ... dcr* • 1l dcr .. .... . . ,, • , ,, ... ,, ... , ,, . , ,, d2 2 13 molecular orbital diagram in figure 1.2. Covalently linking two square planar d8 complexes into a dimer molecule forces an axial interaction between the d2 2 and p2 orbitals of the monomer fragments, which leads to the formation of dcr and pcr bonding and antibonding dimer MOs. Filling this MO scheme with the 16 metal electrons from the two monomer fragments leaves the dcr* level as the compound's highest occupied molecular orbital. The long lived excited states in these complexes are generated by exciting one dcr* electron into a vacant pcr orbital. This electronic configuration yields a singlet and corresponding triplet state as the reactive excited states in these systems. At the inception of this research, our studies regarding excited state electron transfer in d8 -d 8 compounds had centered around a series of iridium pyrazolate bridged COD complexes first prepared by Stobart and coworkers (figure 1.3).23 Marshall et al. conducted a thorough examination of the driving force dependence of the bimolecular triplet electron-transfer quenching reactions in [lr2(Pz)2(COD)2] using a homologous series of pyridinium electron acceptors. 21 d,22i Results from this study are summarized in figure 1.4. Several important conclusions were drawn from these findings: 1) In contrast to the predictions made by the Marcus model, the rate of triplet electron transfer in this photoredox system tends to a diffusion limit at high driving force. We, like others, speculated that our failure to observe inverted behavior in this study was tied to the limitations placed on rapid electron transfer reactions by diffusion. 2) Steady-state quenching experiments showed that the excited state electron-transfer reactivity in this complex was restricted to its triplet state. Because, the singlet state in this compound lies approximately 4000 cm- 1 above its triplet state, in it principle should be a powerful reductant. This lack of reactivity was presumed to be due to the subnanosecond fluorescence lifetime in [lr2(Pz)2(COD)2]. These key observations suggested that our understanding of electron-transfer reactions involving d8-d 8 Aframe compounds could potentially be extended by designing a redox system which eliminated bimolecular diffusion as a rate limiting kinetic process. 14 Figure 1.3: The iridium dimer complex used by Marshall et al. in their bimolecular · quenching studies. This complex possesses terminal chelating COD ligands. 15 ~ IN \ / I r··· r'l ( ············· I r: ~I 16 Figure 1.4: The driving force dependence data of Marshall et al. adapted from reference 22i. 17 ------~---.--r,-3 2 I •5 4• 0.6 > - - - _;0.5 C l- o:: 0.4 -2.0 -1.5 E(A•1Ai, V vs. SSCE -1.0 -0.5 18 In this study we remove the constraints place on excited state electron-transfer reactions due to diffusion, by covalently linking an iridium dimer chromophore (donor) and a pyridinium cation (acceptor) into a single molecular unit. An analogous approach has been recently demonstrated in a number of synthetic 1oa,c, 13e-h,24a-c and biological redox systems.1 Ob, 13a-d,24d Our particular strategy employs a series of redox-active phosphinite ligands. We feel this approach has potential generality, because phosphine ligands are found in a wide variety of inorganic and organometallic compounds. In principle our ligands could be used to prepare and study a number of different inorganic photoredox compounds. Our objectives in this study are; 1) to further characterize the driving force dependence of electron transfer reactions in d8 -d 8 A-frame complexes and look for evidence of inverted behavior, 2) to explore the possibility of observing singlet quenching reactions that might readily occur in an intramolecular electron-transfer complex, and 3) to determine what differences, if any, exist between the factors that govern the triplet, singlet and charge recombination ET processes in these compounds. Evidence for singlet electron-transfer quenching would be of significant interest, because at present the excited state reactivity of d8 -d 8 metal dimers has been exclusively attributed to their long lived triplet states. Our research regarding these issues is organized as follows. Chapter 2 outlines the preparation and characterization of our d8 -d 8 donor-acceptor complexes. Careful consideration is given to their structural properties determined by NMR spectroscopy and from a crystal structure of [lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py+)2](Ph48)2. Chapter 3 summarizes our results concerning the spectroscopic and electrochemical properties of the complexes. These data are used to construct an approximate state diagram for their metal localized and charge-transfer excited states. Finally, in chapter 4, rate constants for their photoinduced electron-transfer and charge recombination reactions are measured using nanosecond and picosecond laser spectroscopy. 19 These data are analyzed with regard to the photophysical properties of the compounds and the driving force dependence of ket• References 1a. Budil, D. E.; Gast, P.; Chang, C.-H.; Schriffer, M.; Norris, J. R., Ann. Rev. Phys. Chem., 1987, 38, 561-583. b. Boxer, S. G., Biochim. Biophys. Acta, 1983, 726, 265-292. c. Deisenhofer, J.; Epp, O.; Miki, K.; Huber, M. R., Mo/. Biol., 1984, 180, 385-398. 2. Hatefi, Y., Ann. Rev. Phys. Chem., 1985, 54, 1015-1069. 3a. Gunner, M. R.; Robertson, D. E.; Dutton, P. L., J. Phys. Chem., 1986, 90, 37833795. b. Ogrodnik, A.; Remy-Richter, N.; Michel-Beyerele, M. E.; Feick, R., Chem. Phys. Lett., 1987, 135, 576-581. 4. Mikkelsen, K. V.; Ratner, M.A., Chem. Rev., 1987, 87, 113-153. Sa. Hansma, P. K.; Tersof, J., J. Appl. Phys., 1987, 61, R1-R23. b. Hansma, P. K.; Elings, V. 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B., manuscript in preparation. 24 Chapter 2 Synthesis and Structure 25 Introduction The basic components of an intramolecular electron-transfer system are an electron donor, an electron acceptor, and a bridging group, which covalently links the two redox partners into a single molecular unit. Each component plays a crucial role in determining the rate of photoinduced and thermal electron transfer in such a system by controlling fundamental electron-transfer parameters such as reaction free energy; donor-acceptor separation, orientation, and electronic coupling; and inner-sphere reorganization energies.1 By varying the chemical and physical properties of these three elements, information concerning the factors that control electron-transfer reactions can be obtained. For example, the driving force dependence of kET has been traditionally investigated by systematically varying the redox energy of a structurally homologous series of electron acceptors. 2 This type of study provides valuable information concerning the reorganization energy controlling the charge-transfer process and probes the system for the inverted behavior predicted by current theoretical models. 1 a Bridging groups can play a number of roles in intramolecular ET systems. In addition to holding the two redox partners at a well-defined separation and orientation, they can also act as a "conducting medium" for transmitting an electron between the donor and acceptor.3 This type of through-bond mechanism may be important in the long range electron-transfer reactions seen in metalloproteins4 and is an area of intense interest. These considerations suggest that a successful donoracceptor system is one that can be systematically modified so that kET can be studied as a function of different donor, acceptor, and bridging group properites. Thus, our goal has been to develop synthetic routes that are flexible enough to allow for these types of 26 Figure 2.1: Structure of the iridium dB-dB donor-acceptor complexes. 27 /N \ R oc/)r---------- I r\'PPh20CH2CH2-~ Py+CH2CH20Ph2P m 28 systematic modifications. This chapter details the preparation and characterization of the series of donoracceptor molecules schematically represented in figure 2.1. These compounds are similar to the d8-d8 compounds used in a previous bimolecular electron-transfer study in that they are based on an lr(I) A-frame metal dimer chromophore. 5 The reaction driving force for excited state electron transfer in these systems can be controlled by varying the aromatic substituents on their pyridinium electron acceptors. These acceptors are covalently atiached to the terminal phosphinite ligands in the complexes via a three atom hydrocarbon spacer. Because the distance dependence of an electrontransfer reaction is difficult to predict apriori , compounds with short simplistic spacers represent the best starting point for initiating studies surrounding a new intramolecular redox system. The short donor-acceptor separations imposed by this bridging group should insure that electron transfer will occur during the lifetime of the iridium dimer's electronic excited states. 6 In the results section of this chapter a detailed account of the preparation of these compounds is given. The complexes have been characterized by high field proton and phosphorous NMR spectroscopy, IR spectroscopy, and x-ray diffraction crystallography. Results from these studies are subsequently discussed in terms of the synthetic routes reported here, the structural properties of the molecules, and the possible solution dynamics of the bridging group. 29 Experimental Materials Solvents All solvents used in the preparation of the phosphinite redox ligands and iridium dimer complexes were of reagent grade quality or better. Acetonitrile was dried over Linde 4A sieves, vacuum transferred to a Schlenk storage flask, and blanketed with dry nitrogen. Tetrahydrofuran and diethyl ether were distilled from sodium/benzophenone. Methylene chloride was distilled from CaH2 and acetone was distilled from activated alumina. Toluene and ethanol were spectral grade in quality and used as received. THF, ethanol, diethyl ether, methylene chloride, and acetone were degassed by evacuation at 80° C for one hour and then stored under a nitrogen atmosphere. Benzene-d6, acetoned6, DMSO-d6 and acetonitrile-d3 were purchased from MSD Isotopes and used without further purification. ligands Chlorodiphenylphosphine, diphenylethoxyphosphine, diphenylmethoxyphosphine, and butoxydiphenylphosphine were purchased from Strem Chemical Company and used as received. Pyridine, 4-methylpyridine, 4-phenylpyridine, and triethylamine were purchased from Aldrich Chemical Company and were used without further purification. 2-Bromoethanol, also purchased from Aldrich, was vacuum distilled prior to use to remove polymeric impurities. Dimethylamine was purchased in 2 lb lecture bottles from Matheson Company and used as received. 3-isobutyl acetylacetone was obtained from Pfaltz and Bauer Chemical Company and used without further purification. 30 3,5-dimethyl-4-isobutylpyrazole was prepared using a published procedure for the preparation of 3,5-dimethylpyrazole.7 Metal Complexes Iridium trichloride trihydrate was purchased from Johnson Matthey (AESAR). 3,5-Dimethylpyrazole and 1,5-cyclooctadiene were purchased from Aldrich Chemical Company. Carbon monoxide was obtained from Matheson Company. Each was used as received. Synthetic Procedures General Procedures All manipulations involving both the phosphinite ligands and iridium dimer complexes were conducted under a nitrogen atmosphere using standard Schlenk techniques or in a Vacuum Atmospheres Co. dry box. 1Hand 31 P NMR spectra were recorded on either a Jeol FX-90-Q or a Jeol GX-400 fourier transform spectrometer. IR spectra were recorded as fluorolube mulls on a Beckman Instruments IR-4240 Spectrometer. Phosphinite Redox Ligands Preparation of (C5Hs)2P-N(CH3)2: N,N-dimethyl-P,P-diphenylphosphine was synthesized using a procedure similar to that reported for related compounds. 8 300 mis of dry deoxygenated diethyl ether was placed in a 1000 ml round bottom Schlenk flask and charged with 10g (45 mmoles) of chlorodiphenylphosphine. The reaction mixture was cooled to 0° C and dimethylamine was slowly bubbled through cold ethereal solution. Almost immediately, dimethylamine hydrochloride began to form as a flocculent white 31 precipitate. The reaction mixture was stirred and dimethylamine was added until hydrochloride salt formation ceased. The resulting suspension was stirred for an additional hour to ensure that all of the starting materials had been consumed. Dimethylamine hydrochloride was separated by cannula filtration and washed with two 100 ml portions of ether. The washings and filtrate were transferred to a second 1000 ml Schlenk flask and rinsed with three 50 ml portions of deoxygenated distilled water to remove any residual hydrochloride salt, which was complexed to the phosphine product. The ether solution was dried first over anhydrous Li2CO3 for 15 minutes and subsequently stirred over CaH2 for 24 hours. CaH2 was separated by filtration through a fine porosity glass frit. The filtrate was concentrated to one-tenth of its original volume and transferred to a 100 ml round bottom flask fitted with a short path distillation head 9. The residual ether was removed in vacuo leaving a white waxy residue. This crude product was purified by vacuum distillation producing a clear viscous liquid which solidified upon cooling {b.p. 76-85 °C/0.04 mm Hg). The product was characterized by 31 P and 1H NMR spectroscopy. 1H: 2.5{d; NCH3) and 7.3{m; {C5H5)2P); 3 1 P: 64.0 ppm {versus ext. H3PO4). Preparation of 1-(2-hydroxyethyl)pyridinium and (2- hydroxyethyl)triethylammonium Salts: With the exception of 1-{2hydroxyethyl)-2,4,6-trimethylpyridinium tetraphenylborate, all other pyridinium salts were synthesized using a procedure analogous to that reported earlier for preparing 1-alkylpyridinium compounds 6. 1-{2-hydroxyethyl)-2,4,6- trimethylpyridinium tetraphenylborate was prepared from 2-aminoethanol and 2,4,6trimethylpyrylium tosylate by the method of Balaban et al. 10 Each compound was characterized by 1H NMR spectroscopy and their spectra were compared to those reported in the literature.11 32 Preparation of [Ph2POCH2CH2-R](Ph4B): The general methodology outlined below was used in the preparation of all the phosphinite redox ligands, except where noted. In all cases a slight excess of N,N-dimethyl-P,P-diphenylphosphine was used to ensure complete consumption of the hydroxyethylpyridinium starting material. 2.0g (9.0 mmoles} of N,N-dimethyl-P,P-diphenylphosphine and 8.0 mmoles of a 1-(2hydroxyethyl}pyridinium salt were dissolved in approximately 150 mis of oxygen free acetonitrile. 0.1 g of benzoic acid was added and the reaction mixture was stirred for four hours at room temperature under an inert atmosphere. The solvent was removed in vacuo leaving an oily residue, which was rinsed with three 20 ml portions of either diethyl ether or toluene until it began to solidify into a crude powder. With the exception of (Ph2POCH2CH2-4Ph-Py}(Ph4B}, the product was purified by recrystallization from acetonitrile/toluene solutions. All of phosphinite ligands were characterized by elemental analysis, and 1H NMR and 31 P spectroscopy. The NMR spectra are summarized in table 2.3a. (Ph2POCH2CH2-NEt3)(Ph48): Anal. Calcd. for C44H49NOBP: C,81.4; H,7.6; N,2.2. Found: C,80.8; H,7.7; N,2.5. (Ph2POCH2CH2-Py)(Ph48): Anal. Calcd. for C43H39NOBP: C,82.3; H,6.3;N,2.2. Found: C,82.7; H,6.5; N,2.4. (Ph2POCH2CH2-4Me-Py)(Ph48): Anal. Calcd. for C44H41 NOBP: C,82.4; H,6.3;N,2.2. Found: C,82.2; H,6.4; N,2.4. (Ph2POCH2CH2-2,4,6Me3-Py)(Ph4B): Anal. Calcd. for C45H45NOBP: C,82.6; H,6.7;N,2.1. Found: C,82.3; H,6.7; N,2.1. (Ph2POCH2CH2-NEt3)(PF5): Anal. Calcd. for C20H29NOF5P: C,49.5; H,5.97; N,2.88. Found: C,50.1; H,6.2; N,3.1. Preparation of (Ph2POCH2CH2-4Ph-Py)(Ph48): Because this compound had greater propensity to form oils in acetonitrile/toluene solutions, the crude product was 33 rinsed with three 50 ml portions of ethanol containing 0.5 mis of dichloromethane. This procedure yielded a crude powder, which was purified by recrystallization from acetone/diethyl ether solutions. (Ph2POCH2CH2-4Ph-Py)(Ph4B): Anal. Calcd. for C49H33BNOP: C,83.8; H,6.1 ;N,2.0. Found: C, 83.8; H, 6.1; N, 1.9. Inorganic Complexes Starting Materials:Di-µ-chloro-bis(1,5-cyclooctadiene)diiridium(I), cyclooctad ien e) bis (µ-3 ,5-di methylpyrazole )di iridium( I), di methylpyrazole )diirid iu m (I), isobutylpyrazole )diiridiu m(I), bis(1,5- tetracarbonylbis(µ-3, 5- bis( 1 ,5-cyclooctad ie ne) bis(µ-3 ,5-di m eth y 1-4and tetracarbonylbis(µ-3 ,5-dimeth yl-4- isobutylpyrazole)diiridium(I) were prepared according to previously established procedures. 1 2,6 The bis µ-3,5-dimethyl-4-isobutyl pyrazolate bridge complexes had not been previously reported and were characterized by elemental analysis and 1H NM R spectroscopy. Bis{ 1, 5-cycl ooctad ie ne) b is(µ-3 ,5-d i met h yl-4isobutylpyrazole)diiridium(I}: Anal. Calcd. for C25H38N4lr2: C,44.93; H,6.60; N,6.16. Found: C,44.97; H,5.98; N,6.15. lH NMR (d6-Benzene): 6 = 2.6 (m; COD), o = 4.0 (m; COD), o= 4.4 (m, COD), 6 =1.85 (m; COD), 6 = 0.8 (d; isobutyl CH3), o =1.5 (septet; isobutyl -CH), 6 = 2.4 (s; pyrazole CH3), o =1.9 (d; isobutyl CH2). Tetra ca rbo ny Ibis (µ-3 ,5-d i met h yl-4-is ob u ty I pyrazol e )di iridium( I): Anal. Calcd. for C9H14N4O4lr2: C,33.07; H,3.78; N,7.01. Found: C,33.09; H,3.79; N,7.01. 1H NMR (d6-Benzene): o = 2.2 (s; pyrazole CH3), 6 =1.9 (d; isobutyl CH2), o =1.45 (septet; isobutyl -CH), 6 = 0.7 (d; isobutyl CH3). Preparation of the [lr2(Pz*)2(CO)2(Ph2POCH2CH2R)2](Ph4B)2 and [lr2(Pz**)2{CO)2(Ph2POCH2CH2R)2](Ph4B)2 Complexes: The same 34 general procedure outlined for preparing [lr2(Pz*)2(CO)2(Ph2POCH2CH2Py)2](Ph4B)2 was used in the synthesis of all the iridium dimer complexes, except where noted. Each of the compounds was characterized by elemental analysis, 1H NMR and 3 1P NMR spectroscopy. Their proton and 31 P NMR spectra are summarized in table 2.3b. [lr2(Pz*)2(C0)2(Ph2POCH2CH2-Py)2](Ph4B)2(1): 0.1g (0.14mmoles) of lr2(Pz*)2(CO)4 was placed in a 250 round bottom Schlenk flask and dissolved in 150 mis of dry THF. A THF (ca. 50 mis) solution containing 0.30 mmoles of (Ph2POCH2CH2-Py)(Ph4B) was added to the reaction mixture dropwise over the course of approximately 2 to 3 minutes. With each addition of phosphinite ligand, carbon monoxide gas was liberated from the solution. The reaction mixture was stirred for four hours under a nitrogen atmosphere to ensure that all of the reactants had been consumed. During this time period the solution changed color from orange to red/orange. THF was removed in vacuo leaving an orange/red residue, which was taken up in 1O mis of acetonitrile. The solution was chilled to 0° C and toluene was slowly added until the product began to crystallize. [lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py)2](Ph4B)2 was isolated by cannula filtration as a bright red solid. Anal. Calcd. for C9aH92B2N5O4P2lr2: C,62.41; H,4.91; N,4.45. Found: C,62.26; H,5.05; N,4.53. [I r2( P z*)2(C0)2( Ph 2 POCH 2 CH 2-4 Me- Py) 2] ( P h4 B) 2 (2): 0.1 g (0.14mmoles) of lr2(Pz*)2(CO)4 was placed in a 250 ml round bottom Schlenk flask and dissolved in 150 mis of THF. 50 mis of THF containing 0.3 mmoles of (Ph2POCH2CH2-4Me-Py)(Ph4B) was added over a period of 2 to 3 minutes. The reaction mixture was sealed under a nitrogen atmosphere and allowed to stand undisturbed for 48 hours during which time the product formed as bright red crystals. The product was separated by cannula filtration and washed with three 20 ml portions of 35 THF. Anal. Calcd. for C1 ooH9sB2NsO4P2lr2: C,63.40; H,5.32; N,4.26. Found: C,63.63; H,5.37; N,4.30. lr2 (Pz*)2(C0)2( P h2 POCH 2C H 3)2(3): 0.1 g(0.14mmoles) of lr2 (Pz*)2 (CO )4 was placed in a 250 round bottom Schlenk flask and dissolved in 150 mis of THF. 0.1 ml (0.7 mmoles) of Ph2POCH2CH3 was added via a gas tight syringe. An immediate liberation of carbon monoxide gas was observed followed by a change in solution color from orange to orange/red. The reaction mixture was stirred for four hours to ensure complete consumption of the starting materials. THF was removed in vacuo leaving an orange/red residue, which was taken up in 20 mis of 2:1 ethanol/dichloromethane. Slow evaporation of this solution under a steady stream of nitrogen afforded the product as bright orange crystals. Anal. Calcd. for C40H44N4O2P2lr2: C,44.04; H,4.03; N,5.13. Found: C,43.74; H,4.08; N,5.06. [lr2(Pz*)2(C0)2(Ph2POCH2CH2-NEt3)2](PF5)2(4): This complex was prepared using the same procedure as that outlined for complex 1 except that a slightly different isolation procedure was employed. After the solvent was removed from the reaction mixture, the orange/red residue was rinsed with three 20 ml portions of ethanol containing 0.1 ml of dichloromethane. This produced a crude solid, which was purified by soxholet extraction into dichloromethane. Anal. Calcd. for C52H72F12NsO4P4lr2: C,39.5; H,5.46; N,5.323. Found: C,39.32; H,5.16; N,4.58. The following complexes were prepared using the procedure outlined for complex 1. [lr2( Pz *)2(CO) 2( Ph 2 POCH 2 CH 2-2,4 ,6Me3-Py)2]( Ph4 B )2 (5): Calcd. for C104H104B2NsO4P2lr2: C,63.40; H,5.32; N,4.26. Anal. Found: C,63.63; H,5.37; N,4.30. [lr2(Pz*)2(C0)2(P h2 POCH 2 CH 2-4Ph-Py)2](Ph4 8)2(6): Anal. Calcd. for C 11 oH 1ooB2NsO4P2lr2: C,64.62; H,4.95; N,4.10. Found: C,63.52; H,4.96; N,3.94. 36 [lr2(Pz*)2(CO)2(Ph2POCH2CH2-NEt3)2](Ph48)2(7): Anal. Calcd. for C1 ooH112B2NsO4P2lr2: C,62.23; H,5.85; N,4.35. Found: C,62.3; H,5.70; N,4.37. The following complexes were prepared using the procedure outline for complex 3. lr2(Pz*)2(CO)2(Ph2POCH2CH2CH2CH3)2(8): Anal. Calcd. for C44H50N4O4P2lr2: C,46.15; H,4.40; N,4.88. Found: C,46.06; H,4.53; N,4.87. lr2(Pz*)2(C0)2(Ph2POCH3)2(9): Anal. Calcd. for C3aH40N4O4P2lr2: C,42.94; H,3.76; N,5.27. Found: C,43.10; H,3.83; N,5.22. lr2(Pz*)2(C0)2(Ph3P)3•CH2Cl2(10): Anal. Calcd. for C49H4sCl2N4O4P2lr2: C,46.27; H,3.62; N,4.41. Found: C,47.50; H,3.95; N,5.38. [I r2(Pz**)2(CO)2( Ph2 POCH 2CH 2-Py)2](Ph4 8)2(11 ): Anal. Calcd. for C1 osH1 oaB2NsO4P2lr2: C,63.60; H,5.69; N,4.20. Found: C,63.58; H,5.36; N,4.23. [I r2( Pz**)2(CO)2(Ph2 POCH 2CH 2-4Me-Py)2]( Ph4 8)2(12): Anal. Calcd. for C1 oaH112B2NsO4P2lr2: C,64.02; H,5.57; N,4.14. Found: C,63.52; H,5.60; N,4.13. [I r2( Pz**)2(CO)2 (Ph 2 POCH2CH2-4 Ph-Py)2](Ph4 8 )2(13): Anal. Calcd. for C11aH11sB2NsO4P2lr2: C,65.94; H,5.40; N,3.91. Found: C,65.80; H,5.59; N,3.55. X-Ray Crystallography Crystals of [lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py)2](Ph4B)2 suitable for xray diffraction studies were grown from DMF solutions layered with dry diethyl ether. An irregularly shaped crystal with approximate dimensions 0.1 0x0.40x0. 70 mm was sealed inside a glass capillary tube under an inert atmosphere and centered on a CAD4 diffractometer equipped with graphite-monochromated MoKa radiation for intensity data collection. Unit cell dimensions were obtained from the angle settings of 25 reflections 37 with 25°< 20<31°. The space group P21/c was chosen based on the systematic absences hOI (1=2n+ 1) and OkO (k=2n+ 1). Three data sets were collected in the quadrants h,k,I; -h,-k,-I; h,-k,I; and -h,k,-I with a total of 18453 reflections and were successfully merged into a final data set having 8019 independent reflections. Check reflections were monitored every 10000 seconds to record crystal decay; only minor variations were observed and the data were corrected accordingly. The data were corrected for absorption, Lorentz, and polarization effects. An absorption coefficient of 26 cm-1 was found to yield the best correction of both the high and low intensity reflections. Special considerations were given to correcting individual reflections for background x-ray scatter. The unusually long c axis of this particular . monoclinic lattice lead to partial overlap between adjacent reflections in approximately 1/3 of the data and precluded the use of standard correction procedures. To circumvent this problem an "average" background correction factor was calculated from the remaining well resolved reflections and was applied to the entire data set. Because systematic errors arising from this procedure should affect the low intensity data to the greatest extent, least squares refinements were carried out, which included and excluded the low intensity reflections. The minor differences observed between values of R, the GOF, and the atomic coordinates for each of these different refinements were taken to indicate that this background correction procedure was valid. Initial coordinates for the four iridium atoms were determined by interpreting Patterson maps. The remaining non-hydrogen atoms were located from structure-factor Fourier calculations. The iridium atoms were given anisotropic thermal factors, while the remaining atoms were treated isotropically. Several cycles of least squares refinement lead to convergence with R of 0.0697 for 7212 reflections. For the strong 38 Table Z.la Coordinates ( x 104 ) z, y, z and U,. 9 a X 104 Atom z y z U,. 9 orB m1 m2 Pl P2 Cl 01 C2 02 NDlA NDlB CDlA CDlB CDlC CDlD CDlE ND2A ND2B CD2A CD2B CD2C CD2D CD2E OAl CAl CBl NPl CPlA CPlB CPlC CPlD CPlE OA2 CA2 CB2 NP2 CP2A CP2B CP2C CP2D CP2E CBlA CBlB CBlC CBlD CBlE 3025 .4l 2296 .4 1995 3001 4015 10) 4683 7) 1031 394 .3~ 580 .3 -267 2 -269 2 -443 9 -1023 6 193 9 -102 7 1382 6 1476 6 2069 8 2192 9 2592 9 2165 10~ 2480 10 1102 6 1144 6 1553 9 1669 8 1918 9 1665 10) 1872 49 477 697 10) -68 -458 -1195 10 -1545 11 -1133 11 -410 10 123 5 618 8 824 8 37 7 -251 9 -1032 -1465 11 -1159 10 -414 -1433 -1836 10 -2756 13 -3199 12 -2893 13 7838 .l~ 7152 .1 8137 1 6832 1 7894 3 7906 2 7085 3 7030 2 7719 2 7427 2 7840 3 7370 3 7631 3 8163 3 7068 4 7587 2 7298 2 7669 3 7200 3 7417 3 7976 4 6878 3 8476 2 8585 3 8896 3 9093 2 9179 3 9344 3 9414 4 9328 4 9170 4 6501 2 6405 3 6079 3 5897 2 5826 3 5675 4 5596 4 5657 3 5813 3 8182 3 8451 4 8475 4 8228 5 317 1 311 1 355 9 327 8 4.2 3 • 5.9 2 • 4.7 3 • 6.7 3 • 3.1 2 • 2.7 2 • 3.6 3 • 4.1 3 • 4.8 3 • 6.0 4 • 6.4 4 • 2.8 2 • 2.8 2 • 4.3 3 • 3.9 3 • 4.8 3 • 6.8 4 • 5.3 3 • 3.8 2 • 5.5 3 • 5.8 4 • 4.2 2 • 4.8 3 • 5.9 4 • 7.4 4 • 7.2 4 • 6.2 4 • 3.1 2 • 3.4 3 • 4.3 3 • 3.7 2 • 5.2 3 • 6.7 4 • 6.9 4 • 6.0 4 • 4.8 3 • 3.9 3 • 6.2 4 • 8.5 5 • 8.1 5 • 8.8 5. ~~ 66 1937 1694 1443 1078 918 10 1578 11 745 12 4052 3791 4977 10 4556 10 5311 11 5457 12 4469 11 2182 6) 3234 11~ 3153 11 3277 8) 2385 10 2481 12 3516 14 4397 13 4283 12 2921 1929 2015 10) 1829 8) 786 11 617 13 1479 13 2517 12 2689 10 2208 10 2420 12 2593 14 2484 14 2243 14 1r 10! H :~ il !~ ll :~ 7949 5 39 Table 2.la (Cont.) Atom z z y CBlF 2148 12) -1969 CB2A 506 -204 CB2B -137 -369 CB2C -1280 -392 CB2D -1779 11 -213 CB2E -1155 12 -27 10) CB2F 15 10 -4 CB3A 2407 9) -1340 CB3B 2446 10 -1759 CB3C 1961 11 -2580 CB3D 1502 12 -2976 10 CB3E 1481 11 -2629 10 CB3F 1921 10 -1773 8 CB4A 4449 9) -517 8 5150 10 CB4B -460 8 CB4C 6248 11 -682 9 CB4D 6670 12 -916 10~ CB4E 6020 13 -987 10 CB4F 4889 11 -770 9 Bl 1474 10 741 9 CHA 2199 734 8 CllB 2666 1493 8 CllC 3360 1468 9 CUD 3656 10 692 9 CUE 3243 10 -73 9 CllF 2519 10 -54 8 C12A 2420 9) 678 8 C12B 2854 10 -113 9 3651 12 C12C -204 C12D 4058 12 562 12 C12E 3748 13 1339 11 C12F 2911 11 1421 10 C13A 620 9) -79 Cl3B -19 10 -285 C13C -771 12 -985 lOl -914 11 -1470 10 C13D C13E -352 11 -1275 C13F 395 10 -569 Cl4A 707 9) 1623 379 11 Cl4B 1903 Cl4C -355 12 2618 Cl4D -707 12 3036 11 C14E -465 12 2790 10 Cl4F 265 11 2054 15()7 11 B2 871 :l lOl Ir !llOl ~~ lOl lOl !l ~l ll ~~ 7940 4 8078 3 8299 3 8239 3 7974 3 7757 3 7805 3 6765 3 6500 3 6439 3 6658 4 6923 3 6975 3 6883 3 6669 3 6731 3 7004 4 7221 4 7155 3 694 3 416 3 311 3 91 3 -16 3 78 3 296 3 972 3 1081 3 1316 4 1434 4 1344 4 1107 3 680 3 421 3 400 4 631 4 900 3 917 3 708 3 971 3 981 4 737 4 469 4 462 3 4313 3 Ueq orB 6.8 4 • 2.7 3 • 3.8 3 • 4.6 3 • 5.4 3 • 5.8 4 • 4.2 3 • 3.0 3 • 4.1 3 • 5.6 4 • 5.9 4 • 5.6 4 • 4.2 3 • 3.1 3 • 4.2 3 • 5.2 3 • 5.7 4 • 6.8 4 • 5.0 3 • 2.6 3 • 3.0 3 • 3.2 3 • 4.4 3 • 4.5 3 • 4.8 3 • 4.0 3 • 3.4 3 • 4.3 3 • 6.1 4 • 7.2 4 • 7.1 4 • 5.5 4 • 3.0 3 • 4.7 3 • 6.2 4 • 5.8 4 • 5.2 3 • 4.1 3 • 3.2 3 • 4.8 3 * 6.7 4 • 6.7 4 • 6.2 4 • 4.9 3 • S.3 3 • 40 Table 2.la (Cont.) z Atom % 11 C21A C21B C21C C21D C21E C21F C22A C22B C22C C22D C22E C22F C23A C23B C23C C23D C23E C23F C24A C24B C24C C24D C24E C24F 4306 9) 4633 10 5348 12 5715 12 5460 12 4764 10 2814 9) 2354 10 1648 10 1375 10 1796 11 2509 10 2546 2020 1131 689 1137 10 2033 10 4320 4978 5701 11 5763 12 5155 11 4448 9) 1550 1879 2614 2993 10 2690 10 1948 9 645 8 1401 8 1387 9 611 9 -149 9 -148 9 731 8 18 46 8 864 8 1599 8 1514 8 -178 7 -436 9 -1160 -1614 10 -1389 -640 ~l lOl ~bl • Ueq ~l lOl lOl ~~ Ueq 4312 3 4047 3 4052 4 4302 4 4560 4 4561 3 4596 3 4704 3 4920 3 5032 3 4937 3 4718 3 4044 3 3901 3 3684 3 3605 3 3750 3 3955 3 4302 3 4553 3 4552 3 4308 4 4053 3 4053 3 orB 3.4 3 * 4.7 3 * 5.9 4 * 6.0 4 * 6.2 4 * 4.4 3 * 3.2 3 * 3.7 3 * 4.5 3 * 5.0 3 * 5.4 4 * 4.5 3 * 3.1 3 * 3.1 3 * 3.4 3 * 4.1 3 * 4.1 3 * 3.7 3 * 2.9 3 * 4.7 3 * 5.5 4 * 6.1 4 * 5.0 3 * 3.3 3 * = ½E, E;[U,;(a;a;)(~ · i;)] •Isotropic displacement pa.rameter, B Table 2.lb Anlstroplc Temperature Factors Atom Uu IRl IR2 Pl P2 U22 Uas Un 34Tl 33Tl 281P -•Ti 345 3 .COl 20l 353 19 310 3 350 21i 322 20 284 4 322 23l 311 22 The form of the displacement factor is: 31 3 -64 17~ 17 16 U1s U2s 62 2 82ri -17 3 74 18 -11 isl 1116l 6116 3Tl exp-2,r 2 (Uuh 2 a•• + U22k 2b•• + Uul2 c•• + 2U12hka•b• + 2U1ahla•c• + 2U23 klb•c•) 41 Table 2.2: Final Crystallographic Parameters Mol. Wt. 1828.98 Cell Dimensions a, A 12.189(1) b, A 15.298(2) C, A 46.372(6) a,deg.90 ~, deg. 90.406(10) y, deg. 90 v, A3 8646(7) 24 p, g/cm3 1.41 µ, cm-1 26.0 Crystal Dimensions 0.1 x 0.4 x O. 7 mm Scan Width ro scans (1 °) No. of Observed Reflections Total 8019 > 0 7212 >3<J 6131 No. of Parameters Varied 477 R 0.0697 R3cr 0.055 GOF2.32 Space Group P21 /c 42 data with F0 2>3crF 0 2, R = 0.055 for 6131 reflections. The goodness of fit was 2.32 for n = 8019 data and P= 477 parameters. Atomic coordinates and final unit cell parameters are found in tables 2.1 and 2.2. Calculational Details Molecular mechanics calculations were carried out on [lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py)2](Ph4B)2 using Biograf Ill version 1.40. 1 3 Initial atom positions were taken from its crystallographic coordinates. Energy minimizations for the three ligand rotational conformations seen in figure 2.16 were carried out using a Dreiding force field, which had been modified to reproduce the experimentally observed bond distances and angles. In each case the coordinates of the Ir, P, CO, and pyrazolate nitrogen atoms were held fixed to preserve their coordination geometries. All calculations were carried out in the absence of atom charges, counterions, and solvent molecules in an extended atom approximation. Rigid geometry conformational mapping involving the four torsional angles seen in figure 2.15 was carried out using procedures described previously by Karplus et a/. 14 . It was found that conformations in which <l>B1 and <l>B2 are both acute were physically impossible. Thus, calculations were carried out in which <l>B1 (cl>B2) was held at 180° and cl>B2(<l>B 1) was varied between 60° and 300° in 120° increments. For pairs of values in <l>B1 and cl>B2, <l>B3 varied between 60° and 300° in 120° increments and <1>B4 varied between 90° and 270° in 30° increments. The van der Waals energy of each conformation was determined using a Lennard-Jones 6-12 potential function of the form 43 E = L r (RJ.. j 2 IJ vdW Ri {RcJ[ Rij ] [ (R * -2 J · .]61 I J Rij (DJ J ij where Do is the van der Waals bond energy in Kcal/mole, Ro is the van der Waals bond length in A and R is the interatomic distance in A. Only those interactions within an interval less that a cutoff distance (Rcut) of 9.0 A were included in this sum. This procedure was repeated for each of the three different lr-P conformations. 44 Results Phosphinite Redox ligands The two step reaction sequence outlined in figure 2.3 was used to synthesize a series of redox phosphinite ligands with different electrochemical reduction potentials (figure 2.2). A number of 1-(2-hydroxyethyl)pyridinium salts were prepared in good yield by reacting pyridine or a para-substituted pyridine with 2-bromoethanol using a previously established procedure (reaction 1a.). 6 1-(2-hydroxyethyl)-2,4,6- trimethylpyridinium tosylate was synthesized using a ring open reaction between a corresponding pyrylium cation and (2-hydroxyethyl)amine, due to the poor nucleophilicity of ortho disubstituted pyridines (reaction 1b.).1 Oa,b Each of the pyridinium salts was metathesized to its corresponding tetraphenylborate salt to enhance the solubility of the phosphinite ligands and iridium dimer complexes in organic solvents. 1H NMR spectra of these compounds compared favorably with spectra found in the literature.11 Step two involved the alcoholysis of N,N-dimethyl-P,P-diphenylphosphine by a 1-(hydroxyethyl)pyridinium compound to produce its corresponding phosphinite ligand (reaction 2.). 15 This transformation was carried out at room temperature in approximately 90% isolated yield by catalyzing the reaction with a small amount of benzoic acid. This approach avoided the higher reaction temperatures and forcing conditions associated with more traditional methods of preparing alkoxy diphenylphosphines and minimized the possibility of Arbuzov rearrangements 15 and side reactions involving the pyridinium cations. 16 Each of the redox ligands prepared in this manner was characterized by 1Hand 31 P NMR spectroscopy. These data are summarized in table 2.3a. 45 Figure 2.2: Structures of the phosphinite redox ligands and their abbreviations. 46 47 Figure 2.3: Synthetic scheme for preparing the phosphinite redox ligands. 48 R HOCH,cH,er + () r/R A • IIOCl½CJ½Nt~ Br" (Reaction 1a.) HNllez (Reaction 2) 49 Table 2.3a: 1H and 31 P NMR Data for Phosphinite Redox Ligands Compound 6/ppma Assignment 1H (Ph2POCH2CH2-Py)(Ph4B) (Ph2POCH2CH2-4-MePy)(Ph4B) Ph2P-Py (ortho H) -Py (para H) -Py (meta H) -OCHaCH2-OCH2CH2- -Py (ortho H) -Py (meta H) -OCHaCH2-OCH2CH2(Ph2POCH2CH2-4-PhPy}(Ph4B) (Ph2POCH2CH2-246Me3-Py)(Ph4B) (Ph2POCH2CH2-NEt3)(Ph4B) (Ph2POCH2CH2-NEt3}(PFs) 116. 1 9.00(d) 8.50(t) 8.00(t) 4.28(dt) 4.84(t) 115.6 Ph2P-Py-CH3 2.S0(s) 8. 70(d) 7. 75(d) 4.25(dt) 4.69(t) 115.1. Ph2P-Py (ortho H) -Py (meta H) -Ph (ortho H) -Ph (meta H) -Ph (para H) -OCHaCH2-OCH2CH2- 8.1 0(d) 7.75(d) 7.80(d) 7.55(m) 7.55(m) 4.20(dt) 4.S0(t) Ph2P-Py (meta H) -Py (ortho CH3) -Py (para CH3) -OCHaCH2-OCH2CH2- 7.S0(s) 2.65(s) 2.40(s) 4.25(dt) 4.69(t) Ph2P-OCHaCH2-OCH2CH2(CH3CH2)(CH3CH2)- 3.52(dt) 4.12(t) 1 .20(t) 3.30(q) Ph2PPF6 -OCH2CH2-OCH2CH2(CH3CH2)(CH3CH2}- 31pb 117.8 11 6. 1 118.5 -143.5 3. 77(dt) 4.27(t} 1.29(t} 3.53(q} a. Spectra were measured in d6-DMSO. b. Chemical shifts are versus ext.H3PO4. 50 NMR spectra of the ligands show proton chemical shifts and multiplicities for the pyridinium aromatic protons and ring substituents, which are similar to those seen in spectra of their 1-(2-hydroxyethyl)pyridinium precursors. The phosphine phenyl groups show up as a pair of complex multiplets at o = 7.6 and o = 7.3 ppm. Notably, the p-methylene protons appear as a doublet of triplets at 4.2 ppm due to protonphosphorous coupling (JPH = 6.50 Hz). In addition, 31 P NMR spectra show ligand phosphorous chemical shifts ranging from o = 115.1 to o = 118.5 ppm. These spectroscopic data identify the ligands as esters of diphenylphosphinous acid. 17 Iridium Dimer Preparation Synthetic Methodology: The series of iridium(I) donor-acceptor complexes and corresponding model compounds reported in this study were prepared using procedures similar to those described previously for a number of analogous rhodium 18 and iridium 19 pyrazolate bridge compounds. Initial attempts at preparing redox phosphinite complexes of diiridium bis(µ-pyrazolate) using the two step method of Stobart and coworkers (figure 2.4) were complicated by the propensity of the products to form as intractable oils. This oil formation was attributed to small amounts of cyclooctadiene released during the preparation of the tetracarbonyl intermediate, lr2(Pz)2(CO)4 (reaction 3a.). lr2(Pz)2(CO)4 could not be isolated and purified for use as a primary starting material because it percipitates from concentrated solutions as a black intractable polymer. However, Nussbaum et ai. 20 had reported a procedure for preparing its bis µ-3,5-dimethylpyrazolate bridge analog in good overall yield. By reacting pure lr2(Pz*)2(CO)4 with two equivalents of a phosphinite ligand of choice, its corresponding bis carbonyl, bis phosphinite iridium dimer was isolated as an orange/red crystalline solid in approximately 90% overall yield (reaction 4.). This 51 Figure 2.4: Synthetic scheme for preparing lr2(Pz*)2(C0)2(Ph2POCH2CH2R)2 complexes. 52 (Reaction 3a.) 2CO '-.J 2CO '-.J 53 approach proved to be general for a variety of phosphine ligands and permitted the preparation of a complete series of donor-acceptor molecules and their corresponding model complexes.21 The donor-acceptor complexes based on the 3,5-dimethylpyrazole bridge framework proved to be soluble in most polar organic solvents, but were fairly insoluble in lower polarity solvents such as dichloromethane. Because some of the electrochemical and spectroscopic studies discussed in later chapters had to be carried out in less polar organic solvents, a second series of compounds was prepared containing 3,5-dimethyl-4-isobutylpyrazole ligands. Each of the complexes was characterized by 1 Hand 31 P NMR spectroscopy. These data are summarized in table 2.3b. lH NMR: Unlike d8-d8 A-frame compounds containing identical terminal ligands, the carbon monoxide phosphine complexes can exist as cis and trans structural isomers as seen in figure 2.5. Based on x-ray crystallographic data available for a number of rhodium and iridium pyrazolate bridge systems, 22 the trans substitutional isomer is favored in the solid state. In solution, however, these two entities could exist in dynamic equilibrium. While Stobart and coworkers have reported that lr2(Pz}2(C0)2(Ph3P)2 exists as its trans isomer in solution and in the solid state, 19 evidence for the presence of both the cis and trans isomers of Rh2(Pz*)2(C0}2(Ph3P)2 has been observed in its proton NMR spectra.2 3 The structural and electronic similarities between these two complexes and the donor-acceptor compounds prepared in this study suggest that structural isomerism should be considered in assigning their NMR spectra. Previous studies have indicated that the bridge substituent magnetic environments in d8-d 8 Aframe complexes are perturbed by the nature and disposition of the dimer's terminal ligands.23, 12, 19 Thus, a careful analysis of the signals due to the pyrazole methyl 54 Table 2.3b: 1 H and 31 P NMR Data for Iridium Dimer Complexes Compound Assignment o(ppm) [lr2(Pz*)2(CO)2(Ph2POCH2CH 2-Py)2b (Ph48)2d op = 97.79° Pz*-CHaA Pz*-CHas Pz*-H -OCH2CH2-OCH~H2- 1.4(s) 2.2(s) 5.35(5) 5.1(m) 4.9(m) 4.3(m) 9.3(d) 9.6(t) 9.2(t) 7.85(d,d) 7.6(d,d) 7.45(t) 7.15(m) 7.35(t) 7.15(m) Py ortho Py para Py meta Ph 2P- ortho Ph~- para Ph 2P- meta lr2(Pz*) 2(CO)2(Ph 2POCH 2CH 2-4CH 3 -Py) 2b · (Ph48)2 op= 100.8 Pz*-CHaA Pz*-CHas Pz*-H -OCH2CH2-OCH~H2- Py ortho Py meta Py-CH3 Ph 2P- ortho Ph2P- meta Ph~- para lr2(Pz*) 2(CO) 2(Ph 2POCH 2CH 2-4Ph-Py) 2b (Ph48)2 op= 99.04 Pz*-CHaA Pz*-CHas Pz*-H -OCH2CHr -OCH2CH2- Py-Ph ortho Py-Ph meta Py-Ph ortho Py-Ph para+meta Ph 2 P- ortho 1.42(s) 2.2(s) 5.4(s) 5.0(m) 4.9(m) 4.25(m) 9.1(d) 8.0(d) 2.6(s) 7.6(d,d) 7.85(d,d) 7.35(t) 7.15(t) 7.45(t) 7.22(t) 2.2(s) 1.4(s) 5.35(s) 5.2(m) 4.4(m) 5.0(m) 9.4(d) 8.65(d) 8.1 ( d) 7.65(m) 7.9(d,d) 7. 65(d,d) 55 Table 2.3b: (cont.) Compound Assignment 6(ppm) Ph 2 P- meta 7.55(d,d) 7.1(d,d) 7.4(d,d) 7.2(d,d) PhaP- para lr2(Pz*)2(CO)2(Ph2POCH2CH2-2,4,6Mea-PY)2a (Ph4B)2 6p = 95.20 Pz*-CH3A Pz*-CH 38 Pz*-H Py-CH3 ortho Py-CH3 para Py-H Ph 2 P- ortho Ph 2 P- para Ph 2 P- meta lr2(Pz*) 2(CO) 2(Ph 2POCH 2CH 2-NEta)2b (Ph4B)2 6p = 104.29 Pz*-CH3A Pz*-CH3s Pz*-H -OCH2CH2-OCHaCH2· -N(CH2CH3)a -N(CHaCHab Ph 2 P- ortho Ph 2 P- para Ph 2 P- meta lr2(Pz*'2(CO) 2(Ph2POCH 3 ) 2c (Ph4B)2 6p = 100.82 Pz*-CH3A Pz*-CH 38 Pz*-H P-O-CH3 Ph 2 P- ortho PhaP- para Ph2 P- meta lr2(Pz*) 2(CO'2(Ph 2POCH 2CH 3 ) 2c (Ph4B)2 6p = 96.15 Pz*-CH3A Pz*-CH3s Pz*-H 1.6(s) 2.25(s) 5.5(s) 2.67(s) 2.45(s) 7.62(s) 7.S(d,d) 7.6(d,d) 7.48 (t) 7 .30(t) 7. 4 (t) 7 .22(t) 1.85(s) 2.3(s) 5.65(s) 3.55(m) 3.65(m) 4.35(m) 4.45(m) 1.2(t) 3.3(q) 8.1(d,d) 7.S(d,d) 7.4 (t) 7.2(m) 7 .3(t) 7.2(m) 2.45(s) 2.10(s) 5.6(s) 3.S(d) 8.2(m) 7.0(t) 6.95(t) 7.1(m) 2.05(s) 2.55(s) 5.60(t) 56 Table 2.3b: (cont.) Compound Assignment 6(ppm) -OCH2CH3 -OCH.aCHa 1.3(t) 4.31 (m) 4.55(m) 8.23(d,d) 8.30(d,d) 6.95(t) 7 .04(t) 7.2(m) Ph 2P- ortho Ph2P- para Ph2P- meta lr2(Pz*)2(CO)2(Ph2POCH2CH2CH2CHa)2c (Ph48)2 6p = 96.11 Pz*-CH3A Pz*-CH3s Pz*-H OCH~H~~CH3 OCH~H2CH_aCHa OCH2CHaCH2CHa OCHaCH~H2CHa Ph 2P- ortho PhaP- para Ph2P- meta lr2(Pz*)2(CO)2(Ph2POCH2CH2-NEta)28 r-(PFs)2 6p = 100.5 Pz*-CH3A Pz*-CH3s Pz*-H -OCH2CH2- -OCH2CH2-N(CH2CH3)a -N(CHaCHa)a Ph 2P- ortho PhaP- para Ph 2P- meta lr2(Pz**)2(CO)2(Ph2POCH2CH2-PY)28 (Ph48)2 6p = 99.17 Pz**-CH2CH( CH3 )2 Pz**-CH2 CH( CHa)2 Pz**-CH2CH(CHa)2 Pz*-CH3A 2.2(s) 2.S(s) 5.6(s) 0.87(t) 1 .44(sextet) 1.68(d,quintet) 4.1 S(m) 4.56(m) 8.37(d,d) 8.23(d,d) 6.95(t) 7.0S(t) 7.15(m) 2.0(s) 2.4(s) 5.55(s) 3.45(m) 3.49(m) 4.6(m) 4.45(m) 1 .3(t) 3.3(q) 8.1 S(d,d) 7.95(d,d) 7.53(t) 7.65(m) 7.47(t) 7.65(m) 0.54(d) 0.39(d) 1 .0S(septet) 1.72(d) 1.S(s) 57 Table 2.3b: (cont.) Compound Assignment 6(ppm) Pz*-CH3s 2.2(s) 4.9(m) 4.41(m) 5.02(m) 7.77(d,d) 7.61 (d,d) 7.34(t) 7.14(t) 7 .08(t) 7.25(t) 8.68(d) 8.45(t) 8.20(t) -OCH2CH2-OCHaCH2- Ph 2 P- ortho PhaP- para Ph 2 P- meta Py- ortho Py- para Py- meta lr2(Pz**)2(CO)2(Ph2POCH2CH2-4Me-Py)28 (Ph4B)2 6p = 99.54 Pz**-CH2CH( CH3 )2 Pz**-CH2CH(CH3)2 Pz**-CH2CH (CH3)2 -OCH2CH2-OCHaCH2- Ph 2 P- ortho Ph 2 P- para Ph 2 P- meta Py-H ortho Py-H meta lr2(Pz**)2(CO)2(Ph2POCH2CH2-4Ph-Py) 28 (Ph48)2 6p = 99.34 Pz**-CH2CH( CH3 )2 Pz**-CH2CH(CH3)2 Pz**-CH2CH(CH3)2 Pz*-CH3A Pz*-CH3s -OCH2CH2-OCHaCH2Ph 2 P- ortho PhaP- para Ph 2 P- meta Py-Ph ortho Py-Ph meta Py-Ph ortho Py-Ph para+meta 0.63(d,d) 1.2(octet) · 1.87(d) 4.6(m) 5.0(m) 4.4(m) 7.89(d,d) 7.77(d,d) 7.47(t) 7.26(m) 7.42(m) 8.23(d) 7.63(d) 0.6(d) 0.65(d) 1.2(m) 1.9(t) 1.6(s) 2.2(s) 4.9(m) 4.6(m) 5.2(m) 8.0(d,d) 7.B(d,d) 7.45(d,d) 7.40(d,d) 7.2(m) 8.55(d) 8.15(d) 7.9(d) 7. 7(m) 58 Table 2.3b: (cont.) Compound I r2 (Pz*) 2 ( CO) 2 (Ph 3 '2c op = 17.56 Assignment o(ppm) Pz*-CH3A Pz*-CH 38 1.8(s) 2.6(s) 5.6(s) 7.8(m) 7.0(s) Pz*-H Ph 3 P ortho Ph 3 P meta+para a. d3-Acetonitrile. b. d6-DMSO c. d6-Benzene. d. Signals for the tetraphenylborate counterions appeared at 6.9(t), 7.0(t), and 7.2(m). e. Chemical shifts are referenced to external H3 P0 4 _ 59 Figure 2.5: Molecular structures for the cis and trans isomers of lr·1(Pz*)2(C0)2(Ph2POCH2CH3)2. The bridging pyrazole substituents are labeled according to their magnetic environments. 60 Trans Cis 61 Figure 2.6: a) 1H NMR spectrum of lr2(Pz*)2(CO)4 in d6-benzene; o = 2.13 ppm (s; pyrazole CH3), o = 5.60 ppm (s; pyrazole C-4 H). b) 1H NMR spectrum of * . lr2(Pz )2(CO)2(Ph2POCH2CH3)2 in d6-benzene. 62 a l ' b . '- I 9 I 8 I 1 I I I 5 I • I J I 2 I I 1 63 groups and C-4 protons in our complexes should disclose their solution coordination geometry. In addition to containing a number of multiplets for the phenyl and ethoxy substituents on the phosphinite ligands, the 1H NMR spectrum of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 (figure 2.6b) shows three signals at o = 5.60, 8 = 2.50, and 8 = 2.05 ppm (integration 1:3:3), which are assigned to the pyrazole C-4 proton and the two pair of inequivalent bridge methyl groups respectively. These spectral data indicate that a solitary isomer of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 is present in solution. This conclusion is substantiated by the presence of a single pair of singlet bands for the pyrazole methyl groups. Spectra of a mixture would show at least four bands for the magnetically inequivalent methyl groups in the cis and trans isomers. The unique singlet band assigned to pyrazole C-4 protons suggests that this complex exists as its trans isomer. In the corresponding cis isomer these protons occupy magnetically inequivalent C-4 sites (figure 2.5). In addition, based on the assignments made for the pyrazolate substituents in lr2(Pz*)2(CO)4 (figure 2.5a), the chemical shifts of the pyrazole methyl groups in lr2(Pz*)2(CO)2{Ph2POCH2CH3)2 can be assigned according to the labeling scheme seen in figure 2.7. Due to the trans configuration of its terminal ligands, lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 lacks internal mirror symmetry. As prepared, this compound is, presumably, a racemic mixture of its two enantiomers. The chiral nature of the complex is manifested in its NMR spectra through the diastereotopic chemical shifts observed for magnetically inequivalent protons bound to the phosphinite ligand alkoxy and phenyl substituents. Figure 2.8a shows the aromatic region of a 400 MHz 1H spectrum of lr2(Pz*}2(CO)2(Ph2POCH2CH3)2 taken in d6-benzene. The 64 Figure 2.7: Chemical shift assignments for the bridging pyrazole substituents in I r2(Pz ")2(C0)2(Ph2POC H 2C H3)2. 65 O(a)= 2.05 ppm O(b)= 2.50 ppm O(c)= 5.60 ppm 66 Figure 2.8: a) 1H NMR spectrum of lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 showing diasteriotopic chemical shifts for the phosphinite phenyl substituents. b) Diasteriotopic chemical shifts for the ethoxy group methylene protons. 67 a ,.s e.o 7.0 7.5 b 1.0 •. o .1.s 68 phenyl group ortho protons appear as a pair of closely spaced doublets at o = 8.23 and o = 8.30 ppm (JpH = 6.25 Hz) along with a pair of slightly second order triplets at o = 6.95 and o = 7.04 ppm assigned to pairs of magnetically distinct para protons. The meta proton chemical shifts are not affected as much by the magnetic inequivalence of the ligand phenyl groups and are observed as a complex multiplet at o = 7.2 ppm. A similar effect is seen in the phosphine ethoxy group p-methylene protons (figure 2.8b.), which appear as a pair of complex multiplets at 01 = 4.30 and 02 = 4.55 ppm. The spectroscopic properties of all of the donor-acceptor complexes reported here are similar to those seen in the lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 (table 2.3b). The proton NMR spectra of [lr2(Pz*)2(C0)2(Ph2POCH2CH2-2,4,6 Me3Py)2](Ph4B)2 in d3-acetonitrile, seen in figures 2.9-2.10, exemplify those spectral features common to this series of compounds. The singlet bands at o= 1.6, o= 2.25, and o = 5.5 ppm, which are assigned to the pyrazole bridge methyl groups and C-4 proton, establish that this complex exists in solution as its trans isomer. This conclusion is further substantiated by the diastereotopic doubling of bands due to protons on the phosphinite ligand phenyl and alkoxy substituents. Figure 2.1 Oa shows the aromatic region of this compound's 400 MHz NMR spectrum. The pair of overlapping doublets at o = 7.8 and o = 7.6 ppm are assigned to phosphinite ligand ortho protons. Signals for the corresponding meta and para protons appear as pairs of second order tiplets at O= 7.4 , o = 7.22 ppm and at o= 7.48, o= 7.30 ppm respectively. Figure 2.1 Ob shows signals for the p and y -methylene protons on the ligand alkoxy group. The p-methylene protons occupy significantly different magnetic environments in the metal complex and are split into a pair of complex mutiplets at o= 4.8 and o= 4.25 ppm. In contrast, the y- methylene protons are not resolved into their individual diastereotopic bands at 400 69 Figure 2.9: 1 H NMR spectrum of [lr2(Pz*)2(C0)2(Ph2POCH2CH2-2,4,6Me3Py)2](Ph4B)2 in d3-acetonitrile. 70 .., .. ,.._ co 71 Figure 2.10: a) 1 H NMR spectrum of [lr2(Pz*)2(C0)2(Ph2POCH2CH2-2,4,6Me3Py)2](Ph4B)2 showing the diasteriotopic chemical shifts for the phosphinite phenyl substituents. The pair of triplets (B = 7.0 and B = 7.15 ppm) and the complex multiplet (B = 7.45 ppm) are due to the tetraphenylborate counterions. b) Diasteriotopic chemical shifts for the alkoxy group methylene protons. 72 a 1!1.0 7.5 7.0 b 1.0 1.0 •. 5 ••• 73 Figure 2.11 a and b: 1 H NMR spectra of lr2(Pz *)2(C0)2(Ph2POCH2CH2CH2CH3)2 showing the change in the magnetic emvironments of the alkoxy group methylene protons as a function of distance from the compound's chiral center. 74 PPH ... o 5.0 a l .0 l.5 b 75 Figure 2.12: a) 1 H NMR spectrum of lr2{Pz**)2{CO)4 in d6-benzene. 6 = 2.20 ppm {s; pyrazole CH3), 6 = 1.90 ppm {d; -CH2 CH{CH3)2), 6 = 1.45 {septet; -CH2C H {CH3)2), 6 = 0.7 ppm {d; -CH2CH{CH3 )2). b) 1 H NMR spectrum of [lr2{Pz*)2{CO)2{Ph2POCH2CH2-4Ph-Py)2]{Ph4B)2 in d6-DMSO showing diasteriotopic chemical shifts for the pyrazole isobutyl substituents. 76 a l b 77 MHz and appear as a slightly second order triplet at 6 = 4.74 ppm. A similar trend is observed for the ~. 'Y , and 6-methylene bands in spectra of lr2(Pz*)2(CO)2(Ph2POCH2CH2CH2CH3)2 (figure 2.11 ). The diastereotopic chemical shifts seen in spectra of the bis (µ-3,5-dimethyl4-isobutylpyrazole) complexes indicate that these compounds also possess a trans configuration of their terminal ligands. Figure 2.12a shows a 400 MHz proton spectrum of [lr2(Pz**)2(CO)4](d6-benzene) and figure 2.12b the same spectral region for [lr2(Pz 0 )2(CO}2(Ph2POCH2CH2-4-Ph-Py)2(Ph4B)2] (d6-DMSO). The familiar doubling of signals for asymmetric ligand substituents seen earlier, is observed for the methyl and ex-methylene protons of the pyrazole bridge isobutyl groups (6 = 0.35 and 6 = 0.23; 6 =1.63 and 6 =1.55 ppm respectively). In addition to the diastereotopic doubling, the methylene protons are split into pairs of doublets most likely indicative of slowly introconverting rotational isomers along the isobutyl chain. IR and 31 P NMR Spectra: Infrared spectra of lr2(Pz*}2(CO)2(Ph2POCH2CH3)2 as fluorolube mulls show two strong bands attributable to CO stretches at vco = 1955 cm- 1 and 201 O cm- 1. The lower energy band is analogous to the CO bands seen in the redox active complexes and previously reported phosphine CO dimer compounds.19,18,22 The higher energy band has not been reported previously and is most likely due to a solid state effect as it is not seen in solution spectra of this complex. Spectra of the remaining complexes all show a single broad CO stretch at 1955 cm- 1. 31 P NMR spectra showed a solitary singlet with chemical shifts between 95.02 and 104.29 ppm consistent with coordination of the phosphinite ligands to an lr(I} metal center. 24 78 Crystallographic Structure To probe the structural properties of the iridium donor-acceptor complexes, the structure of [lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py)2](Ph4B)2 was determined using x-ray crystallographic structure techniques. Results from this study indicate that this complex possesses structural properties similar to those seen in a number of related iridium and rhodium pyrazolate bridge dimers.19, 18,22,25 An ORTEP drawing of this compound with atomic numbering is shown in figure 2.13; selected bond distances and angles are summarized in table 2.4. The complex consists of two d8 monomer fragments held in a face-to-face, Aframe configuration by a pair of 3,5-dimethylpyrazole bridging ligands. These ligands are nearly perpendicular to each other in the complex (8 = 94.2°) and deviate only slightly from a planar geometry. The ND1A, ND1B, CD1A, CD1B, CD1C, CD1D, and CD1 E atoms of the first pyrazole ring are displaced from the mean plane passing through them by 0.0, 0.0, 0.01, 0.03, 0.02, -0.02, and -0.03 A; the ND2A, ND2B, CD2A, CD2B, CD2C, CD2D, CD2E atoms deviate from the plane passing through their pyrazolate ring by 0.0, -0.1, -0.02, 0.0, -0.04, -0.04, and -0.03 A. The torsional angles summarized in table 2.5 indicate that the two iridium atoms and four pyrazole nitrogen atoms define a nearly perfect boat comformation. Interestingly, the two pairs of lr-N-N angles in the boat differ by approximately 5° and are related by the molecule's C2 axis. These minor deviations may reflect the different steric requirements of the complex's terminal CO and phosphine ligands. The metal-metal separation of 3.219(1) A found in this complex is intermediate between those seen in the structures of lr2(Pz)2(CO)2(Ph3)2 and lr2(Pz)2(CO)2(Ph3)2(CsHs) and is indicative of a 79 Figure 2.13: ORTEP drawing of [lr2(Pz*}2(C0}2(Ph2POCH2CH2-Py}2](Ph4B}2. 80 C01C C83F C83E CB1D CB3D 81 Table 2.4 Selected Distances and Angles Iridium Coordination Sphere Distance(A) ffil -IR2 ffil -Cl ffi2 -C2 IRl -Pl ffi2 -P2 IR2-ND1B IRl -NDlA IR2-ND2B IRl -ND2A 3.219 1) 1.759 13) 1.648 13) 2.217 3 2.216 3 2.066 9 2.045 9 2.061 9 2.101 9 Angle( 0 ) NDlA-ffil -ND2A Cl -IRl -Pl ND2A-IR1 -Cl Pl -IRl -NDlA Cl -IRl -NDlA Pl -IRl -ND2A ND1B-IR2 -ND2B C2 -IR2 -P2 ND1B-IR2 -C2 P2 -m2 -ND2B C2 -IR2 -ND2B P2 -IR2 -NDlB IRl -ND2A-ND2B IRl -NDlA-NDlB IR2 -NDlB-NDlA ND2A-ND2B-IR2 CO Ligands Distance(A) Cl-01 C2-02 1.201(16) 1.260(17) Angle( 0 ) 02-C2-IR2 01-Cl-IRl 179.1(12) 174.0(11) Iridium to Pyridlnlum Distances Distance(A) IRl -NPl IR2 -NP! m.2 -NP2 m1 -NP2 5.838!10l 9.003 10 5.841 10 1.971 10 83.0 4 90.l 4 91.0 5 96.3 3 172.6 5 174.4 3 83.2 3 95.1 5 87.9 5 94.1 3 170.4 5 174.3 3 119.2 7 114.2 7 119.5 7 114.2 7 82 Pho1phlne Ligands Distance(!) Pl -OAl Pl -CBIA Pl -CB2A OAl-CAl CAI -CBI CBI -NPl P2 -OA2 P2 -CB3A P2 -CB4A OA2-CA2 CA2-CB2 CB2 -NP2 1.635 8) 1.812 1.809 13l 11 1.478 16 1.49 2) 1.482 18) 1.639 8) 1.803 12 1.795 12 1.454 14 1.557 17 1.474 16 Angle( 0 ) IRl -Pl -OAl 116.1 3 IRl -Pl -CBIA 115.9 4 IRl -Pl -CB2A 120.6 4 CBIA-Pl -CB2A 101.4 6 CBlA-Pl -OAl 100.3 5 CB2A-Pl -OAl 99.2 5 CB2A-Pl -CBlA 101.4 6 Pl -OAl-CAl 118.8 7 OAl -CAI -CBI 106.3 CAI -CBI -NP! 113.8 12 IR2 -P2 -OA2 115.1 3 IR2 -P2 -CB3A 117.9 4 IR2 -P2 -CB4A 118.9 4 CB3A-P2 -CB4A 101.6 5 CB3A-P2 -OA2 101.0 5 CB4A-P2 -OA2 99.1 5 CB4A-P2 -CB3A 101.6 5 P2 -OA2-CA2 116.1 7 OA2 -CA2 -CB2 105.1 9 CA2 -CB2 -NP2 111.5 10) lll Pyrazole Bridging Ligands Distance(!) NDlA-NDlB NDlA-CDlA NDlB;...CDIB CDIA-CD!C CDlA-CDlD CDlB-CDlC CDlB-CDlE ND2A-ND2B ND2A-CD2A ND2B-CD2B CD2A-CD2C CD2A-CD2D CD2B-CD2C CD2B-CD2E 1.359 13l 1.365 15 1.337 16 1.360 18 1.49 2l 1.39 2 1.48 2 1.345 13l 1.340 16 1.347 15 1.39 2l 1.49 2 1.34 2 1.52 2 Angle( 0 ) CDlA-NDlA-IRl CDlA-NDlA-NDlB CDlB-NDlB-NDlA CD1B-ND1B-IR2 CDlC-CDlA-NDlA CDlD-CDlA-NDlA CDlD-CDlA-CDlC CDlC-CDlB-NDlB CDlE-CDlB-NDlB CDlE-CDlB-CDlC CDlB-CDlC-CDlA CD2A-ND2A-ND2B CD2B-ND2B-IR2 CD2B-ND2B-ND2A CD2C-CD2A-ND2A CD2D-CD2A-ND2A CD2D-CD2A-CD2C CD2C-CD2B-ND2B CD2E-CD2B-ND2B CD2E-CD2B-CD2C CD2B-CD2C-CD2A 139.6 8l 105.7 9 109.8 9 130.7 8 110.7 11 118.9 11 130.3 12 108.6 11 121.2 12 130.0 12 105.1 12 110.8 9l 139.3 8 105.6 9 106.6 11 123.9 12 129.4 13 111.1 11 118.6 11 130.4 12 105.9 12 83 Table 2.5 Selected Torsional Angles Atoms Angle (0 ) lr1-P1-OA1-CA1 -22.03 P1-OA 1-CA 1-CB1 -180.74 QA 1-CA 1-CB1-NP1 -285.02 lr2-P2-OA2-CA2 -3 6. 1 9 P2-OA2-CA2-CB2 185.65 OA2-CA2-CB2-NP 272.19 lr1-ND1 A-ND1 B-lr 25.76 lr1-ND2A-ND2B-lr 25.61 ND1 A-ND1 B-lr2-ND2B -68.88 ND2B-ND2A-lr1-ND1 A -68.45 ND2A-ND2B-lr2-ND1 B 58.25 ND1 B-ND1 A-lr1-ND2A 58.18 84 ground state metal-metal interaction similar to that described earlier for a number of d8-d8 metal dimers.2 6 , 19 ,s Notably, the terminal phosphinite and carbon monoxide ligands adopt a trans configuration with respect to the compound's metal-metal axis, which is consistent with the NMR spectroscopic data discussed in the previous section. The coordination environment around each of the two iridium atoms in [lr2(Pz")2(CO)2(Ph2POCH2CH2-Py)2](Ph4B)2 is approximately square planar. The bond angles between adjacent ligands in each of the square planes are close to 90° with the N-lr-N angles consistently smaller than the other L-P-L angles. Both of the d8 monomer fragments deviate from a rigorous planar geometry. The displacements of the lr1, ND1 A, ND2A, C1, P1 and lr2, ND1 B, ND2B, C2, P2 atoms from the mean planes passing through them are -0.015, -0.076, 0.095, -0.087, 0.083 and 0.017, -0.082, 0.061, 0.072, -0.068 A, respectively. The lr1-C1 and lr2-C2 bond lengths differ by approximately 0.1 A. This difference can be attributed to systematic errors in correcting the diffraction intensities for iridium absorption effects based on two observations made during the structure refinement process. Least squares refinements were carried out on two different data sets. One was corrected for heavy atom absorption, while a second data set was left in its uncorrected form. With the exception of the lr1-C1 and lr2-C2 bonds, structures refined from both data sets were nearly identical. It was found that the disparity in the Ir-CO bond lengths was larger in structures refined from uncorrected diffraction data. Secondly, structures refined using only high angle reflections had Ir-CO bond lengths that were the same within their estimated standard deviations. Because low angle reflections are more sensitive to errors incurred from heavy atom absorption, this result further substantiates that the 0.1 A difference in the Ir-CO bond lengths is due to Ir x-ray absorption effects. The bond lengths and angles for comparable substituents on each of the phosphinite ligands in this complex are the same within their estimated standard 85 deviations. In additon, the torsional angles along the ligands' alkoxy subtituents are similar indicating that the molecules possess implicit C2 symmetry in the solid state. Both ligands adopt a rotational conformation about the lr-P bond which avoids less favorable steric interactions between the phosphine phenyl groups and the bridging pyrazole ligands. While this solid state conformation represents a lower energy configuration of the lr-P rotational coordinate, other conformations are most likely accessible in fluid solution (vida infra). The lr-P-O-C and P-O-C-C torsional angles in the ligand alkoxy groups are substantially less than 180° and leave the three atom bridging group linking the pyridinium cation to the metal complex partially extended. This particular solid state conformation occurs, most likely, as a results of crystal packing forces. The 5.9 A through space distance between the two redox partners is intermediate in value between that for a fully extended chain (6.9 A) and for their Van der Waals contact distance. Discussion In the previous sections a detailed description was given of the preparation and characterization of the d8-d8 intramolecular redox molecules, which will form the focus of the subsequent chapters of this thesis. These results will now be examined with regards to: 1) the scope of the synthetic methodologies developed toward preparing a wide variety of donor-acceptor molecules. 2) Structural relationships amongst these complexes and related d8-d8 A-frame complexes. 3) The conformational properties of the redox ligands and the range of intramolecular electron-transfer distances available in these complexes. Synthetic Methodology. A long term goal of this research program is to design, synthesize, and study intramolecular electron-transfer systems to understand kinetic 86 factors such as reaction exoergicity, donor-acceptor separation and orientation, and solvent dynamical effects in electron-transfer reactions. Central to the pursuance of this goal is the development of synthetic methodologies that are flexible enough to allow the redox potentials of the donor and acceptor, the electronic properties of the bridging group, and the donor-acceptor separation to be readily varied within a series of redox complexes. The approach to forming a covalent linkage between an electron donor and acceptor presented in this study posseses two important features for developing a range of different donor-acceptor systems. In contrast to some recently reported excited state donor-acceptor systems based on porphyrins27 ,2b and bipyridyl transition metal complexes,28 the redox active phosphine approach outlined in this study is not restricted to a particular inorganic complex. In principle, redox phosphinite ligands could be used to probe excited state ET reactions in any chromophore containing phosphine ligands. Studies spanning a range of different inorganic chromophores will be important in understanding variations in electron transfer rates as a function of the nature of the excited donor. In addition, the synthetic methodology developed here may prove to be general enough to allow for detailed studies of the medium effects that control excited state ET processes. The range of electronically different bridging groups, which could be incorporated into a series of redox active ligands, should be limited only by the availability of a corresponding haloalcohol. Research involving the preparation and study of new ligands and complexes is currently being pursued in our laboratories. Future studies designed to explore the driving force dependence of electron transfer in the [lr2(Pz*)2(C0)2(Ph2POCH2CH2-Py-R)2] systems will require a series of complexes spanning a range of reaction driving forces. The reaction exoergicity in these complexes can, in principle, be controlled by varying substituents on the pyridinium aromatic ring. The viability of this approach has been demonstrated in bimolecular driving force studies. 5 ,6 While the synthetic methodologies developed in 87 this chapter can be used to prepare ligands containing several different pyridinium cations, there were some notable exceptions, which warrant further discussion. The alcoholysis reaction of N,N-dimethyl-P,P-diphenylphosphine proved useful in synthesizing a number of redox ligands with higher electrochemical reduction potentials, but it could not be extended to the preparation of ligands containing more activated pyridinium cations. Based on the current literature, the difficulties surrounding these reactions were attributed to irreversible ring opening reactions between dimethylamine and the more reactive cyano and carbomethoxy pyridinium cations. 16 Several different R2NH leaving groups were assayed with regard to their reactivity toward 4-cyano-1-methylpyridinium, but none were found to be inert enough to avoid pyridinium ring opening as a rapid side reaction. Thus, reactions that avoid secondary amines as a reaction side product must be developed to prepare ligands with lower electrochemical reduction potentials. Two alternative approaches to preparing diphenylphosphine esters, which would circumvent this problem, are shown in figure 2.14. Reaction 5 is based on the facile alcoholysis reactions of phosphinous acid chlorides. 15 In this particular case, polyvinylpyridine (PVP) acts as a proton sponge for scavanging HCI liberated during the reaction. Features of this approach, which make it attractive, are its mild reaction conditions and the ease with which the products can be isolated from PVP-HCI. Preliminary results along these lines have shown promise. 29 Reaction 6 is based on some recent work published by Batyeva30 and coworkers and represents an extension of Nifant'ev's research on acid catalyzed reactions of phosphrous Ill amides.31 Attractive features of this approach are that it could be carried out under mild reaction conditions and that the dimethylamine liberated during the reaction is sequestered as its corresponding acetamide. Either of these approaches could serve as viable routes to 88 Figure 2.14: Alternative reactions for preparing high driving force redox ligands. 89 PVP (Reaction 5.) (Reaction 6.) 90 ligands with redox potentials outside the range available with the current compounds. Structural Properties: An understanding of intramolecular electron transfer in the donor-acceptor systems prepared here will be developed by making comparisons between the photophysical properties of the redox active complexes and their corresponding "model compounds." An underlying assumption of this approach is that the complexes represent a structurally and electronically homologous series of compounds. The electronic properties of the redox complexes and model compounds are investigated in detail in chapter three. The structural similarity between these molecules follows from the NMR and IR spectroscopic results presented in this chapter. These data clearly establish that all of the d8-d8 complexes contain two terminal carbo.n monoxide and phosphinite ligands arranged in a trans configuration with respect to the metal-metal axis. The crystal structure of [lr2(Pz*)2(C0)2(Ph2POCH2CH2-Py)2](Ph4B)2 shows that the complexes are structurally related to perviously reported da-da Aframe complexes. 38 As seen in table 2.16 the metal-metal separation in this complex is similar to those reported earlier for a number of analogous metal dimers. Previous studies have shown that the unique electronic and photophysical properties of d8 -d8 Aframe metal dimers arise from the pseudoaxial interactions between their lr(I) metal centers.39 These data (table 2.16) suggest that the photophysical models developed for [lr2(Pz-R)2(COD)2] 6 and lr2(Pz*)2(C0)2(Ph3P)2 32 may serve as reference points for understanding the photophysics of the donor-acceptor complexes. This concept will be explored to a greater extent in chapter 3. ET Distance: During the past decade detailed theoretical and experimental studies concerning the electronic factors that control electron-transfer reactions have 91 Table 2.6: Metal-Metal Bond Distances for Selected A-Frame Complexes Compound d(A) [lr2( Pz *)2(CO)2(Ph2POC H2C H2-Py)2](Ph4B)2 3.219(1) aIr2(Pz)2(CO)2(Ph3 P)2 3.163(2) bIr2(Pz)2(COD)2 3.216(1) bIr2( 3,4,5 Me3)2(COD)2 a. See reference 19. b. See reference 6 and references cited there. 3.096(1) 92 indentified donor-acceptor separation and orientation as two primary ET variables. Hopfield has proposed an approximately exponential distance dependence for ET reactions as a consequence of long range overlap between the "wavefunction tails" for the transfering electron. 33 Explicit models directed toward understanding the roles played by bridging groups in donor-acceptor systems have more recently been developed by Beratan and Onuchic within this general framework. 34 Concurrent with developing theories, a number of fixed distance, fixed orientation donor-acceptor systems have appeared in the literature for studying medium effects in electron-transfer reactions. Such systems have employed a variety of bridging groups, ranging from synthetically prepared organic spacers to genetically engineered metalloproteins. 4a Central to this area of electron-transfer research is understanding what constitutes the "electrontransfer medium" and how this medium facilitates the ET process. A detailed understanding of these effects will develop from the design and study of systems that carefully vary molecular ET parameters in a systematic fashion. While the donor-acceptor systems reported in this chapter lack the structural rigidity necessary for studying distance and orientation effects in electron transfer, the three atom linker in these d8-d8 complexes should restrict the distances and orientations for ET to a more limited range of values than those available in bimolecular reactions. A qualitative understanding of this range of values can be developed by using NMR spectra of [lr2(Pz*)2(C0)2(Ph3P)2] in conjunction with results from a molecular graphics and mechanics analysis of [lr2(Pz*)2(C0)2(Ph2POCH2CH2Py)2](Ph4B)2. A consideration of CPK models of [lr2(Pz*)2(C0)2(Ph2POCH2CH2Py)2](Ph4B)2 shows that there are essentially two types of internal rotations that alter the distance and orientation between the iridium atoms and pyridinium aromatic 93 Figure 2.15: Structure showing the torsional angles pertinent to variations in the electron-transfer distance in [lr2(Pz *)2(C0)2(Ph2POC H2C H2-Py)2](Ph4B)2. 94 95 Figure 2.16 a, b, and c: Structures showing three different phosphinite ligand rotational conformations, which are related by a 120° rotation about the lr-P bond (<!>A), R1 and R2 are the through space distances between the lefthand pyridinium nitrogen and its proximal and distal iridium atoms respectively. Cross hatched spheres represent the heteroatoms. Sphere sizes were chosen for the purpose of clarity and should not be interpreted as thermal ellipsoids. In each case <1>B1 = <1>B2 = <1>B3 = 180°; <1>84 = 90°. 96 a R1 = 6.9 A R2 = 9.8 A 97 b R1 = 6.9 A R2 = 9.8 A 98 C R1 = 6.9 A R2 = 9.8 A 99 ring (figure 2.15). Rotations about either of its lr-P bonds (<!>A) drastically affects the relative orientation of the two redox partners. As seen in figures 2.16 a,b,c this rotation essentially allows the pyridinium cation to access different regions of the iridium dimer chromophore. Conformation I (figure 2.16a) probes distances and orientations over one of the square planar ends of the molecule, while conformations II (figure 2.16b) and Ill (figure 2.16c) access regions below the terminal ligands and approximately perpendicular to the pyrazole ligands, respectively. Within each of these three lr-P conformers, rotations about bonds along the linker backbone(<1>s1, <!>82, <l>B3,<l>B4) change the donor-acceptor distance over a region of the iridium dimer dictated by <l>A• Rigid geometry conformational mapping calculations indicate that the through space distance separating the two redox partners varies between limiting values of 6.9 to 5.0 A as a function of <1>B1, <1>B2, <!>B3, and <1>B4· Examples of the different ligand geometries found from this analysis are shown in figures 2.1 ?a-k. In some configurations, the through space separation 35 between each of the complex's iridium metal centers and pyridinium ring are almost identical, while in other conformations these distances are very different. Therefore, distances from a pyridinium nitrogen atom to both of the iridium metal centers are reported with each structure. This analysis indicates that there are a considerable number of orientations and donor-acceptor separations available to the [lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py-R)2] redox systems. The true range of conformers present in solutions of these complexes will depend on the activity of each of their internal rotational degrees of freedom. c 13 NMR T 1 measurements have indicated that internal rotations in aliphatic hydrocarbons produce a distribution of solution conformers that exchange on a picosecond time scale. At the present time, the activity of the spacer group rotations can be inferred from these results. Further studies will be necessary to characterize these degrees of freedom and their effect on photoinduced electron transfer processes in our compounds. Careful 100 consideration of the proton NMR spectrum of [lr2(Pz*)2(C0)2(Ph3P)2] indicates that motion about the lr-P bonds in the [lr2(Pz*)2(C0)2(Ph2POCH2CH2-Py-R)2] complexes must also be important in determining their range of solution conformations. If the triphenylphosphine phenyl groups in [lr2(Pz*)2(C0)2(Ph3P)2] interconvert via intramolecular rotations, NMR spectra of this complex should show at most three groups of multiplets for its ligand aromatic protons. As seen in figure 2.18 the ortho, meta and para protons in this complex appear a complex mutiplet at o = 7.8 ppm (ortho) and a singlet at o= 7.0 ppm (meta+para}, indicating that the ligand phenyl groups exchange magnetic environments on an NMR timescale. Based on Tolman cone angle arguments, 36 rotations about q>A in [lr2(Pz*)2(C0)2(Ph2POCH2CH2-Py])2] should be hindered mainly by its phosphinite phenyl substituents. Because this rotational degree of freedom is active in [lr2(Pz*)2(C0)2(Ph3P)2], it should be active in the donoracceptor complexes as well. In a bimolecular electron-transfer reaction the value of kobs is controlled by the relative rates of electron transfer and molecular diffusion. Under steady-state conditions, "reactive pairs" of redox partners are formed via diffusional processes and are subsequently transformed into products37 . If kEr>> kdiff, as is the case in many excited state ET reactions, then the observed reaction rate becomes diffusion limited. In a "fixed site" donor-acceptor system the redox partners are held at a constant distance and orientation by a rigid covalent linkage. Here, kobs is determined solely by factors that govern ket, because the reactants have been synthetically preassembled into reactant pairs. The structural analysis presented above suggests that electron transfer in these [lr2(Pz*)2(C0)2(Ph2POCH2CH2-Py-R])2] complexes may occur under the influence of a pairwise diffusional process not found in either bimolecular reactions or in fixed site ET systems. The redox partners in these complexes are assembled into 1 01 Figure 2.17 a-d: Structures showing variations in the orientation and distance between the pyridinium cation and iridium dimer as a function of <l>Bi, <1>B2, <1>B3, and <1>B4 for ligand conformation I (figure 2.15a). R1 and R2 are the through space distance between the lefthand pyridinium nitrogen and its proximal and distal iridium atoms respectively. 102 a R1 = 5.8 A R2 = 8.9 A b R1 = 5.9 A R2 = 8.8 A 103 C R1 = 5.5 A R2 = 8.7 A d R1 = 5.0 A R2 = 8.0 A 104 Figure 2.17 e-h: Structures showing variations in the orientation and distance between the pyridinium cation and iridium dimer as a function of <l>B1, <l>B2, <l>B3, and <l>B4 for ligand conformation II (figure 2.1 Sb). R1 and R2 are the through space distance between the lefthand pyridinium nitrogen and its proximal and distal iridium atoms respectively. 105 e R1 = 5.8 A R2 = 7.8 A f R1 = 4.9 A R2 = 6.2 A 106 g R1 = 5.8 A R2 = 7.8 A h R1 = 5.0 A R2 = 6.3 A 107 Figure 2.17 i-k: Structures showing variations in the orientation and distance between the pyridinium cation and iridium dimer as a function of <!>81, <!>82, <!>83, and <!>84 for ligand conformation Ill (figure 2.1 Sc). R1 and R2 are the through space distance between the righthand pyridinium nitrogen and its proximal and distal iridium atoms respectively. 108 R1 = 6.9 A R2 = 8.1 A . J R1 = 5.8 A R2 = 6.2 A 109 k R1 = 5.7 A R2 = 6.8 A 11 0 Figure 2.18: 1 H NMR spectrum of [lr2(Pz*)2(C0)2(Ph3P)2] in d6-benzene. 111 __________ ___ 'I g I 8 I 1 ___. I 6 I 5 I • .,,, I 3 J. \. I I I 2 I l 11 2 reactant pairs as in a fixed site compound, but are free to diffuse within a restricted region of solution defined by the conformational properties of the phosphinite ligands. The effect that intramolecular motion has on the observed rate of electron transfer in these complexes is at present poorly understood and represents an important area for future research. However, as we will see in chapters 3 and 4 the restrictions placed on bimolecular diffusion in these systems will be sufficient for studying excited state ET reactions with rate constants well above kdiff. References 1a. Marcus, R. A.; Sutin, N., Biochim. Biophys. Acta, 1985, 811, 265-322. b. Guarr. T.; Mclendon, G., Coord. Chem. Rev., 1985, 68, 1-52. c. Sutin, N., Prog. /norg. Chem., 1983, 30, 441-498. d. Newton, M. D.; Sutin, N., Ann. Rev. Phys. Chem., 1984, 35, 437-480. e. Meyer, T. J., Prog. lnorg. Chem., 1983, 30, 389-440. f. Sutin, N., Acc. Chem. Res., 1982, 15, 275-282. 2a. Gould, I. R.; Ege, D.; Mattes, S. L.; Farid, S., J. Amer. Chem. Soc., 1987, 109, 3794-3796. b. Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A.; Pewitt, E. B., J. Amer. Chem. Soc., 1985, 107, 1080-1082. c. Closs, G. L.; Miller, J. R., Science, 1988, 240, 440-447. d. Bock, C. R.; Connor, J. A.; Gertierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan,B. P.; Nagel, J. K., J. Amer. Chem. Soc., 1979, 101, 4815-?. 3a. Closs, G. L.; Calcaterra, L. T.; Green, N. J.; Penfield, K. W.; Miller, J. R., J. Phys. Chem., 1986, 90, 3673-3683. i 13 b. Oevering, H.; Paddon-Row, M. N.; Heppener, M.; Oliver, A. M.; Cotsaris, E.; Verhoven, J. W.; Hush, N. S., J. Amer. Chem. Soc., 1987, 109, 3258-3269. 4a. Mayo, S. L.; Ellis, W. R.; Crutchley, R. J.; Gray, H. B., Science, 1986, 233, 948952. b. lsied, S. S., Prog. lnorg. Chem., 1984, 32, 443-?. 5. Marshall, J. L.; Stobart, S. R.; Gray, H. B., J. Amer. Chem. Soc., 1984, 106, 30273029. 6. Marshall, J. L., Ph.D. Dissertation, California Institute of Technology, 1986. 7. Wiley, R.H.; Hexner, P. E., Org. Synth.,1951,31, 43-44. Sa. Ewart, G.; Payne, 0. S.; Porte, A. L.; Lane, A. P., J. Chem. Soc., 1962, 39843990. b. Noth, H.; Vetter, H. J., Chem. Ber., 1963, 96, 1109-1118. c. Maier, L., Helv. Chim. Acta, 1964, 47, 2129-2137. 9. The distillate has a tendency to solidify upon cooling and clog condensers. To avoid this problem, the distillation head and condenser should be wrapped with heating tape and kept warm. 10a. Balaban, A. T.; Boulton, A. J., Org. Synth.,3, 1112-1117. b. Balaban, A. T.; Toma, C., Tetrahedron, Supplement No. 7, 9-25. 11. Katritzky, A. R.; Bopart, J. 8.; Clarament-Elguero, R. M.; Yates, F. S.; Dinculescu, A.; Balaban, A. T.; Chiraleu, F., J. Chem. Res. (M), 1978, 4783-4797. 12. Bushnell, G. W.; Fjeldsted, 0. O.K.; Stobart, S. R.; Zaworotko, M. J.; Knox, S. A. R.; MacPherson, K. A., Organomet., 1985, 4, 1107-1114. 13. Biograf Ill was licensed from Bio-Design Inc., Pasadena, CA. 14a. Gelin, B. R.; Karplus, M., Biochem .. , 1979, 18, 1256-1268. b. Gelin, B. R.; Karplus, M., Proc. Nat. Acad. Sci., 1975, 72, 2002-2006. 1 14 c. Brooks, B.R.; Bruccoleri, R. 8.; Olafson, 8. P.; States, D. J.; Swaminathan, S.; Karplus, M., J. Comp. Chem., 1983, 187-217. 15. Maier, L.; Karsolapoff, G. M., " Organic Phosphorous Compounds", 1972, Wileylnterscience, New York. 16a. Moracci, F. M.; Di Rienzo, B.; Tortorella, S.; Liberatore, F., Tetrahedron, 1980, 36, b. 785-787. Landquist, J. Chem. Soc. Perkin I, 1976, 454-456. c. Moracci, F. M.; Liberatore, F.; Tortorella, S.; Di Rienzo, B., Tetrahedron, 1979, 35, 809-812. 17. Mavel, G., Ann. Rev. NMR Spectr, 1973, 58, 1-89. 18. Powell, J.; Kuksis, A.; Nyberg., S. C.; Ng, W. W., lnorg. Chim. Acta, 1982, 64, L211-L212. 19. Atwood, J. L.; Beveridge, K. A.; Bushnell, G. W.; Dixon, K. R.; Eadie, D. T.; Stobart, S. R.; Zaworotko, M. J., lnorg. Chem., 1984, 23, 4050-4057. 20. Nussbaum, S.; Rettig, S. J.; Storr, A.; Trotter, J., Can. J. Chem., 1985, 63, 692702. 21. Model compounds have ligands, which are similar to those found in the donoracceptor complexes, but lack pyridinium electron acceptors. 22a. Uson, R.; Oro, L. A.; Ciriano, M. A.; Carmona, D.; Tiripicchio, A.; Camellini, M. T., J. Organomet. Chem., 1982, 224, 69-80. b. Uson, R.; Oro, L. A.; Ciriano, M. A.; Pinillos, M. T.; Tiripicchio, A.; Camellini, M. T., J. Organomet. Chem., 1981, 205, 247-257. 23. Banditelli, G.; Bandini, A. L.; Bonati, F.; Mingghetti, G., J. Organomet. Chem., 1981,218, 229-239. 24. Nixon, J. F.; Pidcock, A., Ann. Rev. NMR Spec., 1969, 2, 346-422. 11 5 25a. Coleman, A. W.; Eadie, D. T.; Stobart, S. R., J. Amer. Chem. Soc., 1982, 104, 922-923. b. Beveridge, K. A.; Bushnell, G. W.; Stobart, S. R.; Atwood, J. L.; Zaworotko, M. J., Organomet., 1983, 2, 1447-1451. 26a. Rice, S. F.; Gray, H.B., J. Amer. Chem. Soc., 1981, 103, 1593-1596. b. Dallinger, R. F.; Miskowski, V. M.; Gray, H. B.; Woodruff, W. H.. , J. Amer. Chem. Soc., 1981, 103, 1595-1596. 27a. McMahon, R. J .. ; Force, R. K.; Patterson, H. H.; Wrighton, M. S., J. Amer. Chem. Soc., 1988, 11 b. o, 2670-2672. Gust, D.; Moore, T. A.; Liddell, P.A.; Nemeth, G. A.; Makings, L. R. ; Moore, A. L.; Barrett, D.; Pessiki, P. J.; Bensasson, R. V.; Ronger, M.; Chachaty, C; De Schyver, F. C.; Van der Anwerau, M.; Holzwarth, A. R.; Connolly, J. S., J. Amer. Chem. Soc., 1987, 109, 846-856. 28. Chen, P.; Westmoreland, T. D.; Danielson, E.; Schanze, K, S.; Anthon, D.; Neveux, P. E.; Meyer, T. J., lnorg. Chem., 1987, 26, 1116-1126. 29. Fox, L. S.; Farid, R. S.; Gray, H, B.; unpublished results. 30a. Batyeva, E. S.; Ofitserov, E. N.; lvanyuk, N,. V.; Pydovik, A. N., Zh. Obsh. Khim., 1976, 46, 2282. b. Papers from Zhurnal Obshchei Khimi are cited with pages numbers from their english translations. 31a. Nifant'ev, E. E.; Borisov, E. V., Zh. Obsh. Khim., 1985, 55, 1478-1480. b. Petrov, K. A.; Nifant'ev, E. E.; Lysenko, T. N.; Evdokov, V. P., Zh. Obsh. Khim., 1961, 31, 2214-2217. c. Nifant'ev, E. E.; lvanova, N. L.; Fursenko, I. V., Zh. Obsh. Khim., 1969, 46, 2282. d. Evdakov, V. P.; Beketov, V. P.; Svergun, V. I., Zh. Obsh. Khim., 1973, 43, 51-55. 11 6 e. Evdakov, V. P.; Beketov, V. P.; Bryantsev, B. I., Zh. Obsh. Khim., 1977, 47, 15791583. f. Batyeva, E. S.; Al'fonsov, V. A.; Zamaletdinova, G. U.; Pudivik, A. N,, Zh. Obsh. Khim., 1976, 46, 2120-2123. 32. Smith, T., Ph. D. Dissertation, California Institute of Technology, 1982. 33a. Hopfield, J. J., Biophys. Journ., 1977, 18, 311-321. b. Redi, M.; Hopfield, J. J., J. Chem. Phys., 1980, 72, 6651-6660. c. Beratan, D. N.; Hopfield, J. J., J. Amer. Chem. Soc., 1984, 106, 1584-1594. 34. Onuchic, J. N.; Beratan, D. N., J. Amer. Chem. Soc., 1987, 109, 6771-6778. 35. The through space distance is arbitrarily defined using the iridium metal atoms and pyridinium nitrogen atom. 36. Tolman, C. A., Chem. Rev., 1977, 77, 313-348. 37. Marcus, R. A.; Siders, P. J., J. Phys. Chem., 1982, 86, 622-630. 11 7 Chapter 3 Electronic Spectra and Electrochemistry 11 8 Introduction Having prepared and characterized a series of d8 -d8 donor-acceptor complexes, our attention will now turn toward investigating their photophysical properties. As we saw in the previous chapter, one consequence of covalently binding electron acceptors to an iridium dimer chromophore was to restrict the range of available distances and orientations between the metal centers and acceptors. This binding together of redox partners also has some important photophysical implications. In principle the donor- acceptor complexes represent unique molecular electronic systems. They possess an additional charge-separated excited state that arises solely due to interactions between their redox partners. This state can potentially be accessed from the donor (acceptor) localized levels through radiative and nonradiative processes. Depending upon the nature of the interactions between these charge-transfer and donor (acceptor) localized states, certain photophysical properties of the donor-acceptor molecules will be perturbed. For example, if the charge-transfer state is populated via an electron transfer from a donor localized excited state, then the emission quantum yield and lifetime of the donor state should be diminished relative to an appropriate model compound. Previous studies have established that d8-d 8 metal dimers possess low lying singlet and triplet excited states which arise from a (da*po') electronic configuration. Thus, an important aspect of our studies was to determine how the photophysical properties of these d8-d 8 excited states are perturbed by the presence of a covalently attached electron acceptor. lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 and lr2(Pz*)2(C0)2(Ph2POCH2CH2-NEt3+)2 were employed as two model systems for these investigations. In terms of their composition, both compounds are similar to the donor-acceptor complexes. The later compound contains two non-reducible NEt3+ groups to model the effects that positive charges have on the quantum yields and lifetimes of the metal localized (da*pa) excited states. In the following chapter the electronic and photophysical properties of the donor-acceptor complexes are examined using steady-state absorption and emission 11 9 spectroscopy, and electrochemical techniques. Important objectives of this aspect of our research were; 1) to determine, either spectroscopically or through some indirect means, the relative energies of the (dcr*pa') and charge-transfer states in these complexes, and 2) to obtain photophyscial evidence for excited state electron transfer. Objective number one is clearly important with regard to characterizing the electron transfer reactions in the donor-acceptor complexes as a function of reaction driving force. Cyclic voltammetry and constant potential bulk electrolysis experiments were conducted to determine potentials for the ground state redox processes available in these systems. In the discussion section of this chapter these electrochemical data are used in conjunction with our spectroscopic results to evaluate the energies of the (dcr*pcr) and charge-transfer excited states in these redox systems. Objective number two was important with regard to some of the time-resolved spectroscopic experiments discussed in chapter 4. In interpreting spectra from these studies, it was essential to know which of the (dcr*pcr) excited states was involved in electron-transfer reactions. In the past the reactivity of ds-ds compounds has been exclusively associated with their long lived triplet states. The quantum yield data presented in this chapter represent the first evidence that their shorter lived singlet states are also highly reactive under the appropriate kinetic conditions. In addition, it was important to characterize the spectroscopic properties of the charge-transfer states in these systems. In principle these states should relax ground via both radiative and nonradiative processes. Absorption and emission spectra of the compounds were central to identifying the dominant deactivation pathways available to the charge-transfer states in the complexes. Experimental General Procedures: All manipulations involving the iridium dimer complexes were carried out using standard Schlenk and high vacuum techniques. Solvents used in 120 spectroscopic and electrochemical experiments were reagent grade or better in quality. They were purified as indicated, subjected to a minimum of five freeze/pump/thaw cycles, and stored in a round bottom storage flask fitted with a Kontes teflon stopcock. 2-Methyltetrahydrofuran was purified by distillation from a 0.5% (w/w) suspension of cuprous chloride, followed by distillation from CaH2 and finally from sodium/benzophenone. It was stored under vacuum over sodium/benzophenone as an indicator. Acetonitrile and propionitrile were distilled from CaH2 under nitrogen and stored in vacuo. Methylene chloride was distilled from CaH2, subjected to five freeze/pump/thaw cycles and stored in a Schlenk storage flask under a nitrogen atmosphere. Electronic Absorption Spectroscopy: Electronic Absorption spectra were recorded using a Cary 17 UV-Vis Spectrometer. Room temperature spectra were obtained from 2:1 2-methyltetrahydrofuran/propionitrile solutions that were prepared under vacuum in a 1 cm quartz cuvette attached to a Kontes Teflon stopcock. Low temperature spectra were measured using a quartz optical dewar of local design. Samples were chilled to 77° K in liquid nitrogen and maintained at that temperature via thermal contact with a copper block, which was immersed in a liquid nitrogen bath. Spectra were corrected for absorption due to the solvent, cuvette, and dewar. They were hand digitized using a Calcomp 2000 digitizing pad, an IBM-AT clone PC, and Pro-Design II version 2.5. Extinction coefficients and band widths (fwhm) were determined by fitting the observed band shapes to gaussian envelope functions using Peakfit version 1.2 (EMF Software). Oscillator strengths were calculated using equation 3.1. eq. 3.1 1 21 Electronic Emission Spectroscopy: Electronic emission spectra were recorded on an in-house emission spectrometer whose construction and specifications have been previously described. 5 a Samples were excited with the 436 nm line of an oriel 200 watt Hg/Xe lamp. This line was selected with a Spex 1670 monochromator and filtered with an Oriel 436 nm (#5645) interference filter (fwhm = 80 cm- 1 ) to remove stray excitation frequencies. Sample emission was collimated, focused on the slit of a Spex 1870 monochromator, and detected with a Hamamatsu R955 PMT. Scattered excitation was removed using a Corning 3387 sharp-cut filter. The signal was amplified with an EG&G 182-A lock-in amplifier and transferred to a Horizon North Star computer where it was digitized and corrected for PMT response. The corrected intensity data were transformed to a linear energy scale and plotted as a function of emission frequency. Room temperature spectra were measured of solutions consisting of an iridium dimer complex in dry oxygen-free acetonitrile. Samples were handled in a 1 cm high precision quartz fluorimetery cell that was attached to a Kontes Teflon stopcock. Low temperature spectra were obtained from 2:1 2-methyltetrahydrofuran/propionitrile solutions that were prepared under vacuum in an NMR tube attached to a Kontes Teflon stopcock. The samples were cooled to 77° K by immersing them in a liquid nitrogen bath contained in a quartz finger dewar. Room temperature quantum yields were determined by the optically dilute solution method of Crosby and Demas.37 Integrated emission intensities were determined by the cut and weigh method of integration and used to calculated quantum yields according to equation 3.2. Ru(Bipy)32+ served as a reference.38 The ODs of the samples and reference at 436 nm were adjusted so that they were equivalent within experimental error and were less than or equal to 0.1. 122 Temperature dependent emission spectra of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 were recorded using a CTI-Cryogenics model 70 Cryodyne Cryocooler. Samples were prepared under vacuum as 2-methyltetrahydrofuran solutions and flame sealed in 1 mm path length optical glass cuvettes. Cuvettes were mounted on a copper block, which was attached to the cold stage of the Cryocooler. Thermal contact between the sample and cold stage was achieved via copper grease. The sample temperature was measured using a copper/constantan thermocouple and was controlled with a CTI temperature controller. Samples were allowed to thermally equilibrate at each temperature for 1/2 hour. Excitation Spectroscopy: Electronic excitation spectra of optically dilute solutions were recorded using a Perkin-Elmer model WPF-66 fluorimeter. Intensity data were corrected for variations in the lamp intensity profile using a Rhodamine B quantum counter and were plotted as a function of excitation frequency. Electrochemistry: Cyclic voltammetry (CV) and constant potential bulk electrolysis experiments were performed using an EG&G 173 Potentiostat/Galvanostat, a 175 Universal Programmer, and a 179 Digital Coulometer. CVs were plotted on a Houston Instruments Omnigraphic 2000 XV-Recorder. Two different cell geometries were used to measure cyclic voltammograms and conduct constant potential coulometry experiments. CVs were measured at BAS platinum and glassy carbon electrodes using the electrochemical cell seen in figure 3.1. An SSCE, fitted with a vycor salt bridge, and a platinum basket electrode were employed as reference and counter electrodes respectively. Contact between the reference electrode and working electrode compartment was made through a luggin-capillary with a cracked glass tip. Positioning the luggin-capillary tip close to the surface of the working electrode reduced effects due to IR-drop inherent in electrochemical cells containing organic solvents. The properties of this cell were checked periodically using ferrocene as a test system. This cell 123 Figure 3.1. A schematic drawing of the electrochemical cell used in cyclic voltammetry experiments. The Luggin capillary arrangement used in this cell reduces · effects due to IR drop, which are inherent in electrochemical experiments. 124 Luggin Capillary BAS Electrode 10/30 ef.latinum Basket 14/20 15 mis 125 design and electrode geometry consistently reproduced literature E112 values for ferrocene and showed minor IR drop effects. lnterfacial electron-transfer kinetics between platinum electrodes and the iridium dimers were sensitive to the electrode pretreatment procedure. The following pretreatment suggested by Anson and Long 42 was used to obtain active platinum electrode surfaces. 1) Polish to a metallic luster using 0.3 µm polishing alumina. 2) Sonicate in a 1 :1 methanol/ water solution for 15 minutes. 3) Cycle the electrode in 0.5 M sulfuric acid between 1.0 V and -0.05 V versus SSCE until good H2/O2 characteristics are seen. 4) Sonicate in the solvent used in the experiment at hand for 15 minutes. 5) Cycle the electrode in the supporting electrolyte solution to be used in the experiments until a constant background is observed. Cyclic voltammograms were typically measured on 0.5x10- 3 M solutions of an iridium dimer complex in 10 mis of 0.1 M TBAPFs/CH2Cl2. Measurements were carried out under an argon atmosphere. Constant potential coulometry experiments were conducted in an H-tube type electrolysis cell. Electrolysis experiments were carried out at either a platinum basket or pyrolytic graphite working electrode and a platinum basket counter electrode. The working and counter electrode compartments were separated by a medium porosity glass frit. Potentials were measured with respect to an SSCE, which made contact with the working electrode compartment via a cracked glass luggin-capillary. Measurements were made in 0.1 M TBAPFs/CH2Cl2 solutions under an argon atmosphere. 126 Results Electronic Spectroscopy The electronic structure and excited state properties of d8 -d 8 transition metal complexes are areas of current interest in inorganic photochemistry. 1 Recent research in our laboratories has indicated that the low lying excited states in these compounds display a variety of different chemical reactivities including the excited state oxidative addition of halocarbons, 1b,2 carbon-hydrogen bond activation, 1b,3 and outer-sphere electron transfer. 1b, 4 These diverse photochemical and photophysical properties can be traced to the electronic excited states, which result from the axial metal-metal interactions inherent in these compounds. In the past, electronic spectroscopy has played a central role in understanding the electronic structure and excited state reactivity of these unique inorganic chromophores.s,s Thus, a logical starting point for research directed toward investigating the photophysics and photochemistry of a new class of d8 -d 8 complexes is an examination of their electronic spectra. The current body of spectroscopic data concerning face-to-face d8 -d 8 complexes, such as [Pt2(P20sH2)4] 2· and [Rh2(Bridge)4]2+, indicates that the electronic structure of these systems can be schematically represented by the molecular orbital diagram shown in figure 3.2. Covalently linking two square planar d8 complexes into a dimeric molecule forces an axial interaction between the d2 2 and Pz orbitals of the monomeric fragments, which leads to the formation of dcr and pcr dimer MOs. Filling this MO scheme with the 16 metal electrons from the two monomer fragments leaves the dcr* level as the compound's highest occupied molecular orbital. The low-lying electronic excited states in these complexes are generated by exciting one dcr* electron into a vacant pcr orbital. This model makes some important predictions concerning the electronic 127 Figure 3.2. Molecular orbital diagram showing the metal based orbitals important to the electronic structure of d8 -d8 metal dimers adapted from reference 22. 128 .,.......·· __ ....,.. .... ...•· .,..•· 2 2 dx -y - - _ _ _ Pz _.,..• .,. . ........ _.,. / r_...._.- - - - _.,..-··· 2 2 -y ------------- --L. ---------------------- < .......__ dxz,yz - - dx ------- - - - - - - - - ---======-;,,.. --------· ------· ------------- 1J . da 129 spectra and excited properties of d8-d 8 metal complexes. First, the metal-metal interactions in these compounds should red shift their d ➔ p charge-transfer transitions with respect to transitions localized on a single d8 monomer. Thus, the electronic absorption spectra of d8 -d 8 metal dimers should show low energy d ➔ p bands that are well removed from a manifold of higher energy monomer transitions. In addition, because their low energy excited states involve dcr* ➔ pcr electronic excitations, the strength of the metal-metal bond in these complexes should be greater in their dcr*pcr excited states. Both of these predictions have been verified by recent single crystal spectroscopic studies 5a,b,f,g and excited state resonance raman experiments? involving [Pt2(P2OsH2)4] 2- and [Rh2(Bridge)4] 2+. While the MO model predicts that the energies of the (dcr* ➔ pcr) 1 and (dcr* ➔ pcr)3 electronic transitions in ds-ds complexes are a function of a compound's metal-metal separation, the current body of data concerning face-to-face and A-frame metal dimers indicates that these energies are more intimately related to the type of metal centers, the bridging and terminal ligands, and the metal-metal separations found in these complexes. In this respect some care must be exercised when using this approach to make comparisons between ds-ds dimers with substantially different coordination environments. However, within a structurally homologous series of complexes with identical metal centers and similar ligands, an MO approach should be adequate for understanding the (dcr*pcr) excited states in these chromophores. Recent spectroscopic studies, concerning a series of lr2(µ-pyrazolate)2(COD)2 complexes, have indicated that the molecular orbital model developed for face-to-face d8 -d 8 compounds can be extended to describe the spectroscopic properties of A-frame metai dimers. 8,3d The iow energy excited states in these chromophores have been identified as the (dcr*pcr) 1 and (dcr*pcr)3 states by virtue of their visible absorption bands and fluid solution emission spectra. Because detailed experiments, regarding 130 Table 3.1: Spectral Parameters for the Iridium Donor-Acceptor Complexes. Complexa RoomT 77° K Vmaxf &naxg Vmax cm· 1 M·1cm·1 cm·1 Ph2OCH2CH3 2 2, 1 OO 8900 2400 22,300 12,000 1600 Ph2OCH2CH2-NEt3 21 , 3 0 O 8500 24000 22,000 12,000 1600 Ph2OCH2CH2-Me3Py 21,600 8500 2300 22,000 13,000 1600 Ph2OCH2CH2-4MePyb 21 , 5 0 0 7800 2400 21,500 11,000 1500 Ph2OCH2CH2-Py 21,700 7700 2600 21,800 11,000 1500 Ph2OCH2CH2-4PhPyc 21 , 9 0 0 6800 2500 --d Band I (dc;* ➔ pc;)1 Band II (dc;* ➔ pc;)3 1 9,200 h 19,300 420 1400 Ph2OCH2CH2-4MePy --e 18,600 450 1800 Ph2OCH2CH2-NEt3 --e 19,600 410 1700 Ph2OCH2CH3 Band Ill Ph20CH2CH3 28,200 5000 3800 28,100 5700 3200 Ph2OCH2CH2-NEt3 27,700 5000 4600 28,200 6000 3000 Ph2OCH2CH2-Me3Py 28,300 5600 3700 28,700 8500 4200 Ph2OCH2CH2-4MePy 27,300 5300 5300 27,000 5800 2900 Ph2OCH2CH2-Py 27,600 4000 4500 27,600 --d 4500 4400 4400 --d a. Spectra were measured in 2:1 2-methyltetrahydrofuran/propionitrile solutions unless otherwise indicated. b. A 3,5-dimethyl-4-isobutylpyrazole complex was used due to the poor solubility of the corresponding 3,5-dimethylpyrazole complex. c. Spectra measured in acetonitrile. d. Spectral parameters could not be determined due to the presence of intense Ph4B ➔ 4-PhPy charge-transfer bands. e. Not observed at room temperature. f. +,- 100 cm·1. g. +,- 10%. h. Determined from room temperature excitation spectra. 1 31 Table 3.2: Oscillator Strengths for the (dcr* ➔ pcr) 1 , 3 and Band Ill Electronic Transitions. Complex Room ra f (da* ➔ pcr) 1 Ph20CH2CH3 0.1 0.09 Ph20CH2CH2-NEt3 0.09 0.09 Ph20CH2CH2-Me3Py 0.09 0.10 Ph20CH2CH2-4MePy 0.09 0.08 Ph20CH2CH2-Py 0.09 0.08 Ph20CH2CH2-4PhPy 0.08 --b Ph20CH2CH3 0.09 0.08 Ph20CH2CH2-NEts 0.10 0.08 Ph20CH2CH2-Me3Py 0.09 0.10 Ph20CH2CH2-4MePy 0.10 0.08 Ph20CH2CH2-Py 0.08 --b Ph20CH2CH2-4PhPy 0.09 --b · Band Ill (da* ➔ pa) 3 Ph20CH2CH3 0.03 Ph20CH2CH2-NEts 0.03 Ph20CH2CH2-4MePy 0.04 a. +,- 0.01 b. Not determined due to Ph4B ➔ Py charge-transfer bands which distorted the transition's band shape 132 complexes with terminal phosphine and carbon monoxide ligand have not been carried out , spectroscopic studies of the donor-acceptor and model complexes were undertaken to characterize their low lying excited states. 6 Electronic absorption and emission spectra of the model compounds and donoracceptor complexes were for the most part identical within their experimental uncertainties. Spectroscopic parameters for all of the d8 -d 8 dimers are summarized in tables 3.1 to 3.3. The spectroscopic properties common to this series of compounds are discussed below by referring to absorption, emission, and excitation spectra of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 as illustrative examples. Absorption Spectra: The room temperature absorption spectra of the donor-acceptor complexes and model compounds in 2:1 2-methylTHF/propionitrile solutions show two intense absorption features with maxima at approximately 21,000 cm- 1 (E = 8500) and 27,900 cm- 1 (e = 5000). These transitions are labeled Band I and Band II in figure 3.3a. As seen in figure 3.3b Bands I and II sharpen and blue shift slightly in 77° K glass spectra of the complexes, revealing three lower intensity transitions with maxima at approximately 19,000 cm-1 (Band 111), 24,000 cm- 1 (Band IV), and 26,000 cm- 1 (Band V). Band Ill represents the lowest energy spectroscopic transition in these compounds and is well resolved from the low energy wing of Band I at low temperature. Bands IV and V appear as shoulders on the high energy side of Band I and the low energy side of Band 11, respectively. In terms of the relative intensities and energies of Bands I, 11, and 111, the electronic absorption spectra of the carbon monoxide, phosphine metal dimers, are strikingly similar to spectra of a series of [lr2(Pz)2(COD)2] complexes. 8 Thus, assignments for each of these transitions can be made by analogy to similar spectral features observed for the corresponding cyclooctadiene systems. Based on its intensity and temperature independent oscillator strength, Band I is assigned to a fully 133 Figure 3.3. Absorption spectrum of lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 in 2:1 2methyltetrahydrofuran/propionitrile solutions. a) Room temperature. b) 77° K. 134 0 0 ~ ... Ethaxy Diphenyphaaphine COaplex 295 K a. 0 --o II o U 0 ... ... ~ Band I .,_ .,_ 1111 0 u C 0 ... ... u ...... C 0 Band II 0 0 iO IC "' 0 15000 20000 Frequency 30000 25000 (1/c:111) 0 g Ethaxy Diphenylphasphine COaplex 77 K ... Ill) b. "i u ..... ...:E -. • 0 0 0 0 ... B . . C 0 ... ...u C 0 0 0 IO IC 11,1 Benet Ill 0 20000 frequency U/cal IOOOO 135 allowed 1A ➔ 1 B (dcr* ➔ pcr) electronic transition. Band Ill is logically assigned to the corresponding triplet transition, 1 A ➔ 38 (dcr* ➔ pcr), due to its substantially weaker intensity. These assignments are consistent with the MO diagram seen figure in 3.2 and are supported by previous arguments, regarding the spectra of several A-frame and face-to-face ds-ds complexes.3d,5,6,8,9 The higher energy excited states of d8 -d 8 A-frame metal dimers are poorly understood. In contrast to the face-to-face complexes, definitive assignments for the higher lying electronic transitions in these complexes are precluded by their lower symmetry, their more complicated ligand systems, and a lack of information concerning spectra of their corresponding monomers. 1O In general, these states rapidly internally convert to the lower energy (dcr*pcr) 1,3 states and are not responsible for the photochemical reactivity associated with d8-d8 systems.Sa, 13 While a complete assignment of the higher energy absorption bands in spectra of the donor-acceptor molecules is outside the scope of this thesis, some parallels can be drawn between these bands and analogous features in spectra of related A-frame systems. Absorption features similar to Band II have been observed in spectra of [lr2(Pz)2(COD)2] (381 nm), [lr2(Pz)2(C0)4] (325 nm), and their analogs. These bands have been attributed to (dxz,dyz ➔ pz) transitions in the [lr2(Pz)2(COD)2] systems by analogy to assignments made for Rh2(Bridge) 42+ and lr2 (TMB) 42+.sa,b However, in comparison to the bandshapes observed for the d1t ➔ p transitions in faceto-face d8-d 8 systems, Band II has a much larger half-width at both room temperature and 77° K. This distinction in bandshapes indicates that Band II involves excited state distortions not found in the d1t excited states of face-to-face metal dimers. One possible explanation for the substantial half-widths associated with Band II is that this transition contains a considerable amount of metal-to-pyrazole charge transfer character. This interpretation is suggested by the low energy MLCT transitions seen in orthometallated " lr(ll1) monomers.11 Careful inspection of Bands IV and Vat 77° K shows that they are 136 Figure 3.4. 77° K absorption spectrum of lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 showing a 587 cm· 1 progression in v1r-C superimposed on Bands IV and V. I g ...I t I ..... ..... c., i g 0 ~ 0 138 weakly structured in a 587 cm· 1 vibrational progression (figure 3.4). Similar bands appear in spectra of [lr2(Pz)2(CO)4] and its analogs, but are absent in spectra of the corresponding pyrazolate bridged COD systems. 8 These bands have been tentatively assigned to (d1t ➔ p,1t*CO) transitions based on their 500 cm·1 vibrational progressions and their similarity to features observed in monomer spectra. 12 Their low intensity in spectra of the lr2(Pz*)2(CO)2(Ph2POR)2 systems and absence in spectra of the COD dimers suggests that they are forbidden transitions, which gain substantial intensity via . vibronic coupling to an lr-C vibrational mode. Further detailed spectroscopic studies, involving both the pyrazolate bridged dimers and their corresponding monomers, will be necessary to definitively assign these higher energy bands. Emission Spectra: Electronic emission spectra of the donor-acceptor complexes and model compounds at 77° K show two emission bands in the spectral region between 10,000 and 25,000 cm· 1 with maxima at approximately 19,000 cm·1 and 15,000 cm· 1 (figure 3.5). These bands are logically assigned to 18 ➔ 1A(pcr ➔ dcr*) and 3s ➔ 1A(pcr ➔ dcr*) electronic transitions, respectively, by analogy to emission features observed for a number of A-frame and face-to-face dimer compounds.3d,5,6,8,9 At 77° K the singlet and triplet emission intensities of donor-acceptor compounds were similar to the intensities of the lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 and lr2(Pz*)2(CO)2(Ph2POCH2CH2-NEt3+)2. Further, the 77° K phosphorescence lifetimes of the donor-acceptor and model compounds were the same within their experimental uncertainties. These indicate that rapid photoinduced electron transfer does not take place from either the 1 8 or 38 excited states in a rigid low temperature matrix. Spectral parameters for all the complexes are summarized in table 3.3. The energies and intensities of the 18 ➔ 1A and 38 ➔ 1A emission bands in the model and donor-acceptor compounds change substantially as a function of temperature between 77° K and 295° K. As seen in figure 3.6, at room temperature both emission bands broaden and are red shifted with respect to their 77° K band maxima. Table 3.4 139 Figure 3.5. a) Electronic emission spectrum of lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 at 77° K. b) Higher sensitivity spectrum showing the singlet emission band at 77° K. 140 a. • I • • E • :3 ; ..•• • ! i •. • • . ..• .. .• . I i . ' I I I I I I 10000 l I •i ' 15000 20000 25000 Frlii!quancy Cl /cm) b. I \ ! / i I ! ' \ i i I i ' ! ' ! V' 15000 20000 Fraqusncy Cl/cm) 25000 1 41 Table 3.3: 77° K Emission Parameters for the Iridium Dimer Complexes.a Complex (1B ➔ 1A) (38 ➔ 1A) I r2 ( Pz*) 2 (CO) 2 L2 Vmax cm- 1 V1 /2 cm- 1 Vmax cm-1 V112 cm- 1 Ph2OCH2CH3 19,300 1500 15,500 1500 Ph2OCH2CH2-NEt3 19,200 1500 15,400 1600 Ph2OCH2CH2-Me3Py 19,200 1600 15,400 1600 Ph2OCH2CH2-4MePy 19,300 1500 15,500 1500 Ph2OCH2CH2-Py 19,300 1600 15,500 1500 Ph2OCH2CH2-4PhPy 19,300 1800 15,500 1500 a. Spectra were measured of 2:1 2-methyltetrahydrofuran/propionitrile solutions. b. A 4-isobutyl-3,5-dimethylpyrazole complex was used due to the poor solubility of the corresponding 3,5-dimethylpyrazole complex. 142 Table 3.4: Spectral Data for Selected A-Frame Complexes. Complex lr2(Pz*)2(CO)2(Ph2POCH2CH3)2a lr2(Pz*)2(CO)2(Ph2PO(C H2)2N Et3)2b I r2 ( Pz *) 2 (co) 4 c, d lr2(Pz)2(CO)4c,d I r2(3-M ethyl-Pz)2(CO) 4 C, d lr2(Pz)2(COD)2a a. 2-Methyltetrahydronfuran solutions. b. 2: 1 2-methylTHF/propionitrile solutions c. 2-Methylpentane solutions d. See reference 8. Vmax ( 18 ➔ 1A) cm- 1 Vmax (3 8 ➔ 1A) cm- 1 OK 17,900 13,500 298 19,400 15,500 77 18,200 13,300 298 19,200 15,400 77 18,800 13,600 298 20,000 14,900 77 18,800 13,300 298 19,600 15,400 77 18,800 13,300 298 19,400 14,800 77 17,900 14,600 298 18,000 14,700 77 T 143 Figure 3.6. a) Room temperature emission spectra of (1} lr2(Pz*)2(CO)2(Ph2POC H2C H2N Et3+)2, (2) lr2(Pz*)2(CO)2(Ph2POC H2C H2246M e3Py+)2, (3) lr2(Pz*)2(CO)2(Ph2POCH2CH2-4MePy+)2, and (4) lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py+)2 in acetonitrile comparing their relative intensities and band maxima. Note that the spectrum of lr2(Pz*}2(CO)2(Ph2POCH2CH2NEt3+)2 has been reduced by a factor of five. b) Spectra 2,3, and 4 enlarged. The feature at 500 nm is a mercury line. 144 I - - - ~ UAVCLENGTH M M ~ - • - ae ·- .. ao - WI ftO M MAVELENGTH 145 Figure 3.7. Electronic excitation spectra of lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 in 2methyltetrahydrofuran. a) and b) Room temperature and 77° K singlet spectra. c) and · d) Room temperature and 77° K triplet spectra. 146 - E11I' - - T lintl•t laclteuan lpectrua •• I 0 - uooo aooo aooo Fre..,.ncr It/ad ETDP 77 K llflll•t Eacltetlon &pectrua b. .!... . . ,.. ID £ I. 0 l8000 "'"" ■ncr lt/CIII 147 • ... IETaP floN T Trtplet bc1tet1on llpeetl"UII c. ......... . .s II C II 0 15000 . £TCIP 1111 T1 IC Trt,let bcU.@Uon lpectrue d. ...... .... II i C 0 80000 Pr1111 11nci, Cl/all 148 shows that similar changes as a function of temperatures are observed in emission spectra of [lr2(Pz)2(CO)4) and its analogs, but are curiously absent in spectra of the corresponding [lr2(Pz)2(COD)2] systems. 8 One plausible explanation for this phenomenon is that photoemission in the tetra-CO and CO-phosphine systems occurs from different electronic excited states as a function of temperature. This possibility can be ruled out by examining the excitation spectra of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 seen in figure 3.7. In terms of their band maxima and relative extinctions, the transitions observed in room temperature excitation spectra of this complex correlate well with those observed in its 77° K spectra. In addition, its triplet excitation spectrum is superimposable with its absorption spectrum. These findings indicate that the emission bands found in the high and low temperature spectra of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 both correspond to its (pcr ➔ dcr*) electronic transitions. The intense high energy bands (v >25,000 cm- 1) seen in triplet excitation spectra of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 lose intensity in the correpsonding singlet spectrum, due to efficient intersystem crossing between its (d1tpcr) and 3B excited states. Enhancement in the intersystem crossing rates in this complex can be attributed to spin-orbit effects associated with the iridium metal centers. 13 Similar intensity differences have been observed in spectra of other d8-dB systems. Sa, 14, 13b Further insight into the origin of this "blue shift" effect can be obtained by examining the spectral data for lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 summarized in tables 3.5 and 3.6. Room temperature and 77° K solid state emission spectra of this complex show phosphorescence band maxima that are isoenergetic with those found in its 77° K solution spectrum. In addition, the blue shift in both its singlet and triplet emission band maxima takes place via a completely reversible process over an approximately 10° temperature range between 90° and 100° K. (table 3.6). These data are consistent with a medium induced alteration of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 that modifies its 149 Phase Vmax ( 3 B ➔ 1A) cm- 1 OK T Solid b 15,300 298 Solid --b 15,400 77 Solutiona 17,900 13,500 298 Solution 19,400 15,500 77 a. 2-Methyl THF solutions b. Not observed 150 Table 3.6: Temperature Dependent Emission Data for lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 Vmax (3 8 ➔ 1A) T V112 cm· 1 OK 13,600 1940 302 13,500 1800 270 13,400 1700 230 13,400 1700 200 13,400 1600 180 13,400 1600 160 13,400 1500 150 13,400 1500 140 13,400 1500 120 13,400 1500 100 15,000 1800 90 15,400 1600 80 • 15,500 1600 70 151 emission bandshapes. Analogous effects were first observed by Wrighton and coworkers in the emission spectra of Re(Phen)(C0)3CI and were labeled "rigidochromic luminescence." 15 The origin of luminescence rigidochromism has been discussed in detail by Dellinger and Kasha and can be understood in terms of the potential energy surfaces portrayed in figure 3.a. 16 The extent to which a molecular system can distort in response to any perturbation (i.e., electronic or vibrational) is determined not only by its normal modes and vibrational force constants but also by the ability of its surrounding medium to respond to changes in its overall shape. In a fluid solution the surrounding solvent molecules define a mutable cage, which can readily respond to changes in the shape of an enclosed solute. However, in a low temperature matrix or in a molecular crystal, the surrounding medium is more rigidly fixed and is less able to accommodate changes in a molecule's structure. These Umitations alter the nature of the normal vibrational modes of a solute molecule by restricting certain low frequency molecular distortions. As seen in figure 3.8, these restrictions effectively change the shape of the molecule's potential surface along one or more of its normal coordinates. In terms of its electronic spectra, these changes effect the Frank-Condon factors for emission (absorption) which can drastically change its emission (absorption) band maximum and half width. While our spectroscopic data do not point to a particular molecular distortion as being primarily responsible for the rigidochromic effects in spectra of d8 -d 8 A-frame metal dimers, it can be safely concluded that these distortions must be coupled to the metal-metal coordinate. It is well known from the low temperature single crystal spectra of Rh2(Bridge)4 2+ and Pt2(P20sH2)4 2• that metal-metal vibrations are strongly coupled to their (dcr* ➔ pcr) electronic transitions. 5a,f,g The metal-metal bond distances in A-frame dimer complexes has both an angular as well as a distance 152 Figure 3.8. A schematic representation of a pair of potential energy surfaces illustrating the origin of rigidochromic spectroscopic effects according to the DellingerKasha model. Medium rigidity effects spectroscopically relevant normal modes by restricting the low frequency torsional motions of the chromophore. 153 Fluid Solution E R Rigid Matrix E R 154 dependence. 6 Thus, changes in the metal-metal separation in these systems may involve rearrangement of their bridging pyrazole ligands. This type of torsional motion would be susceptible to medium rigidity effects. 15c Further conclusions, regarding rigidochromic effects in d8 -d 8 A-frame complexes, can be drawn by examining the spectroscopic data summarized in table 3.4. As mentioned earlier, rigidochromic emission appears only in spectra of the [lr2(Pz)2(CO)4] and [lr2(Pz)2(CO)2(Ph2POR)2] systems but is curiously absent in spectra of [lr2(Pz)2(COD)2] and its analogs. These differences can be understood by considering the distinct steric environments provided by the various terminal ligands found in these complexes. Recent x-ray crystallographic results have indicated that the 0.375 A difference in metal-metal distances found in oxidative addition adducts of [lr2(Pz)2(COD)2] and [lr2(Pz}2(CO)2(Ph3P)2] can be attributed in part to the larger steric repulsion between opposing COD ligands in the former compounds. 17 Similar types of steric interactions must also play a role in controlling the excited state metalmetal contractions in these A-frame metal dimers. This premise is supported by comparing the room temperature Stoke's shift seen in [lr2(Pz*)2(CO)4] (5000 cm- 1 ), [lr2(Pz*)2(CO)2(Ph2POCH2CH3}2] (5,800 cm- 1 ), and [lr2(Pz)2(COD)2]. The larger Stoke's shifts in the former two cases are consistent with a larger excited state metal-metal contraction made possible by their less sterically demanding terminal ligands. In contrast to these less sterically hindered systems, contractions along metalmetal coordinate in [lr2(Pz*)2(COD)2] are restricted by steric repulsion between its COD ligands; therefore, this compound does not exhibit rigidochromic effects. In contrast to the low temperature emission data presented earlier, the room temperature emission intensities of both the singlet and triplet excited states in the donor-acceptor complexes are substantially quenched when compared to the corresponding model compounds (figure 3.6).40 This pronounced emission intensity quenching can be attributed to an excited state intramolecular electron-transfer process 155 based on several lines of evidence. In some instances the excited state lifetimes and emission quantum yields of inorganic chromophores can be affected by simple nonchromophoric changes in the nature of their ligands. Nonchromophoric lifetime effects have been previously reported for a series of Mo2X 4 (PR3) 4 complexes 18 and were interpreted within the frame work of Engleman and Jortner's model for nonradiative processes. 19 As seen in table 3.7, the quantum yields and triplet lifetimes of the four model complexes are essentially unperturbed by changes in the phosphinite ligand alkyl substituents. Thus, the intensity quenching seen in spectra of the donoracceptor complexes can not be attributed to changes in knr brought on by the introduction of additional accepting or promoting vibrational modes. This emission quenching must be due to an additional nonradiative deactivation pathway involving intramolecular electron transfer. This conclusion is substantiated by the fact that the decrease in emission intensities across the series of donor-acceptor compounds, presented in figure 3.6, parallels changes in the redox potentials of their covalently attached pyridinium cations (vida infra). Of significant interest is the intensity quenching observed in the singlet emission bands of the donor-acceptor compounds. These changes can be attributed to a direct singlet quenching process rather than a diminished back intersystem crossing yield due to a reactive triplet state. Control experiments show that the singlet emission intensity in lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 is not diminished when > 75% of its triplet emission is quenched via bimolecular energy transfer reactions. Thus, back intersystem crossing is not an active photophyscial process in the donor-acceptor and model complexes. This conclusion is in agreement with equilibrium constants (Keqisc) calculated from the 3700 cm- 1 singlet-triplet energy gap in these complexes. In addition, triplet electron-transfer quenching in these systems can be inferred from the quantum yield data in table 3.7. Because <l>f0 t<l>f < <j>p 0 /<j>p, some of the intensity quenching observed in the triplet 156 Table 3.7: Photophysical Parameters for the Donor-Acceptor Complexes.a Complex 'Pf <I>p lr2(Pz*)2(CO)2L2 1p µs Ph2OCH2CH2-NEtab 0.0015 0.032 1.2 Ph20CH2CH3 0.0023 0.025 1.1 Ph2OCH3 0.0027 0.040 1.2 Ph2O(CH2)3CH3 0.0025 0.030 1.1 Ph2OCH2CH2-Me3Pyc 0.00017 0.0013 Ph2OCH2CH2-4MePy 0.00006 0.0004 Ph2OCH2CH2-Py 0.00003 - _d --d Ph2OCH2CH2-4PhPy a. Quantum yields taken from spectra measured in acetonitrile solutions at room temperature using Ru(Bipy)32+ as a standard. b. Singlet quantum yields for the model compounds are accurate to ±1 0%; triplet quantum yields are accurate to ±30%. c. Quantum yields for the donor-acceptor compounds are accurate to ±80% . d. Emission too weak to measure. 157 emission bands of the donor-acceptor complexes must be due to a direct triplet electrontransfer process.20 To date, the photochemical reactivity of d8 -d 8 complexes has been solely attributed to their long-lived triplet excited states. The spectra, reported here, represent the first evidence for reactivity from the shorter lived singlet states. Analyzing the quantum yield data in table 3.7 along with a singlet lifetime for lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 of 100 ps, places the singlet electron-transfer rates in these complexes between 5x101 a sec-1 and 1012 sec-1. In comparison to the singlet reactivity reported here, the singlet state of [lr2(Pz)2(COD)2] was found to be unreactive in an earlier bimolecular electron-transfer study. 3d, 8 This lack of reactivity was attributed to its subnanosecond fluorescence lifetime. This shorter lived state can be intercepted by a rapid quenching process in the donor-acceptor complexes, presumably because the two redox partners are only several bond rotations away from being within the required distance for rapid electron transfer. Thus, covalently linking pyridinium cations to an iridium dimer chromophore appears to be essential for observing singlet excited state ET reactions. In addition, these reactions must have almost activationless thermal barriers, considering their substantial unimolecular rate constants. A more detailed account of the electron-transfer reactions seen in the donor- acceptor systems will be presented in chapter 4. 158 Electrochemistry Previous studies regarding the electrochemical properties of d8-d 8 metal dimers have played an important role in our understanding of the thermodynamic and kinetic factors that control their ground and excited state redox reactions. Electrochemical studies have lead to the generation and characterization of both d8-d7 and d7 -d7 complexes. These species have been identified as key intermediates in photocatalytic cycles involving Pt2(P205H2)4 2·and lr2(TMB)4 2+. 22 ,1,3 In some instances stable d8 d7 complexes have been generated and characterized by electronic spectroscopy and xray crystallography.21e,f In addition, recent electrochemical studies involving Rh2(TMB)4 2+ have indicated that its one electron oxidized state, Rh2(TMB)43+, is a reactive C-H bond activation catalyst. 22 These results have greatly enhanced our understanding of the excited state properties of d8-d8 complexes by uncovering a new type of ground state reactivity. The electrochemical properties of the donor-acceptor and model compounds were of interest from several different standpoints. An understanding of the reaction exoergicities found .in the donor-acceptor systems is important in characterizing their excited state electron-transfer reactions. Based on arguments presented below, a good estimate of the reaction driving force for photoinduced electron-transfer in our compounds can be obtained from the one electron redox potentials of their iridium metal centers and pyridinium cations and the spectroscopic energies of their 1B, 3B excited states. Thus, one goal of our electrochemical studies was to measure the one electron redox potentials for the iridium dimer chromophores and pyridinium cations. In chapter four, the photophysics of the donor-acceptor complexes is explored using picosecond time-resolved absorption spectroscopy. In principle an iridium dimer cation species, [lr2(Pz*)2(C0)2(Ph2POR)2]+, should be one of several transients produced in these experiments. Identifying this species in time-resolved absorption spectra of the donor-acceptor complexes would be greatly facilitated, if an 159 [lr2+(Pz*)2(C0)2(Ph2PR)2] d8 -d 7 complex could be electrochemically generated and characterized. Finally, it was of interest to determine how covalently linking two pyridinium electron acceptors to an iridium dimer complex perturbs its one electron oxidation potentials. If sufficient ground state electronic coupling exists between the iridium metal centers and pyridinium cations in these systems, the electronic properties of both redox partners should be affected. While evidence for ground state donor-acceptor interactions was not observed in any of the electronic spectra presented earlier, electrochemical experiments offered an additional opportunity for investigating electron coupling effects. Previous studies have indicated that the electrochemical behavior of d8-d 8 dimers strongly depends on the nature of the solvent and supporting electrolyte. 21 d,e,g Much like d8 monomers, these complexes have a propensity to acquire axial ligands upon oxidation. Thus, the presence of axial ligands as either a coordinating solvent or supporting electrolyte can drastically effect the shape of the waves in cyclic voltammograms of these compounds. Cyclic voltammograms (CV) of lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 in 0.1 M TBAPF5/CH3CN showed a single irreversible oxidation wave with Epa= 0.33 V and an irreversible electrode process at Epc= -0.84 V corresponding to the reduction of an [lr2(Pz*)2(C0)2(Ph2POCH2CH3)2r+ species. 43 While this behavior was not investigated in detail, it is presumed that the irreversible nature of both these couples is related to a complex process involving the coordination of one or more acetonitrile molecules to the oxidized iridium dimer. To avoid these complicating factors, CV experiments concerning the donor-acceptor complexes and model compounds were carried out in 0.1 M TBAPs/CH2Cl2 solutions. In some instances complexes containing 3,5-dimethyl-4-isobutyl-pyrazole ligands were used due to the poor solubility of the corresponding 3,5-dimethylpyrazole complexes in methylene chloride. Differences between these compounds and those containing 3,5-dimethylpyrazole bridging ligands 160 will be discussed when necessary. The following section details our results concerning the electrochemistry of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2, lr2(Pz*)2(CO)2(Ph3P)3, and the donor-acceptor complexes as seen in their cyclic voltammograms and bulk electrolysis data. [lr2(Pz*)2(C0)2( Ph2POCH2CH3)2]: Cyclic voltammograms of lr2{Pz*)2{CO)2(Ph2POCH2CH3)2 in 0.1 M TBAPFsfCH2Cl2 solutions, taken at a BAS platinum button working electrode, show two electrochemical couples within the potential window between -1.5 and 1.5 volts (figure 3.9). Values of Epc, Epa, LiE, and E 112 for the first electrode process {Ep,a=0.34 volts) were constant as a function of scan rate(v) for v=20 to 500 mv/sec. Ratios of the anodic and cathodic peak currents for this couple varied between 1.3 and 1.1 and approached unity with increasing scan rate. Similar results were obtained at BAS glassy carbon electrodes. Constant potential bulk electrolysis experiments carried out at a pyrolytic graphite electrode (Eapp=0.6 volts) yielded values of n=1 for this oxidation process. These data are consistent with a quasireversible one electron oxidation of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 at E112=0.30 volts which is followed by a slow chemical reaction. The second electrode process, seen in figure 3.9 at more positive potentials, was found to be irreversible for v=20 to 500 mv/sec. This couple is logically assigned to a second one electron oxidation of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2+ by analogy to results from a recent study concerning [lr2(Pz)2(COD)2].21g Because our interest in this study was focused primarily on the one electron oxidation of the model compounds and donor-acceptor complexes, this couple and its analogs in other systems were not investigated in detail. Additional information regarding the nature of the chemical reactions which follow the one electron oxidation of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 was obtained by examining cyclic voltammograms and infrared spectra of methylene chloride solutions of 1 61 Figure 3.9. Cyclic voltammograms of lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 in 0.1 M TBAPFs/CH2Cl2. This compound shows two single electron oxidations between 0.0 and 1.5 volts versus SSCE. 162 . II') . 0 . II') 0 iii CJ a, a, 0 0 0 . ft-;-, I -!.! C / •:, ! w . II') 0 I 0 . . II') I 163 Figure 3.10. Cyclic voltammogram of lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 following bulk electrolysis at Eapp=0.6 volts. lr2(Pz*)2(C0)2(Ph2POCH2CH3)2+ is not chemically stable on a bulk electrolysis time scale and reacts to form two unidentified · products Epc=-0.23 and Epc=-1.0 volts. t la T 1.5 µA _j_ ..... 0, .r:,.. -1.5 -1.0 -0.5 0.00 E (volts versus SSCE) 0.5 1.0 1.5 165 this complex following bulk electrolysis. Electrochemical oxidation of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 at a pyrolytic graphite electrode (Eapp= +0.6 volts) produces a color change from orange/red to light yellow. The intense visible absorption bands characteristic of the starting material decrease in intensity av.er the course of the experiment and are replaced by a higher energy absorption feature at Amax=376 nm. Cyclic voltammograms of the electrolysis products are substantially different from CVs of lr2(Pz*)2(CO)2(Ph2POCH2CH3}2. As seen in figure 3.10, in addition to the quasireversible couple at E112=+0.30 volts discussed earlier, CVs of the electrolysis products show two irreversible couples at Epc=·0.23 volts and Epc=-1.0 volts. Scanning cathodically over either of these couples in CV experiments leads to the formation of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 as indicated by the appearance of its oxidation wave at 0.3 volts. Bulk electrolysis of product solutions at potentials greater than -1.0 volts regenerates lr2(Pz*)2(CO)2(Ph2POCH2CH3}2 in almost quantitative yield. These data show that the reactions associated with the oxidation of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 are chemically reversible and that both electrolysis products are related to their d8-d8 precursor via what is presumed to be a one electron reduction. Specifically, they indicate that the metal-metal unit in the oxidized complex remains intact. If lr2(Pz*)2(CO)2(Ph2POCH2CH3)2+ fragmented into its corresponding monomers, it is not likely that electrochemically reducing these monomers would efficiently regenerate lr2(Pz*)2(CO)2(Ph2POCH2CH3)2. In addition, the absence of either the -0.23 or -1.0 volt electrode couples in CVs of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 supports the premise that the one electron oxidation of this complex can be characterized by an "ECE type" mechanism where the following chemical step does not significantly perturb the reversible electrode process. 23 166 Figure 3.11. Infrared spectra of lr2(Pz*)2(CO)2(Ph2POCH2C H3)2 in methylene chloride, a) before and b) following bulk electrolysis at Eapp=0.6 volts. Asterisks in part b) label bands due to the solvent. The single CO band in the reactants splits into two separate bands in the products which are assigned to the symmetric and asymmetric CO modes. The stronger interaction between these two modes in spectrum b is consistent with the formation of a partial metal-metal bond in the electrolysis products. 167 b. a. l 2000 I 2200 1 1900 I 2000 I 1900 168 Solution IR spectra of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 and its electrolysis products show features that are consistent with the oxidation of the compound's iridium metal centers. As seen in figure 3.11, the broad band at v=1960 cm- 1, attributed to overlapping symmetric and asymmetric CO vibrational modes in lr2(Pz*)2(CO)2(Ph2POCH2CH3)2, shifts to higher frequency in spectra of the electrolysis products and splits into two well resolved bands with maxima at 2,150 cm1 and 2,000 cm-1. These bands are assigned to the symmetric (VA=2,150 cm· 1) and asymmetric (vs=2,000 cm-1) CO vibrations in the products. 24 ,41 In addition, the larger splitting between the symmetric and asymmetric CO bands in the product is consistent with an increase in the metal-metal bond order that would accompany the oxidation of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2. 21 f Unfortunately, these data say very little concerning the structures of the two electrolysis products. Several possibilities can be offered based on related d8-d8 systems. One plausible reaction, which could follow the oxidation of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2, is the sequential coordination of PFs- counter ions to the dimer's axial coordination sites. The coordination of weak field ligands to d8-d 7 systems has been previously reported in studies involving Rh(Dimen)4 2+ and a series of different counter ions (PFs< CIO4 < Cl). 21 d A more remote possibility would involve the oligomerization of the oxidized metal dimers. A similar dimerization reaction is known to occur in solutions of Rh2(Bridge)43+.25 It is clear, however, that the nature of the electrolysis products formed from lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 is an area that is open to speculation and to further detailed investigations. Donor-Acceptor Complexes: Cyclic voltammograms of the donor-acceptor compounds were, in general, qualitatively similar. Features common to all of these complexes are exemplified by the CVs of 169 Figure 3.12. Cyclic voltammograms of lr2(Pz*)2(C0)2(Ph2POCH2CH2246Me3Py+)2 in 0.1 M TBAPF5/CH2Cl2 solutions. Part a shows all of the redox couples seen in CVs of the donor-acceptor complexes between ±1.5 volts. Part b shows that a return wave for the couple at Epa=0.5 volts can be observed if the potential scan direction is reversed at the foot of the tetraphenylborate wave. 170 0 . . 11D 0 1111 u 0 0 0 C r-:~ = • . • -•2 ::::, ! > IU . 11ft 0 • .... 0 ...• 11ft 1 71 lr2(Pz*)2(CO)2(Ph2POCH2CH2-Me3Py+)2(Ph4B)2 seen in figure 3.12. Final electrochemical parameters for these compounds are summarized in table 3.8. In contrast to lr2(Pz*)2(CO)2(Ph2POCH2CH3)2, CVs of the donor-acceptor compounds show three electrochemical couples in the potential range between + 1.5 and -1.5 volts versus SSCE. The two anodic couples, found at Epa=0.5 volts and Epa=0.95 volts, appear at approximately the same peak potentials in all the donor-acceptor complexes and were observed in CVs of lr2(Pz*)2(CO)2(Ph2POCH2CH2-NEt3+)2(Ph4B)2 as well. The peak potentials of the cathodic couple, found at Epc=-1.47 volts in figure 3.12a, varied as a function of the pyridinium cation in the donor-acceptor complexes and was absent in CVs of lr2(Pz*)2(CO)2(Ph2POCH2CH2-NEt3+)2. This couple is assigned to the one electron reduction of the two covalently bound pyridinium cations in the donor-acceptor systems. The irreversible nature of this electrode process can be attributed to the rapid dimerization reactions of pyridyl radicals. 26 Control experiments involving CVs of (Ph2POCH2CH2-Me3Py+)(Ph4B) and TBA(Ph4B} verified that the irreversible couple at Epa=0.95 volts is due to the electrochemical oxidation of Ph4B-. If the scan direction was reversed at the foot of this wave, a return wave was observed for the couple at Epa=0.5 volts (figure 3.11 b). Scan rate dependence studies in a potential window between 0.0 and 0.6 volts indicated that the values of Epa, Epc, dE, and E112 for this couple were scan rate independent for v=20 to 500 mv/sec. Bulk electrolysis experiments regarding this oxidation process were precluded in the donor-acceptor compounds by its proximity to the oxidation of Ph4B-. However, as seen in figure 3.13, CVs of lr2(Pz*)2(CO)2(Ph2POCH2CH2-NEt3+)2(PFs)2 contain an identical quasireversible electrochemical couple. Bulk electrolysis of this complex at a pyrolytic graphite electrode(Eapp=+0.7 voits) yielded a value of n=1 for this electrode process. Reduction of the electrolysis products at Eapp=-1.0 volts regenerated lr2(Pz*)2(CO)2(Ph2POCH2CH2-NEt3+)2 in close to quantitative yield. These data 1 72 Figure 3.13. Cyclic voltammograms of lr2(Pz*)2(C0)2(Ph2POCH2CH2NEt3+)2(PFs)2 in 0.1 M TBAPFs/CH2Cl2. . t la ..... T ...... w 0.75 µA _L -0.5 0.00 0.5 1.0 E (volts versus SSCE) 1.5 174 indicate that this electrochemical couple in CVs of lr2(Pz*)2(CO)2(Ph2POCH2CH2Me3Py+)2 and lr2(Pz*)2(CO)2(Ph2POCH2CH2-NEt3+)2 corresponds to the one electron oxidation of their iridium metal centers. Analogous quasireversible waves were seen in CVs of lr2(Pz**)2(CO)2(Ph2POCH2CH2-4MePy+)2, Ir2(Pz**)2(CO)2(Ph2POC H2C H2-Py+)2, and lr2(Pz**)2(CO)2( Ph2 POC H2C H24 Ph Py+)2 at E112 =0.42 volts. The 50 mv difference between the oxidation potential of these compounds and lr2(Pz*}2(CO)2(Ph2POCH2CH2-Me3Py+)2 can be attributed to their more electron donating 3,5-dimethyl-4-isobutylpyrazole ligands. Similar trends have been observed in the photoelectron spectra of a series of [lr2(Pz)2(CO)4] complexes (Pz=pyrazole, 3-methylpyrazole, and 3,5-dimethylpyrazole). 27 A careful examination of the redox potentials for the donor-acceptor complexes and model compounds reveals an important trend in their electrochemical properties. As seen in table 3.8, the oxidation potentials of the donor-acceptor complexes lie approximately 150 mv anodic of the oxidation potential of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2. In addition, the oxidation potentials of lr2(Pz*)2(CO)2(Ph2POC H2C H2-N Et3+)2 and lr2(Pz*)2(CO)2(Ph2POC H2CH2Me3Py+)2 are the same within experimental error. These findings indicate that the donor-acceptor compounds and lr2(Pz*)2(CO)2(Ph2POCH2CH2-NEt3+)2 are more difficult to oxidize due to their overall 2+ charge. Our data show little evidence for ground state electronic interactions between the iridium metal centers and pyridinium cations in the donor-acceptor complexes. [lr2(Pz"')2(C0)2( Ph3P)2]: Cyclic voltammograms of lr2(Pz*)2(CO)2(Ph3P)2 show two anodic electrode processes at Epa< 1)=0.2 volts and Epa< 2>=1.3 volts, which are qualitatively similar to the electrochemical couples observed in CVs of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2. Values of Epa, Epc, E112, and LlE for the first electrode process were independent of scan rate and lpa/lpc=1.0 for v=20 to 500 175 Table 3.8: Electrochemical Potentials for the Iridium Dimer Complexes.a e-+ Py+ ➔ Py lr2 ➔ lr2+ + eComplex Epa Epc ~E E 112 lr2(Pz)2(C0)2L2 V V mv V Ph20CH2CH2-NEt3(Ph48) 0.52 0.43 90 0.48 Ph20CH2CH2-NEt3(PFs) 0.53 0.43 100 0.48 Ph20CH2CH3 0.35 0.26 90 0.30 Ph20CH2CH2-Me3Py 0.52 0.42 11 0 0.47 -1 .49 Ph20CH2CH2-4MePyb 0.47 0.37 100 0.42 -1 .32 Ph20CH2CH2-Pyb 0.44 0.36 80 0.40 -1 . 1 7 Epc Ve Ph20CH2CH2-4PhPyb -1 . 1 0 0.47 0.37 100 0.42 a. Measured in 0.1 M TBAPF5/CH2Cl2 versus SSCE. b. Potentials were measured using a 4-isobutyl-3,5-dimethylpyrazole derivative due to the poor solubility of the 3,5-dimethylpyrazole derviative in CH2Cl2. c. Peak potentials measured at a sweep rate of 100 mv/sec. 176 mv/sec. These data indicated that lr2(Pz*)2(CO)2(Ph3P)2 was a suitable system for electrochemically generating and characterizing a stable d8-d 7 A-frame complex. Bulk electrolysis of this complex, in 0.1 M TBAPFs/CH2Cl2 solutions at a platinum basket electrode (Eapp=0.4 volts), produced a color change from red/orange to light yellow and yielded a value of n=1 for this redox process. The intense d ➔ p transitions attributed to the starting material decrease in intensity during the electrolysis experiment and are replaced by absorption bands characteristic of the electrolysis products. As seen in figure 3.14, CVs of the electrolysis products are nearly identical to CVs of lr2(Pz*)2(CO)2(Ph3P)2. Thus, in contrast to [lr2(Pz*)2(CO)2(Ph2POCH2CH3)2]+, [lr2(Pz*)2(CO)2(Ph3P)2]+ is a chemically stable species under the prevailing reaction conditions. IR spectra of this d8-d 7 complex were similar to those seen earlier for the [lr2(Pz*)2(CO)2(Ph2POCH2CH3)2] 01+ system. As in IR spectra of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 and its corresponding cations, vco for [lr2(Pz*)2(CO)2(Ph3P)3]+ is greater than vco for lr2(Pz*)2(CO)2(Ph3P)2 ( figure 3.15 VA=1990 cm-1; vs=1980 cm- 1). Electronic absorption spectra of [lr2(Pz*)2(CO)2(Ph3P)2]+ show four bands with maxima at 373 nm, 460 nm, 500 nm,and 770 nm (figure 3.16). Extinction coefficients for these transitions were estimated from the known concentration of lr2(Pz*)2(CO)2(Ph3P)2 in the electrolysis experiment. The intense band at 373 nm is assigned to a 2B ➔ 2A (dcr ➔ dcr*) electronic transition by analogy to a similar spectroscopic feature found in spectra of Rh2(Bridge)4 2+. 28 The weaker bands at 460 nm and 500 nm are tentatively assigned to ligand field transitions (dxz,dyz ➔ dcr*). The broad low energy transition at 770 nm is not assigned.29 177 Figure 3.14. Cyclic voltammograms of lr2(Pz*)2(C0)2(Ph3P)2 a) before and b) following bulk electrolysis at Eapp=0.4 volts. CVs of the reactant and product of this reaction are identical, indicating that lr2(Pz*)2(C0)2(Ph3P)2+ is a chemically stable species. 178 T 3.0 µA _L t b. 3.0 µA _L -0.5 0.5 0.00 E (volts versus SSCE) 179 Figure 3.15. Infrared spectra of lr2(Pz*)2(C0)2(Ph3P)2 in methylene chloride a)before and b) following bulk electrolysis. 180 b. a. 1990 1980 2000 1900 1800 l 2200 I 2000 I 1900 v(cm- 1 ) 1 81 Figure 3.16. Electronic absorption spectra of lr2(Pz*)2(C0)2(Ph3P)2 a) before and b) following bulk electrolysis. During the electrolysis experiment the 1 A ➔ 18 (do-* ➔ po-) absorption band of the reactants disappears and is replaced by an intense feature at 377 nm which is assigned to a 29 ➔ 2A(do- ➔ dcr*) transition. 182 •• LW LI &I I II I I I II I I I ... b. ~,o,ooo z. ■ &I I LI e=1,0OO e=SOO &I I I I I I I --.-mt-> I I I I 183 Excited State Energetics A central objective of our studies, regarding the electrochemical and spectroscopic properties of the donor-acceptor compounds, was to determine estimates for electron-transfer driving forces. This type of information is important from several stand points. First, reaction exoergicity is known to be a fundamental kinetic parameter in electron-transfer reactions. 30 The driving force dependence is an important characteristic of an electron-transfer process, because exoergicity, along with inner-sphere and outer-sphere reorganization energies, determines the thermal barriers to electron transfer. Secondly, in contrast to a bimolecular ET system, the charge separated state in the donor-acceptor complexes can be thought of as an additional molecular electronic state. Characterizing the relative energies of the metal localized ( 1 B and 3 B) and charge-transfer (CT) states in the donor-acceptor systems is important to understanding their photophysical properties. The reaction exoergicities in the donor-acceptor complexes can be estimated from the one electron oxidation and reduction potentials of their 18 and 3 8 excited states, and pyridinium cations using the spectroscopic and electrochemical data presented in this chapter. 31 ,32 The oxidation potential of the 18 and 39 excited states are given by equations 3.3a and b. eq. 3.3a eq. 3.3b These equations are essentially electrochemical analogs of Forster's equations for excited state acid-base reactions as discussed by Grabowski et a!.33 They assume that ilH(M*/M+)-dH(M/M+)=E 00 (M*) and iiS(M*/M+)-LiS{M*/M+) =0. assumption is reasonable for condensed phase reactions at i atm. The former The validity of the 184 latter assumption depends on the nature M and M*. Differences in the entropy of the ground and excited states of a molecule can arise from several different effects: 1) changes in dipole moment that lead to changes in solvation, 2) changes in internal degrees of freedom, and 3) changes in orbital and spin degeneracies. This last factor can usually be neglected, because it amounts to about 0.03 eV at 25° c.31 Changes in solvation are not likely to contribute to MS in the donor-acceptor systems due to the low polarity of the 18 and 39 metal localized excited states. Entropy effects due to changes in a chromophore's internal degrees of freedom are difficult to predict. However, a good estimate of MS can be obtained from the Stoke's shift between a compound's absorption and emission maxima; larger Stoke's shifts are associated with larger entropy changes. Previous studies have indicated that the triplet state oxidation potentials for [lr2(Pz)2(COD)2], obtained from equation 3b and from bimolecular quenching studies are the same within experimental error; this compound's Stoke's shift is approximately 2500 cm- 1.8 Considering that the Stoke's shift for the donor-acceptor complexes is a factor of two greater than this value suggests that MS contributions in these complexes are negligible as well. Thus, a knowledge of E00 and E112 is required to estimate the excited state redox potentials of the 18 and 38 excited states in these complexes. Values of E00 (18) and E00 ( 38) were estimated from the arithmetic mean of their corresponding emission and absorption maxima. This procedure yields exact values of E00 provided that the ground and excited state vibrational frequencies for the complexes are the same.34 However, the ground and excited state metal-metal vibrational frequencies in da-da complexes are known to differ by as much as 100 cm1. The error incurred by using this approximation was found to be about ±150 cm- 1 by carrying out similar calculations on Pt2(P2OsH2)4 2- and Rh2(8ridge)4 2+. The oxidation potential of the iridium dimer chromophore is approximated as the average of the oxidation potential for lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 and lr2(Pz*)2(CO)2(Ph2POCH2CH2-NEt3+)2 based on the following argument. In contrast 185 to a bimolecular ET reaction, charge in a donor-acceptor system is redistributed with a single molecular unit, rather than transferred from one molecule to another. As seen in equation 3.4, the electron-transfer reactions in the lr2(Pz*)2{C0)2(Ph2POCH2CH2Q+)2 systems can be formally characterized by the redistribution of a positive charge from a single pyridinium cation (Q+) to a pair of iridium metal centers. Q+--lr2--Q+ ~Q+--lr2+--Q eq. 3.4 Within this formalism the oxidation half reaction for the donor-acceptor complexes is represented by the one electron oxidation of their iridium metal centers under the influence of a single pyridinium cation (equation 3.5.). R--lr2--Q+ ~ R--lr2+--Q+ + e- eq. 3.5 The redox potential for this half reaction could be easily determined using a complex that contains one neutral and one charged phosphinite ligand. However, asymmetrically substituted compounds are not readily prepared. It was shown in the previous section that the pyridinium cations alter the oxidation potentials of the donor-acceptor complexes by acting as inert electrostatic charges. Assuming that these effects are additive, a good estimate for the redox potential of reaction 5 is the mean value of the oxidation potential of lr2(Pz*)2(C0)2(Ph2POCH2CH3}2 and Ir2( Pz*) 2(C0}2( Ph2POC H2C H2-N Et3+)2. Determining reduction potentials for the pyridinium cations in the donoracceptor complexes is complicated by the dimerization reactions of pyridyl radical, which distort the pyridinium CV waves.2 6 Earlier studies have established that pyridyl radicals exist in solution in equilibrium with their "4,4'-dihydrobipyridine like" counter parts (equation 3.6.).35 186 2 /=\ R-N \.=I R* •-R* R--N~N---R R* eq. 3.6 At room temperature these reactions take place at approximately 1os M-1 s·1, favoring the dihydrobipyridine adduct. When R is an electron withdrawing substituent, such as CN or a carbomethoxy group, Keq is approximately 105 M- 1 ; For R*= alkyl or H, Keq is greater than 1011 M-1. Electrochemical EC mechanisms, involving dimerization reactions such as those surrounding pyridyl radicals, have been discussed in detail by Saveant et aJ.36 According to the kinetic analysis reported by these authors, the characteristics of an EC electrochemical couple are a function of the sweep rate (v), substrate concentration (C 0 ), and equilibrium constant and rate constant for the dimerization reaction. The dimerization rates for the type of pyridinium cations employed in the donor-acceptor complexes place their electrochemical couples in the pure kinetic region of a log"' versus logx plot. Under these conditions, the value of E112 for their one electron reduction is related to their peak potentials through equation 3.7; where k is the dimerization rate constant. Reduction potentials for the pyridinium cations in the donor-acceptor complexes were calculated from their peak potentials using equation 3.7 and are summarized in table 3.9. This analysis assumes that the dimerization rate constants for pyridyl radicals bound to the donor-acceptor compounds can be approximated by rate constants for free pyridyl radicals. Using the spectroscopic and electrochemical data found in table 3.9, an approximate state diagram for the donor-acceptor systems can be constructed. As seen 187 in figure 3.17, in addition to their 1B and 38 metal localized excited states, each of the donor-acceptor complexes possesses a charge-transfer state whose energy varies with the reduction potential of the compound's covalently attached pyridinium cation. Notably, the charge-transfer states in the complexes are all well below the 3B excited state; However, no evidence for direct electronic transitions between these CT states and the 1A ground state was found in the electronic absorption and emission spectra of the compounds. Such transitions might be loosely thought of as analogs of the MLCT transitions associated with M(CO)s(Py-R) (M=W, R=CN ,Ph, Cl, etc.) or M(Bipy)33+ (M=Ru or Os) complexes39 . Absorption bands corresponding to a direct optical chargetransfer process must be low enough in intensity in the donor-acceptor complexes that they are not detected with our instrumentation. Based on the photophysical data presented in table 3.7, kr for the 38 ➔ 1A transition in these compounds is approximately 104 sec- 1 . The absence of 1A ➔ CT absorption bands in their electronic spectra suggests that kr for this process is less than 1o4 sec-1. In the following chapter we will see that the excited state lifetimes of the CT state in the donor-acceptor compounds ranges from 100 ps to 1O ns. These relatively short lifetimes must correspond to rapid nonradiative charge recombination reactions, which return these systems to their ground states. Thus, charge separation in our donor-acceptor systems occurs via excited state electron transfer, involving the 1 B and 39 excited states of their iridium dimer redox chromophores, and charge recombination occurs by a primarily nonradiative process. 188 Figure 3.17. A state diagram for the donor-acceptor systems, constructed from the spectroscopic and electrochemical data found in table 3.9. While the charge-transfer states in these complexes are energetic enough to have absorption bands in the visible and near IR, such absorption features were not observed. 189 2.5 - - - lr2*•••Q• 2.0 lr2•··•Py• lr2•·••4Ph-Py• 1. 5 eV 1. 0 0.5 - - - lr2---Q+ 190 Table 3.9: Final Energetic Parameters for the Donor-Acceptor Complexes. Transition E(ev) (J 1A ➔ 19 2.5 ±0.02 1A ➔ 3B 2.0 ±0.02 lr2 ➔ lr2+ + e- 0.39 ±0.03 lr2( 1 B) ➔ lr2+ + e- 2.11 ±0.04 lr2( 3 B) ➔ lr2+ + e- 1.61 ±0.04 -1 . 53 ±0.05 e-+ 4Me-Py+ ➔4Me-Py -1. 4 0 ±0.05 e·+ Py+ ➔ Py -1 . 22 ±0.05 e·+ -1.14 ±0.05 e·+ 246Me3-Py+ ➔246Me3-Py 4Ph-Py+ ➔4Ph-Py 1 91 References 1a. Zipp, A. P., Coord. Chem. Rev., 1988, 84, 47-83. b. Marshall, J, L.; Stiegman, A. E.; Gray, H. B., ACS Symp. Ser., 1986, 307, 166176. c. Roundhill, D. M., J. Amer. Chem. Soc., 1985, 107, 4354-4356. d. Che, C.-M.; Lee, W.-M., J. Chem. Soc. Chem. Comm., 1986, 512-513. e. Alexander, K. A.; Stein, P.; Hedden, D. B.; Roundhill, D. M., Polyhedron, 1983, 1389-1392. f. Roundhill, D. M.; Atherton, S. F., submitted for publication. g. Roundhill, D. M.; Atherton, S. J.; Shen, 2.-P, J. Amer. Chem. Soc., 1987, 109, 6076-6079. 2. Caspar, J. V.; Gray, H.B., J. Amer. Chem. Soc., 1984, 106, 3029-3030. 3a. Vlcek, A.; Gray, H. B, lnorg. Chem., 1987, 26, 1997-2001. b. Harvey. E. L.; Stiegman, A. 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Chem., 1976, 15, 1485-1488. b. Bandini, A. L.; Banditelli, G.; Bonati, F.; Minghetti, G.; Demartin, F.; Manasuro, M., J. Organomet. Chem., 1984, 269, 91-105. 193 c. The compound reported in reference b is a monohapto lr(I) pyrazole complex, which might serve as a suitable monomer system for investigating the higher lying excited states of iridium pyrazolate bridge complexes. 11. King, K. A.; Spellane, P. J.; Watts, R. J., J. Amer. Chem. Soc., 1985, 107, 1431- 1432. 12. Geoffroy, G.; lsci, H.; Mason, W. R., lnorg. Chem., 1977, 16, 1950-1955. 13a. The spin-orbit coupling constant for iridium is approximately 4000 cm- 1 . b. Milder, S. J.; Kliger, D.S., J. Phys. Chem., 1985, 89, 4170-4171. 14. V. M. Miskowski, private communication. 15a. Wrighton, M.; Morse, D. L., J. Amer. Chem. Soc., 1974, 96, 998-1003. b. Giordano, P. J.; Redericks, S. M.; Wrighton, M. S.; Morse, D. L., J. Amer. Chem. Soc., 1978, 100, 2257-2259. c. Barigelletti, F.; Belser, P.; Zelewsky, A.; Juris, A.; Balzani V., J. Phys. Chem., 1985, 89, d. 3680-3684. Servaas, P. C.; Dijk, H. K.; Snocek, T. L.; Sterfkens, D. J.; Oskam, A., lnorg. Chem., 1985, 24, 4494-4498. 16a. Dellinger, B.; Kasha, M., Chem. Phys. Lett., 1975, 36, 410-414. b. Dellinger, B.; Kasha, M., Chem. Phys. Lett., 1976, 38, 9-14. c. Gardner, D. J.; Kasha, M., J. Chem. Phys., 1969, 50, 1543-1552. 17a. Bushnell, G. W.; Fjeldsted, D. 0. K.; Stobart, S. R.; Zaworotko, M. J.; Knox, S. A. R.; Macpherson, K. A., Organomet., 1985, 4, 1107-1114. b. Coleman, A. W.; Eadie, D. T.; Stobart, S. R., J. Amer. Chem. Soc., 1982, 104, 922923. c. Beveridge, K. A.; Bushnell, G. W.; Dixon, K, R.; Eadie, D. T.; Stobart, S. R., J. Amer. Chem. Soc.,1982, 104, 920-922. 18. Hopkins, M. D., Ph.D. Dissertation, California Institute of Technology, Pasadena, CA, 1986. 194 19a. Engleman, R.; Jortner, J., J. Mo/. Phys., 1970, 18, 145-164. b. Freed, K. F.; Jortner, J., J. Chem. Phys., 1970, 52, 6272-6291. 20a. Balzani, V.; Moggi, L.; Manfrin, M. F.; Balletta, F.; Laurrence, G. S., Coard. Chem. Rev.,1975, 15, 321-433. b. Kemp, T. J., Prag. React. Kinet., 1980, 10, 301-398. 21 a. Mann, K. R.; Parkinson, 8. A., lnorg. Chem., 1981, 20, 1921-1924. b. Enlow, P. D.; Woods, C., lnorg. Chem., 1985, 24, 1273-1274. c. Womack, D. R.; Enlow, P. D.; Woods, C., lnorg. Chem., 1983, 22, 2653-2656. d. Rhodes, M. R.; Mann, K. R., lnorg. Chem., 1984, 23, 2053-2058. e. Boyd, D. C.; Matsch, P.A.; Mixa, M. M.; Mann, K. R., lnorg. Chem., 1986, 25, 3331-3333. f. Bullock, J. P.; Boyd, D. C.; Mann, K. R., lnorg. Chem., 1987, 26, 3084-3086. g. Boyd, D. C.; Rodman, G. S.; Mann, K. R., J. Amer. Chem. Soc., 1986, 108, 17791784. 22. Smith, D. C., Ph.D. Dissertation, California Institute of Technology, Pasadena, CA, 1989. 23a. Bard, A. J.; Faulkner, L. R., "Electrochemical Methods, Fundamentals and Applications," Chapter 11, Wiley, 1980. b. Nicholson, R. S.; Shain, I., Anal. Chem., 1964, 36, 706-723. 24. Cotton, F. A.; Wilkinson, G., "Advanced Inorganic Chemistry," Wiley-lnterscience, 1980, page 1070 and references therein. 25. Miskowski, V. M.; Sigal, I. S.; Mann. K. R.; Gray, H. B.; Milder, S. J.; Hammond, G. S.; Ryason, P.R., J. Amer. Chem. Soc., 1979, 101, 4383-4385. 26a. Volke, J.; Naarova, M., Coll. Chzech. Chem. Comm., 1972, 37, 3361-3375. b. Raghavan, R.; Iwamoto, R. T., J. Electroanal. Chem., 1978, 92, 101-114. c. Gandiello, J. G.; Larkin, D.; Rason, J. D.; Sosnowski, J. J.; Bancroft, E. E.; Blount, H. N., J. Electroanal. Chem., 1982, 131, 203-214. 195 d. Yu, F. R.; Wang, Y. Y.; Wan, C. C., Electrochem. Acta, 1985, 30, 1693-1701. 27. Lichtenberger, D. L.; Copenhaver, A. S.; Gray, H. B.; Marshall, J. L.; Hopkins, M. D., submitted for publication to lnorg. Chem. 28. Milder, S. J.; Kliger, D. S.; Butler, L. G.; Gray, H. B., J. Phys. Chem., 1986, 90, 5567-5570. 29. This transition has been observed in spectra of several d8 -d7 compounds. Its assignment remains an area of controversy in the literature. 30a. Marcus, R. A.; Sutin, N., Biochim. Biophys. Acta, 1985, 811, 265-322. b. Guarr. T.; Mclendon, G., Coord. Chem. Rev., 1985, 68, 1-52. c. Sutin, N., Prog. lnorg. Chem., 1983, 30, 441-498. d. Newton, M. D.; Sutin, N., Ann. Rev. Phys. Chem., 1984, 35, 437-480. e. Meyer, T. J., Prog. lnorg. Chem., 1983, 30, 389-440. f. Sutin, N., Acc. Chem. Res., 1982, 15, 275-282. 31. Balzani, V.; Bolletta, F.; Gandolfi, M. T.; Maestri, M., Top. Curr. Chem., 1978, 75, 1-64. 32. Chanon, M.; Julliard, M., Chem. Rev., 1983, 83, 425-506. 33a. Grabowski, Z. R.; Grabowski, A., Z. Phys. Chem. Neue Folge, 1976, 101, 197208. b. Grabowski, Z. R.; Rubaszewska, W., J. Chem. Soc. Faraday Trans. I, 1977, 73, 1128. 34. Marcus, R. A., manuscript in preparation. 35a. lkegami, Y ., Rev. Chem. lntermed., 1986, 7, 91-109. b. Akiyama, K.; Kubota, S.; lkegami, Y., Chem. Lett., 1981, 469-472. c. Tero-Kubota, S.; Sano, Y.; lkegami, Y., J. Amer. Chem. Soc., 1982, 104, 37113714. d. Akiyama, K.; Tero-Kubota, S.; lkegami, Y., J. Amer. Chem. Soc., 1983, 105, 36013604. 196 e. Akiyama, K.; lshi, T.; Tero-Kubota, S.; lkegami, Y., Bull. Chem. Soc. Jpn., 1985, 38, 3535-3539. f. Akiyama, K.; Tero-Kubota, S.; lkegami, Y.; lkenone, T., J. Phys. Chem., 1985, 89, 339-342. 36. Andrieux, C. P.; Nadjo, L.; Saveant, J. M., J. Electroanal. Chem. lnterfacial E/ectrochem., 1970, 26, 147-186. 37. Demas, J. N.; Crosby, G. A., J. Phys. Chem., 1971, 75, 991-1024. 38. Caspar, J. V.; Meyer, T. J., J. Amer. Chem. Soc., 1983, 105, 5583-5590. 39. Wrighton, M. S.; Geoffroy, G. L.; "Organometallic Photochemistry," Wiley Inte rscience. 40. Note that the emission spectrum of lr2(Pz*)2(CO)2(Ph2POCH2CH2-NEt3)22+ has been reduced by a factor of five so that all four spectra could be plotted on the same intensity scale. lr2(Pz*)2(CO)2(Ph2POCH2CH2-4PhPy)2 2+ has been omitted in this figure because its low emission quantum yield (< 10-5) precluded measuring its emission spectrum. The weak feature at approximately 500 nm in spectra of the donoracceptor complexes is an artifact due to scattered light from the excitation source. 41. This interpretation is suggested by the similar changes seen in the triphenylphosphine complex. 42. Che, C.-M.; Wong, K.-Y., Anson, F. C., J. Electroanal. Chem., 1987, 226, 211226. 43. Electrochemical couples corresponding to the one electron reduction of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 were not observed. 197 Chapter 4 Time-resolved Experiments 198 Introduction The steady-state spectroscopic results presented in chapter 3 clearly establish that a quenching process involving both singlet and triplet (dcr*pcr) excited states is active in the donor-acceptor complexes. We have proposed that the decrease in the emission intensities from both these states is due to excited state electron transfer. This chapter deals with characterizing the charge transfer reactions of the singlet, triplet, and CT excited states in our intramolecular redox systems using picosecond timeresolved absorption and emission techniques. Results from these experiments provide direct kinetic and spectroscopic information regarding the rates of the charge separation and the charge recombination reactions involving these three excited states. In particular the transient absorption data obtained in this phase of our research conclusively assigns the singlet and triplet quenching processes in our compounds to excited state ET by spectroscopically identifying the charge transfer products. In the discussion section of this chapter the electron transfer rate data are evaluated in terms of their dependence on reaction driving force. Clear evidence for inverted behavior is seen in the charge recombination reactions in our systems. Finally, the photophysical properties of these iridium d8 -d8 donor-acceptor complexes are examined with regard to designing directional charge-transfer.systems Experimental General Procedures: All manipulations involving the iridium dimer complexes were carried out using standard Schlenk and high vacuum techniques. Solvents used in spectroscopic experiments were reagent grade or better in quality. They were purified as indicated, subjected to a minimum of five freeze/pump/thaw cycles, and stored in a round bottom storage flask fitted with a Kontes Teflon stopcock. 2- 199 Methyltetrahydrofuran was purified by distillation from a 0.5% (w/w} suspension of cuprous chloride, followed by distillation from CaH2 and finally from sodium/benzophenone. It was stored under vacuum over sodium/benzophenone as an indicator. Acetonitrile was distilled from CaH2 under nitrogen and stored in vacuo. Kinetics and Spectral Measurements Difference spectra and kinetics as well as emission lifetimes were recorded using two types of laser systems depending on the time scale required for a particular measurement. Both instruments have been fully described elsewhere, 1 ,2 and only their highlights are reported here. Phosphorescence lifetimes were recorded using a nanosecond laser system built at Caltech. The excitation source was a Quanta Ray DCR-1 a-switched Nd:YAG laser, which was frequency doubled and tripled with a Quanta Ray HG-1 harmonic generator (KDP). Doubled and tripled 8 ns (fwhm) laser pulses were separated from the YAG fundamental using a Quanta Ray PHS-1 prism harmonic separator. Light emitted by the sample was collimated at 90° to the excitation beam and focused through a Corning 3483 sharp cut filter onto the entrance slit of a MacPherson 0.35 m monochromator. Luminescence was detected using a Hamamatsu R955 photomultiplier tube, and the signal was amplified with a LeCroy VV101ATM amplifier. Signals were digitized with a Biomation 6500 waveform recorder and transferred to a Digital PDP11/103-L computer. Data sets were analyzed on an IBM-AT PC using kinetics software developed in-house by Dr. Michael Albin. Samples were prepared under vacuum as acetonitrile solutions in an NMR tube which, was attached to a Kontes Teflon stopcock. Picosecond time-resolved absorption and emission experiments were conducted at Brookhaven National Laboratory in collaboration with Dr. Jay R. Winkler. Figure 4.1 shows a schematic representation of the mode-locked Nd:YAG laser used in these experiments. A train of 30 ps (fwhm) pulses was generated in an actively-passively 200 Figure 4.1. Schematic representation of the picosecond laser system used in timeresolved emission and absorption measurements. A train of picosecond pulses is generated in an active-passive mode-locked Nd:YAG laser cavity (aml, osc, pml). A single pulse is selected using a pulse extraction device (pe) and amplified by two single pass YAG amplifiers (amp1, amp2). The amplified pulse is split into two components; one is amplified and used as an excitation source and the other component is used as a probe beam. Probe light is generated by passing the focused probe beam (either 532 nm or 355 nm) through a 10 cm pathlength cell containing 80:20 D20IH20. 201 ~::·····--···--··-··-----··0·---·0·-~, : amp 3 : shg !: thg ~ ----------------------------------------------7' 1---- ------0--------------0------rh-- 80% - - -'1-!" ····--........ ·---..... amp 2 bs -------- ··---···------ ........ --·········------.. /r---rsJ···•···0·------··t------·---~===:J { -------------: .--@]-aml 1 : I ! ~ osc k! -·--·--· --· ·--· · · . . . . . . . . . . . . ."i shg pml □ adj. ' delay .' ·-r·-·····-· ····---·-----·Y◄ ~ ··-·-··-···----------····_t:~~?.~-~----··1·--··-;Y ...._.__.._ _ _._.._y' l-··•=::::..-·----·······P..':~P. --···-·-··-·---- '\-----0- - - --····-·-••--..J:1 s /:. .... . I f. ----- ------------· L;J"- ···-+---rsJ···::: .,,// . .._>/ : : .....__ ~- det 202 mode-locked laser cavity (osc). A single pulse was removed from the pulse train using a Quantel SPS411 Pulse Selector (pe) and amplified twice by two Quantel single pass Nd:YAG amplifiers. The pulse was split into two components; 20% was passed through a third single pass amplifier and frequency doubled or tripled (KDP) for use as a pump beam. The remaining 80% of the pulse intensity was frequency doubled and used to generate picosecond probe light. The probe beam was passed through a series of mirrors and a translation stage to provide time delays between the pump beam and probe. Probe light was generated by focusing the probe beam into a 10 cm quartz cell containing an 80:20 by volume mixture of D2O/H2O and was focused onto a 400 µm-diameter aperture in contact with a sand blasted glass slide. Light from this aperture was collected and transferred to a beam splitter; one third of its intensity was reflected around the sample and used as a reference beam. The probe and reference beams were passed to the entrance slit of a DMC monochromator and then to two photomultiplier tubes. For each delay time and wavelength 100 laser shots were used. Fifty ratios of the probe and reference beams were measured without sample excitation and fifty ratios were measured with sample excitation. The log of the ratio of these ratios is equal to !!A. Picosecond time-resolved emission experiments were conducted using this same laser system except that the sample luminescence was recorded using a Hamamatsu streak camera. Data were transferred to a PDP 11/23 computer and analyzed using kinetics software developed at BNL. Picosecond time-resolved emission data were analyzed using a deconvolution software package developed at Caltech-JPL by Dr. David Brinza. Samples were prepared under vacuum as acetonitrile solutions and were held under argon in the flow cell seen in figure 4.2. Stirring in the main reservoir causes the solution to flow through the 2 mm optical glass cuvette at the bottom of the cell. This procedure assures that a fresh portion of solution is explored in each measurement. Samples were stable to laser radiation under these conditions for several weeks. 203 Figure 4.2. Inert atmosphere cell used in picosecond absorption measurements. The sample is contained under nitrogen in a 250 ml reservoir. When the solution is stirred, the hydrodynamic force created by the stirring action draws the solution through the optical cell. This procedure ensures that each laser shot interrogates a fresh and identical sample solution. 204 205 Results Kinetic and Spectral Data Model Complexes lr2(Pz*)2(C0)2( Ph2POCH2CH3)2 and lr2(Pz*)2(C0)2(Ph2POCH2CH2-NEt3+)2: Figures 4.3a-d show spectral and kinetic results for the two model compounds, lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 and lr2(Pz*)2(CO)2(Ph2POCH2CH2-NEt3+)2, in acetonitrile solutions after a 30 ps laser pulse at 355 nm. Both compounds exhibit a biphasic transient absorption signal, which rises with the laser pulse and returns to baseline at long delay times (t> 1O µs). The shorter component decays with a lifetime of approximately t1 =100 ps; the second transient is much longer lived and decays on a microsecond timescale (t2 =1 .2 µs). Kinetic results for both complexes were independent of A.ex (either 355 nm or 532 nm) and A.obs• Emission lifetimes for the 1B and 3B excited states in these molecules were determined independently to be t( 1B) =100 ps and t(3B) =1.3 µs via time-resolved emission measurements. Thus, the two transients seen in difference spectra of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 and lr2(Pz*)2(CO)2(Ph2POCH2CH2-NEt3+)2 are logically attributed to their singlet and triplet (dcr*pcr) excited states. Figures 4.3a and d show spectral data for the compounds in the wavelength range between 300 nm and 500 nm at delay times immediately following and well after the laser pulse. In addition to a ground state bleach at 450 nm both compounds exhibit a single transient absorption feature at approximately 400 nm. Data for lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 were obtained at probe wavelengths below 380 nm by exciting this complex at 532 nm and generating probe pulses with the 355 nm laser line. Unfortunately, higher energy excited state transitions were not observed in these experiments. Adequate UV probe light could only be generated up to 310 nm and spectral data above 500 nm could not be 206 Figure 4.3. Difference spectra and kinetics of the model compounds. a) Transient spectrum of lr2(Pz*)2(CO)2(Ph2POCH2CH2-NEt3+)2 in acetonitrile (Aex=355 nm, 0: t=0.0 ps, .:l: t=12.35 ns). b) Kinetics for lr2(Pz*)2(CO)2(Ph2POCH2CH2N Et3+)2 (Aex=355 nm, Aobs=420 nm, tfit=93 ps}. c) Kinetics for lr2(Pz*)2(CO)2(Ph2POCH2CH2-NEt3+)2 (Aex=355 nm, Aobs=400 nm, tfit=1.26 µs). d) Transient spectrum of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 (Aex=532 nm, 0: t=0.0 ps, .:l: t=S.0 ns). 207 0,1 t : + a. t • IIJ • u ~ ♦ m 0: ~ m o.o C C1 •• ------------------• ♦ ♦ ♦ • • • t • -0.1 400 450 500 WAVELENGTH, nm 0.10 b. IIJ u ~ ~ 0.05 0 en m C <I o.o o.o 0.5 TIME, ne ,.o 208 c. 0.10 laJ u ~ m 0:: .,, 0 ~ 0.05 <i o.o~~---------------------------------5 0 TIME, i!• •• 0,1 I d• • • +·~ •• •• •• • • 1••~ • •• r~A------.&-------- • -0.1 ~_....., 300 __ _ •__ _ __~_........ ..__ 400 __. ......._ __., 500 WAVELENGTH, nm 600 209 recorded due to luminescence backgrounds. Additional excited state transitions may occur in these complexes, but must lie outside the spectral range available in our experiments. The excited state spectra and kinetics of lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 and lr2(Pz*)2(C0)2(Ph2POCH2CH2-NEt3+)2 are interesting not only as points of reference for investigating electron transfer in the donor-acceptor complexes, but also with regard to the photophysical and excited state properties of d8-d 8 A-frame chromophores. The excited state spectra and kinetics of d8-d 8 compounds have been the focus of several previous studies.3 In compounds such as Rh2(8ridge)4 2+ and Rh2(TM8)4 2+, the 1A2u and 3A2u (dcr*pcr) excited states are easily differentiated by their transient absorption bands. 3c The former excited states show an intense 1A2u ➔ 1A1 9 (dcr*pcr ➔ pcr2) (ca. 450 nm) band in their transient spectrum. This transition is formally spin forbidden from a 3A2u excited state and is too weak to observe in transient difference spectra. The longer lived triplet state characteristically exhibits two transient absorption features which, in Rh2(8ridge)4 2+ and Rh2(TM8}4 2 + have been assigned to 3A2u ➔ 3A1 9 (dcr*pcr ➔ dcrpcr) (ca. 470 nm) and 3A2u ➔ 3 E2g{d1t ➔ pcr) (ca. 420 nm) electronic transitions. Prior to this study, the only d8-d 8 A-frame complex that had been examined using time-resolved spectroscopy was [lr2(Pz)2(COD)2]. 3a In this particular system, only spectra of the 3 8 excited state could be recorded due to its prohibitively short singlet lifetime ('t<30 ps). Thus, the data reported here provide the first opportunity for comparing the spectroscopic properties of the 18 and 3 B excited states in a d8-d8 A-frame complex. In contrast to the transient spectra of [Rh2(8ridge)4] 2 + and [Rh2(TM8)4] 2+, the singlet and triplet excited state spectra in ir2(Pz*)2(C0)2(Ph2POCH2CH3)2 and lr2(Pz*)2(C0)2(Ph2POCH2CH2-NEt3+)2 are remarkably similar. Both states are characterized by an apparently identical absorption feature. At present it is difficult to assign these transitions with any degree of certainty. However, it can be safely 210 concluded that this band does not correspond to a (dcr*pcr ➔ pcr2 ) transition in the triplet spectra of these compounds. Further, based on the transient absorption spectra of [Rh2(Bridge)4] 2+ and [Rh2(TM8)4] 2+, it is unlikely that this transition in the singlet spectrum of lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 or lr2(Pz*)2(C0)2(Ph2POCH2CH2NEt3+)2 would be isoenergetic with either the (dcr*pcr ➔ dcrpcr) or (d1t ➔ pcr) transition originating from its triplet excited state. One plausible explanation for this behavior is that the (dcr*pcr ➔ pcr2) transitions in these complexes are near in energy to their (dcr* ➔ pcr} transitions and are obscured in the bleach region of their transient difference spectra. Under this assumption the two bands at 400 nm are tentatively assigned to (d1t ➔ pcr) and (dcr ➔ dcr*) transitions by analogy to assignments made in spectra of Rh2(Bridge)42+ and Rh2(TM8)42+. Further studies directed toward measuring the extinction coefficients and polarizations of these bands will be necessary to conclusively assign them to specific transitions. With regard to their photophysical properties, lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 and lr2(Pz*)2(C0)2(Ph2POCH2CH2-NEt3+)2 have some of the longest lived singlet and triplet excited states found in A-frame complexes. We propose that the longer lifetimes in these compounds result from their smaller nonradiative decay rates. As seen in table 4.1 , while the radiative rates for a series of different d8 -d8 A-frame complexes are essentially the same, the quantum yields and emission lifetimes of these compounds vary as a function of their terminal and bridging ligands. These trends are indicative of variations in the nonradiative deactivation rates for their (dcr*pcr) excited states due to differences in their coordination environments. While theoretical models for nonradiative decay processes are available,4 the specific molecular parameters, which control these processes in large inorganic complexes are poorly understood. Thus, cause-effect relationships between molecular structure, and nonradiative rate constants in our metal dimers are difficult to evaluate. However, we 211 Table 4.1: Quantum Yield and Lifetime Data for Selected A-Frame Complexes. Complex q,(1B) q,(3 B) -t( 1 B ) ns -t(3 B ) ns [lr2(Pz)2(COD)2]a 0.0001 0.0078 <0.03 250 [I r2( Pz*) 2( COD) 2]a 0 .. 0001 0.0038 <0.03 80 [lr2( Pz) 2( CO) 2( Ph 3P) 2]b <8 [I r2 ( Pz *)2 (CO) 2 ( P h3 P) 2] 0.0003 0.0015 <0.03 60 [lr2(Pz*)2(CO)2 0.0015 0.039 0.09 1200 [lr2(Pz*)2(CO)2(Ph2POC H2C H3)2] 0.0023 0.0250 0.1 1100 Complex kr(1B) sec· 1 kr(3B) sec· 1 [lr2(Pz)2(CO D)2]a 3.0x10 6> 3.0x1 o4 [lr2( Pz*)2(COD) 2]a 3.0x10 6> 4. 75x1 o4 [lr2(Pz*)2(CO)2( Ph3 P)2] 1.0x10 7> 2.5x1 o4 [lr2(Pz*)2(CO)2 1. 7x1 o7 3.25x1 o4 (Ph2POCH2CH2NEt3)2] (Ph2POCH2CH2NEta)2] [lr2(Pz*)2(CO)2(Ph2POC H2C H3)2] 2.3x10 7 a. Marshall, J. L., Ph. D. Dissertation, Caltech, 1987. b. Smith, T. J., Ph. D. Dissertation, Caltech, 1982. 2.3x1 o4 212 can conclude from the data in table 4.1 that the nonradiative rates in d8 -d 8 A-frame complexes are strongly affected by the electronic and steric properties of their terminal and bridging ligands. lr2(Pz*)2(C0)2(Ph2POCH2CH2-4PhPy+)2: Figure 4.4 shows spectral and kinetic data for lr2(Pz*)2(CO)2(Ph2POCH2CH2-4PhPy+)2 in acetonitrile solutions after a 30 ps laser pulse at 532 nm. This particular complex was excited using the frequency doubled Nd:YAG laser line, because strong fluorescence backgrounds from 4Ph-Py+ localized excited states complicated the analysis of spectra obtained with 355 nm excitation. Kinetics measured at 380 nm and in the bleach region were characteristic of a single transient species whose concentration rises on the time scale of the laser pulse and returns to zero at long delay times (t> 500 ps). The kinetic data for this compound were fit to a single exponential intensity function yielding a lifetime of 50 ps for its long lived excited state. Figure 4.4b shows spectral data in the wavelength range between 340 nm and 650nm immediately after the laser pulse. In this case spectroscopic data were readily obtained at wavelengths greater than 500 nm due to this compound's low fluorescence and phosphorescence quantum yields. The spectroscopic properties of lr2(Pz*)2(CO}2(Ph2POCH2CH2-4PhPy+)2 differ substantially from those found in lr2(Pz*}2(CO}2(Ph2POCH2CH3)2 and lr2(Pz*)2(CO}2(Ph2POCH2CH2NEt3+)2. Its difference spectra show two prominent absorption features at 350 nm and 525 nm. These bands can be assigned to transitions localized on a 4-phenyl pyridyl radical, based on previous spectroscopic studies. 5 Our data indicate that the marked luminescence quenching seen in emission spectra of the donor-acceptor complexes can be attributed to an electron-transfer process. Further, electron transfer to form a charge separated state in this particular complex occurs on a timescale equal to or less than the duration of the laser pulse. This conclusion is in line with its low emission quantum yields. An upper limit of 1 ps for its 1B lifetime can be estimated from its fluorescence 213 Figure 4.4. Difference spectra and kinetics of lr2(Pz*)2(C0)2(Ph2POCH2CH24PhPy+)2 in acetonitrile solutions. a) Kinetics (Aex=532 nm, Aobs"'380 nm, 'tfit=50 ps). b) Difference spectrum at t=O.O ps. 214 a. t l&J u z < 0.05 m 0: 0 en m < <3 ♦ 0.0 ------ ------- ------- ------- -----100 0 200 TIME, pe •• b. t+ l&J u z 0.2 < m 0: 0 en m • '+ . ·+ •• • < <3 0.0 •• •• • •••• • • • • • • • 400 • 500 WAVELENGTH, nm 600 215 quantum yield and the quantum yield and singlet lifetime of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2. Thus, the measured 2.0x10 10 sec-1 rate constant in this complex corresponds to electron-hole recombination. lr2(Pz*)2(C0)2( Ph2POCH2CH2-Py+ )2: Figure 4.5 summarizes kinetic and spectral data for lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py+)2 in acetonitrile solutions after an excitation pulse at 355 nm. The fluorescence backgrounds that necessitated exciting lr2(Pz*)2(CO)2(Ph2POCH2CH2-4PhPy+)2 at 532 nm were not observed in experiments involving lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py+)2 and this complex was excited at higher energies where it has a larger absorption cross section. Kinetic results for "-ex=532 nm and "-ex=355 nm were the same within experimental error making comparisons between results for this complex and lr2(Pz*)2(CO)2(Ph2POCH2CH2-4PhPy+)2 possible. Kinetics measured at 400 nm and in the bleach region (450 nm) exhibited a single transient species whose concentration rises on the time scale of the laser pulse and returns to baseline at long delay times (t> 500 ps). The kinetic data for this complex were adequately modeled by an exponential intensity function with a time constant of 150 ps. Figure 4.Sb shows a transient difference spectrum between 31 o nm and 800 nm. In contrast to lr2(Pz*)2(CO)2(Ph2POCH2CH2-4PhPy+)2, lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py+)2 exhibits a single absorption feature at 390 nm. The apparent bleach feature at 620 nm can be attributed to Raman scattering from the probe light generation process. Because N-alkyl pyridyl radicals lack strong absorption bands in the 31 o nm to 700 nm spectral region, this 390 nm band cannot be ascribed to this particular chromophore. 6 Notably, it is very similar to the strong high energy absorption features characteristic of an iridium dimer cation. Recall from chapter 3 that [lr2(Pz*)2(CO)2(Ph3P)3]+ has an intense band at approximately 380 nm. However, this transient absorption feature is also very similar to transitions ascribed to the metal localized 1B and 39 excited states 216 Figure 4.5. Difference spectra and kinetics of lr2(Pz*)2(C0)2(Ph2POCH2CH2Py+)2 in acetonitrile solutions. a) Kinetics (Aex=355 nm, Aobs=400 nm, 'tfit= 150 ps). b) Difference spectrum at t=25 ps (Aex=532 nm). 217 a. 0.10 II.I u z < ID t 0:: .,, 0.05 0 CD ♦ < ~ • 0.0 0.0 0.5 TIME, na 1.0 b• •• 0.1 * ' • •• II.I u z < CD 0:: .,,0 0.0 ~ < ~ ♦ • • • i CD • ••••••• • • • • •••• • • -0.1 400 •• • 600 W_AVELENGTH, nm BOO 218 in the model complexes. Thus, additional information is required to assign this band to a particular excited state species. A 18 excited state lifetime of 2 ps for this complex can be estimated from its fluorescence quantum yield and the quantum yield and singlet lifetime of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2. This analysis indicates that, as for lr2(Pz*)2(CO)2(Ph2POCH2CH2-4PhPy+)2, charge transfer from the 18 state in this complex occurs on a timescale that is shorter than the laser pulse. lr2(Pz*)2(CO)2{Ph2POCH2CH2-Py+)2 has a triplet quantum yield that is less than or equal to 10-5 . Thus, the t=0 yield of formation of its 3 8 excited state should be reduced relative to the yield of formation of the 38 state in a model complex. However, under identical experimental conditions, the yield of formation of the triplet state in lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 and the 390 nm transient in lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py+)2 were the same within experimental error. These data support the conclusion that this absorption feature is due to transitions associated with a transient iridium dimer cation species and can be attributed to the charge separated state in lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py+)2. The measured 6.7x1 o9 sec-1 rate constant in this complex corresponds to a hole-electron charge recombination process. lr2(Pz*)2(C0)2( Ph2POCH2CH2-4MePy+)2: Figure 4.6 shows kinetic and spectral data for lr2(Pz*)2(CO)2(Ph2POCH2CH2-4MePy+)2 in acetonitrile solutions. Flash photolysis of this complex with a 30 ps laser pulse at 355 nm leads to a ground state bleach at 450 nm and the formation of a transient absorption feature at 390 nm, which is similar to the absorption band seen in spectra of lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py+)2. This transient is formed on a timescale that is equivalent to or less than that of the laser pulse and returns to baseline at long delay times (t> 1 ns). Kinetic data for this compound fit well to a biexponential intensity function with time constants of t1 =300 ps and t2=3.0 ns. As with 219 Figure 4.6. Difference spectra and kinetics of lr2(Pz*)2(CO)2(Ph2POCH2CH24MePy+)2 in acetonitrile solutions. a) Kinetics (A..ex=355 nm, A..obs=400 nm, 'tfit1 =300 ps, 'tfit2=3 ns ). b) Difference spectrum at t=1.25 ns (A..ex=355 nm). 220 a. 0.10 laJ u z < m ~ 0 en m 0.05 < ♦ <I 0.0 10 5 0 TIME, ne b. 0,05 laJ u z < m ex t t• t f ♦ ♦ ♦ t 0 en m 0.0 < ♦ <I ... a.as 400 ++++• + tt • + + •+ • 600 WAVELENGTH, nm 800 221 lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py+)2 and lr2(Pz*)2(CO)2(Ph2POCH2CH24PhPy+)2, the singlet lifetime ('t1=2 ps) for this complex is much too short for its 390 nm absorption band to be assigned to a singlet excited state. Its phosphorescence lifetime was found to be 3.0 ns from transient emission measurements. Thus, the longer lived component in transient absorption spectra of lr2(Pz*)2(CO)2(Ph2POCH2CH24MePy+)2 can be assigned to its 3B excited state; the shorter lived component is logically assigned to a charge recombination process involving its CT excited state. lr2( Pz*)2(C0)2(Ph2POCH2CH2-Me3Py+ )2: Figures 4.7 a through j summarize the kinetic and spectral data for lr2(Pz*)2(CO)2(Ph2POCH2CH2-Me3Py+)2 in acetonitrile solutions after a 30 ps excitation pulse at 355 nm. Flash photolysis of this complex leads to a ground state bleach at 450 nm and the formation of a transient absorption feature at 390 nm, which is similar to the absorption bands seen in difference spectra of lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py+)2 and lr2(Pz*)2(CO)2(Ph2POCH2CH2-4MePy+)2. This transient is formed on a timescale equivalent to or less than the duration of the laser pulse and returns to baseline for delay times greater than 500 ns. On a picosecond timescale (figure 4.7b) kinetic data for this complex are adequately modeled using a biexponential intensity function, yielding time constants of 11 ns and 65 ps. The later time constant should be viewed as an upper limit, due to the relative intensities of the short and long lived absorption components. Kinetic data recorded on a nanosecond timescale (figure 4.7i) also fit well to a biexponential decay function with time constants of 14 ns and 144 ns. The former time constant is the same as the 11 ns lifetime found from analyzing the data in figure 4.7b. Thus, the 390 nm transient in this compound's difference spectrum represents three kinetically resolvable processes with time constants of approximately 65 ps, 12 ns, and 144 ns. As with lr2(Pz*)2(CO)2(Ph2POCH2CH2-Py+)2 and 222 Figure 4.7. Difference spectra and kinetics for lr2(Pz*)2(CO)2(Ph2POCH2CH2246Me3Py+)2 in acetonitrile solutions. a) Difference spectra (Aex=355 nm, 0: t=0.0 ps, ~: t=5 ns). b) Picosecond kinetics (Aex=355 nm, A.obs=400 nm, 'Cfit1 =65 ps, 'Cfit2=11 ns). c) and d) Streak camera data showing the temporal and spectral response of the compound's fluorescence after a 30ps (fwhm) laser pulse at 355 nm. e) Response function for our laser system determined by scattering laser pulses from neat acetonitrile. f) Time-resolved emission profile of lr2(Pz*)2(CO)2(Ph2POCH2CH2246 M e3 Py+)2. 223 ••• • . + 0.2 a. • • • •• • • • • • •• • l&I u z < m •• a: 0 en ____ ..;.,.•___ _- - ..... - _.,__• • o.o • m I < <I • • -0.2 400 450 • • • • • 500 WAVELENGTH, nm b. l&I u 0.2 z < m a: 0 en m • ♦ < • <I 0.0 0 5 TIME, na 10 224 c. 475 -50 140 a80 - 1ft f 500 550 Wavel.--gt.h, rwn 600 225 e. 0 0 i 0 f. i1 g .o 1~ J 0 100 Time, pe 200 226 lr2(Pz*)2(C0)2(Ph2POCH2CH2-4MePy+)2 transient absorption features, which distinguish the 1B, 38, and CT excited states in this complex were not observed. Therefore, time-resolved emission studies were conducted to assign these time constants to specific excited state species. In contrast to the donor-acceptor complexes discussed earlier, lr2(Pz*)2(C0)2(Ph2POCH2CH2-Me3Py+)2 exhibited a fluorescence lifetime, which was long enough to be directly determined from piccosecond time-resolved emission measurements. As seen in figures 4.7 c and d, the time-resolved fluorescence spectrum of this complex is identical to its steady-state emission spectrum; thus, its 1 B excited state thermally equilibrates on a subpicosecond timescale. Figures 4.7e and f show the response function of our laser system, measured by scattering its 355 nm line from neat acetonitrile, and time-resolved emission data for lr2(Pz*}2(C0)2(Ph2POCH2CH2Me3Py+)2 (Aex=355 nm ; Aem=550 nm). The emission profile in figure 4.7e is broadened with respect to the temporal width of our system response function, indicating that this compound's singlet excited state decays on a timescale commensurate with the duration of the laser pulse. A fluorescence lifetime of 25 ps was determined by fitting the experimental data to an emission intensity function, constructed from the convolution of our system response function with an exponentially decaying fluorescence intensity. The experimental and theoretical emission profiles are compared in figures 4.7g and h. This analysis indicates that the nominal 65 ps process found in the transient difference spectrum of lr2(Pz*)2(C0)2(Ph2POCH2CH2-Me3Py+)2 can be attributed to its 1 B excited state. As seen in figure 4.7 j, lr2(Pz*}2(C0)2(Ph2POCH2CH2-Me3Py+)2 exhibits a biphasic phosphorescence profiie (Aem=750 nm) with iifetimes of 140 ns and 10 ns. This type of nonexponential behavior indicates that the phosphorescence from this complex is tied to at least two individual molecular species or states. Because the emission 227 Figure 4.7. Difference spectra and kinetics for lr2(Pz*)2(CO)2(Ph2POCH2CH2246Me3Py+)2 in acetonitrile solutions. g) and h) These two figures compare the experimental fluorescence data (g) with a theoretical curve (h) generated by convoluting the system response function with an exponential emission function; 'tf=25 ps. i) Kinetics measured at a longer delay time (Aex=355 nm, Aobs=400 nm, 'tfit1 = 14 ns, 'tfit2=144 ns). j) Time-resolved emission profiles measured at longer delay times (Aex=355 nm, A.obs=700 nm, 'tfit1=10 ns, 'tfit2=139 ns). 228 g. I\• •• • t.l • .,... .. • • • • J • I • •• • .I I • I 0 90 too ... Tl• IP1COHCDftU) h. ..•,., ~ C • ... ~ C •5 0 50 too n• (p tcoHcond■) t!SO 229 I. tj z < m 0.1 ~ .,,m 0 < o.o <I -0.1 5--......,_ _ _ _ _,.___ _ _ ___.__ _ _ _ _.._. 0 500 TIME, na 0 j• • ~ § i -~.z,, ...z -so l&J 500 0 TIME, n41 230 bandshapes and maxima for lr2(Pz*)2(C0)2(Ph2POCH2CH2-Me3Py+)2 are identical to those of the model compounds, its biexponential phosphorescence profiles cannot be attributed to two electronically distinct triplet states. One plausible explanation for this behavior is that two non-interconverting populations of this donor-acceptor molecule with substantially different triplet electron-transfer rates exist in fluid solution. The differences in kET for these two isomers could result from significant differences in their donor-acceptor separations and orientations. Leland and coworkers have recently observed nonexponential emission behavior in a porphyrin-quinone ET system in low temperature matrices.? At 77° K the solvent matrix restricts rotations about the system's three-fold axis and leads to the resolution of kinetically distinct ET processes. Lindsey8 and Bolton9 have observed multiexponential fluorescence decays in porphyrinquinone systems at room temperature in fluid solutions. In both studies, multiple solution conformers were invoked to explain this behavior and were identified in the electronic and NMR spectra of the compounds. While a similar interpretation for lr2(Pz*)2(C0)2(Ph2POCH2CH2-Me3Py+)2 in room temperature acetonitrile solutions is conceptually attractive, it is at odds with the known conformational variability of the redox ligands in this complex. Recall from chapter 2 of this thesis that intramolecular rotations about the lr-P and linker group single bonds make a distribution of donoracceptor separations and orientations available to the iridium dimer compounds in fluid solution. Further, based on the structural similarities between lr2(Pz*)2(C0)2(Ph2POCH2CH2-Me3Py+)2 and lr2(Pz*)2(C0)2(Ph2POCH2CH24MePy+)2 , it is difficult to rationalize profound differences in the distribution of solution conformations available to their redox ligands. Yet as seen earlier, the time resolved emission data for the former compound were readily modeled by a single exponential intensity function. These considerations indicate that a model involving two non-interconverting donor-acceptor complexes is not reasonable for explaining the biphasic emission behavior associated with this complex. 231 A more plausible model, which accounts for the nonexponential emission decays found in lr2(Pz*)2(CO)2(Ph2POCH2CH2-Me3Py+)2, involves the equilibration of this compound's 38 and CT states on a timescale commensurate with their excited state lifetimes (figure 4.8). This kinetic scheme is based on an analogous model for the biphasic emission decays found in exciplex donor-acceptor systems and is in line with the approximate 80 mv separation between the 38 and CT states in this complex reported in chapter 3. 10 Because of this added complication, rates for triplet state electron transfer and charge recombination must be extracted from a detailed analysis of the time resolved emission data in figure 4.7j. To simplify a derivation for the time dependent concentrations of the 38 and CT excited states in lr2(Pz*)2(CO)2(Ph2POCH2CH2Me3Py+)2, we make several fundamental assumptions. The first of these is that electron transfer does not take place from any of the higher-lying excited states in the donoracceptor complexes. This assumption is necessary because the time-resolved emission data for this complex were recorded using 355 nm excitation pulses which populate the compound's higher-lying (dxpcr) states. It will greatly simplify expressions for the yield of formation of triplet and CT state from these higher lying electronic levels, which will play an important role in interpreting the results of our kinetic analysis. This premise is reasonable because a lower limit for k;c in these complexes is 1o13 sec1 and their activationless ET rate is approximately two orders of magnitude slower (vida infra). 11 Further, order of magnitude estimates for the reaction driving force from these (dxpcr} states indicate that ket for these reactions would be well below this activationless limit. 12 A second assumption underlying our kinetic model is that the 3B and CT states are populated on a timescale that is shorter than their excited state lifetimes. This assumption is valid based on the rapid rates of internal conversion and 1B electron transfer in this complex. With these assumptions in place, the kinetic scheme in figure 4.8 indicates that the rate of disappearance of the 38 and CT states in 232 Figure 4.8. A kinetic scheme for lr2(Pz*)2(C0)2(Ph2POCH2CH2-246Me3Py+)2 that includes back electron transfer from the CT to triplet state (T1). Similar models · have been used to describe biphasic emission profiles in exciplex donor-acceptor systems (S1 =lowest energy singlet state, T1=lowest energy triplet state, CT=charge transfer state, Sn=higher energy singlet state being pumped by the laser pulse). 233 Sn 1k S1 I I : i C: I I I I I I I I I I I I k isc 1 I I I I I I I I ket I I I I I I I I I I I I I I I hv k1: I I I I I I I I I I I I I I I I I I I I I I I I I I I I I CT 234 this complex can be written in differential form as seen in equations 4.1 and 4.2. eq. 4.1 Here k1 represents the sum of radiative and nonradiative rates for the triplet state in a model compound. Equations 4.1 and 4.2 are a system of homogeneous first order differential equations with constant coefficients and can be solved using standard techniques.13 Substituting the general exponential functions in 4.3 and 4.4 for the concentration terms in 4.1 and 4.2 leads to a matrix equation relating the fundamental rate constants and emission lifetimes (equation 4.5); eq. 4.3 [CT] = CT0 exp(- At) eq. 4.4 eq. 4.5 where kA=k1 +k2 and ka=k3+k-2. Solving equation 4.5 by requiring that its determinant equal zero produces a Characteristic Equation whose roots are eq. 4.6. 235 It is interesting to note that as k.2 ➔ o, A1 ➔ k1 +k2 and the triplet emission profile becomes monophasic. Thus, the eigenvalues in equation 4.6 tend to their proper limits as the energy gap between T1 and CT increases. Eigenvectors for A1 and A2 can be found ° by substituting each of these eigenvalues into equation 4.5 and solving for T 1 in terms CT 0 · This procedure yields general equations for the time dependent concentrations of T 1 and CT (4.7 and 4.8). eq. 4.7 The coefficients C1 and C2 are determined by the boundary conditions found in equations 4.9 and 4.10. [CT] I =O = Po eq. 4.10 In contrast to exciplex systems where [CT1t=o=0, in our particular case the t=0 populations of the triplet and CT states are not readily determined. 1o This situation arises because these states are populated from higher-lying levels rather than by direct excitation and because they cannot be spectroscopically differentiated in transient absorption data. We will deal with this problem later using order of magnitude arguments regarding the yields of formation of T 1 and CT from upper excited states. Applying these general boundary conditions and solving for C1 and C2 leads to equations 4.11 and 4.12. 236 C = PO -2 k 1 k C = 2 -t (k -11. ) -2 2 1 A 1 A 1 t (k - 11.) - p k O (k _A) 2 A 0 (11. - A) O k -2 (11. 2 - A) 1 -2 ( eq. 4.11 k _ A) A 2 eq. 4.12 These relationships can be further simplified by requiring that the total number of excited molecules at t=O (1 0 ) be equal to the sum of the t=O triplet and CT state populations. to+ Po= Io to <1>1 = -, 0 "' _ Po "'2 - I 0 <1>1 + <1>2 = 1 eq. 4.13 Replacing t0 and Po in 4.11 and 4.12 by their corresponding fractional populations, <1>1 and 1-<1>1 , leads to 4.14 and 4.15. C * = "''1'1 ( k A - 11. 1) - (1 - "''I' 1)k -2 (k - A )1 2 (11.2 -11.1)k_2 Thus, the time dependent concentration of T1is, A 2 0 eq. 4.15 237 Values for k2, k.2, ka, and <1>1 can be determined from the two time constants 0,,1 and A.2), the integrated intensities of the two emission components, and the equilibrium constant k.2/k2 using equations 4.16 through 4.19. A.1 + A2 k 2 k- = k A + k a eq. 4.16 = exp(-L\E) = 0.04384 2 eq. 4.18 11 (1 - <l>1)k -2 - <l>lk A- A.2) ~ = <l>lkA - A1)- k_i1 -<1>1) eq. 4.19 Solving equations 4.16, 4.17, and 4.18 leads to an equation, which is quadratic in k2; its solution yields two positive roots, k2a=6.Sx10 7 sec· 1 and k2b=7.0x106 sec· 1. Sequential substitution of these values into equations 4.16, 4.17, 4.18, and 4.19 produces values for k.2, k3, and <1>1 . <l>2 can be determined from equation 4.13. Numerical values for these fundamental kinetic parameters from each of the roots in k2 are summarized in table 4.2. 238 Table 4.2: Kinetic Parameters for the 2,4,6-Me3Py Complex. Root k2 sec- 1 k-2 sec- 1 k3 sec· 1 <1>1 a 6.5x10 7 2.8x10 6 3.9x10 6 0.046 0.95 b 7.0x10 6 3.1x10 5 6.4x10 7 0.67 0.33 239 Two sets of rate constants, describing different kinetic scenarios, arise from this analysis because the boundary conditions in equations 4.9 and 4.1 o are not uniquely determined. For k2=k2a, k2>k3 and the initial populations of the triplet and CT states are close to their equilibrium values. This situation can be contrasted with a second case where k2<k3 and the initial populations of T1 and CT lie in favor of the triplet state. Because these two cases are readily differentiated by their substantially different values of <!>1 and <1>2, additional information regarding the relative yields of formation of T 1 and CT will identify the correct set of rate constants for the lr2(Pz*}2(C0}2(Ph2POCH2CH2-Me3Py+)2 system. Recall that the luminescence data for lr2(Pz*}2(C0}2(Ph2POCH2CH2Me3Py+)2 were obtained by exciting this complex at 355 nm. Electronic transitions at this wavelength populate one of its higher-lying 1 (d1tpcr} excited states. According to the Jablonski diagram in figure 4.8 and our initial assumptions, this state rapidly relaxes to form a distribution of 1 B and 3B excited states. The singlet states in this initial distribution are subsequently converted to triplet and CT states via intersystem crossing and electron transfer, respectively. Thus, a knowledge of the 1 (d1tpcr) to 1 B and 3B internal conversion and intersystem crossing efficiencies, and the 1 B to 38 intersystem crossing efficiency in this complex is required to fully characterize the initial yields of its triplet and CT states. Information regarding the former two quantities can be obtained by analyzing the excitation spectra of lr2(Pz*)2(C0)2(Ph2POCH2CH3}2 (figure 4.9) using equations 4.20 and 4.21. 1355 I 460 1355 I =E IC eq. 4.20. = E 355 [<1> ~in glet + <I> ~rip let] E 460 460 355 <I> ~in g let E 460 IC IC eq. 4.21. 240 From these relationships, if the conversion efficiencies between the 1{dnpcr) and 1,3(dcr*pcr) excited states in d 8-d8 A-frame complexes are close to unity, then the ratio of the excitation band intensities for these states should be equal to the ratio of their extinction coefficients. As seen in figure 4.9a, in triplet excitation spectra of lr2{Pz*)2{CO)2{Ph2POCH2CH3)2 l355/14so is equal to the ratio of extinction coefficients at these wavelengths. This implies that <l>icsinglet + <l>isctriplet=1. In contrast l355/14so in its singlet excitation spectrum is much less than £355/£4so {figure 4.9b); thus, <l>icsinglet=0.15 and <l>isctriplet=0.85. These results indicate that 1{dnpcr) to 3 B intersystem crossing in ds-ds A-frame complexes is more efficient than internal conversion to their lowest-lying singlet states. Similar results have been reported for Rh2{Bridge)42+ and ascribed to the substantial spin-orbit coupling associated with this ch~omophore's metal centers.1 1 An analogous effect must also play a role in the relaxation of the upper excited states in iridium A-frame compounds. The 1B to 3 8 intersystem crossing efficiency in lr2{Pz*)2{CO)2{Ph2POCH2CH2-Me3Py+)2 can be estimated from equation 4.22; eq. 4.22 where -tO and t are the fluorescence lifetimes of a model complex and lr2{Pz*)2{CO)2{Ph2POCH2CH2-Me3Py+)2, respectively. Here, we assume that kisc is the same for the donor-acceptor and model compounds; a reasonable premise because kisc is a quantity associated with the iridium dimer chromophore. For iridium d8-d 8 complexes <1>0 isc is close to unity. 3b,e Thus, <l>isc is approximately 0.25 in lr2{Pz*)2{CO)2{Ph2POCH2CH2-Me3Py+)2. Using these values for the internal 241 Figure 4.9. Electronic excitation spectra of lr2(Pz*)2(C0)2(Ph2POCH2CH3)2 in 2methyltetrahydrofuran. a) Room temperature triplet spectrum. b) Room temperature singlet spectrum. 242 ... e-8900 0 25000 20000 Frequency (!lea> a, ... ETDP Roo• T Triplet Exc1tetlon Spectrull £-8900 ... 0 30000 243 conversion and intersystem crossing efficiencies, the yields of formation of the 38 and CT excited states are found to be 11triplet=0.88 and 11cr=0.12. The ratio of these two quantities (11crl11triplet=0.14) compares most favorably with <1>2bI<1>1 b=0.49 calculated from the kinetic data in table 4.2. These results indicate that k2b ,k.2b, and k3b are the correct electron-transfer rate constants for this complex and that at t=0 the populations of its triplet and charge-transfer states lie in favor of the 38 state; a situation that is primarily due to efficient intersystem between its 1 (d1tpcr) and 3 8 excited states. 244 Discussion Driving Force Dependence: Previous experimental 14 and theoretical 15 studies have established reaction exoergicity as a fundamental kinetic parameter in electrontransfer reactions. Together with the inner-sphere and outer-sphere reorganization energies, reaction driving force determines the thermal barriers to electron transfer. In the classical theory for outer-sphere electron transfer, first developed by Marcus, 1 6 the reaction rate and activation free energy are described by equations 4.23 to 4.28, _f AG*] ke, = KAa2ex~- RT eq. 4.23 2 AG*= (,.,+AGO') 4 A. AG =A O. Go eq. 4.24 r p + w + W eq. 4.25 eq. 4.27 where Acr2 is an effective solution collision frequency, discussed in detail elsewhere. cr is the average center-to-center distance between the two redox partners in an encounter complex and K is the electronic transmission coefficient. The latter quantity represents 245 the probability that charge transfer will take place once the reactants have reached the transition state. In the Landau-Zener formalism K is related to the electronic interaction between the reactants and their rate of passage through the transition state reg ion .15 a,c When K= 1, charge transfer takes place with unit efficiency and the reaction is termed adiabatic. A K<1 is associated with a less efficient nonadiabatic charge transfer process. The intrinsic thermal barrier (A) is composed of two reorganization components. The inner-sphere component (Ai) describes the energy required to adjust the molecular bond lengths and angles in the two redox partners to their transition state configurations, while l 0 refers to the energy needed to reorient solvent molecules in the surrounding medium. Both are treated classically in the Marcus model as seen in equations 4.26 and 4.27, where ff and fjP are the normal mode force constants for the jth vibration in the reactants and products, respectively, aqj is the change in the jth normal coordinate, a1 and a2 are the hard-sphere radii of the two reactants, and r is their center-to-center separation. wr and wP are the free energies required to bring the reactants and products from r=oo to r=a. According to equation 4.24 the free energy of activation should vary quadratically as a function of the reaction driving force. When the driving force is small compared to the reorganization energy (i.e., the normal region), the reaction rate should increase as a function of aG 0 • and maximize at aG 0 '=l; for aG 0 '>l the reaction rate decreases as a function of driving force into what has been termed "the inverted region." Fundamentally, this model asserts that the total activation free energy for electron transfer is a function of the free energy change of the reaction and the energy required to rearrange the reactant normal coordinates into their transition state configurations. In the normal region the system must extract energy from its surroundings to reach the transition state. As aG 0 ' increases, the thermal barrier to electron transfer decreases until at -aG 0 '=l the reaction proceeds at its quantum mechanical rate ( effectively k=Aa2K). In the inverted region the system must 246 Figure 4.10. Schematic potential energy surfaces illustrating the origin of the normal and invert driving force regions in the Marcus model for electron transfer. Increasing the reaction driving force decreases the energy of the crossing point between the reactant and product surfaces along the reaction coordinate until at .6.G 0 '=A there is no thermal barrier to electron transfer. As the reaction driving force increases further, the reactant and product surfaces cross at larger energies and the rate of electron transfer decreases into the inverted region. 247 R p ' E a p E R a p E a 248 release excess energy into its surroundings to relax from a transition state configuration to one associated with the products and a thermal barrier once again controls the rate of electron transfer. These concepts are pictorially illustrated in figure 4.10. Experimental evidence, collected over the past two decades, has indicated that the Marcus model embodies most of the fundamental principles required to describe adiabatic electron-transfer reactions. 1 Sa In particular, the Marcus equations in conjunction with the Marcus Cross Relation have proven to be powerful relationships for predicting electron-transfer rates in a normal driving force regime. Several models, based on quantum mechanical and semiclassical descriptions, 16 have appeared for interpreting ET kinetics at low temperature and in the inverted region; however, these relationships reduce to the classical Marcus equations on going to a high temperature limit. While the driving force dependence of electron transfer in the normal region is for the most part well understood, reactions in the inverted region have remained largely unexplored. This situation has arisen primarily because until recently, evidence for inverted behavior had not been observed. Failure to observe inverted behavior at high reaction driving forces has been attributed to a number of experimental difficulties including rate limiting effects due to bimolecular diffusion 17 and the formation of electronically excited products at driving forces associated with the inverted region. 18 In addition, theoretical studies have indicated that the driving force dependence found in the inverted region is more gradual than predicted by the Marcus equations due to nuclear tunneling effects. As seen in equation 4.29, developed by Jortner and coworkers, 1Sa,b for hv>>kT the rate of electron transfer at high driving force will be enhanced in comparison to the rate predicted by classical theories due to the formation of vibrationally excited normal modes in the products. This leads to less pronounced variations in kET as a function ~G 0 ' in the inverted region. eq. 4.29 More recently, Ma tag a and coworkers 19 have proposed that the lack of inverted behavior seen in photoinduced charge separation reactions can be attributed to differences in the librational frequencies of solvent molecules surrounding the neutral (charged) reactants and charged (neutral) products. This model challenges the dielectric continuum model for solvent reorganization by suggesting that partial dielectric saturation around charged redox partners is responsible for mitigating inverted behavior. Considering the mechanistic complexity of electron-transfer processes, it is not likely that any single factor controls the driving force behavior of highly exothermic reactions. A greater understanding of the roles played by nuclear tunneling, solvent motion, and electronic excited states in determining the rates of electron-transfer reactions in the inverted region will develop as additional examples of inverted behavior are discovered. In a previous bimolecular study we examined the driving force dependence of triplet electron transfer in [lr2(Pz)2(COD)2] for LiG 0 '=0.13 to 1.1 eV.14a Plots of RTln(ket) versus LiG 0 ' showed a driving force dependent region, where ln(ket) increased as a function of LiG 0 ', and a plateau region where no further driving force dependence was observed. We, like others, have speculated that our failure to observe inverted behavior in this study is tied to the limitations placed on rapid bimolecular electron transfer reactions by diffusion. Our current intramolecu!ar electron-transfer systems have allowed use to measure electron-transfer rates, which are well above the diffusion limit and at driving forces in excess of 1.0 eV. 250 Table 4.3 summarizes the photophysical data for the donor-acceptor complexes and model compounds obtained from the time-resolved spectroscopic experiments discussed in this chapter and from measurements reported in chapter 3. With the exception of the fluorescence lifetimes of lr2(Pz*)2(CO)2(Ph2POCH2CH2-4MePy+)2, 4Ph Py+)2, all of the excited state lifetimes reported here were determined using timeresolved spectroscopic techniques. The three former parameters were calculated from the fluorescence quantum yields of the complexes, and the fluorescence quantum yield and lifetime of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 using equation 4.31. <I> 'to <I>o = 't eq. 4.31. While this procedure is expected to introduce a larger degree of uncertainty in these quantities, similar calculations involving lr2(Pz*)2(CO)2(Ph2POCH2CH2-Me3Py+)2 indicated that reasonable values of 'tf can be obtained using this approach; 'tfalc for ps fluorescence lifetime. These lifetimes carry an accuracy of ±30% based on the uncertainties in their corresponding fluorescence quantum yields. Values of <l>isc for the donor-acceptor complexes were estimated from equation 4.22, assuming that intersystem crossing in the model complexes occurs with near unit efficiency. 3b,e The fluorescence lifetime and intersystem crossing efficiency for negligible emission quantum yields. Electron-transfer rate constants and electrochemical driving forces for the complexes are given in table 4.4. Rate constants 251 Table 4.3: Photophysical Parameters for the Iridium Dimer Complexes. a Complex <I>t <I>p lr2(Pz*)2(CO)2L2 1f <l>isc ps -rp9 µs Ph2OCH2CH2-NEt3b 0.0015 0.032 959 1.2 1 .0 Ph2OCH2CH3 0.0023 0.025 1009 1 .1 1 .0 Ph2OCH3 0.0027 0.040 1 00 1 1 .2 1 .0 Ph2O(CH2)3CH3 0.0025 0.030 1 oot 1.1 1 .0 Ph2OCH2CH2-Me3Pyc 0.00017 0.0013 259 0.012 0.144 0.25 Ph2OC H2C H2-4MePy 0.00006 0.0004 2.6 8 0.003 0.06 Ph2OCH2CH2-Py 0.00003 --d 1.3e 0.03 ~1.oe ~0.02 Ph2OCH2CH2-4PhPy --d a. Quantum yields taken from spectra measured in acetonitrile solutions at room temperature using Ru(Bipy)3 2 + as a standard; lex=436 nm. b. Singlet quantum yields for the model compounds are accurate to ±10%; triplet quantum yields are accurate to ±30%. c. Singlet quantum yields for the donor-acceptor compounds are accurate to ±30%; triplet quantum yields are accurate to ±80%. d. Emission too weak to measure. e. Estimated using quantum yields and the singlet lifetime of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 ; accurate to ±30%. f. Estimated using quantum yields and the singlet lifetime of lr2(Pz*)2(CO)2(Ph2POCH2CH3)2 ; accurate to ±10%. g. Determined from time-resolved emission measurements; accurate to ±10%. 252 Table 4.4: Driving Force and Rate Data. Donor t:\E (eV) 8 keT( sec • 1 ) -0. 0 8 b3.5x10 6 -0.21 b1.7x10 8 -o. 58 b2.0x10 10 •NJ-Me -0.71 c1.5x10 11 •NJ -0. 8 9 c3.2x10 11 •{)-Ph -0. 97 d5.0x10 11 Acceptor Me 3 1r2 1 •N>M• Me 3 1r2 2 •NJ-Me Me 1, r2 3 •N>M• Me 4 1 1r2 5 1, r2 6 1, r2 7 8 9 NJ-Ph •1r2 -1. 5 3 b2.0x10 10 NJ •1r2 -1 . 61 b6.7x10 9 NJ-Me •1r 2 -1. 7 9 b3.3x10 9 •1r 2 -1. 9 2 b6.7x10 7 Me 10 N>M• Me a. Potentials are accurate to ±100 mv versus SSCE in 0.1 M TBAPFs/CH2Cl2. b. Accurate to ±10%. c. Accurate to ±30%. d. Lower limit. 253 for the photoinduced charge separation reactions (#2-6) were determined from equation 4.32, k el = 1/ 1t -1) eq. 4.32 ( -+ 'to assuming that the radiative and nonradiative rates intrinsic to the ds-ds chromophores are not perturbed by the pyridinium cations. This premise is firmly substantiated by the spectroscopic and photophysical data presented in chapter 3 (see tables 3.1, 3.2, and 3.7). The factor of 1/2 in this equation is required by the 1:2 donor-acceptor stoichiometry in these redox systems; it assumes that motions of the quenchers with respect to the iridium dimer chromophores are not correlated. Rate constants for the charge recombination reactions (#7-10) are the reciprocals of the charge transfer state lifetimes. 31 The driving forces found in table 4.4 are taken from the electrochemical data discussed in chapter 3. Peak potentials for the pyridinium reduction waves and iridium dimer oxidation waves are the same in methylene chloride and acetonitrile. However, electrochemical oxidation of the iridium dimer complexes in acetonitriie is an irreversible two electron process, making its one electron oxidation potential less certain in this solvent. Thus, the driving forces in table 4.4 are accurate to only ±100 mv. Figure 4.11 shows a plot of log(ket) versus dG 0 • for the triplet and singlet charge separation reactions and the charge recombination reactions in our systems. The rate of photoinduced charge separation (#1-6) increases as a function of driving force and maximizes near dG 0 '=1.0 eV, consistent with previous estimates for the total reorganization energy associated with iridium dimer/pyridinium redox systems.20 In contrast, rates for the charge recombination processes, which return these compounds 254 Figure 4.11. Plot of Log(ket) versus LiG 0 ' for the charge separation (#1-6) and charge recombination (#7-10) reactions in the donor-acceptor complexes. Driving forces are accurate to ±100 mv; log values are accurate to ±0.05 log units. 255 4 • 11 •7 10 08 Log (k-e) •• 9 2o 8 o1 0 7 •1 6 0 -1 Delta G <•V> -2 -3 256 to their electronic ground states {#7-1 O), decrease by over two orders of magnitude in the driving force range 1.5 to 2.0 eV. This large decrease in ket can be attributed to th8 inverted behavior predicted by classical Marcus theory and by more recent semiclassical and quantum mechanical electron-transfer models. Inverted effects have been observed in a number of different bimolecular14b,f, 21 and donor-acceptor redox systems. 140 ,h Indications of inverted behavior have also been observed in biological donor-acceptor molecules 14g including the photosynthetic reaction center. 140 One common feature found among these chemically diverse systems is that they all have been studied at driving forces well above 1.0 eV. Thus, a principal criterion for observing inverted behavior may be that the redox system probe exoergicities approaching 2.0 eV. This conclusion is borne out in our results as well. Figures 4.12 to 4.14 summarize results from the analysis of our driving force dependence data using the classical and semiclassical models for electron transfer found in equations 4.23 and 4.29. 33 Figure 4.12 shows a fit of our data to a classical Marcus relationship. Recalling that the estimated standard error along the potential axis in this plot is ±100 mv, the data are adequately approximated by this model, with the exception of point #1. This analysis yields a total reorganization energy of 1.0 eV and an electronic coupling matrix element of 35 cm·1. Our value of).. is strikingly similar to the reorganization energies determined in several recent driving force dependence studies on a variety of organic and biological donor-acceptor molecules.14b,c,e,f,g,h It is not likely that Ai would be the same for our compounds and systems based solely on organic donors and acceptors. Therefore, the similarity between our reorganization energy and values found in the literature suggests that ).. =Ao in redox systems whose donors and acceptors are exposed to the solvent. Our value of Hab is comparable to values obtained by Closs and Miller 14e for reactions at substantially longer donoracceptor separations. Because the Closs-Miller systems are known to transfer electrons 257 Figure 4.12. Driving force data fit to a classical Marcus model (solid curve). eV, Hab=35 cm- 1. A.=1.0 258 • ll 10 Log (k-e) 9 8 7 6 0 -l Del ta G <lltYl -2 259 via a through bond mechanism, this comparison suggests that electron transfer in our compounds does not involve through bond pathways. This conclusion is in line with the absence of ICT bands in the electronic spectra of the iridium dimer complexes. 22 Figures 4.13 and 4.14 show results from the analysis of our data using the semiclassical model for electron-transfer found in equation 4.29. This equation includes effects due to nuclear tunneling of high frequency modes in the donor and acceptor by using an average frequency (v) for the tunneling coordinate and by representing the solvent as a dielectric continuum. Closs and Miller, 14e,f Dutton and coworkers, 14c and Farid et a/.1 4b have observed significant nuclear tunneling effects in their driving force dependence studies and have successfully utilized this relationship in analyzing their data. In figures 4.13 and 4.14, we have fixed Hab2=35 cm-1 and A=1.0 eV in accord with our findings using equation 4.23. Ao, Ai, and v were allowed to vary to determine - how changes in the tunneling frequency and inner-sphere and outer-sphere reorganization energies affect the shapes of driving force curves predicted by this model. As seen in figures 4.13a-d for constant values of the inner-sphere and outer-sphere reorganization energies, as v increases from 300 cm-1 to 1000 cm-1, the theoretical driving force curves (solid line) drastically depart from the experimental data; when v=1000 cm- 1 variations in Ao, Ai, and Hab2 do not improve the fit between the model and data. In comparison to the results presented in figure 4.11, the data appear to be better represented by the semiclassical equations when v=300 to 400 cm- 1. It is well known that oxidation of ds-ds metal complexes leads to significant rearrangement of their metal-metal and metal ligand bond lengths. 23 A tunneling frequency of 400 cm- 1 would be consistent with a nuclear tunneling mechanism involving the metal ligand modes in the donor-acceptor compounds.24 Figures 4.14a-h show how emphasizing either Ao or Ai in the total reorganization energy changes the shape of the theoretical driving force curves for several different tunneling frequencies. Here again, good correlations between the model and data are obtained for smaller values of v or larger values of A0 • 260 Due to the 100 mv esds in our values of ~G 0 ', it is difficult to differentiate between a classical or semiclassical kinetic model as being more appropriate for describing electron transfer in the iridium dimer complexes. These results do, however, indicate that in contrast to many of the organic donor-acceptor systems found in the literature, our complexes do not exhibit substantial nuclear tunneling contributions from modes associated with their pyridinium electron acceptors.14b,e, f,h 261 Figure 4.13. Driving force data fit to the semiclassical model of Jortner et al. showing changes in the theoretical curves as a function of the tunneling frequency co for A=1.0 eV and Hab=35. a) C0=300 cm- 1. b) C0=:400 cm- 1 . c) C0=:500 cm- 1 . d) C0=:1000 cm- 1. 262 • a. 11 10 9 a 7 ' 0 -3 -1 Delta G (eV) • b. 11 10 Log (k-e) 9 a 7 0 -1 Delta Q (e'I) -2 -3 263 c. 11 10 Log (lt-e) 9 8 7 6 Delta G (eVl 0 • d. 11 10 Log (lt-e) 9 8 7 6 0 -3 -2 -1 0 -2 264 Figure 4.14. Driving force data fit to the semiclassical model of Jortner et al. showing changes in the theoretical curves as a function of Ai and Ao for A=1.0 eV and Hab=35. a) CO= 300 cm· 1, Ai= 0.2 eV, Ao= 0.8 eV. b) CO= 300 cm· 1, Ai=0.8 eV, Ao= 0.2 eV. c) CO= 400 cm· 1, Ai=0.2 eV, Ao= 0.8 eV. d) 0>=500 cm· 1, Ai=0.80 eV, Ao=0.20 eV. e) 0>=500 cm· 1, Ai=0.2 eV, Ao=0.8 eV. f) 0>=500 cm· 1, Ai=0.8 eV, Ao=0.2 eV. g) 0>=1000 cm· 1, Ai=0.2 eV, Ao=0.8 eV. h) 0>=1000 cm-1, Ai=0.8 eV, A 0 =0.2 eV. 265 a. 11 10 Log (k-e) 9 8 7 6 0 -3 -2 -1 Delta G (eV) • 11 10 Log (k-e) 9 a .7 0 b. 266 c. 11 10 Log (lt-e) 9 8 7 6 0 -3 -2 -1 Delta G (eV) d. 11 10 Log (lt-e) 9 8 7 0 -3 267 • e. 11 10 Log Ck-el 9 8 7 6 0 -3 -2 -l Delta G (eV) • f. 11 10 Log (k-e) 9 I 7 ' 0 -1 Delta G (eY\ 268 g. • 11 10 Log (k-e) g • 8 7 • 6 0 -3 -2 -1 Delta G (eV) • h. 11 10 Log (lt-e) g 8 7 0 -3 -1 Delta G (OV) 269 Our driving force analysis assumes that the reorganization energies and electronic coupling factors for charge separation and charge recombination in these systems are the same. The validity of this assumption is currently an area of significant interest in electron-transfer research.13d, 14h,25 Available data regarding this issue from biological and synthetic donor-acceptor systems suggest that the driving force dependence for charge separation and recombination reactions can be described by the same thermal and electronic barriers; however, further investigation will be necessary to resolve these issues. In this light, the correlations between the experimental data and theoretical models in figures 4.12 to 4.14 are remarkable. From these plots it appears that rate data for the triplet and singlet photoinduced electron transfer reactions and charge recombination reactions in our molecules are adequately represented by the same electronic coupling and reorganization parameters. 32 At present we are particularly intrigued by the premise that the charge separation and recombination reactions in photoredox molecules may be controlled by different reorganization energies, electronic coupling matrix elements, and spin selection rules. An understanding of these factors is clearly important for designing redox systems that perform vectorial photoinduced charge separation. Experiments designed to address these issues are currently in progress in our laboratories. Photophysics and Vectorial Electron Transfer: In addition to providing further evidence for inverted behavior, the quantum yield and lifetime data presented in this chapter also differentiate the photophyscial properties of bimolecular and intramolecular photoredox systems based on iridium d8-d 8 complexes and pyridinium electron acceptors. Much like photoredox systems based on porphyrin electron donors, our iridium donor-acceptor compounds possess a low lying singlet and triplet redox active excited state. Due to the moderate rates for intersystem crossing in the porphyrin based compounds,27 their primary photoredox reactions occur from their singlet excited states. This preferred singlet reactivity is found in synthetic donor- 270 acceptor systems and also in the photosynthetic reaction center. In contrast photoinduced electron transfer in redox systems based on inorganic chromophores largely involves their lowest lying high spin excited states. This apparent change in the excited state reactivities of the spin allowed and spin forbidden states is a direct consequence of the decreased fluorescence lifetimes and increased intersystem crossing rates characteristic of inorganic chromophores. In our earlier bimolecular electron transfer studies, we found that charge transfer occurred solely from the 38 excited states of iridium A-frame complexes.20, 14a However, much like photoredox systems containing porphyrin electron donors, photoinduced electron transfer in our intramolecular ET complexes is primarily a 1 B process. As seen in table 4.3, the intersystem crossing efficiencies for the donor-acceptor compounds are ~0.25; for lr2(Pz*)2(C0)2(Ph2POCH2CH2-4PhPy+)2 intersystem crossing is essentially a negligible singlet deactivation pathway. Thus the initial charge separation reactions in these compounds occur with close to unit efficiency. The recent findings of Closs and Miller 14e, Wasielewski, 14h Gust,28 Dervan,7 and Bolton9 have suggested that one of the primary motives for linking two redox partners into a single molecular unit is to remove diffusion as the rate limiting step in electron transfer. Our results expand upon this notion by indicating that a covalent linkage between donor and acceptor can also allow higher energy, shorter lived excited states to be efficiently intercepted by rapid electron transfer processes. This concept clearly has implications with regard to photochemical energy storage and is discussed in this context below. A second aspect of the photophysical behavior of the donor-acceptor complexes, which differentiates them from their bimolecular analogues, concerns charge recombination to form electronic excited states of the iridium dimer chromophore. In our bimolecular ET studies involving [lr2(Pz)2(COD)2] no evidence for back electron transfer to form its triplet excited state was observed. In particular, values of ket, determined from quantum yield and lifetime quenching experiments, were the same 271 within experimental error, indicating that charge recombination formed exclusively ground state iridium dimer products. In contrast, back electron transfer to form a 3 8 d 8 -d 8 excited state occurs in lr2(Pz*)2(CO)2(Ph2POCH2CH2-Me3Py+)2. It is well known that in some reactions where .1G 00 is greater than the energy of an electronic transition associated with the donor or acceptor, electron transfer leads to the formation of electronically excited products. For example, electrogenerated chemiluminescence has been observed in a number of inorganic redox systems, 29 most notably Ru(Bipy)32+. In addition, back electron transfer to the special pair triplet state is a well characterized process in photosynthetic reaction centers that are missing their ubiquinone electron acceptors.27 This particular process was most likely not observed in our bimolecular studies involving [lr2(Pz)2(COD)2] because the charge transfer state, [lr2(Pz)2(COD)2]+/Py· , is not energetically within kb T of the triplet state. In a highly efficient photochemical energy storage system a photoinduced charge separation must take place with a minimum loss of the captured photon energy and the charge separated state must be maintained long enough to allow bond forming reactions to occur. The realization of well defined molecular systems that execute vectorial charge transfer reactions is an extremely challenging area of current research. In the photosynthetic reaction centers of green plants, transmembrane electron transfer takes place on a picosecond timescale creating an excited state whose energy can be harnessed into chemical bonds (figure 4.15). 27 When all of the intermediate redox partners are in place, directional charge transfer occurs with near unit efficiency. An important kinetic feature of photosynthesis is that it achieves this high charge separation efficiency by sequentially coupling two very rapid electron-transfer processes. A primary photoredox reaction occurs directly from the excited special pair yielding a charge separated state (+P-1OAOs) that is approximately 950 cm- 1 above the special pair triplet state. This transient CT state is transformed by a second rapid reaction into 272 Figure 4.15. A photophysical scheme for the photosynthetic reaction center in Rhodopseudamonas viridis. White arrows correspond to the charge separation pathway in this photoredox system. 273 ,, ' ,,' I , ,, I I I ' I ' I I , I I :20 ns 274 lower energy states that can interact with reaction center redox enzymes. An analogous two step approach has recently been employed by Gust and coworkers to produce long lived charge separated states in synthetic redox triads. 28 The large conversion efficiencies found in these naturally occurring and synthetic photoredox systems can be traced to their ability to rapidly create a substantial spatial separation between a hole and an electron. However, they utilize close to 50% of the total energy made available by their initial donor localized excited states to affect this separation . It is intriguing to consider designing a synthetic photoredox system where a single high energy charge-transfer state is created and then directly utilized to drive bond forming chemical reactions. If efficient, such a system might surpass the photosynthetic reaction center as a light energy conversion device. To our knowledge this issue has not been directly addressed in any of the recent studies concerning synthetic donor-acceptor compounds. The iridium d8-d 8 complexes reported here possess two important characteristics, which suggest that they might be utilized in this capacity. As mentioned earlier, the covalent linkage between the metal complex and pyridinium electron acceptors in these systems allows their more potent singlet excited states to be intercepted by rapid electron-transfer reactions. In addition, we have seen that due to the inverted behavior associated with their charge recombination reactions, these systems possess charge separation and recombination rates, which differ by close to three orders of magnitude. If these two features can be profitably combined, then a CT state with enhanced reactivity relative to the 3 8 state in these systems might be available for subsequent bond forming reactions. Rate constants for the charge separation (kt) and charge recombination steps (kb1 and kb2) in figure 4.16 were calculated as a function of the CT state energy (AGb2) using A.=1.0 eV and Hab=35 cm-1. Results from these calculations reveal several important trends (table 4.5). First, the lifetime of the CT state increases slightly as a function of AGb2 and then decreases to near 100 ps. This trend arises from competition 275 between the parallel recombination processes kb1 and kb2· If charge recombination to form the triplet state were not an active excited state process, the lifetime of the CT state would increase as a function of ~Gb2 to hundreds of nanoseconds, based on the values of kb2 predicted by our calculations. However, back electron transfer to form the 38 state is known to be important in the photophysics of these complexes from earlier discussions regarding the emission properties of lr2(Pz*)2(CO)2(Ph2POCH2CH2Me3Py+)2 and will clearly decrease the lifetime of a high energy CT state. A second and somewhat more limiting trend concerns the lifetime of the 1B state. As the driving force for singlet electron transfer decreases from 0.5 to 0.3 eV, the rate constant for this process decreases to well below the rate of intersystem crossing (1o 10 sec- 1). These changes in kt are paralleled by a substantial decrease in the yield of formation of the CT state. From our calculations, the CT state systems cannot be generated efficiently from the metal localized 1B state when Ecr>0.1 + Etriplet• This analysis indicates that a single high energy, long lived CT state in our donor-acceptor complexes cannot be generated and maintained on the basis of driving force effects alone. Designing a photoredox system with a CT state between the triplet and singlet donor states will require the manipulation of additional electron-transfer parameters. Clearly, one possible avenue of investigation would be to utilize electronic coupling as a second means of controlling the rates of charge separation and recombination in these systems. We are currently investigating the factors that govern through bond ET reactions in these complexes. Our preliminary studies indicate that the singlet and triplet electron-transfer reactions in systems utilizing rigid aromatic spacers have very different distance dependencies. Research along these lines may lead to more efficient photoredox systems. 30 276 Figure 4.16. A Kinetic scheme showing the rate processes important to directional . electron transfer in the iridium d8 -d 8 donor-acceptor complexes. 277 I I I I kisc : I I I 39 ---l'J-r.-__, 278 Table 4.5: Predicted Electron-Transfer Rates and Photophysical Parameters. .1Gf '1Gb1 .1Gb2 eV el eV kt kb1 kb2 'tf tcr ps ns sec· 1 sec· 1 sec· 1 x10 10 x10 9 x10 7 <l>cr -0.5 0 -2.0 2.4 0.003 5.7 30 17 0.70 -0.4 -0 .1 -2 .1 0.72 0.036 0.97 58 22 0.42 -0.3 -0.2 -2.2 0.16 0.28 0.16 86 3.6 0.14 -0.2 -0.3 -2.3 0.028 1.6 0.025 97 0.6 0.03 -0 .1 -0.4 -2.4 0.004 7.2 0.003 100 0.1 0 Calculated for Aj=0.5 eV, Ao=0.5 eV, ro=400 cm· 1. 279 References 1. Nocera, D. G.; Winkler, J. R.; Yocom, K. M.; Bordignon, E.; Gray, H. B., J. Amer. Chem. Soc., 1984, 106, 5145-5150. 2a. Winkler, J. R.; Netzel, T. L.; Creutz, C.; Sutin, N., J. Amer. Chem. Soc., 1987, 109, 2381-2392. b. Winkler, J. R.; Nocera, D. G.; Netzel, T. L., J. Amer. Chem. Soc., 1986, 108, 44514458. 3a. Winkler, J. R.; Marshall, J. L.; Netzel, T. 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Driving forces for (d1tpo-) electron transfer would be greater than 2.0 eV in this complex making ket approximately 108 sec-1. 13. Zill, D. G., "A First Course in Differential Equations with Applications", 1980, Prindle, Weber, Schmidt, Boston, Mass. 14a. Gould, I. R.; Ege, D.; Mattes, S. L; Farid, S., J. Amer. Chem. Soc., 1987, 89, 3675-3679. b. Snadrini, D.; Maestri, M.; Belser, P.; von Zelewsky, A.; Balzani, V., J. Phys. Chem., 1985, 89, 3675-3679. c. Gunner, M. R.; Robertson, D. E.; Dutton, P. L., J. Phys. Chem., 1986, 90, 37833795. d. Meade, T. J.; Gray, H. B.; Winkler, J. R., manuscript in preparation. e. Closs, G. L.; Miller, J. R., Science, 1988, 240, 440-447. 281 f. Miller, J. R.; Beitz, J. V.; Huddleston, R. K., J. Amer. Chem. Soc., 1984, 106, 5057-5068. g. Mclendon, G., Acc. Chem. Res., 1988, 21, 160-167. h. Wasieleski, M. R.; Niemczyk, M. R.; Svec, W. A.; Pewitt, E. 8., J. Amer. Chem. Soc., 1985, 107, 1080-1082. i. Elias, H.; Chou, M. H.; Winkler, J. R., J. Amer. Chem. Soc., 1988, 110, P. j. Meyer, T. J., Prag. lnorg. Chem., 1984, 30, 389-440. 15a. Marcus, R. A.; Sutin, N., Biochim. Biophys. Acta, 1985, 811, 265-322. b. Newton, M. D.; Sutin, N., Ann. Rev. Phys. Chem., 1984, 35, 437-480. c. Sutin, N., Prag. lnorg. Chem., 1984, 30, 441-498. d. Jortner, J., Biochim. Biophys. Acta, 1980, 594, 193-230. 16a. Marcus, R. A., J. Chem. Phys., 1956, 24, 966-978. b. Marcus, R. A., J. Chem. Phys., 1957, 26, 867-871. c. Marcus, R. A., J. Chem. Phys., 1957, 26, 872-877. d. Kestner, N. R.; Logan, J.; Jortner, J., J. Phys. Chem., 1984, 78, 2148-2166. e. Jortner, J., J. Chem. Phys., 1976, 64, 4860-4867. f. Efrima, S.; Sixon, M., Chem. Phys., 1976, 13, 447-460. g. Van Duyne, R. P.; Fischer, S. F., M., Chem. Phys., 1974, 5, 183-197. h. Jortner, J., J. Amer. Chem. Soc., 1980, 102, 6676-6686. 17. Marcus, R. A.; Siders, P., J. Phys. Chem., 1982, 86, 622-630. 18. Siders, P.; Marcus, R. A., J. Amer. Chem. Soc., 1981, 103, 748-752. 19a. Kakitani, T.; Mataga, N., Chem. Phys., 1985, 93, 381-397. b. Kakitani, T.; Mataga, N., J. Phys. Chem., 1985, 89, 4752-4757. c. Kakitani, T.; Mataga, N., J. Phys. Chem., 1985, 89, 8-10. d. Kakitani, T.; Mataga, N., J. Phys. Chem., 1986, 90, 993-995. e. Kakitani, T.; Mataga, N., J. Phys. Chem., 1985, 91, 6277-6285. 282 f. Hatano, Y.; Saito, M.; Kakitani, T.; Mataga, N., J. Phys. Chem., 1988, 92, 10081010. 20. Marshall, J. L., Ph. D. Dissertation, 1987, California Institute of Technology, Pasadena, California. 21a. Ohno, T.; Yoshimura, A.; Shioyama, H.; Mataga, N., J. Phys. Chem., 1987, 91, 4365-4370. b. Ohno, T.; Yoshimura, A.; Mataga, N., J. Phys. Chem., 1986, 90, 3295-3297. 22a. Verhoven, J. W., Pure & Appl. Chem., 1986, 58, 1285-1290. b. Stiegman, A. E.; Miskowski, V. M.; Perry, J. W.; Coulter, D.R., J. Amer. Chem. Soc., 1 087, 109, 5884-5886. 23a. Brost, R. D.; Harrison, D. G.; Fjeldsted, D. 0. K.; Stobart, S. R.; submitted for publication. b. Coleman. A. W.; Eadie, D. T.; Stobart, S. R., J. Amer. Chem. Soc., 1982, 104, 922923. c. Atwood, J. L.; Beveridge, K. A.; Bushnell, G. W.; Dixon, K. R.; Eadie, D. T.; Stobart, S. R.; Zaworotko, M. J., J. Amer. Chem. Soc., 1984, 23, 4050-4057. 24. Nakamoto, K., "Infrared and Raman Spectra of Inorganic Coordination Compounds," 1980, John Wiley and Sons, New York. 25. Schmidt, J. A.; McIntosh, A. R.; Weedon, A. C.; Bolton, J. R.; Connolly, J. S.; Hurly, J. K.; Wasielewski, M. R., J. Amer. Chem. Soc., 1988, 110, 1733-1740. 26. Kikuchi, K.; Kurabayashi, Y,; Kokubun, H.; Kaizer, Y.; Kobayashi, H., J. Photochem. Photobio. A:Chem., 1988, 45, 261-263 and references therein. 27. Boxer, S. G., Biochem. Biophys. Acta, 1983, 726, 265-292. 28.a. Gust, D.; Moore, T. A.; Makings, L. R.; Liddell,-P. A.; Nemeth, G. A.; Moore, A. L., J. Amer. Chem. Soc., 1986, 108, 8028-8031. b. Gust, D.; Moore, T. A.; Liddell, P.A.; Nemeth, G. A.; Makings, L. R. ;Moore, A. L.; Barrett, D.; Pessiki, P. J.; Bensasson, R. V.; Ranger, M.; Chachaty, C; De Schyver, F. C.; 283 Van der Anwerau, M.; Holzwarth, A. R.; Connolly, J. S., J. Amer. Chem. Soc., 1987, 109, 846-856. c. Moore, T. A.; Gust, D.; Mathis, P.; Mialocq, J.-C.; Chachaty, C.; Bensasson R. V.; Land, E. J.; Doizi, D.; Lidell, P.A.; Lehman, W. R.; Nemeth, G. A.; Moore, A. L., Science, 1 984, 30 7, 630-632. d. Liddel, P. A.; Barrett, D.; Makings, L R.; Pessiki, P. J.; Gust, D.; Moore, T. A., J. Amer. Chem. Soc., 1986, 108, 5350-5352. e. Gust, A.; Moore, T. A.; Moore, A. L; Makings, L R.; Gilbert, R. S.; Ma, X.; Trier, T. T.; Gao, F., J. Amer. Chem. Soc., 1988, 110, 7567-7569. 29. Kalyanasundaram, K., Coard. Chem. Rev., 1982, 46, 159-244. 30. Farid, R. S.; Larson, W.; Gray, H. 8., research in progress. 31. Rigorously, 't=0.7/k. However, making this minor correction does not change our values of ket outside their experimental uncertainties. 32. This result suggests that electron transfer must occur by a predominantly through space mechanism. Based on current theories, Habf should not equal Habb if a through bond mechanism is active. 33. Kinetic data were fit to classical and semiclassical models for ket on a Macintosh Plus PC using the program "INVERT" developed by Samir Farid and Ian Gould at the Eastman Kodak Company. 284 Appendix Structure Factor Tables 285 Data appears in the following format: h k Fobs Fcalc ~F 2IO'F2 286 Ir i d i um Dime r -11 0 I 2 982 975 4 -317 -53 6 902 -872 8 1433 1447 10 1798-1801 12 1758 1767 14 1522-1536 16 1222 1262 18 582 -590 20 498 -516 -10 0 4 -186 -287 6 1247 1285 8 374 -29~ 10 1193 1214 12 1419-1402 14 551 51S 16 851 -824 18 -228 143 20 S74 462 22 499 -469 24 1120 1135 26 593 - 563 0 26 -29 ·lS 11 -7 5 8 7 -17 -16 6 -4 6 -23 5 4 11 5 -17 -10 -20 11 -1 -11 20 21 4 -12 -10 0 2 399 519 4 1521 1603 6 1488-1634 8 3000 2930 10 3226-3167 12 1S32 1345 14 971 -937 16 970-1019 18 1541 1556 20 1971- 2014 22 2018 1985 24 1341-1358 26 526 627 28 1078-1205 so 257 434 32 -256 248 34 390 -599 -7 -1 -3 l 2 -225 -182 4 972 -956 6 1895 1883 8 2585-2550 10 2704 2687 12 2144-2194 14 1415 1443 16 554 -633 18 383 -314 20 1305 1309 22 1858-1889 24 2188 2125 26 2034-1969 28 1276 1262 30 665 -715 32 -214 75 -8 - lS l 2 1664-1588 -9 2 -25 8 -4 0 -3 -4 -30 -34 -61 21 17 -4 12 -18 -5 -15 11 -6 -24 -43 -26 -28 ,9 0 2 676 -681 4 1835 1800 6 2166-2230 8 2230 2285 10 2183-2177 12 1488 1480 14 551 -606 -1 14 -25 -21 2 3 -17 1 Page 16 237 - 388 18 1660 1672 20 2155-2192 22 1953 1975 24 1795-1818 26 1449 1492 28 326 -469 30 613 -585 32 1113 1124 34 1307-1387 36 1544 1586 38 1582-1590 -6 -8 -8 -15 -25 6 -3 -26 -lS -2 -20 -14 11 -2 20 58 -14 6 -45 0 -38 -7 -9 3 -36 -32 -32 32 -11 13 0 2 811 874 4 1667-1704 6 2416 2438 8 2332-2378 10 1321 1331 12 -136 182 14 959-1049 16 1365 1363 18 2259-2271 20 2706 2719 22 2008-2003 24 1206 1183 26 756 -778 28 -271 -37 30 1097 1081 32 1404-1533 34 1377 1455 36 1197-1188 38 868 839 40 -15 -430 42 -400 -27 -4 -U 0 2 511 566 4 855 -886 6 3234 3198 8 '2'126-243:s 10 4002 3924 12 2096-1946 14 387 -439 16 2307 2291 18 1873-1982 20 3639 3641 22 2354-2459 24 2449 2470 26 546 -582 28 1210-1201 30 498 649 32 1089-1182 34 1394 1488 36 1334-1229 38 1197 12U 40 1674-1628 -5 -28 -4 -34 -17 -8 -18 -4 -23 -44 1 -4 -4 1 8 -6 -19 5 -46 -26 2 7 -35 -30 0 2 3242-3265 4 3014 2950 6 S699-3590 8 466 467 10 1643-1633 12 232 -192 14 969 954 16 5837-5725 18 4236 4306 20 1614-1776 22 2600 2686 24 1591-1601 26 217 -348 28 1856 1839 30 22S9-2355 32 2452 2400 -7 21 31 0 4 7 7 21 -23 -74 -30 -4 -20 6 .. 40 16 34 1847-1727 36 1478 1493 38 1540-1485 40 1139 1113 42 -232 346 -3 0 l 2 501 612 4 46S -506 6 118 -113 8 1238 l261 10 1042-1059 12 427 -431 14 1317 13S9 16 1237-1273 18 130:S 1325 20 1299-1306 22 940 910 24 300 -320 26 388 -472 28 1245 1220 30 1486-1497 32 1329 1321 34 1054-1081 36 924 893 38 665 -662 40 -436 203 42 -388 72 44 256 -568 -2 0 -62 -21 0 -13 -9 -1 -11 -18 -10 -3 11 -3 -19 9 -3 2 -8 -43 2 35 37 20 38 15 10 9 1 12 27 -6 -13 25 17 49 8 -37 -36 -27 17 2 0 4 618 -686 6 258 320 8 128 -5 10 140 54 12 84 176 14 949 -875 16 1324 1231 18 703 -713 20 -188 245 22 -101 -74 24 -248 -80 26 366 447 28 931 -897 30 1343 1219 32 639 -678 34 -256 261 36 -318 175 38 -234 -164 40 403 312 42 525 -517 44 -62 -36 13 10 -11 34 42 -3 -33 -5 -20 -16 10 41 -9 -28 -27 ~16 11 1 319 0 362 0 1 0 0 2 4 -53 -45 3 -13 15 0 -16 -11 -34 -21 -42 - 51 -45 -23 -30 -32 -38 -35 -24 -18 0 0 5874-5905 2 3163 SUS 4 656 -634 6 5307-5258 8 3226 3232 10 3419-3504 12 4981 5013 14 4248-4186 16 2980 2955 18 362 -328 20 1186-1075 22 2150 2156 24 1084-1145 26 2229 2178 28 2871-2855 30 2644 2700 32 1516-1561 34 -411 -228 36 173 41S S8 2326-2242 40 2750 2775 42 1629-1517 3 30 0 35 39 24 30 0 -29 44 -40 5 3 S8 36 -60 -69 -1 53 -30 48 l 4 665 -725 6 785 843 8 559 -554 10 -113 112 12 250 155 14 138 -128 16 -179 120 18 279 -338 20 84 335 22 208 -341 24 -U9 160 26 -413 94 28 -388 -161 30 -237 -217 32 -359 -73 34 -316 234 36 -423 -40 38 -408 79 40 -349 11 42 -291 -127 2 -4 l 6 3516-3428 8 4868-4867 10 4826 4686 12 4228-4088 14 5793 5678 16 5075-4950 18 2808 2806 20 602 -662 22 1160 1076 24 899 974 26 2592-2578 28 4854 4841 30 2937-2829 32 3141 sou ~i l220-1352 Jd 449 -686 38 810 815 40 2168-2027 42 2394 2474 44 1707-1579 8 0 -46 -30 l 2 5531 5521 4 2593-2506 6 1028 974 8 4677-4593 10 2577-2479 12 1031 1002 14 2223-2197 16 4586 4548 18 4823-4815 20 3921 3872 22 1870-1800 24 527 550 26 1457 1490 28 4766-4646 30 4217 4143 32 3887-3683 34 1651 1626 36 1134-1253 38. -317 -285 40 -366 70 42 694 -604 44 1749 1742 -1 38 -4 17 7 -31 -6 9 16 10 -1 -26 -7 15 8 7 45 -2 -23 17 4 -17 -14 -46 -26 24 -6 32 0 928-1015 605 675 146 245 -56 -40 -22 287 6 285 -359 8 449 -411 10 1253 1246 12 1377-1367 14 1172 1170 16 942 -956 18 218 179 20 -273 -37 22 572 -584 24 888 838 26 678 -627 28 -213 302 30 -421 - 63 32 -70 -263 34 -95 362 36 131 -206 38 -71 364 40 271 -352 4 0 I 0 4975 4795 2 589 -551 4 1574-1604 6 2022 1997 8 4291-4336 10 5116 5004 12 4903-4953 14 358 91 16 3352-3365 18 1719-1653 20 1611 1643 22 2594-2567 24 2865 2795 26 21B-2226 28 2011 1990 30 1624-1717 32 452 625 34 -196 321 36 1590-1627 38 2108 2072 5 0 0 39 17 -15 10 -11 24 -11 45 -4 26 -13 9 21 -40 7 -31 -34 -27 -11 10 1 0 -132 -155 2 336 -401 4 726 751 6 1460-1487 8 1778 1819 10 1406-1410 12 1126 1159 14 -116 -306 16 -182 164 18 428 •so 20 902 -920 22 1431 1479 24 1630-1659 26 1455 1491 28 815 -863 30 -300 299 32 -377 183 S4 455 -650 36 767 872 6 -24 14 3 5 0 -6 5 -27 -3 17 13 -31 -41 -14 -26 -4 -25 -8 -,20 -21 -12 -13 -18 -1 -15 -40 -21 -5 -6 -18 -10 -12 -13 -39 -37 -38 -25 1 0 1094-1129 2 749 -658 4 1182 1196 6 1051-1092 8 3555 3592 10 2467-2512 12 2936 2923 14 1130-1170 16 503 -506 18 2277 2314 20 1494-1516 22 3494 3559 -16 40 -6 -19 -10 -16 4 -16 0 -12 -8 -17 2 Page Iridium Dimer 24 2158-2087 26 922 953 28 280 -539 30 -205 -274 32 -217 340 34 1308-1267 7 0 0 0 0 • 11 0 1 2 -9 -15 -58 -1 6 -10 -37 -17 -3 -38 3 4 -15 - 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