Elucidating the Role of Transition Metal Electronic Structure in Catalysis and Spin Relaxation Citation Luedecke, Kaitlin Mary (2025) Elucidating the Role of Transition Metal Electronic Structure in Catalysis and Spin Relaxation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/7t77-rd65. https://resolver.caltech.edu/CaltechTHESIS:05192025-180916338 Abstract Transition metal complexes are the workhorses of physical inorganic chemistry and have diverse applications in catalysis and quantum information science, especially. The primary descriptor of transition metal complexes, and a good predictor of their utility, is their electronic structure. Notably, rigorous characterization of the spin states, oxidation states, excited states, and magnetic properties of these complexes is necessary to gain mechanistic detail for these applications; this thesis focuses on elucidating the role of transition metal electronic structure in catalysis and spin relaxation. Chapter 1 introduces important transition metal electronic structure considerations and motivates these studies. Part I includes Chapters 2–4 and considers complexes relevant for CO₂ reduction chemistry and cross-coupling reactivity. Chapter 2 investigates the conditions under which a CO₂ reduction catalyst, Fe-p-TMA, undergoes speciation changes and characterizes its excited-state identities and lifetimes. Chapter 3 considers the electrochemical conditions under which highly reduced CO reduction products are generated in an iron porphyrin system, and important connections to photocatalysis are made. Chapter 4 compares the excited-state identities and reactivities of prototypical and tethered Ni(II)–bpy aryl halide complexes. Part 2 includes Chapters 5–6 and focuses on spin relaxation, a key figure of merit in quantum information science. Chapter 5 investigates the effect of structural distortions in S = ½ copper porphyrin systems on their spin-lattice relaxation times, and Chapter 6 moves to identifying the mechanism of spin relaxation in an S = 1 Cr(o-tolyl)₄ system. Together, these compiled studies reveal the nuanced roles of transition metal electronic structure in catalysis and spin relaxation and highlight the importance of their characterization for developing optimized systems. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: Catalysis, spin relaxation Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Hadt, Ryan Thesis Committee: Cushing, Scott (chair) Hadt, Ryan Agapie, Theodor See, Kimberly Defense Date: 1 May 2025 Funders: Funding Agency Grant Number National Science Foundation Graduate Research Fellowship DGE-1745301 U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Fuels from Sunlight Hub DE-SC0021266 U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Condensed Phase and Interfacial Molecular Science DE-SC0022089 Research Corporation for Science Advancement, Cottrell Award 28165 National Institutes of Health (National Institute of General Medical Sciences) R35-GM142595 Record Number: CaltechTHESIS:05192025-180916338 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:05192025-180916338 DOI: 10.7907/7t77-rd65 Related URLs: URL URL Type Description https://doi.org/10.1021/acs.inorgchem.2c03215 DOI Article adapted for chapter 2 https://doi.org/10.1021/acs.inorgchem.3c03822 DOI Article adapted for chapter 4 https://doi.org/10.1039/D3SC05774G DOI Article adapted for chapter 5 https://doi.org/10.1021/acs.jpclett.3c01964 DOI Article adapted for chapter 6 ORCID: Author ORCID Luedecke, Kaitlin Mary 0000-0002-8163-9417 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 17243 Collection: CaltechTHESIS Deposited By: Kaitlin Luedecke Deposited On: 20 May 2025 18:41 Last Modified: 28 May 2025 22:10 Thesis Files PDF (Redacted thesis - Chapter 3 and Appendix B omitted) - Final Version See Usage Policy. 6MB Repository Staff Only: item control page

Elucidating the Role of Transition Metal Electronic Structure in Catalysis and Spin Relaxation Thesis by Kaitlin M. Luedecke In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2025 (Defended May 1, 2025) ii ã 2025 Kaitlin M. Luedecke ORCID: 0000-0002-8163-9417 iii “And on the other side, Why should we deny the truth? We could have less to worry about, honey, I won’t lie to you.” - Andrew John Hozier-Byrne iv ACKNOWLEDGEMENTS This season of my life has been better than I ever could have hoped for—beyond my wildest, wildest dreams. Moving across the country to California for the chance to earn my PhD in chemistry is something that I will never regret. The last few years here at Caltech and in Pasadena have been so life–changing, life–enhancing, and fulfilling. I have been so happy here, and largely, that happiness comes from the people who surround me. Every day I reflect on how grateful (lucky!) I am to be in this specific place at this specific time with these specific people! Here follows my attempt to string together some notes of thanks to those who have made this journey so worthwhile. First, I would like to thank the person who introduced me to the world of inorganic chemistry research and who has remained a mentor to me ever since. I was just 18 years old when I took Prof. Gregory Robinson’s honors general chemistry course at the University of Georgia. Shortly thereafter, Prof. Robinson invited me into his research laboratory, and the rest seems like history. Greg has been the best, most encouraging mentor I have ever had. Thank you for always believing in me (even when I doubt myself) and for pushing me to strive for the best. It has been so wonderful to remain close even after undergrad and throughout graduate school—having you visit Caltech and seeing you at my wedding have been particularly meaningful moments over these last few years. I appreciate your guidance in my life more than you could ever know! Along the line of thanking educators, I would also like to send my appreciation to Stephanie Sisk and Shelly Dowse, my high school AP biology and AP chemistry teachers. Ms. Sisk—you opened my eyes to science in a way that made me think that it could be fun! Your engaging stories about roadkill and pinworms made me laugh in class—and I think that laughter was the key to making science approachable for me. You also were always so encouraging and understanding, two traits that I appreciated a lot during my high school days. Ms. Dowse, thank you for giving me a rigorous introduction to chemistry! I appreciated your teaching style and positive demeanor in class. Thanks also to Barbara Baker, my high school band director. Ms. Baker—you molded me as a person more than I could have anticipated from a band program. From you, I learned to appreciate v other peoples’ experiences and the importance of being a good person and leader. These soft skills have served (and will continue to serve) me well in life! My training as an inorganic chemist has benefited greatly from the mentorship of senior graduate students. Dr. Hunter Hickox was my graduate-student mentor when I was an undergraduate in Prof. Robinson’s lab. Thank you, Hunter, for being such a patient, kind mentor, a good example of a hard worker, and a good friend! I look back fondly on our jams to music in the lab (and that time you tricked me into eating a flower at that conference in Charlotte...). It has been great to keep in touch these last few years, and I am so proud of you! Dr. Ryan Ribson was my first mentor when I was a new graduate student in the Hadt lab. Ryan taught me so much about synthetic chemistry and introduced me to Caltech as a whole. It was always so fun to get margaritas and chat with you! I have benefited a lot from the guidance of various professors at Caltech as well. Thank you to the members of my committee: Prof. Scott Cushing, Prof. Theo Agapie, and Prof. Kim See. I appreciated all of the scientific and nonscientific discussions and feedback from you all! In particular, I would like to thank Scott, the chair of my committee, for not only directing all of my committee meetings, but also for always leading with kindness and consideration. When I was exploring research groups to join as a first year at Caltech, I knew that I could get excited about really any research direction. What was most important to me was working for someone who was kind. In this search, I found Prof. Ryan Hadt’s physical inorganic research group as my home. Ryan, I cannot thank you enough for taking a chance on me all those years ago. We have had quite the journey together these last few years, but through it all, I could always count on you to foster an encouraging lab environment by leading with kindness (the pillars of the Hadt lab, perhaps?). I have found myself mulling (mauling? malling? moling?) over my PhD experience recently, and I can fully say that working in your lab and with you has been the best possible option I ever could have taken. Thank you for always believing in me and supporting me. I appreciate you! Not only is the leader of the Hadt lab kind—but I have had the privilege to work alongside some of the smartest, most good-natured people in the Hadt lab as well. Thank vi you to Dr. Gautam Stroscio, Dr. Alec Follmer, Dr. Ryan Ribson, Dr. Brendon McNicholas, Dr. Daniel Bím, Dr. Thais Scott, Dr. Paul Oyala, Dr. Ruben Mirzoyan, Dr. David Cagan, Nate Lopez, Kevin Alexander, Matthew Cox, Kate Benson, Dr. Stephen DiLuzio, Dr. Erica Sutcliffe, Dr. Stefan Lohaus, Dr. Jake Rothbaum, Dr. Kay Xia, Jonathan Aalto, Maria Blankemeyer, Keon Ha Hwang, Nathanael Kazmierczak, Joyce Kazmierczak, Arshiyan Alam Laaj, Chris (Goose) Totoiu, and Juliette Whiteside for being great coworkers during my time here. In particular, I have co-authored papers and worked on projects with Alec, Daniel, David, Nate, Paul, Stefan, Nathanael, Ryan, Matthew, Kevin, Stephen, Kate, and Keon Ha—thanks to you all for being so helpful and rigorous! Everyone in the group is always so eager to help each other out, discuss a hard problem, etc., and I am so appreciative. Alec, Ryan, and Erica, thank you for teaching me how to use the transient absorption setup in the lab. Daniel, thank you for taking me under your wing to learn computational chemistry and theory—you are a great teacher! Brendon, thank you for teaching me e-chem and for being a good friend as well. It is so great to see you as a professor in Tennessee! Thanks also to Nathanael and Paul for being the most patient at training me on the electron paramagnetic resonance instruments— through frustrations and triumphs! Erica and Maria—working next to you both in the office has been such a treat. Erica, you joining the group meant I was no longer the only woman in the group—and how lovely it has been to work alongside you and have your friendship! You are so kind and smart and humble, and I cannot wait to see what’s next in your future. You deserve the best! And Maria—you are literally one of the smartest people I know. And you’re sooooooo funny and fun and hard–working! I have learned a lot from you. I know that no matter what is in store for your future, you are going to do great things. I am so proud of you! To my cohort, Nathanael and Goose—what a pleasure these last few years have been. Thank you both for your friendship and sanity-checks throughout the years. Nathanael, I have really enjoyed working alongside you on projects and have appreciated your teaching. I always learn so much from you (especially which words of my vocabulary are archaic). It has been so exciting to follow your (and Joyce’s) journey, and I know without vii a shadow of a doubt that you will do meaningful things in Michigan. Goose, I am so grateful for your friendship as well. Hearing you “honk” in the office about Winston Churchill or some other historical figure always lightened the mood! And you were the best inorganic–organometallics seminar co–host. Learning alongside you both and finding our place in the lab has been so meaningful to me. Our lunches and time spent together will be times I look back on fondly. Cheers to us! David, thank you for being a good friend and person. From you, I have had lots of long talks about what it means to be a good scientist and a good person, and I find myself following those discussions still. You deserve everything that you have worked for, and I know that you will be successful on your journey in science. Finally, to the rest of Team LiSA, Stephen and Keon Ha—thank you both for keeping all of us on track and our heads on our shoulders. I am grateful for our discussions and friendships! I must also extend a heartfelt thank you to my other friends at Caltech: Enric Adillon, Sasha Alabugin, Madeline Hicks, Levi Palmer, Jocelyn Mendes, and Dr. Danika Nimlos. Enric and Sasha, I have been “in this” with you guys since the very beginning. It has been so refreshing to have friends who I can fully be myself with and discuss anything with. Thank you both for all of the fun adventures, coffee breaks, and discussions we have had over the last few years. Madeline, thank you for keeping it real. I always appreciate your perspective on problems and topics, and writing letters encouraging people to vote is one of my favorite memories while at Caltech. Levi, Jocelyn, and Danika—my Cushing lab friends! Levi, thank you for always being a leader and such a caring, funny friend. It was so fun to organize an American Chemical Society symposium alongside you! Jocelyn, my wonderful, book-loving, kind friend. Thank you for all of our wine nights and run clubs and book reviews! You are going to absolutely crush whatever is next for you! And Danika, thank you for introducing me to your soccer friends and always being down for a beer or to chat about the world. I would surely not have made it through my PhD with my head screwed on right if not for my communities outside of the immediate Caltech sphere. Thank you to my trivia team, Dr. Paul Oyala, Nate Hart, Dr. John Ovian, Dr. Christian Johansen, Emily Boyd, and Fernando Guerrero. I have looked forward to Tuesday nights at 8 p.m. at T-Boyles viii each week! Thank you to my soccer friends, Michelle Leiker, Lulu Martinez, Jen Chuck, Janel Foo, Danika Nimlos, Jackie Cruz, Hannah Levy, Corey Crossfield, Catalina Combs, Bre Gibson, and Angie Capra. It has been so freeing to bring myself back to playing on a field, and every Monday night with you ladies is such a treat. Finally, I cannot thank my SoulCycle friends—Armineh Yousefian, Lauren Castle, Ana El Beleidy, Angela Tom, Lily Rosenbloom, and Alison Walker—enough for the perspectives they have shown me on life! You all have instilled such confidence in me and reminded me time and time again that being a good person is the most important thing in life. It has been such a privilege to be surrounded by such fun, intelligent, funny, kind, and bada** women. Thank you for riding bikes that go nowhere with me! I have rather found that they take us ~everywhere we need to go~ instead. Thank you to my parents, Patricia Luedecke and James Luedecke, for always encouraging me to do my best in school and making sure I was able to reach my goals. I would not have made it this far if not for my sister, Maddie Luedecke. Maddie, thank you for being the best emotional support/just general supporter of me. I cannot thank you enough for answering any call to talk me through a problem, for constant reassurance, for believing in me, and for going with me to every Hozier concert. It has been so fun to see us become friends these last few years, and I am so excited for our future adventures! (Thank you also for letting me talk to my niece Kailani on FaceTime whenever I’m sad at work!) I am so grateful for the best in-laws that anyone could ever hope for. Sandi and Moe, thank you both for being like second parents to me. And Marissa (and Matt) and Tyler, thank you for treating me like family as well. I am so grateful for all of your love and support, whether it comes as visits to California or career advice! I am so lucky to be married to the best man ever, Josh Trebuchon. Josh has been my rock throughout this entire PhD journey, and I am so grateful for all of his love and support these last few years. Thank you for always celebrating my wins, consoling me when I’m down, and for assuring me that no matter what, everything will be okay. I love you so much and am so excited for this next step of our future together (and for us to get a dog soon)! ix Finally, I must end my acknowledgements with the biggest thank you to my grandparents, John and Anne Cronin. My grandparents immigrated to the United States from Ireland more than 50 years ago, and they have worked so hard to ensure that their family has the best life possible. From my grandparents, I have learned the importance of education and lifelong learning, to appreciate the nature around us, and to enjoy peace. To be quick to praise and to be slow to judge—to be kind in our pursuit of happiness. One of the major reasons I set out to get my PhD was to be the first in the family to be called a “doctor”—to make them proud. Thank you, Grandpa and Grandma, for all you have sacrificed and all you have taught me and done for me. You can call me Dr. Katie, now! I love you all the way. x ABSTRACT Transition metal complexes are the workhorses of physical inorganic chemistry and have diverse applications in catalysis and quantum information science, especially. The primary descriptor of transition metal complexes, and a good predictor of their utility, is their electronic structure. Notably, rigorous characterization of the spin states, oxidation states, excited states, and magnetic properties of these complexes is necessary to gain mechanistic detail for these applications; this thesis focuses on elucidating the role of transition metal electronic structure in catalysis and spin relaxation. Chapter 1 introduces important transition metal electronic structure considerations and motivates these studies. Part I includes Chapters 2–4 and considers complexes relevant for CO2 reduction chemistry and cross-coupling reactivity. Chapter 2 investigates the conditions under which a CO2 reduction catalyst, Fe-pTMA, undergoes speciation changes and characterizes its excited-state identities and lifetimes. Chapter 3 considers the electrochemical conditions under which highly reduced CO reduction products are generated in an iron porphyrin system, and important connections to photocatalysis are made. Chapter 4 compares the excited-state identities and reactivities of prototypical and tethered Ni(II)–bpy aryl halide complexes. Part 2 includes Chapters 5–6 and focuses on spin relaxation, a key figure of merit in quantum information science. Chapter 5 investigates the effect of structural distortions in S = ½ copper porphyrin systems on their spin-lattice relaxation times, and Chapter 6 moves to identifying the mechanism of spin relaxation in an S = 1 Cr(o-tolyl)4 system. Together, these compiled studies reveal the nuanced roles of transition metal electronic structure in catalysis and spin relaxation and highlight the importance of their characterization for developing optimized systems. xi PUBLISHED CONTENT AND CONTRIBUTIONS Follmer, A. H.†; Luedecke, K. M.†; Hadt, R. G. “µ-Oxo Dimerization Effects on a Ground- and Excited-State Properties of a Water-Soluble Iron Porphyrin CO2 Reduction Catalyst.” Inorg. Chem. 2022, 61, 20493. DOI: 10.1021/acs.inorgchem.2c03215. K. M. L. contributed equally to the manuscript with A. H. F. K. M. L. participated in designing the study, synthesized compounds, carried out all experiments, performed data analyses, prepared figures, wrote the manuscript, and was involved in manuscript editing. Kazmierczak, N. P.†; Luedecke, K. M.†; Gallmeier, E. T.; Hadt, R. G. “T1 Anisotropy Elucidates Spin Relaxation Mechanisms in an S = 1 Cr(IV) Optically Addressable Molecular Qubit.” J. Phys. Chem. Lett. 2023, 14, 7658. DOI: 10.1021/acs.jpclett.3c01964. K. M. L. contributed equally to the manuscript with N. P. K. K. M. L. synthesized compounds, prepared all experimental samples, participated in data collection and analysis, contributed to figure generation, and contributed to writing and editing the manuscript. Kazmierczak, N. P.; Lopez, N. E.; Luedecke, K. M.; Hadt, R. G. “Determining the Key Vibrations for Spin Relaxation in Ruffled Cu(II) Porphyrins via Resonance Raman Spectroscopy.” Chem. Sci. 2024, 15, 2380. DOI: 10.1039/D3SC05774G. K. M. L. mentored N. E. L. through the synthesis of compounds and sample preparation, participated in data collection, analysis, and figure generation, and contributed to writing and editing the manuscript. Bím, D.; Luedecke, K. M.; Cagan, D. A.; Hadt, R. G. “Light Activation and Photophysics of a Structurally Constrained Nickel(II)-Bipyridine Aryl Halide Complex.” Inorg. Chem. 2024, 63, 4120. DOI: 10.1021/acs.inorgchem.3c03822. K. M. L. participated in data collection and analysis, including all ultrafast spectroscopic experiments and analyses, participated in figure generation, and contributed to writing and editing the manuscript. xii TABLE OF CONTENTS Acknowledgements ................................................................................................................ iv Abstract.....................................................................................................................................x Published Content and Contributions .................................................................................... xi Table of Contents .................................................................................................................. xii List of Schemes and Figures ................................................................................................. xv List of Tables ...................................................................................................................... xviii List of Equations .................................................................................................................. xix Nomenclature and Abbreviations ......................................................................................... xx Chapter 1: Introduction ..........................................................................................................1 1.1. Introduction ..............................................................................................................2 1.2. Catalysis....................................................................................................................2 1.2.1. COx (x = 1, 2) Reduction with Iron Porphyrin Catalysts .......................2 1.2.2. Cross–coupling Reactions ......................................................................4 1.3. Spin Relaxation ........................................................................................................5 1.3.1. S = ½ Molecules......................................................................................5 1.3.2. S = 1 Molecules.......................................................................................6 1.4. Compound Scope .....................................................................................................7 1.5. References ................................................................................................................8 Part I: Catalysis .................................................................................................................... 12 Chapter 2: µ-oxo Dimerization Effects on Ground- and Excited-State Properties of a Water-Soluble Iron Porphyrin CO2 Reduction Catalyst....................................................... 13 2.1. Introduction ........................................................................................................... 14 2.2. Results and Analysis ............................................................................................. 15 2.2.1. UV-vis Spectroscopy ................................................................................... 15 2.2.2. EPR and MCD Spectroscopy ...................................................................... 18 2.2.3. Ultrafast TA Spectroscopy .......................................................................... 20 2.3. Discussion.............................................................................................................. 25 2.4. Conclusions ........................................................................................................... 29 2.5. References ............................................................................................................. 30 Chapter 3: The Non–innocent Role of Amine Additives in the Electrochemical Generation of Highly Reduced CO Reduction Products in a Molecular Iron Porphyrin System .......... 35 xiii Chapter 4: Photophysics of a Structurally Constrained Ni(II)-Bipyridine Aryl Halide Complex ................................................................................................................................ 60 4.1. Introduction ................................................................................................................ 61 4.2. Results and Analysis .................................................................................................. 64 4.2.1. Steady-State UV-vis Absorption Spectroscopy ........................................... 64 4.2.2. Photochemical Activity................................................................................ 66 4.2.3. Transient Absorption Spectroscopy ............................................................ 71 4.3. Discussion .................................................................................................................. 75 4.4. Conclusions ................................................................................................................ 83 4.5. References .................................................................................................................. 83 Part II: Spin Relaxation ....................................................................................................... 91 Chapter 5: Determining the Key Vibrations for Spin Relaxation in Ruffled Cu(II) Porphyrins via Resonance Raman Spectroscopy ................................................................. 92 5.1. Introduction ........................................................................................................... 93 5.2. Results ................................................................................................................... 97 5.3. Discussion............................................................................................................ 108 5.4. Conclusions ......................................................................................................... 109 5.5. References ........................................................................................................... 110 Chapter 6: T1 Anisotropy Elucidates Spin Relaxation Mechanisms in an S = 1 Cr(IV) Optically Addressable Molecular Qubit ............................................................................. 116 6.1. Introduction ......................................................................................................... 117 6.2. Results and Discussion ........................................................................................ 119 6.3. Conclusions ......................................................................................................... 126 6.4. References ........................................................................................................... 127 Appendix A: Supporting Information for Chapter 2: µ-oxo Dimerization Effects on Ground- and Excited-State Properties of a Water-Soluble Iron Porphyrin CO2 Reduction Catalyst ................................................................................................................................ 132 Appendix B: Supporting Information for Chapter 3: The Non-innocent Role of Amine Additives in the Electrochemical Generation of Highly Reduced CO Reduction Products in a Molecular Iron Porphyrin System .................................................................................... 133 Appendix C: Supporting Information for Chapter 4: Photophysics of a Structurally Constrained Nickel(II)-Bipyridine Aryl Halide COmplex................................................. 153 Appendix D: Supporting Information for Chapter 5: Determining the Key Vibrations for Spin Relaxation in Ruffled Cu(II) Porphyrins via Resonance Raman Spectroscopy ........ 154 xiv Appendix E: Supporting Information for Chapter 6: T1 Anisotropy Elucidates Spin Relaxation Mechanisms in an S = 1 Cr(IV) Optically Addressable Molecular Qubit....... 155 xv LIST OF SCHEMES AND FIGURES Chapter 1 Page 1 Figure 1.1. Redox characterization of iron porphyrin species 3 Figure 1.2. The Zeeman effect, 1/T1, and 1/T2 for an S = ½ system 5 Figure 1.3. Zero-field splitting energy level diagram for an S = 1 system 7 Figure 1.4. Structures of molecules included in this thesis 8 Chapter 2 13 Figure 2.1. UV-vis and kinetics of Fe-p-TMA µ-oxo dimer formation in the 15 presence of NaHCO3 Figure 2.2. UV-vis of Fe-p-TMA in the presence of various electrolytes 16 Figure 2.3. pH titrations of Fe-p-TMA, monitored by UV-vis 18 Figure 2.4. EPR of Fe-p-TMA in various conditions 19 Figure 2.5. MCD of Fe-p-TMA monomer and µ-oxo dimer 20 Figure 2.6. TA excitation wavelengths mapped onto UV-vis of Fe-p-TMA 21 monomer and µ-oxo dimer Figure 2.7. Jablonski diagram describing excited-state relaxation of Fe-p- 22 TMA monomer and µ-oxo dimer Figure 2.8. Normalized EAS wavelength profiles of TA components that 23 described the relaxation of Fe-p-TMA monomer and µ-oxo dimer from 650 nm excitation Figure 2.9. Normalized EAS wavelength profiles of TA components that 25 described the relaxation of Fe-p-TMA monomer and µ-oxo dimer from 550 nm excitation Chapter 3 35 Figure 3.1. Summary of work done toward identifying the origin of CH4 37 formation from photo-/electrochemical CO2 reduction by Fe-p-TMA xvi Scheme 3.1. Relevant reactions that occur under CO or CO2 electro- 38 reductive conditions Scheme 3.2. Common degradation pathways for tertiary alkyl amines in 38 photoredox catalysis Figure 3.2. CV of Fe-p-TMA under Ar and CO 40 Figure 3.3. CV of Fe-p-TMA under Ar and CO in the presence of different 41 acids Figure 3.4. CPE at -2.1 V vs. Fc0/+ of Fe-p-TMA samples containing 45 NH4BF4 and PhOH additives under CO Figure 3.5. Faradaic efficiencies over time for CH4 formation after CPE of 46 NH4BF4, NH4BF4 and Fe-p-TMA, and PhOH and Fe-p-TMA under CO Figure 3.6. Faradaic efficiencies over time after CPE of only NH4BF4 versus 47 NH4BF4 and Fe-p-TMA under CO Figure 3.7. GC-MS spectra after CPE of Fe-p-TMA with PhOH versus Fe- 49 p-TMA with NH4BF4 Chapter 4 60 Figure 4.1. Mechanisms of Ni–C bond homolysis for Ni(II)–bpy aryl 62 halides Figure 4.2. UV-vis of tethered and untethered Ni(II)-bpy aryl halide 65 complexes Figure 4.3. Photoirradiation of tethered and untethered Ni(II)-bpy aryl 67 halide complexes in the presence and absence of mesityl bromide, monitored by UV-vis Figure 4.4. Paramagnetic 1H-NMR and UV-vis characterization of the 70 irradiation product of the tethered Ni(II)–bpy aryl halide complex Figure 4.5. Jablonski diagram describing the excited-state relaxation 73 pathways of the tethered and untethered Ni(II)-bpy aryl halide complexes Figure 4.6. Transient absorption of the tethered Ni(II)-bpy aryl halide complexes 74 xvii Figure 4.7. Cross-coupling scheme describing selectivity in Ni(II)-bpy aryl 77 halide ground- and excited-state catalysis Figure 4.8. Mechanism for reversible Ni–C bond homolysis 79 Figure 4.9. DFT-optimized geometric distortions of tethered and untethered 81 Ni(II)-bpy aryl halide complexes upon 3LF state formation Chapter 5 92 Figure 5.1. Mechanistic studies of spin-lattice relaxation in S = ½ qubits 94 Figure 5.2. Geometries and vibrational modes of ruffled copper porphyrins 96 Figure 5.3. Temperature-dependent T1 of CuOEP, CuTPP, and CuTiPP 100 Figure 5.4. Calculated spin relaxation rates 1/T1 for copper porphyrins 102 Figure 5.5. Calculated vs. experimental 1/T1 log-log slopes for CuOEP and 104 CuTiPP Figure 5.6. 1/T1 vs. temperature and Raman spectra of copper porphyrins; 105 comparison to back-out relevant vibrational mode energies Chapter 6 116 Scheme 6.1. Molecular crystal structures and point groups of S = 1 Cr(o- 118 tolyl)4 and S = ½ Cr(acac)2 Figure 6.1. Temperature-dependent pulse EPR for ZFS determination of 120 Cr(o-tolyl)4 Figure 6.2. T1 anisotropy and functional fitting of S = 1 Cr(o-tolyl)4 and S = 122 ½ Cr(acac)2 Figure 6.3. Temperature dependence of T1 anisotropy contributions for S = 1 Cr(o-tolyl)4 and S = ½ Cr(acac)2 124 xviii LIST OF TABLES Chapter 2 Page 13 Table 2.1. Excited-state lifetimes of Fe-p-TMA monomer and µ-oxo dimer 28 Chapter 5 92 Figure 5.1. Normal coordinate structural decomposition analysis of 98 porphyrin distortions Figure 5.2. Copper porphyrin local mode energies and Raman mixed ligand symmetric stretch peaks 108 xix LIST OF EQUATIONS Chapter 5 Page 92 Equation 5.1. Theoretical prediction of 1/T1 based from molecular spin- 101 phonon coupling considerations Equation 5.2. Local mode fitting of 1/T1 data 105 Chapter 6 116 Figure 6.1. Relationship between sin2(2θ) and T1 anisotropy 125 xx NOMENCLATURE AND ABBREVIATIONS 1/T1 Spin-lattice relaxation rate 3 LF Triplet ligand field AgCl Silver chloride B Externally-applied magnetic field Bpy Bipyridine CE Counter electrode CH4 Methane C2H4 Ethylene C2H6 Ethane C6H6 Benzene COR Carbon monoxide (CO) reduction CO Carbon monoxide CO2 Carbon dioxide CO2R Carbon dioxide (CO2) reduction CO2RR Carbon dioxide (CO2) reduction reaction COx (x = 1, 2) Carbon monoxide (x = 1) or carbon dioxide (x = 2) CPE Controlled potential electrolysis Cr(DTBMS)4 Tetrakis(ditert-butylmethylsiloxide)chromium CrO(HEBA)2- Oxo-chromium bis(2-hydroxy-2-ethylbutyrate) Cr(o-tolyl)4 Tetrakis(o-tolyl)chromium xxi CS2 Carbon disulfide CSD Cambridge structure database Cu(acac)2 Copper acetylacetonate Cu(mnt)22- Copper maleonitriledithiolate CuOEP Copper octraethylporphyrin CuP Copper porphyrin CuPc Copper phthalocyanine CuTiPP Copper tetraisopropylporphyrin CuTPP Copper tetraphenylporphyrin CV Cyclic voltammetry CW Continuous wave D Axial zero-field splitting (parameter) D4h Square planar DFT Density functional theory DMF Dimethylformamide e- Electron E Rhombic zero-field splitting E1/2 Half-wave potential Ep,c Peak cathodic current potential EAS Evolution-associated spectra EDFS Echo-detected field sweep EPR Electron paramagnetic resonance (spectroscopy) xxii ESEEM Electron spin echo envelope modulation EtOH Ethanol Fc Ferrocene FE Faradaic efficiency FeCHOP+ Protonated ferrous carbonyl porphyrin FeCOP Ferrous carbonyl porphyrin FeP Iron porphyrin Fe-p-TMA Iron 5,10,15,20-tetra(para-N,N,N-trimethylanilinium)porphyrin FeTMPyP Iron tetrakis(N-methyl-4-pyridyl)porphyrin FeTPP Iron tetraphenylporphyrin GC Glassy carbon GC Gas chromatography GC-MS Gas chromatography-mass spectrometry GSB Ground state bleach H• Hydrogen atom H+ Proton H2 Hydrogen H2O Water HER Hydrogen evolution reaction IPCC International Panel on Climate Change IR Infrared Ir(ppy)3 Iridium tris(2-phenylpyridine) xxiii ISC Intersystem crossing KCl Potassium chloride KNO3 Potassium nitrate LF Ligand field LMCT Ligand-to-metal charge transfer LSS Ligand symmetric stretch LUMO Lowest unoccupied molecular orbital MCD Magnetic circular dichroism (spectroscopy) MHCO3 Metal bicarbonate MLCT Metal-to-ligand charge transfer MOF Metal organic framework MS Mass spectrometry NH3 Ammonia NH4+ Ammonium NH4BF4 Ammonium tetrafluoroborate NiTiPP Nickel tetraisopropylporphyrin NiTPP Nickel tetraphenylporphyrin NMR Nuclear magnetic resonance (spectroscopy) NSD Normal coordinate structural decomposition NV- Anionic nitrogen vacancy center PCET Proton coupled electron transfer PhOH Phenol xxiv pKa -log10[acid dissociation constant (Ka)] Pd(acac)2 Palladium acetylacetonate PXRD Powder X-ray diffraction Qubit Quantum bit RE Reference electrode RMSD Root-mean-square deviation RPM Revolutions per minute rR Resonance Raman (spectroscopy) S Spin (quantum number) Sn(o-tolyl)4 Tetrakis(o-tolyl)tin T1 Spin-lattice relaxation time T2 Electron spin coherence time Td Tetrahedral Tm Phase memory time TA Transient absorption (spectroscopy) TBABF4 Tetrabutylammonium tetrafluoroborate UV-vis Ultraviolet-visible (spectroscopy) VOPc Vanadyl phthalocyanine VOTPP Vanadyl tetraphenylporphyrin WE Working electrode ZFS Zero-field splitting ZnOEP Zinc octaethylporphyrin xxv ∂g/∂Q Spin-phonon coupling coefficient θ Angle λexc. Excitation wavelength λmax. Wavelength of maximum intensity/absorbance π Pulse duration τ Excited-state lifetime Φ Quantum yield *CO Adsorbed carbon monoxide 1 CHAPTER 1: INTRODUCTION 2 1.1. Introduction Elucidating the role of transition metal electronic structure in various chemical and physical processes has remained a cornerstone thrust of inorganic chemistry. More specifically, the character of specific electrons in transition metal complexes has implications for their reactivity (e.g., catalysis), excited-state chemistry and relaxation, and magnetism. Therefore, there is much interest in understanding the ground states and optical/magnetically excited states of these compounds. This thesis spans a broad space of applications seen in physical inorganic chemistry, from molecular catalysis to quantum bit (qubit) design. However, a unifying theme across all studies discussed herein is the characterization of the electronic structure of target compounds and investigation into the role their configuration plays in achieving specific goals (for e.g., efficient CO2 reduction or realizing high-temperature coherence). Ultimately, transition metal electronic structure dictates the abilities of complexes to act in certain applications, so its characterization is the first step toward understanding what limits a complex’s ability, leads to a complex’s deactivation, or promotes desired behavior in a system. 1.2. Catalysis 1.2.1. COx (x = 1, 2) Reduction with Iron Porphyrin Catalysts Transition metal complexes make up the large majority of known molecular COx (x = 1, 2) reduction catalysts.1,2 The ability of a transition metal center to bind COx and stabilize intermediates en route toward reduced product formation is largely due to the particular electronic structure of the complex. Iron porphyrins demonstrate a good example of the role electronics play in catalysis; iron porphyrin complexes are a well-known class of CO2 reduction catalyst that can generate CO or CH4 both photo- and electrochemically from CO2 feedstocks.3,4 In these systems, a reduced formal Fe(0) state binds CO2 and templates it for proton-coupled electron-transfer (PCET) chemistry to give reduced products.3,5 However, the electronic configuration of this reduced state does not reflect a true Fe(0), d8 configuration; rather, a resonance structure of an Fe(II) center antiferromagnetically coupled to a porphyrin ligand diradical describes the electron density best (Figure 1.1). Indeed, only the first 1 e- reduction of Fe(III) to Fe(II) is metal-centered, with the 3 remaining reduction events taking place on the ligand (Figure 1.1). These electronics 5 play a role in catalysis with iron porphyrin species, especially considering that the radical character of the porphyrin macrocycle may lend itself well to deactivation of the active catalyst via ligand degradation pathways. Formal Fe(I) Example CV of an FeP FeIII/II P0/P-/2Formal Fe(0) Metal-based redox Ligand-centered redox Figure 1.1. Redox characterization of iron porphyrin (FeP) species Further, the Sabatier principle is a key design strategy for the optimal electronic structure of a transition metal catalyst. This principle suggests that maximal activity for COx reduction occurs when the metal center binds COx with sufficient energy to support further chemistry of the substrate, but not too strongly such that intermediate reactivity in disfavored.6 Again, we can use the iron porphyrin’s electronic structure as an example for understanding reactivity. Typically, iron porphyrin species are exceptional CO2-to-CO reduction catalysts, and reactivity beyond CO is rare.7 This trend in reactivity may be explained in part by the very strong binding affinity of Fe(II) for CO, a key intermediate in catalysis.8,9 The dxz and dyz degenerate orbitals of the iron center can engage in π backbonding with the unoccupied CO π* orbitals, and this interaction stabilizes the Fe–C bond while destabilizing the C–O bond. The strong-field CO ligand also pushes the ferrous center to adopt a low-spin configuration, while the unbound ferrous center has a high-spin 10 configuration. 4 Spin state indeed influences COx reduction, with product selectivity differences having been reported for iron porphyrin systems depending on high-spin, intermediate-spin, or low-spin electronic configurations.11 In this thesis, the excited states of an iron porphyrin COx reduction catalyst were characterized in order to determine their relevance for catalysis. For an excited state to react with substrate, it typically must have a lifetime of at least 1 ns to compete with diffusion.10 Indeed, we observed that a µ-oxo dimer form of the iron porphyrin species that was formed in catalytically relevant conditions exhibited excited-state relaxation on that timescale.10 Further, we investigated the viability for COx reduction to give CH4 electrochemically in the same iron porphyrin system. From the electrochemical results, we determined that although the iron porphyrin catalyst was catalytic for the hydrogen evolution reaction, the C–containing reduced products arose from either reduced porphyrin ligand degradation or exogenous acid-based reactivity. 1.2.2. Cross–coupling Reactions In the field of Ni-centered cross-coupling catalysis, the identity of the excited state that allows for the generation of a reactive species has largely been contended.12,13 These Ni(II)–bpy aryl halide complexes have shown the remarkable ability to form C–C, C–N, C–O, and C–F bonds upon photoirradiation, but beyond initial formation of a 1MLCT state, the electronic structure responsible for catalysis was not known until recent careful work from the Hadt laboratory.14 The allowedness of the light-driven promotion of a Ni dyz electron into an unoccupied, high-lying bpy π* orbital ultimately gives rise to an 1 MLCT state that undergoes intersystem crossing and then a final LMCT transition to cleave the Ni–C bond.12 Still, characterization of the 3LF state of the Ni(II)–bpy aryl halide complexes is important in understanding catalysis, particularly for disproving the null hypothesis. For example, synthesis of a covalently-linked aryl-bpy Ni(II)–bpy aryl halide complex turns off 3LF formation, as that electronic state is not able to be stabilized via geometric distortion like in the prototypical example.15 Yet still, similarly to the prototypical example, this tethered complex reacts with electrophile.15 The electronic 5 structure of transition metal complexes can also be leveraged in order to gather mechanistic information about various reactivities. 1.3. Spin Relaxation 1.3.1. S = ½ Molecules The ground-state electronic structure of S = ½ transition metal species is particularly important for understanding factors that give rise to spin relaxation. An ultimate goal of quantum information science is to achieve qubits with high-temperature coherence; minimizing spin relaxation rates is one manner in which that goal may be accomplished.16 In an S = ½ system, the Zeeman effect removes the degeneracy of the MS = + ½ and MS = - ½ in the presence of an externally-applied magnetic field (Figure 1.2). The transition between the lower-lying magnetic sublevel and the excited level gives rise to an “on” state that can be utilized for quantum information science applications. A B 1/T1 Mz t Spin-lattice relaxation C 1/T2 Mx/y t Decoherence time; limited by 1/T1 at high temperatures Figure 1.2. The Zeeman effect (A) as it relates to spin relaxation in an S = ½ system. Spin relaxes longitudinally via spin-lattice relaxation (B) and in the xy plane via decoherence 6 (T2) (C). At high temperatures where the best qubits operate, T2 is limited by T1; T1 is the most important metric for design principles at high temperatures. Various electronic structure considerations may give rise to favorable qubit behavior for an S = ½ qubit. For example, an unpaired electron in an orbital with less orbital angular momentum (e.g., a dz2 orbital with ml = 0) may exhibit longer coherence times; an electron in an orbital with larger orbital angular momentum is predicted to relax faster via Spin Hamiltonian models.17 Additionally, the unpaired electron may interact with the environment via spin-phonon coupling, which contributes to relaxation via specific symmetry rules.18 So, careful consideration of the environment of the unpaired spin on a transition metal complex is necessary in order to develop the most robust qubits. In this thesis, we investigate the symmetry of vibrational modes that may interact with the unpaired spin on copper porphyrin complexes and drive spin relaxation. Totally symmetric vibrational modes (a) are symmetry-allowed contributors to 1/T1,18 and we spectroscopically confirm that the lower-energy, and hence more thermally accessible, vibrational modes with the proper symmetry led to faster relaxation in these model S = ½ systems.19 1.3.2. S = 1 Molecules While S = ½ molecular systems have been extensively studied as qubit candidates, the necessity of large magnetic fields and very cold temperatures for quantitative spin polarization limits their applications.20 However, S = 1 systems may overcome this barrier by leveraging their zero-field splitting, which is borne from their triplet ground-state electronic configuration (Figure 1.3).21,22 Despite this ability to achieve spin polarization at less extreme conditions, spin-lattice relaxation for S = 1 species is typically quite fast, and coherence is usually lost at temperatures above 60 K.23 Therefore, elucidating the mechanisms of relaxation in these optically-addressable qubit candidates is necessary in order to develop design principles to increase their coherence temperatures. 7 S=1 E Energy MS = ±1 MS = -1, 0, +1 MS = +1 MS = -1 D MS = 0 Figure 1.3. Zero-field splitting in the absence of a magnetic field for an S = 1 complex. The ZFS term D refers to the axial ZFS parameter, and the ZFS term E refers to the rhombic ZFS value. Herein, we leveraged variable-temperature and variable-field electron paramagnetic resonance (EPR) in order to decipher the mechanism of spin-lattice relaxation in an S = 1 Cr(o-tolyl)4 quantum bit candidate. We determined that low-energy (pseudo-)acoustic phonon interactions with the spins likely drove relaxation in this system.23 1.4. Compound Scope An overview of the transition metal complexes that are evaluated in this thesis is shown in Figure 1.4. These complexes span applications in catalysis and spin relaxation; their electronic structures are all key factors in understanding their utility. 8 Catalysis N N Cl N N Fe N N N Cl N Ni N Ni Cl N H 3C N N Fe-p-TMA Ni(bpy)ArX NiPh(bpy)X Spin Relaxation Cr Cr(o-tolyl)4 O O Cu O O Cu(acac)2 N N N N Cu N N Cu N N Cu N N N N CuOEP CuTPP CuTiPP Figure 1.4. Molecules evaluated in this thesis 1.5. References (1)Boutin, E.; Merakeb, L.; Ma, B.; Boudy, B.; Wang, M.; Bonin, J.; AnxolabéhèreMallart, E.; Robert, M. Molecular Catalysis of CO2 Reduction: Recent Advances and Perspectives in Electrochemical and Light-Driven Processes with Selected Fe, Ni and Co Aza Macrocyclic and Polypyridine Complexes. Chem. Soc. Rev. 2020, 49 (16), 5772–5809. https://doi.org/10.1039/D0CS00218F. (2)Boutin, E.; Robert, M. Molecular Electrochemical Reduction of CO2 beyond Two Electrons. Trends in Chemistry 2021, 3 (5), 359–372. https://doi.org/10.1016/j.trechm.2021.02.003. (3)Costentin, C.; Robert, M.; Savéant, J.-M.; Tatin, A. Efficient and Selective Molecular Catalyst for the CO 2 -to-CO Electrochemical Conversion in Water. Proc. Natl. Acad. Sci. U.S.A. 2015, 112 (22), 6882–6886. https://doi.org/10.1073/pnas.1507063112. (4)Bonetto, R.; Crisanti, F.; Sartorel, A. Carbon Dioxide Reduction Mediated by Iron Catalysts: Mechanism and Intermediates That Guide Selectivity. ACS Omega 2020, 5 (34), 21309–21319. https://doi.org/10.1021/acsomega.0c02786. (5)Römelt, C.; Song, J.; Tarrago, M.; Rees, J. A.; van Gastel, M.; Weyhermüller, T.; DeBeer, S.; Bill, E.; Neese, F.; Ye, S. Electronic Structure of a Formal Iron(0) 9 Porphyrin Complex Relevant to CO 2 Reduction. Inorg. Chem. 2017, 56 (8), 4745– 4750. https://doi.org/10.1021/acs.inorgchem.7b00401. (6)Zhang, X.; Guo, S.-X.; Gandionco, K. A.; Bond, A. M.; Zhang, J. Electrocatalytic Carbon Dioxide Reduction: From Fundamental Principles to Catalyst Design. Materials Today Advances 2020, 7, 100074. https://doi.org/10.1016/j.mtadv.2020.100074. (7)Rao, H.; Schmidt, L. C.; Bonin, J.; Robert, M. Visible-Light-Driven Methane Formation from CO2 with a Molecular Iron Catalyst. Nature 2017, 548 (7665), 74–77. https://doi.org/10.1038/nature23016. (8)Nagel, R. L.; Gibson, Q. H. Kinetics of the Reaction of Carbon Monoxide with the Hemoglobin-Haptoglobin Complex. Journal of Molecular Biology 1966, 22 (2), 249– 255. https://doi.org/10.1016/0022-2836(66)90130-6. (9)Azcarate, I.; Costentin, C.; Robert, M.; Savéant, J.-M. Dissection of Electronic Substituent Effects in Multielectron–Multistep Molecular Catalysis. Electrochemical CO 2 -to-CO Conversion Catalyzed by Iron Porphyrins. J. Phys. Chem. C 2016, 120 (51), 28951–28960. https://doi.org/10.1021/acs.jpcc.6b09947. (10)Follmer, A. H.; Luedecke, K. M.; Hadt, R. G. μ-Oxo Dimerization Effects on Groundand Excited-State Properties of a Water-Soluble Iron Porphyrin CO 2 Reduction Catalyst. Inorg. Chem. 2022, 61 (50), 20493–20500. https://doi.org/10.1021/acs.inorgchem.2c03215. (11)Patra, S.; Ghosh, S.; Samanta, S.; Nayek, A.; Dey, A. Second Sphere Control of CO2 Reduction Selectivity by Iron Porphyrins: The Role of Spin State. Journal of Organometallic Chemistry 2025, 1023, 123439. https://doi.org/10.1016/j.jorganchem.2024.123439. (12)Cagan, D. A.; Bím, D.; Silva, B.; Kazmierczak, N. P.; McNicholas, B. J.; Hadt, R. G. Elucidating the Mechanism of Excited-State Bond Homolysis in Nickel–Bipyridine Photoredox Catalysts. J. Am. Chem. Soc. 2022, 144 (14), 6516–6531. https://doi.org/10.1021/jacs.2c01356. (13)Ting, S. I.; Garakyaraghi, S.; Taliaferro, C. M.; Shields, B. J.; Scholes, G. D.; Castellano, F. N.; Doyle, A. G. 3 d-d Excited States of Ni(II) Complexes Relevant to 10 Photoredox Catalysis: Spectroscopic Identification and Mechanistic Implications. J. Am. Chem. Soc. 2020, 142 (12), 5800–5810. https://doi.org/10.1021/jacs.0c00781. (14)Cagan, D. A.; Bím, D.; Kazmierczak, N. P.; Hadt, R. G. Mechanisms of Photoredox Catalysis Featuring Nickel–Bipyridine Complexes. ACS Catal. 2024, 14 (11), 9055– 9076. https://doi.org/10.1021/acscatal.4c02036. (15)Bím, D.; Luedecke, K. M.; Cagan, D. A.; Hadt, R. G. Light Activation and Photophysics of a Structurally Constrained Nickel(II)–Bipyridine Aryl Halide Complex. Inorg. Chem. 2024, 63 (9), 4120–4131. https://doi.org/10.1021/acs.inorgchem.3c03822. (16)Wasielewski, M. R.; Forbes, M. D. E.; Frank, N. L.; Kowalski, K.; Scholes, G. D.; Yuen-Zhou, J.; Baldo, M. A.; Freedman, D. E.; Goldsmith, R. H.; Goodson, T.; Kirk, M. L.; McCusker, J. K.; Ogilvie, J. P.; Shultz, D. A.; Stoll, S.; Whaley, K. B. Exploiting Chemistry and Molecular Systems for Quantum Information Science. Nat Rev Chem 2020, 4 (9), 490–504. https://doi.org/10.1038/s41570-020-0200-5. (17)Kazmierczak, N. P.; Xia, K. T.; Sutcliffe, E.; Aalto, J. P.; Hadt, R. G. A Spectrochemical Series for Electron Spin Relaxation. J. Am. Chem. Soc. 2025, 147 (3), 2849–2859. https://doi.org/10.1021/jacs.4c16571. (18)Kazmierczak, N. P.; Hadt, R. G. Illuminating Ligand Field Contributions to Molecular Qubit Spin Relaxation via T 1 Anisotropy. J. Am. Chem. Soc. 2022, 144 (45), 20804–20814. https://doi.org/10.1021/jacs.2c08729. (19)Kazmierczak, N. P.; Lopez, N. E.; Luedecke, K. M.; Hadt, R. G. Determining the Key Vibrations for Spin Relaxation in Ruffled Cu(II) Porphyrins via Resonance Raman Spectroscopy. Chem. Sci. 2024, 15 (7), 2380–2390. https://doi.org/10.1039/D3SC05774G. (20)Harvey, S. M.; Wasielewski, M. R. Photogenerated Spin-Correlated Radical Pairs: From Photosynthetic Energy Transduction to Quantum Information Science. J. Am. Chem. Soc. 2021, 143 (38), 15508–15529. https://doi.org/10.1021/jacs.1c07706. (21)Drago, R. S.; Drago, R. S. Physical Methods for Chemists, 2nd ed.; Saunders College Pub: Ft. Worth, 1992. 11 (22)Deaton, J. C.; Gebhard, M. S.; Solomon, E. I. Transverse and Longitudinal Zeeman Effect on [Tetraphenylphosphonium Tetrachloroferrate(1-)]: Assignment of the Ligand Field Transitions and the Origin of the 6A1 Ground-State Zero-Field Splitting. Inorg. Chem. 1989, 28 (5), 877–889. https://doi.org/10.1021/ic00304a016. (23)Kazmierczak, N. P.; Luedecke, K. M.; Gallmeier, E. T.; Hadt, R. G. T 1 Anisotropy Elucidates Spin Relaxation Mechanisms in an S = 1 Cr(IV) Optically Addressable Molecular Qubit. J. Phys. Chem. https://doi.org/10.1021/acs.jpclett.3c01964. Lett. 2023, 14 (34), 7658–7664. 12 PART I: CATALYSIS 13 CHAPTER 2: µ-OXO DIMERIZATION EFFECTS ON GROUND- AND EXCITED-STATE PROPERTIES OF A WATER-SOLUBLE IRON PORPHYRIN CO2 REDUCTION CATALYST Adapted with permission from: Follmer, A. H.†; Luedecke, K. M.†; Hadt, R. G. “µ-Oxo Dimerization Effects on a Ground- and Excited-State Properties of a Water-Soluble Iron Porphyrin CO2 Reduction Catalyst.” Inorg. Chem. 2022, 61 (50), 20493. DOI: 10.1021/acs.inorgchem.2c03215. † Co-first author. Copyright 2022 American Chemical Society. 14 2.1. Introduction Iron tetraphenylporphyrin (FeTPP) complexes are well studied and versatile catalysts that can accomplish O2 activation, H2 evolution, and CO2 reduction.1-4 Particularly, FeTPP functionalized with trimethyl ammonium appendages at the phenyl para positions is both water soluble and electro/photochemically competent for the CO2 reduction reaction (CO2RR). The Robert and Savéant groups have demonstrated the high stability and selectivity of iron 5,10,15,20-tetra(para-N,N,N-trimethylanilinium)porphyrin (Fe-pTMA) in electro- and photochemical reduction of CO2. In both aqueous and organic solutions without a photosensitizer, Fe-p-TMA can reduce CO2 to CO and ultimately to CH4 in organic solutions upon addition of a photosensitizer.4-9 These capabilities of Fe-pTMA are suggested to arise from the electron-withdrawing nature of the NMe3+ groups that stabilize a key hypothesized catalytic intermediate, an Fe(0)–CO2 adduct, via Coulombic interactions.6 Additionally, the Lewis acidic groups positively shift the FeI/Fe0 couple, effectively lowering the overpotential and making the electrochemical CO2RR more favorable compared to other iron-porphyrin based systems.5 Although proposed catalytic cycles exist for the CO2RR by Fe-p-TMA, the precise identity of many intermediates involved in this conversion is unknown.4,8,9 Previous reports describe the catalyst as monomeric when it enters the catalytic cycle, but it is well known that water-soluble iron porphyrins exhibit complex equilibria with propensities to form μ-oxo diiron species (referred to herein as a μ-oxo dimer for consistency with previous literature) depending on the pH and ionic strength of their aqueous solutions.1016 Indeed, high ionic strength conditions are invoked in both the electro- and photochemical CO2RR by Fe-p-TMA and other catalysts, particularly with metal bicarbonates (MHCO3, M = Na, K, Cs) employed as electrolytes.9,17 Therefore, conditions that favor the CO2RR by Fe-p-TMA also favor its dimerization. However, the relevance of μ-oxo dimer formation in the context of the photocatalytic conditions has not been expressed in previous reports. Here, we utilize a combination of optical and magnetic spectroscopic techniques to characterize Fe-p-TMA and its complex, tunable equilibrium in aqueous conditions. Additionally, we leverage the ability to chemically control and spectrally address these 15 species and their photogenerated states via ultrafast transient absorption (TA) spectroscopy. We find that consideration of dimeric versus monomeric Fe-p-TMA has strong implications for the identities and lifetimes of the excited states generated in photocatalytically relevant solutions. 2.2. Results and Analysis 2.2.1. UV-vis Spectroscopy Water-soluble iron porphyrins are known to undergo μ-oxo dimer formation with increasing pH, ionic strength, and/or concentration.10-16,18 As described further below, the presence of more than two absorption bands in the Q band region and a shoulder in the Soret band indicate that Fe-p-TMA exists as an equilibrium of species at dilute concentrations in low ionic strength and near-neutral pH solutions. Introduction of an electrolyte such as NaHCO3, which increases the ionic strength of the solution, initiates a decrease in intensity and a concomitant blue shift of the Soret peak (arrows in Figure 2.1A). This shift is accompanied by an increase in the intensity of the Q band at 564 nm and a decrease in the features near 650 nm (arrows in Figure 2.1A). These spectral changes are consistent with those observed in the formation of water-soluble μ-oxo binuclear iron porphyrins.16,19-21 As expected, increasing the concentration of bicarbonate leads to faster rates of μ-oxo dimer formation and a greater overall concentration of this species (Figures 2.1B and S6 and Table S1). 16 Figure 2.1. Bicarbonate-induced μ-oxo dimer formation of Fe-p-TMA (pH ∼ 8.3). (A) Evolution of the Fe-p-TMA steady-state absorption spectrum upon addition of 50 mM NaHCO3. Inset: Q band region. (B) Normalized kinetic traces of μ-oxo dimer formation as a function of [NaHCO3] monitored at 412 nm Bicarbonate is a buffering electrolyte and therefore affects both the pH and ionic strength of the Fe-p-TMA solutions. To disambiguate these effects, solutions were prepared using different metal bicarbonates, nonbuffering electrolytes, and across a wide range of pH values. The UV–vis spectra of Fe-p-TMA in the presence of different MHCO3 identities (M = Na, K, Cs) commonly employed in the CO2RR are indistinguishable and support that the origin of the μ-oxo dimerization is an effect induced by the bicarbonate anion (Figure 2.2A). Figure 2.2. (A) Normalized steady-state absorption spectra of Fe-p-TMA in H2O and 500 mM MHCO3 solutions (M = Na, K, Cs). Inset: Q band region. (B) Normalized steadystate absorption spectra of Fe-p-TMA in various electrolyte conditions. Insets: Q band region Non-buffering electrolytes such as KCl and KNO3 induce similar effects on the steady-state absorption spectrum as with the addition of MHCO3 (Figure 2.2B). However, the positions of their respective Soret bands are shifted to higher energies. Both the Soret and Q bands arise from porphyrin-centered π–π* transitions that mix with frontier metal d-orbitals upon metalation.22 Therefore, these transitions are highly sensitive to factors 17 that alter the porphyrin structure and/or metal spin/oxidation states, such as the presence of axial ligands and solvent identity. However, interestingly, in the absorption spectra of Fe-p-TMA, the energy of the Q bands remains unchanged, while the position of the Soret shifts in different electrolyte solutions following the trend in anion: OH– < HCO3– < Cl– < NO3– (arrow in Figure 2.2B). Varying the pH of Fe-p-TMA solutions results in three different UV–vis profiles that correspond to distinct speciation in three pH regimes: <4.5, 4.5–12, and >12 (Figure 2.3). The effects of pH on water-soluble porphyrins have been extensively studied, and Fe-pTMA displays similar behavior.14,15,18,19 In nonbuffered conditions, aqueous FeIII porphyrin systems exist as (1) monomeric in acidic conditions (pH < 4.5), (2) predominately dimeric in basic conditions (pH > 12), and (3) a mixture of monomer and dimer at intermediate pH ranges. At pH < 4.5, we observe multiple monomeric populations with overlapping Soret bands (Figure 2.3A). Literature assignments of other watersoluble iron-based porphyrin systems suggest that in acidic pH solutions (<4.5), the Soret feature has an intense maximum that describes the absorbance of a monoaqua-ligated metalloporphyrin with a shoulder that describes the symmetric six-coordinate diaqualigated monomer.19 As pH approaches neutral conditions, the Soret red-shifts and the Q band region becomes consistent with an equilibrium between one monomeric species and a μ-oxo dimeric species. At very basic pH values (>12), the monomer/dimer equilibrium remains, but the growth of a new feature in the Q band region (∼650 nm) is attributed to a unique hydroxy-ligated monomer.19 In 500 mM NaHCO3, the same trends hold, except that at pHs 4–12, the monomer/dimer equilibrium strongly favors the μ-oxo dimer (Figure 2.3B). However, at high pH values, the growth of a 650 nm feature supports the assignment of a new monomeric species as well. 18 Figure 2.3. pH titrations of Fe-p-TMA in H2O (A) and at a constant ionic strength (500 mM NaHCO3) (B). Insets: Q band region. Spectra have been normalized to the maximum intensity of the Soret. pH was modulated with the addition of NaOH/HCl. 2.2.2. EPR and MCD Spectroscopy Electron paramagnetic resonance (EPR) spectroscopy reveals a marked difference in the magnetic spectra of Fe-p-TMA solutions depending on the electrolyte (Figure 2.4). The 5 K continuous wave (CW) EPR signal in 50/50 H2O/glycerol glass is axial with features gx,y = ∼6 and gz = ∼2, indicating a high-spin FeIII S = 5/2 ground state (Figure S10). Since samples were frozen as glasses, it is possible that they could undergo a change in speciation upon freezing. However, across all spectra that contained an additional electrolyte, there is a common loss of spectral intensity despite identical Fe-p-TMA concentrations. This intensity loss is attributed to μ-oxo dimer formation and resultant antiferromagnetic coupling between the two FeIII centers.23 The remaining signal intensity is attributed to residual monomer population. Additionally, while Fe-p-TMA in KCl solutions exhibits the expected loss of spectral intensity, only the species in NaHCO3 and basic solution induce significant changes in rhombicity. 19 Figure 2.4. 5 K X-band CW EPR spectra of Fe-p-TMA with various 500 mM electrolyte additives and pH conditions To corroborate the assignment of the UV–vis spectra of the monomeric and μ-oxo dimer species, we employed room-temperature magnetic circular dichroism (MCD) spectroscopy taken at 1.4 T. The MCD spectra are consistent with previous spectra obtained for different speciations of FeTPPCl and FeTMPyP, including their μ-oxo forms.24-27 The MCD spectra exhibit dispersion-type A terms with energies consistent with their steady-state absorption spectra/bands (dashed lines in Figure 2.5). As A terms are associated with degenerate excited states, we conclude that the lowest unoccupied molecular orbitals (LUMOs) are degenerate and are interpretable through the Goutermantype four-orbital model of porphyrin absorption spectra. Additionally, the MCD spectra only display differential absorption near 650 nm (∼15,000 cm–1) in water alone (i.e., for the monomeric species). Like the absorption spectra, when bicarbonate is present and μoxo Fe-p-TMA is favored, this feature decreases dramatically (Figure 2.5). 20 Figure 2.5. Top: Electronic absorption spectra of 50 μM Fe-p-TMA in H2O (left) and 500 mM NaHCO3 (right). The Q band region is scaled for a 10× dilution factor consistent with the 50 μM (Soret) and 5 μM (Q band) MCD samples. Bottom: MCD spectra (1.4 T) of Fep-TMA in water (left) and 500 mM (50 mM) NaHCO3 (right) 2.2.3. Ultrafast TA Spectroscopy Direct photoexcitation of Fe-p-TMA at λexc. > 420 nm has been demonstrated to result in the CO2RR.4,8,9,28,29 As these reports utilize MHCO3 to perform catalysis, we studied the impact of the addition of MHCO3 species on the photodynamic properties of Fe-p-TMA. Given the ability to discern between the monomer and dimer species in the steady-state absorption profiles of the Q band region (∼470–700 nm) (Figure 2.6), we performed a wavelength-dependence study using ultrafast TA spectroscopy to delineate the excitedstate behavior of these two species. Differences in relaxation rates between MHCO3 (M = Na, K, Cs) solutions were not observed within error (Figures S12–S26). 21 Figure 2.6. Excitation wavelengths (λexc. = 550 and 650 nm) noted on steady-state absorption spectra of Fe-p-TMA in H2O and 500 mM NaHCO3 solutions. The species in water is best described as an equilibrium of monomer and μ-oxo dimer, whereas NaHCO3 is predominantly μ-oxo dimer. The width of the pump scatter is ∼20 and ∼30 nm on either side of the central wavelength of excitation for 550 and 650 nm, respectively. The steady-state absorption profiles of the monomeric and MHCO3-induced μ-oxo dimeric species display Soret energies ranging from 412 to 407 nm, respectively, which is a function of the interplay of both increased dimer population and perturbed dielectric of the surrounding solution due to the addition of MHCO3 (vide infra). Further, the absorption band at 650 nm is nearly absent in conditions where dimer formation is preferred (i.e., in the presence of bicarbonate and other electrolytes). Note that this band does go away entirely for the dimer species upon addition of KNO3 (Figure 2.2B). This band therefore provides a spectrally addressable, monomer-associated feature, while the blue shifting of the Soret band/ground-state bleach in the TA data provides a handle for the discernment of excited-state dynamics associated with monomeric and μ-oxo dimeric species. Relaxation of monomeric Fe-p-TMA after photoexcitation at 650 nm in neutral pH H2O was best fit to a three-component sequential model (Figure 2.7). With τ1 = ∼333 fs, τ2 = 1.44 ps, and τ3 = 4.66 ps, Fe-p-TMA exhibits a relaxation process analogous to those 30,31 previously described for FeTPP and FeTMPyP monomers. 22 The energy of the ground-state bleach near 412 nm in each of the evolution-associated spectra (EAS) is consistent with its steady-state absorption profile (Figures 2.2 and 2.8). To support this monomeric assignment in H2O with λexc. = 650 nm, we collected TA data of acid-induced monomeric Fe-p-TMA (Figure S35). While the spectral profiles for these two samples differ slightly due to differences in monomeric speciation, the lifetimes of the components are comparable. Figure 2.7. Proposed mechanisms of Fe-p-TMA relaxation after photoexcitation into the Q band region for the monomeric and dimeric forms. In conditions where an equilibrium of monomeric and dimeric species exists, that is, in H2O at mild pH values, both relaxation pathways occur. The oxidation state of the iron centers is depicted at each intermediate state. 23 Figure 2.8. Normalized EAS profiles of Fe-p-TMA in H2O (A) of a three-component sequential model (τ1 = 333 fs, τ2 = 1.44 ps, and τ3 = 4.66 ps) and 500 mM NaHCO3 solution (B), which is best fit to a two-component sequential model (τ1 = 2.04 ps and τ2 = ∼1 ns). The excitation wavelength is λexc. = 650 nm. On the other hand, the addition of MHCO3 to solutions of Fe-p-TMA drastically alters the observed photodynamics. Under these conditions, the μ-oxo dimer form is prevalent, and the difference spectra are best fit to a two-component sequential model with τ1 = 2 ps and τ2 = ∼1 ns (Figures 2.7 and 2.8). The lifetimes and differential TA profiles of the Fe-p-TMA μ-oxo dimer agree well with previous reports of both covalently tethered (Pacman) and nontethered FeIII–μ-oxo-FeIII porphyrin systems.32,33 In these reports, excitation at multiple wavelengths results in the cleavage of a FeIII–O bond and generates transient FeIV═O and FeII porphyrin species (τ1 < 20 ps) that then recombine (or, reclamp) to reform the ground-state dimer (τ2 = ∼1 ns). The EAS profile of component 2 representing the FeIV═O/FeII state should therefore consist of positive features due to the 24 formation of the Fe ═O and Fe species combined with the depletion or subtraction of IV II the dimer spectrum (Figure 2.8B). Indeed, we observe a negative band at 560 nm consistent with the ground-state bleach of the dimer. The ground state dimer spectrum also displays strong positive absorption in the ∼450 nm range, which presents as a negative feature in the TA profile. In combination with a positive absorption at shorter wavelengths, the difference spectrum presents as a negative band at 450 nm. Together with positive features that may be attributed to FeIV═O and FeII species at 420 nm, a broad absorption ∼510 nm, and smaller absorption at ∼590 nm, the spectra become identical to that of a well-characterized FeIV═O/FeII state,34 as depicted in Figure S1 of ref. 34. These bands are distinct from those found in the water alone. We also performed TA spectroscopy with λexc. = 550 nm, where both monomeric and dimeric populations absorb with nearly equal intensity (Figure 2.6). Here, the bicarbonate concentration-dependent photodynamics provided important information about the nature of the second relaxation component. Not only does this lifetime increase upon the increasing concentration of MHCO3 but is also associated with a flattening of the excitedstate absorption feature centered around 430 nm in the EAS profiles (Figure 2.9B). Rury and Sension attributed their τ2 with similar lifetimes to a charge-transfer event,30 and, similarly, Williams and Knappenberger noted a charge transfer τ2 having a dielectric dependence in an FeTMPyP system.31 In an acid-induced monomeric FeTMPyP system, an increase in τ2 was observed by decreasing the dielectric constant of the solution.31 We observe the same so-called dielectric effect on our τ2: increasing the concentration of MHCO3 (i.e., decreasing the dielectric constant of solution) leads to a prolonged τ2 lifetime for the monomeric population (Figure 2.9D). 25 Figure 2.9. Normalized EAS profiles of Fe-p-TMA in H2O and NaHCO3 solutions with λexc. = 550 nm from a three-component sequential model. (A) EAS component 1 (τ1 = ∼500 fs), (B) EAS component 2 (τ2 = 2–4 ps), (C) EAS component 3 (τ3 = ∼1 ns), and (D) τ2 dependence on the position of the steady-state Soret band. With the increasing concentration of NaHCO3, the τ2 excited-state lifetime increases, the EAS profiles flatten in the 430 nm region, and the energy of the steady-state Soret band blue-shifts to higher energy. 2.3. Discussion Previous reports have investigated Fe-p-TMA as an efficient electro- and photocatalyst for the CO2RR.4,8,9,28,29 In these reports, the resting state of the catalyst is proposed to be monomeric. However, we found that Fe-p-TMA speciation is sensitive to its chemical microenvironment. Consequently, in conditions reported for aqueous CO2RR, the catalyst exists predominantly as a μ-oxo porphyrin dimer in its resting state. Since the CO2RR was demonstrated by direct irradiation of Fe-p-TMA at λexc. > 420 nm and both monomeric and μ-oxo dimeric porphyrins display absorption in this region, we utilized optical and magnetic spectroscopic techniques to characterize these monomeric and μ-oxo dimeric species in their ground and excited states. We demonstrate the equilibrium that dictates 26 that speciation depends strongly on the pH and ionic strength of solution, and the achievable photochemical pathways of each species are altered accordingly. Given this propensity for MHCO3 to induce μ-oxo dimer formation in aqueous solutions and that direct photoexcitation of Fe-p-TMA in the presence of MHCO3 at λexc. > 420 nm has been shown to accomplish the CO2RR, we sought to study the tunable photodynamic properties of Fe-p-TMA. The monomer and μ-oxo dimer species are discernible in the steady-state Q band region (470–700 nm) (Figure 2.6). Specifically, the excited-state behavior of monomeric and μ-oxo dimeric populations of Fe-p-TMA was delineated using ultrafast TA spectroscopy by leveraging their differential absorption at 650 nm. This feature is attributed to a monomeric population. In catalytically relevant conditions (i.e., in the presence of an electrolyte, particularly MHCO3), the resting state of Fe-p-TMA is dominated by its μ-oxo dimeric form. Indeed, the formation of this μ-oxo binuclear species is favored and fortified with increasing ionic strength. Increasing the pH from acidic (pH ∼ 4) to basic (pH > 12) results in spectral changes similar to those associated with increasing ionic strength (Figures 2.3 and S2). However, in addition to observing substantial dimeric population in the steady-state absorption spectra at pH > 12, a new spectral feature appears at 650 nm that we attribute to a new monomeric species. It is also observed that the energy difference between the Soret and the Q bands for the μ-oxo binuclear species increases as the anion identity of the electrolyte is modulated from OH– to HCO3– to Cl– to NO3– (Figure 2.2B). This trend likely indicates a change in the solvation environment about the catalyst (e.g., pH or ionic strength) rather than axial ligand exchange, as one would expect simultaneous changes in the position and intensity of the Soret and Q bands upon a change in the primary coordination sphere.35 Given the positive charge of the peripheral appendages of Fe-p-TMA, it is likely that the unique association of these anions with the TMA groups results in these observed differences. The trend in the Soret energies (OH– < HCO3– < Cl– < NO3–) is consistent with decreasing anion basicity. Therefore, it is possible that, as the Lewis basicity of the anionic electrolyte is increased, the electrostatic interaction of the anion with the positive peripheral TMA charges is greater, and, therefore, the stronger base screens the positive porphyrin-based 27 charges more. Consequently, the energy of the π–π*-based Soret band decreases (i.e., higher λSoret wavelength) as a function of the screening ability of the anion. This electrostatic argument is one potential description of the experimental data, but whether the energy shifts arise from electron-withdrawing and/or porphyrin ring distortions is currently unclear. It is also unclear at this point as to whether or not this potential screening will manifest in the relative stabilities of the CO2RR intermediates (e.g., the Fe(0)–CO2 adduct). CW EPR spectroscopy also reveals changes in the Fe-p-TMA ground state upon the addition of different electrolytes. While there is a marked decrease in intensity due to the formation of μ-oxo dimers with antiferromagnetically coupled iron centers, the observed intensity of these spectra arises from residual monomeric populations with high-spin ground states (S = 5/2). The unique changes in rhombicity between these samples likely reflect perturbations to either the electronic structure and/or geometry of the porphyrin ring, which supports the conclusion that the anionic electrolytes are interacting with the TMA substituents. Further, the spectral profile of the residual monomeric population of Fe-p-TMA in the presence of KCl closely resembles the population in H2O alone. However, it is interesting to note that in the presence of MHCO3, which enhances Fe-pTMA’s catalytic capabilities,17 the EPR signal of the complex exhibits a distinctly more rhombic profile. Characterization of the ultrafast photodynamics of monomeric iron porphyrins has been performed on complexes such as FeTPP in organic solvents and the water-soluble FeTMPyP complex.30,31 In those systems, excitation into either the Soret and/or Q bands leads to excited-state dynamics that are best described by a three-component sequential model. These components are attributed to ligand-to-metal charge transfer (LMCT), followed by a metal-to-ligand charge transfer (MLCT), and then a d–d transition with an inferred spin-flip. All of these dynamics are complete in less than 20 ps. In contrast, covalently tethered and noncovalent μ-oxo dimeric iron porphyrin species studied by flash photolysis exhibit excited-state dynamics with lifetimes extending to the nanosecond timescale.33 In these systems, the initial dimer species undergoes a homolytic FeIII–O bond cleavage that leads to the formation of transient FeIV═O and FeII monomeric species. The 28 high-valent photogenerated intermediate can go on to oxidize a variety of substrates. However, if this process is unproductive, that is, in the absence of a substrate, then the transient species ultimately recombine to reform the μ-oxo dimeric ground state.34,36 Here, Fe-p-TMA was found to undergo relaxation processes best described by threeand two-component sequential models for monomeric and μ-oxo dimeric species, respectively. Akin to the relaxation paths reported for FeTPP and FeTMPyP monomers,30,31 Fe-p-TMA monomers undergo LMCT, MLCT, and subsequent relaxation from a ligand field excited state in less than 20 ps. This relaxation mechanism was confirmed by tuning the equilibrium with pH (acidic, 3.58) to favor monomeric species and repeating the TA measurements. These monomer-favoring conditions displayed nearly identical relaxation lifetimes to those found when probing the monomer at neutral pH (Figures S28 and S35). The μ-oxo dimer form, on the other hand, was best fit to a twocomponent sequential model, where cleavage of an FeIII–O bond generates transient FeIV═O and FeII porphyrin monomers (τ1 = 2 ps) that recombine to the ground-state μ-oxo dimer (τ2 = ∼1 ns). The assignment of this long-lived FeIV═O/FeII monomer pair is supported by the spectral similarity to previous reports for covalently and noncovalently linked Fe–porphyrin complexes (Table 2.1 and Figure S1 in reference 34). Table 2.1. Lifetimes Obtained by Global Fittinga Fe-p-TMA τ1 τ2* τ2 monomer ~500 fs 2-4 ps 5 ps µ-oxo dimer 2 ps 1 ns – a For the monomeric form, τ1 and τ2 values were obtained from fits of λexc. = 550 nm data, and τ3 was extracted from λexc. = 650 nm data. *τ2 of the monomer relaxation describes a charge-transfer process that is dependent on [MHCO3] (longer lifetimes with higher concentration). Dimer lifetimes were obtained from fits of λexc. = 650 nm data. The timescale for recombination to regenerate the μ-oxo dimer is sufficiently long to allow for the FeIV═O or FeII porphyrin species to exhibit reactivity. For example, photoexcitation of other μ-oxo iron porphyrin systems has resulted in catalytic oxidation 29 of olefins and sulfides. In that same vein, this prolonged excited state of the Fe-p-TMA 36 dimeric species may help explain the enhanced photocatalyzed CO2RR in aqueous conditions. Further studies are needed to investigate the role of the μ-oxo species in the context of this mechanism. The overlapping absorption at 550 nm was also utilized to assign the identity of the second component of the monomeric relaxation path as MLCT in origin. τ2 found here is highly sensitive to the concentration of MHCO3. Decreasing the dielectric constant of the solution leads to a prolonged τ2 lifetime for the monomeric population and can be related back to the blue shift in the energy of the Soret band (Figure 2.9D). Despite our excitation into the Q band manifold, longer τ2 lifetimes correspond to higher-energy Soret positions and are associated with higher ionic strength (lower dielectric) conditions. These comparisons support our conclusion that the identity of the first two relaxation events upon excitation of Fe-p-TMA at 550 nm can be modeled as monomer-based LMCT and subsequent MLCT processes. It is currently unclear why the LMCT component does not exhibit ionic strength dependence. It should also be noted that the third component of the data collected at 550 nm (Figure 2.9C) does not agree with the monomer-based model but rather is consistent with the second component of μ-oxo dimeric relaxation (Figure 2.8B), reflecting the excitation of both species at this wavelength. 2.4. Conclusions This study has demonstrated the existence of a complex equilibrium between mononuclear and μ-oxo dimeric species in the precatalytic form of a water-soluble iron porphyrin, Fep-TMA, which is capable of carrying out the CO2RR. It is demonstrated how conditions used to perform catalysis with Fe-p-TMA favor μ-oxo dimer formation. Further, as one may expect, the photodynamic pathways accessible to monomeric versus μ-oxo dimeric species are distinct. It is found that Fe-p-TMA forms μ-oxo dimers in the presence of bicarbonate and that the dimeric form exhibits similar excited-state dynamics to previously characterized μ-oxo binuclear iron porphyrin systems used in oxidative chemistry (i.e., FeIV═O or FeII porphyrin species). Using a unique low-energy absorption feature attributed to a monomeric species, the excited-state dynamics of monomer and μ-oxo 30 dimer species were differentiated. Monomeric Fe-p-TMA, regardless of environmental conditions, displayed relaxation dynamics that were complete within 10 ps and were consistent with a three-step relaxation found for other FeIII porphyrin complexes. The assignment of the second state (MLCT) was supported by the bicarbonate dependence of its excited-state profile and lifetime. In this study, optical and magnetic spectroscopic techniques were leveraged to identify and characterize chemical microenvironment effects on the distinct speciation in the ground and excited states of an iron porphyrin CO2RR catalyst. 2.5. References (1) Bhugun, I.; Lexa, D.; Savéant, J.-M. Homogeneous Catalysis of Electrochemical Hydrogen Evolution by Iron(0) Porphyrins. J. Am. Chem. Soc. 1996, 118, 3982– 3983, DOI: 10.1021/ja954326x. (2) Costentin, C.; Dridi, H.; Savéant, J.-M. Molecular Catalysis of O2 Reduction by Iron Porphyrins in Water: Heterogeneous versus Homogeneous Pathways. J. Am. Chem. 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Synthesis of μ2 -Oxo-Bridged Iron(III) Tetraphenylporphyrin–Spacer– Nitroxide Dimers and Their Structural and Dynamics Characterization by Using EPR and MD Simulations. 10.1002/chem.201805016. Chem.─Eur. J. 2019, 25, 2586– 2596, DOI: 33 (24) Kobayashi, H.; Higuchi, T.; Kaizu, Y.; Osada, H.; Aoki, M. Electronic Spectra of Tetraphenylporphinatoiron(III) Methoxide. Bull. Chem. Soc. Jpn. 1975, 48, 3137– 3141, DOI: 10.1246/bcsj.48.3137. (25) Kobayashi, H. Magnetic Circular Dichroism of the Iron (III) Porphyrins. Adv. Biophys. 1976, 8, 191– 222. (26) Kobayashi, N.; Koshiyama, M.; Osa, T.; Kuwana, T. Magnetic Circular Dichroism Studies on [5,10,15,20-Tetrakis(1-Methylpyridinium-4-yl)Porphinato]Iron and Some of Its Derivatives. Inorg. Chem. 1983, 22, 3608– 3614, DOI: 10.1021/ic00166a024. (27) Kobayashi, N. [5,10,15,20-Tetrakis(1-Methylpyridinium-2, -3-, and -4- yl)Porphinato]Iron Complexes as Seen by Electronic Absorption, Magnetic Circular Dichroism, and Electron Paramagnetic Resonance Spectroscopy. Inorg. Chem. 1985, 24, 3324– 3330, DOI: 10.1021/ic00215a005. (28) Torbensen, K.; Han, C.; Boudy, B.; Wolff, N.; Bertail, C.; Braun, W.; Robert, M. Iron Porphyrin Allows Fast and Selective Electrocatalytic Conversion of CO2 to CO in a Flow Cell. Chem.─Eur. J. 2020, 26, 3034– 3038, DOI: 10.1002/chem.202000160. (29) Rao, H.; Lim, C.-H.; Bonin, J.; Miyake, G. M.; Robert, M. Visible-Light-Driven Conversion of CO2 to CH4 with an Organic Sensitizer and an Iron Porphyrin Catalyst. J. Am. Chem. Soc. 2018, 140, 17830– 17834, DOI: 10.1021/jacs.8b09740. (30) Rury, A. S.; Sension, R. J. Broadband Ultrafast Transient Absorption of Iron (III) Tetraphenylporphyrin Chloride in the Condensed Phase. Chem. Phys. 2013, 422, 220– 228, DOI: 10.1016/j.chemphys.2013.01.025. (31) Williams, L. J.; Knappenberger, K. L. Solvent-Dependent Absorption and Electronic Relaxation Dynamics of Iron (III) Tetra-4-N-Methylpyridylporphine. Chem. Phys. Lett. 2017, 671, 100– 106, DOI: 10.1016/j.cplett.2017.01.011. (32) Hodgkiss, J. M.; Chang, C. J.; Pistorio, B. J.; Nocera, D. G. Transient Absorption Studies of the Pacman Effect in Spring-Loaded Diiron(III) μ-Oxo Bisporphyrins. Inorg. Chem. 2003, 42, 8270– 8277, DOI: 10.1021/ic034751o. (33) Guest, C. R.; Straub, K. D.; Hutchinson, J. A.; Rentzepis, P. M. Picosecond Absorption Studies on the Excited State of (µ-Oxo)- 34 Bis[(Tetraphenylporphinato)Iron(III)]. J. Am. Chem. Soc. 1988, 110, 5276– 5280, DOI: 10.1021/ja00224a006. (34) Rosenthal, J.; Luckett, T. D.; Hodgkiss, J. M.; Nocera, D. G. Photocatalytic Oxidation of Hydrocarbons by a Bis-Iron(III)-μ-Oxo Pacman Porphyrin Using O2 and Visible Light. J. Am. Chem. Soc. 2006, 128, 6546– 6547, DOI: 10.1021/ja058731s. (35) Nappa, M.; Valentine, J. S. The Influence of Axial Ligands on Metalloporphyrin Visible Absorption Spectra. Complexes of Tetraphenylporphinatozinc. J. Am. Chem. Soc. 1978, 100, 5075– 5080, DOI: 10.1021/ja00484a027. (36) Rosenthal, J.; Pistorio, B. J.; Chng, L. L.; Nocera, D. G. Aerobic Catalytic Photooxidation of Olefins by an Electron-Deficient Pacman Bisiron(III) μ-Oxo Porphyrin. J. Org. Chem. 2005, 70, 1885– 1888, DOI: 10.1021/jo048570v. 35 CHAPTER 3: THE NON–INNOCENT ROLE OF AMINE ADDITIVES IN THE ELECTROCHEMICAL GENERATION OF HIGHLY REDUCED CO REDUCTION PRODUCTS IN A MOLECULAR IRON PORPHYRIN SYSTEM This chapter is temporarily embargoed. 60 CHAPTER 4: PHOTOPHYSICS OF A STRUCTURALLY CONSTRAINED NI(II)-BIPYRIDINE ARYL HALIDE COMPLEX Adapted with permission from: Bím, D.; Luedecke, K. M.; Cagan, D. A.; Hadt, R. G. “Light Activation and Photophysics of a Structurally Constrained Nickel(II)-Bipyridine Aryl Halide Complex.” Inorg. Chem. 2024, 63 (9), 4120. DOI: 10.1021/acs.inorgchem.3c03822. Copyright 2024 American Chemical Society. 61 4.1. Introduction Photoredox catalysis has motivated modern synthetic organic chemistry through the development of selective bond transformations using light.1-8 At its core, metallaphotoredox catalysis utilizes photoactive transition metal complexes to facilitate intramolecular charge transfer and/or outer-sphere electron transfer processes to generate catalytically reactive intermediates.9-16 Nickel(II)–bipyridine (bpy) aryl halide complexes have emerged as a prominent class of light activated cross-coupling catalysts,7-22 wherein direct photoexcitation leads to population of singlet metal-to-ligand charge transfer (1MLCT) excited-state potential energy surfaces and, ultimately, Ni(II)–C bond homolysis to yield reactive Ni(I) species.19 Two bond homolysis mechanisms are outlined in Figure 4.1a and feature either (1) relaxation to a triplet Ni(II) ligand-field (3LF) state followed by thermal Ni(II)–C bond homolysis19 or (2) intersystem crossing to repulsive ligand-tometal charge transfer (LMCT) states (referred to herein as ISC/LMCT).21-23 Assessing the interplay between these two (or other) possibilities remains challenging, however, as the quantum yields for Ni(II)–C bond homolysis are small, preventing direct observation using transient spectroscopies. Thus, ongoing efforts seek to understand the photophysical processes involved in photoredox catalysis and expand the catalytic repertoire and reactivity of nickel catalysts for specific applications. 62 Figure 4.1. (a) Energy diagrams showing the proposed direct excitation mechanistic pathways for excited-state Ni(II)–C bond homolysis.19,23 (b) Irradiation of untethered Ni(II)–bpy aryl halide yields the catalytically relevant Ni(I)–bpy halide intermediate and a free aryl radical. (c) Irradiation of a tethered Ni(II)–bpy aryl halide complex investigated in this work gives rise to a reversible Ni(II)–C bond homolysis equilibrium. The transient Ni(I)–bpy halide intermediate can be reacted with electrophiles in situ to promote the forward reaction. Previous catalyst optimization efforts explored novel ligand architectures, bpy substitutions, and coordination environments to adjust the electronic properties of organonickel(II) complexes.19-21,24-29 Ni(II)–C bond homolysis serves as a universal photoactive pathway and occurs even upon replacing the imine backbone with aliphatic amines or phosphines.20 However, ligand modifications significantly influence the 63 kinetics of bond homolysis. Upon variation of the bpy and aryl substituents in Ni(II)– bpy aryl halide complexes, rate constants span ∼2 orders of magnitude, trend linearly with ligand Hammett parameters, and exhibit significant dependence on excitation energy.21,22 These observations highlight the ability to tune key aspects of excited-state potential energy surfaces through structural modifications. Additionally, ligand substituents provide essential electronic contributions to the reactivity of photogenerated Ni(I) intermediates24,26,30 and can be finely adjusted to suit different substrates in oxidative addition, to vary the overall solution-phase stability of Ni(I) species to improve selectivity, or to avoid off-cycle reactions. The present study establishes the photochemical and photophysical properties of an underexplored, tethered Ni(II)–bpy aryl halide complex that has yet to be considered in the context of photoredox catalysis. Covalently tethering the aryl and bpy results in a fixed pseudosquare planar coordination geometry.31 Such a structural constraint contrasts traditional untethered analogues and provides a platform to explore how limited ligand flexibility influences excited-state relaxation processes in photoredox catalysis, as we hypothesized that geometric restrictions may modify ground- and excited-state potential energy surfaces and thus the ability to produce reactive intermediates upon photoexcitation. Herein, we have examined the tethered Ni(II)–bpy aryl halide complex (2) using photochemical methods, ultrafast transient absorption (TA) spectroscopy, and quantum chemical calculations. Direct comparisons are made to those of the wellcharacterized, untethered complex (1). Notably, 2 exhibits remarkable stability upon photoexcitation over a broad wavelength range. However, upon introduction of an electrophile during photoirradiation, 2 rapidly converts to a new species. Photochemical behavior studied herein supports the presence of a reversible Ni(II)–C(aryl) ⇄ [Ni(I) + C(aryl)•] photochemical reaction and demonstrates that a photogenerated Ni(I) species can be productively captured by reaction with an electrophile. In addition to their distinct photochemical behavior, 1 and 2 exhibit distinct photophysics. While relaxation to a low-energy 3LF state has been established for 1, structural constraints in 2 prohibit 3LF state formation and restrict excited-state relaxation, as probed by ultrafast TA spectroscopy, to MLCT potential energy surfaces. 64 The photoactivation of 2 in the presence of an electrophile, despite the lack of LF state 3 formation, further suggests that an ISC/LMCT pathway (Figure 4.1a, right) may facilitate excited-state Ni(II)–C bond homolysis. In addition to inhibiting 3LF state formation, structural constraints in 2 prolong 1MLCT and relaxed-1/3MLCT lifetimes by factors of ∼10 and ∼2, respectively. As such, this study holds promise for advancing our understanding of ligand electronic and steric effects in photoredox catalysis and guides approaches to unlocking new potential for the design of efficient and selective photoactive catalysts for organic synthesis. 4.2. Results and Analysis 4.2.1. Steady-State UV-vis Absorption Spectroscopy Untethered (1) and covalently tethered (2) Ni(II)–bpy aryl halide complexes were synthesized and characterized following previous literature reports (Supporting Information section S1.2). UV–vis spectra of 1 and 2 in THF revealed two broad bands in the region of ∼350–600 nm, which were previously assigned to low- (band i) and highenergy (band ii) 1MLCT transitions (Figure 4.2).18,21,31 For 2, band i is red-shifted by ∼900 cm–1 (λi,max = 485 and 507 nm for 1 and 2, respectively) and exhibits a lower extinction coefficient (ε = 4070 M–1 cm–1 and 2800 M–1 cm–1 for 1 and 2, respectively). For 11 corresponding untethered complexes, we have observed a loose trend between increasing ε and longer λi,max, which is the opposite trend observed here for 1 and 2.21 Therefore, we attribute the lower ε in 2 to the different aryl orientations, which influences the relative overlap between donor and acceptor molecular orbitals involved in the MLCT transitions. Furthermore, peak maxima red-shift and decrease in intensity in toluene relative to THF (Figure S2), corroborating the assignment of bands i and ii as mainly 1MLCT electronic transitions with Ni(II)-to-Phbpy π* character. 65 Figure 4.2. (a) UV–vis absorption spectra of 1 and 2 in THF. Spectra recorded in toluene are provided in Figures S2 and S7. (b) Schematic representation of electronic transitions comprising bands i and ii CASSCF/QD-NEVPT2 calculations predict multiconfigurational ground states for 1 and 2, with closed-shell singlet weights in the CAS configuration interaction vector of only ∼56–57%. Remaining configurations consist of 1MLCT states (∼20% and ∼22% for 1 and 2, respectively) and 1LF states (∼17% and ∼16% for 1 and 2, respectively; Tables S4 and S5). The lowest vertical excited triplet states of mainly 3LF character are at ∼9300 cm–1 (∼26.6 kcal mol–1) and ∼10 800 cm–1 (∼30.9 kcal mol–1) for 1 and 2, respectively (Figure S28). However, free energy differences between the optimized (adiabatic) triplet and ground states are significantly different, with ΔG = ∼8 kcal mol–1 (for 1) and ΔG = ∼27 kcal mol–1 (for 2). This difference demonstrates a significant stabilization of the lowest energy 3LF state in 1 due to sterically unencumbered geometric rearrangement, which allows for the formation of a relaxed, pseudo-Td coordination. The same structure is inaccessible in 2 due to steric constraints provided by the covalently tethered aryl group (Figure S32). For 1, calculations predict that bands i and ii in Figure 4.2b are mainly 1MLCT in nature, consisting of excitations between specific Ni 3d donor orbitals and bpy π* acceptor orbitals.19,21,32 The most intense excitations are found for Ni 3d(xz)/3d(yz) donor orbitals, consistent with a previous report.21 However, tethering the aryl group in 2 changes the composition of the electronic excitations. While excited states with the largest oscillator 66 1 1 strengths in 1 are MLCT states, the LF states in 2 have intensities comparable to those of the most intense low-energy 1MLCT band (Figures S29 and S30). We attribute this behavior to a better electronic conjugation and overlap between the tethered aryl and bpy ligand, which increases the bpy character (thus mixing some allowed MLCT intensity) in the unoccupied Ni 3d(x2-y2)/C(sp2)* acceptor orbital (Figure S27). The calculated intensity of the low-energy band in 2 also decreases by ∼2.5 times relative to 1 (Tables S6 and S7), in agreement with the smaller experimental extinction coefficient. In summary, covalently tethering the aryl and bpy groups modulates the electronic properties of 2 relative to 1, as manifested in the experimental and computational changes in the excited-state compositions and intensities of bands i and ii, with band ii being responsible for light-activation in direct excitation nickel photoredox catalysis. In terms of other well-studied Ni(II)–bpy aryl halide complexes, tethered complex 2 provides an important reference for a comprehensive investigation of the influences of electronic and steric perturbations on its photophysical and photochemical characteristics. 4.2.2. Photochemical Activity Irradiating 2 in THF using a 390 nm LED (exciting high-energy 1MLCT band ii) resulted in no significant photochemical activity after 15 h (Figure 4.3a, right). Slow decay of characteristic absorption features occurs after prolonged irradiation, likely due to unproductive photodegradation. 2 is also photoinactive over a broad range of excitation wavelengths (370, 427, 456, 525 nm; Figure S6) and in toluene (Figure S7). This behavior is in stark contrast to 1, which affords almost full conversion to a previously characterized Ni(I)–bpy chloride species after 1 h of photoirradiation (Figure 4.3a, left) using a 390 nm LED with the same power (kobs = 3.8 × 10–2 min–1 and Φ = 0.5 × 10–3; LED power of 170 mW cm–2). 67 Figure 4.3. Photoirradation of 1 and 2 in THF using a 390 nm LED. (a) Time-dependent photolysis profiles of 1 and 2. (b) Time-dependent photolysis profiles of 1 and 2 in the presence of 1000 equiv of mesityl bromide The distinct photochemical behavior of 1 and 2 (Figure 4.3a) indicates that 2 either (a) does not follow the same photoexcitation/relaxation pathways as 1 or (b) excited-state Ni(II)–C bond homolysis, although potentially viable, is reversible due to the covalent tethering of the bpy and aryl groups. We note that the absence of any photoproduct formation following the irradiation of 2 excludes alternate pathways for its irreversible decomposition, such as through Ni(II)–Cl bond homolysis. To distinguish between the two cases above, we have repeated photochemical experiments in the presence of a large excess (1000 equiv) of mesityl bromide, which should react if a Ni(I) intermediate is formed in situ. The steady-state UV–vis spectra do not change appreciably upon the addition of mesityl bromide (Figure S3); no reactivity is observed between Ni(II) and 68 mesityl bromide without irradiation. However, bands i and ii of parent Ni(II) complexes 1 and 2 decay more rapidly upon 390 nm LED irradiation in the presence of mesityl bromide (Figure 4.3b). Compounds 1 and 2 exhibit distinct photochemical differences when irradiated in the presence of a large excess of mesityl bromide. Untethered 1 exhibits precedented reactivity; the accumulation of the Ni(I)–bpy chloride intermediate complex is fully suppressed by mesityl bromide (Figure 4.3b, left). The proposed pathway for this reactivity consists of oxidative addition of mesityl bromide to Ni(I) to yield a high-energy Ni(III) intermediate,30,33-35 which can then undergo comproportionation with another Ni(I) in each reaction cycle to produce a new photoactive Ni(II)–bpy aryl halide and a photoinactive Ni(II)–bpy dihalide in a 1:1 ratio.16,33 With extended irradiation, all of the starting Ni(II)–bpy aryl halide complexes are expected to ultimately convert to a triplet d8 Ni(II)–bpy dihalide (Scheme S8), which does not possess distinctive UV–vis absorption features in the visible region (Figure S4). Note that the sloping UV–vis spectra in Figure 4.3b (left) is indicative of either the precipitation of Ni(II)–bpy dihalide or formation of an off-pathway [Ni(I)–bpy halide]2 binuclear species from two Ni(I)–bpy halide units.30 Conversely, irradiation of 2 with 1000 equiv of mesityl bromide produces a new, stable species with a red-shifted absorption band at 555 nm (Figure 4.3b, right). Interestingly, a species with a similar absorption profile (absorption bands centered at 547, 399, and 352 nm; see Figure S11 for comparison) has been observed by Klein and coworkers via one-electron reductive dissociation of the bromide anion from a related tethered Ni(II)–bpy phenyl bromide complex.36 Based on spectroelectrochemistry, that species was assigned as the tethered, tridentate Ni(I)–bpy phenyl complex. Similar to our photochemical process, no isosbestic points were detected in the spectroelectrochemical conversion of the parent species to the proposed Ni(I)–bpy phenyl intermediate, suggesting more complex multistep reactivity. Our attempts to isolate and characterize this soluble photogenerated species have been unsuccessful due to complications associated with irradiation of larger quantities of starting material (e.g., low solubility and low light penetration) and the presence of a large excess of mesityl bromide, Ni(II)–bpy dihalide (vide infra), and unreacted Ni(II)–bpy aryl 69 halide in the reaction mixture after irradiation. However, according to UV–vis and paramagnetic 1H NMR spectroscopic characterization of the crude reaction mixture, we note that the photogenerated species is paramagnetic (1H NMR peaks at ∼18, ∼23, and ∼69–76 ppm) and stable under high temperature and concentration conditions and reacts rapidly with air (see section S1.4 of the Supporting Information). These findings corroborate the results of Klein and co-workers associating the UV–vis features to a tridentate Ni(I)–bpy phenyl species.36 We also note that calculated (CASSCF/QDNEVPT2) absorption spectra are consistent with this assignment with the tethered, tridentate Ni(I)–bpy phenyl species exhibiting a slight red shift of the main absorption feature from the parent Ni(II) complex (Figure S31). The calculated oscillator strengths further suggest that ∼50% of the parent Ni(II) species is converted to this Ni(I)–bpy phenyl species. Additionally, from the same reaction mixture, we were able to isolate an appreciable amount of sparingly soluble Ni(II)–bpy dihalide; its spectroscopic features resemble an independently synthesized Ni(II)–Phbpy dichloride (Figure 4.4 and section S1.4 of the Supporting Information). We note that the possible presence of mixed chloride and bromide species cannot be ruled out, since the peaks in the paramagnetic region in Figure 4.4 are fairly broad. Moreover, the 1H NMR spectrum of the sample was collected from the crude reaction mixture as a precipitate, washed with THF, and redissolved in D2O for NMR analysis. Therefore, different solubilities of the chloride/bromide complex might result in altered speciation. Nonetheless, this comparison supports photodissociation of the tethered aryl ligand being indeed feasible in 2 as in 1, as irradiation of 2 also yields an intermediate that is captured by mesityl bromide and ultimately results in the formation of a Ni(II)–bpy dihalide complex. 70 Figure 4.4. Spectroscopic characterization and comparison between the independently synthesized Ni(II)–Phbpy dichloride (top) and the isolated product of irradiation of 2 with mesityl bromide (bottom). (a) Paramagnetic 1H NMR spectra measured in D2O. (b) Steady-state UV–vis absorption in H2O Photochemical reactions of 1 and 2 exhibit distinct dependence on mesityl bromide concentrations, with increasing rates observed with increasing mesityl bromide concentrations (Figure S19). In both 1 and 2, the decay of Ni(II) is assisted by the presence of mesityl bromide in solution. As discussed above, this observation is attributed to a reversible equilibrium for Ni(II)–C bond homolysis in 2. For 1, this behavior can likely be attributed to a solvent-caging effect, which prevents aryl radical diffusion from Ni(I), allowing sufficient time for reformation of the Ni(II)–C bond.37 This effect is less pronounced when mesityl bromide is present in sufficiently high concentrations (Figure S19), likely due to limited formation of caged radicals due to a more rapid Ni(I) reaction with the electrophile in solution. Although complex kinetics (especially at higher mesityl 71 bromide concentrations) preclude obtaining well-defined rate constants for the Ni(II)– C bond homolysis step (see further discussion in Supporting Information section S1.5), we note that the irradiations of 1 with and without the addition of quantitative excess mesityl bromide reveal a nearly 2–3-fold increase in Ni(II) conversion. Such estimation offers valuable insights into the quantum yields of Ni(II)–C photochemical bond homolysis and aids in deconvolution between “productive” ligand dissociation and “unproductive” recombination kinetics. It is worth noting that the addition of mesityl bromide does not influence the ultrafast photophysical relaxation pathways as discussed below in section 2.3 and instead facilitates the dissociation of aryl radicals from the [Ni(I)···C(aryl)•] cage. Ongoing work involves detailed quantum chemical molecular dynamics simulations being performed to better understand the behaviors of 1 and 2 after photoexcitation and the impact of mesityl bromide addition on photolysis rate constants. To better understand the nature of the photoinitiated reactivity of 2 with mesityl bromide, we performed irradiations of 2 with a variety of substituted aryl bromides (Figures S14–S18). We find that the soluble products seen after prolonged irradiations exhibit comparable absorption features, consistent with the same tethered Ni(I)–bpy phenyl product seen upon the irradiation of 2 with mesityl bromide. Interestingly, we have observed an electronic influence on the apparent rate of conversion of 2; electron-donating substituents on the aryl bromide contribute to an enhancement of this rate. Steric effects appear to play a lesser role but with a poorly discernible trend. These preliminary results suggest that a concerted oxidative addition mechanism for the activation of aryl bromides via conversion of 2 may not be operative, as it would predict enhanced reactivity with aryl bromides with electron withdrawing substituents and reduced sterics.26,33 Current efforts are underway to thoroughly detail the precise mechanism of the reactivity of aryl halides with light-activated 2. 4.2.3. Transient Absorption Spectroscopy To further understand the photochemical differences between 1 and 2, we examined their excited-state relaxation pathways by using ultrafast TA spectroscopy. The photophysics of 1 and other untethered Ni(II)–bpy aryl halide complexes have been described;18,19 in this study, we include 1 for direct comparison with 2. A consistent relaxation pathway for 72 Ni(II)–bpy aryl halides has been outlined, regardless of whether low-energy MLCT 1 (λexc. = ∼530 nm) or high-energy 1MLCT states (λexc. = ∼400 nm) are accessed (bands i and ii in Figure 4.2b).18,19 However, only the high-energy 1MLCT band ii was proposed to be relevant for catalysis, demonstrating excited-state Ni(II)–C bond homolysis and formation of a reactive Ni(I) intermediate.21,22 This assignment is exemplified by the photochemical activity of 1, which is considerably wavelength-dependent and requires a minimum energy threshold (λexc. < 525 nm) for photolysis (Figure S5). The steady-state absorption profile of 2 is red-shifted relative to that of 1 (Figure 4.2a). This ultimately allows enhanced accessibility of the higher-energy 1MLCT states in 2 (band ii) with lower-energy light. Still, both species have a notable absorption intensity associated with a high-energy 1MLCT state near 400 nm. Therefore, 400 nm excitation has been utilized for TA to probe relaxation pathways from the 1MLCT excited states of 1 and 2, thereby addressing key excited states relevant for catalysis. Direct photoexcitation of 1 with pulsed 400 nm light in THF results in an excitedstate lifetime on the nanosecond time scale (Figure S21). Global analysis revealed a threecomponent sequential decay model composed of a fast time component (τ1 = ∼400 fs), an intermediate component (τ2 = ∼9 ps), and a longer-lived state (τ3 = ∼3.5 ns). This relaxation pathway agrees well with and is analogous to that characterized by Doyle and co-workers for 1 with λexc. = 530 nm (Table S3).19 As illustrated in Figure 4.5, the 1MLCT excited state is first accessed upon photoexcitation. This state decays either into a relaxed 1 MLCT state through vibrational cooling or to a 3MLCT state via intersystem crossing. This process is observable through the blue-shifting of the excited-state absorption (ESA) in the red region of the spectrum (Figure S21B), which occurs on the picosecond time scale. The 1/3MLCT state then relaxes into an excited 3LF state observed through the decay of the ESA feature between a ∼5–20 ps time delay (Figure S21C). Finally, this 3LF excited state undergoes a longer, spin-forbidden relaxation back to the singlet ground state in several nanoseconds (Figure S21D), which is reflected by the recovery of the ground-state bleach (GSB). 73 Figure 4.5. Proposed mechanisms of relaxation for 1 and 2 after photoexcitation into their excited 1MLCT states with 400 nm pulsed light Photoexcitation of 2 into the high-energy 1MLCT state resulted in distinct excitedstate dynamics relative to 1. While the nonzero TA signal for 1 persists into the nanosecond regime, the entire excited-state manifold of 2 relaxes on the tens of picoseconds time scale, a significant shortening of excited-state lifetime by 2–3 orders of magnitude (Figure 4.6). Global analysis of the excited-state dynamics of 2 revealed a two-component sequential model consisting of a shorter (τ1 = ∼3 ps) and longer time component (τ2 = ∼14 ps). Analogous to 1, the first ∼10 ps of the TA difference spectra for 2 are dominated by a blueshifting ESA alongside recovery of the GSB (Figure 4.6c). However, the longer time dynamics differ between the complexes, with the ESA decaying monoexponentially and with the same time constant associated with the recovery of the GSB (Figure 4.6b and d). This isosbestic behavior can be interpreted as recovery of the ground state directly from the relaxed-1MLCT or 3MLCT excited state. In all, the photophysical behavior of 2 does not support the formation of 3LF excited states with an appreciable quantum yield. 74 Figure 4.6. Ultrafast TA spectra of 2 in deoxygenated THF upon 400 nm excitation. (a) Evolution-associated spectra obtained from a two-component, sequential global model of the data. (b) Decay of the ESA feature at 568 nm (13.7 ± 1.8 ps, orange) and recovery of the GSB feature at 506 nm (13.2 ps ±1.8 ps, blue), with their fits to monoexponential functions in black. (c) TA difference spectra for short time delays between the pump and probe. (d) TA difference spectra for longer time delays between pump and probe We also considered the influence of 100 equiv of mesityl bromide on the excitedstate dynamics of 1 and 2. Although UV–vis monitoring provides evidence of reactivity for 1 and 2 with 100 equiv of mesityl bromide during 390 nm LED photoirradiation (Figures S9 and S10), the relaxation pathways of 1 and 2 probed by TA spectroscopy remain largely unaffected by the addition of an electrophile, as relaxation time constants and spectral profiles are consistent between samples with and without mesityl bromide (see section S1.6 in the Supporting Information). These observations indicate that the TA of 1 75 and 2 probe the dominant background relaxation processes and are insensitive to the low quantum yield processes related to light-activated reactivity with electrophiles (vide infra). 4.3. Discussion Significant efforts have been dedicated to investigating the influence of steric and electronic factors on the photoredox reactivity of organonickel(II) complexes. These studies have unveiled structure–function relationships that elucidate how the ligand scaffold or electron-donating or -accepting substituents impact the formation of reactive Ni intermediates in cross-coupling catalysis. However, to the best of our knowledge, no data have been reported regarding organonickel(II) complexes with light-induced activation and dissociation of a coordinated aryl group tethered to the backbone ligand. While a number of previously synthesized tethered Ni(II)–bpy aryl halide complexes have been examined for electrochemical/reductive catalysis, including detailed spectroelectrochemical investigations, an explicit pathway for productive, direct light activation has not been outlined.31,36,38 This study adopted a previously described tethered Ni(II)–bpy aryl chloride complex 2 and explored its photochemical and photophysical properties, particularly through a comparison to untethered Ni(II)–bpy aryl halide photocatalyst 1. In contrast to untethered analogues, 2 exhibits superior stability upon photoirradiation across a broad range of excitation wavelengths spanning ∼370–525 nm. Despite this prolonged stability, the covalently tethered Ni(II) complex can still be lightactivated for reactivity in the presence of electrophile. This reactivity can be attributed to a shift in reversible Ni(II)–C(aryl) ⇄ [Ni(I)···C(aryl)•] photochemical equilibrium, in which the resulting Ni(I) reacts with an electrophile in solution. Such improved stability could potentially be leveraged in photoredox catalysis. For instance, it was previously observed that the presence of electron-withdrawing substituents like methyl or ethyl esters at the bpy 4,4′ positions significantly enhances the photochemical reactivity of 1, while simultaneously shifting the steady-state absorption features to lower energies by ∼2000 cm–1 (λi,max = 532 nm).21 A similar effect may be expected for 2, which would result in even more markedly red-shifted MLCTs, making them accessible not only with lower-energy 76 light but also with the tethered ligand providing stability to the otherwise highly photochemically reactive Ni(II). Although we have studied 2 in terms of its competency for photoredox catalysis, such photochemical robustness also offers the potential for regulating reactivity that is specific to experimental conditions. For example, by tuning the concentrations of other reactive species in solution, the unique photochemistry of 2 could be harnessed for a tailored release of Ni(I), thereby potentially advancing the reaction selectivity (Figure 4.7). This possibility could unveil intriguing and unexplored applications leveraging specific roles of Ni catalysts based on experimental conditions. A similar approach was employed to enhance yields and selectivity in cross-electrophile coupling reactions.39,40 In ref. 39, adjusting the reduction potentials of homogeneous organic reductants allowed for the precise time-release of alkyl radicals from Katritzky salt precursors and, thus, synchronized their production rates with those of the Ni(II)–aryl intermediates proposed for their capture. This strategy minimized the probability of undesired side reactions and Ni catalyst decomposition. Similarly, Reisman and co-workers modulated formation rates of benzyl radicals, which yielded an analogous effect for the cross-coupling of alkenyl and benzyl electrophiles.40 Analogously, the released benzyl radical is intercepted by a Ni(II)–alkenyl resting state, establishing a direct connection between radical generation rate and the selectivity of homo- vs cross-coupling reactivity. Modulation of the rate of Ni(I) formation based on the concentration of the electrophile, as demonstrated in this work, presents an alternative route to a similar strategy. 77 Figure 4.7. Cross-coupling selectivity in the Ni ground- and excited-state catalysis can be improved by synchronizing the generation of alkyl or aryl radicals (R2•) with the rate of formation of the LNi(II)R1X intermediate.39,40 In this work, we propose an alternative strategy of controlling the generation of the LNi(I)X complex from the parent Ni(II) precursor instead, offering an independent handle for the rate of nickel cross-coupling catalysis. Further, regarding the reaction selectivity, 1 and 2 revealed different product formation upon photoirradiation in the presence of mesityl bromide. Prolonged irradiation of 1 culminated in the formation of Ni(II)–bpy dihalide, as monitored by the disappearance of the Ni(II)–bpy aryl halide UV–vis absorption features, which is consistent with the previously proposed oxidative addition and comproportionation reaction mechanism.16,30,33 Contrastingly, 2 yielded a new Ni species with a unique UV–vis spectrum (Figure 4.3b, right) in addition to a Ni(II)–Phbpy dihalide. A species with a similar UV–vis spectrum was accessed via spectroelectrochemical one-electron reduction of the bromine analogue 78 of 2 and was attributed to the tethered, tridentate Ni(I)–bpy phenyl complex. Here, we 36 propose a photochemical pathway to the formation of this reaction intermediate. The formation of Ni(II)–Phbpy dihalide in the reaction mixture also supports the activation of mesityl bromide (Figure 4.4). However, this species is not the dominant product, as the higher stability of the tethered Ni(I)–bpy phenyl species and its lack of reactivity with mesityl bromide terminate the catalytic cycle at this stage. The low reactivity of the Ni(I)– bpy phenyl is likely due to the steric hindrance of the tridentate ligand coordinating and protecting the Ni(I) center. Under catalytic conditions, this stable Ni(I) intermediate could potentially be reactivated via reactivity with radicals,41,42 sterically accessible alkyl bromides,43 or through outer-sphere single-electron transfer processes.44 An examination of mesityl bromide concentration-dependent photochemistry highlighted controllable Ni(II) species decomposition rates (Figure S19), further supporting the notion of reversibility in excited-state bond homolysis. Intriguingly, a similar observation was made for both 2 and 1, wherein reversibility in the Ni(II)–C(aryl) ⇄ [Ni(I)···C(aryl)•] excited-state bond homolysis in the latter was not previously considered. While the concept of solvent caging and retaining the aryl radical in the proximity of Ni(II) has been posited for an oxidative addition of aryl halides reacting through a halogen-atom abstraction mechanism,33 the impact of this reversibility on photochemical reactivity remains unexplored. Through a comparative assessment of the rate constants of photochemical decay of 1 with varying mesityl bromide concentrations, we observed a 5-fold increase in the rate constant. While the experimental data reported here were obtained solely in THF, a thorough investigation of the solvent dependence of this rate enhancement would provide valuable insights for understanding the effect of caged radical recombination in photoredox catalysis.45 Unfortunately, the inherently low quantum yields of Ni(II)–C bond homolysis and very short MLCT excited state lifetimes hinder a direct, comprehensive experimental survey of the excited-state bond homolysis. Traditional computational methods─often static and only probing vertical energies and vertical excited states─are also insufficient.21,23,46,47 Given the complexities of the excitedstate dynamics and bond homolysis behavior, which may also require a multireference 79 treatment, insights derived from the experimental data in this study could provide an important reference for benchmarking more advanced molecular dynamics simulations. When considering the reversibility of C(aryl)• formation, it is noteworthy to emphasize the research by Park and co-workers who observed a similar phenomenon with Ni(II) metallacycle compounds (Figure 4.8).20 Regardless of the specific backbone ligand, metallacycles featuring cycloneophyl (−C6H4-o-C(CH3)2CH2−) and its oxa- (−OC6H4-oC(CH3)2CH2−) and thia- (−SC6H4-o-C(CH3)2CH2−) derivatives have been proposed to undergo light-induced carbon radical generation through Ni(II)–C(sp3) bond homolysis. In the case of thia-metallacycles, radical formation can be followed by C–S bond formation through reductive elimination. In cycloneophyl and its oxa derivative, however, radical formation is followed by recombination to reform the reactant state (Figure 4.8). While the covalent Ni–X linkage in the metallacycles and their overall chemical behavior resemble that of 2, the proposed observation of reversibility in 1 and the photophysics presented in this work suggest that reversible radical recombination to Ni might represent a more general paradigm to consider in Ni photoredox and single-electron chemistry. Figure 4.8. Irradiation of Ni(II) metallacycle compounds results in the formation of transient bound-C(sp3) radicals that can either facilitate reductive elimination or recombine to recover the Ni(II) ground state. Using transient spectroscopies, Doyle and co-workers have provided significant insights into the excited-state dynamics of untethered Ni(II)–bpy aryl halide complexes related to 1.18,19 They observed comparable excited-state dynamics using either 400 or 530 nm excitation and further provided valuable insights into the lifetimes of key MLCT and LF excited states. Their findings established that 1/3MLCT lifetimes are short (<∼15 ps in a series of eight Ni(II)–bpy aryl halide complexes), while 3LF states are significantly 80 longer-lived (∼2–8 ns). From this analysis, in conjunction with corroborating H NMR 1 2D Exchange Spectroscopy (EXSY) and DFT-calculated bond strengths, it was inferred that the excited state responsible for Ni(II)–C(aryl) bond homolysis is the 3LF state. From comparative TA experiments and comparisons to previous literature, we find that 1MLCT states are formed upon initial 400 nm excitation for both 1 and 2.18,21,31 The excited-state dynamics of both complexes share a similar subsequent relaxation step, indicated by a recovery of the GSB and a concomitant blue-shifting in the ESA by ∼20 nm (Figures S21 and S24). Similar relaxation profiles, including the blue-shifting ESA, have been observed in other first- and second-row transition metal complexes, including Ni(II)–bpy aryl halides.19,48 The precise nature of the relaxation process is currently unclear and can be indicative of either vibrational cooling or an intersystem crossing between 1/3MLCT states. Both processes can occur on the time scale consistent with our data, and vibrational cooling can even mediate intersystem crossing to a 3MLCT state. We note that CASSCF/QDNEVPT2 calculations predict multiple overlapping 1/3MLCT states that could be accessible (Figure S28); thus, we cannot unambiguously differentiate vibrational cooling vs intersystem crossing. Notably, the blue-shifting of the ESA has also been observed for Cu(I) bis-phenanthroline complexes and was associated with the flattening of the Td Franck–Condon geometry in the excited-state Cu(II)* on the picosecond time scale, which is also associated with intersystem crossing.49 In the Ni(II) d8 systems studied here, an interconversion between square planar and pseudo-Td geometries near the Franck–Condon point of the excited-state 1MLCT surface might take place. Such behavior could yield significantly distinct 1MLCT lifetimes, as was observed in this work for flexible 1 (τ1 = ∼400 fs) and more constrained 2 (τ1 = ∼3 ps). The excited-state dynamics of 1 and 2 are also distinct after accessing the relaxed 1/3 MLCT excited state. Due to steric constraints in 2, distortion to a pseudo-Td geometry is prohibited (Figures 4.9 and S32), which strongly destabilizes the 3LF state relative to the ground state, likely making it inaccessible from the MLCT excited-state manifold. From quantum chemical calculations, the optimized 3LF state of 1 is ∼19 kcal mol–1 lower in ΔG than the analogous 3LF state in 2. As a consequence, while 1 relaxes from the MLCT excited-state manifold to a longer-lived 3LF state with a time constant of ∼9 ps, 2 exhibits a prolonged 81 MLCT excited state that relaxes back to the singlet ground state with a 1/3 time constant of ∼14 ps. A similar structural control over excited-state dynamics has been quantified for Cu(I) bis-phenanthroline complexes, in which a combined experimental and computational approach determined that steric effects can provide up to ∼20 kcal mol–1 in tuning excited-state potential energy surfaces, which further tunes the MLCT lifetime of those complexes over 4 orders of magnitude.50 Figure 4.9. An overlay of DFT-optimized geometries of 1 and 2 in their ground-state singlet and triplet states, highlighting the prohibited distortion of 2 due to steric constraints. ΔG was obtained from CASSCF/QD-NEVPT2 electronic energies (see section 2.1 of the Supporting Information); RMSD = root-mean-square deviation of atomic positions between superimposed structures of optimized singlet and triplet states. Notably, despite the lack of 3LF state formation due to strong steric constraints, 2 still exhibits reactivity with electrophiles after photoirradiation. This reactivity suggests that access to the 3LF state may not be necessary for the generation of a reactive Ni(I) intermediate. We further note that the excited-state dynamics of 1 and 2 probed by TA are not affected by the addition of mesityl bromide, which indicates that the chemical reactivity with mesityl bromide occurs via a separate process, and reactive species are not observed as part of the excited-state dynamics probed by this experiment. This observation is reminiscent of the nature of the excited-state bond homolysis in compound 1 and related untethered complexes. Due to small quantum yields for homolysis, transient spectroscopies probe effectively unproductive, background excited-state relaxation processes that do not 82 result in Ni(I) intermediates and organic radicals. Similarly for 2, the TA reports on MLCT relaxation but not excited-state bond homolysis. The primary distinction between the unique excited-state dynamics of 1 and 2 lies in their geometric properties and excited-state distortions/constraints, which render 2 incapable of stabilizing 3LF excited states. We can also draw comparisons between the photophysics of 2 and the photophysics of other tethered Ni(II) species. A recent report by Wenger and co-workers investigated the excited-state behavior of a tethered Ni(II)–bpy aryl chloride complex with an N^C^N binding motif rather than C^N^N in 2.51 Analogous to 2, upon 400 nm excitation, the N^C^N tethered complex accesses a 1MLCT excited state, which was proposed to undergo decay through another undefined-spin MLCT state to a 3LF state. The time constant associated with the first decay varies with solvent, ranging from ∼700 to 900 fs. This fast time constant accounts for the relaxation of both singlet and triplet MLCT manifolds. Furthermore, a solvent-dependent time constant associated with LF relaxation to the singlet ground state ranged from ∼9.2 to 14 ps. While the proposed 3 excited-state decay pathway for the N^C^N tethered complex is similar to that proposed for untethered analogues,19 the time constants appear comparable to those observed for 2 in this work. However, the authors hypothesized that the N^C^N isomer does not require distortion from square planar to pseudo-Td to stabilize a 3LF state. Here, we have observed a similar degree of geometric distortion upon optimization of the 3LF state in the N^C^N isomer and 2 (Figure S33). However, the smaller RMSD between the superimposed singlet and triplet structures of the former suggests that the N^C^N ligand remains more planar in the 3LF state, with the main distortion featuring a larger out-of-plane bending of the Ni–Cl bond. Therefore, it remains unclear whether the variance in accessing a 3LF state between the tethered isomers can be attributed to disparities in C^N^N vs N^C^N coordination or potential solvent effects on the excited-state dynamics. In 2, the MLCT-based ESA feature decays monoexponentially with the same time constant as the recovery of the GSB, which allows us to associate the longer-lived time component as MLCT in origin, not 3LF. In both cases, the tethered isomers demonstrate significantly faster relaxation compared to untethered Ni(II)–bpy aryl halide analogues. Ultrafast X-ray spectroscopies may prove 83 useful for obtaining more precise mechanistic information regarding excited-state relaxation dynamics in tethered vs untethered complexes. 4.4. Conclusions We have compared covalently tethered, structurally constrained Ni(II)–bpy aryl halide complex 2 and its untethered analogue 1, exploring the significant distinctions in their photochemical reactivity and excited-state dynamics. Key findings are as follows: (a) Unlike 1, the tethered complex 2 exhibits prolonged stability under photoexcitation across a wide range of LED wavelengths, which indicates that excited-state bond homolysis products from 2 do not effectively accumulate over time. (b) The introduction of an electrophile during photoirradiation of 2 enables productive capture of a species capable of C(sp2)–Br activation, which we propose to be a Ni(I) intermediate based on a comparison to previous literature. This reactivity offers a promising avenue for enhancing nickel photoredox catalysis and refining reaction selectivity. (c) From concentration-dependent photochemical experiments in the presence of an electrophile, both 1 and 2 exhibit behavior consistent with reversible Ni(II)–C(aryl) ⇄ [Ni(I)···C(aryl)•] excited-state bond homolysis. (d) Distinct excited-state relaxation mechanisms occur in 1 vs 2, which can be attributed to steric constraints introduced through the covalent linkage between the phenyl and bpy ligands, prohibiting the formation of a pseudo-Td geometry and access to 3LF excited states. Future investigations should focus on practical applications of the tethered complex in photoredox cross-coupling catalysis to assess its photochemical behavior under catalytic conditions and whether influences on Ni(I)–organic radical cage escape processes play an important role. Relatedly, exploring controlled Ni(I) generation holds the potential for enhancing reaction selectivity in organic synthesis. Further research into the impact of ligand substituents on the tethered bpy backbone, especially electron-withdrawing groups, may enable the use of lower-energy light and improve the stability of highly reactive intermediates, offering opportunities to tune reaction conditions in the nickel ground-state and photoredox catalysis. 4.5. References 84 (1) Chan, A. Y.; Perry, I. B.; Bissonnette, N. B.; Buksh, B. F.; Edwards, G. A.; Frye, L. I.; Garry, O. L.; Lavagnino, M. N.; Li, B. X.; Liang, Y.; Mao, E.; Millet, A.; Oakley, J. V.; Reed, N. L.; Sakai, H. A.; Seath, C. P.; MacMillan, D. 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Vibrational Relaxation and Redistribution Dynamics in Ruthenium(II) Polypyridyl-Based Charge-Transfer Excited States: A Combined Ultrafast Electronic and Infrared Absorption Study. J. Phys. Chem. A 2018, 122 (40), 7941– 7953, DOI: 10.1021/acs.jpca.8b06197. (49) Shaw, G. B.; Grant, C. D.; Shirota, H.; Castner, E. W.; Meyer, G. J.; Chen, L. X. Ultrafast Structural Rearrangements in the MLCT Excited State for Copper(I) BisPhenanthrolines in Solution. J. Am. Chem. Soc. 2007, 129 (7), 2147– 2160, DOI: 10.1021/ja067271f. (50) Stroscio, G. D.; Ribson, R. D.; Hadt, R. G. Quantifying Entatic States in Photophysical Processes: Applications to Copper Photosensitizers. Inorg. Chem. 2019, 58 (24), 16800– 16817, DOI: 10.1021/acs.inorgchem.9b02976. 90 (51) Ogawa, T.; Wenger, O. S. Nickel(II) Analogues of Phosphorescent Platinum(II) Complexes with Picosecond Excited State Decay. Angew. Chemie Int. Ed. 2023, 62 (46), e202312851, DOI: 10.1002/anie.202312851. 91 PART II: SPIN RELAXATION 92 CHAPTER 5: DETERMINING THE KEY VIBRATIONS FOR SPIN RELAXATION IN RUFFLED CU(II) PORPHYRINS VIA RESONANCE RAMAN SPECTROSCOPY Distorted S = ½ Copper Porphyrins EPR + resonance Raman → correct T1 trends 1/T 1/T 1 1 local mode = high energy, totally-symmetric Ee ΔE Eg Temperature Temperature 200 250 300 350 400 450 500 550 -1) Raman Shift Shift (cm (cm-1 Raman ) Adapted with permission from: Kazmierczak, N. P.; Lopez, N. E.; Luedecke, K. M.; Hadt, R. G. “Determining the Key Vibrations for Spin Relaxation in Ruffled Cu(II) Porphyrins via Resonance Raman Spectroscopy.” Chem. Sci. 2024, 15 (7), 2380. DOI: 10.1039/D3SC05774G. Copyright 2024 The Royal Society of Chemistry. 93 5.1. Introduction The rise of molecular quantum information science has placed new importance on developing molecules with long-lasting electron spin coherence times (T2), a parameter which sets the maximum length of time quantum information can be stored and processed.1 At elevated temperatures, vibration-mediated spin relaxation (T1) limits the maximum attainable value of T2,2 implying that vibrations effectively leak quantum information into the environment. While room-temperature electron spin coherence has been measured in a handful of S = ½ systems,3–8 other key classes of molecular qubits, such as the Cr(IV) optically addressable qubits, remain limited to sub-liquid nitrogen temperatures owing to T1-limited T2.9–12 Thus, understanding the relationship between molecular geometric/electronic structure and spin relaxation rates remains a key mechanistic goal of molecular quantum information science. The number of thermallyaccessible vibrational modes that could potentially contribute to spin relaxation renders such investigations theoretically and spectroscopically challenging. One approach to build T1 mechanistic understanding is to employ theoretical models of the spin-relaxation process. Several models based on the easily handled spin Hamiltonian have been proposed, but crucially, multiple expressions have been used for the spin–phonon coupling coefficient (Fig. 5.1A). One can use derivatives of either the gtensor2,13–17 or the hyperfine tensor,15,16 pick different elements of the tensor (on-diagonal only2,5,13,14 vs. including off-diagonal15–17), and employ first13–15 vs. second16,18 derivatives. Depending on the theoretical choices made, different vibrational modes are predicted to dominate spin relaxation, ranging from ultra-low-energy eg rotational modes (<50 cm−1)19 to totally symmetric metal–ligand stretching modes (>200 cm−1).2,13,14 None of these spin Hamiltonian models can be considered mechanistically definitive due to two key issues: (1) derivatives of a spin Hamiltonian tensor do not constitute true matrix elements between spin states,20 no matter how sophisticated the quantum master equation21 used to handle the rate calculations, and (2) no spin Hamiltonian model successfully accounts for T1 anisotropy, which instead requires analysis of spin–orbit wavefunctions.20 At best, spin Hamiltonian models can predict the average temperaturedependent T1 over all molecular orientations, functioning as a proxy for the true spin–orbit 94 mechanism. Further discrepancies arise when comparing the theoretically predicted 20 temperature scaling of T1 to experiment. On the basis of ultra-low-energy modes, contemporary ab initio models commonly predict flat, single-exponent power law behavior19 for temperature-dependent T1. However, experimental log–log plots of 1/T1 vs. temperature display marked curvature for most S = ½ molecular qubits, which has been interpreted by local mode fitting to indicate the contribution of molecular vibrational modes >100 cm−1 that give rise to a nonconstant power law exponent.14,22,23 Experimentally parameterized T1 models have successfully reproduced this curvature for high-symmetry Cu(II) and V(IV) complexes,14 but the curvature is not successfully predicted by ab initio models that emphasize ultra-low-energy modes.19 We conclude that ab initio prediction of T1 in S = ½ molecular qubits has not yet achieved satisfactory agreement with experimental data. Figure 5.1. Mechanistic studies of spin–lattice relaxation in S = ½ qubits. (A) Previous theoretical studies have employed a variety of models for molecular spin–phonon 95 coupling, leading to disparate predictions of modes with different energies and symmetries driving spin relaxation. Figures adapted with permission from refs. 17–20. (B) This work systematically probes the effect of the b1u ruffling distortion on spin relaxation in a series of copper porphyrins, finding that existing theory systematically overestimates the contribution of low-energy ruffling modes. Alternatively, mechanistic understanding may be developed from experimentallydetermined T1 structure–property relationships. While relatively few such relationships have been explored in detail, one of the best-established connections has been demonstrated between the degree of first coordination sphere planarity and T1 in fourcoordinate Cu(II) S = ½ molecular qubits.2 Two limiting geometries are possible: square planar (D4h), in which the opposing L–M–L bond angle is 180°, and tetrahedral (Td), in which the opposing L–M–L bond angle is 109.5°. From the D4h geometry, ligated atoms can undergo distortion along the b2u bending mode, which lowers the point group to D2d and systematically decreases the L–M–L angle from 180° toward 109.5°. It has been experimentally demonstrated that planar structures have longer spin lifetimes (long T1) than b2u-distorted structures.5,23 This result can be understood via application of group theory selection rules to the spin–phonon coupling problem.13,14 In order for a vibrational mode to cause spin relaxation by altering the g value, it must transform as either a totally symmetric vibrational mode (a1 or a1g) or an excited-state Jahn-Teller mode. In the D4h geometry, the b2u bending mode does not satisfy these symmetry requirements. However, once the equilibrium structure has been distorted along the bending motion in the D2d point group, the bending mode now transforms as a1 and can induce spin relaxation, as confirmed by calculation of dg/dQ spin–phonon coupling coefficients.13 This specific relationship illustrates a general principle: when a high-symmetry equilibrium structure is distorted along a non-totally-symmetric vibrational mode, that mode transforms as the totally symmetric representation in the new point group (Figure 5.1B, ESI Section 4C) and can cause spin relaxation.14 Metalloporphyrins offer an attractive platform for extending T1 structure–property relationships to new types of molecular geometries. Depending on the steric hindrance and 96 substitutional pattern of peripheral moieties, porphyrins may adopt equilibrium geometries with saddled (b2u), ruffled (b1u), domed (a2u), or waved (eg) distortions, where the labels indicate the symmetry of the distorting vibrational mode in the D4h point group.24 The saddling distortion is equivalent to the same first coordination sphere bending distortion to D2d described above (Figures 5.2B and C). By contrast, the b1u ruffling distortion (Figures 5.2B and C) also results in a D2d equilibrium geometry, but the first coordination sphere remains completely unaltered (Figure 5.2A). Instead, the porphyrin meso carbons are distorted above and below the plane of the first coordination sphere, rendering ruffling a secondary sphere structural distortion. By analogy to the b2u bending mode argument, the b1u ruffling mode will transform as a1 in the D2d distorted point group, opening up the possibility of contributions to spin relaxation (ESI Section 4C). Ruffling modes in porphyrins exist in the ultra-low-energy range (∼50 cm−1), indicating that ruffled S = ½ porphyrins may show a decisive, unique contribution of low-energy modes to T1. Figure 5.2. Geometries of ruffled metalloporphyrins. (A) Crystal structures demonstrate increasing static ruffling in the series CuOEP < CuTPP < CuTiPP. (B) Representative ruffling and saddling vibrational modes for CuOEP transform as non-totally-symmetric irreducible representations (irreps) due to the planar D4h point group (ethyl groups neglected). Only the saddling mode alters the first coordination sphere geometry. (C) The 97 static distortion of CuTiPP causes the ruffling vibration to transform as the totallysymmetric irrep in the new D2d point group, while the saddling vibration remains nontotally-symmetric. H atoms are omitted for clarity. In this study, we measure temperature-dependent T1 via pulse electron paramagnetic resonance (EPR) on a series of three Cu(II) metalloporphyrins (CuP) (Figures 5.1B and 5.2A): copper octaethylporphyrin (CuOEP), copper tetraphenylporphyrin (CuTPP), and copper tetraisopropylporphyrin (CuTiPP). CuOEP possesses a planar crystal structure, while CuTPP and CuTiPP exhibit increasing degrees of the ruffling distortion (Figure 5.2A). Computational modeling via density functional theory (DFT) suggests that T1 times should decrease with increasing ruffling distortion. However, experimental T1 measurements do not trend with ruffling: the planar CuOEP and highly ruffled CuTiPP display robust room-temperature coherence while the moderately ruffled CuTPP does not. The breakdown of the computational analysis can be attributed to an overemphasis of the lowest-energy ruffling mode. We then employ resonance Raman (rR) spectroscopy to detect the vibrational modes driving spin relaxation across this series and find a set of symmetric stretching vibrations in the 200–300 cm−1 region that trend with the experimentally observed T1. Our results demonstrate that ultra-low-energy modes do not drive spin relaxation in copper porphyrins, thereby distinguishing between conflicting theoretical models of spin relaxation (Figure 5.1A). 5.2. Results To quantify the amount of distortion in a given metalloporphyrin crystal structure, we applied the normal coordinate structural decomposition (NSD) developed by Shelnutt25 and implemented in the program by Kingsbury and Senge.24 CuOEP was chosen as the undistorted control compound, as it displays no tendency towards ruffling and only slight amounts of waving (Table 5.1). The distorted ruffled structures were chosen according to two criteria: (1) ruffling should be by far the largest distortion of the porphyrin, and (2) there must exist a diamagnetic host matrix of comparable distortions for preparation of EPR solid-state dilution samples. CuTPP and CuTiPP satisfy criterion (1), with CuTPP 98 having only a small secondary saddling distortion and CuTiPP having a very small secondary waving distortion. Regarding criterion (2), the corresponding Ni(II) porphyrins both display dominant ruffling distortions. The NiTPP matrix is closely matched to the CuTPP structure in both the primary magnitude of ruffling and the secondary saddling, while the NiTiPP matrix displays somewhat increased ruffling over the Cu structure (2.03 vs. 1.35) and an additional saddling distortion (0.46 vs. 0.00). These metrics indicate that the ruffling distortion increases in the series CuOEP < CuTPP < CuTiPP. Note that metalloporphyrins can crystallize in multiple phases: while the ruffled structure for CuTPP is the more common polymorph in the Cambridge Structural Database,26–28 there also exist two planar polymorphs.29,30 Additionally, we obtained a new ruffled solvate crystal phase for CuTiPP through single crystal X-ray diffraction (ESI Section 2C). We confirmed that all EPR sample preparations in this work adhered to the ruffled (or, for CuOEP and ZnOEP, planar) geometries via powder X-ray diffraction (PXRD) and Rietveld refinement for comparison to the single crystal structures (ESI Section 2B, 4B). Good EPR sample agreement with the polymorphs used for the NSD analysis in Table 5.1 was found in all cases. Table 5.1. Normal coordinate structural decomposition analysis of porphyrin distortions Distortion Ruffling Saddling Doming Waving Waving mode (b1u) (b2u) (a2u) (eg(x)) (eg(y)) CuOEP 0.00 0.00 0.00 0.11 0.05 CuTPP 1.18 0.22 0.00 0.00 0.00 CuTiPP 1.35 0.00 0.00 0.00 0.11 ZnOEP 0.00 0.00 0.00 0.16 0.06 NiTPP 1.27 0.27 0.00 0.00 0.00 NiTiPP 2.03 0.46 0.03 0.09 0.10 Temperature-dependent T1 measurements were acquired for all three CuP species (Figure 5.3), prepared as 1% solid-state powder dilutions in the corresponding isostructural diamagnetic host (Table 5.1). Inversion recovery traces were acquired at field 99 positions corresponding to both perpendicular (Figure 5.3B) and parallel (Figure 5.3C) orientations.20 CuTPP exhibits the fastest spin relaxation of all CuP species at both field positions, with a particularly distinct relaxation trend at the perpendicular position. The comparatively fast spin relaxation is consistent with previous studies on Ti(III)/CuTPP bimetallic and monometallic congeners in solution,31 as well as CuTPP embedded in a metal–organic framework (MOF),32,33 which did not observe room-temperature coherence for CuTPP. CuOEP exhibits the slowest relaxation (i.e., longest T1 time) at the perpendicular position, while CuTiPP exhibits the slowest relaxation at the parallel position; the orientation-averaged 1/T1 values are very similar for CuOEP and CuTiPP (Figure S36). Observer position differences in spin relaxation point to the presence of T1 anisotropy for the three CuP species. T1 anisotropy arises from the presence of anisotropic minority spin contributions in the ground-state spin–orbit wavefunction and how they are modulated by totally symmetric vibrations.20 Note that the T1 values for all three porphyrins are quite similar at 20 K but diverge as temperature increases, indicating that higher-energy molecular vibrational modes are responsible for the differences between the compounds. Note also that CuOEP and CuTiPP display room-temperature (297 K) coherence, with T1 and Tm (the phase memory time2) of 185 ns and 87 ns for CuOEP and 145 ns and 50 ns for CuTiPP at perpendicular orientations. To the best of our knowledge, room-temperature electron spin coherence has only been demonstrated in one previous CuN4 molecular qubit, Cu(tmtaa),5 and is not observed for copper phthalocyanine (CuPc).22 CuTPP displays an extremely weak spin echo at room temperature, with T1 and Tm of 60 and 49 ns at the perpendicular position. No echo was detectable at the CuTPP parallel position, and a room-temperature echo-detected field sweep could not be acquired. As such, we do not consider this signal robust enough for designation of CuTPP as a roomtemperature coherent molecular qubit. 100 Figure 5.3. Temperature-dependent T1 by pulse EPR X-band inversion recovery in 1% solid state powder dilutions of CuOEP in ZnOEP, CuTPP in NiTPP, and CuTiPP in NiTiPP. (A) Echo-detected field sweeps, with field positions selective for parallel and perpendicular molecular orientations indicated for CuOEP. The analogous field positions are chosen for CuTPP and CuTiPP. (B) Perpendicular position spin relaxation rates. (C) Parallel position spin relaxation rates Since 1/T1 increases in the order CuOEP = CuTiPP ≪ CuTPP, while ruffling increases in the order CuOEP < CuTPP < CuTiPP, the results of Figure 5.3 do not support the hypothesis that increased ruffling distortion activates new channels for spin relaxation. To examine the theoretical underpinnings of this notion, we performed molecular spin– phonon coupling calculations according to a previously published procedure to predict 101 14 1/T1 traces (Equation 5.1). The normal-mode derivatives of the principal values of the g-tensor, dg/dQ, were averaged to obtain the spin–phonon coupling coefficients, thereby modeling the average 1/T1 relaxation across all orientations. Thermal weighting was applied via a two-phonon Green's function to model the temperature dependence of the Raman spin relaxation process.16 An experimentally calibrated proportionality constant of A = 1.01 × 105 μs−1 was used to convert the 1/T1 simulations into absolute rates.14 This model has been shown to correctly predict the T1 log–log slope of the molecular qubits CuPc and VOPc, as well as correctly ordering the relative T1 for a series of four Cu(II) and V(IV) sulfur and selenium-ligated qubits.14 ! ' ! $% .*/[1 /3 "] " # $ = % ∑#869 :)! ∑()*,,,- # '$& ( (.*/[1 /3 "]6!)% " ! # # $ (5.1) Theoretical calculations predict that 1/T1 should increase in the order CuOEP < CuTPP < CuTiPP (Figure 5.4A; see Figure S51 for an overlay of theory and experiment). This trend agrees with the order of increasing ruffling, but it does not agree with the experimental T1 ordering. To understand the origin of the trend in the calculations, the individual vibrational mode contributions to 1/T1 are plotted for CuOEP (Figure 5.4B) and CuTiPP (Figure 5.4C); ruffled CuTPP follows similar behavior to ruffled CuTiPP (Figure S52). The contributions from each normal mode are additive towards the total rate of spin relaxation. All mode contributions have the same functional shape in accordance with the thermal weighting function, with two degrees of freedom: (1) a larger energy of the vibrational mode translates the 1/T1 contribution to the right, since the mode will not be thermally populated until higher temperatures, and (2) a larger dgi/dQ value shifts the 1/T1 contribution up on the plot, indicating that it mediates faster spin relaxation. For CuOEP, the calculated relaxation rate from ∼30–70 K is set by low-energy vibrational modes, mostly of an eg rotational character with small amounts of symmetric stretch mixed in due to the waving distortion in CuOEP (Table 5.1). However, at 100 K and above, totally symmetric stretch modes take over the dominant contribution to spin relaxation (green lines, Figures 5.4B and S38–S40, S54A). By contrast, spin relaxation for CuTiPP −1 is predicted to be dominated by a single low-energy ruffling mode at 26 cm 102 (Fig. S46) over the entire temperature range studied (yellow lines, Figure 5.4C), which almost exactly matches the total 1/T1 calculated trace. The contribution from other modes, including the totally symmetric stretch, is predicted to be <15% (Figure S54C). Therefore, molecular spin–phonon coupling calculations predict that the low-energy ruffling modes should dominate the 1/T1 for the ruffled porphyrins, and the absence of these distortionactivated modes in CuOEP should lead to a longer calculated spin lifetime. The roomtemperature coherence of CuTiPP contradicts this prediction. Figure 5.4. Calculated spin relaxation rates 1/T1 for the Cu porphyrin series. (A) Total calibrated relaxation rates. (B) Breakdown of individual vibrational mode contributions for planar CuOEP. Symmetric stretching modes dominate T1 for T ≥ 100 K, while ruffling mode contributions are negligible. (C) Breakdown of individual vibrational mode 103 contributions for ruffled CuTiPP. A single low-energy (26 cm ) ruffling mode −1 dominates the calculated 1/T1 over the entire temperature range. Individual mode contributions are additive on a linear y-axis scale. To determine the source of this discrepancy, we analyzed the temperature-dependent log–log slope of 1/T1. For a single power law process, such as those predicted by the Debye model for acoustic phonon spin relaxation,34 the log–log slope should be constant. This behavior is often observed in experimental T1 for inorganic lattices where the acoustic phonon branches drive T1 over the entire Raman process temperature range.34,35 However, variation in the log–log slope with temperature is common for molecular complexes. This behavior can originate from either (1) a crossover between two different spin relaxation processes (such as acoustic phonon relaxation vs. molecular vibration/local mode relaxation), or (2) the thermal population of higher-energy molecular vibrations, for which the innate spin relaxation contribution does not follow a power law form (Equation 5.1).14 In the first case, a sharp increase occurs in the log–log slope with increasing temperature, as a process that scales weakly with temperature gives way to a process that scales more strongly with temperature. In the second case, a smooth decrease occurs in the log–log slope with increasing temperature due to the thermal weighting functional form. The curvature of the log–log 1/T1 can then be used to pinpoint the energy of the contributing molecular vibration. Higher-energy molecular vibrations display a larger and temperaturedependent log–log slope even at elevated temperatures. In contrast, the log–log slope of a low-energy molecular vibration flattens out towards a constant value of 2 in the high temperature limit (kbT ≫ Evib), owing to the asymptotic behavior of the two-phonon Green's function. Both CuOEP and CuTiPP display a temperature-dependent log–log slope (Figures 5.5A and B), which starts at a minimum below 10 K for the one-phonon direct process, peaks sharply at a value of ∼4 at ∼60 K, and then diminishes gradually towards ∼2.5 at room temperature. The sharp increase from 10–60 K experimentally indicates a change in the relaxation process (case 1), wherein two-phonon scattering from a higher-energy molecular vibration (>60 K) takes over from acoustic/pseudoacoustic phonons (<30 K). 104 The gradual decrease from 60 K towards room temperature indicates that one or more molecular vibrational modes controls spin relaxation over this region (case 2). By comparing these slopes to those of the theoretical calculations (Figure 5.4), the origin of the discrepancies with experiment can be ascertained. The calculated T1 log–log slope for CuOEP matches the experimental behavior, which can be attributed in the calculation to the population of totally symmetric stretch modes above 60 K (Figures 5.4B and 5.5A). Note that the calculations do not include acoustic phonons, so the divergence of the calculated log–log slope below 20 K is expected. However, the calculated T1 for CuTiPP displays a qualitatively incorrect log–log slope of 2 throughout the entire temperature range, while the experimental log–log slope peaks at a value of ∼4.5 at 60 K. The calculated log–log slope of 2 arises from the 26 cm−1 ruffling mode (Figure 5.5B), which is predicted to dominate spin relaxation across all temperatures. A similar disagreement arises for CuTPP due to the same ruffling mode phenomenon (Figures S53 and S55). We conclude that the calculated spin relaxation for CuOEP is in agreement with experiment, but the calculated spin relaxation for CuTiPP and CuTPP is incorrect due to overemphasis on the low-energy ruffling modes. Evidently, the ultra-low-energy modes do not dominate spin relaxation in experiment. 105 Figure 5.5. Calculated vs. experimental 1/T1 log–log slopes for (A) CuOEP and (B) CuTiPP To experimentally ascertain the energy range of the vibrational modes that drive spin relaxation, local mode fits22 to the 1/T1 data (Figures 5.6A–C) were carried out according to Equation 5.2. The least-squares fit was performed on the log–log data for equivalent residuals weighting across the entire temperature range. ! .*/[1 ! &'( /3 "] $ = *+ ; + - (.*/[1 &'( " /3 "]6!)% $ (5.2) Figure 5.6. Determination of the dominant vibrational mode energy for spin relaxation. Local mode fitting at the perpendicular observer position according to eqn (5.1) for (A) CuOEP, (B) CuTiPP, and (C) CuTPP. (D) Resonance Raman spectroscopy obtained via 457.9 nm excitation (Soret preresonance enhancement). CuOEP collected in CS2 at room temperature, while CuTiPP and CuTPP collected in C6H6 at room temperature; all samples are at a concentration of 2 mM. All peak positions are accurate to within 5 cm−1. 106 The power law term accounts for a combination of the one-phonon direct process plus two-phonon Raman relaxation operating on low-energy phonons; combining these reduces the number of free parameters to avoid overfitting. Lower temperature measurements (<10 K) would likely be needed to isolate the direct process contribution. It is well established that the Raman process in molecular solids need not follow the Debye model T9 scaling,36 so a variable exponent n is included. The second term has the same functional form as the two-phonon Green's function used for thermal weighting of molecular vibrational modes. In effect, this fitting postulates a single molecular vibration of energy Eloc capable of explaining the high-temperature T1 values, while allowing for an unresolved collection of weakly-coupled low-energy modes to determine the lowtemperature T1 values. All fits yield a local mode energy between 200–270 cm−1, which fall in the energetic range of bond-stretching vibrational modes. This analysis indicates that a similar type of vibrational mode likely determines T1 in all three porphyrins. The fitted energy for CuTPP is lower than the fitted energies for CuOEP or CuTiPP, matching the room-temperature coherence observation for CuOEP and CuTiPP but not CuTPP. Local mode fitting on T1 values collected at the parallel field position reinforces that the local mode energy for CuTPP is distinct from, and lower than, the mode energies for CuOEP and CuTiPP (Table S7). The local mode energies Eloc correlate better with the observed T1 values than the coupling prefactor b, suggesting that vibrational mode energy shifts are principally responsible for the different T1 values across the compound series (Table S8). rR spectroscopy was employed to identify vibrational modes in this energy range that could participate in spin–phonon coupling. Owing to selection rules of Raman scattering, only gerade modes are visible in centrosymmetric complexes. Furthermore, the A-term mechanism of Soret-band rR intensity enhancement selectively excites totally symmetric a1/a1g modes,37,38 which are precisely those modes predicted by group theory to have the correct symmetry for driving spin relaxation.14 rR spectra for all three compounds display four main bands in the 200–400 cm−1 region, including two bands between 200–300 cm−1 (Figure 5.6D). Crucially, these latter bands appear lower in energy for CuTPP than for CuOEP or CuTiPP, in agreement with the relative rates of spin relaxation. All bands 107 display a depolarization ratio less than 0.75 (Figures S22–S24), indicating at least some component of totally symmetric motion is present. The band positions in the solution-phase data are consistent within 8 cm−1 of rR spectra acquired in the solid state and display similar relative resonance enhancement (Table S2 and Figure S25), indicating these reflect the vibrational structure present in the EPR samples as well. DFT frequency calculations of vibrations with Raman intensity display very good agreement with the experimental spectra and enable the assignment of the 300–400 cm−1 bands to totally symmetric metal–ligand stretching modes (ESI Section 3C). The 200–300 cm−1 bands contain one instance of a mixed ligand symmetric stretch (LSS) and one instance of a mixed metal–ligand gerade mode with a principal contribution from non-totally symmetric motion. At least some symmetric motion must be admixed to account for the observation that all bands are polarized. Note the ordering of these latter two modes changes between the three CuP species. Owing to the spin–phonon coupling group theory selection rule for totally symmetric vibrations, the mixed LSS mode energies were extracted from the rR spectra and compared with the local mode fits. Good agreement is found (Table 5.2), showing a >40 cm−1 distinction between the low-energy CuTPP LSS mode (203 cm−1) and those of CuOEP (271 cm−1) and CuTiPP (244 cm−1). This trend indicates that the energetic positioning of the LSS mode can explain the temperature-dependent T1 results for the three CuP systems. In addition, the presence of a minor saddling distortion for CuTPP but not CuOEP or CuTiPP (Table 5.1) may lead to enhanced spin–phonon coupling, as saddling motions are known to be activated for spin relaxation via a first coordination sphere distortion. Porphyrin saddling contributions to T1 will be analyzed in more detail in a future study. We conclude that high-energy totally symmetric stretch modes, and not low-energy ruffling modes, control the relative T1 ordering for CuOEP, CuTiPP, and CuTPP above 60 K. Table 5.2. Positions of local mode energies and Raman mixed ligand symmetric stretch peaks Perpendicular orientation local 108 Raman mixed LSS mode mode (cm-1) (cm-1) CuOEP 258 271 CuTiPP 251 244 CuTPP 220 203 Compound 5.3. Discussion The failure of computational spin relaxation models to predict the correct high-energy stretching vibrational modes is not a unique feature of ruffled porphyrins. In several cases, molecular spin–phonon coupling models seem to have a bias toward over-emphasizing low-energy modes, leading to predictions of ultra-low-energy vibrations dominating T1. However, such assignments often fail to account for the temperature-dependent T1 curvature and log–log slope changes, such as in the case of ab initio modeling of vanadyl tetraphenylporphyrin (VOTPP).19 Thus, such theoretical claims of low-energy phonon dominance should be treated with significant caution. The origins of this computational low-energy bias are unclear, but likely relate to some of the approximations used to build the models. One possibility is that gas-phase DFT overestimates the amplitudes of low-energy vibrational modes, which should be more constrained in the solid-state. However, condensed-phase phonon calculations retain the same bias toward low-energy modes.19 Additionally, the use of full-g-tensor dgij/dQ values for spin Hamiltonian matrix elements may overestimate the coupling of certain types of low-energy modes, such as eg rotations of the first coordination sphere, that do not dynamically change the total amount of minority spin mixing through the magnitude of spin–orbit coupling. Incorporation of spin–orbit coupling spin-flip matrix elements into ab initio T1 modeling with quantum master equations may remedy this difficulty. Experimentally, this work shows that rR spectroscopy is a powerful tool for building spin dynamics structure–property relationships by leveraging the selectivity for totally symmetric mode energies. Previous works have attempted to empirically correlate vibrations to features of spin relaxation through terahertz spectroscopy39–42 or computation of the vibrational density of states.5,32 However, terahertz or IR absorption spectroscopies 109 rely on electric dipole selection rules that select for ungerade modes in centrosymmetric complexes. Group theory analysis has established a selection rule for spin–phonon coupling, which indicates that only gerade vibrations (predominately a1g modes) are able to couple to the spins in centrosymmetric complexes.14,17 Thus, IR or farIR absorption spectroscopies do not probe the modes of relevance for spin–phonon coupling for square-planar complexes. Furthermore, the full vibrational density of states, or even atom-specific partial density of states, becomes very complicated in macrocyclic qubits like porphyrins, rendering statements about specific vibrational modes challenging. A-term enhanced rR spectra, however, have the key virtue of selectivity for the a1/a1g modes most relevant to spin relaxation, making spectral correlations much more straightforward. In addition, the magnitude of A-term resonance enhancement provided by excitation into a strongly dipole-allowed electronic absorption band is determined by the amount of excited-state distortion through the electronuclear coupling integral.37 This same term arises in the ligand field theory of spin–phonon coupling, where excited-state distortion is required to observe nonzero dgi/dQ.14 Thus, there may be a connection between the magnitude of resonance enhancement and the magnitude of spin–phonon coupling itself. The caveat is that the electronic excited state of relevance is different for g-tensor contributions (d–d transition) vs. rR spectroscopy (charge transfer). In general, one cannot reliably acquire rR spectra for d–d bands, and many molecular qubits feature intense electronic transitions that obscure these transitions in electronic absorption spectra. The excited-state distortions need not be the same between the two types of electronic excited states. Further work will investigate whether reliable information on spin relaxation can be extracted from the magnitude of A-term enhancement in rR spectra of molecular qubits. 5.4. Conclusions This work probes the effect of the ruffling distortion on spin relaxation in a series of three copper porphyrins. Two of the three members of the series (CuOEP and CuTiPP) display room-temperature coherence, indicating the suitability of copper porphyrins as a new class of molecules for room-temperature molecular quantum information science applications. 110 Unlike in the well-studied case of b2u bending/saddling vibrations, increasing b1u ruffling distortion does not correlate to decreased T1 times, despite the mode transforming as a1 in D2d. This unexpected result may indicate that primary coordination sphere distortions are required to materially activate new vibrational modes for spin relaxation, and secondary sphere effects such as ruffling have too weak an influence on angular momentum and spin–orbit coupling to induce spin flips. Computational spin relaxation models fail to account for the insensitivity of T1 to the ruffling distortion, indicating a direction for future theoretical efforts. rR spectroscopy successfully identifies a specific vibrational mode with totally symmetric character that correctly trends with the experimental T1 local mode fits. Thus, this study indicates the primary vibrational modes responsible for S = ½ CuP spin relaxation above ∼60 K correspond to bond stretches with totally symmetric character, not ultra-low-energy modes of rotational character. 5.5. References (1) M. Atzori and R. Sessoli, The Second Quantum Revolution: Role and Challenges of Molecular Chemistry, J. Am. Chem. Soc., 2019, 141(29), 11339–11352, DOI: 10.1021/jacs.9b00984. (2) R. Mirzoyan, N. P. Kazmierczak and R. G. Hadt, Deconvolving Contributions to Decoherence in Molecular Electron Spin Qubits: A Dynamic Ligand Field Approach, Chem.–Eur. J., 2021, 27, 9482–9494, DOI: 10.1002/chem.202100845. (3) K. Bader, D. 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Chem., 2021, 133(5), 2620–2625, DOI: 10.1002/ ange.202009634. 116 CHAPTER 6: T1 ANISOTROPY ELUCIDATES SPIN RELAXATION MECHANISMS IN AN S = 1 CR(IV) OPTICALLY ADDRESSABLE MOLECULAR QUBIT Adapted with permission from: Kazmierczak, N. P.†; Luedecke, K. M.†; Gallmeier, E. T.; Hadt, R. G. “T1 Anisotropy Elucidates Spin Relaxation Mechanisms in an S = 1 Cr(IV) Optically Addressable Molecular Qubit.” J. Phys. Chem. 10.1021/acs.jpclett.3c01964. † Co-first author. Copyright 2023 American Chemical Society. Lett. 2023, 14 (34), 7658. DOI: 117 6.1. Introduction The anionic nitrogen vacancy (NV–) center in diamond constitutes one of the most widely implemented platforms for quantum sensing and imaging.1 Substitutional replacement of two carbons in the diamond lattice with a nitrogen and a vacancy forms an S = 1 paramagnetic defect that exhibits selective photoluminescence behavior depending on the Zeeman sublevel that the spin occupies.2,3 Furthermore, optical excitation of the NV– center accumulates ground-state spin polarization through a spin-selective intersystem crossing mechanism. These two features enable optical initialization and detection of quantum states in the Zeeman sublevels of the S = 1 defect, permitting a much greater degree of spatial localization than can be achieved with direct resonant microwave readout of the Zeeman sublevels. This functionality has enabled quantum sensing in a variety of applications,1 including atomic scale magnetic resonance imaging of nuclear spin clusters,4 probing intracellular molecular dynamics,5 nanometer-scale thermometry with millikelvin accuracy inside a living cell,6 imaging magnetic fields in live magnetotactic bacteria,7 measuring local ion concentrations,8 and strain/pressure sensing.8 The main downsides of NV– centers relate to the large bulk of the diamond lattice, the poor control over where the NV– defects arise in the lattice, and the fixed nature of the NV– center coherence properties.9 Production of molecules exhibiting the same optical initialization and readout capabilities would overcome these limitations of NV– centers and open up molecular in situ and in vivo quantum sensing capabilities on the single nanometer scale. Multiple systems have been investigated on the basis of both S = 1 and S > 1 architectures,10-12 but to date, the most successful molecules have been pseudo-Td S = 1 Cr(IV) tetraaryl and tetraalkyl complexes,13,14 which display optical addressability and prolonged coherence times when diluted in nonisostructural diamagnetic matrices.15 However, owing to fast spin relaxation, Cr(IV) molecular qubits do not display spin coherence at temperatures higher than ∼60 K, at which point Tm becomes spin–lattice relaxation (T1) limited. This behavior is significantly inferior to that of both NV– centers and microwave addressable S = 1/2 molecular qubits such as VOPc and [Cu(mnt)2]2–, which display coherence up to room temperature.16-18 In particular, quantum sensing of biological systems would benefit greatly 118 from the ability to perform room-temperature coherence measurements under ambient biochemical conditions. Therefore, to maximize the full potential of optically addressable molecular qubits, it is essential to identify and remove contributions to fast spin relaxation. Recently, T1 anisotropy has emerged as a novel technique for interrogating spinphonon coupling contributions to spin relaxation and decoherence in S = 1/2 systems.19 This approach can provide information regarding the vibrational contributions and mechanism of spin relaxation that is not accessible through more common temperaturedependent T1 measurements. Here, we apply this methodology to Cr(o-tolyl)4, an S = 1 tetraaryl Cr(IV) optically addressable molecular qubit (Scheme 6.1A). We find qualitatively different T1 anisotropy patterns relative to Cu(acac)2 (Scheme 6.1B) and other copper(II) and vanadium(IV) S = 1/2 systems, indicating unique spin-phonon coupling contributions to spin relaxation. 119 Scheme 6.1. Molecular Crystal Structures and Point Groups of (A) S = 1 Cr(o-tolyl)4 and (B) S = 1/2 Cu(acac)2; (C) Simulated Zeeman Levels for Cr(o-tolyl)4 with D = −0.117 cm–1, as determined in this work.a aIn panels A and B, C = gray, Cr = blue, Cu = orange, and O = red. H atoms omitted for clarity. In panel B, slight distortion from planarity. In panel C, vertical lines denote X-band EPR transitions at 9.6 GHz 6.2. Results and Discussion The concept of T1 anisotropy probes how the spin relaxation rate changes for molecules with different orientations relative to the spectrometer’s applied magnetic field (B0). In general, EPR spectra of powder or frozen solution samples can display resonance positions selective for these different molecular orientations, enabling access to orientation-specific relaxation rates without the need for single crystal experiments.20 Cr(IV) qubits satisfy this requirement for orientation selectivity, as the microwave absorption spectrum of Cr(otolyl)4 is composed of two transitions between the three MS sublevels (Scheme 6.1C), and each spin transition occurs at a different resonance field depending on the molecular orientation (Figure 6.1A, B). By weighting these orientations with the fraction of molecules absorbing at the given field and averaging over the two spin transitions, we can define an average molecular orientation probed by EPR at any given resonance field (Figure 6.1C). At X-band, the pure parallel position can be addressed by performing pulsed EPR at 220 and 480 mT (lines atop Figure 6.1A). While no single position is uniquely selective for the perpendicular orientation, the Pake pattern horns at 280 and 410 mT display an average orientation around 80°, giving a close approximation to the pure perpendicular position behavior. 120 Figure 6.1. Anisotropy in Cr(IV) pulsed EPR. (A) Simulated X-band EDFS for Cr(otolyl)4 showing the contributions from each spin transition in the case of D < 0. (B) Orientation of the molecule with respect to the value of B0. θ = 0° indicates B0 is parallel to the principal symmetry axis of the axial ZFS tensor. (C) Average orientation of the molecule over all spin transitions. (D) Variable-temperature Q-band EDFSs using soft pulses (π = 80 ns) and normalized to the peak at 1160 mT. * likely indicates an artifact due to cross-relaxation or a double-quantum transition (Supporting Information, Section 3). (E) Comparison between experimental powder manifold intensities and simulations for D < 0 and D > 0 The identity of the MS = −1 → 0 and MS = 0 → + 1 transitions depends upon the sign of the axial zero-field splitting (ZFS) parameter D. While a perfectly Td S = 1 molecule cannot exhibit ZFS owing to cubic symmetry and a lack of second-order spin–orbit interactions,21 Cr(o-tolyl)4 crystallizes in the S4 point group.13 The sign of D is sensitive to changes in the electronic structure upon structural distortions from Td to S4, including 121 21,22 splitting of ligand field excited-state energies and anisotropic orbital covalency. Correct computational modeling of the sign of D is therefore essential for spin-phonon coupling calculations to accurately describe the electronic structure. Although the absolute ZFS parameters for Cr(o-tolyl)4 have been reported (|D| = 0.117 cm–1, E ≈ 0 cm–1)13, the sign of the ZFS has yet to be determined experimentally. To achieve this, we acquired variable-temperature Q-band echo-detected field sweeps (EDFSs) from 3.8 to 50 K (Figure 6.1D). Soft microwave pulses were employed to suppress electron spin echo envelope modulation (ESEEM), which can add artifacts to EDFS spectra, and Tm was measured at several field positions and temperatures to ensure anisotropic Tm did not bias the EDFS intensity (Supporting Information, Sections 3–4). As the sample temperature increases, the intensity of the spin transition spanning 1100–1290 mT decreases relative to the transition spanning 1160–1370 mT (arrows in Figure 6.1D). This behavior and corresponding simulation (Figure 6.1E) indicates that the former is the ground-state MS = −1 → 0 transition and that the sign of D is negative for Cr(o-tolyl)4 (D = −0.121 cm–1), consistent with calculations.23,24 Note this sign differs from the pseudo-Td Cr(IV) siloxide complex, Cr(DTBMS)4, which exhibits D > 0.25 This indicates that distortions away from Td can produce categorically distinct electronic structure modifications in Cr(IV) complexes, which may lead to diverging spin-phonon coupling behavior. Inversion recovery T1 measurements at 40 K were conducted on a 1.8% dilution of Cr(o-tolyl)4 into an isostructural diamagnetic Sn(o-tolyl)4 matrix (Figure 6.2A–C). The slowest rates of spin relaxation were recorded at the pure parallel orientations, while the perpendicular orientations also displayed local minima in the spin relaxation rates (Figure 6.2B). Fastest spin relaxation was observed at the intermediate orientations closest to 45°, which are found both immediately to the outside of the Pake pattern horns and also in the very center of the spectrum (Figure 6.1C). The T1 anisotropy pattern can be most nearly described by a sin2(2θ) functional form (Figure 6.2C), where θ is obtained from the orientation analysis in Figure 6.1. The sin2(2θ) function accounts for the slower spin relaxation along the canonical (parallel and perpendicular) orientations and faster spin relaxation along the intermediate orientations. Additionally, a sin2(θ) function and a linear function proportional to B (linear in the magnetic field) were considered. Note that the 122 function linear in B does not directly depend upon the molecular angle θ and is therefore more precisely construed as isotropic field-dependent spin relaxation rather than true anisotropy (Supporting Information, Section 5). This term may be understood as the slope of the Brons–van Vleck spin lifetime field dependence commonly observed in AC magnetometry, and this slope is indeed negative at X-band fields (0.3 T) for common V(IV) qubits.26 A least-squares fit of the inversion recovery data was conducted to quantify the relative contributions of different anisotropy functions (Supporting Information, Section 5.1 and 5.3). The 40 K T1 field dependence of Cr(o-tolyl)4 can be described by this method as composed of 13% sin2(2θ), 6% sin2(θ), and 5% linear in B contributions, with a 76% constant (isotropic) component. This method of T1 anisotropy analysis is applicable to any EPR-addressable S = 1 complex with an anisotropic powder spectrum. 123 Figure 6.2. T1 anisotropy at 40 K of (A–C) Cr(o-tolyl)4 and (D–F) Cu(acac)2. (A, D) X-band EDFSs. (B, E) T1 anisotropy by inversion recovery, overlaid with best fit. (C, F) Orientation functions used to construct the T1 anisotropy fit Crucially, this behavior contrasts with that observed for tetragonal S = 1/2 Cu(II), V(IV), and Cr(V) compounds previously investigated by T1 anisotropy at 100 K, in which the fastest and slowest values of spin relaxation were always found at the canonical orientations.19,20,27 To see if this was an effect of the temperature regime studied, we collected T1 anisotropy at 40 K of 0.1% Cu(acac)2 diluted in the isostructural diamagnetic matrix Pd(acac)2 (Figure 6.2D–F). The 40 K Cu(acac)2 completely follows the sin2(θ) functional form with no apparent significant contributions from sin2(2θ) (Figure 6.2F). The spin relaxation rate at the 45° orientation is simply the average of the rates at the canonical positions. These observations point to a qualitatively different origin of T1 anisotropy in Cr(o-tolyl)4 vs Cu(acac)2. To ascertain the type of vibrations responsible for these distinct patterns, we probed the temperature dependence of the T1 anisotropy for Cr(o-tolyl)4 and Cu(acac)2. We quantify the amount of T1 anisotropy as a fraction of the largest 1/T1 over all field positions (Figure S17). Over the temperature range of 7–60 K, the amount of T1 anisotropy for Cr(otolyl)4 steadily decreases, with the dominant sin2(2θ) anisotropic contribution decreasing from 23% to 10% of the total T1 (Figure 6.3). This decrease with increasing temperature indicates that sin2(2θ) anisotropy arises from very low-energy degrees of freedom, likely acoustic or pseudoacoustic phonons28 (vide infra). Indeed, the isotropic field-dependent contribution to 1/T1 decreases at the same pace over this temperature range, and this contribution is commonly ascribed to the direct process of spin relaxation.26 In contrast, the sin2(θ) T1 anisotropy contribution increases sharply with temperatures for Cu(acac)2, constituting only 0.5% of the total T1 at 20 K but 62% at 100 K (Figure 6.3). Examination of the field-dependent T1 for Cu(acac)2 at 20 K validates that the spin relaxation anisotropy is greatly reduced (Figure S22), while the parallel and perpendicular T1 at 100 K differ by a factor of 2.4, as observed and analyzed previously by the Hadt group.19 This validates that the sin2(θ) anisotropy for Cu(acac)2 arises from higher-energy molecular vibrational 124 modes (optical phonons) that are not thermally populated at 20 K. Thus, the T1 anisotropies for Cr(o-tolyl)4 vs Cu(acac)2 originate from different types of phonons. Note that both anisotropies are temperature dependent in a manner consistent with the effect of thermal population of vibrational modes.29-31 Anisotropic spectral diffusion was ruled out as a principal cause of the observed T1 patterns for both compounds (Supporting Information, Section 5.2). Figure 6.3. Temperature dependence of T1 anisotropy contributions for Cr(o-tolyl)4 and Cu(acac)2 A major attraction of the T1 anisotropy methodology lies in the possibility of correlating the observed functional forms (e.g., sin2(θ) and sin2(2θ)) to their origins in specific molecular degrees of freedom. Previous work has analyzed sin2(θ) spin relaxation anisotropy for Cu(II) and V(IV) molecular vibrations and shown it to be consistent with totally symmetric modes with metal–ligand stretching character29,32,33 inducing relaxation through a modulation of the minority spin component of the ligand field wave function.19 However, a new theoretical analysis is required to explain the sin2(2θ) anisotropy in Cr(otolyl)4. While sin2(2θ) T1 anisotropy has not been previously characterized, Tm anisotropy 125 with a sin (2θ) functional dependence has been observed in several S = 1/2 systems and 2 has been attributed to rotational motion caused by librations.34,36 This induces a change in resonant field position described by dBres/dθ, which is determined by the derivative of the projected g value, dg/dθ. The derivative of the g-tensor dg/dQ has been successfully used as a simplified model for the spin-phonon coupling coefficient describing spin relaxation, where, in this case, the vibrational mode Q is equal to a rotation θ, so there may be a connection between dBres/dθ and spin relaxation in this context as well. Note that here, rotational motion refers specifically to any molecular movement that causes rotation of principal tensor axes of the magnetic Hamiltonian. This does not necessarily imply rigid rotor molecular rotations, and possibilities include coupled rotations of different molecules in the solid-state unit cell (pseudoacoustic or acoustic phonons), glassy librations, or molecular vibrations where the first coordination sphere and the extended ligand framework rotate in opposite directions (Supporting Information, Section 6.1). We therefore hypothesized that rotational motion from low-energy pseudoacoustic or acoustic phonons may explain the sin2(2θ) functional form of the Cr(o-tolyl)4 T1 anisotropy. As the ZFS (D = −0.117 cm–1) is smaller than the Zeeman splitting energy at X-band (0.32 cm–1 for g = 2 at 340 mT), we assume the spins are aligned along the applied magnetic field and treat the ZFS as a first-order perturbation to the Zeeman splitting of energy levels. Treating dBres/dθ as a quantity proportional to the spin-flip matrix element for rotational motion, we obtain (Supporting Information, Section 6): ! "! $< ' >? % ∝ 0 $=)*+ 0 = @A% %% 123' (26) (6.1) Thus, the sin2(2θ) form is consistent with the expected T1 anisotropy due to rotational motion. The decrease in the sin2(2θ) contribution to the T1 anisotropy with increasing temperature also suggests that the rotational motion arises from low-energy acoustic or pseudoacoustic phonons. The latter may carry significant rotational character when there are multiple molecules per unit cell,28 as is the case here (Z = 10).37 Further exploration of 126 structurally diverse S = 1 complexes will be required to ascertain the generality of the rotational sin2(2θ) contributions. The T1 anisotropy for Cr(o-tolyl)4 reduces in magnitude at higher temperatures as molecular vibrations begin to dominate spin relaxation through the Raman process. It remains to be asked why the Cr(o-tolyl)4 higher energy (>100 cm–1) molecular vibrations do not display T1 anisotropy, while the Cu(acac)2 totally symmetric vibrations display strong sin2(θ) anisotropy. This phenomenon is explained by a consideration of the orbital angular momentum matrix elements for a square planar vs. pseudo-Td system. For D4h Cu(acac)2, the d(x2 – y2) ground state has an orbital angular momentum matrix element with the d(xy) excited state of squared modulus 4, while the matrix elements with d(xz) and d(yz) each have a squared modulus of only 1.33 These orbital angular momentum matrix elements control the amount of minority spin in the ground-state wave function. Since the different excited states contribute to the wave function’s minority spin along only some magnetic field orientations, the total minority spin for Cu(acac)2 is greater for the perpendicular orientation than for the parallel orientation, thus giving rise to anisotropic relaxation.19 However, in the cubic symmetry of the Td point group, there can be no difference in matrix elements between the equivalent x, y, and z directions, so there can be no difference in the minority spin along different orientations. The small distortion from Td required to produce nonzero ZFS25,38 is evidently too small to yield a significant sin2(θ) T1 anisotropy in Cr(o-tolyl)4. Similar arguments were proposed to rationalize the presence of T1 anisotropy in tetragonal nitridochromium(V) octaethylporphyrin and nitridochromium(V) tetratolylporphyrin, whereas the rhombic Cr(V)O(HEBA)2– complex did not display appreciable T1 anisotropy.27,39 Therefore, molecular vibrations (optical phonons) are likely to produce isotropic T1 for Cr(o-tolyl)4, in contrast to the previously analyzed tetragonal S = 1/2 systems. 6.3. Conclusions In summary, our findings provide the first direct evidence for ascribing features of spin relaxation to specific types of motion in an optically addressable molecular qubit. These degrees of freedom are distinct from the totally symmetric optical phonons implicated in S 127 = 1/2 microwave addressable molecular systems. 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A 1995, 115 (2), 236– 240, DOI: 132 APPENDIX A: SUPPORTING INFORMATION FOR CHAPTER 2: µOXO DIMERIZATION EFFECTS ON GROUND- AND EXCITED-STATE PROPERTIES OF A WATER-SOLUBLE IRON PORPHYRIN CO2 REDUCTION CATALYST The supporting information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.2c03215 and includes materials and synthesis, high-resolution mass spectrometry as well as additional UV-vis spectra, EPR spectra, MCD spectra, and TA spectra. 133 APPENDIX B: SUPPORTING INFORMATION FOR CHAPTER 3: THE NON-INNOCENT ROLE OF AMINE ADDITIVES IN THE ELECTROCHEMICAL GENERATION OF HIGHLY REDUCED CO REDUCTION PRODUCTS IN A MOLECULAR IRON PORPHYRIN SYSTEM This section is temporarily embargoed. 153 APPENDIX C: SUPPORTING INFORMATION FOR CHAPTER 4: PHOTOPHYSICS OF A STRUCTURALLY CONSTRAINED NICKEL(II)BIPYRIDINE ARYL HALIDE COMPLEX The supporting information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.3c03822 and includes experimental and computational methods, synthetic details, UV-vis/photochemical data, kinetic modeling, transient absorption spectra analysis, NMR spectra, calculated properties, XYZ of the optimize structures, and additional comments. 154 APPENDIX D: SUPPORTING INFORMATION FOR CHAPTER 5: DETERMINING THE KEY VIBRATIONS FOR SPIN RELAXATION IN RUFFLED CU(II) PORPHYRINS VIA RESONANCE RAMAN SPECTROSCOPY The supporting information is available free of charge at https://doi.org/10.1039/d3sc05774g and includes experimental and computational methods, synthetic details, UV-vis data, powder X-ray diffraction data, single crystal x-ray structure determination, resonance Raman data, electron paramagnetic resonance data, and supplemental discussion. 155 APPENDIX E: SUPPORTING INFORMATION FOR CHAPTER 6: T1 ANISOTROPY ELUCIDATES SPIN RELAXATION MECHANISMS IN AN S = 1 CR(IV) OPTICALLY ADDRESSABLE MOLECULAR QUBIT The supporting information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.3c01964 and includes experimental methods, temperature-dependent EPR fitting, T1 anisotropy fitting, and theoretical derivation of T1 anisotropy functional forms.