Structure, Spectroscopy and Photochemistry of Platinum (II) Diimine Complexes

Author: Connick, William Beach

Year: 1997

Degree: Dissertation (Ph.D.)

Advisor: Gray, Harry B.

Committee Members: Bercaw, John E.; Gray, Harry B.; Barton, Jacqueline K.; Marsh, Richard Edward; Okumura, Mitchio

Option: Chemistry

DOI: 10.7907/pz38-8c62

Abstract

Platinum(II) bipyridyl complexes exhibit rich absorption and emission properties in the solid and solution states, which can be understood within the context of their electronic and molecular structures. Depending on the ancillary ligands, the emission from square planar monomers originates from either an intraligand π→π*, ligand field, or MLCT triplet state. Tuning the electron-donating properties of a series of pyrazolate ligands induces energy crossover between the lowest 3π→π* and 3MLCT states.

The Pt···Pt distances determined by X-ray crystallography for nine linear-chain platinum(II) diimine complexes range from 3.24 to 3.49 Å. While we find evidence for interligand interactions influencing these structures, the Pt···Pt bonds are the most important stacking forces. The metal-metal distances are consistent with an electronic structural model in which σ-donor / π-acceptor ligands strengthen Pt···Pt bonding interactions. The intense colors of the linear-chain solids and related binuclear complexes (orange, red, violet, or blue) are due to an electronic transition to the 1MLCT[dσ*(Pt)→π* (diimine)] state, where the HOMO (dσ*) derives from the interaction of the fully occupied 5dz2 levels of adjacent Pt centers and the LUMO (π*) is predominantly centered on the diimine ligand. The intense emission from these materials originates from the corresponding 3MLCT state. Overall, the emission and absorption spectra are consistent with an electronic structural model that accounts for the electronic properties of the ancillary ligands, as well as the Pt···Pt interactions. Interestingly, for the red form of Pt(bpy)Cl2 (bpy=2,2'-bipyridine) the Pt···Pt distance shortens from 3.449(1) to 3.370(2) Å in the interval between 294 and 20 K. The emission maximum red-shifts as the temperature drops (613 nm, 300 K; 651 nm, 10 K), with the peak energy decreasing linearly with the inverse cube of the Pt···Pt separation.

The violet color of Pt(bpy)(bdt) (bdt=1,2-benzenedithiolate) is due to a low-energy transition to a metal-mixed ligand-to-ligand charge-transfer state (LLCT); the emission originates from the corresponding triplet state. Photochemical oxidation of Pt(bpy)(bdt) occurs in the presence of oxygen; this reaction has been investigated by 1H NMR and UV-visible absorption spectroscopy, as well as transient absorption spectroscopy. Singlet oxygen produced by energy transfer from the excited complex is implicated as the active oxygen species, in sequential formation of the sulfinate and disulfinate products. Both products have been characterized by X-ray crystallography.

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