Spectroscopic and theoretical studies on the excited state in diimine dithiolate complexes of platinum(II)

The photophysical properties of a series of Pt(N-N)(S-S) complexes have been studied where (N-N) is either an α,α′-diimine or saturated diamine chelating ligand and (S-S) is either a dithiolate chelating ligand or two monothiolate ligands in order to determine the orbital composition of the excited state. The solvent dependence of the absorption spectra of these complexes and the temperature dependence of their emission intensities and lifetimes have been examined while the ligands have been systematically varied. The electronic spectra are found to be dependent on whether or not the nitrogen chelating ligand is unsaturated (contains a vacant π* orbital). All of the unsaturated diimine complexes show an intense solvatochromic band in the visible region of their electronic spectra which shifts to higher energy with increasing solvent polarity. In the related complexes in which a saturated diamine chelating ligand replaces the unsaturated diimine chelating ligand, no solvatochromism is observed. On the basis of the spectroscopic data, the lowest energy absorption band in the diimine complexes is assigned as a metal-dithiolate to π*(diimine) transition, whereas in the diamine complexes it is assigned as a metal-to-dithiolate MLCT transition. The only room-temperature emissive complexes are those that contain an α,α′-diimine chelating ligand. The nature of the emission in these complexes at all temperatures depends on the dithiolate ligand, and the temperature dependence of the emission spectra has been examined. When (S-S) is the 1,2-dithiolate maleonitriledithiolate (mnt), the emission in rigid glass is structured and shows single exponential behavior with both emission intensity and lifetime, increasing with decreasing temperature. For the other (S-S) complexes studied, the emission in rigid glass shows evidence of multiple emitting states based on the observation that lifetimes increase while emission quantum yields decrease as the temperature is lowered. The nature of the HOMO and LUMO has been examined experimentally using cyclic voltammetry. On the basis of the electrochemical and spectroscopic data, the emission from all of the Pt(diimine)(S-S) complexes except those of mnt is assigned as a 3{d(Pt)/p(S)-π*(diimine)} transition, while, for the mnt complexes, it corresponds to a 3{d(Pt)/p(S)-π*(mnt)} transition. These assignments are supported by extended Hückel molecular orbital calculations.