Excited state properties of quinoxaline-substituted platinum 1,2-enedithiolates

The complexes (dppe)M{S2C2(2-quinoxaline)(R)}, where dppe = (diphenylphosphino)ethane, M = Ni, Pd, and Pt, and R = H and Me, have as their lowest-energy band an intraligand charge transfer transition (ILCT). Excitation of deaerated solutions of (dppe)Pt{S2C2(2-quinoxaline)(R)} lead to emissions from an (1)ILCT* and an (3)ILCT*. The lifetimes of these excited states (tau) and the quantum yields for the emissions (phi) for (dppe)Pt{S2C2(2-quinoxaline)(H)} in CH3CN are (1) tau = 0.16 ns, (1) phi = 0.005 and (3) tau = 3.3 mu s, (3) phi = 0.01, respectively. The (3)ILCT* of these quinoxaline-substituted complexes can undergo a diverse suite of excited state reactions, including electron, proton, and hydrogen atom transfers. The second order rate constants (k(q)) for the quenching of the (3)ILCT* emission by acids increases with the thermodynamic driving force for the excited state proton transfer, an observation consistent with excited state electron and hydrogen atom transfers. Dihydroquinone and p-methoxyphenol are substantially better quenching agents than excited state proton transfer would predict and thermodynamic calculations suggest that they quench the (3)ILCT* by hydrogen atom transfer.