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 1ILCT* and an 3ILCT*. The lifetimes of these excited states (τ) and the quantum yields for the emissions (φ) for (dppe)Pt{S2C2(2-quinoxaline)(H)} in CH3CN are 1τ = 0.16 ns,1φ = 0.005 and 3τ = 3.3 μs, 3φ = 0.01, respectively. The 3ILCT* 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 3ILCT* 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 3ILCT* by hydrogen atom transfer.