Optical and Thermal Electron Transfer Activation of Dioxygen by Viologen Dithiolene Metalates

Abstract

Photoinduced electron transfer activation of dioxygen by redoxactive charge-transfer ion pairs of the type {A2+[Pt(mnt)2]2-} (A2+ = 2,2‘-,4,4‘-bipyridinium or cycloalkylated biimidazolium dication; mnt2- = maleonitriledithiolate) occurs through an optical electron transfer within an ion pair. This affords the primary redox products A•+ and [Pt(mnt)2]- as indicated by laser flash photolysis. Under argon the transients recombine by fast second-order kinetics. Under dioxygen a different behavior is observed. In the case of acceptors with a first reduction potential more positive than −0.6 V back electron transfer prevails. When the potential is more negative however, A•+ reduces O2 by pseudo-first-order kinetics to generate O2•-, while [Pt(mnt)2]- accumulates in the solution. Quantum yields increase with decreasing excitation wavelength. This suggests that internal conversion of the initially populated excited state to the photoreactive ion pair charge-transfer state is more efficient upon excitation to the ...