The Mixed-Valence Missing Link: Direct Observation of Borderline Electron Transfer Dynamics

Three decades of understanding of electron transfer in the borderline between localized and delocalized electronic configurations is based upon steady-state spectroscopy measurements, providing only a qualitative approach. Herein, real-time observation of borderline electron transfer, accomplished using ultrafast IR absorption spectroscopy, is reported. Cyanide-bridge vibrations of a photoinduced mixed-valence system were monitored, providing robust evidence about the evolution of the electronic configuration during equilibration with a rate of 5 × 10¹¹ s^-1 or (2 ps)^-1 in acetonitrile at room temperature, and coexistence of both redox isomers in longer time scales. This study highlights, on one hand, the advantages of photoinduced mixed-valence systems relative to their ground-state analogues. On the other hand, the primordial role of ultrafast vibrational spectroscopy to study the fundamentals of solar-energy conversion and other photoredox processes is demonstrated.