Ultrafast measurements on direct photoinduced electron transfer in a mixed-valence complex

A direct measurement of the kinetics of intramolecular photoinduced metal-metal charge transfer has been made, i.e. (NH₃)₅Ru^IIINCFe^II(CN) ₅^- ⇄_kET^hv (NH₃)₅Ru^IINCFe^III(CN) ₅^- where the solvent is H₂O or D₂O and k_ET signifies the reverse-electron-transfer (ET) rate coefficient. The apparent reverse-electron-transfer kinetics are nonexponential with a limiting rate constant, k_lim = k(t) where t → ∞, equal to (8 ± 3) × 10¹¹ s^-1. This is close to the theoretical predictions from the model of Sumi and Marcus (classical vibrational modes), k_ET,SM = 10¹³ and 5 × 10¹² s^-1, and from the model of Jortner and Bixon (quantum mechanical vibrations), k_ET,JB = 1-2 × 10¹² s^-1, The parameters required for these theories were estimated by resonance Raman spectroscopy, static absorption spectroscopy, and recently published transient Stokes shift measurements.