Solvent Reorganization Energy in Excited-State Electron-Transfer Reactions. Quenching and Geminate-Pair Back Electron Transfer

The temperature dependencies of the quenching rate constants ( k q ) and cage escape yields of the redox products (η ce ) from the electron-transfer reaction of *Ru(bpy) 3 2+ (bpy = 2,2‘-bipyridine) with nine aromatic amines in deaerated 1:1 (v/v) CH 3 CN/H 2 O solutions have been determined. Values of λ, the solvent reorganization energy for electron-transfer quenching and back electron transfer within the solvent cage, have been extracted from plots of log( k q T 1/2 ) vs 1/ T and log((η ce -1 − 1) T 1/2 ) vs 1/ T, respectively. For the quenching process, λ is not a constant value for the series of quenchers; in general, higher values of λ are exhibited by primary amines and lower values by tertiary amines. The structure and size of the quenchers and the nature of the ring substituents contribute to the value of λ. For the back-electron-transfer reaction within the geminate redox pair formed in the quenching process, the more sterically hindered two-ring amines exhibit a higher value of λ (1.1 ± 0.08 eV) than do the majority of the one-ring amines (0.82 ± 0.04 eV). A Marcus plot of log(η ce -1 − 1) vs Δ G ° bt shows a correlation within only the inverted region for systems with the same λ; the earlier identification of the results for the same photosensitizer and quenchers as a bell-shaped curve is due to the coincidental overlap of two independent segments within the inverted region.