On the Role of High-Frequency Intramolecular Vibrations in Ultrafast Back-Electron Transfer Reactions

Abstract

Femtosecond infrared spectroscopy is used to study photoinduced metal-to-metal charge transfer in the mixed-valence complex [(NC)5FeII–CN–PtIV(NH3)4–NC–FeII(CN)5]4– dissolved in D2O. Four intramolecular cyanide stretching (νCN) vibrations create a multidimensional probe of vibrational excitation, redistribution, and relaxation dynamics following ultrafast back-electron transfer (BET). We find that BET to the electronic ground state occurs in 110 ± 10 fs, during which greater than 6 quanta (n > 6) of vibrational energy are directed into the bridging νCN mode (νbridge). Intramolecular vibrational energy redistribution from the νbridge mode excites a solvent-accessible νCN mode on a 630 ± 50 fs time scale. Vibrational cooling to n = 1 and vibrational relaxation ensue on time scales of 1.3 ± 0.1 and 15–20 ps, respectively. These results highlight the important role played by a coupled network of high-frequency vibrations in ultrafast charge transfer processes in solution.