On the Role of High-Frequency Intramolecular Vibrations in Ultrafast Back-Electron Transfer Reactions
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Abstract
Femtosecond infrared spectroscopy is used to study photoinduced metal-to-metal charge transfer in the mixed-valence complex [(NC)<sub>5</sub>Fe<sup>II</sup>–CN–Pt<sup>IV</sup>(NH<sub>3</sub>)<sub>4</sub>–NC–Fe<sup>II</sup>(CN)<sub>5</sub>]<sup>4–</sup> dissolved in D<sub>2</sub>O. Four intramolecular cyanide stretching (ν<sub>CN</sub>) 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 (<i>n</i> > 6) of vibrational energy are directed into the bridging ν<sub>CN</sub> mode (ν<sub>bridge</sub>). Intramolecular vibrational energy redistribution from the ν<sub>bridge</sub> mode excites a solvent-accessible ν<sub>CN</sub> mode on a 630 ± 50 fs time scale. Vibrational cooling to <i>n</i> = 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
