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

Femtosecond infrared spectroscopy is used to study photoinduced metal-to-metal charge transfer in the mixed-valence complex [(NC)(5)Fe(II)-CN-Pt(IV)(NH(3))(4-)NC-Fe(II)(CN)(5)](4-) dissolved in D(2)O. Four intramolecular cyanide stretching (nu(CN)) 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 nu(CN) mode (nu(bridge)). Intramolecular vibrational energy redistribution from the V(bridge) mode excites a solvent-accessible nu(CN) 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.