Ultrafast Electron Transfer and Coupled Vibrational Dynamics in Cyanide Bridged Mixed-Valence Transition-Metal Dimers

Picosecond infrared spectroscopy has been used to investigate electron transfer and vibrational excitation and relaxation in the mixed-valence transition-metal dimer [(NC)₅M^IICNM^III(NH₃)₅]⁻ (M = Ru, Os). Optical excitation into the metal-to-metal charge transfer band results in formation of the excited state redox isomer [(NC)₅M^IIICNM^II(NH₃)₅]⁻. Subsequent back electron transfer results in reformation of the ground state and occurs with τ < 0.5 ps. Upon return to the ground electronic state, large amounts of energy (up to 7 quanta, 14 000 cm⁻¹) are placed into the terminal MC≡N stretching mode. The rate of the subsequent energy relaxation was measured; decay times ranged from <0.5 to 6 ps for the ruthenium analog and from 1.8 to 18 ps for the osmium analog. Results are compared to recent theoretical models of Jortner and Bixon.