Solvent Effects on Vibrational Coherence and Ultrafast Reaction Dynamics in the Multicolor Pump−Probe Spectroscopy of Intervalence Electron Transfer

A detailed investigation of ultrafast photoinduced electron transfer (ET) in a mixed valence compound ((NH3)5RuNCRu(CN)5-; RuRu) in formamide, ethylene glycol, and glycerol was performed using variable wavelength femtosecond pump−probe spectroscopy over a broad range of pump and probe wavelengths. The ET kinetics were monitored by observing recovery of the ground state population. The spectra are highly dynamic, indicating the need for broad spectral coverage to accurately unravel the ET kinetics from excited state and ground state relaxation dynamics. ET is observed to proceed more slowly in slower solvents in a manner consistent with the Hybrid model of solution phase electron transfer. The ET time (1/e) of RuRu in ethylene glycol is 220 fs compared to 100 fs in water. As recently reported for RuRu in water, the ET time is pump wavelength independent. A previously unobserved pump wavelength dependence of the early time dynamics, however, is observed which is assigned to stimulated emission prior to vibr...