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 vibrational relaxation in the excited state. The early time dynamics of the stimulated emission is solvent dependent as demonstrated by the pump wavelength dependence while probing at the red edge of the static absorption spectra. The oscillatory features observed in the pump-probe signal are assigned to vibrational coherence on the ground electronic state created primarily by resonant impulsive stimulated Raman scattering (RISRS). The dephasing time of the vibrational coherence is solvent dependent as are the vibrational population relaxation rates. The wavelength dependent phase and amplitude of the oscillations show differences from typical RISRS signals, which suggest non-Condon contributions to the oscillations. The solvent dependence of the oscillation amplitude suggests the presence of vibrationally impulsive internal conversion for the fastest reaction, RuRu in water. Extremely slow solvent dynamics are observed in the slow solvents despite a very short excited-state lifetime which indicates a solvent response outside the regime of linear response.