Solvent effects on excited-state torsional motion and electronic relaxation of c i s -1,3,5-hexatriene

Resonance Raman spectra of cis-1,3,5-hexatriene, including absolute cross sections, have been obtained in both vapor and solution phases and compared with previous results for the trans isomer. Quantitative modeling of the resonance Raman intensities provides information on the excited-state geometry and dynamics and solvent effects on the excited-state potential surfaces. Cis-hexatriene is nonplanar in both ground and excited states as evidenced by the appearance of out-of-plane fundamentals in the resonance Raman spectra, but the Franck–Condon activity of these modes is weak. Population decay from the lowest allowed singlet state on time scales of approximately 20 fs (cis-hexatriene) and 40 fs (trans-hexatriene) is the major source of diffuseness in the gas-phase absorption spectra of both molecules. Solvation decreases the resonance Raman activity of out-of-plane vibrations, although the effects are smaller in cis-hexatriene than previously observed in the trans isomer. Simulation of the spectra indicates that the allowed excited-state potential surfaces of both isomers undergo an increase in the local force constants for double-bond torsional motion upon solvation in increasingly polarizable environments.