Resonance Raman spectra of t r a n s -1,3,5-hexatriene in solution: Evidence for solvent effects on excited-state torsional motion

Resonance Raman spectra of trans-1,3,5-hexatriene in cyclohexane, hexane, methanol, and perfluorohexane are compared with the corresponding vapor phase spectra. The absolute cross sections in cyclohexane indicate that the solvation induced electronic spectral breadth is partly homogeneous (amplitude level in the Raman process) and partly inhomogeneous. Overtones and combination bands involving torsional modes, particularly the central double bond torsion, are dramatically reduced in intensity upon solvation, the reduction being greatest in solvents that generate the largest red shift of the absorption. Quantitative modeling of the cyclohexane data shows that these intensity changes can be attributed only in part to the preferential damping of low-frequency overtones induced by the increase in electronic homogeneous linewidth upon solvation. The remaining intensity reduction may arise either from a stiffer excited-state potential surface for double bond twisting in solution or from coordinate-dependent dephasing in the upper electronic state. Additionally, time-dependent wave packet propagation techniques are employed to estimate the barrier to double bond twisting in the excited state from the experimental ratio of four-quantum to two-quantum Raman overtone intensities.