SOLVENT SOLUTE INTERACTIONS PROBED BY PICOSECOND TRANSIENT RAMAN-SPECTROSCOPY - BAND ASSIGNMENTS AND VIBRATIONAL DYNAMICS OF S1 TRANS-4,4'-DIPHENYLSTILBENE

We present the ground- and excited-state Raman spectra of trans-4,4'-diphenylstilbene (DPS). We analyze the spectra on the basis of comparison with the Raman spectra of the ground- and excited-state and anion radical spectra of trans-stilbene and biphenyl. The excited-state Raman spectra of DPS in methylene chloride and dioxane exhibit mode-specific, solvent-dependent dynamics. Specifically, the intensities of several vibrational modes associated with the biphenyl portion of DPS are solvent dependent. We attribute the change in intensity to a variation in the Franck-Condon overlap between S(l) and S(n) caused by differences in the planarity of the biphenyl portion of DPS in the two solvents. The lower viscosity solvent, methylene chloride, results in a more planar S(l) structure of DPS than does dioxane. However,the rate at which the S(l) geometry achieves equilibrium is slower in methylene chloride than it is in dioxane. This result suggests that dielectric stabilization of S(l) DPS by the solvent, not viscosity, controls the conformational dynamics.