Solvent-solute interactions probed by picosecond transient Raman spectroscopy: Vibrational relaxation and conformational dynamics in S-1 trans-4,4 '-diphenylstilbene

We present the S-1 picosecond transient Raman spectra of trans-4,4'-dipheny lstilbene (DPS) obtained at excitation wavelengths of 287 and 310 nm and pr obe wavelengths of 630 and 660 nm. We compare spectra at different pump and probe wavelengths in order to evaluate the role of conformational dynamics and vibrational relaxation in S1 DPS. In this study, we find that dioxane and methylene chloride accept excess vibrational energy from the photoexcit ed solute in a different manner. We attribute this variation either to a di fference in the local solvent structure or to the difference in the density of states between methylene chloride and dioxane. We also find that the re lative intensities of certain bands demonstrate a dependence upon the pump and probe wavelengths. From these data, we have established that the change in relative intensity with delay is a marker for the dynamical shift in th e S1 potential energy surface, The dependence of this dynamical change on e xcitation wavelength demonstrates that we are examining a "nonstationary" s tate that evolves on a 10-50 ps time scale.