SOLVENT SOLUTE INTERACTIONS PROBED BY PICOSECOND TRANSIENT RAMAN-SPECTROSCOPY - MODE-SPECIFIC VIBRATIONAL DYNAMICS IN S1 TRANS-STILBENE

We obtain the picosecond transient Raman spectra of S1 trans-stilbene in acetonitrile and n-hexane. We observe mode-specific, solvent-dependent variations in the vibrational spectra. In acetonitrile, the vibrational bands in S1 trans-stilbene associated with the phenyl portion of the molecule shift to higher energy and increase in bandwidth, relative to n-hexane, owing to the increased coupling of the excited state to the more polar solvent. In both solvents, the vibrational motions associated with the olefin portion of the molecule change peak position and bandwidth with delay, while the peak position and bandwidth of the phenyl modes in each solvent remain constant with delay. The change in peak position and bandwidth of the olefin modes depends on the excitation frequency used to excite the molecule. We attribute these changes, in part, to vibrational relaxation via resonance energy exchange involving low-frequency vibrations. Additional factors, likely attributable to conformational changes, appear to give rise to a portion of the effects we observe.