Solvation Dynamics Probed by Femtosecond Transient Absorption Spectroscopy: Vibrational Cooling and Conformational Relaxation in S₁ trans-4,4‘-Diphenylstibene

Abstract

We present the femtosecond transient absorption spectra of trans-4,4‘-diphenylstibene (DPS) in dioxane, methylene chloride, and acetonitrile. The transient absorption spectra feature two bands that are assigned to different electronic transitions. We interpret the spectral changes in the transient absorption spectra in terms of vibrational cooling and conformational dynamics. Vibrational cooling is evidenced by changes in the integrated peak intensity of the transient absorption spectra. For the two transient absorption bands observed in S1 DPS, the vibrational cooling time constants correlate well with the thermal diffusivity of the solvent. The vibrational cooling rates for the two absorption bands are statistically different in all solvents. We attribute this difference to the relative energy exchange rates between the weighted average of the vibrational modes in the Franck−Condon region that contribute to the S1−Sn transition and the weighted average of the vibrational modes that contribute to the S1−Sm transition. On the basis of the present transient absorption data and previous transient Raman data, we suggest that the Franck−Condon region for the S1−Sn transition has a significant contribution from the phenyl−phenyl portion of DPS; the S1−Sm transition is likely more localized on the stilbene portion of DPS. The results from the conformational dynamics are consistent with this interpretation. Conformational dynamics are evidenced by peak position shifts in the transient absorption spectra. Only the S1−Sn transition undergoes significant peak shifts with delay. We attribute the conformational change to the transition from a nonplanar (ground state) to planar (excited state) structure between the two adjacent phenyl rings in DPS.