화학공학소재연구정보센터
Journal of Physical Chemistry A, Vol.106, No.14, 3593-3598, 2002
Solvation dynamics probed by femtosecond transient absorption spectroscopy: Vibrational cooling and conformational relaxation in S-1 trans-4,4'-diphenylstibene
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 S, 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 S-1-S-m transition and the weighted average of the vibrational modes that contribute to the S-1-S-n transition. On the basis of the present transient absorption data and previous transient Raman data, we suggest that the Franck-Condon region for the SI-S, transition has a significant contribution from the phenyl-phenyl portion of DPS; the SI-S 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 SI-S, 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.