KCS

We have studied the photoisomerization reaction of 4,4'-dichloro-trans-stilbene (44DChtS) and 4,4'-dibromo-trans-stilbene (44DBrtS) in the condensed-phase with femtosecond transient absorption and femtosecond stimulated Raman spectroscopy. Coherent oscillation features were observed in the excited-state absorption signal and the frequencies of the coherent motion showing the in-plane bending mode at 150 cm^-1 (44DChtS) and 120 cm^-1 (44DBrtS) are induced by para-halogen-disubstituent. In n-hexane (acetonitrile), photoisomerization lifetime of 44DChtS and 44DBrtS are 314 (124) ps and 85 (65) ps, respectively. In the case of 44DBrtS, we expect another C-Br bond dissociation reaction pathway since TD-DFT calculation shows the low dissociation barrier of C-Br coordinate compared to that of C-Cl. Also, we observed that the C=C stretching mode of transient Raman spectra (1550 cm^-1) depends on the excess energy of excitation pulse. Therefore, we will present the time-resolved structural dynamics of S₁ state in 44DChtS and 44DBrtS comparing the substituent effect to that of trans-stilbene.