Dynamics of trans-cis photoisomerization of large molecules in supersonic jets

In this paper we report on the time-resolved fluorescence decay lifetimes from photoselected vibrational states in the S₁ manifold of trans-stilbene, 4-Cl trans-stilbene and 4-methyl trans-stilbene cooled in supersonic jets and excited by a mode-locked dye laser. Decay lifetimes as short as 100±30 ps were recorded, using a fast single photon counting system. The energy dependence of the nonradiative decay rates, k_nr, of trans-stilbene is invariant with respect to the para-substitution of Cl, whereupon the intersystem crossing to the triplet in 4-Cl trans-stilbene dose not provide the main route for photoisomerization and constitutes a distinct, energy dependent, decay channel. The invariance of k_nr with respect to para-methyl substitution indicates that intrastate vibrational energy redistribution (IVR) does not involve all accessible molecular modes. The experimental data are consistent with a nonadiabatic trans-cis isomerization in the isolated molecule, which involves S₁-S₂ internal conversion in conjunction with IVR.