Dynamics of t r a n s–c i s photoisomerization of large molecules in supersonic jets

Abstract

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.