Mechanism of the direct trans→cis photoisomerization of stilbene. Part 2.—Thermally activated intersystem crossing

Abstract

The importance of the thermally activated intersystem crossing in the direct trans→cis photoisomerization of stilbene is tested on the basis of spin-orbit coupling calculations. A CNDO–CI procedure, with the parametrization suggested by Del Bene and Jaffè, is used to obtain zero-order electronic states, and then the matrix elements of the spin-orbit Hamiltonian are calculated between the lowest excited trans singlet and the nearest triplet states. It is found that the activation energy of the direct phtoconversion compares very well with a small barrier that the molecule must overcome in the lowest excited singlet in order to reach crossover to a quasi-isoenergetic triplet. The calculation of Franck–Condon factors shows that the rate constant of the singlet to triplet relaxation becomes a maximum at the crossing point. However, the non-radiative lifetime of the excited trans singlet, evaluated according to a quantum-mechanical formulation given by Gelbart and Rice, turns out to be excessive (calc. : ∼3 × 10^–9 s; obs. : ∼10^–12 s). These results are discussed with reference to the simplifications of the present theoretical approach.