A theoretical study of the electronic spectrum of cis-stilbene

Abstract

The valence and Rydberg electronic excited singlet states of cis-stilbene have been studied using multiconfigurational second-order perturbation theory (CASPT2). The study includes a geometry determination of the ground state. The geometry of the lowest 1B state has also been optimized, leading to a structure where the two benzyl groups are nearly perpendicular. The calculated vertical spectrum is in agreement with the occurrence of three main bands in the observed one-photon absorption spectrum. Below the relatively intense transition to the 2 1B (HOMO→LUMO) state at 4.61 eV, two weak transitions have been computed at 4.11–4.45 eV, which correspond to transitions to the 1 1B and 2 1A states, respectively. The 3 1A state, placed at 5.56 eV above the ground state, has a pronounced doubly excited character. In addition, the lowest vertical singlet–triplet transition has been characterized. Features of some static aspects along the trans↔ cis photoisomerization process in the singlet manifold are discussed. The lowest singlet excited state at a near-perpendicular conformation is of B symmetry with the next state (of 1A symmetry) 0.4 eV higher.