A theoretical study of the electronic spectrum of cis-stilbene

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 B-1 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(1)B (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(1)B and 2(1)A states, respectively. The 3(1)A 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 tuans<-->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 (1)A symmetry) 0.4 eV higher. (C) 1999 Elsevier Science B.V. All rights reserved.