The electronic spectrum of trans-stilbene in the energy range up to 6 eV has been studied using multiconfigurational second-order perturbation theory (CASPT2). The study includes a geometry determination of the ground state. In all, 12 singlet and one triplet excited states were studied. The calculated spectrum makes it possible to assign the valence excited singlet states corresponding to the three bands observed in the low-energy region of the one-photon absorption spectrum. The most intense feature of the calculated spectrum corresponds to the 1Ag → 21Bu transition at 4.07 eV. The weakly allowed 11Bu state was found 0.3 eV below 21Bu. Transition to the 31Ag state, computed at 4.95 eV, is responsible for the main band observed in the two-photon absorption spectrum. This state has a large weight (around 43%) of doubly excited configurations. The first singlet−singlet Rydberg transition (3s) appears at 5.33 eV and is weak. Implications of the present findings on the trans → cis photoisomerization process in the singlet manifold are discussed. A photoadibatic reaction involving the two lowest states of Bu symmetry seems to be the most plausible mechanism to explain the origin of the small barrier observed on the S1 surface.

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The overall contribution demonstrates that a combined analysis of high-level theoretical results and experimental data can be of great value to perform assignments of detected intermediates in a photocycle.