Theoretical study of the lowest ¹B_U states of trans-stilbene

The results of a theoretical study of the ground state, 1¹A_g, and of the lowest ¹B_u states of trans-stilbene are presented. The vertical and adiabatic excitation energies of the lowest ¹B_u states have been computed using multiconfigurational SCF theory, followed by second-order perturbation theory. It is shown that the two lowest excited states are separated by a small energy gap in the Franck-Condon region. They are the 1¹B_u, characterized by the HOMO→LUMO single excitation substantially localized on the ethylenic moiety, and the 2¹B_u, formed by a combination of one electron excitations localized mainly on the benzene rings. The most intense transition is found to be the lowest in energy when the interaction between different states is included at the level of second-order perturbation theory. The vibronic structure of emission and absorption spectra of the two lowest ¹B_u states have been determined within the Franck-Condon approximation. The spectrum calculated for the 1¹B_u state agrees with the experimental spectrum, while the low intensity band computed for the 2¹B_u state has no experimental counterpart. It is concluded that this band is buried in the strong 1¹B_u absorption and therefore not observed.