Time dependent DFT investigation on the two lowest 1Bu states of the trans isomer of stilbene and stiff-stilbenes

We investigate at TD-DFT/PBE0/6-31+G(d,p) level of theory, the two lowest Bu states of the trans isomer of stilbene and two stiff-stilbenes, whose phenyl rings are blocked in the molecular plane by alkylic bridges, and whose two first Bu bands have been observed. At the Franck–Condon point, computations have been performed both in gas and in condensed phase by PCM method, and the results for the two stiff-stilbenes are in very good agreement with the experiments, predicting a strongly absorbing Bu state with a main HOMO → LUMO character and, about 0.6–0.7 eV higher in energy, a second weak absorbing Bu state, combination of the two HOMO → LUMO + 1 and HOMO-1 → LUMO excitations. A very similar picture is predicted for trans-stilbene in agreement with the experimental data and estimate. The dependence of the vertical excitation energies on the C–C ethylenic bond-distances and on the rotation of the phenyl rings has been explored for trans-stilbene in gas phase.