On the vibronic structure of the S ↔ S 1 transitions in azulene

The semiempirical (quantum chemical consistent force field/π-electron and complete neglect of differential overlap/spectroscopic parametrization) and ab initio methods are used to study the vibronic structure of the optical transitions between the S0 and S1 states of azulene. It is shown that although the excited state is described quite well as arising from the promotion of one electron from the highest occupied molecular orbital to the lowest unoccupied one, the inclusion of doubly excited configurations in description of the wave functions of the two states in question is essential for a proper characterization of the geometry change upon electronic excitation and, therefore, for the resulting Franck–Condon structure of the absorption and emission spectra. The vibronically induced intensities of the b2 fundamentals in the two spectra are calculated and compared with the experimental data, and the problem of correlation between the b2 modes in the two states is solved.