THE VIBRONIC STRUCTURE OF THE S-0[--]S-1 AND S-0[--]S-2 TRANSITIONS IN SIMPLE OLIGOMERS OF THIOPHENE

We present a quantum-chemical study of the electronic spectra of the oligomers of thiophene. Geometries and vibrational force fields of the ground and excited electronic states are obtained by an updated version of the semiempirical quantum consistent force field/pi electron method implemented to describe sulphur atoms and by ab initio Hartre-Fock and configuration interaction singles methods. The displacement parameters of totally symmetric modes are then obtained and used to model the vibrational structure of the electronic spectra. The contribution of sulphur atoms to the description of the excited state is predicted to be negligible both by ab initio and semiempirical methods which, conversely, indicate a close similarity of thiophene oligomers and polyenes. Based on the results of the simulated spectra a reassignment of some of the bands is proposed. It is shown that mode mixing upon excitation, and not large frequency changes, are responsible for the different Franck-Condon structure of the absorption and emission spectra. In addition, a vibronic coupling mechanism analogous to that active in simple polyenes is identified. It accounts for the ''apparent'' frequency increase of the most active a(g) mode upon excitation to the 1B state.