Theoretical studies and spectroscopic investigations of ground and excited electronic states of thiophene oligomers

CNDO/S calculations including single and double excited configurations were performed on thiophene oligomers nT with 2-6 monomer units. In contrast to the results on linear polyenes only single excited configurations are necessary to explain the experimental results. The theoretical energies for the lowest 1(1)B states are in good agreement with the low energy absorption origins and the high energy fluorescence maxima and with the 1(1)B state energies observed at low temperature. The results show that the 2(1)A state lies above the 1(1)B state for 2T-6T. Transient absorption bands measured by picosecond spectroscopy could be assigned to transitions between 1(1)B and higher calculated (1)A states of all studied oligomers nT.