Semiempirical determination of torsional potentials and electronic properties of bithiophene, terthiophene and 3',4'-dihexyl-2,2':5',2''-terthiophene in their ground and first excited singlet and triplet electronic states

We report a detailed study of the conformational behavior of bithiophene (BT), terthiophene (TT) and 3',4'-dihexyl-2,2':5' ,2 ''-terthiophene (DHTT) in their ground (S-0) and first excited singlet (S-1) and triplet (T-1) states. Ground state fully optimized torsional potentials are calculated using the AM1, PM3 and ZINDO/S semiempirical calculation methods. ZINDO/S is also used to obtain the energy of the S-1 <-- S-0 electronic transitions as well as the S-1 and T-1 excited state energies. The calculated singlet-singlet transition energies are compared with UV spectra. It is found that the PM3 method does not reproduce accurately the torsional potential of oligothiophenes. In contrast, AM1 calculations reproduce minima of the BT potential curve at similar dihedral angles to those measured in the vapor phase. AM1 calculations indicate that BT and TT, in their ground state, have slightly twisted trans conformations (torsional angle between adjacent rings approximate to 150 degrees) whereas DHTT is predicted to be much more twisted due to steric hindrance caused by the presence of the hexyl chains, ZINDO/S performed on AM1 optimized conformers suggests that, after excitation, all the molecules relax to planar SI and Ti conformations such that the excited state energy barriers are predicted to be higher than those observed for the ground electronic states of these oligomers.