CONFORMATIONAL-ANALYSIS AND ELECTRONIC-PROPERTIES OF BITHIOPHENE AND TERTHIOPHENE IN THEIR GROUND-STATE AS WELL AS IN THEIR FIRST EXCITED SINGLET AND TRIPLET-STATES

For the first time a joint theoretical and experimental method is prop osed that allows one to elucidate the conformations of oligothiophenes in their respective ground and first excited singlet and tripler stat es. Semiempirical calculations (AM1 and ZINDO/S) are used to obtain th e ground state conformation (S-0) of bithiophene and terthiophene. Sim ilar potential energy curves are obtained for both molecules. A higher rotational barrier between adjacent rings is calculated for terthioph ene suggesting that a higher conjugation length is effective for this molecule, This is confirmed by comparison with experiments. ZINDO/S is used to calculate the energy of the S-1 <-- S-0 electronic transition s as well as the S-1 and T-n excited state energies. These calculation s suggest that, after excitation, both molecules relax to a planar S-1 conformation. The geometry of the molecules in their T-1 states is al so predicted to be planar. From the S-1-T-n energy gaps and the report ed values of the intersystem crossing rate constants (k(isc)) of bithi ophene and terthiophene, it is suggested that the Franck-Condon factor s are not playing an important role in the intersystem crossing probab ility of these thiophene oligomers.