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 proposed that allows one to elucidate the conformations of oligothiophenes in their respective ground and first excited singlet and triplet states. Semiempirical calculations (AM1 and ZINDO/S) are used to obtain the ground state conformation (S0) of bithiophene and terthiophene. Similar potential energy curves are obtained for both molecules. A higher rotational barrier between adjacent rings is calculated for terthiophene 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 S1 ← S0 electronic transitions as well as the S1 and Tn excited state energies. These calculations suggest that, after excitation, both molecules relax to a planar S1 conformation. The geometry of the molecules in their T1 states is also predicted to be planar. From the S1−Tn energy gaps and the reported values of the intersystem crossing rate constants (kisc) of bithiophene and terthiophene, it is suggested that the Franck-Condon factors are not playing an important role in the intersystem crossing probability of these thiophene oligomers.