On the optical properties of thiophene oligomers: configuration interaction study on their ground (S0) and first singlet excited (S1) states

In this paper, oligothiophenes (thiophene)n (n=2-7) were optimized by using Hartree-Fock and restricted configuration interaction/singles (CIS) for their ground states (S0) and first singlet excited states (S1) geometries. This was followed by conformational and optical properties investigations using ab initio method in combination with CIS, TD-B3LYP and ZINDO approaches. It is found that bithiophene (BTP) and terthiophene (TTP) are nonplanar in the S0 states, whereas they almost reach planarity in the S1 states. The geometry relaxation after excitation contribute much to the Stokes shift observed in their absorption and emission spectra. The global potential energy surface (PES) studies show BTP and TTP have high inter-ring trosional flexibilities, in both the ground-states (S0) and first excited-states (S1); the cis-conformers should have evident contribution to the absorption and emission spectra of BTP and TTP molecules. These conclusions are also applicable to larger oligomers by a basic study extended to (thiophene)n (n=4-7).