Theoretical investigations on the electronic and optical properties of bridged oligothiophenes.

A study of the structure, electronic, and optical properties of oligothiophenes is reported. Geometry optimizations of the ground state of derivatives of these molecules were carried out using the density functional theory (DFT) with the B3LYP functional and the 6-31G(d) basis set. Bridged oligothiophenes by C═O, C═S, and C═C(CN)(2) functional groups were found to be planar in their electronic ground states (S(0)). The electronic excitation transitions of the bridged oligothiophenes were investigated using the time-dependent TD-DFT method performed on the ground-state optimized geometries. For all the derivatives, excitation to the S(1) state corresponds mainly to the HOMO → LUMO transition. The excitation energies are found in fair agreement with the experimental values. The optimization (relaxation) of the first singlet excited electronic state (S(1)) has been done using the restricted configuration interaction (singles) (RCIS/6-31G(d) approach. The electronically excited geometries favor a more quinoidic type structure. Emission energies have been obtained from TD-DFT calculations performed on the excited-state optimized geometries S(1). The change of group (C═O, C═S, and C═C(CN)(2)) as well as the incorporation of monomer moieties induce a significant decrease in the excitation and emission energies.