Theoretical investigation of the lowest singlet and triplet excited states in oligo(phenylene vinylene)s and oligothiophenes

By means of a high-level configuration interaction technique, we describe the nature of the lowest singlet and triplet excited states in oligo(phenylene vinylene)s and oligothiophenes. We first calculate the evolution with chain length of the singlet-singlet and triplet-triplet excitation energies, as well as that of the singlet-triplet energy difference. The theoretical results compare favorably with the available experimental data. We then investigate the geometry relaxation phenomena occurring in the lowest excited states of model oligomers. We found that the lowest triplet state is characterized by more pronounced and local lattice distortions than the singlet state, especially in the case of the phenylene vinylene oligomers.