Tight-binding model predicts frontier molecular orbitals and optical gap for conjugated oligomers.

Conjugated organic oligomers are designed as acceptor materials for organic photovoltaic [OPV] cells. OPV optoelectronic properties, including light absorption, intramolecular and intermolecular charge transfer, depends on the properties of frontier molecular orbitals of these conjugated molecules. Recently, we have shown that tight-binding models can predict the energy and wavefunction of heterogeneous oligomers designed for non-polymeric acceptors including IDTBR. The tight-binding parameters from homo-oligomers and alternating co-oligomers of the constituent monomers, without adjusting, were used to define the tight-binding model of IDTBR Here, we extend this model to describe the energy and structure of excitons on oligothiophenes and IDTBR. We show that the tight-binding model predicts the first singlet excitation energy (or optical gap) for a series of oligomers of increasing length, in reasonable agreement with experimental results from UV-Vis spectroscopy.