On the Origin of Small Band Gaps in Alternating Thiophene-Thienopyrazine Oligomers

We have studied experimentally and theoretically the optical and electrochemical properties of small band gap oligo(7,7'-bis(thiophen-2-yl)-5,5'-bisthieno[3,4-b]pyrazine)s with alternating blocks of bithiophene units and bisthienopyrazine units up to a total length of 16 units. The optical absorptions of the ground state, the triplet excited state, and the corresponding radical cation have been identified and shift to lower energy with increasing chain length. The optical absorption correlates well with quantum chemical calculations and the electrochemical band gap. We show that reduction of the band gap with chain length results from a significant rise of the HOMO level and a moderate reduction of the LUMO energy. Comparison of the chain length dependence of the transition energy at maximum absorption (E-max) and of the redox potentials with previously published data on oligothiophenes and related mixed thiophene-thienopyrazine oligomers shows that the reduction of E-max is more easily induced by increasing the number of thienopyrazine units than by extending the chain, mainly because thienopyrazine is both a better donor and a better acceptor than thiophene. Strong interactions between neighboring thienopyrazine units, with some possible admixing of quinoid character, are the main cause of the small band gap in these oligomers.