Oligothiophene radical cations: Polaron structure in hybrid DFT and MP2 calculations

We investigate the geometric and electronic structure of radical cations in oligothiophene chains by performing hybrid DFT (BHandHLYP) calculations on chains with up to 13 thiophene rings and ab initio MP2 calculations. The BHandHLYP-optimized structures indicate a self-localization of charge, spin, and geometric distortion around the middle of the chain, i.e., the formation of a polaron even in the case of the longer chains. This result contrasts with previous pure DFT findings that indicate complete delocalization of the radical cation over the whole chain. The MP2-optimized structure of octathiophene, an oligomer long enough to allow for possible polaron formation, confirms the presence of a well-localized polaron structure, similar to that obtained at the Hartree-Fock level.