Electronic structure of small band gap oligomers based on cyclopentadithiophenes and acceptor units

A combined experimental and theoretical study is presented on a series of well-defined small band gap oligomers. These oligomers comprise two terminal electron-rich cyclopentadithiophene units connected to six different electron deficient aromatic rings that allow tuning the optical band gap from 1.4 to 2.0 eV. Optical absorptions of the ground state, triplet excited state, and radical cation have been investigated. The optical band gaps correlate with the electrochemical oxidation and reduction potentials and are further supported by quantum-chemical calculations at the density functional theory (DFT) level. The optical absorptions of the radical cations show only little variations among the different oligomers, suggesting that the charge is mainly localized on the donor moieties. Triplet energy levels are generally low (<1.2 eV) and the singlet-triplet splitting remains significant when going to smaller band gaps.