The Effects of Acceptor Moiety and Donor-to-Acceptor Ratio on the Geometries and Electronic Properties of Thieno [3,2-b] thiophene-based Donor-Acceptor Alternating Copolymers

Six types of donor-acceptor conjugated polymers were designed by choosing thieno[ 3, 2-b] thiophene (TT) as the electron donor and Benzo[ c][1,2,5] thiadiazole (BT),[1,2,5] thiadiazolo[ 3,4-g] quinoxaline (TdQ) or benzobis[ 1,2,5] thiadiazole (BBT) as the electron acceptor unit. B3LYP method is employed to investigate the electronic structure and properties of the designed TT-based donor-acceptor alternating copolymers. It is found that the electron-withdrawing ability of the acceptor and the ratio of donor to acceptor units (D/A ratio) play a very important role in the geometries and electronic properties for the donor acceptor alternating copolymers. The bridge bond length and the average bond length of carbon-carbon single bond (L-AS) in the donor unit decreases with an increase in the electron-withdrawing ability of the acceptor moiety. However, the average bond length of carbon-carbon double bond (L-AD) in the donor unit shows a same trend as that of the acceptor strength. In addition, the bridge bond length increases with an increase in the D/A ratio for the copolymers. It is of interest to note that the electronic properties of the TT-based copolymers, especially for the copolymers with the D/A ratio of 2 : 1, also exhibit a systematic trend with the acceptor strength. The band gap (E-g) and effective mass of carriers (both hole and electron, m(H), and m(L)) decrease, the bandwidth (both valence and conduction band, W-VB and W-CB) increases systematically with an increase in the electron-withdrawing ability of the acceptor moiety. The theoretical results also suggest that p-TdQ-TT and p-BBT-DT could have potential applications as an electrically conductive material due to their narrow band gap (< 0. 5 eV),large bandwidth and small effective mass of holes and electrons.