Very Small Bandgap π‑Conjugated Polymers with Extended Thienoquinoids

The introduction of quinoidal character to π-conjugated polymers is one of the effective approaches to reducing the bandgap. Here we synthesized new π-conjugated polymers (<b>PBTD4T</b> and <b>PBDTD4T</b>) incorporating thienoquinoids 2,2′-bithiophene-5,5′-dione (BTD) and benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene-2,6-dione (BDTD) as strong electron-deficient (acceptor) units. <b>PBTD4T</b> showed a deep LUMO energy level of −3.77 eV and a small bandgap of 1.28 eV, which are similar to those of the analog using thieno­[3,2-<i>b</i>]­thiophene-2,5-dione (TTD) (<b>PTTD4T</b>). <b>PBDTD4T</b> had a much deeper LUMO energy level of −4.04 eV and a significantly smaller bandgap of 0.88 eV compared to those of the other two polymers. Interestingly, <b>PBDTD4T</b> showed high transparency in the visible region. The very small bandgap of <b>PBDTD4T</b> can be rationalized by the enhanced contribution of the resonance backbone structure in which the <i>p</i>-benzoquinodimethane skeleton in the BDTD unit plays a crucial role. <b>PBTD4T</b> and <b>PBDTD4T</b> exhibited ambipolar charge transport with more balanced mobilities between the hole and the electron than <b>PTTD4T</b>. We believe that the very small bandgap, i.e., the high near-infrared activity, as well as the well-balanced ambipolar property of the π-conjugated polymers based on these units would be of particular interest in the fabrication of next-generation organic devices