Small-bandgap quinoid-based π-conjugated polymers

Small-bandgap π-conjugated polymers are an important class of materials possessing significant potential for their broad application in organic electronics. Incorporating a quinoid structure into the polymer backbone is a powerful strategy for reducing the bandgap. Although this also leads to high-lying HOMO energy levels, resulting in unstable polymers, the use of an electron-deficient quinoid building unit can bring about low-lying HOMO energy levels, thus stabilizing the polymer against oxidation. Recent efforts in molecular design and synthesis have produced a number of fascinating small-bandgap π-conjugated polymers based on quinoid structures, many of which have shown notable performance in organic electronic devices such as transistors and solar cells. This review covers the progress in small-bandgap quinoid-based π-conjugated polymers, focusing on a variety of electron-deficient quinoid building units. We highlight the properties of each quinoid building unit and the polymers along with their structural features and device performances.