An n-type narrow-bandgap organoboron polymer with quinoidal character synthesized by direct arylation polymerization

Narrow bandgap conjugated polymers have received great attention due to their wide applications in optoelectronic devices. Because of the synthetic difficulty, it is greatly challenging to develop n-type conjugated polymers with a narrow bandgap with quinoidal character. Herein, we report an n-type narrow-bandgap organoboron polymer with quinoidal character, PBN-TP. PBN-TP is designed with alternating units of BNBP (double B <- N bridged bipyridine) and quinoidal TP (thieno[3,4-b]pyrazine). Direct arylation polymerization of the di-bromo BNBP unit and the di-hydro TP unit affords the organoboron polymer with a high number-average molecular weight of 28.3 kDa. Compared with the control polymer with alternating units of BNBP and thiophene, PBN-T, the replacement of the thiophene unit with a quinoidal TP unit reduces the bond length alternation (BLA) of carbon-carbon bonds and planarized the polymer backbone of PBN-TP, leading to an enhanced contribution of the polymer quinoidal resonance structure. As a result, compared with PBN-T, PBN-TP shows a much reduced bandgap of 1.50 eV, which is among the lowest reported for n-type organoboron polymers based on BNBP. Moreover, due to the low LUMO energy level, PBN-TP could be readily n-doped and used as an n-type polymer thermoelectric material. This work provides a new n-type narrow bandgap conjugated polymer with quinoidal character.