New Narrow-Bandgap Polymer Composed of Benzobis(1,2,5-thiadiazole) and Thiophenes

摘要

Recently much attention has been focused on development of narrow-bandgap polymers in the field of materials science since they are expected to show interesting physical properties such as intrinsic conductivities and good nonlinear optical properties. Recent studies have shown that copolymerization of aromatic and o-quinoid heterocycles or alternation of donor and acceptor units is effective for lowering bandgaps. In this context we have now designed a new narrow-bandgap polymer, 1, where thiophene is an aromatic donor and benzobis(1,2,5-thiadiazole) is an o-quinoid type acceptor. This system seems to possess several advantages in having a narrow bandgap. First, the benzobis(thiadiazole) unit containing hypervalent sulfur atoms has a high electron affinity. Second, a strong contribution of the quinoid form 1′ is expected since more stable classical 1,2,5-thiadiazole rings are generated in this form. Third, this system has no steric repulsion between the adjacent heterocyclic units, leading to a planar geometry. Fourth, the synthesis of the monomer units is relatively easy. Finally, strong interchain interactions may be caused by short intermolecular S···N contacts which are often observed in 1,2,5-thiadiazole derivatives. We have found that polymer 1 has a bandgap below 0.5 eV that is the lowest value in the reported heterocyclic polymers. The preparation and characterization of polymer 1, the related polymer 2, and their monomers are described here.