Low-band-gap conjugated polymers based on thiophene, benzothiadiazole, and benzobis(thiadiazole)

A series of low-band-gap copolymers of thiophene, benzothiadiazole, and benzobis(thiadiazole) were synthesized. The polymers were synthesized by Stille cross-coupling polymerization of distannylalkyl-thiophenes and dithiophenes with dibromo derivatives of benzothiadiazoles and benzobis(thiadiazole)s. The polymers were characterized using NMR, UV-vis, and size exclusion chromatography (SEC). The molecular weight, solubility, and film-forming ability were highly dependent oil the choice of side chains. 3.7,11-Trimethyldodecyl side chains were found to give polymer products with high molecular weight, good film-forming ability, and good solubility. Band gaps were estimated from UV-vis to be 2.1 - 1.7 eV for polymers based on benzothiadiazole and similar to 0.7 eV for polymers based oil benzobis(thiadiazole). The band gap and electronic Structure of the polymers were determined by a combination of UV-vis spectroscopy and ultraviolet photoelectron spectroscopy (UPS).