Synthesis and properties of poly(heteroaryleneethynylene)s consisting of electron-accepting benzothiadiazole/quinoxaline units and electron-donating alkyl thiophene units

Low-band-gap π-conjugated polymers composed of π-excessive thiophene and π-deficient benzothiadiazole and quinoxaline units were prepared in high yields by a polycondensation method using palladium cross-coupling reactions of alkylthiophene diacetylenes, 4,7-dibromo-2,1,3-benzothiadiazole, and 5,8-dibromo-2,3-dipyridine-2-ylquinoxaline. The copolymers were characterized by NMR, IR, UV, gel permeation chromatography, and elemental analysis. High-molecular-weight (weight-average molecular weight up to 82,600 g/mol), thermostable, soluble, and film-forming materials were obtained. The polymers were photoluminescent in chloroform and showed metallic luster in the solid state. The absorption and emission in solution and in the solid state of the polymers revealed that the polymers generated a π-stacked structure in the solid state, and the polymer molecules in the film were ordered. Thin films of poly[3-dodecylthiophen-2,5-diylethynylene-(benzo[1,2,5]thiadiazole-4,7-diyl) ethynylene] (P-1), poly[3,4-di dodecylthiophen-2,5-diylethynylene-(benzo[1,2,5] thiadiazole-4,7-diyl)ethynylene] (P-2), poly-[3-dodecylthiophene-2,5- diylethynylene-(2,3-dipyridine-2ylquinoxaline-5,8-diyl)ethynylene] (P-3), and poly[3,4-didodecylthiophene-2,5-diylethynylene-(2,3-dipyridine-2-ylquinoxaline- 5,8-diyl)-ethynylene] (P-4) exhibited an optical band gap of ∼1.85-2.08 eV The highest occupied molecular orbital and lowest unoccupied molecular orbital levels of the polymers were determined from electrochemical measurements. In the absorption and emission spectra of these polymers in chloroform/methanol mixtures, all the polymers revealed solvatochromic effects, which were related to the formation of aggregates, as confirmed by temperature-dependence absorption investigations. The absorption spectra of P-2 and P-4 at different temperatures also revealed significant effects of the structure on the molecular interactions.