Synthesis and Characterization of Polythiophenes Bearing Aromatic Groups at the 3-Position

Regioregular poly(3-(4'-(3 '',7 ''-dimethyloctoxy)phenyl)thiophene) (P3PhT) and poly(3-(4'-(3 '',7 ''-dimethyloctoxy)-3'-pyridinyl)thiophene) (P3PyT) were successfully prepared with reasonably high molecular weights and low polydispersity indices by the Grignard metathesis (GRIM) polymerization. These polymers were found to be thermally stable up to 360-390 degrees C, depending on the phenyl or pyridinyl linker in the bristle. Both polymer films revealed a molecularly multilayer structure (i.e., lamellar structure) whose layers stacked normal to the film plane; each lamella consists of two sublayers, namely ordered and amorphous layers. The amorphous sublayer was composed of a bilayer formed from the bristles. The ordered sublayer in P3PhT consisted of laterally stacked 3-phenylthiophene backbone chains, whereas that of P3PyT consisted of thiophene backbone chains without the pyridinyl linker. These ordered sublayer formations led a longer pi-conjugation length. The enhanced pi-conjugation lengths were reflected in their optical and electronic properties, showing that both P3PhT and P3PyT exhibited a lower highest occupied molecular orbital (HOMO) level and lower energy band gap compared to those of poly(3-hexylthiophene) (P3HT). Overall, the structure and properties of P3PhT and P3PyT make them promising materials for advanced polymer solar cells having an excellent performance.