Direct arylation polycondensation for the synthesis of bithiazole-based conjugated polymers and their physical properties

The reaction conditions for direct arylation polycondensation were investigated for the synthesis of bithiazole-based conjugated polymers, which have been difficult to synthesize under previous conditions owing to the low solubility of the product in the reaction solvent. The investigation revealed that the reaction with the Pd(PCy3)(2) precatalyst and N,N-diethylpropanamide (that is, reaction solvent) afforded three types of the bithiazole-based conjugated polymers with high molecular weights. These conditions also effectively suppressed the formation of structural defects, such as a non-alternating structure caused by homocoupling. The structure-property relationships of the obtained polymers were evaluated based on their absorption properties, energy levels, crystallinity and semiconducting properties. The highest occupied molecular orbital/lowest unoccupied molecular orbital levels reasonably reflect their chemical structure. The high crystallinity of the polymers was confirmed by X-ray diffraction analysis. These polymers tend to aggregate even in the solution state. Control of the aggregation characteristics was a key factor for increasing the power conversion efficiency of the organic photovoltaic devices that were fabricated from the polymers. This study expanded the application range of direct arylation polycondensation for the preparation of crystalline polymer materials.