Tuning of electronic properties of novel donor–acceptor polymers containing oligothiophenes with electron-withdrawing ester groups

To investigate the substituent effects introduced into oligothiophene units, a series of novel donor–acceptor conjugated polymers containing ester-substituted oligothiophenes were synthesized by the direct C–H arylation polycondensation of bis(ester-substituted thienyl)benzothiadiazole and dibromo-substituted oligothiophenes. The ultraviolet–visible absorption spectra of three polymers showed two absorption bands in the visible light wavelength region, ascribed to π–π* transition and the intramolecular charge transfer bands. The oxidation potentials of the polymers exhibited a negative shift with an increase in the chain length of the oligothiophene units. By comparing these polymers with alkyl-substituted analogues, it was found that the introduction of electron-withdrawing ester groups induced a negative shift in the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels and affected the LUMO rather than the HOMO energy levels. As a preliminary experiment, organic photovoltaic cells using these polymers were prepared, and their photoelectric conversion characteristics were investigated in relation to their chemical structures.