Effects of branching position of alkyl side chains on ordering structure and charge transport property in thienothiophenedione- and quinacridone-based semiconducting polymers

To investigate the effect of the branching position of the alkyl groups on the side chain of semiconducting polymers, we synthesized two series of semiconducting polymers based on thienothiophene-2,5-dione (PTTD4Ts) and quinacridone (PQA2Ts). 2-Decyltetradecyl, 3-decylpentadecyl, 4-decylhexadecyl, and 5-decylheptadecyl groups were used, and the branching position was systematically varied from the second carbon from the backbone to the fifth carbon. These branched side chains are introduced into the thiophene ring for PTTD4Ts and the quinacridone unit for PQA2Ts. The polymer thin films exhibited small but clear differences in their optical absorption spectra, suggesting that the intermolecular interaction in the solid state varied based on the branching position. The grazing incident X-ray diffraction study revealed that the π-π stacking d-spacing of both polymers decreased when the branching position was moved away from the backbones, indicating that the intermolecular interaction was enhanced. Therefore, regardless of the core where the alkyl groups were introduced, the branching position effectively improved the ordering structure of the polymers, which was most likely due to suppressed steric hindrance. Although PTTD4Ts did not exhibit a clear correlation between the branching position and charge carrier mobility, the mobility of PQA2Ts gradually increased as the branching position moved away from the backbone.