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[Display omitted] • A facile strategy was proposed to overcome the deteriorated crystallinity of π conjugated units. • Local D-A groups destroy the π-localization of charges with huge built-in electric field. • The built-in electric field promotes the exciton dissociation and charge transfer. • The optimized material exhibited excellent photocatalytic activity and stability. Designing a robust built-in electric field (BF) is a charming strategy for enhancing the separation and transportation of charges via introducing large π-conjugated molecules. However, it has flexible or semiflexible geometries, which significantly disorder the crystalline and deteriorated the built-in electric field. Here, a straightforward tactic for creating a cyano-functionalized smaller D (benzene) - A (triazine) units in PDI- triazine based polymer (PDIMB) to enhance intrinsic molecule dipole has been proposed. The density functional theory (DFT) calculation revealed that the modification of smaller D-A groups destroyed the π-localization of charges, which enhanced the molecular dipole and the BF for promoting the exciton dissociation and charge transfer. Moreover, it not only exposed number of active sites, but also enhanced the interfacial molecular interacting. Therefore, PDIMB-2 exhibits high activity (24.5 mmol g-1 h-1) and selectivity (>99%) for the photooxidation of benzylamine to N -benzylidenebenzylamine under mild conditions. Our work offers a potential and simple synthetic option for enhancing the built-in electric field of polymer. [ABSTRACT FROM AUTHOR]