A Carboxylate‐based Hydrophilic Organic Photovoltaic Catalyst with a Large Molecular Dipole Moment for High‐Performance Photocatalytic Hydrogen Evolution

Achieving ultrafast dissociation of photogenerated excitons and efficient charge transport within the photocatalyst is a fundamental issue. Additionally, enhancing the interaction between semiconductors and water is crucial for efficient photocatalytic water splitting. Herein, we synthesized a carboxylate‐based hydrophilic polymer, hPTB7‐Th. Exposed carboxylates enhance semiconductor‐water interfacial compatibility, reducing contact resistance and accelerating charge transfer kinetics. Furthermore, the carboxylate substitution shifts polarity centers, amplifying the molecular dipole moment by 10‐fold. This induces a giant built‐in electric field, enabling ultrafast electron‐transfer process (ca. 0.31 ps) in the hPTB7‐Th:PCBM bulk heterojunction. Consequently, the hPTB7‐Th:PCBM‐based bulk heterojunction nanoparticles exhibit excellent photocatalytic activity, achieving an optimal hydrogen evolution rate of 111.5 mmol g‐1 h‐1, four times over the ester‐based counterpart (PTB7‐Th:PCBM). Moreover, the electrostatic stability imparted by the carboxylates endows hPTB7‐Th:PCBM with outstanding operational stability, maintaining 81% of its initial hydrogen evolution rate after 100 h operation. This result places it among the state‐of‐the‐art organic photovoltaic bulk heterojunction photocatalysts in terms of stability. This work establishes a molecular engineering strategy for high‐performance bulk heterojunction photocatalysts, emphasizing synergistic optimization of hydrophilicity, dipole engineering, and interfacial dynamics.