Achieving Efficient Solar Hydrogen Production via a Three-Motif Molecular Junction with Spatially Separated Dual Reduction Sites

Organic semiconductors are very attractive photocatalysts for the production of solar fuels. However, their development is greatly plagued by limited visible light absorption and severe restriction of photoexcited charge carrier separation and transfer caused by the exciton effect resulting from inherent dielectric constraints. Herein, a three-motif molecular junction hydrogen evolution photocatalyst is constructed by linking a donor-acceptor₁-donor (D-A₁-D) molecule integrating the photosensitizer unit and the redox unit with holey carbon nitride sheets (HCNS) as a second electron acceptor unit (A₂) based on the covalent strategy. The optimized molecular junction (HCNS@BTD-MJ) photocatalyst exhibited an excellent photocatalytic hydrogen evolution rate of 194.9 mmol g^-1 h^-1. Such a rate represents the highest performance among the currently reported carbon nitride-based photocatalysts. Comprehensive experimental results and density functional theory calculations confirm that the designed three-motif molecular junction photocatalyst couples efficient light absorption, photoelectron conversion, and spatially separated catalytic active sites. The bound excitons are split on both sides of the electron acceptors A₁ and A₂ under visible light excitation, thus reducing the reverse charge recombination and weakening the exciton effect, which significantly enhances the charge separation and migration kinetics and contributes to the efficient photocatalytic performance.