Donor–Acceptor Organic Small Molecules for Photocatalytic Hydrogen Evolution: A Review of Recent Advances and Perspectives

Donor–acceptor (D–A) organic small molecules provide a chemically precise platform for solar-driven H2 evolution because their frontier orbital alignment, electronic coupling strength, and dipole distribution can be tuned at the molecular level. Internal electric fields may further emerge from specific D–A molecular packing arrangements or interfacial charge redistribution, thereby influencing charge-separation behavior. In photocatalytic systems, activity is rarely determined by optical absorption or band gap narrowing alone; it reflects a coupled sequence of exciton generation and dissociation, charge-separated-state lifetime, intermolecular transport, cocatalyst contact, sacrificial-donor oxidation, and proton-reduction kinetics. This review summarizes recent advances in D–A organic small molecules for photocatalytic H2 evolution, with emphasis on structure–property-performance relationships. After outlining the thermodynamic and kinetic requirements of D–A photocatalysis, representative systems are classified by acceptor unit, including pyridine-based, fused-ring, perylene diimide, porphyrin, cyano-containing, and organic photovoltaic small-molecule acceptors. Molecular design and regulation strategies are then analyzed for both covalently and noncovalently linked D–A small molecules. Finally, key limitations and future priorities for stand-alone D–A molecular photocatalysts are discussed.