Covalently Locked Donor–Acceptor Block Copolymer Nanoparticles Enable Efficient, Stable and Reproducible Photocatalytic Hydrogen Evolution

ABSTRACT Organic photocatalysts with tailored properties and unique processability offer a sustainable route to solar hydrogen. To maximize light harvesting and charge generation, donor and acceptor materials are usually physically mixed to form D:A heterojunction nanoparticles (NPs). However, weakly constrained interactions can promote phase segregation, morphological evolution, and limited reproducibility. Here, we introduce an all‐conjugated donor–acceptor block copolymer, PBDB‐T‐ b ‐PYT, in which covalent linkage between PBDB‐T and PYT segments constrains nanoscale donor–acceptor organization and promotes intramolecular charge transfer. Compared with nanoparticles prepared from a physical PBDB‐T:PYT blend, PBDB‐T‐ b ‐PYT nanoparticles exhibit stronger photoluminescence quenching, more persistent charge‐associated transient absorption features, and improved photoelectrochemical charge extraction. As a result, hydrogen evolution reaction (HER) rates exceeding 120 mmol cm −2 g −1 h −1 (>450 µmol h −1 ) are achieved with low Pt loading (<3 wt.%). PBDB‐T‐ b ‐PYT NPs retained 95% of the initial HER rate after 96 h of photocatalysis. Conversely, NPs prepared from physically mixed PBDB‐T and PYT show lower activity and faster deactivation, consistent with morphology‐driven losses. PBDB‐T‐ b ‐PYT also maintains high performance across polymer batches, improving reproducibility. These results establish all‐conjugated donor–acceptor block copolymers as a promising design strategy for combining high activity, operational stability, and reproducibility in organic nanoparticle photocatalysis.