Linker-Engineered Dimeric Acceptors Afford Efficient Organic Photocatalytic Hydrogen Evolution via Tailored Nanomorphology for Long-Lived Charge Accumulation.

Organic bulk-heterojunction (BHJ) nanoparticles are promising candidates for solar-to-hydrogen conversion. While the development of organic photocatalysts (OPCs) has leveraged advances in organic photovoltaics (OPVs), molecular design rules tailored to photocatalysis remain underdeveloped. Here we introduce linker-engineered dimeric acceptors that tune self-assembly and thereby control BHJ nanoparticle morphology, enabling high-performance OPCs. Two dimer acceptors, DY1 (unfused linker) and DY2 (fused linker), are synthesised from a monomer analogue (MY), establishing a self-assembly trend of MY > DY2 > DY1. The stronger intermolecular assembly of MY is consistent with a quasi-core-shell morphology that reduces catalytically accessible donor-acceptor interfaces, whereas the weaker intermolecular assembly of DY1 is associated with a more intermixed morphology and increased recombination losses. In contrast, DY2 exhibits a morphology consistent with improved pathway continuity and sufficient donor/acceptor exposure at the particle surface, supporting enhanced accumulation of long-lived, surface-stabilised charges. Consequently, PM6:DY2 OPCs deliver a hydrogen evolution rate of 25.3 µmol h-1 cm-2, outperforming PM6:MY (1.9 µmol h-1 cm-2) and PM6:DY1 (11.9 µmol h-1 cm-2). Notably, this performance trend contrasts with that of the corresponding OPVs, suggesting that photovoltaic design principles do not necessarily translate directly to photocatalysts.