Ultrasmall Organic Nanocrystal Photocatalyst Realizing Highly Efficient Symmetry Breaking Charge Separation and Transport

The high exciton binding energy and short exciton diffusion length (typical 5-10 nm) of organic photocatalysts (OPCs) hinder efficient charge separation and subsequent charge transfer, limiting their potential for solar energy conversion. Inspired by the symmetry breaking charge separation (SBCS) in natural photosystem II, we employed a freeze assembly (FA) strategy to assemble symmetric perylene diimide (PDI) dimers into ultrasmall (sub-5 nm) nanocrystals (NCs) with ordered molecular stacking, exhibiting SBCS characteristics. The SBCS NCs (p-5 nm) showed 12.3-fold enhancement in charge separation efficiency compared to non-SBCS NCs (PDI-5 nm). Furthermore, the charge transfer efficiency in p-5 nm (94.7%) was 1.6 times greater than that of weak SBCS NCs (m-5 nm, 60.4%). Consequently, we achieved a comparable photocatalytic hydrogen evolution rate (1824 μmol h^-1 g^-1) among the PDI-based photocatalysts in p-5 nm. This study highlights the importance of ultrasmall NCs in fulfilling bioinspired SBCS and the potential of the FA strategy for developing high-performance OPCs.