Programming Intraparticle Ordering in Y6-Derived Organic Photocatalyst Nanoparticles for Efficient Solar Hydrogen Evolution

Organic photocatalyst nanoparticles (NPs) dispersed in water provide a scalable platform for solar hydrogen production; however, controlling intraparticle molecular packing remains a major challenge. Here, we regulate the internal ordering of a 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-thiadiazolo[3,4-e]thieno[2″,3″:4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6)-derived nonfullerene acceptor by removing the core alkyl chains to produce YHD-2F. Compared with Y6, YHD-2F NPs exhibit more pronounced vibronic features in their absorption spectra and a smaller Stokes shift, indicating reduced energetic disorder. Powder X-ray diffraction and cryo-TEM directly reveal that YHD-2F retains discernible lattice order even during rapid NP formation driven by rapid aggregation, whereas Y6 shows broadened diffraction features consistent with heterogeneous packing. Molecular dynamics simulations further indicate tighter and more balanced intermolecular packing in YHD-2F. Transient absorption and photoinduced absorption measurements show that YHD-2F generates a larger population of long-lived electrons, which are further enhanced by l-ascorbic acid and quenched by Pt. Consequently, single-component YHD-2F NPs achieve a hydrogen evolution rate of 116.4 mmol g-1 h-1 under AM 1.5G irradiation, more than three times higher than that of Y6 NPs, with an apparent quantum yield of 1.50% at 800 nm. These results demonstrate that core alkyl-chain engineering can program intraparticle ordering in organic photocatalyst NPs, providing a general design principle for morphology-controlled NP photocatalysts toward efficient solar-to-hydrogen energy conversion.