σ–π Hyperconjugation Unlocks Interlayer Charge Separation of Ullazine-Based Supramolecular Nanostructures for Photocatalytic Hydrogen Evolution

This work proposes a σ–π hyperconjugation strategy to establish interlayer charge transport channels (CTC) in supramolecular organic nanostructures. A series of ullazine-based molecular semiconductors were designed and synthesized successfully by engineering end groups to demonstrate the σ–π hyperconjugation that unlocks the quantum confinement of photogenerated charges in π-conjugated planes. Ullazine grafted with tert-butyl (U-t-Bu) showed a J-cross-stacking model in which the cross-stacked U-t-Bu molecular pair smoothly glides along the elongated dimension, forming a Z-schemed interlayer CTC by σ–π hyperconjugations between C–H σ-bonds of tert-butyl end group and π-bonds of ullazines in adjacent layers along the stacking dimension. Consequently, upon photoexcitation of ullazine-based supramolecular nanoaggregates in aqueous solution, the formed Frenkel excitons are dissociated to charge-separated excitons by the interlayer charge separation channels, undergoing an ultrafast charge transfer within 0.58 ps and an ultrafast charge separation within 0.67 ps. The Z-schemed charge separation between adjacent layers leads to a significantly enhanced hydrogen yield over U-t-Bu/PVP/Pt, with a hydrogen evolution rate of 369.9 μmol·g–1·h–1 and an apparent quantum yield of 1.46% at 420 nm. It is 3.8-fold larger than that of ullazine modified with methoxy (U-OMe), without the σ–π hyperconjugation.