Controlled asymmetric aggregation advances n → π* electronic transition and charge separation for enhanced photocatalytic hydrogen synthesis

Photocatalytic water splitting into hydrogen is considered as a promising approach for solar energy storage. The existing organic semiconductor photocatalysts are mainly dominated by pi-conjugated covalent structures featuring pi -> pi* electronic transition. Recent studies found that activating n -> pi* electronic transition is an effective strategy for improving photocatalytic activity of organic semiconductors. Nevertheless, n -> pi* electronic transition is generally forbidden in perfect symmetric structures. Herein, we have successfully synthesized asymmetric trithiocyanuric acid (TA) aggregates by a solvent chemistry strategy. The asymmetric degrees of TA aggregates can be greatly tuned by changing the pulling strength of solvent molecules. The asymmetric characteristic of TA aggregates increases the orbital overlap between the lone pair electrons on sulfur atoms and the adjacent pi* orbitals of triazine, thus allowing n -> pi* electronic transition. Simultaneously, the asymmetric aggregation induces a large dipole moment, significantly improving the separation and transfer of charge carriers. After an assay of a hydrogen evolution reaction, a quantum yield of 42.9 % at 400 nm was recorded for the asymmetric TA aggregates, which is much higher than those of most existing covalent semiconductors. This study unlocks a fresh realm of artificial photosynthesis, which uses asymmetric aggregates featuring n -> pi* electronic transition for efficient photocatalytic hydrogen production.