Charge separation control in organic photosensitizers for photocatalytic water splitting without sacrificial electron donors

Abstract

Photocatalytic hydrogen evolution reaction (photoHER) is one of the most promising approaches towards production of “green” hydrogen. Currently, the state-of-the-art photoHER systems require the use of sacrificial electron donors (SED), because of inefficient charge separation in photosensitizers and thermodynamically challenging water oxidation by the same catalyst. Here, we present a molecular design approach for all-organic photosensitizers with effective intramolecular charge separation, microsecond lifetime of excited states, controllable direction of electron transfer, and ability to oxidize water for recovery of the photocatalytic system to its initial state. Such photosensitizers comprise weakly conjugated strong electron donor and acceptor what enables charge transfer during the light absorption. The excitation energy is stored in long-living triplet states, whose lifetime can be monitored by the thermally activated delayed fluorescence. Additionally, application of heavy-atom effect helps not only to increase the population of triplet state but also to increase its stability and lifetime. When such photosensitizers are attached to the platinized TiO₂, efficient photoHER catalysts are obtained which produce H₂ under irradiation with sunlight. In the presence of SED, the highest turnover number after 24 h (TON_24h) of such systems exceed 3500, whilst in pure water without any SED, TON_24h reaches 2000. Our best system performs photocatalytic SED-free water-splitting for 48 h keeping 100 % of its activity and constant turnover frequency of 26 h⁻¹. The described here investigations reveal that water splitting can be performed by a simple three component system “photosensitizer|TiO₂|Pt” under specific control of 1) the charge separation and its direction, 2) intersystem crossing rate and triplet state lifetime, and 3) favorable water oxidation thermodynamics within a photosensitizer together with 4) appropriate alignment of energy levels to the catalyst.