Water-Dispersed Crystalline Quantum Dots of Ru(II) Complexes for Visible-Light-Driven Photocatalytic Hydrogen Evolution

Solution-accessible ruthenium(II) complexes are promising materials for photocatalytic hydrogen evolution; however, their efficiency is severely limited by the mismatch between the feature size of their aqueous aggregation and charge diffusion distance as a result of poor water dispersibility. Herein, three lipid-soluble complexes ([Ru(bpy)x(dthbpy)y]2+, x/y = 2/1, 1/2, and 0/3, bpy = 2,2′-bipyridyl, dthbpy = 4,4′-bis(5-hexylthiophene-2-yl)-2,2′-bipyridine) with tunable photophysical properties and electronic structures are synthesized and then transformed into water-dispersible crystalline quantum dots (QDs, size: 2.8–4.8 nm). Owing to the facilitated bulk charge transfer and fast charge exchange between the metal-to-ligand charge transfer (MLCT) triplet excited state and aqueous redox species (<854 ps), such QDs beyond traditional powdery amorphous aggregation offer a rate of hydrogen evolution of up to 15.62 mmol·g–1·h–1 under AM 1.5G illumination (100 mW·cm–2, λ > 420 nm). This efficiency surpasses that of previously reported photocatalysts with Ru(II) complexes as the only active component and is 120 times greater than that of the well-studied water-soluble Ru(II)(bpy)3Cl2. To date, this is the only report of crystalline Ru(II) complex QDs serving as the photocatalyst for hydrogen generation. Furthermore, the strategy demonstrated herein seems generalizable to other hydrophobic ionic semiconductors in addition to Ru(II) complexes.