Light-driven catalytic three component systems for the reduction of protons, consisting of a cyclodextrin-appended iridium complex as photosensitizer, a viologen-based electron relay, and cyclodextrin-modified platinum nanoparticles as the catalyst, were found to be capable of producing molecular hydrogen effectively in water, using a sacrificial electron donor. The modular approach introduced in this study allows the generation of several functional photo-active systems by self-assembly from a limited number of building blocks. We established that systems with polypyridine iridium complexes of general formula [Ir(ppy)(2)(pytl-R)]Cl (ppy, 2-phenylpyridine; pytl, 2-(1-substituted-1H-1,2,3-triazol-4-yl)pyridine) as photosensitizers are active in the production of H(2), with yields that under our experimental conditions are 20-35 times higher than those of the classical system with [Ru(bpy)(3)]Cl(2) (bpy, 2,2'-bipyridine), methyl viologen, and Pt. By investigating different photocatalytic systems, it was found that the amount of hydrogen produced was directly proportional to the emission quantum yield of the photosensitizer.

The light‐driven reduction of protons for the production of molecular hydrogen in multicomponent systems depends on electron relays such as viologens. We describe here the effect of viologens appended with guest moieties (adamantane or bile acid) on the efficiency of a system composed of a cyclodextrin‐appended photosensitizer [Ir(ppy)2(pytl‐βCD)]Cl [ppy = 2‐phenylpyridine; pytl = 2‐(1‐substituted‐1H‐1,2,3‐triazol‐4‐yl)pyridine; CD = cyclodextrin], cyclodextrin‐modified Pt nanoparticles as the catalyst, and ethylenediaminetetraacetic acid (EDTA) as the sacrificial donor. The system was designed to self‐assemble in a supramolecular manner in order to promote electron transfer and produce hydrogen. Cyclic voltammetry (CV) measurements in DMF showed that the electron‐donating adamantyl substituent decreases the reduction potential of the viologen. The use of symmetric and asymmetric guest‐appended viologens gave rise to unexpected phenomena in the H2 evolution. The presence of adamantane or bile acid groups on the viologen induced stabilization and aggregation of the radical cations in water, which is disadvantageous for hydrogen formation.

The cover picture shows how light can be used to generate hydrogen gas from water. The light‐harvesting iridium complex, attached to a cyclodextrin, is in the bottom right, and in the top left is the platinum catalyst, with its surface also covered by a cyclodextrin. Responsible for the light‐induced electron transfer are the various viologens found in between; one of them, appended with adamantane and bile acid moieties, is shown to be non‐covalently bound to cyclodextrins on both the light‐harvesting complex and the catalyst. Details of this work are described in the article by M. C. Feiters et al. on p. 6729 ff.