Variation of the Viologen Electron Relay in Cyclodextrin-Based Self-Assembled Systems for Photoinduced Hydrogen Evolution from Water



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.