Enhanced efficiency of the visible-light photocatalytic hydrogen generation by the ruthenium tris(2,2′-bipyridyl)–methyl viologen system in the presence of cucurbit[n]urils

Abstract

Cucurbiturils are cyclic oligomers of glycoluril units whose molecular shape defines an internal hollow space accessible through carbonyl portals. It is known that methyl viologen forms strong host–guest complexes with CB[7] and CB[8]. In the latter case even a dimer can be hosted inside CB[8]. Classical solution systems for visible light H₂ generation use methyl viologen as an electron relay. In this work we have found that the efficiency for H₂ generation of the photocatalytic system comprising EDTA–ruthenium tris(2,2′-bipyridyl)–methyl viologen–Pt increases when cucurbiturils are present in the solution. The enhancement follows the order CB[6] < CB[7] < CB[8]. By means of laser flash photolysis we have established that this efficiency increase arises from the combination of an increase in the relative quantum yield of charge separation (higher efficiency in the formation of MV˙⁺) and a decrease of MV˙⁺ lifetime.