Cyclodextrin-based systems for photoinduced hydrogen evolution

Abstract

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.

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

“…Hydrogen evolution experiments were performed as previously described 13. The components were mixed in aqueous solution to obtain the self‐assembled system presented in Scheme a as well as several other combinations, which are also possible starting from these building blocks.…”

“…As already mentioned, the electron mediator Me‐viol‐Me was used in all of the previously described experiments 13. A better candidate for an electron relay that can improve the efficiency of our system is the modified viologen ada‐viol‐ada, in which both methyl groups of Me‐viol‐Me are replaced by adamantyl moieties.…”

“…The first systems, based on semiconductors and requiring UV light,5–7 were followed by others that used visible light, involving organic dyes and (organo)metallic complexes, including Ru and Ir polypyridine complexes,8–12 as photosensitizers. We have recently reported13 on cyclodextrin‐based self‐assembled systems for photoinduced hydrogen evolution from water, see Scheme . They consist of an iridium complex of general formula [Ir(ppy) 2 (pytl‐R)]Cl as the photosensitizer [ppy, 2‐phenylpyridine; pytl, 2‐(1‐substituted‐1 H ‐1,2,3‐triazol‐4‐yl)pyridine], methyl viologen ([Me‐viol‐Me]Cl 2 ) as the electron relay, and cyclodextrin‐coated platinum nanoparticles as the catalyst.…”

“…The Ir‐CD/Me‐viol‐Me/Pt‐βCD/EDTA system compared favorably to Ru‐bpy/Me‐viol‐Me/Pt‐βCD/EDTA, viz. 137 vs. 3.9 μmol H 2 are produced,13 even though the Ru system contains the same Ru unit {Ru‐bpy = [Ru(bpy) 3 ]Cl 2 , with bpy = 2,2′‐bipyridyl} as the first systems used for photocatalytic production of H 2 from water 6,25. As the viologen electron relay, we used 1,1′‐disubstituted‐4,4′‐bipyridine‐1,1′‐diium species as their chloride salts, in which the nitrogen atoms are located in para positions, and thus have the correct geometry to yield linear self‐assembled structures.…”

“…As the viologen electron relay, we used 1,1′‐disubstituted‐4,4′‐bipyridine‐1,1′‐diium species as their chloride salts, in which the nitrogen atoms are located in para positions, and thus have the correct geometry to yield linear self‐assembled structures. The simplest viologen of the series, the methyl viologen Me‐viol‐Me, also known as paraquat, was used in our studies so far13 and is commercially available. We present here the design and synthesis of two other viologens, one symmetrically and the other asymmetrically appended with moieties that are guests for cyclodextrins, such as adamantane (ada) and bile acid (isoursodeoxycholic acid, here abbreviated as DC) groups, viz.…”