Abstract
A triol-functionalized 2,2′-bipyridine (bpy) derivative has been synthesized and used for the tris-alkoxylation of polyoxometalate (POM) precursors. The resultant POM-bpy conjugates of the Wells–Dawson- and Anderson-type feature a C–C bond as a linkage between the POM and bpy fragments. This structural motif is expected to increase the hydrolytic stability of the compounds. This is of particular relevance with respect to the application of POM-bpy metal complexes, as photocatalysts, in the hydrogen-evolution reaction (HER) in an aqueous environment. Accordingly, Rh(III) and Ir(III) complexes of the POM-bpy ligands have been prepared and characterized. These catalyst-photosensitizer dyads have been analyzed with respect to their electrochemical and photophysical properties. Cyclic and square-wave voltammetry, as well as UV/vis absorption and emission spectroscopy, indicated a negligible electronic interaction of the POM and metal-complex subunits in the ground state. However, emission–quenching experiments suggested an efficient intramolecular electron-transfer process from the photo-excited metal centers to the POM units to account for the non-emissive nature of the dyads (thus, suggesting a strong interaction of the subunits in the excited state). In-depth photophysical investigations, as well as a functional characterization, i.e., the applicability in the HER reaction, are currently ongoing.
Highlights
The molecular metal oxides, the so-called polyoxometalates (POMs), represent an important class of materials with high relevance for a range of potential applications
The two-step synthesis step synthesis of bpy-triol 2 is outlined in Scheme 1
The computed closed-shell ground-state configurations of 5 and 7 feature basically the same localization of the frontier molecular orbitals (FMOs), which is in full agreement with the results presented by Streb and co-workers [10]
Summary
The molecular metal oxides, the so-called polyoxometalates (POMs), represent an important class of materials with high relevance for a range of potential applications Their chemical and electronic properties can be modulated by their chemical composition, and, for example, the pronounced Brønsted acidity can be employed to catalyze a range of organic transformations (e.g., esterification, hydrolysis, etc.) [1]. The same holds true for the photo-/electrochemical reduction of protons to yield H2 as a clean and renewable energy carrier The limitation of this application, the need of hard UV irradiation to activate the POM catalysts, can be overcome by introducing a photosensitizer (PS) into the catalytic system; as a consequence, visible light can be harvested to achieve the photochemical H2 evolution [2,4,5]. As well as transition metal complexes, such as [Ru(bpy)3 ]2+ or [Ir(ppy) (bpy)]+ , have been used in this respect (bpy: 2,20 -bipyridine, ppy: 2-phenylpyridine)
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