Supramolecular Control of Photoinduced Electron Transfer between Tungsten Halide Clusters and Polyoxometalates in Water

Identifying systems composed of earth-abundant elements capable of photoinduced electron-transfer represents a major breakthrough for the development of sustainable photocatalytic processes. Herein, we demonstrate that octahedral tungsten-halide clusters constitute a promising new class of robust photosensitizers, owing to their exceptional chemical stability, intense luminescence, and long-lived triplet excited states. Upon irradiation in aqueous solution, these species transfer an electron to polyoxometalates (POMs), well-known molecular catalysts capable of multielectron redox chemistry. More specifically, we investigate the photophysical properties of supramolecular systems combining the cluster-based motif [{W₆Ii₈}Cla₆]²⁻ and a series of archetypal POMs ([PW₁₂O₄₀]³⁻, [SiW₁₂O₄₀]⁴⁻, [BW₁₂O₄₀]⁵⁻, [P₂W₁₈O₆₂]⁶⁻) as electron-acceptor units, in the presence of γ-cyclodextrin (γ-CD), which acts as both a supramolecular assembly scaffold and a stabilizing agent. The photoinduced electron-transfer process was investigated using steady-state and time-resolved optical spectroscopy, complemented by spectro-electrochemical measurements, which unambiguously confirm electron transfer from the excited cluster to the POMs. Furthermore, we demonstrate that both the ionic charge of the POMs and the presence of γ-CD significantly influence the efficiency of the process. This work highlights the dual role of γ-CD, which facilitates the association of two negatively charged inorganic units and stabilizes the charge-separated state by maintaining an appropriate spatial arrangement within the supramolecular assembly.