Photoinduced electron transfer cascade between Mo- and W-based polyoxometalates

Abstract

The Keggin-type polyoxometalates phosphotungstic acid (PTA, H₃PW₁₂O₄₀) and phosphomolybdic acid (PMA, H₃PMo₁₂O₄₀) can undergo reduction through chemical and photochemical methods. However, the higher electron affinity of Mo(VI) relative to W(VI) and the higher LUMO delocalisation in PTA result in different redox behaviours of the two polyoxometalates. We have identified specific experimental conditions that allow for the photoreduction of PTA in the presence of UV radiation and an electron donor, such as isopropanol (IPA), while PMA does not undergo this reaction. This distinct redox behaviour of polyoxometalates has been leveraged to develop a photoinduced electron transfer cascade from PTA to PMA that can be switched on and off by light. To demonstrate the unique capabilities of this transfer, the mimicry of the early stages of photosynthesis has been achieved by introducing the redox pair benzoquinone (BQ)-hydroquinone (HQ) between PTA and PMA. The transient photoexcitation of the four-component PTA–BQ–HQ–PMA system in the presence of IPA, triggered a cascade of reactions. Initially, PTA was reduced to PTA^red, which, under dark conditions, could reduce BQ to HQ. HQ then reduced PMA to PMA^red. This cascade of reactions, akin to the first stage of photosynthesis, was facilitated by the generation of a proton (H⁺) gradient through the spatial separation of the positive holes created at the PTA terminal and the excited electron migrated across the cascade and localized at the PMA^red terminal. This represents the first example of a unidirectional photo-induced electron transfer cascade between four molecular components.