Insights into the intricate charge photoaccumulation in a polyoxometalate–bodipy covalent hybrid

Abstract

Solar fuel generation relies on the catalysis of multielectron, multiproton reactions facilitated in nature by charge accumulation in electron relays like NADPH or hydroquinone. Here, we demonstrate the light-driven charge accumulation in a noble-metal-free photochemical dyad comprising a bodipy photosensitizer linked to a Dawson polyoxometalate (POM) using triethylamine (TEA) as sacrificial electron donor. Under visible light irradiation, the hybrid dyad accumulates up to two electrons on the POM, achieving complete conversion within few minutes. The first reduction proceeds rapidly and efficiently while the second electron is introduced more slowly through an intricate, multi-pathway mechanism that we inferred through combined spectroscopy, electrochemistry and theoretical calculations. The formation of the two-electron reduced species is enhanced in the presence of trifluoroacetic acid by virtue of proton-coupled electron transfer (PCET) as well as by promoting the dismutation of the one-electron reduced POM. Simultaneously, POM reduction may also take place via a light-independent route involving the reactive TEA radical byproduct, effectively rendering TEA an overall two-electron, one-proton donor. The stored redox equivalents in the POM were demonstrated to activate oxygen but also to be engaged in PCET to substrates such as 1,4-benzoquinone, highlighting the potential utility of POM–photosensitizer hybrids in solar fuel-related transformations.