Yield—not only Lifetime—of the Photoinduced Charge‐Separated State in Iridium Complex–Polyoxometalate Dyads Impact Their Hydrogen Evolution Reactivity

Abstract

Covalently linked photosensitizer–polyoxometalate (PS‐POM) dyads are promising molecular systems for light‐induced energy conversion processes, such as “solar” hydrogen generation. To date, very little is known of their fundamental photophysical properties which affect the catalytic reactivity and stability of the systems. PS‐POM dyads often feature short‐lived photoinduced charge‐separated states, and the lifetimes of these states are considered crucial for the function of PS‐POM dyads in molecular photocatalysis. Hence, strategies have been developed to extend the lifetimes of the photoinduced charge‐separated states, either by tuning the PS photophysics or by tuning the POM redox properties. Recently, some of us reported PS‐POM dyads based on cyclometalated Ir III complexes covalently linked to Anderson‐type polyoxometalate. Distinct hydrogen evolution reactivity (HER) of the dyads was observed, which was tuned by varying the central metal ion M of the POM M ( M =Mn 3+ , Co 3+ , Fe 3+ ). In this manuscript, the photoinduced electron‐transfer processes in the three Ir‐POM M dyads are investigated to rationalize the underlying reasons for the differences in HER activity observed. We report that upon excitation of the Ir III complex, ultrafast (sub‐ps) charge separation occurs, leading to different amounts of the charge‐separated states (Ir .+ ‐POM M .− ) generated in the different dyads. However, in all dyads studied, the resulting Ir .+ ‐POM M .− species are short‐lived (sub‐ns) when compared to reference electron acceptors (e.g. porphyrins or fullerenes) reported in the literature. The reductive quenching of Ir .+ ‐POM M .− by a sacrificial donor, triethyl amine (1 m ), to generate the intermediate Ir‐POM M .− is estimated to be very efficient (70–80 %) for all dyads studied. Based on this analyses, we conclude that the yield instead of the lifetime of the Ir .+ ‐POM M .− charge‐separated state determines the catalytic capacity of the dyads investigated. This new feature in the PS‐POM photophysics could lead to new design criteria for the development of novel PS‐POM dyads.

Covalent Linkage of BODIPY‐Photosensitizers to Anderson‐Type Polyoxometalates Using CLICK Chemistry

“…The team demonstrated that variation of the central heterometal (Fe 3+ , Co 3+ , Mn 3+ ) of the Anderson anion can be used to tune the light‐driven hydrogen evolution of these systems. Further, detailed time‐resolved optical spectroscopy and spectro‐electrochemistry demonstrated the charge‐separation pathways [28] and identified limiting processes for the hydrogen evolution catalysis [29] …”

Controlling Spin‐Correlated Radical Pairs with Donor–Acceptor Dyads: A New Concept to Generate Reduced Metal Complexes for More Efficient Photocatalysis

“…Most studies on visible-light driven photoredoxc atalysis completely ignore the efficiency aspect (i.e.,t he overall quantum yields) and merely focus on the feasibility to use visible light as energy input. In addition to photoredox catalysis, the M À generation is important for many photochemical carbon dioxide reduction [12,[81][82][83] and hydrogen production [13,[84][85][86][87][88][89][90][91][92] mechanisms, demonstrating that the new mechanism introduced in this manuscript has severalp ossible application areas. Our study on the heavily underexplored inherente ffi-ciency of photoinduced electron transfer events might trigger furtherq uantitative investigations that could contribute to a better understanding of how to use photons moree fficiently, [93] which could ultimately result in more sustainability in photochemistry.…”

Multifunctional Polyoxometalate-Platforms for Supramolecular Light-driven Hydrogen Evolution

“…Thus, light-driven catalytic studies were performed in water-free, de-aerated DMF solutions containing the respective catalyst POM-PtX (12.5 µM), the photosensitizer PS (125 M) and triethyl amine/acetic acid (TEA (1.0 M) / HAc (0.2 M)) as sacrificial proton/electron donors; this experimental setup has been adapted from earlier POM-based HER studies. 44,[46][47][48] The samples were irradiated under an Ar atmosphere using monochromatic LEDs (λmax = 470 nm, P ~ 40 mW cm -2 ) at 25°C. Hydrogen evolution was quantified by calibrated gas chromatography.…”