Generation of Photocurrent by Visible-Light Irradiation of Conjugated Dawson Polyoxophosphovanadotungstate-Porphyrin Copolymers

Porphyrin-Polyoxotungstate Molecular Hybrid as a Highly Efficient, Durable, Visible-Light-Responsive Photocatalyst for Aerobic Oxidation Reactions

Organic-polyoxometalate (POM) hybrids have recently attracted considerable interest because of their distinctive properties and wide-ranging applications. For the construction of organic-POM hybrids, porphyrins are promising building units owing to their optical properties and reactivity, including strong visible-light absorption and subsequent singlet-oxygen (1O2*) generation. However, the practical utilization of porphyrins as photocatalysts and photosensitizers is often hindered by their own degradation by 1O2*. Therefore, there is a substantial demand for the development of porphyrin-derived photocatalysts with both high efficiency and durability. Herein, we present a porphyrin-polyoxotungstate molecular hybrid featuring a face-to-face stacked porphyrin dimer (I) fastened by four lacunary polyoxotungstates. Hybrid I exhibited remarkable efficiency and durability in photocatalytic aerobic oxidation reactions, and the selective oxidation of various dienes, alkenes, sulfides, and amines proceeded using just 0.003 mol % of the catalyst. Mechanistic investigations suggested that the high activity of I stems from the efficient generation of 1O2*, resulting from the heavy-atom effect of POMs. Furthermore, despite its high efficiency in 1O2* generation compared to free porphyrins, I exhibited superior durability against 1O2*-induced degradation under photoirradiation.

Designing diverse coordination modes for the covalent attachment of Wells-Dawson type polyoxometalate onto porphyrins

Five covalently bonded polyoxometalate (POM)-porphyrin hybrids were synthesized by reacting the Wells-Dawson type polyoxometalate [N(C4H9)4]5H4P2W15V3O62 with five tris-functionalized porphyrins containing different numbers of tris groups at different peripheral positions. These hybrids were thoroughly characterized using elemental analysis, NMR (1H, 31P, and 51V), mass spectrometry (ESI-MS, MALID-TOF-MS), FT-IR, UV-Vis, and fluorescence spectroscopies. The results proved that different quantities (one, two, and three) of the vanadium-capped Wells-Dawson type metal-oxide cluster P2W15V3O629- can be grafted onto a porphyrin moiety via covalent bonding with different orientations, depending on the number and position of peripheral functional groups on the porphyrin. Interestingly, remarkable fluorescence quenching (60% in 3Py-P@1POM, 75% in trans-2PyP@2POM, 80% in cis-2PyP@2POM, 85% in cis-2PhP@2POM, and 55% in 1Py-P@3POM, as compared to the fluorescence intensity of their corresponding porphyrin precursor) was observed under excitation (λexc = 328 nm), indicating electron transfer from the porphyrin moiety to the POM moiety through covalent linkage.

A photosensitizer-polyoxometalate dyad that enables the decoupling of light and dark reactions for delayed on-demand solar hydrogen production

Decoupling the production of solar hydrogen from the diurnal cycle is a key challenge in solar energy conversion, the success of which could lead to sustainable energy schemes capable of delivering H₂ independent of the time of day. Here, we report a fully integrated photochemical molecular dyad composed of a ruthenium-complex photosensitizer covalently linked to a Dawson polyoxometalate that acts as an electron-storage site and hydrogen-evolving catalyst. Visible-light irradiation of the system in solution leads to charge separation and electron storage on the polyoxometalate, effectively resulting in a liquid fuel. In contrast to related, earlier dyads, this system enables the harvesting, storage and delayed release of solar energy. On-demand hydrogen release is possible by adding a proton donor to the dyad solution. The system is a minimal molecular model for artificial photosynthesis and enables the spatial and temporal separation of light absorption, fuel storage and hydrogen release.

Organo-Polyoxometalate-Based Hydrogen-Bond Catalysis

Several urea-inserted organo-polyoxometalates (POMs) derived from polyoxotungstovanadate [P₂ V₃ W₁₅ O₆₁ ]^9- were prepared. The insertion of the carbonyl into the polyoxometallic framework activates the urea toward Hydrogen-bond catalysis. This was shown on the Friedel-Crafts arylation of trans-β-nitrostyrene. Modelling shows that the most stable form of the organo-POMs features a cis-trans arrangement of the two N-H bonds, but that the likely catalytically active trans-trans form is accessible at room temperature. Finally, it is possible that the oxo substituents next to the vanadium atoms may help the approach of the nucleophile via H-bonding.

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

The covalent attachment of molecular photosensitizers (PS) to polyoxometalates (POMs) opens new pathways to PS‐POM dyads for light‐driven charge‐transfer and charge‐storage. Here, we report a synthetic route for the covalent linkage of BODIPY‐dyes to Anderson‐type polyoxomolybdates by using CLICK chemistry (i. e. copper‐catalyzed azide‐alkyne cycloaddition, CuAAC). Photophysical properties of the dyad were investigated by combined experimental and theoretical methods and highlight the role of both sub‐components for the charge‐separation properties. The study demonstrates how CLICK chemistry can be used for the versatile linkage of organic functional units to molecular metal oxide clusters.

Porphyrins and Polyoxometalate Scaffolds

Polyoxometalates (POMs) can act as unique reservoirs for multiple electron transfers. As POMs display only weak absorption in the visible spectrum, they can be associated with chromophores such as porphyrins and porphyrin antennae. In this Minireview, the research dedicated to the combination of porphyrins and polyoxometalates is put in context and the state of the art identifying the challenges addressed in the optimization of hybrid materials for applications is detailed.

Enhancement of photocurrent by incorporation of Preyssler type polyoxometalate protected nanoparticles in polyporphyrin films

The introduction of nanoparticles (MNPs) at the surface of cationic poly-porphyrin films, obtained by electrostatic interaction between the bis-porphyrin copolymer and the Preyssler type polyoxometalate P5W30@MNPs, enhances the photocurrent (up to 2.5-3 times greater as a function of the used nanoparticle).

Hybrid polyoxometalates as post-functionalization platforms: from fundamentals to emerging applications

Polyoxometalates (POMs) represent an important group of metal-oxo nanoclusters, typically comprised of early transition metals in high oxidation states (mainly V, Mo and W). Many plenary POMs exhibit good pH, solvent, thermal and redox stability, which makes them attractive components for the design of covalently integrated hybrid organic-inorganic molecules, herein referred to as hybrid-POMs. Until now, thousands of organic hybrid-POMs have been reported; however, only a small fraction can be further functionalized using other organic molecules or metal cations. This emerging class of 'post-functionalizable' hybrid-POMs constitute a valuable modular platform that permits coupling of POM properties with different organic and metal cation functionalities, thereby expanding the key physicochemical properties that are relevant for application in (photo)catalysis, bioinorganic chemistry and materials science. The post-functionalizable hybrid-POM platforms offer an opportunity to covalently link multi-electron redox responsive POM cores with virtually any (bio)organic molecule or metal cation, generating a wide range of materials with tailored properties. Over the past few years, these materials have been showcased in the preparation of framework materials, functional surfaces, surfactants, homogeneous and heterogeneous catalysts and light harvesting materials, among others. This review article provides an overview on the state of the art in POM post-functionalization and highlights the key design and structural features that permit the discovery of new hybrid-POM platforms. In doing so, we aim to make the subject more comprehensible, both for chemists and for scientists with different materials science backgrounds interested in the applications of hybrid (POM) materials. The review article goes beyond the realms of polyoxometalate chemistry and encompasses emerging research domains such as reticular materials, surfactants, surface functionalization, light harvesting materials, non-linear optics, charge storing materials, and homogeneous acid-base catalysis among others.

Shining a light on the photo-sensitisation of organic-inorganic hybrid polyoxometalates

Finding new ways of using visible light (or, more specifically, solar irradiation) to drive commercially significant and/or challenging chemical processes is an ongoing research goal. Polyoxometalates (POMs) are discrete, metal-oxide clusters which are cheap, robust and easily synthesised but can also act as versatile molecular building blocks, allowing for astonishing variety in their structures and properties. In particular, the rich redox chemistry and inherent photo-activity of POMs makes them attractive for use in a variety of photochemical applications, however POMs characteristically only absorb strongly in the UV region. In this perspective, we discuss the various strategies which have been employed in order to sensitise POMs to visible light, with a particular focus on hybrid inorganic-organic POM species. We will discuss the two clear photo-activation mechanisms which have been developed to date and provide an outlook on some of the possible future directions of the field.

A mild post-functionalization method for the vanadium substituted P2W15V3 Wells–Dawson polyoxometalate based on a copper catalyzed azide–alkyne cycloaddition

A novel post-functionalization method for the vanadium substituted P₂W₁₅V₃ Wells–Dawson polyoxometalate has been developed using a copper catalyzed azide–alkyne cycloaddition.

Grafting of Secondary Diolamides onto [P2 W15 V3 O62 ](9-) Generates Hybrid Heteropoly Acids

The Dawson tungstovanadate [P2 W15 V3 O62 ](9-) can be grafted to secondary diolamides. The electron-withdrawing character of the polyanion increases the acidity of the amide proton, leading to an organo-polyoxometalate, which can be used as a Brønsted organocatalyst. High-field NMR and DFT modeling indicate that the amide proton stays on the nitrogen and that the exalted acidity derives from the interaction between the organic and inorganic parts of the organo-polyoxometalate. The amide-inserted vanadotungstates thus form a new family of (hybrid) heteropolyacids, offering new perspectives for the application of POM-based catalysis in organic synthesis.

Main-chain polyoxometalate-containing donor–acceptor conjugated copolymers: synthesis, characterization, morphological studies and applications in single-component photovoltaic cells

Single-component solar cells of main-chain polyoxometalate-containing donor–acceptor conjugated copolymers show efficiencies comparable to those conjugated polymers with covalently bonded fullerenes.