Polyoxometalates (POMs) have remarkable properties and a great deal of potential to meet contemporary societal demands regarding health, environment, energy and information technologies. However, implementation of POMs in various functional architectures, devices or materials requires a processing step. Most developments have considered the exchange of POM counterions in an electrostatically driven approach: immobilization of POMs on electrodes and other surfaces including oxides, embedding in polymers, incorporation into Layer-by-Layer assemblies or Langmuir–Blodgett films and hierarchical self-assembly of surfactant-encapsulated POMs have thus been thoroughly investigated. Meanwhile, the field of organic–inorganic POM hybrids has expanded and offers the opportunity to explore the covalent approach for the organization or immobilization of POMs. In this critical review, we focus on the use of POM hybrids in selected fields of applications such as catalysis, energy conversion and molecular nanosciences and we endeavor to discuss the impact of the covalent approach compared to the electrostatic one. The synthesis of organic–inorganic POM hybrids starting from bare POMs, that is the direct functionalization of POMs, is well documented and reliable and efficient synthetic procedures are available. However, as the complexity of the targeted functional system increases a multi-step strategy relying on the post-functionalization of preformed hybrid POM platforms could prove more appealing. In the second part of this review, we thus survey the synthetic methodologies of post-functionalization of POMs and critically discuss the opportunities it offers compared to direct functionalization.

Functionalization and post-functionalization: a step towards polyoxometalate-based materials

Recent progress in stimuli-responsive surfactants is reviewed, covering control of both interfaces and bulk solution properties. Particular attention is devoted to potential future directions and applications.

/pdf/stimuli-responsive-surfactants-4bhlnyfhkf.pdf

Stimuli-responsive surfactants

The Anderson–Evans polyoxometalate: From inorganic building blocks via hybrid organic–inorganic structures to tomorrows “Bio-POM”

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.

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

Recent advances in alkoxylation chemistry of polyoxometalates: From synthetic strategies, structural overviews to functional applications

Smart self-assemblies based on a surfactant-encapsulated photoresponsive polyoxometalate complex

An electroswitchable fluorescence thin-film based on a luminescent polyoxometalate cluster.

A series of BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) derivatives containing ion- and pH-sensory units have been successfully designed and synthesized. One of the compounds was structurally characterized by X-ray crystallography. Owing to the presence of an ICT absorption band, one of the compounds was found to show pronounced solvatochromic behavior in different organic solvents. Their emission energies in various solvents show a linear dependence on the Lippert solvent parameter. The cation-binding properties of the complexes with different metal ions (alkali metal, alkaline earth metal and transition metal ions) have been studied using UV-vis and emission spectroscopies. A 1 : 1 complexation to metal ions (Li+, Na+, Mg2+, Ba2+, Zn2+, Cd2+) was found for the compound with one azacrown moiety in acetonitrile while another one with two azacrown moieties was shown to form 1 : 2 complexes with Zn2+ and Mg2+ cations. Their stability constants have been determined by both UV-vis and emission spectrophotometric methods. By introducing triarylborane moieties into the meso position and the 2-position of the BODIPY skeleton, different electronic absorption spectral changes together with an emission diminution were observed in response to fluoride ions. Ditopic binding study of 5, which was functionalized with both azacrown and triarylborane moieties, showed emission enhancement in the presence of Mg2+ and F−. These findings suggest that these BODIPY derivatives are capable of serving as versatile colorimetric and luminescence probes for pH, cations and F−.

Functionalized BODIPY with various sensory units – a versatile colorimetric and luminescent probe for pH and ions

In this paper, we report the synthesis of a novel ferrocene symmetrically modified Mn-Anderson-type polyoxometalate and its long alkyl chain surfactant-enwrapped complexes and the controlled self-assembly in organic solutions. 1H NMR, FT-IR, ESI-MS and X-ray single crystal structure analysis supported the predicted structure of the polyoxometalate hybrid. The characteristic redox properties of the ferrocene group was found to be maintained in the polyoxometalate, as demonstrated by cyclic voltammetry. The interactions between complexes and between ferrocene and polyoxometalate were confirmed through EPR and XPS measurements. The long alkyl chain surfactant cations covered the ferrocene grafted polyoxometalate giving the complex moderate amphiphilicity, forming spherical self-assemblies. The obtained spherical assemblies underwent structural and morphologic transformations upon the oxidization of inorganic oxidizer Ce(IV) and organic oxidizer 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). The state oxidized by DDQ could be reduced reversibly upon addition of hydrazine, leading to a redox cycle. The reversible control for the assembled structure was checked by the alternate layer spacing and morphologic transformation cycles according to XRD and SEM characterizations.

Zhendong Yin

Papers

Smart self-assemblies based on a surfactant-encapsulated photoresponsive polyoxometalate complex

An electroswitchable fluorescence thin-film based on a luminescent polyoxometalate cluster.

Functionalized BODIPY with various sensory units – a versatile colorimetric and luminescent probe for pH and ions

Synthesis and redox-responsive self-assembly of ferrocene grafted Anderson-type polyoxometalate hybrid complexes