Co2+-Linked [NaP5W30O110]14−: A Redox-Active Metal Oxide Framework with High Electron Density

Enhanced Electrical Conductivity of Polyoxometalates by Bridging with Mixed-Valent Multinuclear Platinum Complexes

Polyoxometalates (POMs) are nanosized molecular metal oxide anion clusters with tuneable structures and functionalities, and they exhibit a redox chemistry and catalytic activity in multielectron redox processes. These are typically poor electrical conductors (<10-10 Scm-1), which is attributed to negligible electronic interactions among anions in the solid state. Since the reduced electrons on the d0 metals in POMs are delocalized, electrical conductivity was improved when judicious pathways for the electrons were created by bridging the POMs. Utilized with the electronic interactions between bridging oxygen atoms with the highest occupied molecular orbital in the POMs and the metal dz2 orbitals in the multinuclear platinum complexes, and three mixed-valent assemblies were synthesized and characterized. Simply mixing Keggin-type or Dawson-type POMs with tetranuclear or trinuclear platinum complexes in solution afforded three single crystals, and all three compounds were paramagnetic with mixed oxidation states and better conductivities at room temperature than the parent compounds.

A hydrogen radical pathway for efficacious electrochemical nitrate reduction to ammonia over an Fe-polyoxometalate/Cu electrocatalyst

Electrochemical nitrate (NO3-) reduction to ammonia (NH3), which is a high value-added chemical or high-energy density carrier in many applications, could become a key process overcoming the disadvantages of the Haber-Bosch process; however, current electrocatalysts have severe drawbacks in terms of activity, selectivity, and stability. Here, we report the hydrogen radical (H*) pathway as a solution to overcome this challenge, as demonstrated by efficacious electrochemical NO3- reduction to NH3 over the Fe-polyoxometalate (Fe-POM)/Cu hybrid electrocatalyst. Fe-POM, composed of Preyssler anions ([NaP5W30O110]14-) and Fe cations, facilitates efficient H* generation via H2O + e- → H* + OH-, and H* transfer to the Cu sites of the Fe-POM/Cu catalyst enables selective NO3- reduction to NH3. Operando spectroelectrochemical spectra substantiate the occurrence of the H* pathway through direct observation of Fe redox related to H* generation and Cu redox related to NO3- binding. With the H* pathway, the Fe-POM/Cu electrodes exhibit high activity for NO3- reduction to NH3 with 1.44 mg cm-2 h-1 in a 500 ppm NO3-/1 M KOH solution at -0.2 V vs. RHE, which is about 36-fold higher than that of the pristine Cu electrocatalyst. Additionally, it attains high selectivity with a faradaic efficiency of up to 97.09% at -0.2 V vs. RHE while exhibiting high catalytic stability over cycles.

Metal-metal bonded pentamolybdate hybrids as electron storage materials

Two electron-rich, metal-metal bonded pentamolybdate hybrids, 2D-[MoIV3MoVI2O10Sr2(H2O)5(C6H4O7)2py3]·3.5H2O (1) and 1D-[MoIV3MoVI2O10Sr(H2O)3(C6H4O7)2py3]·py·2[NH2(CH3)2]·2H2O (2, py = pyridine), were prepared by the partial solvothermal oxidation of [MoIV3O2(O2CCH3)6(H2O)3]ZnCl4·8H2O and citric acid in py/H2O (for 1) or py/H2O/DMF (for 2). Both 1 and 2 feature a triangularly metal-metal bonded incomplete cuboidal [MoIV3O4] unit. Redox-active 6e-[MoIV3O4] units can serve as an "electron sponge" to store/release six electrons reversibly via Δ-bond breakage and re-formation during charging/discharging processes. 1 and 2 further form 3D and 2D supramolecular structures, respectively, through slipped π-π stacking interactions between the pyridine ligands. Both the incorporated 6e-redox active [MoIV3O4] unit and the 3D/2D supramolecular conductive networks in hybrid-POM 1/2 remarkably enhance the electronic conductivity and reversible multi-electron redox ability with the structural integrity retained. Consequently, 1 and 2 exhibited high discharge specific capacities of 236.0 and 277.0 mA h g-1 at 50 mA g-1, respectively, and a good cycling performance at high current density (121.8 mA h g-1, 2 A g-1 for 2), providing a new way for improving POM-based electrode materials.

Aqueous solutions of super reduced polyoxotungstates as electron storage systems

Due to the increasing energy density demands of battery technology, it is vital to develop electrolytes with high electron storage capacity. Polyoxometalate (POM) clusters can act as electron sponges, storing and releasing multiple electrons and have potential as electron storage electrolytes for flow batteries. Despite this rational design of clusters for high storage ability can not yet be achieved as little is known about the features influencing storage ability. Here we report that the large POM clusters, {P5W30} and {P8W48}, can store up to 23 e- and 28 e- per cluster in acidic aqueous solution, respectively. Our investigations reveal key structural and speciation factors influencing the improved behaviour of these POMs over those previously reported (P2W18). We show, using NMR and MS, that for these polyoxotungstates hydrolysis equilibria for the different tungstate salts is key to explaining unexpected storage trends while the performance limit for {P5W30} and {P8W48}, can be attributed to unavoidable hydrogen generation, evidenced by GC. NMR spectroscopy, in combination with the MS analysis, provided experimental evidence for a cation/proton exchange process during the reduction/reoxidation process of {P5W30} which likely occurs due to this hydrogen generation. Our study offers a deeper understanding of the factors affecting the electron storage ability of POMs and provides insights allowing for further development of these materials for energy storage.

TiO2-Embedded Biocompatible Hydrogel Production Assisted with Alginate and Polyoxometalate Polyelectrolytes for Photocatalytic Application

The hybrid hydrogel materials meet important social challenges, including the photocatalytic purification of water and bio-medical applications. Here, we demonstrate two scenarios of polyacrylamide-TiO2 (PAAm@TiO2) composite hydrogel design using calcium alginate (Alg-Ca) or Keplerate-type polyoxometalates (POMs) {Mo132} tuning the polymer network structure. Calcium alginate molding allowed us to produce polyacrylamide-based beads with an interpenetrating network filled with TiO2 nanoparticles Alg-Ca@PAAm@TiO2, demonstrating the photocatalytic activity towards the methyl orange dye bleaching. Contrastingly, in the presence of the POM, the biocompatible PAAm@TiO2@Mo132 composite hydrogel was produced through the photo-polymerization approach (under 365 nm UV light) using vitamin B2 as initiator. For both types of the synthesized hydrogels, the thermodynamic compatibility, swelling and photocatalytic behavior were studied. The influence of the hydrogel composition on its structure and the mesh size of its network were evaluated using the Flory–Rehner equation. The proposed synthetic strategies for the composite hydrogel production can be easily scaled up to the industrial manufacturing of the photocatalytic hydrogel beads suitable for the water treatment purposes or the biocompatible hydrogel patch for medical application.

The missing link between zeolites and polyoxometalates

Open framework materials such as zeolites and metalorganic frameworks are garnering tremendous interest because of their intriguing architecture and attractive functionalities. Thus, new types of open framework materials are highly sought after. Here, we present the discovery of completely new inorganic framework materials, where, in contrast to conventional inorganic open frameworks, the scaffold is not based on tetrahedral EO₄ (E = main group element) but octahedral MO₆ (M = transition metal) building blocks. These structural features place them closer to polyoxometalates than zeolites. The first representatives of this class of materials are [(R)₂₄(NH₄)₁₄(PO(OH)₂)₆]·[M₁₃₄(PO₃(OH,F))₉₆F₁₂₀] (M = Co, R = C₂Py = 1-ethylpyridinium and M = Ni, R = C₄C₁Py = 1-butyl-3-methylpyridinium) featuring interlinked fullerene-like nanosphere cavities. Having a transition metal building up the framework brings about interesting properties, for example, spin-glass behavior, and, with this particular topology, a hedgehog-like spin orientation.

PEG-Infiltrated Polyoxometalate Frameworks with Flexible Form-Factors

We present the synthesis of metal oxide frameworks composed of the Preyssler anion, [NaP₅W₃₀O₁₁₀]^14-, bridged with transition-metal cations and infiltrated with polyethylene glycol. The frameworks can be dissolved in water to form freestanding rigid or flexible films or gels. Powder X-ray diffraction shows that all form-factors maintain the short-range order of the original crystals. Raman spectroscopy reveals that, similar to hydrogels, the macroscopic mechanical properties of these composites are dependent on the water content and the extent of hydrogen-bonding within the water network. The understanding gained from these studies facilitates solution-phase processing of polyoxometalate frameworks into flexible form factors.

Redox-controlled conductance of polyoxometalate molecular junctions

We demonstrate the reversible in situ photoreduction of molecular junctions of a phosphomolybdate [PMo₁₂O₄₀]^3- monolayer self-assembled on flat gold electrodes, connected by the tip of a conductive atomic force microscope. The conductance of the one electron reduced [PMo₁₂O₄₀]^4- molecular junction is increased by ∼10, and this open-shell state is stable in the junction in air at room temperature. The analysis of a large current-voltage dataset by unsupervised machine learning and clustering algorithms reveals that the electron transport in the pristine phosphomolybdate junctions leads to symmetric current-voltage curves, controlled by the lowest unoccupied molecular orbital (LUMO) at 0.6-0.7 eV above the Fermi energy with ∼25% of the junctions having a better electronic coupling to the electrodes than the main part of the dataset. This analysis also shows that a small fraction (∼18% of the dataset) of the molecules is already reduced. The UV light in situ photoreduced phosphomolybdate junctions systematically feature slightly asymmetric current-voltage behaviors, which is ascribed to the electron transport mediated by the single occupied molecular orbital (SOMO) nearly at resonance with the Fermi energy of the electrodes and by a closely located single unoccupied molecular orbital (SUMO) at ∼0.3 eV above the SOMO with a weak electronic coupling to the electrodes (∼50% of the dataset) or at ∼0.4 eV but with a better electrode coupling (∼50% of the dataset). These results shed light on the electronic properties of reversible switchable redox polyoxometalates, a key point for potential applications in nanoelectronic devices.

Decomposition-Reassembly Synthesis of a Silverton-Type Polyoxometalate 3D Framework: Semiconducting Properties and Photocatalytic Applications

Polyoxometalate-based all-inorganic three-dimensional (3D) frameworks have recently attracted attention as a unique class of materials due to their unique physicochemical properties and a wide field of application with excellent prospects. We herein synthesized a novel all-inorganic 3D framework material based on cobalt-substituted Silverton-type polyoxometalate, H₆{Co₆W₁₀O₄₂[Co(H₂O)₄]₃}·2H₂O (Co₉W₁₀), which was successfully constructed using Na₁₂[WCo₃^II(H₂O)₂(Co^IIW₉O₃₄)₂]·46-48H₂O (Co₅W₁₉) and Co(NO₃)₂·6H₂O as starting materials in a hydrothermal reaction via a decomposition-reassembly route together with the rational adjustment of pH values. Co₉W₁₀ has been structurally characterized using single-crystal X-ray diffraction. Photocurrent response, band-gap (E_g) value, and the VB-XPS spectrum have been measured to reveal the semiconducting property of Co₉W₁₀. Furthermore, we synthesized x% PTh/Co₉W₁₀ composites (PTh = polythiophene, x = 0.5, 1, 2, 5) for photodegradation of tetracycline hydrochloride (TH) to evaluate the photocatalytic activities of title composites. Due to the optimal molar ratio of hybrids and matching energy levels, 2% PTh/Co₉W₁₀ composites show the best photocatalytic activities among these composites.

Supramolecular assemblies of organo-functionalised hybrid polyoxometalates: from functional building blocks to hierarchical nanomaterials

This review provides a comprehensive overview of recent advances in the supramolecular organisation and hierarchical self-assembly of organo-functionalised hybrid polyoxometalates (hereafter referred to as hybrid POMs), and their emerging role as multi-functional building blocks in the construction of new nanomaterials. Polyoxometalates have long been studied as a fascinating outgrowth of traditional metal-oxide chemistry, where the unusual position they occupy between individual metal oxoanions and solid-state bulk oxides imbues them with a range of attractive properties (e.g. solubility, high structural modularity and tuneable properties/reactivity). Specifically, the capacity for POMs to be covalently coupled to an effectively limitless range of organic moieties has opened exciting new avenues in their rational design, while the combination of distinct organic and inorganic components facilitates the formation of complex molecular architectures and the emergence of new, unique functionalities. Here, we present a detailed discussion of the design opportunities afforded by hybrid POMs, where fine control over their size, topology and their covalent and non-covalent interactions with a range of other species and/or substrates makes them ideal building blocks in the assembly of a broad range of supramolecular hybrid nanomaterials. We review both direct self-assembly approaches (encompassing both solution and solid-state approaches) and the non-covalent interactions of hybrid POMs with a range of suitable substrates (including cavitands, carbon nanotubes and biological systems), while giving key consideration to the underlying driving forces in each case. Ultimately, this review aims to demonstrate the enormous potential that the rational assembly of hybrid POM clusters shows for the development of next-generation nanomaterials with applications in areas as diverse as catalysis, energy-storage and molecular biology, while providing our perspective on where the next major developments in the field may emerge.

Photochemical reduction of nanocrystalline maghemite to magnetite

We present a method for thephotochemical conversion of the inverse spinel iron oxides in which the mixed-valent magnetite phase (Fe₃O₄) is accessed from the maghemite phase (γ-Fe₂O₃) via a stable, colloidal nanocrystal-to-nanocrystal transformation. Anaerobic UV-irradiation of colloidal γ-Fe₂O₃ nanocrystals in the presence of ethanol as a sacrificial reductant yields reduction of some Fe³⁺ to Fe²⁺, resulting in a topotactic reduction of γ-Fe₂O₃ to Fe₃O₄. This reduction is evidenced by the emergence of charge-transfer absorption and increased d-spacing in UV-irradiated nanocrystals. Redox titrations reveal that ∼43% of Fe in <d> = 4.8 nm nanocrystals can be reduced with this method and comparison of optical data indicates similar reduction levels in <d> = 7.3 and 9.0 nm nanocrystals. Addition of excess acetaldehyde during photoreduction shows that the extent of reduction is likely pinned by the hydrogenation of acetaldehyde back to ethanol and can be increased with the use of an alkylborohydride sacrificial reductant. Photochemical reduction is accompanied by increased magnetization and emergence of magnetic features characteristic of Fe₃O₄. Overall, this work provides a reversible, post-synthetic strategy to obtain Fe₃O₄ nanocrystals with well-controlled Fe²⁺ compositions.

Exploring the Coordination Modes of a Keggin-Type [ZnW12O40]6- Anionic Cluster: Bonding Patterns, Crystal Structure, and Semiconducting Properties

Polyoxometalate-based organic-inorganic hybrid compounds (POIHCs) have been greatly developed due to their wide application prospects, but the pursuit of their directed synthesis via molecular design still remains a challenge. Herein, we demonstrate that the coordination modes of the Keggin-type [ZnW₁₂O₄₀]^6- anion can be tuned, which leads to different semiconductor characteristics. Using the same building block, ligand, and metal ion (ZnW₁₂, phen, Cu²⁺), we synthesized three new POIHCs with different bonding patterns by means of different coordination modes of ZnW₁₂. The three POIHCs (H₂phen){ZnW₁₂O₄₀[Cu(phen)₂]₂}·3H₂O (1), {ZnW₁₂O₄₀[Cu(phen)(H₂O)₂]₂[Cu(phen)(H₂O)]}_n·3H₂O (2), and (Me₄N)₂{ZnW₁₂O₄₀[Cu(phen)(H₂O)]₂}_n·5H₂O (3) (phen = 1,10-phenanthroline) have been structurally characterized by single-crystal X-ray diffraction. Compound 1 appears as a zero-dimensional coordination complex cluster, while compounds 2 and 3 are both 1D chain structures with different Cu²⁺ bridge linkages. Although these three POIHCs possess the same chemical components, their semiconductor properties are different, which is demonstrated by measurements of transient photocurrent and band gap (E_g) values. Furthermore, we carried out comparative experiments on the photoconductivity performance of compounds 1-3 and their photocatalytic reduction from O₂ to H₂O₂, indicating the significant influence of the energy level matching on the photocatalytic activity.

Keggin-type polyoxometalate-containing metal–organic hybrids as friction materials for triboelectric nanogenerators

The POM-based inorganic–organic hybrids with different structures were assembled and used as the friction materials to construct TENGs and the results demonstrated that the output performance was closely related to the dielectric constant.

Directional Bonding in Decaniobate Inorganic Frameworks

Metal-oxo clusters offer an opportunity to assemble inorganic and metal-organic frameworks (MOFs) by a controlled building-block approach, which led to the revolutionary discoveries of zeolites and MOFs. Polyoxometalate clusters are soluble in water, but more challenging to link into frameworks; the inert oxo-caps that provide solubility are resistant to replacement or further connectivity. We demonstrate how the unique directional bonding and varying basicity of the decaniobate ([Nb₁₀ ]) oxo-caps can be exploited to build 1D, 2D, and 3D inorganic frameworks. In nine structures, A⁺ (A=Li, Na, K, Rb and Cs), AE²⁺ (AE=Ca, Sr, Ba) and Mn²⁺ demonstrate that the dimensionality of the obtained material is controlled by cation charge and size. Increased cation charge decreases selectivity for oxo-site bonding, leading to higher dimensional linking. Larger cation radii also decreases bonding selectivity, yielding higher dimensional materials. Ion-exchange studies of the A⁺ -Nb₁₀ family shows exclusive selectivity for Cs⁺ over other alkalis, which is important for radioactive Cs removal and sequestration.

Cation-Controlled Assembly of Polyoxotungstate-Based Coordination Networks

The Preyssler polyoxoanion, [NaP₅ W₃₀ O₁₁₀ ]^14- ({P₅ W₃₀ }), is used as a platform for evaluating the role of nonbridging cations in the formation of transition-metal-bridged polyoxometalate (POM) coordination frameworks. Specifically, the assembly architecture of Co²⁺ -bridged frameworks is shown to be dependent on the identity and amount of alkali or alkaline-earth cations present during crystallization. The inclusion of Li⁺ , Na⁺ , K⁺ , Mg²⁺ , or Ca²⁺ in the framework synthesis is used to selectively synthesize five different Co²⁺ -bridged {P₅ W₃₀ } structures. The influence of the competition between K⁺ and Co²⁺ for binding to {P₅ W₃₀ } in dictating framework assembly is evaluated. The role of ion pairing on framework assembly structure and available void volume is discussed. Overall, these results provide insight into factors governing the ability to achieve controlled assembly of POM-based coordination networks.

Hierarchical nanosheets built from superatomic clusters: properties, exfoliation and single-crystal-to-single-crystal intercalation

Tuning the properties of atomic crystals in the two-dimensional (2D) limit is synthetically challenging, but critical to unlock their potential in fundamental research and nanotechnology alike. 2D crystals assembled using superatomic blocks could provide a route to encrypt desirable functionality, yet strategies to link the inorganic blocks together in predetermined dimensionality or symmetry are scarce. Here, we describe the synthesis of anisotropic van der Waals crystalline frameworks using the designer superatomic nanocluster Co3(py)3Co6Se8L6 (py = pyridine, L = Ph2PN(Tol)), and ditopic linkers. Post-synthetically, the 3D crystals can be mechanically exfoliated into ultrathin flakes (8 to 60 nm), or intercalated with the redox-active guest tetracyanoethylene in a single-crystal-to-single-crystal transformation. Extensive characterization, including by single crystal X-ray diffraction, reveals how intrinsic features of the nanocluster, such as its structure, chirality, redox-activity and magnetic profile, predetermine key properties of the emerging 2D structures. Within the nanosheets, the strict and unusual stereoselectivity of the nanocluster's Co edges for the low symmetry (α,α,β) isomer gives rise to in-plane structural anisotropy, while the helically chiral nanoclusters self-organize into alternating Δ- and Λ-homochiral rows. The nanocluster's high-spin Co edges, and its rich redox profile make the nanosheets both magnetically and electrochemically active, as revealed by solid state magnetic and cyclic voltammetry studies. The length and flexibility of the ditopic linker was varied, and found to have a secondary effect on the structure and stacking of the nanosheets within the 3D crystals. With these results we introduce a deterministic and versatile synthetic entry to programmable functionality and symmetry in 2D superatomic crystals.

Self-Assembly and Ionic-Lattice-like Secondary Structure of a Flexible Linear Polymer of Highly Charged Inorganic Building Blocks

Among molecular building blocks, metal oxide cluster anions and their countercations provide multiple options for the self-assembly of functional materials. Currently, however, rational design concepts are limited to electrostatic interactions with metal or organic countercations or to the attachment and subsequent reactions of functionalized organic ligands. We now demonstrate that bridging μ-oxo linkages can be used to string together a bifunctional Keggin anion building block, [PNb₂Mo₁₀O₄₀]^5- (1), the diniobium(V) analogue of [PV₂Mo₁₀O₄₀]^5- (2). Induction of μ-oxo ligation between the Nb^V═O moieties of 1 in acetonitrile via step-growth polymerization gives linear polymers with entirely inorganic backbones, some comprising over 140 000 repeating units, each with a 3- charge, exceeding that of previously reported organic or inorganic polyelectrolytes. As the chain grows, its flexible μ-oxo-linked backbone, with associated countercations, coils into a compact 270 nm diameter spherical secondary structure as a result of electrostatic interactions not unlike those within ionic lattices. More generally, the findings point to new options for the rational design of multidimensional structures based on μ-oxo linkages between Nb^V═O-functionalized building blocks.

Redox-inactive ions control the redox-activity of molecular vanadium oxides

Polyoxometalates are key materials for energy conversion and storage due to their unique chemical tunability and electrochemical reactivity. Herein, we report that functionalization of molecular vanadium oxides, polyoxovanadates, with redox-inert Ca2+ cations leads to a significant increase in their electron storage capabilities. The electrochemical performance of the Ca2+-functionalized dodecavanadate [Ca2V12O32Cl(DMF)3]2- (={Ca 2 V 12 }) was thus compared with that of the precursor compound (H2NMe2)2[V12O32Cl]3- (={V 12 }). {Ca 2 V 12 } can store up to five electrons per cluster, while {V 12 } only shows one reversible redox transition. In initial studies, we demonstrated that {Ca 2 V 12 } can be used as an active material in lithium-ion cathodes. Our results show how redox-inert cations can be used as structural and electrostatic stabilizers, leading to major changes in the redox-chemistry of polyoxovanadates.

Two new estertin modified tungstosilicates: synthesis, catalytic activity and photoelectrochemical property

Two estertin functionalized tungstosilicates were synthesized and analyzed by DFT. They exhibit good photoelectrocatalytic performance for oxidation of methanol.

Tunable Metal Oxide Frameworks via Coordination Assembly of Preyssler-Type Molecular Clusters

We present the synthesis of metal oxide frameworks composed of [NaP₅W₃₀O₁₁₀]^14- assembled with Mn, Fe, Co, Ni, Cu, or Zn bridging metal ions. X-ray diffraction shows that the frameworks adopt the same assembly regardless of bridging metal ion. Furthermore, our synthesis allows for the assembly of isostructural frameworks with mixed-metal ion bridges, or with clusters that have been doped with Mo, providing a high degree of compositional diversity. This consistent assembly enables investigation into the role of the building blocks in the properties of the metal oxide frameworks. The presence of bridging metal ions leads to increased conductivity compared to unbridged frameworks, and frameworks bridged with Fe have the highest conductivity. Additionally, Mo-doping can be used to enhance the conductivities of the frameworks. Similar structures can be obtained from clusters in which the central Na⁺ has been replaced with Bi³⁺ or Sm³⁺. Overall, the optical and electronic properties are tunable via choice of bridging metal ion and cluster building block and reveal emergent properties in these cluster-based frameworks. These results demonstrate the promise of using polyoxometalate clusters as building blocks for tunable complex metal oxide materials with emergent properties.

Polyoxometalate-Supported Aminocatalyst for the Photocatalytic Direct Synthesis of Imines from Alkenes and Amines

Developing efficient photocatalysts for direct oxidative coupling of alkenes and amines with O₂ under mild conditions is very significant. Herein, ZnW-PYI is well-designed by assembling a [PZnW₁₁O₃₉(H₂O)]^5- photooxidation catalyst and chiral aminocatalyst pyrrolidine-2-ylimidazole (PYI) via a coordination model. ZnW-PYI efficiently catalyzed the synthesis of imines from alkenes and amines using O₂ as the oxidant through nucleophilic catalysis by employing pyrrolidine as an organocatalyst. Combining a polyoxometalate and PYI within one single framework is an effective approach not only for stabilization and heterogenization of the redox-active catalyst and aminocatalyst but also for realization of compatibility between the reaction intermediates and synergy of multiple catalytic cycles.

Proton coupled electron transfer: novel photochromic performance in a host–guest collaborative MOF

Proton coupled electron transfer has been successfully introduced to a host–guest collaborative MOF material, which exhibits novel photochromic properties with reversible, controllable and efficient characteristics.