This review provides a thorough overview of composites with molecular catalysts (polyoxometalates, or organometallic or coordination complexes) immobilised into MOFs via non-covalent interactions.

A unique polyoxometalate complex made up of a tetradecanuclear nickel bisphosphonate cluster capping a {SiW₉} unit has been characterized. This stable compound exhibits a high hydrogen evolution reaction photocatalytic activity under visible light irradiation via a reductive quenching mechanism.

In the reported spiropyran/polyoxometalate/BODIPY triad, the fluorescence of the BODIPY can be highly reversibly quenched by the activation of the photochromic spiropyran.

The first example of ratiometric luminescent thermometers based on lanthanide molecular species encapsulated in a lanthanide metal–organic framework (MOF) is reported. In EuW₁₀@Tb-TATB, the [EuW₁₀O₃₆]⁹⁻ polyoxometalate (POM) is incorporated into the cavities of the mesoporous terbium MOF Tb-TATB with two different POM loadings (9.1 and 19.5 wt%). Eu³⁺ and Tb³⁺ ions of the POM and MOF, respectively, act as dual emitters. Optical measurements evidence efficient Tb³⁺-to-Eu³⁺ energy transfer, suggesting that the EuW₁₀ units are close to terbium centers. These observations are supported by computational investigations whereby a favored localisation of the EuW₁₀ POM in the MOF's pores is found close to Tb-centers evidencing hydrogen bond type interactions between terminal oxygen atoms of the POM and Tb-coordinated water molecules. The reported materials act as temperature sensors in the physiological domain, exhibiting high relative thermal sensitivities of 2.68% K⁻¹ and 2.37% K⁻¹ at 300 K for the 9.1 wt% and 19.5 wt% composites, respectively, with a thermal uncertainty of 0.09 K.

The assembly of two distinct organic photochromic groups and polyoxometalate (POM) within a single molecule has been realized. In the hetero-functionalized Anderson polyanion MnMo₆O₁₈(SP)(SN), one spiropyran and one spironaphthoxazine cap the two faces of a planar {Mn^IIIMo₆ ^VIO₁₈} fragment. A related symmetrical POM where the {Mn^IIIMo₆ ^VIO₁₈} inorganic moiety is connected to two spironaphthoxazines has also been obtained, and isolated as an organoammonium (MnMo₆O₁₈(SN)₂) or a cationic spiropyran ((SP)₃MnMo₆O₁₈(SN)₂) salt. The structures of MnMo₆O₁₈(SN)₂ and MnMo₆O₁₈(SP)(SN) have been solved by single-crystal X-ray diffraction analysis. In solution, both compounds exhibit electrochromic properties with multiple redox coloured states. The solid-state photochromism of all the compounds has also been extensively studied at room temperature. Contrary to the organic spironaphtoxazine derivative SNTris which is very poorly photochromic, the pale yellow colour of MnMo₆O₁₈(SN)₂ shifts to deep blue with high coloration contrast which underlines the positive effect of the grafting onto the inorganic platform. In addition, MnMo₆O₁₈(SN)₂ also shows good cyclability and exhibits a fast bleaching process. Finally, the temperature dependence of the solid-state photochromic properties of MnMo₆O₁₈(SN)₂ is also reported and compared to that of the related MnMo₆O₁₈(SP)₂. The photo-coloration process which is attenuated at low temperatures in both cases is clearly more affected for MnMo₆O₁₈(SN)₂. Moreover, the thermal fading process becomes significant at 340 K for MnMo₆O₁₈(SP)₂ and 260 K for MnMo₆O₁₈(SN)₂ underlining strong different optical behaviours between both materials.

A series of three new polyoxometalate/spiromolecule compounds – including the (TBA)₃[AlMo₆-SN] (SN = spironaphthoxazine) and (TBA)₃[AlMo₆-SP] (SP = spiropyran) Anderson-type polyoxometalates (POMs) and the unprecedented (TBA)₅[P₂W₁₅V₃-SP] Dawson-type species – has been synthesized and characterized using a full panel of techniques. These hybrids have been thoroughly investigated by (spectro)electrochemical techniques, allowing not only to extract the electrochemical data characterizing these POMs but also to deeply investigate the redox mechanisms involved, including the dimerization process occuring during the oxidation step. The solid-state photochromic properties of these species have been fully studied and the photochromic kinetic parameters determined. It revealed that while in (TBA)₅[P₂W₁₅V₃-SP] the merocyanin form is highly stabilized, (TBA)₃[AlMo₆-SN] represents highly robust optical switch. Importantly, the room temperature solid-state luminescent properties of such assemblies have been investigated for the first time. It has been evidenced that (TBA)₃[AlMo₆-SP] exhibits a strong red emission under UV irradiation, while the luminescence of the related (TBA)₃[MnMo₆-SP] is weaker and that (TBA)₅[P₂W₁₅V₃-SP] does not exhibit luminescent properties. These emission intensity differences have been interpreted in terms of differences in photoinduced electron transfers from the excited state of the merocyanine isomer to the inorganic units. A correlation between its efficiency and the electron acceptor ability of the POMs determined from electronic absorption spectroscopic measurement and electrochemical data has been proposed.