Silver(i) chalcogenide/chalcogenolate clusters are promising photofunctional materials for sensing, optoelectronics and solar energy harvesting applications. However, their instability and poor room-temperature luminescent quantum yields have hampered more extensive study. Here, we graft such clusters to adaptable bridging ligands, enabling their interconnection and the formation of rigid metal-organic frameworks. By controlling the spatial separation and orientation of the clusters, they then exhibit enhanced stability (over one year) and quantum yield (12.1%). Ultrafast dual-function fluorescence switching (<1 s) is also achieved, with turn-off triggered by O and multicoloured turn-on by volatile organic compounds. Single-crystal X-ray diffraction of the inclusion materials, obtained by single-crystal-to-single-crystal transformation, enables precise determination of the position of the small molecules within the framework, elucidating the switching mechanism. The work enriches the cluster-based metal-organic framework portfolio, bridges the gap between silver chalcogenide/chalcogenolate clusters and metal-organic frameworks, and provides a foundation for further development of functional silver-cluster-based materials.

Metal-organic frameworks (MOFs) have recently emerged as a type of uniformly and periodically atom-distributed precursor and efficient self-sacrificial template to fabricate hierarchical porous-carbon-related nanostructured functional materials. For the first time, a Cu-based MOF, i.e., Cu-NPMOF is used, whose linkers contain nitrogen and phosphorus heteroatoms, as a single precursor and template to prepare novel Cu P nanoparticles (NPs) coated by a N,P-codoped carbon shell that is extended to a hierarchical porous carbon matrix with identical uniform N and P doping (termed Cu P@NPPC) as an electrocatalyst. Cu P@NPPC demonstrates outstanding activity for both the hydrogen evolution and oxygen reduction reaction, representing the first example of a Cu P-based bifunctional catalyst for energy-conversion reactions. The high performances are ascribed to the high specific surface area, the synergistic effects of the Cu P NPs with intrinsic activity, the protection of the carbon shell, and the hierarchical porous carbon matrix doped by multiheteroatoms. This strategy of using a diverse MOF as a structural and compositional material to create a new multifunctional composite/hybrid may expand the opportunities to explore highly efficient and robust non-noble-metal catalysts for energy-conversion reactions.

Recently, research on metal-organic frameworks (MOFs) serving as a new type of proton conductive material has resulted in many exciting achievements. However, direct observation of a well-established proton-transfer mechanism still remains challenging in MOFs and other crystalline compounds, let alone other conductive materials. Herein we report the solvothermal synthesis of a new proton-conducting MOF, (MeNH)[Eu(L)] (HL = 5-(phosphonomethyl)isophthalic acid). The compound consists of a layered anionic framework [Eu(L)] and interlayer-embedded counter cations (MeNH), which interact with adjacent uncoordinated O atoms of phosphonate groups to form strongly (N-H···O) hydrogen-bonded chains aligned parallel to the c-axis. Facile proton transfer along these chains endows the compound with single-crystal anhydrous conductivity of 1.25 × 10 S·cm at 150 °C, and water-assisted proton conductivity for a compacted pellet of microcrystalline crystals attains 3.76 × 10 S·cm at 100 °C and 98% relative humidity (RH). Proton dynamics (vibrating and transfer) within N-H···O chains of the compound are directly observed using a combination of anisotropic conductivity measurements and control experiments using large single-crystals and pelletized samples, in situ variable-temperature characterization techniques including powder X-ray diffraction (PXRD), single-crystal X-ray diffraction (SCXRD), diffuse reflectance infrared Fourier transform spectrum (DRIFTS), and variable-temperature photoluminescence. In particular, a scarce single-crystal to single-crystal (SCSC) transformation accompanied by proton transfer between an anionic structure (MeNH)[Eu(L)] and an identical neutral framework [Eu(HL)] has been identified.

The tunable inorganic nodes and modifiable organic linkers enable the designable functionality to come true in the new-type porous hybrid materials, namely metal-organic frameworks (MOFs). By using viologen-functionalized mbenzenedicarboxylate as organic linker, a terbium-organic framework embedded with asymmetric viologen species has been constructed. The reversible photochromism from bright yellow to dark green is implemented due to favorable spatial stack of the electron donor and acceptor. The photochromic component of the organic linkers can modulate the luminescence of tetranuclear terbium cluster under irradiation of UV light, which is defined as photoluminescence switch behavior. The different protonation levels of uncoordinated N atoms with varying pH enable the MOF as a potential fluorescent pH sensor. Thus, multi-photofunctionality, viz. photoluminescence, photochromism as well as the derivate fluorescent response to irradiation and pH have been combined in the Tb-MOF, which is the first example in the viologen-based photochromic hybrid materials.

This tutorial review focuses on the modification and assembly of atomically-precise silver clusters by changing shell layers for more stability and functionalities, especially for brighter luminescence.