Mechanoluminescence and aggregation-enhanced emission (AEE) of an In-MOF based on a 9,9'-diphenyl-9H-fluorene tetraacid linker

Recent Advances in Mechanochromism of Metal-Organic Compounds

Smart luminescent materials, which can respond to the changing of external environment (light, electricity, force, temperature, etc.), have always been one of the research hotspots. Mechanochromism refers to the materials whose emission color or intensity can be altered under the stimulation of external mechanical force. This kind of smart materials have been widely used in data storage, information encryption and sensors due to its simple operation, obvious and rapid response. The introduction of metal atoms in metal-organic compounds brings about fascinating metalophilic interactions and results in more interesting and surprising mechanochromic behaviors. In this mini-review, recent advances in mechanochromism of metal-organic compounds, including mono-, di-, multinuclear metal-organic complexes and metallic clusters are summarized. Varies mechanisms are discussed and some design strategies for metal-organic compounds with mechanochromism are also presented.

Zwitterionic Luminescent 2D Metal-Organic Framework Nanosheets (LMONs): Selective Turn-On Fluorescence Sensing of Dihydrogen Phosphate

While a plethora of organic linkers based on carboxylic acids have been utilized in the construction of MOFs, zwitterionic linkers that typify the attributes of naturally occurring amino acids have been exploited only scarcely to the best of our knowledge. Zwitterionic interior characteristics should be expected to impart unique properties to the resultant MOFs with a high potential to interact with guest species through electrostatic interactions. In our investigations with bis(p-carboxyphenyl)imidazolylarenes as a novel class of linkers for the development of functional MOFs, we have found that bisimidazole-tetracarboxylic acid H₄DMBI undergoes metal-assisted self-assembly with Zn(NO₃)₂ to yield a layered MOF (Zn-DMBI). In the latter, the linker serves as a two-connecting linker with imidazoles and carboxylic acids behaving as zwitterions. The layers are offset stacked in the crystal structure and are bound firmly by hydrogen bonds between imidazolium and carboxylate ions. Such a packing precludes fluorescence from being observed due to self-quenching. However, exfoliation into zwitterionic 2D metal-organic nanosheets (MONs) by sonication in methanol for 1 h liberates palpable fluorescence. Furthermore, the suspension of luminescent MONs (LMONs) in methanol permits selective sensing of anions; in particular, dihydrogen phosphate (H₂PO₄^-) that is complementary to the zwitterions in terms of hydrogen bond donor and acceptor sites is observed with fluorescence enhancement by 120%, leading to its detection at a sub-parts-per-million (0.13 ppm) level. Thus, access to zwitterionic 2D MONs and their application for selective anion sensing with "turn-on" fluorescence are demonstrated by a rational de novo bottom-up approach.

Photochromism and photoswitchable luminescence in a Zn7 cluster-based metal-organic framework with an organic guest

Photochromic materials coupled with photoswitchable luminescence functionalities are of particular interest due to their potential applications in switches and optical memory devices. However, the construction of such materials, especially those with two-color emission states, is still challenging. In this context, a rare Zn₇ cluster-based host-guest MOF material, (bbmp)[Zn₇(IPA)₆(OH)₄(H₂O)₂] (1) (H₂IPA = isophthalic acid, bbmp·2I = 4,4'-([1,1'-biphenyl]-4,4'-diyl)bis(1-methylpyridin-1-ium) diiodide), was prepared by encapsulating an organic cation into an anionic MOF produced from zinc cations and isophthalic acid ligands, which exhibits reversible naked detectable photochromic properties varying from yellow to green upon UV-Vis light irradiation. The photoactive guest bbmp²⁺ and the short O⋯N⁺ distances between the oxygen atoms of the carboxylate groups and the pyridine ring play a crucial role in the photochromism of this compound. More interestingly, the luminescence color of this cluster-based host-guest material can be reversibly switched from green to blue upon irradiation, exhibiting photoswitchable luminescence properties.