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

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 HDMBI 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.