Orthogonal self-assembly of a trigonal triptycene triacid: signaling of exfoliation of porous 2D metal-organic layers by fluorescence and selective CO₂ capture by the hydrogen-bonded MOF

Trigonal 3-connecting imidazole-annulated triptycene triacid (H₃TPA) is a molecular module that is programmed for orthogonal self-assembly. Upon treatment with metal salts such as CoCl₂, Mn(NO₃)₂, Zn(NO₃)₂ and Cd(NO₃)₂, highly porous isostructural MOFs are obtained in which the TPA linker undergoes metal-ligand coordination and hydrogen bonding through carboxylate groups and imidazole moieties, respectively. The MOFs are constructed by hydrogen bond-mediated offset stacking of porous (3,3) honeycomb layers formed by the self-assembly of TPA with 3-connecting triangular bimetallic SBUs. We show that the interlayer hydrogen bonds can be disrupted by solvents such as DMSO, leading to solvent-induced delamination of 2D metal-organic nanosheets. The delamination is signaled by turn-on of fluorescence, which is suppressed in the bulk state. Indeed, the extent of exfoliation with different solvents-as reflected from fluorescence quantum yields as well as solvent-induced shifts in emission maxima-can be nicely correlated with Gutmann's solvent DN numbers, which are a measure of the ability of solvents to accept hydrogens in hydrogen bonds. The results demonstrate how the bulk materials with layered structures can be (i) engineered in a 'bottom-up' approach by orthogonal self-assembly of an organic linker created by de novo design and (ii) in turn be exfoliated in a 'top-down' approach by solvent-induced ultrasonication. As bulk materials, the hydrogen-bonded MOFs lend themselves to selective as well as high adsorption of CO₂ under ambient conditions as a result of the nitrogenous environment of pores conferred by the benzimidazole moieties.