Govardhan Savitha

Carbon Dioxide Capture by a Metal–Organic Framework with Nitrogen-Rich Channels Based on Rationally Designed Triazole-Functionalized Tetraacid Organic Linker

A semirigid tetraacid linker H4L functionalized with 1,2,3-triazole was rationally designed and synthesized to access nitrogen-rich MOFs for selective adsorption of CO2. The cadmium MOF, that is, Cd-L, obtained by the reaction of H4L with Cd(NO3)2, is found to be a 3D porous framework structure that is robust to desolvation. Crystal structure analysis reveals channels that are decorated by the triazole moieties of L. Gas adsorption studies show that Cd-L MOF permits remarkable CO2 uptake to the extent of 99 and 1000 cc/g at 1 and 30 bar, respectively, at 0 °C. While literature survey reveals that MIL-112, constructed from a 1,2,3-triazole functionalized linker, exhibits no porosity to gas adsorption due to structural flexibility, the results with Cd-L MOF described herein emphasize how rigidification of the organic linker improves gas uptake properties of the resultant MOF.

Diverse isostructural MOFs by postsynthetic metal node metathesis: anionic-to-cationic framework conversion, luminescence and separation of dyes

An extremely flexible and robust anionic Cd–MOF – accessed with a tetraacid linker, viz., 3,3′,5,5′-tetrakis(p-carboxyphenyl)-2,2′,6,6′-tetramethoxy-1,1′-biphenyl – is shown to exhibit abundant postsynthetic metal exchange (PSME) of metal ion nodes with a number of metal ions that differ vastly in terms of their charges, ionic radii and chemical nature; as many as 16 isostructural new MOFs with transition, lanthanide as well as main group metal ions have been characterized with single crystal X-ray structures determined for 11 of them. The PSME allows unique conversion of the anionic framework structure of the Cd–MOF into a cationic one, when the divalent cadmium metal ions are exchanged by trivalent Ln(III) ions. The Eu@MOF and Tb@MOF, accessed by PSME of a transition metal MOF, are shown to be brilliantly luminescent. The switch-over in the charge characteristics of the framework with PSME has been judiciously exploited for the separation of organic dyes based on their charges. The comprehensive investigations illustrate the broad scope of PSME to access new functional materials that cannot be readily accessed.

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 Gutmanns 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.

A new class of three dimensional D–π–A trigonal cryptand derivatives for second-order nonlinear optics

Synthesis, crystal structures, linear and nonlinear optical properties of tris D–π–A cryptand derivatives with C3 symmetry are reported. Three fold symmetry inherent in the cryptand molecules has been utilized for designing these molecules. Molecular nonlinearities have been measured by hyper-Rayleigh scattering (HRS) experiments. Among the compounds studied, L1 adopts non-centrosymmetric crystal structure. Compounds L1, L2, L3 and L4 show a measurable SHG powder signal. These molecules are more isotropic and have significantly higher melting points than the classical p-nitroaniline based dipolar NLO compounds, making them useful for further device applications. Besides, different acceptor groups can be attached to the cryptand molecules to modulate their NLO properties.

The first D–π–A octupolar cryptand molecule to exhibit bulk non-linearity

The cryptand derivative has H-bond mediated trigonal network structure that leads to octupolar bulk nonlinearity.