Pratap Vishnoi

[3+3] Imine and β-ketoenamine tethered fluorescent covalent-organic frameworks for CO2 uptake and nitroaromatic sensing

Imine and β-ketoenamine based covalent-organic frameworks (COFs) are nitrogen rich organic porous materials which offer enhanced affinity for carbon dioxide. In this article, synthesis, gaseous uptake and chemo-sensing properties of four new 2-D COFs with [3+3] structural motifs have been reported. The COFs have been synthesized from readily available C3-symmetric aldehyde and amine building units via Schiff base condensation. Prior to the synthesis, reactivity and structural integrity of the building blocks were appropriated by a fully characterized model Schiff base (TAPB-Benz) obtained from the condensation of 1,3,5-tris(4′-aminophenyl)benzene (TAPB) and benzaldehyde. Reaction of 1,3,5-tris(4′-aminophenyl)benzene (TAPB) and 1,3,5-tris(4′-amino-3′,5′-isopropylphenyl)benzene (iPrTAPB) with 1,3,5-tris(4′-formylphenyl)benzene (TFPB) and 1,3,5-triformylphluroglucinol (TFP) in dry dioxane and acetic acid (cat.) resulted in the formation of crystalline 2-D frameworks, TAPB-TFPB, iPrTAPB-TFPB, TAPB-TFP and iPrTAPB-TFP. The COFs feature permanent porosity with high surface area and carbon dioxide uptake. Among these, iPrTAPB-TFP revealed the highest surface area of 756 m2 g−1 (Brunauer–Emmett–Teller) and 1515 m2 g−1 (Langmuir) and carbon dioxide uptake of 105 mg g−1 (273 K, 1 atm). Notably with 180 mg g−1 (273 K, 1 atm), TAPB-TFP shows the highest CO2 uptake capacity among all the COFs which is also comparable to previously reported high CO2 uptake capacity COFs. Furthermore, due to the inherent fluorescent capability of triphenylbenzene, the COFs are endowed with fluorescence and fluorescence chemo-sensing ability for polynitroaromatic analytes.

Charge transfer aided selective sensing and capture of picric acid by triphenylbenzenes

A fluorescent chemo-sensor, 1,3,5-tris(4′-(N,N-dimethylamino)phenyl)benzene was synthesized by substituting the N–H protons of 1,3,5-tris(4′-aminophenyl)benzene with methyl groups. The chemo-sensor shows highly selective and remarkable fluorescence quenching in the presence of picric acid with a detection limit of 1.5 ppm. The origin of the selectivity was investigated using absorption, fluorescence emission and 1H NMR spectroscopic techniques. The solid state structure of 1,3,5-tris(4′-(N,N-dimethylamino)phenyl)benzene and its picric acid complex reveals multiple hydrogen bonds (N–H⋯O and C–H⋯O), π–π interactions and electrostatic interactions between 1,3,5-tris(4′-(N,N-dimethylamino)phenyl)benzene and picric acid. The proton transfer process from picric acid to 1,3,5-tris(4′-(N,N-dimethylamino)phenyl)benzene results in the formation of picrate anions and the triply protonated 1,3,5-tris(4′-(N,N-dimethylamino)phenyl)benzene species containing dimethylammonium (–NHMe2+) groups.

An anionic two-dimensional indium carboxylate framework derived from a pseudo C (3)-symmetric semi-flexible tricarboxylic acid

AbstractHydrothermal treatment of indium(III) nitrate with a flexible pseudo C3-symmetric tricarboxylic acid at 115∘C for 5 days in DMF yields a new layered anionic indium carboxylate framework, [(CH3)2 NH 2)][In(L)(HCOO)(DMF)]n (1) (L = 2,4,6-tris[(4′-carboxyphenoxy)methyl]-1,3,5-trimethylbenzene), existing as two-dimensional sheets. The framework solid has been characterized by elemental analysis, FT-IR spectroscopy, TGA, PXRD and single crystal X-ray diffraction studies. DMF undergoes cleavage to dimethyl ammonium and formate ions, which are incorporated in the framework. A slipped stacking of the two dimensional sheets along a–axis in 1 results in a drastic decrease in the anticipated large porosity of the framework. Graphical AbstractA two-fold interpenetrated 2-D In(III)-MOF has been synthesized using hydrothermal treatment of flexible tris-acid ligand H3L, indium nitrate and DMF. The MOF features an anionic uninodal 6-connected sql/Shubnikov plane net (3,6) with (36.46.53) topology. DMF decomposes to (CH3)2NH2+ cations which bridge the 2-D sheets through hydrogen bonds in a staggered fashion.

Dependence of the SBU length on the size of metal ions in alkaline earth MOFs derived from a flexible C3-symmetric tricarboxylic acid

Four new alkaline-earth metal-based metal–organic frameworks, [Mg4(TCMTB)2(OAc)2(DMA)2(H2O)3]n (Mg-TCMTB), {[Ca4(TCMTB)2(OH)(DMF)2(H2O)5]·Cl}n (Ca-TCMTB), [Sr4(TCMTB)2(OH)(OAc)(DMA)6(H2O)]n (Sr-TCMTB) and [Ba9(TCMTB)4(NO3)6(DMA)14]n (Ba-TCMTB) (H3TCMTB = 2,4,6-tris[(4′-carboxyphenoxy)methyl]-1,3,5-trimethylbenzene), have been synthesized and structurally characterized. Structural analysis of the MOFs reveals the presence of diverse structures and topologies in these systems due to the conformational flexibility and multiple coordination sites in H3TCMTB. Coordination polymers Mg-TCMTB, Ca-TCMTB and Ba-TCMTB MOFs are three-dimensional frameworks exhibiting 2-fold interpenetration and one-dimensional hexagonal channels, while Sr-TCMTB is a 2-fold interpenetrated layered MOF. The 2D layers in Sr-TCMTB are interconnected through H–O⋯H hydrogen bonds. Increasing ionic radii and coordination number on moving down the group results in the formation of bi-, tetra- and nona-nuclear M–O–M connected inorganic building units. Owing to its smaller size and lower coordination number, framework Mg-TCMTB gives rise to a moderate surface area of 33.0 m2 g−1 (SABET) and 93.8 m2 g−1 (SALang) which is the highest observed among all the four MOFs. Emission studies of the new MOFs reveal the presence of strong photoluminescence at 380 nm.

Triphenylbenzene Sensor for Selective Detection of Picric Acid

A C3-symmetric triphenylbenzene based photoluminescent compound, 1,3,5-tris(4′-(N-methylamino)phenyl) benzene ([NHMe]3TAPB), has been synthesized by mono-N-methylation of 1,3,5-tris(4′-aminophenyl) benzene (TAPB) and structurally characterized. [NHMe]3TAPB acts as a selective fluorescent sensor for picric acid (PA) with a detection limit as low as 2.25 ppm at a signal to noise ratio of 3. Other related analytes (i.e. TNT, DNT and DNB) show very little effect on the fluorescence intensity of [NHMe]3TAPB. The selectivity is triggered by proton transfer from picric acid to the fluorophore and ground-state complex formation between the protonated fluorophore and picrate anion through hydrogen bonding interactions. The fluorescence lifetime measurements reveal static nature of fluorescence quenching.

Steric group enforced aromatic cyclic trimer conformer in tripodal molecules

A family of tripodal molecules (1–6) with/without steric ethyl groups at the central benzene scaffold and with furan/thiophene/pyridyl group at the 2-position of the benzimidazolyl unit was synthesised. Compounds 1–6 were characterized by elemental analysis and NMR spectroscopy. Compounds 1, 3, and 5 were further characterized by single crystal X-ray diffraction analysis. The molecular structures of 1 and 4 were optimized using density functional theory (DFT) calculations. X-ray and 1H NMR studies reveal that the introduction of three ethyl groups into a central benzene scaffold of furan/thiophene/pyridyl substituted benzimidazolyl based tripodal molecules enhances the edge-to-face C–H⋯π interactions, thereby favouring the aromatic cyclic trimer motif, in solution and the solid state. The unsubstituted central benzene scaffold allows the furan/thiophene substituted benzimidazolyl units in the tripodal molecules to move freely thereby weakening the edge-to-face C–H⋯π interactions between the aromatic cyclic trimer motif. Molecular modelling calculations indicate that the energy minimized structures of the tripodal molecules adopt a symmetric cyclic aromatic motif conformation.

1,3,5-Triphenylbenzene: a versatile photoluminescent chemo-sensor platform and supramolecular building block

Fluorescence chemo-sensors for species of environmental and biological significance have emerged as a major research area in recent years. In this account, we describe fluorescence quenching as well as enhancement-based chemo-sensors obtained by employing C3-symmetric 1,3,5-triphenylbenzene (1,3,5-TPB) as the fluorescence signalling unit. 1,3,5-TPB is a thermally and photochemically stable fluorescent platform with π-electron-rich characteristics. Starting from this platform, supramolecular, discrete, triphenylbenzene-carbazole, covalent-organic framework, covalent-organic polymer and conjugated polymer based sensors have been developed for the selective detection of polynitroaromatic compounds, trinitrotoluene (TNT), dinitrotoluene (DNT) and picric acid (PA). Tris-salicylaldimine Schiff bases have been synthesized for the selective sensing of fluoride ions through a fluorescence turn-on mechanism. It is likely that it should be possible to develop other highly selective and sensitive chemo-sensors by incorporating 1,3,5-TPB as the fluorophore unit.