Dhananjayan Kaleeswaran

[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.

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.

Bulky Isopropyl Group Loaded Tetraaryl Pyrene Based Azo-Linked Covalent Organic Polymer for Nitroaromatics Sensing and CO2 Adsorption

An azo-linked covalent organic polymer, Py-azo-COP, was synthesized by employing a highly blue-fluorescent pyrene derivative that is multiply substituted with bulky isopropyl groups. Py-azo-COP was investigated for its sensing and gas adsorption properties. Py-azo-COP shows selective sensing toward the electron-deficient polynitroaromatic compound picric acid among the many other competing analogs that were investigated. Apart from its chemosensing ability, Py-azo-COP (surface area 700 m2 g–1) exhibits moderate selectivity toward adsorption of CO2 and stores up to 8.5 wt % of CO2 at 1 bar and 18.2 wt % at 15.5 bar at 273 K, although this is limited due to the electron-rich −N=N– linkages being flanked by isopropyl groups. Furthermore, the presence of a large number of isopropyl groups imparts hydrophobicity to Py-azo-COP, as confirmed by the increased adsorption of toluene compared to that of water in the pores of the COP.

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.