Uttam Kumar Das

Supramolecular Synthons in Noncovalent Synthesis of a Class of Gelators Derived from Simple Organic Salts: Instant Gelation of Organic Fluids at Room Temperature via in Situ Synthesis of the Gelators

The supramolecular synthon approach has been employed to synthesize noncovalently a series of low molecular mass organic gelators (LMOGs) derived from benzylammonium salts of variously substituted benzoic acids. The majority of the salts (75%) prepared showed interesting gelation properties. Instant gelation of an organic fluid, namely methyl salicylate, was achieved at room temperature by using most of the gelator salts by in situ synthesis of the gelators. Table top rheology and scanning electron microscopy (SEM) were used to characterize the gels. Single crystal X-ray diffraction studies revealed the presence of both 1D and 2D supramolecular synthons. X-ray powder diffraction (XRPD) studies indicated the presence of various crystalline phases in the fibers of the xerogels. By using these data, a structure-property correlation has been attempted and the working hypothesis for designing the gelator has been reinforced.

Supramolecular Chirality in Organo-, Hydro-, and Metallogels Derived from Bis-amides of L-(+)-Tartaric Acid: Formation of Highly Aligned 1D Silica Fibers and Evidence of 5-c Net SnS Topology in a Metallogel Network

A series of bis-amides derived from L-(+)-tartaric acid was synthesized as potential low-molecular-weight gelators. Out of 14 bis-amides synthesized, 13 displayed organo-, hydro-, and ambidextrous gelation behavior. The gels were characterized by methods including circular dichroism, differential scanning calorimetry, optical and electron microscopy, and rheology. One of the gels derived from di-3-pyridyltartaramide (D-3-PyTA) displayed intriguing nanotubular morphology of the gel network, which was exploited as a template to generate highly aligned 1D silica fibers. The gelator D-3-PyTA was also exploited to generate metallogels by treatment with various Cu(II) /Zn(II) salts under suitable conditions. A structure-property correlation on the basis of single-crystal and powder X-ray diffraction data was attempted to gain insight into the structures of the gel networks in both organo- and metallogels. Such study led to the determination of the gel-network structure of the Cu(II) coordination-polymer-based metallogel, which displayed a 2D sheet architecture made of a chloride-bridged double helix that resembled a 5-c net SnS topology.

Remarkable Shape‐Sustaining, Load‐Bearing, and Self‐Healing Properties Displayed by a Supramolecular Gel Derived from a Bis‐pyridyl‐bis‐amide of L‐Phenyl Alanine

A series of bis-amides decorated with pyridyl and phenyl moieties derived from L-amino acids having an innocent side chain (L-alanine and L-phenyl alanine) were synthesized as potential low-molecular-weight gelators (LMWGs). Both protic and aprotic solvents were found to be gelled by most of the bis-amides with moderate to excellent gelation efficiency (minimum gelator concentration=0.32-4.0 wt.% and gel-sol dissociation temperature T(gel)=52-110 °C). The gels were characterized by rheology, DSC, SEM, TEM, and temperature-variable (1)H NMR measurements. pH-dependent gelation studies revealed that the pyridyl moieties took part in gelation. Structure-property correlation was attempted using single-crystal X-ray and powder X-ray diffraction data. Remarkably, one of the bis-pyridyl bis-amide gelators, namely 3,3-Phe (3-pyridyl bis-amide of L-phenylalanine) displayed outstanding shape-sustaining, load-bearing, and self-healing properties.

Dual colorimetric receptor with logic gate operations: anion induced solvatochromism

A receptor R1 was designed and synthesised for colorimetric detection of F− ions as well as Cu2+ ions via intramolecular charge transfer mechanism. Upon addition of F− ions in dry DMSO, the color of the receptor R1 changed from pale yellow to blue. The receptor showed a unique property of solvatochromism by displaying different coloration with different solvents only in the presence of F− ions, which were applied to determine the percentage composition of binary solvent mixtures. The receptor R1 was able to detect Cu2+ ions colorimetrically where it exhibited a color change from pale yellow to orange-red. In addition, the receptor was subjected to molecular logic gate applications, wherein it showed ‘ON–OFF’ switching operations.

The hierarchies of hydrogen bonds in salts/cocrystals of isoniazid and its Schiff base – a case study

A series of two salts of isoniazid and two cocrystals of its Schiff base have been synthesised and characterized using FT-IR, 1H-NMR, DSC, XRPD and SCXRD. The crystal structures with 2,5-dihydroxybenzoic acid, 2,6-dihydroxy benzoic acid, 3-aminobenzoic acid and o-phthalic acid have been deduced. In all complexes the involvement of pyridine N (proton transfer/robust synthon) is noticed. The unit cell similarity index was calculated between two cocrystals (N-(propan-2-ylidene)isonicotinohydrazide)·(2,5-dihydroxybenzoic acid) and (3-aminobenozic acid)·(N-(propan-2-ylidene)isonicotinohydrazide). Furthermore, the index was compared with the reported complexes and it was found to be close to zero indicating isostructurality. The correlation between the NPyr–H⋯O (acid) hydrogen bond distance for the reported cocrystals and their corresponding pKa values has been deduced and validated.

Aggregation enhanced emission (AEE) in organic salt: A structure-property correlation based on single crystal studies

AbstractSalt formation has been shown as a simple strategy to induce aggregated induced emission or aggregated enhanced emission in primary ammonium salts derived from 9-anthracene carboxylic acid, 1-pyrene carboxylic acid, 3-coumarin carboxylic acid and histamine. All the salts displayed enhanced fluorescence in their solid state compared to that in their solution state. Single crystal structure of the salt of 9-anthracene carboxylic acid i.e., His-anthracene revealed that restricted intramolecular rotation of the fluorphoric moiety (anthracene) was responsible for such radiative pathway leading to enhanced emission. Graphical AbstractA simple strategy of salt formation has been exploited to induce aggregation-induced emission or aggregation-enhanced emission in primary ammonium salts derived from 9-anthracene carboxylic acid, 1-pyrenecarboxylic acid, coumarin-3-carboxylic acid and histamine.

Designing Charge-Assisted Hydrogen Bonded Supramolecular Gelators

Immobilization of solvent(s) by a small amount of solute(s) resulting in a gel is a supramolecular phenomenon. Small molecular compounds (MW < 3,000) capable of displaying such supramolecular properties are known as low molecular weight gelators (LMWGs). Self-assembly of LMWGs driven by strong and directional hydrogen bonding interactions generates 1D fibrous aggregates that entangle to form self-assembled fibrillar networks (SAFiNs) within which the solvent molecules are immobilized (gel formation) due to surface tension or capillary force actions. Research on LMWGs has witnessed an exponential growth in recent years because of the wide range of applications that these materials offer. However, designing LMWGs is a daunting task as precise molecular level information on gelation is still not available and as a consequence, most of the gelators are discovered serendipitously and subsequent design of the next generation of gelators is based on modifying the serendipitously obtained parent gelator molecules. This chapter focuses on the designing aspects and applications of LMWGs derived from various hydrogen-bond-capable molecules. It particularly covers the development of organic salt-based gelators that are designed by following supramolecular synthon approach in the context of crystal engineering-based structure–property correlation.