Parthasarathi Dastidar

The last two decades have witnessed an upsurge of research activities in the area of supramolecular gelators, especially low molecular mass organic gelators (LMOGs), not only for academic interests but also for their potential applications in materials science. However, most of the gelators are serendipitously obtained; their rational design and synthesis is still a major challenge. Wide structural diversities of the molecules known to act as LMOGs and a dearth of molecular level understanding of gelation mechanisms make it difficult to pin-point a particular strategy to achieve rational design of gelators. Nevertheless, some efforts are being made to achieve this goal. Once a gelling agent is serendipitously obtained, new gelling agents with novel properties may be prepared by modifying the parent gelator molecule following a molecular engineering rationale; however, such approach is limited to the same class of gelling agent generated from the parent gelating scaffold. A crystal engineering approach wherein the single-crystal structure of a molecule is correlated with its gelling/nongelling behaviour (structure-property correlation) allows molecular level understandings of the self-assembly of the gelator and nongelator molecules and therefore, provides new insights into the design aspects of supramolecular gelling agents. This tutorial review aims at highlighting some of the developments covering both molecular and crystal engineering approaches in designing LMOGs.

The last two decades have witnessed the research activities in the area of coordination polymers (CPs), which are structurally diverse and functionally intriguing materials. In this endeavor, the most exploited ligand has been a structurally rigid N-donor compound having an innocent backbone (incapable of forming hydrogen bond) namely 4,4'-bipyridine. Much has been achieved by exploiting this wonder ligand in the area of CPs. However, the positional isomers such as 3,3'-bipyridine or 4,3'-bipyridine (which understandably induce diverse ligating topology as compared to their more symmetrical 4,4' counterpart) were not exploited in much detail presumably because of the difficulty in their synthetic accessibility. To get access to such N-donor ditopic ligands having diverse ligating topology, much efforts have been focused in the last decade or so to design such positional isomers of 4,4'-bipyridine having a non-innocent backbone (capable of forming hydrogen bond). The principal focus of such studies is to decipher the effect of diverse ligating topology and the non-innocent backbone of the ligands on the overall supramolecular structures and functions of the resultant CPs. This tutorial review aims at highlighting some of the developments of such structurally diverse and functionally intriguing CPs derived from N-donor ditopic ligands having a non-innocent backbone.

Organic salts based on dicyclohexylamine and substituted/unsubstituted cinnamic acid exhibit efficient gelation of organic fluids, including selective gelation of oil from an oil/water mixture. Among the cinnamate salts, dicyclohexylammonium 4-chlorocinnamate (1), 3-chlorocinnamate (2), 4-bromocinnamate (3), 3-bromocinnamate (4), 4-methylcinnamate (5) and the parent cinnamate (6) are gelators, whereas 2-chlorocinnamate (7), 2-bromocinnamate (8), 3-methylcinnamate (9), 2-methylcinnamate (10) and hydrocinnamate (11) are non-gelators. Non-gelation behaviour of 11 and various benzoate derivatives 12-18 indicate the significance of an unsaturated backbone in the gelation behaviour of the cinnamate salts. A structure-property correlation based on the single-crystal structures of most of the gelators (1, 3, 5 and 6) and non-gelators, such as 7, 8, 10-18, indicates that the prerequisite for the one-dimensional (1D) growth of the gel fibrils is mainly governed by the 1D hydrogen-bonded network involving the ion pair. All the non-gelators show either two- (2D) or zero-dimensional (0D) hydrogen-bonded assemblies involving the ion pair. The molecular packing of the fibres in the xerogels of 1, 3, 5 and 6 has also been established on the basis of their simulated powder diffraction patterns, XRPD of bulk solids and xerogels. Ab initio quantum chemical calculations suggests that pi-pi interactions is not a contributing factor in the gelation process.

A pyridyl urea based low molecular weight supramolecular hydrogelator has been synthesized; crystallized from its gelling solvents, the single crystal structure of the gelator molecule interacting with its gelling solvents reported herein is the first example in the literature.

The supramolecular synthon concept has been exploited to generate a series of new organo gelators derived from salts of a primary amine and various derivatives of cinnamic acid. One such gelator, namely, benzylammonium cinnamate 13 (CIN) displays instant gelation ability at room temperature either with or without a brief exposure to sound. Most interestingly, inflammable commercial fluids such as petrol can be gelled instantly by 13 (CIN) at room temperature, which may find real-life applications in containing oil spill, oil mopping, and convenient transport of oil. Analyses of the single-crystal structures of eight salts suggest that structure−property correlation studies in gel research is quite important so that a rational approach may be undertaken to obtain facile synthesis of new gelator compounds.