Tuning Gel State Properties of Supramolecular Gels by Functional Group Modification

Abstract

The factors affecting the self-assembly process in low molecular weight gelators (LMWGs) were investigated by tuning the gelation properties of a well-known gelator N-(4-pyridyl)isonicotinamide (4PINA). The N―H∙∙∙N interactions responsible for gel formation in 4PINA were disrupted by altering the functional groups of 4PINA, which was achieved by modifying pyridyl moieties of the gelator to pyridyl N-oxides. We synthesized two mono-N-oxides (INO and PNO) and a di-N-oxide (diNO) and the gelation studies revealed selective gelation of diNO in water, but the two mono-N-oxides formed crystals. The mechanical strength and thermal stabilities of the gelators were evaluated by rheology and transition temperature (Tgel) experiments, respectively, and the analysis of the gel strength indicated that diNO formed weak gels compared to 4PINA. The SEM image of diNO xerogels showed fibrous microcrystalline networks compared to the efficient fibrous morphology in 4PINA. Single-crystal X-ray analysis of diNO gelator revealed that a hydrogen-bonded dimer interacts with adjacent dimers via C―H∙∙∙O interactions. The non-gelator with similar dimers interacted via C―H∙∙∙N interaction, which indicates the importance of specific non-bonding interactions in the formation of the gel network. The solvated forms of mono-N-oxides support the fact that these compounds prefer crystalline state rather than gelation due to the increased hydrophilic interactions. The reduced gelation ability (minimum gel concentration (MGC)) and thermal strength of diNO may be attributed to the weak intermolecular C―H∙∙∙O interaction compared to the strong and unidirectional N―H∙∙∙N interactions in 4PINA.

Role of N–Oxide Moieties in Tuning Supramolecular Gel-State Properties

“…We have been successful in identifying the role of various nonbonding interactions in gel formation by comparing the molecular assembly in crystalline and gel/dried gel states [ 30 , 43 , 44 , 45 ]. Although, the xerogel does not represent the exact structure of the gel state since the drying process can lead to a morphological change or phase transition [ 46 ], this approach is still recognized as the most useful strategy to unravel the self-assembly of a gelator [ 3 , 7 , 29 , 30 , 43 , 44 , 45 ]. The self-assembly of L 1 and L 2 in the gel state was correlated with respective crystal structures by comparing the powder X-ray pattern of the bulk crystals and dried gels with the crystal structure.…”

“…The gel to solution transition temperature ( T gel ) was evaluated in different solvent systems using ball-drop method [ 7 , 30 , 44 , 45 ]. The required amount of the gelator was dissolved in the solvent or solvent mixtures by heating the mixture in a sealed vial to obtain a clear solution.…”

“…We have studied the importance of functional groups and the role of nonbonding interactions in tuning gelation properties of LMWGs by altering the intermolecular interactions. This was achieved by modifying the pyridyl groups of N–(4–pyridyl)isonicotinamide (4PINA) [ 29 ] to N –oxide groups, which resulted in two mono– N –oxides and a di– N –oxide compound [ 30 ]. The strong and unidirectional N—H∙∙∙N interactions of the parent gelator was disrupted by the modification of the pyridyl group and the effect of these interactions on the gelation properties was studied.…”

Stimuli responsive dynamic transformations in supramolecular gels

“…[306][307][308] Generally, the binding sites are connected to some associating units on the gelator structure that assist self-assembly involving non-covalent interactions. 18,[309][310][311] Such associating units may contain hydrophobic moieties like cholesterol 169,[312][313][314] and aromatic surface, 315 or hydrophilic functionalities like peptide, 316 sugar 317 etc. Chemical analyte responsive gels have huge potential as molecular sensors, aggregation probes, antibacterial agents, conducting materials and water purifying agents.…”

Making and Breaking of Gels: Stimuli-Responsive Properties of Bis(Pyridyl-N-oxide Urea) Gelators

“…The modification of these functional groups will lead to the alteration of specific interactions arising from the particular functional group, which can be considered as an excellent strategy to evaluate the role of specific interactions in gel formation [30,31]. For example, Zinchuan et al have synthesized a cholesterol-based dendrimer gelator from the parent terminal alkyne-based gelator and have shown that the modified compounds formed gel in polar solvents compared to the parent compound, which only formed gel in nonpolar solvents [32].…”