Tuning Gel State Properties of Supramolecular Gels by Functional Group Modification

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 -(4-pyridyl)isonicotinamide (). The N-H∙∙∙N interactions responsible for gel formation in were disrupted by altering the functional groups of , which was achieved by modifying pyridyl moieties of the gelator to pyridyl -oxides. We synthesized two mono--oxides ( and ) and a di--oxide () and the gelation studies revealed selective gelation of in water, but the two mono--oxides formed crystals. The mechanical strength and thermal stabilities of the gelators were evaluated by rheology and transition temperature () experiments, respectively, and the analysis of the gel strength indicated that formed weak gels compared to . The SEM image of xerogels showed fibrous microcrystalline networks compared to the efficient fibrous morphology in . Single-crystal X-ray analysis of 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--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 may be attributed to the weak intermolecular C-H∙∙∙O interaction compared to the strong and unidirectional N-H∙∙∙N interactions in .