Stimuli-Responsive Properties of Supramolecular Gels Based on Pyridyl-N-oxide Amides

Abstract

The nature of functional groups and their relative position and orientation play an important role in tuning the gelation properties of stimuli-responsive supramolecular gels. In this work, we synthesized and characterized mono-/bis-pyridyl-N-oxide compounds of N-(4-pyridyl)nicotinamide (L1–L3). The gelation properties of these N-oxide compounds were compared with the reported isomeric counterpart mono-/bis-pyridyl-N-oxide compounds of N-(4-pyridyl)isonicotinamide. Hydrogels obtained with L1 and L3 were thermally and mechanically more stable than the corresponding isomeric counterparts. The surface morphology of the xerogels of di-N-oxides (L3 and diNO) obtained from the water was studied using scanning electron microscopy (SEM), which revealed that the relative position of N-oxide moieties did not have a prominent effect on the gel morphology. The solid-state structural analysis was performed using single-crystal X-ray diffraction to understand the key mechanism in gel formation. The versatile nature of N-oxide moieties makes these gels highly responsive toward an external stimulus, and the stimuli-responsive behavior of the gels in water and aqueous mixtures was studied in the presence of various salts. We studied the effect of various salts on the gelation behavior of the hydrogels, and the results indicated that the salts could induce gelation in L1 and L3 below the minimum gelator concentration of the gelators. The mechanical properties were evaluated by rheological experiments, indicating that the modified compounds displayed enhanced gel strength in most cases. Interestingly, cadmium chloride formed supergelator at a very low concentration (0.7 wt% of L3), and robust hydrogels were obtained at higher concentrations of L3. These results show that the relative position of N-oxide moieties is crucial for the effective interaction of the gelator with salts/ions resulting in LMWGs with tunable properties.

The Role of Functional Groups in Tuning the Self‐Assembly Modes and Physical Properties of Multicomponent Gels

“…Supramolecular gels are an excellent class of stimuli‐responsive materials because the gelation process can be turned ON/OFF in the presence of external stimuli such as pH, light, sound, redox, and by the addition of an external entity, for example, salts/ions [20] . The amide moieties are excellent candidates for anions sensing and we have reported the anion sensing properties of LMWGs based on amide moieties [10b,21] . This prompted us to study the stimuli‐responsive properties of multicomponent enantiomeric gels towards anions, which will help to evaluate the effect of functional groups on the anion sensing properties.…”

“…Powder X‐ray diffraction (PXRD) is an important tool to confirm the phase purity of the material and analyze the molecular packing by comparing the simulated pattern of the single‐crystal data with the PXRD pattern of the bulk material or the xerogels [1b,4h,10b,11,18] . PXRD analysis helps to understand the self‐assembly in LMWGs, but this method has some flaws due to possible artifacts during the drying process to prepare the xerogels [10a,19] .…”

Supramolecular Gels Based on C3-Symmetric Amides: Application in Anion-Sensing and Removal of Dyes from Water

“…A positive interaction has the potential to start or enhance the process of gelation, whereas a negative interaction might result in the dissolution or collapse of the gel network [15][16][17][18][19]. LMWGs based on amide moieties are promising candidates for anions sensing [15][16][17][18][19]54], and we have reported the anion-sensing capabilities of amidebased gelators [55,56]. The presence of amide moieties in the gelators prompted us to evaluate the anion-sensing abilities of the N-BTA gelator.…”

Editorial on the Special Issue: “Advances in Xerogels: From Design to Applications”

“…The responsibility of porous materials toward external stimuli is crucial for the obtainment of smart innovative materials, and the chemistry of N-oxide moieties offers a wide range of possibilities. In this context, Trofimov and co-workers report the synthesis of 4-dialkylamino-2,5-dihydroimidazol-1-oxyls with moieties at position 2 and at the exocyclic nitrogen able to act as pH-Sensitive spin labels [ 8 ], while Jayabhavan and colleagues study the stimuli-responsivity supramolecular gels based on pyridyl-N-oxide amides [ 9 ].…”