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

Publisher Copyright: © 2023 by the authors.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 (L 1–L 3). 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 L 1 and L 3 were thermally and mechanically more stable than the corresponding isomeric counterparts. The surface morphology of the xerogels of di-N-oxides (L 3 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 L 1 and L 3 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 L 3), and robust hydrogels were obtained at higher concentrations of L 3. 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.Peer reviewe

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

The structural modification of existing supramolecular architecture is an efficient strategy to design and synthesize supramolecular gels with tunable and predictable properties. In this work, we have modified bis(pyridyl urea) compounds with different linkers, namely hexylene and butylene, to their corresponding bis(pyridyl-N-oxide urea). The gelation properties of both the parent and the modified compounds were studied, and the results indicated that modification of the 3-pyridyl moieties to the corresponding 3-pyridyl-N-oxides induced hydrogelation. The stability of the parent and modified compounds were evaluated by sol-gel transition temperature (Tgel) and rheological measurements, and single-crystal X-ray diffraction was used to analyze the solid-state interactions of the gelators. The morphologies of the dried gels were analyzed by scanning electron microscopy (SEM), which revealed that the structural modification did not induce any prominent effect on the gel morphology. The stimuli-responsive behavior of these gels in the presence of salts in DMSO/water was evaluated by rheological experiments, which indicated that the modified compounds displayed enhanced gel strength in most cases. However, the gel network collapsed in the presence of the chloride salts of aluminum(III), zinc(II), copper(II), and cadmium(II). The mechanical strength of the parent gels decreased in the presence of salts, indicating that the structural modification resulted in robust gels in most cases. The modified compounds formed gels below minimum gel concentration in the presence of various salts, indicating salt-induced gelation. These results show the making and breaking ability of the gel network in the presence of external stimuli (salts), which explains the potential of using LMWGs based on N-oxide moieties as stimuli-responsive materials

An Insight Into the Gelation Properties in Multi-Component Gels

Non peer reviewe

Crystal Habit Modification of Metronidazole by Supramolecular Gels with Complementary Functionality

A series of bis(urea) compounds with complementary functional groups similar to the pharmaceutical drug metronidazole and a structural isomer isometronidazole have been synthesized. The gelation properties of these compounds were studied in various solvent/solvent mixtures. The mechanical strength of the isomeric gelators was compared using rheology, and the morphologies of the xerogels were analyzed by scanning electron microscopy. These gels were used as media for metronidazole crystallization resulting in a marked habit modification of the metronidazole crystals in the drug-mimicking gels. However, crystallization in the nonmimetic isomeric gel resulted in morphologies similar to the solution state. These results indicate that the drug-mimetic gels interact with the surface of the drug crystal giving rise to new morphologies