Reactive organogels based on isoxazole esters: alkali metal ions selective gelation and crystallization

A series of simple ester molecules containing an isoxazole moiety were found to form instant organogels at room temperature in the presence of NaOH without the heating-cooling cycle used for conventional supramolecular gels. The gelation process was triggered due to the hydrolysis of the isoxazole esters and occurred selectively with Na+. When LiOH, NaOH and KOH were separately introduced into the methanol solutions of the isoxazole esters, the solution remained as a solution, transformed to a organogel and a crystal, respectively. With the help of a study on the phase behavior of the corresponding isoxazole acid in the presence of the alkali bases, it was revealed that pi-pi stacking of the isoxazole moiety and the ionic interaction between the carboxylates and Na+ are the main driving forces for the self-assembly and the organogelation. The size of the alkali metal ions will subtly affect the gelation, with the Li+ and K+ ions leading to solution and crystallization, respectively. These results have provided an insight into the balance between the solution, gelation and crystallization with subtle molecular variations.