Supramolecular gels based on boronate esters and imidazolyl donors

Dative boron-nitrogen bonds can be used for the controlled assembly of boronate esters and N-donor ligands. So far, most investigations have focused on crystalline assemblies. In contrast, there are very few reports about soft materials based on dative B-N bonds. Here, we describe the formation of gels by aggregation of diboronate esters and ditopic bisimidazolyl donor ligands. Analyses of low molecular weight model compounds by NMR spectroscopy, isothermal titration calorimetry, and X-ray crystallography reveal that imidazolyl ligands form particularly strong dative bonds with boronate esters. The strong interaction allows gel formation in competitive polar solvents such as THF and acetone. In less polar solvents, very low critical gel concentrations of down to 0.02 wt% are observed. By using a diboronate ester linked by an azobenzene group, it is possible to form a photoresponsive gel.