Exploiting molecular self-assembly: from urea-based organocatalysts to multifunctional supramolecular gels

We describe the self-assembly properties of chiral N,N'-disubstituted urea-based organocatalyst 1 that leads to the formation of hierarchical supramolecular gels in organic solvents at low concentrations. The major driving forces for the gelation are hydrogen bonding and π-π interactions according to FTIR and (1)H NMR spectroscopy, as well as quantum-mechanical studies. The gelation scope could be interpreted based on Kamlet-Taft solvatochromic parameters. TEM, SEM, and AFM imaging revealed that a variety of morphologies including helical, laths, porous, and lamellar nanostructures could be obtained by varying the solvent. Experimental gelation tests and computational structural analysis of various structurally related compounds proved the existence of a unique set of molecular interactions and an optimal hydrophilic/hydrophobic balance in 1 that drive the formation of stable gels. Responses to thermal, mechanical, optical, and chemical stimuli, as well as multifunctionality were demonstrated in some model gel materials. Specifically, 1 could be used for the phase-selective gelation of organic solvent/water mixtures. The gel prepared in glycerol was found to be thixotropic and provided a sensitive colorimetric method for the detection of Ag(I) ions at millimolar concentrations in aqueous solution. Moreover, the gel matrix obtained in toluene served as a nanoreactor for the Friedel-Crafts alkylation of 1H-indole with trans-β-nitrostyrene.