Cyclic Bis-Urea Compounds as Gelators for Organic Solvents

The gelation properties of bis-urea compounds derived from optically pure trans-1,2-diaminocyclohexane and 1,2-diaminobenzene, with pendant aliphatic, aromatic, or ester groups, as well as the structure of the resulting gels, have been studied by differential scanning calorimetry, infrared spectroscopy, small-angle X-ray diffraction, and electron microscopy. These compounds have been found to be very potent gelators for organic solvents, such as aliphatic and aromatic hydrocarbons, esters, ketones, and alcohols, at concentrations well below 1 (w/v)%. Gelation by these compounds is completely thermoreversible, with melting temperatures up to 120 °C, and many of the gels display thixotropic properties. Even at low concentrations these compounds self-assemble into elongated and very thin fibers, which in turn form a three-dimensional network in the solvent. Infrared studies showed that aggregation is accompanied by the formation of a hydrogen-bonded network between urea moieties, and a single-crystal X-ray structure of one of the compounds showed that in crystals the molecules assemble into one-dimensional chains, which are stabilized by the formation of eight hydrogen bonds between the urea groups and adjacent molecules. The molecular arrangement in gels is most likely very similar to that in the crystal, but the complete elucidation of the molecular arrangement in gels is complicated because aggregation of these compounds is prone to polymorphism. It is concluded that the very efficient aggregation of these molecules and the elongated shape of the fibers most likely arise from the highly anisotropic hydrogen-bonding properties of these molecules, which is due to the presence of two coplanar oriented urea moieties in a single molecule. Since the bis-urea compounds presented in this paper are very easy to synthesize and many structural variations are possible without loss of the gelation ability, they are excellent building blocks for the construction of functional gels.