Cyclic bis-urea compounds as gelators for organic solvents

The gelation properties of bis-urea compounds derived from opti- cally 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 scan- ning calorimetry, infrared spectroscopy, small-angle X-ray diffraction, and elec- tron microscopy. These compounds have been found to be very potent gelators for organic solvents, such as aliphatic and aromatic hydrocarbons, esters, ke- tones, and alcohols, at concentrations well below 1 (w/v)%. Gelation by these compounds is completely thermorever- sible, with melting temperatures up to 1208C, and many of the gels display thixotropic properties. Even at low con- centrations these compounds self-as- semble into elongated and very thin fibers, which in turn form a three-dimen- sional network in the solvent. Infrared studies showed that aggregation is ac- companied by the formation of a hydro- gen-bonded network between urea moi- eties, 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 sta- bilized by the formation of eight hydro- gen bonds between the urea groups and adjacent molecules. The molecular ar- rangement in gels is most likely very similar to that in the crystal, but the complete elucidation of the molecular arrangement in gels is complicated be- cause 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 mol- ecule. Since the bis-urea compounds presented in this paper are very easy to synthesize and many structural varia- tions are possible without loss of the gelation ability, they are excellent build- ing blocks for the construction of func- tional gels.