Molecular design and characterizations of new calixarene-based gelators of organic fluids

We have found that certain calix[n]arenes having long acyl groups at the p-positions (e.g., 5,11,17,23,29,35,41,47-octadodecanoylcalix[8]arene-49,50,51, 52,53,54,55,56-octol: 2₈C₁₂) act as excellent and unique gelators of various organic solvents (e.g., toluene, carbon tetrachloride, carbon disulfide, hexane, isopropanol, etc.). The sol-gel phase transition temperatures (T_gel) could be determined by the inverted test-tube method or by the DSC method. The change in the aggregation mode at the phase transition temperature was directly observable by an optical microscope: below T_gel the fibrillar network (diameter ca. 1 μm) appeared whereas above T_gel it 'melted' down. It was shown on the basis of these measurements that the sol-gel phase transition occurs reversibly. The spectroscopic studies using ¹H NMR and IR spectroscopy and the comparative experiments using the analogues of 2₈C₁₂ (nine cyclic and six non-cyclic) established that the prerequisites for the formation of the stable organic gels are (i) the intermolecular C=O⋯HO hygrogen-bonding interaction to form the three-dimensional network and (ii) the moderate affinity of gelators with solvent molecules. The results offer important strategies useful for the molecular design of new gelators of organic fluids.