Low Molecular Mass Gelator: Design, Synthesis And Properties

In recent years, there has been immense interest in studying molecular gels and their self-assembled fibrillar networks (SAFINs). As the number of the reported low-molecular-mass gelators is growing, the study of molecular gels is no longer only focused on the discovery of new gelators. The design and synthesis of gelators with more functional groups and the development of their potential applications have become the major research focuses at the present stage. We choose the widely used trialkoxybenzoyl- and cholesterol-based molecular gel systems. According to the research experience for the stimuli-responsive molecules in our group, we try to introduce some stimuli-responsive molecules into the gelator systems, and build new soft materials with multiple stimuli-responsive properties. Based on the above ideas, we have carried out some work as follows:Firstly, we designed and synthesized four trialkoxybenzoyl-based molecules with similar symmetrical structures, and studied the structure-property relationship of these molecules. It was found that the para-substituted derivatives of p-phenylenediamine and benzidine could form stable gel in many organic solvents, while the meta-substituted derivative of m-phenylenediamine could only form gel in ethyl acetate and n-propanol, and the ortho-substituted derivative of o-phenylenediamine even could not form gel in any test solvents. The formed gels were studied by phase diagrams, SEM, FTIR, XRD, and the main effects of intermolecular hydrogen bonds and van der Waals forces on the self-assembly of gelators into SAFINs were analyzed and discussed.Secondly, we designed and synthesized five A(LS)2-type cholesterol derivatives centered with salen-type bis-salicylaldehyde Schiff-base functional units. After studying the gelability of these molecules in different solvents, we found that only compound G1 was a good gelator. G1 can form a stable gel with volume ratio of 1:1 toluene/alcohol mixed solvent at a very low concentration. The microstructure of the gel was studied by SEM, FTIR and XRD, and it was found that the intermolecular hydrogen bonds, van der Waals forces and the possibleπ-πinteractions played a key role in the self-assembly of the gelator. In addition, we found that G1-gel could be formed either through the traditional thermal-induced method, or under the influence of ultrasound. By exactly examining the gels formed with these two methods, we found that the second method was not the ultrasound-induced gelation as reported in the literature, and in nature, it was the ultrasound-assisted solvent-induced gelation and which was a new gel-forming method. Finally, we also tested the ionic sensitivity of G1 and its gel. G1 was sensitive with many transition metal ions, such as Zn2+, Cu2+, Cd2+, Pb2+ and Hg2+, and some anions, such as OH- and CN-, accompanying with clear spectra and color change. Such a response at the molecular level could be well transcribed to the gel, and G1 gel sould respond to these ions with a gel-to-sol transition, and these responses were quantitative in some degree. These properties might endow G1 and its gel with potential applications as indicator or adsorbent for toxic ions, and the results also confirmed that the intermolecular hydrogen bond plays a key role in the gel formation.And then, we designed and synthesized a stilbazole-based ALS-type cholesterol derivative G6, which showed good stimuli-responsive properties. It could perform photoisomerization and protonation with UV light and HCl, respectively. By choosing the suitable photostationary state as the starting state, and the UV light and HCl as two inputs, we had constructed five two-input logic gates and a 1:2 demultiplexer. But it was found that G6 could not form gel in any test solvents, and showed a certain degree of crystallization. We tried some methods to inhibit the crystallization, and achieved some success. First, we found the 1:1 hydrogen-bonding complex of G6 could form stable gel in some solvents, and the formed gel was characterized in detail. In addition, the hydrogen-bonding complex showed thermotropic mesomorphic behavior. Then, we found that G6 could also form gel with the doping of a A (LS)2-type molecule, and the quantity of the dopant could affect the gel formation and the morphology of formed gel. The effect of the structure of the dopant on the gel formation was discussed, and this might make a new way for the design and synthesis of gelators.