Design, Synthesis And Gelation Behavior Of Calix [4] Aromatic Hydrocarbon Derivatives

As a typical class of soft materials, gels consist of two components at least, a lot of solvents and a very small amount of gelators. Molecular/supramolecular gels based on low-molecular mass gelators (LMMGs) have experienced a rapid development and attracted increasing attention during the last few decades. This is not only because of the very special properties of these gels, such as excellent designable property, reversible stimuli-responsive property, and very high efficiency of gel formation, but also because of their practical applications in template preparation of micro-/nano-and functional materials, controlled release and oil recovery, etc. In suitable solvents, LMMGs self-assemble into primary assemblies, then three-dimensional networks through hydrogen bonding, van der Waals interaction, electrostatic interaction, coordination interaction, host-guest interaction, π-π interaction and other non-covalent interactions. It is the nature of weak interactions that makes the formation and destruction of the molecular gels reversible, and that explains why the gels as discussed possess stimuli-responsive properties.It is well-known that cholesterol-based LMMGs were discovered incidentally by Weiss and coworkers more than 20 years ago. Cholesterol is a unique and widely adopted structural unit of LMMGs because it is equipped with a number of unique properties such as rigid skeleton, multiple chiral centers, strong van der Waals stacking and good biocompatibility, etc. Over the last few years, our group has designed and synthesized a series of cholesterol-based LMMGs exhibiting excellent properties, and have profound accumulation and get rich experience in the field.Although many kinds of cholesterol-based gelators have been produced, few works on LMMGs deriving from macrocyclic-based cholesterol derivatives are reported. It is well known that as the third generation of supramolecular macrocyclic-host compounds, calixarenes have been extensively studied as basic scaffolds for the preparation of functional host molecules due to their easiness in synthesis and modification both at the upper and lower rims, their conformational adaptability, and the hydrophobicity of their cavities. No doubt, combination of calixarenes and cholesterol may produce LMMGs with exceptional structures and properties.Based on discussions above and the works conducted earlier in our lab, two new calix[4]arene derivatives of cholesterol were designed and prepared, their gelation behaviors in suitable solvents, and the structures and properties of gels obtained were studied. As expected, some of the gels exhibit unusual properties. Based on the studies, this thesis mainly consists of the following two parts.In the first section, a calix[4]arene derivative of cholesterol with naphthalene in the linkers (C2N2C) was designed and synthesized. The gelation behavior in suitable solvents, and the gels as obtained were studied in detail. It was demonstrated that the compound could gel both polar and apolar organic solvents, showing remarkable gelling ability. Morphological study indicated that the nature of solvents and the concentration of the gelator had a significant effect on the microstructures of the gels. SEM studies revealed that the compound first aggregated into spherule-like structures, then helical fibers, and then sheets or flakes, finally networked structures with gradually increasing the concentration of the gelator. CD and AFM analyses demonstrated that the molecules of the gelator self-assembled into chiral aggregates with right-hand helical feature. Rheological studies manifested that the gels of benzene and some benzene derivatives possess sensitive, fast and fully reversible thixotropic property. Both tube test and DSC measurements revealed that the Tgl of the benzene gel of the compound could be higher than 140℃, which is close to the double of the boiling point of benzene (-80℃), a result never reported before. It is to be noted that the exceptional thermo-stability of the gel may lay a foundation for it to find some special applications. Temperature and concentration dependent 1H NMR measurements revealed that inter-molecular hydrogen bonding and π-π stacking are two main driving forces for the formation of the gels. XRD study revealed that the gelator in benzene self-assembled into a hexagonal structure.In the second section, based on the results of the first work and related work conducted in our lab, it is realized that increasing the number of cholesteryl fragment and π-conjugated structure not only enhances the gelling ability of the compound but may also improve the structures and properties of the relevant gels. Accordingly, a new compound containing four cholesteryl units and four naphthalene moieties was designed and synthesized in order to create gels with more exceptional properties. Because of time reasons, studies on the gelation behavior of the compound, the gel structures, the gel properties, and the relevant gel formation mechanisms are still in progress.