| In recent years, low-molecular-mass organic gelators (LMOGs) have attracted great attention, because they can self-assemble into physical gels with continuous three-dimensional (3D) networks via supramolecular weak interactions (e.g. hydrogen bonding, van der Waals interactions,π-πstacking, electrostatic interactions, coordination interaction, solvophobic interaction, and host-guest interaction, etc.) in suitable solvents. In the physical gels, the formed network structures can encapsulate and immobilize a large volume of organic solvents via surface tension and capillary forces, and liquid component can support and consolidate the network structure, which makes the obtained gel exhibiting a typical visco-elastic property at macroscopic level. Unlike chemical gels, the formation and deformation of the 3-D network structure in physical gels is a reversible process in most cases, and can be controlled by some physical stimulus, such as heat, pH, photo-irritation, sonication, charge separation, and shear force, etc.. Therefore, these supramolecular properties of molecular gels bring new possibilities for their real-life applications in drug delivery, tissue engineering, templates, sensors, oil industries, light-harvesting materials, and dye sensitized solar cells, etc..Cholesterol-based low-molecular-mass organic gelators (LMOGs) have gained considerable interests due to their versatility in gelation and diversity in structures. Till now, various types of cholesterol-based LMOGs have been designed and synthesized, Generally speaking, cholesterol-based LMOGs reported till now can be classified into four types:ALS, A(LS)2, LS and LS2, where A stands for an aromatic moiety, which is connected to a steroidal (S) group through a linker (L). To the best of our knowledge, the A units of most of the A(LS)2 gelators as reported are aromatic groups with largeπ-conjugated systems. There has been no more report on A(LS)2 gelators with a small aromatic ring, such as a benzene ring, as A units. Recently our group synthesized a series of cholesterol-based A(LS)2 gelators with a benzene ring as the A unit. Gelation test revealed that the compounds are efficient gelators, and these compounds show unusual behaviors, such as gelation at room temperature, phase-selective gelation at room temperature, gel emulsions, etc., and took them into applications of oil/water separation, water purification and the preparation of low density materials in our studies, Moreover, FT-IR and 1H NMR measurements indicated thatπ-πstacking between benzene rings is one of important driving forces for the formation of the gels. However, on the basis of above considerations, and in order to develop the unusual gel system, eight dimeric cholesteryl-based A(LS)2 gelators were specially designed and prepared, in which a benzene ring was employed as the A unit, and two other benzene rings were incorporated into the two L units. Their gelation behaviors in a variety of solvents were also tested.In the first part, eight dimeric cholesteryl-based LS2 gelators were specially designed and prepared, in which a benzene ring was employed as the A unit, and two other benzene rings were incorporated in the two L units. The structures and compositions of all these compounds are characterized by 1H NMR, FT-IR spectroscopy, MS (ESI), and elemental analysis, and satisfied results have been obtained.In the second part, the gelation properties of the eight compounds were examined systematically in common organic solvents. It can be seen that the gelation abilities of the compounds are dependent strongly on the spacer structures of them, compoundⅡ-1 is a typical super-gelator for DMSO with a critical gelation concentration (CGC) of 0.04%(w/v). Interestingly, we have demonstrated a new gelator ofⅡ-3 that can selectively gel the oil phase from a kerosene-water mixture; the spacer structure, the concentration of the gelator and the nature of the solvent play a crucial role in the morphologies of their xerogels, and the microstructures of the xerogels can be controlled by ultrasound treatment; Furthermore, the DMSO gel of this gelator is mechanically much stable than other gels with LMMGs as gelators.In the third part, the formation mechanism of the gels was studied. FT-IR and 1H NMR spectroscopy measurements revealed that hydrogen-bonding andπ-πstacking are two important driving forces for the formation of the gels. XRD analysis shows clearly thatⅡ-1 aggregates into a layered structure in its DMSO gel, a similar structure to that of it in solid state. According to the discussions made above, a structural model was proposed to describe the formation of gel network. |
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