Steroid-based A(LS)3-type gelators: probing the design criteria in creating soft materials

A new series of low molecular weight gelators, namely compounds 1-4, have been synthesized and investigated the gelation ability in various organic solvents for the purpose of probing the rational design criteria in creating optimized steroid-based A(LS)(3)-type gelators. To generate compounds 1-4, we designed identical cholesterol moieties and amide bonds, and fine-tuned the structures of functionalized linkers or aromatic units. The gelation ability indicated that compounds 1 and 4 were poor gelators, and compound 2 was an efficient gelator for some aromatic solvents, while compound 3 was a highly efficient gelator. To facilitate understanding the reason of this phenomenon, a close investigation of the supramolecular structures in the xerogels of compounds 1-4 was carried out using TEM and AFM characterizations. The investigation showed that a slight change in the molecular structure of gelator could greatly affect the gelation ability as well as the morphology of the supramolecular self-assembly. Especially, it was highlighted that an appropriate aromatic unit as a backbone of gelator and a flexible linker were welcomed in order to obtain an effective A(LS)(3) type gelator. The formation mechanism of organogels has also been proposed. Moreover, we explained from a molecular level why the gelling ability as well as thermal stability of organogels formed by compound 3 or 2 was better than that formed by compound 1 or 4. The results described herein possibly provide the rational design criteria in creating optimized steroid-based A(LS)(3)-type gelators toward functional gels with unusual properties.