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.