Stiff-Stilbene-Linked Bis-Cholesterol: Synthesis and Investigation of Its Supramolecular Gelation and Photophysical Behaviors

Here, we report the design, synthesis, and comprehensive characterization of the bis-cholesterol supramolecular gelator, which contains photochromic stiff-stilbene as a bridging unit. The <i>cis</i>-isomer of stiff-stilbene bridged bis-cholesterol (<i>Z</i>-<b>D</b>) was first synthesized with a systematic design, which can be further converted into its <i>trans</i>-isomer (<i>E</i>-<b>D</b>) with a high degree of efficiency (ca. 100%) upon exposure to 385 nm UV light. Unusual gelation behavior was observed for <i>Z</i>-<b>D</b>, which exhibited supergelator properties in mixed solvents of acetonitrile (ACN)/dichloromethane (DCM) (v/v = 1:1), DCM/MeOH (v/v = 1:2), ACN/CHCl₃ (v/v = 2:1), and CHCl₃/MeOH (v/v = 1:2), with minimum gelation concentrations (MGCs) as low as 0.2 w/v%. These gels formed rapidly at room temperature without the aid of any mechanical forces upon the addition of an antisolvent into the vial containing the gelator and its dissolving solvent. The formation of the self-assembled gel was primarily driven by hydrogen bonding, van der Waals forces, and dipole-dipole interactions, as confirmed by ¹H NMR, Fourier transform infrared spectroscopy (FT-IR), and UV-vis spectroscopies. The gelator molecule <i>Z</i>-<b>D</b> entraps organic solvents and organizes itself into three-dimensional (3D) fibrillar networks in various single and mixed solvents, as confirmed by scanning electron microscopy (SEM) analysis. Upon irradiation with 385 nm light, the gel networks disintegrated into a precipitate suspension, resulting in the transformation of the fibrous structures into irregular spherical-like aggregates. This proves that the structural conformation changes in the gelator significantly influence the resulting self-assembled structures. Overall, the findings present in this study pave the way for the future development of novel light-responsive bis-cholesterol-based gelators, especially in their <i>Z</i>-isomeric form.