Solvent effects on structure, photoresponse and speed of gelation of a dicholesterol-linked azobenzene organogel

By introducing methanol as a co-solvent into cyclopentanone, solvent effects on structure, photoresponse and speed of gelation of a dicholesterol-linked azobenzene organogel DCAZO2 have been investigated. Optical microscopy (OM) and scanning electron microscopy (SEM) images reveal that the gel fibers form macroscopic aggregates by adding methanol. Minimum gelation concentration (MGC) of the gels decreases with increasing methanol content, while the gel-to-sol transition temperature (T(gel)) decreases firstly and increases afterwards. For photoisomerization (corresponding to gel-sol transition), the first stage rate constant k(g1) of the gels decreases with increasing methanol content. The photoisomerization process of the gel with 5% methanol is different from the other five samples, whose first stage rate constant k(g1) is greater than its second stage rate constant k(g2). By increasing methanol content from 0% to 30%, gelation time (corresponding to sol-gel transition) of both heated gel and UV-visible light irradiated gel decreases from 7 h to 5 min and 6 h to 0 min (before visible light irradiation is accomplished), respectively. Solubility parameters and Teas plots of the gelator and mixed solvents are systematically calculated to estimate the gelator-solvent interaction. It is found that gelation requires that the gelator-solvent interaction be neither too strong nor too weak, otherwise solution or precipitate are formed.