Multistimuli-responsive organogels based on a chiral azo gelator without hydrogen bond assistance

To investigate multistimuli responses of azobenzene derivatives, predesigned cholesteryl 4-((4-methoxyphenyl)diazenyl)phenyl carbonate (CMA) and cholesteryl 4-((4-methoxypenyl)diazenyl)benzoate (CMAB) were synthesized. To promote both the formation of helical constructions and gelation ability, chiral cholesteryl group was introduced into compounds. Only CMAB shows gelation ability. Molecular simulation shows that CMA and CMAB possess bent and linear structures, respectively. Observations of morphologies and microstructures of gels by SEM and TEM suggest that self-assembly of CMAB forms highly-ordered helical fibers, which are generated by core-centered stacking of cholesteryl group with azobenzene at periphery. Further cross-linking of fibers generates three-dimensional entangled networks. The reversible gel-sol transitions of CMAB due to thermal and UV-irradiation stimuli were confirmed. Photoisomerization of CMAB was studied using UV-vis spectrophotometry. Kinetic analyses show that a supramolecular change is effected by Z-E photoinduced isomerization. A reversible sol-gel transition accompanied by an opaque-to-clear transition caused by external stimuli was observed. Such external-stimuli responsive gels are highly desirable for advanced applications of organogels, such as the sol-gel process, drug delivery, and sensors. Development of such organogels may lead to smart materials.