Photoresponsive organogel and organized nanostructures of cholesterol imide derivatives with azobenzene substituent groups

Two new cholesterol imide derivatives with azobenzene substituent groups have been synthesized. The compounds possess headgroups of hydrogen or methyl units, respectively. The gelation test revealed that a subtle change in the headgroup of azobenzene segment produced a dramatic change in the gelation behavior of both compounds. The compound with headgroups of hydrogen units only gelated N,N-Dimethylformamide (DMF), while the other compound with headgroups of methyl units cannot gelate any solvent tested herein. For DMF gel and solution of the gelator, the reversible photoisomerization transitions by irradiation with ultraviolet (UV) and visible light are investigated by UV-vis absorption spectra, Fourier transform infrared spectrometer, scanning electron microscope, atomic force microscope, and X-ray diffraction analyses. Upon UV irradiation of the gel, trans–cis photoisomerization of the azobenzene groups occurs, the change in molecular polarity leads to the breaking of van der Waals interactions, resulting in the gel–sol transition. The gel can be recovered by the reverse cis–trans photoisomerization after exposure to visible light. Morphological and spectral studies reveal that the gelator molecules self-assemble into one-dimensional fibers, which further crossed-linked to form regular nanobelts. These results afford useful information for the development of new versatile low molecular mass gelators and soft matter.