Rapid and Reversible Gel-Sol Transition of Self-Assembled Gels Induced by Photoisomerization of Dendritic Azobenzenes

An asymmetric bis-dendritic gelator (1) consisting of an azobenzene dendron and an aliphatic amide dendron was synthesized to achieve a photoresponsive self-assembly. The compound gelled in a wide range of organic solvents, even at concentrations as low as 0.02% (w/v) in cyclohexane. The self-assembled fibrillar network structure was confirmed by field-emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) analyses. The rapid and reversible gel-sol transition by irradiation with UV and visible light was investigated by UV-vis and Fourier transform infrared (FT-IR) spectroscopy, FE-SEM, and XRD analyses. Upon irradiation of the gel with UV, trans-to-cis isomerization of the azobenzene groups occurred, and the gel turned into a sol state. The gel was recovered immediately by the reverse cis-to-trans isomerization after the exposure to visible light. The trans-to-cis isomerization of the azobenzenes disrupted the hydrogen bonding of azobenzene amide groups, together with the hydrogen bonding in the aliphatic amide dendron. This facile communication between the two amide dendrons leads to the dissociation of the gel fibers and collapse of the gel.