Photoinduced reversible gel-sol transitions of dicholesterol-linked azobenzene derivatives through breaking and reforming of van der Waals interactions

A series of new symmetric dicholesterol-linked azobenzene gelators with different spacer lengths have been synthesized. The compounds with spacers of zero, two or six methylene units are denoted as DCAZO0, DCAZO2 and DCAZO6, respectively. A gelation test reveals that a subtle change in the length of the spacer can produce a dramatic change in the gelation behavior of the compounds. DCAZO2 obtains the minimum gelation concentration among the three gelators. For cyclopentanone gel of DCAZO2, the reversible gel-sol transitions by irradiation with UV and visible light are investigated by UV-vis absorption and circular dichroism (CD) spectra, SEM, TEM and XRD 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 the exposure to visible light. SEM, TEM and XRD studies reveal that the gelator molecules self-assemble into one-dimensional fibers with diameters 50-100 nm in an anticlockwise direction, which further crossed-linked to form three-dimensional networks.