Study of Aggregation Behavior of Predesigned Azobenzene-Cholesteryl Derivatives in Deep Eutectic Solvents

Abstract

Structurally isomeric cholesteryl-appended azobenzene derivatives (azo-1 to azo-5) with various substituents, such as H/unsubstituted, ether, ester, and nitro at the terminal position of azobenzene units were designed, and synthesized. The gelation ability and aggregation behavior of the above synthesized azobenzene-cholesteryl derivatives in deep eutectic solvents (DES) such as Zinc Chloride: Ethylene Glycol (Zn:EG), Choline Chloride: EG (Ch:EG), Choline Chloride: Urea (Ch: Urea), and Choline Chloride: Glycine (Ch: Gly) were studied. The results revealed that all the azo derivatives formed semi-transparent and strong/hard eutectic gels in at least one DES except azo-4 which formed gel in two DES. The morphological analyses by scanning electron microscopy (SEM) exhibited entangled dense fibrous, flowers, and sheet-like textures, depending on the nature of DES as well as azo derivatives. Like all azobenzene-based organo-gelators, UV-triggered gel-to-sol transition was expected for these eutectic gels. However, these eutectic gels did not undergo the gel-to-sol transition under UV irradiation. This could be due to the hardness of the gel, which arrests the structural transformation from trans-to-cis during photolysis. It was further confirmed by absorption profiles of before and after irradiation of eutectic gels. Regarding application, an attempt has been made to use eutectic gels as a template for the synthesis of nanomaterials and the results revealed that the azo-4 gel can be used to prepare aggregated highly dense nanorods of copper chloride.