부산대학교 도서관

The thermal and optical properties of three different pre-designed eutectic gels were investigated. The gels were obtained by gelation of azobenzene-cholesteryl derivatives by varying substitutions at the terminal position of the azobenzene unit [H/unsubstituted (Azo-1), pyridine (Azo-2) and ester (Azo-3)]. Gelation ability was studied in a deep eutectic solvent of Choline Chloride: Urea (Ch: Urea). The viscosity of the gels was greater than 400 mPa．s and viscosity was found to decrease as temperature increased, due to the weakening of the attractive forces between the molecules. The thermal stability of the eutectic gels, in decreasing order, was Azo-1＞ Azo-2 ＞ Azo-3. Among the gels, Azo-1 and Azo-2 underwent a single-step degradation at 242 and 265 °C, respectively. However, Azo-3 gel underwent a two-stage degradation; the first at 160 °C due to cleavage of the ester attached to the terminal position of the azobenzene, and the second at 245 °C which corresponds to the breaking of the carbonate link [O-C(=O)-O-]. The photolysis of all the gels exhibited obvious absorption maxima at 369 nm, corresponding to the π-π* transition band of the trans-isomers of the azobenzene units. However, the gels did not undergo photo-isomerization under irradiation. It is suspected that this was due to the high viscosity of the eutectic solvent which arrested the transformation of the trans-to-cis isomers. Reflectance spectra results showed that all the gels had a reflectance greater than 30% under visible light (＞ 400 nm) as well as in the near IR region (＞ 700 nm).