| 英文摘要 |
Structurally isomeric cholesteryl-appended azobenzene derivatives with various substituents, such as H/unsubstituted, etherand ester, at the terminal position of azobenzene units were designed, synthesized and characterized spectroscopically. The effectof substituents on the thermal and optical properties of all the derivatives were studied. The results from thermogravimetricanalysis (TGA) revealed that all the derivatives were stable up to 283°C and start to degrade thereafter. In addition, the etherderivatives exhibited lower thermal stability than the other derivatives. It is likely that the presence of ether linkage at the terminalposition introduced greater flexibility to the azobenzene and, consequently, reduced thermal stability. The ether and esterderivatives underwent two-stage decomposition when heated up to 600°C; the first stage between 320-350°C due to the breakingof ether and ester linkages, and the second stage between 440-490°C due to the breaking of carbonate linker which connectazobenzene to the cholesteryl unit. However, the unsubstituted (H-substituted) derivative underwent only one-stage degradation at386°C due to the cleavage of the carbonate linker. Results from the differential scanning calorimetry (DSC) revealed that themelting transition temperature (Tm) of the ester derivative was higher than the other two, supporting the above-said phenomena ofthermal stability. The photolysis of all the derivatives exhibited trans-cis isomerization when irradiated with UV light. The rate ofphotolysis of ether was faster (2 min) than both ester and the unsubstituted derivatives (5 min). These results revealed thatsubstituents at the terminal position of azobenzene are able to influence thermal and optical properties under given conditions. |
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