Synthesis and characterization of azobenzene hydroxypropyl cellulose with photochromic and thermotropic liquid crystal properties

Two types of photochromic azobenzene hydroxypropyl celluloses (azo-HPCs) are prepared by homogeneous etherification of HPC with 4-(2,3-epoxypropoxy)-azobenzene and bromoethoxy-azobenzene in N,N-dimethylacetamide under mild conditions. The degree of substitution of azo moiety of azo-HPCs is tuned in the range of 0.6–1.8 by adjusting the mole ratio of azobenzene to HPC. Their chemical structures and properties are characterized by elemental analysis, infrared spectroscopy, nuclear magnetic resonance, thermogravimetric analysis, and differential scanning calorimetry. Compared to other cellulose ethers and esters with glass transition temperature (Tg) of normally larger than 100 °C, the azo-HPC derivatives display very low Tg of ca. 20–40 °C. The azobenzenoxy-ethoxy-HPC is a thermotropic liquid crystal which displays cholesteric texture at 165 °C while cooling from its isotropic melt. Their solutions show almost fully reversible trans–cis–trans transition upon alternating irradiation of UV and visible light.