Preparation and chiroptical properties of cellulose chlorophenylcarbamate–silica hybrids having a chiral nematic mesomorphic structure

An attempt was made to fabricate optically functional cellulosic–silica hybrid materials wherein a helically ordered mesomorphic structure of the cellulosic component was fixed. Cellulose 3-chlorophenylcarbamate (3Cl-CPC) and cellulose 4-chlorophenylcarbamate (4Cl-CPC) were synthesized by carbanilating reaction of cellulose with 3-chlorophenyl or 4-chlorophenyl isocyanate in a homogeneous solution system. The hydrophobic cellulose derivatives were examined for the solubility and liquid crystallinity in various solvents containing alkoxysilane as a main component. Concentrated (∼40 wt%) solutions of 3Cl-CPC in 3-aminopropyltrimethoxysilane (APTMS) and those of 4Cl-CPC in a mixed solvent of tetramethoxysilane (TMOS)/N,N-dimethylformamide (DMF)/dichloroacetic acid (DCA) formed a chiral nematic mesophase to impart vivid reflection colors. These liquid crystalline solutions were subjected to a sol–gel conversion process of the respective alkoxysilane components in moist air. In the process for the 4Cl-CPC lyotropic system, we utilized a "surface-coating solvent" consisting of TMOS/phenyltrimethoxysilane/DCA to facilitate the sol–gel reaction of TMOS and also to preserve the reflective coloration. Thereby a left-handed chiral nematic hybrid series of 4Cl-CPC–silica was successfully obtained as a monolithic glassy bulk showing iridescent colors. The 3Cl-CPC/APTMS lyotropics were readily converted into a colored solid form without any coating solution, resulting in production of a right-handed chiral nematic series of 3Cl-CPC–silica. Besides investigating the chiroptical characteristics of the liquid crystalline solutions and hybrids, we preliminarily tried separating a racemic compound into two enantiomers by open column chromatography using the cellulosic–silica hybrids as a filler material of the column.