Azo Monomers Exhibiting Low Layer Shrinkage at the SmA-SmC Transition and trans-cis Light-Induced Isomerization

Methacrylic monomers containing a (phenylene)azobenzene unit substituted with a lateral cyano group and alkyl chains of different length are synthesized and characterized by NMR techniques. Their liquid-crystalline properties are studied by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. All monomers exhibit a mesomorphic behavior that extends over wide temperature ranges with nematic and orthogonal or tilted smectic-type mesophases, depending on the length of the terminal chain. The smectic structures are determined to be single-layered with a low layer shrinkage (<5?%) at the SmASmC transition. This atypical behavior is attributed to the combination of a high smectic order promoted by both pp and bond dipolebond dipole interactions between cyano-substituted central cores, and a low correlation between neighboring layers arising from dispersive forces between the end groups (methacrylic group and alkyl chain) of the monomer. On the other hand, the transcis isomerization of monomers is induced in solution by irradiating with a UV lamp. High cis-isomer contents (=96?%) are obtained at the photostationary state, which is reached in a relatively short time (40 s).