title: Azo monomers exhibiting low layer shrinkage at the SmA-SmC transition and trans-cis light-induced isomerization.
abstract: 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 SmA-SmC transition. This atypical behavior is attributed to the combination of a high smectic order promoted by both π-π and bond dipole-bond 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 trans-cis 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).
authors.affiliation: Departamento de Materiales Avanzados, Centro de Investigación en Química Aplicada, Saltillo Coahuila, México.
authors.forename: Tonatiuh
authors.initials: T
authors.lastname: García
authors.forename: Leticia
authors.initials: L
authors.lastname: Larios-López
authors.forename: Rosa Julia
authors.initials: RJ
authors.lastname: Rodríguez-González
authors.forename: José Román
authors.initials: JR
authors.lastname: Torres-Lubián
authors.forename: Dámaso
authors.initials: D
authors.lastname: Navarro-Rodríguez