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).

Photochromic LC–polymer composites containing azobenzene chromophores with thermally stable Z-isomers

“…Such systems may either be formed as mixtures of LCs with azobenzene derivatives, or an azobenzene moiety may be covalently bound in mesogens, thus forming photochromic materials. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The combination of LC order with photochromism provides the possibility of obtaining unique materials that are highly sensitive to light action, which induces reversible E-Z isomerization and signicant structural changes in these systems.…”

“…One of the interesting and promising phenomena observed in such systems is so-called photoinduced isothermal phase transition, resulting from E-Z photoisomerization of the azobenzene groups and formation of bent Z-isomers. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The low anisometry of the Z-form induces a phase transition from an ordered to a less ordered mesophase, or even complete disordering (isotropization) of the sample. As a result of light action, signicant and reversible changes in the optical properties, such as birefringence, dichroism and diffraction efficiency (for photosensitive diffractive gratings), occur.…”

Shape-Persistent Liquid Crystal Elastomers with Cis-Stable Crosslinkers Containing Ortho-Methyl-Substituted Azobenzene

“…Ultraviolet (UV) irradiation of azo-LCEs induces the trans-to-cis isomerization of azobenzene, which reduces the order of the polymer network . Consequently, the azo-LCEs are shortened along the alignment axis and expand in their orthogonal directions, leading to changes in the volume and length of azo-LCE films. , In contrast to the azobenzene isomerization in solution, which occurs on a picosecond timescale, that in solid-state azo-LCEs is prolonged (by orders of magnitude) because it occurs in a restricted network structure . Owing to the spontaneous relaxation of the cis-isomer to its trans counterpart, only short-lived (<24 h) deformation of azo-LCEs can be achieved via UV irradiation .…”