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Azobenzene liquid crystals, possessing two chiral moieties at both of the peripheral ends of the molecular structure, show characteristic chirality-originated liquid-crystalline and physical properties: emergence of stable smectic Q phases, a photoinduced phase transition between 3D liquid crystal phases, and induction of birefringence in the “liquid” phase. A dichiral compound, of which rigidity of the core is reduced by the introduction of ester groups, showed no smectic Q phases, but exhibited the antiferroelectric smectic C phase. The “liquid” phase of the less-rigid azo-compound was found to show electrically induced birefringence. [ABSTRACT FROM PUBLISHER]

Abstract: We investigated photochemical transformation of asymmetric topological defects formed around water droplets dispersed in the azobenzene-containing liquid crystals. It was found that initial structures and the photo-induced phenomena of the defects were clearly affected by chemical structures of the azobenzene compounds. In quite strong homeotropic anchoring obtained by using the azobenzene compound with a long alky chain, the hedgehog was formed around the droplets at the initial state. When the anchoring strength was decreased by changing the carbon number of the alkyl chain, a novel asymmetric defect which will be an intermediate structure between the hedgehog and the Saturn ring was observed. The ionic groups had an influence on the photochemical transformation of the defects. The azobenzene compound with a smaller ionic group induced only a change in the anchoring strength, and a disappearance of the hedgehog was observed on the irradiation with ultra-violet light. In the case of the azobenzene compound with a larger ionic group, photochemical switching of the anchoring and the transformation into the boojums was achieved. The photo-induced phenomena could be qualitative explained by an effectiveness of the cis–trans photoisomerization depending on molecular areas of the azobenzene compounds determined by the ionic groups. [Copyright &y& Elsevier]

Photonic control of defect structures and colloidal chains has been performed in azobenzene-containing liquid crystals dispersed with glycerol or water droplets. When we dispersed the colloidal droplets into the host liquid crystals, we could observe, at an initial state, Saturn ring and hedgehog around the glycerol and water droplets, respectively, indicating that normal alignment of liquid crystal molecules was induced on the droplets by the adsorption of the azobenzene molecules onto the droplets. In the case of the glycerol droplets, we achieved photochemical manipulation of the defect structures between Saturn ring and boojums on irradiation with ultra-violet and visible light. For the water droplets, an inter-droplet distance of the self-organized colloidal chain could be controlled by changes of the defect size on the photoirradiation. Photoinduced structural changes in the topological defects can be explained by the modulation of surface anchoring of the droplets by means of the cis-trans photoisomerization of the adsorbed azobenzene molecules. [ABSTRACT FROM AUTHOR]

We investigated liquid-crystalline properties and photoresponse of a chiral azobenzene compound and found that it shows two kinds of highly-ordered liquid crystal phases. Studies on the optical textures and thermodynamic properties of the azobenzene compound suggest that the both mesophases are classified as the smectic Q (SmQ) phase which has been known as one of the chirality-induced three-dimensional (3D) liquid crystal phases. By the miscibility studies, one of the mesophases was confirmed to be T*I structure among the four structures of the SmQ phase. In a spectroscopic study of the chiral azobenzene compound in a tetrahydrofuran solution, it has been confirmed that a cis-trans photoisomerization can be induced by irradiation with ultra-violet light. Although the 3D structures are not sensitive to the applied electric field, we successfully demonstrate the “photo-switching” of these 3D structures by means of the photoisomerization. [ABSTRACT FROM AUTHOR]

We investigated photoresponsive behavior of azobenzene-containing liquid crystals dispersed with glycerol or water droplets. At an initial state, Saturn ring and hedgehog defects were formed around the glycerol and water droplets, respectively, indicating that homeotropic anchoring was induced by adsorption of the azobenzene molecules onto the droplets. For the glycerol droplets, we observed structural transformation between Saturn ring and boojums on irradiation with ultra-violet and visible light. For the water droplets, an inter-droplet distance varied by changes of the defect size on the irradiation. These phenomena would result from modulation of anchoring conditions by the photoisomerization of the adsorbed dyes. [ABSTRACT FROM AUTHOR]

Abstract: By modulating liquid-crystal alignment on a colloidal sphere, we successfully manipulated topological defects in glycerol-droplet/liquid-crystal emulsions doped with amphiphilic azobenzene derivatives. At an initial state, a disclination loop (Saturn ring) could be observed around the droplet, in which the azobenzene molecules should adsorb onto the droplet and liquid crystal molecules align normally to the surface of the droplet. On irradiation with ultra-violet light (λ =365 nm), the disclination loop was unfastened and transformed into two point defects called boojums. This should be attributed to the alignment change of the liquid crystal molecules from normal to planar arrangement triggered by trans-to-cis photoisomerization of the adsorbed azo-dyes. On irradiation with visible light causing cis-to-trans photoisomerization (λ =435 nm), the boojums went back to the Saturn ring reversibly. [Copyright &y& Elsevier]

Unique characteristics were observed in phase-type gratings which were formed in polymer liquid crystal (PLC) films. We have investigated the formation and the properties of the holographic gratings recorded in PLCs with azobenzene moieties (photoresponsive unit). Two types of PLCs were examined: homopolymers of azobenzene monomers (PALC) and copolymers with a tolane moiety (mesogenic unit with a high birefringence) in the side chain. The diffraction efficiency (η) was enhanced in the LC phase. We obtained the value of η of 31% while it was less than 1% in a glassy state in the Raman-Nath regime. This difference may be due to the modulation of refractive index (Δn) based on alignment change of mesogens in the PLC film. In addition, we attempted the holographic image storage of three-dimensional (3-D) objects in the PLC film. The 3-D object was reconstructed with high resolution (>5,000 lines/mm). [ABSTRACT FROM AUTHOR]