Photocontrolled Phase Transitions and Reflection Behaviors of Smectic Liquid Crystals by a Chiral Azobenzene

Abstract

The photocontrolled phase transitions and reflection behaviors of a smectic liquid crystal, 4-octyl-4'-cyanobiphenyl (8CB), tuned by a chiral azobenzene, are systematically investigated. For the smectic 8CB doped with the chiral azobenzene (1R)-(-)-4-n-hexyl-4'-menthylazobenzene (ABE), the initial smectic phase can be switched to cholesteric and then to isotropic upon UV irradiation due to the trans-to-cis photoisomerization of ABE; however, no reflection band is observed. For the smectic 8CB doped with ABE and the chiral agent (S)-(-)-1,1'-binaphthyl-2,2'-diol (BN), a reflection band located in the short-wavelength infrared region is observed, which disappears after further UV irradiation. For the smectic 8CB doped with ABE and a chiral agent with higher helical twisting power, (S)-2,2'-methylendioxy-1,1'-binaphthalene (DBN), a phototunable system with cholesteric pitch short enough to reflect visible light is demonstrated. With a given concentration of the chiral dopant DBN, a reversible reflection color transition is realized tuned by the isomerization of azobenzene. The reverse phase transition from isotropic to cholesteric and then to smectic can be recovered upon visible irradiation. The photocontrolled phase transitions in smectic liquid crystals and the corresponding changes in reflection band switched by photoisomerization of azobenzene may provide impetus for their practical application in optical memories, displays, and switches.

In Situ Controlled Construction of a Hierarchical Supramolecular Chiral Liquid‐Crystalline Polymer Assembly

“…[28] Generally,arod-like trans-form Azo mesogen can be isomerized to bent-shaped cis-form by UV light irradiation and reversibly transformed to trans state by visible light or heat treatment, accompanying changes in molecular conformation, polarity and other properties. [29] Furthermore,c hirality in Azo liquid-crystalline mesogens [30] has been widely investigated and supramolecular chirality has been frequently introduced in Azo-containing superstructures. [31] Supramolecular chirality of the side chain Azo polymer was usually constructed by postpolymerization assembly of chiral Azo polymers,orachiral Azo polymers triggered by chiral sources, including circularly polarized light (CPL), [32] chiral gelators, [33] and chiral solvation.…”

Photo-Orientation Phenomena in Photochromic Liquid Crystalline Azobenzene-Containing Polymethacrylates with Different Spacer Length

“…[1][2][3][4] Photoisomerization of azobenzene fragments in LC media enables to realize isothermal phase transitions. [5][6][7][8][9][10][11][12] From the other hand, an action of polarized light induces photo-orientation processes and alignment of chromophores in direction perpendicular to the polarization plane of the incident light. [13][14][15][16][17][18][19][20] Another interesting direction of the research is connected with investigation of the surface topography of LC polymer systems and, especially, manipulation of surface relief by light irradiation.…”

“…Several unique advantages of the azobenzene molecules make them the most popular photochrome, such as high quantum yield of photoisomerization, high fatigue resistance, and extreme change in the shape as well as in dipole moment during isomerization . Photoisomerization of azobenzene fragments in LC media enables to realize isothermal phase transitions . From the other hand, an action of polarized light induces photo‐orientation processes and alignment of chromophores in direction perpendicular to the polarization plane of the incident light …”

Light-controllable reflection wavelength of blue phase liquid crystals doped with azobenzene-dimers

“…12 The trans-isomer, which is linear and thermodynamically stable, is beneficial for the stability of LCs; whereas the cis-isomer, which is bent and thermodynamically metastable, destabilizes the LC phase. Depending on the trans-cis transformation of azobenzene, extensive studies have been carried out to control the orientation of LCs, such as inducing smectic phase transition, 13 adjusting the reflection wavelength of the N* phase 14 or Bragg reflection of BPs. 15 Among them, the tuneable wavelength of BPs is attracting since it can be potentially used in three-dimensional photonic crystals and optically addressable BP-LC displays, which was first studied by H. Y. Liu et al 15 However, the adjustment range mentioned in their report is limited to several tens of nanometres and the reflected color of the LC-BP platelet only changed from brown to green, so there is an urgent need to overcome their limitations.…”