Photochemical Phase Transition and Molecular Realignment of Glass-Forming Liquid Crystals Containing Cholesterol/Azobenzene Dimesogenic Compounds

Abstract

We synthesized dimesogenic liquid crystals with both cholesterol and azobenzene groups by esterification of 10,12-docosadiynedioic acid or docosanedioic acid with 4-alkyl-4′hydroxyazobenzene (or 4-hydroxyazobenzene) and cholesterol and studied the thermal, optical, and glass-forming properties. Especially, we focused on the effect of the length of the alkyl chain substituted on the azobenzene unit and the photochemical isomerization of the azobenzene unit on the properties of the mesophases of the compounds or a host cholesteric liquid crystal forming stable glass when using them as a dopant. Two compounds without an alkyl chain on the azobenzene unit show a cholesteric phase, while five compounds with an alkyl chain from butyl to hexadecyl show only a smectic phase. The cholesteric reflection bands of the compounds without an alkyl chain on the azobenzene unit shifted toward shorter wavelengths upon photoisomerization of the azobenzene unit from the trans to cis form. Further photoirradiation induced an isotropic phase in the compounds. The compounds with an alkyl chain showing a smectic phase directly changed to an isotropic phase after the photochemical formation of a certain amount of the cis form. All compounds attained a glassy state by maintaining a molecular order if the compounds between the substrates were rapidly cooled from their liquid-crystalline temperature to 0 °C. When the compounds were doped with 2-10 wt % of a dicholesteryl ester forming a stable cholesteric glass, the cholesteric band of the mixtures shifted toward shorter wavelengths following the photoisomerization from trans to cis of the azobenzene unit on the dopant. The extent of the shift depends on the irradiation energy, temperature, and length of the alkyl chain on the azobenzene unit. The mixture went into the glassy state, keeping the shifted cholesteric reflection band after rapid cooling from the liquid-crystalline temperature to 0 °C. The cholesteric glassy state was stable up to 80 °C. Since the molecular arrangement of the liquid-crystalline compounds and the conformation of the azobenzene unit completely returned to the initial state upon reheating to its isotropic temperature, the photoresponsibility and glass-forming property of the compounds or mixtures are applicable to rewritable recording materials.

Synthesis, Liquid‐Crystalline Properties, and Photo‐optical Studies of Photoresponsive Oligomeric Mesogens as Dopants in a Chiral Glassy Liquid Crystal

“…The images of the LC textures were recorded using a close-up lens and a Sony DXC-950 CCD video camera system. Photoimaging was performed using a laser-scanning instrument, as previously reported [40]. …”

“…[4,40] Figure 7 displays the photoinduced shifts (325 nm, He-Cd laser) of CD8 doped with 3 wt.-% of polymers after UV irradiation. The reflection band shifted to shorter wavelengths upon increasing the irradiation energy and was accompanied by a decrease in the absorption at 360 nm arising from cis-trans isomerization (Fig.…”

Wide‐band reflective polarizers from cholesteric liquid crystals with stable optical properties

“…The correlative methods can be found in patents and research publications. [6][7][8][9][10][11] Glassy cholesteric liquid crystals possess the properties of conventional cholesteric liquid crystals and glassy thermoplastics. By preventing the crystallization of glassy cholesteric liquid crystals, rapid cooling is a very easy way of keeping the molecular helices.…”

Butadienes as Novel Photochromes for Color Tuning of Cholesteric Glasses: Influence of Microscopic Molecular Reorganization within the Helical Superstructure

“…Our earlier studies on similar host±guest systems containing various dopants, including dialkyl-and dialkoxy-substituted azobenzene, azoxybenzene, and biphenyl, have shown that the guest molecules play a crucial role in inducing the formation of smectic domains or clusters within the cholesteric molecular arrangement of the host. [12,24,30] These domains are understood to be consequences of order parameter fluctuations of cholesteric regions to and from a smectic-like arrangement, which are also termed as cybotactic groups. [31,33,34] XRD patterns of unaligned samples of BCn-CD8 mixtures exhibited two diffraction peaks, at 3.15 and 17.1 corresponding to layer spacing of 28.01 and 5.18 .…”

“…[7,8,23] We have recently demonstrated the use of cholesteric glassy liquid crystals as efficient materials for rewritable full-color imaging, based on thermal and photon mode. [19,20,24] Several mechanisms have been proposed to explain the role of dopant molecules in the modulation of macroscopic chirality of CLCs. Broadly these include: i) transfer of chirality from a chiral dopant to a mesogenic host, where the potential of the dopant is characterized by its helical twisting power (HTP), [17,25±29] ii) the impurity effect, where an achiral solute in a CLC medium perturbs the phase-transition temperatures of the host CLC leading to an isothermal modification of the helical pitch, and iii) a dopant-induced formation of smectic domains (smectic cybotactic groups) of the host CLC within its helical superstructure.…”