Photoresponsive vitrifiable chiral dimesogens: photo-thermal modulation of microscopic disordering in helical superstructure and glass-forming properties

Abstract

Thermal, photooptical as well as glass-forming properties of a series of photoresponsive chiral dimesogenic compounds containing cholesterol-azobenzene moieties linked to varying alkyl chain lengths (6-14) are described. All compounds except the dimesogen containing 7 methylene spacers show a cholesteric phase. Selective reflections observed in the cholesteric liquid crystals were found to be blue shifted upon increasing the temperature due to variations in smectic fluctuations. X-Ray diffraction studies of these compounds in the cholesteric phase show a low intensity reflection at small angles, the half widths of which decrease on decreasing the temperature, confirming the presence of smectic cybotactic groups. These materials also exhibit a strong odd-even effect in their reflection wavelength as a function of spacer length. Compounds with even numbers of methylene units show a drastic change in their reflection wavelength (102-225 nm) on temperature change compared to those with odd numbers of methylene units (9-14 nm). UV photolysis, using light of 366 nm wavelength, in the cholesteric super-cooled state exhibits a hypsochromic shift in the reflection wavelength. This can be explained by the formation of the cis isomer destroying the smectic clusters in the helical superstructure due to its bent geometry, which in turn causes a decrease in the reflected wavelength. A maximum wavelength shift of 215 nm (703-488 nm) has been observed on photolysis at 80 uC (cooling cycle) for the dimesogen having 12-methylene spacers. By rapid cooling of the compounds from the cholesteric temperature to 0 uC glassy liquid crystalline films maintaining helical molecular order were obtained. The optical properties of these dimesogens, such as cholesteric reflections, controlled by light and temperature combined with their glass forming properties, can be utilized for the construction of information storage materials.

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

“…[11,[24][25][26][27][28] Recently, we reported that the mechanism of the helical-pitch shift that is induced by achiral dopants (as well as in dimesogenic compounds) is due to enhancement of smectic A (SmA)-like clusters in the cholesteric phases. [29,30] Our interest in this topic is for the design of thermally stable, light-driven, full-color, erasable recording devices. For this purpose, it is necessary to obtain suitable photoresponsive dopants that allow quick writing and provide thermally stable color images.…”

Azobenzene Polymers for Photonic Applications

“…Spin-cast films are typically annealed above the polymer glass transition temperature (T g ) to remove residual solvent and erase any hydrodynamically induced anisotropy. Recently molecular glasses have been investigated as alternatives to amorphous polymer systems (Mallia and Tamaoki, 2003). These monodisperse systems appear to maintain the desirable photomotions and photoswitching properties, while allowing precise control of molecular architecture and thus material properties (Naito and Miura, 1993).…”

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

“…[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 . The broad, large-angle peak is assigned to result from interaction of neighboring parallel molecules and the small-angle peak depicts the smectic domains.…”

“…The relationship between FWHM, of the small-angle peak, and the smectic characteristics of similar systems have been explored earlier. [30,31] Figure 5 shows the effect of alkoxy-chain length on the FWHM of the small-angle XRD peaks of BCn-CD8 mixtures. The FWHM of the small-angle XRD peaks for all BCn-CD8 mixtures were larger than that of pure CD8, although there was a red-shift of the reflectance band in each case when compared to that of pure CD8.…”

“…Recent studies from our group have shown that the changes in the cholesteric light reflectivity could be attributed to microscopic changes of smectic domains in some cholesterol-based CLCs. [30,31] Photoisomerization of dialkyl azobenzene derivatives possessing alkyl chains of suitable length was found to affect the cholesteric pitch of the respective host CLC. [12] The effect of several achiral dopants in affecting cholesteric pitch length of CLC was also studied with dopants having different mesogenic cores.…”