Chiral, Thermally Irreversible and Quasi-Stealth Photochromic Dopant

Liquid crystals (LCs) present a state of matter whereby their properties lie between those of conventional liquids and those of solid crystals. Typically, the temperature of the system, the on/off switching of the electric field, and the interaction with the contacting surfaces have been shown to control the bulk properties of LCs. Additionally, a fourth simulation method to modulate properties of LCs i.e. ‘light’, which comes into play when photo-responsive molecules are mixed into the LC systems or photo-responsive functional groups are incorporated in the LC molecules. Research has shown that when the photo-responsive moiety in a liquid crystal molecule is photochromic, the change in properties of the LC system can also be reversible. So far, the photochemical reversible control of the cholesteric pitch length and, hence, the wavelength of the selective reflection have been examined using several different photochromic compounds. Nonetheless, when these photochromic compounds are applied to displays or bulletin boards, they ought to be thermally irreversible and both photoisomers should not have absorption bands in the visible region. Published literature has thus shown that if the compound employed is thermally reversible, the images will disappear over the course of time. Further, if the photochromic compound absorbs light in the visible region, photochromic reactions visible to the human eye are induced.

In general, researchers have established that reversible azobenzenes and spirooxazines are therefore inadequate for the aforementioned purpose, and overcrowded stilbenes are not recommended since their photochromic reactions occur as a set of photochemical EZ isomerization followed by thermal helix inversion, though both of their stable isomers are mostly colorless. To this end, various attempts have been reported in search for a suitable compound to which mixed results have been presented. Therefore, to address this, researchers from the Yokohama National University in Japan: Engineer Yoshihisa Kurosaki, Dr. Toshiya Sagisaka, Engineer Tomoo Matsushima, Professor Takashi Ubukata and Professor Yasushi Yokoyama, reported the control of the selective reflection color of the cholesteric liquid crystal induced by a thermally irreversible photochromic (R)-binaphthol-condensed benzofurylfulgide 3E/3C photo-conjugate as the photo-responsive chiral dopant. Their work is currently published in the research journal, ChemPhysChem.

In their approach, a chiral and thermally irreversible photochromic fulgide derivative incorporating an (R)-binaphthol unit in its acid anhydride moiety was used for the photoswitching of the pitch length of cholesteric liquid crystals. By acknowledging the fact that absorption maximum wavelengths of both thermally stable photoisomers were nearly in the UV region (quasi-stealth photochromism), the researchers were able to expose it to visible light without inducing photochromic reactions.

The authors reported that the photochromic dopant 3E was chiral, thermally stable and a quasi-stealth photochromic fulgide derivative which possessed an (R)-binaphthol moiety at the acid anhydride moiety, while the absorption maximum wavelengths of both photoisomers were 313 nm (3E) and 398 nm (3C). Further, the photoresponsive cholesteric liquid crystal (PCLC) showed reflection color change between red and blue, which was associated with the photochromism of 3E induced by 313-nm light irradiation and due to the change in the helical twisting power.

In summary, the study demonstrated the preparation of a photoresponsive cholesteric liquid crystal composed of a photoresponsive chiral photochromic dopant 3E, nonphotoresponsive chiral dopant and a base nematic liquid crystal. Remarkably, the researchers were able to successfully regulate the color of cholesteric liquid crystalline cells between red and blue upon UV light irradiation. In a statement to Advances in Engineering, the authors highlighted that they succeeded in realizing reflection light control in the IR region by employing the polymer-dispersed cholesteric LC technique.