Planar Chiral Azobenzenophanes as Chiroptic Switches for Photon Mode Reversible Reflection Color Control in Induced Chiral Nematic Liquid Crystals

Abstract

In this report, for the first time, a planar chiral photoresponsive compound has been employed in commercially available nematic liquid crystals to achieve phototunable reflection colors. We designed an azobenzenophane compound having conformational restriction on the free rotation of naphthalene moiety to impose an element of planar chirality and the corresponding enantiomers were resolved by HPLC on chiral column. We have determined the absolute configuration by comparison of density functional theory (DFT) calculations of its electronic circular dichroism (ECD) spectrum and specific rotation [alpha] D to experimental ECD and [alpha] D data. Enantiomers exhibit photochemically reversible isomerization in solution without undergoing thermal or photoinduced racemization. As chiroptic switches in different host nematic liquid crystals, they exhibit good solubility, moderately high helical twisting power, as well as a large change in helical twisting power due to photoisomerization. A unique feature of these chiral photochromic compounds is that no other auxiliary chiral agents is required to achieve a fast photon mode reversible full-range color control in induced cholesterics, that is, both the hypsochromic and bathochromic shift can be obtained from a single LC formulation by reversible photoisomerization of the single chiral compound.

Photoresponsive Cholesteric Liquid Crystals

“…In 2008, Tamaoki et al reported a planar chiral dopant which was employed in commercially available nematic LCs to achieve phototunable reflection colors [113]. This compound was designed based on an azobenzenophane compound having conformational restriction on the free rotation of naphthalene moiety to impose an element of planar chirality.…”

Design and Synthesis of Chiral Nematic Liquid Crystals

“…The melting points, clearing points, and the N*-SmA* phase transition temperatures are indeed all slightly higher than for the corresponding 2 -fluoro homologs; the SmA* phases all occur enantiotropically, indicating greater thermodynamic stability, unlike the preceding SmA* phases of the 2 -fluoro series (82)(83)(84)(85)(86)(87). This overall result may be partly due to the shielding of the 3-fluoro-substituent by the carboxylate linkage and the fact that the second fluoro-substituent effectively makes the liquid crystal core no broader (i.e., the second fluoro-substituent has little effect on the overall geometrical anisotropy of the molecule).…”

“…This method is useful for measuring long pitch lengths by using very small amounts of material (5 × 10 −5 mg of dopant), and is a simpler alternative to complex chiral GC [72][73][74]. • Chiroptic Liquid Crystal Switches and Motors -here chiral photochemical dopants (chiral molecular motors) have been employed to manipulate the reflection wavelength of induced chiral nematic phases [75][76][77][78][79][80][81][82][83][84][85][86][87][88][89], as optical recording media, and to control the pitch and twist sense of material as a means of controlling viewing angle in LC displays [90].…”

“…Cr ___ N* −6.1 I stress the versatility of these methods, the increased use of which has led to the accessibility of numerous structures throughout liquid crystal chemistry, which were previously thought to be too difficult to synthesize easily. In the first of these fluoro-substituted series, the (R)-1-alkoxy-2-[4-(2 -fluoro-4 -pentylbiphenyl-4-carbonyloxy)phenoxy]propanes (82)(83)(84)(85)(86)(87), it was found that the 2 -fluoro-substituent destabilizes the ability of the molecules to pack efficiently to such a degree that they prefer to form chiral nematic phases over the more ordered smectic phases observed in their nonfluoro-substituted parents (compound 81) [143]. The first five materials show monotropic smectic A* phases at correspondingly low temperatures; furthermore the butyl, pentyl, and hexyl homologs did not crystallize above a temperature of −40 • C (potentially an advantage for microencapsulation).…”

Reversibly tunable helicity induction and inversion in liquid crystal self-assembly by a planar chiroptic trigger molecule

“…8, 12 Recently, we reported the first example of a photoresponsive planar chiral dopant in the form of a monocyclic azophane and successfully achieved a fast photon mode reversible full-range color control in induced cholesterics. 13 Here we report our success in 55 reversibly controlling the helical pitch length as well as the handedness of the induced helix by photochemical and thermal isomerizations of a planar chiral bicyclic azobenzenophane (1) in host nematic LCs. Target molecule was synthesized as pure trans isomer of the racemic compound E,E-1 and the structure was fully 85 characterized by NMR and X-ray crystallographic analysis.…”