Influence of a Change in Helical Twisting Power of Photoresponsive Chiral Dopants on Rotational Manipulation of Micro‐Objects on the Surface of Chiral Nematic Liquid Crystalline Films

Abstract

Herein we report a group of five planar chiral molecules as photon-mode chiral switches for the reversible control of the self-assembled superstructures of doped chiral nematic liquid crystals. The chiral switches are composed of an asymmetrically substituted aromatic moiety and a photoisomerizing azobenzene unit connected in a cyclic manner through methylene spacers of varying lengths. All the molecules show conformational restriction in the rotation of the asymmetrically substituted aromatic moiety in both the E and Z states of the azobenzene units resulting in planar chirality with separable enantiomers. Our newly synthesized compounds in pure enantiomeric form show high helical twisting power (HTP) in addition to an improved change in HTP between the E and Z states. The molecule with a diphenylnaphthalene unit shows the highest ever known initial helical twisting power among chiral dopants with planar chirality. In addition to the reversible tuning of reflection colors, we employed the enantiomers of these five compounds in combination with four nematic liquid crystalline hosts to study their properties as molecular machines; the change in HTP of the chiral dopant upon photoisomerization induces rotation of the texture of the liquid crystal surfaces. Importantly, this study has revealed a linear dependence of the ratio of the difference between HTPs before and after irradiation against the absolute value of the initial HTP, not the absolute value of the change in helical twisting power between two states, on the angle of rotation of micro-objects on chiral nematic liquid crystalline films. This study has also revealed that a change in irradiation intensity does not affect the maximum angle of rotation, but it does affect the speed of rotational reorganization of the cholesteric helix.

Determination of the absolute stereostructure of a cyclic azobenzene from the crystal structure of the precursor containing a heavy element

“…The presence of the bromine substitution in the naphthalene unit gives an added advantage in determining the absolute stereostructure of the separated enantiomers. The determination of the absolute stereostructure of one of the enantiomers of ( E )- 1 by X-ray diffraction has been presented in our previous report [45]. In this study, we employ this enantiomer as a precursor for the determination of the absolute configuration of its reduced product, which is expected as one of the enantiomers of ( E )- 2 .…”

“…1 shows the schematic representation of experiments involved in the separation and reductive debromination of the enantiomer ( E )- 1 B . The enantiomers of molecules ( E )- 1 and ( E )- 2 are previously reported as photocontrolled chiroptical switches for various nematic liquid crystalline hosts for tuning the reflection colors through the entire visible region [45–46]. The molecule ( E )- 1 has been also used to demonstrate the completely photocontrolled rotation of the micro glass rods on the chiral nematic liquid crystalline films induced by the rotational reorganization of the polygonal finger print texture [45].…”

“…Recently our group reported the synthesis of several cyclic azobenzene molecules and their properties [45–50]. All these molecules share a comparable cyclic structure, with a photoisomerizable azobenzene unit linked to a substituted or unsubstituted aromatic unit such as naphthalene or benzene.…”

“…We have explored the property of planar chirality of these molecules and carried out the optical resolution to demonstrate various properties such as molecular brakes, chiroptical switches and chirality sensors for light and solvent etc. [45–50]. Although we could separate the enantiomers of all these chiral cyclic azobenzenes, the experimental determination of the absolute stereostructure by X-ray diffraction was difficult due to the lack of heavy elements.…”

Simple and highly efficient chiral dopant molecules possessing both rod- and arch-like units

“…12 However, the molecules are relatively large, and the molecular structures calculated by a semi-empirical method (AM1) in their report were complicated because of the exibility in the alkyl spacers and central binaphthyl units. Other chiral dopants, such as dithienylperuorocyclopentene, 13 rocked cyclic conformers, 14 17 and chelated borane imine complexes, 18 are known, but none of these chiral dopants possesses a HTP greater than 100 mm À1 . Hence, a simple and effective molecular design strategy for chiral dopants has not yet been established.…”