Optically Active Diarylethenes for Multimode Photoswitching Between Liquid-Crystalline Phases

Abstract

The control of the structure and optical properties of liquid-crystalline (LC) phases by means of light is a major challenge in the development of molecular device and optical data storage systems. [1] For instance the use of photochromic organic molecules for the modulation of mesophases is illustrated by the switching between nematic and isotropic LC phases based on the photochemical cis±trans isomerization of doped azobenzene in nematic media. [2] It is well known that nematic liquid crystals can be converted into chiral nematic (induced cholesteric) LC materials using a chiral dopant. [3] Since nematic and chiral nematic LC phases exhibit distinct optical properties photochemical switching between these phases is highly significant for display technology. In a few cases reversible optical switching between a nematic (N) and chiral nematic (N*) phase has recently been demonstrated. [4±8] We have reported the photomodulation of LC phases using ªchiroptical molecular switchesº. [5,8] Photochemical interconversion of two opposite helices of cis and trans isomers of overcrowded alkenes doped in nematic LCs resulted in switching between two cholesteric phases, depending on the irradiation wavelength employed. As the isomers of the chiral dopant were pseudoenantiomers the pitches of the two phases were opposite in sign. [5] When enantiomers were used instead (racemic mixture), circular polarized light was necessary in order to achieve a small enantiomeric excess, which was amplified by doping the chiral overcrowded alkene photoswitch in a nematic LC phase, which resulted in a chiral nematic phase. [8] It is also conceivable that switching between nematic and cholesteric phases can be achieved by doping with a chiral photoswitch that exhibits large difference in twisting power in its two photostates. [4±7] By doping the nematic LC sample with the amount of dopant needed for the formation of a chiral liquid-crystalline phase only in one photostate a N ® N* or N* ® N LC switch can be produced.Diarylethenes were found to be efficient optical switches (Scheme 1) [9±13] and the two photostates show very different absorption spectra (depending on the substituents). The absorption maximum of the closed-form diarylethene is about 300 nm red shifted from the absorption maximum of the open form, a feature that was used as a non-destructive read-out method. Chiral diarylethenes were shown to undergo asymmetric cyclization, providing one diastereo-_______________________ ± [*] Prof.

Syntheses of Dithienylcyclopentene Optical Molecular Switches

“…The dialdehyde 10 is a precursor for the bis[(phenylethyl)imino] derivative 12. [26] Furthermore a condensation reaction with malonitrile has been carried out to yield 13. [27] Lithiated 1 can also be quenched with a Brønsted acid to give 9.…”

“…Treatment of this compound with a lithiating reagent would result in lithium/halogen exchange of the chlorine atom at the 2-position instead of deprotonation at the 4-position. Therefore this compound was first brominated at the 4-position using Br 2 in chloroform to give 3-bromo-5-chloro-2-methylthiophene (26). Compound 26 was then lithiated at Ϫ78°C in anhydrous diethyl ether using nBuLi.…”

Photoresponsive Cholesteric Liquid Crystals

“…5.9) [43][44][45][46][47][48]. For example, doping 1.4 wt% of 6 into nematic LC ZLI-389 can efficiently induce a cholesteric phase with the characteristic fingerprint texture ( Fig.…”

“…After UV irradiation at 300 nm for 50 s, the cholesteric phase disappeared and a characteristic schlieren texture of nematic phase was observed due to the dramatic HTP decrease of the dopant during photoisomerization. Visible light irradiation of the sample for 30 s resulted in the reappearance of the cholesteric fingerprint texture [43].…”

Phototunable Azobenzene Cholesteric Liquid Crystals with 2000 nm Range

“…[23] Since this original work, other chromophores have been examined including fulgides, [33,34] chiral cyclohexane olefins, [35,36] and diarylethenes. [37][38][39][40][41][42] Azobenzene has been the most commonly studied chromophore, due to its thermal stability, distinguished absorbance spectra of trans and cis conformers, and the compatibility of the structural unit into preexisting mesogen synthetic pathways. Azobenzene CLCs (azo-CLCs) based on azobenzene-NLCs have larger tuning ranges if the clearing temperature of the mixture is increased.…”