Modification of the pitch of chiral nematic liquid crystals by means of photoisomerization of chiral dopants

New photoisomerizable chiral dopants have been studied. The dopants used were menthone derivatives, a chiral stilbene derivative and a nematic copolymer of a menthone derivative and a benzoyloxybenzonitrile derivative. NMR, HPLC and UV results showed that the E-Z-isomerization of all the compounds indicated proceeded rapidly upon UV exposure, without the formation of undesired by-products. Isomerization of the menthone derivatives, including the copolymer, induced a substantial decrease in the helical twisting power. The changes in helical twisting power induced by the isomerization of the chiral stilbene derivative were limited to a factor of 2. In mixtures of the photoisomerizable dopants with commercial dopants of opposite twisting senses and a nematic host mixture, the sign of the twisting sense could be reversed by illuminating the mixture with UV light. The viewing angle dependence of irradiated regions of a 90 -twisted nematic cell was rotated 90 with respect to the viewing angle dependence of the non-irradiated regions. It is expected that this approach may be useful in the preparation of dual domain TN cells with a reduced viewing angle dependence.     

Optical behaviour of photoimageable cholesteric liquid crystal cells with various novel chiral compounds

In theory, both polarity and steric hindrance are basic factors which affect molecular interactions. To investigate the optical properties and steric structures of chiral compounds having different chiral moieties which affect the wavelength of light reflection in liquid crystal (LC) cells, a series of novel chiral compounds and azobenzene derivatives were synthesized. The liquid crystalline phases of the compounds were identified using small angle X‐ray diffraction, differential scanning calorimetry and polarizing optical microscopy. Cholesteric LC cells with various synthesized chiral dopants which selectively reflect visible light were first prepared, the photochemical switching behaviour of colours was then investigated, with special reference to the change in transmittance in cholesteric LC cells containing an azobenzene derivative as a photoisomerizable guest molecule. Reversible isomerization of azobenzene molecules occurred in the cholesteric systems, resulting in a depression of T _ChI and a shift of the selectively reflected wavelength. We discuss the photochemically driven change in the helical pitch of the cholesteric LCs with respect to structural effects involving the chiral moieties. Molecular interactions caused by the added dopants, reliability and stability of the photoisomerization, and UV irradiation effects on the cholesteric LC cells were also investigated. A real image was recorded through a mask on a cholesteric LC cell fabricated in this investigation.     

Optical behaviour of photoimageable cholesteric liquid crystal cells with various novel chiral compounds

In theory, both polarity and steric hindrance are basic factors which affect molecular interactions. To investigate the optical properties and steric structures of chiral compounds having different chiral moieties which affect the wavelength of light reflection in liquid crystal (LC) cells, a series of novel chiral compounds and azobenzene derivatives were synthesized. The liquid crystalline phases of the compounds were identified using small angle X-ray diffraction, differential scanning calorimetry and polarizing optical microscopy. Cholesteric LC cells with various synthesized chiral dopants which selectively reflect visible light were first prepared, the photochemical switching behaviour of colours was then investigated, with special reference to the change in transmittance in cholesteric LC cells containing an azobenzene derivative as a photoisomerizable guest molecule. Reversible isomerization of azobenzene molecules occurred in the cholesteric systems, resulting in a depression of T_ChI and a shift of the selectively reflected wavelength. We discuss the photochemically driven change in the helical pitch of the cholesteric LCs with respect to structural effects involving the chiral moieties. Molecular interactions caused by the added dopants, reliability and stability of the photoisomerization, and UV irradiation effects on the cholesteric LC cells were also investigated. A real image was recorded through a mask on a cholesteric LC cell fabricated in this investigation.     

Photosensitive cholesteric polymers with azobenzene-containing chiral groups and mixtures of cholesteric copolymer with chiral-photochromic dopants

New chiral photochromic cholesteric comb-shaped acrylic copolymers and low molecular mass dopants containing azobenzene photosensitive fragments and chiral groups based on menthol and menthone were synthesized. For the copolymers and their mixtures with low molecular mass dopants, the phase behaviour and optical properties were studied. Under irradiation with UV and visible light, the untwisting of cholesteric helix takes place, and the selective light reflection maximum is shifted to the long wavelength spectral region. This shift is related to the E-Z isomerization of the azobenzene chiral groups. For the copolymers and mixtures of the cholesteric polymer with the menthyl-containing dopant, this process is thermally reversible. The specific features of the kinetics of the forward and the reverse thermal processes were characterized. It was demonstrated, that the copolymers and mixtures of the cholesteric copolymer with the menthyl-containing dopant may be used for coloured reversible recording of optical information. For such materials, their resistance with respect to the repeated 'recording-erasing' cycles was tested, and the fatigue resistance was shown to be rather high.     

Photosensitive cholesteric polymers with azobenzene-containing chiral groups and mixtures of cholesteric copolymer with chiral-photochromic dopants

New chiral photochromic cholesteric comb-shaped acrylic copolymers and low molecular mass dopants containing azobenzene photosensitive fragments and chiral groups based on menthol and menthone were synthesized. For the copolymers and their mixtures with low molecular mass dopants, the phase behaviour and optical properties were studied. Under irradiation with UV and visible light, the untwisting of cholesteric helix takes place, and the selective light reflection maximum is shifted to the long wavelength spectral region. This shift is related to the E-Z isomerization of the azobenzene chiral groups. For the copolymers and mixtures of the cholesteric polymer with the menthyl-containing dopant, this process is thermally reversible. The specific features of the kinetics of the forward and the reverse thermal processes were characterized. It was demonstrated, that the copolymers and mixtures of the cholesteric copolymer with the menthyl-containing dopant may be used for coloured reversible recording of optical information. For such materials, their resistance with respect to the repeated 'recording-erasing' cycles was tested, and the fatigue resistance was shown to be rather high.     

Cholesteric induction power of β-galactose derivatives

Galactose derivatives were explored as chiral dopants, effective for inducing chiral nematic liquid crystal (LC) phases. Galactose bearing O-isopropylidene substituents at the C3 and C4 and butoxybiphenyl substituents at the C2 and C6 positions exhibits a high helical twisting power (HTP) value of ?74.4 μm^?1. Such a high HTP value for the galactose derivative is attributed to (1) linkage of the C3 and C4 carbons with the O-isopropylidene substituent, which places the C2 and C6 substitutes in a skew arrangement at a large angle and (2) enhancement of the affinity with the host nematic LCs by incorporating aromatic substituents at the C2 and C6 positions that are similar to those in the host.     

Preparation of chiral annulated indenes derived from nopinone, verbenone and menthone

A three-step procedure for the synthesis of chiral annulated indenes is described in which nopinone, verbenone and menthone are converted to their enolate form, alkylated with 2-bromomethylbromobenzene, ring-closed with CrCl₂/cat. NiCl₂ and dehydrated with catalytic acid.     

Photoisomerizable chiral compounds derived from isosorbide and cinnamic acid

The syntheses of derivatives of isosorbide and cinnamic acid are described. These chiral compounds are photoisomerizable. The Z-isomers could also be obtained after irradiation of these E-isomeric cinnamic derivatives. The Z-isomers were found to have a much lower helical twisting power than the E-isomers. These compounds are very suitable for use in cholesteric colour filters for liquid crystal displays. This was investigated by functionalizing the E-isomeric derivatives with two acrylate groups. The reflection wavelength of cholesteric layers made with these diacrylates can be tuned by means of UV irradiation because the pitch of the cholesteric layer increases on isomerization to the Z-isomer. Subsequent photopolymerization results in cholesteric films with excellent thermal stability.     

Dynamic photoswitching of helical inversion in liquid crystals containing photoresponsive axially chiral dopants

Chirality switching is intriguing for the dynamic control of the electronic and optical properties in nanoscale materials. The ability to photochemically switch the chirality in liquid crystals (LCs) is especially attractive given their potential applications in electro-optic displays, optical data storage, and the asymmetric synthesis of organic molecules and polymers. Here, we present a dynamic photoswitching of the helical inversion in chiral nematic LCs (N*-LCs) that contain photoresponsive axially chiral dopants. Novel photoresponsive chiral dithienylethene derivatives bearing two axially chiral binaphthyl moieties are synthesized. The dihedral angle of the binaphthyl rings changes via the photoisomerization between the open and closed forms of the dithienylethene moiety. The N*-LCs induced by the dithienylethene derivatives that are used as chiral dopants exhibit reversible photoswitching behaviors, including a helical inversion in the N*-LC and a phase transition between the N*-LC and the nematic LC. The present compounds are the first chiral dopants that induce a helical inversion in N*-LC via the photoisomerization between open and closed forms of the dithienylethene moiety.     

Derivatives of menthothiophene as chiral dopants for nematic liquid crystals

New chiral derivatives of thiophene fused with menthane, camphor or the 3R-3-methylcyclohexane ring were prepared and studied as chiral dopants in cholesteric liquid crystalline mixtures. The helical twisting power of the most effective compounds of this series, menthothiophenes, was strong enough to obtain selective reflection of visible light at 16 wt% concentration of the dopant in a non-chiral nematic host.     

Light-driven reversible handedness inversion in self-organized helical superstructures

We report here a fast-photon-mode reversible handedness inversion of a self-organized helical superstructure (i.e., a cholesteric liquid crystal phase) using photoisomerizable chiral cyclic dopants. The two light-driven cyclic azobenzenophanes with axial chirality show photochemically reversible trans to cis isomerization in solution without undergoing thermal or photoinduced racemization. As chiral inducing agents, they exhibit good solubility, high helical twisting power, and a large change in helical twisting power due to photoisomerization in three commercially available, structurally different achiral liquid crystal hosts. Therefore, we were able to reversibly tune the reflection colors from blue to near-IR by light irradiation from the induced helical superstructure. More interestingly, the different switching states of the two chiral cyclic dopants were found to be able to induce a helical superstructure of opposite handedness. In order to unambiguously determine the helical switching, we employed a new method that allowed us to directly determine the handedness of the long-pitched self-organized cholesteric phase.     

New chiral 2,2′-bipyridine diols as catalysts for enantioselective addition of diethylzinc to benzaldehyde

New C₂-symmetric chiral 2,2′-bipyridine diols were prepared from readily available homochiral materials such as menthone and camphor. Their catalytic activities in the reaction of diethylzinc with benzaldehyde to give 1-phenyl-1-propanol were studied. In all cases, the yields were good and enantioselectivities up to 95% were observed.     

TADDOLs with Unprecedented Helical Twisting Power in Liquid Crystals. Preliminary communication

A large number of TADDOL (α,α,α′, α′-tetraaryl-1,3-dioxolan-4,5-dimethanol) derivatives has been tested as chiral dopants for inducing conversion of nematic to cholesteric phases. With the Merck liquid-crystal materials ZLI-1695 and K15 , it was demonstrated that some TADDOLs have unprecedentedly high helical twisting powers (HTP). Thus, the TADDOL with four 9-phenanthryl α-substituents has a HTP in the achiral mesophase 4-(4-pentylphenyl)benzonitrile of 405 μm^?1 between 24 and 34°. The temperature-dependent HTP measurements have been performed by analyzing Grandjean textures microscopically (Cano method). The structure-dependent HTP of various types of TADDOL dopants is discussed. There are similarities between size and sign of HTP on the one hand, and between degree and relative topicity of enantioselectivity in reactions, on the other hand, as caused by TADDOLs and by 1,1′-binaphthols.     

Light-induced structural changes in chiral liquid crystals

Novel light-sensitive chiral dopants are studied as a light-sensitive component in chiral liquid crystals which may be used in tunable optical devices. Light-induced cis-trans- isomerization of chiral dopants results in changes of helical twisting power which translates into variations of helical pitch. Due to the light absorption in the liquid crystal cell the pitch variation is non-uniform across the cell, which leads, at first, to a deformation of cholesteric layers, and then to the formation of cholesteric bubbles. The sequence of structural changes has a distinct visual pattern and occurs at the surface close to the UV light source. Small deformations of cholesteric layers and bubbles are unstable and disappear after removing UV irradiation. The increasing size of the cholesteric bubbles results in better stability; large bubbles do not disappear after removing UV light. A theoretical model is suggested to describe the undulations of cholesteric layers.     

Novel freebase and zinc bilinone dimers with optically active peripheral groups. Synthesis and application to chiral nematic induction in a nematic mesophase

In order to investigate effective dopants to induce chiral nematic liquid crystalline phases, novel freebase (FbBL) and zinc bilinone (ZnBL) derivatives bearing optically active aliphatic groups ((S)-3,7-dimethyloctyls) at the peripheral positions were prepared. From the CD spectra, it was confirmed that M-helicity in the bilinone frameworks was modestly enriched for ZnBLs, whereas helicity was hardly induced for FbBLs except for the o-xylylene-spaced dimer. When N-(4-methoxybenzylidene)-4-butylaniline (MBBA) was doped with the bilinone derivatives, the chiral nematic phase was effectively induced, and the helical twisting powers (β_Ms) ranged from ?95 to ?159 μm^?1. The control experiment using (S)-3,7-dimethyl-1-phenyloctane (β_M = +14 μm^?1) clearly showed that the induced chiral helical frameworks of FbBL and ZnBL predominantly contribute to chiral nematic induction of MBBA.     

The effect of substituents on the helical twisting power of aldol condensation products of menthone

The helical twisting powers of the E-isomers of aldol condensation products of menthone and aromatic aldehydes are higher than those of the Z-isomers. In order to find out which chiral centre of these menthone derivatives is responsible for the value of the helical twisting in both isomers, the E-isomers of aldol condensation products of 3-methylcyclohexanone and 2-isopropylcyclohexanone were prepared and photoisomerized to form Z-isomers. The physical properties of these species were determined. It was concluded that the strong helical twisting power of the E-isomers of the derivatives of menthone is caused by the chiral carbon atom containing the methyl group in the ring. The relatively low helical twisting power of the Z-isomers and the composition of the E-Z isomers in the photostationary state are determined mainly by the other chiral centre containing the isopropyl group.     

The effect of substituents on the helical twisting power of aldol condensation products of menthone

The helical twisting powers of the E-isomers of aldol condensation products of menthone and aromatic aldehydes are higher than those of the Z-isomers. In order to find out which chiral centre of these menthone derivatives is responsible for the value of the helical twisting in both isomers, the E-isomers of aldol condensation products of 3-methylcyclohexanone and 2-isopropylcyclohexanone were prepared and photoisomerized to form Z-isomers. The physical properties of these species were determined. It was concluded that the strong helical twisting power of the E-isomers of the derivatives of menthone is caused by the chiral carbon atom containing the methyl group in the ring. The relatively low helical twisting power of the Z-isomers and the composition of the E-Z isomers in the photostationary state are determined mainly by the other chiral centre containing the isopropyl group.     

Trans-coplanar rearrangements : The conversion of camphor to nopinone

The conversion of camphor to nopinone was carried out by rearrangement of appropriate glycol monomesylate in a sterospecific manner. The glycol monomesylates were prepared by reaction of camphor and trichloroacetic anhydride to give 1-trichloroacetoxycamphene (2) which on hydrolysis and acylation gave 1-acetoxycamphene (5). Ozonolysis, selective reduction of 6 and conversion to the mesylate gave 1-acetoxy-3,3-dimethylbicyclo[2.2.1]-hept-endo-2-yl mesylate (10) which was rearranged to nopinone.     

Metal-free diastereoselective catalytic hydrogenations of imines using B(C6F5)3

Reductions of chiral ketimines effected under H(2) by catalytic amounts of B(C(6)F(5))(3) result in moderate to excellent diastereoselectivities. In the case of camphor and menthone derived imines, the reductions proceeded with greater than 95% diastereoselectivity.     

Synthesis and properties of chiral stilbene diacrylates

Two chiral and isomerizable liquid crystalline diacrylates were synthesized. The purpose of these compounds was to tune the helical twisting power of cholesteric materials containing these compounds by means of an E–Z photoisomerization of the photoactive group derived from stilbene. The photochemical behaviour of these compounds was studied with the aid of two model compounds containing the same isomerizable mesogenic group. The mesogenic group derived from 4-(4-hydroxybenzoyloxy)-4′-hydroxystilbene decomposes upon irradiation. Its isomer, derived from 4-(4-hydroxyphenoxycarbonyl)-4′-hydroxystilbene, shows a clean E–Zisomerization. The HTP of the chiral diacrylate derived from the latter mesogenic group changes from 7 to 3 μm^-1 in dilute nematic solution. Colour changes in a cholesteric material containing this compound were observed. The effect was very dependent on temperature and concentration due to the strong smectic character of this diacrylate.