WO1998011077A1

WO1998011077A1 - Materials for chirooptical devices

Info

Publication number: WO1998011077A1
Application number: PCT/JP1996/002598
Authority: WO
Inventors: Dietrich Demus; Ryokichi Tarao; Hiroyuki Takeuchi; Kazutoshi Miyazawa
Original assignee: Chisso Corporation
Priority date: 1996-09-12
Filing date: 1996-09-12
Publication date: 1998-03-19
Also published as: AU6944396A; JP2001501186A

Abstract

Chiral compounds of formula (I) where in R1, R2, R3, R4 = equal or unequal = H, F, Cl, R5, R6 or R7; R5 = CN, NCS, CF3, OCF3; R6 = chiral alkyl, alkenyl, alkynyl or fluoroalkyl with chain lengths up to 15 carbon atoms, with one or several asymmetric C atoms substituted with -CH3, -C2H5, -CN, halogen, -CF3, -CHF2, CH2F, -CCl3, -CF2Cl, -CFCl2, -OCF3, -OCHF2, -OCH2F in which one or several CH2 groups independently one to another can be replaced by -S-, -O-, cyclobutane, [1.1.1]bicyclopentane, cyclopropane, oxirane, thiirane, -CO-, -COO-, -OCO-, -OCOO-, -OOCO-; R7 = non chiral alkyl, alkenyl, alkynyl or fluoroalkyl with chain lengths up to 15 carbon atoms, in which one or several C atoms can be substituted with -CH3, -C2H5, -CN, halogen, -CF3, -CHF2, CH2F, -CCl3, -CF2Cl, -CFCl2, -OCF3, -OCHF2, -OCH2F, and in which one or several CH2 groups independently one to another can be replaced by -S-, -O-, cyclobutane, [1.1.1]bicyclopentane, cyclopropane, oxirane, thiirane, -CO-, -COO-, -OCO-, -OCOO-, -OOCO-; ring A1, A2, A3, A4 = independently substituted or unsubstituted 1,4-phenylene ring; X1, X2 = independently S, C(R8 R9); R8, R9 = H, F, Cl, R5, R6, R7; with the proviso that at least one of the groups R1, R2, R3, R4, R8, R9 is equal to R6, and not more than two of the groups R1, R2, R3, R4 are equal to R5. These compounds can be switched between two stable optical states and are suited for use in chirooptical displays.

Description

DESCRIPTION

Materials for chirooptical devices

Technical Field

This invention relates to novel chemical compounds and their use in liquid crystal chirooptical devices. Such devices are useful for storage of information, which is inscribed by irradiation with light.

Background Art

Materials and devices with reversible optical storage capacity are needed for storage of information and data. Optical storage can be applied in optical computers and storage systems for audio and video informations.

For storage of data, materials and devices with reversible optical storage capability are needed. Optical storage can be applied in optical computers, storage systems for audio and video informations. There are many types of optical storage devices, which can be written in only once (write-once-read-many systems = WORM systems ) , using irreversible processes for storage. However they were not applicable for storage systems for computers and, find only limited application in audio and video informations because of their non-reversible properties. For many purposes more advantageous are devices, which can be written in multifold (erasable-direct-read- after-write systems = EDRAW systems) by use of reversible storage processes. For this purpose a survey about materials for optical data storage has been presented by M. Emmelius, G. Pawlowsky and H. W. Vollmann, Angew. Chem. Intern. Ed. 28, 1445 (1989). However because all available media for optical data storage have some shortcomings like storage density, storage rate, reversibility, long time stability, the search for new optical storage materials and devices is quite actual.

Devices of this kind typically consist of a layer of liquid crystal material, contained between two glass slides. By irradiation with light of different wavelenghts, devices containing azo compounds change from the nematic to the isotropic state and reverse (for example, D. Demus, G. Pelzl, F. Kuschel DD WP 134 279).

Especially liquid crystalline polymers containing azo groups have been proposed for such devices (H. Finkelmann, W. Meier and H. Scheuermann, in: Liquid

Crystals. Applications and Uses, ed. by B. Bahadur, World Scientific, Singapore 1992, vol. 3, p. 345-370 ).

The proposed azo compounds have the disadvantage, that the intensity and time of irradiation are quite high, because a large amount of the azo compound has to be transformed to the corresponding isomer. Therefore the switching times are extremely large. Also thermo-optical liquid crystal devices have been proposed, based on thermally induced texture change of cholesteric phases (S. Kobayashi and A. Mochizuki, in: Liquid Crystals. Applications and Uses, ed. by B. Bahadur, World Scientific, Singapore 1992, vol. 3, p. 291-293) or smectic A phases (D. Coates, in: Liquid Crystals. Applications and Uses, ed. by B. Bahadur, World Scientific, Singapore 1990, vol 1, p. 275-303).

Thermo-electrooptic displays use the combined effect of heat and electric fields, in low-molecular glass forming liquid crystals (D. Demus and G. Pelzl, DD WP 242 624 Al ) resp. polymer glass forming liquid crystals (H. Finkel ann, W. Meier and H. Scheuermann, in: Liquid Crystals. Applications and Uses, ed. by B. Bahadur, World Scientific, Singapore 1992, Vol. 3, p. 345-370). However thermo-optical devices generally need a quite large energy for locally heating up the liquid crystal material.

M. Zhang and G. B. Schuster, J. Phys . Chem. 96, 3063-3067 (1992) reported about the photoracemization of chiral binaphthyl derivatives, converting the cholesteric mixture to nematic mixture. Because the reaction is irreversible, the change from nematic to cholesteric is impossible and the material cannot be used for repeated data storage. Holographic technology has been investigated for reversible data storage systems. Several kind of materials have been investigated toward this application. There are three types of materials, inorganic metal compounds, photorefractive polymers and polymer liquid crystals, have been reported. However they all have disadvantages. For materials of optical data storage, photorefractive crystals like lithium niobate ( LiNb0₃ ) , barium titanate (BaTi0₃) and bismuth silicon oxide, have been proposed. However because they are difficult and expensive to grow, and their properties cannot easily be modified, they have not become commercially feasible. Recently in PCT/JP96/01001 chirooptical devices have been described, using mixtures of nematic liquid crystals with chiral materials, which by irradiation with light of two different wavelengths can be switched between two stable states of different chirality, corresponding to two states of different optical properties, useful for storage of data and information.

The chiral materials induce a helical pitch in the nematic liquid crystal, and can be switched by irradiation with light of two different wavelengths to two different isomers or equilibrium mixtures of these isomers, inducing the said two different optical states. For long term storage of the data, these optical states must be stable for long periods without thermal isomerization. On the other hand, for repeated use of the devices, the materials must allow reversible switching between the two different optical states, without deterioration processes in the materials. The chiral materials must be sufficiently soluble in the nematic liquid crystals.

In PCT/JP96/01001 different chiral materials have been proposed. Chiral compounds useful for chirooptical switching are sterically overcrowded chiral olefins, for example formula (a), (B. L. Feringa, W. F. Jager, B. de Lange J. Am. Chem. Soc. 113, 5468-5470 (1991); W. F. Jager, B. de Lange, B. L. Feringa Mol. Cryst. Liq. Cryst. 217, 133-138 (1992); E. W. Meijer, B. L. Feringa, Mol. Cryst. Liq. Cryst. 235, 169-180 (1993); W. F. Jager, J. C. de Jong, B. de Lange, N. P. M. Huck, A. Meetsma and B. L. Feringa Angew. Chem. Int. Ed. Engl. 34, 348-350 (1995)).

(a)

wherein Ra, Rb, Re = alkyl, alkoxy, hydrogen, hydroxy, nitro, dialkylamino,

Xa, Xb = oxygene, sulfur, ethylene.

These sterically overcrowded olefins show limited solubility in nematic liquid crystals, and because of their strong deviation from the rod-like molecular shape they tend to decrease the nematic-isotropic transition temperatures of calamitic liquid crystals.

In PCT/JP96/01001 also azobenzene derivatives of the general formula (b)

with Ya, Yb = independent upon another single bond, oxygene, sulfur, carbonyl, ester group, 1, 2-ethylene, 1, 2-ethenylene, 1, 2-ethynylene, 1, 4-butylene, methyleneoxy, oxymethylene, difluoro ethyleneoxy, ring system like substituted or unsubstituted benzene, cyclohexane, heterocyclic ring, biphenyl, bicyclohexyl;

Rd = elongated flexible group such as alkyl, alkyloxy, alkanoyloxy, alkoxyalkyl, alkenyl, alkynyl, alkenyloxy, alkynyloxy, alkadienyl, alkadienyloxy, haloalkyl, haloalkyloxy; R* = chiral group, were shown.

These azobenzene derivatives allow reversible optically induced switching between the trans and the cis isomers, corresponding to two states of different optical properties. However, as usual in ordinary azo compounds, the cis isomer is less stable and has a strong tendency to thermal iso erization towards the stable trans isomer. This is, the optical state depending on the cis isomer cannot be stored for long periods, and therefore the devices containing simple azobenzene derivatives are not suited for long term storage of data and informations. In a recent publication, H. Rau and D. Rόttger, Mol. Cryst. Liq. Cryst. 246, 143-146 (1994) described new azobenzenes (c) and (d) of the phane type with special optical properties.

wherein Re, Rf = COOC₂H₅

These compounds can be switched between two stable optical states, corresponding to the trans-trans resp. cis-cis isomers of the azo moieties. In these compounds both the isomers are stable for long periods and do not show thermal iso erization. The said compounds, however, possess very limited solubility in nematic liquid crystals, decrease strongly the nematic-isotropic transition temperatures of calamitic liquid crystals and do not show chirality. Therefore they are not suited at all for chirooptical devices.

Disclosure of the Invention — —

We have found that chiral azo compounds of the formula ( 1 )

wherein R_{l r} R₂, R₃, R₄ = equal or unequal, H, F, Cl, R₅, R₆ or

R₇ R₅ = CN, NCS, CF₃, 0CF₃,

R₆ = chiral alkyl, alkenyl, alkynyl or fluoroalkyl with chain lengths up to 15 carbon atoms, with one or several asymmetric C atoms substituted with -CH₃, -C₂H₅, -CN, halogen, -CF₃, -CHF₂, CH2F, -CC1₃, -CF₂C1, -CFC1₂, -0CF₃, -0CHF₂, -OCH₂F in which one or several CH₂ groups independently one to another can be replaced by -S-, -0-, cyclobutane, [1.1.1 jbicyclopentane, cyclopropane, oxirane, thiirane, -CO-, -C00-, -0C0-, -0C00-, -00C0-, R₇ = non chiral alkyl, alkenyl, alkynyl or perfluoroalkyl with chain lengths up to 15 carbon atoms, in which one or several C atoms can be substituted with -CH₃, -C₂H_h, -CN, halogen, -CF₃, -CHF₂, CH₂F, -CC1₃, -CF₂C1, -CFC1₂, -0CF₃, -0CHF₂, -0CH₂F, and in which one or several CH₂ groups independently one to another can be replaced by -S-, -0-, cyclobutane, [1.1.1] bicyclopentane, cyclopropane, oxirane, thiirane, -CO-, -COO-, -0C0-, -OC00-, -00C0-, ring Al, A2, A3, A4 = independently substituted or unsubstituted 1, 4-phenylene ring, X_j, X₂ = independently S, C(R₈ R₉), R₈, R_g = H, F, Cl , R₅, R₆, R₇, with the proviso that at least one of the groups R_{l f} R₂, R₃,

R₄, R₈, R₉ is equal to R₆, and not more than two of the groups R_j, R₂, R₃, R₄ are equal to R₅, can be switched between two stable optical states and are suited for use in chirooptical displays. The novel compounds (1) (hereinafter called chiral azophanes ) can exist as different isomers, with respect to the possibility of cis/trans isomerism of the azo groups (-N=N-): both azo groups trans, abbreviated tt one azo trans, one azo cis, abbreviated ct both azo groups cis, abbreviated cc

The tt and cc isomers are thermally quite stable (half life time at least 1 year) in contrast to the cis-azo-isomer of ordinary azo compounds like azobenzene or 4, 4 '-bis substituted azobenzenes (half life time about 1 day down to some seconds), which under influence of thermal energy return to the trans state, which is more stable from the thermodynamic standpoint . t,t azobenzophanes are characterized by a phane absorption band near 380 nm which is absent in cc and also in ordinary azobenzenes. Irradiation of tt with light of 366 n gives cc. Irradiation of cc with light of 436 ran gives tt again.

The isomers can be switched by irradiation according to the following scheme 1 : Scheme 1

The isomer ct is quite unstable, appears after irradiation of tt or cc, and by irradiation switches to the stable forms cc resp. tt. ct also switches by thermal energy to tt. In the spectral region of 350-370 nm the absorption of tt and cc is very different up to a factor of 75. In addition to the change in absorption, the helical twisting power (HTP) of the chiral azophanes is changed. In nematic liquid crystals, by doping with the chiral azophanes the cholesteric phase is induced. Switching the materials from tt to cc isomeric state changes the pitch of the cholesteric mixtures, and by use in suited arrangements change of an optical contrast can be achieved.

The switching of the isomers occurs with light of high intensity. Probing with light of low intensity causes negligible loss of cc or tt. Storage of the isomers tt or cc is possible for long periods without thermal reaction. Storage of a cc sample at room temperature for several months does not give a change in absorption or HTP. These properties make the system suited for optically switched information storage systems.

For the nematic liquid crystal basic mixtures, all known nematic compounds can be used, provided they do not have an optical absorption band in the respecive regions of irradiation. Ordinarily the following compounds (2-1) - (2-190) can be effectively used.

(wherein R₁₀, R₁₁ = alkyl, alkyloxy, alkenyl, alkynyl and other groups typical for liquid crystals).

Though any sort of nematic liquid crystals can be applied for this invention, preferably the liquid crystal basic mixtures should not have optical absorption bands in the region of the irradiation light switching between the two states. Because switching preferably is done with ultraviolet light, specially non-aromatic liquid crystal materials are useful, preferably compounds of the following general formulas (2-191) - (2-227).

(wherein R₁₀, R = alkyl, alkyloxy, halogene, cyano and other groups typical for liquid crystals, m, n = independently 0 - 10).

Liquid crystals of these general formulas are already known and have been described in the literature.

Also nematic discotic liquid crystals can be used as basic mixtures for verifying the invention. Discotic materials that are listed in LIQCRYST, Database of Liquid Crystalline Compounds for personal Computers, by V. Vill, Hamburg 1995, can be used. Because nematic discotic compounds usually have relatively high melting temperatures, multicomponent mixtures are necessary. Discotic nematic materials are specially compatible with the chiral azophanes. The following compounds (3-1) - (3-11) are suitable discotic materials for this invention.

wherein R₁₂ = alkyl chain or halogen atoms in which one or two methylene moieties can be replaced by 0, C=0, S, C=C, C≡C, and one or two hydrogens can be replaced by CN or halogen atoms. Z represents 0C0, CH₂CH₂, C≡C, CH₂0, OCH₂, CH₂S, SCH₂, NHC0, 0C0CH₂, 0C0CH=CH, CH₂0C0, N=N. The invented chiral azophanes ( 1 ) can be synthesized by combinations of general organic synthesis procedures described in publications. For example the compound ( 1 ) is synthesized by a cross coupling reaction of azophanes according to the literatures, H. Rau and E. Luddecke, J. Am. Chem. Soc. , 104, 1616 (1982), V. Boekelheide, R. A. Hollius, J. Am. Chem. Soc, 95, 3201 (1973) and R. H. Mitchell, V. Boekelheide, J. Am. Chem. Soc, 96, 1547 (1974). For instance,

Scheme 2

A cross coupling reaction of azobenzenes (4) and (5), that can be produced by the method of R. Dabrovski et. al., Mol. Cryst. Liq. Cryst., 61, 61 (1980), using sodium sulfide gives the desired compound (1). The coupling reaction preferably is carried out in aqueous alcoholic solution, such as aqueous ethanol or aqueous methanol. To avoid a homo coupling reaction, the reaction preferably is a high dilution reaction. After the reaction by ordinary work up procedures i.e., a chromatographic separation or a recrystallization, purified (1) can be obtained. The compound ( 1 ) can also be synthesized by a thioetherification of the benzyl bromides (4) and thiols (6) using bases such as, sodium hydroxide, potassium hydroxide, calcium hydride, potassium carbonate, sodium carbonate, in alcoholic solution.

Scheme 3

The thiols (6) can be derived from the bromides (5) by a treatment with thiourea followed by a hydrolysis.

Scheme 4

Introduction of the side chains R_x - R₇ can be done by the following synthetic methods.

The halogen- ( fluorine- or chlorine- ) substituted compounds ( 8 ) can be derived from the benzylhalides ( 7 ) by a bromination under UV irradiation, according to A. Haars, Chem. Ber., 121, 1329 ( 1988 )( Scheme 5). In the scheme, X₃ represents F and Cl, and MG (mesogene) means organic residue. Scheme 5

The compounds ( 10 ) substituted by R₅ can be derived from (9) also by a bromination under UV irradiation (Scheme 6), according to the said literature.

Scheme 6

For example

Chiral or achiral alkyl substituted compounds ( 13 ) can be derived from benzaldehyde (11) by a reaction with nucleophiles followed by a bromination (Scheme 7). Alkyl magnesium bromides or alkyl lithium reagents, derived from corresponding alkylhalides, can be used as the said nucleophiles. The bromination can be done by using normally used bro inating reagents such as PBr₃, SOBr₂, Br₂, NBS (N-bromosuccinimide) etc

Scheme 7

The chiral alkyl bromides (R₆Br) can be derived from corresponding alcohols (R₆OH) that can be derived from natural chiral sources or can be prepared by asymmetric reactions, kinetic resolutions or mechanical resolutions, using chiral catalysts, recrystallizations, biocatalytic reactions or a chiral chromatography . These methods are well-described in the literatures, for example, Asymmetric Synthesis edited by J. D. Morrison, Academic Press (1983). When X_α or X₂ is a disubstituted carbon atom, the compound (1) can be synthesized by an etherification of disubstituted dibromo carbon and diols that can be prepared according to Scheme 7.

All claimed chiral azophanes that are defined by (1) and can be synthesized by the described methods are reasonable materials for light switching devices. In the claimed compounds preferred variants of the invention comprise the following formulas, without restricting it to these variants:

R₆ is preferably 1-m thylpropyl, 1-methylbutyl, 1-methylpentyl, 1-methylhexyl, 1-methylheptyl, 1-methyloctyl, 1-methylnonyl, 2-methylbutyl, 2-methylpentyl, 1-ethylheptyl, 2-ethylheptyl, 1-trifluoromethylheptyl, 2-trifluoromethylheptyl, 2-cyanopentyl, 2-fluorobutyl, 2-fluoropentyl, 2-fluorohexyl, 3-fluorohexyl, 2-chloropentyl, 2-difluromethylpentyl, 2-fluoromethylpentyl, 2-trichlorometylpentyl, 2-fluorodichlorometylhexyl, 2-trifluoromethoxypentyl , 3-trifluoro ethoxyhexyl , R₇ is preferably alkyl, alkyloxy, alkanoyloxy, alkyloxyalkyl, alkenyl, alkynyl, alkenyloxy, alkynyloxy, alkadienyl , alkadienyloxy, haloalkyl, haloalkyloxy, Al, A2, A3 and A4 are preferably 1 , 4-phenylene,

2-fluoro-l,4-phenylene, 2, 3-difluoro-1 , 4-phenylene, 3, 5-difluoro-l,4-phenylene, 2, 5-difluoro-1 , 4-phenylene, 2,3, 5-trifluoro-1, 4-phenylene, 2-chloro-l , 4-phenylene. Brief Explanation of Drawing Fig. 1 is a cross-sectional view of the hybrid cell in which the liquid crystals of the invention are filled.

Fig. 2 is an illustration of the holographic arrangement applied for the invention. Best Mode for Practicing the Invention Example 1 Synthesis of optically active l-( S-2-methylbutyl )-2, 19- dithia [3.3] ( 4, 4 ' )-trans-diphenyldiazeno( 2 )phane (in (1) ^ S-2-methylbutyl, R₂, R₃, R₄ = hydrogen, X_l7 X₂ = sulfur, Al - A4 = 1, 4-phenylene)

Step 1: Synthesis of optically active ( 1-hydroxy-3-methyl )- pentylbenzene ( S-1 ) To a mixture of benzaldehyde (3.2 mol) in 500 ml of tetrahydrofuran (hereinafter called THF),

S-2-methylbutylmagnesium bromide (3.5 mol) that was prepared from S-2-methylbutyl bromide and magnesium was added at temperatures lower than 10^CC and the reaction mixture was stirred at room temperature for 2 hours. With keeping the temperature under 20 C, aqueous ammonium solution was added and the reaction mixture was extracted with diethylether (500 ml x 2), then dried over anhydrous magnesium sulfate. Evaporation of the organic layer gave colorless oil (S-l) (2.9 mol) .

Step 2: Synthesis of optically active (1-t- butyldimethylsilyloxy-3-methyl) pentylbenzene (S-2) A mixture of (S-l) (2.5 mol), t-butyldimethylsilyl chloride (TBDMSC1) (2.7 mol), imidazole (4.0 mol) and 600 ml of dimethylformamide (hereinafter called DMF) was stirred at 15°C for 25 hours. The reaction mixture was poured into 500 g of crashed ice and extracted with hexane ( 500 ml ) . The organic layer was washed with water ( 300 ml x 4 ) , then dried over anhydrous magnesium sulfate. Evaporation of the organic layer followed by a purification with a silica gel column chromatography ( eluted by hexane ) gave colorless oil (S-2) (2.1 mol).

Step 3: Synthesis of optically active 4-( 1-t-butyldi- methylsilyloxy-3-methyl ) pentyl-nitrobenzene (S-3) To a solution of (S-2) (2.1 mol) in acetic acid (550 ml), a mixture of sulfuric acid (2.1 mol) and fuming nitric acid (2.1 mol) was added under - 10°C in 2 hours.

The reaction mixture was poured into ice-cooled water ( 400 ml ) and extracted with toluene ( 500 ml ) . Evaporation of the organic layer gave orange colored tar, which was purified by a silica gel column chromatography (eluted by a mixture of toluene and ethyl acetate) to give 0.6 mol of a yellow colored compound ( S-3 ) . Step 4: Synthesis of optically active 4-(l-t- butyldimethylsilyloxy-3-methyl )pentyl-aminobenzene (S-4) A mixture of (S-3) (0.55 mol), 5% Pd/C and ethanol

( 430 ml ) was stirred in H₂ atmosphere for 5 hours . The catalyst was removed from the reaction mixture by a filtration and the filtrate was evaporated to give crude (S-4), which was purified by a silica gel column chromatography (eluted by a mixture of methylene dichloride and ethanol) to give 0.53 mol of colorless material (S-4). Step 5: Synthesis of optically active 4-( l-hydroxy-3- methyl )pentylaminobenzene (S-5) To a solution of (S-4) (0.52 mol) in 100 ml of THF, 1 M solution of tetrabutylammonium fluoride (hereinafter called TBAF) (0.78 mol) in THF was added at -10°C and the mixture was stirred at the same temperature for 5 hours. Ice-cooled water (500 ml) was added and the mixture was extracted by diethylether (300 ml x 4). The organic layer was washed with brine (50 ml x 3 ) and dried over anhydrous magnesium sulfate. Evaporation followed by a purification by a silica gel column chromatography (eluted by a mixture of methylene dichloride and ethanol) gave 0.50 mol of colorless material (S-5). Step 6: Synthesis of optically active 4-( 1-bromo-3-methyl ) pentyl-aminobenzene (S-6)

To a mixture of the alcohol (S-5) (0.5 mol) and ethylene dichloride (200 ml), triphenylphosphine (0.7 mol) and carbon tetrabromide (0.95 mol) were added at -10°C, and the mixture was stirred for two minutes. After two minutes saturated aqueous NaHC0₃ (300 ml) was added and the separated organic layer was washed with brine ( 100 ml x 2 ) , then dried over anhydrous magnesium sulfate. Evaporation and purification by a silica gel column chromatography (eluted by a mixture of ethyl acetate and diethylether ) gave the captioned bromide (S-6) (0.39 mol).

Step 7: Synthesis of optically active 4-( l-bromo-3-methyl )- pentyl-4 ' -bromomethylazobenzene ( S-7 )

To (S-6) (0.38 mol) in 200 ml of acetic acid 4-nitrosobenzylbromide (0.38 mol) in 100 ml of ethanol was added dropwise at 5°C, and the mixture was heated at 50°C for 1 hour. The reaction mixture was cooled down and poured into ice-cooled water (500 ml), then extracted with toluene ( 300 ml ) . The organic layer was evaporated and purified by a silica gel column chromatography (eluted by a mixture of acetic acid and ethanol) to give a white solid (S-7) (0.29 mol). Step 8: Synthesis of optically active l-(S-2-methylbutyl )-

2, 19-dithia [3.3] (4,4' )-trans-diphenyldiazeno- ( 2 )phane A mixture of (S-7) (0.25 mol) and 4, 4 ' -bisbromo- methylazobenzene (0.25 mol) that was prepared from 4-aminobenzyl bromide and 4-nitrosobenzyl bromide, in 500ml of benzene was added simultaneously with a solution of sodium sulfide nonahydrate (1.0 mol) in 300 ml of 90% aqueous ethanol, to 3 liters of absolute ethanol with vigorous stirring over 4 hours. The solid material that was collected by filtration was subjected to a silica gel column chromatography ( eluted by a mixture of hexane and ethylenedichloride ) to give a slightly yellow solid as a main product (0.015 mol), which was a mixture of diastereomers . The two diastereomers were separated by a silica gel column chromatography (eluted by a mixture of benzene and petroleum ether) to give a more polar fraction (more polar optically active l-( S-2-methylbutyl )- 2, 19-dithia [3.3] (4,4' )-trans-diphenyldiazeno( 2 )phane ) (0.009 mol) and a less polar fraction (less polar optically active l-(S-2-methylbutyl )-2, 19-dithia [3.3] ( 4, 4 ' )-trans- diphenyldiazeno( 2 )phane) (0.005 mol). The desired compounds were determined by elemental analysis, ¹H- and ¹³C-NMR and mass spectroscopy to be the captioned compounds. Example 2 Synthesis of optically active l-(R-4-fluorohexyl )-18-pentyl- 2, 19-dithia[3.3] (4,4' )-trans-diphenyldiazeno(2)phane (in (1) Ri = R-4-fluorohexyl, R₂, R₃ = hydrogen, R₄ = pentyl, X_{l f} X₂ = sulfur and Al - A4 = 1, 4-phenylene )

Step 1: Synthesis of optically active 4-(R-l-bromo-5- fluoroheptyl ) -4 ' -bromomethylazobenzene ( S-8 ) To 4-(R-l-bromo-5-fluoroheptyl)-aminobenzene (0.33 mol ) , that was prepared by the same procedure as Example 1 from R-4-fluorohexylbromide that was derived from S-buteneoxide by a ring opening reaction with HF-pyridine complex and C2 homologation, in 200 ml of acetic acid, a solution of 4-nitrosobenzylbromide (0.35 mol) in 100 ml of ethanol was added dropwise at 5°C, and the mixture was heated at 50^CC for 1 hour. The reaction mixture was cooled down and poured into ice-cooled water ( 500 ml ) , then extracted with toluene (300 ml). The organic layer was evaporated and purified by a silica gel column chromatography (eluted by a mixture of ethyl acetate and ethanol) to give a white solid (S-8) (0.22 mol). Step 2: Synthesis of optically active 1- (R-4-fluorohexyl )- 18-pentyl-2,19-dithia[3.3] (4,4' )-trans- diphenyldiazeno( 2 )phane A mixture of (S-8) (0.20 mol), optically active 4-( 1-bromohexyl )-4' -bromomethylazobenzene (0.20 mol) that was synthesized by the same procedures as Example 1 and step 1 of Example 2 from pentylmagnesiu bromide, and 400 ml of benzene was added simultaneously with a solution of sodium sulfide nonahydrate (0.8 mol) in 300 ml of 90% aqueous ethanol, to 2.5 liter of absolute ethanol with vigorous stirring over 4 hours. The solid material that was collected by filtration was subjected to a silica gel column chromatography ( eluted by a mixture of hexane and ethylene dichloride) to give a slightly yellow solid as a main product, which was determined by elemental analysis, ¹H- and ¹³C-NMR and mass spectroscopy, to be optically active l-(R-4-fluorohexyl)-18-pentyl-2,19-dithia[3.3] (4,4' )-trans- diphenyldiazeno(2)phane (0.015 mol). Example 3

According to the procedures of Examples 1 and 2, the following compounds (1) as specified in Table 1 are synthesized.

Table 1

Table 1 (Cont'd)

Table 1 (Cont'd)

Table 1 (Cont'd)

Table 1 (Cont'd)

Table 1 (Cont'd)

Table 1 (Cont'd)

In the Table, Ph, PhF, Phf, PhFF, PhFf, Phff mean 1, 4-phenylene, 2-fluoro-l, 4-phenylene, 3-fluoro-l, 4- phenylene, 2, 6-difluoro-l, 4-phenylene, 2, 3-difluoro-1, 4- phenylene, 2, 6-difluoro-1 , 4-phenylene, respectively. Example 4

Use of the compounds according to the invention in the chirooptical device

A cholesteric mixture consisting of 99.5 weight% of a nematic mixture (65 mol% 4-n-butylcyclohexanecarboxylic acid + 35 mol% 4-n-hexylcyclohexanecarboxylic acid, clearing temperature = 91°C, Dn = 0.02) and 0.5 weight% of optically active 1-(S-2-methylbutyl )-2, 19-dithia[3.3] (4,4' )- trans-diphenyldiazeno( 2 )phane (the more polar fraction) that was obtained in Example 1, showing cholesteric behaviour at room temperature, was prepared. The cholesteric mixture was filled in a hybrid cell (Fig. 1) consisting of two glass plates, in which the liquid crystals were aligned parallel to one glass plate and perpendicular to the other, the glass plates having ITO layers covered with aligning layers, parallel one to another in a distance of 6 mm. The glass plate showing parallel orientation of the liquid crystal was covered by a rubbed polyimide layer, the glass plate showing perpendicular orientation of the liquid crystal was covered with a thin layer of lecithin. The cell was put in between two polarizers. By adjusting the polarizers in a suitable mutual angle the dark state of the device was obtained which corresponds to an excess of the t,t state of the chiral compound. By irradiation with light of wavelength 366 nm the bright state corresponding to an excess of the c,c state of the chiral compound was obtained, which could be switched back to the dark state by irradiation with light of wavelength 436 nm. The switching is reversible for many times, and both the chirooptical states were stable for long periods. Example 5 Use of the chiral compounds for holographic data storage

A holographic arrangement according to Fig. 2 was prepared, and the liquid crystal cell consisting of two parallel quartz plates with a cell gap of 15 mm, filled with the mixture used in Example 4, was inserted. A laser beam with wavelength 366 nm was split into two beams, one of which was modulated by the modulator, transferring the information to the liquid crystal layer by interference with the second beam in the liquid crystal cell. By irradiation with light of wavelength 436 nm the information could be erased. The prodecure could be repeated many times. Example 6 Use of chiral compounds in discotic nematics

In the device according to Example 4 the following mixture was used: A nematic discotic mixture comprising of

2,3,4,5, 6-pentakis(2-(4-pentylphenyl)ethynyl)-l-( 11- hydroxyundecyloxy)benzene 45 mol%, 2,3, 4, 5, 6-pentakis( 2- ( 4-pentylphenyl )ethynyl ) -1- ( 10-ethoxycarbonyldecyloxy ) - benzene 40 mol%, 2,3, 4, 5, 6-pentakis( 2-(4-pentylphenyl )- ethynyl )-l-( 10-hydroxycarbonyldecyloxy)benzene 15 mol% was prepared. The compounds could be prepared according to the literature, D. Janietz, K. Praefcke and D. Singer,

Liq. Cryst. 13(2), 247 (1993). To this mixture 0.3 weight% of the chiral compound optically active l-( R-4-fluorohexyl )- 18-pentyl-2 , 19-dithia [ 3.3] ( 4 , 4 ' ) -trans-diphenyldiazeno( 2 ) - phane, that was synthesized in Example 2, was added. The chiral compound induced the discotic cholesteric phase.

Switching the chiral compound between the tt and cc states changed the helical pitch of the mixture and changed the optical absorption, so that a good optical contrast was achieved between the two states. The device could be stored in both the optical states for long periods without changes .

Claims

1. A chiral azo compound of the formula ( 1 )

wherein R₁ , R₂ , R₃ , R₄ = equal or unequal = H , F, Cl , R₅ , R₆ or

R₇

R₅ = CN, NCS , CF₃ , OCF₃ ,

R₆ = chiral alkyl, alkenyl, alkynyl or fluoroalkyl with chain lengths up to 15 carbon atoms, with one or several asymmetric C atoms substituted with -CH₃, -C₂H₅, -CN, halogen, -CF₃, -CHF₂, CH₂F, -CC1₃, -CF₂C1, -CFC1₂, -0CF₃, -0CHF₂, -0CH₂F in which one or several CH₂ groups independently one to another can be replaced by -S-, -0-, cyclobutane, [1.1.1] bicyclopentane, cyclopropane, oxirane, thiirane, -CO-, -C00-, -0C0-, -OC00-, -00C0-,

R₇ = non chiral alkyl, alkenyl, alkynyl or fluoroalkyl with chain lengths up to 15 carbon atoms, in which one or several C atoms can be substituted with -CH₃, -C₂H₅, -CN, halogen, -CF₃, -CHF₂, CH₂F, -CCI₃, -CF₂C1, -CFC1₂, -0CF₃, -0CHF₂, -OCH₂F, and in which one or several CH₂ groups independently one to another can be replaced by -S-, -0-, cyclobutane, [1.1.1] bicyclopentane, cyclopropane, oxirane, thiirane, -CO-, -C00-, -0C0-, -OCOO-, -OOCO-, ring Al, A2, A3, A4 = independently substituted or unsubstituted 1, 4-phenylene ring, X_l X₂ = independently S, C(R₈ R₉ ) , Rβ R₉ ⁼ H, F, Cl, R₅, R₆, R₇, with the proviso that at least one of the groups R_{l f} R₂, R₃, R₄, R₈, R₉ is equal to R₆, and not more than two of the groups R₁, R₂, R₃, R₄ are equal to R₅ can be switched between two stable optical states and are suited for use in chirooptical displays.

2. Chirooptical liquid crystal device, consisting of two glass or quartz plates, held parallel one to another in a defined distance, the gap between the plates filled with a liquid crystal mixture, characterized by use of calamitic nematic liquid crystals doped with chiral materials according to claim 1.

3. Chirooptical liquid crystal device, consisting of two glass or quartz plates, held parallel one to another in a defined distance, the gap between the plates filled with a liquid crystal mixture, characterized by use of discotic nematic liquid crystals doped with chiral materials according to claim 1.

4. Chiral liquid crystal mixture comprising of the chiral compounds ( 1 ) according to claim 1 and achiral liquid crystals, useful for chirooptical displays.