WO1998011078A1

WO1998011078A1 - Materials for optical storage devices

Info

Publication number: WO1998011078A1
Application number: PCT/JP1996/002599
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: JP2001501187A; AU6944496A

Abstract

Azophane derivatives of formula (1) wherein R1, R2, R3, R4 = equal or unequal = H, F, Cl, R5, R6 or R7; R5 = CN, NCS, CF3, OCF3; R6 = non 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 exchanged 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, pyridine-2,5-diyl, pyrimidine-2,5-diyl, pyridazine-3,6-diyl, pyrazine-2,5-diyl; X1, X2 = independently S, C(R7 R8); R7, R8 = H, F, Cl, R5, R6; with the proviso that at least one of the groups R1, R2, R3, R4 is equal to R6, and not more than two of the groups R1, R2, R3, R4 are equal to R5. They can be used in a liquid crystal optical storage device.

Description

DESCRIPTION

Materials for optical storage devices

Technical Field

This invention relates to novel chemical compounds and their use in liquid crystal optical storage 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 different types of optical storage devices. Certain types can be written in only once (write-once-read-many systems = WORM systems). They use irreversible chemical or physical processes for storage. They find limited application in audio and video informations, but they cannot be used for computer storage systems, because of their irreversible properties. Certain types of optical storage can be written in multifold, and read multifold (erasable-direct-read- after-write systems = EDRAW systems), since they use reversible chemical or physical process. 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). The available media for optical data storage have some shortcomings like limited storage density, limited storage rate, not complete reversibility, limited long time stability. Therefore the search for new and improved optical storage materials and respective devices is quite actual.

Liquid crystal devices for optical data storage usually consist of two glass slides, comprising a layer of liquid crystal material containing light sensitive components. By irradiation with light of different wavelengths, devices containing simple azo compounds change from the nematic to the isotropic state and reverse (for example, D. De us, 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 devices based on simple azo compounds have several disadvantages. In simple azo compounds the trans isomer is stable, and by irradiation with light of short wavelengths is switched to the less stable cis isomer, which in principle can be switched back to the trans isomer by irradiation with light of larger wavelengths. In competition to the latter, however, by thermal isomerization the cis isomer switches back to the stable trans isomer after a certain period, which depends on the temperature and the detailed chemical nature of the compound. Therefore the simple azo compounds possess only capability for limited storage periods.

In a recent publication, H. Rau and D. Rόttger, Mol. Cryst. Liq. Cryst. 246, 143-146 (1994) described new azobenzenes of the phane type with each two azo groups, with improved optical properties. They can be represented by the formulas A and B.

( wherein Ra, Rb = COOC₂H₅ ) These compounds , azophanes , 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 of the isomers are stable for long periods and do not show thermal isomerization. The said compounds, however, possess very limited solubility in nematic liquid crystals and decrease strongly the nematic-isotropic transition temperatures of calamitic liquid crystals. Disclosure of the Invention

An object of the invention is to obtain optically switchable materials, which are stable in both the trans and cis isomers for long period, do not show thermal isomerization, are sufficiently soluble in liquid crystals and do not depress too much the clearing temperatures of the liquid crystal base mixtures. We have found that azophanes of the formula (1)

wherein R_x, R₂, R₃, R₄ = equal or unequal, H, F, Cl, R₅ or R₆ R₅ = CN, NCS, CF₃, OCF₃,

R₆ = non 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 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-, -0C00- , -00C0-, ring Al , A2, A3, A4 = independently substituted or unsubstituted 1,4-phenylene ring, pyridine-2, 5-diyl, pyrimidine-2, 5-diyl, pyridazine-3, 6-diyl , pyrazine-2, 5-diyl,

X_x, X₂ = independently S, C(R₇ R₈),

R₇, R₈ = H, F, Cl, R₅, R₆, with the proviso that at least one of the groups R_{l f} R₂, R₃, R₄ is equal to R₆, and not more than two of the groups R_{l r} R₂, R₃, R₄ are equal to R₅, can be switched between two stable optical states and are suited for use in optical storage displays.

The novel compounds (1) 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; and 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 ther odynamic standpoint. tt azophanes 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 nm gives cc. Irradiation of cc with light of 436 nm 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 changes in absorption, the helical twisting power ( HTP ) of the chiral materials is changed. In nematic liquid crystals, by doping with the chiral materials 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 respective regions of irradiation. The following compounds (2-1) - (2-190) can be effectively used.

(wherein R₉, R₁₀ = 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) - (2-229).

(wherein R₉, R₁₀ = alkyl, alkyloxy, halogene, cyano and other groups typical for liquid crystals, , n = independently 0 - 10).

Liquid crystals compounds 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 1996, can be used. Because nematic discotic compounds usually have relatively high melting temperatures, multicomponent mixtures are necessary. Discotic materials are specially compatible with the azophanes.

The following compounds (3-1) - (3-11) are suitable discotic materials for this invention.

wherein R_u = 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 a single bond, 0C0, CH₂CH₂, C≡C, CH₂0, 0CH₂, CH₂S, SCH₂, NHCO, 0C0CH₂, 0C0CH=CH, CH₂0C0, N=N.

The invented 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 literature, 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) with 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_: - R₆ 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 residues.

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

Alkyl substituted compounds (13) can be derived from substituted benzaldehydes (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 brominating reagents such as PBr₃, S0Br₂, Br₂, NBS (N-bromosuccinimide) etc.

Scheme 7

When X_j or X₂ is a substituted- or nonsubstituted-carbon atom, the compound (1) can be synthesized by an etherification of substituted- or nonsubstituted-dibromo carbon and diols that can be prepared according to Scheme 7.

All claimed 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 alkyl, alkyloxy, alkanoyloxy, alkyloxyalkyl, alkenyl, alkynyl, alkenyloxy, alkynyloxy, alkadienyl, alkadienyloxy, haloalkyl, haloalkyloxy;

Al , A2, A3 and A4 are preferably 1,4-phenylene,

2-fluoro-1 , 4-phenylene, 2, 3-difluoro-1, 4-phenylene,

3, 5-difluoro-l, 4-phenylene, 2, 5-difluoro-l, 4-phenylene,

2,3, 5-trifluoro-1 , 4-phenylene, 2-chloro-l , 4-phenylene, pyridine-2, 5-diyl, 3-fluoropyridine-2, 5-diyl,

6-fluoropyridine-2 , 5-diyl , pyrimidine-2 , 5-diyl ,

4-fluoropyridine-2, 5-diyl, pyridazine-3, 6-diyl, pyrazine-2, 5-diyl .

Brief Explanation of Drawing Fig. 1 is an illustration of the holographic arrangement applied for the invention.

Best Mode for Practicing the Invention

Example 1

Synthesis of l-pentyl-18-heptyl-2, 19-dithia[3.3] - (4, 4 ' )-trans-diphenyldiazeno( 2 )phane (in (1) R_λ = pentyl, R₄= heptyl, R₂, R₃ = hydrogen, X_:, X₂ = sulfur and Al - A4 =

1 , 4-phenylene )

Step 1: Synthesis of 1-hydroxyhexyl-benzene (S-l)

To a mixture of benzaldehyde (3.0 mol) in 500 ml of tetrahydrofuran (hereinafter called THF), pentylmagnesium bromide (3.2 mol ) that was prepared from pentylbromide and magnesium in THF (300 ml) was added at temperatures lower than 10°C and the reaction mixture was stirred at room temperature for 1 hour. With keeping the temperature under 20°C, aqueous ammonium chloride solution was added and the reaction mixture was extracted with diethylether (500 ml x 3), then dried over anhydrous magnesium sulfate.

Evaporation of the organic layer gave colorless oil (S-l) (2.5 mol).

Step 2: Synthesis of 1-t-butyldimethylsilyloxyhexyl-benzene (S-2) A mixture of (S-l) (2.5 mol), t-butyldimethylsilyl chloride ( TBDMSC1 ) (2.7 mol), imidazole (4.0 mol) and 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 4-( 1-t-butyldimethylsilyloxyhexyl )- nitrobenzene (S-3) To a solution of (S-2) (2.0 mol) in acetic acid (550 ml), a mixture of sulfuric acid (2.0 mol) and fuming nitric acid (2.0 mol) was added at temperatures lower than -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 silicagel column chromatography (eluted by a mixture of toluene and ethyl acetate) to give 0.58 mol of a dark yellow colored compound (S-3). Step 4: Synthesis of 4-( 1-t-butyldimethylsilyloxyhexyl )- aminobenzene (S-4) A mixture of (S-3) (0.55 mol), 5% Pd/C and ethanol (430 ml) was stirred in H₂ atmosphere for 2 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 0.51 mol of colorless material (S-4). Step 5: Synthesis of 4- ( 1-hydroxyhexyl ) -aminobenzene (S-5) To a solution of (S-4) (0.50 mol) in 100 ml of

THF, 1 M solution of tetrabutylammonium fluoride (hereinafter called TBAF) (0.75 mol) in THF was added under -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 0.41 mol of colorless material (S-5).

Step 6: Synthesis of 4- ( 1-bromohexyl ) -aminobenzene (S-6) To a mixture of the alcohol (S-5) (0.4 mol) and ethylene dichloride (150 ml), triphenylphosphine (0.6 mol) and carbon tetrabromide (0.9 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) (0.35 mol). Step 7: Synthesis of 4- ( 1-bromohexyl ) -4 ' -bromomethyl- azobenzene (S-7) To (S-6) (0.35 mol) in 180 ml of acetic acid 4-nitrosobenzylbromide (0.4 mol) in 100 ml of ethanol was added dropwise at 5^CC, 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 by 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) (0.28 mol).

Step 8: Synthesis of l-pentyl-18-heptyl-2, 19- dithia[3.3] (4,4 ' )-trans-diphenyldiazeno( 2 )phane A mixture of (S-7) (0.25 mol) and 4-(l- bromooctyl ) -4 ' -bromomethyl-azobenzene (0.25 mol) that was prepared from benzaldehyde and octyl agnesium bromide according to the methods of steps 1 - 7 of Example 1 , in 500 ml 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 a 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 (0.015 mol), which was recrystallized from benzene for two times, to give a white solid (0.005 mol). The obtained solid was determined by elemental analysis, ¹H- and ¹³C-NMR and mass spectroscopy to be the captioned compound l-pentyl-18-heptyl-2, 19-dithia[3.3] - (4,4' )-trans-diphenyldiazeno( 2 )phane. Example 2 Synthesis of 1 , 3, 18, 20-tetrakis-pentyl-2, 19-dithia[3.3] - (4,4' )-trans-diphenyldiazeno(2 )phane (in (1) R , R_2/ R₃, R₄ = pentyl, X_{l r} X₂ = sulfur and Al - A4 = 1,4-phenylene)

Step 1: Synthesis of 1,3, 18, 20-tetrakis-pentyl-2, 19- dithia[3.3] ( 4, 4 ' )-trans-diphenyldiazeno( 2 )phane A mixture of of 4, 4 ' -bis ( 1-bromohexyl )azobenzene (S-8)(0.2 mol) that was prepared according to Example 1, 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 liters of absolute ethanol with vigorous stirring over 2 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 purified by a recrystallization from benzene, and a white solid was obtained, which was determined by elemental analysis, H- and ¹³C-NMR and mass spectroscopy, to be the captioned compound 1,3,18,20- tetrakis-pentyl-2, 19-dithia[3.3] ( 4, 4 ' )-trans- diphenyldiazeno( 2 )phane (0.03 mol). Example 3

According to the procedures of Examples 1 and 2, the following compounds (1) as specified in Table 1 are synthesized, whereinafter rings Al - A4 are abbriviated as follows:

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)

Table 1 (Cont'd)

Example 4

Use of the compounds according to the invention in the optical storage device

A nematic mixture consisting of 0.5 weight% of l-pentyl-18-heptyl-2,19-dithia[3.3] (4,4' )-trans- diphenyldiazeno( 2 )phane obtained according to the synthesis instruction of Example 1 and 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) was prepared. The mixture is filled in a cell, consisting of two equiplanar glass plates with spacers of 10 mm, the surfaces of the cells being covered with an alignment material ( polyimide ) unidirectionally rubbed in order to obtain a homogenous orientation of the nematic mixture. One polarizer is used in front of the cell, with the polarization direction parallel to the rubbing direction. The cell shows quite strong absorption of visible light.

By irradiation with light of wavelength 366 nm the switching of the azophane is obtained, and the cell comes to its transparent optical state.

This state can be held for long period, without change of the optical properties. By irradiation with light of wavelength 436 n , the switching from transparent state to absorbing state is obtained. The switching is reversible many times in both directions, and both optical states are stable for long periods without fading. Irradiation can be made with a modulating laser, producing changes in the optical properties of the irradiated spots, and allowing the storage of imformation. Another varriant of the devices is to use the polarizer in rear position. Example 5

Use of the compounds according to the invention for holographic data storage Fig. 1 shows the holographic arrangement which is used for demonstrtion of the holographic data storage. The holographic arrangement consists of a laser with wavelength 366 nm, the beam of which is split by a beam splitter into the reference beam and the beam modulated by a spatial light modulator. Both these beams by means of mirrors and lens are sent for interference into the liquid crystal cell, which consists of two quartz plates held in a distance of 15 mm by spacers and filled with the mixture mentioned in Example 4. The cell is storing the holographic imformation. By use of laser light of low intensity the stored hologram can be made visible. By irradiation with light of wavelength 436 nm the hologram can be erased.

Another variant is to start the holographic experiment with the cc state of the azophane, obtained by irradiation of the cell with light of wavelength 366 nm. Then by the aid of a laser with wavelength 436 nm the hologram is produced, which can be read using laser light of low intensity. In this case, erasure of the hologram is obtained by irradiation with light of wavelength 366 nm. Example 6

Use of the compounds according to the invention in discotic nematics

In the cells according to Example 4 the following mixture was used:

A nematic discotic mixture comprising of the discotic materials 2,3,4,5, 6-pentakis( 2-( 4-pentylphenyl )ethynyl )- l-(ll-hydroxyundecyloxy )benzene 45 mol%, 2,3,4,5,6- pentakis( 2-( 4-pentylphenyl )ethynyl )-l-( 10-ethoxycarbonyl- decyloxy)benzene 40 mol%, 2, 3, 4, 5, 6-pentakis( 2-( 4- pentylphenyl )ethynyl )-l-( 10-hydroxycarbonyldecyloxy )- benzene 15 mol%. The compounds can be prepared according to the instruction of the literature, D. Janietz, K. Praefcke and D. Singer, Liq. Cryst. 13 (2), 247 (1993). To this mixture 0.5 weight% of 1, 3, 18, 20-tetrakis-pentyl-2, 19- dithia[3.3] ( 4, 4 ' )-trans-diphenyldiazeno( 2)phane was added, which has been synthesized according to the instruction given in Example 2.

The mixture is put into the cell described in Examples 4 and 5 respectively. Irradiation with light of wavelength 366 nm produces switching to the more transparent cc state, irradiation with light of wavelength 436 nm produces the absorbing tt state. Between both the optical states is a good contrast, stable for quite long periods .

Claims

An azophane derivative of the formula ( 1 )

wherein R_{l f} R₂, R₃, R₄ = equal or unequal = H, F, Cl, R₅, R₆ or R₇

R₅ = CN, NCS, CF₃, 0CF₃,

R₆ = non 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, -OCF₃, -0CHF₂, -0CH₂F in which one or several CH₂ groups independently one to another can be exchanged by -S-, -0-, cyclobutane, [1.1. l]bicyclopentane, cyclopropane, oxirane, thiirane, -CO-, -C00-, -OCO-, -0C00- , -OOCO-, ring Al , A2, A3, A4 = independently substituted or unsubstituted 1,4-phenylene ring, pyridine-2, 5-diyl, pyrimidine-2, 5-diyl, pyridazine-3, 6-diyl, pyrazine-2, 5-diyl, X_{l r} X₂ = independently S, C(R₇ R₈), Rη / R_& ^~ H, F, Cl , R₅, R₆, with the proviso that at least one of the groups R_{l r} R₂, R₃,

R₄ is equal to R₆, and not more than two of the groups R_{l r} R₂, R₃, R₄ are equal to R₅.

2. Liquid crystal optical storage device, consisting of two glass, quartz or plastic plates held parallel one to another in a defined distance by a spacer, the gap between the plates filled with a liquid crystal mixture, characterized by use of calamitic nematic liquid crystals doped with azophane derivatives according to claim 1.

3. Liquid crystal optical storage device, consisting of two glass, quartz or plastic plates held parallel one to another in a defined distance by a spacer, the gap between the plates filled with a liquid crystal mixture, characterized by use of discotic nematic liquid crystals doped with azophane derivatives according to claim 1.

4. Liquid crystal mixture comprising of azophane derivatives ( 1 ) according to claim 1 and achiral liquid crystals, useful for optical storage displays.