EP0364534A1 - Liquid crystal mixtures with low refractive index - Google Patents

Abstract

Liquid crystal mixtures comprise at least two components, the refractive index of the mixture no being less than a selected value in the range between 1.46 and 1.52 and the refractive index of the mixture being at least equal at said value. The components of the mixture do not contain any units of (CF2-CF2) n. These mixtures are useful as covering materials in integrated optics.

Description

 LC mixtures with a low refractive index

The invention relates to liquid crystal mixtures with a low refractive index and their use as overlay materials in integrated optics.

Due to their diverse physical and chemical properties, liquid crystal mixtures have found ever increasing technical application.

Under the influence of electric fields, for example, the optical properties such as light absorption, light scattering, birefringence, reflectivity or color can be changed. In many cases, the targeted selection of the mixture components allows the setting of parameters suitable for a specific technical application.

In general, liquid crystal phases are required for the technical application of these effects in electronic and optical components, which must meet a variety of requirements. The chemical resistance to moisture, air and physical influences such as heat, radiation in the infrared, visible and ultraviolet range and electrical DC and AC fields are particularly important here. The rapid growth of integrated optics makes it necessary to develop new optical components such as switches, couplers, modulators and polarizers. Such components are used in optical data transmission and optical sensor technology.

These components are usually based on electro-optical switching elements. While it is often very difficult to design the waveguide itself to be electro-optically active, the use of evanescent fields in electro-optically active overlay materials enables the desired influencing of the light propagation behavior in a simpler manner.

Due to the high field-induced refractive index changes that can be achieved with liquid-crystalline materials, these are particularly suitable for applications as overlay materials in planar waveguides and as electro-optically active cladding materials in optical glass or polymer fibers. These applications require that at least one of the refractive indices of the liquid crystal be lower than that of the waveguide. In order to achieve optimal efficiency, the refractive indices must also be adapted to the special waveguide configuration.

For the use of liquid crystals in connection with glass waveguides e.g. required that one of the

Refractive indices do not exceed the value of n = 1.457, whereas with polymeric waveguides (e.g. based on PMMA) one of the refractive indices must not be greater than 1.483 at a wavelength of 890 nm.

It is known that fluorinated molecules have smaller refractive indices than the corresponding hydrogenated compounds. Mixtures containing such fluorine th components are described in GB 87 02 126. However, the corresponding compounds are fluorinated to a high degree [(CF ₂ -CF ₂ ) n units with n ≥ 3]. However, adding fluorinated liquid crystal components to liquid crystal mixtures can lead to destabilization of the liquid crystalline phases. Furthermore, these materials are sometimes sparingly soluble to insoluble in conventional LC mixtures containing essentially non-fluorinated liquid crystal compounds. The higher the degree of fluorination of such a mixture component, the greater the disadvantages mentioned.

The invention was therefore based on the object of providing new FIÜSsig crystal materials which, in particular, have favorable n _o and n _e values for use as electro-optically active materials in conjunction with optical glass or polymer waveguides and which do not or only to a lesser extent have the disadvantages described exhibit.

It has surprisingly been found that liquid crystal mixtures, the components of which contain no (CF ₂ -CF ₂ ) _n units, are also outstandingly suitable as overlay materials in integrated optics.

The invention therefore relates to a liquid-crystal mixture free of liquid-crystalline components having (CF ₂ -CF ₂ ) _n units, where n is a natural number, characterized in that the value of the refractive index n of the mixture is smaller and the value of the refractive index n of the mixture is at least as large as a value selected from the interval 1.46-1.52. A preferred object of the invention is a mixture of this type, comprising a dielectrically positive component, selected from the group of the compounds of the formulas I and II,

and optionally a dielectric neutral component selected from the group of the compounds of the formulas III and IV,

R ² -Q ² -Q ³ -R ³ III R ² -Q ² -Q ³ -ZQ ¹ -R ³ IV

wherein

Q ¹ 1,4-cyclohexylene or 1,4-phenylene,

Q ² , Q ³ 1,4-cyclohexylene, one of the rings Q ² or Q ³ also 1,4-cyclohexenylene,

R ¹ , R ² , R ³ non-polar wing groups,

x a polar wing group,

Z a bridge link or a single bond

mean.

The invention further relates to the use of such mixtures as overlay materials in the integrated optics. Finally, the invention relates to an electro-optical switching element, which essentially consists of a waveguide and an optical switching element, the overlay material consisting of at least one such mixture.

Dielectric neutral components here mean compounds whose value for the dielectric anisotropy is -2 ≤ Δε <+2, with dielectrically positive components with a value significantly higher (Δε ≥ +5), preferably ≥ +7.

The radicals R ¹ , R ² , R ³ , X and Z have the meanings customary in LC chemistry. In this context, reference is made to the standard works D. Demus, Liquid Crystals in Tables, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig 1974 (Vol. I) and 1984 (Vol. II) and H. Kelker, R. Hatz, Handbook of Liquid Crystals, Verlag Chemie, Weinheim 1980, referenced, which describes a variety of common and suitable residues.

In the formulas I to IV, R ¹ , R ² and R ³ denote non-polar wing groups, in each case independently of one another preferably an alkyl, alkenyl, alkoxy or one

Oxaalkyl radical with 1 to 12 carbon atoms each.

If R ¹ , R ² and / or R ^{3 are} alkyl radicals and / or alkoxy radicals, these can be straight-chain or branched. They are preferably straight-chain and accordingly are preferably methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, methoxy, ethoxy, propoxy, butoxy, pentoxy, hexoxy or heptoxy, furthermore octyl, nonyl, decyl, undecyl, dodecyl, octoxy, nonoxy , Decoxy, undecoxy. Oxaalkyl preferably means straight-chain 2-oxapropyl (= methoxymethyl), 2- (= ethoxymethyl) or 3-oxabutyl (= 2-methoxyethyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or 5- Oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6 -, 7- or 8-oxanonyl, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.

Branched groups of this type usually contain no more than one chain branch. Preferred branched radicals R ¹ , R ² and R ³ are isopropyl, 2-butyl (= 1-methylpropyl), isobutyl (= 2-methylpropyl),

2-methylbutyl, isopentyl (= 3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 1-methylhexoxy, 1-methylheptoxy.

If R ¹ , R ² and / or R ^{3 are} alkenyl radicals, these can be straight-chain or branched. They are preferably straight-chain and have 2 to 10 carbon atoms. Accordingly, they mean in particular vinyl, prop-1- or prop2-enyl, but-1-, 2- or but-3-enyl, pent-1-, 2-, 3- or pent-4-enyl, hex-1 -, 2-, 3-, 4- or hex-5-enyl, hept-1-, 2-, 3-, 4-, 5- or hept-6-enyl, oct-1-, 2-, 3- , 4-, 5-, 6- or oct-7-enyl, non-1-, 2-, 3-, 4-, 5-, 6-, 7- or non-8-enyl, dec-1-, 2-, 3-, 4-, 5-, 6-, 7-, 8- or Dec-9-enyl.

Q ^{1 represents} 1,4-cyclohexylene or 1,4-phenylene, Q ² and Q ³ 1,4-cyclohexylene and 1,4-cyclohexenylene, but only one of these two in a compound containing Q ² and Q ³ Rings can be 1,4-cyclohexenylene. Among the. Double bond isomers of 1,4-cyclohexenylene are very particularly preferred those in which one of the atoms C ₁ or C _{4 is} part of the double bond. X denotes a polar wing group, preferably -CF ₃ , -NO ₂ , -CN and F. -CF ₃ , -CN and F are particularly preferred. The bridge member Z preferably denotes -CO-O-, -O-CO -, -CH ₂ O-, -OCH ₂ - or -CH ₂ CH ₂ -. Z also means a single bond. -CO-O-, -O-CO-, -CH ₂ CH ₂ - and single bond are particularly preferred.

The dielectric positive compounds of formula I thus correspond to those of sub-formulas Ia and Ib.

I ^{a Ib}

The dielectric positive compounds of formula II thus correspond to those of sub-formulas IIa and IIb.

I ^Ia Ilb

The preferred dielectric neutral compounds of formula III correspond to those of subforms IIIa to IIIc

I ^{IIb II10}

The preferred dielectrically neutral compounds of the formula IV correspond to those of the sub-formulas IVa to IVj. _ _*

- 8 ¬

Accordingly, those compounds of the sub-formulas la, Ib, Ila, Ilb, IIIa to IIIc and IVa to IVj in which R ¹ , R ² , R ³ , X and Z have the preferred meanings given are particularly preferred. Of the compounds of sub-formulas IVa, those of

Sub-formulas IVaa to IVac are particularly preferred.

Of the compounds of sub-formula IVb, those of sub-formulas IVba to IVbc are therefore particularly preferred.

The individual compounds of the formulas I-IV of the liquid-crystal phases according to the invention are either known or their mode of preparation can be readily derived from the prior art for the person skilled in the art, since they are based on standard processes described in the literature.

Corresponding connections are for example in the

DE 27 02 598, 26 36 684, 29 22 236, 32 33 641, 33 21 373, 28 00 553, 32 11 601, 33 17 597 and 30 23 368 and EP 00 94487.

Of the liquid crystal mixtures according to the invention, preference is given in particular to those which contain at least one dielectrically positive component I or II and at least one dielectrically neutral component III or IV, or those which contain at least two dielectrically positive components, at least one of components I or II not having 1, Contains 4-phenyl radical.

In the first case, the mixture preferably contains 2 to 60, in particular 5 to 40% of a dielectrically positive component and 40 to 98, in particular 60 to 95% of one or more dielectrically neutral components.

In the event that the mixture contains at least two dielectrically positive components, at least one of components I or II having no 1,4-phenylene radical contains, so preferably 50 to 95%, in particular 70 to 90% of the 1,4-phenylene-free compound in the mixture.

The liquid crystal mixtures according to the invention are preferably 50 to 100%, in particular 80 to 95%. Contain individual compounds selected from the group of formulas I to IV.

In a particular embodiment of the invention, the mixture contains only fluorine-free components.

The mixtures according to the invention are prepared in a conventional manner. As a rule, the desired amount of the components used in smaller amounts is dissolved in the components which make up the main constituent, advantageously at elevated temperature. It is also possible to use solutions of the components in an organic solvent, e.g. in acetone, chloroform or methanol, and to remove the solvent after mixing, for example by distillation.

The mixtures according to the invention are suitable as overlay materials for polymeric or glass waveguides and are notable for their low viscosity and a wide mesophase range. The usual poly (meth) acrylates, polysiloxanes, polyimides or other materials known to the person skilled in the art can be used as polymers.

Important areas of application for these waveguides covered with overlay materials are electro-optical switches, couplers, modulators and polarizers in integrated optics in the field of data transmission and optical sensor technology. The accompanying drawing (Fig. 1) shows an example of the structure of an electro-optical switching element. For the sake of clarity, components 1 to 6 are not reproduced to one another to scale.

The element consists of a substrate (1) with an inscribed waveguide (2). An electrode (3), which is applied to the substrate surface and is connected to a voltage source (not shown), is arranged on both sides of the waveguide. The space above the waveguide and part of the electrodes is filled by the overlay material, the liquid crystal mixture (4). This space is delimited on the side by spacers (5) and on the top by a glass plate (6). The value of the refractive index of the waveguide material is larger than that of the substrate and lies between the values for no and ne

Liquid crystal mixture, but is at most as large as that for n _e .

This element can be used in different ways. In one application, the longitudinal axes of the liquid crystal molecules are so pre-oriented in the field-free state that the preferred direction between the electrodes is perpendicular to the substrate surface (homeotropic). The application of an electric field causes the molecules to flip in the direction of the field and thus to change the waveguide properties. For example, once the field has been created, a T _E wave arriving in the waveguide is no longer continued. This component can thus serve, for example, as a polarizer for unpolarized light or as an ON / OFF switch for linearly polarized light. If the control is varied, it can also be used as a modulator. Alternatively, the electrodes can also be attached above and below the waveguide and the liquid crystal molecules can also be pre-oriented parallel to the substrate surface. The switching principle remains unchanged.

The following example is intended to illustrate the invention without limiting it.

It means

S-N phase transition temperature smectic-nematic Klp clearing point

All temperatures above and below are given in degrees Celsius. The percentages are percentages by weight.

example 1

A mixture of

15% p-trans-4-propylcyclohexyl benzonitrile

10% trans, trans-4-methoxy-4'-propylcyclohexylcyclohexane 15% trans, trans-4-propoxy-4'-propylcyclohexylcyclohexane 10% trans, trans-4-methoxy-4'-pentylcyclohexylcyclohexane 10% trans, trans-4- Ethoxy-4'-pentylcyclohexylcyclohexane

5% trans, trans-4-propylcyclohexyl-cyclohexane-4'-carboxylic acid-trans-4-propyl-cyclohexyl ester 5% trans, trans-4-propylcyclohexyl-cyclohexane-4'-carboxylic acid trans-4-pentyl-cyclohexyl ester 5% trans, trans-4-butylcyclohexyl-cyclohexane-4'-carboxylic acid-trans-4-propyl-cyclohexyl ester

5% trans, trans-4-butylcyclohexyl-cyclohexane-4'-carboxylic acid-trans-4-pentyl-cyclohexyl ester 5% trans, trans-4-propylcyclohexyl-cyclohexane-4'-carboxylic acid-4-propyl-phenyl ester 5% trans, trans-4-propylcyclohexyl-cyclohexane-4'-carboxylic acid-4-pentyl-phenyl ester 5% trans, trans- 4-Butylcyclohexyl-cyclohexane-4'-carboxylic acid-4-propyl-phenyl ester 5% trans, trans-4-butylcyclohexyl-cyclohexane-4'-carboxylic acid-4-pentyl-phenyl ester

shows SN <-20 °, Klp 91 ° and the following n _o and n _e values, measured at 20 °:

ⁿ o ⁿ e λ [nm]

1.4768 1.5441 589

1.4720 1.5414 633

1.4690 1.5339 900

1.4664 1.5300 1300

Claims

Claims

1. Liquid crystal mixture, free of liquid crystalline components with (CF ₂ -CF ₂ ) _n units, in which n is a natural number, characterized in that the value of the refractive index n _{o of} the mixture is smaller and the value of the refractive index n _{e of} the mixture is at least is as large as a value selected from the interval 1.46-1.52.

2. Mixture according to claim 1, comprising a dielectric positive component selected from the group of compounds of the formulas I and II,

II and optionally a dielectric neutral component selected from the group of the compounds of the formulas III and IV,

R ² -Q ² -Q ³ -R ^{3 III} R ² -Q ² ~ Q ³ -ZQ ¹ -R ³ IV

wherein

Q ¹ 1,4-cyclohexylene or 1,4-phenylene, Q ² , Q ³ 1,4-cyclohexylene, one of the rings Q ² or Q ³ also 1,4-cyclohexenylene,

R ¹ , R ² , R ³ non-polar wing groups,

x a polar wing group,

Z a bridge link or a single bond

mean.

3. Mixture according to claim 2, comprising at least one compound of the formulas I or II, in which Q ^{1 is} 1,4-cyclohexylene, and at least one further dielectrically positive component, selected from the group of the compounds of the formulas I and II.

4. Mixture according to claim 2, comprising at least one dielectric neutral component, selected from the group of compounds of the formulas III and IV.

5. Use of the mixture according to at least one of claims 1, 2, 3 and 4 as overlay materials in the integrated optics.

6. Electro-optical switching element, consisting essentially of a waveguide and an overlay material, characterized in that the overlay material consists of a mixture according to at least one of claims 1, 2, 3 or 4.