CN1328596A - Liquid crystal compound - Google Patents

Abstract

Compounds of formula (I) are provided, wherein R ¹ represents chiral or achiral branched or straight chain C _1-20 alkyl or C _1-20 alkenyl, said group can be optionally replaced by one or more halogen atoms and one or more non-adjacent -CH ₂ -groups can be optionally replaced by -CH=CH-, -O-, -CO-, -COO-, -OOC-, -O-OC- O-or-S-replacement; R ² represents chiral or achiral branched or straight chain C _1-20 alkoxy, C _1-20 alkenyloxy, C _1-20 alkylcarbonyloxy ( Alkyl-COO-) or C _1-20 alkenylcarbonyloxy (alkenyl-COO-), said group may be optionally substituted by one or more halogen atoms and one or more of them are non-adjacent The -CH ₂ - group can optionally be replaced by -CH=CH-, -O-, -CO-, -COO-, -OOC-, -O-OC-O- or -S-; MG ¹ represents the formula ( The group of Ia): -(A-Z ² ) _n -(B-Z ³ ) _m -, wherein A and B are each independently 1,4-phenylene or 1,4-cyclohexylene, said Each of the groups can be optionally substituted by halogen, cyano or nitro or a group of formula R ¹ and one or two non-adjacent -CH- or -CH ₂ - groups can be optionally replaced by nitrogen or oxygen, respectively ;Z ¹ , Z ² and Z ³ independently represent a single bond, -COO- or -OOC-, -CH ₂ CH ₂ -, -CH=CH-, -C≡C-, -OCH ₂ - or -CH ₂ O-; m and n are each independently 0 or 1; and MG ² represents 1,4-phenylene, which may be optionally substituted by halogen, cyano or nitro or a group of formula R ¹ and one or Two non-adjacent -CH- groups are replaced by nitrogen. The compound can be used to prepare ferroelectric liquid crystal devices.

Description

liquid crystal compound

The invention relates to liquid crystal compounds, especially ferroelectric liquid crystal compounds or compounds for preparing liquid crystal mixtures with ferroelectric properties.

The use of ferroelectric liquid crystal phases in the preparation of ferroelectric liquid crystal devices is known. See eg Clark and Largerwall, Appl. Phys. Lett. 1980, 36, 899. Ferroelectric liquid crystal devices (FLCDs) have been found to overcome the limitations of devices based on nematic liquid crystal (LC) compounds and mixtures. These limitations include limited visual range and slower switching times.

The ferroelectric liquid crystal (FLC) phase includes chiral smectic liquid crystal compounds. In the smectic phase, LC molecules are arranged in layers. The regulator can be perpendicular to the layer (eg smectic-A or S _A ) or oblique (smectic-C or S _C ). In order to generate ferroelectricity, at least one component comprising the liquid crystalline phase must be chiral. The chiral S _C phase is denoted S _C ^* . FLC mixtures typically contain one or more achiral components and one or more chiral dopants. Achiral components are generally not required for the preparation of FLC. However, the preparation of FLC mixtures with achiral components means that greater design flexibility is available for LCD fabrication.

The spontaneous polarization of the ferroelectric liquid crystal phase and the strong interaction of this type of liquid crystal phase with an applied electric field means that these materials can provide very fast electro-optical responses, Meyer et al., J.Physique Lett.1975, 36, L-69-L-71. Furthermore, contrary to nematic LCs, ferroelectric liquid crystals can exhibit a linear electro-optic response in an electric field. This means that ferroelectric liquid crystals can be switched on and off at constant speed by reversing the sign of an applied voltage: in contrast to nematic LCDs, the switching of both is driven by an electric field.

The use of pyrimidinyl compounds in liquid crystal applications is well known. US 4,774,020 discloses liquid crystal materials comprising pyrimidinyldioxane derivatives. These compounds have been used in nematic liquid crystal applications.

Examples of pyrimidinyl compounds known to be useful in ferroelectric liquid crystal applications can be found in WO 96/30344. These compounds include halogen-substituted pyrimidinyldioxane derivatives.

The properties of the LC material used to make a ferroelectric liquid crystal (FLC) display have a strong influence on the operation of the display. In addition to requiring that the FLC exhibit a ferroelectric phase over the entire operating temperature range, the FLC material must meet many additional requirements. These requirements include low viscosity, large electro-optic effect, and chemical and photochemical stability, to name but a few.

Therefore, there is a need for further liquid crystal compounds which at least partially meet the above requirements and which can be used for the preparation of ferroelectric liquid crystal phases. The present invention addresses this need.

其中R¹表示手性或非手性的支化或直链C_1-20烷基或C_1-20链烯基，所述基团可任选被一个或多个卤原子取代且其中一个或多个非相邻-CH₂-基团可任选被-CH＝CH-、-O-、-CO-、-COO-、-OOC-、-O-OC-O-或-S-替代；R²表示手性或非手性的支化或直链C_1-20烷氧基、C_1-20链烯氧基、C_1-20烷基羰氧基(烷基-COO-)或C_1-20链烯基羰氧基(链烯基-COO-)，所述基团可任选被一个或多个卤原子取代且其中一个或多个非相邻-CH₂-基团可任选被-CH＝CH-、-O-、-CO-、-COO-、-OOC-、-O-OC-O-或-S-替代；MG¹表示式Ia的基团The first aspect of the present invention provides a compound of formula (I)

Wherein R ¹ represents chiral or achiral branched or straight chain C _1-20 alkyl or C _1-20 alkenyl, said group may be optionally substituted by one or more halogen atoms and one or Multiple non-adjacent _-CH2- groups may optionally be replaced by -CH=CH-, -O-, -CO-, -COO-, -OOC-, -O-OC-O- or -S-; R ² represents chiral or achiral branched or straight chain C _1-20 alkoxy, C _1-20 alkenyloxy, C _1-20 alkylcarbonyloxy (alkyl-COO-) or C _1-20 alkenylcarbonyloxy (alkenyl-COO-), which may be optionally substituted by one or more halogen atoms and wherein one or more non-adjacent -CH ₂ - groups may be optionally Optionally replaced by -CH=CH-, -O-, -CO-, -COO-, -OOC-, -O-OC-O- or -S-; MG ¹ represents a group of formula Ia

-(AZ ² ) _n -(BZ ³ ) _m - Ia

wherein A and B are each independently 1,4-phenylene or 1,4-cyclohexylene, each of which may be optionally substituted by halogen, cyano or nitro or a group of formula ^R and wherein One or two non-adjacent -CH- or -CH ₂ - groups can optionally be replaced by nitrogen or oxygen respectively; Z ¹ , Z ² and Z ³ independently represent a single bond, -COO- or -OOC-, - _CH2CH2- , _-CH =CH-, -C≡C-, _-OCH2- , or _-CH2O- ; m and n are each independently 0 or 1; and MG ² represents 1,4-phenylene , which may be optionally substituted by halogen, cyano or nitro, or a group of formula R ¹ wherein one or two non-adjacent -CH- groups are replaced by nitrogen.

The dioxane-pyridine and -pyrimidine derivatives of the invention may be included in ferroelectric LC mixtures. This allows the skilled artisan to selectively optimize certain mixture parameters. The parameters that can be optimized include: 1. Broad _SC -phase. This is important not only because the addition of new components increases the range of useful phases, but also because it also allows the replacement of existing _SC -components (which are eg unstable) without reducing the _SC -range. 2. Large inclination angle. This is important in applications such as DHF-LCD as it facilitates the fabrication of devices with high brightness. 3. Inhibition of _SA -phase. This is important in applications such as APD-LCDs that require NS _C ^* switching without interfering S _A -phase. 4. Low birefringence Δn. The dioxane-phenyl-pyrimidine compounds of the present invention have a lower birefringence compared to other common wide temperature range _SC -components. Lower birefringence is desirable for most FLC-LCDs because it enables devices to be fabricated with larger cell gaps d, which significantly reduces production difficulty and cost.

It is therefore understood that the use of these compounds alone or in combination with other liquid crystal components leads to the preparation of LCDs with excellent operability.

The group R ¹ is preferably a branched or linear C _5-16 alkyl group.

The group R ² is preferably a branched or linear C _5-16 alkoxy or C _5-16 alkenyloxy group.

The groups R ¹ and R ² may independently of each other be chiral.

The group MG ¹ is preferably 1,4-phenylene, which is optionally substituted by halogen, cyano, nitro, alkyl or alkoxy.

Each of Z ¹ , Z ² and Z ³ is preferably a single bond.

The term "alkyl" is understood to include radicals such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl and dodecyl. group.

The term "alkenyl" is understood to include groups such as E or Z alkenyl as well as alkenyl groups having terminal double bonds. Examples of such groups include allyl, 2-butenyl, 2-pentenyl, 2-hexenyl, 2-heptenyl, 2-octenyl, 2-nonenyl, 2-decenyl Alkenyl, 3-butenyl, 3-pentenyl, 3-hexenyl, 3-heptenyl, 3-octenyl, 3-nonenyl, 3-decenyl, 4-pentenyl , 4-heptenyl, 4-heptenyl, 4-octenyl, 4-nonenyl, 4-decenyl, 5-hexenyl, 6-heptenyl, 7-octenyl, 8- Nonenyl, 9-decenyl, 10-undecenyl, 11-dodecenyl and the like.

The term "alkoxy" is understood to include compounds such as methoxy, ethoxy, n-propoxy, isopropoxy, butoxy, pentyloxy, hexyloxy, heptyloxy, octyloxy, nonyl oxy, decyloxy, undecyloxy and dodecyloxy groups.

The term "alkenyloxy" is understood to include groups such as 2E-alkenyloxy, 3Z-alkenyloxy, 4E-alkenyloxy and alkenyloxy with a terminal double bond. The terms "2E-alkenyloxy", "3Z-alkenyloxy" and "4E-alkenyloxy" include 3 or more, 4 or more and 5 or more A straight-chain alkenyloxy group of carbon atoms, in which the double bonds are located at the 2, 3 and 4 positions, respectively, and E and Z represent the configuration of the double bond. Such groups include 2E-butenyloxy, 2E-pentenyloxy, 2E-hexenyloxy, 2E-heptenyloxy, 2E-octenyloxy, 2E-nonenyloxy, 2E-decenyloxy Alkenyloxy, 3-butenyloxy, 3Z-pentenyloxy, 3Z-hexenyloxy, 3Z-heptenyloxy, 3Z-octenyloxy, 3Z-nonenyloxy, 3Z-decene Oxygen, 4-pentenyloxy, 4E-hexenyloxy, 4E-heptenyloxy, 4E-octenyloxy, 4E-nonenyloxy, 4E-decenyloxy, 5-hexenyloxy 6-heptenyloxy, 7-octenyloxy, 8-nonenyloxy, 9-decenyloxy, 10-undecenyloxy, 11-dodecenyloxy, etc.

The term "alkylcarbonyloxy" is understood to include such groups as methylcarbonyloxy, ethylcarbonyloxy, propylcarbonyloxy, butylcarbonyloxy, pentylcarbonyloxy, hexylcarbonyloxy, octyl Groups such as carbonyloxy, nonylcarbonyloxy, decylcarbonyloxy, undecylcarbonyloxy and dodecylcarbonyloxy.

The term "alkenylcarbonyloxy" is understood to include compounds such as 1E-alkenylcarbonyloxy, 2Z-alkenylcarbonyloxy, 3E-alkenylcarbonyloxy, 4Z-alkenylcarbonyloxy and Groups such as alkenylcarbonyloxy having a terminal double bond. The terms "1E-alkenylcarbonyloxy", "2Z-alkenylcarbonyloxy", "3E-alkenylcarbonyloxy" and "4Z-alkenylcarbonyloxy" include those having 3 or Straight-chain alkenyl groups of more, 4 or more and 5 or more carbon atoms, wherein the double bonds are respectively located at the 2, 3 and 4 positions, and E and Z represent the configuration of the double bonds. Such groups include allylcarbonyloxy, 2Z-butenylcarbonyloxy, 2Z-pentenylcarbonyloxy, 2Z-hexenylcarbonyloxy, 2Z-heptenylcarbonyloxy, 2Z- Octenylcarbonyloxy, 2Z-nonenylcarbonyloxy, 2Z-decenylcarbonyloxy, 3-butenylcarbonyloxy, 3E-pentenylcarbonyloxy, 3E-hexenylcarbonyloxy Base, 3E-heptenylcarbonyloxy, 3E-octenylcarbonyloxy, 3E-nonenylcarbonyloxy, 3E-decenylcarbonyloxy, 4-pentenylcarbonyloxy, 4Z-hexyl Alkenylcarbonyloxy, 4Z-heptenylcarbonyloxy, 4Z-octenylcarbonyloxy, 4Z-nonenylcarbonyloxy, 4Z-decenylcarbonyloxy, 5-hexenylcarbonyloxy , 6-heptenylcarbonyloxy, 7-octenylcarbonyloxy, 8-nonenylcarbonyloxy, 9-decenylcarbonyloxy, 10-undecenylcarbonyloxy, 11- Dodecenylcarbonyloxy and the like.

The term "branched" in relation to the groups R ^{and R} is understood to mean that any of the aforementioned alkyl, alkenyl, alkoxy, alkenyloxy, carboxyalkyl or carboxyalkenyl groups can be further divided as Alkyl or alkenyl substituted as defined above, wherein one or more hydrogen atoms in the substituted group may be replaced by halogen and wherein one or more non-adjacent _-CH2- groups may be replaced by -CH=CH -, -O-, -CO-, -COO-, -OOC-, -O-OC-O-, or -S-substitution.

The term halogen is understood to include radicals such as fluorine, chlorine, bromine and iodine.

其中R¹、R²和Z¹如前对式(I)化合物所定义；L¹-L⁵独立地表示氢、卤素、氰基、硝基、或支化或直链C_1-20烷基或C_1-20链烯基，其可任选被一个或多个卤原子取代且其中一个或多个非相邻-CH₂-基团可任选被-CH＝CH-、-O-、-CO-、-COO-、-OOC-、-O-OC-O-或-S-替代；和X表示-CH-或氮。In a preferred embodiment of the first aspect of the present invention there is provided a compound of formula (II):

Wherein R ¹ , R ² and Z ¹ are as defined above for the compound of formula (I); L ¹ -L ⁵ independently represent hydrogen, halogen, cyano, nitro, or branched or linear C _1-20 alkyl or C _1-20 alkenyl, which may be optionally substituted by one or more halogen atoms and wherein one or more non-adjacent -CH ₂ - groups may be optionally replaced by -CH=CH-, -O-, -CO-, -COO-, -OOC-, -O-OC-O-, or -S- substitute; and X represents -CH- or nitrogen.

Specific preferences for R ¹ , R ² and Z ¹ are the same as those defined above for the compound of formula (I).

Preferably L ¹ -L ⁵ each represent hydrogen.

Preferably X represents nitrogen.

Compounds of the invention are readily synthesized using known methods such as those shown in Schemes 1 and 2. A second aspect of the present invention provides a process for the preparation of a compound of the present invention comprising reacting an ether of 4-(5-hydroxy-2-pyrimidinyl)benzaldehyde with a 2-alkylpropane-1,3-diol to Steps to form a dioxane ring.

It should be understood that it is not necessary to protect the ether oxygen of the starting material of Scheme 1 with a benzyl group. Another method of preparation requires the reaction of long-chain alkyl derivatives with diols to form dioxane rings, thereby eliminating the need for deprotection of hydroxyl groups by catalytic hydrogenation.

方案1其中：In another embodiment of the second aspect of the present invention, there is provided a process for the preparation of compounds of formula (I) and (II), comprising reacting pyrimidine-2-carboxylic acid with 2-hydroxyl of 1,3-dioxane Derivative reactions. This is exemplified in Scenario 2.

Option 1 where:

THF Tetrahydrofuran;

DEAD diethyl azodicarboxylate

TPP Triphenylphosphine

方案2其中：PTS p-toluenesulfonic acid

Option 2 where:

EDC is N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride

DMAP is 4-dimethylaminopyridine

DCM is dichloromethane

方案3其中：The starting materials used in Scheme 1 can be prepared using the procedure shown in Scheme 3.

Option 3 where:

DMF is dimethylformamide

POCl ₃ is phosphorus oxychloride

Et ₃ N is triethylamine

Py is pyridine

MesCl is methanesulfonyl chloride

DIBAH is diisobutylaluminum hydride; and

Tol is toluene

方案4其中：The branched alkyl spacer used in the last step of Scheme 1 was synthesized by the Fouquet Schlosser coupling shown in Scheme 4 (Angew. Chem. 1974, 86(1), 50-51).

Option 4 where:

BTSS bis(trimethylsilyl) sulfate

DCM dichloromethane

The compounds according to the invention can be used alone or as components of ferroelectric liquid crystal mixtures. A third aspect of the present invention therefore provides a ferroelectric liquid crystal mixture comprising a compound of formula (I), the liquid crystal mixture comprising at least two components. Additional components of the liquid crystal mixture may include one or more compounds of formula (I) or may include other suitable known mesogenic or non-mesogenic compounds. The additional liquid crystal components must be miscible with the compound of formula (I). Examples of meso-complexed substances known to be suitable for use with the compounds of the present invention are shown below. Other examples of additional components that may be used in the mixtures of the invention can be found in US 4,898,455; US 4,929,278; US 5,082,589; US 5,238,602; US 5,100,577 and US 5,242,619.

Ferroelectric liquid crystal mixtures are readily prepared by mixing a compound of formula (I) with one or more additional meso-complexed or non-meso-complexed components. The fourth aspect of the present invention therefore provides a method for preparing a ferroelectric liquid crystal mixture, comprising mixing a compound of formula (I) with other meso-complexed or non-meso-complexed compounds. The one or more additional components may comprise other compounds of formula (I) or other suitable meso- or non-meso-complexed components such as those shown above.

The compounds of the present invention can be used to prepare ferroelectric liquid crystal devices. As indicated above, such devices have been found to have excellent optical and electro-optical properties. Therefore, the fifth aspect of the present invention provides the use of the compound of formula (I) in the preparation of ferroelectric liquid crystal devices. The compounds of formula (I) can be used alone, but are most preferably used in the form of the abovementioned liquid crystal mixtures.

In addition to an optical or electro-optical device comprising the liquid crystal mixture described above, the present invention also includes an optical or electro-optical device comprising a compound of formula (I).

The invention will now be illustrated with reference to the following examples. Variations on these examples that fall within the scope of the invention will be apparent to those skilled in the art.

Example 1 (RS)-5-(8-methylundecyloxy)-2-[4-(trans-5-octyl[1,3]dioxan-2-yl)phenyl] Preparation of pyrimidine

0.44g (1.20mmol) 5-hydroxyl-2-[4-(trans-5-octyl[1,3]dioxan-2-yl]phenyl]pyrimidine and 0.25g (1.32mmol) (RS )-8-methylundecan-1-ol was dissolved in 30ml tetrahydrofuran (THF).Add 0.35g (1.32mmol) triphenylphosphine and 0.23g (1.32mmol) diethyl azodicarboxylate (DEAD) And the solution was stirred overnight at room temperature.Then the reaction mixture was poured into 150ml water and extracted 3 times with 50ml ethyl acetate.The combined organic layer was washed twice with water, dried with magnesium sulfate, filtered and evaporated solvent.The crude product was passed through column chromatography Purification on silica gel using cyclohexane/ethyl acetate 7:3 as eluent. Recrystallization from ethanol gave 0.52 g of (RS)-5-(8-methylundecyloxy)-2-[4 -(trans-5-octyl-1,3-dioxan-2-yl)phenyl]pyrimidine; melting point (CS _C ) 98.8°C, phase transition (S _C -N) 135.3°C, clearing point (NI ) 145.6°C.

5-Hydroxy-2-[4-(trans-5-octyl-[1,3]-dioxan-2-yl)phenyl]pyrimidine used as starting material was prepared by:

(a) 10.35 g (55 mmol) of 2-hydroxymethyldec-1-ol and 14.45 g (55 mmol) of 2-(4-formylphenyl)-5-benzyloxypyrimidine were suspended in 470 ml of toluene. Then 0.5 g of p-toluenesulfonic acid were added and the water of reaction was removed by azeotropic distillation. After cooling slightly, the solution was washed 3 times with 120 ml of warm water. The organic layer was evaporated on a rotary evaporator. Diethyl ether was then added and the solution was allowed to crystallize on cooling. The crystals were filtered and washed with ether to give 22.0 (87%) of 5-(benzyloxy)-2-[4-(trans-5-octyl-1,3-dioxan-2-yl)phenyl]pyrimidine , as white crystals.

(b) Suspend 22.0 g (47.7 g) of 5-(benzyloxy)-2-[4-(trans-5-octyl-1,3-dioxan-2-yl)phenyl]pyrimidine in In a mixture containing 500ml of dioxane and 500ml of ethanol. After adding 2.2 ml of triethylamine, the mixture was hydrogenated at room temperature for 90 minutes using 2.2 g of 10% Pd/C as hydrogenation catalyst. Once hydrogen uptake was complete, the reaction mixture was filtered through dcalcite. Then the solvent was removed. Crystallization of the residue from ethanol afforded 9.7 g (66%) of 5-hydroxy-2-[4-(trans-5-octyl[1,3]dioxan-2-yl)phenyl]pyrimidine as white crystals .

其中…表示相邻原子由单键键合The following compounds were prepared in a similar manner:

where ... denotes that adjacent atoms are bonded by a single bond

The preparation of embodiment 2 ferroelectric liquid crystal mixture

表Ia：化合物Ia表II：     N[C]    S_A[C]     S_C ^*[C]     2θ[°] SC0-1014     75.6    67.7     60.6     52.2 15％(Ia)，在SC0-1014中     81.1     -     66.2     65.9 N，S_A和S_C ^*是对应LC-相的温度上限空白相由“-”表示。2θ为在25°观察的切换角，在施加10Vpp/μm²波场(wave field)的测试室中测量。测试室具有ITO电极，低倾度、平行刷涂的聚酰亚胺取向层和10μm的格栅间隙。Prepare the ferroelectric liquid crystal mixture, SC0-1014, whose composition is shown in Table I. The properties of this mixture were compared with those of a (15/85) mixture of compound (Ia) in SCO-1014. The properties of these two mixtures were compared and the results are shown in Table II. Table I: Composition of test mixture SC0-1014

Table Ia: Compound Ia Table II: N[C] S _A [C] S _C ^* [C] 2θ[°] SC0-1014 75.6 67.7 60.6 52.2 15% (Ia) in SC0-1014 81.1 - 66.2 65.9 N, S _A and S _C ^* are the upper temperature limits of the corresponding LC-phases. Blank phases are denoted by "-". 2Θ is the switching angle observed at 25°, measured in a test chamber with an applied 10 Vpp/μm ² wave field. The test chamber has ITO electrodes, low-slope, parallel brushed polyimide alignment layers and a grid gap of 10 μm.

The data in Table II clearly demonstrate the excellent influence of the dioxane-phenyl-pyrimidine compound (Ia) on the mixture parameters of the test mixtures. Specifically: • the upper limit of the _SC ^* -phase increased by 5.6°C; • the clearing point increased by 5.5°C; and • the switching angle increased by 13.7°.

Claims (13)

1. The compound of formula (I) Wherein R ¹ represents chiral or achiral branched or straight chain C _1-20 alkyl or C _1-20 alkenyl, said group may be optionally substituted by one or more halogen atoms and one or Multiple non-adjacent _-CH2- groups may optionally be replaced by -CH=CH-, -O-, -CO-, -COO-, -OOC-, -O-OC-O- or -S-; R ² represents chiral or achiral branched or straight chain C _1-20 alkoxy, C _1-20 alkenyloxy, C _1-20 alkylcarbonyloxy (alkyl-COO-) or C _1-20 alkenylcarbonyloxy (alkenyl-COO-), which may be optionally substituted by one or more halogen atoms and wherein one or more non-adjacent -CH ₂ - groups may be optionally Optionally replaced by -CH=CH-, -O-, -CO-, -COO-, -OOC-, -O-OC-O- or -S-; MG ¹ represents a group of formula Ia -(AZ ² ) _n -(BZ ³ ) _m - Ia wherein A and B are each independently 1,4-phenylene or 1,4-cyclohexylene, each of which may be optionally substituted by halogen, cyano or nitro or a group of formula ^R and wherein One or two non-adjacent -CH- or -CH ₂ - groups can optionally be replaced by nitrogen or oxygen respectively; Z ¹ , Z ² and Z ³ independently represent a single bond, -COO- or -OOC-, - _CH2CH2- , _-CH =CH-, -C≡C-, _-OCH2- , or _-CH2O- ; m and n are each independently 0 or 1; and MG ² represents 1,4-phenylene , which may be optionally substituted by halogen, cyano or nitro, or a group of formula R ¹ wherein one or two non-adjacent -CH- groups are replaced by nitrogen. 2. The compound of formula (II)

Wherein R ¹ represents chiral or achiral branched or straight chain C _1-20 alkyl or C _1-20 alkenyl, said group may be optionally substituted by one or more halogen atoms and one or Multiple non-adjacent _-CH2- groups may optionally be replaced by -CH=CH-, -O-, -CO-, -COO-, -OOC-, -O-OC-O- or -S-; R ² represents chiral or achiral branched or straight chain C _1-20 alkoxy, C _1-20 alkenyloxy, C _1-20 alkylcarbonyloxy (alkyl-COO-) or C _1-20 alkenylcarbonyloxy (alkenyl-COO-), which may be optionally substituted by one or more halogen atoms and wherein one or more non-adjacent -CH ₂ - groups may be optionally Optionally replaced by -CH=CH-, -O-, -CO-, -COO-, -OOC-, -O-OC-O- or -S-; Z ¹ represents a single bond, -COO- or -OOC- , -CH ₂ CH ₂ -, -CH=CH-, -C≡C-, -OCH ₂ - or -CH ₂ O-; L ¹ -L ⁵ represents hydrogen, halogen, cyano, nitro or branched or Straight chain C _1-20 alkyl or C _1-20 alkenyl, which may be optionally substituted by one or more halogen atoms and wherein one or more non-adjacent _-CH groups may optionally be replaced by -CH= CH-, -O-, -CO-, -COO-, -OOC-, -O-OC-O-, or -S- substitute; and X represents -CH- or nitrogen. 3. The compound according to claim 1 or 2, wherein R ¹ is branched or linear C _5-16 alkyl. 4. The compound according to any one of claims 1-3, wherein R ¹ is octyl. 5. A compound according to any preceding claim, wherein the group ^R2 is selected from branched or straight chain _C5-16 alkoxy or _C5-16 alkenyloxy. 6. The compound according to claim 5, wherein R2 is selected from 2E-octeneoxy, 2E-nonenyloxy, 2E-decenyloxy, 2E-undecenyloxy, 2E-dodeceneoxy octyloxy, decyloxy, dodecyloxy, 7-methyldecyloxy, 7-methylundecyloxy, 8-methyldodecyloxy, 9-methyldodecyloxy Alkoxy and 9-methyltridecyloxy. 7. A compound according to any preceding claim, wherein the groups ^R1 and ^R2 are each independently chiral. 8. A compound according to any preceding claim, wherein Z is ^a single bond. 9. A compound according to any preceding claim, wherein ^L1 , ^L2 , ^L3 , ^L4 and ^L5 are hydrogen. 10. A process for preparing a compound of formula (I) or (II), comprising reacting an ether of 4-(5-hydroxy-2-pyrimidinyl)benzaldehyde with 2-alkyl-1,3-propanediol. 11. A ferroelectric liquid crystal mixture comprising at least two components, characterized in that one of the components is a compound of formula (I) or (II) as defined in any one of claims 1-9. 12. Use of compounds of formula (I) or (II) according to any one of claims 1 to 9 or liquid-crystal mixtures according to claim 11 for the production of optical or electro-optical devices. 13. An optical or electro-optical device comprising a compound of formula (I) or (II) as defined in any one of claims 1-9 or a liquid crystal mixture as defined in claim 11.