LIQUID CRYSTAL MEDIUM AND LIQUID CRYSTAL DEVICE

Abstract

The invention relates to a medium comprising at least 60% of one or more compounds of formula I

##STR00001##

wherein R.sup.11, R.sup.12, MG.sup.11, MG.sup.12 and Sp.sup.1 have the meaning given herein below, to the use of such media in liquid crystal devices, in particular in flexoelectric liquid crystal devices, and to a flexoelectric liquid crystal device comprising a liquid crystal medium according to the present invention.

Claims

1. Medium comprising at least 60% of one or more compounds of formula I ##STR00132## wherein R.sup.11 and R.sup.12 are each independently H, F, Cl, CN, NCS or a straight-chain or branched alkyl group with 1 to 25 C atoms, which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH.sub.2 groups to be replaced, in each occurrence independently from one another, by O, S, NH, N(CH.sub.3), CO, COO, OCO, OCOO, SCO, COS, CHCH, CHCF, CFCF or CC in such a manner that oxygen atoms are not linked directly to one another, at least one of R.sup.11 and R.sup.12 is an alkyl group, i.e. a straight-chain or branched alkyl group with 1 to 25 C atoms, which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH.sub.2 groups to be replaced, in each occurrence independently from one another, by O, S, NH, N(CH.sub.3), CO, COO, OCO, OCOO, SCO, COS, CHCH, CHCF, CFCF or CC in such a manner that oxygen atoms are not linked directly to one another, preferably a polar group, in which one CH.sub.2 groups is replaced by CHCH, CHCF, CFCF, but from which OCF.sub.3 and CF.sub.3, are excluded, MG.sup.11 and MG.sup.12 are each independently a mesogenic group, wherein at least one of MG.sup.11 and MG.sup.12 comprises one, two or more 5-atomic and/or 6-atomic rings, in case of comprising two or more 5- and/or 6-atomic rings at least two of these may be linked by a 2-atomic linking group, Sp.sup.1 is a spacer group comprising 1, 3 or 5 to 40 C atoms, wherein one or more non-adjacent and non-terminal CH.sub.2 groups may also be replaced by O, S, NH, N(CH.sub.3), CO, OCO, SCO, OCOO, COS, COO, CH(halogen)-, CH(CN), CHCH or CC, however in such a way that no two O-atoms are adjacent to one another, no two CHCH groups are adjacent to each other and no two groups selected from OCO, SCO, OCOO, COS, COO and CHCH are adjacent to each other, and X.sup.11 and X.sup.12 are independently from one another a linking group selected from COO, OCO, O, CHCH, CC, CF.sub.2O, OCF.sub.2, CF.sub.2CF.sub.2, CH.sub.2O, OCH.sub.2, COS, SCO, CSS, S, and a single bond, under the condition that in X.sup.11-Sp.sup.1-X.sup.12 no two O atoms are adjacent to one another, no two CHCH groups are adjacent to each other and no two groups selected from OCO, SCO, OCOO, COS, COO and CHCH are adjacent to each other.

2. Medium according to claim 1, comprising one or more compounds of formula I wherein at least one of MG.sup.11 and MG.sup.12 comprises one or two 5-atomic rings, and one or more 6-atomic rings, and at least two of these are optionally linked by a 2-atomic group.

3. Medium according to claim 1, comprising one or more compounds of formula I wherein both MG.sup.11 and MG.sup.12 comprise one or two 6-atomic rings.

4. Medium according to claim 1, comprising one or more compounds of formula I wherein R.sup.12 is selected from F, Cl and CN.

5. Medium according to claim 1, comprising one or more compounds of formula I wherein Sp.sup.1 is (CH.sub.2).sub.o and o is 1, 3 or an integer from 5 to 15.

6. Medium according to claim 1, comprising one or more compounds of formula I wherein X.sup.11 and X.sup.12 are independently from one another a linking group selected from COO and a single bond.

7. Medium according to claim 1, characterised in that it optionally comprises one or more compounds selected from the group of the compounds of the formulae III
R.sup.31-MG.sup.31-X.sup.31-Sp.sup.3-X.sup.32-MG.sup.32-R.sup.32III wherein R.sup.31 and R.sup.32 are each independently H, F, Cl, CN, NCS or a straight-chain or branched alkyl group with 1 to 25 C atoms which may be unsubstituted, mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH.sub.2 groups to be replaced, in each case independently from one another, by O, S, NH, N(CH.sub.3), CO, COO, OCO, OCOO, SCO, COS, CHCH, CHCF, CFCF or CC in such a manner that oxygen atoms are not linked directly to one another, MG.sup.31 and MG.sup.32 are each independently a mesogenic group, Sp.sup.3 is a spacer group comprising 5 to 40 C atoms, wherein one or more non-adjacent CH.sub.2 groups may also be replaced by O, S, NH, N(CH.sub.3), CO, OCO, SCO, OCOO, COS, COO, CH(halogen)-, CH(CN), CHCH or CC, and X.sup.31 and X.sup.32 are each independently O, S, CO, COO, OCO, OCOO, CONH, NHCO, CH.sub.2CH.sub.2, OCH.sub.2, CH.sub.2O, SCH.sub.2, CH.sub.2S, CHCH, CHCHCOO, OCOCHCH, CC or a single bond, with the condition that compounds of formula I are excluded.

8. Medium according to claim 1, consisting of two or more compounds of formula I.

9. A method which comprises including a medium according to claim 1, in a liquid crystal device.

10. Liquid crystal device comprising a liquid crystalline medium comprising a medium according to claim 1.

11. Liquid crystal device according to claim 10, characterized in that it is a flexoelectric device.

Description

EXAMPLES

[0243] Typically a 5.6 m thick cell, having an anti-parallel rubbed PI alignment layer, is filled on a hotplate at a temperature at which the flexoelectric mixture in the isotropic phase.

[0244] After the cell has been filled, the phase transitions, including clearing point, are determined using Differential Scanning Calorimetry (DSC) and verified by optical inspection. For optical phase transition measurements, a Mettler FP90 hot-stage controller connected to a FP82 hot-stage is used to control the temperature of the cell. The temperature is increased from ambient temperature at a rate of 5 degrees C. per minute, until the onset of the isotropic phase is observed. The texture change is observed through crossed polarizers using an Olympus BX51 microscope and the respective temperature noted.

[0245] Wires are then attached to the ITO electrodes of the cell using indium metal. The cell is secured in a Linkam THMS600 hot-stage connected to a Linkam TMS93 hot-stage controller. The hot-stage is secured to a rotation stage in an Olympus BX51 microscope.

[0246] The cell is heated until the liquid crystal is completely isotropic. The cell is then cooled under an applied electric field until the sample is completely nematic. The driving waveform is supplied by a Tektronix AFG3021B arbitrary function generator, which is sent through a Newtons4th LPA400 power amplifier before being applied to the cell. The cell response is monitored with a Thorlabs PDA55 photodiode. Both input waveforms and optical response are measured using a Tektronix TDS 2024B digital oscilloscope.

[0247] In order to measure the flexoelectric response of the material, the change in the size of the tilt of the optic axis is measured as a function of increasing voltage. This is achieved by using the equation:

[00001]$\tan .Math..Math.=\frac{P_0}{2.Math..Math.}.Math.\frac{e}{K}.Math.\underset{\_}{E}$

[0248] wherein is the tilt in the optic axis from the original position (i.e. when E=0), E is the applied field, K is the elastic constant (average of K.sub.1 and K.sub.3) and e is the flexoelectric coefficient (where e=e.sub.1+e.sub.3). The applied field is monitored using a HP 34401A multimeter. The tilt angle is measured using the aforementioned microscope and oscilloscope. The undisturbed cholesteric pitch, P.sub.0, is measured using an Ocean Optics USB4000 spectrometer attached to a computer. The selective reflection band is obtained and the pitch determined from the spectral data.

[0249] The mixtures shown in the following examples are well suitable for use in ULH-displays. To that end an appropriate concentration of the chiral dopant or dopants used has to be applied in order to achieve a typical cholesteric pitch of 350 to 275 nm.

[0250] Comparative Mixture Example C-0

[0251] The mixture C-0 is prepared and investigated in particular studying its properties for being aligned.

TABLE-US-00005 Composition Compound No. Abbreviation Conc./% 1 F-PGI-O-9-O-GP-F 25.0 2 F-PGI-O-9-O-PP-N 25.0 3 F-PGI-ZI-9-Z-GP-F 25.0 4 F-PGI-ZI-9-Z-PP-N 25.0 100.0

[0252] The alignment of the mixtures, like mixture C-0, is determined in a test cell with anti-parallel rubbed PI orientation layers, for planar alignment, having a cell gap of 10 m at a wavelength of 550 nm. The optical retardation of the samples is determined using an elipsometer instrument for various angles of incidence ranging from 60 to +40.

[0253] The sample of C-0 shows an optical retardation of 25 nm under perpendicular observation (i.e. at an angle of incidence of 0). This already indicates the presence of a homogeneous alignment. For various angles of incidence, the values of the retardation range from 2 nm to 55 nm. Though they scatter quite significantly as a function of the angle of incidence, there seems to be a trend of the retardation increasing with increasing angle of incidence. However, the significant scatter of the retardation values indicate a rather poor quality of the homeotropic alignment.

MIXTURE EXAMPLES

[0254] The following mixtures are prepared and investigated in particular studying their properties for being aligned.

Mixture Example M-1

[0255]

TABLE-US-00006 Composition Compound No. Abbreviation Conc./% T.sub.NI ( C.) = 72 1 5-P-ZI-5-Z-GP-N 45 2 3-PY-ZI-7-Z-PP-N 35 3 3-CP-5-Z-PP-N 20 100

[0256] The mixture shows very good homeotropic alignment below 51 C. This is indicated by the retardation at normal incidence being close to zero and further the retardation increasing almost symmetrically for positive and negative angles of incidence with increasing absolute value of the angle of incidence.

Mixture Example M-2

[0257]

TABLE-US-00007 Composition Compound No. Abbreviation Conc./% T.sub.NI ( C.) = 65 1 5-P-ZI-5-Z-GP-N 45 2 3-PY-ZI-7-Z-PP-N 35 3 5-P-ZI-5-Z-PP-N 20 100

[0258] The mixture shows very good homeotropic alignment below 54 C. This is indicated by the retardation at normal incidence being close to zero and further the retardation increasing almost symmetrically for positive and negative angles of incidence with increasing absolute value of the angle of incidence.

Mixture Example M-3

[0259]

TABLE-US-00008 Composition Compound No. Abbreviation Conc./% T.sub.NI ( C.) = 58 1 5-P-ZI-5-Z-GP-N 45 2 5-P-ZI-5-Z-PP-N 27.5 3 3-CP-5-Z-PP-N 27.5 100

[0260] The mixture shows very good homeotropic alignment below 52 C. This is indicated by the retardation at normal incidence being close to zero and further the retardation increasing almost symmetrically for positive and negative angles of incidence with increasing absolute value of the angle of incidence.

Mixture Example M-4

[0261]

TABLE-US-00009 Composition Compound No. Abbreviation Conc./% T.sub.NI ( C.) = 91 1 3-PY-ZI-7-Z-PP-N 40 2 5-P-ZI-5-Z-PP-N 30 3 3-CP-5-Z-PP-N 30 100

[0262] The mixture shows very good homeotropic alignment below 67 C. This is indicated by the retardation at normal incidence being close to zero and further the retardation increasing almost symmetrically for positive and negative angles of incidence with increasing absolute value of the angle of incidence.