Chiral smectic Liquid Crystal Mixture

The present invention relates to a chiral smectic liquid crystal mixture and a novel active matrix (AM) liquid crystal device or display containing same. More particularly, it relates to a ferroelectric liquid crystal mixture, which enables the generation of grey levels and full colour representation useful for computer monitors, TV, DVD, Video and other displays. Jn particular these displays are useful for high speed applications and for yielding a strong colour saturation by improved backlight techniques which rely upon a high speed liquid crystal. A further aspect of this invention are active matrix displays containing such a mixture, particularly in a monostable geometry.

Monostable chiral smectic liquid crystal geometry has been proposed to be combined with active matrix technology to simultaneously allow the utilization of a very high 'pixel speed' and show a gray scale, contrary to bistable SSFLC displays.

Takatoh et. al. (6th International Conference on Ferroelectric Liquid Crystals, 1997, 20-24 July, Brest, France; M. Takatoh et al. 1998, SID Digest, 1171-1174. ) have demonstrated an AM display based upon chiral smectics using a very high Ps material driven with an active matrix with polycrystalline Silicon -TFT. Nito etal. (Nito et al., 1993, Journal of the SJD, 1/2, 163-169.) have suggested a monostable AM-FLC-device with much lower Ps, however, with the disadvantage of a stripey FLC texture which is not suitable for high contrast displays without further improvements. Furue et. al. (Furue, H. et al., 1998, JDW '98, 209 - 212) suggested a polymer stabilized SSFLCD with a FELIX® mixture with a material having a moderate Ps value.

Asao et. al. have presented a monostable FLC mode (Y. Asao et al., JLCC 2000, Sendai, and Jpn. J. Appl. Phys. 38, L534-L536, 1999 therein called "half-V-shape FLC" mode; see also T. Nonaka etal., Liquid Crystals 26(11), 1599-1602, 1999, therein called "CDR" mode). Such displays provide, by virtue of their smaller Ps values, solutions for the gray scale problem and the resolution limitation caused by too large Ps values in active matrix panels. However, a smaller Ps value is not sufficient to switch the FLC at lower temperature.

For the practical display application, a wide operating temperature range (large ΔT = HT (high temperature driving limit) - LT (low temperature driving limit)) is required in view of customer demands. Hence the liquid crystal display has to meet both low temperature and high temperature operating demands simultaneously. In general, viscosities of liquid crystals increase drastically with lowering the temperature, causing slower switching response or even non-switching. Decreasing the number of grey scales or a lower frame rate can help to extend the low temperature limit of driveability, but deterioration in image quality is inevitable. A larger Ps value also improves the switching property at low temperature, but is not preferable due to the reason described above. Lowering the viscosity of the chiral smectic mixture by using more 2-ring materials is feasible in order to improve the low temperature switching, but this is at the sacrifice of the high temperature property.

The limit of high temperature driving is defined by the Sc* / Ch phase transition temperature. The driving temperature limit is not identical with the phase transition temperature, but it cannot be above this phase transition temperature. Three-ring (bridged, not-bridged) materials are preferably added to the mixture to raise the high temperature limit of driveability. The chiral smectic liquid crystal mixtures proposed to date for TFT-FLCD applications as disclosed in, e.g. WO99/60441, US 6,551,668, US 6,605,323, WO00/36054, WO00/55684, WO00/69987, US 6,482,479 or EP-A-I 208 182 , are not optimized with respect to the operating temperature range, including both high temperature and low temperature simultaneously.

US 6,551,668 teaches that heterocyclic components (e.g. with a pyrimidine, thiazole or thiadiazole moiety) are inferior to fluoro aromatics as constituents of chiral smectic mixtures for TFT-FLCD applications, whereas US 6,605,323 teaches that fluorinated nitrogen heterocycles (e.g. 2-fluoro-pyridine, 3-fluoro-pyridine or 4-fluoro-pyrimidine) are superior to phenyl pyrimidines and 2,3-difluorophenyl pyrimidines. According to the present invention it was found that a mixture representing a combination of heterocyclic moieties and fluorinated aromatics is superior for fast response driving over an extended working temperature range.

The objective of the present invention was therefore to provide a chiral smectic liquid crystal mixture and an active matrix liquid crystal display containing same with good driveability at low and high temperatures and large ΔT.

The ELC material according to invention is a chiral smectic liquid crystal mixture with a spontaneous polarisation < 20 nC/cm2 and good driveability at low and high temperatures and large ΔT, comprising at least 1 compound from each of the groups A (consisting of Aa and Ab) and B of achiral materials and C of chiral materials R'-A^-R2 R^A^-B-R2 Aa Ab

B

R5-(-A5-M\(-A6-M^i(-AW)e(-A8-MVR6 C with the meaning a, b 0, 1 or 2 , the sum of a and b 2 or 3 c, d, e, f 0 or 1, the sum of c and d and e and f 2 or 3 or 4 R1, R2, R3, R4 independently hydrogen, alkyl or alkyloxy with 1 to 16 C atoms, alkenyl or alkenyloxy with 2 to 16 C atoms, wherein in either case al) one or two non-terminal non-adjacent -CH2- groups can be replaced by -O- , -OC(=O)- , -(C=O), -C(=O)O-, -Si(CHs)2-, -C=C- and wherein also one or several -CH2- groups can be replaced by -CF2- a2) one -CH2- group can be replaced by phenylene-l,4-diyl (optionally substituted once or twice by F ), cyclohexane-l,4-diyl Coptionally substituted by F or CN) oder cylopropane-l,2-diyl with the proviso that within the couples R1 / R2 and R3 / R4, resp., only one can be hydrogen R5 hydrogen, alkyl or alkyloxy with 1 to 16 C atoms, alkenyl or alkenyloxy with 2 to 16 C atoms, wherein one or two non-adjacent -CH2- groups can be replaced by - OC(=O)- or -C(=O)O- or -Si(CH3)2- oder cylopropan-l,2-diyl and wherein also one or several -CH2- groups can be replaced by -CF2-; or R6 R6 a moiety with at least one asymmetric C atom, which is either part of an alkyl group of 3-12 C atoms, wherein also one or two -CH2- groups can be replaced by -O- or -OC(=O) or -C(=O)O- and wherein at least one of the substituents at the asymmetric C atom is selected from -CH3 , -CF3 , -OCH3, -CH3, Cl , F or part of a 3-7 membered carbocycle (wherein also one or two non-adjacent -CH2- groups can replaced by -O- or a -CH2- group can be replaced by -OC(=O)- or -C(=O)O-) B phenylene- 1 ,4-diyl or cyclohexan- 1 ,4-diyl E1 2,3-difluoro-phenylene-l,4-diyl or 2-fluoro-pyridine-3,6-diyl or (l,3,4)thiadiazole-2,5- diyl A1, A2 independently phenylene- 1 ,4-diyl, 2-fluoro-phenylene- 1,4-diyl, 2,3-difluoro- phenylene-1,4- diyl, pyrimidine-2,5-diyl, pyridine~2,5-diyl with the provisos in compound Aa one of A1 / A2 is pyrimidine-2,5-diyl or pyridine-2,5-diyl in compound Ab one of A /A is pyrimidine-2,5-diyl or pyridine-2,5-diyl, the other is phenylene- 1,4-diyl or cyclohexane-l,4-diyl A3, A4 independently phenylene- 1,4-diyl, optionally substituted by one or two F, cyclohexane- 1,4-diyl, optionally substituted by F, CH3 or CN , pyridine-2,5-diyl, pyrimidine- 2,5-diyl, l,3-dioxane-2,5-diyl, l,3-dioxaborinane-2,5-diyl

A independently phenylene- 1,4-diyl, optionally substituted by one or two F, cyclohexane- 1,4-diyl, optionally substituted by F, CH3 or CN, cyclohex-1 -en- 1,4-diyl, optionally substituted by F, CH3 or CN, cyclohex-2-en- 1,4-diyl, optionally substituted by F, CH3 or CN, 1 -alkyl- 1-sila-cyclohexane- 1,4-diyl, bicyclo[2.2.2]octan- 1,4-diyl, indane-2,6- diyl, optionally substituted by one or several F, naphthalene-2,6-diyl, optionally substituted by one or several F, l,2,3s-4-tetrahydronaphthalene-2,6-diyl, optionally substituted by one or several F, phenanthrene-2,7-diyl, optionally substitued by one or several F, 9,10- dihydrophenanthrene-2,5-diyl, optionally substituted by one or several F, fluorene-2,7-diyl, optionally substituted by one or several F, dibenzofuran, optionally substituted by one or several F, pyridine-2,5-diyl, 2-fluoro-pyridine-3,6-diyl, 3-fluoro-pyridine-2,5-diyl, pyrimidine-2,5-diyl, 4-fluoro-pyrimidine-2,5-diyl, thiazole-2,5-diyl, optionally substituted by F, thiazole-2,4-diyl, optionally substituted by F, thiadiazole-2,5-diyl, isoxazole-3,5-diyl, dioxane-2,5-diyl, 1 ,3-dioxaborinane-2,5-diyl

M3, M4 , M5, M6, M7, M8 , independently a single bond, -OC(=O)-, -C(=O)O-, -OC(=O)O-, -CH2O-, -OCH2-, -CF2O-, -OCF2-, -CH2CH2-, -CH=CH-, -CH2-O-CH2CH2-, -CH2CH2CH2CH2- or -C=C-.

When a or b are 2, then each of the two A3-M3 and M4- A4 can be chosen independently, thus independent of the meaning of the other group A3-M3 and M4- A4.

Preferably the mixture comprises at least 3 compounds of group B. Preferably the compounds of group B constitute at least 10 % by weight, specifically 15 to 75 % by weight of the total mixture.

More preferably these at least 3 compounds of group B are selected from the sub groups B2, B3 and B4:

R^-M^E^-A4),, -R4 B2 wherein E2 is 2,3-difluoro-phenylene-l,4~diyl

R3(-A3-M3)a-E3-(M4- A4)b -R4 B3 wherein E3 is 2-fluoro-pyridine-3,6-diyl

R3(-A3-M3)a-E4-(M4-A4)b-R4 B4 wherein E4 is (l,3,4)thiadiazole-2,5-diyl Even more preferably the mixture comprises at least 3, compounds out of two different groups B2, B3 and B4. Specifically it contains at least 5 compounds out of B2, B3 and B4.

Preferably B3 is a compound of the formula B3a and B4 is a compound of the formula B4a

Thus, the present invention provides a chiral smectic liquid crystal mixture that can be driven with low voltage at both low and high temperatures. The mixture according to the invention exhibits a wide operating temperature range. The liquid crystal mixture according to the invention can be used in active matrix liquid crystal displays.

A further object of the invention is a chiral smectic liquid crystal display device, operated in an active matrix panel comprising the above described mixtures, that can be driven at low and high temperatures. The active matrix liquid crystal device comprises the chiral smectic liquid crystal mixture.

A further object of the invention is a chiral smectic liquid crystal display device operated in an active matrix panel using the above described mixtures. With preference this display is a monostable chiral smectic, such as selected from the group consisting of half-V-shape, V- shape, CDR or short pitch FLC displays. Each of these modes can also be combined with "polymer-stabilization" (cp. e.g. Jpn. J. Appl. Physics 38, L534 (1999)).

A further object of the invention is the use of the above described chiral smectic mixtures operated in an active matrix panel, especially if the liquid crystal is in a monostable chiral smectic mode.

The liquid crystal mixtures according to the invention are prepared in a manner which is customary per se. As a rule the components are dissolved in one another, advantageously at elevated temperatures.

Optional additional constituents of the mixtures according to invention are compounds D R1^-A12 -M12-A13-*2 D with the meaning R1, R2 independently as above A11, A12 independently phenylene-l,4-diyl, 2-fluoro-phenylene-l,4-diyl, 2,3-difluoro- phenylene-l,4-diyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl, 3-fluoro-pvridine-2,5-diyl A13 cyclohexane-l,4-diyl, l-cyclohexen-l,-4-diyl, 2-cyclohexen-l,4-diyl, thiophene-2,5- diyl, optionally substituted by F, or thiophene-2,4-diyl, optionally substituted by F M12 -OC(=O)- , -OCH2-, -C(=O)O-, -CH2O- or -OC(=O)O- with the proviso that at least one of A11 , A12 is pyrimidine-2,5-diyl or pyridine-2,5-diyl or 3- fluoro-pyridine-2,5-diyl, of which at least one can be obtained in the liquid crystal mixtures.

Preferably the compounds D constitute 8 to 35% by weight of the total mixture.

Optional additional constituents of the mixtures according to invention are compounds E

R^A^M^-A^-M^-A^M^A17-]*2 E with the meaning R , R independently as above A14, A15, A16, A17 independently phenylene-l,4-diyl, optionally substituted by one or two F, cyclohexane-l,4-diyl, optionally substituted by F, CH3 or CN, cyclohex-l-en-l,4-diyl, optionally substituted by F, CH3 or CN, cyclohex-2-en-l,4-diyl, optionally substituted by F, CH3 or CN, l-alkyl-l-sila-cyclohexane-l,4-diyl, bicyclo[2.2.2]octan-l,4-diyl, indane-2,6- diyl, optionally substituted by one or several F, naphthalene-2,6-diyl, optionally substituted by one or several F, l,2,3,-4-tetrahydronaphthalene-2,6-diyl, optionally substituted by one or several F, phenanthrene-2,7-diyl, optionally substitued by one or several F, 9,10- dmydrophenanthrene-2,5-diyl, optionally substituted by one or several F, fluorene-2,7-diyl, optionally substituted by one or several F, dϊbenzofuran, optionally substituted by one or several F, pyridine-2,5-diyl, 2-fluoro-pyridine-3,6-diyl, 3-fluoro-pyridine-2,5-diyl, pyrimidine-2,5-diyl, 4-fluoro-pyrimidine-2,5-diyl, thiazole-2,-5-diyl, optionally substituted by F, thiazole-2,4-diyl, optionally substituted by F, thiadizole-2,5-diyl, isoxazole-3,5-diyl, dioxane-2,5-diyl, 1 ,3-dioxaborrnane-2,5-diyl M14 , M15, M16 independently a single bond, -OC(=O)-, -C(=O)O~, -OC(=O)O-, -CH2O-, -OCH2-^CF2O-, -OCF2-, -CH2CH2-, -CH=CH-, -CH2-O-CH2CH2 -, -CH2CH2CH2CH2- or -C=C- of which at least one can be contained in the liquid crystal mixtures.

These optional additional constituents of the mixture D and E, resp., can be added to fϊne- tune the mixture with respect to parameters relevant for application, such as smectic C* / Cholesteric (Sc*/Ch) transition temperature, Cholesteric / Isotropic (Ch/T) transition temperature, Crystalline / smectic C* (C/Sc*) transition temperature, dielectrical anisotropy Δε, optical anisotropy Δn.

Preferably the compounds of group E constitute 1 to 10% by weight in the total mixture.

Optional additional constituents of the mixtures according to invention are materials that increase the light stability, specifically UV stabilizers, e.g. of the "benzophenone" type, "benzotriazole" type or HALS (hindered amine light stabilizer) type. Preferably the mixtures may contain 0.001 wt.-% to 5 wt.-% of one or several UV stabilizers; especially preferred are mixtures containing 0.01 wt.-% to 1 wt.-% of one or several UV stabilizers. With this respect reference is made to US 2003 0127627, which is hereby incorporated by reference.

Optional additional constituents of the mixtures according to invention are materials that increase the stability against oxidative degradation (antioxidants, e.g. of the "sterically hindered phenol" type). Preferably the mixtures may contain 0.001 wt.-% to 5 wt.-% of one or several antioxidants; especially preferred are mixtures containing 0.1 wt.-% to 5 wt.-% of one or several antioxidants. With this respect reference is made to US 2003 0127627, which is hereby incorporated by reference.

Optionally the mixtures according to invention may contain a combination of UV stabilizers and antioxidants. Chiral smectic liquid crystal mixtures according to the invention are particularly suitable for use in electro-optical switching and display devices (displays). These displays are usually constructed in such a way that a liquid crystal layer is enclosed on both sides by layers which are usually, in this sequence starting from the LC layer, at least one alignment layer, electrodes and a limiting sheet (for example of glass). In addition, they can contain spacers, adhesive frames, polarizers and, for color displays, thin color-filter layers, or they are operated in the sequential backlight technique. Other possible components are antireflection, passivation, compensation and barrier layers and, for active-matrix displays, electric non¬ linear elements, such as thin-film transistors (TFTs) and metal-insulator-metal (MIM) elements. The structure of liquid crystal displays has already been described in detail in relevant monographs see, for example, T. Tsukuda, "TFT/LCD Liquid crystal displays addressed by thin film transistors", Japanese Technology Reviews, 1996 Gordon and Breach, ISBN 2-919875-01-91. An active matrix liquid crystal display within the scope of this invention can also be an optical element that acts as a light valve or a device used to change the polarization state of light by means of a field-induced re-orientation of the smectic liquid crystal molecules, in particular an FLCOS (ferroelectric liquid crystal on silicon), an optical shutter and the like.

Ih a preferred embodiment the FLC display is operated in the monostable mode with an active matrix panel.

Several documents are cited in this application, e.g. to discuss the state of the art, synthesis of compounds used in the present invention or application of the mixtures according to the invention. All these documents are hereby incorporated by reference. Examples Mixtures are prepared from the following compounds a1 a2

b2 b3 chiral

c1 b4

x3 Experiment A test cell was prepared in the following way. A solution of LQT 120 (Hitachi Kasei) is applied onto glass substrates with ITO by spin coating at 2500 rpm. The electrode area was 0.9 cm2. The substrates are heated at 200°C for 1 hour to form a film. After rubbing the coated film with a nylon cloth in one direction, the substrates are assembled into a cell with spacers having a thickness of 1.5 μm inserted between the substrates in such a manner that the rubbing directions are anti-parallel to each other. All tested mixtures are filled into the cell in the isotropic phase. An uniform monostable alignment follows cooling progressively through the cholesteric and the smectic C* with a small bias dc voltage of 3 V during the cholesteric (Ch) - smectic C* (Sc*) phase transition.

The cell is placed in the Mettler hot stage on a polarizing microscope, equipped with a photodiode to measure the optical transmission between crossed polarisers. The cell is aligned such, that with no voltage applied it appears dark due to the optical axis being arranged parallel to one of the polarizers. Phase transition temperatures (Sc* to Ch and Ch to Isotropic (Iso)) are determined by the microscopic observation with +l°C/min. The Sawyer- Tower method was used to determine the spontaneous polarization value. The test cell has 5 μm thickness and is coated with ITO.

VT-characteristic and temperature dependence A voltage and transmittance (T) curve (VT-characteristics) is obtained by the following manner. 0% transmittance and 100% transmittance are defined as crossed polarizers and parallel polarizers, respectively. A rectangular wave form (180Hz) is applied to measure the VT-characteristics with changing of the applied voltage from OV to 5V. An applied wave form, electrical field (E), and optical response (O) are illustrated in Fig.l. The % transmission (T(%)) is illustrated. The high temperature driving limit is about 5 °C below the phase transition temperature Sc* to Ch due to the beginning of alignment deterioration. The low temperature driving limit for the test material is defined as the temperature where the transmittance value is 50% of the transmittance value that is measured at 300C under 180Hz, 5 V rectangular waveform.

Reference 1 A chiral smectic liquid crystal composition LC-I was prepared by mixing the following compounds in the indicated proportions. LC-I

Physical properties and the high and low temperature driving limits and ΔT are summarized in Table-IA. Table- IB show the material groups for each mixture. LC-I exhibits a Sc* - Ch phase transition temperature of 65 °C, therefore the high temperature driving limit is 60 0C. The low temperature driving limit was measured as 1 0C.

Fig. 2 shows the temperature (Te) dependence of the transmittance (Tr) with rectangular wave form, 180 Hz, 5 V.

Example 1 A chiral smectic liquid crystal composition LC-2 was prepared by mixing the following compounds in the indicated proportions. Physical properties and the high and low temperature driving limits and ΔT are summarized in Table-IA. Table- IB show the material groups for each mixture.

LC-2

Example 2

LC-3 was prepared by mixing the following compounds including materials out of group B3

in the indicated proportions. Physical properties and the high and low temperature driving

limits and ΔT are summarized in Table- IA.

Table-IB show the material groups for each mixture.

LC-3 achieves both a higher phase transition temperature and a lower driving temperature

limit. LC-3

Example 3 . LC-4 was prepared by mixing the following compounds including materials out of group B2 and B3 in Table-2. Physical properties and the high and low temperature driving limits and ΔT are summarized in Table-1 A. Table- IB show the material groups for each mixture.

Example 4 LC-5 was also prepared by mixing the following compounds including materials out of group B2 and B3 in Table-2. Physical properties and the high and low temperature driving limits and ΔT are summarized in Table-1A. Table- IB show the material groups for each mixture. Example 5 LC-6 was also prepared by mixing the following compounds including materials out of group B2 and B3 in Table-2. Physical properties and the high and low temperature driving limits and ΔT are summarized in Table-IA. Table-IB show the material groups for each mixture.

Example 6 LC-7 was prepared by mixing the following compounds including materials out of group B2, B3 and B3 in Table-2. Physical properties and the high and low temperature driving limits and ΔT are summarized in Table- IA. Table- IB show the material groups for each mixture.

Example 7 LC-8 was also prepared by mixing the following compounds including materials out of group B2, B3 and B3 in Table-2. Physical properties and the high and low temperature driving limits and ΔT are summarized in Table- IA. Table- IB show the material groups for each mixture.

Example 8 LC-9 was also prepared by mixing the following compounds including materials out of group B2, B3 and B3 in Table-2. Physical properties and the high and low temperature driving limits and ΔT are summarized in Table-IA. Table- IB show the material groups for each mixture. Table-IA:

HT = High temperature driving limit

LT = Low temperature driving limit

ΔT = HT - LT

Table-1 B:

All examples show, that the mixtures according to the invention exhibit wider operation

temperature ranges and higher resp. lower temperature driving limits. Table-2: