FERROELECTRIC PYRIDINE COMPOUNDS AND LIQUID CRYSTAL COMPOSITIONS CONTAINING THEM

Abstract: Ferroelectric pyridine compounds suitable for use as liquid crystals for display modes utilizing ferroelectric properties have the formula: in which n is 0 or an integer of from 1 to 8; R₁ is a C₂-C₁₈ alkyl group; and R₂ is a C₁-C₁₈ alkyl group.

Description: This invention relates to novel liquid crystal substances and to liquid crystal compositions containing them. At present these are most widely employed, as liquid crystal display elements. TN (Twisted Nematic) type display modes but these are inferior, in respect of response speed, to emissive type display elements such as electroluminescent or plasma display elements. Various attempts have been made to achieve improvements in this respect but nevertheless there does not seem much possibility of a large improvement. Accordingly, various liquid crystal display devices based upon other principles have been proposed. One such device has a display mode utilizing ferroelectric liquid crystals (N.A. Clark et al, Applied Phys. lett., 36,899 (1980). This mode utilizes the chiral smectic C phase (hereinafter referred as the "SC* phase") or chiral smectic H phase (hereinafter referred to as the "SH* phase"). and the temperature range exhibiting these phases is preferably in the vicinity of room temperature. We have now found, in accordance with the present invention, that certain ferroelectric pyridine compounds, as hereinafter defined, are liquid crystal substances suitable for use for this display mode. According to the invention there are provided ferroelectric pyridine compounds of the formula: in which n is 0 or an integer from 1 to 8; R₁ is a C₂-C₁₈ alkyl group; R₂ is a C₁-C₁₈ alkyl group; and, conventionally, the symbol * indicates that carbon atom so labelled is an optically active carbon atom. The invention also provides a liquid crystal composition containing a compound of formula (I) as a component. The phase transition points of representative examples of the compounds of formula (I) are shown in Table 1. In the column in Table 1 showing the phase transition points, C represents the crystalline phase, SH*, the chiral smectic H phase; SG*, the chiral smectic G phase; SF*, the chiral smectic F phase; SA, the smectic A phase; Ch, the cholesteric phase; and I, the transparent phase. The symbol . indicates that the respective phase is exhibited, while the symbol - indicates that the respective phase is not exhibited. Further, the numeral to the right of a symbol . indicates the phase transition point from the phase corresponding to the symbol . to the phase corresponding to a symbol . closest to the numeral. Most of the compounds of formula (I) exhibit an SC* phase suitable to the display mode utilizing the ferroelectric properties at a somewhat higher temperature than room temperature. Particularly the temperature ranges of compounds of sample No. 4, etc. are 20°C or higher and, moreover, the melting point of sample No. 4 is as low as 12.5°C and, also, ferroelectric properties are exhibited in the liquid phases starting from its melting point upto the SC* phase. Further, the values of the spontaneous polarization are large. In the case of the compound of sample No. 4, the vaue is as large as about 4 nC/cm² at 38°C (SC* phase) in spite of its asymmetric carbon atom being apart from the central part of the molecule. Compounds wherein the optically active group on the left side of the formula (I) of the present invention is replaced by a linear chain alkyl, i.e. of the formula: (wherein n is an integer of 1 to 7 and m is 4 or 6) are known (A.I. Pavulchenko et al; Advances in Liquid Crystal Research and Applications; Pergamon Press, 1980, p1007). Most of the known compounds exhibit only an SA phase, and only those wherein n = 5 and m = 6 have been assumed to exhibit an SA phase and SC phase. At any rate, no SC* phase exhibiting ferroelectric properties is exhibited. Since the compounds of formula (I) have a superior compatibility with other compounds such as those exhibiting SC* phase, SH* phase, those exhibiting cholesteric phase, etc., it is possible to broaden the temperature range of the SC* phase by blending with the above other compounds. A chiral smectic liquid crystal composition may be formed only from a plurality of compounds of formula (I), but it is possible to blend compositions of formula (I) with other chrial smectic liquid crystals or achiral smectic liquid crystals to thereby give a liquid crystal composition exhibiting an SC* phase. When the light-switching effect of an SC* phase is supplied to display elements, there are three superior specific features as compared with a TN display mode. Firstly the response is very fast, that is the response time is 1/100 or less of that for elements of a TN display mode. Secondly, there is a memory effect so that multiplex drive is easy in co-operation with the high-speed response properties. The third advantage is as follows. In order to obtain a gray scale according to a TN display mode, this is carried out by controlling the impresses voltage, but there are difficult problems due to the temperature-dependency of the threshold voltage, the voltage-dependency of the response speed, etc. However, when the light switching effect of an SC* phase is applied, it is possible to easily obtain a gray scale by controlling the polarity inversion time; hence it is very suitable to graphic display. As to the display method, two modes are considered, that is, one method is of the birefringence type using two sheets of polarizers, and another of the guest-host type using dichroic dyestuffs. Since SC* phase has a spontaneous polarization, the molecule is inverted around the helical axis as a rotating axis by inverting the polarity of an impressed voltage. A liquid crystal composition having an SC* phase is filled into a liquid crystal cell subjected to aligning treatment so that the liquid crystal molecules can be aligned in parallel to the electrode surfaces, and the resulting cell is placed between two sheets of polarizers arranged so that a director for the liquid crystal molecules and a polarizing plate opposed thereto, followed by impressing a voltage to invert the polarity, whereby a bright field of vision and a dark field of vision (determined by the opposite angle of the polarizers) are obtained. In the operation according to the guest-host system, it is possible to obtain a bright field of vision and a coloured field of vision (determined by the arrangement of the polarizing plate) by inverting the polarity of the impressed voltage. In general, it is difficult to align the liquid crystal molecules in the smectic state in parallel to the glass vall surface, thus, the liquid crystal molecules have been aligned by cooling them very slowly (at a rate of 1° to 2°C/hr) starting from the isotropic liquid thereof in a magnetic field of several tens of kilogauss or more. However, in the case of a liquid crystal substance having a cholesteric phase in the range of temperatures above those at which smectic phase is exhibited, when cooling is carried out starting from the temperatures at which a cholesteric phase is exhibited, down to those at which a smectic phase is exhibited, at a cooling rate of 1°C/min. while a direct current voltage of 50 to 100V is impressed in place of the magnetic field, it is possible to easily obtain a uniformly aligned monodomain state. In addition, as to racemic substances corresponding to compounds of formula (I), when an optically active alcohol used as a raw material in the preparation of the optically active substance (I) shown below is replaced by the corresponding racemic alcohol, the racemic substances are similarly prepared and exhibit almost the same phase transition points as those of the compounds of formula (I). Since the compounds of formula (I) also have an optically active carbon atom, they are capable of inducing a twisted structure when they are added to nematic liquid crystals. Nematic liquid crystals having a twisted structure i.e. chiral nematic liquid crystals do not form the so-called reverse domain (dechiralization lines) of TN type display elements; hence it is possible to use the compounds of formula (I) as an agent for preventing the formation of the reverse domain. The compounds of formula (I) may be prepared as shown in the following reaction scheme: Namely, p-methoxyphenyl-β-chlorovinyl ketone (II) as a known substance is reacted with an ethylamine in the presence of in a solvent and further reacted with perchloric acid to obtain a pyrilium salt (III), which is then reacted with ammonium acetate in a solvent, preferably acetic acid, to obtain a compound (IV), which is then reacted with hydrobromic acid in a solvent, preferably acetic acid, to obtain a compound (V), which is then heated together with an optically active alkylbromide (or an optically active alkyl tosylate) and potassium hydroxide in a solvent, preferably ethanol, to obtain an objective compound of the formula (I). The ferroelectric pyridine compound of the present invention will be described in more detail by way of Examples. Example 1 Preparation of optically active 2-[p-(5-methylheptyloxy)phenyl]-5-heptylpyridine (a compound of the formula (I) wherein R₁=C₂H₅, R₂=C₇H₁₅ and n=4) (sample No.4) (The first step) N-heptenylpiperidine (41.8g, 0.20 mol) and triethylamine (20g, 0.20 mol) were dissolved in ethyl ether (100 mℓ) with stirring, followed by dropwise adding to the resulting solution, a solution of the above compound (II) (30g, 0.20 mol) prepared in advance according to a known method, dissolved in ethyl ether (200 mℓ) while keeping the temperature of the system at 35°C or lower, agitating the resulting mixture at room temperature for 8 hours, adding water (100 mℓ) and toluence (50 mℓ), transferring the mixture into a separatory funnel, twice washing the resulting organic layer with water, distilling off the solvent under reduced pressure, adding 70% perchloric acid (100 mℓ) to the residue, adding water (100 mℓ), heating the mixture under reflux for 10 minutes, cooling the resulting material, washing the resulting crystals with ethyl ether and drying the crystals to obtain 5-heptyl-2-(p-methoxyphenylpyrilium) perchlorate (compound (III), R=C₇H₁₅) (48.0g). (The second step) 5-Heptyl-2-( p-methoxyphenylphrilium) perchlorate obtained in the above first step (48.0g, 0.125 mol) was heated under reflux with stirring together with ammonium acetate (96g, 1.25 mol) and acetic acid (500 mℓ) for 4 hours, followed by pouring the reaction fluid into water, dissolving the resulting crystals in toluene, transferring the resulting solution into a separatory funnel, three times washing with water, distilling off the solvent under reduced pressure and recrystallizing the residue to obtain 5-heptyl-2-(p-methoxyphenyl) pyridine (compound (IV), R=C₇H₁₅) (20g) having a melting point of 54.4 - 56.5°C. In addition, compounds of the formula (IV) wherein R₂=C₈H₁₇, C₉H₁₉ or C₁₀H₂₁ had melting points of 60.7 - 62.2°C, 55.0 - 57.4°C or 61.1 - 62.9°C, respectively. (The third step) 5-Heptyl-2-(p-methoxyphenyl) pyridine obtained in the above second step (20g, 0.071 mol), hydrobromic acid (47%, 130 mℓ) and acetic acid (300 mℓ) were heated under reflux for 30 hours, followed by cooling the mixture, pouring it in water, filtering off the resulting crystals, dissolving the crystals in 2N NaOH aqueous solution, further adding acetic acid to make the solution acidic, filtering off the resulting crystals and recrystallizing them to obtain 5-heptyl-2-(p-hydroxyphenyl) pyridine (compound (V), R=C₇H₁₅) (13g) having a melting point of 105.2 - 105.9°C. In addition, compounds of the formula (V) wherein R=C₆H₁₃, C₈H₁₇, C₉H₁₉ or C₁₀H₂₁ had melting points of 117.6 - 118.0°C, 93.2 - 95.0°C, 85.0 - 87.5°C or 90.4 - 91.8°C, respectively. (The fourth step) 5-Heptyl-2-(4-hydroxyphenyl) pyridine (10g, 0.037 mol) obtained in the above third step, ethanol (100 mℓ), pottasium hydroxide (3.0g, 0.045 mol) and optically active 5-methylheptylbromide (9g, 0.045 mol) were heated under reflux with stirring for 4 hours, followed by cooling the mixture, adding water and toluene, transferring the mixture into a separatory funnel, washing the resulting organic layer with 2N-NaOH aqueous solution, washing it with water, distilling off the solvent under reduced pressure and recrystallizing the residue in a freezer to obtain the objective optically active 2-(5-methylheptyloxyphenyl)-5-heptylpyridine (I)(6.5g). The phase transition points of this product were as follows: C-SG* point: 12.5°C, SG*-SF* point: 23.2°C, SF * -SC* point: 37.5°C and SC*-I point: 58.4°C. Further, its elemental analysis values accorded well with its calculated values as follows: Observed values (%) Calculated values (in terms of C₂₆H₃₉NO) C 81.21 81.84 H 10.1 10.30 N 3.40 3.67 By replacing N-heptenylpiperidine used in the first step by other N-alkenylpiperidines and replacing optically active 5-methylheptyl bromide of the fourth step by various kinds of optically active alkylbromides or optically active alkyl tosylates, other compounds of the formula (I) were obtained in the same manner as the above. The phase transition points of these compounds are shown in Table 1 together with those of Example 1. In addition, a recemic substance corresponding to the compound of sample No.6 was prepared and its phase transition points were measured. The points were as follows: C 30.8°C SG 33.8°C SF 51.0°C SC 62.7°C I. Namely, its phase transition points accorded almost with those of sample No.6 except that it exhibited achiral phases. Example 2 (composition 1) A nematic liquid crystal composition consisting of was filled into a cell having transparent electrodes provided with polyvinyl alcohol (PVA) applied thereonto, followed by rubbing the resulting surfaces to subject the resulting surfaces to parallel aligning treatment and having a distance between the electrodes of 10 µm to prepare a TN type cell, which was then observed under a polarizing microscope, as a result, formation of a reverse twist domain was observed. To the above nematic liquid crystal composition added a compound of the formula (I) of the present invention wherein R₁=C₈H₁₇, n=1 and R₂=C₇H₁₅ i.e. in a quantity of 1% by weight and a TN type cell prepared in the same manner as above was similarly observed. As a result, the above reverse twist domain was dissolved and a uniform nematic phase was observed. Example 3 (composition 2) Using the above sample No.3 of the present invention and another SC* compound, the following equimolecular composition was prepared: The composition was filled in a cell of 2 µm thick provided with transparent electrodes having PVA applied thereonto, followed by rubbing the resulting surfaces to subject the resulting surfaces to parallel aligning treatment, and the resulting liquid crystal cell was provided between two sheets of crossed polarizers, followed by impressing an electric field thereto. Impressing 20V, change in the intensity of transmitted light was observed. The response time was sought from the change in the intensity of transmitted light at that time to give about 70 µsec at 25°C. In addition, with the above composition, the temperature change of the texture was observed under a polarizing microscope. As a result, it was found that a ferroelectric liquid crystal was formed in the temperature range of 10° to 40°C, and its spontaneous polarization value was 20 nC/cm² at 25°C. Example 4 (composition 3) Using the liquid crystal compounds of samples 3, 4 and 8 and other SC* compounds and nematic liquid crystal compounds, a liquid crystal composition having the following components was prepared: The resulting composition was filled in the same cell of 2 µm thick as in Example 3 and the resulting liquid crystal element was placed between two sheets of crossed polarizers. An electric field of 20V was impressed and its intensity of transmitted light was observed. From the change in the intensity of transmitted light, the response time was sought to obtain a value of about 300 µsec at 25°C. In addition, with the above liquid crystal composition, the temperature change of the texture was observed by means of a polarizing microscope. As a result, it was found that a ferroelectric liquid crystal was formed in the temperature range of 5° to 50°C, and its spontaneous polarization value was 13 nC/cm² at 25°C and the tilt angle was 26°.

Claims: A ferroelectric pyridine compound of the formula: in which n is 0 or an integer of from 1 to 8: R₁ is a C₂-C₁₈ alkyl group: and R₂ is a C₁-C₁₈ alkyl group. A compound according to claim 1 in which R₁ is C₂H₅. A compound according to claim 1 in which R₁ is C₆H₁₃. A compound according to claim 1 in wich R₁ is C₈H₁₇. A compound according to any one of the preceding claims in which R₂ is a C₆-C₁₀ alkyl group. A compound according to any one of the preceding claims in which n - is 0 or an integer of from 1 to 5. A chiral smectic liquid crystal composition comprising at least two components at least one of which is a ferroelectric pyridine compound as claimed in any one of the preceding claims. A light-switching element using a liquid crystal composition comprising at least two components at least one of which is a ferroelectrlc pyridine compound as claimed in any one of the preceding claims and exhibiting a chiral smectic C phase.

Inventors: INOUE, HIROMICHI, INUKAI, TAKASHI, MIYAZAWA, KAZUTOSHI, OHNO, KOUJI, SAITO, SHINICHI
Applicants: CHISSO CORPORATION
Application year: 1986
Main classification: C07D213
IPC codes: C09K19, C07D213, G02F1, C07D309