Optically active oxazoline compounds, liquid crystal composition containing the same and optical switching method using the same
BACKGROUND OF THE INVENTION 
     The invention relates to novel optically active oxazoline compounds represented by the formula (I): ##STR2## as well as liquid crystal compositions containing them and an optical switching method using them. 
     So far, conventional liquid crystals have widely been used as materials for display devices. In most cases, these display devices are based on Twisted Nematic (hereinafter, referred to as &#34;TN&#34;) mode using nematic liquid crystals, so that they have a disadvantage that almost all the response speeds of them are extremely low, namely, as slow as several milliseconds at the fastest. 
     In order to eliminate this disadvantage of TN mode, various attempts have been made. For example, a display method using a ferroelectric liquid crystal is disclosed in N. A. Clarks et al., Applied phys. Lett. 36, 899 (1980). This method uses chiral smectic phase, particularly chiral smectic C phase, liquid crystal and attracts attention as a material for a high speed optical switching. Some ferroelectric liquid crystal materials using this principle have already been known [for example, refer to Japanese Patent Application Laid Open No. 70,455 1989]. However, no material having satisfactory properties has been known yet. 
     The present inventors have studied intensively in order to develope a liquid crystal having satisfactory properties to be used for an optical switching method. Finally, they have found that the oxazoline compounds represented by the formula (I) satisfies the requirements. The invention has been completed based on this finding. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an infrared absorption spectrum of an oxazoline compound prepared in Example 1 of the present invention. 
    
    
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide novel oxazoline compounds represented by the formula (I): ##STR3## 
     Another object of the present invention is to provide liquid crystal compositions containing a oxazoline compounds represented by the formula (I). 
     Still another object of the present invention is to provide a liquid crystal material which has high response speed, is chemically stable and suits for an optical switching method. 
     Yet another object of the present invention is to provide an optical switching method using said optically active oxazoline compounds or a liquid crystal composition thereof. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to novel oxazoline compounds represented by the formula (I): ##STR4## wherein R 1  represents an alkoxy group having not more than 20 carbon atoms, R 2  is a low grade alkyl group and * indicates a asymmetric cqrbon atom; as well as a liquid crystal composition containing said compound and an optical switching method using said compound or said composition. 
     The term, &#34;liquid crystal composition&#34; in the present invention means a composition comprising a liquid crystal and another liquid crystal and/or a compound other than a liquid crystal such as a chiral compound and still having a liquid crystal phase. As a &#34;low grade alkyl group&#34;, an alkyl group having 1 to 8 carbon atoms is preferable, the group having 1 to 4 carbon atoms is more preferable, and the group having 1 to 2 carbon atoms is further preferable. 
     The largest characteristic of the oxazoline compounds represented by the formula (I) of the present invention is that they have an asymmetric carbon atom in their oxazoline ring. Since the compounds have an asymmetric carbon atom at α-position of a group having a dipole moment in a direction perpendicular to a molecular axis, their electric properties, such as spontaneous polarization in a liquid crystal phase are improved remarkably. 
     Namely, the optically active oxazoline compounds represented by the formula (I) of the present invention show excellent properties as a ferroelectric liquid crystal. They have high response speed and are chemically stable. As for an optical switching method using the oxazoline compound of the present invention, a switching method having a response speed of not more than 1 m second is preferably used. Moreover, by adding said compound to a nematic liquid crystal, the occurrence of reverse domain in a TN type cell can effectively be prevented. 
     Therefore, the liquid crystal compositions containing the oxazoline compound represented by the formula (I) of the present invention can suitably be used not only for display device, but also as various electro-optic devices such as an electro-optic shutter, an electro-optic iris diaphragm, an optical modulator, a switch for changing optical path for optical communication, a memory and a lens with variable focal length. 
     Practical examples of the optically active oxazoline compounds represented by formula (I) are shown in Table 1. 
     
                       TABLE 1                                                     
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Compound                                                                  
No.            R.sup.1   R.sup.2                                          
______________________________________                                    
 1             C.sub.16 H.sub.33 O--                                      
                         C.sub.2 H.sub.5 --                               
 2             C.sub.14 H.sub.29 O--                                      
                         C.sub.2 H.sub.5 --                               
 3             C.sub.12 H.sub.25 O--                                      
                         C.sub.2 H.sub.5 --                               
 4             C.sub.10 H.sub.21 O--                                      
                         C.sub.2 H.sub.5 --                               
 5             C.sub.9 H.sub.19 O--                                       
                         C.sub.2 H.sub.5 --                               
 6             C.sub.8 H.sub.17 O--                                       
                         C.sub.2 H.sub.5 --                               
 7             C.sub.7 H.sub.15 O--                                       
                         C.sub.2 H.sub.5 --                               
 8             C.sub.6 H.sub.13 O--                                       
                         C.sub.2 H.sub.5 --                               
 9             C.sub.5 H.sub.11 O--                                       
                         C.sub.2 H.sub.5 --                               
10             C.sub.16 H.sub.33 O--                                      
                         CH.sub.3 --                                      
11             C.sub.14 H.sub.29 O--                                      
                         CH.sub.3 --                                      
12             C.sub.12 H.sub.25 O--                                      
                         CH.sub.3 --                                      
13             C.sub.10 H.sub.21 O--                                      
                         CH.sub.3 --                                      
14             C.sub.9 H.sub.19 O--                                       
                         CH.sub.3 --                                      
15             C.sub.8 H.sub.17 O--                                       
                         CH.sub.3 --                                      
16             C.sub.7 H.sub.15 O--                                       
                         CH.sub.3 --                                      
17             C.sub.6 H.sub.13 O--                                       
                         CH.sub.3 --                                      
18             C.sub.5 H.sub.11 O--                                       
                         CH.sub.3 --                                      
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     These compounds of the formula (I) can generally be obtained by reacting a nitrile compound of the formula (II) with an optically active 2-amino-alcohol compound of the formula (III) in the presence of a catalyst. 
     As a catalyst to be used for the reaction, cadmium acetate, cadmium chloride, zinc chloride and cobalt chloride can be exemplified. The catalyst is added preferably in an amount of 10 -5  to 10 -1  mol per 1 mol of the nitrile compound. 
     The reaction can usually be carried out without any solvent or in an inert solvent such as an aromatic hydrocarbon. The reaction is carried out at a temperature of 80° to 200° C., preferably 100° to 150° C. ##STR5## wherein, R 1  represents an alkoxy group having not more than 20 carbon atoms. ##STR6## wherein, R 2  is a low grade alkyl group, and * indicates an asymmetric carbon atom. 
     The optically active oxazoline compounds represented by the formula (I) of the present invention show excellent properties as a ferroelectric liquid crystal material. They have high response speed and are chemically stable. 
     The novel optically active oxazoline compound represented by the formula (I) of the present invention can be used by itself as a ferroelectric liquid crystal material, but, it can also be mixed with a smectic liquid crystal which in itself is not chiral, or with another ferroelectric liquid crystal material, to obtain a liquid crystal composition having an improved performance. 
     Furthermore, by adding the optically active oxazoline compound represented by the formula (I) to a nematic liquid crystal, the occurrence of reverse domain can effectively be prevented. In this case, said oxazoline compound is used preferably at an amount of 0.01 to 50% by weight of the liquid crystal composition to be obtained. 
     Moreover, by adding said oxazoline compound to a nematic liquid crystal or to a chiral nematic liquid crystal, it can be used as a chiral nematic liquid crystal composition for a phase transition type liquid crystal device or a guest-host type liquid crystal device. In this case, said oxazoline compound is used preferably at an amount of 0.01 to 80% by weight of the liquid crystal composition to be obtained. 
     The liquid crystal composition containing the compound of the present invention can suitably be used in the electro-optic devices such as an electro-optic shutter, an electro-optic iris diaphragm, an optical modulator, a switch for changing optical path for optical communication, a memory and a lens with variable focal length. 
     The compound of the present invention can be used in combination with various liquid crystal compounds, but, it is preferably used in combination with those showing ferroelectric chiral smectic phase, and more preferably in combination with those listed in Table 2. 
     
         TABLE 2-I
  No. Chemical Structure and Name of Compound Phase Transition Temperature
  (°C.)
      1
  ##STR7##
  ##STR8##
  2
 ##STR9##
  ##STR10##
  3
 ##STR11##
  ##STR12##
  4
 ##STR13##
  ##STR14##
  5
 ##STR15##
  ##STR16##
  6
 ##STR17##
  ##STR18##
  7
 ##STR19##
  ##STR20##
  8
 ##STR21##
  ##STR22##
  9
 ##STR23##
  ##STR24##
   10
  ##STR25##
  ##STR26##
  11
 ##STR27##
  ##STR28##
  12
 ##STR29##
  ##STR30##
  13
 ##STR31##
  ##STR32##
  14
 ##STR33##
  ##STR34##
  15
 ##STR35##
  ##STR36##
 Foot Note:
 SmC*; Ferroelectric smectic C phase
 SmA: Smectic A phase
 SmH*; Ferroelectric smectic H phase
 Sm3: Smectic phase
 Iso; Isotropic phase
 Cr; Crystalline phase
 Cho; Choleteric phase
 
    
     EXAMPLES 
     The invention will be explained more in detail with reference to the following examples, but, the invention is not limited thereto. As to phase transition temperatures in the examples, the values thereof vary slightly depending on a measurement method and a purity of the compounds. 
     EXAMPLE 1 
     Preparation of optically active 2-[4&#39;-(4&#34;-n-dodecyloxyphenyl)phenyl]-5-ethyl-2-oxyazoline [Compound No. 3 in Table 1] 
     3.65 g (0.01 mol) of 4-n-dodecyloxy-4&#39;-cyanobiphenyl, 1.14 g (0.015 mol) of (R)-2-amino-1-butanol and 0.27 g of cadmium acetate dihydrate were heated and refluxed for 30 hours in toluene. After cooling to room temperature, an inorganic substance was removed by filtration. A residue obtained by distilling out the toluene was treated with a silica gel column chromatography using 1,2-dichloroethane as a developing solvent. The thus obtained crude objected substance was recrystallized three times in ethanol to obtain 2.32 g of pure objected substance [Compound No. 3]. 
     As an angle of rotation of compound [α] D  =+24.50 (in chloroform) was observed. The structure of this compound was confirmed by an infrared absorption spectrum, which is shown in FIG. 1. 
     EXAMPLES 2 to 6 
     Compounds 4 to 8 in Table 1 were synthesized by the same method as in EXAMPLE 1 except that R 1  in the compound of formula (II) was changed accordingly. The chemical structure (only R 1  is indicated), elemental analysis value and the phase transition temperature of each of the compounds are listed in Table 3 together with those of Compound 3 of EXAMPLE 1. 
     
                                           TABLE 3-I                               
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 ##STR37##                                                                
                Elemental Analysis                                        
   Compound     Measured Value                                            
                         Calculated Value                                 
                                  Phase Transition                        
Ex.                                                                       
   No.   R.sup.1                                                          
                (%)      (%)      Temperature (°C.)                
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1  (3)   n-C.sub.12 H.sub.25 O                                            
                C79.98 H C79.95 H  9.49 N  3.22                           
                                   ##STR38##                              
2  (4)   n-C.sub.10 H.sub.21 O                                            
                C79.44 H C79.56 H  9.15 N  3.44                           
                                   ##STR39##                              
3  (5)   n-C.sub.9 H.sub.19 O                                             
                C79.29 H C79.35 H  8.96 N  3.56                           
                                   ##STR40##                              
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                                           TABLE 3-II                              
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 ##STR41##                                                                
                Elemental Analysis                                        
   Compound     Measured Value                                            
                         Calculated                                       
Ex.                                                                       
   No.   R.sup.1                                                          
                (%)      Value (%)                                        
                               Phase Transition Temperature               
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                               (°C.)                               
4  (6)   n-C.sub.8 H.sub.17 O                                             
                C79.12 H C78.98 H  8.96 N  3.90                           
                                ##STR42##                                 
5  (7)   n-C.sub.7 H.sub.15 O                                             
                C78.64 H C78.86 H  8.55 N  3.44                           
                                ##STR43##                                 
6  (8)   n-C.sub.6 H.sub.13 O                                             
                C78.64 H C78.60 H  8.32 N  3.99                           
                                ##STR44##                                 
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     [In the above Table 3, Cryst. means crystalline state, Iso means isotropic liquid, Sc* means chiral smectic C phase and Sx means a smectic phase but can not clearly identified.] 
     EXAMPLE 7 
     Preparation of optically active 2-[4&#39;-(4&#34;-n-decyloxyphenyl)phenyl]-6-methyl-2-oxazoline 
     [Compound No. 13 in Table 1] 
     3.35 g (0.01 mol) of 4-n-decyloxy-4&#39;-cyanobiphenyl, 1.13 g (0.015 mol) of (S)-2-aminopropanol and 0.27 g of cadmium acetate dihydrate were heated and refluxed in toluene for 30 hours. After cooling to room temperature, an inorganic substance was removed by filtration. A residue obtained by distilling out the toluene was treated with a silica gel column chromatography using 1,2-dichloroethane as a developing solvent to obtain a crude objected substance. The crude substance was recrystallized three times in ethanol to obtain 1.42 g of pure objected substance [Compound No. 13]. 
     The structure of this compound was confirmed by an infrared absorption spectrum. 
     EXAMPLES 8 to 10 
     Compounds 14 to 16 in Table 1 were synthesized by the same method as in EXAMPLE 7 except that R 1  in the nitrile compound of the formula (II) was changed accordingly. The chemical structure (only R 1  is indicated), elemental analysis value and the phase transition temperature of each of the compounds are listed in Table 4 together with those of Compound No. 13 of EXAMPLE 7. 
     
                                           TABLE 4                                 
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 ##STR45##                                                                
                Elemental Analysis                                        
   Compound     Measured Value                                            
                         Calculated Value                                 
Ex.                                                                       
   No.   R.sup.1                                                          
                (%)      (%)      Phase transition Temperature            
                                  (°C.)                            
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7  (13)  n-C.sub.10 H.sub.21 O                                            
                C79.22 H C79.35 H  8.96 N  3.56                           
                                   ##STR46##                              
8  (14)  n-C.sub.9 H.sub.19 O                                             
                C78.98 H C79.12 H  8.76 N  3.69                           
                                   ##STR47##                              
9  (15)  n-C.sub.8 H.sub.17 O                                             
                C78.84 H C78.87 H  8.55 N  3.83                           
                                   ##STR48##                              
10 (16)  n-C.sub.7 H.sub.15 O                                             
                C78.40 H C78.60 H  8.32 N  3.99                           
                                   ##STR49##                              
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     [In the above Table 4, Cryst. means crystalline state, Iso. means isotropic liquid and Sc* means chiral smectic C phase.] 
     APPLICATION EXAMPLES 
     APPLICATION EXAMPLE 1 
     Two transparent electrodes were coated on their surface with polyvinyl alcohol (PVA). The surfaces was then subjected to a parallel orientation treatments by means of rubbing. The electrodes were set facing each other with a distance of 3 μm formed a cell making the PVA films inner sides and the optically active oxazoline compound of EXAMPLE 4 [compound No. 6] was injected into the cell. The cell obtained was placed between two polarizers which were placed perpendicular to each other and obtained a liquid crystal display device. When a voltage of 30 V was applied between the electrodes, a switching action of a high response speed (ca. 100μ second) was observed. 
     The spontaneous polarization of this liquid crystal was 28.5 nc/cm 2  at 100° C. 
     APPLICATION EXAMPLE 2 
     A composition of the compound No. 3 prepared in Example 1 was prepared by mixing 21% by weight of p-octyloxy-benzoic acid-p&#39;-n-hexyloxyphenylester. This composition had the following phase transition points: ##STR50## Wherein Cryst: Crystalline state, 
     SmC*: Ferroelectric smectic C phase, 
     Ch: Cholesteric phase 
     Iso: Isotropic liquid.