CA1051421A - Nematic liquid crystal compounds and compositions - Google Patents

Abstract

Abstract of the Disclosure:
New nematic liquid crystal compounds with positive dielectric anisotrophy, represented by the formula wherein R is n-butyl, n-pentyl, n-hexyl, n-heptyl or n-octyl group are obtained by condensing p-n-alkylnitrosobenzenes with p-cyanoanilines and oxidizing the resulting p-n-alkyl-p'-cyanoazobenzenes with peracetic acid. Nematic liquid crystal compositions with positive dielectric anisotrophy, having a nematic liquid crystal phase over a broad temperature range are obtained by mixing two or more of p-n-alkyl-p'-cyanoazoxybenzenes of the above formula.
Also, an useful nematic liquid crystal composition with positive dielectric anisotrophy which exhibits its nematic liquid crystal phase over a broad temperature range, including room temperature can be obtained by mixing at least one of Schiff base type or azoxy type compounds in a suitable ratio.
The liquid crystals disclosed may be used for a torsion effect type display device which exhibits a nematic liquid crystal phase over a broad temperature range including room temperature.

Description

~C~5~4Z~

Background of the Invention:
This invention relates to a new nematic liquid crystal compound or composition having positive dielectric anisotropy and a process for the preparation of said new nematic liquid crystal compound.
Nematic liquid crystal compounds or nematic liquid crystal compositions (hereinafter referred to as N-liquid ~0 ~S~4Z~

crystal compounds or N-liquid crystal composition~) are used for electro-optical devices~ utilizing the phenomenon that their optical properties are changed if an electric field is applied thereto. Electro-optical display devices with use of the N-liquid cry~tal material may be divided into two groups acc~rding to dielectric anisotropy of the N-liquid crystal material used. One group comprl~es devices in which there i~ employed a dynamic scattering mode (DSM), i.e. an optical scattering phenomenon caused by colllsion of an ion and an N-liquid cry~tal domain when an eloctric field is applied to an N-liquid crystal material having larger dielectric con~tant in a direction perpondicular to longitudinal axe~ of th~ molecules than dielectric constant in the longitudinal direction, i.e. nematic liquid cry~tal material having negative dielectric ani~otropy (hereinafter referred to a~ Nn-liquid cryotal material). Another group compri~e~ deYiceo in which there i8 employed an electric field offect mode (FEM)~ i.e. a chanBe in lean or torsion of N-liquid crystal molecules cau~od by applying an electric field to an N-liquid cry~tal material having larger dielectric con~tant in a longitud~l direction of the molecule~ than ; that in a dir-ction perpendicular to longitudinal axe~ o~ the molocules~ i.e. nematic liquid cry~tal material ha~ing po~lbiYe dielectric ani~otropy (hereinafter referred to as Np~ uid cry~tal material). Mbny of the lattor FE~-type ; devices are 30-callod ~tor~ion effec*-type display devices"
(h~r~inafter refo~r~d to a~ TN-type di~play device) wherein a ohange ~n tor~ion of the molecules ~o~o of the electric field effect modos 3 i8 utilized. The Np-liquid cry~tal compeund or Np-liquid crystal composition of this invention ,

- 2 -1~51~
i~ utilized for the TN-type display device.
Summary ~f ~- In~
An ob~ect of thi~ invention is to provide a new Np-liquid crystal compound for the TN-type display device and a process for the pr0paration thereof.
A further ob~ect of thi~ invention is to provide a new Np-liquid crystal composition for use in the TN-type display device.
A ~till further object of this invention i~ to provide a new Np-liquid crystal compo~it~on for the TN-type display device, which exhibit~ the nematic liquid crystal phase over a broad temperature range, including room temperature.
In accordance with this invention, there i~ provided a new Np-liquid crystal compound, p-n-alkyl-p~-cyanoazoxybenzene~ having the formula I~

R- ~ N = N ~ CN (I) O

wherein R i8 n-butyl, n-pentyl~ n-hexyl~ n-heptyl or n-octyl group. With mixing of two or more of ~aid p-n-alkyl-p~-cyanoa~ox~benzenes, an Np-liquid cry~tal composition can be obtained~ of which the n~matic liquid crystal phase temperature range or mesomorphic range is broader than that Or each componen~ of said composition. Further9 a new Np-liquid ory~tal co~position which exhibit~ a broad mesomorphic rangc includ~n~ room t~mperature is obtained by mixing (A) at lea~t one of p-n-alkyl-p~-cyanoa~oxybenzenes and ~B) at lea~k one of Schiff base type compounds represented by the formula, ~ 3 lQSl~Zl Rl ~ CH = N ~ R2 (II) wherein Rl iB n-alkoxy of 1 to 8 carbon atom~ and R2 ~
n-alkyl of 1 to 8 carbon atom~ or Rl i~ n-alkyl of 2 to 8 carbon atoms and R2 i~ n-alkoxy of 1 to 8 carbon atoms in a weight ratio of 0.05 ~ A/A~B ~ 1, or by mixing ~aid component (A) and (C) at least one of azoxy typ~ compound~
represented by the formula,

3 ~ N ~ N- ~ R4 (III) vhore~l any one of ~3 and R4 i~ n-alkyl Or 1 to 8 carbon atom~ and the other i~ n-alkoxy Or 1 to 8 carbon atoms or each of R3 and R~ i~ n-alkyl of 3 to 8 carbon atoms in a w-ight ratio Or 0.05 ~ A/A~C ~ 1.
Brief Explanation of Drawin~ss Fig. 1 repre~ent~ an in~rarod ab~orption ~pectrum Or p-n-butyl-p~-cyanoazosybenzene.
Fig, 2 repre~ents a gas chromatogra~ of p-n-butyl-p~-cyanoazoxybenzene.
Fig. 3 represent~ a ma~ ~pectrum of p-n-butyl-p~-cyanoazoxybenzene .
2S ~ig. 4 repre~nt~ a phas~ d~agra~ of an Nn-liquid cry~ta~ compo~ition con~i~ting of a mis~ure of p-. : ~
;; ~ metho~benzylldene-p~-n_butylaniline (MBnA) and p-:~ e*hoxybenærlidene-p~ butylanil~ne (EB~A).

~: Fig~ 5 repr~ent~ a pha~e diagr~m of an Nn-liquid c~ys~al compositio~ cons~ ~t~ng of a mixture of ~BA, EBBA

; - 4 .

., 1~5~

and p-ethoxyb~nzylidene-p~-n-heptylaniline (EBHpA)~
Fig. 6 repre~ents a pha~e diagram of a Np-liquid crystal compo~ition of thi~ invention consisting of a mixture of an Nn-liquid cry~tal co~po~ition obtained by mixing MB~A, EBBA and E~HpA in equal part~ by wei~ht and p-n-butyl-p~-cyanoazoxybenzene.
~ lg~ 7 represents a phaso diagram of a Np-liquld cry~tal compo~ition of thi~ invention con~ist~ng of a mixture of an Nn-liquid crystal composition obtained by mixing M~A, EBBA and E~HpA in equal parts by weight ~nd p-n-hexyl-p~-cyanoazoxy~enze~e.
Fig. 8 repre~ent a pha-e diagram of an Np~ uid cry~tal compo~ition of this i~ention consi~ting of a mixture of an Nn-liquid crystal cQmposition ~Compo~ition ) obtained by mixlng p-mothoxy-p~-n-butylazoxybenzen~ p-m0thosy-p~-n-h0xylazosybenzone~ p-methosy-p~
octylazoxybenzono and p-ethoxy-p~-n-butylazosybenzene in equal part~ by wei~ht and p-n-butyl-p~-Gyanoazo~ybenzeno.
Flg. 9 rsproJent8 a pha80 dlagrum of an Np-liquid cry~tal compo~ltion of thl~ ~nventlon consi~ting of a mixture of ~ald Compound a ~nd p-n-hexyl-pt-cyanoazoxyben~ene.

The new ~p-l~quid cry~tal oompound of ~ormula I msy ~e obtai~ed b~ co~den~i~g p-n-alkyl~itrosobenzenQs repre~ented ~5 b~ the ~onmNla, R ~ N0 wh~o~a R is a~ defin~d ~bove wi~h p-cyanoanll~ne to form p-~alkyl-p~-cy~noazo~enzenes r~pr~sented by the fQrmula~

lnsl~zl R ~ N = N ~ CN

wherein R is a~ defined above, and then oxidizing the resulting p-n-alkyl-p'-cyanoazobenzene with peracetic acid which may be obtained by mixing hydrogen peroxide ~olution and acetic acid.
The foregoing is represented by the reaction formulae to follow:
R ~ N0 I NH2 ~ CN- --> R ~ N = N ~ CN + H20 R ~ ~ H202 ~ CH3COOH ~ N - N ~ CN

p-n-alkylnitro~obenzones used as the ~tarting material may be formed by oubJecting n-alkylbenzens~ to nitration or by osidizing p-n-alkylanilines, converting the re~ulting p-n-alkylnitrob~nzene~ to p-hydroxyamino-n-al~ylbenzenes with u~e of a powdered sinc and am~onium chloride and I sub~ecting the oame to nitroeoation. Tho crude p-n-~, alkylnitro~obenzene thu~ obtained may be purif~od, for ~xa~plef by ~t~am di~tillation. The oxidation reaction with peracetic acld i~ conven~ently effected at temperatures of 70 to 80C for 6 to 8 hours. If ths react~on temperature i~ Iowerod below the abo~e range, the reaction ti~e become~
~ ~ ~ory lo~e~ On the contrary, if the temperature i~ slevated ov~r the abo~e ra~g~ by-produots are form~d. Tho crude p-n-al~rl-p~-cy~noazoxybonzen~s are purifiod by 1~51~
recrystallization from ethyl alcohol.
The compounds of Formula I were identified with infrared absorption spectrum, gas chromatogram and mas~
spectrum. I`ig. 1, Fig. 2 and Fig. 3 repre~ent an lnfrared absorption ~pectrum, gas chromatogrlm and ma~s spectrum re~pecti~ely, of p-n-butyl-pl-eyanoazoxybenze~e. According to the gas chromatography on the p-n-alkyl~p~-cyanoazoxybenzene, it was confirmed that there are pre~ent t~o i~omer~ of the following formulae in about the equal parts by weight.

R ~ N = N- ~ CN

R- ~ N = N ~ N

~n the F~g. 2, though it i~ unclear what peak corresponds to what isomer~ the welght ratio of the two isomers is about 1 t 1. As to p-n-pentyl-p~-cyanoazoxybenzen~, p-n-hexyl-p~-cya~oazoxybenzene, p-n-heptyl-p~-cyanoazoxybenzene, and p-n-octyl-p~-cyanoazoxybenzene~ the same re~ult~ as in the p-n-butyl-p~-cyanoazoYybenzene Yere obtained.
- The Np-l~quid cry~tal compound~ of the ~ormula I and the Np-liquid crystal compo~ition obtained by mixing two or more o~ Gompound~ of the ~o~mula I exhibit a broad mesomorphic range.
A c~ystal-ne~atic liqu~d crystal pha~e transition point (CN

po~nt~ and nematic li~u~d cry~tal-i~otropic pha3e transition point (NI point) in rc~pect of thsse Np-liquid cry~tal compou~d~ a~d compQsitions are gi~en in Table 1 and Table 2.

~ 7 ~

lQ5142~
Table 1 CN Points, NI Points, ComDounds C _ C

C~{3 (CH2)3 ~N = N ~CN 56 110 CH3 (CH2)4 ~N . N ~CN 66 118 CH3 (CH2)5 ~ ~ ~CN 68 106 CH3 ~ 2)6 ~N 1~ ~CN 98 119 CH3 ^ ~CH2)7 ~N N ~CN 65 100 l~S~4'~1 o ~ U~ o ~ o ,, o , o H ~
z O c~
P. O ~ U~ ~o U'~
g :Z Z :~; Z

N
~ ~ :Z Z X
~ llto ~0 I~to 11~3 O~
O

~ _ _ _ _ N
C~
_ _ _ _ 3 ~
"~o o :C

2~

o ~, C~l r oo ~ o ,, ,~ o o H ,1 _I ,1 ~1 ~0 t~ U~ O O 'I
~ O ~ t~
V

~1 r~ ~ ~
C~l N -1 _ ..

~ o ~o _ _ _ _ _ _ :~

~ .. .. .. ..
C ~ ~

1~ ~ 0 ~ ~ 0 r~ V
_~ .
::~ V~

, . . .
.
., ~ . ..
. '' ~ ~- . ,' ' ' ~14;~
.,, o V ~ U~ , D~ ~ ~ O O
H ~
z o~
o ~ a~
o ~ ~ ~r Z

~ ~ ,1 :Z Z

Z ~ O Z ~ O

_ _ _ _ V V
~ _ _~
C~
O ~ ~
U ~ 5 _ O C~
~1 ~1 el ~ .. .. ..
Z X Z
~1 C~

~ ~0 Z ~O Z ~0 ~11 _ _ _ ~ C~
o C~ $~ 3 C~
.
C~ C~

Z ~ O Z

C`! C~2 ~ c~
` - - ~
~ -u ~

-~5~42~

As seen from Table 2, a broader ~esomorphic range is obtained by mixing two or more of the p-n-alkyl-p~-cyanoazoxybenzene~. The Np-liquid cryst~l compo~itions are not critical in a mixing ratio of the component~ and any ratio may be used.
The compounds of the Formula I and the compositions thereof can be used as Np-liquid crystal materials for the TN type display device. However~ the me~omorphic range~
of these Np-liquid cry~tal materials are on a high temperature side over room temperature as indicated in the Tables 1 and 2. It ha~ al~o beon found that a broader mesomorphic range including room temperature can be obta~ned by mixing at leaot one of the compounds of Formula I ~ith at lea~t one of the Schiff ba~e type compounds of Formula II or of the azoxy type compound- of Formula III and that the compounds of the Formula I have good mi~cibility with the compound6 of the Formulao II and III. The Np-liquid cry~tal compo~itions thu~ obtained are al~o included in the scope of thiM
in~ention.
The Schiff ba~e type compound~ of the ~ormula II are the well-known Nn-liquid cry~tal compound~.
The CN point~ and NI point~ Or the typical Schiff baae ~ype co~pound~ are set forth in Tablo 3.

~5~Zl CN NI
Point~ Point~
Compounds C C
-Cl13~ CH = N~ ( C1~2 ) 2 C 3 5 61 3 (~ CH N-(~ ( CH2 ) 3 CH3 22 45 2 5 ~ CH - N ~ t CH2 ) 3 ~ CH3 36 80 C H50-~CH = N~Cll2~5-cH3 41 80 2 5 ~ CH = N -~ ~ CH~ ) 7 ~ CH3 5 83 CH3- (CH~30-~ CH ~ N~C2H5 5 65 CH3- (CH2)3o~cH ~ N-~(C~l2)3-CH3 43 7 CH3- (CH2)3o~cN ~ N-~(CH2)6 CH3 56 82 CH3-(CH2)sO-(~CH D N~CH3 57 7 CH3 ~ ( CH2 ~ 70 - ~ CH :: N -~ CH3 67 7 S

e~l3-(C~12~2 ~H ~ N~O-C2H5 80 94 CH34~cH2)3 ~CH :- N~0C2H5 ~1 75 ~(lf5~42~

Tabler 3 ~cont~d) CN NI
Point~ Points Com~ounds C C

CH3-(CH2)5 ~ CH = N ~ 3 7 CH3-(CH2)5 ~ CH = ~ ~ 0 C2H5 7 82 Nn-liquid cryfstal compof~itions obtained by mi~ing two or more of the Schif~ baae typ~ compoundf~ are in general lowered in the CN point due to the frsezing point depres~ion and therefore~ are broadened in the mesomorphic range af compared with each comffponent of fsaid compositionfof.
Among the Schi$f baf~ffffa type compof~itionf~ an Nn-liquid cry~tal compositio~ obtained by mixing MBRA and EBBA in a ~ultable ratio exhibitf~ a broad meff3fomorphic range including room temperature. Afs if~ff apparent from ~ig. 4~ an Nn-liquid cry~tal compo~ition having a weight ratio of MHfBA to EBBA
. of from 7 s 3 to 4 t 6 exhibit~ a broad mesomorphic range and particularly~ in case- of the mixing ratio of 1~ s 1~
. .
there is pro~ided the broadest mef~omorphic range (-15 to ~ 58C). According to the pre~erred embodiment of thi f in~ention, an Np-liquid cr~f~ftal compofsfition which exhibit~
a broad mcf30morphic range inclufding room temperatur0 is obtained by mixing at leaJt one of p-n-alkyl-p~-cyanoazoxybenz0nefs with the Nn-liguid cryf~tal compof3ition : , :~ ' confYi~*ing of a mixtur~ o~ ~BBA and EBBA i~ which the weight ratio iB comprised between 7 : 3 and 4 s 6.
. ' .

_ 14 o .. . .. ..

3~5J.~2i It is kr~own that if a proper ~mount of EBllpA i~ added to a ~ixture of e~lual parts by weight of MBBA and EBBA, th~
CN point i~ lowered and OIl th~ other hand, ~he NI point i~
elevated ~ and thu~, there i9 obtained a me~omorphic range broader than that of the mixture of eq~ual part~ by weight of`
MB~A and E~BA .
A~ clearly ~howrl in E`ig. 5 ~ a CN point of' the Nn-liquid crystal compoJition obta~ned by incorporating l~BHpA lnto th~
Nn-llquid cry~tal compo~ition con~i~ting of th~ mixturc Or equal parts by weight of MBBA and EBB~ io lower than ~ha$
of the latter N~--liqu:Ld cs~ystsl compositlon~, if EBHpA/MBBA~EBBA
i~ bolow about 1 (MBl~ s EBBA ~ EBHpA ~ 2) . 0~ the oth0r hand~ e~ NI pol~t ia ~levated a~ an~ount o~ B}~pA addod i~ incr-a-ed. Wlth du0 r~Bard *o the CN po~t ~ it i~ noted 'chat ~ lf EB~pA/MB~A~EBBJ~ i~ bolow sbout 1 (MBBA s E}~BA s EBHpA
1 t 1 t 2) ~ tho ~o~omorplllc ran~o Or tho Nn--llquld crr~tal coelpo~itlcn con~1-t:lng of the D-i~turo of oqual partE~ by ~elght of ~ ~d EBBA and EBHpA i~ practically moro larger than that of tho Nh--liquid cry~tal compo~itlon conalsting of' .
th~ cturo of equal part~ b~r weiBht Or MBBA and EBBA.
~rticularly~ ~ practioally more preferrod mo~omos~phic ra~ge c4~ be o~ta~ned lf' ~ EBBA Or ~he N~llquld cry~tal co~po~ ion ia rou~hly ln tho rango of from 1/4 (~BA s l:B~ 2 E}~pA ~ 2 s 2 t 1) to 9/11 (MBBA t EBE1A ~ E:~3HpA ~
11 s 11 s 18) . A far ~or~ preforred iJ a~l Nh--liquid cry~tal ~ompol-ition ~ ~h:lch EBHpAtP~BA ~ EBBA i~ th~ range of ~rom abou~ 3/7 (MB~A s EBB~ s EBHpA - 7 s 7 s 6) to 2/3 ~ s ~ ~A 3 3 ~ 3 4~ . m~ mo3t pr~f~rred io an N~-liquld csy$tal co~po~ition in which EB~lpA/MBBA~EBB~
1~ about 1/2 (~ s EBBA s EB~pA - 1 : 1 t 1), whi;:h haB

~ 15 -~514'~

the lowe~t CN point and which ~xhibits a broad me~omorphic range including room temperatur~. According to the pr~ferred embodiment of this in~ention~ Np-liquid cry~tal compo~itions which e~l~bit a broad mesomorphic range including room temperature can be obtalned by mix~ng a proper amount of at lea~t one of p-n-alkyl-p'-cyanoazoxybenz~ne~ Yith tho Nn-liquid cryatal compo~ition of MBBA, EBBA and EBHpA in t~
ratio EBHpA/MBBA~EBBA of les~ than about 1 (MBBA s EBBA s EBHpA = 1 : 1 s 2).
The azosy typo compound~ of the Formuln III ar~ al~o~
the ~ell-known Nn-liquid cry~tsl compounds. The CN point~
and NI points of the typlcsl azoxy type compound~ are ind~cat&d in Table 4.

Table 4 CN Points~ NI Po~nts~
_ ComE~unds C C

20CH30- ~ N ~ N- ~ (CH2)3.CH3 28 80 3 ~ __ ~ (CH2)4-CH3 44 94 5CH30- ~ N Y N- ~ (CH2)5-cH3 29 82 O

3 ~ ~ ~ (C~2)6 CH3 41 90 Z~

Table 4 ( cont ~ d) CN Point~, NI Po~nt~ t Co~Dounds _ ; C _ _ C

3 ~N = N -~( 2 ) 7 3 32 83 2 5 (~N ~ N-~(CI~2)3-CH3 42 119 o C2}ISO~ ~ N-~(CH2)4-cH3 32 117 O

C;ZHSO~ N ~(CH2)5~CH3 38 100 ~S O

2 5 ~N - Il~(C~2)6 CH3 38 O

~ 20 C;~Hso~ ~ N-~(CH2)7-CH3 39 97 ;

~H3~C}I2)20-~N ~ ~(C~2~2'C~3 47 87 C~

~H3 (~2~3 ~ ~ ~ ~2)2 3 9 o l~S1~2~

CN Points ~ NI Polnts ComlDounds C C

CH3- (CH2) ~0-~NN-~(C~2)2 CH3 58 ~ES

CH3- (CH2)2-~N ~N~(~)2 CH3 60 66 0 ( ~ionotropio ) CH3-(CH2)3~N D ~ ( 2)3 3 16 27 C~3 ( CH2 ) 4 ~~ ~ ~( C~2 ) 4 CH3 20 72 CH3~ (CH2)S-~ .N ~( 2)5 3 20 4B

C~3- (C1~2)6-~N c N ~(~ H2)6 C~3 ~5 71 o C}~3~ (CH2~7~N sl N ~(CH2)7~CH3 66 76 O
2,5 i~S~

Comp~sitions obtain~d by mixing at least one of the compounds of the Formula I with the azoxy type compound of the Formula III or mixtures thereof are also included in the Np-liquid crystal composition of this invention.
Among the azoxy type Nn-liquid cry~tal composition, Nn-liquid crystal composition~ obtained by mixing p-methoxy-p'-n-butyl~zoxybenzene~ p-methoxy-p'-n-hexylazoxybenzene and p-metho~y-p'-n-octylazoxyben~ene are lowered in CN point due to the freezing point depression and therefore, exhibit a broad mesomorphic range including room temperature. ~or example, a compo~ition obtai~ed by mixing the above-mentioned, three azoxy type compounds in equal parts by ~eight has a mesomorphic range of -6 to 79C. Al~o, a compo~ition obtained by mixing p-metho~y-p'-n-butylazoxybenzene~ p-methoxy-p'-n-hexylazoxybenzene, p-methoxy-p'-n-octylazoxybenzene and p-ethoxy-p'-n-butylazoxyben~ene in suitable amounts is lowered in it~ CN point dua to the freezing point d~preJsion and exhibits a broad me~omorphic range including room temperature.
By way of exampl~, a compo~itio~ obtained by mi~ing the abov~-m~ntion~d~ four azo~y type compounds in equal parts by : weight ha~ a m~omorphic range of -8 to 81C, The Np-li~uid crystal compo~itiona of thi~ invention obtained by mixing a proper amount of at l~ast one of pn-alkyl-pl-cyanoa~oxybenzene with th~ N~-liquid ory8tal composition con~i~ting of the abo~0 e~umerated~ three azo~y type co~pounda or of the above-e~umerated~ four azosy type compound~ exhibit a broad m~omorphic ran~e including room temper~lture and therefore, are u#0ful Np~ uid cry~tal material~ for the TN type display devlce~
3Q Accordi~g to thi~ inv~ntis~, the compounds of the ~5~
For~lula I or mixtures thereof serve to impart po~itive dielectric anisotropy to mixtures with the Nn-liq~id crystal materials and accordingly, may be called a positive dielectric ani~otropy-imparting agent. In Np-liquid crystal compositions consisting of a mixture of p-n-alkyl-p~-cyanoazoxybenzene(s) and Nn-liquid crystal material~ of the Schiff ba~e type or azoxy type, p-n-alkyl-p~-cyanoazoxybenzene(s) is to be sdded in an amount of not less than 5~ by weight~ If tho amount i8 le~
than 5% by weight, N-liquid crystal compo~itions having positiv~ dielectric ani~otropy can not be obtained~
G~nerally, p-n-alkyl-p~-cyanoazoxybenzenes are higher in the temperature range tha~ the Schiff base type and azoxy type compounds. Accordingly, me~omorphic rangas of the Np-liquid cry~tal compo~itions ~hift to high temperature ranges as an amount of p-n-alkyl-p~cyanoazoxybenzene iB increased. In view of this point, tho amount of p-n-alkyl-p~-cyanoazoxybenzcne(~) in the Np-liquid crystal compo~itions i~ preferably not more than 30% by weight~ more preferably not more than 20% by woight.
This invention will be illu~trated by the following non-limitativc example~.
Esam~le 1 O.2 mols of p-cyanoanilin~ were di~solvod in 240 ml of g~acial acotic acid at 40C and dissolved in 0.22 mols of ~$ p-n~butylnitro~oben~ene which had previously been purified by ~toa~ di~tillation. After ~ooling to room temperature and allowing to stand i~ the dark for 4 days, a conden~ation reaction of p-cyanoaniline with p-n-butylnitrosobenzene ~a~
effect~d~ After completion of the roaction, crude p-n-butyl~p~-cyanoazoben~ene i~ depo~ited by addition of di~tilled - 2~ _ 1~51~

water a~d filtered of~. After washing by di~tilled water and removing acetic acid, the purified p-n-butyl-p'-cyanoazobenzene was obtained by recrystallization from ethanol (~I P~ 100C, exhibiting mesomorphic pha~e on lowering of temperature).
Next, 0.1 mol~ of p-n-butyl-p'-cyanoazob~nzone were dis~olved in 500 ml of glacial acetic acid and 31 ml of a 3 ~ hydrogen peroxide solution were added~ and then stirring was effectod while heating at 74 to 76C for 7 hour~. Then, after adding distilled water whilo stirring and cooling, p-n-butyl-p'-cyanoazoxybenzene was cry~tallized and filtered off . After wa~hing with di~tilled water and removing acetic acid, purification wa~ effected by recrystallization from ethanol, A me~omorphic rango of the product thus obta~ned wa~ 56 to 110C. This compound was identified with infrared ab~orption ~pectrum, gas chromato ram and masa ~pectrum as ~hown ln Fig. 1, Fig. 2 and Fig. 3.
ExamDl- 2 0.2 mol~ of p-cyanoaniline were dissol~ed in 240 ~1 of glacial acetic acid at 40C and dissolved in 0.22 mols of p-n-pentylnitrooobenzene which had previously been purified by ~toam distillation. After cooling to room temperature and allowing to stand in the dark for 4 day~, a condensation reaction of p-cyanoanilino with p-n-pentylnitro~obenzene was effected. After completion of the reaction, crude p-n-pentyl-p~-cy~noazobenzene 1~ depoaited by addition of distill~d water a~d filtered off. After washin~ by distilled water and remo~ing acetic ac~d~ the purified p-n-pentyl-p'-cyanoazobenzene wa~ obtained by recry-ctalliza~ion from ethanol (M.P. 94C, exhibiting ~esomorphic pha~e o~ lowering of temperature).

1~5~

Next, 0.1 ~lol~ of p-n-pentyl-pl-cyanoazobenzene were dissolved in 500 ml of glacial acetic acid and 31 ml of a 30% hydrogen peroxide solution were added, ~nd then stirring was effected while heating at 74 to 76C for 7 hour~.
Then, after adding distilled water while ~tirring and cooling, p-n-pentyl-p~-cyanoazoxybenzene wa~ cry~tallized and filtsred off. After wa~hing with distilled water and removing acetic acid~ purification wa9 effected by recry-~tallization from ethanol. A me~omorphic range of the lQ product thus obtained was 66 to 118C. Thi~ compound was identified with infrared ab~orption spectru~, gas chromatogram and mass ~pectrum.

O~Z mol~ of p-cyanoaniline were di~olved in 240 ml of glacial acetic acid at 40C and diesolved in 0.22 mols of p-n-hexylnitrosobenzene which had previou~ly been purified by ~team distillation. After cooling to room temperature and allowing to stand in the dark for 4 daya, a condensation react~on of p-cyanoaniline with p-n-hexylnitrosobenzene wa~
~ffected. After completion of the reaction, crude p-n-hexyl-p~-cyanoazobenzene i~ depo~ited by addition of distilled wate~ and fi~tered off. After washlng by distilled water and re~o~ing acetic acid, the purlfied p-n-hexyl-p~-cya~oazobenzene was obtai~ed by recrystallization from ethanol (M.P. 91 C~ e~hibiting me40morphic phase on lowering of temperature).
N~xt, 0.1 mol5 o~ p-n-hexyl-p~-cyanoa~obenzene were diseol~ed in 50~ ml o~ glacial acetic acid and 31 ml of a 30~ ~ydrog0n ~eroxide ~olution were added, and then ~tirring wa~ effected while heating at 74 to 76C for 7 hours.

~ 2:2 -~514'~1 Then, after adding distilled water while stirring and cooling, p-n-hexyl-p'-cyanoazoxybenzene was crystallized and filtered off. After washing with dîstilled water and removing acetic acid, purification was effected by recrystalli-zation from ethanol. A mesomorphic range of the product thusobtained was 68 to 106C. Thi~ compound wa~ identified with infrared absorption spectrum~ ga~ chromatogram and mass spectrum.
Example 4 0.2 mol~ of p-cyanoaniline were di~solved in 240 ml o~
glacial acetic acid at 40G and dissolved in 0.22 mol~ of p-n-heptylnitrosobenzene which had previoualy been purified by steam distillation. After cooling to room temperature and allowing to ~tand in the dark for 4 day~, a condensation reaction of p-cyanoaniline with p-n-heptylnitrosobenzene was effected. The reaction mixture was added with distilled water and ~ub~ected to extraction with ether, and after removing the solvent~ ~ub~ected to column chromatography (200 mesh silica ~el - benzene). ~inally~ with recrystallization from ethanol~ the purified p-n-heptyl-p~-cyanoazobenzene was obtained (M.P. 90C, exhibiting mesomorphic phase on lowering ; of temperature).
Next~ 0.1 mol~ of p-n-heptyl-p~-cyanoazobenzene were dl~solved in 500 ml of glacial acetic acid and 31 ml o~ a 30% hydrogen peroxide ~olution ~ero added~ and then ~tlrring was e~r~cted while heating at 74 to 76C for 7 hours. Then, after adding di~illed water ~hile stirring and cooling~
p-n h0ptyl-p~-oyaneazoxybenzene was cry~tallizQd and filtered o~. A~ter wa~h~g with distilled water and removing acetic acid~ purification wa~ e~fected by recry~tallization from ethanol. A me3cmorphic ran~e of the product thus obtained lQ~4'~

was 98 to 119 C. This compound was identified with in~rared absorption spectrum, gas chromatogram and mass spectrum.
Example 5 0.2 mol~ of ~-cyanoaniline were dis~olved in 240 ml of glacial acetic acid at 40C and dissolved in 0.22 mols of p-n-octylnitrosobenzene which had pre~iously been purified by steam di~tillation. After cooling to room temperature and allowing to stand in the dar~ for 4 days, a conden~stion reaction of p-cyanoaniline with p-n-octylnitro~o-benzene ~as ~fected. The reaction mixture was added with distilled water and ~ub~ected to extraction with ether~ and after removing the solve~t~ ~ubJected to column chromatography ~200 mesh ~ilica gel - benzene). Finally, with recry~tallization from ethanol~ the purified p-n-octyl-p~-cyanoazobenzeno wa~ ob~a~nod (M.P. 88C~ exhibiting moeomorph~c phase on lowering of tempe~aturc).
Mcxt~ 0.1 mols Or p-n-octyl-p~-cyanoa~obenzene were dioeolved in 500 ml Or glacial acetic acid and 31 m~ of a 3 ~ hydrog~n peroxide solution ~er~ added~ and then ~tirring was ef~ected while hcating at 74 to 76C for 7 hours.
; Aft~r adding diatilled watcr while stirring and cooling, p-n-octyl-p~-cyanoazoxyben~ene was crystallized and filtered off. After wa~hing with distilled water and remo~ng acetic acid~ purification was e~fected by rccrystallization from oth~nol. A mesomorphic range of the product thu~ obtained wa~ 65 to 100CCo Thi~ compound wa~ identified with infrared absorption ap~ctrum, gas chromatogram and ma~o ~pectrum.
, ~
A Np-liquid cry~tal composition having it~ mcsomorphic r~nge Or 57 to 115~C wa~ obta~ned by heatinB at 130C equal -- 2~ _ .

l~Sl~

parts of weight of each of p-n-butyl-p'-cyanoazoxybenzene and p-n-pentyl-p'-cyanoazoxybenzelle to make them an i~otropic phase, and mixing them while stirring Similarly, variou~ Np-liquid crystal compositions were obtained by mixing two or more of p-n-butyl-p'-cyanoazoxybenzene, p-n-pentyl-p'-cyanoazoxybenzene, p-n-hexyl-p'-cyanoazoxybenzene, p-n-heptyl-p'-cyanoazoxybenzene and p-n-octyl-p'-cyanoazoxybenzene. Mesomorphic rangeo of th~e compositions are listed in the afore~entioned Table 2.
Exam~le 7 Np-li~uid cry~tal composition~ having a me~omorph~c range of 20 to 45C and 29 to 56C, re~pectively wer~
obtained by mixing 20~ by weight and 3 ~ by ~eight of p-n-butyl-p'-cyanoazoxybenzene ~ith 8 ~ by weight and 70%
by weight Or MBBA (me-omorphic ranges 21 - 45C), respectively.
ExamDle 8 A Np-liquid cry~tal composition having its mesomorphic range of 7 to 59C waa obtained by mising 2 ~ by weight of p-n-butyl-p'-cyanoazo~ybenzene with 8 ~ by weight of an Nn-liquid cry~tal compoffitlon (mesomorphic ranges 5 - 59C) which i~ a mi~tur~ of equal part~ by weight of MBBA and ., p-n-butylbonzylid~ne-p'--ethosyaniline (m so~orphic ranges 61 - 75C).
E~ampla 2 A Np-liquid crystal compo~ition having its me~omorphic range of 28 to ~0C wa~ obta~ned by mixing 2 ~ by weight of p-n-butyl-p'-cy~oazo~ybenzenc with 8 ~ by ~eight of an ~n-li~uid crr~al compoJltion (~e~omorphic ranees 27 - 70C) which i~ a mixture of 80% by weight of p-n-butylbenzylidene-~ ethoxyaniline and 2 ~ by weight of p~n-butylbenzylidene-- .

4'~1 p'-methoxy~niline (exhibiting a meaomorphic phase at 4~ - 80C
on lowering of temperature).
ExamPle 10 Np~ uid crystal co~poaitions hnving a me~omorphic range of -13 to 60C and -2 to 65C~ respec~iYely were obta~ned by ~ixing 8 ~ by weleht ~nd 7 ~ by weight o~ an Nn-liquid crrstal compo~ltion (mosomorphic ranges -15 ~o 58C) ~hich i~ a mi~ture of equal part~ by ~ei~ht o~ MBBA
and EBBA with 2 ~ by woight and 3 ~ by woight 9 reapoctivol o~ a Np-liquid cryatsl compoaition (meao~orphic ra~ge~ 46 -105&) which i~ a mixture of oqual past~ by wolght of p-n-butyl-p'-cyanoazo~sybonz~no, p-n-pon~rl-p~-cyanoazoxybsn~e~o and p- n - oc tyl-p~-cyanoazoxybonzan~.
~' 1~ A Np-liquld crystal compocitlon havlng lt~ m~co~orphic rAnec Or 3 to 67C ~a~ obtained by ~ixing 7 ~ by wcight of tho Nn-liqu~d cry-tal compo~i~lon of E~a~plo 10 with 3~
by ~olght of a Np-liquid cry~tRl COmpoJitiOn (m-~o~orphic rangos 62 - 113C) con-lsting of oqual part- by woleht of p-n-b~tyl-p~-cyaDoazoxrbenz~ne and p-n-hoptyl-p~-¢yanoa~osybonsons.
E~am~l~ 12 Np-llquid crystal compo~itlon~ w~re obtai~ed by mising an ~h-liquid cry~tal compo~itlon (~e~omorphic range~ -32 to ~8C) a~d p~n-but~l-p~-cy~noazosr~enz~ne ~n w~leht ratio~ A~
indlcate~ ~ Table 5~ Me~omorphic ranges o~ tho compo~it~on~
obtain~d are also ~von ~n tho Table 5.

.:
;

- 26 _ 1~51~
Table 5 Weight ratios of Nn-liquid cry~tal Me~omorphic compo~ition to Np-liquid crystal ranges, compound C

~5 : 5 -28 - 68 90 : 10 -26 - 68 80 s 20 -21 - 69 7 : 30 4 _ 69 60 s 40 20 - 70 ~0 : 70 45 - 85 10 go 5 - 108 The pha~o diagram givon in Fig. 6 is pr~pared from 1~ the data of tha Table 5. It i8 clear from the Fig. 6 that when p-n-butyl-p~-cyanoazoxyben~ene i~ added ~n amounts of not more than about 2 ~ by weight~ u~eful Np-liquid CryJtal CompoSitionJ ha~ing a very broad me40morphic range including room temperature are obtained.
Examvle 13 Np~ uid crystal composition~ wero obtained by mixing the ~n-liquid cry~tal compo~ition of Example 12 and p-n-: hexyl-p'-cyanoazoxybonzene (me~omorphic ranges 68 - 106C) in weight ratios a~ ~et forth ~n Table 6. Me~omorphic ranBes of the compo~ition~ obtained are al~o given in the Table 6.
:

.

lQ~

Table 6 Weight ratio~ of Nn-liquid crystal Me~omorphic compo~ition to Np-liquid crystal range~, compound C

95 s 5 -2B - 68 9 : 10 -25 - 68 80 : 20 -20 - 69 7 : 30 6 _ 70 60 : 40 23 _ 71 10 s 90 61 - 100 ~ho pha~e diagram given in Fig. 7 i~ preparod from the data o~ the Table 6. A~ i8 apparent ~rom the Fig. 7~ when p-n-hexyl-p~-cyanoazoxybenzeno i~ added in amount~ of not more than about 2 ~ by weight, useful Np-liquid cry~tal compo~ition~ having a very broad me~omorphic range inc~uding room te~perature are obtained.
ExamDle 14 Np-liquid cry~tal compo~ition~ having a meaomorphic rang~ o~ 34 to 68C and 46 to 76C roJp~ctively were obtained by mixing 2 ~ by weight and 3 ~ br weight of p-~-butyl-p~-cyanoazoxybenzeno with 8 ~ by weigh~ and 7 by ~cight re~p~ct~vely, of p~p~-di-n-pentylazoxybenzene (mo~omorph~c rango~ 20 - 72C).

~ p-llquid c~yatal compo~ition~ having a meeomorphic ran~e of 11 t~ 80C and ~4 to 83C respectivcly were 3~ obtained by mixing 2 ~ by weight and 3 ~ by woight of _ 2~3 ~

- . .
: :

~ ~S~

p-n-butyl-p~-cya~oazoxybenzene with 80~ by weight and 70%
by weight respectively, of p-methoxy-p'-n-butylazoxybenzene (mesomorphic range: 28 - 80C).
Example 16 Three Nn-liquid crystal compo~ition~ were obtained by mixing 70~ by weight, 50~ by weight and 30~ by weight of p,p'-di-n-pentylazoxybenzene with 30~ by weight, 5 ~ by weight and 70~ by weight respectively~ o~ p-ethoxy-p'-n-hexylazoxybenzene (me~omorphic range: 38 - 100 C).
Mecomorphic ranges of the three compositions obtained were 8 - 73C~ 20 - 77C and 29 - 80C re~pectively. Np-liquid crystal compositions having a mesomorphic range of 10 - 74 C, 23 - 79C and 30 -- 81C re~pectively were obtained by mixing 80~ by weight of each of the above ,Nn-liquid cry~tal compo~itions with 20% by welght of p-n-butyl-p'-cyanoazoxybenzene~
~samDle 17 Np-liquid crystal compo~itions having a mesomorphic range of -5 to 80C and 5 to 82C respectively uere obtained by mixi~g 8 ~ by welght and 70% by weight of an Nn-liquid crystal compo~ition (mesomorphic ranges -6 - 79C) which is a mixture of equal part~ by weight of p-methoxy-p'-n-butylazoxybenzene, p-~ethoxy-~'-n-hexylazoxybenzene and p-metho~y-p'-n-octylazoYybenzene with 2 ~ by weight and 3 ~:
by weight respectively~ of a Np-liquld crystal composition (me~o~orphic range: 46 - 105C) which is a mlxture of equal part~ by w~ight Or p-~-butyl-p'-cyanoazoxybenzene, p-n-pentyl-p'-eyanoazoYybenzene and p-n-octyl-p'-cyanoazoxybenzene.

A Np-liquid cry~tal co~po3ition havlng it~ me~omorphic range of 6 - 84C wa~ obtained by mixing 7 ~ by weight of _ 2'3 ~

:

1~51~

the Nn-liquid cry tal composition o~ Example 17 w~th 30~
by weigllt of a Np-liquid cryatal compoaition ~mesomorphic ran~e: 62 - 113C) conaistiDg o~ equal parta ~y ~e~ght of p-n-butyl-p~-cyanoazoYybenzone and p-n-heptyl-p~-cyanoa~oxybenzene.
ExamDle 19 Np-liquid cry~al compoHitlon~ ~ere obtained by mixinB an Nn~ uid ¢ry~tal composition (mesomorphic ranBe~ -8 - 81C, hereinafter reforrod to as Co~po-ltion a) cons$~ting of equal part~ by ~eieht of p-methoxy-p~-n-butylazoxybonzen~, p-m~tho~y - p '-n-hexylaso~yben~ene, p-methoxy-p~-n-octylazoxybenz~ne ~nd p - ~thoxy - p ~ - n -b~tylazoxybons~ne ~ith p-n-butyl-p~-cy~noazoxybonzen~ ln ; ~ieht ratio~ as set forth in Table 7. Moaomorph~c range~
15 o~ th compoalt~ ons ol~tain-d are alao indioated ln the Tal~lo 7 .

I

~5 , , . 3~

3 ~

1~51~
Table 7 . -. ~
Weight ratio of Composition a to Me~omorphic ranges Np-liquid crystal compound C
. .
95 s 5 -8 - 81 9 : 10 -7 - 81 $ : 20 -5 - 82 7 s 30 4 - ~5 60 : 40 17 - 88 3 : 70 4~ - ~8 10 : 90 51 - $o6 The pha~e diagram gi~en in Fig. 8 is prcpared from the dat~ of the Table 7. Aa i~ apparent from the ~ig. 8, when p-n-butyl-p~-cyanoazoxybe~zene i8 added in amounts o~ not more than about 20~ by weight~ u~eful Np-liquid cry~tal compo~ition~ which have a broad mesomorphic range including rvom temperature are obtained~
~
Np-liquid crystal compositions were obtained by mixing the Compo~ition a o~ Example 19 and p-n-hexyl-p~-: cyanoazoxybenzene (mesomorphic range: 68 - 106C) in weight ratio~ as indicated in Table 8. Mesomorphic range~ Or the composition~ obtained are al~o given in the Table 8.

3~- -.

~OS14;~1 Table 8 Weight ratios of Composition to Mesomorphic ranges Np-liquid crystal compound C
9 5 : 5 -8 - 81 :10 -7 - 81 :20 _4 _ 82 :30 6 - 83 : 4 24 - 86 : 7 5 - 93 : 9 61 - loO

~he phase diagram of Fig. 9 are prepared from the data of the Table 8. It i9 clear from the Fig. 9 that when p-n-hexyl-p~-cyanoazoxybenzene i8 added in amounts of not more than about 20% by weight, useful Np-liquid crystal compositions which have a broad mesomorphic range including .-room temperature are obtained.

. : :

Claims (17)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Nematic liquid crystal compounds having positive dielectric anisotropy, represented by Formula I, wherein R is n-butyl, n-pentyl, n-hexyl, n-heptyl or n-octyl.

2. A process for preparing p-n-alkyl-p'-cyanoazoxybenzenes of the Forumla I as defined in claim 1 which comprises condensing p-n-alkylnitrosobenzene of the formula, wherein R is as defined above with p-cyanoaniline and oxidizing the resulting p-n-alkyl-p'-cyanoazobenzene with peracetic acid.

3. Nematic liquid crystal compositions having positive dielectric anisotropy, consisting of a mixture of two or more of p-n-alkyl-p'-cyanoazoxybenzenes of Formula I as defined in claim 1.

4. Nematic liquid crystal compositions having positive dielectric anisotropy, consisting of a mixture of (A) at least one of p-n-alkyl-p'-cyanoazoxybenzenes of the Formula I
and (B) at least one of Schiff base type compounds represented by Formula II, II

wherein R1 is n-alkoxy of 1 to 8 carbon atoms and R2 is n-alkyl of 1 to 8 carbon atoms, or R1 is n-alkyl of 2 to 8 carbons and R2 is n-alkoxy of 1 to 8 carbon atoms, the two components A and B being present in a weight ratio of 0.05 ? A/A+B < 1.

5. Nematic liquid crystal compositions of claim 4 wherein said two components A and B are present in a weight ratio of 0.05 ? A/A+B ? 0.3.

6. Nematic liquid crystal compositions of claim 4 wherein said two components A and B are present in a weight ratio of 0.05 ? A/A+B ? 0.2.

7. Nematic liquid crystal compositions of claim 6 wherein said component B is a Schiff base type composition consisting of a mixture of p-methoxybenzylidene-p'-n-butylaniline and p-ethoxybenzylidene-p'-n-butylaniline, the weight ratio of p-methoxybenzylidene-p'-n-butylaniline to p-ethoxybenzylidene-p'-n-butylaniline being comprised between 7 : 3 and 4 : 6.

8. Nematic liquid crystal compositions of claim 6 wherein said component B is a Schiff base type composition consisting of a mixture of equal parts by weight of p-methoxy-benzylidene-p'-n-butylaniline and p-ethoxybenzylidene-p'-n-butylaniline.

9. Nematic liquid crystal compositions of claim 6 wherein said component B is a Schiff base type composition consisting of a mixture of p-methoxybenzylidene-p'-n-butylaniline, p-ethoxybenzylidene-p'-n-butylaniline and p-ethoxybenzylidene-p'-n-heptylaniline, the weight ratio of p-methoxybenzylidene-p'-n-butylaniline to p-ethoxybenzylidene-p'-n-butylaniline being 1 : 1 and the weight ratio of p-ethoxybenzylidene-p'-n-heptyl-aniline to p-methoxybenzylidene-p'-n-butylaniline and p-ethoxy-benzylidene-p'-n-butylaniline being not more than 1 : 1.

10. Nematic liquid crystal compositions of claim 6 wherein said component B is a Schiff base type composition consisting of a mixture of p-methoxybenzylidene-p'-n-butylaniline, p-ethoxybenzylidene-p'-n-butylaniline and p-ethoxybenzylidene-p'-n-heptylaniline, the weight ratio of p-methoxybenzylidene-p'-n-butylaniline to p-ethoxybenzylidene-p'-n-butylaniline being 1 : 1 and the weight ratio of p-ethoxybenzylidene-p'-n-heptylaniline to p-methoxybenzylidene-p'-n-butylaniline and p-ethoxybenzylidene-p'-n-butylaniline being comprised between 1 : 4 and 9 : 11.

11. Nematic liquid crystal compositions of claim 6 wherein said component B is a Schiff base type composition consisting of a mixture of p-methoxybenzylidene-p'-n-butylaniline, p-ethoxybenzylidene-p'-n-butylaniline and p-ethoxybenzylidene-p'-n-heptylaniline, the weight ratio of p-methoxybenzylidene-p'-n-butylaniline to p-ethoxybenzylidene-p'-n-butylaniline being 1 : 1 and the weight ratio of p-ethoxybenzylidene-p'-n-heptyl-aniline to p-methoxybenzylidene-p'-n-butylaniline and p-ethoxy-benzylidene-p'-n-butylaniline being comprised between 3 : 7 and 2 : 3.

12. Nematic liquid crystal compositions of claim 6 wherein said component B is a Schiff base type composition consisting of a mixture of equal parts by weight of p-methoxy-benzylidene-p'-n-butylaniline, p-ethoxybenzylidene-p'-n-butylaniline and p-ethoxybenzylidene-p'-n-heptylaniline.

13. Nematic liquid crystal compositions having positive dielectric anisotropy, consisting of a mixture of (A) at least one of p-n-alkyl-p'-cyanoazoxybenzenes of the Formula I
as defined in claim 1 and (C) at least one of azoxy type compounds represented by Formula III, III

wherein any one of R3 and R4 is n-alkyl of 1 to 8 carbon atoms and the other is n-alkoxy of 1 to 8 carbon atoms or each of R3 and R4 is n-alkyl of 3 to 8 carbon atoms, the two components A and C being present in a weight ratio of 0.05 ? A/A+C < 1.

14. Nematic liquid crystal compositions of claim 13 wherein said two components A and C arc present in a weight ratio of 0.05 ? A/A+C ? 0.3.

15. Nematic liquid crystal compositions of claim 13 wherein said two components A and C are present in a weight ratio of 0.05 ? A/A+C ? 0.2.

16. Nematic liquid crystal compositions of claim 15 wherein said component C is an azoxy type composition consisting of a mixture of equal parts by weight of p-methoxy-p'-n-butylazoxybenzene, p-methoxy-p'-n-hexylazoxybenzene and p-methoxy-p'-n-octylazoxybenzene.

17. Nematic liquid crystal compositions of claim 15 wherein said component C is an azoxy type composition consisting of a mixture of equal parts by weight of p-methoxy-p'-n-butylazoxybenzene, p-methoxy-p'-n-hexylazoxybenzene, p-methoxy-p'-n-octylazoxybenzene and p-ethoxy-p'-n-butylazoxy-benzene.