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NEW SYNTHESES.THE LIQUID CRYSTAL PROPERTIES OF SOME NEW MESOGENS
G. Gray
To cite this version:
G. Gray. NEW SYNTHESES.THE LIQUID CRYSTAL PROPERTIES OF SOME NEW MESO- GENS. Journal de Physique Colloques, 1975, 36 (C1), pp.C1-337-C1-347. �10.1051/jphyscol:1975157�.
�jpa-00216236�
NEW SYNTHESES.,
Classification Physics Absrracrs
7.130
THE LIQUID CRYSTAL PROPERTIES OF SOME NEW MESOGENS
G. W. GRAY
Department of Chemistry, T h e University of Hull, HU6 7RX, England
Resume. - Les 4'-n-alkyle-4-cyanobiphCnyles et 4'-n-alkyloxy-4-cyanobiphenyles donnent des phases mCsomorphes sans couleur qui sont stables chimiquement et photochimiquement. Dans beaucoup de cas, les phases mtsomorphes se forment aux basses temperatures et en eKet deux mem- bres de la serie n-alkyle sont nematiques a la temperature du laboratoire. Les melanges eutectiques qu'on prepare des deux series donnent des plages nematiques larges, par exemple, environ
- 2 O C a plus de 60 OC. Tous les composes possbdent une anisotropie dielectrique qui est forte-
ment positive et les composes purs ou leurs melanges doment des phases nematiques qui fonctionnent bien en tlements Blectro-optiques qui utilisent l'effet nematique torsade. Les plages nematiques ont Bte etendues, sans mauvais effet sur les stabilites ou les proprietks electriques, par ['addition de certains membres de la serie de 4"-n-alkyle-4-cyano-p-terphenyles. Par exemple, un melange eutec- tique est nematique de 5 O CA 91 @C.
Deux analogues chiraux des cyanobiphenyles et cyano-p-terphenyles ont Ctt prepares en utilisant une chaine arborescente en groupe alkyle ou alkyloxy. L'incorporation de ces composes en melanges avec les 4'-n-alkyle et 4'-n-alkyloxy-4cyanobiphenyles donne des phases cholesteriques avec un grand pas d'enroulement, qui existent a la temperature ambiante et qui sont sans couleur et stables chimiquement et photochimiquement. Ces phases fonctionnent bien en elements electro-optiques, en utilisant Ie changement de phase cholesterique a nkmatique. On peut aussi utiliser ces composCs chiraux aux basses concentrations (1 %) pour prevenir la formation des regions avec les torsions renversks dans des elements electro-optiques qui utilisent l'effet nkmatique torsade.
Le 4'-n-octyl-4-cyanobiphenyle donne une phase smectique k la temperature ambiante. Par la methode de miscibilite, nous avons montre que cette phase et celles des autres homologues plus eleves des deux series sont des phases smectiques A. Cependant, les phases smectiques donnent des rksultats inhabituels aux rayons X.
La polymorphie smectique des 4"-n-alkanoyle-4-bromo-p-terphknyles a Cte etudih ; le compost heptanoyle donne seulement le troisibme exemple d'une transition directe de l'etat smectique A I'Ctat nkmatique.
Les quatre series de 4-(4'-cyanobenza1amino)cinnamates de 3"-chlor-, 3"-methyle, 4"-chlor- et 4"-methyle-w-phenylealkyle ont ete BtudiCes. Nous avons conclu que les alternations extrCmement fortes qu'on observe pour les transitions de l'etat nematique au liquide amorphe quand on monte dans les series s'expliquent par les changements de la distribution spatiale de I'anneau terminal du groupe ester, dktermines par la stereochimie de la chaine alkylbne.
Abstract. - The 4'-n-alkyl-4-cyanobiphenyls and 4'-n-alkoxy-4-cyanobiphenyls provide colour- less liquid crystal phases which are chemically and photochemically stable. In many cases, the meso- phases are produced a t quite low temperatures, and indeed two members of the n-alkyl series are nematic a t room temperature. Eutectic mixtures prepared from members of the two series give wide nematic ranges, e. g., from about - 2 OC to above 60 O C . All the compounds are of high positive dielectric anisotropy and either individually or in admixture their nematic phases perform well in twisted nematic displays. The nematic ranges have been further extended, without adversely affecting the stability characteristics or the electrical properties by incorporating certain members of the series of 4"-n-alkyl-4-cyano-p-terphenyls. For example, one eutectic mixture is nematic from 5 OC to 91 O C .
Two optically active analogues of the cyanobiphenyls and cyano-p-terphenyls have been prepared by using a branched chain as the alkyl or alkoxy group. Incorporation of these compounds in mixtures with the 4'-n-akyl- and 4'-rr-alkoxy-4-cyanobiphenyls gives long pitch cholesteric phases which exist in the room temperature range, and which are colourless and both chemically and photochemically stable. These phases perform well in cholesteric-nematic phase change devices.
These chiral solutes used in low concentration (1 %) also prevent areas of reversed twist in twisted nematic displays.
4'-n-octyl-4-cyanobiphenyl gives a room temperature smectic phase. This phase and those shown by other higher homologues of the two series have been shown by miscibility to be smectic A.
However, the smectic phases have unusual X-ray characteristics.
The smectic polymorphism of the 4"-n-alkanoyl-4-bromo-p-terphenyls has been studied ; the heptanoyl compound provides only the third known example of a direct Sn-N transition.
The four series of 4"-chloro-, 4"-methyl-, 3"-chloro- and 3"-methyl-w-phenylalkyl 4-(4'-cyano- benzy1ideneamino)cinnamates have been studied. It is concluded that the extremely large alterna- tions in the N-I transition temperatures as the series are ascended are explained in terms of changes in the spatial distribution of the terminal ring of the ester function which are determined by the stereochcmistry of the alkylene chain.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975157
'21-338 G. W. GRAY
1 . Introduction.
-
In this lecture I have decided not to give a review of the more general type relating to the recent literature on the synthesis of new mesogens, but to concentrate rather on three lines of work which we have been investigating in the last year or so. My reasons for doing this are simply that our own results are in themselves of general interest because they relate to three important areas of liquid crystal research- materials for electro-optic displays, smectic poly- morphism and molecular geometry of mesogens
-
and, as such, they would seem to constitute suitable subject matter for a plenary lecture of this kind.
2. Room-temperature liquid crystals derived from substituted biphenyls and p-terphenyls. - Some eigh- teen months ago, we began to investigate a range of 4,4'-disubstituted biphenyls ( I ) carrying suitable, but simple substituents X and Y, with the object of obtaining room temperature liquid crystals.
I
Some of the results have already been published in short communications 11, 2, 3, 4, 51 or have been presented at recent conferences 16, 71 whose proceed- ings have not been published a t the time of writing.
I felt it would be of use therefore to summarise the work in a coherent fashion and present some of the most recent, as yet unpublished findings.
Our reasons for selecting 4,4'-disubstituted biphe- nyls were simple :
( a ) From much earlier studies [ 8 ] , we knew that biphenyl derivatives have a strong tendency to form liquid crystals.
(b) Since the molecules contain no central group linking the two p-phenylene rings, such as the -CH=N- group in Schiff's bases like MBBA or the -N =N- group in azo-compounds, etc., the compounds should be chemically and photochemi- cally more stable.
(c) The less extended conjugated aromatic system, compared with that in ~chiff's bases, azo-compounds or azoxy-compounds should result in colourless or less coloured materials.
As the group Y we selected either a nitro-group or a cyano-group, because both strongly promote nematic properties, and because we were primarily interested in liquid crystals of strongly positive dielectric anisotropy for use in twisted nematic displays. Both groups are also quite stable.
As the group X we chose either the inert n-alkyl- or n-alkoxy-groups in order to lengthen the molecules.
Preliminary investigations quickly revealed that the nitro-compounds were unsatisfactory [ 6 ] . They are pale yellow and as the results in Table I show, the liquid crystal phases of the 4'-n-alkoxy-4-nitrobiphenyls are
of only short range or are monotropic. As a conse- quence, the 4'-n-alkyl-4-nitrobiphenyls would exhibit liquid crystal phases with even lower N-I transition temperatures, and these were not investigated.
C-S, N or I S-N N-I
X Temp. OC Temp. OC Temp. OC
- - - -
n - C 5 H 1 1 0 54.5 - (< 42)
n - C 6 H 1 3 0 67 - (32.5)
n-C7H 1 5 0 36.5 (30.5) 38.5
n-CsH 1 7 0 1491 51.5 49.5 51.5
C = crystal ; S
-
smectic ; N = nematic ; I-
isotropic liquid.Temperatures in round brackets are for monotropic transi- tions.
The temperature in square brackets is for a lower melting polymorphic solid.
It is convenient to mention here that later in the work [6] we investigated the replacement of the biphe- nyl by the fluorene ring system, which has sometimes been found [9] to give nematic phases of enhanced thermal stabilities. However, as shown by the results in Table I1 for two 7-substituted 2-cyanofluorenes, the results were disappointing, the mesogenic tendencies being very low or non-existent.
C- I N-I
X Temp. OC Temp. OC
- - -
n-C,H, 63.5 (- 23)
n-C,H,
3O
106.5 (< 8 2 ) ( * ) C = crystal ; N = nematic ; I = isotropic liquid.Temperatures in parenthesis are for monotropic transitions.
(*) Crystallisation occurred a t 82 OC - no mesophase being observed.
However, as shown by the data in Table 111, our choice of the 4'-n-alkyl- and 4'-n-alkoxy-4-cyano- biphenyls proved to be good.
In relation to the results in Table Il1;the following observations may usefully be made.
( a ) A11 the compounds are colourless.
(b) Of the two classes of compound listed, the 4'-n- alkyl-compounds have the lower melting points and N-I transition temperatures. The n-pentyl-compound provides a colourless, stable (see (d) below), single
THE LIQUID CRYSTAL PROPERTIES OF SOME NEW MESOGENS Cl-339
Transition temperatures and enthalpies of melting for 4'-n-alkyl and 4'-n-alkoxy-4-cyanobiphenyls
C-S, N or I S-N N-I (*)
X Temp. (OC) AH (kcal mol-
'
(* *) Temp. (OC) Temp. (OC)C = crystal ; N = nematic ; S = smectic ; I = isotropic liquid.
Temperatures in square brackets relate to the m. p. s. (C-S or N) for metastable solids so far identified. For the compound with X = CSHl ,O, the metastable solid apparently exists independently and does not revert even on long standing to the higher melting form.
Temperatures in round brackets are for monotropic N-I transitions.
(*) Enthalpies for the N-I transitions were typically in the range 0.1-0.3 kcal mol-'.
(**) Enthalpy values relate to the higher melting stable solids where dimorphism occurs.
component, room temperature nematic with an enan- tiotropic range of 12.5 OC and the ability to supercool in the nematic state for long periods down to about 4 OC. The n-octyl compound likewise provides a room temperature smectic liquid crystal which appears to be of the smectic A type, but with a difference, as men- tioned later.
(c) The 4'-n-alkoxy-compounds are higher melting, but give considerably higher N-I transition tempera- tures.
(6)
All the compounds are chemically and photo- themically stable. 4'-n-pentyl-4-cyanobiphenyl (5 CB for short), one of the first compounds to be produced, was particularly rigorously tested [I]. Samples of this material in the nematic state were exposed as thin uncovered films to moisture, light and air (laboratory atmosphere) and also to radiation from a mercury vapour lamp or fluorescent lighting ; the samples remained colourless, and the N-I temperature fell by less than 1 oC. Other samples were placed in test dis- play cells, crudely sealed with epoxy resins such as araldite or Torr seal and kept in an atmosphere of air saturated with water vapour at 40 OC for a period of one week. The 5 CB was unaffected, whereas with simi- larly treated cells containing MBBA, the contents were found to be isotropic at room temperature.The series of 4'-n-alkyl-4-cyanobiphenyls had there- fore made available both a room temperature smectic
and a room temperature nematic material, both of which were colourless, chemically stable, photo- chemically stable and suitable for a wide range of physical experimental studies.
We then established that the materials had suitable electrical properties for use in displays, the prelimi- nary studies again being made with 5 CB which is nematic at room temperature [2]. The dielectric aniso- tropy measured using aligned samples of the nematic phase was strongly positive (c,, = 17 ; cI= 6 ; E,=
+
11).Using 5 CB which had been purified only in the routine manner employed for all the materials in Table 111, i. e., by column chromatography and distillation under vacuum, resistivities of 1010-1011 Rcm at 100 Hz were obtained. Resistivity values greater by at least an order of magnitude are readily obtained however by using ion-exchange resins.
These electrical characteristics of 5 CB seemed suitable for its use in twisted nematic displays. The parallel alignment of the nematic director to the glass electrodes was readily achieved and good contrast ratios were obtained on switching 12 pm thick cells, using either a parallel or a perpendicular relative orien- tation of the polariser and analyser. The threshold voltage was 1.1 Vrms ; rise times of 100 ms a t 3 Vrms and 10 ms at 10 Vrms (10 kHz) were obtained. Decay times were longer (250 ms), although to the human eye, decay appeared shorter (ca. 100 ms). Response times
CI-340 G . W. GRAY
are of course intimately related to details of cell fabri- cation, and superior values to these could now be quoted.
Since the above mentioned properties of 5 CB relating to colour, stability and electrical characte- ristics are shared by the other conlpounds in Table 111, the next step was to obtain both lower melting points and higher N-I temperatures by preparing eutectic mixtures. In this way, the lower melting characteristics of the n-alkyl compounds and the higher N-I tempera- tures of the n-alkoxy compounds listed in Table 111 could be used to the maximum advantage. Prelimi- nary experiments with non-eutectic compositions showed that wide range mixtures could be obtained, e. g., C-N 0.5 OC ; N-I, 40 OC for a simple binary mixture [I].
To avoid the time consuming task of establishing eutectic compositions by experiment, the following equation due to Le Chatelier [lo], Schroder [ l l ] and van Laar [I21 was used
log, X , = AHA -
R
( i - 9
where X A is the mol. fraction of a component A having an enthalpy of fusion AHA and a melting point of T, in a mixture of melting point T. A simple procedure of approximations was used [3] to handle the N such equations for an N component mixture. This involved selecting a temperature T and evaluating the mol.
fractions using the data in Table 111. When the sum of the mol. fractions is unity, T is the eutectic melting point, and XA,
...,
X , define the eutectic composition.On average, about five successive approximations gave the predicted values for the eutectic mixture. N-I temperatures were estimated by making linear extra- polations of individual N-I values weighted by the eutectic mol. fraction.
In Table 1V are listed a few results [3] selected from those obtained from many mixtures prepared and examined experimentally. The predicted compositions and transition temperatures for the eutectic systems are recorded together with the observed transition tempe- ratures. Over many results, the agreement was always good, but the observed melting points were often slightly lower than the predicted values. The examples quoted represent about the maximum limits for the nematic range that can be obtained using mixtures of the biphenyl mesogens in Table 111, i. e., from a few degrees below 0 OC to just over 60 OC. All mixtures supercool strongly, and the nematic melts persist for long periods at - 20 OC or below. Often it is difficult to obtain eutectic mixtures solid in order to determine their melting temperatures.
The mixtures have the same colourless, stable characteristics as the pure mesogens in Table 111, and similar electrical characteristics ; they therefore make available wide range nematics that perform well in twisted nematic displays. Displays containing these materials are unlikely to crystallise under ordinary conditions of use or storage, although response times of course increase when the tempe- rature falls much below room temperature. Life tests conducted on such cells have now been conducted for about 7 000 hours without failure, the cells being switched at 1 second intervals with bursts of sine wave AC.
To further improve the nematic ranges of the mix- tures, without adding materials which would detract from the excellent stability characteristics, we syn- thesised a number of 4"-n-alkyl-4-cyano-p-terphe- nyls [5, 71. The transition temperatures for these aro- matic homologues of the biphenyls are given in Table V ; the smectic phases were classified by optical microscopy and miscibility studies.
Data for some predicted eutectic mixtures o f 4'-substituted Ccyanobiphenyls and the observed transition temperatures
Composition Predicted Observed
(mol
%>
C-N (OC) N-I (OC) C-N (OC) N-I (OC)THE LIQUID CRYSTAL PROPERTIES OF S O M E NEW MESOGENS
C-N or S ,
X Temp. OC
-
-n-C3H7 182
n - C ~ H ~ 1 130 n - C 6 H ~ , 125
n-C7H~5 134
n-CsH1 7 127
Transition temperatures for 4"-n-alkyl-4-cyano-p-terphenyls
SE-SB SB-SA
Temp. OC Temp. OC
SA-N Temp. OC
N-I Temp. OC
- 257.5 239 228 222 216 C = crystal ; S = smectic ; N = nematic ; I = isotropic liquid.
The temperature in round brackets is for a monotropic transition.
Although the melting points of these colourless, stable mesogens are high, their enthalpies of melting are quite low, even when allowance is made for the complex solid polymorphism exhibited by some of the compounds, e. g., the total enthalpy of melting for 4"-n-pentyl-4-cyano-p-terphenyl is 4.06 kcal moI-', including the enthalpy for a crystal-crystal change.
Using the same procedure as that for the biphenyl mixtures [3], eutectic compositions were calculated for mixtures of the biphenyls and p-terphenyls in Tables 111 and V. Table VI gives results for three eutectic mix- tures incorporating one of these terphenyl mesogens, 4"-n-pentyl-4-cyano-p-terphenyl.
The second mixture (I) shows that a nematic range comparable with the best obtained with a quinary biphenyl mixture can be achieved with a simpler ter-
( I ) It is noted that the second mixture quoted in Table I1 of
reference [S], does not in fact represent a eutectic composition.
nary mixture, and the third mixture shows that qui- nary compositions can give very wide ranges. This last mixture has never been obtained crystalline. Although the other two mixtures in Table VI solidify slowly on cooling to
-
20 OC or below, the third mixture sets to a clear glass, which has become a freely mobile nematic fluid by 5 OC.It should be noted that, because of the lower solu- bilities of the 4-n-alkyl4cyano-p-terphenyls, any excess of the terphenyl component(s) over the eutectic amount will segregate. Calculated eutectic composi- tions must be closely adhered t o in practice. Segrega- tion is not a problem with purely biphenyl mixtures, even for compositions well off the eutectic compo- sitions.
These mixtures involving terphenyl additives again perform well in twisted nematic displays and appear to provide the wider ranges of temperature desirable for some display applications.
Data for some predicted eutectic mixtures of 4"-n-pentyl-4-cyano-p-terphenyl (5 and 4'-substituted (X) 4-cyanobiphenyls and the observed transition temperatures
Mixture Composition Predicted Observed
( ~ 1 5 CT) (mol
%I
C-N (OC) N-I (OC) C-N (OC) N-I (OC)C = crystal ; N = nematic ; 1 = isotropic liquid.
C 1-342 G . W. GRAY It should be noted that full experimental details of
the methods of preparing both the 4'-n-alkyl- and -alkoxy-4-cyanobiphenyls and the 4-n-alkyl-4-cyano- p-terphenyls are in the course of publication [6, 71. All compounds given in Tables 111 and V were purified by column chromatography on silicic acid and distilled or sublimed under vacuum. The products (single spot by t. 1. c.) had a purity not less than 99.6
%
by g. 1. c., gave the correct molecular weight by mass spectro- metry and gave satisfactory microanalytical data. A range of the alkyl- and alkoxy-cyanobiphenyls and their mixtures is now commercially available.T o widen the sphere of usefulness of the materials under discussion in other forms of electro-optic dis- play, we investigated the incorporation of optically active solutes to obtain long pitch cholesteric mixtures, stable at room temperature, which could be used for cholesteric-nematic phase change displays. Prelimi- nary experiments using 10
%
of cholesteryl chloride in 5 CB gave a long pitch cholesteric of positive dielec- tric anisotropy which gave satisfactory performance-
a film 6-7 Fm thick gave completion of the phase change at 2.8 Vrms (10 kHz sine wave signal) with a rise time of 100 ms and a decay time of 20 ms.To avoid the use of labile chiral solutes in the stable nematic mixtures, we prepared the two optically active compounds given in Table VII. Since both are structu- rally similar to the mesogens in Tables 111 and V, their use as optically active solutes detracts from neither the stabilities of the mixtures nor their electrical characte- ristics. The compounds were prepared by the methods used for the mesogens in Tables I11 or V. In the case of the terphenyl compound, commercially availabe (-)- 2-methylbutan-1-01 was converted into (-)-2-methyl-
buty I bromide which was then interacted in the usual way with diethyl malonate and the resulting ester
hydrolysed. The acid was converted into (-)-4-methyl- hexanoyl chloride which was used in the Friedel Crafts reaction on 4-bromo-p-terphenyl. Reduction of the carbonyl group and replacement of bron~o- by cyano- gave (-)-4-(4"'-methylhexy1)-4-cyano-p-terphenyl.
As can be seen from Table VII, compound I1 is not a mesogen, although it has latent mesomorphic tenden- cies. Compound 111 gives an enantiotropic cholesteric phase which reflects coloured light. Several mixtures of compounds I1 or 111 with materials from Table I11 were prepared. The twisting power of compound I1 is less than that of compound Ill, and to obtain a suitable pitch length, 40 mol
%
of compound 11 was used in mixture I in Table VIlI ; this mixture is not therefore a eutectic mixture, but segregation does not occur.Suitable pitch lengths to achieve the phase change effect were obtained using the eutectic compositions of compound IT1 and other materials from Table III- mixtures 2 and 3 in Table VIII. The total enthalpy of fusion of compound 111 is 5.1 kcal mol-', including solid-solid changes.
All the mixtures in Table VlZI crystallised only with great difficulty, remaining cholesteric at temperatures as low as - 55 OC for long periods and crystallising only when seeded. The mixtures performed well in phase change displays ; their electrical characteristics were similar, e. g., for mixture I, the phase change was complete a t 26 Vrms for a 50 pm thick layer (5.2 x lo5 Vm-').
Finally on this topic, it is noted that these stable, colourless optically active solutes are also of value for addition in low concentration (I %) in order to bestow a pitch of about 100 pm on a nematic which is required for twisted nematic displays. This prevents the forma- tion in the display cells of areas of reverse twist which, if present, can give the films a patchy appearance.
TABLE VII
Transition temperatures of two optically active compourzds (11) and (111)
C-T, 53.5 OC ; I-C, 26 OC (No mesophase observed)
Virtual Ch-I, 9
+
3 OC - by extrapolation of transition temperatures for mixtures [4]C-S2, 120 OC ; S2-S1, 137 OC ; S,-Ch, 163 OC ; Ch-I, 186 OC
Phases S, and S, not yet confirmed in type ; S, is probably S,
THE LIQUID CRYSTAL PROPERTIES OF SOME NEW MESOGENS C 1 -343
TABLE VIII
Mixtures of compound (11) or (111) (Table VII) with 4'-substituted (X) 4-cyanobiplzenyls (Table 111) and relevant data
Mixture Composition Predicted Observed Pitch
(XI11 or 111) (mol
%)
C-Ch Ch-I ("C) C-Ch Ch-I (OC) ( ~ m )3. New smectogens. - As noted earlier, 4'-n-octyl-4-cyanobiphenyl (8 CB) is a room tempe- rature smectic material, and two of the other mesogens in Table I11 also exhibit smectic phases. Preliminary studies of these phases showed that they had a very marked tendency to be homeotropic and had positive uniaxial properties. Using 8 CB, we carried out some miscibility studies which appeared to show that the smectic phase was immiscible with SA, SB or SC phases ; however, it was noted [6, 71 that standard materials with comparable transition temperatures with 8 CB were not available, and the validity of the conclusions was doubtful.
Later we prepared two of the higher homologues, as shown in Table IX, which also includes the transition temperatures for 8 CB for comparison.
ca. 2
As it seemed unlikely that the S phases of the lower homologues of the 4'-n-alkyl- and 4'-n-alkoxy-4-cyano- biphenyls were different in type from the dodecyl homologues, we repeated miscibility studies on 8 CB.
Using the contact method and D. T. A. methods, we have now shown that 8 CB has a S phase which is separately miscible with the known S, phases of compound IV, the 4'-n-dodecyl-compound and the 4'-n-dodecyloxy-compound. Also, the S phases of all three smectic compounds in Table 111 are co-miscible.
Independent verification of these results has been obtained by Professor J. Billard [I41 who showed that the S phases of 8 CB and of a standard S, compound (V) were completely miscible.
TABLE IX Me2CHCH2CH2CH2 w c 4 c H 2 ) 7 M e
Transition temperatures for
!I
some 4'-substituted (X) 4-cyanobiphenyls 0
v
C-S S-N S or N-I
The smectic phases of the two dodecyl compounds in Table IX gave typical focal-conic or fan textures and had a much smaller tendency to be homeotropic.
The smectic textures appeared typical of S, phases and both smectic phases were readily shown to be separa- tely miscible in all proportions with the known smec- tic A phase of compound IV [13].
On the basis of miscibility therefore, the S phases of the alkyl- and alkoxy-cyanobiphenyls are S, in type.
However, as already reported briefly [6, 7, 151, X-ray investigations of a preliminary kind have shown that the diffraction patterns of these S phases differ from those of ordinary SA phases in that the former have a second diffuse outer ring at 8.6
A.
Moreover, the layer spacing is intermediate between one and two molecular lengths, implying an interdigitated bilayer arrange- ment. It is too early to be certain of the situation, but it appears from the X-ray data that we are dealing with a new kind of smectic phase or, if the results of classifi- cation by miscibility are accepted, a special kind of S, phase. In either case, these results show that the infor- mation obtained from miscibility experiments should be treated with some caution ; such experiments would seem to provide a guide to similarity of smectic type,G . W. GRAY
Transition temperatures for the compounds 4"-R . CO
.
C6H4. C6H4. C6H4. Br-4C-S, or N SE-SB SB-SA S, or SB-N
R ("C) ("C) ( " 0 PC)
N-I ("C)
C = crystal ; S,, S,, S, = smectic A, B, E ; N = nematic ; I = isotropic liquid.
All snlectic phases were classified by optical microscopy and miscibility.
but not a firm criterion of structural identity of the phases.
In relation to smectic polymorphism, it is also of interest to record the behaviour of the intermediate ketones, the 4-n-alkanoyl-4-bromo-p-terphenyls, pre- pared during the synthesis of the 4"-n-alkyl-4-cyano-p- terphenyls. The results for these mesogens are given in Table X.
The interesting feature of these results is the injection in the homologous series of a n additional high tempe- rature S, phase in the last member so far studied. The injection of additional high temperature S phases in homologous series is not unique [16], but is not common. This comment also applies to the compound with R = n-C6Hl, which shows a direct SB-N transi- tion. Hitherto, only two compounds exhibiting such transitions 1211 have been recorded ; in both cases,
rather more complex molecules are involved. The n-hexyl ketone in Table X therefore probably provides the most readily accessible example of such a direct S,-N transition.
At the N-S, transition, a change from a largely homeotropic nematic to a schlieren S, occurs (Plate 1) ; the SB phase appears to be positive biaxial and pre- sumably involves a tilted molecular arrangement. The
SE
phase appears very suddenly, on further cooling, as a poorly defined mosaic texture.The classification of the phases was carried out by miscibility studies using the following standard mate- rials :
(1) n-propyl 4
-
(4' - phenylbenzy1ideneamino)cinna- mate 1171S E + S B + S A + N 4 1 TABLE XI
Transition temperatures for the substituted phenyl and o-phenylalkyl4-(4'-cyanobenzylideneamino)cinnamates.
n Substitution
L -
0 Unsubstituted 4"-CI
4"-Me 3"-Cl 3"-Me 1 Unsubstituted
4-C1 4"-Me 3"-Cl 3"-Me
C-N or I (OC) - 168.9 193 165 166 162.6 160.5 146.8 156 131 127.1
N-I ("C) n Substitution
-
- 2 Unsubstituted4-C1
@-Me 3"-CI 3"-Me 3 Unsubstituted
4"-C1 4"-Me 3"-C1 3"-Me
C-N or I (OC) 146 177.5 178.2 142.4 116
95 96 106.2
93.7 91
N-I ("C)
Monotropic transition temperatures are in parenthesis.
(*) No phase actually observed : C = crystal ; N = nematic ; I = isotropic liquid.
THE LIQUID CRYSTAL PROPERTIES OF SOME NEW MESOGENS C1-345
PLATE 1. -The transition N-SB on cooling. The schlieren texture of the smectic B phasc is seen forming in the top right hand corner of the picture and invading the homeotropicnematic texture (optically extinct). As the temperature falls further, the whole field of view adopts the schlieren Sn texture, including the area in the lower left hand comer of the picture whichis showing a birefringent nematic texture. Crossed polarisers (magnification
X 120).
(2) methyl 4-(4'-phenylbenzy1ideneamino)cinna- mate [17]
(3) 4"-chlorobenzyl 4-(4'-phenylbenzylideneamino) cinnamate [I 81
(S,)
*
-t S,+ I*
monotropicIt will be noted that the last of these three standard materials involves a unique S,-I transition which is the subject of a separate paper [18] presented at this meeting.
Using compounds 1 or 3 with the n-hexyl ketone from Table X, the S , phases gave a broad region of co-miscibility extending fully across the phase dia- gram. With compounds 1 or 2, the range of the S, phase narrowed progressively on approaching 100
%
of the n-hexyl ketone. For example, at a concentration of 98
%
of the ketone and 2%
of standard compound 2, a S, phase of only 1 OC range was detectableand the S,-N temperature was 1 OC below that of the S,-N transition of the ketone, i. e., the S, phase is comple- tely suppressed in the ketone, and the S, phase changes directly to the N phase. The S, phase of the ketone was separately miscible in all proportions with the S, phases of standard compounds 1 and 3.4. Phenyl and o-phenylalkyl esters of 4-(4'-cyano- benzylideneamino)cinnamic acid. - Some time ago, we reported [19] on the mesomorphic properties of the phenyl and o-phenylalkyl 4-(4-substituted benzyli- deneamin0)cinnamates (VI).
//-<cH = CH = CH-CO
.
O(CH,),x-, \d"
With X=CN, the compounds having n = 0-4 were purely nematic and the series exhibited an enormously high alternation of the nematic-isotropic liquid (N-I) transition temperatures as n changed from even to odd.
The N-I temperatures were as follows : n = 0,280.2 OC ; n = 1 (38.5 OC) ; n = 2, 187.7 O C ; n = 3 (36.4 OC) ; n = 4,126 OC, the nematic phases changing from enan- tiotropic to monotropic (values in parenthesis) as n changed its value from even to odd.
This behaviour was attributed to the possibility that the terminal ring of the ester function moves in and out of line with the long axis of the rest of the molecule as the axis of propagation of the alkylene chain changes its direction on passing from even to odd values of n.
The effect is illustrated in figure 1.
Thus, the molecules of esters with even n values are linear, whereas those of esters with odd n values deviate appreciably from linearity and are somewhat L-shaped. The alternation does diminish in magnitude as n increases, and this may be consistent with a greater ability of longer alkylene chains to bend into line with the molecular long axis. Nonetheless, the alternation remains very large, compared with that in other series, e. g., in homologous series of n-alkyl ethers, etc., even
FIG. 1.
-
Diagrammatic representation of the structures of substituted and unsubstituted phcnyl and benzyl esters of 4-(4'-cyanobenzylideneamino)cinnamic acid. The upper struc- ture applies to higher homologues with even values of n (see structure VI in text) and the lower structure to higher homologues with odd values of n. Theaxis lines represent themajor axis of the entire elongated molecule (even values of n) or of the majorelongated part of the molecule (odd values of n).
when n has values of 3 and 4, and this suggests that although the molecules as whole units may be rotating around their axes in the nematic phase, individual covalent. bonds within the molecules attempt to main- tain their stable stereochemical arrangements.
It seemed important to verify this on-axis : ojf-axis hypothesis, particularly as a similar behaviour was found independently [20] for the cholesteric-isotropic transition temperatures of some steryl o-phenyl- alkanoates.
We have done this by preparing the esters containing 4"-chloro, 4"-methyl, 3"-chloro or 3"-methyl substi- tuents in the ester phenyl ring. Transition temperatures for the sixteen new esters, together with those of the unsubstituted esters [19] are given in Table XI. The methods of preparation and purification of these esters were identical to those described elsewhere [18] for related esters in which the substituent in the benzyli- dene ring was phenyl rather than cyano. All inter- mediates in the syntheses were checked for purity by appropriate methods, and the final products listed in Table XI gave satisfactory elemental analytical data.
The spatial positions which the chloro and methyl substituents occupy are shown in figure 1 by the num- bers 3" and 4 . It should be noted that if rotation about the O-ring or CH2-ring bonds is possible, two extreme positions arise for 3"-substituents (for both even and odd values of rr) when the ring is co-planar with the rest of the molecule.
An analysis of the transition temperatures for these substituted phenyl and o-phenylalkyl esters (Table XI), with reference to figure 1, leads to certain conclusions :
Relative to the ester having the same value of n, but no substituent in the ester ring :
(1) When n is even (0 or 2), a 4"-Me or 4-C1 substi- tuent enhances the N-I temperature, consistent with the lath-shaped molecule being lengthened by the terminal 4-substituent ;
(2) When n is odd (1 or 3), a 4 - M e or 4-C1 substi- tuent diminishes the N-1 temperature, consistent with the molecular broadening effect of the 4"-substituent because of the 08-axis position which the ester ring occupies (here in efiect we have an apparent terminal substituent broadening the molecule) ;
(3) When n is even (0 or 2), a 3"-Me or 3"-C1 substi- tucnt lowers the N-I temperature, consistent with the broadening of the lath-shaped nlolecules which will occur irrespective of which of the extreme spatial posi- tions the 3"-substituent occupies, see figure I , n = even values ;
(4) When n is odd (1 or 3), a 3"-Me or 3"-C1 substi- tuent diminishes the N-I temperature in three cases and increases it in one case (see Table XI). As can be seen from figure 1, n = odd values, a 3"-substituent will not have the same effect if it occupies one or other of the two extreme positions indicated. In one position, the molecule is effectively lengthened and in the other it is broadened. The fact that three decreases and one increase in N-1 temperatures are observed suggests that the amounts of different rotational conformers can differ with substituent type and alkylene chain length. Thus for 3"-Cl and n = 1, the 3"-position in space which effectively lengthens the molecule appears to be favoured. Since a consistent effect for 3"-Me and
3"-C1 substituents, independent of whether n = 1 or 3 is not observed, it does not appear that continuous free rotation about the CH2-ring bond occurs in the nematic phase. From conclusions (1) and (2) above, we see that 4"-substituents enhance the on-axis : of-axis situation and the result is an even higher alternation in the N-I temperatures than was originally observ- ed [19] for the unsubstituted esters. For example, as shown in figure 2 for the 4"-chloro series, the N-I temperature decreases from 338 OC for n = 0 to ( <
-
30 OC) for n = 1 and back to 217.8 OC for n = 2.n
FIG. 2. - Plot of the transition temperatures, C-N or l ( 0 ) and N-I (A) for the compounds
4'-NC . CsH4. CH : N . CcjH4. CH :
CH
.
CO. O ( C H ~ ) ~ C S H ~ . C1-4D (see Table XI) against n, the nu~nber of methylene units in the alkylene chain. Notc the enormously high alternation of theN-I tempcratures.
Therefore the results lend strong support to the original explanation of the high alternation in terms of the spatial disposition of the terminal ester ring as determined by the stereochemistry of thealkylenechain.
The results also serve to warn against assessments of the mesogenic potential of a system based on a casual inspection of molecular structure. Substituents which appear to be terminal using linear molecular formulae may not be so considered when proper account is taken of the stereochemical situation. Likewise, substituents which might be judged to be lateral may in their effects act as terminal groups.
5. Experimental note.
-
All transition ternpera- tures reported in the tables and text of this lecture were measured using either a Mettler FP 52 microscope hot stage and control unit or, in the case of transition temperatures below ca 25 OC, a Kofler microscope cold stage. All recorded transitions were confirmed by differential thermal analysis using a Stanton Redcroft low temperature thermal analyser (model 671B).THE LIQUID CRYSTAL PROPERTIES OF SOME NEW MESOGENS Cl-347 6. Acknowledgments. - The author wishes to
acknowledge the essential experimental contributions of his colleagues Dr. K. J. Harrison, Dr. J. A . Nash and Dr. D. Coates to the results presented in this lec- ture. The work on the new biphenyl and p-terphenyl mesogens was conducted under a research contract from the Ministry of Defence and the relevant results are published by permission of the Director of the
Royal Radar Establishment. The author wishes to thank his colleagues at the Royal Radar Establishment, particularly Dr. J. Kirton and Dr. E. P. Raynes, for their co-operation and experimental work connected with electrical measurements on the new materials.
A maintenance grant from the Science Research Council to D. Coates is acknowledged with grati- tude.
References
[I] GRAY, G. W., HARRISON, K . J. and NASH, J. A . , Electron.
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[2] ASHFORD, A., CONSTANT, J., KIRTON, J. and RAYNES, E. P., Electron. Lett. 9 (1 973) 1 18.
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[I01 LE CHATELIER, H., C . R. Hebd. Sean. Acad. Sci. 100 (1885) 50.
[I 1] SCHRODER, I., Z. Phys. Chem. 11 (1893) 449.
[I21 VAN LAAR, J. J., Arch. ngerl. 118 (1903) 264.
[13] BERING, A., DEMUS, D., GRAY, G. W. and SACKMANN, H., Mol. Cryst. Liqu. Cryst. (1974), in press.
[I41 BILLARD, J., private communication.
[IS] GRAY, G. W. and LYWN, J. E., Nature 252 (1974) 221.
[16] DEMUS, D., KUNICKE, G., NRELSON, J. and SACKMANN, H., 2. Naturforsch. 23a (1968) 84.
[I71 COATES, D., HARRISON, K. J. and GRAY, G. W., Ma[. Cryst.
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[I81 COATES, D. and GKAY, G. W., paper presented at the Fifth International Liquid Crystal Conference, Stockholm, June 1974.
[I91 GRAY, G. W. and HARRISON, K. J., Symposium of the Fara- day Sac. 5 (1971) 54.
[20] ENNULAT, R. D. and BROWN, A. J., Mol. Cryst. Liqu. Cryst.
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