Textures and structure of the low temperature liquid crystal phases of HOBACPC [(R-) hexyloxybenzylidene p'-amino-2-chloropropyl cinnamate]

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Textures and structure of the low temperature liquid crystal phases of HOBACPC [(R-) hexyloxybenzylidene

p’-amino-2-chloropropyl cinnamate]

P.E. Cladis, H.R. Brand, P. Keller, P.L. Finn

To cite this version:

P.E. Cladis, H.R. Brand, P. Keller, P.L. Finn. Textures and structure of the low temperature liquid

crystal phases of HOBACPC [(R-) hexyloxybenzylidene p’-amino-2-chloropropyl cinnamate]. Journal

de Physique, 1985, 46 (12), pp.2151-2160. �10.1051/jphys:0198500460120215100�. �jpa-00210164�

2151

Textures and structure

of the low temperature liquid crystal phases ^{of HOBACPC} [(R-) hexyloxybenzylidene p’-amino-2-chloropropyl ^cinnamate]

P. E. Cladis (+), ^{H. R.} Brand (+ +), ^P. Keller(++ +) ^{and P. L.} Finn (+)

(+) AT & T Bell Laboratories, Murray Hill, ^N.J. 07974, ^U.S.A.

(+ +) AT & T Bell Laboratories, Murray Hill, ^N.J. 07974, U.S.A., ^and ESPCI, 10, ^rue Vauquelin, ^F75231 Paris,

France

(+++) Laboratoire L. Brillouin, ^C.E.N. Saclay, ^F91191 Gif-sur-Yvette, ^France

(Reçu le 29 octobre 1984, ^{révisé le} 23 juillet ^1985, accepté ^le 24 juillet 1985)

Résumé.

HOBACPC possède ³ phases héliélectriques déjà identifiées sous les notations C*, ^{I* et} (G’)* ^{ou J*.}

Dans ^cette étude, ^nous proposons qu’il ^existe également, ^à plus ^basse température, ^une phase qui ^est intrinsèque-

ment ferroélectrique.

La polarisation spontanée ^des phases héliélectriques ^est moyennée à zéro par leur ^structure hélicoïdale. Dans des échantillons massifs, ^{elles ne sont} habituellement pas bistables (par bistable, ^nous indiquons que la configu-

ration induite _{par le} champ ^subsiste lorsque ^le champ ^est annulé), c’est-à-dire que la ^structure hélicoïdale se réins- talle quand ^le champ électrique ^{E est} annulé. Au contraire, les corrélations à longue portée ^{entre couches} suppri-

ment la structure hélicoïdale dans le smectique X, qui ^{est de ce} fait bistable. Nous soutenons que le passage d’un comportement héliélectrique ^ou ferroélectrique ^a lieu dans la phase (G’), puisqu’elle ^est caractérisée par des ^cor- rélations entre couches de portée ^finie produisant ^{une structure} hélicoïdale non uniforme.

Comme plusieurs phases smectiques penchées ^ont été découvertes récemment avec de nombreux changements dans la nomenclature, nous avons trouvé utile de rassembler dans un appendice ^notre description actuelle des caractères structuraux de ces phases, ^en particulier ^{dans le cas} de HOBACPC.

Abstract

HOBACPC is known to exhibit three helielectric phases identified as C*, ^{I* and} (G’)* ^{or J*. Here}

we present evidence that a truly ferroelectric phase, X, ^{exists at lower} temperatures than these phases.

Owing ^to their helicoidal structure, ^the spontaneous polarization of helielectric phases globally ^{averages to}

zero. In bulk samples they ^{are not} usually ^bistable (by ^{bistable we mean that the} field induced configuration ^remains

when the field is removed) ^so that when the field E is turned off, the helicoidal structure returns. In contrast, long

range inter-layer correlations suppress the helicoidal ^structure in smectic X and it is bistable. We _argue that the

cross-over from helielectric to ferroelectric behaviour takes place ^{in the} (G’)* phase since it is characterized by

finite inter-layer correlations resulting ^{in a} non-uniform helicoidal structure.

Because many tilted smectic phases have been discovered recently and there have been numerous changes ⁱⁿ nomenclature, ^we find it useful to review our current understanding of the structural features of these phases ^as they ^may apply ^{to HOBACPC in an} appendix.

J. Physique ⁴⁶ (1985) ^{2151-2160 DTCEMBPE} 1985,

Classification

Physics ^Abstracts

61. 30G - 64. 70M - 77 . 80

1. Introduction.

Chiral smectic liquid crystal phases ^have recently [1]

attracted considerable attention mainly because of their potential usefulness as electro optic ^devices.

Here, we emphasize ^the opportunity they provide ^to study ^the effect of chirality ^{on the} large variety ^of spatial ordering ^exhibited by ^the many tilted smectic

phases ^formed by chiral molecules. In an appendix

we review the structural features of those phases

relevant to HOBACPC (see Fig. 1) ^the compound ^we

chose to study. ^{We chose} HOBACPC because it exhibits many novel ^textures in the polarizing ^micro-

scope illustrating ^the competition ^between increasing positional ^{order and} chirality.

A point symmetry argument [2] ^is that the lack of reflection symmetry ^demanded by chirality, ^{results in}

the appearance of a polarization, P, perpendicular ^to

the plane spanned by ^the layer ^normal and the direc- tor, n, the preferred direction for molecular alignment.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198500460120215100

This symmetry argument ^cannot predict ^{that the com-}

bination of chirality ^{and tilt creates a} helicoidal struc- ture where the director rotates uniformly ^{about the} layer normal with a pitch typically ^{2-4 gm. The} pola-

rization is rigidly coupled ^to the director so that, ⁱⁿ

the absence of an applied ^{field, there is no net} pola-

rization. A linear coupling ^{to external electric} fields, E, of the form E P results in the growth ^of regions

where P is parallel ^{to E at the} expense of regions

where P is not parallel ^{to E} [3, 4]. ^{The two states, P and}

-

P, correspond ^{to two} different orientations of ⁿ

separated by ^an angle-of 2 ^0, where 0 is the tilt angle.

This electrooptic response is the basis for devices

using these materials [5]. Large ^{fields are needed to}

erase all trace of regions ^{where P is} antiparallel ^{to E} consequently ^a threshold is not associated with the

switching process [4]. ^These phases ^are commonly

known as ferroelectric [1] ] ^{but we have} suggested

helielectric [6] ^{as a more} appropriate ^{name since that}

describes their zero field ground ^{state and} distinguishes

them from a recently discovered [7, 8] truly ^ferroelec-

tric liquid crystal phase called smectic X. In smectic X, the in-plane translational order is rectangular, ^to

ensure an easy axis for the tilt direction [6], ^but long

range inter-layer correlations _suppress the helix

resulting ⁱⁿ ferroelectricity.

Despite ^the 3-dimensional long range positional

order of this phase (~), ^a ferroelectric _response was

observed for voltages, ^V, of ~ 20-100 V/12 gm.

The _response time, ^i, is several seconds at temperatures well below the transition to a helielectric phase ^but

decreases near that transition. V In T was found to scale linearly ^with temperature difference ^{from the} X-G’* transition temperature, ^{T -} TX _ G-,, ^extrapo- lating ^{to zero at T =} T X-G’.. ^Unlike its helielectric

analogues, ^bulk samples of smectic X are bistable.

Fig. 1. - The molecular structure of HOBACPC [(R-) hexyloxybenzylidene p’-amino-2-chloropropyl cinnamate].

Fully extended HOBACPC is 31 A long.

In this _paper, our objective ^{is to establish the iden-} tity of smectic X in HOBACPC. To do this, ^{our stra-} tegy ^{is to} show that its textures and electrooptic ^res-

ponse ^are different from crystal ^and helielectric phases.

In our scenario, supported by observations in the

(~) ^Molecular crystals, ^as opposed ^{to atomic} crystals,

have many internal degrees ^of freedom thus although posi-

tional ordering ^is long-range, ^not all rotational degrees ^of

freedom have condensed.

polarizing microscope, ^{the crossover} from helielectric to ferroelectric behaviour takes place ^{in the} (G’)*

(recently ^renamed [9] ^the J*) phase. ^It is in this phase

that inter-layer correlations build _up to become long

range ^at the transition to smectic X.

In the next section we present ^our observations and in a final section, ^our conclusions.

2. Observations.

HOBACPC is an interesting compound because it has one of the largest polarizations ^{known to date}

for helielectric and ferroelectric liquid crystals ^{as well}

as many smectic phases. ^It has been studied by ^several groups [10-14] ^{since it was first} synthesized ⁱⁿ 1976[15].

Three helielectric phases ^have been identified as C*,

I* and (G’)* ^{or J*. Here we} present evidence that a

truly ferroelectric phase, X, ^{exists at lower} tempera-

tures than these phases.

Z .1 DIFFERENTIAL SCANNING CALORIMETRY.

Figures 2(a) ^and (b) ^{show both} cooling ^and heating

differential scanning calorimetry measurements of chiral HOBACPC and racemic HOBACPC. Although

the heating ^scan for chiral HOBACPC is well known

[12, 13], the data for racemic HOBACPC as well as

the cooling ^{curves have not been} published previously.

Table I summarizes the magnitudes of the heats and temperatures of transition. The following points

emerge from this data :

1. A remarkable supercooling ^is observed for the transition to the solid state : ~ 40 °C for the chiral

species ^{and more} than 50 °C for the racemate. Apart

from crystallization, ^{there is} little difference between the heats of transition and transition temperatures ^of chiral and racemic HOBACPC.

2. A substantial heat of transition of ~ 1.5 cal/g

is associated with the onset of 2-d BOO (bond ^orien-

tational order, ^see appendix) ^{or 2-d} crystallization ^at

the C-I and C*-I* transitions.

3. Although chiral HOBACPC shows a small

peak ^{at the} I*-(G’)* transition, nothing ^{is observed}

at the corresponding transition of the racemate con-

sistent with a second order character of the onset of

inter-layer correlations.

4. There is no latent heat associated with the (G’)*

to X transition or the analogous transition of the race- mate. This shows that (G’)* ^doesn’t supercool ^{and is}

consistent with finite inter-layer correlations in this

phase. When these become long-range, ^{a new} phase, X, necessarily ^results.

In contrast to crystal phases, ^thick freely suspended

films of smectic X are stable. Figure ^{3 shows the texture}

of such a film observed in the X phase ^{with a} polarizing microscope ^{and for} comparison ^the very different crystal ^{texture of the same} film. The rate at which a

crystalline film breaks is directly ^{related to its rate of}

2153

Fig. ^2.

a) Differential scanning calorimetric traces of 9.64 mg of chiral HOBACPC on heating (top) ^and cooling (bottom) ^and b) ^5.9 mg racemic HOBACPC.

Table I.

Transition temperatures and ^heats of ^tran-

sition for ^both species of HOBACPC. The ^star refers

to the phases of the ^chiral species which have a helicoidal structure. The positive ^heats of transition are asso-

ciated with data taken in heating ^{and the} negative ^heats of transition with data taken on cooling. ^The scanning

rate was 5 OC/min ^{in both cases.}

crystallization. By holding ^{the film at 36.3 °C over-} night, crystallization ^{set in} slowly and the film broke

slowly (seen ^on the left of the figure).

2.2 X-RAY DIFFRACTION.

Doucet et al. [12] ^deduced

the in-plane ^structure of the low temperature liquid crystal phases of the chiral species of HOBACPC by indexing ^{a wide} angle powder pattern. ^Their samples

were oriented in an electric field at high temperatures then cooled to about 50 °C. They found, ^for phases

below I*, that the director tilted towards the short side of the rectangular in-plane lattice, identifying ^the

first smectic (G’)* phase (see appendix for details of this structure).

X-ray diffraction photographs of the I* phase ^of

HOBACPC taken in an electric field, ^{show 6 crescent} shaped spots. ^{These are} equally spaced ^{on a} plane perpendicular ^{to the director} (2). ^{Doucet et al.} [12]

report ^{that on} cooling through ^the phase ^we identify

as (G’)* ^{to the} phase ^we identify ^as X, ^the spots ^shar- pen and remain when the field is switched off. Addi- tional wide angle spots appear in X, doubling ^the

reflections perpendicular ^{to the} applied field, ^{E. This}

is strong evidence that the in-plane ^{structure of the} (2) Recently, ^{J. W.} Goodby, ^{J. S.} Patel and T. M. Leslie in Ferroelectrics 59 (1984) ^{121 made a} simple ^{error in inter-} preting ^{these spots to mean that the} in-plane ^{lattice of}

HOBACPC is hexagonal, instead of rectangular.

Fig. ^3.

^{X at 43.6 °C} (left). ^A thick film in the (X) phase is stable for long ^times providing additional evidence for the liquid crystallinity ^{of X.} Crystal ^{at 36.3 °C} (right). ^The film is in the process of breaking (seen in the lower left hand corner of the

picture).

X phase of HOBACPC is similar to H’ or K [9] ^where

there is a doubling of the number of molecules _per unit cell compared ^{to the G’} phase (see appendix ^for

a discussion of the different liquid crystal phases

relevant to HOBACPC).

Although ^its powder pattern ^{was identical to that} of the chiral species, ^{the racemate did not show} spots

even in fields as large ^{as 1.3 x 104} V/cm (presumably applied ^{in the C} phase then cooled to G’). ^Their suggestion ^{was that the} sample ^had separated ^into

small left and right handed domains.

Recently, direct observation in the optical ^micro-

scope of the low temperature phases ^{of 8 SI*} (S-( +)- (4-2’-methylbutyl)phenyl 4’-n-octylbiphenyl ^{4’ car-} boxylate) ^{in an} electric field [8] ^{showed a remarkable} separation ^into nearly periodic ^{sets of} planes, ^each

set composed of molecules with the same chirality.

The conclusion is that solidification drives the phase separation of different chiral species because in a given layer, ^the polarization associated with one species ^and

tilt is opposite ⁱⁿ sign for the other species ^{with the}

same tilt. Thus, ^to minimize the competition ^between polarization ^and free volume effects, ^{the two} species phase separate. Entropic ^terms optimize the size of each domain so that they ^{can be} visible in the polarizing microscope when the concentration of each species ^is large enough.

Our measurements of the layer spacing ^{of chiral}

and racemic HOBACPC lend support ^{to the above} observations. Figures 4(a) ^and (b) ^{show that the} layer spacing ^is identical for liquid crystal phases ^whereas

the racemic crystal phase ^{is about 1 A} shorter than the crystal phase of the chiral species. The racemic

phase ^{forms more} compact domains after phase separation whereas chiral HOBACPC, ^{with about} 10 % ^{of the} opposite hand, ^{is not able to} separate ^as

successfully ^{and its} packing ^is slightly ^{more inflated.}

The observation of spots [12] perpendicular ^{to the}

director could imply ^that ferroelectric switching ^invol-

ves a displacement of the molecule within each layer

if the layers ^are uncorrelated in zero field In figure ^5a

we sketch this idea and _propose that it is responsible

for the transient strain lines observed after switching

a thick sample of HOBACPC in the low temperature helielectric phase (Fig. 5b) ^{which we discuss next.}

The implication ^{of a} hexagonal array of spots ^on the X-ray pattern perpendicular ^{to the} long ^molecular

axis is that the molecules ^are lined _up in strings ^as

shown in figure ^5a. Hexagonal ^{bundles are seen when}

viewed along ^the long axis, and the end to end posi-

tional correlation is large. Since short range inter-

layer correlation is implied by the helielectric struc- ture in the absence of a field, shown in the middle cartoon of figure 5a, ^we propose in-plane ^molecular

motion results when the field is turned on as the molecules line _up in strings. Evidence for this conten- tion is provided by transient strain lines shown cross

hatched in figure ^5a and observed in figure ^{5b. The}

contrast of these lines is weak when the polarizer ^is perpendicular ^to the director verifying ^that they ^are

indeed parallel ^to the director. Furthermore, ^{the two}

sets of lines shown in figure ^{5b are related to each}

other by ^{the tilt} angle.

2.3 TEXTURES IN THE OPTICAL MICROSCOPE.

Although ^HOBACPC and 8SI* share the same phases,

2155

Fig. ^4.

^The X-ray layer spacing ^{as a} function of tempe-

rature. a) chiral HOBACPC and b) racemic HOBACPC.

the textures of C*, I*, (G’)* ^{and X in the} optical ^mi-

croscope ^are frequently strikingly ^different.

1. Pitch lines are not seen in the C* phase ^{of HO-}

BACPC whereas they ^are easily ^{observed in the C*}

phase ^{of 8SI*.}

2. In freely suspended films, ^{the I*} phase appears

as disc-like objects ^{in HOBACPC that} disappear ^once

the phase ^is established. This occurs in 8SI* in such

a small temperature interval it’s hardly ^{ever seen.}

3. Figure ^{6 shows the} striking fine lines observed at the onset of the G’ phase of racemic HOBACPC.

Chiral HOBACPC also exhibits these lines whereas 8SI* does not.

We interpret the lines in figure ^{6 as} kinks that result

as the helicoidal structure is suppressed. ^{A helicoidal}

structure is a three dimensional entity ^{and cannot be} simply destroyed ^without allowing ^{the director to}

escape into the direction parallel ^{to the helix. In} layered phases ^this requires ^an energetically costly

increase in layer spacing. Thus, the helix is unlikely

to unwind in smectics whereas it can easily ^{do so in}

cholesteric materials where this constraint does not exist [4].

Figure ⁷ illustrates smectic (G’)* ^{as a mixture of}

correlated and uncorrelated layers. The helix is unwound for the correlated layers but, ^{the twist}

cannot escape ^so the uncorrelated layers ^{twist even} tighter, forming ^{a kink. As} inter-layer correlations

build, the 2 ~ rotation _per pitch ^is distributed over

fewer layers. ^The only topological constraint is that a

total 2 x rotation of the director _per pitch ^{must be}

conserved. Once inter-layer correlations are the size of the pitch everywhere, they ^are effectively long-range owing ^to the initial periodicity ^{of the I*} phase ^from

which G’* _grows and the lines disappear ^{at the tran-}

sition to smectic X. This picture ^accounts for the small

supercooling of (G’)* ^{as well as the} appearance of the lines in the racemate which phase separates ^{into left} and right ^handed layers [8].

HOBACPC may exhibit these lines whereas 8 SI*

doesn’t, because of the different nature of the in-plane ordering of their I* phases. ^For example, ^{the I*} phase

of HOBACPC _may be crystalline ^{in the} plane ^{of the} layers with uncorrelated layers [16] whereas that of 8 SI* has only ^BOO [17]. ^{In this} respect, ^the (G’)*

phase of HOBACPC is a crossover phase ^{from 2 to 3}

dimensional long range positional order. Aniso-

tropic solidification would account for the differences in its textures from those of 8 SI* for which the build _up of positional correlations is more isotropic.

The two X phases ^{have the same} degree ^of positional ordering only ^the anisotropy of the solidification process is different.

2.4 ELECTROOPTIC RESPONSE.

Recently, ^Brand

and Cladis [7, 8] found that for a given voltage, V, close to a threshold voltage, Vt, ^the ferroelectric domains of 8 SI* switched in a time T such that V scaled linearly ^with temperature ^{and was} smallest at the transition to the G’* phase.

In X, ^{there are two stable states relative to the in-} plane rectangular ^{lattice. One} of these orientations is associated with a + P state and the other with a

-

P state, say. An activation _energy is,, required ^to change the orientation of ⁿ from one of these states to the other. In an applied field, V, reorientation starts at grain boundaries and walls move across the

sample. ^{When the} applied field is turned off, the walls stop moving. ^{Walls move} parallel ^{to the} layers ^{in the}

X phase ^and perpendicular ^to them in helielectric

phases.

Preparing samples ^between transparent electrodes,

Fig. ^5.

a) Transient lines that rotate by ^{the tilt} angle, ² 0, ^{when the} applied electric field is switched from + E to - E.

The sample ^{is 50 jLmi thick.} The distance between the lines is irregular but about 25 _ym. b) ^{There is no contrast} of transient lines in the photographs taken in the (G’)* phase of 50 um thick samples ^{when the} polarizer ^is perpendicular ^to the lines and maximum contrast when parallel. ^{These two} observations show that the director is parallel ^{to the cross hatched} strips ^of

the figure.

2157

Fig. ^6.

Fine lines observed at the I-G’ transition of HO- BACPC racemate. These are identical to the ones observed at the I*-(G’)* transition of HOBACPC.

Fig. ^7.

Scenario for the anisotropic solidification process which accounts for the unusual textures of the liquid crystal phases of HOBACPC. The distance between kinks in the

(G’)* phase ^{need not be constant.}

using the method of Kondo [18], ^we measured the time to ^switch the director at a given voltage ^{in the}

low temperature liquid crystal phases of HOBACPC.

Figure ⁸ shows that the parameter ^{V In i measured} in the vicinity of the threshold voltage ^{for the} 50 J.1m and 12 _J.1m thick samples is linear in temperature ⁱⁿ

Fig. ^8.

^The product ^{of the} voltage V ^{and In} ~, the time to switch from one polarization ^{state to} another in a thick

sample (50 ym) ^{and a thin one} (12 gm). The helicoidal structure of the (G’)* phase slowly ^{returned once the field}

was removed. The area observed in the 100 _um sample ^was

500 ym ^x 500 ym and in the 12 Ilm sample ^{it was} 250 um ^x 250 _urn.

the X phase, ^as expected, but, surprisingly, ^{in the}

thicker sample, ^{this rule also} applies ,xo ^the (G)*

phase. ^{Does this mean} that the 2-d crystallinity plus

the field strengthened inter-layer correlations (see Fig. 5a) ^are sufficient to make (G’)* bistable ? The

answer is no because when the field is turned off, (G’)* ^{relaxes to its} ground ^state after several minutes.

Again, (G’)* appears ^{as a} cross-over phase ^between

true helielectric phases, like C* and I*, ^{and ferro-}

electric phases, ^{like X.}

Since P is large, ^an antibistable transient (1) ^was

observed in helielectric phases ^because polarization charges accumulate at the electrode surfaces [19].

Instead of preserving ^{the state P} parallel ^{to E when}

the field is turned off, ^the polarization jumps ^{to the} anti-parallel state, - ^P. When the field E is turned

off, the non-uniform helicoidal structure (proposed

in Figure ^{7 for} (G’)*) ^returns [20]. ^{In further} support of the difference between the (G’)* phase ^and X, ^the anti-bistable transient became slower on approaching

the X phase and, finally, ^{was no} longer observed in the X phase.

Our conclusion, ^{based on the above} evidence, ^is

that smectic X is a separate phase ^{from smectic} (G’)*.

Smectic X is truly bistable because its inter-layer

correlations are long range. Smectic (G’)* ^{exhibits a} switching response that shares characteristics com- mon to helielectric and ferroelectric phases, ^{but it is}

not bistable. This is consistent with finite inter-layer

correlations.

3. Conclusions.

We have presented evidence that chiral HOBACPC exhibits the following sequence of transitions :

58 OC, ^{heat of} melting = ^8.5 cal/g ^{where X}

is a ferroelectric liquid crystal phase (4) characterized

by ^{a bistable} electrooptic response. A helicoidal structure characterizes helielectric phases ^{which are} designated ^{with a star « * ».} The helix is responsible

for their lack of bistability.

The transitions of HOBACPC fit well into the

picture ^{of an I*} phase composed ^of uncorrelated,

2-dimensional solid layers ^{with the onset of} layer

correlations at the (G’)* transition. At the (G’)*-X

transition (Fig. 7) these correlations become long-

range. Textures associated with the transitions of HOBACPC observed in the optical microscope may be different from those of 8 SI*, ^even though they

both exhibit the same sequence of transitions [7, 8],

because its solidification _process is more anisotropic.

Thus, ^{kinks are a} manifestation of the build _up of

positional correlations between solid layers ^{and are}

therefore observed in HOBACPC (Fig. 6). Layers

with less well-defined 2-dimensional structure do not exhibit kinks and they ^{are not observed in 8 SI*.}

(3) Antihistability ^{has recently} been observed in HOBACPC by ^{N. A. Clark} and S. T. Lagerwall (to ^be published ^{in the} proceedings ^{of the SID} Conference, Paris, France, September (1984)). ^However, these authors

interpret the effect as , ^{a «} confirmation ^of bistability », ^a

point ^{of view we do not} share. In our view, antibistability

occurs only ^{in the absence of a} threshold field and therefore it is a diagnostic ^for helielectricity ^or absence of bistability.

(~) ^The I*-(G’)* ^and (G’)*-(X) ^{are broad} compared ^to

transitions in simpler materials. HOBACPC is a compli- cated molecule (Fig. 1) and in its purest’ state, several iso- meric forms co-exist. Isomers of HOBAC share the same

chemical formula but their spatial ^and electronic ^configu-

rations make them chemically different. This complication

is further aggravated by ^{the creation of} decomposition pro- ducts as the material deteriorates.

Acknowledgments.

It is a pleasure ^to acknowledge interesting ^discussion

with A. Kmetz. HRB also thanks Deutsche Fors-

chungsgemeinschaft ^for support of his work.

Appendix.

STRUCTURE OF TILTED SMECTIC PHASES RELEVANT TO

HOBACPC. - In the last few years many different tilted smectic phases have been discovered. Some of these phases have been renamed several times as

researchers strive to develop ^a consistent organiza-

tional scheme for the bewildering variety ^of phases.

In all likelihood, many ^more will be discovered in the coming years. Here ^we present ^our view of these

fascinating phases bearing in mind that their struc- ture and properties ^are still under active investiga- tion, consequently, ^{our ideas are still} evolving.

Smectic phases ^are layered. They ^{differ from each}

other not only in the kind and degree ^of in-plane

order but also the _range of positional correlations between layers. ^{In tilted} smectics, ^the director is inclined relative to the layer ^normal. Rectangular positional ^{order is} associated with the more ordered tilted smectic phases ^{with the two known} preferred

orientations for director tilt being parallel ^{to the short}

side or to the long ^{side of the} rectangle. Brand et al. [6]

have argued ^{that the} in-plane ^{order for tilted} phases

is necessarily rectangular ^to provide ^an easy axis ^for

the tilt direction. The hexagonal ordering ^some

authors [21] ^have mistakenly attributed to HOBACPC does not provide ^{such an} easy axis.

Starting ^{with the} high temperature phases, ^smectic

C is fluid but anisotropic within the layers. ^There

are no positional correlations between layers. ^{In the} plane ^{of the} layers, smectic A is a 2-dimensional

isotropic liquid and smectic C is a 2-dimensional

nematic

2159

Conflicting evidence has been found for the struc- ture of smectic I. In the no. m series (n-alkoxybenzyli-

dene m-alkylaniline) Benattar et al. [16] found their data for the smectic I phases compatible ^{with a 2-d} rectangular lattice in the layers ^{and no} positional

correlation between layers. ^{In the I} phase, the tilt is

parallel ^to the short side of the rectangle. ^Budai

et al. [17] found that the I phase ^{of 8} SI*, ^{is an uncor-}

related stack of layers ^with rectangular ^{BOO in the} plane ^{of the} layers. Benattar et al. [16] ^{found this}

structure for the F phases ^{of the no.} m, with the dif- ference that the tilt is parallel ^{to the} long side of the

rectangle ^{for F} and the short side for I. As we point

out in the text, ^both kinds of smectic _{I appear} to exist determining ^the anisotropy of the solidification process.

The picture Brand et al. suggest ^{for BOO} (~) ^is

one in which molecular motion in the layers ^{iS res-}

tricted to a rectangular grid. ^{There is a} substantial heat of transition associated with the C to I transition

( ~ 1.5-2.0) cal/g ^{which seems to be a reasonable}

amount for either 2-d crystallization ^{or BOO.}

The G’ phase ^{has known} many aliases, ^{its most}

recent being ^J [9]. The molecules that moved freely

on tracks provided by ^the rectangular grid ^{in smec-}

tics I or F are now localized at the intersections of the grid. Intra-layer positional correlations start in the G’ phase which is consistent with the second order or weakly discontinuous nature of this transi- tion. In the G’ phase the director tilt is parallel ^to

the short side of the in-plane ^centred rectangular

order. The G phase ^is like the G’ phase except ^the

tilt is parallel ^{to the} long side of the rectangle. ^Budai

et al. [17] have identified the in-plane ^{structure of}

smectic X to be similar to the G phase.

In the H’ phase, ^{also known as smectic K} [9],

molecular rotations have frozen out so that the in-

plane ^{structure is now} simple rectangular ^{with two}

molecules _per unit cell. The H’ tilt is parallel ^{to the}

short side whereas the H phase ^{tilt is} parallel ^{to the} long side. Since the in-plane tilt of HOBACPC ^was determined by ^{Doucet et al.} [12] ^{to be} parallel ^{to the}

short side of the in-plane rectangle, ^{its smectic X} phase may be similar (H’) particularly ^since they

observe a doubling of the wide angle reflections in this phase.

For chiral molecules, the above phases ^{with short}

range inter-layer correlations adopt ^{a helicoidal}

structure as described in the text. We adopt ^the convention, which is different from that of reference [9],

that the letter designation ^{for the} particular phase

is then given ^{a star} attribute, thus, ^{C*. For} phases

which do not exhibit the helicoidal structure there is no change ⁱⁿ designation ^or structure, thus, ^{smectic A} remains smectic A. Smectic X presents ^{a dilemma} because it exhibits a macroscopic polarization. Sup-

pose the in-plane order of smectic X happens ^{to be}

similar to smectic G, say. Then, because there is no

helicoidal structure, ^{we do not} say smectic G*. But, it exhibits a polarization which smectic G phases ^of

non-chiral molecules do not. The solution of Brand and Cladis [7] ^{to this} dilemma is to call the ferro- electric G phase G, preserving information about in-

plane ^{structure and} chirality.

References

[1] ^{See for} example ^{the recent} special ^{issues on} liquid crystal ferroelectrics edited by BLINC, R., CLARK,

N. A., GOODBY, J., PIKIN, S. A. and YOSHINO, K., published in Ferroelectrics 58-59 (1984).

See also : DE GENNES, P. G., ^The Physics of Liquid Crystals (Clarendon ^Press, Oxford) ^1975.

[2] MEYER, ^R. B., LIEBERT, L., STRZELECKI, ^{L. and} KELLER, P., ^J. Physique ^{Lett. 36} (1975) ^L-69.

[3] MEYER, ^R. B., ^Mol. Cryst. Liq. Cryst. ⁴⁰ (1977) ^33.

[4] CLADIS, ^P. E., BRAND, H. R. and FINN, ^P. L., Phys.

Rev. A 28 (1983) 512. The absence of threshold is due to the continuous degeneracy ^of the allowed orientations of P provided by the helicoidal struc- ture.

See also for example : ^{DE GENNES, P. G. and} PINCUS, P., Solid State Commun. 7 (1969) ^339.

[5] CLARK, ^{N. A. and} LAGERWALL, ^S. T., Appl. Phys. ^Lett.

36 (1980) ^899. Their idea is that helielectric

samples ~ ¹

2 03BCm thick in the direction per-

pendicular ^{to the twist axis} may be bistable.

[6] ^{BRAND, H.} R., CLADIS, P. E. and FINN, ^P. L., Phys.

Rev. A 31 (1985) ^361.

[7] BRAND, ^{H. R. and} CLADIS, ^P. E., ^J. Physique ^{Lett. 45} (1984) L-217.

[8] BRAND, H. R. and CLADIS, ^P. E., ^Mol. Cryst. Liq.

Cryst. 114 (1984) ^207.

[9] GOODBY, ^J. W., ^Mol. Cryst. Liq. Cryst. ^{Lett. 92} (1984)

171.

[10] MARTINOT-LAGARDE, Ph., DUKE, ^{R. and} DURAND, G.,

Mol. Cryst. Liq. Cryst. ⁷⁵ (1981) 249 and referen-

ces found therein.

[11] BERESNEV, ^L. A., BAIKALOV, V. A., BLINOV, ^L. M., POZHIDAEV, E. P. and PURVANETSKAS, ^G. V., JETP Lett. 33 (1981) ^536.

[12] DOUCET, J., KELLER, P., LEVELUT, A. ^{M. and} PORQUET, P., ^J. Physique ³⁹ (1978) ^548.

[13] JAIN, S. C. and WAHL, J., ^J. Physique ^{Lett. 44} (1983)

L-201.

[14] LIEN, ^S. C., VINER, ^J. M., HUANG, ^{C. C. and} CLARK, ^N.

A., ^Mol. Cryst. Liq. Cryst. ¹⁰⁰ (1983) ^145.

[15] KELLER, P., JUGÉ, S., LIEBERT, ^{L. and} STRZELECKI, L.,

C.R. Hebd. Séan. Acad. Sci. 282C (1976) ^639.

[16] ^{BENATTAR, J.} J., MOUSSA, ^{F. and} LAMBERT, M., ^J.

Chim. Phys. ⁸⁰ (1983) ^99.

[17] BUDAI, J., PINDAK, R., GOODBY, ^{J. W. and} DAVEY, ^S. C., J. Physique ^{Lett. 45} (1984) ^L-1053.

[18] KONDO, K., KOBAYASHI, F., FUKUDA, ^{A. and} KUZE, E., Japan ^J. Appl. Phys. ²⁰ (1981) ^1773.

[19] SPROKEL, G. J., ^Mol. Cryst. Liq. Crys. ²¹ (1973) ^249.

[20] ^We have also observed the antibistable transient in the I* phase ^{of DOBAMBC} (p-decyloxybenzili-

dene p’-amino-2-methylbutylcinnamate) ^{and the}

helielectric phases ^{of DOBA 1 MBC} (p-decyloxy-

benzilidene p’-amino-1-methylbutylcinnamate)

whereas the helielectric phases of 8 SI* and MORA12 (S-4-0-(6 methyl)octylresorcylidene-4’- dodecylaniline) ^{did not} exhibit this effect consis- tent with the lower polarization ^{of the last two} compounds.

[21] GOODBY, ^J. W., PATEL, J. S. and LESLIE, ^T. M., ^Ferro-

electrics 59 (1984) ^121.