Structural and electro-optical studies of the smectic polymorphism of a homologous series of chiral molecules

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Structural and electro-optical studies of the smectic polymorphism of a homologous series of chiral molecules

Patrick Hamelin, Anne-Marie Levelut, Philippe Martinot-Lagarde, Claude Germain, Lionel Liebert

To cite this version:

Patrick Hamelin, Anne-Marie Levelut, Philippe Martinot-Lagarde, Claude Germain, Lionel Liebert.

Structural and electro-optical studies of the smectic polymorphism of a homologous series of chiral molecules. Journal de Physique II, EDP Sciences, 1993, 3 (5), pp.681-696. �10.1051/jp2:1993160�.

�jpa-00247864�

Classification

Physics

Abstracts

61.30E 64.70M

Structural and electro-optical studies of the smectic

polymorphism of

_a

homologous ^series of chiral molecules

Patrick

Hamelin,

Anne-Marie Levelut,

Philippe Martinot-Lagarde,

Claude Germain and Lionel Liebert

(*)

Laboratoire de

Physique

des Solides, Universit6 de Paris-Sud, Bfitiment 5IO, 91405

Orsay,

France

(Received 28 October 1992, accepted in

final

form 5 February 1993)

Abstract. The molecular

organization

in bulk

samples

of the smectic O

mesophase

of

2-heptyl

and

2-octyl 1,4-terephthalydene-bis-4-aminocinnamates

mixtures is studied by various methods.

The

phase diagrams

_are obtained

by thermo-optical

and microcalorimetric

analysis.

The structures are

investigated by X-ray

diffraction and the electrical

properties (spontaneous polarization,

dielectric constants) _are studied in detail. The smectic O

pha8e

has _a lamellar structure in which the molecules _are tilted in the

layers.

The order inside the

layers

is

liquid-like

but, contrariwise _to the smectic C phase, where it is uniform, the azimuth tums between _two

adjacent layers

with _an

angle

of

nearly

180°. We observe the smectic O-smectic C transition

by varying

temperature or

by applying

_an electric field. Moreover, the smectic C-smectic O-smectic C sequence is obtained in _a

narrow composition _range. Our results _are in good agreement ^{with those}

previously

reported

obtained _on thin films growing at the isotropic liquid-smectic interface.

Some

mesophases

of chiral

mesogenic

molecules have

specific symmetry properties

related to the twisted

configuration

of the molecular directors which form _a helicoidal array with _a

pitch

which is

generally

of the order of I _~Lm. In this field the studies of chiral smectic C

(Sc«)

which shows ferroelectric

properties

have been

developed

in view of

electro-optical applications [I].

In the _course of the search for chiral materials with

good electro-optical properties

two _new

mesophases

the smectic O

(So)

and the smectic

Q (S~)

were observed in 1983

[2],

in _a

compound

which in addition shows _an unusual

mesomorphic

behaviour. The most

striking

feature is the difference between the

polymorphism

of the racemic mixture and that of the

optically

active pure enantiomer since the first _one has _a clarification temperature ^{25 K}

higher

(*) Nous tenons h

exprimer

ici _tout

ce que nous devons h Lionel Liebert, l'initiateur _au sein du laboratoire de nombreuses collaborations _entre chimistes _et

physiciens.

Son enthousiasme de

longue

date pour les

propridtds

fascinantes des

composds

ddcrits ici _a dtd pour nous extrEmement stimulant, _et reste

encore trbs

pr6sent

dans notre mdmoire bien

qu'une

annde _se soit

ddjh

6coulde

depuis

sa

disparition

brutale.

than the second. Moreover the

S~ ^mesophase

^has no

longer

_a lamellar structure but is

organized

with _a 3D

large

size

tetragonal

lattice

(1422)

and it exists

only

for

quite

_pure

enantiomers and _over 2.5 K below the clarification temperature.

Recently optical

observations of the racemic mixture have shown that the

So

_grows in successive

parallel

molecular

layers

at the

isotropic liquid

smectic interface. In each

layer

the molecules _are

organized

as in _a smectic C

layer,

with _a

liquid

like

ordering

of the centers of _mass and _a

unique

orientation of the director which is tilted with

respect

to the normal to the smectic

planes

_; however the

symmetry

^{of the}

phase

is different since there _are mirror

planes

at each interface between

successive molecular

layers implying

therefore that the translation

period

is twice the thickness of _one

layer [3].

This

organization

which _{was seen} at the

So-liquid

^{interface and} under the

action of _a weak electric

field,

is consistent with

X-ray

diffraction

pattems

^{of the racemic} mixture in _an oriented state

[2].

However the behaviour of the

optically

active mixtures is still

questionable

at least for bulk

samples

since in _a molecular twisted array with _{a non}

periodic

in-

layer ordering,

^the smectic

period

_as measured

by X-ray diffraction,

is not modified

by

the

occurrence of _a twist axis nomJal _to the

layer planes

and

by

the way for the twisted state of _an

antiferroelectric array of the successive

layers (Sc.

with a

pitch

close to twice the

layer thickness).

During

the last two years different kinds of chiral smectics have been discovered and

analysed.

On the _one hand transitions between several fluid smectic

phases

were observed in _a pure chiral

compound

and it has been

suggested

that _some of these

phases

have _a fern- _{or an} antiferroelectric character

[4].

On the other hand the existence of _a chiral smectic A

(S~~)

_was asserted from

optical

and

X-ray

diffraction data

[5].

The molecular

organization

of this

phase

_appears to be consistent with

previously predicted

^{models of} the twist

grain boundary phase (TGB) [6].

In this

phase

the helix axis is _no

longer

normal to the

layer planes.

Therefore better information _on the

thermodynamical,

structural and

electro-optical

properties

of the

So phase

will

clarify

^the

problem

of the electric

properties

of the chiral form.

In order _to

improve

_our

knowledge

_we have follow the

impact

of the

optical activity,

of the

length

of the molecules of the _same

series,

and of the electric field upon the

stability

and the

physical properties

of the

So phase.

Experiments.

Up

to now the

So phase

has

only

been detected in _one

compound,

the

2-octyl 1,4- terephthalydene-bis-4-amino

cinnamate

[2].

Shorter chain derivatives of the _same series have been

prepared, they _present

_a smectic

mesophase

the structure of which has not

yet

been

unambiguously

established

[7].

SYNTHESIS. The

2-alkyl 1, 4-terephthalydene-bis-4-aminocinnamates

were

prepared

accord-

ing

to the usual way

[8, 9].

Esterification of the different alcohols

by

^the

p-nitrocinnamoyl

chloride

(from

acid ₊

SOCI2),

^{then reduction}

(Fecl~

₊

NH~ )

of the nitroester into the aminoester followed

by

its condensation with _a

half-equivalent

of

terephthaldehyde.

Ethanol has been used for _most of

recrystallizations.

The

2-alkyl terephthalydene-bis-aminocinnamates

_present three different stereo isomers. The pure active

compounds (R, R)

_or

(S, S)

_are obtained from

commercially

available

(Aldrich)

R _or S alcohols. The reaction of _a racemic mixture of alcohols

provides

_an

optically

inactive mixture of 0.25

(R, R),

0.25

(S, S)

and 0.5

(R, S) molecules,

this last _one

cannot be obtained

separately.

We have focussed _our attention _on two

compounds

of this series _: the

octyl (Cg)

and the

heptyl(C7)

derivatives. One

important problem

is the

activity

of the

commercially

available alcohols which drives the

purity

of the

mesogenic compounds.

^In fact the

2-heptyl

alcohols

(R)

and

(S) provided by

Aldrich have _a

higher optical activity

than the

(R)

_one used in _a

previous study [7].

Moreover the

specific

rotation is

nearly

the _same

(in modulus)

for the four alcohols of two different chain

lengths

used in _our

synthesis.

Therefore _{we assume} that all _our

derivatives _are

optically

_pure. The NMR spectra ^{of the final}

products

of the

synthesis

assert also the absence of any other kind of

impurities.

CHARACTERISATION oF THE MESOPHASES.

Optical microscopy

and differential

scanning calorimetry (D.S.C.)

^have been

performed

_{on pure}

compounds

and _on

binary

and temary

mixtures in order _to detect

phase

transitions. Moreover _we have also observed contact

preparations [10],

in order to cover some ranges of concentration which _seem of

peculiar

interest.

Optical properties

such _as

specific

rotation of

polarization

^and

pitch

of the helical array ^{have also been measured.}

ELECTRO-OPTICAL MEASUREMENTS.

Optical microscopy

between crossed

polarizers

have been

performed

in a Mettler

heating

_stage. Furthermore the textural

changes

under

application

of _an electric field _are studied _on

samples

sandwiched between two

glass plates

coated with ITO. The director orientation is

generally planar

and the

sample

thickness of 20 _~L. The spontaneous

polarization

is measured

by

_a direct method

already

described

[I ii

^the same

device allows measurements of the _a-c- dielectric constant.

X-RAY DIFFRACTION EXPERIMENTS. Two kinds of

experiments

have been

perforrned.

Powder pattems

using

a Guinier _camera

equipped

with _a

heating

device allows _an evaluation of the smectic

layer periodicity.

Under the action of _a

magnetic

field it is

possible

to orient the

samples, provided

that the

mesophase

is not too viscous in _case of _a twisted array the field

strength

must be

higher

than _a threshold value which

depends

on the

pitch

and _on the twist elastic constant. In _our device _we have _a

magnetic

field of 1.7 T

acting

_{on a}

sample

held in 1.5 _mm

glass

tube and _we have encountered _some difficulties _on

aligning

chiral smectic

phases.

In both

experiments

the X rays are issued from _a reflection _{on a}

crystal, selecting

the

CUK«j

radiation for

powder patterns

^{and the CUK« doublet for oriented}

patterns.

Phase

diagram.

Figure

I presents the

phase diagrams

of the

binary

mixtures of the _two

optically

active

antipodes C7

and

C8.

At _a first

glance they

_appear to be very similar _: in both _cases the solidus line

gives

the evidence of _a solid racemate, the

melting

temperature ^{of this} racemate is about 112 °C for

Cg

and 105 °C for the

C~

derivatives while the

melting

temperature ^{of the}

antipodes

are 95 and 85 °C

respectively.

^Both

systems present

a

positive

_azeotrope for the clarification

temperature

^{which is}

higher

of 25 and 22 K for the

C~

and

Cg

racemic

mixtures, respectively,

than for the

corresponding

_pure active derivatives.

The

tridimensionally

ordered

tetragonal phase S~

^is

only

_seen for the pure active

compounds Cg

of _more than 90 §b of

optical purity

and within 1-3 K

just

below the

clearing point.

The C~

compound presents

a

single

fluid smectic

phase

_{at every}

composition

but it appears,

looking

at contact

preparations

that the smectic

phase

is different for the racemic mixture and for the pure

antipodes,

the

separation

between the _two

phases

is _a

straight

line

parallel

to the

temperature

^{axis and}

corresponds

to an

optical activity

_±

Fo(70 ~Fo

_< 60

§b).

This

phase separation

^line crosses the clarification

spindle

without

disturbing significantly

its

topology

however a clear

jump

is _seen in the

dependence

of the

clearing enthalpy

_versus

composition

in

C~(R, R)

₊

C~(S, S) (Fig. 2).

In order _to

identify

these two smectic

phases

_we have studied

binary

^and temary mixtures of derivatives of different chain

lengths. Figure

3

gives

the

behaviour of the temary

diagram C~(R, R), C~(S, S), Cg(R, R).

It appears that the _two

different kinds of

binary

mixtures of

C~

^and

Cg

present

only

the

So phase,

_{except a} small

T1°cj Tj°c)

160 ~

160

L

140 140

S~ So

120

SO

ioo ioo

K

C7(R,R) C7(S,S) C8(S,S) C8(R,R)

Fig.

I.

Binary phases

diagrams of the

antipodes

of

2-heptyl

(left) and

2-octyl

(right)

terephthalydene-

bis-aminocinamates K _: crystal, L

isotropic liquid, Sc, So.

smectic phases,

S~.

mesophase with _a

tetragonal

lattice, T

Temperature,

The hatched _areas

correspond

to

biphasic

_zones.

AH

(cavmole)

loco

soo

o

F

Fig.

2.

Clearing enthalpy jump

AH _versus the

optical activity

F of the C7 derivative.

(°c)

C~R,R)

'~° Sc

~

na2 n~4 c~

sc

'>

'

' _,

.'

~O ,' ,

,' ,

,' , ,

c~js,Sj c~S,s)

Fig.

3.-

Projection

onto the

composition plane

(mole fraction) of the ternary phase

diagram

of C7(R, RI,

C7(S,

S)

C8(S,

S). The vertical hatched _area is _{a zone} of

polymorphism So-S~

while the

obliquely

hatched _area is _{a zone} of

polymorphism So-Sc.

The

azeotropic clearing points

are located _on the dashed line. The

interrupted

line

corresponds

to mixtures of racemic C7 with

Cs(S,

S). The temperature

behaviour of these mixtures are shown (see the insert), in _{a narrow} range of

composition, including

the temary ^mixture To.

S~

area and _a very limited

biphasic

area

containing

two smectic

phases

for ~0.7 moles

C7(R,

R

)

+ 0.25 moles

Cg(S,

S

)

and within 5 K in the

vicinity

of the

melting point. Temary

mixtures _can

present

a smectic

polymorphism

ⁱⁿ _a ^wide range of

composition (Fig. 3),

moreover a reentrant sequence

involving

_a central

So phase

has been observed in _a temary

mixture

To

0.375

[C7 (R,

^R +

C7 (S,

S

)]

+

0.230(Cg (R, R))

moles. A quaternary ^racemic mixture

Ro

0.35

[C7(R,

R ₊

C7(S,

S

)]

₊ 0.15

[Cg(R, R)

₊

Cg(S,

S

)]

moles has _a similar reentrant sequence. In both _{cases an}

enthalpy peak

is _seen in D.S.C. traces

only

^{for the} _upper transition

temperature

^{between the} two

smectics,

this

enthalpy

is of lo

cal/mole

for the temary mixture and 14 cal/mole for the racemic quatemary ^{mixture. The low}

temperature

^{transition is}

spread

over a few

degrees

and _no measurable

enthalpy

_can be associated to this transition

(Fig. 4a).

Structural studies.

The structural studies of the

Cg

derivatives have been

already published

therefore _we will _not discuss here the

phase

of

tetragonal

(1

422)

lattice

[2].

If _we

except

^this

phase, powder pattems

of the _two derivatives and of their mixtures all

correspond

to fluid lamellar

phases

with _a

layer

periodicity significantly

smaller than the molecular

length

and close to 30

A

_{for all the}

samples.

The smectic

periodicity

of the reentrant mixtures does not show any

discontinuity

at the smectic-smectic transitions but _a

jump

of the

expansion

coefficient

corresponds

to the lower temperature ^transition

(Fig. 4b).

In order _to make _a structural distinction between the two

smectic

phase

we examined

magnetically

oriented

samples

of various racemic mixtures.

Figure

5

gives

the

X-ray

diffraction pattems obtained after _a slow

cooling

of the

samples

of different mixtures from the

isotropic liquid. Figure

5a for the racemic

C7

^mixture is

typical

of _a

AH

(Kcavmole.K~

~

36o 40o 44o

T(K~

a)

d

jAi

31.5

31

433 TlK~

~°

too 120 14o loo T

b)

Fig.

4. Thermal behaviour of the Ro mixture 0.70 _rac C7 ₊ 0.30 _rac C8. al DSC trace AH _versus temperature ^T. Among the two transitions

So-Sc

(marked down by the arrows), only that of

higher

temperature

provides

_a measurable

enthalpy peak.

b) layer thickness d dependence versus temperature T showing _a dilatation jump at the lower temperature So-Sc transition.

">

~'

~~

,

( i] _b"

h Id

~

~

""

~ '~~

,

-~ _'

-'~~j~

j fG~~ '~~

l'~, f.).(, _)~"' ]Ii

~

-'» ] / '~

=~- Q~

~~

a) c)

b)

d)

f~ e)

Fig.

5. X-ray diffraction pattems ^of samples

aligned

with _a magnetic field of 1.7 T. Racemic mixtures of a) C7, b)

C8,

_c, d, e)_To, at

respectively

T

= 110 °C, 140 °C, 155 °C. In 0 the small

angle

area for _a mixture of 0.5 mole C7 + 0.5 mole Cs. The magnetic field is

along

the vertical direction.

single

domain of

Sc phase

: the director is

nearly parallel

to the

magnetic

field while several orders of reflection _on the

layer planes

_are

aligned

in _a direction at about 45° of that of the

magnetic

field. In fact the number of visible reflections

(three)

^is unusual for _a

Sc

^{and is}

indicative of _a well stratified structure. We get a

single

domain

probably

because the smectic

phase

is obtained

directly

from the

isotropic phase

and grows

easily

in

large

domains.

Figure

5b is obtained for the

Cg

racemic

mixture,

the

layers

have _a

unique

orientation while the director takes two different orientations each at 45° of the

layer

normal the number of orders of reflection is still

high, therefore,

the

layer

character is

likely

to be the _same in the _two

phases.

Figures 5c, d,

e represent ^{three different} patterns of the quatemary ^mixture

Ro respectively

_at

I lo

°C,

140

°C,

155 °C.

Figures

5c and 5e _are similar _to

figure

5a and therefore appear

typical

of _a

Sc phase

while

figure

5d is similar to

figure

5b _; let _{us note} that the orientation of the

layer planes

does _not

change significantly through

the smectic-smectic transitions while the director has _a

unique

direction with respect ^{to the normal} to the

layer (corresponding

_{to a}

Sc phase)

at I lo °C and 155

°C,

and reorients at least in two directions

equally

tilted with

respect

to the

layer

normal _at 140 °C in the

So phase.

Moreover the

X-ray pattem

of _a mixture of 0.5 moles

C7(R, R)

+ 0.5 moles

Cg(S, S)

is

similar to

figures

5b and 5d. Therefore _{we can} characterize

unambiguously

the diffraction pattern ^of an

aligned sample

of _an

So phase.

The

layer periodicity

is the _same in

Sc

and

So phase

the molecules _are in _a

liquid

_state inside the

layers

and their director is tilted with respect to the

layer

normal with _an

angle

of 45°. The

only

difference is that the director

points

ⁱⁿ a

unique

direction in the

Sc phase, involving

_a

2/m point

_{group symmetry}

(racemic mixtures)

^{and in} two directions

lying

in the _same

plane, involving

_a 2/m 2/m 2/m

point

_group symmetry, ^for a non-chiral

So phase.

It has been shown

by optical

observations of

So

films

floating

on an

isotropic droplet

that the director takes two definite directions

making

the _same

angle

with the

layer

normal, and

corresponding

_to

opposite

azimuthal

orientations,

uniform in each

layer

and

altemating

from _one

layer

_to the _{next one.} This altemation _can

extend _over loo

layers.

A similar structure _can be found in

Cg compound

of 0.91

optical

activity

^but an overall twist of

large pitch

0.36 _~cm is

superimposed

above this local order

[12].

The

X-ray

pattems ^of

So

racemic mixtures _are consistent with the fact

that,

in bulk

samples,

the director takes two directions

equally

tilted with respect to the

layer plane

but with

opposite

azimuthal directions. Since the reentrant mixture

clearly

shows that the

Sc-So

transition involves reorientations of the director without modification of the

layer

texture, the double director orientation is

actually

_a structural feature and _{not a} textural _one. The two director

orientations _can be distributed either in the _same

layer,

_on each side of

w walls in _a

periodic

array, or in successive

layers,

then

inducing

_a smectic superstructure. ^However on

powder

and oriented patterns we have _no detected any

Bragg peak

issued from _a modulation of the

density

either in _a direction

parallel

to the

layer plane

with _{a wave vector} smaller than 200

A

or

in _a direction

perpendicular

to the

layer plane

at least with _{a wave vector}

larger

than the

layer

thickness and smaller than

15001

_{(~ 50}

_{layers )-. Moreover,}

on the pattem ^of a

mixture of 0.5 mole

C7 (R,

R ₊ 0.5 mole

Cg (S,

^S

)

there is _no diffuse

scattering corresponding

to any fluctuations of _wave vector

larger

than the

layer periodicity (Fig. 5fj.

Therefore the

structure described in reference

[3]

is

fairly

consistent with the

X-ray

pattem ^{of bulk}

samples,

since _{we must assume} that the different director orientations _are linked

by

_a twist axis

parallel

to the

layer

normal. For the racemic

mixtures,

this _{structure can} be described

by

the space group

2/c 2/m 21/m,

with _a

symmetry

^of translation

(periodicity) only along

the _z

direction,

the

period

c

being equal

to twice the

layer thickness,

I-e. 60

A.

Such _{a structure} has also been

proposed

in order to

explain

the

polymorphism

of some chiral

compounds describing

_an antiferroelectric smectic

phase S~c [4].

Let _us remark that the

dipole

array is antiferroelectric in _an unwound

So~ sample

_:

however,

for _a racemic

mixtures,

if

(R)

and

IS) species

_are

equally

distributed in each

layer,

_as it _{seems to} be from _our observation of _a 0.5 moles

C7 (R,

^R

)

₊ 0.5 moles

Cs(S,

S

mixture,

the

Sc

and

So phases

are both

paraelectric.

Therefore _we

prefer

to

keep

the label

So

rather than

S~c [4].

Furthermore in order _{to test} the ferro- and antiferroelectric behaviour of various mixtures _we have

performed electro-optical

experiments.

Electro-optic

behaviour.

On

slowly cooling samples

sandwiched between two

glass plates,

from the

isotropic phase

we

obtained _a

planar

orientation of the director unless the

sample

thickness is very thin

(<

3

~Lm).

These thin

homeotropic preparations

have

only

been used for

specific

rotation measurements, the

samples

are too thin for the observation of

conoscopic

_pattems, and

by

the

way we have not been able _{to measure} the

birefringence

_tensor. The

specific activity

of the two

Sc

^and

So phases

have the _same absolute value but

opposite signs

in each side of the

phase separation line,

when

looking

at contact

preparation

of

C7

^{mixtures and of} the reentrant temary

mixture

To.

When the thickness of the

sample

exceeds 20 _~cm the

sample

takes _an overall

planar

orientation with _a

juxtaposition

of domains with different director orientations

(Fig. 6).

The behaviour of

samples

of

binary

mixtures of

C7

enantiomers under electric field is different _on each side of lines

corresponding

_to the

optical activity

_±

Fo.

for mixtures with

[F

_< F

o the behaviour is

typical

of _a

Sc~ phase,

the helicoidal array is

unwound

by

_an electric field

higher

than _a threshold value

(1-5 x105Vm-')

and _a

spontaneous

polarization

_appears. In the unwound

sample

the

optical

_{axes are at} about 45° to the

layer

normal

(Fig. 6g)

and their direction switches each time the electric field is reversed

for mixtures with

[F

_~

Fo

the director takes the _same orientation with respect to the

layer

but _at

higher

values of the threshold

(~

5 _x

10~

Vm~ ^'

). Beyond

this threshold value

large

areas of the

sample

_are oriented with the director at about 45° of the

layer

normal _;

by

increasing

the field _an uniform

Sc

texture can be obtained and

then,

the spontaneous

polarization

measured. In _a

decreasing

electric field dechiralisation lines _can be observed and then _a clear transition appears, the final state

corresponds

to neutral lines

parallel

or

perpendicular

to the

layer planes.

Therefore the average director orientation is

perpendicular

to the

layer plane,

and since molecules _are tilted with respect to the

layer normal,

the director is twisted with _a

pitch

that is small

compared

to the

light wavelength. By applying

an electric field on contact

preparations

of the _two

C7

enantiomers it is shown that the electric field induces _a

So~-Sc

transition since the

separation

line between the _two

phases

is

displaced

in such _a way that the

So~

area is reduced

(Fig. 6g).

The behaviour of the chiral reentrant mixture

(To)

under electric field is similar _: below 130 °C and above 150 °C, the electric field induces _a

Sc~-Sc

second order transition

(Figs.

6a, b,

g)

and _a

So~-Sc

first order transition

(Figs. 6c,

d,

e). Figure

7

gives

the electric

field/temperature phase diagram

of this

optically

active temary ^{mixture. The threshold field is}

independent

of the

temperature

^{for the}

Sc~-Sc

transition and it increases

rapidly

when

going

into the

So~

temperature

stability

_range, its maximum value is 3 _x 106 V/m at T

= 143 °C.

Therefore the

So~-Sc

^transition can be obtained

by varying

the

temperature,

concentration and electric field. However _we have _to notice that

So~ samples

of mixtures of the

Cg antipods

could

not be unwound

by

an electric field since

electrohydrodynamical phenomena

take

place

beyond

the threshold

field,

at least if their

optical activity

is _too low

( [F

< 0.8

).

The _same

situation

prevails

for mixtures of

C7(R,R)+Cg(S,S)

for _a molar concentration in

C7(R, R) comprised

between 0.25 and 0.5.

In order to have _a better characterization of the

electro-optical properties

of _our

samples

_we

have also measured the dielectric

susceptibility

associated with the Goldstone

mode,

the

pitch

of the helix and the electric spontaneous

polarization.

The relative dielectric

susceptibility

ⁱⁿ the smectic

plane,

_e~, of the _reentrant temary ^mixture To was measured _{as a} function of temperature

(Fig. 8).

To lower the ionic conduction effect the

measurement

frequency (300 Hz)

was chosen _near the relaxation

frequency

of the low

frequency

dielectric mode due to the helix deformation _: the Goldstone mode

[19].

The

JOURNAL DE PHYSIQUE T 1N'S MAY1091 27

Fig.

6.

Optical microscopy images

between crossed

polarisers

(vertical and

horizontal),

the electric field E is

perpendicular

_to the

preparation.

Temary mixture To in the

Sc phase,

a) at 100°C, E

= 0 b) _same temperature ^E ₌ ^0.I MV/m

m the threshold _;f~ at lsl °C, E

= 0. Behaviour ofthe _same

-

~imv/mi

3

. s

, ~

2

,

Sc.

i

Sc

S~

_i

iSc*

o

too im 14o 1llo

Fig.

7. Electric field E _versus temperature ^T

phase diagram

of the To ^{mixture. The hatched} area is _a coexistence _zone of

So

with Sc~ or

Sc.

^{The full line}

corresponds

to the measured threshold field and the dashed line to the limit of the coexistence _zone of

So

with

Sc.

iso

' O

'

"

'

"

'

"

W'

~

Tocj

lW 110 140 100

Fig.

8.

Temperature-T-dependence

of the relative dielectric

susceptibility

_e~, ⁱⁿ a direction

parallel

to the

layer planes,

for the To ^{mixture. The full circle is} a measure with _an additional DC field

larger

than the

threshold value (this point could be

compared

to the value of e, in the

So Phase).

Fig.

6 (continued).

sample in the

So phase

at 140 °C with successively _; cl E

= 0, d) E

=

5 MV/m, and e) retuming to

E=0;

g)

contact

preparation

^{of the} C7

antipodes

mixtures with _an

applied

^electric field

E

= 0.33 MV/m, the optical

activity

decrease from the left side _to the right one, crossing ^the frontier between the unwound

Sc

state and the So~ state arrows points smectic C domains in the

biphasic

area

which assert the

anisotropy

of the field effect.

continuity

of _p~ from _one smectic C*

phase

to the other C*

phase

_can ^be remarked. At both

phase

transitions between

Sc~

and

So

the _p~

jump

^is

significant (Ap,

_m

70).

This difference in e~ ^at the

phase

transition

corresponds

^{well with} the difference obtained when _at 120 °C _one

passes from the

sample

in the smectic C*

phase

to the

sample

measured under _a

5 _x 106 V/m dc

applied field,

which field unwinds the helix and thus suppresses the Goldstone mode

(e~

^value

represented by

full

circle).

In order to estimate the elastic energy involved

by

^{the helical} _array, an estimate of the

pitch

of the

mesophase

is useful _; this _can be obtained either

by measuring

the

periodicity

of dechiralisation lines _on

planar samples (for

the

Sc~),

_or

by measuring

the

specific

rotation _on

homeotropic preparations (provided

the refraction indices _are

known). Figure

9

gives

the temperature

dependence

for the

specific

rotation of _a

Cs sample

with _a

0.91optical activity

and for A

=

0.546 _~cm, the

corresponding

value of the

pitch

deduced from these measurements is also

reported

_on the _same

figure.

The

pitch

is

nearly

constant between loo and 128° and

equal

to 2 _~Lm, then it decreases

quickly

close to the

clearing point.

^The low temperature ^value fits well with the dechiralisation line

spacing, while,

close to the

clearing point,

the _{curve can} be

reasonably extrapolated

towards the value of

0.36~Lm

estimated

by

Galeme and Liebert

[10]

_on smectic films _at the contact of the

isotropic liquid.

We have

reported

above that the

specific

rotation

changes

its

sign

_on

crossing

the

So~-Sc~line

of transition while

keeping

the

same absolute

value,

this _means that the two

phases

_are twisted with similar

pitches. However,

since the helical array is _a modulation of the

layer periodicity,

the

pitch

is not

always

unambiguously

defined. In fact the

So~ pitch

is close to the smectic

periodicity (I.e.

60

A)

if

one chooses the smallest definition.

The spontaneous

polarization

is measured in the

Sc phase

_on

applying

_an electric field

larger

than the threshold value. For pure

C7(S, S)

or

Cg(S, S)

at

l10°,

far from the

clearing

temperature, ^this

polarization

^is of about I

Debye/molecule.

^{This is} a rather

high,

but not

unusual, value for _a

Sc~ phase.

At a constant temperature, ^the spontaneous

polarization

of the

C7

enantiomer mixtures increases

linearly

with the

optical purity.

The mixtures of

C~

enantiomers _cannot be unwound with _a field lower than the

sample

breakdown,

consequently

the

spontaneous polarization

cannot be measured in these mixtures. The spontaneous

polarization

of mixtures of

C7

and

C~

has _a non-monotonic

dependence

_i<ersus ^their

composition (Fig. 10).

The

spontaneous polarization

of mixtures of

C7(R, R)

and

C~(R, R)

goes

through

a minimum value for _a mixture I/I with _a ratio

P~~~/P~,~

^of about I.4. In mixtures of

C7(R, R)

and

C~(S, S)

the spontaneous

polarization

is

quite

linear for less than

' '

' '

'

ioo ix

Fig.

9.

Temperature-T-dependence

of the

specific

rotation fl and of the

pitch

P (estimated from _a

typical birefringence

of 0.15, wavelength of the

light

0.546 ~m).

(Debyewole)

C~jS,Sj

°

o-m o-is i

C~ (R,R)

Fig.

IO.

Spontaneous polarisation P~

_versus mole

composition

in C~

Csis.

Sl _{mixtures squares}

and dashed line for mixtures

C7(S,S)+C8(S,S)i

circles and full line for mixtures

C7(R, RI

+Cs(S,S),

between 0.25 and 0.5

C7(R,R)

mole the heli; _cannot be unwound and

consequently,

the polarization cannot be measured.

0.25

C7

mole, and it _can be

extrapolated

to zero for this concentration. As _I;e ha,~e

pointed

out

above, the

polarization

cannot be measured between 0.25 and 0.5 C~ mole since the helix cannot be unwound. In the range 0.5-1

C~

^{mole the}

polarization

^is

negative

and goes from 0 for 0.5

C7(R,R)

moles to

lDebye/mol

for the pure

C7(R.R),

with _a

nearly

^linear

dependence.

Discussion.

The symmetry ^{of the}

So phase

deduced from

X-ray experiments

on

Cs

and

Ro

^{racemic mixture}

is orthorhombic

2/c2/m2j/m

with _an absence of

periodicity

ⁱⁿ the ab

plane

and _a lD

periodicity along

_c

(Fig.

I

la).

Based upon

electro-optical

observations _on films

[12],

we can

assume that the local array derived from this symmetry ^is

preserved

in the

So~ phase.

All the

mirror

planes disappear,

^{the twist axis normal} to the

layer plane

remains but the

pitch

associated _to this axis becomes incommensurate with the smectic

periodicity

while

being

close to it. Therefore the twisted

So~ phase

can be described _as a twisted

Sc~

with _a

pitch nearly equal

to twice the

layer

thickness

(Fig. I16).

In fact the

electro-optical properties

^of the twisted

So~

are not consistent with the elastic model

I]

established for the twisted

Sc~, extrapolated

at

small values of the

pitch

since in such _{a case} the transition toward the SmC will _occur at a very

g g

Fig.

II. Schematic

representation

of the

So

structure in its racemic state (al and in _an

optically

active state (b).

high

value of the eIectric field threshold E~:

E~ =

[(ar/4)~

_(K@

_{~)/p~] q~}

,

where K is the twist elastic constant, p~ the spontaneous

polarization,

and q the _wavevector of

the helicoidal array. If _{one assumes} that K has the _same order of

magnitude

in both

So~ ^and

Sc~ phases,

the threshold field E~ ^{would be six orders of}

magnitude higher

than it is

actually

ⁱⁿ our

experiments.

In fact the elastic interaction between two

adjacent layers

of the

So~ phase

has to be reconsidered. The elastic interaction between

neighbouring layers

ⁱⁿ an

antiferroelectric structure has been

already

modelled

by

^Orihara and Ishibashi

[13]. They proposed

a

phenomenological

model in order to describe _{a more}

complex

_sequence

SCA~-Sc~~-Sc~

^{where the structure of} SCA is ^{that of} our

So~,

^and

Sc~~

^is a ferrielectric smectic

phase. Actually taking

^into account the fact that in _{our case} the

layer

thickness and the tilt

angles

do not vary

throughout

the

So~-Sc~

transition _a

simpler

model _can be

proposed.

This is

an

Ising-like

model which has been

developed

^elsewhere

[14].

We recall it

shortly

here _: the free energy is

expressed

_as a function of

Ap

where _p is the azimutal

angle

of the director in the

layer plane,

^and Ap ^{its variation between} two

adjacent layers.

The free energy is

developed

into Fourier components ^of Ap,

taking only

the first two harmonics into _account. This model is consistent with the

experimental

data _on the reentrant mixture

To.

The spontaneous

polarization

in the

Sc phase

has to be consistent with the molecular

properties

of the series here studied. The molecular

dipole

is estimated for different conformations of the aromatic _{core :} it varies between 0.I and 5.2

Debye depending

_on the relative orientations of the

carboxylic

and Schiff base groups which

give

the main

dipolar

contributions. We note that the

highest

values

correspond

to a syn- conformation of the

terephthalidene

_core. Therefore in order to

explain

the value of the spontaneous

polarization

in

pure

C~

or

C8

active

compounds

_we must admit that syn- conformations _are favoured in the smectic

phase,

as it is the _case, in the _same temperature range, for the

alkyl-terephthalidene- bis-butyanilines

which have _a similar core

[15].

Moreover the rotation of the molecule around its

long

axis is

widely hindered,

at least _at the _core level. This last

point

confirms _our first X-

ray observation about the

in-plane anisotropy

of the local array

[2].

We _note that _our

experimental

value of the spontaneous

polarization

at 110 °C is _one order of

magnitude higher

than the _one

measured,

at the

clearing temperature,

on

So

thin films

by

Galeme and

Liebert[12].

However the difference of

temperature (high

temperature

increasing

the molecular

disorder),

and _a too low estimate of the elastic constants could

explain

this

discrepancy (I).

The Goldstone mode

amplitude

measurement

gives

_an estimate of

P~/K.

_This

estimate is

compatible

^with our value of the

polarization

and _an elastic _{constant one} order of

magnitude higher,

but is _not

compatible

^with a ten times lower

polarization. Finally

incoherent

quasi-elastic

neutron

scattering experiments

_seem to confirm both the

anisotropy

of the rotational motion of the molecule and the

predominence

of syn conformations

[16].

The behaviour of the spontaneous

polarization

_versus the

composition

in mixtures of

C~

antipodes

shows that the conformation of individual molecules is _not affected

by

_some

specific

interaction between the two enantiomers. We have also

shown, looking

at

X-ray

pattems ^of mixtures of the two

homologues,

that _no

segregation

of molecules of different

chirality

and chain

length

takes

place

_{even at a} short scale. However the decrease of the spontaneous

polarization

in mixtures

C~(S,

S

)

+

C8 (S,

S

)

can be related to an

increasing

rotational disorder

induced

by

^the difference in chain

lengths.

If _{now we} tum our attention _to

C~(R, RI

+

C8(S, S) mixtures,

it follows that a small amount of short chains in

longer

_ones increases

dramatically

the molecular rotational and conformational

disorder,

while _a small _amount of

long

chains in shorter _ones does not

change

this molecular disorder

significantly.

This _can be

understood _as holes _{are more} effective in

creating

disorder than

bumps

but since

only

0.25 moles of

C~

can cancel the

spontaneous polarization

of _a

C~(R, R)

₊

C~(S, S) mixture,

each

C~

^{molecule disturbs} a

large

number of

C8

^{molecules which} means that _a

C~(R, R)

molecule is surrounded

preferentially by C8(S, S)

molecules. This _must be understood _{as an}

ordering

interaction between molecules of

opposite chirality

rather than between molecules with different chain

lengths, explaining by

the way the

positive azeotropic

behaviour of the

binary phase diagrams

of

antipodes

and the wide difference in the behaviour of

C~(S, S)

+

C8(S, S)

in

comparison

with

C~(R, R)

+

C8(S,

S

)

mixtures.

Conclusion.

The series of

2-alkyl 1,4 terephthalydene-bis-4-aminocinamates

_{presents a very} unusual

polymorphism

_: these derivatives

belong

to _a class of

compounds

with _a ferro-antiferroelectric transition between fluid smectic

phases.

We have

provided

evidence for _a reentrant sequence in temary ^{chiral mixtures and} quatemary ^racemic

(pseudo binary)

mixtures. The strutural

study

of this last mixture

brings

a confirmation of the existence of _a

zigzag

_array of molecules

(21

axis

perpendicular

to the smectic

planes)

in the bulk

So phase.

^{Moreover the}

pitch

in this

phase

is

always

_very close _to twice the

layer thickness, reaching

this value

only

for the racemic mixtures. This molecular

organization

is the _same for

optically

_pure

compounds

and for every

composition

of the two enantiomers _as

well,

in fact

implying,

that the

So~ phase

is _{not a true} antiferroelectric

phase.

The

electro-optical properties

of the smectic

phases

_are consistent with the structural observations. Moreover the unusual behaviour of the

clearing

temperature ^of

(1) Indeed the

in-plane anisotropy

allows _{us to} think that the elastic _constants in the smectic plane for this

compound

_may be much

higher

than the value taken in that paper

by

analogy with usual

Sc.

binary

mixtures of

optical antipodes

_can be related to

intra-layer

local interactions between molecules of different

chirality.

Let _us remark that the increase of the

clearing point

with _a

decreasing optical activity

is

nearly

the _same for the

C~

and the

C8

^mixtures

despite

their different

polymorphism. Finally

the addition of the _meso stereoisomer

(R, S)

does not disturb

significantly

the structural and

thermodynamical properties

of these systems

[2, 7].

It _seems therefore that the existence of _a well stratified structure could

explain

the weak influence of the

stereoisomery

_upon the smectic

polymorphism.

Since the

So~ phase

has _a

pitch

of the order of 60

I,

_the three-dimensional

tetragonal phase

of unit cell 75. 5 _x 75.5 _x 68.4

I (1422)

which exists

just

below the

clearing

temperature ^{in the}

quasi

_pure ^active

C8

derivatives _can be

compared

to the blue

phases II 7]

since the lattice cell

constants scale _as the

pitch

of the

So~ phase.

Therefore the

vicinity

of this

phase

introduces the

question

of the existence of defects in the

So phase.

If there is _no evidence _at all of _a

regular

array of

w walls

through X-ray

diffraction and

optical experiments,

however isolated lines

corresponding

to a ar

change

of the azimutal

angles

_can exist inside

layers (such

lines _{are seen}

on thin

films) [3].

Moreover _at the

transition,

the

growing

_process of the

Sc phase

inside the

So phase

is faster when the

layer planes

are oriented

perpendicularly

to the _mean interface between the _two

phases,

^thus

implying

_a coexistence of the two

phases

at a

microscopic

scale and _a defect mediated

growth (Fig. fig). Finally

_{we can}

point

out that _we have _seen

anisotropic

diffuse streaks around the first-order

Bragg peak

_on

X-ray pattems

^{of the}

To

^mixture as well in the

So

as in the

Sc phases.

These streaks remind _one of similar streaks _{seen on} the

X-ray

pattern ^{of the}

SA phase

of _some side chain

liquid crystalline polymers.

Therefore it _seems that _a

great density

of dislocations is _at the

origin

of these diffuse streaks in _our system as it has been

shown for the _case of the

polymers [18].

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Phys.

Lett. France 36 (1975) 69.

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