Anomalies of periodicity in some liquid crystalline cyano derivatives

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Anomalies of periodicity in some liquid crystalline cyano derivatives

F. Hardouin, A.M. Levelut, G. Sigaud

To cite this version:

F. Hardouin, A.M. Levelut, G. Sigaud. Anomalies of periodicity in some liquid crystalline cyano

derivatives. Journal de Physique, 1981, 42 (1), pp.71-77. �10.1051/jphys:0198100420107100�. �jpa-

00208993�

Anomalies of periodicity ^{in some} liquid crystalline cyano derivatives

F. Hardouin, A. M. Levelut (*) ^{and G.} Sigaud

C.R.P.P., Université de Bordeaux I, ³³⁴⁰⁵ Talence, ^France (Reçu ^le 1 er juillet 1980, accepté ^{le 15} septembre 1980)

Résumé.

Dans les phases nématique ^et smectique ^de composés mésomorphes possédant ^un groupe terminal

nitrile, nous avons constaté des anomalies dans les fluctuations de la densité massique. ^En particulier, ^nous révélons,

pour la première ^{fois dans un} corps pur, ^une transition SA-SA.

Dans la phase SA ^haute température (SA1), ^une modulation à grande distance de périodicité égale ^{à 1,6} longueur moléculaire, ^{coexiste avec une} modulation à courte distance de période égale ^{à la} longueur moléculaire. La phase SA2 ^basse température correspond ^à l’apparition de deux taches de Bragg qui ^{sont les deux} premiers ^{ordres de}

diffraction par ^une modulation de période bimoléculaire. Par ailleurs, ^{un autre dérivé} cyané qui possède ^{à la fois}

une phase nématique rentrante et une phase smectique ^{A rentrante révèle} l’existence simultanée et la compétition

dans ces phases ^de deux modulations de densité. Toutefois, l’apparition ^d’une phase ^{rentrante ne} semble pas due à un blocage commensurable ou non de deux longueurs ^d’onde caractéristiques.

Abstract.

Unusual modes for mass density fluctuations are observed by X-ray experiments in nematic and

smectic phases ^{of some} liquid crystalline materials with cyano end group. In particular, ^{for the first time, we} give

evidence for a SA-SA transition in a pure compound (DB7) : ^{in the} high temperature SA1 phase ^a long-range ^den- sity ^{wave with a} wavelength equal ^to 1.6 molecular length coexists with a short-range wavelength ^{of about one}

molecular length. ^{At low} temperature ^a transition towards an SA2 phase ^{occurs and we see two} sharp Bragg spots which correspond ^{to the two} first-orders of diffraction by ^a modulation of two molecular length periodicity.

Further another cyano derivative which exhibits both ^reentrant nematic and reentrant smectic A phases ^reveals

the simultaneous existence and the competition ^{of two} modulations in these phases. ^{But the} occurrence of a reen- trant phase ^{does not seem to} be connected to a commensurate or incommensurate lock-in of the two characteristic

wavelengths.

Classification

Physics ^Abstracts

61. 30

64. 70 E

Introduction.

From the study ^{of the} phase ^tran-

sitions in compounds ^{with a} polar nitril end group,

we have revealed these last two years ^{two new} pheno-

mena investigated ^{in detail} by X-ray scattering. ^We

gave evidence :

-

For a true bimolecular smectic A with a layer spacing ^d quasi-equal ^{to two molecular} lengths 1 (d ro..I 1.9 1). Adding ^non polar impurities together

with increasing temperature, ^a phase transition arises

giving ^a smectic A with a half-layer spacing, ^{i.e. a}

monomolecular smectic A [1, 2, 3].

-

For a doubly ^reentrant polymorphism ^nematic-

smectic A-reentrant nematic-reentrant smectic A in a

pure compound ^at atmospheric pressure [4]. ^{In the}

high temperature SA phase, ^the layer spacing ^is larger

than one molecular length (d - ^1.2 1), ^as frequent ^for

cyano compounds [5], ^{while the reentrant} SA ^exhibits

a layer thickness close to 1 and also a modulated

superlattice ^discussed furtherhere [6].

These two original results have been obtained from two basic compounds : ^the cyanobenzoyloxybenzoate

of pentyl phenyl (« ^DB5 ») ^{in the first case and the} octyloxybenzoyloxycyano ^stilbene (« ^T8 ») ^{in the}

second one.

We _carry on our X-ray study ^{on new} derivatives

recently synthesized ^{in our} laboratory ^and chemically closely ^{related to either reference} compounds, although they ^{lead to rather different} behaviours. We analyse

the X-ray patterns ^{obtained from} samples ^orientated

in nematic phase by ^a magnet ^{and with a} X-ray ^beam perpendicular ^to the field direction (CuKa

^1.54 À,

distance from the sample ^to the film : 67 mm). ^The

relative _accuracy on d is 5 ^x 10-3.

(*) Laboratoire de Physique ^des Solides, Université de Paris- Sud, ⁹¹⁴⁰⁵ Orsay, ^France.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198100420107100

72

1. Compounds analogous ^{to the « DB5 ».}

First,

let us recall briefly ^the analysis ^{of the DB5} patterns [3] :

-

In the nematic phase, ^{we note at small} angles

two diffuse spots ^with nearly ^{the same} intensity ^and

with wavevectors ql and _q2 such as q2

2 _ql.

-

In the smectic A, these two diffuse scatterings

are condensed in two intense Bragg reflections 001 and 002, ^which correspond ^{to a} bimolecular modu- lation : d - 1.9 l.

1.1 4-CYANO BENZOYLOXY-4’ PENTYL AZOBENZENE : ^:

« C5 AZO » (Table I).

One of the benzoate linkage

is now replaced by an azo one. This compound

exhibits a N-SA transition at 151 OC.

As previously observed in the DB5, in the nematic

phase ^{two diffuse} spots appear ^at small angles (Fig. la) :

one, ^more intense, corresponds ^{to a} damped ^modu-

lation with a wavelength ^{close to one molecular} length (q2) ; the other diffuse scattering (ql) corresponds

in this case to mass density fluctuations of distinct

wavelength q2 =1= 2 qb q2lql - ^1.5 (T ’" 160 °C).

Also in contrast with the DB5, in the smectic A

phase, ^{the most intense spot is alone condensed} (q2) indicating ^a layer spacing ^near the molecular length (Fig. 1 b). The second diffuse spot :

-

First, splits ^{out the} [001] ^row (Z ^{axis corres-} ponding in the real _space to a direction normal to Table 1.

X-ray results for ^{the various} investigated cyano compounds.l : length of the molecule in its most extended

conformation (Dreiding stereomodels). ² n/q2 : fluctuation wavelengths of the « monomolecular » periodicities

in a direction perpendicular ^{to the} layers. ² n/qlz :fluctuation wavelengths ^{at smaller} angles ^{in a direction} per-

pendicular ^{to the} layers. ² nlqlx : fluctuation wavelengths of the modulation in the smectic planes. ^{The transition} temperatures ^{in OC.}

(*) ^Reentrant SA.

Fig. ^1.

X-ray patterns ^{of the} 4-cyano benzoyloxy-4’ pentyl azobenzene (« ^{C5 AZO} ») : a) nematic phase, ^T

160°C; b) ^smec-

tic A phase, ^T

^118°C.

the layers) ^{into two diffuse} spots ^{with a} large angular spread.

-

And second, ^the wavevector component along Z(qlZ) ^{tends to} decrease with decreasing temperature :

at T - 120°C, q2/ q lZ /"OV ^1.7.

1.2 4-CYANO BENZOYLOXY-4’ PENTYL STILBENE : :

« C5 STILBENE » (Table I).

^{The same benzoate}

group is now replaced by ^{a stilbene} linkage. ^This

material presents ^{a more} complex polymorphism ^with

a nematic, ^{a smectic A and a low} temperature ^{smectic B} phase. ^As already ^mentioned [7] ^{an ordered} phase ⁱⁿ

aromatic compounds ^{with a} polar cyano head group is seldom observed. Despite ^{the near} crystallization,

the SB ^structure has been confirmed by ^{means of} powder diagrams performed ^{with a Guinier camera.}

In nematic and smectic A phases, ^the X-ray photo- graphs ^{are similar to} those obtained with the azo

compound (Fig. 2a).

In the SB phase, ^{the diffuse} scattering ^{of small}

wavevector (q 1 ) ^remains clearly split ^and along ^the

Z axis we still find q21qlz "-1 ^1.7 (Fig. 2b).

Thus, ^{it seems} that these singular fluctuations modes

Fig. ^2.

X-ray patterns of ^the 4-cyano benzoyloxy-4’ pentyl

stilbene (« C5 STILBENE ») : a) ^{smectic A} phase, ^T

182oC;

b) smectic B phase, ^T

^135°C.

can take place whatever the thermodynamically ^stable mesophase is, ^{and in} particular they ^{are not at all ruled}

out by ^the arising ^{of a} translational order of a S.

type within the layers. This last remark is supported . by ^recent X-ray observations carried out in the SE phase of another _cyano compound by ^{G. J.} Brownsey

and A. J. Leadbetter [8].

Furthermore, the dumbell shape fluctuations could recall either skewed cybotactic groups of smectic C type ^or the existence of a modulation more or less

damped within the layers.

1.3 CYANOBENZOYLOXYBENZOATE OF HEXYL PHE- NYL « DB6 » (Table I) ^AND CYANOBENZOYLOXY- BENZOATE OF HEPTYL PHENYL « DB7 » (Table I). - Referring ^{to the DB5, the} change ^{of a} linkage ^between

two benzenic rings clearly disturbs the nature of the smectic A phase. ^{One can} consider that the polarity plays ^an essential role in the stability ^{of the} bimolecular

SA phase and that it will be less disturbed if the rigid

core is unchanged whereas the aliphatic ^chain length only ^varies.

The DB6 has a NSA transition at 165.5°C. Indeed

74

the X-ray photographs show that these phases ^are

similar to those of the DB5 : the SA phase ^with

is bimolecular (2 ¹

⁵⁶ A). By comparison ^{with the} DB5, the second-order Bragg reflection remains

intense, nevertheless in this case weaker than the first- order one (Fig. 3).

Fig. ^3.

X-ray pattern ^{of the} cyanobenzoyloxybenzoate ^of hexyl phenyl (DB6) ^{smectic A} phase (T

¹⁵⁰ °C).

With one more methylene group (DB7), ^{we observe}

a strengthening ⁱⁿ intensity of the diffuse spot ^cor- responding ^{to the} higher wavevector q i ^at the expense of the other one (q2). ^In addition, ^we find that in the nematic phase ^{two collinear} wavelengths ^are present because q2 =1= ² ql (qllq2 - 1.8).

For the DB7 the striking ^{feature is} the existence

(for the first time in a pure compound) ^{of a} SA-SA

transition : as for some binary mixtures of the DB5- TBBA system [1], only ^a nematic-smectic A transition is detected by microscopic observations even though

D.S.C. experiments ^{reveal two} small heat peaks, ^the

first connected to the N-SA transition (172°C) ^and

the second one a few degrees ^lower ( 168 °C).

Moreover, ^we observe ^on the X-ray patterns ^that below the nematic phase (T - ¹⁷⁰ °C) ^{the intense}

diffuse spot ^{is condensed} giving ^a Bragg ^reflection

characteristic of ^a smectic A phase ^with

The second diffuse scattering ^{is not modified and in} particular ^{the ratio} q2lql remains close to 1.8 (Fig. 4a).

We note that, if the nematic phase of the DB7 gives

the same patterns ^{as the C5 AZO and C5} STILBENE,

a contrario in the SA phase ^{of these two last} compounds

the short- and long-range modulations are different from the DB7 high temperature SA ^ones.

At lower temperature (T 168 °C), ^the first-order

Fig. ^4.

X-ray patterns ^{of the} cyanobenzoyloxybenzoate of heptyl phenyl (DB7) : a) high temperature ^{smectic A} phase (T

¹⁷⁰ °C) ; b) ^lower temperature ^{smectic A} phase (T ^{= 161} °C).

Bragg reflection is shifted : 2 nlql

^{54 A} (dfl - 1.9), q2 ^is unchanged ^{and we have now} q2 = 2 ql and this commensurate lock-in leads to an intense Bragg

reflection 002 (Fig. 4b) instead of a diffuse spot.

Thus, ^{at T 168 °C we find} quite ^{the same} bimolecular

SA ^phase of the DB5 (or DB6).

Thus in this case we reveal a SA-SA transition type which does not appear ^as only ^an order-disorder transition of the dipoles changing ^a bimolecular

layering ^{into a} monomolecular one (as ^{proposed for}

the DB5-TBBA system [3]).

In fact, for the DB7 the mass density fluctuations have two incommensurate wavelengths ^{in both nema-}

tic and high temperature ^{smectic A} phases. ^The SA-SA

transition phase ^{is due to the} adjustment ^{of the} large wavelength ² nlql ^to the other 2 nlq2 - 1, ^{that is to}

say ^a commensurate lock-in of these two wavelengths

with a ratio equal 2, giving only ^one modulation with d - 2 1 : ^we label this phase ^as SA2. The successive

analysis ^{of the DB5, DB6, DB7} compounds merely suggests ^{that the} SA-SAZ transition in the DB7 is connected to the aptitude ^{for the} long aliphatic ^chains

to overlap ^{in the} longitudinal ^{direction at} high ^tem-

perature.

1.4 4-CYANOBENZOATE OF 4 HEXYL BENZOYLOXY PHENYL : « DB6s » (Table I).

^{From DB5 or DB6} scheme, ^we modify ^{the sense of the} longitudinal dipole ^{of the} linkage group the more distant from the nitril [9]. Contrary ^to the C5 AZO or C5 STILBENE the mass density fluctuations are hence characterized

by only ^one wavelength such that d f"tooI 0.9 1 in the SA phase (Fig. 5). ^{Therefore we} conclude for the funda- mental role played by ^each longitudinal dipole ^{of the} rigid ^{aromatic core to} generate ^two wavelengths ^{so we}

understand that the SA2 ^{phase is only observed in the}

DB5 family ^{due to its} very high sensitivity ^{to mole-}

cular structure modifications.

Contrary, ^we have elsewhere pointed ^out [10] ^that

reentrant phases ^{can appear in numerous series of}

Fig. ^5.

X-ray pattern ^{of the} 4-cyanobenzoate ^{of 4} hexyl benzoyloxy phenyl (DB6s) ^{smectic A} phase (T

¹⁴⁷ OC).

triaromatic _cyano compounds, with various linkage

groups. Nevertheless, ^{we shall see that a} slight change

in the chain with regard ^{to the reference} compound

T8 induces some structural modifications of the reentrant phases.

2. Compound analogous ^{to the « T8 » : :} 4 cyano 4’ nonanoyloxy benzoyloxy ^stilbene (« ^alkanoate », Table n (1).

^{In this} compound ^the octyloxy ^chain

of the T8 is replaced by ^a nonanoyloxy ^one [11].

The polymorphism is similar to the T8 one with both nematic and smectic A reentrant phases :

N-SA-reentrant N-reentrant SA (enantiotropic poly- morphism).

In the following, ^{we achieve a} comparison ^{of the} X-ray patterns of this last compound with those

previously published for the T8 [6].

In the high temperature SA, ^we find also a layer spacing larger ^{than one molecular} length

but this distance does not decrease with temperature in contrast with the T8 (Fig. 6).

In the reentrant nematic in which this modulation

(ql) ^becomes again highly damped ^{a second diffuse} scattering ^of higher wavevector q2 appears. Its inten-

sity ^{increases as the} temperature ^decreases contrary (1) Communication at the Liquid Crystals ^of

^{and two-}

dimensional order, Garmisch-Partenkirchen, January ¹⁹⁸⁰ (unpu-

blished results).

Fig. ^6.

Thermal evolution of the modulation wavevectors in the 4 cyano 4’ nonanoyloxy benzoyloxy ^stilbene

I1, 12 : intensity corresponding ^{to the} wavevector ql and q2 ;

A, . : Bragg spots ; 0, +,

diffuse scatterings.

76

Fig. ^7.

X-ray patterns of ^{the 4} cyano 4’ nonanoyloxy benzoyloxy

stilbene : a) ^reentrant nematic, ^T

161 °C ; b) ^reentrant nematic,

T

112°C ; c) ^{reentrant nematic at lower} temperature,

T

103 °C ; d) ^{reentrant smectic A} phase, ^T

^{101 °C.}

to the diffuse scattering ^of wavevectors ql (Fig. 7a-b).

Thus, ^we give evidence for simultaneous existence and

competition ^{of two} modulation wavelengths ^with q2lql - 1.3 in this reentrant nematic phase (Fig. 6).

In the low temperature ^{smectic A} phase, ^{the diffuse}

spot ^of higher wavevector q2 ^{is alone condensed and} the layer spacing is hence close to the molecular length :

d

2 nlq2 - 0.97 , ^{as for T8.}

It is important ^{to note} that from the reentrant nematic phase ^{at low} temperature ^{the diffuse} spot ^of

ql wavevector splits ^{off the Z axis} (Fig. 7c) ^{and that}

in the reentrant SA phase ^this splitting ^becomes sharper (Fig. 7d). ^For the T8 this dumbell shape ^is only ^{distinct at low} temperature ^{in the reentrant} SA

and this behaviour is also clearly ^{marked in the} SA phase ^{of another} cyano compound ^{which does not}

exhibit reentrant phases [8].

On over-exposed patterns, we can see that the modulations which .exist in the ALKANOATE as

well as in the T8 are not independent : ^{a third diffuse} scattering ^{is visible} split ^{off the Z} axis and its com-

ponent ^{on this axis} q3Z is such as :

Thus the existence of a superlattice ^{connected to}

the fundamental modulation is evidenced, ^{a tentative}

model of which was proposed ^{in the case of the T8} [6].

.

3. Conclusion. - The influence of the temperature

on the ^mass density fluctuations seems to be qualita- tively ^{the same in the} CS AZO, C5 STILBENE

compounds presented ⁱⁿ part ^{1 as well as} in the T8 and ALKANOATE compounds. Indeed for the _cyano

compounds ^{above mentioned the} longitudinal ^fluc-

tuations are characterized at high temperature by ^two wavelengths. ^Their ratio, apparently incommensurate, only varies from one compound ^{to the other}

Nevertheless if the T8 and ALKANOATE struc- ture is clearly modulated in this longitudinal ^direction parallel ^{to the} long ^molecular axis, in fact for the C5 AZO and C5 STILBENE _may be they ^{are inde-} pendent coexisting density ^waves.

Moreover, ^the experimental study ^{of these four} compounds reveal that the larger modulation wave-

length ^{loses its} unidimensional form with decreasing temperature. Consequently, ^{if we refer to T8 or}

ALKANOATE we think that large wavelength ^fluc-

tuations (> ⁴⁰ Á) ^take place ^{in the} layers; ^these

fluctuations are less and less damped ^{as the} tempe-

rature decreases. This last argument ^{will be} specified

and emphasized ⁱⁿ forthcoming paper conceming

binary system [12].

On the other hand, in the nematic and smectic A

phases of DB5 and DB6, the mass density modulations

correspond ^to only ^one wavelength ^{close to two}

molecular lengths. Nevertheless the study ^{of the} DB7,

the first _pure compound which exhibits a SA-SA2

transition, shows that the SAZ ^{phase can be considered}

as the limit of the commensurate lock-in with a ratio

equal ^{2 of two collinear} wavelengths.

At the opposite ^the occurrence of reentrant phases

does not seem to be due to a commensurate or incom- mensurate lock-in of these two wavelengths.

All these experimental ^facts closely agree with the J. Prost’s theoretical basic arguments [13, 14] ^which consider two incommensurate wavelengths ^{to describe}

the nematic and smectic A phases ^of systems ^of molecules involving ^{a strong} longitudinal dipole.

Acknowledgments. ^{- The authors are indebted to}

Drs. Nguyen Huu Tinh and C. Destrade for the

synthesis ^{of the} compounds ^{and Dr.} J. Prost for valuable discussions.

References

[1] SIGAUD, G., HARDOUIN, F., ACHARD, ^M. F., GASPAROUX, H., J. Physique Colloq. ⁴⁰ (1979) ^C3-356.

[2] SIGAUD, G., HARDOUIN, F., ACHARD, ^M. F., Phys. ^{Lett. 72A} (1979) ^24.

[3] HARDOUIN, F., LEVELUT, ^A. M., BENATTAR, J. J., SIGAUD, G., Solid State Commun. 33 (1980) ^337.

[4] HARDOUIN, F., SIGAUD, G., ACHARD, ^M. F., GASPAROUX, H., Phys. ^{Lett. 71A} (1979) 347 ; Solid State Commun. 30

(1979) ^265.

[5] LEADBETTER, ^A. J., FROST, J. C., GAUGHAN, J. P., GRAY, G. W., MOSLEY, A., J. Physique ⁴⁰ (1979) ^375.

[6] HARDOUIN, F., LEVELUT, A. M., J. Physique ⁴¹ (1980) ^41.

[7] ^{NGUYEN HUU} TINH, HARDOUIN, F., SIGAUD, G., ^Mol. Cryst.

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[8] BROWNSEY, ^G. J., LEADBETTER, ^A. J., Phys. ^Rev. Lett., ^to be

published.

[9] ^{NGUYEN HUU} TINH, DESTRADE, C., ^{to be} published.

[10] SIGAUD, G., NGUYEN HUU TINH, HARDOUIN, F., GASPA- ROUX, H., ^{to be} published.

[11] NGUYEN HUU TINH, DESTRADE, C., JOUSSOT-DUBIEN, M., Mol. Cryst. Liq. Cryst. Lett., ^{to be} published.

[12] SIGAUD, G., NGUYEN HUU TINH, HARDOUIN, F., ACHARD, M.F., LEVELUT, A. M., 8th International Liquid Crystals Confe-

rence, Kyoto ^1980.

[13] PROST, J., Proceedings of ^the Liquid Crystals of

^{and two-}

dimensional order conference, Garmisch-Partenkirchen, 1980.

[14] PROST, J., J. Physique ⁴⁰ (1979) ^581.