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X-ray investigations and magnetic field effect on anematic phase of disc-like molecules

A.M. Levelut, F. Hardouin, H. Gasparoux, C. Destrade, Nguyen Huu Tinh

To cite this version:A.M. Levelut, F. Hardouin, H. Gasparoux, C. Destrade, Nguyen Huu Tinh. X-ray investigations andmagnetic field effect on a nematic phase of disc-like molecules. Journal de Physique, 1981, 42 (1),pp.147-152. �10.1051/jphys:01981004201014700�. �jpa-00208983�

147

X-ray investigations and magnetic field effect on a nematic phaseof disc-like molecules

A. M. Levelut, F. Hardouin*

Laboratoire de Physique des Solides, Université de Paris-Sud, 91405 Orsay, France

H. Gasparoux, C. Destrade and Nguyen Huu Tinh

Centre de Recherche Paul-Pascal, Université de Bordeaux I, 33405 Talence, France

(Reçu le 14 avril 1980, révisé le Il septembre, accepté le 15 septembre 1980)

Résumé. 2014 Les diagrammes de diffraction des rayons X d’échantillons non orientés de la mésophase fluide demolécules en plateau, les hexaalkoxybenzoates de triphénylène, sont identiques à la figure de diffraction d’unephase nématique non orientée de molécules en bâtonnets. De plus, nous montrons le caractère uniaxe de ce milieuanisotrope qui possède une anisotropie diamagnétique négative, car les molécules plates s’orientent parallèlementles unes aux autres sous l’action d’un champ magnétique tournant. Enfin, les diagrammes de diffraction d’échan-tillons orientés du dérivé hexahexyloxybenzoate montrent l’existence d’un ordre prétransitionnel analogue à celuiqui est observé dans la phase nématique et au voisinage de la transition nématique SmC. Ici, cet effet prétransi-tionnel est relié à l’existence d’une phase de basse température que nous décrivons et où les molécules sont empiléesen colonnes, leur plan moyen étant oblique par rapport à l’axe de la colonne.

Abstract. 2014 The X-ray diffraction patterns of powder samples of the fluid mesophase of the hexaalkoxybenzoatesof triphenylene which is a disc-like molecule are similar to the diffraction patterns of non orientated nematic phaseof rod-like molecules. Furthermore, we give evidence for a uniaxial character of this anisotropic medium with anegative diamagnetic anisotropy, the flat molecules are orientated parallel each other by a rotating magnetic field.In addition, the X-ray patterns of orientated samples of the hexyloxy compound revealed pretransitional effectsin this nematic phase of disc-like molecules (ND). This short-range translational order, sort of skewed cybotacticgroups originates from the existence of a low temperature columnar phase with an original tilted packing heredescribed.

J. Physique 42 (1981) 147-152 JANVIER 1981,

Classification -

Physics Abstracts61.30 - 64.70E

1. Introduction. - Recently a new mesogenicseries [1] of disc-like molecules has been discoveredwhich exhibits a highly fluid mesophase, the textureof which is similar to the classical nematic one’.

Moreover, it can be compared to the textures seen ithe carbonaceous mesophase which is a mesomorphistate of a complex mixture containing polyaromaticmolecules with some aliphatic parts [2]. Therefore,the new phase of disc-like molecules could be a purecomponent simple model for the carbonaceous mesot-phase.We have performed X-ray and magnetic investi-

gations in order to obtain some information on thestructure and the symmetry properties of this newfluid anisotropic phase.

2. Experimental technics. - We have studied twoderivatives of the hexaalkoxybenzoates of tripheny-lene (Fig. 1) with alkyl chains of six and eleven carbons.Each compound exhibits two mesophases : the higbtemperature phase is fluid (with similar textures) forboth compounds. The low temperature phase is muchmore viscous. The texture of the C6 derivative is amosaic one [3] which is observed for the first time withdisc-like compounds while the texture of the C11derivative is similar to the texture of the D1 phasesof the alkanoates of triphenylene [4, 5].

X-ray investigations have been made on powdersamples with a Guinier camera using the CuKàradiation. At the same time, samples held in a Lin-deman glass capillary have been studied within a3 kG magnetic field and illuminated with a point-focusing X-ray beam CuK« perpendicular to the

magnetic field.* Permanent address : Centre de Recherche Paul-Pascal, Univer-

sité de Bordeaux 1, 33405 Talence, France.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01981004201014700

148

Fig. 1. - Chemica.l formula of the hexaalkoxybenzoates of tri-

phenylene.

To determine to what extent an extemal magnetiefield is apt to induce a single domain (in the bulk)of the highly fluid mesophase, we have also performedmagnetic measurements. First, by means of the classi-cal Faraday method [6] (the magnetic field has avertical gradient) the magnetic susceptibilities arçmeasured. In addition, we have undertaken someexperiments with a homogeneous rotating magneticfield of about 11 kG [7]. This latter technique is necesrsary to define the sign of the diamagnetic anisotropy.

3. Results. - 3.1 X-RAY DIFFRACTION PATTERNS

OF THE FLUID MESOPHASE. - X-ray diffraction pat-terns of samples held in a capillary tube outside ofthe magnetic field in the fluid mesophase (Fig. 2) arévery similar to the diffraction patterns of the isotropeliquid phase of the C11 derivative or other disc-likemesogenic compounds (we have not attempted toreach the isotropic phase for the C6 compoundT > 274°C). We have two broad diffraction rings,

Fig. 2. - X-ray diffraction pattern of the fluid mesophase of a nonorientated sample (C6 derivative).

the inner ring being of high intensity compared to theouter ring. The outer ring lies at

while the inner is related to the molecular size and

corresponds to d = 20.5 A for the C6 and d = 26.5 Àfor the Cl,. This is very similar to the diffraction by anon orientated nematic phase. Nevertheless a strikingdifférence lies in the relative intensity of the two rings :the inner is of higher intensity in the case of this flui4mesophase of disc-like molecules while the outer ismore intense for nematics of rod-like molecules.Moreover, the 3 kG magnetic field has no effect on thefluid mesophase of the Cl 1 derivative.Going down from the isotropic phase up to the

columnar phase appears, we observed only a slightnarrowing of the inner ring for X-ray patterns in thefluid phase domain. In contrast, in the case of the C6derivative the magnetic field orientates the sample(Fig. 3). We have to notice that it is necessary to heatat least 20 °C above the transition temperature from

Fig. 3. - X-ray difïraction pattern of the fluid mesophase of anorientated sample of hexyloxybenzoate of triphenylene (the arrowindicates the direction of the director). a) 3 hours - 220 °C ; b)1 hour - 205 °C.

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the columnar phase in order to have a good alignmentof the sample. Thus, the viscosity of the mesophase haslikely an influence upon the sample alignment obtainedby a rather weak magnetic field (- 3 kG).The best orientation effects are obtained after a

rotation of 7c/2 of the capillary tube around its axiswhich is perpendicular to the magnetic field. The sameprocess of orientation has been successful for thenematic lyotropic mesophase of type II [8] which issupposed to have identical symmetry and magneticproperties as the disc-like nematic phase (see 3.4).Further rotation of the sample does not induce 4change in the diffraction pattern. We develop furtherthe effect of a rotating magnetic field on this phase(see 3.4). 1

By changing the angle between the X-ray beam andthe magnetic field we have established that the diffrac..tion pattern of the C6 fluid phase (Fig. 3) has an infmiterotational symmetry axis parallel to the axis of thecapillary tube. One can see two broad discs at

1

while the inner ring is split into four broad spots.The structure of the inner ring is similar to the dif-fraction patterns of a nematic phase of elongatedmolecules with skewed cybotactic groups [9] howevetthe angular extension of the four spots on the innefring is greater in the disc-like compound (these diffusespots are poorly visible on the over-exposed patternsof the figure 3). The presence of such spots led us tô study the columnar phase in order to check if they areindicative of a pretransitional local order. Neverthe-

less, the two outer discs lying at u - 1 A -1 areY g d .characteristic of a preferred orientation of the molecu.lar lateral substituents, these elongated substituentslying in the plane of the applied magnetic field (planeperpendicular to the axis of the capillary tube).

3.2 X-RAY DIFFRACTION PATTERNS OF THE COLUM-NAR PHASES. - Non orientated sample of the columnarphase has been obtained from the Cl 1 derivative.The powder pattern of this mesophase is similar

to the powder pattern of the Dl phase of alkanoatesderivatives of triphenylene [10]. Thereforé, this phaseis likely constituted by columns of molecules in arectangular two dimensional lattice (space group Pgg)the lattice parameters are a = 51.8 Á, b = 32.6 À(table I) and the specific area AIM = 2.58 x 10’ cm2 g- 1(A is the area of a column and M the molecular mass)and this specific area is similar to that of the dode-canate of triphenylene in the Dl phase [10]. We recallthat in this phase the molecules are stacked in column$with their plane perpendicular to the column axis,the columns forming a herring-bone pseudo-hexagonalarray.

In the case of the C6 derivative the orientationobtained in the fluid phase largely remains in thé

Table I. - Reticular distances observed on powderpatterns of the columnar phases.

columnar phase by slow cooling. Although thèse

samples are not at all single domains, there is gene-rally one principal domain in the X-ray beam. Indeed,rotating the sample, the magnetic field of the X-raydevice is unable to reorient the molecules in this veryviscous phase. Then it is possible to obtain variousX-ray patterns corresponding to successive rotationsof the capillary tube around its axis and describingthe successive orientations of a same principal domain.One example of such an X-ray pattern is given figure 4afor which we can analyse qualitatively the structure.

Fig. 4a. - X-ray diffraction pattern of the tilted columnar phase ofthe hexyloxybenzoate of triphenylene. The intense 110 reflectionsare pointed out by arrows. The other spots are at least one order ofmagnitude less intense and originate from other domains.

Two sets of two broad reflections are observed at

The first one indicates a liquid-like order of thesubstituents and the second, which is only visible forthe columnar phase, is characteristic of a linear

stacking order of the cores of the molecules.This is clearly understood if one considers the

diffraction pattern of a liquid-like line of points : thiswould be constituted of two diffuse planes perpen-dicular to the column axis ; the distance between thesetwo planes is related to the mean distance betweenthe points on the line. Now, if we have disc-likemolecules instead of the points, the intensity in the

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diffuse plane is modulated by the molecular structurefactor which in this case has a cylindrical symmetry.Consequently the intensity scattered by one columnis localized into two diffuse discs, the axis joining thécentres of the discs being perpendicular to the mole-cular plane. The distance between the two discs givesthe mean distance between two molecules in a column.At last, me diameter of the diffuse disc is related to thesize of the disc-like scattering entity and in this mannerwe can see in our case that the central part of themolecules alone contributes to such kind of broad

reflections. In addition, the location of the 1 A- 4.5

spots indicates that the lateral groups are more or lessin the same plane as the molecular cores. Near thecentre of the pattern (Fig. 4a) two intense équivalentpoints are seen on reciprocal rows going through thçorigin. These reflections originate from the twodimensional lattice formed by the arrangement of thecolumns. The line joining these two spots gives thédirection of a plane perpendicular to the column axig.Therefore, remembering what was described for thédiffuse discs, we see on figure 4 that the plane of thë

Fig. 4b. - Projections of the reciprocal space on the (001) and (110)planes : 1 : equatorial section ofthe Ewald sphère ; 2 : section of the

Ewald sphere at the level 1 o A-1; 3 : localization of thescattered intensity by the molecular cores.

molecules is not perpendicular to the column axis andwe conclude for the first time in favour of a tiltedcolumnar phase. Unfortunately we are unable tgdeduce the reciprocal lattice of column arrays fromsuch samples since they are too badly oriented forthis purpose. Powder patterns enable us to assign acentred rectangular lattice for the column array ;the lattice parameters are, in a plane perpendicularto thé column axis (Fig. 5) a = 30.7 À, b = 28.4 À(table I). The symmetry properties of this phase can be

described by the three dimensional monoclinic spacegroup Cm or C2/m (depending on the molecularconformation and on an eventual orientational disor-

der) in which we suppress the periodic translationalorder along the c direction. We have also to point outthat as there is no periodicity along c, the column

Fig. 5. - Schematic representation of the lattice of the tiltedcolumnar phase of the hexyloxybenzoate of triphenylene.

lattice is described in a plane perpendicular to thecolumn axis that is to say perpendicular to the cdirection. We assume also that the shorter axis b is

perpendicular to the mirror plane because this

assumption ensures a better packing of the columns.Thus it is not the conventional description for amonoclinic cell.

_

Figure 4b shows the projection of the reciprocalspace on the a* b* plane and its intersection with theEwald sphere when the X-ray beam is parallel to thisplane and perpendicular to the 110 ) direction.The X-ray pattern of figure 4a corresponds to anorientation which differs from the scheme 4b by arotation ( N 30°) of the sample around the 110 >row. From the figure 4a, one can estimate the angle acbetween the core plane and the 100 direction : thisangle a is of the order of 550 (Fig. 5).The specific area of a molecule can be deduced from

the area per column :

since there are two columns per rectangular lattice.The area of a molecule is taken in a plane parallel to

the molecular core Amal = 435.9 Â2 ; and from thecos 55°mass of one molecule 2.94 x 10-21 g we deduce aspecific area of 2.58 x 10+’ cm2 g-1, therefore weobtain the same value for the C11 and C6 compounds.

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3. 3 PRETRANSITIONAL LOCAL ORDER IN THE FLUIDMESOPHASE. - From the structure of this phase weare now able to understand the diffraction pattern ofthe C6 nematic phase. The four diffuse spots in theinner ring are at the same distance as the 110 reflectionsof the columnar phase. In the nematic phase smalldomains with a columnar order are present : as thedomains are small we only observe the first order ofreflection and moreover it is broad. In fact, the fourspots correspond to the intersection of two circleswith the photographic plate ; these circles are perpen-dicular to the capillary axis and we can assert thatall the discs are parallel, the order of the centres ofmass is liquid-like whereas the column directions ofthe different domains are on a cone of 420 of half

appex.The axis perpendicular to the molecular plane is

perpendicular to the plane defmed by the magneticfield and the rotation of the capillary and we canmeasure the apparent diameter of the molecule lookingat the first intensity maxima in the direction of themagnetic field (c.-- 23 Â).

3.4 MAGNETIC MEASUREMENTS. - The magneticexperiments were performed on different samples ofthe same compounds using two different apparatus.The Faraday device is equipped with a magnet provid-ing fields up to 15 kG. Moreover the rotating magnetgives a maximum field of 11 kG.

First, in order to check the influence of the magneticfield upon the degree of orientation of the nematicphase we have studied the diamagnetic susceptibilityof these compounds. The thermal evolutions of thisparameter x (measured in the direction of the magneticfield, during slow cooling from the isotropic phase)corroborate the existence of a spontaneous orientatingeffect of the applied field at the isotropic-fluid meso,phase transition for both C6 and C11 compounds(i.e. a sharp decrease of the absolute value of thediamagnetic susceptibility, figure 6). In addition,we have to note that, contrarily to the columnar phasesof triphenylene derivatives investigated elsewhere [ 11],this effect as a function of the magnetic field value issaturated for H >, 5 kG.

Furthermore, the study of the behaviour of a samplehung by a quartz wire in a homogeneous rotatingmagnetic field of about 11 kG [7] shows that the fluidmesophase is isotropic in the rotation plane of themagnetic field : no magnetic torque occurs and thesample remains in its initial position. Thus, a singledomain of this fluid mesophase is obtained with a

rotating magnetic field : this medium is magneticallyuniaxial with the preferred axis perpendicular to themagnetic field rotation plane. For an uniaxial system,the magnetic anisotropy is defmed by

where X~ Il is connected to the magnetic susceptibilitymeasured in a direction parallel to the main axis and

Fig. 6. - Discontinuity of the magnetic susceptibility at the iso-tropic-fluid mesophase transition.

xi to the susceptibility measured perpendicularly tothis axis.

In the case of this fluid mesophase xl is the diama-gnetic susceptibility in the magnetic field direction

thus | X~ |1 > 1 Xl. 1 (x ~ and X, 0) and the diama-gnetic anisotropy in this mesophase is hence negative :.

We can note the strong tendency for this mesophaseto give extinction between crossed polarizers : thisalso suggested an optically uniaxial mesophase.Moreover, the upper X-ray study has pointed out thatthe aromatic cores of the molecules are parallel to therotation plane of the magnetic field. Thus the pre-ferred axis (i.e. the director) is perpendicular to themolecular core in good agreement with the disc-likemolecule symmetry and with the chemical structureof the molecule (Fig. 1) (the diamagnetism essentiallydepends on the aromatic part and thus the largestmagnetic susceptibility in absolute value is likely per-pendicular to the polycondensed rings).

In order to determine the magnetic anisotropy ofthis phase, we have to notice first that it is not sure toobtain a single domain in a static magnetic field.Indeed, if A x is negative, the director can be orientatednot only in one direction but can point everywhere inthe plane perpendicular to the magnetic field (as it isthe case for the nematic of rod-like molecules of thealkyl bicyclohexyl carbonitriles [12] and for the

lyotropic mesophase of type II [8]).

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Nevertheless, in the limit of our experimentalaccuracy, the value of the susceptibility along themagnetic field direction is unchanged after turningthe sample around a vertical axis in order to improvealignment. Therefore, the diamagnetic susceptibilitymeasured in the direction of the static magnetic fieldby the Faraday method seems to correspond to thesusceptibility perpendicular to the director (i.e. /J.

If we assume, like for rod-like nematogenic com-pounds, that the average magnetic susceptibility doesnot depend on the temperature and on the state of thematerial we can write :

and then :

Thus, as for a classical nematic [6], the magneticsusceptibility measurement in the direction of the held,respectively in the isotropic phase and in the fluidmesophase, lead to determine the magnetic anisotropyof this latter phase. In fact, very small amounts offerromagnetic impurities prevent from obtaining thçintrinsic thermal evolution of the disc-like compoundanisotropy although they do not disturb the discon-tinuity at the transition. Thus near the isotropic phasewe find :

We can explain the different values of OxM betweenthe C6 and the C11 compounds - first consideringthat larger the aromatic over aliphatic parts ratio is,larger the molecular anisotropy is - secondly, maybe the degree of orientational order at the isotropicphase - fluid mesophase transition has to be takenin account. This difference in the magnetic anisotropiesassociated to a higher viscosity of the Cl, couldexplain that we cannot get orientated X-ray patterns(under 3 kG magnetic field) with this compoundcontrarily to what is observed for the C6 compound.

4. Conclusion. - To conclude, our X-ray resultsagree with the first microscopic observations of thefluid mesophase of disc-like molecules. Indeed, thismesophase can be considered as a nematic phase ofdisc-like molecules (ND for short) because there is notranslational order and the flat molecules are more orless parallel each other (from the X-ray patterns it isdifficult to precise the molecular fluctuations).

In addition, we give evidence for an uniaxial cha-racter of this medium with a negative diamagneticanisotropy, this fluid mesophase is spontaneouslyorientated in the bulk by a rotating magnetic field.As for rod-like mesogenic compounds, the X-ray

patterns of orientated samples have revealed pre-transitional effects in the ND phase. Also, as for seriesof rod-like materials, the low temperature orderedphase which succeeds to the nematic phase can exhibitdifferent structures according to the length of thesubstituents. Thus, the possibilities of investigationssupplied by the nematic phase of disc-like moleculesseem to be as large as the expected ones from theclassical nematic phase a long time ago.

References

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[2] GASPAROUX, H., DESTRADE, C., FUG, G., Mol. Cryst. Liq.Cryst. (1980), to be published.

[3] DESTRADE, C., BERNAUD, M. C., GASPAROUX, H., LEVE-

LUT, A. M., NGUYEN HUU TINH, Proceedings of BangaloreInternational Conference (1979), to be published.

[4] DESTRADE, C., MONDON, M. C., MALTHETE, J., J. PhysiqueColloq. 40 (1979) C3-17.

[5] DESTRADE, C., MONDON-BERNAUD, M. C., NGUYEN HUU TINH,Mol. Cryst. Liq. Cryst. Lett. 49 (1979) 169.

[6] GASPAROUX, H., REGAYA, B., PROST, J., C. R. Hebd. Séan.Acad. Sci. Paris 372 (1971) 1168.

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TINH, Liquid Crystals of one and two-dimensional order,Springer Series in Chemical Physics (Springer, Berlin,Heidelberg, New York) 1980.

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