THE STRUCTURE OF THE CRYSTAL, SMECTIC E AND SMECTIC B FORMS OF IBPBAC

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THE STRUCTURE OF THE CRYSTAL, SMECTIC E AND SMECTIC B FORMS OF IBPBAC

A. Leadbetter, J. Frost, J. Gaughan, M. Mazid

To cite this version:

A. Leadbetter, J. Frost, J. Gaughan, M. Mazid. THE STRUCTURE OF THE CRYSTAL, SMECTIC E AND SMECTIC B FORMS OF IBPBAC. Journal de Physique Colloques, 1979, 40 (C3), pp.C3- 185-C3-192. �10.1051/jphyscol:1979337�. �jpa-00218733�

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JOURNAL DE PHYSIQUE Colloque C3, supplPment au no 4, Tome 40, Avril 1979, page C3-185

THE STRUCTURE OF THE CRYSTAL,

SMECTIC E AND SMECTIC B FORMS OF IBPBAC

A. J. LEADBETTER, J. FROST, J. P. GAUGHAN a n d M. A. M A Z I D Department of Chemistry, University of Exeter, Exeter, England

RBsumB. - Une Ctude structurale des phases cristallines et smectiques ordonnkes de l'isobutyl-4- (4' phCnylbenzylidbne amino) cinnamate (IBPBAC) a Ctt faite par diffusion des rayons X.

Nous avons affint complktement la structure de phase cristalline : la structure est une bi-couche oh les molCcules sont inclinkes t i 400 par rapport 21 la normale aux couches. A I'intCrieur des couches, les molCcules sont dispostes suivant un rtseau hexagonal dtformt.

A la transition cristal -, S, l'orientation des couches ne change pas. La structure de phase S, est probablement aussi une bi-couche rnais est peut-&tre monoclinique avec j? = 950 : une ambigui'te due au dtsordre intvitable dans les tchantillons apparait. Dans chacun des cas, la structure est difftrente de ce que l'on a prtvu prCcCdemment. Dans les couches, les moltcules sont dispostes suivant un rtseau hexagonal deform6 en accord avec le travail antCrieur.

Dans la phase S,, nous confirmons que les molCcules dans les couches sont dispostes suivant un rtseau vraiment hexagonal. Cependant les molCcules sont peut-&tre inclintes de

-

60 par rapport

?i la normale aux couches. L'ordre du rtseau hexagonal s'Ctend 9 grande distance dans une couche, mais dans la direction normale aux couches, I'ordre s'Ctend jusqu'a 1-2 couches et ce dtsordre conEre

?i la phase un caractkre optique uniaxe. Une seconde possibilite poui'lfbrdie local est une tendance vers un arrangement du type ABC.

..

des couches hexagonales.

Abstract. - Detailed structural work has been carried out by X-ray diffraction techniques on the crystal and ordered smectic phases of isobutyl-4-(4' phenylbenzylideneamino) cinnamate (IBPBAC).

The crystal structure has been fully refined and is a bilayer structure with the molecules tilted at

-

400 to the layer normal and packed in each half layer in a distorted hexagonal arrangement.

The layers maintain their orientation on transforming to the S, phase while the molecules straighten.

The structure of the S, is probably a bilayer but could be monoclinic with a tilt angle of

-

^{50, the}

ambiguity arising because of intrinsic disorder in the specimens. In either case the structure is different from previous suggestions. The molecular packing is distorted hexagonal similar to that proposed previously. The packing in the S, phase is confirmed as truly hexagonal but the molecules may be tilted about 6O relative to the layer normal. The hexagonal net has long range order but the molecular and net displacements reduce the correlation length of planes normal to the layers to 1-2 layers so that the phase is macroscopically uniaxial. An alternative possibility for the local order is a tendency to an ABC ... packing of the hexagonal layers.

1. Introduction. ^-Compounds in the series

were the first t o have the structure of the S, phase characterised in some detail [I].

W e have made extensive studies of two members of this series ; structural investigations by X-ray diffraction a n d molecular dynamics by neutron scattering. The two compounds are n-butyl-(BPBAC)- and isobutyl-(1BPBAC)-4-(4' phenylbenzylideneamino) cinnamate. T h e phase behaviour of these compounds is as follows (temperatures in oC) :

BPBAC Crystal 77 S, 108 S, 172

S, 206 Isotropic Liquid.

IBPBAC Crystal 86 S, 114 S , 164 S, 207 N 214 I.

2. The crystal structure of IBPBAC. - T h e crystal a n d molecular structure of IBPBAC has been determined by direct methods. T h e structure was refined by fullmatrix least-squares calculations. T h e crystal has a head to tail bi-layer structure with the molecular long axes a t a n angle o f

-

400 to the layer normal.

The packing of the molecules in the layers is approximately hexagonal as for the Smectic E a n d B phases. T h e CH=N groups a r e nearly coplanar a n d appear, therefore, t o be important in determining the layer structure.

The changes taking place a t the transitions will be discussed below. Crystal data are given in table I.

Figure 1 shows t h e asymmetric unit of the unit cell looking down the b axis. Figure 2 shows the molecular packing a s seen along b, a a n d c axes respectively.

T h e full details of the structure determination will be published elsewhere.

Samples were supplied by Dr. G. W. Gray and 3. The crystal ^oS, transition. - By cooling from used without further purification. the isotropic liquid phase in a 2 T field it is possible

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979337

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A. J. LEADBETTER, J. FROST, J. P. GAUGHAN AND M. A. MAZID

Crystal data for IBPBAC

Molecular formula C,,H,,NO,

Monoclinic F.W. 382.26

- -

a = 20.115 (2) A Z = 8

b = 5.589 (1) A q000) = 1 632

c = 37.816 (4) A D (calculated) = 1.21 g cm-3

b = 97.53 (1) A D (measured) = 1.19 g cm-3 1 (CuKx) = 1.541 8 A

Systematic absences :

hkl reflections h

+

k = 2 n

+

1 p (CuKa) = 5.20 cm-'

hol l = 2 n + l

w axis : b Space group Cc or C 2/c

Cc is confirmed by structure determination

FIG. 1. -The two-molecule asymmetric unit of the IBPBAC crystal structure.

to prepare highly aligned specimens as shown by the diffraction pattern of a S, sample in X-ray photograph 1. The sample is however more or less randomly

C

FIG. 2. -Packing diagrams of the crystal structure. a) Looking down the b axis ; b) Looking down the a axis ; c) A half layer of the unit cell (a single layer of molecules) looking down the c axis.

The arrow indicates the direction of tilt of the molecules and up and down orientations are indicated by

+

^and^-.

oriented about the layer normal (C*). On crystal- lisation the diffraction pattern of photograph 2 is obtained which shows that the crystal layers retain the same orientation as the smectic layers. The layer spacing changes of course due to the tilted bilayer structure of the crystal (Fig. 2). The principal change is a tilting of the molecules by

-

⁴⁰⁰which is shown together with the random azimuthal orientation of the sample, by the four strong reflections which must come from planes containing the molecular long axes (see Fig. 2). In the S, phase on the other hand these outer reflections are essentially orthogonal to the layer reflections.

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STRUCTURE OF CRYSTAL. SMECTIC E AND B FORMS C3-187

PHOTO 1. - SE aligned by cooling in a field of 2 T, incident beam parallel to Layers.

layers retain their orientation on forming the S, phase, the,molecules straightening up relative to the layer ,ndrmal.

The transition has also been studied in the reverse direction by careful melting of true single crystals to give the S, phase. A typical X-ray diffraction photograph of the S, phase taken with the incident beam along the original crystal C direction is shown in plate 3. This is clearly almost perpendicular to the smectic layers. A photograph taken after rotation of the S, sample by

-

^90°about the original crystal a direction is shown in plate 4 which is essentially identical to 1 and these results show that the crystal

4. The structure of the S , phase. - First consider the results obtained by experiments on the S, phase with the incident beam along the previous crystal C direction. One of two types of diffraction pattern was always obtained in many different experiments.

The first is shown in photograph 3 and the second in 5.

These can be explained in terms of a particular disordering of domains as follows. First the observed diffraction patterns can be built from a rectangular (hk) reciprocal lattice net with

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C3-188 A. J . LEADBETTER, J . FROST, J. P. GAUGHAN A N D M. A. MAZID

PHOTO 2. - Crystal obtained from aligned S,.

PHOTO 4. - SE from single crystal, incident beam down original crystal b direction.

PHOTO 3. - SE from melting of single crystal, incident beam down original crystal c direction.

The disorder required for the simpler pattern (5) comprises two sets of cells with coincident (110) planes giving the sharp diffraction spots with the more arc-like reflections arising from two further domains produced by a -- 300 anticlockwise rotation of the first two. The sharp spots of the more complex pattern (3), which is the one most usually obtained, requires a third set of domains having their [OlO]

direction along the mutual [I101 direction of the original two. The more disordered reflections require 3 domains produced by rotation of the first three anticlockwise through 300. This is illustrated in figure 3 which shows all six possible configurations.

The orientation of the original crystal unit cell is also shown. The various possible domains appear to

PHOTO 5 . - SE from crystal incident beam down c (fewer domains than in 3, see text).

grow from the crystal at the crystal-S, transition with roughly equal probability and no change was detected after holding in the E phase for times of order of a day. It thus appears to be extremely dif- ficult to obtain a true monodomain of this S, phase.

The results so far are in agreement with the conclusions of Doucet et al. [l] about the structure of the S, phase, based on experiments on other members of this series of compounds but using powders or less well-aligned monodomains. However, on viewing the S, phase with the X-ray beam parallel to the layers we find diffraction patterns not compatible with the orthorhombic monolayer structure suggested by their results. With the incident beam along the original crystal b direction, i.e. taking a section through the

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STRUCTURE O F CRYSTAL. SMECTIC E AND B FORMS C3-189

'Or

+

100

q0

FIG. 3. - Orientations of the ab plane of the smectic E unit cell required to produce the observed diffraction patterns of photographs 3 and 5. The heavy solid lines show the two or three orientations producing the sharp diffraction spots (photos 5 and 3) while the lighter dashed lines show the cells which are less well ordered and give the arc-like reflections. The arrows denote the symmetry directions of orientation 1. The a and b axes of the pre-

cursor crystal are also shown.

colinear sharp spots of (3), patterns like that of photograph 4 are always found. Similar results are often obtained with a section at 900 to this through the diffuse arcs (but see below) while similar results (photo 1) are obtained with a flat sample in an alu- minium-walled container cooled in a 2 T field.

Because a true monodomain is unavailable the interpretation is still not unambiguous and there are two possible structures. In the first place the unit cell could be monoclinic with a tilt of 50 about the [020] direction and the observed reflections can be indexed with the following unit cell :

based on the reflections 001, 002, 003, 004, 005, 110, 111, 112, 020, 021, 022, 120, 200 (201, 130 ?). Note that in order to conform to the standard convention that the b axis is unique a and b are interchanged from Doucet's original indexing. If the cell is indeed monoclinic it implies an 8- or 12- fold disorder about C* rather than the 4- or 6- possibilities discussed above because rotation by ⁷¹about C* now produces a different configuration. It is of course this disorder which produces the orthogonal symmetry of the diffraction pattern.

The space group for this structure from observed reflections is P2 l/a. Since Z ⁼2, the molecules must be placed on the centre of symmetry with four fold disorder. The molecular packing is very close to that already proposed [I] except that we necessarily have head-to-tail disorder and the molecules are tilted

-

50 relative to the layer normal. This is compatible with both the crystal packing (a distorted hexagonal net with a 400 tilt of the molecules) and the

Smectic B packing (a true hexagonal packing).

The second possible structure is a bilayer with the observed reflections indexed as follows :

024, 121,200, 123, with 1 10 and 210 now absent

.

The unit cell is orthorhombic

The space group is not uniquely determined but certain possibilities can be excluded on general physical grounds, e.g. Bma2 would involve a gross change of packing within the layer not consistent with an intermediate structure between crystal and S,.

Bba2 implies all molecules with long axes pointing in the same direction which is not feasible although head-to-tail disorder such that on average both ends of the average molecule are identical, is of course still possible. The packing of each half layer is then identical to that proposed by Doucet with a lateral translation of b/2 between each half layer.

Another possibility is a space group such as Bbam which involves a four-fold disorder of the molecules (head-to-tail and a centre of symmetry) and a distorted hexagonal packing in each half layer similar to that

FIG. 4. - Symmetry of packing in the S, layers for the possible monoclinic structure or half layer packing for molecular centres in some possible bilayer orthorhombic structures (see text).

already shown in figure 4, except that the position of the average molecule in the cell centre is related to that of a corner by a 2-fold axis and a b-glide.

This means that a chevron-like packing is no longer an appropriate description but in any case the average molecule now resembles a rod rather than a lath.

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C3-190 A. J. LEADBETTER, J. FROST, J. P. GAUGHAN A N D M. A. MAZID The relationship between the positions of molecules

in the two half layers is then simply a translation of a12 or b/2.

The establishment of the detailed structure must await analysis of intensity measurements (at least) but we believe a bilayer structure to be the more likely of the two possibilities discussed above for the following reasons. First it would be a remarkable coincidence for the tilt of the monoclinic cell to be such as to give reflections exactly at the half-layer positions; second, the precursor crystal itself is a pronounced bilayer ; and finally the tilt direction would have to be different from that in the S, phase (see below) which seems unlikely. In any case the structure is certainly not a simple orthorhombic monolayer.

It is worth noting that the layer (or half layer) spacing exceeds the molecular length by > 1.5

A

suggesting that the local head-to-tail arrange@knt of the crystal with the bulky isobutyl group oveflapping the biphenyls (Fig. 2) is maintained in the 5, phase.

We now note photographs 6 and 7 which show diffraction patterns obtained (6) from a melted single crystal looking at a section through the diffuse arcs (900 to (4)) and (7) from a sample in a Lindemann tube cooled in a 2 T field, both inherently are more disordered specimens than those discussed above.

They show apparently simple monolayer orthorhombic patterns but careful inspection shows that the patterns are not symmetrical about the zero level so we are not looking exactly paralled to the layers.

Indeed we may often have a specimen of many domains tilted a few degrees about a and or b. In confirmation that these patterns are indeed a disturbed view of the reciprocal lattice we observe that with a sample of type (5) oriented as for (6) we have been able to obtain patterns like (4) with incident beam exactly parallel to the layers but change to patterns

PHOTO 7 . - SE in Lindemann tube cooled in field of 2 T.

like those of (6) by tilting the sample a few degrees in either direction.

Even for a bilayer structure the question of the possible tilt of the molecules remains to be established because the diffraction experiments have so far given only the unit cell symmetry. In view of the results for the B phase (below) it seems likely that the molecules have a slight tilt (a few degrees) relative to the layer normal. This view is supported by experiments on supercooled BPBAC where it is possible to obtain a glassy S, phase at 80 K. The (monolayer) spacings (d) and the molecular length (I) are

which implies a molecular tilt of

-

200 (but with the same cell svmmetrv) when the thermal motion is - ,

frozen out at 80 K. An analysis of the intensities of scattering from BPBAC at 80 K gave rms displacements of the molecules in the layers of

and normal to the layers of

These results indicate the highly disordered nature of the S, phase even when most thermal motion is quenched.

Finally it should be emphasized that the sub- tleties of structure discussed above would not gene- rally be observed with a powder or poorly aligned specimen so that the detailed generic structure of S, materials must be regarded again as an open question.

Much more work is required to establish whether PHOTO 6. - SE from crystal incident beam down crystal a direc- the features found here are common to all SE

tion (900 to that in 4) but not exactly parallel to all the layers. compounds.

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STRUCTURE OF CRYSTAL, SMECTIC E AND B FORMS C3-191

5. The S, structure. -X-ray photographs were taken from well-aligned samples of S, phase prepared by various means :

a) Single crystal ^-+S, ^-+S,.

b) I + N + S A + S B

2 T Magnetic field, flat plate sample or the tube sample.

c) I + N + S A + S B + S E - + S B 2 T Magnetic field, flat plate or tube sample.

A preliminary report on results obtained with samples of type b) and c) has already been given [2].

'x-ray photographs taken looking down the layer normal (photograph 8) on samples a) prepared from a single crystal, showed twelve spots, from two kinds of domains with difference in orientation of 30".

This is characteristic of the hexagonal molecular packing within layers. The formation of two sets of domains misoriented by 300 must inevitably occur

due to the disorder already present in the S, phase. PHOTO 9. - SB with ~ncident beam parallel to layers.

8. - SB prepared from single crystal, with incident beam perpendicular to layers.

On viewing the S, phase parallel to the layers with any kind of oriented sample X-ray photographs like that in photograph 9 are observed. The hexagonal spots are in fact elongated bars or rods of scattering of half length

--

C*. This shows that the coherence length of the hko planes in the C*-direction is only 1-2 layers due to displacement of the hexagonal nets in adjacent layers. The strong diffuse scattering around the positions of the 21-0 reflections shows that the local order is like that in the S, phase where this scattering condenses to Bragg spots.

Careful investigation of the scattering rods shows (photograph 10) that their intensity is not uniform but consists of four spots superimposed on a strong diffuse rod of scattering. These four spots are not always resolved but usually 2 maxima in intensity can be resolved. One plausible interpretation is that the hexagonal packing (the molecules) is tilted

PHOTO 10. - SB parallel to layers, lower exposure than 9.

at

-

60 relative to the layer normal - as shown in figure 5 to give a local monoclinic cell.

For a sample prepared by cooling from S, the orientation is random about the layer normal while for a sample obtained from a crystal via S, there is six-fold disorder (and two different sets of domains).

In either case this means that effectively all the hko spots are observed in a single photograph giving a pattern as shown in figure 5.

Hence the local S, structure would be monoclinic with

a = 8.9

A

^, ^b⁼^5.1

A ,

c = 27.1

A

and

B

= 96O.

It is worth noting that despite the poor correlations

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C3-192 A. J. LEADBETTER, J. FROST, J. P. GAUGHAN AND M. A. MAZID

FIG. 5. - Reciprocal lattice for S, : a) hko net single domain ; b) observed net parallel to layers arising from 6 O tilt about b*

and disorder about c*.

in the C * direction of the hko planes (see above) the hexagonal net has very long range coherence, extending throughout a bulk sample. This means that the layers are not free to slide over each other.

A consequence of the poor correlations in the C*

direction discussed above is that the tilt direction is on average random and a S, sample will therefore be on average a hexagonal, uniaxial phase although locally monoclinic (*).

6. Conclusions. - The crystal structure of IBPBAC is a tilted bilayer with a distorted hexagonal packing of the molecules in each half layer.

The S, and S, structures are significantly different from those previously proposed, although the packing of molecules within the layers is confirmed as respectively distorted and true hexagonal. However, the S, structure is probably an orthorhombic bilayer while the S, is possibly locally monoclinic with a 60 tilt angle and poor interlayer correlation resulting in an overall uniaxial structure or alternatively a locally trilayer (ABC.. .) packing of the hexagonal layers but with considerable disorder.

Note added in proof: - Further work on the smectic B phase of both IBPBAC and a number of other compounds strongly suggests an alternative description of the S , structure of IBPBAC. The diffraction spots indexed 110 (i10) and 200 (300) in figure 5 could also be indexed with I = 113 and 213 and this suggests a trilayer structure arising from an ABC..

.

type packing of the hexagonal layers. The strong diffuse scattering indicates considerable disorder in the packing and in many specimens this is dominant so that the regular (and presumably lowest energy) packing does not form. This will be discussed more fully in a letter to be submitted to J. Physique.

References

[I] DOUCET, J., LEVELUT, A. M., LAMBERT, M., LIEBERT, L. and STRZELECKI, L., J. Physique Colloq. 36 (1975) C1-13.

[2] RICHARDSON, R. M., LEADBETTER, A. J. and FROST, J. C., Ann. Phys. 3 (1978) 177.