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The unclassified smectic phase ofN-(4-n-pentyloxybenzylidene)-4-n-hexylaniline (50.6)

J.W. Goodby, G.W. Gray, A.J. Leadbetter, M.A. Mazid

To cite this version:J.W. Goodby, G.W. Gray, A.J. Leadbetter, M.A. Mazid. The unclassified smectic phase of N-(4-n-pentyloxybenzylidene)-4-n-hexylaniline (50.6). Journal de Physique, 1980, 41 (6), pp.591-595.<10.1051/jphys:01980004106059100>. <jpa-00209284>

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The unclassified smectic phase of N-(4-n-pentyloxybenzylidene)-4-n-hexylaniline (50.6)

J. W. Goodby, G. W. Gray

Chemistry Department, The University, Hull, HU6 7RX, U.K.

A. J. Leadbetter and M. A. Mazid

Chemistry Department, The University, Exeter EX4 4QD, U.K.

(Reçu le 14 dgcembre 1979, révisé le 18 fevrier, accepté le 25 fevrier 1980)

Résumé. 2014 Pendant plusieurs années, le N-(4-n-pentoxybenzylidene)-4-n-hexylaniline (50.6) a donné lieu à unephase smectique non classée. Cette phase est située entre une phase smectique B et une phase smectique G, etles transitions observées en fonction de la température sont réversibles. L’examen approfondi de cette phasenon classée fait ici montre que c’est une phase smectique F. Ainsi, (50.6) donne lieu aux séquences des phasesénantiotropiques N, SA, SC, SB, SF, SG; les trois demières phases de cette séquence correspondent aux passages,en fonction de la température, d’une phase smectique ordonnée à une phase moins ordonnée puis à une phasesmectique plus ordonnée.

Abstract. 2014 For many years, N-(4-n-pentyloxybenzylidene)-4-n-hexylaniline (50.6) has been known to exhibitan unclassified smectic phase. This phase has been shown to occur between a smectic B and a smectic G phase,and the transitions to and from the phase on both heating and cooling have been shown to be truly reversible.The nature of the unclassified phase has now been investigated fully, and we have shown that it has the classifi-cation-smectic F. Therefore, 50.6 exhibits the sequence (1) of enantiotropic phases : N, SA, SC, SB, SF, SG and thelast three phases in this sequence show a change from an ordered to a less ordered to a more ordered smecticphase with temperature change.

J. Physique 41 (1980) 591-595 JUIN 1980,

Classification

Physics Abstracts61. 30

1. Introduction. - N-(4-n-pentyloxybenzylidene)-4-n-hexylaniline (50.6) (I) was initially prepared bySmith, Gardlund, and Curtis [1, 2]. They notedthat this compound exhibited five smectic phases andreported the transition temperatures (in °C) as

follows

(1) Following discussions between Sackmann, Demus, Grayand Goodby at Halle and as announced by G. W. Gray in theopening lecture of the recent European Conference at Garmisch-Partenkirchen (January, 1980), a unified nomenclature systemfor smectic phases has been recommended and is used in this paper.SG is now used to describe what has previously been called a tiltedSB phase or a SH phase. Conversely, SH is now used for a moreordered smectic phase, e.g., in TBBA the sequence is N, SA, Sc,SG, SH on cooling.

Since this report in 1973, the nO.m series of

compounds, of which 50.6 is a member, have beenthe subject of a variety of investigations. Demus andRichter [3] studied the phases of 50.6 by opticalmicroscopy and miscibility techniques and concludedthat it exhibited N, SA, Sc, SB, S4 and SG phases;phase S4 remained unclassified. In a later X-rayinvestigation by Doucet and Levelut [5], the S4phase was again unclassified, but they suggested thatit might be a mixture of two phases.

In the present work, we have examined the smecticpolymorphic modifications of 50.6 by optical micro-scopy, calorimetry, miscibility methods, and X-raydiffraction. Our results show that the S4 phaseexhibits the properties of a smectics F phase, and wetherefore conclude that the phase sequence exhibitedby this compound is N, SA, Sc, SB, SF, SG-

2. Results. - 2.1 OPTICAL MICROSCOPY. - Obser-vation of microscopic textures and measurementsof transition temperatures were made using a Nikon

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01980004106059100

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L-Ke polarizing microscope in conjunction with aMettler FP52 hot-stage and control unit.On cooling a sample of 50.6 from the isotropic

liquid, six liquid crystal phases were observed. Thetransition temperatures obtained (in °C) were as

follows :

Optical microscopy revealed that a typical nematicphase was formed from the isotropic liquid on cooling.Further cooling produced first a smectic A phase,which exhibited typical focal-conic fan and homeo-tropic textures (Plate 1), and then a smectic C phase,which exhibited the usual broken fan and schlierentextures (Plate 2). Further reduction in temperatureproduced a smectic B phase which again showedtypical focal-conic fan and homeotropic textures

(Plate 3). Cooling of this phase produced a phasewhich showed a broken fan texture and a fine mosaictexture (Plate 4). The fans are lightly chequered,and some arcs run laterally across them, as in asmectic E phase. However the longitudinal fissures

Plate 1. - Focal-conic fan texture and homeotropic areas (black)of the smectic A phase of 50.6 (crossed polarizers, x 300).

Plate 2. - Broken fan texture and schlieren areas of the smecticC phase of 50.6 (crossed polarizers, x 300).

Plate 3. - Focal-conic fan texture and homeotropic areas (black)of the smectic B phase of 50.6 (crossed polarizers, x 300).

Plate 4. - Focal-conic fan texture (broken, lightly chequeredfans) and fine mosaic areas of the smectic 4 (smectic F) phaseof 50.6 (crossed polarizers, x 300).

rule out the possibility that the phase is of the E type.Only the smectic F phase shows chequered patcheson the backs of the fans, although these are usuallybetter defined than in this particular case. The mosaicareas formed from the homeotropic areas are verysmall, and they form in chains, in an almost hexago-

Plate 5. - Focal-conic fan texture and mosaic areas of the smecticG phase of 50.6 (crossed polarizers, x 300).

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nal array. The mosaic areas are however too small to

yield much information about the phase. On cooling,this phase gives rise to a smectic G phase whichexhibits typical textures (Plate 5).

2.2 MISCIBILITY STUDIES. - The smectic A, C, B,and G phases were each shown to be separatelymiscible with the corresponding phases of the standardmaterial N-(4-n-pentyloxybenzylidene)-4-n-heptyl ani-line (50.7) (N, SA, Sc, SB, SG phases) [5].The smectic 4 phase was shown to be miscible

with the smectic F phase of the standard materialN-(4-n-nonyloxybenzylidene)-4-n-butylaniline (90.4)(SA, SF, and SG phases) [6].

2.3 CALORIMETRY. - The phase-behaviour hasbeen investigated using a Perkin-Elmer differential

scanning calorimeter (DSC 2). All the transitionslisted above were observed at temperatures withinabout a degree of those quoted in section (1) andrepeated below.The SC-SA transition was observed as a weak

shoulder on the high temperature side of the SB -+ Sctransition, consistent with a weak (perhaps 2nd order)transition and the enthalpy change could not bedetermined. In all other cases AH was measured,and comparisons with previous results for other

compounds like TBBA suggest an accuracy of ± 10 %for the following values of AH/kJ mol-1 (transition.temperatures in °C)

The SG-SF enthalpy is similar to values found forother compounds [7] and shows that the structuralchange at this transition is small.

2.4 X-RAY DIFFRACTION. - The samples were

contained in 0.7-1.0 mm Lindemann glass tubes.Oriented specimens were prepared by cooling in a2 T magnetic field from the isotropic liquid phaseinto the Sc or SB phase. Thereafter, good alignmentwas maintained without the field. X-ray diffractionphotographs were obtained using graphite-mono-chromated CuKcx radiation and simple, stationarysample, flat film techniques. Measurements were

made on the smectic C, B and G phases as well ason the unknown smectic 4 phase. Precision of tem-perature control and relative temperature measure-ment was ± 0.1 K and the accuracy with whichthe sample temperature is known is ~ ± 1 K.

Typical X-ray photographs are shown in plate 6for the B, G and smectic 4 phases. The quality ofthe results for SG was less good than for some othern0. m compounds due to the formation of severaldomains of slightly different orientation on formationof the phase.At temperatures greater than about 45 °C, the

smectic B structure is a bilayer with the ABAB...type packing configuration that has been established

Plate 6. - X-ray photographs of the smectic B, F and G phasesof 50.6. a) SB, 48 OC (ABA... packing); b) SB, 44 °C (ABCA...packing); c) SF, 42°C; d) SG, 36 °C. Different sample-filmdistances were used for the three phases.

for several members of the n0.m series [8, 9, 10].This is shown by the microdensitometer trace (Fig. 1)taken along the OOl ) (or C*) direction for thebar of scattering corresponding to the lowest orderreciprocal lattice points (100, 110 etc.) of the hexa-gonal lattice. This shows reflections 1 = ± 3/2, ± 1,+ 1/2 and 0 relative to the 001 reflections designated001 and 002, together with a strong diffuse back-ground and/or broadened peaks. Because this diffusescattering is relatively much stronger than that forthe SB phase of the free film of compound 40.8 studiedby Moncton and Pindak [10], this probably meansthat samples prepared by cooling in a field have amuch greater stacking disorder of the layers thanthat for free films. On cooling below about 45 °Cthe interlayer stacking changes to ABCA... typeas shown by the diffraction pattern along 001 &#x3E;showing maxima at I = ± 1/3 and ± 2/3 and at

about 1 degree above the Se-Sp transition this revertsto an ABAB... packing although this contains moredisorder than that at higher temperatures. Similartransitions have been seen for a number of othern0. m compounds [10, 11] ] and it appears that such

594

Fig. 1. - Scattering intensities in the SB phase of 50.6 along the 001 &#x3E; row (at h and/or k :0 0) at : a) T = 48 °C showing anABAB... packing and b) T = 44 °C showing an ABCA... packing,taken from microdensitometer traces of photographs like thoseof plate 6.

changes, which are not observed calorimetrically,but which are reproducible, are common featuresof these compounds and show both that the inter-layer ordering energy is weak and that relativelylong range forces are important.Throughout the SB phase satellite reflections are

found associated with the 001 (and 002) spot andsituated on the 001 (and 002) plane at Q.,.,IQIOO - 1/18.These satellites consist of a ring of scattering whichincreases in intensity with falling temperature andwhich must arise from modulations of the SB layersby transverse waves of well-defined wavelength(~ 18 times the 100 spacing) involving longitudinaldisplacements of the molecular long axes, as dis-cussed briefly elsewhere for 50.7 [9].

For 50.6 alone among the n0.m compounds sofar studied, a new phase appears between the SBand SG phases. The diffraction pattern clearly showsthis to be an SF phase. This is shown by the photo-graphs of plate 6 and by the microdensitometertraces (Fig. 2) of the lowest order hk0 ring for theSe, SF and SB(SG) phases. The results for SB show theoverall experimental resolution (plus wings of diffusescattering) and the net width of the diffraction peakfor the SF phase is about 3 times narrower than forSc. The profile is Lorentzian and its width gives acorrelation length çp ~ 30 A (or about 7 molecules).The width of the diffraction peak along C* is approxi-mately the same as I C* I showing that there is essen-tially no correlation of molecular position betweenlayers. Comparison of the diffraction patterns ofSB and SF shows that the transition occurs by arelative motion of molecules along C (as for the

SB modulation), their long axis direction remainingunchanged, resulting in a tilting of the layers in the

Fig. 2. - Scattering intensity of the first equatorial (h and/ork = 0, 1 = 0) peak for smectic C, F and B (or G) phases of 50.6,arbitrarily scaled to the same peak height and the SB peak position.Profiles were obtained from microdensitometer traces of X-rayphotographs.

SF phase with a tilt angle of 240. We suggest thatthe SB structure becomes unstable to these displace-ments when their amplitude reaches some criticalvalue and the stable tilted structure is formed. At~ 38 °C, the SF phase changes to a SG phase whichinvolves the reappearance of long range order inthe hexagonal packing. The structure of the SGphase is monoclinic, comprising a tilted hexagonalmonolayer packing of the molecular long axes,as described elsewhere [9,12]. However, the strong 100,110 reflections in the SG phase are associated withstrong diffuse scattering, similar to the total scatteringfor the SF phase (Plate 6), indicating that considerableregions of disorder remain.The orientation of the long axes does not change

at the SF-SG transition and the tilt angle in the SGphase is 260.

2. 5 DIscussIoN. - The newly established sequenceof phases is quite remarkable because we may inferfrom this and earlier work that both the SB and SGphases have essentially long range 3 dimensional

(3-D) order. The intermediate SF phase, by analogywith more extensive work on other SF phases [13, 14]has long range order of the tilt direction (as do Scphases) and also long range bond orientationalorder, but relatively short range positional order.This kind of situation has been discussed by Bir-

geneau and Litster [15] as a model for SB phases,and while it seems not to be appropriate for theseessentially crystal-like phases, it may be useful for

describing the SF phase. Thus SF has exponentialdecay of positional order (çp ’" 30 A) (short rangeorder, SRO), but long range bond orientational

order, corresponding to weakly coupled 2-D layers,the weak coupling having induced this long rangeorder on the intrinsic algebraic decay of bond orienta-tional order for the true 2-D layer. We speculatethat this combination of long range orientational,but short range positional order must be associatedwith extensive dislocations or grain boundaries inthe layers.

595

The outstanding problem is why the SF phase,which lacks 3-D long range order, appears betweentwo phases which possess it. We cannot yet answerthis question but note that the unusual phase sequencein 50.6 may be described in two ways. First, comparedwith other n0. m compounds so far studied, a

decoupling of the layers occurs between SB and SGphases. The difference between these phases lies bothin the average relative longitudinal displacement (Al)between molecules (finite in SG giving tilted layers,and zero in SB giving an orthogonal structure) andin the type of the interlayer correlations (AAA... in

SG and ABA... in SB). Second, compared with othercompounds exhibiting SF phases the displacementAl disappears (SF --+ SB) before the complete meltingof the molecular positions within the layers (SF -+ Sc).We speculate that the important molecular para-meter might be the relatively symmetrical molecularshape because C5H11O - is nearly the same lengthas C6HI3 but we shall not pursue this further here.

Acknowledgments. - We are grateful to SRC forfinancial support and to Mr. P. C. Walters for makingthe DSC measurements.

References

[1] SMITH, G. W., GARDLUND, Z. G. and CURTIS, R. J., Mol.Cryst. Liq. Cryst. 19 (1973) 327.

[2] SMITH, G. W. and GARDLUND, Z. G., J. Chem. Phys. 59(1973) 3214.

[3] DEMUS, D. and RICHTER, L., Textures of Liquid Crystals(Verlag Chemie, West Germany), 1978.

[4] GOODBY, J. W. and GRAY, G. W., Mol. Cryst. Liq. Cryst. Lett.49 (1979) 217.

[5] DOUCET, J. and LEVELUT, A.-M., J. Physique 38 (1977) 1163.[6] GOODBY, J. W. and GRAY, G. W., Mol. Cryst. Liq. Cryst.

Lett. 56 (1979) 43. [7] LEADBETTER, A. J. and WALTERS, P., unpublished work.[8] LEADBETTER, A. J., FROST, J. C. and MAZID, M. A., J. Physi-

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[9] LEADBETTER, A. J., MAZID, M. A., KELLY, B. A., GOODBY, J.W. and GRAY, G. W., Phys. Rev. Lett. 43 (1979) 630.

[10] MONCTON, D. E. and PINDAK, R., Phys. Rev. Lett. 43 (1979)701.

[11] LEADBETTER, A. J., MAZID, M. A. and RICHARDSON, R. M.,Proc. Bangalore Conf. on Liquid Crystals, December 1979.To be published (Hayden &#x26; Sons, London) 1980.

[12] DOUCET, J. and LEVELUT, A.-M., J. Physique 38 (1977) 1163.[13] LEADBETTER, A. J., GAUGHAN, J. P., KELLY, B. A., GRAY, G. W.,

GOODBY, J. W., J. Physique Colloq. 40 (1979) C3-178.[14] DOUCET, J. and LEVELUT, A.-M., Phys. Rev., in press.[15] BIRGENEAU, R. J. and LITSTER, J. D., J. Physique Lett. 39

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