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

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The unclassified smectic phase of

N-(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�

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.

Pendant plusieurs ^{années, le} N-(4-n-pentoxybenzylidene)-4-n-hexylaniline ^{(50.6) a donné lieu à une} phase smectique ^non classée. Cette phase ^{est située entre une} phase smectique ^{B et une} phase smectique ^{G, et}

les transitions observées en fonction de la température ^sont réversibles. L’examen approfondi ^{de cette} phase

non classée fait ici montre que c’est une phase smectique ^F. Ainsi, (50.6) donne lieu aux séquences ^des phases énantiotropiques ^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} phase smectique plus ^ordonnée.

Abstract.

For many years, N-(4-n-pentyloxybenzylidene)-4-n-hexylaniline ^(50.6) has been known to exhibit

an 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 the}

last three phases ⁱⁿ this sequence show ^a change ^{from an ordered to a} less ordered to a more ordered smectic phase ^with temperature change.

Classification

Physics ^Abstracts

61. 30

1. Introduction.

N-(4-n-pentyloxybenzylidene)- 4-n-hexylaniline (50.6) (I) ^was initially prepared by Smith, Gardlund, and Curtis [1, 2]. They ^noted

that this compound exhibited five smectic phases ^and reported the transition temperatures (in °C) ^as

follows

(1) Following discussions between Sackmann, Demus, Gray

and Goodby ^at Halle and as announced by ^{G. W.} Gray ^{in the} opening lecture of the recent European Conference at Garmisch- Partenkirchen (January, 1980), ^a unified nomenclature system for smectic phases has been recommended and is used in this _paper.

SG ^{is now used to} describe what has previously been called a tilted

SB phase ^{or a} SH phase. Conversely, SH ^{is now used for a more}

ordered 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 been the subject ^{of a} variety ^of investigations. ^{Demus and}

Richter [3] ^{studied the} phases ^{of 50.6} by optical microscopy ^and miscibility techniques ^{and concluded}

that it exhibited N, SA, Sc, SB, S4 ^and SG phases;

phase S4 ^remained unclassified. In a later X-ray investigation by Doucet and Levelut [5], ^the S4 phase ^was again unclassified, ^but they suggested ^that

it might ^{be a} mixture of two phases.

In the present work, ^{we have} examined the smectic

polymorphic modifications of 50.6 by optical ^micro-

scopy, calorimetry, miscibility methods, ^and X-ray

diffraction. Our results show that the S4 phase

exhibits the properties ^{of a smectics F} phase, ^{and we}

therefore conclude that the phase sequence exhibited

by ^this compound ^is N, SA, Sc, SB, SF, SG-

2. Results.

2.1 OPTICAL MICROSCOPY.

Obser- vation of microscopic ^{textures and} measurements

of transition temperatures ^{were made} using ^{a Nikon}

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

592

L-Ke polarizing microscope ⁱⁿ conjunction ^{with a}

Mettler FP52 hot-stage ^and control unit.

On cooling ^a sample ^of 50.6 from the isotropic liquid, ^six liquid crystal phases ^were observed. The transition temperatures ^obtained (in °C) ^{were as}

follows :

Optical microscopy revealed that a typical ^nematic phase ^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 schlieren textures (Plate 2). Further reduction in temperature produced ^{a smectic B} phase ^which again ^showed typical focal-conic fan and homeotropic ^textures (Plate 3). Cooling ^{of this} phase produced ^a phase

which showed a broken fan texture and a fine mosaic texture (Plate 4). ^{The fans are} lightly chequered,

and some arcs run laterally ^across them, ^{as in a} smectic 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 smectic C 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 chequered fans) ^{and fine mosaic areas} of the smectic 4 (smectic F) phase

of 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 patches

on the backs of the fans, although ^{these are} usually

better defined than in this particular ^{case. The mosaic}

areas formed from the homeotropic areas are very

small, ^and they ^{form in} chains, ^{in an almost} hexago-

Plate 5.

Focal-conic fan texture and mosaic areas of the smectic

G phase ^{of 50.6} (crossed polarizers, ^x 300).

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 ^which

exhibits typical ^textures (Plate 5).

2.2 MISCIBILITY STUDIES.

The smectic A, C, B, and G phases ^{were each shown to be} separately

miscible with the corresponding phases ^{of the standard} material 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 material

N-(4-n-nonyloxybenzylidene)-4-n-butylaniline (90.4) (SA, SF, ^and SG phases) [6].

2.3 CALORIMETRY.

The phase-behaviour ^has

been investigated using ^a Perkin-Elmer differential scanning calorimeter (DSC 2). All the transitions listed above were observed at temperatures ^within about a degree ^{of those} quoted ^{in section} (1) ^and repeated ^below.

The SC-SA transition was observed as a weak shoulder on the high temperature side of the SB -+ Sc transition, consistent with a weak (perhaps ^2nd order)

transition and the enthalpy change ^{could not be}

determined. 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 for other compounds [7] and shows that the structural

change ^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 a}

2 T magnetic ^{field from the} isotropic liquid phase

into the Sc ^or SB phase. Thereafter, good alignment

was maintained without the field. X-ray diffraction

photographs ^{were obtained} using graphite-mono-

chromated CuKcx ^{radiation and} simple, stationary sample, ^{flat film} techniques. Measurements were

made on the smectic C, ^{B and G} phases ^{as well as}

on the unknown smectic 4 phase. ^{Precision of tem-} perature control and relative temperature ^measure-

ment was ± 0.1 ^K and the _accuracy with which the sample temperature is known is

± ^{1 K.}

Typical X-ray photographs ^{are shown in} plate ⁶

for the B, ^{G and smectic 4} phases. ^The quality ^of

the results for SG ^{was less} good ^{than for some other}

n0. m compounds ^{due to the} formation of several domains of slightly ^different orientation ^on formation of 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} phases

of 50.6. a) SB, ^{48 OC} (ABA... packing); b) SB, ^{44 °C} (ABCA...

packing); c) SF, 42°C; d) SG, ^{36 °C.} Different sample-film

distances 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 the}

bar of scattering corresponding to ^the lowest order

reciprocal ^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 designated

001 and 002, together ^{with a} strong diffuse back-

ground and/or ^broadened peaks. Because this diffuse

scattering ^is relatively ^much stronger than that for the SB phase ^{of the free film of} compound 40.8 studied

by Moncton and Pindak [10], ^this probably ^means

that samples prepared by cooling ^{in a} field have a

much greater stacking disorder of the layers ^than

that for free films. On cooling below about 45 °C the interlayer stacking changes ^{to ABCA...} type

as shown by the diffraction pattern along 001 >

showing ^{maxima at I}

^± 1/3 and ± 2/3 ^{and at}

about 1 degree ^{above the} Se-Sp transition this reverts to an ABAB... packing although this contains more

disorder than that ^at higher temperatures. ^Similar

transitions have been seen for a number of other

n0. m compounds [10, 11] ] ^and it appears that such

594

Fig. ^1.

Scattering intensities in the SB phase ^{of 50.6} along ^the 001 > ^row (at h and/or k :0 0) ^{at :} a) ^T

^{48 °C} showing ^an

ABAB... packing ^and b) ^T

^{44 °C} showing ^{an ABCA...} packing,

taken from microdensitometer traces of photographs like those of plate ^6.

changes, ^{which are not observed} calorimetrically,

but which are reproducible, are common features of these compounds and show both that the inter-

layer ordering energy is weak and that relatively long range forces are important.

Throughout ^the SB phase ^satellite reflections are

found associated with the 001 (and 002) spot ^and situated on the 001 (and 002) plane ^at Q.,.,IQIOO - ^1/18.

These satellites consist ^{of a} ring ^of scattering ^which

increases in intensity ^with falling temperature ^and which must arise from modulations of the SB layers by transverse waves of well-defined wavelength (~ ^{18 times the 100} spacing) involving longitudinal displacements of the molecular long ^{axes, as dis-}

cussed briefly ^{elsewhere for 50.7} [9].

For 50.6 alone _among the n0.m compounds ^so

far studied, ^{a new} phase appears between the SB

and SG phases. ^The diffraction pattern clearly ^shows

this to be an SF phase. This is shown by ^the photo- graphs ^of plate ^{6 and} by ^the microdensitometer

traces (Fig. 2) ^{of the lowest order hk0} ring ^{for the} Se, SF ^and SB(SG) phases. ^{The results for} SB ^{show the} overall experimental resolution (plus wings ^{of diffuse} scattering) ^{and the net} width of the diffraction peak

for the SF phase is about 3 times narrower than for Sc. ^The profile is Lorentzian and its width gives ^a

correlation 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 ^between layers. Comparison of the diffraction patterns ^of SB ^and SF shows that the transition occurs by ^a

relative motion of molecules along ^{C (as for the} SB modulation), ^their long axis direction remaining unchanged, resulting ^{in a} tilting ^{of the} layers ^{in the}

Fig. ^2.

Scattering intensity of the first equatorial (h and/or k = 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-ray photographs.

SF phase ^{with a tilt} angle of 240. We suggest ^that the SB ^structure becomes unstable to these displace-

ments when their amplitude ^{reaches some critical}

value and the stable tilted structure is formed. At

~

38 °C, ^the SF phase changes ^{to a} SG phase ^which

involves the reappearance of long range order in the hexagonal packing. ^{The structure of the} SG phase ^is monoclinic, comprising ^{a tilted} hexagonal monolayer packing of the molecular long ^axes,

as described elsewhere [9,12]. However, ^the strong 100, 110 reflections in the SG phase ^are associated with strong ^diffuse scattering, ^{similar to the total} scattering

for the SF phase ^(Plate 6), indicating that considerable

regions ^{of disorder remain.}

The orientation of the long ^{axes does not} change

at the SF-SG transition and the tilt angle ^{in the} SG phase ^{is 260.}

2. 5 DIscussIoN.

The newly established _sequence of phases ^is quite remarkable because we may infer from this and earlier work that both the SB ^and SG phases ^have essentially long range ³ dimensional

(3-D) order. The intermediate SF phase, by analogy

with more extensive work ^on other SF phases [13, 14]

has long range order of the tilt direction (as ^do Sc phases) ^{and also} long ^{range bond} orientational order, ^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 these} essentially crystal-like phases, ^it may be useful for

describing ^the SF phase. ^Thus SF ^has exponential decay ^of positional ^order (çp ’" ³⁰ A) (short range order, SRO), ^but long range bond orientational order, corresponding ^to weakly coupled ^2-D layers,

the weak coupling having induced this long range order on the intrinsic algebraic decay of bond orienta- tional order for the true 2-D layer. ^We speculate

that this combination of long ^range orientational,

but short _range positional ^{order must} be associated

with extensive dislocations or grain boundaries in

the layers.

The outstanding problem ^is why ^the SF phase,

which lacks 3-D long range order, appears between two phases ^which possess it. We cannot yet ^answer this question ^{but note} that the unusual phase sequence in 50.6 _may be described in two ways. First, compared

with other n0. m compounds ^{so far studied, a} decoupling ^{of the} layers ^{occurs between} SB ^and SG phases. ^The difference between these phases ^{lies both}

in the average relative longitudinal displacement (Al)

between molecules (finite ⁱⁿ SG giving ^tilted layers,

and zero in SB giving ^an orthogonal structure) ^and

in the type ^{of the} interlayer correlations (AAA... ⁱⁿ

SG and ABA... in SB). Second, compared ^{with other} compounds exhibiting ^SF phases ^the displacement

Al disappears (SF --+ SB) before ^the complete melting

of the molecular positions within the layers (SF -+ Sc).

We speculate ^{that the} important ^molecular para- meter might ^{be the} relatively symmetrical ^molecular shape ^because C5H11O - ^is nearly ^{the same} length

as C6HI3 ^{but we shall not} pursue this further here.

Acknowledgments.

^{We are} grateful ^{to SRC for}

financial support ^{and to Mr. P.} C. Walters for making

the 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-

que Lett. 40 (1979) ^L-325.

[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 & 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}

(1978) ^L-399.