Chemically-induced discotic liquid crystals. Structural, studies with NMR spectroscopy

Article abstract of DOI:10.1080/15421400390213311

The use of nuclear magnetic resonance (NMR) spectrometry for the structural studies of chemically-induced discotic liquid crystals was discussed. The synthesis of the phase inductor with two deutrons in the phenyl ring was accomplished in a single step using the catalytic exchange procedure. The reaction was quenched by the addition of 5 ml D₂O, the organic layer was left to evaporate and 0.38 g of the product was obtained. The degree of deuteriation was found by proton NMR to be approximately 70%.

Full text of DOI:10.1080/15421400390213311

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Chemically-Induced Discotic
Liquid Crystals. Structural
Studies with Nmr Spectroscopy
J. R. Hughes a , G. R. Luckhurst a , K. Praefcke b , D.
Singer c & W. M. Tearle a
a Department of Chemistry and Southampton
Liquid Crystal Institute, University of Southampton,
Southampton, SO171BJ, United Kingdom
b Technische Universität Berlin, J437, Strasse des 17
Juni 135, Berlin, 10623, Germany
c Institut für Organische Chemie, Technische
Universität Berlin, Strasse des 17 Juni, D-10623,
Germany
Published online: 18 Oct 2010.
To cite this article: J. R. Hughes , G. R. Luckhurst , K. Praefcke , D. Singer & W.
M. Tearle (2003) Chemically-Induced Discotic Liquid Crystals. Structural Studies
with Nmr Spectroscopy, Molecular Crystals and Liquid Crystals, 396:1, 187-225, DOI:
10.1080/15421400390213311
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Mol. Cryst. Liq. Cryst., Vol. 396, pp. 187–225, 2003
Copyright Taylor & Francis Inc.
#
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421400390213311
CHEMICALLY-INDUCED DISCOTIC LIQUID
CRYSTALS. STRUCTURAL STUDIES WITH NMR
SPECTROSCOPY
J. R. Hughes* and G. R. Luckhurst
Department of Chemistry and Southampton Liquid Crystal
Institute, University of Southampton, Southampton SO17 1BJ,
United Kingdom
K. Praefcke
Technische Universita¨t Berlin, J437, Strasse des 17 Juni 135,
10623 Berlin, Germany
D. Singer**
Institut fu¨r Organische Chemie, Technische Universita¨t Berlin,
Strasse des 17 Juni, D-10623, Germany
W. M. Tearle***
Department of Chemistry and Southampton Liquid Crystal
Institute, University of Southampton, Southampton SO17 1BJ,
United Kingdom
The contribution by Professor Dr.-Ing. K. Praefcke was made possible by the support of the
¨
Technische Universitat Berlin and by Professor Dr.-Ing. H. G. Wagemann and his group in the
Institute of Solid State Electronics of this University, which are both gratefully acknowledged.
Professor G. R. Luckhurst is grateful to the former Science and Engineering Research Council,
now the Engineering and Physical Sciences Research Council, for their financial support which
made the purchase of the Bruker MSL200 NMR spectromer used for the studies reported here
possible. Drs. J. R. Hughes and W. M. Tearle also want to thank the Research Council for the
award of Research Studentships. Finally we wish to thank Dr S. J. Perkins (University of
Southampton) for assistance in the synthesis of D102-d2.
Present address: *IBM ITS Technical Support, Alencon House, Basingstoke RG1 7EJ,
United Kingdom.
**Rhenus-Kleyling, Speditions GmbH & Co. KG, Haffenstrasse 28, DE 79576 Weil am
Rhein, Germany.
***Enviros Consulting Ltd., Culham Science Centre, Abingdon, OX14 3DB, United
Kingdom.
187

188
J. R. Hughes et al.
Deuterium NMR spectroscopy provides an especially important technique with
which to investigate a wide range of liquid crystal behaviour. Here, we describe
its use in the structural study of liquid crystals formed from discotic or disc-like
molecules and, to illustrate the breadth and power of the NMR method, we have
selected two systems in which the liquid crystal phases are chemically induced.
The binary mixtures are similar in that one component is a radial multiyne but
differ significantly in the nature of the chemical inductor. In one case the
inductor is a nitrated fluorenone which has a similar anisotropy to the radial
multiyne while for the second system the inductor, a substituted o-xylene is
considerably smaller and far less anisotropic. By measuring the deuterium
NMR spectra of suitably deuteriated versions of the components we show how it
is possible to investigate the orientational order of the components in the var-
ious induced phases, their effective molecular symmetry, the molecular geo-
metry, the chain flexibility and orientational order together with the phase
symmetry. The results of these investigations are described and explained.
Keywords: discotic liquid crystals; chemically-induced phases; deuterium NMR spectroscopy
1. INTRODUCTION
The defining characteristic of a liquid crystal is its long range orientational
order; that is one or more axes set in the constituent molecules tend to be
highly correlated, even when separated by many millions of molecules. It
follows, therefore, that a prime requirement for the formation of a liquid
crystal is an anisotropy in the molecular shape. Of course, attractive forces
are also important in influencing the formation of a liquid crystal, but for
the moment we shall concentrate on the shape. Following the observation
by Reinitzer [1] that cholesteryl benzoate formed a chiral nematic phase,
the materials which were subsequently found to exhibit liquid crystal
phases were made of essentially rod-like molecules. It was the long axes or
near symmetry axes which tended to be orientationally correlated over
large distances. There is, however, no apparent reason why molecules with
different shape anisotropies should not form liquid crystal phases. One of
the most obvious shapes is that which is at the opposite extreme of a rod,
namely a disc, molecules with such a shape are commonly referred to as
discotic. However, it was not until the seminal work by Chandrasekhar and
his colleagues [2] that a liquid crystal mesophase exhibited by discotic
molecules was first observed for a hexasubstituted benzene. Since then
numerous other materials with discotic molecules have been shown to form
liquid crystal phases [3]. It seems as if once it had been demonstrated that
liquid crystals are indeed formed by discotic molecules then chemists were
given the confidence to hunt for, and find, many more examples. As addi-
tional discotic materials were prepared, so the range of liquid crystal
phases formed by such disc-like molecules grew. It is now clear that there is

Chemically induced DLCs
189
a rich polymorphism with nematic and a significant number of columnar
phases having been characterised [4].
NMR spectroscopy in general, and deuterium NMR spectroscopy in
particular, has been shown to be a very powerful technique for the study of
liquid crystals composed of rod-like molecules [5]. There was every reason
to believe that the technique would be as valuable for the study of
mesophases formed from discotic molecules and this has certainly proved
to be the case. Indeed, just a few years after their discovery, the first NMR
investigation of a discotic liquid crystal appeared [6], and since then many
other studies have been undertaken [7]. Here we illustrate the power of the
NMR technique for discotic systems by considering two particular exam-
ples for which the orientational order, molecular geometry chain flexibility
and phase symmetry have been determined. However, before we come to
such studies we shall first consider, in the following section, the basic
features of deuterium NMR spectroscopy and its use in the investigation of
liquid crystals. In sections 3 and 4 we describe our results for the structure
of two quite different discotic liquid crystals formed by chemical induction,
but with very different molecules inducing the mesophases. Our conclu-
sions are given in section 5 and the syntheses of the deuteriated materials,
essential for such NMR investigations, are described in section 7.
2. DEUTERIUM NMR SPECTROSCOPY: THE BASICS
Although there are several magnetic nuclei whose NMR spectra could be
used to investigate the liquid crystal phase, we shall only be concerned with
deuterium primarily because the spectra are relatively simple, straight-
forward to analyse and extremely informative [5]. We begin by considering
a molecule containing a single deuteron. In the isotropic phase the spec-
trum consists of a single line made up of a pair of degenerate transitions,
since the spin quantum number of deuterium is one. In a liquid crystal
phase the degeneracy of the transitions is lifted and the NMR spectrum
contains two lines. The degeneracy is removed because of the quadrupolar
interaction experienced by the deuteron which, unlike the isotropic phase,
is not averaged to zero by the rapid tumbling of the molecule because of the
long range orientational order of the liquid crystal phase. The observation
of just two spectral lines requires a further condition, namely that the
director of the phase, assuming that it is uniaxial, is uniformly aligned with
respect to the magnetic field inherent in the NMR spectrometer. This is
usually easy to achieve, at least for a nematic since the director will be
~
~
aligned either parallel to the field ðDw > 0Þ or perpendicular to it ðDw < 0Þ.
~
For discotic materials the diamagnetic anisotropy, Dw, is usually negative
and so the director is aligned perpendicular to the magnetic field. We shall

190
J. R. Hughes et al.
discuss shortly what form the spectrum takes if the director is not uni-
formly aligned.
The magnitude of the quadrupolar splitting depends on the orientational
order and when the director is orthogonal to the field is given by
~
Dn ¼ ꢀð3=4ÞqCDSCD
:
ð1Þ
This simple expression results because the magnetic quadrupolar inter-
action for a deuteron attached to a carbon atom approximates closely to
being cylindrically symmetric. Here qCD is the quadrupolar coupling con-
stant and if the magnetic field of the spectrometer was applied along the
C-D bond direction, the observed quadrupolar splitting would be (3/2)qCD
.
The magnitude of qCD varies slightly with the chemical structure; it is about
183 kHz for an aromatic deuteron and 168 kHz for an aliphatic deuteron.
The orientational order parameter, SCD, is defined as
SCD ¼ ð3cos2 bCD ꢀ 1Þ=2;
ð2Þ
where the bar indicates an average over all molecular orientations and bCD
is the angle between the C-D bond and the director. When the molecule has
cylindrical symmetry, its orientational order at the second rank level is
defined by the Tsvetkov order parameter [8]
S ¼ ð3cos2 b ꢀ 1Þ=2;
ð3Þ
where b is the angle between the molecular symmetry axis and the
director. The two order parameters are related by
SCD ¼ Sð3 cos2 g ꢀ 1Þ=2;
ð4Þ
where g is the angle between the C-D bond and the molecular symmetry
axis. This angle is often 90ꢁ as we shall see and so S is just minus twice the
order parameter SCD available from the quadrupolar splitting.
Real discotic molecules are rarely cylindrically symmetric [3] and so their
orientational order cannot be described with a single order parameter. What
is needed is a matrix of order parameters which was introduced by Saupe
[8]. For a rigid molecule an element of the ordering matrix is given by
Sab ¼ ð3cos ba cos bb ꢀ dabÞ=2;
ð5Þ
where a denotes a molecular axis, ba is the angle between this axis and the
director while dab is the Kroenecker delta. In the principal axis system, only
the diagonal elements of S are non-zero and because the matrix is traceless
there are just two independent order parameters [8]. These are generally
taken to be Szz, the major order parameter, and (Sxx – Syy), the biaxial
order parameter, where the axis labels are chosen so that (Sxx – Syy) is

Chemically induced DLCs
191
positive and smaller than |Szz|. It is important to recognise that (Sxx – Syy)
refers to the biaxial ordering of the molecule with respect to the director
and does not reflect the symmetry of the phase. The C-D bond order
parameter is related to the elements of the Saupe ordering matrix and in
the principal axis system for S
SCD ¼ fð3 cos2 gz ꢀ 1Þ=2gSzz þ ðcos2 gx ꢀ cos2 gyÞðSxx ꢀ SyyÞ;
ð6Þ
where, for example, gz is the angle between the C-D bond and the z axis.
Clearly, at least two quadrupolar splittings together with a knowledge of the
molecular geometry and the location of the principal axes are needed to
determine the major and biaxial order parameters.
So far we have been concerned with a monodomain sample of the liquid
crystal, that is one in which the director is uniformly oriented. When the
director adopts a range of orientations with respect to the magnetic field,
the observed spectrum is a sum of spectra from the various director
orientations. The quadrupolar splitting varies with the director orientation
according to
2
~
~
DnðyÞ ¼ Dn0ð3 cos y ꢀ 1Þ=2;
ð7Þ
where y is the angle made by the director with the magnetic field and D~n0 is
the splitting when the director is parallel to the field. It is then the largest
splitting and as the angle, y, increases, so the splitting decreases, passes
pffiffiffi
through zero at the magic angle ðy ¼ cosꢀ1 1= 3 ¼ 54:74ꢁÞ and increases
to half the original value when the director is orthogonal to the magnetic
~
field. Strictly the quadrupolar splitting is ꢀDn0=2 although its sign is not
readily available from the NMR experiment. The powder pattern obtained
by adding spectra from all director orientations with a totally random
distribution is shown in Figure 1(a). It has two distinct features, firstly the
horns in the centre of the spectrum which are associated with those regions
of the sample where the director is perpendicular to the magnetic field.
Secondly, there are weaker shoulders at the extremes of the spectrum
which come from the director being parallel to the field. The fact that the
spectrum only contains these two sorts of features and that the splittings
associated with them are in the ratio of 2:1 is a result of the uniaxial
symmetry of the phase. For this system the component of the quadrupolar
~
~
tensor along the director is qjj, giving a splitting of ð3=2Þqjj while the
~
component perpendicular to the director is q?. Since the tensor is also
traceless, q? is simply ꢀqjj=2. This symmetry for the quadrupolar tensor,
partially averaged by the rotational motion of the molecules in the liquid
crystal phase, follows from the uniaxial symmetry of the phase. Indeed, the
~
~
~
uniaxial symmetry for q obtains whatever the shape of the molecule con-
taining the deuterium nuclei.

192
J. R. Hughes et al.
FIGURE 1 The simulated deuterium NMR spectrum for a system containing a
single deuteron for which the directors are randomly distributed with respect to the
magnetic field for (a) a uniaxial phase, and (b) a biaxial phase.
It follows from this discussion that if the liquid crystal phase is
biaxial, as say for a rectangular columnar phase, then the partially
averaged quadrupolar tensor will also be biaxial. That is, the three
~
principal components of q will be different, however because the tensor
is traceless there are just two independent elements which are expres-
~
sed as the major element, qZZ, and the biaxiality which is represented
by the reduced quantity [9]
~
~
~
~
Z ¼ ðqXX ꢀ qYY Þ=qZZ
:
ð8Þ

Chemically induced DLCs
193
~
This parameter Z provides a pragmatic characterisation of the phase
biaxiality but is not uniquely defined for it is likely to vary, for example, with
the location of the deuteron in the molecule [10]. When the magnetic field is
parallel to a principal axis or equivalently one of the directors the observed
~
quadrupolar splitting is just three halves of the component of q associated
with that axis. In general if the field is at an arbitrary orientation with
respect to the three directors, then the quadrupolar splitting is given by
2
2
~
~
~
Dnðy; fÞ ¼ ð3=2ÞqZZfð3 cos y ꢀ 1Þ=2 þ ðZ=2Þ sin y cos 2fg;
ð9Þ
where y and f are the spherical polar angles made by the field with the
~
principal axes of q. It is, however, extremely difficult to prepare a mono-
domain sample of a biaxial liquid crystal whose orientation can then be
varied with respect to the magnetic field of the NMR spectrometer. The
alternative is to study a sample in which the directors are randomly
arranged. An example of such a powder pattern for a biaxial liquid crystal
~
with a biaxiality parameter Z of 0.20 is shown in Figure 1(b). It is clear that
this NMR spectrum contains three pairs of features, a central pair of horns
and two sets of shoulders, which are associated with the different quad-
rupolar splittings occurring when the field is parallel to the three directors.
The pattern for the biaxial phase clearly contrasts with that for the uniaxial
phase which has only two pairs of spectral features; it should, therefore, be
easy to identify a biaxial liquid crystal phase using deuterium NMR spec-
troscopy and this is certainly the case, as we shall see.
3. SYSTEM 1: ORIENTATIONAL ORDER AND MOLECULAR
GEOMETRY
One of the intriguing features of discotic liquid crystals is that, in addition
to being formed by pure materials, these phases can also be induced by
mixing two materials. Such chemically-induced liquid crystal phases clearly
result from a specific interaction between the two components which
favours the formation of the induced phase. The existence of these induced
phases provides further clear evidence that molecular shape is not the only
factor responsible for the formation of liquid crystals. For the discotic
systems chosen to illustrate the power of deuterium NMR spectroscopy in
the study of liquid crystals, we have selected two in which the phases are
formed by chemical induction.
In the first example the radial multiyne, pentakis(phenylethynyl)phenyl-
decyl ether (D102) is mixed with 2,4,7-trinitrofluorenone (TNF) [11]; the
chemical structures of both materials are shown in Figure 2. The code used
to denote this multiyne and the other which we have studied were intro-
duced by the person who synthesised them; they indicate his first name

194
J. R. Hughes et al.
FIGURE 2 The phase diagram for a binary mixture of a radial multiyne (D102) and
2,4,7-trinitroflurorenone (TNF); the chemical structures of both components are
given.
(Dirk) and the order in which the compounds were prepared. The phase
diagram for the binary mixture is also given in this figure and we can see
that whereas the pure components are not mesomorphic their equimolar
mixture exhibits two enantiotropic phases, a columnar nematic phase, NCol
,

Chemically induced DLCs
195
and the other a hexagonal columnar phase [11]; there is also a monotropic
columnar phase, ColX, whose structure has not been determined. The
nematic phase, NCol, is unlike a conventional nematic formed from discotic
molecules in that the D102 and TNF molecules are stacked, on average,
alternately in short columns [11] giving a structure in which these
aggregates resemble the rod-like molecules of a calamitic nematic. The
mixtures have a striking reddish-orange colour which contrasts with the
pale yellow appearance of the pure components. It results from a charge-
transfer band caused by the electron-rich character of D102 and the high
electron affinity of TNF. Although the existence of the charge-transfer band
is not in doubt, the role this plays in stabilising the induced phases is
questionable. The weakness of the charge-transfer interaction has led to
the suggestion that a quadrupolar electrostatic interaction, not to be
confused with the quadrupolar spin interaction in NMR spectroscopy, could
be responsible for the chemical induction of the liquid crystal phases [12].
This suggestion is supported by the results of a computer simulation study
for a mixture of Gay-Berne discs, each with an embedded point quadrupole,
but of opposite sign. Not only is a columnar phase induced but a nematic
phase also appears which exhibits a local columnar structure in which, on
average, the components alternate along the column [12]. A more general
model for such phase induction in which the charge is distributed over the
two molecules rather than representing it as a point quadrupole has sub-
sequently been developed [13].
In order to study the system using deuterium NMR spectroscopy, we
have prepared perdeuteriated TNF-d5, D102 with the five phenyl rings
deuteriated, D102-d25 and D102 with the decyloxy chain perdeuteriated,
D102-d21. The availability of these deuteriated materials is clearly essential
for such spectroscopic investigations and their synthesis is described in
section 7. Here D102-d25 is denoted by 2, D102-d21 by 1 and TNF-d5 by 4.
In these studies we have concentrated on the equimolar mixture since this
has the longest mesophase range (see Fig. 2). We begin with the
results for TNF-d5 and the NMR spectra of this in the NCol and Colho phases
are shown in Figure 3. In the nematic phase, close to the nematic-isotropic
transition there are two doublets with very similar splittings coming from
the ordered nematic together with a single peak originating from the iso-
tropic phase. It shows immediately that the system exhibits a biphasic
region with co-existing nematic and isotropic phases, as expected for a two
component mixture. What is unexpected is that the quadrupolar splittings
for the deuterons in TNF are so similar. As we have seen the quadrupolar
splittings are proportional to the order parameter for the C-D bonds which,
in turn, are determined by the principal components of the ordering matrix
and the molecular geometry (see Eq. (6)). If TNF is taken to be planar then
one of the principal axes, say z, must be orthogonal to the molecular plane.

196
J. R. Hughes et al.
FIGURE 3 The deuterium NMR spectrum of TNF-d5 in an equimolar mixture of
D102 and TNF at (a) TNI – T ¼ 5ꢁC in the NCol phase, and (b) at TNI – T ¼ 53ꢁC in
the Colho phase.
The non-symmetric substitution pattern of the nitro groups makes the
location of the remaining principal axes less definite. However, if we ignore
this difficulty then we could locate one axis along the C¼O bond and the
other orthogonal to this and in the plane. Such an intuitive identification of
the principal axes for S is consistent with their more theoretical location
based on the surface tensor model [14] which places the in-plane principal
axes just 8ꢁ from our more approximate identification [15]. With this
approximation, the five deuterons divide into a geometrically equivalent
group of three, in positions 1, 5, 8, and a group of two at positions 3, 6.
The C-D bonds in these groups make significantly different angles with the
x, y principal axes but the same angle, of 90ꢁ, with the z axis. It follows

Chemically induced DLCs
197
from Eq. (6), therefore, that the only way in which the quadrupolar
splittings for the two groups of deuterons can be the same is if the biaxiality
(Sxx – Syy) is zero. It seems, therefore, that although TNF is certainly
biaxial in shape it finds itself in an environment in which it can rotate about
its z axis unhindered. This would be consistent with it being sandwiched in
the local nematic columns between the aromatic cores of two D102
molecules. In marked contrast, if TNF-d5 is dissolved in a calamitic
nematic, then the ordering matrix is found to have a large biaxiality [15],
presumably because the environment of TNF is significantly less uniform
that in a columnar nematic phase. On lowering the temperature for the
equimolar mixture of TNF and D102, the orientational order increases
significantly as is apparent from the increase in the quadrupolar splitting
shown by the spectrum in Figure 3(b). On entering the ColX phase, the
spectrum disappears and we shall return to this unusual observation shortly.
The temperature dependence of the quadrupolar splitting for TNF-d5
is shown in Figure 4 for both the Ncol and Colho phases. We see that there is
a small temperature range where the two phases coexist, as expected for a
binary mixture. Within the nematic phase the quadrupolar splitting exhibits
FIGURE 4 The dependence on the shifted temperature, TNI – T, of the quadrupolar
splittings for TNF-d5 and D102-d25 in an equimolar mixture of D102 and TNF. Also
shown are the order parameters, Szz, for an axis perpendicular to the molecular
planes for the two components, (j) TNF-d5 and (u) D102-d25. For comparison the
equivalent parameter for the pure mesogen D13 is also shown as (ꢂ).

198
J. R. Hughes et al.
a strong temperature dependence but in the more ordered columnar phase,
the splitting is far less sensitive to temperature. The measurement of the
quadrupolar splitting is especially important because it provides a route to
the orientational order parameter. As we have seen, for a uniaxial disc in a
phase with the director aligned at right angles to the field, it follows from
Eqs. (1) and (4) that the splitting is directly proportional to the order
parameter for the symmetry axis of the disc. We have converted the
quadrupolar splitting for TNF-d5 to the order parameter, which we denote
by Szz, and the results are shown in Figure 4.
A knowledge of the order parameter is important, not only to char-
acterise the liquid crystal phase but also to test theories of the phase. For
nematic liquid crystals the Maier-Saupe molecular field theory [6] provides,
almost certainly, the best description even though the theory is especially
simple. One of the predictions of this theory is that uniaxial particles
whether disk-like or rod-like forming a nematic phase should have the same
orientational order parameters. Our results allow this intriguing prediction
to be tested. Thus, at the nematic-isotropic transition the order parameter,
SzNzI , for TNF-d5 is found to be 0.41 which compares favourably with the
value of about 0.35 which is typical of that determined for nematics com-
posed of rod-like molecules. In addition, the transitional order parameter is
predicted by the Maier-Saupe theory to be 0.429 [6] in remarkably close
agreement with experiment. There is a significant jump in the orientational
order parameter at the Colho – NCol transition consistent with the first order
nature of this. However, by analogy with the SmA-N transition, it might
have been expected that the wide nematic range would have made the
transition far weaker. The orientational order parameter in the columnar
phase is between 0.8 and 0.9 depending on the temperature (see Fig. 4)
which indicates that there is a high degree of order in good agreement with
the computer simulation results for the generic quadrupolar Gay-Berne
model of this system [12].
We now turn to the other component of the binary mixture, namely
D102–d25, which was perdeuteriated in the five phenyl rings. From our
results for TNF–d5 we might have anticipated that all of the 25 deuterons
would have had the same quadrupolar splittings. However, this is clearly
not the case as we can see from the spectra of D102–d25 shown in Figure 5
for the nematic and hexagonal columnar phase of the equimolar mixture.
The spectral resolution is better in the columnar phase, presumably
because the director fluctuations which can broaden the lines in the
columnar nematic phase are largely quenched in the hexagonal columnar
phase. In any event, we can clearly see four distinct quadrupolar splittings
together with a weak central peak coming from an isotropic phase. It is not
at all apparent why there should be this small amount of isotropic phase so
deep into the columnar phase. We would note, however, that an isotropic

Chemically induced DLCs
199
FIGURE 5 The deuterium NMR spectrum of D102-d25 in an equimolar mixture of
TNF and D102 at (a) TNI – T ¼ 20ꢁC in the NCol phase, and (b) TNI – T ¼ 50ꢁC in
the Colho phase.
peak is also found for the nematic phase even though the system is some
20ꢁC from the nematic-isotropic transition. The four quadrupolar splittings
observed for the columnar phase become three in the nematic phase
because the difference in the two larger splittings is small and the line-
widths have increased. Given the marked inequivalence of the deuterons
we are faced with the problem of assigning the splittings to particular

200
J. R. Hughes et al.
deuterons. This is a difficult task which could only be solved unambigu-
ously by specific deuteriation of the rings but this would involve some
difficult chemical synthesis. Here, we shall use a more heuristic approach
and begin by recalling that the area under a spectral line is proportional to
the number of nuclei giving that peak. Integration reveals that in the
nematic phase the largest splitting comes from five deuterons, the next
largest from twelve and the smallest from eight. There are, of course, just
five para deuterons and it seems reasonable to associate the largest
splitting with these. The fact that the splittings for these deuterons are
essentially the same suggests that, as for TNF–d5, the biaxiality in the
ordering matrix for D102–d5 is also negligibly small. If this is so then why
are the splittings for the other deuterons so different? The answer would
seem to be that the rings are twisted out of the molecular plane unlike the
structure of TNF which is necessarily planar. This view is supported by an
X-ray crystallographic study of the analogous multiyne with six phenyl
acetylene groups attached to a central phenyl ring [16]. If the rings are
twisted, probably to different extents, then the C-D bonds for the ortho and
meta deuterons will make different angles with the effective symmetry axis
of the molecule in contrast to the para deuterons and so will give different
splittings dependent on the twist angle (see Eqs (1) and (4)). Thus, the
para deuterons for the five phenyl rings will be in the average plane of
the molecule and so will be orthogonal to its symmetry axis and will have
the same quadrupolar splittings.
This partial assignment now allows us to determine the order parameter,
Szz, for D102-d25 using the splittings for the five para deuterons. The
results for this in the NCol, Colho and ColX phases are shown in Figure 4,
again as a function of the shifted temperature. The order parameter at the
nematic-isotropic transition, SzNzI , is 0.48 which is very similar to that of
TNF-d5 especially given the quite different molecular structures of these
two components of the mesogenic mixture. It would seem, therefore, that
the orientations of the two components are highly correlated. We see, from
Figure 4, that the orientational order parameter is more or less constant
over a small temperature region which would be consistent with the
existence of a biphasic region with coexisting isotropic and nematic phases
[17]. It then grows with decreasing temperature to a value of 0.71 at the
transition to the Colho phase; this again is very similar to the result found
for TNF-d5. Within the hexagonal columnar phase the orientational order
parameter determined from the averaged splitting for the para deuterons is
found to grow from 0.81 to 0.85, which is slightly smaller than that found
for TNF-d5. In view of the significant difference in the molecular structure
of the two components it is not really possible to explain these small dif-
ferences. Surprisingly we were able to record a spectrum for D102-d25 in
the unidentified columnar phase which is in stark contrast to the absence of

Chemically induced DLCs
201
a spectrum for TNF-d5. The orientational order parameter in the ColX
phase is about 0.9 which is somewhat higher than for the preceding Colho
phase; this is perhaps to be expected since it is a lower temperature phase.
We now come to the molecular geometry of D102-d25, in particular the
twist angles for the phenyl rings which cause the molecule to deviate
from planarity although the conformationally averaged structure should
be planar. From the structure of D102 it is to be expected that two pairs
of rings would be equivalent as indicated by the labelling shown in Figure
2 with one unique ring. The twist angles for this need not be the same
and so we should see three different quadrupolar splittings for the ortho/
meta deuterons. This follows because as we have seen the quadrupolar
splittings are related to the order parameter, Szz and the angle made by
the C-D bond with the z axis. However, only two quadrupolar splittings
are observed coming from the twenty ortho/meta deuterons in the five
rings. This means that the twist angles for three rings are the same but
different to those for the two remaining rings. If we now assume that the
phenyl rings have a regular hexagonal structure and denote the twist
angle by e, that is the angle between the normal to the central phenyl ring
and one attached to it, then
cos g ¼ ð3=4Þ1=2 sin e:
ð10Þ
This gives the quadrupolar splittings for the ortho/meta deuterons, which
are the same for a given ring, as
Dno=m ¼ ð3=32ÞqCDSzzð9 sin2 e ꢀ 4Þ;
ð11Þ
~
where Szz is available from the quadrupolar splitting for the para-deuter-
ons. This can, in fact, be used to write Eq. (11) as
D~no=m=D~np ¼ ð9 sin2 e ꢀ 4Þ:
ð12Þ
There is now one complication which relates to the fact that the signs of the
quadrupolar splittings are not available from spectra such as those in
~
~
Figure 5. We have, therefore, to consider both signs for the ratio Dno=m=Dnp
which will give two values for the twist angle, e. In addition we have two
splittings for the ortho/meta deuterons, the larger associated with twelve
deuterons and so coming from three phenyl rings and the smaller asso-
ciated with eight deuterons and so coming from two phenyl rings. Clearly
one of the rings in the set of three must be the unique phenyl in D102-d25
but we are not able to make any assignment for the remaining four rings.
The twist angles calculated from the quadrupolar splittings are shown in
~
~
Figure 6. For the ratio of the splittings, Dno=m=Dnp, taken to be negative the
twist angles are approximately 30ꢁ whereas if the splittings are positive, the
angles are much larger at about 55ꢁ.

202
J. R. Hughes et al.
FIGURE 6 The dependence of the twist angles, e, for the five phenyl rings in D102
on the shifted temperature, TNI – T, for the two choices of the sign of the ratio
~
~
Dno=m=Dnp, (a) positive, and (b) negative.
We see that the twist angles, for both choices of relative sign, are
essentially temperature independent in the nematic and hexagonal
columnar phase. In addition they do not change at the Colho-Ncol phase
transition which seems physically reasonable. However, for the smaller
twist angles on going to lower temperatures the angles decrease by a small
amount making the molecule slightly more planar which is also physically
realistic. In contrast, the larger twist angles increase on lowering the
temperature which is unexpected. It should be stressed, however, that in
both cases the changes are small. It might be expected that p-conjugation
would favour a planar structure for D102 and so we expect the rings to
adopt the smaller of the two sets of twists. This would also be in accord
with the X-ray structure of the analogous molecule [16]. However, the
deviation from planarity was found to be just 11ꢁ which is significantly
smaller than that given by NMR. This may result simply from the difference
in the phase, crystal versus liquid crystal, or it could result from the per-
turbation of the structure by the decyloxy chain in D102-d25. This differ-
ence in the twist angles between the liquid crystal and crystal phases is
consistent with that found for biphenyl. In the crystal phase the rings are
planar whereas in the nematic phase biphenyl has the rings twisted by
about 35ꢁ with respect to each other [18]. Again steric forces between
neighbouring protons are taken to be responsible for the twist of the rings.
In any event the twist of the phenyl rings out of the molecular plane of

Chemically induced DLCs
203
FIGURE 7 The deuterium NMR spectrum of D102-d21 in the Colho phase formed
by an equimolar mixture of TNF and D102 at a shifted temperature, TNI – T, of
about 41ꢁC. An expanded version of half of this symmetric spectrum is also shown.
D102 is almost certainly caused by steric overlap between the ortho deu-
terons on neighbouring rings.
We have attempted to explore the geometry of the chain in D102 by
studying the system with the perdeuteriated chain. In addition, this will
also give the orientational order parameters for the C-D bonds along the
chain. This information is important, not only as a way of characterising the
order but also because it allows the influence of chain flexibility on its
orientational order to be evaluated and compared with theoretical pre-
dictions. The beautiful spectrum of D102-d21 is shown in Figure 7 for the
hexagonal columnar phase formed by the equimolar mixture of TNF and
D102. A similar spectrum was obtained for the columnar nematic phase but
as we have seen, the linewidths are somewhat larger in this phase and so
the resolution is not quite so impressive as in the Colho phase. There are
nine well-resolved quadrupolar doublets but we expect to see ten from the
ten groups of equivalent deuterons in the decyloxy chain. This implies that
two of the groups of deuterons have very similar quadrupolar splittings and
by expanding the spectrum and showing just half in Figure 7 we can see

204
J. R. Hughes et al.
that the peak labelled f has a shoulder which is just discernible. Before we
consider a more detailed analysis, two features are immediately observable
from the spectra in Figure 7. The first of these is the presence of the iso-
tropic peak for the sample which is in the columnar phase far from the
nematic-isotropic transition. This could result from a spatial variation in the
concentration of the components but the observation of such an isotropic
peak for different samples makes this explanation seem unlikely. The other
spectral feature is that the quadrupolar splittings are significantly smaller
than those for the deuterons attached to the phenyl rings in D102-d25. This
implies that further motional averaging of the quadrupolar interactions is
taking place and that this is associated with the conformational changes in
the decyloxy chain [8].
We now return to the observation of the complete set of quadrupolar
splittings and the difficult task of assigning these to positions in the chain.
Of course, this could be achieved by preparing D102 with a specifically
deuteriated chain but only with considerable difficulty and so we shall first
explore simpler procedures. As we have seen, we can use the areas of the
peaks to determine the number of deuterons responsible for them. This
shows that whereas most peaks are associated with two deuterons, the
composite peak, f, has four as we had already implied and i has an intensity
coming from three deuterons. It is reasonable to identify this quadrupolar
splitting as coming from the methyl deuterons which is consistent with this
being the smallest splitting, as found for other systems [19]. Examination of
the expanded half spectrum in Figure 7 reveals that with the exception of
peak h all of the others are symmetrically spaced with respect to the centre
of the spectrum. This suggests that the deuterons responsible for peak h
have a slightly different chemical shift to the others. The only reason for
this difference would be the location of the CD2 group next to the oxygen
whose high electronegativity would cause the chemical shift to change. If
this analysis is correct then it would mean that the next smallest splitting
comes from the methylene group closest to the aromatic core of D102. This
would be a remarkable result for it is generally observed that such a
methylene group would have the largest splitting [19].
In an attempt to confirm this unusual result and also to assign all of
the quadrupolar splittings to positions in the chain, we have adopted the
following approach. It might be expected that the orientational order
would decrease monotonically along the chain as a result of conforma-
tional changes. This would seem to imply that the splittings should also
decrease in the same way but numerous studies of both calamitic and
discotic mesogens have revealed that the splittings often alternate along
the chain. However, although the order may not decrease monotonically
along the chain, it has been suggested that the chain dynamics should
increase monotonically [20]. Since the NMR spin-lattice relaxation time,

Chemically induced DLCs
205
FIGURE 8 The spectra measured as a function of time during the inversion-
recovery experiment for D102-d21 in the hexagonal columnar phase of the equi-
molar mixture of TNF and D102 at a shifted temperature of 43ꢁC; only half of the
symmetric spectrum is shown.

206
J. R. Hughes et al.
T1, is inversely proportional to the correlation time determining the
conformational dynamics, it is expected that T1 should increase mono-
tonically along the chain from the point of attachment to the core to its
end. This behaviour is in agreement with the results of a number of
studies [21]. We have determined the spin-lattice relaxation times using
an inversion-recovery experiment in which the populations of the spin
levels are inverted. As a consequence the spectral intensities are also
inverted and we monitor the intensities of all the spectral lines as a
function of time as they return to their equilibrium values. The spectra
observed in such an experiment for D102-d21 in its hexagonal columnar
phase at a shifted temperature of 43ꢁC are shown in Figure 8. The first
spectrum, or rather half of the symmetric spectrum, recorded just 1ms
after the initial inversion shows that all of the lines have been inverted.
As time increases we expect the intensities to grow, pass through zero
and reach an equivalent positive intensity to the initial negative value.
This process is readily seen for the outer spectral line a, corresponding to
the largest quadrupolar splitting. In contrast the line labelled j retains its
negative intensity, indicating a long spin-lattice relaxation time in com-
parison to those for other peaks. However, this is to be expected because
peak j corresponds to the methyl deuterons and so should indeed have
FIGURE 9 The variation of the modulus of the order parameter, |SCD| for the
deuterons in the decyloxy chain of D102-d21 with their position in the chain for the
equimolar mixture of TNF and D102 in the Colho phase at a shifted temperature of
43ꢁC.

Chemically induced DLCs
207
the longest value of T1. In marked contrast the intensity of peak j cor-
responding to the next smallest splitting increases rapidly and, in fact,
has the shortest spin relaxation time. This is exactly what is to be
expected if the deuterons giving this peak were at the end of the chain
closest to the core, as we had previously speculated. To be absolutely
certain of this especially unusual result we have prepared D102-d2 with
the a-position specifically deuteriated. The deuterium NMR spectrum of
this material in an equimolar mixture with TNF consists of the expected
single quadrupolar doublet, and the magnitude of the quadrupolar split-
ting is equal at the same shifted temperature to that of the next smallest
splitting found for the perdeuteriated decylocy chain [15]. Given this
valuable confirmation of the assignment of the next smallest splitting to
the pair of deuterons in the a-methylene group in the decyloxy chain, we
have used the values of T1 to assign the remaining splittings and hence
the C-D bond order parameters to the other groups in the chain. This
assignment is shown in Figure 9 where the position along the chain is
labelled with the letters a, b, . . . .k. We see that the modulus of the order
parameter |SCD| alternates along the chain although this alteration is
attenuated until there is a large drop in |SCD| for the methyl deuterons.
This final reduction is a consequence of the additional averaging of the
quadrupolar interaction caused by the rotation of the methyl group about
the terminal C-C bond.
The results shown in Figure 9 present a challenge to theories of chain
ordering in liquid crystalline systems [22]. We shall not deal with our
attempts to fit the results for D102 here but note that the agreement
between theory and experiment is convincing [15] even though the chain is
in an usually inhomogeneous local environment with two phenyl rings on
each side of it, unlike the situation for other discotic materials, where there
are other chains on either side [19]. What is important for our studies is that
the order parameter |SCD| for the a-position is particularly sensitive to the
torsional angle defined as that between the normal to the central phenyl
ring and the normal to the CarOCal plane. For a zero torsional angle the
a-carbon atom lies in the plane of the ring and for a 90ꢁ torsional angle
the carbon atom is immediately above the ring. For ether linked chains, the
torsional angle is usually zero [16]. What we find for D102 is that to account
for the very small order parameter of the deuterons on the first methylene
group, the torsional angle needs to be about 60ꢁ. If the chain were twisted
just in this way then the two C-D bonds of the a-methylene group would
make significantly different angles with the normal to the molecular plane
and so would have different quadrupolar splittings (see Eq. (4)). The fact
that these different splittings are not observed in the spectra of D102-d21
(see Fig. 7) clearly demonstrates that the torsional angle for the chain must
jump between equal but opposite values. This dynamic process will then

208
J. R. Hughes et al.
average the different quadrupolar splittings and so restore the equivalence
of the a-deuterons. The large alternating value of the torsional angle
undoubtedly results from the steric interactions of the chain with the
neighbouring phenyl rings. This arrangement for the chain causes D102 to
deviate from a disc-like shape; indeed it has a scorpion-like appearance
which could help explain the absence of a liquid crystal phase for the pure
material and the formation of a chemically-induced nematic phase and not
just a columnar phase. This situation obtains because the formation of a
columnar phase results, in part, from a strong interaction between the
discotic molecules when they are in a face-to-face configuration. This is
most likely to occur when as many atoms are as close to each other as
possible. Molecules with a planar core such as the triphenylenes will allow
this to happen. In contrast for discotic molecules such as D102 the twist of
the phenyl rings, especially if this is not regular, together with that for the
decyloxy chain, will reduce the number of effective atomic contacts.
4. SYSTEM 2: ORIENTATIONAL ORDER AND PHASE
SYMMETRY
The other binary mixture with which we have chosen to illustrate the
importance of NMR spectroscopy shows quite remarkable behaviour. One
component is the radial multiyne, 2,3,4,6,7,8-hexakis(4-nonylphenyl
ethynyl)naphthalene. The phase sequence, namely Cr-N – I, can be changed
dramatically by the addition of relatively small molecules such as 4,5-
dibromo-o-xylene (DBOX), as the phase diagram in Figure 10 shows [23].
Thus for 75 mol % of D13 the mixture exhibits a hexagonal columnar phase, a
rectangular columnar phase and a re-entrant nematic phase on cooling. This
change in phase behaviour seems really quite remarkable although it should
be noted that reducing the length of the terminal chains from nine to eight
carbon atoms produces the same phase sequence [23]. The phase behaviour
of the equimolar mixture of D13 and DBOX is very different to that for the
previous mixture in that only a hexagonal columnar phase is formed. The
precise role of the small molecules in inducing the phases is not clear.
However, what is known is that the molecule must possess an electric dipole
moment, for example 1,2,4,5-tetramethylbenzene which is isosteric to
DBOX does not induce a phase but simply depresses the nematic-isotropic
transition. In addition, the larger the dipole moment the higher is the
maximum temperature for the hexagonal columnar to isotropic transition. It
has also been suggested that the ability of the phase inductors to fit into the
gaps left near the two unsubstituted positions on the naphthalene core is
also important. It is clearly of interest to investigate the orientational order
of the two components especially to see how they compare with each other

Chemically induced DLCs
209
FIGURE 10 The phase diagram for a binary mixture of D13 and DBOX; the che-
mical structures of the two components are also given.

210
J. R. Hughes et al.
and with other discotic systems. To this end we have specifically deuteriated
D13 at the two vacant positions 1 and 5 in the naphthalene core, D13-d2; and
also at the two unsubstituted positions, 3 and 6, on the phenyl ring of DBOX
to give DBOX-d2. The syntheses of these specifically deuteriated compounds
is described in section 7 where D13-d2 is denoted by 3 and DBOX-d2 by 5.
We begin with the pure mesogen D13-d2; this gave the expected simple
NMR spectrum containing a single quadrupolar doublet since the two
deuterons are equivalent. It is not possible, therefore, to estimate the
biaxiality in the ordering matrix of D13 but if we assume that, like D102,
the ordering matrix is uniaxial then we can calculate the order parameter
for the axis orthogonal to the plane of the naphthalene core; this gives the
results shown in Figure 4. We can see from this that the transitional order
parameter, SzNzI , is 0.46 which is essentially the same as that found for D102
in the equimolar mixture of TNF and D102. It would seem, therefore, that
the local columnar structure in the NCol phase has essentially no influence
on the orientational order parameters of the components in comparison
with a conventional nematic formed from disc-like molecules. In contrast,
the entropy change, DS/R, at the nematic-isotropic transition does appear
FIGURE 11 The deuterium NMR spectrum of an equimolar mixture of D13-d2 and
DBOX-d2 in the hexagonal columnar phase at a shifted temperature, TI – T, of 50ꢁC.

Chemically induced DLCs
211
to be sensitive to this structure. Thus for D13, the transitional entropy is
0.03 [21] which is very small in comparison with the value of 0.16 found for
the columnar nematic-isotropic transition exhibited by the equimolar
mixture of TNF and D102 [13]. It is also of interest that DS/R for nematics
formed by rod-like molecules are higher than those formed from disc-like
molecules, being typically about 0.3.
We now consider the orientational order of the two components in the
equimolar mixture of D13 and DBOX which, as we can see from Figure 10,
forms just a Colho phase over a wide temperature range. The NMR spec-
trum of the columnar phase 50ꢁC from the transition to the isotropic phase
is shown in Figure 11. Since both components are partially deuteriated, the
spectrum contains two quadrupolar doublets; the strong peaks with a small
quadrupolar splitting come from DBOX-d2 while the doublet from D13-d2
has a small height and a large splitting. The dramatic difference in line
heights for the two doublets originates in the significant difference in
linewidths. This difference is probably caused by the faster reorientation of
DBOX in comparison with D13 because of the considerably larger size of
D13. The lines broaden further as the temperature is lowered but differ-
entially so that the spectral lines for DBOX-d2 are still discernible after
those from D13-d2 are no longer apparent. The fact that the spectrum in
Figure 11 contains just simple quadrupolar doublets means that the
director in the sample is uniformly aligned with respect to the field. This is
somewhat surprising because it shows that the magnetic field of the
spectrometer is able to align the column axes despite the high viscosity of
FIGURE 12 The dependence of the quadrupolar splittings and the associated order
parameters for (j) D13-d2 and (r) DBOX-d2 on the shifted temperature, TI – T.

212
J. R. Hughes et al.
this ordered phase. This may occur because the sample was prepared by
cooling from the isotropic phase; it passes, therefore, through a biphasic
region in which columnar and isotropic phases coexist and that facilities
alignment by the magnetic field.
The temperature dependence of the two quadrupolar splittings together
with the order parameters calculated from them are shown in Figure 12.
The most striking feature of these results is the dramatic difference in the
orientational order of the two components of this chemically-induced
phase. Thus while the order parameter for D13-d2 in the columnar phase is
typically about 0.8, which compares well with that for D102 in the columnar
phase formed by its equimolar mixture with TNF (see Fig. 4), the order
parameter for DBOX-d2 is only about 0.02. This huge difference certainly
suggests that DBOX is not sandwiched between the D13 discs unlike TNF
with D102. This result provides further evidence for the suggestion that
DBOX and similar phase inductors reside in the bay regions of D13 where
they tumble more or less freely [23]. The filling of these regions by the small
molecules implies a steric mechanism for the phase induction but the need
for the inductors to possess an electric dipole moment shows that attrac-
tive forces must also be involved.
Another curious feature of this, and indeed the other system exhibiting
chemically-induced liquid crystal phases, is the observation of an isotropic
peak in the spectra over a wide temperature range. Since the isotropic peak
is relatively sharp in comparison with the quadrupolar doublets coming
from the coexisting liquid crystal phases, NMR spectroscopy is especially
sensitive to the occurrence of the isotropic phase; indeed this would be
extremely difficult to detect with other techniques. For the equimolar
mixture of D13 and DBOX, the peak from the coexisting isotropic phase
can be seen over about 50ꢁC as is apparent from the results shown in
Figure 12. Such an extended biphasic region contrasts with the behaviour
for conventional mixtures where the biphasic region results from the first
order character of the phase transition and the phases coexist over just a
few degrees. However, during the NMR experiments we have noticed that
as the sample is held at relatively high temperatures (see Fig. 10), that its
colour passes from pale yellow, to dark yellow and eventually to brown.
Associated with this colour change, presumably caused by the thermal
decomposition of D13, was an increase in the intensity of the isotropic peak
in the NMR spectrum. The decomposition of D13 might result in the
creation of concentration gradients within the sample and so result in the
appearance of small isotropic domains.
The mixture containing 75 mol% of D13 and 25 mol% of DBOX is fas-
cinating because it exhibits a reentrant nematic phase and a rectangular
columnar phase in addition to the hexagonal columnar phase. Here we use
this system to illustrate how NMR spectroscopy can be used to establish

Chemically induced DLCs
213
the symmetry of a liquid crystal phase since the rectangular phase is biaxial
whereas its hexagonal counterpart is uniaxial. As we have seen this can be
achieved by measuring the principal components of a partially averaged
quadrupolar tensor. These components can be determined by recording the
spectrum of a sample in which the three directors are randomly aligned
with respect to the magnetic field of the spectrometer. Here we show that
this need not be the case, provided two of the directors are randomly
aligned.
Since the deuterium NMR spectrum of DBOX-d2 is so much stronger
than that of D13-d2 it is this deuteriated component which was used to
explore the phase symmetry of the columnar phases. We started in the
reentrant nematic phase of the mixture of D13 (75 mol%) and DBOX-d2
(25 mol%). This is especially convenient because the director is aligned
orthogonal to the field as was clearly apparent from the simple quadrupolar
doublet in the spectrum. As the temperature was increased a broad feature
appeared between the two nematic peaks corresponding to the formation
of the Colro phase. The spectra of DBOX-d2 recorded as a function of
temperature within this rectangular columnar are shown in Figure 13
together with one at 385 K just inside the hexagonal columnar phase. All of
the spectra contain, as we have come to expect, an isotropic component
but this represents a small fraction of the sample; we shall ignore it in
future discussions of these spectra. The powder-like features of the spectra
show that the directors of the biaxial rectangular columnar phase are not
uniformly aligned with respect to the magnetic field. In addition the
absence of central horns in the spectrum (see Fig. 1) suggests that it is just
two of the three directors which are randomly oriented with respect to the
field. This is to be expected because the Colro phase is formed through a
relatively weak transition from the nematic phase which would allow the
director that was orthogonal to the field to remain so aligned and to
become the column axis in the Colro phase. Then, as the discs tilt within a
column, the columnar cross-section becomes elliptical creating a biaxiality
which is also responsible for the rectangular organisation of the columns.
From the appearance of the spectra it would seem that the two directors
are randomly arranged with respect to the magnetic field, presumably the
tilt of the molecular discs within a column takes place without any pre-
ference for the tilt direction. This analysis is supported by the observation
that, as the temperature is increased, so the width of the two broad fea-
tures decreased until an essentially sharp quadrupolar doublet appeared on
entering the uniaxial hexagonal columnar phase. However, the observation
of a random arrangement of the two directors with respect to the magnetic
field is somewhat surprising because it might have been expected that the
director associated with the most negative component of the magnetic
susceptibility would be aligned parallel to the magnetic field. As a result the

214
J. R. Hughes et al.
FIGURE 13 The deuterium NMR spectra of DBOX-d2 recorded as a function of
temperature within the Colro phase and just inside the Colho phase at 385 K.