Oligomeric rod-disc nematic liquid crystals

Article abstract of DOI:10.1039/b614922g

Six new oligomeric nematic liquid crystals are reported consisting of a triphenylene-based core attached to which are six 4-cyanobiphenyl units via flexible alkyl spacers. The Royal Society of Chemistry.

Full text of DOI:10.1039/b614922g

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Oligomeric rod–disc nematic liquid crystals
Corrie T. Imrie,*a Zhibao Lu,a Stephen J. Pickenb and Zeynep Yildirimb
Received (in Cambridge, UK) 13th October 2006, Accepted 22nd November 2006
First published as an Advance Article on the web 8th December 2006
DOI: 10.1039/b614922g
are attached to a central triphenylene core via flexible alkyl spacers
Six new oligomeric nematic liquid crystals are reported
consisting of a triphenylene-based core attached to which are
six 4-cyanobiphenyl units via flexible alkyl spacers.
(Fig. 1).{ The triphenylene core was chosen because it is known to
promote liquid crystalline behaviour in a wide range of molecular
architectures and has a very versatile synthetic chemistry.17,18 The
acronym used to refer to these compounds is TP(nCB)6 where n
refers to the number of methylene units linking the triphenylene
and 4-cyanobiphenyl units.
One of the most active areas of research in liquid crystal science in
recent years has been the search for the elusive biaxial nematic
phase.1,2 In the uniaxial nematic phase, the molecules are arranged
such that there are no positional correlations between their centres
of mass but the unique axes of the molecules are arranged about a
common direction known as the director. By comparison in the
biaxial nematic phase there is also a correlation of the molecules in
a direction perpendicular to the director. The existence of the
biaxial nematic phase was predicted over 30 years ago by Freiser3
and there have been many claims for its discovery in low molar
mass thermotropic liquid crystals, although experimental difficul-
ties in unambiguously identifying the symmetry of these phases
raise questions concerning these assignments.4,5
The thermal behaviour of the TP(nCB)6 series was determined
using optical polarizing microscopy and differential scanning
calorimetry and the transition temperatures and associated entropy
changes, expressed as the dimensionless quantity DS/R, are listed
in Table 1. A classical schlieren texture indicative of a nematic
mesophase but consisting of a 3–4 fold excess of ¡K over integer
disclinations was observed for all members of the TP(nCB)6 series
when viewed under a polarizing optical microscope (Fig. 2); this
was observed on both heating and cooling each homologue with
the exception of TP(5CB)6 for which the nematic phase was
observed only on cooling the isotropic melt.
Theoretical studies have shown that mixtures of rods and discs
can exhibit the biaxial nematic phase.6,7 In such a mixture the
optimum packing arrangement has the long axes of the rods
arranged perpendicularly to the short axes of the discs and hence,
the system has two directors. In these simulations an attractive
interaction is required between the rods and discs to prevent phase
separation into two uniaxial nematic phases and in real systems
phase separation does indeed occur. To overcome this difficulty a
number of authors have attached rod-like and disc-like units via
flexible alkyl spacers.8–15 In these molecular systems a single rod
and disc have been interconnected and such dimers have recently
been the subject of a computer simulation study,16 or one, two or
three rods have been attached to a single disc. We have extended
this approach and report here the synthesis and characterisation of
a series of molecules in which six rod-like 4-cyanobiphenyl moieties
On cooling the nematic phase for each member, crystallisation
was not observed and instead vitrification occurred. On reheating,
each member exhibited a glass transition and a nematic–isotropic
transition, except for TP(5CB)6 which underwent cold crystal-
lisation and a subsequent crystal–isotropic transition at 171 uC.
The glass transition temperatures initially decrease on increasing
the spacer length but appear to reach a limiting value, after which
Tg is no longer sensitive to changes in spacer length.
On increasing the length of the flexible spacers, the nematic–
isotropic transition temperature falls sharply by 58 uC on passing
from TP(5CB)6 to TP(6CB)6, decreases by 14 uC going to
TP(7CB)6 while further increasing the spacer length increases the
transition temperature. The entropy change associated with the
nematic–isotropic transition reveals a similar dependence on
the length of the flexible spacers although a more marked relative
increase is observed for the higher homologues. These values of the
entropy change are particularly high for a nematic–isotropic
Table 1 Transitional properties of the TP(nCB)6 series extracted
from a second heating DSC tracea
n
Tg/uC
TNI/uC
DSNI/R
5
6
7
8
9
50
49
33
30
31
32
[167]b
109
95
116
115
123
2.08
1.38
1.23
2.25
3.04
4.05
Fig. 1 Structure of the TP(nCB)6 series.
aChemistry, School of Engineering and Physical Sciences, Meston
Building, University of Aberdeen, Old Aberdeen, UK AB24 3UE.
E-mail: c.t.imrie@abdn.ac.uk; Fax: 44 (0)1224 272 921;
Tel: 44 (0)1224 272910
10
a
Tg glass transition temperature; TNI nematic–isotropic transition
temperature; DSNI/R scaled nematic–isotropic entropy; [ ] denotes a
bNanoStructured Materials, Faculty of Applied Sciences, Delft
University of Technology, Julianalaan 136, 2628BL Delft, The
Netherlands. E-mail: S.J.Picken@tnw.tudelft.nl; Fax: 31 15 278 7415;
Tel: 31 15 278 6946/1828
b
monotropic transition. Undergoes cold crystallisation on heating.
All measurements were made at a heating rate of 10 uC min21
.
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Fig. 4 Schematic representation of the local molecular arrangement in
the nematic phase.
Fig. 2 Schlieren texture of the nematic phase of TP(10CB)6 (120 uC).
the same plane forming a large discotic entity (Fig. 4). Thus the
aligned ND diffraction pattern suggests that the overall molecular
geometry in the nematic phase is disc-like, and even the presence of
the six 4-cyanobiphenyl groups in the tails has not given rise to a
more rod-like geometry.
transition but it may be argued that they should be scaled by the
number of interacting mesogenic units per molecule.19 This would
give values more comparable to those seen for conventional low
molar mass liquid crystals. For conventional oligomeric liquid
crystals consisting of molecules containing rod-like units inter-
connected via flexible alkyl spacers varying the length and parity of
the flexible spacers normally gives rise to pronounced alternations
in both the transition temperatures and associated entropy
changes.20–22 Such behaviour is most commonly interpreted in
terms of the change in the average molecular shape on varying the
length of the spacer while the observed behaviour of the TP(nCB)6
series implies such pronounced shape changes do not occur for
these oligomers.
As the temperature is increased in the discotic nematic phase, a
remarkable shift to smaller angles is observed in the interdisc
spacing. This indicates that the interdisc spacing increases,
which we believe is caused by the thermal motion and increasing
disorder of the calamitic 4-cyanobiphenyl tails. This shift is not
normally observed in columnar discotic phases. To keep the
overall density constant, the intercolumnar distance has to
compensate for this change by shifting to higher angles, i.e.
smaller distances (Fig. 5 and 6). Thus, on increasing temperature
the discs are becoming thicker due to the increased fluctuations of
the 4-cyanobiphenyl moieties and therefore, less wide. We should
note that the apparent peak at about 27u in the X-ray
diffraction profiles (Fig. 5) is in fact an artefact. If one draws
a baseline at 10–12u 2-theta, this coincides with the level at about
24–25u. The ‘‘shoulder’’ at 27u is due to the integration procedure
because at wide angles some parts of the signal are outside the
detector grid (see Fig. 3).
The nematic phase exhibited by TP(10CB)6 was studied using
X-ray diffraction{ and a typical diffraction pattern obtained for a
magnetically aligned nematic sample is shown in Fig. 3. The
pattern is characteristic of a discotic nematic phase. The sharp
small angle peak indicates the presence of local columnar-like
orientational fluctuations within the nematic phase and corre-
˚
sponds to a spacing or intercolumnar distance of about 32 A. The
˚
calculated disc diameter is ca. 56 A assuming the alkyl chains are in
The combination of rod–disc-like moieties in the TP(nCB)6
series has not sufficiently perturbed the average molecular shape to
yield calamitic mesophases. The average molecular shape is disc-
like for these homologues. It is noteworthy, however, that these
compounds show the widest temperature range discotic nematic
phases yet to be reported in the literature and columnar
mesophases are not observed. This indicates that although the
rod-like units in the tail have not significantly changed the overall
shape, their thermal and orientational fluctuations inhibit the
molecular packing required to observe columnar mesophases.
extended all trans conformations. This implies that the side chains
are essentially fully interdigitated. The simple diffraction pattern
also suggests that all the aromatic rings, including the core
triphenylene disc and the 4-cyanobiphenyl units in the tails, lie in
Fig. 3 X-ray diffraction pattern of an aligned nematic phase of
TP(10CB)6 (90 uC); the magnetic field direction is along the equator.
Fig. 5 X-ray diffraction profiles for TP(10CB)6.
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Notes and references
{ First, 1,2-di-(v-bromoalkanoxy)benzene was prepared from the reaction
of catechol and a large excess of the appropriate a,v-dibromoalkane (5–
10 fold) with potassium carbonate in refluxing acetone overnight. The
product was trimerised using ferric chloride in dicholomethane to give
hexakis(v-bromoalkanoxy)triphenylene in around 20% yield, which was
reacted with 4-hydroxy-49-biphenylcarbonitrile in the presence of potassium
carbonate in refluxing acetone to give the expected product.
TP(10CB)6: 1H-NMR (in CDCl3): 7.82 (s, 6H, ArH from triphenylene),
7.62 (dd, 24H, ArH from biphenyl, J 5 8.8 Hz), 7.48 (d, 12H, ArH from
biphenyl, J 5 8.8 Hz), 6.94 (d, 12H, ArH from biphenyl, J 5 8.8 Hz), 4.21
(t, 12H, ArOCH2, J 5 6.4 Hz), 3.96 (t, 12H, ArOCH2, J 5 6.4 Hz), 1.93
(m, 12H, CH2), 1.78 (m, 12H, CH2), 1.70–1.25 (m, 72H, CH2).
{ The samples were analysed with an X-ray diffractometer D8-Discover
from Bruker-nonius with a GADDS 2-D detector. A high strength
magnetic field of about 4 T is applied using NdFeB permanent magnets
with focussing polecaps. The temperature of the sample in a standard glass
0.7 mm X-ray capillary is controlled using a small graphite tube heating
element which is mounted between the polecaps of the magnet (10 mK
stability, 20–350 uC range, .100 K min21 heating and cooling rate).
Fig. 6 Intercolumnar and interdisc spacings for TP(10CB)6 (120 uC).
These also affect the interdisc spacing in the nematic phase which
increases on increasing temperature as well as prevent the
crystallisation of the nematic phases and instead stable glassy
nematic phases are seen. To our knowledge only two compounds
with similar molecular structures have been reported in the
literature.23,24 Shimizu et al.23 reported a compound containing a
triphenylene core attached to which were six azobenzene-based
mesogens. In this system the length of the terminal chains on each
rod-like mesogen considerably exceeded that of the spacer
connecting the rod and disc. Consequently, the interdigitation we
observed here between the discs was not possible and only a
columnar phase was seen. Rahman et al.24 described the properties
of a triphenylene-based compound containing six nitroazobenzene
units attached via hexamethylene spacers. This compound
exhibited a nematic phase and the authors considered that the
triphenylene moiety acted only as a linking unit connecting the
rod-like groups rather than as a disc-like unit which would
promote columnar phase formation. By comparison, we have
shown that the molecules are indeed disc-like in shape and a
discotic nematic phase is formed. Thus, the triphenylene core
effectively controls the overall molecular shape and hence the
transitional behaviour. It is interesting to note that for other
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cyanobiphenyl units attached to a central unit, for example,
substituted siloxysilanes25 or functionalised fullerenes,26 conven-
tional smectic A phases are seen. This behaviour has been
interpreted in terms of the formation of rod-like structures by these
supermolecules,27 indicating that the central units in these
structures do not control the average molecular shape in the
same way as the core triphenylene group in the compounds
reported here.
27 C. Tschierske, J. Mater. Chem., 2001, 11, 2647.
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