Toward the biaxial nematic phase of low molar mass thermotropic mesogens: Substantial molecular biaxiality in covalently linked rod-disk dimers [9]

Full text of DOI:10.1021/ja015943q

J. Am. Chem. Soc. 2001, 123, 10115-10116
Toward the Biaxial Nematic Phase of Low Molar
10115
Mass Thermotropic Mesogens: Substantial
Molecular Biaxiality in Covalently Linked Rod-Disk
Dimers
Jonathan J. Hunt, Richard W. Date, Bakir A. Timimi,†
Geoffrey R. Luckhurst,*,† and Duncan W. Bruce*
School of Chemistry, UniVersity of Exeter
Stocker Road, Exeter EX4 4QD, UK
Department of Chemistry and Southampton Liquid
Crystal Institute, UniVersity of Southampton
Southampton SO17 1BJ, UK
Figure 1. Representation of a biaxial nematic phase
ReceiVed April 4, 2001
Since the prediction of the biaxial nematic phase by Freiser,1
there has been considerable interest in the demonstration of this
phase as a physical reality, and extensive debate in the literature
concerning the validity, or otherwise, of putative examples.2 In
the uniaxial nematic phase (Nu) of liquid crystals, there is no
positional order, and the unique axes of the molecules are oriented
about a director, n. In the biaxial nematic phase (Nb) there is,
additionally, a correlation of the molecules in a direction
perpendicular to n; therefore, whereas in the Nu phase the physical
properties in the plane perpendicular to n are angle-independent,
in the Nb phase this is not the case. A simple schematic
representation of the Nb phase composed of lozenge-shaped
molecules is shown as Figure 1.
Figure 2. Rodlike moiety attached end-on (A) and side-on (B) to disk.
In the extended Onsager approach adopted by Vanakaras et
al., the attractive interactions used in the simulation had a strength
of the order of hydrogen bonds, but an alternative strategy, and
one which is guaranteed to avoid phase separation, is to link the
rod and disk covalently. One such example is known11 based on
a pentayne disk and a cyanobiphenyl moiety which were linked
according to Figure 2A, although it showed only a strongly
monotropic mesophase. However, with this idea in mind and
taking account of the seemingly successful approach adopted by
Hessel and Finkelmann, we undertook the design and synthesis
of a rod-disk mesogen in which the rod would be attached
laterally to the disk according to Figure 2B. Thus, the molecule,
shown in Figure 3 (X ) H), contains a pentayne disk as described
by Janietz et al.12 joined to the lateral rod described by Hessel
and Finkelmann.5
Examination by polarized optical microscopy shows that the
material possesses a monotropic nematic phase which is very
viscous and does not readily crystallize. There is evidence of a
schlieren texture, and two-brush defects are clearly seen, although
the presence of four-brush defects is less obvious. Chandrasekhar
has previously suggested that the observation of only two-brush
defects can confirm the presence of a biaxial nematic phase,
although this is in doubt, especially with the recent observation
of this texture in the biaxial smectic A phase.13 Evaluation of the
thermal characteristics of these transitions by differential scanning
calorimetry showed typical values of the molar enthalpy and
entropy changes on melting, but the same parameters measured
on passing from the nematic to the isotropic phase were very
small (Figure 3). Molecular field theory predicts that the biaxial-
nematic-to-isotropic phase transition is second order,14 and
therefore the small thermal parameters observed are rather
encouraging.
In 1980, Yu and Saupe3 published what is accepted as a genuine
example of the biaxial nematic phase formed by a ternary
lyotropic system, although some doubts about even this system
have been expressed.4 Then, Hessel and Finkelman5 published a
paper describing a side-chain liquid crystal polymer in which the
mesogenic group was attached laterally. This material was also
claimed to have a Nb phase, and this claim has not, to our
knowledge, been disputed. However, in low-molar-mass thermo-
tropic liquid crystals, unequivocal proof of a Nb phase remains
to be given, despite several claims to the contrary. One of the
principal issues centers around how a biaxial nematic phase may
be identified; this is reviewed by Galerne.6
In considering a molecular design for a putative biaxial material
which can display three, orthogonal directors, several options are
apparent. One is to employ a shape biaxiality,7 another is to design
in molecular biaxiality (i.e. the extent to which the molecule
and/or its interactions deviate from those associated with cylindri-
cal symmetry), while another is somehow to mix rods and disks.
Using this last approach, theoretical work by Sharma et al.8 and
by Vanakaras et al.9 has shown that rod-disk mixtures can lead
to Nb phases if the rod and the disk are attracted more to one
another than to each other. This attractive interaction is required
otherwise the mixtures simply separate into two uniaxial
nematics.10
To determine the symmetry of the nematic phase, we undertook
some deuterium NMR experiments; thus, we synthesized a
material in which the protons of the phenyl groups which are
ortho to the alkyne groups were selectively deuterated (Figure 3,
X ) D). We first tried to obtain a 2H NMR spectrum of the neat
† University of Southampton.
(1) Freiser, M. J. Phys. ReV. Lett. 1970, 24, 1041.
(2) Hughes, J. R.; Kothe, G.; Luckhurst, G. R.; Maltheˆte, J.; Neubert, M.
E. Shenouda, I.; Timimi, B. A.; Tittlebach, M. J. Chem. Phys. 1997, 107,
9252.
(3) Yu, L. J.; Saupe, A. Phys. ReV. Lett. 1980, 45, 1000.
(4) Berejnov, V.; Cabuil, V.; Perzynski, R.; Raikher, Yu. J. Phys. Chem.
1998, 102, 7132.
(10) Palffy-Muhoray, P.; de Bruyn, J. R.; Dunmur, D. A. J. Chem. Phys.
1985, 82, 5294; Pratibha R.; Madhusudana, N. V. Mol. Cryst. Liq. Cryst. Lett.
1985, 1, 111; Hashim, R.; Luckhurst; G. R.; Romano, S. Mol. Phys. 1984,
53, 1535; Hashim, R.; Luckhurst; G. R.; Prata; F.; Romano, S. Liq. Cryst.
1993, 15, 283.
(5) Hessel, P.; Finkelmann, H. Polym. Bull. 1986, 15, 349.
(6) Galerne, Y. Mol. Cryst., Liq. Cryst. 1998, 323, 211.
(7) Praefcke, K.; Kohne, B.; Gu¨ndogan, B.; Singer, D.; Demus, D.; Diele,
S.;Pelzl, G.; Bakowsky, U. Mol. Cryst., Liq. Cryst. 1991, 198, 393.
(8) Sharma, S. R.; Palffy-Muhoray, P.; Bergersen, B.; Dunmur, D. A. Phys.
ReV. A 1985, 32, 3752.
(11) Fletcher, I. D.; Luckhurst, G. R. Liq. Cryst. 1995, 18, 175.
(12) Janietz, D.; Praefcke, K.; Singer, D. Liq. Cryst. 1993, 13, 247.
(13) Hegmann, T.; Kain, J.; Diele, S.; Pelze, G.; Tschierske, C. Angew.
Chem., Int. Ed. 2001, 40, 887.
(9) Vanakaras, A. G.; McGrother, S. J.; Jackson, G.; Photinos, D. J. Mol.
Cryst., Liq. Cryst. 1998, 323, 199.
(14) Boccara, N.; Mejdaniand, R.; De Seze, L. J. Phys. 1997, 38, 149.
10.1021/ja015943q CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/20/2001

10116 J. Am. Chem. Soc., Vol. 123, No. 41, 2001
Communications to the Editor
Figure 4. 2H NMR spectrum of the dimer dissolved in E7.
essentially four times that of the larger quadrupolar splitting. These
are interpreted as follows.
First, we start by recognizing that E7 possesses a positive
diamagnetic anisotropy so that the rodlike molecules which
constitute the mixture will align parallel to the magnetic field.
Second, we make the assumption that the rod part of the dimer
will align itself with the director of E7. Thus, we have to address
the orientation of the pentayne ring. We begin our analysis by
assuming that the average plane of the disk is parallel to the
orientation of the rodlike part and, hence, to the rods of E7. We
then attribute the smaller splitting of 16 734 Hz to the deuterium
nuclei located on disk phenyl groups situated in positions 2-, 3-,
5-, and 6- relative to the central phenyl ring because of its stronger
intensity. Similarly, the larger splitting must be associated with
the deuterons in the single phenyl ring at the 4-position. The fact
that the splittings for the two classes of deuterons have these
relative magnitudes is consistent with the pentayne disk being
arranged with the 1,4-axis tending to be aligned perpendicular to
the director. In other words, the average conformation of the dimer
tends to be like that sketched in Figure 2B which will align in a
calamitic nematic with the rod parallel to the director, the
symmetry axis of the disk orthogonal to it, and the 1,4-axis also
orthogonal to the director.
To determine the principal components of the Saupe ordering
matrix, S, for the disk from the two quadrupolar splittings requires
that we know their signs, but these are not available from the
NMR spectra. We have assumed, therefore, that the effective
shape of the rod-disk dimer is indeed that shown in Figure 2B
which will align in the calamitic nematic such that the larger
splitting is positive, while the smaller splitting is negative. Now
to evaluate the principal components of S we need the effective
geometry of the pentayne disk and take the rings to be twisted
out of the plane of the central phenyl ring by 30°.15 With these
assumptions we find that at 295 K, the average ordering matrix
for the disk unit has a major component, Szz ) 0.553 and a biaxial
part, Sxx - Syy ) 0.379. This biaxiality in the ordering matrix is
relatively large16 and indicates that the combination of rod and
disk units in a single molecular entity has created a highly biaxial
molecule. This approach could well provide an example of a low-
molar-mass, thermotropic biaxial nematic.
Figure 3. Synthesis and structure of the rod-disk dimer and its thermal
behavior. (i) HOC6Br5/NaH/DMF; (ii) C5H11C6H2X2CtCH/CuI/[PdCl2-
(PPh3)2]/PPh3. Details in the Supporting Information.
material in its nematic phase, but its viscosity is so high that only
a single, broad line was observed and not the quadrupolar doublets
found for conventional nematics. This would be consistent with
the low degree of orientational order expected from the small
transitional entropy. Next, we attempted to characterize the degree
of molecular biaxiality, as opposed to phase biaxiality, by
dissolving the dimer (5 wt %) in the eutectic mixture of
cyanobiphenyls known as E7 (from Merck). At this concentration,
the dimer will not perturb the symmetry of the Nu phase of E7.
From this nematic solution, we obtained a spectrum (Figure 4)
showing two quadrupolar splittings of 56 337 Hz and 16 734 Hz
at 295 K. The intensity of the smaller quadrupolar splitting is
Acknowledgment. We thank Dr. Mark Goulding Merck NB-SC for
discissions and Merck NB-SC, NEDO and EPSRC for support.
Supporting Information Available: Experimental details (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
JA015943Q
(15) Tearle, W. M. Ph.D. Thesis, University of Southampton, 1995.
(16) Emsley, J. W.; Hamilton, K.; Luckhurst, G. R.; Sundholm, F.; Timimi,
B. A.; Turner, D. L. Chem Phys Lett. 1984, 104, 136.