Shape-persistent V-shaped mesogens - Formation of nematic phases with biaxial order

Article abstract of DOI:10.1039/b605718g

A homologous series of shape-persistent V-shaped molecules has been designed to form the biaxial nematic phase. Phenyleneethynylene moieties are attached to a bent fluorenone unit to create an apex angle of about 90°, which is determined from the single c

Full text of DOI:10.1039/b605718g

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Shape-persistent V-shaped mesogens—formation of nematic phases with
biaxial order{
Matthias Lehmann,*a Shin-Woong Kang,b Christiane Ko¨hn,a So¨nke Haseloh,c Ute Kolb,c
Dieter Schollmeyer,d QingBing Wangb and Satyendra Kumare
Received 21st April 2006, Accepted 7th August 2006
First published as an Advance Article on the web 19th September 2006
DOI: 10.1039/b605718g
A homologous series of shape-persistent V-shaped molecules has been designed to form the biaxial
nematic phase. Phenyleneethynylene moieties are attached to a bent fluorenone unit to create an
apex angle of about 90u, which is determined from the single crystal structure. Two mesogens, one
symmetric and another unsymmetric, have been synthesized by attaching a cyano group to one or
both of the peripheral phenyl units, respectively. These groups introduce local dipoles essential for
the formation of the nematic phases. The tendency to form a crystalline phase is reduced by
laterally substituted hexyloxy chains which allow the nematic phase to be supercooled to a glassy
state. Two of the three fluorenone derivatives exhibit a transition from the uniaxial nematic to the
biaxial nematic phase. This transition has an undetectably small transition enthalpy, but the
X-ray diffraction, polarizing optical microscopy, and conoscopy reveal the presence of the biaxial
order in the low temperature nematic phase.
that nematogens with a tetrahedral angle (i.e., y109u) between
the two arms of the V-shaped molecule were the most
promising candidates. However, the thermotropic V-shaped
mesogens forming the thermotropic Nb phase,1,7 have apex
angles of about 140u and 120u. The effect of a local dipole
moment that these mesogens possess was not considered in the
theoretical models and, as has been suggested,8 may play an
important role in the organisation of the mesogens and
establishment of the biaxial order. In addition to the Nb
phase, biaxial smectic9 phases also have been reported in
banana shaped non-symmetric mesogens with dipoles.
Therefore, we envisage the new shape-persistent V-shaped
mesogen 1 to be a good system10 to study the dependence of
the nature of resulting nematic phases on the molecular
architecture. These mesogens are based on rigid oligophenyl-
eneethynylene wings attached to different core units at
various angles. The presence of alkoxy chains in the
2,5-position of the central benzene ring appears to favour the
formation of nematic LC phases as evident from the study of
stilbenoid oligomers11 and polymers.12 In these materials, local
dipoles may be introduced at the central and peripheral
aromatic units by electron-withdrawing substituents. Here,
we present the synthesis and characterization of the phases
formed by V-shaped mesogens with hexyloxy chains and a
fluorenone core (1a–c). A model for their supramolecular self-
assembly is proposed.
Introduction
Recently, V-shaped mesogens based on an oxazoline centre
came into the limelight, because they exhibit1 the ever so
elusive biaxial nematic (Nb) liquid crystal phase. Since the
prediction of its existence by Freiser2 and the first verification
of the biaxial nematic phase in a lyotropic system by Yu and
Saupe,3 many attempts have been made to synthesize biaxial
nematogens. Most of the synthetic effort has focussed on
board-shaped (or, sanidic) mesogens. Practically none of the
claims, before 2002, of the discovery of the Nb phase in small
molecule mesogens could be substantiated.4 In addition to the
synthesis and study of board-shaped mesogens in the hope
of realizing the Nb phase, theoretical and experimental
investigations of mixtures of disk- and rod-shaped mesogens,
such as mesogenic oligomers and polymers, have also been
undertaken.5
Recently discovered rigid V-shaped mesogens were con-
sidered as good candidates for the formation of the biaxial
nematic phase due to their inherent molecular biaxiality. A
number of theoretical and simulation studies investigated
suitable geometric shapes and calculated the phase diagrams
for such materials. Luckhurst4a and Masters et al.6 predicted
aChemnitz University of Technology, Strasse der Nationen 62, 09111,
Chemnitz, Germany. E-mail: Matthias.Lehmann@chemie.tu-
chemnitz.de; Fax: (+) 49/371 531 1839; Tel: +49 371 531 1205
bDepartment of Physics, Kent State University, Kent, OH 44242, USA
cInstitute of Physical Chemistry, Johannes Gutenberg-University Mainz,
Welderweg 11, 55099 Mainz, Germany
Results and discussion
dInstitute of Organic Chemistry, Johannes Gutenberg-University Mainz,
Duesbergweg 10-14, 55099 Mainz, Germany
Synthesis
eNational Science Foundation, 4201 Wilson Blvd., Arlington, VA 22230,
USA
An optimised strategy, shown in Scheme 1, was applied for the
synthesis of unsymmetric phenyleneethynylene oligomers. The
key precursor in the synthesis scheme is the 4-bromo-2,5-
dihydroxyiodobenzene 3.13 After etherification with hexyl-
bromide, unsymmetric compound 5 can be synthesised using a
{ Electronic supplementary information (ESI) available: Comparision
of 13C NMR data, X-ray of oriented samples, simulated powder
pattern, view along the c-axis of the single crystal of 1a with
characteristic distances. See DOI: 10.1039/b605718g
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Fig. 1 X-Ray structure of 1a, crystallised from acetone. The molecule
lies with C1–O1 on a twofold axis. The molecular axes are defined as B
(along the bisector), T (in the direction of normal of the fluorenone
plane; thickness of the mesogen; which in the nematic phase is parallel
to the director n) and L (along the length of the molecule perpendicular
to the bisector). B: The view along the c-axis. Molecules are aligned in
an anti-parallel manner. The alkyl chains are omitted for clarity.
may be the driving force for the anti-parallel organisation of
mesogens in the crystal phase. In compound 1b, the net dipole
moment (y2.35 D) parallel to the bisector is in the opposite
direction due to the large contribution of the two cyano
groups. However, the lateral components of the dipoles cancel
because of the molecular C2v symmetry. The situation is
different in the case of the non-C2-symmetric compound 1c.
Here, the single cyano group on one arm not only reduces the
molecular dipole moment (to approx. 0.52 D) along the
bisector, but also establishes a lateral component of y2.86 D,
which may favour the alignment of the second molecular
axis (L).9,18
Scheme 1 Synthesis of V-shaped, shape-persistent mesogens 1.
Reagents and conditions: (i) BBr3, CH2Cl2, 77%; (ii) KOH, DMF,
C6H13Br, 82%; (iii) 1) piperidine, Pd(PPh3)4, CuI, HCMCSi(iPr)3; 2)
HCMCSi(CH3)3; 3) K2CO3, MeOH, THF, 92%; (iv) 1) piperidine,
Pd(PPh3)4, CuI, PhI; 2) (H9C4)4NF, THF, 81%; (v) 1) piperidine,
Pd(PPh3)4, CuI, 4-bromobenzonitrile; 2) (H9C4)4NF, THF, 77%; (vi)
piperidine, Pd(PPh3)4, CuI, 33–52%.
one-pot reaction procedure,10,14 followed by selective cleavage
of the trimethylsilyl group. The peripheral aromatic units
were cross-linked with 5 and the remaining protecting group
was detached to yield the individual arms 6a,b.10 These arms
have been coupled to the fluorenone dibromide 715 to yield
symmetrical V-shaped molecules 1a and 1b.10 The non-C2-
symmetrical compound 1c was prepared in a stoichiometric
reaction with 6a and 6b (ESI{). The target molecules were
fully characterised by NMR, FT-IR, mass spectroscopy and
elemental analysis.
Thermotropic mesomorphic properties
The mesomorphic properties of these materials have been
investigated by means of polarizing optical microscopy
(POM), differential scanning calorimetry (DSC), and X-ray
diffraction (XRD). Table 1 summarises the results. Derivative
1a, without cyano substituents, does not possess mesomorphic
properties. It only melts into the isotropic phase upon heating
and then freezes into a glassy state when cooled. However, the
other two mesogens 1b and 1c bearing cyano groups on the
peripheral phenylene units, form nematic phases. Compound
1b shows a stable, enantiotropic nematic phase above 161.3 uC,
Molecular structure
A single crystal analysis of 1a revealed the geometry of the
rigid V-shaped mesogens.16 Although the structure of the 3,6-
disubstituted fluorenone core, which was energetically mini-
mised by the semiempirical AM1 method, should have a bend
angle h of about 82u, it is found to be approximately 90u
(Fig. 1) in the crystal phase. The rigid scaffold has a length of
Table 1 Mesomorphic properties of 1a–c
˚
˚
L = 24.3 A and a width of B = 15.4 A along the bisector. The
fluorenone cores orient in anti-parallel manner and column-
like stacks. Mesogens 1a–c have different local terminal dipole
moments. The keto group contributes to the dipole moment
along the bisector of the molecule, whereas the cyano groups
provide a dipole moment with components in opposite and
orthogonal directions to the keto group. Using simple addition
of local dipoles,17 compound 1a is estimated to have a dipole
moment of approximately 3.38 D along the bisector which
DSC results from second heating at a rate
of 10 uC min21 (onset/uC; DH/kJ mol21 a
Compound
)
1a
1b
1c
a
g 29.7 (Tg) I (melting at 104.3 uC)b
g 34.4 (Tg) N 178.0/0.7 I (melting at 161.3 uC)b
g 26.4 (Tg) N 121.8/0.4 I (melting at 132.3 uC)b
b
Cr crystal, g glass, N nematic phase, I isotropic phase. Phases
reversibly observed at a heating rate of 10 uC min21. Annealing
above the glass transition in the nematic or isotropic phases and
below the given melting temperatures results in slow crystallisation.
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but crystallizes slowly upon annealing at temperatures below
161.3 uC and above the glass transition. Mesogen 1c exhibits
only monotropic nematic phases. The transition enthalpies of
0.7 kJ mol21 (DS/R = 0.19) for 1b and 0.4 kJ mol21 (DS/R =
0.11) for 1c, provide evidence of the first order nature of the
isotropic–nematic transition. The nematic phase of both
compounds freezes to a nematic glass near room temperature
(RT), which prevents crystallisation. This effect is attributed to
the lateral attachment of short alkoxy chains. The side chains
at 2,5-positions not only prevent facile crystallisation but also
nanosegregation and thus make the formation of mesophases
of higher supramolecular order (columnar or lamellar phases),
which are often observed in conventional nematogens,4
improbable. Therefore, this property should help the forma-
tion of the biaxial nematic phase at lower temperatures.
Optical textures and interesting electro-optical properties
are observed in cells with a 10–15 mm gap prepared with
untreated and ITO-coated glass substrates treated with two
different polyimides. One of them is known to generate
homogeneous, while the other produces homeotropic bound-
ary conditions for calamitic mesogens. Surprisingly, surfaces
with homeotropic anchoring conditions produce planar
director orientation for these bent-core materials, while
homogeneous surface conditions produce homeotropic align-
ment. In cells with homeotropic anchoring, sample 1c shows a
typical planar orientation with s = K (i.e., 2-brush) disclina-
tions, Fig. 2a,b, throughout the nematic temperature range. As
the temperature is lowered below the clearing point in cells
with homogeneous boundary conditions (and homeotropic
alignment), the transition to the Nu phase appears via the
formation of a typical but transient nematic Schlieren texture
(Fig. 2c). The nematic director n (parallel to the molecular axis
T; cf. next paragraph and Fig. 1) then immediately adopts a
homeotropic orientation (Fig. 2d) and the sample appears dark
between crossed polarizers. In this phase, rotation of the
sample between crossed polarizers does not give rise to any
variation in the intensity of transmitted light. Evidently, no
optical anisotropy was present in the plane perpendicular to
the director n, pointing clearly to the uniaxial nature of this
phase. The optical texture becomes gradually brighter below
80 uC as the birefringence associated with biaxiality develops
in the plane perpendicular to the director n. The cell is
brightest when the rubbing direction, R, is at 45u with respect
to the polarizer/analyzer axes (Fig. 2e). This marks the onset of
the Nb phase and indicates that the secondary director m aligns
along R, which we assume to be parallel to the axis B. The
transmitted intensity is minimum when the rubbing direction is
parallel to either the polarizer or the analyzer axis (Fig. 2f).
The brightness of the texture continues to increase with
increasing biaxial order at lower temperatures. The results of
the optical retardation measurements made on a thin homo-
geneously aligned cell as a function of temperature are shown
in Fig. 3. The measured retardation is proportional to the
optical birefringence in the place of the cell. The data show a
change in temperature dependence of the birefringence at the
onset of biaxiality (ESI{19 at 80 uC (Fig. 3) as is expected.
Conoscopy is a method that has traditionally been used to
investigate the orientation of optic axes in a crystalline sample
and is often used to infer the phase biaxiality. It requires well
aligned single domain samples.4,19,20 A systematic conoscopic
study of
a homeotropically aligned sample of 1c was
performed. The symmetric conoscopic cross shown in Fig. 4a
in the nematic phase at high temperatures affirms its uniaxial
nature. It practically remains unchanged from 120 uC down to
approximately 75 uC in the Nu phase, below which it slowly
evolves into the shape of a dagger and moves off-centre,
Fig. 2 POM textures of 1c in a cell with homeotropic boundary
conditions: (a) at the I to Nu transition and (b) at 50 uC. (c–f) Optical
texture of sample 1c in a cell with homogeneous boundary conditions.
The rubbing direction R is at 45u to the polarizer and analyzer
directions. (c) Schlieren texture (118.2 uC) near the I to Nu transition
during cooling. (d) The Nu phase adopts homeotropic orientation for
the director n. (e) Birefringent texture of the Nb phase at 25 uC with n
perpendicular to the cell and m most likely oriented along the rubbing
direction, and (f) with m parallel to polarizer axis.
Fig. 3 Phase retardation (3 birefringence) of 1c upon cooling from
the isotropic phase. Arrows show temperatures with changes in the
slope of the curve. The inset provides a magnified view of the data in
the proximity of 80 uC. The slope changes at the onset of the biaxial
nematic phase.
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Fig. 5 (a) Homeotropic alignment of the director n in the Nu phase of
sample 1c at 115 uC with R at 45u to the polarizer/analyzer axes and no
applied field; (b) a transient Schlieren texture appears with planar
orientation of n when a 2 V mm21 square wave of 1 kHz is applied; (c)
stable homogeneous state reached after approx. 2 s with n in the plane
of the substrate. (d) Rotation yields the darkest state when R is parallel
to the polarizer axis proving that n orients perpendicular to the field in
these dielectrically negative materials.
Fig. 4 (a) Conoscopic picture of homeotropically aligned Nu phase of
1c at 114 uC; (b) asymmetric conoscopic cross in the biaxial nematic
phase at 28 uC; (c) sample rotated by 45u with respect to the polariser
axis; and (d) conoscopy with circular polarisers inserted above and
below the sample, the two dark spots provide a view of the two optic
axes of the Nb phase.
parallel to either the polarizer or the analyzer axis. At lower
temperatures in the Nb phase, the viscosity becomes too
large for the applied field to have any effect on the director
orientation.
suggesting the onset of biaxiality. While the shift in the
position of the cross may, in part, be attributable to a slight tilt
of the director n relative to the microscope’s optic axis, the
change in its shape is entirely due to phase biaxiality. Fig. 4b
shows an asymmetric conoscopic cross at 28 uC in the Nb
phase. The change in the shape of the cross cannot be caused
by mere tilting of nematic director n of the Nu phase. If only
the director n tilts, then the cross will move off-center but will
maintain its symmetric shape of a ‘‘plus’’ sign, which it clearly
does not. As the cell is rotated by 45u in between the polarizers,
the off-center cross changes to two arcs (isogyres), Fig. 4c. The
separation between the two isogyres increases at lower
temperatures and is proportional to the degree of biaxiality
of the lower temperature Nb phase. A standard technique to
visualize the direction of the two optic axes of a biaxial crystal
is to place two quarter-wave plates, one before and the second
after the sample. Under these conditions, two dark spots are
seen, Fig. 4d, which mark the orientations of the two optic
axes of the Nb phase and confirm its biaxiality.
XRD measurements are carried out on both powder and
aligned samples21 filled in glass capillaries of 2 mm diameter
(ESI{). In the isotropic phase of compounds 1, the diffraction
pattern consists of two diffuse rings. The inner and outer rings
are each composed of two reflections, which are not fully
resolved. However, the length scales corresponding to these
˚
reflections are estimated to be 22.1, 14.4, 5.1 and 4.2 A. The
presence of four X-ray reflections suggests the biaxial nature of
the mesogenic molecules (cf. calamitic mesogens possess only
two distinct length scales, i.e., the length and lateral dimension
of the molecules). The diffraction patterns of samples 1b and
1c at 30 uC exhibit four diffuse arcs (Fig. 6a). Two of them are
˚
˚
at wide angles; the reflections at 4.6 A (1b) and 4.5 A (1c) are
attributed to the mean distance of liquid-like aliphatic chains
˚
˚
while the other two, at 3.7 A (1b) and 3.8 A (1c), correspond to
the typical mean separation of p-stacks. At small angles, there
˚
are also two broad diffuse reflections at 13.9 A (14.5 A)
˚
22
˚
˚
and 20.3 A (21.6 A) for the two compounds. The diffuse
character of all reflections is due to liquid-like positional
correlations in the nematic phase which becomes glassy at low
temperatures.23 The temperature dependence of the wide-angle
reflections demonstrates decreasing p–p distance at lower
temperatures (Fig. 6b). This leads us to conclude that the
phenomenon driving the transition from the uniaxial to the
biaxial nematic phase is the gradually increasing p-interaction
which gives rise to increasing hindrance of molecular rotations
about the n director (see Fig. 1). The fact that the Nu–Nb
transition is not observed in conventional DSC scans is con-
sistent with the theoretical prediction of it being second order.
When the sample is cooled into the nematic region in the
presence of a magnetic field, the intensity of the small and
large angle rings gets concentrated in two pairs of arc
The electro-optical response of the homeotropically aligned
Nu phase of sample 1c (Fig. 5a) was investigated at 115 uC.
When a square-wave field of 20 V and 1 kHz is applied, a
Schlieren texture, shown in Fig. 5b, with both, s = 1 and K
disclinations appears immediately as the director n begins to
reorient perpendicular to the field and takes on a planar
alignment confirming the negative dielectric nature of these
mesogens. However, the texture continues to slowly change
and eventually adopt a homogeneous orientation in about two
seconds, with n parallel to the rubbing direction, Fig. 5c.
Homogeneous director alignment in the plane of the cell is
confirmed by monitoring the transmitted intensity as a
function of the angle between R and the polarizer axis. One
obtains minimum light transmission through the cell when R is
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arcs is not evident at high temperatures but becomes clear
upon entering the Nb phase, as seen in Fig. 7 at 71 uC. The two
intensity plots in Fig. 7c are obtained by partially masking the
diffraction pattern, along orthogonal directions, to best show
the splitting. The two outer reflections can now be clearly
distinguished. One of these gives rise to X-ray diffraction
primarily along the direction perpendicular to the field and,
thus, is attributed to director n, while the second weak
reflection appears to have no angular concentration of
intensity in the plane of the detector. The diffraction angle
for this ring can be better estimated via an equatorial scan. The
˚
large angle reflections correspond to 5.13 and 4.20 A.
In the small angle region of the diffraction pattern, the two
diffuse signals are attributed to intermolecular distances along
the bisector of the molecule. Comparison of measured lengths
with the molecular dimensions of 1a shows, that the length
scales along the directors m and l are larger than the distances
calculated from the diffuse peaks in the nematic glass (compare
with Fig. 1). This suggests that the molecules are interdigi-
tated. A close inspection of the integrated intensities for 1b and
˚
1c shows a more intense second halo at approximately 13.9 A
for the symmetric compound 1b compared to the non-
˚
˚
˚
symmetric 1c (14.5 A). The size of the halo at 20.3 A (21.6 A)
does not change significantly (Fig. 6a), thus the scattered
intensity likely arises from periodic electron density which is
not affected by the change in the symmetry of the molecule.
This could be related to higher electron density in 2,5-dialkoxy
substituted aromatic units, which should not differ signifi-
cantly for 1b and 1c. The resulting model, shown in Fig. 8,
illustrates essential features inferred from the X-ray patterns of
1b and 1c. The aromatic planes are oriented parallel to the
Fig. 6 (a) Powder X-ray diffraction pattern of 1b and 1c. (b)
Temperature dependent measurements in the wide angle range of 1b.
˚
surface and the magnetic field B (halo at 3.7 A corresponds to
reflections (Fig. 7). The inner arcs consist of two reflections in
the direction of the magnetic field corresponding to the length
˚
scales of 20.3 and 14.3 A. The separation between these two
the orientation of director n). In the biaxial phase, additional
orientation ordering of the bisector (director l) is obtained.
Case A shows mesogens (1c) where adjacent molecules
present different peripheral aromatic units along the bisector
Fig. 7 (a) Diffraction pattern from sample 1c in the Nb phase 71 uC;
(b) Small angle diffraction pattern on expanded scale shows the
presence of two reflections; (c) Intensity vs. 2h peaks generated clearly
reveal four peaks marked by the arrows.
Fig. 8 Model of mean separation of mesogens 1c in the nematic
phase. (A) Cyano-groups of next neighbours, having the carbonyl
groups aligned along the same direction of the bisect, pointing to the
opposite site and (B) to the same site.
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l (anti-parallel alignment of lateral dipoles). No scattering is
expected from these molecular parts (red dotted line), whereas
the 2,5-dialkoxy substituted aromatic units and the fluorenone
obtained from DSC (onset temperature, first heating) or by
POM observations.
X-Ray diffraction (XRD) measurements were carried out
on both powder and aligned samples filled in glass capillaries
of 2 mm diameter. The samples filled in capillary were aligned
by cooling from the isotropic phase to RT in a 0.25 T magnetic
field applied perpendicular to the X-ray beam. Other samples
were aligned by simple unidirectional shearing and by drawing
a fibre from the nematic melt (for results see also Supporting
Information{, Fig. S1). Diffraction experiments on sheared
and fiber samples were performed using a copper anode
(2.2 KW) source with pinhole collimation, an X-ray mirror
(Osmic typ CMF15-sCu6), and a Bruker detector (High-star)
with 1024 6 1024 pixels. X-Ray patterns were analyzed with
the software package Datasqueeze25 written by Prof. Paul
Heiney. Some of the measurements on magnetically aligned
samples were performed at the MUCAT’s sector of the APS.
There, XRD patterns were acquired using a high-resolution
(3450 6 3450 pixels) image plate detector MAR-345 (MAR
Research) placed at 1158.9 mm from the sample, and l =
˚
centre should still contribute to the peak at 14.5 A (green
dotted line). If all molecules were oriented parallel along the
molecular axis L and the bisector (case B), scattered intensity
˚
at 14.5 A along the orientation direction of the bisector will
increase. The latter should always be the case for a symmetric
molecule like 1b, and may be the reason for the relative large
˚
halo at approximately 13.9 A in Fig. 6a. The signal of constant
˚
strength at approximately 20.3 or 21.6 A along the same
direction can arise from the symmetrically substituted parts
(i.e., the intermediate 2,5-dialkoxy substituted aromatic units)
of molecules aligned in an anti-parallel manner, as shown in
Fig. 8B for mesogen 1c.24
Summary and conclusions
To summarize, two nematic mesophases are observed exclu-
sively for the mesogens with at least one lateral cyano
substituent in the homologous series of rigid nematogens
having a fluorenone core with an apex angle of 90u. Although
theory predicts uniaxial nematic phases for this value of the
apex angle, orthoscopic and conoscopic investigation of the
non-symmetric mesogen 1c clearly indicates a biaxiality at
about 40 uC below the clearing point. Results of X-ray
diffraction experiments suggest anti-parallel stacking of meso-
gens along the bisectors and their long axis, as previously
proposed for different mesogens in a biaxial SmA phase.9b The
second order transformation from the uniaxial to biaxial
nematic liquid crystal appears to be mediated by increasing
p-interaction with decreasing temperature.
˚
0.7653 A. The 2-D XRD patterns were analyzed using the
software package FIT2D26 written by A. P. Hammersley of the
European Synchrotron Radiation Facility.
Synthesis of 1-bromo-4-iodo-2,5-dimethoxybenzene 2.
Compound 2 was synthesised earlier by Hu¨nig et al.,27
however, only a yield as high as 29% has been obtained.
Therefore, we used a synthetic strategy analogous to the
synthesis reported by Ho¨ger et al.13
1-Bromo-2,5-dimethoxybenzene (9.4 g, 43 mmol), iodine
(8.3 g, 33 mmol) and KIO3 (3.8 g, 18 mmol) were added to a
mixture of 50 ml glacial acetic acid, 5 ml sulfuric acid and
12 ml CCl4 and heated for 5 h to 80 uC. The mixture was
poured onto ice and the precipitate was washed with water
and methanol. The crude brownish solid was recrystallized
from methanol to give 12.4 g (84%) of colourless crystals, mp
142.8 uC (POM; lit.27 138 uC). 1H NMR (300 MHz, CDCl3) d =
3.84, 3.85 (2s, 6H, OCH3), 7.01, 7.29 (2s, 2H) ppm; 13C NMR
(75 MHz, CDCl3) d = 57.4, 57.5 (OCH3), 84.4, 112.2 (Cq),
116.3, 123.2 (Ct), 151.2 (Cq), 153.4 (Cq) ppm.
Experimental
General methods
Solvents and reagents were purchased from Aldrich and used
as received. Column chromatography was performed on silica
gel (Merck silica gel 60, mesh size 0.2–0.5 mm). NMR spectra
were recorded in CDCl3 on a Bruker Avance 300 with the
solvent signal as an internal standard. Mass spectra were
recorded on a Finnigan MAT 95 instrument (FD-MS).
Elemental analysis was completed in the Microlab of the
University of Mainz. Differential scanning calorimetry (DSC)
was done on a Mettler Toledo DSC 821 and heating/cooling
scans were recorded at 10 uC min21 unless stated otherwise.
POM observations were made with a Zeiss Axioscop 40
equipped with a Linkam THMS600 hot stage. Orthoscopy and
conoscopy observations were made on cells with a 10–15 mm
gap. These were prepared with untreated glass or ITO
substrates treated with two different polyimides which are
proprietary. One of the polyimides is designed for homeotropic
orientation of calamitic liquid crystals. However, the com-
pounds reported here, exhibited opposite alignment, i.e., they
aligned in a planar manner. The second rubbed polyimide that
aligns calamitic mesogens homogeneously, aligned these
materials homogeneously. The electro-optical properties were
studied by applying a 1 kHz square wave electric field of
variable strength. The melting points reported here were
Synthesis of 1-bromo-4-iodohydroquinone 3. The target
compound 3 has been published earlier by Ho¨ger et al.13 It
has been prepared starting from propyloxy derivatives. We
developed a synthetic strategy starting from a dimethoxy
derivative, since after step two of the reaction path the
monobromination product (1-bromo-2,5-dimethoxybenzene)
can be easily purified from the dibrominated and non-
brominated side products by a simple distillation.
12.0 g (35 mmol) 1-bromo-4-iodo-2,5-dimethoxybenzene
dissolved in 140 ml CH2Cl2 were cooled to 278 uC. 9.9 ml
(105 mmol) BBr3 in 70 ml CH2Cl2 were added to the reaction
mixture. Ice was added to the suspension after stirring for 1 h
at RT. The precipitate was then filtered and recrystallised from
methanol to yield 8.5 g (77%) of a colourless product, mp
165.4 uC (POM) under decomposition (lit.13 186 uC). 1H NMR
(400 MHz, DMSO-d6) d = 7.21, 6.94 (2s, 2H, CH), 9.75, 9.91
(2s br., 2H, CH) ppm; 13C NMR (100 MHz, DMSO-d6) d =
83.3, 109.2 (Cq), 117.9, 125.0 (CH, Ar), 147.6, 150.1 (Cq) ppm.
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 4326–4334 | 4331

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Synthesis of 1-bromo-4-iodo-2,5-dihexyloxybenzene 4. 5.5 g
(17.4 mmol) 3 and 3.1 g (47.6 mmol) KOH were dissolved in
100 ml dry DMF and heated to 60 uC. After addition of 8.0 ml
(56.7 mmol) n-hexylbromide the mixture was stirred for 3 h
and poured onto ice. The aqueous layer was extracted with
hexane. Evaporation of the solvent yielded white crystals,
which were washed with water and ethanol to afford 6.9 g
(14.3 mmol, 82%) of a colourless solid, mp 48.5 uC (POM). 1H
1 : 2) afforded 0.92 g (77%, two steps) of a slightly yellow solid,
mp 97.5 uC (POM). 1H NMR (300 MHz, CDCl3) d = 0.89,
0.91 (2t, 6H, CH3), 1.34 (m, 4H, CH2), 1.51 (m, 4H, CH2), 1.82
(m, 4H, CH2), 3.37 (s, 1H, CCH), 3.99, 4.00 (2t, 4H, OCH2),
6.98, 7.00 (2s, 4H, CH), 7.58, 7.63 (AA9BB9, 6H, CH) ppm;
13C NMR (75 MHz, CDCl3) d = 14.0 (CH3), 22.5, 22.6, 25.6,
25.7, 29.1, 29.2, 31.5 (CH2), 69.5, 69.7 (OCH2), 82.8, 90.3, 93.0
(CMC), 111.4, 113.3, 113.7 (Cq, Ar), 116.9, 117.6 (CH, Ar),
118.5 (Cq, Ar) ppm; 131.9, 132.0 (CH, Ar), 153.7, 154.1 (Cq,
Ar) ppm; FT-IR (KBr) n (cm21) = 3254, 2925, 2855, 2224,
1601, 1221. MS (FD) m/z (%) = 427.3 (100, M+); EA
(C29H33NO2) calcd C 81.46, H 7.78, N 3.28, found C 81.31,
H 7.87, N 3.29%.
3
NMR (400 MHz, CDCl3) d = 0.90 (t, 3H, CH3, J = 6.9 Hz),
0.91 (t, 3H, CH3, 3J = 6.9 Hz), 1.34 (m, 8H, CH2), 1.49 (m, 4H,
CH2), 1.79 (m, 4H, CH2), 3.92 (t, 2H, OCH2, 3J = 6.6 Hz), 3.93
(t, 2H, OCH2, 3J = 6.6 Hz), 6.98 (s, 1H, CH), 7.27 (s, 1H, CH)
ppm; 13C NMR (100 MHz, CDCl3) d = 14.0 (CH3), 22.5
(CH2), 25.5 (CH2), 29.0 (CH2), 31.4 (CH2), 70.2 (OCH2), 84.7,
112.4 (Cq), 116.9, 117.0 (CH, Ar), 150.3, 152.4 (Cq, C–O) ppm;
FT-IR (KBr) n (cm21) = 2920, 2856, 1452, 1212; MS (FD) m/z
(%) = 482.1 (100, M+), 483.1 (20, M + 1), 484.1 (98, M + 2);
EA (C18H28BrIO2) calcd C 44.74, H 5.84, found C 44.70, H
5.82%.
Synthesis of 3,6-bis{2-[2,5-dihexyloxy-4-(2-phenylethynyl)
phenyl]ethynyl}-9H-fluoren-9-one 1a. 79 mg (0.23 mmol) 3,6-
dibromo-9H-fluoren-9-one, 30 mg Pd(PPh3)4 and 10 mg CuI
were dissolved in 5 ml piperidine. Oxygen was removed
by a freeze–pump–thaw procedure and afterwards 200 mg
(0.49 mmol) of 6a was added to the mixture. The reaction was
stirred at RT for 12 h. After evaporation of the solvent, the
compound was purified by column chromatography (silica,
hexane : CH2Cl2 = 1 : 1). Yield 100 mg (44%), mp 104.3 uC
Synthesis of 2-[2,5-dihexyloxy-4-(2-phenylethynyl)phenyl]-
ethyne 6a. 0.97 g (4.8 mmol) iodobenzene, 60 mg (0.5 mmol)
Pd(PPh3)4 and 25 mg (13.0 mmol) CuI were mixed with 20 ml
piperidine. Oxygen was removed by a freeze–pump–thaw
procedure and afterwards 1.20 g (2.5 mmol) of 510,14 was
added to the stirred reaction mixture. After 12 h the solvent
was evaporated, followed by a chromatographic purification
(silica, hexane : CH2Cl2 = 5 : 1) to yield 1.23 g of the silyl
protected derivative of 6a. The protecting group was cleaved
by adding 650 mg (0.0024 mol) (C4H9)4NF to the latter
material dissolved in 10 ml THF. After stirring 2 h at RT, the
reaction mixture was filtered over florosil. Chromatographic
purification (silica, CH2Cl2 : hexane 1 : 4) afforded 0.81 g
(81%, two steps) of a slightly yellow solid, mp 67.9 uC (POM).
1H NMR (300 MHz, CDCl3) d = 0.89, 0.91 (2t, 6H, CH3), 1.35
(m, 4H, CH2), 1.51 (m, 4H, CH2), 1.82 (m, 4H, CH2), 3.34 (s,
1H, CCH), 4.00, 4.01 (2t, 4H, OCH2), 6.98, 7.00 (2s, 4H, CH),
7.35 (m, 6H, CH), 7.54 (m, 4H, CH) ppm; 13C NMR (75 MHz,
CDCl3) d = 14.0 (CH3), 22.6, 25.6, 25.7, 29.1, 29.2, 31.5, 31.6
(CH2), 69.6 (OCH2), 80.0, 82.2, 85.6, 94.9 (CMC), 112.5, 114.6
(Cq, Ar), 116.8, 117.8 (CH, Ar), 123.3 (Cq, Ar), 128.3 (CH,
Ar), 131.6 (CH, Ar), 153.4, 154.1 (Cq, Ar) ppm; FT-IR (KBr)
n (cm21) = 3286, 2942, 2853, 1505, 1217; MS (FD) m/z (%) =
403.2 (100, [M + H]+); EA (C28H34O2) calcd C 83.54, H 8.51,
found C 83.55, H 8.58%.
1
(DSC data). H NMR (300 MHz, CDCl3) d = 0.91 (m, 12H,
CH3), 1.37 (m, 8H, CH2), 1.57 (m, 8H, CH2), 1.88 (m, 8H,
CH2), 4.06 (m, 8H, OCH2), 7.04, 7.05 (2s, 4H, CH), 7.36 (m,
3
6H, CH), 7.49 (dd, ABC, J = 7.7 Hz, J = 1.2 Hz, 2H, CH),
4
4
5
7.55 (m, 4H, CH), 7.669 (dd, ABC, J = 1.2 Hz, J = 0.6 Hz,
3
5
2H, CH), 7.673 (dd, ABC, J = 7.7 Hz, J = 0.6 Hz, 2H, CH)
ppm;28 13C NMR (75 MHz, CDCl3) d = 14.0, 14.1 (CH3), 22.6,
22.7, 25.7, 25.8, 29.3, 31.6, 31.6, 53.4 (CH2), 69.6, 69.7 (OCH2),
85.8, 90.1, 94.3, 95.3 (Cq, CMC), 113.0, 115.0 (Cq, Ar), 116.8,
117.0, 123.1 (CH, Ar), 123.3 (Cq, Ar), 124.3, 128.3, 128.4 (CH,
Ar), 130.0 (Cq, Ar), 131.6, 132.7 (CH, Ar), 133.6, 143.7, 153.6,
153.9 (Cq, Ar), 191.9 (Cq, CO) ppm; FT-IR (KBr) n (cm21) =
3054, 2925, 2855, 2200, 1716, 1610, 1599, 1506, 1216, 1026,
917, 876, 851, 782, 753, 689, 668; MS (FD) m/z (%) = 980.6
(100, M+); EA (C69H72O5) calcd C 84.45, H 7.40, found C
83.92, H 7.99%.
Synthesis of 3,6-bis(2-{2,5-dihexyloxy-4-[2-(4-cyanophenyl)
ethynyl]phenyl}ethynyl)-9H-fluoren-9-one 1b. Procedure analo-
gous to the synthesis of 1a, using 6b as the ethyne component
afforded 118 mg (52%) of an orange solid, melting at 161.3 uC
and clearing at 178.1 uC (DSC data).1H NMR (300 MHz,
CDCl3) d = 0.91 (m, 12H, CH3), 1.38 (m, 8H, CH2), 1.57 (m,
8H, CH2), 1.88 (m, 8H, CH2), 4.06 (m, 8H, OCH2), 7.03, 7.04
Synthesis of 2-{2,5-dihexyloxy-4-[2-(4-cyanophenyl)-ethynyl]-
phenyl}ethyne 6b. 0.75 g (4.1 mmol) 4-bromobenzonitrile,
60 mg (0.5 mmol) Pd(PPh3)4 and 25 mg (13.0 mmol) CuI were
mixed with 15 ml piperidine. Oxygen was removed by a freeze–
pump–thaw procedure and afterwards 1.35 g (2.8 mmol) of
510,14 were added to the stirred reaction mixture. After 12 h
the solvent was evaporated, followed by a chromatographic
purification (silica, hexane : CH2Cl2 = 5 : 1) to yield 1.23 g of
the silyl protected derivative of 6b. The protecting group was
cleaved by adding 650 mg (0.0024 mol) (C4H9)4NF to a
solution of the latter material dissolved in 10 ml THF. After
stirring at RT for 2 h, the reaction mixture was filtered over
florosil. Chromatographic purification (silica, CH2Cl2 : hexane
3
4
(2s, 4H, CH), 7.49 (dd, ABC, J = 7.7 Hz, J = 1.2 Hz, 2H,
CH), 7.60, 7.65 (AA9BB9, 8H, CH), 7.66 (dd, ABC, 4J =
5
3
1.2 Hz, J = 0.6 Hz, 2H, CH), 7.67 (dd, ABC, J = 7.7 Hz,
5J = 0.6 Hz, 2H, CH) ppm;28 13C NMR (75 MHz, CDCl3) d =
14.0, 14.1 (CH3), 22.6, 22.7, 25.7, 25.8, 29.2, 31.5, 31.6, 53.4
(CH2), 69.5, 69.6 (OCH2), 89.7, 90.3, 93.4, 94.7 (Cq, CMC),
111.5, 113.6, 114.1 (Cq, Ar), 116.7 (CH, Ar), 118.5 (Cq, CN),
123.1, 124.3 (CH, Ar), 128.2, 129.8 (Cq, Ar), 131.96, 132.0,
132.7 (CH, Ar), 133.7, 143.7, 153.8, 153.9 (Cq, Ar), 191.8
(Cq, CO) ppm; FT-IR (KBr) n (cm21) = 3055, 2926, 2855,
2224, 2206, 1708, 1601, 1510, 1217, 1027, 919, 880, 854,
783, 689, 553; MS (FD) m/z (%) = 1030.4 (100, M+); EA
4332 | J. Mater. Chem., 2006, 16, 4326–4334
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View Article Online
2 M. J. Freiser, Phys. Rev. Lett., 1970, 24, 1041.
(C71H70N2O5) calcd C 82.69, H 6.84, N 2.72, found C 82.59, H
6.77, N 2.63%.
3 L. J. Yu and A. Saupe, Phys. Rev. Lett., 1980, 45, 1000.
4 (a) G. R. Luckhurst, Thin Solid Films, 2001, 393, 40; (b)
K. Praefcke, Braz. J. Phys., 2002, 32, 564.
Synthesis of 3-{2-[2,5-dihexyloxy-4-(2-phenylethynyl)phenyl]
ethynyl}-6-(2-{2,5-dimethoxy-4-[2-(4-cyanophenyl)-ethynyl]phe-
nyl}ethynyl)-9H-fluoren-9-one 1c. Procedure analogous to the
synthesis of 1a, using a 1 : 1 mixture of 6a and 6b as the ethyne
component afforded 81 mg (33%) of an orange solid, mp
5 (a) R. Alben, Phys. Rev. Lett., 1973, 30, 778; (b) R. Alben, J. Chem.
Phys., 1973, 59, 4299; (c) J. P. Straley, Phys. Rev., 1974, A10, 1881;
(d) H. H. Wensink, G. J. Vroege and H. N. W. Lekkerkerker, Phys.
Rev., 2002, E66, 041704.
6 (a) P. J. Champ, M. P. Allen and A. J. Masters, J. Chem. Phys.,
1999, 111, 9871; (b) P. I. C. Teixeira, A. J. Masters and
B. M. Mulder, Mol. Cryst. Liq. Cryst., 1999, 323, 167.
7 V. Prasad, S.-W. Kang, K. A. Suresh, L. Joshi, Q. Wang and
S. Kumar, J. Am. Chem. Soc., 2005, 127, 17224.
8 (a) G. R. Luckhurst, Angew. Chem., 2005, 117, 2894, (Angew.
Chem., Int. Ed., 2005, 44, 2834); (b) G. R. Luckhurst, Nature, 2004,
430, 413.
9 (a) R. A. Reddy and B. K. Sadashiva, J. Mater. Chem., 2004, 14,
310; (b) B. K. Sadashiva, R. A. Reddy, R. Pratibha and N. V.
Madhusudana, J. Mater. Chem., 2002, 12, 943; (c) B. K. Sadashiva,
R. A. Reddy, R. Pratibha and N. V. Madhusudana, Chem.
Commun., 2001, 2140.
10 M. Lehmann and J. Levin, Mol. Cryst. Liq. Cryst., 2004, 411, 273.
11 H. Meier, M. Lehmann, H. C. Holst and D. Schwo¨ppe,
Tetrahedron, 2004, 60, 6881.
12 (a) Z. Bao, Y. Chen, R. Cai and L. Yu, Macromolecules, 1993, 26,
5281; (b) S. H. Chen, A. C. Su, S. R. Han, S. A. Chen and Y. Z. Lee,
Macromolecules, 2004, 37, 181.
13 S. Ho¨ ger, K. Bonrad, G. Scha¨ fer and V. Enkelmann,
Z. Naturforsch., B: Chem. Sci., 1998, 53b, 960. In contrast to the
reference compound 3 was obtained from dimethyl ether 2 as the
starting material.
14 S. Ho¨ger, K. Bonrad, A. Mourran, U. Beginn and M. Mo¨ller,
J. Am. Chem. Soc., 2001, 123, 5651.
1
132.3 uC (DSC data). H NMR (300 MHz, CDCl3) d = 0.91
(m, 12H, CH3), 1.38 (m, 8H, CH2), 1.57 (m, 8H, CH2), 1.88 (m,
8H, CH2), 4.06 (m, 8H, OCH2), 7.03, 7.05 (2s, 4H, CH), 7.49
3
4
(dd, ABC, J = 7.7 Hz, J = 1.2 Hz, 2H, CH), 7.55 (m, 4H,
CH), 7.60, 7.65 (AA9BB9, 8H, CH), 7.67 (dd, ABC, 4J = 1.2 Hz,
5J = 0.6 Hz, 2H, CH), 7.68 (dd, ABC, 3J = 7.7 Hz, 5J = 0.6 Hz,
2H, CH) ppm;28 13C NMR (75 MHz, CDCl3) d = 14.0, 14.1
(CH3), 22.6, 22.7, 25.7, 25.8, 29.2, 31.5, 31.6, 53.4 (CH2), 69.5,
69.6, 69.7 (OCH2), 85.5, 89.7, 90.1, 90.4, 93.4, 94.3, 94.7, 95.3
(Cq, CMC), 111.4, 112.9, 113.5, 114.1, 115.0 (Cq, Ar), 116.8,
117.0 (CH, Ar), 118.5 (Cq, CN), 123.1 (CH, Ar), 123.3 (Cq,
Ar), 124.3, 128.3, 128.4 (CH, Ar), 129.8, 130.0 (Cq, Ar), 131.6,
131.9, 132.0, 132.7 (CH, Ar), 133.5, 133.7, 143.66, 143.71,
153.6, 153.8, 153.9 (Cq, Ar), 191.8 (Cq, CO) ppm; FT-IR (KBr)
n (cm21) = 3057, 2929, 2858, 2223, 2207, 1708, 1601, 1508,
1218, 1028, 919, 881, 853, 838, 783, 754, 690, 670, 554; MS
(FD) m/z (%) = 1005.8 (100, M+); EA (C70H71NO5) calcd C
83.55, H 7.11, N 1.39, found C 83.58, H 7.37, N 1.31%.
15 J. Ipatschki, R. Hosseinzadeh, P. Schlaf, E. Dreiseidler and
R. Goddard, Helv. Chim. Acta, 1998, 81, 1821.
16 Crystal data: C69H72O5, M = 981.27, monoclinic, C2/c, a =
Acknowledgements
˚
˚
˚
16.0034(9) A, b = 33.0383(7) A, c = 10.6224(5) A, b = 97.142(2)u,
We are grateful to the Communaute´ Francaise de Belgique
(ARC nu 00/05-257), the Bundesministerium fu¨r Bildung
und Forschung (BMBF) and to the US National Science
Foundation (grant DMR-03-12792) for their financial sup-
port. We thank Prof. Herbert Meier for making the FD mass
and elemental analysis at the University of Mainz available
to us and to Prof. Yves Geerts for support and helpful
discussions. We are grateful to Michael Bach and Prof. Jochen
Gutmann for their help during X-ray measurements at the
Max Planck Institute for Polymer Research at Mainz. We
thank Dr N. E. Zafeiropoulos at the Institute for Polymer
Research Dresden for the help with additional X-ray
measurements and fruitful discussions. Use of the Advanced
Photon Source (APS) was supported by the U.S. Department
of Energy (DOE), Basic Energy Sciences (BES), Office of
Science, under Contract No. W-31-109-Eng-38. The
Midwestern Universities Collaborative Access Team’s
(MUCAT) sector at the APS is supported by the U.S. DOE,
BES, Office of Science, through the Ames Laboratory under
Contract No. W-7405-Eng-82. Any opinions, findings, and
conclusions or recommendations expressed in this publication
are those of the author(s) and do not necessarily reflect the
views of the National Science Foundation.
V = 5572.8(4) A , z = 4, rcalc. = 1.42 g cm23, crystal size 0.1 6
0.2 6 1.0 mm , Cu-Ka radiation l = 1.54178 A, m = 0.558 mm
3
˚
3
21
,
˚
scan type: v/2h, scan range: 2 , h , 74u, T = 193 K, 6052
reflections measured, 5636 unique, Lp – correction, 5636 reflec-
tions refined, Rint = 0.0269, structure solution with direct methods
(program SIR9729), refinement against F2 using full matrix least
square refinement (program SHELXL9730), 358 refined para-
meters, wR2 = 0.1207, R1 (for 4561 refl. |F|/s(F) . 4) = 0.0453,
hydrogen atom at calculated positions and refined using riding
motion model with isotropic displacement parameters, peaks in
final differential Fourier calculation: 0.27, 20.18 e A23. CCDC
reference number 294987. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b605718g.
˚
17 For the estimation of dipole moments, the experimental values for
fluorenone 3.38 D (D. A. Pitt and C. P. Smyth, J. Am. Chem. Soc.,
1959, 80, 1061) and benzonitrile 4.05 D (K. B. Everard, L. Kumar
and L. E. Sutton, J. Chem. Soc., 1951, 2807) in benzene were used.
18 K. Praefcke, Mol. Cryst. Liq. Cryst., 2001, 364, 15.
19 Y. Galerne, Mol. Cryst. Liq. Cryst., 1998, 323, 211.
20 Surface effects may induce phase biaxiality. However, it was
suggested that in thick films such influence should be vanishingly
small (B. K. Sadashiva, personal communication). The films
investigated had a thickness of 10–20 mm.
21 Samples were aligned in a magnetic field of 0.25 T, perpendicular
to the X-ray beam. Slow cooling of the material in a strong
magnetic field (7 T) induced a crystallisation process, due to the
metastable nature of the monotropic nematic phase of 1c and the
supercooled nematic phase of 1b.
22 These d values can be compared with distances found in the
crystal structure of 1a. For more details see the Supporting
Information{.
References
23 Correlation lengths have been estimated from the half-width of
˚
1 (a) B. R. Acharya and S. Kumar, Bull. Am. Phys. Soc., 2000, 45,
101; (b) B. R. Acharya, A. Primak, T. J. Dingemans, E. T. Samulski
and S. Kumar, Pramana, 2003, 61, 231; (c) B. R. Acharya,
A. Primak and S. Kumar, Phys. Rev. Lett., 2004, 92, 145506; (d)
L. A. Madsen, T. J. Dingemans, M. Nakata and E. T. Samulski,
Phys. Rev. Lett., 2004, 92, 145505.
˚
˚
the signals for 1b: 47.5 A (2.3 molecules ($20.3 A)); 75.5 A
˚
˚
˚
˚
($5.4 molecules (13.9 A)); 12.2 A ($2.6 CH2 (4.7 A)); 40.6 A
˚
˚
($11.0 molecules (3.7 A)). For 1c: 84.9 A ($3.9 molecules
(21.6 A)); 58.9 A ($4.1 molecules (14.5 A)); 13.2 A ($2.9 CH2
˚
˚
˚
˚
˚
˚
˚
(4.5 A)); 40.1 A ($10.6 molecules (3.8 A)).
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 4326–4334 | 4333

View Article Online
24 Another explanation for the observed X-ray pattern may
include a model where different biaxial domains align with their
director n or director m perpendicular to the X-ray beam, i.e.
parallel to the magnetic field. A similar pattern has already been
observed by Diele et al.: K. Praefke, B. Kohne, B. Gundogan,
D. Demus, S. Diele and G. Pelzl, Mol. Cryst. Liq. Cryst. Lett.
Sect., 1990, 7, 27.
25 P. Heiney, Datasqueeze 2.0.7, Datasqueeze software, Wayne, PA,
USA, 2006.
26 A. P. Hammersley, Fit2d version 12.040, ESRF, Grenoble, France,
2006.
27 S. Hu¨nig, R. Bau, M. Kemmer, H. Meixner, T. Metzenthin,
K. Peters, K. Sinzger and J. Gulbis, Eur. J. Org. Chem., 1998,
335–348.
28 Coupling constants and chemical shifts of the ABC system were
obtained by the best fit of the ABC pattern using the program
MestRe–C 2.3a.
29 A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano,
C. Giacovazzo, A. Guagliardi, A. G. G. Moliterni, G. Polidori
and R. Spagna, J. Appl. Crystallogr., 1999, 32, 115.
30 G. M. Sheldrick, SHELXL-97, Program for refinement of crystal
structures, University of Go¨ttingen, Germany, 1997.
4334 | J. Mater. Chem., 2006, 16, 4326–4334
This journal is ß The Royal Society of Chemistry 2006