Synthesis and properties of oxadiazole based V-shaped, shape persistent nematogens

Article abstract of DOI:10.1039/b718348h

A series of biaxial V-shaped, shape-persistent molecules has been synthesised by stepwise coupling of phenylene ethynylene arms to an oxadiazole bending unit. Studies of their thermotropic nematic phases point to phase biaxiality. The Royal Society of Chemistry.

Full text of DOI:10.1039/b718348h

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Synthesis and properties of oxadiazole based V-shaped, shape persistent
nematogensw
a
a
b
b
¨
Matthias Lehmann,* Christiane Kohn, Horst Kresse and Zinaida Vakhovskaya
Received (in Cambridge, UK) 28th November 2007, Accepted 18th January 2008
First published as an Advance Article on the web 12th February 2008
DOI: 10.1039/b718348h
A series of biaxial V-shaped, shape-persistent molecules has
been synthesised by stepwise coupling of phenylene ethynylene
arms to an oxadiazole bending unit. Studies of their
thermotropic nematic phases point to phase biaxiality.
tions of arms 26b with oxadiazole derivative 39 (Scheme 1). The
different reactivities of the iodo and bromo functionalities of
compound 3 provide the possibility of the stepwise linkage of
two different arms to obtain non C2-symmetric molecules 1a
and 1b.
The phase behaviour of all the materials investigated by
polarizing optical microscopy (POM) and differential scanning
calorimetry (DSC) is summarized in Table 1. Compounds 1a–c
show nematic phases; the only exception is 1d which lacks a
peripheral CN functionality. All these nematic phases are
monotropic. The most stable liquid crystal phase is formed
from 1a, which can be supercooled into a nematic glass.10 We
attribute this behaviour to the meta-cyano group which in-
troduces additional attractive intermolecular interactions, but
the rotation of the peripheral aromatic ring also results in a
variation of the molecular dipole moment, which might be the
origin of the low crystallisation tendency compared to 1b.
The nematic phases of 1a–c are characterized by schlieren
textures at the I–N transition (Fig. 2a) and very low transition
enthalpies (0.10–0.56 kJ molÀ1) and entropies (0.16–1.34
J KÀ1 molÀ1). Such small values and the almost exclusive
observation of two brushed disclinations in Fig. 2a have been
reported to indicate a possible transition to a biaxial phase.11
Only compound 1a could be aligned between two glass plates
with polyimide alignment layers by cooling from the isotropic
liquid to the nematic phase (Fig. 2b) without crystallisation of
the material.9 Rotation of the sample gives maximum birefrin-
gence with the rubbing direction at 451 to the polariser or
analyser (Fig. 2c). The low birefringence can be explained by a
small film thickness of approximately 7.5 mm.12 Investigation
in the conoscopy mode reveals separation of isogyres and two
optical axes (Fig. 2d and 2e). Only annealing at room temera-
ture (rt) for several days discloses a beginning crystallisation.
X-Ray diffraction patterns on an aligned sample of 1a were
recorded at rt (Fig. 3). The nematic phase shows an equatorial
diffuse arc (iii) at wide angles (d = 3.7 A), attributed to
V-shaped molecules are in the focus of liquid crystal research
not only for their intriguing properties in banana phases,1 but
also owing to their potential to form the so elusive thermo-
tropic biaxial nematic phase.2 After the theoretical
predictions,2a,3 many V-shaped mesogens have been proposed
to possess a biaxial nematic mesophase (Nb).4–6 Theory pre-
dicts that the formation of this phase depends on the angle y
between the two wings of the molecule. Without a permanent
molecular dipole, the optimum angle y is expected to be
tetrahedral;2a however, this might vary if dipoles are intro-
duced.7 The biaxial character of the nematic phase in a series
of oxadiazole derivatives has been evidenced by 2H solid-state
NMR and X-ray diffraction and, therefore, the compounds
are widely recognised as the first low molecular thermotropic
biaxial nematic materials.4 Recently, it has been claimed that
switching about their molecular bisect is about 100 times faster
than that about their long axis.8 Most of the molecular
structures suggested to form Nb phases possess rather flexible
backbones, without a well-defined bending angle y, which
prompted us to synthesise shape persistent mesogens based
on a phenylene ethynylene scaffold 1 (Fig. 1). A shape-
persistent fluorenone derivative has been shown to exhibit a
uniaxial–biaxial nematic phase transition about 40 1C below
the clearing temperature.6 These findings are in agreement
with theoretical phase diagrams, where biaxial nematic phases
are almost always predicted at low temperature if crystal-
lisation or transitions to high ordered liquid crystal phases can
be prevented. The latter processes can be hindered by laterally
attached aliphatic chains (cf. Fig. 1). For these reasons it is
highly attractive to combine the oxadiazole bending unit, as a
leading structure, with shape persistent phenylene ethynylene
arms containing lateral alkyl chains.
The synthesisw of the target molecules was performed by
two consecutive Sonogashira–Hagihara cross-coupling reac-
a Institute of Chemistry, Chemnitz University of Technology, Straße
der Nationen 62, 09111 Chemnitz, Germany. E-mail:
Matthias.Lehmann@chemie.tu-chemnitz.de; Fax: +49 (0)371
53121229; Tel: +49 (0)371 53131205
b Institute of Physical Chemistry, University of Halle-Wittenberg,
Muhlpforte 1, 06108 Halle, Germany
¨
w Electronic supplementary information (ESI) available: Experimental
data, including synthesis, DSC curves and X-Ray of a shear aligned
sample. See DOI: 10.1039/b718348h
Fig. 1 Design of V-shaped, shape persistent nematogens.
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Fig. 2 POM investigations of 1a. (a) Schlieren texture near the I–N
transition at 68.5 1C during cooling. (b) Aligned sample with rubbing
direction R parallel and (c) at 451 to polariser P and analyser A in the
nematic phase at 64 1C. (d) Splitted isogyres in the conoscopic mode.
(e) Two optical axes by using the circular polariser.
Scheme 1 Synthesis of oxadiazole based molecules.
p–p-interactions, a halo (iv) at 4.9 A, corresponding to the
liquid-like aliphatic chains and two broad reflections centred
on the meridian at (i) 14.9 A and (ii) 10.7 A. The latter are
assigned to periodical structures along the bisect of the
molecule, as shown in the model of Fig. 3b. The molecular
arrangement compares well with the one of fluorenone
derivatives;6b however, the distances along the bisect have
decreased due to the larger angle y of approximately 1401.
The X-ray experiment shows the alignment of two molecular
axes along directors and, thus, confirms the biaxial nature of
the thermotropic nematic phase.
Fig. 3 (a) X-Ray diffractogram of an in the magnetic field oriented
sample 1a at 25 1C (the white arrow indicates the direction of the
magnetic field). (b) Possible model rationalising reflections (i) and (ii)
with short range order along the bisect of the molecules. The anti-
parallel aligned mesogen is placed approximately 3.7 A below the
reference plane. Correlation lengths estimated by the Scherrer
formula13 amounts to 3–6 molecules.14
In order to study the molecular motion of 1a with a strong
dipole along the CN group and the oxadiazole unit, we
performed dielectric spectroscopy15 on a sample aligned in a
magnetic field. Dielectric data measured parallel and perpen-
dicular to the director are presented in Fig. 4a and 4b. Two
processes, a low frequency and a high frequency process
overlapping each other, can be distinguished. The first is
attributed to the reorientation of the molecular long axis
and appears in both measurements, but is lower in intensity
when measured perpendicular to the director. Consequently,
the substance was not completely oriented, because, according
to the classical experiments and the model by Maier and
Meier,16 the low frequency relaxation mode should be seen
only in the parallel direction. The second process at higher
frequency is very broad and pronounced when measured
perpendicular to the director. The shape of the absorption
and its frequency, which is smaller than the frequency typically
attributed to intramolecular motions or rotations of typical
calamitic mesogens about their long axis,17 point to a complex
process involving most probably the rotation of the bisect, i.e.
the V-shaped molecule turns about the molecular long axis.
The observation of both processes with different intensities at
altered molecular orientations demonstrates that, in principal,
Table 1 Mesomorphic properties of 1a–1d
Rate 10 1C minÀ1
Compound
(Onset [1C]/DH [kJ molÀ1])
DSN/J KÀ1 molÀ1
1a
1b
1c
1d
a
Cr 130.1/18.4b (g 25.0a N 73.5/0.15) I 0.43
Cr 123.7/36.2 (N 104.0/À0.06) I
Cr 120 Ic
0.16
1.34
—
Fig. 4 Raw data of the complex dielectric permittivity of 1a measured
in the nematic phase (a) at 60 1C and (b) at 45 1C parallel or
perpendicular to the director. (c) Relaxation frequencies. The stronger
decreasing frequencies with decreasing temperature for process one
can be rationalised by the approaching glass transition at 25 1C.w
Cr 144.9/47.2 (N 127.0/À0.56) I
b
Glass transition temperature. Several overlapping melting transi-
c
tions between 130–140 1C. Determined by POM.
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the V-shaped molecules can be reoriented about their two
different principal molecular axes when perfectly aligned.
In summary, new shape-persistent V-shaped compounds
with an oxadiazole core have been synthesised. The molecular
design, 2,5-alkyloxy substitution at the middle benzene ring
and peripheral cyano substituents, leads to low temperature,
monotropic nematic mesophases. Indications for phase biaxi-
ality of the nematic mesophase of 1a were found in POM and
X-ray investigations. Two processes are detected in dielectric
studies, which are attributed to the rotations about the
molecular short and long axes. Work is in progress to sub-
stantiate these findings by 2H solid-state NMR techniques and
to synthesise low temperature enantiotropic nematic phases
applying the presented design principle.
8 J.-H. Lee, T.-K. Lim, W.-T. Kim and J.-I. Jin, J. Appl. Phys., 2007,
101, 034105.
9 Y. Cheng and T. Luh, Macromolecules, 2005, 38, 4563.
10 Compounds 1b and 1c show a transition at a higher temperature
than 1a and no additional transition upon cooling in DSC with a
rate of 10 1C minÀ1. Thus, their LC (liquid crystal) temperature
intervals seem to be larger than that of 1a. However, upon heating
of 1b and 1c, the nematic phase crystallises rapidly at 58 1C (1b)
and 107 1C (2c) and, therefore, the nematic phase is clearly
monotropic. These compounds also crystallise when slowly cooled
in order to orient the materials for conoscopy or X-ray diffraction.
In contrast, compound 1a can be kept cooling for at least one hour
in the vicinity of the transition temperature without crystallisation
and consequently, 1a was studied in detail.
11 P. J. H. Kouwer and G. H. Mehl, J. Am. Chem. Soc., 2003, 125,
11172.
12 The birefringence for the aligned sample in Fig. 2c was estimated
by means of POM and amounts to Dn = 0.023. The sample is
optical negative, which indicates that the short axis of the optical
indicatrix (smallest refractive index) is aligned orthogonal to the
surface of the substrate. Thus, the value for Dn is presumably not
the maximum birefringence of the sample. This is one reason for
the higher birefringence in non-aligned samples, as in Fig. 2a.
However, in addition, the sample thickness for non-aligned sam-
ples was not rigorously controlled. The thickness typically varies
between 20–30 mm.
Notes and references
1 (a) G. Pelzl, S. Diele and W. Weissflog, Adv. Mater., 1999, 11, 707;
(b) W. Weissflog, H. N. S. Murthy, S. Diele and G. Pelzl, Philos.
Trans. R. Soc. London, Ser. A, 2006, 364, 2657.
2 (a) G. R. Luckhurst, Angew. Chem., 2005, 117, 2894 (Angew.
Chem., Int. Ed., 2005, 44, 2834); (b) G. R. Luckhurst, Nature,
2004, 430, 413; (c) K. Praefcke, Braz. J. Phys., 2002, 32, 564; (d) B.
K. Sadashiva, in Handbook of Liquid Crystals, ed. D. Demus, G.
W. Gray, H.-W. Spiess and V. Vill, VCH, Weinheim, 1998, vol. 2B,
p. 933.
3 (a) G. R. Luckhurst, Thin Solid Films, 2001, 393, 40; (b) P. I. C.
Teixeira, A. J. Masters and B. M. Mulder, Mol. Cryst. Liq. Cryst.,
1998, 323, 167.
4 (a) B. R. Acharya, A. Primak and S. Kumar, Phys. Rev. Lett.,
2004, 92, 145506; (b) L. A. Madsen, T. J. Dingemans, M. Kakata
and E. T. Samulski, Phys. Rev. Lett., 2004, 92, 145505.
5 V. Prasad, S.-W. Kang, K. A. Suresh, L. Joshi, Q. Wang and S.
Kumar, J. Am. Chem. Soc., 2005, 127, 17224.
13 (a) R. Jenkens and R. L. Snyder, in Chemical Analysis: Introduc-
tion to X-ray Powder Diffractometry, Wiley, New York, 1996, vol.
138; (b) P. Scherrer, Nachr. Ges. Wiss. Gottingen, Math.–Phys. Kl.,
1918, 2, 96.
¨
14 The correlation length for reflection (i) amounts to x(i) = 66 A;
this means that approximately 3–5 molecules are correlated in the
direction of the bisect. The correlation length x(iii) = 22 A in the
p–p-stacking direction, thus, about 6 molecules are correlated.
These values are of short range and in agreement with a nematic
phase.
15 H. Kresse, H. Schlacken, U. Dunemann, M. W. Schroeder and
G. Pelzl, Liq. Cryst., 2002, 29, 1509.
16 (a) W. Maier and G. Meier, Z. Naturforsch., A: Astrophys., Phys.
Phys. Chem., 1961, 16a, 262; (b) W. Maier and G. Meier, Z.
Naturforsch., A: Astrophys., Phys. Phys. Chem., 1961, 16a, 470.
17 H. Kresse, Dynamic dielectric properties of nematics, in Physical
properties of liquid crystals: nematics-(EMIS Datareviews series,
no. 25), ed. D. A. Dunmur, A. Fukuda and G. R. Luckhurst,
INSPEC, London, 2001, pp. 277–287.
6 (a) M. Lehmann, S.-W. Kang, Ch. Kohn, S. Haseloh, U. Kolb, D.
¨
Schollmeyer, Q. Wang and S. Kumar, J. Mater. Chem., 2006, 16,
4326; (b) M. Lehmann and J. Levin, Mol. Cryst. Liq. Cryst., 2004,
411, 273.
7 M. A. Bates and G. R. Luckhurst, Phys. Rev. E: Stat., Nonlinear,
Soft Matter Phys., 2005, 72, 051702.
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1770 | Chem. Commun., 2008, 1768–1770