V-shaped nematogens with the "magic bent angle"

Article abstract of DOI:10.1039/c1cc10577a

V-shaped nematogens 1a-c and 2a-b with benzodithiophene bending units have been synthesised. The derivatives 1a-c comprise a flat core with a bending angle of 109°, which is almost the tetrahedral angle proposed to be optimal in the realization of mesogen

Full text of DOI:10.1039/c1cc10577a

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V-shaped nematogens with the ‘‘magic bent angle’’w
¨
Jens Seltmann,c Kathrin Muller,c Susanne Kleinb and Matthias Lehmann*ac
Received 29th January 2011, Accepted 12th April 2011
DOI: 10.1039/c1cc10577a
V-shaped nematogens 1a–c and 2a–b with benzodithiophene
bending units have been synthesised. The derivatives 1a–c comprise
a flat core with a bending angle of 1091, which is almost the
tetrahedral angle proposed to be optimal in the realization of
mesogens forming a biaxial nematic thermotropic mesophase.
Additionally, heteroatoms may generate dipole moments along
the bisector of the bending unit which can be favourable for
direct formation of the Nb from the isotropic liquid (I).
We succeeded to identify a potential candidate among the
benzodithiophenes. Whereas AM1 and DFT (B3-LYP/TZVPP
with TURBOMOLE package)13 energy minimised bending unit
B2 revealed a bending angle of 1341, the structure of derivative
B1 showed an angle of 108.91 which is extremely close to the
desired value (Fig. 1).
V-shaped mesogens are one principal target structure thought to
be successful in order to find a real thermotropic biaxial nematic
phase (Nb) for low molecular mass compounds at lower
temperature. Such mesophases are highly interesting with respect
to theory and application, however, whether they exist or not is
strongly debated. So far there have been only few examples, for
which a number of different techniques provide evidence for this
elusive phase at relatively high temperatures.1–3 Theory predicts
an optimum angle for V-shaped mesogens without dipoles to
form the Nb phase at a critical point (Landau point). This special
angle is proposed to be the tetrahedral angle (109.41),4,5 to which
we will refer as a ‘‘magic bent angle’’. A recent publication,6
however, shows that such a point can broaden to a line if dipoles
along the bisector are introduced. The tetrahedral angle may
apparently be achieved by using a diphenylmethane bending
unit. However, early X-ray studies of a 4,40-diiodo derivative
demonstrated a much larger angle of about 1181.7 Moreover, in
our approach towards V-shaped nematogens with shape-
persistent oligophenyleneethynylene scaffolds, the diphenyl-
methane core did not generate liquid crystalline materials, pre-
sumably because of the non-planarity originating from the steric
congestion of the ortho-hydrogens.8 Benzophenone or diphenyl-
ether bending units are close to the tetrahedral angle but do not
allow for a planar conformation of the mesogen obviously
needed for mesomorphic behaviour. Rarely, non-shape-
persistent banana mesogens form nematic phases from molecules
with an angle close to the tetrahedral value, but only at relatively
high temperatures.9–11 Moreover, banana molecules tend to form
lamellar phases and show a strong tendency for biaxial clustering
in the temperature range of the nematic phases.9,12
The synthesis of the V-shaped mesogens 1 and 2 (Fig. 1)
starting from benzodithiophene cores14,15 and the terminal
alkynes8,16–18 is outlined in Schemes S1,S2. All compounds 1
and 2 were thoroughly purified and characterised with NMR
techniques, elemental analysis and mass spectrometry (ESIw).
The thermotropic properties of all compounds were investi-
gated by polarised optical microscopy (POM) and differential
scanning calorimetry (DSC) and are collected in Table 1.
Schlieren textures (Fig. S1, ESIw) of highly fluid phases revealed
the exclusive formation of nematic liquid crystals with the present
molecular design as stressed earlier for other bending units.14,17
The phases are monotropic but kinetically, extremely stable in
the liquid crystal temperature range. This stability can be
enhanced by the position of cyano-substituents on the terminal
aromatic unit and the type of aliphatic lateral chains.17,19 Thus
compound 1c with meta-CN substituents and a mixture of lateral
pentyl and heptyl chains revealed the most kinetically stable
monotropic nematic phase in the series, which could be annealed
close to the clearing point (TNI) for more than 24 hours without
any sign of crystallisation. The transition enthalpies DH and
entropies DS at TNI are small pointing to a weak first order
transition (Table 1).20 This observation does not exclude a
possible direct I–Nb transition.6
Therefore, we were seeking for heterocyclic cores with the ideal
angle between coupling sites which guarantee planarity.
a Institute of Organic Chemistry, University of Wurzburg,
¨
Am Hubland, 97074 Wurzburg, Germany.
¨
E-mail: Matthias.Lehmann@uni-wuerzburg.de;
Fax: +49 (0)931 31 87350; Tel: +49 (0)931 31 83708
b HP Labs, Long Down Avenue, Stoke Gifford, Bristol BS34 8QZ, UK
c Institute of Chemistry, Chemnitz University of Technology,
Straße der Nationen 62, 09111 Chemnitz, Germany
w Electronic supplementary information (ESI) available: Experimental
details, analytical data and models. See DOI: 10.1039/c1cc10577a
Fig. 1 Calculated (DFT) angles y/1 and dipole moments m/D and
structure of V-shaped mesogens 1a–c and 2a–b.
c
6680 Chem. Commun., 2011, 47, 6680–6682
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Table 1 DSC data of 1a–c, 2a–b for the first heating (Cr–I) and the
second cooling (I–N)a
Rate 10 1C minꢀ1
Compound
(onset [1C]/DH [kJ molꢀ1])
DSNI/JKꢀ1 molꢀ1
1a
1b
1c
2a
2b
Cr 128.1/37.3 (N 104.4/ꢀ0.3) I
Cr 145b (N 80.0/ꢀ0.3) I
0.8
1.4
1.2
0.8
1.1
Cr 129.9/45.0a (N 66.8/ꢀ0.4) I
Cr 150.0/69.3 (N 118.2/ꢀ0.3) I
Cr 113.0/45.1 (N 86.1/ꢀ0.4) I
a
b
Cr: crystal, N: nematic, I: isotropic phase. Determined by POM.
All compounds 1 and 2 aligned homeotropically spontaneously
in an optically negative nematic phase, i.e. with the director n
orthogonal to the aromatic planes and normal to the substrate
(see Fig. S7 (ESIw) and discussion below), between two glass
plates and in LC cells for homogenous alignment.z Therefore, the
materials could be studied by conoscopy21 in high quality LC cells
(1) with a 23 mm and (2) with a 50 mm gap and planar
antiparallely rubbed alignment layers (Nissan 130). Fig. 2 shows
representative results for magic bent angle compound 1c in the
50 mm cell (2). After cooling the sample to the nematic LC at
TNIꢀT = 1.5 1C, a strongly birefringent texture with randomly
oriented domains appeared (A, B). After seven hours annealing,
this phase transformed to a birefringent uniformly aligned film
(D).22 The large and almost symmetric splitting of the optical axes
in conoscopic investigations points to phase biaxiality (E). How-
ever, further annealing for 14 h afforded a multidomain texture
visualised by the different colours when the l-compensation plate
was inserted (F, see also Fig. S7, ESIw). These areas of the LC cell
with disordered biaxial microdomains give in sum a uniaxial
response when investigated by conoscopy, which is evidenced by a
slightly tilted conoscopic cross and only one optical axis. How-
ever, at the sample edge the material appears to be still uniformly
oriented and exhibits biaxiality. Fig. 2F shows a multidomain
structure which was uniaxial at 60 1C. Upon cooling this area
aligned macroscopically and at room temperature (rt) the sample
revealed maximum biaxiality (Fig. 2H). The director n was
uniformly oriented but tilted by approximately 61 from the
normal position with the substrate and oblique (azimuthal angle
approx. 751) with respect to the splitting of the optical axes:
sample tilt by 61 along the rubbing direction centred the images
(Fig. S2, ESIw). Various isochromes are visible at rt, which is a
consequence of the sample thickness and the increasing density of
the material with decreasing temperature. The transition from the
uniaxial multidomain structure to the biaxial domain is reversible.
Interestingly, in most of the well oriented areas the large refractive
index is oriented parallel to the air–LC interface. This may be
rationalised with the orientation of the lateral aliphatic chains
towards the air interface which is favourable for most liquid
crystals.23 Thus the molecular long axes should be aligned parallel
with the sample edge, which is in perfect agreement with the
experimental observations. Conoscopy reveals also the optical
negative anisotropy for all materials 1 and 2. Thus in the homeo-
tropically aligned samples the aromatic planes are oriented
parallel with the surfaces (Fig. S7, ESIw). However, we have no
evidence whether the rubbing direction has any effect on the
domain orientation. Consequently, the observed slow formation
of Nb at the I–N transition is transient, not thermodynamically
stable and breaks down in small biaxial domains, since the rubbed
Fig. 2 (A–E) POM of 1c at TNIꢀT = 1.5 1C in cell (2) between crossed
polarisers (P, A). Texture after transition to the nematic phase at t = 0 (A)
and t = 3 h (B). (C and D) Texture at normal (01) and diagonal position
(451) after 7 h annealing. (E) Conoscopic images at 01 and 451 and optical
axes visualised using a circular polariser at 451. F: multi-microdomain
structure observed applying the l compensation plate at 60 1C. In the
green part the largest refractive index of the sample adds on the largest
refractive index of the l plate (white arrow), indicating that the molecular
long axes are oriented parallel with the same direction. G: same area
uniformly oriented. H: conoscopic images at 01 (left) and 451 (right) and
optical axes visualised with a circular polariser at 01 (middle).
polyimide layer does obviously not orient the second director of
the nematic phase at a large (millimetre) scale. Upon cooling the
anchoring to the interfaces becomes stronger which is presumably
the driving force to the uniformly aligned areas at the sample
edge. In principle these observations are in agreement with the
generic cluster theory which predicts that molecules form biaxial
clusters or small biaxial domains in the nematic phase which can
be subsequently oriented by external forces such as electric,
magnetic fields or surface anchoring.9 Note, however, that in
the present case the small biaxial domains are already macro-
scopic (Fig. 2F).
The results indicate biaxiality of the phase, which cannot
originate solely from surface effects as sometimes suggested.
However, certain director field distributions were proposed to
exhibit signs of biaxiality similar to a real biaxial phase (see
Fig. S3, ESIw).24,25 In the present case, the director n is tilted by 61
oblique to the direction of the separation of the effective optical
axes. To explain this effect other than by biaxiality we would need
a special director field with opposite tilt of the molecules at the
substrate. This is highly unlikely, because the cell was assembled
to achieve a constant pretilt across the bulk of the liquid crystal
(see ESIw). Moreover, the sign of biaxiality does not vanish in very
thick LC cells (2), in which the uniformly aligned uniaxial bulk
should dominate. Therefore, the results point to a real biaxial
nature of the nematic phase already at the I–N transition. In
subsequent POM studies of the mesogen 2b with the 1341 angle
we did expect to find only a uniaxial phase. However, the sample
revealed the same properties in cell (1) than compounds 1 (Fig. S4,
ESIw). This points to a direct I–Nb transition also in the series of
large angle derivatives 2.
Magnetic field aligned samples were investigated by
temperature-dependent wide angle X-ray scattering (WAXS) to
get more detailed structural information. A typical diffraction
pattern when probed with X-ray radiation perpendicular to the
c
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thicknesses close to the phase transition N–I. The optical
negative phases align uniformly (n almost normal to the
substrate). The second director is aligned in some areas at
the air–LC interface and POM indicates a biaxial nature of the
nematic phases. WAXS studies confirm the biaxiality of the
microdomains. These results indicate that the angle is not
important for this mesogen family to form a biaxial phase
directly from the isotropic phase. Further studies are in
progress to confirm and substantiate the phase biaxiality and
the structure–property relationship.
Notes and references
z Homeotropic orientation is defined for uniaxial phases, when the
director n and consequently the optical axis is normal to the substrate
and light becomes extinct in POM. In a biaxial phase the same
alignment appears to be birefringent, see P. Collings, M. Hird,
Introduction to Liquid Crystals—Chemistry and Physics, Taylor and
Francis, London 1997.
Fig. 3 (A) WAXS pattern of 1b at 50 1C recorded with the X-ray
beam orthogonal to the direction of the previously applied magnetic
field. (B) Model of the short range order in the N phase. The vectors N,
M and L show the direction of the different molecular axes. (C)
Integration along the equator. (D) Orientation of biaxial domains
with directors n, m and l consistent with the diffraction pattern.
1 L. A. Madsen, T. J. Dingemans, M. Nakata and E. T. Samulski, Phys.
Rev. Lett., 2004, 92, 145505; B. R. Acharya, A. Primak and S. Kumar,
Phys. Rev. Lett., 2004, 92, 145506; C. D. Southern, P. D. Brimicombe,
S. D. Siemianowski, S. Jaradat, N. Roberts, V. Gortz, J. W. Goodby
¨
and H. F. Gleeson, EPL, 2008, 82, 56001; M. Nagaraj, K. Merkel,
J. K. Vij and A. Kocot, EPL, 2010, 91, 66002.
2 H.-G. Yoon, S.-W. Kang, R. Y. Dong, A. Marini, K. A. Suresh,
M. Srinivasarao and S. Kumar, Phys. Rev. E, 2010, 81, 051706.
3 The biaxial phase has been demonstrated for LC polymers and
tetrapodes (high molar mass mesogens): K. Severing and
previously applied magnetic field B is shown in Fig. 3A. Three
very diffuse signals, (i) at small angles at the equator d(i) =
14.0 A, (ii) at wide angles at the meridian d(ii) = 3.7 A and (iii)
the halo d(iii) = 4.4 A, correspond to the mean distance along the
bisect, between the p-faces and the liquid-like aliphatic chains.
There are two additional diffuse peaks (iv) and (v), which are only
apparent when integrating the WAXS pattern along the equator
(see Fig. 3C and Fig. S5, ESIw). The origin of these two signals at
d(iv) = 6.6 A, d(v) = 5.2 A is not yet known, but may be
rationalised by intra- and intermolecular sulfur separations
(Fig. 3B). All signals possess very low scattering intensities and
are broad, with liquid-like correlation lengths x/d in the range of a
few molecules, pointing clearly to nematic liquid crystalline nature
of the phases. Analogous to the microscopical studies, a multi-
domain structure was expected to be formed in the magnetic field
treated samples. Possible orientations of the domains are shown
in Fig. 3D. Biaxial domains are rotationally distributed about the
director m or n, thus the diamagnetic anisotropy forces the
aromatic scaffold to align parallel with the magnetic field. More
detailed analysis reveals that the molecular axis M orients
preferentially along the magnetic field (see Fig. S6, ESIw).
Therefore the p–p-distance reveals only at the meridian, whereas
the distances along the bisector are visible exclusively at the
equator of the diffraction pattern. Similar results have been
recently obtained for other biaxial optically negative nematics.16,17
Interestingly, there is no important influence of the angle on
the mean separation of the molecules along the bisector,
when mesogens with the same length of lateral chains are
compared.16,17,19 This becomes also apparent for derivatives 1b
(d(i) = 13.9 A) and 2b (d(i) = 14.4 A) which reveal almost no
difference with the change of the bending angle from 1091 to 1341.
Note that these X-ray results do not prove the macroscopic
biaxiality of the nematic phase. They show only the alignment
of two directors in small biaxial microdomains.
K. Saalwachter, in Thermotropic Liquid Crystals: Recent Advances,
¨
ed. A. Ramamoorthy, Springer, Netherland, 2007, ch. 5 and ref. 9.
4 P. I. C. Teixeira, A. J. Masters and B. M. Mulder, Mol. Cryst. Liq.
Cryst., 1998, 323, 167.
5 G. R. Luckhurst, Thin Solid Films, 2001, 393, 40.
6 M. A. Bates, Chem. Phys. Lett., 2007, 437, 189.
7 O. Kratky and W. Worthmann, Monatsh. Chem., 1946, 76, 264.
8 M. Lehmann and J. Levin, Mol. Cryst. Liq. Cryst., 2004, 411, 273.
9 C. Tschierske and D. J. Photinos, J. Mater. Chem., 2010, 20, 4263.
10 J. Thisayukta, Y. Nakayama and J. Watanabe, Liq. Cryst., 2000,
27, 1129.
11 N. Gimeno, M. J. Clemente, P. Forcen, J. L. Serrano and
M. B. Ros, New J. Chem., 2009, 33, 2007.
12 C. Keith, A. Lehmann, U. Baumeister, M. Prehm and
C. Tschierske, Soft Matter, 2010, 6, 1704.
¨ ¨ ¨
13 R. Ahlrichs, M. Bar, M. Haser, H. Horn and C. Kolmel, Chem.
Phys. Lett., 1989, 162, 165; O. Treutler and R. Ahlrichs, J. Chem.
Phys., 1995, 102, 346; M. v. Arnim and R. Ahlrichs, J. Comput.
Chem., 1998, 19, 1746.
14 K. Starcevic, D. W. Boykin and G. Karminsksi-Zamola, Heteroat.
Chem., 2003, 14, 218; J. Larson and K. Bechgaard, Acta Chem.
Scand., 1996, 50, 71.
15 S. Yoshida, M. Fujii, Y. Aso, T. Otsubo and F. Ogura, J. Org.
Chem., 1994, 59, 2526.
16 M. Lehmann, Ch. Kohn, C. Cruz, J. L. Figueirinhas, G. Feio and
¨
R. Dong, Chem. Eur. J., 2010, 16, 8275 and references therein.
17 M. Lehmann, Ch. Kohn, H. Kresse and Z. Vakhovskaya, Chem.
¨
Commun., 2008, 1768.
18 J. Seltmann and M. Lehmann, Liq. Cryst., 2011, 38, 407.
19 M. Lehmann, J. Seltmann, A. A. Auer, E. Prochnow and
U. Bendikt, J. Mater. Chem., 2009, 19, 1978.
20 DH, DS values are usually in the range of 1–2 kJ molꢀ1 and
2–5 JKꢀ1 molꢀ1 and are equally observed for banana mesogens:
G. Pelzl, S. Diele and W. Weissflog, Adv. Mater., 1999, 11, 707.
21 M. Born and E. Wolf, Principles of Optics, Cambridge University
Press, Cambrigde, 7 edn, 1999.
22 The long annealing for alignment originates from the relatively high
viscosity of the nematic phase and the large thickness of the LC cell.
23 K. Kawata, Chem. Rec., 2002, 2, 59.
24 Y. Galerne, Mol. Cryst. Liq. Cryst., 1998, 323, 211.
25 Y. A. Nastishin, O. B. Dovgyi and O. G. Vlokh, Ukr. J. Phys. Opt.,
2001, 2, 98–106.
In conclusion, the two series of newly prepared benzodithio-
phene shape-persistent V-shaped nematogens with the ‘‘magic
bent angle’’ 1 and the larger 1341 angle 2 exhibit optical biaxial
areas and multi-microsized domain areas in LC cells of various
c
6682 Chem. Commun., 2011, 47, 6680–6682
This journal is The Royal Society of Chemistry 2011