Tuning the mesomorphic properties of liquid-crystalline dibenzotetraaza[14]annulenes - Discotic nematic phases of tetraalkoxy- substituted derivatives

Article abstract of DOI:10.1016/j.tetlet.2012.07.053

A homologous series of five tetrasubstituted dibenzotetraaza[14]annulene- based discotic mesogens was prepared and studied using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). A nematic phase was identified for the product bearing two peripheral isobutoxy and two dodecyloxy substituents.

Full text of DOI:10.1016/j.tetlet.2012.07.053

Tetrahedron Letters 53 (2012) 5127–5130
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Tetrahedron Letters
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Tuning the mesomorphic properties of liquid-crystalline
dibenzotetraaza[14]annulenes — discotic nematic phases
of tetraalkoxy-substituted derivatives
⇑
Jarosław Grolik, Łukasz Dudek, Julita Eilmes
Department of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
A homologous series of five tetrasubstituted dibenzotetraaza[14]annulene-based discotic mesogens was
prepared and studied using polarizing optical microscopy (POM) and differential scanning calorimetry
(DSC). A nematic phase was identified for the product bearing two peripheral isobutoxy and two dode-
cyloxy substituents.
Received 14 May 2012
Revised 22 June 2012
Accepted 11 July 2012
Available online 20 July 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Liquid crystals
Dibenzotetraaza[14]annulene
Nematic phase
Polarizing optical microscopy
Differential scanning calorimetry
The majority of interest in discotic liquid crystals is due to the
unique physical properties exhibited by their columnar mesopha-
ses. For example, the one-dimensional charge and energy transport
abilities of these materials make them interesting candidates for
practical applications in a variety of devices, such as one-dimen-
sional conductors, photoconductors, field effect transistors, and
sensors.1
In recent years, the potential of discotic nematic liquid crystals
has been recognized in the field of liquid crystal display (LCD) tech-
nology. Due to their more suitable viewing angle characteristics, it
has been shown that discotic nematic liquid crystals can be em-
ployed instead of the commonly used calamitic nematics.2 Alterna-
tively, these discotic nematics can be used in compensation films to
improve the viewing angle performance and contrast ratio of con-
ventional twisted angle nematic liquid crystal displays (TNLCD),
which are made of calamitic materials.3
and to control the thermal behavior of the mesomorphic molecular
aggregates. A number of papers have been published which show
that formation of the nematic assembly, rather than columnar
stacking of the discotic molecules, can be achieved effectively by
reducing
p–p inter-core interactions through careful synthetic
control of the size, shape, and flexibility of discotic units.4 The
number of peripheral side chains, their structures, and their attach-
ment positions to the central core have also been suggested to be
important factors in the successful design of discotic nematics.2b,4,5
Although remarkable progress has been achieved in recent
years in the design of nematic discogens, few of the known discotic
systems display a discotic nematic phase. Among these phases are
materials that have been successfully prepared through appropri-
ate structural modifications of typical disc-like cores, such as ben-
zene,5,6 triphenylene,4,7 naphthalene,8 and truxene.9 For example,
the first room temperature discotic nematic liquid crystals exhibit-
ing wide viewing angle characteristics were prepared based on
carefully designed substituted alkynylbenzenes.2b In addition,
due to reduced inter-core interactions, peripherally substituted tri-
phenylene dimers appeared to display a nematic mesophase over a
wide temperature range.4 It has also been found that modification
of discotic molecules with appropriate rod-like counterparts re-
sults in materials with a stable nematic phase.10
However, in contrast to rod-shaped molecules that form cala-
mitic nematic liquid crystals, discotic mesogens rarely exhibit
nematic phases. Moreover, nematic mesophases formed by discotic
molecules are often characterized by high-temperature phase tran-
sitions and narrow mesophase temperature ranges, and therefore
are of limited application.
Significant effort has been applied to the search for methods
that would allow rational influence of the self-assembly processes
We recently developed a new discotic liquid crystal based on
the dibenzotetraaza[14]annulene (DBTAA) macrocycle bearing
two meso 2-hydroxybenzoyl substituents and four 3,7-dimethy-
loctoxy groups attached to o-phenylene moieties (Fig. 1).11 The
⇑
Corresponding author. Tel.: +48 12 6632294; fax: +48 12 6340515.
E-mail address: jeilmes@chemia.uj.edu.pl (J. Eilmes).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.07.053

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J. Grolik et al. / Tetrahedron Letters 53 (2012) 5127–5130
OH
O
O
O
N
N
N
N
O
O
H
H
O
HO
Figure 1. Liquid crystalline dibenzotetraaza[14]annulenes.11,12
optically pure analogues of these materials were also synthesized,
and a lamello-columnar mesomorphism with helical organization
of the mesophase was assigned to them based on POM, DSC, and
small-angle X-ray diffractometry (SA-XRD) supported by molecu-
lar dynamics simulations.12
In this paper, we describe new DBTAA-based discotics in which
four promesogenic 3,7-dimethyloctoxy side chains have been re-
placed by two isobutoxy and two unbranched n-alkoxy groups of
varying lengths. A series of five products was prepared (Scheme
1) and the temperature behavior of the compounds was tested
using polarizing optical microscopy (POM) and differential scan-
ning calorimetry (DSC). We report the first example of a discotic
nematic phase in the liquid crystalline derivatives of DBTAA.
The synthetic procedure that we employed was analogous to
that described earlier and is based on the condensation of o-phen-
ylenediamine with 3-formylchromone to yield meso-bis(2-hydrox-
ybenzoyl)dibenzotetraaza[14]annulene.13 In the case of the
unsymmetrically substituted o-phenylenediamines 13–17, how-
ever, the synthesis led to equimolar mixtures of isomers A and B
of products 18–22 (Scheme 1). The preparation started with the
Figure 2. Optical textures observed under polarized light microscopy (crossed
polarizers): (a) 18 at 180 °C (annealing on heating), magnification 50Â; (b) 19 at
170 °C (annealing on heating), magnification 20Â.
3
n=3
OH
OH
OH
O
O
O
O
i
4 n=5
5 n=7
ii
iii
O
OH
6
n=9
7 n=11
n
1
2
3-7
O2N
O2N
O
H2N
H2N
O
O
v
iv
O
n
n
8-12
13-17
O
O
O
vi
OH
OH
O
B
O
A
O
O
N HN
NH N
O
O
O
N HN
NH N
O
O
+
n
O
n
n
n
18-22 (mix 1:1)
O
O
18 n=3
19 n=5
20 n=7
21 n=9
22 n=11
OH
OH
Scheme 1. Reagents and conditions: (i) 3,4-dihydro-2H-pyran, PPTS, CH2Cl2, rt, 3 h, (89%); (ii) 1-bromo-2-methylpropane, K2CO3, DMF, 80 °C, 4 h; EtOH, PPTS, 55 °C, 2 h,
(67%); (iii) alkyl bromides, K2CO3, DMF, 80 °C, 4 h, (81–87%); (iv) CH2Cl2/HNO3, H2SO4, rt, 1.5 h, (75–85%); (v) MeOH, 10% Pd/C, NaBH4; 65 °C, 30 min (vi) MeOH, reflux, 1 h,
(66–86%).

J. Grolik et al. / Tetrahedron Letters 53 (2012) 5127–5130
5129
Figure 4. (a) Optical texture of 22 observed under polarized light microscopy, at
159.7 °C (crossed polarizers, magnification 20Â, cooling 1 °C/min).
Table 1
Phase transition temperatures of compounds 18–22 (DSC data)
Compound
Heating (°C)
Cooling (°C)
18
19
20
21
22
Cr 184 Col
Cr 152 Col
Cr 155 Col
Cr 149 Col 166 Col
Cr 157 N 168 I
Col 135 Cr
Col 99 Cr
Col 73 Cr
Col 114 Col 92 Cr
I 161 N 136 Col 109 Col 85 Cr
Cr– crystal; Col– discotic columnar phase; N– nematic phase; I– isotropic phase.
The thermal behavior of products 18–22 was studied using DSC
and POM, and the results were analyzed as a function of the length
of the n-alkoxy substituent attached ortho to the isobutoxy group.
The photographs were taken during heating and on slow cooling
from the isotropic liquid (in the case of compound 22), or from
the temperature close to decomposition (compounds 18–21). All
samples were found to exhibit liquid crystalline textures under
polarized light microscopy. The textures of compounds 18–21
were typical of discotic columnar phases,15 and representative
examples of pseudo focal conic textures are shown in Figure 2a
and b. The product with the longest aliphatic side chain (22,
n = 11) displayed well-defined schlieren textures typical of discotic
nematic mesophases upon both heating and cooling (Fig. 3c and d).
In addition, droplets, recognized as highly characteristic of nematic
discotic phases16 were observed in a slightly thicker sample of 22,
immediately below the clearing point (Fig. 4f). The texture shown
in Figure 3c contains four-brush defects (|s| = 1), whereas that in
Figure 3d reveals almost exclusively two-brush disclinations of
strength |s| = ½. According to a test of biaxiality of the nematics,
as formulated by Chandrasekhar, textures that exhibit only two
brush defects, such as that shown in Figure 3d, usually correspond
to the biaxial nematic phase Nb 17. Interestingly, the dimensions of
the DBTAA macrocyclic core were in good agreement with those
reported as optimal for biaxial nematics.18 According to theoretical
predictions, optimal molecular dimensions for the broadest tem-
perature range of the nematic phases corresponded to a length-
to-breadth-to-width ratio (L:B:W) of approximately 5:3:1. Assum-
ing that the average width of the molecules ranges from 3.35 to
3.5 Å,12 using the above criterion gives 16.75–17.50 Å and 10.05–
10.50 Å for the length and breadth, respectively. These values ap-
pear to be in good agreement with the DBTAA core dimensions.11
It is worth noting that compound 22 shows fairly good thermal
stability below the isotropization temperature. On heating, this
compound melts at 157 °C to a nematic phase represented by
texture (d) (Fig. 3) and clears at approximately 168 °C without
Figure 3. Optical textures of 22 observed under polarized light microscopy (crossed
polarizers, magnification 20Â): (a) at 153 °C (cooling 5 °C/min); (b) at 162 °C
(heating 5 °C/min); (c) at 136 °C (cooling 5 °C/min).
protection of one OH group of catechol using 3,4-dihydro-2H-pyr-
an and alkylation of the other OH group with 1-bromo-2-methyl-
propane. In the next step, the phenolic OH was deprotected and
was subsequently alkylated with the appropriate alkyl bromide
CH3(CH2)nBr (n = 3,5,7,9, and 11) to yield the dialkylated catechols
3–7. The dinitro derivatives 8–12 were synthesized according to
the general procedure reported earlier,14 and were subsequently
reduced using NaBH4/methanol in the presence of 10% Pd/C. Next,
diamines 13–17 were condensed with 3-formylchromone to give
the final products 18–22 in reasonable yields of 66–86%. The
new products 3–12 and 18–22 were fully characterized by IR, 1H,
and 13C NMR spectroscopy, mass spectra, and elemental analyses.
Detailed procedures, analytical and spectroscopic characterization
of the new compounds are given in the Supplementary data.

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J. Grolik et al. / Tetrahedron Letters 53 (2012) 5127–5130
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before becoming isotropic liquids, as judged from the results of
TLC tests performed on the corresponding samples. On cooling
from the isotropic liquid, the nematic phase of 22 appears at
161 °C and crystallizes at 85 °C. At 136 °C, a fan-like texture of
the discotic columnar phase was identified using polarizing optical
microscopy (Fig. 3e). The phase-transition temperatures of prod-
ucts 18–22, as determined by means of differential scanning calo-
rimetry, are given in Table 1, and the corresponding DSC traces are
collected in the Supplementary data.
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Acknowledgements
Financial Support from the Polish Ministry of Science and High-
er Education (Project NN204 333437) is gratefully acknowledged.
The research was carried out with equipment purchased thanks
to the financial support of the European Regional Development
Fund in the framework of the Polish Innovation Economy Opera-
tional Program (contract no. POIG.02.01.00-12-023/08).
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
07.053.
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