Discotic liquid crystals: Synthesis and characterization of radial polyalkynylbenzene derivatives

Article abstract of DOI:10.1246/bcsj.81.163

New radial polyalkynylbenzene derivatives, which consist of 4-phenylacetylene unit connected with a central phenyl or phenoxy or tolyl linking moiety, are presented. The mesomorphic properties of these compounds in their pure form and as donor-accepter (D

Full text of DOI:10.1246/bcsj.81.163

Ó 2008 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 81, No. 1, 163–167 (2008)
163
Discotic Liquid Crystals: Synthesis and Characterization
of Radial Polyalkynylbenzene Derivatives
Ã1
Sanjay Kumar Varshney, Hideo Takezoe,2 and Doddamane Sreenivasamurthy Shankar Rao1
1Centre for Liquid Crystal Research, Post Box No. 1329, Jalahalli, Bangalore-560013, India
2Department of Organic and Polymeric Materials, Tokyo Institute of Technology,
O-okyama, Meguro-ku, Tokyo 152-8552
Received May 25, 2007; E-mail: skv14@lycos.com
New radial polyalkynylbenzene derivatives, which consist of 4-phenylacetylene unit connected with a central
phenyl or phenoxy or tolyl linking moiety, are presented. The mesomorphic properties of these compounds in their pure
form and as donor–accepter (D–A) complexes with 2,4,7-trinitrofluoren-9-one (TNF) were investigated with polarizing
optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD) studies. The hexa-
alkynylbenzene derivatives in a pure form had a columnar mesophase texture, and pentaalkynylphenyl and pentaalky-
nyltoluene derivatives remained in a highly viscous liquid state even at room temperature, whereas D–A complexes
showed a the columnar hexagonal (Colh) mesophase. The unexpected mesophase behavior of these radial polyalkynyl-
benzene derivatives comes from the molecular architecture arising from the branched aliphatic chain.
The six-fold Sonogashira coupling reaction of some ter-
minal phenyl acetylene with a hexabromobenzene is known
to form a new class of discotic liquid crystals (DLCs).1 The
penta- and hexaalkynylbenzene derivatives regularly show
nematic mesophase that is not very common for DLCs. The
synthesis of symmetrical and unsymmetrical polyalkynylben-
zene derivatives has been reported. Due to the absence of long
aliphatic chains, a mesophase was not found. Later, first liquid
crystalline polyalkynylbenzene derivative has been reported
by Kohne and Praefcke.1b Several penta- and hexaalkynyl-
benzene derivatives with aliphatic and branched aliphatic
chains have been reported. They all show stable discotic nem-
atic mesophase above and below room temperature.2–4 A few
penta- and hexakis(biphenylethynyl)benzene and naphthalene
derivatives exhibiting discotic nematic and columnar meso-
phases at higher temperature have been reported.5,6
DLCs have many promising applications. In recent times.
they have been found to form negative retardation film which
enhances their viewing angle behavior that is an important part
of twisted nematic liquid crystal display (TNLCD).7 In the
columnar mesophase, remarkable high charge carrier mobility
(0.71 cm2 VÀ1 sÀ1) has been reported, which is comparable to
the amorphous silicon (1 cm2 VÀ1 sÀ1).8 It offers potential ap-
plications as an organic charge-transport material in different
types of electronic devices, such as one-dimensional conduc-
tors, photovoltaic solar cells, field effect transistors, etc. The
advantages of columnar mesophase of DLCs over inorganic
material are easy alignment by external forces, such as shear
and electrical field, their ability of self-healing, etc.9–12
rigid aromatic cores is the main deriving force for their self-
assembly.
Various intermolecular interactions are very effective for
forming the mesophase morphology. In addition to the inter-
molecular forces, such as hydrogen bonding, dipole–dipole
interactions, and charge-transfer interactions, and steric and
ion-dipole interactions can import the phase formation and self-
organization mechanisms at several length scales.16 Investiga-
tion of charge-transfer interactions in binary mixtures of donor
and acceptor has proven to be an effective tool to manipulate
or induce liquid crystalline properties and already has been in-
corporated into self-assembled systems.17 D–A complexes of
disk-shaped molecules or discotic liquid crystals with TNF
as an electron-acceptor moiety have been extensively studied
to achieve the unexpected mesophase.18 D–A complexes based
on triphenylene or pentaalkynylbenzene derivatives displaying
novel types of mesomorphic behavior have been reported.18b A
variety of D–A complexes, such as composite form or a twin,
in which donor moiety is connected with a acceptor moiety via
a flexible spacer, has been prepared in different methods and
their thermal behavior has been reported. The aim of this work
was to study the effect of the branched aliphatic chain (special-
ly the second carbon atom substituted) on the mesophase mor-
phology of polyalkynylbenzene super-disk-based discotic liq-
uid crystals. The particular focus was to synthesize these mole-
cules, to prepare D–A complexes with TNF and to study the
thermal properties.
Experimental
DLCs consist of peripheral branched aliphatic chains con-
nected with a central unit via different functional groups and
often decrease the melting and clearing temperatures of the
mesogens, but the phase behavior is often not affected.13–15
The segregation of the peripheral aliphatic chains from the
The synthesis of the new polyalkynylbenzene derivatives 5 was
carried out in four steps starting from 4-bromophenyl (1) to 4-
ethynylphenyl (4), followed by coupling with hexabromobenzene,
or 1,2,3,4,5-pentabromo-6-(2-ethylhexyloxy)benzene or pentabro-
motoluene to achieve the target molecules. Hexabromobenzene
Published on the web January 10, 2008; doi:10.1246/bcsj.81.163

164 Bull. Chem. Soc. Jpn. Vol. 81, No. 1 (2008)
Discotic LCs with Polyalkynylbenzenes
OR
X
OR
X
X
X
(i)
(iii)
I
(ii)
HO
I
RO
1
4
3
2
H
OH
(a): R = CH2CH(C2H5)(CH2)3CH3 , X = H
(b): R = CH2CH(C2H5)(CH2)3CH3 , X = CH3
Reagents and reaction conditions:
(i) BrCH2CH(C2H5)(CH2)3CH3, CsCO3, MEK, 60 °C /20h
(ii) 2-methyl-3-butyne-2-ol, Pd(PPh3)2Cl2, CuI, TPP, Et3N, 80 °C /18h
(iii) KOH , PhMe, 110 °C / 2h
Scheme 1. The synthesis of phenylacetylenes.
R
R
X
R
X
Br
Br
Br
Br
Pd(PPh3)2Cl2, CuI, TPP, Et3N,
5
4
+
80 °C / 36h
R
Br
R
CH3
5(a): R = X =
5(b): R = X =
O
O
CH3
O
5(c): R =
X = CH3
O
5(d): R =
X = CH3
CH3
O
5(e): R =
5(f): R =
O
X = OCH2CH(C2H5)(CH2)3CH3
X = OCH2CH(C2H5)(CH2)3CH3
Scheme 2. Synthesis of radial polyalkynylbenzene derivatives by Sonogashira coupling.
J gÀ1). Derivative 5b was crystalline yellow solid, which
melted into a columnar mesophase at 64.3 ꢀC (ꢀH 34.2 J gÀ1),
and converted into isotropic liquid phase at 199.9 ꢀC (ꢀH
2.9 J gÀ1). On cooling, the columnar mesophase appeared at
188.7 ꢀC (ꢀH À1:7 J gÀ1) with dendritic growth (see Fig. 1b).
The texture remained unaltered even after several hours at
ambient temperature.
The thermal stability of the mesophase was rather low. By
heating the materials above 190 ꢀC for few minutes, extended
domains formed and the mesophase texture became surround-
ed by brown domains, which are indicative of material decom-
position. Compound 5b is relatively more stable than com-
pound 5a. The transition temperatures of the derivatives 5
are summarized in Table 1. According to the results of micro-
scopic observations (Fig. 1), DSC data and literature, the
mesophase was easily determined to be columnar mesophase.
Finally, the mesophase was confirmed by X-ray analysis.
On cooling, the mesophase transition temperature was lower in
the comparison to the heating transition, which could be due to
the supramolecular assembly or the partial decomposition.
Pure samples of phenoxy and tolyl derivatives 5c–5f were
highly viscous liquid at room temperature. POM study showed
no birefringence on a plain glass plate, and no transition peak
and pentabromotoluene were purchased from Aldrich (10-713-1)
and Acros organic (344080250), respectively. 1,2,3,4,5-Penta-
bromo-6-(2-ethylhexyloxy)benzene was prepared by alkylation
of pentabromophenol with 2-ethylhexyl bromide. The two key
phenylacetylenes were prepared by using reported methods.19
All of the derivatives of 5 had 2-ethylhexyl as a branched aliphatic
chain. The TNF was prepared as reported earlier.20 The prepara-
tion of the phenylacetylenes is as shown in Scheme 1.
All of the target molecules reporting here were synthesized by
using a multifold Sonogashira cross-coupling reaction as shown in
Scheme 2.
Results and Discussion
All of the derivatives showed an unexpected columnar
mesophase. The hexaalkynylbenzene derivatives were solid
at room temperature, whereas pentaalkynylbenzene and tolyl
derivatives remained in a highly viscous liquid state.
Compound 5a was yellow crystalline solid at room temper-
ature and melted into a columnar mesophase at 78.3 ꢀC (ꢀH
47.8 J gÀ1). On further heating, the mesophase clears to an
isotropic liquid state at 231 ꢀC (ꢀH 4.1 J gÀ1). Identical meso-
phase texture was observed on cooling at 189.2 ꢀC (ꢀH À2:8
J gÀ1) (see Fig. 1a), and it crystallized at 63.7 ꢀC (ꢀH À4:3

S. K. Varshney et al.
Bull. Chem. Soc. Jpn. Vol. 81, No. 1 (2008)
165
Fig. 1. Columnar mesophase textures observed by POM
cooling from isotropic liquid: (a) 5a at 156 ꢀC (b) 5b at
142 ꢀC.
Fig. 2. Columnar hexagonal mesophase textures observed
by POM: (a) 5e/TNF at 43 ꢀC, (b) 5f/TNF at 83 ꢀC. Both
the images are taken on cooling from the isotropic liquid.
Table 1. Thermal Properties and X-ray Results of 5a–5f and Their D–A Complexes with TNF
˚
X-ray results/A
lattice parameter/stacking distance
Compda)
Thermal behaviorb)
5a
5b
5c/TNFc)
5d/TNFc)
5e/TNFc)
5f/TNFc)
Cr 78.3 (47.8) Col 231 (4.1) I 189 (À2:8) Col 63.7 (À4:3) Cr
Cr 64.3 (34.2) Col 199.9 (2.9) I 188.7 (À1:7) Cold)
75–81 I 70 M
d)
81–92 I 83 Colh
58–62 I 57 Colh
110–160 I 150 Colh
22.24/3.43
19.34/3.46
20.67/3.45
d)
d)
a) Molar ratio of TNF to is 1.0. b) Cr = crystal, Col = columnar, Colh = columnar hexagonal, M = unidentified
mesophase, I = isotropic. Phase-transition temperatures (ꢀC) and the enthalpies (J gÀ1) in parentheses. c) Transition
was observed by POM. d) No crystallization observed at ambient temperature.
was observed even in the DSC scans. The D–A complexes of
tolyl derivative 5c showed an unidentified mesophase at 70 ꢀC
on cooling, and it was assigned to be an M mesophase, where-
as derivative 5d showed a columnar hexagonal phase. The
D–A complexes of phenoxy derivatives 5e–5f shows the focal
conic texture of the columnar hexagonal phase as shown in
Fig. 2. The transition temperatures are summarized in Table 1.
On cooling, the columnar phase that appeared, remained
unchanged till room temperature. The mesophase texture re-
mained unaltered even after several days. The D–A complexes
of the tolyl derivatives show a broad isotropic transition. The
results of these D–A complexes are in the initial stage of char-
acterization, and some of them are still not fully understood.
Detailed investigations on these compounds are necessary to
determine their intricate order and to understand their induced
phase behaviour.
the D–A complex of 5f. A Sharp reflection in the low-angle
region corresponds to the disc diameter (or inter-columnar dis-
˚
tance, 20.67 A). In the wide-angle region two diffuse reflec-
tions were observed. The broad one corresponds to the liquid
like order of the aliphatic chains, whereas the relatively sharp-
er one observed at high 2ꢀ values, which is well separated
from the broad one, is due to the stacking of the aromatic cores
in the columnar fashion (see Fig. 3b). The core–core distance
21
˚
is 3.45 A. The fixed intra-columnar distance indicates the for-
mation of an alternately stacked structure. Similar features of
the D–A complexes of 5c and 5e (see Supporting Information)
were observed. XRD results of pure compound 5b showed
a sharp reflection in low-angle region that indicate a 2D co-
lumnar lattice, whereas the wide-angle ring is very broad (see
Supporting Information). We do not know the reason for this,
but speculate that it could be due to the decomposition of the
sample upon heating it to the isotropic phase. Thus, the X-ray
results clearly indicate a columnar order of the aromatic cores,
suggesting a columnar mesophase.
XRD experiments were carried out on derivatives 5b and on
D–A complexes of 5c, 5e, and 5f. Figure 3a shows a represen-
tative 2D diffraction pattern and 1D intensity vs. 2ꢀ profile for

166 Bull. Chem. Soc. Jpn. Vol. 81, No. 1 (2008)
Discotic LCs with Polyalkynylbenzenes
mesomorphism will be revealed in due course. This approach
to organic materials or ‘‘soft-materials,’’ such as discotic liquid
crystals, makes the induction of columnar hexagonal phase via
D–A interactions of interest for technical applications.
SKV is very grateful to Prof. K. A. Suresh for helpful dis-
cussion and keen support for the work. SKV and DSSR thank-
ful to Dr S. K. Prasad for significant discussion.
Supporting Information
Synthesis procedure, characterization of the compounds and
XRD patterns of D–A complexes. This material is available free
of charge on the web at http://www.csj.jp/journals/bcsj.
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