Star-shaped discotic nematic liquid crystal containing 1,3,5-triethynylbenzene and oxadiazole-based rigid arms

Article abstract of DOI:10.1016/S0040-4039(01)00245-3

A novel three-armed discotic liquid crystal based on 1,3,5-triethynylbenzene as a core and 2,5-diphenyloxadiazole as rigid arms has been synthesized, which is the first star-shaped molecule exhibiting a discotic nematic phase.

Full text of DOI:10.1016/S0040-4039(01)00245-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2697–2699
Star-shaped discotic nematic liquid crystal containing
1,3,5-triethynylbenzene and oxadiazole-based rigid arms
Bong Gi Kim, Sehoon Kim and Soo Young Park*
School of Materials Science and Engineering, Seoul National University, San 56-1, Shillim-Dong, Kwanak-Ku,
Seoul 151-742, South Korea
Received 18 December 2000; revised 6 February 2001; accepted 9 February 2001
Abstract—A novel three-armed discotic liquid crystal based on 1,3,5-triethynylbenzene as a core and 2,5-diphenyloxadiazole as
rigid arms has been synthesized, which is the first star-shaped molecule exhibiting a discotic nematic phase. © 2001 Elsevier
Science Ltd. All rights reserved.
Discotic liquid crystals (LCs) have been a research
subject of great interest from theoretical and practical
viewpoints since their discovery.1 LC assembly of disk-
like molecules is influenced not only by the mesogenic
shape anisotropy, but also by the inter-core interaction
along the disk normal. Because the conventional disco-
genic cores are usually very flat structures consisting of
p-electron-rich aromatic rings2 with strong inter-core
interaction, the columnar phase is normally predomi-
nant over nematic phase for discotic LCs. Actually,
there are only a limited number of reported examples
for discotic nematic phase including derivatives of
triphenylene,3,4 naphthalene,5 hexa- and penta-alkynyl
benzene.2 Due to the higher viscosity and multidomain
scattering of columnar phase, nematic phase is often
preferred for the electrooptic application of discotic
LCs. Considering that the reduced inter-core interac-
tion is necessary for the nematic phase, discotic LCs
with smaller core seem to be an appropriate candidate
for this purpose. Recently, three-armed, planar star
molecules composed of small polar core have been
reported to exhibit discotic LC phases.6 These noncon-
ventional discotic LCs, however, tended to form colum-
nar phase rather than discotic nematic phase due to the
polar inter-core interaction along the disk normal. In
this contribution, we present the first report on discotic
nematic phase in a C3h-symmetric star molecule with
unusual shape of a small core and three extended rigid
arms.
Scheme 1 shows the structure and synthetic routes of a
discotic LC studied in this work, which are character-
ized by 1,3,5-triethynylbenzene unit linked with 2,5-
diphenyloxadiazoles. The synthesis followed the general
methodology of oxadiazole preparation and palladium-
catalyzed ethynylation. 4-Hydroxy ethylbenzoate was
alkylated with 1-bromohexane, followed by the hydroly-
sis of ester and conversion into the corresponding acyl
chloride (1), which were subsequently reacted with 5-
fold hydrazine monohydrate to give phenyl hydrazide
(2). The obtained compound 2 was reacted with 4-bro-
mobenzoyl chloride, followed by dehydration in POCl3
to form oxadiazole ring (3). 1,3,5-Triethynylbenzene (4)
prepared by literature procedure7 was reacted with
compound 3 through palladium-catalyzed coupling and
we could get the desired star-shaped molecule contain-
ing oxadiazole moiety in the middle of its arms in good
yield (54%). Final product 5 was isolated using column
chromatography on silica gel and successive reprecipita-
tion in n-hexane was performed for high purity. Com-
pounds 5 showed good solubility in common organic
solvent. The key compounds 3 and 5 were carefully
identified by 1H NMR and MALDI-TOF mass
spectroscopy.8
The thermal behavior of 5 was checked by polarized
optical microscopy (POM), differential scanning
calorimetry (DSC) and X-ray diffraction (XRD). As
summarized in Table 1, 5 showed crystal to LC phase
transition at 189°C and LC to isotropic phase transi-
tion at 200°C in the heating trace. In the subsequent
cooling cycle, however, isotropic to LC phase transition
with very small enthalpy change (−0.1 J g−1) was only
observed with no further changes down to room tem-
Keywords: liquid crystals; oxadiazoles; X-ray crystal structures; Heck
reaction.
* Corresponding author. Tel.:+(82) 2-880-8330; fax:+(82) 2-886-8331;
e-mail: parksy@plaza.snu.ac.kr
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00245-3

2698
B. G. Kim et al. / Tetrahedron Letters 42 (2001) 2697–2699
O
O
C
i, NH2NH2 H2O
H2NHN C
OC6H13
Cl
OC6H13
2
1
O
1) i
O
Br
OC6H13
Br
C Cl
N N
2) POCl3
3
Br
OC6H13
Br
Br
N
N
1)
O
2) iii
Si
ii, 70 oC
H
H
3
ii, 100 oC
N
O
N
O
H
H
5
N
4
N
C6H13O
OC6H13
Scheme 1. Reagents and conditions; (i) THF, triethylamine, 0°C; (ii) triethylamine, CuI, PdCl2(PPh3)2, PPh3; (iii) K2CO3,
methanol, room temperature.
perature. In this LC state, Schlieren texture, typical of
nematic phase, was observed between the crossed polar-
izer as shown in Fig. 1. The X-ray diffractogram of 5
taken at 140°C showed two diffuse haloes correspond-
ing to spacing of 44–35 A and 6.3–5.0 A, attributable to
the lateral and vertical disk-to-disk distances of discotic
nematic phase, respectively. To our best knowledge,
this is the first observation of a discotic nematic phase
in three-armed small-core discotic LCs. The driving
force for the nematic phase can be found in the peculiar
molecular shape of 5, that is, fairly long rigid arms
comparable to the calamitic mesogen in C3h symmetry.
Since these linearly elongated arms are attached to a
small nonpolar core, i.e. triethynylbenzene, large free
space favorable to arm-to-arm interaction between
neighboring molecules exists. In addition to the appro-
priate arm-to-arm interaction, the negligible core-to-
core interaction between small nonpolar cores seems to
be an important factor for the formation of discotic
nematic assembly rather than columnar stacking in 5.
,
,
Table 1. Phase transitions of star-shaped discotic liquid
crystal containing 1,3,5-triethynylbenzene unit linked with
2,3-diphenyloxadiazolesa
Compound
T/°C (DH/J g−1
)
Heating
Cooling
5
1st heating, K1 125 (−2.7), K2
189, ND 200 Ib
I 149 (−0.1),
ND 110 FNc
2nd heating, FN 127 (−10.8) K
190, ND 200 Ib
K, crystalline phase; ND, discotic nematic mesophase; I, isotropic
liquid; FN, frozen nematic state.
a Transition temperatures and enthalpies were determined by DSC
(scan rate, 10°Cmin−1).
b Enthalpy change could not be determined due to the peak overlap.
c Transition observed only by POM.
Figure 1. Polarized optical microscopic image of discotic
mesogen (Table 1).

B. G. Kim et al. / Tetrahedron Letters 42 (2001) 2697–2699
2699
Actually, it should be noted that the lateral disk-to-disk
References
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1
8. Compound 3: yield 56%, H NMR (CDCl3, 300 MHz): l
Acknowledgements
8.03 (d, 2H), 8.01 (d, 2H), 7.01 (d, 2H), 7.67 (d, 2H), 4.04
(t, 2H); Compound 4: yield 54%, 1H NMR (CDCl3, 300
MHz): l 8.15 (d, 6H), 8.08 (d, 6H), 7.75 (s, 3H), 7.69 (d,
6H), 7.04 (d, 6H), 4.05 (t, 6H); MALDI-TOF (reflector,
positive) m/z 1112.
This work was supported by CRM-COSEF and BK-21
program. X-ray diffraction study was performed in
Pohang Accelerator Laboratory (beam-line 4C1).
.
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