Synthesis and liquid crystalline studies of disc-shaped molecule on azo-bridged benzothiazole-phenyl ethers

Article abstract of DOI:10.1016/j.molliq.2016.08.110

A new series of triphenylene core attached by six peripheral arms containing benzothiazole-azo with terminal ethoxy unit have successfully been synthesized and their liquid crystalline properties were characterized. The members in this series differ by the length of the methylene unit of [sbnd](CH₂)_n[sbnd], where n, varied following ascending order from 5 to 10. The molecular structure was characterized by elemental (C, H and N) analysis and spectroscopic methods including Fourier Transform Infrared (FT-IR) and Proton Nuclear Magnetic Resonance (¹H NMR). The texture and the phase transition temperatures were investigated by using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The disc-shape molecule with shorter spacer (n?=?5) showed monotropic nematic phase. The higher homologues with n?=?8–10 exhibited enantiotropic nematic phase and monotropic smectic C phase. The XRD diffraction studies further confirm the presence of smectic C without any discotic columnar phase.

Full text of DOI:10.1016/j.molliq.2016.08.110

Synthesis and liquid crystalline studies of disc-shaped molecule on azo-
bridged benzothiazole-phenyl ethers
Guan-Yeow Yeap, Mursyida Abdul Rahim, Chien-Min Lin, Hong-Cheu
Lin, Nobuyuki Maeta, Masato M. Ito
PII:
DOI:
Reference:
S0167-7322(16)30260-4
doi: 10.1016/j.molliq.2016.08.110
MOLLIQ 6271
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
29 January 2016
29 August 2016
30 August 2016
Please cite this article as: Guan-Yeow Yeap, Mursyida Abdul Rahim, Chien-Min Lin,
Hong-Cheu Lin, Nobuyuki Maeta, Masato M. Ito, Synthesis and liquid crystalline studies
of disc-shaped molecule on azo-bridged benzothiazole-phenyl ethers, Journal of Molecular
Liquids (2016), doi: 10.1016/j.molliq.2016.08.110
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ACCEPTED MANUSCRIPT
Synthesis and liquid crystalline studies of disc-shaped molecule on
azo-bridged benzothiazole-phenyl ethers
Guan-Yeow Yeapa*, Mursyida Abdul Rahim a, Chien-Min Linb, Hong-Cheu Linb,
Nobuyuki Maetac and Masato M Itoc
a Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains
Malaysia 11800 Penang, Malaysia
bDepartment of Materials Science and Engineering, National Chiao Tung University,
Hsinchu 300, Taiwan, Republic of China
cFaculty of Science & Engineering, Soka University, 1-236 Tangi-cho, Hachioji, Tokyo 192-
8577, Japan
ABSTRACT
A new series of triphenylene core attached by six peripheral arms containing benzothiazole-
azo with terminal ethoxy unit have successfully been synthesized and their liquid crystalline
properties were characterized. The members in this series differ by the length of the
methylene unit of -(CH2)n-, where n, varied following ascending order from 5 to 10. The
molecular structure was characterized by elemental (C, H and N) analysis and spectroscopic
methods including Fourier Transform Infrared (FT-IR) and Proton Nuclear Magnetic
Resonance (1H NMR). The texture and the phase transition temperatures were investigated by
using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The
disc-shape molecule with shorter spacer (n = 5) showed monotropic nematic phase. The
higher homologues with n = 8-10 exhibited enantiotropic nematic phase and monotropic
smectic C phase. The XRD diffraction studies further confirm the presence of smectic C
without any discotic columnar phase.
Keywords: Triphenylene; discotic liquid crystal; nematic phase; smectic C phase; phase
transition
_____________________
Corresponding author. G.Y.Yeap. Email: gyyeap@usm.my. Fax : +60-4-6574854

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1.
Introduction
Discotic liquid crystals have attracted much attention among chemists as well as
physicists due to their unique structural features and unusual material characteristics [1].
Discotic liquid crystals show a number of potential applications in a variety of devices such
as one-dimensional conductors, photo-conductors, field effect transistors and photovoltaic
solar cells [1-3]. The most intensively studied discotic liquid crystals are triphenylene
derivatives [4] since the triphenylene core is readily accessible through established synthetic
routes, and is excellent at conferring liquid crystalline properties [5]. Triphenylene also can
combine synthetic versatility with the molecular robustness required for device applications
[6]. Hexa-esters and hexa-ethers of triphenylene are the most widely investigated discotic
materials since they possess a large conjugated aromatic core [7] and have a strong tendency
to form columnar mesophases [6-9], which are of interest for one-dimensional energy and
electron transport. They are also useful in optical compensation films particularly in opto-
electronic devices [5,10-11].
Current efforts have been directed towards designing novel molecules which in some
ways combine the features of rod-shape and disc-like molecules having photoactive
properties. However, the combination of calamitic moieties was incompatible with the
molecular topology and induced the formation of smectic phase. Compatible combination of
discotic and calamitic mesogens results in the formation of a nematic phase [8]. Recent
studies by Shimizu et al. on a triphenylene mesogen with peripheral azobenzene units, was
found to show kinetically controlled bimesomorphism with metastable smectic and stable
hexagonal columnar phases created by the rod–disc alternation of the molecular shape on
cooling. This molecule comprised a triphenylene core with six azobenzene units linked via
short (C3) alkyl chains [12]. Azobenzene-containing materials have potential technological
applications such as in high density optical data storage, optical image processing, dynamic
holography, optical computing and pattern recognition. In addition, the aromatic azo
compounds undergo the cis-trans isomerization in response to light or heat and thus offer
many opportunities in photonic liquid crystal applications [8,13,17].
In this report, we incorporate azo-bridged benzothiazole-phenyl ethers moieties into
triphenylene molecule via respective alkyl chains giving triphenylene-based discotic liquid
crystals with six peripheral rod-like azobenzene units. The spacers with odd and even-parity

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ranging from C5H10 to C10H20 are introduced into this discotic system. The synthetic route
used for the preparation of final compounds 5a-5f is summarized in Scheme 1.
.
Scheme 1. Synthesis route toward formation of the compounds 5a-5f
2.
Experimental
2.1
Materials
All solvents and reagents were purchased commercially and used without any further
purification. 2-amino-6-ethoxybenzothiazole and 1,2-dimethoxybenzene were purchased
from Acros Organics and TCI. Dibromoalkane (CnH2nBr2, where n = 5, 6, 7, 8, 9, 10) was

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obtained from TCI, Merck and Sigma-Aldrich. Sodium nitrite, phenol, anhydrous FeCl3 ,
boron tribromide were obtained from Sigma-Aldrich.
2.2
Measurements
The infrared (IR) spectra for all intermediates and final compounds were recorded
using the Perkin Elmer 2000 FT-IR spectrophotometer, in which the samples were prepared
as KBr disks. 1H- and 13C-NMR spectra were recorded in CDCl3 and deuterated DMSO using
a Bruker 500 MHz Ultrashield TMFT-NMR spectrometer. Tetramethylsilane (TMS) was used
as an internal standard. Elemental analyses were carried out on a Perkin Elmer 2400 LS
Series CHNS/O analyser.
The phase transition temperature and associated enthalpy values were determined
using a differential scanning calorimeter (DSC) (Seiko DSC6200R), in which the samples
were heated and cooled at the rates of ±5◦C min−1. Texture observation of the mesophase was
carried out by using polarizing optical microscopy (POM) (Carl Zeiss Axioskop 40 polarizing
microscope) equipped with a hot stage (Linkam LTS350) and a temperature controller
(TMS94).
Synchrotron powder X-ray diffraction (XRD) measurements were performed at
beamline BL17A of the National Synchrotron Radiation Research Center, Taiwan, where the
wavelength of X-ray was 1.33366 Å. The XRD data were collected using imaging plates
(area = 20 × 40 cm2 with a pixel resolution of 100) curved with a radius equivalent to a
sample-toimage plate distance of 280 mm, and the diffraction signals were accumulated for 3
min. The powder samples were packed into capillary tubes heated by a heat gun, whose
temperature controller was programmable by a computer with a proportional, integral and
differential feedback system. The scattering angle θ was calibrated by a mixture of silver
behenate and silicon.
2.3
Synthesis
2.3.1. Synthesis of 4-((5-ethoxybenzothiazol-2-yl)diazenyl)phenol (1)
Compound 1 was prepared according to the method described in earlier report [13]. A
solution containing 20 ml diluted sulphuric acid (60 %) and 6-ethoxy-2-aminobenzothiazole
(1 g, 5.1 mmol), which was earlier dissolved in 15 ml glacial acetic acid, was cooled down to
5˚C. Sodium nitrite (0.35 g, 5.1 mmol) in 12.5 ml water was added dropwise to the cooled

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mixture and stirred for 1h in an ice bath. Phenol (0.47 g, 5.1 mmol) dissolved in 15 ml
ethanol was cooled to 5˚C and the diazonium salt solution was added dropwise at a
temperature below 5˚C. The mixture was stirred for 1h. The pH was subsequently increased
to 6-7 by the addition of 1 N NaOH and the mixture was stirred for 1h in an ice bath. Finally,
water was added to the resulting mixture and the red crystals were collected by filtration. The
product was recrystallized from ethanol. Yield: 75 %. IR (KBr) ʋ/cm-1: 3300–2600 (OH
broad), 3042 (C–H aromatic), 1605, 1579 (C=C aromatic, C=N thiazole), 2943 (C–H
1
aliphatic), 1057 (benzothiazole), 1135 (C–O). H-NMR (DMSO-d6) δ/ppm: 1.39 (t, 3H, –
CH3), 4.1 (q, 2H, –OCH2), 7.01 (d, 2H, ArH), 7.1 (d, 1H, ArH), 7.6 (d, 1H, ArH), 7.8 (d, 2H,
Ar), 8.0 (s, 1H, ArH).
2.3.2. Synthesis of 4-((6-ethoxybenzothiazole-2-yl)diazenyl)bromo-pentanoxyphenyl (2a)
The mixture consisting of compound 1 (1.0 g, 3.34 mmol) in 150 ml dry acetone,
potassium carbonate (K2CO3) (4.61 g, 33.4 mmol), a catalytic amount of potassium iodide
(100 mg), and a five-fold excess of 1,5-dibromopentane (3.82 g, 16.7 mmol) was refluxed for
24 h. K2CO3 was removed by filtration while it was still hot and the acetone was removed
under reduced pressure. The product was recrystallized three times from ethanol whereupon
the pure compound was isolated giving orange solid. Yield: 60 %. IR (KBr) ʋ/cm-1: 3071 (C-
H aromatic), 2973, 2860 (C-H aliphatic), 1600 (C=C aromatic), 1579 (C=N thiazole), 1264
(Ar-O-R ether), 1052 (C-S-C thiazole). 1H-NMR (CDCl3) δ/ppm: 1.47 (t, 3H, CH3–), 1.53–
1.55 (m, 2H, –(CH2)2–), 1.8–1.9 (m, 4H, – (CH2)2–), 3.46 (t, 2H, –CH2Br), 4.09–4.14 (m, 4H,
(CH2)O–), 7.01 (d, 2H, Ar–H), 7.08 (d-d, 1H, Ar–H), 7.26 (d, 1H, Ar–H), 7.9 (s, 1H, Ar–H),
8.0 (d, 2H, Ar–H).
2.3.3. Synthesis 2,3,6,7,10,11-Hexamethoxytriphenylene (3)
A solution of 1,2-dimethoxybenzene (10 g, 72.4 mmol) in dichloromethane (50 ml)
was added dropwise to a suspension of anhydrous FeCl3 (35.22 g, 217.2 mmol) in
dichloromethane (100 ml) and concentrated sulphuric acid (0.5 ml). After complete addition
(15 min), the reaction mixture was further stirred for 3 h at room temperature. 200 ml of
methanol were then slowly added under vigorous stirring. The obtained mixture was further
stirred for additional 30 min. and the precipitate was filtered off, washed with methanol (5 x

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1
100 ml) and dried under reduced pressure to give a purple solid. Yield: 80 %, H-NMR
(CDCl3) δ/ppm: 4.10 (s, 18H, OCH3), 7.80 (s, 6H, ArH)
2.3.4. Synthesis 2,3,6,7,10,11-Hexahydroxytriphenylene (4)
The preparation of compound 4 is similar to that reported in the literature [14].
Hexamethoxytriphenylene (7 g, 17.14 mmol) was dissolved in dichoromethane (50 ml) and
the solution thus obtained was cooled in ice bath. A solution of boron tribromide (11.47 ml)
was then added slowly to the reaction mixture over a period of 30 min. After complete
addition, the reaction was left stirring overnight in room temperature. The reaction mixture
was then slowly poured into crushed ice (100 g) and the obtained mixture was stirred
vigorously until the ice melted. It was then extracted with ethyl acetate (6 x 150 ml), dried
over anhydrous sodium sulphate and evaporated to dryness giving dark purple solid. The
yield was quantitative and this compound was used without further purification.
2.3.5. Synthesis of compound 5a
Similar procedure with equal amount of molar ratio of intermediate 4 was used to
synthesize the derivatives 5a-5f. Derivative 5a is chosen as a representative compound for
detailed description. In round bottom flask, compound 4 (0.038 g, 0.12 mmol) was dissolved
in 50 ml of N-N-dimethylformamide. K2CO3 (0.15 g, 1.2 mmol) and a catalytic amount of
potassium iodide (50 mg) was added to the solution and the reaction mixture was heated at
refluxed for 2 h. Compound 2a (0.52 g, 1.2 mmol) was dissolved in minimal amount of N,N-
dimethylformamide and was subsequently added to the reaction mixture and refluxed at
100˚C for 120 h. The reaction was monitored by TLC. The resulting solution was poured into
the water (100 ml) whereupon the precipitate will form. The precipitate was filtered out and
was purified by silica gel column chromatography with chloroform as the mobile phase to
afford orange solid. Yield: 68 %. Elemental analysis: Calculated for C138H138N18O18S6: C
65.47, H 5.46, N 9.96%; found: C 65.72, H 5.58, N 10.09%. IR (KBr) ʋ/cm-1: 3065 (C-H
aromatic), 2938, 2868 (C-H aliphatic), 1594 (C=C aromatic), 1578 (C=N thiazole), 1249 (Ar-
1
O-R ether), 1055 (C-S-C thiazole). H-NMR (CDCl3) δ/ppm: 1.45 (t, 3H, CH3-), 1.72-1.79
(m, 2H, -(CH2)2-),1.9-2.0 (m, 4H, -(CH2)2-), 3.94-4.0 (m, 4H, CH2O-), 4.29 (t, 2H, CH2O-
Ar), 6.95 (d, 2H, Ar-H), 7.05 (d-d, 1H, Ar-H), 7.21 (d, 1H, Ar-H), 7.87 (s, 1H, Ar-H), 7.94
(d, 2H, Ar-H), 7.98 (s, 1H, Ar-H). 13C-NMR (CDCl3) δ/ppm: 173.88 (C=N thiazole), 163.39-

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104.96 (Ar-C), 69.43, 68.41 (O-CH2-CH2), 64.10 (O-CH2-CH3), 29.21-22.95 (-(CH2)3-),
14.77 (-CH3).
The physical results, IR, 1H- and 13C-NMR along with elemental analysis data for the
compound 5b-5f are shown in Sections 2.3.6-2.3.10.
2.3.6. Synthesis of compound 5b
Yield: 52 %. Elemental analysis: Calculated for C144H150N18O18S6: C 66.12, H 5.74, N
9.64%; found: C 66.41, H 5.85, N 9.78%. IR (KBr) ʋ/cm-1: 3068 (C-H aromatic), 2935, 2865
(C-H aliphatic), 1595 (C=C aromatic), 1578 (C=N thiazole), 1253 (Ar-O-R ether), 1056 (C-
1
S-C thiazole). H-NMR (CDCl3) δ/ppm: 1.45 (t, 3H, CH3), 1.65-1.75 (m, 4H, -(CH2)2-),1.85-
2.0 (m, 4H, -(CH2)2-), 4.04 (t, 2H, CH2O-), 4.06-4.11 (m, 2H, CH2O-), 4.26 (t, 2H, CH2O-
Ar), 6.95 (d, 2H, Ar-H), 7.06 (d-d, 1H, Ar-H), 7.22 (d, 1H, Ar-H), 7.86 (s, 1H, Ar-H), 7.95
(d, 2H, Ar-H), 7.98 (s, 1H, Ar-H). 13C-NMR (CDCl3) δ/ppm: 173.89 (C=N thiazole), 163.45-
104.97 (Ar-C), 69.50, 68.39 (O-CH2-CH2), 64.11 (O-CH2-CH3), 29.42-25.86 (-(CH2)4-),
14.77 (-CH3).
2.3.7. Synthesis of compound 5c
Yield: 63 %. Elemental analysis: Calculated for C150H162N18O18S6: C 66.73, H 6.01, N
9.34%; found: C 67.04, H 6.12, N 9.45%. IR (KBr) ʋ/cm-1: 3066 (C-H aromatic), 2931, 2863
(C-H aliphatic), 1595 (C=C aromatic), 1577 (C=N thiazole), 1252 (Ar-O-R ether), 1056 (C-
1
S-C thiazole). H-NMR (CDCl3) δ/ppm: 1.48 (t, 3H, CH3), 1.40-1.46 (m, 6H, -(CH2)2-),1.82-
1.96 (m, 4H, -(CH2)2-), 4.07 (t, 2H, CH2O-), 4.10-4.16 (m, 2H, CH2O-), 4.25 (t, 2H, CH2O-
Ar), 6.95 (d, 2H, Ar-H), 7.09 (d-d, 1H, Ar-H), 7.23 (d, 1H, Ar-H), 7.84 (s, 1H, Ar-H), 7.96
(d, 2H, Ar-H), 7.98 (s, 1H, Ar-H). 13C-NMR (CDCl3) δ/ppm: 173.90 (C=N thiazole), 163.49-
104.99 (Ar-C), 69.50, 68.43 (O-CH2-CH2), 64.11 (O-CH2-CH3), 29.51-25.65 (-(CH2)5-),
14.78 (-CH3).
2.3.8. Synthesis of compound 5d
Yield: 58 %. Elemental analysis: Calculated for C156H174N18O18S6: C 67.31, H 6.26, N
9.06%; found: C 67.55, H 6.41, N 9.15%. IR (KBr) ʋ/cm-1: 3070 (C-H aromatic), 2933, 2857
(C-H aliphatic), 1594 (C=C aromatic), 1578 (C=N thiazole), 1252 (Ar-O-R ether), 1055 (C-

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1
S-C thiazole). H-NMR (CDCl3) δ/ppm: 1.39 (t, 3H, CH3), 1.42-1.50 (m, 8H, -(CH2)2-),1.80-
1.98 (m, 4H, -(CH2)2-), 4.02 (t, 2H, CH2O-), 4.06-4.12 (m, 2H, CH2O-), 4.24 (t, 2H, CH2O-
Ar), 6.96 (d, 2H, Ar-H), 7.06 (d-d, 1H, Ar-H), 7.24 (d, 1H, Ar-H), 7.84 (s, 1H, Ar-H), 7.97
(d, 2H, Ar-H), 7.99 (s, 1H, Ar-H). 13C-NMR (CDCl3) δ/ppm: 173.92 (C=N thiazole), 163.55-
105.01 (Ar-C), 69.52, 68.43 (O-CH2-CH2), 64.12 (O-CH2-CH3), 29.81-25.35 (-(CH2)6-),
14.79 (-CH3).
2.3.9. Synthesis of compound 5e
Yield: 51 %. Elemental analysis: Calculated for C162H186N18O18S6: C 67.84, H 6.49, N
8.79%; found: C 68.12, H 6.62, N 8.89%. IR (KBr) ʋ/cm-1: 3071 (C-H aromatic), 2933, 2856
(C-H aliphatic), 1595 (C=C aromatic), 1577 (C=N thiazole), 1253 (Ar-O-R ether), 1056 (C-
S-C thiazole). 1H-NMR (CDCl3) δ/ppm: 1.47 (t, 3H, CH3), 1.32-1.41 (q, 10H, -(CH2)2-),1.78-
1.96 (m, 4H, -(CH2)2-), 4.03 (t, 2H, CH2O-), 4.05-4.14 (m, 2H, CH2O-), 4.23 (t, 2H, CH2O-
Ar), 6.95 (d, 2H, Ar-H), 7.05 (d-d, 1H, Ar-H), 7.22 (d, 1H, Ar-H), 7.88 (s, 1H, Ar-H), 7.96
(d, 2H, Ar-H), 7.99 (s, 1H, Ar-H). 13C-NMR (CDCl3) δ/ppm: 173.92 (C=N thiazole), 163.64-
105.03 (Ar-C), 69.54, 68.45 (O-CH2-CH2), 64.14 (O-CH2-CH3), 29.89-24.95 (-(CH2)7-),
14.79 (-CH3).
2.3.10. Synthesis of compound 5f
Yield: 69 %. Elemental analysis: Calculated for C168H198N18O18S6: C 68.36, H 6.71, N
8.54%; found: C 68.64, H 6.67, N 8.42%. IR (KBr) ʋ/cm-1: 3070 (C-H aromatic), 2923, 2852
(C-H aliphatic), 1595 (C=C aromatic), 1578 (C=N thiazole), 1252 (Ar-O-R ether), 1055 (C-
S-C thiazole). 1H-NMR (CDCl3) δ/ppm: 1.48 (t, 3H, CH3), 1.30-1.42 (q, 12H, -(CH2)2-),1.78-
1.98 (m, 4H,-(CH2)2-), 4.02 (t, 2H, CH2O-), 4.07-4.14 (m, 2H, CH2O-), 4.24 (t, 2H, CH2O-
Ar), 6.97 (d, 2H, Ar-H), 7.07 (d-d, 1H, Ar-H), 7.25 (d, 1H, Ar-H), 7.83 (s, 1H, Ar-H), 7.97
(d, 2H, Ar-H), 7.99 (s, 1H, Ar-H). 13C-NMR (CDCl3) δ/ppm: 173.92 (C=N thiazole), 163.69-
105.05 (Ar-C), 69.55, 68.47 (O-CH2-CH2), 64.15 (O-CH2-CH3), 29.95-24.85 (-(CH2)8-),
14.79 (-CH3).
3.
Result and Discussion
3.1
Mesomorphic behaviour

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The transition temperatures and associated enthalpy changes determined by
differential scanning calorimetry (DSC) for compounds 5a-5f are listed in Table 1. On
cooling the homologue 5a with n = 5 showed monotropic nematic phase. Compounds 5b and
5c displayed enantiotropic nematic phase. However, the homologues with the spacer length n
= 8-10 exhibit enantiotropic nematic phase and on cooling monotropic smectic C (SmC)
phase was observed. Observation on 5e under the polarized microscope upon cooling from its
isotropic liquid phase showed the presence of two mesophases. Upon cooling the isotropic
liquid of 5e, Schlieren texture (Figure 1(a)) of nematic phase appeared at 142˚C. Upon further
cooling, the emergence of the smectic C phase (Figure 1(b)) was observed at 112˚C.
Table 1. Phase transition temperature (˚C) and associated enthalpy (in kJ mol-1) (in brackets)
for compounds 5a-5f upon heating and cooling.
Compound
5a
Cr












SmC
N
I

215.3 (31.2)
147.4 (38.9)
128.2 (18.6)
56.8 (23.6)
133.6 (40.6)
71.3 (25.8)
136.2 (31.8)
90.8 (30.6)
139.2 (28.9)
65.5 (26.3)
127.7 (22.3)
51.3 (18.8)
o










210.4 (0.50)
184.7 (0.69)
180.3 (1.14)
189.7 (0.66)
172.6 (0.94)
178.4 (0.88)
166.7 (1.32)
173.8 (1.12)
156.2 (0.98)
166.4 (0.81)
159.5 (0.25)











5b
5c
5d
5e
5f
o
o
o
135.8 (1.12)
122.2 (0.64)
122.1 (0.49)
Cr = crystalline phase; SmC = smectic C phase; N = nematic phase; I = isotropic; • =
enantiotropic phase; ᴏ = monotropic phase
(b)
(a)
Figure 1. Optical photomicrographs of 5e during cooling cycle. On cooling from the isotropic
liquid, Schlieren texture (a) of nematic phase appeared at 142˚C. On further cooling, broken
fan-shaped texture of smectic C (b) observed at 112˚C.

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The DSC heating-cooling traces for representative compound 5f are given in Figure
2. Upon heating, two endothermic peaks were observed at 127.7˚C and 166.4˚C
with enthalpies ∆H =22.3 kJ mol-1 and 0.81 kJ mol-1, which represent the crystal-
nematic and nematic-isotropic transitions, respectively. However, on cooling three
exothermic peaks are observed at 159.5˚C, 122.1˚C and 51.3˚C, with respective
enthalpies values of 0.25 kJ mol-1, 0.49 kJ mol-1 and 18.8 kJ mol-1 assignable to the
isotropic-nematic, nematic-smectic C and smectic C-crystal transitions.
Figure 2. DSC thermograms of 5f upon heating and cooling.
The dependence of the transition temperatures on spacer length (n) for compounds 5a-5f is
shown in Figure 3. The melting point dropped from 5a to 5b and then gradually increased
from 5b to 5e. As the length of spacer increased, the melting point continued to decrease
from 5e (n = 9) to 5f (n = 10). However, the clearing temperature increased from 5b to 5c and
then descended when the length of carbon chain is increased. This observation could be due
to the dilution of the mesogenic core resulted from the flexibility of the spacer [15].

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Figure 3. Dependence of the transition temperatures on the number of carbon atoms linking
with the 2,3,6,7,10,11-hexahydroxytriphenylene comprising benzothiazole moieties with azo
linkage, n, in the compounds 5a-5f upon heating.
3.2
XRD analysis
XRD study has been carried out on a representative compound 5f (n=10) in order to
provide more detailed information on the liquid-crystalline phase structures of the rod-disc
shape mesogen. Based on XRD data of 5f, the presence of N phase is confirmed from the
XRD pattern at 140˚C (Figure 4) whereby no peak is recorded at small angle but a broad peak
is recorded in wide-angle region wherein 2θ=13.53˚, thus indicating a liquid-like order of N
phase. Figure 5 demonstrate a typical XRD pattern of the SmC phase exhibited by 5f. At
100˚C, an obvious sharp peak is shown in the low angle 2θ=0.763˚ (with the d-spacing value
of 7.703 nm), which could verify the tilted smectic layered structures of the SmC phase in
compound 5f. The broad peak at the wide angle of 2θ=13.82˚ corresponds to the average
distances between rigid rods of SmC phase, which should be irrelevant to their molecular
chain lengths [18]. The formation of smectic phase by rod-disc shaped mesogen is
presumably due to a side-by-side organization of azobenzene base peripheral units which lie
parallel to each other. The layer distance is a bit less than one half of the molecular length in
a conformation as shown in Figure 6 derived from MM2 computations of CS Chem Draw 3D
(version 5) program.

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7000
6000
5000
4000
3000
2000
1000
0
0
5
10
15
20
2 theta
Figure 4. The intensity versus 2θ profile for the X-ray diffraction pattern of compounds 5f at
140˚C during the cooling process.
6000
5000
4000
3000
2000
1000
0
0
5
10
15
20
2 theta
Figure 5. The intensity versus 2θ profile for the X-ray diffraction pattern of compounds 5f at
100˚C during the cooling process.
The aromatic rod-like cores are organized in common layers together with the disc-like
triphenylene cores. It can be deduced that the rod-like peripheral arms will determine the type
of mesophase because the alkyl spacer (n=5 until 10) linking the linearly benzothiazole azo
and triphenylene core is flexible enough to allow the pheripheral to rotate freely [18]. Hence,
the smectic and nematic layers are formed by these rod-like units and the triphenylene unit
acts only as linking unit interconnecting the rods. A related azobenzene triphenylene linked
by flexible chain (Figure 7) had been reported by Shimizu et al.

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Figure 6. Molecular model of compound 5b in which six rod-like benzothiazole azo as the
peripheral units are connected to a triphenylene core
Figure 7. The structure of compound 2,3,6,7,10,11-hexakis(4-n-decyloxyphenylazophenyl-
4-propylenecarboxy)triphenylene, reported by Shimizu et al [12].
In the present compounds 5a-5f, the triphenylene core and peripheral benzothiazole moieties
with azo unit are linked by more flexible spacers. This system allows conformational change.
Thus, the rod-shape segment could easily align in parallel arrangement resulting in layered
structure characteristics of smectic phase.

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3.3
Absorption spectra
The preliminary studies of the photochemical properties were carried out on the
compounds 5a-5f consisting of azo –N=N- linkage. The UV-Visible absorption spectra for all
the compounds are depicted in Figure 8 and the absorption maximums are listed in Table 9.
The absorption spectra for all the six compounds are found to be almost identical. This can be
attributed to the fact that the compounds possess almost the same molecular structures with
only the variation of the length of methylene unit -(CH2)n-, where n=5-10. One of the
remarkable trend that we can infer from the absorption spectra is that for the peaks which
appeared in high-intensity at 427.50-430.50 nm can be associated with the π-π* transition of
the trans-configuration of azobenzene. This observation complies with the previously
reported rod-disc shaped liquid crystals containing triphenylene and azobenzene with trans-
configuration of which the absorption fell within the range from 392 to 460 nm [16].
Figure 8. UV-Visible absorption spectra of compound 5a-5f in chloroform
Table 9. UV-Visible results of compounds 5a-5f
Absorbance, nm
Compounds
5a (n=5)
5b (n=6)
5c (n=7)
5d (n=8)
5e (n=9)
5f (n=10)
λmax
427.50
429.00
430.00
431.50
431.50
430.00

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4.
Conclusion
A new series which consists of both discotic and calamatic bearing a triphenylene as central
core and six rod-like benzothiazole azo as the peripheral units was prepared. The combination
of calamitic and discotic units led to a nematic phase although the molecule is disc-shaped. It
seems that the triphenylene unit acts only as a linking unit, interconnecting the rod-like units
(oligomer effect) rather than as a disc-like unit, which would lead to a columnar organisation.
The homologue with n = 5 displayed monotropic nematic phase. However, the higher
homologues (n = 8 to 10) exhibited nematic phase and upon cooling showed monotropic
smectic C phase.
Acknowledgement
Main author (G.-Y. Yeap) would like to thank Universiti Sains Malaysia for providing the
research facilities and partial financial support through Research University RU Grant No.
1001/PKIMIA/811223.
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Highlights





Newly derived triphenylene armed by peripheral azo-bridged benzothiazole-phenyl ethers.
The member with shorter spacer (n=5) exhibits predominantly monotropic N phase.
The members with longer spcaer (n=8-10) show enantiotropic N and monotropic SmC phase.
Central core of triphenylene acts as a linking unit rather than as a disc-like entity.
XRD data reveals the presence of smectic C without any discotic columnar phase