Synthesis of triphenylene discotic liquid crystals possessing nine alkyl chains: Influence of molecular symmetry and chain length on mesomorphism

Article abstract of DOI:10.1080/15421406.2013.781489

A series of new triphenylene discotic liquid crystals has been synthesized by a straightway and the influence of molecular symmetry and chain length on the mesomorphism investigated. The symmetrical compounds 1, 3,7,11- trimethoxytriphenylene-2,6,10-triyl tris(3,4,5-tris(alkyloxy)benzoate), with alkyl chain lengths of C7, C8, C10, and C12, and the unsymmetrical compounds 2 were studied by using polarized optical microscopy (POM), differential scanning calorimetry (DSC) and by wide-angle X-ray diffraction (XRD). The results showed that the symmetrical compounds 1a (C7) and 1b (C8) exhibit a plastic columnar mesophase, whereas the other compounds display a hexagonal columnar mesophase over a wide temperature range. All symmetrical compounds 1 display higher isotropization temperatures than that of the unsymmetrical 2. The clearing points are lowered as the alkyl chain lengthened.

Full text of DOI:10.1080/15421406.2013.781489

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Synthesis of Triphenylene Discotic Liquid
Crystals Possessing Nine Alkyl Chains:
Influence of Molecular Symmetry and
Chain Length on Mesomorphism
Bo Xiao a , Zhao-Jun Liu a , Bi-Qin Wang a , Ping Hu a , Carl Redshaw a
b & Ke-Qing Zhao a
a College of Chemistry and Materials Science , Sichuan Normal
University , Chengdu , China
b Department of Chemistry , The University of Hull , Hull , HU6
Published online: 09 Jul 2013.
To cite this article: Bo Xiao , Zhao-Jun Liu , Bi-Qin Wang , Ping Hu , Carl Redshaw & Ke-Qing Zhao
(2013) Synthesis of Triphenylene Discotic Liquid Crystals Possessing Nine Alkyl Chains: Influence of
Molecular Symmetry and Chain Length on Mesomorphism, Molecular Crystals and Liquid Crystals,
577:1, 25-35, DOI: 10.1080/15421406.2013.781489
To link to this article: http://dx.doi.org/10.1080/15421406.2013.781489
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Mol. Cryst. Liq. Cryst., Vol. 577: pp. 25–35, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.781489
Synthesis of Triphenylene Discotic Liquid Crystals
Possessing Nine Alkyl Chains: Influence
of Molecular Symmetry and Chain Length
on Mesomorphism
BO XIAO,1 ZHAO-JUN LIU,1 BI-QIN WANG,1 PING HU,1
CARL REDSHAW,1,2 AND KE-QING ZHAO1,∗
1College of Chemistry and Materials Science, Sichuan Normal University,
Chengdu, China
2Department of Chemistry, The University of Hull, Hull, HU6
A series of new triphenylene discotic liquid crystals has been synthesized by a straight-
way and the influence of molecular symmetry and chain length on the mesomorphism
investigated. The symmetrical compounds 1, 3,7,11-trimethoxytriphenylene-2,6,10-triyl
tris(3,4,5-tris(alkyloxy)benzoate), with alkyl chain lengths of C7, C8, C10, and C12, and
the unsymmetrical compounds 2 were studied by using polarized optical microscopy
(POM), differential scanning calorimetry (DSC) and by wide-angle X-ray diffraction
(XRD). The results showed that the symmetrical compounds 1a (C7) and 1b (C8) ex-
hibit a plastic columnar mesophase, whereas the other compounds display a hexagonal
columnar mesophase over a wide temperature range. All symmetrical compounds 1 dis-
play higher isotropization temperatures than that of the unsymmetrical 2. The clearing
points are lowered as the alkyl chain lengthened.
Keywords Discotic liquid crystals; hexagonal columnar mesophase; molecular sym-
metry; triphenylene; plastic columnar phase.
1. Introduction
Liquid crystals (LCs) have emerged as a new kind of organic semiconductor [1–5], which
when compared with established inorganic semiconductors show favorable features in terms
of cost, solubility, device fabrication, charge carrier mobility, and tunability of physical
properties by molecular design.
Discotic LCs (DLCs) self-organize into a columnar mesophase [6], and charged carriers
hop favorably along the column axis [7], and the highest charge mobility rate for DLCs
has reached 1.5 cm2/v s [8]. Furthermore, DLCs have been widely investigated as active
materials in organic field effect transistors [9,10], organic light-emitting diodes [11], and
organic photovoltaic solar cells [12–19].
∗Address correspondence to Ke-Qing Zhao, College of Chemistry and Materials Science,
Sichuan Normal University, Chengdu 610066, China. Tel.: +86-28-84764743; Fax: +86-28-
84764743. E-mail: kqzhao@sicnu.edu.cn
25

26
B. Xiao et al.
Triphenylene derivatives are regarded as one of the most important DLCs [20–25] and
have been commercially used as optical compensation films for widening the view an-
gle of LC displays [2]. 2,3,6,7,10,11-Hexakis(alkyloxy)triphenylene displays a hexagonal
columnar (Colh) mesophase with low melting points and clearing points, whereas triph-
enylene with six ester chains show higher phase transition temperatures; however, both
exhibit narrow mesophase ranges for practical applications. Bayer et al. [26] proposed that
triphenylene DLCs with ether and ester mixed tails would have improved mesophase and
physical properties. It was noted that the subtle balance between the aromatic core size and
the peripheral alkyl chains was an important factor in determining the type of mesophase,
the phase transition temperature, and also the stability of the mesophase.
Recently, we have reported DLCs as molecular templates [27–31] to construct ordered
2-dimentional frameworks, which host different number of functional molecules, such as
porphyrin, phthalocyanine, and fullerene. These molecular frameworks are controlled by
the factors of shape of molecule, symmetry of molecules, the size of rigid core, and the
length of peripheral flexible chain.
In this report, we have investigated the liquid crystalline properties of the above-
mentioned triphenylene DLC template molecules. Compared with hexakis(alkyloxy)
triphenylene, these compounds contain a larger rigid core for columnar mesophase sta-
bilization, and nine flexible peripheral chains for lowering the melting point; therefore,
wide mesophase temperature ranges would be expected. We highlight the simplified syn-
thesis and separation of the eight new DLC 1 and 2, and discuss the influence of molecular
symmetry and chain length on mesomorphism. Scheme 1 shows the synthesis of the targeted
DLCs.
2. Experimental
2.1. General
1
1
Triphenylene DLCs were characterized using H NMR and IR spectroscopy. H NMR
spectra were recorded using a 400 MHz Varian spectrometer with Me4Si as an internal
standard. IR spectra were obtained with KBr pellets on a BRUKER VERTEX 70 FT-IR
spectrometer. The microanalysis was carried out on a Thermal Flash 2000 instrument.
The phase transition temperatures and enthalpies were measured by a differential scanning
calorimeter (DSC, TA Instrument, 2920MDSC), and the optical textures of the mesophases
were observed and recorded on a polarizing optical microscope (POM, Olympus, BH-2),
with a hot stage (Mettler, FP80HT) and controller (Mettler, FP80HT). X-ray diffraction
(XRD) studies were carried out on a PHILIPS 3 KW X-ray diffractometer.
The reagents and solvents were commercial products and were used without further
purification.
2.2. Synthesis
2.2.1. 2,6,10-Trimethoxyltriphenylene-3,7,11-triol, 3 and 2,6,11-Trimethoxyl-3,7,10-trih
ydroxy-triphenylene, 4. A mixture of 2-methoxyphenol (5.0 g, 40.3 mmol), concentrated
H2SO4 (2 drops), FeCl3 (20.0 g, 123.1 mmol), and CH2Cl2 (50 mL) was stirred for 45
minutes. Then cold CH3OH (30 mL) was carefully added and further stirred for 1 hour.
The solid precipitate was collected by filtration and washed with cold ethanol and wa-
ter. The mixture of 2,6,10-trimethoxyltriphenylene-3,7,11-triol, 3 and 2,6,11-trimethoxyl
triphenylene-3,7,10-triol, 4 was prepared in a total yield of 40%.

Synthesis of Triphenylene Discotic Liquid Crystals
27
R
O
H
O
H
O
H
3CO
H
3CO
l
COC
R
O
H
OC
H
O
e
H
O
1 F Cl CH Cl
,
3
( )
3
2
2
5
R
O
co
e
2
( )
ld M OH
H
O
H
OC
H
OC
3
3
NEt3
H
l
C
2C 2
,
H
O
H
O
3
4
H
OC
H
OC
3
3
m x ure
i t
R
O
R
O
R
O
R
O
R
O
R
O
O
O
O
O
H
3CO
H
3CO
O
H
OC
O
3
R
O
O
O
O
O
C
R
O
R
R
O
R
R
R
R
H
3
O
O
O
O
O
R
O
H
OC
H
OC
3
32
O
O
O
1
R
O
R
O
R
O
uns mme r ca com oun
t i
l
d
s mme r ca com oun
t i
y
p
l
d
y
p
oo so u
l bilit
an se ara e
co umn c roma o ra
d
d
t d b
l
h
t
h
p y
g
y
p
y
g
=
R
H15
H17
H21
H25
C12
,
C7
C8
C10
,
,
a
a
c
c
1
1b
2b
1
1d
2
2
2d
Scheme 1. The synthesis of triphenylene DLCs with nine alkyl chains.
2.2.2. The Synthesis of 3,4,5-Tris(alkoxy)benzoyl Chloride. A mixture of 3,4,5-
trihydroxybenzoic acid (50 g, 294.1 mmol), MeOH (200 mL), and H2SO4 (5 mL) was
refluxed for overnight. Then the cooled mixture was poured into ice water and extracted
with CH2Cl2. The organic extract was washed with 5% aqueous Na2CO3 solution and
water, dried, and organic solvent was removed. Crystallization from toluene resulted in
methyl 3,4,5-trihydroxybenzoate as a white solid with a yield of 83%.
The mixture of methyl 3,4,5-trihydroxybenzoate (2 g, 11.1 mmol), K2CO3, and 1-
bromododecane (9.5 g, 38.2 mmol) in ethanol was refluxed for 24 hours. Methyl 3,4,5-
tris(dodecyloxy)benzoate was collected from ethanol as a white solid with a yield of 80%.
Methyl 3,4,5-tris(dodecyloxy)benzoate (2 g, 2.9 mmol) was hydrolyzed by using KOH
(1 g, 17.9 mmol) in ethanol (95%, 20 mL). 3,4,5-Tris(dodecyloxy)benzoic acid was col-
lected as a white solid after acidification and recrystallization from ethanol with a yield of
90%.
3,4,5-Tris(dodecyloxy)benzoic acid (1.1 g, 1.6 mmol) was refluxed with excess thionyl
chloride (5 mL) and N,N-dimethylformamide (1 drop) for 5 hours. After the reaction fin-
ished, the excess thionyl chloride was distilled, and 3,4,5-tris(dodecyloxy)benzoyl chloride
was ready for use.
2.2.3. 3,7,11-Trimethoxytriphenylene-2,6,10-triyl tris(3,4,5-tris(dodecyloxy)benzoate),
1d, and 3,7,10-Trimethoxytriphenylene-2,6,11-triyl tris(3,4,5-tris(dodecyloxy)benzoate),

28
B. Xiao et al.
2d. A mixture of 3,7,11-trimethoxyltriphenylene-2,6,10-triol (3) and 3,7,10-tri-
methoxyltriphenylene-2,6,11-triol (4) (0.15 g, 0.4 mmol) was dissolved in CH2Cl2 (30 mL)
and Et3N (0.3 g, 3.0 mmol) was added, and a solution of 3,4,5-tris(dodecyloxy)benzoyl
chloride (1.13 g, 1.6 mmol) in CH2Cl2 was added dropwise at 0◦C. After stirring for
2 hours, the reaction mixture was refluxed for 5 hours, then poured into aqueous HCl
(5%, 20 mL), and extracted with CH2Cl2. The organic extract was dried over anhydrous
magnesium sulfate and concentrated in vacuum. The residue was separated by column
chromatography (SiO2), eluted with a mixture of solvents (CH2Cl2:petroleum ether = 3:1,
V/V) followed by crystallization in ethyl acetate resulting in 1d as a white solid with a
yield of 6% and 2d as a white solid with a yield of 80%.
The symmetrical compounds 1a, 1b, and 1c and asymmetrical compounds 2a, 2b, and
2c were prepared using the method described above.
1a, 3,7,11-Trimethoxytriphenylene-2,6,10-triyl tris-(3,4,5-tris(heptyloxy)benzoate):
1
White solid 35 mg, yield 5%, spectral data: H NMR (CDCl3, TMS, 400 Hz, δ):8.23
(s, 3H, ArH), 7.71–7.72 (m, 3H, ArH), 7.26–7.32 (m, 6H, ArH), 3.95–4.03 (m, 27H,
ArOCH2, ArOCH3), 1.79–1.81 (m, 18H, CH2), 1.32–1.54 (m, 72H, CH2), 0.89 (t, J =
6.40 Hz, 27H, CH3). IR (KBr) ν: 3101, 2956, 2928, 2857, 1723, 1625, 1588, 1500, 1468,
1451, 1430, 1385, 1338, 1269, 1114, 1045, 940, 843, 723 cm−1
.
2a, 3,7,10-Trimethoxytriphenylene-2,6,11-triyl tris-(3,4,5-tris(heptyloxy)benzoate):
1
White solid 584 mg, yield 83%, spectral data: H NMR (CDCl3, TMS, 400 Hz, δ):8.27
(s, 1H, ArH), 7.85–7.94 (m, 4H, ArH), 7.70–7.75 (m, 1H, ArH), 7.36–7.47 (m, 6H,
ArH), 3.96–4.06 (m, 27H, ArOCH2, ArOCH3), 1.73–1.82 (m, 18H, CH2), 1.31–1.56 (m,
72H, CH2), 0.89 (t, J = 2.00 Hz, 27H, CH3). IR (KBr) ν: 3101, 2955, 2928, 2856,
1728, 1623, 1587, 1517, 1501, 1468, 1430, 1380, 1337, 1268, 1115, 1048, 938, 840,
723 cm−1
.
1b, 3,7,11-Trimethoxytriphenylene-2,6,10-triyl tris-(3,4,5-tris(octyloxy)benzoate):
1
White solid 55 mg, yield 7%, spectral data: H NMR (CDCl3, TMS, 400 Hz, δ):8.23
(s, 3H, ArH), 7.65–7.68 (m, 4H, ArH), 7.26–7.31 (m, 5H, ArH), 3.95–4.03 (m, 27H,
ArOCH2, ArOCH3), 1.75–1.80 (m, 18H, CH2), 1.31–1.56 (m, 90H, CH2), 0.86–0.91 (m,
27H, CH3). IR(KBr) ν: 3101, 2958, 2926, 2855, 1722, 1625, 1588, 1520, 1500, 1468, 1452,
1430, 1386, 1338, 1267, 1112, 1045, 948, 843, 722 cm−1
.
2b, 3,7,10-Trimethoxytriphenylene-2,6,11-triyl tris-(3,4,5-tris(octyloxy)benzoate):
White solid 692 mg, yield 92%, spectral data: 1H NMR (CDCl3, TMS, 400 Hz, δ):8.27 (s,
1H, ArH), 7.67–7.88 (m, 5H, ArH), 7.26–7.47 (m, 6H, ArH), 3.95–4.06 (m, 27H, ArOCH2,
ArOCH3), 1.73–1.82(m, 18H, CH2), 1.30–1.57(m, 90H, CH2), 0.88(t, J = 4.00 Hz, 27H,
CH3). IR (KBr) ν: 3102, 2955, 2925, 2855, 1727, 1623, 1587, 1518, 1501, 1468, 1430,
1381, 1337, 1269, 1115, 1048, 949, 804, 722 cm−1
.
1c, 3,7,11-Trimethoxytriphenylene-2,6,10-triyl tris-(3,4,5-tris(decyloxy)benzoate):
1
White solid 43 mg, yield 5%, spectral data: H NMR (CDCl3, TMS, 400 Hz, δ):8.23(s,
3H, ArH), 7.61–7.64(m, 3H, ArH), 7.26–7.52 (m, 6H, ArH), 3.77–4.02 (m, 27H, ArOCH2,
ArOCH3), 1.74–1.79 (m, 18H, CH2), 1.28–1.57 (m, 126H, CH2), 0.85–0.91 (m, 27H, CH3).
IR (KBr) ν: 3101, 2955, 2924, 2853, 1722, 1625, 1588, 1521, 1500, 1468, 1452, 1431,
1388, 1338, 1269, 1116, 1045, 948, 843, 721 cm−1
.
2c, 3,7,10-Trimethoxytriphenylene-2,6,11-triyl tris-(3,4,5-tris(decyloxy)benzoate):
1
White solid 593 mg, yield 69%, spectral data: H NMR (CDCl3, TMS, 400 Hz, δ):8.27
(s, 1H, ArH), 7.85–7.90 (m, 4H, ArH), 7.68–7.73 (m, 1H, ArH), 7.36–7.42 (m, 6H, ArH),
3.96–4.06 (m, 27H, ArOCH2, ArOCH3), 1.76–1.82 (m, 18H, CH2), 1.27–1.56 (m, 126H,
CH2), 0.88(d, J = 5.60 Hz, 27H, CH3). IR (KBr) ν: 2924, 2853, 1722, 1623, 1586, 1518,
1501, 1468, 1430, 1385, 1337, 1268, 1116, 1049, 948, 833, 721 cm−1
.

Synthesis of Triphenylene Discotic Liquid Crystals
29
Figure 1. Polarising photomicrographs of triphenylene discogens. (a) 1a, Colp, cooling to 270◦C
from isotropic liquid; (b) 2a, Colh, cooling to 200◦C from isotropic liquid; (c) 1b, Colp, cooling
to 200◦C from isotropic liquid; (d) 2b, Colh, cooling to 180◦C from isotropic liquid; (e) 1d, Colhd,
cooling to 190◦C from isotropic liquid; (f) 1d, Colho, cooling to 153◦C from Colhd.
1d, 3,7,11-Trimethoxytriphenylene-2,6,10-triyl tris-(3,4,5-tris(dodecyloxy)benzoate):
White solid 62 mg, yield 6%, spectral data: 1H NMR (CDCl3, TMS, 400 Hz, δ) δ:8.23 (s,
3H, ArH), 7.52–7.61 (m, 3H, ArH), 7.26–7.42 (m, 6H, ArH), 3.93–4.02 (m, 27H, ArOCH2,
ArOCH3), 1.77–1.79 (m, 18H, CH2), 1.25–1.59 (m, 162H, CH2), 0.86(t, J = 6.80Hz, 27H,
CH3). IR (KBr) ν: 2923, 2853, 1723, 1624, 1588, 1520, 1500, 1467, 1431, 1389, 1337,

30
B. Xiao et al.
A
B
315 oC
Colp
Iso
310 oC
Colp
Colh
1a
1a
Iso
Iso
231oC
2a
1b
2b
Iso
Colh
Iso
230oC
2a
1b
289oC
Colp
Colh
Iso
Colp
291 oC
213oC
Iso
Iso
Iso
Colh
2b
1c
2c
211 oC
1c
2c
230 oC
Iso
Colh
Colh
Iso
227 oC
Iso
o
Colh
177 C
Colh
o
Cry
176 oC
Colho
9 C
Col
hd Iso
Cry
4oC
1d
198oC
o
169 C
Colhd
Colho
Iso
1d
o
o
o
157 oC
Cry
195 C
Colh 139 C
-21 C
Cry
2d
-2 oC
Iso
Colh
Iso
137 oC
First cooling
2d
Second heating
Temperature (oC)
Figure 2. The DSC traces of triphenylene DLCs 1a–2d.
Temperature (oC)
1268, 1118, 1044, 947, 844, 721 cm−1. Anal. Calcd. for C150H246O18: C 77.07, H 10.61;
found C 78.06, H 10.64.
2d, 3,7,10-Trimethoxytriphenylene-2,6,11-triyl tris-(3,4,5-tris(dodecyloxy)benzoate):
1
White solid 768 mg, yield 80%, spectral data: H NMR (CDCl3, TMS, 400 Hz, δ)
δ:8.28 (s, 1H, ArH), 7.87–8.00 (m, 4H, ArH), 7.72–7.73 (m, 1H, ArH), 7.36–7.43 (m,
6H, ArH), 3.96–4.06 (m, 27H, ArOCH2, ArOCH3), 1.74–1.82 (m, 18H, CH2), 1.25–1.59
(m, 162H, CH2), 0.85–0.88 (m, 27H, CH3). IR (KBr) ν: 2923, 2853, 1727, 1624, 1587,
1518, 1501, 1467, 1430, 1385, 1337, 1270, 1116, 1048, 950, 884, 721 cm−1. Anal. Calcd.
for C150H246O18: C 77.07, H 10.61; found C 78.16, H 10.89.
3. Results and Discussion
3.1. Synthesis and Characterization
The one-pot syntheses of 3, 2,6,10-trimethoxytriphenylene-3,7,11-triol and 4, 2,6,11-
trimethoxytriphenylene-3,7,10-triol were achieved by the direct oxidation of 2-
methoxyphenol with FeCl3 in CH2Cl2 followed by treatment with cold MeOH. The mixture
of 3 and 4 displayed low solubility in common organic solvents and it was difficult to sepa-
rate them from each other, and thus they were used directly in the subsequent esterification
reaction as a mixture.

Synthesis of Triphenylene Discotic Liquid Crystals
31
Table 1. Thermotropic liquid crystalline properties of triphenylene DLCs 1a–2d
Mesophases, transition temperature and enthalpy changes
Second heating /◦C (ꢀH, First cooling / ◦C (ꢀH,
Compd
(Cn)
kJ .mol−1 kJ .mol−1
)
)
1a
2a
1b
2b
1c
2c
1d
(C7)
(C7)
(C8)
Colp 315 (68.06) Iso
Colh 231 (22.46) Iso
Colp 289 (37.70) Iso
Colh 213 (20.26) Iso
Colh 230 (17.86) Iso
Colh 177 (13.34) Iso
Cry 9 (68.62) Colho169
(9.03) Colhd198 (2.96) Iso
Cry −21(57.25) Colh 139
(8.96) Iso
Iso 310 (43.37) Colp
Iso 230 (20.67) Colh
Iso 291 (40.19) Colp
Iso 211 (17.86) Colh
Iso 227 (19.30) Colh
Iso 176 (13.19) Colh
Iso195 (3.04) Colhd157
(7.54) Colho4 (72.12) Cry
Iso 137 (8.88) Colh
−2(42.48) Cry
(C8)
(C10)
(C10)
(C12)
2d
(C12)
Cry, crystal state; Col, discotic columnar phase; Iso, isotropic liquid.
3,4,5-Tris(alkoxy)benzoyl chloride, 5 was prepared by starting from 3,4,5-
trihydroxybenzoic acid through an acid catalyzed esterification in MeOH. Specifically,
the Williamson ether synthesis was employed, namely, the reaction of RBr with phenol in
the presence of K2CO3, hydrolysis of the ester to acid, and finally transformation of the
acid to acid chloride by the reaction with SOCl2.
Target compounds 1 and 2 were synthesized by the reaction of 3 and 4 with 5 in the
presence of Et3N. The mixture of 1 and 2 showed good solubility in common organic
solvents, and they were separated by column chromatography and collected in the pure
form. However, we note that the yields of the symmetrical compounds were much lower
than those of the unsymmetrical compounds. 1a–2d were characterized by using 400 MHz
1H NMR and IR spectroscopy, and two compounds were further characterized by using
microanalysis.
A
315
115.72
B
120
100
80
60
40
20
0
sym
asym col iso
col iso
sym
col iso
asym col iso
289
300
250
200
150
100
50
231
230
213
198
67.06
177
139
44.55
41.67
35.14
29.63
11.99
1d
8.96
0
1a
2a
1b
2b
1c
2c
1d
2d
1a
2a
1b
2b
1c
2c
2d
Compound
Compound
Figure 3. A, Phase diagram of triphenylene discogens 1a–2d from Col phase to Iso. B, Entropy
change of triphenylene discogens 1a–2d from Col to Iso. (entropy change ꢀS = ꢀH/T).

32
B. Xiao et al.
Figure 4. X-ray diffraction patterns of three triphenylene DLCs measured at room temperature.
A, 2c (unsym., C10); B, 2d (unsym., C12); C, 1d (sym., C12).
3.2. Mesomorphism
3.2.1. POM Results. Initially, we measured the liquid crystalline properties of the nine-
chain triphenylenes by using POM with a heating plate and temperature controller, and part
of the photomicrographs is shown in Fig. 1. It was noted that the symmetrical compounds
1a (sym. C7) and 1b (sym. C8) with shorter chain length displayed the typical textures
of a plastic columnar mesophase (Colp), whereas all other compounds exhibited textures
associated with a discotic columnar mesophase (Col). Interestingly, we found that 1d (sym.
C12) exhibited homeotropic alignment behavior on the untreated glass plate by slowly
cooling from its isotropic liquid state. It is known that good homeotropic molecular self-
assembly behavior of DLC is a prerequisite for application in organic electronic devices,
such as OLEDs and OPVs.
3.2.2. DSC Results. The mesomorphism of the synthesized triphenylene DLCs was further
studied by using DSC. The DSC traces of all compounds are displayed in Fig. 2, and the
resulting data are summarized in Table 1. It was noted that for the longer chain DLC
1d (sym. C12) and 2d (unsym. C12), both melting point and isotropization temperature
were tested, whereas for the shorter chain DLCs, only isotropization temperatures were
measured. Interestingly, 1d exhibited two enantiotropic mesophases, which are assigned
as the disordered hexagonal columnar mesophase (Colhd) at high temperature zone and the

Synthesis of Triphenylene Discotic Liquid Crystals
33
ordered hexagonal columnar mesophase (Colho) at low temperature; the latter was further
confirmed by using XRD at room temperature.
We further summarized the correlation between clearing point temperatures and the
alkyl chain length, see Fig. 3(A), and entropy change (ꢀSCol→Iso) with chain length into
Fig. 3(B). Figure 3(A) clearly shows that series 1 (sym.) has higher clearing points than
does series 2 (unsym.), whereas Fig. 3(B) reveals that series 1 had higher entropy change
values for the phase transition from Col. to Iso. than does series 2. Based on these results,
we conclude that the symmetrical DLCs (series 1) have more ordered molecular columnar
stacking than does the unsymmetrical DLCs (series 2).
3.2.3. XRD Results. The mesophases of three DLCs, namely, 2c (unsym. C10), 2d (unsym.
C12), and 1c (sym. C12) were further investigated by using XRD, and the results are shown
in Fig. 4 and Table 2. The XRD diffraction patterns of the three compounds show that they
present as Colho at room temperature. At the small angle region, there is a strong peak
correlated to (100) and weak peaks of (200), (210), (300), and (220) or (310). At the wide
angle region, the broad halo at 4.3 Å–4.4 Å suggests that the flexible alkyl chains are in a
molten state. The small peak at 3.3 Å–3.4 Å demonstrates that the discotic molecules stack
on top of each other in a weak ordered form in the columns.
Table 2. Miller indices and lattice parameters for Colh mesophase of three DLCs
Compound
dobs(Å)a
hklb
dcalcd(Å)a
Lattice parameter
28.8
14.6
11.0
9.7
8.3
4.3
(100)
(200)
(210)
(300)
(220)
28.8
14.4
10.9
9.6
a = 33.3 Å
2c (unsym. C10)
2d (unsym. C12)
8.3
broad halo
(001)
3.4
32.6
16.1
12.3
10.6
9.1
(100)
(210)
(300)
(220)
32.6
16.3
12.3
10.8
9.0
a = 37.6 Å
a = 36.6 Å
(310)
4.4
3.4
broad halo
(001)
1d (sym. C12)
31.7
16.0
12.3
10.7
8.9
(100)
(200)
(210)
(300)
31.7
15.8
12.0
10.6
8.8
(310)
4.4
3.3
broad halo
(001)
adobs and dcalcd denote the observed and the calculated diffraction spacing, respectively. bhkl are the
Miller indices of the 2D hexagonal lattice [7–9].

34
B. Xiao et al.
4. Conclusions
In summary, two series of triphenylene DLCs 1 and 2 have been synthesized and success-
fully separated by column chromatography. All eight triphenylene derivatives displayed
wide plastic or Col mesophase temperature ranges. 1a (C7) and 1b (C8) exhibited a plastic
columnar phase (Colp), whereas the other compounds exhibited a Colh phase. The symmet-
rical compounds (series 1) displayed higher isotropization temperatures and more stable Col
phases than did the unsymmetrical compounds (series 2). 1d exhibits the two enantiotropic
mesophases Colho and Colhd. We anticipate that these new DLCs, possessing homeotropic
alignment behavior at nanometer to micrometer scale, have potential application in organic
solar cells as active additives.
Acknowledgements
This research was supported by the National Natural Science Foundation of China (Contract
Nos 50973076, 51073112 and 51273133).
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