Novel triphenylenoimidazole discotic liquid crystals

Article abstract of DOI:10.1016/j.tetlet.2011.08.041

A novel discotic core was constructed by fusing imidazole unit with well-known triphenylene discotic core. Two new imidazole fused unsymmetrically substituted triphenylene derivatives were prepared and characterized. While the molecular structures of the new compounds were verified by ¹H NMR, UV, MS and elemental analysis, their liquid crystalline properties were determined by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction studies. These triphenylenoimidazole derivatives were found to exhibit hexagonal columnar mesomorphism over a wide temperature range.

Full text of DOI:10.1016/j.tetlet.2011.08.041

Tetrahedron Letters 52 (2011) 5363–5367
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journal homepage: www.elsevier.com/locate/tetlet
Novel triphenylenoimidazole discotic liquid crystals
⇑
Sandeep Kumar , Satyam Kumar Gupta
Raman Research Institute, C. V. Raman Avenue, Sadashivanagar, Bangalore 560 080, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel discotic core was constructed by fusing imidazole unit with well-known triphenylene discotic
core. Two new imidazole fused unsymmetrically substituted triphenylene derivatives were prepared
and characterized. While the molecular structures of the new compounds were verified by 1H NMR,
UV, MS and elemental analysis, their liquid crystalline properties were determined by polarizing optical
microscopy, differential scanning calorimetry and X-ray diffraction studies. These triphenylenoimidazole
derivatives were found to exhibit hexagonal columnar mesomorphism over a wide temperature range.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 15 June 2011
Revised 4 August 2011
Accepted 7 August 2011
Available online 16 August 2011
Keywords:
Discotic liquid crystal
Triphenylenoimidazole
Triphenylene
Imidazole
Columnar phase
One of the largest and most important classes of molecules that
exhibit columnar liquid crystal phases is represented by triphenyl-
ene derivatives. Triphenylenes are, to date, the most widely
explored materials out of all discotic mesogens since the triphenyl-
ene core is easily accessible through recognized synthetic routes and
is excellent at granting mesomorphic properties.1 Research over the
past decade has demonstrated the potential of these and other
related discotic materials as active components in organic semicon-
ductor devices such as LEDs, solar cells, field effect transistors, etc.2
Hexasubstituted triphenylene derivatives are the most broadly
examined discotic materials because they have a strong tendency
to form columnar mesophases, which are of interest in one-dimen-
sional energy and electron transport properties. Structure–property
relationships of triphenylene-based discotic mesomorphic materi-
als have been well exploredbecause of their considerable technolog-
ical importance.3
Molecular shape is one of the most imperative factors which
determines the self-assembly of molecules into mesomorphic
phases. Expansion of triphenylene ring is expected to lead signifi-
cant changes not only in mesomorphic properties but also in elec-
tronic properties. Such supramolecular macro discotics are
anticipated to be mesomorphic over broad temperature ranges4
and exhibit high charge carrier mobilities5 in their columnar
phases. Thus efforts have been made to enlarge the triphenylene
core to generate phenanthrophenazine,6 hexabenzotriphenylene,7
triphenylenophthalocyanine,8 etc., derivatives. On the other hand,
imidazole-based materials have a great prospective in the fields
of biological and materials sciences.9–15 Imidazole moiety is also
personalized for hydrogen bond formation which has played an
important role in the creation of supramolecular mesomorphic
aggregates.16 There have been reports of design and construction
of supramolecular mesomorphic assembly on the basis of imidaz-
ole moiety.17 Strong intermolecular hydrogen bonding resulted in
formation of supramolecular bilayers of smectogens by the assem-
bly of linear imidazole-containing Schiff’s bases.18 Kadkin et al.
reported smectic mesomorphism in imidazole-based biforked
amphiphiles.19 The structure of smectic phase had bilayered struc-
ture with a continuous network of hydrogen bonds. Imidazole has
also been used as ligand for complexation with metal to achieve
metallomesogens.18,20
However, most of the liquid crystalline materials based on
imidazole skeleton come from the category of ionic liquids; the
mesomorphic materials based on molecular imidazole derivatives
have been less investigated. Imidazole based bis(imidazole)-annu-
lated discotic terphenyl derivatives have been reported by Pisula et
al.21 Columnar order with helical stacking of disks within the
column was observed in these compounds. The isotropization tem-
perature was lowered on increasing the bulkiness of peripheral
alkyl chain. Very recently, the effect of extension of electron defi-
cient p-conjugated perylene core has been investigated in perylene
diester benzimidazole derivatives. Wicklein et al. prepared a series
of compounds in which one side of perylene ring had diester and
the anhydride ring of the second side was fused with benz-imidaz-
ole unit.22 Extension of
p-conjugation due to fusion with benz-
imidazole was confirmed by a red shift in absorption spectra as
compared to perylene bisimide. Imidazole group has been con-
nected via a single bond to tetraalkoxymonomethoxy triphenylene
by Kimura et al.23 In its pure form this compound was non-liquid
⇑
Corresponding author. Tel.: +91 80 23610122; fax: +91 80 23610492.
E-mail addresses: skumar@rri.res.in, skumar.sandeep@gmail.com (S. Kumar).
crystalline, while
a mixture of this triphenylene-imidazole
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.08.041

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S. Kumar, S. K. Gupta / Tetrahedron Letters 52 (2011) 5363–5367
compound with benzene-1,3,5-tricarboxylic acid in 1:3 molar ratio
displayed mesomorphism over a broad thermal range. On the basis
of lattice constant values and the approximate diameter of single
molecule, it was assumed that each disk of the column was
constructed from arrangement of three molecules, connected to
central benzene-1,3,5-tricarboxylic acid via hydrogen bonding.
Also long range intracolumnar order was verified by the presence
of a sharp peak in wide angel region.
In present investigation, we have looked over unsymmetrical
examples of imidazole-fused-triphenylene derivatives for the ef-
fect of extension of discotic core on the mesomorphic behavior.
Here, we report the extension of triphenylene ring with a nitrogen
containing heterocycle, that is, imidazole. We adopted this
enthalpy data obtained from the heating and cooling cycles of
DSC or POM are collected in Table 1. The peak temperatures are
given in °C and the numbers in parentheses indicate the transition
enthalpy (D
H) in kcal molꢀ1. Compound 5a was found to be liquid
crystalline from 110 to 154 °C on heating. On slow cooling from the
isotropic liquid, the microscopic preparation revealed a texture
which is typical of that of a hexagonal columnar mesophase with
large monodomains (Fig. 1a). On cooling, the crystallization was
confirmed by the slow appearance of small needle like domains
into the liquid crystal texture. It is notable that this transformation
occurred at a much lower temperature than the crystal-to-meso-
phase transformation determined on heating. The microscopic
results were in agreement with the DSC results. Two endothermic
peaks were observed during the first heating from ꢀ30 to 160 °C
(Fig. 2). The peak at 118.9 °C corresponds to the melting of the
crystalline phase into the mesophase, followed by a small one at
147.3 °C, associated to the clearing temperature. On first cooling
two exothermic peaks were seen, the first one at 143 °C corre-
sponds to isotropic-to-columnar transition and the second one at
30.6 °C. Further heating and cooling scans of the sample gave
almost similar thermal behavior. On the other hand, compound
5b exhibits fan-like optical texture (Fig. 1b) of a hexagonal colum-
nar mesophase upon cooling from the isotropic liquid (146 °C). No
sign of crystallization could be detected on cooling up to room
temperature; however, the material became very viscous at lower
temperature. Probably it vitrifies at certain temperature. Upon
heating, the material is shearable at about 116 °C and the DSC
approach to increase
p-electron conjugation as well as to reduce
overall symmetry of triphenylene ring. We discuss the synthesis,
characterization and mesomorphism of novel triphenylene-imid-
azole-fused mesogens. The number of peripheral alkyl chains
around triphenylene has been varied between 5 and 6. All the
newly synthesized compounds were characterized using spectral
techniques and elemental analysis. The mesophase behavior of
both compounds was investigated by polarizing optical micros-
copy and differential scanning calorimetry. The mesophase struc-
ture of these compounds was established with the help of X-ray
diffractometry.
The synthesis of triphenylenoimidazole derivatives 5 is outlined
in Scheme 1. Monohydroxytriphenylene 3 was prepared starting
from catechol as reported previously.24 Oxidation of the 2-hydro-
xy-3,6,7,10,11-pentakis(hexyloxy)triphenylene 3 with ceric ammo-
nium nitrate resulted in the formation of o-quinone derivatives, that
is, 3,6,7,10,11-pentakis(hexyloxy)triphenylene-1,2-diones 4.25 Cou-
pling of this o-quinone with ammonia and aldehyde in presence of
acetic acid under reflux condition produces the desired triphenyle-
noimidazoles 5.26 Appearance of a new red shifted peak at 371 nm
in UV clearly indicates the extension of triphenylene core. The chem-
ical structure and purity of the newly synthesized materials were
confirmed by spectral and elemental analysis.26
shows
a glass-like transition at 108.4 °C. On cooling, one
exothermic peak was detected at 140.9 °C corresponding to isotro-
pic-to-columnar phase transition. No other transition (such as
crystallization or glass transition) was seen down to ꢀ30 °C. It
may be noted that the melting as well as isotropization tempera-
ture of these compounds is much higher as compared to that
reported for hexahexyloxytriphenylene (HAT6).27 This behavior
can be explained in terms of increased ratio of rigid-to-flexible por-
tions in the mesogen due to extension of triphenylene ring into
triphenylene-imidazole ring without increasing the number or
length of peripheral alkyl chains. The possibility of intermolecular
H-bonded dimer formation, which can also enhance the melting
The thermal behavior of both the compounds was investigated
by polarizing optical microscopy (POM) and differential scanning
calorimetry (DSC). The transition temperature and associated
OC6H13
OC6H13
C6H13O
C6H13O
C6H13
O
OC6H13
OC6H13
OC6H13
OH
(i)
(ii)
C6H13O
1
C6H13
O
C6H13O
OC6H13
OC6H13
2
3
(iii)
OC6H13
OC6H13
C6H13O
C6H13O
OC6H13
OC6H13
O
(iv)
H
N
N
O
C6H13
O
C6H13O
R'
OC6H13
OC6H13
4
5a
5b
: R' = H
: R' = C6H13
Scheme 1. Synthesis of triphenylenoimidazole mesogens. Reagents: (i) VOCl3, CH2Cl2; (ii) catechol boron bromide, CH2Cl2; (iii) CAN, CH3CN (iv) R0CHO, CH3COONH4,
CH3COOH, reflux.

S. Kumar, S. K. Gupta / Tetrahedron Letters 52 (2011) 5363–5367
5365
Table 1
Phase transition temperatures (peak, °C) and associated enthalpy changes (kcal molꢀ1
compounds (see Scheme 1 for chemical structures)
,
in parentheses) of triphenylenoimidazole
First cooling scan
Compound
First heating scan
5a
5b
DSC
POM
Cr 118.9 (15.86) Colh 147.3 (0.34) I
Cr 110 Colh 154 I
I 143.0 (0.34) Colh 30.6 (9.32) Cr
I 151 Colh 51 Cr
DSC
POM
g 108.4 (0.17) Colh
g 116 Colh 146 I
I
I 140.9 (0.18) Colh
I 137 Colh
5a
32
24
50
100
Temperature/ C
150
Figure 2. DSC thermogram of 5a on heating and cooling cycles (scan rate
10 °C minꢀ1).
small angle region, two peaks were observed, one very strong
and second weak peak. Taken in the ascending order of diffraction
angle, the d-spacings of the first reflection (lowest angle and high-
p
est intensity) to the second one is in the ratio of 1:1/ 3. These
values correspond to those expected from a two-dimensional hex-
agonal lattice. In the wide angle region there is one broad peak at h
ꢁ10°. This broad peak with a d-spacing of ꢁ4.7 Å was due to the
liquid like packing of the aliphatic chains. The intercolumnar
distances, a, calculated by using the relation a = d10/(cos 30°),
where d10 is the spacing corresponding to the strongest peak in
the small angle region, was found to be 20.45 Å. All the features
fit into the well-known model for the Colh phase in which the
disc-like core of the molecules stack one on top of the other to form
columns surrounded by alkyl chains and these columns in turn ar-
range themselves in a two-dimensional hexagonal lattice. Figure 4
represents X-ray diffraction pattern and one-dimensional intensity
versus theta (h) graph of compound 5a at various temperatures
while heating, as may be seen from the figure that the pattern of
the compound has numerous sharp peaks in low and wide angle
at 100 °C. This indicates that the material is in crystalline phase
at this temperature. Whereas at higher temperatures (125,
140 °C) the material exhibited liquid crystalline behavior. This
behavior is in accordance with DSC and POM results.
Figure 1. Optical photomicrograph (a) of 5a at 138 °C (upper photograph, crossed
polarizers, magnification ꢂ200) and (b) of 5b at 130 °C on cooling from the isotropic
liquid (lower photograph, crossed polarizers, magnification ꢂ500).
point, was ruled out as the IR spectrum of 5a does not display any
H-bonded N–H absorption. Probably, the steric hindrance of the
long aliphatic peripheral chains around the core prevents the
molecule to form H-bonded dimer. The compound 5a crystallizes
at lower temperature, whereas 5b remains mesomorphic up to
room temperature. This indicates that attachment of additional
flexible hexyl chain to imidazole-fused triphenylene discotic sup-
presses crystallization and favors liquid crystallinity.
In order to reveal the mesophase structure and hence the supra-
molecular organization of these compounds, X-ray diffraction
experiments were carried out using unoriented samples. X-ray dif-
fraction patterns for compounds were recorded in the columnar
phase around 10 °C below the clearing temperature while cooling
from the isotropic phase. The X-ray diffraction patterns of the
mesophase exhibited by samples are supportive of a discotic hex-
agonal columnar arrangement. The X-ray diffraction pattern of
compound 5a and its one-dimensional intensity versus theta (h)
graph derived from the pattern are shown in the Figure 3. In the
Compound 5b shows similar XRD behavior. The intercolumnar
distance for 5b was found to be 21.33 Å. It is evident that as
number of alkyl chains increases the diameter of the cylindrical
columns formed by the discotic molecules also increases (Table
2). The intercolumnar distance of hexahexyloxytriphenylene
(H6TP) was found, as expected, in between of 5a and 5b.
In conclusion, the molecular architecture obtained by fusing
imidazole with triphenylene may be considered as a new discotic
core whose full potential has yet to be realized in discotic field.
The synthesis, characterization and mesomorphic behavior of

5366
S. Kumar, S. K. Gupta / Tetrahedron Letters 52 (2011) 5363–5367
Figure 3. X-ray diffraction pattern and intensity versus h profile of 5a at 130 °C.
100
125
140
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Shirai, H. Chem. Mater. 2010, 22, 5732.
Figure 4. Intensity versus h profile of 5a at 100, 125 and 140 °C while heating.
Table 2
Intercolumnar distances (Å) of 5 derived from their diffraction patterns
Compound
d-Spacing (Å)
Intercolumnar distance (Å)
5a
17.71
18.47
18.34
20.45
21.33
21.17
5b
H6TP27a
two novel triphenylenoimidazole derivatives are presented here.
One derivative 5a is having five peripheral hexyloxy chains around
the discotic core, while the other one 5b contains one additional
peripheral hexyl chain at the periphery. Liquid crystalline behavior
of these two imidazole fused triphenylene derivatives was
confirmed by polarizing optical microscopy and differential scan-
ning calorimetry. They exhibit columnar mesophase over a wide
range of temperature. Hexagonal columnar structure of the meso-
phase of these compounds was confirmed by X-ray diffraction
studies.
24. Kumar, S.; Manickam, M. Synthesis 1998, 1119.
25. Kumar, S.; Manickam, M.; Varshney, S. K.; Shankar Rao, D. S.; Prasad, S. K. J.
Mater. Chem. 2000, 10, 2483.
26. Ammonium ceric nitrate (405 mg, 0.739 mmol) was added to a stirred solution
of monohydroxytriphenylene 3 (500 mg, 0.671 mmol) in acetonitrile at room
temperature. The reaction mixture was stirred at room temperature for about
3 min. The product was separated from the solvent by normal filtration. The
residue was purified by column chromatography over silica gel (eluant: 10%
ethyl acetate in hexane) to yield 4 as a dark blue colored solid (332.8 mg,
65.33%).
A stirred mixture of 4 (332.8 mg, 0.438 mmol), formaldehyde
(0.0711 ml, 37 wt %), glacial acetic acid (2 ml) and ammonium acetate
(701.7 mg, 9.1 mmol) was refluxed for 4.5 h. After cooling the reaction
mixture was diluted with water. Concentrated aqueous ammonia solution
(sp. gr 0.91, 25–28 wt %) was added to neutralize the solution up to pH 7. A
brown precipitate was formed. It was filtered, washed with distilled water. The
residue was extracted with a mixture of chloroform and distilled water. The
organic extract was dried over anhydrous sodium sulfate, concentrated and the
product was purified by repeated column chromatography over silica gel
(eluant: 5% ethyl acetate in hexane). Solvent was then removed in rotary
evaporator. The residue left was now dissolved in dichloromethane and the
resulting solution was added to cold methanol to afford 5a as gray color solid
(90 mg, 27%). Similarly, 5b was obtained by reacting 4 with heptanal in 28%
yield. Selected data for compound 4: 1H NMR (400 MHz, CDCl3): d 8.96 (s, 1H),
References and notes
1. Kumar, S. Liq. Cryst. 2004, 31, 1037.
2. Kumar, S. Chem. Soc. Rev. 2006, 35, 83.
3. (a) Kumar, S. Chemistry of Discotic Liquid Crystals: From Monomers to Polymers;
CRC Press: Boca Raton, FL, 2011; (b) Kong, X.; He, Z.; Zhang, Y.; Mu, L.; Liang, C.;
Chen, B.; Jing, X.; Cammidge, A. N. Org. Lett. 2011, 13, 764; (c) Chen, H. -M.;
Zhao, K.-Q.; Wang, L.; Hu, P.; Wang, B.-Q. Soft Materials 2011, 9, 359; (d) Kaller,

S. Kumar, S. K. Gupta / Tetrahedron Letters 52 (2011) 5363–5367
5367
7.67 (d, 2H), 7.43 (s, 1H), 7.06 (s, 1H), 4.21 (m, 10H), 1.93 (m, 10H), 1.3–1.6 (m,
30H), 0.93 (m, 15H). UV–vis (CHCl3): kmax 271, 339, 383, 561 nm. Compound
5a: 1H NMR (400 MHz, CDCl3): d 8.78 (s, 1H), 8.31 (s, 1H), 7.8–8.0 (m, 5H), 4.49
(t, J = 6.6, 2H), 4.32 (t, J = 6.6, 2H), 4.26 (m, 6H), 1.96 (m, 10H), 1.2–1.7 (m, 30H),
0.94 (m, 15H). 13C NMR (100 MHz, CDCl3): d 150.8, 149.9, 149.3, 149.0, 148.7,
129.6, 128.5, 123.3, 123.1, 121.5, 110.1, 108.3, 107.1, 106.9, 101.9, 69.8, 69.6,
69.2, 31.7, 29.4, 25.9, 22.7, 14.0. UV–vis (CHCl3): kmax 282, 353, 371 nm. IR
(KBr): 3120, 2955, 2931, 2920, 2854, 1614, 1584, 1518, 1504, 1425, 1385, 1265,
1174, 925, 839 cmꢀ1. Elemental Anal. Calcd for C49H72N2O5: C, 76.52; H, 9.44;
N, 3.64. Found: C, 76.63; H, 9.64; N, 3.69. HRMS (Methanol+Chloroform, m/z):
769.55 (M)+, 770.55 (M+H)+, 771.56 (M+2H)+, 647.41, 643.38, 615.36, 409.19,
361.12, 306.19, 305.19, 187.13, 131.06. Compound 5b: 1H NMR (400 MHz,
CDCl3): d 8.79 (s, 1H), 7.8–7.9 (m, 5H), 4.46 (t, J = 6.8, 2H), 4.32 (t, J = 6.7, 2H),
4.26 (m, 6H), 3.13 (t, J = 7.8, 2H), 1.96 (m, 10H), 1.2–1.7 (m, 38H), 0.94 (m,
18H). UV–vis (CHCl3): kmax 283, 351, 369 nm. Elemental Anal. Calcd for
C55H84N2O5: C, 77.42; H, 9.92; N, 3.28. Found: C, 77.03; H, 10.17; N,
3.06.
27. (a) Paraschiv, I.; Delforterie, P.; Giesbers, M.; Posthumus, M. A.; Marcelis, A. T.
M.; Zuilhof, H.; Sudholter, E. J. R. Liq. Cryst. 2005, 32, 977; (b) Stackhouse, P. J.;
Hird, M. Liq. Cryst. 2008, 35, 597.