Design, synthesis, and self-assembly behavior of C3-symmetry discotic molecules via click chemistry

Article abstract of DOI:10.1039/b921685e

We prepared a series of C₃-symmetry discogens with three 1,2,3-triazoles and a central benzene ring by a copper-catalyzed "click reaction" between 1,3,5-triethynylbenzene and 3,4,5-trialkoxybenzyl azides. According to the differential scanning

Full text of DOI:10.1039/b921685e

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Design, synthesis, and self-assembly behavior of C3-symmetry discotic
molecules via click chemistry
*
Mi-Hee Ryu, Jin-Woo Choi and Byoung-Ki Cho
Received 19th October 2009, Accepted 7th December 2009
First published as an Advance Article on the web 15th January 2010
DOI: 10.1039/b921685e
We prepared a series of C3-symmetry discogens with three 1,2,3-triazoles and a central benzene ring by
a copper-catalyzed ‘‘click reaction’’ between 1,3,5-triethynylbenzene and 3,4,5-trialkoxybenzyl azides.
According to the differential scanning calorimetry (DSC) data, compounds with octyl and dodecyl
peripheries showed stable liquid crystalline (LC) phases at room temperature, and upon heating
ꢀ
underwent an isotropization at 105.3 C anꢀd 92.2 ꢀC, respectively. Compound with octadecyl
ꢀ
peripheries melted into a LC phase at 63.5 C, and changed into a disordered liquid at 84.4 C. As
characterized by optical polarized microscopy (POM) and X-ray scattering techniques, all LC phases
revealed hexagonally packed columns, the axes of which are aligned vertically on the glass substrate.
Interestingly, the experimentally accessible crystalline phase of the compound with octadecyl chains
kept the 2-D hexagonal columnar structure, although the interdisc distance within the columns changed
at the transition from the LC to crystalline phase.
discotic molecules consist of the conjugated aromatic core con-
Introduction
taining three 1,2,3-triazoles connected to the central benzene
ring, and to this disc-type mesogen, three 3,4,5-trialkoxybenzyl
units are linked. It is interesting note that the three heterocylic
triazoles are formed by one-step click chemistry from non-
mesogenic azide and alkyne units, thus the extension to the dis-
cotic mesogen can be easily accomplished. In this article, we
report the synthesis, and self-assembling behavior of discotic
molecules A-C, as characterized by optical polarized microscopy
(POM) and X-ray scattering experiments.
Discotic liquid crystals, referred as discogens, consisting of a flat
aromatic core and flexible peripheral chains are well-known to
form columnar mesophases by the stacking of anisotropic disc-
like cores. The self-assembling characteristic of discogens has
been attracting growing interest because it is able to provide
a one-dimensional conducting pathway for electrons, photons or
energy via the large p-overlap of adjacent aromatic cores.1 In this
regard, it would be interesting to prepare novel discotic liquid
crystals, and investigate their mesomorphic behavior.
Click chemistry, i.e., a copper catalyzed [2 + 3] cycloaddition
reaction between organic azides and alkynes, has been increas-
ingly used in various chemistry fields, due to its outstanding
merits such as high conversion and no side products.2 Recently,
click chemistry has become of great importance to macromo-
lecular chemistry, because a number of research groups have
employed this chemistry in the preparation of block copolymers
and cyclic, side chain and dendritic polymers.3
Synthesis
The synthesis began with the alkylation of propyl gallate with
octyl, dodecyl, and octadecyl bromides for 2a-c, respectively.
Subsequent reduction, bromination/or chlorination, and azida-
tion reactions yielded azide-terminated precursors (4a-c) for
a click reaction (Scheme 1(a)).5 During these reactions, benzyl
halide intermediates seemed to be labile during the silica column
chromatographies, thus without performing the complete
Meanwhile, click chemistry could be a powerful tool for the
design of liquid crystal (LC) molecules, because the formed
heterocyclic 1,2,3-triazole has a planar structure and high
chemical/thermal stability. Thus, by direct conjugation with
other aromatic rings, a molecular anisotropy (a pre-requisite for
LC formation) can be achieved. Nevertheless, only a few LC
molecules created through the use of click chemistry have been
reported, and all of them were calamitic molecules showing
smectic or nematic LC phases.4 Particularly, as far as we know,
there is no discotic LC example based upon the efficient click
chemistry.
With this in mind, we designed C3-symmetry discotic mole-
cules A-C assisted by click chemistry. As shown in Fig. 1, the
Department of Chemistry, Dankook University, Gyeonggi-Do, 448-701,
Korea. E-mail: chobk@dankook.ac.kr; Fax: + 82 31 8005 3148; Tel:
+ 82 31 8005 3153
Fig. 1 Design concept via click chemistry, and molecular structures of
A-C with the extended triazole-based disc.
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Scheme 1 Synthesis of (a) the azide precursor, and (b) the alkyne
precursor for click reaction.
purification of them, the azidation reaction with sodium azide
was implemented. The aromatic core precursor, 1,3,5-trie-
thylnylbenzene (7), was prepared by the Sonogashira coupling
reaction of 1,3,5-triiodobenzene (5) with 2-methylbut-3-yn-2-ol,
and subsequent deprotection of compound 6 (Scheme 1(b)).6 The
final discotic molecules were obtained by a click reaction of the
obtained azide and alkyne precursors using Cu(I)Br and
2,20-bipyridine as catalysts, as shown in Fig. 1.7
The discotic molecules were characterized by 1H- and 13C-NMR
spectroscopies, elemental analysis, and gel permeation chroma-
tography. The 1H-NMR spectrum of compound A displayed
a signal for the protons in the triazoles at 7.86 ppm, and other
aromatic protons and benzyl methylene protons could be assigned
as might be expected (Fig. 2(a)). In addition, eight distinct carbon
signals appeared in the aromatic region of the 13C-NMR spectrum
of A (Fig. 2(b)), which corroborates the formation of the designed
extended aromatic disc. All discotic molecules showed narrow
polydispersities (Mw/Mn) of 1.01 in gel permeation chromatog-
raphy (GPC) data, indicative of high purities (Fig. 2(c)).
Fig. 2 (a) 1H- and (b) 13C-NMR spectra of A, and (c) the GPC elugrams
of A-C.
Thermotropic behavior
The phase transitions of A-C were detected by differential
scanning calorimetry (DSC) and POM observations. Fig. 3
represents the second heating and first cooling DSC thermo-
grams of A-C in a temperature range of ꢁ60 ꢀC–120 ꢀC.
Increasing the length of the alkyl peripheries leads to a gradual
decrease of the LC-to-liquid phase transition temperature. On
the other hand, the crystal (K)-to-LC phase transition tempera-
ture steeply increases as the alkyl chain length increases.
Compounds A and B showed stable LC phases at room
temperature, and particularly, the former did noꢀt crystallize even
Fig. 3 DSC traces of A-C. K ¼ crystalline, Col ¼ hexagonal columnar, I
¼ isotropic liquid. Temperature is given in ꢀC, and the values in the
parentheses are the corresponding enthalpy changes (kJ molꢁ1).
ꢀ
at ꢁ60 C, and its LC phase persisted to 105.3 C.
To identify the LC phases, we observed the POM textures of
A-C in the melt, as represented in Fig. 4(a)–(c). On cooling from
the isotropic liquid to the LC phase, they all showed similar
optical textures with fan-like or dendritic domains in the dark
background. These optical textures are typically found in the
hexagonal columnar LC phase of discotic liquid crystals.8
Interestingly, on further cooling, the dark background did not
become birefringent in the LC phases, suggesting that the
columns occupying the background prefer the orientation
perpendicular to the glass substrate.
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angle X-ray scattering (SAXS) data of A-C. Compound A at
25 ꢀC exhibited three reflections with q-spacing ratios of 1:O3:O4,
which can be interpreted as a 2-dimensional hexagonal columnar
structure with the intercolumnar distance (a) of 3.05 nm.
However, compund B displayed only one strong reflection at
2.04 nmꢁ1, with which the structure was not accurately assigned.
Thus, to identify it, we plotted the intercolumnar distances (a) as
a function of [molecular weight]1/2. As observed in Fig. 5(b),
the plot showed a linear dependence and the extrapolation to
a ¼ 0 gave [molecular weight]1/2 ¼ 0.11 This suggested that the LC
phase of B analogous to the hexagonal structure shown in A.
Together with the pseudo-focal conic texture of B from the POM
observation, it was concluded that the LC phase of B is a hex-
gaonal columnar structure.
Likewise, compound C in the LC phase at 65 ꢀC showed
a hexagonal columnar structure, the intercolumnar distance (a)
of which is estimated to be ꢀ4.10 nm. Interestingly, on cooling to
the crystalline phase at 25 C, the SAXS pattern did not alter,
indicating that the hexagonal ordering is maintained over the
entire ordered state, which is consistent with the POM observa-
tions. This supramolecular structural retention in the crystalline
phase is particularly important when considering the optical and
electrical applications of discotic molecules.12
Fig. 4 The optical textures of (a) A at 80 ꢀC, (b) B at 70 ꢀC, and (c,d) C
at 70 ꢀC and 40 ꢀC.
Compound C has an experimentally accessible crystalline
phase, thus we investigated the variation of the optical texture at
the transition from the LC to crystalline phase. Upon the crys-
tallization, the dark areas became birefringent, which confirms
the homeotropic alignment of columns in the dark area between
two glass, as observed in A and B. Several discotic LC molecules
containing heteroatoms on aromatic cores/or side chains have
been reported to have a strong tendency to form such a homeo-
tropic alignment, and the driving force of which might be the
polar interaction of the heteroatoms with the glass surface.9 On
the other hand, the fan-like domains in the hexagonal columnar
LC phase still persisted in the crystalline phase (Fig. 4(d)). This
suggests that the supramolecular hexagonal ordering is kept at
the phase transition.10
Along with the SAXS investigation in the crystalline and LC
phases of compound C, we performed wide angle X-ray scat-
tering (WAXS) experiments to examine the interdisc distance in
each phase (Fig. 6(a)). According to the WAXS data in the
crystalline phase, discs within a column were interpreted to stack
ꢀ
with a periodic distance of 4.0 A which is due to the p–p stacking
of aromatic cores. On the other hand, only a diffused halo at
ꢀ
4.5 A was detected in the LC phase. This dimension is charac-
teristic of the liquid-like aliphatic chains.13 From these WAXS
results, we speculated that this variation of interdisc distance
upon melting may significantly influence the intercolumnar
distance determined from the SAXS data. In this regard, we
examined the principal d-spacing, d(10), of compound C as
a function of temperature (Fig. 6(b)). In the crystalline phase
(K), d(10) increased as temperature increased. This can be eluci-
dated by the fact that upon heating the amorphous region of
X-Ray scattering characterization
In order to characterize the microstructural details, we carried
out X-ray scattering experiments. Fig. 5(a) represents the small
Fig. 5 (a) Small angle X-ray scattering (SAXS) data of A-C, and (b) the
dependence of the intercolumnar distance (a) upon M1/2 (M ¼ molecular
weight).
Fig. 6 (a) Wide angle X-ray scattering (WAXS) data of C at various
temperatures and (b) the dependence of principal d-spacing, d(10), upon
temperature for compound C.
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semi-crystalline octadecyl chains 2-dimensionally expands by
ꢀ
keeping the constant interdisc distance of 4.0 A. In contrast to
Synthesis
Analogous/or idendtical compounds to 2a-c, 3a-c, 6 and 7 were
synthesized previously, and the related experimental data were
described elsewhere.5d,6b Here, we presented the synthetic
procedures and spectroscopic data of 4a-c and the final
compounds (A-C).
the crystalline phase, d(10) in the LC phase decreased with
increasing temperature. This might be because the interdisc
distance is not fixed in the molten state, but it may be increased
by flexible alkyl chains which tend to have less stretched
conformations at higher temperatures.14 Consequently, it can be
said that in the LC phase, the intercolumnar distance decreases
as the interdisc distance increases along the columnar axis.
Synthesis of 3,4,5-trialkoxybenzyl-N3 (4a-c)
Although 4a and 4b-c were mediated via bromination and
chlorination, respectively, the synthesis procedures for 4a-c were
almost identical, thus 4a is described here as a representative
example. To a stirred solution of 3,4,5-trioctyloxybenzyl alcohol
(3a) (5.3 g, 10.8 mmol) in dichloromethane (20 mL) was added
a solution of PBr3 (5.8 g, 21.5 mmol) in dichloromethane
(21.5 mL) at 0 ꢀC. The reaction mixture was stirred 3 h at room
temperature, and then quenched with water. The mixture was
extracted with dichloromethane and deionized water three times,
and then dried over MgSO4. Without further purification, the
brominated compound (2.7 g, 4.9 mmol) and sodium azide (1.6 g,
24.3 mmol) were dissolved in 5 mL of anhydrous DMF. The
reaction mixture was heated at 100 ꢀC for 3 days under N2
atmosphere. The reaction mixture was extracted with dichloro-
methane and deionized water three times, and then dried over
MgSO4. After removing dichloromethane in a rotary evaporator,
the resulting mixture was purified by a silica gel column chro-
matography to hexane–ethyl acetate ¼ 20 : 1 as the eluent, to
yield 1.84 g (73.1%). 1H-NMR (CDCl3, d, ppm): 6.48 (s, Ar–H),
4.24 (s, CH2N3), 3.91–4.00 (m, CH2OAr), 1.70–1.84 (m,
CH2CH2OAr), 1.29–1.46 (m, CH3(CH2)5CH3), 0.88 (t, J ¼
6.2 Hz, (CH2)7CH3).
Conclusions
A series of C3-symmetry discotic molecules A-C with octyl,
dodecyl and octadecyl chains, respectively, were synthesized with
the aid of an efficient click reaction. By performing the click
reaction once, the extension to a conjugate discotic mesogen
consisting of three 1,2,3-triazoles and a central benzene ring was
easily achieved. Their thermotropic properties were character-
ized by DSC technique. As the peripheral alkyl chain was elon-
gated, the isotropization temperature decreased, while the crystal
melting temperature increased. Discotic molecules A-C exhibited
stable LC phases on both heating and cooling scans. The
observed LC phases were revealed as 2-D hexagonal columnar
structures. Interestingly, A-C all showed a strong tendency to
align homeotropically on the glass substrate, which may be
potentially utilized for pattern-applications on distinctly chemi-
cally modified surfaces.15 Unlike A and B, the crystalline phase of
C was experimentally accessible, and thus characterized by
SAXS and WAXS techniques. Interestingly, the crystalline phase
maintained the 2-D hexagonal columnar structure like its LC
phase, although the interdisc distance varied at the transition
from LC to crystalline phase. It should be noted that the LC
molecules in this study, to the best of our knowledge, are the first
example to apply the ‘‘click chemistry’’ for the preparation of
‘‘discotic LC’’ materials showing columnar assemblies.
Currently, structural modifications using the triazole-based dis-
cotic motif shown in this study are in progress for various
materials applications.
3,4,5-Tridodecyloxybenzyl-N3 (4b). Yield: 75.1% 1H-NMR
(CDCl3, d, ppm): 6.48 (s, Ar–H), 4.24 (s, CH2N3), 3.91–4.00
(m, CH2OAr), 1.67–1.83 (m, CH2CH2OAr), 1.26–1.46
(m, CH3(CH2)5CH3), 0.88 (t, J ¼ 6.4 Hz, (CH2)7CH3).
3,4,5-Trioctadecyloxybenzyl-N3 (4c). Yield: 69.7% 1H-NMR
(CDCl3, d, ppm): 6.48 (s, Ar–H), 4.24 (s, CH2N3), 3.91–4.00
(m, CH2OAr), 1.67–1.83 (m, CH2CH2OAr), 1.26–1.46
(m, CH3(CH2)5CH3), 0.88 (t, J ¼ 6.6 Hz, (CH2)7CH3).
Experimental
General methods
1H-, and 13C-NMR spectra were recorded from a CDCl3 solution
using Varian 200 and Bruker AM 500 spectrometers. The purity
of the products was checked by thin-layer chromatography
(TLC; Merck, silica gel 60). Gel permeation chromatography
(GPC) measurements were conducted in THF and N,N0-dime-
thylacetamide (99.9%) (98 : 2 volume ratio) using a Waters 401
instrument equipped with Stragel HR 2,3 columns and Shodex
AT-8045 at a flow rate of 1.0 mL minꢁ1. X-Ray scattering
measurements were performed in transmission mode with
synchrotron radiation at the 10C1 beam line of the Pohang
Accelerator Laboratory (PAL), Korea. The sample was held in
an aluminium sample holder with films on both sides. Micro-
analyses were performed with a Perkin Elmer 240 elemental
analyzer at the Organic Chemistry Research Center, Sogang
University, Korea. The compounds were purified by column
chromatography (silica gel) and prep-HPLC (Japan Analytical
Instrument).
Synthesis of dicotic molecules (A-C)
B and C were prepared in a similar manner, thus A is described
here as a representative example. 1,3,5-Triethynylbenzene (7)
(0.12 g, 0.8 mmol), azide-functionalized compound (4) (1.32 g,
2.56 mmol), 2,20-dipyridyl (1.25 g, 7.99 mmol) and copper(I)-
bromide (0.92 g, 6.39 mmol) were dissolved in 5 mL of anhydrous
THF. The reaction mixture was degassed three times by freeze-
pump-thaw cycles. The mixture was heated at 50 ꢀC for two days
with stirring. The mixture was extracted with deionized water
and dichloromethane. The solvent was removed by a rotary
evaporator, and the remaining azide-functionalized compound
(4) was removed by the silica gel column chromatography using
dichloromethane–ethyl acetate ¼ 8 : 1 as the eluent, and the
crude product was then purified by a HPLC (Japan Analytical
Industry), to yield 0.88 g (64.7%). 1H-NMR (500 MHz, CDCl3, d,
ppm): 8.24 (s, Ar-H), 7.86 (s, H-triazole), 6.52 (s, benzyl-H), 5.46
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(s,
ArCH2),
3.94–3.98
(m,
CH2OAr),
1.74–1.82
(m, CH2CH2OAr), 1.44–1.50 (m, CH2(CH2)4CH3), 1.28–1.34
(m, CH2(CH2)4CH3), 0.88 (t, J ¼ 11.5 Hz, (CH2)7CH3).
13C-NMR (125 MHz, CDCl3, d, ppm): 153.96, 147.6, 138.9,
132.0, 129.3, 122.4, 120.3, 107.0, 73.7, 69.5, 55.0, 32.1, 32.0, 30.6,
29.8, 29.6, 29.5, 26.3, 22.9, 22.9, 14.3. Anal. Calcd for
C105H171N9O9: C, 74.03; H, 10.12; N, 7.40, Found: C, 73.97; H,
ꢁ
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10.17; N, 7.60. Mw/Mn ¼ 1.01 (GPC).
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Discotic molecule B. Yield: 48.0%. 1H-NMR (500 MHz,
CDCl3, d, ppm): 8.26 (s, Ar–H), 7.88 (s, H-triazole), 6.52 (s,
benzyl-H), 5.46 (s, ArCH2), 3.94–3.98 (m, CH2OAr), 1.72–1.82
(m, CH2CH2OAr), 1.44–1.50 (m, CH2(CH2)8CH3), 1.26–1.34 (m,
CH2(CH2)8CH3), 0.88 (t, J ¼ 7.0 Hz, (CH2)7CH3). 13C-NMR
(125 MHz, CDCl3, d, ppm): 153.7, 147.4, 138.6, 131.5, 129.0,
122.3, 120.3, 106.7, 73.5, 69.2, 54.9, 32.0, 31.9, 30.4, 29.78, 29.76,
29.71, 29.68, 29.66, 29.47, 29.41, 29.38, 26.14, 26.11, 22.7, 14.12.
Anal. Calcd for C141H243N9O9: C, 76.68; H, 11.09; N, 5.71,
Found: C, 76.76; H, 10.99; N, 5.40. Mw/Mn ¼ 1.01 (GPC).
6 (a) C. Mechtler, M. Zirngast, W. Gaderbauer, A. Wallner,
J. Baumgartner and C. Marschner, J. Organomet. Chem., 2006, 691,
Discotic molecules C. Yield: 38.0%. 1H-NMR (500 MHz,
CDCl3, d, ppm): 8.24 (s, Ar–H), 7.86 (s, H-triazole), 6.52 (s,
benzyl-H), 5.46 (s, ArCH2), 3.94–3.98 (m, CH2OAr), 1.74–1.82
(m, CH2CH2OAr), 1.44–1.50 (m, CH2(CH2)8CH3), 1.26–1.32 (m,
CH2(CH2)14CH3), 0.89 (t, J ¼ 7.0 Hz, (CH2)7CH3). 13C-NMR
(125 MHz, CDCl3, d, ppm): 153.7, 147.4, 138.6, 131.7, 129.0,
122.2, 120.1, 106.7, 73.5, 69.2, 54.8, 31.9, 30.4, 29.80, 29.78,
29.74, 29.68, 29.67, 29.48, 29.41, 29.38, 26.14, 26.12, 22.7, 14.1.
Anal. Calcd for C195H351N9O9: C, 78.97; H, 11.93; N, 4.25,
Found: C, 78.98; H, 12.23; N, 4.13. Mw/Mn ¼ 1.01 (GPC).
´
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Acknowledgements
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This work was supported by Basic Science Research Program
(2009-0070798) from the National Research Foundation (NRF),
Core Research Program (2009-0084501) from the NRF funded by
the Korean government, and the GRRC program (2009-A01) of
Gyeonggi province. We thank the Pohang Accelarator Labora-
tory (Beamline 10C1), Korea, for use of synchrotron radiation.
€
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