A hydrogen-bonded supramolecular hexagonal columnar liquid crystal composed of a tricarboxylic triphenylene and monopyridyl dendrons

Article abstract of DOI:10.1246/cl.2007.282

Tricarboxylic triphenylene (TPC5) and monopyridyl dendron (DenC12) were mixed in 1:2,1:3, and 1:4 molar ratios, and investigation by IR, DSC, and XRD studies proved that TPC5 and DenC12 self-assembled to form a hexagonal columnar liquid crystal with 1:3 m

Full text of DOI:10.1246/cl.2007.282

282
Chemistry Letters Vol.36, No.2 (2007)
A Hydrogen-bonded Supramolecular Hexagonal Columnar Liquid Crystal
Composed of a Tricarboxylic Triphenylene and Monopyridyl Dendrons
Shinsuke Ishihara, Yuusuke Furuki, and Shinji TakeokaÃ
Department of Applied Chemistry, Graduate School of Science and Engineering, Waseda University,
3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555
(Received October 13, 2006; CL-061205; E-mail: takeoka@waseda.jp)
Tricarboxylic triphenylene (TPC5) and monopyridyl
gen-bonding system, but also in rotational symmetry.
dendron (DenC12) were mixed in 1:2, 1:3, and 1:4 molar ratios,
and investigation by IR, DSC, and XRD studies proved that
TPC5 and DenC12 self-assembled to form a hexagonal colum-
nar liquid crystal with 1:3 molar stoichiometry via complemen-
tary hydrogen-bonds.
TPC5 was mixed with DenC12 in 1:2, 1:3, and 1:4 molar
ratios, and then the mixtures were dissolved in benzene/metha-
nol, heated at 40 ꢀC, and treated with sonication for 5 min. The
resulting homogeneous solutions were dried in vacuo, and the
complexes TPC5–DenC12 1/2, TPC5–DenC12 1/3, and
TPC5–DenC12 1/4 were obtained.
The formation of complementary hydrogen bonds between
the carboxylic acid in TPC5 and the pyridine in DenC12 was
confirmed by infrared (IR) spectrometry. The IR absorption of
TPC5 at 1729 cmÀ1 assigned to the C=O stretching vibration
mode of the free carboxylic acid decreased slightly after being
mixed with more than three equivalents of DenC12, at which
time an IR absorption at 1638 cmÀ1 assigned to the hydrogen-
bonded C=O stretching vibration mode subsequently appeared.9
In addition, the solubility of TPC5 in benzene was drastically
improved upon mixing with more than three equivalents of
DenC12, which supports that TPC5 forms a hydrogen-bonded
complex with three equivalents of DenC12, with the hydropho-
bic dodecyloxy chains directed toward the outside of the result-
ing disc-like assemblies.
The precise stoichiometry forming the most suitable hydro-
gen-bonded complex was confirmed by a differential scanning
calorimetry (DSC) analysis. The DSC thermogram in Figure 2
indicated that the components of TPC5–DenC12 1/3 were com-
pletely compatibilized, with the subsequent thermally reversible
mesophase appearing from 66.9 to 1.1 ꢀC upon cooling and from
7.2 to 76.7 ꢀC upon heating. The DSC thermogram of TPC5–
DenC12 1/2 in Figure 2 is almost similar to that of TPC5–
DenC12 1/3, because the pure TPC5 region due to the forma-
tion of the most appropriate 1:3 assembly had no thermal phase
transition from À20 to 140 ꢀC. On the other hand, TPC5–
DenC12 1/4 revealed an extra endothermal peak at 94.6 ꢀC,
Recently, uniform dispersion of low molecular weight com-
pounds at the center of an array of surrounding heterogeneous
molecules1 has been attracting attention in the fabrication of
supramolecular nanomaterials such as nanowires,2 nanorings,3
and nanoporous polymers.4 In particular, several reports have
focused on supramolecular columnar liquid crystals composed
of an aromatic core and surrounding fan-shaped molecules;5
however, derivatives of triphenylene,6 which have great poten-
tial in the development of optoelectric nanodevices, have re-
ceived little attention as core molecules.
In order to fabricate a supramolecular columnar liquid
crystal composed of a triphenylene core and surrounding
fan-shaped molecules, a symmetric structured triphenylene de-
rivative, 2,6,10-tris(carboxymethoxy)-3,7,11-tris(pentyloxy)tri-
phenylene (TPC5), was designed with three carboxymethoxy
groups for hydrogen-bonding and three pentyloxy groups to in-
crease compatibility with the surrounding fan-shaped molecules
(Figure 1). Incidentally, a symmetric-structured triphenylene is
considered promising as a core molecule, because the alternation
of the pentyloxy group in TPC5 with a chiral alkoxy group,
a semi-fluoroalkoxy group, or a stimuli-responsive side chain
will lead to amplification of the core molecule’s properties
throughout the entire supramolecular columnar liquid crystal.
As a complementary companion for TPC5, the fan-shaped
dendron 3,5-bis(3,4-bisdodecyloxybenzyloxy)-N-(pyridin-4-yl)-
benzamide (DenC12) was prepared, having a pyridyl group for
hydrogen-bonding between the carboxylic acid groups in
TPC57 (Figure 1). DenC12 belongs to a class of second gener-
ation dendrons and has a wide, flat structure, and the ꢀ value of
the dendron, assigned to the number of molecules per disc of the
column, is estimated to be 3 or 4, considering the structurally
similar dendrons reported by Percec et al.8 Therefore, TPC5
and DenC12 are quite complementary, not only in the hydro-
(a)10
(b) 10
9
8
7
6
5
4
3
2
1
0
9
8
7
6
5
4
3
2
1
0
DenC12
TPC5
TPC5-DenC12 1/4
Cr
Iso
Iso
LC
Cr
TPC5-DenC12 1/3
Cr
Cr
TPC5-DenC12 1/2
LC
Iso
Iso
TPC5-DenC12 1/2
Cr
LC
Iso
TPC5-DenC12 1/3
LC
Cr
Cr
LC
LC
Iso
Iso
TPC5
Cr
OC12H25
OH
TPC5-DenC12 1/4
O
OC12H25
DenC12
Cr
O
OC5H11
Iso
Cr
40
O
O
O
O
-20
0
20
40
60
80 100 120 140
-20
0
20
60
80 100 120 140
N
NH
+
C5H11
O
O
OH
Temp/ °C
Temp/ °C
O
OC5H11
OC12H25
OC12H25
Figure 2. DSC thermograms of TPC5, DenC12, TPC5–
DenC12 1/2, TPC5–DenC12 1/3, and TPC5–DenC12 1/4
upon (a) 2nd heating and (b) 2nd cooling at a scan rate of
10 ꢀC/min.
HO
DenC12
TPC5
O
Figure 1. Chemical structures of TPC5 and DenC12, and a
self-assembled structure of TPC5–DenC12 1/3.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.2 (2007)
283
40000
the most suitable supramolecular complex was estimated to be
a 1:3 ratio. The TPC5–DenC12 1/3 reported here is the first ex-
ample of a supramolecular columnar liquid crystal composed of
a triphenylene and dendrons organized by noncovalent binding,
which can serve as a simple and fundamental model in the fab-
rication of novel supramolecular nanomaterials. For example,
based on this model, triphenylene derivatives having chiral
alkoxy, semi-fluoroalkoxy, or stimuli-responsive side chains
instead of the pentyloxy group of TPC5 are being developed
to evaluate dynamic transformation throughout the entire supra-
molecular structure. Furthermore, transcription of structural
information of the triphenylene derivatives at the center of the
surrounding dendrons, and fabrication of triphenylene nanorods
or nanoporous dendron films by molecular imprinting techniques
are also now in progress.
(a)
38.4 Å, d100
38.4 Å
TPC5-DenC12 1/3
30000
DenC12
TPC5
34.5 Å
26.9 Å
34.5 Å,
d100
20000
10000
0
0
1
2
3
4
5
26.9 Å,
d100
2θ
15.4 Å, d110
0
5
10
15
20
25
30
2
θ
40000
30000
20000
10000
0
(b)
38.4 Å, d100
38.4 Å
TPC5-DenC12 1/4
TPC5-DenC12 1/3
TPC5-DenC12 1/2
26.9 Å
This work was partially supported by the 21COE ‘‘Practical
Nano-Chemistry’’ and ‘‘Consolidated Research Institute for
Advanced Science and Medical Care’’ from MEXT, Japan.
The authors thank Mr. S. Enomoto (M.C.C.L. at Waseda Univ.)
for his kind assistance with XRD experiments.
0
1
2
3
4
5
2θ
26.9 Å,
d100
0
5
10
15
20
25
30
2θ
References and Notes
1
Figure 3. XRD patterns of (a) TPC5, DenC12, and TPC5–
DenC12 1/3; (b) TPC5–DenC12 1/2, TPC5–DenC12 1/3,
and TPC5–DenC12 1/4 at mesophase states upon cooling.
a) K. Binnemans, Chem. Rev. 2005, 105, 4148. b) T. Kato, N.
Mizoshita, K. Kishimoto, Angew. Chem., Int. Ed. 2006, 45,
38. c) U. Beginn, G. Zipp, M. Moller, Adv. Mater. 2000,
¨
12, 510. d) M. Yoshio, T. Mukai, H. Ohno, T. Kato, J. Am.
Chem. Soc. 2004, 126, 994. e) M. Katoh, S. Ueharar, S.
Kohmoto, K. Kishikawa, Chem. Lett. 2006, 35, 322.
V. Percec, M. Glodde, T. K. Bera, Y. Miura, I.
Shiyanovskaya, K. D. Singer, V. S. K. Balagurusamy,
P. A. Heiney, I. Schnell, A. Rapp, H.-W. Spiess, S. D.
Hudson, H. Duan, Nature 2002, 417, 384.
which was assigned to the melting of the pure DenC12 region.
Therefore, the DSC results prove that TPC5 and DenC12
self-organized to form a supramolecular complex only in the
1:3 ratio, and this result agreed well with the stoichiometry of
the hydrogen-bonding units.
2
The X-ray diffraction (XRD) patterns of DenC12 and
TPC5–DenC12 1/3 shown in Figure 3a gave strong diffraction
3
4
S.-H. Jung, W. Pisula, A. Rouhanipour, H. J. Rader, J. Jacob,
¨
peaks from d100 and weak diffraction peaks from d110 and d200
,
K. Mullen, Angew. Chem., Int. Ed. 2006, 45, 4685.
¨
which are typical to a disordered hexagonal columnar lattice.
The diameters of the columns were calculated to be 31.4, 39.8,
˚
and 44.4 A, respectively. The degree of change in the extended
˚
diameter of the column by 4.6 A from DenC12 to TPC5–
DenC12 1/3 is somewhat smaller than the molecular size of
a) D. L. Gin, W. Gu, B. A. Pindzola, W.-J. Zhou, Acc. Chem.
Res. 2001, 34, 973. b) H.-K. Lee, H. Lee, Y. H. Ko, Y. J.
Chang, N.-K. Oh, W.-C. Zin, K. Kim, Angew. Chem., Int.
Ed. 2001, 40, 2669. c) Y. Ishida, S. Amano, K. Saigo, Chem.
Commun. 2003, 2338.
˚
TPC5 (approximately 14 A) owing to the buffering effect of al-
5
6
a) A. Kraft, A. Reichert, R. Kleppinger, Chem. Commun.
2000, 1015. b) J. Kadam, C. F. J. Faul, Ulli Scherf, Chem.
Mater. 2004, 16, 3867.
D. Adam, P. Schuhmacher, J. Simmerer, L. Haussling,
K. Siemensmeyer, K. H. Etzbach, K. Ringsdorf, D. Haarer,
Nature 1994, 371, 141.
C. M. Paleos, D. Tsiouvas, Angew. Chem., Int. Ed. Engl.
1995, 34, 1696.
a) V. Percec, J. A. Heck, D. Tomazos, G. Ungar, J. Chem.
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Cho, G. Ungar, D. J. P. Yeardley, Angew. Chem., Int. Ed.
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kyl chain packing. The stronger XRD peak intensity of TPC5–
DenC12 1/3 compared to that of TPC5–DenC12 1/2 or
TPC5–DenC12 1/4 indicates that TPC5–DenC12 1/3 is the
most structured assembly. The XRD pattern of TPC5–DenC12
1/2 in Figure 3b is obviously derived from the two segregational
domains of TPC5–DenC12 1/3 and pure TPC5. On the other
hand, the XRD pattern of TPC5–DenC12 1/4 in Figure 3b is
almost identical to the patterns of TPC5–DenC12 1/3, because
the segregational domain of pure crystalline DenC12 is compat-
ible with the adjacent liquid crystalline state of TPC5–DenC12
1/3 at 60 ꢀC on cooling from a hot melt state, as is shown in the
DSC thermogram of TPC5–DenC12 1/4 in Figure 2.
7
8
9
Synthesis, IR spectra, polarized optical textures, and XRD
were shown in the supporting information. Supporting
Information is available electronically on the CSJ-Journal
Web site: http://www.csj.jp/journals/chem-lett/index.html.
In conclusion, fabrication of a hydrogen-bonded hexagonal
columnar liquid crystal composed of TPC5 and DenC12 has
been effected, and the precise stoichiometry for constructing
Published on the web (Advance View) January 18, 2007; doi:10.1246/cl.2007.282