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Chemistry Letters Vol.38, No.9 (2009)
One-dimensional Stacks of Triphenylenes Stabilized
by a Peripheral Hydrogen-bonding Network
Yoko Tatewaki,1 Tatsuya Hatanaka,2 Mutsumi Kimura,ꢀ1;2 and Hirofusa Shirai1
1Collaborative Innovation Center of Nanotech FIBER (nanoFIC), Shinshu University, Ueda 386-8567
2Department of Functional Polymer Science, Faculty of Textile Science and Technology,
Shinshu University, Ueda 386-8567
(Received June 29, 2009; CL-090608; E-mail: mkimura@shinshu-u.ac.jp)
Triphenylenes bearing chiral amide termini were found to
form self-assembled nanofibers by the combination of ꢀ–ꢀ
stacking of triphenylene moieties with a hydrogen-bond network
of amide terminates.
Since the discovery of discotic liquid crystals by
Chandrasekhar,1 self-assembled one-dimensional stacks com-
posed of polyaromatic molecules have attracted increasing atten-
tion as potential molecular components in organic electronic and
optoelectronic devices due to their fast one-dimensional charge-
and energy-transfer capabilities.2 Supramolecular conductors
have great advantages in anisotropy, processibility, and self-
healing characteristics of structural defects. The properties of or-
ganized states strongly depend on the ordering of aromatic disks
in the stacks and the length of stacks. Shinkai et al. reported the
eclipsed overlap formation of triphenylene moieties within the
stacks due to hydrogen-bonding interactions among amides.3
The combination of intermolecular noncovalent bonds with ꢀ–
ꢀ stacking between ꢀ-conjugated triphenylenes enables control
of the ordering and molecular arrangement of triphenylenes
within the stacks.4 Here we describe the stabilization of one-di-
mensional stacks of triphenylenes by a peripheral hydrogen net-
work.
Figure 1. TEM (a) and AFM (b) images of fibrous aggregates
made of S-1.
IR spectra, the characteristic peaks appeared at 3268 and 1658
cmꢂ1, which are assignable to N–H and C=O intermolecular hy-
drogen-bonding stretching vibrations, respectively.5 The forma-
tion of intermolecular hydrogen bonds between terminate amide
units is one of the driving forces in gelation. Thermal properties
were investigated using DSC and temperature-controlled polar-
ized optical microscopy (TPOM). The DSC trace of S-1 and S-2
exhibited one transition peak at 171 and 124 ꢁC, and S-1 and S-2
did not display any liquid crystalline textures in TPOM. Thus,
the transition peaks observed in DSC correspond to the melting
points of S-1 and S-2.
The gelation in organic solvents suggests that supramolecu-
lar aggregates form through weak intermolecular interactions.7
The morphology of the aggregates was examined by TEM and
AFM (Figure 1). TEM and AFM images of S-1 gel showed twist-
ed fibrous structures with a length of several micrometers and a
minimum width of about 30 nm. The gels of R-1 and S-2 had a
similar morphology. The XRD pattern of dried S-1 gel was char-
acterized by a broad reflection of 1.8 nm, which could be attrib-
uted to the average distance between one-dimensional stacks. In
addition, there was a weak peak at 0.35 nm due to the stacking
between triphenylene moieties within the stacks.
To obtain detailed information about the molecular arrange-
ment of triphenylene within the aggregates, the temperature-de-
pendent UV–vis, fluorescence, and CD spectra of S-1 were
measured in 2-methoxyethanol at a concentration of 6.1 mM
(1.0 wt/vol %). The gel was subjected to UV–vis, fluorescence,
and CD spectra at temperatures ranging from 25 to 75 ꢁC. With
heating, the intensity of the absorption bands from 250 to 340 nm
increased (Figure 2a). The fluorescence spectrum of S-1 in 2-me-
thoxylethanol at 25 ꢁC exhibited two maxima at ꢁ ¼ 380 and
405 nm with a red-shifted shoulder between 425–550 nm. Upon
increasing temperature, the fluorescence intensity of the emis-
sion at 380 nm increased and the intensity of the shoulder de-
creased (Figure 2b). These spectral changes are attributed to
the stacks of triphenylene rings through intermolecular ꢀ–ꢀ in-
teraction within the fibrous aggregates observed in TEM and
Triphenylenes bearing six R- and S-methylbenzylamide ter-
mini were prepared from 2,3,6,7,10,11-hexahydroxytriphenyl-
ene in three synthetic steps (Scheme 1).5,10 Compounds R-1, S-
1, and S-2 were obtained by the reaction of hexaacids with chiral
phenylethylamine in the presence of 1,10-carbonylbis-1H-imi-
1
dazole, and they were fully characterized by H and 13C NMR
spectroscopy and MALDI-TOF mass spectroscopy. The physical
gels were formed when a mixture of powdered compound and
organic solvent was heated to form a homogeneous fluid and
then allowed to cool at 25 ꢁC.6 The gelation properties were in-
vestigated in various organic solvents. Compound S-1 can gelate
in 2-methoxyethanol, anisol, DMSO, and NMP. The critical ge-
lation concentrations of S-1 and S-2 for 2-methoxyethanol were
0.8 and 2.1 wt/vol %, suggesting that S-1, having shorter
spacers, forms a more stable physical gel than S-2. In the FT-
Scheme 1.
Copyright Ó 2009 The Chemical Society of Japan
Tatewaki, Yoko
