core have been synthesized and characterized. The morphology
of TMDA and TPDA gels in cyclohexane was fibrillar and
globular, respectively, as characterized by SEM and TEM. The
self-assembled nano-architectures were mainly governed by the
dramatic interplay between the hydrophobic interactions of
the alkyl chains and the intermolecular hydrogen bonding of
the amide groups, as confirmed by XRD and IR measurements,
respectively. Solid state NMR studies showed that TMDA has a
higher structural order than TPDA. This work represents an
example of how substitution patterns (para vs. meta) of the
functional peripherals can influence the intermolecular inter-
actions, which eventually lead to different nanostructures.
We thank the financial support from National Science Council
of Taiwan (NSC 98-2112-M-007-028-MY3, 98-2119-M-002-
007-MY3), technical assistant of Technology Commons, College
of Life Science and Precision Instrumentation Center, NTU and
Dr. Tsung-Han Lin for PXRD.
Fig. 4 Schematic illustration of the formation process of different
nanostructures for TPDA and TMDA.
TMDA nanofibers originates from the intermolecular hydrogen
bonding of the neighboring amide groups.
1
Temperature-dependent H NMR (in cyclohexane-d12) experi-
ments were performed to further study the intermolecular inter-
actions (Fig. S6, ESIw). Apparently, at 293 K, the 1H NMR
spectra of TPDA and TMDA exhibited broad and ill-resolved
signals for both the aromatic and aliphatic protons, indicating the
immobilization and extensive aggregation of the organic molecules
in the gel state. As the temperature increased, the NMR signals of
the phenoxyl C–H protons of the gelator TMDA not only became
sharp and strong, but also shifted downfield slightly, whereas the
signals of TPDA were still broad at the same temperature (345 K).
We then carried out the solid-state NMR studies (Fig. S7 and S8,
ESIw) to investigate the packing order at the molecular level and
the driving force derived from TPDA and TMDA to form
spherical and fibril structures, respectively. On the basis of the
13C{1H} cross-polarization magic-angle spinning spectra (Fig. S9
and S10, ESIw), TMDA showed a higher structural order than
TPDA, and the molecular geometry of the truxene region was not
significantly perturbed by the alkylation when precursor 1c or 2c
was transformed to TPDA or TMDA. With reference to the XRD
data, there was a diffraction peak corresponding to the inter-
digitated packing of the alkyl chains (ca. 4.3 A) for both TMDA
and TPDA. In addition, IR measurements suggested that strong
hydrogen bonds were formed among the amide groups. Altogether,
we conclude that the formation of fibrillar and globular structures
for TMDA and TPDA, respectively, reflects the dramatic interplay
of the hydrophobic interactions among the alkyl chains and the
intermolecular hydrogen bonding interactions of the amide
groups. Based on these findings, the molecular packing models
of the TMDA and TPDA gels are proposed (Fig. 4). Apparently,
the overall molecular configurations of the truxene-based gelators
TMDA and TPDA are quite different due to the different
substitution patterns. We speculate that the alkyl chains stretch
from the rigid truxene core to make TMDA adopt a propeller-
shaped conformation. The intermolecular interactions in this
conformation lead to an ordered sheet, followed by suitable
packing to a lamellar structure that entangles into fibrillar bundles,
and finally transforms into reticular nanofibers and gels at high
concentrations. In contrast, the alkyl chains of the para-substituted
counterpart TPDA stretch to give a dumbbell-shaped configu-
ration, which tends to form vesicular nanostructures.12 Accordingly,
the dumbbell-shaped TPDA molecules self-assemble into multi-
layered sheets with curvature, which close to give hollow nano-
spheres. As the concentration increases, the nanospheres cohere
with one another to generate giant aggregates.
Notes and references
1 (a) P. Dastidar, Chem. Soc. Rev., 2008, 37, 2699; (b) A. Dawn,
T. Shiraki, S. Haraguchi, S.-i. Tamaru and S. Shinkai, Chem.–Asian J.,
2011, 6, 266; (c) J. W. Steed, Chem. Commun., 2011, 47, 1379.
2 (a) F. Camerel, L. Bonardi, M. Schmutz and R. Ziessel, J. Am.
Chem. Soc., 2006, 128, 4548; (b) T. Shu, J. Wu, M. Lu, L. Chen,
T. Yi, F. Li and C. Huang, J. Mater. Chem., 2008, 18, 886.
3 (a) K. Kuroiwa, T. Shibata, A. Takada, N. Nemoto and N. Kimizuka,
J. Am. Chem. Soc., 2004, 126, 2016; (b) O. J. Dautel, M. Robitzer,
J.-P. Lere-Porte, F. Serein-Spirau and J. J. E. Moreau, J. Am.
Chem. Soc., 2006, 128, 16213.
4 A. Ajayaghosh, V. K. Praveen and C. Vijayakumar, Chem. Soc. Rev.,
2008, 37, 109.
5 K. Y. Lee and D. J. Mooney, Chem. Rev., 2001, 101, 1869.
6 (a) S. Srinivasan, S. S. Babu, V. K. Praveen and A. Ajayaghosh,
Angew. Chem., Int. Ed., 2008, 47, 5746; (b) R. Abbel, C. Grenier,
M. J. Pouderoijen, J. W. Stouwdam, P. E. L. G. Leclere,
R. P. Sijbesma, E. W. Meijer and A. P. H. J. Schenning, J. Am.
Chem. Soc., 2009, 131, 833; (c) F. S. Schoonbeek, J. H. van Esch,
B. Wegewijs, D. B. A. Rep, M. P. de Haas, T. M. Klapwijk,
R. M. Kellogg and B. L. Feringa, Angew. Chem., Int. Ed., 1999,
38, 1393; (d) T. Kishida, N. Fujita, K. Sada and S. Shinkai,
Langmuir, 2005, 21, 9432; (e) F. Wurthner, C. Bauer,
¨
V. Stepanenko and S. Yagai, Adv. Mater., 2008, 20, 1695;
(f) C. Giansante, G. Raffy, C. Schafer, H. Rahma, M.-T. Kao,
¨
A. G. L. Olive and A. Del Guerzo, J. Am. Chem. Soc., 2011,
133, 316; (g) T. Akutagawa, K. Kakiuchi, T. Hasegawa, S.-i. Noro,
T. Nakamura, H. Hasegawa, S. Mashiko and J. Becher, Angew.
Chem., Int. Ed., 2005, 44, 7283; (h) L. S. Birchall, S. Roy,
V. Jayawarna, M. Hughes, E. Irvine, G. T. Okorogheye,
N. Saudi, E. De Santis, T. Tuttle, A. A. Edwards and
R. V. Ulijn, Chem. Sci., 2011, 2, 1349.
7 (a) C.-C. Tsou and S.-S. Sun, Org. Lett., 2006, 8, 387; (b) X. Yang,
R. Lu, F. Gai, P. Xue and Y. Zhan, Chem. Commun., 2010, 46, 1088.
8 (a) J. Luo, T. Lei, X. Xu, F.-M. Li, Y. Ma, K. Wu and J. Pei,
Chem.–Eur. J., 2008, 14, 3860; (b) J. Luo, T. Lei, L. Wang, Y. Ma,
Y. Cao, J. Wang and J. Pei, J. Am. Chem. Soc., 2009, 131, 2076.
9 T. Ishi-i, T. Hirayama, K.-i. Murakami, H. Tashiro, T. Thiemann,
K. Kubo, A. Mori, S. Yamasaki, T. Akao, A. Tsuboyama,
T. Mukaide, K. Ueno and S. Mataka, Langmuir, 2005, 21, 1261.
10 M.-T. Kao, J.-H. Chen, Y.-Y. Chu, K.-P. Tseng, C.-H. Hsu,
K.-T. Wong, C.-W. Chang, C.-P. Hsu and Y.-H. Liu, Org. Lett.,
2011, 13, 1714.
11 (a) A. J. Lampkins, O. Abdul-Rahim, H. Li and R. K. Castellano,
Org. Lett., 2005, 7, 4471; (b) H.-F. Hsu, M.-C. Lin, W.-C. Lin,
Y.-H. Lai and S.-Y. Lin, Chem. Mater., 2005, 15, 2115.
12 (a) A. Ajayaghosh, R. Varghese, V. K. Praveen and S. Mahesh,
Angew. Chem., Int. Ed., 2006, 45, 3261; (b) J.-K. Kim, E. Lee,
Y.-b. Lim and M. Lee, Angew. Chem., Int. Ed., 2008, 47, 4662;
(c) J. Xu, X. Liu, J. Lv, M. Zhu, C. Huang, W. Zhou, X. Yin,
H. Liu, Y. Li and J. Ye, Langmuir, 2008, 24, 4231.
In summary, two new organic gelators TMDA and TPDA
stemming from an unusual 3-dimensional aryl-substituted truxene
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3515–3517 3517