Uncommon hexagonal microtubule based gel from a simple trimesic amide

Article abstract of DOI:10.1039/b804455d

A small molecule of N,N′,N″-tris(3-methylpyridyl)-trimesic amide was assembled into a novel hexagonal microtube through intermolecular hydrogen bonds, and simultaneously formed a gel system in H₂O-THF mixed solvent. Tuning gelator concentration or the preparation method can effectively control the size of the hexagonal tubes. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008.

Full text of DOI:10.1039/b804455d

View Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/njc | New Journal of Chemistry
Uncommon hexagonal microtubule based gel from a simple
trimesic amidew
Naien Shi,ab Gui Yin,a Hongbian Li,a Min Hana and Zheng Xu*a
Received (in Monpellier, France) 17th March 2008, Accepted 8th May 2008
First published as an Advance Article on the web 17th July 2008
DOI: 10.1039/b804455d
A small molecule of N,N0,N00-tris(3-methylpyridyl)-trimesic amide was assembled into a novel
hexagonal microtube through intermolecular hydrogen bonds, and simultaneously formed a gel
system in H2O–THF mixed solvent. Tuning gelator concentration or the preparation method can
effectively control the size of the hexagonal tubes.
(4 : 1) mixed solvents by heating in a screwed tube, and
Introduction
allowed to stand to cool in a refrigerator (10 1C), it self-
assembled into hexagonal microtubes. Even after the solvent
was dried in vacuum, the structure of the tube was kept stable.
Fig. 1 shows the corresponding microtubes formed in H2O–
THF (5 : 1, v/v) mixture at a concentration of 1.36 wt%. From
these figures, we can clearly see that TMPTA self-assembled
into long microtubes with various sizes of 1–6 mm in diameter
and hundreds of micrometres in length (Fig. 1a). The corres-
ponding high magnification image clearly shows the open ends
with a hexagonal cross-section (Fig. 1b). Furthermore, the
dark edges and relatively bright center in the optical micros-
copy image further evidenced that they are of tubular struc-
tures (Fig. 1c). Similarly, hexagonal microtubes also existed in
the H2O–EtOH mixed solvent (Fig. S1w).
The need to improve miniaturization and device performance
in information technology greatly stimulates the research
interests in the self-assembly of organic functional materials,
because of its tailorability, multifunctionality and low cost.1
Micro- and nanotubes can perform diverse functions and have
various applications in chemistry, biochemistry, and materials
science.2 However, in comparison with the extensively studied
carbon nanotubes and inorganic nanotubes, such as metal and
oxide ones, nanotubes of small molecular organic compounds
have relatively few reports.3 Self-assembled organic micro-
tubules (1–100 mm) in solution are even rarer,2,4 although these
low-molecular-weight compounds offer great variety and
flexibility in molecular design with potential benefits for the
fundamental theory of non-covalent self-assembly.
Herein, we report a simple C3-symmetric compound of
N,N0,N00-tris(3-methylpyridyl)-trimesic
Rheology measurement
amide
(TMPTA,
During the formation of the microtubes, the solvent turned
into a gel like state. The microtubes would not be separated
from the solvent under shaken or shear operations. Moreover,
the solvent in the inverted hexagonal-microtube-based system
would not flow down on the force of gravity (Fig. S2w). That
is, the microtubes give a remarkable gelled rheology property
to the solvent. Some kinds of low-molecular-weight
compounds can self-assemble into fibrous aggregates in the
solvent through non-covalent interactions (c o 5.0 wt%).
During the aggregation, the fibrous aggregates entangle into
a meshlike matrix that holds the solvent by interfacial tension
and supramolecular interactions, which results in a supra-
molecular gel.5,6 Supramolecular gels have been attracting
much interest because of their potential applications in biology
Scheme 1) which can assemble into hexagonal microtubes
and simultaneously form a gel system in diluted THF–H2O
solution at a concentration of B1.0–2.0 wt%.
Results and discussions
Synthesis and morphology characterization
TMPTA was synthesized by amidation of trimesic chloride
with 3-aminomethyl pyridine in the presence of pyridine as the
catalyst (Experimental section in ESIw). It is insoluble in
water, but can dissolve in tetrahydrofuran (THF), ethanol
(EtOH) or methanol (MeOH). When TMPTA was dissolved
in H2O–THF (5 : 1, v/v), H2O–EtOH (4 : 1), or H2O–MeOH
a State Key Laboratory of Coordination Chemistry, National
Laboratory of Solid State Microstructures, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093,
P. R. China. E-mail: zhengxu@netra.nju.edu.cn;
Fax: +86-25-83594502; Tel: +86-25-83593133
b Jiangsu Key Laboratory of Organic Electronics & Information
Displays and Institute of Advanced Materials, Nanjing University of
Posts and Telecommunications, Nanjing 210046, P. R. China
w Electronic supplementary information (ESI) available: Experimental
details, SEM image of the tubes obtained in a mixed solvent of H2O–
EtOH, direct observation of the system in H2O–THF (5 : 1, v/v) mixture,
crystal structure of TPTA reported, X-ray diffraction patterns and
proposed formation process of the hexagonal microtubes. For ESI see
DOI: 10.1039/b804455d
Scheme 1 The chemical structure of TMPTA (left) and TPTA (right).
ꢀc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008 New J. Chem., 2008, 32, 2011–2015 | 2011

View Online
Fig. 3 SEM images of the hexagonal microtubes obtained in the
solvent of H2O–THF = 3 : 1 (v/v) at higher concentration of 2.0 wt%
(a) corresponding low magnification image, (b) enlarged view of the
broken tube, (c) higher magnification of the cross-section face of
the broken tube, (d) higher magnification of the fractured face of the
broken tube.
Fig. 1 (a) SEM image of the microtubes obtained in solvent of
H2O–THF = 5 : 1 (v/v, 1.36 wt%), (b) higher magnification of (a),
(c) corresponding optical microscopy image (ꢁ400).
organogels are less than 500 nm in diameter, and the corres-
ponding minimum gelation concentration (MGC) is less than
1.0 wt%.5–8 Such a hexagonal microtube based gel system is
very rare, and other trimesamide-based gelators were reported
to form fibril structures.11
arising from their special biodegradability.7 Typically, two
criteria of rheology and structural features for supramolecular
gel should be included: (i) that it appears solid-like and yet is
predominately composed of a liquid at the microscopic scale;
(ii) that the material returns to its original form when an
applied stress is removed (below a certain limit).8 The rheology
measurement of the system was performed and corresponding
viscoelastic stress sweep response was shown in Fig. 2. We can
see that the value of the storage modulus G0 (the elastic
response) exceeds that of G00 (viscous response) by one order
of magnitude. The G0 value of this system is at the magnitude
of 6 ꢁ 104 Pa while the G0 value of some peptide hydrogels is
50 Pa at a frequency of 1 Hz.9 The corresponding yield stress
(the critical applied stress after which the gel starts to flow10) is
about 108 Pa. All these indicate the typical features of a gel for
this system. Almost all kinds of nanofibers in hydrogels or
Coarse-texture of the microtubes
Larger tubes were obtained if increasing the concentration of
TMPTA. Fig. 3a showed the microstructures obtained from
H2O–THF (3 : 1) mixture at a concentration of 2.0 wt%. The
microtubes have larger diameters of 5–30 mm in comparison
with that of 1.36 wt% (Fig. 3a and 1). Most of the tubes are
hexagonal, and rectangular microtubes occasionally occurred.
The special tubular structures generate many more small
channels in the tubes, which provide enough compartments
and interfacial tension to hold the solvent. Hydrophilic pyridyl
groups of TMPTA also fortify the binding of solvent molecules.
In the middle of Fig. 3a, there is a broken microtube. The reason
for the brake is still not clear, and perhaps the rapid evaporation
of solvent trapped inside the tube led to the crack when dried
under vacuum. Fig. 3b–d shows the enlarged view of the broken
open end, cross-section, and fractured face of the tube, respec-
tively. We can see that the hexagonal tube was composed of
many belt-like moieties. It should have been assembled by non-
covalent interactions since it is composed of small molecules.
Intermolecular interactions by FT-IR studies
This gel is stable in neutralized conditions and switches to sol by
acidifying to pH o 3.0 using HCl (1 N). It switches back to the
gel state by neutralizing to pH = 7.0 using NaOH (1 N). This
transition is reversible. Since pyridyl groups can be protonated by
acid, it is reasonable that protonation of pyridyl groups breaks
the hydrogen bonds and therefore damages the microtubular
structure, and then the trapped solvent is released out to form a
sol. Therefore, pH-responsive properties of the system indicated
the role of hydrogen bonding interactions from N–Hꢂ ꢂ ꢂPy.
Fig. 2 Plots of storage modulus G0 and loss modulus G00 versus stress
for the system in H2O–THF (5 : 1, v/v, 1.36 wt%).
ꢀc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008
2012 | New J. Chem., 2008, 32, 2011–2015

View Online
hydrogen-bonded u-NH band 3245 cmꢃ1 significantly
decreased (Fig. 4b). This means 3245 cmꢃ1 in FT-IR spectrum
of the microtubes mainly comes from hydrogen bonding of
N–Hꢂ ꢂ ꢂPy. Therefore, FT-IR results confirm that the TMPTA
hexagonal tube is stabilized by hydrogen bonds of N–Hꢂ ꢂ ꢂPy.
Formation process
However, it is very difficult to get the crystal structure of the
microtube because the crystal is too thin to determine by X-ray
crystallography. Hexagonal crystals of a similar trimesic
amide of 3-aminopyridine, N,N0,N00-tris(3-pyridyl)-trimesic
amide (TPTA) has been reported by Meijer et al.13 In their
report, the obtained hexagonal crystal of TPTA is affiliated to
ꢀ
the space group P3. The molecules self-assembled to form a
2-D sheet architecture along x and y direction of the basic unit
of six amide molecules stabilized by twelve hydrogen bonds of
N–Hꢂ ꢂ ꢂPy (Fig. S3w). The 2-D sheet further stacked along its
z-axis to form a hexagonal crystal. In this structure, all the
amide N–H groups are hydrogen bonded to pyridyl groups.
This is in accordance with its FT-IR spectrum (Fig. 4c). As
seen, only the hydrogen-bonded u-NH band at 3276 cmꢃ1
occurred. Comparing X-ray diffraction patterns of TPTA
hexagonal crystals and our TMPTA hexagonal microtubes,
we can see that both of their strongest peaks are much stronger
than others and located at about 71 (Fig. S4w), which indicates
a similar preferred growth direction for them. Based on the
above structure, the organization mode of TMPTA may be as
follows. Six amide molecules are assembled into a rosette basic
unit through six hydrogen bonds of N–Hꢂ ꢂ ꢂPy (black line
indicated) shown in Fig. 5. Six basic units are further
assembled to form a small hexagonal 2-D sheet. Then
six assemblies were organized into a C3-symmetric small
hexagonal tube. The small tubes further assembled into
belt-like moieties, which is the basic component of the large
microtubes. Similar assembly mode has been proposed in the
assembled tubes of a cyclic peptide.14 The belt-like moieties
further parallel stacked to form the large hexagonal microtube
Fig. 4 FT-IR spectra of (a) TMPTA hexagonal microtubes obtained
in H2O–THF = 5 : 1 (v/v), (b) protonated TMPTA, (c) TPTA
hexagonal crystals.
FT-IR experiments were also done to characterize the
hydrogen bonds. As is known to us all, u-NH bands of
3200–3300 cmꢃ1 are an indicator of a hydrogen bonded N–H
group, while the free vibration band of u-NH B 3400 cmꢃ1
,
which indicates there is absence of hydrogen bonds or specifi-
cally here, the hydrogen bonds (N–Hꢂ ꢂ ꢂPy) are broken.12 As
seen in the FT-IR spectrum of TMPTA hexagonal microtubes
(Fig. 4a), there are both the two bands in u-NH region centered
at 3245 cmꢃ1 and 3408 cmꢃ1
, respectively. When the
gel is acidified and broken by HCl, vibration band for free
N–H at B3400 cmꢃ1 greatly increased and simultaneously
Fig. 5 Proposed formation process of the hexagonal microtubes. This scheme describes the assembly mode of the moieties in the hexagonal
microtube, and it doesn’t show the definite diameter of the microtube.
ꢀc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008 New J. Chem., 2008, 32, 2011–2015 | 2013

View Online
Fig. 6 (a) SEM image of the product from crystallization of a drop of TMPTA THF solution on a glass slide, (b) enlarged view of (a), (c) SEM
image of the tubule obtained by the reprecipitation method, (d) corresponding TEM image of the tubule obtained by the reprecipitation method.
under a crystal packing force. In the obtained postulated
structure of the TMPTA microtube, only half the N–H groups
are hydrogen bonded because the additional methylene group
changes the position of the N–H donor and pyridyl nitrogen
acceptor. This was also consistent with the FT-IR
results, which showed both the hydrogen-bonded u-NH band
(3245 cmꢃ1) and free N–H band (B3400 cmꢃ1) in its IR
spectrum (Fig. 4a), while as mentioned above, there is only
the hydrogen-bonded N–H band at 3276 cmꢃ1 in FT-IR
spectrum of TPTA crystal (Fig. 4c). Additionally, the melting
point of TPTA crystal is 276–277 1C, while only 190–192 1C
for the TMPTA microtubes although the latter has larger
molecular weight. This also confirmed that the intermolecular
interactions in TMPTA hexagonal microtubes were much
smaller than that in the TPTA hexagonal crystals, which is
concordant with the above analysis on hydrogen bonding
interactions.
of THF, EtOH or MeOH as the co-solvent. It is quite an
uncommon hexagonal microtube based gel system compared
to others reported. Otherwise, tuning the gelator concentra-
tion or preparation method can effectively control the size of
the hexagonal tubes. This may greatly benefit the large-
scale production of organic functional materials with special
microstructures.
Experimental
Synthesis of TMPTA
Trimesic acid (5 g, 23.8 mmol) was put in SOCl2 (10 mL) in the
presence of DMF (0.2 mL) and stirred for 1 h at room
temperature, then refluxed for 6 h until it all dissolved and
reacted. Then the solvent was completely removed. The
trimesic chloride was obtained as a solid at room temperature.
Then, trimesic chloride (0.92 g, 3.4 mmol) was added in freshly
distilled anhydrous THF (20 mL) and 3-aminomethylpyridine
(1.44 g, 10.6 mmol) in the presence of pyridine (1.5 mL). The
obtained precipitate was washed by water. Finally a white
product was obtained. MS m/z (%): 481.3 (M+1,100). 1H
NMR/d (ppm): 9.36 (brs, 3H, N–H); 8.58 (s, 3H, H-2,4,6);
8.50 (s, 36H, H-20); 8.46 (d, 3H, H-60); 7.74 (d, 3H, H-40); 7.36
(dd, 3H, H-50); 4.52 (d, 6H , CH2). EA: anal. calcd. for
C27H24N6O3 C, 67.49; H, 5.03; N, 17.49; found C, 66.98;
H, 5.34; N, 17.01.
In addition, the formation of hexagonal tubes doesn’t depend
on the solvent or method. Dropping a drop of THF solution of
TMPTA on a glass slide at room temperature, a cluster of
hexagonal tubes was formed after the THF completely evapo-
rated (Fig. 6a). An enlarged SEM image (Fig. 6b) shows that
these tubes are B500 nm in diameter. Additionally, using the
reprecipitation method, which is usually utilized to get organic
nanoparticles,15 hexagonal nanotubes can also be obtained
(Fig. 6c, 500–800, nm in diameter). Corresponding TEM image
clearly shows the outer wall and inner hollow structure of the
hexagonal nanotubes (Fig. 6d).
Preparation of TMPTA hexagonal microtubes
A proper amount of TMPTA was dissolved in a H2O–THF
(5 : 1, v/v), H2O–EtOH (4 : 1), or H2O–MeOH (4 : 1) mixed
solvent by heating in a screwed tube, and allowed to stand in a
refrigerator (10 1C). After cooling the liquid didn’t flow down
through the tube when the tube was turned upside down, at a
TMPTA concentration of 1–2 wt%. The hexagonal tubes were
obtained by centrifugation and evaporating the solvent in vacuum.
Conclusions
In conclusion, a small molecule of N,N0,N00-tris(3-methyl-
pyridyl)-trimesic amide was assembled into novel hexagonal
microtubes through intermolecular hydrogen bonds, and
simultaneously formed a gel in water utilizing a small amount
ꢀc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008
2014 | New J. Chem., 2008, 32, 2011–2015

View Online
Preparation of TMPTA tubes by the reprecipitation method
References
A solution of TMPTA in EtOH (0.08 M, 100 mL) was dropped
into 5 mL aqueous solutions under vigorous stirring for one
minute. The suspended solid material was dropped on the sub-
strate and sputtered a layer of platinum for SEM characterization.
1 Y. S. Zhao, W. S. Yang, D. B. Xiao, X. H. Sheng, X. Yang,
Z. G. Shuai, Y. Luo and J. N. Yao, Chem. Mater., 2005, 17,
6430.
2 X. J. Zhang, X. H. Zhang, W. S. Shi, X. M. Meng, C. Lee and S. T.
Lee, Angew. Chem., Int. Ed., 2007, 46, 1525.
3 (a) T. Shimizu, M. Masuda and H. Minamikawa, Chem. Rev.,
2005, 105, 1401; (b) F. Mallet, S. Petit, S. Lafont, P. Billot, D.
Lemarchand and G. Coquerel, Cryst. Growth Des., 2004, 4, 965; (c)
J. S. Hu, Y. G. Guo, H. P. Liang, L. J. Wan and L. Jiang, J. Am.
Chem. Soc., 2005, 127, 17090; (d) R. Ghosh, A. Chakraborty, D.
K. Maiti and V. G. Puranik, Org. Lett., 2006, 8, 1061; (e) C. L.
Zhan, P. Gao and M. H. Liu, Chem. Commun., 2005, 4, 462.
4 (a) H. Y. Lee, S. R. Nam and J.-I. Hong, J. Am. Chem. Soc., 2007,
129, 1040; (b) T. Shimizu, M. Kosigo and M. Masuda, J. Am.
Chem. Soc., 1997, 119, 6209; (c) J.-P. Douliez, C. Gaillard, L.
Navailles and F. Nallet, Langmuir, 2006, 22, 2942.
5 J. H. Jung, S. Shinkai and T. Shimizu, Chem.–Eur. J., 2002, 8,
2684.
6 S. Bhattacharya, A. Srivastava and A. Pal, Angew. Chem., Int. Ed.,
2006, 45, 2934.
7 L. A. Estrofff and A. D. Hamilton, Chem. Rev., 2004, 104,
1201.
8 (a) D. J. Abdallha and R. G. Weiss, Adv. Mater., 2000, 12, 1237;
(b) T. Kitahara, M. Shirakawa, S. Kawano, U. Beginn, N. Fujita
and S. Shinkai, J. Am. Chem. Soc., 2005, 127, 14980.
9 H. Yokoi, T. Kinoshita and S. Zhang, Proc. Natl. Acad. Sci. U. S.
A., 2005, 102, 8414.
Characterization
FT-IR spectra were recorded on a Bruker Vector 22 FT-IR
spectrophotometer. ESIMS result was obtained on LCQTM
electrospray ionization mass spectrometer (Finnigan, American).
NMR spectra were collected on AVANCE DAX-500 (Bruker,
Sweden) NMR spectrometers. The SEM images were taken
on a JEOL JSM-5610LV scanning electron microscope, and
JEO-1500VP field emission scanning electron microscope. For
the rheology measurement, a German Haake RS-600 rheometer
with a cone-plate (C60/1 Ti) was used. The gap distance between
the cone and the plate was fixed at 0.052 mm. The hydrogel was
placed on the plate of the rheometer. A stress-amplitude sweep
experiment was performed at a constant oscillation frequency of
1 Hz at 10 1C. The rheometer had a built-in computer to convert
the torque measurements into either G0 (storage modulus) or
G00 (loss modulus) in oscillatory shear experiments.
10 F. M. Menger and A. V. Peresypkin, J. Am. Chem. Soc., 2003, 125,
5340.
11 (a) N. E. Shi, H. Dong, G. Yin, Z. Xu and S. H. Li, Adv. Funct.
Mater., 2007, 17, 1837; (b) D. K. Kumar, D. A. Jose, P. Dastidar
and A. Das, Chem. Mater., 2004, 16, 2332; (c) K. Hanabusa, C.
Koto, M. Kimura, H. Shirai and A. Kakehi, Chem. Lett., 1997,
429; (d) Y. Yasuda, E. Iishi, H. Inada and Y. Shirota, Chem. Lett.,
1996, 575.
Acknowledgements
Financial support from National Natural Science Foundation
of China project No. 90606005, No. 20490210, No. 20371026,
No. 20571040 and NY207042 is greatly appreciated. And great
thanks to the contribution in the chemical synthesis from Mr
Xiangda Yu, and structure characterization from Prof. Yizhi
Li in Nanjing University as well as Prof. Ziyu Wu in Chinese
Academy of Sciences.
12 V. Caplar, M. Zinic, J.-L. Pozzo, F. Fages, G. Mieden-Gundert
´
and F. Vogtle, Eur. J. Org. Chem., 2004, 4048.
¨
13 A. R. A. Palmans, J. A. J. M. Vekemans, H. Kooijman, A. L. Spek
and E. W. Meijer, Chem. Commun., 1997, 22, 2247.
14 S. Leclair, P. Baillargeon, R. Skouta, D. Gauthier, Y. Zhao and
Y. L. Dory, Angew. Chem., Int. Ed., 2004, 43, 349.
15 D. B. Xiao, W. S. Yang, J. N. Yao, L. Xi, X. Yang and Z. G.
Shuai, J. Am. Chem. Soc., 2004, 126, 15439.
ꢀc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008 New J. Chem., 2008, 32, 2011–2015 | 2015