this kind of chiral H-bond in the cooling process. This is supported
by the FT-IR spectra of the xerogel II. The amide-II band at 1552
cm21 suggests that the NH group forms H-bonds, confirmed by the
red-shift of nNH to 3310 cm21, whereas the strong amide-I band,
occurring at 1650 cm21, indicates that the CO group is free of H-
bonds. The broad band of nCNO with two peaks at 1745 and 1717
cm21 indicates the existence of free, laterally H-bonded and
bifurcated –COOH groups,9 being possible for formation of the
intra- and inter-layered H-bonds between intermolecular –COOH
and/or amide groups. The FT-IR spectrum of gel I is very different
from that of gel II. The amide-I and -II bands appear at 1643 and
1541 cm21, respectively, suggesting that both the CO- and NH-
water, the C18-Glu molecules form intermolecular chiral H-bonds
between the NH group of amide and the CO group of the –COOH
unit which is directly linked to the chiral carbon atom,7,8 and then
assemble into a spiral molecular bilayer, e.g. one layered nanodisk.
Moreover, part of the free –COOH groups can further form intra-
and inter-layered intermolecular H-bonds between the adjacent
–COOH groups in the spiral structure.11 However, in chloroform,
the intramolecular H-bonds were favored and the formation of both
the intramolecular H-bonds between the amide and –COOH units12
and then the intermolecular H-bonds between –COOH units would
drive the C18-Glu molecules to self-assemble into nanofibers
through hydrophobic and hydrophilic interactions,11 just as many
reported fiber-mediated amphiphiles do.1
groups all form H-bonds. The nCNO bands at 1727 and 1683 cm21
,
respectively, indicate that the –COOH groups simultaneously form
lateral inter- and intra-molecular H-bonds, which is supported by
the broad, strong vibration band at 3500–2800 cm21 with a peak at
3060 cm21. The FT-IR data suggest that C18-Glu forms inter-
molecular H-bonds in 1:1 mixed ethanol/water, whereas it forms
intra- and inter-molecular H-bonds in chloroform. This is further
comfirmed by temperature-dependent 1H-NMR spectra.
In conclusion, the change from hydrophobic to hydrophilic
surroundings depending on the solvents used would switch the
intramolecular/intermolecular H-bonding styles and then control
the morphology of the self-assembled nanostructures for C18-Glu,
an MHB amphiphile.
Authors would like to thank Professor Yalin Tang for his help in
the NMR spectra. This work is supported by the NSFC (No.
20303024, 50172049, 90301010), the Major State Basic Research
Development Program (No. G2000078103, 2002CCA03100).
As shown in Fig. 2, there is a slight downfield shift of the proton
of the amide upon heating each of the gels in CDCl3. This clearly
reveals the formation of H-bonds for the –NH units in the gels.10
Significant differences are observed for the protons of –COOH in
the two gels. At a lower temperature of 325 K, a broad peak
ascribed to the protons of –COOH was observed at ca. 3–7 and ca.
1–4 ppm, repestively, for gels I and II, suggesting H-bonding of
–COOH in the gels, which is further supported by the following
downfield shift upon heating. However, the shifted pattern was very
different. In the gel I, only one broad peak corresponding to two
protons was observed at each temperature, suggesting that the two
protons of –COOH have similar environments in gel I. On the other
hand, in gel II, three peaks were observed at 6.6, 7.2 and 9.3 ppm
at 334 K, indicating that there are three different kinds of protons
with different surroundings for –COOH in gel II. Moreover, the
three peaks are relative to two protons. These strongly suggest that
there exist two kinds of H-bonding protons and one kind of free
proton for the two protons of carboxylic acid.
Notes and references
1 J. H. van Esch and B. L. Feringa, Angew. Chem., Int. Ed., 2000, 39,
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2 For examples of template synthesis, see: E. D. Sone, E. R. Zubarev and
S. I. Stupp, Angew. Chem., Int. Ed., 2002, 41, 1705; S. Kobayashi, N.
Hamasaki, M. Suzuki, M. Kimura, H. Shinkai and K. Hanabusa, J. Am.
Chem. Soc., 2002, 124, 6550; C. Zhan, J. Wang, J. Yuan, H. Gong, Y.
Liu and M. Liu, Langmuir, 2003, 19, 9440.
3 For examples of functional materials, see: F. S. Schoonbeek, J. H. van
Esch, B. Wagewijs, D. B. A. Rep, M. P. de Haas, T. M. Klapwijk, R. M.
Kellog and B. L. Feringa, Angew. Chem., Int. Ed., 1999, 38, 1393; F.
Placin, J.-P. Desvergne and J.-C. Lassegues, Chem. Mater., 2001, 13,
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N. Reinhoud, J. Am. Chem. Soc., 2002, 124, 10 754; A. Shumburo and
M. C. Biewer, Chem. Mater., 2002, 14, 3745; M. George, S. L. Snyder,
P. Terech, C. L. Glinka and R. G. Weiss, J. Am. Chem. Soc., 2003, 125,
10 275; K. Inoue, Y. Ono, Y. Kanekiyo, T. Ishi-I, K. Yoshihara and S.
Shinkai, J. Org. Chem., 1999, 64, 2933.
Based on the above results, models can be proposed to explain
the formation of the different nanostructured organogels formed in
the different solvents, as shown in Fig. 1c. In 1:1 mixed ethanol/
4 J. H. Jung, S. Shinkai and T. Shimizu, Nano Lett., 2002, 2, 17.
5 J. Song, Q. Cheng, S. Kopta and R. C. Stevens, J. Am. Chem. Soc., 2001,
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10 M. Suzuki, M. Yumoto, M. Kimura, H. Shirai and K. Hanabusa, Chem.
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11 These models were supported by the CD sepctra of the xerogels I and II
(ESI,† Fig. 5), the stronger CD signal of nanodisks indicates the
formation of the intermolecular chiral H-bonds in the nanodisks.
12 Preliminary investigation by using PCMODEL minimizing indicates
that the –COOH-to-amide intramolecular H-bonds are more stable than
those between –COOH units in molecular energy (ESI,† Fig. 6).
Fig. 2 1H-NMR spectra of gel I with CDCl3 (a), and II suspended in
CDCl3 (b), respectively. a = aA = 325 K; b = bA = 330 K; c = cA = 334
K.
C h e m . C o m m u n . , 2 0 0 4 , 1 1 7 4 – 1 1 7 5
1175