Self-assembly of amphoteric azopyridine carboxylic acids: Organized structures and macroscopic organized morphology influenced by heat, pH change, and light

Article abstract of DOI:10.1021/ja001790f

In this paper, we describe the synthesis of novel amphoteric azopyridine carboxylic acids (2b, 3a, 3b, 4a, and 4b), having both a carboxyl group as a hydrogen donor and a pyridyl group as an acceptor at each molecular terminus, and their self-organization, which is markedly affected by external stimuli including heat, pH changes, and light. The amphoteric compounds form intermolecular hydrogen bonds between pyridyl and carboxyl groups in a head-to-tail manner in the solid state to give linear pseudopolymer structures, as supported by FT-IR analysis. Heating and cooling across their melting points induced thermoreversible supramolecular depolymerization to and polymerization from small molecular components of monomers and the corresponding carboxylic acid dimers. In alkaline aqueous media, these amphoteric compounds, dissolved as carboxylate anions, were gradually neutralized by atmospheric carbon dioxide, leading to their deposition as novel fibrous materials from 3b and 4b, substituted with a propyl group at the phenyl ring, and as leaflet crystals from 3a and 4a bearing no substituent. FT-IR and X-ray diffraction measurements supported the conclusion that the formation of fibrous materials from 3b and 4b arises from their intermolecular hydrogen bonding in a head-to-tail manner as well as the suppressive effect of propyl substitution on the π-π stacking of the molecules. UV irradiation of alkaline solutions of 4b resulted in the modification of the morphology of fibrous materials, probably because the photoisomerized Z-isomer of 4b affected the nucleation process in the fibrous formation. These results suggest that morphological properties of these macroscopic self-assemblages are tunable by appropriate choices of environmental stimuli such as heat, pH, and light.

Full text of DOI:10.1021/ja001790f

J. Am. Chem. Soc. 2000, 122, 10997-11004
10997
Self-Assembly of Amphoteric Azopyridine Carboxylic Acids:
Organized Structures and Macroscopic Organized Morphology
Influenced by Heat, pH Change, and Light
Ken’ichi Aoki, Masaru Nakagawa, and Kunihiro Ichimura*
Contribution from the Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta,
Midori-ku, Yokohama 226-8503, Japan
ReceiVed May 22, 2000
Abstract: In this paper, we describe the synthesis of novel amphoteric azopyridine carboxylic acids (2b, 3a,
3b, 4a, and 4b), having both a carboxyl group as a hydrogen donor and a pyridyl group as an acceptor at each
molecular terminus, and their self-organization, which is markedly affected by external stimuli including heat,
pH changes, and light. The amphoteric compounds form intermolecular hydrogen bonds between pyridyl and
carboxyl groups in a head-to-tail manner in the solid state to give linear pseudopolymer structures, as supported
by FT-IR analysis. Heating and cooling across their melting points induced thermoreversible supramolecular
depolymerization to and polymerization from small molecular components of monomers and the corresponding
carboxylic acid dimers. In alkaline aqueous media, these amphoteric compounds, dissolved as carboxylate
anions, were gradually neutralized by atmospheric carbon dioxide, leading to their deposition as novel fibrous
materials from 3b and 4b, substituted with a propyl group at the phenyl ring, and as leaflet crystals from 3a
and 4a bearing no substituent. FT-IR and X-ray diffraction measurements supported the conclusion that the
formation of fibrous materials from 3b and 4b arises from their intermolecular hydrogen bonding in a head-
to-tail manner as well as the suppressive effect of propyl substitution on the π-π stacking of the molecules.
UV irradiation of alkaline solutions of 4b resulted in the modification of the morphology of fibrous materials,
probably because the photoisomerized Z-isomer of 4b affected the nucleation process in the fibrous formation.
These results suggest that morphological properties of these macroscopic self-assemblages are tunable by
appropriate choices of environmental stimuli such as heat, pH, and light.
Introduction
nano- or microscale structures. Kunitake et al. studied previously
the self-organization of various kinds of synthetic single- and
double-chain ammonium amphiphiles to discuss the relationship
between aggregate morphology and chemical structure of the
amphiphiles in aqueous media.3 Several kinds of structural
components, including hydrocarbon flexible tails and spacers,
mesogenic rigid segments, and hydrophilic headgroups, deter-
mine the aggregates morphology of amphiphiles and their
stability.3e Later, Fuhrhop et al. reported for the first time the
formation of stable fibrous assemblages made from amphiphiles
with an aldonamide headgroup,4 demonstrating that the inter-
molecular hydrogen bonding of secondary amide groups mark-
edly stabilizes the molecular assemblages.5 That work was
followed more recently by reports on the analogous solidlike
The levels of noncovalent intermolecular interactions such
as hydrogen bondings, π-π stackings, metal-ligand interac-
tions, charge-transfer interactions, and so on, which are em-
ployed and designed to assemble supramolecular architectures,
are determined to a significant extent not only by the chemical
structures of the component molecules, but also by environ-
mental factors, including the properties of the media in which
the starting molecules are dissolved. As seen in biological
systems such as deoxyribonucleic acids, optimizing such non-
covalent molecular interactions by environmental factors has
the most significant effect in the creation of intricate supramo-
lecular assemblages and the emergence of highly sophisticated
functionalities.1 Such noncovalent intermolecular interactions
have been extensively utilized in the field of supramolecular
chemistry to construct many kinds of supramolecules, such as
complexes of crown ethers, cryptands, and so forth with selective
alkali metal cations.2 Further advances in supramolecular
chemistry have allowed the formation of assorted three-
dimensional architectures, which may enable us to create fine
(3) (a) Okahata, Y.; Kunitake, T. J. Am. Chem. Soc. 1979, 101, 5231.
(b) Kunitake, T.; Okahata, Y. J. Am. Chem. Soc. 1980, 102, 549. (c)
Kunitake, T.; Nakashima, N.; Morimitsu, K. Chem. Lett. 1980, 1347. (d)
Kunitake, T.; Nakashima, N.; Simomura, M.; Okahata, Y. J. Am. Chem.
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Yasunami, S.; Takarabe, K. J. Am. Chem. Soc. 1981, 103, 5401. (f)
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* To whom correspondence should be addressed. Telephone: +81-45-
924-5266. Fax: +81-45-924-5276. E-mail: kichimur@res.titech.ac.jp.
(1) (a) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford
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10.1021/ja001790f CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/21/2000

10998 J. Am. Chem. Soc., Vol. 122, No. 44, 2000
Aoki et al.
fibrous materials formed from other amphiphiles of phospholipid
derivatives,6 glucosamide bolaamphiphiles,7 amino acid deriva-
tives,8 and others.9 Moreover, Shimizu et al. reported on
precisely controlled microtubes encapsulating a number of
vesicular assemblages inside their aqueous compartment, using
bolaamphiphiles with carboxylic headgroups at both molecular
ends.10 These molecular assemblages are ultimately associated
with natural biological systems.
For the construction of the well-defined molecular as-
semblages mentioned above, hydrogen bonds are regarded as
an essential driving force because their moderate and reversible
nature differs intrinsically from that of covalent bonds. Such
characteristics of hydrogen bonding give rise to specific
functionalities of artificial materials as well as biological
materials. Kato et al.11 and Griffin et al.12 reported on hydrogen-
bonded supramolecular liquid crystals and liquid crystalline
polymers with the goal of their application to optical elements.
In the other cases, the formation of hydrogen-bonded fibrous
nanoscale networks gave rise to the gelation of a variety of
solvents.13 Without such hydrogen bonding, anthracene deriva-
tives,14 steroid derivatives,15 and steroidal and condensed
aromatic rings16 form gels with various kinds of organic
solvents,17 suggesting that intermolecular interactions involving
Figure 1. Azopyridine carboxylic acids (2b, 3a, 3b, 4a, and 4b).
van der Waals interactions among aromatic rings or hydrophobic
cyclic hydrocarbons also play an important role in molecular
assembly.
Knowing that the self-organization of molecular assemblages
possessing highly ordered structures is decisively governed by
the chemical structures of the original low-molecular-weight
molecules, we anticipate that the macroscopic morphological
shapes of the molecular assemblages should be modulated by
the modification of original molecules by external physical or
chemical stimuli such as light, heat, and pH changes. This is
because molecular-level alteration by external stimuli should
be amplified in molecular assemblages at the macroscopic level
during the self-organization process through multiple intermo-
lecular interactions.18 Indeed, several studies have shown that
morphological features of organic tubes (length distribution and
thickness of wall) and of low-molecular-weight gels can be
controlled by varying the preparation conditions such as
concentrations of lipids, solvents, cooling rates, and light.16c,19
Our investigation has been concentrated on the control of the
macroscopic morphology of self-organized materials by external
physical stimuli such as light.
We started by designing simple organic molecules to give
molecular assemblages which might lead to a better understand-
ing of the relationships between monomolecular chemical
structure and macroscopic morphological features. Kato et al.
demonstrated that novel supramolecular liquid crystals are
formed through a pyridyl/carboxyl hydrogen bond when two
kinds of organic molecules possessing either a pyridyl group
as a hydrogen acceptor or a carboxyl group as a hydrogen donor
are mixed.11 Griffin et al. reported that binary mixtures of bola-
form organic compounds bearing either two carboxyl groups
or pyridyl groups at both molecular termini give rise to
supramolecular polymerization through formation of hydrogen
bonds between their carboxyl and pyridyl groups, and that the
resulting hydrogen-bonded materials exhibit rheological proper-
ties similar to those of conventional linear polymers formed
through covalent bonds.12 Taking these facts into consideration,
we designed a novel family of amphoteric azopyridine car-
boxylic acids possessing both a carboxyl group as a hydrogen
donor and a pyridyl group as a hydrogen acceptor at each
molecular terminus, as shown in Figure 1 on the basis of the
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Self-Assembly of Amphoteric Azopyridine Carboxylic Acids
J. Am. Chem. Soc., Vol. 122, No. 44, 2000 10999
following. First, the amphoteric compounds should form
pseudolinear polymers, based on intermolecular hydrogen bonds
forming between the pyridyl and carboxyl groups in a head-
to-tail manner. Second, a dipole moment arising intrinsically
from the asymmetrical molecular structure of the amphoteric
molecules should enhance dipole-dipole interactions among the
molecules, leading to their molecular stacking, extending the
information that may be obtained about the monomolecular
chemical structure to three-dimensional macroscopic molecular
aggregates. Third, it is likely that the occurrence of intermo-
lecular hydrogen bonds in a head-to-tail manner is significantly
dependent on the pH value of the medium because the
amphoteric compounds have an ionizable carboxyl group.
Fourth, since aromatic azo compounds undergo E/Z photo-
isomerization,20 it is anticipated that the photoinduced alteration
of their molecular geometry causes the transformation of
macroscopic molecular assemblages. Consequently, the ampho-
teric azopyridine carboxylic acids should respond to a variety
of external stimuli such as light, pH change of the medium,
and heat, leading to the on-demand control of macroscopic
molecular assemblages. In addition, these compounds offer
advantages in the simplicity of their monomolecular chemical
structure when compared with those in a majority of the previous
reports. This fact should serve to deepen the understanding of
the relationship between the monomolecular chemical structure
and the morphology of organized materials. We report first the
formation of novel fibrous materials starting from two azo-
pyridine carboxylic acids, which are substituted with a propyl
group at one of the aromatic rings, followed by the structural
analysis of molecular assemblages to reveal the contribution of
the propyl side-chain substituent. Subsequently, the possibility
to control the formation of molecular assemblages by controlling
external stimuli including heat application, pH changes, and light
irradiation is discussed.
During our extensive studies on amphoteric compounds of
this kind since our first report,21 closely related work has been
recently reported on the thin-film organization of linear poly-
meric assemblages formed from a one-component molecule in
a head-to-tail hydrogen-bonded manner, which is fabricated by
organic molecular beam deposition methods.22 The major
purpose of these papers is to reveal second harmonic generation
arising from the asymmetrical character of the organized thin-
film materials.
Figure 2. FT-IR spectra of 4a at (a) 50, (b) 170, and (c) 180 °C on
heating and at (d) 170 and (c) 50 °C on cooling.
for acidic or basic solvents such as acetic acid and pyridine,
while the other derivatives, with a propyl substituent on an
aromatic ring of the azopyridine moiety (2b, 3b, and 4b),
exhibited improved solubility in common organic solvents such
as tetrahydrofuran, acetone, and chloroform. The thermal
properties of each compound were investigated by means of
DSC analysis. As observed for 3b and 4b (mp 172 °C for 3b
and 152 °C for 4b), the introduction of a propyl substituent to
3a and 4a led to a significant lowering of their melting points
(229 °C for 3a and 173 °C for 4a), presumably due to the
alteration of intermolecular interactions involving π-π stacking,
dipole-dipole interactions, and hydrogen bonding arising from
the side-chain substituent effect.
Results
Synthesis and Thermal Properties of Azopyridine Car-
boxylic Acids. Amphoteric azopyridine carboxylic acids were
synthesized in three steps, involving the azo coupling of a phenol
with 4-aminopyridine, the Williamson ether synthesis with
ω-bromoalkanoate, and subsequent alkaline hydrolysis. The
azopyridine carboxylic acids without a side-chain substituent
(3a and 4a) showed little solubility in organic solvents except
(20) (a) Crano, J. C.; Guglielmetti, R. J. Organic Photochromic and
Thermochromic Compounds, Volume 2; Kluwer Academic/Plenum Publish-
ers: New York, 1999; pp 9-63. (b) Du¨rr, H.; Bouas-Laurent, H.
Photochromism, Molecules and Systems; Elsevier: Amsterdam, 1990; pp
165-192.
Temperature-dependent FT-IR spectra provide valuable in-
formation concerning thermally induced changes in the as-
sembled molecular structures of hydrogen-bonded materials.11
As shown in Figure 2a, the FT-IR spectrum of 4a at 50 °C
exhibits νCdO, νOH, and its Fermi resonance bands at 1707,
2500, and 1930 cm-1, respectively. The same infrared spectral
features have been reported for several hydrogen-bonded
complexes formed from a binary mixture of pyridine and
carboxylic acid derivatives.23 Therefore, these characteristic
bands are ascribed to the formation of consecutive intermo-
lecular hydrogen bonds in a head-to-tail manner between the
(21) (a) Aoki, K.; Nakagawa, M.; Morino, S.; Seki, T.; Ichimura, K.
Prepr. Chem. Soc. Jpn. 1998, 1390. (b) Aoki, K.; Nakagawa, M.; Ichimura,
K. Chem. Lett. 1999, 1205.
(22) (a) Cai, C.; Bo¨sch, M. M.; Tao, Y.; Mu¨ller, B.; Ku¨ndig, A.;
Bosshard, C.; Gu¨nter, P. Polym. Prepr. (Am. Chem. Soc., DiV. Polym. Chem.)
1998, 39, 1069. (b) Cai, C.; Bo¨sch, M. M.; Tao, Y.; Mu¨ller, B.; Gan, Z.;
Bosshard, C.; Liakatas, I.; Ja¨ger, M.; Gu¨nter, P. J. Am. Chem. Soc. 1998,
120, 8563. (c) Cai, C.; Mu¨ller, B.; Weckesser, J.; Barth, J. V.; Tao, Y.;
Bo¨sch, M. M.; Ku¨ndig, A.; Bosshard, C.; Biaggio, I.; Gu¨nter, P. AdV. Mater.
1999, 11, 750. (d) Mu¨ller, B.; Cai, C.; Ku¨ndig, A.; Tao, Y.; Bo¨sch, M.;
Ja¨ger, M.; Bosshard, C.; Gu¨nter, P. Appl. Phys. Lett. 1999, 74, 3110.
(23) Johnson, S. L.; Rumon, K. A. J. Phys. Chem. 1965, 69, 74.

11000 J. Am. Chem. Soc., Vol. 122, No. 44, 2000
Aoki et al.
Figure 3. Optical microscopy images of fibrous materials formed from (a) 3b and (b) 4b; polarized optical microscopy images of leaflet crystals
formed from (c) 3a and (d) 4a (scale bar: 10 µm); and AFM images of microfibers formed from (e) 3b (5 × 5 µm) and (f) 4b (2 × 2 µm).
pyridyl and carboxyl groups of 4a in the solid state, leading to
the self-organization of a pseudopolymer structure by a single-
component molecule, as depicted in Figure 1. Similar spectral
features were also observed for the other amphoteric azopyridine
carboxylic acids of 3a, 3b, and 4b in the solid state. No
significant spectral change was observed in Figure 2b upon
heating from 50 to 170 °C, while further heating to 180 °C,
beyond the melting point of 4a (at 173 °C), caused abrupt
spectral changes involving the disappearance of νOH and its
Fermi resonance bands and the shift of the νCdO band to a higher
wavenumber (Figure 2c). The spectral changes were ac-
companied by the appearance of a broad band around 3000
cm-1, assignable to a νOH band derived from the formation of
the carboxylic acid dimer of 4a. These spectral changes were
completely reversible upon heating and cooling across the
melting point, suggesting that the pseudopolymer chains formed
through hydrogen bonding between the pyridyl and carboxyl
groups in a head-to-tail manner undergo thermoreversible
polymerization from and depolymerization to small component
molecules such as monomeric 4a and/or its dimers.
Self-Organization in Aqueous Media. A. Self-Organization
Behavior. When a 1 mmol dm-3 aqueous solution of either 3b
or 4b containing a 10 molar equivalent amount of NaOH was
left to stand for several days under atmospheric conditions, self-
organization into well-defined microfibers occurred. Parts a and
b of Figure 3 show optical microscopy images of typical
microfibers made from 3b and 4b, respectively. The microfibers
are more than several hundred micrometers in length and 1 µm
in diameter. The microfibers of 3b and 4b were formed at about
pH 9 and 10, respectively, and their morphology was highly
stable in the aqueous medium for several months. Both of the
yield and the shape of the microfibers were independent of the
initial concentration of 4b in the range of 2 × 10-5-10-2 mol
dm-3. AFM images of dried fibers, shown in Figure 4e,f,
indicate that the microfibers of 3b and 4b are composed of
bundles of submicrofibers with an almost uniform diameter of
350 and 200 nm, respectively. In contrast to 3b and 4b,
azopyridine carboxylic acid, with a shorter spacer length (2b),
failed to self-organize into fibrous assemblages. The azopyridine
carboxylic acid without the propyl substituent (3a) formed

Self-Assembly of Amphoteric Azopyridine Carboxylic Acids
J. Am. Chem. Soc., Vol. 122, No. 44, 2000 11001
Figure 5. Polarized FT-IR spectra of a microfiber self-organized from
4b at (a) 0°, (b) 45° and (c) 90° angles between the polarization plane
of the probe light and the long axis of the fiber.
pure compounds recrystallized from organic solvents. This
indicates the formation of consecutive intermolecular hydrogen
bonds in a head-to-tail manner between the pyridyl and carboxyl
groups, leading to the self-organization of a pseudopolymer
structure by a single-component molecule, as depicted in Figure
1. In contrast, FT-IR spectra of the leaflet crystals formed from
-
an alkaline solution of 4a (Figure 4c) appeared νas,COO ) 1554
cm-1, ascribable to the sodium salt, as supported by the
elemental analysis. These results indicate that the fibrous
materials of 3b and 4b and the leaflet crystals of 3a are
comprised of pseudopolymer structures as a result of hydrogen
bond formation between the pyridyl and carboxyl groups in a
head-to-tail manner, while the leaflet crystals of 4a consist of
the corresponding sodium salt.
Figure 4. FT-IR spectra of the molecular assemblages formed from
(a) 3a, (b) 3b, (c) 4a, and (d) 4b.
Polarized FT-IR microscopy measurements were performed
to obtain further information about the molecular orientations
in a microfiber of 4b. As shown in Figure 5, FT-IR spectra
displayed a marked dependence of the absorption band intensity
on the angle (θ) made by the long axis of the fiber and the
polarization plane of probing light. The maximum for νφ-O and
the minimum for νCdO appear at θ ) 0°, respectively. As
presented in Figure 1, the transition moment of the band for
νφ-O lies in parallel with the long molecular axis of 4b, while
the νCdO band has the transition moment perpendicular to the
molecular axis on average. These results indicate decisively that
the molecular long axis of the azopyridine carboxylic acid 4b
is aligned parallel to the fiber axis.
crystalline leaflets under the same conditions, as shown in Figure
3c. 4a gave also leaflet crystals (Figure 3d) from the alkaline
solution of the mixture of water and THF (3:1 v/v). The
influence of THF on the formation of the microfibers of 4b
was ignored since microfibers with an analogous morphological
feature were deposited from the mixed solvent of water and
THF. Taking the difference in morphological features between
3a and 3b or 4a and 4b into consideration, we can conclude
that the propyl side-chain substituent on the azopyridine moiety
is necessary in order for the microfibers to be formed.
B. Elemental Analysis. The elemental analysis of fibers dried
in vacuo gave a chemical composition of C, 67.24%; H, 6.82%;
N, 11.75% for 3b and C, 70.12%; H, 8.28%; N, 9.97% for 4b.
These results are consistent with those for pure samples of 3b
(C, 67.58%; H, 7.09%; N, 11.82%) and 4b (C, 70.56%; H,
8.29%; N, 9.87%), revealing that the fibrous materials are hardly
contaminated by sodium ion, even when formed in alkaline
aqueous solutions. The elemental analysis of leaflet crystals of
3a and 4a exhibited a chemical composition of C, 65.14%; H,
5.99%; N, 13.37% for 3a and C, 65.30%; H, 7.37%; N, 10.40%
for 4a. These values are in good agreement with those of the
pure compound 3a and the corresponding sodium salt of 4a,
respectively. (Calcd for C17H19N3O3 of 3a: C, 65.16%; H,
6.11%, N, 13.41%. Calcd for C22H28N3 NaO3 of 4a: C, 65.17%;
H, 6.96%; N, 10.36%).
D. Powder X-ray Diffraction Analysis. Powder X-ray
diffraction (XRD) measurements were carried out to investigate
the molecular structures of the resulting self-organized materials.
Figure 6a,b shows the XRD patterns observed for fibrous
materials of 3b and for leaflet crystals of 3a, respectively. Lattice
spacings d calculated according to the Bragg equation are also
presented in Figure 6. A prominent feature of XRD patterns of
the fibrous materials is that the broad peak appears in the region
of 2θ > 20° (d < 0.44 nm) for the fibrous materials of 3b with
a propyl side-chain substituent as compared to the case for the
leaflet crystals of 3a without the substituent. Analogous dif-
fraction patterns were obtained for the fibrous assemblages of
4b and leaflet crystals of 4a. These results support the conclusion
that the fibrous materials formed from 3b and 4b are not made
of crystals and that the pseudopolymer structures are formed
as a result of the consecutive formation of intermolecular
hydrogen bonds of the azopyridine carboxylic acids in a uniaxial
manner, presumably because of the reduction of lateral ordering
among pseudopolymer chains. It is obvious that the propyl
substituent plays a critical role in the suppression of π-π
C. FT-IR Analysis. FT-IR measurements were carried out
to elucidate the molecular structures of self-organized azo-
pyridine carboxylic acids deposited from alkaline aqueous
solutions. The results are shown in Figure 4. Leaflet crystals
formed from 3a (Figure 4a) and fibrous materials made from
3b (Figure 4b) and 4b (Figure 4d) exhibited the same bands
ascribed to νCdO, νOH, and the Fermi resonance, at ca. 1707,
2500, and 1930 cm-1, as those observed for the corresponding

11002 J. Am. Chem. Soc., Vol. 122, No. 44, 2000
Aoki et al.
Figure 6. Powder X-ray diffraction patterns of microfibers of (a) 3b
and (b) leaflet crystals of 3a in a wet state.
stacking among aromatic azopyridine moieties in the self-
organized fibrous materials.
Photoinduced Morphological Changes of Self-Organized
Materials. It is of great interest to investigate the influence of
photoirradiation on the self-organization behavior of the am-
photeric azopyridine carboxylic acids, because E-Z photo-
isomerization of aromatic azo-compounds generally brings about
the transformation of their chemical structure from the rodlike
E-isomer to the bent Z-isomer. The amphoteric azopyridine
carboxylic acid 4b, dissolved in an aqueous NaOH solution,
exhibits an absorption maximum centered at 364 nm due to the
π-π* transition of the azopyridine moiety. Photoirradiation of
the aqueous solution of 4b with monochromatic 365-nm light
caused E-to-Z photoisomerization of 4b, displaying a decrease
in the absorbance at 364 nm and an increase in the absorbance
at 440 nm due to the n-π* transition in addition to the
appearance of an isobestic point at 425 nm. When the solution
was illuminated with light of >350 nm, the proportion of
Z-isomer at the photostationary state was reduced because of
the simultaneous n-π* excitation. E-to-Z photoisomerizability
on irradiation with 365- and >350-nm light was estimated to
be 38 and 18%, respectively.24 The Z-to-E thermal reversion
was completed within a few hours in the dark at room
temperature. On the other hand, no spectral change due to E-Z
photoisomerization occurred, even when 4b dissolved in an
acidic solvent like acetic acid was exposed to 365-nm light.
These results are in agreement with previous reports on the
photoisomerization behavior of phenylazopyridines and azobis-
(pyridine)s,25 indicating that either protonated or alkylated
pyridinium moiety as a strong electron-withdrawing group
markedly destabilizes Z-isomers due to their electronic structure,
similar to a push-pull type of azobenzenes.
Figure 7. Optical microscopy images of molecular assemblages formed
from 4b (a) in the dark, (b) under irradiation with light at >350 nm,
and (c) with UV light at 365 nm, respectively (scale bar: 100 µm).
with needlelike fibers radiating from each single point, as shown
in Figure 7c. The morphology of the assemblages is quite
different from that deposited from an aqueous alkaline E-isomer
of 4b in the dark (Figure 7a). Careful comparison of part a of
Figure 7 with part c indicates that the growth of needlelike
materials along with the propagation direction of the microfibers
is suppressed under the photoirradiation. The illumination with
>350-nm light to give a photostationary state of a lower level
of Z-isomer caused intermediate morphological features of
fibrous materials, as shown in Figure 7b. Consequently, it may
be concluded that morphological features of the molecular
assemblages are controllable by light.
To reveal the effect of the coexistence of Z-isomer on the
formation of the microfibers, aqueous alkaline solutions of 4b
were exposed to light of 365 or >350 nm throughout the
deposition of self-organized materials, since the Z-isomer of
4b is reversed to the E-isomer within a few hours. When a 0.2
mmol dm-3 aqueous solution (0.7 mL) of 4b containing 20
mmol dm-3 NaOH was left to stand for 13-15 h under
irradiation with 365-nm light, unique assemblages were formed,
(24) E-to-Z photoisomerizability was defined as (A0 - A)/A0, where A0
and A represent the absorbance at 364 nm (absorption maxima) before and
after photoirradiation, respectively.
Discussion
(25) (a) Brown, E. V.; Granneman, G. R. J. Am. Chem. Soc. 1975, 97,
621. (b) Nakagawa, M.; Rikukawa, M.; Watanabe, M.; Sanui, K.; Ogata,
N. Bull. Chem. Soc. Jpn. 1997, 70, 737.
The amphoteric azopyridine carboxylic acids self-organize
into macroscopically well-defined molecular assemblages due

Self-Assembly of Amphoteric Azopyridine Carboxylic Acids
J. Am. Chem. Soc., Vol. 122, No. 44, 2000 11003
to cooperative intermolecular interactions despite the simplicity
of their chemical structure. It is reasonable to conclude that the
formation of the molecular assemblages is driven by intermo-
lecular interactions, including the head-to-tail hydrogen bonding
between the carboxyl and pyridyl groups, the π-π stacking and
the dipole-dipole interactions among the aromatic chro-
mophores, and hydrophobic interactions among the spacer
chains, as depicted in Figure 1. Due to the involvement of
pyridyl, carboxyl, and azo groups in the molecules, the
azopyridine carboxylic acids are sensitive to external stimuli
such as heat, pH alteration, and light, which accordingly
influence molecular assembling processes decisively as follows.
Concerning the effect of heat treatment, supramolecular struc-
tures exhibiting pseudopolymer chains formed by consecutive
hydrogen bonds in a head-to-tail manner undergo reversible
depolymerization to and polymerization from small molecular
species such as a monomer and/or a dimer of the azopyridine
carboxylic acid upon heating. The azopyridine carboxylic acids
dissolve in an aqueous alkaline solution as carboxylate anions,
which are neutralized gradually by atmospheric carbon dioxide
to give fibrous materials as supramolecular assemblages when
a propyl substituent is introduced at the aromatic ring. Photo-
irradiation of the alkaline solution influences the self-organiza-
tion process to deform the morphology of the fibrous materials.
A detailed discussion on the effect of these factors on molecular
assemblages of the azopyridine carboxylic acids is given below.
chemical structure of the azopyridine carboxylic acids. 3b and
4b with a propyl side chain gave fibrous materials exhibiting
less crystallinity, while leaflet crystals were obtained from 3a
and 4a, both of which bear no substituent, as shown in Figure
3a-d.
It is worth noting that the fibrous materials and the leaflet
crystals of 4a had quite different chemical compositions. The
FT-IR measurements and elemental analyses indicate clearly
that the fibrous materials of 3b and 4b and the leaflet crystals
of 3a consist of pseudopolymers of azopyridine carboxylic acids
as a result of head-to-tail coupled hydrogen bonding, whereas
the leaflet crystals made from 4a consist of sodium salts of
azopyridine carboxylate. For the self-organization process of
fibrous materials, it is assumed that the azopyridine carboxylate
anions in an alkaline solution are neutralized gradually by
atmospheric carbon dioxide to form the intermolecular hydrogen
bonds. Indeed, the deposition of molecular assemblages took
place at pH 6 for 2b, pH 9 for 3b, and pH 10 for 4b. Imae8
and Shimizu10b reported analogous results for the unusually
facile protonation of carboxylic acid derivatives when the length
of their methylene chains was increased. In addition, Shimizu
et al. reported that the neutralization of an alkaline aqueous
solution containing a bolaamphiphile with two carboxyl groups
by atmospheric carbon dioxide leads to the formation of
hydrogen-bonded microtubes.10 These facts are consistent with
our observation, indicating that precursor aggregates of the
azopyridine carboxylate anions were formed probably due to
hydrophobic interactions among the methylene spacers. These
precursors presumably act as nuclei, leading to the one-
dimensional growth of the head-to-tail coupled hydrogen bonds
as a consequence of neutralization by carbon dioxide in the air.
Organized Structure Affected by Heat. As revealed by FT-
IR measurements presented in Figure 2, heating of 3a at
temperatures above the melting point (173 °C) results in the
disappearance of the absorption bands at 1707, 1930, and 2500
cm-1 due to the formation of consecutive hydrogen bonds
between the carboxyl and pyridyl groups, leading to the
appearance of the band at ca. 3000 cm-1 ascribable to carboxylic
acid dimers (Figure 2c). Upon cooling below the melting point,
the thermally generated dimeric species are transformed again
to give the head-to-tail coupled and hydrogen-bonded pseudopoly-
mer structure. Closely related works were reported previously
on binary systems consisting of pairs of pyridine and carboxylic
acid derivatives to give supramolecular liquid crystals and liquid
crystalline polymers.11,12 The self-organized liquid crystalline
materials formed through hydrogen bonds in this way display
the cleavage of the hydrogen bonds over a phase transition
temperature between a mesophase and an isotropic phase. To
the authors’ knowledge, the present observation is the first
example of the thermoreversible structuring of pseudopolymers
starting from single compounds. The consecutive formation of
hydrogen bonds in a head-to-tail manner to give thermodynami-
cally stable pseudopolymer structures plays an essential role in
the self-organization of azopyridine carboxylic acids.
The next process involves the self-assemblages of the linear
pseudopolymers into submicron fibers with a diameter of ca.
350 nm for 3b and of ca. 200 nm for 4b as shown in Figure
3e,f. It is anticipated that the formation of the hydrogen bonds
results in the enlargement of dipole-dipole interactions among
the pseudopolymer chains, which is essential in the self-
organization of the pseudopolymer chains in the lateral directions
to give the fibrous materials. This anticipation is supported by
the polarized FT-IR microscopy measurements, which reveal
that the maximum intensity of νφ-O as the probe band for the
molecular axis lies at the direction parallel to the fiber axis
(Figure 5).
The absence of the propyl substituent in the azopyridine
carboxylic acids (3a and 4a) causes crystallization to give
leaflets as a result of strong π-π stacking forces and dipole-
dipole interactions among the azopyridine chromophores. In
other words, the propyl substituent plays a crucial role in
reducing the molecular interactions in the lateral directions to
suppress the crystallization. The occurrence of the strong π-π
stacking in leaflet crystals of 3a and 4a was confirmed by
powder X-ray diffraction measurements, as shown in Figure 6,
exhibiting clear peaks in the wide-angle region (2θ > 22°, d <
0.4 nm). It should be mentioned here that crystalline leaflets of
4a are composed of the corresponding sodium salt, whereas the
crystalline leaflets of 3a obtained from an alkaline solution are
identical with the pure 3a recrystallized from ethanol. The
marked difference stems from the lower solubility of 4a in an
alkaline aqueous solution due to the strong molecular interac-
tions, including the π-π stacking of the aromatic rings, the
dipole-dipole interactions, and hydrophobic interactions among
the decamethylene spacer chains. Because THF was added to
an alkaline aqueous solution to obtain a homogeneous solution
of 4a, the evaporation of THF occurred more rapidly when
Self-Organization Affected by pH Change. The amphoteric
azopyridine carboxylic acids are soluble in basic and acidic
organic solvents such as pyridine and acetic acid. Such
amphoteric compounds, except for 4a, are readily dissolved also
in alkaline aqueous solutions, whereas they show poor solubility
in a neutral aqueous solution. This makes it possible to control
the deposition of the azopyridine carboxylic acids from alkaline
solutions by gradual neutralization to form molecular as-
semblages through the formation of consecutive hydrogen bonds.
In fact, the azopyridine carboxylic acids (3a, 3b, and 4b),
dissolved in alkaline aqueous solutions as carboxylate anions,
were deposited by aging under atmospheric conditions for
neutralization with atmospheric carbon dioxide to give molecular
assemblages containing no alkaline metal salt. The morphology
of the molecular assemblages was remarkably dependent on the

11004 J. Am. Chem. Soc., Vol. 122, No. 44, 2000
Aoki et al.
compared with the neutralization with atmospheric carbon
dioxide, to result in recrystallization of the sodium salt.
Changes in Organization Morphology by Light. Since the
moderation of molecular interactions of the azopyridine car-
boxylic acids in the lateral directions is a key to assembling
fibrous materials as supramolecular aggregates, as discussed
above, the photoconversion into the Z-isomer is an alternative
way to modulate the lateral molecular interactions. Three kinds
of alkaline aqueous solutions of 4b with different levels of the
photoisomerization were subjected to gradual deposition of
insoluble materials. As shown in Figure 7, it was found that
the growth of the resulting supramolecular assemblages was
hindered efficiently by the increment of Z-isomer contents. It
should be stressed that the materials deposited under the
photoirradiation exhibit XRD patterns identical with those of
the fibrous materials made from the E-isomer of 4b in the dark,
irrespective of their different morphologies. If we accept, in the
foregoing formation process, that fluidlike micelles or bilayers
nucleate the growth of the submicrofibers, photoinduced Z-
isomer should affect the shapes of the nucleating species. This
is very likely to be the case, because the molecular structure of
azopyridine carboxylate anions composing the nuclear species
can be modified by E-to-Z photoisomerization upon UV light
irradiation, and because the photoinduced Z-isomer possesses
a different basicity in comparison with the E-isomer,25a altering
the strength and direction of hydrogen bonding. Attention has
to be given to the fact that the protonation of the Z-isomer of
4b leads to rapid Z-to-E thermal reversion in solutions. Since
the short lifetime of the photoinduced Z-isomer prevents our
further studying the nucleation process in detail, another system
is under investigation.
interactions involving hydrogen bonding, π-π stacking, dipole-
dipole interactions, and hydrophobic interactions. Heat treatment
of the linear pseudopolymers arising from head-to-tail coupled
hydrogen bonding between pyridyl and carboxyl residues caused
reversible supramolecular polymerization. The gradual neutral-
ization of alkaline aqueous solutions of the azopyridine car-
boxylic acids (3b and 4b) with the propyl substituent induced
by atmospheric carbon dioxide brought about the self-organiza-
tion of well-defined submicrofibers. E-to-Z photoisomerization
of 4b affects morphological features of the fibrous materials
such as the length of the fibers and the number of branching
points, depending on the level of Z-isomer content. That is to
say, molecular-level modifications arising from external stimuli
including heat, pH, and light are amplified by the self-
organization process through cooperative effects of multiple
noncovalent intermolecular interactions on their organization
morphology at the macroscopic level. We expect that these
systems develop further to create progressive molecular as-
semblages having the desired morphology and functionality on
demand.
Acknowledgment. The authors are grateful to Mr. Kimura
and Mr. Nakano of Nippon Bio-Rad Laboratories for polarized
FT-IR microscope measurements. We are grateful to Professor
Takashi Kato of the University of Tokyo and Professor Takahiro
Seki of Tokyo Institute of Technology for their helpful
discussions.
Supporting Information Available: Experimental section
(material, deposition of an azopyridine carboxylic acid from
an alkaline solution, physical measurement, and photoirradiation)
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
Conclusion
Amphoteric azopyridine carboxylic acids display character-
istic self-organization behavior due to multiple intermolecular
JA001790F