Supramolecular fibers and microbelts from a phthalhydrazide derivative of crown ether with alkyl chains

Article abstract of DOI:10.1246/cl.2006.1256

Assemlbing behaviors of some crown ether derivatives with alkyl chains were investigated. It was found that a phthalhydrazide derivative was unique among them in that it assembles into fibers in solution, which transform into microbelts on the surface of

Full text of DOI:10.1246/cl.2006.1256

1256
Chemistry Letters Vol.35, No.11 (2006)
Supramolecular Fibers and Microbelts from a Phthalhydrazide Derivative
of Crown Ether with Alkyl Chains
Joe Otsuki,ꢀ1 Yusuke Okabe,1 Satoshi Eitaki,1 Yoshihisa Sei,2 and Kentaro Yamaguchi2
1College of Science and Technology, Nihon University, 1-8-14 Kanda Surugadai, Chiyoda-ku, Tokyo 101-8308
2Department of Pharmaceutical Technology, Tokushima Bunri University, Shido, Sanuki 769-2193
(Received August 28, 2006; CL-060974; E-mail: otsuki@chem.cst.nihon-u.ac.jp)
Assemlbing behaviors of some crown ether derivatives with
due to hydrogen-bond formation. The 1H NMR spectrum for
PHC, which has a common phthalhydrazide head group as
PHCA but lacks alkyl chains, showed that the aromatic protons
and crown ether protons close to the head group were broadened,
while the farthest protons from the head group remained sharp.
The spectral behavior indicates that the aggregation is due to
the head group association. The comparison of the spectra for
PHCA and PHC reveals that alkyl chains also play an important
role in the formation of larger aggregates.
PHCA gelated toluene when a 1 mM or more concentrated
solution made at an elevated temperature was cooled down to
room temperature. Any other compounds listed in Chart 1 did
not gelate toluene. Gelation by PHCA implies that a fibrous
network is formed in solution that traps solvent molecules. A
transmission electron microscopy image for a sample cast and
air-dried from 0.1 mM toluene solution on a carbon-coated
TEM-grid is shown in Figure 1. Fibers with various diameters
in a range from several nanometers to tens of nanometers are
observed.
To gain insight into the unique behavior of PHCA, solutions
of PHCA were subjected to electrospray ionization mass spec-
trometry. The spectrum for 0.1 mM solution of PHCA in CHCl3,
with a spray temperature of 80 ꢁC, showed peaks at m=z ¼ 746:9
([PHCA + Na]þ), 1472.4 ([PHCA2 + Na]þ), and 2196.9
([PHCA3 + Na]þ) with relative intensities being 100, 8, and
2. The mass spectrum under lower temperature conditions
with a spray temperature of 23 ꢁC (coldspray ionization mass
spectrometry)6 gave the intensity ratios of 100, 16, and 18, with
additional small peaks for higher oligomers as shown in SI.4 The
marked increase in intensity for the trimer is indicative of an
exceptional stability of the trimeric assembly as compared to
other aggregates.
Phthalhydrazides can, in principle, exist as an equilibrium
mixture of three tautomeric forms: lactam–lactam, lactim–
lactam, and lactim–lactim forms. The lactim–lactam isomer,
which is the most stable,7 can assemble into a trimer via multiple
complementary hydrogen bonds. The trimeric structure is found
in the crystal of luminol.8 Phthalhydrazides modified with
alkyl chains were investigated. It was found that a phthalhydra-
zide derivative was unique among them in that it assembles into
fibers in solution, which transform into microbelts on the surface
of highly oriented pyrolytic graphite.
Nature uses ions, rather than electrons, for information proc-
essing. Artificial molecular devices may also be envisaged,
which are established on the basis of ions as information carriers.
In this context, nano/microarchitectures, especially fibers, con-
taining crown ethers merit special attention for their potential
use in ion selective components. To date, very limited classes
of compounds containing a crown ether moiety are known to
form fibers,1–3 phthalocyanine derivatives being prominent ex-
amples.1 Herein, we present our finding that a phthalhydrazide
derivative of crown ether bearing alkyl chains form fibers by
self-assembly, which transform into microbelts on a surface.
We examined four dibenzocrown ether derivatives bearing
alkyl chains on one benzene ring and various functionalities
on the other, two of them having hydrogen-bonding capability,
as shown in Chart 1 (see Supporting Information (SI) for prepa-
ration).4 We also used a model compound with a hydrogen-
bonding capability but without alkyl chains (PHC).5
1H NMR was employed to probe the association behavior in
CDCl3 solution (1 mM). All proton peaks for DBCA and PNCA,
which lack hydrogen-bonding capability, were sharp. Also, the
peaks for PICA were sharp regardless of the hydrogen-bonding
capability of the imide group. However, the peaks in both of the
aromatic and crown ether parts for PHCA were significantly
broadened, as shown in SI.4 A similar spectral shape was ob-
served for a 0.1 mM solution as well. The broadened spectra
suggest that aggregation of the molecules involving the whole
crown ether moiety occurs in the solution. The spectrum sharp-
ened up upon addition of an aliquot of CD3OD to the CDCl3 so-
lution, which provides evidence that the aggregation is primarily
O
O
O
O
O
X
X
C10H21
C10H21
O
O
O
O
O
O
O
O
NH
N
OH
O
PHC
O
NH
X
X
−CN
−CN
−Br
−Br
=
NH
N
OH
PHCA
O
PICA
PNCA DBCA
Chart 1. Crown ether derivatives used in this study. In the
codes, ‘‘PH’’, ‘‘PI’’, ‘‘PN’’, ‘‘DB’’, ‘‘C’’, and ‘‘A’’ represent
phthalhydrazide, phthalimide, phthalonitrile, dibromo, crown
ether, and alkyl groups, respectively.
Figure 1. TEM image of the fibers from PHCA. The grey
fibrous features are the molecular fibers. The black shadow
encircling the image is the carbon-coated TEM grid.
Copyright ꢀ 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.11 (2006)
1257
C10H21 C10H21
O
O
O
O
O
O
O
O
H
N
N
H
H
O
H
O
N
N
N
O
H
N
O
O
O
O
O
O
O
O
H
O
O
O
O
O
C10H21
C10H21
C10H21
C10H21
Figure 2. Likely structure of the trimer of PHCA in solution.
Figure 3. Representative DFM images (5 ꢃ 5 mm2) of the as-
semblies of PHCA on HOPG. (a) Fibers and layers. The height
profile between the pair of arrows A and B is given. (b) A region
where only the first layer is present. The arrows indicate steps of
the HOPG substrate. The height profile along the straight line is
given.
dendrons or alkyl chains also form trimers.9 The trimer structure
consistent with the present data and literature results8,9 is shown
in Figure 2. The diameter of the circle encompassing the extend-
ed alkyl chains is estimated to be ca. 5 nm by MM3 calculations.
Fibers were also observed on highly oriented pyrolytic
graphite (HOPG). A drop of toluene solutions with a concentra-
tion in a range of 0.1–1 mM was cast onto HOPG, which was
then air-dried. Samples thus prepared were observed by means
of dynamic force mode atomic force microscopy (DFM).
Representative DFM images are shown in Figure 3. In some
places, fibers similar to those observed in TEM were observed
as shown in Figure 3a. The diameter of the fibers, as judged from
the DFM height, varied in a wide range, with the thinnest fibers
being ca. 4 nm. The same procedure using PHC or a 1:1 mixture
of PHCA and PHC resulted only in globular aggregates. These
model experiments show that the alkyl chains are indispensable
for the fiber formation. The coincidence between the diameters
of the trimer model and the thinnest fibers suggests that the trim-
er units stack into the fibers with the aid of intermolecular alkyl–
alkyl interactions.
It is noted that the background of fibers in Figure 3a is not
the surface of HOPG, but is covered by a molecular layer. The
analysis of the image indicates that the layer is extremely flat,
the heights being multiples of a unit thickness of 3:3 ꢂ 0:4 nm,
as the height profile taken along the line between the pair of
arrows A and B across a fiber clearly shows. The fiber is located
just on the border of the first layer on the A side and the second
layer on the B side. It appears that the fiber was transforming
into the layer at this site, when the solvent dried up. It is likely
that the affinity of the alkyl chains toward the HOPG surface
promotes the transformation.
complementary hydrogen bonds. The fibers, which are formed
in solution and gelate toluene, transform into microbelts on the
HOPG surface. Ion uptaking and transporting properties of these
assemblies are of interest as materials for molecular ionics.
We thank Dr. A. Tsukamoto of Nihon University for help
in TEM measurements. This work was partially supported by
the MEXT, the High-Tech Research Center Project for Private
Universities.
References and Notes
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Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
Figure 3b shows one of the places where only the first layer
is observed. The long meandering morphology suggests that the
fibers formed in solution merge into wider belt-like first layer.
The crevices observed in the belts are directed parallel to the
belts, which supports the picture that the belts are made of
aligned fibers. Also, belts are not disrupted at the step edges of
the basal HOPG surface, as indicated by the arrows in the figure,
evidencing that the formation of the belts are not initiated at the
surface but rather preformed fibers are deposited and trans-
formed on the surface.
5
6
J. Otsuki, K. C. Russell, J.-M. Lehn, Bull. Chem. Soc. Jpn.
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a) S. Sakamoto, M. Fujita, K. Kim, K. Yamaguchi, Tetra-
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Summarizing, we have examined the aggregation behaviors
of several structurally related compounds containing a crown
ether moiety. The phthalhydrazide derivative is unique among
them in that it has a strong tendency to aggregate into fibers in
solution, likely through the formation of trimers via multiple
9
Published on the web (Advance View) October 7, 2006; doi:10.1246/cl.2006.1256