Supramolecular J-aggregate assembly of a covalently linked zinc porphyrin-β-cyclodextrin conjugate in a water/ethanol binary mixture

Article abstract of DOI:10.1021/jp905904h

The aggregation behavior of the zinc porphyrin-β-CD conjugate in a water/ethanol binary mixed solution was investigated. The spectroscopic data and the atomic force microscopy (AFM) image strongly suggest that a part of a Zn porphyrin is included in the β

Full text of DOI:10.1021/jp905904h

11560
2009, 113, 11560–11563
Published on Web 08/04/2009
Supramolecular J-Aggregate Assembly of a Covalently Linked Zinc
Porphyrin-ꢀ-cyclodextrin Conjugate in a Water/Ethanol Binary Mixture
Takayuki Kiba, Hiroki Suzuki, Kiyotada Hosokawa, Hirohisa Kobayashi, Shingo Baba,
Toyoji Kakuchi, and Shin-ichiro Sato*
DiVision of Biotechnology and Macromolecular Chemistry, Graduate School of Engineering,
Hokkaido UniVersity, Sapporo 060-8628, Japan
ReceiVed: June 23, 2009; ReVised Manuscript ReceiVed: July 27, 2009
The aggregation behavior of the zinc porphyrin-ꢀ-CD conjugate in a water/ethanol binary mixed solution
was investigated. The spectroscopic data and the atomic force microscopy (AFM) image strongly suggest
that a part of a Zn porphyrin is included in the ꢀ-CD nanocavity of another ZnP-ꢀ-CD conjugate at certain
concentrations, leading to the formation of Zn porphyrin J-aggregates.
Porphyrins and metalloporphyrins play key roles in biomi-
metic photosynthesis and other photon-driven processes in
photophysical chemistry. Porphyrin J-aggregates have attracted
attention because of their potential application as nonlinear
optical materials,1 and as model substances for aggregates of
the light-harvesting antenna chlorophyll with a “storage-ring”
configuration in photosynthesis.2 Accordingly, there have been
many studies of the porphyrin J-aggregates and their photo-
physical properties. For example, the water-soluble tetrakis-
(4-sulfonatophenyl)porphyrin (TPPS4) has received attention
because it can easily form highly ordered J-aggregates due to
its Coulombic interactions in acidic aqueous solution.3-6 On the
Figure 1. Chemical structure of ZnP-ꢀ-CD.
other hand, until now, there have been few reports of J-aggregate
formation of neutral porphyrins.7-9
The porphyrin-cyclodextrin (Por-CD) conjugate is a very
leading to the formation of highly ordered J-aggregates. We
attractive supramolecular system, since it possesses functions
also conducted atomic force microscopy (AFM) measurements
closely mimicking those of biological systems, and it has
potential use as a scaffold for supramolecular assemblies. CDs
in order to obtain direct evidence of ZnP-ꢀ-CD J-aggregate
formation.
(R-, ꢀ-, and γ-CD), which are oligosaccharides with hydrophobic
Zinc tetraphenylporphyrin-linked ꢀ-cyclodextrin (ZnP-ꢀ-CD)
interiors and hydrophilic exteriors, can encapsulate organic and
was synthesized as described elsewhere.10 Briefly, the freebase
inorganic molecules in aqueous solution. Por-CD conjugates
composite (H2P-ꢀ-CD) was prepared by a condensation reac-
have been synthesized and their properties investigated by
tion of H2PCOOH with 6-deoxy-6-amino-ꢀ-cyclodextrin in dry
several researchers.10-16 Kano et al. reported a Por-CD
DMF/THF (1:1) mixed solvent containing a small amount of
conjugate that can be used as a host/guest unit for forming
1,3-dicyclohexylcarbodiimide, 1-hydroxybenzotriazole hydrate,
heteroporphyrin arrays through noncovalent bonding simply by
and triethylamine. H2PCOOH was obtained by dealkylation of
mixing with another water-soluble porphyrin in aqueous solu-
5-(4-methoxycarbonylphenyl)-10,15,20-triphenyl-21H, 23H-por-
tion.12
phine (Tokyo Chemical Industry Company, Tokyo, Japan).
Herein, we report on self-association behaviors of the zinc
H2P-ꢀ-CD was then reacted with zinc acetate to yield the
tetraphenylporphyrin linked ꢀ-CD conjugate (Figure 1)10 in a
ZnP-ꢀ-CD by stirring in a chloroform/methanol (5:1) mixed
water/ethanol binary mixed solution. We investigated the
solvent for 4 days. The formation of the ZnP-ꢀ-CD was
concentration dependence of the steady-state absorption spectra
confirmed by fast atom bombardment (FAB) mass spectrometry,
of ZnP-ꢀ-CD in the presence and absence of 1-adamantanol,
1H NMR spectroscopy, and UV/vis spectroscopy.
which is well-known to be incorporated into the CD cavity with
1-Adamantanol (ADM) was purchased from Aldrich
high affinity and thereby to displace the encapsulated porphyrin
Chemical Co. (Milwaukee, WI). All sample solutions were
moiety. These spectroscopic data strongly suggest that a part
prepared by mixing ZnP-ꢀ-CD in water/ethanol (7:3)
of the porphyrin moiety is included in the ꢀ-CD nanocavity of
another ZnP-ꢀ-CD conjugate at a specific concentration range,
solution, and stirred for 12 h. A Milli-Q water purification
system (Millipore, Bedford, MA) was used for purification
of water. Ethanol (Kanto Kagaku, Tokyo, Japan) was used
without further purification. The concentrations of sample
* To whom correspondence should be addressed. E-mail: s-sato@
eng.hokudai.ac.jp. Phone/Fax: +81-11-706-6607.
10.1021/jp905904h CCC: $40.75  2009 American Chemical Society

Letters
J. Phys. Chem. B, Vol. 113, No. 34, 2009 11561
Figure 2. Concentration dependence of the absorption spectra of
ZnP-ꢀ-CD in a water/ethanol (7:3) binary mixture. The concentration
of ZnP-ꢀ-CD was varied from 1.0 × 10-6 to 3.0 × 10-5 M (a-g).
The inset shows the absorption spectra of the Soret region normalized
at 422 nm.
Figure 3. Absorption spectra of ZnP-ꢀ-CD in a water/ethanol (7:3)
solution with 1.4 × 10-3 M ADM (solid line) and without ADM (dashed
line).
solutions varied from 1.0 × 10-6 to 3.0 × 10-5 M. Steady-
state absorption and fluorescence spectra were measured with
a U-3010 spectrophotometer (Hitachi, Tokyo, Japan). All
measurements were carried out using freshly prepared
solutions in a 10 mm cuvette cell at 293 K.
Figure 2 shows the concentration dependence of the absorp-
tion spectra of ZnP-ꢀ-CD in a water/ethanol (7:3) solution.
The ZnP-ꢀ-CD concentration was varied from 1.0 × 10-6 to
3.0 × 10-5 M. At the lowest concentration (1.0 × 10-6 M),
only a monomeric band at 422 nm was observed in the Soret
region, which indicates that ZnP-ꢀ-CD exists as a monomer.
As the concentration of ZnP-ꢀ-CD was increased, interestingly,
a sharp red-shifted band (band I) appeared at around 450 nm.
The intensity of this band relative to the monomeric Soret band
(422 nm) was enhanced with increasing concentration, and the
sharp band shape was visible until the concentration reached
ca. 1.0 × 10-5 M. Above 1.0 × 10-5 M, spectral broadening
was observed for both the Soret band and band I. The inset in
Figure 2 shows the normalized spectra of these bands. We can
see that two bands newly appeared as a shoulder around the
Soret band, the red-shifted one (band II) at 435-445 nm and
the blue-shifted one (band III) at 405-415 nm. Band I observed
at relatively lower concentrations (<1.0 × 10-5 M) may be
assigned to a porphyrin J-aggregate, judging from its sharp band
shape and the red-shift value. Yamaguchi et al. reported the
J-aggregate formation of dendritic zinc porphyrins driven by
hydrogen bonds,8 and the position (453 nm) and the line shape
of its J-band agreed well with our observation. In our case, the
driving force behind the formation of J-aggregates may have
been the hydrophobic interactions that occurred by encapsulation
of the ZnP moiety of ZnP-ꢀ-CD into the CD moiety of another
ZnP-ꢀ-CD, which could have allowed ZnP-ꢀ-CD to form
consecutive ZnP J-aggregates.
Figure 4. AFM image of ZnP-ꢀ-CD spin-coated onto HOPG (upper
panel). The cross section analysis of the green line in the AFM image
is presented in the lower panel.
is known to be incorporated into the CD cavity with high affinity
and thereby to displace the encapsulated porphyrin moiety.
Figure 3 shows the spectral change in the Soret region of
ZnP-ꢀ-CD (4.0 × 10-6 M) in the presence and absence of 1.4
× 10-3 M ADM. The disappearance of the J-band by the
addition of ADM strongly suggests that the J-aggregation of
ZnP-ꢀ-CD involves the CD inclusion phenomenon. Further-
more, the enhancement of the monomeric Soret band clearly
indicates that the ZnP J-aggregates dissociate and exist as
monomers by the ADM addition.
To obtain more direct evidence of the consecutive inclusion
structure of the ZnP-ꢀ-CD J-aggregate, atomic force micros-
copy (AFM) measurements were carried out. AFM images were
recorded on a Nanoscope IIIa (Digital Instruments, Santa
Barbara, CA) at the OPEN FACILITY, Hokkaido University
Sousei Hall. Topographic images were taken at a tapping mode
in air using a silicon cantilever with a rotated tip whose typical
radius was 10 nm (MPP-21100-10, Veeco). For AFM measure-
ments, 1.0 × 10-5 M ZnP-ꢀ-CD solution was spin-coated at
1500 rpm onto freshly cleaved highly oriented pyrolytic graphite
(HOPG). A typical AFM image is shown in Figure 4. There
Above 1.0 × 10-5 M, the CD encapsulation may not be
sufficient to stabilize the insolubility of porphyrin moieties, and
thus ZnP-ꢀ-CD begins to form other aggregates in which the
porphyrin moieties associate. Li et al. reported the dimer
formation of ZnTPP in an acetonitrile solution, whose absorption
bands were located at around 436 and 413 nm.9 Thus, band II
(∼440 nm) and band III (∼410 nm) observed for ZnP-ꢀ-CD
at higher concentrations may be attributable to the dimer
absorption of a weakly (compared to J-aggregates) associated
dimer.
The fact that the CD cavity plays a key role in the J-aggregate
formation was evidenced by using 1-adamantanol (ADM), which

11562 J. Phys. Chem. B, Vol. 113, No. 34, 2009
Letters
SCHEME 1: Schematic Representation and Molecular Modeling of the Proposed Structures for ZnP-ꢀ-CD
J-Aggregatesa
a Parts a and c show aggregates in linear form, and parts b and d show aggregates in bent (zigzag) form.
are some rod-like features with a length of 100-300 nm. The
heights of the observed rod-like features ranged from 1.0 to
3.0 nm. The width of the rod-like structures should be shorter
than the AFM horizontal resolution of about 20 nm, which
depends on the curvature of the AFM tip. On the basis of these
observations, the rod-like structures appear to correspond to
consecutive supramolecular aggregates of ZnP-ꢀ-CD, which
were linked together by cyclodextrin inclusion.
than the distances between porphyrin moieties in the linear type,
and there would be enough overlaps between porphyrin moieties
to yield a sharp J-band. Therefore, the bent structure is more
likely to be representative of the supramolecular ZnP-ꢀ-CD
aggregates in the present system. Castricano et al. investigated
the aggregate formation of anionic tetrakis(4-sulfonatophe-
nyl)porphyrin (TPPS4) in aqueous microemulsions, and reported
that the interplane distance between TPPS4 molecules was 3.6
Å, and the J-band splitting was ca. 4000 cm-1 17
. In our case,
the interplane distance of ZnP was much longer than that of
TPPS4, and the J-band splitting (ca. 1400 cm-1) was much
smaller than that of TPPS4.
Here, we estimate the structure of supramolecular J-ag-
gregates of ZnP-ꢀ-CD based on the spectroscopic data and
AFM images. The appearance of a relatively sharp J-band in
the absorption spectra clearly indicates the formation of highly
ordered J-aggregates in which many porphyrin units interact
via π-π interactions. The disappearance of the J-band upon
addition of ADM suggests that the formation of porphyrin
J-aggregates involves CD inclusion phenomenon. On the basis
of molecular modeling, we propose two types of aggregate
structures of ZnP-ꢀ-CD, a “linear” type and “bent” (zigzag)
type, which are shown in Scheme 1. In the case of the linear
type, which is the simpler structure, the distance between
porphyrin π-electron clouds was not sufficient to yield a so-
called sharp J-band, although the tube-like supramolecular
structure was achieved by using CD inclusion. That is, the
distance between center zinc atoms of porphyrin moieties was
estimated to be ca. 21 Å, and the interplane distance of
porphyrins was ca. 8.5 Å, assuming the geometry shown in
Scheme 1c. Therefore, the linear type may not be a likely
structure that can account for the red shift observed for our
J-aggregates. On the other hand, in the case of the bent type,
the Zn-Zn and interplane distances between porphyrin moieties
are ca. 9.1 and 6.4 Å, respectively, which are much smaller
In summary, the aggregation behavior of hydrophobic por-
phyrin-ꢀ-CD conjugates in a water/ethanol binary mixed
solution was investigated. The observation of red-shifted sharp
absorption and the AFM image at certain concentrations strongly
suggest the formation of J-aggregates, where the zinc porphyrin
moieties are included within the ꢀ-CD nanocavity of another
ZnP-ꢀ-CD conjugate. As far as we know, this is the first report
of the formation of J-aggregates by means of a covalently linked
porphyrin-cyclodextrin conjugate. Most of the previously
reported Por-CD conjugates have used water-soluble hydro-
philic porphyrins. Therefore, the binding force between the
porphyrin moiety and the CD cavity of another conjugate would
not be sufficient to form J-aggregates. In our case, the
supramolecular assembly of J-aggregates was achieved by using
the hydrophobic porphyrin. Our results suggest that the su-
pramolecular architecture using CD inclusion can be successfully
applied to the dye J-aggregate formation.

Letters
Acknowledgment. This work was supported by a Grant-in-
Aid for Scientific Research (C) (No. 20550001) from MEXT
Japan to S.-i.S., and by Grant-in-Aid for JSPS Fellows (No.
20·2850) from JSPS to T.K. This work was partly supported
by the Global COE Program (Project No. B01: Catalysis as the
Basis for Innovation in Materials Science) from MEXT, Japan.
J. Phys. Chem. B, Vol. 113, No. 34, 2009 11563
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