Design, syntheses, and studies of supramolecular porphyrin-fullerene conjugates, using bis-18-crown-6 appended porphyrins and pyridine or alkyl ammonium functionalized fullerenes

Article abstract of DOI:10.1021/jp057547q

Photoinduced electron-transfer processes in cis and trans functionalized bis-18-crown-6 porphyrin self-assembled with fullerene functionalized with pyridine or alkylammonium cation entities are reported. The structural integrity of the newly formed supramolecular conjugates was accomplished by optical absorption and emission, electron spray ionization mass, electrochemistry, and semiempirical PM3 calculations. A 1:2 stoichiometry of the supramolecular porphyrin:fullerene conjugates was deduced from these studies. The conjugates revealed stable two-point ' binding involving metal-ligand coordination and alkylammonium cation-crown ether binding or only the latter type of binding depending upon the functionality of the fullerene and metal ion in the porphyrin cavity. The effect of the variation on free energy changes of charge separation and the charge recombination was achieved by varying the metal ion in the porphyrin cavity. The charge-separation rates (k_CS) determined from the picosecond time-resolved emission studies were generally higher for the cis biscrown functionalized porphyrins than those of the corresponding trans ones. A comparison of the k_CS values reported earlier for 1:1 porphyrin-fullerene conjugates with a similar self-assembly mechanism suggested , that employing a higher number of acceptor entities improves the electron-transfer rates. The calculated chargerecombination rates (k_CR) were 2-3 orders of magnitude smaller than the k_CS values, suggesting the occurrence of the charge recombination process in the Marcus inverted region. The lifetimes of the radical ion pair (TRIP) ranged between 46 and 233 ns indicating charge stabilization in the studied conjugates.

Full text of DOI:10.1021/jp057547q

J. Phys. Chem. B 2006, 110, 5905-5913
5905
Design, Syntheses, and Studies of Supramolecular Porphyrin-Fullerene Conjugates, Using
Bis-18-crown-6 Appended Porphyrins and Pyridine or Alkyl Ammonium Functionalized
Fullerenes
Francis D’Souza,*,† Raghu Chitta,† Suresh Gadde,† Amy L. McCarty,† Paul A. Karr,†
Melvin E. Zandler,† Atula S. D. Sandanayaka,‡ Yasuyaki Araki,‡ and Osamu Ito*,‡
Department of Chemistry, Wichita State UniVersity, 1845 Fairmount, Wichita, Kansas 67260-0051, and
Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity,
Katahira, Sendai 980-8577, Japan
ReceiVed: December 28, 2005; In Final Form: February 7, 2006
Photoinduced electron-transfer processes in cis and trans functionalized bis-18-crown-6 porphyrin self-assembled
with fullerene functionalized with pyridine or alkylammonium cation entities are reported. The structural
integrity of the newly formed supramolecular conjugates was accomplished by optical absorption and emission,
electron spray ionization mass, electrochemistry, and semiempirical PM3 calculations. A 1:2 stoichiometry
of the supramolecular porphyrin:fullerene conjugates was deduced from these studies. The conjugates revealed
stable “two-point”’ binding involving metal-ligand coordination and alkylammonium cation-crown ether
binding or only the latter type of binding depending upon the functionality of the fullerene and metal ion in
the porphyrin cavity. The effect of the variation on free energy changes of charge separation and the charge
recombination was achieved by varying the metal ion in the porphyrin cavity. The charge-separation rates
(kCS) determined from the picosecond time-resolved emission studies were generally higher for the cis bis-
crown functionalized porphyrins than those of the corresponding trans ones. A comparison of the kCS values
reported earlier for 1:1 porphyrin-fullerene conjugates with a similar self-assembly mechanism suggested
that employing a higher number of acceptor entities improves the electron-transfer rates. The calculated charge-
recombination rates (kCR) were 2-3 orders of magnitude smaller than the kCS values, suggesting the occurrence
of the charge recombination process in the Marcus inverted region. The lifetimes of the radical ion pair (τRIP
)
ranged between 46 and 233 ns indicating charge stabilization in the studied conjugates.
Introduction
emerged as exceptional three-dimensional electron acceptors.1
Macrocycles such as porphyrins, phthalocyanines, and metal
polypyridyl have been successfully used as electron donors.1
The advantage of these donors lies in the minimal structural
changes when electrons are released from them. Studies
performed have shown that the nature of the linker between
the donor and acceptor entities controls the electronic coupling
and thus small structural variations affect the overall photody-
namics of charge separation.1 However, it is often difficult to
control the distance and orientation in noncovalently bound
donor-acceptor systems primarily due to weak interactions and
the associated equilibrium processes. We have addressed this
issue by employing the “two-point” binding approach in
porphyrin-fullerene dyads which resulted in dyads with defined
distance and orientation.12 Our methodology involved covalent-
coordination,12a coordination-coordination,12b coordination-
hydrogen bonding,12c and coordination-alkylammonium cation
crown ether complexation12d approaches. In the coordination-
alkylammonium cation crown ether complexation approach, zinc
porphyrins were functionalized with one or four 18-crown-6
entities at different positions of the porphyrin ring. The acceptor,
fullerene, was functionalized to possess pyridine and/or an
alkylammonium cation entity. Two-point binding involving
coordination of pyridine to zinc, and crown ether-alkyl cation
complexation resulted in stable donor-acceptor dyads where
the photoinduced electron-transfer processes were possible to
monitor in a polar solvent, benzonitrile.12d,13
Development of self-assembled supramolecular architectures
hosting donor and acceptor entities capable of undergoing light-
induced electron/energy transfer is a topic of current interest
owing to their applications to light energy harvesting and
electronic devices.1-7 Although a significant amount of progress
has been made in this area of research, challenges still exist in
building supramolecules with defined geometry and orientation,
and possessing more than one donor or acceptor entity, viz.,
forming triads, tetrads, etc. Building complicated supramolecular
architectures with less effort, including structures that otherwise
cannot be built by covalent chemistry, is another aspect of
supramolecular chemistry. Noncovalent interactions comprised
of ion-ion, ion-dipole, hydrogen-bond, dipole-dipole, and
π-π stacking interactions with energies ranging from a few to
several hundred kilojoules per mole are available for building
such supramolecules.8 The occurrence of photoinduced electron
transfer in these model donor-acceptor systems held by
noncovalent interactions has been demonstrated in several
instances.
Fullerenes, C60 and C70,9 due to their inherent properties such
as reversible, stepwise addition of up to six electrons10 and low
reorganization energy accompanying electron transfer,11 have
* Address correspondence to these author. F.D.: e-mail francis.dsouza@
wichita.edu. O.T.: e-mail ito@tagen.tohoku.ac.jp.
† Wichita State University.
‡ Tohoku University.
10.1021/jp057547q CCC: $33.50 © 2006 American Chemical Society
Published on Web 03/08/2006

5906 J. Phys. Chem. B, Vol. 110, No. 12, 2006
D’Souza et al.
SCHEME 1: Structures of the Bis-benzo-18-crown-6 Appended Porphyrins, 1-6, and the Fullerene Derivatives, 7
+
(PyC60NH3+)-8 (PhC60NH3
)
Here, we have extended our studies on building supramo-
lecular conjugates using bis-18-crown-6 functionalized porphy-
rins, 1-6 (both cis and trans isomers), and fullerene function-
alized to possess both a pyridine and an alkylammonium cation
entity, 7 (PyC60NH3+), or only an alkylammonium cation, 8
(PhC60NH3+) (Scheme 1). The cis-isomer is functionalized at
the 5,10 positions of the porphyrin ring while the trans-isomer
is functionalized at the 5,15 positions of the porphyrin ring with
the 18-crown-6 entities. Free-base, zinc, and magnesium metal
porphyrins have been employed since they would form conju-
gates with different geometry and energetics in electron-transfer
transfer. Systematic studies have been performed to visualize
these trends and photoinduced electron transfer with steady-
state and time-resolved spectroscopy performed on the supra-
molecular triads in the present study.
Results and Discussion
UV-Visible Spectral Studies and Ground-State Interac-
tions. The optical absorption behavior of the cis and trans
derivatized with bis-benzo-18-crown-6 appended metallopor-
phyrins was found to be similar to that of the corresponding
pristine metallotetraphenylporphyrins.16 That is, they exhibited
an intense Soret band in the 415-430 nm range and less intense
bands in the 500-700 nm region. No apparent absorption band
in the 350-700 nm region corresponding to the crown ether
entities was observed. Addition of 7 (PyC60NH3+) or 8 (PhC60-
NH3+) to a solution containing any of porphyrins 1-6 revealed
spectral changes. However, the spectral changes were drastic
when 7 (PyC60NH3+) was titrated with 2 (trans-ZnP), 3 (trans-
MgP), 5 (cis-ZnP), or 6 (cis-MgP) due to the axial coordination
of the pyridine entity of 7 (PyC60NH3+) binding to the metal
center.17 Typical spectral changes observed for 3 (trans-MgP)
in the presence of increasing amounts 7 (PyC60NH3+) are shown
in Figure 1. Job’s plots of continuous variation also confirmed
the 1:2 molecular stoichiometry of the porphyrin:(fullerene)2
supramolecular conjugates. It is important to note that the
spectral changes observed for the magnesium porphyrin inter-
acting with fullerene 7 (PyC60NH3+) are that of the bis-pyridine
coordinated species.17c,d This is in contrast to the earlier reported
dyad involving magnesium tetraphenylporphyrin coordinating
to imidazole appended fullerene where only a 1:1 complex was
obtained.14 In that study, the anticipated 1:2 complex could not
be obtained due to the large amount of fullerene derivative
needed to accomplish this task. This example nicely demon-
strates the success of the “two-point” binding approach to
accomplish structures that are otherwise difficult to form.
reactions. That is, the crown ether appended free-base porphy-
+
rins, 1 (trans-H2P) or 4 (cis-H2P), would bind to 7 (PyC60NH3
)
or 8 (PhC60NH3+) through crown ether alkylammonium cation
complexation with the possibility of additional π-π interactions
(Scheme 2a). This is also going to be the case when 8 (PhC60-
NH3+) binds to any of the porphyrins, 1-6. In the case of 2
+
(trans-ZnP) and 5 (cis-ZnP), addition of excess 7 (PyC60NH3
)
would result in a conjugate in which the pyridine of one of two
fullerenes will be axial-coordinated to the zinc center and
cation-crown ether complexation as shown in Scheme 2b.
Interestingly, the magnesium porphyrins are expected to form
bis-axial ligated complexes in addition to cation-crown ether
complexation as shown in Scheme 2c,d. It may be mentioned
here that when imidazole appended fullerene was utilized to
form a self-assembled complex via the axial coordination
approach, the magnesium porphyrin yielded only the pentaco-
ordinated complex instead of the anticipated six-coordinated
complex due to the large amount of fullerene derivative needed
to accomplish the six-coordination.14 By adopting the present
“two-point” binding strategy, it is expected to yield such six-
coordinated complexes due to the resulting higher stability of
the complexes. Additionally, the oxidation potentials of the
porphyrins generally follow the trend MgP < ZnP < H2P.15
This property, combined with the emission properties of the
porphyrins, is expected to modulate the energetics of electron

Supramolecular Porphyrin-Fullerene Conjugates
J. Phys. Chem. B, Vol. 110, No. 12, 2006 5907
SCHEME 2: Structures of the Supramolecular Bis-fullerene-Porphyrin Conjugates According to the Metal Ion Present
in the Porphyrin Cavity
By using the spectral changes the binding constants, K, were
evaluated with the Benesi-Hildebrand method.18 However, we
could not obtain reliable binding constants for conjugates formed
by only crown ether-ammonium cation binding but with no
axial coordination, that is, the conjugates formed from 1 (trans-
H2P) or 4 (cis-H2P) with fullerene 8 (PyC60NH3+), and all of
the conjugates formed by fullerene 8 (PhC60NH3+) having no
pyridine functionality. This was due to the small spectral
changes associated with the conjugate formation and appreciable
absorbance of fullerene at the monitoring wavelength of
porphyrin. As discussed in the subsequent section, by using
fluorescence quenching data, it was possible to obtain reliable
binding constants for all of the conjugates. Extending the
absorption wavelength into the near-IR region (650-1000 nm)
did not reveal new absorption bands corresponding to π-π type
interactions that would result in the case of conjugates shown
in Scheme 2a,b. This is probably due to the extended free-base
porphyrin absorption bands (up to 700 nm) into this wavelength
region and weaker π-π interactions in these conjugates.
Furthermore, ESI-mass spectral studies were performed to
characterize the supramolecular conjugates. As shown in Figure
2, for a representative case of 3 (trans-MgP) interacting with 7
(PyC60NH3+), a molecular ion peak corresponding to the 1:2
conjugate was observed. Very similar results were obtained for
the other studied supramolecular conjugates. Recently, Solladie`
et al.19 also characterized 1:2 complexes of the type 1:82 and
4:82, that is, bis-crown ether appended free-base porphyrins and
ammonium cation functionalized fullerene, and obtained similar
ESI-mass spectral results.
Steady-State Fluorescence Studies. The singlet excited state
quenching of cis and trans bis-crown ether appended porphyrins
by functionalized fullerenes was investigated to obtain the
binding constants of the self-assembled conjugates and the
overall quenching behavior. The emission behavior of the bis-
crown ether appended porphyrins was found to be similar to
that of the corresponding pristine porphyrins with two emission
bands. The position of the two crown ether entities had little or
no effect on the wavelength maxima. The respective emission
bands were located at 615 and 664 nm for the zinc porphyrins,
620 and 672 nm for the magnesium porphyrins, and 658 and
+
721 nm for the free-base porphyrins. Addition of 7 (PyC60NH3
)
or 8 (PhC60NH3+) to a solution of any of the porphyrins, 1-6,
revealed fluorescence quenching. However, the quenching
efficiency was found to be higher for 7 (PyC60NH3+) binding

5908 J. Phys. Chem. B, Vol. 110, No. 12, 2006
D’Souza et al.
Figure 3. (a) Fluorescence spectral changes observed on increasing
addition of 7 (PyC60NH3+) (1.3 µM each addition) to a solution of
porphyrin, 3 (trans-MgP, 1.4 µM) in benzonitrile. λex ) 567.5 nm. (b)
Stern-Volmer plots for fluorescence quenching of (i) 5 (cis-ZnP) with
7 (PyC60NH3+), (ii) 2 (trans-ZnP) with 7 (PyC60NH3+), (iii) 1 (trans-
H2P) with 7 (PyC60NH3+), (iv) 6 (cis-MgP) with 7 (PyC60NH3+), (v) 4
(cis-H2P) with 7 (PyC60NH3+), (vi) 3 (trans-MgP) with 7 (PyC60NH3+),
(vii) mono-18-crown-6 appended free-base porphyrin with 7 (PyC60-
NH3+), and (viii) mono-18-crown-6 appended free-base porphyrin with
8 (PhC60NH3+) in benzonitrile.
Figure 1. UV-visible spectral changes observed during the titration
of 3 (trans-MgP) (1.1 µM) with 7 (PyC60NH3+) (0.7 µM each addition)
in benzonitrile. The figure inset shows a Benesi-Hildebrand plot
constructed to obtain the binding constant with fluorescence data.
over 1011 M-1 s-1, which are up to 3 orders of magnitude higher
than that expected for diffusion controlled-bimolecular quench-
ing processes in the studied solvents (∼5.0 × 109 M-1 s-1
)
suggesting that the faster intrasupramolecular processes are
responsible for the fluorescence quenching. The tendency of
increasing in the KSV values was in good agreement with those
evaluated K values.
Semiempirical PM3 Calculations. To visualize the geometry
of the bis-crown ether appended porphyrin-fullerene conjugates,
PM3 calculations were performed. Representative examples of
the space-filling model structures of the self-assembled conju-
gates are shown in Figure 4. It may be noted here that we could
not perform higher level calculations due to the large size and
flexible nature of these conjugates, although B3LYP/3-21G(*)
calculations are expected to provide better geometry and
electronic structure.20 In all of the studied conjugates, the two
crown ether entities formed stable complexes with the alky-
lammonium group of 7 (PyC60NH3+) or 8 (PhC60NH3+).
However, different geometries were possible to achieve due to
the additional pyridine-metal coordination. When 1 (trans-H2P)
and 3 (cis-H2P) were self-assembled with 2 equiv of either of
7 (PyC60NH3+) or 8 (PhC60NH3+), a structure similar to that
shown in Figure 4a was obtained. In these structures, one of
the fullerene entities was close to the porphyrin π-system
revealing some interactions while the second fullerene was far
enough away to provide appreciable interactions with the
porphyrin ring. This was also the case when the zinc and
magnesium porphyrin derivatives were self-assembled with 8
(PhC60NH3+) as shown in Figure 4b for the representative 2:82
conjugate. When 7 (PyC60NH3+) possessing both pyridine and
alkylammonium cation functionalities was employed, only one
pyridine-fullerene was coordinated to the zinc center, leaving
the second pyridine-fullerene with only a crown ether-
ammonium bond. Interestingly, when 3 (trans-MgP) and 6 (cis-
MgP) were allowed to interact with 7 (PyC60NH3+), bis-
coordinated magnesium conjugates were obtained as shown in
Figure 4c,d for the respective trans and cis isomers. These
predictions are in agreement with the experimentally character-
Figure 2. ESI-mass spectrum of the supramolecular triad formed by
3 (trans-MgP) and 7 (PyC60NH3+) in dichloromethane.
to zinc or magnesium derivatives of the porphyrin derivatives
due to two modes of binding. Representative emission changes
of 3 (trans-MgP) on increasing addition of 7 (PyC60NH3+) in
benzonitrile are shown in Figure 3a. By using the emission data,
the binding constants (Kapp ) K1K2) for the formation of self-
assembled triads were obtained by constructing Benesi-
Hildebrand plots18 and the Kapp values are listed in Table 1. It
may be mentioned here that the K values for mono-18-crown-6
appended porphyrin binding to either 7 or 8 were in the range
104-105 M-1 13
. A comparison of this K with Kapp suggests that
the magnitude of K2 is close to that of K1 in the investigated
triads. The Kapp values are higher for Zn or Mg porphyrins
binding to 7 due to mono- or bis-axial coordination.
The Stern-Volmer plots for the fluorescence intensity
quenching of the crown ether appended free-base porphyrins
by 7 and 8 were constructed (Figure 3b). The calculated Stern-
Volmer constant, KSV, values were as high as 105 M-1. On
employing the excited state lifetime of the porphyrins, the
fluorescence quenching rate constants, kq, were evaluated to be

Supramolecular Porphyrin-Fullerene Conjugates
J. Phys. Chem. B, Vol. 110, No. 12, 2006 5909
TABLE 1: Electrochemical Half-Wave Redox Potentials (E1/2 vs Fc/Fc+) and Driving Forces for Forward and Reverse Electron
Transfer for the Bis-crown Ether Appended Porphyrin-Fullerene Conjugates in the Presence of 0.1 M (n-Bu)4NClO4 in
Benzonitrile
compounda
RCC/Å
Kappb/M-2
M
0/•+/V
C600/•-/V
M
0/-/V
-∆GCS/eV
-∆GCR/eV
1 (trans-H2P)
1:8 (1:2)
4 (cis-H2P)
4:8 (1:2)
2 (trans-ZnP)
2:8 (1:2)
2:7 (1:2)
5 (cis-ZnP)
5:8 (1:2)
5:7 (1:2)
3 (trans-MgP)
3:8 (1:2)
3:7 (1:2)
0.50
0.52
0.50
0.51
0.28
0.27
0.25
0.28
0.25
0.25
0.18
0.15
0.12
0.19
0.16
0.12
-1.65
-1.65
-1.63
-1.63
-1.83
-1.85
-1.85
-1.83
-1.85
-1.85
-1.92
-1.96
-1.96
-1.91
-1.95
-1.95
9.1
1.16 × 1010
2.08 × 1010
-1.04
-1.04
0.44
0.45
1.51
1.49
10.0
11.1
10.2
2.50 × 1010
-1.03
-1.03
0.80
0.82
1.24
1.22
6.10 × 1010
10.0
10.0
4.27 × 1010
1.38 × 1011
-1.03
-1.03
0.82
0.82
1.22
1.22
10.9
10.5
2.07 × 1010
-1.02
-1.02
0.90
0.93
1.12
1.09
2.50 × 1010
6 (cis-MgP)
6:8 (1:2)
6:7 (1:2)
10.0
10.2
2.30 × 1010
4.01 × 1010
-1.02
-1.02
0.90
0.94
1.12
1.08
a See Scheme 1 for structures. Kapp ) K1K2 (data from fluorescence quenching studies).
b
Figure 5. Cyclic voltammograms of 6 (solid dark line) on addition of
0.5 (red dash), 1.0 (magenta dot), and 2.0 equiv (solid blue) of 7
(PyC60NH3+) in benzonitrile containing 0.1 M (n-Bu)4NClO4. Scan rate
) 100 mV/s.
Figure 4. Semiempirical PM3 optimized space-filling model structures
of (a) 1 (trans-H2P):82 (PhC60NH3+)2, (b) 2 (trans-ZnP):82 (PhC60-
NH3+)2, (c) 3 (trans-MgP):72 (PyC60NH3+)2, and (d) 6 (cis-MgP):72
(PyC60NH3+)2 supramolecular conjugates.
+
magnesium porphyrins binding to PyC60NH3 (up to 70 mV),
as a result of the axial coordination. To verify whether the
resulting shifts are upon formation of 1:1 or 1:2 complexation,
titrations were carried out by stepwise addition of 7 (0.5 equiv
each addition) to the benzonitrile solution containing MgP. As
shown in Figure 5, the anodic shift was complete with only
one equivalent addition of 7 (PyC60NH3+). The second equiva-
lent addition of fullerene did not result in any additional potential
shifts. On addition of 8 (PhC60NH3+) having no axial coordinat-
ing ability, the observed potential shifts were less than 30 mV.
The driving forces for charge recombination (-∆GCR) and
charge separation (-∆GCS) were calculated according to eqs 1
and 2, using the electrochemical redox data:22
ized bis-pyridine-coordinated magnesium complex shown in
Figure 4c,d. The center-to-center distance (RCt-Ct) between the
metal and the center of the axially coordinated fullerene in these
conjugates was about 10.5 Å, while the distance for the π-π
interacting porphyrin-fullerene was about 5.6 Å.
Electrochemical Studies. Cyclic voltammetric studies were
performed to evaluate the redox potentials of the conjugates
and to establish the free-energy changes for electron-transfer
processes in the studied solvent. All of the investigated bis-
crown ether appended porphyrins, 1-6, revealed a one-electron
oxidation process corresponding to the formation of MP•+ (M
) 2H, Zn, or Mg), and two one-electron reductions correspond-
ing to the formation of MP•- and ΜP2-, respectively. Extending
the potential window beyond the first oxidation process resulted
in a large anodic wave, perhaps due to the oxidation of the
appended crown-ether entities.21 The first oxidation potential
followed the trend MgP < ZnP < H2P, while an opposite trend
was observed for the first reduction process. Fullerenes 7 (PyC60-
NH3+) and 8 (PhC60NH3+) upon binding to the porphyrins
revealed no significant changes in their reduction potentials.
However, depending upon the mode of binding the oxidation
potentials of the porphyrins revealed cathodic shifts, as listed
in Table 1. Maximum potential shifts were observed for
-∆GCR ) Eox - Ered - ∆Gs
-∆GCS ) E0,0 - (-∆GCR
(1)
(2)
)
where Eox is the first oxidation potential of the porphyrin
(MP0/•+), Ered is the first reduction potential of the fullerene
(C600/•-), ∆E0,0 is the energy of the 0-0 transition between the
lowest excited state and the ground state of the porphyrin
evaluated from the fluorescence emission peaks, and ∆Gs refers
to the static energy, calculated by using the “Dielectric
Continuum Model”22 according to eq 3.
∆Gs ) e2/(4πꢀ0ꢀsRCt-Ct
)
(3)

5910 J. Phys. Chem. B, Vol. 110, No. 12, 2006
D’Souza et al.
long lifetime components are attributed to the uncomplexed
porphyrin emission.
The charge-separated rates (kSCS) and quantum yields (ΦS
)
CS
were evaluated from the (τf)complex values according to eqs 4
and 5, a procedure commonly adopted for intramolecular
electron-transfer process.23
kSCS ) (1/τf )complex - (1/τf )MP
(4)
(5)
ΦSCS ) [(1/τf )complex - (1/τf )MP]/(1/τf )complex
As listed in Table 2, higher values of kS and ΦS were
CS
CS
obtained for all of the dyads indicating the occurrence of
efficient charge separation irrespective of the location of the
crown ether entities on the porphyrin macrocycle and the type
of employed fullerene derivative for conjugate formation.
However, specific trends could be observed from these kinetic
results. These include the following: (i) For a given porphyrin
derivative interacting with 8 (PhC60NH3+), the kCS and ΦCS
followed the trend H2P < ZnP < MgP. That is, they tracked
the free-energy values given in Table 1. (ii) The kCS and ΦCS
values are higher for ZnP and MgP interacting with 7 (PyC60-
NH3+) due to the two-point binding and the resulting structural
rigidity. (iii) The kCS and ΦCS values are higher for the cis bis-
crown compared to the trans bis-crown derivative porphyrins.
This trend agrees readily with the binding and steady-state
quenching observations (Figure 3b). (iv) The kCS and ΦCS values
are higher for 2:72, 2:82, 5:72, and 5:82 compared to the earlier
reported 1:1 mono-crown ether appended zinc porphyrin:
fullerene dyads involving 7 (PyC60NH3+) and 8 (PhC60NH3+).
This observation clearly indicates that employing a higher
number of acceptor entities improves the charge-separation
efficiency. Furthermore, nanosecond transient absorption spectral
studies were also performed to characterize the charge-separated
states and evaluate the kinetic results of the charge-recombina-
tion process.
Figure 6. Fluorescence decays for the following: (A) (a) 1 (trans-
H2P) (0.07 mM), (b) 1 (trans-H2P) (0.07 mM) + 8 (PhC60NH3+) (0.07
mM), and (c) 1 (trans-H2P) (0.07 mM) + 8 (PhC60NH3+) (0.14 mM);
(B) (a) 2 (trans-ZnP) (0.07 mM), (b) 2 (trans-ZnP) (0.07 mM) + 8
(PhC60NH3+) (0.07 mM), and (c) 8 (PhC60NH3+) (0.14 mM); (C) (a) 3
(trans-MgP) (0.07 mM), (b) 3 (trans-MgP) (0.07 mM) + 7 (0.07 mM),
and (c) 3 (trans-MgP) (0.07 mM) + 7 (PyC60NH3+) (0.14 mM); (D)
(a) 6 (cis-MgP) (0.07 mM), (b) 6 (cis-MgP) (0.07 mM) + 7(PyC60-
+
NH3+) (0.07 mM), and (c) 6 (cis-MgP) (0.07 mM) + 7 (PyC60NH3
)
(0.14 mM). The porphyrins were excited at 400 nm in benzonitrile
solvent.
The symbols ꢀ0 and ꢀs represent the vacuum permittivity and
the dielectric constant of the solvent benzonitrile, respectively.
Values of RCt-Ct were based on the computed structures shown
in Figure 4 and discussed in the previous section.
The calculated free-energy changes in Table 1 reveal that the
charge separation from the singlet excited porphyrin to fullerene
in these dyads is exothermic and follows the order H2P < ZnP
< MgP of a given porphyrin derivative. These results predict
the occurrence of rapid charge separation in these supramo-
lecular dyads since the ∆GCS values are almost on the top region
of the Marcus parabola. On the other hand, the charge
recombination in the charge-separated state (MP•+:C60•-) of the
supramolecular conjugates is predicted to be a slower process
since the highly exothermic ∆GCR values lie on the inverted
region of the Marcus parabola.
Picosecond Time-Resolved Emission. The time-resolved
emission studies of the self-assembled conjugates tracked those
of steady-state quenching measurements. Figure 6 shows the
emission decay profiles of the representative bis-crown ether
appended porphyrins in the absence and presence of fullerene
derivatives. The bis-crown appended porphyrins revealed mono-
exponential decays with lifetimes of 9.0, 1.9, and 5.1 ns
respectively for H2P, ZnP, and MgP. The appended crown ether
moieties had a small quenching effect on the lifetimes of the
singlet excited porphyrins. Addition of fullerene to the bis-crown
ether appended porphyrins caused rapid fluorescence decay in
addition to a slow decaying tail as shown in curves b (for one
equivalent addition) and c (for two equivalent additions) in
Figure 6A-D. The porphyrin emission decay in the dyads could
be fitted satisfactorily by a biexponential decay curve, the
lifetimes (τf) of which are summarized in Table 2. The short
lifetimes of MP were predominantly due to charge separation
within the supramolecular conjugates ((τf)complex), whereas the
Nanosecond Transient Absorption Studies. The nanosecond
transient spectra recorded after 532 or 550 nm laser irradiation
of the bis-crown ether appended porphyrins, 1-6, revealed
absorption peaks corresponding to their excited triplet state of
porphyrins.24 Fullerenes 7 (PyC60NH3+) and 8 (PhC60NH3
)
+
showed a band around 700 nm corresponding to their excited
triplet states.25,26 Figure 7 shows the transient absorption spectra
of the representative conjugates. In these spectra, in addition to
the peaks corresponding to the triplet excited free-base porphyrin
and fullerene, a peak around 1020 nm corresponding to the
formation of fullerene anion radical was observed. The absorp-
tion band of the radical cation of the porphyrins, expected to
appear in the 600-700 nm region as a counterpart of the
fullerene anion radical, was difficult to observe due to the strong
emission of the porphyrins in this wavelength region. However,
the spectral features of fullerene anion radical provided an
experimental proof for the generation of the charge-separated
state (MP•+:C60•-) as a consequence of fluorescence quenching.
As shown in the inset of Figure 7A-F, the time profile of
the C60•- transient band at 1020 nm was monitored for obtaining
kinetic information for the charge recombination process. In
each time profile, an initial decay followed by slow decay was
observed. It may be mentioned here that the initial time profiles
of these peaks for all of the studied conjugates followed the
first-order decay kinetics, once again suggesting the occurrence
of the intramolecular charge-recombination process of the radical
ion pair. The lifetimes of the radial ion pairs, τRIP, evaluated as
the reciprocal of the charge-recombination rates (kCR), are listed

Supramolecular Porphyrin-Fullerene Conjugates
J. Phys. Chem. B, Vol. 110, No. 12, 2006 5911
TABLE 2: Fluorescence Lifetimes (τf), Charge-Separation Rate Constants (kSCS),a Charge-Separation Quantum Yields (ΦSCS),a
Charge-recombination Rate Constants (kCR), and Lifetime of the Radical Ion Pairs of the Investigated Bis-crown Ether
Appended Porphyrin-C60 Conjugates in Benzonitrile
compounda
τf/ps
kSCS/s-1
ΦS
kCRb/s-1
τRIPb/ns
CS
1:8 (1:1)
1:8 (1:2)
4:8 (1:1)
4:8 (1:2)
2:8 (1:1)
2:8 (1:2)
2:7 (1:1)
2:7 (1:2)
5:8 (1:1)
5:8 (1:2)
5:7 (1:1)
5:7 (1:2)
3:8 (1:1)
3:8 (1:2)
3:7 (1:1)
3:7 (1:2)
6:8 (1:1)
6:8 (1:2)
6:7 (1:1)
6:7 (1:2)
1860 (55%), 7500 (45%)
1670 (68%), 6400 (32%)
1000 (58%), 7800 (42%)
970 (66%), 6100 (34%)
366 (55%), 1500 (45%)
265 (60%), 1400 (40%)
252 (60%), 1550 (40%)
220 (65%), 1450 (35%)
297 (56%), 1600 (44%)
235 (60%), 1500 (40%)
209 (58%), 1500 (42%)
198 (69%), 1450 (31%)
320 (67%), 3500 (33%)
280 (72%), 2600 (28%)
315 (60%), 3200 (40%)
225 (68%), 2600 (32%)
305 (60%), 2720 (40%)
250 (62%), 2900 (38%)
240 (71%), 2700 (29%)
200 (74%), 2300 (26%)
4.2 × 108
4.9 × 108
8.9 × 108
9.2 × 108
2.2 × 109
3.3 × 109
3.4 × 109
4.0 × 109
2.8 × 109
3.7 × 109
4.3 × 109
4.5 × 109
2.9 × 109
3.4 × 109
3.0 × 109
4.3 × 109
3.1 × 109
3.8 × 109
3.9 × 109
4.8 × 109
0.79
0.81
0.89
0.89
0.81
0.86
0.87
0.88
0.84
0.88
0.89
0.90
0.94
0.95
0.94
0.96
0.94
0.95
0.95
0.96
4.61 × 106
6.86 × 106
8.28 × 106
1.06 × 107
4.30 × 106
8.30 × 106
6.10 × 106
9.20 × 106
5.33 × 106
7.37 × 106
7.00 × 106
9.60 × 106
7.50 × 106
9.50 × 106
1.45 × 107
2.20 × 107
6.54 × 106
9.04 × 106
8.35 × 106
1.20 × 107
217
146
121
94
233
121
164
109
188
137
143
104
133
105
69
46
153
111
120
83
a The single exponential τ values of free-base, zinc, and magnesium porphyrins were 9000, 1900, and 5100 ps, respectively. b See text for details
in Table 2. The calculated kCR values were 2-3 orders of
magnitude smaller than the kCS values, suggesting the occurrence
of the charge recombination process in the Marcus inverted
region. The τRIP values range between 46 and 233 ns indicating
charge stabilization in the studied conjugates and higher values
were observed for conjugates formed by 8 (PhC60NH3+). No
specific trend with respect to different porphyrins could be
observed from the present results. As for the slow decay part,
a portion may be attributed to the absorption tail of the triplet
states of the porphyrin and fullerene entities with lifetimes on
the order of 50-100 µs in the long time-scale measurements.
Some part of the decay was prolonged beyond the triplet state
lifetimes suggesting the occurrence of intermolecular electron
transfer between uncomplexed components via the triplet states,
since the slow rise until ca. 50 µs was observed with longer
lifetimes in the order of 300-500 µs.
Summary
Free-base, zinc, and magnesium porphyrins possessing two
18-crown-6 entities at the cis or trans positions of the porphyrin
ring have been synthesized and characterized. These porphyrins
formed self-assembled conjugates with fullerenes functionalized
with alkylammonium and/or pyridine functionalized fullerenes
with well-defined structures. “Two-point” binding was observed
in the case of fullerenes functionalized with pyridine and alkyl-
ammonium groups with metalloporphyrins. A 1:2 stoichiometry
of the supramolecular porphyrin:fullerene conjugates was
deduced from the optical absorption and emission, electron spray
ionization mass, electrochemistry, and semiempirical PM3
calculations. In general, the kCS and ΦCS values were higher
for the present 1:2 porphyrin:fullerene conjugates as compared
to the earlier reported 1:1 porphyrin:fullerene conjugates using
the same binding methodologies. The experimentally determined
Figure 7. Nanosecond transient absorption spectra of the following:
kCR values from nanosecond transient absorption studies were
2-3 orders of magnitude smaller than the kCS values, suggesting
the occurrence of the charge recombination process in the
Marcus inverted region, and charge stabilization in the studied
conjugates.
(A) 2 (trans-ZnP) (0.1 mM) in the presence of 8 (PhC60NH3+) (0.1
+
mM); (B) 2 (trans-ZnP) (0.1 mM) in the presence of 8 (PhC60NH3
)
(0.2 mM); (C) 3 (trans-MgP) (0.1 mM) in the presence of 8
(PhC60NH3+) (0.2 mM); (D) 3 (trans-MgP) (0.1 mM) in the presence
of 7 (PyC60NH3+) (0.2 mM); (E) 6 (cis-MgP) (0.1 mM) in the presence
of 8 (PhC60NH3+) (0.2 mM); (F) 6 (cis-MgP) (0.1 mM) in the presence
of 7 (PyC60NH3+) (0.2 mM). All the spectra were recorded at 100 (b)
and 1000 ns (O) in benzonitrile after the 550 nm laser irradiation. The
figure insets show the time profiles of the transient bands at the indicated
wavelengths.
Experimental Section
Chemicals. Fullerene C60 was from SES Research, Houston,
TX. The syntheses and characterization of 7 (PyC60NH3+) and

5912 J. Phys. Chem. B, Vol. 110, No. 12, 2006
D’Souza et al.
8 (PhC60NH3+) are given elsewhere13 and were freshly purified
over silica gel column prior to the spectral studies. All the
chromatographic materials and solvents were procured from
Fisher Scientific and were used as received. Tetra-n-butylam-
monium perchlorate, (n-C4H9)4NClO4, was from Fluka Chemi-
cals. All other chemicals utilized in the synthesis were from
Aldrich Chemicals (Milwaukee, WI) and were used as received.
(d, 2 H, benzocrownphenyl H), 4.46-4.40 (m, 4 H, crowneth-
ylene H), 4.32-4.26 (m, 4 H, crownethylene H), 4.13-4.07
(m, 4 H, crownethylene H), 3.99-3.87 (m, 8 H, crownethylene
H), 3.85-3.73 (m, 20 H, crownethylene H), -2.79 (br s, 2 H,
imino H). UV/vis (toluene) λmax 425.5, 518.5, 554.5, 592.5, 651
nm.
5,15-Di(benzo[18]crown-6)-10,20-diphenylporphyrinatozinc-
(II) (trans-2). The free-base porphyrin 1 (10 mg, 0.009 mmol)
was dissolved in CHCl3 (10 mL), a saturated solution of zinc
acetate in methanol was added to the solution, and the resulting
mixture was refluxed for 2 h. The course of the reaction was
followed spectrophotometrically by monitoring the disappear-
ance of the 516 nm band of 2. At the end, the reaction mixture
was washed with water and dried over anhydrous Na2SO4.
Chromatography on neutral alumina column by using hexanes:
CHCl3 (30:70 v/v) gave the title compound. Yield 9.3 mg
Synthesis of 4′-Formylbenzo[18-crown-6]. The title com-
pound was synthesized by two different procedures.
Method A: A mixture of 3,4-dihydroxybenzaldehyde (1 g,
7.24 mmol) and K2CO3 (5 g, 36.2 mmol) was taken in DMF
(10 mL). Pentaethyleneglycol di-p-toluenesulfonate (4.354 g,
3.483 mL, 7.964 mmol) was added to it with stirring and the
solution was refluxed under argon for 18 h. The cooled mixture
was acidified with dilute HCl and then filtered. The filtrate was
evaporated to dryness and the residue was extracted with CHCl3
3 or 4 times. The extract was evaporated to yield a viscous oil
that was purified over silica gel column initially with toluene:
CHCl3 (40:60 v/v) and finally with CHCl3:MeOH (98:2 v/v).
On trituration with diethyl ether a white fibrous product was
obtained, which on crystallization from ethanol gave the desired
product. Yield: 1.8 g (75%).
1
(∼90%). H NMR (400 MHz, CDCl3, 25 °C, TMS) δ (ppm)
8.96 (dd, 8 H, â-pyrrolic H), 8.22 (d, 4 H, o-phenyl H), 7.80-
7.68 (m, 10 H, m,p-phenyl H (6 H) and benzocrownphenyl H(4
H)), 7.17 (d, 2 H, benzocrownphenyl H), 4.34-4.29 (m, 4 H,
crownethylene H), 4.18-4.11 (m, 4 H, crownethylene H), 3.92-
3.87 (m, 4 H, crownethylene H), 3.77-3.56 (m, 28 H,
crownethylene H). UV/vis (toluene) λmax 432, 560, 603 nm. ESI
mass in CH2Cl2 m/z (%) calcd 1146.62, found 1146.2 (85) [M]+,
1144.2 (100).
Method B: Benzo-18-crown-6 (2.52 g, 8.07 mmol) and
hexamethylenetetramine (1.2 g, 8.5 mmol) were mixed with
trifluoroacetic acid (6.1 mL, 78.6 mmol) and the reaction
mixture was heated to 100 °C under N2 for 24 h.27 The resulting
dark red mixture was cooled to 5 °C, mixed with ice, and stirred
for 1 h. The product was extracted with chloroform and the
organic layer was dried over Na2SO4. The organic extract was
evaporated to yield a viscous oil that was purified through
column chromatography over silica gel, using CHCl3:MeOH
(98:2 v/v) as eluent. Evaporation of the solvent yielded pale-
yellow oil, which upon trituration with diethyl ether and
refrigeration gave the desired product as a white solid. Yield
1.64 g (60%). 1H NMR (400 MHz, CDCl3, 25 C, TMS) δ (ppm)
9.84 (s, 1H), 7.44 (dd, 1H, phenyl H), 7.39 (d, 1H, phenyl H ),
6.95 (d, 1H, phenyl H ), 4.28-4.18 (m, 4H, crownethylene H),
4.02-3.90 (m, 4H, crownethylene H), 3.82-3.71 (m, 8H,
crownethylene H), 3.69 (s, 4H, crownethylene H).
10-Di(benzo[18]crown-6)-15, 20-diphenylporphyrinatozinc-
(II) (cis-5). This compound was prepared from porphyrin 3 (10
mg, 0.009 mmol) according to the procedure outlined above
1
for 4. Yield:9.3 mg (∼90%). H NMR (400 MHz, CDCl3, 25
°C, TMS) δ (ppm) 8.99-8.90 (m, 8 H, â-pyrrolic H), 8.25-
8.18 (m, 4 H, o-phenyl H), 7.79-7.65 (m, 10 H, m,p-phenyl H
(6 H) and benzocrownphenyl H(4 H)), 7.12-7.06 (m, 2 H,
benzocrownphenyl H), 4.19-4.12 (m, 4 H, crownethylene H),
4.07-3.98 (m, 4 H, crownethylene H), 3.71-3.61 (m, 4 H,
crownethylene H), 3.60-3.38 (m, 28 H, crownethylene H). UV/
vis (toluene) λmax 431.5, 560.5, 602.5 nm. ESI mass in CH2Cl2
m/z (%) calcd 1146.62, found 1146.3 (100) [M]+, 1177.2 (92)
[M + MeOH]+.
5,15-Di(benzo[18]crown-6)-10, 20-diphenylporphyrinato-
magnesium(II) (trans-3). This compound was prepared ac-
cording to a general procedure developed by Lindsey and
Woodford28 for Mg porphyrin synthesis. A sample of 10 mg
(0.00925 mmol) of 2 was dissolved in 10 mL of CH2Cl2. Then
0.026 mL (18.72 mg, 0.19 mmol) of triethylamine was added
followed by 23.9 mg (0.093 mmol) of MgBr2‚O(Et)2. The
mixture was stirred for 20 min at room temperature. The course
of the reaction was monitored by absorption spectroscopy. The
mixture was diluted with 20 mL of CH2Cl2, washed with 5%
NaHCO3, and dried over anhydrous Na2SO4, then the filtrate
was evaporated. Chromatography on neutral alumina column
with CHCl3 as eluent yielded residual 2. Elution with CHCl3:
methanol (98:2 v/v) yielded a greenish purple fraction of the
desired compound. Yield: 8 mg (∼80%). 1H NMR (400 MHz,
CDCl3, 25 °C, TMS) δ (ppm) 8.96-8.85 (m, 8 H, â-pyrrolic
H), 8.25-8.12 (m, 4 H, o-phenyl H), 7.75-7.56 (m, 10 H, m,p-
phenyl H (6 H) and benzocrownphenyl H(4 H)), 6.98-6.88 (m,
2 H, benzocrownphenyl H), 3.91-3.33 (m, 8 H, crownethylene
H), 3.28-2.53 (m, 32 H, crownethylene H). UV/vis (toluene)
λmax 432, 568, 609 nm. ESI mass in CH2Cl2 m/z (%) calcd
1105.54, found 1105.54 (60) [M]+, 1127.8 (100) [M + Na]+.
H2-trans- and cis-Di(benzo[18]crown-6)diphenylporphy-
rins, 1 and 4. A mixture of 4′-formylbenzo[18]crown-6 (1) (600
mg, 1.763 mmol), benzaldehyde (179.23 µL, 187.11 mg, 1.763
mmol), and pyrrole (244 µL, 236.24 mg, 3.53 mmol) was
refluxed in propionic acid (200 mL) for 3.5 h. The propionic
acid was removed under reduced pressure. Column chroma-
tography on neutral alumina with CHCl3 as eluent gave a
mixture of five porphyrins including 1 and 4. Subsequent
column chromatography on neutral alumina yielded 1 (hexanes:
CHCl3 ) 30:70 v/v) and 4 (hexanes:CHCl3 ) 20:80 v/v) as
third and fourth fractions, respectively. H2-5,15-Di(benzo[18]-
crown-6)-10,20-diphenylporphyrin (trans-1): Yield 13.7 mg
1
(∼1%). H NMR (400 MHz, CDCl3, 25 °C, TMS) δ (ppm)
8.87 (dd, 8 H, â-pyrrolic H), 8.22(d, 4 H, o-phenyl H), 7.82-
7.69 (m, 10 H, m,p-phenyl H (6 H) and benzocrownphenyl H(4
H)), 7.21 (d, 2 H, benzocrownphenyl H), 4.48-4.42 (m, 4 H,
crownethylene H), 4.31-4.26 (m, 4 H, crownethylene H), 4.14-
4.08 (m, 4 H, crownethylene H), 3.99-3.88 (m, 8 H, crown-
ethylene H), 3.85-3.73 (m, 20 H, crownethylene H), -2.79
(br s, 2 H, imino H). UV/vis (toluene) λmax (nm) 426, 519, 554.5,
592.5, 651 nm. H2-5,10-di-(benzo-[18]-crown-6)-15, 20-diphe-
nylporphyrin (cis-4): Yield 24.9 mg (∼1.3%). 1H NMR (400
MHz, CDCl3, 25 °C, TMS) δ (ppm) 8.94-8.82 (m, 8 H,
â-pyrrolic H), 8.25-8.18 (m, 4 H, o-phenyl H), 7.81-7.68 (m,
10 H, m,p-phenyl H (6 H) and benzocrownphenyl H(4 H)), 7.19
5,10-Di(benzo[18]crown-6)-15, 20-diphenylporphyrinato-
magnesium(II) (cis-6). This compound was prepared from
porphyrin 3 (10 mg, 0.009 mmol) according to the procedure
outlined above for 6. Yield 8 mg (∼80%). 1H NMR (400 MHz,

Supramolecular Porphyrin-Fullerene Conjugates
J. Phys. Chem. B, Vol. 110, No. 12, 2006 5913
(6) (a) Gust D.; Moore, T. A. In The Porphyrin Handbook; Kadish,
K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: Burlington, MA,
2000; Vol. 8, pp 153-190. (b) Imahori, H.; Sakata, Y. AdV. Mater. 1997,
9, 537. (c) Prato, M. J. Mater. Chem. 1997, 7, 1097. (d) Mart´ın, N.; Sa´nchez,
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CDCl3, 25 °C, TMS) δ (ppm) 9.04-8.84 (m, 8 H, â-pyrrolic
H), 8.28-8.15 (m, 4 H, o-phenyl H), 7.80-7.53 (m, 10 H, m,p-
phenyl H (6 H) and benzocrownphenyl H(4 H)), 6.93-6.83 (m,
2 H, benzocrownphenyl H), 3.85-3.33 (m, 8 H, crownethylene
H), 3.25-2.44 (m, 32 H, crownethylene H). UV/vis (toluene)
λmax 432, 568, 609 nm. ESI mass in CH2Cl2 m/z (%) calcd
1105.54, found 1105.6 (100) [M]+.
Instrumentation. The UV-visible spectral measurements
were carried out with a Shimadzu UV-1650PC spectrophotom-
eter. The fluorescence emission was monitored by using either
Spex Fluorolog-tau or Varian Cary Eclipse spectrometers. The
right angle method was utilized. Cyclic voltammograms were
recorded on a EG&G Model 263A potentiostat using a three-
electrode system. A platinum or glassy carbon electrode was
used as the working electrode. A platinum wire served as the
counter electrode and a Ag/AgCl electrode was used as the
reference electrode. A ferrocene/ferrocenium redox couple was
used as an internal standard. All the solutions were purged prior
to electrochemical and spectral measurements with argon gas.
The computational calculations were performed by using
semiempirical PM3 methods with the GAUSSIAN 03 software
package29 on high-speed computers.
Time-Resolved Emission and Transient Absorption Mea-
surements. The picosecond time-resolved fluorescence spectra
were measured by using an argon-ion pumped Ti:sapphire laser
(Tsunami; pulse width ) 2 ps) and a streak scope (Hamamatsu
Photonics; response time ) 10 ps). The details of the experi-
mental setup are described elsewhere.30 Nanosecond transient
absorption spectra in the NIR region were measured by means
of laser-flash photolysis; 532 or 550 nm light from a Nd:YAG
laser (pulse width ) 6 ns) was used as the exciting source and
a Ge-avalanche-photodiode module was used for detecting the
monitoring light from a pulsed Xe-lamp for shorter time scale
measurements than 5 µs. For longer time scale measurements
than 5 µs, the InGaAs photodiode detector was used to detect
the monitoring light from a continuous Xe lamp.
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Acknowledgment. The authors are thankful to Travis
Cooper for recording the ESI-mass spectra. This work is
supported by the National Science Foundation (Grant no.
0453464 to F.D.), the donors of the Petroleum Research Fund,
administered by the American Chemical Society, and Grants-
in-Aid for Scientific Research on Primary Area (417) from the
Ministry of Education, Science, Sport and Culture of Japan (to
O.I. add Y.A.).
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