Efficient photoinduced electron transfer in a porphyrin tripod-Fullerene supramolecular complex via π-π interactions in nonpolar media

Article abstract of DOI:10.1021/ja100192x

A novel porphyrin tripod (TPZn₃) was synthesized via click chemistry . Three porphyrin moieties of TPZn₃ are geometrically close and linked by a flexible linker. The electron-transfer oxidation of TPZn₃ results in intramolecular π-dimer formation between porphyrin moieties as indicated by electrochemical, vis-NIR, and ESR measurements. The cyclic voltammogram of TPZn3 exhibited stepwise one-electron oxidation processes of three porphyrin moieties In the range from 0.58 to 0.73 V (vs SCE in CH₂Cl₂). When TPZn₃ was oxidized by tris(2,2'-bipyridyl)-ruthenlum(lll) ([Ru(bpy)₃]³⁺), the oxidized species (TPZn₃)ⁿ⁺ (0 < n < 3) exhibited a charge resonance band In the NIR region due to the π-dimer formation between porphyrin moieties. A supramolecular electron donor-acceptor system was also constructed using TPZn₃. The flexible conformation of TPZn ₃ makes It possible to capture a fullerene derivative containing a pyridine moiety (PyC₆₀) inside the cavity by π-π interactions as well as the coordination bond between Zn₂₊ and the pyridine moiety. The formation of a 1:1 supramolecular complex of TPZn₃ with PyC₆₀ (TPZn₃-PyC₆₀) was indicated In the UV-vis and ¹H NMR spectra in nonpolar solvents. The association constant of TPZn₃ with PyC₆₀ (1.1 × 10⁵M ^-1 in toluene) is much larger as compared with those of the corresponding monomer (MPZn) and dimer porphyrin (DPZn₂). The dynamics of photoinduced electron transfer from the singlet excited state of TPZn₃ to PyC₆₀ was examined by laser flash photolysis measurements. The efficient intracomplex photoinduced electron transfer in TPZn₃-PyC₆₀ occurred in nonpolar solvents, resulting from the π-π interactions between the porphyrin and fullerene moieties, together with intramolecular π-bond formation between the porphyrin radical cation and the neutral porphyrin In TPZn₃⁺.

Full text of DOI:10.1021/ja100192x

Published on Web 03/04/2010
Efficient Photoinduced Electron Transfer in a Porphyrin
Tripod-Fullerene Supramolecular Complex via π-π
Interactions in Nonpolar Media
Atsuro Takai,† Mohammed Chkounda,‡ Antoine Eggenspiller,‡ Claude P. Gros,‡
Mohammed Lachkar,§ Jean-Michel Barbe,*,‡ and Shunichi Fukuzumi*,†,|
Department of Material and Life Science, DiVision of AdVanced Science and Biotechnology,
Graduate School of Engineering, Osaka UniVersity, SORST, Japan Science and Technology
Agency (JST), Suita, Osaka 565-0871, Japan, ICMUB, UMR CNRS 5260, UniVersite´ de
Bourgogne, 9 AVenue Alain SaVary, BP 47870, 21078 Dijon Cedex, France, UniVersite´ Sidi
Mohamed Ben Abdellah, Faculte´ des Sciences Dhar El Mahraz, Laboratoire d’Inge´nierie des
Mate´riaux Organome´talliques et Mole´culaires “L.I.M.O.M.”, De´partement de Chimie, B.P. 1796
(Atlas), 30000 Fe`s, Maroc, Department of Bioinspired Science, Ewha Womans UniVersity,
Seoul 120-750, Korea
Received January 9, 2010; E-mail: fukuzumi@chem.eng.osaka-u.ac.jp; Jean-Michel.Barbe@u-bourgogne.fr
Abstract: A novel porphyrin tripod (TPZn3) was synthesized via “click chemistry”. Three porphyrin moieties
of TPZn3 are geometrically close and linked by a flexible linker. The electron-transfer oxidation of TPZn3
results in intramolecular π-dimer formation between porphyrin moieties as indicated by electrochemical,
vis-NIR, and ESR measurements. The cyclic voltammogram of TPZn3 exhibited stepwise one-electron
oxidation processes of three porphyrin moieties in the range from 0.58 to 0.73 V (vs SCE in CH2Cl2).
When TPZn3 was oxidized by tris(2,2′-bipyridyl)-ruthenium(III) ([Ru(bpy)3]3+), the oxidized species (TPZn3)n+
(0 < n e 3) exhibited a charge resonance band in the NIR region due to the π-dimer formation between
porphyrin moieties. A supramolecular electron donor-acceptor system was also constructed using TPZn3.
The flexible conformation of TPZn3 makes it possible to capture a fullerene derivative containing a pyridine
moiety (PyC60) inside the cavity by π-π interactions as well as the coordination bond between Zn2+ and
the pyridine moiety. The formation of a 1:1 supramolecular complex of TPZn3 with PyC60 (TPZn3-PyC60)
was indicated in the UV-vis and 1H NMR spectra in nonpolar solvents. The association constant of TPZn3
with PyC60 (1.1 × 105 M-1 in toluene) is much larger as compared with those of the corresponding monomer
(MPZn) and dimer porphyrin (DPZn2). The dynamics of photoinduced electron transfer from the singlet
excited state of TPZn3 to PyC60 was examined by laser flash photolysis measurements. The efficient
intracomplex photoinduced electron transfer in TPZn3-PyC60 occurred in nonpolar solvents, resulting from
the π-π interactions between the porphyrin and fullerene moieties, together with intramolecular π-bond
•+
formation between the porphyrin radical cation and the neutral porphyrin in TPZn3
.
Introduction
to easier accessibility as compared to covalently linked electron
donor-acceptor ensembles. Extensive efforts have so far been
devoted toward the design of such supramolecular electron
donor-acceptor composites.2-16 Porphyrins are particularly
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process, photosynthetic components are maintained by nonco-
valent interactions in relatively nonpolar membrane proteins.
Thus, it is quite beneficial to utilize noncovalent interaction in
order to construct electron donor-acceptor composites from the
viewpoint of mimicking the biological photosystems in addition
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‡ Universite´ de Bourgogne.
§ Universite´ Sidi Mohamed Ben Abdellah.
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10.1021/ja100192x  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 4477–4489 4477

A R T I C L E S
Takai et al.
Scheme 1
attractive building blocks for construction of supramolecular
electron donor-acceptor composites using π-π interaction due
to their excellent photophysical and electron-transfer proper-
ties.2-10,12a,16,17 However, noncovalent binding between highly
π-conjugated compounds such as porphyrins and fullerenes is
not strong enough in polar solvents which are generally used
for studies on photoinduced electron-transfer reactions.9a,12a,18-21
Thus, two-point binding sites have been introduced for por-
phyrin-fullerene systems:11-13 e.g., axial coordination (zinc ion
of porphyrin and pyridyl or imidazole moieties of fullerene
derivatives)andionicinteraction(cation-crownethercomplex).12,13
Porphyrin dimers have also been employed to enhance the π-π
interactions with electron acceptors.14,22
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We report herein the design and synthesis of a novel
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transfer properties, and construction of a highly organized
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Efficient Photoinduced Electron Transfer
A R T I C L E S
Scheme 2
electron donor-acceptor system. The electron-transfer properties
of the corresponding dimer DPZn2 and monomer MPZn were
also examined for comparison.
appears from the analysis of the spectrum, which leads to the
equivalency of each of the three arms attached to the nitrogen
atom. For example, the methylene protons close to the tertiary
amine group appear as a broad singlet at 4.10 ppm. Conversely,
the CH2 protons located between the triazole motif and the
phenyl group of the porphyrin ring are deshielded to be observed
at 5.88 ppm. The multiplicities of the porphyrin proton signals
are similar to those observed for the same protons for the starting
azido porphyrin. However, all signals are slightly shielded
compared to the corresponding ones for the azido derivative
by approximately 0.5 ppm (see Experimental Section). This
could account for some interaction between the porphyrin rings
The electron-transfer properties of TPZn3, DPZn2, and MPZn
were evaluated based on electrochemical (CV and DPV),
vis-NIR, and ESR measurements. A supramolecular complex
of TPZn3 was formed with a fullerene derivative containing a
pyridine moiety (PyC60), which can be accommodated inside
the cavity by π-π interaction as well as by a coordination bond
formed between Zn2+ and the pyridine moiety. The photody-
namics of the supramolecular complex (TPZn3-PyC60) was
investigated by femtosecond and nanosecond laser flash pho-
tolysis measurements in nonpolar media. The present study
provides the first example for investigations on π-bond forma-
tion in the oxidized multiporphyrins, which facilitates photo-
induced electron transfer in a supramolecular complex of
multiporphyrins with an electron acceptor in nonpolar media.21a
1
in the tripod under these solution conditions. The H NMR
spectrum is given in the Supporting Information (Figure S2).
MPZn27 and DPZn2 were also synthesized for comparison
purposes. Physicochemical data for DPZn2 are given in the
Experimental Section.
Results and Discussion
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Synthesis and Physicochemical Characterization of Tripod
Porphyrin TPZn3. We used click chemistry for the preparation
of TPZn3, because it is known to be a very selective and
effective synthetic method, which can be performed under mild
and clean conditions.25 Many reports have already mentioned
the utility of click chemistry of an azide and an ethynyl
functionalized moiety for elaboration of sophisticated structures
involving porphyrin as a backbone.26 The synthetic procedure
for the preparation of TPZn3 is given in Scheme 2. The coupling
reaction of 3 equiv of the azido zinc porphyrin and 1 equiv of
commercially available tripropargylamine in the presence of
CuSO4 ·5H2O and sodium ascorbate affords the tripod porphyrin.
The formation of TPZn3 is clearly evidenced by the appear-
ance on the MALDI/TOF mass spectrum of an ionic pattern at
2061.15 Da, which corresponds roughly to the radical cation
species. Moreover, HR-MS measurements were performed using
an ESI-QTOF technique on the isolated derivative, showing the
presence of only two ions attributed to the mono- and disodium
adducts. In both cases, the perfect match between experimental
and simulated ionic patterns undoubtedly confirmed the structure
of TPZn3 (see Figure S1 in the Supporting Information for
spectra). For example, the calculated mass for the monosodium
adduct is equal to 2083.7838 Da, agreeing well with the
experimental value found at 2083.7860 Da.
Characterization of the tripod porphyrin was also carried out
1
using H NMR. At 500 MHz, the spectrum is well-resolved
and quite easy to interpret if recorded at 328 K. A C3 symmetry
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A R T I C L E S
Takai et al.
Figure 1. UV-vis spectra of MPZn (- -), DPZn2 (- - -), and TPZn3 (s)
at 3.0 × 10-6 M in CH2Cl2 at 298 K.
Figure 2. Optimized structure of TPZn3 calculated by DFT at the BLYP/
3-21G(*) level. ꢀ-Substituents of TPZn3 are replaced by protons for
simplicity.
Figure 3. Cyclic voltammograms (50 mV s-1) of (a) MPZn, (b) DPZn2,
and (c) TPZn3 in CH2Cl2 (5 × 10-4 M) containing 0.1 M TBAPF6. (d)
Differential pulse voltammograms of TPZn3 under the same conditions as
CV. The red line is the simulation curve using two discrete Gaussian-shape
oxidation waves (blue and green lines).
Intramolecular π-π Interaction between Porphyrin Rings
of the Oxidized Tripod Porphyrins. The intramolecular π-π
interaction between porphyrin moieties of the oxidized TPZn3
was investigated by means of electrochemical (CV and DPV),
UV-vis-NIR, ESR measurements, together with computational
studies.
Figure 1 shows the UV-vis spectra of TPZn3, DPZn2, and
MPZn. Both the Soret and Q bands of TPZn3 are slightly shifted
as compared to those of MPZn. Particularly, TPZn3 exhibits a
shoulder of Soret band at around 390 nm, which indicates the
π-electron overlap between porphyrins in face-to-face confor-
mation.28
voltammograms and differential pulse voltammograms of MPZn,
DPZn2, and TPZn3 in CH2Cl2, respectively. MPZn exhibits two
reversible redox waves which are typical for the sequential
oxidation processes for zinc porphyrin: MPZn/MPZn•+ and
MPZn•+/MPZn2+. Similar redox waves are observed in the case
of DPZn2. Such redox behaviors indicate that each porphyrin
in DPZn2 behaves as a monomer porphyrin because it is
geometrically independent, when the initial oxidation of two
porphyrin moieties occurs at the same potential.
In contrast to the independent oxidation of two porphyrin
moieties in DPZn2 at the same potential, the initial three-electron
oxidation of TPZn3 is split into different steps. Because the
sequential redox waves are close to each other, discrete redox
waves are not observed even at a slow scan rate (10 mV s-1).
The initial oxidation wave obtained from DPV measurements
can be simulated by multi Gaussian-shape oxidation waves,
which ranged from 0.58 to 0.73 V. The next three-electron
oxidation of TPZn3 took place at the same potential (1.08 V).
All electrochemical data for MPZn, DPZn2 and TPZn3 are
summarized in Table 1. The first oxidation potential of TPZn3
(0.58 V in CH2Cl2) is lower than that of MPZn (0.71 V). This
The optimized structure of TPZn3 is shown in Figure 2 (see
Experimental Section for the calculation method). Each por-
phyrin in TPZn3 is close to each other. Especially, two of three
porphyrins in TPZn3 have a face-to-face conformation. This
optimized structure reflects the π-π interaction as expected
from the UV-vis spectrum of TPZn3.
Electrochemical studies were performed by using cyclic
voltammetry (CV) and differential pulse voltammetry (DPV)
to investigate possible electronic interactions between porphyrin
moieties in the oxidized states. Figure 3 shows the cyclic
(28) Satake, A.; Kobuke, Y. Org. Biomol. Chem. 2007, 5, 1679.
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4480 J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010

Efficient Photoinduced Electron Transfer
A R T I C L E S
Table 1. Oxidation Potentials of Porphyrins in CH2Cl2 and PhCN
Eox, V vs SCEa
MPZn
DPZn2
TPZn3
CH2Cl2
PhCN
0.71
1.05
0.52
0.88
0.71
1.03
0.52
0.87
0.58-0.73
1.08
0.49
0.90
a Measured in CH2Cl2 and PhCN containing 0.1 M TBAPF6 at 298
K. The oxidation potentials of ferrocene as external standard are 0.52
and 0.34 V in CH2Cl2 and PhCN, respectively.
Figure 5. Plots of absorption changes at 660 nm (dashed line with circle)
and 1030 nm (solid line with square) vs concentration of oxidized TPZn3
(1.7 × 10-5 M) in CH2Cl2 at 298 K.
Figure 4. vis-NIR spectral changes upon addition of [Ru(bpy)3]3+ (up to
3 equiv) to a CH2Cl2 solution of TPZn3 (1.7 × 10-5 M) at 298 K.
Figure 6. vis-NIR spectral changes of a CH2Cl2 solution of TPZn3 (1.4
× 10-5 M) in the presence of 2 equiv of [Ru(bpy)3]3+ at different
temperatures.
is because the radical cation of trimer porphyrins is stabilized
as compared to MPZn•+ due to the π-π interaction between
two porphyrin rings. Meanwhile, the rest of the oxidation
potentials of TPZn3 are higher than those of MPZn because of
the electrostatic repulsion between porphyrin cations. The
oxidation potentials in PhCN are also given in Table 1. It should
be noted that no splitting of the initial redox waves of the trimer
porphyrins was observed in PhCN (Figure S3), since PhCN can
coordinate to zinc porphyrin to preclude any electronic com-
munication between porphyrin rings.
the so-called charge-resonance band, indicates the existence of
electronic interaction between porphyrin radical cations.31,32
In sharp contrast to what is observed for TPZn3, there is no
change in the NIR region when MPZn and DPZn2 are oxidized
by [Ru(bpy)3]3+ under the same conditions (Figures S4).33
Figure 5 shows the absorbance changes at 660 and 1030 nm.
The absorbance at 1030 nm slightly increases until the addition
of ca. 1.5 equiv of [Ru(bpy)3]3+, followed by a large increase
to give the largest value at [[Ru(bpy)3]3+]/[TPZn33+] ) 3, where
all of the porphyrins are one-electron oxidized. Thus, the NIR
band is attributed to the electronic interaction among porphyrin
radical cations. In such a case, there are two possibilities to
show the charge resonance band: the charge delocalization (i)
among three porphyrin rings and (ii) between two of three
porphyrin rings. In order to clarify this point, the vis-NIR and
The vis-NIR spectral titration of the trimer porphyrin was
examined by using a one-electron oxidant, [Ru(bpy)3]3+. Upon
addition of [Ru(bpy)3]3+ in MeCN to a CH2Cl2 solution of
TPZn3 at 298 K, new absorption bands at around 590 nm, 670
nm, and 1030 nm appear, and the absorption maximum at 670
nm is shifted to 660 nm, as shown in Figure 4. The absorption
band at around 660 nm is typically assigned to the monomeric
porphyrin radical cation (Figure S4).29 This indicates that at
the initial stage of electron-transfer oxidation of TPZn3, the one-
electron oxidized porphyrin interacts with the other neutral
porphyrins,30 which is consistent with the negative shift of the
first oxidation potential of TPZn3 as compared to that of MPZn.
The appearance of NIR absorption bands (λmax ) 1030 nm),
2+
ESR spectra of TPZn3 were measured under the same
conditions. Figures 6 and 7 depict the temperature dependence
of vis-NIR and ESR spectra, respectively. As the temperature
2+
of a CH2Cl2 solution of TPZn3 decreases, the absorbance at
660 nm derived from the monomeric porphyrin radical cation
decreases, while the charge resonance band (λmax ) 1030 nm)
and Q bands of neutral TPZn3 increase (Figure 6).
(29) (a) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J. Am.
Chem. Soc. 1970, 92, 3451. (b) Fuhrhop, J. H.; Wasser, P.; Riesner,
D.; Mauzerall, D. J. Am. Chem. Soc. 1972, 94, 7996.
(31) Kochi, J. K.; Rathore, R.; Magueres, P. L. J. Org. Chem. 2000, 65,
6826.
(32) Both of the NIR absorption and visible absorption due to the
monomeric porphyrin radical cation disappear when more than 3 equiv
of a MeCN solution of [Ru(bpy)3]3+ is added (see Figure S5 in the
Supporting Information).
(30) The characteristic NIR absorption (λmax ≈ 3000 nm) due to the
interaction between neutral porphyrin and radical cation was not clearly
observed by vis-NIR spectroscopy, because electron-transfer oxidation
2+
states of TPZn3 are in equilibria to afford stable TPZn3 prior to
(33) The ratio of absorbance at 660 and 1030 nm of TPZn3 is the same
irrespective of the concentration. These results indicate that the NIR
TPZn3•+, and the absorption coefficient of TPZn3•+ in NIR region may
be too small (see ref 24).
n+
band of TPZn3 (0 < n e 3) is due to intramolecular interaction.
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J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010 4481

A R T I C L E S
Takai et al.
Figure 7. (a) ESR spectral changes of a CH2Cl2 solution of TPZn3 (1.6 × 10-4 M) in the presence of 2 equiv of [Ru(bpy)3]3+ at different temperatures. (b)
Dependence of the ratio of the ESR intensity (IP/IG, where IP and IG is the ESR intensity of TPZn32+ and galvinoxyl radical, respectively) on the temperature.
The ESR intensity was determined by double integration of the corresponding first-derivative ESR spectrum. Inset: Dependence of the ESR intensity (IP,
2+
solid line and IG, dashed line) on the temperature. Both of the concentrations of TPZn3 and galvinoxyl radical are 1.6 × 10-4 M and 3.2 × 10-4 M in
CH2Cl2, respectively.
Scheme 3
2+
This suggests that at low temperature, the charge of TPZn3
is delocalized between two porphyrins only to form a dimeric
radical cation, and the other porphyrin behaves like a neutral
By combining the above results with the electrochemical data
obtained by CV and DPV, the electron-transfer oxidation process
of TPZn3 can be summarized as illustrated in Scheme 3. First,
the one-electron oxidized porphyrin interacts with the adjacent
porphyrin to form a π-bond, which stabilizes the dimer radical
cation as compared to monomer porphyrin radical cation (Figure
S7a). The negative shift of the first oxidation potential of TPZn3
proves the existence of such an interaction, together with the
shift of the absorption maximum from 670 to 660 nm. The two-
electron oxidized state, TPZn32+, exists in equilibrium between
the following two states: (i) one electron is delocalized between
two adjacent porphyrins and the other electron is localized on
the porphyrin which is sterically independent, and (ii) two
electrons are delocalized between two adjacent porphyrins.
The latter is more favorable at lower temperature, contrary to
the case of meso-meso linked porphyrin dimer reported in the
literature.36,37 Finally, in the three-electron oxidized state, all
of the porphyrins of TPZn3 are one-electron oxidized to afford
monomeric and dimeric porphyrin radical cations (Scheme 3c).
To the best of our knowledge, this is the first example to show
the stepwise electron-transfer oxidation and π-bond formation
of tripod porphyrin system.
2+
monomer porphyrin. In addition, the ESR intensity of TPZn3
changes reversibly and decreases precipitously when lowering
the temperature, as shown in Figure 7. The quantitative analysis
of the ESR intensity was performed to distinguish the inherent
2+
radical concentrations of TPZn3 from the Curie law and
instrumental factors (see Experimental Section for details). The
IP/IG value at 293 K (0.62) indicates that 38% of the oxidized
porphyrins form diamagnetic species in TPZn32+, which is fairly
consistent with the vis-NIR measurements as shown in Figures
4 and 5.34 If the charge of TPZn3 were delocalized among
2+
three porphyrin rings, the minimum ESR intensity should be
one-third of the initial ESR intensity. Therefore, the major drop
in signal intensity can be assigned to the formation of
diamagnetic ESR-silent species, according to the spin coupling
between two porphyrin radical cations (for molecular orbital
diagrams, see Figure S7b).35 Such a temperature dependence
of vis-NIR and ESR spectra also indicates the existence of an
2+
equilibrium between two electronic states of TPZn3 as
described later. The strong interaction between two of the three
porphyrins is consistent with the optimized structure obtained
by DFT calculation (Figure 2).
Complex Formation of Tripod Porphyrin with p-Pyridyl
Fulleropyrrolidine. The electrochemical (CV and DPV), vis-NIR,
and ESR spectroscopic studies depicted the interchromophoric
interactions of TPZn3 through π-π interactions, particularly in
(34) Comparison of the ESR intensities of [TPZn3]n+ (0 < n e 3) is shown
in Figure S6 (in the Supporting Information). The ESR intensities of
TPZn32+/3+ are quite smaller relative to the values expected from the
linear increase of ESR intensity of TPZn3•+, which is again consistent
with the vis-NIR measurements as shown in Figures 4 and 5.
(35) Lu¨, J.-M.; Rosokha, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2003, 125,
12161.
(36) Segawa, H.; Senshu, Y.; Nakazaki, J.; Susumu, K. J. Am. Chem. Soc.
2004, 126, 1354.
n+
(37) In any oxidized states of TPZn3 (0 < n e 3), no triplet signal was
observed by ESR spectroscopy at 77 K.
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4482 J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010

Efficient Photoinduced Electron Transfer
A R T I C L E S
the one, two, and three electron-oxidized states. The flexible
tripod linker allows such electronic interactions. Additionally,
the geometric feature of TPZn3 makes it possible to capture of
guest molecules inside the cavity of the tripod efficiently through
noncovalent interactions and coordination bond.20f,38,39 Here,
we demonstrate the complexation behavior of TPZn3 with
N-methyl-2-(p-pyridyl)fulleropyrrolidine (PyC60) in comparison
with MPZn and DPZn2 (Scheme 4). Fullerene derivatives
Scheme 4
generally exhibit noteworthy electron acceptor properties in
combination with appropriate π-conjugated electron donors,40
and PyC60 is known to bind with monomer zinc porphyrin by
axial coordination in nonpolar media such as toluene and
o-dichlorobenzene.8a
Figure 8. UV-vis spectral changes upon addition of PyC60 (0 to 4.8 ×
10-5 M) to an o-dichlorobenzene solution of TPZn3 (3.0 × 10-6 M) at 298
K.
∆Abs at 410 nm in the absence and presence of PyC60, the
association constant K is determined to be 9.4 × 104 M-1 in
o-dichlorobenzene at 298 K.
UV-vis spectral changes were recorded during the complex-
ation of TPZn3 with PyC60 in o-dichlorobenzene as shown in
Figure 8. The original Soret band of TPZn3 (λmax ) 410 nm)
decreases in intensity and all the visible bands due to TPZn3
are red-shifted. The red shift of the Soret band is widely known
for the formation of the coordination bond.8a The absorbance
change of the Soret band at 410 nm is shown in Figure 9a.
Once the absorption decreases, the absorption increases linearly
due to the absorption of PyC60. The inset in Figure 9a exhibits
the Soret band spectral change subtracted from the absorption
of PyC60. Job’s plot of continuous variation method was run to
confirm the stoichiometry of the complex (Figure S9). The plot
exhibits a broad curve with a maximum value, 0.46. This result
indicates that a 1:1 complex (theoretical maximum is 0.50) is
formed as a major product.41 The association constant K of
TPZn3-PyC60 (1:1) complex is determined from the ∆Abs at
410 nm.42 From a linear plot of [TPZn3]/(∆A0 - ∆A) versus
[PyC60]-1 at 410 nm (Figure 9b), where ∆A0 and ∆A are the
As TPZn3 forms the supramolecular complex with PyC60, the
fluorescence emission of TPZn3 is quenched by intracomplex
electron transfer from the singlet excited state of TPZn3 moiety to
PyC60 in nonpolar media (Figure S10a). The association constant
K of TPZn3-PyC60 complex is also obtained as 5.5 × 104 M-1
from the change in the fluorescence intensity at 577 nm (Figure
S10b). The K value estimated from the fluorescence quenching is
slightly smaller than that determined from the UV-vis spectral
change, because the intermolecular dynamic quenching process
between TPZn3 moiety and free PyC60 is not negligible at relatively
high concentrations of PyC60, and the absorption of TPZn3 at the
excitation wavelength is overlapped with that of PyC60 to decrease
the original fluorescence emission of TPZn3.
UV-vis measurements were similarly examined for MPZn and
DPZn2 to determine the association constants K with PyC60. The
K value of DPZn2 in each solvent (toluene and o-dichlorobenzene)
is much smaller as compared with that of TPZn3. In the case of
MPZn, UV-vis spectral change during the complexation with
PyC60 is too small to determine the K value accurately. Thus, the
(38) Tong, L. H.; Pascu, S. I.; Jarrosson, T.; Sanders, J. K. Chem. Commun.
2006, 1085.
(39) The complex behavior between TPZn3 and pristine C60 was investigated
with 1H NMR spectroscopy. The protons of TPZn3 in CDCl3/CS2 (1:1
V/V) slightly changed upon addition of pristine C60 as shown in Figure
S8a-c, while that of MPZn exhibited little change (Figure S8d-f). It
is difficult to determine the association constant between TPZn3 and
C60 because of its small value and limited solubility. Nevertheless,
1
association constants were determined by means of H NMR
titration. Figures 10 and S11 depict the spectral changes of TPZn3
and MPZn, respectively, upon addition of PyC60 to a CDCl3/CS2
1
(1:1 V/V) mixture at room temperature. The H NMR signals of
1
the H NMR signals of meso-protons of TPZn3 (9.52 and 9.46 ppm)
TPZn3 exhibit downfield shifts upon complexation with PyC60 as
shown in Figure 10. Meanwhile, the pyridyl protons of PyC60
exhibit large upfield shifts by the complexation, which is ascribed
to the influence of the large porphyrin aromatic ring current. This
result clearly shows that the pyridyl group of PyC60 coordinates to
the central zinc ions of TPZn3. The signal of the free TPZn3 and
the complexed TPZn3 always coalesce into a single signal. This
indicates that the complexation and exchange between different
zinc ions takes place faster than the NMR time scale. The
association constant of TPZn3-PyC60 complex is determined from
the peak shift of the meso-proton signals of TPZn3 using eq 143
tended to be simplified into a single signal by the addition of C60, as
was observed for the complexation between TPZn3 and PyC60. Thus,
pristine C60 is also incorporated inside the cavity of TPZn3. These
results indicate that π-π interactions between TPZn3 and fullerene
π-sphere strongly assist the coordination bond between the zinc ion
of TPZn3 and the pyridyl group of PyC60.
(40) (a) Guldi, D. M.; Rahman, G. M. A.; Marczak, R.; Matsuo, Y.;
Yamanaka, M.; Nakamura, E. J. Am. Chem. Soc. 2006, 128, 9420.
(b) Marczak, R.; Wielopolski, M.; Gayathri, S. S.; Guldi, D. M.;
Matsuo, Y.; Matsuo, K.; Tahara, K.; Nakamura, E. J. Am. Chem. Soc.
2008, 130, 16207. (c) Matsuo, Y.; Maruyama, M.; Gayathri, S. S.;
Uchida, T.; Guldi, D. M.; Kishida, H.; Nakamura, A.; Nakamura, E.
J. Am. Chem. Soc. 2009, 131, 12643. (d) Sandanayaka, A. S. D.; Ito,
O.; Tanaka, T.; Isobe, H.; Nakamura, E.; Yudasaka, M.; Iijima, S.
New J. Chem. 2009, 33, 2261. (e) Guldi, D. M.; Illescas, B. M.;
Atienza, C. M.; Wielopolskia, M.; Martin, N. Chem. Soc. ReV. 2009,
38, 1587, and references therein.
δobs ) δH + (δHG - δH) ×
([Ht] + [Gt] + 1/K) - {([Ht] + [Gt] + 1/K)2 - 4[Ht][Gt]}1/2
(41) A 1:2 complex (theoretical maximum is 0.33) may be also formed as
a minor species.
(42) Fukuzumi, S.; Kondo, Y.; Mochizuki, S.; Tanaka, T. J. Chem. Soc.,
Perkin Trans. 2 1989, 1753.
2[Ht]
(1)
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A R T I C L E S
Takai et al.
Figure 9. (a) Original spectral change at 410 nm due to TPZn3 with increasing concentration of PyC60 in o-dichlorobenzene at 298 K. Inset: ∆Abs at 410
nm. (b) Plot of [TPZn3]0/(A0 - A) vs [PyC60]-1
.
Table 2. Association Constants (K, M-1) of Porphyrins with PyC60
K, M-1
MPZn
DPZn2
TPZn3
CDCl3/CS2 (1:1 V/V)
o-dichlorobenzene
toluene
9.4 × 102
2.3 × 103
5.9 × 103
3.9 × 104
1.1 × 104
9.4 × 104
1.1 × 105
a
a
a Little UV-vis spectral changes were observed to preclude the
determination of the K values.
Figure 11. Optimized structure of TPZn3-PyC60 complex calculated by
DFT at the BLYP/3-21G(*) level: (a) top view and (b) side view.
ꢀ-Substituents of TPZn3 are replaced by protons for simplicity.
for MPZn) and the results are given in Table 2. It should be noted
that the signals of MPZn and DPZn2 exhibit upfield shifts during
the complexation, whereas the signals of TPZn3 exhibit downfield
shifts as mentioned above. If PyC60 bound to TPZn3 only by axial
coordination, the signals of TPZn3 should shift in a similar way to
those of MPZn. Thus, PyC60 is captured inside the cavity of TPZn3
not only by a coordination bond but also π-π interactions, which
cause the downfield shifts of protons in TPZn3.20f,22a,43
Figure 10. 1H NMR spectra of (a) PyC60, (b) TPZn3 (1.0 × 10-3 M), and
(c and d) TPZn3 (1.0 × 10-3 M) in the presence of PyC60; (c) 1.2 × 10-3
M and (d) 3.7 × 10-3 M in CDCl3/CS2 (1:1 V/V). The dashed lines represents
the spectral changes due to TPZn3, whereas the dotted lines represents the
spectral changes due to pyridine moieties of PyC60. (e) Change in the chem-
ical shift of the meso-protons of TPZn3 upon addition of PyC60. The curve
represents the best fit according to eq 1.
The K values increase in the order MPZn < DPZn2 < TPZn3.
The enhanced binding ability of TPZn3 again results from π-π
interactions between TPZn3 and PyC60 which is embedded into
the cavity of TPZn3.
The DFT calculation (B3LYP/3-21G(*) basis set) also
supports the encapsulation of PyC60 into the cavity of TPZn3
(see Figure 11). The distance of axial coordination bond (Zn-N)
is found to be 2.00 Å, which is quite similar to the values
reported previously.8a The minimum distances between the other
porphyrin planes and fullerene are found to be 2.58 and 2.63
where [Ht] and [Gt] are total host and guest concentrations,
respectively, δobs is the observed shift, δH is the chemical shift of
the host, δHG is the chemical shift of the host-guest complex, and
K is the association constant. From a fitting curve shown in Figure
10e, the K value of TPZn3 is determined to be 1.1 × 104 M-1 in
CDCl3/CS2 (1:1 V/V) at 298 K. Likewise, the K values of DPZn2
and MPZn were evaluated from 1H NMR titration (see Figure S11
(43) Miyazaki, S.; Ohkubo, K.; Kojima, T.; Fukuzumi, S. Angew. Chem.,
Int. Ed. 2007, 46, 905.
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4484 J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010

Efficient Photoinduced Electron Transfer
A R T I C L E S
Figure 12. Differential transient absorption spectra of (a) TPZn3 (7.0 × 10-6 M) and (b) MPZn (1.1 × 10-5 M) in the presense of PyC60 (2.3 × 10-5 M)
obtained at 2 (black), 62 (red), and 2800 ps (green) after femtosecond laser pulse irradiation at 410 nm in deaerated toluene at 298 K.
Figure 13. (a) Time profiles of differential transient absorption spectra for TPZn3 (7.0 × 10-6 M) in the presense of PyC60 (2.3 × 10-5 M) at 0-15 ps time
intervals. (b) Time profiles of differential transient absorption spectra for TPZn3 in the presense of PyC60 at 0-3000 ps time intervals. Each curve monitored
at 670 (black), 820 (blue), and 1000 nm (red) represents the best fit to the first-order rise and decay.
Å, respectively. These values of spatial separation suggest
possible π-π interactions, which are consistent with the
crystallographic studies of porphyrin-fullerene systems.20f,44
to the monofunctionalized fullerene radical anion.8a,46 The
accompanying absorption band at 670 nm corresponds to the
absorption spectrum of one-electron oxidized species of TPZn3
(TPZn3•+) which is observed by vis-NIR spectroscopy (Figure
4).8a,46d-g The rise of the absorptions at 670 and 1000 nm
affords an apparent first-order rate constant (kET) of 6.4 × 1011
s-1 (Figure 13a). In addition, the decay of the band at 820 nm
Photoinduced Electron Transfer from Tripod Porphyrin
to PyC60. Time-resolved transient absorption spectra of TPZn3
and MPZn in the presence of PyC60 were recorded by femto-
second laser flash photolysis in deaerated toluene, as shown in
Figure 12. From UV-vis spectral changes, TPZn3 mainly exists
as a complex TPZn3-PyC60, whereas MPZn scarcely forms
complex with PyC60 because of its small association constant.
The absorption spectrum observed 2 ps after the laser pulse
(black line) exhibits absorption bands which are unambiguously
assigned to the singlet excited states of the porphyrin moieties,
1
due to TPZn3* is related to the rise of the absorptions at 670
and 1000 nm. All these observations are in accordance with
the intracomplex electron transfer from 1TPZn3* to PyC60
affording the radical ion pair state (TPZn3•+-PyC60•-). At 2800
ps, the absorption at 670 and 1000 nm decay with a first-order
rate constant of 1.9 × 109 s-1 (Figure 13b). We assigned this
•-
process to the intracomplex back electron transfer from PyC60
1
1TPZn3*-PyC60 (Figure 12a) and MPZn* (Figure 12b), re-
spectively.45 The transient absorption spectrum due to
1TPZn3*-PyC60 changes as time elapses to afford another
absorption band at 62 ps after the laser pulse (red line). The
absorption band observed at λmax ) 1000 nm is clearly attributed
(46) (a) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695. (b)
Fukuzumi, S.; Ohkubo, K.; Imahori, H.; Shao, J.; Ou, Z.; Zheng, G.;
Chen, Y.; Pandey, R. K.; Fujitsuka, M.; Ito, O.; Kadish, K. M. J. Am.
Chem. Soc. 2001, 123, 10676. (c) Ohkubo, K.; Kotani, H.; Shao, J.;
Ou, Z.; Kadish, K. M.; Li, G.; Pandey, R. K.; Fujitsuka, M.; Ito, O.;
Imahori, H.; Fukuzumi, S. Angew. Chem., Int. Ed. 2004, 43, 853. (d)
Schuster, D. I.; Loa, K.; Guldi, D. M.; Palkar, A.; Echegoyen, L.;
Stanisky, C.; Cross, R. J.; Niemi, M.; Tkachenko, N. V.; Lemmetyinen,
H. J. Am. Chem. Soc. 2007, 129, 15973. (e) Imahori, H.; Tamaki, K.;
Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito, O.; Sakata, Y.; Fukuzumi,
S. J. Am. Chem. Soc. 2001, 123, 2607. (f) Imahori, H.; Guldi, D. M.;
Tamaki, K.; Yoshida, Y.; Luo, C.; Sakata, Y.; Fukuzumi, S. J. Am.
Chem. Soc. 2001, 123, 6617. (g) D’Souza, F.; Maligaspe, E.; Karr,
P. A.; Schumacher, A. L.; Ojaimi, M. E.; Gros, C. P.; Barbe, J.-M.;
Ohkubo, K.; Fukuzumi, S. Chem.sEur. J. 2008, 14, 674.
(44) (a) Sun, D.; Tham, F. S.; Reed, C. A.; Chaker, L.; Boyd, P. D. J. Am.
Chem. Soc. 2002, 124, 6604. (b) Olmstead, M. M.; Costa, D. A.;
Maitra, K.; Noll, B. C.; Phillips, S. L.; Van Calcar, P. M.; Balch,
A. L. J. Am. Chem. Soc. 1999, 121, 7090. (c) Boyd, P. D. W.;
Hodgson, M. C.; Rickard, C. E. F.; Oliver, A. G.; Chaker, L.; Brothers,
P. J.; Bolskar, R. D.; Tham, F. S.; Reed, C. A. J. Am. Chem. Soc.
1999, 121, 10487.
(45) Rodriguez, J.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1989, 111,
6500.
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A R T I C L E S
Takai et al.
Table 3. Luminescence Data for MPZn, DPZn2, and TPZn3
is determined to be τCS ) 0.53 ns. On the other hand, only
1
E(S1), eVa
τfl, nsb
Φflb
E(T1), eVc
energy transfer from MPZn* to PyC60 takes place to produce
compound
3
1PyC60*, in competition with intersystem crossing to MPZn*,
MPZn
DPZn2
TPZn3
2.16
2.16
2.16
2.24 ( 0.12
1.92 ( 0.06
2.10 ( 0.07
0.024
0.022
0.016
1.77
1.76
1.76
following the formation of 3PyC60*. The intermolecular electron
transfer from MPZn to PyC60 was not observed in nanosecond
laser flash photolysis measurements.
a Determined from the absorption and fluorescence spectra in toluene
at 298 K. b Measured in toluene at 298 K. c Measured in 2-MeTHF at
77 K.
TPZn3 can bind PyC60 much stronger than MPZn or DPZn2,
which allows a strong electron donor-acceptor complex to form.
The intracomplex electron transfer from 1TPZn3* to PyC60
1
occurs, whereas no electron transfer from MPZn* to PyC60
•+
to TPZn3 to recover the ground state of the complex
occurs under the same experimental conditions due to the weak
binding of PyC60 to MPZn. Such enhanced electron-transfer
properties of TPZn3 relative to MPZn and DPZn2 may be derived
from the π-π interactions among three porphyrin π-planes of
TPZn3 and π-sphere of PyC60, together with intramolecular
TPZn3-PyC60. The slightly remained absorption spectrum
(highlighted in green in Figure 12a) may be derived from the
triplet excited states of uncomplexed free TPZn3 and PyC60.
In sharp contrast to the TPZn3-PyC60 complex system, the
transient absorption spectrum of MPZn in the presence of PyC60
(Figure 12b) exhibits no characteristic absorption at 670 nor
1000 nm assigned to MPZn•+ and PyC60•-, respectively. The
transient absorption spectrum obtained 62 ps after the laser pulse
(red line in Figure 12b) exhibits the slight increase of the broad
absorption in visible/NIR region and decrease of the absorption
at around 800 nm. Such a spectral change from 1MPZn* (black
line in Figure 12b) can be assigned to the energy transfer from
π-bond formation between the porphyrin radical cation and the
•+
neutral porphyrin in TPZn3
.
Summary and Conclusions
The present study has demonstrated unequivocally the
electronic communication among porphyrins in the novel tripod
porphyrin TPZn3. In the absence of guest molecules, the π-bonds
are formed between porphyrin rings in the oxidized states. In
the presence of fullerene derivatives bearing with the π-sphere
(PyC60), TPZn3 captures PyC60 inside the cavity of the tripod.
Efficient photoinduced electron transfer from TPZn3 to PyC60
occurs in the supramolecular complex, whereas no electron
transfer from MPZn to PyC60 occurs under the same experi-
mental conditions because of its smaller association constants
with PyC60. The present study provides an excellent opportunity
to clarify the difference among trimer (TPZn3), dimer (DPZn2),
and monomer (MPZn) porphyrins for photoinduced electron
transfer in noncovalently linked electron donor-acceptor sys-
tems. The π-π interaction plays an important role to construct
efficient electron donor-acceptor systems by controlling their
electron-transfer properties.
1
MPZn to PyC60 to give the singlet excited state PyC60* (1.76
eV: determined from the absorption and fluorescence spectra).46e
1
1
The MPZn* as well as PyC60* result in the formation of the
3
3
triplet excited states MPZn* and PyC60* at 2800 ps (green
line in Figure 12b), accompanied by the recovery of the ground
state.
The singlet energy levels of porphyrins (E(S1)) are listed in
Table 3, together with the fluorescence lifetimes (τfl), the
fluorescence quantum yields (Φfl), and the triplet energy levels
(E(T1)). The cyclic voltammograms of TPZn3 in the absence
and presence of PyC60 in CH2Cl2/o-dichlorobenzene (1:1 V/V)
are shown in Figure S12. Interestingly, in the presence of PyC60,
only the first oxidation process of TPZn3 exhibits reversible
redox waves, and the first oxidation of TPZn3 moiety and first
reduction of PyC60 moiety are found at +0.63 and -0.61 V,
respectively. The first oxidation potential (0.63 V) is similar to
that of TPZn3 in the absence of PyC60 (0.61 V). This indicates
that the one-electron oxidized state of the TPZn3-PyC60
complex (TPZn3•+-PyC60) is stabilized by electronic interaction
between porphyrins even in the presence of PyC60. Electro-
chemical measurement was similarly examined for MPZn in
the presence of PyC60, and reversible redox waves were observed
at +0.73 and -0.61 V, respectively.
Experimental Section
Chemicals and Reagents. Silica gel (Merck; 70-120 mm) was
used for column chromatography. Analytical thin layer chroma-
tography was performed using Merck 60 F254 silica gel (precoated
sheets, 0.2 mm thick). Reactions were monitored by thin-layer
chromatography, UV-vis spectroscopy, and MALDI/TOF mass
spectrometry. 4-(Azidomethyl)-benzaldehyde was synthesized as
already described.50 The preparation of MPZn was described
elsewhere.27 Tris(2,2′-bipyridyl)ruthenium(III) hexafluorophosphate
Based on the photophysical and electrochemical data of
porphyrins as well as fullerene derivatives, the energy diagrams
of photochemical events for TPZn3 and MPZn in the presence
of PyC60 in toluene are summarized in Scheme 5, parts a and
b, respectively. When the solvent is different from the one used
in the electrochemical determinations, the electrochemical
potentials are corrected by using the Born dielectric continuum
model.47,48 In the present case, εs ) 2.4 is used as the dielectric
constant of toluene. The εp value is taken as 9.6, an average of
the dielectric constant of CH2Cl2 (8.9) and of o-dichlorobenzene
(10.4). Thus, the energy level of the charge-separated states
(TPZn3•+-PyC60•-) is calculated as 1.49 eV in toluene, which is
lower than the energy level of the triplet excited state of PyC60
moieties (1.56 eV).49 The kET value is sufficiently large relative
to the rate constant of intersystem crossing, which enables the
photoinduced electron transfer from TPZn3 to PyC60. From the
(47) The electrochemical potentials are corrected by taking into account
the Coulombic interaction term and the ion solvation energy based
on the Born dielectric continuum model, according to the following
equation; ∆GCS ) Eox- Ered-(e2/4πꢀ0) [1/(RDAꢀs)-{1/(2rD) + 1/(2rA)}(1/
ꢀs-1/ꢀp)], where e is elementary charge and ꢀ0 is vacuum permittivity.
In the present case, rD and rA are radii of electron donor (3.5 Å) and
electron acceptor (3.7 Å) and RDA is center-to-center donor-acceptor
distance (6 Å). ꢀs ) 2.4 and ꢀp ) 9.6 represent the dielectric constant
of toluene and o-dichlorobenzene/CH2Cl2.
(48) (a) Arnold, B. R.; Farid, S.; Goodman, J. L.; Gould, I. R. J. Am. Chem.
Soc. 1996, 118, 5482. (b) Flamigni, L.; Wyrostek, D.; Voloshchuk,
R.; Gryko, D. T. Phys. Chem. Chem. Phys. 2010, 12, 474.
(49) (a) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.;
Diederich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Phys.
Chem. 1991, 95, 11. (b) Hung, R. R.; Grabowski, J. J. J. Phys. Chem.
1991, 95, 6073. (c) Ziessel, R.; Allen, B. D.; Rewinska, D. B.;
Harriman, A. Chem.sEur. J. 2009, 15, 7382.
(50) (a) Barbe, J.-M.; Canard, G.; Brande`s, S.; Guilard, R. Eur. J. Org.
Chem. 2005, 21, 4601. (b) Felts, A. S.; Siegel, B. S.; Young, S. M.;
Moth, C. W.; Lybrand, T. P.; Dannenberg, A. J.; Marnett, L. J.;
Subbaramaiah, K. J. Med. Chem. 2008, 51, 4911.
rate constant of back electron transfer (kBET ) 1.9 × 109 s-1),
the lifetime of the charge-separated state (TPZn3•+-PyC60
)
•-
9
4486 J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010

Efficient Photoinduced Electron Transfer
A R T I C L E S
Scheme 5
[Ru(bpy)3](PF6)3 was prepared from tris(2,2′-bipyridyl)-ruthe-
nium(II) chloride hexahydrate by the oxidation with PbO2.51
N-Methyl-2-(p-pyridyl)fulleropyrrolidine was prepared according
to the literature.52 Absolute dichloromethane (CH2Cl2: Carlo Erba)
for synthesis, CH2Cl2 and toluene spectroscopic grade (Nacalai
Tesque, Inc.) for analysis, CS2 (Wako Pure Chemical Industries,
Ltd.), o-dichlorobenzene and galvinoxyl free radical (Aldrich
Chemical Co.), and 2-MeTHF (Tokyo Chemical Industry Co., Ltd.)
were obtained commercially and used without further purification.
Benzonitrile (PhCN) was purchased from Wako Pure Chemical
Industries, Ltd., and distilled with P2O5 under reduced pressure.53
various temperatures. Fluorescence and phosphorescence spectra were
recorded on a Shimadzu spectrofluorophotometer (RF-5300PC). The
fluorescence lifetimes of porphyrins were measured by a Photon
Technology International GL-3300 with a Photon Technology Inter-
national GL-302, nitrogen laser/pumped dye laser system, equipped
with a four-channel digital delay/pulse generator (Stanford Research
System Inc. DG535) and a motor driver (Photon Technology Inter-
national MD-5020). The excitation wavelength was 415 nm.
Laser Flash Photolysis Measurements. Femtosecond transient
absorption spectroscopy experiments were conducted using a Clark-
MXR 2010 laser system and an optical detection system provided
by Ultrafast Systems (Helios). The source for the pump and probe
pulses was derived from the fundamental output of Integra-C (780
nm, 2 mJ/pulse and fwhm ) 130 fs) at a repetition rate of 1 kHz.
The pump beam was attenuated at 5 µJ/pulse with a spot size of 1
mm diameter at the sample cell, where it was merged with the white
probe pulse at a close angle (<10°). The probe beam, after passing
through the 2 mm sample cell, was focused on a 200 µm fiber
optic cable which was connected to a CCD spectrograph. Typically,
2500 excitation pulses were averaged to obtain the transient
spectrum at a set delay time. Kinetic traces at appropriate
wavelengths were assembled from the time-resolved spectral data.
All measurements were recorded at 298 K.
Nanosecond time-resolved transient absorption measurements
were carried out according to the following procedure. Experiments
employed the pulses (λ ) 540 nm with the power of 2.5 mJ per
pulse) from a Panther OPO pumped by a Nd:YAG laser (Con-
tinuum, SLII-10, 4-6 ns fwhm) at 298 K. The transient absorption
spectra were monitored by continuous exposure to a xenon lamp
(150 W) as a probe light and a photomultiplier tube (Hamamatsu
2949) as a detector. Toluene solutions containing porphyrins were
prepared at concentrations such that the absorbance were around
0.4 at the excitation wavelength, and deoxygenated by argon
purging for 10 min prior to use.
1
Physicochemical Characterization of Compounds. H NMR
spectra were recorded on a Bruker Avance II 300 (300 MHz) or on a
Bruker Avance DRX 500 (500 MHz) spectrometers at the “Plateforme
d’Analyse Chimique et de Synthe`se Mole´culaire de l’UniVersite´ de
Bourgogne (PACSMUB)”; chemical shifts are expressed in ppm
relative to chloroform (7.26 ppm) or pyridine (7.22, 7.58, and 8.74
ppm). UV-vis spectra were recorded on a Varian Cary 1 spectro-
photometer. Mass spectra and accurate mass measurements (HR-MS)
were obtained on a Bruker Daltonics Ultraflex II spectrometer in the
MALDI/TOF reflectron mode using dithranol as a matrix or on a
Bruker MicroQTof instrument in ESI mode. Both measurements were
made at the “Plateforme d’Analyse Chimique et de Synthe`se Mole´cu-
laire de l’UniVersite´ de Bourgogne (PACSMUB)”.
Electrochemical Measurements. Cyclic voltammetry (CV) mea-
surements were carried out with a BAS 100W electrochemical analyzer
in a deaerated solvent containing 0.10 M TBAPF6 as a supporting
electrolyte at 298 K. A conventional three-electrode cell was used with
a platinum working electrode and a platinum wire as a counter
electrode. The redox potentials were measured with respect to the Ag/
AgNO3 (1.0 × 10-2 M) reference electrode. The potential values (vs
Ag/AgNO3) are converted to those vs SCE by adding 0.29 V.54
Photophysical Measurements. UV-vis-NIR spectra were re-
corded on a Hewlett-Packard 8453 diode array spectrophotometer at
Electron Spin Resonance (ESR) Measurements. The ESR
spectra were measured at various temperatures with a JEOL X-band
spectrometer (JES-RE1XE). The ESR spectra were recorded under
nonsaturating microwave power conditions. The magnitude of the
modulation was chosen to optimize the resolution and the signal-to-
noise (S/N) ratio of the observed spectra. The g values were calibrated
(51) Desimone, R. E.; Drago, R. S. J. Am. Chem. Soc. 1970, 92, 2343.
(52) Maggini, M.; Scorrano, G. J. Am. Chem. Soc. 1993, 115, 9798.
(53) Purification of Laboratory Chemicals, 5th ed.; Armarego, W. L. F.,
Chai, C. L. L., Eds.; Butterworth-Heinemann: Amsterdam, 2003.
(54) Electrochemical Reactions in Nonaqueous Systems; Mann, C. K.,
Barnes, K. K., Eds.; Marcel Dekker: New York, 1970.
9
J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010 4487

A R T I C L E S
Takai et al.
with an Mn2+ marker. CH2Cl2 solutions containing porphyrins were
deaerated by argon purging for 10 min prior to use. The concentrations
of radical species were qualitatively determined by double integration
of the first-derivative ESR signal in reference to that of a known
amount of a stable radical, galvinoxyl free radical, under the same
experimental conditions. Since the radical concentration of the galvi-
noxyl radical is the same over the entire temperature range, variations
of the ESR intensity only depend on the Curie law and instrumental
factors such as cavity Q-factor. Thus, by comparing the ratio of the
ESR intensity of TPZn32+ (IP) with that of galvinoxyl radical (IG), the
quantitative radical concentrations of TPZn32+ can be determined at
various temperatures.
Theoretical Calculations. Density-functional theory (DFT)
calculations were performed on a COMPAQ DS20E computer.
Geometry optimizations were carried out using the Becke3LYP
functional and 3-21G(*) basis set with the Gaussian 03 program,
revision C.02.55 The graphical output of the optimized structure
was generated with the Gauss View software program (version 3.09)
developed by Semichem, Inc.56
(10). HR-MS (MALDI/TOF): m/z ) 643.2416 [M]•+, 643.2402
calcd. for C37H37N7Zn.
Diporphyrin Biszinc DPZn2. 2,3,7,8,12,18-Hexamethyl-5-(4-
azidomethylphenyl)-13,17-diethylporphyrin (31 mg, 48.2 µmol) and
5-(4-ethynylphenyl)-13,17-diethyl-2,3,7,8,12,18-hexamethylporphy-
rin zinc (30 mg, 49.1 µmol) were dissolved in 15 mL of THF in a
100 mL round-bottom flask fitted with a reflux condenser. Sodium
ascorbate (15.7 mg, 79 µmol) dissolved in 1 mL of water and
CuSO4 ·5H2O (6.7 mg, 30 µmol) were added to the solution. The
reaction mixture was stirred at 70 °C for 64 h. The solution was
then evaporated under reduced pressure and dissolved in 30 mL of
methylene chloride. The solution was washed twice with a saturated
NH4Cl solution then dried over anhydrous MgSO4. After filtration
over a silica plug (CH2Cl2/Heptane, 50/50), the unreacted reagents
were eliminated. Further purification by silica gel chromatography
(first CH2Cl2 then CH2Cl2/MeOH, 96/4) afforded the title compound
in 43% yield (26 mg, 20.6 µmol) as a purple solid. 1H NMR
(Pyridine-D5, 298 K): δ (ppm) ) 1.79 (t, J ) 7.5 Hz, 12H, CH3),
2.50 (m, 12H, CH3), 3.48 (m, 24H, CH3), 4.04 (q, J ) 7.5 Hz, 8H,
CH2), 6.15 (s, 2H, CH2-Ar), 7.77 (d, J ) 7.8 Hz, 2H, HAr), 8.12
(d, J ) 7.8 Hz, 2H, HAr), 8.25 (d, J ) 7.8 Hz, 2H, HAr), 8.61 (d,
J ) 7.8 Hz, 2H, HAr), 8.83 (s, 1H, Htriazole), 10.32 (s, 2H, Hmeso),
10.39 (s, 4H, Hmeso). UV-vis (CH2Cl2): λmax (nm) (ꢀ × 10-3 M-1
cm-1) ) 405.0 (420), 535.0 (21), 571.0 (19.5). MS (MALDI/TOF):
m/z ) 1255.43 [M]•+, 1255.46 calcd. for C75H73N11Zn2. HR-MS
(ESI): m/z ) 1255.4410 [M]•+, 1255.4633 calcd. for C75H73N11Zn2;
m/z ) 1278.4461 [M+Na]+, 1278.4531 calcd. for C75H73N11NaZn2.
Trisporphyrin Triszinc TPZn3. 2,3,7,8,12,18-Hexamethyl-5-
(4-azidomethylphenyl)-13,17-diethylporphyrin (45 mg, 66 µmol,
3 equiv) and tripropargylamine (2.91 mg, 22.2 µmol) were dissolved
in 15 mL of THF in a 100 mL round-bottom flask fitted with a
reflux condenser. Sodium ascorbate (43.9 mg, 222 µmol, 10 equiv)
dissolved in 1 mL of water and CuSO4 ·5H2O (22.2 mg, 111 µmol,
5 equiv) were added to the solution. The reaction mixture was stirred
at 70 °C for 48 h. The solution was then evaporated under reduced
pressure and dissolved in 30 mL of methylene chloride. The solution
was washed twice with a saturated NH4Cl solution then dried over
anhydrous MgSO4. After purification over a silica plug (CH2Cl2/
Heptane, 50/50), the unreacted reagents were eliminated. Further
purification by silica gel chromatography (first CH2Cl2 then CH2Cl2/
MeOH, 95/5) afforded the title compound in 29% yield (13 mg,
6.3 µmol) as a purple solid. 1H NMR (CDCl3, 298 K): δ (ppm) )
1.67 (t, J ) 8.3 Hz, 18H, CH3), 1.96 (s, 18H, ꢀ-CH3), 3.08 (s,
18H, ꢀ-CH3), 3.29 (s, 18H, ꢀ-CH3), 3.74 (q, J ) 8.3 Hz, 12H,
CH2), 4.10 (s, 6H, N-CH2-Triazole), 5.88 (s, 6H, Ph-CH2-
Triazole), 7.42 (m, 12H, Ph), 8.06 (s, 3H, Htriazole), 9.32 (s, 3H,
2,3,7,8,12,18-Hexamethyl-5-(4-azidomethylphenyl)-13,17-dieth-
ylporphyrin. A mixture of 270 mg (1.68 mmol) of 4-(azidomethyl)-
benzaldehyde and 1.00 g (1.67 mmol) of a,c-biladiene dihydro-
bromide was dissolved in 100 mL of hot absolute ethanol.
p-Toluenesulfonic acid (1.7 g, 9.87 mmol) in 35 mL of ethanol
was slowly added over 12 h, and the mixture was stirred under
reflux for 48 h. The mixture was cooled to room temperature, and
the solution was evaporated under vacuum. The residue was
dissolved in dichloromethane (200 mL), neutralized with a saturated
solution of NaHCO3, and then washed thoroughly three times with
500 mL of water. The organic phase was dried over MgSO4 and
filtered, the solvent was removed in vacuo. The residue was purified
by silica column chromatography (CH2Cl2/MeOH, 98/2). After
crystallization from CH2Cl2/MeOH, the title compound was isolated
1
in 23% yield (223 mg, 0.38 mmol) as a purple solid. H NMR
(CDCl3, 298 K): δ (ppm) ) -3.31 (s, 2H, NH-imino), 1.89 (t, J )
8.2 Hz, 6H, CH3), 2.38 (s, 6H, ꢀ-CH3), 3.50 (s, 6H, ꢀ-CH3), 3.60
(s, 6H, ꢀ-CH3), 4.04 (q, J ) 8.2 Hz, 4H, CH2), 4.58 (s, 2H,
CH2-N3), 7.56 (d, J ) 8.2 Hz, 2H, Ph), 7.96 (d, J ) 8.2 Hz, 2H,
Ph), 9.91 (s, 1H, Hmeso), 10.11 (s, 2H, Hmeso). UV-vis (CH2Cl2):
λmax (nm) (ꢀ × 10-3 M-1 cm-1) ) 403.0 (341), 500.0 (16), 533.9
(13), 571.0 (12). HR-MS (MALDI/TOF): m/z ) 581.3225 [M]•+,
581.3267 calcd. for C37H39N7.
2,3,7,8,12,18-Hexamethyl-5-(4-azidomethylphenyl)-13,17-dieth-
ylporphyrinato-Zn (II). 2,3,7,8,12,18-Hexamethyl-5-(4-azidom-
ethylphenyl)-13,17-diethylporphyrin (200 mg, 0.352 mmol) and zinc
acetate tetrahydrate (155 mg, 0.704 mmol, 2 equiv) were dissolved
in chloroform (35 mL) and methanol (15 mL) and refluxed for 90
min in the presence of sodium acetate (289 mg, 3.25 mmol). The
metalation reaction was monitored by UV-vis spectroscopy and
MALDI/TOF mass spectrometry. At the end of the reaction, the
solvent was removed under reduced pressure and the crude product
was dissolved in CH2Cl2 (50 mL), washed with water (3 × 250
mL) and dried over MgSO4. The solvent was removed in vacuo
and the residue was purified by silica column chromatography
(CH2Cl2 as eluent). After evaporation, the title compound was
obtained in 91% yield (206 mg, 0.32 mmol) as a red microcrys-
talline solid. 1H NMR (CDCl3, 298 K): δ (ppm) ) 1.83 (t, J ) 7.4
Hz, 6H, CH3), 2.40 (s, 6H, ꢀ-CH3), 3.48 (s, 6H, ꢀ-CH3), 3.57 (s,
6H, ꢀ-CH3), 4.01 (q, J ) 7.4 Hz, 4H, CH3-CH2), 4.67 (s, 2H,
CH2-N3), 7.66 (d, J ) 8.0 Hz, 2H, Ph), 8.05 (d, J ) 8.0 Hz, 2H,
Ph), 9.89 (s, 1H, Hmeso), 10.04 (s, 2H, Hmeso). UV-vis (CH2Cl2):
λmax (nm) (ꢀ × 10-3 M-1 cm-1) ) 404.0 (215), 534.0 (11), 570.0
H
meso), 9.39 (s, 6H, Hmeso). UV-vis (CH2Cl2): λmax (nm) (ꢀ × 10-3
M-1 cm-1) ) 404.9 (375), 537.0 (26), 571.9 (24). MS (MALDI/
TOF): m/z ) 2061.15 [M]•+, 2060.79 calcd. for C120H121N22Zn3.
HR-MS (ESI): m/z ) 2083.7860 [M+Na]+, m/z ) 1041.8950
[M+Na]2+ calcd. for C120H120N22Na1Zn3: 2083.7838 (monocation),
1041.8913 (dication).
Acknowledgment. This research was supported by KOSEF/
MEST through WCU project (R31-2008-000-10010-0) and Grants-
in-Aid (No. 20108010) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. A.T. appreciates support
from the Global COE program “Global Education and Research
Center for Bio-Environmental Chemistry” of Osaka University
(S.F.) and JSPS fellowship for young scientists. The Centre National
de Recherche Scientifique (CNRS, UMR 5260) and “Re´gion
Bourgogne” are also acknowledged for funding.
Supporting Information Available: ESI HR-MS spectrum of
TPZn3 (S1), H NMR spectrum of TPZn3 (S2), cyclic voltam-
mograms of MPZn, DPZn2, and TPZn3 in PhCN (S3), vis-NIR
spectral changes upon addition of [Ru(bpy)3]3+ to a CH2Cl2
solution of MPZn and DPZn2 (S4), vis-NIR spectral changes
(55) Frisch, M. J.; Gaussian 03, ReVision C.02; Gaussian, Inc.: Wallingford,
CT, 2004. The full list of authors is given in the Supporting
Information.
(56) Dennington R., II; Keith, T.; Millam, J.; Eppinnett, K.; Hovell, W. L.;
Gilliland, R. Gauss View, version 3.09; Semichem, Inc.: Shawnee
Mission, KS, 2003.
1
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4488 J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010

Efficient Photoinduced Electron Transfer
A R T I C L E S
upon addition of [Ru(bpy)3]3+ (3-6 equiv) to a CH2Cl2 solution
of TPZn3 (S5), ESR spectra of TPZn3 in the presence of
[Ru(bpy)3]3+ and dependence of the ESR intensity (IESR) on the
ratio of concentration of [Ru(bpy)3]3+ to TPZn3 (S6), Molecular
orbital diagrams for the radical cation and the radical cation
dimer (S7), 1H NMR spectra of TPZn3 and MPZn in the absence
and presence of C60 (S8), Job’s plot for the complexation
between TPZn3 and PyC60 in o-dichlorobenzene (S9), Fluores-
cence spectral changes of TPZn3 upon addition of PyC60 and
change in the fluorescence intensity at 577 nm (S10), 1H NMR
spectra of PyC60 and MPZn in the absence and presence of PyC60
for the determination of association constant between MPZn
and PyC60 (S11), cyclic voltammograms of TPZn3 in the absence
and presence of PyC60 (S12), and full author list of ref 55 (S13).
This material is available free of charge via the Internet at http://
pubs.acs.org.
JA100192X
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