Fullerene- and pyromellitdiimide-appended tripodal ligands embedded in light-harvesting porphyrin macrorings

Article abstract of DOI:10.1021/ic201250q

Three new tripyridyl tripodal ligands appended with either fullerene or pyromellitdiimide moieties, named C₆₀-s-Tripod, C₆₀-l- Tripod, and PI-Tripod, were synthesized and introduced into a porphyrin macroring N-(1-Zn)₃ (where 1-Zn = trisporphyrinatozinc(II)). From UV-vis absorption and fluorescence titration data, the binding constants of C₆₀-s-Tripod, C₆₀-l-Tripod, and PI-Tripod with N-(1-Zn)₃ in benzonitrile were estimated to be 3 × 10 ⁸, 1 × 10⁷, and 2 × 10⁷ M ^-1, respectively. These large binding constants denote multiple interactions of the ligands to N-(1-Zn)₃. The binding constants of the longer ligand (C₆₀-l-Tripod) and the pyromellitdiimide ligand (PI-Tripod) are almost the same as those without the fullerene or pyromellitdiimide groups, indicating that they interact via three pyridyl groups to the porphyrinatozinc(II) coordination. In contrast, the larger binding constants and the almost complete fluorescence quenching in the case of the shorter ligand (C₆₀-s-Tripod) indicate that the interaction with N-(1-Zn)₃ is via two pyridyl groups to the porphyrinatozinc(II) coordination and a π-π interaction of the fullerene to the porphyrin(s). The fluorescence of N-(1-Zn)₃ was quenched by up to 80% by the interaction of C₆₀-l-Tripod. The nanosecond transient absorption spectra showed only the excited triplet peak of the fullerene on selective excitation of the macrocyclic porphyrins, indicating that energy transfer from the excited N-(1-Zn)₃ group to the fullerenyl moiety occurs in the C₆₀-l-Tripod/N-(1-Zn)₃ composite. In the case of PI-Tripod, the fluorescence of N-(1-Zn)₃ was quenched by 45%. It seems that the fluorescence quenching probably originates from electron transfer from the excited N-(1-Zn)₃ group to the pyromellitdiimide moiety.

Full text of DOI:10.1021/ic201250q

ARTICLE
pubs.acs.org/IC
Fullerene- and Pyromellitdiimide-Appended Tripodal Ligands
Embedded in Light-Harvesting Porphyrin Macrorings
Yusuke Kuramochi,† Akiharu Satake,† Atula S. D. Sandanayaka,‡ Yasuyuki Araki,‡ Osamu Ito,‡ and
Yoshiaki Kobuke*,†,§
†Graduate School of Materials Science, Nara Institute of Science and Technology, Takayama 8916-5, Ikoma, Nara 630-0101, Japan
‡Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Sendai, 980-8577, Japan
S Supporting Information
b
ABSTRACT:
Three new tripyridyl tripodal ligands appended with either fullerene or pyromellitdiimide moieties, named C60ꢀsꢀTripod,
C60ꢀlꢀTripod, and PIꢀTripod, were synthesized and introduced into a porphyrin macroring Nꢀ(1ꢀZn)3 (where 1ꢀZn =
trisporphyrinatozinc(II)). From UVꢀvis absorption and fluorescence titration data, the binding constants of C60ꢀsꢀTripod,
C60ꢀlꢀTripod, and PIꢀTripod with Nꢀ(1ꢀZn)3 in benzonitrile were estimated to be 3 ꢁ 108, 1 ꢁ 107, and 2 ꢁ 107 Mꢀ1
,
respectively. These large binding constants denote multiple interactions of the ligands to Nꢀ(1ꢀZn)3. The binding constants of the
longer ligand (C60ꢀlꢀTripod) and the pyromellitdiimide ligand (PIꢀTripod) are almost the same as those without the fullerene or
pyromellitdiimide groups, indicating that they interact via three pyridyl groups to the porphyrinatozinc(II) coordination. In
contrast, the larger binding constants and the almost complete fluorescence quenching in the case of the shorter ligand
(C60ꢀsꢀTripod) indicate that the interaction with Nꢀ(1ꢀZn)3 is via two pyridyl groups to the porphyrinatozinc(II) coordination
and a πꢀπ interaction of the fullerene to the porphyrin(s). The fluorescence of Nꢀ(1ꢀZn)3 was quenched by up to 80% by the
interaction of C60ꢀlꢀTripod. The nanosecond transient absorption spectra showed only the excited triplet peak of the fullerene on
selective excitation of the macrocyclic porphyrins, indicating that energy transfer from the excited Nꢀ(1ꢀZn)3 group to the
fullerenyl moiety occurs in the C60ꢀlꢀTripod/Nꢀ(1ꢀZn)3 composite. In the case of PIꢀTripod, the fluorescence of
Nꢀ(1ꢀZn)3 was quenched by 45%. It seems that the fluorescence quenching probably originates from electron transfer from
the excited Nꢀ(1ꢀZn)3 group to the pyromellitdiimide moiety.
’ INTRODUCTION
attention, not only from a pure scientific interest but also from
potential applications of photosynthesis in solar energy conver-
sion systems, which could help resolve the energy and environ-
mental problems of the world. Porphyrin is a good chromophore
for use in artificial photosynthesis because of its structural and
functional similarities to natural chlorophyll. Thus, a number of
researchers have studied various types of multiporphyrins and
porphyrinꢀelectron acceptor conjugated systems as models for
LH and RC systems.5ꢀ7
In photosynthesis, sunlight is converted into biochemical
energy with a high efficiency. These light reactions begin with
the capture of dilute photons from the sun by a network of
chromophores, known as light-harvesting (LH) complexes.
Photosynthetic purple bacteria have well-defined cyclic bacterio-
chlorophyll as architectures, such as LH1 and LH2,1,2 and carry
out isotropic energy migration among the LHs in the photo-
synthetic membrane.3 The excited energy migrates along the
LHs until it finally reaches a special pair of the reaction center
(RC) that can produce a stable transmembrane-spanned charge-
separated state through a multistep photoinduced electron
transfer process.4 The efficient collection of sunlight and con-
version of energy into electrochemical potential enable the
photosynthetic living organism to use solar energy for metabolic
reactions with a quantum yield of almost 100%. The construction
of artificial photosynthetic systems has attracted considerable
We have reported on LH porphyrin macrorings8 using a
supramolecular methodology based on the complementary co-
ordination of spacer-linked bisimidazolylporphyrinatozinc(II),9
and have developed further the host macroring Nꢀ(1ꢀZn)3
having three noncoordinated porphyrinatozinc(II) units as
Received: June 10, 2011
Published: September 22, 2011
r
2011 American Chemical Society
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Figure 1. Molecular structures of the macroring Nꢀ(1ꢀZn)3 and tripyridyl tripodal ligands. The distances were estimated from molecular models
using the Material Studio software package.13
scaffolds that can accommodate the tripyridyl tripodal ligands.10,11
We employed a tripodal pyridyl ligand with an energy acceptor
porphyrin appended to a fullerene (C60ꢀZnPꢀTripod) as the
specific guest of Nꢀ(1ꢀZn)3.11c The composite exhibited a
smooth singletꢀsinglet energy transfer process from the excited
Nꢀ(1ꢀZn)3 group to the acceptor porphyrin, followed by the
production of a stable charge-separated state between the
acceptor porphyrin and the appended fullerene in C60ꢀZnPꢀ
Tripod. We also synthesized a tripodal ligand with a fullerene
linked via an amide (C60ꢀTripod), and incorporated it into
Nꢀ(1ꢀZn)3, aiming to induce the direct electron transfer from
the excited Nꢀ(1ꢀZn)3 unit to the fullerenyl moiety.11a
Although C60ꢀTripod was smoothly accommodated into Nꢀ
(1ꢀZn)3 with a binding constant up to 3 ꢁ 108 Mꢀ1, even in a
polar benzonitrile environment, and the fluorescence of Nꢀ
(1ꢀZn)3 was almost completely quenched, no peaks from the
radical ion were detected in either the nanosecond or the
picosecond transient absorption spectral measurements. These
results suggest that the fullerenyl moiety of C60ꢀTripod is in
direct contact with the porphyrin plane(s) of Nꢀ(1ꢀZn)3
through a fullereneꢀporphyrin πꢀπ interaction along with
coordination with two pyridyl groups, resulting in fast back
electron transfer.12
interaction between the fullerenyl moiety and the porphyrin groups
of Nꢀ(1ꢀZn)3, which causes a fast back electron transfer.
C60ꢀsꢀTripod was the reference compound, as it has a short
phenylene linker, where the distance from the pyridyl nitrogen atom
to the fullerenyl edge is small enough for the fullerenyl moiety to be
enclosed by the Nꢀ(1ꢀZn)3 group. PIꢀTripod has a pyromellit-
diimide as an electron acceptor. In a PIꢀTripod/Nꢀ(1ꢀZn)3
composite, a three-point coordination from the pyridyl groups to
the porphyrinatozinc(II) groups is expected to be an exclusive
binding mode, because the πꢀπ interaction between the pyromel-
litdiimide and the porphyrin would be weak, and furthermore, a
πꢀπ stacking between PIꢀTripod and Nꢀ(1ꢀZn)3 is structurally
not possible. By comparing the properties of these novel tripodal
ligands, we attempted to construct supramolecular composites that
would perform an optimal photoinduced electron transfer from LH
macrorings to a central acceptor.
’ RESULTS AND DISCUSSION
Synthesis of C60ꢀsꢀTripod, C60ꢀlꢀTripod, and PIꢀ
Tripod. The C60ꢀsꢀTripod, C60ꢀlꢀTripod, and PIꢀTripod were
prepared according to Schemes 1, 2, and 3, respectively. C60ꢀ
sꢀTripod was synthesized from tetrakis(4-iodophenyl)-
methane14 in four steps. The cyanation of tetrakis(4-iodophenyl)-
methane was carried using 1 equiv of CuCN in dimethylforma-
mide (DMF) to give a mixture of 1 (35%), unreacted starting
material (50%), and a dicyanated byproduct (15%). The crude
mixture was purified using a silica gel column to afford 1 in a yield
of 34%. Cyanide 1 was reduced by DIBAL-H to the correspond-
ing aldehyde 2 in a yield of 79%. The cross-coupling reaction of 2
with 4-ethynylpyridine15 was carried out in the presence of a
Here, we report on the synthesis of three novel tripodal
acceptor ligands: C60ꢀlꢀTripod, C60ꢀsꢀTripod, and PIꢀTripod
(Figure 1). C60ꢀlꢀTripod has a long phenyleneꢀethynyleneꢀ
phenylene linker in between a fullerenylꢀpyrrolidine and a tris-
[4-{2-(4-pyridyl)ethynyl}phenyl]methane part. In C60ꢀlꢀTripod,
the distance from the pyridyl nitrogen atom to the fullerenyl edge is
large enough to prevent the fullerenyl moiety from being enclosed
by the Nꢀ(1ꢀZn)3 groups, and also to prevent a direct πꢀπ
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Scheme 1. Synthetic Route of C60ꢀsꢀTripod
Scheme 2. Synthetic Route of C60ꢀlꢀTripod
Pd(PPh3)2Cl2/CuI catalyst16 to afford sꢀTripod in a yield of
78%. The succeeding 1,3-dipolar cycloaddition with C60 and
N-methylglycine17 afforded C60ꢀsꢀTripod in a yield of 41%.
C60ꢀlꢀTripod was synthesized from tetrakis(4-iodophenyl)-
methane in three steps. lꢀTripod was synthesized from tetrakis-
(4-iodophenyl)methane in a total yield of 21%, as has been
described elsewhere.11c C60ꢀlꢀTripod was synthesized using a
1,3-dipolar cycloaddition employing lꢀTripod, C60 and N-methyl-
glycine in a yield of 57%. PIꢀTripod was synthesized according
to Scheme 3. Because the starting material 4-(triphenyl-
methyl)aniline has an NH2 group that strongly activates electro-
philic substitution at the position ortho to the NH2 group in the
aniline moiety, pyromellitic monoanhydride was first introduced
at the para-positions of the three other phenyl groups before
halogenation.18 The condensation of 4-(triphenylmethyl)aniline14
and pyromellitic monoanhydride19 in DMF afforded 3 in a yield
of 74%. In the final cross-coupling reaction for PIꢀTripod, the
reactivity of aryl iodide was expected to be much higher than that
of aryl bromide.20 Thus, we first attempted to synthesize the
iodinated precursor for PIꢀTripod. Iodination at the para-
positions of the three phenyl groups of 3 was first attempted
using bis(trifluoroacetoxy)iodobenzene (3ꢀ5 equiv) and iodine
(3ꢀ5 equiv) in CCl4.14 However, the iodination reaction gave a
mixture of para- and meta-iodinated products (Scheme 3). The
meta byproduct could not be removed using column chromatog-
raphy, even after the successful coupling reaction with 4-ethynyl-
pyridine. On the other hand, the bromination reaction only
proceeded at the targeted para-positions of the three phenyl
groups after treatment with neat bromine at room temperature.21
Purification using a silica gel column gave 4 in a yield of 89%. The
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Scheme 3. Synthetic Route of PIꢀTripod
PIꢀTripod, there was no absorption extending to the visible
region (>450 nm).
Binding Modes of C60ꢀsꢀTripod, C60ꢀlꢀTripod, and
PIꢀTripod to Nꢀ(1ꢀZn)3. Incorporation of the acceptor tripo-
dal ligands into the Nꢀ(1ꢀZn)3 group was determined using
UVꢀvis absorption and fluorescence titration. The experiments
were performed in benzonitrile as a suitable solvent to stabilize
the charge-separated species. The binding mode of the tripodal
ligands to Nꢀ(1ꢀZn)3 was estimated by comparing their
binding constants with the corresponding tripodal ligand without
their acceptors. If almost the same values were observed between
the two tripodal ligands, with and without the acceptor, the
binding mode was concluded to maintain the three-point co-
ordination of the pyridyl group to the zinc atom. The changes in
the UVꢀvis absorption spectra are shown in Supporting Infor-
mation, Figures S15ꢀS17. In all three spectra, a small decrease
and a red-shift of the Sorbet band region was observed. No
change in the Q-band was observed, because benzonitrile was
already coordinated to the uncoordinated porphyrinatozinc-
(II).11a The change in the fluorescence spectra of Nꢀ(1ꢀZn)3
on addition of the acceptor ligands is shown in Figure 3. The
fluorescence of Nꢀ(1ꢀZn)3 (4.0 ꢁ 10ꢀ7 M) is efficiently quen-
ched by 98% with 2.8 equiv of C60ꢀsꢀTripod (Figure 3A, inset).
The binding constant estimated from the UVꢀvis absorption
and fluorescence titration data was almost same: 3 ꢁ 108 and 4 ꢁ
108 Mꢀ1, respectively.22 These values are 10 times the value of
2.9 ꢁ 107 Mꢀ1 of the precursor sꢀTripod, suggesting a switch of
the binding mode to a two-point coordination of the pyridyl group
to the zinc atom and a πꢀπ interaction between the fullerenyl
Figure 2. UVꢀvis absorption spectra of C60ꢀsꢀTripod (CHCl3),
C60ꢀlꢀTripod (benzonitrile), and PIꢀTripod (CHCl3) at 25 °C.
cross-coupling reaction of 4 with 4-ethynylpyridine hardly
proceeded when Pd(PPh3)2Cl2 and CuI were used as catalysts.
To overcome this problem, the bulky and strong electron-
donating phosphine ligand (P(t-Bu)3) was used instead of
PPh3.20 The coupling reaction proceeded at room tempera-
ture in the presence of Pd(PhCN)2Cl2, ((t-Bu)3PH)BF4
and CuI, and afforded PIꢀTripod in a yield of 44%. C60ꢀ
sꢀTripod, C60ꢀlꢀTripod, and PIꢀTripod were characterized
1
using H and 13C NMR, Matrix-Assisted Laser Desorption/
Ionization-Time-of-Flight (MALDI-TOF) mass spectrometry,
high-resolution mass spectrometry, and UVꢀvis absorption
spectroscopy.
The UVꢀvis absorption spectra of C60ꢀsꢀTripod, C60ꢀ
lꢀTripod, and PIꢀTripod are shown in Figure 2. The spectra
of C60ꢀsꢀTripod and C60ꢀlꢀTripod show a strong absorp-
tion in the UV region accompanied by a weak absorption that
extended continuously to the visible region, which originated
from the absorption of the fullerenyl moiety. In the case of
moiety and the porphyrin(s) in Nꢀ(1ꢀZn)3, as was observed
11a
previously.
The almost complete fluorescence quenching is
similar to the previous case, consistent with this binding mode.
On the titration of C60ꢀlꢀTripod, the binding constant was
estimated from the UVꢀvis absorption and fluorescence titration
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Figure 3. Steady-state fluorescence quenching of Nꢀ(1ꢀZn)3 from the addition of: (A) C60ꢀsꢀTripod, (B) C60ꢀlꢀTripod, and (C) PIꢀTripod in
benzonitrile at 25 °C. When excited at 567 nm: (A) [Nꢀ(1ꢀZn)3] = 4.0 ꢁ 10ꢀ7 M, [C60ꢀsꢀTripod] = 0ꢀ2.8 equiv, (B) [Nꢀ(1ꢀZn)3] = 2.5 ꢁ 10ꢀ7 M,
[C60ꢀlꢀTripod] = 0ꢀ4.3 equiv, (C) [Nꢀ(1ꢀZn)3] = 3.9 ꢁ 10ꢀ7 M and [PIꢀTripod] = 0ꢀ4.6 equiv. The insets show the change in fluorescence intensity
with the signal integration (b) and the theoretical curves (blue lines) for: (A) Ka = 4 ꢁ 108 Mꢀ1, (B) Ka = 1.3 ꢁ 107 Mꢀ1, and (C) Ka = 2.0 ꢁ 107 Mꢀ1
.
Table 1. Binding Constants, Ka, and Fluorescence Quenching Efficiencies of Nꢀ(1ꢀZn)3 of Tripodal Ligands in Benzonitrile at
25 °C, and Distances from the Pyridyl Nitrogen Atom and the Fullerene Edge of the Tripodal Ligands
sꢀTripod
C60ꢀsꢀTripod
C60ꢀTripoda
3.4 ꢁ 108
3.1 ꢁ 108
97%
lꢀTripod
C60ꢀlꢀTripod
PIꢀTripod
Ka/Mꢀ1
fluorescenceb
UVꢀvis
4 ꢁ 108e
3 ꢁ 108e
98%
1.3 ꢁ 107
1.0 ꢁ 107
80%
2.0 ꢁ 107
2.3 ꢁ 107
45%
2.9 ꢁ 107
f
1.1 ꢁ 107
f
quenching
efficiencyb,c
distanced
24.7 Å
25.6 Å
28.9 Å
a Ref 11a. b The data were obtained using an excitation wavelength at 567 nm. c These values were calculated assuming a complexation rate of 100%. d See
Figure 1. e Ref 22. f No fluorescence quenching was observed.
data to be 1.0 ꢁ 107 and 1.3 ꢁ 107 Mꢀ1, respectively. The
fluorescence of Nꢀ(1ꢀZn)3 was quenched by 80% (Figure 3B,
inset). The binding constant of C60ꢀlꢀTripod was similar to
that of the precursor, lꢀTripod (1.1 ꢁ 107 Mꢀ1), indicating that
it maintained the three-point coordination of the pyridyl group
to the zinc atom.
Fluorescence titration using PIꢀTripod in benzonitrile is
shown in Figure 3C. From the UVꢀvis absorption and the
fluorescence titration curves, the binding constants were esti-
mated to be 2.3 ꢁ 107 and 2.0 ꢁ 107 Mꢀ1, respectively. The
fluorescence of Nꢀ(1ꢀZn)3 was quenched by 45% (Figure 3C,
inset). The binding constant of PIꢀTripod was almost the same
as that of the tripodal ligands,10,23 indicating a three-point
coordination of the pyridyl group to the zinc atom.
All the binding constants and the fluorescence quenching
efficiencies in benzonitrile are summarized in Table 1. Judging
from these data, it was estimated that C60ꢀsꢀTripod interacted
with the Nꢀ(1ꢀZn)3 group as a two-point pyridyl coordination
along with a fullereneꢀporphyrin contact, whereas the other two
ligands interacted as a three-point coordination.
According to the molecular model created using the Material
Studio software package,13 the distance in C60ꢀsꢀTripod from
the pyridyl nitrogen atom to the fullerenyl edge was estimated to
be 24.7 Å, which is smaller than the cavity of Nꢀ(1ꢀZn)3
(Figure 1) based on the coordination bond length (approximately
2.1 Å)24 between the pyridyl nitrogen atom and the porphyr-
inatozinc atom. Thus, the fullerenyl moiety is incorporated inside
the Nꢀ(1ꢀZn)3 ring and so gains a high degree of entropy from
the release of constricted solvent molecules located inside the
cavity.23 In the case of C60ꢀlꢀTripod, the distance from the
pyridyl nitrogen atom to the fullerenyl edge was estimated to be
28.9 Å, which is too large for the fullerenyl moiety to be enclosed
by the ring. The pyromellitimide moiety of PIꢀTripod cannot be
located inside the cavity, because weak πꢀπ interaction between
the pyromellitdiimide and the porphyrin ring cannot replace the
three cooperative pyridylꢀporphyrinatozinc coordinations. The
prospective structures of their composites are shown in Figure 4.
Photodynamic Properties of Macroring/Tripodal Ligand
Composites. The results of the binding constant and the
fluorescence quenching experiments suggest that the binding
mode of C60ꢀsꢀTripod is the same as that of C60ꢀTripod.
Thus, a stable charge-separated state is not expected, as observed
previously.11a On the other hand, the fullerenyl moiety in the
C60ꢀlꢀTripod/Nꢀ(1ꢀZn)3 composite must locate not inside,
but over the cavity. Because the nearest edge-to-edge distance
between the porphyrin and the fullerenyl moiety in this compo-
site is estimated to be 13 Å, the formation of a stable charge-
separated state is expected if the photoinduced electron transfer
from the excited Nꢀ(1ꢀZn)3 group to the fullerenyl moiety
occurs.6
The time-resolved fluorescence decay and nanosecond tran-
sient absorption were measured to investigate the photophysical
dynamics of the C60ꢀlꢀTripod/Nꢀ(1ꢀZn)3 composite. Un-
der these conditions, the binding constants ensured that more
than 95% of the macroring existed as a complex with C60ꢀ
lꢀTripod. The fluorescence time profile of the C60ꢀlꢀTripod/
Nꢀ(1ꢀZn)3 composite decayed biexponentially with a time
constant of 184 ps (70%) and 1220 ps (30%). The minor longer-
lived component may arise from impurities or decomposed
products. The quenching rate (kq) and the quantum yield
(Φq) were evaluated to be 5.0 ꢁ 109 sꢀ1 and 0.9 from the
shorter-lifetime component using the lifetime of 2100 ps of free
Nꢀ(1ꢀZn)3.11a The nanosecond transient absorption spectra
were measured to elucidate the quenching mechanism of the
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Figure 4. Prospective structures of C60ꢀsꢀTripod, C60ꢀlꢀTripod, and PIꢀTripod accommodated by Nꢀ(1ꢀZn)3. (The left image is depicted from
the top. The others are side views).
F€orster equation:
8:8 ꢁ 10ꢀ25k2Φ
kTS
¼
ð1Þ
n4R6τ
where k2, Φ, τ, R, and J are the orientation factor of the donor
and acceptor, the fluorescence quantum yield, the fluorescence
lifetime of the donor, the distance between the donor and the
acceptor, and the spectral integral of the fluorescence of the
donor and the absorption of the acceptor, respectively. The
orientation factor of k2 = 2/3 was used for randomly orientated
dipoles25 to estimate the energy transfer rate. The fluorescence
quantum yield of Φ = 0.039 and the fluorescence lifetime of τ =
2.1 ns were obtained from the experiments.11a The distance
between the center of the acceptor and the center of the nearest
donor, R = 16 Å, was estimated from molecular modeling using
the Material Studio software package, and the spectral integral
was calculated to be J ≈ 5 ꢁ 10ꢀ15 cm6 mmolꢀ1 from the
experimental data. The energy transfer rate, kTS, from Nꢀ
(1ꢀZn)3 to the fullerenyl moiety was calculated to be approxi-
mately 6 ꢁ 108 sꢀ1 using eq 1. The calculated value is comparable
to the experimental value (5.0 ꢁ 109 sꢀ1). The difference
between the two values may come from contributions of
Dexter-type through-bond energy transfers or higher-order
multipoleꢀmultipole interactions, which is neglected in the
F€orster approximation,26 otherwise, of electron transfer from
C60/1Nꢀ(1ꢀZn)3* to yield C60•ꢀ/Nꢀ(1ꢀZn)3•+. The three-
point coordination from the pyridyl groups to the porphyrins
makes fullerenyl moiety of C60ꢀlꢀTripod to be fixed tightly over
the center of the cavity of the Nꢀ(1ꢀZn)3. Since the through-
bond pathway from the porphyrins to the fullerene moiety is very
long, the electron transfer is expected to occur via the through-
space pathway. However, the superexchange interaction invol-
ving only solvent molecules would be too small for the electron
transfer to compete the energy transfer, resulting in the singletꢀ
singlet energy transfer mostly.
Figure 5. Nanosecond transient absorption spectra of the composite
formed by mixing Nꢀ(1ꢀZn)3 (1.2 ꢁ 10ꢀ6 M) and C60ꢀlꢀTripod
(2.6 ꢁ 10ꢀ6 M) in benzonitrile at 0.1 μs (b) and 1.0 μs (O) after laser
irradiation at 532 nm. Inset = absorption-time profiles at 720 nm in Ar-
and O2-saturated benzonitrile.
excited singlet state of Nꢀ(1ꢀZn)3. The spectra were obtained
using laser irradiation at 532 nm so that the Nꢀ(1ꢀZn)3 group
was excited selectively. The transient absorption spectra showed
a peak occurring at 720 nm without a signal occurring around
1000 nm corresponding to the fullerenyl radical anion (Figure 5),
whose spectrum is similar to that observed in only C60ꢀ
lꢀTripod (Supporting Information, Figure S18). The decay
profile at 720 nm was reduced markedly on exposure to oxygen
(Figure 5 inset). These results indicate that the species resulting
from photoexcitation of the macroring porphyrins is the excited
triplet state of fullerene formed by intersystem crossing (ISC)
1
from C60*, which is produced after the singletꢀsinglet energy
transfer from Nꢀ(1ꢀZn)3 to fullerene. Such energy transfer and
ISC processes may occur competitively with the photoinduced
1
electron transfer from C60/1Nꢀ(1ꢀZn)3* and C60*/Nꢀ
(1ꢀZn)3* to yield C60•ꢀ/Nꢀ (1ꢀZn)3•+, which is thermody-
namically possible. Absence of a clear absorption peak from
In the PIꢀTripod/Nꢀ(1ꢀZn)3 composite, the possibility of
a through-space singletꢀsinglet energy transfer from an excited
Nꢀ(1ꢀZn)3 group to a pyromellitdiimide moiety can be
excluded owing to both the zero absorption of PIꢀTripod
in the visible region where the fluorescence of Nꢀ(1ꢀZn)3
•ꢀ
C60 at 1000 nm, even in the spectrum at 0.1 μs in Figure 5,
suggests that C60•ꢀ/Nꢀ (1ꢀZn)3•+ has a considerably shorter
lifetime, and mostly returns to the neutral ground state within a
period of 0.1 μs, leaving only 3C60*.
1
The energy transfer process is also supported by the result that
the fluorescence quenching efficiency of Nꢀ(1ꢀZn)3 by the
addition of C60ꢀlꢀTripod in less polar benzene is almost same
(83%) as that in benzonitrile (80%). Since fullerene has a weak,
but steady absorption band in the visible region (Supporting In-
formation, Figure S19), the through-space singletꢀsinglet en-
ergy transfer rate, kTS, from Nꢀ(1ꢀZn)3 to the fullerenyl
moiety of C60ꢀlꢀTripod can be calculated using the following
occurs (Figure 2) and the higher energy of PI*/Nꢀ(1ꢀZn)3
(approximately 3.7 eV) than PI/1Nꢀ(1ꢀZn)3* (2.0 eV). The
energy level of the ion-pair state, PI•ꢀ/Nꢀ(1ꢀZn)3•+, was
estimated to be 1.34 eV above the ground state.28 Therefore,
the photoinduced electron transfer from PI/1Nꢀ(1ꢀZn)3* to
yield PI•ꢀ/Nꢀ(1ꢀZn)3 should have sufficient driving force.
•+
Table 2 shows the solvent dependency of the binding constant of
PIꢀTripod and the fluorescence quenching. The fluorescence
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Table 2. Solvent Dependency of the Steady-State Fluorescence Quenching Rate of Pl-Tripod/Nꢀ(1ꢀZn)3 Compositea
Nꢀ(1ꢀZn)3
solvent
toluene
chloroformh
εb
binding constantc/Mꢀ1
Φfd
τfe/ns
quenching efficiencyf
quenching rateg/sꢀl
2.38
4.81
25.2
>108 (>4 ꢁ 108)
5.1%
3.4%
3.9%
2.2
2.0
2.1
31%
38%
45%
2.0 ꢁ 108
3.1 ꢁ 108
3.9 ꢁ 108
4.5 ꢁ 107 (3.0 ꢁ 107)
2.0 ꢁ 107 (2.3 ꢁ 107)
benzonitrile
a Excited at 567 nm. b Dielectric constant. c Estimated from fluorescence titration. The values in parentheses were estimated from UVꢀvis titration data.
d The fluorescence quantum yield of free Nꢀ(1ꢀZn)3 was determined using ZnTPP of = 3.3% as the standard value.27 e Fluorescence lifetime of free
Nꢀ(1ꢀZn)3. f These values were calculated assuming a complexation rate of 100%. g The quenching rate constant was calculated using the following
equation, kq = 1/τf ꢁ Φq/(1 ꢀ Φq), where τf is the fluorescence lifetime of free Nꢀ(1ꢀZn)3 and Φq is the steady-state fluorescence quenching
efficiency. h Chloroform containing 0.5% EtOH as a stabilizer.
quenching was more efficient in a polar solvent, such as benzoni-
trile. This observation suggests that photoinduced electron trans-
fer occurs from Nꢀ(1ꢀZn)3 to the pyromellitdiimide moiety.
Since the center-to-center distance between the nearest porphyr-
in ring and the pyromellitdiimide moiety in the composite was
estimated to be 16 Å, photoinduced electron transfer in the
PIꢀTripod/Nꢀ(1ꢀZn)3 composite is considered most likely to
be the result of a superexchange coupling via the linker moieties
of PIꢀTripod. The transient absorption spectrum at 0.1 μs after
laser irradiation showed neither a definitive charge-separated
species nor a triplet excited state of Nꢀ(1ꢀZn)3 formed by ISC
emission spectra were recorded on a Hitachi F-4500 spectrometer. The
fluorescence intensities were normalized at the absorption of their
excitation wavelengths. UVꢀvis λmax (log ε) values are reported in
nanometers (nm). Quantum yields were determined by corrected
integrated ratios of steady-state fluorescence spectra relative to that of
ZnTPP (Φf = 3.3% in benzene; tetraphenylporphyrinatozinc(II)).27
MALDI-TOF mass spectra were obtained on PerSeptive Biosystems
Voyager DE-STR with dithranol (Aldrich) as the matrix. Reactions were
monitored on silica gel 60 F254 TLC plates (Merck). The silica gel
utilized for column chromatography was purchased from Kanto Chemi-
cal Co. Inc.: Silica Gel 60N (Spherical, Neutral) 60ꢀ210 μm and
40ꢀ210 μm (Flash). The nanosecond transient absorption spectra in
the NIR region were measured by means of laser-flash photolysis;
565 nm light from Nd:YAG laser was used as the exciting source, and
a Ge-avalanche-photodiode module was used for detecting the monitor-
ing light from a pulsed Xe-lamp. The details of the experimental setup
are described elsewhere.29
1
from Nꢀ(1ꢀZn)3*. This strongly indicates that a fast charge
recombination to the ground state occurred after the initial
charge separation.
’ CONCLUSIONS
We have synthesized three new acceptor tripodal ligands
having fullerene or pyromellitdiimide moieties. The tripodal
ligands having fullerene through a long linker (C60ꢀlꢀTripod)
and the tripodal ligand containing pyromellitdiimide (PIꢀTripod)
were accommodated via a three-point coordination from a
pyridyl group to a porphyrinatozinc(II) group. In a C60ꢀlꢀ
Tripod/Nꢀ(1ꢀZn)3 composite, the singletꢀsinglet energy trans-
fer from the excited porphyrin in Nꢀ(1ꢀZn)3 to the fullerene
occurs in preference to electron transfer, even in polar benzoni-
trile. In this case, the fullerene works as a good energy acceptor
because the fullerenyl moiety of C60ꢀlꢀTripod is tightly fixed
over the center of the cavity of the Nꢀ(1ꢀZn)3 group with a
lower electronic orbital coupling with the donor Nꢀ(1ꢀZn)3
group, and the fullerenyl moiety has an absorption peak in the
region where the fluorescence of the Nꢀ(1ꢀZn)3 group
appears. In the case of the PIꢀTripod/Nꢀ(1ꢀZn)3 composite,
where the pyromellitdiimide moiety has no absorption in the
visible region, a solvent dependence of the quenching rate was
observed, indicating that electron transfer from the excited
Nꢀ(1ꢀZn)3 group to the pyromellitdiimide moiety occurred.
4-Cyanophenyl-tris(4-iodophenyl)methane (1). In a 300 mL
three necked flask equipped with a dropping funnel, a reflux condenser,
and a N2 balloon were placed tetrakis(4-iodophenyl)methane (1 g,
1.2 ꢁ 10ꢀ3 mol)14 and dry DMF (100 mL) under N2 atmosphere. The
mixture was heated at 140 °C to become a homogeneous solution. CuCN
(78 mg, 1.2 ꢁ 10ꢀ3 mol) dissolved in dry DMF (40 mL) was then added
dropwise. The reaction mixture was stirred for 24 h at 140 °C. After the
solvent was evaporated, toluene (approximately 100 mL) and aqueous
ammonia solution (approximately 100 mL) were added and stirred
overnight. The organic layer was collected, washed with aqueous
ammonia solution and water, and dried over anhydrous Na2SO4. The
residue obtained by evaporation of the solvent was purified with a short
silica gel column chromatography (CHCl3/hexane 1:1 to CHCl3). The
fraction elutedwith CHCl3 was collected and evaporated to afford 295 mg
(34%) of the titled compound: TLC (silica gel, CHCl3/hexane 1:2)
Rf = 0.2, (silica gel, CHCl3) Rf = 0.7; 1H NMR (600 MHz, CDCl3, 25 °C,
TMS) δ 7.60 (d, J = 8.4 Hz, 6H, Ph), 7.56 (d, J = 8.4 Hz, 2H, Ph-CN),
7.29 (d, J = 8.4 Hz, 2H, Ph-CN), 6.86 (d, J = 8.4 Hz, 6H, Ph).
4-Formylphenyl-tris(4-iodophenyl)methane (2). In a 50 mL
flask was placed 1 (295 mg, 4.1 ꢁ 10ꢀ4 mol), and the flask was evacuated
and replaced with Ar gas. Dry benzene (5 mL) was added to it, and the
solution was cooled to 0 °C, and then 0.95 M hexane solution of
diisobutylaluminum hydride (DIBAL-H, 1.2 mL, 1.1 ꢁ 10ꢀ3 mol) was
added at 0 °C. The reaction mixture was warmed to room temperature
and stirred for 1.5 h. The reaction was quenched by addition of 5% H2SO4
aqueous solution. The organic layer was collected, and the aqueous layer
was extracted with benzene several times (Benzene is a cancer suspect
agent and flammable liquid. Toluene should be substituted for
benzene.). The combined organic layer was washed with water and
dried over anhydrous Na2SO4, and then the solvent was evaporated. The
residue was purified with a silica gel column chromatography (CHCl3),
giving 235 mg (79%) of the titled compound: TLC (silica gel, CHCl3)
’ EXPERIMENTAL SECTION
General Procedure. All chemicals and solvents were of commer-
cial reagent grade, and used without further purification unless otherwise
stated. Dry tetrahydrofuran (THF), 1,4-dioxane, benzene, and toluene
were prepared by distillation over benzophenone-Na. Dry dimethylfor-
mamide (DMF) and triethylamine (Et3N) were prepared by distillation
over CaH2. 1H NMR spectra (600 MHz) were recorded on JEOL ECP-
600, and chemical shifts were recorded in parts per million (ppm)
relative to tetramethylsilane. UVꢀvis absorption spectra were recorded
on a Shimadzu UV-3100PC spectrometer. Steady-state fluorescence
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1
Rf = 0.6; H NMR (600 MHz, CDCl3, 25 °C, TMS) δ 9.99 (s, 1H,
compound: TLC (silica gel, CHCl3/MeOH 9:1) Rf = 0.8; MALDI-TOF
MS found m/z 1500.0 (MH+); HRMS (ESI) calcd for C117H39N4
CHO), 7.78 (d, J = 7.0 Hz, 2H, Ph-CHO), 7.60 (d, J = 7.3 Hz, 6H, Ph),
7.35 (d, J = 7.0 Hz, 2H, Ph-CHO), 6.91 (d, J = 7.3 Hz, 6H, Ph).
4-Formylphenyl-tris[4-{2-(4-pyridyl)ethynyl}phenyl]-
methane (sꢀTripod). In a Schlenk flask were placed 2 (116 mg, 1.6 ꢁ
10ꢀ4 mol), 4-ethynylpyridine (82 mg, 8.0 ꢁ 10ꢀ4 mol),15 Pd(PPh3)2Cl2
(13 mg, 1.9 ꢁ 10ꢀ5 mol), and CuI (3.7 mg, 1.9 ꢁ 10ꢀ5 mol) under Ar
atmosphere. Dry Et3N (0.5 mL) and dry THF (0.5 mL) were added to it,
and the mixture was degassedbyfreezeꢀthaw cycles. The reaction mixture
was stirred at room temperature under Ar atmosphere. After stirring for
1 h, further dry THF (1 mL) was added to the reaction mixture to
dissolve the insoluble solid. After stirring for 20 h, the reaction mixture
was diluted with chloroform, washed with water and dried over
anhydrous Na2SO4. The residue obtained by evaporation of the solvent
was purified with a silica gel column chromatography (CHCl3/MeOH
9:1). The fractions showing the signals of the target compound on
MALDI-TOF mass spectra were concentrated and reprecipitated with
hexane to remove a byproduct of 1,4-bis(4-pyridyl)butadiyne. Further
purification was performed with a silica gel column chromatography
(CHCl3/MeOH 9:1), giving 80.9 mg (78%) of the titled compound:
TLC (silica gel, CHCl3/MeOH 9:1) Rf = 0.3; MALDI-TOF MS found
m/z 652.6 (MH+); HRMS (FAB) calcd for C47H30N3O (MH+)
652.2389, found 652.2383; 1H NMR (600 MHz, CDCl3, 25 °C, TMS)
δ 10.02 (s, 1H, CHO), 8.60 (d, J = 6.0 Hz, 6H, Py), 7.83 (d, J = 8.2 Hz,
2H, Ph-CHO), 7.50 (d, J = 8.5 Hz, 6H, Ph), 7.43 (d, J = 8.2 Hz, 2H, Ph-
CHO), 7.37 (d, J = 6.0 Hz, 6H, Py), 7.25 (d. J = 8.5 Hz, 6H, Ph); 13C
NMR (150 MHz, CDCl3, 25 °C) δ 191.5, 152.0, 149.8, 146.1, 134.6,
131.6, 131.3, 131.1, 130.8, 129.3, 125.5, 120.6, 93.16, 87.31, 65.36.
C60ꢀsꢀTripod. In a Schlenk flask were placed sꢀTripod (15 mg,
2.3 ꢁ 10ꢀ5 mol), C60 (83 mg, 1.2 ꢁ 10ꢀ4 mol), and N-methylglycine
(107 mg, 1.2 ꢁ 10ꢀ3 mol). The flask was evacuated and replaced with Ar
gas. Toluene (10 mL) was added to it, and the mixture was heated at
100 °C and stirred for 21 h. The solvent was evaporated to give a crude
residue, which was passed through a short silica gel column by using
toluene, and then, CHCl3/MeOH 9:1 as eluents to remove excess C60.
The fractions eluted with CHCl3/MeOH 9:1 was collected and evapo-
rated. Further purification was performed with a silica gel column
chromatography (CHCl3/MeOH 99:1 to 98:2), giving 13 mg (41%)
of the titled compound: TLC (silica gel, CHCl3/MeOH 9:1) Rf = 0.6;
MALDI-TOF MS found m/z 1400.6 (MH+); HRMS (ESI) calcd for
C109H35N4 (MH+) 1399.2862, found 1399.2861; 1H NMR (600 MHz,
CDCl3, 25 °C, TMS) δ 8.60 (brs, 6H, Py), 7.52ꢀ7.77 (brs, 2H, Ph-
pyrrolidine), 7.41 (d, J = 8.2 Hz, 6H, Ph), 7.34 (brd, J = 4.8 Hz, 6H, Py),
7.25 (brs, Ph-pyrrolidine), 7.19 (d. J = 8.2 Hz, 6H, Ph), 4.98 (d, J = 9.6 Hz,
1H, pyrrolidine CH2), 4.96 (s, 1H, pyrrolidine CH), 4.28 (d, J = 9.6 Hz,
1H, pyrrolidine CH2), 2.90 (s, 3H, NꢀCH3); 13C NMR (150 MHz,
CDCl3, 25 °C) δ 156.19, 153.85, 153.29, 152.96, 149.76, 147.32, 147.28,
146.89, 146.65, 146.43, 146.27, 146.20, 146.11, 146.09, 145.95, 145.88,
145.72, 145.56, 145.52, 145.41, 145.36, 145.29, 145.21, 145.14, 144.98,
144.70, 144.68, 144.40 144.30, 143.16, 143.0,1 142.69, 142.58, 142.44,
142.21, 142.15, 142.07, 142.03, 141.92, 141.72, 141.67, 141.46, 140.21,
140.12, 139.71, 139.06, 136.50, 136.41, 135.97, 135.78, 135.72, 131.42,
131.26, 130.82, 128.79, 125.54, 120.20, 93.54, 87.18, 82.78, 77.37, 70.04,
68.93, 65.11, 40.05; UVꢀvis (CHCl3) 257 (5.17), 292 (5.08), 311
(5.06), 430 (3.56), 703.5 (2.60).
1
1499.3175 (MH+), found 1499.3170; H NMR (600 MHz, CDCl3,
25 °C, TMS) δ 8.60 (d, J = 5.4 Hz, 6H, Py), 7.81 (brs, 2H, Ph-
pyrrolidine), 7.59 (d, J = 7.8 Hz, 2H, Ph-pyrrolidine), 7.47 (d, J = 9.0 Hz,
6H, Ph), 7.44 (d, J = 9.0 Hz, 2H, Ph0), 7.36 (d, J = 5.4 Hz, 6H, Py), 7.23
(d, J = 9.0 Hz, 6H, Ph), 7.18 (d, J = 9.0 Hz, 2H, Ph0), 4.99 (d, J = 9.6 Hz,
1H, pyrrolidine CH2), 4.95 (s, 1H, pyrrolidine CH), 4.27 (J = 9.6 Hz, 1H,
pyrrolidine CH2), 2.81 (s, 3H, NꢀCH3); 13C NMR (150 MHz, CDCl3,
25 °C) δ 156.1, 153.9, 153.1, 153.0, 149.8, 147.33, 147.31, 146.6, 146.4,
146.32, 146.30, 146.23, 146.20, 146.15, 146.01, 145.95, 145.7, 145.6,
145.52, 145.48, 145.42, 145.37, 145.29, 145.27, 145.24, 145.18, 144.7,
144.6, 144.42, 144.36, 143.2, 143.0, 142.7, 142.60, 142.56, 142.25,
142.24, 142.18, 142.14, 142.10, 142.05, 141.99, 141.9, 141.8, 141.7,
141.6, 140.21, 140.19, 139.9, 139.5, 140.22, 140.19, 139.9, 139.5, 137.6,
136.9, 136.4, 135.9, 135.7, 131.9, 131.5, 131.3, 131.2, 130.9, 130.8, 125.5,
123.1, 121.5, 120.4, 93.37, 89.83, 89.45, 87.19, 83.27, 70.00, 69.06, 65.03,
40.00; UVꢀvis (benzonitrile) 314 (5.02), 432 (3.42).
N-{4-(Triphenylmethyl)phenyl}-N0-(n-octyl)pyromellitdi-
imide (3). In a 20 mL flask were placed N-(n-octyl)pyromellitic
monoanhydride (100 mg, 3.0 ꢁ 10ꢀ4 mol),19 4-(triphenylmethyl)-
aniline (102 mg, 3.0 ꢁ 10ꢀ4 mol), and dry DMF (2 mL). The mixture
was purged with N2 gas and then heated at 130 °C for 6 h. A white
solid generated during cooling was collected by filtration and washed
with hexane, affording 117.6 mg (60%) of the titled compound. The
filtrate of the reaction mixture was evaporated and the residue was
recrystallized in acetic anhydride. The crystal was collected by
filtration and dried in vacuo, giving 26.7 mg (14%) of the titled
compound. Total yield was 144.3 mg (74%): TLC (silica gel, CHCl3)
Rf = 0.5; 1H NMR (600 MHz, CDCl3, 25 °C, TMS) δ 8.57 (s, 2H, Ph-
PI), 7.39 (d, J = 9.0 Hz, 2H, Ph0), 7.36 (d, J = 9.0 Hz, 2H, Ph0),
7.20ꢀ7.29 (m, 15H, Ph), 3.75 (t, J = 7.5 Hz, 2H, CαH2), 1.71
(quintet, J = 6.9 Hz, 2H, CβH2), 1.24ꢀ1.34 (m, 10H, CH2), 0.87 (t,
J = 6.9 Hz, 3H, CH3); 13C NMR (150 MHz, CDCl3, 25 °C) δ 166.2,
165.3, 147.3, 146.3, 137.6, 136.7, 131.9, 131.1, 128.8, 127.6, 126.1,
125.0, 118.7, 64.86, 38.82, 31.72, 29.10, 29.06, 28.41, 26.83,
22.58, 14.04.
N-[4-{Tris(4-bromophenyl)methyl}phenyl]-N0-(n-octyl)-
pyromellitdiimide (4). In a 20 mL flask were placed 3 (165 mg, 2.6 ꢁ
10ꢀ4 mol) and neat bromine (1 mL), and then the reaction mixture was
stirred for 30 min at room temperature. The reaction mixture was diluted
with CHCl3 (20 mL) and washed successively with an aqueous sodium
bisulfite solution and water. The organic layer was dried over anhydrous
Na2SO4, and the solvent was evaporated. The residue was purified with
silica gel column chromatography (CHCl3/hexane 1:2 to CHCl3/MeOH
9:1) to afford 201 mg (89%) of the titled compound: TLC (silica gel,
CHCl3) Rf = 0.8; 1H NMR (600 MHz, CDCl3, 25 °C, TMS) δ 8.37 (s,
2H, Ph-PI), 7.41 (d, J = 8.2 Hz, 8H, Ph and Ph0), 7.32 (d, J = 8.5 Hz, 2H,
Ph0), 7.08 (d, J = 8.2 Hz, 6H, Ph), 3.75 (t, J = 7.1 Hz, 2H, CαH2), 1.71
(brquintet, 2H, CβH2), 1.26ꢀ1.34 (m, 10H, CH2), 0.87 (t, J = 6.6 Hz,
3H, CH3); 13C NMR (150 MHz, CDCl3, 25 °C) δ 166.0, 165.1, 145.6,
144.4, 137.7, 136.6, 132.4, 131.4, 131.1, 129.4, 125.4, 120.8, 118.7, 63.80,
38.80, 31.69, 29.06, 29.02, 28.37, 26.78, 22.55, 14.02.
PIꢀTripod. Pd(PhCN)2Cl2 (2.6 mg, 6.8 ꢁ 10ꢀ6 mol), [(t-Bu)3-
PH]BF4 (3.9 mg, 1.4 ꢁ 10ꢀ5 mol), and CuI (1.3 mg, 6.8 ꢁ 10ꢀ6 mol)
were placed in a Schlenk flask and dissolved in dry dioxane (3 mL) and
dry (i-Pr)2NH (100 μL, 6.9 ꢁ 10ꢀ4 mol) under Ar atmosphere. The
solution was degassed by freezeꢀthaw cycles, and then 4 (120 mg, 1.4 ꢁ
10ꢀ4 mol) and 4-ethynylpyridine (70 mg, 6.8 ꢁ 10ꢀ4 mol)15 were added
to it. The reaction mixture was stirred for 11 h at room temperature. The
solvent was evaporated in vacuo, and the residue was extracted with
CHCl3 and washed with water. The solvent was evaporated, and the
residue was purified with flash silica gel column chromatography
(CHCl3/MeOH 95:5) to give a mixture including the target compound.
C60ꢀlꢀTripod. In a test tube with screw cap were placed lꢀTripod
(4.2 mg, 5.6 ꢁ 10ꢀ6 mol),11c C60 (20 mg, 2.8 ꢁ 10ꢀ5 mol), N-methyl-
glycine (25 mg, 2.8 ꢁ 10ꢀ4 mol), and toluene (1 mL), and the tube was
purged with N2 gas. The reaction mixture was heated to 100 °C and
stirred for 32 h. The resulting solution was passed through a short silica
gel column (toluene to CHCl3/MeOH 9:1) to remove excess C60. The
fraction eluted with CHCl3/MeOH 9:1 was collected and evaporated.
Further purification was performed with a silica gel column chromatog-
raphy (CHCl3/MeOH 99:1 to 98:2), giving 4.8 mg (57%) of the titled
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Further purification of the mixture by reprecipitation with hexane
afforded 56.7 mg (44%) of the titled compound: MALDI-TOF MS
found m/z 951.2 (MH+); HRMS (FAB) calcd for C64H48N5O4
950.3706, found 950.3713; 1H NMR (600 MHz, CDCl3, 25 °C, TMS)
δ 8.64 (brs, 6H, Py), 8.39 (s, 2H, Ph-PI), 7.50 (d, J = 8.5 Hz, 6H, Ph),
7.45 (d, J = 8.8 Hz, 2H, Ph0), 7.38ꢀ7.40 (m, 8H, Py and Ph0), 7.28 (d. J =
8.2 Hz, 6H, Ph), 3.76 (t, J = 7.4 Hz, 2H, CαH2), 1.72 (quintet, J = 7.0 Hz,
2H, CβH2), 1.25ꢀ1.34 (m, 10H, CH2), 0.87 (t, J = 7.0 Hz, 3H, CH3);
13C NMR (150 MHz, CDCl3, 25 °C) δ 166.0, 165.1, 149.7, 146.5, 145.5,
137.7, 136.6, 131.54, 131.49, 131.2, 130.9, 129.5, 125.5, 120.5, 118.7,
93.32, 87.21, 64.90, 38.82, 31.68, 29.06, 29.02, 28.38, 26.79, 22.55, 14.01;
UVꢀvis. (CHCl3) 292.0 (4.97), 311.8 (4.94).
(h) Satake, A.; Kobuke, Y. Org. Biomol. Chem. 2007, 5, 1679. (i) Beletskaya,
I.; Tyurin, V. S.; Tsivadze, A. Y.; Guilard, R.; Stern, C. Chem. Rev. 2009,
109, 1659.
(6) For reviews of porphyrin and fullerene conjugate, see: (a)
Wasielewski, M. R. Chem. Rev. 1992, 92, 435. (b) Ward, M. D. Chem.
Soc. Rev. 1997, 26, 365. (c) Gust, D.; Moore, T. A. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: New York, 2000; Vol. 8, pp 153. (d) Fukuzumi, S.; Guldi, D. M. In
Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim,
Germany, 2000; Vol. 2, p 270. (e) Imahori, H. Org. Biomol. Chem. 2004,
2, 1425. (f) D’Souza, F.; Ito, O. Chem. Commun. 2009, 4913.
(7) For most recent reports of cyclic porphyrin array and multi-
porphyin and fullerene conjugate, see: (a) Hoffmann, M.; Wilson, C. J.;
Odell, B.; Anderson, H. L. Angew. Chem., Int. Ed. 2007, 46, 3122. (b)
Kelley, R. F.; Lee, S. J.; Wilson, T. M.; Nakamura, Y.; Tiede, D. M.;
Osuka, A.; Hupp, J. T.; Wasielewski, M. R. J. Am. Chem. Soc. 2008,
130, 4277. (c) Jensen, R. A.; Kelley, R. F.; Lee, S. J.; Wasielewski, M. R.
Chem. Commun. 2008, 1886. (d) Flamigni, L.; Ventura, B.; Oliva, A. I.;
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’ ASSOCIATED CONTENT
1
Supporting Information. Listings of H, 13C NMR of
S
b
1ꢀ4, sꢀTripod, C60ꢀsꢀTripod, C60ꢀlꢀTripod, and PIꢀ
Tripod, UVꢀvis spectral changes of Nꢀ(1ꢀZn)3 by bindings of
C60ꢀsꢀTripod, C60ꢀlꢀTripod, and PIꢀTripod, nanosecond
transient absorption spectra of C60ꢀlꢀTripod, spectral overlap
of the absorption of C60ꢀlꢀTripod, and the fluorescence of
Nꢀ(1ꢀZn)3. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Fax: (+81)774-38-3508. E-mail: kobuke@iae.kyoto-u.ac.jp.
(8) (a) Takahashi, R.; Kobuke, Y. J. Am. Chem. Soc. 2003, 125, 2372.
(b) Shoji, O.; Okada, S.; Satake, A.; Kobuke, Y. J. Am. Chem. Soc. 2005,
127, 2201. (c) Hajjaj, F.; Yoon, Z. S.; Yoon, M.-C.; Park, J.; Satake, A.;
Kim, D.; Kobuke, Y. J. Am. Chem. Soc. 2006, 128, 4612. (d) Fujisawa, K.;
Satake, A.; Hirota, S.; Kobuke, Y. Chem.—Eur. J. 2008, 14, 10735.
(9) Kobuke, Y.; Miyaji, H. J. Am. Chem. Soc. 1994, 116, 4111.
(10) The Job plot of the tripyridyl tripodal ligand and Nꢀ(1ꢀZn)3
showed a maximum at a mole fraction value of 0.5, supporting a 1:1
stoichiometry of binding, see: Kuramochi, Y.; Satake, A.; Kobuke, Y.
J. Am. Chem. Soc. 2004, 126, 8668.
(11) (a) Kuramochi, Y.; Satake, A.; Itou, M.; Ogawa, K.; Araki, Y.; Ito,
O.; Kobuke, Y. Chem.—Eur. J. 2008, 14, 2827. (b) Nagata, N.; Kuramochi,
Y.; Kobuke, Y. J. Am. Chem. Soc. 2009, 131, 10. (c) Kuramochi, Y.;
Sandanayaka, A. S. D.; Satake, A.; Araki, Y.; Ogawa, K.; Ito, O.; Kobuke, Y.
Chem.—Eur. J. 2009, 15, 2317. (d) Satake, A.; Azuma, S.; Kuramochi, Y.;
Hirota, S.; Kobuke, Y. Chem.—Eur. J. 2011, 17, 855.
(12) A lifetime of charge separated state on πꢀπ stacked fullere-
neꢀporphyrin system is 38 ps in benzonitrile, see: Guldi, D. M.; Hirsch,
A.; Scheloske, M.; Dietel, E.; Troisi, A.; Zerbetto, F.; Prato, M. Chem.—
Eur. J. 2003, 9, 4968. In the previous report (ref 11a), we also observed a
lifetime of 77 ps in the πꢀπ stacked fullereneꢀmacroring system.
(13) Material Studio (version 4.1/ Forcite/ Force Field UNI-
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mine in CHCl3 at room temperature quantitatively afforded 2,6-
dibromo-4-(triphenylmethyl)aniline, which was selectively brominated
at the ortho-position of the NH2 group.
Present Addresses
§Institute of Advanced Energy, Kyoto University, Gokasho, Uji,
Kyoto 611ꢀ0011, Japan.
’ ACKNOWLEDGMENT
This work was supported by Grants-in-Aid for Scientific
Research (A) (No. 18750118) (Y.K.) from the Japan Society
for the Promotion of Science (JSPS). We thank Mrs. Y. Nishikawa,
and Mrs. M. Yamamura, technical staff of NAIST for HRMS
measurements.
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Inorganic Chemistry
ARTICLE
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