Charge separation in a novel artificial photosynthetic reaction center lives 380 ms

Article abstract of DOI:10.1021/ja004123v

An extremely long-lived charge-separated state has been achieved successfully using a ferrocene-zincporphyrin-freebaseporphyrin-fullerene tetrad which reveals a cascade of photoinduced energy transfer and multistep electron transfer within a molecule in f

Full text of DOI:10.1021/ja004123v

J. Am. Chem. Soc. 2001, 123, 6617-6628
6617
Charge Separation in a Novel Artificial Photosynthetic Reaction
Center Lives 380 ms
Hiroshi Imahori,* Dirk M. Guldi,*,† Koichi Tamaki,‡ Yutaka Yoshida, Chuping Luo,†
Yoshiteru Sakata,‡ and Shunichi Fukuzumi*
Contribution from the Department of Material and Life Science, Graduate School of Engineering,
Osaka UniVersity, CREST, Japan Science and Technology Corporation, Suita, Osaka 565-0871, Japan,
Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556, and The Institute of
Scientific and Industrial Research, Osaka UniVersity, 8-1 Mihoga-oka, Ibaraki, Osaka 567-0047, Japan.
ReceiVed NoVember 30, 2000. ReVised Manuscript ReceiVed May 2, 2001
Abstract: An extremely long-lived charge-separated state has been achieved successfully using a ferrocene-
zincporphyrin-freebaseporphyrin-fullerene tetrad which reveals a cascade of photoinduced energy transfer
and multistep electron transfer within a molecule in frozen media as well as in solutions. The lifetime of the
resulting charge-separated state (i.e., ferricenium ion-C60 radical anion pair) in a frozen benzonitrile is
determined as 0.38 s, which is more than one order of magnitude longer than any other intramolecular charge
recombination processes of synthetic systems, and is comparable to that observed for the bacterial photosynthetic
reaction center. Such an extremely long lifetime of the tetrad system has been well correlated with the charge-
separated lifetimes of two homologous series of porphyrin-fullerene dyad and triad systems.
Electron transfer (ET) chemistry in biological and artificial
systems has been among the most intensively studied topics
over the past decades.1-10 Natural photosynthesis applies ET
systems, where a relay of ET reactions evolves among chloro-
phyll and quinone moieties embedded in a transmembrane
protein matrix. Ultimately the product of this cascade is
conversion of light into usable chemical energy.11 The mimicry
of these complex and highly versatile processes has prompted
the design of synthetic donor-acceptor linked ensembles such
as triads, tetrads, and pentads.6-10 However, achieving the long
lifetime and the high efficiency of charge separation (CS) like
natural photosynthesis has been mainly hampered by synthetic
difficulties. A specific challenge involves attaining a fine-tuned
and directed redox gradient along donor-acceptor linked arrays.
With the advent of fullerenes, a new three-dimensional
electron acceptor became available, exhibiting attractive char-
acteristics.12 In particular, fullerenes have remarkably small
reorganization energies of ET, which result from the π electron
systems delocalized over the three-dimensional surface together
* Author to whom correspondence should be addressed. E-mail:
imahori@ap.chem.eng.osaka-u.ac.jp; guldi.1@nd.edu; fukuzumi@ap.chem.
eng.osaka-u.ac.jp.
† University of Notre Dame.
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10.1021/ja004123v CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/13/2001

6618 J. Am. Chem. Soc., Vol. 123, No. 27, 2001
Imahori et al.
Figure 1. Structures of porphyrin-fullerene linked tetrad and the reference compounds.
with the rigid, confined structure of the aromatic π sphere.10,13,14
In this regard, it has been observed frequently that photoinduced
CS is accelerated and charge recombination (CR) is decelerated
in donor-linked fullerenes, as compared to those with conven-
tional acceptors such as quinones and imides.10,13 The primary
ET processes of the photosynthesis, on the other hand, are also
characterized by an extremely small reorganization energy (λ
) ∼0.2 eV). This is essential to achieve the ultrafast forward
ET and to retard the energy-wasting back ET, which is highly
not only more than one-order of magnitude larger than any
earlier reported intramolecular CR processes in synthetic
systems,6-10,15-26 but more importantly, it is comparable to that
seen for the bacterial photosynthetic reaction centers.11
Results and Discussion
Synthesis of the Fc-ZnP-H2P-C60 Tetrad. The preparation
of Fc-ZnP-H2P-C60 was carried out as shown in Scheme 1.
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exergonic (-∆G0
) 1.1 eV) and thereby deeply in the
BET
Marcus inverted region.11 Taking these facts into concert, a
significant impact, concerning the lifetime of CS, is expected
without, however, lowering the overall efficiency of CS in
fullerene-containing multicomponent systems. Along this line
we and others have prepared a variety of donor-fullerene linked
dyads and triads,10,15-23 some of which have successfully
mimicked photosynthetic ET on electrodes23 as well as in
solutions.
In this work we wish to report the longest lifetime of a charge-
separated state ever achieved in any other artificial reaction
center. In particular, in a newly designed ferrocene (Fc)-
zincporphyrin (ZnP)-freebaseporphyrin (H2P)-C60 tetrad
(Fc-ZnP-H2P-C60; Figure 1) a cascade of photoinduced energy
transfer (EN), coupled to a multistep ET, occurs in frozen as
well as in condensed liquid media. The lifetime of the resulting
charge-separated state (i.e., ferricenium ion (Fc+)-C60 radical
anion (C60•-) pair), as determined in a frozen benzonitrile
(PhCN) matrix, amounts to a remarkable 0.38 s. This value is
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Charge Separation in a Photosynthetic Reaction Center
J. Am. Chem. Soc., Vol. 123, No. 27, 2001 6619
The important synthetic intermediate 3 was prepared by
condensation of dipyrromethane 127 with 3,5-di-tert-butylben-
zaldehyde28 in the presence of BF3OEt2, followed by reduction
of the nitro group in 2. Condensation of phenylferrocene acid
chloride29 with diaminoporphyrin 3, followed by treatment with
zinc acetate, gave ferrocene-zincporphyrin 4 in 43% yield.
Cross-condensation of porphyrin bis(acid chloride) 522e with 4
and 4-tert-butyldimethylsilyloxymethylaniline22e in benzene,
followed by deprotection of the tert-butyldimethylsilyl group
with n-Bu4NF, afforded ferrocene-zincporphyrin-freebase-
porphyrin 6 in 41% yield. The triad 6 was then converted to
the Fc-ZnP-H2P-C60 tetrad using oxidation with MnO2 and
subsequent 1,3-dipolar cycloaddition30 of 7 with C60 and
N-methylglycine in 45% yield (two steps).
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ZnP-H2P-C60,22e Fc-ZnP-C60,22f Fc-H2P-C60,22f ZnP-C60,22e
Fc-ref, and C60-ref22f (Figures 1 and 2) were prepared by
following the same procedures as described previously. Struc-
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Ed.; Wiley-VCH: Weinheim, 2001; Vol. 2, pp 927-975.
1
analysis including H NMR, IR, and FAB mass spectra. The
absorption spectrum of Fc-ZnP-H2P-C60 in PhCN is virtually
the superposition of the spectra of the individual chromophores,
indicating the lack of strong interaction between the chro-
mophores in the ground state. Similar results were obtained in
THF and DMF.
One-Electron Redox Potentials and ET Driving Force. An
accurate determination of the driving force (-∆G0ET) for all
the intramolecular ET processes required measuring the redox
potentials of Fc-ZnP-H2P-C60 and the reference chromophores
(Fc-ref and C60-ref) in various solvents. The differential pulse
voltammetry was performed in THF, PhCN, and DMF solu-
tions containing the same supporting electrolyte (i.e., 0.1 M
n-Bu4NPF6). Table 1 summarizes all the redox potentials of the
investigated compounds. The first one-electron oxidation po-
tential (E0ox) of Fc-ref (0.07 V vs ferrocene/ferricenium
(Fc/Fc+)) and the first one-electron reduction potential (E0
)
red
of C60-ref (-1.04 V vs Fc/Fc+) in benzonitrile are the same as
those of Fc-ZnP-H2P-C60 in benzonitrile. This further implies
that electronic interaction between the ferrocene and the C60 is
negligible in the ground state. The first one-electron oxida-
tion potential of Fc-ref remains nearly constant, despite the
substantial increase in solvent polarity.31 On the other hand,
C60-ref exhibits positive shifts for the underlying first one-
electron reduction potentials.22f,32
The driving forces (-∆G0ET(CR) in eV) for the intramolecular
CR processes from the C60 radical anion (C60•-) to the freebase
porphyrin radical cation (H2P•+), the zincporphyrin radical cation
(ZnP•+), and the ferricenium ion (Fc+) in Fc-ZnP-H2P-C60 were
calculated by eq 1, where e stands for the elementary charge.
-∆G0ET(CR) ) e[E0ox(D•+/D) - E0red(A/A•-)]
(1)
The -∆G0
values (in eV), thus obtained in THF,
benzonitrile, and DMF, are listed in Table 2. By contrast, the
ET(CR)
driving force for the intramolecular CS processes (-∆G0
in eV) from the freebase porphyrin singlet excited state to the
ET(CS)
(24) (a) Gust, D.; Moore, T. A.; Moore, A. L.; Lee, S.-J.; Bittersmann,
E.; Luttrull, D. K.; Rehms, A. A.; DeGraziano, J. M.; Ma, X. C.; Gao, F.;
Belford, R. E.; Trier, T. T. Science 1990, 248, 199. (b) Gust, D.; Moore, T.
A.; Moore, A. L.; Macpherson, A. N.; Lopez, A.; DeGraziano, J. M.; Gouni,
I.; Bittersmann, E.; Seely, G. R.; Gao, F.; Nieman, R. A.; Ma, X. C.;
Demanche, L. J.; Hung, S.-C.; Luttrull, D. K.; Lee, S.-J.; Kerrigan, P. K.
J. Am. Chem. Soc. 1993, 115, 11141.
(25) (a) Wasielewski, M. R.; Gains, G. L., III; Wiederrecht, G. P.; Svec,
W. A.; Niemczyk, M. P. J. Am. Chem. Soc. 1993, 115, 10442. (b)
Wasielewski, M. R.; Wiederrecht, G. P.; Svec, W. A.; Niemczyk, M. P.
Sol. Energy Mater. Solar Cells 1995, 38, 127.
(26) (a) Lendzian, F.; Schlu¨pmann, J.; von Gersdorff, J.; Mo¨bius, K.;
Kurreck, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 1461. (b) Hasharoni,
K.; Levanon, H.; von Gersdorff, J.; Kurreck, H.; Mo¨bius, D. J. Chem. Phys.
1993, 98, 2916.
(27) Bru¨ckner, C.; Karunaratne, V.; Rettig, S. J.; Dolphin, D. Can. J.
Chem. 1996, 74, 2182.
C
60 was determined by eq 2, where ∆E0-0 is the energy of the
-∆G0ET(CS) ) ∆E0-0 + ∆G0
(2)
ET(CR)
0-0 transition energy gap between the lowest excited state and
the ground state, which is determined by the 0-0* absorption
(29) Little, W. F.; Reilley, C. N.; Johnson, J. D.; Lynn, K. N.; Sanders,
A. P. J. Am. Chem. Soc. 1964, 86, 1376.
(30) Maggini, M.; Scorrano, G.; Prato, M. J. Am. Chem. Soc. 1993, 115,
9798.
(31) Gagne´, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980,
19, 2855.
(32) Dubois, D.; Moninot, G.; Kutner, W.; Jones, M. T.; Kadish, K. M.
J. Phys. Chem. 1992, 96, 7137.
(28) Newman, M. S.; Lee, L. F. J. Org. Chem. 1972, 37, 4468.

6620 J. Am. Chem. Soc., Vol. 123, No. 27, 2001
Imahori et al.
Scheme 1
•-
(Scheme 2). The final charge separated state (Fc+ and C60
and 0*-0 fluorescence maxima in condensed media (the asterisk
denotes the excited state). The -∆G0ET(CS) values are given in
Table 2. The driving forces for intramolecular charge-shift
(CSH) processes (-∆G0ET(CSH) in eV) from the ZnP to the H2P•+
and from the Fc to the ZnP•+ in Fc-ZnP-H2P-C60 were deter-
mined by subtracting the energy of the final state from that of
the initial state (Table 2). It should be noted that the Coulombic
terms in the present donor-acceptor systems are negligible, es-
pecially in solvents with moderate or high polarity, because of
the relatively long edge-to-edge distance (Ree > 11 Å) employed.
Photodynamics of Fc-ZnP-H2P-C60. Successful mimicry of
pair) is characterized by a large Ree value of 48.9 Å. Time-
resolved transient absorption spectra, following picosecond and
nanosecond laser pulses, were employed to examine the
photodynamics of Fc-ZnP-H2P-C60 (vide infra).
At first, the photodynamics of the triad systems (ZnP-
H2P-C60 and Fc-ZnP-C60) are described for the better under-
standing of the more complex tetrad system. We reported the
photophysics of ZnP-H2P-C60 in benzonitrile in detail (Scheme
3).22e An initial singlet-singlet EN from ZnP* (2.04 eV) to
1
H2P (1.89 eV) (kEN ) 1.5 × 1010 s-1) is followed by a sequential
ET relay evolving from the ZnP-1H2P*-C60 (CS1) to yield
22e
primary events in photosynthesis using ZnP-H2P-C60 and
22d,f
•-
Fc-ZnP-C60
encouraged us to combine these two systems
ZnP-H2P•+-C60•- (1.63 eV) and subsequently ZnP•+-H2P-C60
into an integrated single system (Fc-ZnP-H2P-C60). Since a
ferrocene unit is tethered at the end of ZnP-H2P-C60, the tetrad
will display coupled photoinduced EN and ET, namely,
Fc-1ZnP*-H2P-C60 (2.04 eV) f Fc-ZnP-1H2P*-C60 (1.89 eV)
(1.34 eV) via CSH1. Thereby, the individual rate constants
kET(CS1) and kET(CSH1) were determined as 7.0 × 109 and 2.2 ×
109 s-1, respectively.22e The final charge-separated state, formed
in moderate quantum yields (0.40), decays directly to the ground
state with a rate constant (kET(CR2)) of 4.8 × 104 s-1 as analyzed
•-
•-
f Fc-ZnP-H2P•+-C60 (1.63 eV) f Fc-ZnP•+-H2P-C60
•-
•-
(1.34 eV) f Fc+-ZnP-H2P-C60 (1.11 eV) in polar media
from the decay kinetics of the C60 absorption (ca. 1000

Charge Separation in a Photosynthetic Reaction Center
J. Am. Chem. Soc., Vol. 123, No. 27, 2001 6621
states (i.e., the porphyrin singlet excited state (1ZnP* (2.04 eV)),
the porphyrin triplet excited state (3ZnP* (1.53 eV)), the C60
singlet excited state (1C60* (1.75 eV)), and the C60 triplet excited
*
state (3C60 (1.50 eV))) to the counterpart within the molecule
to produce Fc-ZnP•+-C60 (1.21-1.42 eV).22f The photody-
•-
namical behavior is similar to that of ZnP-C60.21g The CS rates
from 1ZnP*, 1C60*, and 3C60* in ZnP-C60, together with the CR
•-
rates of ZnP•+-C60 in various solvents, were reported pre-
viously21g,22f and are summarized in Table 3. The resulting
transient Fc-ZnP•+-C60•- state undergoes a subsequent and rapid
•-
CSH to generate the final Fc+-ZnP-C60 state with a rate
constant of 2.8 × 109 s-1 in benzonitrile.22f The efficiency of
CSH from the Fc to the ZnP•+ in Fc-ZnP•+-C60•- was found to
be nearly unity.22f A similar photodynamical behavior was noted
in THF and DMF solutions.22d,f By analyzing the decay kinetics
of the C60 fingerprint at 1000 nm,22e,33 the ET rate constants
•-
for CR in Fc+-ZnP-C60•- were determined as 2.7 × 105 (THF),
1.3 × 105 (benzonitrile), and 6.3 × 104 s-1 (DMF) (Table 3).22d,f
The total quantum yields of CS (ΦCS(total)) in Fc-ZnP-C60 were
found to be 0.74 in THF, 0.85 in PhCN, and 0.36 in DMF by
the comparative method22e from the corresponding transient
absorption spectra.22f Fc-H2P-C60 exhibits similar photodynami-
cal behavior in THF, benzonitrile, and DMF.22f The CS and
CR rates and the corresponding driving forces in Fc-ZnP-C60
and Fc-H2P-C60 were reported previously22d,f and are sum-
marized in Table 3.
Taking into account the similarity in molecular structures of
Fc-ZnP-H2P-C60 (Figure 1), ZnP-H2P-C60, and Fc-ZnP-C60
(Figure 2), we can expect similar photodynamics in Fc-ZnP-
H2P-C60 (Scheme 2). Transient absorption spectra, recorded at
various time delays following a short 18 ps laser excitation in
deoxygenated benzonitrile, shed light onto the cascade of
intramolecular reactions in photoexcited Fc-ZnP-H2P-C60 (Fig-
ure 3). The absorption ratio (ZnP:H2P ) 1:2) of the two
porphyrin chromophores (i.e., ZnP and H2P) at the 532 nm laser
excitation leads unavoidably to the population of the singlet
excited state of both porphyrin moieties, ZnP (2.04 eV) and
H2P (1.89 eV). Considering the energy gap between the two
singlet excited states (0.15 eV) with the lower state being that
of H2P, a rapid singlet-singlet EN prevails with a time constant
of kEN ) 2.6 × 1010 s-1. Importantly, the measured EN rate
closely resembles the dynamics of ZnP-H2P-C60 (vide supra)
and the time-resolved fluorescence measurements (2.2 × 1010
s-1).22c Kinetic evidence for this transfer stems from the grow-
in of the singlet excited-state absorption features of H2P around
650-760 nm,22e which took place in two distinguishable steps
(Figure 3a,b). The faster process, which is masked by the
apparatus’ time resolution (<20 ps), refers to the direct route,
which is the excitation of H2P. By contrast, the slower, indirect
route results from the actual EN event between the two
porphyrin moieties. The 760 nm kinetic trace (Figure 3b)
illustrates the grow-in of the H2P singlet-singlet absorption.
In the wavelength region between 500 and 530 nm, the singlet-
singlet absorption of the H2P chromophore is, however, weaker
than that of the ZnP precursor. Here the actual energy transfer
event is associated with a loss of absorption, as shown in the
525 nm kinetic trace of Figure 3b.
Figure 2. Structures of porphyrin-fullerene linked triads and dyad.
Table 1. One-Electron Redox Potentials (vs Fc/Fc+)a of Tetrad
and the References in Various Solvents
E 0ox/V
H2P•+ ZnP•+
E 0red/V
Fc+/
C60•/-
/
/
compound
solvent
H2P
ZnP
Fc C60
Fc-ZnP-H2P-C60 benzonitrile
0.59
0.30
0.07 -1.04
0.05 -1.02b
0.07 -1.04b
0.06 -0.92b
Fc-ref, C60-ref
Fc-ref, C60-ref
Fc-ref, C60-ref
THF
benzonitrile
DMF
a The redox potentials were measured by differential pulse voltam-
metry in THF, benzonitrile, and DMF using 0.1 M n-Bu4NPF6 as a
supporting electrolyte with a sweep rate of 10 mV s-1 b From ref 22f.
.
nm22e,33). By analyzing the decay kinetics of the C60•- fingerprint
•-
at 1000 nm, the ET rate constants for CR in ZnP•+-H2P-C60
(kET(CR2)) were also determined as 2.9 × 104 (THF) and 5.0 ×
104 s-1 (DMF) (Table 3).22f The total quantum yields of CS in
ZnP-H2P-C60 (ΦCS(total)) were estimated from the transient
absorption spectra as 0.26 in THF and 0.21 in DMF,
respectively.22f The relatively low quantum yields (0.21-0.40)
can be explained by the competition of the CSH from ZnP-
H2P•+-C60 (1.63 eV) to ZnP•+-H2P-C60 (1.34 eV) versus
•-
•-
the decay of ZnP-H2P•+-C60 to the triplet excited states of
•-
the freebase porphyrin (3H2P* (1.40 eV)) and the C60 (3C60* (1.50
•-
eV)).22f The decay of ZnP-H2P•+-C60 to the ground state is
negligible because of the slow CR1 rate (∼104 s-1) estimated
The conclusion of the initial singlet excited-state energy
from the Marcus parabola for ZnP-C60 (vide infra).
1
migration is a high-lying and very reactive H2P* (1.89 eV),
We also reported the photodynamics of Fc-ZnP-C60 in
various solvents.22d,f Based on the energy level of each state in
benzonitrile, initial photoinduced CS occurs from all the excited
which is located adjacent to the electron and energy accepting
1
fullerene. H2P* is the starting point for the formation of Fc-
ZnP-H2P•+-C60 (1.63 eV) via CS1, as in the case of ZnP-
•-
H2P-C60. CS from Fc-ZnP-H2P-1C60* could not be confirmed
(33) Guldi, D. M.; Hungerbu¨hler, H.; Asmus, K.-D. J. Phys. Chem. 1995,
99, 9380.
by fluorescence lifetime measurements, because of the extensive

6622 J. Am. Chem. Soc., Vol. 123, No. 27, 2001
Imahori et al.
Table 2. Rate Constants (kET and kEN) and the Driving Forces (-∆G0 and -∆G0EN) of Fc-ZnP-H2P-C60 in Solutions
ET
solvent
initial statea
final statea
-∆G0/eV
k/s-1,b
kET(CR3) < 102
Φ
CS(total)c
0.20
Fc+-ZnP-H2P-C60
(1.07 eV)
Fc-ZnP-H2P-C60
1.07
•-
THF
(ꢀs ) 7.58)
benzonitrile
(ꢀs ) 25.2)
Fc-1ZnP*-H2P-C60
(2.04 eV)
Fc-ZnP-1H2P*-C60
(1.89 eV)
0.15
0.26
0.29
0.23
1.11
0.98
kEN ) 2.6 × 1010
kET(CS1) ) 6.9 × 109
kET(CSH1) ) 2.5 × 109
kET(CSH2) ) 1.7 × 108
kET(CR3) < 102
•-
Fc-ZnP-1H2P*-C60
(1.89 eV)
Fc-ZnP-H2P•+-C60
(1.63 eV)
Fc-ZnP-H2P•+-C60
(1.63 eV)
Fc-ZnP•+-H2P-C60
(1.34 eV)
•-
•-
Fc-ZnP•+-H2P-C60
(1.34 eV)
Fc+-ZnP-H2P-C60
(1.11 eV)
•-
•-
Fc+-ZnP-H2P-C60
(1.11 eV)
Fc-ZnP-H2P-C60
0.24
0.17
•-
•-
DMF
(ꢀs ) 36.7)
Fc+-ZnP-H2P-C60
(0.98 eV)
Fc-ZnP-H2P-C60
kET(CR3) < 102
a The energy of each state relative to the ground state is given in parentheses. b The kET values for CS, CR, and CSH and the kEN value were
determined by transient absorption spectroscopy. c The total quantum yield of CS based on the comparative method.22e
Scheme 2. Reaction Scheme and Energy Diagram for Fc-ZnP-H2P-C60 in Benzonitrile
Scheme 3. Reaction Scheme and Energy Diagram for ZnP-H2P-C60 in Benzonitrile
overlap between the free base porphyrin emission (λmax ) 655,
720 nm) and the C60 emission (λmax ) 720 nm). However, the
fullerene singlet excited state (1.75 eV) may also undergo CS
from the H2P to generate Fc-ZnP-H2P•+-C60•-, which is a minor
process judging from the relative ratio of the C60 absorption
(<10%) at 532 nm.21g,22e,f Spectroscopically, the oxidized donor,
H2P•+, was identified by a strong absorption in the visible (i.e.,
600-800 nm, see Figure 4a).22e,34 Hereby, the formation kinetics
of the π-radical cation in the visible and the fullerene π-radical
anion in the near-infrared (i.e., 1000 nm)22e,33 are valuable aids
in determining the absolute rate constants, providing similar
values (kET(CS1)) of 7.1 × 109 and 6.7 × 109 s-1, respectively.
Exemplifying time-absorption profiles in the visible are given
in Figure 3b, showing the first CS step at 525 and 760 nm.
Precisely at 760 nm CS is associated with a monoexponential
decay of the 1H2P* absorption to the weaker absorbing Fc-ZnP-
H2P•+-C60•-, while at 525 nm a biexponential rate law was
found to govern the kinetics in the time window up to 250 ps.
Here, the biexponential decay is constituted by the fast EN (kEN
) 2.6 × 1010 s-1) and slower CS (kET(CS1) ) 7.1 × 109 s-1).
The absorption of the H2P π-radical cation, as it emerged
from the CS process, is subject to a rapid intramolecular decay.
(34) Gasyna, Z.; Browett, W. R.; Stillman, M. J. Inorg. Chem. 1985,
24, 2440.

Charge Separation in a Photosynthetic Reaction Center
Table 3. Free Energy Changes (-∆G0ET) and ET Rate Constants (kET) for CS and CR in Fullerene-Based Dyad, Triads, and Tetrad at 298 K
THF (ꢀs ) 7.58) benzonitrile (ꢀs ) 25.2) DMF (ꢀs ) 36.7)
-∆G0ET/eV kET/s-1
J. Am. Chem. Soc., Vol. 123, No. 27, 2001 6623
initial state
-∆G0ET/eV
kET/s-1
-∆G0ET/eV
kET/s-1
1ZnP*-C60
0.65a
0.33a
0.08a
1.42a
1.00a
d
1.3 × 1010 b
5.1 × 108 a
1.6 × 107 a
3.7 × 105 a
2.7 × 105 e
d
0.66a
0.37a
0.12a
1.38a
1.03a
1.03a
1.34a
1.11
9.5 × 109 c
5.5 × 108 c
1.5 × 107 c
1.3 × 106 c
1.3 × 105 e
1.2 × 105 a
4.8 × 104 f
2.9g
0.85a
d
1.3 × 1010 b
ZnP-1C60*
ZnP-3C60*
d
d
d
•-
ZnP•+-C60
1.21a
0.91a
0.91a
1.17a
0.98
1.8 × 106 a
6.3 × 104 e
5.3 × 104 a
5.0 × 104 a
3.7g
Fc+-ZnP-C60
•-
Fc+-H2P-C60
•-
•-
ZnP•+-H2P-C60
1.37a
1.07
2.9 × 104 a
d
Fc+-ZnP-H2P-C60
•-
a From ref 22f. b From ref 21f. c From ref 21g. d Not determined. e From ref 22d. f From ref 22e. g Extrapolated using eq 5.
Figure 3. Differential absorption spectra obtained upon picosecond
Figure 4. Differential absorption spectra obtained upon picosecond
flash photolysis (532 nm) of ∼10-5 M solutions of Fc-ZnP-H2P-C60
tetrad in nitrogen saturated benzonitrile with a time delay of (a) 80
(solid circle), 300 (open circle), and 1100 ps (cross) and (b) time-
absorption profiles at 555 (open circle) and 650 nm (solid circle).
flash photolysis (532 nm) of ∼10-5 M solutions of Fc-ZnP-H2P-C60
tetrad in nitrogen-saturated benzonitrile with a time delay of (a) -50
(cross), 40 (open circle), and 80 ps (solid circle) and (b) time-
absorption profiles at 525 (solid circle) and 760 nm (open circle).
The product of this deactivation (kET(CSH1) ) 2.5 × 109 s-1) is
a differential absorption spectrum, which displays the ZnP
π-radical cation absorption with a maximum at 650 nm (Figure
4a).22e,35 Time-absorption profiles (Figure 4b), which unravel
both the formation (ZnP•+) and also the decay (H2P•+) kinetics
at 650 and 555 nm, respectively, corroborate that the underlying
rates are nearly identical. Furthermore, the new transient (Figure
4a) is identical with that noted for the final charge-separated
radical pair (ZnP•+-H2P-C60•-) in the above-described ZnP-
H2P-C60 triad.22e Taking the spectroscopic and kinetic evidence
in concert, we reach the conclusion that the resulting species
evolves from a first CSH between the two porphyrin donors
of ZnP, H2P, and C60, thereby ruling out the subsequent
formation of an excited state as a product of CR. In fact, the
long-lived ZnP•+-H2P-C60•- (kET(CR2) ) 4.8 × 104 s-1), which
can be taken as a suitable model, is known to regenerate the
singlet ground state quantitatively.
In line with the thermodynamics, which predict a second,
exothermic (0.23 eV) CSH reaction from the end-terminus Fc
to ZnP•+, the 650 nm maximum of ZnP•+ decreases further
(Figure 5a). Importantly, the final photoproduct, Fc+-ZnP-H2P-
C60•-, bears no spectral resemblance to the transient intermedi-
ates constituted in part by either of the strongly absorbing ZnP•+
or H2P•+ π-radical cations in Fc-ZnP•+-H2P-C60•- and Fc-ZnP-
H2P•+-C60•-, respectively. Similarly, the bleaching of the ZnP
and H2P Q-band absorption (i.e., 500-600 nm), which is an
additional attribute of the one-electron oxidized porphyrins, is
completely lacking. The actual rate constant for this second CSH
reaction (kET(CSH2) ) 1.7 × 108 s-1) could be derived with
reasonable accuracy from the time-absorption profiles. The
yielding the Fc-ZnP•+-H2P-C60 radical pair (1.34 eV). It is
•-
imperative to note that this energy level (1.34 eV) falls
substantially below that of any singlet or triplet excited states
(35) (a) Fuhrhop, J.-H.; Mauzerall, D. J. Am. Chem. Soc. 1969, 91, 4174.
(b) Chosrowjan, H.; Taniguchi, S.; Okada, T.; Takagi, S.; Arai, T.;
Tokumaru, K. Chem. Phys. Lett. 1995, 242, 644.

6624 J. Am. Chem. Soc., Vol. 123, No. 27, 2001
Imahori et al.
Figure 5. (a) Differential absorption spectra obtained upon picosecond
flash photolysis (532 nm) of ∼10-5 M solutions of Fc-ZnP-H2P-C60
tetrad in nitrogen-saturated benzonitrile with a time delay of 6 ns and
(b) differential absorption spectra obtained upon nanosecond flash
photolysis (532 nm) of 7.2 × 10-6 M solutions of Fc-ZnP-H2P-C60
tetrad in nitrogen-saturated benzonitrile with a time delay of 50 ns at
298 K.
Figure 6. (a) The time profile of absorbance at 1000 nm obtained by
nanosecond time-resolved absorption spectra of Fc-ZnP-H2P-C60 tetrad
(3.6 × 10-6 M) in argon-saturated DMF excited at 532 nm at 298 K.
(b) Second-order plot derived from the absorption change at 1000 nm.
CSH2 rate is slower by 1 order of magnitude than the
corresponding CSH value (2.8 × 109 s-1) of Fc-ZnP-C60 in
PhCN.22f The reversal of the amide bond between the ferrocene
and the zincporphyrin in Fc-ZnP-H2P-C60 and Fc-ZnP-C60 has
its impact on the driving force (0.23 eV (Fc-ZnP-H2P-C60)
versus 0.35 eV (Fc-ZnP-C60)). Thus, the attenuation of the
CSH2 rate in Fc-ZnP-H2P-C60 can be explained by the
difference in the driving force. On the other hand, the spectral
changes in the near-infrared, due to the fullerene π-radical anion,
remained constant, throughout the cascade of CSH reactions.
Nanosecond transient absorption studies following laser
excitation of the ZnP moiety, the total quantum yields (ΦCS
(total)) for CS of 0.20 (THF), 0.24 (PhCN), and 0.17 (DMF)
were determined (Table 2), by applying the comparative method
-
from the nanosecond time-resolved transient spectra [mono-
•-
functionalized C60 (ꢀ1000
) 4700 M-1 cm-1)].22e The
nm
relatively low quantum yields (0.17-0.24) can be rationalized
by the competition of the CSH from Fc-ZnP-H2P•+-C60•- (1.63
•-
eV) to Fc-ZnP•+-H2P-C60 (1.34 eV) versus the decay of
•-
Fc-ZnP-H2P•+-C60 to the triplet states of the freebase por-
phyrin (1.40 eV) and the C60 (1.50 eV), which is similar to that
of ZnP-H2P-C60 (vide supra).
•-
excitation of Fc-ZnP-H2P-C60 reveal unambiguously the C60
In contrast to the primary building blocks of the present tetrad
(ZnP-C60, Fc-ZnP-C60, and ZnP-H2P-C60), the decay dynamics
of the charge-separated radical pair (Fc+-ZnP-H2P-C60•-) does
not obey first-order kinetics. The time profiles at 1000 nm (i.e.,
at the maximum of the C60•- absorption) obey instead second-
order kinetics, as shown in Figure 6. The second-order rate law
was verified by changing the effective radical ion pair concen-
tration by employing different laser power over a wide range
(i.e., increments to reach a 5-fold increase in laser intensity)
and also by changing the initial Fc-ZnP-H2P-C60 concen-
tration (i.e., (2.0-20) × 10-6 M). From the best fits intermo-
lecular ET dynamics were calculated that are nearly diffusion
controlled (PhCN, 5.4 × 109 M-1 s-1; DMF, 2.5 × 109 M-1
s-1). To the point that this analysis is correct a complement-
ary investigation in a solvent with an even higher viscosity
(DMF: η ) 0.924 [10-3 Pa s]) was deemed important, which
led us to probe Triton X-100 (η ) 7.2 [10-3 Pa s]). Despite
succeeding with respect to slowing-down the diffusion-
fingerprint (ca. 1000 nm), which matches quite reasonably that
known for the π-radical anion of a N-methylfulleropyrrolidine
(Figure 5b).22e,33 In regard to the oxidized donor part, the weak
absorption features of the ferricenium ion (Fc+: λmax∼ 800 nm
(ꢀ ∼ 1000 M-1 cm-1))36 have prevented its direct detection.
As such, the UV-visible changes are dominated by the
differential absorption changes, which are associated with the
one-electron reduction of the fullerene, especially around 400
nm. Since no absorption bands due to the ZnP•+ 35 or H2P•+ 34
were detected together with the absorption due to the reduced
part (C60•-), the oxidized part must be Fc+ (Scheme 2). In fact,
the transient absorption spectra of Fc-ZnP-H2P-C60 (Figure 5)
are virtually the same as those of Fc+-ZnP-C60•- and Fc+-H2P-
•- 22f,g
C60
.
Fc-ZnP-H2P-C60 exhibits similar photodynamical
behavior in THF and DMF. On the basis of the predominant
(36) Fukuzumi, S.; Mochizuki, S.; Tanaka, T. Inorg. Chem. 1989, 28,
2459.

Charge Separation in a Photosynthetic Reaction Center
J. Am. Chem. Soc., Vol. 123, No. 27, 2001 6625
for the intramolecular CR process (kET(CR3)). Furthermore, under
similar experimental conditions, the ESR response of
Fc-ZnP-H2P-C60 was much larger than those of ZnP-C60 and
ZnP-H2P-C60 despite the lower quantum yield of CS for
Fc-ZnP-H2P-C60 (0.24) in PhCN solution relative to those of
ZnP-C60 (0.8522e) and ZnP-H2P-C60 (0.4022e). The latter exhib-
ited multiexponential decay profiles (Figure 7b), which sug-
gests the involvement of intermolecular ET in ZnP-C60 and
ZnP-H2P-C60.
The clean monoexponential dynamics corroborate that the
origin of the Fc-ZnP-H2P-C60 ESR signal is indeed the
intramolecular radical ion pair (Fc+-ZnP-H2P-C60•-).39 In
contrast, an intermolecular ET, as it may prevail between radical
ion pairs located in close proximity, should exhibit multiexpo-
nential decay profiles, depending on the relative distance
between the radical ion pairs. Under the present experimental
conditions this contribution was found to be negligible. To
further test this hypothesis, the concentration was increased
systematically up to 1.0 × 10-4 M. In fact, at higher concentra-
tions the first-order decay started to be mixed with a slower
multiexponential decay due to the intermolecular ET.
The temperature dependence of kET(CR3) reveals only a
moderate change (2.6-3.0 s-1) upon varying the temperature
between 163 and 203 K. Similar results were obtained in DMF
(kET(CR3) ) 2.9-3.4 s-1) within a range of 163-193 K. The
weak temperature dependence suggests that a stepwise intramo-
Figure 7. (a) ESR spectrum of Fc-ZnP-H2P-C60 (1.0 × 10-5 M) in a
frozen deaerated PhCN solution observed at 203 K under irradiation
of ultraviolet-visible light from a high-pressure Hg lamp and (b) signal
intensity response of Fc-ZnP-H2P-C60 (1.0 × 10-5 M, top) and
ZnP-C60 (1.0 × 10-5 M, bottom) in a frozen deaerated PhCN solu-
•-
lecular CR process of Fc+-ZnP-H2P-C60 via a transient
•-
tion at the maximum of the ESR signal intensity due to the C60
.
Fc-ZnP•+-H2P-C60•- intermediate can be ruled out with a high
degree of confidence.
First-order plot for the decay of the ESR signal intensity (I) in
Fc-ZnP-H2P-C60 (kET(CR3) ) 3.0 s-1) is shown in the right panel.
To analyze the ET reaction of Fc-ZnP-H2P-C60 in light of
the thermodynamic parameters, the following equations (eq 3
and 4) were employed:1
controlled limit by 1 order of magnitude, we still obtained
dynamics that were best fitted to an intermolecular process with
an associated rate constant of 6.9 × 108 M-1 s-1. This confirms
(∆G0ET + λ)2
1/2
•-
that the intramolecular ET in Fc+-ZnP-H2P-C60 (kET(CR3)
<
4π3
kET
)
V2 exp -
(3)
102 s-1) is indeed too slow to compete with the diffusion-limited
intermolecular ET even in a highly viscous solvent (Table 2).
Electron Spin Resonance Measurements under Irradia-
tion. To segregate the intermolecular ET from the intramo-
lecular ET processes in Fc+-ZnP-H2P-C60•-, electron spin
resonance (ESR) measurements were performed in a frozen
matrix at variable temperatures using a low concentration in
PhCN (1.0 × 10-5 M) under irradiation. The ESR spectrum
(Figure 7a) under irradiation at 203 K shows a characteristic
broad signal attributable to C60•- (g ) 2.0004).37 C60•- was also
produced via chemical reduction of C60-ref (Figure 1) with
tetramethylsemiquinone radical anion, yielding essentially an
identical ESR signal. As far as direct ESR evidence for the
(
)
h2λkBT
[
]
4λkBT
(∆G0ET + λ)2
4λ
∆G# )
(4)
with λ being the reorganization energy, ∆G0ET the free energy
change of ET, V the electronic coupling matrix element, T the
absolute temperature, and ∆G# the activation barrier for the ET
reaction. The “Marcus equation” (eq 3) was then transformed
to its linear form as given in eq 5, which, in turn, enabled
evaluation of ∆G#, λ, and V.
(∆G0ET + λ)2
4λkBT
2π3/2V2
ln(kETT1/2) ) ln
-
(5)
ferricenium ion (Fc+) in the Fc+-ZnP-H2P-C60 pair is
•-
1/2
(
)
h(λkB)
concerned, its lack is rationalized in terms of well-known line-
broadening of the Fc+ ESR signal.38
The Arrhenius plots, that is, the ET rate (kET(CR3)) as a
reciprocal function of the absolute temperature, reveal no
appreciable deviation from the best-fitted straight line (Figure
8). Importantly, the small activation barriers (∆G#) of 0.012
and 0.016 eV in PhCN and DMF, respectively, infer that these
The ESR signal was monitored at a fixed magnetic field to
•-
obtain maximal intensity of the C60 response and to enable
performance of the following lifetime experiments in frozen
PhCN. As shown in Figure 7b, the ESR signal grows-in imme-
diately upon “turning on” the irradiation of Fc-ZnP-H2P-C60.
Only a slight amplification was noted during illumination.
However, upon “turning off” the irradiation source, the ESR
signal did not diminish instantly. Instead, it exhibited a slow
and nearly seconds lasting decay, which was best fitted by first-
order kinetics yielding a rate constant of 3.0 s-1 (Figure 7b)
(39) Gust et al.15h reported that freebase porphyrin-C60 dyad reveals
photoinduced ET from the porphyrin excited singlet state to the C60 even
in the rigid glass down to at least 8 K. They suggested that the energy of
the C60 radical anion was less sensitive to solvents effect than that of
semiquinone radical anion, as the C60 radical anion is a relatively large,
spherical ion having the charge spread over the three-dimensional carbon
framework. Thus, destabilization of the C60 radical anion by frozen solvents
should be mimimal. The unique behavior was rationalized by our proposal
that fullerenes have small reorganization energies in ET.13,14 Overall, the
(37) Fukuzumi, S.; Mori, H.; Suenobu, T.; Imahori, H.; Gao, X.; Kadish,
K. M. J. Phys. Chem. A 2000, 104, 10688.
•-
(38) (Fc-ref)+ produced by the oxidation with Ru(bpy)33+ (bpy ) 2,2′-
bipyridine) exhibits no significant ESR signal under the same experimental
conditions.
Fc-ZnP-H2P•+-C60 state may be less destabilized even in frozen ben-
zonitrile or DMF, thereby leading to the production of the final charge-
separated state (Fc+-ZnP-H2P-C60•-).

6626 J. Am. Chem. Soc., Vol. 123, No. 27, 2001
Imahori et al.
Figure 9. Driving force (-∆G0ET) dependence of intramolecular ET
rate constants in ZnP-C60 (CS, open circles; CR, open squares),
Fc-ZnP-C60 (solid circles), Fc-H2P-C60 (solid triangles), ZnP-
H2P-C60 (solid squares), and Fc-ZnP-H2P-C60 (open triangles). The
lines represent the best fit to eq 3 (ZnP-C60, λ ) 0.66 eV, V ) 3.9
cm-1, 22f; Fc-ZnP-C60, Fc-H2P-C60, and ZnP-H2P-C60, λ ) 1.09 eV, V
) 0.019 cm-1, 22f; Fc-ZnP-H2P-C60, λ ) 1.32 eV, V ) 0.00017 cm-1).
Figure 8. Arrhenius analyses of the temperature-dependent ET rate
constants (kET(CR3)) for Fc-ZnP-H2P-C60 in frozen PhCN (circles) and
DMF (triangles). The Arrhenius plots of the ET rates against the
reciprocal of absolute temperature gave a straight line with the intercept
(PhCN, 4.38; DMF, 4.80) and the slope (PhCN, 0.142; DMF, 0.191)
according to eq 5.
ET processes occur in the top region of the “Marcus curve” to
give the optimal rate constant (kET(optimal)) (Figure 9). From a
closer inspection of the Arrhenius plots, the reorganization
energy (λ) and the electronic coupling matrix element (V) were
determined as 1.37 eV in PhCN and 1.27 eV in DMF and 1.6
× 10-4 cm-1 in PhCN, and 1.9 × 10-4 cm-1 in DMF,
respectively.
Finally, to shed light onto the intramolecular ET process,
the kET(optimal) values of Fc-ZnP-H2P-C60 (Ree ) 48.9 Å), Fc-
ZnP-C60, Fc-H2P-C60, ZnP-H2P-C60 (Ree ) 30.3 Å), and ZnP-
C60 (Ree ) 11.9 Å) (Table 3 and Figure 9) are correlated with
the edge-to-edge distance (Ree), separating the radical ions,
according to eq 6 and 7:
1/2
1/2
4π3
4π3
kET(optimal)
)
V2 )
V02 exp(-âRee) (6)
Figure 10. Edge-to-edge distance (Ree) dependence of optimal electron-
transfer rate constants (ln[kET(optimal)/s-1]) in ZnP-C60 dyad (square),
Fc-ZnP-C60 triad (circle), Fc-H2P-C60 triad (diamond), ZnP-H2P-C60
triad (cross), and Fc-ZnP-H2P-C60 tetrad (triangle). The line represents
the best fit to eq 7 (â ) 0.60 Å-1, V0 ) 210 cm-1).
2
2
(
)
(
)
h λkBT
h λkBT
2π3/2V0
2
ln(kET(optimal)) ) ln
- âRee
(7)
1/2
(
)
h(λkBT)
lifetimes (∼1 s-1) of the bacteriochlorophyll dimer radical cation
((Bchl)2•+)-secondary quinone radical anion (QB•-) ion pair
in the bacteria photosynthetic reaction centers.11 Such an
extremely long lifetime of a CS state could only be determined
in frozen media, since in condensed media intermolecular
dynamics dominate the back ET. It should also be emphasized
that such an extremely long lifetime of the tetrad system has
been well correlated with the charge-separated lifetimes of two
homologous series of porphyrin-fullerene dyad and triad
systems.
Hereby, V0 refers to the maximal electronic coupling element,
while â is the decay coefficient factor (damping factor), which
depends primarily on the nature of the bridging molecule. The
plots are fitted well with a straight line, including the extremely
slow CR process of Fc-ZnP-H2P-C60 tetrad to yield V0 ) 210
cm-1 and â ) 0.60 Å-1 (Figure 10). This â value is located
within the boundaries of nonadiabatic ET reactions for saturated
hydrocarbon bridges (0.8-1.0 Å-1) and unsaturated phenylene
bridges (0.4 Å-1).1,40
In summary, the present study has demonstrated unequivo-
cally occurrence of an intramolecular CR process in the Fc-
ZnP-H2P-C60 tetrad, observable by ESR measurements under
irradiation. More importantly, the lifetimes (0.38 s in PhCN at
193 K and 0.34 s in DMF at 173 K) are by far the longest
values ever reported for intramolecular CR in a donor-acceptor
ensemble (with earlier values at 340 µs in solutions24 and 12.7
ms at 77 K25). They are also comparable, for example, to the
Experimental Section
General. Melting points were recorded on a Yanagimoto micro-
melting point apparatus and not corrected. 1H NMR spectra were mea-
sured on a JEOL EX-270 or a JEOL JNM-LA400. Fast atom bom-
bardment mass spectra (FABMS) were obtained on a JEOL JMS-
DX300. Matrix-assisted laser desorption/ionization (MALDI) time-of-
flight (TOF) mass spectra were measured on a Kratos Compact MALDI
I (Shimadzu). IR spectra were measured on a Perkin-Elmer FT-IR 2000
spectrophotometer as KBr disks. Steady-state absorption spectra were
(40) Helms, A.; Heiler, D.; McLendon, G. J. Am. Chem. Soc. 1992, 114,
6227.

Charge Separation in a Photosynthetic Reaction Center
J. Am. Chem. Soc., Vol. 123, No. 27, 2001 6627
measured on a Shimadzu UV3000 spectrometer. Elemental analyses
were performed on a Perkin-Elmer Model 240C elemental analyzer.
The edge-to-edge distances (Ree) were determined from CPK modeling
using CAChe (version 3.7 CAChe Scientific, 1994).
J ) 5 Hz, 2H), 8.92 (s, 4H), 8.97 (d, J ) 5 Hz, 2H); FABMS m/z
1221 (M + H+).
6: A solution of 522e (153 mg, 0.165 mmol), thionyl chloride (0.370
mL, 5.07 mmol), and pyridine (0.20 mL) in benzene (30.0 mL) was
refluxed for 2 h under nitrogen atmosphere. Excess reagent and solvents
were removed under reduced pressure, and the residue was redissolved
in a mixture of toluene (20 mL) and pyridine (1.0 mL). To the reaction
mixture were added a solution of 4 (99.8 mg, 0.0818 mmol) and 4-tert-
butyldimethylsilyloxymethylaniline22e (60.8 mg, 0.256 mmol) in toluene
(30.0 mL) and pyridine (1.0 mL). The solution was allowed to stir
overnight at room temperature under nitrogen atmosphere. TLC showed
three products (Rf ) 0.60, 0.45, and 0.40, benzene/ethyl acetate ) 9/1)
and the second band was separated by flash column chromatography
on silica gel (chloroform/triethylamine ) 100/0.25) and subsequent
reprecipitation from benzene-CH3CN gave a deep violet solid. To a
solution of the deep violet solid in THF (40 mL) was added 0.640 mL
of tetrabutylammonium fluoride (1 M in THF), then the mixture was
stirred for 3.5 h at room temperature under nitrogen atmosphere. The
solvents were removed under reduced pressure. Flash column chro-
matography on silica gel (toluene/ethyl acetate ) 9/1) and subsequent
reprecipitation from chloroform-acetonitrile afforded 6 as a deep purple
solid (74.9 mg, 0.0335 mmol, 41% yield). Mp >300 °C; 1H NMR (400
MHz, CDCl3; some of the signals could not be assigned because of
the poor solubility) δ -2.68 (br s, 2H), 1.56 (s, 72H), 4.11 (s, 5H),
4.45 (s, 2H), 4.68 (d, J ) 5 Hz, 2H), 4.79 (s, 2H), 7.5-9.1 (m, 55 H);
MALDI-TOFMS (positive mode) m/z 2236 (M + H+).
Materials. All solvents and chemicals were of reagent grade quality,
obtained commercially and used without further purification except as
noted below. Tetrabutylammonium hexafluorophosphate used as a
supporting electrolyte for the electrochemical measurements was
obtained from Tokyo Kasei Organic Chemicals. THF, benzonitrile, and
DMF were purchased from Wako Pure Chemical Ind., Ltd., and purified
by successive distillation over calcium hydride. Thin-layer chroma-
tography (TLC) and flash column chromatography were performed with
Art. 5554 DC-Alufolien Kieselgel 60 F254 (Merck) and Fujisilicia
BW300, respectively.
Synthesis of Tetrads. 2: A solution of dipyrromethane 127 (15.62
g, 58.44 mmol) and 3,5-di-tert-butylbenzaldehyde28 (12.76 g, 58.44
mmol) in chloroform (4000 mL) was degassed by bubbling with
nitrogen for 1 h. The reaction vessel was shielded from ambient light.
Then boron trifluoride diethyl etherate (7.43 mL, 58.6 mmol) was added
at one portion. The solution was stirred for 2 h at room temperature
under nitrogen atmosphere. To the reddish black reaction mixture was
added p-chloranil (21.55 g, 87.64 mmol) with overnight stirring.
Triethylamine (25.2 mL, 181 mmol) was added and then the reaction
mixture was concentrated. Flash column chromatography on silica gel
with chloroform as the eluent gave a mixture of several porphyrins.
Subsequent flash column chromatography on silica gel (toluene) yielded
the desired porphyrin from the mixture as the third major band.
Reprecipitation with chloroform-methanol afforded 2 as a vivid reddish
7: To a solution of 6 (39.9 mg, 0.0179 mol) in chloroform (65 mL)
was added activated manganese dioxide (398 mg) in one portion, and
then the suspended mixture was stirred for 3.5 h at room temperature
under nitrogen atmosphere. The catalyst was removed by filtration and
the filtrate was concentrated under reduced pressure. Flash column
chromatography on silica gel (benzene/ethyl acetate ) 9/1) and
subsequent reprecipitation from chloroform-acetonitrile afforded 7 as
a deep purple solid (30.9 mg, 0.0138 mmol, 77% yield). Mp >300 °C;
1H NMR (270 MHz, CDCl3) δ -2.67 (br s, 2H), 1.55 (s, 72H), 4.10
(s, 5H), 4.44 (s, 2H), 4.77 (s, 2H), 7.64 (s, 2H), 7.8-8.5 (m, 37H),
8.7-9.2 (m, 16H), 9.98 (s, 1H); MALDI-TOFMS (positive mode) m/z
2234 (M + H+).
1
purple solid (3.14 g, 3.38 mmol, 12% yield). Mp >300 °C; H NMR
(270 MHz, CDCl3) δ -2.75 (br s, 2H), 1.53 (s, 36H), 7.82 (t, J ) 2
Hz, 2H), 8.08 (d, J ) 2 Hz, 4H), 8.41 (d, J ) 8 Hz, 4H), 8.64 (d, J )
8 Hz, 4H), 8.75 (d, J ) 5 Hz, 4H), 8.95 (d, J ) 5 Hz, 4H); FABMS
m/z 929 (M + H+).
3: To a suspension of 2 (201 mg, 0.217 mmol) in concentrated HCl
solution (50 mL) was added SnCl2 (276 mg, 1.46 mmol). The reaction
mixture was stirred for 45 min at room temperature and maintained at
95-100 °C for an additional 30 min. After cooling, the reaction was
quenched by the slow addition of 25% ammonia solution. The organic
layer was extracted with CHCl3 and washed with water. After standing
over anhydrous Na2SO4, the organic extract was concentrated to dryness.
Flash column chromatography on silica gel with toluene-ethyl ace-
tate (30:1) as the eluent and subsequent reprecipitation from CHCl3-
CH3CN gave 3 as a deep violet solid (123 mg, 0.142 mmol, 65%). Mp
Fc-ZnP-H2P-C60: A solution of 7 (17.2 mg, 0.00770 mmol), C60
(27.8 mg, 0.0386 mmol), and N-methylglycine (35.1 mg, 0.394 mmol)
in toluene (40 mL) was refluxed under nitrogen atmosphere until TLC
analysis showed no detection of 7. The reaction mixture was poured
on the top of a flash chromatography column (silica gel) that was
washed with toluene as an eluate to remove unreacted C60. Then the
column was washed with chloroform, eluting the desired product.
Reprecipitation from chloroform-methanol afforded Fc-ZnP-H2P-C60
as a dark violet solid (13.3 mg, 0.00446 mmol, 58% yield). Mp >300
1
>300 °C; H NMR (270 MHz, CDCl3) δ -2.71 (br s, 2H), 1.53 (s,
36H), 4.02 (br s, 4H), 7.06 (d, J ) 8 Hz, 4H), 7.79 (t, J ) 2 Hz, 2H),
8.00 (d, J ) 8 Hz, 4H), 8.08 (d, J ) 2 Hz, 4H), 8.87 (d, J ) 5 Hz,
4H), 8.92 (d, J ) 5 Hz, 4H); FABMS m/z 869 (M + H+).
1
°C; H NMR (400 MHz, CDCl2CDCl2, 60 °C) δ -2.65 (s, 2H), 1.57
4: Oxalyl chloride (0.190 mL, 2.18 mmol) was added to a stirred
solution of sodium 4-ferrocenylbenzoate29 (70.5 mg, 0.215 mmol) in
dry CH2Cl2 (15 mL) and pyridine (0.01 mL). The resulting solution
was stirred for 13 h at room temperature and then refluxed for 5 h.
Excess oxalyl chloride and solvents were removed under reduced
pressure and the residue was redissolved in dry toluene (20 mL). This
solution was added to a stirred solution of 3 (373 mg, 0.430 mmol)
and pyridine (6.0 mL) in dry toluene (150 mL), and the solution was
stirred for 18 h. The reaction mixture was evaporated. Flash column
chromatography on silica gel with toluene-ethyl acetate (80:1) as the
eluent and subsequent reprecipitation from CHCl3-CH3CN gave a deep
violet solid. A saturated methanol solution of Zn(OAc)2 (5.0 mL) was
added to a solution of the deep violet solid in CHCl3 (50 mL) and
refluxed for 30 min. After cooling, the reaction mixture was washed
with water twice and dried over anhydrous Na2SO4, and then the solvent
was evaporated. Flash column chromatography on silica gel with CHCl3
as an eluent and subsequent reprecipitation from chloroform-meth-
anol gave 4 as a deep violet solid (113 mg, 0.0926 mol, 43% yield).
Mp >300 °C; 1H NMR (270 MHz, CDCl3 + pyridine-d5) δ 1.52
(s, 36H), 4.08 (s, 5H), 4.41 (t, J ) 2 Hz, 2H), 4.75 (t, J ) 2 Hz,
2H), 7.03 (d, J ) 8 Hz, 2H), 7.61 (d, J ) 8 Hz, 2H), 7.77 (t, J ) 2 Hz,
2H), 7.98 (d, J ) 8 Hz, 2H), 7.99 (d, J ) 8 Hz, 2H), 8.05 (d, J ) 8
Hz, 2H), 8.06 (d, J ) 2 Hz, 4H), 8.21 (d, J ) 8 Hz, 2H), 8.91 (d,
(s, 36H), 1.59 (s, 36H), 2.86 (s, 3H), 4.13 (s, 5H), 4.20 (d, J ) 10 Hz,
1H), 4.46 (s, 2H), 4.79 (s, 2H), 4.90 (s, 1H), 4.94 (d, J ) 10 Hz, 1H),
7.68 (d, J ) 8 Hz, 2H), 7.84 (t, J ) 2 Hz, 4H), 7.90 (s, 4H), 7.98 (d,
J ) 8 Hz, 2H), 8.07 (d, J ) 8 Hz, 2H), 8.11 (d, J ) 2H, 4H), 8.14 (d,
J ) 2 Hz, 4H), 8.17 (br s, 1H), 8.22 (d, J ) 8 Hz, 2H), 8.26 (d, J )
8 Hz, 2H), 8.28 (br s, 1H), 8.30 (d, J ) 8 Hz, 2H), 8.36 (d, J ) 8 Hz,
2H), 8.38 (d, J ) 8 Hz, 2H), 8.45 (d, J ) 8 Hz, 2H), 8.50 (d, J ) 8
Hz, 2H), 8.52 (br s, 1H), 8.83 (d, J ) 5 Hz, 2H), 8.92 (d, J ) 5 Hz,
2H), 8.96 (d, J ) 5 Hz, 2H), 8.99 (d, J ) 5 Hz, 2H), 9.05 (d, J ) 5
Hz, 2H), 9.07 (d, J ) 5 Hz, 2H), 9.10 (d, J ) 5 Hz, 2H), 9.12 (d, J )
5 Hz, 2H); MALDI-TOFMS (positive mode) m/z 2981 (M + H+), 2260
[M - C60]+; UV-vis (PhCN) λmax (log ꢀ) 432 (5.91), 519 (4.40), 557
(4.52), 600 (4.22), 650 (3.78).
Fc-ref: Oxalyl chloride (1.0 mL, 11 mmol) was added to a stirred
solution of sodium 4-ferrocenylbenzoate (0.600 g, 1.83 mmol)29 in dry
CH2Cl2 (50 mL) and pyridine (0.1 mL). The resultant solution was
stirred for 14 h at room temperature and then refluxed for 5 h. The
excess oxalyl chloride and solvents were removed under a reduced
pressure and the residue was redissolved in dry CH2Cl2 (25 mL). This
solution was added to a stirred solution of aniline (1.0 mL, 11 mmol)
in dry CH2Cl2 (25 mL) and then stirred for 28 h. The reaction mix-
ture was poured into 150 mL of water, extracted with CHCl3, dried
over anhydrous Na2SO4, and evaporated. Recrystallization from tol-

6628 J. Am. Chem. Soc., Vol. 123, No. 27, 2001
Imahori et al.
1
uene gave Fc-ref as orange needles (0.580 g, 1.52 mmol, 83%). H
NMR (270 MHz, CDCl3) δ 7.80 (d, J ) 8 Hz, 2H), 7.79 (s, 1H), 7.66
(d, J ) 8 Hz, 2H), 7.57 (d, J ) 8 Hz, 2H), 7.39 (t, J ) 8 Hz, 2H), 7.16
(t, J ) 8 Hz, 1H), 4.72 (t, J ) 2 Hz, 2H), 4.40 (t, J ) 2 Hz, 2H), 4.05
(s, 5H); MS (FAB) 382 (M + H+); UV-vis (CH2Cl2) λmax (log ꢀ) 263
(4.22), 300 (4.37), 362 (3.52), 455 (3.04). Anal. Calcd for C23H19-
FeNO: C, 72.46; H, 5.02; N, 3.67. Found: C, 72.73; H, 4.83; N, 3.68.
Flash Photolysis Experiments. For flash-photolysis studies, the
fullerene concentrations were prepared to exhibit an optical density of
at least 0.02 at 532 nm, the wavelength of irradiation. Picosecond laser
flash photolysis experiments were carried out with 532-nm laser pulses
from a mode-locked, Q-switched Quantel YG-501 DP ND:YAG laser
system (pulse width ∼ 18 ps, 2-3 mJ/pulse).22e The white continuum
picosecond probe pulse was generated by passing the fundamental
output through a D2O/H2O solution. Nano- to millisecond laser flash
photolysis experiments were performed with laser pulses from a Qunta-
Ray CDR Nd:YAG system (532 nm, 6 ns pulse width) in a front face
excitation geometry.22e A Xe lamp was triggered synchronously with
the laser. A monochromator (SPEX) in combination with either a Hama-
matsu R 5108 photomultiplier or a fast InGaAs-diode was employed
to monitor transient absorption spectra. The quantum yields were mea-
sured using the comparative method.22e In particular, the strong fullerene
ESR Measurements under Photoirradiation. A quartz ESR tube
(internal diameter: 4.5-mm) containing a deaerated PhCN solution of
porphyrin-fullerene linked compounds (1.0 × 10-5 M) was irradiated
in the cavity of the ESR spectrometer with the focused light of a
1000-W high-pressure Hg lamp (Ushio-USH1005D) through an aqueous
filter. The internal diameter of the ESR tube is 4.5 mm, which is small
enough to fill the ESR cavity but large enough to obtain good signal-
to-noise ratios during the ESR measurements under photoirradiation
at low temperatures. The ESR spectra in frozen PhCN or DMF were
measured under nonsaturating microwave power conditions at various
temperatures (163 to 203 K) with a JEOL X-band spectrometer (JES-
RE1XE) using an attached variable-temperature apparatus.
Acknowledgment. This work was supported by Grant-in-
Aids for COE Research and Scientific Research on Priority Area
of Electrochemistry of Ordered Interfaces and Creation of
Delocalized Electronic Systems from the Ministry of Education,
Culture, Sports, Science and Technology, Japan, a Millennium
Project from the Science and Technology Agency, Japan (No.
12310), and the Office of Basic Energy Sciences of the U.S.
Department of Energy, U.S.A. H.I. thanks the Sumitomo
Foundation for financial support. This is document NDRL-4273
from the Notre Dame Radiation Laboratory.
triplet-triplet absorption (ꢀ700
) 16100 M-1 cm-1; ΦTRIPLET
)
nm
0.98)22e served as probe to obtain the quantum yields for the CS state,
especially for the fullerene π-radical anion (ꢀ1000
) 4700 M-1
nm
cm-1).22e
JA004123V