Ultrafast photodynamics of exciplex formation and photoinduced electron transfer in porphyrin-fullerene dyads linked at close proximity

Article abstract of DOI:10.1021/jp035412j

The ultrafast photodynamics of porphyrin-fullerene dyads in which the distance between the porphyrin and C₆₀ moieties is varied systematically at close proximity has been examined using fluorescence up-conversion and pump-probe transient absorption techniques with time resolutions of ca. 100 fs. The porphyrin-fullerene dyads examined are MP-D-C₆₀ (M = Zn and 2H) in which the C₆₀ moiety is directly connected with the porphyrin ring at the meso position and MP-O-C₆₀, MP-M-C₆₀, and MP-P-C₆₀ in which the C₆₀ moiety is linked with porphyrin moieties through the benzene ring at the ortho, meta, and para positions, respectively. The charge transfer (CT) bands are observed for MP-D-C₆₀ and MP-O-C₆₀, whereas no CT band is seen or MP-M-C₆₀ and MP-P-C₆₀. Time-resolved absorption spectral measurements indicate that the photoexcitation of ZnP-D-C₆₀ in benzonitrile (PhCN) results in formation of the exciplex, which decays to the ground state without forming the charge-separated state. The strong interaction between the ZnP and the C₆₀ moieties due to the short linkage distance in ZnP-D-C₆₀ as indicated by the observation of the strongest CT band at the ground state results in formation of the exciplex. The energy of the exciplex is lower than that of the charge-separated state even in a polar solvent such as PhCN. In contrast, the photoexcitation of the dyad with longer linkage, ZnP-O-C₆₀, in PhCN results in formation of the charge-separated state via the exciplex formation, which is higher in energy than the charge-separated state. The photodynamics of exciplex formation of porphyrin-C₆₀-linked dyads with a short linkage is characterized by the extremely fast formation rate from the singlet excited states of porphyrins involving both the second and first excited states due to the interaction between the porphyrin and C₆₀ moieties, which are placed at close proximity. In the case of MP-D-C₆₀, the exciplex formation from the first singlet excited state of MP occurs at an ultrafast time scale with a time constant of 160 fs and that from the second singlet excited state occurs faster with a time constant less than 50 fs.

Full text of DOI:10.1021/jp035412j

8834
J. Phys. Chem. A 2003, 107, 8834-8844
Ultrafast Photodynamics of Exciplex Formation and Photoinduced Electron Transfer in
Porphyrin-Fullerene Dyads Linked at Close Proximity
Nikolai V. Tkachenko,*,‡ Helge Lemmetyinen,*,‡ Junko Sonoda,† Kei Ohkubo,† Tomoo Sato,
Hiroshi Imahori,*,§ and Shunichi Fukuzumi*,†
Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity, CREST, Japan
Science and Technology Corporation, Suita, Osaka 565-0871, Japan, Institute of Materials Chemistry,
Tampere UniVersity of Technology, P.O. Box 541, FIN-33101 Tampere, Finland, Department of Chemistry,
UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan, and Department of Molecular Engineering,
Graduate School of Engineering, Kyoto UniVersity, PRESTO, Japan Science and Technology Corporation
(JST), Katsura, Nishikyo-ku, Kyoto 615-8510, Japan and Fukui Institute for Fundamental Chemistry,
Kyoto UniVersity, 34-4, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan
ReceiVed: May 22, 2003; In Final Form: August 6, 2003
The ultrafast photodynamics of porphyrin-fullerene dyads in which the distance between the porphyrin and
C
60 moieties is varied systematically at close proximity has been examined using fluorescence up-conversion
and pump-probe transient absorption techniques with time resolutions of ca. 100 fs. The porphyrin-fullerene
dyads examined are MP-D-C60 (M ) Zn and 2H) in which the C60 moiety is directly connected with the
porphyrin ring at the meso position and MP-O-C60, MP-M-C60, and MP-P-C60 in which the C60 moiety is
linked with porphyrin moieties through the benzene ring at the ortho, meta, and para positions, respectively.
The charge transfer (CT) bands are observed for MP-D-C60 and MP-O-C60, whereas no CT band is seen for
MP-M-C60 and MP-P-C60. Time-resolved absorption spectral measurements indicate that the photoexcitation
of ZnP-D-C60 in benzonitrile (PhCN) results in formation of the exciplex, which decays to the ground state
without forming the charge-separated state. The strong interaction between the ZnP and the C60 moieties due
to the short linkage distance in ZnP-D-C60 as indicated by the observation of the strongest CT band at the
ground state results in formation of the exciplex. The energy of the exciplex is lower than that of the charge-
separated state even in a polar solvent such as PhCN. In contrast, the photoexcitation of the dyad with longer
linkage, ZnP-O-C60, in PhCN results in formation of the charge-separated state via the exciplex formation,
which is higher in energy than the charge-separated state. The photodynamics of exciplex formation of
porphyrin-C60-linked dyads with a short linkage is characterized by the extremely fast formation rate from
the singlet excited states of porphyrins involving both the second and first excited states due to the interaction
between the porphyrin and C60 moieties, which are placed at close proximity. In the case of MP-D-C60, the
exciplex formation from the first singlet excited state of MP occurs at an ultrafast time scale with a time
constant of 160 fs and that from the second singlet excited state occurs faster with a time constant less than
50 fs.
Introduction
lar, the combination of porphyrins and fullerenes has been
employed to attain long-lived photoinduced charge-separated
During the past half-century, the field of electron-transfer
chemistry has undergone a remarkable expansion by the detailed
analytic theory developed by R. A. Marcus1 and also by
introduction of new technology such as lasers that have
expanded the electron-transfer systems that could be studied,
extending to ultrafast reactions in even the femtosecond
regime.2-6 Electron transfer plays a pivotal role not only in
chemical processes but also in biological redox processes that
are essential for life, such as photosynthesis and respiration.2,7
Extensive efforts have thereby been devoted toward the prepara-
tion of electron donor-acceptor linked-compounds to construct
efficient photoinduced electron-transfer systems.8-15 In particu-
(CS) states in high quantum yields, as achieved in the
photosynthetic reaction center.14-17 Porphyrins contain an
extensively conjugated two-dimensional π system, which is
suitable for efficient electron transfer because the uptake or
release of electrons results in minimal structural change upon
electron transfer.18 On the other hand, fullerenes contain an
extensively conjugated three-dimensional π system, which is
also suitable for efficient electron transfer.14-17,19 Rates of
electron transfer of a variety of electron donor-acceptor systems
can be well predicted in light of the Marcus theory of electron
transfer, once the fundamental electron-transfer properties of
electron donors and acceptors such as the one-electron redox
potentials and the reorganization energies of electron transfer
are determined.1,20 In these cases, the interaction between the
donor and acceptor moieties is minimized when the type of
electron transfer is classified as an outer-sphere electron
transfer.1,20 When the C60 moiety is folded onto the porphyrin
* To whom correspondence should be addressed. E-mail addresses:
nikolai.tkachenko@tut.fi; fukuzumi@ap.chem.eng.osaka-u.ac.jp; imahori@
scl.kyoto-u.ac.jp; helge.lemmetyinen@tut.fi.
† Osaka University.
‡ Tampere University of Technology.
University of Tsukuba.
§ Kyoto University.
10.1021/jp035412j CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/25/2003

Ultrafast Photodynamics of Exciplex Formation
J. Phys. Chem. A, Vol. 107, No. 42, 2003 8835
CHART 1
plane, the distance between the C60 and the porphyrin moieties
(ZnP-O34-C60 center-to-center distance (Rcc) ) 7.6 Å) is close
enough to observe the charge-transfer (CT) interaction at both
the ground and excited state as indicated by the CT absorption
and emission bands,21 which are not observed for other
porphyrin-fullerene dyads in which the porphyrin and the C60
were separated apart in longer distances (Rcc ) 18.6, 14.4, and
12.5 Å).22 This indicates that the close contact between the
porphyrin plane and the C60 moiety is essential for the
observation of the CT absorption and emission bands.
the stronger interaction between the donor and acceptor moieties
in the exciplex.26
In contrast to such extensive studies on intermolecular
formation of exciplexes and radical ion pairs, there have been
few studies on intramolecular formation of exciplexes in
comparison with the electron-transfer quenching of the excited
states in donor-acceptor linked systems.27 The ultrafast pho-
todynamics of donor-acceptor-linked dyads including exciplex
formation and decay and the relation with the charge-separated
state has yet to be scrutinized.27
There have been a number of examples in which charge-
transfer excitation of electron donor-acceptor (EDA) complexes
resulted in formation of exciplexes, which exhibit high degrees
of charge transfer similar to the radical ion pairs.23-25 The radical
ion pair is also formed by electron-transfer quenching of the
excited state of an electron donor or acceptor molecule in the
EDA complex.23-25 The important difference between the
exciplex formed by charge-transfer excitation and the radical
ion pair formed by electron-transfer quenching is their relaxation
dynamics.26 The back electron transfer in the exciplex is
generally much faster than that in the radical ion pair formed
by electron-transfer quenching of the excited state because of
We report herein the first comprehensive study on the ultrafast
photodynamics of porphyrin-fullerene dyads in which the
distance between the porphyrin and C60 moieties is varied
systematically. The structures of the porphyrin-fullerene dyads
examined in this study are shown in Chart 1, in which the dyads
are named as ZnP-D-C60, H2P-D-C60, ZnP-O-C60, H2P-O-C60,
ZnP-M-C60, H2P-M-C60, ZnP-P-C60, and H2P-P-C60. Reference
compounds (ZnP-D-ref, H2P-D-ref, ZnP-ref, H2P-ref, and C60-
ref) are also given in Chart 1. In the ZnP-D-C60 and H2P-D-C60
dyads, the C60 moiety is directly connected with the porphyrin
ring at the meso position. The other dyads have the linkage
between the C60 and porphyrin moieties through the benzene

8836 J. Phys. Chem. A, Vol. 107, No. 42, 2003
Tkachenko et al.
SCHEME 1
ring at the ortho, meta, and para positions. The photodynamics
of ZnP-P-C60 and H2P-P-C60 dyads have been reported previ-
ously and compared with the results in the present study.27 The
ultrafast dynamics of these porphyrin-fullerene dyads are
reported using fluorescence up-conversion and pump-probe
transient absorption techniques with time resolutions of ca. 100
fs. The decay-associated spectra, which are employed to
reconstruct the sample transient absorption with compensated
probe pulse dispersion, allow us to examine the dynamic
behavior of the exciplex formation and decay in relation with
the dynamics of the formation and decay of the charge-separated
state. Thus, the present study provides valuable insight into the
relation between the exciplex and the charge-separated state in
intramolecular electron transfer in the donor-acceptor linked
systems.
Compound 1. A solution of meso-unsubstituted dipyr-
romethane29 (2.31 g, 15.8 mmol), 3,5-di-tert-butylbenzalde-
hyde22 (5.86 g, 26.9 mmol), and 3,5-di-tert-butylphenyl-
substituted dipyrromethane30 (4.49 g, 13.41 mmol) in CHCl3
(2200 mL) was degassed by bubbling with nitrogen for 30 min.
Then, trifluoroacetic acid was added (3.0 mL, 39.2 mmol). The
solution was stirred for 2 h at room temperature under N2. To
the reddish black reaction mixture was added p-chloranil (12.4
g, 50.4 mmol), and the resulting mixture was concentrated. Flash
column chromatography on silica gel with CHCl3 as the eluent
and preparative SEC column chromatography with toluene as
the eluent gave 1 as a purple solid (1.56 g, 1.82 mmol, 11.5%).
1
Mp > 300 °C; H NMR (300 MHz, CDCl3) δ 10.2 (s, 1H),
9.33 (d, J ) 5 Hz, 2H), 9.05 (d, J ) 5 Hz, 2H), 8.95 (d, J )
5 Hz, 2H), 8.91 (d, J ) 5 Hz, 2H), 8.11 (d, J ) 2 Hz, 4H),
8.06 (d, J ) 2 Hz, 2H), 1.55 (s, 54H), -2.90 (s, 2H); MALDI-
TOFMS (positive mode) m/z 876 (M + H+).
Experimental Section
General. Melting points were recorded on a Yanagimoto
micro-melting point apparatus and not corrected. 1H NMR
spectra were measured on a JEOL JNM-AL300. Matrix-assisted
laser desorption/ionization (MALDI) time-of-flight mass spectra
(TOF) were measured on a Kratos Compact MALDI I (Shi-
madzu). Steady-state absorption spectra in the visible and near-
IR regions were measured on a Shimadzu UV-3100PC.
Materials. All solvents and chemicals were of reagent grade
quality, obtained commercially and used without further puri-
fication except as noted below. Tetrabutylammonium hexafluo-
rophosphate used as a supporting electrolyte for the electro-
chemical measurements was obtained from Tokyo Kasei
Organic Chemicals. Benzene, toluene, THF, benzonitrile, and
DMF were purchased from Wako Pure Chemical Ind., Ltd., and
purified by successive distillation over calcium hydride.28 Thin-
layer chromatography (TLC) and flash column chromatography
were performed with Art. 5554 DC-Alufolien Kieselgel 60 F254
(Merck), and Fujisilicia BW300, respectively. Preparative-scale
size-exclusive chromatography (SEC) was performed using
BioRad Bio-Beads SX-1 (Bio-Rad).
Compound 2. A mixture of 1 (1.40 g, 1.60 mmol) and nickel-
(II) acetate tetrahydrate saturated-methanol solution (20 mL)
in CHCl3 (100 mL) was heated at reflux for 20 h. The solution
was washed with water, dried over anhydrous sodium sulfate,
and filtered, and the solvent was completely removed. Then
the residue was passed through short silica gel using CHCl3
eluent. Porphyrin 2 was isolated as a purple-red solid (1.40 g,
1.50 mmol, 94%). Mp > 300 °C; 1H NMR (300 MHz, CDCl3)
δ 9.83 (s, 1H), 9.13 (d, J ) 5 Hz, 2H), 8.93 (d, J ) 5 Hz, 2H),
8.83 (s, 4H), 7.89 (dd, J ) 7 Hz, J ) 2 Hz, 6H), 7.72 (dt, J )
8 Hz, J ) 2 Hz, 3H), 1.49 (s, 54H); MALDI-TOFMS (positive
mode) m/z 932 (M + H+).
Compound 3. POCl3 (12 mL, 128 mmol) was added dropwise
to DMF (10.3 mL, 128 mmol) at 0 °C and stirred for 30 min at
room temperature. Then, 2 (1.40 g, 1.50 mmol) in 1,2-
dichloroethane (470 mL) was poured at 60 °C and stirred for 1
h. Sodium acetate aqueous solution was added dropwise to the
mixture and stirred for 2 h. The reaction mixture was extracted
with CHCl3. The organic phase was washed with water and
dried over Na2SO4. After evaporation, the residue was purified
Synthesis of MP-D-C60. The synthetic procedures of MP-
D-C60 are summarized in Scheme 1.

Ultrafast Photodynamics of Exciplex Formation
J. Phys. Chem. A, Vol. 107, No. 42, 2003 8837
by column chromatography (SiO2, 30% CHCl3/hexane) to give
3 as a purple-red solid (1.10 g, 1.15 mmol, 76%). Mp > 300
°C; 1H NMR (300 MHz, CDCl3/DMSO ) 5:2) δ 12.0 (s, 1H),
9.79 (d, J ) 5 Hz, 2H), 8.88 (d, J ) 5 Hz, 2H), 8.69 (d, J )
5 Hz, 2H), 8.62 (d, J ) 5 Hz, 2H), 7.80 (d, J ) 1 Hz, 6H),
7.71 (dt, J ) 8 Hz, J ) 2 Hz, 3H), 1.47 (s, 54H); MALDI-
TOFMS (positive mode) m/z 960 (M + H+).
Compound 4. A solution 3 (1.00 g, 1.04 mmol) in concen-
trated sulfuric acid (98%, 10 mL) and TFA (10 mL) was stirred
for 30 min. Then NaHCO3(aq) (0.1 M) was poured onto the
solution at 0 °C. The resulting mixture was poured into CHCl3
(100 mL), and the organic phase was separated. The organic
layer was washed with water, dried over anhydrous sodium
sulfate, and filtered, and the solvent was removed. The residue
was purified by column chromatography over silica using a
benzene-hexane mixture (1:2) as an eluent to give 4 as a purple
solid (0.75 g, 0.831 mmol, 80%). Mp > 300 °C; 1H NMR (300
MHz, CDCl3) δ 12.51 (s, 1H), 10.01 (d, J ) 5 Hz, 2H), 9.00
(d, J ) 5 Hz, 2H), 8.80 (d, J ) 5 Hz, 2H), 8.73 (d, J ) 5 Hz,
2H), 8.02 (dd, J ) 6 Hz, J ) 2 Hz, 6H), 7.81 (dt, J ) 9 Hz, J
) 2 Hz, 3H), 1.53 (s, 54H), -1.83 (s, 2H); MALDI-TOFMS
(positive mode) m/z 904 (M + H+).
Spectral Measurements. Steady-state fluorescence spectra
were measured on a Fluorolog 3 spectrofluorimeter (ISA Inc.)
equipped with a cooled IR-sensitive photomultiplier (R2658,
Hamamatsu Inc.). The excitation wavelength was at the
maximum of the Soret band (410-430 nm region). The emission
spectra were recorded in the wavelength range from 600 to 1000
nm with the detection monochromator slits set to 2 nm and the
accumulation time set to 1 s. The background counts were
subtracted, and the spectra were corrected using the correction
function supplied with the instrument (accounting for both
photomultiplier quantum efficiency and monochromator through-
put spectra). Ultrafast fluorescence decays were measured by
an up-conversion method as described previously.32 The instru-
ment (FOG100, CDP, Moscow, Russia) utilizes the second
harmonic (420 nm) of 50 fs pulsed Ti:sapphire laser (TiF50,
CDP, Moscow, Russia) pumped by an Ar ion laser (Innova
316P, Coherent). The samples were placed into rotating disk-
shaped 1 mm cuvettes. Typical time resolution for the instrument
was 150 fs (fwhm). Subnanosecond emission decays were
studied using a time-correlated single-photon-counting instru-
ment described elsewhere.32 The excitation wavelength was 590
nm, and the time resolution was ca. 100 ps. Femtosecond to
picosecond time-resolved absorption spectra were collected
using a pump-probe technique as described elsewhere.32b The
femtosecond pulses of the Ti:sapphire generator were amplified
by using a multipass amplifier (CDP-Avesta, Moscow, Russia)
pumped by a second harmonic of the Nd:YAG Q-switched laser
(model LF114, Solar TII, Minsk, Belorussia). The amplified
pulses were used to generate second harmonic (420 nm) for
sample excitation (pump beam) and white continuum for time-
resolved spectrum detection (probe beam). The samples were
placed into 1 mm rotating cuvettes, and averaging of 100 pulsed
at 10 Hz repetition rate was used to improve signal-to-noise
ratio. The wavelength range for a single measurement was 227
nm, and typically three regions were studied, 450-670, 550-
780, and 820-1050 nm. Typical response time of the instrument
was 150 fs (fwhm). A global multiexponential fitting procedure
was applied to process the data. The procedure takes into
account the instrument time response function and the group
velocity dispersion of the white continuum and allowed calcula-
tion of the decay time constants and dispersion-compensated
transient absorption spectra.
H2P-D-C60. Porphyrin-linked fullerene H2P-D-C60 was ob-
tained by the 1,3-dipolar cycloaddition of azomethine ylide to
fullerene. A mixture of 4 (133 mg, 0.13 mmol), fullerene (0.74
g, 7 equiv), and N-methylglycine (0.91 g, 60 equiv) in dry
toluene (80 mL) was stirred for 30 min under N2 and heated at
reflux for 70 h. After evaporation, the residue was purified by
column chromatography over silica using a benzene-hexane
mixture (1:1) as an eluent to give H2P-D-C60 as a dark violet
1
solid (46 mg, 0.0279 mmol, 20%). Mp > 300 °C; H NMR
(300 MHz, CDCl3) δ 11.58 (d, J ) 5 Hz, 1H), 9.92 (d, J ) 5
Hz, 1H), 8.96 (d, J ) 5 Hz, 1H), 8.93 (d, J ) 5 Hz, 1H), 8.88
(d, J ) 5 Hz, 1H), 8.84 (d, J ) 5 Hz, 1H), 8.82 (d, J ) 5 Hz,
1H), 8.78 (d, J ) 5 Hz, 1H), 8.13 (s, 1H), 8.05 (s, 1H), 8.05 (s,
1H), 8.03 (s, 1H), 7.99 (s, 1H), 7.81 (s, 2H), 7.78 (t, J ) 2 Hz,
1H), 7.78 (t, J ) 2 Hz, 1H), 7.75 (t, J ) 2 Hz, 1H), 5.55 (d, J
) 10 Hz, 1H), 4.85 (d, J ) 10 Hz, 1H), 3.13 (s, 3H), 1.56 (s,
9H), 1.53 (s, 9H), 1.50 (s, 18H), 1.49 (s, 18H), -2.53 (s, 2H);
MALDI-TOFMS (positive mode) m/z 1653 (M + H+).
ZnP-D-C60. A saturated methanol solution of Zn(OAc)2 (10
mL) was added to a solution of H2P-D-C60 (20 mg, 0.012 mmol)
in CHCl3 (50 mL) and refluxed for 12 h. 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-methanol gave
ZnP-D-C60 as a dark violet solid (98% yield, 20 mg, 0.012
Electrochemical Measurements. The differential pulse vol-
tammetry measurements were performed on a BAS 50W
electrochemical analyzer in a deaerated PhCN solution contain-
ing 0.10 M n-Bu4NPF6 as a supporting electrolyte at 298 K (10
mV s-1). The platinum working electrode was polished with
BAS polishing alumina suspension and rinsed with acetone
before use. The counter electrode was a platinum wire. The
measured potentials were recorded with respect to a Ag/AgNO3
(0.01 M) reference electrode. Ferrocene/ferricenium was used
as an external standard. All potentials (vs Ag/Ag+) were
converted to values vs SCE by adding 0.29 V.33 All electro-
chemical measurements were carried out under an atmospheric
pressure of argon.
1
mmol). Mp > 300 °C; H NMR (300 MHz, CDCl3) δ 11.80
(d, J ) 5 Hz, 1H), 10.13 (d, J ) 5 Hz, 1H), 9.11 (d, J ) 5 Hz,
1H), 9.06 (d, J ) 5 Hz, 1H), 8.96 (d, J ) 5 Hz, 1H), 8.94 (d,
J ) 5 Hz, 1H), 8.84 (d, J ) 5 Hz, 1H), 8.83 (d, J ) 5 Hz, 1H),
8.15 (s, 1H), 8.07 (s, 1H), 8.06 (s, 1H), 8.01 (s, 2H), 7.92 (s,
1H), 7.80 (s, 1H), 7.80 (t, J ) 2 Hz, 1H), 7.80 (t, J ) 2 Hz,
1H), 7.80 (t, J ) 2 Hz, 1H), 5.55 (d, J ) 10 Hz, 1H), 4.86 (d,
J ) 10 Hz, 1H), 3.13 (s, 3H), 1.54 (s, 18H), 1.51 (s, 18H),
1.51 (s, 18H); MALDI-TOFMS (positive mode) m/z 1715 (M
+ H+).
Theoretical Calculations. Theoretical calculations were
performed on a COMPAQ DS20E computer. The PM3 Hamil-
tonian was used for the semiempirical MO calculations.34 Final
geometries and energetics were obtained by optimizing the total
molecular energy with respect to all structural variables. The
heats of formation (∆Hf) were calculated with the restricted
Hartree-Fock (RHF) formalism.
MP-O-C60 and MP-M-C60 and Reference Compounds. The
synthetic procedures and characterization of MP-O-C60, MP-
M-C60,27 and MP-D-ref (MP ) ZnP or H2P) are given in
Supporting Information. ZnP-ref,31 H2P-ref,31 and C60-ref17 were
prepared by following the same procedures as described
previously.

8838 J. Phys. Chem. A, Vol. 107, No. 42, 2003
Tkachenko et al.
Figure 2. Absorption spectra (a) of ZnP-D-C60 (solid line), C60-ref
(dashed line), and ZnP-D-ref (dotted line) in benzene (the CT absorption
(dash-dotted line) of ZnP-D-C60 has been obtained by subtracting the
absorptions of ZnP-D-ref and C60-ref from the dyad absorption) and
(b) of ZnP-O-C60 (solid line), C60-ref (dashed line), and ZnP-ref (dotted
line) in toluene (the CT absorption (dash-dotted line) of ZnP-O-C60
has been obtained by subtracting the absorptions of ZnP-ref and C60-
ref from the dyad absorption).
of ZnP-D-C60 is broader and the Soret band is red-shifted as
compared with that of the ZnP-D-ref reference porphyrin as
shown in Figure 2. Enlarged absorption spectra (650-800 nm)
of ZnP-D-C60, ZnP-O-C60, and the reference compounds (ZnP-
D-ref, ZnP-ref, and C60-ref) in nonpolar solvents are shown in
insets of Figure 2, in which the CT absorption spectra are
obtained by subtracting the spectra of porphyrin reference and
fullerene reference from those of porphyrin-fullerene dyads.
Thus, the ground-state interaction is observed for ZnP-D-C60
and ZnP-O-C60, whereas no such interaction is detectable for
ZnP-M-C60 and ZnP-P-C60. This is ascribed to the shorter
distance between the porphyrin and C60 moieties for the former
cases as compared with the latter.
Figure 1. 1H NMR spectrum (300 Hz) of H2P-D-C60 in CDCl3 at room
temperature.
Results and Discussion
Ground-State Interaction between Porphyrin and C60 in
Dyads. The synthesized dyads were well-characterized by
1
spectroscopic analysis including H NMR and MALDI-TOF
mass spectra (see Experimental Section). The 1H NMR spectrum
of H2P-D-C60 shows that all eight â protons are magnetically
unequivalent and two of them are shifted to downfield as shown
in Figure 1. These two doublets (a and h in Figure 1) can be
assigned to the â protons vicinal to the meso carbon connected
to the pyrrolidine ring. The large downfield shifts for the two
of eight â protons are due to the deshielding effect that results
from C60 π-electrons. The methyne proton of the pyrrolidine
ring (s in Figure 1) also exhibits a downfield shift by about 2
ppm with respect to those of the usual pyrrolidine ring owing
to the ring current effect of the porphyrin.
The calculated structures of H2P-D-C60 and H2P-O-C60 are
shown in Figure 3 for comparison. The center-to-center distance
between the porphyrin and the C60 moieties increases in the
order: H2P-D-C60 (8.9 Å) < H2P-O-C60 (9.7 Å) < H2P-M-C60
(10.9 Å) < H2P-P-C60 (12.6 Å).
Excited-State Interaction between Porphyrin and C60 in
Dyads. The excited-state interaction between porphyrin and C60
is expected to be the strongest for ZnP-D-C60, as is the case of
the ground-state interaction (vide infra). Figure 4 shows the
transient absorption component and time-resolved spectra of
ZnP-D-C60 in toluene. The transient absorption spectra show
broad and featureless absorption bands in the 600-800 nm
region extending over 1000 nm and in time scale ranging from
the resolution limit of the instrument (∼100 fs) to 1 ns (the
longest delay available). Such broad and featureless absorption
bands extending over 1000 nm are characteristic of the exciplex
in which the porphyrin excited state forms the complex with
the C60 moiety.27 The first detected state has lifetime of ∼300
All dyads were studied in two solvents: toluene and ben-
zonitrile. In ZnP-M-C60, H2P-M-C60, ZnP-P-C60, and H2P-P-
C60, the absorption spectra of the dyads are well reproduced as
a sum of the spectra of two chromophores: porphyrin and
fullerene. This indicates that there are no observable ground-
state interactions between the chromophores.
In contrast, a considerable ground-state interaction of the
porphyrin and fullerene moieties is seen in the absorption
spectrum of ZnP-D-C60 and ZnP-O-C60 in nonpolar solvents.
The same phenomenon was reported for the zinc derivative of
the dyad ZnP-O34-C60, in which the ZnP and the C60 moieties
are in close proximity (Rcc ) 7.6 Å).17 The absorption spectrum

Ultrafast Photodynamics of Exciplex Formation
J. Phys. Chem. A, Vol. 107, No. 42, 2003 8839
Figure 3. Structures of H2P-D-C60, H2P-O-C60, H2P-M-C60, and H2P-P-C60 obtained by the PM3 calculations. The center-to-center distances (edge-
to-edge distances), Rcc (Ree), of H2P-D-C60, H2P-O-C60, H2P-M-C60, and H2P-P-C60 are 8.9 (2.6), 9.7 (3.4), 10.9 (3.7), and 12.6 Å (5.2 Å), respectively.
Figure 4. Transient absorption component spectra of ZnP-D-C60 in
toluene obtained from global three-exponential fit of the data (bottom)
and corresponding reconstructed time-resolved spectra (top). The fitted
time constants and selected delay times are indicated on the plots.
Figure 5. Steady-state emission spectrum of ZnP-D-C60 in toluene.
Inset shows the emission decay at 730 nm (the major decay lifetime is
1.6 ns).
fs and will be discussed later. The second decay component,
16 ps, has only minor impact on the absorption spectrum in
visible and IR regions. Overall lifetime of the exciplex is longer
than the time scale measured, that is, >1 ns.
The exciplex emission of ZnP-D-C60 in toluene is observed
as a steady-state emission at λmax ) 730 nm when excited at
430 nm as shown in Figure 5. Subnanosecond emission decays
were measured using a time-correlated single-photon-counting
instrument (see Experimental Section). At wavelengths longer
than 720 nm, the decay profiles were almost monoexponential
with dominating time constant of 1.6 ns (as shown in the inset
of Figure 5), which is the exciplex lifetime for ZnP-D-C60 in
toluene.
In a much more polar solvent (PhCN) than toluene, virtually
the same transient absorption spectra are obtained as the case
in toluene (compare the spectra in Figure 6 with those in Figure
4). The broad and featureless absorption bands in the 600-800
nm region extending over 1000 nm, which are characteristic of
the exciplex, are also observed in Figure 6, and no sharp
absorption band at 640 nm35,36 due to ZnP•+ is detected.
Figure 6. Transient absorption component spectra of ZnP-D-C60 in
PhCN obtained from global three-exponential fit of the data (bottom)
and corresponding reconstructed time-resolved spectra (top). The fitted
time constants and selected delay times are indicated on the plots.
The dynamics of the exciplex formation and decay at 690
nm is shown in Figure 7. The initial instantaneous rise (0-0.2
ps) is followed by a slower rise (0.5-5 ps), as seen in the inset
of Figure 7. The initial fast formation of the exciplex is

8840 J. Phys. Chem. A, Vol. 107, No. 42, 2003
Tkachenko et al.
Figure 7. Transient absorption time profile for ZnP-D-C60 in PhCN
at 690 nm, which shows formation and decay of the exciplex.
Figure 9. Transient absorption component spectra of ZnP-O-C60 in
PhCN obtained from global three-exponential fit of the data (bottom)
and corresponding reconstructed time-resolved spectra (top). The fitted
time constants and selected delay times are indicated on the plots.
Figure 10. Transient absorption time profile for ZnP-O-C60 in PhCN
at 630 nm.
SCHEME 2
Figure 8. Porphyrin fluorescence decays at 600 nm for ZnP-D-C60 in
(a) toluene and (b) PhCN measured by up-conversion method (dots).
The solid lines show fits of the data with decay time of 160 fs and
instrument response time of 150 fs.
consistent with the fast emission decay of the porphyrin locally
excited singlet state of ZnP-D-C60 in PhCN (lifetime of 160 fs)
measured by the up-conversion method (Figure 8). The exciplex
decays with a lifetime of 50 ps to the ground state (Figure 7),
which is much shorter than that in toluene.
state of ZnP (2SZnP*-D-C60), because the direct photoexcitation
of the CT band of ZnP-D-C60 is highly unlikely because of the
much smaller CT absorption than the Soret band at the excitation
wavelength (415 nm).41 The photodynamics of the exciplex
formation and decay is summarized in Scheme 2.
The primary excited state of ZnP-D-C60 is the second singlet
excited state of ZnP (2SZnP*-D-C60).37-39 The internal conver-
sion of the second singlet excited state to the first singlet excited
state is known to occur on a time scale of 1 ps,3b,37-39 which is
certainly much slower than the fast emission rise and decay of
the 1SZnP* fluorescence in Figure 8 (160 fs). This indicates
that the 2SZnP*-D-C60 state is relaxed to the exciplex (ZnP-D-
C60)* in competition to the first singlet excited state (1SZnP*-
D-C60), which is also converted to the exciplex. The energy
transfer from 1SZnP* may also occur to the C60 moiety to
produce (ZnP-D-1SC60*).27 The subsequent slower rise in the
absorption due to the exciplex is thereby ascribed to the exciplex
formation from ZnP-D-1SC60*.40 In addition, the instant appear-
ance of the exciplex upon photoexcitation (most pronounced
in toluene, see time-resolved spectrum at t ) 0 ps, Figure 4)
indicates that the exciplex is also formed from the second excited
A slight elongation of the distance between the ZnP and C60
moieties in the dyad results in a change in the energy balance
between the exciplex and the charge-separated state. The
transient absorption spectra of ZnP-O-C60 in PhCN are shown
in Figure 9, in which the sharp absorption band at 640 nm due
to the ZnP•+ moiety is observed together with the absorption
band at 1000 nm42-44 due to the C60 for the component that
•-
has the longest lifetime (60 ps). It is worthy of notice that the
transient absorption spectra of ZnP-O-C60 and ZnP-D-C60 at a
short delay time (compare spectra at t ) 1 ps in Figures 6 and
9) are virtually identical to each other. The difference in spectra
appears at the completion of the picosecond processes, when
the charge-separated state of ZnP-O-C60 is formed.
The time profile of the absorbance at 630 nm is shown in
Figure 10. The initial instant rise (0-0.2 ps) is followed by a

Ultrafast Photodynamics of Exciplex Formation
J. Phys. Chem. A, Vol. 107, No. 42, 2003 8841
Figure 11. Fluorescence decay of ZnP-O-C60 in PhCN measured with
up-conversion method (dots). The solid line shows a fit of the data
where the fast decay lifetime is 180 fs and the instrument response
time is 160 fs.
Figure 12. Transient absorption component spectra of ZnP-O-C60 in
toluene obtained from global three-exponential fit of the data (bottom)
and corresponding reconstructed time-resolved spectra (top).
SCHEME 3
Figure 13. Transient absorption component spectra of ZnP-M-C60 in
PhCN obtained from global three-exponential fit of the data (bottom)
and corresponding reconstructed time-resolved spectra (top).
slower rise (τ2 ) 6 ps), as seen in the inset of Figure 10. The
transient absorption spectrum at 1 ps delay (Figure 9) indicates
that at this time the exciplex is already formed. The initial
ultrafast formation of the exciplex is consistent with the ultrafast
emission decay of ZnP-O-C60 in PhCN (lifetime of 180 fs)
measured by the up-conversion method (Figure 11). The mirror
image between the decay component spectra of 6 and 60 ps in
Figure 9 indicates that the charge-separated state (ZnP•+-O-
C60•-) is formed with a time constant of 6 ps and has lifetime
of 60 ps.
The photodynamics of ZnP-O-C60 in PhCN is summarized
in Scheme 3. The second singlet excited state is formed upon
photoexcitation and is relaxed to the exciplex and the first singlet
excited state, which is also converted to the exciplex with the
lifetime of 180 fs. The exciplex formation from the second
singlet excited state occurs within instrumental resolution (<50
fs). The exciplex is then converted to the charge-separated state
with the time constant of 6 ps. The charge-separated state decays
to the ground state with the lifetime of 60 ps.
When benzonitrile is replaced by toluene, the charge-separated
state is not observed any more and instead only the exciplex is
formed, as shown in Figure 12, in which the transient absorption
component spectra of ZnP-O-C60 in toluene have only broad
and featureless absorption bands in the 600-800 nm region
extending over 1000 nm, which are characteristic of the exciplex.
The transient absorption spectra of ZnP-D-C60 and ZnP-O-C60
in toluene are very much alike, and the photodynamics of ZnP-
O-C60 can be well presented by Scheme 2.
Figure 14. Fluorescence decay of ZnP-M-C60 in PhCN measured with
up-conversion method (dots). The solid line shows a monoexponential
fit of the data with time constant of 0.9 ( 0.2 ps (the instrument
response time is 160 fs).
of the charge-separated state is 90 ps, and the time constant for
the transition from exciplex to charge-separated state is 11 ps.
Also the lifetime of the singlet excited state of porphyrin
chromophore is 0.9 ps for ZnP-M-C60, as indicated by up-
conversion measurements (Figure 14). In contrast to the results
in PhCN, only a long-lived exciplex was observed for ZnP-M-
C60 in toluene, similar to that of ZnP-O-C60 and ZnP-D-C60 (see
Supporting Information).
ZnP-P-C60 dyad studied previously fits perfectly well into
the observed tendency in this study.27 In PhCN, the charge-
separated state is formed via the exciplex, that is, the energy of
the charge-separated state is lower than that of the exciplex,
whereas in toluene only the exciplex was observed. Also the
time constants of the reactions are the slowest for ZnP-P-C60
as compared to other dyads involved in this study.
Further elongation of the distance between the donor and
acceptor results in a further stabilization of the charge-separated
state relative to the exciplex. The transient absorption measure-
ments of ZnP-M-C60 in PhCN (Figure 13) show all key features
observed for ZnP-O-C60 in PhCN (Figure 9), but the time
constants of the reactions are gradually increased. The lifetime

8842 J. Phys. Chem. A, Vol. 107, No. 42, 2003
Tkachenko et al.
TABLE 1: Rate Constants for Exciplex Formation and Decay and Decay Rate Constants of Charge-Separated States (When
Observed)a
dyad
solvent
k
p2x,b s-1
kpx,c s-1
kcx, s-1
kxg, s-1
kxi, s-1
kig, s-1
ZnP-D-C60
toluene
PhCN
toluene
PhCN
toluene
PhCN
toluene
PhCN
toluene
PhCN
toluene
PhCN
toluene
PhCN
g2 × 1013
g2 × 1013
g2 × 1013
g2 × 1013
∼6 × 1012
∼7 × 1012
∼6 × 1012
∼6 × 1012
1.1 × 1012
1.1 × 1012
5 × 1011
6.3 × 1010
2.3 × 1011
1.7 × 1010
∼3 × 1010
0.6 × 109
2 × 1010
6.3 × 108
6.3 × 108
7.7 × 108
ZnP-O-C60
ZnP-M-C60
ZnP-P-C60
H2P-D-C60
H2P-O-C60
H2P-M-C60
H2P-P-C60
1.7 × 1011
9.1 × 1010
8.3 × 1010
1.7 × 1010
1.1 × 1010
7.7 × 109
5 × 1011
5 × 1012
6 × 108
5 × 1012
∼ 5 × 1010
5 × 109
5 × 1012
7 × 1012
4 × 1010
1.5 × 1010
2.9 × 109
1.1 × 109
1.1 × 109
1.3 × 1011
1 × 1011
∼1010 d
toluene
3 × 1010
4 × 109 d
PhCNe
a The rate constants are marked according to notation used in Schemes 2 and 3. b Two pathways contribute to the relaxation of the second
excited state: internal conversion (kic) and exciplex formation (kp2x). Assuming kic ≈ 1012 s-1 37-39
d The exciplex decays to fullerene singlet excited state. e Exciplex was not observed.
,
the fast component is mainly due to exciplex
formation. c A competing process is intermolecular energy transfer (kpc), which is a minor relaxation channel and has been neglected in calculations.
The observed trends in the exciplex and charge-transfer
photodynamics indicate that both the distance between the ZnP
and C60 moieties and solvent polarity are important in determin-
ing the energy balance between the charge-separated state and
the exciplex.45 In the case of ZnP-D-C60 in PhCN, the strong
interaction between the ZnP and the C60 moieties due to the
short linkage distance as indicated by the observation of the
strongest CT band in the ground state results in lowering the
energy of the exciplex, which becomes lower than the charge-
separated state even in a polar solvent such as PhCN (Scheme
2). Already in the case of ZnP-O-C60, the interaction in the
exciplex is not strong enough to make the energy lower than
that of the charge-separated state, which is stabilized by polar
solvent molecules (Scheme 3). Virtually the same results are
obtained for the dyads with a longer linkage (ZnP-M-C60 and
ZnP-P-C60), which afford the charge-separated states via the
exciplex formation in PhCN. In toluene, the solvent stabilization
of the charge-separated state is not as strong as that in PhCN
and the charge-separated state becomes higher in energy than
the exciplex.
Qualitatively similar tendency was observed for free-base
dyads. The transient absorption spectra of H2P-D-C60 and H2P-
O-C60 in PhCN are presented in Figure 15. In the case of H2P-
D-C60 only a broad and featureless absorption band of the
exciplex is seen in the red and infrared regions. In contrast, the
transient data for H2P-O-C60 in PhCN shows reshaping of the
spectra due to the transition from the exciplex to the charge-
separated state. As in the case of zinc porphyrin-C60 dyads,
the charge-separated state was not observed in toluene for free-
base porphyrin-C60 dyads. Moreover, for H2P-P-C60 in toluene,
there was no direct evidence of exciplex formation,27 and for
H2P-M-C60, the exciplex energy appeared to be higher than the
energy of the singlet excited fullerene chromophore, which
resulted in efficient energy transfer from porphyrin to fullerene
mediated by the exciplex (see Supporting Information for
details). Unlike for zinc porphyrins, internal conversion of the
second excited state to the first excited state is faster than 100
fs. Therefore the formation of exciplex directly from the
porphyrin second singlet excited state should be relatively
inefficient for free-base porphyrin-C60 dyads but cannot be
excluded totally.
Figure 15. Results of the pump-probe studies of (a) H2P-D-C60 and
(b) H2P-O-C60 in PhCN. The lower plots show absorption component
spectra obtained from the global three-exponential fits, and the upper
plots show corresponding reconstructed time-resolved spectra.
photodynamics of MP-D-C60, MP-O-C60, MP-M-C60, and MP-
P-C60 dyads (M ) Zn and 2H) in PhCN and toluene. The results
are summarized in Table 1. The ultrafast formation of the
exciplex from the porphyrin second excited state (2SP*) deserves
a special comment. This feature is the most pronounced in
toluene for ZnP-D-C60 and ZnP-O-C60 (and their free-base
analogues), where transient absorption spectra hold the exciplex
character immediately after excitation (spectra at t ) 0 ps in
Figures 4, 12, and 15a). Although for these samples some
emission from the porphyrin first singlet excited state was
observed, the emission intensity was very weak (e.g., compare
Figures 8 and 14) and the lifetime was extremely short (Figures
8 and 11). This means that the quantum yield of the internal
The data of emission decay by the up-conversion methods
and transient absorption spectra afford the rate constants of

Ultrafast Photodynamics of Exciplex Formation
J. Phys. Chem. A, Vol. 107, No. 42, 2003 8843
References and Notes
conversion is reduced gradually and there is an alternative
pathway for the second excited-state relaxation. This alternative
pathway is exciplex formation, as revealed by the transient
spectra at t ) 0 ps (Figures 4 and 12), and because the reaction
was not time-resolved in our measurements, its time constant
is beyond the time resolution of the instrument; it is shorter
than 50 fs.
Another noticeable result is that the rate constants of the
exciplex formation from the porphyrin first singlet excited state
(kpx) are not affected by the solvent polarity but they are very
sensitive to the donor-acceptor distance and mutual orientation.
In terms of center-to-center distance (Rcc), the distance increase
by 3 Å (ZnP-O-C60 vs ZnP-P-C60)46 results in more than 10-
fold decrease in the exciplex formation rate. For comparison,
the ratio of the charge recombination rates (kig) for the same
compounds is only 2.2. A similar trend is seen for the free-
base porphyrin-C60. This sharp difference in distance depend-
ences for the exciplex and charge-separated state indicates that
these two states have rather different electronic configurations,
that is, the corresponding electronic coupling matrix elements
have very different distance/orientation dependence.
It is interesting to note that in PhCN the lifetimes of the
exciplex for ZnP-D-C60 and the charge-separated state for ZnP-
O-C60 are virtually the same: 55 and 60 ps, respectively. ZnP-
D-C60 is characterized by a shorter DA distance as compared
with ZnP-O-C60, and under other equal conditions, the decay
rate of the exciplex of ZnP-D-C60 is expected to be greater than
the charge recombination rate of the charge-separated state of
ZnP-O-C60.47 However, the exciplex results in a smaller
reorganization energy as compared to the charge-separated state,
and accounting for the fact that the charge recombination takes
place in inverted Marcus region, the recombination of a state
with greater reorganization energy (the charge-separated state)
should be faster than that of the exciplex. In this particular case,
these two opposite effects (increase of the rate due to increase
in reorganization energy and decrease of the rate due to distance
increase) may compensate each other resulting in similar
recombination rates.
In conclusion, the photodynamics of exciplex formation of
porphyrin-C60-linked dyads is characterized by the extremely
fast formation from the singlet excited states of porphyrins
involving both the second and first excited states due to the
interaction between the porphyrin and C60 moieties, which are
placed at close proximity. In the case of ZnP-D-C60, which has
the shortest linkage, the strong interaction between the ZnP and
C60 moieties results in lowering the energy of the exciplex,
which becomes lower than the charge-separated state even in a
polar solvent such as PhCN (Scheme 2).
(1) (a) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 155. (b)
Marcus, R. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111. (c) Marcus,
R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265.
(2) (a) Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH:
Weinheim, Germany, 2001. (b) Mataga, N.; Miyasaka, H. Prog. React.
Kinet. 1994, 19, 317. (c) Gauduel, Y. J. Mol. Liq. 1995, 63, 1. (d) Wynne,
K.; Hochstrasser, R. M. AdV. Chem. Phys. 1999, 107, 263. (e) Zhang, J. Z.
J. Phys. Chem. B 2000, 104, 7239.
(3) (a) Mataga, N.; Miyasaka, H. AdV. Chem. Phys. 1999, 107, 431.
(b) Mataga, N.; Chosrowjan, H.; Shibata, Y.; Yoshida, N.; Osuka, A.;
Kikuzawa, T.; Okada, T. J. Am. Chem. Soc. 2001, 123, 12422.
(4) (a) Chiorboli, C.; Rodgers, M. A. J.; Scandola, F. J. Am. Chem.
Soc. 2003, 125, 483. (b) Reid, G. D.; Whittaker, D. J.; Day, M. A.; Creely,
C. M.; Tuite, E. M.; Kelly, J. M.; Beddard, G. S. J. Am. Chem. Soc. 2001,
123, 6953. (c) Kambhampati, P.; Son, D. H.; Kee, T. W.; Barbara, P. F. J.
Phys. Chem. A 2000, 104, 10637.
(5) (a) Wan, C.; Fiebig, T.; Schiemann, O.; Barton, J. K.; Zewail, A.
H. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 14052. (b) Kononov, A. I.;
Moroshkina, E. B.; Tkachenko, N. V.; Lemmetyinen, H. J. Phys. Chem. B
2001, 105, 535. (c) Lewis, F. D.; Wu, T.; Liu, X.; Letsinger, R. L.;
Greenfield, S. R.; Miller, S. E.; Wasielewski, M. R. J. Am. Chem. Soc.
2000, 122, 2889.
(6) (a) Asbury, J. B.; Ghosh, H. N.; Ellingson, R. J.; Ferrere, S.; Nozik,
A. J.; Lian, T. Ultrafast Phenomena XI; Springer Series in Chemical Physics;
Springer: Berlin, Germany, 1998; p 639. (b) Asbury, J. B.; Hao, E.; Wang,
Y.; Ghosh, H. N.; Lian, T. J. Phys. Chem. B 2001, 105, 4545. (c)
Zimmermann, C.; Willig, F.; Ramakrishna, S.; Burfeindt, B.; Pettinger, B.;
Eichberger, R.; Storch, W. J. Phys. Chem. B 2001, 105, 9245.
(7) Kaim, W.; Schwederski, B. Bioinorganic Chemistry: Inorganic
Elements in the Chemistry of Life; Wiley: Chichester, U.K., 1994.
(8) (a) Paddon-Row, M. N. Acc. Chem. Res. 1994. 27, 18. (b) Jordan,
K. D.; Paddon-Row, M. N. Chem. ReV. 1992, 92, 395. (c) Paddon-Row,
M. N. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH:
Weinheim, Germany, 2001; Vol. 3, pp 179-271.
(9) (a) Wasielewski, M. R. In Photoinduced Electron Transfer; Fox,
M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part A, pp 161-
206. (b) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (c) Wasielewski,
M. R.; Wiederrecht, G. P.; Svec, W. A.; Niemczyk, M. P. Sol. Energy Mater.
Sol. Cells 1995, 38, 127.
(10) Verhoeven, J. W. AdV. Chem. Phys. 1999, 106, 603.
(11) (a) Osuka, A.; Mataga, N.; Okada, T. Pure Appl. Chem. 1997, 69,
797. (b) Sun, L.; Hammarstro¨m, L.; Åkermark, B.; Styring, S. Chem. Soc.
ReV. 2001, 30, 36. (c) Scandola, F.; Chiorboli, C.; Indelli, M. T.; Rampi,
M. A. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH:
Weinheim, Germany, 2001; Vol. 3, pp 337-408.
(12) (a) Harriman, A.; Sauvage, J.-P. Chem. Soc. ReV. 1996, 25, 41. (b)
Blanco, M.-J.; Jime´nez, M. C.; Chambron, J.-C.; Heitz, V.; Linke, M.;
Sauvage, J.-P. Chem. Soc. ReV. 1999, 28, 293. (c) Chambron, J. C.; Collin,
J. P.; Dalbavie, J. O.; Dietrich-Buchecker, C. O.; Heitz, V.; Odobel, F.;
Solladie, N.; Sauvage, J. P. Coord. Chem. ReV. 1998, 180, 1299.
(13) (a) Lewis, F. D.; Liu, X. Y.; Liu, J. Q.; Miller, S. E.; Wasielewski,
M. R. Nature 2000, 406, 51. (b) Lewis, F. D.; Letsinger, R. L.; Wasielewski,
M. R. Acc. Chem. Res. 2001, 34, 159.
(14) (a) Imahori, H.; Sakata, Y. AdV. Mater. 1997, 9, 537. (b) Imahori,
H.; Sakata, Y. Eur. J. Org. Chem. 1999, 64, 2445. (c) Fukuzumi, S.; Imahori,
H. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH:
Weinheim, Germany, 2001; Vol. 2, pp 927-975. (d) Imahori, H.; Mori,
Y.; Matano, Y. Photochem. Photobiol. C 2003, 4, 51.
(15) (a) Gust, D.; Moore, T. A. In The Porphyrin Handbook; Kadish,
K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA,
2000; Vol. 8, pp 153-190. (b) Gust, D.; Moore, T. A.; Moore, A. L. Acc.
Chem. Res. 2001, 34, 40. (c) Gust, D.; Moore, T. A.; Moore, A. L. In
Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim,
Germany, 2001; Vol. 3, pp 272-336.
(16) (a) Imahori, H.; Yamada, K.; Hasegawa, M.; Taniguchi, S.; Okada,
T.; Sakata, Y. Angew. Chem., Int. Ed. Engl. 1997, 36, 2626. (b) Imahori,
H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito, O.; Sakata, Y.;
Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 2607. (c) Fukuzumi, S.; Imahori,
H.; Yamada, H.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O.; Guldi, D. M. J.
Am. Chem. Soc. 2001, 123, 2571.
Acknowledgment. This work was partially supported by
Grants-in-Aid for Scientific Research on Priority Area (Grant
Nos. 11228205 and 13440216) and Grant-in-Aid for the
Development of Innovative Technology (Grant No. 12310) from
the Ministry of Education, Culture, Sports, Science and Tech-
nology, Japan, and by the Academy of Finland and the National
Technology Agency of Finland. H.I. also thanks Grant-in-Aid
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan (21st Century COE on Kyoto University
Alliance for Chemistry) for financial support.
(17) Imahori, H.; Guldi, D. M.; Tamaki, K.; Yoshida, Y.; Luo, C.;
Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 6617.
(18) Fukuzumi, S. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 8,
pp 115-151.
Supporting Information Available: Experimental details
including synthetic procedures and characterization, emission
and transient absorption spectra, and cyclic voltammetry in
benzonitrile. This material is available free of charge via the
Internet at http://pubs.acs.org.
(19) (a) Fukuzumi, S.; Guldi, D. M. In Electron Transfer in Chemistry;
Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Vol. 2, pp 270-
337. (b) Guldi, D. M. Chem. Soc. ReV. 2002, 31, 22.
(20) Eberson, L. Electron-Transfer Reactions in Organic Chemistry;
ReactiVity and Structure; Springer: Heidelberg, Germany, 1987; Vol. 25.

8844 J. Phys. Chem. A, Vol. 107, No. 42, 2003
Tkachenko et al.
(21) Imahori, H.; Tkachenko, N. V.; Vehmanen, V.; Tamaki, K.;
Lemmetyinen, H.; Sakata, Y.; Fukuzumi, S. J. Phys. Chem. A 2001, 105,
1750.
(22) Imahori, H.; Hagiwara, K.; Aoki, M.; Akiyama, T.; Taniguchi, S.;
Okada, T.; Shirakawa, M.; Sakata, Y. J. Am. Chem. Soc. 1996, 118, 11771.
Akimoto, S.; Yamazaki, I.; Miyasaka, H.; Itaya, A.; Murakami, M.; Osuka,
A. J. Phys. Chem. A 2001, 105, 4822.
(38) (a) Andersson, M.; Davidsson, J.; Hammarstro¨m, L.; Korppi-
Tomola, J.; Peltola, T. J. Phys. Chem. B 1999, 103, 3258. (b) Harriman,
A.; Hissler, M.; Trompette, O.; Ziessel, R. J. Am. Chem. Soc. 1999, 121,
2516.
(23) Gould, I. R.; Farid, S. Acc. Chem. Res. 1996, 29, 522.
(39) Kesti, T.; Tkachenko, N.; Yamada, H.; Imahori, H.; Fukuzumi, S.;
Lemmetyinen, H. Photochem. Photobiol. Sci. 2003, 2, 251.
(40) The maximum bleaching of the porphyrin ground-state absorption
is achieved at 5-15 ps delay time (Figure 6). Also the 4.3 ps component
(Figure 6) has a sharp band at 560 nm, which allows associating it with
Q-band bleaching. Therefore, this component, 4.3 ps, can be attributed to
formation of the exciplex from fullerene singlet excited state. Noticeably,
this relaxation channel gives only minor absorption increase in PhCN. In
toluene, its contribution is even less visible giving no observable signal
increase at the same wavelengths.
(41) The extinction of the CT band is roughly 1000 times lower than
that of the Soret band, which is right at the excitation wavelength. It is
hardly possible to achieve any sensible amount of directly excited CT. On
the other hand, relaxation of the second excited state to exciplex is an
extremely fast reaction and will be observed as almost instant formation of
the exciplex.
(24) (a) Hilinski, E. F.; Masnovi, J. M.; Amatore, C.; Kochi, J. K.;
Rentzepis, P. M. J. Am. Chem. Soc. 1983, 105, 6167. (b) Hilinski, E. F.;
Masnovi, J. M.; Kochi, J. K.; Rentzepis, P. M. J. Am. Chem. Soc. 1984,
106, 8071.
(25) (a) Hubig, S. M.; Bockman, T. M.; Kochi, J. K. J. Am. Chem. Soc.
1996, 118, 3842. (b) Hubig, S. M.; Bockman, T. M.; Kochi, J. K. J. Am.
Chem. Soc. 1997, 119, 2926. (c) Rathore, R.; Hubig, S. M.; Kochi, J. K. J.
Am. Chem. Soc. 1997, 119, 11468. (d) Bockman, T. M.; Hubig, S. M.;
Kochi, J. K. J. Am. Chem. Soc. 1998, 120, 6542. (e) Hubig, S. M.; Kochi,
J. K. J. Am. Chem. Soc. 1999, 121, 1688.
(26) (a) Ojima, S.; Miyasaka, H.; Mataga, N. J. Phys. Chem. 1990, 94,
4147, 5834, 7534. (b) Asahi, T.; Ohkohchi, M.; Mataga, N. J. Phys. Chem.
1993, 97, 13132. (c) Asahi, T.; Mataga, N. J. Phys. Chem. 1991, 95, 1956.
(27) Kesti, T. J.; Tkachenko, N. V.; Vehmanen, V.; Yamada, H.; Imahori,
H.; Fukuzumi, S.; Lemmetyinen, H. J. Am. Chem. Soc. 2002, 124, 8067.
(42) (a) Fukuzumi, S.; Suenobu, T.; Patz, M.; Hirasaka, T.; Itoh, S.;
Fujitsuka, M.; Ito, O. J. Am. Chem. Soc. 1998, 120, 8060. (b) Fukuzumi,
S.; Suenobu, T.; Hirasaka, T.; Sakurada, N.; Arakawa, R.; Fujitsuka, M.;
Ito, O. J. Phys. Chem. A 1999, 103, 5935.
(43) Imahori, H.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O.; Sakata, Y.;
Fukuzumi, S. J. Phys. Chem. A 2001, 105, 325.
(44) (a) Kadish, K. M.; Gao, X.; Van Caemelbecke, E.; Suenobu, T.;
Fukuzumi, S. J. Phys. Chem. A 2000, 104, 3878. (b) Guldi, D. M.; Asmus,
K.-D. J. Phys. Chem. A 1997, 101, 1472. (c) Guldi, D. M.; Hungerbu¨hler,
H.; Asmus, K.-D. J. Phys. Chem. 1995, 99, 9380.
(28) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R., Purification of
Laboratory Chemicals, 4th ed.; Pergamon Press: Elmsford, NY, 1996.
(29) Imahori, H.; Tamaki, K.; Araki, Y.; Sekiguchi, Y.; Ito, O.; Sakata,
Y.; Fukuzumi, S. J. Am. Chem. Soc. 2002, 124, 5165.
(30) Imahori, H.; Ozawa, S.; Ushida, K.; Takahashi, M.; Azuma, T.;
Ajavakom, A.; Akiyama, T.; Hasegawa, M.; Taniguchi, S.; Okada, T.;
Sakata, Y. Bull. Chem. Soc. Jpn. 1999, 72, 485.
(31) Takagi, S.; Miyamoto, T. K.; Sakai, Y. Bull. Chem. Soc. Jpn. 1986,
59, 2371.
(45) The third important parameter is the mutual orientation of the
reactant. However, the molecular modeling (Figure 3) indicates only slight
and unregular variation in the orientation factor for this series of the dyad,
whereas the distance changes from almost van der Waals contact to 5.2 Å
(edge-to-edge).
(46) The molecular modeling was performed for free base dyads, Figure
3. However, one can expect a similar relation in center-to-center distances
for the Zn dyads.
(47) The exciplex of ZnP-D-C60 can be regarded as the charge-separated
state with a strong interaction. However, the exact difference between the
exciplex and charge-separated state remains to be clarified, although they
are clearly differentiated by their transient absorption spectra as shown in
this study.
(32) (a) Tkachenko, N. V.; Tauber, A. Y.; Grandell, D.; Hynninen, P.
H.; Lemmetyinen, H. J. Phys. Chem. A 1999, 103, 3646. (b) Tkachenko,
N. V.; Rantala, L.; Tauber, A. Y.; Helaja, J.; Hynninen, P. H.; Lemmetyinen,
H. J. Am. Chem. Soc. 1999, 121, 9378.
(33) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in Non-
aqueous Systems; Marcel Dekker: New York, 1990.
(34) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209, 221.
(35) Fuhrhop, J.-H.; Mauzerall, D. J. Am. Chem. Soc. 1969, 91, 4174.
(36) Luo, C.; Guldi, D. M.; Imahori, H.; Tamaki, K.; Sakata, Y. J. Am.
Chem. Soc. 2000, 122, 6535.
(37) (a) Akimoto, S.; Yamazaki, T.; Yamazaki, I.; Osuka, A. Chem.
Phys. Lett. 1999, 309, 177. (b) Nakano, A.; Yasuda, Y.; Yamazaki, T.;