Solvent dependence of charge separation and charge recombination rates in porphyrin-fullerene dyad

Article abstract of DOI:10.1021/jp002158b

Photoinduced processes in zinc porphyrin-C₆₀ dyad (ZnP-C₆₀) in different organic solvents have been investigated by fluorescence lifetime measurements and pico- and nanosecond time-resolved transient absorption spectroscopies. Irresp

Full text of DOI:10.1021/jp002158b

J. Phys. Chem. A 2001, 105, 325-332
325
Solvent Dependence of Charge Separation and Charge Recombination Rates in
Porphyrin-Fullerene Dyad
Hiroshi Imahori,* Mohamed E. El-Khouly,† Mamoru Fujitsuka,† Osamu Ito,†,*
Yoshiteru Sakata,‡ 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 for Chemical Reaction Science,
Tohoku UniVersity, Katahira, Aoba-ku, Sendai 980-8577, Japan, and The Institute of Scientific and Industrial
Research, Osaka UniVersity, 8-1 Mihoga-oka, Ibaraki, Osaka 567-0047, Japan
ReceiVed: June 15, 2000; In Final Form: October 27, 2000
Photoinduced processes in zinc porphyrin-C60 dyad (ZnP-C60) in different organic solvents have been
investigated by fluorescence lifetime measurements and pico- and nanosecond time-resolved transient absorption
spectroscopies. Irrespective of the solvent polarity, the charge-separated state (ZnP•+-C60•-) is formed via
photoinduced electron transfer from the excited singlet state of the porphyrin to the C60 moiety. However, the
resulting charge-separated state decays to different energy states depending on the energy level of the charge-
separated state relative to the singlet and triplet excited states of the C60 moiety. In nonpolar solvents such as
benzene (ꢀs ) 2.28), the charge-separated state undergoes charge recombination to yield the C60 singlet excited
state, followed by intersystem crossing to the C60 triplet excited state, since the energy level of the charge-
separated state is higher than that of the C60 singlet excited state (1.75 eV). More polar solvents such as
anisole (ꢀs ) 4.33) render the energy level of the charge-separated state lower than the C60 singlet excited
state, resulting in the direct formation of the C60 triplet excited state (1.50 eV) from the charge-separated
state, formed by the photoinduced charge separation from the porphyrin to the C60 singlet excited state as
well as from the porphyrin excited singlet state to the C60. In polar solvents such as benzonitrile (ꢀs ) 25.2),
where the energy level of the charge-separated state (1.38 eV) is low compared with the C60 triplet excited
state, the charge-separated state, produced upon excitation of the both chromophores, decays directly to the
ground state. Such solvent dependence of charge recombination processes in ZnP-C60 can be rationalized by
small reorganization energies of porphyrins and fullerenes in electron-transfer processes.
Introduction
been reported to produce the charge-separated states even in
nonpolar solvents,5a-c,6e,8c,9b,11,13c most other systems have
Excited-state properties and photoinduced electron transfer
(ET) reactions of covalently linked donor-acceptor systems
have been extensively studied to mimic photosynthetic ET.1 The
fundamental strategy in mimicking photosynthesis involves
application of multistep ET and control of reorganization energy
(λ).2 In particular, it seems of fundamental significance to
achieve small reorganization energy in ET systems, since the
charge recombination (CR) processes can be markedly deceler-
ated by shifting the energetics deep into the Marcus inverted
region, where the driving force (-∆G0et) is larger than the
reorganization energy of ET.3 In this context, utilization of
fullerenes as an electron acceptor has been demonstrated to be
very effective,4 because of the small reorganization energies of
fullerenes in ET.5 The most extensively studied fullerene-based
systems are porphyrin-linked fullerenes,5-15 where a porphyrin
is used as an electron donor as well as a sensitizer. In these
studies, both photoinduced energy transfer (EN) and ET have
been reported to occur. It is well established for porphyrin-
C60 systems that the charge-separated state is formed in polar
solvents via photoinduced ET.5-15 Although some systems have
exhibited direct singlet-singlet EN from the porphyrin to the
fullerene moiety in nonpolar solvents such as toluene (ꢀs ) 2.38)
and benzene (ꢀs ) 2.28). Thus, a systematic study on the solvent-
dependent change in the charge separation (CS) and CR
processes in porphyrin-fullerene linked systems has yet to be
reported.
This study reports detailed solvent dependence of both CS
and CR processes in porphyrin-C60 dyad (ZnP-C60) (Figure
1). A porphyrin is connected to C60, with a relatively rigid spacer
[the edge-to-edge distance (Ree) ) 11.9 Å and the center-to-
center distance (Rcc) ) 18.0 Å] so that the photodynamics can
be analyzed at a fixed distance between the porphyrin and C60
moieties. The photodynamical processes have been studied by
fluorescence lifetime measurements and pico- and nanosecond
laser flash photolysis. These flash photolysis experiments will
reveal the spectral characteristics of the zinc porphyrin radical
cation (ZnP•+) and the C60 radical anion (C60•-) moieties in the
charge-separated state in benzene, anisole, and benzonitrile. In
addition, the C60 and porphyrin triplet excited states can be
monitored by nanosecond flash photolysis. Complementary
information on the excited singlet states of the porphyrin and
the C60 will be provided by the fluorescence lifetime experi-
ments.
* Authors for correspondence. E-mail: imahori@ap.chem.eng.osaka-
u.ac.jp, ito@icrs.tohoku.ac.jp, fukuzumi@ap.chem.eng.osaka-u.ac.jp.
† Institute for Chemical Reaction Science.
‡ Institute of Scientific and Industrial Research.
10.1021/jp002158b CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/20/2000

326 J. Phys. Chem. A, Vol. 105, No. 2, 2001
Imahori et al.
1146 (M + H+); Fourier transform IR (KBr) 3059, 2962, 2905,
2869, 2364, 2345, 1804, 1686, 1658, 1593, 1519, 1493, 1477,
1425, 1406, 1398, 1363, 1340, 1316, 1290, 1247, 1219, 1205,
1069, 1001, 947, 930, 899, 883, 823, 797, 759, 745, 718, 508
cm-1
.
Spectral Measurements. Time-resolved fluorescence spectra
were measured by a single-photon counting method using a
second harmonic generation (SHG, 410 nm) of a Ti:sapphire
laser [Spectra-Physics, Tsunami 3950-L2S, 1.5 ps full width
at half-maximum (fwhm)] and a streakscope (Hamamatsu
Photonics, C4334-01) equipped with a polychromator (Acton
Research, SpectraPro 150) as an excitation source and a detector,
respectively.
Picosecond laser flash photolysis was carried out using SHG
(532 nm) of active/passive mode-locked Nd:YAG laser (Con-
tinuum, PY61C-10, fwhm 35 ps) as the excitation light. A white
continuum pulse generated by focusing the fundamental of the
Nd:YAG laser on a D2O/H2O (1:1 volume) cell was used as
monitoring light. The visible monitoring light transmitted
through the sample was detected with a dual MOS detector
(Hamamatsu Photonics, C6140) equipped with a polychromator
(Acton Research, SpectraPro 150). For detection of the near-
IR light, an InGaAs linear image sensor (Hamamatsu Photonics,
C5890-128) was used as a detector. The spectra were obtained
by averaging 80 events on a microcomputer. The time resolution
of the present system was ca. 35 ps.
Nanosecond transient absorption measurements were carried
out using SHG (532 nm) of Nd:YAG laser (Spectra-Physics,
Quanta-Ray GCR-130, fwhm 6 ns) as an excitation source. For
transient absorption spectra in the near-IR region (600-1600
nm), monitoring light from a pulsed Xe lamp was detected with
a Ge-avalanche photodiode (Hamamatsu Photonics, B2834).
Photoinduced events in micro- and millisecond time regions
were estimated by using a continuous Xe lamp (150 W) and an
InGaAs-PIN photodiode (Hamamatsu Photonics, G5125-10)
as a probe light and a detector, respectively. Details of the
transient absorption measurements were described elsewhere.17
All the samples in a quartz cell (1 × 1 cm) were deaerated by
bubbling argon through the solution for 15 min.
Steady-state absorption spectra in the visible and near-IR
regions were measured on a Jasco V570 DS spectrophotometer.
Fluorescence spectra were measured on Hitachi 850 spectro-
fluorophotometer.
Electrochemical Measurements. The cyclic voltammetry
measurements were performed on a BAS 50 W electrochemical
analyzer in deaerated benzonitrile solution containing 0.10 M
Bu4NPF6 as a supporting electrolyte at 298 K (100 mV s-1).
The glassy carbon working electrode was polished with BAS
polishing alumina suspension and rinsed with acetone before
use. The counterelectrode was a platinum wire. The measured
potentials were recorded with respect to an Ag/AgCl (saturated
KCl) reference electrode.
Figure 1. Structures of molecules.
Experimental Section
Materials. ZnP-C6015 was obtained by 1,3-dipolar cycload-
dition using the corresponding porphyrin aldehyde.16 Porphyrin
reference (ZnP-ref) was prepared by the coupling reaction of
5-(4-carboxyphenyl)-10,15,20-tris(3,5-di-tert-butylphenyl)por-
phyrin and 4-methylaniline, followed by treatment with Zn-
(OAc)2. C60 reference (C60-ref13f) was synthesized from 3,5-
di-tert-butylbenzaldehyde, N-methylglycine, and C60 by the same
method as described for ZnP-C60.
Structures of all new compounds were confirmed by spec-
1
troscopic analysis including H NMR, infrared (IR), and fast
atom bombardment (FAB) mass spectra.15 Tetrabutylammonium
hexafluorophosphate used as a supporting electrolyte for the
electrochemical measurements was obtained from Tokyo Kasei
Organic Chemicals. Benzene, anisole, and benzonitrile were
purchased from Wako Pure Chemical Ind., and purified by
successive distillation over calcium hydride.
ZnP-Ref. A solution of 5-(4-carboxyphenyl)-10,15,20-tris-
(3,5-di-tert-butylphenyl)porphyrin15 (200 mg, 0.207 mmol),
pyridine (0.33 mL, 4.32 mmol), and thionyl chloride (0.095 mL,
0.648 mmol) in benzene (30 mL) was refluxed for 1 h. The
excess thionyl chloride and solvents were removed under
reduced pressure and the residue was redissolved in benzene
(20 mL). This solution was added to a stirred solution of
4-methylaniline (23.0 mg, 0.215 mmol) in benzene (10 mL)
and pyridine (1 mL), and the solution was stirred for 12 h. The
solvent was removed under reduced pressure. Flash column
chromatography on silica gel with benzene (Rf ) 0.35) as an
eluent and subsequent reprecipitation from benzene-acetonitrile
gave a vivid reddish-purple solid. A saturated methanol solution
of Zn(OAc)2 (3 mL) was added to a solution of the resultant
solid in CHCl3 (30 mL) and refluxed for 30 min. After cooling,
the reaction mixture was washed with water twice, dried over
anhydrous Na2SO4, and then the solvent was evaporated. Flash
column chromatography on silica gel with benzene (Rf ) 0.30)
as an eluent and subsequent reprecipitation from benzene-
acetonitrile gave ZnP-ref as a deep red-purple solid (72% yield,
Results and Discussion
Steady-State Absorption and Fluorescence Studies. Figure
2 shows absorption spectra of ZnP-C60 and the references
(ZnP-ref and C60-ref) in benzonitrile. The absorption spectrum
of ZnP-C60 in benzonitrile is virtually the superposition of the
spectra of the individual chromophores, indicating that there is
no evidence for strong interaction between the chromophores
in the ground state. Similar results were obtained in benzene
and anisole.
1
171 mg, 0.149 mmol); mp > 300 °C; H NMR (270 MHz,
CDCl3) δ 9.02 (d, J ) 5 Hz, 2H), 9.01 (s, 4H), 8.90 (d, J ) 5
Hz, 2H), 8.37 (d, J ) 8 Hz, 2H), 8.22 (d, J ) 8 Hz, 2H), 8.09
(m, 7H), 7.79 (m, 3H), 7.66 (d, J ) 8 Hz, 2H), 7.27 (d, J ) 8
Hz, 2H), 2.40 (s, 3H), 1.53 (s, 54H); mass spectrometry (FAB)
Steady-state fluorescence spectra of ZnP-C60 and the refer-
ences (ZnP-ref and C60-ref) were measured in benzene,

Porphyrin-Fullerene Dyad
J. Phys. Chem. A, Vol. 105, No. 2, 2001 327
Figure 3. Steady-state fluorescence spectra of ZnP-C60 (solid line)
and ZnP-ref (dotted line) in benzene (λex ) 410 nm, absorption ratio
of ZnP:C60 ) 92:8). The spectra are normalized at the peak position
for comparison.
TABLE 1: Fluorescence Lifetimes (τ) of ZnP-C60 and the
Reference Compounds in Benzene, Anisole, and Benzonitrilea
lifetime (τ)/ps
benzene
anisole
benzonitrile
(ꢀs ) 2.28)
(ꢀs ) 4.33)
(ꢀs ) 25.2)
compound
λobs
610 nm 720 nm 610 nm 720 nm 610 nm 720 nm
ZnP-C60
ZnP-ref
C60-ref
200
2000
1200
120
2100
1100
100
2000
760
1300
1300
1300
a Absorption ratio of ZnP and C60 in ZnP-C60 at excitation
wavelength (λex ) 410 nm); ZnP:C60 ) 92:8 (benzene), 90:10 (anisole
and benzonitrile).
(1ZnP*) occurs by the attached C60 moiety through ET or EN
or both, judging from the results reported for zinc porphyrin-
C60 dyads.5-15 On the other hand, the fluorescence lifetimes of
ZnP-C60 at 720 nm [τ ) 760 ps (benzonitrile), 1100 ps
(anisole), 1200 ps (benzene)] become longer as the solvent
polarity decreases, approaching the lifetime of C60-ref (1300
ps). This suggests that the excited singlet state of the C60 (1C60*)
is less quenched by the attached porphyrin in nonpolar solvents
through ET, which will be discussed in a later section. It should
be noted here that a rise component of the fluorescence at 750
nm with a rate constant of 2.5 × 109 s-1, where the emission
is due to the C60 solely, was clearly detected in benzene, as
shown in Figure 4. This is in sharp contrast with no appearance
of the fluorescence rise in anisole and benzonitrile. Because
the rise rate (rate constant of 2.5 × 109 s-1) at 750 nm due to
1C60* in benzene does not match the decay rate (rate constant
of 5.0 × 109 s-1) at 610 nm due to 1ZnP*, it is strongly
suggested that the charge-separated state (ZnP•+-C60•-) in
benzene, produced via photoinduced ET, undergoes the CR to
Figure 2. (a) Absorption spectra of ZnP-C60 (solid line), ZnP-ref
(dotted line), and C60-ref (dashed line) in benzonitrile. (b) Spectra are
expanded by a factor of 10 along vertical axis. Spectrum of a
superposition of ZnP-ref and C60-ref (broken line) is shown for
comparison.
anisole, and benzonitrile with excitation at 410 nm, where the
absorption ratios of the porphyrin and C60 moieties are 92:8 in
benzene and 90:10 in anisole and benzonitrile. A typical example
in benzene is shown in Figure 3. Emissions from both the
porphyrin moiety (λmax ) 600, 645 nm)5a-c,13 and the C60 moiety
(λmax ) 710 nm)5a-c,13 are clearly observed for ZnP-C60 in
benzene, anisole, and benzonitrile. However, the fluorescence
spectra of ZnP-C60 are quenched more strongly than those of
ZnP-ref under the same conditions (relative intensity ) 0.027),
indicating that 1ZnP* is quenched by the C60 moiety. It is
interesting to note that the emission intensity of the C60 moiety
relative to the porphyrin decreases with increasing solvent
polarity.
Fluorescence Lifetime Measurements. The fluorescence
lifetimes (τ) of ZnP-C60, ZnP-ref, and C60-ref were measured
by a time-correlated single-photon-counting apparatus with
excitation at 410 nm, and monitored at 610 and 720 nm, where
the emission is due to the porphyrin and the C60 moiety,
respectively. The results are summarized in Table 1. The
fluorescence decay curve was well fitted as a single exponential.
The fluorescence lifetimes of ZnP-C60 at 610 nm [τ ) 100 ps
(benzonitrile), 120 ps (anisole), 200 ps (benzene)] are much
shorter than those of ZnP-ref (2000-2100 ps). This indicates
that the rapid quenching of the porphyrin singlet excited state
1
yield C60*. Such recombination of charge-separated state
1
(ZnP•+-C60•-) to C60* has also been reported in porphyrin-
C60 dyads.5a-c,6e,13c
Time-Resolved Transient Absorption Spectra. Time-
resolved transient absorption spectra of ZnP-C60 in various
solvents were measured by pico- and nanosecond laser pho-
tolysis with excitation wavelength at 532 nm, where the
absorption ratios of the porphyrin and the C60 moieties are
85:15 in benzene, 76:24 in anisole, and 77:23 in benzonitrile.
Figure 5 displays picosecond time-resolved absorption spectra
of ZnP-C60 in benzene. The Sn r S1 difference spectrum taken
immediately after excitation with 532-nm laser pulse (50-ps

328 J. Phys. Chem. A, Vol. 105, No. 2, 2001
Imahori et al.
TABLE 2: CS and CR Rate Constants in ZnP-C60
-∆G0
/
kET
/
ET
eVa
solvent
benzene
initial state
1ZnP*
final state
s-1 b
Φc
•-
ZnP•+-C60
1C60*
<0.33
<0.33
4.5 × 109 0.90
(ꢀs ) 2.28) ZnP•+-C60
2.5 × 109 ∼1
•-
•-
•-
anisole
(ꢀs ) 4.33) C60*
1ZnP*
ZnP•+-C60
ZnP•+-C60
3C60*
0.32-0.57 7.8 × 109 0.94
1
<0.25
<0.25
0.66
1.4 × 108 0.15
3.0 × 108 ∼1d
9.5 × 109 0.95
5.5 × 108 0.42
1.5 × 107 0.99
•-
ZnP•+-C60
•-
•-
•-
benzonitrile
(ꢀs ) 25.2) C60*
1ZnP*
ZnP•+-C60
ZnP•+-C60
ZnP•+-C60
ZnP-C60
1
0.37
0.12
1.38
3C60*
ZnP•+-C60
1.3 × 106
1
•-
a -∆G0 ) E0 (D/D•+) - E0 (A/A•-), -∆G0 ) ∆E0-0
-
CR
ox
red
CS
(-∆G0CR), where E0ox (D/D•+) and E0red (A/A•-) are the first oxidation
potential of donor and the first reduction potential of acceptor,
respectively, and ∆E0-0 is energy of the 0-0 transition between the
lowest excited state and the ground state. The redox potentials were
measured by differential pulse voltammetry in benzonitrile using 0.1
M Bu4NPF6 as supporting electrolyte. The Coulombic term in polar
solvents such as benzonitrile is negligible. On the other hand, in
nonpolar solvents such as anisole and benzene it was impossible to
determine -∆G0ET with accuracy, because of the difficulty in estimating
the Coulombic term and the redox potentials in nonpolar solvents.
b Charge separation (CS) rates from 1ZnP* and 1C60* were determined
Figure 4. Time profiles of fluorescence intensity for ZnP-C60 in argon-
saturated benzene monitored at 750 nm (λex ) 410 nm). The time profile
was fitted by rise (rate constant of 2.5 × 109 s-1) and decay (rate
constant of 7.7 × 108 s-1) components, respectively.
from the following equation using the fluorescence lifetimes: kCS
)
[1/τ (ZnP-C60)] - [1/τ (ZnP-ref or C60-ref)]. The other CS and charge
recombination rates were determined by analyzing the rise or decay of
•-
C60 at 1000-1010 nm. c The quantum yields (Φ) for each pathway
were estimated on a basis of Schemes 1-3. d Assuming that contribu-
tion of the deactivation pathway from ZnP•+-C60•- to 3ZnP* (1.53 eV)15
is negligible.
SCHEME 1: Schematic Energy Levels and Relaxation
1
1
Pathways for ZnP* and C60* in ZnP-C60 in Benzene;
k0 ) 5.0 × 108 s-1, kCS1 ) 4.5 × 109 s-1, kCR1 < 1.3 ×
106 s-1, kCR2 ) 2.5 × 109 s-1, kISC ) 7.7 × 108 s-1
,
kT ) 2.2 × 105 s-1
Figure 5. (a) Picosecond time-resolved absorption spectra of ZnP-
C60 in argon-saturated benzene excited at 532 nm (absorption ratio of
ZnP:C60 ) 85:15) and (b) time dependence of the absorbance at 1010
nm. The solid line shows a simulated curve using a rate constant of
anisole and benzonitrile. The values of ET rates and quantum
yields are summarized in Table 2. The CS rates from
1ZnP* to C60 in ZnP-C60 are nearly the same in benzene,
anisole, and benzonitrile. This may result from an increase in
both the reorganization energy and driving force for CS with
increasing solvent polarity, leading to similar CS irrespective
of the solvent polarity.5a,13c The decay profile at 1010 nm in
benzene was analyzed by a single exponential with a rate
constant of 2.5 × 109 s-1, as shown in Figure 5b. The time
constant (kCR2 ) 2.5 × 109 s-1) agrees well with that of the
fluorescence rise from the C60 moiety at 750 nm (2.5 × 109
s-1) (Figure 4).
2.5 × 109 s-1
.
delay) is characterized by the bleaching of the ground-state
porphyrin absorption at 560 and 600 nm of the Q-bands, the
stimulating emission from the porphyrin at 650 nm, and the
1
broad absorption of C60* at near-IR region. With increasing
delay time (200-ps delay), a new absorption band at 1020 nm
•- 15,18
due to the C60 radical anion (C60
)
appears clearly,
accompanied by the rise in a broad absorption around 650 nm
due to zinc porphyrin radical cation (ZnP•+).13c,19 These results
1
clearly show that photoinduced ET takes place from ZnP* to
C60 in ZnP-C60, resulting in formation of the charge-separated
The energy level of the charge-separated state is known to
be pushed up in nonpolar solvents such as benzene (ꢀs ) 2.28)
and toluene (ꢀs ) 2.38), as compared with polar solvents. Thus,
the charge-separated state (ZnP•+-C60•-) is located between
1ZnP* (2.08 eV) and 1C60* (1.75 eV) in benzene, as illustrated
state. On the basis of the fluorescence lifetimes, the CS rate
1
(kCS1) from ZnP* and the quantum yield (Φ) of formation of
the charge-separated state were estimated as 4.5 × 109 s-1 and
0.90, respectively (Table 2). Similar transient absorption spectra,
exhibiting the formation of ZnP•+ and C60•-, were obtained in
1
in Scheme 1.5a,13c This is consistent with the fact that C60* is

Porphyrin-Fullerene Dyad
J. Phys. Chem. A, Vol. 105, No. 2, 2001 329
Figure 6. Nanosecond transient absorption spectra obtained by 532-
nm (absorption ratio of ZnP:C60 ) 85:15) laser photolysis of ZnP-
C60 (0.1 mM) in argon-saturated benzene.
almost unquenched by ZnP in ZnP-C60 (Table 1). The
distinctobservation of the charge-separated state in both nonpolar
and polar solvents contrasts with previous reports that efficient
singlet-singlet EN from porphyrin to C60 occurs in nonpolar
solvents such as benzene and toluene.6a-c,7,10c,11 It is well
established that intramolecular EN rates are not susceptible to
solvent polarity.20 If the EN process occurs with a rate constant
of ∼1010 s-1 in nonpolar solvents, the EN process with the
similar rate constant would be observed in polar solvents as
well. However, no observation of such EN process in polar
solvents precludes such possibility. Because CR rates in donor-
acceptor linked dyads are known to decrease with decreasing
solvent polarity,21 the CR rate (kCR1) from the charge-separated
state to the ground state in nonpolar solvents would be slower
than the CR rate (1.3 × 106 s-1) in benzonitrile (vide infra, see
1
Table 2). Therefore, the quantum yield of formation of C60*
from the charge-separated state in benzene can be estimated as
almost unity (∼1). The reorganization energies of porphyrin-
fullerene systems are known to be small,5 and driving force for
the CR to the ground state (>1.75 eV) is much larger than that
for the CR to the 1C60* (<0.33 eV). Thus, the driving force for
Figure 7. (a) Nanosecond time-resolved absorption spectra obtained
by 532-nm (absorption ratio of ZnP:C60 ) 76:24) laser photolysis of
ZnP-C60 (0.1 mM) in argon-saturated anisole and (b) time profiles of
absorbance at 700 and 1000 nm.
1
the CR to C60* is located in the normal or top region of the
Marcus parabola, whereas the driving force for the CR to the
ground state goes deep into the Marcus inverted region.
Consequently, the CR to1C60* in benzene is dominant over the
CR to the ground state.
using the fluorescence lifetimes (Table 2). However, with
increasing the solvent polarity from benzene (ꢀs ) 2.28) to
anisole (ꢀs ) 4.33), the deactivation pathway from the charge-
separated state changes drastically. Figure 7a shows nanosecond
Formation of the C60 triplet excited state (3C60*) in argon-
saturated benzene is also confirmed by the complementary
nanosecond experiments, as shown in Figure 6, which exhibits
a characteristic triplet-triplet absorption maximum at 700 nm.
In addition, the weak transient absorption due to the porphyrin
triplet excited state (3ZnP*) appears at 740 and 840 nm.15,22
time-resolved transient absorption spectra of ZnP-C60 in
3
anisole. Both transient absorption bands due to C60* (λmax
)
•-
700 nm) and C60 (λmax ) 1020 nm) appear within the time
3
resolution (10 ns). The unambiguous detection of ZnP* was
somewhat obscured in anisole, since the absorption due to 3ZnP*
overlaps with the 3C60* absorption extensively at 600-900 nm.
3
3
The ZnP* (1.53 eV)15 may be produced from the intersystem
However, the contribution of ZnP*, if any, would be minor,
crossing from 1ZnP* in ZnP-C60. Considering the molar
as is the case in benzene (Figure 6). The absorption band at
3
absorption coefficient of C60* (ꢀ ) 4200 M-1 cm-1 23
)
and
1020 nm due to C60•- decays rapidly to leave the characteristic
3ZnP* (ꢀ ) 8500 M-1 cm-1),22 it is concluded that the 3C60* is
generated predominately via the intersystem crossing from 1C60*
with a rate constant of 7.7 × 108 s-1 (Scheme 1).15 The resulting
3C60* decays to the ground state with a rate constant of 2.2 ×
band at 700 nm due to the C60* (Figure 7b). The decay rate
3
(kCR2) at 1020 nm was determined as 3.0 × 108 s-1 using
picosecond transient absorption spectroscopy (Table 2). Because
no rise component due to the formation of 1C60* was observed
in the fluorescence lifetime measurement in anisole, the ZnP•+-
C60•- state recombines to generate 3C60* in anisole, as illustrated
105 s-1
.
1
In anisole, photoinduced ET also occurs from ZnP* to C60
to yield the charge-separated state with a rate constant of 7.8
× 109 s-1 and the quantum yield of 0.94, which are obtained
3
in Scheme 2. The quantum yield of formation of C60* from
the charge-separated state can be estimated as almost unity (∼1),

330 J. Phys. Chem. A, Vol. 105, No. 2, 2001
Imahori et al.
1
1
SCHEME 2: Schematic Energy Levels and Relaxation Pathways for ZnP* and C60* in ZnP-C60 in Anisole; k0 )
4.8 × 108 s-1, kCS1 ) 7.8 × 109 s-1, kCS2 ) 1.4 × 108 s-1, kCR1 < 1.3 × 106 s-1, kCR2 ) 3.0 × 108 s-1, kISC ) 7.7 × 108 s-1
kT ) 1.2 × 105 s-1
,
because the CR to the ground state (<1.3 × 106 s-1) is much
slower than the CR to the 3C60* (3.0 × 108 s-1). Judging from
the fact that the driving force for the former (>1.50 eV) is much
larger than that for the latter (<0.25 eV), the CR processes to
3
the ground state and to the C60* are located in the Marcus
inverted region and normal region, respectively. Accordingly,
the preference for the latter process over the former one may
be explained by the small reorganization energy (<0.8 eV in
anisole) of porphyrin-C60 linked systems (vide supra). It is
interesting to note that the charge-separated state is likely to
3
3
yield C60* (1.50 eV) rather than ZnP* (1.53 eV) in anisole.
3
3
The predominant formation of the C60* over ZnP* may be
accommodated by smaller reorganization energy of the C60 over
the porphyrin, in addition to the slightly larger driving force
for the former (<0.25 eV) against the latter (<0.22 eV) in the
Marcus normal region. The resulting 3C60*, generated from both
the ZnP•+-C60•- state and 1C60* (kISC ) 7.7 × 108 s-1), decays
to the ground state with a rate constant (kT) of 1.2 × 105 s-1 at
700 nm (Scheme 2). The rate constant (kCR2 ) 3.0 × 108 s-1
)
in anisole is one order of magnitude smaller than kCR2 (2.5 ×
109 s-1) for CR from ZnP•+-C60•- to the 1C60* in benzene. In
both cases CR occurs through a spin-conserving process.
Namely, the CR from the singlet charge-separated state to the
3C60* in anisole takes place through the singlet-triplet mixing
of the charge-separated state, whereas the CR from the singlet
1
charge-separated state to the C60* in benzene requires no
singlet-triplet mixing of the charge-separated state. If the
singlet-triplet mixing process in anisole is a rate-determining
step, the slow rate of the mixing may be responsible for the
3
slower CR rate from the charge-separated state to the C60* in
anisole, as compared with the CR rate to the 1C60* in benzene.
Such a difference in the CR rates would also result from the
difference in the driving force. Overall, the energy level of the
ZnP•+-C60 in anisole becomes lower than the level in
•-
benzene, thereby being located between the enegy levels of
Figure 8. (a) Nanosecond time-resolved absorption spectra obtained
by 532-nm (absorption ratio of ZnP:C60 ) 77:23) laser photolysis of
ZnP-C60 (0.1 mM) in argon-saturated benzonitrile and (b) time profiles
of absorbance at 1010 nm.
3
1C60* (1.75 eV) and C60* (1.50 eV). This is consistent with
the fact that the fluorescence lifetimes of C60 (1100 ps) in ZnP-
C
60 in anisole is slightly shorter than that of C60-ref (1300 ps)
1
because of the slightly exergonic ET from ZnP to C60* (kCS2
in Scheme 2). The kCS2 value and the quantum yield were
determined as 1.4 × 108 s-1 and 0.15, respectively, on the basis
of the fluorescence lifetimes (Table 2).
moiety to give rise to the charge-separated state with a rate
constant of 5.5 × 108 s-1 and the quantum yield of 0.42.
Complementary nanosecond experiments, performed with ZnP-
C60, also confirmed a characteristic absorption maximum of
In benzonitrile (ꢀs ) 25.2) as well, photoinduced ET takes
1
•-
place from ZnP* to C60 in ZnP-C60 upon photoexcitation of
C60 at 1010 nm, together with the broad absorption due to
the ZnP moiety, leading to the production of the charge-
separated state with a rate constant of 9.5 × 109 s-1 and the
quantum yield of 0.95 (Table 2). In addition, CS also occurs
from ZnP to 1C60* in ZnP-C60 upon photoexcitation of the C60
ZnP•+ around 650 and 850 nm (Figure 8a). The time profile of
ZnP-C60 at 1010 nm reveals the rise and decay components
with rate constants of 1.5 × 107 s-1 and 1.3 × 106 s-1
,
respectively (Figure 8b). Judging from the energy level diagram

Porphyrin-Fullerene Dyad
J. Phys. Chem. A, Vol. 105, No. 2, 2001 331
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SCHEME 3: Schematic Energy Levels and Relaxation
1
1
Pathways for ZnP* and C60* in ZnP-C60 in
Benzonitrile; k0 ) 5.0 × 108 s-1, kCS1 ) 9.5 × 109 s-1
,
kCS2 )5.5 × 108 s-1, kCS3 )1.5 × 107 s-1, kCR1 ) 1.3 ×
106 s-1, kISC ) 7.7 × 108 s-1, kT ) 1.1 × 105 s-1
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in benzonitrile (Scheme 3), photoinduced CS from ZnP to 3C60*
also takes place to yield the charge-separated state, which
corresponds to the rise in absorbance at 1010 nm with a rate
constant of 1.5 × 107 s-1. Given the rate constant for the decay
of 3C60* in benzonitrile (1.1 × 105 s-1), the quantum yield for
formation of the charge-separated state from 3C60* is 0.99 (Table
2). Accordingly, the photoinduced CS occurs from 1ZnP*, 1C60*,
and 3C60*, resulting in formation of the same charge-separated
state with a total quantum yield of almost unity.24 This is
consistent with the quantum yield (Φ ) 0.85)15 determined using
the comparative method (ꢀ1000
) 4700 M-1 cm-1).25 The
nm
resulting charge-separated state recombines to regenerate the
ground state with a rate constant of 1.3 × 106 s-1, which is
about four orders of magnitude smaller than the CS rate from
1ZnP* (9.5 × 109 s-1). This is in sharp contrast with conven-
tional porphyrin-quinone dyads, where the CR rates are much
faster than the CS rates in polar solvents.1,21a-c The accelerated
CS and decelerated CR can be rationalized by the small
reorganization energy of porphyrin-C60 dyads, as compared
with those of conventional donor-acceptor linked systems.
In conclusion, solvent dependence of CS and CR processes
in porphyrin-fullerene dyad has been well established in the
present systems. In particular, solvent dependence of decay
pathway from the charge-separated state to the excited states
of C60 versus the ground state has been clarified in detail. The
resulting ET dynamics can be rationalized by the small
reorganization energy of porphyrin-fullerene systems. Such
information will be helpful for understanding fundamental
properties of ET on donor-acceptor systems and, in turn,
developing artificial photosynthetic systems.
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Acknowledgment. This work was supported by Grants-in-
Aid for COE Research and Scientific Research on Priority Area
of Electrochemistry of Ordered Interfaces and Creation of
Delocalized Electronic Systems from Ministry of Education,
Science, Sports and Culture, Japan. H.I. thanks the Sumitomo
Foundation for financial support.
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(24) Because the energy level of 3ZnP* (1.53 eV) is slightly higher
3
than that of C60* (1.50 eV), the charge-separated state may also be
formed via 3ZnP* through ET with a rate constant of >1.5 × 107 s-1
.
Considering the high quantum yield of formation of the ZnP triplet
excited state (0.88) and C60 triplet excited state (0.98) and the slow
decay rate constant (3ZnP*: 2.3 × 104 s-1
;
3C60*: 4.0 × 104 s-1),15
the total quantum yield of formation of the charge-separated state
from 1ZnP*, 3ZnP*, 1C60*, and 3C60* in benzonitrile is estimated as almost
unity [Φ ) (0.95 + 0.05 × 0.88) × 0.77 + (0.42 + 0.58 × 0.98 × 0.99)
× 0.23 ) 0.99].
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