Control of photoinduced energy- and electron-transfer steps in zinc porphyrin-oligothiophene-fullerene linked triads with solvent polarity

Article abstract of DOI:10.1021/jp044316v

The dramatic changes of the lifetimes of the charge-separated (CS) states were confirmed in zinc porphyrin (ZnP)-oligothiophene (nT)-fullerene (C ₆₀) linked triads (ZnP-nT-C₆₀) with the solvent polarity. After the selective excitation of the ZnP moiety of ZnP-nT-C₆₀, an energy transfer took place from the ¹ZnP* moiety to the C ₆₀ moiety, generating ZnP-nT-¹C₆₀*. In polar solvents, the CS process also took place directly via the ¹ZnP* moiety, generating ZnP^ì?+-nT-C ₆₀^ì?-, as well as the energy transfer to the C ₆₀ moiety. After this energy transfer, an indirect CS process took place from the ¹C₆₀* moiety. In the less polar solvent anisole, the radical cation (hole) of ZnP^ì?+-nT-C ₆₀^ì?- shifted to the nT moiety; thus, the nT moiety behaves as a cation trapper, and the rates of the hole shift were evaluated to be in the order of 10⁸ s^-1; then, the final CS states ZnP-nT^ì?+-C₆₀^ì?- were lasting for 6-7 ??s. In the medium polar solvent o-dichlorobenzene (o-DCB), ZnP-nT ^ì?+-C₆₀^ì?- and ZnP^ì?+-nT- C₆₀^ì?- were present as an equilibrium, because both states have almost the same thermodynamic stability. This equilibrium resulted in quite long lifetimes of the CS states (450-910 ??s) in o-DCB. In the more polar benzonitrile, the generation of ZnP-nT^ì?+-C ₆₀^ì?- was confirmed with apparent short lifetimes (0.6-0.8 ??s), which can be explained by the fast hole shift to more stable ZnP^ì?+-nT-C₆₀^ì?- followed by the faster charge recombination. It was revealed that the relation between the energy levels of two CS states, which strongly depend on the solvent polarity, causes dramatic changes of the lifetimes of the CS states in ZnP-nT-C₆₀; that is, the most appropriate solvents for the long-lived CS state are intermediately polar solvents such as o-DCB. Compared with our previous data for H₂P-nT-C₆₀, in which H₂P is free-base porphyrin, the lifetimes of the CS states of ZnP-nT-C₆₀ are a??30 times longer than those in o-DCB. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp044316v

J. Phys. Chem. B 2005, 109, 14365-14374
14365
Control of Photoinduced Energy- and Electron-Transfer Steps in Zinc
Porphyrin-Oligothiophene-Fullerene Linked Triads with Solvent Polarity
Takumi Nakamura,† Jun-ya Ikemoto,‡ Mamoru Fujitsuka,†,§ Yasuyuki Araki,† Osamu Ito,*,†
Kazuo Takimiya,‡ Yoshio Aso,‡,§ and Tetsuo Otsubo*,‡
Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, CREST (JST), Katahira,
Sendai, Miyagi 980-8577, Japan, and Department of Applied Chemistry, Graduate School of Engineering,
Hiroshima UniVersity, Higashi-Hiroshima 739-8527, Japan
ReceiVed: December 14, 2004; In Final Form: March 25, 2005
The dramatic changes of the lifetimes of the charge-separated (CS) states were confirmed in zinc porphyrin
(ZnP)-oligothiophene (nT)-fullerene (C60) linked triads (ZnP-nT-C60) with the solvent polarity. After the
selective excitation of the ZnP moiety of ZnP-nT-C60, an energy transfer took place from the 1ZnP* moiety
to the C60 moiety, generating ZnP-nT-1C60*. In polar solvents, the CS process also took place directly via
the ZnP* moiety, generating ZnP•+-nT-C60•-, as well as the energy transfer to the C60 moiety. After this
1
energy transfer, an indirect CS process took place from the 1C60* moiety. In the less polar solvent anisole, the
•-
radical cation (hole) of ZnP•+-nT-C60 shifted to the nT moiety; thus, the nT moiety behaves as a cation
trapper, and the rates of the hole shift were evaluated to be in the order of 108 s-1; then, the final CS states
•-
ZnP-nT•+-C60 were lasting for 6-7 µs. In the medium polar solvent o-dichlorobenzene (o-DCB), ZnP-
nT•+-C60•- and ZnP•+-nT-C60•- were present as an equilibrium, because both states have almost the same
thermodynamic stability. This equilibrium resulted in quite long lifetimes of the CS states (450-910 µs) in
•-
o-DCB. In the more polar benzonitrile, the generation of ZnP-nT•+-C60 was confirmed with apparent
•-
short lifetimes (0.6-0.8 µs), which can be explained by the fast hole shift to more stable ZnP•+-nT-C60
followed by the faster charge recombination. It was revealed that the relation between the energy levels of
two CS states, which strongly depend on the solvent polarity, causes dramatic changes of the lifetimes of the
CS states in ZnP-nT-C60; that is, the most appropriate solvents for the long-lived CS state are intermediately
polar solvents such as o-DCB. Compared with our previous data for H2P-nT-C60, in which H2P is free-base
porphyrin, the lifetimes of the CS states of ZnP-nT-C60 are ∼30 times longer than those in o-DCB.
Introduction
nT-C60, the H2P moiety acts as an antenna and an electron
donor, the nT moiety acts as a linker, and the C60 moiety acts
Photoinduced electron transfer is one of the most interesting
research fields attracting much attention. An electron-transfer
rate is mainly governed by factors such as the redox potentials
(Eredox) of the electron acceptor and donor, the reorganization
energy (λ), the electronic coupling (V), and the distance between
the acceptor and donor moieties (R(D-A)).1-10 According to
the Marcus theory, a charge-separated (CS) state lives for a
longer time when the charge-recombination (CR) process occurs
at the deep position in the inverted region.11 Taking these points
into consideration, many research groups have designed in-
tramolecular electron-transfer systems for long-lived CS states.
Because fullerene (C60) has unique characteristics such as a low
reduction potential (Ered) and a small λ value,12-15 many research
groups have reported the photophysical and photochemical
properties of fullerene-donor linked dyad molecules.16 A lot
of groups have designed intramolecular multistep electron-
transfer systems using various donors and acceptors.9,17-21 In
the previous study, we synthesized triads with free-base por-
phyrin (H2P), oligothiophenes (nT), and C60, which were
abbreviated as H2P-nT-C60 in our previous paper.22a,b In H2P-
as an electron acceptor. In H2P-nT-C60 triads, the fluorescence
of the H2P moiety was quenched by introduction of the C60
moiety, comparing with H2P-nT dyads,22a,b which suggests the
direct generation of CS states (H2P•+-nT-C60•- ) from 1H2P*-
nT-C60; after hole shift (HS) from H2P•+-nT-C60•-, H2P-
nT•+-C60•- was confirmed as the final CS state with lifetimes
shorter than 20 µs in polar solvents such as benzonitrile (PhCN)
and o-dichlorobenzene (o-DCB).22a To maintain the CS states
for a longer time, it is necessary to use the porphyrin moiety
(Por) having a lower oxidation potential (Eox) than the nT
moiety. Then, it is expected that the relative stability (∆GRIP
)
•-
of two CS states such as Por-nT•+-C60•- and Por•+-nT-C60
is changeable by solvent polarity, because the energies of these
CS states strongly depend on the distance between the donor
and the acceptor and their radii, as suggested by Weller’s
equation.23,24 Hence, considering the oxidation potential (Eox),
the zinc porphyrin (ZnP) must be more proper for the terminal
electron donor than H2P (Figure 1), because three cases of the
relative stability (∆GRIP) of ZnP-nT•+-C60•- and ZnP•+-nT-
•-
C60 are possible by changing the solvent polarity: ∆GRIP
-
(ZnP-nT•+-C60•-) < ∆GRIP(ZnP•+-nT-C60•-), ∆GRIP(ZnP-
nT•+-C60•-) ) ∆GRIP(ZnP•+-nT-C60•-), and ∆GRIP(ZnP-
nT•+-C60•-) > ∆GRIP(ZnP•+-nT-C60•-) in less, medium, and
more polar solvents, respectively. In the present study, we
examined the CS and CR processes as well as the energy-
* To whom correspondence should be addressed. E-mail: ito@
tagen.tohoku.ac.jp (O.I.); otsubo@hiroshima-u.ac.jp (T.O.).
† Tohoku University.
‡ Hiroshima University.
§ Present address: The Institute of Scientific and Industrial Research,
Osaka University, Ibaraki, Osaka 567-0047, Japan.
10.1021/jp044316v CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/12/2005

14366 J. Phys. Chem. B, Vol. 109, No. 30, 2005
Nakamura et al.
J ) 9.64 Hz, 1H), 3.73 (d, J ) 9.64 Hz, 1H), 3.09 (t, J ) 7.82,
2H), 2.82 (s, 3H), 2.74 (t, J ) 7.82 Hz, 2H), 1.90-1.97 (m,
2H), 1.53-1.62 (m, 2H), 1.19-1.47 (m, 12H), 0.79-0.92 (m,
6H); MS(MALDI-TOF) m/z 1873.9 (M+, calcd 1873.3).
ZnP-8T-C60. Yield 68% from H2P-8T-C60; brown pow-
der; mp 195 °C (decomp); 1H NMR (400 MHz, CDCl3) δ 9.33
(d, J ) 4.88 Hz, 2H), 8.97 (d, J ) 4.88 Hz, 2H), 8.91 (s, 4H),
8.18-8.21 (m, 6H), 7.74-7.78 (m, 9H), 7.75 (s, 1H), 7.28 (d,
J ) 3.90 Hz, 1H), 7.21 (d, J ) 3.90 Hz, 1H), 7.16 (d, J ) 3.88
Hz, 1H), 7.13 (s, 1H), 7.07 (d, J ) 3.78 Hz, 2H), 7.03 (d, J )
3.88 Hz, 1H), 6.99 (d, J ) 3.78 Hz, 2H), 6.97 (s, 2H), 4.95
(s,1H), 4.73 (d, J ) 9.76 Hz, 1H), 3.95 (d, J ) 9.76 Hz, 1H),
3.13 (t, J ) 7.82 Hz, 2H), 2.89 (s, 3H), 2.70-2.77 (m, 6H),
1.94-1.99 (m, 2H), 1.59-1.68 (m, 6H), 1.21-1.44 (m, 24H),
0.80-0.95 (m, 12H); MS(MALDI-TOF) m/z 2371.6 (M+, calcd
2371.5).
Figure 1. Molecular structures of ZnP-nT-C60.
transfer (EN) process in ZnP-nT-C60 by time-resolved absorp-
tion and fluorescence measurements in four solventsstoluene,
anisole, o-DCB, and PhCN. In the last three polar solvents, the
CS processes are expected to occur when the ZnP moiety is
photoexcited. Especially in o-DCB, the energy of ZnP•+-nT-
•-
C60 is equal to that of ZnP-nT•+-C60•-, which is produced
via the HS process or the indirect CS process via ZnP-nT-
1C60*. Our main goal of the present study is to maintain the CS
states for a long time by controlling the driving force for the
CR process by changing the solvent polarity. In addition, to
investigate the distance dependence of the efficiencies of the
CS and EN processes, we compared two triads including
different nT (n ) 4 and 8), which are expected to act as a
molecular wire and as a positive charge trapper ()electron
donor) with respect to ZnP•+. It is well understood that the Eox
values of nT decrease with their lengths, because the π-conjuga-
tion extends along the nT chain.25 These triads including the
ZnP moiety are expected to absorb visible light more efficiently
than the nT-C60 dyads; thus, the photoinduced processes with
high light harvesting ability initiated by the excitation of the
ZnP moiety have been investigated in the present study.
Methods. Molecular orbital calculations were performed
using density functional theory (DFT) at the B3LYP/3-21G(*)
level.26
Square wave volumetric measurements were carried out with
a potentiostat (BAS CV50W) and a cell equipped with a
platinum working electrode, an Ag/Ag+ reference electrode, and
a platinum counter electrode. All electrochemical measurements
were performed in anisole, o-DCB, and PhCN containing 0.10
M tetra-n-butylammonium hexafluorophosphate ((n-C4H9)4-
NPF6, Nacalai Tesque) at room temperature. The redox poten-
tials (Eredox) were corrected against the ferrocene/ferrocenium
(Fc/Fc+) couple.
Steady-state absorption spectra were measured on a JASCO
V-530 UV-vis spectrophotometer. Steady-state fluorescence
spectra were measured on a Shimadzu RF-5300 PC spectro-
fluorophotometer.
Experimental Section
The fluorescence lifetimes were measured by a single-photon
counting method with a streak scope (Hamamatsu Photonics,
C4334-01) using the second harmonic generation (SHG, 400
nm) of a Ti:sapphire laser (Spectra-Physics, Tsunami 3950-L2S,
fwhm 150 fs) as an excitation source.
Materials. Syntheses of ZnP-nT-C60 were performed by
the insertion of the Zn atom into the corresponding H2P-nT-
C60,22b with refluxing them in the presence of Zn(OAc)2.
Components such as nT (n ) 4 and 8) and N-methylpyrrolidino-
C
60 (NMPC60) were prepared according to the methods described
The picosecond transient absorption spectra were measured
by the pump and probe method using a Ti:sapphire regenerative
amplifier seeded by the SHG of an Er-doped fiber laser (Clark-
MXR CPA-2001 plus, 1 kHz, fwhm 150 fs). A white continuum
pulse used as a monitoring light was generated by focusing the
fundamental of the amplifier on a rotating H2O cell. The samples
were excited by the SHG (388 nm) of fundamental or output
of OPA (Clark-MXR vis-OPA, 560 nm). The monitoring light
transmitted through the sample in a rotating cell was detected
with a dual MOS detector (Hamamatsu Photonics, C6140)
equipped with a polychromator for the visible region or an
InGaAs linear image sensor (Hamamatsu Photonics, C5890-
128) for the near-IR region.
The nanosecond transient absorption spectra were measured
using the pulsed laser light from an optical parametric oscillation
(Continuum Surelite OPO, fwhm 4 ns) pumped by a Nd:YAG
laser (Continuum, Surelite II-10). For the measurements of the
transient absorption spectra in the near-IR region for a time scale
shorter than 5 µs, a Ge avalanche photodiode (APD, Hamamatsu
Photonics, B2834) was used as a detector for monitoring light
from the pulsed xenon lamp. For a time scale longer than 5 µs,
an InGaAs photodiode was used as a detector for monitoring
light from the continuous xenon lamp.
in the literature.22b Other chemicals such as zinc tetraphenylpor-
phyrin (ZnTPP) and the solvents (anisole, benzonitrile (PhCN),
o-dichlorobenzene (o-DCB), and toluene) were of the best
commercial grade available.
General. All chemicals are of reagent grade. The melting
1
points are uncorrected. H NMR spectra were measured on a
JEOL Lambda 400 spectrometer using deuteriochloroform or
deuteriodimethyl sulfoxide as solvent and tetramethylsilane as
internal standard. MS spectra were recorded on a Shimadzu
KOMPACT-MALDI PROBE spectrometer using a dithranol
matrix.
ZnP-4T-C60 and ZnP-8T-C60. A typical experimental
procedure is described for the synthesis of ZnP-4T-C60 as
follows: A solution of H2P-4T-C60 (31.4 mg, 0.017 mmol)
in chloroform (5 mL) was mixed with a solution of zinc acetate
(26.6 mg, 0.12 mmol) in methanol (2 mL). The mixture was
refluxed for 1 h in a nitrogen atmosphere and washed with water
(10 mL × 3). After dryness over anhydrous sodium sulfate,
the organic layer was directly subjected to column chromatog-
raphy on alumina with 1:1 hexane-dichloromethane as an eluent
to give a brown powder of ZnP-4T-C60 (23.7 mg).
ZnP-4T-C60. Yield 71% from H2P-4T-C60; mp 265 °C
1
(decomp); H NMR (400 MHz, CDCl3) δ 9.30 (d, J ) 4.88
Results and Dscussion.
Hz, 2H), 8.94 (d, J ) 4.88 Hz, 2H), 8.91 (s, 4H), 8.16-8.20
(m, 6H), 7.73-7.75 (m, 9H), 7.75 (s, 1H), 7.24 (d, J ) 3.78
Hz, 1H), 7.16 (d, J ) 3.78 Hz, 1H), 7.12 (d, J ) 3.80 Hz, 1H),
7.05 (s, 1H), 7.00 (d, J ) 3.80 Hz, 1H), 4.77 (s, 1H), 4.53 (d,
Steady-State Absorption Spectra. Absorption spectra of
ZnP-4T-C60 and the components ZnTPP, 4T, and NMPC60
in PhCN are shown in Figure 2a. The B-band of the ZnP moiety

Energy and Electron Transfer in ZnP-nT-C60 Linked Triads
J. Phys. Chem. B, Vol. 109, No. 30, 2005 14367
where R(D-A) refers to the center-to-center distance between
the donor and the acceptor and ∆E0-0 refers to the energies of
1
1
the ZnP* and C60* moieties.
To obtain insight into the optimized structures of ZnP-nT-
C60, computational studies were performed using density
functional theory (DFT) at the B3LYP/3-21G(*) level (Sup-
porting Information Figures S1 and S2).26 From the DFT-
optimized structures of ZnP-4T-C60 and ZnP-8T-C60, the
R(D-A) values were determined as follows: R(ZnP-C60) )
25.3 Å, R(4T-C60) ) 12.8 Å, and R(ZnP-4T) ) 12.5 Å for
ZnP-4T-C60 and R(ZnP-C60) ) 40.5 Å, R(8T-C60) ) 20.4
Å, and R(ZnP-8T) ) 20.1 Å for ZnP-8T-C60, respectively.
According to eqs 1 and 2, the driving force for the CS process
is governed by ꢀS; therefore, the driving force can be controlled
by changing the solvent. It is also notable that the energy levels
of ZnP•+-8T-C60 vary more widely than those of ZnP-
•-
8T•+-C60 with the variation of the solvent polarity, because
•-
of the R(D-A) term in eq 1 (Supporting Information Figure
S3). The ∆GCR values are summarized in Table 2.
From these thermodynamic values and electronic transition
energies, the energy diagram for ZnP-8T-C60 is illustrated in
Figure 3. A similar energy diagram is obtained for ZnP-4T-
C60.
Steady-State Fluorescence Measurements. Figure 4 shows
the fluorescence spectra of ZnP-4T-C60 and ZnP-4T obtained
by excitation of the ZnP moiety with 420 nm light in PhCN
and toluene, in which the sample concentrations were adjusted
so that the same numbers of photons were absorbed by the ZnP
moiety. The fluorescence band at 625 nm with a shoulder around
Figure 2. Steady-state absorption spectra of (a) ZnP-4T-C60 and (b)
ZnP-8T-C60 in PhCN.
1
670 nm of ZnP-4T can be attributed to the ZnP* moiety.
Because the fluorescence intensity of ZnP-4T in PhCN was
slightly weaker than that in toluene, a small amount of the CS
process takes place between the ZnP and 4T moieties. From
the negative ∆GCS values (Supporting Information Table S1),
both CS processes generating ZnP•--4T•+ and ZnP•+-4T•- via
1ZnP*-4T are possible in PhCN. In toluene, the fluorescence
intensity of the 1ZnP* moiety of ZnP-4T-C60 decreased
compared with that of ZnP-4T and a fluorescence peak of the
1C60* moiety was confirmed around 700 nm (vide infra),
suggesting the EN process to the C60 moiety in ZnP-4T-C60.
In PhCN, further decreases of the fluorescence intensities of
in ZnP-nT-C60 shifted slightly to a longer wavelength and
became broader than that of ZnTPP. Similar absorption spectra
were observed for ZnP-8T-C60, as shown in Figure 2b. These
findings suggest that the symmetry of the ZnP moiety in ZnP-
nT-C60 becomes lower than the highly symmetric ZnTPP, but
appreciable interaction between the ZnP moiety and the nT
moiety in ZnP-nT-C60 may not be present.
One-Electron Redox Potentials and ET Driving Force. The
E
ox and Ered values of ZnP-nT-C60 were evaluated by square
wave voltammetry in anisole, o-DCB, and PhCN, as summarized
in Table 1. The Eox values shifted progressively to a more
negative direction as the solvent polarity was increased: anisole
(ꢀS ) 4.33) < o-DCB (ꢀS ) 9.93) < PhCN (ꢀS ) 25.2),27 where
ꢀS refers to the solvent dielectric constant. Negative shifts in
oxidation potentials are principally rationalized by a stronger
solvation of the resulting radical cations of the ZnP and nT
moieties in polar solvents. On the other hand, the reduction
potentials shifted to a positive direction in polar solvents, which
was due to solvation of the radical anion in polar solvents.
The Eox values of the ZnP moieties are lower than those of
the nT moieties in the dyads and triads. Therefore, the ZnP
moiety mainly works as an electron donor. Because the Eox value
of the 4T moiety is similar to that of the 8T moiety in the dyads
and triads, the differences in kinetic data (vide infra) must be
attributed to the length of the nT moiety rather than the
electronic factors. From the Ered values, the order of the electron-
acceptor abilities is ZnP < nT , C60.
1
1
the ZnP* and C60* moieties were observed compared with
that in toluene, indicating the CS process to ZnP•+-4T-C60
•-
from 1ZnP*-4T-C60 and to ZnP-4T•+-C60•- via ZnP-4T-
1C60* after the EN process. These CS processes are supported
by the negative ∆GCS value in PhCN (-0.70 and -0.34 eV,
respectively, from eq 2 using ∆GCR in Table 2). In anisole and
o-DCB, fluorescence quenching behavior similar to that in PhCN
was observed, indicating that both CS processes and the EN
process were included. For ZnP-8T and ZnP-8T-C60, similar
tendencies to ZnP-4T and ZnP-4T-C60 were observed in all
solvents, respectively.
Fluorescence Lifetime Measurements in Toluene. Figure
5 shows the fluorescence temporal profiles of ZnP-8T and
ZnP-8T-C60 after the excitation of the ZnP moiety in toluene
and PhCN; for ZnP-4T and ZnP-4T-C60, similar fluorescence
temporal profiles to ZnP-8T and ZnP-8T-C60 were observed,
respectively. The temporal profiles of 1ZnP*-nT in toluene can
be fitted with a single-exponential function, from which the
The free-energy changes for the CS and CR processes
(-∆GCS and -∆GCR) were calculated by Weller’s eqs 1 and
2;23
1
fluorescence lifetime (τF) of the ZnP* moiety was evaluated
to be 1.2 ns. Because the τF values for 1ZnP*-nT are
independent from the nT moieties (Table 3), this value (1.2 ns)
is a characteristic fluorescence lifetime (τ0) of the ZnP moiety
without the CS and EN processes in the dyads. τ0-1 was defined
-∆GCR ) Eox - Ered - e2/(ꢀSR(D-A))
-∆GCS ) ∆E0-0 - (-∆GCR
(1)
(2)
)

14368 J. Phys. Chem. B, Vol. 109, No. 30, 2005
Nakamura et al.
TABLE 1: Redox Potentials in Various Solvents
a
E/V (vs Fc/Fc•+
)
•-
material
solvent
anisole
(ꢀS ) 4.33)
ZnP/ZnP•+
nT/nT•+
C60/C60
ZnP/ZnP•-
nT/nT•-
ZnP-4T
0.41
0.42
0.39
0.40
0.38
0.38
0.36
0.37
0.31
0.32
0.30
0.31
0.49
0.48
0.48
0.49
0.47
0.47
0.46
0.46
0.46
0.46
0.39
0.39
-1.70
-1.70
-1.71
-1.72
-1.70
-1.67
-1.68
-1.68
-1.67
-1.63
-1.60
-1.60
-1.58
-1.60
-1.55
-1.60
-1.59
-1.57
-1.53
-1.55
-1.54
-1.55
-1.50
-1.51
ZnP-4T-C60
ZnP-8T
-1.18
-1.18
-1.13
-1.13
-1.04
ZnP-8T-C60
ZnP-4T
o-DCB
(ꢀS ) 9.93)
ZnP-4T-C60
ZnP-8T
ZnP-8T-C60
ZnP-4T
PhCN
(ꢀS ) 25.2)
ZnP-4T-C60
ZnP-8T
ZnP-8T-C60
NMPC60
-1.04
-1.04
a An electrochemical cell equipped with a platinum working electrode, an Ag/Ag+ reference electrode, and a Pt counter electrode. The electrolyte
was 0.10 M (n-Bu)4NPF6. The Eredox values were corrected against the Fc/Fc+ couple.
TABLE 2: Driving Forces (-∆GCR) of the CR Process of
the CS States between ZnP and C60 or between nT and C60
in ZnP-nT-C60 in Toluene, Anisole, o-DCB, and PhCN
-∆GCRa/eV
solvent
toluene
initial state
final state
n ) 4
n ) 8
ZnP-nT•+-C60
ZnP•+-nT-C60
ZnP-nT•+-C60
ZnP•+-nT-C60
ZnP-nT•+-C60
ZnP•+-nT-C60
ZnP-nT•+-C60
ZnP•+-nT-C60
ZnP-nT-C60
ZnP-nT-C60
ZnP-nT-C60
ZnP-nT-C60
ZnP-nT-C60
ZnP-nT-C60
ZnP-nT-C60
ZnP-nT-C60
1.83b
2.13b
1.40
1.48
1.46
1.49
1.45
1.34
2.11b
2.38b
1.43
1.50
1.49
1.47
1.42
1.33
•-
•-
•-
•-
•-
•-
•-
•-
anisole
o-DCB
PhCN
a From eq 1 except in toluene. b Estimated using -∆GCR ) Eox
-
Ered + (e2/(4πꢀ0))[(1/(2R+) + 1/(2R-) - 1/R(D-A))/ꢀS - (1/(2R+) +
1/(2R-))/ꢀR], where R+ and R- are the radii of the cation and anion,
respectively (4T, 7.5 Å; 8T, 14.5 Å; ZnP, 5.0 Å; NMPC60, 4.2 Å).
R(D-A) is the center-to-center distance between the donor and the
acceptor. ꢀ0 and ꢀR refer to the vacuum permittivity and dielectric
constant in the reaction. Eox and Ered are the redox potentials estimated
experimentally in o-DCB.
Figure 4. Fluorescence spectra of ZnP-8T and ZnP-8T-C60 in
toluene and PhCN. Excitation wavelength 420 nm.
Figure 5. Fluorescence decay profiles around 650 nm of ZnP-8T
and ZnP-8T-C60 in toluene and PhCN. Excitation wavelength 400
nm.
1
Figure 3. Schematic energy diagram and main processes for ZnP-
8T-C60; almost the same energy diagram was obtained for ZnP-4T-
C60.
The rise of the fluorescence due to the C60* moiety was
confirmed around 710 nm in toluene, indicating the EN process
from the ZnP* moiety to the C60 moiety (eq 3), which is in
1
good agreement with the appearance of the fluorescence of the
1C60* moiety by the excitation of the 1ZnP* moiety in the steady-
state fluorescence spectra (Figure 4):
as kd ) (kIC + kRAD + kP*(ISC)), in which the rate constants refer
to the internal conversion (IC), radiative (RAD), and intersystem
crossing (ISC) processes of the ZnP moiety, respectively. In
the case of 1ZnP*-nT-C60, the fluorescence intensity decreased
while obeying almost a single-exponential function (> 95%);
the shorter lifetime of 1ZnP*-nT-C60 than that of 1ZnP*-nT
was estimated, as summarized in Table 3. Moreover, the τF
kEN(P*-C)
1ZnP*-nT-C60
8 ZnP-nT-1C60*
(3)
The CS process via 1ZnP*-nT-C60 is impossible in toluene
from the positive ∆GCS(P*-C) values, which can be calculated
from eq 2 using ∆GCR in Table 2; therefore, this shortened τF
1
values of ZnP*-nT-C60 varied depending on the length of
the nT moiety (Table 3).

Energy and Electron Transfer in ZnP-nT-C60 Linked Triads
J. Phys. Chem. B, Vol. 109, No. 30, 2005 14369
1
108 s-1 by changing from 4T to 8T, as listed in Table 5. The
CS process generating distant CS states, ZnP•+-nT-C60•-, via
1ZnP*-nT-C60 has negative ∆GCS(P*-C) values in polar sol-
vents. In this distant CS process, the nT moiety behaves like a
molecular wire.22a For the processes (X) such as CS(P*-C) and
CS(P*-T), the ΦX values were evaluated by eq 8:
TABLE 3: Fluorescence Lifetimes (τF) of ZnP* in Various
Solvents
toluene
anisole
o-DCB
τF/ps
PhCN
materials
τF/ps
τF/ps
τF/ps
ZnP-4T
1200
50
1200
460
1100
40
1100
400
1000
40
920
300
1100
40
980
330
a
ZnP-4T-C60
ZnP-8T
-1
ZnP-8T-C60
ΦX ) kX/[(τ
) ]
(8)
F,triad
a Estimation error of 5%.
When kx for the triad is set equal to kCS(P*-T),dyad in eq 8, the
quantum yields for ZnP•--nT•+-C60 or ZnP•+-nT•--C60 in
the triad (ΦCS(P*-T)) can be evaluated to be less than 0.08
(Supporting Information Table S2), suggesting that the contribu-
tion of such vicinal CS processes in the triads is quite small
even in polar solvents. Possible CS processes of triads in polar
solvents are shown in eqs 9 and 10:
TABLE 4: Rate Constants (kEN(P*-C)) and Quantum Yields
1
(ΦEN(P*-C)) for Energy Transfer from ZnP*-nT-C60 to
ZnP-nT-1C60*a
solvent
k
EN(P*-C) (ΦEN(P*-C)
)
n ) 4
n ) 8
kEN(P*-C)
1.9 × 1010
1.3 × 109
b
c
toluene
anisole
o-DCB
PhCN
ΦEN(P*-C)
0.95
0.61
c
ΦEN(P*-C)
0.76
0.76
0.76
0.52
0.39
0.43
c
ΦEN(P*-C)
c
ΦEN(P*-C)
a The energy difference between 1ZnP* and 1C60* was evaluated to
be 0.28 eV from the fluorescence bands. b The kEN(P*-C) values were
estimated from τF of 1ZnP*-nT-C60 in toluene from eq 4. c The
Φ
EN(P*-C) values in toluene and in polar solvents were evaluated from
eq 5, assuming that the kEN values in polar solvents are the same as
Transient Absorption Studies in PhCN. Figure 6a shows
the picosecond transient absorption spectra of ZnP-8T-C60 in
PhCN with the excitation of the ZnP moiety using a 560 nm
laser light pulse. At 20 ps after the laser irradiation, the
that in toluene.
1
value is due to the EN process from the ZnP* moiety to the
C60 moiety. The rate constant (kEN(P*-C)) and quantum yield
(ΦEN(P*-C)) for the EN process (eq 3) were estimated according
to eqs 4 and 5 from the τ0 and τF values of the triad (τF,triad) in
toluene.
1
absorption bands of the ZnP* moiety were confirmed around
520 and 580 nm with an absorption tail ranging to 1100 nm.
1
Accompanying the decay of the ZnP* absorption band, the
absorptions appeared around 840 and 1050 nm, which are
attributed to the 8T•+ moiety16e,22,29 and the C60 moiety,
•-
kEN(P*-C) ) (τF,triad
)
-1 - (τ0)-1
(4)
(5)
3
respectively.30 Additionally, the absorption band of the ZnP*
moiety appeared at 780 nm, which was overlapped with the
absorption band of the 8T•+ moiety. In Figure 6b, the temporal
profile at 820 nm was curve-fitted by three components: the
-1
ΦEN(P*-C) ) kEN(P*-C)/(τF,triad
)
Both the kEN(P*-C) and ΦEN(P*-C) values drastically decreased
from 1.9 × 1010 to 1.3 × 109 s-1 and from 0.95 to 0.61,
respectively, by changing the nT moiety from 4T to 8T (Table
4). Thus, the nT moiety acts as the spacer for the EN process.
Fluorescence Lifetime Measurements in Polar Solvents.
The kEN values can be considered to be almost the same in
solvents with similar refractive indices, which usually affects
the kEN values.28 Thus, we assumed that the kEN(P*-C) values
are identical in the solvents employed in the present study. The
ΦEN(P*-C) values were evaluated from eq 5 using the τF,triad
values in polar solvents (Table 4).
1
3
decay of ZnP*-8T-C60, the rise and decay of ZnP*-8T-
C60, and the formation of ZnP-8T•+-C60•-. The rate constant
of the decay of the ZnP* moiety, which can be considered to
be the same as the rise of the ZnP* moiety, was employed
1
3
-1
from the (τF,triad
)
value in PhCN (3.0 × 109 s-1); thus, the
time profile of 8T•+ was evaluated by subtracting these two
lines from the observed time profile.
The slow generation rate of ZnP-8T•+-C60 suggests the
•-
presence of some intermediate processes from 1ZnP*-8T-C60
•-
to ZnP-8T•+-C60 (eq 11); for such processes, the energeti-
cally possible intermediate process is the EN process generating
ZnP-8T-1C60* (eq 3) with ΦEN(P*-C) ) 0.43 in PhCN (Table
4). From the curve indicating the generation of ZnP-8T•+-
The rate constant for the CS process between the 1ZnP* and
nT moieties in the triad (kCS(P*-T),triad) was evaluated by eq 6,
assuming that kCS(P*-T),triad is equal to that of the dyad
(kCS(P*-T),dyad), because appreciable interaction is absent between
the nT and C60 moieties, as suggested by the steady-state
absorption spectra reported previously.22a
C60•-, the kCS(T-C*) value was evaluated to be 1.3 × 108 s-1
.
kCS(T-C*)
ZnP-nT-1C60*
8 ZnP-nT•+-C60
(11)
•-
-1
kCS(P*-T),triad ) kCS(P*-T),dyad ) (τF,dyad
)
-1 - τ0
(6)
The vicinal CS(P*-T) process (eq 10) is thermodynamically
possible; however, this process is ignorable because of low
ΦCS(P*-T) ()0.06 (Supporting Information Table S2)).
The kCS(P*-T),triad values for ZnP-4T-C60 and ZnP-8T-C60
are <10 × 107 s-1 and (0.9-2.6) × 108 s-1, respectively
(Supporting Information Table S2).
1
In the case of ZnP-4T-C60, the fast decay of the ZnP*
absorption band was also observed around 530 nm and the
absorption band of the 4T•+ moiety appeared slowly around
700 nm.16d,22a,28 This result implies that the generation process
of ZnP-4T•+-C60•- is mainly attributed to eq 11 after the EN
Finally, the rate for the CS process from 1ZnP* to C60
(kCS(P*-C)) can be evaluated from eq 7:
-1
-1 - (τF,triad,toluene
)
-
1
process from ZnP*-4T-C60 to ZnP-4T-1C60* (eq 3).
kCS(P*-C) ) (τF,triad,polar solvent
)
Figure 7a shows the nanosecond transient absorption spectra
of ZnP-8T-C60 in Ar-saturated PhCN after the nanosecond
laser excitation at 560 nm. The characteristic transient absorption
kCS(P*-T),triad (7)
The kCS(P*-C) value decreased from 5.0 × 109 to (2.4-9.4) ×

14370 J. Phys. Chem. B, Vol. 109, No. 30, 2005
Nakamura et al.
TABLE 5: Driving Forces (-∆GCS, -∆GEN, and -∆GHS) and Rate Constants (kCS(P*-C), kCS(T-C*), and kHS(P-T)) of
ZnP-nT-C60 in Anisole, o-DCB, and PhCN
n ) 4
k/s-1
n ) 8
k/s-1
solvent
anisole
initial state
final state
-∆Ga/eV
Φb
-∆Ga/eV
Φb
•-
•-
•-
•-
•-
•-
1ZnP*-nT-C60
ZnP•+-nT-C60
ZnP-nT•+-C60
ZnP-nT•+-C60
ZnP•+-nT-C60
ZnP•+-nT-C60
ZnP-nT•+-C60
kCS(P*-C)
kHS(P-T)
kCS(T-C*)
kCS(P*-C)
kCS(P*-C)
kCS(T-C*)
0.56
0.08
0.36c
0.55
0.70
0.31c
5.0 × 109
0.20
0.54
0.07
0.33c
0.57
0.62
0.34c
2.4 × 108
1.6 × 108
2.4 × 109
9.4 × 108
6.8 × 108
1.3 × 108 d
0.11
ZnP•+-nT-C60
ZnP-nT-1C60*
1ZnP*-nT-C60
1ZnP*-nT-C60
ZnP-nT-1C60*
6.6 × 108
•-
0.16
0.28
0.23
o-DCB
PhCN
5.0 × 109
5.0 × 109
4.0 × 109 d
0.20
0.20
a From eq 2 in the text and -∆GHS(P-T) ) -∆GCR(T-C) - (-∆GCR(P-C)). b From eq 8. c The ∆E0-0 value for 1C60* is 1.76 eV in eq 2. d From eqs
15 and 16.
Figure 6. (a) Picosecond transient absorption spectra of ZnP-8T-
C60 in PhCN (2.0 × 10-4 M) at 50 ps and 1.9 ns after 560 nm laser
irradiation; (b) absorption time profiles at 820 nm.
bands of 8T•+ were observed at 840 nm and around 1520
nm16e,22a as well as the C60 moiety at 1000 nm. Therefore,
•-
the CS state persisting in this time region was attributed to ZnP-
8T•+-C60•-. The absorption band around 700-800 nm was
3
3
attributed to ZnP*, indicating that ZnP*-8T-C60 was also
generated via an ISC process (Supporting Information Table
S2 (kd and Φd)). From the decay of the C60 moiety (Figure
•-
Figure 7. (a) Nanosecond transient absorption spectra of ZnP-8T-
C60 (1.0 × 10-4 M) in PhCN at 250 ns (b) and 2.5 µs (O) after 560
nm laser irradiation; (b and c) absorption-time profiles at 1020 and
1520 nm, respectively.
7b), the charge-recombination rate can be evaluated, which was
equal to the rate for the absorption decay of 8T•+ at 1520 nm
(Figure 7c). In the case of ZnP-4T-C60, the transient absorption
bands were observed at 700-780, 1000, and 1140 nm, which
•-
C60 was expected to occur as fast as the HS process from
were attributed to ZnP*, C60•-, and 4T•+, respectively.16d,22a
3
nT•+ to ZnP, as shown in eq 12, in which the CR process of
ZnP•+-nT-C60•- may be even faster. Thus, the observed decay
rates of ZnP-nT•+-C60•- are attributed to the apparent charge-
recombination rates of ZnP-nT•+-C60•-, which is the rate-
determining step as listed in Table 6.
The lifetimes of ZnP-nT•+-C60 (τRIP) were evaluated to
•-
be 0.63 µs for nT ) 4T and 0.83 µs for nT ) 8T (Table 6),
which were shorter than those of H2P-nT•+-C60•- (2.4 µs for
nT ) 4T and 1.9 µs for nT ) 8T).22a This difference in these
τ
RIP values was due to the relation of the energy levels of Por-
nT•+-C60•- and Por•+-nT-C60•-. The energy of ZnP•+-nT-
kHS(T-P)
kCR(P-C)
C60 became lower than that of ZnP-nT•+-C60 in PhCN
(Table 2), while the energy of H2P•+-nT-C60•- is higher than
that of H2P-nT•+-C60•-. In the case of ZnP-nT-C60, since
the final CS state (ZnP•+-nT-C60•-) was not confirmed in the
transient absorption spectra, the CR process of ZnP•+-nT-
•-
•-
ZnP-nT•+-C60
8 ZnP•+-nT-C60
8
•-
•-
ZnP-nT-C60
k
CR(T-C) Z kHS(T-P) < kCR(P-C)
(12)

Energy and Electron Transfer in ZnP-nT-C60 Linked Triads
J. Phys. Chem. B, Vol. 109, No. 30, 2005 14371
TABLE 6: Driving Forces (-∆GCR and -∆GHS(P-T)) of the CR and HS Processes, Rate Constants (kCR) for the CR Process,
and Lifetimes of the Radical Ion Pair (τRIP) of ZnP-nT-C60 in Anisole, o-DCB, and PhCN
a The final CS state is ZnP-nT-C60. b From eq 2 in the text and -∆GHS(P-T) ) -∆GCR(T-C) - (-∆GCR(P-C)). c At infinite dilution. Intermolecular
CR rate constants at higher laser power: 8.5 × 109 M-1 s-1 for 4T and 1.1 × 109 M-1 s-1 for 8T.
In general, the singlet-triplet energy gap in the radical ion-
pair state is characterized by the exchange interaction, which
strongly depends on the distance between the two spins.31 In
ZnP•+-nT-C60•-, they are separated by the nT moiety, which
results in the small energy gap between the singlet and triplet
spin states, mixing each other. Consequently, the lifetime of
ZnP•+-nT-C60•- due to the charge recombination through the
singlet spin state may be shorter than that of ZnP-nT•+-C60
•-
with the triplet spin state due to the relatively large exchange
interaction.
Transient Absorption Studies in Anisole. When the ZnP
moiety of ZnP-8T-C60 was excited with 560 nm laser light
in anisole, the absorption bands were observed at 600-650 nm
and at 1000-1100 nm, as shown in Figure 8a. The ZnP•+ moiety
(600-650 nm)32 showed the rise and decay (Figure 8b), while
•-
the absorption band of the C60 moiety showed a monotonic
rise, as shown in Figure 8c. This finding suggests that ZnP•+-
8T-C60•- was directly produced from 1ZnP*-8T-C60 (eq 9).
Similarly, the direct generation of a remote CS state was also
confirmed for ZnP-4T-C60. As shown in Figure 8b, the
temporal profile at 650 nm consists of three components, that
is, two decay curves and one rise curve. When one of the decay
curves is attributed to the 1ZnP* moiety with the rate as
evaluated from (τF,triad
)
-1 being 2.5 × 109 s-1, the deconvolution
of the temporal profile gave the rising temporal profile of the
ZnP•+ moiety at 650 nm with the rate constant, krise(ZnP•+) )
2.3 × 109 s-1. The kCS(P*-C) values were evaluated from the
rise of the ZnP•+ moiety by eq 13:
kCS(P*-C) ) ΦCS(P*-C)krise(ZnP•+)
(13)
By substituting ΦCS(P*-C) ) 0.11 (Table 5), the rate constant
(kCS(P*-C)) for the generation of the ZnP•+ moiety from the
1ZnP* moiety was evaluated to be 2.5 × 108 s-1. Moreover,
the decay rate of the ZnP•+ moiety gave the rate constant
(kHS(P-T)) for the HS process from ZnP•+ to 8T (eq 14), which
was evaluated to be 1.6 × 108 s-1. The nT moiety acted as a
cation trapper from the ZnP•+ moiety similar to H2P-nT•+-
Figure 8. (a) Picosecond transient absorption spectra of ZnP-8T-
C
60 in anisole (2.0 × 10-4 M) at 250-350 ps and 1.8 ns after 560 nm
•- 22a
C60
.
laser irradiation; (b and c) absorption-time profiles at 650 and 1030
nm, respectively.
kHS(P-T)
ZnP•+-nT-C60
8 ZnP-nT•+-C60
(14)
•-
•-
which is almost the same rate as the rise of the ZnP•+ moiety
(kdirect(C60•-) in Figure 8c), while the latter is the CS process
via ZnP-nT-1C60* (kindirect(C60•-)) which was calculated by
subtraction of the superposition of the decay of 1ZnP* and the
direct generation from the best-fitted curve, giving an apparent
rate of 3.2 × 109 s-1. Direct observation of the rise and decay
of the 1C60* moiety was difficult, because of its broad absorption
In Figure 8c, the temporal profile at 1030 nm consists of three
components, that is, one decay curve and two rise curves. The
1
decay profile is attributed to the absorption tail of the ZnP*
moiety, while the rise components are due to the C60•- and 1C60*
moieties. The C60•- moiety was formed directly and indirectly;
1
the former is the direct CS process via ZnP*-nT-C60 (eq 9)

14372 J. Phys. Chem. B, Vol. 109, No. 30, 2005
Nakamura et al.
in the 900-1000 nm region, with which the absorption tails of
•-
1ZnP* and C60•- were overlapped. Furthermore, the kindirect(C60
)
1
value was larger than the rate for the generation of the C60*
moiety (1.3 × 109 s-1): kindirect(C60•-) > kEN(P*-C). However,
taking the fluorescence lifetime of NMPC60 (1.30 ns) into
consideration, the values of kCS(T-C*) (2.4 × 109 s-1) and
ΦCS(T-C*) (0.16) were calculated using eqs 15 and 16.
kCS(T-C*) ) kobs(C60•-) - τF(NMPC60)-1
(15)
ΦCS(T-C*) ) ΦET(P*-C)[kCS(T-C*)/kindirect(C60•-)] (16)
Comparing the absorption temporal profile at 650 nm with
•-
that at 1030 nm, the absorption of the C60 moiety remains
after 0.5 ns, while the ZnP•+ moiety begins to decay. This
finding supports that the HS process occurs from the ZnP•+
moiety to the 8T moiety (eq 14).
In the case of ZnP-4T-C60, the analysis of the absorption
temporal profile at 1000 nm was difficult, because the absorption
1
1
bands of the C60* and ZnP* moieties were overlapped with
•-
that of the C60 moiety.
In the microsecond region, the absorption bands due to the
C60•-, 8T•+, and 3ZnP* moieties were confirmed, indicating the
generation of ZnP-8T•+-C60 and 3ZnP*-8T-C60. The
•-
kCR(T-C) value was evaluated, as listed in Table 6, from which
the τRIP values were estimated to be 7.7 and 9.1 µs for ZnP-
4T•+-C60 and ZnP-8T•+-C60•-, respectively, which were
•-
almost the same as those of H2P-nT•+-C60 (5.3 µs for nT
•-
Figure 9. (a) Nanosecond transient absorption spectra of ZnP-8T-
C60 (1.0 × 10-4 M) in o-DCB at 50 µs in Ar-saturated (b) and O2-
saturated (O) after 560 nm laser irradiation; (b) absorption-time profiles
at 1520 nm. Inset: laser-power dependence of the CR rate.
) 4T and 4.5 µs for nT ) 8T, Supporting Information Table
•-
S3). Since ZnP-nT•+-C60 is more stable than ZnP•+-nT-
•-
C60 in anisole (Table 2), it is indicated that the final CR
process takes place via ZnP-nT•+-C60 to the neutral triads
•-
18. This was supported by the similarity of the energy of ZnP-
•- 22a
(eq 17) similar to that of H2P-nT•+-C60
.
•-
•-
nT•+-C60 to that of ZnP•+-nT-C60
.
kCR(T-C)
kCR
8
[ZnP-nT•+-C60•- a ZnP•+-nT-C60
]
•-
ZnP-nT•+-C60
8 ZnP-nT-C60
(17)
•-
ZnP-nT-C60 (18)
Transient Absorption Studies in o-DCB. In o-DCB, the
picosecond transient absorption spectra were not obtained,
because the ZnP moiety was degraded seriously by the intense
femtosecond laser in solvents including halogen atoms. On the
other hand, the nanosecond transient absorption spectra in
o-DCB were observed without degradation of the ZnP moiety,
because of the low laser power of the nanosecond laser pulse.
Figure 9a shows the nanosecond transient absorption spectra
of ZnP-8T-C60 observed at 50 µs in Ar- and O2-saturated
o-DCB. In the Ar-saturated solution, the absorption bands of
the ZnP•+, 8T•+, C60•-, and 3ZnP* moieties were confirmed. In
O2-saturated o-DCB, the absorption bands of the 3ZnP* moiety
vanished, while the long-lived ZnP•+, 8T•+, and C60•- moieties
remained, as seen in Figure 9a. Thus, the kinetic analysis can
be performed without interference of the huge absorption bands
of the 3ZnP* moiety. The absorption band attributed to the 8T•+
moiety (>1300 nm) becomes broad with a red shift compared
with that in PhCN; similarly, a red shift was confirmed in the
ZnP•+ moiety (680 nm), where ZnTPP•+ appeared around 620
nm.32
The temporal profile of the 8T•+ moiety in Ar-saturated o-DCB
showed two step-decays: the intramolecular CR (CRintra) process
and the intermolecular back electron-transfer (BETinter) process
(eq 19). The apparent decay rate strongly depended on the laser
power, which varies the concentration of the radical ions.
inter
kBET
(ZnP-nT)•+-C60•- + (ZnP-nT)•+-C60
8
•-
ZnP-nT-C60 + ZnP-nT-C60 (19)
Hence, the rate constants were estimated from the decay of
the nT•+ moiety by eq 20.
-d[ln(∆A0)]/dt ) ∆kfirst ) kCRintra + (2kBETinter/ꢀnT)∆A0
(20)
Hereby, ∆A0 refers to the absorbance at t ) 0, kfirst refers to the
first-order rate constant evaluated in the initial part of the decay,
intra
inter
kCR
and kBET
refer to the rate constants for the intramo-
lecular CR process between the C60•- and nT•+/ZnP•+ moieties
and intermolecular back electron transfer, respectively, and ꢀnT
refers to an extinction coefficient of the absorption band of
nT•+.22 From the laser-power dependence of the ∆A0 and kfirst
values, a linear relation between the kfirst and ∆A0 values was
In O2-saturated o-DCB, initial decays of the absorption bands
ZnP•+, 8T•+, and C60 seem to be faster than those in Ar-saturated
solution; the slow decay component of the 8T•+ moiety remains
over 2-3 ms in both Ar-saturated and O2-saturated o-DCB, as
shown in Figure 9b. It is notable that the ZnP•+, 8T•+, and C60
obtained, as shown in the inset of Figure 9b. From the intercept,
•-
value was estimated to be 1.1 × 103 s-1, which
intra
moieties decayed at the same rate. Thus, it was concluded that
the kCR
•-
•-
ZnP-nT•+-C60 and ZnP•+-nT-C60 were present as an
corresponds to τRIP ) 910 µs. From the slope and ꢀnT, the
equilibrium mixture in the singlet spin state, as shown by eq
kBETinter value was estimated to be 1.1 × 109 M-1 s-1 for ZnP-

Energy and Electron Transfer in ZnP-nT-C60 Linked Triads
J. Phys. Chem. B, Vol. 109, No. 30, 2005 14373
(4) (a) Davis, W. B.; Ratner, M. A.; Wasielewski, M. R. Chem. Phys.
2002, 281, 333. (b) Miller, S. E.; Lukas, A. S.; Marsh, E.; Bushard, P.;
Wasielewski, M. R. J. Am. Chem. Soc. 2000, 122, 7802. (c) Gosztola, D.;
Wang, B.; Wasielewski, M. R. J. Photochem. Photobiol., A 1996, 102, 71.
(d) Wasielewski, M. R. Chem. ReV. 1992, 92, 395.
8T-C60 in o-DCB. The intermolecular BET process takes place
near the diffusion-controlled limit (kdiff ) 5.0 × 109 M-1 s-1
in o-DCB).28
Comparison with H2P-nT-C60. In the case of ZnP-nT-
C60, the final CS states depended on the solvent polarity, while
(5) Verhoeven, J. W.; Scherer, T.; Willemse, R. J. Pure Appl. Chem.
1993, 65, 1717.
the final CS state for H2P-nT-C60 was H2P-nT•+-C60
•-
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those for H2P-nT•+-C60•- (14-27 µs). Considering the relation
402, 47.
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of the energy levels of the CS states (∆GRIP(ZnP-nT•+-C60
)
•-
) ∆GRIP(ZnP•+-nT-C60•-) and ∆GRIP(H2P-nT•+-C60•-) <
∆GRIP(H2P•+-nT-C60•-)), this difference in the τRIP values is
•-
due to the equilibrium of ZnP-nT•+-C60 and ZnP•+-nT-
C60•-, as described in eq 18.
In anisole, the τRIP values of ZnP-nT•+-C60•- (7.7-9.1 µs)
are almost the same values as those of H2P-nT•+-C60•- (5.3-
4.5 µs), since ∆GRIP(ZnP-nT•+-C60•-) < ∆GRIP(ZnP•+-nT-
C60•-) is the same tendency with the ∆GRIP values for H2P-
•-
nT-C60. The final CR process via ZnP-nT•+-C60 may be
relatively slow because of the Marcus inverted region similar
•- 22a
to that of H2P-nT•+-C60
.
Furthermore, the triplet spin
character of this vicinal radical ion pair may also prolong the
lifetimes.31
In PhCN, on the other hand, the τRIP values of ZnP-nT•+-
C60 (0.63-0.83 µs) were shorter than those of H2P-nT•+-
•-
•-
C60 (1.9-2.4 µs), indicating that the CR process may occur
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Conclusion
The charge separation of ZnP-nT-C60 by excitation of the
1ZnP* moiety gave two CS sates; one is ZnP•+-nT-C60
•-
1
generated by the direct CS process via ZnP* with a wirelike
performance of the nT moiety, and the other is CS ZnP-nT•+-
•-
1
C60 generated by the indirect CS process via C60* after the
EN process from ZnP* to C60. ZnP-nT•+-C60 was also
generated by the HS process from ZnP•+-nT-C60•-. The
electron-transfer processes were successfully controlled by the
solvent polarity. In the case of o-DCB, the τRIP values were
estimated to be extremely long compared with those in the other
solvents and H2P-nT•+-C60•- in o-DCB. These long τRIP values
1
•-
•-
may be caused by the equilibrium between ZnP-nT•+-C60
and ZnP•+-nT-C60•- with similar energies in o-DCB. The roles
of the nT moieties vary much with the length, electronic factors,
and medium effects.
Acknowledgment. The present work was partly supported
by a Grant-in-Aid for Scientific Research on Priority Area (417)
from the Ministry of Education, Science, Sports and Culture of
Japan.
Supporting Information Available: Optimized structures
and the HOMO and LUMO. Tables for free-energy changes
for charge separation and charge recombination. Table for rate
constants and quantum yields. Calculation of energy-transfer
rate constants. Charge-recombination data of H2P-nT-C60 in
anisole. This material is available free of charge via the Internet
at http://pubs.acs.org.
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