Linkage dependent charge separation and charge recombination in porphyrin-pyromellitimide-fullerene triads

Article abstract of DOI:10.1021/jp014433f

A homologous series of zincporphyrin (ZnP)-pyromellitimide (Im)-C₆₀ linked triads where the pyromellitimide (Im) moiety is incorporated as an intermediate acceptor between the above two chromophores with a linkage of different spacers, ZnP-Im-CH₂-C₆₀, ZnP-Im-C₆₀, and ZnP-CH₂-Im-C₆₀ as well as the reference dyads (ZnP-Im-CH₂-ref, ZnP-Im-ref, and ZnP-CH₂-Im-ref) have been prepared to investigate linkage dependence of photoinduced electron transfer (ET) and back ET to the ground state in the triads. Time-resolved transient absorption spectra of the triads measured by picosecond laser photolysis as well as the fluorescence lifetimes in THF reveal the occurrence of photoinduced ET from the singlet excited state of the ZnP to the Im moiety to give the initial charge-separated state, i.e., the zincporphyrin radical cation (ZnP^.+)-imide radical anion (Im^.-) pair, followed by a charge shift (CSH) to produce the final charge-separated state, the ZnP^.+-C₆₀^.- pair. The rate constants of photoinduced ETs in ZnP-Im-C₆₀ (1.8 × 10¹⁰ s^-1) and ZnP-Im-CH₂-C₆₀ (1.3 × 10¹⁰ s^-1) in THF are much larger than those in ZnP-CH₂-Im-C₆₀ (2.9 × 10⁹ s^-1) and ZnP-CH₂-Im-ref (1.9 × 10⁹ s^-1). The larger charge separation (CS) rates in the former case are ascribed to the relatively strong electronic coupling because of the absence of a methylene linkage between the ZnP and the Im moieties in ZnP-Im-C₆₀ and ZnP-Im-CH₂-C₆₀ as compared to the triad and dyad with the methylene linkage. The transient absorption spectra of the final charge-separated state, the ZnP^.+-C₆₀^.- pair, have been also measured by nanosecond laser photolysis. It has been found that the rate constants of charge recombination (CR) of ZnP^.+-Im-C₆₀^.- are temperature independent, but that the CR rate constants of ZnP^.+-Im-CH₂-C₆₀^.- exhibit an Arrhenius-like temperature dependence with an activation energy of 0.13 eV which corresponds to the energy difference between ZnP^.+-Im^.--CH₂-C₆₀ and ZnP^.+-Im-CH₂-C₆₀^.-. This indicates that the relatively strong electronic coupling without methylene linkage in ZnP-Im-C₆₀ results in the preference of the tunneling superexchange ET over the sequential ET in the CR process which requires the thermal activation to reach the higher energy state (i.e., ZnP^.+-Im^.--C₆₀), whereas the sequential ET predominates in the triads with the methylene linkage.

Full text of DOI:10.1021/jp014433f

J. Phys. Chem. A 2002, 106, 2803-2814
2803
Linkage Dependent Charge Separation and Charge Recombination in
Porphyrin-Pyromellitimide-Fullerene Triads
Hiroshi Imahori,*,† Koichi Tamaki,‡ Yasuyuki Araki,§ Taku Hasobe,| Osamu Ito,*,§
Akihisa Shimomura, Santi Kundu, Tadashi Okada,*, Yoshiteru Sakata,‡ and
Shunichi Fukuzumi*,|
Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity, CREST,
Japan Science and Technology Corporation (JST), Suita, Osaka 565-0871, Japan, and The Institute of
Scientific and Industrial Research, Osaka UniVersity, 8-1 Mihoga-oka, Ibaraki, Osaka 567-0047, Japan, and
Institute for Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, CREST, Japan Science and
Technology Corporation (JST), Katahira, Aoba-ku, Sendai 980-8577, Japan, and Department of Chemistry,
Graduate School of Engineering Science and Research Center for Materials Science at Extreme Conditions,
Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan
ReceiVed: December 5, 2001
A homologous series of zincporphyrin (ZnP)-pyromellitimide (Im)-C60 linked triads where the pyromellitimide
(Im) moiety is incorporated as an intermediate acceptor between the above two chromophores with a linkage
of different spacers, ZnP-Im-CH2-C60, ZnP-Im-C60, and ZnP-CH2-Im-C60 as well as the reference dyads
(ZnP-Im-CH2-ref, ZnP-Im-ref, and ZnP-CH2-Im-ref) have been prepared to investigate linkage dependence
of photoinduced electron transfer (ET) and back ET to the ground state in the triads. Time-resolved transient
absorption spectra of the triads measured by picosecond laser photolysis as well as the fluorescence lifetimes
in THF reveal the occurrence of photoinduced ET from the singlet excited state of the ZnP to the Im moiety
to give the initial charge-separated state, i.e., the zincporphyrin radical cation (ZnP•+)-imide radical anion
•-
(Im•-) pair, followed by a charge shift (CSH) to produce the final charge-separated state, the ZnP•+-C60
pair. The rate constants of photoinduced ETs in ZnP-Im-C60 (1.8 × 1010 s-1) and ZnP-Im-CH2-C60 (1.3 ×
1010 s-1) in THF are much larger than those in ZnP-CH2-Im-C60 (2.9 × 109 s-1) and ZnP-CH2-Im-ref (1.9
× 109 s-1). The larger charge separation (CS) rates in the former case are ascribed to the relatively strong
electronic coupling because of the absence of a methylene linkage between the ZnP and the Im moieties in
ZnP-Im-C60 and ZnP-Im-CH2-C60 as compared to the triad and dyad with the methylene linkage. The transient
•-
absorption spectra of the final charge-separated state, the ZnP•+-C60 pair, have been also measured by
nanosecond laser photolysis. It has been found that the rate constants of charge recombination (CR) of ZnP•+-
•-
•-
Im-C60 are temperature independent, but that the CR rate constants of ZnP•+-Im-CH2-C60 exhibit an
Arrhenius-like temperature dependence with an activation energy of 0.13 eV which corresponds to the energy
difference between ZnP•+-Im•--CH2-C60 and ZnP•+-Im-CH2-C60•-. This indicates that the relatively strong
electronic coupling without methylene linkage in ZnP-Im-C60 results in the preference of the tunneling
superexchange ET over the sequential ET in the CR process which requires the thermal activation to reach
the higher energy state (i.e., ZnP•+-Im•--C60), whereas the sequential ET predominates in the triads with the
methylene linkage.
Introduction
nates over coherent superexchange ET when ET from D to B
as well as from B to A is exothermic, whereas the relative
There has been considerable interest and discussion about
the relative importance of a direct superexchange (coherent)
process in electron transfer (ET) between electron donors (D)
and acceptors (A) linked by bridges (B) in D-B-A systems
(in which D•+-B•--A plays the role of a virtual state) versus
a sequential (incoherent) process which involves a bona fide
intermediate D•+-B•--A.1-3 Incoherent sequential ET domi-
importance of sequential versus superexchange ET may be
altered by a subtle change in the driving force and the electronic
coupling in each ET step when ET from D to B is slightly
endothermic or thermoneutral and ET from B to A is exother-
mic. Such an argument will be also applicable to charge
recombination (CR) processes from A•- to D•+. Namely, direct
superexchange-mediated ET occurs from A•- to D•+ or sequen-
tial ET takes place from A•- to D•+ via D•+-B•--A. However,
a switch between sequential and superexchange ET, especially
for CR processes, has yet to be fully studied in a homologous
series of D-B-A systems.2-14
* To whom correspondence should be addressed. E-mail: imahori@
mee3.moleng.kyoto-u.ac.jp; ito@tagen.tohoku.ac.jp; okada@chem.es.
osaka-u.ac.jp; fukuzumi@ap.chem.eng.osaka-u.ac.jp.
† Kyoto University.
‡ The Institute of Scientific and Industrial Research, Osaka University.
§ Tohoku University.
We report herein such a system to demonstrate a switch
between sequential and superexchange ET using a homologous
series of zincporphyrin (ZnP)-C60 linked triads where the
pyromellitimide (Im) moiety is incorporated as an intermediate
| Graduate School of Engineering, Osaka University.
Science and Research Center for Materials Science at Extreme
Conditions, Osaka University.
10.1021/jp014433f CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/05/2002

2804 J. Phys. Chem. A, Vol. 106, No. 12, 2002
Imahori et al.
Results and Discussion
Synthesis and Characterization. The synthetic routes to
ZnP-Im-CH2-C60, ZnP-Im-C60,15 and ZnP-CH2-Im-C60 are
shown in Scheme 1. Cross-condensation of aminoporphyrin
1a,23b pyromellitic dianhydride 2, and formyl-protected benzyl-
amine 3a29 or aniline 3b,30 followed by acid hydrolysis, gave
pyromellitimide-substituted porphyrins 4a and 4b in 25 and 19%
yield, respectively. Similar cross-condensation of aminometh-
ylporphyrins 1b, 2, and 3b and subsequent acid hydrolysis
afforded pyromellitimide-substituted porphyrin 4c in 13%. The
zincporphyrin-pyromellitimide-C60 triads (ZnP-Im-CH2-C60,
ZnP-Im-C60, and ZnP-CH2-Im-C60) were obtained in 23-42%
yield by 1,3-dipolar cycloadditions31 with 4a-c, C60, and
N-methylglycine in toluene and subsequent treatment with zinc
acetate. Porphyrin-pyromellitimide dyads [ZnP-Im-CH2-ref
(m)0, n)1, RdCH3), ZnP-Im-ref (m, n)0, RdC16H33), and
ZnP-CH2-Im-ref (m)1, n)0, RdC16H33)] as well as porphyrin
and C60 references [ZnP-ref23b (m)0), ZnP-CH2-ref (m)1),
and C60-ref25a] were also prepared (Figure 2). Structures of all
new compounds were confirmed by spectroscopic analysis
including 1H NMR, IR, and FAB mass spectra (see the
Experimental Section). The absorption spectra of ZnP-Im-CH2-
C60, ZnP-Im-C60, and ZnP-CH2-Im-C60 in THF are virtually
linear combinations of the absorption spectra of each chro-
mophore, giving no indication for strong interactions between
these chromophores in the ground state.
Figure 1. Porphyrin-pyromellitimide-C60 triads.
acceptor between the above two chromophores with a linkage
of different spacers (ZnP-Im-CH2-C60 (m ) 0 and n ) 1)),
ZnP-Im-C60 (m and n ) 0)15 and ZnP-CH2-Im-C60 (m ) 1
and n ) 0), as shown in Figure 1. The energy gradient of each
state in ZnP-(CH2)m-Im-(CH2)n-C60 is well tuned to display
sequential ET within a molecule (i.e., 1ZnP*-(CH2)m-Im-(CH2)n-
C60 f ZnP•+-(CH2)m-Im•--(CH2)n-C60 f ZnP•+-(CH2)m-Im-
(CH2)n-C60•-). This type of sequential ET, which is similar to
initial charge separation (CS) event in photosynthesis, has not
been reported in donor-linked fullerenes.15-27
Because fullerenes exhibit small reorganization energies (λ)
in ET,23-28 photoinduced CS and CR are accelerated and
decelerated, respectively, which have been observed frequently
in donor-linked fullerenes.23-27 Thus, the CR rates from the
C60 to the ZnP•+ in the resulting final charge-separated state
•-
Similar results were obtained in 1,4-dioxane. The center-to-
center distance (Rcc) and edge-to-edge distance (Ree) were
estimated by molecular mechanic calculations (MacroModel,
version 6.0) and summarized in Table 1.
of the present triads can be markedly slowed by shifting them
deep into the inverted region of the Marcus parabola. Taking
into account the fact that the first reduction potentials of the
Im are more negative by ∼0.1 V than those of the C60,15 the
CR processes will be affected greatly by the Im moiety as a
“stepping stone”. In this study, the effects of linkage on the
CR processes as well as the CS processes in the triads are
examined in detail by time-resolved transient absorption spec-
troscopy and fluorescence lifetime measurements.
Photophysics of Porphyrin-Pyromellitimide Dyads. At first,
the photodynamics of the dyad systems (ZnP-Im-CH2-ref, ZnP-
Im-ref, and ZnP-CH2-Im-ref) were investigated for the better
understanding of the more complex triad systems. Time-resolved
transient absorption spectra of the dyads were measured by
picosecond laser photolysis. Upon excitation of ZnP-Im-CH2-
SCHEME 1

Porphyrin-Pyromellitimide-Fullerene Triads
J. Phys. Chem. A, Vol. 106, No. 12, 2002 2805
TABLE 1: One-Electron Redox Potentials (vs Fc/Fc+)a of Triads in CH2Cl2 and Edge-to-Edge Distance (Ree) and
Center-to-Center Distance (Rcc) between Each Chromophoreb
edge-to-edge distance
redox potential
(center-to-center distance)b
Ree (Rcc/Å)/Å
E 0ox/V
ZnP•+/ZnP
+0.31
E0red/V
•-
compound
C60/C60
Im/Im•-
ZnP, Im
Im, C60
ZnP, C60
ZnP-Im-CH2-C60
-1.13
-1.13
-1.14
-1.26
5.7
(12.5)
5.7
(12.5)
6.4
7.6
(13.4)
6.5
(12.3)
6.5
18.6
(25.3)
18.4
(24.2)
16.3
ZnP-Im-C60
+0.31
+0.32
-1.21
-1.26
ZnP-CH2-Im-C60
(11.7)
(12.3)
(19.8)
a
Measured by differential pulse voltammetry in CH2Cl2 using 0.1 M n-Bu4NPF6 as a supporting electrolyte with a sweep rate of 10 mV s-1
.
b Estimated using molecular mechanics calculations (MacroModel, version 6.0).
Figure 2. Porphyrin-pyromellitimide dyads and porphyrin and C60
references.
Figure 4. Time profiles in the transient absorption spectra of ZnP-
Im-CH2-ref in THF at 460 (top), 620 (middle), and 710 nm (bottom)
in THF. The solid lines are the simulated curves by convolution of
exciting pulse, transient absorption, and probing pulse with the lifetime
of 85 and <5 ps (S1 and S2).
charge-separated state, i.e., the ZnP•+-Im•- radical ion pair via
1
photoinduced ET from the ZnP* to the Im moiety.
Figure 3. Differential absorption spectra obtained upon picosecond
flash photolysis (590 nm) of ∼10-4 M solutions of ZnP-Im-CH2-ref
in argon saturated THF with a time delay of 25 ps.
The time profile of the absorbance at 460 nm, which
corresponds to the 1ZnP* and ZnP•+, was analyzed by a single-
exponential decay with a lifetime of 85 ( 9 ps (Figure 4). The
time profiles of the absorbance at 620 and 710 nm, which are
due to ZnP•+ and Im•-, respectively, were fitted by a fast rise
(<5 ps) and a decay (85 ( 9 ps) component (Figure 4). The
fitting procedure of the kinetic data are given in the Supporting
Information (S1 and S2). The fluorescence lifetimes of the
porphyrin-pyromellitimide dyads were also measured by a
picosecond single-photon counting technique (see the Experi-
mental Section). The samples in THF were excited at 524 nm,
where the porphyrin moiety absorbs mainly. The fluorescence
decay was monitored at 605 nm, where the emission is due to
ref in THF with a picosecond pulse at 590 nm (absorption ratio
of porphyrin:C60)14:1), transient absorption bands at 460, 620,
and 710 nm appear, accompanied by bleaching of the ground-
state porphyrin absorption at 560 nm, as shown in Figure 3.
The transient absorption bands at 460 and 620 nm are
characteristic of those of the zincporphyrin radical cation
(ZnP•+),32 and the band at 710 nm is diagnostic of the imide
radical anion (Im•-).33 The absorption that is due to the
porphyrin excited singlet state (1ZnP*) is overlapped with the
band at 460 nm. These results clearly reveal formation of the

2806 J. Phys. Chem. A, Vol. 106, No. 12, 2002
Imahori et al.
TABLE 2: CS (kET(CS)), CSH (kET(CSH)), and CR (kET(CR)) Rate Constants of Triads and Dyads Obtained Using Transient
Absorption Spectroscopy, Quantum Yield (O1, O2, Ototal), and Activation Energy (Ea)
k
ET(CS)/s-1a
k
ET(CSH)/s-1a
k
ET(CR)/s-1a
(φ1)b
(φ2)b
(φtotal ) φ1 × φ2)b
Ea/eV d
THF
compound
THF
dioxane
THF
dioxane
THF
dioxane
ZnP-Im-CH2-C60
1.3 × 1010
4.5 × 109
7.0 × 1010
2.8 × 109
2.9 × 106
>2 × 1011
c
0.13d
(0.97)
(0.90)
(<0.26)
(0.11)
(<0.25)
(0.10)
ZnP-Im-CH2-ref
ZnP-Im-C60
1.1 × 1010
(0.96)
4.5 × 109
(0.90)
2.2 × 1010
1.8 × 1010
(0.97)
7.8 × 109
(0.94)
1.0 × 1011
1.6 × 1010
8.3 × 105
c
∼0 d
(0.50)
(0.48)
(0.49)
(0.45)
ZnP-Im-ref
1.1 × 1010
(0.92)
5.1 × 109
(0.91)
1.0 × 1011
1.7 × 1010
ZnP-CH2-Im-C60
ZnP-CH2-Im-ref
2.9 × 109
(0.88)
1.7 × 109
(0.59)
1.7 × 1010
4.4 × 108
8.3 × 105
c
0.07 d
(0.25)
(0.37)
(0.22)
(0.22)
1.9 × 109
(0.79)
8.0 × 108
(0.32)
5.0 × 1010
7.4 × 108
ZnP-C60
3.7 × 105 e
e
(0.99)
e
ZnP-H2P-C60
2.9 × 104 e
(0.26)
a The kET values were determined by analyzing the decay and rise of transient species (i.e., 1ZnP*, ZnP•+, Im•-, and C60•-) at 460, 620, 710, and
1000 nm. b The quantum yields of formation of charge-separated states were determined according to Schemes 2 and 3 [φ1 ) kET(CS1)/(kET(CS1) + k0
+ kISC), φ2 ) kET(CSH)/(kET(CSH) + kET(CR1))]. c The decay of the charge-separated state to the triplet excited states is much faster than that to the
ground state. d The Ea values were determined from Arrhenius analyses of the temperature-dependent CR rates. e From ref 24c,e.
SCHEME 2: Reaction Scheme and Energy Diagram for
ZnP-Im-CH2-ref in THF
Section and the Supporting Information, S3).36 The driving
forces (-∆G0ET(CR) in eV) for the intramolecular CR processes
from the imide radical anion (Im•-) to the zincporphyrin radical
cation (ZnP•+) in the dyads are determined by eq 2, where e
stands for the elementary charge:
-∆G0ET(CR) ) e[E0ox (D•+/D) - E0red (A/A•-)]
(2)
The driving forces for the intramolecular CS processes
(-∆G0ET(CS) in eV) from the zinc porphyrin singlet excited state
to the Im was determined by eq 3:
-∆G0ET(CS) ) ∆E0-0 + ∆G0
(3)
ET(CR)
where ∆E0-0 is the energy of the 0-0 transition energy gap
between the lowest excited state and the ground state, which is
determined by the 0-0* absorption and 0*-0 fluorescence
maxima. It should be noted that the Coulombic terms in the
present donor-acceptor systems are assumed to be negligible
in solvents with moderate polarity (i.e., THF), because of the
relatively long edge-to-edge distance employed.36
The photodynamical behavior of ZnP-Im-ref and ZnP-CH2-
Im-ref in THF was similar to those described in ZnP-Im-CH2-
ref. The rate constants of the CS (kET(CS1)) and the CR (kET(CR1))
only the porphyrin moiety. The decay curves were fitted by a
single exponential. On the basis of the fluorescence lifetimes
of the dyads (τ) and the reference (ZnP-ref: τ0), the CS rate
constant (kET(CS1)) from the 1ZnP* to the Im moiety was
determined using eq 1. The fluorescence lifetime (τ)
kET(CS1) ) τ-1 - τ0
(1)
-1
1
of the ZnP* moiety in ZnP-Im-CH2-ref (77 ( 8 ps) agrees
within experimental error with the lifetime of the radical ion
pair (85 ( 9 ps) in Figure 4. Such an agreement indicates that
the formation of the radical ion pair (ZnP•+-Im•--CH2-ref) via
were determined by analyzing the decay and rise of the
absorbance as in the case of ZnP-Im-CH2-ref (Figure 4). The
quantum yields of formation of CS states (φ1) were determined
from the fluorescence lifetimes according to Scheme 2. The
values of kET(CS1), kET(CR1), and φ1 thus determined are sum-
marized in Table 2.
1
photoinduced ET from the ZnP* to the Im moiety is the rate-
determining step, followed by the fast CR to the ground state
(<5 ps). In such a case, the concentration of ZnP•+-Im•--CH2-
ref rapidly reaches a maximum within 5 ps and decays as 1ZnP*
disappears via the photoinduced ET with the same time constant
as observed in Figure 4. Similar kinetic behavior has been
reported in dyads consisting of porphyrin linked quinones34 and
imides.35 Overall, photoinduced ET takes place from the
porphyrin excited singlet state to the Im moiety, and in turn,
the resulting ZnP•+-Im•- ion pair recombines to regenerate the
ground state as shown in Scheme 2.
The kET(CS1) (1.1 × 1010 s-1) and kET(CR1) (1.0 × 1011 s-1
)
values of ZnP-Im-ref without a methylene linkage are signifi-
cantly larger than the kET(CS1) (1.9 × 109 s-1) and kET(CR1) (5.0
× 1010 s-1) values of ZnP-CH2-Im-ref with a methylene
linkage, respectively. The incorporation of one methylene unit
between the porphyrin and imide moieties renders the electronic
coupling between the two chromophores smaller, thereby
attenuating the ET rates of the latter as compared to those of
the former. The photodynamical behavior of ZnP-Im-CH2-ref,
ZnP-Im-ref, and ZnP-CH2-Im-ref in 1,4-dioxane was similar
to those in THF, and the data are also given in Table 2.37 It
The energetics shown in Scheme 2 are obtained from the one-
electron redox potentials and the excitation energies of the
electron donor and acceptor moieties (see the Experimental

Porphyrin-Pyromellitimide-Fullerene Triads
J. Phys. Chem. A, Vol. 106, No. 12, 2002 2807
Figure 6. Differential absorption spectra obtained upon nanosecond
flash photolysis (550 nm) of ∼10-4 M solutions of ZnP-Im-CH2-C60
in argon saturated THF with a time delay of 200 ns (black circles) and
2 µs (open circles). The time profile of absorbance at 1000 nm at 273
K is shown as the inset.
Figure 5. Differential absorption spectra obtained upon picosecond
flash photolysis (590 nm) of ∼10-4 M solutions of ZnP-Im-CH2-C60
in argon saturated THF with a time delay of 5, 25, 60, and 100 ps.
SCHEME 3: Reaction Scheme and Energy Diagram for
ZnP-Im-CH2-C60 in THF
in THF. This is virtually the same as that described in the dyads
(vide supra). Similar deceleration in the CS1 rates of ZnP-CH2-
Im-C60 (1.7 × 109 s-1) was observed in 1,4-dioxane as
compared to those of ZnP-Im-C60 (7.8 × 109 s-1) and ZnP-
Im-CH2-C60 (4.5 × 109 s-1) in 1,4-dioxane (see the Supporting
Information, S4-S6). Thus, the attenuation of the CS1 rate
results from the weaker electronic coupling between the
porphyrin and the imide moieties because of the existence of
the additional methylene group within the spacer.
The CSH rate constant of ZnP-Im-CH2-C60 in THF (7.0 ×
1010 s-1) is smaller than that of ZnP-Im-C60 in THF (1.0 ×
1011 s-1) despite of the slightly large driving force (-∆GET(CSH)
) 0.13 eV) for the former relative to the latter (-∆GET(CSH)
)
0.08 eV). This can be accommodated by the small electronic
coupling of the former as compared to the latter because of the
incorporation of an additional methylene group between the Im
and the C60 moieties.40 It should be noted here that, despite the
small driving force for the CSH (0.08-0.13 eV), the CSH
process competes well with the CR1 process, resulting in the
formation of the final charge-separated state (i.e., ZnP•+-
(CH2)m-Im-(CH2)n-C60•-). Osuka et al. reported sequential ET
in similar zincporphyrin-pyromellitimide-quinone (ZnP-Im-Q)
triads where a short methylene spacer is employed between the
imide and the quinone moieties.33 Provided that there is a similar
driving force for the CSH reaction (0.07-0.24 eV) with the
short methylene spacer, the CSH rate from the imide to the
quinone in THF is estimated as 109-1010 s-1, which is
comparable to the CSH value (1.7 × 1010 s-1) of the corre-
sponding triad (ZnP-CH2-Im-C60) in THF. Taking into account
the stronger electronic coupling of the CSH process in ZnP-
Im-Q versus ZnP-CH2-Im-C60, the fast CSH in the present
systems can be attributed to the small reorganization energy of
the C60 relative to the imide.27c
should be noted here that the relative ratios of kET(CR1) vs kET(CS1)
in THF are much larger than those in 1,4-dioxane. The driving
force (-∆GET(CR)) for the CR process decreases, and the
reorganization energy increases with increasing the solvent
polarity, rendering the CR rate larger in polar solvents. In
contrast, both of the ET parameters for the CS process increase
with increasing the solvent polarity, leaving the CS rate rather
unchanged.34 Therefore, the photodynamics of the triads were
not examined in more polar solvents such as benzonitrile.
Photophysics of Porphyrin-Pyromellitimide-Fullerene Tri-
ads. The energy levels of the triads in THF are obtained from
the one-electron redox potentials and the excitation energies of
the electron donor and acceptor moieties (see the Experimental
Section) as shown in Scheme 3 (ZnP-Im-CH2-C60 as a
representative example).
Figure 5 depicts the picosecond absorption spectra of ZnP-
Im-CH2-C60 in a THF solution (absorption ratio of ZnP:C60)14:
1). First, a fast step occurs, which involves CS from the 1ZnP*
to the Im moiety (kET(CS1) ) 1.3 × 1010 s-1, φ1 ) 0.97), and
second, a CSH reaction between the Im•- and the C60 moieties
occurs (kET(CSH) ) 7.0 × 1010 s-1, φ2 < 0.26).38 The total
quantum yield of the formation of the charge-separated state
(φtotal ) φ1 × φ2) is determined as <0.25 in THF. Similar
photodynamical behavior was noted for ZnP-Im-C60 and ZnP-
CH2-Im-C60, and the results are summarized in Table 2.39
•-
Formation of C60 (1000 nm) and ZnP•+ (broad absorption
around 620 nm) in ZnP-Im-CH2-C60 was substantiated by a
set of complementary nanosecond laser photolysis experiments
(550 nm), as shown in Figure 6. The decay of both the
absorption bands (1000 and 620 nm) obeys clean first-order
kinetics with decay rate constants that are virtually identical
for both radical species. No characteristic absorption because
of the Im•- was detected as an intermediate species. This
The CS1 rate constant of ZnP-CH2-Im-C60 (2.9 × 109 s-1
)
•-
in THF is smaller by 1 order of magnitude than those of ZnP-
demonstrates that an intramolecular CR from C60 to ZnP•+
Im-C60 (1.8 × 1010 s-1) and ZnP-Im-CH2-C60 (1.3 × 1010 s-1
)
governs the fate of ZnP•+-Im-CH2-C60•-. The CR rate constants

2808 J. Phys. Chem. A, Vol. 106, No. 12, 2002
Imahori et al.
CHART 1
(kET(CR2)) of ZnP-Im-CH2-C60, ZnP-Im-C60, and ZnP-CH2-
Im-C60 in THF at 273 K thus determined are listed in Table 2.
In contrast to the CR process in THF, no formation of C60
Figure 7. Arrhenius plots of kCR for the intramolecular CR process
from C60•- to ZnP•+ in (a) ZnP-Im-CH2-C60, (b) ZnP-Im-C60, and (c)
ZnP-CH2-Im-C60 in THF.
•-
(1000 nm) and ZnP•+ (broad absorption around 620 nm) in the
longer time scale than 10 ns was detected for ZnP-Im-CH2-
C60 in 1,4-dioxane (the dielectric constant: ꢀs ) 2.2) which is
less polar than THF (ꢀs ) 7.58).41 In 1,4-dioxane, the energy
level of the radical ion pair, ZnP•+-Im-CH2-C60•-, is higher
than the triplet excited states of C60 (3C60*) and ZnP (3ZnP*),
•-
the C60 to the ZnP•+ in the triads was examined in THF.
Arrhenius plots of the kCR2 values display a drastic trend in the
triads, as shown in Figure 7. There is a good linear correlation
between ln(kCR2) and T-1 for ZnP•+-Im-CH2-C60•- (Figure 7a).
The activation energy (Ea) derived from the slope (0.13 eV)
agrees with the difference in the energy level between ZnP•+-
*
and thus, the decay of the CS state results in formation of 3C60
and 3ZnP*, which are detected as the triplet-triplet absorption
spectra of 3C60* (700 nm)23,42 and 3ZnP* (740 and 840 nm)23,42
upon nanosecond laser flash photolysis of ZnP-Im-CH2-C60
in 1,4-dioxane (See the Supporting Information, S6).43
•-
Im•--CH2-C60 and ZnP•+-Im-CH2-C60 (0.13 eV in Scheme
3). Such an agreement strongly indicates that the CR2 process
proceeds via a sequential ET pathway through the intermediate,
ZnP•+-Im•--CH2-C60, which is by 0.13 eV higher in energy
than ZnP•+-Im-CH2-C60•- followed by ET from Im•- to ZnP•+.
Such a sequential ET may be preferred because of the weak
electronic coupling at the relatively long distance (Ree ) 18.6
Å; vide infra).
The kET(CR2) values of ZnP-Im-C60 (8.3 × 105 s-1) and ZnP-
CH2-Im-C60 (8.3 × 105 s-1) in THF are identical, but they are
smaller by a factor of 3 than that of ZnP-Im-CH2-C60 (2.9 ×
106 s-1). We have reported the relationship between the edge-
to-edge distance (Ree) and the optimal ET rate constant
(kET(optimal)) in a homologous series of porphyrin-linked fullerene
systems [i.e., zincporphyrin-fullerene dyad (ZnP-C60 (Ree)11.9
Å)), zincporphyrin-freebase porphyrin-fullerene triad (ZnP-
H2P-C60 (Ree)30.3 Å)), and ferrocene-zincporphyrin-freebase
porphyrin-fullerene triad (Fc-ZnP-H2P-C60 (Ree)48.6 Å));
Chart 1].24c,e The kET(optimal) values are correlated with the Ree
values to yield the decay coefficient factor [damping factor (â)
) 0.60 Å-1], which is located within the boundaries of
nonadiabatic ET reactions for saturated hydrocarbon bridges
(0.8-1.0 Å-1) and unsaturated phenylene bridges (0.4 Å-1).44
Taking into account the above experimental results, we can
estimate the kCR2 values in the present systems (Ree ) 16.3-
18.6 Å) as (0.6-2) × 105 s-1, which is smaller by 1 order of
magnitude than the experimental values [(0.83-2.9) × 106 s-1].
This indicates that the CR mechanism in porphyrin-pyromel-
litimide-fullelere triads is different from that reported for
porphyrin-linked fullerene systems24c,e,45 (vide infra).
In contrast to such a thermally activated CR, the kCR2 value
of ZnP-Im-C60 remains a nearly temperature-independent value
(Figure 7b). Such a temperature independence of kCR2 strongly
suggests that the CR2 process in ZnP-Im-C60 does not proceeds
over a thermally activated barrier but proceeds directly through
electron tunneling via superexchange.45 The relatively strong
electron coupling without methylene linkage in ZnP-Im-C60
may result in such preference of the superexchange ET over
the sequential ET which requires the thermal activation to reach
the higher energy state of ZnP•+-Im•--C60 (vide infra). On the
other hand, the temperature dependence of kCR2 of ZnP-CH2-
Im-C60 shows both behaviors: there is an Arrhenius-like
temperature dependence at higher temperatures, but below 230
K, the kCR2 value becomes a temperature-independent value
(Figure 7c). The temperature-independent kCR2 value of ZnP-
CH2-Im-C60 is smaller than the value of ZnP-Im-C60 because
of the weaker electron coupling with the methylene linkage.46
Assuming that the Franck-Condon factor is similar through-
out the series and the CR takes place mainly through the σ and
Sequential vs Superexchange Charge Recombination.
Temperature dependence of the intramolecular CR rates from

Porphyrin-Pyromellitimide-Fullerene Triads
J. Phys. Chem. A, Vol. 106, No. 12, 2002 2809
π bonds of the linkage, the observed temperature dependence
of kCR2 can be rationalized by the following superexchange
mechanism.47,48 The superexchange coupling, Vse, between a
donor (D) state (d) and an acceptor (A) state (a) via a bridge
(B) state (b) is given by eq 4:
mmol) in dry DMF (15 mL) were heated at reflux under an
atmospheric pressure of nitrogen in the dark for 16 h. The
reaction mixture was allowed to cool to room temperature and
then evaporated to dryness at a reduced pressure. TLC showed
several porphyrin bands (benzene), and the fourth band (Rf )
0.9, chloroform/methanol (9:1)) was separated by flash column
chromatography. The fraction was concentrated and dissolved
in a mixture of chloroform (10 mL), trifluoroacetic acid (4 mL),
and 5% aqueous sulfuric acid (3 mL). After it was stirred for
12 h, the mixture was poured into 30 mL of water and extracted
with CHCl3. The organic layer was washed with saturated
NaHCO3 aqueous solution, dried over anhydrous Na2SO4, and
evaporated. Flash column chromatography on silica gel with
chloroform as an eluent (Rf ) 0.45, chloroform) and subsequent
reprecipitation from benzene/methanol gave 4b as a black red
Vse ) (Vdb Vba)/∆Edb
(4)
where Vdb and Vba are respective electronic coupling between d
and b and b and a and ∆Edb is the energy difference between
the states d and b. The bridged state b is a virtual state which
simply increases the electronic coupling between the D and A
states by mixing with them. In superexchange-mediated CR of
the present systems, the process could involve the following
transfers: an ET from the SOMO of the C60•- (D) to the SOMO
of the ZnP•+ (A), mediated by the LUMO of the Im (B).
On the basis of the -∆GET(CSH) values in THF, ∆Edb values
1
solid (24.1 mg, 0.0190 mmol, 19%). mp > 300 °C; H NMR
(270 MHz, CDCl3) δ 10.11 (s, 1H), 8.94 (d, J ) 5 Hz, 2H),
8.93 (s, 4H), 8.91 (d, J ) 5 Hz, 2H), 8.43 (d, J ) 8 Hz, 2H),
8.31 (s, 2H), 8.13 (d, J ) 2 Hz, 4H), 8.11 (d, J ) 2 Hz, 2H),
8.08 (d, J ) 8 Hz, 2H), 7.88 (d, J ) 8 Hz, 2H), 7.82 (t, J ) 2
Hz, 2H), 7.81 (t, J ) 2 Hz, 1H), 7.75 (d, J ) 8 Hz, 2H), 1.54
(s, 54H), -2.76 (br.s, 2H); MS(FAB) 1271 (M + H+); FTIR
(KBr) 3737, 1734, 1654, 1559, 1542, 1508, 1474, 1362, 802,
are in the order of ZnP-Im-CH2-C60 ≈ ZnP-CH2-Im-C60
>
ZnP-Im-C60. Taking into account the effect of a methylene
group on the electronic coupling, Vdb and Vba are in the order
of ZnP-CH2-Im-C60 ≈ ZnP-Im-C60 > ZnP-Im-CH2-C60 and
ZnP-Im-CH2-C60 ≈ ZnP-Im-C60 > ZnP-CH2-Im-C60, respec-
tively. Thus, the Vse value of ZnP-Im-C60 is much larger than
those of ZnP-CH2-Im-C60 and ZnP-Im-CH2-C60. This is
consistent with the experimental results in that no temperature
dependence of the CR2 rate in ZnP-Im-C60 was observed.
In conclusion, this study has demonstrated for the first time
that a subtle change of the linkage between the donor and
acceptor moieties causes a drastic switch between the sequential
and superexchange-mediated CR as well as the modulation of
the CS process.
669 cm-1
.
ZnP-Im-C60. C60 (37.4 mg, 0.0520 mmol), 4b (22.0 mg,
0.0173 mmol), and N-methylglycine (46.3 mg, 0.520 mmol) in
dry toluene (11 mL) were heated at reflux under an atmospheric
pressure of nitrogen in the dark for 6 h. The reaction mixture
was allowed to cool to room temperature and then evaporated
to dryness at a reduced pressure. Flash column chromatography
on silica gel with chloroform as an eluent (Rf ) 0.35,
chloroform) and subsequent reprecipitation from benzene/
methanol gave the desired product as a black red solid (11.5
Experimental Section
1
mg, 5.71 µmol, 33%). mp > 300 °C; H NMR (270 MHz,
General. Melting points were recorded on a Yanagimoto
micro-melting-point apparatus and not corrected. 1H NMR
spectra were measured on a JEOL EX-270. Fast atom bombard-
ment mass spectra (FABMS) were obtained on a JEOL JMS-
DX300. Matrix-assisted laser desorption/ionization (MALDI)
time-of-flight mass spectra (TOF) were measured on a Kratos
Compact MALDI I (Shimadzu). IR spectra were measured on
a Perkin-Elmer FT-IR 2000 spectrophotometer as KBr disks.
Steady-state absorption spectra in the visible and near-IR regions
were measured on a Jasco V570 DS spectrophotometer.
Elemental analyses were performed on a Perkin-Elmer model
240C elemental analyzer. The edge-to-edge (Ree) and center-
to-center (Rcc) distances were determined from molecular
modeling using MM3 (MacroModel, version 6.0).
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. THF and DMF were purchased from Wako
Pure Chemical Ind., Ltd., and purified by successive distillation
over calcium hydride. Dichloromethane was refluxed and
distilled from P2O5. Thin-layer chromatography (TLC) and flash
column chromatography were performed with Art. 5554 DC-
Alufolien Kieselgel 60 F254 (Merck) and Fujisilicia BW300,
respectively.
CDCl3) δ 8.95 (d, J ) 5 Hz, 2H), 8.92 (d, J ) 5 Hz, 4H), 8.89
(d, J ) 5 Hz, 2H), 8.44 (s, 2H), 8.36 (d, J ) 8 Hz, 2H), 8.12
(d, J ) 2 Hz, 4H), 8.06 (d, J ) 2 Hz, 2H), 7.92 (d, J ) 8 Hz,
2H), 7.83 (br.s, 2H), 7.81 (t, J ) 2 Hz, 2H), 7.78 (t, J ) 2 Hz,
1H), 7.64 (d, J ) 8 Hz, 2H), 4.63 (d, J ) 10 Hz, 1H), 4.52 (s,
1H), 3.85 (d, J ) 10 Hz, 1H), 2.72 (s, 3H), 1.55 (s, 54H), -2.71
(br.s, 2H); MS (FAB) MS (FAB) 721 (C60), 1298 [M - C60]+;
FTIR (KBr) 3747, 1731, 1591, 1514, 1455, 1361, 1089, 1001,
797, 718, 527 cm-1 13C NMR (68 MHz, CDCl3, The symbol
;
* means several superimposed signals.) δ 164.92, 164.70,
148.70, 148.53, 145.5-143.9*, 140.3-141.7*, 138.49*, 137.02*,
135.06, 133.77, 130.8-131.8*, 130.87, 130.39, 129.95, 129.55,
125.99, 124.45, 121.83, 121.56, 121.00, 118.88. 118.63, 117.90,
82.41 (-CH-Ph), 77.13 (C60-sp3), 69.41 (-CH2-), 67.89 (C60-
sp3), 40.08 (N-CH3), 35.20 (-C(CH3)3), 35.13 (-C(CH3)3),
31.95 (-C(CH3)3), 31.89 (-C(CH3)3). Some of the 13C NMR
signals could not be assigned because of the overlapping with
the solvent peaks. A saturated methanol solution of Zn(OAc)2
(3 mL) was added to a solution of the sample of the C60 adduct
(10.0 mg, 4.96 µmol) in CHCl3 (30 mL) and heated at reflux
for 30 min. After cooling, the reaction mixture was washed with
water twice and dried over anhydrous Na2SO4, and then the
solvent was evaporated. Flash column chromatography on silica
gel with CHCl3 as an eluent and subsequent reprecipitation from
benzene-methanol gave ZnP-Im-C60 as a brown solid (100%
yield, 10.3 mg, 4.96 µmol). mp > 300 °C; 1H NMR (270 MHz,
CDCl3) δ 9.05 (d, J ) 5 Hz, 4H), 9.03 (d, J ) 5 Hz, 2H), 9.01
(d, J ) 5 Hz, 2H), 8.97 (d, J ) 5 Hz, 2H), 8.40 (s, 2H), 8.33
(d, J ) 8 Hz, 2H), 8.11 (d, J ) 2 Hz, 4H), 8.04 (d, J ) 2 Hz,
2H), 7.92 (d, J ) 8 Hz, 2H), 7.81 (t, J ) 2 Hz, 2H), 7.76 (t, J
Synthesis of Porphyrin-Fullerene Linked Molecules and
the References. 4b. 5-(4-Aminophenyl)-10,15,20-tris(3,5-di-
tert-butylphenyl)porphyrin 1a (96.5 mg, 0.100 mmol), pyro-
mellitic dianhydride 2 (22.0 mg, 0.100 mmol), and 2-(4-
aminophenyl)-5,5-dimethyl-1,3-dioxane 3b (21.0 mg, 0.100

2810 J. Phys. Chem. A, Vol. 106, No. 12, 2002
Imahori et al.
) 2 Hz, 1H), 7.72 (br.s, 2H), 7.62 (d, J ) 8 Hz, 2H), 4.42 (d,
J ) 10 Hz, 1H), 4.25 (s, 1H), 3.63 (d, J ) 10 Hz, 1H), 2.66 (s,
3H), 1.56 (s, 54H); MS (FAB) 721 (C60), 1360 [M - C60]+; IR
(KBr) 1731, 1591, 1514, 1455, 1361, 1089, 1001, 797, 718,
3H); MS (FAB) 222 (M + H+); IR (KBr) 1611, 1471, 1381,
1315, 1214, 1033, 825, 790, 764, 712 cm-1. Found: C, 70.7;
H, 8.54; N, 5.96%. Calcd for C14H17NO2: C, 70.6; H, 8.65; N,
6.33%.
527 cm-1 13C NMR (68 MHz, CDCl3) δ 165.38, 165.10,
;
4a. This compound was synthesized from 1a, 2, and 3a by
the same method as that described for 4b. 4a as a black red
150.56, 150.44, 150.36, 149.68, 148.64, 148.52, 146.75, 145.70,
145.52, 145.31, 144.78, 144.53, 143.71, 141.83, 141.09, 137.30,
137.27, 135.07, 135.07, 132.64, 132.44, 132.44, 132.28, 131.52,
129.89, 129.54, 128.34, 126.13, 124.41, 122.66, 120.87, 120.82,
119.17, 82.63 (-CH-Ph), 77.76 (C60-sp3), 69.65 (-CH2-),
68.39 (C60-sp3), 40.05 (N-CH3), 35.06 (-C(CH3)3), 32.03
(-C(CH3)3).
1
solid from benzene/methanol (25% yield); mp > 300 °C; H
NMR (270 MHz, CDCl3) δ 9.99 (s, 1H), 8.93 (d, J ) 5 Hz,
2H), 8.92 (s, 4H), 8.90 (d, J ) 5 Hz, 2H), 8.41 (d, J ) 8 Hz,
2H), 8.16 (s, 2H), 8.13 (s, 4H), 8.11 (s, 2H), 7.87 (d, J ) 8 Hz,
2H), 7.86 (d, J ) 8 Hz, 2H), 7.81 (s, 3H), 7.57 (d, J ) 8 Hz,
2H), 4.89 (s, 2H), 1.54 (s, 54H), -2.78 (br.s, 2H); MS(FAB)
1284 (M + H+); FTIR (KBr) 1730, 1476, 1362, 1254, 1095,
ZnP-Im-ref. Aminoporphyrin 1a (193 mg, 0.200 mmol),
pyromellitic dianhydride 2 (44.0 mg, 0.200 mmol), and 4-hexa-
decylaniline (63.0 mg, 0.200 mmol) in dry DMF (30 mL) were
heated at reflux under an atmospheric pressure of nitrogen in
the dark for 22 h. The reaction mixture was allowed to cool to
room temperature and then evaporated to dryness at a reduced
pressure. TLC showed two porphyrin bands (Rf ) 0.55 and 0.45,
benzene), and the second band was separated by flash column
chromatography. Reprecipitation from benzene/methanol gave
the desired product as a black red solid (76.5 mg, 0.0523 mmol,
26%). mp > 300 °C; 1H NMR (270 MHz, CDCl3) δ 8.93 (d, J
) 5 Hz, 2H), 8.92 (s, 4H), 8.90 (d, J ) 5 Hz, 2H), 8.49 (s,
2H), 8.42 (d, J ) 8 Hz, 2H), 8.11 (d, J ) 2 Hz, 4H), 8.09 (d,
J ) 2 Hz, 2H), 7.90 (d, J ) 8 Hz, 2H), 7.81 (t, J ) 2 Hz, 2H),
7.80 (t, J ) 2 Hz, 1H), 7.39 (d, J ) 8 Hz, 2H), 7.37 (d, J ) 8
Hz, 2H), 2.69 (t, J ) 8 Hz, 2H),1.68 (m, 2H), 1.56 (s, 54H),
1.27 (m, 26H), 0.89 (t, J ) 8 Hz, 3H), -2.72 (br.s, 2H); MS
(FAB) 1467 (M + H+); FTIR (KBr) 3733, 1732, 1559, 1593,
1557, 1515, 1471, 1363, 1247, 1094, 973, 914, 880, 802, 722,
668 cm-1. A saturated methanol solution of Zn(OAc)2 (3 mL)
was added to a solution of the sample of the dyad (10.0 mg,
6.83 µmol) in CHCl3 (30 mL) and heated at reflux for 30 min.
After cooling, the reaction mixture was washed with water twice
and dried over anhydrous Na2SO4, and then the solvent was
evaporated. Flash column chromatography on silica gel with
CHCl3 as an eluent and subsequent reprecipitation from
chloroform/methanol gave ZnP-Im-ref as a black red solid
974, 917, 914, 899, 882, 802, 717 cm-1
.
ZnP-Im-CH2-C60. C60 (56.3 mg, 0.0781 mmol), 4a (33.0 mg,
0.0257 mmol), and N-methylglycine (69.4 mg, 0.779 mmol) in
dry toluene (20 mL) were heated at reflux under an atmospheric
pressure of nitrogen in the dark for 19 h. The reaction mixture
was allowed to cool to room temperature and then evaporated
to dryness at a reduced pressure. Flash column chromatography
on silica gel with chloroform as an eluent (Rf ) 0.15,
chloroform) and subsequent reprecipitation from chloroform/
methanol gave the desired product as a dark grayish violet solid
1
(25.0 mg, 0.0153 mmol, 60%). mp > 300 °C; H NMR (270
MHz, CDCl3) δ 8.91 (s, 8H), 8.44 (d, J ) 8 Hz, 2H), 8.39 (s,
2H), 8.09 (s, 6H), 7.88 (d, J ) 8 Hz, 2H), 7.80 (s, 3H), 7.76
(br.s, 2H), 7.53 (d, J ) 8 Hz, 2H), 4.94 (s, 1H), 4.90 (d, J ) 10
Hz, 1H), 4.17 (d, J ) 10 Hz, 1H), 2.77 (s, 3H), 1.54 (s, 54H),
-2.72 (br.s, 2H); MS (MALDI-TOF) 2026 (M + H+); FTIR
(KBr) 3745, 1725, 1592, 1457, 1363, 1095, 1002, 998, 718,
527 cm-1. A saturated methanol solution of Zn(OAc)2 (3 mL)
was added to a solution of the sample of the C60 adduct (10.3
mg, 5.07 µmol) in CHCl3 (30 mL) and heated at reflux for 30
min. After cooling, the reaction mixture was washed with water
twice and dried over anhydrous Na2SO4, and then the solvent
was evaporated. Flash column chromatography on silica gel with
CHCl3 as an eluent (Rf ) 0.15, chloroform) and subsequent
reprecipitation from chloroform/methanol gave ZnP-Im-CH2-
C60 as a dark violet solid (38% yield, 4.0 mg, 1.9 µmol). mp >
300 °C; 1H NMR (270 MHz, CDCl3) δ 9.02 (d, J ) 5 Hz, 2H),
9.00 (s, 4H), 8.99 (d, J ) 5 Hz, 2H), 8.48 (s, 2H), 8.41 (d, J )
8 Hz, 2H), 8.08 (d, J ) 2 Hz, 6H), 7.91 (d, J ) 8 Hz, 2H),
7.80 (t, J ) 2 Hz, 2H), 7.79 (t, J ) 2 Hz, 1H), 7.72 (br.s, 2H),
7.54 (t, J ) 8 Hz, 1H), 4.94 (s, 2H), 4.81 (d, J ) 10 Hz, 1H),
4.79 (s, 1H), 4.08 (d, J ) 10 Hz, 1H), 2.76 (s, 3H), 1.53 (s,
54H); MS (MALDI-TOF) 2095 (M + H+); FTIR (KBr) 1725,
1
(100% yield, 10.3 mg, 6.83 µmol). mp > 300 °C; H NMR
(270 MHz, CDCl3) δ 9.04 (d, J ) 5 Hz, 2H), 9.03 (s, 4H), 9.01
(d, J ) 5 Hz, 2), 8.55 (s, 2H), 8.43 (d, J ) 8 Hz, 2H), 8.12 (d,
J ) 2 Hz, 4H), 8.10 (d, J ) 2 Hz, 2H), 7.90 (d, J ) 8 Hz, 2H),
7.81 (t, J ) 2 Hz, 2H), 7.79 (t, J ) 2 Hz, 1H), 7.39 (d, J ) 8
Hz, 2H), 7.38 (d, J ) 8 Hz, 2H), 2.69 (t, J ) 8 Hz, 2H), 1.66
(m, 2H), 1.56 (s, 54H), 1.34 (m, 26H), 0.89 (t, J ) 8 Hz, 3H);
MS (FAB) 1531 (M + H+); IR (KBr) 1731, 1593, 1514, 1466,
1364, 1290, 1248, 1219, 1094, 1002, 930, 880, 824, 799, 723
1591, 1457, 1362, 1091, 1002, 798, 718, 527 cm-1
.
ZnP-Im-CH2-ref. Aminoporphyrin 1a (71.9 mg, 0.0744
mmol), pyromellitic dianhydride 2 (17.3 mg, 0.0793 mmol),
and 4-methylbenzylamine (10.2 mg, 0.0842 mmol) in dry DMF
(11 mL) were heated at reflux under an atmospheric pressure
of nitrogen in the dark for 25 h. The reaction mixture was
allowed to cool to room temperature and then evaporated to
dryness at a reduced pressure. TLC showed three porphyrin
bands (Rf ) 0.65, 0.15, and 0.05, chloroform), and the second
band was separated by flash column chromatography. Repre-
cipitation from benzene/methanol gave the desired product as
a dark violet solid (20.4 mg, 0.0161 mmol, 22%). mp > 300
cm-1
.
3a. To a stirred solution of LiAlH4 (5.57 g, 149 mmol) in
dry THF (100 mL) was added a solution of 2-(4-cyanophenyl)-
5,5-dimethyl-1,3-dioxane29 (5.56 g, 25.6 mmol) in dry THF (40
mL) dropwise at 0 °C, and stirring was continued at 0 °C for 2
h. The reaction mixture was quenched by addition of 5 mL of
ethyl acetate followed by addition of 60 mL of 3 M HCl aqueous
solution. The organic layer was separated, washed with saturated
Na2CO3, and dried over anhydrous Na2SO4, and then the solvent
was evaporated. Flash column chromatography on silica gel with
ethyl acetate/ethanol (4:1) as an eluent and subsequent repre-
cipitation from hexane/benzene gave 3a as a pale yellow solid
(73% yield, 4.16 g, 4.16 mmol). mp 115.2-116.2 °C; 1H NMR
(270 MHz, CDCl3) δ 7.48 (d, J ) 8 Hz, 2H), 7.31 (d, J ) 8
Hz, 2H), 5.39 (s, 1H), 3.86 (s, 2H), 3.77 (d, J ) 11 Hz, 2H),
3.65 (d, J ) 11 Hz, 2H), 1.57 (s, 2H), 1.30 (s, 3H), 0.80 (s,
1
°C; H NMR (270 MHz, CDCl3) δ 8.93 (d, J ) 5 Hz, 2H),
8.92 (s, 4H), 8.91 (d, J ) 5 Hz, 2H), 8.41 (d, J ) 8 Hz, 2H),
8.32 (s, 2H), 8.10 (d, J ) 2 Hz, 4H), 8.09 (d, J ) 2 Hz, 2H),
7.86 (d, J ) 8 Hz, 2H), 7.80 (t, J ) 2 Hz, 2H), 7.79 (t, J ) 2
Hz, 1H), 7.37 (d, J ) 8 Hz, 2H), 7.17 (d, J ) 8 Hz, 2H), 4.87
(s, 2H), 2.34 (s, 3H), 1.54 (s, 54H), -2.74 (br.s, 2H); MS (FAB)

Porphyrin-Pyromellitimide-Fullerene Triads
J. Phys. Chem. A, Vol. 106, No. 12, 2002 2811
1270 (M + H+); FTIR (KBr) 3774, 1725, 1593, 1476, 1370,
1247, 1095, 973, 917, 882, 802, 721 cm-1. A saturated methanol
solution of Zn(OAc)2 (3 mL) was added to a solution of the
sample of the dyad (40.5 mg, 0.0319 mmol) in CHCl3 (190
mL) and heated at reflux for 30 min. After cooling, the reaction
mixture was washed with water twice and dried over anhydrous
Na2SO4, and then the solvent was evaporated. Flash column
chromatography on silica gel with CHCl3 as an eluent (Rf )
0.25, chloroform) and subsequent reprecipitation from chloroform/
methanol gave ZnP-Im-CH2-ref as a black red solid (75% yield,
SO4, and evaporated. Flash column chromatography on silica
gel with chloroform as an eluent (Rf ) 0.10, benzene/ethyl
acetate (30:1)) and subsequent reprecipitation from chloroform/
methanol gave 4c as a black red solid (50.4 mg, 0.0393 mmol,
1
13%). mp > 300 °C; H NMR (270 MHz, CDCl3) δ 10.09 (s,
1H), 8.88 (s, 4H), 8.85 (d, J ) 5 Hz, 2H), 8.77 (d, J ) 5 Hz,
2H), 8.30 (s, 2H), 8.22 (d, J ) 8 Hz, 2H), 8.08 (s, 4H), 8.07 (s,
2H), 8.05 (d, J ) 8 Hz, 2H), 7.83 (d, J ) 8 Hz, 2H), 7.78 (s,
3H), 7.71 (d, J ) 8 Hz, 2H), 5.24 (s, 2H), 1.55 (s, 54H), -2.79
(br.s, 2H); MS(FAB) 1284 (M + H+); FTIR (KBr) 3742, 1730,
1593, 1508, 1475, 1425, 1364, 1247, 1205 1092, 1000, 972,
1
31.7 mg, 0.0238 mmol). mp > 300 °C; H NMR (270 MHz,
915, 883, 802, 724 cm-1
.
CDCl3) δ 9.03 (d, J ) 5 Hz, 2H), 9.02 (s, 4H), 9.00 (d, J ) 5
Hz, 2H), 8.45 (s, 2H), 8.41 (d, J ) 8 Hz, 2H), 8.10 (d, J ) 2
Hz, 4H), 8.09 (d, J ) 2 Hz, 2H), 7.87 (d, J ) 8 Hz, 2H), 7.80
(t, J ) 2 Hz, 2H), 7.79 (t, J ) 2 Hz, 1H), 7.39 (d, J ) 8 Hz,
2H), 7.39 (d, J ) 8 Hz, 2H), 7.18 (d, J ) 8 Hz, 2H), 4.90 (s,
2H), 2.34 (s, 3H), 1.53 (s, 54H); MS (FAB) 1332 (M + H+);
FTIR (KBr) 1730, 1591, 1363, 1290, 1249, 1219, 1097, 1002,
ZnP-CH2-Im-C60. C60 (145.6 mg, 0.202 mmol), 4c (85.8 mg,
0.0668 mmol), and N-methylglycine (179 mg, 2.01 mmol) in
dry toluene (20 mL) were heated at reflux under an atmospheric
pressure of nitrogen in the dark for 10 h. The reaction mixture
was allowed to cool to room temperature and then evaporated
to dryness at a reduced pressure. Flash column chromatography
on silica gel with chloroform as an eluent (Rf ) 0.10,
chloroform) and subsequent reprecipitation from chloroform/
methanol gave the desired product as a dark yellowish brown
929, 884, 800, 720 cm-1
.
5-(4-Cyanophenyl)-10,15,20-tris(3,5-di-tert-butylphenyl)-
porphyrin. 3,5-Di-tert-butylbenzaldehyde (16.20 g, 74.20 mmol),
4-cyanobenzaldehyde (3.23 g, 24.6 mmol), and pyrrole (7.30
mL, 105 mmol) were dissolved in dry CHCl3 (2600 mL). The
mixture was degassed with nitrogen for 30 min. Then Et2O‚
BF3 (4.10 mL, 32.4 mmol) was added to the solution; this
mixture was stirred for 2 h under nitrogen atmosphere. Chloranil
(18.2 g, 74.0 mmol) was added to the reaction mixture, and
this was stirred overnight. The solvent was removed under
reduced pressure. Flash column chromatography on silica gel
with hexane/benzene (1:1) as an eluent (Rf ) 0.25) and
subsequent reprecipitation from chloroform/methanol gave
desired compound as a dark red solid (3.79 g, 3.88 mmol, 16%).
1
solid (66.1 mg, 0.0325 mmol, 49%). mp > 300 °C; H NMR
(270 MHz, CDCl3) δ 8.88 (s, 4H), 8.84 (d, J ) 5 Hz, 2H), 8.76
(d, J ) 5 Hz, 2H), 8.37 (s, 2H), 8.20 (d, J ) 8 Hz, 2H), 8.07
(d, J ) 2 Hz, 2H), 8.06 (d, J ) 2 Hz, 4H), 7.86 (br.s, 2H), 7.81
(d, J ) 8 Hz, 2H), 7.77 (t, J ) 2 Hz, 2H), 7.76 (t, J ) 2 Hz,
1H), 7.57 (d, J ) 8 Hz, 2H), 5.22 (s, 2H), 4.82 (d, J ) 10 Hz,
2H), 4.80 (s, 1H), 4.07 (d, J ) 10 Hz, 1H), 2.78 (s, 3H), 1.50
(s, 54H), -2.77 (br.s, 2H); MS (MALDI-TOF) 2026 (M +
H+); FTIR (KBr) 3735, 1592, 1459, 1363, 1095, 800, 725, 527
cm-1. A saturated methanol solution of Zn(OAc)2 (3 mL) was
added to a solution of the sample of the C60 adduct (66.1 mg,
0.0325 mmol) in CHCl3 (180 mL) and heated at reflux for 30
min. After cooling, the reaction mixture was washed with water
twice and dried over anhydrous Na2SO4, and then the solvent
was evaporated. Flash column chromatography on silica gel with
CHCl3 as an eluent (Rf ) 0.45, benzene/ethyl acetate (19:1))
and subsequent reprecipitation from chloroform-methanol gave
ZnP-Im-CH2-C60 as a dark brownish violet solid (85% yield,
1
mp > 300 °C; H NMR (270 MHz, CDCl3) δ 8.92 (d, J ) 5
Hz, 2H), 8.91 (s, 4H), 8.72 (d, J ) 5 Hz, 2H), 8.36 (d, J ) 8
Hz, 2H), 8.08 (d, J ) 2 Hz, 4H), 8.07 (d, J ) 2 Hz, 2H), 8.06
(d, J ) 8 Hz, 2H), 7.80 (t, J ) 2 Hz, 2H), 7.79 (t, J ) 2 Hz,
1H), 2.72 (s, 3H), 1.52 (s, 54H), -2.72 (br.s, 2H); MS (FAB)
976 (M + H+); FTIR (KBr) 3732, 1593, 1475, 1394, 1362,
1246, 970, 916, 882, 860, 804, 730, 715, 668 cm-1
.
1
57.8 mg, 0.0276 mmol). mp > 300 °C; H NMR (270 MHz,
1b. This compound was synthesized from 5-(4-cyanophenyl)-
10,15,20-tris(3,5-di-tert-butylphenyl)porphyrin by the same
method as that described for 3a. 1b was obtained as a black
red solid from chloroform/methanol (61% yield); mp > 300
CDCl3) δ 8.98 (s, 4H), 8.94 (d, J ) 5 Hz, 2H), 8.87 (d, J ) 5
Hz, 2H), 8.43 (s, 2H), 8.20 (d, J ) 8 Hz, 2H), 8.06 (s, 6H),
7.81 (d, J ) 8 Hz, 4H), 7.77 (s, 3H), 7.57 (d, J ) 8 Hz, 2H),
5.23 (s, 2H), 4.70 (d, J ) 10 Hz, 1H), 4.69 (s, 1H), 3.94 (d, J
) 10 Hz, 1H), 2.76 (s, 3H), 1.53 (s, 54H); MS (MALDI-TOF)
2095 (M + H+); FTIR (KBr) 1730, 1592, 1475, 1363, 1092,
1
°C; H NMR (270 MHz, CDCl3) δ 8.93 (d, J ) 5 Hz, 2H),
8.88 (s, 4H), 8.87 (d, J ) 5 Hz, 2H), 8.08 (d, J ) 2 Hz, 4H),
8.07 (d, J ) 2 Hz, 2H), 8.01 (d, J ) 8 Hz, 2H), 7.78 (t, J ) 2
Hz, 2H), 7.77 (t, J ) 2 Hz, 1H), 7.07 (d, J ) 8 Hz, 2H), 4.22
(s, 2H), 1.60 (s, 2H), 1.54 (s, 54H), -2.71 (br.s, 2H); MS(FAB)
980 (M + H+); FTIR (KBr) 3746, 1695, 1476, 1394, 1366,
1002, 797, 718, 527 cm-1
.
ZnP-CH2-Im-ref. Aminomethylporphyrin 1b (302 mg, 0.308
mmol), pyromellitic dianhydride 2 (135 mg, 0.618 mmol), and
4-hexadecylaniline (223 mg, 0.703 mmol) in dry DMF (46 mL)
were heated at reflux under an atmospheric pressure of nitrogen
in the dark for 11 h. The reaction mixture was allowed to cool
to room temperature and then evaporated to dryness at a reduced
pressure. TLC showed three porphyrin bands (Rf ) 0.65, 0.30,
and 0.05, benzene), and the second band was separated by flash
column chromatography. Reprecipitation from benzene-aceto-
nitrile gave the desired product as a dark grayish purple solid
(176 mg, 0.119 mmol, 39%). mp > 300 °C; 1H NMR (270 MHz,
CDCl3) δ 8.88 (s, 4H), 8.84 (d, J ) 5 Hz, 2H), 8.78 (d, J ) 5
Hz, 2H), 8.37 (s, 2H), 8.21 (d, J ) 8 Hz, 2H), 8.07 (s, 6H),
7.81 (d, J ) 8 Hz, 2H), 7.79 (s, 3H), 7.35 (s, 2H), 5.23 (s, 2H),
2.67 (t, J ) 8 Hz, 2H), 1.7-1.2 (m, 28H), 1.54 (m, 54H), 0.89
(t, J ) 8 Hz, 3H), -2.75 (br.s, 2H); MS (FAB) 1480 (M +
H+); FTIR (KBr) 3745, 1729, 1593, 1515, 1476, 1362, 1247,
1247, 973, 917, 883, 803, 733, 715, 668 cm-1
.
4c: Aminomethylporphyrin 1b (295.7 mg, 0.302 mmol),
pyromellitic dianhydride 2 (135 mg, 0.619 mmol), and 3b (195
mg, 0.940 mmol) in dry DMF (45 mL) were heated at reflux
under an atmospheric pressure of nitrogen in the dark for 5 h.
The reaction mixture was allowed to cool to room temperature
and then evaporated to dryness at a reduced pressure. TLC
showed several porphyrin bands (benzene), and the fourth band
(Rf ) 0.50, benzene/ethyl acetate (15:1)) was separated by flash
column chromatography. The fraction was concentrated and
dissolved in a mixture of chloroform (53 mL), trifluoroacetic
acid (21 mL), and 5% aqueous sulfuric acid (16 mL). After
stirring for 16 h, the mixture was poured into 100 mL of water
and extracted with CHCl3. The organic layer was washed with
saturated NaHCO3 aqueous solution, dried over anhydrous Na2-

2812 J. Phys. Chem. A, Vol. 106, No. 12, 2002
Imahori et al.
1098, 973, 915, 883, 803, 724 cm-1. A saturated methanol
solution of Zn(OAc)2 (3 mL) was added to a solution of the
sample of the dyad (176 mg, 0.119 mmol) in CHCl3 (200 mL)
and heated at reflux for 30 min. After cooling, the reaction
mixture was washed with water twice and dried over anhydrous
Na2SO4, and then the solvent was evaporated. Flash column
chromatography on silica gel with CHCl3 as an eluent (Rf )
0.35, benzene) and subsequent reprecipitation from chloroform/
methanol gave ZnP-Im-CH2-ref as a dark grayish solid (90%
yield, 165 mg, 0.107 mmol). mp > 300 °C; 1H NMR (270 MHz,
CDCl3) δ 9.00 (s, 4H), 8.96 (d, J ) 5 Hz, 2H), 8.88 (d, J ) 5
Hz, 2H), 8.40 (s, 2H), 8.21 (d, J ) 8 Hz, 2H), 8.08 (s, 6H),
7.79 (d, J ) 8 Hz, 2H), 7.78 (s, 3H), 7.36 (d, J ) 8 Hz, 2H),
7.34 (d, J ) 8 Hz, 2H), 5.23 (s, 2H), 2.68 (t, J ) 8 Hz, 2H),
1.7-1.2 (m, 28H), 1.51 (s, 54H), 0.88 (t, J ) 8 Hz, 3H); MS
(FAB) 1542 (M + H+); FTIR (KBr) 1730, 1591, 1363, 1290,
Details of the transient absorption measurements were described
elsewhere.23f All of the samples (10-4∼10-5 M) in a quartz cell
(1 × 1 cm) were deaerated by bubbling argon through the
solution for 15 min.
Electrochemical Measurements. The differential pulse vol-
tammetry (DPV) measurements of the dyads and triads were
performed on a BAS 50W electrochemical analyzer in a
deaerated CH2Cl2 solution containing 0.10 M n-Bu4NPF6 as a
supporting electrolyte at a sweep rate of 10 mV s-1 at 298 K
(see Supporting Information, S3). The glassy carbon 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 an Ag/AgCl (saturated KCl) reference electrode.
Ferrocene/ferricenium was used as an external standard.
1249, 1219, 1097, 1002, 929, 884, 800, 724 cm-1
.
Acknowledgment. This work was supported by COE, Grant-
in-Aid for Scientific Research, Specially Promoted Research
(No. 10102007), and the Development of Innovative Technology
(No.12310) from the Ministry of Education, Culture, Sports,
Science and Technology of Japan.
ZnP-CH2-ref. To a stirred solution of 1b (101 mg, 0.103
mmol) in dry CH2Cl2 (10 mL) was added acetic anhydride
(0.040 mL, 0.420 mmol) dropwise at 0 °C, and stirring was
continued at 0 °C for 20 min. The reaction mixture was allowed
to cool to room temperature and then evaporated to dryness at
a reduced pressure. Flash column chromatography on silica gel
with toluene/ethyl acetate (1:1) as an eluent (Rf ) 0.35) and
subsequent reprecipitation from benzene/acetonitrile gave the
desired product as a dark grayish brown solid (80.7 mg, 0.0789
Supporting Information Available: Kinetic analysis of the
time profile of transient absorption (S1,S2), DPV voltammogram
of ZnP-CH2-Im-C60 (S3), differential absorption spectra ob-
tained upon picosecond flash photolysis of ZnP-Im-CH2-C60
in 1,4-dioxane (S4), time-profile of transient absorbance of ZnP-
Im-CH2-C60 in 1,4-dioxane (S5), differential absorption spectra
obtained upon nanosecond flash photolysis of ZnP-Im-CH2-
C60 in 1,4-dioxane (S6; 6 page, print/PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
1
mmol, 77%). mp > 300 °C; H NMR (270 MHz, CDCl3) δ
8.89 (s, 4H), 8.88 (d, J ) 5 Hz, 2H), 8.81 (d, J ) 5 Hz, 2H),
8.20 (d, J ) 8 Hz, 2H), 8.08 (s, 6H), 7.79 (s, 3H), 6.02 (t, J )
8 Hz, 1H), 4.78 (d, J ) 8 Hz, 2H), 2.18 (s, 3H), 1.52 (s, 54H),
-2.71 (br.s, 2H); MS (FAB) 1022 (M + H+); IR (KBr) 1260,
1096, 1015, 922, 795, 706 cm-1. A saturated methanol solution
of Zn(OAc)2 (3 mL) was added to a solution of the sample of
the porphyrin (80.7 mg, 0.0789 mmol) in CHCl3 (200 mL) and
heated at reflux for 30 min. After cooling, the reaction mixture
was washed with water twice and dried over anhydrous Na2-
SO4, and then the solvent was evaporated. Flash column
chromatography on silica gel with CHCl3 as an eluent (Rf )
0.35, benzene) and subsequent reprecipitation from benzene-
acetonitrile gave Zn-CH2-ref as a deep purple solid (81% yield,
References and Notes
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69.3 mg, 0.0639 mmol). mp > 300 °C; H NMR (270 MHz,
CDCl3) δ 9.00 (s, 4H), 8.99 (d, J ) 5 Hz, 2H), 8.91 (d, J ) 5
Hz, 2H), 8.20 (d, J ) 8 Hz, 2H), 8.09 (d, J ) 2 Hz, 4H), 8.08
(d, J ) 2 Hz, 2H), 7.79 (t, J ) 2 Hz, 2H), 7.78 (t, J ) 2 Hz,
1), 6.00 (t, J ) 8 Hz, 1H), 4.74 (d, J ) 8 Hz, 2H), 2.13 (s, 3H),
1.52 (s, 54H); MS (FAB) 1085 (M + H+); IR (KBr) 1247, 1217,
1001, 947, 930, 880, 823, 797, 716 cm-1
.
Spectral Measurements. Time-resolved fluorescence spectra
of the porphyrin moiety of the dyads and triads were measured
by a single-photon counting method using a second harmonic
generation (SHG, 524 nm) of a semiconductor YLF laser
(Lightwave 131-1047-300, fwhm ) 20 ps) as an excitation
source.
Picosecond transient absorption spectra were measured by
means of a picosecond dye laser (fwhm 12 ps) pumped by the
second harmonic of a repetitive mode-locked Nd3+:YAG laser
(Quantel, Picochrome YG-503 C/PTL-10).23b The 590 nm output
of the dye laser (Rhodamine 6G) was used for excitation.
Nanosecond transient absorption measurements were carried
out using SHG (550 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).

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a positive charge in the porphyrin radical cation in the former relative to
the latter.
(41) Physical Methods of Chemistry; Rossiter, B. W., Hamilton, J. F.,
Eds.; Wiley-International: New York, 1986; Vol IIIB, p 114.
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(43) The decay of charge-separated state to porphyrin and C60 excited
triplet states rather than to the ground state has been reported in porphyrin-
fullerene linked systems.23f This can be rationalized by the small reorganiza-
tion energies of ET.26,27 Namely, on the Marcus parabolic curve, the former
ET processes are located near to the top region, whereas the latter process
to the ground state is forced to be shifted deeply into the inverted region.
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(36) -∆G0 values are assumed to be similar in CH2Cl2 and THF,
ET
because of the ꢀs values are largely the same in CH2Cl2 (8.9) and THF
(7.58).
(37) The CR rates of the dyads are larger than the corresponding CS
rates in both 1,4-dioxane and THF, except the case of ZnP-CH2-Im-ref in
1,4-dioxane. Because the Ree value (6.4 Å) of ZnP-CH2-Im-ref is slightly
larger than that (5.7 Å) of ZnP-Im-CH2-ref and ZnP-Im-ref, the charge-
separated state of the former in 1,4-dioxane may be destabilized as compared
to those of the latter, resulting to the comparable ET rates for CS1 and
CR1.
(45) On the basis of Marcus plot for ET processes of ZnP-C60 dyad (λ
) 0.66 eV in THF) and ZnP-H2P-C60 triad (λ ) 1.09 eV in THF),24c,e the
λ value in the present systems is estimated to be ∼0.9 eV, which is smaller
to the ZnP•+. This implies that the CR process of ZnP-Im-C60 is located
deeply into the inverted region of the Marcus parabola, thus ruling out the
possibility of an activation less process near the top region of the Marcus
parabola.
•-
by 0.5-0.6 eV than the driving force for CR2 (1.44-1.46 eV) from C60
(38) The initial CS rates (kET(CS1)) in the triads without a methylene
spacer between the Im and C60 moieties (i.e., ZnP-CH2-Im-C60 and ZnP-
Im-C60) are by ∼50-110% larger than the CS rates of the corresponding
porphyrin-imide dyads, whereas the kET(CS1) values in the triad with a
methylene spacer between the Im and C60 moieties are virtually the same
as those of the corresponding dyad. Thus, the acceleration effect may result
from the electron-withdrawing character of the C60 moiety attached to the
Im moeity.15 There is also another possibility that direct ET from 1ZnP* to
(46) The apparent Ea value of ZnP-CH2-Im-C60 (0.07 eV) is smaller
than the difference in the energy level between ZnP•+-CH2-Im•--C60 and
•-
ZnP•+-CH2-Im-C60 (0.12 eV) probably because of some contribution
of the temperature independent process in the examined temperature
range.
(47) The two systems (ZnP-Im-CH2-C60 and ZnP-CH2-Im-C60) that
show the temperature dependence have a larger degree of flexibility because
of the methylene group. Thus, there is another possibility that different
conformations give rise to different temperature dependence. Such a
possibility is ruled out at least in the former case, because of no deviation
from the straight line for the plot (Figure 7a). In contrast, a conformational
change might occur with decreasing the temperature in the latter case,
leading to the deviation from the straight line in the plot (Figure 7c).
However, such a conformational change is unlikely to occur because all of
the decay processes can be analyzed by a single-exponential component at
various temperatures.34
1
C60 occurs in addition to the sequential ET from ZnP* to C60 via the Im
moiety. The somewhat larger kET(CS1) values of the triads as compared with
the values of dyads without C60 (Table 2) may be ascribed to the occurrence
of such a direct ET pathway.
(39) The total quantum yield of the CS (0.49) for ZnP-Im-C60 in THF
agrees largely with the corresponding value (0.46) obtained by the transient
absorption spectrum [ꢀ ) 13 500 M-1 cm-1 (ZnP•+) at 620 nm].
(40) The CSH rates of ZnP-CH2-Im-C60 in 1,4-dioxane and THF are
smaller than those of ZnP-Im-C60, respectively. Because the Rcc value of
ZnP-CH2-Im-C60 (11.7 Å) is small as compared to that of ZnP-Im-C60
(12.5 Å), the retardation of CSH in the former may result from the strong
Coulombic interaction between an electron in the imide radical anion and
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J.; Michel-Beyerle, M. E.; Ogrodnik, A. Biochim. Biophys. Acta 1989, 977,
273.