Photoinduced intramolecular electron-transfer processes in [60]fullerene-(spacer)-N,N-bis(biphenylyl)aniline dyad in solutions

Article abstract of DOI:10.1021/jp046879c

Intramolecular photoinduced charge-separation and charge-recombination processes in a covalently connected C₆₀-(spacer)-N,N-bis(biphenylyl) aniline (C₆₀-spacer-BBA) dyad, in which the center-to-center distance of the electron accepto

Full text of DOI:10.1021/jp046879c

2428
J. Phys. Chem. A 2005, 109, 2428-2435
Photoinduced Intramolecular Electron-Transfer Processes in
[60]Fullerene-(Spacer)-N,N-Bis(biphenylyl)aniline Dyad in Solutions
G. Abraham Rajkumar,†,¶ Atula S. D. Sandanayaka,‡ Kei-ichiro Ikeshita,† Mitsunari Itou,‡
Yasuyuki Araki,‡ Yoshio Furusho,†,£ Nobuhiro Kihara,† Osamu Ito,‡,* and
Toshikazu Takata*,†,¶
Department of Applied Chemistry, Osaka Prefecture UniVersity, Gakuen-cho, Sakai-shi, Osaka 599-8531,
Japan, Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama, Meguro,
Tokyo 152-8552, Japan, Yashima Super Structured-Helix Project, JST, Moriyama-ku, Nagoya 463-0003, Japan,
and Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira 2-1-1,
Aoba-ku, Sendai 980-8577, Japan
ReceiVed: July 14, 2004; In Final Form: NoVember 30, 2004
Intramolecular photoinduced charge-separation and charge-recombination processes in a covalently connected
C60-(spacer)-N,N-bis(biphenylyl)aniline (C60-spacer-BBA) dyad, in which the center-to-center distance of the
electron acceptor and electron donor is 15 Å, have been studied by time-resolved fluorescence and transient
absorption methods. The observed low fluorescence intensity and the short fluorescence lifetime of the
C60 moiety of the dyad in PhCN and THF indicate that charge separation takes place via the excited singlet
state of the C60 moiety at a quite fast rate and a high efficiency. The nanosecond transient absorption
spectra in PhCN and THF showed the broad absorption bands at 880 and 1100 nm, which were attributed to
C60•--spacer-BBA•+. The charge-separated state decays with a lifetime of 330-360 ns in PhCN and THF at
room temperature. From temperature dependence of the charge-recombination rate constants, the reorganization
energy was evaluated to be 0.77-0.87 eV, which indicates that the charge-recombination process is in the
inverted region of the Marcus parabola. With lowering temperature, the contribution of charge separation via
the excited triplet state of the C60 moiety increases due to an increase in solvation of C60•--spacer-BBA•+.
Introduction
Because [60]fullerene, C60, has been revealed as a very
attractive electron acceptor with unique photophysical and
electrochemical properties,1-3 considerable efforts have been
devoted in recent years to develop the systems in which C60 is
covalently linked to electron donors,4-12 in addition to the
mixture systems of C60 and donor.14-21 Molecular systems
including C60 are of particular interest, because they can exhibit
characteristic electronic properties in the excited states in
addition to its small reorganization energy due to spherical
molecular shape;22 thus, a lot of research has been conducted
to the photoinduced electron-transfer processes including C60.23-25
These phenomena open the potential applications in the realiza-
tion of new artificial photosynthetic systems, molecular elec-
tronic devices, and photovoltaic cells.4,6,7
Among the wide variety of donor molecules that can be
covalently linked to C60, one of the fascinating donors is
aromatic amines; various aromatic amine-connected C60 dyads
have been synthesized and the photoinduced processes were
revealed.5 In the previous report, it was reported that C60-bis-
(biphenylyl)aniline (BBA) dyad with a short linkage with a
center-to-center distance (RCC of C60-BBA) of 10 Å generates
a quite persistent charge-separated state in PhCN even in such
a short distance between the radical anion and radical cation.26
Figure 1. Molecular structures of C60-spacer-BBA and reference
samples.
* Author for correspondence. E-mail: takata@chem.osakafu-u.ac.jp.
E-mail: ito@tagen.tohoku.ac.jp.
† Osaka Prefecture University.
In the present study, we report synthesis of C60-spacer-BBA
dyad with a longer linkage (RCC ) 15 Å), as shown in Figure
1 and the charge-separation and charge-recombination processes
¶ Tokyo Institute Technology.
£ Yashima Super Structured Helix Project.
‡ Tohoku University.
10.1021/jp046879c CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/19/2005

ET in C60-Spacer-N,N-Bis(biphenylyl)aniline
J. Phys. Chem. A, Vol. 109, No. 11, 2005 2429
SCHEME 1
investigated by the time-resolved fluorescence and absorption
spectra in the visible and near-IR regions. It would be expected
that the temperature effects on the rate constants of the charge-
separation and charge-recombination processes afford valuable
information about the Marcus parameters of the electron-transfer
processes of this dyad.27
BBA was almost the same as the Eox values of ref-BBA (Eox )
+0.35 V vs Fc/Fc+), suggesting the absence of interaction
between BBA and C60 in the ground state.
The reduction potential (Ered) of C60-spacer-BBA was evalu-
ated to be -1.02 V vs Fc/Fc+; this value is corresponding to
E
red of the C60 moiety, because a quite similar value was reported
for the ref-C60 (Ered ) -1.10 V vs Fc/Fc+).7c From these Eox
and Ered values for BBA-spacer-C60, the free-energy changes
Results and Discussion
for charge separation (∆GCS) and charge recombination (∆GCR
)
Synthesis of C60-Spacer-BBA. C60-spacer-BBA was syn-
thesized as shown in Scheme 1. The Ullmann coupling of
4-nitroaniline 1 with 4-tert-butyl-4′-iodobiphenyl (2) in 1,2,4-
trichlorobenzene using Cu powder as a catalyst afforded
triarylamine 3 in 84% yield. Nitroarene 3 was subjected to the
catalytic hydrogenation using Pd/C to give amine 4 in 92% yield.
Reaction of 4 with acid chloride 5 gave amide 6 in 57% yield.
Diels-Alder reaction of 6 and C60 in refluxing toluene afforded
C60-spacer-BBA in 22% yield. The structure of C60-spacer-BBA
was fully characterized by 1H and 13C NMR, IR, and elemental
analysis. Ref-BBA was prepared according to the literature
method.28 Reaction of 4 with acetyl chloride in the presence of
an amine gave ref-BBA in 75% yield. Ref-C60, which is
4-methoxycarbonyl-o-quinodimethane adduct with C60, was
prepared according to the method described in the literature.29
Details are described in the Experimental Section.
Molecular Orbital Calculations. Figure 2 shows the opti-
mized structure, the electron densities of the lowest unoccupied
molecular orbital (LUMO), and the highest occupied mol-
ecular orbital (HOMO) of C60-spacer-BBA dyad calculated by
GAUSSIAN 98 at the B3LYP/3-21G level.30 The optimized
structure shows that relative positions of BBA and C60 with
RCC ) 15 Å, in which the BBA plane is in the same plain with
the bridge bonds, but not face-to-face alignment with the C60
sphere. The calculated electron densities of the HOMO indicate
the delocalization of electrons in the BBA moiety, whereas the
electron densities in the LUMO are all located on the C60 moiety,
suggesting that the BBA and C60 moieties act as an electron-
donor and an electron-acceptor, respectively.
can be calculated from the Weller equations:31
-∆GCS ) ∆E0-0 - (-∆GCR
)
(1)
(2)
-∆GCR ) Eox - Ered + ∆GS
-∆GS )
e2
4πꢀ0
1
1
2R-
1
1
1
1
2R-
1
+
-
-
+
+
(3)
+
(
)( )
(
)
( )
[
]
RCC ꢀs
ꢀr
2R
2R
where ∆E0-0 is the energy of the 0-0 transition of C60, R+ is
the radii of the radical cation of BBA, and R- is the radii of
the radical anion of C60 (Table 1); e, ꢀ0, ꢀs, and ꢀr refer to
elementary charge, vacuum permittivity, and static permittivities
of the solvents used for rate measurements and redox potential
measurements, respectively. The ∆GCS and ∆GCR values for
C60-spacer-BBA are summarized in Table 1. In a polar solvent
such as PhCN and THF, charge separation via the excited singlet
state of the C60 (1C60*) is exothermic and occurs easily, whereas
in toluene, this process is endothermic and hardly occurs. The
charge-separation process via the excited triplet state of the C60
(3C60*) is slightly exothermic in THF and PhCN.
Steady-State Absorption Measurements. Steady-state ab-
sorption spectra of BBA-spacer-C60 and its components in
toluene are shown in Figure 4. The broad absorption bands at
640 and 710 nm of C60-spacer-BBA are characteristic of the
58 conjugated π-electrons of the C60 moiety. The BBA moiety
shows the absorption at shorter wavelength than 400 nm. For
C60-spacer-BBA, the absorption intensity in longer wavelength
than 400 nm was almost the same as that of ref-C60, indicating
that no appreciable interaction in the ground state. From the
MO calculation, the charge-transfer band would be anticipated
in the longer wavelength region than 700 nm; however, the
absorption intensity is too low to observe explicitly.
Electrochemical Measurements. The cyclic voltammogram
of C60-spacer-BBA dyad in PhCN is shown in Figure 3. Almost
reversible pattern was observed, suggesting the stability of the
radical ions. The Eox value (0.33 V vs Fc/Fc+) of C60-spacer-

2430 J. Phys. Chem. A, Vol. 109, No. 11, 2005
Rajkumar et al.
Figure 4. Steady-state absorption spectra of C60-spacer-BBA (0.1 mM),
ref-C60 (0.1 mM), and ref-BBA (0.1 mM) in toluene.
Figure 5. Steady-state fluorescence spectra of C60-spacer-BBA (0.1
mM) in toluene, THF, and PhCN and ref-C60 in toluene; excitation at
500 nm, where the absorbance of both compounds was matched.
Figure 2. Optimized structures and electron distributions of LUMO
and HOMO of C60-spacer-BBA dyad calculated at B3LYP/3-21G level.
Figure 6. Fluorescence decay profiles around 710-730 nm of C60-
spacer-BBA dyad (0.1 mM) in toluene and PhCN after 410 nm laser
irradiation. Filled curve indicates laser profile. Solid lines are fitting
curves.
Figure 3. Cyclic voltammogram of C60-spacer-BBA dyad (0.2 mM)
in Ar-saturated PhCN containing Bu4NClO4 (0.05 M) as a supporting
electrolyte at scan rate of 0.1 V s-1
.
fluorescence of C60-spacer-BBA is attributed to the C60 moiety.
From the cross point of fluorescence band and absorption band
after normalizing both intensities, the lowest excited singlet
energy (E0-0) of the C60 moiety was estimated to be 1.72 eV.
The fluorescence intensity of C60-spacer-BBA in toluene is
almost the same as that of ref-C60, when the absorbance at
excitation wavelength was matched.
TABLE 1: Free-Energy Changes for Charge Separation
(-∆GCS) and Charge Recombination (-∆GCR) of
C60-Spacer-BBA
solvent
-∆GSCS/eVa
-∆GTCS/eVa
-∆GCR/eVa
toluene
THF
PhCN
-0.20
0.27
0.41
-0.40
0.07
0.21
1.92
1.45
1.31
In polar solvents such as THF and PhCN, the fluorescence
intensity of C60-spacer-BBA becomes lower than that in toluene,
as shown in Figure 5. Such a decrease in fluorescence intensity
of the C60-spacer-BBA in polar solvent suggests the electron
1
a Calculated from eqs 1-3 employing ∆E0-0 ) 1.72 eV for C60*,
∆E0-0 ) 1.52 eV for 3C60*, Eox ) 0.33 V for BBA, and Ered ) -1.02
V for C60 vs Fc/Fc+ in PhCN. R+ ) 7.5 Å for BBA, and R- ) 4.0 Å
for C60, and RCC ) 15 Å, as evaluated from Figure 2. Permittivities of
toluene, THF, and PhCN are 2.38, 7.58, and 25.2, respectively.
1
transfer via the C60* moiety.
Fluorescence Lifetimes. The time profiles of the fluorescence
intensity at the peak position of the 1C60* moiety in C60-spacer-
BBA in toluene and PhCN are shown in Figure 6. In toluene,
Steady-State Fluorescence Measurements. Steady-state
fluorescence spectrum of C60-spacer-BBA in toluene ex-
hibits the fluorescence peak (λf) at 720 nm as shown in Figure
5. Because the spectral shape of the fluorescence band of
C60-spacer-BBA is almost the same as that of ref-C60 in the
same solvent (λf ) 717 nm), the origin of the observed
1
the fluorescence decay of the C60* moiety obeys a single-
exponential function, yielding the fluorescence lifetime of 1400
ps, which is the same as that of ref-C60.17a This finding indicates
that both energy and electron transfers do not take place for

ET in C60-Spacer-N,N-Bis(biphenylyl)aniline
J. Phys. Chem. A, Vol. 109, No. 11, 2005 2431
TABLE 2: Fluorescence Lifetime (τf at 710-730 nm),
1
Charge-Separation Rate Constant via C60* (kSCS), Quantum
1
Yield for Charge Separation (ΦSCS) via C60*,
Charge-Recombination Rate Constant (kCR) of
C60-Spacer-BBA at Room Temperature
solvent τf/ps
kSCS/s-1
ΦS
kTCS/s-1 a
k
CR/s-1
τRIP/ns
CS
toluene 1400
0.00
THF
PhCN
340 2.2 × 109 0.76 3.4 × 107 2.8 × 106
360
330
270 3.0 × 109 0.81 4.2 × 107 3.1 × 106
a The kTCS (charge separation rate constant via 3C60*) values at -10
°C.
C60-spacer-BBA in toluene. In PhCN, the fluorescence intensity
of the C60* moiety in C60-spacer-BBA shows fast decay
obeying a single-exponential kinetics, giving a short fluorescence
lifetime (τf ) 370 ps). In THF, τf ) 340 ps was similarly
evaluated as listed in Table 2.
Figure 7. Nanosecond transient absorption spectra of C60-spacer-BBA
dyad (0.1 mM) observed by 532 nm laser irradiation in at 0.1 µs (b)
and 1.0 µs (O) in PhCN at room temperature. Inset: absorption-time
profiles at 860 and 1000 nm in PhCN.
1
The difference between the τf values evaluated from the main
decays in 710-730 nm in polar and nonpolar solvents can be
1
attributed predominantly to charge separation via the C60*
moiety in C60-spacer-BBA, generating the radical ion-pair state
(C60•--spacer-BBA•+) in polar solvents.
1
From the τf value of the C60* moiety in C60-spacer-BBA,
the intramolecular charge-separation rate constant (kSCS) in THF
and PhCN was evaluated, as summarized in Table 2 using
kSCS ) (1/τf)sample - (1/τf)ref
(4)
where (τf)ref is the fluorescence lifetime of ref-C60 in toluene.17e
Thus, the kSCS values were evaluated to be (2.2-3.0) × 109 s-1
in polar solvents, which indicate that charge separation via the
1C60* moiety is an effective process in polar solvents. The
quantum yields of charge separation (ΦSCS) via the 1C60* moiety
in the C60-spacer-BBA in polar solvents were evaluated from
Figure 8. Nanosecond transient absorption spectra of C60-spacer-BBA
dyad (0.1 mM) observed by 532 nm laser irradiation in at 0.1 µs (b)
and 1.0 µs (O) in toluene at room temperature. Inset: absorption-
time profiles at 720 nm in toluene.
ΦSCS ) [(1/τf)sample - (1/τf)ref]/(1/τ)sample
(5)
Thus, ΦSCS ) 0.76 and 0.81 were calculated for the C60-spacer-
BBA in THF and PhCN, respectively, which indicates that
1
charge separation via the C60* moiety is competitive with the
intersystem crossing (ISC) process to the 3C60* moiety even in
PhCN.
Thus, the kS and ΦS values for C60-spacer-BBA are
CS
CS
smaller than those reported for C60-BBA with short linkage,26
indicating that the long distance between the donor and acceptor
for C60-spacer-BBA is disadvantageous for the charge-separation
system in polar solvents.
Figure 9. Semiclassical Marcus (modified Arrhenius) plots of the
temperature dependence of kCR for C60-spacer-BBA in PhCN and THF.
Nanosecond Transient Absorption Measurements. Tran-
sient absorption spectra observed by the nanosecond laser
excitation (532 nm) of C60-spacer-BBA in PhCN are shown in
Figure 7. The broad absorption bands were observed in the
region of 600-1200 nm, which are the overlapping regions of
lifetime more than 40 µs.17e This observation was supported by
the positive ∆GS value in toluene (Table 1), suggesting
CS
difficulty of charge separation.
Temperature Effect. The energy barriers for the charge-
separation and charge-recombination processes can be estimated
•-
the absorption bands of BBA•+ (880 nm)26 and C60 (1000
by measuring the temperature dependence of the kS and kCR
nm).17e From the decay time profile at 880 nm, the charge-
recombination rate constant (kCR) was evaluated to be 3.1 ×
106 s-1, from which the lifetime (τRIP) of the charge-separated
state was calculated to be 320 ns at room temperature.
In THF, similar transient spectra to those in PhCN were
observed (Supporting Information; Figure S2), showing the
generation of the charge-separated state, which persists with
τRIP ) 360 ns.
CS
values. From the semiclassical Marcus equation,27 the electron-
transfer rate constant (k) can be described as follows:
2
2π3/2|V|
h λk
∆Gq
kBT
x
ln(k T) ) ln
-
(6)
(
)
x
B
where T, |V|, h, λ, kB, and ∆Gq are absolute temperature, the
electron coupling matrix element, Planck’s constant, the reor-
ganization energy, Boltzman’s constant, and Gibb’s activation
energy in the Marcus theory,27 respectively. Figure 9 shows the
modified Arrhenius plot for the semiclassical Marcus eq 6,27
In toluene, transient absorption band was observed around
700 nm at 100 ns (Figure 8). The possible species of the
3
11b
absorption band is the C60* moiety (700 nm),
which did
not show the appreciable decay during 1 µs, because of its long

2432 J. Phys. Chem. A, Vol. 109, No. 11, 2005
Rajkumar et al.
Figure 10. Nanosecond transient absorption spectra of C60-spacer-
BBA dyad (0.1 mM) observed by 532 nm laser irradiation in at 0.1 µs
(b) and 1.0 µs (O) in PhCN at low temperature (-10 °C). Inset:
absorption-time profiles at 860 and 1000 nm in PhCN.
TABLE 3: Gibbs Activation Energy (∆GqCR),
Reorganization Energy (λCR), and Electron Coupling Matrix
Element (|VCR|) for Charge-Recombination Process of
C60•--Spacer-BBA•+ in THF and PhCN
solvent
∆GqCR/eV
λ
CR/eV
|VCR|/cm-1
THF
PhCN
0.10
0.09
0.87
0.77
0.64
0.60
Figure 11. Schematic energy diagram for electron-transfer processes
of C60-spacer-BBA in toluene, THF, and PhCN.
TABLE 4: Gibbs Activation Energy (∆GqCS),
Reorganization Energy (λCS), and Electron Coupling Matrix
Element (|VCS|) for Charge-Separation Process of
C60-Spacer-BBA in PhCN
moiety. From the rise time profile of the C60•- moiety, the rate
constant (kTCS) for the charge separation can be evaluated to be
4.2 × 107 s-1 at -10 °C. In PhCN, the ∆GTCS value is negative
CS process
∆GqCS/eV
λ
CS/eV
|VCS|/cm-1
12.9
1.61
3
via 1C60*
via 3C60*
0.09
0.11
0.17
0.05
allowing the charge separation via C60*. In THF, similar
phenomena were observed giving kTCS ) 3.4 × 107 s-1 at -10
°C, because the ∆GT value is slightly negative. Probably,
CS
1/2
which shows a linear relation between ln(k
T
CR
) values and
C60•--spacer-BBA•+ may be stabilized in these polar solvents
by lowering temperature, increasing the ∆GTCS value negatively.
From the temperature dependency of the rise temporal profiles
1/T. The observed changes in kCR value in the temperature region
shown in Figure 9 were larger than the experimental error ((5%;
Supporting Information, Figure S3). From the slope, i.e.,
of the C60 moiety, the ∆GqTCS value in PhCN was evaluated
•-
(-∆GqCR/kB), the ∆Gq value was estimated to be 0.098 and
to be 0.11 eV, from which λT ) 0.05 eV and |VTCS| ) 1.61
CR
CS
0.094 eV in THF and PhCN, respectively. The λ values for the
charge-recombination process (λCR) were calculated to be 0.87
and 0.77 eV in THF and PhCN,32 respectively, from the
following relation:27
cm-1. From ∆GqT
) -0.11 eV, the charge-separation
CS
3
processes via the C60* moiety are considered to take place at
almost the top region of the Marcus parabola.27 Indeed, the kT
CS
values are slow at -10 °C.
Energy Diagram. Figure 11 shows an energy diagram of
C60-spacer-BBA when the 532 nm laser light is used as the
excitation light. Energy levels of the radical ion-pairs were cited
from Table 1. In THF and PhCN, the charge separation takes
place via 1C60*-spacer-BBA, as indicated by the weak fluores-
cence intensity and short fluorescence lifetime, because the
energy levels of C60•--spacer-BBA•+ were lower than those for
1C60*-spacer-BBA. In toluene, on the other hand, charge
(∆GCR + λ)2
∆Gq )
(7)
4λ
From the comparison of the λCR values with the ∆GCR values
in PhCN and THF (-1.31 and -1.45 eV, respectively), the
charge-recombination processes are considered to take place in
the Marcus inverted region.27 This clearly explains the longer
lifetimes of C60•--spacer-BBA•+ compared with those of other
dyads.5 The |VCR| values in PhCN and THF were calculated to
be 0.60 and 0.64 cm-1, respectively, from the intercept of the
slope in Figure 9. These small |VCR| values are reasonable to
explain the slow charge recombination of C60•--spacer-BBA•+
in polar solvents. From the temperature dependency of fluo-
rescence temporal profiles (Supporting Information; Figure S4),
the ∆GqSCS value in PhCN was evaluated to be 0.09 eV, giving
1
3
separation is impossible via C60*, only generating C60* via
the ISC process. Thus, the generation of 3C60*-spacer-BBA with
the ISC process from 1C60*-spacer-BBA is also possible, because
the ΦSCS values are less than unity in polar solvents. The charge
3
separation via C60*-spacer-BBA becomes prominent at low
temperature, probably because solvation of C60•--spacer-BBA•+
increased its stabilization.
Comparison with Other Systems. For C60-BBA with short
λSCS ) 0.17 eV and |VSCS| ) 12.9 cm-1. Compared with ∆GS
linkage (10 Å), the kS values ((3.4-5.0) × 1010 s-1) are 1
CS
CS
) -0.41 eV, the charge-separation processes via 1C60* moiety
are also considered to take place in the Marcus inverted region.33
Indeed, the kSCS values are moderate in the range of (2.2-3.0)
× 109 s-1 at room temperature. On lowering the temperature,
the rise of the radical ions seems to become slowed, as shown
in Figure 10 for C60•--spacer-BBA•+ in PhCN at -10 °C,
suggesting the charge separation also takes place via the 3C60*
order larger than those for C60-spacer-BBA; the ΦS values
CS
(0.95-0.99) are also larger than those for C60-spacer-BBA.
These observations indicate the charge separation is effective
with short linkage. The lifetime of C60•--spacer-BBA•+ was
evaluated to be 330 ns in PhCN, which is slightly longer than
that of C60•--BBA•+ (220 ns in PhCN at room temperature),26
probably because of the smaller |VCR| values, which are usually

ET in C60-Spacer-N,N-Bis(biphenylyl)aniline
J. Phys. Chem. A, Vol. 109, No. 11, 2005 2433
inverted proportional to the distance between the electron and
the hole connected by covalent bonds.
Synthesis of 6. To a solution of 4 (500 mg, 0.95 mmol) and
Et3N (0.18 mL, 1.28 mmol) in THF (30 mL) was added
dropwise a solution of 5 (320 mg, 0.98 mmol) in THF (20 mL)
at 0 °C. After being stirred at 0 °C for 1 h, the reaction mixture
was stirred at room temperature for 3 h. Then the reaction
mixture was diluted with 70 mL of CHCl3 and washed with 1
M HCl (2 × 10 mL), brine (20 mL), and water (20 mL).
Evaporation of the solvent gave the crude product, which was
purified by preparative HPLC to give 5 as a bright green solid
(440 mg, 57%). 1H NMR (CDCl3, 500 MHz): δ 7.89 (bs, 1H,
NH), 7.79-7.77 (m, 2H, Ar-H), 7.56-7.44 (m, 15H, Ar-H),
7.20-7.16 (m, 6H, Ar-H), 4.80-4.79 (m, 2H, CH2Br), 4.69-
4.68 (m, 2H, CH2Br), 1.36 (s, 18H, tert-Bu).
Compared with aromatic amine donors such as dimethyl-
aniline and diphenylaniline, the BBA moiety has excellent donor
ability, giving persistent radical ion-pair, irrespective the distance
between the amine and C60 moieties. For example, for NMPC60-
TPA dyad, the τRIP value was less than 10 ns.34 In the case of
a triad composed of NMPC60-quterthiophene-TPA, the τRIP value
was evaluated to be 18 ns in DMF at room temperature.35b The
τRIP value for a triad composed of the C60-fluorene-diphenyl-
amine was evaluated to be 150 ns in DMF at room temperature.35a
In the case of C60-bridge-dimethylaniline systems, the τRIP values
in the range of 8-250 ns were reported, depending on the kinds
and lengths of the bridge molecules.5a,5b Compared with these
Synthesis of C60-spacer-BBA. A mixture of 6 (60 mg, 0.074
mmol), KI (87 mg, 0.52 mmol), 18-crown-6 (79 mg, 0.30
mmol), and C60 (69 mg, 0.096 mmol) in dry toluene (25 mL)
was refluxed for 16 h under argon atmosphere. The reaction
mixture was evaporated to dryness, and the residue was
subjected to silica gel column chromatography (from 50%
toluene in CHCl3 to 100% CHCl3) to afford the crude product
(96 mg). The crude product was subjected to preparative HPLC
C
60 and amine systems, C60•--spacer-BBA•+ showed long τRIP
value.
Summary
For C60-spacer-BBA, the photoinduced charge separation via
1C60*-spacer-BBA was observed in polar solvents at room
temperature. By lowering the temperature, the contribution of
charge separation via 3C60*-spacer-BBA increased. The lifetimes
of 320-360 ns were evaluated for the C60•--spacer-BBA•+ in
PhCN and THF, respectively, at room temperature. Furthe-
rmore, it is revealed that a smaller λCR value than the absolute
values of ∆GCR leads to the inverted region of the Marcus
parabola, resulting in slow the charge-recombination rate.
The C60•--spacer-BBA•+ gains a chance to mix with the
3C60*-spacer-BBA, which makes the lifetime of C60•--spacer-
BBA•+ long. With an increase of distance between donor and
acceptor from 10 to 15 Å, the lifetimes increase slightly. These
findings afford a guideline to design new dyads.
1
to afford C60-spacer-BBA (22 mg, 22%). Mp: >300°C. H
NMR (CDCl3, 500 MHz): δ 7.89 (bs, 1H, NH), 7.79-7.77 (m,
2H, Ar-H), 7.56-7.44 (m, 15H, Ar-H), 7.20-7.16 (m, 6H,
Ar-H) 4.80-4.79 (bs, 2H, CH2), 4.69-4.68 (bs, 2H, CH2),
1.36 (s, 18H, tert-Bu). 13C NMR (CDCl3, 125 MHz): δ 165.41,
156.28, 155.94, 149.68, 147.60, 146.46, 146.39, 146.16, 145.36,
144.83, 144.59, 144.57, 144.15, 143.06, 142.50, 142.10, 142.03,
141.49, 140.14, 138.81, 137.57, 135.14, 134.63, 133.05, 128.33,
127.61, 126.60, 126.22, 125.60, 125.15, 123.87, 121.64, 65.63,
65.59, 45.06, 34.57, 31.45, 29.77. FT-IR (KBr): ν 3433 (N-
H), 1652 (CO-NH), 1493, 1261, 1098, 1020 cm-1. Calcd for
(C107H46N2O)(CHCl3)0.5(H20)4.0: C, 85.65; H, 3.65; N, 1.86%.
Found: C, 85.86; H, 3.79; N, 1.88.
Experimental Section
Synthesis of ref-BBA. To a solution of 4 (27.8 mg, 53.0
µmol) and triethylamine (8.00 µL, 57.4 µmol) in chloroform
(1.00 mL) was added acetyl chloride (5.00 µL, 70.3 µmol) at 0
°C. The mixture was stirred at room temperature overnight. The
reaction mixture was evaporated to dryness, and the residue was
subjected to silica gel column chromatography (chloroform/ethyl
acetate ) 19/1, Rf ) 0.22) to afford the crude product (28.1
mg). The crude product was subjected to preparative HPLC to
afford ref-BBA as a white solid (22.4 mg, 40.0 µmol, 75%).
Materials. [60]Fullerene (C60) was obtained from MER
Corporation, USA (99.5% purity). PhCN, toluene, n-hexane,
benzene-d6, N-methylglycine, and R-cyano-4-hydroxycinnamic
acid were purchased from Aldrich Chemicals (Milwaukee, WI).
Synthesis of 3. A mixture of 4-nitroaniline (310 mg, 2.24
mmol), 4-tert-butyl-4′-iodobiphenyl (3.02 g, 4.49 mmol), Cu
powder (856 mg, 13.4 mmol), K2CO3 (3.70 g, 26.8 mmol), and
18-crown-6 (650 mg, 2.46 mmol) in 1,2,4-trichlorobenzene (30
mL) was heated at 165 °C under Ar atmosphere for 24 h. The
reaction mixture was filtered, and the filtrate was passed over
a short column on SiO2. Elution with hexane removed 1,2,4-
trichlorobenzene followed by elution with CHCl3-hexane (ca.
1/1) to afford the crude product. Further purification by SiO2
column chromatography (eluent: CHCl3/hexane (1/4)) afforded
1
Mp: 146-150 °C. H NMR (CDCl3, 400 MHz): δ 7.63 (s,
1H), 7.52-7.39 (m, 16H), 6.85 (br s, 4H), 2.24 (s, 3H), 1.35
(s, 18H) ppm. IR (KBr): 2961, 1655, 1601, 1508, 1497, 1319,
1273, 820, 527 cm-1. FAB-MS (matrix: mNBA): m/z 566 [M]+.
Anal. Calcd for C40H42N2O: C, 84.77; H, 7.47; N, 4.94.
Found: C, 84.33; H, 7.74; N, 5.14.
1
3 as a bright orange crystal (1.05 g, 84% yield). H NMR
(CDCl3, 500 MHz): δ 8.08-8.07 (2H, m, Ar-H), 7.59-7.58
(4H, m, Ar-H), 7.54-7.52 (4H, m, Ar-H), 7.48-7.46 (4H,
m, Ar-H), 7.27-7.19 (4H, m, Ar-H), 7.05-7.03 (2H, m, Ar-
H), 1.37 (18H, s, tert-Bu).
Synthesis of 4. A mixture of 3 (600 mg, 1.08 mmol) and
5% Pd-C (131 mg) in THF (40 mL) was stirred under H2
atmosphere at room temperature for 24 h. The reaction mixture
was filtered, and the filtrate was evaporated. The residue was
purified by SiO2 column chromatography (eluent: CHCl3/
hexane (1/3)) to afford amine 4 (520 mg, 92%) as a bright red-
brown crystal.
Measurements
Electrochemical Measurements. Reduction potentials (Ered
)
and oxidation potentials (Eox) were measured by a cyclic
voltammetry with a potentiostat (BAS CV50W) in a conven-
tional three electrode-cell equipped with Pt-working and counter
electrodes with an Ag/Ag+ reference electrode at scan rate of
100 mV/s. The Ered and Eox were expressed vs ferrocene/
ferrocenium (Fc/Fc+) used as internal reference. In each case,
a solution containing 0.2 mM of a sample with 0.05 M of n-Bu4-
NClO4 (Fluka purest quality) was deaerated with argon bubbling
before measurements.
1H NMR (CDCl3, 270 MHz): δ 7.51-7.49 (m, 4H, Ar-H),
7.45-7.42 (m, 8H, Ar-H), 7.13 (d, 4H, Ar-H, J ) 8.5 Hz),
7.05 (d, 2H, Ar-H, J ) 8.5 Hz), 6.69 (d, 2H, Ar-H, J ) 8.5
Hz), 1.36 (s, 18H, tert-Bu).
Steady-State Measurements. Steady-state absorption spectra
in the visible and near-IR regions were measured on a Jasco
V570 DS spectrophotometer. Steady-state fluorescence spectra

2434 J. Phys. Chem. A, Vol. 109, No. 11, 2005
Rajkumar et al.
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were measured on a Shimadzu RF-5300 PC spectrofluoropho-
tometer equipped with a photomultiplier tube having high
sensitivity in the 700-800 nm region.
Time-Resolved Fluorescence Measurements. The time-
resolved fluorescence spectra were measured by single photon
counting method using a streakscope (Hamamatsu Photonics,
C4334-01) as a detector and the laser light (second harmonic
generation (SHG), 410 nm) of a Ti:sapphire laser (Spectra-
Physics, Tsunami 3950-L2S, 1.5 ps fwhm) as an excitation
source.36 Lifetimes were evaluated with software attached to
the equipment.
Nanosecond Transient Absorption Measurements. Nano-
second transient absorption measurements were carried out using
SHG (532 nm) of a Nd:YAG laser (Spectra-Physics, Quanta-
Ray GCR-130, 5 ns fwhm) as an excitation source. For transient
absorption spectra in the near-IR region (600-1200 nm) and
the time profiles, monitoring light from a pulsed Xe lamp was
detected with a Ge-APD (Hamamatsu Photonics, B2834). For
the measurements in the visible region (400-1000 nm), a Si-
PIN photodiode (Hamamatsu Photonics, S1722-02) was used
as a detector.17,36
Molecular Orbital Calculations. The optimized structure,
energy levels of the molecular orbitals, and electron densities
were calculated by GAUSSIAN 98 (B3LYP/3-21G level).30
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Acknowledgment. This present work was supported by a
Grants-in-Aid on Scientific Research on Priority Areas (417)
from the Ministry of Education, Culture, Sports, Science and
Technology of Japan.
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Supporting Information Available: Space-filled molecular
structure, nanosecond transient spectra in THF, and time profiles
with changing temperature in THF, semiclassical Marcus plots.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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ET in C60-Spacer-N,N-Bis(biphenylyl)aniline
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(32) Although from eq 7 two solutions were given for the λCR values,
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(33) For C60-BBA with Rcc ) 10 Å, from ∆GqS ) 0.15 eV, λS
)
CS
CS
0.02 and 0.62 eV were obtained corresponding to |VSCS| ) 7.93 and 17.98
cm-1 in anisole. In the previous paper,26 the larger values were discussed;
however, the smaller values are also comparable with the values in PhCN
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