Synthesis and photoinduced intermolecular electronic acceptor ability of pyrazolo[60]fullerenes vs tetrathiafulvalene

Article abstract of DOI:10.1246/bcsj.78.1500

The synthesis of three novel pyrazolo[60]fullerene derivatives containing electron-withdrawing substituents has been described. High solubility in ordinary organic solvents was proved. Their electrochemical properties of pyrazolo-[60]fullerenes showed bet

Full text of DOI:10.1246/bcsj.78.1500

1500 Bull. Chem. Soc. Jpn., 78, 1500–1507 (2005)
ꢀ 2005 The Chemical Society of Japan
Synthesis and Photoinduced Intermolecular Electronic Acceptor
Ability of Pyrazolo[60]fullerenes vs Tetrathiafulvalene
ꢀ
´
Juan Luis Delgado, Pilar de la Cruz, Vicente Lopez-Arza, Fernando Langa,
ꢀ;1
Zhenhai Gan,1 Yasuyuki Araki,1 and Osamu Ito
Facultad de Ciencias del Medio Ambiente, Universidad de Castilla-La Mancha, 45071, Toledo, Spain
1Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
Katahira, Aoba-ku, Sendai 980-8577
Received January 12, 2005; E-mail: Fernando.LPuente@uclm.es, ito@tagen.tohoku.ac.jp
The synthesis of three novel pyrazolo[60]fullerene derivatives containing electron-withdrawing substituents has
been described. High solubility in ordinary organic solvents was proved. Their electrochemical properties of pyrazolo-
[60]fullerenes showed better electron affinity than pristine C60 in the ground state. The nanosecond transient absorption
spectra showed clear evidence for photoinduced electron transfer from TTF to the excited triplet state of the pyrazolo-
[60]fullerenes, in which stronger electron-withdrawing substituents accelerate the rates of the photoinduced electron-
transfer, indicating a high electron-acceptor ability of the excited triplet state of pyrazolo[60]fullerenes.
One of the most outstanding applications of fullerenes is the
preparation of photovoltaic cells as a result of their electron ac-
ceptor ability;1,2 composite films of polymers derived from
poly(phenylenevinylene) (PPV) and C60 present high efficien-
cy,3 which is caused by intermolecular photoinduced electron
transfer (PET) from the donor to C60 (the acceptor) as a key
process.4 Nevertheless, the limited solubility of pristine C60
in organic solvents and its tendency to crystallize during film
formation prevent its use in high-concentration blends.5 To
overcome these problems, a series of soluble C60 derivatives
has been developed;6 however, most of the C60 derivatives de-
crease the electron-acceptor properties compared with C60 as a
result of the saturation of a double bond of the C60 cage, rais-
ing the LUMO energy.7 Thus, attempts to increase the electron
affinity of C60, looking for a more efficient behavior in the
charge-transfer and photoinduced electron-transfer processes,
have been performed by many research groups, and better
electron-affinities than pristine C60 have been found in several
types of C60-compounds.8,9 On the other hand, we have shown
that the 1,3-dipolar cycloaddition of nitrile imines, generated
in situ from hydrazones,10 is a general and versatile procedure
to prepare pyrazolo[60]fullerenes (pyrazolo-C60). In contrast
with other families of fullerene derivatives, molecules based
on pyrazolo-C60 derivatives present a similar electron affinity
to pristine C60 as a consequence of the ꢁI effect of the nitro-
gen atom close to the C60 cage.11
ties of 2a–c were measured by means of cyclic voltammetry
(CV) and Osteryoung square-wave voltammetry (OSWV)
techniques, which gave useful information about the acceptor
ability of pyrazolo-C60 derivatives in the ground state, when
compared with the parent C60. In addition, we report a photo-
physical evidence that shows efficient intermolecular electron-
transfer of the excited triplet states of these derivatives from
TTF, by measuring the transient absorption spectra in the vis
and near-IR regions after nanosecond laser light irradiation
in a polar solvent.
Results and Discussion
Synthesis of Pyrazolo[60]fullerenes. Pyrazolo[60]fuller-
enes 2a–c were synthesized in two steps from the correspond-
ing aldehydes, as shown in Scheme 1. Hydrazones 1a–c were
prepared in 68–83% yield by a reaction of the corresponding
aldehyde with 4-nitrophenylhydrazine in the presence of acetic
acid using ethanol as a solvent. Cycloadducts 2a–c were pre-
pared by a modification of our previously described proce-
dure.12 The corresponding hydrazone was treated with N-
bromosuccinimide (NBS) in chloroform at room temperature
for 30 min; after removing the solvent in vacuo, the resulting
bromo-derivative was reacted with Et3N and C60 in toluene
(see Experimental Section). The pyrazolo-C60 derivatives
2a–c were obtained in 25–42% yield after purification by col-
umn chromatography (silica gel, toluene), followed by centri-
fugation in methanol and diethyl ether.
The structures of hydrazones 1a–c and the C60-cycloadducts
2a–c were confirmed by analytical and spectroscopic data. The
1H NMR spectra of 2a–c in CDCl3 exhibit all of the expected
signals corresponding to the organic addend. A low field shift
for the ortho protons (ꢀꢀ ¼ 1:0{1:1 ppm in the N-aryl group
and 0.6–0.7 ppm in the C-aryl group) compared to the corre-
sponding hydrazone was observed. Although it has been sug-
gested that these low-field shifts are due to an intramolecular
In the present study, we focus on the preparation of a series
of soluble pyrazolo-C60 derivatives (2a–c) bearing electron-
withdrawing substituents, with the aim of improving the elec-
tron-acceptor ability of the C60 cage. The electrochemical and
photophysical properties of the newly prepared pyrazolo-C60
derivatives 2a–c have been studied in the both absence and
presence of strong electron donors, such as the tetrathiafulva-
lene (TTF), in order to test their ability to be used as the accep-
tor partner in photovoltaic cells. The electrochemical proper-
Published on the web August 4, 2005; DOI 10.1246/bcsj.78.1500

J. L. Delgado et al.
Bull. Chem. Soc. Jpn., 78, No. 8 (2005) 1501
Scheme 1.
Table 1. Solubility, in mol dmꢁ3, of 2a–c and C60 in Several Solvents at Room Temperature
Compounds
Solvents
a)
C60
3:6 ꢂ 10ꢁ4 a)
2a
1:0 ꢂ 10ꢁ3
2b
2c
Dichloromethane
Acetone
Diethyl Ether
Methanol
1:6 ꢂ 10ꢁ3
5:8 ꢂ 10ꢁ5
4:5 ꢂ 10ꢁ5
2:6 ꢂ 10ꢁ3
1:2 ꢂ 10ꢁ4
4:7 ꢂ 10ꢁ4
2:9 ꢂ 10ꢁ5
1:4 ꢂ 10ꢁ6
—
—
b)
b)
b)
—
—
b)
b)
b)
—
—
a) According to Ref. 15b. b) Less than 1:0 ꢂ 10ꢁ6 mol dmꢁ3
.
Solubility. It is well documented that the solubility of C60
is quite low in most common solvents.15 In fact, C60 is totally
insoluble in polar solvents, like as acetone or methanol.15 This
low solubility limits its use in various devices, such as photo-
voltaic cells, due to its strong tendency to crystallize in the
procedures to prepare films. To overcome this problem, solu-
ble fullerene derivatives are needed. Interesting studies have
been made by several groups in order to obtain highly soluble
fullerene derivatives in water or other solvents.16 In the present
study, we determined the solubility of the pyrazolo-C60 de-
rivatives 2a–c in polar and nonpolar solvents by UV–vis spec-
troscopy. Standard solutions of 2a–c in the studied solvents
were prepared to obtain a linear calibration line of absorbance
vs concentration under dilute concentration. Saturated solu-
tions of all compounds were then prepared and centrifugated
to obtain clear solutions. After dilution, the solubility was de-
termined by interpolation using the calibration line; the results
are collected in Table 1. In dichloromethane, in which pristine
C60 shows the best solubility among the ordinal organic sol-
vents, better solubility of 2a–c by factors of 3–7 was observed.
In polar acetone, which shows low solubility for pristine C60,
2b and 2c showed higher solubility by factors of 40–80. In
diethyl ether and methanol, in which pristine C60 is scarcely
soluble (<10ꢁ6 mol dmꢁ3), a solubility of more than 2:9 ꢂ
10ꢁ5–4:7 ꢂ 10ꢁ4 mol dmꢁ3 was observed for 2c. For 2c, the
solubility increases in the order of methanol < acetone <
diethyl ether < dichloromethane. The high solubility of the
pyrazolo-C60 derivatives in these solvents should be very
valuable in preparing homogeneous films in devices.
Fig. 1. 13C NMR spectrum of 2b in CDCl3.
charge-transfer (CT) interaction between the benzene ring and
the C60 cage,13 it seems that the strong anisotropic effect of the
fullerene is the origin of this low field-shift.14 The electron-
withdrawing nature of the substituents in 2a–c supports this
hypothesis. The observed signals in the 13C NMR spectra are
in agreement with the proposed structures showing the signals
originated from the substituted pyrazoline moiety as well as
most from the C60 system with the appearance of the sp3 car-
bons of the C60 cage in the range of 81–91 ppm (Fig. 1). The
structures of 2a–c were also confirmed by MALDI-TOF mass
spectra, which show the expected MHþ peaks at m=z ¼ 1063
(2a), 1005 (2b), and 1096 (2c), together with several fragmen-
tation peaks. The absorption spectra of 2a–c are similar to
those described for other pyrazolo-C60 derivatives showing
the band at 427 nm, which is typical of [6,6]adducts.11
Electrochemistry. The electrochemical behavior of the
newly synthesized pyrazolo-C60 derivatives was examined
using cyclic voltammetry (CV) and Osteryoung square-wave

1502 Bull. Chem. Soc. Jpn., 78, No. 8 (2005)
Synthesis and PET of Pyrazolo-C60
Fig. 2. OSWV (left) and cyclic voltammogram (right) of compound 2c in o-dichlorobenzene/acetonitrile (4:1) (V vs Ag/AgNO3;
scan rate was 100 mV sꢁ1).
Table 2. Reduction Potentials (V)a) Determined by OSWV of 2a–c and C60 Measured in o-Dichlo-
robenzene/Acetonitrile (4:1)
E1
E2
E3
E4
E5
E6
red
b)
b)
Compound
red
red
red
red
red
C60
2a
2b
2c
ꢁ0:960
ꢁ0:910
ꢁ0:927
ꢁ0:892
ꢁ1:368
ꢁ1:318
ꢁ1:331
ꢁ1:310
ꢁ1:837
ꢁ1:932
ꢁ1:862
ꢁ1:839
ꢁ2:303
ꢁ2:381
ꢁ2:175
ꢁ2:230
ꢁ1:512
ꢁ1:722
ꢁ1:780
ꢁ1:660
a) V vs Ag/AgNO3; GCE as working electrode; 0.1 mol dmꢁ3 TBAP; scan rate was 100 mV sꢁ1
b) Irreversible according to cyclic voltammetry, corresponds to the nitrobenzene group.
.
voltammetry (OSWV) techniques at room temperature, as
shown in Fig. 2 for 2c. These studies were carried out in o-di-
chlorobenzene/acetonitrile (4:1) as a solvent and using tetra-
butylammonium perchlorate (TBAP) as a supporting electro-
lyte (see Experimental Section). Data obtained by the OSWV
technique are collected in Table 2 along with those of pristine
C60 for the sake of a comparison; 2b and 2c show four quasi-
reversible reduction potentials, which are assigned to the C60
cage.17 For 2a, additional non-reversible reduction potentials
are situated between the second and third potentials of the
C60 cage (Fig. 2); this additional reduction wave is assigned
to the reduction of the nitrophenyl group. Pyrazolo-C60 deriva-
tives 2a–c show values of the first reduction potential with
slightly less negative values (33–66 mV) than that of C60,
indicating a higher electron affinity than pristine C60 in the
Fig. 3. Steady-state absorption spectra of 2a (0.05 mmol
dmꢁ3), TTF (0.5 mmol dmꢁ3), and mixture of 2a (0.05
mmol dmꢁ3) and TTF (0.5 mmol dmꢁ3), and added spectra
(2a þ TTF) in o-dichlorobenzene.
ground states. The E1 values shift to less negative in the
red
order of 2c (CF3) < 2a (NO2) < 2b (Cl).
Steady-State Absorption and Fluorescence Spectra. The
UV–vis spectra of 2a and the mixture of 2a with TTF in o-di-
chlorobenzene are shown in Fig. 3; the weak peak at 685 nm
and broad absorption band in the 600–500 nm region with in-
termediate intensity are characteristic of C60 derivatives. The
absorption bands in 350–500 nm are an overlapping of the
C60 and nitrobenzene moieties. Almost the same absorption
spectra were observed for 2b and 2c. The solvent-polarity ef-
fect on these absorption spectra was small. Upon the addition
of TTF, the absorption bands at 460 nm and 355 nm are attrib-
uted to TTF (Fig. 3). In the 510–720 nm region, no appreciable
change was observed upon the addition of TTF. This suggests
that the interaction between 2a (0.05 mmol dmꢁ3) and excess
TTF (0.5 mmol dmꢁ3) is quite weak. Similar absorption spec-
tra were observed for a mixture of 2b/2c and TTF in o-dichlo-
robenzene and in benzonitrile. The absorption maxima in the
longest wavelength region are listed in Table 3. In the fluores-
cence spectra and transient absorption experiments, as describ-
ed in a later part of this report, a 532-nm laser-light was em-
ployed to selectively excite the C60 moiety of 2a–c without
the excitation of TTF.
The fluorescence spectra of 2a and a mixture of 2a with
TTF in o-dichlorobenzene observed by 532 nm light excitation
are shown in Fig. 4. For 2a, a fluorescence peak was observed
at 695 nm with a shoulder at 770 nm; the fluorescence peaks
are almost a mirror image of the absorption in the 650–700
nm region. Thus, the fluorescence peak at 695 nm with a
shoulder at 770 nm arises from the C60 moiety of 2a. Almost
the same fluorescence spectra were observed for 2b and 2c in
o-dichlorobenzene and in other organic solvents. The fluores-
cence peaks are summarized in Table 3. Upon the addition
of excess TTF, no appreciable change was observed (Fig. 4).

J. L. Delgado et al.
Bull. Chem. Soc. Jpn., 78, No. 8 (2005) 1503
Table 3. Spectroscopic Data for 2a–c and C60; Longest Absorption Maxima (ꢂabs,max), Fluores-
cence Peak (ꢂF,max), and Lifetime (ꢁF) in o-Dichlorobenzene; Triplet–Triplet Absorption Maxima
(ꢂabs,maxT) in Toluene and Absorption Maxima of Radical Anion (ꢂabs,maxA) in Benzonitrile
Compound
ꢂ
abs,max/nm
ꢂ
F,max/nm
ꢁF/ns
ꢂabs,maxT/nm
ꢂabs,maxA/nm
C60
2a
2b
2c
624
691
691
686
697
695
697
686
1.30
1.46
1.43
1.48
740
700
700
700
1080
1060
1080
1040
Fig. 4. Steady-state fluorescence spectra of 2a (0.05
mmol dmꢁ3; solid line) and mixture (dotted line) of 2a
(0.05 mmol dmꢁ3) and TTF (0.5 mmol dmꢁ3) observed
with excitation wavelength at 532 nm in o-dichloroben-
zene.
Fig. 6. Nanosecond transient absorption spectra observed
by the excitation of 2c (0.5 mmol dmꢁ3) with 532 nm
laser-light in Ar-saturated benzonitrile.
state fluorescence measurements. From the time profiles,
which obey a single exponential function, the fluorescence
lifetimes (ꢁF) of 2a–2c were evaluated, as listed in Table 3.
The ꢁF values of 2a–2c are almost the same as that of pristine
C60 and the reported ꢁF values for other C60 derivatives.18
Nanosecond Transient Absorption Spectra. Figure 6
shows the transient absorption spectra observed by the excita-
tion of 2c with the 532 nm laser light in toluene. The peak at
700 nm is characteristic of the triplet–triplet transition of the
C60 moiety.19 In an Ar-saturated toluene solution, the decay
at 700 nm was slow living until ca. 50 ms; from longer time-
scale measurements, the lifetime of the triplet state of the
C60 moiety of 2c (32c ) was evaluated to be ca. 30 ms. Upon
ꢀ
the addition of O2, the 700 nm band decayed quickly, indicat-
ing that the triplet energy of 2c transfers to O2. Similar transi-
ent spectra and time profiles were observed for 2a and 2b.
These observations confirmed the formation of 32ꢀ, which
may be produced via intersystem crossing (ISC) from the ex-
cited singlet states of 2a–c (12ꢀ) having a shorter lifetime than
a nanosecond laser pulse (6 ns in our instrument). In 2a–c,
without an electron-donating group, intramolecular charge
separation does not take place, although for pyrazolo-C60 de-
rivatives with the electron-donating groups, the intramolecular
charge separation was observed.20 The absorption maxima of
32ꢀ are listed in Table 3.
Fig. 5. Time-resolved fluorescence spectra of 2a (0.05
mmol dmꢁ3) observed with excitation wavelength at 400
nm in THF. Inset: Time profile at 700 nm.
This suggests that in such a dilute solution (2a–c, 0.05
mmol dmꢁ3 and TTF, 0.5 mmol dmꢁ3), the interaction be-
tween the excited singlet state of 2a and excess TTF is quite
weak in o-dichlorobenzene. A similar behavior was observed
for a mixture of 2b–c and TTF in o-dichlorobenzene and in
benzonitrile.
Time-Resolve Fluorescence Measurements.
Time-re-
Figure 7 shows the transient absorption spectra observed by
the excitation of 2c with 532-nm laser-light in the presence of
TTF in Ar-saturated benzonitrile. Immediately after laser-light
excitation, the transient absorption band of 2c was mainly
observed. With a concomitant decay of the 700 nm band, a
new absorption band appeared at 1040 nm, which can be at-
tributed to the radical anion of the C60 moiety (2cꢃꢁ); the ab-
solved fluorescence spectra and time profiles of 2a–2c were
observed. In Fig. 5, the fluorescence spectra and time profiles
of 2a are shown. The fluorescence spectrum is almost the same
at longer wavelengths than the peak region (680–850 nm),
while in a shorter wavelength region than 675 nm, a slight dif-
ference was observed, probably due to stray light in the steady-
3
ꢀ

1504 Bull. Chem. Soc. Jpn., 78, No. 8 (2005)
Synthesis and PET of Pyrazolo-C60
Fig. 7. Nanosecond transient absorption spectra observed
by the excitation of 2c (0.5 mmol dmꢁ3) with 532 nm
laser-light in the presence of TTF (0.5 mmol dmꢁ3) in
Ar-saturated benzonitrile; ( ) 100 ns and ( ) 1000 ns.
Inset: Time profiles at 700 and 1040 nm.
Fig. 8. Time profiles at (A) 700 nm and (B) 1040 nm for
2c with changing TTF concentrations; [TTF] = (a) 0.2,
(b) 0.5, and (c) 1.0 mmol dmꢁ3. Other conditions were
the same as Fig. 6.
Scheme 2.
sorption peaks of 2cꢃꢁ are listed in Table 3. These observa-
tions clearly indicated that electron transfer takes place via
32c as Eq. 1 and Scheme 2.19 In the transient spectrum ob-
ꢀ
served at 1.0 ms, an absorption tail was observed in the 600–
640 nm region, which may be attributed to the radical cation
of TTF (TTFꢃ).21
kET
32ꢀ þ TTF ꢁꢁ! 2ꢃꢁ þ TTFꢃþ
:
ð1Þ
3
ꢀ
In the time profiles inserted in Fig. 7, the decay of 2c in-
creased on the addition of TTF; with this rapid decay, a rise
of 2cꢃꢁ was observed. These decays and rise rates increased
with the concentration of TTF, as shown in Fig. 8.
These decays and rises were curve-fitted with a single expo-
nential, from which the first-order rate constants (kfirst-order
)
were evaluated. The kfirst-order values increased linearly with
the TTF concentration, as shown in Fig. 9 for the 2c–TTF so-
3
ꢀ
Fig. 9. Plots of kfirst-order vs [TTF] for the decay of 2c at
700 nm and rise of 2cꢃꢁ at 1040 nm.
ꢀ
lution. The plots for the decay rates of 32c are almost the
same as those for the rise rates of 2cꢃꢁ within the experimental
errors, giving the same slope, from which the second-order rate
constants (kq) were evaluated as 2:2 ꢂ 109 molꢁ1 dm3 sꢁ1. For
2a–TTF and 2b–TTF solutions, the kq values were evaluated
as listed in Table 4. The kq values increase slightly in the order
of 2a (NO2) < 2b (Cl) < 2c (CF3).
Table 4. Quenching Rate Constants (kq) of 32ꢀ by TTF,
Quantum Yield (ꢁET) and Rate Constant (kET) for
Forward Electron Transfer via 32ꢀ from TTF, and Back
Electron-Transfer Rate-Constants (kBET) between 2ꢃꢁ
and TTFꢃþ in Benzonitrile
3 ꢀ
The efficiency of electron transfer via 2 can be evaluated
by the ratio of the maximal absorbance of 2ꢃꢁ (A2 ,max) to
ꢃꢁ
kq/
molꢁ1 dm3 sꢁ1
kET
/
kBET/
3 ꢀ
3 ꢀ
the initial absorbance of 2 (A 2 ,initial). This ratio increases
Acceptor
ꢁET
molꢁ1 dm3 sꢁ1 molꢁ1 dm3 sꢁ1
with the TTF concentration, saturating at TTF = ca. 1.0
mmol dmꢁ3, as shown in Fig. 10. Upon assuming the molar ex-
tinction coefficients for the absorption band of 2ꢃꢁ at ca. 1000
nm ("A) and the absorption band of 32ꢀ at ca. 700 nm ("T) to
2a
2b
2c
1:8 ꢂ 109
2:0 ꢂ 109
2:2 ꢂ 109
0.54
0.66
0.70
1:0 ꢂ 109
1:3 ꢂ 109
1:5 ꢂ 109
8:6 ꢂ 109
8:3 ꢂ 109
8:2 ꢂ 109

J. L. Delgado et al.
Bull. Chem. Soc. Jpn., 78, No. 8 (2005) 1505
By laser flash photolysis, the electron-transfer process from
TTF to the excited triplet states of pyrazolo[60]fullerenes
was confirmed; the rate constants for electron transfer increase
in the order of 2a (NO2) < 2b (Cl) < 2c (CF3).
Experimental
General Remarks.
All cycloaddition reactions were per-
formed under argon. C60 was purchased from MER Corporation
(Tucson, AZ); TLC using Merck silica gel 60-F254 was employed
to monitor cycloaddition reactions. 1H NMR and 13C NMR spectra
were recorded on a Varian Mercury 200 apparatus. UV–vis ab-
sorption spectra were obtained on a Shimadzu spectrophotometer.
FT-IR spectra were recorded on a Nicolet Impact 410 spectropho-
tometer using KBr disks. MALDI-TOF mass spectra were ob-
tained on a Bruker ReflexIII spectrometer. Cyclic voltammetry
measurements were carried out on an Autolab PGSTAT 30 poten-
tiostat using a BAS MF-2062 Ag/0.01 mol dmꢁ3 AgNO3 as a ref-
erence electrode, an auxiliary electrode consisting of a Pt wire,
and a Metrohm 6.1247.000 conventional glassy carbon electrode
(GCE, 3 mm o.d.) as a working electrode, directly immersed
in the o-dichlorobenzene/acetonitrile solution containing 0.1
mol dmꢁ3 TBAP as an electrolyte. A 10 mL electrochemical cell
from BAS, Model VC-2, was also used. The reference potential
was shifted by 290 mV towards a more negative potential com-
pared with the Ag/AgCl scale. E1=2 values were taken as the aver-
age of the anodic and cathodic peak potentials. The scan rate was
vs [TTF]; for 32ꢀ at
,initial
ꢃꢁ
Fig. 10. Plots of A2 ,max/A 2
3
ꢀ
700 nm and for 2ꢃꢁ at 1040–1080 nm.
100 mV sꢁ1
.
Steady-state absorption spectra were measured on a JASCO
V-530 UV–vis spectrophotometer. Steady-state fluorescence spec-
tra were measured on a Shimadzu RF-5300 PC spectrofluoropho-
tometer. Fluorescence lifetimes were measured by a single-photon
counting method using a second harmonic generation (SHG, 410
nm) of a Tisapphire laser [Spectra–Physics, Tsunami 3950-L2S,
1.5 ps full width at half-maximum (fwhm)] and a streak scope
(Hamamatsu Photonics, C4334-01) equipped with a polychroma-
tor (Action Research, SpectraPro 150) as an excitation source and
a detector, respectively. Nanosecond transient absorption spectra
were measured using pulsed laser light from an optical parametric
oscillation (Continuum Surelite OPO, fwhm 6 ns) pumped by a
Nd:YAG laser (Continuun, Surelite II-10). For measurements of
the transient absorption spectra in the near-IR region, a Ge avalan-
che photodiode (APD, Hamamatsu Photonics, B2834) was used as
a detector for monitoring light from a pulsed xenon lamp.22
General Synthesis Procedure of Hydrazones. A solution of
the corresponding aldehyde (6.52 mmol), 4-nitrophenylhydrazine
(6.52 mmol) and two drops of acetic acid in 10 mL of ethanol
was heated under reflux for 10 min. After the mixture was cooled
to room temperature, the solid was filtered. Then, the solid was
recrystallized from ethanol.
Fig. 11. Longer time-scale of decay curves at 1040 nm
of 2cꢃꢁ generated by laser-light excitation of 2c (0.1
mmol dmꢁ3) in the presence of TTF: [TTF] = (a) 0.1,
(b) 0.5, and (c) 1.0 mmol dmꢁ3 in Ar-saturated benzoni-
trile. Inset: Plot of second-order kinetics.
be 5000 and 6000 cmꢁ1 molꢁ1 dm3, respectively,18 the quan-
tum yields (ꢁET) of electron transfer via 32ꢀ (ꢁET) were eval-
uated to be 0.54–0.70. The ꢁET values increase in the order
of 2a (NO2) < 2b (Cl) < 2c (CF3). Thus, the kET values were
finally obtained from the relation kET ¼ ꢁET kq, as listed
ꢄ
in Table 4. The kET values increase in the order of 2a
(NO2) < 2b (Cl) < 2c (CF3).
In long time-scale measurements, 2ꢃꢁ began to decay, obey-
ing second-order kinetics (inset in Fig. 11), which indicates
that the back electron transfer takes place between 2ꢃꢁ and
TTFꢃþ after both radical ions are solvated by benzonitrile.
The time profiles are almost the same with increasing TTF
concentration, as shown in Fig. 11. The slope of the second-
order plot gave the ratio of the back electron transfer rate con-
stant (kBET) to the "A values. Upon employing "A ¼ 5000
cmꢁ1 molꢁ1 dm3, the kBET values close to the diffusion-con-
trolled limit in benzonitrile were evaluated (Table 4). This
finding indicates that the photoinduced processes complete
without side reactions.
4-Nitrobenzaldehyde 4-Nitrophenylhydrazone (1a). Yield:
ꢅ
83%. mp 245–247 C. FT-IR (KBr) ꢃ/cmꢁ1 526, 685, 745, 844,
1
1268, 1494, 1560, 1687, 2362. H NMR (CDCl3) ꢀ/ppm 7.2 (d,
2H, J ¼ 8:8 Hz), 7.84 (d, 2H, J ¼ 8:4 Hz), 8.23 (d, 2H, J ¼ 8:4
Hz), 8.26 (s, 1H), 8.28 (d, 2H, J ¼ 8:8 Hz). 13C NMR (DMSO)
ꢀ/ppm 111.3, 123.3, 125.4, 126.4, 138.2, 138.5, 140.4, 146.2,
149.1. MALDI MS: m=z: 285.8. Anal. calcd for C13H10N4O4:
C, 54.55; H, 3.52; N, 19.57%. Found: C, 54.15; H, 3.31; N,
19.72%.
Conclusion
A series of newly synthesized pyrazolo[60]fullerenes bear-
ing electron-withdrawing substituents showed an improved
solubility and electron affinity compared with pristine C60.
2,3,6-Trichlorobenzaldehyde 4-Nitrophenylhydrazone (1b).
Yield 68%. mp 225–227 ꢅC. FT-IR (KBr) ꢃ/cmꢁ1 592, 751,
851, 871, 957, 1461, 1560, 1786, 1859, 1918. H NMR (CDCl3)
1

1506 Bull. Chem. Soc. Jpn., 78, No. 8 (2005)
Synthesis and PET of Pyrazolo-C60
ꢀ/ppm 7.2 (d, 2H, J ¼ 9:2 Hz), 7.3 (d, 1H, J ¼ 8:8 Hz), 7.4 (d,
1H, J ¼ 8:8 Hz), 8.1 (s, 1H), 8.2 (d, 2H, J ¼ 9:2 Hz), 8.3 (s,
1H), 8.4 (s, 1H). 13C NMR (DMSO) ꢀ/ppm 111.7, 123.8, 125.8,
126.8, 126.9, 138.6, 138.9, 146.6, 149.6. Anal. calcd for
C13H8Cl3N3O2: C, 45.31; H, 2.34; N, 12.19%. Found: C, 45.47;
H, 2.16; N, 12.03%.
nitrile) ꢂmax/nm (log ") 322 (4.56), 392 (4.28), 490 (3.19), 682
(2.16). MS m=z: 1095.9 (M þ 1), 720 (C60).
Financial support for this work was provided by grants from
´
Ministerio de Educacion y Ciencia of Spain and Feder funds
(Project CTQ2004-00364/BQU) and Junta de Comunidades
de Castilla-La Mancha (Project PAI-02-023). The authors
(Y. A. and O. I.) are also grateful to financial supports by
Grant-in-Aid for Scientific Researches on Priority Area (417)
from Ministry of Education, Culture, Sports, Science and
Technology of Japan.
3,5-Bis(trifluoromethyl)benzaldehyde 4-Nitrophenylhydra-
ꢅ
zone (1c). Yield: 71%. mp 259–261 C. FT-IR (KBr) ꢃ/cmꢁ1
526, 692, 745, 831, 990, 1110, 1196, 1474, 1534, 1587, 1706.
1H NMR (CDCl3) ꢀ/ppm 7.2 (d, 2H, J ¼ 9:2 Hz), 7.8 (s, 1H),
8.1 (s, 2H), 8.2 (d, 2H, J ¼ 9:2 Hz), 8.45 (s, 1H). 13C NMR
(CDCl3) ꢀ/ppm 111.7, 125.7, 125.8, 126.1, 130.1, 130.8, 137.4,
137.8, 138.8, 149.8. Anal. calcd for C15H9F6N3O2: C, 47.76; H,
2.40; N, 11.14; O, 8.48%. Found: C, 47.69; H, 2.42; N, 11.08%.
General Synthesis Procedure of Cycloadducts. A solution
of NBS (0.26 mmol) and the corresponding hydrazone (0.17
mmol) in CHCl3 was stirred at room temperature for 30 min.
The solvent was removed in vacuo, and a solution of C60 (0.13
mmol) and Et3N (0.21 mmol) in toluene (50 mL) was added to
the solid. The mixture was treated according to the described pro-
cess for each compound. After the corresponding reaction time,
the solvent was removed under reduced pressure. The resulting
solid was purified by silica-gel flash chromatography using tol-
uene as eluent. Centrifuging three times with methanol and once
with diethyl ether accomplished further purification of the solid.
10,30-Bis(4-nitrophenyl)pyrazolo[40,50-a][60]fullerene (2a).
The solution was irradiated by microwave for 30 min at 210 W
of power. Yield 25% (72% based on recovered C60). FT-IR
(KBr) ꢃ/cmꢁ1 527, 718, 1182, 1421, 1494, 1547, 1653, 1699.
1H NMR (CDCl3) ꢀ/ppm 8.3 (d, 2H, J ¼ 9:5 Hz), 8.38 (d, 2H,
J ¼ 9:5 Hz), 8.41 (d, 2H, J ¼ 8:7 Hz), 8.5 (d, 2H, J ¼ 8:7 Hz).
13C NMR (CDCl3) ꢀ/ppm 81.5, 91.0, 109.4, 119.7, 123.8,
125.0, 129.7, 136.2, 136.7, 137.5, 139.4, 140.4, 141.7, 141.8,
142.0, 142.2, 142.3, 142.8, 143.0, 143.94, 144.1, 144.2, 144.8,
145.0, 145.1, 145.3, 145.4, 145.8, 145.9, 146.0, 146.2, 146.3,
147.0, 147.4, 147.9, 148.5. UV–vis (benzonitrile) ꢂmax/nm
(log ") 323 (4.58), 415 (4.26), 682 (2.17). MS m=z: 1005.0
(M þ 1), 720.1 (C60).
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