The Isoindazole Nucleus as a Donor in Fullerene-Based Dyads. Evidence for Electron Transfer

Article abstract of DOI:10.1021/jo0499017

A series of isoindazole-C₆₀ dyads 4a-c based on pyrazolino[60]fullerene have been prepared by 1,3-dipolar cycloadditions of the nitrile imines, generated in situ from hydrazones 3a-c, to C₆₀. Molecular orbital calculations for 4b revealed that the electron distribution of the HOMO is located on the isoindazole moiety, while the electron distribution of the LUMO is located on the C₆₀ moiety. Electrochemical properties of the new dyads 4a-c show a similar electron affinity with respect to C₆₀. Charge-transfer interactions in the ground state between the isoindazole ring and the fullerene cage are predicted by the molecular orbital calculations and confirmed by electrochemical studies in 4a,b. Steady-state fluorescence emission spectra of dyads 4a-c show that fluorescence intensities in polar benzonitrile solvent decrease with increasing electron-donating ability of the substituent attached on the isoindazole group. This was confirmed by the shortening of fluorescence lifetimes, from which intramolecular charge-separation rates and efficiencies via the excited singlet states of the fullerene moiety were evaluated. The yields of the triplet states in polar solvent decrease with the electron-donating ability, supporting the competitive formation of the charge-separated state with the intersystem crossing from the excited states. Thus, isoindazole[60]fullerene 4b can be considered a molecular switch with an AND logic gate.

Full text of DOI:10.1021/jo0499017

Th e Isoin d a zole Nu cleu s a s a Don or in F u ller en e-Ba sed Dya d s.
Evid en ce for Electr on Tr a n sfer
J uan L. Delgado, Pilar de la Cruz, Vicente Lo´pez-Arza, and Fernando Langa*
Facultad de Ciencias del Medio Ambiente, Universidad de Castilla-La Mancha, 45071, Toledo, Spain
David B. Kimball and Michael M. Haley*
Department of Chemistry and the Materials Science Institute, University of Oregon,
Eugene, Oregon 97403-1253
Yasuyuki Araki and Osamu Ito*
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University,
Katahira, Sendai, Miyagi 980-8577, J apan
fernando.lpuente@uclm.es
Received J anuary 16, 2004
A series of isoindazole-C60 dyads 4a -c based on pyrazolino[60]fullerene have been prepared by
1,3-dipolar cycloadditions of the nitrile imines, generated in situ from hydrazones 3a -c, to C60.
Molecular orbital calculations for 4b revealed that the electron distribution of the HOMO is located
on the isoindazole moiety, while the electron distribution of the LUMO is located on the C60 moiety.
Electrochemical properties of the new dyads 4a -c show a similar electron affinity with respect to
C60. Charge-transfer interactions in the ground state between the isoindazole ring and the fullerene
cage are predicted by the molecular orbital calculations and confirmed by electrochemical studies
in 4a ,b. Steady-state fluorescence emission spectra of dyads 4a -c show that fluorescence intensities
in polar benzonitrile solvent decrease with increasing electron-donating ability of the substituent
attached on the isoindazole group. This was confirmed by the shortening of fluorescence lifetimes,
from which intramolecular charge-separation rates and efficiencies via the excited singlet states
of the fullerene moiety were evaluated. The yields of the triplet states in polar solvent decrease
with the electron-donating ability, supporting the competitive formation of the charge-separated
state with the intersystem crossing from the excited states. Thus, isoindazole[60]fullerene 4b can
be considered a molecular switch with an AND logic gate.
In tr od u ction
ties and to further the understanding of the process.
Although derivatization of C60 decreases, in general, the
electron affinity of the all-carbon sphere as a consequence
of the loss of conjugation,7 we have shown that pyrazolino-
[60]fullerenes present an improved electron affinity
During the past five years, electron donor-acceptor
(DA) dyads based on [60]fullerene, which upon photoex-
citation give rise to photoinduced charge-separated states,
have been the focus of intensive research as candidates
for artificial photosynthetic reaction centers.1,2 A wide
variety of donor fragments such as porphyrin,3 tetrathi-
afulvalene,4 ferrocene,5 and amines6 have been used both
in the search for improved electron-transfer (ET) proper-
(3) (a) Gust, D.; Moore, T. H.; Moore, A. L. Acc. Chem. Res. 2001,
34, 40-48. (b) Guldi, D. M. Chem. Soc. Rev. 2002, 31, 22-36. (c)
Liddell, P. A.; Kodis, G.; Moore, A. L.; Moore, T. A.; Gust, D. J . Am.
Chem. Soc. 2002, 124, 7668-7669. (d) Wilson, S. R.; MacMahon, S.;
Tat, F. T.; J arowski, P. D.; Schuster, D. I. Chem. Commun. 2003, 226-
227.
(4) (a) Llacay, J .; Veciana, J .; Gancedo, J . V.; Bourdelande, J . L.;
Moreno, R. G.; Rovira, C. J . Org. Chem. 1998, 63, 5201-5210. (b) Pe´rez,
I.; Liu, S.; Mart´ın, N.; Echegoyen, L. J . Org. Chem. 2000, 65, 3796-
3803. (c) Segura, J . L.; Priego, E. M.; Mart´ın, N. Tetrahedron Lett. 2000,
41, 7737-7741.
(5) (a) Guldi, D. M.; Magini, M.; Scorrano, G.; Prato, M. J . Am.
Chem. Soc. 1997, 119, 974-980. (b) D’Souza, F.; Zandler, M. E.; Smith,
P. M.; Deviprasad, K. A.; Fujitsuka, M.; Ito, O. J . Phys. Chem. A 2002,
106, 649-656. (c) Zandler, M. E.; Smith, P. M.; Fujitsuka, M.; Ito, O.;
D’Souza, F. J . Org. Chem. 2002, 67, 9122-9129. (d) Fujitsuka, M.;
Tsuboya, N.; Hamasaki, R.; Ito, M.; Onodera, S.; Ito, O.; Yamamoto,
Y. J . Phys. Chem. A 2003, 107, 1452-1458.
(1) Bracher, P. J .; Schuster, D. I. Electron Transfer in Functionalized
Fullerenes. In Fullerenes: From Synthesis to Optoelectronic Properties;
Guldi, D. M., Martin, N., Eds.; Kluwer Academic Publishers: Norwell,
MA, 2002; Chapter 6.
(2) (a) Imahori, H.; Sakata, Y. Adv. Mater. 1997, 9, 537-546. (b)
Mart´ın, N.; Sa´nchez, L.; Illescas, B.; Pe´rez. I. Chem. Rev. 1998, 98,
2527-2548. (c) Imahori, H.; Sakata, Y. Eur. J . Org. Chem. 1999, 2445-
2457. (d) Guldi, D. M. Chem. Commun. 2000, 321-327. (e) Guldi, D.
M.; Prato, M. Acc. Chem. Res. 2000, 33, 695-703. (f) Nierengarten, J .
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9, 37-41.
10.1021/jo0499017 CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/13/2004
J . Org. Chem. 2004, 69, 2661-2668
2661

Delgado et al.
SCHEME 1
8
compared to the parent C60 (up to 70 mV) because of
the inductive effect of the pyrazoline ring. Several DA
dyads based on pyrazolino[60]fullerenes have been pre-
pared, and efficient intramolecular photoinduced electron-
transfer (PET) processes have been observed.9 The recent
report of an easy, high-yield synthesis of isoindazole
aldehydes by heating (2-ethynylphenyl)triazenes with
CuCl in 1,2-dichloroethane (DCE)10 led us to consider this
heterocycle as a donor unit in our continuing investiga-
tions. Herein, we report the synthesis, theoretical calcu-
lations, and the electrochemical and photophysical prop-
erties of a series of pyrazolino[60]fullerene dyads in which
a 2-aminoisoindazolyl moiety is acting as a donor.
tion,11 which in turn can be generated in situ from the
corresponding hydrazones 3a -c. This methodology has
proven to be a powerful tool for the functionalization of
C60 due to the availability of the starting materials and
the fact that no stereogenic centers are produced during
the reaction.9a Isoindazole aldehydes 2a -c were prepared
by Cu-promoted cyclization of 1a -c as previously
described.10c Reaction of 2a -c with 4-nitrophenylhydra-
zine in EtOH under reflux for 10 min afforded hydrazones
3a -c in very good yields (73-83%). Reaction of 3a -c
with NBS in CHCl3 at room temperature and subsequent
addition of C60 and Et3N in toluene and irradiation in a
focused microwave reactor at 210 W for 25 min afforded
cycloadducts 4a -c in 21-27% isolated yield after column
chromatography (silica gel, toluene) followed by centrifu-
gation in hexane, MeOH, and Et2O. Cycloadduct 5, used
as a reference, was prepared by a similar procedure from
acetaldehyde 4-nitrophenylhydrazone (6) and C60 in 42%
yield.
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion . The synthetic
approach to preparing isoindazolylpyrazolino[60]fullerenes
4a -c (Scheme 1) relies upon the 1,3-dipolar cycloaddition
of isoindazolyl nitrile imines under microwave irradia-
The structures of hydrazones 3a -c, cycloadducts 4a -c
and the reference compound 5 were confirmed by ana-
1
lytical and spectroscopic data. The H NMR spectra of
(6) (a) Williams, R. M.; Zwier, J . M.; Verhoeven, J . W. J . Am. Chem.
Soc. 1995, 117, 4093-4099. (b) Sun, Y. P.; Ma, B.; Bunker, C. E. J .
Phys. Chem. 1998, 102, 7580-7590. (c) Liu, S. G.; Shu, L.; Raimundo,
J . M.; Roncali, J .; Gorgues, A.; Echegoyen, L. J . Org. Chem. 1999, 64,
4884-4886. (d) Guldi, D. M.; Swartz, A.; Luo, C.; Go´mez, R.; Segura,
J . L.; Mart´ın, N. J . Am. Chem. Soc. 2002, 124, 10875-10886.
(7) Echegoyen, L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31, 593-
601.
(8) Langa, F.; de la Cruz, P.; Esp´ıldora, E.; de la Hoz, A.; Bourde-
lande, J . L.; Sa´nchez, L.; Mart´ın. N. J . Org. Chem. 2001, 66, 5033-
5041.
(9) (a) Langa, F.; Go´mez-Escalonilla, M. J .; Diez-Barra, E.; Garc´ıa-
Mart´ınez, J . C.; de la Hoz, A.; Rodr´ıguez-Lo´pez, J .; Gonza´lez-Corte´s,
A.; Lo´pez-Arza, V. Tetrahedron Lett. 2001, 42, 3435-3438. (b) Langa,
F.; de la Cruz, P.; Delgado, J . L.; Go´mez-Escalonilla, M. J .; Gonza´lez-
Corte´s, A.; de la Hoz, A.; Lo´pez-Arza, V. New. J . Chem. 2002, 26, 76-
80. (c) Esp´ıldora, E.; Delgado, J . L.; de la Cruz, P.; de la Hoz, A.; Lo´pez-
Arza, V.; Langa F. Tetrahedron 2002, 58, 5821-5826.
4a -c and 5 in CDCl3 exhibit all of the expected signals
corresponding to the organic addendum. The observed
signals in the 13C NMR spectra are in agreement with
the proposed structures showing the signals originating
from the isoindazole moiety as well as most of those from
the fullerene system with the appearance of the sp3
carbons of the C60 cage in the range of 82-91 ppm. The
structures of 4a -c and 5 were also confirmed by MALDI-
TOF mass spectra, which show the expected MH+ peaks
at m/z ) 1071 (4a ), 1127 (4b), 1096 (4c), and 898 (5),
together with several fragmentation peaks. Accurate
masses of 4a -c (FAB+ technique, see Experimental
Section) confirmed the proposed structure.
(10) (a) Kimball, D. B.; Hayes, A. G.; Haley, M. M. Org. Lett. 2000,
2, 3825-3827. (b) Kimball, D. B.; Herges, R.; Haley, M. M. J . Am.
Chem. Soc. 2002, 124, 1572-1573. (c) Kimball, D. B.; Weakley, T. J .
R.; Haley, M. M. J . Org. Chem. 2002, 67, 6395-6405. (d) Kimball, D.
B.; Weakley, T. J . R.; Herges, R.; Haley, M. M. J . Am. Chem. Soc. 2002,
124, 13463-13473.
Molecu la r Or bita l Ca lcu la tion s. The geometries of
dyads 4a -c were optimized by semiempirical AM1
(11) de la Cruz, P.; D´ıaz-Ortiz, A.; Garc´ıa, J . J .; Go´mez-Escalonilla,
M. J .; de la Hoz, A.; Langa, F. Tetrahedron Lett. 1999, 40, 1587-1590.
2662 J . Org. Chem., Vol. 69, No. 8, 2004

The Isoindazole Nucleus as a Donor in Fullerene-Based Dyads
TABLE 1. Red ox P oten tia ls (OSWV) of Or ga n ofu ller -
a
en es 4a -c a n d 5, Hyd r a zon es 3a -c, 6, a n d C60
R
E1
E2
E3
E4
E5
E6
E1
E2
ox
red
red
red
red
red
red
ox
b
4a -0.63 -1.04 -1.45b -1.59 -1.80
-1.99 0.87c
4b -0.63 -1.04 -1.46b -1.59 -1.80b -2.00 0.87c
4c -0.61 -1.03 -1.44b -1.58 -1.76b -1.99 0.86c
C60 -0.64 -1.05
-1.51
-1.96
-1.97
3a
3b
3c
5
6
-1.36b
-1.38b
-1.32b
-1.81b
-1.82b
-1.73b
0.79c 1.18c
0.75c 1.25c
0.87c 1.36c
-0.63 -1.04 -1.45b -1.64
-1.44b
1.22c
a
V vs Ag/AgNO3; GCE as the working electrode; 0.1 M TBAP;
b
ODCB/MeCN (4:1); scan rate ) 100 mV/s. Irreversible according
to CV. c Measured by CV.
irreversible CV peaks such as E3 are assigned to the
red
p-nitrophenyl moiety and E5 to the isoindazole group
red
by comparison with those of the starting hydrazones 3a -
c. Similar to other pyrazolino[60]fullerene derivatives,8,9
the first reduction potentials (E1red) are analogous to that
of pure C60 and shifted to more positive values in
comparison to other fullerene derivatives due to the -I
effect of the pyrazoline ring. The first reduction potential
is more shifted toward a positive value in 4c (30 mV with
respect to C60) as a consequence of the presence of the
electron-withdrawing cyano group.
F IGURE 1. (a) Optimized structure, (b) LUMO, and (c)
HOMO of 4b calculated by Gaussian03 at the HF/6-31G level.
calculations. Figure 1 shows the most stable conformation
of cycloadduct 4b. The isoindazole ring is twisted by 50.1°
with respect to the pyrazoline ring, whereas the p-
nitrophenyl group is twisted by 25.3°. The calculated
distance between the closest C-atom of the isoindazole
ring and the C60 surface is 3.27 Å, shorter than the van
der Waals radii for C-C interaction (3.40 Å),12 suggesting
that through-space electronic interaction can take place
between both moieties.
On the oxidation side, dyads 4a -c present remarkable
donor strength, as a one-electron irreversible wave was
observed at around +0.87 V. Hydrazones 3a -c show two
oxidation potentials at around 0.8 and 1.2 V (see Table
1). The first one can be assigned to the isoindazole group,
and E2 must be due to the hydrazone moiety by
ox
comparison with the oxidation potential of the hydrazone
6. When the oxidation potentials of 4a -c are compared
with those in hydrazones 3a -c (see Table 1), the poten-
tial values of 4a and 4b are anodically shifted relative
to the corresponding hydrazones 3a and 3b by 80 and
120 mV, respectively. This indicates that some degree of
charge-transfer exists in the ground state in 4a and 4b
from the donating isoindazole moiety to the acceptor C60
as the molecular orbital calculations predicted. Since this
shift is not observed in 4c as a consequence of the
electron-withdrawing effect of the cyano group, it can be
concluded that this compound does not show charge-
transfer interactions.
UV-Vis Sp ectr a . UV-vis spectra in toluene and
benzonitrile of dyads 4a -c are shown in Figure 2. The
absorption peaks appearing at 690, 600, 550, and 470-
490 nm in the inset of Figure 2 are characteristic of the
fullerene moieties.
With changing the isoindazole moieties and solvents,
the absorption intensities and peak position varied
appreciably. In the region of wavelengths shorter than
430 nm, the main peaks were observed at 330 nm with
a shoulder at 380-400 nm. In this region, the absorption
of the fullerene moieties and the isoindazole moieties may
be overlapped. In addition, the typical absorption band
of 1,1-dihydrofullerenes was observed at 426 nm. In
nonpolar toluene, shoulders appeared at 400 nm, while
this shoulder shifted to a shorter wavelength in polar
benzonitrile.
The electron distributions were calculated by Gaussian
03 at the HF/6-31G level. The electron distributions of
the HOMOs of dyads 4a -c are localized on the isoinda-
zole moiety, and the energy levels (4a , -8.66 eV; 4b,
-8.56 eV; 4c, -8.95 eV) are higher in energy relative to
pure C60 (-9.48 eV). An example of the HOMO is shown
in Figure 1c for 4b. The LUMOs are located in the
fullerene cage. An example of the LUMO is shown in
Figure 1b for 4b. The LUMO energy levels of these
systems (4a , -3.05 eV; 4b, -3.04 eV; 4c, -3.10 eV) are
lower than that of C60 (-2.88 eV). Consequently, these
systems present a small HOMO-LUMO gap (4a , 5.61
eV; 4b, 5.52 eV; 4c, 5.85 eV) enabling intramolecular
electron-transfer processes, particularly in the case of
4a ,b.
Electr och em istr y. The electrochemical behavior of
hydrazones 3a -c and dyads 4a -c was studied using
cyclic voltammetry (CV) and Osteryoung square-wave
voltammetry (OSWV) techniques at room temperature
in ODCB/CH3CN (4:1) as a solvent and using Bu4N+ClO4
-
(TBAP) as the supporting electrolyte. The redox poten-
tials of 4a -c and 5 are collected in Table 1 (see the plots
in Supporting Information) and compared with those of
the precursor hydrazones 3a -c and 6 and C60 as a
reference.
As a general feature on the reduction side, dyads 4a -c
give rise to six one-electron reduction waves. Four of
them (E1red, E2red, E4red, and E6red) are quasi-reversible,
corresponding to the fullerene core, while the other two
Interestingly, when the spectrum of 4a was recorded
in CH2Cl2 (Figure 3), a broad band between 340 and 450
nm appears in comparison with the spectrum in toluene,
(12) Bondi, A. J . J . Phys. Chem. 1964, 68, 441-451.
J . Org. Chem, Vol. 69, No. 8, 2004 2663

Delgado et al.
F IGUR E 4. Absorption spectra of hydrazone 3b (0.1 mM)
and addition effect of trifluoroacetic acid in benzonitrile.
Inset: Fluorescence spectra of 3b (0.1 mM) (λexc: 310 nm).
solvents such as CH2Cl2, as suggested by electrochemical
studies (vide supra) in more polar solvents.
For 3b, the absorption spectra are shown in Figure 4,
in which the peak was observed at 435 nm. However, the
corresponding absorption bands were not appreciably
observed as shown in Figures 2 and 3. This observation
suggests that the hydrazones 3a -c have quite different
electronic properties from the isoindazole group attached
to the C60 cage. On addition of trifluoroacetic acid (TFA)
to the hydrazones 3a -c in toluene, appreciable change
was not observed in the absorption intensity, peak, and
shape as shown in Figure 4. This finding suggests that
the hydrazones 3a -c do not have the ability to accept a
proton from TFA in toluene.
Stea d y-Sta te F lu or escen ce Sp ectr a . The room-
temperature fluorescence spectra of hydrazone 3b in
benzonitrile are shown in the inset of Figure 4, in which
the fluorescence peak was observed at 580 nm. On
addition of TFA in benzonitrile, the fluorescence peak and
shape were not changed appreciably. Selected data are
summarized in Table 2. In toluene, on the other hand,
an appreciable increase in the fluorescence intensity of
3b was observed.
F IGURE 2. Absorption spectra of 4a -c and 5 (0.1 mM) in
(a) benzonitrile and (b) toluene. Inset: Enlarged illustration
of absorbance.
Fluorescence spectra of the dyads 4a -c and the model
N-methylpyrrolidino[60]fullerene 514 as a reference were
measured in toluene and benzonitrile as shown in
Figures 5a and 5b, respectively. In toluene, peaks ap-
peared at 697 nm due to the fullerene moiety.15
Since the fluoresence in the 500-600 nm region was
not observed for 4a -c, energy transfer takes place
efficiently from the excited singlet state of the isoindazole
(13) (a) Nakamura, Y.; Minowa, T.; Tobita, S.; Shizuka, H.; Nish-
imura. J . J . Chem. Soc., Perkin Trans. 2 1995, 2351-2357. (b) Guldi,
D. M.; Maggini, M.; Scorrano, G.; Prato, M. J . Am. Chem. Soc. 1997,
119, 974-980. (c) Llacay, J .; Veciana, J .; Gancedo, J . V.; Bourdelande,
J . L.; Moreno, R. G.; Rovira, C. J . Org. Chem. 1998, 63, 5201-5210.
(d) Fong, R., II; Schuster, D. I.; Wilson, S. R. Org. Lett. 1999, 1, 729-
732. (e) Liu, S. G.; Shu, L.; Rivera, J .; Liu, H.; Raimundo, J . M.; Roncali,
J .; Gorgues, A.; Echegoyen, L. J . Org. Chem. 1999, 64, 4884-4886. (f)
Ohno, T.; Moriwaki, K.; Minaya, T. J . Org. Chem. 2001, 66, 3397-
3401.
(14) Maggini, M.; Scorrano, G.; Prato, M. J . Am. Chem. Soc. 1993,
115, 9798-9799.
(15) At this wavelength, both the isoindazole and the fullerene
moieties should be excited according to the UV-vis spectra.
F IGURE 3. UV-vis spectra of 4a (0.1 mM) in cyclohexane
(-) and dichloromethane (- - -).
which revealed some interactions in the ground state13
between the isoindazole group and the C60 cage in polar
2664 J . Org. Chem., Vol. 69, No. 8, 2004

The Isoindazole Nucleus as a Donor in Fullerene-Based Dyads
TABLE 2. F lu or escen ce P ea k (λF ) a n d In ten sity of
Com p ou n d s 3a -c, 4a -c, a n d 5
λF (nm)/intensity (a.u.)
compda
toluene
benzonitrile
3a
3b
3c
4a
4b
4c
5
565/43
565/45
570/42
697/110
967/110
699/108
699/112
697/62
697/100
697/38
697/114
a
λexc ) 400 nm in toluene and benzonitrile at the same
absorbance.
F IGURE 6. Fluorescence time profiles of 4a -c and 5 in
benzonitrile; λexc ) 400 nm.
TABLE 3. F lu or escen ce Lifetim es (τF ), Qu en ch in g Ra te
Con sta n ts (kq), Qu en ch in g Qu a n tu m Yield s (Φq), a n d
F r ee-En er gy Ch a n ges for Ch a r ge Sep a r a tion (∆GCS) of
Com p ou n d s 4a -c a n d 5
b
compd
solvent
τf (ns)a
kq (s-1
)
Φq
∆GCS (eV)c
4a
4a
4b
4b
4c
4c
5
toluene
PhCN
toluene
PhCN
toluene
PhCN
toluene
PhCN
1.51
0.98
1.46
0.59
1.46
1.33
1.53
1.50
8.7 × 106
3.5 × 108
3.1 × 107
1.0 × 109
3.1 × 107
8.5 × 107
0.01
0.35
0.05
0.60
0.05
0.11
-0.36
-0.36
-0.39
5
a
F IGURE 5. Fluorescence spectra of dyad 4a -c and 5 in (a)
toluene and (b) in benzonitrile; λexc ) 400 nm.
λexc ) 400 nm in toluene and benzonitrile at the same
b S
-1
absorbance. Calculated by the following equations; kq ) τF
-
-1
S
-1
τ0 and Φq ) (τF - τ0-1)/τF-1, where lifetimes of references (5)
c
are referred to τ0. ∆GCSS values were calculated from the Weller
(or hydrazone) moiety to the fullerene moieties, which
exhibit peaks at 697 nm. In toluene, an appreciable
substituent effect was not observed among 4a -c. In
benzonitrile, on the other hand, the fluorescence intensi-
ties at 697 nm of the fullerene moieties vary considerably
from reference 5 and 4c to 4a and 4b (Figure 5b). With
increasing electron-donating ability of the substituents
of the isoindazole moiety, the fluorescence intensity
decreased significantly.
equation (∆GCS ) Eox (TPA) - Ered(C60) - E0 - Ec),34 employing
1
the ES energy level of C60* (1.75 eV), the Ered(C60), and the Eox
(4a -c) shown in Table 1.
values, the quenching rate constants (kq) and quantum
yields (Φq) were calculated as listed in Table 3.
Since the kq and Φq values increase with the electron-
donating substituents of the isoindazole moiety in polar
solvents, we can conclude that the intramolecular charge
separation takes place via the excited singlet state of
fullerenes; thus, the kq and Φq values can be put equal
Tim e P r ofiles of F lu or escen ce Mea su r em en ts.
Time profiles of fluorescence intensities were measured
for reference 5 and 4a -c in benzonitrile as shown in
Figure 6. Fluorescence decay of 4c is as slow as that of
reference 5, while the fluorescence decay rates increase
for 4a and 4c, suggesting that quenching occurs via the
excited singlet states of fullerene moieties. These obser-
vations are in good agreement with the fluorescence
intensities. In toluene, the fluorescence time profiles are
all the same among 4a -c and 5, suggesting that nothing
occurs via the excited singlet state of the fullerenes in
toluene. All of these fluorescence time profiles are curve-
fitted with a single-exponential function, giving the
lifetimes (τF) as summarized in Table 3. From these τF
to the kCS and Φ values in benzonitrile, which can be
CS
supported by the electron distribution of HOMO and
LUMO in Figures 1b and 1c. The charge separation
generates the radical cation in the isoindazole moiety as
supposed by the hole distribution in the HOMO, while
the radical anion is generated on the C60 moiety as the
electron distribution of the LUMO.
Tr a n sien t Absor p tion Mea su r em en ts. To confirm
the charge-separation state, we employed nanosecond
transient absorption spectra, because the kCS values are
on the nanosecond time scale. The observed transient
spectra for 4a -c and 5 in benzonitrile are shown in
J . Org. Chem, Vol. 69, No. 8, 2004 2665

Delgado et al.
F IGURE 8. Schematic energy diagrams.
Acid Ad d ition Effect. Addition of increasing amounts
of TFA to a benzonitrile solution of 4b causes no change
in the absorption spectra, as shown in Figure 9a in the
region of wavelengths longer than 400 nm. In the
fluorescence spectra shown in Figure 9b, an appreciable
change occurs in the fluorescence spectrum in the 650-
850 nm region due to the fluorescence of the C60 cage,
which is in good agreement with the absence of change
in the absorption spectra. On the other hand, the
fluorescence peak observed at 435 nm in the absence of
TFA increased its intensity with a blue shift. The origin
of the 435 nm fluorescence peak may be the isoindazole
moiety, although the fluorescence peak shifted from the
corresponding hydrazone 3b probably because of the
addition to the C60 cage. Besides such fluorescence
changes with addition of TFA, no change was observed
in the fluorescence peak of the C60 cage, which suggests
that the donor ability of the isoindazole moiety does not
decrease due to the protonation of the isoindazole moi-
ety.
F IGURE 7. Nanosecond transient absorption spectra ob-
served by the 532 nm laser light excitation of 4a -c and 5 in
deaerated benzonitrile.
TABLE 4. T-T Absor p tion P ea k (λT), Qu a n tu m Yield s
(ΦT), a n d F r ee-En er gy Ch a n ges for Ch a r ge
Recom bin a tion (∆GCR) of Com p ou n d s 4a -c a n d 5
compda
solvent
λT (nm)
ΦT
∆GCR (eV)
4a
4a
4b
4b
4c
4c
5
toluene
PhCN
toluene
PhCN
toluene
PhCN
toluene
PhCN
720
720
720
720
720
720
720
720
0.92
0.57
0.92
0.47
0.92
0.90
0.92
0.90
-1.42
-1.42
-1.39
0
0
5
a
λexc ) 532 nm in toluene and benzonitrile.
The observed appreciable recovery of the fluorescence
in 350-450 nm (Figure 9b) can be utilized as an AND
logic gate (Figure 10) for this compound.18 Only fluores-
cence at 380 nm (switching ON) can be observed when
both inputs are 1 (in the presence of irradiation and TFA
addition); when one of either inputs is 0, the molecular
switch is OFF.
Figure 7. A similar transient absorption band was
observed centered at 720 nm for 4a -c and 5; the 720
nm band was attributed to the triplet excited state of the
fullerene moieties. However, the intensity of the 720 nm
peak in benzonitrile decreases in the order of electron-
donating ability of the substituents on the isoindazole
moiety, which implies that the decreased amount of the
triplet state is consumed at the excited singlet state as
the charge separation before coming to the triplet state
via intersystem crossing. The quantum yields (ΦT) are
listed in Table 4, which can have tendencies opposite to
the ΦCS values via the excited singlet state of the
fullerene moiety.
These observed phenomena can be reasonably ex-
plained by energy diagrams as shown in Figure 8, in
which the charge separation via the excited singlet state
is a competitive process with the intersystem crossing
with high quantum yields. The quantum yield for the
triplet formation (ΦT) can be represented by the ratio of
Con clu sion
In summary, we have synthesized a novel series of
isoindazole-C60 dyads 4a -c that show an improved
electron affinity with respect to other fullerene deriva-
tives. Good agreement is found between theoretical
calculations and electrochemical studies of charge-
transfer interactions in the ground state between the
isoindazole ring in 4a ,b. Steady-state fluorescence emis-
sion spectra and fluorescence lifetimes of dyads 4a -c
(16) Solutions with the same absorbance were employed so that the
fluorescence and the quantum yields could be correlated.
(17) Armaroli, N. Photoinduced Energy Transfer Processes in Func-
tionalized Fullerenes. In Fullerenes: From Synthesis to Optoelectronic
Properties; Guldi, D. M., Martin, N., Eds.; Kluwer Academic Publish-
ers: Norwell, MA, 2002; Chapter 5.
(18) Prasanna de Silva, A.; McClenaghan, N. D.; McCoy, C. P.
Molecular Logic Systems. In Molecular Switches; Feringa, B. L., Ed.
Wiley-VCH: Weinheim, 2001; Chapter 11.
k
isc/(kcs + kics). From Tables 3 and 4, this relation is
satisfied. In toluene, the CS state becomes unstable, lying
higher than the excited singlet state of the fullerene;
thus, the CS process does not take place, but the ISC to
the triplet state predominates.
2666 J . Org. Chem., Vol. 69, No. 8, 2004

The Isoindazole Nucleus as a Donor in Fullerene-Based Dyads
Exp er im en ta l Section
2-Diet h yla m in o-2H-in d a zole-3-ca r b oxa ld eh yd e 4-n i-
tr op h en ylh yd r a zon e (3a ): yield 76%; mp 184-186 °C; FT-
IR (KBr) ν (cm-1) 3191, 2993, 1586, 1275, 1109, 990, 751, 625;
1H NMR (CD2Cl2) δ 0.82 (t, 6H, J ) 7.3 Hz), 3.18 (m, 2H),
3.41 (m, 2H), 7.20 (d, 2H, J ) 9.5 Hz), 7.26 (dd, 1 H, J ) 8.1
Hz, 6.6 Hz), 7.38 (dd, 1H, J ) 8.8 Hz, 6.6 Hz), 7.67 (dd, 1H, J
) 8.8 Hz), 8.21 (d, 2H, J ) 9.5 Hz), 8.26 (d, 1H, J ) 8.1 Hz),
8.41 (s, 1H, NH), 8.42 (s, 1H); 13C NMR (CDCl3) δ 11.9, 52.4,
111.7, 117.4, 122.1, 123.4, 126.2, 127.1, 131.3, 132.5, 140.0,
146.3, 149.5; UV-vis (CH2Cl2) λmax (nm) (log ꢀ) 416.0 (4.8).
Anal. Calcd for C18H20O2N6: C, 61.35; H, 5.72; N, 23.85.
Found: C, 60.65; H, 5.45; N, 23.93.
5-ter t-Bu tyl-2-d ieth yla m in o-2H-in d a zole-3-ca r boxa ld e-
h yd e 4-Nitr op h en ylh yd r a zon e (3b): yield 83%; mp 214-
216 °C; FT-IR (KBr) ν (cm-1) 3417, 1494, 1308, 1268, 1103,
831; 1H NMR (CDCl3) δ 0.84 (t, 6H, J ) 7.0 Hz), 1.52 (s, 9H),
3.15 (m, 2H), 3.36 (m, 2H), 7.20 (d, 2H, J ) 9.1 Hz), 7.53 (dd,
1H, J ) 9.1 Hz, 2.2 Hz), 7.69 (d, 1H, J ) 9.1 Hz), 8.23 (d, 1H,
J ) 2.2 Hz), 8.25 (d, 2H, J ) 9.1 Hz), 8.27 (s, 1H, NH), 8.40 (s,
1H); 13C NMR (CDCl3) δ 12.0, 31.2, 35.0, 52.4, 111.6, 116.3,
117.2, 117.4, 126.3, 126.7, 131.0, 132.4, 140.3, 145.1, 146.5,
149.4; UV-vis (CH2Cl2) λmax (nm) (log ꢀ) 419.0 (4.7). Anal.
Calcd for C22H28O2N6: C, 64.68; H, 6.91; N, 20.57. Found: C,
64.10; H, 6.73; N, 19.41.
5-Ca r bon itr ile-2-d ieth yla m in o-2H-in d a zole-3-ca r box-
a ld eh yd e 4-Nitr op h en ylh yd r a zon e (3c): yield 73%; mp
280-282 °C; FT-IR (KBr) ν (cm-1) 3417, 1600, 1481, 1109, 851,
513; 1H NMR (CD2Cl2) δ 0.38 (t, 6H, J ) 7.0 Hz), 3.20 (m,
2H), 3.39 (m, 2H), 7.22 (d, 2 H, J ) 9.5 Hz), 7.51 (d, 1H, J )
8.8 Hz), 7.75 (d, 1H, J ) 8.8 Hz), 8.24 (d, 2H, J ) 9.5 Hz,),
8.40 (s, 1H), 8.52 (s,1H, NH), 8.68 (s, 1H); 13C NMR (DMSO)
δ 7.1, 47.2, 100.6, 107.3, 111.1, 114.5, 115.2, 121.7, 123.2, 124.8,
127.2, 129.0, 134.7, 141.4, 145.0; UV-vis (CH2Cl2) λmax (nm)
(log ꢀ) 416.0 (4.6), 427.5 (4.6). Anal. Calcd for C19H19O2N7: C,
60.47; H, 5.07; N, 25.98. Found: C, 60.16; H, 5.06; N, 25.63.
3′-(2-Dieth yla m in o-in d a zolyl)-1′-(4-n itr op h en yl)p yr a -
zolin o[60]fu ller en e (4a ): yield 27% (62% based on recovered
C
60); FT-IR (KBr) ν (cm-1) 1587, 1494, 1096, 1043, 871, 838,
1
738, 519; H NMR (CDCl3) δ 1.1 (t, 6H, J ) 7.3 Hz), 3.4 (q,
4H, J ) 7.3 Hz), 7.13 (dd, 1H, J ) 8.2 Hz, 6.6 Hz) 7.3 (dd, 1H,
J ) 8.8 Hz, 6.6 Hz), 7.6 (d, 1H, J ) 8.2 Hz), 7.7 (d, 1H, J ) 8.8
Hz), 8.28 (d, 2H, J ) 9.2 Hz), 8.37 (d, 2H, J ) 9.2 Hz); 13C
NMR (CDCl3) δ 12.4, 52.3, 82.8, 89.6, 118.1, 119.1, 120.2, 121.1,
123.3, 124.0, 125.4, 126.4, 135.8, 137.2, 138.4, 139.4, 140.8,
141.9, 142.0, 142.2, 142.3, 142.4, 142.5, 142.9, 142.9, 143.2,
144.0, 144.3, 144.6, 145.1, 145.2, 145.3, 145.4, 145.5, 146.0,
146.1, 146.2, 146.4, 147.2, 147.7, 149.5; UV-vis (CH2Cl2) λmax
(nm) (log ꢀ) 255.0 (5.1), 322.0 (4.8), 387.0 (4.5); MS m/z 1071
(M + 1), 720 (C60). Accurate mass (m/z): 1071.1597 (calcd for
F IGURE 9. (a) Absorption spectra of 4b (0.1 mM) in ben-
zonitrile and trifluoroacetic acid addition effect. Inset: En-
larged illustration of absorbance in longer wavelngth region.
(b) Fluorescence spectra of 4b (0.1 mM, λexc ) 300 nm) in
benzonitrile upon increasing the amount of trifluoroacetic acid.
Inset: Fluorescence spectra at shorter wavelengths.
C
78H19N6O2: 1071.1569).
3′-(2-Die t h yla m in o-5-t er t -b u t yl-in d a zolyl)-1′-(4-n i-
tr op h en yl)p yr a zolin o[60]fu ller en e (4b): yield 26% (71%
based on recovered C60); FT-IR (KBr) ν (cm-1) 1580, 1474, 1103,
1
1030, 871, 831, 784, 745, 513; H NMR (CDCl3) δ 1.12 (t, 6H,
J ) 7.3 Hz), 1.28 (s, 9H), 3.44 (q, 4H, J ) 7.3 Hz), 7.45 (dd,
1H, J ) 9.2 Hz, 1.8 Hz), 7.59 (dd, 1H, J ) 1.8 Hz, 1.1 Hz),
7.72 (dd, 1H, J ) 9.2 Hz, 1.1 Hz), 8.30 (d, 2H, J ) 9.5 Hz),
8.38 (d, 2H, J ) 9.5 Hz); 13C NMR (CDCl3) δ 12.3, 31.1, 34.7,
52.4, 83.5, 89.2, 115.0, 117.7, 118.8, 120.6, 123.6, 125.3, 125.8,
135.6, 137.0, 138.1, 139.3, 140.7, 141.8, 141.9, 142.1, 142.2,
142.3, 142.8, 143.1, 143.9, 144.2, 144.5, 144.7, 145.1, 145.2,
145.2, 145.3, 145.4, 145.5, 145.8, 146.0, 146.1, 146.3, 147.0,
147.6, 149.3; UV-vis (CH2Cl2) λmax (nm) (log ꢀ) 256.0 (5.2),
321.0 (4.8), 394 (3.5); MS m/z: 1127 (M + 1), 720 (C60). Ac-
curate mass (m/z): 1127.2164 (calcd for C82H27N6O2: 1127.2195).
3′-(2-Die t h yla m in o-5-ca r b on it r ile -in d a zolyl)-1′-(4-
n itr op h en yl)p yr a zolin o[60]fu ller en e (4c): yield 21% (82%
based on recovered C60); FT-IR (KBr) ν (cm-1) 3417, 1600, 1481,
F IGURE 10. AND logic gate and its corresponding truth table
for 4b (I1, Irradiation at 430 nm; I2, addition of TFA; O,
fluorescence; 1 ) yes, O ) no.
1
show the occurrence of photoinduced electron-transfer
processes in benzonitrile. Isoindazole[60]fullerene 4b can
be consider a molecular switch with an AND logic gate.
1109, 851, 745, 513; H NMR (CDCl3) δ 1.11 (t, 6H, J ) 7.3
Hz), 3.42 (q, 4H, J ) 7.3 Hz), 7.48 (d, 1H, J ) 9.2 Hz), 7.84 (d,
1H, J ) 9.2 Hz), 8.18 (s, 1H), 8.25 (d, 2H, J ) 9.2 Hz), 8.38 (d,
J . Org. Chem, Vol. 69, No. 8, 2004 2667

Delgado et al.
2H, J ) 9.2 Hz); 13C NMR (CDCl3) δ 12.5, 52.7, 83.4, 89.4,
107.1, 119.7, 119.9, 120.6, 125.8, 127.4, 128.3, 128.5, 129.3,
131.2, 136.2, 137.2, 137.4, 139.8, 141.1, 141.8, 142.2, 142.3,
142.4, 142.5, 142.7, 143.3, 143.5, 144.3, 144.6, 144.7, 144.9,
145.5, 145.7, 146.3, 146.4, 146.6, 146.7, 147.5, 149.4; UV-vis
(CH2Cl2) λmax (nm) (log ꢀ) 258.0 (5.3), 321.5 (5.0), 390.0 (4.6);
MS m/z 1096 (M + 1), 720 (C60). Accurate mass (m/z):
1096.1513 (calcd for C79H18N7O2: 1096.1521).
3′-Meth yl-1′-(4-n itr oph en yl)pyr azolin o[60]fu ller en e (5).
An Ar-blanketed solution of NBS (0.10 mmol) and the acetal-
dehyde 4-nitrophenylhydrazone21 (0.10 mmol) in dry CHCl3
(30 mL) was stirred at room temperature for 30 min. The
solvent was removed in vacuo, and a solution of C60 (0.10
mmol) and NEt3 (0.15 mmol) in dry toluene (50 mL) was added
to the solid. The solution was stirred at room temperature for
10 min. The solvent was removed under reduced pressure. The
resulting solid was purified by silica gel flash chromatography
using toluene as an eluent. Centrifuging three times with
MeOH and once with Et2O accomplished further purification
of the solid: yield 42% (60% based on recovered C60); FT-IR
(KBr) ν (cm-1) 1593, 1500, 1310, 1231, 1104, 838, 746, 525;
1H NMR (CDCl3) δ 2.87 (s, 3H), 8.18 (d, 2H, J ) 9.53 Hz),
8.32 (d, 2H, J ) 9.53 Hz); 13C NMR (CDCl3) δ 15.7, 83.7, 88.5,
118.4, 125.5, 136.1, 137.0, 139.2, 141.1, 141.7, 141.9, 142.0,
142.1, 142.3, 142.4, 142.5, 142.9, 143.0, 143.2, 143.7, 144.2,
144.4, 144.6, 145.1, 145.2, 145.3, 145.4, 145.5, 145.9, 146.1,
146.2, 146.3, 146.4, 146.5, 146.6, 147.2, 147.8, 150.2; UV-vis
(CH2Cl2) λmax (nm) (log ꢀ) 229.0 (4.9), 255.0 (5.5), 322.0 (4.5);
MS m/z 898.1 (M + 1), 720.0 (C60).
Ack n ow led gm en t. Financial support for this work
was provided by a grant from Ministerio de Ciencia y
Tecnolog´ıa of Spain and FEDER funds (Project BQU2-
001-1512), the J unta de Comunidades de Castilla-La
Mancha (Project PAI-02-023), and the National Science
Foundation (CHE-0104854). The authors (Y.A. and O.I.)
are also grateful for the financial support from Core
Research for Evolutional Science and Technology
(CREST) of J apan Science and Technology Agency and
a Grant-in-Aid for Scientific Research (Nos. 10207202,
11740380, and 12875163).
(19) (a) Komamine, S.; Fujitsuka, M.; Ito, O.; Morikawa, K.; Miyata,
T.; Ohno, T. J . Phys. Chem. A 2000, 104, 11497. (b) Fujitsuka, M.; Ito,
O.; Yamashiro, T.; Aso, Y.; Otsubo, T. J . Phys. Chem. A 2000, 104,
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Ito, O.; Sasaki, Y.; Yoshikawa, Y.; Watanabe, A. J . Phys. Chem. 1995,
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Su p p or tin g In for m a tion Ava ila ble: General remarks,
experimental procedures, copies of 1H NMR and 13C NMR
spectra, HPLC plots for all compounds, as well as plots of cyclic
voltammetries of 4a -c. This material is available free of
charge via the Internet at http://pubs.acs.org.
(21) McElvain, S. M. The Characterization of Organic Compound,
3rd ed.; The MacMillan: New York, 1953.
J O0499017
2668 J . Org. Chem., Vol. 69, No. 8, 2004