Selective one-electron and two-electron reduction of C60 with NADH and NAD dimer analogues via photoinduced electron transfer

Article abstract of DOI:10.1021/ja9813459

The selective one-electron reduction of C₆₀ to C₆₀^.- is attained through photoinduced electron transfer from an NADH analogue, 1-benzyl-1,4- dihydronicotinamide (BNAH), and the dimer analogue [(BNA)₂] to the tri

Full text of DOI:10.1021/ja9813459

8060
J. Am. Chem. Soc. 1998, 120, 8060-8068
Selective One-Electron and Two-Electron Reduction of C60 with
NADH and NAD Dimer Analogues via Photoinduced Electron
Transfer
Shunichi Fukuzumi,*,† Tomoyoshi Suenobu,† Matthias Patz,† Takeomi Hirasaka,†
Shinobu Itoh,† Mamoru Fujitsuka,‡ and Osamu Ito*,‡
Contribution from the Department of Material and Life Science, Graduate School of Engineering,
Osaka UniVersity, Suita, Osaka 565-0871, Japan, and Institute for Chemical Reaction Science,
Tohoku UniVersity, Sendai, Miyagi 980-8577, Japan
ReceiVed April 20, 1998
Abstract: The selective one-electron reduction of C60 to C60•- is attained through photoinduced electron transfer
from an NADH analogue, 1-benzyl-1,4-dihydronicotinamide (BNAH), and the dimer analogue [(BNA)2] to
•-
the triplet excited state of C60. The limiting quantum yield for formation of C60 in the case of (BNA)2
exceeds unity; Φ∞ ) 1.3. In this case, the initial electron transfer from (BNA)2 to the triplet excited state
•+
(3C60*) is followed by fast C-C bond cleavage in the resulting (BNA)2 to give BNA• and BNA+ and the
second electron transfer from BNA• to C60 yields BNA+ and C60•-, when (BNA)2 acts as a two-electron donor
•-
to produce 2 equiv of C60
.
When BNAH is replaced by 4-tert-butylated BNAH (t-BuBNAH), the
photochemical reaction with C60 yields not C60 but instead the tert-butylated anion (t-BuC60-) selectively.
•-
In this case, the initial electron transfer from t-BuBNAH to 3C60* is also followed by fast C-C bond cleavage
in t-BuBNAH•+ to give t-Bu•, which is coupled with C60•- produced in the electron transfer to yield t-BuC60
.
-
The selective two-electron reduction of C60 to 1,2-dihydro[60]fullerene (1,2-C60H2) is also attained with the
use of another NADH analogue, 10-methyl-9,10-dihydroacridine (AcrH2), under visible light irradiation in
deaerated benzonitrile solution containing trifluoroacetic acid. The studies on the quantum yields, the kinetic
deuterium isotope effects, and the quenching of the triplet-triplet absorption of C60 by AcrH2 have revealed
that the photochemical reduction proceeds via photoinduced electron transfer from 10-methyl-9,10-
•-
dihydroacridine to the triplet excited state of C60, which is followed by proton transfer from AcrH2•+ to C60
and a second electron transfer from the deprotonated acridinyl radical (AcrH•) to C60H• in the presence of
trifluoroacetic acid to yield the final products 10-methylacridinium ion (AcrH+) and 1,2-C60H2. The transient
spectra of the radical ion pair formed in the photoinduced electron transfer have been detected successfully in
laser flash photolysis of each NADH analogue-C60 system. The mechanistic difference between the selective
one- and two-electron reductions of C60 is discussed on the basis of the difference in the redox and acid-base
properties of NADH and the dimer analogues.
Introduction
et al.5-9 We have recently reported that the photoinduced
electron-transfer process from ketene silyl acetals to 3C60* gives
the fullerene with an ester functionality.10 The selective
photochemical allylation of C60 with allylic stannanes has also
been established.11 It is now well-known that the triplet excited
(5) Arbogast, J. W.; Foote, C. S.; Kao, M. J. Am. Chem. Soc. 1992,
114, 2277. Foote, C. S. Top. Curr. Chem. 1994, 169, 347.
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1995, 117, 4093. Guldi, D. M.; Maggini, M.; Scorrano, G.; Prato, M. J.
Am. Chem. Soc. 1997, 119, 974.
Buckminsterfullerene (C60) and its homologues are known
to act as electrophiles, and thus much attention has been focused
on their functionalization with various nucleophiles.1-4 The use
of the photoexcited state of C60 has further expanded the scope
of the reactions with nucleophiles since the early work by Foote
† Osaka University.
‡ Tohoku University.
(1) Taylor, R.; Walton, D. R. M. Nature 1993, 363, 685. Wudl, F. Acc.
Chem. Res. 1992, 25, 157. Diederich, F.; Thilgen, C. Science 1996, 271,
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The Chemistry of the Fullerenes; Georg Thieme Verlag: New York, 1994.
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F.; Thilgen, C. Science 1996, 271, 317.
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Nagase, S.; Kobayashi, K.; Yamamoto, K.; Funasaka, H.; Takahashi, T.
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Am. Chem. Soc. 1995, 117, 554. Siedschlag, C.; Luftmann, H.; Wolff, C.;
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Soc. 1993, 115, 8495. Akasaka, T.; Ando, W.; Kobayashi, K.; Nagase, S.
J. Am. Chem. Soc. 1993, 115, 10366. Akasaka, T.; Mitsuhida, E.; Ando,
W.; Kobayashi, K.; Nagase, S. J. Am. Chem. Soc. 1994, 116, 2627.
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Kaprinidis, N.; Wu, Y.; Jensen, A. W.; Lu, Q.; Wang, H.; Wilson, S. R. J.
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S0002-7863(98)01345-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/01/1998

SelectiVe Reduction of C60
J. Am. Chem. Soc., Vol. 120, No. 32, 1998 8061
The dideuterated compound, 10-methyl[9,9′-2H2]acridine (AcrD2) was
prepared from 10-methylacridone by reduction with LiAlD4.21 Prepara-
tion of 1-benzyl-1,4-dihydronicotinamide (BNAH) was described
previously.21 The tert-butylated BNAH (t-BuBNAH) was prepared by
a Grignard reaction with BNA+Cl-.22,23 The BNA dimer was prepared
by the following procedure according to the literature.24,25 To a stirred
solution of 12 g of Zn dust in 20 mL of water, 4 g of cupric sulfate in
40 mL of water and then 20 mL of concentrated NH3 and 100 mL of
MeOH are added. While the solution was being stirred strongly, 10 g
of BNA+Cl- in 40 mL of water is added. The solution immediately
turns yellow. After 20 min, the mixture is filtered and the precipitate
is extracted four times with 40 mL of hot ethanol under N2. From the
combined EtOH solutions EtOH is removed at 313-323 K under
reduced pressure until the product starts to fall out. The solution is
cooled to 253 K, and the product is filtered (still under N2). Yield:
17.5%, light yellow crystals. The dimer is very sensitive to acid and
somewhat sensitive to light and oxygen, especially in solution. UV
(MeOH): 268 nm (ꢀ ) 6.3 × 103 M-1 cm-1), 348 nm (ꢀ ) 7.3 × 103
M-1 cm-1). Benzonitrile (PhCN, 99.9%) was purchased from Aldrich
and purified by successive distillation over P2O5 prior to use.
state of C60 is formed by efficient intersystem crossing.5,12 The
triplet excited state of C60 has a reduction potential of E0
)
red
1.14 V versus SCE and can therefore be reduced by a variety
of organic compounds giving the C60 radical anion.5,13,14
A
long-lived transient of C60•- has been reported to be formed in
photoinduced electron transfer from ZnO or TiO2 semiconductor
colloids to C60.15 In homogeneous systems, however, the
•-
lifetime of the generated C60 is generally extremely short as
a result of fast back electron transfer to the reactant pair,
•- 16,17
resulting in no net formation of C60
.
We report herein
that the photoinduced electron transfer from an NADH analogue,
1-benzyl-1,4-dihydronicotinamide (BNAH), and the dimer
analogue [(BNA)2] to the triplet excited state of C60 (3C60*)
•-
yields stable C60 in benzonitrile solution with a surprisingly
high quantum yield, exceeding unity in the latter case; Φ )
1.3. We also report the selective two-electron reduction of C60
to the tert-butylated C60 anion (t-BuC60-) and 1,2-dihydro[C60]-
fullerene (1,2-C60H2) via photoinduced electron transfer from
4-tert-butyl-1-benzyl-1,4-dihydronicotinamide (t-BuBNAH) and
10-methyl-9,10-dihydroacridine (AcrH2) to 3C60*, respectively.18
The reduction of C60 has so far been achieved by the use of
strong reductants such as BH3, which yields not only C60H2
but also polyhydride mixtures.19,20 The use of the triplet excited
state of C60 has enabled us to attain the selective two-electron
reduction of C60 to 1,2-C60H2 by using AcrH2, which is a mild
hydride donor. In this study we could observe the transient
absorption spectra in the visible and near-IR region to confirm
the formation of the radical ion pair produced upon photo-
induced electron transfer from NADH analogues to the triplet
excited state of C60. This study provides an excellent op-
portunity to develop mechanistic insight into the selective one-
and two-electron reductions of C60 depending on different
NADH analogues.
Reaction Procedure. Typically, to a solution of C60 (10.1 mg, 0.014
mmol) in deaerated PhCN (50 mL) under an atmospheric pressure of
argon was added 10-methyl-9,10-dihydroacridine (2.7 mg, 0.014 mmol),
and the solution was irradiated with a Xe lamp (λ > 540 nm) equipped
with a Toshiba O-54 cut filter for 30 min. After the solvent was
evaporated under reduced pressure, the residue was separated by
washing it with acetonitrile and centrifuged to give 1,2-C60H2 in 70%
yield, and the recovered C60 was measured by HPLC equipped with
an analytical “Buckyclutcher I” column (Regis, Morton Grove, IL). A
hexane-toluene mixture was used as the eluent with a flow rate of 2
mL min-1. The product was monitored at 434 nm with a UV-vis
detector. 1H NMR spectra were measured on a JEOL GSX-400 (400
1
MHz) spectrometer. Chemical shifts of H NMR were expressed in
parts per million downfield from tetramethylsilane as an internal
standard (δ ) 0). 1H NMR (400 MHz, C6D6): δ 5.91 (s, 2H). The
UV-vis spectra were measured on a Hewlett-Packard 8453 photodiode
array spectrophotometer. UV-vis (λmax, PhCN): 434, 714 nm.
Similarly, a deaerated PhCN solution (50 mL) containing C60 (10.1
mg, 0.014 mmol) and t-BuBNAH (0.014 mmol) was irradiated with a
Xe lamp (λ > 370 nm) equipped with a Toshiba UV-37 cut filter for
1 h. The successive reaction with CF3COOH (0.018 mmol) in deaerated
PhCN at room temperature gave 1-tert-butyl-1,2-dihydro[60]fullerene
(1,2-t-BuC60H), which was isolated on an aluminum column using
hexane as an eluent. 1,2-t-BuC60H (typically 60% yield) along with
unreacted C60 was obtained. FAB-MS: mass calcd for C64H10, 778.8;
found, 778.7. 1H NMR (CS2/CDCl3 3:1 v/v, 298 K): δ 2.06 (s, 9H),
6.64 (s, H). UV-vis (λmax, CS2): 437 nm.
Addition of PhCH2Br instead of CF3COOH to the photolyzed PhCN
solution of C60 and t-BuBNAH gave 1-tert-butyl-4-benzyl-1,4-dihydro-
[60]fullerene [1,4-t-Bu(PhCH2)C60]. The final product was isolated and
characterized by FAB-MS and 1H NMR spectroscopy. FAB-MS: mass
calcd for C71H16, 868.9; found, 868.7. 1H NMR (CS2/CDCl3 3:1 v/v,
298 K): δ 1.98 (s, 9H), 4.29 (d, 1H, Jab ) 13.2 Hz), 4.51 (d, 1H, Jab
Experimental Section
Materials. C60 (>99.95% pure) was purchased from Science
Laboratories Co., Ltd., Japan, and used as received. C60 of 99.99%
purity was obtained from Texas Fullerenes Corp. and used for the
spectral measurements. 10-Methyl-9,10-dihydroacridine (AcrH2) was
prepared from 10-methylacridinium iodide (AcrH+I-) by reduction with
NaBH4 in methanol and was purified by recrystallization from ethanol.21
(12) Dimitrijevic´, N. M.; Kamat, P. V. J. Phys. Chem. 1992, 96, 4811.
Fraelich, M. R.; Weisman, R. B. J. Phys. Chem. 1993, 97, 11145. Etheridge,
H. T., III; Averitt, R. D.; Halas, N. J.; Weisman, R. B. J. Phys. Chem.
1995, 99, 11306. Guldi, D. M.; Asmus, K.-D. J. Phys. Chem. A 1997, 101,
1472.
(13) Steren, C. A.; van Willigen, H.; Biczo´k, L.; Gupta, N.; Linschitz,
H. J. Phys. Chem. 1996, 100, 8920.
(14) For photoinduced electron transfer to C70, see: Osaki, T.; Tai, Y.;
Tazawa, M.; Tanemura, S.; Inukai, K.; Ishiguro, K.; Sawaki, Y.; Saito, Y.;
Shinohara, H.; Nagashima, H. Chem. Lett. 1993, 789.
(15) Kamat, P. V. J. Am. Chem. Soc. 1991, 113, 9705. Kamat, P. V.;
Bedja, I.; Hotchandani, S. J. Phys. Chem. 1994, 98, 9137.
(16) Sension, R. J.; Szarka, A. Z.; Smith, G. R.; Hochstrasser, R. M.
Chem. Phys. Lett. 1991, 185, 179. Ghosh, H. N.; Pal, H.; Sapre, A. V.;
Mittal, J. P. J. Am. Chem. Soc. 1993, 115, 11722.
(17) Ito, O.; Sasaki, Y.; Watanabe, A.; Hoffmann, R.; Siedschlag, C.;
Mattay, J. J. Chem. Soc., Perkin Trans. 2 1997, 1007.
(18) A preliminary report for the photochemical reaction of C60 with
AcrH2 has appeared: Fukuzumi, S.; Suenobu, T.; Kawamura, S.; Ishida,
A.; Mikami, K. Chem. Commun. 1997, 291.
(19) Henderson, C. C.; Cahill, P. A. Science 1993, 259, 1885. Becker,
L.; Evans, T. P.; Bada, J. L. J. Org. Chem. 1993, 58, 7630. Ballenweg, S.;
Gleiter, R.; Kra¨tschmer, W. Tetrahedron Lett. 1993, 34, 3737. Guarr, T.
F.; Meier, M. S.; Vance, V. K.; Clayton, M. J. Am. Chem. Soc. 1993, 115,
9862. Fedorovna, G. N.; Petrovich, M. A. Russ. Chem. ReV. 1997, 66, 323.
(20) Mandrus, D.; Kele, M.; Hettich, R. L.; Guiochon, G.; Sales, B. C.;
Boatner, L. A. J. Phys. Chem. B 1997, 101, 123. Meier, M. S.; Weedon, B.
R.; Spielmann, H. P. J. Am. Chem. Soc. 1996, 118, 11682.
(21) Fukuzumi, S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J. Am. Chem.
Soc. 1987, 109, 305.
) 13.2 Hz), 7.26 (m, 1H), 7.32 (m, 2H), 7.50 (m, 2H). UV-vis (λmax
,
CS2): 447 nm.
Quantum Yield Determinations. A standard actinometer (potas-
sium ferrioxalate)26 was used for the quantum yield determination of
the photoreduction of C60 by electron donors. Square quartz cuvettes
(10 mm i.d.) that contained a deaerated PhCN solution (3.0 cm3) of
C60 (3.0 × 10-4 M) with NADH and the dimer analogues at various
concentrations were irradiated with monochromatized light of λ ) 546
nm from a Shimadzu RF-5000 fluorescence spectrophotometer. Under
the conditions of actinometry experiments, the actinometer and C60
absorbed essentially all the incident light of λ ) 546 nm. The light
(22) Anne, A. Heterocycles 1992, 34, 2331.
(23) Takada, N.; Itoh, S.; Fukuzumi, S. Chem. Lett. 1996, 1103.
(24) Wallenfels, K.; Gellerich, M. Chem. Ber. 1959, 92, 1406.
(25) Patz, M.; Kuwahara, Y.; Suenobu, T.; Fukuzumi, S. Chem. Lett.
1997, 567.
(26) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956,
235, 518.

8062 J. Am. Chem. Soc., Vol. 120, No. 32, 1998
Fukuzumi et al.
intensity of the monochromatized light of λ ) 546 nm was determined
to be 4.99 × 10-9 einstein s-1 with a slit width of 5 nm. The
photochemical reaction was monitored using a Hewlett-Packard 8452A
diode-array spectrophotometer. The quantum yields were determined
from the increase in absorbance due to the C60 adducts at 434 nm and
C60•- at 1080 nm, where the C60 absorbance at 434 nm has been taken
into account. To avoid the contribution of light absorption of the
products, only the initial rates were determined for determination of
the quantum yields.
Laser Flash Photolysis. The procedure and the equipment for the
measurements of the triplet-triplet absorption spectrum of 3C60* were
described previously.10 The detailed procedures for the measurements
3
•-
of transient absorption spectra of C60* and C60 are available in
Supporting Information.
ESR Measurements. ESR spectra of the photolyzed PhCN solution
of C60 and (BNA)2 were taken on a JEOL JES-RE1XE and were
recorded under nonsaturating microwave power conditions. The
magnitude of the modulation was chosen to optimize the resolution
and the signal-to-noise ratio (S/N) of the observed spectra. The g values
were calibrated using a Mn2+ marker.
Figure 1. Dependence of the quantum yields on [(BNA)2] for the
photoreduction of C60 (2.8 × 10-4 M) by (BNA)2 in deaerated PhCN
at 298 K.
Results and Discussion
Selective One-Electron Reduction of C60 via Photoinduced
Electron Transfer. When a dimeric NADH analogue [(BNA)2]
is used as an electron donor, irradiation of a PhCN solution
containing (BNA)2 and C60 with daylight results in efficient one-
The quantum yields (Φ) for the one-electron photoreduction
of C60 were determined from an increase in absorbance due to
C60•- by using a ferrioxalate actinometer26 under irradiation of
monochromatized light of λ ) 546 nm. The Φ value for the
photoreduction of C60 by (BNA)2 in PhCN increases with an
increase in the concentration of (BNA)2 to reach a limiting value
(Φ∞) as shown in Figure 1. It should be noted that the Φ∞
value exceeds unity; Φ∞ ) 1.3. Such a large quantum yield
exceeding unity is consistent with the stoichiometry in eq 1,
where (BNA)2 can reduce 2 equiv of C60. The dependence of
Φ on the BNAH concentration was also examined, and the Φ∞
values for (BNA)2 and BNAH are listed in Table 1.
•-
electron reduction of C60 to C60 (eq 1). No reaction occurs
•-
in the dark. The formation of C60 is detected by the typical
NIR spectrum (λmax ) 1080 nm).27 The C60•- generated in the
photochemical reaction is stable in deaerated PhCN, and the
stoichiometry of the reaction is established as shown in eq 1,
where (BNA)2 acts as a two-electron donor to reduce 2 equiv
•- 28
•-
of C60 to C60
.
The formation of C60 was also confirmed
by ESR spectroscopy after the photochemical reaction of C60
with (BNA)2. A characteristic broad signal at g ) 2.0000 is
The singlet excited state of C60 produced initially upon
irradiation is known to be efficiently converted to the triplet
excited state by the fast intersystem crossing.5,12 The transient
absorption spectra in the visible and near-IR region are observed
by the laser flash photolysis of a deaerated PhCN solution of
C60 containing BNAH with 532 nm laser light as shown in
3
Figure 2. The triplet-triplet absorption band of C60* at 740
nm appearing immediately after nanosecond laser exposure
decays accompanied by concomitant appearance of new absorp-
tion bands at 600 and 1080 nm (Figure 2). The absorption band
observed together with a sharp spike signal, which is always
at 1080 nm in the near-IR region in Figure 2 is readily assigned
•- 29
•- 27
observed in the ESR spectrum of C60
.
Similarly, the
to C60
.
The absorption band at 600 nm in the visible region
can be assigned as BNAH•+, since the radical cation of an
photochemical reaction of C60 with the monomeric NADH
analogue, 1-benzyl-1,4-dihydronicotinamide (BNAH), occurs to
yield C60•- efficiently under irradiation with visible light from
a Xe lamp (λ > 540 nm) equipped with a Toshiba O-54 cut
filter (eq 2).
NADH analogue has a similar absorption band.30,31 The
transient absorption spectrum of C60 is also observed by the
•-
laser flash photolysis of a deaerated PhCN solution of C60
containing (BNA)2. The lifetime of the transient 3C60* triplet-
triplet (T-T) absorption at λmax ) 740 nm is significantly
reduced by the presence of (BNA)2 or BNAH. The bimolecular
3
quenching rate constants (kq) of C60* by (BNA)2 and BNAH
were determined from the decay kinetics of transient T-T
absorption at 740 nm. In each case, the first-order decay rate
constant of 3C60* (kd) increases linearly with an increase in the
concentration of (BNA)2 or BNAH as shown in Figure 3 for
(BNA)2.
(27) Lawson, D. R.; Feldheim, D. L.; Foss, C. A.; Dorhout, P. K.; Elliot,
C. M.; Martin, C. R.; Parkinson, B. J. Electrochem. Soc. 1992, 139, L68.
(28) The stoichiometry of the photochemical reaction of C60 with (BNA)2
was confirmed by the spectral titration.
(29) Allemand, P.-M.; Srdanov, G.; Koch, A.; Khemani, K.; Wudl, F.;
Rubin, Y.; Diederich, F.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J.
Am. Chem. Soc. 1991, 113, 2780. Dubois, D.; Kadish, K. M.; Flanagan, S.;
Haufler, R. E.; Chibante, L. P. F.; Wilson, L. J. J. Am. Chem. Soc. 1991,
113, 4364. Stinchcombe, J.; Pe´nicaud, A.; Bhyrappa, P.; Boyd, P. D. W.;
Reed, C. A. J. Am. Chem. Soc. 1993, 115, 5212.
The kq values of (BNA)2 and BNAH are listed in Table 1,
where the oxidation potentials (E0ox) of (BNA)225 and BNAH21
(30) Fukuzumi, S.; Tokuda, Y.; Kitano, T.; Okamoto, T.; Otera, J. J.
Am. Chem. Soc. 1993, 115, 8960.
(31) Fukuzumi, S.; Tanaka, T. In Photoinduced Electron Transfer; Fox,
M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part C, pp 578-
635.

SelectiVe Reduction of C60
J. Am. Chem. Soc., Vol. 120, No. 32, 1998 8063
Table 1. Free Energy Change ∆G0 and Rate Constants ket of
et
3
Photoinduced Electron Transfer from NADH Analogues to C60*,
Triplet Quenching Rate Constants kq, and Limiting Quantum Yields
Φ∞ in the Photoreduction of C60 by NADH Analogues in PhCN at
298 K
3
Figure 3. Plots of kd vs [(BNA)2] for the quenching of C60* (2.8 ×
10-5 M) by (BNA)2 in deaerated PhCN at 298 K.
has well been established as given by the Rehm-Weller free-
energy relation (eq 4),32,33 where ∆Gq is the intrinsic barrier
0
∆Gqet ) (∆G0 /2) + [(∆G0 /2)2 + (∆Gq )2]1/2
(4)
et
et
0
that represents the activation free energy when the driving force
a Obtained from the E0 values of NADH analogues21,25 and the
of electron transfer is zero, that is, ∆Gqet ) ∆Gq0 at ∆G0et ) 0.
ox
3
E0 value of C60* (1.14 V vs SCE)5 by using the equation ∆G0
)
The ∆Gq values are related to the rate constant of electron
red
et
et
F(E0ox - E0red). b Experimental error is (5%. c Evaluated by using the
transfer (ket) as given by eq 5, where Z is the collision frequency
Rehm-Weller equation (eq 4).32 The ∆G* values of the electron
0
that is taken as 1 × 1011 M-1 s-1; F is the Faraday constant,
transfer are taken as 4.0, 2.6, 4.0, and 5.5 kcal mol-1 for t-BuBNAH,
AcrH2, BNAH, and (BNA)2, respectively.21,25 The diffusion-limited
∆Gqet ) 2.3RT log[Z(ket-1 - kdiff-1)]
(5)
electron-transfer rate constant is taken as 5.6 × 109 M-1 s-1 5
.
and kdiff is the diffusion rate constant in PhCN (5.6 × 109 M-1
s-1).5 Then, the ket values can be calculated from the ∆G0
et
and ∆Gq0 values21,25 by using eqs 4 and 5. The ket values thus
evaluated are listed in Table 1 and agree well with the observed
kq values (Table 1).
Such an agreement between the ket and kq values as well as
the direct observation of the products of electron transfer
indicates that the photochemical reaction proceeds via photo-
3
induced electron transfer to C60* as shown in Scheme 1 for
the case of (BNA)2. The photoinduced electron transfer from
(BNA)2 to 3C60* gives (BNA)2•+ and C60•- in competition with
the decay to the ground state. This step is followed by a fast
cleavage of the C-C bond of the dimer (kc) to produce
N-benzylnicotinamide radical (BNA•) and BNA+.25 The sub-
sequent second electron transfer from BNA• to C60 should be
faster by far than the first, as BNA• is a strong reductant (E0
ox
) -1.08 V vs SCE).21 Thus, once photoinduced electron
transfer from (BNA)2 to 3C60* occurs, two C60•- molecules are
produced.34
Figure 2. Transient absorption spectra observed in the photoreduction
of C60 (1.0 × 10-4 M) by BNAH (1.1 × 10-3 M) at 100 ns (b) and 1
µs (O) after laser excitation in deaerated PhCN at 295 K.
By application of the steady-state approximation to the
reactive species, 3C60*, (BNA)2•+, and BNA• in Scheme 1, the
dependence of Φ on [(BNA)2] can be derived as given by eq 6,
which agrees with the observed dependence of Φ on [(BNA)2]
in Figure 2. The limiting quantum yield Φ∞ corresponds to
as well as the free energy change of electron transfer from the
3
donor to C60* (∆G0et) are also given. The ∆G0 value is
et
obtained from the one-electron oxidation potential of the donor
3
(E0ox) and the one-electron reduction potential of C60* (E0
)
red
by eq 3. The E0 values (vs SCE) of (BNA)2 (0.26 V)25 and
ox
BNAH (0.57 V)21 have been reported previously, and the E0
red
Φ ) Φ∞ketτT[(BNA)2]/(1 + ketτT[(BNA)2])
(6)
∆G0et ) F(E0ox - E0
)
(3)
2kc/(kc + kb). Thus, the Φ∞ value exceeding unity, Φ∞ ) 1.3,
red
(32) Rehm, A.; Weller, A. Isr. J. Chem. 1970, 8, 259.
(33) Fukuzumi, S.; Fujita, M.; Otera, J.; Fujita, Y. J. Am. Chem. Soc.
1992, 114, 10271.
value of 3C60* is known to be 1.14 V.5 In each case the ∆G0
et
value is highly negative, suggesting that the rate of the electron-
transfer reaction may be diffusion-limited. The dependence of
the activation free energy of photoinduced electron transfer
(34) Although electron transfer from BNA• to C60•- may occur to produce
C602-, the fast comproportionation between C602- and C60 should take place
to yield C60•- as the final product. Once all C60 is converted to C60•-, there
∆Gq on the free energy change of electron transfer (∆G0
)
would be no further photoinduced electron transfer from (BNA)2 to C60
.
•-
et
et

8064 J. Am. Chem. Soc., Vol. 120, No. 32, 1998
Fukuzumi et al.
Scheme 1
Figure 4. Electronic absorption spectra observed in the photochemical
reaction of C60 (2.8 × 10-4 M) with t-BuBNAH (2.8 × 10-4 M) in
deaerated PhCN under irradiation of visible light (λ > 540 nm) at 298
K.
Scheme 3
Scheme 2
subsequent trap of the photoproduct by CF3COOH and PhCH2-
Br gave t-BuC60H and t-Bu(PhCH2)C60, respectively (Scheme
3, see Experimental Section), as reported for the reactions of
t-BuC60- with electrophiles.37 The isolated t-Bu(PhCH2)C60 has
a broad absorption band at 447 nm, which is similar to that
seen in the spectrum of 1,4-(PhCH2)2C60, the X-ray structure
of which has been disclosed recently.38 The 1H NMR spectrum
of the isolated t-BuC60H agrees with that of 1,2-t-BuC60H, which
is readily distinguished from that of 1,4-t-BuC60H (see Experi-
may be ascribed to the efficient competition of C-C bond
cleavage in (BNA)2•+ (kc) with the back electron transfer process
(kb). Once C60 is produced, it is stable under the present
•-
experimental conditions.
The one-electron photoreduction of C60 by BNAH may also
proceed via photoinduced electron transfer from BNAH to 3C60*
-
mental Section).39,40 The t-BuC60 has so far been prepared
(Scheme 2). In this case, the deprotonation of BNAH•+ gives
by using a strong alkylating reagent, that is, tert-butyllithium
•-
BNA•, which can further reduce C60 to C60
.
in toluene.37 Thus, the photochemical reaction (eq 7) provides
-
-
Selective Two-Electron Photoreduction of C60 to t-BuC60
.
a unique and new way to prepare t-BuC60 with use of
When BNAH is replaced by 4-tert-butylated BNAH (t-BuB-
NAH), no one-electron reduction of C60 by t-BuBNAH occurs,
but instead the selective two-electron reduction of C60 to the
tert-butylated anion (t-BuC60-) is attained accompanied by the
two-electron oxidation of BNAH to BNA+ (eq 7). Figure 4
shows the visible-near-IR spectral change observed in the
photochemical reaction of t-BuBNAH with C60 in deaerated
t-BuBNAH, which is a mild alkylating reagent under neutral
conditions.
The quantum yields (Φ) for the photochemical formation of
t-BuC60- were determined from an increase in absorbance due
-
to t-BuC60 under irradiation by monochromatized light of λ
) 546 nm. The Φ value for the photoreduction of C60 by
t-BuBNAH in PhCN increases with an increase in the concen-
tration of t-BuBNAH to reach a limiting value (Φ∞) as in the
(36) Kitagawa, T.; Tanaka, T.; Takata, Y.; Takeuchi, K.; Komatsu, K.
Tetrahedron 1997, 53, 9965. Murata, Y.; Motoyama, K.; Komatsu, K.; Wan,
T. S. M. Tetrahedron 1996, 52, 5077.
(37) Hirsch, A.; Soi, A.; Karfunkel, H. R. Angew. Chem., Int. Ed. Engl.
1992, 31, 766. Kitagawa, T.; Tanaka, T.; Takata, Y.; Takeuchi, K. J. Org.
Chem. 1995, 60, 1490. Tanaka, T.; Kitagawa, T.; Komatsu, K.; Takeuchi,
K. J. Am. Chem. Soc. 1997, 119, 9313.
(38) Subramanian, R.; Kadish, K. M.; Vijayashree, M. N.; Gao, X.; Jones,
M. T.; Miller, M. D.; Krause, K. L.; Suenobu, T.; Fukuzumi, S. J. Phys.
Chem. 1996, 100, 16327.
PhCN. The new absorption bands at 660 and 955 nm seen in
- 35,36
Figure 4 agree with those of t-BuC60
.
In fact, the
(35) Fukuzumi, S.; Suenobu, T.; Hirasaka, T.; Gao, X.; Van Caemel-
becke, E.; Kadish, K. M. In Recent AdVances in the Chemistry and Physics
of Fullerenes; Ruoff, R. S., Kadish, K. M., Eds.; Pennington: NJ, 1997;
Vol. 4, pp 173-185.
(39) Banim, F.; Cardin, D. J.; Heath, P. Chem. Commun. 1997, 25.
(40) The initial product of 1,4-t-BuC60H was rearranged to the 1,2-isomer
during the isolation procedure to give the 1,2-isomer exclusively (see ref
39).

SelectiVe Reduction of C60
J. Am. Chem. Soc., Vol. 120, No. 32, 1998 8065
Scheme 4
Scheme 5
t-BuBNAH has recently been confirmed by applying a rapid-
mixing flow electron spin resonance (ESR) technique.42 Thus,
the photoinduced electron transfer from t-BuBNAH to 3C60* to
give t-BuBNAH•+ and C60 may be followed by the facile
•-
C(4)-C bond cleavage of t-BuBNAH•+, giving t-Bu• that is
•-
coupled immediately with C60 to yield the final product (t-
Figure 5. Decay of the absorbance at 740 nm due to 3C60* (a) and the
BuC60-) in competition with the back electron transfer as shown
in Scheme 5.
•-
rise of the absorbance at 1080 nm due to C60 (b) observed in the
photoreduction of C60 (1.0 × 10-4 M) by t-BuBNAH (2.0 × 10-3 M)
Selective Two-Electron Reduction of C60 to C60H2 via
Photoinduced Electron Transfer. When another NADH
analogue, 10-methyl-9,10-dihydroacridine (AcrH2), is employed
in the photochemical reaction with C60, the selective two-
electron reduction of C60 to C60H2 is attained accompanied by
the two-electron oxidation of AcrH2 to AcrH+ in the presence
of trifluoroacetic acid (eq 8). Irradiation of a solution of C60
(10.1 mg, 0.014 mmol) in deaerated PhCN (50 mL), 10-methyl-
9,10-dihydroacridine (AcrH2, 2.7 mg, 0.014 mmol), and
CF3COOH (0.014 mmol) with a Xe lamp (λ > 540 nm)
equipped with a O-54 cut filter for 30 min resulted in the
formation of 1,2-C60H2 exclusively in 70% yield (see Experi-
mental Section). In the dark, however, no reaction has occurred
after laser excitation in deaerated PhCN at 295 K.
case of (BNA)2 in Figure 2. The Φ∞ value for t-BuBNAH is
also listed in Table 1.
Although no one-electron reduction of C60 to C60 occurs
•-
under a steady-state visible light irradiation (Figure 4), the
formation of C60•- is detected as a reactive intermediate in the
-
two-electron reduction of C60 by t-BuBNAH to t-BuC60 by
the laser flash photolysis of a deaerated PhCN solution of C60
in the presence of t-BuBNAH. The decay of the absorbance at
740 nm due to 3C60* obeys pseudo-first-order kinetics, coincid-
ing with the rise of the absorbance at 1080 nm due to C60•- as
shown in Figure 5. Thus, it is confirmed that the photochemical
reaction of C60 with t-BuBNAH proceeds via photoinduced
3
electron transfer from t-BuBNAH to C60*. The rate constant
for the reaction of 3C60* with BNAH was determined as 2.1 ×
109 M-1 s-1 as listed in Table 1.
We have previously reported that cleavage of the C(9)-C
bond of the radical cation of an NADH analogue, 9-tert-butyl-
10-methyl-9,10-dihydroacridine (AcrH(t-Bu)), occurs selectively
rather than the cleavage of the C(9)-H bond in the electron-
transfer oxidation of AcrH(t-Bu) by Fe3+ complexes.30 Save´ant
et al. have also reported that the electrochemical oxidation of
t-BuBNAH results in the selective C(4)-C bond cleavage of
t-BuBNAH•.41 Although there are two possible modes of the
carbon-carbon bond cleavage in such reactions to generate (a)
t-Bu• and BNA+ or (b) t-Bu+ and BNA• as shown in Scheme
4, the formation of t-Bu• in the one-electron oxidation of
(42) In the presence of O2, t-Bu• can be trapped efficiently by O2 to
yield tert-butylperoxyl radical (t-BuOO•), the formation of which, upon the
oxidation of t-BuBNAH (5.0 × 10-4 M) by [Fe(phen)3]3+ (5.0 × 10-4 M)
in aerated MeCN, was confirmed by the ESR spectrum (g ) 2.016) with a
rapid mixing flow apparatus; see ref 23.
(41) Anne, A.; Moiroux, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1993, 115,
10224.

8066 J. Am. Chem. Soc., Vol. 120, No. 32, 1998
Fukuzumi et al.
Figure 6. Electronic absorption spectra observed in the photoreduction
of C60 (2.8 × 10-4 M) by AcrH2 (2.8 × 10-4 M) in the presence of
CF3COOH (5.6 × 10-4 M) under irradiation of visible light (λ > 540
nm) in deaerated PhCN at 298 K. The inset shows the time dependence
of the concentration of C60H2 in the absence and presence of NaI (1.9
× 10-2 M).
Figure 7. Dependence of the quantum yields on [AcrH2] (a) and
[AcrD2] (b) for the photoreduction of C60 (2.8 × 10-4 M) by AcrH2
and AcrD2, respectively, in the presence of CF3COOH (5.6 × 10-4 M)
in PhCN at 298 K.
even at high temperatures (e.g., 373 K). No appreciable
amounts of polyadducts were obtained even after the prolonged
irradiation time under the present experimental conditions. The
1H NMR signal at δ 5.91 (s, 2H) of 1,2-C60H2 (in C6D6) agrees
well with that reported previously.19 Irradiation of the absorp-
tion band of C60 in PhCN solution containing AcrH2 and
CF3COOH results in an increase in the absorbances at λmax
)
434 and 714 nm, which is known as a fingerprint of the 1,2-
monoadduct such as 1,2-C60H2 resulting from 1,2-addition to a
6-6 bond.1 A typical example of the electronic absorption
spectra observed in the photoreduction of C60 by AcrH2 in the
presence of CF3COOH in deaerated PhCN is shown in Figure
4. When the photoreduction of C60 is carried out in the pres-
ence of NaI, which is a well-known triplet quencher,43 the
reaction is strongly inhibited by NaI as shown in the inset in
Figure 6.
The quantum yields (Φ) for the photoreduction of C60 by
AcrH2 were determined from an increase in absorbance due to
1,2-C60H2 under irradiation of monochromatized light of λ )
546 nm. The Φ value increases with an increase in the
concentration of AcrH2 to reach a limiting value (Φ∞) as shown
in Figure 7a. When AcrH2 is replaced by the dideuterated
compound (AcrD2), the deuterium isotope effect is observed
Figure 8. Dependence of the quantum yield on [CF3COOH] in the
photoreduction of C60 (2.8 × 10-4 M) by AcrH2 (2.8 × 10-4 M)
under irradiation of visible light (λ ) 546 nm) in deaerated PhCN at
298 K.
•-
to C60•-. The rate constant for formation of C60 determined
from the rise of the absorbance at 1080 nm (3.7 × 109 M-1
for the limiting quantum yield (Φ∞H/Φ∞ ) 1.2); compare
D
s-1) agrees within experimental error with the kq value
Figure 7a with Figure 7b. The effect of CF3COOH on the
quantum yields was also examined as shown in Figure 8, where
the quantum yield is constant with variation of the CF3COOH
concentration.
3
determined from the quenching of C60* by AcrH2 (4.3 × 109
M-1 s-1 in Table 1). At the prolonged monitoring time (0-16
•-
µs) the absorbance at 1080 nm due to C60 decays gradually
(ca. 10%). The addition of CF3COOH (1.0 × 10-3 M) to the
The transient absorption spectra of the radical ion pair
•+
•-) in the visible and near-IR regions are observed
AcrH2-C60 system resulted in no appreciable effect on the
(AcrH2
C
60
•-
formation or decay rate of C60
.
by the laser flash photolysis of a deaerated PhCN solution of
C60 in the presence of AcrH2 and CF3COOH with 532 nm laser
light as shown in Figure 9. The absorption band at 640 nm in
the visible region agrees with that reported for AcrH2•+ observed
as a transient spectrum in the electron-transfer oxidation of
When AcrH2 was replaced by AcrD2, essentially the same kq
value was obtained as for AcrH2. Thus, there is no kinetic
isotope effect on the quenching process. The rate constant of
3
photoinduced electron transfer from AcrH2 to C60* (ket) can
AcrH2 by Fe(phen)3 (phen ) 1,10-phenanthroline).30 The
3+
also be evaluated from the ∆G0 and ∆Gq values21 using eqs
et
4 and 5. The ket value thus evaluated (4.3 ×0109 M-1 s-1) agrees
with the kq value (4.3 × 109 M-1 s-1) listed in Table 1. The
ket value for AcrH2 is the largest despite it having the least
decay of the absorbance at 740 nm due to 3C60* obeys pseudo-
first-order kinetics, coinciding with the rise of the absorbance
at 640 nm due to AcrH2•+ and the absorbance at 1080 nm due
negative ∆G0 value. This results from it having the smallest
et
(43) Fukuzumi, S.; Kuroda, S.; Tanaka, T. J. Am. Chem. Soc. 1985, 107,
3020.
∆Gq value (2.6 kcal mol-1).21
0

SelectiVe Reduction of C60
J. Am. Chem. Soc., Vol. 120, No. 32, 1998 8067
produced in the initial photoinduced electron transfer from
AcrH2 to C60 could be protonated by the acid, and hydrogen
•+
transfer from AcrH2 to C60H• gives the final products.
However, no increase in the quantum yield was observed with
an increase in the CF3COOH concentration. In fact, the first
reduction potential of C60 has not been affected by the presence
of CF3COOH in PhCN (see Experimental Section). Thus, the
protonation process may not be involved in the rate-determining
step as shown in Scheme 6. The theoretical calculations suggest
that 1,2-C60H2 is the most stable form among 23 different
regioisomers,48 and it is thereby obtained selectively in the
photoreduction of C60 by AcrH2.
By application of the steady-state approximation to the
3
reactive species, C60* and the radical ion pair in Scheme 6,
the dependence of Φ on the donor concentration [AcrH2] can
be derived as given by eq 9, which agrees with the observed
dependence of Φ on [AcrH2] in Figure 7. The limiting quantum
Figure 9. Transient absorption spectra observed in the photoreduction
of C60 (1.0 × 10-4 M) by AcrH2 (1.0 × 10-3 M) at 100 ns (b) and 1
µs (O) after laser excitation in deaerated PhCN at 295 K.
Φ ) [kp/(kp + kb)]ketτT[AcrH2]/(1 + ketτT[AcrH2]) (9)
Scheme 6
yield Φ∞ corresponds to kp/(kp + kb). The observed isotope
effect in Φ∞ (Figure 7) is thereby ascribed to the proton-transfer
•+
process from AcrH2 to C60•-. The small isotope effect is
consistent with the Φ∞ value being significantly smaller than
unity, when the back electron transfer (kb) may be much faster
than the proton transfer (kp), that is, kp , kb.
The Mechanistic Difference in One-Electron and Two-
Electron Reduction of C60. The one-electron reduction of C60
by (BNA)2 (Scheme 1) and BNAH (Scheme 2) as well as the
two-electron reduction by AcrH2 (Scheme 6) proceeds via
photoinduced electron transfer from the electron donor to 3C60*.
In the case of (BNA)2 in Scheme 1, the facile C-C bond
cleavage of (BNA)2•+ to produce BNA• that can reduce C60 to
•-
C60 is responsible for the selective one-electron reduction of
C60. On the other hand, the C-C bond cleavage of t-
BuBNAH•+ to produce t-Bu• that cannot reduce C60 to C60
•-
but instead is coupled with C60•- to yield t-BuC60- (Scheme 5)
is responsible for the selective two-electron reduction of C60 to
t-BuC60-. In the case of AcrH2 in Scheme 6, the strong acidity
The agreement between the ket and kq values together with
the direct observation of the radical ion pair by the laser flash
photolysis indicates that a photoinduced electron-transfer mech-
anism may also be applied to the photoreduction of C60 by
AcrH2 as shown in Scheme 6. The photoinduced electron
•+
•+
•-
of AcrH2 causes a proton transfer from AcrH2 to C60 to
produce a strong reductant-oxidant pair (AcrH•C60H•) fol-
lowed by the second electron transfer to yield two-electron
reduction of C60 to 1,2-C60H2 in the presence of an acid. Since
the pKa value of BNAH•+ (3.6)21,49 is significantly larger than
that of AcrH2•+ (2.0),21,30 the proton transfer from BNAH•+ to
C60•- may be much slower than that from AcrH2•+. In addition,
BNA• produced by the deprotonation of BNAH•+ is a much
stronger one-electron reductant than AcrH•, judging from the
3
transfer from AcrH2 to C60* (ket) gives the radical ion pair
•+
(AcrH2
C
60
•-) in competition with the decay to the ground state
(kT ) τT-1). The pKa of singly reduced C60 (C60H•) encapsulated
in γ-cyclodextrin (γ-CD) and dissolved in water-propane-2-
ol has recently been determined as 4.5 on the basis of a specific
IR absorption band for C60•--γ-CD.44 On the other hand, the
largely negative E0 value of BNA• (-1.08 V)21 as com-
•+
ox
pKa of AcrH2 in water has previously been determined as
pared with that of AcrH• (-0.43 V).21 In such a case, the
•+
•-
2.0.21,30 Thus, proton transfer from AcrH2 to C60 (kH) is
significantly exergonic,45,46 and it may occur efficiently in the
radical ion pair in competition with the back electron transfer
to the reactant pair (kb), to give C60H•, which is converted to
1,2-C60H2 by the fast electron transfer from AcrH• in the
deprotonation of BNAH•+ may be followed by the fast electron
transfer from BNA• to C60 to yield BNA+ and C60 as shown
•-
in Scheme 2. Thus, the difference in the redox and acid-base
properties of the radical cations produced in the photoinduced
electron transfer may determine the subsequent reaction path-
way, leading to the one-electron reduction or the two-electron
reduction.
presence of CF3COOH (Scheme 6).47 Alternatively, C60
•-
(44) Ohlendorf, V.; Willnow, A.; Hungerbu¨hler, H.; Guldi, D. M.; Asmus,
K.-D. J. Chem. Soc., Chem. Commun. 1995, 759.
(45) The pKa value in an aprotic solvent is expected to be much larger
than that in water because of the strong solvation of water to protons. The
pKa value of AcrH2•+ in acetonitrile is determined as in the range 6.8-8.1,
depending on the H2O concentration.30 On the other hand, the pKa value of
C60H• has been estimated as 9 (see ref 46). Thus, the proton transfer from
AcrH2•+ to C60•- in an aprotic solvent may also be significantly exergonic.
(46) Niyazymbetov, M. E.; Evans, D. H.; Lerke, S. A.; Cahill, P. A.;
Henderson, C. C. J. Phys. Chem. 1994, 98, 13093.
(47) In the absence of CF3COOH, 1,2-C60H2 (434 nm) was not formed,
but instead a new broad absorption at 450 nm, which may be assigned to
a radical adduct, appears in the photochemical reaction. However, the radical
adduct remains to be characterized.
(48) Matsuzawa, N.; Dixon, D. A.; Fukunaga, T. J. Phys. Chem. 1992,
96, 7594.
(49) Fukuzumi, S.; Kondo, Y.; Tanaka, T. J. Chem. Soc., Perkin Trans.
2 1984, 673.

8068 J. Am. Chem. Soc., Vol. 120, No. 32, 1998
Fukuzumi et al.
Acknowledgment. We are grateful to Professor K. Mikami,
Dr. A. Ishida, and Mr. S. Kawamura for their valuable
contributions in the early stage of the present work. This work
was partially supported by a Grant-in-Aid for Scientific Research
Priority Area (Nos. 09237239, 09231226, 10125220, and
10131242) from the Ministry of Education, Science, Culture,
and Sports, Japan.
Supporting Information Available: Experimental proce-
dures of laser flash photolysis (2 pages, print/PDF). See any
current masthead page for ordering information and Web access
instructions.
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