Photooxidation of olefins sensitized by bisazafullerene (C59N)2 and hydroazafullerene C59HN: Product analysis, emission of singlet oxygen, and transient absorption spectroscopy

Article abstract of DOI:10.1021/jo0104678

The photooxidation reactions of olefins sensitized by the excited triplet states of bisazafullerene (C₅₉N)₂ and hydroazafullerene C₅₉HN have been studied. Oxidation yields were compared with those of pristine C₆₀. The singlet oxygen yields are also determined directly from the emission intensities, which are in good agreement with the oxidation yields. The triplet states of (C₅₉N)₂ and C₅₉HN have been identified by the time-resolved spectroscopic method by observing the triplet - triplet absorption spectra, which decay in the presence of oxygen. It has been proven that (C₅₉N)₂ and C₅₉HN have the ability to sensitize the reactions via singlet oxygen in about half of the efficiency of that of pristine C₆₀. For both azafullerenes, the triplet lifetimes are shorter than that of pristine C₆₀, which may be related to the nitrogen atom embedded in the C₆₀ moiety.

Full text of DOI:10.1021/jo0104678

8
026
J . Org. Chem. 2001, 66, 8026-8029
P h otooxid a tion of Olefin s Sen sitized by Bisa za fu ller en e (C59N)
a n d Hyd r oa za fu ller en e C59HN: P r od u ct An a lysis, Em ission of
Sin glet Oxygen , a n d Tr a n sien t Absor p tion Sp ectr oscop y
2
Nikos Tagmatarchis and Hisanori Shinohara*
Department of Chemistry, Nagoya University, Nagoya 464-8602, J apan
Mamoru Fujitsuka and Osamu Ito*
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira,
Aoba-ku, Sendai 980-8577, J apan
nori@nano.chem.nagoya-u.ac.jp
Received May 7, 2001
The photooxidation reactions of olefins sensitized by the excited triplet states of bisazafullerene
and hydroazafullerene C59HN have been studied. Oxidation yields were compared with
those of pristine C60. The singlet oxygen yields are also determined directly from the emission
(C59N)
2
intensities, which are in good agreement with the oxidation yields. The triplet states of (C59N)
2
and C59HN have been identified by the time-resolved spectroscopic method by observing the triplet-
triplet absorption spectra, which decay in the presence of oxygen. It has been proven that (C59N)
2
and C59HN have the ability to sensitize the reactions via singlet oxygen in about half of the efficiency
of that of pristine C60. For both azafullerenes, the triplet lifetimes are shorter than that of pristine
C
60, which may be related to the nitrogen atom embedded in the C60 moiety.
In tr od u ction
The incorporation of a nitrogen atom into the fullerene
skeleton strongly perturbs not only the structural char-
acter of the parent molecule but also its electronic and
physical properties.1 The macroscopic synthesis of the
simplest azafullerene, namely, C59N, was achieved by a
,2
3
,4
three-step organic reaction sequence starting from C60
.
•
The resulting azafullerenyl radical C59N , a product of
the difference in valence between a tetravalent carbon
atom and a trivalent nitrogen, is a highly reactive species.
Consequently, the radical dimerizes yielding bisazaful-
59 2
F igu r e 1. Molecular structures of bisazafullerene (C N) and
hydroazafullerene C59HN.
5
lerene (C59N)
2
.
If the synthetic procedure is slightly
modified, hydroazafullerene C59HN is formed (Figure
6
,7
studies have been performed for various fullerenes and
1
).
their derivatives,1
1,12
only a few preliminary results have
The studies on the photochemical and physical proper-
ties of fullerenes shed light on their electronic structures
and photoexcited states and can lead to their potential
been reported so far for the family of the azafullerene
analogues,13 mainly because of the difficulty in their
synthesis. Also, detailed photochemical and physical
applications in materials science ranging from supercon-
ductivity to nanostructured devices.8-10 Although such
2 2
studies on the dimeric structures of (C60) and (C60) O
have already appeared in the literature.1
Very recently, we showed from the product analysis of
the reaction mixture that both (C59N) and C59HN pho-
4,15
*
Corresponding authors. H. Shinohara: fax, +81-52-789-3660.
O. Ito: fax, +81-22-217-5608; e-mail, ito@tagen.tohoku.ac.jp.
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2
1
6
(
tosensitize the reaction of olefins with oxygen. In that
study, however, the generation of singlet oxygen upon
1
(
(
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W.; Duclos, S. J .; Lyons, K. B.; Miller, B.; Rosamilia, J . M.; Fleming,
R. M.; Kortan, A. R.; Glarum, S. H.; Makhija, A. V.; Muller, A. J .; Eick,
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359.
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Chem. A 1999, 103, 7155.
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Ito, O. J . Phys. Chem. A 2001, 105, 675.
1
(
(
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F. Science 1995, 269, 1554.
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(
Srdanov, V. I.; Collins, T. G.; Hummelen, J . C.; Bellavia-Lund, C.;
Pavlovich, J .; Wudl, F.; Holczer, K. Nature 1996, 383, 147.
(
7) Tagmatarchis, N.; Pichler, T.; Krause, M.; Kuzmany, H.; Shi-
nohara, H. J . Chem. Soc., Perkin Trans. 2 2000, 2361.
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(
(
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1
0.1021/jo0104678 CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/27/2001

Photooxidation of Olefins
J . Org. Chem., Vol. 66, No. 24, 2001 8027
Ta ble 1. Sen sitized P h otooxid a tion Yield s of
2
-Meth yl-2-bu ten e a n d r-Ter p in en e a n d Excited
Tr ip let-Sta te P r op er ties of Hyd r oa za fu ller en e C59HN a n d
Bisa za fu ller en e (C59N)2 As Com p a r ed w ith Th ose of
P r istin e C60
C
59HN
(C59N)
85
2
C
60
sensitized oxidation yield of 90
-methyl-2-butene (%)a
95
95
40
2
sensitized oxidation yield of 80
70
R-terpinene (%)a
triplet-state lifetime τ
(µs)b
5
10
2.0 × 10
T
9
9
9
quenching rate constant kO2 2.0 × 10
1.9 × 10
-1
-1 c
(M
s )
d
singlet oxygen yield ΦIO2
triplet-triplet absorption
nm)
0.48 ( 0.05 0.48 ( 0.05 0.96 ( 0.02
750 680 740
F igu r e 2. Ene and Diels-Alder photooxygenations of 2-meth-
yl-2-butene and R-terpinene in the presence of (C59N) or
2
(
C
59HN as a sensitizer.
a
Yields were determined by integrating the appropriate area
1
of the corresponding olefinic protons on the H NMR spectrum of
the reaction mixture of the sensitized photooxidation, and they
were optimized regarding the molecular concentration of the
azafullerene photosensitizer (1 order of magnitude higher as
UV irradiation of azafullerenes was presumed only by
the formation and subsequent identification of the cor-
responding products in the examined photochemical
reactions (Figure 2). In this article, we present for the
first time direct experimental evidence for the formation
of singlet oxygen based on the transient absorption
spectroscopy of these azafullerenes. From the transient
absorption spectra in the visible and near-IR regions, the
triplet-state formations were identified and their life-
times were estimated as well as their quenching rates
by oxygen. These spectroscopic data were compared with
those of pristine C60. In addition, the observation of
luminescence in the near-IR region further confirms the
generation of singlet oxygen. Hydroperoxides 1 and 2 as
well as endoperoxide 3 were formed upon UV irradiation
of 2-methyl-2-butene and R-terpinene, respectively, in the
presence of azafullerenes. The yields of conversion of
these photooxygenations were compared with those
obtained when C60 was used as the photosensitizer, and
the results were discussed in terms of the efficiency of
the singlet oxygen production between the azafullerenes
and C60. Finally, steady-state absorption measurements
were performed before and after the laser irradiation of
these azafullerenes to see whether or not transformation
to different species occurred during the light irradiation
measurements.
b
compared to those of C60). In deaerated toluene solution, τT )
1
/kT. c In oxygenated toluene solution. d From luminescence mea-
surements Φisc ≈ ΦIO2.
pulsed Xe lamp was detected with a Ge avalanche photodiode
Hamamatsu Photonics, B2834). For the measurements in the
(
visible region (400-800 nm), a Si-PIN photodiode (Hamamat-
su Photonics, S1722-02) was used as the detector.
Lu m in escen ce Mea su r em en ts. Emission of singlet oxy-
gen was observed by the laser excitation of 514 nm of
2
azafullerenes in the O -saturated toluene solution. An Ar-ion
laser (Spectra-Physics, BeamLok 2060-10-AS, ca. 200 mW) was
used as the excitation source. InGaAs, a PIN photodiode (New
Focus, 2153) was used as a detector in the near-IR region.
Stea d y-Sta te Absor p tion Sp ectr a . Absorption spectra in
the visible and the near-IR regions were measured with a
J ASCO V-570DS spectrometer at room temperature.
Resu lts a n d Discu ssion
P h otosen sitiza tion of Olefin s. When a deuterated
benzene solution of 2-methyl-2-butene or R-terpinene was
irradiated under a stream of pure oxygen in the presence
of either bisazafullerene (C59N)
59HN, the identification of the corresponding peroxy-
genated products was carried out by means of H NMR
analysis of the reaction mixture (Figure 2). The ene
photosensitization reaction of 2-methyl-2-butene with
singlet oxygen afforded hydroperoxides 1 and 2 in the
presence of both azafullerenes. As expected, the second-
ary and tertiary hydroperoxides were formed in an almost
2
or hydroazafullerene
C
1
Exp er im en ta l Section
2
Ma ter ia ls. The synthesis of bisazafullerene (C59N) and
hydroazafullerene C59HN was accomplished in a three-step
sequence starting from C60 and has already been reported.5
The materials were purified by high-performance liquid chro-
matography (5PYE column, toluene as the eluent, 10 mL/min
flow rate) and characterized by elemental methods. Soot
containing pristine C60 was obtained by direct-current arc
discharge of graphite/metal composite rods that were used for
the production of various endohedral metallofullerenes, which
was followed by Soxhlet extraction and HPLC purification, as
previously described. The photooxygenations of 2-methyl-2-
butene and R-terpinene were performed with a UV lamp
equipped with a Kapton filter (cutoff wavelength < 400 nm),
as described previously. All other commercial materials were
used as received and were of the best commercial grade
available.
-7
1
:1 ratio, which means that there is no Markovnikov-
type effect upon the oxygen addition, as with the case
when conventional sensitizers were used. The total yields
of 1 and 2 are summarized in Table 1, in which the yield
3
obtained via (C HN)* is slightly higher than that via
59
3
[
(C59N)
those obtained when C60 was used as the photosensitizer
see footnote a in Table 1). Importantly, a considerable
amount of (C59N) was found to be transformed into
monomeric hydroazafullerene species as well as oxygen-
ated adducts under the applied UV irradiation conditions,
as revealed by HPLC analysis, denoting the ease of the
C-C interdimer bond disruption. This is not surprising
as we already expected that, as soon as the first excited
2
]*, although they are both slightly lower than
1
7
(
2
1
6
Sp ectr oscop ic Mea su r em en ts. All measured samples
were placed in a quartz cell (1 × 1 cm) and deaerated by
bubbling Ar through the solution for 15 min. Transient
absorption spectra in the near-IR region (600-1600 nm) were
measured by using the third-harmonic generation (THG, 355
nm) of a Nd:YAG laser (Spectra-Physics, Quanta-Ray GCR-
singlet state was populated, some fractions of (C59N)
2
could intersystem-cross to the triplet state and subse-
quently generate singlet oxygen while some others might
follow the pathway that leads to the breakage of the
dimeric structure (competitive process, see discussion
below).
1
30, 6 ns fwhm) as an excitation source. Probing light from a
(17) Shinohara, H. Rep. Prog. Phys. 2000, 63, 843.

8
028 J . Org. Chem., Vol. 66, No. 24, 2001
Tagmatarchis et al.
F igu r e 3. Transient absorption spectrum of C59HN in toluene
solution observed by 355 nm laser irradiation. Inset: absorp-
2
F igu r e 4. Transient absorption spectrum of (C59N) in toluene
solution observed by 355 nm laser irradiation. Inset: absorp-
tion-time profile of a transient absorption band at 750 nm of
tion-time profile of a transient absorption band at 680 nm of
[(C59N) ]*.
2
3
3
[C
59HN]*.
calculated to be 2 × 10 M s-1. The overall transient
spectrum is very similar to that of the triplet-triplet
absorption of C , indicating that the presence of nitrogen
9
-1
Similarly, the Diels-Alder photooxygenation of R-ter-
pinene did not proceed smoothly. Only after prolonged
irradiation periods (i.e., 1 h) and/or higher molecular
concentrations of the azafullerenes (i.e., 0.01 mol %) was
the formation of endoperoxide 3 achieved in relatively
high yield and decreased in the order of C60 > C59HN >
6
0
in the fullerene skeleton has little effect on the triplet
absorption spectrum. Moreover, oxygen quenches the
triplet excited state of C60 as well as the triplet states of
other structurally related iminofullerenes with similar
rate constants.2
2
(C59N) (Table 1). However, optimum conversion of the
1,22
examined alkenes to the corresponding peroxygenated
products was achieved when the molecular concentration
of azafullerenes was at least 1 order of magnitude higher
than that of C60. Finally, when we repeated the ene and
Diels-Alder photoxygenations in the presence of a
catalytic amount of 1,4-diazabicyclo[2.2.2]octane (DAB-
CO),18 we hardly observed any progress of the reactions.
This further verifies that singlet oxygen is the species
responsible for the photooxygenations of these alkenes
because DABCO is well-known as a singlet oxygen
quencher.18 In contrast, when C60 was used as the
photosensitizer, both 2-methyl-2-butene and R-terpinene
were transformed quantitatively into the corresponding
products; more importantly, not only was a smaller
For (C N) , the transient absorption spectrum is much
different from that of C HN. There is a maximum
5
9
2
5
9
transient absorption band at 680 nm (Figure 4). This
absorption maximum was blue-shifted with respect to
those of C60 and hydroazafullerene, as previously de-
scribed. In addition, the appearance of other absorption
bands and shoulders at 500, 850, 1020, and 1230 nm
strongly supports the hypothesis that interactions exist
between azafullerenyl C N cages in the triplet states.
There are 61 π-electrons in each C N radical (i.e., this
radical combines in pairs to form bisazafullerene (C N) ),
and consequently, it is isoelectronic to the π-radical anion
of C . Moreover, the replacement of a carbon atom with
5
9
5
9
5
9
2
60
amount of sensitizer used for the transformation but a
a nitrogen atom resulted in the reduction of a [6, 6]
double bond in the parent C60 skeleton and, therefore, in
a smaller conjugated area. Thus, interaction between two
C N moieties may shift the triplet absorption to longer
shorter reaction time was also applied (Table 1).1
9,20
This
is consistent with our experimental findings from the
transient absorption spectroscopy study regarding the
lower yields of singlet oxygen production of azafullerenes
compared to that of C60 (see results below).
5
9
wavelengths.
The lifetime of (C N) at the 680 nm band was
5
9
2
Excited Tr ip let Sta tes. Irradiation of a toluene
solution of hydroazafullerene C59HN with the nanosecond
laser light at 355 nm generated transient absorption
bands at 750 nm along with a shoulder at 1050 nm
59
estimated to be twice as long as that of C HN and decays
in 10 µs in the absence of oxygen. In oxygenated toluene
solution, the second-order rate constant was calculated
9
-1 -1
to be identical to that of C HN, 2 × 10 M
5
9
s
(inset in
(Figure 3). By the photoexcitation, the lowest singlet
Figure 4), a value typical for the triplet excited states of
aromatic hydrocarbons. For both azafullerenes, the triplet
lifetimes are shorter than that of C60 or even some
fulleropyrrolidene derivatives.23
excited state was generated first, followed by intersystem
crossing to the corresponding triplet excited state. Since
the transient absorption bands were observed in the
microsecond range (inset in Figure 3), they can be
assigned to the triplet-triplet absorption bands. In
deaerated toluene solutions, the lifetime of the triplet
state was estimated to be 5 µs. In the presence of oxygen,
all these transient bands were quenched, and therefore
it was confirmed that these bands are attributable to the
As mentioned previously, the triplet states of both
3
3
2
azafullerenes, namely, [(C59N) ]* and [C59HN]*, were
efficiently quenched by molecular oxygen. Generation of
the singlet oxygen was further confirmed by the observa-
tion of the luminescence in the near-IR region (Figure
5
). The luminescence intensity of the singlet oxygen
triplet excited state of hydroazafullerene, namely,
3
3
2
derived from either [(C59N) ]* or [C59HN]* is less than
3
[C59HN]*. The rate of the quenching was subsequently
(
21) Guldi, D. M.; Hungerbuhler, H.; Carmichael, I.; Asmus, K.-D.;
(18) 1,4-Diazabicyclo[2.2.2]octane (DABCO) is an excellent singlet
Maggini, M. J . Phys. Chem. A 2000, 104, 8601.
oxygen quencher. See: Foote, C. S. In Singlet Oxygen; Wasserman, H.
H., Murray, R. W., Eds.; Academic Press: New York, 1979; p 139.
(22) Arbogast, J . W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.;
Diederich, F. N.; Alvarez, M. M.; Anz, S. J .; Whetten, R. L. J . Phys.
Chem. 1991, 95, 11.
(23) Luo, C.; Fujitsuka, M.; Watanabe, A.; Ito, O.; Gan, L.; Huza,
Y.; Huang, C.-H. J . Chem. Soc., Faraday Trans. 1998, 94, 527.
(
19) Orfanopoulos, M.; Kambourakis, S. Tetrahedron Lett. 1994, 35,
945.
20) Tokuyama, H.; Nakamura, E. J . Org. Chem. 1994, 59, 1135.
1
(

Photooxidation of Olefins
J . Org. Chem., Vol. 66, No. 24, 2001 8029
observed. This implies that (C59N)
by the photolysis of C59HN, as (C59N)
transient bands (Figure 4). In the case of (C59N)
other hand, it has already been shown by ESR studies
that the interdimer C-C bond of bisazafullerene (C59N)
2
was not accumulated
shows broad
, on the
2
2
2
2
4-26
was broken either photochemically or thermally.
There are two possible paths for the interdimer C-C
bond cleavage of (C59N)
tion of C59N takes place via the excited singlet state of
2
upon its excitation: the forma-
•
(C
59N)
2
immediately after excitation or via the triplet
excited state of (C59N) after intersystem crossing from
the excited singlet state. Since the Φisc value of (C59N)
2
2
was similar to that of C59HN, it is reasonable to assume
that as soon as the first excited singlet states were
populated, some fractions of the species intersystem-
crossed to the triplet states while some others followed
the pathway that led to the formation of the azafullerenyl
F igu r e 5. Luminescence spectra of singlet oxygen in oxygen-
saturated toluene solution generated by Ar ion-laser excitation
2
(matched at 514 nm) of (C59N) (solid line), C59HN (dotted line),
and pristine C60 (dashed line) as a standard.
•
radicals C59N . However, in this context, it is extremely
•
half than that derived from pristine C60 (the absorbance
was matched at the excitation wavelength at 514 nm).
important to mention that most C N radicals return to
5
9
(C N) by the radical coupling reaction, which implies
5
9
2
•
Subsequently, the quantum yield of singlet oxygen (Φ1O2
)
,
59
that the reactivity of C N with a solvent such as toluene
was estimated to be 0.48 for both C59HN and (C59N)
2
is quite low.
taking Φ1O2 ) 0.96 for C60 as a standard.22
The quantum yield for the singlet oxygen generation
1O2 can be expressed with the following equation:
Con clu sion
Φ
Ene and Diels-Alder photosensitized oxygenations in
the presence of the examined azafullerenes were con-
firmed, although the yields of the photooxygenation
reactions of 2-methyl-2-butene and R-terpinene in the
presence of azafullerenes are smaller than those obtained
when C60 was used as the photosensitizer. Excited states
ΦIO2 ) Φ ×(k [O ]/(k + k [O ]))
isc
O2
2
T
O2
2
where Φisc is the quantum yield for the intersystem
crossing, kO2 is the bimolecular quenching rate constant
of the triplet state by oxygen, [O
of oxygen, and k is the decay rate of the triplet state in
the absence of oxygen. Thus, the term (kO2[O ]/(k
])) corresponds to the efficiency of the singlet oxygen
generation. It is worth mentioning at this point that in
the nonpolar toluene employed in the present study, any
electron-transfer process with oxygen can be neglected
for bimolecular reactions. For hydroazafullerene C59HN,
2
] is the concentration
2
of bisazafullerene (C59N) and hydroazafullerene C59HN
T
were easily generated upon laser irradiation as identified
by transient spectroscopy. The triplet excited azafullere-
nyl moiety C59N in bisazafullerene was influenced by the
adjacent π-electron system of the neighboring azaful-
lerenyl group. The quantum yield for singlet oxygen
photosensitization was estimated from the luminescence
spectrum for both azafullerenes to be almost half of that
of C60, which verified the yields of photosensitized oxida-
2
T
+ kO2-
[O
2
5
7
-1
k
T
and kO2[O
then, the term (kO2[O
Therefore, Φisc was evaluated to be 0.48. As for the case
of bisazafullerene (C59N) , the terms k and kO2[O ] are
× 10 and 2 × 10 s , respectively. Therefore, (kO2[O ]/
+ kO2[O ])) is equal to 0.99, and subsequently, in a
similar way as previously performed, the value of Φisc
for (C59N) is equal to 0.48. At this point, it has yet to be
2
] are 2 × 10 and 2 × 10 s , respectively;
1
6
2
]/(k ])) is equal to 0.99.
T
+ kO2[O
2
tion by azafullerenes and C60
.
As the mechanical and
photochemical properties of azafullerenes and fullerenes
are rather similar,27 we expect to use them as building
blocks for novel nanostructured architectures in which
both the position and the electronic properties of each
component could be well specified.
2
T
2
5
7
-1
1
2
(k
T
2
2
Ack n ow led gm en t. N.T. thanks the J apan Society
for the Promotion of Science (J SPS) for a Postdoctoral
Fellowship for Foreign Researchers. H.S. thanks J SPS
for the Future Program on New Carbon Nanomaterials.
considered to what extent the lower intersystem-crossing
quantum yield for azafullerenes should be attributed to
the introduction of nitrogen into the fullerene skeleton.
The resultant perturbation of the electronic structure of
the materials and/or a different pathway they follow from
their singlet excited states (i.e., formation of azafullerenyl
radicals) may contribute independently to the intersystem-
crossing quantum yield.
Su p p or tin g In for m a tion Ava ila ble: Steady-state ab-
sorption spectra before and after laser flash photolysis experi-
ments. This material is available free of charge via the Internet
at http://pubs.acs.org.
P h otosta bility. The UV-vis-NIR absorption spectra
J O0104678
2
of (C59N) and C59HN have previously been reported and
5
-7
extensively analyzed. We found that under the experi-
mentally applied conditions for the current photochemical
studies, the steady-state absorption spectra remain un-
changed after the laser irradiation (see Supporting
Information). In the transient absorption spectra, it has
been presumed that C59HN is quite stable under laser
irradiation, as the sharp transient absorption bands were
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Prassides, K. J . Phys. Chem. A 1999, 103, 6969.
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G.; Wudl, F. J . Am. Chem. Soc. 1997, 119, 11128.
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