Remote control on the photochemical rearrangement of 1,6-(N-aryl)aza-[60] fulleroids to 1,2-(N-arylaziridino)-[60]fullerenes by N-substituted aryl groups

Article abstract of DOI:10.1021/jp0472717

Photochemical rearrangement of 1,6-(N-aryl)aza-[60]fulleroids (1) to 1,2-(N-arylaziridino)-[60]fullerenes (2) depends on the N-aryl substituents remote from the reaction center. A systematic kinetic study of the N-substituents discloses a decrease in the reaction rates of the photochemical rearrangement in the order 1-naphthyl (1b) > 1-pyrenyl (1d) > phenyl (1a) > 2-naphthyl (1e). The large substituent effect in the rates, which vary by ca. 2200-fold, is interpreted in terms of changes in the reaction mechanisms. The fast photochemical rearrangement of derivatives 1b,d proceeds through the normal triplet states of 1; in the case of 1b, triplet sensitization by the product 2b also operates. For the slow rearrangement rates of 1a.c, nanosecond transient absorption spectroscopy reveals that different triplet states participate, namely, electron transfer between the N-aryl substituent and the fullerene.

Full text of DOI:10.1021/jp0472717

9584
J. Phys. Chem. A 2004, 108, 9584-9592
Remote Control on the Photochemical Rearrangement of 1,6-(N-Aryl)aza-[60]fulleroids to
1,2-(N-Arylaziridino)-[60]fullerenes by N-Substituted Aryl Groups
Akihiko Ouchi,*,† Bahlul Z. S. Awen,† Ryota Hatsuda,† Reiko Ogura,‡ Tadahiro Ishii,‡
Yasuyuki Araki,§ and Osamu Ito*,§
National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan;
Faculty of Science, Tokyo UniVersity of Science, Kagurazaka, Tokyo 162-8601, Japan; and the Institute of
Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Sendai, Miyagi 980-8577, Japan
ReceiVed: June 23, 2004; In Final Form: August 25, 2004
Photochemical rearrangement of 1,6-(N-aryl)aza-[60]fulleroids (1) to 1,2-(N-arylaziridino)-[60]fullerenes (2)
depends on the N-aryl substituents remote from the reaction center. A systematic kinetic study of the
N-substituents discloses a decrease in the reaction rates of the photochemical rearrangement in the order
1-naphthyl (1b) > 1-pyrenyl (1d) > phenyl (1a) > 2-naphthyl (1c). The large substituent effect in the rates,
which vary by ca. 2200-fold, is interpreted in terms of changes in the reaction mechanisms. The fast
photochemical rearrangement of derivatives 1b,d proceeds through the normal triplet states of 1; in the case
of 1b, triplet sensitization by the product 2b also operates. For the slow rearrangement rates of 1a,c, nanosecond
transient absorption spectroscopy reveals that different triplet states participate, namely, electron transfer between
the N-aryl substituent and the fullerene.
Introduction
SCHEME 1
In recent years, chemical modification of C60 became increas-
ingly important as a tool for the functionalization of fullerenes
to add new functions to the parent fullerene molecules. Many
thermal and photochemical reactions have been developed for
the efficient chemical modification of fullerenes.1,2 One of the
widely used reactions is that of fullerenes with organic azides,3,4
which has been extensively employed for the preparation of
functional fullerene derivatives5 and for the incorporation of
fullerenes into various polymers.6 Both thermal and photo-
chemical processes have been reported for this reaction. For
C60 molecules, the thermal reaction proceeds by initial 1,3-
dipolar cycloaddition of the azide to the double bonds of C60,
followed by nitrogen elimination to form 1,6-(N-substituted)-
aza-[60]fulleroids (1),3 whereas the photochemical reaction
proceeds by the initial generation of nitrenes and their addition
to the double bonds of C60 to form 1,2-(N-substituted-aziridino)-
[60]fullerenes (2).3
The photochemical conversion 1 f 2 (Scheme 1) is also often
used in the chemical modification of C60,3-6 but the details on
the scope and limitations of this reaction are not known. A
spectroscopic study of 1 and 2 with alkyl substituent, e.g., the
methoxyethoxymethyl (MEM) group, has been conducted, and
an interaction between the electron pair of the nitrogen atom
and the fullerene π-electron system was found.7 If the interaction
between the nitrogen atom and the C60 moiety of 1 and 2 is
due to the nitrogen electron pair, a similar reactivity would be
expected for the photochemical rearrangement 1 f 2, regardless
of the type of N-substituents used. Nonetheless, a striking effect
of the N-substituent has been reported in the photochemical
rearrangement, e.g., whereas 1,6-(N-methoxyethoxymethyl)aza-
[60]fulleroid (1, Ar ) MEM) is photochemically inactive,3c 1,6-
(N-phenyl)aza-[60]fulleroid (1a) rearranges photochemically into
2a.3g This result indicates additional features that play an
important role in the photochemical reactivity of 1. Although
photochemical rearrangements of the carbon analogues, i.e.,
fulleroids to methanofullerenes,8 have been carried out, sys-
tematic studies on the substituent effects of the bridged carbon
atom have not been reported so far. This remote control of the
reaction is most likely due to interactions between the N-aryl
substituents that are not directly connected to the reaction center
and the C60 moiety.
For this reaction to be useful in the preparation of novel
materials, it is necessary to understand the nature of this remote
controlling effect of the N-substituted aryl groups. If the
photochemical properties of the fullerene derivatives may be
manipulated by appropriate N-substituents, such fullerene
derivatives may find valuable applications in material science.
Therefore, we have conducted a systematic study on the
photochemical rearrangement 1 f 2 with different N-aryl
substituents (Scheme 1),9 namely, the phenyl (a),3g 1-naphthyl
(b), 2-naphthyl (c), and 1-pyrenyl (d) groups. Indeed, the
difference in the photochemical rearrangement rates was found
to be more than 2000-fold between the fastest (1b) and slowest
(1c). This rate effect is attributed to switching between two
† National Institute of Advanced Industrial Science and Technology
(AIST).
‡ Tokyo University of Science.
§ Tohoku University.
* Corresponding author. E-mail: ouchi.akihiko@aist.go.jp.
10.1021/jp0472717 CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/08/2004

1,2-(N-Arylaziridino)-[60]fullerenes
J. Phys. Chem. A, Vol. 108, No. 44, 2004 9585
Figure 1. (A) Absorption spectra of 1a-d and C60 and (B) 2a-d and
C60. Concentration: 10-5 and 10-4 M (insets) in toluene. Codification
of substrates and products: 1a, 2a (s); 1b, 2b (- - -); 1c, 2c (- ‚ -); 1d,
2d (- ‚ ‚ -); C60 (‚ ‚ ‚).
Figure 2. (A) Fluorescence spectra of 1a-d and C60 and (B) 2a-d
and C60 in toluene at room temperature. Concentration: 10-4 M (1a-
c, 2a-d, and C60) and saturated solution (1d, between 5 × 10-5 and
10-4 M). Excitation wavelength: 500 nm with bandpath 15 nm and
emission bandpath 20 nm. All emission spectra were recorded by setting
580 nm cut filter in the emission pathway. Codification of substrates
and products: 1a, 2a (s); 1b, 2b (- - -); 1c, 2c (- ‚ -); 1d, 2d (- ‚ ‚ -);
C60 (‚ ‚ ‚).
different excited states due to the structural difference of the
N-substituents. Such a phenomenon may be applied to molecules
other than fullerene derivatives and has potential for exploring
new chemistry. We presently report on the details of our triplet
sensitization, spectroscopic, and kinetic experiments.
Results
Steady-State Absorption and Fluorescence Spectra of 1
and 2. Compounds 1 and 2 were synthesized from C60 and the
corresponding aryl azides as reported previously.9 Parts A and
B of Figure 1 show absorption spectra of 1a-d and 2a-d,
respectively, together with that of C60. Both figures show strong
absorptions at <430 nm and weak absorptions at >430 nm. In
analogy to C60,11 the absorptions of 1a-d and 2a-d at <430
nm can be assigned to the allowed and the weak absorptions at
>430 nm to the forbidden transitions.
On comparison of these spectra with that of the parent C60,
a considerable increase in the absorption intensity of the
forbidden transition bands was observed for both 1 and 2. The
increase of the absorption intensities of 2a-d at >400 nm was
1.8-2.7-fold compared with that of the parent C60. This increase
is due to the interactions caused by direct bonding of C60 and
arylamine groups because mixtures of N,N-dimethyl- or N,N-
diethylanilines and C60 in decalin do not show any considerable
change in their absorption intensities.12 One of the effects of
the direct bonding is the decrease of the symmetry of C60. The
pattern of the absorption bands, for both 1 and 2, is also slightly
influenced by the difference of the substituents.
The absorption spectra of 1 at >430 nm also showed a
considerable red shift compared with those of 2, particularly in
the case of 1d, which can be rationalized by the increase of the
number of π-electrons from 60 to 62 electrons.7 The small sharp
absorption that corresponds to the λmax at 407 nm of C60 was
not observed in 1a-d, but the corresponding absorption
appeared at ca. 424 nm for 2.
Figure 3. (A) Decrease of 1a-c and formation of 2a-c,14c (B) of 1b
and 2b,14d (C) of 1a,d and 2a,d,14c and (D) of 1d and 2d14c in degassed
solutions as a function of irradiation time. Codification of substrates
1a (O), 1b (]), 1c (0), 1d (4) and products 2a (b), 2b ([), 2c (9),
2d (2). Concentration: (A, B) 5 × 10-4 M and (C, D) 1 × 10-5 M 1
in toluene; light source: 500 W Xe lamp, fitted with a water filter and
a Toshiba UV-29 filter (>290 nm light).
derivatives, such as pyrrolidinofullerenes.13 In contrast, the
fluorescence spectra of 1a and 1c were quite different from the
others; the spectra had emission maxima at 760 and 800 nm
and a shoulder at 695 nm. This red shift of the fluorescence
emission was also observed in the N-MEM-substituted 1.7
Photochemical Rearrangement 1 f 2. Figure 3 shows the
time profiles of photochemical rearrangement 1 f 2 by
irradiation with >290 nm light under nitrogen atmosphere,
which were determined by HPLC analyses. The figure shows
the decrease of 1a-d and the increase of 2a-d as a function
of irradiation time. The reaction proceeded quantitatively for
Parts A and B of Figure 2 show fluorescence spectra of 1a-d
and 2a-d, respectively. Although the intensity of the fluores-
cence spectra was different, the shapes of the spectra of 1b,d
and 2a-d were very similar to each other. The spectra showed
an emission maximum at 695 nm and shoulders at 760 and 800
nm, which is very similar to those of 1,2-substituted C60

9586 J. Phys. Chem. A, Vol. 108, No. 44, 2004
Ouchi et al.
all substituents without the formation of byproducts. The light
is mainly absorbed by the allowed transitions of 1a-d at 290-
400 nm, and as seen in Figure 1a, the molar absorption
coefficients (ꢀ) of 1a-d were similar to each other in this
wavelength region: ꢀ of 1a-d at 290 nm and λmax were 46 800
and 46 100 M-1 cm-1 (336 nm) for 1a, 49 200 and 48 000 (334
nm) M-1 cm-1 for 1b, 50 500 and 42 300 (334 nm) M-1 cm-1
for 1c, and 51 200 and 52 900 (347 nm) M-1 cm-1 for 1d.
As seen in Figure 3A, the consumption of 1a,c and the forma-
tion of 2a,c showed exponential-like decay and rise, which is
an indication for a unimolecular process. The reaction was com-
pleted after 6 h for 1a and 8 h for 1c, whereas for 1b the reaction
was complete in only 13 s. In addition, an induction period was
observed in the reaction of 1b, which exhibited nonexponential-
like curves for the consumption of 1b and the formation of 2b
(cf. Figure 3B). Control experiments showed no decomposition
of 2a-d under the same photolysis conditions as those of 1a-
d, which indicates that the yield of 2a-d was not affected by
the secondary reactions of 2a-d or by the reverse reactions 2
f 1. The photolysis of 1d had to be conducted at lower
concentrations because of its low solubility; thus, for comparison
(cf. Figure 3C), the photolysis of 1a was also run at low
concentration. As seen in Figure 3C, the reaction was completed
after 60 min for 1a and 10 min for 1d. Figure 3D also displays
the exponential-like decay and rise in the photolysis of 1d, which
is an indication for a unimolecular process. The required reaction
time for complete consumption of 1 followed the relative order
1a:1b:1c:1d ) 1660:1:2220:280.
The photolyses of 1a-d in air-saturated solutions showed a
large decrease in the rate of photochemical rearrangement 1 f
2.15 The photolyses were very sensitive to small amounts of
oxygen; bubbling of nitrogen gas for 2 h through the reactant
solution prior to the photolysis was not sufficient for the
complete removal of oxygen, and the photolysis rate was
considerably decreased.15 This indicates that the photolysis
proceeded through triplet states of 1, which is consistent with
the report that the intersystem crossing (ISC) of C60 and its
derivatives to their triplet states is very fast and efficient2b,16
and also that the oxygen quenching is diffusion-controlled.13
Figure 4. (A) Decrease of 1a-c and formation of 2a-c, (B) of 1b
and 2b, (C) of 1a,d and 2a,d, and (D) of 1d and 2d in degassed
solutions as a function of irradiation time.14a Codification of substrates
1a (O), 1b (]), 1c (0), 1d (4) and products 2a (b), 2b ([), 2c (9),
2d (2). Concentration: (A, B) 5 × 10-4 M and (C, D) 1 × 10-5 M 1
in toluene; light source: 500 W Xe lamp, fitted with a water filter and
a Toshiba R-60 filter (>600 nm light).
(cf. parts C and D of Figure 4). The relative order of the required
reaction time for complete rearrangement of 1 is 1a:1b:1c:1d
) 1440:1:2160:312, which is similar to the photolysis with
>290 nm light. The photolysis of 1a-d with >450 nm light
also shows the same trend.15
Sensitization Effects in Photochemical Rearrangement 1
f 2. Figures 3B and 4B show that the photochemical rear-
rangement 1b f 2b is not a simple unimolecular reaction. The
absence of secondary reactions of 2a-d was confirmed under
the same photolysis condition as those for 1a-d. Therefore,
we have interpreted the nonexponential-like feature of the
reaction as triplet sensitization by the product 2b. To confirm
the sensitization effect of 2 and to estimate the relative triplet-
state energy levels of 1 and 2, we have conducted experiments
on the effect of addition of authentic 2 in the photochemical
rearrangement 1 f 2.
Figure 5 shows the consumption of 1 and the formation of 2
as a function of the ratio of the amount of initially added
authentic 2 to 1. In the case of 1b, a large increase in the
consumption of 1b and the formation of 2b is observed, which
depends linearly on the amount of initially added authentic 2b.
Thus, the rate of the rearrangement increases with the progress
of the reaction, which is consistent with the time profile of the
rearrangement 1b f 2b (cf. Figures 3B and 4B). The same
effect was also observed when the photolysis was conducted
with >600 nm light.15 On the contrary, in the cases of 1a,c,d,
the addition of authentic 2a,c,d showed no effect on the
consumption of 1a,c,d and the formation of 2a,c,d.
The relative triplet energy levels of 1 and 2 were determined
by the photosensitized reactions using 2; some of the results
are exhibited in Figures 6-8. The addition of 2b-d and C60
does not affect the photochemical rearrangement 1a f 2a.15 In
contrast, Figure 6 shows that the addition of C60 accelerates
considerably the photochemical rearrangement 1b f 2b, but
2a,c,d do not.15 The consumption of 1b and the formation of
2b depended linearly on the amount of initially added C60.
Figure 7B also displays a considerable acceleration of the
photochemical rearrangement 1c f 2c when C60 was added,
and the rate was slightly accelerated in the addition of 2b (Figure
Wavelength Effect in the Photochemical Rearrangement
1 f 2. The photolysis of 1 by >290 nm light proceeds mainly
through the excitation to the allowed electronic transition states
at 290-400 nm. The irradiations were also conducted by >600
nm light to investigate how the excitation through forbidden
transitions affects the reaction. As seen in Figure 1A, >600
nm light is mainly absorbed in the longer wavelength region of
the forbidden transition. The maximum molar absorption
coefficients (ꢀ) of 1a-d at >600 nm are 1830 M-1 cm-1 for
1a, 1780 M-1 cm-1 for 1b, 2130 M-1 cm-1 for 1c, and 2590
M-1 cm-1 for 1d; thus, the absorption coefficients are similar
at this wavelength.
Figure 4 shows the time profiles for the photochemical
rearrangement 1 f 2 by >600 nm light irradiation under a
nitrogen atmosphere. The figure shows the decrease of 1a-d
and the increase of 2a-d as a function of irradiation time; the
features are similar to the reaction with >290 nm light, except
for the slow reaction rate. The slow rate may be explained by
the smaller ꢀ values compared with those for >290 nm light.
The exponential-like reaction was completed after 20 h for 1a
and 30 h for 1c (Figure 4A), whereas a fast nonexponential-
like reaction was observed for 1b, which required only 50 s
irradiation time for completion (Figure 4B). The photolyses of
1a and 1d were conducted at lower concentration, whose
reaction time for completion was 6 h for 1a and 1.3 h for 1d

1,2-(N-Arylaziridino)-[60]fullerenes
J. Phys. Chem. A, Vol. 108, No. 44, 2004 9587
Figure 7. Yield of 2c and remaining 1c in degassed solutions as a
function of the ratio of the amount of initially added authentic 2b to
1c (A)14a and C60 to 1c (B).14a Codification of substrate 1c (0) and
product 2c (9). Irradiation time: 1 h. Concentration: 5 × 10-4 M 1c
in toluene. Light source: 500 W Xe lamp, fitted with a water filter
and a Toshiba UV-29 filter (>290 nm light).
Figure 5. (A) Yield of 2a and remaining 1a, (B) of 2b and 1b, (C) of
2c and 1c, and (D) of 2d and 1d in degassed solutions as a function of
the ratio of the amount of initially added authentic 2 to 1.14a Codification
of substrates 1a (O), 1b (]), 1c (0), 1d (4) and products 2a (b), 2b
([), 2c (9), 2d (2). Irradiation time: (A) 20 min, (B) 6 s, (C) 1 h, and
(D) 50 s. Concentration: (A-C) 5 × 10-4 and (D) 1 × 10-5 M 1 in
toluene. Light source: 500 W Xe lamp, fitted with a water filter and
a Toshiba UV-29 filter (>290 nm light).
Figure 8. Yield of 2d and remaining 1d in degassed solutions as a
function of the ratio of the amount of initially added authentic 2a to
1d (A), 2b to 1d (B), 2c to 1d (C), and C60 to 1d (D).14a Codification
of substrate 1d (4) and product 2d (2). Irradiation time: 50 s.
Concentration: 1 × 10-5 M 1d in toluene. Light source: 500 W Xe
lamp, fitted with a water filter and a Toshiba UV-29 filter (>290 nm
light).
Figure 6. Yield of 2b and remaining 1b in degassed solutions as a
function of the ratio of the amount of initially added authentic C60 to
1b.14a Codification of substrate 1b (]) and product 2b ([). Irradiation
time: 6 s. Concentration: 5 × 10-4 M 1a in toluene. Light source:
500 W Xe lamp, fitted with a water filter and a Toshiba UV-29 filter
(>290 nm light).
7A); however, no rate effect was observed during the addition
of 2a,d.15 Figure 8 reveals in the photochemical rearrangement
1d f 2d a large acceleration for the addition of 2b and C60
(the rearrangement terminates by 50 s irradiation when the initial
ratio C60/1d is >0.15), a small one for 2c, and no effect for 2a.
Transient Spectra of Substrate 1 and Product 2. The
difference of the reaction rates between 1a-d, shown in Figures
3 and 4, cannot be explained by the molar extinction coefficients
(ꢀ) because the ꢀ values are very similar in the >290 nm region
and in the >600 nm region they vary only ca. 1.5-fold (Figure
1). Therefore, the large variation in the reactivity of 1a-d
suggested differences in the type of excited states involved. To
scrutinize this possibility, we have measured nanosecond
transient spectra of 1 and 2 in toluene (Figures 9 and 10).
Figure 9B-1 is the transient absorption spectrum of 1b, which
displays a normal triplet peak at 700 nm; the position and the
shape of the peak were the same as those of reported fullerene
derivatives.17,18 This fact suggests that the fast photochemical
rearrangement 1b f 2b proceeds through normal triplet state(s).
Derivative 1d also rearranged fast, but it was difficult to measure
the transient spectrum and to observe the expected triplet
absorption because of the low substrate solubility. In contrast,
the derivatives 1a,c (Figure 9A-1,C-1) possess strong absorp-
tions at 420, 650, and 1050 nm and a broad band at >1300
nm. The first two peaks are similar to those reported for the
radical cations of arylamines,19 and the third peak matches the
absorption of the radical anion of fullerene derivatives.18,20
Furthermore, the decay profile of the transient absorption
disclosed oxygen quenching of the excited state.15 On the basis
of these results, the transient spectra were tentatively attributed
to the ion-pair states with triplet character, ion-pair triplet states.
From the decay profiles, the lifetimes of the transient species
of 1a and 1c were determined to be 4.5 and 7.4 µs (cf. parts
A-2 and C-2 of Figure 9); the lifetime of 1b could not be
obtained because its decay profile was too noisy.
Figure 10 shows the transient spectra of the products 2a-d
in toluene, whose general features are quite similar to the spectra
of 1a-d. Derivatives 2b and 2d absorb at 720 nm (normal triplet
state), whose lifetimes were determined to be 12 and 5.0 µs for
2b and 2d (cf. parts B-2 and D-2 of Figure 10). In contrast,

9588 J. Phys. Chem. A, Vol. 108, No. 44, 2004
Ouchi et al.
SCHEME 2
photochemical rearrangement 1 f 2 was observed. These results
are tentatively explained in terms of the various photochemical
pathways in Scheme 2.
Direct Photochemical Rearrangement. Despite the large
difference in the photochemical reactivities observed for the
substrates 1a-d, the computed ground-state heats of formation
and the charge distribution on the nitrogen atom were similar
in the PM3 level.15 Thus, the large variation in the photochemi-
cal reactivity implies considerable differences in the nature of
the excited states that are involved, which are most likely due
to electronic and/or structural effects of the substituents. Since
the reaction proceeds through triplet states, transient absorption
spectra showed that 1b, and most likely also 1d, possess normal
triplet states. In contrast, the excited states of 1a,c are assigned
to ion-pair triplet states because the transient absorption spectra
of 1a,c are similar to those of fullerene derivatives with
intramolecular charge separation18,20 and the excited states are
quenched by oxygen.15
Figure 9. Nanosecond transient absorption spectra of 1a (A-1), 1b
(B-1), and 1c (C-1) and the decay profiles of the transient absorption
peaks of 1a (A-2) and 1c (C-2). Excitation wavelength: 530 nm.
Concentration: 10-4 M in toluene. Transient absorption spectra were
measured at 200 ns (A, C) and 100 ns (B) after the laser pulse.
The fast photochemical rearrangement of 1b,d proceeds
through the triplet reaction path 1(S0) f 1(S) f 1(T, normal)
f 2(S0), shown in bold solid arrows in Scheme 2. The slow
reactions of 1a,c follow the same reaction path, but with a
different triplet state, namely 1(S0) f 1(S) f 1(T, ion-pair) f
2(S0). The photochemical experiments indicate that for 1a,c
the step 1(T, ion-pair) f 2(S0) is inefficient and the major step
is 1(T, ion-pair) f 1(S0), as shown by the broken arrow in
Scheme 2.
Triplet-Sensitized Photochemical Rearrangement and the
Triplet Energy Levels. In contrast to the exponential reaction
rates of 1a,c,d, substrate 1b proceeds nonexponentially (cf.
Figures 3 and 4). Presumably, the nonexponential reaction
involves triplet sensitization by the product 2b since the energy
of the triplet state of 2 is reported to be slightly higher than
that of 1.7 This triplet-sensitization mechanism of 2b is coded
with light solid arrows in Scheme 2.
The sensitization experiments allow us to derive the relative
triplet energy levels of the substrates 1a-d, the products 2a-
d, and C60. For example, from Figure 5, we conclude that the
relative order of the triplet energy levels is 2a < (1a, 1b) <
(2b, 2c) < 1c, and 2d < 1d because 2b sensitizes the
photochemical rearrangement 1b f 2b, but there is no
sensitization by 2a, 2c, and 2d in the rearrangement 1a, c, d f
2a, c, d. Similarly, the relative order (2b, 2c, 2d, C60) < 1a
may be obtained from the sensitization experiments on 1a; (2a,
2c, 2d) < 1b < C60 from Figure 6 and the sensitization
experiments on 1b; (2a, 2b, 2d) < 1c < C60 from Figure 7 and
the sensitization experiments on 1c; (2a, 2d) < 1d < (2b, 2c,
C60) from Figure 8, whereas 2a < 2d is determined from the
Figure 10. Nanosecond transient absorption spectra of 2a (A-1), 2b
(B-1), 2c (C-1), 2d (D-1), and the corresponding decay profiles of
the transient absorption peaks of 2a (A-2), 2b (B-2), 2c (C-2), and 2d
(D-2). Excitation wavelength: 530 nm (A, C, D) and 532 nm (B).
Concentration: 10-4 M in toluene. Transient absorption spectra were
measured at 200 ns after the laser pulse.
products 2a and 2c, which have been tentatively assigned to
ion-pair triplet states, possess peaks at 450, 650, and 800 nm
and a broad absorption at 1250 nm. The lifetimes of 2a and 2c
were determined to be 2.1 and 1.9 µs (cf. parts A-2 and C-2 of
Figure 10), which are much shorter than the normal triplet states
of 2b and 2d.
Discussion
Despite the small difference in the structure of the N-
substituted aryl groups, a large difference in the rate of

1,2-(N-Arylaziridino)-[60]fullerenes
J. Phys. Chem. A, Vol. 108, No. 44, 2004 9589
Figure 11. Relative ordering of the triplet energy levels of substrates
1a-d, products 2a-d, and C60.
kinetic analysis of the data in Figures 3 and 4. The relative
ordering of the triplet energy levels for all the compounds is
summarized in Figure 11.
Although 2d possesses a normal triplet state according to
transient absorption, the lack of sensitization by 2d may be due
to the higher triplet energy level of 1d than 2d, as displayed in
Figure 11. Alternatively, other energy dissipation paths may
operate for 1d and 2d.
Figure 12. Decrease of 1a-d as a function of irradiation time with
curve fitting according to eq 2. Codification of substrate: 1a (O), 1b
(]), 1c (0), d (4). Initial concentration ([1]0): 5 × 10-4 (A, B) and 1
× 10-5 M (C, D) in toluene; light source: 500 W Xe lamp, fitted with
a water filter and a Toshiba UV-29 filter (>290 nm light).
The transient spectra of 2a and 2c are similar to those of 1a
and 1c; therefore, the excited states of 2a,c are also tentatively
assigned to the ion-pair triplet states. The absence of triplet
sensitization in the photochemical rearrangement of 1a,c
indicates that the triplet energies of 2a and 2c (T, ion-pair) are
lower than of 1a and 2c (T, ion-pair).
Although the normal triplet energy level of the rearrangement
product 2 is reported to be higher than that of substrate 1,7 the
relative ordering in Figure 11 reveals that the triplet energy of
2 is not always higher than that of the corresponding 1. This
may be due to a considerable decrease in the triplet energy level
by the ion-pair triplet.
Contribution of the Direct and Sensitized Processes. To
understand the substituent effect on the photochemical rear-
rangement, we have conducted a kinetic analysis to assess the
relative contribution of the direct and sensitized processes in
the rearrangement. As photochemical conversion 1 f 2
proceeded quantitatively, the consumption of 1 is presented by
the kinetic expression in eq 1, where [1]t and [2]t are the
concentrations of 1 and 2 at time t, and kd and ks are the rate
constants of the direct and sensitized processes. The first term
of the right side of eq 1 represents the consumption of 1 by the
direct photolysis and the second term for the sensitized process.
Since [2]t ) [1]0 - [1]t, where [1]0 is the initial concentration
of 1, the integrated form of eq 1 is given by eq 2.21
TABLE 1: Experimental and Curve-Fitted kd and ks Values
of the 1a-d Derivatives According to Eq 2
a
wavelength initial concn
kd
ksa
entry
(nm)
(×10-4 M) substrate (×10-5 s-1
)
(s-1 M-1
)
1
2
3
4
5
6
7
8
9
>290
5.0
1a
1b
1c
1a
1d
1a
1b
1c
1a
1d
14 (1)
500 (36)
7 (0.5)
91 (1)
471 (5.2)
5 (1)
290 (58)
2 (0.4) 0.020 (0.7)
11 (1)
36 (3.3)
0.28 (1)
1040 (3714)
0.16 (0.6)
27 (1)
443 (16)
0.028 (1)
230 (8214)
0.1
5.0
>600
0.1
12 (1)
63 (5.3)
10
a The numbers in the parentheses are the values relative to 1a.
The rate constants kd and ks depend on the molar absorption
coefficient (ꢀ). In the cases of >290 nm light photolysis, the
light is mostly absorbed by the allowed transitions of 1a-d
and 2a-d at λmax 350 nm, whose ꢀ values are similar in this
wavelength region (cf. Figure 1). Therefore, when the same
photolysis conditions were used, kd and ks reflect the initial
photochemical reactivity of 1a-d. The kd values of entries 1-3
in Table 1 are 36- and 72-fold faster than those of 1a and 1c
for the direct photolysis of 1b. The ks values indicate that the
sensitized photolysis of 1b proceeded 3700- and 6200-fold faster
than those of 1a and 1c. The contribution of the triplet
sensitization in the photochemical rearrangement 1 f 2 matches
the triplet energy in Figure 11.
d[1]
-
t ) kd[1]t + ks[1]t[2]t
(1)
dt
Similarly, the entries 4 and 5 indicate that the direct and
sensitized reactions of 1d proceed 5.2- and 16-fold faster than
those of 1a. Despite the fact that the nonexponential nature of
the reaction (evidence for the triplet sensitization) was not
definite in Figure 3, the ks value of entry 5 (relative value is
16) indicates some triplet sensitization in the photochemical
rearrangement 1d f 2d; however, the contribution of the
sensitization in the rearrangement 1d f 2d is only about 0.4%
of that of 1b f 2b.
Entries 6-10 give the kd and ks values in the photolysis with
>600 nm light, whose relative magnitudes vary about 100- and
10 000-fold. The absorption by 1a-d and 2a-d in this
wavelength region is due to the forbidden transitions, and as
seen in Figure 1, the ꢀ values differs in this wavelength region.
1
[1]t )
ks
[1]0 kd + ks[1]0
ks
1
-
exp{(kd + ks[1]0)t} +
(
)
kd + ks[1]0
(2)
The results of the kinetic analysis by means of eq 2 are
displayed in Figure 12. The symbols give the experimental
consumption of 1a-d by >290 nm light irradiation, and the
lines are obtained by curve fittings22 of the data points according
to eq 2; the kd and ks values are summarized in Table 1. Table
1 also shows the values kd and ks obtained similarly from the
data of the >600 nm light irradiations.15,22

9590 J. Phys. Chem. A, Vol. 108, No. 44, 2004
Ouchi et al.
This fact implies that 1a-d have different effect of ꢀ on the kd
and ks values so that the difference in the relative values cannot
be explained only by the wavelength effects. However, obtained
relative values were similar to those in the photolysis with >290
nm light, which indicates that the effect of ꢀ on kd and ks values
is quite small.
Our results show that the small structural differences in the
N-substituent of fulleroids are responsible for the involvement
of distinct excited states, which induce large differences in the
reactivity (more than 2200-fold) of fulleroids, whose reaction
center is remote from the N-substituent. This first-time observa-
tion in fullerene chemistry may be the consequence of the
relative spatial orientation of the N-amino aryl and fullerene
functionalities. This fact may be generalized to other molecules
that have two distinct chromophores in different spatial orienta-
tions, a phenomenon that may have potential for developing
novel chemistry, e.g., the application of fullerene derivatives
as nanoswitches, in which the switching is accomplished through
small structural variations of the molecule by varying remote
substituents.
appropriate time. The stirred solution was then irradiated with
a 500 W xenon lamp (USHIO Optical ModuleX SX-UI500XQ),
supplied with an 18 cm water filter and a Toshiba UV-29 filter
under a nitrogen gas atmosphere in a Pyrex vessel. The reaction
mixture was then submitted to silica gel chromatography (1:1
mixture of CS2:n-hexane as eluent) to give pure 2. The
experimental details, analytical data, and the spectral data of
2a-d are given in the Supporting Information.
Photolysis of 1,6-(N-Aryl)aza-[60]fulleroids (1a-d). The
toluene (spectral grade) solutions of 1 [5 × 10-4 M (1a-c)
and 1 × 10-5 M (1a, d)] were prepared and degassed by three
freeze-pump-thaw cycles. The photolysis of 1 was conducted
in a 20 mL Pyrex round-bottomed flask under a nitrogen-gas
atmosphere while stirring. A 500 W xenon lamp, fitted with an
18 cm water filter (output energy after the water filter was 9.6-
9.8 mW cm-2) and a Toshiba UV-29, Y-45, or R-60 filter, was
used for the photolysis. The photolyses of 1 in air-saturated
and nitrogen-purged solutions were conducted under the same
experimental conditions; the solution were stirred under the air
or nitrogen gas for 2 h. The consumption of 1 and the formation
of 2 were determined by HPLC analysis [RP-18e column (4
mm i.d. × 250 mm); UV/vis detector wavelength: 300 nm],
relative to authentic samples.
Conclusion
In conclusion, the photochemical rearrangement of 1,6-(N-
aryl)aza-[60]fulleroids (1) to 1,2-(N-arylaziridino)-[60]fullerenes
(2) depends on the type of N-aryl substituents, remote from the
reaction center. A systematic study on the N-substituent effect
by phenyl (1a), 1-naphthyl (1b), 2-naphthyl (1c), and 1-pyrenyl
(1d) groups shows a large substituent effect on the rates of the
photochemical rearrangement, in which the rates between the
fastest (1b) and the slowest (1c) differs by ca. 2200-fold. The
decreasing order of the reaction rates is 1-naphthyl (1b) >
1-pyrenyl (1d) > phenyl (1a) > 2-naphthyl (1c). The fast
photochemical rearrangement proceeds through noraml triplet
states of 1 and triplet sensitization by the product 2b in the
case of 1b. The rate constants for the direct photochemical and
sensitized rearrangement were obtained by kinetic analysis of
the reaction. The slow reaction of 1a,c is interpreted by the
participation of different triplet states from the normal ones,
which tentatively assigned to ion-pair triplet states. The relative
ordering of the triplet energy levels of 1a-d, 2a-d, and C60,
obtained by sensitization, increases in the order 2a < 2d < 1d
< 2c < 1b < 2b < 1c < C60 < 1a. These results indicate that
a small structural difference of the substrate amplifies enor-
mously the reactivity (more than 2200-fold) through switching
between the two distinct triplet excited states.
Photolysis of 1,6-(N-Aryl)aza-[60]fulleroids (1a-d) in the
Presence of 1,2-(N-Arylaziridino)-[60]fullerenes (2a-d) or
C60. The toluene (spectral grade) solutions of 1 [5 × 10-4
M
(1a-c) and 1 × 10-5 M (1d)] and 2 [0-1.5 × 10-4 M (2a-c)
and 0-3 × 10-6 M (2d)] were prepared and degassed by three
freeze-pump-thaw cycles. The photolysis was conducted by
using the same experimental conditions as these for the
photolysis of 1. The consumption of 1 and the formation of 2
were determined by the same analytical system as that used for
the photolysis of 1. The results are the average of three or four
independent runs.
Flash Photolysis of 1 and 2. Transient absorption spectra in
the visible/near-IR regions were observed by the laser-flash
photolysis spectroscopy. The C60 derivatives were excited with
530-532 nm light from the OPO system, attached to a
Nd:YAG laser (6 ns fwhm). A Si-PIN photodiode module
(400-600 nm) and a Ge-APD module (600-1600 nm) were
employed as detectors for monitoring the light from a pulsed
Xe lamp.18f
Acknowledgment. We thank Mr. S. Kawabata, Mr. M.
Yamaguchi, and Mr. A. Tominaga, Shimadzu Co., for MALDI-
TOFMS measurements, Dr. H. Sakai for 500 MHz NMR
measurements, and Dr. P. K. Drechsel for the preparation of
1a. We thank Professor W. Adam for going through the
manuscript. Y.A. and O.I. are grateful for financial support by
a Grant-in-Aid of Scientific Research on Priority Areas (417)
from the Ministry of Education, Culture, Sports, Science, and
Technology of Japan.
Experimental Section
Synthesis of 1,6-(N-Aryl)aza-[60]fulleroids (1). General
Procedure. A sample of 500 mg of C60 was dissolved in 30
mL of 1,2-dichlorobenzene (DCB), and 1.5 equiv of the aryl
azide in DCB was added dropwise to the stirred solution at room
temperature under a nitrogen atmosphere. The reaction mixture
was then stirred at 80 °C, after cooling to room temperature,
130 mL of acetonitrile was added, the precipitate was separated
by centrifugation, and the solvent was passed through a PTFE
membrane filter (pore size 0.45 µm) to collect any remaining
small particles. The centrifuged and filtered precipitates were
combined and then submitted to silica gel chromatography (1:1
mixture of CS2:n-hexane as eluent) to give pure 1 and recovered
C60. Experimental details, analytical data, and spectral data of
1a-d are given in the Supporting Information.
Supporting Information Available: Preparation of phenyl
azide, 1- and 2-naphthyl azides, and 1-pyrenyl azide; experi-
mental details, analytical data, and spectral data of 1a-d and
2a-d; derivation of eq 2 from eq 1; results on the photochemical
rearrangement of 1a-d under air-saturated conditions, photoly-
sis of 1a-c under nitrogen-purged conditions, and photolysis
of 1a-d with >450 nm light under degassed conditions; the
effects of 2b addition in the photochemical rearrangement 1b
f 2b with >600 nm light, of 2b-d and C60 addition in the
rearrangement 1a f 2a with >290 nm light, of 2a,c,d and C60
addition in the rearrangement 1b f 2b with >290 nm light,
and of 2a,b,d and C60 addition in the rearrangement 1c f 2c
Synthesis of 1,2-(N-Arylaziridino)-[60]fullerenes (2). Gen-
eral Procedure. A sample of 1 was dissolved in toluene, and
nitrogen gas was passed through the stirred solution for an

1,2-(N-Arylaziridino)-[60]fullerenes
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1a-d by >600 nm light irradiation according to eq 2; PM3
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(21) Derivation of eq 2 from eq 1 is given in the Supporting Information.
(22) Curve fitting was conducted by using the Levenberg-Marquardt
algorithm with the ORIGIN version 6.1J computer program, produced by
MICROCAL Software, Inc.
(23) The corresponding aryl azides were synthesized according to the
reported procedures.15