Drastic change in the rate of photochemical rearrangement of 1,6-(N-phenyl)aza-[60]fulleroids by switching the excited states through simple methyl substitution on the phenyl group

Article abstract of DOI:10.1016/j.tetlet.2005.07.133

The reaction rate for the photochemical rearrangement of 1,6-N-(substituted-phenyl)aza-[60]fulleroid 1 to 1,2-N-(substituted-phenyl) aziridino-[60]fullerene 2 differed ca. 3000-fold depending on the position and number of methyl substituents on the N-phen

Full text of DOI:10.1016/j.tetlet.2005.07.133

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 6713–6716
Drastic change in the rate of photochemical rearrangement of
,6-(N-phenyl)aza-[60]fulleroids by switching the excited
states through simple methyl substitution on the phenyl group
1
a,
a
b
b
b,
*
*
Akihiko Ouchi, Bahlul Z. S. Awen, Hongxia Luo, Yasuyuki Araki and Osamu Ito
a
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi 980-8577, Japan
b
Received 17 June 2005; revised 20 July 2005; accepted 26 July 2005
Available online 10 August 2005
Abstract—The reaction rate for the photochemical rearrangement of 1,6-N-(substituted-phenyl)aza-[60]fulleroid 1 to 1,2-N-(substi-
tuted-phenyl)aziridino-[60]fullerene 2 differed ca. 3000-fold depending on the position and number of methyl substituents on the N-
phenyl group. The required time for the completion of the reaction decreased in the order 2,6-dimethylphenyl (1d) < 2-methylphenyl
(
1b) < phenyl (1a) < 4-methylphenyl (1c). The difference was mainly due to switching of the excited states between normal (fast reac-
tions) and charge-separated (slow reactions) triplet states, which was induced by steric interactions between the N-phenyl group and
the C60 moiety.
ꢀ
2005 Elsevier Ltd. All rights reserved.
Molecules showing twisted intramolecular charge trans-
fer (TICT) have been well studied and it is known that
their spectroscopic properties are largely affected by
NAr
NAr
hν (> 450 nm)
1
small structural modifications of the molecules. An
toluene, N2
explanation for this phenomenon is the switching be-
tween two excited states by intramolecular steric interac-
tions. If this switching is linked to a chemical reaction in
which the rate of the reaction is very different between
the two excited states, a molecule that amplifies a small
structural difference into a large chemical reactivity can
be constructed.
1
2
Me
Me
Me
Ar =
Me
a
b
c
d
Scheme 1. Photochemical rearrangement 1 ! 2.
We report here a photochemical rearrangement of 1,6-
Absorption spectra of 1a–d and 2a–d were similar to
(
N-phenyl)aza-[60]fulleroids (1) to 1,2-(N-phenyl)aziri-
that of parent C60, though a slight difference among
their molar absorption coefficients (e) was observed.
The absorptions of 1a–d and 2a–d at <430 nm can be as-
signed to allowed transitions and the weak absorptions
2
,3
4
dino-[60]fullerenes (2) with different substitutions on
the phenyl group (1a–d) (Scheme 1), which provides an
example for the above mentioned amplification systems;
a small difference in the methyl substitution on the phe-
nyl group is found to induce an approximate 3000-fold
acceleration in the reaction rates between the slowest
at >430 nm to orbital forbidden electronic transitions,
5
by analogy with those of C60
.
(
1
1c) and the fastest (1d) photochemical rearrangement
! 2.
Figure 1 shows the time profiles of photochemical rear-
rangement 1 ! 2 by >450 nm light irradiation under a
nitrogen atmosphere. The light is mainly absorbed by
the forbidden transitions of 1a–d. The reactions were
very sensitive to oxygen and a significant retardation
of the reactions was observed in the presence of even a
small amount of oxygen. As seen in Figure 1, the
conversion of 1 to 2 was quantitative and Figure 1A
Keywords: Amino compounds; Fullerenes; Photochemical reactions;
Reactivity; Rearrangements.
*
Corresponding authors. Tel.: +81 29 861 4550; fax: +81 29 861 4511
A.O.); e-mail: ouchi.akihiko@aist.go.jp
(
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.07.133

6714
A. Ouchi et al. / Tetrahedron Letters 46 (2005) 6713–6716
Figure 1. Decrease of 1 and yield of 2 for (A) 1a,c and 2a,c, and (B) 1b,d and 2b,d as a function of irradiation time. Substrates and products; 1a: j,
a: h, 1b: d, 2b: s, 1c: m, 2c: n, 1d: r, 2d: }. Initial concentration: 10ꢀ5 M 1 in toluene; irradiated light: >450 nm.
6
2
shows that the required times for complete consumption
of 1a and 1c were 3.5 and 13 h, respectively, whereas
Figure 1B demonstrates a very fast reaction of 1b and
trast, the transient absorption spectra of slow reacting
1a and 1c show absorption bands at <420, 680,
1050 nm, and a broad absorption at >1300 nm (Fig. 2,
A1 and C1), which can be attributed to a charge-sepa-
1
d which only took 25 and 15 s, respectively. The ratio
of the required time for the complete consumption of
was 1a/1b/1c/1d = 790:1.6:2930:1.
3
b,8–10
rated triplet state.
Interestingly, the transient spec-
1
trum of 1b also shows absorptions corresponding to the
charge-separated triplet state in addition to the normal
triplet absorptions.
Figure 2 shows nanosecond transient absorption spectra
of 1a–d and 2a–d. The transient absorption spectra of
fast reacting 1b and 1d show a main absorption band
at 720 nm (Fig. 2, B1 and D1), which is characteristic
These results indicate a switching of the excited states
between the normal and charge-separated triplet states,
which leads to the fast and slow reactions, respectively.
7
,8
of the excited triplet state of C60 derivatives. In con-
3
3
Figure 2. Nanosecond transient absorption spectra of (A1) 1a, (A2) 2a, (B1) 1b, (B2) 2b, (C1) 1c, (C2) 2c, (D1) 1d, and (D2) 2d. Excitation
ꢀ4
wavelength: 530 (A1) and 532 (A2, B–D) nm. Concentration: 10 M in toluene. Measured times of the spectra after the laser pulse are shown in the
figures.

A. Ouchi et al. / Tetrahedron Letters 46 (2005) 6713–6716
6715
Scheme 2 indicates the presence of a large steric interac-
tion between the ortho-methyl-substituted phenyl group
and C60 moiety in the case of fast reactions (1b,d), par-
ticularly when the phenyl group rotates around the N–
Ph bond, whereas smaller interactions are expected for
the slow reactions (1a,c). The Ar group without the
ortho-methyl-substituent may give the best angle with
respect to the N-lone pair that enables facile electron
transfer of the lone pair electron to the C60 moiety,
which is most probably due to the rupture of efficient
delocalization between the lone pair electrons and the
p-electrons of the phenyl ring. The generation of the po-
sitive charge on the nitrogen atom might be the reason
for the decrease in the rate of the rearrangement in the
case of charge-separated triplet states.
as triplet sensitizers, which is consistent with the fact
that rearrangement 1 ! 2 proceeded by a unimolecular
process. In contrast, it is reported that the triplet energy
1
1
of 2d is higher than that of 1d so that 2d can act as a
triplet sensitizer showing a large k value and nonexpo-
s
nential feature in the rearrangement. Although 2b has a
normal triplet character, the sensitization effect is not
apparently observed in the reaction; this is due to the
fact that the k value was 1/4.5 but the k value was
s
d
11.7-fold larger than that of 1d. This inefficient triplet
sensitization of 2b can be rationalized by 2b having sim-
ilar triplet energy to that of 1b.
Although the rearrangement rate is mostly controlled by
the steric effect, the electronic factor of the substituents
also affected the rate of the reaction. In the case of the
unimolecular process, kd values decreased with the
introduction of the electron-donating group to the phe-
nyl group; the rate decreased to 1/3.2 with para-methyl
(1c vs 1a) and to 1/11.7 with ortho-methyl (1b vs 1d) sub-
stitutions. The result on the para-methyl substitution
can be explained by the facilitation of charge transfer
from the electron-donating N-lone pair to the electron-
accepting C60 moiety due to the increase in the electron
density of the N-lone pair. In contrast, a 4.5-fold in-
Besides rearrangement rates, Figure 1 also demonstrates
a difference in the reaction mechanisms. The consump-
tion of 1a–c and the formation of 2a–c showed single
exponential-like decay and rise, which is an indication
of a unimolecular process. However, the reaction of 1d
shows nonexponential curves for the consumption of
1
d and the formation of 2d. This is explained by the trip-
3
let sensitization of the reaction by product 2d. To deter-
mine the contribution of the unimolecular and sensitized
processes, the rate constants of both processes were ob-
tained similar to the reported procedure with consider-
ation of the difference in e values of 1a–d and 2a–d.
crease in k value was observed with ortho-methyl substi-
s
3
b
tution (1b vs 1d), which is rationalized by the facilitation
of the triplet energy transfer from 2 to 1 due to the
increase in triplet energy of 2d.
4
Thus, the rate constants (normalized with e) obtained
for the unimolecular (k ) and the sensitized (k ) pro-
d
s
ꢀ
7
ꢀ1
ꢀ3
ꢀ1
cesses are 1.5 · 10 cm M s
and 7.3 · 10 cm s
In summary, the reaction rate of the photochemical
rearrangement 1 ! 2 differed ca. 3000-fold depending
on the position and number of the methyl substituents
on the N-phenyl group. The required time for the com-
pletion of the reaction decreased in the order
1d < 1b < 1a < 1c. The difference was mainly due to a
switching of the excited states between normal (fast
reactions) and charge-separated (slow reactions) triplet
states, which was caused by steric interactions between
the N-phenyl group and the C60 moiety. The large differ-
ence in the reaction rates can be considered as an ampli-
fication of small structural differences into large
chemical reactivities by the switching between the two
different excited states, which may be extended to the
development of molecular switches.
ꢀ
4
ꢀ1
ꢀ1
for 1a, 1.4 · 10 cm M s and 5.8 cm s for 1b, 4.7 ·
ꢀ
8
ꢀ1
ꢀ17
ꢀ1
ꢀ1
1
1
0
cm M s
and 6.3 · 10
cm s
for 1c, and
ꢀ
5
ꢀ1
4
.2 · 10 cm M s and 2.6 · 10 cm s for 1d.
The result of the sensitized reactions can be explained by
the relative triplet energies of 1a–d and 2a–d (Scheme 3).
The transient spectra of 2a–d are similar to those of 1a–d
so that those of 2a and 2c are assigned to the charge-sep-
arated triplet state and 2b and 2d to the normal triplet
state. As the triplet energy of charge-separated 2 is esti-
3
b
mated to be lower than that of 1, 2a and 2c cannot act
H
H(Me)
(Me)H
N
N
H
C H
H
H
Acknowledgements
We thank Mr. Shin-ichirou Kawabata and Mr. Akio
Tominaga, Shimadzu Co., for MALDI-TOFMS mea-
surements, Dr. Y. Xiao for the preparation of 1b–d
and 2b–d, and Dr. T. Oishi for the purification and spec-
troscopic measurements of 1 and 2.
slow reaction
fast reaction
Scheme 2. Steric interaction between N-aryl and C60 moieties.
normal triplet
Supplementary data
charge-separated
triplet
Experimental details for the synthesis, analytical data,
spectral data, steady-state photolyses, and flash photo-
lyses of 1 and 2. UV spectra of 1a–d and 2a–d, and deter-
mination of rate constants k and k . Supplementary
2d
1d
1b
2b
1a,c 2a,c
Scheme 3. Schematic diagram for the relative triplet energy of 1a–d
and 2a–d.
d
s

6716
A. Ouchi et al. / Tetrahedron Letters 46 (2005) 6713–6716
data associated with this article can be found, in the
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. Average of three independent runs.