A diarylethene with two nitronyl nitroxides: Photoswitching of intramolecular magnetic interaction

Article abstract of DOI:10.1021/ja000605v

A diarylethene having two nitronyl nitroxides, 1,2-bis[6-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin- 2-yl)-2-methyl-1-benzothiophen-3-yl]hexafluorocyclopentene (2a), was synthesized in an attempt to control the intramolecular magnetic interaction by photoirradiation. The photochemical conversions from open-ring isomer 2a to closed-ring isomer 2b and from 2b to 2a were both almost 100%. Magnetic measurement revealed the antiferromagnetic interaction between two nitronyl nitroxides remarkably increased from 2J/k(B) = -2.2 K to 2J/k(B) = -11.6 K when the diarylethene spin coupler was switched from the open-ring isomer 2a to the closed-ring isomer 2b. ESR measurements were also carried out for both 2a and 2b in benzene solutions at room temperature and in MTHF solid solutions at cryogenic temperature. Both ESR and magnetic measurement indicated that the intramolecular interaction was switched by the photochromic spin coupler.

Full text of DOI:10.1021/ja000605v

J. Am. Chem. Soc. 2000, 122, 7195-7201
7195
A Diarylethene with Two Nitronyl Nitroxides: Photoswitching of
Intramolecular Magnetic Interaction
Kenji Matsuda* and Masahiro Irie*
Contribution from Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu
UniVersity, and CREST, Japan Science and Technology Corporation, 6-10-1 Hakozaki, Higashi-ku,
Fukuoka, 812-8581, Japan
ReceiVed February 18, 2000. ReVised Manuscript ReceiVed May 1, 2000
Abstract: A diarylethene having two nitronyl nitroxides, 1,2-bis[6-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-
2-yl)-2-methyl-1-benzothiophen-3-yl]hexafluorocyclopentene (2a), was synthesized in an attempt to control
the intramolecular magnetic interaction by photoirradiation. The photochemical conversions from open-ring
isomer 2a to closed-ring isomer 2b and from 2b to 2a were both almost 100%. Magnetic measurement revealed
the antiferromagnetic interaction between two nitronyl nitroxides remarkably increased from 2J/kB ) -2.2 K
to 2J/kB ) -11.6 K when the diarylethene spin coupler was switched from the open-ring isomer 2a to the
closed-ring isomer 2b. ESR measurements were also carried out for both 2a and 2b in benzene solutions at
room temperature and in MTHF solid solutions at cryogenic temperature. Both ESR and magnetic measurement
indicated that the intramolecular interaction was switched by the photochromic spin coupler.
Introduction
Photochromism is defined as light-induced reversible trans-
formation of chemical species between two isomers having
different absorption spectra.1 The two isomers differ from each
other not only in the absorption spectra but also in various
physical and chemical properties, such as geometrical structure,
refractive index, dielectric constant, and oxidation/reduction
potentials. These property changes have been utilized to control
functions of photoresponsive molecules and polymers, such as
host-guest interactions,2 various physical properties of poly-
mers,3 alignment of liquid crystals,4 nonlinear optical properties,5
and electronic conduction.6
Figure 1. Photoswitching of magnetic interaction.
metal ion,9 or by intercalation of a photochromic compound
with metallic compounds,10 photocontrol of intramolecular
magnetic interaction in a molecule has not yet been achieved.11
When two open-shell moieties were placed at the edges of a
π-conjugative molecule, two electronic spins interact magneti-
cally through the π-conjugative framework by exchange interac-
tion J. The sign of the exchange interaction depends on the
topology of the π-system. If we can change the strength of the
coupler by external stimulation, we can control the magnetism
of the system. In this paper, we report on the photoswitching
of the intramolecular magnetism by incorporating two radical
moieties into a photochromic diarylethene spin coupler (Figure
1).
Molecular magnetism7 can also be photocontrolled by incor-
porating a photochromic moiety into the system. Although so
far, intermolecular magnetic interaction has been photochemi-
cally controlled by electron transfer,8 by spin crossover of the
(1) (a) Brown, G. H. Photochromism; Wiley-Interscience: New York,
1971. (b) Du¨rr, H.; Bouas-Laurent, H. Photochromism: Molecules and
Systems; Elsevier: Amsterdam, 1990. (c) Irie, M.; Uchida, K. Bull. Chem.
Soc. Jpn. 1998, 71, 985.
(2) (a) Desvergne, J.-P.; Bouas-Laurent, H. J. Chem. Soc., Chem.
Commun. 1978, 403. (b) Shinkai, S. Pure Appl. Chem. 1987, 59, 425. (c)
Blank, M.; Soo, L. M.; Wasserman, N. H.; Erlanger, B. F. Science 1981,
214, 70. (d) Irie, M.; Kato, M.; J. Am. Chem. Soc. 1985, 107, 1024. (e)
Takeshita, M.; Irie, M. J. Org. Chem. 1998, 63, 6643.
(3) (a) Irie, M.; Hirano, Y.; Hashimoto, S.; Hayashi, K. Macromolecules
1981, 14, 262. (b) Irie, M. AdV. Polym. Sci. 1990, 94, 27. (c) Irie, M. AdV.
Polym. Sci. 1993, 110, 49.
(4) (a) Ichimura, K.; Suzuki, Y.; Seki, T.; Hosoki, A.; Aoki, K. Langmuir
1988, 4, 1214. (b) Ikeda, T.; Sasaki, T.; Ichimura, K. Nature 1993, 361,
428.
(5) Atassi, Y.; Delaire, J. A.; Nakatani, K. J. Phys. Chem. 1995, 99,
16320.
(6) (a) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. J. Chem. Soc., Chem.
Commun. 1993, 1439. (b) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. Chem.
Eur. J. 1995, 1, 275. (c) Kawai, T.; Kunitake, T.; Irie, M. Chem. Lett. 1999,
905.
(7) Kahn, O. Molecular Magnetism; VCH: New York, 1993.
(8) (a) Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. Science 1996,
272, 704. (b) Gu, Z.-Z.; Sato, O.; Iyoda, T.; Hashimoto, K.; Fujishima, A.
J. Phys. Chem. 1996, 100, 18289.
Results and Discussion
Molecular Design and Synthesis. Diarylethenes undergo a
hexatriene-cyclohexadiene-type photochromic reaction (Scheme
1). They exhibit excellent photochromic performance: thermal
stability of both isomers even at 100 °C, high fatigue resistance
(>104 coloration/decoloration cycles), and very rapid response
(∼1 ps). The diarylethene family is one of the most promising
photochromic materials for applications to optoelectronics.
(9) (a) Boillot, M.-L.; Roux, C.; Audie`re, J.-P.; Dausse, A.; Zarembow-
itch, J. Inorg. Chem. 1996, 35, 3975. (b) Boillot, M.-L.; Chantraine, S.;
Zarembowitch, J.; Lallemand, J.-Y.; Prunet, J. New J. Chem. 1999, 179.
(10) Fujita, W.; Awaga, K. J. Am. Chem. Soc. 1997, 119, 4563.
(11) (a) Hamachi, K.; Matsuda, K.; Itoh, T.; Iwamura, H. Bull. Chem.
Soc. Jpn. 1998, 71, 2937. (b) Matsuda, K.; Irie, M. Chem. Lett. 2000, 16,
a preliminary communication of this paper.
10.1021/ja000605v CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/11/2000

7196 J. Am. Chem. Soc., Vol. 122, No. 30, 2000
Scheme 1. Photochromic Diarylethenes
Matsuda and Irie
There is a characteristic feature in the electronic structural
changes of diarylethenes. Scheme 2 shows the open-ring isomer
3a and closed-ring isomer 3b of the radical-substituted diaryl-
ethenes. While there is no resonant closed-shell structure for
Figure 2b. The normal bond alternation is clearly shown,
indicating this molecule is a normal Kelule´ molecule.
Scheme 2a
a Reagents and conditions: (a) dichloromethyl methyl ether, AlCl3,
nitrobenzene, 82%. (b) 2,3-dimethyl-2,3-bis(hydroxyamino)butane sul-
fate, methanol and then NaIO4, CH2Cl2, 16%.
3a, there exists 3b′ as the resonant quinoid-type closed-shell
structure for 3b. 3a is classified as a non-Kekule´ diradical, and
3b is classified as a normal Kekule´ molecule. In other words,
3a has two unpaired electrons and 3b has no unpaired electrons.
To know the exchange interaction between two spins of 3a,
the shape of two singly occupied molecular orbitals (SOMOs)
was calculated by the MNDO UHF method (Figure 2a).12 The
two SOMOs are separated in the molecule and there is no
overlap. This configuration is a typical disjoint diradical,13 such
as tetramethyleneethane, in which intramolecular radical-radical
interaction is weak. In the open-ring isomer, bond alternation
is disconnected at the 3-position of the thiophene rings. This is
the origin of the disjointed nature of the electronic configuration
of 3a.
Figure 2. (a) Shape of the two SOMOs of 3a calculated at the MNDO
UHF level. (b) Shape of HOMOs of 3b′ calculated at the MNDO RHF
level.
The electronic structural changes of radical-substituted dia-
rylethenes along with the photoisomerization is the change from
a disjointed non-Kekule´ structure to a closed-shell Kekule´
structure. It is inferred from the above consideration that the
interaction between spins in the open-ring isomer of diarylethene
is weak, while significant antiferromagnetic interaction takes
place in the closed-ring isomer. In other words, the open-ring
isomer is the “off” state and the closed-ring isomer is the “on”
state.
We chose 1,2-bis(2-methyl-1-benzothiophen-3-yl)perfluoro-
cyclopentene (1a) as a photochromic spin coupler (Scheme 1).
1a is one of the most fatigue-resistant diarylethenes.14 Nitronyl
nitroxide was chosen for the spin source, because this radical
is π-conjugative. Thus, we designed molecule 2a, which is one
of the concrete models of 3a (Scheme 1).
The MNDO RHF calculation was also carried out for the
closed-ring isomer 3b′. The shape of the HOMO is shown in
(12) Calculation and visualization of molecular orbital was performed
on a MOPAC 97 program in WinMOPAC ver. 2 of Fujitsu Co. at Chiba,
Japan.
(13) The term “disjoint” means that two SOMO of diradical are placed
in such a configuration that the two spins cannot have exchange interaction.
See: (a) Borden, W. T.; Davidson, E. R. J. Am. Chem. Soc. 1977, 99, 4587.
(b) Matsuda, K.; Iwamura, H. J. Am. Chem. Soc. 1997, 119, 7412.
(14) (a) Uchida, K.; Nakayama, Y.; Irie, M. Bull. Chem. Soc. Jpn. 1990,
63, 1311. (b) Hanazawa, M.; Sumiya, R.; Horikawa, Y.; Irie, M. J. Chem.
Soc., Chem. Commun. 1992, 206.

A Diarylethene with Two Nitronyl Nitroxides
J. Am. Chem. Soc., Vol. 122, No. 30, 2000 7197
Table 1. Quantum Yields and Conversions of Photochromic
Reactions of 1a, 1b, 2a, and 2b in Ethyl Acetate
Scheme 3
diarylethene
Φ
conversion/%
1a f 1b
1b f 1a
2a f 2b
2b f 2a
0.31a
43a
100b
100a
100b
0.28b
0.040a
0.0010b
a Measured at 313 nm. b Measured at 517 nm.
The synthesis was performed according to Scheme 3. 1a was
formylated by dichloromethyl methyl ether to give diformyl
compound 4. Then, 4 was treated with 2,3-bis(hydroxyamino)-
2,3-dimethylbutane sulfate followed by sodium periodate. 2a
was obtained and purified by column chromatography and gel
permeation chromatography (GPC). Recrystallization from
hexane-CH2Cl2 gave a dark blue plate crystal of 2a. The
structure of 2a was confirmed by ESR and mass spectroscopy
and finally by X-ray crystallography (vide infra).
irradiation, the intense absorption at 565 nm grew and after 5
min it reached the photostationary state. A clear isosbestic point
was observed at 334 nm. Then the sample was irradiated with
578-nm light for 60 min. The spectrum converted back to the
original one with retention of the isosbestic point at 334 nm.
Although the radical moiety has absorption around 550-700
nm, it did not prohibit the photochromic reaction.
Similar photochromic behavior was also observed starting
from closed-ring isomer 2b. Upon irradiation with 578-nm light,
the absorbance at 565 nm almost disappeared. The remaining
absorption is due to the radicals. After that the solution was
irradiated with 313-nm light, the absorption returned perfectly
to the initial closed-ring isomer 2b, suggesting the conversion
from 2a to 2b is almost 100%. For practical usage of
photochromic materials, high conversion is one of the important
characteristics.
The color of the solution was interconverted from blue in 2a
to red-purple in 2b. The effect of the radical moiety, which
possesses blue color, will be discussed in the next section.
Effect of Radical on Quantum Yield. It is indispensable to
investigate the quantum yields of photochromic reactions for
the design of high-sensitive and high-conversion photochromic
units. The quantum yields of both cyclization and cycloreversion
reactions of 1a and 2a and conversions in the photostationary
state were measured in ethyl acetate. The results are summarized
in Table 1.
1
The quantum yield of cyclization reaction of 2a was ∼ /8 of
the value of 1a. The cycloreversion reaction was also strongly
suppressed. This is due to the contribution of the resonant
quinoid structure 2b′ (Scheme 4). The low cycloreversion
quantum yield resulted in the high conversion from open-ring
isomer 2a to closed-ring isomer 2b. Lehn et al. reported that
the closed-ring isomer of bis-phenolic diarylethenes could be
electrochemically oxidized to the quinoid form and the photo-
chromism was locked.6b In our case, the contribution of the
quinoid structure was not so large. Therefore, a reversible
photochromic reaction took place.
Switching of Molecular Structure. Both isomers 2a and 2b
could be isolated as crystals. The crystal structure was deter-
mined by X-ray crystallographic analysis. The crystallographic
data are summarized in Table 2. Figure 4 shows ORTEP
drawings of the open-ring isomer 2a and the closed-ring isomer
2b. 2a had a twisted structure. The dihedral angle between
benzothiophene ring and perfluorocyclopentene ring was 86.1°,
suggesting disconnection of π-conjugation between the ben-
zothiophene ring and the ethene moiety. The nearest intermo-
Figure 3. UV-visible absorption spectra measured at different stages
of photochromic reaction: (a) starting from open-ring isomer 2a (1.7
× 10-5 M AcOEt solution). (1) Initial and (2) after irradiation with
313-nm light for 1, (3) 5, and (4) 10 min (5) after irradiation with
578-nm light for 5, (6) 30, and (7) 60 min. (b) Starting from closed-
ring isomer 2b (1.9 × 10-5 M AcOEt solution). (1) Initial and (2) after
irradiation with 578-nm light for 60 min (3) after irradiation with 313-
nm light for 5 min.
Photochromic Reaction. Figure 3 shows the photochromic
interconversion between 2a and 2b. The ethyl acetate solution
of 2a (1.7 × 10-5 M) was irradiated with 313-nm light. Upon
Scheme 4

7198 J. Am. Chem. Soc., Vol. 122, No. 30, 2000
Matsuda and Irie
Table 2. Crystallographic Parameters for 2a and 2b
2a
2b
formula
formula weight
crystal color
crystal system
space group
a/Å
C37H36F6N4O4S2 C38H38Cl2F6N4O4S2
778.82
863.74
black
monoclinic
P21/c
dark blue
triclinic
P1h
11.537(5)
12.256(5)
14.568(6)
85.023(8)
72.660(8)
71.030(7)
1859.3(14)
2
12.2767(18)
23.850(4)
26.549(4)
b/Å
c/Å
R/deg
â/deg
90.513(3)
γ/deg
V/Å3
7773(2)
8
Figure 5. øT-T plot for (O) 2a and (4) 2b measured at 5000 G.
Solid line represents the theoretical curve (see text).
Z
data/restraints/parameters 8638/0/476
8143/0/1001
0.1153
0.2893
Table 3. Fitting Parameters of the Magnetic Data for Diradical 2a
and 2b
R1
0.0859
0.2112
wR2
diradical
2J/kBK
θ/K
f
2a
2b
-2.2 ( 0.04
-11.6 ( 0.4
0.24 ( 0.01
-2.4 ( 0.3
0.91
1.14
indicating that the contribution of the quinoid structure is small.
The intermolecular contact of nitronyl nitroxide was 3.62 Å,
which is rather short to ignore the intermolecular interaction.
The effect of intermolecular interaction was taken into account
in the analysis of magnetic interactions shown below.
Switching of Magnetism. The magnetic susceptibilities of
2a and 2b were measured on a SQUID susceptometer in
microcrystalline form. øT-T plots are shown in Figure 5. The
data were analyzed in terms of a modified singlet-triplet two-
spin model (the Bleaney-Bowers type) in which two spins (S
) 1/2) couple antiferromagnetically within a biradical molecule
by exchange interaction J (eq 1).16 θ indicates a Weiss constant
2Ng2µB T
2
1
ømolT ) f
(1)
kB(T - θ) 3 + exp(-2J/kBT)
employed to describe the additional intermolecular interaction
by a mean field theory. Correction factor f was introduced for
correction of the experimental error. The best-fit parameters by
means of a least-squares method were 2J/kB ) -2.2 ( 0.04 K,
θ ) 0.24 ( 0.01 K, and f ) 0.91 for 2a and 2J/kB ) -11.6 (
0.4 K, θ ) -2.4 ( 0.3 K, and f ) 1.14 for 2b (Table 3).17
Although the interaction (2J/kB ) -2.2 K) between two spins
in the open-ring isomer 2a was small, spins of 2b have a
remarkable antiferromagnetic interaction (2J/kB ) -11.6 K).
We measured the magnetic susceptibility of 2b (3% w/w)
doped in poly(n-butyl methacrylate) to eliminate the contribution
of the intermolecular interaction in the closed-ring form crystal.
The measured intramolecular antiferromagnetic interaction in
the polymer dispersed system (2J/kB ) -12.5 ( 0.5 K) was
similar to that observed in the crystal. This clearly indicates
that the increase in |2J/kB| is not due to intermolecular
interaction but due to the intramolecular interaction.
The open-ring isomer 2a had a twisted molecular structure
and disjointed electronic configuration. On the other hand, the
closed-ring isomer 2b had a planar molecular structure and
nondisjointed electronic configuration. The photoinduced change
in magnetism agreed well with the expected change; the open-
ring isomer has the “off” state and the closed-ring isomer has
the “on” state.
Figure 4. ORTEP drawings of (a) 2a and (b) 2b with thermal ellipsoids
(50% probability). The hydrogen atoms are omitted for clarity. For
2b, the solvent molecule is omitted. For 2b, one of the two identical
molecules is shown.
lecular contact between oxygen atoms of nitroxides was 4.38
Å, suggesting intermolecular magnetic interaction is negligible.
The dihedral angle between the imidazoline ring and ben-
zothiophene ring was 30.0°, suggesting π-conjugation between
them. The distance between reactive carbons was 4.72 Å, which
is too long to photochemically react in the crystalline phase.
Photochromism in the crystalline phase was not observed.15
There were two crystallographically independent molecules
in the crystal of 2b. The two molecules were almost identical
except for the dihedral angle between nitronyl nitroxide and
the benzothiophene ring. The crystal contained one dichlo-
romethane per one 2b molecule. The dihedral angle between
the benzothiophene ring and perfluorocyclopentene ring was
2.6°. π-Electrons were delocalized throughout the molecule. The
bond length could be interpreted by the structure of 2b not 2b′,
(15) (a) Kobatake, S.; Yamada, T.; Uchida, K.; Kato, N.; Irie, M. J. Am.
Chem. Soc. 1999, 121, 2380. (b) Kobatake, S.; Yamada, M.; Yamada, T.;
Irie, M. J. Am. Chem. Soc. 1999, 121, 8450.
(16) Bleaney, B.; Bowers, K. D. Proc. R. Soc. London 1952, A214, 451.
(17) The diamagnetic contribution was corrected by using Pascal’s
constants. The correction for the solvent in the crystal was also performed.

A Diarylethene with Two Nitronyl Nitroxides
J. Am. Chem. Soc., Vol. 122, No. 30, 2000 7199
Figure 6. X-band ESR spectra of diradicals in solution at room temperature and in a glass matrix at cryogenic temperature. (a) 2a (1.3 mM in
benzene, room temperature, 9.32 GHz) and (b) 2b (1.6 mM in benzene, room temperature, 9.32 GHz). For both 2a and 2b, hyperfine coupling
constant |aN| and Lande´ g factor were determined as |aN| ) 3.7 G and g ) 2.006. (c) 2a (1 mM in MTHF, 8.0 K, 9.33 GHz) and (d) 2b (1 mM
in MTHF, 7.2 K, 9.33 GHz). For both 2a and 2b, zero-field splitting parameter |D/hc| ) 0.0026 cm-1
.
2a and 2b implies the distance between spins did not change
by isomerization.
The temperature dependence of the signal intensity at ∆ms
) 1 was measured and plotted as intensity vs reciprocal
temperature (Curie plot, Figure 7). While the data of 2a were
straight because of the absence of strong interaction between
spins, the data of 2b were curved due to the notable exchange
interaction. The data were consistent with the theoretical curve
derived from the exchange interaction obtained from magnetic
measurement. This result indicates that the intramolecular
interaction was switched by photochromic spin coupler.
Switching of Electrochemical Properties. The electrochemi-
cal properties of diarylethenes 1a, 1b, 2a, and 2b was studied
by means of cyclic voltammetry. The voltammograms are shown
in Figure 8 and the numerical data are summarized in Table 4.
Figure 7. Intensities of the ESR signal at the ∆ms ) 1 region for (O)
2a and (4) 2b. The solid and dotted lines indicate the theoretical curve
of 2a (2J/kB ) -2.2 K) and 2b (2J/kB ) -11.6 K), respectively.
Switching of ESR Spectra. The ESR measurement was
carried out for both 2a and 2b in benzene solution at room
temperature and MTHF solid solution at cryogenic temperature
(Figure 6). In solution at room temperature, both 2a and 2b
showed nine lines centered at g ) 2.006 with a hyperfine
coupling constant |aN| of 3.7 G due to four equivalent nitrogen
atoms, indicating that the exchange interactions between radicals
were larger than the hyperfine coupling constant |aN|. Since the
hyperfine coupling constant is very small (the energy is
equivalent to ∆E/kB ≈ 1 × 10-3 K), there exist large interactions
to give nine lines even in open-ring isomer 2a.
Table 4. Electrochemical Data for 1a, 1b, 2a, and 2b in
Acetonitrile (V vs Ag/Ag+)
diarylethene•-
diarylethene Epc
/
NN•/
diarylethene/
diarylethene
NN+ E1/2
diarylethene•+ Epa
1a
1b
2a
2b
-1.33
-1.07
+1.05
+1.40
+0.46
+0.52
The orbital diagrams are also shown in Figure 9. While the
open-ring isomer 1a did not show any remarkable oxidation or
reduction waves in the region from -2.2 to +1.6 V (vs Ag/
Ag+), the closed-ring isomer 1b showed two quasi-reversible
redox potentials at -1.33 and +1.05 V. The absorption
wavelength calculated from the HOMO-LUMO gap was 520
nm, which is in good agreement with the value obtained by
UV-visible absorption spectroscopy.14
ESR spectra of 2a at 7.2 K and 2b at 8.0 K showed broad
lines with fine structures characteristic of triplet species in the
∆ms ) 1 region for both open- and closed-ring isomers (|D/hc|
) 0.0026 cm-1). A half-field signal at the ∆ms ) 2 region was
not observed for either 2a or 2b. By point dipole approximation,
the distance between radical centers was calculated to be 10 Å
for both 2a and 2b. This distance agrees with the X-ray data.
Since D values depend on dipole-dipole interaction between
spins, not directly on exchange interaction, D values reflect
directly spatial distance. No difference in the D values between
2a had one reversible oxidation wave at +0.46 V due to the
oxidation of nitronyl nitroxide. On the other hand, 2b showed
three redox potentials at -1.07, +0.52, +1.40 V. The oxidation

7200 J. Am. Chem. Soc., Vol. 122, No. 30, 2000
Matsuda and Irie
Figure 9. Potential diagram of 1a, 1b, 2a and 2b.
changes the intramolecular magnetic interaction by photoirra-
diation. The change of magnetization was attributed to the
change of the π-system. This is the first example of reversible
switching of intramolecular magnetism. This is a potential
photoswitch applicable to logic operation circuits.
Experimental Section
A. Materials. 1H NMR spectra were recorded on a Bruker AC-
250P instrument. UV-visible spectra were recorded on a Hitachi
U-3500 spectrophotometer. Mass spectra were obtained with JEOL
JMS-HX110A instruments. Melting points are not corrected.
All reactions were performed under an atmosphere of dry argon
unless otherwise specified. The reactions were monitored by thin-layer
chromatography carried out on 0.2-mm E. Merck silica gel plates (60F-
254). Column chromatography was performed on silica gel (E. Merck,
70-230 mesh).
1,2-Bis(6-formyl-2-methyl-1-benzothiophen-3-yl)hexafluorocyclo-
pentene (4). To a stirred solution of 1,2-bis(2-methyl-1-benzothiophen-
3-yl)hexafluorocyclopentene (1a; 2.35 g, 5.02 mmol) in nitrobenzene
(25 mL) was added dichloromethyl methyl ether (7.5 mL) and
anhydrous aluminum chloride (2.8 g) at 0 °C and stirred for 15 h at
room temperature under argon atmosphere. Water was poured into the
reaction mixture and then the product was extracted with ethyl acetate,
washed with water, dried with magnesium sulfate, and concentrated.
After evaporation of nitrobenzene in a vacuum, column chromatography
(Hex:Et2O ) 3:1-1:1) gave diformyl compound 4 (2.16 g, 82%) as a
Figure 8. Cyclic voltammograms of(a) 1a, (b) 1b, (c) 2a, and (d) 2b
in acetonitrile (0.1 M TBAP) on a platinum electrode (vs Ag/Ag+).
Scan rates were 200 mV/s.
potential of the diarylethene moiety of 2b (+1.40 V) was 0.35
V more positive than that of 1b (+1.05 V). This shift is
attributable to the oxidation of the nitronyl nitroxide moiety of
2b preceding the oxidation of the diarylethene moiety.
The oxidation at nitronyl nitroxide of 2b occurred at almost
the same potential as 2a because the mixing of the SOMO of
nitronyl nitroxide and HOMO of diarylethene was negligible
because the SOMO of nitronyl nitroxide is localized at the
ONCNO moiety of the radical.18 The reduction potential of the
diarylethene moiety of 2b (-1.07 V) was 0.26 V less negative
than that of 1b (-1.33 V). This shift is attributable to the mixing
of the LUMO of diarylethene with the LUMO of nitronyl
nitroxide. The small shift of the oxidation potential of the
nitronyl nitroxide between 2a and 2b indicates that the orbital
interaction between nitronyl nitroxide and diarylethene is small.
When the radicals of larger interaction with diarylethene are
used, this shift should be larger.
1
white powder: mp 183.0-184.0 °C; H NMR (CDCl3, 250 MHz) δ
2.31 and 2.56 (s 2, 6 H), 7.68-8.20 (m, 6 H), 9.95 and 10.05 (s 2, 2
H). Anal. Calcd for C25H14F6O2S2: C, 57.25; H, 2.69. Found: C, 57.22;
H, 2.70.
1,2-Bis[6-(1-oxyl-3-oxide 4,4,5,5-tetramethylimidazolin-2-yl)-2-
methyl-1-benzothiophen-3-yl]hexafluorocyclopentene (2a). A solu-
tion of 4 (2.0 g, 3.8 mmol), 2,3-bis(hydroxyamino)-2,3-dimethylbutane
sulfate (4.75 g, 19.3 mmol), and potassium carbonate (2.85 g, 20.6
mmol) in methanol (40 mL) was refluxed for 24 h. The reaction mixture
was poured into water, extracted with ethyl acetate, washed with water,
dried over magnesium sulfate, and concentrated to give tetrahydroxyl-
amine as yellow oil. Purification was not performed.
To a solution of tetrahydroxylamine in dichloromethane (300 mL)
was added a solution of sodium periodate (2.45 g, 11.5 mmol) in water
(500 mL), and the resultant mixture was stirred for 1 h in the open air.
The organic layer was separated, washed with water, dried over
magnesium sulfate, and concentrated. Purification was performed by
column chromatography (silica, hexane:Et2O ) 1:1-0:1) followed by
GPC and then the residue was recrystallized from CH2Cl2/hexane by
the diffusion method in the dark. 2a was obtained as dark blue
microcrystalline (470 mg, 16%): mp 231.0-232.0 °C (dec); UV-vis
(AcOEt) λmax(ꢀ) 300 (sh), 309 (3.4 × 104), 359 (sh), 377 (1.6 × 104),
553 (sh), 598 (6.3 × 102), 646 (6.3 × 102), 706 (sh); ESR (benzene)
Conclusions
We have developed a photochromic system, which reversibly
(18) Kumai, R.; Matsushita, M. M.; Izuoka, A.; Sugawara, T. J. Am.
Chem. Soc. 1994, 116, 4523.

A Diarylethene with Two Nitronyl Nitroxides
J. Am. Chem. Soc., Vol. 122, No. 30, 2000 7201
1:4:10:16:19:16:10:4:1, 9 lines, g ) 2.007, |aN| ) 3.7 G; FAB HRMS
(m/z) [M + H]+ calcd for C37H37F6N4O4S2, 779.2160; found, 779.2173.
Corresponding closed-ring isomer 2b: black microcrystalline; UV-
vis (AcOEt) λmax(ꢀ) 280 (1.7 × 104), 348 (1.9 × 104), 385 (sh), 400
(2.0 × 104), 565 (1.5 × 104); ESR (benzene) 1:4:10:16:19:16:10:4:1,
9 lines, g ) 2.007, |aN| ) 3.7 G.
maintained at high vacuum by a turbomolecular/rotary pump set. The
sample was dissolved in solvent (∼1 mM) and degassed with argon
bubbling for 5 min. The ESR intensities for Curie plots in the
temperature range 7.2-50 K were measured at appropriate power
attenuation calibrated to exclude saturation effect. The temperatures
were stepped up from 7.2 to 50 K with intervals of ∼3 K.
E. Magnetic Measurement. A fine crystalline and polymer samples
(>97.5% purity checked by HPLC) were mounted in a capsule and
measured on a Quantum Design MPMS-5S SQUID susceptometer at
5000 G. Corrections for the diamagnetic contribution were made by
using Pascal’s constants.
F. Electrochemical Measurement. The measurement of electro-
chemical properties was performed on a BAS CV-50W electrochemical
analyzer. Measurements were carried out in acetonitrile solution (∼1
mM) containing tetra(n-butyl)ammonium perchlorate (TBAP, 100 mM)
as a supporting electrolyte. A three-electrode assembly (BAS) was used
which was equipped with a platinum working electrode, a platinum
coil as the counter electrode, and a Ag/Ag+ (TBAP/acetonitrile)
electrode as the reference electrode.
B. Photochemical Measurement. Absorption spectra were mea-
sured on a spectrophotometer (Hitachi U-3500). Photoirradiation
was carried out by using a USHIO 500-W superhigh-pressure mer-
cury lamp or a USHIO 500-W xenon lamp. Mercury lines of 313 and
578 nm were isolated by passing the light through a combination of
Toshiba band-pass filter (UV-D33S) or sharp cut filter (Y-48, Y-52)
and monochromator (Ritsu MC-20L).
C. Crystallography. The data collection was performed on a Bruker
SMART1000 CCD-based diffractometer (50 kV, 40 mA) with Mo KR
radiation. The data were collected as a series of ω-scan frames, each
with a width of 0.3°/frame. The crystal-to-detector distance was 5.118
cm. Crystal decay was monitored by repeating the 50 initial frames at
the end data collection and analyzing the duplicate reflections. Data
reduction was performed using SAINT software, which corrects for
Lorentz and polarization effects and decay. The cell constants were
calculated by the global refinement. The structure was solved by direct
methods using SHELXS-8619 and refined by full least-squares on F2
using SHELXL-97.20 The positions of all hydrogen atoms were
calculated geometrically and refined by the riding model. The disordered
part was refined isotropically.
Acknowledgment. This work was supported by CREST of
Japan Science and Technology Corp. and by a Grant-in Aid
for Scientific Research on Priority Area “Creation of Delocalized
Electronic Systems” (No. 12020244) from the Ministry of
Education, Science, Culture, and Sports, Japan.
D. ESR Spectroscopy. A Bruker ESP 300E spectrometer was used
to obtain X-band ESR spectra. Temperatures were controlled by an
RMC CT-470-ESR cryogenic temperature controller. The cryostat was
Supporting Information Available: X-ray structural infor-
mation on 2a and 2b (PDF). An X-ray crystallographic file
(CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
(19) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467-473.
(20) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
Refinement; Universita¨t Go¨ttingen: Go¨ttingen, 1997.
JA000605V