New photoswitching unit for magnetic interaction: Diarylethene with 2,5-bis(arylethynyl)-3-thienyl group

Article abstract of DOI:10.1021/ja053200p

Photoswitching of the intramolecular magnetic interaction was demonstrated using diarylethenes with 2,5-bis(arylethynyl)-4-methyl-3-thienyl side group. Two nitroxide radicals were placed at each end of the 2,5-bis(arylethynyl)-4- methyl-3-thienyl group. T

Full text of DOI:10.1021/ja053200p

Published on Web 09/03/2005
New Photoswitching Unit for Magnetic Interaction:
Diarylethene with 2,5-Bis(arylethynyl)-3-thienyl Group
Naoki Tanifuji,† Masahiro Irie,‡ and Kenji Matsuda*,†,‡
Contribution from the Precursory Research for Embryonic Science and Technology (PRESTO),
Japan Science and Technology Agency (JST), and Department of Chemistry and Biochemistry,
Graduate School of Engineering, Kyushu UniVersity, 6-10-1 Hakozaki, Higashi-ku,
Fukuoka 812-8581, Japan
Received May 17, 2005; E-mail: kmatsuda@cstf.kyushu-u.ac.jp
Abstract: Photoswitching of the intramolecular magnetic interaction was demonstrated using diarylethenes
with 2,5-bis(arylethynyl)-4-methyl-3-thienyl side group. Two nitroxide radicals were placed at each end of
the 2,5-bis(arylethynyl)-4-methyl-3-thienyl group. Three kinds of aryl groups, 2,5-thienylene, p-phenylene,
and m-phenylene groups, were used in the arylethynyl moiety. The diarylethene photoswitching units have
an extended π-conjugated chain on one side of the diarylethene. The photochromic reactivity was dependent
on the arylethynyl group. Diarylethenes with m-phenylene group showed an efficient photochromic reactivity.
Along with the photochromic reaction the diarylethenes showed photoswitching of an ESR spectrum
originating from the change in the magnetic interaction between two unpaired electrons. The open-ring
isomer showed stronger exchange interaction than the photogenerated closed-ring isomer. The magnetic
interaction between two radicals via the π-conjugated chain was altered by photocyclization due to the
change of the hybrid orbital at the 2-position of the thiophene ring from sp2 to sp3.
Introduction
Molecular magnetism can be photocontrolled by using spin
crossover phenomena, which are light-induced excited spin state
trapping (LIESST), light-induced thermal hysteresis (LITH), and
ligand-driven light-induced spin change (LD-LISC).10 In addi-
tion to spin crossover systems, several systems using photo-
Photochromic compounds undergo reversible isomerization
between two discrete states that have different colors by
irradiation with light of appropriate wavelengths. The bistable
molecules can be used as photoswitching units to control
chemical and physical properties of the molecular systems.1
Among various types of photochromic compounds, diarylethene
is superior to other photochromic compounds with respect to
their fatigue-resistant and thermally irreversible photochromic
performance.2 Photoswitching of diarylethene derivatives has
been applied to control not only the absorption spectrum but
also many various properties, such as liquid-crystalline phases,3
sol-gel transition,4 surface morphology of single crystals,5
fluorescence resonance energy transfer (FRET),6 fluorescence,7
electric conductivity,8 and refractive index.9
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† PRESTO, JST.
‡ Kyushu University.
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10.1021/ja053200p CCC: $30.25 © 2005 American Chemical Society

Photoswitching Unit for Magnetic Interaction
A R T I C L E S
Scheme 1
Scheme 2. Photochromism of Diarylethene Having 2,5-Arylethynyl-3-thienyl Unit
chromic units are also reported.11 Among them, the most
effective photoswitching of intramolecular magnetic interaction
has been observed by using diarylethene derivatives as photo-
switching units (Figure 1).12 It has been demonstrated that the
interaction was found to be more than 150-fold between 1a and
1b (Scheme 1).12h
In these photoswitches, radical units are placed at each side
of the diarylethene photoswitching unit and separated by an
extended π-conjugated chain. When the π-conjugated chain
length between the radicals becomes longer, both photocycliza-
tion and cycloreversion reactivities are reduced. This can be
attributed to the reduced excitation density at the central
diarylethene unit.13 The excitation density localized at the center
of both sides of the π-conjugated aryl unit such as oligothio-
phene. To solve the problem, it is necessary to develop new
switching systems, in which the excitation density at the
switching unit is not strongly reduced. The proposed new
switching molecule has its switching unit located in the middle
of the π-conjugated chain.
In this paper, we propose a new switching unit, in which
two radicals are placed in the same aryl unit and the π-conju-
gated chain is extended from 2- and 5-positions of the thiophene
ring in one aryl unit of the diarylethene. The photochromic
reactivity and magnetic switching of the new diarylethene
derivatives will be discussed.
Figure 1. Photoswitching of the intramolecular magnetic interaction.
exchange interaction between two nitronyl nitroxide radicals,
which are located at each end of diarylethene, is photoswitched
reversibly by alternate irradiation with ultraviolet and visible
light. While the conventional spin crossover system has a
bistability in the spin state of the metal center, the molecular
systems have a bistability based on molecular photoisomeriza-
tion.
The switching of molecular magnetism is based on the
molecular structure change of the diarylethene unit; the open-
ring isomer of the biradical has a disjoint configuration while
the closed-ring isomer has a resonant quinoid structure. This
difference in the electronic structure brought about the change
in the exchange interaction. The change of the exchange
Results and Discussion
Molecular Design. Scheme 2 shows the photochromic
behavior of the newly developed diarylethenes. The photocy-
clization reaction of the diarylethene unit breaks the π-conjuga-
tion in the 2,5-bis(arylethynyl)-3-thienyl unit due to the change
of the orbital hybridization from sp2 to sp3 at the 2-position of
the thiophene ring. Branda et al. used the switching of the orbital
hybridization for the control of the absorption spectrum.14
Nitronyl or imino nitroxide radicals are introduced at both ends
of the 2,5-bis(arylethynyl)thiophene π-conjugated chain. The
designed molecules 2a-5a are listed in Chart 1. The magnetic
interaction between the two radicals via the π-conjugated chain
can be altered by the photocyclization. The open-ring isomer
represents the “ON” state because the π-conjugated system is
delocalized between two radicals, while the closed-ring isomer
represents the “OFF” state because the π-conjugated system is
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S.; Ojima, T.; Akutsu, H.; Yamada, J.-i. J. Org. Chem. 2002, 67, 916-
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M. Tetrahedron Lett. 2000, 41, 2577-2580. (c) Matsuda, K.; Irie, M. J.
Am. Chem. Soc. 2000, 122, 7195-7201. (d) Matsuda, K.; Irie, M. J. Am.
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66, 8799-8803. (i) Matsuda, K.; Matsuo, M.; Mizoguti, S.; Higashiguchi,
K.; Irie, M. J. Phys. Chem. B 2002, 106, 11218-11225. (j) Takayama, K.;
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A R T I C L E S
Chart 1
Tanifuji et al.
Scheme 3 a
a Reagents and conditions: (a) I2, H5IO6, CH3COOH, H2SO4, H2O, 96%; (b) Pd(PPh3)2Cl2, CuI, trimethylsilylacetylene, Et3N, 40%; (c) n-BuLi, THF,
and then perfluorocyclopentene, 81%; (d) THF, n-BuLi, and then 9, 79%; (e) THF, n-BuLi, and then 9, 90%.
disconnected at the 2-position of the thiophene ring. This
switching direction is the opposite of the regular diarylethenes
such as 1a.
2,5-Thienylene (2a), p-phenylene (3a), and m-phenylene (4a
and 5a) were chosen as the spacer between the radicals and
reactive center. 2a, 3a, and 4a have nitronyl nitroxides and 5a
has imino nitroxides. 5a has a methoxy group at the reactive
carbon. These spacers have different features in respect to
conjugation and magnetic interaction. While 2,5-thienylene and
p-phenylene are strong antiferromagnetic spin couplers, m-
phenylene is a ferromagnetic spin coupler. The absolute value
of the exchange interaction decreases in the following order:
2,5-thienylene, p-phenylene, and m-phenylene. In other words,
the π-conjugation in 2,5-thienylene is more effective than
m-phenylene. In addition, there is a possible resonant quinoid
structure for the spacer of 2,5-thienylene and p-phenylene. The
quinoid structure plays an important role in the photoreactivity.
Synthesis. The syntheses of 2a-5a were performed according
to Schemes 3 and 4. (4-Methyl-2,5-bis(trimethylsilylethynyl)-
3-thienyl)heptafluorocyclopentene 9 was prepared from 3-bromo-
9
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Photoswitching Unit for Magnetic Interaction
A R T I C L E S
Scheme 4 a
a Reagents and conditions: (a) 20% KOH aq., MeOH, THF, 86%; (b) Pd(PPh3)2Cl2, CuI, 2-bromo-5-thiophenecarboxaldehyde, diisopropylamine, 20%;
(c) Pd(PPh3)2Cl2, CuI, 4-bromobenzaldehyde, diisopropylamine, 38%; (d) Pd(PPh3)2Cl2, CuI, 3-bromobenzaldehyde, diisopropylamine, 54%; (e) 2,3-dimethyl-
2,3-bis(hydroxyamino)butane sulfate and methanol and then NaIO4 and CH2Cl2, 9-25%.
4-methylthiophene 6 by two steps. 3-Bromo-2,4-dimethyl-5-
phenylthiopene 1015 and 3-bromo-2-methoxy-4-methyl-5-
phenylthiopene 1216 were prepared according to the method
described in the literature. Unsymmetrical diarylethenes, 2′-
methyl derivative 11 and 2′-methoxy derivative 13, were
synthesized by the coupling of 9 with lithiated 10 and 12 in
79% and 90% yield, respectively. After desilylation by KOH,
Sonogashira coupling with bromoformyl compounds gave
bisformylated diarylethenes 14-17. Formyl derivatives 14-
17 were converted into nitroxide radicals 2a-5a in 9-25%
yield. When the substituent at the 2′-position of the thiophene
ring is a methyl group, a major product of the final step was a
nitronyl nitroxide radical. But when the substituent at the 2′-
position of the thiophene ring is a methoxy group, a major
product was an imino nitroxide radical. In general the oxidation
of bishydroxylamine affords both nitronyl and imino nitroxide
radicals. Therefore, the difference in the products between
methyl- and methoxy-substituted radicals was simply the
difference in the product ratio between nitronyl and imino
nitroxide. The structures of the synthesized biradicals were
confirmed by UV, ESR, and high-resolution mass spectroscopy.
The structure of the key intermediate 11 was also confirmed
by X-ray crystallography.17
Photochromic Reactions. The photochromic reactivity of
diarylethene 2a-5a was studied in ethyl acetate solution (Figure
2). Diarylethene 2a did not show any photoreactivity by
irradiation with light of any wavelength. On the other hand,
compounds 3a-5a underwent photochromic reactions upon
alternate irradiation with UV and visible light. Before irradiation,
the solutions showed pale green, pale blue, and pale yellow for
3a, 4a, and 5a, respectively. The solutions of 4a and 5a turned
blue by irradiation with 365 nm light. This blue color is due to
the formation of the closed-ring isomers 4b and 5b. The blue
color was bleached by irradiation with 578 nm light. The color
change of the solution of 3a was much less than the solutions
of 4a and 5a. Absorption maxima, coefficients, and conversions
from the open- to the closed-ring isomers under irradiation with
(17) Crystallographic data for compound 11: C32H32F6OS2Si2, FW ) 650.89,
monoclinic P21/n, a ) 17.833(4) Å, b ) 9.664(2) Å, c ) 20.097(4) Å,
â ) 104.716(4)°, V ) 3349.8(13) Å3, Z ) 4, R1 ) 0.062 (I > 2σ), wR2)
0.157 (all data). For the detail of the structure, see Supporting Information.
(15) Kobatake, S.; Irie, M. Tetrahedron 2003, 59, 8359-8364.
(16) Fukaminato, T.; Sasaki, T.; Kawai, T.; Tamai, N.; Irie, M. J. Am. Chem.
Soc. 2004, 126, 14843-14849.
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A R T I C L E S
Tanifuji et al.
Figure 2. Absorption spectra of diarylethenes along with photochromism in ethyl acetate: (a) 2 (8.7 × 10-6 M); (b) 3 (1.3 × 10-5 M); (c) 4 (1.9 ×
10-6 M); (d) 5 (1.0 × 10-5 M). The open-ring isomer (solid line), the closed-ring isomer (dashed line) and in the photostationary state under irradiation with
365 nm light (dotted line).
Table 1. Absorption Maxima of the Open- and the Closed-Ring Isomers and the Conversion from the Open- to the Closed-Ring Isomers
under Irradiation with 365 nm Light
compound
open-ring isomer
λmax (ꢀ)/nm
closed-ring isomer
λmax (ꢀ)/nm
conversion/%
2
3
4
5
409 (3.9 × 104), 646 (450), 708 (500)
383 (4.9 × 104), 628 (420), 674 (360)
361 (5.5 × 104), 590 (500), 633 (300)
356 (3.3 × 104), 550 (sh)
N/A
608 (N/A)
0
<10
58
606 (1.6 × 104), 410 (sh)
616 (1.2 × 104), 410 (sh)
82
Table 2. Cyclization and Cycloreversion Quantum Yields of
4 and 5
and nitronyl nitroxide radical units, has a cycloreversion
quantum yield larger than a cyclization quantum yield. This is
the reason why the conversion is as low as 58%.
quantum yield
compound
ΦO C (365 nm)
ΦC O (578 nm)
f
f
In the case of compound 5, which has a methoxy group at
the reactive carbon, it has a cyclization quantum yield larger
than a cycloreversion quantum yield. The methoxy substituent
suppresses the cycloreversion reaction.19 Therefore compound
5 has a smaller cycloreversion quantum yield than that of
compound 4, resulting in the high conversion of 82%. Com-
pound 5 also has a larger cyclization quantum yield than that
of compound 4. Since the methoxy substitution is known not
to affect the cyclization quantum yield,19 the difference in
cyclization quantum yield is attributed to the difference in the
radical species. Compound 5 has two imino nitroxide radicals
instead of nitronyl nitroxide radicals. Teki et al. reported 9,10-
bis[3-(4,4,5,5-tetramethyl-1-yloxy-imidazoline-2-yl)phenyl]an-
thracene as a photoswitchable molecule for spin alignment in a
π-conjugated spin system.20 When nitronyl nitroxide groups
were used as radical sources instead of two imino nitroxides,
the photoswitching of ESR spectrum was not observed because
4
5
0.011
0.055
0.048
0.0041
365 nm light are summarized in Table 1. Thiophene-2,5-diyl
has the strongest π-conjugation so that the absorption maximum
of the open-ring isomer was the longest. The conversion from
the open- to the closed-ring isomers decreases as the π-conjuga-
tion between radicals gets stronger. The low conversion is
ascribed to the low cyclization quantum yields or the high
cycloreversion quantum yield. In the next section the relationship
between the quantum yield and the structure will be discussed.
Quantum-Yield Measurements. Cyclization and cyclo-
reversion quantum yields were measured in ethyl acetate using
1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene as a
reference.18 The quantum yields were summarized in Table 2.
Compound 4, which has a methyl group at the reactive carbon
(19) Shibata, K.; Kobatake, S.; Irie, M. Chem. Lett. 2001, 618-619.
(20) Teki, Y.; Miyamoto, S.; Nakatsuji, M.; Miura, Y. J. Am. Chem. Soc. 2001,
123, 294-305.
(18) Irie, M.; Lifka, T.; Kobatake, S.; Kato, N. J. Am. Chem. Soc. 2000, 122,
4871-4876.
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Photoswitching Unit for Magnetic Interaction
A R T I C L E S
Figure 3. Absorption spectra of 14 and 15 along with photochromic reaction: (a) 14; (b) 15. The solid line denotes the open-ring isomer, and the dotted
line denotes the spectra in the photostationary state under irradiation with 365 nm light.
Scheme 5. Possible Resonant Quinoid Structure of 2a and 3a
of the short lifetime of the excited state of the anthracene.21
With regard to the πfπ* transition band around 350 nm, the
nitronyl nitroxide radical has a larger absorption band than that
of the imino nitroxide radical. Therefore, nitronyl nitroxide
radicals are anticipated to quench the exited state more ef-
fectively than imino nitroxide radicals. In other words, the
energy transfer from the diarylethene to the nitronyl nitroxide
radicals is more efficient than to the imino nitroxide.
radical centers becomes longer, the biradical structure becomes
more stable and the quinoidal effect becomes weaker. Although
both nitronyl and imino nitroxides inherently do not have a
strong quinoidal effect because of their allyl-like structure, the
cyclization quantum yield was strongly suppressed when the
radical is nitronyl nitroxide.
Switching of Magnetic Interaction. ESR spectroscopy is
the most convenient way to detect weak changes in the magnetic
interaction. Nitronyl nitroxide has two equivalent nitrogen atoms
to give a 5-line ESR spectrum with a relative intensity of
1:2:3:2:1 and 7.5-G spacing. When two nitronyl nitroxide
radicals are magnetically coupled with exchange interaction, the
biradical gives a 9-line ESR spectrum with a relative intensity
of 1:4:10:16:10:4:1 and 3.7-G spacing.23 Imino nitroxide has a
7-line ESR spectrum for the isolated state due to the two
inequivalent nitrogen atoms. When two imino nitroxide radicals
interact, the spectrum gives 13 lines. The spectra of the specific
interaction can be simulated, so that the ESR spectral change
can be used as a probe to detect the exchange interaction.24
The changes in the ESR spectra along with photochromism
were examined in toluene solution for diarylethenes 2-5. The
The open-ring isomer 2a and 3a has a possible resonant
quinoid structure 2a′ and 3a′ as shown in Scheme 5, while 4a
and 5a cannot take such a quinoid structure. In a previous paper,
it has been reported that the resonant quinoid structure sup-
presses the photocycloreversion quantum yield, and this effect
is not discerned for the monosubstituted diarylethene.12b By
analogy, in 2a and 3a the quinoid structure in the open-ring
isomer is anticipated to suppress the cyclization reaction.12i The
low conversion of 2a and 3a is due to the contribution of the
quinoid structure. In the case of precursors 14 and 15, which
are indifferent to the quinoid structure, the photocyclization
reaction efficiently took place (Figure 3).
It has been discussed intensively whether a specific biradical
adopts a biradical or quinoid structure.22 As the distance between
(23) (a) Wautelet, P.; Moigne, J. L.; Videva, V.; Turek, P. J. Org. Chem. 2003,
68, 8025-8036. (b) Zoppellaro, G.; Enkelmann, V.; Geies, A.; Baumgarten,
M. Org. Lett. 2004, 6, 4929-4932.
(24) (a) Brie`re, R.; Dupeyre, R.-M.; Lemaire, H.; Morat, C.; Rassat, A.; Rey,
P. Bull. Soc. Chim. France 1965, 11, 3290-3297. (b) Glarum, S. H.;
Marshall, J. H. J. Chem. Phys. 1967, 47, 1374-1378.
(21) Private communication from Prof. Yoshio Teki, Osaka City University,
Japan.
(22) Guihery, N.; Maynau, D.; Malrieu, J.-P. Chem. Phys. Lett. 1996, 248, 2199-
206.
9
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A R T I C L E S
Tanifuji et al.
Figure 4. ESR spectra of biradicals measured in toluene at room temperature (9.32 GHz): (a) 2a; (b) 3a.
Figure 5. ESR spectra of biradicals measured in toluene at room temperature (9.33 GHz): (a) 4a; (b) 4b; (c) after irradiation with 365 nm light to 4a.
ESR spectra of 2a and 3a are shown in Figure 4. Both spectra
showed 9 lines, indicating that the exchange interaction between
two radicals in 2a and 3a is larger than the coupling constant
(|2J/kB| > 0.04 K). As mentioned in the previous section,
diarylethenes 2a did not show any photoreactivity and the
reactivity of 3a was also limited. Therefore, the ESR spectra of
2a and 3a did not show any spectral change upon irradiation
with UV light.
Figure 5 shows the ESR spectra of the open-ring isomer 4a,
the isolated closed-ring isomer 4b, and the spectra after
irradiation with 365 nm light to the open-ring isomer 4a. The
open-ring isomer has 9 lines, while the closed-ring isomer has
5 lines. This suggests that the exchange interaction between two
radicals is much smaller in the closed-ring isomer (|2J/kB| <
3 × 10-4 K) than in the open-ring isomer (|2J/kB| > 0.04 K).
Simulation of these spectra suggests that the change is more
than 150-fold. The small interaction in the closed-ring isomer
is attributed to the disconnection of the π-conjugated chain at
the sp3 carbon of the reactive center. In the photostationary state,
the spectrum showed distorted 9 lines. This means that the
spectrum after irradiation with UV light is the superposition of
the 9-line and 5-line spectra. The low conversion ratio by
irradiation with UV light could not cause obvious spectral
change of the open-ring isomer.
3 × 10-4 K). Upon irradiation with 578 nm light, as the
generation of the open-ring isomer 5a, a 13-line spectrum with
the ratio of 1:2:5:6:10:10:13:10:10:6:5:2:1 generated. Upon
further irradiation, the spectrum was completely converted to
the 13-line spectrum. The 13-line spectrum indicates that the
exchange interaction takes place between the two radicals
(|2J/kB| > 0.04 K). Subsequent irradiation with 365 nm light
regenerated the 7-line spectrum along with the regeneration of
the closed-ring isomer 5b. The double integral values of the
spectra at initial and photostationary state were almost identical,
indicating that there is no gain or loss of the amount of the
spins during the photochromic reaction. The difference in the
exchange interaction between the open- and the closed-ring
isomers was estimated to be larger than 150-fold. The difference
in the exchange interaction is attributed to the change of the
hybridization of the carbon atom from an sp2 to sp3 atom at the
reaction center. The interconversion between 7 and 13 lines can
be repeated 10 times without degradation.
Conclusions
Control of the photoreactivity and switching of the magnetic
interaction in diarylethenes with 2,5-bis(arylethynyl)-3-thienyl
group and nitroxide radical were studied. The photochromic
reactivity of the diarylethenes is dependent on the arylethynyl
group. Diarylethenes with m-phenylene group showed an
efficient photochromic reactivity, and diarylethenes with a
methoxy substituent showed enhanced conversion. The diaryl-
ethenes with m-phenylene groups showed photoswitching of the
ESR spectrum. As inferred from the ESR spectral changes, the
open-ring isomer showed stronger exchange interaction than the
ESR spectral change of the toluene solution containing
compound 5 was followed with keeping the sample in the ESR
cavity during irradiation with UV and visible light (Figure 6).
The photoreaction was started from the isolated closed-ring
isomer 5b. The closed-ring isomer 5b showed a 7-line spectrum,
which is a spectrum of an isolated imino nitroxide (|2J/kB| <
9
13350 J. AM. CHEM. SOC. VOL. 127, NO. 38, 2005

Photoswitching Unit for Magnetic Interaction
A R T I C L E S
room temperature and stirred for 4 h at 60 °C. The reaction mixture
was cooled to room temperature, filtered out brown solid. The solution
was poured into water, extracted with ether, washed with water, dried
over magnesium sulfate, and concentrated. Column chromatography
(silica, hexane) gave compound 8 (3.42 g, 42%) as a colorless needle:
1
mp 104.5-106.5 °C; H NMR (CDCl3, 200 MHz) δ 0.26 (s, 9 H),
0.27 (s, 9 H), 2.25 (s, 3 H). Anal. Calcd for C15H21BrSSi2: C, 48.76;
H, 5.73. Found: C, 48.78; H, 5.73.
1-[4-Methyl-2,5-bis(trimethylsilylethynyl)-3-thienyl]heptafluoro-
cyclopentene (9). n-Butyllithium in hexane (1.6 N, 4.59 mL, 7.04
mmol) was added to a solution of bromothiophene derivative 8 (2.59
g, 7.0 mmol) in THF (200 mL) at -78 °C under argon atmosphere.
After the mixture was stirred for 30 min, perfluorocyclopentene (4.58
g, 21 mmol) was added in one portion. The solution was allowed to
warm to 10 °C with stirring. Water was added to the reaction mixture.
The reaction mixture was extracted with ether, washed with water, dried
over magnesium sulfate, and concentrated. Column chromatography
(silica, hexane) gave compound 9 (2.75 g, 79%) as a colorless wax:
1H NMR (CDCl3, 200 MHz) δ 0.21 (s, 9 H), 0.26 (s, 9 H), 2.21 (s, 3
H). Anal. Calcd for C20H21F7SSi2: C, 49.77; H, 4.39. Found: C, 50.07;
H, 4.50.
1-[2,4-Dimethyl-5-phenyl-3-thienyl]-2-[4-methyl-2,5-bis(trimeth-
ylsilylethynyl)-3-thienyl]hexafluorocyclopentene (11). n-Butyllithium
in hexane (1.6 N, 3.03 mL, 4.84 mmol) was added to a solution of
3-bromo-2,4-dimethyl-5-phenylthiophene 10 (2.59 g, 4.4 mmol) in THF
(25 mL) at -78 °C under argon atmosphere. After the mixture was
stirred for 30 min, a solution of monosubstituted perfluorocyclopentene
9 (1.93 g, 3.95 mmol) in THF (10 mL) was added dropwise. The
solution was allowed to warm to 10 °C with stirring. Water was added
to the reaction mixture. The reaction mixture was extracted with ether,
washed with water, dried over magnesium sulfate, and concentrated.
Column chromatography (silica, hexane/CH2Cl2 ) 10:1) gave com-
pound 11 (2.35 g, 76%) as a colorless powder: mp 57.5-59.5 °C; 1H
NMR (CDCl3, 200 MHz) δ 0.09-0.15 (m, 18 H), 1.95-2.52 (m, 9
H), 7.24-7.44 (m, 5 H). Anal. Calcd for C32H32F6S2Si2: C, 59.05; H,
4.96. Found: C, 58.80; H, 4.99.
1-(2-Methoxy-4-methyl-5-phenyl-3-thienyl)-2-(4-methyl-2,5-bis-
(trimethylsilylethynyl)-3-thienyl)hexafluorocyclopentene (13). n-
Butyllithium in hexane (1.6 N, 3.03 mL, 4.84 mmol) was added to a
solution of 3-bromo-2-methoxy-4-methyl-5-phenyl thiophene 12 (566
mg, 2.0 mmol) in THF (10 mL) at -78 °C under argon atmosphere.
After 30 min, the mixture was transferred to syringe and added dropwise
to a solution of 9 (977 mg, 2.0 mmol) in THF (10 mL) at -78 °C
under argon atmosphere. The solution was allowed to warm to 10 °C
with stirring. Water was added to the reaction mixture. The reaction
mixture was extracted with ether, washed with water, dried over
magnesium sulfate, and concentrated. Column chromatography (silica,
hexane:ethyl acetate ) 10:1) gave compound 13 (1.19 g, 90%) as
colorless powder: mp 114.0-115.5 °C; 1H NMR(CDCl3, 200 MHz) δ
0.19 (s, 9 H), 0.24 (s, 9 H), 2.07 (s, 3 H), 2.14 (s, 3 H), 3.85 (s, 3 H),
7.25-7.42 (m, 5 H). Anal. Calcd for C32H32F6OS2Si2: C, 57.63; H,
4.84. Found: C, 57.83 H, 4.91.
Figure 6. ESR spectra of biradical 5 along with photochromic reaction
measured in toluene at room temperature (9.32 GHz): (a) the closed-ring
isomer 5b; (b) after irradiation with 578 nm light for 5 min; (c) for 7 min;
(d) for 10 min, which is identical with the spectrum of 5a; (e) after irradiation
with 365 nm light for 1 min; (f) for 3 min; (g) for 5 min, which is identical
with the spectrum of 5b.
photogenerated closed-ring isomer. The magnetic interaction
between two radicals via the π-conjugated chain was altered
by photocyclization due to the change of the hybrid orbital at
the 2-position of the thiophene ring from sp2 to sp3.
Experimental Section
A. Materials. IR spectra were recorded on a Perkin-Elmer Spectrum
1
One instrument by an ATR method. H NMR spectra were recorded
on a Varian Gemini 200 instrument. UV-vis spectra were recorded
on a Hitachi U-3310 spectrophotometer. Mass spectra were obtained
on a JEOL JMS-GCmate II. Melting points were not corrected.
All reactions were monitored by thin-layer chromatography carried
out on 0.2-mm E. Merck silica gel plates (60F-254). Column chroma-
tography was performed on silica gel (Kanto, 63-210 mesh).
3-Bromo-2,5-diiodo-4-methylthiophene (7). To a solution of
3-bromo-4-methylthiophene 6 (5.0 g, 28.2 mmol) in acetic acid (30
mL) were added water (22 mL), concentrated H2SO4 (5.2 mL),
orthoperiodic acid (2.0 g, 8.8 mmol), and iodine (6.76 g, 26.6 mmol).
The reaction mixture was refluxed for 3 h and cooled to room
temperature. After the addition of ice water, the reaction mixture was
neutralized with sodium hydroxide, poured into water, extracted with
ethyl acetate, washed with water, dried over magnesium sulfate, and
concentrated. Column chromatography (silica, hexane/ethyl acetate )
5:1) gave compound 7 (11.19 g, 92%) as a colorless needle: mp 79.5-
80.5 °C; 1H NMR (CDCl3, 200 MHz) δ 2.34 (s, 3 H). Anal. Calcd for
C6H3BrI2S: C, 14.00; H, 0.71. Found: C, 14.05; H, 0.70.
1-[2,5-Bis(5-formyl-2-thienylethynyl)-4-methyl-3-thienyl]-2-[2,4-
dimethyl-5-phenyl-3-thienyl]hexafluorocyclopentene (14). To a solu-
tion of diarylethene 11 (651 mg, 1.0 mmol) in THF (20 mL) and
methanol (10 mL) was added potassium hydroxide in water (20 wt %,
0.25 mL) with stirring. After the mixture was stirred for 30 min, the
reaction mixture was extracted with ether, washed with water, dried
over magnesium sulfate, and concentrated. Column chromatography
(silica, hexane) gave the desilylated compound (461 mg, 86%) as a
dark brown wax.
3-Bromo-4-methyl-2,5-bis(trimethylsilylethynyl)thiophene (8). To
a solution of diiodinated compound 7 (9.43 g, 22 mmol) in triethylamine
(40 mL) were added Pd(Ph3P)2Cl2 (0.93 g, 1.3 mmol) and CuI (0.34 g,
1.8 mmol). A solution of trimethylsilylacetylene (4.32 g, 44 mmol) in
triethylamine (440 mL) was added dropwise to the reaction mixture at
A solution of desilylated compound (461 mg, 0.86 mmol) in
diisopropylamine (20 mL) was added to a solution of 2-bromo-5-
thiophenecarboxaldehyde (172 mg, 0.9 mmol), Pd(PPh3)2Cl2 (151 mg,
0.22 mmol), and copper iodide (41 mg, 0.22 mmol) in diisopropylamine
(10 mL) at room temperature under argon atmosphere and then stirred
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J. AM. CHEM. SOC. VOL. 127, NO. 38, 2005 13351

A R T I C L E S
Tanifuji et al.
for 5 h at 60 °C. The reaction mixture was cooled to room temperature,
and a brown solid was filtered out. The solution was poured into water,
extracted with ether, washed with water, dried over magnesium sulfate,
and concentrated. Column chromatography (silica, hexane/ethyl acetate
) 3:2) gave compound 14 (129 g, 20%) as a light green powder: mp
383 (4.9 × 104), 628 (420), 674 (360) nm. FAB HRMS (m/z) [M+2H]+
Calcd for C48H42F6N4O4S4: 970.3021. Found: 970.3047.
1-[2,5-Bis(3-formylphenylethynyl)-4-methyl-3-thienyl]-2-[2,4-di-
methyl-5-phenyl-3-thienyl]hexafluorocyclopentene (16). Desilylated
diarylethene, which was prepared from TMS-protected 11 (268 mg,
0.5 mmol) by the same procedure of 14, in diisopropylamine (25 mL)
was added to a solution of 3-iodobenzaldehyde (348 mg, 1.5 mmol),
Pd(PPh3)2Cl2 (70.2 mg, 0.1 mmol) and copper iodide (19 mg, 0.1 mmol)
in diisopropylamine (20 mL) at room temperature under argon
atmosphere and then stirred for 5 h at 60 °C. The reaction mixture was
cooled to room temperature, filtered out brown solid. The solution was
poured into water, extracted with ether, washed with water, dried over
magnesium sulfate, and concentrated. Column chromatography (silica,
hexane:ethyl acetate ) 3:1) gave compound 16 (196 g, 54%) as a light
green powder: mp 51.5-53.5 °C; 1H NMR (CDCl3, 200 MHz) δ 2.08-
2.35 (m, 6 H), 2.38 (s, 3 H), 7.22-7.44 (m, 5 H), 7.56-7.74 (m, 4 H),
7.84-7.94 (m, 4 H), 10.00-10.10 (m, 2 H); FAB HRMS [M]+ Calcd
For C40H24F6O2S2: 714.1122. Found 714.1131.
1
60.5-62.5 °C; H NMR (CDCl3, 200 MHz) δ 2.05-2.55 (m, 9 H),
7.20-7.50 (m, 7 H), 7.69 (d, J ) 4.2 Hz, 2 H), 9.89 (s, 2 H). FAB
HRMS [M]+ Calcd For C36H20F6O2S4: 726.0250. Found 726.0231.
1-[2,4-Dimethyl-5-phenyl-3-thienyl]-2-[4-methyl-2,5-bis(5-(1-oxyl-
3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)thienylethynyl)-3-thienyl]-
hexafluorocyclopentene (2a). A solution of bisformylated derivative
14 (129 mg, 0.18 mmol), 2,3-bis(hydroxyamino)-2,3-dimethylbutane
sulfate (262 mg, 1.07 mmol), and potassium carbonate (152 mg, 1.10
mmol) in dry methanol (10 mL) and benzene (5 mL) was refluxed for
18 h. The reaction mixture was poured into water, extracted with ethyl
acetate, washed with water, dried over magnesium sulfate, and
concentrated to give tetrahydroxylamine as an orange solid. Purification
was not performed.
To a solution of tetrahydroxylamine in dichloromethane (10 mL)
was added a solution of sodium periodate (34 mg, 0.16 mmol) in water
(20 mL). The mixture was stirred for 30 min in the open air. The organic
layer was separated, washed with water, dried over magnesium sulfate,
and concentrated. Purification was performed by column chromatog-
raphy (silica, CH2Cl2/ethyl acetate ) 100:1). 2a was obtained as a green
amorphous solid (15 mg, 9%): mp 98.5-100.5 °C; IR (Ge ATR) 2953,
2932, 2925, 1371, 1275, 1134 cm-1; ESR (toluene) 1:4:10:16:19:16:
10:4:1, 9 lines, g ) 2.0065, aN ) 3.7 G (open-ring isomer 2a); UV-
vis (AcOEt) λmax(ꢀ) 409 (3.9 × 104), 646 (450), 708 (500) nm. FAB
HRMS (m/z) [M + 2]+ Calcd for C48H44F6N4O4S4: 982.2150. Found:
982.2166.
1-[2,4-Dimethyl-5-phenyl-3-thienyl]-2-[2,5-bis(3-(1-oxyl-3-oxide-
4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)-4-methyl-3-thien-
yl]hexafluorocyclopentene (4a). A solution of bisformylated diaryl-
ethene 16 (143 mg, 0.2 mmol), 2,3-bis(hydroxyamino)-2,3-dimethylbutane
sulfate (296 mg, 1.2 mmol), and potassium carbonate (180 mg, 1.3
mmol) in dry methanol (10 mL) and benzene (5 mL) was refluxed for
18 h. The reaction mixture was poured into water, extracted with ethyl
acetate, washed with water, dried over magnesium sulfate, and
concentrated to give tetrahydroxylamine as an orange solid. Purification
was not performed.
To a solution of tetrahydroxylamine in dichloromethane (10 mL)
was added a solution of sodium periodate (150 mg, 0.7 mmol) in water
(20 mL). The mixture was stirred for 30 min in the open air. The organic
layer was separated, washed with water, dried over magnesium sulfate,
and concentrated. Purification was performed by column chromatog-
raphy (silica, CH2Cl2/ethyl acetate ) 100:1). 4a was obtained as a dark-
blue amorphous solid (31 mg, 16%): mp 127.5-129 °C; IR (Ge ATR)
2987, 2929, 2853, 1363, 1274, 1143 cm-1; ESR (toluene) 1:4:10:16:
19:16:10:4:1, 9 lines (open isomer 4a), g ) 2.0065, aN ) 3.7 G; UV-
vis (AcOEt) λmax(ꢀ) 361 (5.5 × 104), 590 (500), 633 (300) nm. FAB
HRMS (m/z) [M + 2H]+ Calcd for C48H42F6N4O4S4: 970.3021.
Found: 970.3025.
1-[2,5-Bis(4-formylphenylethynyl)-4-methyl-3-thienyl]-2-[2,4-di-
methyl-5-phenyl-3-thienyl]hexafluorocyclopentene (15). Desilylated
diarylethene, which was prepared from TMS-protected 11 (198 mg,
0.37 mmol) according to the same procedure for 14, in diisopropylamine
(20 mL) was added to a solution of 4-bromobenzaldehyde (205 mg,
1.11 mmol), Pd(PPh3)2Cl2 (52 mg, 0.07 mmol), and copper iodide (10
mg, 0.07 mmol) in diisopropylamine (20 mL) at room temperature under
argon atmosphere and then stirred for 5 h at 60 °C. The reaction mixture
was cooled to room temperature, and a brown solid was filtered out.
The solution was poured into water, extracted with ether, washed with
water, dried over magnesium sulfate, and concentrated. Column
chromatography (silica, hexane/ethyl acetate ) 4:1) gave compound
1-[2,5-Bis(3-formylphenyletynyl)-4-methyl-3-thienyl]-2-(2-meth-
oxy-4-methyl-5-phenyl-3-thienyl)hexafluorocyclopentene (17). A
solution of desilylated compound, which was prepared from 13 (551.
mg, 1.0 mmol) by the same synthetic method of 14, in diisopropylamine
(50 mL) was added dropwise to a solution of 4-iodobenzaldehyde (945
mg, 3.0 mmol), Pd(PPh3)2Cl2 (140 mg, 0.2 mmol), and copper iodide
(38 mg, 0.2 mmol) in diisopropylamine (50 mL) at room temperature
under argon atmosphere and then stirred for 12 h. The reaction mixture
was filtered out to obtain a brown solid. The solution was poured into
water, extracted with ether, washed with water, dried over magnesium
sulfate, and concentrated. Column chromatography (silica, CH2Cl2) gave
compound 17 (310 mg, 42%) as a green powder: mp 173.5-175.5
°C; 1H NMR (CDCl3, 200 MHz) δ 2.10 (s, 3H), 2.24 (s, 3H), 7.78 (s,
1H), 3.78 (s, 3H), 7.22-7.46 (m, 3H), 7.48-8.05 (m, 10H), 9.98 (s,
1H), 10.02 (s, 1H). Anal. Calcd for C40H24F6O3S2: C, 66.12; H, 3.52.
Found: C, 65.84; H, 3.54.
1
15 (100 mg, 38%) as a light green powder: mp 80.0-81.5 °C; H
NMR (CDCl3, 200 MHz) δ 2.00-2.56 (m, 9H), 7.20-7.50 (m, 5H),
7.50-7-98 (m, 8H), 10.03 (s, 2H). FAB HRMS [M]+ Calcd For
C40H24F6O2S2: 714.1122. Found 714.1117.
1-[2,4-Dimethyl-5-phenyl-3-thyenyl]-2-[2,5-bis(4-(1-oxyl-3-oxide-
4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)-4-methyl-3-thien-
yl]hexafluorocyclopentene (3a). A solution of bisformylated diaryl-
ethene 15 (100 mg, 0.14 mmol), 2,3-bis(hydroxyamino)-2,3-di-
methylbutane sulfate (208 mg, 1.07 mmol), and potassium carbonate
(124 mg, 0.9 mmol) in dry methanol (10 mL) and benzene (5 mL)
was refluxed for 18 h. The reaction mixture was poured into water,
extracted with ethyl acetate, washed with water, dried over magnesium
sulfate, and concentrated to give tetrahydroxylamine as an orange solid.
Purification was not performed.
To a solution of tetrahydroxylamine in dichloromethane (10 mL)
was added a solution of sodium periodate (107 mg, 0.5 mmol) in water
(20 mL). The mixture was stirred for 30 min in the open air. The organic
layer was separated, washed with water, dried over magnesium sulfate,
and concentrated. Purification was performed by column chromatog-
raphy (silica, CH2Cl2:ethyl acetate ) 100:1). 3 was obtained as a dark-
green wax (35 mg, 25%): IR (Ge ATR) 2676, 2925, 2853, 1368, 1275,
1140 cm-1; ESR (toluene) 1:4:10:16:19:16:10:4:1, nine lines (open-
ring isomer 3a), g ) 2.0065, aN ) 3.7 G; UV-vis (AcOEt) λmax(ꢀ)
1-[2-Methoxy-4-methyl-5-phenyl-3-thienyl]-2-[2,5-bis(3-(1-oxyl-
4,4,5,5-tetramethylimidazolin-2-yl)phenyl)-4-methyl-3-thienyl]hexaflu-
orocyclopentene (5a). A solution of bisformylated diarylethene 17 (600
mg, 0.84 mmol), 2,3-bis(hydroxyamino)-2,3-dimethylbutane sulfate
(1.24 g, 5.02 mmol), and potassium carbonate (760 mg, 5.50 mmol) in
dry MeOH (20 mL) was refluxed for 72 h. The reaction mixture was
poured into water, extracted with ethyl acetate, washed with water,
dried over magnesium sulfate, and concentrated to give tetrahydrox-
ylamine as an orange solid. Purification was not performed.
9
13352 J. AM. CHEM. SOC. VOL. 127, NO. 38, 2005

Photoswitching Unit for Magnetic Interaction
A R T I C L E S
To a solution of tetrahydroxylamine in dichloromethane (10 mL)
was added a solution of sodium periodate (718 mg, 3.36 mmol) in
water (20 mL). The mixture was stirred for 2 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, CH2Cl2/ethyl acetate ) 3:2). 5a was obtained
as a yellow brown solid (178 mg, 22%): mp 98.5-100.5 °C; IR (Ge
ATR) 2973, 2931, 2864, 1274, 1140 cm-1; ESR (toluene) 1:2:5:6:10:
10:13:10:10:6:5:2:1, 13 lines, g ) 2.0065, aN ) 9.0 G, 4.5 G; UV-vis
(AcOEt) λmax(ꢀ) 356 (3.3 × 104), 550 (sh) nm. FAB HRMS (m/z)
[M + 2H]+ Calcd for C52H48F6N4O3S2: 954.3072. Found: 954.3078.
B. Photochemical Measurements. Absorption spectra were mea-
sured on a spectrophotometer (Hitachi U-3500). Photoirradiation was
carried out by using a USHIO 500 W super high-pressure mercury
lamp or a USHIO 500-W xenon lamp with a combination of an optical
filter and monochromator (Ritsu MC-20L).
Conditions for the separation of the isomers 4a and 4b were as
follows: Pump, Hitachi L-2130; Detector, Hitachi L-2420; Column,
Wakosil5SIL (Wako Chemical) 250-3 mm; Eluent, hexane/AcOEt )
1:1.
C. ESR Measurements. A Bruker ESP 300E spectrometer was used
to obtain X-band ESR spectra. The sample was dissolved in toluene
and degassed with Ar bubbling for 5 min.
Acknowledgment. This work was supported by PRESTO,
JST and by a Grant-in-Aid for Scientific Research (S) (No.
15105006) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Supporting Information Available: Crystal Structures of 11
(PDF). X-ray structural data for 11 (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
Conditions for the separation of the isomers 3a and 3b were as
follows: Pump, Hitachi L-2130; Detector, Hitachi L-2420; Column,
Wakosil5SIL (Wako Chemical) 250-3 mm; Eluent, hexane/AcOEt )
98.5:1.5.
JA053200P
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J. AM. CHEM. SOC. VOL. 127, NO. 38, 2005 13353