Reversed photoswitching of intramolecular magnetic interaction using a photochromic Bis(2-thienyl)ethene spin coupler

Article abstract of DOI:10.1021/jp026562h

Bis(3-hienyl)- and bis(2-thienyl)ethenes having nitronyl nitroxide radicals at both ends of the molecules were synthesized. Bis(3-thienyl)ethene diradicals underwent a reversible photochromic reaction by irradiation with UV and visible light. Photoswitching of intramolecular magnetic interaction was proved by EPR spectroscopy. By choosing an appropriate spacer, the intramolecular exchange interaction in the closed-ring isomer was detected to be more than 150 times larger than that in the open-ring isomer. In the case of bis(2-thienyl)ethene diradicals, the photocyclization reaction was prohibited, and only the cycloreversion reaction took place by irradiation with visible light. The closed-ring isomers of the diradicals were synthesized from the closed-ring isomers of the precursors. From the analysis of the EPR spectra of the system with an appropriate spacer, it was found that on/off switching behavior was reversed by changing the substitution position of the thiophene rings to the ethene moiety from the 3- to the 2-position. The open-ring isomer had a larger interaction than the closed-ring isomer. These results were rationally explained by the switching of the connectivity of the ?? bonds.

Full text of DOI:10.1021/jp026562h

11218
J. Phys. Chem. B 2002, 106, 11218-11225
Reversed Photoswitching of Intramolecular Magnetic Interaction Using A Photochromic
Bis(2-thienyl)ethene Spin Coupler
Kenji Matsuda,* Mitsuyoshi Matsuo, Shinji Mizoguti, Kenji Higashiguchi, and Masahiro Irie*
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: July 18, 2002; In Final Form: August 24, 2002
Bis(3-thienyl)- and bis(2-thienyl)ethenes having nitronyl nitroxide radicals at both ends of the molecules
were synthesized. Bis(3-thienyl)ethene diradicals underwent a reversible photochromic reaction by irradiation
with UV and visible light. Photoswitching of intramolecular magnetic interaction was proved by EPR
spectroscopy. By choosing an appropriate spacer, the intramolecular exchange interaction in the closed-ring
isomer was detected to be more than 150 times larger than that in the open-ring isomer. In the case of bis-
(2-thienyl)ethene diradicals, the photocyclization reaction was prohibited, and only the cycloreversion reaction
took place by irradiation with visible light. The closed-ring isomers of the diradicals were synthesized from
the closed-ring isomers of the precursors. From the analysis of the EPR spectra of the system with an appropriate
spacer, it was found that on/off switching behavior was reversed by changing the substitution position of the
thiophene rings to the ethene moiety from the 3- to the 2-position. The open-ring isomer had a larger interaction
than the closed-ring isomer. These results were rationally explained by the switching of the connectivity of
the π bonds.
Introduction
The use of photochromic compounds to photocontrol the
physical properties of materials has started to attract the interest
of many chemists.1 Photochromic compounds are potential
molecular-scale photoswitches. Among these compounds, di-
arylethenes are the most promising for several applications
because of their fatigue-resistant and thermally irreversible
photochromic performance.2 The open-ring and closed-ring
isomers of the diarylethenes differ from each other not only in
their absorption spectra but also in various physical and chemical
properties such as geometrical structure, refractive index,
dielectric constant, and oxidation/reduction potentials.
Figure 1. Photoswitching of the magnetic interaction.
In this work, we have synthesized bis(3-thienyl)- and bis(2-
thienyl)ethenes having nitronyl nitroxide radicals at both ends
of the molecule to reveal the effect of the substitution position
of the thiophene rings to the ethene moiety on the photoreactivity
and switching of the magnetic interaction.
When an unpaired electron is placed at both ends of a
π-conjugated chain, the two spins of the unpaired electrons
interact magnetically.3 On the basis of this principle, several
kinds of molecule-based magnetic materials have been synthe-
sized. Among them, photofunctionalized magnetic materials
have attracted attention in recent years.4 If a π-conjugated chain
length can be switched using a photochromic spin coupler, then
the magnetic interaction is expected to be controlled by
irradiation with light. For this purpose, diarylethenes are very
effective because the π-conjugated chain length is switched by
photoirradiation. π systems of the two aryl rings are separated
in the open-ring isomer, whereas the π system is delocalized
throughout the molecule in the closed-ring isomer. On the basis
of this idea, we have demonstrated the photoswitching of
intramolecular magnetic interaction by several detection methods
(i.e., temperature dependence of the magnetic susceptibilities,
temperature dependence of the EPR signal intensities at
cryogenic temperatures, and EPR spectral changes at room
temperature) (Figure 1).5
Results and Discussion
Molecular Design and Synthesis. We chose two kinds of
diarylethenes as switching units. Scheme 1 shows the two kinds
of diarylethene: 1,2-bis(2,4-dimethyl-3-thienyl)perfluorocyclo-
pentene (1a) and 1,2-bis(3,4-dimethyl-2-thienyl)perfluorocy-
clopentene (2a). The thiophene rings are substituted onto the
perfluorocyclopentene ring at the 3-position in 1a and at the
2-position in 2a.6
In the case of bis(3-thienyl)ethenes, the bond alternation is
discontinued in the open-ring isomer, but in the closed-ring
isomer, the π electron is delocalized throughout the molecule.
So far we have reported the photoswitching of the magnetic
interaction using bis(3-thienyl)ethenes as the spin coupler.5
When two unpaired electrons are placed at both 5-positions of
the thiophene rings, the magnetic interaction in the closed-ring
isomer is much larger than that in the open-ring isomer. In other
words, the open-ring isomer is in the OFF state because of the
disconnection of the π system, and the closed-ring isomer is in
the ON state because of the delocalization of the π conjugated
system.
Because the magnetic interaction depends on the π conjuga-
tion length of the coupler, the on/off switching is regulated by
cyclization/cycloreversion reactions of the diarylethene units.
The situation is reversed when thiophene rings are substituted
onto the ethene moiety at the 2-position. The bond alternation
is continued throughout the molecule in the open-ring isomer,
whereas in the closed-ring isomer, two aryl rings are separated
* Corresponding authors. E-mail: kmatsuda@cstf.kyushu-u.ac.jp; irie@
cstf.kyushu-u.ac.jp.
10.1021/jp026562h CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/05/2002

Reversed Photoswitching of Magnetic Interaction
J. Phys. Chem. B, Vol. 106, No. 43, 2002 11219
SCHEME 1: Photochromism of Bis(3-thienyl)ethene 1a
and Bis(2-thienyl)ethene 2a
The synthesis was performed according to Scheme 2. 5a and
6a were synthesized from 1,2-bis(2,4-dimethyl-5-iodo-3-thi-
enyl)hexafluorocyclopentene (9). Suzuki coupling with 4-formyl-
phenylboronic acid gave diformyl compound 10. Transformation
of 9 to diboronic acid followed by Suzuki coupling with
4-formyl-4′-iodobiphenyl gave diformyl compound 11. Con-
densation of diformyl compounds with 2,3-bis(hydroxyamino)-
2,3-dimethylbutane sulfate and succeeding oxidation gave
nitronyl nitroxides 5a and 6a. 7a and 8a were synthesized from
1,2-bis(3,4-dimethyl-5-iodo-2-thienyl)hexafluorocyclopentene (12).
Suzuki coupling with 4-formylphenylboronic acid gave diformyl
compound 13a. Transformation of 12 to diboronic acid followed
by Suzuki coupling with 4-formyl-4′-iodobiphenyl gave diformyl
compound 14a. The same treatment was employed for formyl
compounds to convert nitronyl nitroxide radicals 7a and 8a.
For all compounds, the structures were confirmed by NMR,
EPR, and then elemental analysis or mass spectrometry.
by the sp3 carbon and the sulfur atom. The magnetic interaction
between the two unpaired electrons at the 5-positions in the
closed-ring isomer is expected to become much smaller than
that in the open-ring isomer. In other words, the open-ring
isomer is in the ON state, and the closed-ring isomer is in the
OFF state.
Photochromic Reactions. 5a and 6a underwent photochro-
mic reactions in ethyl acetate solution. Upon irradiation with
UV light, the pale-blue solution turned blue-purple with the
retention of isosbestic points at 337 and 341 nm (Figure 2).
The color change is due to the formation of closed-ring isomers
5b and 6b. This blue-purple color disappeared upon irradiation
with visible light. The absorption maximum of the closed-ring
isomer 6b was 583 nm, which is 5 nm shorter than that of 5b.
A hypsochromic shift with an increase in the π-conjugated chain
length was observed. The conversion in the photostationary state
under irradiation with UV light was 76% for 5 and 48% for 6.
The conversion was lowered with the increase in the π-conju-
gated chain length. Similar phenomena were also observed in
3a and 4a and have been discussed in detail.5e
The nitronyl nitroxide radical was chosen for the spin source
because this radical is π-conjugative. For the detection of
magnetic switching, EPR spectroscopy was employed. Nitronyl
nitroxide itself has two equivalent nitrogen atoms to give a five-
line EPR spectrum with relative intensities of 1:2:3:2:1 and a
7.5-G spacing. When two nitronyl nitroxides are magnetically
coupled with exchange interaction, the diradical gives a nine-
line EPR spectrum with relative intensities of 1:4:10:16:19:16:
10:4:1 and a 3.7-G spacing. If the exchange interaction is smaller
than the hyperfine coupling in the diradical, two nitroxide
radicals are magnetically independent and give the same
spectrum as the isolated monoradical. In intermediate situations,
Although bis(3-thienyl)ethene 5a and 6a showed reversible
photochromic reactions, bis(2-thienyl)ethene 7a and 8a did not
show any photoreactivity. Their formyl precursors 13a and 14a,
in which there is no radical substituent, underwent normal
photochromic reactions. The radical substituents are considered
to suppress photochromic reactivity. Therefore, the closed-ring
isomers 7b and 8b were synthesized from the closed-ring
isomers of the formyl precursors 13b and 14b (Scheme 3). The
synthesized closed-ring isomers 7b and 8b underwent photo-
cycloreversion reactions upon irradiation with visible light. The
orange closed-ring isomers turned to pale green open-ring
isomers. The bleached spectra were identical to the spectra of
the corresponding open-ring isomers 7a and 8a. Figure 3 showed
the absorption spectra of 7 and 8 before and after the cyclo-
reversion reaction. The open-ring isomers of diarylethenes 7a
and 8a had absorption bands at longer wavelengths than those
of the corresponding open-ring isomers of bis(3-thienyl)ethenes
5a and 6a. However, the absorption bands of the closed-ring
isomers 7b and 8b showed hypsochromic shifts in comparison
with those of the corresponding closed-ring isomers of bis(3-
thienyl)ethenes 5b and 6b.
7
the spectrum becomes complex.
We have already reported the photoswitching of the intramo-
lecular magnetic interaction in 3a and 4a. 4a has p-phenylene
spacers at both ends of the molecule, which modifies the
exchange interaction. Any photoinduced EPR spectral change
was not observed in 3a, whereas in 4a, an obvious photoinduced
EPR spectral change was observed by alternate irradiation with
UV and visible light.5c The π-conjugated chain length in 3a is
too short, and the exchange interaction is strong enough even
in the open-ring isomer state. However, the chain length of 4a
is long enough for us to detect the exchange interaction.
Therefore, we have designed 5a-8a, whose chain lengths are
all longer than 3a, to investigate the photoswitching of the
magnetic interaction by EPR spectroscopy.
As discussed in previous papers, the radical-containing closed-
ring isomers of bis(3-thienyl)ethenes have a resonant quinoid
structure.5b,e For example, the closed-ring isomer 5b has the
resonant quinoid structure 5b′, as shown in Scheme 4. The
contribution of the quinoid structure stabilizes the closed-ring
isomer and decreases the cycloreversion quantum yield. This
effect gets weaker when the π-conjugated chain length becomes
longer. The decrease in conversion in the photostationary state
and the hypsochromic shift of the absorption maxima with the
increase in the chain length from 5b to 6b can be explained as
the decrease of the contribution of the quinoid structure. In the
case of bis(2-thienyl)ethene, the open-ring isomer has the

11220 J. Phys. Chem. B, Vol. 106, No. 43, 2002
Matsuda et al.
a
SCHEME 2
a Reagents and conditions: (a) Pd(PPh3)4, 4-formylphenylboronic acid, Na2CO3, THF, and H2O, 78-83%. (b) n-BuLi, B(OBu)3 and then Pd(PPh3)4,
4-formyl-4′-iodobiphenyl, Na2CO3, THF, and H2O, 21-26%. (c) 2,3-Dimethyl-2,3-bis(hydroxyamino)butane sulfate and methanol and then NaIO4
and CH2Cl2, 6-35%.
resonant quinoid structure. For example, the open-ring isomer
7a has the resonant quinoid structure 7a′, as shown in Scheme
4. However, the formyl precursor 13a does not have such a
resonant quinoid structure. The existence of the resonant quinoid
structure in 7a and 8a may be the reason that 7a and 8a do not
photocyclize. Closed-ring isomers 7b and 8b do not have such
resonance structure. Therefore, they undergo normal photocy-
cloreversion reactions. The photoreactivity of the open-shell
diarylethenes was found to be largely affected by the substitution
position of the thiophene rings.
by EPR spectroscopy. The change in the exchange interaction
was estimated to be more than 150-fold.
The EPR spectra of 7 and 8 were also measured. In the case
of 7, both the open- and closed-ring isomer had a nine-line
spectrum, suggesting the exchange interaction is too strong to
be detected by EPR spectroscopy. Because 7 and 5 have the
same number of carbon atoms between two nitronyl nitroxides,
similar EPR spectra are expected, as observed in Figures 4a
and 5a.
8a showed a complicated 15-line spectrum. In this case, the
exchange interaction is comparable to the hyperfine coupling
constant. This spectrum can be simulated by the superposition
of two kinds of exchange interactions.9 The open-ring isomer
of diarylethene has two atropisomers, which are parallel and
antiparallel, and they should have different exchange interac-
tions.10 The values were estimated to be |2J/gµB| ) 24 G (|2J/
kB| ) 0.0032 K) and |2J/gµB| < 2 G (|2J/kB| < 3 × 10-4 K).
Switching of EPR Spectra. The exchange interaction
between two radical centers was investigated by means of EPR
spectroscopy (Figures 4 and 5).8 In the case of 5, any EPR
spectral change was not observed along with the photochromism.
Both the open- and closed-ring isomers of 5 have a nine-line
spectrum, suggesting that in both isomers the exchange interac-
tions are stronger than the hyperfine coupling constant. By the
simulation of the spectrum, the exchange interaction |2J/kB| was
estimated to be larger than 0.04 K. To see the change in the
exchange interaction by EPR spectroscopy, the π-conjugated
chain length should be longer than 5. In the case of 6a, a drastic
spectral change was observed. The open-ring isomer 6a showed
a five-line spectrum, suggesting that the exchange interaction
is smaller than the hyperfine coupling constant (|2J/kB| < 3 ×
10-4 K). On the contrary, the closed-ring isomer 6b showed a
nine-line spectrum. The π-conjugated chain length of 6 is
appropriate for the detection of the exchange interaction change
Although 8a and 6a have the same number of carbon atoms
between two nitronyl nitroxides, the EPR spectrum of 8a showed
a stronger interaction than that of 6a. The open-ring isomer of
bis(2-thienyl)ethene has a delocalized π-conjugated system;
therefore, it has a stronger interaction than bis(3-thienyl)ethene.
Meanwhile, the closed-ring isomer 8b had a five-line EPR
spectrum. In this case, the exchange interaction is much smaller
than the hyperfine coupling constant. The small interaction is
due to the fact that the two nitronyl nitroxides are separated by
four phenyl rings and by the sp3 carbon and sulfur atoms.

Reversed Photoswitching of Magnetic Interaction
J. Phys. Chem. B, Vol. 106, No. 43, 2002 11221
demonstrated that the exchange interaction changed more than
150-fold upon alternate irradiation with UV and visible light
when an appropriate spacer was chosen. In the case of bis(2-
thienyl)ethene diradicals, only cycloreversion reactions took
place by irradiation with visible light. The closed-ring isomers
of the diradicals were prepared from the closed-ring isomers of
the precursors. When a biphenyl spacer was employed, it was
found that the switching behavior is reversed by changing the
substitution position of the thiophene rings to the ethene moiety.
Although the bis(2-thienyl)ethene diradicals did not show
reversibility, these results expand the possibility of using
diarylethenes as photoswitching units in molecular devices.
Experimental Section
1
A. Materials. H NMR spectra were recorded on Varian
Gemini 200, Bruker AC-250P, and JEOL GSX-400 instruments.
UV-vis spectra were recorded on a Hitachi U-3500 spectro-
photometer. Mass spectra were obtained on JEOL JMS-HX110A
and Applied Biosystems Voyager DE instruments. 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 chromatography was performed on silica gel (E. Merck,
70-230 mesh).
1,2-Bis(5-formylphenyl)-2,4-dimethyl-3-thienyl)hexafluo-
rocyclopentene (10). To a solution of 9 (400 mg, 0.62 mmol)
in THF (10 mL) was added Pd(PPh3)4 (70 mg, 0.06 mmol),
Na2CO3 (2.5 g), water (10 mL), and 4-formylphenylboronic acid
(360 mg, 2.4 mmol). The reaction mixture was refluxed for 24
h and was then poured into water, extracted with Et2O, washed
with water, dried over magnesium sulfate, and concentrated.
Column chromatography (silica, hexane/Et2O 3:1) gave diformyl
compound 10 (290 mg, 78%) as a white powder: mp 143.9-
Figure 2. (a) UV-vis absorption spectra of 5 (2.6 × 10-5 M ethyl
acetate solution): open-ring isomer 5a (s), closed-ring isomer 5b
(- - -), and in the photostationary state under irradiation with 313-
nm light (‚‚‚). (b) UV-vis absorption spectra of 6 (2.6 × 10-6 M ethyl
acetate solution): open-ring isomer 6a (s), closed-ring isomer 6b
(- - -), and in the photostationary state under irradiation with 366-
nm light (‚‚‚).
1
145.7 °C; H NMR (CDCl3, 250 MHz): δ 2.16 (s, 6H), 2.40
(s, 6 H), 7.54 (d, J ) 8 Hz, 4H), 7.90 (d, J ) 8 Hz, 4H), 10.03
(s, 2H). Anal. Calcd for C31H22F6O2S2: C, 61.58; H, 3.67.
Found: C, 61.50; H, 3.84.
The EPR spectral change and the estimated exchange
interaction are summarized in Table 1 along with the absorption
maxima. When a spacer of appropriate length was chosen, the
change in the exchange interaction was detected by EPR. The
exchange interaction change between the open- and closed-ring
isomers is dependent on the substitution positions of the
thiophene rings onto the ethene moiety. The closed-ring isomer
has a larger interaction than the open-ring isomer in bis(3-
thienyl)ethenes, whereas in bis(2-thienyl)ethenes, the open-ring
isomer has a larger interaction. Although in both cases the
closed-ring isomers had absorption maxima at longer wave-
lengths than the open-ring isomers, the magnetic interaction
showed the reverse effect. This result indicates that the magnetic
interaction and the absorption spectra have a different depen-
dence on the π-conjugated structure. The bond alternation is
connected throughout the molecule in both 6b and 8a, but 6b
had a stronger interaction than 8a. This is attributable the fact
that the planarity of the closed-ring isomer 6b is higher than
that of 8a.
1,2-Bis[5-(4-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-
2-yl)phenyl)-2,4-dimethyl-3-thienyl]hexafluorocyclopen-
tene (5a). A solution of 10 (1.4 g, 2.3 mmol), 2,3-bis-
(hydroxyamino)-2,3-dimethylbutane sulfate (2.9 g, 12 mmol),
and potassium carbonate (1.7 g, 12 mmol) in methanol (140
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 tetrahydroxyl-
amine as a yellow oil. Purification was not performed.
To a solution of tetrahydroxylamine in dichloromethane (300
mL) was added a solution of sodium periodate (1.7 g, 7.7 mmol)
in water (300 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 chromatography (silica, chloroform/
AcOEt ) 10:1). 5a was obtained as dark-blue microcrystals
(105 mg, 6%): mp 218.5-220.4 °C (dec); UV-vis (AcOEt)
λ
max, nm (ꢀ): 314 (3.5 × 104), 376 (1.5 × 104), 549 (sh), 600
(5.3 × 102), 645 (4.9 × 102), 718 (sh); EPR (benzene) 1:4:10:
16:19:16:10:4:1, nine lines, g ) 2.006, aN ) 3.4 G. Anal. Calcd
for C43H44F6N4O4S2: C, 60.13; H, 5.16; N, 6.52. Found: C,
60.40; H, 5.43; N, 6.27.
Conclusions
Bis(3-thienyl)ethenes and bis(2-thienyl)ethenes having ni-
tronyl nitroxide radicals at both ends of the molecule were
synthesized, and the photochromic performance and EPR
spectral change upon photoirradiation were investigated. Bis-
(3-thienyl)ethene diradicals underwent photochromic reactions
by irradiation with UV and visible light. EPR spectral analysis
Corresponding Closed-Ring Isomer 5b. UV-vis (AcOEt)
λ
max, nm (ꢀ): 294 (2.0 × 104), 337 (2.3 × 104), 352 (sh), 588
(9.5 × 103); EPR (benzene) 1:4:10:16:19:16:10:4:1, nine lines,
g ) 2.006, aN ) 3.6 G.

11222 J. Phys. Chem. B, Vol. 106, No. 43, 2002
Matsuda et al.
a
SCHEME 3
a Reagents and conditions: (a) UV light (340 nm < λ < 440 nm), AcOEt. (b) 2,3-Dimethyl-2,3-bis(hydroxyamino)butane sulfate and methanol
and then NaIO4 and CH2Cl2, 5-17%.
SCHEME 4: Possible Resonant Quinoid Structure of 5b
and 7a
dried over magnesium sulfate, and concentrated. Column
chromatography (hexane/dichloromethane 20:1) gave compound
11 (100 mg, 21%) as a white wax: 1H NMR (CDCl3, 200
MHz): δ 2.17 (s, 6H), 2.41 (s, 6H), 7.50 (d, J ) 8 Hz, 4H),
7.68 (d, J ) 8 Hz, 4H), 7.78 (d, J ) 8 Hz, 4H), 7.98 (d, J )
8 Hz, 4H), 10.03 (s, 2H); FAB HRMS (m/z): [M]+ calcd for
C43H30F6O2S2: 756.1591; found: 756.1596.
1,2-Bis[5-(4-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazo-
lin-2-yl)phenyl)-2,4-dimethyl-3-thienyl]hexafluorocyclopen-
tene (6a). A solution of 11 (100 mg, 0.13 mmol), 2,3-bis-
(hydroxyamino)-2,3-dimethylbutane sulfate (320 mg, 1.3 mmol),
and potassium carbonate (180 mg, 1.3 mmol) in benzene (10
mL) and methanol (2 mL) was refluxed for 15 h. The reaction
mixture was poured into water, extracted with ethyl acetate,
washed with water, dried over magnesium sulfate, and concen-
trated to give tetrahydroxylamine as a yellow oil. Purification
was not performed.
Figure 3. (a) UV-vis absorption spectra of 7 (2.3 × 10-5 M ethyl
To a solution of tetrahydroxylamine in dichloromethane (30
acetate solution): open-ring isomer 7a (s) and closed-ring isomer 7b
(- - -). (b) UV-vis absorption spectra of 8 (1.4 × 10-5 M ethyl
acetate solution): open-ring isomer 8a (s) and closed-ring isomer 8b
(- - -).
mL) was added a solution of sodium periodate (85 mg, 0.40
mmol) in water (30 mL); the mixture was stirred for 15 min in
the open air. The organic layer was separated, washed with
water, dried over magnesium sulfate, and concentrated. Purifica-
tion was performed by column chromatography (silica, chlo-
roform/Et2O 10:1). 6a was obtained as a dark-green wax (40
mg, 30%): UV-vis (AcOEt) λmax, nm (ꢀ): 319 (6.4 × 104),
376 (1.8 × 104), 560 (sh), 599 (8.4 × 102), 650 (sh), 710 (sh);
EPR (benzene) 1:2:3:2:1, five lines, g ) 2.007, aN ) 7.5 G;
FAB MS (m/z): [M+H]+ calcd for C55H53F6N4O4S2: 1011;
found: 1011.
1,2-Bis(5-(4-(4-formylphenyl)phenyl)-2,4-dimethyl-3-thi-
enyl)hexafluorocyclopentene (11). To a solution of 9 (410 mg,
0.63 mmol) in THF (10 mL) was added 1.6 M n-butyllithium
in hexane (0.9 mL, 1.6 mmol) at -78 °C. After the mixture
was stirred for 1 h at -78°C, tri-n-butylborate (0.8 mL, 3.0
mmol) was added. The solution was allowed to warm to 10 °C
with stirring. Water (3 mL) was added to the reaction mixture
followed by the addition of Pd(PPh3)4 (80 mg, 0.07 mmol), Na2-
CO3 (2.4 g), water (10 mL), and 4-formyl-4′-iodobiphenyl (1.1
g, 3.6 mmol). The reaction mixture was refluxed for 12 h and
then poured into water, extracted with Et2O, washed with water,
Corresponding Closed-Ring Isomer 6b. UV-vis (AcOEt)
λ
max, nm (ꢀ): 348 (4.4 × 104), 583 (1.5 × 104); EPR (benzene)
1:4:10:16:19:16:10:4:1, nine lines, g ) 2.007, aN ) 3.7 G.
1,2-Bis(5-iodo-3,4-dimethyl-2-thienyl)hexafluorocyclopen-
tene (12). To a stirred solution of 1,2-bis(3,4-dimethyl-2-

Reversed Photoswitching of Magnetic Interaction
J. Phys. Chem. B, Vol. 106, No. 43, 2002 11223
Figure 4. X-band EPR spectra of the diradical in benzene solution at room temperature (9.32 GHz). (a) 5a. (b) 5b. (c) 6a. (d) 6b.
Figure 5. X-band EPR spectra of the diradical in benzene solution at room temperature (9.32 GHz). (a) 7a. (b) 7b. (c) 8a. (d) 8b.
TABLE 1: EPR Line Shapes and Exchange Interactions
MgSO4. The solvent was evaporated, and the residue was
purified by short-path column chromatography (silica, hexane).
Diiodo compound 12 (2.2 g, 71%) was obtained as a yellow
solid: mp 99.1-100.8 °C; 1H NMR (CDCl3, 200 MHz): δ 1.75
(s, 6 H), 2.09 (s, 6 H); FAB HRMS (m/z): [M]+ calcd for
C17H12F6I2S2: 647.8374; found: 647.8399.
(|2J/kB K|) of Diradicals
open-ring isomer a
closed-ring isomer b
EPR line
|2J/kB K| λmax/nm shape
EPR line
shape
|2J/kB K| λmax/nm
5
6
7
8
9 lines >0.04
5 lines <3 × 10-4
9 lines >0.04
15 lines 0.0032,
<3 × 10-4
314
319
375
377
9 lines >0.04
588
583
469
467
9 lines >0.04
9 lines >0.04
5 lines <3 × 10-4
1,2-Bis(5-(4-formylphenyl)-3,4-dimethyl-2-thienyl)hexaflu-
orocyclopentene (13a). To a stirred solution of 12 (1.0 g, 1.5
mmol) in THF (15 mL) was added Pd(PPh3)4 (300 mg, 0.3
mmol), Na2CO3 (2.3 g), water (15 mL), and 4-formylphenyl-
boronic acid (1.5 g, 10 mmol). The reaction mixture was
refluxed for 30 h and then poured into water, extracted with
Et2O, washed with water, dried over magnesium sulfate, and
concentrated. Column chromatography (silica, chloroform) gave
diformyl compound 13a (750 mg, 83%) as pale-yellow crys-
tals: mp 151.3-152.9 °C; 1H NMR (CDCl3, 200 MHz): δ 1.79
(s, 6 H), 2.20 (s, 6 H), 7.63 (d, J ) 7 Hz, 4 H), 7.95 (d, J ) 8
thienyl)hexafluorocyclopentene (1.9 g, 4.8 mmol) in acetic acid
(28 mL), sulfuric acid (5.2 mL), and water (12 mL) was added
iodine (2.0 g, 7.9 mmol) and H5IO6 (0.8 g, 3.5 mmol), and the
mixture was stirred for 5 h at 70 °C in the open air. The reaction
mixture was poured into ice water and extracted with Et2O, and
the organic layer was washed with water, NaHCO3 aqueous
solution, and sodium thiosulfate aqueous solution and dried over

11224 J. Phys. Chem. B, Vol. 106, No. 43, 2002
Matsuda et al.
Hz, 4 H), 10.07 (s, 2 H); UV-vis (AcOEt) λmax, nm: 301, 368;
FAB HRMS (m/z): [M+H]+ calcd for C31H23F6O2S2: 605.1044;
found: 605.1047. For the corresponding closed-ring isomer
13b: UV-vis (AcOEt) λmax, nm: 300, 476.
1,2-Bis[5-(4-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-
2-yl)phenyl)-3,4-dimethyl-2-thienyl]hexafluorocyclopen-
tene (7a). A solution of 13a (500 mg, 0.8 mmol), 2,3-
bis(hydroxyamino)-2,3-dimethylbutane sulfate (1.0 g, 4.0 mmol),
and potassium carbonate (0.8 g, 5.5 mmol) in methanol (20 mL)
was refluxed for 15 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 a yellow oil. Purification was not performed.
found: 756. Anal. Calcd for C43H30F6O2S2: C, 68.24; H, 4.00.
Found: C, 68.13; H, 4.07. Corresponding closed-ring isomer
14b: UV-vis (AcOEt) λmax, nm: 345, 468.
1,2-Bis[5-(4-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-
2-yl)phenyl)-3,4-dimethyl-2-thienyl]hexafluorocyclopen-
tene (8a). A solution of 14a (340 mg, 0.45 mmol), 2,3-
bis(hydroxyamino)-2,3-dimethylbutane sulfate (800 mg, 3.2
mmol), and potassium carbonate (0.6 g, 4.3 mmol) in a mixed
solvent of benzene (50 mL) and methanol (10 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 a yellow
oil. Purification was not performed.
To a solution of tetrahydroxylamine in dichloromethane (50
mL) was added a solution of sodium periodate (0.42 g, 2.0
mmol) in water (75 mL); the mixture was stirred for 15 min in
the open air. The organic layer was separated, washed with
water, dried over magnesium sulfate, and concentrated. Purifica-
tion was performed by column chromatography (silica, chlo-
roform/Et2O 1:1). 7a was obtained as dark-green microcrystals
(240 mg, 35%): mp 207 °C (dec); UV-vis (AcOEt) λmax, nm
(ꢀ): 299 (3.2 × 104), 318 (sh), 375 (4.0 × 104), 556 (sh), 603
(6.1 × 102), 646 (5.7 × 102), 720 (sh); EPR (benzene) 1:4:10:
16:19:16:10:4:1, nine lines, g ) 2.006, aN ) 3.6 G. Anal. Calcd
for C43H44F6N4O4S2: C, 60.13; H, 5.16; N, 6.52. Found: C,
60.45; H, 5.28; N, 6.54.
To a solution of tetrahydroxylamine in dichloromethane (50
mL) was added a solution of sodium periodate (0.4 g, 2.0 mmol)
in water (80 mL); the mixture was stirred for 15 min 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, chloroform/
Et2O 9:1). 8a was obtained as a dark-green wax (100 mg,
21%): UV-vis (CH2Cl2) λmax, nm (ꢀ): 311 (5.6 × 104), 377
(5.2 × 104), 545 (sh), 597 (8.9 × 102), 630 (sh), 703 (sh); EPR
(benzene) complicated 15 lines, g ) 2.007. Anal. Calcd for
C55H52F6N4O4S2: C, 65.35; H, 5.27; N. 5.53. Found: C, 65.33;
H, 5.18; N, 5.54.
Corresponding Closed-Ring Isomer 8b. The following
procedure was performed under red light. A solution of 14b
(140 mg, 0.19 mmol), 2,3-bis(hydroxyamino)-2,3-dimethylbu-
tane sulfate (0.15 g, 0.61 mmol), and potassium carbonate (0.10
g, 0.72 mmol) in methanol (50 mL) and benzene (50 mL) was
refluxed for 40 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 a yellow oil. Purification was not performed.
Corresponding Closed-Ring Isomer 7b. The following
procedure was performed under red light. A solution of 13b
(100 mg, 0.17 mmol), 2,3-bis(hydroxyamino)-2,3-dimethylbu-
tane sulfate (0.15 g, 0.61 mmol), and potassium carbonate (0.10
g, 0.72 mmol) in methanol (50 mL) was refluxed for 25 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 a yellow oil.
Purification was not performed. To a solution of tetrahydroxyl-
amine in dichloromethane (50 mL) was added a solution of
sodium periodate (0.15 g, 0.70 mmol) in water (50 mL); the
mixture stirred for 15 min 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, dichloromethane/Et2O 1:1). 7b was
To a solution of tetrahydroxylamine in dichloromethane (100
mL) was added a solution of sodium periodate (0.15 g, 0.70
mmol) in water (100 mL); the mixture was stirred for 15 min
in the open air. The organic layer was separated, washed with
water, dried over magnesium sulfate, and concentrated. Purifica-
tion was performed by column chromatography (silica, dichlo-
romethane/Et2O 1:1). 8b was obtained as a brown wax (10 mg,
5.3%): UV-vis (AcOEt) λmax, nm (ꢀ): 313 (6.9 × 104), 376
(2.1 × 104), 467 (5.0 × 103), 602 (750); EPR (benzene) 1:4:
10:16:19:16:10:4:1, nine lines, g ) 2.008, aN ) 7.5 G. FAB
HRMS (m/z): [M+H]+ calcd for C55H53F6N4O4S2: 1011;
found: 1011.
obtained as a brown wax (25 mg, 17%): UV-vis (AcOEt) λmax
,
nm (ꢀ): 300 (3.8 × 104), 351 (2.0 × 104), 372 (2.1 × 104),
469(4.9 × 103); EPR (benzene) 1:4:10:16:19:16:10:4:1, nine
lines, g ) 2.008, aN ) 3.6 G; MALDI-TOF MS (m/z):
[M+H]+ calcd for C43H45F6N4O4S2: 859; found: 859.
1,2-Bis(5-(4-(4-formylphenyl)phenyl)-3,4-dimethyl-2-thi-
enyl)hexafluorocyclopentene (14a). To a solution of 12 (500
mg, 0.75 mmol) in THF (10 mL) was added 1.6 M n-
butyllithium in hexane (1.0 mL, 1.6 mmol) at -78 °C. After
the mixture was stirred for 1 h at -78°C, tri-n-butylborate (0.8
mL, 3.0 mmol) was added. The solution was allowed to warm
to 10 °C with stirring. Water (3 mL) was added to the reaction
mixture followed by the addition of Pd(PPh3)4 (80 mg, 0.07
mmol), Na2CO3 (2.4 g), water (10 mL), and 4-formyl-4′-
iodobiphenyl (600 mg, 1.9 mmol). The reaction mixture was
refluxed 12 h and then poured into water, extracted with Et2O,
washed with water, dried over magnesium sulfate, and concen-
trated. Column chromatography (hexane/dichloromethane ) 9:1)
gave compound 14a (150 mg, 26%) as a yellow wax: 1H NMR
(CDCl3, 400 MHz): δ 1.80 (s, 6H), 2.21 (s, 6H), 7.57 (d, J )
8 Hz, 4H), 7.71 (d, J ) 8 Hz, 4H), 7.79 (d, J ) 8 Hz, 4H),
B. Photochemical Measurements. Absorption spectra were
measured on a spectrophotometer (Hitachi U-3500). Photoir-
radiation 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 optical filters and monochromator (Ritsu MC-
20L).
C. EPR Spectroscopy. A Bruker ESP 300E spectrometer
was used to obtain X-band EPR spectra. Samples were dissolved
in benzene and degassed with Ar bubbling for 5 min.
Acknowledgment. We thank Professor Paul M. Lahti and
Professor B. Kirste for making theBIRADG program available
to us. This work was supported by CREST of Japan Science
and Technology Corporation and by a Grant-in Aid for Scientific
Research on the Priority Area “Creation of Delocalized Elec-
tronic Systems” (no. 12020244) from the Ministry of Education,
Science, Culture, and Sports, Japan.
7.98 (d, J ) 8 Hz, 4H), 10.07 (s, 2H); UV-vis (AcOEt) λmax
,
nm: 315, 367; MS (m/z): [M]+ calcd for C43H30F6O2S2: 756;

Reversed Photoswitching of Magnetic Interaction
J. Phys. Chem. B, Vol. 106, No. 43, 2002 11225
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