Photoswitching of intramolecular magnetic interaction using a photochromic spin coupler: An ESR study [6]

Full text of DOI:10.1021/ja001767u

J. Am. Chem. Soc. 2000, 122, 8309-8310
Photoswitching of Intramolecular Magnetic
8309
Interaction Using a Photochromic Spin Coupler: An
ESR Study
Figure 1. Photoswitching of magnetic interaction.
Kenji Matsuda* and Masahiro Irie*
Department of Chemistry and Biochemistry
Graduate School of Engineering, Kyushu UniVersity
CREST, Japan Science and Technology Corporation
6-10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581 Japan
ReceiVed May 19, 2000
ReVised Manuscript ReceiVed July 6, 2000
Photochromic compounds reversibly change their absorption
spectra by irradiation with light of appropriate wavelengths.1-3
Not only absorption spectra but also molecular geometric and
electronic structures are altered along with the photochromism.
This feature can be applied to fabricating molecular-scale switch-
ing devices.4-6 When two unpaired electrons are placed at both
ends of a π-conjugated chain, the two spins of the unpaired
electrons interact magnetically. If the π-conjugated chain length
can be switched when using a photochromic spin coupler, the
magnetic interaction can be controlled by irradiation with light
(Figure 1).
Figure 2. UV-vis absorption spectra measured at different stages of
the photochromic reaction starting from open-ring isomer 1a (1.3 × 10-5
M AcOEt solution). (a) Initial, (b) after irradiation with 313 nm light for
2 min, (c) 10 min, (d) after irradiation with 578 nm light for 2 min, (e)
20 min.
On the basis of this concept, we have designed 1a, in which
two nitronyl nitroxides were located at the both ends of 1,2-bis-
(6-phenyl-2-methyl-1-benzothiophen-3-yl)hexafluorocyclopen-
tene (Scheme 1). To detect the change of the exchange interaction
by ESR spectroscopy, the value of the interaction should be
comparable to the hyperfine coupling constant. Therefore, the
system should have a long π-conjugated chain. Two p-phenylene
groups are introduced for this purpose.
We have reported the photoswitching of the intramolecular
magnetic interaction in 2a, in which there is no p-phenylene
spacer. Although 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 open-ring isomer 2a to closed-ring isomer 2b
(Scheme 1), any photoinduced ESR spectral change was not
observed, because the exchange interaction between the two
radicals was much stronger than the hyperfine coupling constant.7,8
In this work we have demonstrated photoswitching of intramo-
lecular magnetic interaction, which can be evidenced by ESR
spectra at room temperature.
The synthesis of 1a was performed starting from 1,2-bis(2-
methyl-1-benzothiophen-3-yl)hexafluorocyclopentene.9 1,2-bis-
(2-methyl-1-benzothiophen-3-yl)hexafluorocyclopentene was di-
iodinated by iodine and orthoperiodic acid to give diiodinated
compound at 6,6′-position in a 76% yield. Suzuki coupling with
4-formylphenylboronic acid afforded diformyl derivative in a 49%
yield. This diformyl derivative was refluxed in methanol with
2,3-bis(hydroxyamino)-2,3-dimethylbutane sulfate in the presence
Figure 3. X-band ESR spectra measured at room temperature at different
stages of the photochromic reaction starting from open-ring isomer 1a
(1.1 × 10-4 M benzene solution, 9.32 GHz). (a) Initial, (b) after irradiation
with 366 nm light for 1 min, (c) 4 min, (d) after irradiation with λ > 520
nm light for 20 min, (e) 50 min.
of K2CO3 then oxidized with sodium periodate in dichloromethane
and water to give nitronyl nitroxides 1a in a 13% yield.10
Figure 2 shows the photochromic interconversion between 1a
and 1b. The ethyl acetate solution of 1a (1.3 × 10-5 M) was
irradiated with 313 nm light. Upon irradiation the intense
absorption at 553 nm grew and after 10 min it reached the
photostationary state. The color of the solution was changed from
pale blue to red purple. Clear isosbestic points were observed at
346 nm and 285 nm. The red purple solution was irradiated with
578 nm light for 20 min. The spectrum returned to the original
one with retention of the isosbestic points at 346 nm and 285
nm. Although the radical moiety has absorption around 550-
700 nm, the existence did not prohibit the photochromic reaction.
The conversion at the photostationary state was 99%. The high
conversion was estimated by comparison with the isolated closed-
ring isomer.11
(1) G. H. Brown, Photochromism; Wiley-Interscience: New York, 1971.
(2) Du¨rr, H.; Bouas-Laurent, H. Photochromism: Molecules and Systems;
Elsevier: Amsterdam, 1990.
(3) (a) Irie, M.; Uchida, K. Bull. Chem. Soc. Jpn. 1998, 71, 985. (b) Irie,
M. Chem. ReV. 2000, 100, 1685.
(4) Saika, T.; Irie, M.; Shimidzu, T. J. Chem. Soc., Chem. Commun. 1994,
2123.
(5) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. Chem. Eur. J. 1995, 1, 275.
(6) Endtner, J. M.; Effenberger, F.; Hartschuh A.; Port, H. J. Am. Chem.
Soc. 2000, 122, 3037.
(10) 1a: blue solid; mp 210.0-211.0 °C dec; UV-vis (AcOEt) (ꢀ) 313
(7.6 × 104), 377 (2.2 × 104), 555 (sh), 600 (7.9 × 102), 644 (7.4 × 102), 718
(sh); ESR (benzene) complicated 15 lines, g ) 2.007; FAB HRMS (m/z) [M
+ H]+ calcd for C49H45F6N4O4S2: 931.2786, found: 931.2739; Anal. Calcd
for C49H44F6N4O4S2: C 63.2, H 4.8, N, 6.0; found: C 63.5, H 5.2, N 5.7.
(7) Matsuda, K.; Irie, M. Chem. Lett. 2000, 16.
(8) Matsuda, K.; Irie, M. J. Am. Chem. Soc. 2000, 122, 7195.
(9) Hanazawa, M.; Sumiya, R.; Horikawa, Y.; Irie, M. J. Chem. Soc. Chem.,
Commun. 1992, 206.
10.1021/ja001767u CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/11/2000

8310 J. Am. Chem. Soc., Vol. 122, No. 34, 2000
Communications to the Editor
Scheme 1
Figure 3 shows the ESR spectra of 1a at room temperature at
different stages of the photochromic reaction. The irradiation was
performed in the ESR cavity. The ESR spectrum of 1a showed
a complex of 15 lines. This result suggests that the two spins of
nitronyl nitroxide are coupled by the exchange interaction that is
comparable to the hyperfine coupling constant. Upon irradiation
with 366 nm light, the signal of 9 lines at the center grew, and
after 4 min the spectrum reached the photostationary state. This
9-line spectrum corresponds to closed-ring isomer 1b. The 9-line
spectrum indicates that the exchange interaction between the two
spins in 1b is much larger than the hyperfine coupling constant.
At the photostationary state the 15-line spectrum originating
from 1a disappeared, which is consistent with the high conversion
proved by the measurement of UV-vis absorption spectra. When
the sample was irradiated with λ > 520 nm light, the spectrum
completely returned to the original one. The double integrals 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 (Figure 4).
To estimate the value of the exchange interaction, the simula-
tion of the spectra was performed according to the formulation
of Rassat et al.12,13 It was revealed that the antiferromagnetic
interaction between two nitronyl nitroxides remarkably increased
from |2J/gµB| ≈ 8 G (|2J/kB| ≈ 1 × 10-3 K) to |2J/gµB| > 600
G (|2J/kB| > 0.08 K) when the diarylethene spin coupler was
switched from the open-ring isomer 1a to the closed-ring isomer
1b. The strength of exchange interaction was switched by more
than 2 orders of magnitude.
Figure 4. ESR spectra of (a) closed-ring isomer 1b and (b) open-ring
isomer 1a with double integrals.
In conclusion the photoswitching of intramolecular magnetic
interaction was detected by ESR spectroscopy at room temperature
in solution. By introducing a p-phenylene spacer to the diben-
zothienylethene aryl groups, the strength of the exchange interac-
tion was successfully regulated so that the switching could be
detected by ESR spectroscopy. This method is applicable to
develop more complicated logic circuits.
Acknowledgment. This work was supported by CREST of Japan
Science and Technology Corporation 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.
(11) 1b: UV-vis (AcOEt) (ꢀ) 280 (4.3 × 104), 367 (4.4 × 104), 553 (2.6
× 104); ESR (benzene) 1:4:10:16:19:16:10:4:1, 9 lines, g ) 2.007, aN ) 3.7
G.
(12) Brie`re, R.; Dupeyre R.-M.; Lemaire, H.; Morat, C.; Rassat, A.; Rey,
P. Bull. Soc. Chim. Fr. 1965, 11, 3290.
(13) Glarum, S. H.; Marshall, J. H. J. Chem. Phys. 1967, 47, 1374.
JA001767U