Photochromism of diarylethenes on porous aluminum oxide: Fatigue resistance and redox potentials of the photochromes

Article abstract of DOI:10.1246/cl.2001.366

The photochromism of three diarylethene derivatives was examined on porous aluminum oxide. Upon alternate irradiation with UV and visible light, the diarylethenes having thiophene rings decomposed easily, while a derivative having benzothiophene rings underwent a reversible photochromic reaction. The decomposition started from the closed-ring isomers of the dithienylethenes. The decomposition mechanism was discussed based on the oxidation potentials of the compounds.

Full text of DOI:10.1246/cl.2001.366

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Chemistry Letters 2001
Photochromism of Diarylethenes on Porous Aluminum Oxide:
Fatigue Resistance and Redox Potentials of the Photochromes
†
††
Kingo Uchida,* Masayuki Fujita, Yoshifumi Aoi, Mitsunori Saito, and Masahiro Irie
Department of Materials Chemistry, Faculty of Science and Technology, Ryukoku University, Seta, Otsu 520-2194
Department of Electronics and Informatics, Faculty of Science and Technology, Ryukoku University, Seta, Otsu 520-2194
†
††
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University and
CREST, Japan Science and Technology Corporation (JST), 6-10-1 Hakozaki, Fukuoka 812-8581
(Received January 26, 2001; CL-010081)
The photochromism of three diarylethene derivatives was
diation and regenerated the open-ring isomers 1a, 2a, and 3a.
The coloration–decoloration cycles of the films doped with 1 and
2 were poor, while the film contains 3 showed excellent fatigue
resistance. In order to know which isomer is responsible for the
decomposition on the alumina surface, isolated open and closed-
ring isomers were absorbed on the porous alumina films and
time-dependence of spectral changes were observed in the dark.
examined on porous aluminum oxide. Upon alternate irradiation
with UV and visible light, the diarylethenes having thiophene
rings decomposed easily, while a derivative having benzothio-
phene rings underwent a reversible photochromic reaction. The
decomposition started from the closed-ring isomers of the
dithienylethenes. The decomposition mechanism was discussed
based on the oxidation potentials of the compounds.
Photochromic molecules attract much attention from both
fundamental as well as practical points of view because of their
potential for the application to the optical devices such as optical
1
memories and switches.
Among these compounds,
diarylethenes are regarded as the best candidate for the applica-
tion, because of the thermal stability of their both isomers and the
2–5
fatigue resistant property.
It is essential for the practical appli-
cation to study the medium effect on the photochromic reactions.
On the other hand, anodic alumina (Al O ) films, which con-
2
3
tain columnar micropores, are an attractive material for use in
optical devices.6 Various optical functions, e.g., spectral selec-
tion, polarization, and optical switching, were realized by filling
the pores with such guest materials as metal, polymer, and liquid
crystal.7 In this paper, we have examined the photochromic reac-
tions of diarylethenes on the porous alumina film as a solid medi-
um.
Commercially available anodic alumina films (Whatman
ANODISC; pore size: 0.2 µm) were used without further treat-
ment. The diarylethenes 1,2-bis(2,5-dimethylthiophen-3-yl)per-
2
fluorocyclopentene (1a), 1,2-bis(2,4,5-trimethylthiophen-3-
2
yl)perfluorocyclopentene (2a), and 1,2-bis(2-methylbenzo[b]-
8
thiophen-3-yl)perfluorocyclopentene (3a) were dissolved in
hexane and the films were dipped into the solution and used after
drying in vacuo. Absorption spectral changes of the dye on
anodic films were observed by reflective spectrometry.
Upon UV irradiation, the films doped with 1–3 turned to red
and new bands appeared at 504, 526, and 515 nm, respectively.
They are due to the formation of closed-ring isomers 1b, 2b, and
3b. The red color was bleached upon visible light (>500 nm) irra-
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
367
Absorption spectra of 1a, 2a, 3a and 3b were stable, while the
spectra of 1b and 2b were changed on anodic alumina even at
room temperature as shown in Figure 1.
In order to know the decomposition mechanism, the oxida-
tion potentials of open- and closed-ring isomers of 1–3 were
peaks were observed for 1b at 0.61 V and 2b at 0.51 V (vs
Ag/Ag ), while no oxidation peaks were observed for 3b and all
+
open-ring isomers. These results indicate that the decomposi-
tion of the diarylethenes due to the oxidation on anodic alumina
film. Diarylethene 3 showed stable photochromism because of
its high oxidation potential. We failed to identify the decompo-
sition products by oxidation because of the difficulty to extract
the products from the film.
measured in CH CN in a double compartment cell with a stan-
3
dard three-electrode system. The working electrode was in a
compartment separated from the rest by sintered glass. Platinum
wire and Ag/AgCl (0.1 M tetrabutylammonium chloride in
The compound 3 was also introduced into the pores of alu-
mina film by melting. The melting point was 151 °C. Although
CH CN) were used as the counter and reference electrodes,
3
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respectively.
3 does not show photochromism in the crystalline phase, the
The cyclic voltammograms for oxidation of the closed-ring
isomers of diarylethenes are shown in Figure 2. Oxidation
film containing 3 showed a reversible photochromic reaction.
This indicates that 3 in the porous film is in the amorphous
phase. The amorphous photochromic composite film will be
applicable for wave-guide and novel recording media.
1
0
This work was supported by Murata Science Foundation, a
Grant-in-Aid for Scientific Research (C) No. 12640567 from
the Ministry of Education, Science, Sports, and Culture and by
CREST and ACT of Japan Science and Technology
Cooperation (JST).
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