Photochromism of a diarylethene with methoxymethyl groups at reactive carbons: Thermal irreversible reaction of the closed-ring isomer

Article abstract of DOI:10.1246/cl.2011.93

A photochromic diarylethene having methoxymethyl groups at the reactive carbons was synthesized, and the thermal stability of the closed-ring isomer was examined. The closed-ring isomer of the diarylethene caused both the thermal cycloreversion and a thermally irreversible reaction at 120°C. The structure of the thermal product was determined by ¹H NMR, MS, and X-ray crystallographic analysis.

Full text of DOI:10.1246/cl.2011.93

doi:10.1246/cl.2011.93
Published on the web December 16, 2010
93
Photochromism of a Diarylethene with Methoxymethyl Groups at Reactive Carbons:
Thermal Irreversible Reaction of the Closed-ring Isomer
Daichi Kitagawa and Seiya Kobatake*
Department of Applied Chemistry, Graduate School of Engineering, Osaka City University,
3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585
(Received November 5, 2010; CL-100931; E-mail: kobatake@a-chem.eng.osaka-cu.ac.jp)
A photochromic diarylethene having methoxymethyl groups
at the reactive carbons was synthesized, and the thermal stability
of the closed-ring isomer was examined. The closed-ring isomer
of the diarylethene caused both the thermal cycloreversion and a
thermally irreversible reaction at 120 °C. The structure of the
F
F
F F
F
F
F
F
F
F
F
F
UV
R
R
Vis.
S
S
S
S
R
R
1
thermal product was determined by H NMR, MS, and X-ray
crystallographic analysis.
1a: R = CH2OCH3
1b: R = CH2OCH3
Scheme 1.
Photochromism refers to chromatic change upon photoirra-
diation.1,2 Photochromic compounds change not only their mo-
lecular structures but also their physical properties including
absorption spectra, refractive indices, dielectric constants, and
oxidation-reduction potentials. Various types of photochromic
compounds have been synthesized and researched in an attempt to
apply them to optical memories and photooptical switching
devices. Among them, diarylethene derivatives are the most
promising compounds for application because of some excellent
characteristics, such as the thermal stability of both isomers and
fatigue-resistant properties.3-7 Such materials can potentially be
used in applications such as optical memory media,8-10 switching
devices,11-17 display materials,18,19 and photomechanical actua-
tors,20-23 because of the thermal irreversibility at room temperature.
The substituents at the reactive carbons of diarylethenes can
control the thermal and photochemical cycloreversion reactiv-
ities of the closed-ring isomers.24-32 For example, introduction
of bulky substituents at the reactive carbons enhances the
thermal cycloreversion reactivity.24-27 The photocycloreversion
quantum yield is strongly suppressed by introduction of
methoxy groups at the reactive carbons.28 By introduction of
cyclohexyloxy groups instead of methoxy groups, the photo-
stable diarylethene-closed-ring isomers return to their initial
colorless open-ring isomers by heating at temperatures above
100 °C.29,30 Cyano groups contribute to the increase in the
photocycloreversion quantum yield.31 When trimethylsilyl
groups are introduced at the reactive carbons, the colored
isomer immediately and irreversibly changes to a colorless by-
product by heating at 100 °C.32 As various types of substituents
have been already introduced at the reactive carbons, introduc-
tion of substituents at the reactive carbons of diarylethenes gives
us encouragement to make novel photochromic functions.
Here, we introduce methoxymethyl groups at the reactive
carbons of diarylethenes. 1,2-Bis(2-methoxymethyl-5-phenyl-3-
thienyl)perfluorocyclopentene (1a) was newly synthesized and
examined for the thermal stability of the closed-ring isomer 1b.
During the course of the study on the thermal stability of the
closed-ring isomer, we found that it changes to another product,
which is different from the open-ring isomer, by heating at
120 °C. The structure of the by-product is clearly discussed in
this letter.
Figure 1. Absorption spectral change of diarylethene 1a
(1.4 © 10¹5 mol dm¹3) in hexane upon irradiation with 313-nm
light. Upon UV irradiation for 540 s, the solution of 1a reached
in the photostationary state.
Diarylethene 1a showed reversible photoisomerizations
upon alternating irradiation with ultraviolet (UV) and visible
light at room temperature (Scheme 1). Figure 1 shows the
absorption spectral change of 1a. Diarylethene 1a has the
absorption maximum at 280 nm in hexane. Upon irradiation with
313-nm light, the colorless solution turned blue, in which the
visible absorption maximum was observed at 580 nm. This
spectral change is ascribed to the photoisomerization from the
open-ring isomer to the closed-ring isomer. The blue color
disappeared by irradiation with visible light (- > 500 nm) and
the absorption spectrum returned to that of 1a. The photo-
cyclization and photocycloreversion quantum yields were
determined to be 0.53 and 0.0055, respectively.33 The photo-
isomerization conversion from 1a to 1b was 95% in hexane
upon irradiation with 313-nm light. The open-ring isomer 1a and
the closed-ring isomer 1b were stable at room temperature.
These photochromic properties are similar to those of diaryl-
ethene having methyl groups at the reactive carbons.34
Next, the thermal stability of the closed-ring isomer was
examined. The closed-ring isomer 1b35 in toluene, which was
Chem. Lett. 2011, 40, 93-95
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

94
Figure 2. Decay curve and the first-order plot for thermal
reaction of 1b at 120 °C.
Figure 3. HPLC of the concentrate after heating 1b for 70 h at
120 °C, followed by irradiation with visible light in toluene. The
peak intensity was detected by absorbance at 254 nm in hexane/
ethyl acetate (9:1).
degassed and sealed off under vacuum in an optical quartz cell,
was heated at 120 °C. Figure 2 shows a decay curve of
absorbance of 1b at 120 °C. The decay curve followed the
first-order kinetics. The rate constant (k) of the thermal bleaching
reaction was estimated to be 9.7 © 10¹6 s¹1 from the slope of the
first-order kinetic plots. The value is similar to that of 1,2-bis(2-
methoxy-5-phenyl-3-thienyl)perfluorocyclopentene.29
When the thermal bleaching solution was irradiated with
UV light again, the performance of the coloration was reduced.
This indicates that a by-product was produced by repeating the
cycle of UV irradiation and heating at 120 °C. The closed-ring
isomer 1b was heated in toluene for 70 h at 120 °C. Then the
blue color was largely bleached by the thermal reactions. The
disappearance of the closed-ring isomer was estimated to be ca.
92% conversion from the change in absorbance of 1b at 580 nm.
When the remaining blue color was bleached by irradiation with
visible light to give the open-ring isomer 1a, the color of the
solution turned to pale yellow, which is different from those of
1a and 1b. This also indicates that the by-product was produced
by heating the closed-ring isomer 1b at 120 °C.
To identify the structure of the thermally produced product,
the solution was analyzed by high-performance liquid chroma-
tography (HPLC) connected with a normal phase silica gel
column using a hexane/ethyl acetate (9:1) mixture as the eluent.
Figure 3 shows the chromatograph of the solution after heating
for 70 h at 120 °C, followed by irradiation with visible light in
toluene. The open-ring isomer 1a was eluted at 20 min, and the
new peak appeared at 42 min, which is different from the closed-
ring isomer 1b. This indicates that the closed-ring isomer 1b
gave the open-ring isomer 1a and the by-product 1c. The by-
product eluted at 42 min was separated by HPLC to identify the
structure.
Figure 4. ORTEP drawing of the by-product 1c showing 50%
probability displacement ellipsoids.
F
F
F
F
F
F
F
F
F
F
F
F
120 °C
R
R
S
S
S
S
OCH3
1a: R = CH2OCH3
OCH3
1c: R = CH2OCH3
Figure 5. The reaction mechanism proposed for formation of
1c.
The structure of the by-product 1c was analyzed by
1H NMR, MS, and X-ray crystallographic analysis (see Support-
ing Information).36 Two types of methoxymethyl groups exist in
the 1H NMR spectrum of 1c. The mass spectrum of 1c indicates
that the by-product has the same mass as the open- and closed-
ring isomers. However, the structure of 1c could not be
determined conclusively by NMR and mass spectra. Finally,
we established the structure of 1c by X-ray crystallographic
analysis.
cleavage of the C-S bond is confirmed. This result is very
similar to that of the trimethylsilyl derivative.32 Cleavage of the
C-S bond results in the formation of a benzene ring at the central
position, followed by a methoxymethyl rearrangement. The by-
product is stable under heating as well as photoirradiation due to
the aromatic stability. Although the mechanism of formation of
1c is not clear, we propose that 1c is produced by a C-S bond
cleavage, as shown in Figure 5.
Figure 4 shows an ORTEP drawing of 1c. One of the
methoxymethyl groups is attached to the sulfur atom, and the
In conclusion, we have synthesized a new diarylethene with
methoxymethyl groups at the reactive carbons. Upon alternating
Chem. Lett. 2011, 40, 93-95
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

95
UV and visible light irradiation, the diarylethene showed
photoreversible photochromism in solution. In addition to the
photochromic reaction, we found that the closed-ring isomer 1b
irreversibly changed to a pale yellow by-product 1c by heating at
120 °C, which is a new type of by-product. The closed-ring
isomer 1b gave 58% of 1a and 42% of 1c by the thermal
reaction of 1b for 70 h at 120 °C. The by-product 1c was stable
against both UV and visible light irradiation. A similar reaction
was not observed in diarylethene bearing propyl groups at the
reactive carbons, while the closed-ring isomer returned to the
initial open-ring isomer by heating at 120 °C.37 The results from
this work can be useful for a molecular design of photochromic
diarylethenes, which have novel photochromic functions.
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This work was partly supported by a Grant-in-Aid for
Scientific Research on Priority Area “Strong Photon-Molecule
Coupling Fields” (470) (No. 21020032) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
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Chem. Lett. 2011, 40, 93-95
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/