The irreversible thermo-bleaching function of a photochromic diarylethene having trimethylsilyl groups

Article abstract of DOI:10.1039/b822372f

A new function of a photochromic diarylethene, having trimethylsilyl groups at the reactive positions, has been developed. Upon alternating ultraviolet and visible light irradiation, the diarylethene showed photoreversible photochromism in a polymer film, as well as in solution. The colored state changed to colorless immediately upon heating at 100 °C. The colorless state, which is different from the open-ring isomer, was stable under both ultraviolet and visible light irradiation. Such photochromic materials could potentially be used in applications as secret display materials. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009.

Full text of DOI:10.1039/b822372f

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www.rsc.org/njc | New Journal of Chemistry
The irreversible thermo-bleaching function of a photochromic
diarylethene having trimethylsilyl groupsw
Seiya Kobatake,*ab Hiroyuki Imagawa,a Hidenori Nakatania
and Seiichiro Nakashimaa
Received (in Montpellier, France) 15th December 2008, Accepted 5th February 2009
First published as an Advance Article on the web 19th March 2009
DOI: 10.1039/b822372f
A new function of a photochromic diarylethene, having trimethylsilyl groups at the reactive
positions, has been developed. Upon alternating ultraviolet and visible light irradiation, the
diarylethene showed photoreversible photochromism in a polymer film, as well as in solution. The
colored state changed to colorless immediately upon heating at 100 1C. The colorless state, which
is different from the open-ring isomer, was stable under both ultraviolet and visible light
irradiation. Such photochromic materials could potentially be used in applications as secret
display materials.
methoxy groups at the reactive positions.27 Cyano groups
Introduction
contributed to a large photocycloreversion quantum yield.28
The photostable colored isomers returned to their initial
colorless isomers thermally at high temperatures above 100 1C
by the introduction of cyclohexyloxy groups instead of methoxy
groups.29,30
Photochromism is referred to as a photoinduced reversible
transformation reaction between two isomers with different
absorption spectra upon irradiation with light of an appropriate
wavelength.1,2 Photochromic compounds exhibit changes to
various chemical and physical properties, such as their absorp-
tion spectra, refractive indices, dielectric constants,
oxidation–reduction potentials and geometrical structures.
They are typically classified into two types, P-type (thermally
stable photogenerated isomer) and T-type (thermally unstable
photogenerated isomer).1,3 Among the various photochromic
compounds, diarylethenes with heterocyclic aryl rings have
been developed as P-type photochromic compounds because
of some excellent characteristics, such as the thermal stability of
both isomers and fatigue-resistant properties.4–8 Such materials
could potentially be used in applications such as optical
memory media,9–11 switching devices,12–14 display materials15,16
and photomechanical actuators.17–20
The introduction of substituents at the reactive positions
gave us encouragement to make novel photochromic
functions. For the purpose of developing new functionalized
photochromic diarylethenes, we have designed and syn-
thesized a diarylethene having trimethylsilyl (TMS) groups
at the reactive positions (1a) (Scheme 1). In this work, we show
a new reaction system, in addition to the usual photochromic
reactions of diarylethenes. Diarylethene 1a, prepared in this
work, showed reversible photoisomerizations upon alternating
irradiation with ultraviolet (UV) and visible light, and the
colored isomer was thermally bleached to give other photo-
stable molecules. The chemical and physical properties of the
diarylethene were examined in a polymer film, as well as in
solution.
We have recently designed and synthesized diarylethenes for
novel material applications. P-type photochromic diarylethenes
containing a diethylamino group can switch to a T-type photo-
chromic system by the addition of an acid as an external
stimulus.21 The electron donating character of the substituent
varied the electron accepting character by protonation,
resulting in a T-type photochromic system.22,23 Enhancement
of the thermal cycloreversion reaction was realized by the
introduction of bulky substituents at the reactive 2- and
20-positions of the thiophene rings.24–26 The photodecoloration
quantum yield was strongly suppressed by the introduction of
Scheme 1 Photochromic reactions of diarylethene 1a.
Results and discussion
a Department of Applied Chemistry, Graduate School of Engineering,
Osaka City University, Sugimoto, Sumiyoshi-ku, Osaka 558-8585,
Japan. E-mail: kobatake@a-chem.eng.osaka-cu.ac.jp;
Fax: +81 6-6605-2797; Tel: +81 6-6605-2797
Synthesis of diarylethene 1a
Diarylethene 1a was synthesized according to the synthetic
route described in Scheme 2. 2-Bromo-5-phenylthiophene was
prepared in 83% yield by the bromination of 2-phenyl-
thiophene.31 3-Bromo-5-phenyl-2-trimethylsilylthiophene was
synthesized in 87% yield by the Halogen dance reaction32,33
of 2-bromo-5-phenylthiophene using lithium diisopropylamide
b Precursory Research for Embryonic Science and Technology
(PRESTO), Japan Science and Technology Agency (JST),
Kawaguchi Center Building, 4-1-8 Honcho, Kawaguchi 332-0012,
Japan
w CCDC 719459 (1a), 719460 (1c) and 719461 (1d). For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b822372f
ꢀc
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closed-ring isomer appeared at 615 nm. The blue-colored
solution returned to colorless by irradiation with visible light,
and the photobleached colorless solution was confirmed to be
due to open-ring isomer 1a. This indicates that diarylethene 1a
shows photoreversible photochromism in hexane. The absorp-
tion spectral changes were also observed in acetonitrile, as
shown in Fig. 1(b). Upon irradiation with 254 nm light, the
solution turned blue and a new peak appeared at 625 nm. The
colored solution returned to colorless by irradiation with
visible light at a rate similar to that seen in hexane. Such
photochromic behavior was also observed in a poly(methyl
methacrylate) (PMMA) film, as shown in Fig. 1(c).
Scheme 2 Synthetic scheme leading to diarylethene 1a, having TMS
groups.
(LDA). Diarylethene 1a was obtained by a coupling reac-
tion between 3-bromo-5-phenyl-2-trimethylsilylthiophene and
octafluorocyclopentene. The structure of the product was
identified by its 1H NMR spectrum, mass spectrum and
X-ray crystallographic analysis (see Experimental section).
Thermal bleaching reaction
The blue-colored solution, produced by irradiating with UV
light, was gradually bleached to colorless in hexane, even at
room temperature. The bleaching rate was accelerated by
increasing the temperature. Fig. 2(a) shows the absorption
spectral changes of the blue-colored solution in hexane when
heated at 50 1C. After heating for 60 min at this temperature,
the solution became colorless. The thermo-bleached product
Photochromism of diarylethene 1a
Upon irradiation with UV light, a hexane solution of 1a
turned blue. Fig. 1(a) shows the UV-visible absorption spectral
changes in hexane upon irradiation with 254 nm light. Upon
irradiation with UV light, the new absorption band of the
Fig. 2 Absorption spectral changes of the blue-colored solution in
(a) hexane, (b) acetonitrile and (c) a PMMA film when heated at 50 1C.
The blue-colored solution was obtained by irradiation of 1a with
254 nm light.
Fig. 1 Absorption spectral changes of 1a in (a) hexane, (b) aceto-
nitrile and (c) a PMMA film upon irradiation with 254 nm light.
ꢀc
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had a spectrum different to that of 1a. The colorless solution
was stable upon irradiation with either UV or visible light.
This indicates that 1b changed to products distinct from 1a.
The thermal bleaching was also examined in acetonitrile.
After heating for 60 min at 50 1C, the solution became color-
less; the spectral changes are shown in Fig. 2(b). It was found
that the bleaching rate depended on the solvent used, and that
a polar solvent accelerated the thermal reaction.
It was noticed that the absorption spectra of the thermal
bleaching products in hexane and acetonitrile were different,
as can be seen in Fig. 2. This means that the thermal bleaching
products may consist of more than two kinds of compound,
and that their content depends on the solvent. Fig. 2(c) shows
the absorption spectral changes of the thermal bleaching of a
PMMA film. After thermal bleaching, the spectrum was
similar to that in hexane.
Identification of thermal products
In order to identify the thermal bleaching products, the
bleached solution was checked by HPLC. The acetonitrile
solution at the photostationary state was analyzed by HPLC
with a reversed phase column, using acetonitrile as the eluent;
1a and 1b eluted at 31 and 29 min, respectively. Closed-ring
isomer 1b could be separated by HPLC and was used
immediately in the next experiment. Fig. 3 shows chromato-
graphs of 1b and the solution after heating at 60 1C in
acetonitrile and hexane. As can be seen from the charts, we
realized that 1b was converted to two different structures,
referred to as 1c and 1d, and that 1a was not produced upon
heating. In acetonitrile, the main product was 1d, the forma-
tion of which was preferred in this polar solvent. On the other
hand, 1c was mainly produced from a hexane solution.
Compounds 1c and 1d are thermally stable and did not change
upon heating or photoirradiation. The reaction mechanism
leading to the formation of 1c and 1d may be different.
Fig. 4 ORTEP drawings of (a) 1c and (b) 1d, showing 50%
probability displacement ellipsoids. 1d has a disordered structure for
the cyclopentadienyl ring. Only one of these structures is illustrated for
clarity.
1
during the thermal reaction of 1b. The H NMR spectrum of
1d indicates that both of the TMS groups had been eliminated.
Furthermore, two fluorine atoms are confirmed to have been
1
eliminated, as seen from its mass spectrum; the H NMR and
mass spectroscopy data of 1c and 1d appear in the
Experimental section. However, the structures of 1c and 1d
could not be determined conclusively by NMR and mass
spectroscopy, and they were finally established by the X-ray
crystallographic analysis of single crystals.
In order to determine the molecular structures of the
thermally-produced materials, we isolated them by separative
HPLC connected with a reversed phase column using aceto-
Fig. 4 shows ORTEP drawings of 1c and 1d; one of the TMS
groups in 1c is no longer present, and the cleavage of the C–S
bond is confirmed. On the other hand, both TMS groups in 1d
are no longer present. The mass spectra of these compounds
identify them as structures 1c and 1d.
1
nitrile. Isolated 1c and 1d were analyzed by H NMR, mass
spectroscopy and X-ray crystallographic analysis.
1H NMR spectroscopy and mass spectrometry of 1c
indicate that one of the two TMS groups had been eliminated
The proposed reaction mechanism is shown in Fig. 5.
Cleavage of the Si–C bond results in the formation of a
benzene ring at the central position, followed by cleavage of
Fig. 3 HPLC charts of (a) 1b, and the solution after the thermal
bleaching reaction of 1b in (b) acetonitrile for 0.5 h and (c) hexane for
1 h at 60 1C. The peak intensity was detected by absorbance at 300 nm
in acetonitrile.
Fig. 5 The reaction mechanism proposed for formation of 1c and 1d.
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the C–S bond to give 1c. Compound 1d shows the elimination
of both TMS groups and two fluorine atoms. The reaction is
proposed to proceed in two steps; the first is elimination of the
TMS group and one fluorine atom. It is presumed that the
second step takes place at a fast rate because of the aromatic
stability of the product. The intermediate product was not
observed under our reaction conditions.
Thermal bleaching reactivity
The thermal bleaching reaction of 1b was examined in hexane,
acetonitrile and on a PMMA film. The thermal bleaching
reaction rates of 1b were followed at various temperatures by
absorption spectral measurements in the visible region. Fig. 6
the shows decay curves of the absorbance of 1b; the decay
curves followed first-order kinetics. The absorbance of 1b
Fig. 7 The temperature dependence of the rate constant for the
thermal bleaching reaction of 1b in hexane (’), acetonitrile (K)
and a PMMA film (J).
solvent. Extrapolation of the temperature dependence indicates
1
that the t of 1b at 100 1C is 5.6, 4.8 and 4.5 s in hexane, acetonitrile
and PMMA film, respectively. This indicates that a fast thermal
2
1
decreased slowly at 30 1C, and the half-life time, t , in hexane
at this temperature was estimated to be 3.3 h. The reaction in
2
bleaching reaction is accomplished above 100 1C.
1
acetonitrile (t = 1.8 h) was faster than that in hexane. The
2
Secret image recordings
difference between the reactivities in hexane and acetonitrile
may be ascribed to the difference in their thermal bleaching
products. The reaction even proceeded in a PMMA film; at
Diarylethene 1a showed photoreversible photochromism and
a rapid thermal bleaching reaction of the colored state (1b) to
give the other colorless compounds (1c and 1d), which are
stable to both UV and visible light irradiation. This irrever-
sible thermo-bleaching function can therefore be added to the
P-type photochromic function.
1
higher temperatures, the decay was accelerated. The t was 5.0
2
and 3.3 min at 60 1C in hexane and acetonitrile, respectively.
Fig. 7 shows the temperature dependence of the bleaching
reaction. From the plots, the activation energy (Ea) and frequency
factor (A) were determined: Ea = 103 kJ molꢁ1 and A = 3.3 ꢂ
Fig. 8 shows secret image recordings using the irreversible
thermo-bleaching function. When a colorless state paper
surface was irradiated with UV light through a photomask,
a colored pattern was observed. The pattern was stable for a
few hours at room temperature. When the pattern was heated
for 10 s at 100 1C, it was completely bleached and the image
recorded through the photomask could not be seen on the
surface. In this state, the recording is secret. To see the secret
image, the entire surface was irradiated with UV light. When
UV light irradiation was carried out on the whole surface, the
pattern appeared as the opposite image. The secret state and
the image patterning state can be switched between for more
than 100 cycles by alternating UV and visible light irradiation.
1013
s
ꢁ1 in hexane, Ea = 98 kJ molꢁ1 and A = 7.7 ꢂ 1012
s
ꢁ1 in
acetonitrile, and Ea = 102 kJ molꢁ1 and A = 3.0 ꢂ 1013
s
ꢁ1 in the
PMMA film; these parameters depended on the polarity of the
Conclusions
We have synthesized a new functionalized diarylethene that
has TMS groups at the reactive positions. Upon alternating
UV and visible light irradiation, the diarylethene showed
photoreversible photochromism in a polymer film, as well as
in solution. In addition to the photochromic reactions, we
found that the colored state immediately and irreversibly
changed to a colorless state by heating at 100 1C, and that
the colorless state was stable to both UV and visible light
irradiation. Such photochromic materials could potentially be
used in applications as secret display materials.
Experimental section
General
1H (400 MHz) and 13C (100 MHz) NMR spectra were
recorded using a JEOL JNM A400 spectrometer. Tetramethyl-
silane was used as an internal standard. Mass spectra
were obtained with a JEOL JMS-700/700S mass spectrometer.
Fig. 6 Decay curves of 1b in (a) hexane, (b) acetonitrile and (c) a
PMMA film.
ꢀc
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Fig. 8 Secret image recordings using a photomask by UV irradiation and heating at 100 1C. Diarylethene 1a is present on the paper surface.
UV-visible absorption spectra in solution were measured with
JASCO V-560 or JASCO V-630 absorption spectro-
photometers. The quartz cell was heated by a JASCO
ETC-505 thermostatic cell holder. Spectroscopic grade
solvents were used after purification by distillation. The
solvents were dried over 4A molecular sieves and used without
de-gassing. The absorption spectra of polymer films on quartz
glass plates were measured using a Nikon E600POL micro-
scope connected to a Hamamatsu PMA-11 photodetector. A
Mettler-Toledo FP-90/FP82HT hot stage was used to main-
tain the constant temperature of the polymer film. Photo-
irradiation was carried out using a 200 W mercury–xenon
lamp (Moritex MUV-202) as the light source. Monochromatic
light was obtained by passing the light through a mono-
chromator and a UV filter. Monochromatic light was also
obtained by using a Keyence UV-400/UV-50 LED lamp
(365 nm) or a handy TLC UV lamp (254 nm). The PMMA
films were prepared by casting a toluene solution of the
polymer onto a quartz glass plate. X-Ray crystallographic
analyses were carried out using a Rigaku RAXIS RAPID imaging
plate diffractometer with Mo-Ka radiation (l = 0.71073 A)
(50 kV, 40 mA) monochromated by graphite. Data collections
were performed by direct methods using SHELXS-97, and
refined by the full-matrix least-squares method on F2 using
SHELXL-97, with anisotropic displacement parameters for
the non-hydrogen atoms.34 The positions of all the hydrogen
atoms were calculated geometrically and refined using a riding
model. The bond lengths and geometry of the disordered parts
of molecules were restrained in the refinement.
trimethylsilyl chloride (2.64 mL, 0.0209 mol) was added to
the reaction mixture, and it was stirred for 30 min. The
reaction mixture was then washed with water, extracted with
ether and the organic layer dried over MgSO4. The filtrate was
concentrated and purified by column chromatography on
silica gel using hexane/ethyl acetate (95 : 5) as the eluent.
Yield: 4.49 g (86.8%). 1H NMR (400 MHz, CDCl3, TMS):
d = 0.42 (s, 9H, TMS), 7.28 (s, 1H, Ar-H), 7.30 (t, J = 7.6 Hz,
1H, Ar-H), 7.38 (t, J = 7.6 Hz, 2H, Ar-H) and 7.56 (d, J = 7.6 Hz,
2H, Ar-H). MS: m/z = 310 (M+).
1,2-Bis(5-phenyl-2-trimethylsilyl-3-thienyl)perfluorocyclo-
pentene (1a). To a stirred solution of 3-bromo-5-phenyl-2-
trimethylsilylthiophene (0.910 g, 2.92 mmol) in dry ether
(12 mL) was added dropwise a 1.6 M n-BuLi/hexane solution
(2.0 mL, 3.2 mmol) at ꢁ78 1C under an argon atmosphere, and
stirring was continued for 1 h at this low temperature.
Octafluorocyclopentene (0.20 mL, 1.49 mmol, Nippon Zeon)
was slowly added to the reaction mixture at ꢁ78 1C, and the
mixture was stirred for 4 h at this temperature. The reaction
was stopped by the addition of water, the product extracted
with ether, and the organic layer washed with a 1 N HCl
aqueous solution and water. The organic layer was dried over
MgSO4, filtered and evaporated. The crude product was
purified by column chromatography on SiO2 using hexane as
the eluent. Yield: 0.459 g of 1a (49.3%) as colorless crystals.
1H NMR (400 MHz, CDCl3): d = 0.10 (s, 18H, TMS), 7.32
(t, J = 7.2 Hz, 2H, Ar-H), 7.38 (s, 2H, Ar-H), 7.39 (t, J = 7.2 Hz,
4H, Ar-H) and 7.55 (d, J = 7.2 Hz, 4H, Ar-H). 13C NMR
(100 MHz, CDCl3): d = 0.13, 111.09 (t of quintet, JC–F = 271
and 25 Hz), 116.00 (tt, JC–F = 257 and 24 Hz), 126.24, 126.76,
128.35, 129.17, 133.41, 133.50, 139.33 (t, JC–F = 25 Hz),
142.66 and 149.77. MS: m/z = 636 (M+). Crystal data:
C31H30F6S2Si2, M = 636.85, T = 296(2) K, triclinic, P-1,
a = 8.959(16), b = 10.52(2), c = 17.69(3) A, a = 97.56(9),
b = 97.02(8), g = 107.21(8)1, V = 1556(5) A3, Z = 2, R1
(I 4 2s(I)) = 0.0628, wR2 (all data) = 0.1682. CCDC
719459.w
Materials
3-Bromo-5-phenyl-2-trimethylsilylthiophene. Into
a flask
containing diisopropylamine (2.70 g, 0.0267 mol) in dry
THF (150 mL) was added dropwise a 1.6 M n-BuLi/hexane
solution (12.7 mL, 0.0203 mol) at ꢁ30 1C under an argon
atmosphere. The solution was stirred for 1 h at this tempera-
ture. To the reaction mixture was added a dry THF (30 mL)
solution of 2-bromo-5-phenylthiophene31 (4.00 g, 0.0167 mol)
at ꢁ78 1C in one batch. The reaction mixture was stirred for
2 h at this temperature. A dry THF solution containing
1
1c. H NMR (400 MHz, CDCl3, TMS): d = 0.57 (s, 9H,
TMS), 2.90 (s, 1H, SH), 6.96 (s, 1H, Ar-H), 7.44 (t, J = 7.2 Hz,
ꢀc
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4H, Ar-H), 7.50 (t, J = 7.2 Hz, 2H, Ar-H), 7.67 (d,
J = 7.2 Hz, 2H, Ar-H), 7.81 (d, J = 7.2 Hz, 2H, Ar-H) and
7.87 (s, 1H, Ar-H). MS: m/z = 564 (M+). Crystal data:
C28H22F6S2Si, M = 564.67, T = 123(2) K, monoclinic,
C2/c, a = 13.994(12), b = 16.162(15), c = 23.68(2) A,
b = 94.73(4)1, V = 5337(8) A3, Z = 8, R1 (I 4 2s(I)) = 0.1238,
wR2 (all data) = 0.3875. CCDC 719460.w
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15 J. Yao, K. Hashimoto and A. Fujishima, Nature, 1992, 355, 624.
16 A. Fernandez-Acebes and J.-M. Lehn, Adv. Mater., 1999, 11, 910.
17 M. Irie, S. Kobatake and M. Horichi, Science, 2001, 291, 1769.
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1d. 1H NMR (400 MHz, CDCl3, TMS): d = 7.41 (t,
J = 6.9 Hz, 2H, Ar-H), 7.48 (t, J = 6.9, 8.0 Hz, 4H, Ar-H),
7.71 (s, 1H, Ar-H), 7.77 (d, J = 8.0 Hz, 4H, Ar-H) and 7.80
(d, JH–F = 18 Hz, 1H, Ar-H). MS: m/z = 452 (M+). Crystal
data: C25H12F4S2, M = 452.47, T = 123(2) K, orthorhombic,
Pba2, a = 19.245(10), b = 24.970(11), c = 3.9399(19) A,
V = 1893.3(16) A3, Z = 4, R1 (I 4 2s(I)) = 0.0589, wR2
(all data) = 0.1149, Flack x = 0.11(11). CCDC 719461.w
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de Jong, N. Katsonis, Y. Kojima, S. Nakamura, J. Areephong,
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Acknowledgments
This work was partly supported by a Grant-in-Aid for
Scientific Research (C) (no. 19550142) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan,
and PRESTO, Japan Science and Technology Agency. We
also thank Nippon Zeon Co., Ltd. for their supply of
octafluorocyclopentene.
24 S. Kobatake, K. Shibata, K. Uchida and M. Irie, J. Am. Chem.
Soc., 2000, 122, 12135.
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009 New J. Chem., 2009, 33, 1362–1367 | 1367