Molecular design for a write-by-light/erase-by-heat recording system using photochromic diarylethenes with thermal cycloreversion

Article abstract of DOI:10.1016/j.tet.2019.130487

We designed and synthesized photochromic diarylethenes for a write-by-light/erase-by-heat recording system. The introduction of polar substituents at both sides of the diarylethene maintained the photocyclization and photocycloreversion reactivities, but

Full text of DOI:10.1016/j.tet.2019.130487

Accepted Manuscript
Molecular design for a write-by-light/erase-by-heat recording system using
photochromic diarylethenes with thermal cycloreversion
Yuta Sato, Daichi Kitagawa, Seiya Kobatake
PII:
S0040-4020(19)30814-2
DOI:
https://doi.org/10.1016/j.tet.2019.130487
Article Number: 130487
Reference:
TET 130487
To appear in: Tetrahedron
Received Date: 3 July 2019
Revised Date: 20 July 2019
Accepted Date: 27 July 2019
Please cite this article as: Sato Y, Kitagawa D, Kobatake S, Molecular design for a write-by-light/erase-
by-heat recording system using photochromic diarylethenes with thermal cycloreversion, Tetrahedron
(2019), doi: https://doi.org/10.1016/j.tet.2019.130487.
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Molecular design for a write-by-light/erase-
by-heat recording system using
photochromic diarylethenes with thermal
cycloreversion
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Yuta Sato, 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, Japan

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Tetrahedron
journal homepage: www.elsevier.com
Molecular design for a write-by-light/erase-by-heat recording system using
photochromic diarylethenes with thermal cycloreversion
Yuta Sato, 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, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
We designed and synthesized photochromic diarylethenes for a write-by-light/erase-by-heat
recording system. The introduction of polar substituents at both sides of the diarylethene
maintained the photocyclization and photocycloreversion reactivities, but significantly changed
the thermal cycloreversion reactivity. The introduction of electron-withdrawing substituents
accelerated the thermal reaction and the introduction of electron-donating substituents
suppressed the thermal reaction. The rate constants of the thermal reaction were well correlated
with Brown-Okamoto's substituent constant σ+ that is a modified value of Hammett's
substituent constant σ. The large rate constants are ascribed to the lower activation energy for
the thermal reaction. These results provide new knowledge for the molecular design of
diarylethenes for a write-by-light/erase-by-heat recording system.
Available online
Keywords:
Photochromism
Diarylethene
Substituent Effect
Thermal Reactivity
Substituent Constant
2019 Elsevier Ltd. All rights reserved
.
the substituent (R1) at the reacting positions of a typical
diarylethene derivative changes from methyl (1o) to methoxy
groups (2o), the photocycloreversion quantum yields
extraordinarily decrease by a factor of 1000, whereas the
cyclization quantum yields are almost constant [16]. On the other
hand, the introduction of the bulky substituents at the reacting
positions leads to accelerating the thermal cycloreversion
reaction [17]. As a result, a diarylethene derivative with the
cyclohexyloxy group at the reacting positions has specific
photochromic properties [18]. Thus, 1,2-bis(2-cyclohexyloxy-5-
phenyl-3-thienyl)perfluorocyclopentene (3o) undergoes the
photocyclization reaction with a moderate quantum yield of 0.43.
In contrast, the photocycloreversion reaction is prohibited as can
be seen from the extraordinarily low quantum yield of 6.4×10−4.
However, the thermal cycloreversion reaction takes place with a
half-life time of 45 s at 160 °C, whereas the colored isomer is
significantly stable at room temperature. These properties are an
1. Introduction
Materials that can reversibly switch their color have attracted
much attention because of the various possibilities of application
for rewritable media and switching devices. Photochromism is
defined as a photoreversible transformation between two isomers
having different absorption bands [1]. Photochromic compounds
can be switched between two states upon photoirradiation. There
are two types in the photochromic systems: T- and P-types. T-
type photochromic compounds undergo thermal back reactions of
the photogenerated isomers. Azobenzene [2], spiropyran [3,4],
spirooxazine [4], naphthopyran [5], and hexaarylbiimidazole [6]
are known as the representative T-type photochromic
compounds. T-type photochromic compounds can be used for
photomodulated materials such as ophthalmic lenses [7]. In
contrast, P-type photochromic compounds are thermally stable
for both isomers. The photogenerated isomer returns to the initial
isomer only by photoreaction. The representative P-type
photochromic compounds are diarylethene [8], furylfulgide [9],
and phenoxynaphthacenequinones [10]. Among them,
diarylethenes have the most excellent properties such as fatigue
resistance, high sensitivity, and rapid response, and are expected
to be applicable in optical memories [11], photooptical switches
[12], displays [13], nonlinear optics [14], and photoresponsive
actuators [15].
advantage for application to
a write-by-light/erase-by-heat
recording system. It is desirable to design diarylethene molecules
whose closed-ring isomers exhibit the thermal cycloreversion
reaction at temperatures much lower than 160 °C but exist
enough stable at room temperature.
An introduction of electron-withdrawing substituents on both
sides in the π-conjugation of diarylethene leads to the fast
thermal cycloreversion reaction [19]. Here, trifluoromethyl,
cyano, and formyl groups have been introduced as the electron-
The photochemical and thermal properties of diarylethenes
can be modified by changing the substituent. For example, when
———
Corresponding author. Tel.: +81-6-6605-2797; fax: +81-6-6605-2797; e-mail: kobatake@a-chem.eng.osaka-cu.ac.jp

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withdrawing substituent at both sides of diarylethene. For
comparison, methoxy groups have been introduced as the
electron-donating substituent and trifluoromethyl groups at two
m-positions have also been introduced. Thermal cycloreversion
reactivities of the closed-ring isomers were correlated using
Hammett’s polarity parameters to reveal the effect of inductive
and resonance of the substituents to the thermal cycloreversion
reactions.
Fig. 1. Absorption spectral change of (a) 4 (2.19×10−5 M in n-hexane),
(b) 5 (1.63×10−5 M in dichloromethane), (c) 6 (1.84×10−5 M in
dichloromethane), (d) 7 (1.71×10−5 M in n-hexane), and (e) 8 (1.66×10−5
M in n-hexane) by photoirradiation: open-ring isomer (black solid line),
solution in the photostationary state under irradiation with 313 nm light
(blue dashed line), and closed-ring isomer (blue solid line).
Scheme 1. Molecular structure of diarylethenes used in this work.
λ
max, molar absorption coefficient, and photocyclization and
2. Results and discussion
photocyclization quantum yields. The Φo→c for 4−8 were
determined to be 0.38, 0.45, 0.46, 0.33, and 0.35, respectively.
2.1. Photochromism
There is hardly any change in Φo→c compared to that of 3 (Φo→c
=
0.43) [18], which indicates that high photocyclization reactivity
was maintained. Moreover, the Φc→o values for 4−8 were
determined to be 4.6×10−4, 1.5×10−4, 8.6×10−5, 1.5×10−4, and
5.9×10−4, respectively. The Φc→o hardly changed in comparison
with that of 3 (Φc→o = 6.4×10−4) [18], which means that high
photostability was also maintained. Thus, it was found that the
Φo→c and Φc→o were hardly affected even when substituents were
introduced into the terminal phenyl groups of the diarylethene
having the cyclohexyloxy group at the reactive positions. Thus,
diarylethenes 4−8 have the good photocoloration reactivity and
the high photostability in the colored state enough for use as the
write-by-light/erase-by-heat recording system.
Diarylethenes 4o−8o prepared in this work undergo
photocyclization reactions as well as diarylethenes 1o−3o upon
irradiation with ultraviolet (UV) light. Fig. 1 shows absorption
spectral changes of 4−8 in n-hexane or dichloromethane. The
open-ring isomers 4o−8o are colorless and have absorption
maximum wavelength (λmax) at the range from 310 to 369 nm.
Upon irradiation with 313 nm light, a new absorption band
derived from the closed-ring isomers appeared in the visible
region and the solution color turned to blue. The λmax of 4c−8c
appeared at 635, 640, 664, 673, 645, and 638 nm, respectively.
The photogenerated closed-ring isomers were stable at room
temperature and they were possible to be isolated by HPLC.
Upon irradiation with visible light, the closed-ring isomers
returned to the open-ring isomers very slowly. The
photocyclization and photocycloreversion quantum yields (Φo→c
and Φc→o) of 4−8 were examined to quantitatively discuss the
photochemical reactivity of the diarylethenes. Table 1 shows
2.2. Thermal cycloreversion reactivity
The thermal reactivity of 4c−8c in toluene was investigated.
Table 1
Optical properties in n-hexane.
Open-ring isomer
Closed-ring isomer
Diarylethene
Φo→c
Φc→o
Ref.
λmax/nm
ε/M−1cm−1
35600
33000
34000
42700
59700
56000
41500
39300
λmax/nm
575
ε/M−1cm−1
15600
15000
13000
13500
14200
17500
17900
12700
1
2
3
280
309
313
326
347
369
310
327
0.59
0.44
0.43
0.38
0.45
0.46
0.33
0.35
0.0083
1.7×10−5
6.4×10−4
4.6×10−4
1.5×10−4
0.86×10−4
1.5×10−4
5.9×10−4
20,21
16,18
18
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625
635
4
640
664
5 a)
6 a)
673
645
638
7
8
a) In dichloromethane.

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Fig. 3. Temperature dependence of the rate constant for the thermal
bleaching reaction of 4c ( ○ ), 5c ( ● ), 6c ( □ ), 7c ( ■ ), and 8c ( ▲ ) in
toluene.
closed-ring isomers at λmax decreased slowly above 60 °C. The
decay curves followed the first-order kinetics plots, as also
shown in Fig. 2. The rate constants (k) for the thermal
cycloreversion reactions of 4c−8c were determined from the
slope of the first-order kinetic plot
Fig. 3 shows the temperature dependence of k for the thermal
bleaching reaction of 4c−8c. The kinetic parameters (activation
energy (Ea) and pre-exponential factor (A)) of the thermal
bleaching reaction were calculated from the slope and the
intercept of the linear relationship and are summarized in Table
2. The Ea values for 4−8 were determined to be 114, 109, 113,
131, and 113 kJ mol−1, respectively. The Ea values for the
diarylethene derivatives introduced electron-withdrawing
substituents (4, 5, 6, and 8) are smaller than that of 3 (Ea = 120 kJ
mol−1), while the Ea value for diarylethene introduced electron-
donating substituent (7) is higher. This result indicates that the
introduction of the electron-withdrawing substituents leads to a
decrease in Ea, which is a preferable condition for the
acceleration of the thermal bleaching reactivity. On the other
hand, the A values for 4−8 were determined to be 2.2×1012,
8.8×1011, 3.0×1012, 2.4×1013, and 4.2×1012 s−1, respectively. The
A values for diarylethenes introduced electron-withdrawing
substituents (4, 5, 6, and 8) are also smaller than that of 3 (A =
4.4×1012 s−1), while the A value for diarylethene electron-
donating substituent (7) is higher. This result indicates that the
introduction of the electron-withdrawing substituents also leads
to a decrease in A, which is an unpreferable condition for the
acceleration of the thermal bleaching reactivity. To compare the
thermal bleaching reactivity, the k values for 3−8 at 120 °C were
calculated to be 5.0×10−4, 1.6×10−3, 2.9×10−3, 2.9×10−3, 9.4×10−5,
and 4.1×10−3 s−1, respectively, using the kinetic parameters. The
introduction of electron-withdrawing substituents accelerated the
Fig. 2. Thermal bleaching behavior of (a) 4c, (c) 5c, (e) 6c, (g) 7c, and
(i) 8c in toluene. Absorbance (A) was monitored at the maximum
wavelength of the closed-ring isomer. The first-order plots for the decay
curves of 4c, 5c, 6c, 7c, and 8c are shown in (b), (d), (f), (h), and (j),
respectively.
The closed-ring isomers 4c−8c underwent thermal bleaching
reactions from blue to colorless. The absorption spectra after the
thermal bleaching reaction remained almost the same in the
shape and intensity compared with the absorption spectra of the
open-ring isomers. This indicates that the thermal bleaching is
ascribed to the cycloreversion reaction from the closed-ring
isomer to the open-ring isomer. Fig. 2 shows decay curves of the
absorption peaks of 4c−8c in toluene. The absorbance of the
Table 2
Thermal bleaching properties of 1c-8c in toluene.
k/s−1
t1/2/month
t1/2/min
Diarylethene
Ea/kJ mol−1
A/s−1
Reference
at 25 °C
46000
13000
67
at 120 °C
4.4×10−6
1.3×10−5
5.0×10−4
1.6×10−3
2.9×10−3
2.9×10−3
9.4×10−5
4.1×10−3
at 120 °C
2600
880
23
1c a)
139
137
120
114
109
113
131
113
1.3×1013
2.1×1013
4.4×1012
2.2×1012
8.8×1011
3.0×1012
2.4×1013
4.2×1012
20
16,18
2c
18
3c
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4c
7.4
12
5c
4.0
4.0
6c
4.0
5.9
7c
120
2.8
1050
4.2
8c
a) In 3-methylpentane.

4
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of the inductive effect and the resonance effect, and the σ+ values
Table 3
proposed by Brown-Okamoto have been widely adopted in the
electrophilic reaction. Furthermore, using the equation proposed
by Yukawa-Tsuno, the electronic effect of the substituents can be
separated into a contribution of the inductive and resonance
effects.
The polarity parameters of the substituents and the kinetic
parameters for the thermal cycloreversion reaction
Substituent
R2
log(k/kH)
at 120 °C
σ
σ+
Ea/kJ mol−1 log(A/s−1)
H
0
0
0
120
114
109
113
131
113
12.6
12.3
11.9
12.5
13.4
12.6
Hammett's plot was performed as shown in Fig. 4a. The
natural logarithm of the rate constant of the substituted
diarylethenes relative to the unsubstituted diarylethene (log(k/kH))
at 120 °C was plotted for the σ value. Although the electron-
withdrawing substituents exhibited a linear relation to the σ
value, the electron-donating substituent was out of the relation.
Thus, the σ value was replaced into the σ+ value. The
relationship using the σ+ value showed a linear relationship for
both the electron-withdrawing and the electron-donating
substituent. It means that the thermal cycloreversion reaction is
affected by not only the inductive effect but also the resonance
effect. However, the resonance effect of the methoxy group
negatively affected the reaction. Thus, the large resonance effect
of the methoxy group is considered to contribute to the
stabilization of the energy level of the closed-ring isomer,
resulting in the small reactivity. The normal logarithm of the pre-
exponential factor (log(A)) and the Ea values were plotted versus
σ+. All of the values are linearly related to the substituent
constant. As the substituent constant value increases, the Ea and
log(A) values decrease, while the log(k/kH) increases. This result
also indicates that the magnitude of the electron-withdrawing
property of the substituent has a large influence on the Ea as
mentioned above, resulting in an increase of the rate constant.
The relationship between the substituent constant and the rate
constant would provide a guideline for molecular design to
synthesize molecules exhibiting a desired thermal bleaching
reactivity.
p-CF3
0.54
0.61
0.505
0.763
0.763
−0.726
0.914
p-CN
0.66
0.66
p-CHO
p-OCH3
m,m-(CF3)2
0.42
0.73
−0.27
2×0.43
−0.78
2×0.52
reaction and the introduction of electron-donating substituents
suppressed the reaction. This result clearly indicates that the
thermal cycloreversion reactivity is largely influenced by the Ea
value than the A value.
When these materials are applied as a rewritable paper, the
stability of the colored state at room temperature is important.
The half-life time (t1/2) of the thermal cycloreversion reaction at
room temperature (25 °C) was at least 4 months for the
diarylethenes synthesized in this work. These results indicate that
these diarylethenes have sufficient stability for common use as a
write-by-light/erase-by-heat recording system.
2.3. Relationship between the substituent and the thermal
cycloreversion reactivity
The thermal cycloreversion reaction is expected to be
influenced by both the inductive effect and resonance effect of
the substituent R2 because the substituent affects the electron
density in the π-conjugated system of the diarylethene closed-
ring isomer. To elucidate the relationship between the introduced
substituents and the thermal properties, Hammett's substituent
constant (σ) [22] and Brown-Okamoto's substituent constant (σ+)
[22,23] were employed. The σ+ values correspond to r = 1 in the
Yukawa-Tsuno equation [24]. The σ and σ+ values include not
only the inductive effect but also the resonance effect. The
contribution of the resonance effect for the σ+ value is larger than
that of the σ value. Table 3 shows σ and σ+ together with the
kinetic parameters for the thermal cycloreversion reactions. The
difference between σ and σ+ is the difference between the content
2.4. Application
For a write-by-light/erase-by-heat recording system, we
investigated the thermal bleaching reactivity in the filter paper.
Fig. 5. Photocoloration and thermal and photochemical bleaching of 1
(upper left), 3 (upper right), 4 (bottom left), and 8 (bottom right) in the
filter paper. (a) Before UV irradiation and (b) after UV irradiation.
When the colored sample was kept for 10 min at 120 °C, 140 °C, and
160 °C in the dark, it changed to sample (c), (d), and (e), respectively.
The blue color of 3c, 4c, and 8c disappeared at an appropriate
temperature, while the blue color of 1c remained the original color.
When the colored sample was irradiated with visible light (> 500 nm)
for 1 min, it changed to (f). The blue color of 1c disappeared, while the
color of 3c, 4c, and 8c remained.
Fig. 4. The relationship between (a) traditional Hammett's parameter (σ)
or (b) Brown-Okamoto's parameter (σ+) and log(k/kH) at 120 °C. (c) and
(d) show the relationship between σ+ and Ea (c) and log(A) (d).

5
4.3. Thermal bleaching reaction
Fig. 5 shows the color changes of 1, 3, 4, and 8 by light and heat
ACCEPTED MANUSCRIPT
in the filter paper. To disperse the diarylethene at the molecular
level in the filter paper, the diarylethene was mixed with
poly(methyl methacrylate) (PMMA). The sample was prepared
by dipping the toluene solution containing diarylethene and
PMMA to the filter paper and drying. The colorless open-ring
isomers (Fig. 5a) tuned blue upon UV irradiation due to the
generation of the closed-ring isomers (Fig. 5b). Then, the
samples were kept at 120 °C in the dark. After 10 min, the blue
color of 8c disappeared, while the blue color of 1c, 3c, and 4c
remained (Fig. 5c). When the samples were kept for 10 min at
140 °C in the dark, the blue color of 4c and 8c disappeared, while
the blue color of 1c and 3c remained (Fig. 5d). Moreover, when
the samples were kept for 10 min at 160 °C in the dark, the blue
color of 3c, 4c, and 8c disappeared, while the blue color of 1c
remained (Figure 5e). On the other hand, when the colored
sample (Fig. 5b) was irradiated with visible light (> 500 nm) for
1 min, the blue color of 1c disappeared, while the color of 3c, 4c,
and 8c remained (Fig. 5f). Thus, diarylethenes 4 and 8 that we
synthesized in this work showed high photostability and thermal
cycloreversion reactivity even in solid medium. We succeeded in
tuning the thermal cycloreversion reactivity by introducing
various substituents into the terminal phenyl groups.
The thermal bleaching reaction of diarylethene closed-ring
isomers was carried out in toluene as follows. The diarylethene
open-ring isomer was put in an optical quartz cell degassed and
sealed under vacuum. The solution in the cell was irradiated with
313 nm light to give the closed-ring isomer. The cell was placed
in a constant temperature chamber (ESPEC ST-110) during the
thermal bleaching reaction. The reaction yields were periodically
determined by absorption spectroscopic measurements.
4.4. Preparation of photochromic papers and thermal
cycloreversion reaction in the papers
PMMA was dissolved in toluene to prepare a 10 wt% solution.
Diarylethene (ca. 1 wt% relative to the polymer) was added to the
polymer solution. The mixed solution was poured into a petri
dish, and a filter paper was dipped into the dish. The paper was
pulled out using tweezers and dried with a dryer. The filter paper
was cut into squares. The paper was irradiated with 313 nm light.
The colored filter paper after the photoirradiation was heated in a
constant temperature chamber (ESPEC ST-110) during the
thermal cycloreversion reaction. The same papers were used
repeatedly.
3. Conclusion
5. Acknowledgments
We have synthesized diarylethenes for a write-by-light/erase-
by-heat recording system. The introduction of polar substituents
at both sides of the diarylethene significantly changed the
thermal cycloreversion reactivity. The introduction of electron-
withdrawing substituents accelerated the reaction and the
introduction of electron-donating substituents suppressed the
reaction. The rate constants, k, were well correlated with Brown-
Okamoto's substituent constant σ+ that is a modified value of
Hammett's substituent constant σ. The increase in the k value is
ascribed to lower Ea. These results provide new knowledge for
molecular design of diarylethenes for a write-by-light/erase-by-
heat recording system.
This work was partly supported by JSPS KAKENHI Grant
Number JP26107013 in Scientific Research on Innovative Areas
"Photosynergetics" to S.K. and JSPS KAKENHI Grant Number
JP16K17896 in Scientific Research for Young Scientists (B) to
D.K. The authors also thank Nippon Zeon Co., Ltd. for providing
octafluorocyclopentene.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tet.****.**.***.
4. Experimental section
4.1. General
References and notes
Solvents used were of spectroscopic grade and purified by
distillation before use. 1H NMR (300 MHz) spectra were
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ACCEPTED MANUSCRIPT
Highlights
We designed photochromic diarylethenes for a write-by-light/erase-by-heat recording
system.
Diarylethenes having polar substituents at both sides were newly synthesized.
Photocyclization and photocycloreversion quantum yields remained constant even in the
introduction of the polar substituents.
Thermal cycloreversion reactivities were significantly accelerated by introduction of the
electron-withdrawing substituents.
We demonstrated a write-by-light/erase-by-heat recording system in the filter paper with
the ability to erase even at 120 °C.