Photochromic polymers bearing various diarylethene chromophores as the pendant: Synthesis, optical properties, and multicolor photochromism

Article abstract of DOI:10.1039/c1jm12707a

Photochromic polymers with various diarylethene derivatives were synthesized by a conventional radical polymerization of styrene derivatives having diarylethene chromophores as the pendant. All the polymers exhibited reversible photochromism in the film a

Full text of DOI:10.1039/c1jm12707a

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PAPER
Photochromic polymers bearing various diarylethene chromophores as the
pendant: synthesis, optical properties, and multicolor photochromism†
*
Hiroyasu Nishi, Tomoko Namari and Seiya Kobatake
Received 13th June 2011, Accepted 23rd August 2011
DOI: 10.1039/c1jm12707a
Photochromic polymers with various diarylethene derivatives were synthesized by a conventional
radical polymerization of styrene derivatives having diarylethene chromophores as the pendant. All the
polymers exhibited reversible photochromism in the film as well as in solution, while the
photocyclization conversion in the film decreased in comparison with that in solution because of
a restriction of the conformational structure in the solid state. Although the photocyclization and
photocycloreversion quantum yields at the initial stage of the reactions in the film were comparable to
those in solution, the apparent cycloreversion quantum yield decreased along with the reaction, which
is derived from a distribution of the quantum yields in the solid state. Finally, the photochromic
terpolymers consisting of three diarylethene monomers which show photochromism changing color to
cyan, magenta, and yellow were synthesized. The terpolymers were demonstrated to show bright colors
including black, green, red, and blue both in solution and in the solid state by selective bleaching
processes. Such multicolor photochromic polymers composed of diarylethene derivatives have
a potential for rewritable photochromic display devices.
medium can be reversibly written and erased by photo-
Introduction
irradiation, and the written information does not disappear in
the dark. In order to design and develop the photochromic full-
color rewritable materials, it is necessary to examine more
detailed information on the photochromic reactivities, thermal
stability, color tones, and fatigue-resistance. In addition, it is also
required to develop photochromic polymers.
Rewritable media are of great benefit not only to our lives but
also in terms of limited resources and environmental problems
that we have been confronted with. Although plenty of rewrit-
able data storage and display media have been developed with
significant progress of personal computers, piles of paper media
are still used in a variety of situations. One reason for the great
use of the paper is that the medium has an advantage over
electronic display media in terms of convenience and readability,
and is useful for a temporary recording medium rather than for
permanent data storage. Meanwhile, most of them are single-use
and end up in the garbage. From this point of view, a lot of
paper-like rewritable display devices such as electronic papers
and thermosensitive rewritable papers have also been developed.
Among them, a paper-like and full-color display medium using
three photochromic fulgide derivatives changing color to yellow
(Y), cyan (C), and magenta (M) has been reported.1 Fulgide is
one of the photochromic compounds whose color can be
reversibly changed upon alternating irradiation with ultraviolet
(UV) and visible light, and most of the fulgide derivatives never
have bleached color at room temperature in the dark unlike
azobenzene and spiropyran derivatives.2 Thus, the photochromic
Diarylethene is known as a thermally stable photochromic
compound that undergoes back-and-forth photoisomerization
between a colorless open-ring form and a colored closed-ring
form upon alternating irradiation with UV and visible light.3,4
The photochromic reaction of a diarylethene derivative can be
repeated more than 10 000 cycles, and the color can be modu-
lated by changing the structure or additional substituent.3 These
excellent properties are suitable for the rewritable display
medium. Along these lines, there are several reports focusing on
multicolor photochromic materials using the diarylethene
derivatives. For instance, a fused diarylethene trimer consisting
of three different diarylethene frameworks,5 multicolor two- or
three-component diarylethene mixed crystals,6,7 diarylethene
mixture on a silica plate,8 and multicolor polymer materials9 have
been reported.
Diarylethene derivatives dispersed in a matrix,8,10 incorporated
into the main chain of a polymer,11–14 or connected to the poly-
mer chain9,15–21 are useful in terms of application for the rewrit-
able display devices because they can directly be coated on paper
or any other solid surfaces. However, in most cases, the reactivity
of the diarylethene derivatives in the polymer matrix is different
from that in solution. Especially, the movement of the molecules
Department of Applied Chemistry, Graduate School of Engineering, Osaka
City University, 3-3-138 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
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1jm12707a
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depends on the glass transition temperature (Tg) and the free
volume in the polymer matrix. Therefore, the photocyclization
and cycloreversion quantum yields are affected by Tg of
the polymer matrix.21 A distribution of the quantum yields of
diarylethenes in a polymer matrix has been evaluated by
numerical calculation.22,23 Namely, the photochromic reaction of
the diarylethene derivatives in the polymer matrix is affected by
the ambient surrounding such as stiffness of the matrix and the
situation of the chromophores embedded in the matrix. An
understanding of the photochromic reaction behavior in the solid
state is still an important topic to develop the rewritable and
paper-like display media.
calorimetry (DSC) was performed on a SEIKO Instruments
EXSTAR 6000 DSC6200. Absorption spectra were measured
using a UV/visible absorption spectrophotometer (JASCO V-
560). Photoirradiation was conducted using a 200 W mercury–
xenon lamp (MORITEX MUV-202) or a 300 W xenon lamp
(Asahi Spectra MAX-301) as a light source. Monochromatic
light was obtained by passing the light through a mono-
chromator (Jobin Yvon H10 UV or JASCO CT-10) and glass
filters. Photocyclization conversions of diarylethene chromo-
1
phores were determined by H NMR and absorption spectros-
copies. Photocyclization and cycloreversion quantum yields were
determined as relative quantum yields using diarylethene deriv-
atives whose quantum yields have already been determined.25
Molar absorption coefficients of the diarylethene polymer films
are assumed to be the same as those in toluene.
Color-modulation is, of course, essential for the full-color
photochromic display media. Multicomponent, multi-
addressable polymers composed of diarylethenes showing blue
(B), red (R), and yellow (Y) have already been synthesized by
ring-opening metathesis polymerization.9 Although the polymers
showed a wide range of bright colors, they did not entirely
demonstrate all the colors from a single polymer. A lot of
diarylethene derivatives have been so far reported. However,
there is no report on the completely full-color photochromism
showing all the colors in the CMY system in a single component.
If we can incorporate three diarylethene moieties showing C, M,
and Y in the appropriate ratio, all the colors including black (K),
green (G), and so forth can be demonstrated in the system.
We have previously synthesized a photochromic polymer with
a diarylethene moiety as a pendant (poly(2a) in Scheme 1) by
a conventional radical polymerization.24 Such a pendant-type
photochromic polymer has a large advantage to contain high
density (76 wt% for poly(2a)) of the chromophores in the poly-
mer, which can display vivid and high-contrast images. In
addition, poly(2a) exhibited reversible photochromism with high
cyclization conversion (92%) even in the film to change color to
B. In order to develop the rewritable full-color display devices,
we have to prepare the photochromic polymers showing all the
colors in the CMY system.
Materials
2,20-Azobisisobutyronitrile (AIBN) was purified by recrystalli-
zation from methanol. Toluene was purified by distillation. All
other reagents were commercially available and used without
further purification.
Synthesis of diarylethene monomers
Diarylethene monomer 2a was prepared by the method described
in our previous paper.24 1a and 3a–8a were synthesized from the
corresponding diarylethenes with a formyl group as well as 2a.
The synthetic scheme of all the diarylethene monomers is shown
in Scheme S1 in the ESI†. The details are described in the ESI†.
Synthesis of diarylethene homo- and terpolymers
Diarylethene homo- and terpolymers were prepared by
a conventional radical polymerization of the corresponding
ꢀ
monomers using AIBN as the initiator in toluene at 60 C. The
polymers were obtained by precipitation in methanol for poly
(1a)–poly(7a) or hexane for poly(8a) and terpolymers. The
resulting polymers and terpolymers were purified by
reprecipitation.
In this study, we designed, synthesized, and characterized
novel diarylethene homopolymers (poly(1a)–poly(8a)) as shown
in Scheme 1. All the homopolymers exhibited reversible photo-
chromism in the solid state as well as in solution with relatively
high cyclization conversion. The cyclization and cycloreversion
quantum yields of the polymers were examined both in solution
and in the film. Finally, we chose three components from di-
arylethene monomers 1a–8a to display all the colors in the CMY
system and synthesized their diarylethene terpolymers to
demonstrate the rewritable process on paper media by
photoirradiation.
Synthesis of diarylethene derivatives
Diarylethene derivatives (1a0–8a0) as references were synthesized
according to the methods similar to the diarylethene monomers.
The details are described in the ESI†.
Results and discussion
Polymerization
Experimental
Table 1 shows polymerization conditions, yields, number-
average molecular weight (Mn), polydispersity (Mw/Mn), and
glass transition temperature (Tg) of the diarylethene homopoly-
mers. The polymerization reactivities of 1a–8a are similar to each
other independent of the molecular structure of the diarylethene
moieties in the side chain. Tg of the polymers was 77–120 ꢀC
depending on the molecular structure of the side chain. Fig. 1
shows the relationship between Tg and molar mass of the
pendant group of the polymers. Tg increased with an increase in
the molar mass of the pendant group. It is known that Tg values
Measurements
1H NMR spectra were recorded on
a JEOL A-400 or
a BRUKER AV-300N spectrometer. Mass spectra were taken
with a JEOL JMS-700T mass spectrometer. Gel-permeation
chromatography (GPC) was performed using a Tosoh ꢀ8000
series GPC system equipped with TSK-gel columns at 40 C in
tetrahydrofuran (THF) as the eluent. Standard polystyrenes were
used as the calibration standard. Differential scanning
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Scheme 1 Diarylethene homopolymers and reference compounds used in this work.
Table 1 Polymerization conditions and characterization of poly(1a)–(8a)
Monomer
[Monomer]/M
[AIBN]/M
Time/h
Yield (%)
Mn
Mw/Mn
Tg/ꢀC
1a
2aa
3a
4a
5a
6a
7a
8a
0.37
0.37
0.37
0.37
0.37
0.37
0.37
0.37
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
14.5
10
10
10
10
10
10
10
43
59
70
79
83
53
73
68
34 000
22 400
24 000
41 900
25 900
21 400
28 600
24 400
2.51
2.18
2.26
2.24
2.21
2.31
1.99
1.76
120
106
104
102
93
112
82
77
a
Ref. 24
of polystyrene derivatives depend on the molecular weight of the
polymer26 and side chain structure.27,28 Because there is no
significant difference in Mn values of the polymers in the present
work, it is considered that bulkiness of the diarylethene moieties
in the side chain directly affects cooperative motion of the chain
segments. The Tg values of the diarylethene polymers in the
present work are much higher than room temperature as well as
polystyrene. Therefore, they can be used as a film in the solid
state at ambient temperature.
Photochromism of diarylethene homopolymers in solution
Fig. 2 shows absorption spectral changes of poly(1a) in toluene
upon irradiation at 313 nm. The new absorption band appeared
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Fig. 1 Relationship between Tg and molar mass of the pendant group in
poly(1a)–poly(8a).
in the visible region, which can be assignable to the generation of
poly(1b). The spectrum reverted to the initial shape upon irra-
diation with visible light because of the photocycloreversion
reaction to poly(1a). Such photochromic behavior was observed
in all the diarylethene homopolymers. The absorption spectra of
poly(1a)–poly(8a) in toluene in the photostationary state
(PSS) upon irradiation at 313 nm are shown in Fig. 3a. The
diarylethene polymers exhibited various colors depending on the
structure of the diarylethene moiety in the side chain and ranging
in the visible region from 400 nm to 750 nm. Therefore, we
succeeded in preparing the photochromic polymer materials
using the diarylethene derivatives with absorption spectra
covering the entire visible region. Table 2 summarizes the
Fig. 3 Normalized absorption spectra of poly(1a)–poly(8a) (a) in
toluene and (b) in the film in the PSS upon irradiation at 313 nm.
the photochromic reaction in high photocyclization conversion
in solution as well as polymer-free diarylethenes (1a0–8a0).
absorption maximum (lmax
) in the visible region, molar
absorption coefficient (3) at lmax, photocyclization conversions
in the PSS, photocyclization quantum yields upon irradiation at
313 nm (Fo/c), and photocycloreversion quantum yields upon
irradiation at lmax (Fc/o) for poly(1a)–poly(8a) in toluene.
Table 3 shows those for 1a0–8a0 as the reference compounds in
hexane.29,30 One can notice that the optical properties of the
diarylethene polymers are almost the same as those of the cor-
responding reference compounds. This result indicates that the
diarylethene moieties attached to the polymer chain can undergo
Photochromism of diarylethene homopolymers in the solid state
Fig.
4 shows photographs of poly(1a)–poly(8a) powders
before and after UV light irradiation. All the polymer powders
exhibited bright colors upon irradiation with UV light, and the
color disappeared upon irradiation with visible light. These
results indicate that poly(1a)–poly(8a) exhibited reversible
photochromism in the solid state as well as in solution. Fig. 3b
shows absorption spectra of poly(1a)–poly(8a) films cast on the
quartz glass in the PSS upon irradiation at 313 nm. The cast films
were fabricated by casting the toluene solution of the diary-
lethene polymers on the quartz glass. The absorption spectra in
the cast films are similar to those in the solution, while the
cyclization conversions decreased to 40–70% of those in the
solution except for poly(2a) as can be seen from Table 2. The
decrease of the cyclization conversion is considered to be mainly
due to a structural restriction of the diarylethene moiety in the
solid state. The open-ring form of diarylethene derivatives has
two conformations: parallel conformation (P form) and anti-
parallel conformation (AP form) in which the aryl rings are in
mirror symmetry and C2 symmetry, respectively.30 The cycliza-
tion reaction takes place only from the AP form. In solution, the
ratio of the AP form to the P form is ca. 1 : 1 in most cases unless
the diarylethene derivative is characteristically designed to form
the AP form.31,32 In addition, the diarylethene derivative can
thermally transform to each conformation in solution at room
temperature. This is the reason why the cyclization conversion in
Fig. 2 Absorption spectral changes of poly(1a) in toluene upon irradi-
ation at 313 nm: [poly(1a)] ¼ 1.8 ꢁ 10ꢂ5 M.
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Table 2 Absorption maximum in the visible region (lmax), molar absorption coefficient at lmax (3), photocyclization conversions upon irradiation at 313
nm, photocyclization quantum yields upon irradiation at 313 nm (Fo/c), and photocycloreversion quantum yields upon irradiation at lmax (Fc/o) of
poly(1a)–poly(8a) in toluene and in the film
Conversion (%)
In toluene
Fo/c
Fc/o
Diarylethene polymer
lmax/nm (3/Mꢂ1 cmꢂ1) in toluene
In film
In toluene
In filmb
In toluene
In filmc
Poly(1a)
Poly(2a)a
Poly(3a)
Poly(4a)
Poly(5a)
Poly(6a)
Poly(7a)
Poly(8a)
612 (22 700)
592 (16 900)
576 (10 300)
534 (14 200)
534 (14 300)
547 (12 800)
470 (10 300)
434 (5900)
96
97
79
88
80
90
71
69
56
92
40
63
44
64
47
28
0.50
0.54
0.57
0.47
0.52
0.55
0.38
0.43
0.46
0.53
0.34
0.45
0.47
0.44
0.35
0.39
0.00033
0.0044
0.013
0.014
0.0045
0.031
0.091
0.57
0.00031
0.0034
0.0085
0.010
0.0023
0.020
0.075
0.49
a
b
Ref. 24. At the initial stage of photocyclization from the open-ring form. c At the initial stage of photocycloreversion from the PSS.
Table 3 Optical properties of 1a0–8a0 in hexane
Diarylethene
lmax/nm (3/Mꢂ1 cmꢂ1
)
Conversion (%)
Fo/c
Fc/o
Ref.
1a0
2a0
3a0
4a0
5a0
6a0
7a0
8a0
602 (24 900)
575 (15 600)
562 (11 000)
524 (14 900)
528 (17 200)
537 (15 500)
459 (11 200)
423 (6470)
97
97
79
87
93
81
77
77
0.55
0.59
0.46
0.48
0.54
0.55
0.40
0.45
0.00038
0.0083
0.015
0.018
0.0083
0.090
0.11
This work
29
30
This work
This work
This work
This work
This work
0.57
Fig. 4 Photographs of (a) poly(1a), (b) poly(2a), (c) poly(3a), (d) poly(4a), (e) poly(5a), (f) poly(6a), (g) poly(7a), and (h) poly(8a) powders before (left)
and after (right) irradiation with UV light.
solution upon irradiation with UV light is relatively high (69–
97%) while the photocyclization quantum yield is around 50%. In
the film whose Tg is higher than room temperature, however, it is
difficult for the diarylethene moiety to convert to each form
during the two conformations because the chromophore is
confined in the solid state. Thus, the photocyclization conver-
sions of poly(1a)–poly(8a) in the film were lower than those in the
solution. In fact, the cyclization quantum yields in the film at the
initial stage of the reaction are comparable to those in the solu-
tion as shown in Table 2. These results indicate that the decrease
of the cyclization conversion was ascribed not to the decrease of
the quantum yield, but to the structural restriction of the chro-
mophore in the solid state. It is noteworthy that the poly(2a) film
exceptionally exhibited high cyclization conversion.24 It is
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possible that each conformation (P and AP forms) of the
diarylethene moiety in poly(2a) can easily transform into another
form even in the film. This fact is considered to arise from
a structural character of poly(2a). In the solid state, it is
concluded that the cyclization conversions for poly(1a)–poly(8a)
are high enough to exhibit brilliant colors.
decrease derived from the structural restriction of the chromo-
phore in the solid state was observed as the reaction proceeds.
These results are consistent with a system reported previously in
which diarylethene derivatives are dispersed in polymer
matrices.22 As seen in Fig. 5, a trend of the decrease of the
apparent Fc/o along with the cycloreversion reaction is similar
to each other. It is worth mentioning that the molecular structure
and the optical property of the diarylethene moiety in the side
chain have a significant effect on the apparent Fc/o distribution.
We could show that the photocycloreversion reactivities of the
diarylethene derivatives tethered to polymer chain as the pendant
are also affected by the structural restriction in the film as well as
the diarylethene derivatives dispersed in the polymer matrix.
Cycloreversion quantum yields of diarylethene homopolymers in
the solid state
In order to discuss the photochromic reactivity of the diarylethene
homopolymers in the solid state, the photocycloreversion
quantum yields in the films of poly(1a)–poly(8a) were examined
using the films in the PSS upon irradiation at 313 nm. The
cycloreversion quantum yields (Fc/o) can be determined using
the following equation:25
Design of multicolor diarylethene terpolymer
In order to express multicolor photochromism based on the
CMY system, we should prepare at least three diarylethene
components showing C, M, and Y. As described above, we
prepared various types of diarylethene polymers and examined
their optical properties. According to their properties, poly(1a),
poly(4a), and poly(8a) were chosen as the C, M, and Y compo-
nents, respectively. Although the multicolor photochromism can
be achieved by a polymer blend consisting of poly(1a), poly(4a),
and poly(8a), we aimed to create more sophisticated photo-
chromic materials, terpolymers composed of three diarylethene
monomers, 1a, 4a, and 8a. Using such terpolymers we can
demonstrate all the colors from only a single polymer compo-
nent, which leads to no concern with phase separation of the
chromophores or homopolymers, ease of the coating process to
fabricate the full-color devices, and writing more microscopic
images. If the terpolymers can exhibit K upon irradiation with
UV light, various colors and images will be expressed by selective
bleaching to apply to multicolor displays, inks, and barcodes. In
order to demonstrate K, the three diarylethene chromophores in
the terpolymer should show similar extent of color (absorbance)
log (10A(t) ꢂ 1) ¼ ꢂFc/o3It + log (10A(0) ꢂ 1)
(1)
where A(0) and A(t) are absorbances at the irradiation wave-
length before and after irradiation for time t, respectively. 3 and I
are the molar absorption coefficient and intensity of the incident
light, respectively, at the wavelength. The plot of t against the
left-hand side term in eqn (1) shows a linear relationship as far as
the Fc/o value is constant. The Fc/o values for poly(1a)–poly
(8a) and 1a0–8a0 in the solution can be unambiguously deter-
mined from the slope as shown in Table 2 because of the linear
relationship. In the film, however, the plots between the left-hand
side term in eqn (1) and irradiation time did not show the linear
relationship. It means that the cycloreversion quantum yields
change depending on the conversion. The relationship between
the apparent Fc/o determined from the slope of the tangential
line to eqn (1) fitted to the experimental data and the content of
the closed-ring form in the polymer is shown in Fig. 5. The Fc/o
value at the initial stage of the reaction in the film shown in Table
2 is comparable to that in the solution, while a significant
Fig. 5 Relationship between the apparent cycloreversion quantum yield and the content of the closed-ring form for poly(1a)–poly(8a).
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in the polymer after UV light irradiation, as shown in Fig. 6. The
ideal molar ratio of three components in the terpolymer ([1a] :
[4a] : [8a] ¼ 10 : 14: 76) can be calculated by considering the
cyclization conversion in the film and the molar absorption
coefficient shown in Table 2: (0.56 ꢁ 22 700 ꢁ [1a]) : (0.63 ꢁ
14 200 ꢁ [4a]) : (0.28 ꢁ 5900 ꢁ [8a]) ¼ 1 : 1 : 1.
least more than 50 times in toluene without being degassed,
keeping the photochromic performance in more than 80%.3
The appearance of the multicolor photochromism of
terpolymer 1 in toluene by selective bleaching is shown in Fig. 8.
The colorless solution turned to K upon irradiation at 313 nm.
The K solution changed into vivid R, G, and B solutions upon
irradiation at >680, 550, and 434 nm, which is due to the
disappearance of C, M, and Y, respectively. Then, Y, C, and M
solutions were obtained by exposure of >580, 434, and >680 nm
light to the R, G, and B solutions, respectively. We successfully
prepared a multicolor photochromic terpolymer showing all the
colors in the CMY system. To the best of our knowledge, such
multicolor photochromism including K using only a single
polymer component have not been so far reported.
The polymerization conditions including the ideal one dis-
cussed above, yields, Mn, Mw/Mn, and Tg of the terpolymers are
shown in Table 4. The molar ratio of [1a] : [4a] : [8a] in the
1
terpolymer was determined by H NMR spectroscopy, and was
almost the same as that in the feed. In addition, yield, Mn, and
Mw/Mn of the terpolymers were similar to each other. These
results also indicate that the diarylethene monomers have similar
polymerization reactivities independent of the side-chain struc-
ture and we can prepare photochromic terpolymers consisting of
the desirable diarylethene monomers in an arbitrary ratio. Tg of
the terpolymers were 85–88 ꢀC, which is well reproduced by Fox
and Pochan equations33 using weight fractions of 1a, 4a, and 8a
in the terpolymer and Tg of the homopolymers. Thus, the molar
ratio and the weight fraction of the chromophores in the polymer
chain, Tg, solubility, and so on can be controlled by copoly-
merization in the presence of the various diarylethene monomers
and other versatile monomers.
Photochromism of diarylethene terpolymer in solid state
To apply the photochromic polymer as the full-color display
medium, the photochromic polymer should be used in the solid
state such as film, paper, and glass. Thus, in this section, the
photochromic reaction of terpolymer 1 was investigated in the
film and was demonstrated for the multicolor photochromism on
a paper medium. Fig. 9a and b show the changes in difference
spectra of terpolymer 1 in the film upon irradiation with UV and
visible light. A new absorption band appeared upon irradiation
at 313 nm and the C, M, and Y components were selectively
bleached upon irradiation at >680, >580, and >450 nm, respec-
tively as well as in the solution. The absorption spectra of poly
(1b), poly(4b), and poly(8b) segments in the PSS are summarized
in Fig. 9c. The three spectral components show almost the same
absorption intensity as designed in the previous section. In
addition, the spectral shape of the terpolymer in the PSS is
similar to the simulated one expected in Fig. 6. These results
indicate that we succeeded in designing and preparing the
photochromic terpolymer with an ideal ratio of diarylethene
chromophores.
Photochromism of diarylethene terpolymer in solution
Fig. 7a and b show absorption spectral changes of terpolymer 1
in toluene upon irradiation with UV and visible light. A new
absorption band ranging from 400 to 750 nm appeared upon
irradiation at 313 nm. The absorption band assigned to poly(1b)
segment disappeared upon irradiation at >680 nm because of the
cycloreversion reaction to poly(1a). In the same manner,
absorption bands derived from poly(4b) and poly(8b) segments
also disappeared upon irradiation at >580 nm and >450 nm,
respectively, and the absorption spectrum reverted to the original
position as shown in Fig. 7b. The absorption spectra corre-
sponding to poly(1b), poly(4b), and poly(8b) are summarized in
Fig. 7c. The spectral components have almost the same shape as
those of homopolymers. It means that the diarylethene chro-
mophores exhibited their original color even in the terpolymer.
The coloration and decoloration of the terpolymer upon alter-
nating irradiation at 313 nm and >480 nm could be repeated at
Finally, we fabricated a rewritable photochromic display
medium using a filter paper to demonstrate multicolor photo-
chromism in the solid state. The filter paper (55 mm in diameter)
was impregnated with terpolymer 1 in CHCl3 and dried under
air. Fig. 10 shows the demonstration of the multicolor photo-
chromism on the filter paper upon photoirradiation. Upon
irradiation with UV light, the filter paper turned to complete K,
and the color G appeared as a spot by partial irradiation with G
light (550 nm). In the same manner, a partial exposure of R and B
light (>680 and 434 nm, respectively) generated R and B spots on
the paper. The image could be selectively or completely removed
upon irradiation with visible light (>480 nm), and a new image
could be rewritten on the filter paper. The photochromic
terpolymer consisting of 1a, 4a, and 8a exhibited vivid colors in
the CMY system even in the solid state. Furthermore, the image
ꢀ
has never been bleached even at 100 C for a week. Our photo-
chromic terpolymer has appropriate properties such as thermal
stability, fatigue-resistance, high content of the chromophore in
the polymer chain (ca. 72 wt% for terpolymer 1), and brilliant
colors based on the CMY system to be used as the rewritable
display device. This work will contribute to fabricate the full-
color rewritable display device.
Fig. 6 Normalized absorption spectra of poly(1a) (---), poly(4a) (
),
and poly(8a) ( ) in toluene in the PSS upon irradiation at 313 nm
and the sum of these spectra (solid line).
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Table 4 Polymerization conditions and characterization of the resultant diarylethene terpolymersa
Mw/Mn [1a] : [4a] : [8a] in terpolymer Tg/ꢀC (Fox)b Tg/ꢀC (Pochan)c Tg/ꢀC (Observed)
Entry [1a] : [4a] : [8a] in feed Yield (%) Mn
1
2
3
10 : 14 : 76
6 : 16 : 78
3 : 10 : 86
64
71
61
24 200 1.98
23 900 2.05
23 400 1.98
8 : 14 : 78
7 : 18 : 75
4 : 11 : 85
84
84
81
84
85
82
85
88
85
a
[Monomers] ¼ 0.37 M, [AIBN] ¼ 0.020 M, in toluene, 60 ꢀC, 10 h. b Calculated by the Fox equation. c Calculated by the Pochan equation.
Fig. 7 Absorption spectral changes of terpolymer 1 in toluene (a) upon irradiation at 313 nm, (b) upon subsequent irradiation at >680 nm (----), >580
nm ( ), and >480 nm ( ), and (c) difference spectrum before and after irradiation at 313 nm (solid line) and spectral decrease upon the
subsequent irradiation at >680 nm (---), >580 nm ( ), and >480 nm ( ).
Fig. 8 Multicolor photochromism of terpolymer 1 in toluene.
Fig. 9 Changes in difference spectra of terpolymer 1 in the film (a) upon irradiation at 313 nm, (b) upon subsequent irradiation at >680 nm (----), >580
nm (
), and >480 nm (
), and (c) difference spectra before and after irradiation at 313 nm (solid line) and spectral decrease upon the
subsequent irradiation at >680 nm (---), >580 nm (
), and >480 nm (
).
17256 | J. Mater. Chem., 2011, 21, 17249–17258
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Fig. 10 Multicolor photochromism of filter paper impregnated with terpolymer 1 upon irradiation at 313 nm and selective irradiation with visible light.
€
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We synthesized various types of diarylethene homopolymers and
terpolymers by a conventional radical polymerization. The
photochromic polymers exhibited reversible photochromism in
the film as well as in solution. In the film, however, the photo-
cyclization quantum yield of diarylethene moieties in the poly-
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