Fast photochrome polymers carrying [2.2]paracyclophane-bridged imidazole dimer

Article abstract of DOI:10.1021/ma100197z

The synthesis and photochromic behavior of the fast photochromic polymers carrying [2.2]paracyclophane-bridged imidazole dimer are demonstrated. A significant feature of this synthetic strategy is that we can modify the photochromic properties such as coloration/decoloration rate, coloring, and photosensitivity via the stepwise synthetic approach of the imidazole dimer system. Notably, the photochromic behavior of the polymers is not affected by the environment around the photochromes and copolymerization with other monomers in both solution and film, which cannot be realized in any other conventional photochromic systems. The comparable photochromic behavior of the homopolymers and. copolymers in solution and film indicates that the photochromic unit is independent from the local environment, which allows effective molecular design of the photochromic monomer unit to accomplish desired photochromic properties of the polymer.

Full text of DOI:10.1021/ma100197z

3764 Macromolecules 2010, 43, 3764–3769
DOI: 10.1021/ma100197z
Fast Photochromic Polymers Carrying [2.2]Paracyclophane-Bridged
Imidazole Dimer
Atsushi Kimoto,† Atsuhiro Tokita,‡ Takeru Horino,‡ Toyoji Oshima,‡ and Jiro Abe*,†
†Department of Chemistry, School of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe,
Sagamihara, Kanagawa 229-8558, Japan, and ‡Tokyo Research Laboratory,
Mitsubishi Gas Chemical Company, Inc., 6-1-1 Niijuku, Katsushika, Tokyo 125-0051, Japan
Received January 26, 2010; Revised Manuscript Received March 17, 2010
ABSTRACT: The synthesis and photochromic behavior of the fast photochromic polymers carrying [2.2]para-
cyclophane-bridged imidazole dimer are demonstrated. A significant feature of this synthetic strategy is that we
can modify the photochromic properties such as coloration/decoloration rate, coloring, and photosensitivity via
the stepwise synthetic approach of the imidazole dimer system. Notably, the photochromic behavior of the
polymers is not affected by the environment around the photochromes and copolymerization with other mono-
mers in both solution and film, which cannot be realized in any other conventional photochromic systems. The
comparable photochromic behavior of the homopolymers and copolymers in solution and film indicates that
the photochromic unit is independent from the local environment, which allows effective molecular design of the
photochromic monomer unit to accomplish desired photochromic properties of the polymer.
Introduction
by any other currently available photochromic systems. Develop-
ing into bulk photochromism, investigation of polymeric system is
Controlling photochromic property in bulk phase is an impor-
tant and challenging issue for investigating the high-performance
photochromic systems and photomechanical properties.1,2 How-
ever, realizing universally applicable photochromic system in both
solution and film includes difficulties because the photochromic
reaction is generally affected by local environments, such as rigi-
dity, viscosity, polarity, and free volume. Thus, elaborate molecular
design of monomers does not always give desired photochromic
properties, especially coloration/decoloration rate, in bulk phase.
Applying photochromic compounds as a phototriggered switch3-8
into stimuli-responsive materials (intelligent materials),9-11 the
equality of the photochromic behavior in solution and film is
strongly required, and an enthusiastic search for such compounds
is still ongoing. This article introduces a unique millisecond-order
polymer photochromic system characterized by smaller interaction
with other components and polymer matrices, allowing versatile
functional designs. The fast photochromic polymers can be ob-
tained via rational synthetic procedure, which enables to accom-
plish designer photoinduced switching polymer materials.
We recently investigated a unique series of photochromic mole-
cules based on [2.2]paracyclophane-bridged imidazole dimer,
which allow instantaneous coloration upon exposure to UV light
and rapid decoloration in the dark.12-15 The half-life of the
colored species derived from the homolytic photocleavage of the
C-N bond of imidazole dimer is on the order of milliseconds
in solution. Another peculiar aspect of the photochromism of
the imidazole dimer systems is photogeneration of imidazolyl
radicals with an unpaired electron, which could modulate the
magnetic property simply by turning the optical stimulation on
and off.16-18 The important aspect of paramagnetic character
includes an intrinsically large molar absorption coefficient, which
brings about a significant change in refractive index. This fast
optical switching in both colors and spin states cannot be realized
of critical importance to achieve rapid photomodulation of
various physicochemical properties.19-22
From standpoint of synthetic approach, [2.2]paracyclophane-
bridged imidazoledimer can be prepared viaa stepwise formation
of two imidazole rings by the condensation reaction between
aromatic 1,2-diketones and two aldehyde groups of pseudogem-
bisformyl[2.2]paracyclophane as a starting material (Scheme 1).
Recently, we proposed that this synthetic strategy allows rational
design for achieving highly functionalized photochromes with
desired performances such as thermal-bleaching rate, coloring,
and photosensitivity. Herein, the stepwise synthetic approach of
[2.2]paracyclophane-bridged imidazole dimer to yield a fast
photochromic polymer is demonstrated.
Experimental Section
Materials. All reactions were monitored by thin-layer chro-
matography carried out on 0.2 mm E. Merck silica gel plates
(60F-254). Column chromatography was performed on silica gel
(Wakogel C-300). All reagents were purchased from TCI, Wako
Co. Ltd., Aldrich Chemical Co., Inc., and Acros Organics and
were used without further purification. All reaction solvents
were distilled on the appropriate drying reagents prior to use.
Measurements. The NMR spectra were recorded on a JMN-
ECP500A (JEOL) spectrometer, and the chemical shifts are quoted
in ppm relative to tetramethylsilane. The UV-vis absorption spec-
tra were measured on a UV-3150 (Shimadzu) spectrometer. The
FAB mass spectra were measured with an MStation MS-700
(JEOL) spectrometer by using 3-nitrobenzyl alcohol as a matrix.
Molecular weights and polydispersities of the polymers were esti-
mated with gel permeation chromatography (GPC) analysis per-
formed in tetrahydrofuran (THF, 1.0 mL/min) at 40 °C using a
Shimadzu GPC apparatus, with a Shimadzu RID-10A detector and
a Shodex KF-806 L column. The GPC was calibrated with nar-
row polydispersity polystyrene standards (Shodex STANDARD
SM-105). The laser flash photolysis experiments were carried out
with a Unisoku TSP-1000 time-resolved spectrophotometer.
*Corresponding author: Tel þ81-(0)42-759-6225; Fax þ81-(0)42-759-
6225; e-mail jiro_abe@chem.aoyama.ac.jp.
pubs.acs.org/Macromolecules
Published on Web 03/26/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 8, 2010 3765
Scheme 1. Synthetic Approach of Functionalized [2.2]Paracyclophane-
Scheme 2. Synthesis of Methacrylate-benzila
Bridged Imidazole Dimer
a Reagents and conditions: (i) 6-chloro-1-hexanol, potassium carbo-
nate, potassium iodide, DMF, 80 °C, 12 h; (ii) methacryloyl chloride,
triethylamine, CH2Cl2, rt, 6 h.
6.98-6.94 (m, 4H), 6.09 (s, 1H), 5.55 (s, 1H), 4.16 (t, J = 7.0
Hz, 2H), 4.04 (t, J = 7.0 Hz, 2H), 3.89 (s, 3H), 1.94 (s, 3H),
1.84-1.73 (m, 2H), 1.57-1.39 (m, 4H). 13C NMR (125 MHz,
CDCl3) δ: 193.87, 193.82, 167.85, 165.18, 164.75, 136.82, 132.71,
126.69, 126.48, 125.57, 115.04, 114.62, 68.58, 64.90, 55.96, 29.25,
28.87, 26.08, 25.99, 18.66. FAB-MS: m/z 425 [M þ H]þ.
Synthesis of H-CHO or OMe-CHO. Pseudogem-bisformyl-
[2.2]paracyclophane (1 equiv), benzil derivative (1 equiv), and
ammonium acetate (15 equiv) were heated at 80 °C in acetic acid
for 12 h. The reaction mixture was cooled to room temperature and
was neutralized with aqueous NH3. The resulting precipitate was
filtered and washed with water. The crude mixture was purified
with silica gel column chromatography using THF/hexane=1/2
as eluent to give the target compound as yellow powders.
A Continuum Minilite II Nd:YAG (Q-switched) laser with the
third harmonic at 355 nm (ca. 8 mJ per 5 ns pulse) was employed
for the excitation light. The probe beam from an OSRAM
HLX64623 halogen lamp was guided with an optical fiber scope
to be arranged in an orientation perpendicular to the exciting laser
beam. The probe beam was monitored with a Hamamatsu R2949
photomultiplier tube through a prior to the laser flash photolysis
experiments. The vis-NIR absorption spectra of the absorption
spectra of the colored species in film state were obtained using
Fastevert S-2600 multichannel spectrometer (Soma Optics Ltd.)
using a halogen lamp as the probe beam. The sample was cooled to
253 K using a liquid nitrogen cryostat USP-203-A (Unisoku, Inc.)
in order to decelerate the recombination reaction. After UV
irradiation was carried out using a Keyence UV-400 series UV-
LED (UV-50H type), equipped with a UV-L6 lens unit (365 nm,
irradiation power 300 mW/cm2) for 2 s, the absorption spectrum
was immediately measured.
Synthesis of OH-benzil. To a suspension of 4-hydroxy-40-
methoxybenzil (4.00 g, 15.6 mmol), potassium carbonate (3.02 g,
21.8 mmol), and potassium iodide (0.052 g, 0.312 mmol) in
degassed N,N-dimethylformamide (55 mL) was added 6-chloro-
1-hexanol (3.1 mL, 23.4 mmol). The reaction mixture was heated
under a N2 atmosphere at 80 °C for 12 h and poured into water.
After extraction using ethyl acetate, the organic layer was washed
with water and brine and dried over Na2SO4. To a solution of the
crude mixture in dichloromethane, a large amount of hexane was
added to give OH-benzil as pale yellow powders (5.47 g, 98%). 1H
NMR (500 MHz, CDCl3) δ: 7.95-7.92 (m, 4H), 6.97-6.94 (m,
4H), 4.04 (t, J = 6.5 Hz, 2H), 3.88 (s, 3H), 3.66 (t, J = 6.5 Hz, 2H),
1.84-1.81 (m, 2H), 1.62-1.59 (m, 2H), 1.52-1.44 (m, 4H), 1.32 (s,
1H). 13C NMR (125 MHz, CDCl3) δ: 193.88, 193.83, 165.19,
164.79, 132.71, 126.70, 126.46, 115.06, 114.62, 68.65, 63.17, 55.97,
32.96, 29.33, 26.13, 25.84. FAB-MS: m/z 357 [M þ H]þ.
H-CHO. 1H NMR (500 MHz, DMSO-d6) δ: 12.14 (s, 1H),
9.55 (s, 1H), 7.58 (d, J= 7.5 Hz, 2H), 7.51 (d, J= 7.0 Hz, 2H), 7.47
(dd, J = 7.5 Hz, 2H), 7.40 (t, J = 7.0 Hz, 1H), 7.31 (dd, J = 7.0
Hz, 2H), 7.16 (t, J = 7.0 Hz, 1H), 7.10 (s, 1H), 6.96 (s, 1H), 6.84 (d,
J= 7.5 Hz, 1H), 6.71 (d, J= 8.0 Hz, 1H), 6.66 (d, J=7.5Hz, 2H),
4.49-4.45 (m, 1H), 3.95-3.92 (m, 1H), 3.15-2.97 (m, 6H). 13C
NMR (125 MHz, CDCl3) δ: 190.45, 145.29, 140.38, 140.17,
138.52, 138.15, 138.10, 136.78, 136.57, 136.10, 135.37, 133.24,
132.84, 131.44, 131.10, 130.85, 129.20, 128.61, 128.30, 127.93,
127.14, 35.28, 35.24, 35.09, 31.82. FAB-MS: m/z 455 [M þ H]þ.
1
OMe-CHO. H NMR (500 MHz, CDCl3) δ: 9.82 (s, 1H),
9.19 (s, 1H), 7.32-7.26 (m, 2H), 7.13-7.12 (m, 1H), 7.09-7.08
(m, 2H), 6.89-6.83 (m, 2H), 6.79-6.77 (m, 1H), 6.69-6.68 (m,
2H), 6.63-6.61 (m, 2H), 4.27-4.25 (m, 1H), 4.02-4.00 (m, 1H),
3.90 (s, 6H), 3.88 (s, 6H), 3.13-3.03 (m, 6H). 13C NMR (125
MHz, CDCl3) δ: 190.44, 149.24, 149.12, 148.96, 148.28, 144.73,
144.27, 140.38, 140.12, 138.23, 138.06, 137.87, 136.74, 136.47,
136.10, 132.86, 132.67, 131.23, 130.88, 128.45, 127.51, 124.19,
121.18, 120.24, 112.21, 111.63, 111.43, 111.33, 56.25, 56.22,
56.02, 35.33, 35.20, 35.06, 31.58. FAB-MS: m/z 575 [M þ H]þ.
Synthesis of AH. Methacrylate-benzil (1.03 g, 2.4 mmol),
H-CHO (1.11 g, 2.4 mmol), p-methoxyphenol (0.15 g), and
ammonium acetate (2.82 g, 36 mmol) were heated at 80 °C
in acetic acid (8 mL) for 4 days. The reaction mixture was
cooled to room temperature and was neutralized with aqueous
NH3. The resulting precipitate was filtered and washed with
water. The crude mixture was purified with silica gel column
chromatography using THF/hexane = 1/2 as eluent to give AH
as colorless powders (1.78 g, 85%). 1H NMR (500 MHz, CDCl3)
δ: 9.06 (br, 2H), 7.39-7.30 (br, 3H), 7.14-6.98 (m, 12H),
6.73-6.61 (m, 9H), 6.10 (s, 1H), 5.55 (s, 1H), 4.23-4.15 (m,
4H), 3.91-3.88 (m, 4H), 3.76 (s, 3H), 3.20-3.12 (m, 6H), 1.94 (s,
3H), 1.85-1.71 (m, 4H), 1.54-1.43 (m, 4H). 13C NMR (125
MHz, CDCl3) δ: 167.49, 146.21, 145.51, 139.66, 137.51, 136.65,
136.28, 133.27, 132.95, 130.46, 128.88, 128.08, 125.39, 67.81,
64.81, 55.29, 35.31, 35.13, 29.36, 28.72, 25.96, 18.48. FAB-MS:
m/z 860 [M þ H]þ.
Synthesis of Methacrylate-benzil. To a solution of OH-benzil
(3.56 g, 10.0 mmol), triethylamine (3.0 mL, 22.0 mmol), and
p-hydroquinone (22 mg, 0.20 mmol) in dichloromethane (20 mL)
was added methacryloyl chloride (2 mL, 20.0 mmol) at 0 °C. The
reaction mixture was stirred under a N2 atmosphere at room
temperature for 6 h and poured into water. After extraction using
dichloromethane, the organic layer was washed with water and
brine and dried over Na2SO4. The crude mixture was purified
with silica gel column chromatography using dichloromethane as
eluent to give Methacrylate-benzil as pale yellow powders (4.71 g,
75%). 1H NMR (500 MHz, CDCl3) δ: 7.96-7.92 (m, 4H),

3766 Macromolecules, Vol. 43, No. 8, 2010
Scheme 3. Synthesis of the Photochromic Monomers (A and B)a
Kimoto et al.
a Reagents and conditions: (i) NH4OAc, AcOH, p-methoxyphenol, 80 °C, 4 days; (ii) K3Fe(CN)6, KOH, benzene, rt, 30 min.
Chart 1. Structures of the Photochromic Monomers
Scheme 4. Polymerization of [2.2]Paracyclophane-Bridged
Imidazole Dimer
Synthesis of BH. Methacrylate-benzil (0.216 g, 0.52 mmol),
OMe-CHO (0.28 mg, 0.49 mmol), p-methoxyphenol (30 mg),
and ammonium acetate (0.563 g, 7.3 mmol) were heated at 80 °C
in acetic acid (4 mL) for 4 days. The reaction mixture was cooled
to room temperatureand was neutralized withaqueous NH3. The
resulting precipitate was filtered and washed with water. The
crude mixture was purified with silica gel column chromato-
graphy using THF/hexane = 1/2 as eluent to give BH as pale
yellow powders (0.249 g, 53%). 1H NMR (500 MHz, DMSO-d6)
δ: 11.47-11.42 (m, 2H), 7.13-7.08 (m, 4H), 7.01-6.98 (m, 1H),
6.92-6.86 (m, 3H), 6.78-6.76 (m, 3H), 6.69-6.51 (m, 9H), 6.02
(br, 1H), 5.66 (br, 1H), 4.61-4.45 (m, 2H), 4.11 (t, J = 6.0 Hz,
2H), 3.89-3.79 (m, 2H), 3.75-3.58 (m, 15H), 3.13-3.01 (m, 6H),
1.88 (s, 3H), 1.76-1.65 (m, 4H), 1.50-1.35 (m, 4H). 13C NMR
(125 MHz, CDCl3) δ: 167.88, 148.91, 148.03, 145.93, 145.78,
139.94, 136.85, 136.63, 136.42, 133.54, 133.13, 130.71, 129.59,
128.67, 126.85, 125.86, 125.58, 123.91, 120.73, 120.34, 114.74,
114.19, 113.58, 111.72, 111.33, 111.17, 68.30, 68.02, 64.98, 56.10,
55.95, 55.45, 35.75, 35.58, 35.04, 34.57, 30.66, 29.55, 28.93, 26.19,
25.95, 18.68. FAB-MS: m/z 980 [M þ H]þ.
was vigorously stirred for 30 min. The organic layer was
separated, exhaustively washed with water, and concentrated
in vacuo. The crude mixture was purified with silica gel column
chromatography using THF/hexane = 1/2 as eluent to give A as
colorless powders (0.646 g, 80%). 1H NMR (500 MHz, CDCl3)
δ: 7.47 (br, 1H), 7.31-7.03 (m, 15H), 6.92 (br, 1H), 6.82 (dd, J =
9.0 Hz, 2H), 6.71 (dd, J = 9.5 Hz, 2H), 6.64-6.61 (m, 2H), 6.48
(d, J = 7.5 Hz, 1H), 6.42 (d, J = 8.0 Hz, 1H), 6.09 (br, 1H), 5.54
(br, 1H), 4.54-4.51 (m, 1H), 4.18-4.15 (m, 2H), 4.00-3.93 (m,
2H), 3.84-3.80 (m, 3H), 3.33-2.94 (m, 7H), 1.94 (m, 3H),
1.83-1.70 (m, 4H), 1.55-1.42 (m, 4H). 13C NMR (125 MHz,
CDCl3) δ: 167.95, 164.69, 147.42, 141.21, 138.34, 137.48, 136.84,
136.34, 134.98, 134.76, 133.78, 133.19, 132.82, 132.56, 132.05,
131.81, 128.52, 128.39, 128.29, 128.09, 127.19, 126.37, 125.57,
114.06, 113.71, 113.62, 113.26, 111.91, 68.17, 64.92, 55.68, 55.61,
35.65, 35.57, 34.78, 34.46, 29.42, 28.89, 26.14, 18.67. FAB-MS:
m/z 858 [M þ H]þ.
Synthesis of the Monomer A. All manipulations were carried
out with the exclusion of light. Under nitrogen, to a solution of
AH (0.812 g, 0.95 mmol) in benzene (150 mL) was added the
solution of potassium ferricyanide (15.57 g, 47 mmol) and KOH
(5.31 g, 95 mmol) in water (105 mL), and the reaction mixture
Synthesis of the Monomer B. All manipulations were carried
out with the exclusion of light. Under nitrogen, to a solution of
BH (0.564 g, 0.64 mmol) in benzene (100 mL) was added the

Article
Macromolecules, Vol. 43, No. 8, 2010 3767
Typical Procedure for Polymerization. The monomer A and
2,20-azobis(2,4-dimethylvaleronitrile) (1.5 mol % of monomer)
were dissolved in dry THF (0.35 mmol/L) and placed in a
polymerization tube. After several freeze-pump-thaw cycles,
the tube was sealed under vacuum. The mixture was stirred and
heated in a sealed tube at 60 °C for 3 days. The cooled solution
was poured into methanol with stirring. The precipitation was
collected and dried under vacuum for 12 h to obtain the target
polymer.
Results and Discussion
Two photochromic monomers with [2.2]paracyclophane-bridged
imidazole dimer (A and B, Chart 1) possessing a methacrylate
moiety at their terminus were designed in this study. The
methacrylate-terminated benzil (Methacrylate-benzil) was pre-
pared by the etherification between 4-hydroxy-40-methoxy-
benzil23 and 6-chloro-1-hexanol in the presence of potassium
carbonate, followed by reaction with methacryloyl chloride to
give Methacrylate-benzil (Scheme 2). The benzil was reacted with
the corresponding [2.2]paracyclophane aldehyde (H-CHO or
OMe-CHO) to form the monomer precursor (AH and BH),
which was allowed to be oxidized to yield the photochromic
monomer (A and B) (Scheme 3). The structural feature of these
monomers is that polymerization unit and photochromic con-
trolling unit is separately introduced into a different imidazole
ring. Irradiating UV light to dichloromethane solution of the
monomers, rapid photochromism from colorless to blue (A) or
green (B) is observed. Under continuous irradiation, the dichloro-
methane solution of the methacrylate monomers reaches the
photostationary equilibrium, and the absorption decays rapidly
following first-order thermal-bleaching kinetics after ceasing the
illumination. While the half-life of the colored species is 7.2 ms
(A) at 298 K in dichloromethane, the half-life of the colored
species is 52 ms (B), which has 3,4-dimethoxyphenyl moiety, with
∼8 times longer than that of the unsubstituted monomer, A
(Figure 1a). The transient vis-NIR absorption spectra of the
monomers in degassed dichloromethane solution at 298 K are
shown in the inset of Figure 1b. A characteristic absorption
around 400 nm and a broad absorption from 500 to 1000 nm can
be attributed to the colored species. Compared with monomer A,
a spectral red shift can be observed for B (Figures 1b and 2).
These substituent effects on decoloration rate and coloring
suggest that this monomer design approach is useful to control
the photochromic property of methacrylate monomers.
Figure 1. (a) Decay profiles of the colored species generated from A, B,
1a, and 2a monitored at 400 nm in degassed dichloromethane solution
(A and B: 2.0 ꢀ 10-4 M; 1a and 2a: 50 μg/mL, 10 mm light path length)
at 298 K with a nanosecond laser pulse (excitation wavelength, 355 nm;
pulse width, 5 ns; power, 8 mJ/pulse). (b) Transient vis-NIR absorp-
tion spectra of 1a and 2a in degassed dichloromethane solution (50 μg/
mL), normalized at 600 nm. Inset shows transient vis-NIR absorption
spectra of A and B in dichloromethane (2.0 ꢀ 10-4 M, 10 mm light path
length), normalized at 600 nm.
The homopolymers were prepared by radical polymerization
using a low-temperature initiator, 2,20-azobis(2,4-dimethylvaler-
onitrile) (Scheme 4). 1a-1c and 2a-2b have been prepared
starting from the monomer A and B, respectively. The polymer-
ization was performed at 60 °C for 3 days to yield the polymers
(Table 1). Photochromic properties of the polymers in solution
were investigated in order to study the influence of the polymer-
ization. The polymers readily show fast photochromism, which
implies that no decomposition of the imidazole dimer unit
occurred under the radical polymerization conditions. In dichloro-
methane at 298 K, the half-life of the colored species is 17 ms (1a)
and 108 ms (2a), and the decoloration of 1a finishes within 200
ms, whereas that of 2a within 800 ms, indicating that the
substituent effect by methoxy moieties is maintained even after
the polymerization (Figure 1a). We should note that the thermal-
bleaching reaction is independent from molecular weight of the
polymers (see Supporting Information Figures S2 and S3) and
obeys first-order kinetics after ceasing the illumination (see
Supporting Information Figure S4). This observed kinetics for
decoloration of the homopolymers is attributed to the overall
first-order recombination reaction for each chromophore with
various local environments. The recombination reaction of the
homopolymers, 1a and 2a, is slower than that of each monomer,
Figure 2. UV-vis absorption spectra of A and B in dichloromethane
(2.0 ꢀ 10-4 M, 1 mm light path length) at 298 K.
solution of potassium ferricyanide (10.6 g, 32 mmol) and KOH
(3.61 g, 64 mmol) in water (72 mL), and the reaction mixture was
vigorously stirred for 30 min. The organic layer was separated,
exhaustively washed with water, and concentrated in vacuo. The
crude mixture was purified with silica gel column chromato-
graphy using THF/hexane = 1/3 to 1/1 as eluent to give B as
pale yellow powders (0.469 g, 83%). 1H NMR (500 MHz,
CDCl3) δ: 7.30-6.42 (m, 22H), 6.10 (br, 1H), 5.55 (br, 1H),
4.61-4.45 (m, 1H), 4.17-4.12 (m, 2H), 3.99-3.89 (m, 2H),
3.91-3.55 (m, 15H), 3.35-2.90 (m, 7H), 1.94 (m, 3H),
1.85-1.62 (m, 4H), 1.50-1.35 (m, 4H). 13C NMR (125 MHz,
CDCl3) δ: 167.86, 151.85, 141.74, 136.83, 136.49, 136.36, 136.12,
132.44, 132.23, 131.07, 131.00, 130.90, 129.51, 129.16, 128.83,
128.59, 128.55, 126.92, 126.44, 125.93, 125.85, 125.59, 115.08,
114.98, 114.81, 114.66, 114.37, 64.95, 64.90, 55.63, 55.59, 54.51,
35.41, 35.03, 34.85, 34.56, 33.17, 31.92, 30.66, 29.45, 29.39,
28.91, 28.87, 26.17, 26.08, 26.06, 22.98, 21.52, 18.67, 14.45.
FAB-MS: m/z 979 [M þ H]þ.

3768 Macromolecules, Vol. 43, No. 8, 2010
Kimoto et al.
Table 1. Polymerization Characteristics of Poly([2.2]paracyclophane-bridged imidazole dimer)s and Their Copolymers
initiatora
(mol %)
concentration
(mmol/L)
reaction time
(days)
feed ratio
(mol/mol)
yield
(%)
b
sample
Mw
Mw/Mn
1a
1b
1c
2a
2b
3a
3b
4
1.5
3.0
5.0
1.5
3.0
3.0
3.0
3.0
0.35
0.35
0.35
0.35
0.35
0.35
0.35
0.70
3
3
3
3
3
3
3
6
55
42
55
37
50
70
62
37
28 400
19 800
19 100
9 500
2.15
1.93
1.76
1.60
1.41
1.65
1.59
1.79
7 800
A/MMA = 1/1
A/MMA = 1/4
A/B = 1/1
11 200
9 600
11 400
a 2,20-Azobis(2,4-dimethylvaleronitrile) was used as an initiator. b Determined by GPC analysis (polystyrene calibration standard) using THF as eluent.
Figure 4. (a) Decay profile of the colored species generated from 4
Figure 3. (a) Decay profiles of the colored species generated from the
monitored at 400 nm in degassed dichloromethane solution (50 μg/mL,
cast film of 1a, 3a, and 3b monitored at 400 nm. Inset shows decay
10 mm light path length) at 298 K with a nanosecond laser pulse
profiles of the colored species generated from 1a, 3a, and 3b monitored
(excitation wavelength, 355 nm; pulse width, 5 ns; power, 8 mJ/pulse).
at 400 nm in degassed dichloromethane solution (50 μg/mL). (b)
Inset shows the first-order kinetic plots for 1a, 2a, and 4. (b) Transient
Vis-NIR absorption spectra of the cast film of 1a and 2a after UV
vis-NIR absorption spectra of 4 in degassed dichloromethane solution
irradiation at 253 K, normalized at 600 nm.
(50 μg/mL, 10 mm light path length). Inset shows magnified view of the
region from 425 to 575 nm.
A and B, respectively. This deceleration of thermal bleaching for
[2.2]paracyclophane-bridged imidazole dimer as a pendant of the
polymer is supported by the idea that larger structural change
during the photochromic reaction of the polymer is required than
that of the monomer. Absorption spectra of the colored species of
the polymers, 1a and 2a, are quite similar to those of the monomers,
A and B, respectively (Figure 1b). The transient spectrum of the
colored species depends on the electronic state of the chromo-
phores. In solution, there is no specific change on the transient spec-
trum after the polymerization because the photochromic unit is
apart from the polymer main chain with small electronic effects.
We prepared copolymers of A and MMA (methyl methacrylate)
with a different copolymerization ratio (3a and 3b) in order to
study the influence on the photochromism by copolymerization
with another monomer. The copolymerization ratio (3a: m/n=1/
1.3 and 3b: m/n=1/4.6) was estimated from the 1H NMR measure-
ment. In dichloromethane solution, the thermal-bleaching beha-
vior of the copolymers, 3a and 3b, is almost the same as that of
the homopolymer, 1a, even though MMA unit is incorporated
(Figure 3a, inset). The decoloration completes within 200 ms, and
the half-life is 16 ms (3a) and 13 ms (3b) at 298 K. The photo-
chromic behaviors of the cast film of 1a, 3a, and 3b are also ana-
logous to those observed in solution (Figure 3a). Considering that
the reaction rate of photochromism is generally influenced by the
environment around the photochrome,24-29 the photochromic
properties of the film cannot be predicted from those in solutions.
On the other hand, the similarity of the photochromic properties
between the homopolymers and copolymers of [2.2]paracyclo-
phane-bridged imidazole dimer allows further versatile functional-
ization by copolymerization with various monomers, which does
not affect the fast photochromism in solution. In the cast film,
although the absorption decays do not follow first-order thermal-
bleaching kinetics, the bleaching behavior of the fast component
seems to be similar to that in solution. The half-life for the film is
defined as the time with 50% of the normalized ΔOD (optical
density) and estimated to be ∼30 ms. The slow bleaching compo-
nent also exists during the thermal bleaching, which is commonly
generated in other polymeric photochromic systems. Also, the
absorption spectra of the colored species are quite similar to those
in dichloromethane and characteristic absorption bands around
600 and 800 nm are observed (Figures 1b and 3b). In other words,
the fast photochromism of [2.2]paracyclophane-bridged imidazole
dimer remains intact even in the film state.
In order to ensure the advantage of functionalization via
the copolymerization approach, we preliminarily prepared the

Article
Macromolecules, Vol. 43, No. 8, 2010 3769
copolymer of A and B (4, m/n = 1/0.44) and investigated its
photochromic behavior (Figure 4 and Figure S5). In dichloro-
methane solution, the thermal bleaching finishes within 800 ms,
which is identical to that of the slower photochromic unit, 2a
(Figure 4a). The thermal back-reaction reveals the existence of two
components of the first-order reaction, and there is an inflection
point around 50 ms. Notably, the slope of the decay of 4 over the
time range from 200 ms, where the recombination of A has
completed, is identical to that of 2a (Figure 4a, inset). As proceed-
ing the recombination reaction, the absorption spectrum of the
photogenerated colored species eventually varies (Figure 4b). This
result suggests that even introduction of the other photochromic
unit maintains the photochromic properties, and no reactions
between the chromophores occur, which is similar to the photo-
chromic behavior of homopolymers. The structural difference
between the homopolymers, 1a and 2a, and the copolymer, 4, is
composition of the photochromic units on the polymer main chain.
All the photochromic polymers in this study are classified into
pendant-type photochromic polymers; the photochromic behavior
of 4 is essentially the same as that of the homopolymers. Consider-
ing the decay profile of the colored species of the homopolymers
obeys the first-order plot (Figure 4a, inset), the recombination
reaction between the imidazolyl radicals of the individual chromo-
phore in these pendant-type photochromic polymers takes place
without cross-recombination between neighboring chromophores;
the decay profile of the copolymer (4) is the sum of recombination
reaction in the individual chromophores with the first-order
kinetics.
modules into photo-oriented systems. By copolymerizing with
various functional monomers, we can open up possibilities of this
fast photochromic system.
Acknowledgment. This work was supported by a Grant-in-
Aid for Science Research in a Priority Area “New Frontiers in
Photochromism (No. 471)” from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan and
by a High-Tech Research Center project for private universities
with the matching fund subsidy from MEXT.
Supporting Information Available: Typical 1H NMR spec-
trum of the photochromic polymer and photochromic proper-
ties for each polymer. This material is available free of charge via
the Internet at http://pubs.acs.org.
References and Notes
(1) Crano, J. C.; Guglielmetti, R. J. Organic Photochromic and Thermo-
chromic Compounds; Plenum Press: New York, 1999.
€
(2) Durr, H.; Bouas-Laurent, H. Photochromism: Molecules and Sys-
tems; Elsevier: Amsterdam, 2003.
(3) Yu, Y.; Nakano, M.; Ikeda, T. Nature 2003, 425, 145.
(4) Yamada, M.; Kondo, M.; Mamiya, J.; Yu, Y.; Kinoshita, M.;
Barrett, C.; Ikeda, T. Angew. Chem., Int. Ed. 2008, 47, 4986.
(5) Kobatake, S.; Takami, S.; Muto, H.; Ishikawa, T.; Irie, M. Nature
2007, 446, 778.
(6) Kawai, T.; Nakashima, Y.; Irie, M. Adv. Mater. 2005, 17, 309.
(7) Irie, M.; Fukaminato, T.; Sasaki, T.; Tamai, N.; Kawai, T. Nature
2002, 420, 759.
(8) Matsuda, K.; Irie, M. J. Am. Chem. Soc. 2000, 122, 7195.
(9) Sangeetha, N. M.; Maitra, U. Chem. Soc. Rev. 2005, 34, 821.
(10) Kato, T.; Hirai, Y.; Nakaso, S.; Moriyama, M. Chem. Soc. Rev.
2007, 36, 1857.
Conclusion
In conclusion, the synthesis of the fast photochromic polymers
carrying [2.2]paracyclophane-bridged imidazole dimer is demon-
strated. We can modify the photochromic properties such as
coloration/decoloration rate, coloring, and photosensitivity via
the stepwise synthetic approach of the imidazole dimer system.
Notably, the photochromic behavior is not affected by the
environment around the photochromes and copolymerization
with other monomers both in solution and film, which cannot be
realized in any other conventional photochromic systems. The
comparable photochromic behavior of the homopolymers and
copolymers in solution and film indicates that the photochromic
unit is independent from the local environment, which allows
effective molecular design of the photochromic monomer unit to
accomplish desired photochromic properties of the polymer.
Generally, advantage of polymeric materials is comprised of
their good film-forming property with assembled and concen-
trated functions in polymer chain, which cannot be realized by
simple doping into the host polymer matrices because phase
separation and precipitation of the dopant takes place at higher
doping level. When developing functional polymers into electro-
nic applications, the cooperativity between functional units
derived from “polymer effect” is favored. On the other hand, in
the photochromic polymer system, interaction with other com-
ponents or host polymer matrices deteriorates the photochromic
property as usually seen in common photochromic polymer
systems. Hence, the independent photochromic behavior in
polymeric [2.2]paracyclophane-bridged imidazole dimers is ex-
pected as a quite reasonable system for various applications,
especially in bulk phase. This photochromic system is proved to
be a powerful candidate for applying various phototriggered
(11) Sagara, Y.; Kato, T. Nat. Chem. 2009, 1, 605.
(12) Iwahori, F.; Hatano, S.; Abe, J. J. Phys. Org. Chem. 2007, 20, 857.
(13) Fujita, K.; Hatano, S.; Kato, D.; Abe, J. Org. Lett. 2008, 10, 3105.
(14) Kishimoto, Y.; Abe, J. J. Am. Chem. Soc. 2009, 131, 4227.
(15) Harada, Y.; Hatano, S.; Kimoto, A.; Abe, J. J. Phys. Chem. Lett.
2010, 1, 1112.
(16) Caspar, J. V.; Khudyakov, I. V.; Turro, N. J.; Weed, G. C.
Macromolecules 1995, 28, 636.
(17) Kawano, M.; Sano, T.; Abe, J.; Ohashi, Y. J. Am. Chem. Soc. 1999,
121, 8106.
(18) Abe, J.; Sano, T.; Kawano, M.; Ohashi, Y.; Matsushita, M. M.;
Iyoda, T. Angew. Chem., Int. Ed. 2001, 40, 580.
(19) Fukushima, K.; Vandenbos, A. J.; Fujiwara, T. Chem. Mater.
2007, 19, 644.
(20) Zhu, L.; Wu, W.; Zhu, M.-Q.; Han, J. J.; Hurst, J. K.; Li, A. D. Q.
J. Am. Chem. Soc. 2007, 129, 3524.
(21) Lee, H.-i.; Wu, W.; Oh, J. K.; Mueller, L.; Sherwood, G.; Peteanu,
L.; Kowaleski, T.; Matyjaszewski, K. Angew. Chem., Int. Ed. 2007,
46, 2453.
(22) Iwamura, T.; Sakaguchi, M. Macromolecules 2008, 41, 8995.
€
(23) Simburger, H.; Kerna, W.; Hummel, K.; Hagg, C. Polymer 2000,
41, 7883.
(24) Evans, R. A.; Hanley, T. L.; Skidmore, M. A.; Davis, T. P.; Such,
G. K.; Yee, L. H.; Ball, G. E.; Lewis, D. A. Nat. Mater. 2005, 4, 249.
(25) Such, G. K.; Evans, R. A.; Davis, T. P. Macromolecules 2006, 39,
1391.
ꢀ
(26) Sriprom, W.; Neel, M.; Gabbutt, C. D.; Heron, B. M.; Perrier, S.
J. Mater. Chem. 2007, 17, 1885.
(27) Ercole, F.; Malic, N.; Davis, T. P.; Evans, R. A. J. Mater. Chem.
2009, 19, 5612.
(28) Norikane, Y.; Davis, R.; Tamaoki, N. New J. Chem. 2009, 33,
1327.
(29) Ercole, F.; Davis, T. P.; Evans, R. A. Macromolecules 2009, 42,
1500.