Synthesis of a novel family of photochromic amorphous molecular materials based on dithienylethene, their photochromic properties and application for dual image formation

Article abstract of DOI:10.1039/b205201f

A novel family of photochromic amorphous molecular materials containing a dithienylethene moiety, 1-{5-[4-(di-p-tolylamino)phenyl]-2-methylthiophen-3-yl}-2- (2,5-dimethylthiophen-3-yl)-3,3,4,4,5,5-hexafluorocyclopentene, 1-{5-[4-(di-p-tolylamino)phenyl]-2-methylthiophen-3-yl}-2- (2-methylbenzo[b]thiophen-3-yl)-3, 3,4,4,5,5-hexafluorocyclopentene and 1,2-bis{5-[4-(di-p-tolylamino)phenyl]-2-methylthiophen-3-yl}-3, 3,4,4,5,5-hexafluorocyclopentene, have been designed and synthesised. These compounds, together with their photocyclised products, were found to readily form amorphous glasses with well-defined glass transition temperatures and to undergo photochromism as amorphous films as well as in solution. These compounds are characterised by high quantum yields for the photocyclisation reactions, very low quantum yields for the reverse ring-opening reactions and, hence, almost 100% fractions of the photocyclised form at the photostationary state in solution. These results suggest that the anti-parallel conformer is more populated than the parallel conformer. Optical dichroism was induced by irradiation of coloured films of the photocyclised compounds with linearly polarised red light, and dual image formation at the same location was realised by utilising this phenomenon.

Full text of DOI:10.1039/b205201f

View Article Online / Journal Homepage / Table of Contents for this issue
Synthesis of a novel family of photochromic amorphous molecular
materials based on dithienylethene, their photochromic properties
and application for dual image formation
Hisayuki Utsumi, Daisuke Nagahama, Hideyuki Nakano and Yasuhiko Shirota*
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka,
Suita, Osaka 565-0871, Japan. E-mail: shirota@ap.chem.eng.osaka-u.ac.jp
Received 29th May 2002, Accepted 21st June 2002
First published as an Advance Article on the web 5th August 2002
A novel family of photochromic amorphous molecular materials containing a dithienylethene moiety,
1-{5-[4-(di-p-tolylamino)phenyl]-2-methylthiophen-3-yl}-2-(2,5-dimethylthiophen-3-yl)-3,3,4,4,5,5-
hexafluorocyclopentene, 1-{5-[4-(di-p-tolylamino)phenyl]-2-methylthiophen-3-yl}-2-(2-methylbenzo[b]thiophen-3-
yl)-3,3,4,4,5,5-hexafluorocyclopentene and 1,2-bis{5-[4-(di-p-tolylamino)phenyl]-2-methylthiophen-3-yl}-
3,3,4,4,5,5-hexafluorocyclopentene, have been designed and synthesised. These compounds, together with their
photocyclised products, were found to readily form amorphous glasses with well-defined glass transition
temperatures and to undergo photochromism as amorphous films as well as in solution. These compounds are
characterised by high quantum yields for the photocyclisation reactions, very low quantum yields for the
reverse ring-opening reactions and, hence, almost 100% fractions of the photocyclised form at the
photostationary state in solution. These results suggest that the anti-parallel conformer is more populated than
the parallel conformer. Optical dichroism was induced by irradiation of coloured films of the photocyclised
compounds with linearly polarised red light, and dual image formation at the same location was realised by
utilising this phenomenon.
Photochromic materials have recently attracted renewed
interest in view of their potential technological applications
for visible image formation, optical data storage and optical
switching. Photochromic materials applied for such practical
uses may be used in the form of solid films and, hence, extensive
studies have been made of photochromic polymers and
molecularly dispersed polymer systems, where low molecular
weight photochromic compounds are dispersed in polymer
binders.1–4 In contrast to polymers, low molecular weight
organic compounds generally do not form smooth and uniform
films, since they tend to crystallise readily. We have performed
a series of studies on the creation of low molecular weight
organic compounds that readily form stable amorphous glasses
above room temperature, namely amorphous molecular mater-
ials, and investigated their structures, reactions, properties and
applications.5 We have proposed several new concepts for
photo- and electroactive amorphous molecular materials, which
include electrically conducting amorphous molecular mater-
ials,6,7 photochromic amorphous molecular materials,8–10
molecular resists for nanolithography11–14 and materials for
photovoltaic devices15–17 and organic light-emitting diodes.18–25
Photochromic amorphous molecular materials constitute a
novel class of photochromic molecular materials that form
uniform amorphous thin films by themselves in the absence of
any polymer binder. They have the advantage that there is
no dilution of photochromic chromophores relative to photo-
chromic polymers and composite polymer systems, where
low molecular weight organic photochromic compounds may
crystallise at high concentration. Based on this concept, we
have created a novel family of photochromic amorphous
molecular materials containing an azobenzene moiety.8,9
It is of great interest and significance to create photochromic
amorphous molecular materials containing a dithienylethene
moiety, since dithienylethene derivatives have recently received
attention as promising photochromic compounds with thermal
stability and excellent fatigue-resistant properties.26–31 We report
here the synthesis of a novel family of photochromic amorphous
molecular materials containing a dithienylethene moiety, 1-{5-[4-
(di-p-tolylamino)phenyl]-2-methylthiophen-3-yl}-2-(2,5-dimethyl-
thiophen-3-yl)-3,3,4,4,5,5-hexafluorocyclopentene (TPTTC),
1-{5-[4-(di-p-tolylamino)phenyl]-2-methylthiophen-3-yl}-2-(2-
methylbenzo[b]thiophen-3-yl)-3,3,4,4,5,5-hexafluorocyclopentene
(TPTBC) and 1,2-bis{5-[4-(di-p-tolylamino)phenyl]-2-methyl-
thiophen-3-yl}-3,3,4,4,5,5-hexafluorocyclopentene (BTPTC),
their glass-forming and photochromic properties, and applica-
tion for dual image formation at the same location. Part of
this work has been reported in a previous communication.10
Experimental
Materials
Tetrakis(triphenylphosphine)palladium(0), 1.6 N hexane solu-
tion of n-butyllithium and 1,2,3,3,4,4,5,5-octafluorocyclopen-
tene are commercially available and were used without further
purification. 3-Bromo-2-methylthiophene-5-boronic acid29 and
3-bromo-2,5-dimethylthiophene32 were prepared as described
in the literature.
Synthesis of 1-{5-[4-(di-p-tolylamino)phenyl]-2-methylthiophen-
3-yl}-2-(2,5-dimethylthiophen-3-yl)-3,3,4,4,5,5-
hexafluorocyclopentene (TPTTC)
A THF solution (70 ml) containing N,N-bis(4-methylphenyl)-
4-bromoaniline (11.0 g, 31 mmol) and tetrakis(triphenylphos-
phine)palladium(0) (3.35 g, 2.9 mmol) was slowly added to a
THF solution (75 ml) of 3-bromo-2-methylthiophene-5-boro-
nic acid (10.1 g, 46 mmol). To this was added an aqueous
solution (100 ml) of potassium carbonate (1.0 mol dm23) and
the resulting mixture was refluxed for 10 h. Then, the solution
was extracted with benzene, washed with water, and dried over
sodium sulfate. After removal of the solvent, the product was
purified by silica gel column chromatography using a mixture
2612
J. Mater. Chem., 2002, 12, 2612–2619
This journal is # The Royal Society of Chemistry 2002
DOI: 10.1039/b205201f

View Article Online
of hexane and toluene (10 : 1 v/v) as the eluent, followed by
recrystallisation from a mixed solvent of hexane and toluene
to give 3-bromo-5-{4-[bis(4-methylphenyl)amino]phenyl}-2-
methylthiophene (1) as a colourless powder. Yield: 10.3 g
(35 ml) of 3 (2.2 g, 4.0 mmol) at 278 uC and the mixture stirred
for 24 h. Dilute hydrochloric acid was then added and the
solution washed with water. After removal of the solvent, the
product was purified by silica gel column chromatography
using a mixture of hexane and toluene (10 : 1 v/v) and gel
permeation chromatography (GPC) using benzene as the
eluent, followed by recrystallisation from a mixture of THF
and ethanol to give a pale yellow powder. Yield: 0.79 g (30%).
1
(50%). M.p.: 123 uC. H NMR (750 MHz, CDCl3) d (ppm):
2.32 (s, 6H), 2.40 (s, 3H), 6.97 (d, 2H, J ~ 8.7), 6.98 (s, 1H),
7.00 (d, 4H, J ~ 8.1), 7.08 (d, 4H, J ~ 8.1), 7.30 (d, 2H, J ~
8.7 Hz). MS(EI) m/z: 448 (M1). Found: C, 66.77; H, 4.80; N,
3.08; calcd. for C25H22NBrS: C, 66.96; H, 4.95; N, 3.12%.
A hexane solution of n-butyllithium (20 mmol) was added to
a THF solution (50 ml) of 1 (6.6 g, 15 mmol) at 278 uC under a
nitrogen atmosphere and the mixture stirred for 1 h to produce
3-lithio-5-{4-[bis(4-methylphenyl)amino]phenyl}-2-methylthio-
phene (2). To this was added 1,2,3,3,4,4,5,5-octafluorocyclo-
pentene (3.5 ml, 26 mmol) at 278 uC and the solution was
stirred for 24 h. The reaction mixture was extracted with ether
and then washed successively with dilute hydrochloric acid and
water. After removal of the solvent, the product was purified by
silica gel column chromatography using a mixture of hexane
and toluene (10 : 1 v/v) as the eluent, followed by recrystal-
lisation from a mixed solvent of THF and ethanol to give
1-(5-{4-[bis(4-methylphenyl)amino]phenyl}-2-methylthiophen-
3-yl)-2,3,3,4,4,5,5-heptafluorocyclopentene (3) as a pale yellow
powder. Yield: 2.25 g (28%). M.p.: 129 uC. 1H NMR (750 MHz,
CDCl3) d (ppm): 2.32 (s, 6H), 2.45 (s, 3H), 7.00 (d, 2H, J ~
8.9), 7.01 (d, 4H, J ~ 8.5), 7.08 (d, 4H, J ~ 8.5), 7.11 (s, 1H),
7.35 (d, 2H, J ~ 8.9 Hz). MS(EI) m/z: 561 (M1). Found : C,
64.20; H, 3.97; N, 2.51; calcd. for C30H22F7NS: C, 64.17; H,
3.95; N, 2.49%.
1
M.p.: 158 uC. H NMR (750 MHz, THF-d8) d (ppm): 1.89 (s,
3H), 1.91 (s, 3H), 2.28 (s, 6H), 2.41 (s, 3H), 6.80 (s, 1H), 6.94 (d,
2H, J ~ 8.5), 6.96 (d, 4H, J ~ 8.1), 7.06 (d, 4H, J ~ 8.1), 7.24
(s, 1H), 7.40 (d, 2H, J ~ 8.5 Hz). MS(EI) m/z: 653 (M1).
Found: C, 65.94; H, 4.51; N, 2.13; calcd. for C36H29F6NS2: C,
66.14; H, 4.47; N, 2.14%.
Synthesis of 1-{5-[4-(di-p-tolylamino)phenyl]-2-methylthiophen-
3-yl}-2-(2-methylbenzo[b]thiophen-3-yl)-3,3,4,4,5,5-
hexafluorocyclopentene (TPTBC)
A hexane solution of n-butyllithium (11 mmol) was added to an
ether solution (200 ml) of 3-bromo-2-methylbenzo[b]thiophene
(3.4 g, 15 mmol) at 278 uC under a nitrogen atmosphere and
the solution stirred for 1 h. To this was added 1,2,3,3,4,4,5,5-
octafluorocyclopentene (3.2 ml, 24 mmol) at 278 uC and the
the mixture stirred for 24 h. Dilute hydrochloric acid was then
added and the resulting solution washed with water. After
removal of the solvent, the product was purified by sublimation
(57 uC, 2.0 mmHg) to give 1-(2-methylbenzo[b]thiophen-3-yl)-
2,3,3,4,4,5,5-heptafluorocyclopentene (4) as colourless prisms.
Yield: 3.6 g (72%). M.p.: 74 uC. 1H NMR (750 MHz, CDCl3) d
(ppm): 2.35 (s, 3H), 7.36 (t, 1H, J ~ 8.0), 7.40 (t, 1H, J ~ 8.0),
7.49 (d, 1H, J ~ 8.0), 7.79 (d, 1H, J ~ 8.0 Hz). MS(EI) m/z:
A hexane solution of n-butyllithium (14 mmol) was added
to a THF solution (30 ml) of 3-bromo-2,5-dimethylthiophene
(1.6 g, 8.4 mmol) at 278 uC under a nitrogen atmosphere and
the mixture stirred for 1 h. To this was added a THF solution
J. Mater. Chem., 2002, 12, 2612–2619
2613

View Article Online
340 (M1). Found: C, 49.36; H, 2.07; calcd. for C14H7F7S: C,
49.42; H, 2.07%.
BTPTC-c. 1H NMR (600 MHz, benzene-d6) d (ppm): 2.09
(s, 6H), 2.19 (s, 12H), 6.67 (s, 2H), 6.89 (d, 4H, J ~ 8.9), 6.91
(d, 8H, J ~ 8.2), 7.02 (d, 8H, J ~ 8.2), 7.12 (d, 4H, J ~
8.9 Hz). MS(EI) m/z: 919 (M1). Found: C, 72.28; H, 4.94; N,
2.95; calcd. for C55H44N2F6S2: C, 72.51; H, 4.87; N, 3.07%.
To a solution of 2, prepared by the reaction of 1 (3.5 g,
7.8 mmol) with n-butyllithium (9.6 mmol) in THF (50 ml), was
added a THF solution (30 ml) of 4 (3.0 g, 8.8 mmol) at 278 uC
under a nitrogen atmosphere and the mixture stirred for 24 h.
Dilute hydrochloric acid was then added and the resulting
solution washed with water. After removal of the solvent, the
product was purified by silica gel column chromatography
using hexane as the eluent, followed by recrystallisation from a
mixture of THF and ethanol to give a pale yellow powder.
Yield: 1.8 g (33%). M.p.: 158 uC. 1H NMR (600 MHz, benzene-
d6) d (ppm): 1.65 (s, 3H), 1.97 (s, 3H), 2.10 (s, 6H), 6.91 (d, 4H,
J ~ 8.3), 6.92 (t, 1H, J ~ 8.0), 7.00 (d, 2H, J ~ 8.9), 7.05 (t,
1H, J ~ 8.0), 7.05 (d, 4H, J ~ 8.3), 7.15 (d, 2H, J ~ 8.9), 7.18
(s, 1H), 7.30 (d, 1H, J ~ 8.0), 7.64 (d, 1H, J ~ 8.0 Hz). MS(EI)
m/z: 653 (M1). Found: C, 67.83; H, 4.26; N, 2.04; calcd. for
C39H29NF6S2: C, 67.91; H, 4.24; N, 2.03%.
Preparation of amorphous films
Amorphous films of the synthesised compounds were prepared
on transparent glass substrates by spin coating from their
benzene solutions (ca. 1–5 6 1022 mol dm23), dried overnight
under reduced pressure and used for the measurements of
electronic absorption spectra.
Photochromic reactions
The synthesised compounds, TPTTC, TPTBC and BTPTC,
were irradiated in benzene solution and as amorphous films
with UV light (365 nm) from a 500 W super-high pressure
mercury lamp through a combination of cut-off glass filters
(UV-35 and UV-D33S, Toshiba) and an aqueous solution of
NiSO4. The photocyclised compounds, TPTTC-c, TPTBC-c
and BTPTC-c, were irradiated in toluene solution and as
amorphous films with visible light (w580 nm) from a 500 W
Xenon lamp through a cut-off glass filter (O-58, Toshiba).
Synthesis of 1,2-bis{5-[4-(di-p-tolylamino)phenyl]-2-
methylthiophen-3-yl}-3,3,4,4,5,5-hexafluorocyclopentene
(BTPTC)
To a solution of 2, prepared by the reaction of 1 (5.4 g,
12 mmol) with n-butyllithium (18 mmol) in THF (50 ml),
was added 1,2,3,3,4,4,5,5-octafluorocyclopentene (1.25 ml,
9.3 mmol) at 278 uC under a nitrogen atmosphere and the
mixture stirred for 24 h. Dilute hydrochloric acid was then
added and the resulting solution washed with water. After
removal of the solvent, the product was purified by silica gel
column chromatography and GPC using a mixture of hexane
and toluene (4 : 1 v/v) and benzene, respectively, as the eluents,
followed by recrystallisation from a mixture of THF and
ethanol to give a pale yellow powder. Yield: 2.5 g (45%). M.p.:
207 uC. 1H NMR (600 MHz, benzene-d6) d (ppm): 1.72 (s, 6H),
2.11 (s, 12H), 6.92 (d, 8H, J ~ 8.4), 7.04 (d, 4H, J ~ 8.6), 7.07
(d, 8H, J ~ 8.4), 7.25 (d, 4H, J ~ 8.6 Hz), 7.28 (s, 2H). MS(EI)
m/z: 919 (M1). Found: C, 72.41; H, 4.94; N, 3.12; calcd. for
C55H44N2F6S2: C, 72.51; H, 4.87; N, 3.07%.
Measurements and apparatus
Gel permeation chromatography (GPC) was performed by
using an LC-908 recycling preparative HPLC (Japan Analy-
tical Industry Co., Ltd.) with two columns for the separation of
low molecular weight organic compounds, JAIGEL-1H and
JAIGEL-2H (a kind of styrene polymer gel).
Quantum yields for the photocyclisation reactions were
measured with a potassium ferrioxalate actinometer by irra-
diation with 365 nm light. Quantum yields for the photo-
chemical ring-opening reactions were measured with an optical
power meter (ML910A, Anritsu) by irradiation with 600 nm
light with a bandwidth of 10 nm from a 500 W Xenon lamp
through an interference filter (IF-S 600, Vacuum Optics Co.).
Photochromic reactions were analyzed by monitoring the
change of the electronic absorption spectra with a spectro-
photometer (U-3200, Hitachi).
Synthesis of the photocyclised isomers TPTTC-c, TPTBC-c and
BTPTC-c
Differential scanning calorimetry (DSC) was carried out
with a Seiko DSC220C instrument. X-Ray diffraction (XRD)
measurements were carried out with a M18XHF-SRA dif-
fractometer (MAC Science Co., Ltd.). Polarising microscopy
was performed with a OPTIPHOT X2 (Nikon) microscope
fitted with a TH-600PM hot stage (Linkam) and crossed
polarisers.
A benzene solution (ca. 300 ml) of TPTTC (ca. 40 mg) was
irradiated with 365 nm light from a 500W super-high pressure
mercury lamp for ca. 10 min. After removal of the solvent,
the resulting photocyclised isomer TPTTC-c was purified by
GPC using benzene as the eluent, followed by reprecipitation
from THF–acetonitrile. The other photocyclised isomers,
TPTBC-c and BTPTC-c, were prepared and purified by similar
procedures.
Results and discussion
Molecular design and synthesis of photochromic amorphous
moloecular materials based on dithienylethene
TPTTC-c. 1H NMR (750 MHz, THF-d8) d (ppm): 2.07 (s,
3H), 2.08 (s, 3H), 2.22 (s, 3H), 2.30 (s, 6H), 6.11 (s, 1H), 6.64 (s,
1H), 6.92 (d, 2H, J ~ 8.9), 7.00 (d, 4H, J ~ 8.2), 7.11 (d, 4H,
J ~ 8.2), 7.43 (d, 2H, J ~ 8.9 Hz). MS(EI) m/z: 653 (M1).
Found: C, 65.97; H, 4.68; N, 2.12; calcd. for C36H29F6NS2: C,
66.14; H, 4.47; N, 2.14%.
For the purpose of developing a novel class of photochromic
amorphous molecular materials based on dithienylethene,
we have designed and synthesised TPTTC, TPTBC and
BTPTC. The synthetic procedures for the preparation these
compounds are shown in Scheme 1. These new compounds
were synthesised using 3-bromo-5-{4-[bis(4-methylphenyl)-
amino]phenyl}-2-methylthiophene (1) prepared by the reaction
of N,N-bis(4-methylphenyl)-4-bromoaniline with 3-bromo-
2-methylthiophene-5-boronic acid in the presence of palla-
dium catalyst. TPTTC was obtained by the reaction of
1,2,3,3,4,4,5,5-octafluorocyclopentene with an equimolar amount
of 3-lithio-5-{4-[bis(4-methylphenyl)amino]phenyl}-2-methyl-
thiophene (2), prepared by lithiation of 1, followed by reaction
with 3-lithio-2,5-dimethylthiophene, prepared by lithiation of
3-bromo-2,5-dimethylthiophene. TPTBC was synthesised by
TPTBC-c. 1H NMR (600 MHz, benzene-d6) d (ppm): 1.99
(s, 3H), 2.00 (s, 3H), 2.09 (s, 6H), 6.64 (s, 1H), 6.71 (t, 1H, J ~
7.4), 6.73 (t, 1H, J ~ 7.4), 6.82 (d, 1H, J ~ 7.4), 6.86 (d, 2H,
J ~ 8.9), 6.91 (d, 4H, J ~ 8.9), 7.00 (d, 4H, J ~ 8.6), 7.09 (d,
2H, J ~ 8.9), 7.99 (d, 1H, J ~ 7.4 Hz). MS(EI) m/z: 653 (M1).
Found: C, 68.07; H, 4.26; N, 2.09; calcd. for C39H29NF6S2: C,
67.91; H, 4.24; N, 2.03%.
2614
J. Mater. Chem., 2002, 12, 2612–2619

View Article Online
Scheme 1 Synthetic procedures for the preparation of new compounds containing a dithienylethene moiety.
the reaction of 2 with an equimolar amount of 1-(2-methyl-
benzo[b]thiophen-3-yl)-2,3,3,4,4,5,5-heptafluorocyclopentene
(4) derived from 3-bromo-2-methylbenzo[b]thiophene. BTPTC
was synthesised by the reaction of 1,2,3,3,4,4,5,5-octafluoro-
cyclopentene with two molar equivalents of 2. They were
obtained as polycrystals by recrystallisation from solution.
Their photocyclised isomers, TPTTC-c, TPTBC-c and BTPTC-
c, were obtained by irradiation of the corresponding open
forms in solution with 365 nm light. They were obtained as
amorphous solids, in spite of attempted recrystallisation from
solution.
an endothermic peak due to melting was observed at 207 uC.
When the isotropic liquid was cooled on standing in air, an
amorphous glass was spontaneously formed via a supercooled
Glass-forming properties of TPTTC, TPTBC, BTPTC and their
photocyclised compounds
The synthesised new compounds containing a dithienylethene
moiety, TPTTC, TPTBC and BTPTC, and their photocyclised
isomers were found to readily form stable amorphous glasses
with well-defined glass transition temperatures (Tg), as evi-
denced by DSC, XRD and polarising microscopy. Fig. 1 shows
DSC curves for BTPTC. When a crystalline sample, obtained
by recrystallisation from THF–ethanol solution, was heated,
Fig. 1 DSC curves for BTPTC. (a) Crystalline sample obtained by
recrystallisation from solution. (b) Glass sample obtained by cooling
the melt. Heating rate: 5 uC min21
.
J. Mater. Chem., 2002, 12, 2612–2619
2615

View Article Online
Fig. 3 Electronic absorption spectra of (a) TPTBC and (b) TPTBC-c in
benzene solution (1.0 6 1025 mol dm23).
Table 2 Electronic absorption maxima (lmax) of photochromic amor-
phous molecular materials in benzene, together with their molar
extinction coefficients (e)
Compound
l )
max/nm (e/104 dm3 mol21 cm21
TPTTC
TPTTC-c
TPTBC
TPTBC-c
BTPTC
BTPTC-c
304 (2.3)
298 (2.2)
302 (2.2)
303 (2.8)
304 (3.9)
300 (3.6)
358 (3.3)
428 (1.3)
357 (2.9)
455 (1.3)
362 (6.2)
442 (2.6)
575 (2.4)
580 (3.1)
635 (3.9)
Fig. 2 XRD patterns of BTPTC. (a) Crystalline sample obtained by
recrystallisation from solution. (b) Glass sample obtained by cooling
the melt.
Electronic absorption spectra of TPTTC, TPTBC and BTPTC
Fig. 3 shows the electronic absorption spectra of TPTBC and
its photocyclised isomer TPTBC-c. Table 2 summarises the
electronic absorption maxima and the molar extinction
coefficients of TPTTC, TPTBC, BTPTC and their photo-
cyclised isomers in benzene solution. The absorption bands of
the cyclised forms in the visible region undergo increasing red
shifts in the order TPTTC-c v TPTBC-c v BTPTC-c; this
may be due to the increasing extent of p-conjugation in this
series. The electronic absorption spectra of these compounds as
amorphous films were found to be similar to those in solution.
liquid state. When the glass sample was heated again, a glass
transition phenomenon was observed at 94 uC. No crystal-
listion was observed on further heating above the Tg. Similar
results were obtained for TPTTC and TPTBC.
Formation of an amorphous glass was also confirmed by
XRD. As Fig. 2 shows, the XRD pattern of a polycrystalline
sample of BTPTC obtained by recrystallisation from solution
displays sharp peaks characteristic of crystals, whereas that of
the glass sample obtained by cooling a BTPTC melt sample
shows a broad halo. Similar results were obtained for TPTTC
and TPTBC.
Table 1 summarises the Tg and melting points (Tm) of
TPTTC, TPTBC and BTPTC, and their photocyclised isomers,
TPTTC-c, TPTBC-c and BTPTC-c. These photochromic
amorphous molecular materials have relatively high Tg
values. It is of interest to note that the Tg values of the
photocyclised isomers are higher than those of the correspond-
ing open forms. It is understood that the molecular structures
of the cyclised forms are more rigid than those of the
corresponding open forms, leading to higher Tg values for
the cyclised forms. Increases in Tg values as a result of
incorporation of rigid moieties has been reported for other
amorphous molecular materials.5,8,33–35 All these compounds
form uniform amorphous films by spin coating.
Photochromic properties of TPTTC, TPTBC and BTPTC
The new compounds containing a dithienylethene moiety,
TPTTC, TPTBC and BTPTC, were found to exhibit photo-
chromism as amorphous films as well as in solution. Fig. 4
shows the electronic absorption spectral changes of an
amorphous film of TPTBC prepared by spin coating from
benzene solution. Upon irradiation with 365 nm light, the band
with a maximum at around 360 nm gradually decreased in
intensity and new absorption bands with maxima at around
455 and 580 nm appeared in the visible region due to the
Table 1 Glass transition temperatures (Tg) and melting points (Tm)
of photochromic amorphous molecular materials based on dithienyl-
ethene
Compound
Tg/uC
Tm/uC
TPTTC
TPTTC-c
TPTBC
TPTBC-c
BTPTC
BTPTC-c
51
57
66
73
94
104
158
—
158
—
207
—
Fig. 4 Electronic absorption spectral changes of an amorphous film of
TPTBC. (a) Before photoirradiation. (b) Photostationary state upon
irradiation with 365 nm light.
2616
J. Mater. Chem., 2002, 12, 2612–2619

View Article Online
Table 3 Quantum yields for the photocyclisation (Wo-c) and the reverse
photochemical ring-opening (Wc-o) reactions together with the fractions
(Ypss) of the photocyclised forms at the photostationary state
On the other hand, the quantum yields for the reverse ring-
opening reactions (Wo-c) of TPTTC-c, TPTBC-c and BTPTC-c
to regenerate the corresponding open forms in solution were
approximately two orders of magnitude smaller than those
reported for other cyclised dithienylethene derivatives, e.g.
1,2-bis(2,4-dimethylthiophen-3-yl)-3,3,4,4,5,5-hexafluorocyclo-
pentene and 1,2-bis(2-methylbenzo[b]thiophen-3-yl)-3,3,4,4,5,5-
hexafluorocyclopentene (0.13 and 0.41, respectively).36,39 The
very low quantum yields observed for TPTTC-c, TPTBC-c and
BTPTC-c are in accord with the report that the presence of
extended p-conjugation significantly reduces the quantum yield
for the photochemical ring-opening reaction, as observed
for 1,2-bis(3,5-dimethyl-5@-cyano-2:2’,5’:2@-terthiophen-4-yl)-
3,3,4,4,5,5-hexafluorocyclopentene (1.3 6 1024).39 The quan-
tum yields for the reverse ring-opening reactions of TPTTC-c,
TPTBC-c and BTPTC-c as amorphous films were similar to
those for solution. It is thought that the ring-opening reaction
does not require significant molecular motion.
System
Wo-c
Wc-o
Ypss
TPTTC in benzene solution
TPTTC as amorphous film
0.81
0.28
0.001
0.001
1.00
0.68
TPTBC in benzene solution
TPTBC as amorphous film
0.79
0.33
0.015
0.010
1.00
0.36
BTPTC in benzene solution
BTPTC as amorphous film
0.61
0.33
0.001
0.001
1.00
0.77
transformation of the open form into the closed form via
photocyclisation; the reaction system finally reached a photo-
stationary state (irradiated for ca. 3 min at 0.5 mW cm22).
Similar spectral changes were observed for amorphous films of
TPTTC and BTPTC. The reverse ring-opening reaction of the
cyclised form to regenerate the open form took place on
irradiation with visible light of wavelength longer than 580 nm
for all the compounds.
The fraction of the photocyclised form at the photosta-
tionary state, Ypss, is theoretically determined from eqn. 2.
Table 3 summarises quantum yields for the photocyclisation
(Wo-c) and the reverse photochemical ring-opening (Wc-o) reac-
tions, together with the fractions (Ypss) of the photocyclised
form at the photostationary state for all the compounds. The
fraction Ypss represents the molar ratio of the photocyclised
molecule to the total amount of the starting and photocyclised
molecules at the photostationary state upon irradiation with
365 nm light, which is experimentally determined from eqn. 1,
Ypss ~ eoWo-c/(eoWo-c 1 ecWc-o
)
(2)
The Ypss values for TPTTC, TPTBC and BTPTC in solution
were almost 100% because of the high Wo-c and the very low
c-o. The Ypss values for the amorphous films of TPTTC and
W
BTPTC were greater than 0.5, as confirmed by HPLC of
the benzene solution obtained by dissolving the film at the
photostationary state. Since it is thought that the transforma-
tion between the p- and ap-conformer does not take place in the
amorphous film, the values of Ypss being greater than 0.5 for
the amorphous films of TPTTC and BTPTC suggests that
the ap-conformer is more populated than the p-conformer in
the amorphous films. The Ypss value of TPTBC is less than 0.5,
but it is thought that TPTBC also preferentially takes the
ap-conformer, as described later.
Ypss ~ (1 2 Apss/A0)/(1 2 ec/eo)
(1)
where A0 and Apss represent the absorbances of the film before
photoirradiation and at the photostationary state, respecively,
and eo and ec stand for the molar extinction coefficients of the
open and cyclised forms at the wavelength of incident light,
respectively.
In order to gain further information on the population of
the ap-conformer, the value of Ypss was measured for the
amorphous films of the open form with only the ap-
conformation, which were obtained from the 100% cyclised
amorphous films of TPTTC-c, TPTBC-c and BTPTC-c by
irradiation with visible light (w580 nm). The results show that
the Ypss values obtained for these films were 0.69, 0.38 and 0.79
for TPTTC, TPTBC and BTPTC, respectively, being almost
the same as those for the corresponding films obtained by spin
coating from solutions of the unirradiated compounds in the
open forms (Table 3). These results indicate that almost all
molecules take up the ap-conformation in the amorphous films.
The result that the Ypss values for the amorphous films do not
reach 1.0 suggests that there are ap-conformers which cannot
undergo photocyclisation in the film, probably because the
local free volume around the remaining molecules is less than
that required for the photocyclisation reaction. The lower Ypss
value for TPTBC than those for TPTTC and BTPTC is
probably due to the requirement of a larger free volume for
the photocyclisation of TPTBC, as suggested from the CPK
models of these molecules. That is, while the photocyclisation
of TPTTC and BTPTC requires only partial rotation of
the thiophene moiety around the single bond connecting
the thienyl group and cyclopentene ring in the ap-conformer,
the photocyclisation of TPTBC requires partial rotation of the
larger benzothiophene moiety.
As Table 3 shows, the quantum yields for the photocyclisa-
tion reactions (Wo-c) of TPTTC, TPTBC and BTPTC in
solution were found to be much higher than those reported for
other dithienylethene derivatives in solution, which are mostly
in the range 0.3 to 0.5 as reported for e.g. 1,2-bis(5-phenyl-
2,4-dimethylthiophen-3-yl)-3,3,4,4,5,5-hexafluorocyclopentene
(0.41) and 1,2-bis(2-methylbenzo[b]thiophen-3-yl)-3,3,4,4,5,5-
hexafluorocyclopentene (0.35).36,37 It is known that dithienyl-
ethene derivatives assume two conformers, i.e. an ‘‘anti-
parallel’’ (ap) conformer having two thienyl moieties with
C2 symmetry and a ‘‘parallel’’ (p) conformer having two
thienyl moieties with mirror symmetry, and that only the
ap-conformer undergoes the photocyclisation reaction.26,27 It is
thought that dithienylethene derivatives usually take up the p-
and ap-conformations in a ca. 1 : 1 molar ratio in solution and,
hence, the quantum yields for the photocyclisation reactions of
these compounds are less than 0.5. The fact that the quantum
yields for the photocyclisation reactions of TPTTC, TPTBC
and BTPTC in solution are much higher than 0.5 suggests that
the ap-conformer is more populated than the p-conformer
for these molecules, probably due to the presence of the bulky
bis(4-methylphenyl)aminophenyl substituents. A high quan-
tum yield has also been reported for the photocyclisation
reaction of a photochromic polymer with a dithienylethene
backbone (0.86); this result is attributed to the existence of only
the ap-conformation in the polymer backbone.38 The values of
W
o-c for the amorphous films were found to be lower than those
Polarised light-induced dichromism and dual image formation
for solutions of all the compounds. This result can be explained
in terms of molecular motions for the cyclisation reaction being
restricted due to the presence of surrounding molecules in the
amorphous film. In addition, intermolecular interactions in the
solid state may cause enhanced radiationless deactivation,
decreasing Wo-c for the amorphous films relative to solution.
Photoinduced dichroism and birefringence of photochromic
solids is the subject of considerable interest. There are two
types of polarised light-induced dichroism and birefringence.
One is due to a change in the molecular alignment by polarised
light irradiation, which is known as the ‘‘Weigert effect’’.40
J. Mater. Chem., 2002, 12, 2612–2619
2617

View Article Online
Utilising this effect, modulation of the molecular alignment of
polymer liquid crystals41,42 and liquid crystal monolayers43
containing azobenzene chromophores has been performed.
The other type of polarised light-induced dichroism is due to
the bleaching of photochromic molecules lying in a specific
direction in isotropic solids by irradiation with polarised light.
This was first demonstrated for fulgide-doped poly(methyl
methacrylate).44,45 Recently, polarised light-induced dichroism
and birefringence have also been reported for a sol–gel glass
containing a dithienylethene moiety.46 In the present study, the
polarised light-induced dichroism of the dithienylethene-based
photochromic amorphous molecular materials as amorphous
films was investigated.
Irradiation of the amorphous films of open form TPTBC and
BTPTC on glass substrates with non-polarised 365 nm light at
ca. 20 mW cm22 caused a colour change from almost colourless
to blue and green for TPTBC and BTPTC, respectively, due to
the photocyclisation reactions to give TPTBC-c and BTPTC-c.
After the system had reached the photostationary state, the
coloured film was then irradiated with linearly polarised light
of wavelength longer than 580 nm at ca. 20 mW cm22 to induce
the reverse photochemical ring-opening reaction.
Fig. 5(a) shows the electronic absorption spectrum of the
coloured film obtained by irradiation of a TPTBC film with
non-polarised 365 nm light. When this coloured film was
irradiated with linearly polarised red light to induce the reverse
ring-opening reaction of TPTBC-c, the colour of the film
gradually faded. Fig. 5(b) and (c) show the absorption spectra
of the film after irradiation with polarised red light for 2 min.
The intensity of the absorption band at 580 nm depended on
the angle (h) between the polarisation directions of the
irradiated red light and probe light, as seen from the spectra
for h ~ 0 [Fig. 5(b)] and 90u [Fig. 5(c)]. Similar phenomena
were observed for the amorphous film of BTPTC, although
colour fading took place more slowly relative to the amorphous
film of TPTBC because of the one order of magnitude smaller
quantum yield for the photoinduced ring-opening reaction of
BTPTC-c. Fig. 6 shows the dependence of the angle h on the
absorbances at 580 and 630 nm for the amorphous films
prepared by irradiation of the coloured films of TPTBC and
BTPTC with polarised red light for 2 and 60 min, respectively.
The absorbances of the films of TPTBC and BTPTC at 580 and
630 nm, respectively, increased with increasing h, being least for
h ~ 0u and greatest for h ~ 90u. The transition moment for the
absorption giving rise to bands peaking at 580 and 630 nm for
TPTBC-c and BTPTC-c is thought to be in a direction almost
parallel to the longest molecular axis (horizontal direction for
the molecules drawn in Scheme 1). The closed isomers, which
are in random orientations, absorb red light more effectively
Fig. 6 Dependence of the angle h on the absorbances at 580 and 630 nm
for amorphous films of (a) TPTBC and (b) BTPTC after irradiation
with polarised red light for 2 and 60 min, respectively.
when the angle between the irradiated polarised red light
and the longest molecular axis is small. Thus, the number of
coloured molecules remaining unfaded becomes larger with
increasing h.
Dichroism was evaluated according to eqn. 3,46
D ~ DI/I0 ~ 102A 2 102A
//
^
(3)
where I0 represents the intensity of the incident probe light, DI
the difference in the intensity of the transmitted probe light
between h ~ 0 and 90u, and A// and A^ the absorbances of the
film for h ~ 0 and 90u, respectively. The maximum values of D
for the amorphous films of TPTBC and BTPTC were ca. 0.10
and ca. 0.11, respectively, which were larger than that for the
dithienylethene-doped sol–gel glass (ca. 0.06).46
Utilising the phenomenon of linearly polarised light-induced
dichroism, dual image formation at the same location of the
amorphous film was demonstrated. The amorphous films of
TPTBC and BTPTC with a thickness of 5 to 10 mm were
prepared by sandwiching a melt sample between two glass
substrates and then cooling in air. The amorphous film of
TPTBC was irradiated with non-polarised 365 nm light for 5
min and then the resulting blue-coloured film was irradiated
with polarised red light of horizontal polarisation for 30 s
through a mask of a letter ‘‘H’’, where the area other than the
letter H is exposed, followed by irradiation with polarised red
light of vertical polarisation for another 30 s through another
mask of a letter ‘‘V’’, where the area other than the letter V is
exposed.
Fig. 7 shows photographs of the dual images formed at the
same location on the TPTBC amorphous film. When the film is
viewed through a polariser with h ~ 0u, the letter ‘‘H’’ is visible
[Fig. 7(a)], while the letter ‘‘V’’ is visible through a polariser
with h ~ 90u [Fig. 7(b)]. Such dual image formation was also
Fig. 5 (a) Electronic absorption spectrum of the coloured film obtained
by irradiation of the TPTBC film with non-polarised 365 nm light.
Electronic absorption spectra of the film after irradiation with polarised
red light for 2 min with h ~ 0 (b) and 90u (c).
2618
J. Mater. Chem., 2002, 12, 2612–2619

View Article Online
8
9
Y. Shirota, K. Moriwaki, S. Yoshikawa, T. Ujike and H. Nakano,
J. Mater. Chem., 1998, 8, 2579.
T. Ujike, K. Moriwaki, H. Nakano and Y. Shirota, J. Photopolym.
Sci. Technol., 1998, 11, 33.
10 H. Utsumi, D. Nagahama, H. Nakano and Y. Shirota, J. Mater.
Chem., 2000, 10, 2436.
11 M. Yoshiiwa, H. Kageyama, Y. Shirota, F. Wakaya, K. Gamo
and M. Takai, Appl. Phys. Lett., 1996, 69, 2605.
12 T. Kadota, H. Kageyama, F. Wakaya, K. Gamo and Y. Shirota,
J. Photopolym. Sci. Technol., 2000, 13, 203.
13 T. Kadota, M. Yoshiiwa, H. Kageyama, F. Wakaya, K. Gamo
and Y. Shirota, SPIE-Int. Soc. Opt. Eng., 2001, 4345, 891.
14 T. Kadota, H. Kageyama, F. Wakaya, K. Gamo and Y. Shirota,
Mater. Sci. Eng., C, 2001, 16, 91.
15 Y. Shirota, T. Kobata and N. Noma, Chem. Lett., 1989, 1145.
16 M. Kinoshita, N. Fujii, T. Tsuzuki and Y. Shirota, Synth. Met.,
2001, 121, 1571.
17 Y. Shirota, N. Noma, K. Namba, T. Tsuzuki, N. Fujii and
M. Kinoshita, SPIE-Int. Soc. Opt. Eng., 2001, 4108, 17.
18 Y. Kuwabara, H. Ogawa, H. Inada, N. Noma and Y. Shirota,
Adv. Mater., 1994, 6, 677.
19 Y. Shirota, Y. Kuwabara, H. Inada, T. Wakimoto, H. Nakada,
Y. Yonemoto, S. Kawami and K. Imai, Appl. Phys. Lett., 1994, 65,
807.
Fig. 7 Photographs of dual images formed on an amorphous film of
TPTBC, viewed through polarisers with h ~ 0 (a) and 90u (b).
realised for a BTPTC film. These images were stable for more
than three months in the dark at room temperature. These
two images could be erased by irradiation with intense non-
polarised red light, and new dual images could be formed
repeatedly by the same procedures described above. The
formation of dual images at the same location can be applied
for the formation of a stereo image.
20 T. Noda, H. Ogawa, N. Noma and Y. Shirota, Appl. Phys. Lett.,
1997, 70, 699.
21 T. Noda, I. Imae, N. Noma and Y. Shirota, Adv. Mater., 1997, 9,
239.
Summary
A new class of photochromic amorphous molecular materials
containing a dithienylethene moiety, TPTTC, TPTBC and
BTPTC, were designed and synthesised. These compounds,
together with their photocyclised products, were found to
readily form amorphous glasses with well-defined glass trans-
ition temperatures and to undergo photochromism as amor-
phous films as well as in solution. While the quantum yields for
the photocyclisation reactions (Wo-c) of these compounds in
solution were very high, those for the ring-opening reactions
(Wc-o) of the photocyclised compounds were very low. Because
of the high Wo-c and the very low Wc-o, the fractions (Ypss) of the
photocyclised forms for these compounds in solution were
almost 100%. The results that the Wo-c of these compounds in
solution are much larger than 0.5 and the Ypss of TPTTC and
BTPTC as amorphous films upon irradiation with 365 nm
light are greater than 50% suggest that the anti-parallel con-
former is more populated than the parallel conformer for the
present molecules.
22 T. Noda, I. Imae, N. Noma and Y. Shirota, Adv. Mater., 1997, 9,
720.
23 T. Noda and Y. Shirota, J. Am. Chem. Soc., 1998, 120, 9714.
24 Y. Shirota and K. Okumoto, SPIE-Int. Soc. Opt. Eng., 2000, 4105,
158 and references therein.
25 Y. Shirota, M. Kinoshita, T. Noda, K. Okumoto and T. Ohara,
J. Am. Chem. Soc., 2000, 122, 11021.
26 M. Irie and M. Mohri, J. Org. Chem., 1988, 53, 803.
27 K. Uchida, Y. Nakayama and M. Irie, Bull. Chem. Soc. Jpn., 1990,
63, 1311.
28 S. L. Gilat, A. H. Kawai and J.-M. Lehn, J. Chem. Soc., Chem.
Commun., 1993, 1239.
29 M. Takeshita and M. Irie, J. Org. Chem., 1998, 63, 6643.
30 A. Ferna´ndez-Acebes and J.-M. Lehn, Chem. Eur. J., 1999, 5,
3285.
31 M. Fukudome, K. Kamiyama, T. Kawai and M. Irie, Chem. Lett.,
2001, 70.
32 R. Lantz and A.-B. Ho¨rnfeldt, Chem. Scr., 1972, 2, 9.
33 A. Higuchi, H. Inada, T. Kobata and Y. Shirota, Adv. Mater.,
1991, 3, 549.
34 Y. Kuwabara, H. Ogawa, H. Inada, N. Noma and Y. Shirota,
Adv. Mater., 1994, 6, 677.
35 K. Okumoto and Y. Shirota, Chem. Lett., 2000, 1034.
36 M. Hanazawa, R. Sumiya, Y. Horikawa and M. Irie, J. Chem.
Soc., Chem. Commun., 1992, 206.
Polarised light-induced dichroism in amorphous films of
the dithienylethene-based photochromic amorphous molecular
materials was investigated, and dual image formation at
the same location was realised by taking acvantage of this
phenomenon.
37 M. Irie, K. Sakemura, M. Okinaka and K. Uchida, J. Org. Chem.,
1995, 60, 8305.
38 F. Stellacci, C. Bertarelli, F. Toscano, M. C. Gallazzi, G. Zotti and
G. Zerbi, Adv. Mater., 1999, 11, 292.
39 M. Irie, T. Eriguchi, T. Takada and K. Uchida, Tetrahedron, 1997,
53, 214.
40 F. Weigert, Naturwissenschaften, 1921, 29, 583.
41 Y. Wu, Y. Demachi, O. Tsutsumi, A. Kanazawa, T. Shiono and
T. Ikeda, Macromolecules, 1998, 31, 349.
42 Y. Wu, J. Mamiya, A. Kanazawa, T. Shiono, T. Ikeda and
Q. Zhang, Macromolecules, 1999, 32, 8829.
43 K. Ichimura, Chem. Rev., 2000, 100, 1847.
44 P. Jones, P. Darcy, G. S. Attard, W. J. Jones and G. Williams,
Mol. Phys., 1989, 67, 1053.
45 A. Kozak and G. Williams, Mol. Phys., 1989, 67, 1065.
46 J. Peretti, J. Biteau, J.-P. Boilot, F. Chapu, V. I. Safarov,
J.-M. Lehn and A. Ferna´ndez-Acebes, Appl. Phys. Lett., 1999, 74,
1657.
References
1
2
K. Horie and I. Mita, Adv. Polym. Sci., 1989, 88, 79.
H. Du¨rr and H. Bouas-Laurant, Photochromism, Molecules and
Systems, Elsevier, Amsterdam, 1990.
3
4
C. B. McArdle, Applied Photochromic Polymer Systems, Blackie &
Son Ltd., New York, 1992.
K. Ichimura, in Organic Photochromic and Thermochromic
Compounds, ed. J. C. Crano and R. J. Guglielmetti, Plenum
Press, New York and London, 1999, vol. 2, p. 9.
Y. Shirota, J. Mater. Chem., 2000, 10, 1 and references therein.
A. Higuchi, H. Inada, T. Kobata and Y. Shirota, Adv. Mater.,
1991, 3, 549.
5
6
7
M. Ishihara, T. Noda, H. Kageyama, H. Nakano and Y. Shirota,
Synth. Met., 2001, 120, 795.
J. Mater. Chem., 2002, 12, 2612–2619
2619