New photochromic diarylethenes with a six-membered aryl unit

Article abstract of DOI:10.1016/j.tet.2008.07.073

A new class of photochromic diarylethenes bearing a six-membered aryl unit has been developed. They underwent reversible cyclization and cycloreversion reactions upon alternating irradiation with UV and visible light both in solution and in PMMA film. However, diarylethenes 1, 2, and 4 showed no photochromism in the crystalline phase although they packed with an anti-parallel conformation and the distances between the two reactive carbon atoms were both less than 4.2 ?.

Full text of DOI:10.1016/j.tet.2008.07.073

Tetrahedron 64 (2008) 9464–9470
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Tetrahedron
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New photochromic diarylethenes with a six-membered aryl unit
*
Shouzhi Pu , Congbin Fan, Wenjuan Miao, Gang Liu
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University, Nanchang 330013, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 May 2008
Received in revised form 17 July 2008
Accepted 18 July 2008
Available online 23 July 2008
A new class of photochromic diarylethenes bearing a six-membered aryl unit has been developed. They
underwent reversible cyclization and cycloreversion reactions upon alternating irradiation with UV and
visible light both in solution and in PMMA film. However, diarylethenes 1, 2, and 4 showed no photo-
chromism in the crystalline phase although they packed with an anti-parallel conformation and the
distances between the two reactive carbon atoms were both less than 4.2 Å.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
membered aryl ring and a vinyl group. The results are very in-
teresting and important, and they contribute to a broad un-
Photochromic diarylethenes have been extensively studied for
optoelectronic applications because of their notable thermally
irreversible photochromic behavior and remarkable fatigue re-
sistance.1 Up to date, design and synthesis of new photochromic
compounds is an active area of research, and many publications
concerning synthesis and investigation of the properties of diaryl-
ethenes with the heterocyclic aryl rings have been reported. Among
diarylethenes hitherto reported, most of the heteroaryl moieties
have been thiophene or benzothiophene rings,2 with just a few re-
portsconcerningotherheteroaryl moieties, such as furan,3 thiazole,4
indole,5 benzofuran,6 pyrrole,7 and pyrazole,8 etc. The photochromic
performance of each kind of diarylethene is strongly dependent on
the nature of the aryl moieties. For example, diarylethenes with
thiophene or benzothiophene moieties exhibit excellent thermal
stability and outstanding fatigue resistance,1 whereas diarylethenes
with pyrrole rings are thermally unstable and return to the open-
ring isomers even in the dark.7b Moreover, Kawai et al.9 reported the
triangular terthiophene derivatives that showed reversible photo-
chromic reactions with high cyclization quantum yields. Yamaguchi
derstanding of the photochromism of diarylethene derivatives with
various aryl groups. At the same time, the results have also given us
some good suggestions to design some new photochromic diaryl-
ethene systems. We predicted that diarylethene would have some
novel characteristics when replacing a five-membered heterocyclic
ring with a six-membered aryl ring. As far as we are aware, there
are many reports on diarylethene derivatives with heteroaromatic
rings as both thermally irreversible and reversible photochromic
compounds, but in the case of six-membered rings as aryl moieties,
only a few diphenyl-perfluorocyclopentene derivatives have been
reported.12 The majority of these compounds are thermally
reversible with poor photochromism. To the best of our knowledge,
photochromic hybrid diarylethene derivatives bearing both five-
membered and six-membered moieties have not been reported.
Herein, using a benzene ring as an arylmoiety, wehavedesigned and
prepared a new class of photochromic diarylethene derivatives, i.e.,
1-(2-methyl-5-phenyl-3-thienyl)-2-(2-methoxylphenyl)perfluoro-
cyclopentene (1a), 1-(2-methyl-5-phenyl-3-thienyl)-2-(2-methyl-
phenyl)perfluorocyclopentene (2a),1-(2-methyl-5-phenyl-3-thienyl)-
2-(2-cyanophenyl)perfluorocyclopentene (3a), and 1-(2-methyl-5-
phenyl-3-thienyl)-2-(2-trifluoromethylphenyl)perfluorocyclopentene
(4a). All of these diarylethenes showed good reversible photochro-
mism both in solution and in PMMA amorphous film. They are, to the
best of our knowledge, the first class example of photochromic diary-
lethene derivatives bearing both five-membered and six-membered
moieties. The photochromic scheme of 1a–4a is shown in Scheme 1.
et al.10 developed a new type of 6
p conjugated photochromic system
with a bis(2,30-benzothienyl) unit showing efficient photochromism
and thermal stability for the colored isomers. Peters et al.11 syn-
thesized a new class of photochromic compounds based on a modi-
fied hexatriene skeleton. This novel molecular scaffold offers the
chance to decorate photoresponsive systems with a wide range of
functional groups without sacrificing photochromic behavior.
As described above, it can be easily concluded that these reports
have
a common merit, i.e., the hexatriene backbone of all
2. Results and discussion
photochromic perfluorocyclopentene systems is composed of
five-membered heterocyclic rings or the combination of a five-
2.1. Synthesis of the diarylethenes 1a–4a
The synthetic route for diarylethenes 1a–4a is shown in Scheme
2. Compounds 1a–4a were prepared from 2-methyl-5-phenyl-3-
* Corresponding author. Tel./fax: þ86 791 3805212.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.07.073

S. Pu et al. / Tetrahedron 64 (2008) 9464–9470
9465
F
F
A
B
C
0.8
0.6
0.4
0.2
0.0
F
F
F
F
F
F
F
F
F
UV
Vis
Vis
F
UV
S
R
R
S
1b: R = OCH3
2b: R = CH3
1a: R = OCH3
2a: R = CH3
CN
3b: R =
CN
3a: R =
4b: R = CF3
4a: R = CF3
Scheme 1. Photochromism of diarylethenes 1a–4a.
UV
Vis
F
F
F
F
F
F
Br
F
F
F
F
F
F
R
300
400
500
Wavelength/nm
600
700
800
R
S
n-BuLi, THF
-78 °C
1a: R = OCH3
S
2a: R = CH3
0.8
0.6
0.4
0.2
0.0
CN
3a: R =
4a: R = CF3
2a
3a
4a
Scheme 2. Synthetic route of diarylethenes 1a–4a.
thienyl-perfluorocyclopentene11,13 in a one-step coupling reaction
with 2-bromoanisole, 2-bromotoluene, 2-bromobenzonitrile, and
2-bromobenzotrifluoride, respectively. Their structures were con-
firmed by NMR, MS, elemental analysis, and IR (see Section 4).
Compounds 1a, 2a, and 4a were also confirmed by X-ray single-
crystal diffraction.
4b
3b
2b
2.2. Photochromic reactions both in solution
and in PMMA amorphous film
300
400
500
Wavelength/nm
600
700
800
The photochromic behaviors of diarylethenes 1–4 induced by
photoirradiation at room temperature were measured both in
hexane and in PMMA amorphous film. In hexane, the absorption
spectral and color changes of diarylethenes 1–4 induced by alter-
nating irradiation with UV light and visible light with appropriate
wavelength are shown in Figure 1. As shown in Figure 1A, compound
1a exhibited a sharp absorption peak at 275 nm in hexane, which
was arisen from p/p
*
transition.14 Upon irradiation with 297 nm
light, a new visible absorption band centered at 581 nm emerged
while the original peak at 275 nm decreased, indicating the for-
mation of the closed-ring isomer 1b. Correspondingly, the colorless
solution of 1a turned blue owing to produce the closed-ring isomer
1b. The blue colored solution turned colorless by irradiation with
visible light (l>500 nm), indicating that 1b returned to the initial
state 1a and a clear isosbestic point was observed at 309 nm.
Just as diarylethene 1, diarylethenes 2–4 also showed photo-
chromism in hexane. Upon irradiation with 297 nm light, absorp-
tion bands in visible region appeared and all solutions containing
2a, 3a, and 4a, respectively, turned magenta as a result of the cy-
clization reactions to produce the closed-ring isomers 2b, 3b, and
4b, which the absorption maxima of them were observed at 554,
545, and 544 nm, respectively (Fig. 1B). The color changes of
diarylethenes 1–4 upon photoirradiation in hexane are shown in
Figure 1C. All the solutions of 2b, 3b, and 4b can be decolorized
Figure 1. Absorption spectral and color changes of diarylethenes 1–4 upon alternating
irradiation with UV and visible light in hexane (3.0ꢀ10ꢁ5 M) at room temperature; (A)
spectral changes for compound 1; (B) spectral changes for compounds 2–4; (C) color
changes for compounds 1–4.
upon irradiation with visible light (l>450 nm) attributable to
reproduce the open-ring isomers 2a, 3a, and 4a. In the photosta-
tionary state, the isosbestic points for diarylethenes 2, 3, and 4 were
observed at 300, 301, and 297 nm, respectively. The absorption
spectral features of these compounds are summarized in Table 1.
The cyclization and cycloreversion quantum yields of diaryl-
ethenes 1–4 were measured in hexane at room temperature, and
the results are also summarized in Table 1. As can be seen from
these data, different substituents at 2-position of the benzene ring
had a significant effect on the photochromic features of diaryl-
ethenes 1–4, including the absorption maxima, molar absorption
coefficients, and quantum yields. The results indicated that the
cycloreversion quantum yields enhanced significantly when
the electron-donating groups were substituted at the 2-position of

9466
S. Pu et al. / Tetrahedron 64 (2008) 9464–9470
Table 1
A
Absorption spectral properties of diarylethenes 1–4 in hexane at 3.0ꢀ10ꢁ5 mol/L
4
3
2
1
0
a
UV
Compound lo,max (nm)
lc,maxb (nm)
Fc
Vis
(3
/L molꢁ1 cmꢁ1
)
(3
/L molꢁ1 cmꢁ1
)
Hexane
PMMA film Hexane PMMA film Fo–c Fc–o
1
2
3
4
275(2.5ꢀ104) 333
264(2.3ꢀ104) 312
278(2.1ꢀ104) 329
274(1.6ꢀ104) 290
581(3.8ꢀ103) 585
0.26 0.15
0.25 0.31
0.64 0.019
0.65 0.011
552(4.8ꢀ103) 564
548(2.3ꢀ103) 554
539(4.7ꢀ103) 560
a
b
c
Absorption maxima of open-ring forms.
Absorption maxima of closed-ring forms.
Quantum yields of cyclization reaction (Fo–c) and cycloreversion reaction (Fc–o),
respectively.
UV
Vis
the benzene ring of these diarylethenes, while the cyclization
quantum yields improved remarkably when the electron-with-
drawing groups were substituted at the same position. The ab-
sorption maxima of diarylethenes 1–4 displayed a well regular
change in hexane, i.e., they showed a remarkable blue shift
(581 nm/539 nm) with the increase of electron-withdrawing
ability. On the one hand, the absorption maxima of both open-ring
and closed-ring isomers of diarylethene 1 are much longer than
those of diarylethene 2, as a result of the electron-donating ability
of methoxyl group is larger than that of methyl group;15 but the
molar absorption coefficient of 1b is smaller than that of 2b.
The cyclization quantum yields of the two compounds are almost
equal. However, there is a relatively big difference of the cyclo-
reversion quantum yield between compounds 1 and 2. The cyclo-
reversion quantum yield of 2 (Fc–o¼0.31) is two-fold larger than
that of 1 (Fc–o¼0.15), indicating that the cycloreversion quantum
yield was remarkably dependent on the electron-donating sub-
stituents and it was dramatically increased with the decrease of
electron-donating ability. On the other hand, the electron-with-
drawing substituents (cyano and trifluoromethyl groups) signifi-
cantly shifted the absorption maxima of diarylethenes 3b and 4b to
a shorter wavelength, and their molar absorption coefficients
increased with the increasing electron-withdrawing ability. The
result is contrary to those reported previously.15a,16 In addition,
electron-withdrawing groups can also enhance notably the cycli-
zation quantum yield and depress heavily the cycloreversion
quantum yield. From Table 1, it can be easily found that the cycli-
zation quantum yields of diarylethenes 3 and 4 bearing electron-
withdrawing groups are much higher than those of diarylethenes 1
and 2 bearing electron-donating groups, but a reverse case for the
cycloreversion quantum yields. Furthermore, the cycloreversion
quantum yield of 3 (Fc–o¼0.019) was almost equal to that of 5
300
400
500
Wavelength(nm)
600
700
800
B
4
3
2
1
0
3a
4a
3b
2b
4b
2a
300
400
500
Wavelength/nm
600
700
800
C
(
Fc–o¼0.011). The result suggested that the ability of the electron-
withdrawing substituents did not influence remarkably the
cycloreversion quantum yield.15a
For practical applications in optical devices, it is very important
that photochromic materials can keep good photochromism in
a polymer film, such as the PMMA film.17 Dissolved ultrasonically
10 mg diarylethene sample and 100 mg PMMA into 1.0 mL chlo-
roform, the film was prepared by spin-coating method. In PMMA
amorphous film, diarylethenes 1–4 also showed good photochro-
mism (Fig. 2) as similar to those in solution. The photochromic
properties of diarylethenes 1–4 in PMMA film are also summarized
in Table 1. Upon irradiation with 313 nm light, the colorless
diarylethene 1a/PMMA film turned blue for which the absorption
maxima were observed at 585 nm, as the closed-ring isomer 1b was
generated; while the colors of other three diarylethene/PMMA
films changed from colorless to magenta with the appearance of
a new broad absorption band at 564, 554, and 560 nm, respectively,
which was assigned to the formation of the closed-ring isomers 2b,
3b, and 4b. All colored diarylethene/PMMA films can revert to
Figure 2. Absorption spectral and color changes of diarylethenes 1–4 upon alternating
irradiation with UV and visible light in PMMA film (10% w/w) at room temperature; (A)
spectral changes for compound 1; (B) spectral changes for compounds 2–4; (C) color
changes for compounds 1–4.
been observed for most of the reported diarylethenes,18 the maxi-
mum absorption peaks of both the open-ring and the closed-ring
isomers in PMMA film are longer than those in hexane solution. The
red shift values of the absorption maxima of the open-ring isomers
are 58 nm for 1a, 48 nm for 2a, 51 nm for 3a, and 16 nm for 4a, and
those of the closed-ring isomers are 4 nm for 1b,12 nm for 2b, 6 nm
for 3b, and 21 nm for 4b, respectively. The red shift phenomena
colorless upon irradiation with visible light (l>450 nm). As has

S. Pu et al. / Tetrahedron 64 (2008) 9464–9470
9467
may be ascribed to the stabilization of molecular arrangement in
solid state.19
The thermal stabilities of the open-ring and closed-ring isomers
A
1.0
0.8
0.6
0.4
0.2
of diarylethenes 1–4 were tested in hexane both at room temper-
ature and at 80 ꢂC. Storing these solutions in hexane at room
temperature in the dark and then exposing them to air for more
than 10 days, we found that no changes in the UV/vis spectra of
diarylethenes 2–4 were observed and, indeed, no decomposition
was detected when the three compounds were exposed to air for
more than a year. For diarylethene 1, no decomposition was also
detected for the open-ring isomer 1a when exposed it to air for
more than a year, but its closed-ring isomer 1b returned to 1a when
exposed it to air in the dark for only 10 days. That is to say, the
thermal stability of 1b is very weak. At 80 ꢂC, diarylethenes 2–4 still
showed good thermal stabilities for more than 100 min, but the
blue color of 1b disappeared completely after 3 min. The result
suggests that the thermal stability of diarylethenes with a six-
membered ring is very different from that of photochromic
dithienylethene derivative.1a,16 In general, dithienylethene de-
rivatives bearing electron-donating substituents show good ther-
mal stability, but those with strong electron-withdrawing
substituents, such as dicyanovinyl group,1a,16 show thermal in-
stability. The possible reason is that the strong electron-donating
ability of methoxyl group leads to the high-density electron cloud
distribution of the cyclohexadiene moiety in 1b system, resulting in
its thermal instability.
1a
2a
3a
4a
0
2
4
Cyclization/cycloreversion Cycles
6
8
10
12
B
1.0
0.8
0.6
0.4
0.2
1a
2a
3a
4a
Fatigue-resistant property, i.e., how may times photocyclization
and cycloreversion reaction cycles can be repeated without loss of
performance,1a,7b is an very important factor for practical applica-
tions in optical devices.1a,20 The fatigue resistances of diarylethenes
1–4 were examined both in hexane and in PMMA amorphous film
at room temperature, as shown in Figure 3. In hexane, diaryl-
ethenes 1–4 were irradiated alternatively with 297 nm and visible
light (l>450 nm), respectively. The irradiation time was long
0
50
100
Cyclization/cycloreversion cycles
150
200
enough for coloration to reach the photostationary state and for the
color to be completely bleached. As shown in Figure 3A, the fatigue-
resistant characteristics of 1 and 2 in solution indicated that w70%
of each of 1b and 2b was destroyed after 10 repeat cycles, but only
27% of 3b and 12% of 4b were destroyed after 11 repeat cycles at the
same condition. This may be ascribed to degradation resulting from
the formation of an epoxide.21 Similarly, the fatigue resistances of
diarylethenes 1–4 were tested in PMMA film by alternatively irra-
diating with 313 nm and visible light, respectively. As shown in
Figure 3B, the fatigue resistances of diarylethenes 1–4 in the solid
state (PMMA amorphous film) are much stronger than those in
solution. After 200 repeated cycles, these compounds still showed
good photochromism with only w37% degradation of 1b, 22% of 2b,
9% of 3b, and 7% of 4b, respectively. This remarkable improvement
may result from effectively suppressing the oxygen diffusion.1a As
described above, it can be easily concluded that the fatigue re-
sistances of 3 and 4 are much stronger than those of 1 and 2, both in
solution and in PMMA amorphous film, which may be attributed to
the different electron-withdrawing/electron-donating substituent
effects.
Figure 3. Fatigue resistance of diarylethenes 1–4 in hexane and in PMMA film in air
atmosphere at room temperature. Initial absorbance of the sample was fixed to 1.0; (A)
in hexane; (B) in PMMA film.
in the crystalline phase. The distances between the two reactive
carbons of 1a (C10/C22), 2a (C4/C21), and 4a (C1/C18) were
3.672(4), 3.817(3), and 4.089(7) Å, respectively. In general, the
molecule undergoes the photocyclization reaction if the molecule is
fixed in an anti-parallel mode and the distance between reacting
carbon atoms on the aryl rings is less than 4.2 Å.24 Therefore, 1a, 2a,
and 4a should undergo photochromism in the crystalline phase.
However, the fact is contrary to the above analysis. All of the three
compounds showed no photochromism in the crystalline phase.
This is verified by the fact that irradiating single crystals of 1a, 2a,
and 4a with UV light for 12 h resulted in no observable color
change. When these crystals were dissolved in hexane, the solu-
tions showed colorless and their absorption spectra were the same
as those of the open-ring isomers 1a, 2a, and 4a. Thus, the impor-
tant rule described above24 should restrict its applied condition: it
wasn’t applicable to diarylethene systems bearing a six-membered
aryl ring reported in this paper at least.
2.3. Non-photochromism in the crystalline phase
Among diarylethenes 1a–4a, single crystals of diarylethenes 1a,
2a, and 4a were obtained by recrystallization from diethyl ether. To
know better, the relation between the conformation and the pho-
tochromic behaviors of diarylethenes 1a, 2a, and 4a in the crys-
talline phase, their final structural conformations were provided by
X-ray crystallographic analysis. The ORTEP drawings of diaryl-
ethenes 1a, 2a, and 4a are shown in Figure 4, and the X-ray crys-
tallographic analysis data are listed in Ref. 22. As shown in Figure 4,
these three molecules were packed in anti-parallel conformations23
2.4. Fluorescence of diarylethenes 1–4
The fluorescence spectra of diarylethenes 1a–4a both in hexane
and in PMMA amorphous film at room temperature are illustrated
in Figure 5. All of them showed good fluorescence both in hexane
and in PMMA film. As shown in Figure 5, we could clearly see that
the fluorescent emissions of 1a, 2a, 3a, and 4a were at 353, 409,

9468
S. Pu et al. / Tetrahedron 64 (2008) 9464–9470
A
1a
1.0
0.8
0.6
0.4
0.2
2a
4a
3a
0.0
320
400
480
Wavelength/nm
B
1a
2a
1.0
0.8
0.6
0.4
0.2
4a
3a
0.0
350
400
450
Wavelength/nm
500
550
Figure 5. Fluorescence emission spectra of diarylethenes 1–4 both in hexane solution
(2.0ꢀ10ꢁ5 mol/L) and in PMMA film (10% w/w) at room temperature; (A) emission
spectra in hexane, excited at 285 nm; (B) emission spectra in PMMA film, excited at
295 nm.
constant in PMMA film. Among compounds 1a, 2a, 3a, and 4a, the
emission intensity of 1a is the strongest and that of 3a is the
weakest both in hexane solution and in PMMA film. The result
suggests that different substituents attached at the 2-position of
the benzene ring has a significant effect not only on the emission
peak but also on the emission intensity.
Generally, the open-ring isomers of diarylethenes have been
known to exhibit fluorescence, while the fluorescence of the
closed-ring isomers of diarylethenes is inactive.25 As has been
observed for most of the reported diarylethenes,26 diarylethenes
1–4 exhibited fluorescent switches during the process of photo-
isomerization both in solution and in PMMA film. When irradiated
by UV light, the photocyclization reaction occurred and the emis-
sion intensity of diarylethenes 1–4 decreased gradually, because of
forming the non-fluorescence closed-ring isomers. The back
irradiation by visible light with an appropriate wavelength regen-
erated their open-ring isomers and the original emission intensity.
In hexane, the emission intensity of diarylethenes 1–4 was de-
creased upon irradiation with 297 nm light, and it was quenched to
ca. 90% for 1, 47% for 2, 65% for 3, and 29% for 4, respectively, when
arrived at a photostationary state. The existence of parallel con-
formations and weakly fluorescent anti-parallel conformations of
diarylethenes 1a, 2a, and 3a in hexane may be the main cause for
the lower fluorescent change induced by photoirradiation.18a,b
Similarly, upon irradiation with 313 nm light, the emission
Figure 4. ORTEP drawings of crystals 1a, 2a, and 4a showing 35% probability dis-
placement ellipsoids: (A) 1a, (B) 2a, and (C) 4a.
475, and 436 nm when excited at 285 nm; while those of them
were observed at 435, 417, 471, and 445 nm when excited at 295 nm
in PMMA film. By using anthracene (0.27 in acetonitrile) as the
reference, the fluorescence quantum yields of 1a, 2a, 3a, and 4a
were determined to be 0.047, 0.021, 0.010, and 0.041, respectively.
Thus, it is clear that the introduction of methoxyl group at
2-position of benzene ring of diarylethene system can significantly
enhance the fluorescence quantum yield. Compared to those in
hexane, the emission peaks of diarylethenes 1a, 2a, and 4a showed
a remarkable bathochromic shift in PMMA film, which is well
consistent with those of their maxima absorption wavelengths.
Compared to that in hexane, the emission peak of 3a is almost

S. Pu et al. / Tetrahedron 64 (2008) 9464–9470
9469
intensity of diarylethenes 1–4 was also decreased remarkably in
PMMA film. When arrived at its photostationary state, the emission
intensity of diarylethenes 1–4 was quenched to ca. 15%, 28%, 58%,
and 22%, respectively. The result showed that the fluorescent
switchable efficiencies of diarylethenes 1–4 in PMMA film were
much bigger than those in solution. For example, the fluorescent
switchable efficiency of diarylethene 1 in PMMA film was enhanced
about 75% more than that in hexane solution, and those of other
three compounds increased remarkably in PMMA film. Among
and evaporated in vacuo, then purified by column chromatography
(petroleum ether/acetyl acetate mixture, v/v¼5:1) to give 1a (1.2 g)
in 53% yield. The compound was recrystallized from diethyl ether at
room temperature and produced the crystals suitable for X-ray
analysis. Mp 98.3–98.7 ꢂC; 1H NMR (400 MHz, CDCl3, ppm):
d 1.93
(s, 3H, –CH3), 3.53 (s, 3H, –CH3), 6.85, 6.87 (d, 1H, J¼8.0 Hz, ben-
zene–H), 7.02 (t, 1H, J¼8.0 Hz, benzene–H), 7.25 (s, 1H, thiophene–
H), 7.30 (t, 1H, J¼8.0 Hz, benzene–H), 7.39 (t, 4H, benzene–H), 7.53,
7.55 (d, 2H, J¼8.0 Hz, benzene–H); 13C NMR(100 MHz, CDCl3, ppm):
these compounds, diarylethene
4
showed
a
big fluorescent
d 13.57, 54.72, 110.98, 116.94, 120.31, 122.55, 124.98, 126.17, 127.14,
switchable efficiency both in solution and in PMMA film, suggest-
ing that it is the most promising candidate for application on
photoswitchable devices, such as optical memory and fluorescent
modulation switches.27
128.46, 129.45, 131.27, 133.20, 139.24, 140.67, 156.62; IR (KBr, n,
cmꢁ1): 757, 867, 894, 988, 1023, 1128, 1191, 1269, 1339, 1462, 1497,
1600, 2836, 2935. Anal. Calcd for C23H16F6OS: C, 60.79, H, 3.55.
Found: C, 60.71, H, 3.46. MS m/z (Mþ) 455.1. Compound 1b: 1H NMR
(400 MHz, CDCl3, ppm): d 1.18 (s, 3H, –CH3), 2.75 (s, 3H, –CH3), 6.76,
3. Conclusion
6.78 (d, 1H, J¼8.0 Hz, benzene–H), 7.00–7.08 (m, 3H, benzene–H),
7.14 (s,1H, thiophene–H), 7.58–7.74 (m, 3H, benzene–H), 8.19, 8.21 (d,
1H, J¼8.0 Hz, benzene–H), 8.29, 8.31 (d, 1H, J¼8.0 Hz, benzene–H).
In conclusion, we have succeeded in preparing four new pho-
tochromic diarylethenes based on a six-membered aryl ring skel-
eton and investigated their spectral properties both in solution and
in solid media. This new photochromic system showed good pho-
tochromism, strong fatigue resistance, and remarkable fluorescent
switch in PMMA film. The result indicated that the substituents at
2-position of the benzene ring had a significant effect on the
properties of these diarylethene derivatives. The six-membered
aryl moiety induced some new dramatic properties differing from
other diarylethenes with five-membered aryl moieties reported so
far. The results provide a useful design strategy for the tuning of
photochromic properties for further potential applications.
4.2.2. Synthesis of 2a
Using 2-bromotoluene as the starting material, compound 2a
was prepared by a method similar to that used for 1a and obtained
as white solid in 57% yield. Mp 86.4–86.7 ꢂC; 1H NMR (400 MHz,
CDCl3, ppm):
d 1.99 (s, 3H, –CH3), 2.02 (s, 3H, –CH3), 7.19 (t, 2H,
J¼8.0 Hz, benzene–H), 7.28 (s, 1H, thiophene–H), 7.34, 7.36 (d, 2H,
J¼8.0 Hz, benzene–H), 7.38 (t, 3H, J¼8.0 Hz, benzene–H), 7.49, 7.51
(d, 2H, J¼8.0 Hz, benzene–H); 13C NMR(100 MHz, CDCl3, ppm):
d
13.69, 18.65, 121.68, 124.35, 124.57, 125.09, 126.65, 126.80, 127.94,
128.05, 128.92, 129.39, 132.39, 135.96, 140.03, 140.87; IR (KBr, n,
cmꢁ1): 748, 827, 861, 895, 988, 1062, 1199, 1278, 1341, 1439, 1500,
1600, 1640, 2917. Anal. Calcd for C23H16F6S: C, 63.01, H, 3.68. Found:
C, 63.05, H, 3.59. MS m/z (Mþ) 439.3. Compound 2b: 1H NMR
4. Experimental
4.1. General methods
(400 MHz, CDCl3, ppm): d 1.93 (s, 3H, –CH3), 2.14 (s, 3H, –CH3),
5.92–5.95 (m, 2H, benzene–H), 5.99 (t, 1H, J¼8.0 Hz, benzene–
H), 6.39, 6.41 (d,1H, J¼8.0 Hz, benzene–H), 7.12 (s,1H, thiophene–H),
7.32 (t, 1H, J¼7.2 Hz, benzene–H), 7.44 (t, 2H, J¼7.2 Hz, benzene–H),
7.56–7.58 (m, 2H, benzene–H).
NMR spectra were recorded on Bruker AV400 (400 MHz) spec-
trometer with CDCl3 as the solvent and tetramethylsilane as an
internal standard. IR spectra were recorded on Bruker Vertex-70
spectrometer and mass spectra were measured with Agilent MS
Trap VL spectrometer. The elemental analysis was measured with
PE CHN 2400. The absorption spectra were measured using Agilent
8453 UV/VIS spectrometer. Photoirradiation was carried out using
SHG-200 UV lamp, CX-21 ultraviolet fluorescence analysis cabinet,
and BMH-250 Visible lamp. Light of appropriate wavelengths was
isolated by different light filters. The X-ray experiment of the single
crystal was performed on Bruker SMART APEX2 CCD area-detector
4.2.3. Synthesis of 3a
Using 2-bromobenzonitrile as the starting material, compound
3a was prepared by a method similar to that used for 1a and
obtained as yellowish oil liquid in 20% yield. 1H NMR (400 MHz,
CDCl3, ppm): d 2.06 (s, 3H, –CH3), 7.18 (s, 1H, thiophene–H), 7.29 (s,
1H, benzene–H), 7.35 (t, 2H, J¼8.0 Hz, benzene–H), 7.46, 7.48 (d, 2H,
J¼8.0 Hz, benzene–H), 7.55 (t, 1H, J¼8.0 Hz, benzene–H), 7.62, 7.64
(d, 1H, J¼8.0 Hz, benzene–H), 7.64–7.74 (m, 2H, benzene–H); 13C
equipped with graphite monochromatized Mo Ka radiation at room
temperature (290ꢃ2 K). The fluorescent property was measured
using a Hitachi F-4500 spectrophotometer, and the breadths of
excitation and emission slit were selected both 10 nm.
THF was distilled over lithium aluminum hydride under argon.
All other solvents were used as received. 2-Methyl-5-phenyl-3-
thienyl-perfluorocyclopentene (MPTF) was synthesized according
to Refs. 11 and 12. The other reagents were purchased from
Shanghai Reagents Ltd.
NMR (100 MHz, CDCl3, TMS): d 14.51, 116.21, 125.73, 127.86, 128.91,
129.75, 130.41, 131.75, 132.91, 133.30, 133.88, 141.24, 142.18; IR (n,
KBr, cmꢁ1): 758, 863, 894, 991, 1065, 1137, 1197, 1270, 1340, 1384,
1445, 1470, 1600, 1637, 2230, 2860, 2926, 3073. Anal. Calcd for
C
23H13F6NS: C, 61.47, H, 2.92, N, 3.12. Found: C, 61.35, H, 2.89, N,
3.07. MS m/z (Mþ) 450.1. Compound 3b: 1H NMR (400 MHz, CDCl3,
ppm):
d
1.93 (s, 3H, –CH3), 6.40, 6.42 (d, 1H, J¼8.0 Hz, benzene–H),
6.86, 6.88 (d, 1H, J¼8.0 Hz, benzene–H), 7.10–7.14 (m, 2H, benzene–
H), 7.15 (s, 1H, thiophene–H), 7.62, 7.64 (d, 1H, J¼8.0 Hz, benzene–
H), 8.26, 8.28 (d, 1H, J¼8.0 Hz, benzene–H), 7.73–7.90 (m, 2H,
benzene–H), 8.58, 8.60 (d, 1H, J¼8.0 Hz, benzene–H).
4.2. Synthesis of diarylethenes 1a–4a
The syntheticroute fordiarylethenes1a–4a isshownin Scheme2.
4.2.4. Synthesis of 4a
4.2.1. Synthesis of 1a
Using 2-bromobenzotrifluoride as the starting material, com-
pound 4a was prepared by a method similar to that used for 1a and
obtained as white solid in 28% yield. Mp 92.7–93.5 ꢂC; 1H NMR
To a stirred THF solution (30 mL) of 2-bromoanisole (0.94 g,
5.0 mmol) was slowly added a 2.5 M n-BuLi/hexane solution
(2.0 mL, 5.0 mmol) at ꢁ78ꢂC under an argon atmosphere. After
30 min, MPTF (1.83 g, 5.0 mmol) was added and the mixture was
stirred for 2 h at this temperature. The reaction mixture was
extracted with diethyl ether, dried with anhydrous MgSO4, filtered,
(400 MHz, CDCl3, ppm): d 2.19(s, 3H, –CH3), 6.98 (s, H, thiophene–H),
7.24 (t, 1H, J¼8.0 Hz, benzene–H), 7.32 (t, 2H, benzene–H), 7.40–
7.45 (m, 3H, benzene–H), 7.54 (t, 1H, J¼8.0 Hz, benzene–H), 7.62 (t,
1H, J¼8.0 Hz, benzene–H), 7.67, 7.69 (d, 1H, J¼8.0 Hz, benzene–H);

9470
S. Pu et al. / Tetrahedron 64 (2008) 9464–9470
14. Li, Z. X.; Liao, L. Y.; Sun, W.; Xu, C. H.; Zhang, C.; Fang, C. J.; Yan, C. H. J. Phys.
13C NMR (100 MHz, CDCl3):
d
¼14.70, 121.92, 123.85, 124.64, 125.55,
Chem. C 2008, 112, 5190–5196.
125.70, 127.60, 127.80, 128.95, 129.66, 129.98, 130.33, 131.03, 131.78,
133.35, 140.76, 141.60; IR (
, KBr, cmꢁ1): 752, 859, 989, 1067, 1126,
1266, 1316, 1445, 1581, 1654, 1985, 2859, 2923. Anal. Calcd for
23H13F9S: C, 56.10, H, 2.66. Found: C, 56.15, H, 2.73. MS m/z (Mþ)
493.1. Compound 4b: 1H NMR (400 MHz, CDCl3, ppm):
1.96 (s,
15. (a) Irie, M.; Sakemura, K.; Okinaka, M.; Uchida, K. J. Org. Chem. 1995, 60,
8305–8309; (b) Uchida, K.; Matsuoka, T.; Kobatake, S.; Yamaguchi, T.; Irie, M.
Tetrahedron 2001, 57, 4559–4565.
n
16. Pu, S. Z.; Zheng, C. H.; Le, Z. G.; Liu, G.; Fan, C. B. Tetrahedron 2008, 64,
2576–2585.
17. (a) Pu, S. Z.; Zhang, F. S.; Xu, J. K.; Shen, L.; Xiao, Q.; Chen, B. Mater. Lett. 2006, 60,
485–489; (b) Pu, S. Z.; Tang, H. H.; Chen, B.; Xu, J. K.; Huang, W. H. Mater. Lett.
2006, 60, 3553–3557; (c) Pu, S. Z.; Yang, T. S.; Yao, B. L.; Wang, Y. L.; Lei, M.; Xu,
J. K. Mater. Lett. 2007, 61, 855–859.
C
d
3H, –CH3), 5.66, 5.68 (d, 1H, J¼8.0 Hz, benzene–H), 5.95 (t, 1H,
J¼8.0 Hz, benzene–H), 6.21–6.23 (m, 2H, benzene–H), 6.96 (s,
1H, thiophene–H), 7.31 (t, 1H, J¼8.0 Hz, benzene–H), 7.40–7.45 (m,
2H, benzene–H), 7.57–7.63 (m, 2H, benzene–H).
18. (a) Fan, C. B.; Pu, S. Z.; Liu, G.; Yang, T. S. J. Photochem. Photobiol., A: Chem. 2008,
194, 333–343; (b) Fan, C. B.; Pu, S. Z.; Liu, G.; Yang, T. S. J. Photochem. Photobiol.,
A: Chem. 2008, 197, 415–425; (c) Pu, S. Z.; Yang, T. S.; Xu, J. K.; Shen, L.; Li, G. Z.;
Xiao, Q.; Chen, B. Tetrahedron 2005, 61, 6623–6629; (d) Tsujioka, T.; Kume, M.;
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Acknowledgements
This work was supported by the Project of National Natural
Science Foundation of China (20564001), the Key Scientific Project
from Education Ministry of China (208069), the Project of Jiangxi
Youth Scientist, and the Science Funds of the Education Office of
Jiangxi, China (GJJ08365).
20. Tian, H.; Wang, S. Chem. Commun. 2007, 781–792.
21. Higashiguchi, K.; Matsuda, K.; Yamada, T.; Kawai, T.; Irie, M. Chem. Lett. 2000,
1358–1359.
22. Crystal data for 1a:
C
23H16F6OS: M¼454.42, monoclinic (P21/n), a¼7.
6537(15) Å, b¼21.900(4) Å, c¼12.403(2) Å,
a
¼90ꢂ,
b
¼90.574(4)ꢂ,
g
¼90ꢂ, V¼
2079.0(7) Å3, Z¼4,
m
¼0.221 mmꢁ1, R1[I>2
s
(I)]¼0.0473, wR2[I>2 (I)]¼0.1084,
s
R1 (all data)¼0.0910, wR2 (all data)¼0.1336, GOF¼1.028. The CCDC number of
1a is 615079. Crystal data for 2a: C23H16F6S: M¼438.42, triclinic (P-1), a¼8.
References and notes
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a
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b
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g
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R1[I>2
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