Photochromism of new unsymmetrical diarylethenes with an indazole moiety

Article abstract of DOI:10.1016/j.tetlet.2015.07.052

A new class of photochromic diarylethenes with an indazole moiety has been firstly synthesized, and their photochromic and fluorescence properties have been investigated. The indazole moiety was connected directly to the central cyclopentene ring as one h

Full text of DOI:10.1016/j.tetlet.2015.07.052

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Please cite this article as: Liu, J., Liu, H., Pu, S., Photochromism of new unsymmetrical diarylethenes with an
indazole moiety, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.07.052
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Photochromism of new unsymmetrical
diarylethenes with an indazole moiety
Jingjing Liu, Hongliang Liu, Shouzhi Pu
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang
330013, PR China

Tetrahedron Letters
1
TETRAHEDRON
LETTERS
Pergamon
Photochromism of new unsymmetrical diarylethenes with an indazole moiety
Jingjing Liu, Hongliang Liu, Shouzhi Pu
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, PR China
A new class of photochromic diarylethenes with an indazole moiety has been synthesized firstly, and their photochromic and
fluorescence properties have been investigated. The indazole moiety was connected directly to the central cyclopentene ring
as one heteroaryl unit and availably participated in the photoinduced cyclization reaction in solution, amorphorous film, as
well as in the crystalline phase. These diarylethenes exhibited excellent photochromism with good thermal stability and
remarkable fatigue resistance. They also functioned as a notable fluorescence switch in both solution and amorphorous films.
In addition, the different substituents at the para-position of the terminal benzene ring had a significant effect on their
properties: the electron-donating methoxy group could enhance the quantum yields of cyclization and cycloreversion and
fluorescence quantum yield, but the electron-withdrawing trifluoromethyl had opposite effect.1
© 2015 Elsevier Science. All rights reserved
———
Keyword: Photochromism; Diarylethenes; Indazole moiety; Substituent effect; Single crystal.
Corresponding author. Tel.& Fax: +86-791-83831996; e-mail: pushouzhi@tsinghua.org.cn (S. Pu)

2
Tetrahedron Letters
Research on photochromic compounds has received much The detailed experimental procedures and data are
attention because of their potential application to summarized in the Supplementary Information (SI).
optoelectronics devices.1 Among the wide variety of
H
photochromic compounds (azobenzenes,2 spiropyrans,3
N
N
NaH, CH3
DMF
I
N
F
F
N
fulgides,4 etc.), diarylethenes are one of most promising
thermally irreversible photochromic molecules developed by
Irie et al.5 and Lehn et al.6 These compounds are
characterized by efficient photoisomerization with excellent
thermal stability, remarkable fatigue resistance, high
sensitivity, fast response, and non-destructive readout
capability.7
F
F
Br
Br
F
F
1o: R =
2o: R =
OCH3
H
4
F
5
n-BuLi
195 K
F
3o: R =
CF3
F
F
THF,
S
6a: R =
6b: R =
6c: R =
F
F
OCH3
H
CF3
N
R
N
F
S
R
Scheme 2. Synthetic route to diarylethenes 1o–3o.
Design and synthesis of new diarylethenes with different
heteroaryl moieties have become a hot topic in the research
field because the heteroaryl moieties can effectively
modulate their photochromic characteristics.8 For example,
diarylethenes with thiophene or benzothiophene moieties
exhibit excellent thermal stability and the ones with indole
units show effective fluorescent photoswitches.9 However,
diarylethenes with pyrrole rings are thermally unstable and
can return to the open-ring isomers even in the dark.10 In
addition, functional substituents also have important effects
on the photochromic properties of diarylethenes, such as the
absorption maxima, absorption molar coefficients, and the
quantum yields of cyclization and cycloreversion. 11 To date,
the hexatriene backbones of the reported diarylethene
systems are mainly confined to the five-membered
heteroaryl rings,12 with just a few reports concerning other
six-membered heteroaryl moieties, such as benzene,13
naphthalene,14,11b pyridine,15 and pyrimidine.16 Indazole is an
attractive heteroaryl unit due to its low aromatic stabilization
energy and a broad range of biological activities.17 However,
photochromic diarylethene derivatives with a six-membered
heteroaryl indazole moiety have not hitherto been reported.
0.6
Vis
UV
0.4
UV Vis
UV Vis
0.2
0.0
270
405
540
675
Wavelength (nm)
Fig. 1. Absorption spectral changes of 1 upon alternating irradiation with
UV and visible light in hexane (2.0 × 10-5 mol L-1).
Fig. 2. The color changes of 1–3 by photoirradiation in different media at
room temperature: (A) in hexane, (B) in PMMA films.
The photochromic properties of 1–3 induced by
photoirradiation were investigated in both hexane (2.0 × 10-5
mol L-1) and PMMA films (10%, w/w). The absorption
spectral changes of diarylethene 1 in hexane are shown in
Fig. 1, and the color changes of 1–3 by photoirradiation are
shown in Fig. 2. In hexane, 1o showed a sharp absorption
peak at 295 nm (ε, 4.89 × 104 L mol-1 cm-1) due to π→π*
transition.19 Upon irradiation with 297 nm light, a new
visible absorption band centered at 566 nm (ε, 8.95 × 103 L
mol-1 cm-1) was observed with a notable color change from
colorless to violet due to the formation of the closed-ring
isomer 1c. Reversely, the colored solution turned colorless
upon irradiation with visible light of appropriate wavelength
(λ > 500 nm). Diarylethenes 2 and 3 exhibited similar
photochromism as observed for 1 in hexane, and their
absorption maxima were observed at 560 nm for 2c and 558
nm for 3c (Fig. S1). In the photostationary state, the
isosbestic points were observed at 312 nm for 1, 301 nm for
2, and 320 nm for 3. Moreover, the photoconversion ratios
of 1–3 were measured by HPLC analysis in the
photostationary state, with the value of 53% for 1, 58% for 2,
and 39% for 3 (Fig. S2). In PMMA films, 1–3 also showed
similar photochromism as observed in hexane, and their
In this paper, we designed and synthesized a new class of
diarylethenes with both indazole and thiophene moieties
(1o–3o). The indazole moiety is connected directly to the
central ethene unit as a six-membered heteroaryl moiety and
participates in the photochromic hexatriene–cyclohexadiene
reaction (Scheme 1). All of these diarylethenes exhibited
thermally irreversible photochormism in solution, PMMA
films, and even in the single crystalline phase. The
photochromism of diarylethenes 1o–3o is shown in Scheme
1.
F
F
F
F
F
F
F
F
F
F
F
F
UV
Vis
S
S
N
N
R
R
N
N
1o: R =
2o: R =
3o: R = CF3
1c: R =
2c: R =
3c: R =
OCH3
H
OCH3
H
CF3
Scheme 1. Photochromism of diarylethenes 1o–3o.
The synthetic route for diarylethenes 1o–3o is shown in
Scheme 2. N-methylation of 5-bromo-4-methyl-1H-indazole
and coupling with iodomethane gave compound 5.18 spectral changes are shown in Fig. S3. Upon irradiation with
Compound 5 was lithiated and then separately coupled with
6a–c16 to give diarylethenes 1o–3o. The structures of 1o–3o
were confirmed by elemental analysis, NMR, IR, and MS.
UV light, the colors of the diarylethene/PMMA films 1–3
changed from colorless to violet, with the appearance of a
new broad absorption band centered at 579 nm for 1c, 568

Tetrahedron Letters
3
nm for 2c, and 576 nm for 3c. Upon irradiation with with a benzene13 or pyrimidine moiety.16 The results
appropriate visible light, these colored films reverted to indicated that the indazole moiety could be effective to shift
colorless due to the reproduction of 1o–3o. Compared to the absorption maximum of diarylethene to a longer
those in hexane, the absorption maxima of the closed-ring wavelength.
isomers 1c–3c in PMMA films exhibited a notable
bathochromic shift, with a redshift value of 13 nm for 1c, 8
nm for 2c, and 18 nm for 3c. The polar effect of the polymer
matrix and the stabilization of molecular in solid state may
be resulted in this redshift phenomenon.20
Table 1. Absorption spectral properties of diarylethenes 1–3 in hexane (2.0
× 10-5 mol L-1) and PMMA films (10%, w/w) at room temperature.
λo,max/nma
λc,max/nmb
Φc
(ε/L mol-1 cm-1)
(ε/L mol-1 cm-1)
PRd
(%)
Compd
PMMA
film
PMMA
film
hexane
295
hexane
566
Φo-c
Φc-o
298
579
0.34
0.16
53
58
39
1
2
3
(4.89 ×
104)
(8.95 ×
103)
Fig. 3. ORTEP drawings of crystals and color changes of 1o–3o: (A) 1o, (B)
2o, (C) 3o, (D) photographs demonstrating their photochromic processes in
the crystalline phase.
295
560
299
301
568
576
0.27
0.24
0.10
0.07
(3.76 ×
104)
(7.33 ×
103)
To know better the relation between the conformation and
the photochromic behaviors of these diarylethenes in the
crystalline phase, the structural conformations of the crystals
of 1o–3o were provided by X-ray crystallographic analysis.
Their ORTEP drawings and color changes induced by
photoirradiation are shown in Fig. 3. Their packing views
along the x direction are shown in Fig. 4S, and the X-ray
crystallographic analysis data are listed in Tables S1 and S2.
The three indazole-containing diarylethenes crystallized with
photoactive anti-parallel conformations in the crystalline
phase, and the distances between the two reactive carbon
atoms are 3.793 Å for 1o (C11…C19), 3.787 Å for 2o
(C8…C16), and 4.070 Å for 3o (C11…C23). Therefore,
they could be expected to undergo photochromism in the
single crystalline phase.22 As expected, the crystals of 1o–
3o display favourable photochromism by photoirradiation in
the single crystalline phase. Upon irradiation with 297 nm
light, the colorless crystal of 1o–3o turned violet, and the
colored crystals reverted to colorless upon irradiation with
visible light (λ > 500 nm). The result is consistent with that
of the pyridine-containing diarylethenes,15 but it is contrary
to that of naphthalene-containing diarylethenes.14
301
558
(6.59 ×
104)
(9.04 ×
103)
a The absorption maxima of the open-ring isomers.
b The absorption maxima of the closed-ring isomers.
c Quantum yields of cyclization (Φo-c) and cycloreversion (Φc-o).
d Photoconversion ratios in the photostationary state.
The absorption spectral parameters of 1–3 in both hexane
and PMMA films are listed in Table 1. These data showed
that different substituents at para-position of the terminal
benzene ring had significant effects on the photochromic
features of 1–3, including the absorption maxima, molar
absorption coefficients, and quantum yields of cyclization
and cycloreversion. Among the three diarylethenes, the
molar absorption coefficients of the two isomers of the
unsubstituted parent compound 2 are the smallest. Replacing
the hydrogen atom at the para-position of the terminal
benzene ring with either an electron-donating methoxy
group (compound 1) or with an electron-withdrawing
trifluoromethyl group (compound 3) resulted in bigger molar
absorption coefficients. The result was consistent with that
of diarylethenes with a pyridine,15 pyrimidine,16 thiazole,12c
or isoxazole moiety.21 As shown in Table 1, the quantum
yields of unsubstituted parent diarylethene 2 were 0.27 for
the cyclization reaction and 0.10 for the cycloreversion
reaction. When the electron-donating methoxy group was
substituted at the para-position of the terminal benzene ring,
the quantum yields of cyclization and cycloreversion
(compound 1) increased significantly. However, a reverse
trend was observed when the electron-withdrawing
trifluoromethyl group was introduced at the same position
(compound 3). The result was in good agreement with that
of the reported thiazole-containing diarylethenes.12c In
addition, the absorption maxima of the closed-ring isomers
1c–3c were observed at the range of 558–579 nm, which
were much longer than those of analogous diarylethenes
The thermal stability and fatigue resistance of photochromic
compounds are crucial factors for practical applications in
optical devices.23 The thermal stability of the open-ring and
closed-ring isomers of 1–3 was evaluated in hexane by
UVvis spectroscopy both at room temperature and at 343 K.
Storing these solutions in hexane at room temperature in the
dark and then exposing them to air for more than 10 days,
we found no changes in the UVvis spectra of diarylethenes
1–3. At 343 K, diarylethenes 1–3 still showed excellent
thermal stability for more than 2 hr. The results suggested
that the indazole-containing diarylethenes had good thermal
stability at both room temperature and higher temperature.
The fatigue resistance of 1–3 was tested in both hexane and
PMMA films by alternative irradiation with UV and visible

4
Tetrahedron Letters
light in air at room temperature, as shown in Fig. 4. In
hexane, the coloration and decoloration cycles of 1–3 were
repeated 10 times with the degradation of 24% for 1c, 18%
for 2c, and 25% for 3c, respectively. The relatively large
degradation may result from the formation of epoxides.12b In
PMMA films, the fatigue resistance of 1–3 enhanced notably,
as compared to that in solution. After 100 cycles, the
degradation percentages of 1–3 in PMMA films were
determined to be 5% for 1c, 4% for 2c, and 6% for 3c. This
remarkable improvement may result from suppression of O2
diffusion in the solid medium.24 Compared to diarylethenes
with a pyrimidine moiety,16 the fatigue resistance of 1–3 was
much better in both solution and PMMA films. But their
fatigue resistance was much weaker that that of the
diarylethenes with a naphthalene moiety.14
1o
1o
4000
3000
2000
1000
0
5200
3900
2600
1300
0
3o
3o
2o
2o
400
450
500
550
424
477
530
583
Wavelength (nm)
Wavelength (nm)
(A)
(B)
Fig. 5. Emission spectra of diarylethenes 1–3 in hexane and PMMA films at
room temperature: (A) in hexane (2.0 × 10−5 mol L-1), (B) in PMMA films
(10%, w/w).
As has been observed for most of the reported
diarylethenes,26 1–3 show notable fluorescence switches
upon photoirradiation in both hexane and PMMA films. Fig.
6 shows the fluorescence spectral changes of 1 by alternating
irradiation with UV and visible light in hexane and a PMMA
film. Upon irradiation with 297 nm UV light, its emission
intensity was quenched to ca. 34% in hexane and 37% in a
PMMA film when reached the photostationary state. Back
irradiation with visible light restored the original emission
spectrum due to the formation of the open-ring isomer 1o.
Similarly, the emission intensity of 2 and 3 was quenched ca.
26% and 21% in hexane, and 41% for 2 and 44% for 3 in
PMMA films, respectively (Fig. S5 and S6). The result
exhibited that the fluorescent modulation efficiency of 1–3
in hexane was much larger than that in PMMA films. The
result is in agreement with that of diarylethenes with an
pyrimidine16 or pyrrole moiety,27 but contrary to that of
diarylethenes with a benzene moiety.13 Therefore, the
diarylethenes could be potentially used as photoswitchable
devices, such as optical memory and photo-controllable
switches.25a,26a,28
1.0
1.00
0.9
0.95
0.8
0.90
0.85
0.80
1o
2o
3o
1o
2o
3o
0.7
0.6
0.5
0
2
4
6
8
10
0
10 20 30 40 50 60 70 80 90 100
Reapeat cycles
Reapeat cycles
(A)
(B)
Fig. 4. Fatigue resistance of 1–3 in hexane and in PMMA films in air
atmosphere at room temperature: (A) in hexane, (B) in PMMA films. Initial
absorptance of the sample was fixed to 1.0.
The fluorescence switching properties based on the
photochromism of diarylethenes have attracted much
attention because of their potential applications in
molecularscale optoelectronics, ion-sensors, and digital
photoswitches.25 The fluorescence spectra of diarylethenes
1o–3o both in hexane (2.0 × 10-5 mol L-1) and in PMMA
films (10%, w/w) were measured at room temperature, as
shown in Fig. 5. In hexane, the emission peaks were
observed at 463 nm for 1o, 429 nm for 2o, and 372 nm for
3o when excited at 300 nm, and were observed at 469 nm
(λex, 319 nm) for 1o, 442 nm (λex, 300 nm) for 2o, and 418
nm (λex, 291 nm) for 3o in PMMA films. Compared to those
in hexane, the emission peaks of 1o–3o showed a notable
bathochromic shift in PMMA films, with a redshift value of
15 nm for 1, 17 nm for 2, and 12 nm for 3. Compared to the
unsubstituted parent compound 2o, either the electron-
donating methoxy group of 1o or the trifluoromethyl group
of 3o shifted the emission peak to a shorter wavelength and
greatly increased the emission intensity. The fluorescence
quantum yields of diarylethenes 1o–3o were measured by
Absolute PL quantum yield spectrometer C11347, with the
values of 0.014 for 1o, 0.010 for 2o, and 0.011 for 3o, which
were larger than those of the isoxazole-containing
diarylethenes.21 In addition, the electron-donating methoxy
group could enhance the fluorescence quantum yield of the
diarylethenes with an indazole moiety to a certain extent.
The result is agreed with those of diarylethenes with a
pyrimidine moiety,16 but different from those of
diarylethenes with a naphthalene or pyridine moiety, whose
fluorescence quantum yields enhanced with the increase of
electron-withdrawing ability of the substituents.14,15
UV Vis
UV Vis
5000
4000
3000
2000
1000
0
3400
2550
1700
850
0
400
440
480
520
400
450
500
550
Wavelength (nm)
Wavelength (nm)
(A)
(B)
Fig. 6. Fluorescence spectral changes of 1 by photoirradiation at room
temperature: (A) in hexane (2.0 × 10-5 mol L-1), excited at 300 nm, (B) in
PMMA films (10%, w/w), excited at 319 nm.
In summary, three new unsymmetrical diarylethenes with an
indazole moietiy were synthesized, and their structures were
determined by single-crystal X-ray diffraction analysis. The
diarylethenes exhibited favorable photochromism with good
thermal stability and functioned as a notable fluorescence
photoswitch in solution and PMMA films. The behaviors of
the three indazole-containing diarylethenes were different
from each other due to the substitution effects in the terminal
benzene ring. The indazole moiety induced new
photochromic properties differing from other diarylethenes
with a six-membered aryl moiety reported so far. Our
experimental results may be useful for design and synthesis

Tetrahedron Letters
5
2013, 54, 646–650.
of new photochromic diarylethenes based on different six-
membered aryl moieties.
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Acknowledgments
This work was supported by the National Natural Science
Foundation of China (51373072, 21162011), the Project of
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Jiangxi
Advantage
Sci-Tech
Innovative
Team
(20142BCB24012), and the Science Funds of Natural
Science Foundation of Jiangxi Province (20132BAB203005).
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