Synthesis and photochromism of novel unsymmetrical diarylethenes with an azaindole unit

Article abstract of DOI:10.1016/j.tetlet.2014.03.004

A new class of unsymmetrical photochromic diarylethenes with an azaindole moiety has been firstly synthesized. Their properties, including photochromism, crystal structure, as well as fluorescence, were investigated systematically. The azaindole was conne

Full text of DOI:10.1016/j.tetlet.2014.03.004

Tetrahedron Letters 55 (2014) 2471–2475
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Tetrahedron Letters
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Synthesis and photochromism of novel unsymmetrical diarylethenes
with an azaindole unit
⇑
Zhiyuan Sun, Hui Li, Shouzhi Pu , Gang Liu, Bing Chen
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi 330013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of unsymmetrical photochromic diarylethenes with an azaindole moiety has been firstly
synthesized. Their properties, including photochromism, crystal structure, as well as fluorescence, were
investigated systematically. The azaindole was connected directly to the central cyclopentene ring as a
heteroaryl moiety and available to participate in the photoisomerization reaction. Each of the diaryleth-
enes exhibited favorable photochromism, good thermal stability, remarkable fatigue resistance, and
notable fluorescence switches in both solution and solid media. The substituents at the para-position
of the terminal benzene ring affected evidently their properties: the electron-donating methoxy could
be effective to enhance the cyclization quantum yield, while the electron-withdrawing cyano could shift
the absorption maximum to a longer wavelength in both hexane and solid film. The results revealed
that the introduction of azaindole moieties and different substituents played an important role in the
photoisomerization process of these diarylethenes.
Received 8 December 2013
Revised 25 February 2014
Accepted 2 March 2014
Available online 11 March 2014
Keywords:
Photochromism
Diarylethene
Azaindole moiety
Substituent effect
Ó 2014 Elsevier Ltd. All rights reserved.
Photochromic compounds have attracted much attention
because of their potential applications to photonic devices, such
as optical recording and photoswitches.1 To date, a large amount
of photochromic compounds have been explored, including spiro-
pyrans,2 azobenzenes,3 fulgides,4 and diarylethenes.5 Among all
these photochromic compounds, diarylethenes are the best
candidates due to their good thermally irreversible stability and
remarkable fatigue resistance.6
Nowadays, the design of new diarylethene skeletons with
different aryl moieties has become an active area of research.7
Among the diarylethenes hitherto reported, most of the heteroaryl
moieties have been thiophene or benzothiophene rings,8 with just
a few reports concerning other heteroaryl moieties, including isox-
azole,9 pyrrole,10 thiazole,11 pyrimidine,12 pyridine,13 naphtha-
lene,14 benzofuran,15 etc. Diarylethenes with indole moiety are
well known to exhibit strong fluorescence.16 However, the indole
moiety is rarely used in diarylethenes.17 The reason is that its
bromide can be easily oxidized when exposed to the air and its
high aromatic stabilization energy results in low thermal stability
of the closed-ring forms.17b Azaindole is an attractive aryl
unit because of its importance of polyaza tetracyclic compounds
as DNA intercalators and possible anticancer agents,18 wide
existence in nature,19 and its similar structure to that of indole.
F
F
F
F
F
F
F
F
F
F
F
F
UV
Vis
N
S
N
S
N
N
R
R
OCH3
H
1o: R =
2o: R =
3o: R =
OCH3
H
CN
1c: R =
2c: R =
3c: R =
CN
Scheme 1. Photochromism of diarylethenes 1–3.
Consequently, introduction of an azaindole ring into the photo-
chromic diarylethene system can be expected to undergo favorable
photochromism with good photochromic properties. However, to
the best of our knowledge, photochromic diarylethenes with an
azaindole moiety have not hitherto been reported.
In this Letter, we explored a new class of diarylethenes
with an azaindole moiety (1o–3o) and systematically investigated
their properties. The synthesized compounds are 1-(1,2-dimethyl-
7-azaindole-3-yl)-2-[2-methyl-5-(4-methoxyphenyl)-3-thienyl]
perfluorocyclopentene (1o), 1-(1,2-dimethyl-7-azaindole-3-yl)-
2-[2-methyl-5-(4-phenyl)-3-thienyl]perfluorocyclopentene (2o),
and 1-(1,2-dimethyl-7-azaindole-3-yl)-2-[2-methyl-5-(4-cyano-
phenyl)-3-thienyl]perfluorocyclopentene (3o). All of these
diarylethene derivatives exhibit good thermal irreversible photo-
chormism in solution, in PMMA amorphous films, and even in
⇑
Corresponding author. Tel./fax: +86 791 83831996.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
http://dx.doi.org/10.1016/j.tetlet.2014.03.004
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

2472
Z. Sun et al. / Tetrahedron Letters 55 (2014) 2471–2475
F
F
F
F
F
F
F
F
F
F
F
F
Br
n-BuLi/THF
+
F
N
195 K
N
S
N
N
S
R
R
4
5a: R =
5b: R =
OCH3
H
OCH3
1o: R =
2o: R =
H
5c: R = CN
3o: R = CN
Scheme 2. Synthetic route for diarylethenes 1o–3o.
the single crystalline phase. The photochromic scheme of
diarylethenes 1o–3o is illustrated in Scheme 1.
UV
Vis
The synthetic route for diarylethenes 1o–3o is shown in
Scheme 2. First, 1,2-dimethyl-3-bromine-7-azaindole (4) was
derived from 7-azaindole by the procedures of acylation, methyla-
tion, and bromination.20 Compounds 5a–c were synthesized by the
reported method.12 Compound 4 was lithiated and then separately
coupled with 5a–c to afford the target diarylethenes 1o–3o,
respectively. The structures of 1o–3o were confirmed by elemental
analysis, NMR, IR, and mass spectrum. The detailed experimental
procedures and data are summarized in the Supplementary
Information (SI).
0.45
0.30
0.15
0.00
Vis
UV
Diarylethenes 1o–3o showed excellent photochromism and
could be toggled between the colorless open-ring isomers
(1o–3o) and the colored closed-ring isomers (1c–3c) by alternating
irradiation with UV light and appropriate wavelength visible light
(Fig. S1). The absorption spectral changes of diarylethene 1 in
hexane are shown in Figure 1. The maximum absorption of 1o
300
400
500
600
700
Wavelength (nm)
Figure 1. Absorption spectral changes of 1 upon alternating irradiation with UV
and visible light in hexane (2.0 Â 10À5 mol LÀ1) at room temperature.
was observed at 296 nm (
e
, 2.63 Â 104 L molÀ1 cmÀ1) due to
p?
p⁄ transition.21 Upon irradiation with 297 nm light, a new
visible absorption band centered at 581 nm (
e
, 1.95 Â 104 L molÀ1
cmÀ1) emerged and the original peak decreased due to the forma-
tion of the closed-ring isomer 1c. This could be seen with the naked
eye, as the colorless solution of 1o turned blue (Fig. 2). Reversely,
the blue solution turned colorless upon irradiation with visible
light of appropriate wavelength (k >450 nm). Diarylethenes 2 and
3 exhibited similar photochromism as observed for 1 in hexane,
and their absorption maxima were observed at 579 nm for 2c
and 587 nm for 3c. Their color changes by photoirradiation are also
shown in Figure 2. In the photostationary state, the isosbestic
points were observed at 321 nm for 1, 355 nm for 2, and 360 nm
for 3, respectively. Moreover, the photoconversion ratios of 1–3
were measured by HPLC analysis in the photostationary state, with
the value of 74% for 1, 61% for 2, and 68% for 3 (Fig. S2). In PMMA
films, 1–3 also showed similar photochromism as observed in
hexane (Fig. S3). Upon irradiation with UV light, the colors of the
diarylethene/PMMA films 1–3 changed from colorless to blue
(Fig. 2), with the appearance of a new broad absorption band
centered at 598 nm, 592 nm, and 605 nm, respectively. Upon
irradiation with visible light, these colored diarylethene/PMMA
Figure 2. The color changes of diarylethenes 1–3 by photoirradiation in both
hexane (A) and PMMA amorphous films (B) at room temperature.
films reverted to colorless. Compared to those in hexane, the
absorption maxima of the closed-ring isomers 1c–3c in PMMA
films are found at longer wavelengths, with the redshift value of
17 nm for 1c, 13 nm for 2c, and 18 nm for 3c. The polar effect of
the polymer matrix and the stabilization of molecular arrangement
in solid state may be resulted in this redshift phenomenon.22
The absorption spectral features of 1–3 in both hexane and
PMMA films are summarized in Table 1. Among 1–3, the absorp-
tion maximum of the unsubstituted parent diarylthene 2 was at
Table 1
Absorption characteristics and photochromic reactivity of diarylethenes 1–3 in hexane (2.0 Â 10À5 mol LÀ1) and in PMMA films (10%, w/w)
Compd
ko,max/nma
kc,max/nmb
Uc
PRd (%)
(e
/L molÀ1 cmÀ1
)
(e
/L molÀ1 cmÀ1
)
Hexane
PMMA film
Hexane
PMMA film
Uo–c
Uc–o
1
2
3
296 (2.63 Â 104)
291 (2.42 Â 104)
316 (2.86 Â 104)
296
293
321
581 (1.66 Â 104)
579 (1.63 Â 104)
587 (1.53 Â 104)
598
592
605
0.45
0.25
0.39
0.13
0.28
0.15
74
61
68
a
b
c
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of open-ring (Uo–c) and closed-ring isomers (Uc–o), respectively.
d
Photoconversion ratios in the photostationary state.

Z. Sun et al. / Tetrahedron Letters 55 (2014) 2471–2475
2473
the shortest wavelength in both solution and a PMMA film. Replac-
ing the hydrogen atom at the para-position of the terminal ben-
zene ring with either an electron-donating substituent (methoxy,
such as in compound 1) or with an electron-withdrawing substit-
uents (cyano, such as in compound 3) resulted in a minor redshift
wavelength. Compared with the unsubstituted parent diarylethene
2, the molar absorption coefficients of diarylethene 1 with a meth-
oxy group increased notably for its open-ring and closed-ring
isomers. As shown in Table 1, the cyclization quantum yield
increased and the cycloreversion quantum yield decreased
significantly when the methoxy or cyano group was substituted
at the para-position of the terminal benzene moiety. As a result,
the unsubstituted parent 2 has the smallest cyclization quantum
yields (Uo–c, 2 = 0.25) and the largest cycloreversion quantum
yields (Uc–o, 2 = 0.28). This is in good agreement with that of
diarylethenes with a pyridine moiety.13 In addition, the absorption
maxima of the closed-ring isomers 1c–3c were observed at 579–
605 nm, which were much longer than those of analogous diary-
lethenes with a pyridine,13 benzothiophene,8a benzofuran,15b or
naphthalene moiety.14 This indicated that the azaindole moiety
could be effective to shift the absorption maximum of diarylethene
to a longer wavelength.
Single crystals of 1o–3o were obtained via slow evaporation of
their solutions and subjected to X-ray diffraction analysis. Their
ORTEP drawings and photochromic processes in the crystalline
phase are shown in Figure 3. The three azaindole-containing diary-
lethenes crystallized with photoactive anti-parallel conformations
in crystalline phase, and the distances between the two reactive
carbons were shorter than 4.2 Å (3.755 Å for 1o (C7. . .C13),
4.059 Å for 2o (C8. . .C16), and 3.682 Å for 3o (C11. . .C19)) (Tables
S1 and S2). As expected, the crystals of 1o–3o exhibited a notable
photochromism upon irradiation with UV and visible light.23 In
fact, the three crystals showed favorable photochromism in
accordance with the expected analysis in the crystalline phase.
The colorless crystals of 1o–3o turned blue quickly upon irradia-
tion with 365 nm light. When the colored crystals were dissolved
in hexane, intense absorption maxima were observed at the same
wavelength as those of their respective closed-ring isomers in
solution. In contrast, exposure to visible light entirely bleached
the colors of 1c–3c. The result is consistent with that of the
pyridine-containing diarylethenes,13 but is contrary to that of
benzene-containing diarylethenes.24
The thermal stability and fatigue resistance of diarylethene are
crucial factors for practical applications in optical devices,25 and
they are mainly dependent on the aromatic stabilization energy
of the aryl group.10b Thus, we examined the thermal stability of
the closed-ring isomers 1–3 in ethanol at room temperature and
351 K. Storing these solutions in the dark and then exposing them
to air for more than 30 days, we did not observe decoloration of
these diarylethenes. At 351 K, diarylethenes 1–3 still showed
excellent thermal stability for more than 15 days. The result
suggested that these diarylethene derivatives had very excellent
thermally irreversible photochromic behaviors. The fatigue resis-
tances of 1–3 were measured in both hexane and PMMA films by
alternative irradiation with UV and visible light in air at room tem-
perature, and the result is shown in Figure 4. In hexane, the color-
ation and decoloration cycles of 1–3 could be repeated 100 cycles
with only 7% degradation of 1c, 10% degradation of 2c, and 12%
degradation of 3c. The degradation may be ascribed to the forma-
tion of epoxide.26 In PMMA films, they could be repeated 200
cycles with only 11% degradation of 1c, 5% degradation of 2c, and
9% degradation of 3c. This improvement may be resulted from
suppression of O2 diffusion in solid medium.17c Compared to
diarylethenes with an indole moiety,17d the fatigue resistance of
diarylethenes with an azaindole moiety was enhanced remarkably
in both solution and PMMA films, indicating that the azaindole
moiety could effectively enhance the fatigue resistance of
diarylethenes.
The fluorescence modulation is widely applied to molecular-
scale optoelectronics, ion-sensors, and digital photoswitches.27
The fluorescence spectra of 1o–3o were measured in both hexane
(2.0 Â 10À5 mol LÀ1) and PMMA films (10%, w/w) at room temper-
ature. As shown in Figure 5, the emission peaks of 1o–3o in hexane
were observed at 437 nm for 1o (kex, 289 nm), 428 nm for 2o
(kex, 290 nm), 431 nm for 3o (kex, 328 nm), and were observed at
452 nm for 1o (kex, 342 nm), 445 nm for 2o (kex, 340 nm), and
443 nm for 3o (kex, 351 nm) in PMMA films. The result revealed
that the emission peaks in PMMA films were much longer than
those in hexane, and the redshift values were 15 nm for 1, 17 nm
for 2, and 12 nm for 3. Compared to the unsubstituted parent 2,
both the electron-donating methoxy group and the electron-
withdrawing cyano group decreased remarkably the emission
intensity in both solution and PMMA films. The results indicated
that different substituents attached at the terminal of benzene ring
Figure 3. ORTEP drawings of crystals and color changes of 1o, 2o, and 3o: (A) 1o, (B) 2o, (C) 3o, (D) photographs demonstrating their photochromic processes in the
crystalline phase.

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Z. Sun et al. / Tetrahedron Letters 55 (2014) 2471–2475
1.0
1.0
0.8
0.6
0.4
0.2
0.0
0.8
1
2
3
1
2
3
0.6
0.4
0.2
0.0
0
50
100
150
200
0
20
40
60
80
100
Repeat cycles
Repeat cycles
(A)
(B)
Figure 4. Fatigue resistance of diarylethenes 1–3 in hexane (A) and in PMMA films (B) in air atmosphere at room temperature. Initial absorptance of the sample was fixed to
1.0.
4000
3000
2000
1000
0
3o
3o
4000
3000
2000
1000
0
1o
2o
3o
1o
2o
3o
2o
2o
1o
1o
400
450
500
550
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
(A)
(B)
Figure 5. Emission spectra of diarylethenes 1–3 at room temperature: (A) in hexane (2.0 Â 10À5 mol LÀ1), (B) in PMMA films (10%, w/w).
had a significant effect not only on the emission peak but also on
the emission intensity.
hexane and a PMMA film (10%, w/w) at room temperature. Upon
irradiation with 297 nm UV light, the fluorescence modulation
efficiency of 1 was 76% in hexane and 88% in a PMMA film when
arrived at the photostationary state. Similarly, the fluorescence
modulation efficiency of the other two diarylethenes was 72% for
2 and 70% for 3, and was 94% for 2 and 96% for 3 in PMMA films
As has been observed for most of the reported diarylethenes,28
1–3 exhibited notable fluorescent switches by photoirradiation in
both hexane and PMMA films (Figs. S5 and S6). Figure 6 shows
the fluorescence spectral changes of 1 by photoirradiation in
1000
800
UV
Vis
UV
Vis
800
600
400
200
0
600
400
200
0
400
450
500
550
600
400
450
500
550
Wavelength (nm)
Wavelength (nm)
(A)
(B)
Figure 6. Emission intensity changes of 1 by photoirradiation at room temperature: (A) in hexane (2.0 Â 10À5 mol LÀ1), excited at 289 nm, (B) in PMMA films (10%, w/w),
excited at 342 nm.

Z. Sun et al. / Tetrahedron Letters 55 (2014) 2471–2475
2. Han, S. L.; Chen, Y. Dyes Pigm. 2011, 88, 235–239.
2475
in the photostationary state. The result indicated that the fluores-
cence modulation efficiency of 1–3 in PMMA films was much larger
than that in hexane. The result is in good agreement with that of
diarylethenes with an isoxazole, naphthalene, or pyridine moi-
ety.9a,13,14b Compared to the pyridine/naphthalene-containing
diarylethenes with the similar molecular skeleton,13,14b the
fluorescent modulation efficiency of 1–3 enhanced notably during
the process of photoisomerization. Therefore, these diarylethenes
can be potentially used for optical memories with fluorescence
readout method or fluorescent photoswitches.29
In summary, three new unsymmetrical diarylethenes based on
the hybrid of azaindole and thiophene moieties were synthesized
and their structures were determined by single-crystal X-ray
diffraction analysis. All these diarylethenes exhibited favorable
photochromism and functioned as notable fluorescence photos-
witches in solution, PMMA films, and even in the crystalline phase.
The behaviors of the three azaindole-containing diarylethenes
were significantly different from each other, which might be attrib-
uted to the effects of different substituents in the terminal phenyl
group. The results of this study may be useful for a new strategy in
developing photoactive diarylethenes based on azaindole and
other aryl units.
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Acknowledgments
This work was supported by the National Natural Science
Foundation of China (51373072, 21362013), the Project of Jiangxi
Advantage Sci-Tech Innovative Team (20113BCB24023), the
Science Funds of Natural Science Foundation of Jiangxi Province
(20122BAB213004, 20114BAB203007, 20132BAB203005), and
the Project of the Science Funds of Jiangxi Education Office
(KJLD12035, GJJ12587).
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.03.
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