The effects of heteroaryl ring on the photochromism of diarylethenes with a naphthalene moiety

Article abstract of DOI:10.1016/j.tetlet.2013.07.094

Three new asymmetrical naphthalene-containing diarylethenes with different heteroaryl groups have been synthesized to investigate the heteroaryl effects on their properties. The three diarylethenes exhibited distinctive photochromism with good thermal stability, which may be attributed to the different heteroaryl effects. Their cycloreversion quantum yields increased in the order of 2-methylbenzofuran < 2-methylbenzothiophene < 1,2-dimethylindole, while the cyclization quantum yields exhibited a reverse trend. Compared to indole and benzothiophene, the benzofuran moiety could effectively shift the absorption maximum to a shorter wavelength and notably enhance the cyclization quantum yield and fluorescence quantum yield of the diarylethene. The results indicated that the category of heteroaryl groups played a vital role during the process of photoisomerization of naphthalene-containing diarylethene derivatives.

Full text of DOI:10.1016/j.tetlet.2013.07.094

Tetrahedron Letters 54 (2013) 5307–5310
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
The effects of heteroaryl ring on the photochromism of diarylethenes
with a naphthalene moiety
⇑
Renjie Wang, Shouzhi Pu , Gang Liu, Shiqiang Cui, Hui Li
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:
Three new asymmetrical naphthalene-containing diarylethenes with different heteroaryl groups have
been synthesized to investigate the heteroaryl effects on their properties. The three diarylethenes exhib-
ited distinctive photochromism with good thermal stability, which may be attributed to the different het-
eroaryl effects. Their cycloreversion quantum yields increased in the order of 2-methylbenzofuran < 2-
methylbenzothiophene < 1,2-dimethylindole, while the cyclization quantum yields exhibited a reverse
trend. Compared to indole and benzothiophene, the benzofuran moiety could effectively shift the absorp-
tion maximum to a shorter wavelength and notably enhance the cyclization quantum yield and fluores-
cence quantum yield of the diarylethene. The results indicated that the category of heteroaryl groups
played a vital role during the process of photoisomerization of naphthalene-containing diarylethene
derivatives.
Received 13 May 2013
Revised 11 July 2013
Accepted 18 July 2013
Available online 26 July 2013
Keywords:
Photochromism
Diarylethene
Naphthalene moiety
Heteroaryl effect
Optical property
Ó 2013 Elsevier Ltd. All rights reserved.
Photochromism is the term used for a reversible photo-induced
transformation of a molecule between two isomers whose absorp-
tion spectra are of distinguishable difference.1 During the revers-
ible photoisomerization, the physical properties of photochromic
compounds, such as absorption spectra, fluorescence, conjugation,
and refractive index can be tuned by light. These reversible
changes can be potentially used for optical data storage,
photoswitching devices, and chemical sensors.2 Among the
reported photochromic systems, diarylethenes are the most
promising candidates for optoelectronic devices due to their good
thermal stability, excellent fatigue resistance, and high reactivity.3
So far, reports on the synthesis and properties of symmetrical
photochromic diarylethenes have been extensively reported.4 In
general, the photochromic reactivity of diarylethenes can be mod-
ulated by introducing different substituents or changing hetero-
aryl moieties on the hexatriene moiety.5 Zhou et al., reported
that diarylethenes with different substituents on the 5-position
of thiophene ring could influence evidently the electrochemical
open-ring reaction.6 Morimitsu et al., and Takami et al., found that
the bulky alkoxy substituents on the 2- and 20-positions of the thi-
ophene ring could strongly suppress cycloreversion quantum
yields.7 In previous works, we reported that many hybrid diary-
lethenes with different substituents on the terminal phenyl exhib-
ited distinguishable properties due to the substituent effects.8
Apart from the fact that the substituent effects can affect the pho-
tochromic property, the heteroaryl groups have more important
role to play in modifying the photoreactivity of diarylethenes,
and even completely changing their photochromism. For example,
diarylethenes with five-membered heteroaryl rings showed
remarkable thermal stability and excellent fatigue resistance,
whereas diarylethenes with a six-membered heteroaryl ring were
thermally unstable.1,2a,9 The correlation between the aromatic sta-
bilization energy (ASE) of heteroaryl groups and thermal stability
of diarylethenes by Irie exerts a great impact on the preference
for low ASE aryl units such as thiazolyl and thienyl groups in the
design of symmetrical diarylethenes.2a In a recent Letter, we fur-
ther proved the validity of this principle in more complicated
asymmetrical systems.10
Benzofuran and benzothiophene are fascinating aryls to design
and synthesis diarylethenes because of their low aromatic stabil-
ization energies.4a,11 As another intriguing aryl, indole can notably
enhance the fluorescence modulation efficiency of diarylethenes.12
The three heteroaryl rings have similar chemical structures with
different electron cloud distribution so that they are representative
candidates for investigating the heteroaryl effects on the photo-
chromic properties of diarylethenes. In this Letter, we synthesized
three new asymmetrical naphthalene-containing diarylethenes
and discussed the heteroaryl effects on their physicochemical
properties. Diarylethenes 1o–3o showed favorable photochromism
in both solution and poly(methyl methacrylate) (PMMA) films. The
photochromic scheme of 1–3 is shown in Scheme 1.
The synthesis route for diarylethenes 1o–3o is shown in
Scheme 2. First, NBS was added into the THF solutions of
compound 4a–c at 273 K to produce brominated heteroaryl
compounds 5a–c. Compounds 5a–c were lithiated and then
⇑
Corresponding author. Tel./fax: +86 791 3831996.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.07.094

5308
R. Wang et al. / Tetrahedron Letters 54 (2013) 5307–5310
3 also showed favorable photochromism in hexane. Upon irradi-
ation with 297 nm light, the colorless solutions of 2o and 3o
turned orange and yellow. The absorption maxima were observed
at 494 and 455 nm, respectively (Fig. S1, SI). The colored solutions
of 2c and 3c could be decolorized upon irradiation with visible
light (k >500 nm). In the photostationary state (PSS), the photo-
conversion ratios from the open-ring to the closed-ring isomers
of 1–3 were evaluated by HPLC analysis (Fig. S2, SI), with the val-
ues of 32% for 1, 36% for 2, and 77% for 3. In PMMA films, 1–3
showed similar photochromism. But the absorption maxima of
the closed-ring isomers 1c–3c in PMMA films are much longer
than those in hexane. The red shift values are 13 nm for 1c,
12 nm for 2c, and 29 nm for 3c. The phenomena may be ascribed
to more planar structures and stabilization of molecular arrange-
ment of 1–3 in solid state.15
The photochromic features of 1–3 in both hexane and PMMA
films are summarized in Table 1. The results showed that the het-
eroaryl moiety significantly affected their photochromic proper-
ties, such as the absorption maxima, molar absorption
coefficients, and cyclization/cycloreversion quantum yields.
Among 1–3, the absorption maxima of the closed-ring isomers
remarkably increased in the order of benzofuryl < benzothie-
nyl < indolyl. Among the three analogs, the diarylethene with an
indole moiety (1) has the biggest visible absorption peak, molar
absorption coefficient, and cycloreversion quantum yield, but the
lowest cyclization quantum yield. Replacing the indole moiety
with a benzothiophene moiety (2), the absorption maximum and
the cycloreversion quantum yield decrease evidently, but the cycli-
zation quantum yield showed a minor increase. When the same
position has a benzofuran moiety (3), the molar absorption coeffi-
cient was still significantly lower than that of diarylethene 1. Its
absorption maximum and the cycloreversion quantum yield fur-
ther decreased whereas the cyclization quantum yield was notably
enhanced. Compared with the indole moiety (1), the absorption
maximum of 3 showed a notably hypsochromic shift with 80 nm.
The results indicated that the benzofuran ring could effectively
shift the absorption maximum to a short wavelength. In addition,
the molar absorption coefficients of both the open-ring and
closed-ring isomers varied with the same trend. They decreased
in the order of 1 > 3 > 2. Compared to those of 1, the cyclization
quantum yield of 3 was two fold larger but its cycloreversion quan-
tum yield was only one third of 1. As mentioned above, the photo-
chromic features of 1–3 were distinctly different. Compared with
the substituent effects, the heteroaryl group played a vital role dur-
ing the process of photoisomerization for the diarylethenes with a
similar photochromic skeleton. The possible reason may be that
the heteroaryl groups are directly connected to the central cyclo-
pentene ring, and can be available to participate in the photoin-
duced cyclization reaction. Additionally, in comparison with
symmetrical diarylethenes,4a,11f,12,16 the naphthalene-containing
diarylethenes with different heteroaryl moiety could significantly
decrease the absorption maximum of the closed-ring isomers and
the cycloreversion quantum yield. The results could be a useful
strategy for developing photochromic diarylethenes with shorter
absorption wavelength.
Scheme 1. Photochromism and color changes of diarylethenes 1–3 in hexane
(C = 5.0 ꢀ 10ꢁ5 mol Lꢁ1).
Br
NBS,THF
273 K
R
F
F
R
4a: R = N
4b: R = S
4c: R = O
F
F
F
F
5a: R = N
5b: R = S
5c: R = O
n-BuLi/THF
195 K
F
F
F
F
F
F
Br
R
1o: R = N
2o: R = S
3o: R = O
C5F8,195 K
n-BuLi/THF
F
6
7
Scheme 2. Synthetic route for diarylethenes 1o–3o.
coupled with the compound 78d to give diarylethenes 1o–3o,
respectively. Their structures were confirmed by elemental analy-
sis, NMR, and IR (Supplementary Information, SI).13
Diarylethenes 1–3 exhibited typical absorption changes upon
alternating irradiation with UV and visible light in both hexane
and polymer matrix at room temperature. As shown in Figure 1,
1o exhibited a sharp absorption peak at 281 nm (
e
, 1.22 ꢀ 104 -
L molꢁ1 cmꢁ1), which was arisen from p⁄ transition.14 Upon
p
?
irradiation with 297 nm UV light, the colorless solution of 1o
gradually turned purple (Scheme 1), and a new absorption max-
imum was observed at 535 nm (
e
, 1.09 ꢀ 104 L molꢁ1 cmꢁ1) due
to the formation of the closed-ring isomer 1c. Upon exposure to
visible light (k >500 nm), the purple color was bleached entirely,
indicating that 1c returned to the initial state 1o. Similarly, 2 and
0.6
3o
2o
0.4
The thermal stability of diarylethenes with two naphthalene
moieties is very excellent, and the absorptance of the closed-ring
isomers remains almost constant for more than 500 h even at
343 K.16 Therefore, it can be expected that the unsymmetrical
diarylethenes with a naphthalene moiety also exhibit good ther-
mal stability. The thermal stability of the open-ring and closed-ring
isomers of 1–3 was examined at both room temperature and
351 K. Storing these solutions in the dark and then exposing them
to air for more than a week at room temperature, no changes in
color and absorption spectra of 1–3 were observed. When these
ethanol solutions were heated under reflux (351 K) for 8 h in
PSS 3
1o
PSS 1
PSS 2
0.2
0.0
300
400
500
600
Wavelength (nm)
Figure 1. Absorption spectral changes of diarylethenes 1–3 by photoirradiation in
hexane (C = 5.0 ꢀ 10ꢁ5 mol Lꢁ1) at room temperature.

R. Wang et al. / Tetrahedron Letters 54 (2013) 5307–5310
5309
Table 1
Absorption spectral properties of diarylethenes 1–3 in hexane (5.0 ꢀ 10ꢁ5 mol Lꢁ1) and in PMMA films (10%, w/w) at room temperature
Compd
ko,max/nma
(e
/L molꢁ1 cmꢁ1
)
kc,max/nmb
(e
/L molꢁ1 cmꢁ1
)
Uc
PRd (%)
Hexane
PMMA
Hexane
PMMA
Uo–c
Uc–o
1
2
3
281(1.22 ꢀ 104)
281(7.48 ꢀ 103)
275(1.06 ꢀ 104)
282
281
282
535(1.09 ꢀ 104)
491(4.72 ꢀ 103)
455(5.58 ꢀ 103)
548
503
484
0.10
0.15
0.22
0.27
0.13
0.09
32
36
77
a
b
c
The maximum absorption peak of the open-ring isomers.
The maximum absorption peak of the closed-ring isomers.
Quantum yields of cyclization (Uo–c) and cycloreversion (Uc–o), respectively.
Photoconversion ratios of diarylethenes 1–3 (PR%) in the photostationary state.
d
6000
8000
6000
darkness, no decomposition was detected by UV–vis absorption
and NMR spectroscopy. The result indicated that these diaryleth-
enes exhibited good thermally irreversible photochromism. The
thermal cycloreversion kinetics of 1c–3c at various temperatures
was measured in hexane (Fig. S3, SI). According to the Arrhenius
equation, the activation energy of the thermal reversion of 1c–3c
was calculated with the values of 36.9 kJ molꢁ1 for 1c,
49.8 kJ molꢁ1 for 2c, and 51.9 kJ molꢁ1 for 3c. The half-life time
of 1c–3c at 298 K was 6 h for 1c, 9.8 h for 2c, and 12.5 h for 3c.
Compared to the reported diarylethene with two pyrrole rings
(32.5 kJ molꢁ1 at 298 K), diarylethenes 1c–3c showed elevated
activation energy, but it was lower than that of the 1,2-di-(2-naph-
tyl)cyclopentene (121.0 kJ molꢁ1 at 298 K).17
A
B
3o
4500
3000
1500
0
3o
4000
2000
0
1o
1o
2o
2o
400
480
560
640
360
420
480
540
Wavelength (nm)
Wavelength (nm)
Figure 3. Emission spectra of diarylethenes 13 in both hexane (C = 5.0 ꢀ 10ꢁ5
mol Lꢁ1) and PMMA films (10%, w/w) at rt: (A) in hexane, excited at 300 nm, (B) in
PMMA films, excited at 307 nm.
Fatigue resistance, which is defined as the number of
photocyclization and cycloreversion reaction cycles are repeated
without loss of performance, is a critical factor for practical appli-
cations in optical devices.2a,b,17 The fatigue resistance of 1–3 was
examined in both hexane and PMMA films in air at room temper-
ature. As shown in Figure 2, the coloration and decoloration cycles
of 1–3 can be repeated for at least 100 times with 21% degradation
of 1c, 55% of 2c, and 23% of 3c in hexane, which may be ascribed to
the degradation resulting from the formation of an epoxide.18 In
PMMA films, 1–3 still exhibited favorable photochromism after
200 cycles with only 25% degradation of 1c, 61% degradation of
2c, and 35% of 3c. Therefore, the fatigue resistance of 3 was much
weaker than that of 1 and 2 in both hexane and PMMA films due to
the different effects of the heteroaryl moieties.
The fluorescence spectra of 1o–3o in both hexane and PMMA
films at room temperature are illustrated in Figure 3. The emission
peaks of 1o–3o were observed at 503, 443, and 433 nm upon exci-
tation at 300 nm in hexane, and were observed at 461, 431, and
437 nm when excited at 307 nm in PMMA films. When compared
with those in hexane, the emission peaks of 1o and 2o showed a
remarkable hypochromatic shift in PMMA films, with the value
of 42 nm for 1, and 12 nm for 2. However, the emission peak of
3o exhibited a minor bathochromic shift with the value of 4 nm
in a PMMA film. Compared to the benzothiophene derivative 2o,
the indole-substituted derivative 1o shifted the emission peak to
a longer wavelength and evidently increased its emission intensity.
However, the benzofuran-substituted 3o shifted the emission peak
to a shorter wavelength and significantly increased its emission
intensity. Using anthracene (Uf = 0.27) as a reference, the fluores-
cence quantum yields of 1o–3o in acetonitrile were determined
to be 0.088, 0.039, and 0.105, respectively. The results suggested
that the different heteroaryl groups had a significant effect on
the fluorescence property of the naphthalene-containing
diarylethenes.
As has been observed for most of the reported diarylethenes,19
diarylethenes 1–3 functioned as fluorescent switches in both hex-
ane and PMMA films. Upon being irradiated by 297 nm UV light,
the emission intensity of 1–3 was decreased in both hexane and
PMMA films due to the formation of the non-fluorescent closed-ring
isomers 1c–3c. The back irradiation by appropriate visible light
regenerated the open-ring isomers 1o–3o and recovered the origi-
nal emission intensity. As shown in Figure 4, the emission intensity
of 1 was quenched to ca. 36% in hexane and 75% in a PMMA film in
the photostationary state. Similarly, the emission intensity of 2 and
3 in PSS was quenched to ca. 60% for 2o and 44% for 3o in hexane,
and that in PMMA films was quenched to ca. 73% for 2o and 49%
for 3o. (Figs. S4 and S5). The residues of fluorescence for diaryleth-
enes 1–3 in PSS may be attributed to the incomplete cyclization
reaction and the existence of parallel conformations.20 Among the
three analogs, 3 exhibited a greater fluorescent switch property in
solid medium due to its higher potoconversion efficiency in PSS (Ta-
1.00
1.00
2800
2100
1400
3600
2700
1800
A
B
A
B
0.75
0.50
0.25
0.00
0.75
0.50
0.25
0.00
Vis
UV
Vis
UV
1o
2o
3o
1o
2o
3o
700
0
900
0
0
50
100
150
200
0
25
50
75
100
400
480
Wavelength (nm)
560
640
350
420
490
560
Repeat Cycles
Repeat cycles
Wavelength (nm)
Figure 2. Fatigue resistance of diarylethenes 1–3 in both hexane and PMMA films
in air atmosphere at rt: (A) in hexane, (B) in PMMA films. Initial absorbance of the
sample was fixed to 1.0.
Figure 4. Emission intensity changes of diarylethene 1 in both hexane
(C = 5.0 ꢀ 10ꢁ5 mol Lꢁ1) and a PMMA film (10%, w/w) upon irradiation with UV
light at rt: (A) in hexane, excited at 300 nm, (B) in a PMMA film, excited at 307 nm.

5310
R. Wang et al. / Tetrahedron Letters 54 (2013) 5307–5310
9, 2133–2142; (c) Yumoto, K.; Irie, M.; Matsuda, K. Org. Lett. 2008, 10, 2051–
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ble 1). Compared with the reported diarylethenes with five-mem-
bered moieties,8a,c the fluorescent modulation efficiencies of 1–3
were much smaller in both solution and solid media. The main rea-
son is that the photoconversion ratios of diarylethenes with a naph-
thalene moiety are much lower than those of the diarylethenes with
five-membered rings.
In conclusion, three new naphthalene-containing diarylethenes
with different heteroaryl moieties have been synthesized and their
properties have been investigated. The results indicated that the
heteroaryl groups had significant effects on the properties of these
diarylethenes. The present work provides a useful design strategy
for effectively tuning the photochromic properties of diarylethenes
and seeking for their further applications in different optoelec-
tronic devices.
10. Liu, G.; Pu, S. Z.; Wang, R. J. Org. Lett. 2013, 15, 980–983.
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Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (21162011, 21262015), the Project of Jiangxi
Advantage Sci-Tech Innovative Team (20113BCB24023), the Pro-
ject of Jiangxi Natural Science Foundation (20114BAB203007,
20132BAB203005), and the Project of the Science Funds of Jiangxi
Education Office (KJLD12035, GJJ11026, GJJ12587).
12. Yagi, K.; Irie, M. Bull. Chem. Soc. Jpn. 2003, 76, 1625–1628.
13. Data for 1o: Mp 464-465 K; Calcd for C26H19F6N: Calcd C, 67.97; H, 4.17; N,
3.05. Found C, 67.93; H, 4.12; N, 3.11; 1H NMR (400 MHz, CDCl3, TMS): d 1.94
(s, 3H, –CH3), 2.32 (s, 3H, –CH3), 3.39 (s, 3H, –CH3), 7.06–7.12 (m, 3H, phenyl–
H), 7.28 (d, 1H, phenyl-H, J = 8.0 Hz), 7.40–7.50 (m, 2H, naphthalene-H),
7.61–7.63 (d, 1H, naphthalene-H, J = 8.0 Hz); 7.68–7.75 (m, 2H, naphthalene-
H); 7.87 (d, 1H, naphthalene-H, J = 8.0 Hz); 13C NMR (100 MHz, CDCl3, TMS):
d = 11.96, 20.86, 29.55, 99.49, 108.93, 110.68, 119.41, 120.30, 121.50, 123.85,
125.32, 125.65, 126.34, 128.05, 128.62, 129.44, 131.81, 135.88, 136.64,
m
, cmꢁ1): 744, 772, 816, 872, 972, 992, 1041, 1096, 1139,
Supplementary data
137.20. IR (KBr,
1188, 1274, 1338, 1476, 1766, 3418; Data for 2o: mp 347–348 K; Calcd for
25H16F6S: Calcd C, 64.92; H, 3.49. Found C, 64.86; H, 3.42; 1H NMR
C
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
07.094.
(400 MHz, CDCl3, TMS): d 1.99 (s, 3H, –CH3), 2.32 (s, 3H, –CH3), 7.14–7.21 (m,
2H, phenyl-H), 7.23–7.29 (m, 2H, phenyl-H), 7.43–7.53 (m, 3H, naphthalene-
H), 7.73–7.78 (m, 3H, naphthalene-H); 13C NMR (100 MHz, CDCl3, TMS):
d = 15.72, 21.26, 121.66,121.97, 122.53, 124.20, 124.34, 125.55, 126.40,
126.49, 126.63, 128.20, 128.59, 129.86, 131.48, 131.73, 135.30, 135.84,
m
, cmꢁ1): 750, 814, 984, 1138, 1190, 1270, 1338,
References and notes
138.19, 138.68; IR (KBr,
1401, 1631, 3202, 3440; Data for 3o: Mp 377–379 K; Calcd for C25H16F6O:
Calcd C, 67.27; H, 3.61. Found C, 67.21; H, 3.57; 1H NMR (400 MHz, CDCl3,
TMS): d 1.99 (s, 3H, –CH3), 2.32 (s, 3H, –CH3), 7.15–7.19 (m, 2H, phenyl-H),
7.22–7.30 (m, 2H, phenyl-H), 7.42–7.53 (m, 3H, naphthalene-H), 7.77 (d, 3H,
naphthalene–H, J = 8.0 Hz); 13C NMR (100 MHz, CDCl3, TMS): d = 13.64, 20.69,
104.12, 110.88, 119.90, 121.94, 123.18, 124.21, 125.11, 125.60, 126.37,
126.73, 127.22, 128.26, 129.97, 131.85, 135.56, 146.61, 153.86, 155.33; IR
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