Photochromism of novel isomeric diarylethenes with benzofuran and pyridine moieties

Article abstract of DOI:10.1016/j.dyepig.2014.01.019

A new class of isomeric diarylethenes with benzofuran and pyridine moieties was synthesized to investigate the effects of the position of the nitrogen atom in pyridine moiety on their photochromism, acidichromism, and fluorescence. The six-membered pyridine moiety was connected directly to the central cyclopentene ring as an aryl moiety and participated in the photoisomerization reaction in both solution and solid media. These isomeric diarylethenes exhibited multi-addressable switching behavior by the stimulation of acid/base and light. Addition of trifluoroacetic acid to the solutions of these compounds produced the corresponding protonated derivatives. The protonated derivatives exhibited excellent photochromism and notable acidichromism, and the different nitrogen atom position resulted in distinguishable absorption spectra and color changes. The results revealed that the effects of the nitrogen atom position in pyridine moiety played a very important role during the photoisomerization process of these isomeric diarylethenes.

Full text of DOI:10.1016/j.dyepig.2014.01.019

Dyes and Pigments 105 (2014) 47e56
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Photochromism of novel isomeric diarylethenes with benzofuran
and pyridine moieties
*
Xiaoting Li, Shouzhi Pu , Hui Li, Gang Liu
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 isomeric diarylethenes with benzofuran and pyridine moieties was synthesized to
investigate the effects of the position of the nitrogen atom in pyridine moiety on their photochromism,
acidichromism, and fluorescence. The six-membered pyridine moiety was connected directly to the
central cyclopentene ring as an aryl moiety and participated in the photoisomerization reaction in both
solution and solid media. These isomeric diarylethenes exhibited multi-addressable switching behavior
by the stimulation of acid/base and light. Addition of trifluoroacetic acid to the solutions of these
compounds produced the corresponding protonated derivatives. The protonated derivatives exhibited
excellent photochromism and notable acidichromism, and the different nitrogen atom position resulted
in distinguishable absorption spectra and color changes. The results revealed that the effects of the ni-
trogen atom position in pyridine moiety played a very important role during the photoisomerization
process of these isomeric diarylethenes.
Received 8 December 2013
Received in revised form
20 January 2014
Accepted 23 January 2014
Available online 1 February 2014
Keywords:
Diarylethene
Photochromism
Benzofuran
Pyridine
Acidichromism
Fluorescence switch
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
candidates for the multiple responsive switching systems because
of their sensitive response of hydrogen and metal ions [22e25]. For
In the last few decades, many photochromic molecules have
been developed in order to investigate their photoelectrical prop-
erties aimed at finding more promising optical memory media and
photo-optical switching devices [1e4]. Among such compounds,
photochromic diarylethenes are of special concern because of their
excellent thermal stability of the respective isomers, notable fa-
tigue resistance, and high reactivity in the solid state [5e8].
The pyridine ring has attracted much attention because of its
unique nucleophilicity and reactivity [9]. Some examples of diary-
lethenes with pyridine groups have been previously explored. In
these diarylethene systems, pyridine groups are mainly connected
by two ways: one way is that the pyridine ring connects with other
aromatic moieties as a side substituent [10e12], and the other is
that pyridine ring directly connects with the ethene moiety as a
heteroaryl moiety to participate in photoisomerization reaction
[13e15]. Moreover, some pyridine derivatives can be used to detect
metal ions or synthesize complexes [16e18]. Recently, multiple
responsive switching systems have attracted more and more
attention based on modulating the photochromism of diary-
lethenes [19e21]. Diarylethenes with a pyridine group are good
example, Tian and co-workers demonstrated that near-Infrared
photochromic diarylethene Iridium (III) complex exhibited excel-
lent near-infrared photochromic behavior accompanied by efficient
quenching of phosphorescence emission [20]. Ordronneau et al.
reported several new rhenium and ruthenium complexes coordi-
nating to the photochromic diarylethene, which could be modu-
lated by linear and nonlinear optical properties [22]. Yi et al.
developed a multiresponsive fluorescence switch as a detector for
the biological process of metal ion transmembrane transport based
on a diarylethene with terpyridine units [23]. In our previous work
we reported a multiple responsive diarylethene with a pyridine
unit and found it could be simultaneously modulated by light and
chemical stimuli [24]. The valuable achievements have contributed
to a broad understanding of the specific photochromic character-
istics of diarylethenes with pyridine units.
In general, the substituent position effects have a vital role to
modify the optoelectronic behavior of diarylethene derivatives
[26e28]. Although many publications concerning the substituent
position effects on the photochromic properties have been hitherto
reported, the results mainly focus on effects of a certain substituent
attached at different positions of the terminal benzene ring on their
photochromic properties [29e31]. Previously, we have reported the
nitrogen atom position effect of pyridine ring on the properties of
diarylethenes [32e34]. The results revealed that the nitrogen atom
* Corresponding author. Tel./fax: þ86 791 83831996.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
0143-7208/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2014.01.019

48
X. Li et al. / Dyes and Pigments 105 (2014) 47e56
position of pyridine ring had a significant effect on the photo-
chromism of these diarylethenes, although the pyridine ring didn’t
directly participate in photoisomerization reaction. To the best of
our knowledge, the nitrogen atom position effect of a pyridine ring
directly linked with the ethene moiety of a diarylethene derivative
has not been hitherto reported. Therefore, we designed a series of
new unsymmetrical isomeric diarylethenes with both benzofuran
and pyridine moieties to investigate the nitrogen atom position
effect of pyridine ring on the properties of these derivatives in this
work. The photoisomerization of diarylethenes with nitrogen atom
at ortho-, meta-, and para-position of the methylpyridyl moiety (1e
3) along with a diarylethene with a benzene moiety as a reference
compound (4) is illustrated in Fig. 1.
from J&K Scientific LTD without further purification. All reactions
were monitored by thin-layer chromatography carried out on
0.20e0.25 mm silica gel plates (GF-254). Column chromatography
was performed on silica gel (300e400 mesh). Dissolved ultrason-
ically 10 mg of diarylethene sample and 100 mg PMMA into 1.0 mL
chloroform, the diarylethene/PMMA film was prepared by spin-
coating on the surface of quartz substrate.
2.2. Synthesis
The synthetic route to prepare diarylethenes 1oe4o is shown in
Fig. 2. The mono-substituted (2-methyl-3-benzofuranyl)per-
fluorocyclopentene (6) was synthesized by the reported method
using 3-bromo-2-methylbenzofuran (5) as raw material [35,36].
Compounds 7aed were lithiated and then separately coupled with
6 to give diarylethenes 1oe4o, respectively. The structures of 1oe
4o were confirmed by elemental analysis, NMR, and IR
spectroscopy.
2. Results and discussion
2.1. General methods
NMR spectra were recorded on a Bruker AV400 (400 MHz)
spectrometer with CDCl3 as solvents and tetramethylsilane as an
internal standard. Infrared spectra (IR) were carried out on a Bruker
Vertex-70 spectrometer. Fluorescence spectra were measured on a
Hitachi 4600 fluorescence spectrophotometer. Melting points were
measured on a WRS-1B melting point apparatus. UVeVis absorp-
tion spectra were measured using an Agilent 8453 UVeVis spec-
trometer. Photoconversion ratios in the photostationary state were
measured using an Agilent 1100 HPLC chromatographic analyzer.
Photo-irradiation was carried out with an SHG-200 UV lamp, a CX-
21 ultraviolet fluorescence analysis cabinet, and a BMH-250 visible
lamp. Lights of appropriate wavelengths were isolated by different
light filters. All solvents used were of spectrograde and were pu-
rified by distillation prior to use. All other reagents were obtained
2.2.1. (2-Methyl-3-benzofuranyl)perfluorocyclopentene (6)
To a stirred THF solution (80.00 mL) of compound 5 (3.17 g,
15.00 mmol) was added dropwise a 2.4 mol Lꢀ1 n-BuLi/hexane
solution (8.90 mL, 16.50 mmol) at 195 K under nitrogen atmo-
sphere. Stirring was continued for 30 min at 195 K, octa-
fluorocyclopentene (C5F8) (2.30 mL, 16.50 mmol) was slowly
added and the reaction mixture was stirred for 2 h at this low
temperature. The reaction was quenched by water. After being
extracted with ether, the organic layer was washed with
1.0 mol Lꢀ1 aqueous HCl and water. The organic layer dried over
anhydrous MgSO4, filtered and evaporated. The crude product was
purified by column chromatography on silica gel using petroleum
ether as the eluent to give 4.25 g 6 as pale yellow liquid in 87%
yield. IR (
y
, KBr, cmꢀ1): 564, 744, 800, 972, 1041, 1130, 1205, 1282,
2.02 (s,
1456, 1601, 1700, 2970, 3676; 1H NMR (400 MHz, CDCl3):
d
3H, eCH3), 7.20e7.25 (m, 2H, benzofuraneH), 7.29e7.34 (m, 1H,
benzofuraneH), 7.43e7.45 (m, 1H, benzofuraneH); 13C NMR
(100 MHz, CDCl3):
d 13.8, 111.2, 122.6, 124.9, 126.2, 129.0, 133.4,
154.3, 154.4; MS (ESIþ): m/z calcd for C14H7F7O [M þ H]þ 325.1;
found 325.0.
2.2.2. 1-(2-Methyl-3-benzofuranyl)-2-(2-methyl-3-pyridyl)
perfluorocyclopentene (1o)
To a stirred anhydrous THF (40 mL) solution of compound 7a
(0.69 g, 4.00 mmol) was added dropwise a 2.4 mol Lꢀ1 n-BuLi/
hexane solution (1.67 mL, 4.00 mmol) at 195 K under argon at-
mosphere. After 30 min, THF (10 mL) containing compound 6
(1.43 g, 4.40 mmol) was added and the reaction mixture was
stirred for 2 h at this low temperature. The reaction was allowed
to slowly warm to the room temperature and quenched by water.
The product was extracted with ether. The combined organic
layers was dried over MgSO4, filtered and concentrated. The crude
product was purified by column chromatography on silica gel
using petroleum ether/ethyl acetate (v/v ¼ 3/1) as the eluent to
give 0.12 g diarylethene 1o as a pale red solid in 30% yield. mp 69e
70 ꢁC; IR (
1195, 1250, 1278, 1317, 1342, 1401, 1455, 1648, 3131, 3436; 1H NMR
(400 MHz, CDCl3): 2.10 (s, 3H, eCH3), 2.22 (s, 3H, eCH3), 7.22e
y
, KBr, cmꢀ1): 785, 843, 879, 992, 1067, 1125, 1140, 1156,
d
7.29 (m, 3H, benzofuraneH, pyridineeH), 7.39 (d, 1H, J ¼ 8.0 Hz,
benzofuraneH), 7.46 (d, 1H, J ¼ 7.6 Hz, benzofuraneH), 7.70 (d, 1H,
J ¼ 8.0 Hz, pyridineeH), 8.51e8.52 (m, 1H, pyridineeH); 13C NMR
(100 MHz, CDCl3):
d 13.4, 22.8, 104.4, 111.2, 119.7, 120.8, 123.1,
123.6, 124.7, 126.3, 136.8, 150.4, 154.2, 155.5, 156.7; Calcd for
20H13F6NO (%): Calcd C, 60.46; H, 3.30; N, 3.53. Found C, 60.57; H,
3.35; N, 3.51.
C
Fig. 1. Photochromism of diarylethenes 1e4.

X. Li et al. / Dyes and Pigments 105 (2014) 47e56
49
Fig. 2. Synthetic route for diarylethenes 1oe4o.
2.2.3. 1-(2-Methyl-3-benzofuranyl)-2-(3-methyl-2-pyridyl)
perfluorocyclopentene (2o)
purified by column chromatography on silica gel using petroleum
ether as the eluent to give 0.86 g 4o as a pale yellow solid in 54%
Diarylethene 2o was prepared according to the method used for
diarylethene 1o from 3-bromo-4-methylpyridine (7b). The crude
product was purified by column chromatography on silica gel using
petroleum ether/ethyl acetate (v/v ¼ 6/1) as the eluent to give
yield. mp 82e83 ꢁC; IR ( , KBr, cmꢀ1): 764, 835, 865, 985,1074,1126,
y
1171, 1193, 1249, 1270, 1316, 1336, 1400, 1454, 1608, 1652, 3138,
3433; 1H NMR (400 MHz, CDCl3):
d
1.97 (s, 3H, eCH3), 2.06 (s, 3H, e
CH3), 7.09e7.11 (m, 1H, phenyleH), 7.20e7.27 (m, 4H, benzofurane
H, phenyleH), 7.35e7.38 (m, 2H, benzofuraneH), 7.51 (d, 1H,
0.34 g 2o as a pale red solid in 8% yield. mp 98e99 ꢁC; IR (
y, KBr,
cmꢀ1): 795, 844, 878, 989, 1081, 1129, 1193, 1249, 1269, 1282, 1321,
1338, 1400, 1455, 3133, 3468; 1H NMR (400 MHz, CDCl3):
1.95 (s,
J ¼ 7.6 Hz, phenyleH); 13C NMR (100 MHz, CDCl3):
d 13.4, 19.6,
d
104.7, 111.0, 120.0, 123.5, 124.4, 126.1, 126.7, 127.2, 128.9, 129.9,
130.9, 136.9, 154.1, 155.7; Calcd for C21H14F6O (%): Calcd C, 63.64; H,
3.56. Found C, 63.73; H, 3.61.
3H, eCH3), 2.13 (s, 3H, eCH3), 7.20e7.26 (m, 3H, benzofuraneH,
pyridineeH), 7.36e7.42 (m, 2H, benzofuraneH), 7.51 (d, 1H,
J ¼ 7.6 Hz, pyridineeH), 8.61 (d, 1H, J ¼ 4.4 Hz, pyridineeH); 13C
NMR (100 MHz, CDCl3):
d 13.2, 18.3, 104.4, 111.0, 120.1, 123.6, 124.1,
3. Results and discussion
124.5, 126.4, 133.4, 138.5, 147.3, 147.7, 154.1, 156.7; Calcd for
20H13F6NO (%): Calcd C, 60.46; H, 3.30; N, 3.53. Found C, 60.54; H,
3.37; N, 3.59.
C
3.1. Photochromism of diarylethenes
The photochromic behavior of diarylethenes 1e4 induced by
photoirradiation at room temperature were measured in both
hexane (5.0 ꢂ 10ꢀ5 mol Lꢀ1) and PMMA films (10%, w/w). The ab-
sorption spectral changes of 1 and the color changes of 1e4 upon
alternating irradiation with UV and visible light in hexane at room
temperature are shown in Fig. 3. The absorption band of the open-
ring isomer 1o appeared at 242 nm in hexane, which arose from a
2.2.4. 1-(2-Methyl-3-benzofuranyl)-2-(4-methyl-3-pyridyl)
perfluorocyclopentene (3o)
Diarylethene 3o was prepared according to the method used for
diarylethene 1o from 3-bromo-2-methylpyridine (7c). The crude
product was purified by column chromatography on silica gel using
petroleum ether/ethyl acetate (v/v ¼ 6/1) as the eluent to give
0.47 g 3o as a pale red solid in 21% yield. mp 71e72 ꢁC; IR (
y, KBr,
pep* transition [37]. Upon irradiation with 297 nm UV light, the
cmꢀ1): 797, 843, 870, 982, 1061, 1080, 1126, 1195, 1272, 1323, 1400,
1457, 1651, 3119, 3447; 1H NMR (400 MHz, CDCl3):
2.01 (s, 3H, e
colorless solution of 1o turned into orange red and produced a new
visible absorption band centered at 496 nm due to the formation of
the closed-ring isomer 1c. Alternatively, the orange red solution
could be bleached completely upon irradiation with visible light
d
CH3), 2.12 (s, 3H, eCH3), 6.06 (d, 1H, J ¼ 5.2 Hz, pyridineeH), 7.22e
7.29 (m, 2H, benzofuraneH), 7.39 (d, 1H, J ¼ 7.6 Hz, benzofuraneH),
7.47 (d, 1H, J ¼ 8.4 Hz, benzofuraneH), 8.46 (d, 1H, J ¼ 5.6 Hz,
pyridineeH), 8.56 (s, 1H, pyridineeH); 13C NMR (100 MHz, CDCl3):
(l > 450 nm), indicating that 1c returned to the initial state 1o. The
coloration/decoloration cycle could be repeated more than 20
times and a clear isosbestic point was observed at 263 nm, indi-
cating a reversible two-component photochromic reaction scheme
[38]. Diarylethenes 2e4 exhibited similar photochromism to that
observed for 1 in hexane solution. Upon irradiation with 297 nm UV
light, absorption bands in the visible region appeared and the
colorless solutions 2oe4o turned orange red due to the formation
of the closed-ring isomers 2ce4c (Fig. 3). Their absorption maxima
were observed at 493 nm for 2, 491 nm for 3, and 489 nm for 4. All
d
13.4, 19.3, 104.3, 111.2, 119.7, 123.7, 124.3, 124.7, 125.5, 126.3, 146.3,
149.3, 150.7, 154.1, 155.7; Calcd for C20H13F6NO (%): Calcd C, 60.46;
H, 3.30; N, 3.53. Found C, 60.53; H, 3.33; N, 3.58.
2.2.5. 1-(2-Methyl-3-benzofuranyl)-2-(2-methylphenyl)
perfluorocyclopentene (4o)
Diarylethene 4o was prepared according to the method used for
diarylethene 1o from 2-bromotoluene (7d). The crude product was

50
X. Li et al. / Dyes and Pigments 105 (2014) 47e56
diarylethenes 1e4 were calculated with the value of 49% for 1, 37%
for 2, 43% for 3, and 17% for 4. Compared to the diarylethene with a
benzene moiety (4), the photoconversion ratios of the three
isomeric diarylethenes with
a pyridine (1e3) were notably
increased in hexane. In PMMA films, diarylethenes 1e4 also
showed similar photochromism as observed in hexane. The ab-
sorption spectral changes of 1 and the color changes of diary-
lethenes 1e4 are shown in Fig. 5. Upon irradiation with 297 nm UV
light, the colorless diarylethene 1o/PMMA film turned orange red
with a new visible absorption band at 499 nm due to the formation
of the closed-ring isomer 1c. The other three films changed from
colorless to orange red for 2, purple red for 3, and orange red for 4.
In PMMA films, the absorption maxima of 2ce4c were observed at
501 nm, 516 nm, and 497 nm, respectively. All colored diary-
lethene/PMMA films could be bleached completely by irradiating of
visible light with appropriate wavelength. Compared with those in
solution, the absorption maxima of the closed-ring isomers 1ce4c
exhibited an evidently bathochromic shift in PMMA films. The red
shifts of the absorption maxima of 1ce4c were 3 nm for 1c, 8 nm
for 2c, 25 nm for 3c, and 8 nm for 4c. This red shift phenomenon
may be attributed to the polar effect of the polymer matrix in the
amorphous polymer state as observed for the majority of the re-
ported diarylethenes [28,39,40].
The photochromic properties of diarylethenes 1e4 in hexane
and PMMA films are summarized in Table 1. The data indicated that
Fig. 3. Absorption spectral changes of diarylethene 1 (A) and color changes of diary-
lethenes 1e4 (B) upon alternating irradiation with UV and visible light in hexane
(5.0 ꢂ 10ꢀ5 mol/L) at room temperature.
of the orange red solutions 2ce4c could be decolorized by irradi-
ation with visible light of wavelength longer than 450 nm. In the
photostationary state, the isosbestic points of 2e4 in hexane were
observed at 264, 264, and 263 nm, respectively. In addition, the
photoconversion ratios from open-ring to closed-ring isomers of
1e4 in the photostationary state in hexane were measured by HPLC
analysis. As shown in Fig. 4, the photoconversion ratios of
Fig. 5. Absorption spectral changes of diarylethene 1 (A) and the color changes of
diarylethenes 1e4 (B) upon alternating irradiation with UV and visible light in PMMA
films (10%, w/w) at room temperature.
Fig. 4. The photoconversion ratios of diarylethenes 1e4 in the photostationary state in
hexane by HPLC analysis.

X. Li et al. / Dyes and Pigments 105 (2014) 47e56
51
Table 1
(1e3), the molar absorption coefficients of the open-ring isomers
increased in the order of meta- < ortho- < para-substitution by the
nitrogen atom in methylpyridyl moiety; however, those of the
closed-ring isomers increased in the order of para- < meta- < ortho-
substitution by the nitrogen atom in methylpyridyl moiety. The
nitrogen atom position in pyridine ring had no significant effect on
the absorption maxima of both the open-ring and closed-ring iso-
mers of 1e3. In hexane, the absorption maxima only changed
slightly both for their open-ring and closed-ring isomers when the
position of nitrogen atom was changed. In PMMA films, no changes
in the absorption maxima of the open-ring isomers 1oe3o were
observed, but were observed for the closed-ring isomers 1ce3c.
The absorption maximum of 3c is at the longest wavelength and
resulted in the appearance of purple red color in its PMMA film.
Compared to the diarylethene with a benzene moiety (4c), the
absorption maxima of the three isomeric diarylethenes with a
pyridine (1ce3c) exhibited a bathochromic shift in both solution
and PMMA films, but their molar absorption coefficients were
notably decreased in hexane. The cyclization quantum yields
increased in order of 3 (0.077) < 2 (0.10) < 1 (0.13), whereas the
Absorption parameters and photochromic reactivity of diarylethenes 1e4 in hexane
(5.0 ꢂ 10ꢀ5 mol Lꢀ1) and PMMA films (10%, w/w).
Compd lo,max/Nma
/L molꢀ1 cmꢀ1
lc,max/Nmb
/L molꢀ1 cmꢀ1
)
Fc
PRd
(%)
(
3
)
(
3
Hexane
PMMA Hexane
PMMA Foec Fceo
film
film
1
2
3
4
242 (1.27 ꢂ 104) 243
240 (1.03 ꢂ 104) 243
241 (1.38 ꢂ 104) 243
242 (1.40 ꢂ 104) 244
496 (7.12 ꢂ 103) 499
493 (6.86 ꢂ 103) 501
491 (5.42 ꢂ 103) 516
489 (1.54 ꢂ 104) 497
0.13 0.31 49
0.10 0.32 37
0.077 0.27 43
0.11 0.20 17
a
b
c
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of cyclization reaction (Fo-c) and cycloreversion reaction (Fc-o),
respectively.
d
Photoconversion ratio at PSS in hexane.
the pyridine moiety and its nitrogen position had a significant effect
on the photochromic properties of these diarylethenes, such as
molar absorption coefficients, cyclization quantum yields, and
cycloreversion quantum yields. For the three isomeric derivatives
Fig. 6. Fatigue resistance of diarylethenes 1e4 in hexane (A) and in PMMA films (B) in air atmosphere at room temperature. Initial absorptance of the sample was fixed to 1.0.

52
X. Li et al. / Dyes and Pigments 105 (2014) 47e56
cycloreversion quantum yields decreased in order of 2 (0.32) > 1
(0.31) > 3 (0.27). Consequently, the ortho-substituted derivative 1
has the largest cyclization quantum yield, and the para-substituted
derivative 3 has the smallest quantum yields of cyclization and
cycloreversion. Compared to 4, no significant changes in the cycli-
zation quantum yields of 1e3 were observed, but were observed for
their cycloreversion quantum yields. The cycloreversion quantum
yields of 1e3 were larger than that of 4, indicating that the pyridine
moiety could effectively enhance the cycloreversion quantum yield.
Compared to the diarylethenes with a pyridine substituent [32e
34], the cyclization quantum yields of diarylethenes 1e3
decreased and their cycloreversion quantum yields increased
notably. This may be ascribed to the higher aromatic stabilization
energy of the six-membered pyridine ring resulting in the lower
cyclization quantum yield [14].
The thermal stability of the open-ring and closed-ring isomers
of 1e4 was tested in hexane at room temperature and at 341 K.
Storing these solutions in hexane at room temperature in darkness
and then exposing them to air for more than 100 h, no changes in
the UV/vis spectra of diarylethenes 1e4 were observed. When
these hexane solutions were heated under reflux (341 K) for more
than 2 h in darkness, no decomposition was detected by UV/vis
spectroscopy. The fatigue resistance is a very important factor for
practical applications in optical devices [41,42]. Upon alternating
irradiation with UV and visible light in air at room temperature, the
fatigue resistance of 1e4 was examined in both hexane and PMMA
films, and the result is depicted in Fig. 6. In hexane, the coloration
and decoloration cycles of diarylethenes 1e4 could be repeated
more than 100 times with ca. 62% degradation of 1c, 13% degra-
dation of 2c, 74% degradation of 3c, and 29% degradation of 4c. In
PMMA films, they also exhibit favorable photochromism with ca.
61% degradation of 1c, 6% degradation of 2c, 39% degradation of 3c,
and 43% degradation of 4c after 100 repeated cycles. Therefore, the
fatigue resistance of diarylethene 2 was the best in both solution
and PMMA films. The degradation of the absorption intensity may
be ascribed to the undesirable side reactions resulting in generation
of photo-inactive byproducts [41]. Compared to the diarylethenes
with a pyridine substituent [32e34], the fatigue resistance of dia-
rylethenes 1e3 became much weaker when the six-membered
pyridine ring was directly connected with the ethene moiety as a
heteroaryl moiety to participate in photoisomerization reaction
[14].
3.2. Acidichromism of diarylethenes
Fig. 7. The structural changes between diarylethenes 1 and 10 (A) and the color
changes of diarylethenes 1e3 (B) by light and acid/base stimuli.
It is a hot topic to develop multiple responsive switching diary-
lethenes. The derivatives integrate several switchable functions into a
single molecule in order to miniaturize the components of machinery
andelectronicsdowntothemolecularlevel[43,44]. Here, themultiple
switching behaviors of diarylethenes 1e3 were studied by the stim-
ulation of acid/base and light. The structuralchanges of diarylethene 1
could be conducted by stimulating with acid/base. The absorption
spectrum and colorchanges of1ce3cbytheadditionoftrifluoroacetic
acid in the photostationary state are shown in Fig. 8. When tri-
fluoroacetic acid was added to the solution of 1c, the absorption
maximum shifted from 496 to 567 nm due to producing the proton-
ated 1c0, and this process was accompanied by a notable color change
of solution from orange red to blue. The redshift phenomenon was in
agreement with that of the reported pyridine-containing diary-
lethenes [15,33,45]. Similarly, the absorption maximum of 3c shifted
from 491 to 602 nm accompanied with a notable color change from
orange red to blue. However, for 2c, the absorption maximum shifted
from 493 to 458 nmwhen trifluoroacetic acid was added. This process
wasaccompaniedbyanotablecolorchangefrom orangeredtoyellow.
The result revealed that the nitrogen atom position in the pyridine
ring had a significant effect on the acidichromism of these isomeric
diarylethene.
andcolorchanges of diarylethenes 1e3 inhexane (5.0 ꢂ10ꢀ5 mol Lꢀ1
)
were illustrated in Fig. 7. Addition of trifluoroacetic acid in hexane
solution (1.0
m
L, 6.8 ꢂ 10ꢀ3 mol Lꢀ1) to the solutions of 1oe3o pro-
duced the protonated diarylethenes 1o0e3o0. The compounds 1o0e
3o0 could return to the initial state 1oe3o by neutralization with
triethylamine base (5.0
m
L, 3.6 ꢂ10ꢀ3 mol Lꢀ1). Diarylethenes 1o0e3o0
also exhibited notable photochromism by photoirradiation. Upon
irradiationwith UV light, the colorless solution of 1o0e3o0 turned into
different colors, indicating the formation of N-protonated closed-ring
isomers 1c0e3c0. Their absorption maxima in the visible region were
observed at 567 nm for 1c0 (
3
¼ 5.33 ꢂ 103 L molꢀ1 cmꢀ1), 458 nm for
2c0
(
3
¼
4.02 ꢂ 103
L
molꢀ1 cmꢀ1), and 602 nm for 3c0
(
3
¼ 5.43 ꢂ 103 L molꢀ1 cmꢀ1). Alternatively, a reversible trans-
formation between the colored diarylethenes 1ce3c and 1c0e3c0

X. Li et al. / Dyes and Pigments 105 (2014) 47e56
53
3.3. Fluorescence of diarylethenes
1ce4c. They could go back to their open-ring isomers by irradiation
of appropriate wavelength visible light and recovered their original
emission intensity. The emission intensity changes of 1oe4o upon
irradiation with UV and visible light at room temperature in hexane
and in PMMA films are shown in Figs. 10 and 11, respectively. When
arrived at the photostationary state, the emission intensity of 1o
was quenched to ca. 39% in hexane and 14% in a PMMA film.
Similarly, the emission intensity of diarylethenes 2e4 in hexane
were quenched to ca. 46% for 2o, 46% for 3o, and 36% for 4o, and
that in PMMA films was quenched to ca. 18% for 2o, 11% for 3o, and
25% for 4o in the photostationary state. The result showed that the
fluorescent modulation efficiency of 1e4 in PMMA films was much
higher than that in the respective solutions. In addition, the fluo-
rescence of 1e4 could not be quenched completely in the photo-
stationary state, which may be ascribed to the incomplete
cyclization reaction and the existence of parallel conformations
[32,49]. For practical applications in optoelectronic devices, it is
very important that diarylethenes can function as an effective
fluorescent switch in a solid state, such as PMMA film. Therefore,
these diarylethene derivatives could be potentially applied to op-
tical memories with fluorescence readout method or fluorescent
photoswitches [50e52].
The fluorescence properties of diarylethenes 1oe4o in both
solution (5.0 ꢂ 10ꢀ5 mol Lꢀ1) and PMMA films (10%, w/w) at room
temperature are shown in Fig. 9. In hexane, the emission peaks of
1oe4o were observed at 407 nm for 1o, 408 nm for 2o, 407 nm for
3o, and 401 nm for 4o when excited at 291 nm, and those in PMMA
films were observed at 430 nm for 1o, 426 nm for 2o, 434 nm for
3o, and 412 nm for 4o when excited at 288 nm. The result indicated
that the nitrogen atom position in pyridine ring had no significant
effect on the emission peaks of the three isomeric derivatives.
Compared to those in hexane, the emission peaks of 1oe4o in
PMMA films showed an evident bathochromic shift with values of
23 nm for 1o, 18 nm for 2o, 27 nm for 3o, and 11 nm for 4o. This
phenomenon is consistent with most of the reported diarylethenes
[27,29,30]. The emission intensity of 1oe3o increased in order of
2o < 3o < 1o in both hexane and PMMA films, indicating that the
emission intensity of 2o was the lowest and that of 1o was the
highest in both hexane and PMMA films. Compared to 4o, the
emission peaks of 1oe3o were at a longer wavelength and the
emission intensity was lower. This result indicated that the pyridine
moiety could effectively enhance the emission peak wavelength
and decrease the emission intensity, as compared with the benzene
moiety.
As has been observed for most of the reported diarylethenes
[46e48], 1oe4o exhibited an excellent fluorescent switch on
changing from the open-ring isomers to closed-ring isomers by
photoirradiation in both hexane and PMMA films. When irradiated
by UV light, the emission intensity of 1oe4o quenched evidently
due to the formation of the non-fluorescent closed-ring isomers
Fig. 9. Emission spectra of diarylethenes 1e4 in hexane (5.0 ꢂ 10ꢀ5 mol Lꢀ1) when
excited at 291 nm (A) and in PMMA films (10%, w/w) when excited at 288 nm (B) at
room temperature.
Fig. 8. The absorption changes of diarylethenes 1e3 with addition of acid in the
photostationary state: (A) 1; (B) 2; (C) 3.

Fig. 10. Emission intensity changes of diarylethenes 1e4 upon irradiation with UV and visible light at room temperature in hexane (5.0 ꢂ 10ꢀ5 mol Lꢀ1) excited at 291 nm: (A) 1; (B)
2; (C) 3; (D) 4.
Fig. 11. Emissionintensitychanges of diarylethenes 1e4 uponirradiationwith UVandvisiblelight atroom temperature inPMMA films (10%, w/w) excitedat288nm: (A) 1; (B)2; (C)3; (D)4.

X. Li et al. / Dyes and Pigments 105 (2014) 47e56
55
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4. Conclusion
Three new isomeric diarylethenes with a six-member pyridine
moiety were synthesized and their properties were systematically
investigated. Each of these diarylethenes showed favorable
photochromism and functioned as notable fluorescence switches in
both solution and PMMA films. Compared to the diarylethene with
a benzene moiety, diarylethenes with a pyridine moiety had larger
cycloreversion quantum yields and photoconversion ratios, but
smaller molar absorption coefficients and emission intensity.
Moreover, diarylethenes with a pyridine moiety also performed a
reversible isomerization by acid/base stimuli, and their closed-ring
isomers exhibited a notable absorption spectral and color changes.
The behavior of the three isomeric diarylethenes were notably
different from each other, which might be attributed to the effects
of nitrogen atom position in methylpyridyl moiety. The results will
be helpful in understanding the nitrogen atom position effect on
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This work was supported by the National Natural Science
Foundation of China (21262015, 51373072), the Project of Jiangxi
Advantage Sci-Tech Innovative Team (20113BCB24023), the Science
Funds of Natural Science Foundation of Jiangxi Province (20122
BAB213004, 20132BAB203005), and the Project of the Science Funds
of Jiangxi Education Office (KJLD12035, GJJ12587).
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