Photochromism of new diarylethene derivatives based on the hybrid photochromic skeleton of benzofuran and benzene moieties

Article abstract of DOI:10.1016/j.dyepig.2013.09.026

Four new photochromic diarylethenes with both benzofuran and benzene moieties were synthesized, and the structures of three diarylethenes were determined by single-crystal X-ray diffraction analysis. The six-membered benzene ring was connected directly to the central perfluorocyclopentene unit as an aryl moiety and available to participate in photoisomerization reaction. All the diarylethenes exhibit favorable photochromism and function as notable fluorescence switches in both hexane and poly(methyl methacrylate) films. The electron-donating substituents effectively enhanced the cyclization quantum yields, while the electron-withdrawing substituents notably shifted the fluorescent emission peaks to a longer wavelength in both solution and poly(methyl methacrylate) films. The result indicated that the different substituents linked at the reactive carbon atom of the benzene moiety had a significant effect on their optical and electrochemical properties.

Full text of DOI:10.1016/j.dyepig.2013.09.026

Accepted Manuscript
Photochromism of new diarylethene derivatives based on the hybrid photochromic
skeleton of benzofuran and benzene moieties
Hui Li, Shouzhi Pu, Gang Liu, Bing Chen
PII:
S0143-7208(13)00351-3
10.1016/j.dyepig.2013.09.026
DYPI 4100
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 30 July 2013
Revised Date: 16 September 2013
Accepted Date: 17 September 2013
Please cite this article as: Li H, Pu S, Liu G, Chen B, Photochromism of new diarylethene derivatives
based on the hybrid photochromic skeleton of benzofuran and benzene moieties, Dyes and Pigments
(2013), doi: 10.1016/j.dyepig.2013.09.026.
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ACCEPTED MANUSCRIPT
Graphical Abstract
Photochromism of new diarylethene derivatives based on the hybrid
photochromic skeleton of benzofuran and benzene moieties
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
*Corresponding author: E-mail: pushouzhi@tsinghua.org.cn (S. Pu); Tel./Fax: +86-791-83831996.
A new class of unsymmetrical diarylethenes with both benzofuran and benzene moieties was
synthesized to inevestigate the substituent effects linked at the reactive carbon atom of benzene
moiety on their optoelectronic properties.
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Photochromism of new diarylethene derivatives based on the hybrid
photochromic skeleton of benzofuran and benzene moieties
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
*Corresponding author: E-mail: pushouzhi@tsinghua.org.cn (S. Pu); Tel./Fax: +86-791-83831996.
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Abstract: Four new photochromic diarylethenes with both benzofuran and benzene moieties
were synthesized, and the structures of three diarylethenes were determined by single-crystal
X-ray diffraction analysis. The six-membered benzene ring was connected directly to the central
perfluorocyclopentene unit as an aryl moiety and available to participate in photoisomerization
reaction. All the diarylethenes exhibit favorable photochromism and function as notable
fluorescence switches in both hexane and poly(methyl methacrylate) films. The electron-donating
substituents effectively enhanced the cyclization quantum yields, while the electron-withdrawing
substituents notably shifted the fluorescent emission peaks to a longer wavelength in both solution
and poly(methyl methacrylate) films. The result indicated that the different substituents linked at
the reactive carbon atom of the benzene moiety had a significant effect on their optical and
electrochemical properties.
Keywords: Photochromism; Diarylethene; Benzofuran and benzene; Crystal structure;
Fluorescence; Electrochemical property.
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1. Introduction
Photochromic compounds have attracted considerable attention due to their wide applications in
photonic devices, such as optical memory, photo-switches, and full-color displays [1-3]. Among
various photochromic compounds, diarylethenes are one of the most promising candidates for the
applications mentioned above due to their excellent thermal stability of the respective isomers,
notable fatigue resistance, and high reactivity in the solid state [4-7].
In recent years, many efforts have been made to investigate diarylethene derivatives, including
molecular design principle, new synthetic method, and their optoelectric properties. In order to
promote the development of new diarylethenes with excellent optical and electrical characteristics,
a wide structural diversity of photochromic systems has been extensively explored [8-13].
Nowadays, the successful synthestic method for diarylethene derivatives with various heteroaryl
units, such as thiophene/benzothiophene [3,4,6], furan [14], isoxazole [15,16], naphthalene [17],
thiazole [18-21], indole [22-24], pyrazole [25], and pyrrole [26,27], has been established.
Benzofuran is an attractive aryl unit which has low aromatic stabilization energy, and its
molecular structure is similar to that of benzothiophene. Therefore, introduction of a benzofuran
ring into the heteroaryl moieties of diarylethene derivatives can be expected to undergo excellent
photochromic reactions [28]. Up to date, there are a few examples of photochromic diarylethene
derivatives bearing benzofuran rings [28-32]. All the reported diarylethenes with benzofuran
moieties are symmetrical compounds and their hexatriene backbones are composed of
five-membered heterocyclic rings. To the best of our knowledge, photochromic hybrid
diarylethene derivatives with both benzofuran and six-membered aryl ring have not hitherto been
reported.
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Apart from the fact that the heteroaryl moieties have an important role to play in modifying the
properties of diarylethenes, the substituent effects can also affect their properties significantly. To
date, although many reports concerning the substituent effects on the photochromic properties
have been reported, the results mainly focus on effects of different substituents linked at the
terminal benzene ring on their photochromic properties [15,32,33]. However, the effects of
substituents linked at the reactive carbon atoms on the photochromic properties of diarylethenes
have been rarely reported [34,35].
In this work, a new class of photochromic diarylethenes with both benzofuran and benzene
moieties were firstly explored to investigate effects of substituents linked at the reactive carbon
atoms on photochromism of these diarylethenes. The synthesized diarylethenes are
1-(2-ethyl-3-benzofuranyl)-2-(2-methoxyphenyl)perfluorocyclopentene
1-(2-ethyl-3-benzofuranyl)-2-(2-methylphenyl)perfluorocyclopentene
1-(2-ethyl-3-benzofuranyl)-2-(2-cyanophenyl)perfluorocyclopentene
(1o),
(2o),
and
(3o),
1-(2-ethyl-3-benzofuranyl)-2-(2-trifluoromethylphenyl)perfluorocyclopentene (4o). All of these
diarylethenes showed excellent photochromism in both solution and PMMA amorphous films, and
diarylethenes 1 and 4 showed photochromism even in the crystalline phase. The photochromic
scheme of 1–4 is shown in Fig. 1.
<Fig. 1>
2. Experimental
2.1. General methods
Absorption spectra were measured using an Agilent 8453 UV/Vis spectrophotometer. NMR
spectra were recorded on a Bruker AV400 (400 MHz) spectrometer with CDCl3 as the solvent and
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tetramethylsilane as an internal standard. Elemental analysis was carried out with a PE CHN 2400
analyzer. IR spectra were recorded on a Bruker Vertex-70 spectrometer. Fluorescence spectra
were measured on a HITACHI 4600 fluorescence spectrophotometer. 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 spectro-grade and purified by distillation prior to use.
Suitable crystals of 1o, 3o, and 4o were obtained by slow evaporation of hexane solutions of the
three diarylethenes. All the measurements were collected by a Bruker SMART APEX II CCD
diffractometer using a MULTI scan technique at room temperature using Mo Kα radiation. The
structures were solved by direct methods and refined by full-matrix least-squares procedures on F2
by full-matrix least-squares techniques using SHELXTL-97 program. Further details on the crystal
structure investigation have been deposited with The Cambridge Crystallographic Data Centre as
supplementary publication CCDC 941982 for 1o, 941983 for 3o, and 941786 for 4o. Copies of the
data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: +44 0 1223 336033 or e-mail:deposit@ccdc.cam.ac.uk).
Electrochemical examinations were performed in a one-compartment cell by using a Model 263
potentiostat−galvanostat (EG&G Princeton Applied Research) under computer control at room
temperature. Platinum-electrodes (diameter 0.5 mm) served as working electrode and counter
electrode. Platinum wire served as a quasi reference electrode. It was calibrated using the
ferrocene (Fc/Fc+) redox couple which has a formal potential E1/2 = +0.35 V versus platinum wire.
The typical electrolyte was acetonitrile (5.0 mL) containing 0.1 mol/L tetrabutylammonium
tetrafuoroborate ((TBA)BF4) and 4.0 × 10-3 mol L-1 diarylethene sample. All solutions were
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deaerated by bubbling with a dry argon stream and maintained at a slight argon overpressure
during electro-chemical experiments.
2.2. Synthesis of diarylethene derivatives
2.2.1. 2-Ethylbenzofuran (6)
To a stirred anhydrous THF solution (100.0 mL) containing benzofuran (5) (6.1 g, 52 mmol) was
slowly added n-BuLi/hexane (20.0 mL of 2.84 mol L-1) at 195 K under nitrogen atmosphere, and
the solution was stirred for 45 min. Then the ethyl bromide (4.6 mL, 62 mmol) was added slowly
to the reaction mixture, and the reaction mixture was warmed to room temperature, kept stirring
for another 16 h. Then the reaction mixture was poured into concentrated sodium chloride solution
and extracted with diethyl ether. The organic layer was dried, filtrated, and concentrated. The
residue was purified by column chromatography using petroleum ether as the eluent to give 6.3 g
of 6 as colorless liquid in 84% yield. 1H NMR (400 MHz, CDCl3, ppm): δ 1.33 (t, 3H, J = 7.4 Hz,
-CH3), 2.76–2.82 (m, 2H, -CH2-), 6.36 (s, 1H, thienyl-H), 7.15–7.23 (m, 2H, phenyl-H), 7.40 (d,
1H, J = 8.0 Hz, phenyl-H), 7.47 (d, 1H, J = 6.8 Hz, phenyl-H); 13C NMR(100 MHz, CDCl3, ppm):
δ 11.94, 21.91, 101.17, 110.86, 120.39, 122.56, 123.26, 129.24, 154.91, 161.16; IR (v, KBr, cm-1):
749, 807, 881, 930, 971, 1009, 1172, 1254, 1455, 1597, 2849, 2878, 2939, 2974, 3056.
2.2.2. 3-Bromo-2-ethyl-benzofuran (7)
To a stirred THF solution (100.0 mL) containing compound 6 (3.0 g, 20 mmol) was slowly added
4.02 g N-bromosuccinimide (23 mmol) at 273 K. The reaction mixture was stirred for 24 h, then
was poured into sodium thiosulfate solution and extracted with diethyl ether. The organic layer
was dried, filtrated, and concentrated. The residue was purified by column chromatography using
petroleum ether as the eluent to give 3.6 g of 7 as a pale brown liquid in 75% yield. 1H NMR (400
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MHz, CDCl3, ppm): δ 1.32 (t, 3H, J = 7.6 Hz, -CH3), 2.82–2.87 (m, 2H, -CH2-), 7.24–7.29 (m, 2H,
phenyl-H), 7.39–7.45 (m, 2H, phenyl-H); 13C NMR(100 MHz, CDCl3, ppm): δ 12.07, 20.36,
93.59, 111.18, 119.22, 123.21, 124.49, 128.54, 153.59, 156.71; IR (v, KBr, cm-1): 743, 838, 896,
1000, 1010, 1062, 1083, 1105, 1174, 1231, 1261, 1308, 1451, 1597, 2938, 2977.
2.2.3. (2-Ethyl-1-benzofuran-3-yl) perfluorocyclopentene (8)
To a stirred THF solution (100.0 mL) containing compound 7 (3.0 g, 13 mmol) was slowly added
n-BuLi (5.2 mL of 2.84 mol L-1) at 195 K under nitrogen atmosphere. After 40 min, the
perfluorocyclopentene (2.2 mL) was added quickly to the reaction mixture, and kept stirring for 2
h at 195 K under nitrogen atmosphere. Then the reaction mixture was extracted with diethyl ether
and evaporated in vacuum. The residue was purified by column chromatography using petroleum
1
ether as the eluent to give 2.6 g of 8 as yellow oil liquid in 61%. H NMR (400 MHz, CDCl3,
ppm): δ 1.36 (t, 3H, J = 7.6 Hz, -CH3), 2.76–2.82 (m, 2H, -CH2-), 7.27–7.36 (m, 2H, phenyl-H),
7.49–7.54 (m, 2H, phenyl-H); 13C NMR(100 MHz, CDCl3, ppm): δ 11.82, 21.22, 111.28, 120.19,
122.37, 123.75, 124.85, 133.58, 154.42, 161.94; IR (v, KBr, cm-1): 565, 747, 804, 841, 974, 1015,
1028, 1057, 1152, 1203, 1256, 1280, 1330, 1364, 1400, 1456, 2930, 2985, 3150, 3459.
2.2.4. 1-(2-Ethyl-3-benzofuranyl)-2-(2-methoxyphenyl) perfluorocyclopentene (1o)
To a stirred THF solution (50 mL) of 2-bromoanisole (0.4 g, 2 mmol) was slowly added
n-BuLi/hexane solution (0.7 mL of 2.93 mol L-1) at 195 K under an argon atmosphere. After 40
min, compound 8 (0.7 g, 2 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, and evaporated in vacuo, then purified by column chromatography (petroleum ether) to
give 1o as a pale yellow solid (0.3 g) in 29% yield. Calcd for C22H16F6O2: Calcd C, 61.98; H, 3.78.
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Found C, 61.86; H, 3.85; Mp: 337–338 K; 1H NMR (400 MHz, CDCl3, ppm): δ 0.86 (t, 3H, J =
7.5 Hz, -CH3), 2.35–2.38 (m, 2H, -CH2-), 3.22 (s, 3H, -OCH3), 6.74 (d, 1H, J = 8.4 Hz,
benzene-H), 6.97 (t, 1H, J = 7.5 Hz, benzene-H), 7.24–7.26 (m, 2H, benzene-H), 7.32 (t, 1H, J =
7.6 Hz, benzene-H), 7.36–7.39 (m, 2H, benzene-H), 7.55 (d, 1H, J = 4.7 Hz, benzene-H); 13C
NMR(100 MHz, CDCl3, ppm): δ 11.47, 20.36, 54.92, 104.89, 110.81, 111.32, 116.85, 120.42,
120.69, 123.14, 124.07, 127.64, 129.78, 131.66, 154.18, 157.03, 159.09; IR (v, KBr, cm-1): 750,
804, 834, 856, 870, 989, 1026, 1047, 1080, 1126, 1194, 1271, 1340, 1398, 1436, 1458, 1495, 1602,
2943, 2981.
2.2.5. 1-(2-Ethyl-3-benzofuranyl)-2-(2-methylphenyl)perfluorocyclopentene (2o)
Diarylethene 2o was prepared by a method similar to that used for 1o and obtained as a pale
yellow oil liquid in 34% yield. Calcd for C22H16F6O: Calcd C, 64.39; H, 3.93. Found C, 64.28; H,
1
3.85; H NMR (400 MHz, CDCl3, ppm): δ 0.89 (t, 3H, J = 7.5 Hz, -CH3), 1.98 (s, 3H, -CH3),
2.38–2.43 (m, 2H, -CH2-), 7.10 (t, 1H, J = 8.0 Hz, benzene-H), 7.22–7.29 (m, 4H, benzene-H),
7.35 (d, 1H, J = 6.2 Hz, benzene-H), 7.39 (d, 1H, J = 8.4 Hz, benzene-H), 7.54 (d, 1H, J = 7.0 Hz,
benzene-H); 13C NMR(100 MHz, CDCl3, ppm): δ 11.49, 19.63, 20.84, 103.55, 111.06, 120.12,
123.39, 124.37, 125.97, 126.76, 127.15, 128.96, 129.83, 130.81, 136.96, 154.21, 160.08; IR (v,
KBr, cm-1): 748, 780, 806, 835, 865, 990, 1014, 1046, 1080, 1128, 1194, 1272, 1313, 1341, 1456,
1593, 2882, 2943, 2983.
2.2.6. 1-(2-Ethyl-3-benzofuranyl)-2-(2-cyanophenyl)perfluorocyclopentene (3o)
Diarylethene 3o was prepared by a method similar to that used for 1o and obtained as a pale
yellow solid in 22% yield. Calcd for C22H13F6NO: Calcd C, 62.71; H, 3.11; N, 3.32. Found C,
62.69; H, 3.08; N, 3.25; Mp: 374–375 K; 1H NMR (400 MHz, CDCl3, ppm): δ 1.04 (t, 3H, J = 7.6
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Hz, -CH3), 2.52–2.58 (m, 2H, -CH2-), 7.17–7.25 (m, 2H, benzene-H), 7.39 (d, 1H, J = 8.0 Hz,
benzene-H), 7.43 (d, 1H, J = 7.6 Hz, benzene-H), 7.49 (t, 1H, J = 7.6 Hz, benzene-H), 7.60 (d, 1H,
J = 8.0 Hz, benzene-H), 7.63 (d, 1H, J = 8.0 Hz, benzene-H), 7.69 (t, 1H, J = 7.6 Hz, benzene-H);
13CNMR (100 MHz, CDCl3, ppm): δ 11.58, 21.13, 102.83, 110.98, 113.23, 116.16, 120.46, 123.34,
124.56, 126.49, 129.69, 130.43, 131.41, 132.86, 133.83, 154.25, 160.74; IR (v, KBr, cm-1): 749,
807, 834, 855, 867, 990, 1014, 1048, 1080, 1142, 1197, 1283, 1340, 1455, 1588, 2231, 2944,
2983.
2.2.7. 1-(2-Ethyl-3-benzofuranyl)-2-(2-trifluoromethylphenyl)perfluorocyclopentene (4o)
Diarylethene 4o was prepared by a method similar to that used for 1o and obtained as a colorless
solid in 29% yield. Calcd for C22H13F9O: Calcd C, 56.91; H, 2.82. Found C, 56.86; H, 2.78; Mp:
348–349 K; 1H NMR (400 MHz, CDCl3, ppm): δ 1.05 (t, 3H, J = 7.5 Hz, -CH3), 2.53–2.56 (m, 2H,
-CH2), 7.20–7.24 (m, 2H, benzene-H), 7.35 (d, 1H, J = 8.6 Hz, benzene-H), 7.45 (t, 2H, J = 8.5 Hz,
benzene-H), 7.52 (t, 1H, J = 7.6 Hz, benzene-H), 7.59 (d, 1H, J = 7.5 Hz, benzene-H), 7.63 (d, 1H,
J = 8.0 Hz, benzene-H); 13C NMR (100 MHz, CDCl3, ppm): δ 11.77, 20.90, 102.84, 110.83,
120.21, 121.83, 123.21, 124.30, 124.55, 125.28, 127.44, 130.29, 130.85, 131.65, 154.10, 159.73;
IR (v, KBr, cm-1): 749, 770, 807, 835, 866, 992, 1013, 1044, 1060, 1083, 1137, 1183, 1281, 1316,
1341, 1392, 1455, 1584, 2945, 2985.
3. Results and discussion
3.1 Discussion of the synthesis of diarylethenes
The
synthesis
route
for
diarylethenes
1o–4o
is
shown
in
Fig.
2.
1-(2-Ethyl-1-benzofuran-3-yl)perfluorocyclopentene (8) was prepared by alkylation, bromination,
and lithiation reactions from benzofuran. Finally, compounds 9a–9d was separately lithiated and
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then coupled with the mono-substituted compound 8 to give the unsymmetrical diarylethenes
1o–4o, respectively. The structures of 1o–4o were confirmed by elemental analysis, NMR, and IR.
<Fig. 2>
3.2 Photochromism of diarylethenes
Diarylethenes 1–4 were observed to undergo reversible photoisomerization in both hexane and
PMMA amorphous films at room temperature. Fig. 3 shows the absorption spectral changes of
diarylethene 1 and color changes of diarylethenes 1–4. Upon irradiation with 297 nm light, the
colorless solution of 1o turned purple red and a new visible absorption band was observed at 527
nm (ε, 7.11 × 103 L mol-1 cm-1), while the original peak at 242 nm (ε, 9.96 × 103 L mol-1 cm-1)
decreased due to the formation of the closed-ring isomer 1c. Alternatively, the purple red colored
solution was completely bleached to colorless upon irradiation with visible light (λ > 450 nm) due
to the reproduction of the original open-ring isomer 1o. Just as 1, 2–4 showed similar
photochromism in hexane, and their color changes by photoirradiation are shown in Fig. 3B. Upon
irradiation with 297 nm light, the colorless solutions of 2o–4o turned orange due to the formation
of the closed-ring isomers 2c–4c, whose absorption maxima was observed at 501, 493, and 500
nm, respectively. When arrived at the photostationary state, the isosbestic points of diarylethenes
1–4 were observed at 265, 263, 256, and 255 nm, respectively. The photoconversion ratios of 1–4
were analyzed by HPLC in photstationary state, and the values were 32% for 1, 29% for 2, 19%
for 3, and 27% for 4. Compared to diarylethenes with five-membered aryl moieties [27,32], the
photoconversion ratios of 1–4 with a six-membered benzene moiety decreased notably in hexane.
In PMMA films, 1–4 also showed similar photochromism to that in hexane, and the absorption
spectral changes of 1 and color changes of 1–4 in PMMA films are shown in Fig. 4. Upon
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irradiation with 297 nm light, the colors of the diarylethene/PMMA films 1–4 changed from
colorless to purple red for 1 and to red for 2, 3, and 4. The absorption maxima of 1c–4c in PMMA
films were observed at 530, 517, 504, and 514 nm, respectively. Compare to those in hexane, the
absorption maxima of 1c–4c in PMMA films showed a bathochromic shift, and the redshift values
were 3 nm for 1c, 16 nm for 2c, 11 nm for 3c, and 14 nm for 4c. The red shift phenomena may be
attributed to the polar effect of the polymer matrix and the stabilization of molecular arrangement
in the solid state [36,37].
<Fig. 3>
<Fig. 4>
The photochromic parameters of 1–4 in hexane and PMMA films are summarized in Table 1. The
result showed that the substituents linked at the reactive carbon atoms of the benzene moiety had a
significant effect on the photochromic features of diarylethenes 1–4, including the absorption
maxima, molar absorption coefficients, and quantum yields. For the open-ring isomers 1o–4o, the
absorption maxima only changed slightly in both hexane and PMMA films when different
substituents were attached at the reactive carbon atoms of the benzene moiety. However, the
absorption maxima of the closed-ring isomers 1c–4c changed notably in both hexane and PMMA
films when different substituents were introduced into the reactive carbon atoms. Compared to
diarylethenes with an electron-withdrawing group (cyano or trifluoromethyl group, such as in
compound 3 or 4), the absorption maxima of diarylethenes with an electron-donating (methoxy or
methyl group, such as in compound 1 or 2) were observed at a longer wavelength in both hexane
and PMMA films. In additon, all the absorption maxima of the closed-ring isomers 1c–4c were
observed at 490-530 nm, which were much shorter than that of thiophene-containing diarylethenes
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with the similar molecular skeleton [34]. This indicated that the benzofuran moiety could be
effective to shift the absorption maximum of diarylethene to a shorter wavelength. The molar
absorption coefficients varied with the same trend in hexane. Diarylethenes with an
electron-donating (1 or 2) had bigger molar absorption coefficients both for their open-ring and
closed-ring isomers, as compared to the diarylethenes with an electron-withdrawing group (3 or 4).
The result is quite different from that of the reported benzene-containing diarylethene derivatives
[21,34]. As shown in Table 1, the cyclization quantum yields of 1 and 2 with an electron-donating
group were much bigger than those of 3 and 4 with an electron-withdrawing group. The result
indicated that the cyclization quantum yields were enhanced significantly when the
electron-donating groups were introduced into the reactive carbon atom of the benzene moiety of
these diarylethenes. However, for diarylethenes 1–4, their cycloreversion quantum yields showed
an irrigular changing trend when different substituent were linked at the reactive carbon atom.
Among the four diarylethene derviatives, 1 has the biggest cyclization quantum yield (Φo-c, 1
=
0.20) and the smallest cycloreversion quantum yield (Φc-o, 1 = 0.014). The result is quite different
from that of the reported thiophene-containing diarylethenes with the similar molecular skeleton,
where the diarylethene with an electron-withdrawing trifluoromethyl group has the biggest
cyclization quantum yield and the smallest cycloreversion quantum yield [34].
<Table 1>
The thermal stability of diarylethenes is an indispensable property, and it is mainly dependent on
the aromatic stabilization energy of the aryl group [38]. In general, the benzofuran moiety has low
aromatic stabilization energy and its structure is similar to benzothiophene, which can be expected
to undergo thermally irreversible photochromic reactions [39]. Thus, we examined the thermal
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stability both of the open-ring and closed-ring isomers of diarylethenes 1–4 in hexane at room
temperature and 341 K. Storing these solutions in the dark and then exposing them to air for more
than 30 days at room temperature, we found that no changes in the UV/vis spectra of diarylethenes
2–4 were observed. Moreover, no decomposition was detected when the three compounds were
exposed to air for more than a half year. For diarylethene 1, no decomposition for its open-ring
isomer was detected when exposed to air for more than a half year. But, its closed-ring isomer 1c
returned to 1o when exposed to air in the dark for only 20 days. Therefore, the thermal stability of
1c is much weaker than that of 2c–4c. At 341 K, diarylethenes 2–4 still showed good thermal
stability for more than 120 min. However, the colored 1c was entirely bleached only 8 min later.
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 1c system, resulting in its
thermal instability [34].
Fatigue resistance is a very important factor for practical applications in optical devices [21].
Fatigue resistance of diarylethenes 1–4 was examined in both hexane and PMMA films by
alternating irradiating with UV and visible light in air at room temperature. In hexane, the
coloration and decoloration cycles of 1–4 could repeat 10 times with ca. 81% degradation of 1c,
60% degradation of 2c, 52% degradation of 3c, and 39% degradation of 4c. The degradation may
be attributed to degradation resulting from the formation of an epoxide [40]. However, the fatigue
resistance in PMMA films was much better than that in hexane. After 10 repeat cycles,
diarylethenes 1–4 still showed favorable photochromism with ca. 42% degradation of 1c, 33%
degradation of 2c, 16% degradation of 3c, and 11% degradation of 4c. This remarkable
improvement may result from effectively suppressing the oxygen diffusion in the solid medium
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[4]. As mentioned above, the fatigue resistance of diarylethenes with electron-withdrawing groups
(such as 3 and 4) was much stronger than that of diarylethenes with electron-donating groups
(such as 1 and 2) in both hexane and PMMA films. The result is well in agreement with that
previously reported for analogues with a thiophene or a thiazole moiety, where the fatigue
resistance increased remarkably with the increase of electron-withdrawing ability [21,34].
<Fig. 5>
<Table 2>
<Table 3>
In general, diarylethene has two conformations with two heterocyclic rings in C2 symmetry
(anti-parallel conformation) and in mirror symmetry (parallel conformation). It is well known that
only anti-parallel conformation can undergo effective photocyclization reaction by a conrotatory
mechanism, whereas the parallel conformation is photochemically inactive [4]. Single crystals of
1o, 3o, and 4o were obtained by slow evaporation of a hexane solution. To gain a deeper
understanding of the relation between the conformation and the photochromic behaviors of these
diarylethenes in the crystalline phase, the final structural conformations were provided by X-ray
crystallographic analysis. The ORTEP drawings and photochromic processes in the crystalline
phase are shown in Fig. 5, and the X-ray crystallographic analysis data are listed in Table 2. The
three derivatives crystallized with photoactive anti-parallel conformation in crystalline phase. For
1o, the dihedral angles between the hexafluorocyclopentene ring and the two adjacent heteroaryl
rings are 57.4° for O2/C19–C20 and 64.0° for C6/C1–C5. The corresponding data of 3o, and 4o
are summarized in Table 3. The distance between the two reactive carbon atoms is 3.888 Å for 1o
(C16…C20), 4.036 Å for 2o (C5…C20), and 4.123 Å for 3o (C6…C16). Based on the empirical
15

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rule that 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 [41,42-45], The crystals of 1o, 3o, and 4o could potentially display favorable
photochromism by photoirradiation in the single crystalline phase. In fact, the crystals of 1o and
4o showed favorable photochromism in accordance with the expected analysis upon irradiation
with UV light. The colorless crystals of 1o and 4o turned red quickly upon irradiation with 297 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 and 4c. Although the crystal of
3o was also packed in an anti-parallel conformation and the distance between bond-forming
carbons was less than 4.2 Å, irradiating the crystal of 3o with UV light for 1 h resulted in no
observable color change, indicating that 3o showed no photochromism in the crystalline phase.
When the crystal was dissolved in hexane, the solution remained colorless and the absorption
spectrum was the same as that of 3o.
As shown in Fig. 6, 3o exhibits distinctively different photochromic reactivity in solution and
crystalline phase. This result could be possibly ascribed to the strong intermolecular interactions
between the diarylethene molecules in crystal cell. For the crystal of 3o, the molecules are packed
in the crystal by stacking interactions with the intermolecular hydrogen bonds of C−HꢀꢀꢀF and
C−HꢀꢀꢀN each other (Table 4). On the other hand, an intramolecular C−HꢀꢀꢀO interactions are
observed between benzofuran and methyl units, with HꢀꢀꢀO distances of 2.45Å. These interactions
might be understood to be electrostatic interactions between the electron-rich N, S, F atom and the
electron-deficient carbon atom. Compared to 3o, 1o and 4o only had the intermolecular C−HꢀꢀꢀF
16

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(Table 4), suggesting that the C−HꢀꢀꢀN and C−HꢀꢀꢀO interactions in the crystal of 3o played a vital
role in the inhibition of photocyclization. The observation that the photochromic reactivity of 3o
was significantly lower in an amorphous PMMA film than that in solution was also in accordance
with the proposed explanation. In solution phase, the solvent molecules disrupted the
intermolecular H-bondings of 3o. In contrast, the restriction of intermolecular H-bondings of 3o
became pronouced in a PMMA film due to the lack of the solvation effect.
<Fig. 6>
<Table 4>
3.3 Fluorescence of diarylethenes
The fluorescence properties of 1–4 were evaluated in both hexane and PMMA films at room
temperature. The fluorescence spectra of diarylethenes 1o–4o in hexane (5.0 × 10-5 mol L-1) and
PMMA films (10%, w/w) at room temperature are shown in Fig. 7. The emission peaks of 1o–4o
in hexane were observed at 404 nm for 1o, 389 nm for 2o, 449 nm for 3o, and 414 nm nm for 4o
when excited at 280 nm, while those in PMMA films were observed at 416 nm for 1o, 411 nm for
2o, 459 nm for 3o, and 440 nm for 4o when excited at 320 nm. Compared to those in hexane, the
emission peaks of 1o–4o in PMMA films showed a bathochromic shift with values of 12 nm for
1o, 22 nm for 2o, 10 nm for 3o, and 26 nm for 4o. By using anthracene as a reference, the
fluorescence quantum yields of 1o–4o were determined to be 0.0069, 0.0074, 0.0077, and 0.0073,
respectively. The cyano-substituted derivative 3o has the largest fluorescence quantum yield, as
compared with other three derivatives. The results indicated that the substituents linked to the
reactive carbon atom had a significant effect on the fluorescence properties of these diarylethene
derivatives.
17

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<Fig. 7>
Typically, the fluorescence of diarylethenes could be reversibly modulated by photoirradiation
during the process of photoisomerization [46-49]. This characteristic can be applied in
molecular-scale optoelectronics and photoswitchable probe for imaging living cells [50]. Upon
irradiation with 297 nm light, the emission intensity of 1o–4o was notably decreased in both
hexane and PMMA films due to the formation of the weak fluorescent closed-ring isomers . The
back irradiation by appropriate visible light regenerated the open-ring isomers 1o–4o and
recovered the original emission intensity. As shown in Fig. 8, the emission intensity of 1 was
quenched to ca. 48% in hexane and 23% in a PMMA film in the photostationary state. Similarly,
the emission intensity of 2–4 in the photostationary state was quenched to ca. 47% for 2o, 66% for
3o, and 58% for 4o in hexane, and that in PMMA films was quenched to ca. 19% for 2o, 44% for
3o, and 36% for 4o. That is to say, the fluorescent modulation efficiency of 1–4 was 52% for 1,
53% for 2, 34% for 3, and 42% for 4 in hexane, and that in PMMA films was 77% for 1, 81% for
2, 56% for 3, and 64% for 4. The result showed that the fluorescent modulation efficiency in
PMMA films was much higher than that in their respective solutions. In addition, the fluorescent
modulation efficiency of diarylethenes with an electron-donating substituent (such as 1 and 2) was
much bigger than that of diarylethenes with an electron-withdrawing substituent (3 and 4) in both
hexane and PMMA films. Compared to the thiophene-containing diarylethenes with the similar
molecular skeleton [34], the fluorescent modulation efficiency of 1–4 enhanced notably during the
process of photoisomerization. Therefore, these diarylethenes can be potentially suitable for use as
an optical memory with fluorescence readout method or a fluorescent photoswitch [51,52].
<Fig. 8>
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3.4 Electrochemistry of diarylethenes
To date, the electric properties of photochromic molecules have been extensively reported [53-56].
It is well known that the hexatriene–cyclohexadiene reaction of diarylethene derivatives can be
initiated either by light irradiation or by an electrochemical or chemical redox process [57,58].
During the process of isomerization, the electrical properties of diarylethenes, such as ionization
potential, electron affinity, and the hole transport property can reversibly change [59-61]. In order
to investigate of 1−4, the electrochemical tests were performed by cyclic voltammetry (CV) under
identical experimental conditions at a scanning rate of 50 mV/s. The CV curves of diarylethenes
1−4 are shown in Fig. 9. The oxidation onsets (Eonset) of the open-ring isomes 1o−4o were initiated
at 1.76, 1.83, 1.84, and 1.83 V, and those of the closed-ring isomers 1c−4c were observed at 1.72,
1.66, 1.78, and 1.77 V, respectively. The difference of oxidation onset between the open-ring and
closed-ring isomers of 1−4 (ꢁVo-c) was 0.05 V for 1, 0.17 V for 2, 0.06 V for 3, and 0.06V for 4.
The result indicated that the oxidation process for the closed-ring isomers 1c−4c occured at lower
potentials than in the corresponding open-ring isomers 1o−4o. This may be ascribed to the longer
conjugation length of the closed-ring isomers which generally leads to a less positive potential
[62,63]. After the cyclization reaction, the π-conjugation of the closed-ring isomers extends across
the perfluorocyclopentene ring causing a lower oxidation onset. Based on the reported method
[33], the band-gaps Eg of 1−4 can be determined, and the corresponding values are summarized in
Table 5. Among these compounds, the Eg was the smallest in 3c, which illustrated the charge
transfers must be faster in 3c compared to that in others [64]. Moreover, the electronic current and
polarization curve shapes between the open-ring and closed-ring isomers of 1–4 at the scanned
voltage region were distinctively different, suggesting that different substituents had a remarkably
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effect on the electrochemical behaviors of these diarylethene.
<Fig. 9>
<Table 5>
4. Conclusion
A new class of unsymmetrical diarylethenes based on the hybrid of benzofuran and benzene
moieties has been developed to investigate the substituent effects linked at the reactive carbon
atom of the benzene moiety on their properties. The diarylethenes showed favorable
photochromism in both solution and PMMA films, and two of them exhibited photochromism
even in the single crystalline phase. Different substituents of the benzene moiety had a significant
effect on the properties of these diarylethenes, such as photochromism, thermal stability, fatigue
resistance, fluorescence, as well as electrochemistry. The results will be helpful in designing and
the synthesis of new photochromic diarylethene sytstems with tunable photochromic
charatteristics for further potential application in optoelectronic devices.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21162011,
21262015), 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, GJJ11026, GJJ12587).
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Table 1. Absorption characteristics and photochromic reactivity of diarylethenes 1−4 in hexane
(5.0 × 10-5 mol L-1) and in PMMA films (10%, w/w).
λo,max/nma (ε/L mol-1 cm-1)
λc,max/nmb (ε/L mol-1 cm-1)
Φc
Compd
PRd (%)
PMMA
PMMA
hexane
film
hexane
film
Φo-c
Φc-o
1
2
3
4
242 (9.96 × 103)
244 (1.12 × 104)
241 (9.10 × 103)
239 (7.22 × 103)
243
245
244
241
527 (7.11 × 103)
501 (5.78 × 103)
493 (4.63 × 103)
500 (4.34 × 103)
530
517
504
514
0.20
0.16
0.14
0.12
0.014
0.052
0.048
0.022
32
29
19
27
a Absorption maxima of open-ring isomers.
b Absorption maxima of closed-ring isomers.
c Quantum yields of open-ring (Φo-c) and closed-ring isomers (Φc-o), respectively.
d Photoconversion ratios of diarylethenes 1−4 in the photostationary state.
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Table 2. Crystal data for diarylethenes 1o, 3o, and 4o.
Compd 1o
C22H16F6O2
Compd 3o
C22H13F6NO
420.33
Compd 4o
C22H13F9O
464.32
Formula
Formula weight
Temperature
426.35
296(2)
296(2)
296(2)
Crystal system
Space group
Monoclinic
P2(1)/n
Monoclinic
P2(1)/c
Monoclinic
P2(1)/n
Unit cell dimensions
a (Å)
8.6823(13)
22.308(3)
10.0364(15)
90.00
12.4236(16)
10.6643(14)
15.318(2)
90.00
9.3031(5)
21.5857(12)
21.5857(12)
90.00
b (Å)
c (Å)
α (o)
β (o)
γ (o)
98.131(2)
90.00
100.597(2)
90.00
100.9590(10)
90.00
Volume (Å3)
Z
1924.4(5)
4
1994.8(4)
4
1997.72(18)
4
Reflections collected
Reflections observed
Number of parameters
ꢂ (mm-1)
3583
3708
3710
2932
2420
2701
273
273
290
0.132
0.125
0.152
Density (calcd.) (g/cm3)
Goodness-of-fit on F2
Radiation (Å)
1.472
1.400
1.544
1.042
1.866
1.042
0.71073
0.0639
0.1554
0.0752
0.1692
0.71073
0.1319
0.4184
0.1608
0.4567
0.71073
0.0868
0.2319
0.1100
0.2548
Final R1[I >2s(I)]
wR2[I >2s(I)]
R1 (all data)
wR2 (all data)
29