Photochromic diarylethenes with a naphthalene moiety: Synthesis, photochromism, and substitution effects

Article abstract of DOI:10.1016/j.tet.2013.04.080

Five unsymmetrical diarylethenes with a naphthalene moiety were synthesized, and the structures of four diarylethenes were determined by single crystal X-ray diffraction analysis. The naphthalene was connected directly to the central perfluorocyclopentene

Full text of DOI:10.1016/j.tet.2013.04.080

Tetrahedron 69 (2013) 5537e5544
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Tetrahedron
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Photochromic diarylethenes with a naphthalene moiety: synthesis,
photochromism, and substitution effects
*
Renjie Wang, Shouzhi Pu , Gang Liu, Bing Chen
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University, Nanchang 330013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Five unsymmetrical diarylethenes with a naphthalene moiety were synthesized, and the structures of
four diarylethenes were determined by single crystal X-ray diffraction analysis. The naphthalene was
connected directly to the central perfluorocyclopentene ring as an aryl moiety and available to partici-
pate in photoisomerization reaction. Their properties, such as photochromism, thermal stability, fatigue
resistance, and fluorescence, were investigated systematically. All five diarylethenes, which are thermally
stable, exhibit favorable photochromism, and function as notable fluorescence switches in both solution
and solid media. The electron-donating substituents effectively suppressed the cycloreversion quantum
yield, whereas the electron-withdrawing substituents evidently enhanced the fluorescence quantum
yield of diarylethenes with a naphthalene moiety. The results indicated that the naphthalene moiety and
the substituent effects played an important role during the process of photoisomerization reaction for
these diarylethene derivatives.
Received 29 January 2013
Received in revised form 6 April 2013
Accepted 17 April 2013
Available online 30 April 2013
Keywords:
Photochromism
Diarylethene
Naphthalene
Substitution effect
Fluorescence switch
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
even in the dark.8 Moreover, Tanifuji et al. reported the effect of
radical substituents on photochromic reactivity of bis(3-
Upon stimulation with light, photochromic compounds can in-
terconvert their molecular structures between two isomers with
different properties including color, refractive indices, dipole mo-
ments, as well as oxidation/reduction potentials, and electro-con-
ductivities.1 Photochromic compounds are the most promising
candidates for photoelectronic applications, such as optical mem-
ory, chemical sensor, and molecular switching, because of their
thermally-irreversible and fatigue-resistant photoisomerization
performances.2 To date, a large amount of organic photochromic
molecular systems were reported, including spiropyrans,3 azo-
benzenes,4 fulgides,2b diarylethenes.5 Among these photochromic
molecules, diarylethenes have been extensively reported due to
their rapid response rate and high photochromic reactivity.2b,5,6
Generally, the photochromic properties of diarylethenes mainly
depend on several factors, such as nature of heterocycle moieties,
electron-withdrawing/donating substituents, conformation of
benzothiophene)perfluorocyclopentene,9 and Yamaguchi et al. in-
vestigated the substitution effects on photochromic properties of
diarylethenes bearing two benzofuran groups.10 In our previous
publications, we have explored several classes of diarylethenes and
discussed the effects of substituents and their positions on photo-
chromic properties.11 The thermal stability of diarylethene plays an
important role in application of optical memory media. According
to different thermally stable of photo-generated isomers, diary-
lethenes are typically classified into two types, P-type (thermally
stable photogenerated isomer) and T-type (thermally unstable
photogenerated isomer).12 During the past decade, a huge number
of diarylethenes containing five-membered aryl rings, such as
thiophene,13 benzothiophene,14 oxazole,15 pyrrole,16 thiazole,17
have been developed. The diarylethenes with six-membered aryl
rings were rarely reported due to their poor thermal stability and
fatigue resistance.11b,18 To date, few research on the diarylethenes
containing six-membered aryl rings have been reported.19 In pre-
vious research, we reported diarylethenes with either benzene or
pyridine moiety, and found that all of them exhibited favorable
photochromism.11b,20 However, reports on unsymmetrical diary-
lethenes with a naphthalene moiety are still rare.21
open-ring isomer, and
p
-conjugation length of aryl groups.7 For
instance, diarylethenes bearing thiophene/benzothiophene moie-
ties exhibit excellent thermal stability and outstanding fatigue
resistance,2b,5 whereas symmetrical diarylethene with two pyrrole
rings is thermally unstable and returns to the open-ring isomer
In this work, five unsymmetrical diarylethenes with a naphtha-
lene moiety were synthesized. In order to discuss the substitution
effects on their physicochemical properties, different substituents
were introduced into meta-position of the terminal benzene. The
* Corresponding author. Tel./fax: þ86 791 83831996; e-mail address: pushouzhi@
tsinghua.org.cn (S. Pu).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.04.080

5538
R. Wang et al. / Tetrahedron 69 (2013) 5537e5544
synthesized diarylethenes are 1-(2-methyl-1-naphthyl)-2-[2-
methyl-5-(3-cyanophenyl)-3-thienyl]perfluorocyclopentene (1o),
1-(2-methyl-1-naphthyl)-2-[2-methyl-5-(3-fluorophenyl)-3-
thienyl]perfluorocyclopentene (2o), 1-(2-methyl-1-naphthyl)-2-
[2-methyl-5-(3-chlorophenyl)-3-thienyl]perfluorocyclopentene
(3o), 1-(2-methyl-1-naphthyl)-2-[2-methyl-5-(3-methylphenyl)-3-
thienyl]perfluorocyclopentene (4o), and 1-(2-methyl-1-naphthyl)-
2-(2-methyl-5-(3-methoxyphenyl)-3-thienyl)per-
lamp, a CX-21 ultraviolet fluorescence analysis cabinet, and a BMH-
250 visible lamp. The required wavelength was isolated by the use
of the appropriate filters. Fluorescence spectra were measured
using a Hitachi F-4500 spectrophotometer. 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 in-
vestigation have been deposited with The Cambridge Crystallo-
graphic Data Centre as supplementary publication CCDC 909429 for
1o, 909430 for 2o, 909431 or 3o, and 909432 for 5o. Copies of the
data can be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: þ44 0 1223 336033 or e-
mail:deposit@ccdc.cam.ac.uk).
fluorocyclopentene (5o). The photochromic scheme of diary-
lethenes 1oe5o is shown in Scheme 1.
F
F
F
F
F
F
F
F
F
F
UV
Vis
F
F
2.2. Synthesis
S
S
The synthesis route for diarylethenes 1oe5o is shown in Scheme
2. First, the phenylthiophene derivatives (7aee) were prepared by
Suzuki coupling of five bromobenzene derivatives with a thiophene
boronic acid (6).22 Then, 1-bromo-2-methylnaphthalene (8) was
lithated and coupled with octafluorocyclopentene to give (2-
methyl-1-naphthyl)perfluorocyclopentene (9).21a Finally, com-
pounds 7aee was separately lithiated and then coupled with the
compound 9 to give the unsymmetrical diarylethene derivatives
1oe5o, respectively. The structures of 1oe5o were confirmed by
elemental analysis, NMR, and IR.
1o: R =
2o: R =
3o: R =
4o: R =
1c: R =
2c: R =
3c: R =
4c: R =
CN
F
CN
F
R
R
Cl
Cl
CH3
CH3
5o: R = OCH3
5c: R = OCH3
Scheme 1. Photochromism of diarylethenes 1e5.
2. Experimental
2.1. General
2.2.1. 3-Bromo-2-methyl-5-(3-cyanophenyl)thiophene
(7a). Compound 7a was prepared by reacting 3-bromo-2-methyl-
5-thienylboronic acid (3.0 g, 11.3 mmol) with 3-bromobenzonitrile
(2.47 g, 13.6 mmol) in the presence of Pd(PPh3)4 (0.15 g, 0.01 mmol)
and Na2CO3 (2.0 mol Lꢀ1, 50 mL) in tetrahydrofuran (THF) (80 mL
containing 10% water). After refluxing for 16 h, the product was
extracted with diethyl ether. The organic layer was dried over
MgSO4, filtrated, and evaporated. The crude product was purified
by column chromatography on SiO2 using petroleum ether as the
eluent and 2.0 g of 7a obtained as a yellowish solid in 53% yield. Mp
All solvents used were spectroscopic grade and were purified by
distillation before use. NMR spectra were recorded on a Bruker
AV400 (400 MHz) spectrometer with CDCl3 as the solvent and
tetramethylsilane as an internal standard. Infrared spectra (IR)
were recorded on a Bruker Vertex-70 spectrometer. Melting point
was taken on a WRS-1B melting point apparatus. Absorption
spectra were measured using an Agilent 8453 UV/VIS spectropho-
tometer. Photoirradiation was carried out using an SHG-200 UV
377e378 K; 1H NMR (400 MHz, CDCl3, ppm):
d 2.46 (s, 3H, eCH3),
R
Br
Br
CN
7a: R =
7b: R =
7c: R = Cl
CH3
7e: R = OCH3
F
Pd(PPh3)4, Na2CO3,aq
S
7
B(OH)2
S
6
7d: R =
R
F
F
Br
F
F
F
1) n-BuLi/THF
F
F
2) C5F8,-78°C
8
9
F
F
F
F
F
F
n-BuLi/THF
1o: R = CN
2o: R =
3o: R =
4o: R = CH3
5o: R = OCH3
-78°C
F
S
Cl
R
Scheme 2. Synthetic route to diarylethenes 1e5.

R. Wang et al. / Tetrahedron 69 (2013) 5537e5544
5539
7.18 (s,1H, thiopheneeH), 7.50 (t,1H, benzeneeH, J¼8.0 Hz), 7.57 (d,
1H, benzeneeH, J¼8.0 Hz), 7.73 (d, 1H, benzeneeH, J¼8.0 Hz), 7.79
(s, 1H, benzeneeH).
naphthaleneeH), 7.86 (t, 2H, naphthaleneeH, J¼8.0 Hz); 13C NMR
(100 MHz, CDCl3, TMS):
d
¼14.90, 20.46, 113.27, 118.23, 123.25,
124.83, 124.96, 125.32, 125.80, 127.15, 128.44, 128.57, 128.67, 129.35,
129.71, 130.18, 130.73, 131.90, 134.57, 135.49, 138.44, 142.468; IR (n,
2.2.2. 3-Bromo-2-methyl-5-(3-fluorophenyl)thiophene
(7b). Compound 7b was prepared by an analogous method similar
to that used for 7a and obtained as a light green solid in 67% yield.
KBr, cmꢀ1): 622, 677, 742, 788, 811, 871, 894, 981, 1049, 1107, 1133,
1193,1273,1339,1438,1494,1581, 2230, 2918, 3692; HRMS (ESI): m/
z calcd for C28H17F6NS [MþH]þ 513.0986; found 513.0993.
Mp 312e313 K; 1H NMR (400 MHz, CDCl3, ppm):
d 2.44 (s, 3H,
eCH3), 6.99 (s, 1H, thiopheneeH), 7.22 (d, 1H, benzeneeH,
J¼8.0 Hz), 7.30e7.37 (m, 3H, benzeneeH).
2.2.8. 1-(2-Methyl-1-naphthyl)-2-[2-methyl-5-(3-fluorophenyl)-3-
thienyl]perfluorocyclopentene (2o). Diarylethene 2o was prepared
by a method similar to that used for 1o. The crude product was
purified by column chromatography on SiO2 using petroleum ether
as eluent to give 2o (0.98 g, 53%) as a yellow solid. Calcd for
C27H17F7S (%): calcd C, 64.03; H, 3.38. Found C, 64.13; H, 3.43; mp
2.2.3. 3-Bromo-2-methyl-5-(3-chlorophenyl)thiophene
(7c). Compound 7c was prepared by an analogous method similar
to that used for 7a and obtained as a pale yellow liquid in 66% yield.
1H NMR (400 MHz, CDCl3, TMS):
d
2.44 (s, 3H, eCH3), 7.12 (s, 1H,
357e358 K; 1H NMR (400 MHz, CDCl3, ppm):
d 2.21 (s, 3H, eCH3),
thiopheneeH), 7.28 (t, 2H, benzeneeH), 7.45 (d, 1H, benzeneeH,
J¼8.0 Hz), 7.62 (d, 1H, benzeneeH, J¼8.0 Hz).
2.37 (s, 3H, eCH3), 6.87 (s, 1H, thiopheneeH), 6.90 (d, 2H,
benzeneeH, J¼8.0 Hz), 6.99 (d, 1H, benzeneeH, J¼8.0 Hz), 7.23 (t,
1H, benzeneeH, J¼8.0 Hz), 7.35 (d, 1H, naphthaleneeH, J¼8.0 Hz),
7.50e7.56 (m, 2H, naphthaleneeH), 7.71 (d, 1H, naphthaleneeH,
J¼8.0 Hz), 7.84 (t, 2H, naphthaleneeH, J¼8.0 Hz); 13C NMR
2.2.4. 3-Bromo-2-methyl-5-(3-methylphenyl)thiophene
(7d). Compound 7d was prepared by an analogous method similar
to that used for 7a and obtained as a light green liquid in 80% yield.
(100 MHz, CDCl3, TMS):
d¼14.86, 20.43, 112.06, 112.29, 114.28,
1H NMR (400 MHz, CDCl3, ppm):
d
2.38 (s, 3H, eCH3), 2.43 (s, 3H,
114.49, 120.99, 123.27, 124.10, 124.85, 125.86, 127.02, 128.34, 128.53,
130.09, 130.33, 130.41, 131.85, 135.49, 137.42, 139.42, 139.78, 141.21,
eCH3), 7.11 (s, 1H, thiopheneeH), 7.14 (t, 1H, benzeneeH, J¼8.0 Hz),
7.28 (t, 1H, benzeneeH, J¼8.0 Hz), 7.32 (t, 1H, benzeneeH,
J¼8.0 Hz), 7.34 (s, 1H, benzeneeH).
141.56, 148.60; IR (n
, KBr, cmꢀ1): 676, 734, 776, 811, 863, 897, 981,
1047, 1135, 1192, 1273, 1338, 1401, 1464, 1500, 1633, 3437; HRMS
(ESI): m/z calcd for C27H17F7S [MþH]þ 506.0939; found 506.0945.
2.2.5. 3-Bromo-2-methyl-5-(3-methoxyphenyl)thiophene
(7e). Compound 7e was prepared by an analogous method similar
to that used for 7a and obtained as a light green liquid in 53% yield.
2.2.9. 1-(2-Methyl-1-naphthyl)-2-[2-methyl-5-(3-chlorophenyl)-3-
thienyl]perfluorocyclopentene (3o). Diarylethene 3o was prepared
by a method similar to that used for 1o. The crude product was
purified by column chromatography on SiO2 using petroleum ether
as the eluent to give 0.34 g 3o as a colorless solid in 38% yield. Calcd
for C27H17ClF6S (%): calcd C, 62.01; H, 3.28. Found C, 62.31; H, 3.36;
1H NMR (400 MHz, CDCl3, ppm):
d 2.34 (s, 3H, eCH3), 3.86 (s, 3H,
eCH3), 6.84 (d, 1H, thiopheneeH), 7.05 (s, 1H, benzeneeH), 7.12 (s,
2H, benzeneeH), 7.30 (t, 1H, benzeneeH, J¼8.0 Hz).
2.2.6. (2-Methyl-1-naphthyl)perfluorocyclopentene (9). To a stirred
THF solution (60 mL) of compound 8 (3.0 g, 13.5 mmol) was added
dropwise a 2.5 mol Lꢀ1 n-BuLi/hexane solution (6.0 mL, 14.9 mmol)
at ꢀ78 ꢁC under argon atmosphere. After 30 min, octafluorocyclo
pentene (C5F8) (2.0 mL, 14.9 mmol) was slowly added and the re-
action mixture was stirred for 2.0 h at this low temperature. The
reaction was quenched by water. The product was extracted with
diethyl ether. The organic layer was washed with saturated sodium
chloride aqueous and water, and then dried over anhydrous MgSO4,
filtrated and evaporated in vacuo. The crude product was purified
by column chromatography on silica gel using petroleum ether as
the eluent to give 3.9 g compound 9 obtained as colorless crystal-
line in 87% yield. Mp 335e336 K; 1H NMR (400 MHz, CDCl3, ppm):
mp 349e350 K; 1H NMR (400 MHz, CDCl3, ppm):
d 2.18 (s, 3H,
eCH3), 2.34 (s, 3H, eCH3), 6.85 (s, 1H, thiopheneeH), 7.05e7.07 (m,
1H, benzeneeH), 7.12e7.18 (m, 3H, benzeneeH), 7.33 (d, 1H,
naphthaleneeH, J¼8.0 Hz), 7.46e7.56 (m, 2H, naphthaleneeH), 7.69
(d, 1H, naphthaleneeH, J¼8.0 Hz), 7.84 (t, 2H, naphthaleneeH,
J¼8.0 Hz); 13C NMR (100 MHz, CDCl3, TMS):
¼15.30, 20.48, 123.29,
d
123.43, 124.21, 124.86, 125.00, 125.36, 125.70, 127.05, 127.54, 128.37,
128.56, 130.11, 131.86, 134.78, 134.96, 135.49, 139.51, 141.69; IR (n,
KBr, cmꢀ1): 743, 773, 811, 832, 870, 893, 976, 987, 1048, 1083, 1105,
1133, 1192, 1274, 1341, 1425, 1497, 1572, 1596, 3061, 3632; HRMS
(ESI): m/z calcd for C27H17ClF6S [MþH]þ 522.0644; found 522.0652.
2.2.10. 1-(2-Methyl-1-naphthyl)-2-[2-methyl-5-(3-methylphenyl)-3-
thienyl]perfluorocyclopentene (4o). Diarylethene 4o was prepared
by a method similar to that used for 1o. The crude product was
purified by column chromatography on SiO2 using petroleum ether
as eluent to give 4o (0.57 g, 42%) as a yellow-green solid. Calcd for
C28H20F6S (%): calcd C, 66.92; H, 4.01. Found C, 67.03; H, 4.12; mp
d
2.45 (s, 3H, eCH3), 7.45 (s, 1H, naphthaleneeH), 7.52e7.56 (m, 3H,
naphthaleneeH), 7.88e7.94 (m, 2H, naphthaleneeH).
2.2.7. 1-(2-Methyl-1-naphthyl)-2-[2-methyl-5-(3-cyanophenyl)-3-
thienyl]perfluorocyclopentene (1o). To a stirred anhydrous THF
containing 7a (1.37 g, 4.9 mmol) was added dropwise a 2.5 mol Lꢀ1
n-BuLi/hexane solution (2.2 mL, 5.4 mmol) at ꢀ78 ꢁC under argon
atmosphere. After the mixture has been stirred for 30 min, com-
pound 9 (1.6 g, 4.9 mmol) in solvent of anhydrous THF was added.
The reaction was further stirred at ꢀ78 ꢁC for 2 h, and the reaction
was allowed to slowly warm to the room temperature. The reaction
was quenched with distilled water. The product was extracted with
diethyl ether, dried with MgSO4, and concentrated under reduced
pressure. The crude product was purified by column chromatogra-
phy using petroleum ether as the eluent to afford to 0.87 g com-
pound 1o as a white powder solid in 43% yield. Calcd for C28H17F6NS
(%): calcd C, 65.49; H, 3.34; N, 2.73. Found C, 65.55; H, 3.38; N, 2.77;
323e324 K; 1H NMR (400 MHz, CDCl3, ppm):
d 2.21 (s, 3H, eCH3),
2.30 (s, 3H, eCH3), 2.36 (s, 3H, eCH3), 6.83 (s, 1H, thiopheneeH),
7.06 (t, 1H, benzeneeH), 7.10e7.16 (m, 2H, benzeneeH), 7.32 (d, 1H,
benzeneeH, J¼8.0 Hz), 7.47e7.55 (m, 3H, naphthaleneeH), 7.70 (d,
1H, naphthaleneeH), 7.81 (t, 2H, naphthaleneeH, J¼8.0 Hz); 13C
NMR (100 MHz, CDCl3, TMS):
d
¼14.89, 20.47, 21.32, 122.46, 122.63,
123.33, 124.79, 124.99, 125.02, 125.62, 126.08, 126.23, 126.98,
128.30, 128.42, 128.58, 128.74, 130.01, 131.87, 131.93, 133.14, 135.55,
138.52, 140.77, 141.27; IR (n
, KBr, cmꢀ1): 684, 743, 775, 811, 870, 894,
984, 1049, 1105, 1134, 1191, 1274, 1341, 1509, 1924, 3299, 3694;
HRMS (ESI): m/z calcd for C28H20F6S [MþH]þ 502.1190; found
502.1195.
mp 422e423 K; 1H NMR (400 MHz, CDCl3, ppm):
d 2.25 (s, 3H,
eCH3), 2.37 (s, 3H, eCH3), 6.89 (s, 1H, thiopheneeH), 7.35e7.40 (m,
4H, benzeneeH), 7.48e7.52 (m, 3H, naphthaleneeH), 7.54 (d, 1H,
2.2.11. 1-(2-Methyl-1-naphthyl)-2-(2-methyl-5-(3-methoxyphenyl)-
3-thienyl)perfluorocyclopentene
(5o). Diarylethene
5o
was

5540
R. Wang et al. / Tetrahedron 69 (2013) 5537e5544
prepared by a method similar to that used for 1o. The crude product
was purified by column chromatography on SiO2 using petroleum
ether as eluent to give 5o (0.87 g, 58%) as a yellow solid. Calcd for
C28H20F6OS (%): calcd C, 64.86; H, 3.89. Found C, 64.99; H, 3.94; mp
were 55% for 1, 65% for 2, 62% for 3, 52% for 4, and 78% for 5 in
photostationary state. Compared to diarylethenes with a pyrrole
moiety,24 the photoconversion rations of 1e5 decreased re-
markably in hexane. In PMMA films, 1e5 also showed similar
photochromism to that in hexane, and the color changes are also
shown in Fig. 2. Compared to those in solution, the absorption
maxima of 1ce5c were observed at a longer wavelength in PMMA
films. The redshift values are 8 nm for 1c, 8 nm for 2c, 9 nm for 3c,
12 nm for 4c, and 14 nm for 5c. The phenomena may be ascribed to
more planar structure and stabilization of molecular arrangement
in solid state.25
The photochromic features of 1e5 in hexane and PMMA films
are summarized in Table 1. In hexane, the absorption maxima only
changed slightly, when different substituents were attached at the
meta-position of the terminal benzene. But other photochromic
features including molar absorption coefficients and cyclization/
cycloreversion quantum yields were dependent on the sub-
stituents. When strong electron-withdrawing groups (cyano, fluo-
rine, and chlorine group, 1e3) were introduced to the meta-
position of the terminal benzene, the cyclization quantum yields
362e363 K; 1H NMR (400 MHz, CDCl3, ppm):
d 2.22 (s, 3H, eCH3),
2.34 (s, 3H, eCH3), 3.71 (s, 3H, eCH3), 6.61 (s, 1H, thiopheneeH),
6.74 (d, 1H, benzeneeH, J¼8.0 Hz), 6.79 (d, 2H, benzeneeH,
J¼8.0 Hz), 7.15 (t, 1H, benzeneeH, J¼8.0 Hz), 7.33 (d, 1H, naph-
thaleneeH, J¼8.0 Hz), 7.47 (t, 1H, naphthaleneeH, J¼8.0 Hz), 7.52
(d, 1H, naphthaleneeH, J¼8.0 Hz), 7.79 (d, 1H, naphthaleneeH),
7.81 (t, 2H, naphthaleneeH, J¼8.0 Hz); 13C NMR (100 MHz, CDCl3,
TMS):
d
¼14.91, 20.42, 55.18, 110.48, 113.54, 117.89, 123.66, 125.00,
125.63, 126.99, 128.30, 128.59, 129.87, 130.04, 130.00, 131.67, 133.26,
135.68,140.96,141.46; IR (
n
, KBr, cmꢀ1): 678, 742, 767, 810, 872, 894,
982, 1046, 1101, 1132, 1190, 1257, 1342, 1437, 1468, 1503, 1582, 1608,
1904, 3300, 3628; HRMS (ESI): m/z calcd for C28H20F6OS [MþH]þ
518.1139; found 518.1147.
3. Results and discussion
(from F3,
¼0.23 to F1,
¼0.49) and cycloreversion quantum
3.1. Photoisomerization of diarylethenes
oec
oec
yields (from F3, ceo¼0.12 to F1, ceo¼0.25) gradually increased ac-
companied with the increase of electron-withdrawing ability.
Similarly, when electron-donating groups (methyl and methoxy, 4
and 5) were introduced, the cyclization quantum yields (from F4,
The photochromic behaviors of 1e5 induced by photoirradiation
were measured at room temperature in both hexane
(2.0ꢂ10ꢀ5 mol Lꢀ1) and ploy (methyl methacrylate) (PMMA) films
(10%, w/w). The absorption spectral changes of 1 in both hexane
and a PMMA film upon photoirradiation are shown in Fig. 1. In
hexane, 1o exhibited a sharp absorption peak at 264 nm (ε,
¼0.42 to F5, oec¼0.55) and cycloreversion quantum yields (from
oec
F4,
¼0.09 to F5,
¼0.13) increased with the increase of
ceo
ceo
electron-donating ability. As a result, 5 has the largest cyclization
quantum yield and 4 has the smallest cycloreversion quantum
yield. Furthermore, all cyclization quantum yields were higher than
their cycloreversion quantum yields for 1e5. The result is quite
different from that of diarylethenes with a pyrazole moiety, whose
cyclization quantum yields were much smaller than their cyclo-
reversion quantum yields.26 The cycloreversion quantum yields of
1e3 were much higher than those of 4 and 5, suggesting that the
electron-donating substituents could effectively suppress the
cycloreversion quantum yields, which was well consistent with the
diarylethenes with pyridine and thiophene moieties.19b But this
result is different from that of diarylethenes with a benzene moiety,
which their cycloreversion quantum yields significantly decreased
accompanied with the increase of the electron-withdrawing
ability.11b,27
2.45ꢂ104 L molꢀ1 cmꢀ1) due to a
p/p
* transition.23 Upon irradi-
ation with 297 nm light, the colorless solution of 1o turned red and
a new visible absorption band was observed at 513 nm (ε,
1.89ꢂ104 L molꢀ1 cmꢀ1), while the intensity of the original peak at
264 nm decreased due to the formation of the closed-ring isomer
1c. Upon irradiation with visible light (l>500 nm), the red colored
solution was entirely bleached to colorless due to the reproduction
of the original open-ring isomer 1o. In photostationary state, a clear
isosbestic point of 1 was observed at 275 nm, which supported the
reversible two-component photochromic reaction scheme.6b 2e5
showed similar photochromism in hexane. The color changes of
1e5 by photoirradiation are shown in Fig. 2. Upon irradiation with
297 nm light, the colorless solutions of 2oe5o turned red due to the
formation of the closed-ring isomers 2ce5c, which the absorption
maxima appeared at 511, 510, 514, and 514 nm, respectively. All the
red solutions of 2ce5c were decolorized upon irradiation with
The thermal stability of the open-ring and closed-ring isomers
of 1e5 was tested in hexane at room temperature and in ethanol at
351 K. The hexane solutions were stored at room temperature in
the dark, and exposed to air for more than one month. No changes
were detected in UV/Vis spectra for 1e5. At 351 K, 1e5 also showed
excellent thermal stability for more than 8 h in ethanol. As a key
factor for practical applications in optical devices,5,28 fatigue
visible light (l>500 nm). When arrived at photostationary state,
the isosbestic points of 2e5 were observed at 285, 286, 291, and
290 nm, respectively. As shown in Fig. 3, the photoconversion ratios
from open-ring to closed-ring isomers of 1e5 were analyzed by
HPLC in photostationary state. The photoconversion ratios of 1e5
1.2
B
A
0.5
Vis
Vis
UV
UV
0.4
0.8
0.4
0.0
0.3
UV
Vis
0.2
UV
0.1
0.0
Vis
300
400
500
600
300
400
500
600
Wavelength (nm)
Wavelength (nm)
Fig. 1. Absorption spectral changes of diarylethene 1 by photoirradiation at room temperature: (A) in hexane (2.0ꢂ10ꢀ5 mol Lꢀ1), (B) in a PMMA film (10%, w/w).

R. Wang et al. / Tetrahedron 69 (2013) 5537e5544
5541
Fig. 2. Color changes of 1e5 by photoirradiation at room temperature: (A) in hexane, (B) in PMMA films.
Initial States
hexane is much stronger than that in PMMA films. Compared to
diarylethenes with a benzene moiety,11b,20 the fatigue resistance of
the diarylethenes with a naphthalene moiety is superior in both
hexane and PMMA films. In addition, the fatigue resistance of
diarylethene bearing an electron-donating methoxy group was the
best among these compounds in PMMA films. The results were well
in agreement with that of diarylethenes with thiophene and pyri-
dine moieties,19b but are completely contrary to that of the reported
analogs with a benzene moiety.11b
PSS States
2o
1o
2c
1c
4.4
4.8
5.2
3o
5.6
4
5
6
7
4o
3c
4c
4
5
6
7
4
5
6
7
3.2. Non-photochromism in crystalline phase
5o
5c
8
In general, diarylethene has two conformations with two het-
erocyclic 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 con-
formation is photochemically inactive.5 Therefore, in order to know
10
12
T/min
Fig. 3. The photoconversion ratios of 1e5 in photostationary state by HPLC analysis.
A
B
1.00
0.75
0.50
0.25
0.00
1.0
0.8
0.6
0.4
0.2
0.0
0
25
50
75
100
0
50
100
150
200
Repeat cycles
Repeat cycles
Fig. 4. Color changes of 1e5 by photoirradiation at room temperature: (A) in hexane, (B) in PMMA films.
resistance of 1e5 was examined in both hexane and PMMA films by
photoirradiation at room temperature (Fig. 4). In hexane, the col-
oration/decoloration cycles of 1e5 could be repeated for more than
100 times with only 10% degradation of 1c, 12% degradation of 2c,
14% degradation of 3c, 18% degradation of 4c, and 12% degradation
of 5c. The degradation may be ascribed to the formation of an
epoxide.18d Similarly, the fatigue resistance of diarylethenes 1e5
was tested in PMMA films by alternatively irradiating with 297 nm
and visible light, which could be repeated for more than 200 times
with the degradation ca. 27% of 1c, 54% of 2c, 25% of 3c, 26% of 4c,
and 21% of 5c. As mentioned above, the fatigue resistance of 1e5 in
the relation between conformation and photochromic behaviors of
diarylethenes in crystalline phase, it is necessary to acquire diary-
lethene crystals. In this work, the single crystals of 1o, 2o, 3o, and
5o were obtained by slow evaporation of hexane solutions. Their
structures were provided by X-ray crystallographic analysis. Their
ORTEP drawings are shown in Fig. 5, and X-ray crystallographic
analysis data are listed in Table 2. The four derivatives crystallized
with photoactive anti-parallel conformation in crystalline phase.
For 1o, the double bond (C13eC17) of hexafluorocyclopentene ring
ꢀ
is 1.321 A, being significantly shorter than other carbonecarbon
ꢀ
ꢀ
single bonds (1.499 A to 1.550 A) of the ring. The naphthalene and

5542
R. Wang et al. / Tetrahedron 69 (2013) 5537e5544
Table 1
Absorption spectral properties of 1e5 in hexane (2.0ꢂ10ꢀ5 mol Lꢀ1) and in PMMA films (10%, w/w) at room temperature
Compound
lo,max/nma (ε/L molꢀ1 cmꢀ1
)
lc,max/nmb (ε/L molꢀ1 cmꢀ1
)
Fc
Conversion at
PSS in hexane
Hexane
PMMA
Hexane
PMMA
Foec
Fceo
film
film
1
2
3
4
5
264 (2.45ꢂ104)
262 (2.32ꢂ104)
263 (3.00ꢂ104)
264 (3.06ꢂ104)
264 (3.22ꢂ104)
265
288
265
266
265
513 (1.89ꢂ104)
511 (2.18ꢂ104)
510 (2.00ꢂ104)
514 (2.79ꢂ104)
514 (2.10ꢂ104)
521
519
519
526
528
0.49
0.47
0.23
0.42
0.55
0.25
0.24
0.12
0.09
0.13
55
65
62
52
78
a
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of cyclization reaction (Fo-c) and cycloreversion reaction (Fceo).
b
c
Table 2
thiophene moieties are linked by the C13]C17 double bond. The
dihedral angles between the hexafluorocyclopentene ring and the
two heteroaryl rings are 76.3ꢁfor C18eC27 and 64.0ꢁ for S1/C8eC11,
and that between the thiophene ring and the linked benzene ring is
3.7ꢁ. The corresponding data of compounds 2o, 3o, and 5o are
summarized in Table 3. The distance between the two reactive
Crystal data for diarylethenes 1o, 2o, 3o and 5o
Compounds
1o
2o
3o
5o
Formula
C28H17F6NS C27H17F7S C27H17ClF6S
513.49
296(2)
Monoclinic Monoclinic Orthorhombic Orthorhombic
P2(1)/c
C28H20F6OS
518.50
296(2)
Formula weight
Temperature
Crystal system
Space group
Unit cell
506.47
296(2)
522.92
293(2)
ꢀ
ꢀ
carbon atoms is 4.185 A for 1o (C11/C19), 4.484 A for 2o
ꢀ
ꢀ
(C10/C26), 4.104
A for 3o (C2/C28), and 4.017 A for 5o
P2(1)/c
Pbcn
Pbcn
13.9192(13)
(C11/C27). Based on the empirical rule that the molecule un-
dergoes the photocyclization reaction if the molecule is fixed in an
anti-parallel mode and the distance between reacting carbon atoms
20.291(9) 18.472(7) 14.309(2)
ꢀ
dimensions a (A)
ꢀ
b (A)
14.114(6) 14.477(5) 8.3799(13)
8.500(4)
90.00
100.796(6) 102.821(5) 90.00
90.00 90.00 90.00
2391.1(18) 2330.2(15) 4797.0(12)
8.4148(8)
41.020(4)
90.00
90.00
90.00
4804.5(8)
8
1.434
on the aryl rings is less than 4.2 A,29 crystals 2o should not exhibit
ꢀ
ꢀ
c (A)
8.936(3)
90.00
40.006(6)
90.00
a
b
g
(o)
(o)
(o)
photochromism but 1o, 3o and 5o could potentially display favor-
able photochromism by photoirradiation in the single crystalline
phase. In fact, the crystals 2o showed no photochromism in ac-
cordance with the expected analysis upon irradiation with UV light.
However, the crystals 1o, 3o and 5o also showed no photochro-
mism in the crystalline phase, in which was contrary to the theo-
retical analysis. That is to say, the four crystals showed no
photochromism in crystalline phase. This is verified by the fact that
irradiating crystals of 1o, 2o, 3o, and 5o with UV light for 2 h
resulted in no observable color change. When these crystals were
dissolved in hexane, the solutions showed colorless and their
3
ꢀ
Volume (A )
Z
4
4
8
Density (calcd) (g/cm3) 1.426
1.444
0.974
0.0730
0.1544
0.1087
0.1762
1.448
0.942
0.2475
0.3611
0.0961
0.2344
Goodness-of-fit on F2 1.062
0.952
Final R1[I>2s(I)]
wR2[I>2s(I)]
R1 (all data)
0.1319
0.2956
0.2173
0.3755
0.0708
0.1675
0.1089
0.1958
wR2 (all data)
Fig. 5. ORTEP drawings of crystals of 1o, 2o, 3o and 5o, showing about 30% probability displacement ellipsoids: (A) 1o, (B) 2o, (C) 3o, and (D) 5o.

R. Wang et al. / Tetrahedron 69 (2013) 5537e5544
5543
Table 3
with those in hexane solutions, the emission peaks of 1oe5o
showed a remarkable bathochromic shift in PMMA films, with the
values of 2 nm for 1, 12 nm for 2, 27 nm for 3, 20 nm for 4, and
20 nm for 5. The emission intensity of diarylethenes with electron-
withdrawing groups (cyano and fluorine, 1 and 2) was much
stronger than that of diarylethenes with electron-donating groups
(methyl and methoxy, 4 and 5) in hexane. Consequently, the
emission intensity of 1o is the strongest in both hexane and
a PMMA film. By using anthracene as the reference in acetonitrile,
the fluorescence quantum yields of 1oe5o were determined to
be 0.042 for 1o, 0.027 for 2o, 0.143 for 3o, 0.013 for 4o, and 0.012
for 5o. The results indicated that the electron-withdrawing sub-
stituents could effectively enhance the fluorescence quantum yield
of diarylethene with a naphthalene unit. This observation is well
agreement with that of diarylethenes with a pyridine moiety,19b
while contrary to that of reported diarylethenes with a benzene
moiety, whose fluorescence quantum yields increased with the
increase of the electron-donating ability.11b,27
q
(ꢁ) of di-
ꢀ
Distances between the reacting carbon atoms d (A) and dihedral angles
arylethenes 1o, 2o, 3o and 5o
q
(ꢁ)a
ꢀ
Compounds
d (A)
q1
q2
q3
1o
2o
3o
5o
C11/C19
C10/C26
C2/C18
4.185
4.484
4.104
4.017
76.3
96.7
75.3
70.8
64.0
79.0
58.5
53.3
3.7
1.1
4.8
7.0
C11/C27
a
q1, Dihedral angle between the cyclopentene ring and the naphthalene ring; q2
,
dihedral angle between the cyclopentene ring and the thiophene ring; q3, dihedral
angle between the thiophene ring and the adjacent benzene ring.
absorption spectra were the same as those of their open-ring iso-
mers. The results indicated that the diarylethene systems with
a naphthalene moiety showed no photochromism in crystalline
phase although in accordance with the reported empirical rule.
As has been observed for most reported diarylethenes,31 1e5
exhibited a notable fluorescence switch from the open-ring isomers
to the closed-ring isomers by photoirradiation in both hexane and
PMMA films. Upon irradiation with UV light, the fluorescence of 1e5
effectively decreased along with photocyclization due to the pro-
duction of the non-fluorescence closed-ring isomers 1ce5c. The
back irradiation with visible light of appropriate wavelength
regenerated the open-ring isomers 1oe5o and recovered original
emission intensity. The emission intensity changes of 1 by photo-
irradiation in both hexane (2.0ꢂ10ꢀ5 mol Lꢀ1) and a PMMA film
(10%, w/w) at room temperature are shown in Fig. 7. When arrived at
photostationary state, the emission intensity of 1 was quenched to
ca. 26% in hexane and 38% in a PMMA film. Its fluorescence modu-
lation efficiency in photostationary state was 74% in hexane and 62%
3.3. Fluorescence of diarylethenes
The fluorescence modulation is becoming an intriguing ap-
proach, which can be widely applied to molecular-scale optoelec-
tronics, ion-sensors, and digital photoswitches.12a,13a As most
reported diarylethenes,30 1e5 also exhibited notable fluorescence
in both solution (2.0ꢂ10ꢀ5 mol Lꢀ1) and PMMA films (10%, w/w).
Their fluorescence spectra were measured at room temperature
using a Hitachi F-4500 spectrophotometer, and the result is shown
in Fig. 6. In hexane, the emission peaks of 1oe5o were observed at
422, 419, 402, 418, and 415 nm, when excited at 298 nm. The
emission peaks of 1oe5o were observed at 424, 431, 429, 438, and
425 nm, when excited at 310 nm in PMMA films. In comparison
B
8000
6000
4000
2000
0
A
3200
DT-1o
DT-1o
DT-2o
DT-2o
2400
DT-5o
DT-5o
DT-4o
1600
DT-4o
DT-3o
DT-3o
800
0
350
350
400
450
500
550
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Fig. 6. Fluorescence emission spectra of 1e5 at room temperature: (A) in hexane (2.0ꢂ10ꢀ5 mol Lꢀ1), excited at 298 nm; (B) in PMMA films (10%, w/w), excited at 310 nm.
8000
B
A
3200
6000
Vis
2400
UV
Vis
UV
4000
2000
0
1600
800
0
400
450
500
550
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Fig. 7. Emission intensity changes of diarylethene 1 upon irradiation with UV light at room temperature: (A) in hexane (2.0ꢂ10ꢀ5 mol Lꢀ1), excited at 298 nm, (B) in a PMMA film
(10%, w/w), excited at 310 nm.

5544
R. Wang et al. / Tetrahedron 69 (2013) 5537e5544
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in a PMMA film. Similarly, the fluorescence modulation efficiencies
of the other four diarylethenes were 75% for 2, 43% for 3, 74% for 4,
and 76% for 5 in hexane, and were 63% for 2, 60% for 3, 69% for 4, and
70% for 5 in PMMA films in photostationary state. Therefore, the
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than those in PMMA films except for 3. The result is consistent with
that of diarylethenes with a pyrrole or thiazole moiety.17b,24 Com-
pared with diarylethenes bearing two thiophene moieties,32 the
fluorescence modulation efficiencies of 1e5 were significantly en-
hanced in both solution and solid media. Among 1e5, 5 showed
the largest fluorescence modulation efficiency in both solution
and a PMMA film, suggesting that it is the most promising candidate
for application in photoswitchable devices, such as optical
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4. Conclusions
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Five unsymmetrical diarylethenes with a naphthalene moiety
were synthesized, and their photochromic and fluorescent prop-
erties were investigated systematically. All of these diarylethene
derivatives exhibited favorable photochromism and acted as no-
table fluorescent switches in both solution and PMMA films. Dif-
ferent substituents at the meta-position of the terminal benzene
showed significant effects on the properties of these diarylethenes.
The naphthalene moiety induced some new features, which were
distinguishable from those of the reported diarylethenes bearing
benzene, thiophene, pyrazole, or pyrrole rings. The results will be
helpful in designing new diarylethene systems with tunable pho-
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This work was supported by the National Natural Science
Foundation of China (21162011, 21262015), the Project of Jiangxi
Academic and Technological leader (2009DD00100), the Project of
Jiangxi Advantage Sci-Tech Innovative Team (20113BCB24023), the
Project of Jiangxi Youth Scientist, the Project of the Science Funds of
Jiangxi Education Office (KJLD12035, GJJ11026, GJJ12587), and the
Science Funds of Jiangxi Science & Technology Normal University
(Ky2012zz03).
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