Photochromism of new 3,5-position hybrid diarylethene derivatives bearing both thiophene and thiazole moieties

Article abstract of DOI:10.1016/j.tet.2010.09.066

Five new diarylethenes based on a hybrid structure of bis(5-thiazolyl) ethene and bis(3-thienyl)ethene were synthesized, and the structures of the four compounds were determined by single-crystal X-ray diffraction analysis. The properties of these diaryle

Full text of DOI:10.1016/j.tet.2010.09.066

Tetrahedron 66 (2010) 8862e8871
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Photochromism of new 3,5-position hybrid diarylethene derivatives bearing both
thiophene and thiazole moieties
,
,
b,
Gang Liu a b, Shouzhi Pu a , Xiaomei Wang
*
*
a Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University, Fenlin Street, Nanchang 330013, PR China
b College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 June 2010
Received in revised form 17 September 2010
Accepted 20 September 2010
Available online 24 September 2010
Five new diarylethenes based on a hybrid structure of bis(5-thiazolyl)ethene and bis(3-thienyl)ethene
were synthesized, and the structures of the four compounds were determined by single-crystal X-ray
diffraction analysis. The properties of these diarylethenes, such as photochromism, fluorescence, and
electrochemical properties were investigated in detail. All of these compounds showed good photo-
chromism and fluorescence both in solution and in PMMA films. The electron-donating substituents
could effectively increase the cyclization and cycloreversion quantum yields, and the fluorescence
emission peaks, whereas the electron-withdrawing groups functionalized an inverse action for these
diarylethene derivatives. Cyclic voltammetry revealed that great differences existed amongst the elec-
trochemical behaviors of these compounds. The oxidation potentials and the band gaps of these diary-
lethenes increased remarkably with the increase in electron-withdrawing ability. All results suggested
that the effects of substitution have a significant effect on the photochemical and electrochemical be-
haviors of these diarylethene derivatives.
Keywords:
Photochromism
Diarylethene
Thiazole moiety
Substituent effect
Optical and electrochemical property
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
capacity is proportional to the recording laser wavelength.6 That is
to say, the shorter the absorption wavelength of diarylethene
Photochromic compounds have attracted much attention be-
cause of their potential application to photonic devices, such as
high-density optical recording materials and photoswitches.1
Among various types of photochromic compounds, diarylethene
derivatives with heterocyclic aryl rings are the most promising
candidates for photoelectronic applications because of the excel-
lent thermal stability of the respective isomers, notable fatigue
resistance, and rapid response, and high reactivity in solid state.2
Among the diarylethenes hitherto reported, most of them have
an absorption peak with the region between 550 and 750 nm
wavelengths,3 with just a few reports concerning photochromic
diarylethenes whose absorption peaks beyond this wavelength
range. The longest wavelength of diarylethene so far reported was
observed at 828 nm,4 and the shortest one showed its longest ab-
sorption bands in the UV light region (260 nm).5 For the application
to optical recording, it is very important to develop photochromic
diarylethenes with different absorption wavelengths, which can
expect to match perfectly with the wavelength of recording laser.
Especially, it is indispensable to prepare diarylethene derivatives
with a shorter absorption wavelength because the recording
contains, the higher recording density is expected to achieve be-
cause the light can be focused more sharply. Therefore, developing
diarylethene derivatives with an absorption band around 500 nm
or even shorter for making use of short-wavelength optical re-
cording materials is very necessary.
Guided by this aim, several approaches to shift the absorption
maximum of the closed-ring isomer to a shorter wavelength are to
attach the thiophene rings to the ethene moiety at the 2-posi-
tion.3,7,2d As we all known, the absorption spectrum is dependent on
the substituent effects8 and the
p
-conjugation length in a diary-
lethene molecule.9 In bis(2-thienyl)ethene photochromic system,
the -conjugation in closed-ring isomer is only localized in a cyclo-
p
hexadiene structure, shifting the absorption to shorter wavelength,
as compared with the closed-ring isomer of bis(3-thienyl)ethene
photochromic system where the p-conjugation extends throughout
the whole molecule.3,7b For instance, the absorption maximum of
the closed-ring diarylethene with a structure of 1,2-bis(2-thienyl)
ethene was observed at 425 nm,10 while that of the reported dia-
rylethene with a structure of 2,3-position hybrid thiophene rings
was observed at 469 nm.11 However, the absorption maximum of the
closed-ring isomer with a structure of 1,2-bis(3-thienyl)ethene was
observed at 534 nm.10,12 Compared with the parent analogues,
diarylethene derivatives with oxidized thiophene/benzothiophene
rings could also significantly shift their absorption band to a shorter
* Corresponding authors. Tel./fax: þ86 791 3831996; e-mail address: pushouzhi@
tsinghua.org.cn (S. Pu).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.09.066

G. Liu et al. / Tetrahedron 66 (2010) 8862e8871
8863
wavelength during the process of photocyclization.5,13,14 Another
attempt is to introduce the thiazole rings into the diarylethene
system as the aryl moieties.15 When the thiophene rings of 1,2-bis(2-
methyl-5-phenyl-3-thienyl)perfluorocyclopentene16 are replaced
with thiazole rings, the absorption maximum of the closed-ring
isomer shifts from 575 nm to 525 nm.15a,d Similarly, when the
thiophene rings of 1,2-bis(3,5-dimethyl-2-thienyl)perfluorocyc-
lopentene10,17 are replaced with thiazole rings, the absorption
maximum shifts from 432 nm to 391 nm.15a
From the reports described above, it can be easily concluded
that most of diarylethenes with thiazole rings reported to date are
symmetrical compounds or the thiazole ring is attached to the
ethene moiety at the 4-position. Report concerning diarylethenes
with 5-thiazolyl moieties is very rare. To the best of our knowledge,
diarylethenes with a hybrid structure of 5-thiazolyl and 3-thienyl
moieties have not hitherto been reported. In this study, we have
synthesized a new class of photochromic diarylethene derivatives
bearing both 5-thiazolyl and 3-thienyl moieties (1oe5o). All of
these diarylethenes showed good photochromism both in solution
and in PMMA amorphous films, and most of them also showed
good photochromism in the crystalline phase. The photochromic
scheme of diarylethenes 1oe5o is shown in Scheme 1.
a dry argon stream and maintained at a slight argon overpressure
during electrochemical experiments.
2.2. Synthesis of diarylethenes
The synthesis route for the diarylethenes 1oe5o is shown in
Scheme 2. Suzuki coupling of the five bromobenzene derivatives
with a thiophene boronic acid18 gave the alkylphenylthiophene
derivatives (7aee). Separately, 1-(2,4-dimethyl-5-thiazolyl)per-
fluorocyclopentene (10) was synthesized by bromination and lith-
iation reactions from 2,4-dimethylthiazole. Finally, compounds
7aee were separately lithiated and then coupled with compound
10 to give diarylethenes 1oe5o, respectively. The structures of
1oe5o were confirmed by elemental analysis, NMR, and IR.
7a: R =
7b: R =
7c: R =
7d: R =
H
Br
Br
OCH3
CH3
F
Br
R
S
B(OH)2
S
Pd(PPh3)4, Na2CO3,aq.
7e: R =
CN
R
6
F
F
F
F
N
Br2
N
9
n-BuLi/THF
C5F8, -78 oC
F
F
F
S
Br
S
S
10
8
N
F
F
F
F
F
F
F
F
F
F
F
F
1o: R =
2o: R =
H
OCH3
UV
Vis
F
F
n-BuLi/THF
-78 oC
F
F
F
F
3o: R = CH3
S
S
S
4o: R =
5o: R =
F
S
N
CN
S
R
S
N
N
R
R
Scheme 2. Synthetic route for the target compounds.
1c: R =
2c: R =
3c: R =
4c: R =
5c: R =
H
1o: R =
2o: R =
3o: R =
4o: R =
5o: R =
H
OCH3
CH3
F
OCH3
CH3
F
2.2.1. 3-Bromo-2-methyl-5-phenyl-thiophene (7a). This compound
was synthesized by the same method as that reported in a refer-
ence.15e Compound 7a was prepared by reacting 3-bromo-2-methyl-
5-thienylboronic acid18 (2.90 g, 13.10 mmol) with bromobenzene
(2.06 g,13.10 mmol) in the presence of Pd(PPh3)4 (0.27 g, 0.23 mmol)
and Na2CO3 (6.36 g, 60.00 mmol) in tetrahydrofuran (THF) (80 mL
containing 10% water). After refluxing for 15 h at 70 ꢂC, the product
was allowed to slowly warm to the room temperature and then
extracted with ether. The organic layer was collected and dried over
MgSO4, filtrated, and evaporated. The crude product was purified by
column chromatography on SiO2 using hexane as an eluent resulting
in 2.76 g of 7a being obtained as a buff solid in 83% yield. Mp
CN
CN
Scheme 1. Photochromism of diarylethenes 1e5.
2. Experimental
2.1. General methods
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 tet-
ramethylsilane as an internal standard. Infrared spectra (IR) were
recorded on a Bruker Vertex-70 spectrometer. Elemental analysis
was measured with PE CHN 2400 analyzer. Melting point was taken
on a WRS-1B melting point apparatus. Absorption spectra were
measured using an Agilent 8453 UV/vis spectrophotometer. Photo-
irradiation was carried out using a SHG-200 UV lamp, CX-21 ultra-
violet 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. Electrochemical examinations were performed
in a one-compartment cell by using a Model 263 potentios-
tategalvanostat (EG&G Princeton Applied Research) under com-
puter control at room temperature. Platinum wires (diameter
0.5 mm) served as the working electrode and counter electrode.
Platinum wire (diameter 0.5 mm) in the supporting electrolyte so-
lution served as a quasi-reference electrode, which was calibrated
using an internal ferrocene (Fc/Fþc ) standard with a formal potential
of E1/2¼þ0.35 V versus platinum wire in the same electrolyte. The
typical electrolyte was acetonitrile (5 mL) containing 0.1 mol/L tet-
rabutylammonium tetrafluoroborate((TBA)BF4) and1.0ꢀ10ꢁ3 mol/L
diarylethene sample. All solutions were deaerated by bubbling with
66e68 ꢂC; 1H NMR (400 MHz, CDCl3):
d 2.34 (s, 3H, eCH3), 7.02 (s,
1H, thienyl-H), 7.20 (d, 1H, J¼8.0 Hz, phenyl-H), 7.29 (t, 2H, J¼8.4 Hz,
phenyl-H), 7.42 (d, 2H, J¼8.0 Hz, phenyl-H).
2.2.2. 3-Bromo-2-methyl-5-(4-methoxyphenyl)thiophene
(7b). Compound 7b was prepared by a method similar to that used
for 7a. The crude product was purified by column chromatography
on SiO2 using petroleum ether as an eluent to give 7b (2.63 g, 79%) as
a buff solid. Mp 106e107 ꢂC; 1H NMR (400 MHz, CDCl3):
d 2.41 (s, 3H,
eCH3), 3.91 (s, 3H, eOCH3), 6.95e6.99 (m, 2H, phenyl-H), 7.27 (s, 1H,
thienyl-H), 7.30 (s, 1H, phenyl-H), 7.54 (d, 1H, J¼8.0 Hz, phenyl-H).
2.2.3. 3-Bromo-2-methyl-5-(4-methylphenyl)thiophene
(7c). Compound 7c was prepared by a method similar to that used
for 7a. The crude product was purified by column chromatography
on SiO2 using petroleum ether as an eluent to give 7c (2.66 g, 80%)
as a buff solid. Mp 68e69 ꢂC; 1H NMR (400 MHz, CDCl3, TMS):
d 2.37
(s, 3H, eCH3), 2.41 (s, 3H, eCH3), 7.01 (s, 1H, thienyl-H), 7.19 (d, 2H,
J¼8.0 Hz, phenyl-H), 7.38 (d, 2H, J¼8.0 Hz, phenyl-H).
2.2.4. 3-Bromo-2-methyl-5-(4-fluorophenyl)thiophene
(7d). Compound 7d was prepared by a method similar to that used

8864
G. Liu et al. / Tetrahedron 66 (2010) 8862e8871
for 7a. The crude product was purified by column chromatography
on SiO2 using petroleum ether as an eluent to give 7d (2.54 g, 77%)
as a baby-yellow solid. Mp 61e63 ꢂC; 1H NMR (400 MHz, CDCl3,
prepared by a method similar to that used for 1o. The crude product
was purified by column chromatography on SiO2 using petroleum
ether as an eluent to give 2o (0.16 g, 52%) as a white solid. Mp
71e72 ꢂC; calcd for C22H17F6NOS2 (%): calcd C, 53.98; H, 3.50; N,
2.86. Found C, 54.04; H, 3.61; N, 2.82; 1H NMR (400 MHz, CDCl3,
TMS):
d
2.45 (s, 3H, eCH3), 7.18 (s, 1H, thienyl-H), 7.50 (t, J¼7.6 Hz,
1H, phenyl-H), 7.56 (d, 1H, J¼8.0 Hz, phenyl-H), 7.73 (d, 1H,
J¼8.0 Hz, phenyl-H), 7.79 (s, 1H, phenyl-H).
TMS):
d 1.89 (s, 3H, eCH3), 1.91 (s, 3H, eCH3), 2.61 (s, 3H, eCH3),
3.77 (s, 3H, eOCH3), 6.84 (d, 2H, J¼7.6 Hz, phenyl-H), 7.04 (s, 1H,
2.2.5. 3-Bromo-5-(4-cyanophenyl)-2-methylthiophene
(7e). Compound 7e was prepared by a method similar to that used
for 7a. The crude product was purified by column chromatography
on SiO2 using petroleum ether as an eluent to give 7e (2.33 g, 74%)
as a yellow solid. Mp 79e80 ꢂC; 1H NMR (400 MHz, CDCl3, TMS):
thienyl-H), 7.40 (d, 2H, J¼8.0 Hz, phenyl-H); 13C NMR (100 MHz,
CDCl3, TMS):
d 14.02, 16.13, 18.71, 54.90, 113.42, 113.92, 116.48,
120.69, 124.78, 125.58, 126.48, 130.88, 139.84, 142.18, 154.26, 159.10,
168.09; IR (
n
, KBr, cmꢁ1): 738, 790, 833, 891, 985, 1059, 1106, 1136,
1272, 1338, 1438, 1472, 1514, 1612.
d
2.41 (s, 3H, eCH3), 7.03 (s, 1H, thienyl-H), 7.06 (t, 2H, J¼8.0 Hz,
phenyl-H), 7.44e7.48 (m, 2H, phenyl-H).
2.2.10. 1-(2,4-Dimethyl-5-thiazolyl)-2-[2-methyl-5-(4-methyl-
phenyl)-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 an eluent to give 3o (0.31 g, 54%) as a yellow solid. Mp
87e89 ꢂC; calcd for C22H17F6NS2 (%): calcd C, 55.81; H, 3.62; N, 2.96.
Found C, 55.90; H, 3.78; N, 2.92; 1H NMR (400 MHz, CDCl3, TMS):
2.2.6. 5-Bromo-2,4-dimethylthiazole (9). Bromine in acetic acid
(v/v¼29/40) 69 mL was slowly added to a stirred solution of 2,4-
dimethylthiazole (3.00 g, 26.50 mmol) in acetic acid (100 mL) at
0
ꢂC. The reaction mixture was stirred overnight at this temper-
ature. The reaction was quenched by the addition of water 20 mL.
The mixture was neutralized to pHꢃ9.0 with Na2CO3 solid and
extracted with ether. The ether extract was dried, filtrated, and
evaporated in vacuo. The residue was purified by column chro-
matography on SiO2 using trichloromethane as an eluent to give
compound 9 (4.24 g, 83%) as a red solid. Mp 135e136 ꢂC; 1H NMR
d
1.90 (s, 6H, eCH3), 2.30 (s, 3H, eCH3), 2.61 (s, 3H, eCH3), 7.12 (s,
1H, thienyl-H), 7.14 (s, 2H, phenyl-H), 7.37 (d, 2H, J¼8.0 Hz, phenyl-
H); 13C NMR (100 MHz, CDCl3, TMS):
13.50, 14.54, 16.61, 19.20,
116.94, 121.77, 125.30, 125.30, 129.66, 130.47, 138.01, 140.76, 142.90,
154.76, 168.59; IR (
, KBr, cmꢁ1): 740, 815, 841, 893, 987, 1026, 1058,
d
n
(400 MHz, CDCl3, TMS):
d
2.46 (s, 3H, eCH3), 2.79 (s, 3H, eCH3).
1105, 1137, 1189, 1290, 1336, 1434, 1476, 1521, 1641, 1807.
2.2.7. 1-(2,4-Dimethyl-5-thiazolyl)perfluorocyclopentene
(10). Compound 9 (4.90 g, 25.50 mmol) in anhydrous THF (80 mL)
was added dropwise to a 2.5 M n-BuLi/hexane solution (10.80 mL)
at ꢁ78 ꢂC under a nitrogen atmosphere. Stirring was continued for
30 min at this low temperature. Perfluorocyclopentene (C5F8,
3.50 mL, 25.50 mmol) was slowly added to the reaction mixture at
ꢁ78 ꢂC, and the mixture was stirred for another 2 h at this low
temperature. The reaction was quenched by the addition of
methanol (5 mL). The product was extracted for three times with
ether (150 mL). The organic layer was washed with 1 M aqueous
NaCl and water, respectively. The organic layer was dried over
MgSO4, filtrated, and evaporated. The crude product was purified
by column chromatography on silica gel using hexane as an eluent
to give 10 (1.11 g, 59%) as a yellow oil. 1H NMR (400 MHz, CDCl3,
2.2.11. 1-(2,4-Dimethyl-5-thiazolyl)-2-[2-methyl-5-(4-fluorophenyl)-
3-thienyl]perfluorocyclopentene (4o). Diarylethene 4o was pre-
pared by a method similar to that used for 1o. The crude product
was purified by column chromatography on SiO2 using hexane as
an eluent to give 4o (0.61 g, 57%) as a white solid. Mp 107e109 ꢂC;
calcd for C21H14F7NS2 (%): calcd C, 52.83; H, 2.96; N, 2.93. Found C,
52.89; H, 3.02; N, 2.97; 1H NMR (400 MHz, CDCl3, TMS):
d 1.91 (s,
6H, eCH3), 2.62 (s, 3H, eCH3), 7.02 (t, 2H, J¼8.0 Hz, phenyl-H), 7.09
(s, 1H, thienyl-H), 7.43(t, 2H, J¼8.0 Hz, phenyl-H); 13C NMR
(100 MHz, CDCl3, TMS):
d 14.01, 16.16, 18.74, 115.62, 116.40, 121.80,
124.95, 126.88, 129.02, 140.81, 141.16, 154.28, 160.84, 163.31, 168.22;
IR (n
, KBr, cmꢁ1): 738, 820, 846, 897, 948, 987, 1026, 1057, 1106, 1187,
1271, 1335, 1434, 1492, 1551, 2218.
TMS):
d
2.51 (s, 3H, eCH3), 2.86 (s, 3H, eCH3).
2.2.12. 1-(2,4-Dimethyl-5-thiazolyl)-2-[2-methyl-5-(4-cyano-
phenyl)-3-thienyl]perfluorocyclopentene (5o). Diarylethene 5o was
prepared by a method similar to that used for 1o. The crude product
was purified by column chromatography on SiO2 using hexane as
an eluent to give 5o (0.59 g, 47%) as a white solid. Mp 125e127 ꢂC;
calcd for C22H14F6N2S2 (%): calcd C, 54.54; H, 2.91; N, 5.78. Found C,
2.2.8. 1-(2,4-Dimethyl-5-thiazolyl)-2-(2-methyl-5-phenyl-3-thienyl)
perfluorocyclopentene (1o). To a stirred anhydrous THF containing 7a
(0.45 g, 1.77 mmol) was added dropwise a 2.5 mol/L n-BuLi/hexane
solution (0.75 mL, 1.87 mmol) at ꢁ78 ꢂC under argon atmosphere.
After the mixture has been stirred for 30 min, compound 10 (0.52 g,
1.70 mmol) in solvent of anhydrous THF was added. The reaction was
further stirred at ꢁ78 ꢂC 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 ether, dried
over MgSO4, and concentrated under reduced pressure. The crude
product was purified by column chromatography using petroleum
ether as an eluent to afford to 0.38 g in 49% yield of 1o as white solid.
Mp 106e107 ꢂC; calcd for C21H15F6NS2 (%): calcd C, 54.89; H, 3.29; N,
3.05. Found C, 54.93; H, 3.34; N, 3.08; 1H NMR (400 MHz, CDCl3,
54.64; H, 2.97; N, 5.83; 1H NMR (400 MHz, CDCl3, TMS):
d 1.92 (s,
3H, eCH3),1.96 (s, 3H, eCH3), 2.62 (s, 3H, eCH3), 7.27 (s,1H, thienyl-
H), 7.56 (d, 2H, J¼8.0 Hz, phenyl-H), 7.61 (d, 2H, J¼7.6 Hz, phenyl-
H); 13C NMR (100 MHz, CDCl3, TMS):
d 14.17, 16.16, 18.71, 112.40,
116.21, 118.01, 124.01, 125.41, 132.34, 136.88, 139.89, 143.01, 154.36,
168.40; IR (
n
, KBr, cmꢁ1): 741, 792, 836, 891, 984, 1057, 1104, 1136,
1188, 1272, 1330, 1435, 1470, 1517, 1606, 2231, 2922.
2.3. Determination of crystal structures
TMS):
d
2.01 (s, 6H, eCH3), 2.71 (s, 3H, eCH3), 7.26 (s, 1H, thienyl-H),
Suitable crystals of 1o, 2o, 4o, and 5o were obtained by slow
evaporation of a hexane solution. All the crystal data were made on
a Bruker SMART APEX II CCD diffractometer using a MULTI scan
technique at room temperature. 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. The
7.33 (t, 1H, J¼8.0 Hz, phenyl-H), 7.42 (t, 2H, J¼8.0 Hz, phenyl-H), 7.57
(d, 2H, J¼7.6 Hz, phenyl-H); 13C NMR (100 MHz, CDCl3, TMS):
d 14.09,
16.14, 18.72, 116.43, 121.82, 124.90, 125.16, 127.52, 128.52, 132.72,
140.81, 142.26, 154.28, 168.17; IR (n
, KBr, cmꢁ1): 696, 737, 761, 790,
841, 896, 988, 1060, 1106, 1138, 1189, 1273, 1341, 1437, 1471, 1623.
linear absorption coefficients,
for Mo K
radiation are 3.22, 3.11, 3.31, and 3.15 cmꢁ1. All non-hy-
drogen atoms were refined anisotropically. All hydrogen atoms were
m, of diarylethenes 1o, 2o, 4o, and 5o
2.2.9. 1-(2,4-Dimethyl-5-thiazolyl)-2-[2-methyl-5-(4-methox-
yphenyl)-3-thienyl]perfluorocyclopentene (2o). Diarylethene 2o was
a

G. Liu et al. / Tetrahedron 66 (2010) 8862e8871
8865
generated geometrically with CeH bond distances of 0.93e0.97 Å
3. Results and discussion
according to the criteria described in the SHELXTL manual. They were
included in the refinement with Uiso(H)¼1.2Ueq(C) or 1.5Ueq(methyl
C). The X-ray crystallographic analysis data are listed in Table 1. Fur-
ther details on the crystal structures have been deposited in the
Cambridge Crystallographic Data Centre as supplemental publication
CCDC 774964 for 1o, CCDC 774967 for 2o, CCDC 774965 for 4o, and
CCDC774966 for 5o. Copies of thedatacanbeobtained, freeofcharge,
on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
þ44 1223 336033 or email: deposit@ccdc.cam.ac.uk).
3.1. Photochromic behaviors
The photochromic behaviors of diarylethenes 1e5 induced by
photoirradiation were measured at room temperature both in
hexane (2.0ꢀ10ꢁ5 mol/L) and in PMMA amorphous films (10%, w/
w). Fig. 1 shows the changes in the absorption spectra and color of
diarylethenes 1e5 in hexane, induced by alternating irradiation
with UV and visible light of appropriate wavelength. As shown in
Table 1
Crystal data for diarylethenes 1o, 2o, 4o, and 5o
Compound 1o
Compound 2o
Compound 4o
Compound 5o
Formula
C21H15F6NS2
459.46
296(2)
Monoclinic
P2(1)/c
C22H17F6NOS2
489.49
296(2)
Triclinic
Pꢁ1
C21H14F7NS2
477.45
296(2)
Monoclinic
P2(1)/c
C22H14F6N2S2
484.47
296(2)
Triclinic
Pꢁ1
Formula weight
Temperature
Crystal system
Space group
Unit cell dimensions
a (Å)
13.622(7)
18.270(10)
8.557(5)
90.00
106.637(6)
90.00
8.6535(15)
11.454(2)
12.138(2)
110.388(2)
103.990(2)
91.912(2)
1085.0(3)
2
13.588(5)
18.323(7)
8.589(3)
90.00
106.594(4)
90.00
8.7185(16)
11.360(2)
11.846(2)
107.661(2)
105.759(2)
93.836(2)
1061.6(4)
2
b (Å)
c (Å)
a
b
g
(ꢂ)
(ꢂ)
(ꢂ)
Volume (Å3)
2040.4(19)
4
2049.5(13)
4
Z
Reflections collected
Reflections observed
Number of parameters
14989
3803
274
0.322
1.496
8341
4011
293
0.311
1.498
1.017
0.71073
0.0442
0.1016
0.0794
0.1213
14712
3725
283
0.331
1.547
8115
3926
292
0.315
1.516
1.031
0.71073
0.0346
0.0914
0.0408
0.0964
m
(mmꢁ1
)
Density (calcd) (g/cm3)
Goodness-of-fit on F2
Radiation (Å)
1.042
1.456
0.71073
0.0328
0.0805
0.0426
0.0867
0.71073
0.1731
0.3899
0.2270
0.4278
Final R1[I>2s(I)]
wR2[I>2s(I)]
R1 (all data)
wR2 (all data)
Fig. 1. Absorption spectral change of diarylethene 1 (A) and color changes of diarylethenes 1e5 (B) upon alternating irradiation with UV and visible light in hexane (2.0ꢀ10ꢁ5 mol/L)
at room temperature.

8866
G. Liu et al. / Tetrahedron 66 (2010) 8862e8871
Fig. 1A, compound 1o exhibited a sharp absorption peak at 291 nm
in hexane, as a result of a
transition.11 Upon irradiation with
hydrogen atom at the para-position of the terminal benzene ring
with an electron-donating substituent (methoxy or methyl group),
the cyclization, and cycloreversion quantum yields increase evi-
dently with the increase in electron-donating ability in hexane.
When the same position instead contains an electron-withdrawing
substituent (fluorine or cyano group), the quantum yields decrease
to some extent with the increase in electron-withdrawing ability.
As a result, both the cyclization and cycloreversion quantum yields
of diarylethenes 2 and 3 bearing electron-donating groups are
much higher than those of 4 and 5 bearing electron-withdrawing
groups. The result is quite different from those reported for dia-
rylethenes bearing a phenyl moiety, where the cyclization quan-
tum yield significantly increased with the increase of electron-
withdrawing ability and cycloreversion quantum yield showed the
reverse trend.19
For practical applications in optical devices, it is very important
that photochromic materials can keep good photochromism in
a polymer film, such as PMMA.1b,21 Dissolved ultrasonically 10 mg
diarylethene sample and 100 mg PMMA into 1.0 mL chloroform, the
filmwas prepared by spin-coating on the surface of quartz substrate.
In PMMA amorphous film, diarylethenes 1e5 also showed good
photochromism similar to samples in solution. The absorption
spectral change of 1 and colorchanges of 1e5 are shownin Fig. 2, and
the data are listed in Table 2. Upon irradiation with 297 nm light, the
colors of the diarylethene/PMMA films 1e5 changed from colorless
to red, with the appearance of anewbroad absorption bandcentered
at 496, 499, 498, 496, and 505 nm, respectively, which was assigned
p
/p
*
297 nm light, a new visible absorption band centered at 483 nm
emerged while the original peak at 291 nm decreased, indicating
the formation of the closed-ring isomer 1c. This could be seen with
the naked eye, as the colorless solution of 1o turned red. Alterna-
tively, the red colored solution could be bleached completely to
colorless upon irradiation with visible light (l>450 nm), indicating
that 1c returned to the initial state 1o. As same as diarylethene 1,
diarylethenes 2oe5o also showed photochromism in hexane.
Upon irradiation with 297 nm light, absorption bands in visible
region appeared and the solutions 2oe5o turned red (Fig. 1B) as
a result of the cyclization reactions leading to produce the closed-
ring isomers 2ce5c, which their absorption maxima were observed
at 491, 489, 485, and 496 nm, respectively. All the solutions of
2ce5c can be decolorized by irradiation with appropriate wave-
length visible light, indicating return to the open-ring isomers
2oe5o. The absorption spectral features of these compounds are
summarized in Table 2. The results showed that different sub-
stituents at the para-positions of the benzene ring had a significant
effect on the photochromic features of diarylethenes 1e5, in-
cluding the absorption maxima, molar absorption coefficients, and
quantum yields. In hexane, the unsubstituted parent compound 1
has the smallest absorption maximum among diarylethenes 1e5,
both for its open-ring and closed-ring isomers. No matter replacing
the hydrogen atom at the para-position of the terminal benzene
ring with an electron-donating (methoxy or methyl group, such as
in compound 2 or 3) or with an electron-withdrawing substituent
(fluorine or cyano group, such as in compound 4 or 5), the ab-
sorption maxima have notably increased with the increase in both
electron-donating and electron-withdrawing ability. That is to say,
the stronger the electron-donating/withdrawing ability of the
substituent is, the greater the absorption maximum of diary-
lethene is achieved both in the open-ring and the closed-ring iso-
mers. Therefore, the absorption maximum of 2 is greater than that
of diarylethene 3 due to the stronger electron-donating ability of
methoxy group as compared to a methyl group, and the similar
change occurs in between 5 and 4 because of the stronger electron-
withdrawing ability of cyano group as compared to a fluorine
group. The result is in agreement with that of diarylethenes bearing
a six-membered aryl unit.19 In addition, all the absorption maxima
of the closed-ring isomers 1ce5c were observed at 480e500 nm,
which were much shorter than that of diarylethenes with the
similar molecular skeleton including thiophene, pyrazole or ben-
zene moiety.19,20 This indicated that the thiazole moiety could be
effective to shift the absorption maximum of diarylethene to
a shorter wavelength. As shown in Table 2, the cyclization and
cycloreversion quantum yields of the unsubstituted parent com-
pound 1 are 0.39 and 0.11, respectively. When replacing the
Table 2
Absorption spectral properties of diarylethenes 1e5 in hexane at 2.0ꢀ10ꢁ5 mol/L
Compound lo,max (nm)a
/L molꢁ1 cmꢁ1
Hexane
lc,max (nm)b
Fc
(3
)
(
3
/L molꢁ1 cmꢁ1
)
PMMA Hexane
PMMA Foꢁc Fcꢁo
film
film
1
2
3
4
5
291 (1.9ꢀ104) 297
297 (2.3ꢀ104) 302
293 (1.8ꢀ104) 297
292 (1.8ꢀ104) 294
310 (3.2ꢀ104) 313
483 (8.1ꢀ103) 496
491 (1.1ꢀ104) 499
489 (7.0ꢀ103) 498
485 (7.3ꢀ103) 496
496 (1.2ꢀ104) 505
0.39 0.11
0.43 0.17
0.41 0.15
0.23 0.11
0.21 0.07
a
b
c
Absorption maxima of open-ring forms.
Absorption maxima of closed-ring forms.
Quantum yields of cyclization reaction
Fig. 2. Absorption spectral change of diarylethene 1 (A) and color changes of diary-
lethenes 1e5 (B) upon alternating irradiation with UV and visible light in PMMA film
(10%, w/w) at room temperature.
(
Foꢁc
)
and cycloreversion reaction
(
Fcꢁo), respectively.

G. Liu et al. / Tetrahedron 66 (2010) 8862e8871
8867
to the formation of the closed-ring isomers 1ce5c. All colored dia-
rylethene/PMMA films can revert to colorless upon irradiation with
visible light (l>450 nm). As has been observed for most of the
C12eC16eC17eC18 [ꢁ43.7(3)ꢂ] and are thus trans with respect to
the double bond. This kind conformation is crucial for the compound
to exhibit photochromic and photo-induced properties.25 The
reported diarylethenes,22 the maximum absorption peaks of both
the open-ring and the closed-ring isomers in PMMA film are longer
than those in hexane solution. The red shift values of the absorption
maxima for the open-ring isomers are 6 nm for 1o, 5 nm for 2o, 4 nm
for 3o, 2 nm for 4o, and 3 nm for 5o, and those for the closed-ring
isomers are 13 nm for 1c, 8 nm for 2c, 9 nm for 3c, 11 nm for 4c, and
9 nm for 5c, respectively. The red shift phenomenamaybe attributed
to the polar effect of the polymer matrix and the stabilization of
molecular arrangement in solid state.23
Single crystals of diarylethenes 1o, 2o, 4o, and 5o were obtained
byslowevaporation ofahexanesolution. Toknow bettertherelation
between the conformation and the photochromic behaviors of these
diarylethene derivatives in the crystalline phase, their final struc-
tural confirmations were provided by X-ray crystallographic analy-
sis. The ORTEP drawings of the single crystals are shown in Fig. 3.
These molecules crystallize with an appropriate C2 symmetry with
intramolecular distance between the two reactive C atoms
(C10eC18) is 3.593(4) Å. The corresponding data of compounds 2o,
4o, and 5o are summarized in Table 3. For these compounds, both of
them also adopt the anti-parallel conformation in the crystalline
phase, and the distance between the two reactive C atoms is 3.589
(2) Å for 2o, 3.554(7) Å for 4o, and 3.562 (3) Å for 5o. As a result, all
molecules of crystals 1o, 2o, 4o, and 5o are fixed in an anti-parallel
mode in the crystalline phase and the distances of the two reactive C
atoms are less than 4.2 Å, which is close enough for the photo-
cyclization reaction totake place.24c,25,26 Infact, crystalsof1o, 2o, 4o,
and 5o showed good photochromism, in accordance with the
expected ring closure, to form 1c, 2c, 4c, and 5c upon irradiationwith
UV light. The color changes of crystals 1o, 2o, 4o, and 5o upon
photoirradiation in the crystalline phase are shown in Fig. 4. The
colorless crystals of these compounds turned red upon irradiation
with 297 nm UV light. When the red crystals were dissolved in
Fig. 3. ORTEP drawings of crystals 1o, 2o, 4o, and 5o, showing about 30% probability displacement ellipsoids: (A) 1o, (B) 2o, (C) 4o, (D) 5o.
Table 3
the photoactive anti-parallel conformation in the crystalline phase,
Distances between the reacting carbon atoms (d, Å) and dihedral angles (q,
ꢂ) of 1o,
which can undergo photocyclization.24 For diarylethene 1o, the di-
2o, 4o, and 5o
hedral angles between the hexafluorocyclopentene ring and the two
Compound
d (Å)
q
(ꢂ)a
heteroaryl rings are 46.4(3)ꢂ for S1/C7-C10, and 42.0(3)ꢂ for S2/C17,
C18/N1/C20, and that between the thiophene ring and the linked
benzene ring is 36.9(3)ꢂ. In hexafluorocyclopentene ring, the dis-
tances clearly show that the C12eC16 bond (1.353(2) Å) is a double
bond, being significantly shorter than other carbonecarbon single
bonds (1.505(3) Åe1.531(3) Å) of the ring. The thiophene and thia-
zole moieties are linked by the C12]C16 double bond. The two
methyl groups are located on opposite sides of the double bond,
reflected in the torsion angles C16eC12eC9eC10 [ꢁ42.7(3)ꢂ] and
q1
q2
q3
1o
2o
4o
5o
C10/C18
C11/C19
C6/C19
C7/C11
3.593(4)
3.589(2)
3.554(7)
3.562(3)
42.0(3)
36.5(2)
41.2(5)
37.8(6)
46.4(3)
46.4(2)
46.4(5)
47.0(6)
36.9(3)
28.8(2)
35.9(5)
28.4(6)
a
q1, Dihedral angle between the hexafluorocyclopentene ring and 2,4-dime-
thylthiazole ring; q2, dihedral angle between the hexafluorocyclopentene ring and
the thiophene ring attached a benzene ring; q3, dihedral angel between the thio-
phene ring and the adjacent benzene ring.

8868
G. Liu et al. / Tetrahedron 66 (2010) 8862e8871
hexane. After 200 repeat cycles, diarylethene 1 still showed good
photochromism with only ca. 6% degradation of 1c in PMMA film.
Similarly, the fatigue resistances of diarylethenes 2e5 in PMMA films
are also much stronger than those in solution. After 200 repeat cycles,
they showed good photochromism with only ca. 10% degradation of
2c, 13% of 3c, 19% of 4c, and 22% of 5c, respectively. But, the degra-
dation percents of diarylethenes 2e5 in hexane are 23% of 2c, 18% of
3c, 32% of 4c, and 39% of 5c, respectively. This remarkable improve-
ment may result from effectively suppressing the oxygen diffusion in
thesolidmedium.2g Theresultindicatedthatthefatigueresistancesof
diarylethenes with an electron-donating group (such as in 2 and 3)
are much better than those of diarylethenes with an electron-with-
drawing group (such asin 4 and 5) both in hexane and in PMMA films.
The result is contrary to that of diarylethenes bearing both thiazole
and benzene moieties whose fatigue resistance markedly increased
with the increase in electron-withdrawing ability.30
Fig. 4. Photographs of photochromic processes of diarylethenes 1, 2, 4, and 5 in the
crystalline phase.
3.2. Fluorescence of diarylethenes
Fluorescent properties can be useful not only in molecular-scale
optoelectronics, but for digital photoswitching of fluorescence.31
The fluorescence modulation is a particularly intriguing approach
due to the stabilization of diarylethene and versatility in materials
selection.32 Until now, many diarylethene derivatives and their
fluorescent properties have been reported.33 In this work, the
fluorescence properties of the five diarylethenes both in solution
and in PMMA film were measured using a Hitachi F-4500 fluo-
rimeter. The fluorescence emission spectra of 1oe5o at room
temperature are illustrated in Fig. 6. When exited at 330 nm, the
emission peaks of diarylethenes 1oe5o were observed at 429, 448,
434, 424, and 411 nm in hexane, and were observed at 435, 458,
437, 426, and 419 nm in PMMA films. Compared to those in hexane,
the emission peaks of diarylethenes 1oe5o showed a remarkable
bathochromic shift in PMMA film consistently across their maxima
absorption wavelengths with values of 6 nm for 1, 10 nm for 2, 3 nm
for 3, 2 nm for 4, and 8 nm for 5, respectively. When going from
electron-donating to electron-withdrawing groups, the emission
peaks of diarylethenes 1oe5o gradually decreased both in hexane
(from 448 to 411 nm) and in PMMA films (from 458 to 419 nm), and
their emission intensity also decreased evidently with the increase
in electron-withdrawing ability. Therefore, the emission peak and
the emission intensity of 2o are both the biggest and those of 5o are
the smallest in hexane and in PMMA film. The result is completely
contrary to that of diarylethenes bearing a biphenyl moiety.34
Diarylethenes 1e5 exhibited a very good fluorescent switch on
changing from the open-ring isomers to closed-ring isomers by
photoirradiation both in hexane and in PMMA film. When irradi-
ated by UV light, the photocyclization reaction was occurred and
the emission intensity of diarylethenes 1oe5o decreased
hexane, an intense absorption maximum was observed at the same
wavelength as thatof their respective ring-closed isomer in solution.
Alternatively, the red-colored crystals returned to colorless upon
irradiation with the appropriate visible light (l>450 nm). Further-
more, these diarylethene crystals exhibited remarkable fatigue-re-
sistance greater than 200 cyclization/cycloreversion repeated cycles
and their ring-closed isomers remained stable for more than 1000 h
in the dark at room temperature. So, they will be good candidates for
the construction of certain optoelectronic devices.27
The thermal stabilities of the open-ring and closed-ring isomers of
diarylethenes 1e5 were examined in hexane both at room temper-
ature and at 80 ꢂC. Stored these solutions in the dark and then ex-
posing them to air for more than 1000 h at room temperature, no
changesintheUV/visspectraofdiarylethenes1e5wereobservedand
indeed, no decomposition was detected when the five compounds
were exposed to air for more than a year. At 80 ꢂC, diarylethenes 1e5
still showed good thermal stabilities for more than 100 h. Fatigue-
resistant property, i.e., how many times photocyclization and cyclo-
reversion reaction cycles can be repeated without loss of perfor-
mance, is a very important factor for practical applications in optical
devices.2g,28 The fatigue resistances of diarylethenes 1e5 were ex-
amined both in hexane and in PMMA films at room temperature, as
shown in Fig. 5. Diarylethenes 1e5 were irradiated alternatively by
297 nm UV and visible light (l>450 nm), respectively. The irradiation
time was long enough for coloration to reach the photostationary
state and for the color to be completely bleached. In hexane, 27% of 1c
was destroyed after 20 repeat cycles, which may be attributed to
degradation resulting from the formation of an epoxide.29 However,
its fatigue resistance in PMMA film is much stronger than that in
A
B
1.0
1.0
0.8
0.8
0.6
0.6
1
1
2
0.4
2
3
3
0.4
4
4
0.2
5
5
0.2
0.0
0
4
8
12
16
20
0
40
80
120
160
200
Repeat cycles
Repeat cycles
Fig. 5. Fatigue resistance of diarylethenes 1e5 in hexane (A) and in PMMA film (B) in air atmosphere at room temperature. Initial absorbance of the sample was fixed to 1.0.

G. Liu et al. / Tetrahedron 66 (2010) 8862e8871
8869
A
B
3500
3000
2400
1800
1200
600
3o
3o
1o
2800
1o
2o
2o
4o
5o
2100
4o
5o
1400
700
0
0
400
450
500
550
600
390
420
450
480
510
540
570
Wavelength(nm)
Wavelength(nm)
Fig. 6. Fluorescence emission spectra of diarylethenes 1e5 both in hexane solution (1.0ꢀ10ꢁ4 mol/L) (A) and in PMMA film (10%, w/w) (B) at room temperature, excited at 330 nm.
significantly due to producing the non-fluorescence closed-ring
isomers 1ce5c. The back irradiation by appropriate wavelength
visible light regenerated their open-ring isomers 1oe5o and re-
covered the original emission intensity. Fig. 7 shows the fluores-
cence changes of diarylethene 1 by photoirradiation in hexane
(1.0ꢀ10ꢁ4 mol/L) and in PMMA film (10%, w/w). As shown in Fig. 7,
the emission intensity of 1o was quenched to ca. 17% in hexane and
18% in PMMA film when arrived at the photostationary state. That is
to say, its fluorescent modulation efficiency at photostationary state
was 83% in hexane and 82% in PMMA film. Just as diarylethene 1,
the fluorescent modulation efficiencies of other four diarylethenes
in hexane were 82% for 2, 81% for 3, 85% for 4, and 89% for 5, and
those in PMMA films were 79% for 2, 69% for 3, 67% for 4, and 79%
for 5 when arrived at the photostationary state. Therefore, the
fluorescent modulation efficiencies of diarylethenes 1e5 in hexane
power, indicating that the switching effect is indeed photo-
chemical.2b,15e Therefore, these diarylethene compounds could be
potentially applied to optical memory with fluorescence readout
method and fluorescence modulation switches.36
3.3. Electrochemistry of diarylethenes
The electrochemical properties of diarylethene can be potentially
applied to molecular-scale electronic switches.37 The electrochemical
behavior of diarylethene derivatives has attracted much attention.38
Herein, cyclic voltammograms (CV) were performed on the diary-
lethenes 1e5 under identical experimental conditions at a scanning
rate of 50 mV/s. The CV curves of diarylethenes 1e5 are shown in
Fig. 8. The oxidation onsets of the open-ring isomers 1oe5o were
initiated at 1.39, 0.95, 1.15, 1.31, and 1.39 V, and their closed-ring
Fig. 7. Emission intensity changes of diarylethene 1 upon irradiation with 297 nm UV light at room temperature, excited at 330 nm: (A) in hexane (1.0ꢀ10ꢁ4 mol/L); (B) in PMMA
film (10%, w/w).
were much higher than those in PMMA film. For example, the
fluorescent modulation efficiency of diarylethene 4 in PMMA film
was reduced about 18% more than that in hexane, and those of
other four compounds decreased partly in PMMA films. The result
is completely contrary to those of the unsymmetrical dithienyle-
thene derivatives whose fluorescent modulation efficiencies in
hexane are much lower than those in PMMA film.18a,20b Compared
with diarylethenes bearing two thiophene moieties,22 the fluores-
cent modulation efficiencies of diarylethenes 1e5 were signifi-
cantly enhanced both in the liquid and solid states. This indicated
that diarylethenes bearing a thiazole unit have the potential use as
fluorescent modulation switches.35
isomers 1ce5c were initiated at 0.74, 0.60, 0.66, 0.66, and 0.67 V,
respectively. The difference of oxidation onset between the open-ring
and closed-ringisomers ofdiarylethenes 1e5 (DVoꢁc)was0.65V for1,
0.35 V for 2, 0.49 V for 3, 0.65 V for 4, and 0.72 V for 5. The result in-
dicates that the oxidation process for the open-ring isomers 1oe5o
occurs at higher potentials than in the corresponding closed-ring
isomers 1ce5c. This is because the longer conjugation length of the
closed-ring isomers generallyleads to a less positive potential.39 After
the cyclization reaction, the p-conjugation of the closed-ring isomers
extends across the perfluorocyclopentene ring causing a lower oxi-
dation onset.
For the reduction process, the values of reduction onsets are not
very clear from the CV tests. In order to confirm the reduction onsets
of CV results, differential pulse voltammetry (DPV) was performed
on the diarylethenes 1e5 under identical experimental conditions at
In addition, the ‘on’ and ‘off’ state of the fluorescence was
measured by changing the power of the UV and visible light. The
average ‘on’ and ‘off’ times shortened with the increase in light

8870
G. Liu et al. / Tetrahedron 66 (2010) 8862e8871
synthesized, and their photochemical and electrochemical prop-
1c
2c
erties were investigated in detail. All of these compounds showed
good photochromism and acted as a remarkable fluorescent switch
both in solution and in PMMA film. It has been demonstrated that
the thiazole moiety can decrease significantly the absorption
maxima of the closed-ring isomers compared with thiophene or
pyrazole moiety in the similar photochromic molecular skeleton.
These diarylethene derivatives show distinguishable optical and
electrochemical characteristics, which may be attributed to the
different substituent effects. The results will contribute to un-
derstanding of the substituent effects and tuning the optoelectronic
properties of photochromic diarylethenes bearing both thiophene
and thiazole moieties for further potential applications.
1o
2o
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
3c
4c
3o
4o
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
5c
5o
Acknowledgements
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
Potential/V
This work was supported by Program for the NSFC of China
(20962008), New Century Excellent Talents in University (NCET-08-
0702), the Key Scientific Project from Education Ministry of China
(208069), the Project of Jiangxi Academic and Technological leader
(2009DD00100), National Natural Science Foundation of Jiangxi
Province (2008GZH0020, 2009GZH0034) and the Project of Jiangxi
Youth Scientist.
Fig. 8. Cyclic voltammetry (second scan) of 1e5 in acetonitrile with the scanning rate
of 50 mV sꢁ1
.
a scanning rate of 50 mV/s according to the reported method.40 The
results showed that the reduction onsets of the open-ring isomers
1oe5o were initiated at ꢁ0.84, ꢁ1.07, ꢁ0.97, ꢁ0.99, and ꢁ1.04 V, and
their closed-ring isomers 1ce5c were initiated at ꢁ0.98, ꢁ1.18,
ꢁ1.08, ꢁ1.16, and ꢁ1.18 V, respectively. The results indicated that the
values of the DPV tests are almost the same as those of the CV tests.
So, the band gaps (Eg,) of diarylethenes 1e5 can be estimated by
using the energy of ferrocene as reference,41 and the corresponding
values are summarized in Table 4. The data showed that the band-
gaps of the closed-ring isomers were much lower than those of their
corresponding open-ring isomers. Among these compounds, the Eg
was the smallest in 1c, implying that the charge transfer must be
faster in 1c compared to that in others.42 Furthermore, the oxidation
onsets and the Egs of diarylethenes 1e5 increased remarkably with
the increase in electron-withdrawing ability. Both the oxidation
onsets and the band-gaps of diarylethenes 2 and 3 bearing electron-
donating groups are much lower than those of diarylethenes 4 and 5
bearing electron-withdrawing groups. There are great differences
amongst the electronic current and polarization curve shapes be-
tween the open-ring and closed-ring isomers of diarylethenes 1e5
at the scanned voltage region. The results suggest that different
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Table 4
Electrochemical properties of diarylethenes 1e5 in acetonitrile
Compound
Oxidiation
Reduction
Band gap
Eonset (V)
IP (eV)
Eonset (V)
EA (eV)
Eg
1o
1c
2o
2c
3o
3c
4o
4c
5o
5c
1.39
0.74
0.95
0.60
1.15
0.66
1.31
0.66
1.39
0.67
ꢁ6.19
ꢁ5.54
ꢁ5.75
ꢁ5.40
ꢁ5.95
ꢁ5.46
ꢁ6.11
ꢁ5.46
ꢁ6.19
ꢁ5.47
ꢁ0.84
ꢁ0.98
ꢁ1.07
ꢁ1.18
ꢁ0.97
ꢁ1.08
ꢁ0.99
ꢁ1.16
ꢁ1.04
ꢁ1.18
ꢁ3.96
ꢁ3.82
ꢁ3.73
ꢁ3.62
ꢁ3.83
ꢁ3.72
ꢁ3.81
ꢁ3.54
ꢁ3.76
ꢁ3.62
2.23
1.72
2.02
1.78
2.12
1.74
2.30
1.82
2.43
1.85
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