Electron-donating methoxyl group position effect on properties of diarylethene derivatives having a pyrazole unit

Article abstract of DOI:10.1139/V06-182

New types of asymmetrical photochromic diarylethene derivatives having a pyrazole unit, namely 1-[1,3,5-trimethyl-1-pyrazol-4-yl],2-[2-methyl-5-(4- methoxylphenyl)-1-thien-3-yl]perfluorocyclopentene (1a), 1-[1,3,5- trimethylpyrazol-4-yl],2-[2-methyl-5-(3-methoxylphenyl)-1-thien-3-yl] perfluorocyclopentene (2a), 1-[1,3,5-trimethyl-pyrazol-4-yl],2-[2-methyl-5-(2- methoxylphenyl)-1-thien-3-yl]perfluorocyclopentene (3a), and 1-[1,3,5-trimethyl-pyrazol-4-yl],2-[2-methyl-5-phenyl-1-thien-3-yl] perfluorocyclopentene (4a), were synthesized. Their optical and electrochemical properties, such as photochromism, photochromic cyclization-cycloreversion kinetics, and fluorescence and electrochemical properties were investigated in detail. The results show that all of these- compounds have good photochromism, high cycloreversion quantum yield, and relatively strong fluorescence. Their cyclization-cycloreversion processes were determined to be zeroth to first order reactions. The oxidations of diarylethenes 1a-4a were initiated at 0.73, 1.11, 0.79, and 1.03 V, respectively. Furthermore, introduction of the electron-donating methoxyl group at different positions of the terminal phenyl ring was found to strongly influence these optical and electrochemical properties.

Full text of DOI:10.1139/V06-182

12
Electron-donating methoxyl group position effect
on properties of diarylethene derivatives having a
pyrazole unit
Tianshe Yang, Shouzhi Pu, Bing Chen, Jingkun Xu
Abstract: New types of asymmetrical photochromic diarylethene derivatives having a pyrazole unit, namely 1-[1,3,5-
trimethyl-1-pyrazol-4-yl],2-[2-methyl-5-(4-methoxylphenyl)-1-thien-3-yl]perfluorocyclopentene (1a), 1-[1,3,5-
trimethylpyrazol-4-yl],2-[2-methyl-5-(3-methoxylphenyl)-1-thien-3-yl] perfluorocyclopentene (2a), 1-[1,3,5-trimethyl-
pyrazol-4-yl],2-[2-methyl-5-(2-methoxylphenyl)-1-thien-3-yl]perfluorocyclopentene (3a), and 1-[1,3,5-trimethyl-pyrazol-
4-yl],2-[2-methyl-5-phenyl-1-thien-3-yl]perfluorocyclopentene (4a), were synthesized. Their optical and electrochemical
properties, such as photochromism, photochromic cyclization–cycloreversion kinetics, and fluorescence and electro-
chemical properties were investigated in detail. The results show that all of these compounds have good photochrom-
ism, high cycloreversion quantum yield, and relatively strong fluorescence. Their cyclization–cycloreversion processes
were determined to be zeroth to first order reactions. The oxidations of diarylethenes 1a–4a were initiated at 0.73,
1.11, 0.79, and 1.03 V, respectively. Furthermore, introduction of the electron-donating methoxyl group at different po-
sitions of the terminal phenyl ring was found to strongly influence these optical and electrochemical properties.
Key words: photochromism, diarylethene, electron-donating group, optical and electrochemical properties.
Résumé : On a effectué la synthèse de nouveaux types de dérivés diaryléthènes asymétriques et photochomiques com-
portant une unité pyrazole, soit le 1-[1,3,5-triméthyl-1-pyrazol-4-yl]-2-[2-méthyl-5-(4-méthoxyphényl)-1-thién-3-yl]per-
fluorocyclopentène (1a), le 1-[1,3,5-triméthyl-1-pyrazol-4-yl]-2-[2-méthyl-5-(3-méthoxyphényl)-1-thién-3-
yl]perfluorocyclopentène (2a), le 1-[1,3,5-triméthyl-1-pyrazol-4-yl]-2-[2-méthyl-5-(2-méthoxyphényl)-1-thién-3-yl]per-
fluorocyclopentène (3a) et le 1-[1,3,5-triméthyl-1-pyrazol-4-yl]-2-[2-méthyl-5-phényl-1-thién-3-yl]perfluorocyclopentène
(4a). Les propriétés optiques et électrochimiques, telle la photochromie, la cinétique de la cyclisation/cycloréversion
photochromique, la fluorescence et les propriétés électrochimiques ont toutes été étudiées en détail. Les résultats mon-
trent que tous ces composés ont une bonne photochromie, un rendement quantique de cycloréversion élevé et une fluo-
rescence relativement forte. On a déterminé que le processus des réactions cyclisation/cycloréversion est d’ordre
zéro/un. Les oxydations des diaryléthènes 1a–4a sont respectivement initiées à 0,73, 1,11, 0,79 et 1,03 V. De plus, on
a observé que l’introduction du groupe méthoxy électrodonneur dans différentes positions du noyau benzénique termi-
nal influence fortement les propriétés optiques et électrochimiques examinées plus haut.
Mots-clés : photochromie, diaryléthène, groupe électrodonneur, propriétés optiques et électrochimiques.
[Traduit par la Rédaction] Yang et al. 20
Introduction
tronic devices, photochromic compounds should have good
thermal stability at room temperature and high sensitivity. In
addition, it is required to further develop thermally irrevers-
ible photochromic compounds whose photogenerated col-
ored isomers are very stable and never return to the initial
colorless isomers in the dark (4). Recently, several thermally
irreversible photochromic compounds, such as furylfulgides,
phenoxynaphthacene-quinones, and diarylethenes have been
developed (5). Of these compounds, diarylethene derivatives
are the most promising candidates for these applications be-
cause of their excellent thermal stability in both isomers, fa-
tigue-resistant character, rapid response, and high reactivity
in the solid state (6, 7).
To date, a large number of results concerning the synthe-
sis and photochromic properties of diarylethenes with hetero-
cyclic aryl rings have been reported. Among diarylethene
derivatives synthesized to date, most are composed of
thiophene or benzothiophene hetero-aryl groups (6–13).
These compounds are known to exhibit excellent thermal
Photochromic compounds have attracted considerable at-
tention because of their potential application in photonic de-
vices such as optical memories, photo-switches, and full-
color displays (1–3). For practical application in optoelec-
Received 12 October 2006. Accepted 29 November 2006.
Published on the NRC Research Press Web site at
http://canjchem.nrc.ca on 10 January 2007.
S. Pu.1 Jiangxi Key Lab of Organic Chemistry, Jiangxi
Science and Technology Normal University, Nanchang
330013, China; School of Chemistry & Chemical
Engineering, Jiangsu Teachers University of Technology,
Changzhou 213001, P.R. China.
T. Yang, B. Chen, and J. Xu. Jiangxi Key Lab of Organic
Chemistry, Jiangxi Science and Technology Normal
University, Nanchang 330013, P.R. China.
1Corresponding author (e-mail: pushouzhi@tsinghua.org.cn).
Can. J. Chem. 85: 12–20 (2007)
doi:10.1139/V06-182
© 2007 NRC Canada

Yang et al.
13
Scheme 1. Photochromism of diarylethenes 1a–4a.
stability of both isomers and fatigue-resistant characteristics
(6). In particular, diarylethenes with thiophene ring systems
bearing terminal phenyl groups have been of considerable
interest because the end group can be substituted by an elec-
tron-donating or -withdrawing group. These groups can in-
evitably influence the optoelectronic properties of
corresponding diarylethenes (14). Besides diarylethene de-
rivatives with thiophene or benzothiophene moieties, other
hetero-aryl moieties, such as thiazole, (15–17), pyrrole (18),
indole (19, 20), indene (21), benzofuran (22), crysothio-
phene (23) have also been reported. The photochromic per-
formance of each kind of diarylethene derivative is strongly
dependent on the aryl moieties. For example, diarylethenes
with indole rings exhibit strong fluorescence (19), whereas
those with pyrrole rings are thermally unstable and return to
open-ring isomers even in the dark (22). When the aryl
groups have low aromatic stabilization energy, the deriva-
tives undergo thermally irreversible photochromic reactions
(21). Pyrazole is an attractive aryl unit because of its low ar-
omatic stabilization energy and the structure is similar to
isothiazole and so is expected to undergo thermally irrevers-
ible photochromic reactions. Until now, photochromic hy-
brid diarylethene derivatives with pyrazole and thiophene
moieties have not been reported, with the exception of one
letter published by us (24).
F
F
F
F
F
F
F
F
UV—vis
F
F
F
F
N
N
N
S
N
S
R
R
1a: R = para-OCH3
2a: R = meta-OCH3
3a: R = ortho-OCH3
4a: R = H
1b: R = para-OCH3
2b: R = meta-OCH3
3b: R = ortho-OCH3
4b: R = H
263 potentiostat–galvanostat (EG&G Princeton Applied Re-
search) under computer control at RT. Platinum electrodes
(diameter 0.5 mm) served as working electrode and counter
electrode. Platinum wire was used as a quasireference elec-
trode. It was calibrated using the ferrocene (Fc – Fc+) redox
couple that has a formal potential, E1/2, of +0.35 V vs. plati-
num wire. The typical electrolyte was acetonitrile (5.0 mL),
which contained LiClO4 (0.10 mol/L) and diarylethene (4.0
× 10–3 mol/L). All solutions were deaerated by a dry argon
stream and maintained at a slight argon overpressure during
electrochemical experiments.
Synthesis
For this paper, one of our research goals was to develop a
new class of diarylethene derivatives that show good
photochromic behavior. We also want to discuss the effect of
electron-donating methoxyl group position on the optical
and electrochemical properties of diarylethenes. Therefore,
we have synthesized four asymmetrical diarylethene deriva-
tives bearing a pyrazole unit and a thiophene moiety with
methoxyl group at the para, meta, and ortho positions of ter-
minal phenyl group, i. e. 1-[1,3,5-trimethyl-1-pyrazol-4-yl],
2-[2-methyl-5-(4-methoxylphenyl)-1-thien-3-yl]perfluorocyclo-
pentene (1a), 1-[1,3,5- trimethylpyrazol-4-yl],2-[2-methyl-5-
(3-methoxylphenyl)-1-thien-3-yl]perfluorocyclopentene (2a),
1-[1,3,5-trimethyl-pyrazol-4-yl],2-[2-methyl-5-(2-methoxyl-
phenyl)-1-thien-3-yl]perfluorocyclopentene (3a), 1-[1,3,5-tri-
methyl-pyrazol-4-yl],2-[2-methyl-5-phenyl-1-thien-3-yl]
perfluorocyclopentene (4a). The photochromism of
diarylethenes 1a–4a, which are discussed in this study, is
shown in Scheme 1.
The four diarylethenes 1a–4a were prepared by the
method shown in Scheme 2, and experimental details are
given in the following.
Synthesis of 3,5-dibromo-2-methylthiophene (5)
To a stirred solution of 2-methylthiophene (24.0 g,
244.8 mmol) in acetic acid (100 mL) at 0 °C was slowly
added a Br2–acetic acid solution (v/v, 29:40). The reaction
mixture was stirred overnight at this temperature. The reac-
tion was stopped by the addition of water. The mixture was
neutralized by Na2CO3 and extracted with ether. The ether
extract was dried, filtrated, and concentrated. The residue
was purified by distillation in vacuo. Compound 2 was ob-
1
tained as a colorless oil, 50.7 g in 81.2% yield. H NMR
(400 MHz, CDCl3, TMS) δ: 2.94 (s, 3H), 7.61 (s, 1H).
Synthesis of 3-bromo-2-methyl-5-thienylboronic acid (6)
To a stirred solution of 2 (16.3 g, 63.7 mmol) in dry ether
(150 mL) was added a 1.6 mol/L n-BuLi–hexane solution
(40.6 mL, 65.0 mmol) at –78 °C under nitrogen atmosphere.
Stirring continued for 30 min, and boric acid tri-butyl ester
(18.8 mL) was quickly added to the reaction mixture. After
increasing the temperature to RT, the reaction was stopped
by the addition of 4% HCl (10 mL). The mixture was ex-
tracted with 4% NaOH (100 mL), and the aq. NaOH solu-
tion was neutralized by 10% HCl. The residue was washed,
filtrated, and dried. The product 3 was obtained as yellowish
Experimental
General methods
All solvents were purified by distillation before usage.
NMR spectra were recorded on a Bruker AV400 (400 MHz)
spectrometer using CDCl3 as solvent and tetramethylsilane
as an internal standard. IR spectra were recorded on a
Bruker Vertex-70 spectrometer and mass spectra were mea-
sured with an Agilent MS Trap VL spectrometer. The
absorption spectra were measured using an Agilent 8453
UV–vis spectrometer. Photo-irradiation was carried out us-
ing an SHG-200 UV lamp, a CX-21 UV fluorescence analy-
sis cabinet, and a BMH-250 visible lamp. Light of
appropriate wavelengths was isolated by different light
filters. Fluorescence spectra were measured using a Hitachi
F-4500 spectrophotometer. Electrochemical examinations
were performed in a one-compartment cell using a Model
1
solid, 12.0 g in 85.5% yield. H NMR (400 MHz, CDCl3,
TMS) δ: 2.51 (s, 3H), 4.59 (s, 2H), 7.33 (s, 1H).
3-Bromo-2-methyl-5-(4-methoxylphenyl)thiophene (7c)
Compound 7c was prepared by reacting compound 6
(5.0 g, 22.6 mmol) with 4-bromoanisole (4.23 g, 22.6 mmol)
in the presence of Pd(PPh3)4 (0.8 g) and Na2CO3 (6.40 g,
60 mmol) in tetrahydrofuran (THF) (80 mL containing 10%
water) for 15 h at 70 °C. Compound 7c was purified by col-
© 2007 NRC Canada

14
Can. J. Chem. Vol. 85, 2007
Scheme 2. Synthetic route for diarylethenes 1a–4a.
Br
Br
1 ) n-BuLi,–78 °C
2 ) B(OBu)3
Br2
CH3COOH
S
Br
S
5
B(OH)2
S
6
Br
R
Br
S
R
F
F
Pd(Pph3)4
Na2CO3
F
F
7c: R =
7d: R =
7e: R =
7f : R =
para-OCH3
meta-OCH3
ortho-OCH3
H
F
F
n-BuLi,–78 °C
N
F
F
N
S
Br
F
F
1 ) n-BuLi,–78 °C
F
F
1a: R = para-OCH3
N
R
2 ) C5F8
N
2a: R = meta-OCH3
3a: R = ortho-OCH3
4a: R = H
F
N
N
8
9
umn chromatography on SiO2 using hexane as the eluent
and obtained as baby-yellow solid in 78.3% yield. H NMR
(400 MHz, CDCl3) δ: 2.40 (s, 3H, -CH3), 3.83 (s, 3H, -CH3),
6.89–6.91 (d, 2H, J = 8.8 Hz, phenyl-H), 6.99 (s, 1H,
thienyl-H), 7.42–7.44 (d, 2H, J = 8.8 Hz, phenyl-H).
was dried over MgSO4, filtrated, and evaporated. The crude
product was purified by column chromatography on SiO2
using petroleum ether – acetic ether (4:1) as the eluent to
give 2.98 g of compound 9 as a yellow oil in 65.8% yield.
1H NMR (400 MHz, CDCl3) δ: 2.20 (s, 3H, -CH3), 2.22 (s,
3H, -CH3), 3.77 (s, 3H, -NCH3). 19F NMR (400 MHz,
CDCl3) δ: –107.61 (2F), –118.36 (2F), –129.22 (1F), –130.51
(2F).
1
3-Bromo-2-methyl-5-(3-methoxylphenyl)thiophene (7d)
Compound 7d was prepared by a method similar to that
1
used for 7c and obtained as a buff solid in 75.3% yield. H
NMR (400 MHz, CDCl3) δ: 2.41 (s, 3H), 3.83 (s, 3H), 6.82–
6.84 (d, 1H, J = 8.0 Hz, phenyl-H), 7.03 (s, 1H, thienyl-H),
7.08–7.10 (t, 2H, J = 4.0 Hz, phenyl-H), 7.25–7.29 (t, 1H,
J = 8.0 Hz, phenyl-H).
1-[1,3,5-Trimethyl-1-pyrazol-4-yl],2-[2-methyl-5-(4-
methoxylphenyl)-1-thien-3-yl]perfluorocyclopentene (1a)
To stirred anhyd. THF (30 mL) containing 7c (0.568 g,
1.66 mmol) was added dropwise a 1.6 mol/L n-BuLi solu-
tion (1.05 mL) at –78 °C under argon atmosphere. After the
mixture was stirred for 30 min at –78 °C, compound 9
(0.5 g, 1.66 mmol), dissolved in anhyd. THF solvent, was
added quickly. The reaction mixture was continuously
stirred at –78 °C for 2 h, and the reaction was allowed to
slowly warm to RT. The reaction was quenched by the addi-
tion of water. The product was extracted with ether, dried
with MgSO4, and concentrated in vacuo. The crude product
was purified by column chromatography (petroleum ether –
acetic ether, 4:1) to afford 0.35 g of diarylethene 1a as yel-
3-Bromo-2-methyl-5-(2-methoxylphenyl)thiophene (7e)
Compound 7e was prepared by a method similar to that
1
used for 7c and obtained as a buff solid in 70.2% yield. H
NMR (400 MHz, CDCl3) δ: 2.41 (s, 3H, -CH3), 3.91 (s, 3H,
-CH3), 6.95–6.99 (m, 2H, J = 7.6 Hz, phenyl-H), 7.27 (s,
1H, thienyl-H), 7.30 (s, 1H, phenyl-H), 7.53–7.55 (d, 1H,
J = 8.0 Hz, phenyl-H).
3-Bromo-2-methyl-5-phenylthiophene (7f)
Compound 7f was prepared by a method similar to that
1
1
low solid in 45.0% yield. H NMR (400 MHz, CDCl3) δ:
used for 7c and obtained as a buff solid in 70.0% yield. H
1.94 (s, 3H, -CH3), 1.97 (s, 3H, -CH3), 2.03 (s, 3H, -CH3),
3.70 (s, 3H, -OCH3), 3.84 (s, 3H, -NCH3), 6.90–6.92 (d, 2H,
J = 8.4 Hz, phenyl-H), 7.13 (s, 1H, thiophene-H), 7.44–7.47
(d, 2H, J = 8.4 Hz, phenyl-H). 13C NMR (400 MHz, CDCl3)
δ: 10.54, 12.86, 14.30, 36.27, 55.40, 106.29, 114.38, 121.43,
122.87, 125.72, 126.23, 139.17, 139.63, 141.92,
146.46 159.48. IR (KBr, cm–1) ν: 800, 821 (benzene 1,4-
substituted), 1124 (-CF2), 1255 (C-N, st), 1341, 1477, 1550
(thiophenene bond), 1611 (benzene skeleton), 2841 (-OCH3),
2927, 2960 (-CH3). 19F NMR (400 MHz, CDCl3) δ: –109.43
(2F), –110.52 (2F), –132.58 (2F). MS m/z (M+): 487.2 (+H),
509.2 (+Na)
NMR (400 MHz, CDCl3) δ: 2.28 (s, 3H, -CH3), 6.84 (s, 1H,
phenyl-H), 7.12 (s, 1H, thienyl-H), 7.33–7.35 (d, 2H, J =
8.0 Hz, phenyl-H), 7.56–7.58 (d, 2H, J = 8.0 Hz, phenyl-H).
1-(1,3,5-Trimethylpyrazole)perfluorocyclopentene (9)
To a stirred solution of 4-bromo-1,3,5-trimethylpyrazole
(8) (2.84 g, 15 mmol) in 50 mL of THF was added dropwise
a 1.6 mol/L n-BuLi solution (9.4 mL, 15 mmol) at –78 °C
under argon atmosphere. Stirring was continued for 30 min
at the low temperature. Perfluorocyclopentene (2.04 mL,
15 mmol) was added to the reaction mixture at –78 °C, the
mixture was stirred for another 1 h at this temperature, and
then the reaction temperature was increased spontaneously
to RT. The reaction was stopped by the addition of water.
The mixture was extracted with ether and washed with aq.
HCl (1 mol/L) and water, respectively. The organic layer
1-[1,3,5-Trimethyl-1-pyrazol-4-yl],2-[2-methyl-5-(3-
methoxylphenyl)-1-thien-3-yl]perfluorocyclopentene (2a)
Diarylethene 2a was prepared by a method similar to that
© 2007 NRC Canada

Yang et al.
15
Fig. 1. Absorption spectral changes of diarylethenes 1–4 in hexane solution at RT (2.0 × 10–5 mol/L): (A) 1, (B) 2, (C) 3, and (D) 4.
0.6
(A)
(C)
0.6
0.4
0.2
0.0
0.4
0.2
0.0
540 s
360
120
80
UV
720 s
240
100
45
UV
45
20
30
10
10
0
0
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
0.6
0.4
0.2
0.0
(B)
(D)
0.6
0.4
0.2
0.0
720 s
UV
360
240
180
120
60
360 s
150
60
UV
30
10
20
0
0
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
1
used for 1a and obtained as a solid in 38.0% yield. H NMR
(400 MHz, CDCl3) δ: 1.96 (s, 3H, -CH3), 1.97 (s, 3H, -CH3),
2.03 (s, 3H, -CH3), 3.70 (s, 3H, -OCH3), 3.86 (s, 3H,
-NCH3), 6.84–6.86 (d, 1H, J = 8.0 Hz, phenyl-H), 7.05 (s,
1H, thiophene-H), 7.11–7.13 (d, 1H, J = 7.6 Hz, phenyl-H),
7.24 (s, 1H, phenyl-H), 7.28–7.32 (t, 1H, J = 8.0 Hz,
phenyl-H). 13C NMR (400 MHz, CDCl3) δ: 10.01, 12.33,
13.98, 35.67, 54.85, 105.73, 110.92, 112.71, 117.67, 122.27,
125.29, 129.54, 134.17, 138.10, 138.65, 140.17, 141.32,
145.92, 159.54. IR (KBr, cm–1) ν: 687, 770, 799, 882 (ben-
zene 1,3-substituted), 1121 (-CF2), 1271 (C-N, st),
1336,1434 (thiophenene bond), 1602 (benzene skeleton),
2841 (-OCH3), 2948, 2966 (-CH3, st). 19F NMR (400 MHz,
CDCl3) δ: –109.45 (2F), –110.51 (2F), –132.58 (2F). MS m/z
(M+): 487.2 (+H), 509.1 (+Na).
19F NMR (400 MHz, CDCl3) δ: –109.46 (2F), –110.51 (2F),
–132.57 (2F). MS m/z (M+): 487.2 (+H), 509.2 (+Na), 467.2
(-F).
1-[1,3,5-Trimethyl-1-pyrazol-4-yl],2-[2-methyl-5-phenyl-1-
thien-3-yl]perfluorocyclopentene (4a)
Diarylethene 4a was prepared by a method similar to that
1
used for 1a and obtained as solid in 44.1% yield. H NMR
(400 MHz, CDCl3) δ: 1.96 (s, 3H, -CH3), 1.97 (s, 3H, -CH3),
2.03 (s, 3H, -CH3), 3.84 (s, 3H, -NCH3), 7.26 (s, 1H,
thiophene-H), 7.30–7.32 (d, 1H, J = 7.6 Hz, phenyl-H),
7.37–7.40 (d, 2H, J = 7.6 Hz, phenyl-H), 7.52–7.54 (d, 2H,
J = 7.6 Hz, phenyl-H). 13C NMR (400 MHz, CDCl3) δ:
10.54, 12.87, 14.51, 36.29, 106.25, 122.55, 125.60, 125.85,
127.85, 129.00, 133.37, 139.17, 140.61, 142.00, 146.45. IR
(KBr, cm–1) ν: 689, 756, 894 (benzene 1-substituted), 1124
(-CF2), 1277 (C-N, st), 1341, 1443 (thiophenene bond), 1626
(benzene skeleton), 2928, 2960 (-CH3). 19F NMR (400 MHz,
CDCl3) δ: –109.45 (2F), –110.52 (2F), –132.58 (2F). MS m/z
(M+): 457.2 (+H), 479.2 (+Na), 437.3 (-F), (M–) 413.4
(-3CH3).
1-[1,3,5-Trimethyl-1-pyrazol-4-yl],2-[2-methyl-5-(2-
methoxylphenyl)-1-thien-3-yl]perfluorocyclopentene (3a)
Diarylethene 3a was prepared by a method similar to that
1
used for 1a and obtained as solid in 41.2% yield. H NMR
(400 MHz, CDCl3) δ: 1.96 (s, 3H, -CH3), 1.97 (s, 3H, -CH3),
2.04 (s, 3H, -CH3), 3.69 (s, 3H,, -OCH3), 3.92 (s, 3H,
-NCH3), 6.96–7.02 (dd, 2H, J = 8.0 Hz, phenyl-H), 7.27–
7.29 (d, 1H, J = 8.0 Hz, phenyl-H), 7.42 (s, 1H, thiophene-
H), 7.57–7.59 (dd, 1H, J = 7.6 Hz, phenyl-H). 13C NMR
(400 MHz, CDCl3) δ: 10.49, 12.86, 14.21, 36.25, 55.53,
106.36, 111.69, 113.90, 121.01, 122.36, 124.59, 124.84,
127.86, 128.73, 137.05, 139.19, 140.96, 146.48, 146.95,
155.49. IR (KBr, cm–1) ν: 751 (benzene 1,2-substituted),
1127 (-CF2), 1274 (C-N, st), 1341, 1438 (thiophenene bond),
1635 (benzene skelteton), 2839 (-OCH3), 2942 (-CH3).
Results and discussion
Photochromism of diarylethenes 1–4
In hexane solution, the photochromic reactivity of
diarylethenes 1–4 was examined. Figure 1 shows their ab-
sorption spectral changes induced by UV light at RT (2.0 ×
10–5 mol/L). Compounds 1a–4a exhibited good photochromic
properties and can be toggled between their colorless open-
ring isomers (1a–4a) and colored closed-ring isomers (1b–
© 2007 NRC Canada

16
Can. J. Chem. Vol. 85, 2007
Table 1. Absorption spectral properties of diarylethenes 1–4 in hexane at 2.0 × 10–5 mol/L.
Φc
λo,max (nm)a,
λc,max (nm)b,
Compound
ε (L mol–1 cm–1)
ε (L mol–1 cm–1)
Φo-c
Φc-o
1
2
3
4
291, 2.9 × 104
284, 2.5 × 104
278, 2.7 × 104
287, 3.2 × 104
573, 1.0 × 104
573, 7.9 × 103
573, 1.1 × 104
575, 8.8 × 103
0.47
0.45
0.43
0.27
0.57
0.53
0.46
0.40
aAbsorption maxima of open-ring forms.
bAbsorption maxima of closed-ring forms.
cQuantum yields of cyclization reaction, Φo-c, and cycloreversion reaction, Φc-o, respectively.
4b) by alternating irradiation with UV and visible light, as
monitored using UV–vis absorption and H NMR spectros-
pyrazole unit and is a significant difference between the
diarylethenes reported here and other diarylethenes reported
to date.
1
copy. Upon irradiation with 313 nm light, absorption bands
in the visible region appear and the solutions turns blue as a
result of the cyclization reactions to produce 1b–4b (Fig. 1
and Table 1). In the photostationary state, the content of the
closed-ring isomer can be easily analyzed with 1H NMR
spectroscopy by comparing the relative intensities of thio-
The property of fatigue resistance, i.e., the number of
times photocyclization and ring-opening reaction cycles can
be repeated without loss of performance, was examined at
RT according to ref. 18. The hexane solution of compound 1
was irradiated alternatively with UV light (313 nm) and visi-
ble light (510 nm). The irradiation time was long enough for
coloration to reach the photostationary state and for the
color to be completely bleached. The absorbance of the
open-ring form remained almost constant even after 200 col-
oration–decoloration cycles and a clear isosbestic point was
observed at 315 nm after 200 cycles. Compounds 2–4
showed similar properties to that of 1. The results suggested
that the diarylethene derivatives bearing heterocyclic
pyrazole ring show excellent fatigue resistance.
1
phene proton signals. According to this method, H NMR
spectroscopic studies performed on CDCl3 (1.0 × 10–3 mol/L)
solutions of 1a–4a reveal that the contents of the closed-ring
isomers (1b–4b) in photostationary states are 67% for 1b,
69% for 2b, 68% for 3b, and 69% for 4b, respectively. All
the solutions of 1b–4b can be decolorized by irradiating
them with visible light of wavelengths greater than 500 nm
to induce the cycloreversion reactions and reproduce 1a–4a.
The coloration–decoloration cycles of the four compounds
could be repeated more than 20 times and a clear isosbestic
point was observed at 315, 318, 327, and 309 nm for
diarylethenes 1, 2, 3, and 4, respectively. The absorption
spectral properties of the four compounds are summarized in
Table 1. The quantum yields were determined by comparing
the reaction yields of diarylethenes 1–4 in hexane against
1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene in
hexane (25), and the results are also shown in Table 1.
These data suggest that the maximum absorption wave-
lengths of closed-ring forms and the molar absorption coef-
ficients of both open- and closed-ring forms of the four
diarylethene compounds are not evidently different. How-
ever, the maximum wavelengths of open-ring forms and
their cyclization–cycloreversion quantum yields are relatively
remarkable. Among diarylethenes 1–4, the quantum yields
of cyclization and cycloreversion reactions of the para-
Photochromic cyclization–cycloreversion kinetics of
diarylethenes 1–4
The photochromic cyclization and cycloreversion kinetics
of diarylethenes 1–4 in hexane were investigated by irradia-
tion by UV–vis (313 nm, from open to closed form) or visi-
ble light (λ > 500 nm, closed to open form), as shown in
Fig. 2. The concentration of all solutions was 2.0 × 10–5 mol/L.
All absorption data were collected at the maximum wave-
length of every solution, i.e., at the maximum optical den-
sity. As shown in Fig. 2a, the cyclization kinetics of
diarylethenes 1–4 in hexane show that the photochromic
cyclization reaction follows zeroth-order kinetics; the rela-
tionships between absorbance and exposure time (A vs. t)
show good linearity (linear correction factors >0.99) upon ir-
radiation with UV light (313 nm) in all solutions. The slopes
of the A vs. t lines gave the zeroth order rate constant, ko-c
.
methoxyl substituted derivative (1) are the greatest (ΦO-C
0.47, ΦC-O = 0.57), while those of the the non-methoxyl
substituent derivative (4) are the smallest (ΦO-C = 0.27, ΦC-O
=
According to this method, all the k values for the cyclization
process (ko-c) of diarylethes 1–4 can be readily obtained in
hexane and are 2.18 × 10–3, 1.25 × 10–3, 1.89 × 10–3, and
0.89 × 10–3 mol/L s–1, respectively. Similarly, the cyclo-
reversion kinetic curves shown in Fig. 2b can be plotted ac-
cording to eq. [1] (26),
=
0.40). The values of the other two compounds are between
those of diarylethenes 1 and 4. As shown in Table 1, all
cyclization–cycloreversion quanta of diarylethene deriva-
tives with methoxyl group substituent (1a–3a) are much
higher than that of the diarylethene with no methoxyl group
(4a). The results indicate that the cyclization–cycloreversion
quantum yield could be effectively increased when the elec-
tron-donating methoxyl group was introduced into the ben-
zene rings. In addition for diarylethenes 1–4, the values of
their cycloreversion quantum yields are much higher than
those of cyclization quantum yields (Table 1). This is an in-
teresting characteristic of diarylethene derivatives having a




A∞ − A
0 
[1]
kt = ln
A∞ − A
t

where k is the first-order rate constant, A0, A∞, and At are the
observed absorption data measured at the beginning, at the
end, and at time t of the reaction, respectively. The data in
Fig. 2b represent a perfect linear fit, indicating that the
© 2007 NRC Canada

Yang et al.
17
Fig. 3. (a) Fluorescence spectra of diarylethenes 1a–4a in hex-
Fig. 2. The cyclization–cycloreversion kinetics of diarylethenes
1–4 in hexane (2.0 × 10–5 mol/L): (A) Cyclization kinetics upon
irradiation with light (λ = 254 nm), (B) Cycloreversion kinetics,
upon irradiation with visible light (λ > 510 nm).
ane solution (5.0 × 10–5 mol/L) at RT, excited at 280, 317, 315,
and 320 nm, respectively. (b) Fluorescence spectra of diarylethene
1a in various concentrations in hexane at RT, monitored at
280 nm (c) Fluorescence spectra of diarylethene 1a in different
solutions at RT, monitored at 320 nm.
5000
3a
(a)
4000
3000
2a
4a
2000
1000
1a
0
350
400
450
500
550
600
Wavelength (nm)
600
500
400
300
200
100
0
-6
7
(b)
A 2X10 mol/L
E
-6
6
5
4
3
B 5X10 mol/L
H
-5
C 1X10 mol/L
D
C
-5
D 2X10 mol/L
-5
E 5X10 mol/L
300 350 400 450 500 550
-4
F 1X10 mol/L
-4
F
G 2X10 mol/L
-3
H 1X10 mol/L
B
A
G
H
300
350
400
450
500
550
Wavelength (nm)
600
400
200
0
photochromic cycloreversion process follows first-order re-
action kinetics. From Fig. 2b, the rate constant k of the
cycloreversion process (kc-o) of diarylethenes 1–4 in hexane
were calculated to be 6.25 × 10–2, 4.17 × 10–2, 5.44 × 10–2,
and 6.55 × 10–2 mol/L s–1, respectively. Of the four com-
pounds, the ko-c of diarylethene 4 in hexane is the smallest,
while its kc-o is the greatest in hexane. The result showed
that the methoxyl group can effectively accelerate the
photochromic cyclization reaction and suppress the
cycloreversion reaction.
1
2
3
4
5
6
Hexane
(c)
2
Dichloromethane
Ethyl acetate
DMF
1
3
6
Acetonitrile
Ethanol
4
5
Fluorescence of diarylethenes 1–4
The fluorescence spectra of diarylethenes 1a–4a (5.0 ×
10–5 mol/L) in hexane at RT are illustrated in Fig. 3a. All
exhibited good fluorescence at different wavelengths in hex-
ane solution, when excited by their respective excitation
wavelength. From Fig. 3a, we can clearly see that the hex-
ane solutions containing diarylethenes 1a–4a showed rela-
tively strong fluorescence at 416, 438, 441, and 439 nm
when excited at 280, 317, 315, and 320 nm, respectively.
350
400
450
500
550
600
Wavelength (nm)
© 2007 NRC Canada

18
Can. J. Chem. Vol. 85, 2007
Table 2. Fluorescence spectral properties of diarylethenes 1a–4a at various concentrations in hex-
ane at RT.
λem, max (nm), (fluorescence intensity)
Concn. (mol/L)
1a
2a
3a
4a
2 × 10–6
5 × 10–6
1 × 10–5
2 × 10–5
5 × 10–5
1 × 10–4
2 × 10–4
1 × 10–3
414 (102)
418 (193)
412 (320)
420 (394)
416 (514)
421 (228)
421 (48)
442 (7)
438 (219)
438 (484)
436 (816)
440 (1323)
438 (2180)
439 (2005)
443 (1084)
445 (24)
440 (699)
441 (1529)
441 (2573)
441 (3394)
441 (4366)
442 (2637)
442 (641)
446 (29)
422 (273)
420 (519)
423 (791)
423 (1172)
422 (1064)
421 (573)
a
—
425 (10)
aNo fluorescence peak.
The fluorescence maxima of the four compounds were ob-
served to be between 416 and 441 nm, indicating that the ef-
fect on the fluorescence peak of the electron-donating
methoxyl group and its position was relatively significant.
On the other hand, the fluorescence intensities of
diarylethenes 1a–4a changed dramatically with the
substituent position. The fluorescence intensities increased
rapidly from the para-substituted 1a to the ortho-substituted
3a derivatives. The value of non-methoxyl-substituted
diarylethene 4a is located between those of the para-
methoxyl-substituted 1a and the meta-methoxly-substituted
2a derivatives. Among the four compounds, the relative flu-
orescence intensity of the ortho-substituted derivative is the
strongest, and that of the para-substituted derivative is the
weakest. The result is exactly contrary to that reported in our
previous paper (27). In that paper, the electro-withdrawing
fluorine substituent position did not significantly affect the
fluorescence peak of dithienylethene, but had a remarkable
effect on the fluorescence intensity; the fluorescence inten-
sity of the ortho-substituted derivative was the weakest, and
that of the para-substituted derivative was the strongest. The
possible reasons are due to the electro-donating nature of the
methoxyl substituent of the terminal phenyl group and that
the pyrazole ring can also possibly affect the fluorescence
properties of the three diarylethenes. Upon irradiation with
313 nm light, their fluorescence intensity decreased, along
with the photochromism, from open-ring form to closed-ring
form, suggesting that all of the closed-ring forms of these
four diarylethenes showed no fluorescence or that their fluo-
rescence was very weak. These results are consistent with
those of most reported diarylethenes (28–31).
The concentration dependence of fluorescence spectra of
1a was measured in hexane solution at RT, shown in Fig. 3b.
With the concentration increased from 2.0 × 10–6 to 5.0 ×
10–5 mol/L, the maximum emission occurred almost at
416 nm when excited at 280 nm, and the relative fluores-
cence intensity increased dramatically. However, when the
concentration was increased further from 5.0 × 10–5 to 1.0 ×
10–3 mol/L, the maximum emission appeared redshifted
from 416 to 442 nm, and the relative fluorescence intensity
decreased remarkably. The hexane solution showed very
weak fluorescence when the concentration was increased to
1.0 × 10–3 mol/L. This self-quench phenomenon perhaps re-
sulted from the formation of excimers and (or) exciplexes in
high concentration solutions so that a deactivation effect
could occur during the excited state lifetime (32). Moreover,
the concentration effect on the fluorescence spectra of
diarylethenes 2a–4a were also measured, and the results are
summarized in Table 2. As shown in Table 2, compounds
2a–4a showed similar fluorescent properties as 1a, depend-
ing on the concentration employed. At high concentration,
e.g., 1.0 × 10–3 mol/L, the fluorescence peak of each
diarylethene was obviously redshifted comparing to other
concentrations. For all four compounds, the relative fluores-
cence intensity showed a remarkable initial increase with
subsequent dramatic decrease with increasing concentration.
The greatest and the smallest values were obtained at 5.0 ×
10–5 and 1.0 × 10–3 mol/L, respectively. The systematic
redshift at high concentrations and the relatively remarkable
fluorescence intensity decrease with increased concentration
demonstrated that molecular aggregation and fluorescence
quench may occur at higher concentration (31, 33).
The effects of the solvent on the fluorescence spectra of
diarylethenes 1a–4a were also investigated. Figure 3c shows
the fluorescence spectra of 1a (5.0 × 10–5 mol/L) in different
solvents at RT. As shown in Fig. 3c, when excited at
320 nm, the fluorescence peaks of diarylethene 1a were ob-
served at 418, 440, 443, 456, 470, and 426 nm in hexane, di-
chloromethane, ethyl acetate, DMF, acetonitrile, and ethanol,
respectively. The solvent effect on the fluorescence of
diarylethenes 2a–4a was also measured, and the fluores-
cence properties of these four diarylethenes in various sol-
vents at RT are summarized in Table 3.
From these data, one can readily see that the fluorescence
spectra depended remarkably on the polarity of solvent. The
maximum fluorescence peaks of the four compounds ap-
peared to be redshifted as the polarity of the solvent in-
creased, except for ethanol for which the contrary occurred.
The reason may be possibly attributed to its specific sol-
vent–solute interaction such as hydrogen bonding with the
fluorine atoms of the diarylethenes. In addition, the solvent
effect on the relative fluorescence intensity showed rela-
tively complex and irregular changes (Table 3). Under the
same experimental conditions, the fluorescence intensities in
ethanol are the strongest, except for diarylethene 1a, for
which the strongest intensity appeared in dichloromethane.
Electrochemistry of diarylethenes 1–4
It is reported that the ring-opening and -closing transfor-
mation of some diarylethenes can be initiated not only by
© 2007 NRC Canada

Yang et al.
19
Table 3. Fluorescence properties of diarylethenes 1a–4a in different solvents at 5.0 × 10–5 mol/L.
λem, max (nm), (fluorescence intensity)a
Solvent
1a
2a
3a
4a
Hexane
416 (514)
440 (549)
443 (433)
456 (376)
470 (330)
426 (415)
438 (2180)
450 (2739)
452 (2196)
460 (2225)
460 (2130)
449 (2833)
441 (4366)
452 (6354)
452 (5560)
460 (5376)
463 (5629)
451 (7032)
439 (1630)
451 (2003)
—
461 (1531)
462 (1601)
448 (2176)
Dichloromethane
Ethyl acetate
DMF
Acetonitrile
Ethanol
b
aFluorescent maxima monitored at 320, 330, 333, and 320 nm for 1a, 2a, 3a, and 4a, respectively.
bNo spectrum peaks.
Fig. 4. The anodic polarization curves of diarylethenes 1–4.
UV or visible light irradiation but also by electrochemical or
chemical, i.e., electrochromic, oxidation (34–36). The elec-
trochemical properties of diarylethenes can be potentially
applied to molecular scale electronic switches. Electrochem-
ical examinations by the linear sweep method under the
same experimental conditions using diarylethenes 1–4 were
performed, and the results are shown in Fig. 4. As shown in
Fig. 4, para-methoxyl and ortho-methoxyl substituted
diarylethenes (1 and 3) showed similar electrochemical be-
havior. The oxidation onsets of 1a, 1b, 3a, and 3b initiated
at 0.79, 1.00, 0.73, and 0.78 V, respectively. Both of the oxi-
dation onsets of the open-ring forms of compounds 1 and 3
are lower than those of the corresponding closed-ring forms.
This indicates that the electron-donating effect of the
methoxyl group is more significant in the open-ring form.
However, the oxidation onsets of compounds 2 and 4 were
opposite to those of compounds 1 and 3; the oxidation
onsets of 2a, 2b, 4a, and 4b were 1.11, 0.87, 1.03, and 0.94
V, respectively. The oxidation onsets of the open-ring forms
are higher than those of the closed-ring forms. This implies
that the electron-donating effect of the methoxyl group or
hydrogen atom is more significant in the closed-ring form of
compounds 2 or 4, respectively. Moreover, the differences
between the open- and closed-ring forms of diarylethenes 1–
4 (∆Va–b) were 0.21 for 1, 0.24 for 2, 0.05 for 3, and 0.09 for
4, respectively. At the same applied potential, the current
density of 2b was higher than that of 2a, which is exactly
opposite to the characterisitcs of 1a and 1b. The electronic
current of compounds 3 and 4 at the same applied potential
intersected. From the previous description together with
Fig. 4, a reasonable conclusion can be drawn that the substi-
tution of the electron-donating methoxyl group has a great
effect on 1–3. For ortho or meta position substitution of the
methoxyl group, the oxidation potential onset showed a re-
markable difference with ∆Va–b greater than 0.2 V for 1 and
2. On the other hand, when substituted at the ortho position,
the electron-donating effect together with its steric effect
plays a main role, which made the oxidation potential onsets
of 3a and 3b similar to each other (∆Va–b = 0.05 V). A simi-
lar result was observed when the ortho-methoxyl group was
substituted by a hydrogen atom (4a and 4b, ∆Va–b = 0.09 V).
In addition, three oxidation processes can be readily ob-
served during the anodic oxidation of 2a and 2b, while only
two oxidation processes can be observed during anodic oxi-
dation of the other three compounds. The three oxidation
processes may involve the oxidation of a pyrazole ring, a
thiophene ring, together with the benzene ring of compound
180
1a
1b
150
2a
2b
120
3a
3b
90
4a
4b
60
30
0
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Potential (V) vs. Pt
2. However, the substitution of the methoxyl group, or
hydrogen atom, at the para and ortho position made the oxi-
dation process of benzene unclear for 1a, 1b, 3a, 3b, 4a, and
4b. All these electrochemical results indicated that the sub-
stitution of the methoxyl group on the benzene and pyrazole
units had great effect on both the open- and closed-ring
forms of the three diarylethene derivatives. A detailed expla-
nation requires further investigation.
Conclusions
Four new asymmetrical diarylethenes bearing a pyrazole
unit were synthesized for the investigation of their optical
and electrochemical properties. The effect of different sub-
stitution positions on the optical and electrochemical perfor-
mances of diarylethenes was investigated. The results
suggested that the optical and electrochemical properties of
these diarylethene derivatives, such as photochromic proper-
ties, fluorescence, and their electrochemical characteristics,
were significantly dependent on the effect of the electron-
donating methoxyl substituent position and the pyrazole
unit. The reason may be attributed to the different electron-
donating effects when the methoxyl group was substituted
on the different positions of the terminal benzene ring and
the different aromatic stabilization energy of pyrazole unit
from that of thiophene unit. All the diarylethenes reported in
this study exhibited good photochromism and strong fluores-
cence. Significantly, their cycloreversion quantum yields are
much higher than those of their cyclization quantum yields.
© 2007 NRC Canada

20
Can. J. Chem. Vol. 85, 2007
14. S.Z. Pu, T.S. Yang, J.K. Xu, L. Shen, G.Z. Li, Q. Xiao, and B.
Chen. Tetrahedron, 61, 6623 (2005).
15. K. Uchida, T. Ishikawa, M. Takeshita, and M. Irie. Tetrahe-
The results are very useful for the design of efficient
photoactive and diarylethene compounds with excellent
characteristics.
dron, 54, 6627 (1998).
16. S. Takami and M. Irie. Tetrahedron, 60, 6155 (2004).
17. S. Takami, S. Kobatake, T. Kawai, and M. Irie. Chem. Lett. 32,
892 (2003).
Acknowledgements
This work was supported by the Project of National Natu-
ral Science Foundation of China (grant No. 20564001), the
Natural Science Foundation of Jiangxi, China (grant
No.050017), the Science Funds of the Education Office of
Jiangxi, China (grant No. [2005] 140), and the Projects of
Fundamental Study & Applied Fundamental Study Funds of
Jiangsu Teachers University of Technology.
18. K. Uchida, T. Matsuoka, K. Sayo, M. Iwamoto, S. Hayashi,
and M. Irie. Chem. Lett. 28, 835 (1999).
19. K. Yagi and M. Irie. Bull. Chem. Soc. Jpn. 76, 1625 (2003).
20. M. Irie. Supramol. Sci. 3, 87 (1996).
21. T. Yamaguchi and M. Irie. Tetrahedron Lett. 47, 1267 (2006).
22. T. Yamaguchi and M. Irie. J. Org. Chem. 70, 10323 (2005).
23. T. Yamaguchi, Y. Fujita, H. Nakazumi, S. Kobatake, and M.
Irie. Tetrahedron, 60, 9863 (2004).
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6473 (2006).
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© 2007 NRC Canada