Syntheses and properties of new photochromic diarylethene derivatives having a pyrazole unit

Article abstract of DOI:10.1016/j.tetlet.2006.06.073

New photochromic diarylethenes 1a, 2a and 3a bearing a pyrazole unit have been synthesized. Their properties, including photochromic reactivity, fluorescence and electrochemical properties were investigated. These compounds showed good photochromic proper

Full text of DOI:10.1016/j.tetlet.2006.06.073

Tetrahedron Letters 47 (2006) 6473–6477
Syntheses and properties of new photochromic
diarylethene derivatives having a pyrazole unit
*
Shouzhi Pu, Tianshe Yang, Jingkun Xu and Bing Chen
Jiangxi Key Lab of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, PR China
Received 30 March 2006; revised 3 June 2006; accepted 6 June 2006
Abstract—New photochromic diarylethenes 1a, 2a and 3a bearing a pyrazole unit have been synthesized. Their properties, including
photochromic reactivity, fluorescence and electrochemical properties were investigated. These compounds showed good photochro-
mic properties, high cycloreversion quantum yield and relatively strong fluorescence. The cycloreversion quantum yields of 1a, 2a
and 3a are 0.46, 0.53 and 0.57, respectively, which are larger than those of their cyclization quantum yields (0.43, 0.45 and 0.47,
respectively). The oxidations of diarylethenes 1a, 2a and 3a were initiated at 0.73, 1.11 and 0.79 V, respectively. Moreover, the posi-
tion of the methoxyl substituent had remarkable impacts on the above optical and electrochemical properties.
ꢀ
2006 Published by Elsevier Ltd.
The design and synthesis of photochromic compounds
is an area of intense research because of their wide-
spread potential application in photonic devices such
as high-density optical recording materials and photo-
are thermally unstable and return to the open-ring iso-
2
4
mers even in the dark. When the aryl groups have
low aromatic stabilization energy, the derivatives under-
2
7
go thermally irreversible photochromic reactions. Pyr-
azole is an attractive aryl unit because of its low
aromatic stabilization energy and the structure is similar
to isothiazole, expecting to undergo thermally irrevers-
ible photochromic reactions. Until now, photochromic
hybrid diarylethene derivatives with pyrazole and thio-
phene moieties have not yet been reported.
1
–6
switches.
Among various types of photochromic
compounds, photochromic diarylethene derivatives with
heterocyclic aryl rings are the most promising organic
photochromic compounds for photoelectronic applica-
tions because of the excellent thermal stability of both
of the two isomers, fatigue resistant character, rapid
7
–10
response and high reactivity in solid state.
In the present work, we have synthesized three new
diarylethenes having a pyrazole unit, 1-(1,3,5-trimethyl-
1-pyrazol-4-yl)-2-[2-methyl-5-(2-methoxylphenyl)-1-thien-
3-yl]perfluorocyclopentene (1a), 1-(1,3,5-trimethyl-1-pyr-
azol-4-yl)-2-[2-methyl-5-(3-methoxylphenyl)-1-thien-3-yl]-
perfluorocyclopentene (2a) and 1-(1,3,5-trimethyl-1-pyr-
azol-4-yl)-2-[2-methyl-5-(4-methoxylphenyl)-1-thien-3-yl]-
perfluorocyclopentene (3a). The effect of electron-donat-
ing methoxyl substituent position on the properties of
photochromic diarylethenes was investigated based on
the absorption spectra, fluorescence and electrochemi-
cal properties. The photochromism of diarylethenes
1a–3a which are discussed in this letter, is shown in
Scheme 1.
To date, a large number of publications concerning syn-
thesis and investigation of their photochromic proper-
ties of diarylethenes with heterocyclic aryl rings have
been reported. Among diarylethene derivatives so
far synthesized, most of the hetero-aryl moieties have
1
0–19
been thiophene or benzothiophene rings,
and other
2
0,21
22,23
hetero-aryl moieties, such as thiazole,
indole,
2
4
25
26
27
benzofuran, crysothiophene, pyrrole and indene,
have also been reported partially. The photochromic
performance of each kind of diarylethene is strongly
dependent on the aryl moieties. Diarylethenes with thio-
phene or benzothiophene moieties, for instance, exhibit
excellent thermal stability and remarkable fatigue resis-
1
0
tance; diarylethenes with indole rings exhibit strong
2
2
fluorescence; whereas diarylethenes with pyrrole rings
The synthetic route for diarylethenes 1a, 2a and 3a is
shown in Scheme 2. At first, 1-(1,3,5-trimethyl-1-pyr-
azol-4-yl)perfluorocyclopentene (5) was synthesized
2
8
*
Corresponding author. Tel.: +86 791 3805183; fax: +86 791
805212; e-mail: pushouzhi@tsinghua.org.cn
according to the similar procedure of Peters et al.
solution of 4-bromo-1,3,5-trimethylpyrazole (4)
3
A
0040-4039/$ - see front matter ꢀ 2006 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2006.06.073

6474
S. Pu et al. / Tetrahedron Letters 47 (2006) 6473–6477
F
F
0.6
0.5
F
F
F
F
F
F
UV
Vis
F
F
F
F
UV
0
0
0
0
0
.4
.3
.2
.1
.0
N
N
N
S
N
S
O
O
1
2
3
b: ortho-OCH3
b: meta-OCH
b: para-OCH
1a: ortho-OCH3
2a: meta-OCH
3a: para-OCH
3
3
3
3
UV
Scheme 1. Photochromism of diarylethenes 1a–3a.
F
F
Br
300
400
500
600
700
800
F
F
1
) n-BuLi,-78 °C
2 ) C
Wavelength (nm)
F
F
N
N
5 8
F
Figure 1. Absorption spectra of diarylethenes 1a in hexane solution
C = 2.0 · 10 mol/L).
F
À5
(
N
4
N
5
7
: R=ortho-OCH3
O
Table 1. Absorption spectral properties of diarylethenes 1–3 in hexane
at 2.0 · 10 mol/L
Br
Br
8: R=meta-OCH
9
Br
3
À5
: R=para-OCH
3
S
B(OH)
2
Pd(Pph ) , THF
3 4
S
a
b
c
Compound
k
o,max (nm)
k
c, max (nm)
U
Na CO , aq.
2
3
À1
À1
À1
À1
)
6
O
(e/Lmol cm
)
(e/Lmol cm
U
o–c
U
c–o
F
F
F
F
4
4
1
2
3
278 (2.7 · 10 )
573 (1.1 · 10 )
0.43 0.46
0.45 0.53
0.47 0.57
F
F
4
3
1
2
3
a: R=ortho-OCH3
a: R=meta-OCH3
a: R=para-OCH
284 (2.5 · 10 )
573 (7.9 · 10 )
n-BuLi
78 °C
4
4
291 (2.9 · 10 )
573 (1.0 · 10 )
-
N
3
a
b
c
Absorption maxima of open-ring forms.
Absorption maxima of closed-ring forms.
Quantum yields of cyclization reaction (Uo–c) and cycloreversion
reaction (Uc–o), respectively.
N
S
O
Scheme 2. Synthetic route for the target compounds.
(
2.0 g, 10.6 mmol) in an anhydrous THF (30 mL) cooled
to À78 ꢁC was treated with n-BuLi/hexane solution
6.6 mL, 1.6 M) dropwise under an argon atmosphere.
tral properties are summarized in Table 1. The quantum
yields for diarylethenes 1–3 were measured in hexane
(
using
1,2-bis(2-methyl-5-phenyl-thien-3-yl)perfluoro-
3
1
After 30 min, perfluorocyclopentene (1.45 mL, 10.7
mmol) was slowly added to the reaction mixture. The
reaction mixture was stirred at this low temperature
for 3 h, then was extracted with diethyl ether and evap-
orated in vacuo, and purified by column chromatogra-
phy (chloroform) to give compound (5) in 65.8% yield.
Then, Suzuki coupling of three isomeric bromobenzenes
cyclopentene as a reference, and the results are also
shown in Table 1. From these data, we can see that the
maxima wavelengths of closed-ring forms and the molar
absorption coefficients, both open-ring and closed-ring
forms, of the three diarylethene compounds are not evi-
dently different. However, the maxima wavelengths of
open-ring forms and their quantum yields of cyclization
reactions and cycloreversion reactions are relatively
remarkable. Among diarylethenes 1–3, the maxima
wavelength of the open-ring form and its quantum yields
of cyclization and cycloreversion reactions of the ortho-
substituted derivative are the smallest (kO,max = 278 nm,
2
9
with thiophene boronic acid (6) gave methoxylphenyl-
thiophene derivatives (7–9). Finally, they were lithiated
and then coupled with compound 5 to give methoxyl-
substituted diarylethene derivatives (1a–3a). The struc-
3
0
1
tures of compounds 1a–3a were confirmed by
H
1
9
NMR and F NMR spectroscopy, mass spectrometry,
IR and UV–vis spectroscopy.
UO–C = 0.43, UC–O = 0.46); while those of the para-
substituted derivative are the biggest (kO,max = 291 nm,
UO–C = 0.47, UC–O = 0.57). The values of the meta-
substituted derivative are in between those of the ortho-
and para-substituted derivatives (kO,max = 284 nm,
Figure 1 shows the absorption spectral change of
diarylethene 1a by photoirradiation in hexane (C =
À5
2
.0 · 10 mol/L). In hexane solution, the absorption
UO–C = 0.45, UC–O = 0.53). In addition, for diaryleth-
maximum of 1a was observed at 278 nm (e =
enes 1–3, all of the cycloreversion quantum yields are
higher than their cyclization quantum yields (Table 1).
This is a unique characteristic of diarylethene derivatives
having a pyrazole unit, and it is a significant difference
between diarylethenes reported herein and all other
diarylethenes reported so far.
4
À1
À1
2
.7 · 10 Lmol cm ). Upon irradiation with 313 nm
light, the color of the hexane solution turned blue, in
which the absorption maximum was observed at
3
À1
À1
5
73 nm (e = 7.6 · 10 Lmol cm ). The blue solution
turned colorless, upon irradiation with visible light
k > 500 nm).
(
The fluorescence spectra of diarylethenes 1a–3a
À5
The spectral changes of diarylethenes 2a and 3a were
similar to that of diarylethene 1a. Their absorption spec-
(C = 2.0 · 10 mol/L) in hexane at room temperature
are illustrated in Figure 2. All of them showed good

S. Pu et al. / Tetrahedron Letters 47 (2006) 6473–6477
6475
5
4
3
2
1
000
000
000
000
000
0
4500
-
6
2
.0x10 mol/L
30
1
a
1.0x10-3 mol/L
-
6
5
.0x10 mol/L
25
4
3
3
2
2
1
1
000
500
000
500
000
500
000
-5
1
.0x10 mol/L
-5
20
15
10
5
2.0x10 mol/L
-5
5
.0x10 mol/L
-
4
1
.0x10 mol/L
-4
2
.0x10 mol/L
-3
1.0x10 mol/L
0
3
50 400 450 500 550 600
2a
3a
500
0
350
400
450
500
550
600
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Figure 2. Fluorescence spectra of diarylethenes 1a–3a in hexane
solution (C = 5.0 · 10 mol/L) at room temperature, excited at 315,
Figure 3. Fluorescence spectra of diarylethene 1a in various concen-
trations in hexane at room temperature, monitored at 315 nm.
À5
317 and 280 nm, respectively.
room temperature, as shown in Figure 3. When the
fluorescence at their respective excitation wavelengths,
and their fluorescence intensity decreased dramatically
along with the photochromism from open-ring isomer
to closed-ring isomer upon irradiation with 313 nm
concentration of diarylethene 1a in hexane was
À6
À5
increased from 2.0 · 10 to 5.0 · 10 mol/L, the maxi-
mum emission almost arose at 441 nm when excited at
315 nm, and the relative fluorescence intensity increased
3
2,33
light.
As shown in Figure 2, we can clearly see that
dramatically. However, when the concentration in-
À5
À3
the hexane solutions of diarylethenes 1a–3a showed
strong fluorescence at 441, 438 and 416 nm when excited
at 315, 317 and 280 nm, respectively. When irradiated
by light of 313 nm, the photocyclization reactions were
carried out and the non-fluorescent closed-ring forms
of these three compounds were produced. The back irra-
diation by appropriate wavelength visible light regener-
ated the open-ring forms of diarylethenes 1a–3a and
recovered the original emission spectra. The phenomena
are useful for application as the fluorescence
creased from 5.0 · 10 to 1.0 · 10 mol/L, the maxi-
mum emission appeared in a minor red shift from 441
to 446 nm, and the relative fluorescence intensity de-
creased remarkably. The hexane solution showed very
weak fluorescence when the concentration was increased
À3
to 1.0 · 10 mol/L. Just as for 1a, compounds 2a and
3a showed similar fluorescent properties depending on
the concentration employed. The results are summa-
rized in Table 2. These data show that all fluorescences
of the three compounds are remarkably concentration
dependent. The fluorescence peaks showed a minor
red shift upon increasing the concentration. However,
the relative fluorescence intensity showed relatively
complex change. They showed a remarkable initial in-
crease with subsequent dramatic decrease with increas-
ing concentration. The biggest and the smallest values
3
4,35
switches.
The fluorescence maxima of diarylethenes
1
a–3a were observed at 416 and 441 nm, indicating that
the electron-donating methoxyl substituent position
effect on the fluorescence peak is significant. On the other
hand, the fluorescence intensities changed dramatically
along with the substituent position. The values
decreased rapidly from ortho-substituted to para-
substituted diarylethene derivative. Among the three
compounds, the relative fluorescence intensity of ortho-
substituted derivative is the strongest, and that of
para-substituted derivative is the weakest. The values
of the meta-substituted derivative are located in between
those of the ortho- and para-substituted derivatives. The
result is exactly contrary to that reported in our previous
À5
À3
were obtained at 5.0 · 10
and 1.0 · 10 mol/L,
respectively. The results also demonstrated that mole-
cular aggregation and the fluorescence quenching may
occur when the concentration increases.
3
7
The solvent effect on the fluorescence spectra of diaryl-
ethenes 1a–3a was also investigated. Fluorescence
3
6
letter. In that letter, the electron-withdrawing fluorine
substituent position does not affect significantly on the
fluorescence peak of dithienylethene, but can affect
remarkably on the fluorescence intensity, with the fluo-
rescence intensity of ortho-substituted derivative being
the weakest, and that of para-substituted derivative the
strongest. The possible reason is ascribed to the elec-
tron-donating nature of methoxyl substituent of the ter-
minal phenyl group, and the pyrazole ring can also
possibly affect the fluorescence properties of the three
diarylethenes.
Table 2. The concentration effect on the fluorescence spectra of
diarylethenes 1a–3a at room temperature in hexane, monitored at
2
90 nm
Concentration (mol/L)
kem, max (Relative intensity)
1a
2a
3a
À6
2
5
.0 · 10
.0 · 10
440 (699)
441 (1529)
441 (2573)
441 (3394)
441 (4366)
442 (2637)
442 (641)
446 (29)
438 (219)
438 (484)
436 (816)
440 (1323)
438 (2180)
439 (2005)
443 (1084)
445 (24)
414 (102)
418 (193)
412 (320)
420 (396)
416 (514)
421 (228)
421 (48)
441 (7)
À6
À5
À5
À5
À4
À4
À3
1.0 · 10
2.0 · 10
5.0 · 10
.0 · 10
.0 · 10
.0 · 10
1
2
1
The concentration dependence on the fluorescence spec-
trum of diarylethene 1a was measured in hexane at

6476
S. Pu et al. / Tetrahedron Letters 47 (2006) 6473–6477
1
80
150
20
Hexane
7
6
5
4
3
2
1
000
000
000
000
000
000
000
0
Dichloromethane
Ethyl acetate
DMF
2b
3a
Acetonitrile
1
1a
9
6
3
0
0
0
0
1b
2a
3b
350
400
450
500
550
600
0
.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Wavelength (nm)
Potential / V vs. Pt
Figure 5. The anodic polarization curves of diarylethene 1–3.
Figure 4. Fluorescence spectra of diarylethene 1a in various solvents in
À5
5.0 · 10 mol/L at room temperature, monitored at 333 nm.
spectra of 1a in different solvents at room temperature
are shown as in Figure 4, and the fluorescent properties
of these three compounds in various solvents at room
temperature are summarized in Table 3. From these
data, we can see that the fluorescence spectra depended
on the polarity of the solvent. It gave a systematic red
shift when the polarity of the solvent increased. How-
ever, the relative fluorescence intensity is irregular upon
increasing the polarity of the solvent.
title compounds. For ortho-position substitution of
methoxyl group of 1a and 1b, the electron-donating
effect together with its steric effect plays a main role,
which made the oxidation potential onset of 1a and 1b
almost similar to each other. On the other hand, when
substituted at para-position, the oxidation potential
onset of closed-ring form (3b) is higher than that of
open-ring form (3a). This indicates that the electron-
donating effect of methoxyl group is more significant
in open-ring form. However, when substituted at meta-
position, opposite results can be easily observed. The
oxidation potential onset of closed-ring form (2b) is low-
er than that of open-ring form (2a). This implies that the
electron-donating effect of methoxyl group is more sig-
nificant in closed-ring form of compound 2. In addition,
three oxidation processes can be easily observed during
anodic oxidation of 2a and 2b, while only two oxidation
processes can be observed during anodic oxidation of
the other two compounds. The three oxidation processes
may involve the oxidation of pyrazole ring, thiophene
ring, together with the benzene ring of compound 2.
However, the substitution of methoxyl group at para-
and ortho-position made the oxidation process of benz-
ene not very clear for 1a, 1b, 3a and 3b. All these
electrochemical results indicated that the substitution
of methoxyl group on benzene and the pyrazole unit
had great effect on both the open-ring and closed-ring
forms of the three diarylethene derivatives. The detailed
explanation requires further investigation.
The electrochemical properties of diarylethene are being
used for molecular switching and can also be potentially
3
6
applied to molecular-scale electronic switches. In order
to investigate the electrochemical properties of diaryl-
ethenes having a pyrazole unit, we performed electro-
chemical examinations by linear sweep method under
the same experimental conditions using diarylethenes
1
a, 2a and 3a, respectively. Experimental method and
condition were the same as that described in our previ-
3
6
ous letter. The typical electrolyte was acetonitrile
5 mL) containing 0.15 mol/L LiClO4 and
.0 · 10 mol/L dithienylethene. All solutions were
(
4
À3
deaerated by a dry argon stream and maintained at a
slight argon overpressure during electrochemical experi-
ments. Figure 5 showed the anodic polarization curves
of 1a, 1b, 2a, 2b, 3a and 3b on Platinum electrode. It
can be clearly seen from Figure 5 that the oxidation of
1
a, 1b, 2a, 2b, 3a and 3b was initiated at 0.73, 0.78,
1
.11, 0.87, 0.79 and 1.00 V, respectively. From these
data together with those shown in Figure 5, a conclusion
can be reasonably drawn that the substitution of elec-
tron-donating methoxyl group has great effect on the
In conclusion, new types of photochromic diarylethenes
having a pyrazole unit were synthesized and their optical
and electrochemical properties were investigated. The
results showed that the photochromic reactivity, fluores-
cence and electrochemical properties were remarkably
dependent on the substituent position effect of methoxyl
group and the pyrazole unit. The reason may be attrib-
uted to the different electron-donating effects when the
methoxyl group was substituted on the different posi-
tions of the terminal benzene ring and the different aro-
matic stabilization energies of pyrazole unit from that of
thiophene unit. The results of this study are useful for
the design of efficient photoactive and excellent charac-
teristic diarylethene compounds.
Table 3. Fluorescent properties of diarylethenes 1a–3a in various
À5
solutions at 5.0 · 10 mol/L
a
Solvent
k,max (nm)
1
a
2a
3a
Hexane
441 (4366)
452 (6354)
452 (5560)
460 (5376)
463 (5629)
438 (2180)
450 (2739)
452 (2196)
460 (2225)
460 (2130)
416 (514)
440 (549)
443 (433)
456 (376)
470 (330)
Dichloromethane
Ethyl acetate
DMF
Acetonitrile
a
Fluorescence maxima, excited at 333, 330 and 320 nm for 1a, 2a and
a, respectively.
3

S. Pu et al. / Tetrahedron Letters 47 (2006) 6473–6477
6477
Acknowledgements
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2
This work was supported by the Projects of National
Natural Science Foundation of China (Grant No.
0564001), the Natural Science Foundation of Jiangxi,
China (Grant No. 050017) and the Science Funds of
the Education Office of Jiangxi, China (Grant No.
1
2
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. Fan, M. G.; Yu, L.; Zhao, W. In Organic Photochromic
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1
2
3
2
46, 115–125.
1
3
0. Selected data for 1a–3a: Compound 1a: H NMR
(
400 MHz, CDCl
3
): d 1.96 (s, 3H, –CH
3
), 1.97 (s, 3H,
–
CH ), 2.04 (s, 3H, –CH ), 3.69 (s, 3H, –OCH ), 3.92
3
3
3
(
3
s, 3H, –NCH ), 6.98 (dd, 2H, J = 8.0 Hz, benzene–H),
d 7.28 (d, 1H, J = 8.0 Hz, benzene–H), d 7.42 (s, 1H,
thiophene–H), d 7.58 (dd, 1H, J = 7.6 Hz, benzene–H);
1
1
9
F NMR (100 MHz, CDCl ) d À109.46 (2F), À110.51
4
5
6
3
+
À1
)
(
2F), À132.57 (2F); MS m/z (M ) 487.2; IR (KBr, cm
2
7
2
1
–
51, 1127, 1274, 1341,1438, 1635, 2839, 2942; Compound
a: H NMR (400 MHz, CDCl ): d 1.96 (s, 3H, –CH ),
.97 (s, 3H, –CH
OCH
1
3
3
3
), 2.03 (s, 3H, –CH
3
), 3.70 (s, 3H,
3
3
), 3.86 (s, 3H, –NCH ), 6.85 (d, 1H, J = 8.0 Hz,
benzene–H), 7.05 (s, 1H, thiophene–H), 7.12 (d, 1H,
7
8
. Tian, H.; Yang, S. J. Chem. Soc. Rev. 2004, 33, 85–97.
. Matsuda, K.; Irie, M. J. Photochem. Photobiol. C:
Photoch. Rev. 2004, 5, 169–182.
. Morimoto, M.; Irie, M. Chem. Commun. 2005, 3895–3905.
0. Irie, M. Chem. Rev. 2000, 100, 1685–1716.
J = 7.6 Hz, benzene–H), 7.24 (s, 1H, benzene–H), 7.30
1
9
(
t, 1H, J = 8.0 Hz, benzene–H); F NMR (100 MHz,
CDCl
3
) d À109.45 (2F), À110.51 (2F), À132.58 (2F); MS
9
1
1
+
À1
m/z (M ) 487.2; IR (KBr, cm ) 687, 770, 799, 882, 1121,
271, 1336,1434, 1602, 2841, 2948, 2966; Compound 3a:
1
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1
H NMR (400 MHz, CDCl
s, 3H, –CH ), 2.03 (s, 3H, –CH
OCH ), 3.84 (s, 3H, –NCH ), 6.91 (d, 2H, J = 8.4 Hz,
3
) d 1.94 (s, 3H, –CH
3
), 1.97
2
10.
(
–
3
3
), 3.70 (s, 3H,
1
1
1
1
1
1
1
1
2
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3
3
benzene–H), 7.13 (s, 1H, thiophene–H), 7.45 (d, 2H,
1
9
J = 8.4 Hz, benzene–H); F NMR (100 MHz, CDCl
3
)
d À109.43 (2F), À110.52 (2F), À132.58 (2F); MS m/z
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+
À1
(
1
M ) 487.2; IR (KBr, cm ) 800, 821, 1124, 1255,
341,1477, 1550, 1611, 2841, 2927, 2960.
3
777–3780.
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