The effect of the fluorine atom position upon isomeric photochromic diarylethenes bearing a pyrrole unit

Article abstract of DOI:10.1016/j.jphotochem.2013.04.023

A new class of isomeric diarylethenes bearing a pyrrole moiety have been synthesized, and their properties including photochromism, fluorescence, and electrochemical properties have been discussed systematically. Each of the diarylethenes exhibited eviden

Full text of DOI:10.1016/j.jphotochem.2013.04.023

Journal of Photochemistry and Photobiology A: Chemistry 263 (2013) 41–49
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
The effect of the fluorine atom position upon isomeric photochromic
diarylethenes bearing a pyrrole unit
Gang Liu, Shouzhi Pu∗, Bing Chen
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science & Technology Normal University, Nanchang 330013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of isomeric diarylethenes bearing a pyrrole moiety have been synthesized, and their prop-
erties including photochromism, fluorescence, and electrochemical properties have been discussed
systematically. Each of the diarylethenes exhibited evident photochromism and functioned as a remark-
able fluorescent switch in both solution and PMMA films. Their photoconversion ratios were larger than
90%, and the fluorescent modulation efficiencies were greater than 88% in the photostationary state.
The absorption maxima, quantum yields of cyclization and cycloreversion, and band-gaps of the closed-
ring isomers increased whereas the emission intensity and band-gaps of the open-ring isomers greatly
decreased when the fluorine atom was attached at any of the three positions on the terminal benzene
ring. Cyclic voltammograms suggested that the oxidation onsets and band-gaps of the open-ring isomers
were much bigger than those of the closed-ring isomers. The fluorine atom and its substituted position
could availably modulate their optical and electrochemical behaviors.
Received 4 December 2012
Received in revised form 20 April 2013
Accepted 25 April 2013
Available online 6 May 2013
Keywords:
Photochromism
Diarylethene
Pyrrole moiety
Substituent position effect
Fluorine atom
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
conformations of open-ring isomers, the categories of heteroaryl
moieties, the electron donor/acceptor substituents, as well as the
Various types of photochromic organic compounds, including
stilbene [1], azobenzenes [2], furylfulgides [3,4], spirooxazines [5],
and diarylethenes [6,7], have been so far developed from the
view points of fundamental organic photochemistry and practi-
cal applications such as light-modulating and optical recording
materials. During the past several decades, there have been
important achievements in the synthesis of new families of
organic photochromic molecules [8–10]. Among these molecules,
diarylethenes with heterocyclic aryl rings are regarded as the
best candidates for photoelectronic applications because of their
excellent thermal stability, remarkable fatigue resistance, and high
coloration sensitivity [11,12].
The photochromic process of diarylethenes is based on a
reversible transformation between the open-ring isomer with a
hexatriene structure and the closed-ring isomer with a cyclohexa-
diene structure, according to the Woodward–Hoffmann rule [11].
The open-ring isomer has two conformations, anti-parallel and par-
allel conformations, which exchange even at room temperature
[13]. In general, the diarylethene undergoes a photocyclization
reaction if it is orientated in an anti-parallel array and if the
distance between two reactive carbon atoms on the aryl moi-
ety is within 0.42 nm [14]. The photochromic characteristics of
diarylethenes mainly depend on several factors, including the
conjugation length of heteroaryl systems [15–24]. Particularly, the
categories of heteroaryl moieties and substituents on the reac-
tive position greatly influence the photoreactivity of diarylethene
derivatives. On the one hand, the nature of the heteroaryl moieties
greatly influences the photo-reactivity and the distinguishable fea-
tures of diarylethenes. For instance, diarylethenes with benzofuran
moieties have outstanding fatigue resistance and small photocon-
version ratio [15], whereas diarylethenes bearing both thiazole
and benzene moieties have relatively weak fatigue resistance
[24]. Unsymmetrical diarylethenes bearing pyrrole groups exhib-
ited excellent thermal stability and strong fluorescence switch
[23], whereas symmetrical diarylethenes bearing two pyrrole
groups were thermally unstable and returned to the open-ring
isomers even in the dark [25]. Moreover, pyrazole derivatives
had a big absorption maxima and high cycloreversion quantum
yield [26], whereas oxazole derivatives had a small absorption
maxima and showed good pH sensitivity and strong fluores-
cence switch [27]. On the other hand, the electron donor/acceptor
substituents and their substituted positions have also a signifi-
cant effect on the properties of diarylethene derivatives [23,28].
For instance, the electron-donating substituents of the bis(3-
thienyl)ethene diarylethenes could be effective to increase the
absorption coefficient of the closed-ring isomers and decrease the
cycloreversion quantum yield [29,30]. However, those attached
bis(2-thienyl)ethene diarylethenes could increase the absorption
maxima of the open-ring isomers and reduce the cyclization quan-
tum yield [31].
∗
Corresponding author. Tel.: +86 791 83831996; fax: +86 791 83831996.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2013.04.023

42
G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 263 (2013) 41–49
standard with a formal potential of E1/2 = +0.35 V versus platinum
F
F
F
F
F
F
wire. The typical electrolyte was acetonitrile (5 mL) containing
0.1 mol L−1 tetrabutylammonium tetrafluoroborate ((TBA)BF4) and
1.0 × 10−3 mol L−1 diarylethene sample. All solutions were deae-
rated by bubbling with a dry argon stream and maintained at
a slight argon overpressure during electrochemical experiments.
Solvents used were spectroscopic grade and were purified by dis-
tillation.
F
F
F
UV
Vis
F
F
F
S
R
NC
N
S
R
NC
N
1c: R = ortho
2c: R = meta
3c: R = para
4c: R = H
F
1o: R = ortho
F
F
F
F
2o: R = meta
3o: R = para
4o: R = H
2.2. Synthesis
F
The synthesis route for diarylethenes 1o–4o is shown in
Scheme 2. Compounds (5a–5d) [18] were separately lithiated and
then coupled with 1-(2-cyano-1,5-dimethyl-4-pyrryl) perfluorocy-
clopentene (6) [23] to give diarylethenes 1o–4o, respectively.
Scheme 1. Photochromism of diarylethenes 1–4.
Pyrrole is an attractive aryl unit due to its biological character-
istics [28], and its structure is similar to those of thiophene and
furan. However, reports of diarylethene derivatives with a pyrrole
unit are very rare [23,32]. Previously, we have reported the effect of
methoxy/cyano substituent position on the properties of unsym-
metrical diarylethenes with a pyrrole unit [23,33]. We have also
investigated the effect of fluorine atom position on the properties
of dithienylethenes [18,20]. The results indicated that the sub-
stituent position had a significant effect on the properties of these
diarylethene derivatives. However, as far as we are aware, the effect
of fluorine atom position on the properties of diarylethenes with
a pyrrole unit has not hitherto been reported. In order to realize
this idea, we have synthesized four hybrid diarylethenes bearing
both pyrrole and thiophene moieties in this work. The synthesized
diarylethenes are 1-[2-methyl-5-(2-fluorophenyl)-3-thienyl]-
2.2.1. Synthesis of 1-[2-methyl-5-(2-fluorophenyl)-3-thienyl]-2-
(2-cyano-1,5-dimethyl-4-pyrryl)perfluorocyclopentene
(1o)
To a stirred anhydrous THF (80 mL) of compound 5a (0.76 g,
2.82 mmol)[18] was added dropwise a 2.5 mol L−1 n-BuLi/hexane
solution (1.20 mL, 3.00 mmol) at 195 K under argon atmosphere.
After 30 min, 15 mL THF containing compound 6 (0.86 g, 2.75 mmol)
[23] was added and the reaction mixture was stirred for 2 h at
this low temperature and quenched by water. The product was
extracted with diethyl ether, and then dried with MgSO4, filtered,
and evaporated in vacuo. The crude product was purified by column
chromatography using petroleum ether as the eluent to give 0.51 g
compound 1o as a colorless solid in 38% yield. M.p. 342–343 K; anal.
calcd. for C23H15F7N2S (%): C, 57.02; H, 3.12; N, 5.78. Found C, 57.11;
H, 3.04; N, 5.88; 1H NMR (400 MHz, CDCl3): ı 1.77 (s, 3H, CH3),
2.01 (s, 3H, CH3), 3.61 (s, 3H, CH3), 6.93 (s, 1H, pyrrole-H), 7.13 (d,
1H, J = 8.0 Hz, phenyl-H), 7.17 (s, 1H, thiophene-H), 7.26–7.29 (m,
1H, phenyl-H), 7.37 (s, 1H, phenyl-H), 7.37 (t, 1H, J = 6.8 Hz, phenyl-
H). 13C NMR (400 MHz, CDCl3): ı 11.3, 14.3, 33.0, 41.1, 105.6, 111.0,
112.9, 116.6, 118.8, 121.3, 124.6, 126.0, 128.3, 129.3, 135.1, 141.2,
157.8, 160.3; IR (ꢀ, KBr, cm−1): 757, 842, 896, 984, 1054, 1110, 1187,
1273, 1332, 1383, 1436, 1482, 1551, 1635, 2216, 2920.
2-(2-cyano-1,5-dimethyl-4-pyrryl)perfluorocyclopentene
(1o),
1-[2-methyl-5-(3-fluorophenyl)-3-thienyl]-2-(2-cyano-1,5-dime-
thyl-4-pyrryl) perfluorocyclopentene (2o), 1-[2-methyl-5-(4-
fluorophenyl)-3-thienyl]-2-(2-cyano-1,5-dimethyl-4-pyrryl)
perfluorocyclopentene
(3o),
and
1-(2-methyl-5-phenyl-3-
thienyl)-2-(2-cyano-1,5-dimethyl-4-pyrryl)perfluorocyclopentene
(4o) of which, 1o, 2o, and 3o are new compounds. The pho-
tochromic process of these derivatives is shown in Scheme 1.
Although diarylethene 4o has been reported previously [23,33],
it is presented here for comparison with those of other three
diarylethene derivatives.
2.2.2. Synthesis of 1-[2-methyl-5-(3-fluorophenyl)-3-thienyl]-2-
(2-cyano-1,5-dimethyl-4-pyrryl)perfluorocyclopentene
(2o)
2. Experimental
Diarylethene 2o was prepared by
a method similar to
that used for diarylethene 1o using 3-bromo-2-methyl-5-(3-
fluorophenyl)thiophene (5b) instead of 5a, and was obtained as a
colorless solid 0.44 g in 40% yield. M.p. 403−404 K; anal. calcd. for
C23H15F7N2S (%): C, 57.02; H, 3.12; N, 5.78. Found C, 57.09; H, 3.06;
N, 5.87; 1H NMR (400 MHz, CDCl3): ı 1.77 (s, 3H, CH3), 1.96 (s, 3H,
CH3), 3.60 (s, 3H, CH3), 6.85 (d, 1H, J = 6.8 Hz, phenyl-H), 6.92
(s, 1H, pyrrole-H), 7.06 (s, 1H, thiophene-H), 7.12 (d, 1H, J = 7.8 Hz,
phenyl-H), 7.21 (s, 1H, phenyl-H), 7.28 (t, 1H, phenyl-H). 13C NMR
(400 MHz, CDCl3): ı 11.3, 14.5, 33.0, 55.4, 105.5, 109.9, 111.4, 112.9,
113.3, 118.1, 118.8, 122.8, 125.9, 130.1, 134.6, 135.9, 140.4, 142.3,
160.1; IR (ꢀ, KBr, cm−1): 740, 777, 842, 900, 983, 1032, 1056, 1114,
1137, 1185, 1274, 1335, 1384, 1441, 1488, 1585, 1612, 1656, 2222,
2370, 2921.
2.1. General method
NMR spectra were recorded on a Bruker AV400 (400 MHz)
spectrometer with CDCl3 as the solvent and tetramethylsilane
as an internal standard. Infrared spectra (IR) were performed
using a Bruker Vertex-70 spectrometer. Elemental analysis was
measured with an elemental analyzer labeled the PE 2400 CHN
analyzer. Melting point was taken on a WRS-1B melting point
apparatus. Absorption spectra were measured using an Agilent
8453 UV/VIS spectrophotometer. Photoconversion ratios from the
open-ring to the closed-ring isomers in the photostationary state
were measured using an Shimadzu HPLC 10AVP chromatographic
analyzer. Fluorescence spectra were measured using a Hitachi
F-4500 fluorimeter. Photo-irradiation was carried out using an
SHG-200 UV lamp, a CX-21 ultraviolet fluorescence analysis cab-
inet, and a BMH-250 visible lamp. The required wavelength was
isolated by the use of the appropriate filters. Electrochemical exam-
inations were performed in a one-compartment cell by using
a Model 263 potentiostat–galvanostat (EG&G Princeton Applied
Research) under computer control at room temperature. Platinum-
electrodes (diameter 0.5 mm) served as the working electrode
and counter electrode. Platinum wire served as a quasireference
electrode. It was calibrated using an internal ferrocene (Fc/Fc+)
2.2.3. Synthesis of 1-[2-methyl-5-(4-fluorophenyl)-3-thienyl]-2-
(2-cyano-1,5-dimethyl-4-pyrryl)perfluorocyclopentene
(3o)
Diarylethene 3o was prepared by
a method similar to
that used for diarylethene 1o using 3-bromo-2-methyl-5-(4-
fluorophenyl)thiophene (5c) instead of 5a, and was obtained as
a colorless solid 0.39 g in 34% yield. M.p. 434–435 K; anal. calcd.
for C23H15F7N2S (%): C, 57.02; H, 3.12; N, 5.78. Found C, 57.07; H,
3.08; N, 5.86; 1H NMR (400 MHz, CDCl3): ı 1.71 (s, 3H, CH3), 1.88
(s, 3H, CH3), 3.55 (s, 3H, CH3), 6.86 (s, 1H, pyrrole-H), 7.02 (t,

G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 263 (2013) 41–49
43
F
F
F
F
F
F
F
F
F
Br
F
F
F
F
+
n-BuLi/THF
S
R
S
NC
R
N
195 K
NC
N
1o: R = ortho
F
F
F
5a: R = ortho
5b: R = meta
F
6
2o: R = meta
3o: R = para
4o: R = H
F
5c: R = para
F
5d: R = H
Scheme 2. Synthetic route for diarylethenes 1o–4o.
2H, J = 8.4 Hz, phenyl-H), 7.10 (s, 1H, thiophene-H), 7.42–7.45 (m,
2H, phenyl-H). 13C NMR (400 MHz, CDCl3): ı 11.3, 14.5, 77.0, 33.1,
105.5, 109.9, 112.9, 115.9, 116.1, 118.8, 122.5, 126.0, 127.3, 127.4,
129.5, 135.9, 140.3, 141.4, 161.3, 163.8; IR (ꢀ, KBr, cm−1): 741, 807,
828, 894, 987, 1056, 1095, 1183, 1231, 1275, 1335, 1397, 1469,
1513, 1548, 1623, 2221, 3123.
2.2.4. Synthesis of 1-(2-methyl-5-phenyl-3-thienyl)-2-(2-cyano-
1,5-dimethyl-4-pyrryl)perfluorocyclopentene
(4o)
Diarylethene 4o was prepared by the reported method [23]
using 3-bromo-2-methyl-5-phenyl-thiophene (5d) as raw mate-
rials, and was obtained as a yellow solid 0.47 g in 36% yield. M.p.
Fig. 1. Absorption spectral and color changes of diarylethenes 1–4 upon alternating irradiation with UV and visible light in hexane (2.0 × 10−5 mol L−1) at room temperature:
(A) spectral changes for 1, (B) spectral changes for 2, (C) spectral changes for 3, (D) spectral changes for 4, and (E) color changes for 1–4.

44
G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 263 (2013) 41–49
413–414 K; anal. calcd. for C23H16F6N2S (%): C, 59.22; H, 3.46; N,
6.01. Found C, 59.27; H, 3.51; N, 6.03; 1H NMR (400 MHz, CDCl3):
ı 1.77 (s, 3H, CH3), 1.96 (s, 3H, CH3), 3.61 (s, 3H, CH3), 6.93
(s, 1H, pyrrole-H), 7.23 (s, 1H, thiophene-H), 7.39 (t, 1H, J = 8.0 Hz,
benzene-H), 7.53–7.56 (m, 2H, benzene-H), 7.54 (d, 2H, J = 7.6 Hz,
benzene-H); 13C NMR (400 MHz, CDCl3): ı 10.7, 13.9, 32.4, 76.1,
76.4, 76.6, 76.7, 104.9, 109.4, 112.03, 125.0, 125.4, 127.4, 128.5,
132.7, 135.4, 139.7, 142.0; IR (ꢀ, KBr, cm−1): 756, 845, 987, 1097,
1183, 1277, 1334, 1385, 1548, 1624, 2221, 2372, 2920, 3125, 3450.
initial state
PSS state
2o
1o
1c
2c
4c
0
0
2
2
4
4
6
6
8
10
0
2
4
4
6
6
8
4o
8
10
10
initial state
PSS state
3o
3. Results and discussion
3c
3.1. Photochromic behaviors of diarylethenes
8
10
0
2
The photochromic behaviors of diarylethenes 1–4 induced by
photoirradiation at room temperature were measured both in
hexane (2.0 × 10−5 mol L−1) and in PMMA amorphous films (10%,
w/w). In hexane, the absorption spectral and color changes of
diarylethene 1–4 induced by alternating irradiation with UV light
and visible light with appropriate wavelength are shown in Fig. 1.
Diarylethene 1o exhibited a sharp absorption peak at 295 nm in
hexane, which was arisen from ␲→␲* transition [34]. Upon irra-
diation with 297 nm light, a new visible absorption band centered
at 589 nm emerged while the original peak at 295 nm decreased
due to the formation of the closed-ring isomer 1c. This could be
seen with the naked eye, as the colorless solution of 1o turned
blue. Alternatively, the blue colored solution could be bleached to
colorless upon irradiation with visible light (ꢁ > 450 nm), indicat-
ing 1c returned to the initial state 1o. The coloration–decoloration
cycle could be repeated more than 50 times and a clear isos-
bestic point was observed at 309 nm. As with diarylethene 1,
diarylethenes 2o–4o also showed evident photochromism in hex-
ane. Upon irradiation with 297 nm light, absorption bands in visible
region appeared and the solutions containing 2o–4o turned blue
as a result of the cyclization reactions to produce the closed-ring
isomers 2c–4c, which the absorption maxima were observed at
587 nm for 2c, 586 nm for 3c, and 584 nm for 4c. The blue col-
ored solutions of 2c–4c can be decolorized upon irradiation with
visible light (ꢁ > 450 nm) again because of reproducing the open-
ring isomers 2o–4o, respectively. In the photostationary state, the
isosbestic point was observed at 335 nm for 2c, 321 nm for 3c, and
334 nm for 4c. When arrived at the photostationary state, the pho-
toconversion ratios from the open-ring to the closed-ring isomers
of these derivatives were analyzed under UV irradiation in hexane
by HPLC method, and the results are shown in Fig. 2. It can be easily
calculated the photoconversion ratios of these diarylethene deriva-
tives in the photostationary sate, with the value of 92% for 1, 94%
for 2, 91% for 3, and 90% for 4, respectively (Table 1).
For practical applications in optical devices, it is very important
that photochromic materials can keep good photochromism in a
polymer film, such as the PMMA film [35]. The PMMA films were
prepared by dissolving 10 mg diarylethene sample and 100 mg
poly(methylmethacrylate) (PMMA) into chloroform (1.0 mL), with
the aid of ultrasound, then spin coating the homogeneous solu-
tion on a quartz substrate (10 mm × 10 mm × 1 mm). The thickness
of these PMMA films was determined to be ca. 20 ␮m. In PMMA
amorphous films, diarylethenes 1–4 also showed similar pho-
tochromism as samples in solution. The absorption spectral and
color changes of diarylethenes 1–4 are shown in Fig. 3. Compared to
those in hexane, the maximum absorption peaks of both the open-
ring and the closed-ring isomers of diarylethenes 1–4 in PMMA
films are at a longer wavelength than that in hexane. The red
shift values of the absorption maxima of the open-ring isomers are
16 nm for 1o, 13 nm for 2o, 13 nm for 3o, and 12 nm for 4o, and those
of the closed-ring isomers are 36 nm for 1c, 36 nm for 2c, 23 nm for
Time (m)
Fig. 2. The photoconversion ratios of diarylethenes 1–4 in hexane in the photosta-
tionary state by HPLC analysis.
3c, and 23 nm for 4c, respectively. The red shift phenomena are
consistent with those of the reported diarylethenes [23,33], which
may be attributed to the polar effect of the polymer matrix in the
amorphous solid state [36,37].
The photochromic features of diarylethenes 1–4 in hexane
and in PMMA films are summarized in Table 1. The results
showed that the fluorine atom position had a significant effect
on the photochromic properties of these diarylethene derivatives,
including the absorption maxima, molar absorption coefficients,
photoconversion ratios, and quantum yields of cyclization and
cycloreversion. Among these derivatives, the unsubstituted par-
ent diarylethene 4 had the smallest absorption maxima, quantum
yields of cyclization and cycloreversion, as well as the photo-
conversion ratio. When introduction of the electron-withdrawing
fluorine atom into any position of the terminal benzene ring, the
values of these photochromic parameters of diarylethenes 1–3
clearly increased. The result is in good agreement with that of
the reported diarylethenes bearing a methoxy group [23]. For
isomeric diarylethenes 1–3, the absorption maxima and molar
absorption coefficients of both the open-ring and the closed-
ring isomers varied with the same trend, i.e., they decreased
in the order of ortho- > meta- > para-substituent by the fluorine
atom. Therefore, the absorption maxima and molar absorption
coefficients of the ortho-substituted derivative 1 are the biggest;
while those of the para-substituted derivative 3 are the smallest.
The result is quite different from that of diarylethenes bear-
ing a methoxy group, where the absorption maxima and molar
absorption coefficients of the meta-substituted derivative are the
smallest [23]. Furthermore, the cyclization quantum yields and
the photoconversion ratios of these isomeric compounds varied
with the same trend, i.e., they decreased in the order of meta-
> ortho- > para-substituent by the fluorine atom. However, the
cycloreversion quantum yields showed a reverse changing trend,
and they increased in the order of meta- < ortho- < para-substituent
by the fluorine atom. Consequently, the meta-substituted deriva-
tive 2 had the biggest cyclization quantum yield and the smallest
cycloreversion quantum yield, while the para-substituted deriva-
tive 3 had the smallest cyclization quantum yield and the biggest
cycloreversion quantum yield. This varying pattern is different
from those of isomeric diarylethenes reported previously [18,23].
In addition, for diarylethenes 1–4, all of the cyclization quantum
yields were higher than their respective cycloreversion quantum
yields (Table 1), which is consistent with isomeric diarylethenes
reported previously [23,33]. The results indicated that the cate-
gories of heteroaryl moieties and substituents greatly influenced

G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 263 (2013) 41–49
45
Table 1
Absorption spectral properties of diarylethenes 1–4 in hexane (2.0 × 10−5 mol L−1) and in PMMA films (10 %, w/w) at room temperature.
c
Compound
ꢁo,max/nma (ε/L mol−1 cm−1
)
ꢁc,max/nmb (ε/L mol−1 cm−1
)
˚
Photoconversion ratios at PSS
Hexane
PMMA film
Hexane
PMMA film
˚
o–c
˚
c–o
1
2
3
4
295 (3.29 × 104)
291 (2.93 × 104)
290 (2.11 × 104)
290 (2.49 × 104)
311
304
303
302
589 (1.41 × 104)
587 (1.15 × 104)
586 (7.19 × 103)
584 (1.00 × 104)
625
623
609
607
0.33
0.40
0.26
0.20
0.072
0.067
0.098
0.054
92%
94%
91%
90%
a
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of cyclization reaction (˚o–c) and cycloreversion reaction (˚c–o), respectively.
b
c
Fig. 3. Absorption spectral and color changes of diarylethenes 1–4 upon alternating irradiation with UV and visible light in PMMA films (10%, w/w) at room temperature:
(A) spectral changes for 1, (B) spectral changes for 2, (C) spectral changes for 3, (D) spectral changes for 4, and (E) color changes for 1–4.

46
G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 263 (2013) 41–49
6000
4800
3600
2400
1200
0
6000
4o
4o
1o
1o
2o
3o
4800
3600
2400
1200
0
2o
3o
400
450
500
550
400
450
550
Wavelength5(0n0m)
Wavelength (nm)
(A)
(B)
Fig. 4. Emission spectra of diarylethenes 1–4 in hexane (5.0 × 10−5 mol L−1) and in PMMA films (10%, w/w) at room temperature: (A) emission spectra in hexane, excited at
340 nm; (B) emission spectra in PMMA films, excited at 350 nm.
the optoelectronic properties of diarylethenes. For example, when
replacing the pyrrole moiety with the thiophene moiety in the
same molecular skeleton of photochromic diarylethenes, the pho-
tochromic parameters including the absorption maxima, molar
absorption coefficients, and the cycloreversion quantum yields
decreased, but the cyclization quantum yields increased evidently
[18]. When replacing the fluorine atom with the methoxy group
in the same molecular skeleton, the absorption maxima and the
cycloreversion quantum yields increased significantly [10].
The thermal stabilities of the open-ring and the closed-ring iso-
mers of 1–4 were tested in hexane both at room temperature and at
342 K. Storing these solutions in hexane in the dark and then expos-
ing them to air both at room temperature and at 342 K for more
than 72 h, no changes in their UV/vis spectra were observed, sug-
gesting that no decomposition was detected. The result indicates
that these unsymmetrical diarylethenes bearing a pyrrole unit are
thermal stable.
17 nm for 4o. This is completely contrary to those of the reported
diarylethenes whose emission peaks in PMMA films are much
longer than those in solution [18,42]. For the isomeric diarylethenes
1–3, the emission peak and emission intensity varied with the
same trend in both hexane and PMMA films, i.e., they decreased
in the order of ortho- > meta- > para-substituent by the fluorine
atom. The result is in good agreement with that of the reported
diarylethenes bearing a methoxy group [23]. However, it is quite
different from that of diarylethenes bearing a thiophene moiety
[18]. Compared to isomeric diarylethenes 1o–3o, the unsubsti-
tuted parent diarylethene 4o showed the biggest emission peak
and the strongest emission intensity in both hexane and a PMMA
film. The results indicated that the fluorine atom could be effec-
tive to decrease the emission intensity and shift the emission peak
to a shorter wavelength direction, which may be attributed to the
steric hindrance of the fluorine group and its electron-withdrawing
nature. By using anthracene (0.27 in acetonitrile) as the refer-
ence, the fluorescence quantum yields of diarylethenes 1o–4o were
determined to be 0.033, 0.030, 0.028, and 0.010, respectively. The
result indicated that introduction of electron-withdrawing fluorine
group into any position of the terminal benzene ring could improve
significantly the fluorescence quantum yields of the diarylethene
derivatives.
As has been observed for most of the reported diarylethenes
[18,22,23], diarylethenes 1–4 exhibited a notable fluorescent
switch on changing from the open-ring to the closed-ring isomers.
When irradiated by UV light, the photocyclization occurred and
the fluorescence of diarylethenes 1o–4o greatly quenched due to
producing the non-fluorescence closed-ring isomers 1c–4c. The
back irradiation by appropriate wavelength visible light regen-
erated their respective open-ring isomers and recovered the
original emission spectra. During the process of photoisomeri-
zation, the emission spectral changes of diarylethene 1–4 in hexane
(5.0 × 10−5 mol L−1) and in PMMA films (10%, w/w) are shown
in Figs. 5 and 6, respectively. The intensity of the fluorescence
decreased to 8% in hexane and 9% in a PMMA film of initial value
by UV irradiation and recovered to the initial intensity upon visible
light irradiation. Therefore, the fluorescent modulation efficiency
of diarylethene 1 was 92% in hexane and 91% in PMMA film.
Just like diarylethene 1, the fluorescent modulation efficiencies
of diarylethenes 2–4 were 88% for 2, 90% for 3, and 93% for 4 in
hexane, and were 79% for 2, 74% for 3, and 83% for 4 in PMMA
films when arrived at the photostationary state. For diarylethenes
1–4, their fluorescent modulation efficiencies in hexane were much
bigger than those in PMMA films. Compared with diarylethenes
bearing two thiophene moieties [18,42,43], the fluorescent modu-
lation efficiencies of diarylethenes 1–4 were significantly enhanced
in both hexane and PMMA films. Therefore, diarylethenes bearing a
3.2. Fluorescence of diarylethenes
Fluorescent properties can be useful in molecular-scale opto-
electronics and digital photoswitching of fluorescence [38–40].
The fluorescence modulation is a particularly intriguing approach
due to the stabilization of diarylethene and versatility in mate-
rials selection [41]. In this work, the fluorescence properties of
the four diarylethenes in hexane and in PMMA films were mea-
sured at room temperature. The fluorescence emission spectra of
diarylethenes 1o–4o in hexane (5.0 × 10−5 mol L−1) and in PMMA
films (10%, w/w) at room temperature are illustrated in Fig. 4, and
the data are listed in Table 2. In hexane, the emission peaks of
diarylethenes 1o–4o were observed at 437, 426, 414, and 440 nm
when excited at 340 nm, and were observed at 422, 416, 409,
and 423 nm when monitored at 350 nm in PMMA films. Com-
pared to samples in hexane, the emission peaks of diarylethenes
1o–4o in PMMA films consistently exhibit a hypsochromic shift
with the value of 15 nm for 1o, 10 nm for 2o, 5 nm for 3o, and
Table 2
The fluorescent emission behaviors of diarylethenes 1–4 at room temperature in
hexane (5.0 × 10−5 mol L−1) and in PMMA films (10%, w/w).
Compound
ꢁem,max (emission intensity)
Fluorescent modulation
efficiency (%)
Hexane
PMMA film
Hexane
PMMA
film
1
2
3
4
437 (5127)
426 (4524)
414 (3057)
440 (5779)
422 (5192)
416 (4672)
409 (1372)
423 (5836)
92
88
90
93
91
79
74
83

G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 263 (2013) 41–49
47
6000
4800
3600
2400
1200
0
5000
Vis
Vis
UV
4000
3000
2000
1000
0
UV
400
450
550
Wavelength5(0n0m)
400
450
500
550
Wavelength (nm)
(A)
(B)
3500
2800
2100
1400
700
7000
Vis
UV
6000
5000
4000
3000
2000
1000
0
Vis
UV
0
400
450
500
550
400
450
500
550
Wavelength (nm)
Wavelength (nm)
(C)
(D)
Fig. 5. Emission intensity changes of diarylethenes 1–4 upon irradiation with 297 nm UV light at room temperature in hexane (5.0 × 10−5 mol L−1), excited at 340 nm: (A) 1,
(B) 2, (C) 3, and (D) 4.
pyrrole unit have one of the most promising candidates for fluores-
cent switching materials [44–48].
(ꢂVo–c) was 0.86 V for 1, 0.84 V for 2, 0.85 V for 3, and 0.91 V for
4. The result indicated that the oxidation process of the open-ring
isomers 1o–4o occurs at higher potentials than that of the cor-
responding closed-ring isomers 1c–4c. This is in accordance with
the theory that the longer conjugation length generally leads to
a less positive potential [46,53]. After the cyclization reaction, the
␲-conjugation of the closed-ring isomers extends across the perflu-
orocyclopentene ring causing a lower oxidation onset. Compared to
isomeric diarylethenes 1–3, the unsubstituted parent diarylethene
4o showed the biggest oxidation onsets both for the open-ring and
the closed-ring isomers. The oxidation onsets of diarylethenes 1–3
notably decreased when the fluorine group was attached at any of
the three positions on the terminal benzene ring.
According to the reported method [54–56], the highest occupied
molecular orbitals (HOMO) and the lowest unoccupied molecular
orbitals (LUMO) energy levels can be estimated by using the energy
level of ferrocene as reference. Based on the HOMO and LUMO
energy level, the band-gap (Eg) of each compound can be calcu-
lated approximately and the data are summarized in Table 3. The
result showed that the band-gaps of the closed-ring isomers 1c–4c
are much lower than those of the open-ring isomers 1o–4o. Among
these compounds, the Eg of 4o is the highest (Eg = 2.60 eV) and that
of 4c is the lowest (Eg = 1.35 eV), implying that the charge trans-
3.3. Electrochemistry of diarylethenes
The electrochemical properties of diarylethenes can be poten-
tially applied to molecular scale electronic switches [49]. The
oxidative cyclization and cycloreversion and the reductive elec-
trochemical cyclization of some diarylethene derivatives have
been reported [50–52]. Herein, cyclic voltammograms (CV) were
performed on the diarylethenes 1–4 under identical experimen-
tal conditions at a scanning rate of 50 mV s−1. The CV curves
of diarylethenes 1–4 are shown in Fig. 7. The oxidation onsets
of 1o–4o were observed at +1.47, +154, +1.52, and +1.63 V, and
those of 1c–4c were observed at +0.61, +0.70, +0.67, and +0.72 V,
respectively. Therefore, the difference of oxidation onset between
the open-ring and the closed-ring isomers of diarylethenes 1–4
Table 3
Electrochemical properties of diarylethenes 1–4 in acetonitrile.
Compound
Oxidation
Reduction
Band gap (Eg)
Eonset (V)
IP (eV)
Eonset (V)
EA (eV)
fer
must be faster in 4c compared to that in others [52]. Compared
with the unsubstituted parent diarylethene 4, the band-gaps of the
open-ring isomers 1o–3o decreased, and those of the closed-ring
isomers 1c–3c increased significantly. The results indicated that
the fluorine atom and its substituted position had a great effect on
the electrochemical properties of these diarylethene compounds
but further work is required to quantify these effects. It should be
noted here that calculation absolute HOMO and LUMO levels from
1o
1c
2o
2c
3o
3c
4o
4c
1.47
0.61
1.54
0.70
1.52
0.67
1.63
0.72
−6.27
−5.41
−6.34
−5.50
−6.32
−5.47
−6.43
−5.52
−1.04
−0.88
−1.05
−0.91
−1.04
−0.75
−0.97
−0.63
−3.76
−3.92
−3.75
−3.89
−3.76
−4.05
−3.83
−4.17
2.54
1.49
2.59
1.61
2.56
1.42
2.60
1.35

48
G. Liu et al. / Journal of Photochemistry and Photobiology A: Chemistry 263 (2013) 41–49
6000
4800
3600
2400
1200
0
5000
Vis
Vis
4000
3000
2000
1000
0
UV
UV
400
450
500
550
400
450
500
550
Wavelength (nm)
Wavelength (nm)
(A)
(B)
1500
1200
900
600
300
0
6000
4500
3000
1500
0
Vis
UV
Vis
UV
400
450
500
550
400
450
500
550
Wavelength (nm)
Wavelength (nm)
(C)
(D)
Fig. 6. Emission intensity changes of diarylethenes 1–4 upon irradiation with 297 nm UV light at room temperature in PMMA films (10%, w/w), excited at 350 nm: (A) 1, (B)
2, (C) 3, and (D) 4.
properties. In comparison of the unsubstituted parent diarylethene
4, the absorption maxima, quantum yields of cyclization and
2c
cycloreversion, and band-gaps of the closed-ring isomers of these
2o
1o
isomeric diarylethenes increased whereas their emission intensity
and the band-gaps of the open-ring isomers decreased remarkably.
The introduction of the pyrrole moiety induced new features which
differed from those of diarylethenes bearing thiophene or pyrazole
moieties. The results in this study will be useful for understanding
the substituent position effect on the properties of diarylethenes
bearing a pyrrole moiety and for the design of efficient photoactive
isomeric diarylethene derivatives with tunable properties.
1c
-2.4 -1.6 -0.8 0.0 0.8 1.6 2.4
-1.6 -0.8 0.0 0.8 1.6 2.4
4c
4o
3o
3c
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (21262015 and 21162011), the Project of Jiangxi
Advantage Sci-Tech Innovative Team (20113BCB24023), the Project
of Jiangxi Youth Scientist, and the Project of the Science Funds of
Jiangxi Education Office (KJLD12035, GJJ11026, and GJJ12587).
-1.6 -0.8 0.0
0.8
1.6
-1.6 -0.8 0.0 0.8 1.6
Potential/V
Fig. 7. Cyclic voltammetry (second scan) of diarylethene 1–4 in acetonitrile with
the scanning rate of 50 mV s−1
.
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