Photochromism of asymmetrical diarylethenes with a pyrimidine unit: Synthesis and substituent effects

Article abstract of DOI:10.1016/j.dyepig.2013.10.047

Abstract Five photochromic diarylethenes with a six-membered pyrimidine moiety were synthesized to investigate the effects of the substituents on their photochromic behaviors, and the structures of four of the diarylethenes were determined by single-cryst

Full text of DOI:10.1016/j.dyepig.2013.10.047

Dyes and Pigments 102 (2014) 159e168
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Photochromism of asymmetrical diarylethenes with a pyrimidine
unit: Synthesis and substituent effects
*
Hongliang Liu, Shouzhi Pu , Gang Liu, Bing Chen
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University, Nanchang 330013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Five photochromic diarylethenes with a six-membered pyrimidine moiety were synthesized to inves-
tigate the effects of the substituents on their photochromic behaviors, and the structures of four of the
diarylethenes were determined by single-crystal X-ray diffraction analysis. The pyrimidine moiety was
connected directly to the central cyclopentene ring as an aryl moiety to participate the photo-
isomerization reaction in solution, solid amorphous films, and crystalline phase. All of the diarylethene
derivatives showed favorable photochromism and functioned as notable fluorescent photo-switches in
both solution and solid media. The electron-donating substituents enhanced their cyclization quantum
yields, fatigue resistance, and fluorescence quantum yields, whereas the electron-withdrawing groups
exerted inversed actions on the diarylethenes. The results revealed that the pyrimidine moiety and
substituents played a very important role during the process of photoisomerization reactions.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 3 September 2013
Received in revised form
16 October 2013
Accepted 31 October 2013
Available online 8 November 2013
Keywords:
Photochromism
Diarylethene
Fluorescence
Pyrimidine moiety
Substituent effect
Crystal structure
1. Introduction
properties. For example, diarylethenes having two thiophenes,
furans, or thiazoles exhibit excellent thermal stability. The ones
Photochromism is a photo-induced reversible transformation of
a chemical species between two isomers [1e3]. During the
reversible photoisomerization process, some physicochemical
properties of photochromic compounds, such as absorption
spectra, fluorescence, electrochemical property, magnetic proper-
ties, dipole interaction, as well as refractive indice could be tuned
by light [4e6]. The instant property changes by photoirradiation
lead to their wide applications in optical memory and photo-optical
switching devices [5e8]. Among various photochromic materials,
the diarylethenes with heteroaryl groups are the most promising
candidates for their thermal irreversibility and high photochromic
reactivity [9e11].
In the past several decades, numerous studies have focused on
molecular design, especially the synthesis of symmetrical and
asymmetrical frameworks in diarylethenes with different hetero-
aryl units such as thiophene, benzothiophene [4e6], furan [12],
thiazole [13e15], indole [16,17], benzofuran [18], indene [19], pyr-
azole [20,21], pyrrole [22], benzene [23,24], chryso[b]thiophene
[25], etc. It has been revealed that the aryl moieties and functional
substituents impose significant influence on their photochromic
with two indoles, pyrroles, or benzenes are thermally unstable
[4,5,12,26]. In a previous work, we revealed that the thiazole moiety
could decrease the absorption maxima of the closed-ring isomers
compared with the thiophene or pyrazole moiety [15]. Irie et al.
reported that the introduction of alkoxy groups at the reactive
carbon sites in diarylethenes remarkably suppressed the cyclo-
reversion quantum yields [27]. Nevertheless, the hexatriene back-
bones necessary for the versatility of the diarylethenes reported so
far have been mostly limited to the five-membered aryl rings. The
diarylethenes with six-membered aryl rings were rarely reported
due to the poor photochromic activity and thermal stability [28e
30]. For a more detailed research on the effects of six-membered
aryl rings as the hexatriene backbones, we synthesized a new
class of diarylethenes with both five-membered and six-membered
moieties, which exhibited favorable photochromism, fluorescence
switching properties, and excellent thermal stability [15,23,31,32].
To date, the six-membered aryl rings used in known diarylethenes
include benzene, pyridine, and naphthalene. Pyrimidine as a critical
intermediates for drugs and pesticides [33], however, has rarely
been reported in the synthesis of diarylethenes [34].
In this work, five photochromic diarylethenes with pyrimidine
and thiophene moieties were synthesized. In order to discuss the
substituent effects on their physicochemical properties, different
substituents were introduced into para-position of the terminal
* Corresponding author. Tel./fax: þ86 791 83831996.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.10.047

160
H. Liu et al. / Dyes and Pigments 102 (2014) 159e168
benzene. All of these diarylethenes showed good photochromism
in solution, ploy(methylmethacrylate) (PMMA) films, and crystal-
line phase. The photochromic scheme of diarylethenes 1e5 by
photoirradiation is illustrated as shown in Fig. 1.
2.2.1. 3-Bromo-2-methyl-5-(4-methoxyphenyl)thiophene (7a)
7a was prepared by reacting compound 6 (4.00 g, 18.1 mmol)
with 4-bromoanisole (3.38 g, 18.1 mmol) in the presence of
Pd(PPh3)4 and Na2CO3 (6.40 g, 60 mmol) in THF (80 mL containing
10% water). After refluxing for 15 h at 368 K, the product was
extracted with CH2Cl2. The organic layer was dried over MgSO4,
filtrated, and evaporated. Column chromatography on SiO2 with
petroleum ether as the eluent afforded 3.99 g of 7a as a yellowish
solid in 78% yield. M.p. 378e379 K; 1H NMR (400 MHz, CDCl3)
2. Experimental
2.1. General methods
d
2.40 (s, 3H, eCH3), 3.83 (s, 3H, eOCH3), 6.90 (d, 2H, J ¼ 8.0 Hz,
Melting points were measured with a WRS-1B melting point
apparatus. NMR spectra were recorded on a Bruker AV400
(400 MHz) spectrometer with CDCl3 as the solvent and tetra-
methylsilane as an internal standard. Infrared spectra (IR) were
recorded on a Bruker Vertex-70 spectrometer. Mass spectra
were obtained on an Agilent 1100 ion trap MSD spectrometer.
Fluorescence spectra were measured on a HITACHI 4500 fluo-
rescence spectrophotometer. Absorption spectra were
measured using an Agilent 8453 UV/Vis spectrometer. Photo-
irradiation was carried out using an SHG-200 UV lamp, a CX-
21 ultraviolet fluorescence analysis cabinet and a BMH-250
visible lamp. All solvents were dried and distilled according to
standard procedures. Other reagents were used without addi-
tional purification.
phenyl-H), 6.99 (s, 1H, thienyl-H), 7.43 (d, 2H, J ¼ 8.0 Hz, phenyl-H).
2.2.2. 3-Bromo-2-methyl-5-(4-methylphenyl)thiophene (7b)
7b was prepared by a method similar to that used for 7a. Column
chromatography on SiO2 with petroleum ether as the eluent
afforded 3.86 g of 7b as a pale yellow solid in 80% yield. M.p. 340e
341 K; 1H NMR (400 MHz, CDCl3):
d 2.36 (s, 3H, eCH3), 2.41 (s, 3H, e
CH3), 7.06 (s, 1H, thienyl-H), 7.17 (d, 2H, J ¼ 8.0 Hz, phenyl-H), 7.39
(d, 2H, J ¼ 8.0 Hz, phenyl-H).
2.2.3. 3-Bromo-2-methyl-5-phenyl-thiophene (7c)
7c was prepared by a method similar to that used for 7a. Column
chromatography on SiO2 with petroleum ether as the eluent
afforded 3.52 g of as a pale yellow solid in 77% yield. M.p. 339e
Suitable crystals of 1o, 2o, 3o and 5o were obtained by slow
evaporation in hexane. All the measurements were collected by a
Bruker SMART APEX II CCD diffractometer using a MULTI scan
340 K; 1H NMR (400 MHz, CDCl3):
d 2.34 (s, 3H, eCH3), 7.02 (s, 1H,
thienyl-H), 7.20 (d, 1H, J ¼ 8.0 Hz, phenyl-H), 7.29 (t, 2H, J ¼ 8.4 Hz,
phenyl-H), 7.42 (d, 2H, J ¼ 8.0 Hz, phenyl-H).
technique at room temperature using Mo K
a radiation. The
structures of 1o, 2o, 3o and 5o were solved by direct methods and
refined by full-matrix least-squares procedures on F2 by full-
matrix least-squares techniques using SHELXTL-97 program.
Further details on the crystal structure investigation have been
deposited with The Cambridge Crystallographic Data Center as
supplementary publication CCDC 950116 for 1o, 950117 for 2o,
950118 for 3o, and 950119 for 5o. Copies of the data can be ob-
tained, free of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: þ44 0 1223 336033 or e-mail:de-
posit@ccdc.cam.ac.uk).
2.2.4. 3-Bromo-2-methyl-5-(4-fluorophenyl)thiophene (7d)
Compound 7d was prepared by a method similar to that used for
7a. Column chromatography on SiO2 with petroleum ether as the
eluent afforded 3.77 g of 7d as a pale yellow solid in 77% yield. M.p.
334e335 K; 1H NMR (400 MHz, CDCl3):
d 2.43 (s, 3H), 7.05 (s, 1H),
7.09 (d, 2H, J ¼ 8.0 Hz, phenyl-H), 7.46e7.50 (m, 2H, phenyl-H).
2.2.5. 3-Bromo-2-methyl-5-(4-trifluoromethylphenyl)thiophene
(7e)
Compound 7e was prepared by a method similar to that used for
7a. Column chromatography on SiO2 with petroleum ether as the
eluent afforded 3.85 g of 7e as a pale yellow solid in 79% yield. M.p.
2.2. Synthesis of diarylethene derivatives
359e360 K; 1H NMR (400 MHz, CDCl3):
d 2.44 (s, 3H, eCH3), 7.02 (s,
The synthetic route for diarylethenes 1oe5o is shown in
Fig. 2. The phenylthiophene derivatives 7aee were prepared by
Suzuki coupling of five bromobenzene derivatives with a thio-
phene boronic acid 6 [35,36]. Then, compounds 7aee were
lithiated and coupled with perfluorocyclopentene to give mono-
substituted perfluorocyclopentene 8aee, respectively. Finally, 5-
bromo-2,4-dimethoxypyrimidine was lithiated and coupled
with 8aee to give the unsymmetrical 1oe5o, respectively. The
structures of 1oe5o were confirmed by elemental analysis, NMR,
IR, and MS.
1H, thienyl-H), 7.58e7.64 (m, 4H, phenyl-H).
2.2.6. [2-Methyl-5-(4-methoxyphenyl)-3-thenyl]
perfluorocyclopentene (8a)
To a stirred anhydrous THF containing 3-bromo-2-methyl-5-(4-
methoxyphenyl)thiophene (7a) (1.00 g, 3.5 mmol) was added
dropwise a 2.5 M n-BuLi hexane solution (1.5 mL, 3.9 mmol) at
195 K under an argon atmosphere. Stirring was continued for
30 min. Excess octafluorocyclopentene (0.6 mL, 3.8 mmol) was then
added. The reaction was quenched with 20 mL water after 2 h. The
mixture was warmed to room temperature and extracted with
ether. The organic layer was collected and washed with saturated
salt water (30 mL) and then water (50 mL). The organic layer was
dried over MgSO4, filtrated, and evaporated. Column chromatog-
raphy on SiO2 with petroleum ether as the eluent afforded 0.60 g of
8a as a colorless solid in 43% yield. 1H NMR (400 MHz, CDCl3):
d 2.46
(s, 3H, eCH3), 3.84 (s, 3H, eOCH3), 6.92 (d, 2H, J ¼ 8.0 Hz, phenyl-H),
7.13 (s, 1H, thienyl-H), 7.47 (d, 2H, J ¼ 8.0 Hz, phenyl-H).
2.2.7. [2-Methyl-5-(4-methylphenyl)-3-thienyl]
perfluorocyclopentene (8b)
8b was prepared by a method similar to that used for 8a. Column
chromatography on SiO2 with petroleum ether as the eluent
afforded 0.86 g of 8b as a yellow solid in 60% yield. 1H NMR
Fig. 1. Photochromism of diarylethenes 1e5.

H. Liu et al. / Dyes and Pigments 102 (2014) 159e168
161
F
F
Br
F
F
Br
R
Br
F
F
n-BuLi, 195K
S
F
C5F8
B(OH)2
Pd(PPh3)4
Na2CO3,aq.
S
S
R
6
7a: R = OCH3
8a: R = OCH3
R
7b: R =
CH3
7c: R =
H
8b: R =
CH3
8c: R =
H
7d: R =
F
8d: R =
F
7e: R =
CF3
8e: R =
CF3
F
F
F
F
Br
195K
n-BuLi/THF,
F
F
OCH3
CH3
O
1o: R =
2o: R =
OCH3
8a: R =
8b: R =
8c: R =
8d: R =
8e: R =
H
3o: R =
4o: R =
5o: R =
N
N
N
CH3
H
F
O
S
N
CF3
F
O
O
CF3
R
1o-5o
Fig. 2. Synthetic route for diarylethenes 1e5.
(400 MHz, CDCl3):
(m, 3H, thienyl and phenyl-H), 7.43 (d, 2H, J ¼ 8.0 Hz, phenyl-H).
d
2.37 (s, 3H, eCH3), 2.47 (s, 3H, eCH3), 7.17e7.20
368 K; Calcd for C23H18F6N2O3S (%): Calcd C, 53.49; H, 3.51; N, 5.42.
Found C, 53.53; H, 3.54; N, 5.47; 1H NMR (400 MHz, CDCl3):
2.01
d
(s, 3 H, -CH3), 3.74 (s, 3H, eOCH3), 3.84 (s, 3H, eOCH3), 4.01 (s, 3H, e
OCH3), 6.91 (d, 2H, J ¼ 8.0 Hz, benzene-H), 7.06 (s, 1H, thiophene-
H), 7.45 (d, 2H, J ¼ 8.0 Hz, benzene-H), 8.33 (s, 1H, pyrimidine-H);
2.2.8. (2-Methyl-5-phenyl-3-thienyl)perfluorocyclopentene (8c)
Compound 8c was prepared by a method similar to that used for
8a. Column chromatography on SiO2 with petroleum ether as the
eluent afforded 0.81 g of 8c as a buff solid in 55% yield. 1H NMR
13C NMR (100 MHz, CDCl3):
d 14.08, 54.16, 55.21, 55.30, 104.13,
114.36, 121.38, 125.91, 126.16, 126.84, 138.58, 142.05, 159.04, 159.47,
166.05, 168.03; IR (KBr, v, cmꢀ1): 739, 758, 799, 822, 841, 891, 986,
1003, 1034, 1074, 1123, 1198, 1257, 1298, 1337, 1371, 1408, 1476, 1518,
1549, 1595, 1647, 2843, 2941, 2964, 3014, 3412, 3688; LRMS, ESIþ m/
z 517.1 (MHþ, [C23H19F6N2O3S]þ requires 517.1).
(400 MHz, CDCl3): d 2.46 (s, 3H, eCH3), 7.25 (d, 1H, thienyl-H), 7.31
(t, 1H, J ¼ 8.0 Hz, phenyl-H), 7.39 (t, 2H, J ¼ 8.0 Hz, phenyl-H), 7.55
(d, 2H, phenyl-H).
2.2.9. [2-Methyl-5-(4-fluorophenyl)-3-thienyl]
perfluorocyclopentene (8d)
2.2.12. 1-(2,4-Dimethoxy-5-pyrimidinyl)-2-[2-methyl-5-(4-
methylphenyl)-3-thienyl]perfluorocyclopentene (2o)
2o was prepared by a method to that used for 1o using 8b
instead of 8a. Column chromatography on SiO2 with petroleum
ether and ethyl acetate (v/v ¼ 5/1) as the eluent afforded 0.31 g of
2o as a colorless solid in 48% yield. M.p. 378e379 K; Calcd for
Compound 8d was prepared by a method similar to that used for
8a. Column chromatography on SiO2 with petroleum ether as the
eluent afforded 0.72 g of 8d as a colorless solid in 49% yield. 1H NMR
(400 MHz, CDCl3):
d 2.48 (s, 3H, eCH3), 6.71 (s, 3H, thienyl-H),
7.03e7.06 (m, 3H, phenyl-H), 7.48e7.51 (m, 2H, phenyl-H).
C
23H18F6N2O2S (%): Calcd C, 55.20; H, 3.63; N, 5.60. Found C, 55.27;
2.2.10. [2-Methyl-5-(4-trifluoromethylphenyl)-3-thienyl]
perfluorocyclopentene (8e)
Compound 8e was prepared by a method similar to that used for
8a. Column chromatography on SiO2 with petroleum ether as the
eluent afforded 0.87 g of 8e as a buff solid in 57% yield. 1H NMR
H, 3.65; N,5.64; 1H NMR (400 MHz, CDCl3):
d
2.03 (s, 3 H, eCH3),
2.37 (s, 3H, eCH3), 3.73 (s, 3H, eOCH3), 4.01 (s, 3H, eOCH3), 7.14 (s,
1H, thiophene-H), 7.19 (d, 2H, J ¼ 8.0 Hz, benzene-H), 7.42 (d, 2H,
J ¼ 8.0 Hz, benzene-H), 8.34 (s, 1H, pyrimidine-H); 13C NMR (100
MHz, CDCl3):
d 14.15, 21.13, 54.19, 55.24, 104.15, 122.00, 125.48,
(400 MHz, CDCl3):
7.59e7.61 (m, 4H, phenyl-H).
d
2.44 (s, 3H, eCH3), 7.20 (d, 1H, thienyl-H),
125.99, 129.66, 120.62, 137.88, 139.04, 142.29, 159.06, 166.09,
168.05; IR (KBr, v, cmꢀ1): 737, 804, 854, 893, 981, 1015, 1067, 1123,
1196, 1242, 1269, 1292, 1333, 1404, 1476, 1555, 1593, 1647, 2868,
2962, 2993, 3024, 3439; LRMS, ESIþ m/z 501.1 (MHþ,
[C23H19F6N2O3S]þ requires 501.1).
2.2.11. 1-(2,4-Dimethoxy-5-pyrimidinyl)-2-[2-methyl-5-(4-
methoxyphenyl)-3-thienyl]perfluorocyclopentene (1o)
To
a
stirred anhydrous THF containing 5-bromo-2,4-
dimethoxypyrimidine (0.29 g, 1.3 mmol) was added dropwise a
2.5 M n-BuLi/hexane solution (0.6 mL, 1.4 mmol) at 195 K under
argon atmosphere. After 30 min, 8a (0.52 g, 1.3 mmol) was added
and the reaction mixture was stirred for 2 h at this temperature.
The reaction was allowed to warm to room temperature and
quenched by addition of water. The product was extracted with
diethyl ether. Then the combined organic layers were dried over
MgSO4, filtered and concentrated. Column chromatography on SiO2
with petroleum ether and ethyl acetate (v/v ¼ 6/1) as the eluent
afforded 0.35 g of 1o as a colorless solid in 52% yield. M.p. 367e
2.2.13. 1-(2,4-Dimethoxy-5-pyrimidinyl)-2-(2-methyl-5-phenyl-3-
thienyl)perfluorocyclopentene (3o)
Diarylethene 3o was prepared by a method to that used for 1o
using 8c instead of 8a. Column chromatography on SiO2 with pe-
troleum ether and ethyl acetate (v/v ¼ 5/1) as the eluent afforded
0.40 g of 3o as a colorless solid in 56% yield. M.p. 358e359 K; Calcd
for C22H16F6N2O2S (%): Calcd C, 54.32; H, 3.32; N, 5.76. Found C,
54.43; H, 3.37; N, 5.81; 1H NMR (400 MHz, CDCl3):
d 2.04 (s, 3H, e
CH3), 3.74 (s, 3H, eOCH3), 4.01 (s, 3H, eOCH3), 7.19 (s, 1H,
thiophene-H), 7.29 (t, 1H, J ¼ 7.6 Hz, benzene-H), 7.39 (t, 2H,

162
H. Liu et al. / Dyes and Pigments 102 (2014) 159e168
J ¼ 7.6 Hz, benzene-H), 7.53 (d, 2H, J ¼ 7.6 Hz, benzene-H), 8.35 (s,
1H, pyrimidine-H); 13C NMR (100 MHz, CDCl3):
14.22, 54.23,
d
55.30, 104.10, 122.47, 125.53, 126.05, 127.88, 129.00, 133.31, 142.09,
146.34, 159.04, 166.04, 167.98; IR (KBr, v, cmꢀ1): 762, 799, 827, 845,
856, 893, 934, 984, 1003, 1074, 1134, 1194, 1248, 1296, 1335, 1375,
1410, 1479, 1545, 1599, 1649, 1599, 1649, 2868, 2966, 3020, 3306,
3689; LRMS, ESIþ m/z 487.1 (MHþ, [C22H17F6N2O2S]þ requires
487.1).
2.2.14. 1-(2,4-Dimethoxy-5-pyrimidinyl)-2-[2-methyl-5-(4-
fluorophenyl)-3-thienyl]perfluorocyclopentene (4o)
Diarylethene 4o was prepared by a method similar to that used
for 1o using 8d instead of 8a. Column chromatography on SiO2 with
petroleum ether and ethyl acetate (v/v ¼ 6/1) as the eluent afforded
0.35 g of 4o as a colorless solid in 53% yield. M.p. 365e366 K; Calcd
for C22H15F7N2O2S (%): Calcd C, 52.38; H, 3.00; N, 5.55. Found C,
52.45; H, 3.03; N, 5.57; 1H NMR (400 MHz, CDCl3):
d 2.04 (s, 3H, e
CH3), 3.75 (s, 3H, eOCH3), 4.02 (s, 3H, eOCH3), 7.06e7.10 (m, 2H,
benzene-H), 7.11 (t, 1H, thiophene-H), 7.43e7.56 (m, 2H, benzene-
H), 8.33 (s, 1H, pyrimidine-H); 13C NMR (100 MHz, CDCl3):
d
14.13, 54.20, 55.27, 104.05, 116.00, 122.54, 126.09, 127.29, 129.64,
139.60, 141.05, 159.10, 162.50, 166.12, 168.05; IR (KBr, v, cmꢀ1): 737,
800, 826, 893, 982, 1005, 1072, 1126, 1194, 1248, 1296, 1333, 1377,
1406, 1477, 1514, 1547, 1599, 1726, 2868, 2963, 3024, 3446; LRMS,
ESIþ m/z 505.1 (MHþ, [C22H16F7N2O2S]þ requires 505.1).
2.2.15. 1-(2,4-Dimethoxy-5-pyrimidinyl)-2-[2-methyl-5-(4-
trifluoromethylphenyl)-3-thienyl]perfluorocyclopentene (5o)
Diarylethene 5o was prepared by a method similar to that used
for 1o using 8e instead of 8a. Column chromatography on SiO2 with
petroleum ether and ethyl acetate (v/v ¼ 5/1) as the eluent afforded
0.46 g of 5o as a colorless solid in 47% yield. M.p. 406e407 K; Calcd
for C23H15F9N2O2S (%): Calcd C, 49.83; H, 2.73; N, 5.05. Found C,
49.95; H, 2.77; N, 5.07; 1H NMR (400 MHz, CDCl3):
d 2.06 (s, 3H, e
CH3), 3.73 (s, 3H, eOCH3), 4.01 (s, 3H, eOCH3), 7.27 (s, 1H,
thiophene-H), 7.63 (s, 4H, benzene-H), 8.34 (s, 1H, pyrimidine-H);
13C NMR (100 MHz, CDCl3):
d 14.22, 54.20, 55.29, 103.90, 123.98,
124.0, 126.04, 126.44, 129.70, 136.66, 140.30, 141.04, 159.10, 166.17,
168.00; IR (KBr, v, cmꢀ1): 737, 797, 826, 854, 893, 984, 1015, 1067,
1128, 1177, 1198, 1242, 1271, 1298, 1323, 1377, 1410, 1476, 1493, 1555,
1591, 1651, 2937, 2963, 2993, 3416, 3470; LRMS, ESIþ m/z 555.1
(MHþ, [C23H16F9N2O2S]þ requires 555.1).
Fig. 3. Absorption spectral changes of diarylethene 1 and color changes of diary-
lethenes 1ꢀ5 upon alternating irradiation with UV and visible light at room temper-
ature: (A) spectral changes for 1 in hexane (2.0 ꢁ 10ꢀ5 mol Lꢀ1); (B) spectral changes
for 1 in a PMMA film (10%, w/w); (C) color changes for 1ꢀ5 in both hexane and PMMA
films. (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
3. Results and discussion
3.1. Photochromism of diarylethenes 1e5
The photochromic behaviors of 1e5 induced by photoirradiation
were measured in both hexane (2.0 ꢁ 10ꢀ5 mol Lꢀ1) and PMMA
films (10%, w/w) at room temperature. The absorption spectral
change of 1 and color changes of diarylethenes 1ꢀ5 induced by
alternating irradiation with UV and visible light in hexane and
PMMA films are shown in Fig. 3. In hexane,1o was found to show an
intense absorption band at 290 nm (ε, 3.92 ꢁ 104 L molꢀ1 cmꢀ1) due
electronic transitions are observed in the visible region [38,39].
2ꢀ5 exhibited photochromism similar to 1 in hexane. Upon irra-
diation with 297 nm light, the solutions containing 2oe5o turned
red as a result of the cyclization reactions to produce the closed-
ring isomers 2oe5o, whose absorption maxima were observed at
526, 522, 517 and 518 nm, respectively. The red colored solutions of
2oe5o could again be bleached by irradiation with visible light
because the compounds returned to the open-ring isomers 2oe5o
respectively. As shown in Fig. 4, the photoconversion ratios from
open-ring to closed-ring isomers of 1e5 were analyzed by HPLC in
the photostationary state (PSS). The photoconversion ratios of 1ꢀ5
were 80% for 1, 63% for 2, 72% for 3, 47% for 4, and 84% for 5.
Compared with diarylethenes with a pyrrole moiety [40], the
photoconversion ratios of 1e5 were significantly decreased in so-
lution. However, they were approximately equal to those of diary-
lethenes with pyridine or isoxazole moieties [32,41]. In PMMA
to
pep* transition [37], and give a colorless solution without any
absorption bands in the visible range. Upon UV irradiation at
297 nm, the closed-ring isomer 1c was produced by the typical
photocyclization as the colorless solution changed to red, thereby
resulting in broad absorption bands centered at 533 nm (ε,
1.95 ꢁ 104 L molꢀ1 cmꢀ1). Reversely, the red solution of 1c could be
completely bleached upon irradiation with visible light
(
l
> 450 nm). The significant difference in absorption bands of the
closed-ring isomers compared to their open-ring isomers is mainly
due to the increase in conjugation, which dramatically changes
the electronic structure as a whole in such a way that new
p

H. Liu et al. / Dyes and Pigments 102 (2014) 159e168
163
Initial State
2o
1o
PSS State
1c
2c
0
2
4
6
8
0
2
4
6
3o
4o
4c
3c
0
2
4
6
0
2
4
6
5o
5c
0
2
4
6
Time (min)
Fig. 4. The photoconversion ratios of diarylethenes 1ꢀ5 in the photostationary state in hexane by HPLC analysis.
films, 1e5 also showed similar phototchromism as observed in
hexane (Fig. 3B). Compared to those in hexane, the maximum ab-
sorption peaks of 1ꢀ5 in PMMA films shifted to a longer wave-
length. The redshift values of the closed-ring isomers were 3 nm for
1c, 9 nm for 2c, 15 nm for 3c, 9 nm for 4c, and 10 nm for 5c. The
redshift phenomena were consistent with those of reported dia-
rylethenes [23,42e45], and may be attributed to the polar effect of
the polymer matrix in amorphous solid state.
The photochromic features of 1e5 in both hexane and PMMA
films are summarized in Table 1. As expected, the different sub-
stituents at the para-position of the benzene ring had remarkable
effects on the photochromic properties of these diarylethenes,
including the absorption maxima, molar absorption coefficients,
and the quantum yields of cyclization and cycloreversion. As shown
in Table 1, when an electron-donating substituent (methoxy or
methyl group, 1 or 2) was attached at the para-position of the
phenyl ring, the molar absorption coefficients of the closed-ring
isomers and the cyclization quantum yields rose with the in-
crease of electron-donating ability, but the cycloreversion quantum
yields decreased [46]. However, the molar absorption coefficients
of the closed-ring isomers and the cyclization quantum yields did
not change much when an electron-withdrawing substituent
(fluorine or trifluoromethyl group, 4 or 5) was attached. As a result,
1 had the largest molar absorption coefficient of the closed-ring
0.0
-0.1
-0.2
-0.3
-0.4
1c
2c
3c
4c
5c
0
200
400
600
800
1000
Time (s)
Fig. 5. Thermal fading of 1cꢀ5c at 293 K.
isomers and the cyclization quantum yield. This indicated that
the electron-donating substituent could effectively enhance the
molar absorption coefficients of closed-ring isomers and the
cyclization quantum yields. The result was well consistent with the
Table 1
Absorption spectral properties and photochromic reactivity of diarylethenes 1ꢀ5 in hexane (2.0 ꢁ 10ꢀ5 mol Lꢀ1) and in PMMA films (10%, w/w) at room temperature.
Compd
lo,max/nma (ε/L molꢀ1 cmꢀ1
)
lc,max/nmb (ε/L molꢀ1 cmꢀ1
)
Fc
PRd (%)
Hexane
PMMA film
Hexane
PMMA film
Foec
Fceo
1
2
3
4
5
290 (3.92 ꢁ 104)
287 (3.56 ꢁ 104)
285 (4.02 ꢁ 104)
284 (3.56 ꢁ 104)
291 (3.46 ꢁ 104)
291
290
287
285
294
533 (1.95 ꢁ 104)
526 (1.47 ꢁ 104)
522 (1.00 ꢁ 104)
517 (1.07 ꢁ 104)
518 (1.01 ꢁ 104)
536
535
537
526
528
0.51
0.29
0.17
0.14
0.15
0.10
0.19
0.26
0.16
0.29
80%
63%
72%
47%
84%
a
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of cyclization reaction (Foec) and cycloreversion reaction (Fceo), respectively.
b
c
d
Photoconversion ratio from the open-ring to closed-ring isomers in the photostationary state.

164
H. Liu et al. / Dyes and Pigments 102 (2014) 159e168
As a key factor for practical applications in optical devices [49],
A
1.0
0.8
0.6
0.4
0.2
0.0
the fatigue resistance of 1e5 was examined in both hexane and
PMMA films by alternative irradiation with UV and visible light in
air at room temperature, and the result is shown in Fig. 6. In hexane,
the coloration and decoloration cycles of 1e5 were repeated 20
times with 31% degradation of 1c, 55% of 2c, 74% for 3c, 71% for 5c,
and 87% of 5c. The degradation may be ascribed to the formation of
an epoxide [50]. The fatigue resistance of 1e5 in PMMA films is
much stronger than that in solution. After 60 repeat cycles, the
degradation percentages of 1e5 in PMMA films were 52% for 1c,
54% for 2c, 58% for 3c, 59% for 4c, and 61% for 5c. This improvement
may result from suppression of oxygen diffusion in solid medium
[49]. In addition, the fatigue resistance of the diarylethene with an
electron-donating group was better than that of diarylethene with
an electron-withdrawing group in both hexane and PMMA films.
The result is in accordance with those of reported diarylethenes
with a pyridine or thiazole moiety [32,44], but contrary to that of
diarylethenes with a naphthalene moiety [31].
1
2
3
4
5
0
5
10
15
20
Repeat cycles
B
1.0
0.8
0.6
0.4
0.2
0.0
The structural conformations of the crystals of 1o, 2o, 3o, and 5o
were provided by X-ray crystallographic analysis. Their ORTEP
drawings are shown in Fig. 7, and the X-ray crystallographic anal-
ysis data are listed in Tables 2 and 3. In crystalline phase, the four
diarylethenes crystallized in photoactive anti-parallel conforma-
tions, which can be expected to undergo photocyclization [51]. For
the crystal of 3o, there were two independent molecules in the
asymmetric unit and both of them adopted an approximate C2
symmetry. The two planar aryl moieties exhibited similar geome-
tries in each molecule, with dihedral angles between the cyclo-
pentene ring and the adjacent aryl rings of 48.2ꢂ for N1eN2/C17e
C20 and 61.8ꢂ for S1/C7eC10 in one molecule. In the other mole-
cule, the dihedral angles between the cyclopentene ring and the
adjacent aryl rings are 44.5ꢂ for N3eN4/C39eC42 and 64.2ꢂ for S2/
C29eC32. The distances between the photoactive carbons
(C10∙∙∙C20 and C32∙∙∙C42) in each molecule were 3.628 and
1
2
3
4
5
0
15
30
45
60
Repeat cycles
ꢀ
3.702 A, respectively. The distances between the reactive carbons
Fig. 6. Fatigue resistance of diarylethenes 1e5 in air atmosphere at room temperature:
(A) in hexane; (B) in PMMA films. Initial absorbance of the sample was fixed to 1.0.
ꢀ
d (A) and dihedral angles
q
(ꢂ) of 1o, 2o, and 5o were shown in
Table 3. As expected, the colorless crystal of 1o, 2o, 3o, and 5o
turned red upon irradiation with 297 nm light, and the colored
crystals reverted to a colorless state upon irradiation with visible
diarylethenes with a naphthalene, pyridine, isoxazole, or thiazole
moiety [31,32,41,47]. In addition, the absorption maxima of 1ce5c
were observed between 518 and 533 nm in hexane, which were
much shorter than those of the diarylethenes with benzene, pyri-
dine, pyrazole or thiophene moieties [20,23,32,48]. The results
suggested that the pyrimidine moiety can effectively shift the ab-
sorption maximum of diarylethene to a shorter wavelength.
The thermal stability of the open-ring and closed-ring isomers
of 1e5 was examined in hexane at both room temperature and
341 K. Storage of the solutions of 1oe5o in the darkness at room
temperature and exposure to air for more than 10 days caused no
changes in their UV/Vis spectra. But 1ce5c returned to 1oe5o
when they were exposed to air in the dark for only 8, 7, 6.5, 5, and
4 h, respectively. At 341 K, the red color of 1ce5c faded completely
after 100, 90, 80, 75, and 50 s, respectively. The results revealed that
the thermal stability of diarylethenes with a pyrimidine unit is
much weaker than that of diarylethenes with five-membered
heteroaryl rings due to the higher aromatic stabilization energy
of six-membered pyrimidine [4,23,41,47]. The greatly enhanced
ground-state energy difference between the open-ring and closed-
ring isomers resulted in thermal instability of the closed-ring iso-
mers of 1e5 [38,39]. As shown in Fig. 5, the thermal cycloreversion
rates of 1ce5c at 293 K accelerated when the electron-donating
groups were changed with electron-withdrawing groups. This
result indicated that the electron-donating groups could effectively
enhance the thermal stability of diarylethenes with a pyrimidine
moiety.
light (l > 450 nm). The color changes of the four derivatives in the
crystalline phase by photoirradiation are shown in Fig. 8. After 100
repeat cycles, these crystals still exhibited favorable photochro-
mism in the crystalline phase, suggesting that they can be poten-
tially used for the construction of optoelectronic devices [52].
3.2. Fluorescence of diarylethenes
The fluorescence of 1oe5o was measured in both hexane
(1.0 ꢁ 10ꢀ4 mol Lꢀ1) and PMMA films (10%, w/w) at room tem-
perature, and the result is shown in Fig. 9 and Table 4. The emission
peaks of 1oꢀ5o were observed at 449, 439, 427, 426, and 413 nm in
hexane, and were observed at 469, 467, 442, 433 and 418 nm in
PMMA films. Compared to those in hexane, the emission peaks of
1oe5o in PMMA films showed a bathochromic shift, with the value
of 20 nm for 1, 28 nm for 2, 15 nm for 3, 7 nm for 4, and 5 nm for 5.
When going from electron-donating to electron-withdrawing
groups, the emission peaks of 1oꢀ5o gradually shifted to a
shorter wavelength in both hexane (from 449 to 413 nm) and
PMMA films (from 469 to 418 nm), and their emission intensity also
enhanced evidently with the increase in electron-withdrawing
ability. The result is consistent with that of diarylethenes with a
pyridine, isoxazole or thiazole moiety [32,40,47], but contrary to
that of diarylethenes with a biphenyl moiety [24]. The fluorescence
quantum yields of 1oe5o were determined to be 0.0179, 0.0075,
0.0065, 0.0056, and 0.0047, respectively, indicating that the

H. Liu et al. / Dyes and Pigments 102 (2014) 159e168
165
Fig. 7. ORTEP drawings of 1o, 2o, 3o, and 5o, showing about 30% probability displacement ellipsoids: (A) 1o, (B) 2o, (C) 3oemolecule I, (D) 3oemolecule II, and (E) 5o.
Table 2
Crystal data for diarylethenes 1o, 2o, 3o, and 5o.
1ꢂ
2ꢂ
3ꢂ
5ꢂ
Formula
C23H18F6N2O3S
516.45
293(2)
Monoclinic
P2(1)/c
C23H18F6N2O2S
500.45
296(2)
Monoclinic
P2(1)/c
C22H16F6N2O2S
486.43
296(2)
Triclinic
P-1
C23H15F9N2O2S
554.43
296(2)
Triclinic
P-1
Formula weight
Temperature (K)
Crystal system
Space group
Unit cell dimensions
a (Ǻ)
13.0363(7)
8.3736(6)
21.4912(12)
90.00
93.283(5)
90.00
2342.15
4
1.469
13.0447(10)
8.2222(8)
21.6787(15)
90.00
91.814(7)
90.00
2324.0(3)
4
1.430
8.5948(10)
13.1124(15)
19.894(2)
80.9400(10)
87.6290(10)
76.9200(10)
2156.5(4)
2
8.1811(17)
9.4444(19)
15.778(3)
95.693(2)
100.976(2)
96.637(2)
1179.5(4)
2
b (Ǻ)
c (Ǻ)
a
b
g
(ꢂ)
(ꢂ)
(ꢂ)
Volume (Ǻ3)
Z
Density (calcd.) (g/cm3)
1.498
0.957
1.561
1.063
Goodness-of-fit on F2
0.999
1.034
Final R indices [I/2s(I)]
R1
0.0552
0.1164
0.0690
0.1910
0.0453
0.0955
0.0674
0.2019
u
R1
R indices (all data)
R1
0.1217
0.1538
0.1182
0.2450
0.0675
0.1105
0.0832
0.2207
u
R1

166
H. Liu et al. / Dyes and Pigments 102 (2014) 159e168
Table 3
A
q
(ꢂ) of 1o,
ꢀ
Distances between the reacting carbon atoms d (A) and dihedral angles
2o, 3o, and 5o.
1500
1200
900
600
300
0
5o
4o
3o
q
(ꢂ)a
ꢀ
Compound
d (A)
q1
q2
q3
1o
2o
3o
C1.C11
3.635
3.595
3.628
3.702
3.630
44.7
44.7
48.2
44.5
51.0
60.2
60.4
61.8
64.2
58.8
13.6
11.0
20.4
14.4
25.3
2o
1o
C11.C19
C10.C20
C32.C42
C11.C18
5o
a
q1, Dihedral angle between the cyclopentene ring and 2,4-dimethoxypyrimidine
ring, q2, dihedral angle between the cyclopentene ring and thiophene ring, q3
dihedral angel between the thiophene ring and the adjacent benzene ring.
,
350
400
450
500
550
Wavelength/nm
B
5000
4000
3000
2000
1000
0
5o
4o
3o
2o
1o
350
400
450
500
550
600
Wavelength/nm
Fig. 8. Photographs demonstrating the photochromic processes of diarylethenes 1, 2,
3, and 5 in the crystalline phase.
Fig. 9. Emission spectra of diarylethenes 1e5 in both hexane solution
(1.0 ꢁ 10ꢀ4 mol Lꢀ1) and PMMA films (10%, w/w) at room temperature: (A) in hexane;
(B) in PMMA films.
fluorescence quantum yield decreased notably when going from
electron-donating to electron-withdrawing groups. The result is
different from those of the diarylethenes with a naphthalene or
pyridine moiety whose fluorescence quantum yields increased
with the increase of electron-withdrawing ability [31,32].
and their photochromic and fluorescent properties were investi-
gated systematically. The pyrimidine attached directly to the cen-
tral cyclopentene ring participated in the photoinduced cyclization
reaction in hexane, PMMA films, and crystalline phase. Different
substituents at the para-position of the terminal benzene ring had
significant effects on their properties. The pyrimidine moiety
induced some new characteristics which were different from those
of reported diarylethenes with other six-membered aryl moieties.
The results will be helpful to understand the substituent effects on
the properties of diarylethenes with a pyrimidine moiety and
synthesize novel diarylethenes with tunable photochromic
behaviors.
Similar to most reported diarylethenes [53e57], 1e5 exhibited
notable fluorescent switches by photoirradiation in both hexane
and PMMA films, and the result is shown in Fig.10. Upon irradiation
with 297 nm UV light, the fluorescence modulation efficiency of 1
was 83% in hexane and 34% in a PMMA film when arrived at the
photostationary state (PSS). Similarly, the fluorescence modulation
efficiency of the other four diarylethenes was 70% for 2, 75% for 3,
48% for 4, and 82% for 5 in hexane, and was 34% for 2, 39% for 3, 32%
for 4, and 30% for 5 in PMMA films in the photostationary state.
Therefore, the fluorescence modulation efficiency of 1e5 in hexane
was much larger than those in PMMA films. The result is contrary to
that of diarylethenes with a naphthalene, pyridine or isoxazole
moiety whose fluorescence modulation efficiency in hexane was
much lower than that in PMMA films [32,32,41]. In addition, the
fluorescent modulation efficiency of 1e5 was notably enhanced in
hexane as compared with that of all reported diarylethenes with a
six-membered aryl rings [15,23,31,32], suggesting that diary-
Table 4
Fluorescence parameters of diarylethenes 1e5 in both hexane (1.0 ꢁ 10ꢀ4 mol Lꢀ1
and PMMA films.
)
Hexane
PMMA
Ff
lem/nma
(
Ifc
Hd
(%)
lem/nm
If
H
(%)
lex/nmb)
(
lex/nm)
1o
2o
3o
4o
5o
449 (327)
439 (324)
427 (319)
426 (317)
413 (317)
751
83
70
75
48
82
469 (319)
467 (311)
442 (300)
433 (300)
418 (291)
1650
2197
2623
2887
5220
34
34
39
32
30
0.0179
0.0075
0.0065
0.0056
0.0047
lethenes with
a six-membered pyrimidine moiety could be
1002
1078
1281
1552
potentially applied in certain photoswitchable devices [58,59].
4. Conclusions
a
Emission peak.
Excitation wavelength.
Emission intensity.
b
c
A new class of asymmetrical diarylethenes based on the hybrid
skeleton of pyrimidine and thiophene moieties was synthesized,
d
Fluorescence modulation efficiency in the photostationary state.

H. Liu et al. / Dyes and Pigments 102 (2014) 159e168
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Acknowledgments
This work was supported by the National Natural Science
Foundation of China (21162011, 21262015), the Project of Jiangxi
Advantage Sci-Tech Innovative Team (20113BCB24023), the Science
Funds of Natural Science Foundation of Jiangxi Province
(20122BAB213004, 20114BAB203007, 20132BAB203005), and the
Project of the Science Funds of Jiangxi Education Office (KJLD12035,
GJJ11026, GJJ12587).
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optical and electrochemical properties of a new unsymmetrical diarylethene.
J Mol Struct 2007;833:23e9.
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