New photochromic diarylethenes bearing a pyridine moiety

Article abstract of DOI:10.1016/j.tetlet.2010.11.011

A new class of diarylethenes bearing a pyridine unit has been firstly developed and their properties such as photochromism, fatigue resistance, and fluorescence have been discussed. The pyridine unit was connected directly to the central cyclopentene ring

Full text of DOI:10.1016/j.tetlet.2010.11.011

Tetrahedron Letters 52 (2011) 143–147
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
New photochromic diarylethenes bearing a pyridine moiety
⇑
Shouzhi Pu , Peijian Yan, Gang Liu, Wenjuan Miao, Weijun Liu
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:
A new class of diarylethenes bearing a pyridine unit has been firstly developed and their properties such
as photochromism, fatigue resistance, and fluorescence have been discussed. The pyridine unit was con-
nected directly to the central cyclopentene ring as one aryl moiety and availably participated in the pho-
toinduced cyclization reaction even in the crystalline phase. All of these diarylethenes exhibited excellent
photochromism, remarkable fatigue resistance, and notable fluorescence photoswitches both in solution
and in poly(methylmethacrylate) films. Moreover, the different substituents had a significant effect on
their properties. The results indicated that the electron-donating substituent could significantly enhance
the cyclization quantum yield and depress the cycloreversion quantum yield while the electron-
withdrawing group had a notable contribution to the cycloreversion quantum yield and fluorescence
quantum yield for these diarylethenes.
Received 17 August 2010
Revised 27 October 2010
Accepted 2 November 2010
Available online 5 November 2010
Keywords:
Diarylethene
Pyridine moiety
Photochromism
Optical property
Ó 2010 Elsevier Ltd. All rights reserved.
Due to the widespread potential application in photonic devices
such as recording media and photoswitches, the exploration of
photochromic diarylethenes has aroused a surge of interest in
materials science.1 Among diarylethenes hitherto reported, there
are many theses on cationic diarylethenes containing pyridinium
cation units as the side group connected to the photoreactive thi-
enyl moieties.2 It is because pyridine groups are widely used for
the interaction with carboxylic acids to form intermolecular hydro-
gen bonds,3 which would result in the generation of novel struc-
tures such as organogels,4 nanostructures,5 and optical textures
in liquid crystal materials.3c–e,6
Nowadays, the design of new diarylethene skeletons with
different aryl moieties has become an active area of research. As
is well known, studies on the hexatriene backbone of diarylethenes
are mainly confined to the five-membered heteroaryl moieties, and
the diarylethene designs are generally limited to the addition of
functional units to the aryl rings and the substitution of the central
ethene group.7 In the case of six-membered rings as aryl moieties,
only a few symmetrical diarylethene derivatives bearing two phe-
nyl/naphthyl groups have been reported.8 The majority of these
derivatives are thermally reversible with poor photochromism. In
previous works,9 we reported a new class of diarylethene deriva-
tives bearing both five-membered and six-membered moieties,
which showed some new characteristics differing from diaryleth-
enes containing five-membered aryl rings. Among the diaryleth-
enes bearing a six-membered aryl moiety hitherto reported, the
six-membered unit has been only limited to the benzene ring.9
Although they showed good photochromism both in solution and
in poly(methylmethacrylate) (PMMA) film, they exhibited no
photochromism in the crystalline phase.9a As far as we are aware,
photochromic hybrid diarylethenes bearing other six-membered
moieties, which exhibit good photochromism in the crystalline
phase have not hitherto been reported.
Based on the above aspects, in this Letter, we have developed
a new class of photochromic diarylethene derivatives in which
a six-membered pyridine group is connected directly to the central
ethene unit as an aryl moiety and participates in the photochro-
mic hexatriene–cyclohexadiene reaction (Scheme 1). The synthe-
sized diarylethenes are 1-(3-methyl-2-pyridyl)-2-[2-methyl-5-(4-
methoxyphenyl)-3-thienyl]perfluorocyclopentene (1o), 1-(3-methyl-
2-pyridyl)-2-(2-methyl-5-phenyl-3-thienyl)per-fluorocyclopentene
(2o), and 1-(3-methyl-2-pyridyl)-2-[2-methyl-5-(4-cyanophenyl)-
3-thienyl]perfluorocyclopentene (3o). All of these diarylethenes
showed good photochromism in solution, in amorphous PMMA
films and in the crystalline phase. They are, to the best of our
knowledge, the first example of photochromic diarylethene deriv-
atives bearing a pyridine moiety.
F
F
F
F
F
F
F
F
UV
Vis
F
F
F
F
N
N
S
S
R
OCH3
R
1c: R =
2c: R =
3c: R =
OCH3
1o: R =
2o: R =
3o: R =
H
H
CN
CN
⇑
Corresponding author. Tel./fax: +86 791 3831996.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
Scheme 1. Photochromism of diarylethenes 1o–3o.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.11.011

144
S. Pu et al. / Tetrahedron Letters 52 (2011) 143–147
F
F
Br
F
F
F
F
F
F
N
Br
F
F
F
F
1 ) n-BuLi, 195K
N
F
S
n-BuLi, 195 K
R
2 ) C5F8
R
S
S
OCH3
OCH3
OCH3
1o: R =
2o: R =
3o: R =
4a: R =
4b: R =
4c: R =
5a: R =
5b: R =
5c: R =
R
H
H
H
CN
CN
CN
Scheme 2. Synthetic route for diarylethenes 1o–3o.
0.5
0.4
0.3
0.2
0.1
0.0
Vis
UV
UV
410s
UV
140
75
40
15
0
Vis
Figure 2. The color changes of diarylethenes 1–3 by photoirradiation in different
media at room temperature: (A) in hexane, (B) in PMMA film, and (C) in crystalline
phase.
300
400
500
600
700
800
Wavelength (nm)
Figure 1. Absorption spectral changes of diarylethene 1 by photoirradiation in
hexane (2.0 Â 10À5 mol LÀ1) at room temperature.
amorphous film, diarylethenes 1–3 also showed good photochro-
mism similar to samples in solution (Fig. 2B). Upon irradiation with
313 nm light, the colors of the diarylethene/PMMA films 1–3 chan-
ged from colorless to violet, with the appearance of a new broad
absorption band centered at 581, 569 and 571 nm, respectively.
This band was assigned to the formation of the closed-ring isomers
1c–3c. All colored diarylethene/PMMA films reverted to colorless
upon irradiation with visible light (k >450 nm). As has been
observed for most of the reported diarylethenes,14 the maximum
absorption peaks of 1c–3c are found at longer wavelengths in
PMMA film than those in hexane solution. This red shift phenome-
non may be attributed to the polar effect of the polymer matrix in
solid state.15 The spectral properties of diarylethenes 1–3 are sum-
marized in Table 1. Also, we have tested the response of the mole-
cules to acidic condition according to the references.16 The
photochromic reactivity of diarylethenes 1–3 is controlled by the
addition of acid (trifluoroacetic acid) and base (diethylamine) in
corresponding CH3CN solutions. With the addition of acid to the
solutions of 1c–3c in photostationary state, the colors changed to
green for 1c and blue to 2c and 3c by protonation, and their absorp-
tion maxima exhibited a remarkable bathochromic shift similar to
that of the reported diarylethenes.16 The neutralization with base
could regenerate the original absorption spectra and colors of
1c–3c. It should be noted here that diarylethenes 1o–3o showed
excellent photochromism in the crystalline phase, too (Fig. 2C).
They could be colored and bleached repeatedly more than 200
times by alternating irradiation with UV light and visible light with
appropriate wavelength. The result is completely reverse to that re-
ported for diarylethenes bearing a benzene moiety, where they
showed no photochromism in the crystalline phase.9a To the best
of our knowledge, this is the first class of diarylethenes with a
six-membered ring moiety which they could exhibit good photo-
chromism in the crystalline phase.
The synthetic route for diarylethenes 1o–3o is shown in
Scheme 2. First, three phenylthiophene derivatives (4a–4c)10 were
lithiated and coupled with octafluorocyclopentene to give three
mono-perfluorocyclopentene derivatives (5a–5c). Finally, 2-bro-
mo-3-methylpyridine was lithiated and then separately coupled
with 5a–5c to give diarylethenes 1o–3o, respectively. The
structures of 1o–3o were confirmed by elemental analysis, NMR,
and IR.11
Reversible absorption spectral changes of diarylethenes 1o–3o
were observed in hexane at room temperature, and each of them
presented an isosbestic point which supported the reversible two
component photochromic reaction schemes.12 Figure 1 shows the
absorption spectral changes of diarylethene 1 with the photochro-
mic reaction as a typical example. In hexane, compound 1o exhib-
ited a sharp absorption peak at 291 nm (
transition.13 Upon irradiation with
*
in hexane, as a result of a p?p
e
, 2.13 Â 104 L molÀ1 cmÀ1
)
297 nm light, a new visible absorption band centered at 563 nm
(e
, 1.07 Â 104 L molÀ1 cmÀ1) emerged while the original peak at
291 nm decreased, indicating the formation of the closed-ring iso-
mer 1c. This could be seen with the naked eyes, as the colorless
solution of 1o turned violet (Fig. 2A). The photostationary state is
achieved upon irradiation with 297 nm light for 410 s. Alterna-
tively, 1c could return to the initial state 1o upon irradiation with
visible light (k >450 nm) for 60 s. Just like diarylethene 1, the spec-
tral changes of diarylethenes 2o and 3o were similar to that of 1o.
The absorption maxima of diarylethenes 2c and 3c were observed
at both 555 nm, which was 8 nm shorter than that of 1c. In the
photostationary state, the isosbestic points of diarylethenes 1–3
were observed at 313, 298, and 335 nm, respectively. The photosta-
tionary state is achieved by irradiation 425 s for 2 and 420 s for 3,
and the bleaching time is 40 s for 2 and 33 s for 3, respectively. In
addition, the photoconversion ratios of the three derivatives were
analyzed by HPLC in the photostationary state, with the value of
80% for 1, 74% for 2, and 85% for 3, respectively (Table 1). In PMMA
The quantum yields were determined by comparing the
reaction yields of diarylethenes 1–3 against 1,2-bis(2-methyl-5-
phenyl-3-thienyl)perfluorocyclopentene in hexane at room

S. Pu et al. / Tetrahedron Letters 52 (2011) 143–147
145
Table 1
Absorption spectral properties of diarylethenes 1–3 in hexane (2.0 Â 10À5 mol LÀ1) and in PMMA film (10%, w/w) at room temperature
Compd
ko,max /nma
(
e
/L molÀ1 cmÀ1
)
kc,max /nmb
(
e
/L molÀ1 cmÀ1
)
Uc
PR/%d
Hexane
PMMA
Hexane
PMMA
Uo–c
Uc–o
1
2
3
291(2.13 Â 104)
284(1.70 Â 104)
311(2.66 Â 104)
291
285
313
563(1.07 Â 104)
555(6.59 Â 103)
555(8.25 Â 103)
581
569
571
0.43
0.33
0.37
0.088
0.096
0.16
80
74
85
a
b
c
The maximum absorption peak of the open-ring isomers.
The maximum absorption peak of the closed-ring isomers.
Quantum yields of cyclization (Uo–c) and cycloreversion (Uc–o), respectively.
Photoconversion ratios of diarylethenes 1–3 (PR%) in the photostationary state.
d
B 1.0
0.8
A
1.0
0.8
1
2
3
1
0.6
0.6
0.4
0.2
0.0
2
3
(%)
()%
0
0
A
A
A/
A/
0.4
0.2
0.0
0
50
100
Repeat Cycles
150
200
0
20
40
60
80
100
Repeat Cycles
Figure 3. Fatigue resistance of diarylethenes 1–3 in hexane and in PMMA film in air atmosphere at room temperature: (A) in hexane, (B) in PMMA film. Initial absorbance of
the sample was fixed to 1.0.
temperature,17 and the results are also listed in Table 1. These data
3000
2500
2000
1500
1000
500
3o
illuminated that altering the substituents had a significant effect
on the photochromic features of diarylethenes 1–3 such as the
absorption maxima, molar absorption coefficients and quantum
yields. Among diarylethenes 1–3, the molar absorption coefficients
of the two isomers and its cyclization quantum yield of the unsub-
stituted parent compound 2 are the smallest. When replacing with
a methoxy group at the para-position of the terminal phenyl ring,
the molar absorption coefficients and the cyclization quantum
yield increased notably, but the cycloreversion quantum yield
showed a slight reduction. When the same position instead con-
tains a cyano group, all of the photochromic parameters except
the absorption maxima of the closed-ring isomer have notably in-
creased. The result indicated that the electron-donating methoxy
group could be effective to increase the molar absorption coeffi-
cient and to decrease the cycloreversion quantum yield, which is
consistent with that of dithienylethenes reported by Irie et al.18
Compared to that of 3c, the absorption maximum of 1c is red-
shifted by 8 nm and its molar absorption coefficient is larger,
which may be attributed to the more marked intramolecular
charge transition (ICT).10a Moreover, the pyridine moiety has also
a significant effect on the photochromic properties of diarylethenes
1–3. When replacing the pyridine moiety with the benzene moiety
in the same molecular skeleton of diarylethenes, the cyclization
quantum yield markedly decreased and the cycloreversion quan-
tum yield showed the reverse trend.9a Similarly, when the same
molecular skeleton instead contains other aryl moieties such as
thiophene, thiazole, pyrazole, and pyrrole, among others,9,10,19
their photochromic features are very different in comparison with
those of diarylethenes bearing a pyridine moiety.
2o
1o
0
350
400
450
500
550
Wavelength (nm)
Figure 4. Emission spectra of diarylethenes 1–3 in hexane (2.0 Â 10À5 mol LÀ1) at
room temperature when excited at 300 nm.
in the dark and then exposing them to air for more than 10 days, we
found that no changes in the UV/vis spectra of diarylethenes 1–3
were observed. At 80 °C, diarylethenes 1–3 still showed good ther-
mal stabilities for more than 5 h. The result suggests that these
diarylethene derivatives have notable thermally irreversible photo-
chromic behaviors, which are different from the dithienylethenes
having pyridinium ions that undergo thermally reversible photo-
chromic reactions.7a Besides this, the photogenerated closed-ring
isomers of diarylethenes with six-membered benzene rings are also
thermally unstable, which may be attributed to the relatively high
aromatic stabilization energy.7a,9a
Thethermalstabilitiesoftheopen-ringandclosed-ringisomersof
diarylethenes 1–3 were tested in hexane both at room temperature
and at 80 °C. Storing these solutions in hexane at room temperature
The fatigue resistances of diarylethenes 1–3 were examined
both in hexane and in PMMA films by alternating irradiating with

146
S. Pu et al. / Tetrahedron Letters 52 (2011) 143–147
800
600
400
200
0
2000
A
B
Vis
UV
Vis
1500
UV
1000
500
0
400
450
500
550
420
450
480
510
540
Wavelength (nm)
Wavelength (nm)
Figure 5. Emission intensity changes of diarylethenes 1 both in hexane (2.0 Â 10À5 mol LÀ1) and in PMMA film (10%, w/w) by photoirradiation at room temperature: (A) in
hexane (kex = 300 nm), (B) in PMMA film (kex = 350 nm).
UV and visible light in air at room temperature, as shown in
Figure 3. In hexane, the coloration and decoloration cycles of diary-
lethenes 1–3 can be repeated at least 10 times with almost no pho-
todegradation. However, 28% of 1c and 31% of both 2c and 3c were
destroyed after 100 repeat cycles in the same condition, which may
be ascribed to degradation resulting from the formation of an
epoxide.20 The fatigue resistance of diarylethenes 1–3 in PMMA
films is much stronger than that in solution. After 200 repeated
cycles, these compounds still showed good photochromism with
only ca. 15% degradation of 1c, 27% of 2c, and 17% of 3c, respec-
tively. This remarkable improvement may result from effectively
suppressing the oxygen diffusion.7a Compared to diarylethenes
bearing a benzene moiety,9 the fatigue resistances of the diaryleth-
enes bearing a pyridine moiety are much better both in hexane and
in PMMA films, which may be attributed to the lower aromatic sta-
bilization energy of the pyridine ring. In addition, the fatigue resis-
tance of diarylethene with an electron-donating group (such as in
compound 1) is slightly stronger than that of diarylethene with an
electron-withdrawing group (such as in compound 3) both in hex-
ane and in PMMA films. The result is completely contrary to that
reported for analogs bearing a benzene moiety, where fatigue
resistance markedly increased with the increase of electron-with-
drawing ability.9
The fluorescence spectra of diarylethenes 1o–3o both in hexane
(2.0 Â 10À5 mol LÀ1) and in PMMA films (10%, w/w) were mea-
sured at room temperature. The fluorescent emission peaks of
diarylethenes 1o–3o were observed at 463, 429 and 372 nm when
excited at 300 nm in hexane (Fig. 4), and were observed at 460,
452, and 465 nm when excited at 350 nm in PMMA films. Com-
pared to those in hexane, the emission peaks of diarylethenes 2o
and 3o showed a remarkable bathochromic shift in PMMA film,
which is well consistent with those of their maxima absorption
wavelengths. But to the diarylethene 1o, its emission peak is al-
most constant in PMMA film. As shown in Figure 4, it could be
clearly seen that the electron-donating methoxy group shifted
the emission peak to longer wavelength and notably decreased
the emission intensity; while the cyano group could be effective
to increase the emission intensity and shifted the emission peak
to a shorter wavelength, compared with the unsubstituted parent
diarylethene 2o. By using anthracene as the reference, the fluores-
cence quantum yields of diarylethenes 1o–3o were determined to
be 0.0078, 0.0083 and 0.015, respectively. The results suggested
that the electron-withdrawing cyano substituent could effectively
increase the fluorescence quantum of diarylethene bearing a pyri-
dine unit. This is completely contrary to those of the diarylethenes
bearing a biphenyl unit whose fluorescence quantums increased
with the electron-donating ability.21
Diarylethenes 1–3 exhibited a notable fluorescent photoswitch
on changing from the open-ring isomers to closed-ring isomers by
photoirradiation both in hexane and in PMMA film, just like most
of the reported diarylethenes.1c,d,19 During the process of photoiso-
merization, diarylethene 1 exhibited changes in its fluorescence
both in hexane and in PMMA film as shown in Figure 5. Upon irra-
diation with 297 nm UV light, the sample arrived at the photosta-
tionary state, in which its emission intensity quenched to ca. 75%
in hexane and 34% in PMMA film, respectively. Back irradiation
of the appropriate wavelength of visible light regenerated the
open-ring isomer 1o and duplicated the original emission spectra.
The phenomena are useful for application as the fluorescence
switches.22 Similarly, diarylethenes 2 and 3 also functioned as a
notable fluorescent switch upon photoirradiation both in hexane
and in PMMA films. In the photostationary state, their emission
intensities were quenched ca. 46% for 2 and 48% for 3 in solution
and ca. 32% for 2 and 38% for 3 in PMMA films. The incomplete
cyclization reaction and the existence of parallel conformations
with relatively strong fluorescence may be the main cause for
the lower fluorescent conversion.14
In conclusion, three new unsymmetrical diarylethenes based on
a six-membered pyridine ring have been firstly developed and
their properties have been discussed. This new photochromic sys-
tem showed good photochromism in solution, in PMMA films, and
in crystalline phase. This is the first example of diarylethenes bear-
ing a six-membered moiety which exhibited good photochromism
in the crystalline phase. The pyridine moiety induced some new
dramatic properties differing from other diarylethenes with
five-membered and six-memered aryl moieties reported so far.
The results will be helpful for the synthesis of efficient photoactive
diarylethene derivatives with new molecular skeletons and to
design new photochromic systems for further potential applica-
tions in optoelectronic devices.
Acknowledgments
This work was supported by Program for the NSFC of China
(20962008), New Century Excellent Talents in University (NCET-
08-0702), the Project of Jiangxi Academic and Technological leader
(2009DD00100), the Key Scientific Project from Education Ministry
of China (208069), National Natural Science Foundation of Jiangxi
Province (2008GZH0020, 2009GZH0034) and the Science Funds
of the Education Office of Jiangxi, China (GJJ09646, GJJ09567).

S. Pu et al. / Tetrahedron Letters 52 (2011) 143–147
147
H), 7.49 (d, 1H, J = 8.0 Hz, pyridyl–H), 8.63 (d, 1H, J = 4.0 Hz, pyridyl–H); 13C
NMR(100 MHz, CDCl3, ppm): d 14.4, 18.3, 55.4, 114.4, 121.3, 124.1, 124.9,
References and notes
126.1, 126.9, 133.5, 138.5, 141.0, 142.1, 147.7, 159.5; IR (KBr, m
, cmÀ1): 738,
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792, 825, 876, 951, 992, 1037, 1069, 1109, 1131, 1185, 1257, 1340, 1383, 1456,
1516, 1572, 1609, 1655, 1703, 2046, 2838, 2931, 2962, 3011, 3133, 3186, 3261,
3331, 3429, 3690. Data for 2o: mp 117–118 °C; Calcd for C22H15F6NS: C, 60.13;
H, 3.44; N, 3.19. Found: C, 60.21; H, 3.39; N, 3.15; 1H NMR (400 MHz, CDCl3,
ppm): d 1.98 (s, 3H, –CH3), 1.99 (s, 3H, –CH3), 7.17 (s, 1H, thienyl–H), 7.23–7.30
(m, 2H, Ar–H), 7.36 (t, 2H, J = 7.5 Hz, Ar–H), 7.48 (d, 3H, J = 8.0 Hz, Ar–H), 8.63
(d, 1H, J = 4.3 Hz, pyridyl–H); 13C NMR (100 MHz, CDCl3, ppm): d 14.4, 18.3,
122.4, 124.2, 125.6, 127.9, 129.0, 132.9, 133.5 138.6, 141.9, 142.3, 147.8; IR
(KBr, m
, cmÀ1): 787, 882, 995, 1072, 1139, 1168, 1278, 1344, 1401, 1454, 1613,
1663, 2316, 2376, 2620, 3188, 3336, 3438, 3745. Data for 3o: mp 176–177 °C;
Calcd for C23H14F6N2S: C, 59.48; H, 3.04; N, 6.03. Found: C, 59.69; H, 3.00; N,
6.12; 1H NMR (400 MHz, CDCl3, ppm): d 1.99 (s, 3H, –CH3), 2.03 (s, 3H, –CH3),
7.27 (t, 1H, pyridyl–H), 7.30 (s, 1H, thienyl–H), 7.50 (d, 1H, J = 8.0 Hz, pyridyl–
H), 7.57 (d, 2H, J = 8.0 Hz, phenyl–H), 7.65 (d, 2H, J = 8.0 Hz, phenyl–H), 8.64 (d,
1H, J = 4.4 Hz, pyridyl–H); 13CNMR (100 MHz, CDCl3, TMS): d 14.6, 18.3, 111.1,
118.6, 124.4, 124.7, 125.8, 132.8, 133.3, 137.4, 138.7, 139.8, 147.9; IR (KBr, m,
cmÀ1): 738, 801, 832, 874, 993, 1068, 1137, 1191, 1276, 1341, 1379, 1472,
1511, 1629, 1749, 2226, 3136, 3230, 3305, 3377, 3511. Data for 1c: 1H NMR
(400 MHz, CDCl3, ppm): d 1.89 (s, 3H, –CH3), 2.08 (s, 3H, –CH3), 3.87 (s, 3H, –
OCH3), 5.98 (d, 1H, J = 7.2 Hz, pyridyl–H), 6.25 (d, 1H, J = 5.4 Hz, pyridyl–H),
6.60 (s, 1H, thienyl–H), 6.93 (d, 2H, J = 8.8 Hz, phenyl–H), 7.53 (d, 2H, J = 8.4 Hz,
phenyl–H), 7.60 (s, 1H, pyridyl–H); Data for 2c: 1H NMR (400 MHz, CDCl3,
ppm): d 1.89 (s, 3H, –CH3), 2.09 (s, 3H, –CH3), 5.99 (d, 1H, J = 6.4 Hz, pyridyl–H),
6.27 (d, 1H, J = 9.6 Hz, pyridyl–H), 6.70 (s, 1H, thienyl–H), 7.42–7.45 (m, 3H,
phenyl–H), 7.56–7.59 (m, 2H, phenyl–H), 7.63 (s, 1H, pyridyl–H); Data for 3c:
1H NMR (400 MHz, CDCl3, ppm): d 1.90 (s, 3H, –CH3), 2.10 (s, 3H, –CH3), 6.03 (d,
1H, J = 6.4 Hz, pyridyl–H), 6.29 (d, 1H, J = 5.6 Hz, pyridyl–H), 6.76 (s, 1H,
thienyl–H), 7.64 (d, 1H, J = 2.4 Hz, phenyl–H), 7.66 (d, 1H, J = 2.4 Hz, phenyl–H),
7.68 (s, 1H, phenyl–H), 7.70 (s, 1H, phenyl–H), 7.72 (s, 1H, pyridyl–H).
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11. Data for 1o: mp 103–104 °C; calcd for C23H17F6NOS: C, 58.85; H, 3.65; N, 2.98.
Found: C, 58.89; H, 3.72; N, 2.94; 1H NMR (400 MHz, CDCl3, ppm): d 1.95 (s, 3H,
–CH3), 1.97 (s, 3H, –CH3), 3.83 (s, 3H, –OCH3), 6.89 (d, 2H, J = 8.0 Hz, phenyl–H),
7.06 (s, 1H, thienyl–H), 7.25 (t, 1H, pyridyl–H), 7.41 (d, 2H, J = 8.0 Hz, phenyl–