New photochromic 1,2-diarylperfluorocyclopentenes bearing unsymmetrical six-membered aryl units

Article abstract of DOI:10.1016/j.tetlet.2011.11.035

A new class of unsymmetrical photochromic diarylethenes based on the six-membered naphthalene-benzene and naphthalene-pyridine backbones has been firstly developed and their properties have been discussed. The two different six-membered aryl moieties were

Full text of DOI:10.1016/j.tetlet.2011.11.035

Tetrahedron Letters 53 (2012) 320–324
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Tetrahedron Letters
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New photochromic 1,2-diarylperfluorocyclopentenes bearing unsymmetrical
six-membered aryl units
⇑
Renjie Wang, Shouzhi Pu , Gang Liu, Shiqiang Cui, Weijun Liu
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 unsymmetrical photochromic diarylethenes based on the six-membered naphthalene-
benzene and naphthalene-pyridine backbones has been firstly developed and their properties have been
discussed. The two different six-membered aryl moieties were connected directly to the central cyclo-
pentene ring and available to participate in the photoinduced cyclization reaction. The three diaryleth-
enes exhibit distinctly different photochromism by photoirradiation in solution: they turn red, yellow,
and orange upon photocyclization, which may be resulted from the different substituent and six-
membered aryl moiety effects. Compared with the analog bearing a benzene unit, diarylethene bearing
a pyridine moiety has a shorter absorption maximum and a smaller fluorescent quantum yield, and it
shows a dual-addressable photo-switch by photoirradiation and acid/base stimulation.
Received 4 September 2011
Revised 29 October 2011
Accepted 8 November 2011
Available online 15 November 2011
Keywords:
Photochromism
Diarylethene
Unsymmetrical six-membered aryl unit
Optical property
Ó 2011 Elsevier Ltd. All rights reserved.
Organic photochromic materials have received intense interest
due to their potential promising photonic applications in optical
data storage and switching devices.1 Among the various photo-
chromic systems, dithienyl perfluorocyclopentenes are the most
representative class of photochromic compounds with thermally
irreversible and high fatigue resistance, which have received most
attention because they can be potentially applied in optical
switches and high-density optical recording materials.2,3 So far,
many studies concerning the photochromic properties of dithieny-
lethene derivatives have been reported.4
For the application to optical recording, it is very important to
develop photochromic diarylethenes with different absorption
wavelengths, especially the shorter absorption wavelength because
the recording capacity is proportional to the recording laser wave-
length.5 However, among the diarylethenes hitherto reported, most
of them have an absorption peak with the region between 550 and
750 nm,6 and just a few reports concerning photochromic diaryleth-
enes whose absorption peaks below this wavelength range. There-
fore, developing diarylethene derivatives with absorption band
around 500 nm or even shorter for making use of short-wavelength
optical recording materials is very necessary. One approach to
achieve this expected goal is to attach the naphthalene ring to the
ethene moiety, for example, the absorption maximum of the
closed-ring isomer of diarylethene bearing two naphthalene rings
is observed at 471 nm.7
Among the reported diarylethenes, their hexatriene backbones are
mainly confined to the five-membered heteroaryl moieties.2a,c In
the case of six-membered aryl rings, only a few symmetrical
diarylethenes bearing two phenyl/naphthyl groups have been
reported.7,8 The majority of these derivatives are thermally revers-
ible with poor photochromism. Previously, we reported diaryleth-
enes bearing both naphthalene and thiophene moieties, in which
the absorption peak band around 510 nm.9 In this Letter, one of
the main purposes was to develop a new class of diarylethenes bear-
ing unsymmetrical six-membered aryl units with evident
photochromism. The synthesized diarylethenes are 1-(2-methyl-
naphthyl)-2-(1-methoxyphenyl)perfluorocyclopentene (1o), 1-(2-
methylnaphthyl)-2-(1-methylphenyl)perfluorocyclopentene (2o),
and 1-(2-methylnaphthyl)-2-(3-methyl-2-pyridyl)perfluorocycl-
opentene (3o). Each of the diarylethenes exhibited notable photo-
chromism and fluorescent photo-switch in solution; to the best of
our knowledge, they are the first examples of photochromic diary-
lethenes bearing unsymmetrical six-membered aryl moieties. The
photochromic scheme and the color changes of diarylethenes 1–3
by photoirradiation are illustrated in Scheme 1.
The synthetic routes for diarylethenes 1o–3o are shown in
Scheme 2. 1-Bromo-2-methoxybenzene, 1-bromo-2-methylben-
zene, and 2-bromo-3-methylpyridine were separately lithiated,
and thencoupled with2-methyl-1-naphthyl-perfluorocyclopenten-
e 59 to give diarylethenes 1o–3o. The synthetic details were de-
scribed in Supplementary Information (SI). Their structures were
confirmed by elemental analysis, NMR, and IR.10
Presently, the synthesis of new photochromic diarylethene with
different aryl moieties has become an active area of research.
The absorption spectral changes of diarylethenes 1–3 upon
alternating irradiation with UV and visible light in hexane
(5.0 Â 10À5 mol/L) are shown in Figure 1. In hexane, diarylethene
⇑
Corresponding author. Tel./fax: +86 791 3831996.
E-mail address: pushouzhi@tsinghua.org.cn (S. Pu).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.11.035

R. Wang et al. / Tetrahedron Letters 53 (2012) 320–324
321
1o exhibited a sharp absorption peak at 280 nm in hexane due to a
p⁄ transition.4j,11 Upon irradiation with UV light, a new visible
p?
absorption band centered at 480 nm, corresponding to the color
change of the solution from colorless to red, as a result of the for-
mation of the closed-ring isomer 1c. The red-colored solution of 1c
can be decolorized completely upon irradiation with visible light
(k > 400 nm) again, due to the conversion of the compound back
to the open-ring isomer 1o. As with diarylethene 1, diarylethenes
2, and 3 also showed photochromism in hexane. Upon irradiation
with UV light, the colorless solution of 2o and 3o turned to yellow
and orange, and their absorption maxima centered at 469 for 2c
and 406 nm for 3c. The photochromic features of diarylethenes
1–3 in hexane are summarized in Table 1. The absorption maxima
of both open- and closed-ring isomers of diarylethene 1 are much
longer than those of diarylethene 2, which may be attributed to the
stronger electron-donating ability of methoxy group as compared
to a methyl group. Compared with the analogous diarylethene
bearing a benzene unit (such as in 1 or 2), the introduction of pyr-
idine unit (such as in 3) can be effective to shift the absorption
maximum to a shorter wavelength. The cyclization and cyclorever-
sion quantum yields of diarylethenes 1–3 were measured by the
reported method.12 The cyclization quantum yields of diaryleth-
enes 1–3 were determined to be 0.16 for 1, 0.06 for 2, and 0.11
for 3, and the cycloreversion quantum yields were 0.05 for 1,
0.03 for 2, and 0.13 for 3. The result suggests that the photochro-
mic reaction of diarylethene can be effectively modulated by the
introduction of the different substituent or six-membered aryl
unit. For diarylethenes bearing a benzene unit (1 and 2), the cycli-
zation quantum yields are greater than their respective cyclorever-
sion quantum yields. However, the cyclization quantum yield of
diarylethene 3 is lower than its cycloreversion quantum yield.
Among these derivatives, the cyclization quantum yield of diary-
lethene 1 is the biggest, and the cycloreversion quantum yield of
3 is the biggest. In addition, the photoconversion ratios of the three
diarylethenes were analyzed by HPLC in the photostationary state,
with the value of 33% for 1, 22% for 2, and 17% for 3, respectively
(Table 1). Compared to the reported analogs,9 the photoconversion
ratios of the three diarylethenes were significantly decreased in
hexane, which may be attributed to the higher aromatic stabiliza-
tion energy of the six-membered aromatic ring.2a
Scheme 1. Photochromism and the color changes of diarylethenes 1–3 in hexane.
F
F
F
F
Br
O
F
F
n-BuLi,THF
195 K
O
1o
F
F
F
F
F
F
F
F
Br
F
F
F
F
F
n-BuLi,THF
195 K
5
2o
F
F
F
F
Br
N
F
Furthermore, the switching properties between 3 and 4 bearing
a pyridine moiety were also investigated by protonation of 3o and
3c with trifluoroacetic acid in acetonitrile (1.3 Â 10À3 mol/L) and
neutralization of 4o and 4c with triethylamine base (7.2 Â 10À3
mol/L), respectively. The molecular structural and color changes
between 3 and 4 are illustrated in Scheme 3. Addition of trifluoro-
F
N
n-BuLi,THF
195 K
3o
Scheme 2. Synthetic route for diarylethenes 1o–3o.
acetic acid (20 lL) to the solution of 3o in acetonitrile produces a
protonated diarylethene 4o, and 4o can return to 3o by neutraliza-
tion with triethylamine base. Similarly, addition of trifluoroacetic
acid to the solution of 3c produced a protonated diarylethene 4c
in photostationary state. As has been observed for the reported
diarylethenes,13 the yellow solution (3c) changed to a reddish
one (4c) by protonation, and the absorption maximum bathochro-
mic shifted from 444 to 479 nm. The reddish solution of 4c either
returns completely back to orange (3c) after neutralization with
the amine or gets bleached to colorless (4o) upon irradiation with
visible light.
The thermal stabilities of the open- and closed-ring isomers of
diarylethenes 1–3 were tested in ethanol at room temperature
and 371 K, respectively. Storing these solutions in ethanol at room
temperature in the dark and then exposing them to air for more
than 10 days, no changes in their colors and spectra were observed
and, indeed, no decomposition was detected when these deriva-
tives were exposed to air for more than three months. Compared
with the reported diarylethenes bearing two benzene ring whose
closed-ring isomers gradually return to the open-ring isomers at
0.52
0.39
0.26
0.13
0.00
3o
2o
1o
3c
1c
2c
300
400
500
600
Wavelength (nm)
Figure 1. Absorption spectral changes of diarylethenes 1–3 by photoirradiation in
hexane (5.0 Â 10À5 mol/L) at room temperature.

322
R. Wang et al. / Tetrahedron Letters 53 (2012) 320–324
Table 1
shown in Figure 2. In hexane, the coloration and decoloration
cycles of diarylethenes 1–3 were irradiated alternatively with
297 nm and visible light, and the irradiation time was long enough
for coloration to reach the photostationary state. The fatigue resis-
tant characteristics of diarylethenes 1–3 in solution indicated that
10% of 1c, 12% of 2c, and 8% of 3c were destroyed after 10 repeat
cycles, respectively. After 50 repeat cycles, diarylethene 3 still
exhibited good photochromism with only 12% degradation of 3c.
However, 61% of 1c and 40% of 2c were destroyed after 50 repeat
cycles at the same experimental condition. Therefore, the fatigue
resistance of diarylethene 3 bearing a pyridine moiety is much bet-
ter than that of diarylethene 1 or 2 bearing a benzene moiety,
which may be attributed to the lower aromatic stabilization energy
of pyridine ring. The result is well in agreement with that of the re-
ported analogs bearing both six-membered and five-membered
aryl moieties, where the fatigue resistances of diarylethenes bear-
ing a pyridine moiety are much better than those of diarylethenes
bearing a benzene moiety.12,14
Photochromic parameters of diarylethenes 1–3 in hexane (5.0 Â 10À5 mol/L) at room
temperature
Compound
ko,max /nma
/L molÀ1 cmÀ1
Hexane
kc,max /nmb
/L molÀ1 cmÀ1
Hexane
Uc
PR/%d
(e
)
(e
)
Uo–c Uc–o
1
2
3
282 (7.65 Â 103)
278 (6.51 Â 103)
271 (8.48 Â 103)
480 (7.82 Â 103)
469 (3.46 Â 103)
406 (6.79 Â 103)
0.16
0.06
0.11
0.05
0.03
0.13
33
22
17
a
b
c
Absorption maxima of open-ring isomers.
Absorption maxima of 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
Fluorescence is widely applicable to molecular-scale optoelec-
tronics, ion-sensors, and digital fluorescent photoswitches.17 So
far, the fluorescent properties of many diarylethene derivatives
have been extensively studied.18 In this work, the fluorescent
features of diarylethenes 1–3 in hexane (5.0 Â 10À5 mol/L) were
measured using a Hitachi F-4500 fluorimeter at room temperature.
As shown in Figure 3, the emission peaks of 1o–3o were observed
at 430, 422 and 424 nm when excited at 290 nm. Although there is
not remarkable difference among the emission peaks of diaryleth-
enes 1o–3o, the emission intensities of them exist significant dif-
ference. The emission intensity of diarylethene 1o or 2o is
notably stronger than that of 3o. Using anthracene as a reference,
the fluorescence quantum yields of diarylethenes 1o, 2o, and 3o
were determined as 0.057, 0.048, and 0.033, respectively. The
result indicates that the six-membered aryl ring attached at ethene
moiety has a significant effect on the emission intensity and fluo-
rescence quantum yield of diarylethene. Compared to the reported
analogs,9 the six-membered aryl ring could notably increase the
fluorescence quantum yield .
Scheme 3. Structural and color changes between 3o, 3c, 4o, and 4c in acetonitrile
(1.3 Â 10À3 mol/L).
1.00
0.75
As has been observed for most of the reported diarylethenes,19
diarylethenes 1–3 exhibited changes in the fluorescence between
the open-ring isomers and the closed-ring isomers during the
process of photoisomerization by photoirradiation in hexane. The
fluorescent changes of diarylethene 1 during the process of photo-
isomerization are shown in Figure 4. Upon irradiation with 297 nm
UV light, the photocyclization reaction was occurred and the emis-
sion intensity of diarylethene decreased significantly when excited
at 290 nm. When the samples arrived at the photostationary state
by irradiation with UV light, the emission intensity of diarylethene
0
0.50
/A
1o
2o
3o
0.25
0.00
0
10
20
30
40
50
Repeat Cycles
2000
Figure 2. Fatigue resistant characteristics of diarylethenes 1–3 in hexane in air
atmosphere at rt. Initial absorbance of the sample was fixed to 1.0.
1o
1500
1000
500
0
ambient temperature,8 the thermal stabilities of diarylethenes 1–3
are very good at room temperature. However, storing these solu-
tions in ethanol at 371 K, the colored solutions of 1c–3c gradually
disappeared after 5 min. Hence the thermal stabilities of
diarylethenes 1–3 are relatively weak at high temperature, com-
pared with the reported diarylethenes bearing five-membered aryl
moieties,2a,14
2o
3o
Fatigue resistance of photochromic diarylethene is a critical fac-
tor for practical applications in optical devices.2a,15 As with the re-
ported method,2a,16 the fatigue resistant characteristics of
diarylethenes 1–3 were measured in hexane by alternating irradi-
ating with UV and visible light in air at room temperature, as
350
400
450
500
550
Wavelength (nm)
Figure 3. Emission spectra of diarylethenes 1–3 in hexane (5.0 Â 10À5 mol/L) at rt
when excited at 290 nm.

R. Wang et al. / Tetrahedron Letters 53 (2012) 320–324
323
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Vis
UV
350
400
450
Wavelength (nm)
500
550
Figure 4. Emission intensity changes of diarylethene 1 in hexane (5.0 Â 10À5 mol/
L) by photoirradiation at rt when excited at 290 nm.
1 was quenched to ca. 49%. Back irradiation of the appropriate
wavelength of visible light regenerated the open-ring isomer 1o
and duplicated the original emission spectra. As with diarylethene
1, diarylethenes 2 and 3 also exhibited the similar fluorescent
switching properties upon photoirradiation. In the photostationary
state, their emission intensities were quenched to ca. 67% for 2 and
64% for 3. The result shows that diarylethene 1 exhibits a more
marked change in fluorescence upon photocyclization than 2 and
3 because of its higher photoconversion efficiency. Upon irradia-
tion with appropriate wavelength visible light, their open-ring iso-
mers were regenerated and the original emission intensities were
recovered. Compared with diarylethenes bearing two thiophene
moieties,4g,20 the fluorescent modulation efficiencies of diaryleth-
enes 1–3 were significantly decreased in solution. One reason is
the lower photoconversion efficiency resulted from the relatively
high aromatic stabilization energy of the six-membered aryl back-
bone.2a,7 The other reason for the lower fluorescent change induced
by photoirradiation may be attributed to the existence of parallel
conformations of 1o, 2o, and 3o in the photostationary state.20,21
In conclusion, three new photochromic diarylethenes based on
the unsymmetrical six-membered aryl backbone have been devel-
oped and their properties have been investigated. The new photo-
chromic system showed evident photochromism and acted as a
remarkable fluorescent switch in solution at room temperature.
It has been demonstrated that the categories of six-membered aryl
units and substituents have a significant effect on the properties of
these diarylethene derivatives. The results of this work may be
useful for new strategy in exploring photochromic diarylethenes
based on different six-membered aryl units.
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10. Data for 1o: mp 79–80 °C; Calcd for C23H16F6O (%): Calcd C, 65.41; H, 3.82.
Found C, 65.46; H, 3.79; 1H NMR (400 MHz, CDCl3, ppm): d 2.31(s, 3H, ÀCH3),
3.36 (s, 3H, ÀCH3), 6.66 (d, 1H, benzeneÀH), 6.80 (t, 1H, benzeneÀH, J = 8.0 Hz),
7.18 (s, 1H, benzeneÀH, J = 8.0 Hz), 7.24 (t, 2H, naphthaleneÀH, J = 8.0 Hz), 7.36
(t, 1H, benzeneÀH, J = 8.0 Hz), 7.43 (t, 1H, naphthaleneÀH, J = 8.0 Hz), 7.76 (m,
3H, naphthaleneÀH, J = 8.0 Hz); 13C NMR (100 MHz, CDCl3, TMS): d = 20.16,
54.70, 110.92, 116.85, 120.24, 123.71, 125.26, 125.87, 126.24, 127.39, 128.33,
129.47, 129.93, 131.66, 131.97, 135.25, 142.68, 157.28; IR (KBr, m
, cmÀ1): 517,
588, 750, 771, 817, 848, 863, 875, 981, 1021, 1090, 1128, 1193, 1265, 1337,
1461, 1498, 1602. Data for 2o: mp 86–87 °C; C23H16F6 (%):Calcd C, 67.98; H,
3.97. Found C, 67.92; H, 3.94; 1H NMR (400 MHz, CDCl3, TMS): d 2.22 (s, 3H, –
CH3), 2.44 (s, 3H, –CH3), 7.02 (m, 2H, benzene–H, J = 8.0 Hz), 7.11 (m, 1H,
benzene–H, J = 8.0 Hz), 7.14 (m, 1H, naphthalene–H, J = 8.0 Hz), 7.25 (t, 1H,
benzene–H, J = 8.0 Hz), 7.44 (t, 1H, naphthalene–H, J = 8.0 Hz), 7.52 (t, 1H,
naphthalene–H, J = 8.0 Hz), 7.75 (t, 3H, naphthalene–H, J = 8.0 Hz); 13C NMR
(100 MHz, CDCl3, TMS): d = 20.35, 20.86, 122.61, 125.28, 125.44, 125.52,
126.64, 126.84, 128.21, 128.35, 129.62, 129.84, 130.56, 131.71, 131.78, 135.47,
136.91; IR (KBr, m
, cmÀ1): 462, 495, 535, 572, 744, 783, 813, 838, 857, 986,
1056, 1095, 1110, 1132, 1192, 1274, 1344, 1400, 1453, 1511, 1637. Data for 3o:
M.p. 131–132 °C; Calcd for C22H15F6N (%): Calcd C, 64.87; H, 3.71. Found C,
64.93; H, 3.95; 1H NMR (400 MHz, CDCl3, ppm): d 2.13 (s, 3H, ÀCH3), 2.50 (s,
3H, ÀCH3), 7.06 (m, 1H, naphthaleneÀH), 7.29 (t, 2H, naphthaleneÀH,
J = 8.0 Hz), 7.42 (t, 1H, pyridineÀH, J = 8.0 Hz), 7.50 (t, 1H, pyridineÀH,
J = 8.0 Hz), 7.77 (d, 3H, naphthaleneÀH, J = 8.0 Hz), 8.40 (s, 1H, pyridineÀH,
J = 8.0 Hz); 13C NMR (100 MHz, CDCl3, TMS): d = 18.71, 20.98, 122.26, 124.01,
125.40, 125.47, 126.58, 128.07, 128.55, 129.91, 131.35, 131.57, 133.25, 136.50,
138.15, 146.48, 147.05; IR (KBr, m
, cmÀ1): 509, 536, 571, 586, 732, 753, 788,
816, 850, 865, 880, 984, 1054, 1136, 1191, 1271, 1338, 1450, 1573.
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Acknowledgments
This work was supported by the Program for the NSFC of China
(21162011, 20962008), the Project of Jiangxi Academic and Tech-
nological leader (2009DD00100), and the Science Funds of the Edu-
cation Office of Jiangxi, China (GJJ09646, GJJ11026).
Supplementary data
Supplementary data associated with this article can be found, in
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