In situ preparation of highly fluorescent dyes upon photoirradiation

Article abstract of DOI:10.1021/ja204583e

Photoswitchable or photoactivatable fluorescent dyes are potentially applicable to ultrahigh density optical memory media as well as super-resolution fluorescence imaging when the dyes are highly fluorescent and have large absorption coefficients. Here, w

Full text of DOI:10.1021/ja204583e

ARTICLE
pubs.acs.org/JACS
In Situ Preparation of Highly Fluorescent Dyes upon Photoirradiation
Kakishi Uno,† Hiroyuki Niikura,† Masakazu Morimoto,† Yukihide Ishibashi,‡ Hiroshi Miyasaka,‡ and
Masahiro Irie*,†
†Department of Chemistry and Research Center for Smart Molecules, Rikkyo University, Nishi-Ikebukuro 3-34-1, Toshima-ku,
Tokyo 171-8501, Japan
‡Division of Frontier Materials Science, Graduate School of Engineering Science and Center for Quantum Science and Technology
under Extreme Conditions, Osaka University, Toyonaka, Osaka 560-8531, Japan
S Supporting Information
b
ABSTRACT: Photoswitchable or photoactivatable fluorescent
dyes are potentially applicable to ultrahigh density optical
memory media as well as super-resolution fluorescence imaging
when the dyes are highly fluorescent and have large absorption
coefficients. Here, we report on highly fluorescent photochro-
mic dyes, which are initially nonluminous in solution under
irradiation with visible light but activated to emit green or red
fluorescence upon irradiation with ultraviolet (UV) light. The
dyes 5aꢀ9a are sulfone derivatives of 1,2-bis(2-ethyl-6-phenyl(or thienyl)-1-benzothiophen-3-yl)perfluorocyclopentene. It was
found that substitution of phenyl or thiophene rings at 6 and 60 positions of the benzothiophene-1,1-dioxide groups is effective to
increase the fluorescence quantum yields of the closed-ring isomers over 0.7 and absorption coefficients over 4 ꢁ 104 Mꢀ1 cmꢀ1
.
The phenyl-substituted derivatives 5aꢀ7a undergo photocyclization reactions to produce yellow closed-ring isomers 5bꢀ7b, which
emit brilliant green fluorescence at around 550 nm (ΦF = 0.87ꢀ0.88) under irradiation with 488 nm light. Any absorption intensity
change of the closed-ring isomers was not observed even after 100 h storage in the dark at 80 °C. The closed-ring isomers slowly
returned to the initial open-ring isomers upon irradiation with visible (λ > 480 nm) light. The ring-opening quantum yields (ΦCfO
)
were measured to be (1.6ꢀ4.0) ꢁ 10ꢀ4. When the phenyl substituents are replaced with thiophene rings, such as compounds 8a and
9a, the absorption bands of the closed-ring isomers shift to longer than 500 nm. The closed-ring isomers exhibit brilliant red
fluorescences at around 620 nm (ΦF = 0.61ꢀ0.78) under irradiation with 532 nm light. The ring-opening reactions are very slow
(ΦCfO < 1 ꢁ 10ꢀ5). The fluorescence lifetimes of these sulfone derivatives were measured to be around 2ꢀ3 ns, which is much
longer than the value of the closed-ring isomer of 1,2-bis(2-methyl-1-benzothiophen-3-yl)perfluorocyclopentene (τF = 4 and 22 ps).
The closed-ring isomer 8b in 1,4-dioxane exhibits excellent fatigue resistant property under irradiation with visible light
(λ > 440 nm) superior to the stability of Rhodamine 101 in ethanol.
’ INTRODUCTION
Even limited to molecules having a photochromic diarylethene
derivative as a photoswitching unit, a number of derivatives having a
fluorescent unit, such as triphenyl imidazole,29 anthracene,17,28,30,31 per-
ylenebisimide,20,21,25,32 or 4,4-difluoro-8-(40-iodophenyl)-1,3,5,
7-tetramethyl-4-bora-3a,4a-diaza-s-indacene,26 have been pre-
pared. In these fluorescent photochromic molecules, the fluo-
rescence is switched off by energy transfer or electron transfer
processes when the diarylethene unit converts from the open- to
the closed-ring isomer. Initially the molecules are fluorescent,
while they convert to nonfluorescent states upon irradiation with
UV light. The fluorescent switching from fluorescent to non-
fluorescent states can be applied to optical memory media but is
hardly applicable to super-resolution fluorescence imaging, such
as PALM (photoactivatable localization microscopy) or STORM
(stochastic optical reconstruction microscopy), because the
imaging method requires initial dark background.11,12,33
Fluorescence is the most convenient tool to detect small
amounts of molecules. Even single molecules can be detected
using the fluorescence.1ꢀ7 To make use of the high sensitivity,
various types of fluorescent molecules have been developed and
widely applied to microanalysis,8,9 bioimaging,10ꢀ16 and memory
media.17ꢀ21 Fluorescence spectra and/or intensity changes by
external stimuli, such as chemicals, electrons (or holes), or
photons, are adopted for the analysis and the imaging. When
the fluorescence can be switched on and off upon photoirradia-
tion, the fluorescence modulation is potentially applied to ultra-
high density optical memory media17ꢀ21 and also to super-
resolution fluorescence imaging.14,16,22 It is strongly desired to
explore for highly fluorescent dyes, which can be efficiently and
instantaneously prepared or activated upon photoirradiation.
The photoswitchable fluorescent dyes can be designed and con-
structed by combining both photochromic and fluorescent chromo-
phores in a molecule. On the basis of this idea, various fluorescent
photochromic molecules have been synthesized.17,18,20,21,23ꢀ28
Received: May 18, 2011
Published: August 05, 2011
r
2011 American Chemical Society
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Scheme 1. Photochromism of Diarylethenes 1ꢀ4a
Scheme 2. Photochromism of Diarylethenes 5ꢀ9 Having
Phenyl and Thiophene Substituents
a The closed-ring isomers are fluorescent.
Another type of fluorescent photochromic diarylethenes is sul-
fone derivatives of 1,2-bis(2-methyl-1-benzothiophen-3-yl)-
perfluorocyclopentene.34,35 They are initially nonluminous and
dark in solution under irradiation with visible (λ > 400 nm) light,
while they are activated to emit blue fluorescence upon UV
irradiation. Although the sulfone derivatives fulfill the PALM
(or STORM) switching requirement to produce fluorescent dyes
in the dark background, the quantum yields and the absorption
coefficients are too low to be practically used for the ultrahigh
density optical memory media and the PALM fluorescence
imaging, both of which are, in principle, based on detection of
single molecules. Our aim is to create photoswitchable fluores-
cent dyes, which have the fluorescence quantum yields close to
0.9 and high absorption coefficients36 by chemical modification
of the sulfone derivatives.
’ RESULTS AND DISCUSSION
1,2-Bis(2-methyl-1-benzothiophene-1,1-dioxide-3-yl)perfluo-
rocyclpentene (1a) (Scheme 1) is known to undergo a reversible
photochromic reaction and exhibit fluorescence in the closed-
ring isomer 1b.34,35 1a is instantaneously activated to the
fluorescent isomer 1b upon UV irradiation. The fluorescence
quantum yield and peak fluorescence wavelength were found to
vary depending on alkyl substituents at the reactive carbons.37
When the methyl substituents of 1a are replaced with ethyl,
propyl, and butyl substituents, such as compounds 2a, 3a, and 4a,
the peak wavelength shifts to longer wavelength region from
490 nm (1b) to 498 nm (2b), 500 nm (3b), and 499 nm (4b),
and the yield increases from 0.01 (1b) to 0.12 (2b), 0.09 (3b),
and 0.08 (4b) in ethyl acetate upon excitation at 398 nm. Among
the four substituents, ethyl substituents are the most effective to
increase the fluorescence quantum yield. Although the reason of
the substituent effect is not known at the present stage of
research, we employed ethyl substituents in the following
experiments.
The fluorescence quantum yields and the absorption coeffi-
cients of the closed-ring isomers of 1,2-bis(2-alkyl-1-benzothio-
phene-1,1-dioxide-3-yl)perfluorocyclopentenes34,35,37 and a poly-
mer derivative38 are insufficiently low.36,39 To increase the
quantum yield, we examined various substituent effects not only
at the reactive carbons but also at other parts of the molecule and
found that substitution of phenyl or thiophene rings at 6 and 60
positions of the benzothiophene-1,1-dioxide groups was effective
to dramatically increase the yield and the absorption coefficient.
Figure 1. Absorption spectra of 5a (black dashed line), 5b (black solid
line), and photostationary state under irradiation with 313 nm light
(black dotted line) in 1,4-dioxane (2.0 ꢁ 10ꢀ5 M) and fluorescence
spectrum of 5b (green solid line) under irradiation with 488 nm light.
Phenyl- or thiophene-substituted derivatives 5aꢀ9a (Scheme 2)
were prepared, and their fluorescent properties were examined.
SuzukiꢀMiyaura coupling reaction using a 6,60-diiodo derivative
of 2a was employed to prepare the derivatives. The details of the
synthetic procedures are described in the Experimental Section.
Figure 1 shows the absorption spectra of 5a (λ1max, 298 nm; ε,
2.0 ꢁ 104 Mꢀ1 cmꢀ1; λ2max, 336 nm; ε, 1.7 ꢁ 104 Mꢀ1 cmꢀ1), 5b
(λmax, 456 nm; ε, 4.6 ꢁ 104 Mꢀ1 cmꢀ1), and photostationary
state under irradiation with 313 nm light in 1,4-dioxane. 5b was
isolated from the photostationary solution using HPLC. 5a has
no absorption in the visible region (λ > 400 nm). Upon
irradiation with UV light (λ = 313 nm), a new absorption appears
at 456 nm. The new absorption is ascribed to the closed-ring
isomer 5b. The photocyclization quantum yield was determined
to be 0.42.
The closed-ring isomer 5b exhibits brilliant green fluorescence
at around 550 nm. The fluorescence excitation spectrum agrees
to the absorption spectrum of 5b. The fluorescence quantum
yield was measured to be 0.87 in 1,4-dioxane upon excitation at
488 nm. The quantum yield is extremely high and 7 times larger
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Table 1. Photophysical Properties of 2, 5ꢀ9 in 1,4-Dioxanea
open-ring isomer a
closed-ring isomer b
λmax/nm (ε/104 Mꢀ1 cmꢀ1
)
ΦOfC
λmax/nm (ε/104 Mꢀ1 cmꢀ1
)
ΦCfO
ΦF
τF/ns
2
5
6
7
8
9
275 (0.52), 310 (0.52)
298 (2.0), 336 (1.7)
267 (4.1), 300 (3.2)
301 (2.2), 340 (1.9)
330 (2.6), 374 (2.6)
363 (4.1)
0.28 b,c
0.42 c
0.51 c
0.43 c
0.21 e
0.32 e
414 (1.8)
456 (4.6)
456 (4.7)
463 (5.0)
506 (6.2)
485 (6.3)
0.18 b,d
0.22
0.87
0.87
0.88
0.78
0.61
1.2
3.0
2.8
2.9
2.6
1.9
4.0 ꢁ 10ꢀ4f
3.6 ꢁ 10ꢀ4f
1.6 ꢁ 10ꢀ4f
<1.0 ꢁ 10ꢀ5f
<1.0 ꢁ 10ꢀ5f
a λmax, absorption maximum; ε, absorption coefficient; ΦOfC, cyclization quantum yield; ΦCfO, cycloreversion quantum yield; ΦF, fluorescence
quantum yield; τF, fluorescence lifetime. b 1 was used as a reference.35 c Under irradiation with 313 nm light. d Under irradiation with 405 nm light.
e Under irradiation with 365 nm light. f Under irradiation with 488 nm light.
than the value of 2b in ethyl acetate. The closed-ring isomer 5b
has a large absorption coefficient and is thermally stable. Any
absorption intensity decrease was not observed even after 100 h
storage in the dark at 80 °C. The absorption at 456 nm and the
fluorescence at around 550 nm decrease and return to the initial
state upon irradiation with visible (λ > 480 nm) light. The ring-
opening quantum yield (ΦCfO) of 5b was measured to be 4.0 ꢁ
10ꢀ4, which is much lower than the value of 1b (ΦCfO = 0.061)
and 2b (ΦCfO = 0.18).35 Solvent dependence of the fluores-
cence quantum yield was also measured. The yields were
determined to be 0.89 (in dichloromethane), 0.79 (in THF),
and 0.76 (in ethanol). Although the yield has a tendency to
decrease with increasing solvent polarity, the high yield still
remains even in polar ethanol.
Figure 2. Absorption spectra of 8a (black dashed line), 8b (black solid
line), and photostationary state under irradiation with 365 nm light in
1,4-dioxane (1.0 ꢁ 10ꢀ5 M) and fluorescence spectrum of 8b (red solid
line) under irradiation with 532 nm light. The photostationary spectrum
overlaps with the spectrum of 8b. The absorption spectra of 2a (blue
dashed line) and 2b (blue solid line) in 1,4-dioxane (1.0 ꢁ 10ꢀ5 M) were
also shown as a reference.
The highly fluorescent and thermally stable dye is instanta-
neously prepared from the open-ring isomer 5a upon irradiation
with UV light. The open-ring isomer has no absorption in the
visible region (λ > 400 nm), which means that the solution is
initially dark under irradiation with 488 nm light. Upon UV
irradiation, a new visible absorption due to the closed-ring isomer
appears and excitation of the band by irradiation with 488 nm
light exhibits brilliant fluorescence. Therefore, the signal ratio bet-
ween the fluorescent state and dark background becomes huge.
Similar fluorescence behavior was observed for 6 and 7 having
acetophenone and benzylalcohol substituents, respectively (see
Figures S1 and S2). The acetyl group is reported to be effective to
increase the fluorescence quantum yield,35 and the hydroxy
group is useful to introduce a reactive tag to proteins and others.
Contrary to the expectation, increase in the fluorescence quan-
tum yield was not observed by the acetyl modification. The
absorption coefficients, cyclization/cycloreversion quantum
yields, and fluorescence quantum yields of 6 and 7 are summar-
ized in Table 1.
The above phenyl-substituted derivatives can be excited using
488 nm laser light and exhibit green fluorescence. Another useful
laser light locates at 532 nm. To shift the absorption bands of the
closed-ring isomer to longer than 500 nm, the phenyl substituent
was replaced with thiophene substituent as compounds 8a and
9a. Figure 2 shows the absorption spectra of 8a (λ1max, 330 nm; ε,
2.6 ꢁ 104 Mꢀ1 cmꢀ1; λ2max, 374 nm; ε, 2.6 ꢁ 104 Mꢀ1 cmꢀ1), 8b
(λmax, 506 nm; ε, 6.2 ꢁ 104 Mꢀ1 cmꢀ1), and photostationary
state under irradiation with 365 nm light in 1,4-dioxane. The
photostationary spectrum overlaps with that of 8b. 8a has no
absorption longer than 450 nm. Upon irradiation with UV light
(λ = 365 nm), a new absorption appears at 506 nm. The new
absorption is ascribed to the closed-ring isomer 8b. The photo-
cyclization quantum yield was determined to be 0.21. The closed-
ring isomer exhibits brilliant red fluorescence at around 620 nm
under irradiation with 532 nm light. The fluorescence quantum
yield was measured to be 0.78 in 1,4-dioxane. The closed-ring
isomer is thermally as well as photochemically stable and hardly
returns to the initial state. Any absorption intensity change of the
closed-ring isomer was not observed even after 100 h storage in
the dark at 80 °C.
The acetyl thiophene-substituted derivative 9b also exhibits
brilliant red fluorescence upon irradiation with 532 nm light, and
the fluorescence quantum yield was measured to be 0.61, which is
lower than the value of the methyl thiophene derivative 8b.
Although the ring-opening reactions of these thiophene-substi-
tuted derivatives 8b and 9b are very slow under irradiation with
visible light (λ > 480 nm), the absorption at around 500 nm is
substantially bleached. The ring-opening quantum yields were
estimated to be less than 1 ꢁ 10ꢀ5
.
Figure 3 shows photographs of fluorescence changes of 1,4-
dioxane solutions containing 5 and 8 upon irradiation with
365 nm light under irradiation with 488 nm blue light (left-side
solution containing 5) and 532 nm green light (right-side
solution containing 8). Before irradiation with UV light, both
solutions are dark, and any luminescence was not observed.
Upon 365 nm light irradiation, brilliant green and red fluores-
cences instantaneously appeared. These fluorescences are
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Figure 4. The absorbance changes of 5b and 8b in 1,4-dioxane
solutions (4 ꢁ 10ꢀ6 M) upon irradiation with visible light
(λ > 440 nm) and weak 365 nm light. The 365 nm light was applied
to convert the photogenerated open-ring isomers to the closed-ring
isomers. The absorbance changes of Rhodamine 101 in ethanol and
fluorescein in water (4 ꢁ 10ꢀ6 M) were also shown as references.
nonradiative decay processes including the cycloreversion reac-
tion are strongly suppressed by the substitution, and this is the
reason why the fluorescence quantum yield increases.
In conclusion, we have prepared diarylethene derivatives,
which can be activated to fluorescent isomers upon irradiation
with UV light. The isomers exhibit brilliant and fatigue resistant
fluorescence upon irradiation with 488 or 532 nm light.
Figure 3. Photographs of 1,4-dioxane solutions containing 5 and 8
before (a) and after (b) irradiation with 365 nm light under irradiation
with 488 nm blue light (left-side solution containing 5) and 532 nm
green light (right-side solution containing 8).
’ EXPERIMENTAL SECTION
attributed to the formation of 5b and 8b upon irradiation with
365 nm light.
General Procedures. Commercially available reagents and sol-
vents for syntheses were of reagent grade and used without further
purification. Solvents for spectral measurements were of spectroscopic
grade. 1H and 13C NMR spectra were measured with a NMR spectro-
meter (Bruker, Avance 400). Tetramethylsilane was used as an internal
standard. In the NMR data, (ap) and (p) indicate the proton signals of
antiparallel and parallel conformations, respectively. Mass spectra were
measured with a gas chromatographyꢀmass spectrometer (Shimadzu,
GCMS-QP2010Plus). Elemental analysis was carried out with an
elemental micro analysis system (Elementar, Vario MICRO Cube).
The closed-ring isomers were isolated from the photostationary solu-
tions using HPLC (Hitachi L-2130 pump system, L-2420 detector,
Wakosil 5SIL). Absorption and fluorescence spectra were measured
with an absorption spectrophotometer (Hitachi, U-4100) and a fluor-
escence spectrophotometer (Hitachi, F-2500), respectively. Fluores-
cence spectra were corrected using the fluorescence of acridine orange as
a reference.43 Fluorescent quantum yields were measured with an
absolute PL quantum yield measurement system (Hamamatsu,
C9920-02G) and were confirmed by comparing the yields with those
of reference samples, fluorescein and Rhodamine 6G. The absorption
maxima were used as the excitation wavelengths of the fluorescence
quantum yield measurement. For the measurement of the fluorescence
time profiles, time-correlated single-photon counting (TCSPC) method
using a picosecond Nd3+:YAG laser (DPM-1000&SBR-5080-FAP,
Coherent, 532 nm) with 8 MHz repetition rate and a femtosecond
broad band Ti: Sapphire laser (SHG 450 nm) with 8 MHz repetition rate
were employed. A photomultiplier tube (Hamamatsu Photonics,
R3809U-50) with an amplifier (Hamamatsu Photonics C5594) and a
counting board (PicoQuanta, PicoHarp 300) were used for the signal
detection. The instrumental response function was estimated by the
fwhm of the scattered light from a colloidal solution for the excitation
light pulse. In the present measurements, it was 32 ps. Photoirradiation
for photoreactions was carried out using a 500 W xenon lamp (Ushio,
SX-UI501XQ) or a 300 W xenon lamp (Asahi spectra, MAX-302).
Another characteristic feature of these sulfone derivatives
having phenyl or thiophene substituents is fatigue resistant
property. Figure 4 shows the decrease of the absorption maxima
of 5b and 8b in 1,4-dioxane upon irradiation with visible light
(λ > 440 nm) and weak 365 nm light. The 365 nm light was applied
to convert photogenerated open-ring isomers to the closed-ring
isomers. The decay curves of absorbances of Rhodamine 101 in
ethanol and fluorescein in water are also shown as references. Any
appreciate decay of the absorbance was not observed for 8b, while
appreciable decrease was observed in the absorbances of 5b and
Rhodamine 101 upon irradiation with visible light (λ > 440 nm).
The absorption band of fluorescein disappeared in less than 2 h
under the present illumination condition. The fatigue resistant
property of 8b is excellent and superior to the stability of
Rhodamine 101, while the stability of 5b is moderate.
The fluorescence lifetimes of 2b, 5bꢀ9b were measured and
are shown in Table 1. The derivatives having phenyl or thiophene
substituents exhibit similar lifetimes of 2ꢀ3 ns, while the lifetime
of 2b is shorter (1.2 ns). These lifetimes are, however, much
longer than the lifetime of 1,2-bis(2-methyl-1-benzothiophen-3-
yl)perfluorocyclopentene (τF = 4 and 22 ps).40,41
To reveal the reason why the fluorescence quantum yields of
the derivatives having phenyl or thiophene substituent are large,
we estimated the fluorescence decay rate constant (kf) and
nonradiative decay rate constant (knr) on the basis of the
fluorescence quantum yields and the lifetimes. The rate constants
for unsubstituted derivative 2b and 5b having phenyl substitu-
ents were calculated to be 1.8 ꢁ 108 sꢀ1 (kf) and 6.5 ꢁ 108 sꢀ1
(knr), and 2.9 ꢁ 108 sꢀ1 (kf) and 4.3 ꢁ 107 sꢀ1 (knr),
respectively.42 Change in kf is not significant by the substitution,
but knr remarkably decreases in 5b. The result indicates that
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Scheme 3. Synthesis of Compounds 5aꢀ9a
Wavelength of the light was selected using band-pass or cutoff optical
filters and a monochromator (Ritsu, MC-10N). Typical intensity of
365 nm light used for the cyclization was 3ꢀ10 mW/cm2. Photocycliza-
tion and photocycloreversion quantum yields were determined using
furyl fulgide44 as a reference.
1,2-Bis(2-ethyl-6-phenyl-1-benzothiophen-1,1-dioxide-3-yl)per-
fluorocyclopentene (5a). To a THF solution (10 mL) containing 11
(150 mg, 0.185 mmol) and phenylboronic acid (57 mg, 0.47 mmol) were
added saturated aqueous K2CO3 (10 mL), tris(dibenzylideneacetone)-
dipalladium(0) (30 mg, 0.033 mmol), and 18% tricyclohexylphosphine
toluene solution (0.1 mL), and the mixture was stirred at room
temperature for 20 min. The resulting mixture was neutralized with
diluted hydrochloric acid and extracted with chloroform. The organic
layer was evaporated in vacuo, and the residue was purified by silica gel
column chromatography (hexane:ethyl acetate = 9:1 f 7:1) to give 5a
(126 mg, 0.177 mmol, 96% yield) as a white solid. 5a: mp 199ꢀ200 °C;
1H NMR (400 MHz, 1,4-dioxane-d8) δ 1.04 (t, J = 7.6 Hz, 3.7H (ap)),
1.37 (t, J = 7.6 Hz, 2.3H (p)), 2.45ꢀ2.51 (m, 1.6H (p)), 2.59ꢀ2.70 (m,
2.4H (ap)), 7.34ꢀ7.50 (m, 8H), 7.62 (d, J = 7.2 Hz, 1.6H (p)), 7.67 (d, J
= 7.2 Hz, 0.8H (p)), 7.71 (d, J = 7.2 Hz, 2.4H (ap)), 7.87 (d, J = 7.2 Hz,
1.2H (ap)), 8.16 (s, 0.8H (p)), 8.22 (s, 1.2H (ap)); 13C NMR (100
MHz, CDCl3) δ 11.64, 11.92, 19.17, 19.29, 121.05, 122.88, 123.08,
123.25, 123.34, 127.07, 127.11, 127.99, 128.07, 128.96, 129.04, 129.22,
129.29, 131.80, 132.17, 136.51, 138.08, 144.39, 144.52, 148.15, 148.56;
MS (EI) m/z (M+) 712. Anal. Calcd for C37H26F6O4S2: C, 62.35; H,
3.68; S, 9.00. Found: C, 62.55; H, 4.02; S, 8.92.
1,2-Bis(2-ethyl-6-(4-acetylphenyl)-1-benzothiophen-1,1-dioxide-3-
yl)perfluorocyclopentene (6a). To a THF solution (10 mL) containing
11 (150 mg, 0.185 mmol) and 4-acetylphenylboronic acid (74 mg, 0.45
mmol) were added saturated aqueous K2CO3 (10 mL), tris-
(dibenzylideneacetone)dipalladium(0) (30 mg, 0.033 mmol), and
18% tricyclohexylphosphine toluene solution (0.1 mL), and the mixture
was stirred at room temperature for 20 min. The resulting mixture was
neutralized with diluted hydrochloric acid and extracted with chloro-
form. The organic layer was evaporated in vacuo, and the residue was
purified by silica gel column chromatography (hexane:ethyl acetate = 8:2
f 6:4) to give 6a (137 mg, 0.172 mmol, 93% yield) as a white solid. 6a:
mp 218ꢀ219 °C; 1H NMR (400 MHz, 1,4-dioxane-d8) δ 1.06 (t, J = 7.6
Hz, 3.9H (ap)), 1.38 (t, J = 7.6 Hz, 2.1H (p)), 2.46ꢀ2.52 (m, 1.4H (p)),
2.54 (s, 2.1H (p)), 2.58 (s, 3.9H (ap)), 2.61ꢀ2.70 (m, 2.6H (ap)), 7.37
(d, J = 8.0 Hz, 2H), 7.73 (m, 2.1H), 7.83 (d, J = 8.4 Hz, 2.6H (ap)), 7.94
(d, J = 8.0 Hz, 1.3H (ap)), 8.01 (d, J = 8.4 Hz, 1.4H (p)), 8.07 (d, J = 8.4
Synthesis. Compounds 5aꢀ9a were prepared as shown in Scheme 3.
1,2-Bis(2-ethyl-6-iodo-1-benzothiophen-1,1-dioxide-3-yl)perfluoro-
cyclopentene (11). A solution of 1,2-bis(2-ethyl-1-benzothiophen-3-
yl)perfluorocyclopentene (10)45 (4.0 g, 8.1 mmol) in acetic acid
(200 mL) was heated to the boiling temperature (118 °C). Twenty
milliliters of 50% hydrogen peroxide solution was slowly added to the
solution, and the mixture was refluxed for 30 min. The resulting mixture
was poured into cold water (200 mL) to give a precipitate. The
precipitate was filtered, washed with distilled water several times, and
dried in vacuo. The white powder product was added into cold sulfuric
acid (80 mL), and the mixture was stirred vigorously at ꢀ5 °C for 20 min
to dissolve the powder perfectly. Orthoperiodic acid (2.3 g, 10 mmol)
and iodine (4.5 g, 18 mmol) were added to the solution, and the mixture
was stirred at ꢀ5 °C for 2 h. After the pale green mixture turned dark
purple, the mixture was warmed to 0 °C and further stirred for 2 h, and
then carefully poured into cold ice water (160 mL). The mixture was
extracted with chloroform (300 mL) three times. Additionally, the
aqueous layer was extracted with ethyl acetate (200 mL) twice. The
organic layer (chloroform and ethyl acetate) was washed with aqueous
NaHCO3, Na2S2O3, and brine, dried over anhydrous MgSO4, and
evaporated in vacuo. The crude product was purified by silica gel column
chromatography (hexane:ethyl acetate = 10:1 f 8:1) to give 11 (4.7 g,
5.8 mmol, 72% yield) as a white solid. 11: mp 123ꢀ124 °C; 1H NMR
(400 MHz, CDCl3) δ 1.06 (t, J = 7.6 Hz, 3.4H (ap)), 1.39 (t, J = 7.6 Hz,
2.6H (p)), 2.29ꢀ2.37 (m, 1.7H (p)), 2.42ꢀ2.59 (m, 2.3H (ap)), 6.80
(d, J = 8.4 Hz, 0.9H (p)), 6.91 (d, J = 8.4 Hz, 1.1H (ap)), 7.79 (dd, J = 8.4
and 1.6 Hz, 0.9H (p)), 7.95 (dd, J = 8.4 and 1.6 Hz, 1.1H (ap)), 8.00 (d, J
= 1.6 Hz, 0.9H (p)), 8.06 (d, J = 1.6 Hz, 1.1H (ap)); MS (EI) m/z (M+)
812; 13C NMR (100 MHz, CDCl3) δ 11.45, 11.68, 19.08, 19.24, 96.37,
122.68, 122.84, 123.49, 123.87, 128.67, 128.79, 131.30, 131.33, 136.94,
142.42, 142.76, 148.00, 148.48. Anal. Calcd for C25H16F6I2O4S2: C,
36.96; H, 1.99; S, 7.89. Found: C, 36.97; H, 1.91; S, 7.85.
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ARTICLE
Hz, 2.6H (ap)), 8.24 (s, 0.7H (p)), 8.29 (s, 1.3H (p)); 13C NMR (100
MHz, CDCl3) δ 11.65, 11.87, 19.27, 19.37, 26.70, 121.19, 122.95,
123.20, 127.27, 127.34, 128.84, 129.21, 129.28, 132.04, 132.42, 136.68,
137.25, 142.30, 143.01, 143.15, 148.71, 149.25, 197.29,; MS (EI) m/z
(M+) 796. Anal. Calcd for C41H30F6O6S2: C, 61.80; H, 3.79; S, 8.05.
Found: C, 61.47; H, 3.52; S, 8.44.
1,4-dioxane-d8) δ 1.03 (t, J = 7.6 Hz, 3.7H (ap)), 1.36 (t, J = 7.6 Hz, 2.3H
(p)), 2.48ꢀ2.69 (s, 10H), 7.28 (d, J = 8.0 Hz, 0.8H (p)), 7.32 (d, J = 8.0
Hz, 1.2H (ap)), 7.56 (d, J = 4.0 Hz, 0.8H (p)), 7.64 (d, J = 4.0 Hz, 1.2H
(ap)), 7.71ꢀ7.77 (m, 2.8H), 7.92 (d, J = 8.0 Hz, 1.2H (ap)), 8.24
(s, 0.8H (p)), 8.31 (s, 1.2H (ap)); 13C NMR (100 MHz, CDCl3) δ
11.62, 11.86, 19.25, 19.37, 26.61, 26.66, 119.80, 122.93, 123.04, 123.13,
123.41, 125.97, 126.01, 128.92, 129.02, 130.79, 131.09, 133.24, 133.28,
136.42, 136.48, 136.84, 136.88, 145.30, 145.37, 148.14, 148.18, 148.84,
149.28, 190.28; MS (EI) m/z (M+) 808. Anal. Calcd for C37H26F6O6S4:
C, 54.94; H, 3.24; S, 15.86. Found: C, 55.09; H, 3.23; S, 15.99.
1,2-Bis(2-ethyl-6-(4-(hydroxymethyl)phenyl)-1-benzothiophen-1,1-
dioxide-3-yl)perfluorocyclopentene (7a). To a THF solution (10 mL)
containing 11 (150 mg, 0.185 mmol) and 4-(hydroxymethyl)-
phenylboronic acid (68 mg, 0.45 mmol) were added saturated aqueous
K2CO3 (10 mL), tris(dibenzylideneacetone)dipalladium(0) (30 mg,
0.033 mmol), and 18% tricyclohexylphosphine toluene solution (0.1 mL),
and the mixture was stirred at room temperature for 20 min. The re-
sulting mixture was neutralized with diluted hydrochloric acid and
extracted with chloroform. The organic layer was evaporated in vacuo,
and the residue was purified by silica gel column chromatography
(hexane:ethyl acetate = 6:4 f 2:8) to give 7a (133 mg, 0.172 mmol,
93% yield) as a white solid. 7a: mp 205ꢀ206 °C; 1H NMR (400 MHz,
1,4-dioxane-d8) δ 1.03 (t, J = 7.6 Hz, 3.9H (ap)), 1.37 (t, J = 7.6 Hz, 2.1H
(p)), 2.40ꢀ2.50 (m, 1.4H (p)), 2.56ꢀ2.70 (m, 2.6H (ap)), 3.78ꢀ3.84
(m, 2H), 4.59 (d, J = 6.0 Hz, 1.4H (p)), 4.62 (d, J = 6.0 Hz, 2.6H (ap)),
7.34 (d, J = 8.0 Hz, 2H), 7.40 (d, J = 8.0 Hz, 1.4H (p)), 7.46 (d, J = 8.0
Hz, 2.6H (ap)), 7.61 (d, J = 8.0 Hz, 1.4H (p)), 7.70 (m, J = 8.0 Hz, 3.3H),
7.88 (d, J = 8.0 Hz, 1.3H (ap)), 8.17 (s, 0.7H (p)), 8.23 (s, 1.3H (ap));
13C NMR (100 MHz, CDCl3) δ 11.65, 11.91, 19.17, 19.29, 64.70, 64.75,
120.94, 122.91, 123.12, 127.21, 127.27, 127.66, 127.72, 128.07, 131.69,
132.07, 136.53, 137.30, 141.92, 144.14, 148.14; MS (EI) m/z (M+) 772.
Anal. Calcd for C39H30F6O6S2: C, 60.62; H, 3.91; S, 8.30. Found: C,
60.79; H, 3.94; S, 8.40.
’ ASSOCIATED CONTENT
S
Supporting Information. Absorption and fluorescence
b
spectra of 6, 7, and 9. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
iriem@rikkyo.ac.jp
’ ACKNOWLEDGMENT
The present work was supported by a Grant-in-Aid for
Scientific Research on Priority Areas “New Frontiers in Photo-
chromism (471)” (No. 1950008) from the ministry of Educa-
tion, Culture, Sports, Science, and Technology, Japan. We wish
to express our thanks to Prof. K.-H. Ahn and Dr. Y.-C. Jeong of
Kyung Hee University for their kind suggestions and discussions.
1,2-Bis(2-ethyl-6-(5-methylthiophen-2-yl)-1-benzothiophen-1,1-di-
oxide-3-yl)perfluorocyclopentene (8a). To a THF solution (10 mL)
containing 11 (150 mg, 0.185 mmol) and 5-methylthiophen-2-ylboro-
nic acid (65 mg, 0.46 mmol) were added saturated aqueous K2CO3
(10 mL), tris(dibenzylideneacetone)dipalladium(0) (30 mg, 0.033 mmol),
and 18% tricyclohexylphosphine toluene solution (0.1 mL), and the
mixture was stirred at room temperature for 20 min. The resulting
mixture was neutralized with diluted hydrochloric acid and extracted
with chloroform. The organic layer was evaporated in vacuo, and the
residue was purified by silica gel column chromatography (hexane:ethyl
acetate = 10:1 f 7:3) to give 8a (132 mg, 0.175 mmol, 95% yield) as
a pale yellow solid. 8a: mp 204ꢀ205 °C; 1H NMR (400 MHz,
1,4-dioxane-d8) δ 0.98 (t, J = 7.6 Hz, 3.9H (ap)), 1.34 (t, J = 7.6 Hz,
2.1H (p)), 2.37ꢀ2.69 (m, 10H), 6.76 (d, J = 3.6 Hz, 0.7H (p)), 6.81 (d,
J = 3.6 Hz, 1.3H (ap)), 7.19 (d, J = 8.4 Hz, 0.7H (p)), 7.26 (d, J = 8.4 Hz,
1.3H (ap)), 7.32 (d, J = 3.6 Hz, 0.7H (p)), 7.41 (d, J = 3.6 Hz, 1.3H (ap)),
7.53 (dd, J = 8.0 and 1.6 Hz, 0.7H), 7.73 (dd, J = 8.0 and 1.6 Hz, 1.3H
(ap)), 8.06 (d, J = 1.6 Hz, 0.7H (p)), 8.14 (d, J = 1.6 Hz, 1.3H (ap)); 13C
NMR (100 MHz, CDCl3) δ 11.64, 11.92, 15.51, 15.55, 19.02, 19.21,
118.89, 118.94, 122.90, 123.20, 123.31, 123.38, 125.39, 125.46, 126.92,
127.01, 129.39, 129.67, 136.57, 136.63, 137.76, 137.83, 138.42, 142.51,
142.65, 147.51, 147.83; MS (EI) m/z (M+) 752. Anal. Calcd for C35H26-
F6O4S4: C, 55.84; H, 3.48; S, 17.04. Found: C, 56.13; H, 3.48; S, 17.35.
1,2-Bis(2-ethyl-6-(5-acetylthiophen-2-yl)-1-benzothiophen-1,1-di-
oxide-3-yl)perfluorocyclopentene (9a). To a THF solution (10 mL)
containing 11 (150 mg, 0.185 mmol) and 5-acetylthiophen-2-ylboronic
acid (76 mg, 0.45 mmol) were added saturated aqueous K2CO3
(10 mL), tris(dibenzylideneacetone)dipalladium(0) (30 mg, 0.033
mmol), and 18% tricyclohexylphosphine toluene solution (0.1 mL),
and the mixture was stirred at room temperature for 20 min. The
resulting mixture was neutralized with diluted hydrochloric acid and
extracted with chloroform. The organic layer was evaporated in vacuo,
and the residue was purified by silica gel column chromatography
(hexane: ethyl acetate = 7:3 f 4:6) to give 9a (134 mg, 0.166 mmol,
90% yield) as a white solid. 9a: mp 251ꢀ252 °C; 1H NMR (400 MHz,
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