Photoswitchable "turn-on" Fluorescence Diarylethenes: Substituent Effects on Photochemical Properties and Electrochromism

Article abstract of DOI:10.1002/chem.201504131

A series of "turn-on" fluorescence diarylethenes derived from 2,3-bis(2-methylbenzo[b]thiophen-3-yl)-5,6-dihydro-4H-thieno[2,3-b]thiopyran-4-one (1) with alkyl and acetyl substituents were synthesized. The photochemical and photophysical properties of these derivatives, including the photoreaction of crystalline 1, were thoroughly investigated to reveal substituent effects on their properties. The results indicated that alkyl substituents did not significantly affect the absorption and emission spectra of the diarylethenes. However, large absorption and emission wavelength shifts were observed for the diarylethene with an acetyl substituent due to extension of π-π conjugation. Significantly, all of the fluorescent ring-closed forms of the compounds isomerized to their ring-open forms in the presence of Cu²⁺ in the dark. EPR results provide clear evidence for the formation of the compound 1 radical cation intermediate that might be generated in the reaction between c-1 and Cu²⁺. DFT calculations found that the ground-state activation energy for ring-opening of 1^.+ was approximately 9.2 kcal mol^-1 lower than that of 1 without Cu²⁺, such that a Cu²⁺-catalyzed oxidative cycloreversion reaction at room temperature might be possible.

Full text of DOI:10.1002/chem.201504131

DOI: 10.1002/chem.201504131
Full Paper
&
Photochemistry
Photoswitchable “Turn-on” Fluorescence Diarylethenes:
Substituent Effects on Photochemical Properties and
Electrochromism
Qi Ai, Shichong Pang, and Kwang-Hyun Ahn*[a]
Abstract: A series of “turn-on” fluorescence diarylethenes
derived from 2,3-bis(2-methylbenzo[b]thiophen-3-yl)-5,6-di-
hydro-4H-thieno[2,3-b]thiopyran-4-one (1) with alkyl and
acetyl substituents were synthesized. The photochemical
and photophysical properties of these derivatives, including
the photoreaction of crystalline 1, were thoroughly investi-
gated to reveal substituent effects on their properties. The
results indicated that alkyl substituents did not significantly
affect the absorption and emission spectra of the diaryle-
thenes. However, large absorption and emission wavelength
shifts were observed for the diarylethene with an acetyl sub-
stituent due to extension of p–p conjugation. Significantly,
all of the fluorescent ring-closed forms of the compounds
isomerized to their ring-open forms in the presence of Cu2+
in the dark. EPR results provide clear evidence for the forma-
tion of the compound 1 radical cation intermediate that
might be generated in the reaction between c-1 and Cu2+
DFT calculations found that the ground-state activation
.
+
C
energy for ring-opening of 1 was approximately 9.2 kcal
molÀ1 lower than that of 1 without Cu2+, such that a Cu2+
-catalyzed oxidative cycloreversion reaction at room temper-
ature might be possible.
chromism[13] in that it provides a pathway to bypass the large
ground-state thermodynamic barrier required for isomeriza-
tion. For diarylethenes, electrochromism offers a valuable addi-
tional means of accessing the two switching states, and this
unique dual chromism behavior has been exploited for molec-
ular switches.[14]
Introduction
In recent years, fluorescence regulation of fluorescent mole-
cules by external stimuli such as light has received a great deal
of attention because light is non-invasive, and it is easily tuned
and focused to provide a high degree of spatial and temporal
control.[1] Among these fluorescent molecules, “turn-on” fluo-
rescence diarylethenes, the initial non-fluorescent open iso-
mers of which are converted to closed isomers that emit green
or red fluorescence upon UV irradiation, have attracted much
attention.[2] Such derivatives may be useful for PALM[3] (photo-
activated localization microscopy), STORM[4] (stochastic optical
reconstruction microscopy), and RESOLFT[5] (reversible satura-
ble optical fluorescence transition) applications.
Recently, we reported a novel “turn-on” fluorescent diaryle-
thene—2,3-bis(2-methylbenzo[b]thiophen-3-yl)-5,6-dihydro-4H-
thieno[2,3-b]thiopyran-4-one (1)—and its successful application
to live cell imaging (Scheme 1).[15] The fluorescence of the com-
pound was thought to be due to the bridge unit containing
the six-membered ring with sulfur and carbonyl groups form-
ing a “push–pull” system.[16] Thus, to improve the photochro-
mic properties, and especially the fluorescence of diarylethene
1, a series of derivatives, 2–7, retaining the “push–pull system”
was synthesized, and their photophysical properties were stud-
ied. Interestingly, during our study with 1–7, we unexpectedly
found that the diarylethenes had electrochromic properties.
The ring-closed forms of the diarylethenes readily transformed
to ring-open forms in the presence of an external oxidant. De-
tailed results of this study are described herein.
Photochemical properties of diarylethenes are strongly de-
pendent upon the particular substituents.[6–8] Information relat-
ed to substituent effects may contribute to a broad under-
standing of the specific photochromic characteristics of diaryle-
thenes. In a recent study, a few effective “turn-on” fluorescence
diarylethenes with various substituents were developed.[9]
Besides photochemical reactions, many diarylethenes are re-
ported to undergo cyclization and cycloreversion reactions in
electrochemical oxidation or reduction processes.[10] The ring-
opening reaction can also take place in the presence of oxidiz-
ing reagents.[11] Such electrochromism[12] resembles photo- Results and discussion
Synthesis
[a] Dr. Q. Ai, Dr. S. Pang, Prof. Dr. K.-H. Ahn
Department of Applied Chemistry
Kyung Hee University, Yongin, 446-701 (Republic of Korea)
E-mail: khahn@khu.ac.kr
To investigate substituent effects on the fluorescence of com-
pound 1, various diarylethenes (2–7) with alkyl and acetyl
groups at the R1, R2, and R3 positions were prepared according
to methods described in Schemes 2 and 3.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504131.
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A single crystal of 1 in its open form was obtained from its
solution in hexane and ethyl acetate.[17] Interestingly, the crys-
tal showed “turn-on” fluorescence behavior in the solution
with UV or visible light irradiation. The yellow non-fluorescent
ring-open form was converted to the red fluorescent ring-
closed form upon irradiation with 365 nm light. The red color
of the crystal reverted to yellow after exposure to visible light
(Figure 1b). The crystal structure was solved by X-ray crystallo-
graphic analysis. The ORTEP drawing of the crystal structure in
Figure 1a exhibits the open isomer of 1 with approximately C2
symmetry. It was packed in a photoactive anti-parallel confor-
mation in the crystalline phase, suitable for the photocycliza-
tion reaction. It has been reported that photocyclization of a di-
arylethene molecule in the single crystalline phase was possi-
ble when the distance between the two reacting carbon
atoms packed in the crystal was less than 4.0 Š.[18] The distance
between C8 and C24 of the open isomer of 1 was determined
to be 3.58 Š, which is close enough for the photocyclization re-
action in the single-crystalline phase to occur.
Scheme 1. Photochromic reactions of 1 and derivatives.
Compound 4 was synthesized from 1 in a reaction with
acetyl chloride and AlCl3 in 90% yield (Scheme 2b). Since 1 has
an asymmetric structure, electronic effects due to substituents
at either the R2 or R3 position were expected to be different.
Thus, compounds with asymmetric substituents, such as 6 and
7, were prepared. To prepare these two compounds
(Scheme 3a), stepwise Suzuki coupling reactions were em-
ployed with 2,3-dibromothiophene to form the 2-heteroaryl-3-
bromothiophene intermediates. To confirm the structures of
the intermediates, debromination of 6b was performed with
Photophysical properties
1
n-butyl lithium to prepare 6’b (Scheme 3b). The H NMR spec-
The photophysical properties of diarylethenes 2–7 were inves-
tigated in ethyl acetate (1.0”10À5 m) at room temperature.
Figure 2 shows absorption spectra of the open and closed di-
arylethenes in ethyl acetate. All of the closed isomers showed
a broad band near 520–560 nm. The reddish color of the solu-
tions, which represents ring-closed isomers, disappeared after
irradiation with visible light due to the ring-opening reaction.
trum of 6’a prepared from 3-bromothiophene showed a dis-
tinct singlet peak for the thiophene ring. Meanwhile, the
1H NMR of 6’b had no such singlet peak, indicating that the ini-
tial Suzuki coupling with 2,3-dibromothiophene took place re-
gioselectively at the 2-position. The chemical structures of 2–7
1
were confirmed by H and 13C NMR spectroscopy, and HRMS.
Scheme 2. a) Synthesis of compounds 2 and 3. b) Synthesis of compound 4.
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Scheme 3. a) Synthesis of compounds 5–7. b) Synthesis of compounds 6’b and 6’a.
The photochromic properties of these compounds are sum-
marized in Table 1. The results showed that alkyl substituents
at the R1, R2, and R3 positions had a slight effect on the photo-
chromic features of diarylethenes, including absorption
maxima, molar absorption coefficients, and quantum yields of
the photoreaction. However, a large absorption shift of 33 nm
was observed in the absorption spectrum of 4 compared to
1 due to extension of the p–p conjunction of the benzothio-
phene with the carbonyl substituent, and the photoconversion
ratio of 4 increased to 82%. These results indicate that the car-
bonyl group had a significant effect on the photochromic
properties of 1.
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Figure 2. Absorption spectra for 1–7 in ethyl acetate (1.0”10À5 m) at room
temperature. Cycling between the open and closed isomers was performed
by alternating illumination with 312 nm and visible light.
Figure 1. a) ORTEP drawing of the open-ring isomer of 1 with 30% probabil-
ity ellipsoids. b) Photochromism of 1 in the crystalline phase.
Table 1. Photophysical properties of diarylethene derivatives.
[b,c]
[b,d]
AP:P[a] Fo!c
Fc!o
conv-
ersion open ring
lmax [nm]/e [103 mÀ1 cmÀ1
closed ring
]
Fluorescence of diarylethenes
1
2
3
4
65:35
73:27
–
0.25
0.35
0.20
0.16
0.17
0.17
0.07
50
35
40
82
252/27.2
252/30.3
252/30.8
254/44.1
286/26.7
253/27.6
252/28.9
253/32.8
520/3.2
524/4.0
526/3.7
553/13.4
The fluorescence properties of diarylethenes 2–7 close form
were measured in ethyl acetate (1.0”10À5 m) at room tempera-
ture. Because of the bridge unit forming a “push–pull” balance
with the sulfur atom and carbonyl group, the ring-closed form
for all of the diarylethenes showed good fluorescence in solu-
tion upon UV irradiation. The fluorescence spectra of diaryl-
ethenes at their photostationary states are shown in Figure 3
and Table 2.
[e]
60:40
0.016
5
6
7
70:30
65:36
66:34
0.21
0.24
0.14
0.14
0.17
0.084
35
48
57
527/3.3
523/3.6
523/6.6
[a] Antiparallel (AP) to parallel (P) ratio as observed in 1H NMR spectra in
CDCl3 (5.0”10À3 m). [b] BTF6 was used as a reference. [c] Cyclization quan-
tum yield, measured at 312 nm in ethyl acetate. [d] Ring-opening quan-
tum yield in ethyl acetate measured at the lmax of the closed-ring isomer.
[e] Not determined due to complex peaks.
As presented in Table 2, emission peaks of 2 and 3 upon
photoexcitation at the absorption maxima were observed at
594 and 597 nm, respectively, which are approximately 14 and
17 nm bathochromic shifts compared with 1. At the same
time, the fluorescence quantum yields decreased to 0.050 and
0.044, respectively. These results indicate that an increase in
the alkyl chain length at R1 is accompanied by a red-shift of
emission maxima and a decrease in emission intensities. For 4,
the emission maximum was red-shifted by 31 nm. Although,
a carbonyl substituent lowered the fluorescence quantum
yield, it may offer a new method for the synthesis of “turn-on”
fluorescence NIR photoswitching materials for biological probe
applications.
As for 6 and 7 bearing an ethyl substituent at different posi-
tions, compound 7 had a better fluorescence quantum yield
compared with 6. However, compound 5, which contains ethyl
groups at both R2 and R3, exhibited the highest fluorescence
quantum yield. This result demonstrates that the fluorescence
quantum yield is quite sensitive to the position of alkyl group
attachment.
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Figure 3. Normalized fluorescence spectra upon photo-excitation at the ab-
sorption maxima for the photostationary state of 1–7 close form in ethyl
acetate (1.0”10À5 m) at room temperature.
Table 2. Fluorescence properties of diarylethene derivatives 1–7.
lmax [nm][a]
FF[b]
lmax [nm][a]
FF[b]
1
2
3
4
580
594
597
611
0.110
0.050
0.044
0.024
5
6
7
582
581
582
0.115
0.056
0.079
[a] lmax of the emission band following photo-excitation at absorption
maxima. [b] Determined using rhodamine B (0.70:510 nm photoexcitation
in EtOH) as the reference.
Electrochromism
It was recently reported that Cu2+ ions can oxidize the ring-
closed form of certain diarylethenes to promote their ring-
opening.[11] In our study of 1, it was thought that fluorescence
quenching by Cu2+ resulting from ring-opening of fluorescent
ring-closed 1 (c-1) would be very interesting for the design of
a logic gate.[18,19]
Since 1 is electron rich, one-electron oxidation of c-1 by
Cu2+ was presumed to be possible. As expected, the addition
of Cu2+ (0.5 equiv) to a solution of c-1 (1.0”10À5 m, CH3CN) at
its photostationary state in the dark promoted an immediate
color change of the solution from red to colorless, accompa-
nied by disappearance of the absorption band at 520 nm indi-
cating that the ring-opening reaction of c-1 had occurred. Fur-
thermore, photoexcitation of the solution of ring-opened 1 (o-
1) and Cu2+ with 365 nm light did not drive the open form
into the ring-closed form, as evidenced by no clear change in
the absorption spectra, that is, the photoisomerization of
1 from the open to closed form was completely inhibited by
Cu2+, representing a specific “lock” gate role of Cu2+ for the
open form. Interestingly, the gate behavior was reversible.
Quantitative titration of Cu2+ with EDTA recovered the photo-
switching behavior of 1 (Figure 4), and the photochemical
Figure 4. a) Absorption changes for the open form of 1 in CH3CN (black
line), close form (red line), upon addition of Cu(ClO4)2 (10À3 m) (blue line),
and finally upon irradiation with 365 nm light (cyan line) after adding EDTA
(10À3 m). b) Absorption switching of 1 monitored at 520 nm upon alternating
additions of Cu2+ and EDTA in CH3CN. c) Fluorescence switching of 1 moni-
tored at 580 nm with excitation at 520 nm, upon alternating additions of
Cu2+ and EDTA in CH3CN.
open/close cycle with Cu2+ and EDTA was repeated five times
without a significant decrease in the absorption at 520 nm,
which represents the concentration of c-1 during the cycle.[20]
Apparently, EDTA could be considered a “key” to the “lock”.
Further investigation with diarylethenes, 2–7 showed similar
results.
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The electrochromic ring-opening of c-1 by Cu2+ is assumed
to occur through the radical cation intermediate of 1. Interest-
ingly, the red c-1 turned green upon the addition of Cu2+ at
near À408C and the color was disappeared at room tempera-
ture, indicating the completion of ring-opening process (Fig-
ure S1 in the Supporting Information). The green intermediate
radical cation (c-1 +) is the rate-limiting step of the electrochro-
C
mic cycloreversion process, because of the fast oxidation and
reduction steps.
+
C
The activation energy for the ring-opening of c-1 calculat-
ed theoretically (DFT, uB3LYP/6-31+G(d)) was 16.6 kcalmolÀ1.
This activation energy is much lower than that of the ground-
state ring-opening of c-1, which is 25.8 kcalmolÀ1. The large
difference in activation energies between the two steps sup-
ports the mechanism that reasonably rationalizes the fast
room-temperature ring-opening of c-1 in the presence of Cu2+
through an electrochromic cycloreversion process without irra-
diation from visible light.
was assumed to be 1 +. However, to get the EPR spectrum of
C
the intermediate that might provide the clear evidence for the
radical character, another one-electron oxidant was needed,
because the strong EPR signal of copper itself might interrupt
the observation of organic radical. We were lucky to find that
ceric ammonium nitrate (CAN), which is a good one-electron
oxidant frequently used in various organic reactions, including
EPR studies for observation of organic radical cation intermedi-
ates,[21] promoted the ring-opening of c-1 through the green
intermediate in a similar fashion to Cu2+. The EPR spectrum of
the green solution obtained with c-1 and CAN at 110 K exhibit-
ed a single isotropic line at g=2.0041 (Figure 5) indicating the
radical character of the intermediate.
Conclusion
We have synthesized a series of “turn-on” fluorescence diaryle-
thene derivatives (2–7) with alkyl and acetyl groups at the re-
active carbon and benzothiophene positions. The photochemi-
cal and photophysical properties of these derivatives were
thoroughly investigated to reveal substituent effects on their
properties. The results showed that alkyl substituents did not
have a significant effect on the absorption and emission spec-
tra of the diarylethenes. Meanwhile, a large absorption shift of
33 nm was observed in the absorption spectrum of the acetyl-
substituted diarylethene (4) compared to the absorption of
1 due to the extension of p–p conjugation of the benzothio-
phene with the carbonyl substituent.
Interestingly, all of the ring-closed forms of diarylethenes 1–
7 underwent cycloreversion upon addition of Cu2+ in the dark.
EPR results provide clear evidence for the formation of the
compound c-1 radical cation intermediate that might be gen-
erated in the reaction between 1 and Cu2+. DFT indicated that
+
C
the ground-state activation energy for ring-opening of 1 was
approximately 9.2 kcalmolÀ1 lower than that of 1 without
Cu2+, such that a Cu2+-catalyzed oxidative cycloreversion reac-
tion at room temperature might be possible.
Figure 5. EPR spectrum of the green intermediate prepared from the reac-
tion between c-1 and CAN (1:1) in CH3CN.
Acknowledgements
The proposed mechanism for the electrochromic cyclorever-
sion of diarylethene 1 is illustrated in Scheme 4, which is simi-
lar to the mechanism suggested by You et al. based on system-
atic kinetic studies with dithienylcyclopentene (DTE).[11] The
main idea of the mechanism is the ring-opening step of c-1
This work was supported by a grant from Kyung Hee Universi-
tyin 2013 (KHU-20130571).
Keywords: diarylethenes · electrochromism · fluorescence ·
photochromism · substituent effects
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Published online on December 4, 2015
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