Di(benzothienyl)cyclobutenes: Toward Strained Photoswitchable Fluorophores

Article abstract of DOI:10.1002/cplu.202000481

Bis(benzothienyl)ethene sulfones are very interesting molecules for super-resolution microscopy due to their photoswitching properties. However, functionalization of the ‘classical’ bis(benzothienyl)ethene sulfones with a five-membered central ring leads to significant decrease of quantum yields of photoconversion of the fluorescent closed form of the dye to the non-fluorescent open form that limits their application in microscopy. Here, we designed and synthesized diarylethenes with a fluorinated four-membered central ring that adds extra strain to the closed form of the dye. The reaction mechanism of their formation was studied, and byproducts formed upon structural rearrangement of the benzothiophene fragment were characterized. The photochromic properties of the new molecules were investigated by NMR and absorption spectroscopy. Some of these compounds show enhanced tendency to ring opening and have quantum yields of the ring-opening reaction in the range of 0.2–0.5.

Full text of DOI:10.1002/cplu.202000481

A Multidisciplinary Journal Centering on Chemistry
Accepted Article
Title: Di(benzothienyl)cyclobutenes: Toward Strained Photoswitchable
Fluorophores
Authors: Dmytro Sysoiev, Eliška Procházková, Aleksander
Semenenko, Radek Pohl, Svitlana Shishkina, Blanka
Klepetářová, Volodymyr Shvadchak, and Dmytro A
Yushchenko
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To be cited as: ChemPlusChem 10.1002/cplu.202000481
Link to VoR: https://doi.org/10.1002/cplu.202000481
01/2020

10.1002/cplu.202000481
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Di(benzothienyl)cyclobutenes: Toward Strained Photoswitchable
Fluorophores
Dmytro Sysoiev,*[a] Eliška Procházková,[a] Aleksander Semenenko,[b] Radek Pohl,[a] Svitlana
Shishkina,[b] Blanka Klepetářová,[a] Volodymyr Shvadchak,[a] and Dmytro A. Yushchenko*[a], [c]
[a]
Dr. D. Sysoiev, Dr. E. Procházková, Dr. R. Pohl, Dr. B. Klepetářová, Dr. V. Shvadchak, Dr. D. A. Yushchenko
Institute of Organic Chemistry and Biochemistry, Czech Academy of Sciences
Flemingovo nam. 2, 16610 Prague (Czech Republic)
E-mail: dmytro.sysoiev@uochb.cas.cz, dmytro.yushchenko@uochb.cas.cz
Dr. A. Semenenko, Prof. Dr. S. Shishkina
SSI “Institute for Single Crystals” of National Academy of Sciences of Ukraine
Nauky Ave. 60, 61072 Kharkiv (Ukraine)
Dr. D. A. Yushchenko, Miltenyi Biotec B.V. & Co. KG, Friedrich-Ebert-Straße 68, 51429 Bergisch Gladbach, Germany
E-Mail: dmytroy@miltenyi.com
[b]
[c]
Supporting information for this article is given via a link at the end of the document.
Abstract: Bis(benzothienyl)ethene sulfones are very interesting
result in a significant decrease of quantum yields (QYs) of the ring
opening photoreaction down to 10-4–10-5 (compared to 0.1–0.2 of
non-substituted DAEs).[8] This complicates switching-off the
fluorophores upon ring-opening reaction and, therefore, limits
application of DAEs in super-resolution fluorescence microscopy.
To compensate the upcoming decrease in QY arising from
substitution in 6,6’-positions, DAE sulfones with enhanced ring
strain in their closed form can offer accelerated ring-opening. This
direct modification of the switching core[9] offers an alternative to
the known approaches of ring opening efficiency control in
DAEs.[6, 7b]
molecules for super-resolution microscopy due to their photoswitching
properties.
However,
functionalization
of
the
‘classical’
bis(benzothienyl)ethene sulfones with five-membered central ring
leads to significant decrease of quantum yields of photoconversion of
the fluorescent closed form of the dye to the non-fluorescent open
form that limits their application in microscopy. Here, we designed and
synthesized diarylethenes with fluorinated four-membered central ring
that adds extra strain to the closed form of the dye. We studied the
reaction mechanism of their formation and characterized byproducts
formed upon structural rearrangement of the benzothiophene
fragment. The photochromic properties of new molecules were
investigated by means of NMR and absorption spectroscopy. We
found that some of them show enhanced tendency to ring opening
and have quantum yields of the ring-opening reaction in the range of
0.2-0.5.
Introduction
Recent rapid development of super-resolution fluorescence
microscopy techniques paved a road to better understanding of
the cell structure and created a demand for new fluorophores that
are optimized for imaging, and allow using all advantages of novel
microscopy methods.[1] Photoactivated localization microscopy
(PALM) based on controlling numbers of fluorophores in imaging
area by light illumination and reversible switchable optical linear
fluorescence transition microscopy (RESOLFT) essentially
require suitable photoswitchable fluorophores.[2]
Figure 1. Chemical structure of benzothiophene-based DAEs. Previously
published compounds with fluorinated five-membered central unit (left) and the
DAEs studied in this work with four-membered ring that display higher tension
in the closed form leading to faster ring opening (right).
Diarylethenes (DAEs) are among the main classes of dyes
applicable for super-resolution microscopy techniques. The most
useful are DAEs with benzo[b]thiophene dioxide (sulfone)
substituents (Figure 1) in which ethene double bond is a part of
polyfluorinated cyclopentene ring.[3] They can interconvert
between dark open form and fluorescent closed form under UV or
visible light illumination.[4],[5] These DAEs are thermally stable,
display high fatigue resistance and, which is the most important
aspect for imaging, exhibit a large difference between the
fluorescence intensity in the open and the closed forms.[6] The
absorption maxima of DAEs as well as the photoswitching
wavelengths can be finely tuned by substitution at 2,2’- and 6,6’-
positions (Figure 1).[7] However, the 6,6’-modifications usually
In this work, we report on new fluorescent DAEs with a structurally
constrained four-membered cyclobutenyl ring that show
enhanced ring-opening performance induced by visible light
(Figure 2). We synthetized new DAE derivatives, determined their
structure by NMR spectroscopy and XRD analysis and studied
their photochemical behavior. We monitored the fast ring-opening
reaction by advanced NMR spectroscopy[10] with in situ irradiation,
where the light is guided directly into the NMR tube via an optical
fiber.[11] This setup enabled reaction monitoring in real time and
estimation of the ring-opening QYs which were consequently
compared to the values determined by optical methods. The best
candidate was used for visualization of cell membranes.
1
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Interestingly, we detected an ambivalent reactivity of
benzothiophene anion in DAE synthesis and we proposed a
mechanism of the alternative pathway, which has not been
described yet.
Photochromic diarylcyclobutenes 1-Me and 1-Et were prepared
using reaction of hexafluorocyclobutene with 3-bromo-2-
alkylbenzothiophenes (4) (Scheme 1), as reported for
cyclopentene analogues.[8] Diarylcyclobutene derivatives 1-Me
and 1-Et were isolated as white solids, however the reaction
yields were lower (40–50%) than in case of the known DAE
analogues C5-1-Me and C5-1-Et (>70%).
The standard procedure with boiling acetic acid/hydrogen
peroxide solution might cause thermal cyclobutene ring-opening
reaction. Therefore, while preparing the fluorescent sulfones 2-
Me and 2-Et, we kept the oxidation under mild conditions (m-
chloroperbenzoic acid, (mCPBA), at room temperature) despite
required extra purification step. However, the mild conditions with
stoichiometric amount of the oxidant led to the mono-oxidated
byproduct, 2-mono-Me (Section S-IV). Similar behavior was
observed by Jeong et al where they studied stepwise oxidation of
(benzo)thiophene-based DAEs to sulfones in detail.[17] The bi-
oxidized desired final products 2-Me and 2-Et were obtained
using 2.5 eq of mCPBA and isolated as pale-yellowish solids
(details in the SI).
Figure 2. Photoconversion of the DAEs designed and prepared in this study.
Thermodynamically more stable open form (left) converts into the closed form
(right) upon UV light illumination with photostationary state (PSS) strongly
depending on the X and R substituents and the wavelength used. Central ring
contraction to cyclobutene adds extra strain to the DAE closed form.
Interestingly, during purification of the target compounds 1-Me
and 1-Et, we collected a mixture of byproducts with similar polarity
and the same molecular weight as the desired product 1 (e.g., m/z
Results and Discussion
=
446.0790 and 446.0782 for 1-Et and byproduct 8-Et,
Design and synthesis of cyclobutene derivatives
respectively). A wide range of NMR experiments were used for
structure elucidation of all the obtained products (Section IV in the
SI). The NMR spectra of 8-Et showed a single 19F signal and one
set of 1H signals of 1,2-disubstituted benzene ring suggesting
symmetrical structure. H,C-HMBC experiment revealed long-
range interactions of protons from both ethyl and phenyl moieties
(1.27, 2.48 and 7.42 ppm) to alkyne 13C resonances (77.53 and
99.09 ppm). On the other hand, 1H NMR spectrum of 9-Me
isolated during preparation of 1-Me provided two sets of signals
corresponding to 1,2-disubstituted benzene ring protons with non-
The aim of this work was to prepare new DAE derivatives with
less stable closed form in order to speed-up the ring-opening
reaction. Benzothiophene-based DAEs with perfluorinated
cycloalkene ring are stable towards thermal- and
photodegradation while in the closed form.[12] They react readily
with various electrophilic reagents,[13] thus allowing facile
functionalization of both benzene rings towards push-pull and
near-infrared switchable fluorophores. Only a few examples of
readily accessible photochromic[14] fluorinated diarylcyclobutenes
have been reported so far.[12, 15]
symmetrically substituted tetrafluorocyclobutene pattern. The 13
C
We designed derivatives with cyclobutene ring destabilizing the
closed form by contraction of the DAE central ring. We prepared
two DAE derivatives differing in R substituent, 1-Me[12] and 1-Et
(bearing methyl and ethyl group in positions 2,2’, respectively).
Methyl and ethyl groups at the reactive carbon atoms were
introduced to prevent dehydrogenation of cyclohexadiene ring
after photocyclization. Consequently, we oxidized them to the
corresponding sulfones 2-Me and 2-Et (Figure 2). These sulfones
are fluorescent in the closed form upon excitation with visible light
and can be used for super-resolution imaging, whereas the non-
oxidized derivatives 1-Me and 1-Et remain interesting for
molecular electronics.[16]
NMR spectra of 9-Me and oxidized 9-Et uncovered both
benzothienyl and alkynylphenylthio moieties in the structure, and
the signal assignment was based on F,C-HMQC and F,C-HMBC
2D experiments (Figures S-IV-25/26 in the SI). In addition, IR
spectra of both byproducts contained 2235 cm-1 band
characteristic for an alkyne group, which was clearly absent in the
IR spectra of 1-Me and 1-Et. From all collected analytical data,
we proposed the structures of bis(arylthio)cyclobutenes 8 and
1-arylthio-2-arylcyclobutenes 9[18] which was then confirmed by
XRD analysis (Figure 3).
Scheme 1. Synthesis of the studied bis(benzothienyl)cyclobutenes 1 and 2.
2
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lithiated thianaphthenes 5 with hexafluorocyclobutene proceeds
via two competitive pathways as demonstrated in Scheme 2: a)
sulphur atom interaction[19] with electrophilic cyclobutene double
bond giving the intermediate 6, or b) addition-elimination
sequence from carboanion yielding the intermediate 7. We have
not observed pathway a) in analogous reaction of
perfluorocyclopentene with 3-lithiated 2-ethylthianaphthene.[20]
Hexafluorocyclobutene was reported to react with various alkyl-
and aryllithium compounds and other nucleophiles[21] yielding 1,2-
disubstituted tetrafluorocyclobutenes. Importantly, such ring
opening at low temperature has not been reported by other
groups actively working with similar thianaphthene-based
diarylethenes C5-1-Me and C5-1-Et[22] yet. We believe that after
metal-halogen exchange in brominated alkylthianaphtene
at -78 °C with BuLi no rapid thiophene ring cleavage[23] occurs.
This behavior can be explained by the enhanced affinity of
hexafluorocyclobutene towards S-nucleophilic center which has
been studied by Timperley.[24] Knowledge about this side reaction
helps in the design and optimization of multistep synthesis
yielding photoswitchable sulfone-based fluorophores.
Moreover, both S-bound derivatives 8 and 9 mimic the target DAE
1 in their chemical behavior and can be easily misinterpreted and
oxidized in the next step to the hardly separable mixtures of S,S-
dioxides (e.g., isolated oxidized 9-Et), where only a certain
fraction (40–50%) corresponds to the target products 2-Me and 2-
Et. IR spectroscopy was used as a fast diagnostic method to
examine composition of the reaction mixture and to separate the
byproduct-containing fractions (characteristic 2235 cm-1 band of
alkyne triple bond) upon purification.
1-Me
9-Me
2-Et
oxidized 9-Et
Figure 3. Structural rearrangement of thianaphthene ring confirmed by XRD
analysis. ORTEP single-crystal XRD plots of DAEs 1-Me and 2-Et are shown
together with their corresponding byproducts 9-Me and oxidized 9-Et.
Based on the structure of the undesired side products, we
proposed mechanism of the alternative reaction pathway
(Scheme 2), which should be considered when working with
similar systems. Under the same conditions, anion 5 reacted
smoothly with other electrophile to give the corresponding boronic
acid ester, and no S-alkylated byproducts were detected (Section
VIII in the SI). Our experimental findings suggest that reaction of
Scheme 2. The proposed mechanism of byproduct formation. The starting anion 5 exists as a resonance hybrid leading to two competitive reaction pathways a)
and b). Only the pathway b) (in green) provides the desired final product 1.
3
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0.18
0.15
0.12
0.09
0.06
0.03
0.00
0.18
0.15
0.12
0.09
0.06
0.03
0.00
1-Me
1-Et
open
PSS
open
PSS
300
350
400
450
500
550
600
650
700
300
350
400
450
500
2-Et
550
600
650
700
5
4
3
2
1
0
0.25
0.20
0.15
0.10
0.05
0.00
0.18
0.15
0.12
0.09
0.06
0.03
0.00
2-Me
open
PSS
open
PSS
3
2
1
0
Fluorescence
Fluorescence
300
350
400
450
500
550
600
650
700
300
350
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Figure 4. Absorption spectra of 1-Me, 2-Me, 1-Et, 2-Et measured in CDCl3 at room temperature. Open forms are in black, and photostationary state (PSS) upon
310 nm light illumination (20 min) is in red. Blue curves represent emission spectra of the corresponding fluorescent closed forms of 2.
Photochromic properties
of QYs of the ring-opening reaction (C→O). At least four curves
were measured in order to collect the data for statistics, as an
example, representative decay curve is shown in Figure S-III-1 in
the SI. The five-membered structural analogue C5-1-Et was used
as a standard for QYs extraction. The QYs of 1-Me and 1-Et were
determined to be 0.57 and 0.28, respectively (Table 1).
Open forms of non-oxidized compounds 1-Me and 1-Et exhibited
a short wavelength absorption band with maximum at 326 and
316 nm, respectively (Figure 4). Upon UV irradiation, a new band
corresponding to the closed form appears at 532 and 567 nm,
respectively. As expected, the oxidation to sulfones 2-Me and
2-Et did not affect the absorption spectra of the open forms,
significantly, whereas the absorption bands of the closed forms
blue-shifted over 100 nm (Figure 4). Upon excitation at 420 nm,
both sulfones exhibited fluorescence at 500 nm (Figure 4, blue
lines).
Photochromic properties were investigated by NMR spectroscopy
with in situ irradiation (Figure 5), where H (or 19F) NMR spectra
were recorded during the light irradiation in real time, and by
optical methods. Thermodynamically more stable open form is
closed upon irradiation by UV light, which leads to
photostationary state (PSS), a mixture of the open and closed
forms. The open/closed form ratio at PSS depends on irradiation
wavelength and on the QYs ratio of ring closure and ring-opening
reactions (O→C and C→O, respectively). The structure of the
closed forms and PSS composition was unambiguously
determined by NMR spectroscopy as the NMR signals of both
forms are very well separated and, therefore, easily integrated
without mutual overlaps (Section S-IV in the SI).
In the case of non-oxidized 1-Me and 1-Et DAEs, irradiation at
310 nm provided up to 35% of the closed form, whereas the 365
nm irradiation led to approximately 5% of the closed form in PSS
(Figures S-III-3 to S-III-6 in the SI). The significant decrease of the
accumulated closed form can be explained by stretching of its
absorption band over the available irradiation wavelength (Figure
4). The closed forms of 1-Me and 1-Et (1-Me-closed and 1-Et-
closed, respectively) were quantitatively converted back to the
open form by irradiation with visible light (470/505 nm). NMR
spectroscopy with continuous irradiation showed very high rate of
ring-opening process in the tens-of-seconds time scale (Figure S-
III-2 in the SI). The kinetic curves were then used for estimation
On the other hand, the oxidized DAE derivatives 2-Me and 2-Et
provided significantly higher concentration of the closed form
upon 254 nm irradiation (40 and 15%, respectively) than upon
irradiation at higher wavelengths (>300 nm). The ring-opening
photoreaction induced by visible light (405 nm) was nicely
monitored and after integrating methyl signals, kinetic curves
were extracted as shown in Figure 5. The five-membered
structural analogue C5-2-Et was used as a standard for
estimating the ring-opening QYs of 2-Me-closed and 2-Et-closed
to be 0.42 and 0.17, respectively and were significantly lower than
that determined for the non-oxidized analogues 1-Me and 1-Et
(0.57 and 0.28, respectively). This nicely fits with the published
QY values of the standard derivatives C5-2-Et and C5-1-Et (0.22
and 0.35, respectively). Interestingly, the methyl derivatives 1-Me
and 2-Me displayed higher QYs compared to their ethylated
counterparts, which nominated them for further modification and
application.
For comparison, the QYs were also determined by an optical
method. Photoswitching of compounds 1-Me and 1-Et was
studied via the decrease of absorbance of the closed form, which
provided quantum yields 0.54 and 0.48, respectively, in a good
agreement with the values extracted from NMR (0.57 and 0.28,
as shown in Table 1). As sulfones 2-Me and 2-Et are fluorescent
in the closed form, their photoswitching was also studied by the
decay of emission intensity (Fig. S-II-1, Table S-II-1). The
obtained values slightly differ from the QYs determined by NMR
spectroscopy (Table 1) but still show about twofold faster ring-
opening of new compounds compared to the reference.
1
a
4
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367 s irrad.
77 s irrad.
12% closed form
1% closed form
264 s irrad.
56 s irrad.
19% closed form
3% closed form
171 s irrad.
35 s irrad.
26% closed form
6% closed form
80 s irrad.
35% closed form
7 s irrad.
11% closed form
405 nm
405 nm
Prior to irrad.
Prior to irrad.
40% closed form
12% closed form
8
7
6
5
4
3
2
1
0
8
7
6
5
4
3
2
1
0
Chemical shift(ppm)
Chemical shift (ppm)
Chemical shift (ppm)
Time (s)
Figure 5. Ring-opening reaction of 2-Me and 2-Et monitored by 1H NMR spectroscopy upon continuous irradiation by visible light (405 nm) in CDCl3 at room
temperature. Kinetic curves were constructed from integrals of the signals corresponding to methyl groups (highlighted in yellow frames).
Determination of QYs by optical methods was complicated by the
fact that PSS composition could not be estimated directly from
UV-Vis spectra. Therefore, QY extraction includes errors of the
extinction coefficients of investigated DAEs and of the reference.
On the other hand, the deviation can be caused by integration
error of the NMR signals. As the ring-opening reaction is very fast,
low-scans NMR spectra with very low signal-to-noise ratio could
be recorded. The low quality of the NMR spectra causes
integration error of ca. 15–25%, the lower for integrating methyl
single signal (in 2-Me) and higher when integrating triplet signal
(in 2-Et). This is in a good agreement with the fact that the
difference of the QY estimated by NMR and determined by UV/Vis
spectroscopy is lower for 2-Me and higher for 2-Et (Table 1).
Importantly, we did not observe any signs of decomposition during
the kinetic experiments of 1 and 2. Moreover, the on-off
photoswitching of DAEs 1 and 2 can be performed several times
(Figure S-II-2), which is crucial for application in microscopy and
cell imaging.
photochromic dyes with superior properties by modifying with
appropriate aryl substituents at 6 and 6’ positions which will be a
subject of our further work in future.
Table 1. Ring-closure and ring-opening quantum yields of 1-Me, 1-Et, 2-Me,
and 2-Et (determined), and of their cyclopentene analogues. [a]
DAE
ΦO C (UV)
→
ΦC O (NMR)
→
ΦC O (UV)
→
ΦC O (fl)
→
1-Me
1-Et
0.41 ± 0.04
0.56 ± 0.05
0.39[25]
0.57 ± 0.08
0.28 ± 0.08
0.35[25]
0.54 ± 0.05
0.48 ± 0.05
0.35[25]
---
---
C5-1-Et
2-Me
2-Et
---
0.22 ± 0.02
0.17 ± 0.02
0.30[b]
0.42 ± 0.06
0.17 ± 0.05
0.22[b]
0.41 ± 0.05
0.33 ± 0.05
0.22[26]
0.33 ± 0.01
0.31 ± 0.01
---
C5-2-Et
[a] Measurements performed in CDCl3 using 470 nm light for DAEs 1 and 405
nm light for sulfones 2. ΦO→C ring-closure quantum yield; ΦC→O ring-opening
quantum yield calculated from optical density (UV), fluorescence (fl), and NMR
integral intensity decay data, where C5-1-Et and C5-2-Et in CDCl3 were used
as a standard with already published QY values 0.35 and 0.22 correspondingly.
[b] In toluene.
Based on the QY results, we selected 2-Me derivative as the best
candidate for application. The fluorescent closed form (500 nm)
can be obtained in a high concentration (40%) and displayed
significantly higher QY of ring-opening reaction (0.4). Therefore,
we used 2-Me DAE for visualization of cell membranes (Figure S-
VII-1 in the SI).
Experimental Section
Methods
Conclusion
Optical spectroscopy
In this work, we designed and prepared a series of compounds
with fluorinated cyclobutene ring displaying increased rates of the
ring opening reaction upon visible light illumination. Diarylethene
sulfones 2-Me and 2-Et displayed fluorescence at 500 nm and
exhibited photoswitching behavior similar to their cyclopentene
analogues. Compound 2-Me with higher concentration of the
closed form at PSS and higher QY of ring-opening reaction was
used for cell visualization. We believe that new sulfones with
enhanced fluorescence switching-off due to the additional strain
in the switching unit can be used for the construction of
Absorption and fluorescence spectra were recorded on Duetta
spectrometer (Horiba Scientific).
For fluorescence measurements excitation and emission slits were 1 and
3
nm respectively, integration time 5 s per spectrum. Absorption
(transmission) spectra were recorded with 2 nm bandpass.
To record absorption spectra of photostationary state, the samples
containing open form were irradiated at absorption maxima (~310 nm) for
5
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few minutes. Then preliminary absorption spectra of mixture of closed and
open forms were recorded to select the optimal wavelength for the
photoswitching (see Table S-IV-1). After that the samples were irradiated
at the selected wavelength until reaching PSS (no further change of the
absorption spectra detected).
Mass spectrometry
High resolution mass spectrometry was performed on LTQ Orbitrap XL
(APCI, ESI) and Waters GCT Premier (EI) instruments. GC/MS was
performed on an Agilent GC/MS 7890A/5975C instrument (EI, 70 eV).
All spectra were corrected for Raman scattering. Measurements were
performed at room temperature in 1 cm path length quartz cuvettes.
Samples of photochromic compounds pre-dissolved in chloroform to 1 mM
concentration were used as stock solutions for measurements at
concentration ~ 1 μM.
Determination of photoreaction quantum yield using optical methods
Solutions of compounds 1-Me, 1-Et, C5-1-Et, 2-Me, 2-Et, and C5-2-Et in
CDCl3 with OD≈0.05 were irradiated at 310 nm to reach PSS. The sulfone
samples were irradiated at 405 nm (±2 nm). The decrease of closed form
fraction upon photoreaction was monitored each 1 s using fluorescence
intensity at 550 nm and the absorbance at 405 nm. The non-oxidized DAE
samples were irradiated at 470 nm (±2 nm). The decrease of closed form
fraction upon photoreaction was monitored each 1 s using absorbance at
470 nm. The curves were fitted to equation: C=C0exp(-k ΦC→O), where k is
a constant depending on the lamp intensity stable for all measurements. It
was calculated based on the known ΦC→O of the standard samples (0.35
for C5-1-Et[25] and 0.22 for C5-2-Et[26]). The experiment was performed on
Fluoromax-4 and Duetta spectrometers (Horiba Scientific), using 4 mm
light-pass quartz cuvettes with stirring. Photocyclization QYs were
determined in the same setup upon irradiation of samples at 313 nm (±2
nm). The increase of closed form fraction upon photoreaction was
monitored using absorbance at 425 nm for sulfones and at 525 nm for the
non-oxidized DAEs. QYs were calculated based on the known ΦO→C of the
standard samples (0.39 for C5-1-Et[25] and 0.30 for C5-2-Et[26]).
Fluorescence quantum yields were calculated using solution of C5-2-Et in
dioxane as a reference with published QY=0.22.[8a]
IR spectra were recorded on a Perkin-Elmer 100 Series FT-IR and Bruker
Alpha-P spectrometers using neat compounds.
NMR spectroscopy
NMR spectra were recorded on a Bruker Avance III HD spectrometer with
a broad-band Prodigy cryo probe with ATM module (5 mm CPBBO BB-
1H/19F/D Z-GRD) operating at 400 MHz for 1H and 100 MHz for 13C.
Double-decoupled 13C and 2D 13C-19F heteronuclear experiments were
performed on a JEOL JNM-ECZ 500R instrument (500 MHz for 1H, 126
MHz for 13C, 471 MHz for 19F). 1H NMR chemical shifts were referenced to
residual solvent signal (CDCl3: δ = 7.26 ppm). For 13C NMR following
reference value was used: CDCl3: δ = 77.16 ppm. The signals were
assigned based on 2D-NMR experiments (COSY, HSQC, HMBC and
NOESY). All the NMR spectra obtained upon light irradiation were
recorded on a Bruker Avance III spectrometer with a broad-band cryo
probe with ATM module (5 mm CPBBO BB-1H/19F/15N/D Z-GRD) operating
Determination of photoreaction quantum yield using NMR
spectroscopy with in situ irradiation
For the NMR experiments with in situ irradiation, we used light emitting
diodes (LEDs) at 365, 405, 470 and 505 nm from Thorlabs, Germany. The
light was guided directly into the NMR tube via a coaxial insert with a
multimode silica optical fiber with 1 mm diameter, 0.39 NA, high amount of
OH from Thorlabs, Germany. The kinetic curves needed for QYs extraction
were constructed from at least 8 points obtained by signals integration.
The relative concentration was then re-calculated to the real concentration
in order to extract the reaction rate of the initial slope. C5-2-Et was used
as an actinometer for sulfones 2-Me and 2-Et with the ring-opening QY =
0.22.[26] C5-1-Et with QY = 0.35 was used as a reference for estimation of
ring-opening QYs of non-oxidized derivatives 1-Me and 1-Et.
1
at 500 MHz for H, 125.7 MHz for 13C and 471 MHz for 19F measured in
CDCl3 at room temperature. Atom numbering for NMR signals assignment
is given in SI (Table S-IV-1).
X-Ray diffraction
X-Ray diffraction studies for compounds 1-Me, 1-Et, and oxidized 9-Et
were performed on an automatic «Xcalibur 3» diffractometer (graphite
monochromated MoKα radiation, CCD-detector, -scanning). The
structures were solved by direct method using SHELXTL package.[27]
Positions of the hydrogen atoms were located from electron density
difference maps and refined by “riding” model with Uiso = nUeq of the carrier
atom (n = 1.5 for methyl and hydroxyl groups and n = 1.2 for other
hydrogen atoms). Bond lengths of the disordered fragments were refined
using standard values (Csp3–Csp3 bond length is 1.54 Å, Csp3–F bond
length is 1.35 Å). The crystallographic data and experimental parameters
are listed in Table S-I-2.
Synthesis
All operations with air- and moisture-sensitive compounds were performed
under nitrogen atmosphere using standard Schlenk techniques. Reagents,
metal catalysts and solvents obtained from commercial sources were used
as received. Hexafluorocyclobutene was purchased from ABCR and
dissolved in dry THF (5
g in 20 ml) prior to use. 3-Bromo-2-
methylbenzo[b]thiophene 4-Me and 3-bromo-2-ethylbenzo[b]thiophene
4-Et were prepared according to the reported procedure.[31] Closed forms
of DAEs were studied in PSS mixtures and were not isolated individually.
Deposition Numbers 2001508 (for 1-Me), 2001509 (for 1-Et), 1999926 (for
2-Et), 1999927 (for 9-Me) and 1993163 (for oxidized 9-Et) contain the
supplementary crystallographic data for this paper. These data are
provided free of charge by the joint Cambridge Crystallographic Data
Centre and Fachinformationszentrum Karlsruhe Access Structures service
www.ccdc.cam.ac.uk/structures.
Chromatography was performed on MN Kieselgel 60 M (silica gel,
Macherey-Nagel, Germany).
3,3'-(Perfluorocyclobut-1-ene-1,2-diyl)bis(2-methylbenzo[b]thiophene)
Single-crystal diffraction data of 2-Et and 9-Me were collected on Xcalibur
PX diffractometer with monochromated CuKα radiation (λ=1.54180 Å) at
200K. CrysAlisProCCD was used for data collection, cell refinement and
data reduction. The structures were solved by direct methods with
SIR92[28] (2-Et) and by charge flipping with SUPERFLIP[29] (9-Me) and
were refined by full-matrix least-squares on F with CRYSTALS.[30] The
positional and anisotropic thermal parameters of all non-hydrogen atoms
were refined. All hydrogen atoms were found from a Fourier difference
map, then recalculated into idealized positions, and refined with riding
constraints.
(1-Me[12]
)
3-Bromo-2-methylbenzo[b]thiophene 4-Me (5.0 g, 22 mmol) was dissolved
in dry THF (150 ml) at –78 °C under nitrogen atmosphere. nBuLi (8.8 ml,
22 mmol, 2.5M) was added dropwise, and the mixture was stirred for
further 40 min. Hexafluorocyclopentene solution in THF (containing 1.78 g,
11 mmol of hexafluorocyclobutene) was added via pre-cooled syringe, and
the reaction mixture was allowed to warm up to room temperature
overnight. After quenching with water, THF was evaporated, and the
residue was extracted with diethyl ether (3x150 ml). Organic phase was
6
This article is protected by copyright. All rights reserved.

10.1002/cplu.202000481
ChemPlusChem
FULL PAPER
separated, dried over MgSO4, and the solvent was evaporated. Oily
residue was purified by column chromatography (silica gel,
hexanes/EtOAc 9:1) yielding DAE 1-Me as a light-yellowish solid (2.0 g,
43%) and byproduct 9-Me as a yellow oil (1.23 g, 26%) together with 2-
methylbenzo[b]thiophene (0.3 g, 9%). Single crystals of 1-Me were grown
from its saturated dichloromethane solution upon addition of threefold
excess of hexane. 1H NMR spectrum (400 MHz, CDCl3, 25 °C): δ = 2.25
(s, 6H; CH3 (C1)), 7.31 (m, 4H; CHAr), 7.74 ppm (m, 4H; CHAr). 13C NMR
spectrum (100 MHz, CDCl3, 25 °C): δ = 15.69 (C1), 119.28 (m, C11),
121.72 (C3), 122.17 (C8), 122.42 (m, C5), 124.78 (C7), 125.36 (C6),
137.96 (C4), 137.99 (C9), 141.94 (m, C10), 145.39 ppm (C2). 19F NMR
(376 MHz, CDCl3, 25 °C): –106.89 ppm (s, 4F; CF2). IR: υ = 1526, 1431,
1305 cm-1. MS (EI, 70 eV): 418 [M]. HRMS (EI): found 418.04670,
calculated for C22H14F4S2 418.04676 [M].
3,3'-(Perfluorocyclobut-1-ene-1,2-diyl)bis(2-methylbenzo[b]thiophene 1,1-
dioxide) (2-Me)
DAE 1-Me (2.0 g, 4.8 mmol) was dissolved in dichloromethane (100 ml) at
0 °C. mCPBA (4.9 g, 29 mmol) was added in one portion, and the mixture
was stirred overnight at room temperature. Reaction mixture was
neutralized with aqueous potassium carbonate solution, organic phase
was separated, dried over MgSO4, and the solvent was evaporated. After
purification
by
column
chromatography
(silica
gel,
hexane/dichloromethane 1:1) 2-Me was obtained as a colorless solid (1.9
g, 82%). 1H NMR spectrum (400 MHz, CDCl3, 25 °C): δ = 2.14 (s, 6H; CH3
(C1)), 7.40 (m, 2H; CH (C5)), 7.53–7.61 (m, 4H; CH (C6 and C7)), 7.82
ppm (m, 2H; CH (C8)). 13C NMR spectrum (100 MHz, CDCl3, 25 °C): δ =
9.09 (C1), 118.00 (m, C11), 122.83 (C8), 123.09 (t, J5-F = 1.7 Hz, C5),
124.99 (C3), 128.95 (C4), 130.93 (C7), 134.58 (C6), 135.29 (C9), 144.63
(m, C10), 145.00 ppm (C2). 19F NMR (376 MHz, CDCl3, 25 °C): -107.27
ppm (s, 4F; CF2). IR: υ = 1453, 1436, 1303 cm-1. HRMS (ESI): found
505.01568, calculated for C24H18F4S2 505.01618 [M+Na].
1-Me-closed. 1H NMR (500 MHz, CDCl3, 25 °C): δ = 2.00 (s, 6H; CH3 (C1)),
7.14–7.18 (m, 2H; CHAr), 7.29–7.31 (m, 4H; 2 x CHAr), 7.46–7.50 ppm (m,
2H; CHAr). 13C NMR (125 MHz, CDCl3, 25 °C): δ = 25.04 (C1), 66.63 (C2),
123.94 (CHAr), 125.84 (CHAr), 126.37 (CHAr), 132.07 (CHAr), 143.4 ppm
(C3, from HMBC), C10, C11 not detected. 19F NMR (471 MHz, CDCl3,
25 °C): -108.80 – -110.53 (m, 2F; CF2), -111.65 – -113.38 ppm (m, 2F;
CF2).
2-Me-closed. 1H NMR (500 MHz, CDCl3, 25 °C): δ = 1.84 (s, 6H; CH3 (C1)),
7.74–7.81 (m, 2H; H7), 7.80–7.85 (m, 4H; H5, H6), 7.96–7.98 ppm (m, 2H;
H8). 13C NMR (125 MHz, CDCl3, 25 °C): δ = 18.49 (C1), 68.02 (C2), 123.46
(C8), 126.74 (C5), 127.86 (C4), 134.09 (C7), 134.79 (C6), 136.39 (C3),
141.10 ppm (C9), C10, C11 not detected. 19F NMR (471 MHz, CDCl3,
25 °C): -108.50 – -109.05 (m, 2F; CF2) -111.21 – -111.76 ppm (m, 2F; CF2).
3,3'-(Perfluorocyclobut-1-ene-1,2-diyl)bis(2-ethylbenzo[b]thiophene)
(1-Et)
3-Bromo-2-ethylbenzo[b]thiophene 4-Et (5.0 g, 21 mmol) was dissolved in
dry THF (150 ml) at –78 °C under nitrogen atmosphere. nBuLi (8.4 ml, 21
mmol, 2.5M) was added dropwise, and the mixture was stirred for further
40 min. Hexafluorocyclopentene solution in THF (containing 1.68 g, 10
mmol of hexafluorocyclobutene) was added via pre-cooled syringe, and
the reaction mixture was allowed to warm up to room temperature
overnight. After quenching with water, THF was evaporated, and the
residue was extracted with diethyl ether (3x150 ml). Organic phase was
separated, dried over MgSO4, and the solvent was evaporated. Oily
residue was purified by column chromatography (silica gel, hexane)
yielding 1-Et as a colorless solid (2.5 g, 54%) and a mixture (1.4 g) of
isomeric 9-Et and 8-Et with 2-ethylbenzo[b]thiophene. Single crystals of
1-Et were grown from its saturated dichloromethane solution upon addition
of threefold excess of hexane. 1H NMR spectrum (400 MHz, CDCl3, 25 °C):
δ = 1.09 (t, 3J1-2 = 7.5 Hz, 6H; CH3 (C1)), 2.68 (q, 3J2-1 = 7.5 Hz, 4H; CH2
(C2)), 7.29–7.35 (m, 4H; H7, H8), 7.72–7.76 (m, 2H; H6), 7.77–7.80 ppm
(m, 2H, H9). 13C NMR spectrum (100 MHz, CDCl3, 25 °C): δ = 15.33 (C1),
23.28 (C2), 119.41 (m, C12), 120.00 (C4), 122.33 (C9), 122.41-122.49 (m,
C6), 124.66 (C8), 125.25 (C7), 137.85 (C5), 137.99 (C10), 143.33 (dt, 2J11-
F = 19.2 Hz, C11), 152.64 ppm (C3). 19F NMR (376 MHz, CDCl3, 25 °C): -
107.84 ppm (s, 4F; CF2). IR: υ = 2971, 1530, 1304 cm-1. MS (EI, 70 eV):
446 [M]. HRMS (EI): found 446.0790, calculated for C24H18F4S2 446.0786
[M].
3,3'-(Perfluorocyclobut-1-ene-1,2-diyl)bis(2-ethylbenzo[b]thiophene 1,1-
dioxide) (2-Et)
DAE 1-Et (2.3 g, 5.1 mmol) was dissolved in dichloromethane (100 ml) at
0 °C. mCPBA (5.3 g, 31 mmol) was added in one portion, and the mixture
was stirred overnight at room temperature. Reaction mixture was
neutralized with aqueous potassium carbonate solution, organic phase
was separated, dried over MgSO4, and the solvent was evaporated. After
purification
by
column
chromatography
(silica
gel,
hexane/dichloromethane 1:1) 2-Et was obtained as a colorless solid (1.7
1
g, 65%). H NMR spectrum (400 MHz, CDCl3, 25 °C): δ = 1.24 (t, 3J1-2
=
7.6 Hz, 6H; CH3 (C1)), 2.61 (q, 3J2-1 = 7.6 Hz, 4H; CH2 (C2)), 7.32 (m, 2H;
CH (C6)), 7.55–7.60 (m, 4H; CH (C7 and C8)), 7.77 ppm (m, 2H; CH (C9)).
13C NMR spectrum (100 MHz, CDCl3, 25 °C): δ = 12.46 (C1), 18.52 (C2),
118.11 (m, C12), 122.58 (C9), 122.93 (br, C6), 124.10 (C4), 128.97 (C5),
130.98 (C8), 134.37 (C7), 135.53 (C10), 145.39 (m, C11), 149.16 ppm
(C3). 19F NMR (376 MHz, CDCl3, 25 °C): -108.53 ppm (s, 4F; CF2). IR: υ
= 2981, 2933, 1457, 1298 cm-1. HRMS (EI): found 510.0584, calculated
for C24H18F4O4S2 510.0583 [M].
2-Et-closed. 1H NMR (500 MHz, CDCl3, 25 °C): δ = 0.61 (t, 3J1-2 = 7.4 Hz,
6H; CH3 (C1)), 2.46 (m, 2H; CHa (C2)), 3.05 (m, 2H; CHb (C2)), 7.71–7.76
(m, 2H; CHAr), 7.78–7.83 (m, 4H; H6 and CHAr), 7.93–7.95 ppm (m, 2H;
H9). 13C NMR (125 MHz, CDCl3, 25 °C): δ = 10.40 (C1), 23.28 (C2), 73.62
(C3), 122.77 (C9), 125.53 (C7), 129.97 (C5), 133.89 (C6), 134.83 (C8),
135.03 (C4), 143.43 ppm (C10). C11, C12 not detected. 19F NMR (471
MHz, CDCl3, 25 °C): -107.75 – -108.36 (m, 2F; CF2), -111.98 – -112.56
ppm (m, 2F; CF2).
1-Et-closed. 1H NMR (500 MHz, CDCl3, 25 °C): δ = 0.94 (t, 3J1-2 = 7.4 Hz,
6H; CH3 (C1)), 2.37 (m, 2H; CHa (C2)), 2.99 (m, 2H; CHb (C2)), 7.07–7.11
(m, 2H; CHAr), 7.16–7.23 (m, 4H; 2 x CHAr), 7.36–7.39 ppm (m, 2H, H6).
13C NMR (125 MHz, CDCl3, 25 °C): δ = 9.83 (C1), 30.55 (C2), 71.4 (C3,
from HMBC), 123.02 (C8), 125.13 (CHAr), 125.68 (CHAr), 131.91 (C8),
142.4 (C4, from HMBC), 148.0 ppm (C10, from HMBC), C11, C12 not
detected. 19F NMR (471 MHz, CDCl3, 25 °C): -108.94 – -109.62 (m, 2F,
CF2), -113.33 – -113.88 ppm (m, 2F; CF2).
C5-2-Et-closed. 1H NMR (500 MHz, CDCl3, 25 °C): δ = 0.61 (t, 3J1-2 = 7.4
Hz, 6H; H1), 2.31 (m, 2H; CHa (C2)), 3.11 (m, 2H; CHb (C2)), 7.76–7.80
(m, 2H; H8), 7.81–7.85 (m, 2H; H7), 7.99–8.01 (m, 2H; H9)), 8.22–8.24
ppm (m, 2H; H6). 13C NMR (125 MHz, CDCl3, 25 °C): δ = 10.30 (C1), 22.58
(C2), 71.92 (C3), 123.03 (C9), 127.21 (C6)), 129.51 (C5), 134.26 (C8),
134.80 (C7), 140.05 (C11), 141.96 (C4), 144.37 ppm (C10), C12 and C13
not detected. 19F NMR (471 MHz, CDCl3, 25 °C): -109.18 – -110.81 (m,
4F; CF2), -131.28 – -133.06 ppm (m, 2F; CF2).
C5-1-Et-closed. 1H NMR (400 MHz, CDCl3, 25 °C): δ = 0.93 (t, 3J1-2 = 7.4
Hz, 6H; CH3 (C1)), 2.39 (m, 2H; CHa (C2)), 2.93 (m, 2H; CHb (C2)), 7.08–
7.12 (m, 2H; H7), 7.18–7.26 (m, 4H; H8, H9), 7.80–7.84 ppm (m, 2H, H6).
13C NMR (100 MHz, CDCl3, 25 °C): δ = 9.48 (C1), 29.85 (C2), 70.17 (C3),
123.05 (C8), 125.57 (C7), 127.07 (C6), 132.35 (C9), 148.6 ppm (C4, from
HMBC), C5, C10-C13 were not detected. 19F NMR (376 MHz, CDCl3,
25 °C): -109.01 – -109.73 (m, 2F, CF2), -115.43 – -116.15 (m, 2F, CF2), -
132.44 – -132.51 ppm (m, 2F; CF2).
2-Methyl-3-(3,3,4,4-tetrafluoro-2-(2-methylbenzo[b]thiophen-3-
yl)cyclobut-1-en-1-yl)benzo[b]thiophene 1,1-dioxide (2mono-Me)
7
This article is protected by copyright. All rights reserved.

10.1002/cplu.202000481
ChemPlusChem
FULL PAPER
Isolated as byproduct from the oxidation reaction of DAE 1-Me with
stoichiometric amount of mCPBA. 1H NMR spectrum (400 MHz, CDCl3,
25 °C): δ = 1.93 (s, 3H; CH3 (C1)), 2.43 (s, 3H; CH3 (C10)), 7.34–7.42 (m,
2H; CH (C15 and C16)), 7.43 (m, 1H; CH (C5)), 7.49–7.55 (m, 2H; CH (C6
and C7)), 7.75–7.82 ppm (m, 3H; CH (C8, C14 and C17)). 13C NMR
spectrum (100 MHz, CDCl3, 25 °C): δ = 8.74 (C1), 16.17 (C10), 120.61
(C12), 122.10 (t, JCF = 4.0 Hz, C14), 122.29 (C17), 122.43 (C8), 123.57 (t,
JCF = 3.8 Hz, C5), 125.41 (C16), 125.93 (C15), 126.88 (C3), 129.85 (C4),
130.37 (C7), 134.35 (C6), 135.57 (C9), 137.30 (C13), 137.87 (C18),
142.96 (C2), 148.13 ppm (C11). 19F NMR (376 MHz, CDCl3, 25 °C): -
106.46 (m, 2F; CF2), -107.24 ppm (m, 2F; CF2).
(m, C11). 19F NMR (376 MHz, CDCl3, 25 °C): -111.52 ppm (m, 4F; CF2).
IR: υ = 2977, 2235, 1467, 1303, 1236 cm-1. MS (EI, 70 eV): 446 [M]. HRMS
(EI): found 446.0782, calculated for C24H18F4S2 446.0786 [M].
Acknowledgements
The work was supported by Czech Science Foundation (GAČR)
grant 19-21318S and IOCB postdoctoral fellowship to DS. We
thank Maksym Galkin for help with microscopy experiments and
Mass Spectrometry Department at IOCB for HR-MS spectra.
Valuable comments on the QY determination procedure from Dr.
Tomaš Slanina are greatly acknowledged.
2-Methyl-3-(3,3,4,4-tetrafluoro-2-((2-(prop-1-yn-1-yl)phenyl)thio)cyclobut-
1-en-1-yl)benzo[b]thiophene (9-Me)
Isolated as isomeric byproduct from synthesis of DAE 1-Me. 1H NMR
spectrum (400 MHz, CDCl3, 25 °C): δ = 2.12 (s, 3H; CH3 (C10)), 2.49 (s,
3H; CH3 (C1)), 6.75 (td, 3J16-17 = 3J16-15 7.6, 4J16-14 = 1.2 Hz, 1H; CH (C16)),
6.98 (td, 3J15-16 = 3J15-14 =7.6, 4J15-17 = 1.2 Hz, 1H; CH (C15)), 7.10 (dd, 3J14-
15 = 7.6, 4J14-16 = 1.2 Hz, 1H; CH (C14)), 7.23 (dd, 3J17-16 = 7.6, 4J17-15 = 1.2
Hz, 1H; CH (C17)), 7.26 (td, 3J7-6 = 3J7-8 = 7.9, 4J7-5 = 1.0 Hz, 1H; CH (C7)),
7.34 (td, 3J6-7 = 3J6-5 = 7.9, 4J6-8 = 1.1 Hz, 1H; CH (C6)), 7.59 (d, 3J8-7 = 7.9
Keywords: cyclobutenes • diarylethene sulfones •
dyes/pigments • photochromic reactions • photoswitches
[1]
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3
Hz, 1H; CH (C8)), 7.63 ppm (d, J5-6 = 7.9 Hz, 1H; CH (C5)). 13C NMR
spectrum (100 MHz, CDCl3, 25 °C): δ = 4.62 (C10), 15.26 (C1), 77.62
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[5]
[6]
[7]
Y.-C. Jeong, S. I. Yang, K.-H. Ahn, E. Kim, Chem.
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(C12), 93.71 (C11), 119.51 (C3), 119.78 (C20 and C21), 121.75 (C8),
5
122.61 (t, C5, JCF = 1.8 Hz), 124.26 (C7), 124.59 (C6), 126.49 (C18),
127.24 (C16), 128.53 (C13), 129.57 (C15), 132.50 (C14), 134.48 (C17),
138.02 (C9), 138.43 (C4), 142.54 ppm (C2). 19F NMR (376 MHz, CDCl3,
25 °C): -107.84 – -108.07 (m, 2F; C20-F2), -112.53 – -112.75 ppm (m, 2F;
C21-F2). HRMS (APCI): found 418.0466, calculated for C22H14F4S2
418.0467 [M].
3-(2-((2-(But-1-yn-1-yl)phenyl)sulfonyl)-3,3,4,4-tetrafluorocyclobut-1-en-
1-yl)-2-ethylbenzo[b]thiophene 1,1-dioxide (oxidized 9-Et)
Isolated in form of beige solid after oxidation of the crude mixture of
1-Et/8-Et/9-Et with excess amounts of mCPBA. Single crystals of oxidized
9-Et were grown from its saturated dichloromethane solution. 1H NMR
[8]
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a) K. Uno, H. Niikura, M. Morimoto, Y. Ishibashi, H.
Miyasaka, M. Irie, J. Am. Chem. Soc. 2011, 133, 13558-
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A. V. Metelitsa, V. Z. Shirinian, J. Org. Chem. 2017, 82,
1477-1486.
3
spectrum (400 MHz, CDCl3, 25 °C): δ = 1.28 (t, J11-12 = 7.5 Hz, 3H; CH3
(C11)), 1.33 (t, 3J1-2 = 7.6 Hz, 3H; CH3 (C1)), 2.50 (q, 3J12-11 = 7.5 Hz, 2H;
CH2 (C12)), 2.59 (q, 3J2-1 = 7.6 Hz, 2H; CH2 (C2)), 7.15 (d, 3J6-7 = 7.4 Hz,
1H; CH (C6)), 7.17 (ddd, 3J18-19 = 8.1, 3J18-17 = 7.6, 4J18-16 = 1.3 Hz, 1H; CH
(C18)), 7.28 (dd, 3J16-17 = 7.8, 4J16-18 = 1.3 Hz, 1H; CH (C16)), 7.35 (ddd,
[10]
[11]
F. G. Erko, J. Berthet, H. Ogawa, Y. Yokoyama, S.
Delbaere, Tetrahedron Lett. 2013, 54, 6366-6369.
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3J17-16 = 7.8, 3J17-18= 7.6, 4J17-19 = 1.3 Hz, 1H; CH (C17)), 7.48 (ddd, 3J8-7
7.5, 3J8-9 = 7.4, 4J8-6 = 1.2 Hz, 1H; CH (C8)), 7.54 (ddd, 3J7-8 = 7.5, 3J7-6
7.4, 4J7-9 = 1.4 Hz, 1H; CH (C7)), 7.59 (ddd, 3J9-8 = 7.4, 4J9-7 = 1.4, 5J9-6
=
=
=
0.4 Hz, 1H; CH (C9)), 7.71 ppm (dd, 3J19-18 = 8.1, 4J19-17 = 1.3 Hz, 1H; CH
(C19)). 13C NMR spectrum (100 MHz, CDCl3, 25 °C): δ = 12.43 (C1), 12.98
(C11), 13.58 (C12), 18.62 (C2), 75.39 (C14), 105.44 (C13), 117.16 (m,
C23), 118.15 (m, C22), 121.81 (C9, C4), 123.32 (C6), 124.32 (C15),
128.13 (C18), 128.71 (C5), 130.05 (C19), 130.57 (C8), 133.80 (C7),
135.00 (C16), 135.19 (C17), 136.50 (C20), 145.13 (m, C21), 148.19 (C3),
151.98 ppm (m, C24). 19F NMR (376 MHz, CDCl3, 25 °C): -109.82 – -
110.06 (m, 2F; CF2), --110.39 – -110.75 ppm (m, 2F; CF2). IR: υ = 2988,
2236, 1466, 1315 cm-1. HRMS (APCI): found 511.06485, calculated for
C24H18F4O4S2 511.06554 [M+H].
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= 7.5 Hz, 6H; CH3 (C1)), 2.48 (q, J2-1 = 7.5 Hz, 4H; CH2 (C2)), 7.22 (m,
2H; CH (C8)), 7.28 (m, 2H; CH (C7)), 7.42 (m, 2H; CH (C6)), 7.44 ppm (m,
2H; CH (C9)). 13C NMR spectrum (100 MHz, CDCl3, 25 °C): δ = 13.45 (C2),
13.78 (C1), 77.53 (C4), 99.09 (C3), 118.92 (m, C12), 128.14 (C8), 128.54
(C10), 128.65 (C5), 129.47 (C7), 133.02 (C6), 134.23 (C9), 140.43 ppm
8
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Entry for the Table of Contents
Fluorescent photoswitchable dyes are essential for several super-resolution microscopy techniques. We developed dyes with
enhanced photoswitching performance based on diarylethenes containing sterically constrained perfluorocyclobutene ring. The
mechanism of the reaction of thianaphthenes with hexafluorocyclobutene leading to the target dyes was studied and formed
byproducts were characterized. The photoswitching reaction was investigated using both optical and NMR-based methods and
quantum yields of photoconversion were measured.
Institute and/or researcher Twitter usernames: @IOCBPrague
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