It's a Trap: Thiol-Michael Chemistry on a DASA Photoswitch

Article abstract of DOI:10.1002/chem.201904770

Donor–acceptor Stenhouse adducts (DASA) are popular photoswitches capable of toggling between two isomers depending on the light and temperature of the system. The cyclized polar form is accessed by visible-light irradiation, whereas the linear nonpolar f

Full text of DOI:10.1002/chem.201904770

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Accepted Article
Title: It’s a Trap: Thiol-Michael Chemistry on a DASA Photoswitch
Authors: Jessica Alves, Sandra Wiedbrauk, David Gräfe, Sarah L.
Walden, James P. Blinco, and Christopher Barner-Kowollik
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To be cited as: Chem. Eur. J. 10.1002/chem.201904770
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It’s a Trap: Thiol-Michael Chemistry on a DASA Photoswitch
Jessica Alves, §[a] Sandra Wiedbrauk, §[a] David Gräfe,[a] Sarah L. Walden,[a] James P. Blinco,*[a] and
Christopher Barner-Kowollik*[a]
Abstract: Donor-Acceptor Stenhouse Adducts (DASA) are popular
photoswitches capable of toggling between two isomers depending
on the light and temperature of the system. The cyclized polar form is
accessed via visible light irradiation, while the linear nonpolar form is
recovered in the dark. Upon the formation of the cyclized form, the
DASA contains a double bond featuring a -carbon prone to
nucleophilic attack. Here, we present an isomer selective thiol-
Michael reaction between the cyclized DASA and a base-activated
thiol. The thiol-Michael addition was carried out with an alkyl (1-
butanethiol) and an aromatic thiol (p-bromothiophenol) as reaction
partners, both in the presence of base. Under optimized conditions,
the reaction proceeds preferentially in the presence of light and base.
The current study demonstrates that DASAs can be selectively
trapped in their cyclized state.
by a bond rotation, and finally a thermal conrotatory 4
electrocyclization leading to the cyclized ring, commonly
stabilized by its zwitterionic form through a proton transfer.[10–
13] Clarification of the cyclization mechanism resulted in a pool
of DASA photoswitches bearing a variety of acceptor and
donor functionalities as well as an improved knowledge of
factors influencing the switching process.[8,9,14,15]
Molecular photoswitches undergo reversible isomerization
upon irradiation with light, and sometimes in combination with
a different stimulus, e.g. temperature.[1] Light, as a readily
available and adjustable reaction trigger, is particularly
interesting for implementing spatiotemporal control on
reaction outcomes. For example, photoswitches are
employed for biomedical and biochemical processes including
targeted drug delivery and chemical/biological sensing
applications.[2–5] Photoswitches, such as derivatives of the
Donor-Acceptor Stenhouse Adduct (DASA), coexist in an
equilibrium between two isomeric forms. One isomer is
constituted of an intensely colored (blue, purple or orange)
nonpolar linear form, while the other isomer is cyclized, polar,
colorless and commonly zwitterionic (Scheme 1).[6–9] The
equilibrium between the two forms is determined by the
solvent environment and the structural motifs of the DASA
derivatives, consisting of an electron donor as well as an
acceptor moiety, resulting in a push-pull system. Upon
irradiation, the equilibrium is shifted towards the ring cyclized
isomer (cyclized form) and — upon thermal relaxation —
returns to the initial conjugated state (linear form). The
cyclization mechanism has been clarified in a careful study by
Feringa and Buma where density functional theory
calculations and transient absorption kinetics measurements
were employed.[32] The current consensus on the cyclization
process involves isomerization of the double bonds, followed
Scheme 1. The photoswitching of DASA between the linear and cyclized form,
depicted for a general structural motif. The acceptor group may be altered by
substituting X and Y, while the donor properties are adjustable via the amino
functionality R.
The inherent advantages of DASA photoswitches, which
includes negative photochromism, inexpensive starting
materials, modular synthesis, tunable absorption spectrum,
visible light switches, and good fatigue resistance, are
appealing for a range of applications. To date, polarity and
color are the main adjustable properties of DASAs explored
for applications including micelles formation targeting
biomedical applications,[4,16] pH and temperature sensors,[5,17]
and visible light responsive devices.[18–20]
The isomerization of DASAs is dependent on the
prevailing conditions, such as solvent, temperature,
concentration or pH.[21] The effect of the solvent on the
switching behavior of the DASA has been previous reported
in the literature.[7,10,21] DASAs are commonly categorized
according to the different generations. The first generation
DASAs — the focus of the present study — are stabilized in
the cyclized form with protic solvents, whereas halogenated
solvents encourage the linear form.[10,21] For first generation
DASAs, aromatic solvents are known for allowing reversible
switching upon irradiation, followed by a fast, thermally
induced reversion to the linear form in the absence of
light.[10,21] Second generation DASAs have been reported in
the literature, however, the equilibrium in the dark consists of
a mixture between linear and cyclized form without allowing
the control of equilibrium ratio with light. Therefore, the
present work focuses exclusively on a first generation DASA.
Precisely, we used D1 in this study.
[a]
J. Alves, Dr. S. Wiedbrauk, Dr. D. Gräfe, Dr. S. L. Walden, Dr. J. P.
Blinco, and Prof. Dr. C. Barner-Kowollik
School of Chemistry, Physics and Mechanical Engineering
Queensland University of Technology
2 George Street, Brisbane, QLD 4000 Australia
E-mail: christopher.barnerkowollik@qut.edu.au; j.blinco@qut.edu.au
To the best of our knowledge, ligation reactions of DASA
photoswitches have not been reported so far and the reactivity
as well as selectivity of this molecule class remain unknown.
Selectively gated reactions are uncommon, with only a few
successful systems known.[22–24] The ability of molecules to
§
These authors contributed equally to the work
Supporting information for this article is given via a link at the end of
the document.
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react selectively in only one of its isomeric states is promising
for the development of advanced materials, e.g. for the use as
inks in direct laser writing.[22] Upon irradiation, the conjugated
linear DASA forms a substituted cyclopentenone. The
formation of an ,-unsaturated carbonyl makes the -carbon
prone to nucleophilic attack in a (1,4) addition such as the
thiol-Michael reaction. Since its first report in 1964 by Allen et
al.[25] the thiol-Michael addition, being classified as a ‘click-
reaction’, has been extensively used in organic and polymer
synthesis.[26–29] The reaction commonly does not form any
side-product, features a high selectivity and proceeds to high
yields.[26–28] As the DASA isomerization and the thiol-Michael
addition requires specific conditions (e.g. base and light), a
screening of the reaction conditions is required to assess the
potential of a respective ligation reaction. Herein, we show
that the DASA photoswitches are a suitable class of
compounds for a selective reaction with a thiol via a thiol-
Michael addition of the DASA in its cyclized form (Scheme 2).
The synthesis of the DASA photoswitch employed for the
current study was adapted from the literature.[7,30] The
photoswitch was obtained in quantitative yields as a red solid.
The reaction of D1 with the model thiols 1-butanethiol (T1) or
p-bromothiophenol (T2) in the presence of a base, either
pyridine or triethylamine (Et3N), leads to the formation of A1
and A2, respectively, as depicted in Scheme 3.
The photoinduced ligation reactions were performed in
different solvents such as toluene, DCM, or methanol. In
methanol the cyclization undergoes independently of light, in
DCM reversible switching behavior is possible.[7,10,21] Toluene
was selected as the solvent as it is well known to enable
reversibility of the switching process of DASA compounds
upon irradiation.[7] Further, the dark equilibrium of D1 in
toluene and DCM lies almost completely on the linear form
(100% and 96%, respectively). The ligations were performed
in toluene with 1.3 eq. of thiol under constant irradiation with
green LEDs (max = 525 nm, 3 mW·cm-2). Structural proof for
the successful ligation was obtained after column
chromatography via nuclear magnetic resonance (NMR)
spectroscopy and is supported via liquid chromatography
coupled to electrospray ionization mass spectrometry (LC-
MS).
LC-MS results of the unpurified reaction mixture between
D1 and T1, after irradiation for 2 hours, is shown in Figure 1
and indicates the formation of the thiol-Michael product A1.
Scheme 2. A DASA photoswitch in the cyclized form can undergo a thiol-
Michael addition at the activated double bond of the furan ring. Herein, reactivity
was observed with both aromatic and alkyl thiols R’-SH.
Scheme 3. Switching of the DASA, initially in the linear form (D1linear) upon light irradiation allows the formation of a cyclized species containing a cyclopentenone
(D1cyclized). The double bond formed is a suitable reaction partner for a thiol-Michael reaction in presence of pyridine or Et3N with either (a) 1-butanethiol (T1) or (b)
p-bromothiophenol (T2). The resulting thiol-Michael adducts are depicted as A1 and A2, respectively.
Structural proof for A1 was obtained via 1H NMR
spectroscopy (Figure 2). A recent study showed that the
DASA switching was affected when the DASA concentration
changed from 1 M to 0.003 M in toluene, demonstrating that
the DASA switch is drastically inhibited at higher
concentrations.[31] As a conventional 1H NMR spectrum
requires a concentration of approximately 0.01 M, the
switching yield towards the cyclized form is limited. Therefore,
to ensure sufficient conversion to the thiol-Michael product A1
even at high DASA concentrations, the reaction was
performed in methanol (MeOH), where the cyclized form is
favored, even in the dark and with a greater excess of T1 and
Figure 1. LC-MS analysis of the reaction between the DASA D1 and 1-
butanethiol (T1) upon irradiation (max = 525 nm, 2 hours, 3 mW·cm-2) in
presence of pyridine in DCM. (a) Chromatogram and (b) MS spectrum at RT =
10.2 – 10.4 min of the thiol-Michael product A1. Refer to supplementary
information for details on the mass accuracy.
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Et3N. The reaction consisted of dissolving 70.6 μmol of D1,
5.6 eq. of T1 and 8.5 eq. of Et3N in 5 mL methanol. The
reaction mixture was stirred under ambient conditions for
48 hours. The thiol-Michael product A1 was isolated by
reverse phase column chromatography using a mixture of
MeOH and acetonitrile (ACN).
significantly affects the equilibrium between the linear and
cyclized forms in DCM.[8] Further to this, we observed that the
basicity of the solution, due to the quantity of base present,
also affected the switching. A high pKa base (e.g., Et3N), in
particular at high concentrations, strongly affects the
formation of A1. In Figure 4a-c, three solutions of D1 in CD2Cl2
(33.33 μmol·mL-1) with 1.3 eq. T1 and 1 eq., 5 eq. or 9 eq. of
Et3N were prepared. Irradiation was performed for 6 hours
(shaded area in Figure 4a-d). When 1 eq. of base is used, the
rate of ring closure is reduced, and the thiol-Michael product
is not formed (Figure 4a). Due to precipitation of salt formation
of D1cyclized with the base the overall yield is around 20%. Little
effect is observed when the base equivalence is increased
from 5 eq. to 9 eq. (Figure 4b and c, respectively). In both
cases, nearly full conversion of A1 was observed after
irradiation. A comparison of the abundance of product A1 in
all three situations is presented in Figure 4d. The findings
suggest that — at high concentrations of DASA — non-
stoichiometric equivalence of base is required for A1
formation. Considering the dependence of the DASA
switching on the solvent and the base equivalence, and also
recalling that the base needs to be of a sufficient pKa to
deprotonate the thiol, one can tune the reactivity of the DASA.
In addition to the ligation with 1-butanethiol (T1), p-
bromothiophenol (T2) also led to successful formation of the
product using the standard procedure. Figure 5 shows the LC-
MS analysis of D1 reaction with T2 under different conditions.
The reaction in the presence of base and light proved to be
most efficient, leading to efficient formation of the expected
product A2 (Figure 5a, bottom spectrum). Since there are two
stable forms of D1, which elute at different times from the
column, two D1 peaks are observed in the LC trace. One
around 3 minutes corresponding to the cyclized isomer and
another close to 13 minutes, assigned to the linear isomer.
Figure 2. 1H NMR spectrum (600 MHz, 25 °C) assignment in CDCl3 of A1. The
reaction between the DASA (D1) and 1-butanethiol (T1) in the presence of Et3N
under ambient conditions for 48 hours and purified by reverse column
chromatography results in A1.
After purification of the thiol-Michael product A1, the
reappearance of D1linear and D1cyclized was observed after
some time. To investigate this, the stability of A1 was
1
assessed via H NMR spectroscopy (Figure 3). The product
was obtained via the aforementioned procedure using
methanol as the solvent. 1H NMR spectra were recorded as a
function of time. Initially, the product A1 comprises 94 % of
the total species in the solution (D1linear, and D1cyclized). Over
a period of more than 50 hours, an equilibrium between the
species, A1, D1cyclized and D1linear, was observed. D1linear
,
initially non-existent, was reformed while the quantity of
D1cyclized increases substantially (from 5 % to 20 % in the
same timeframe). D1cyclized is known to exist in several
different isomers, shown by Feringa and Buma et al.
recently.[32] So far little is known about the individual stability
of these different isomers. It is hypothesized that the reaction
of T1 with one particular isomer of D1cyclized is not stable in
solution. Therefore, the bond once formed between T1 and
the less stable D1cyclized is cleaved in solution, leading to the
formation of both starting materials. However, further research
is needed to full understanding.
Figure 4. Irradiation (max = 525 nm, 6 mW·cm-2, 6 hours) of the DASA D1 with
1-butanethiol T1 (1.3 eq.) in presence of (a) 1 eq., (b) 5 eq. or (c) 9 eq. Et3N in
CD2Cl2 affects the product A1 formation. A1 is not formed when 1 eq. of Et3N is
used. (d) Comparison of A1 formation with respect to different base
equivalencies shows that an excess of base is required. CD2Cl2 was the
selected solvent as it does not affect the switching behavior and allows high
solubility of the species. The integrals were calculated using an internal
standard (1,3,5-trimethoxybenzene). The error bars were determined by the
percentage error between the two standard signals for each NMR spectrum and
applied for each integral of D1linear, D1cyclized and A1. Note that the lines in the
graphs are a guide for the eye only.
Figure 3. Reaction between the DASA D1 and 1-butanethiol (T1) upon
irradiation (max = 525 nm, 3 mW·cm-2) in the presence of Et3N led to the
formation of adduct A1. The stability of A1 was assessed by 1H NMR
spectroscopy in CDCl3. Details on the 1H NMR signals used are available in the
supplementary information.
Hemmer et al. observed previously that the dissociation
constant (determined via the pKa) of the precursor amine
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Control experiments of the ligation reaction with T2 were
performed to determine the robustness of the thiol-Michael
reaction. Without light (max = 525 nm), the UV-Vis trace at
280 nm of the respective LC-MS measurement showed no
conversion of A2 within a 3 hour period (Figure 5a, top
spectrum). Upon irradiation, but without base, the formation of
A2 was limited to 40 % of the fraction initially obtained when
base is present — even after additional reaction time (Figure
5a, blue line spectrum). In this case, A2 formation can still
occur possibly due to the basicity of the DASA moiety and the
low pKa value of p-bromothiophenol T2. However, the
combination of light and base is essential for high yields of the
desired adduct A2. The stability of A2 was tested via HPLC
measurements, where two different isomers were observed.
The stability was tested over 20 hours in acetonitrile and a
decrease about 58% was observed of one isomer. The other
isomer was stable without any noticeable degradation over 20
hours.
prospect of fine tuning, not just the switching, but also the
reactivity of the DASA. We thus highlight a pathway for
selectively controlling the reactivity of a cyclized DASA and its
subsequent trapping.
Experimental Section
The thiols used, 1-butanethiol (T1) and p-bromothiophenol
(T2), as well as the bases used: pyridine and Et3N, were
obtained from commercially available sources. The standard
procedure for the reaction consists of dissolving 1.7 μmol of
D1, 1.3 eq. of the thiol and 1.3 eq. of base in 0.9 mL of dry
toluene, constantly stirred during irradiation with 8 x 525 nm
green LEDs in a photoreactor for a minimum of 2 hours. For
the detailed description, refer to supplementary information.
Acknowledgements
C.B.-K. acknowledges the Australian Research Council (ARC) for
funding in the form of a Laureate Fellowship underpinning his
photo-chemical research program as well as the Queensland
University of Technology (QUT) for key continued support. D.G.
acknowledges the German Research Foundation (DFG) for
funding his post-doctoral studies. C.B.-K, J.P.B. and S.L.W.
acknowledge support via an ARC Discovery project targeted at
red-shifting photoligation chemistry. The authors acknowledge
the facilities and the scientific and technical assistance at the
Central Analytical Research Facility (CARF) operated by the
Institute for Future Environments (IFE). The authors acknowledge
Dr. Neil Mallo (UNSW) for insightful discussions.
Conflict of interest
The authors declare no conflict of interest.
Figure 5. Reactivity of the DASA D1 and the p-bromothiophenol (T2) assessed
via liquid chromatography coupled to mass spectrometry (LC-MS). In (a), the
experimental conditions — without light (red, c = 7.5 μmol·mL-1), without base
(pyridine, blue, c = 4.1 μmol·mL-1), and irradiation with base (pyridine, black, c =
2.1 μmol·mL-1) — were changed to access the effect on product formation of
A2. Note that the peak corresponding to the product A2 is split due to the
presence of diastereomers. The m/z values for the peaks shown in (a)
correspond to those expected for (b) D1 (RT = 2.5 – 3 min and 12.5 – 13 min)
and (c) A2 (RT = 13.5 – 14 min).
Keywords: selective reactions • thiol-Michael addition • molecular
switch • Donor-Acceptor Stenhouse Adducts • photogated
reactivity
References
In summary, the reactivity of the cyclized form D1cyclized
was assessed. A first generation DASA was successfully
trapped with 1-butanethiol (T1) and p-bromothiophenol (T2)
leading to the respective products at high yields. The light
promotes the cyclisation of the DASA at low concentrations,
which subsequently reacts with a thiol in the presence of a
base towards the thiol-Michael product. At high
concentrations, the base has a significant effect on the
product formation and switching rate. In solution, the product
A1 has an inherent equilibrium with the DASA starting material.
These new ligations properties, in addition to the well-
established tunability of the DASA properties based on donor
and acceptor type and solvent environment allow for the
[1]
[2]
[3]
M.-M. Russew, S. Hecht, Adv. Mater. 2010, 22, 3348–3360.
Z. L. Pianowski, Chem. - A Eur. J. 2019, 25, 5128–5144.
W. A. Velema, W. Szymanski, B. L. Feringa, J. Am. Chem. Soc. 2014,
136, 2178–2191.
[4]
S. O. Poelma, S. S. Oh, S. Helmy, A. S. Knight, G. L. Burnett, H. T.
Soh, C. J. Hawker, J. Read de Alaniz, Chem. Commun. 2016, 52,
10525–10528.
[5]
[6]
D. Zhong, Z. Cao, B. Wu, Q. Zhang, G. Wang, Sensors Actuators B
Chem. 2018, 254, 385–392.
P. Šafář, F. Považanec, N. Prónayová, P. Baran, G. Kickelbick, J.
Kožíšek, M. Breza, Collect. Czechoslov. Chem. Commun. 2000, 65,
This article is protected by copyright. All rights reserved.

10.1002/chem.201904770
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1911–1938.
[19]
O. Rifaie-Graham, S. Ulrich, N. F. B. Galensowske, S. Balog, M.
Chami, D. Rentsch, J. R. Hemmer, J. Read De Alaniz, L. F. Boesel,
N. Bruns, J. Am. Chem. Soc. 2018, 140, 8027–8036.
A. Balamurugan, H. Il Lee, Macromolecules 2016, 49, 2568–2574.
M. M. Lerch, W. Szymański, B. L. Feringa, Chem. Soc. Rev. 2018,
47, 1910–1937.
[7]
[8]
S. Helmy, S. Oh, F. A. Leibfarth, C. J. Hawker, J. Read de Alaniz, J.
Org. Chem. 2014, 79, 11316–11329.
J. R. Hemmer, S. O. Poelma, N. Treat, Z. A. Page, N. D. Dolinski, Y.
J. Diaz, W. Tomlinson, K. D. Clark, J. P. Hooper, C. Hawker, et al., J.
Am. Chem. Soc. 2016, 138, 13960–13966.
[20]
[21]
[9]
N. Mallo, P. T. Brown, H. Iranmanesh, T. S. C. MacDonald, M. J.
Teusner, J. B. Harper, G. E. Ball, J. E. Beves, Chem. Commun. 2016,
52, 13576–13579.
[22]
P. Mueller, M. M. Zieger, B. Richter, A. S. Quick, J. Fischer, J. B.
Mueller, L. Zhou, G. U. Nienhaus, M. Bastmeyer, C. Barner-Kowollik,
et al., ACS Nano 2017, 11, 6396–6403.
[10]
[11]
M. M. Lerch, S. J. Wezenberg, W. Szymanski, B. L. Feringa, J. Am.
Chem. Soc. 2016, 138, 6344–6347.
[23]
[24]
H. Vijayamohanan, E. F. Palermo, C. K. Ullal, Chem. Mater. 2017, 29,
4754–4760.
M. Di Donato, M. M. Lerch, A. Lapini, A. D. Laurent, A. Iagatti, L.
Bussotti, S. P. Ihrig, M. Medved’, D. Jacquemin, W. Szymański, et al.,
J. Am. Chem. Soc. 2017, 139, 15596–15599.
V. Lemieux, S. Gauthier, N. R. Branda, Angew. Chemie - Int. Ed.
2006, 45, 6820–6824.
[25]
[26]
C. F. H. Allen, G. P. Happ, Can. J. Chem. 1964, 42, 641–649.
C. E. Hoyle, C. N. Bowman, Angew. Chemie - Int. Ed. 2010, 49,
1540–1573.
[12]
[13]
J. N. Bull, E. Carrascosa, N. Mallo, M. S. Scholz, G. da Silva, J. E.
Beves, E. J. Bieske, J. Phys. Chem. Lett. 2018, 9, 665–671.
M. M. Lerch, M. Medved, A. Lapini, A. D. Laurent, A. Iagatti, L.
Bussotti, W. Szymański, W. J. Buma, P. Foggi, M. Di Donato, et al.,
J. Phys. Chem. A 2018, 122, 955–964.
[27]
[28]
[29]
[30]
[31]
[32]
D. P. Nair, M. Podgórski, S. Chatani, T. Gong, W. Xi, C. R. Fenoli, C.
N. Bowman, Chem. Mater. 2014, 26, 724–744.
C. E. Hoyle, T. Y. Lee, T. Roper, J. Polym. Sci. Part A Polym. Chem.
2004, 42, 5301–5338.
[14]
J. R. Hemmer, Z. A. Page, K. D. Clark, F. Stricker, N. D. Dolinski, C.
J. Hawker, J. Read de Alaniz, J. Am. Chem. Soc. 2018, 140, 10425–
10429.
T. O. Machado, C. Sayer, P. H. H. Araujo, Eur. Polym. J. 2017, 86,
200–215.
[15]
[16]
[17]
N. Mallo, E. D. Foley, H. Iranmanesh, A. D. W. Kennedy, E. T. Luis,
J. Ho, J. B. Harper, J. E. Beves, Chem. Sci. 2018.
F. Bigi, S. Carloni, L. Ferrari, R. Maggi, A. Mazzacani, G. Sartori,
Tetrahedron Lett. 2001, 42, 5203–5205.
S. Helmy, F. A. Leibfarth, S. Oh, J. E. Poelma, C. J. Hawker, J. Read
de Alaniz, J. Am. Chem. Soc. 2014, 136, 8169–8172.
B. P. Mason, M. Whittaker, J. Hemmer, S. Arora, A. Harper, S.
Alnemrat, A. McEachen, S. Helmy, J. Read de Alaniz, J. P. Hooper,
Appl. Phys. Lett. 2016, 108, 41906.
B. F. Lui, N. T. Tierce, F. Tong, M. M. Sroda, H. Lu, J. Read de Alaniz,
C. J. Bardeen, Photochem. Photobiol. Sci. 2019, 18, 1587–1595.
H. Zulfikri, M. A. J. Koenis, M. M. Lerch, M. Di Donato, W. Szymański,
C. Filippi, B. L. Feringa, W. J. Buma, J. Am. Chem. Soc. 2019, 141,
7376–7384.
[18]
G. Sinawang, B. Wu, J. Wang, S. Li, Y. He, Macromol. Chem. Phys.
2016, 217, 2409–2414.
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Entry for the Table of Contents
COMMUNICATION
Jessica Alves, Sandra Wiedbrauk, David
Gräfe, Sarah L. Walden, James P.
Blinco,* Christopher Barner-Kowollik*
Page No. – Page No.
It’s a Trap: Thiol-Michael Chemistry
on a DASA photoswitch
It’s a trap: DASAs are well-known photoswitches that reversibly isomerize from a
strongly colored linear to a colorless cyclized form upon visible light irradiation. We
herein show that the cyclized form can be readily trapped via a base-mediated thiol-
Michael addition. The reactivity of the closed DASA towards thiol addition was found
to be strongly dependent on the reaction conditions. Tuning of the conditions allows
to selectively control the DASA’s reactivity in its cyclized form.
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