Angewandte
Communications
Chemie
Dynamic Covalent Chemistry
Control of Imine Exchange Kinetics with Photoswitches to Modulate
Self-Healing in Polysiloxane Networks by Light Illumination
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Michael Kathan , Petr Kova rˇ ꢀ cˇ ek , Christoph Jurissek, Antti Senf, Andre Dallmann,
Andreas F. Thꢁnemann, and Stefan Hecht*
Abstract: Various aldehyde-containing photoswitches have
been developed whose reactivity toward amines can be
controlled externally. A thermally stable bifunctional diary-
lethene, which in its ring-closed form exhibits imine formation
accelerated by one order of magnitude, was used as a photo-
switchable crosslinker and mixed with a commercially avail-
able amino-functionalized polysiloxane to yield a rubbery
material with viscoelastic and self-healing properties that can
be reversibly tuned by irradiation.
with aldehydes, which should allow for convenient tuning of
their electrophilicity by photoisomerization reactions.
[12]
Herein, we describe how light can control the kinetics of
general imine exchange reactions. To demonstrate the
applicability of our approach, a photoswitchable dynamic
covalent crosslinker was used to modulate the self-healing
ability of a polymer network after illumination with UV and
visible light.
Our initial work focused on the design of a photoswitch,
which can be used to modify the reactivity of an appended
aldehyde group (Figure 1a). For this purpose we have
prepared and studied aldehyde-functionalized photo-
S
upramolecular and macromolecular systems that are able
to structurally respond to changes in their environment can be
realized by the introduction of reversible linkages on the
[
13,14]
switches
belonging to the three most commonly used
[
1,2]
[3]
[15]
[16]
molecular level
or dynamic covalent bonds. The latter allow for strong yet
reversible connections between building blocks leading to
based on either noncovalent interactions
photochromic families: spiropyran SP, azobenzene AB,
and diarylethene DAE (Figure 1b).
[4]
[17]
Upon illumination
with UV light, these specific photoswitches are readily
converted to their corresponding isomers, which display an
[
5]
constitutional adaptation. Among the most attractive stim-
[6]
[12]
uli to trigger a materialꢀs response is light, as it can be
applied in an external, noninvasive fashion with high spatial
and temporal resolution. There are several ways in which light
can control the connection or disconnection of molecular
building blocks, involving photodissociation and (retro)-
photodimerization as well as photoisomerization of sub-
enhanced carbonyl group reactivity and can be returned to
their less reactive forms by exposure to visible light and/or
heat. The mechanism of modulating the carbonyl reactivity
differs among the three designed photoswitches: On the one
hand, photoisomerization of ABt and SP unmasks an
[
7]
[
8]
[18]
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activating ortho-hydroxyl group
in ABc and [MC-H] ,
[
9]
strates and/or products in thermal cycloadditions. In all of
these cases the photoreaction is directly linked to the
respectively. On the other hand, the ring-open DAEo bears
two cross-conjugated and hence decoupled thiophene moi-
eties and can be converted to the ring-closed DAEc, in which
p-conjugation of both electron-withdrawing terminal formyl
groups leads to their synergistic activation.
(
reversible) covalent bond formation/scission, leading to
a significant limitation of the substrate scope.
In an attempt to truly separate the primary photoevent
from the covalent (thermal) reaction in the most widely
applicable sense, we sought to modulate the electrophilicity of
a carbonyl compound, which is known to engage in dynamic
The difference in reactivity of the aldehyde electrophiles
in all three pairs of photoisomers was investigated in
a condensation reaction with different amine-based nucleo-
philes. The AB and SP derivatives were treated with pyridine-
4-carbohydrazide, and the formation of the corresponding
acylhydrazones was monitored by UV/Vis absorption spec-
troscopy. Upon continuous illumination, leading to the
[10]
covalent imine (hydrazone, oxime etc.) exchange reactions,
by light irradiation. This particular connection was chosen as
it operates under ambient conditions (room temperature, air,
[11]
and humidity) without a catalyst
and involves highly
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abundant (aliphatic) amine nucleophiles in combination
formation of the photoisomers ABc and [MC-H] , the
condensation rates were significantly higher than those of
the corresponding nonirradiated samples composed solely of
ABt or SP. In the case of AB, the rate of the condensation is
increased by a factor of 2.4 in the photostationary state (PSS,
see Figure S1 in the Supporting Information), containing ca.
[
+]
[+]
[
*] M. Kathan, Dr. P. Kova ˇr ꢀ cˇ ek, C. Jurissek, A. Senf, Dr. A. Dallmann,
Prof. S. Hecht
Department of Chemistry & IRIS Adlershof
Humboldt-Universitꢁt zu Berlin
Brook-Taylor Strasse 2, 12489 Berlin (Germany)
E-mail: sh@chemie.hu-berlin.de
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2% of the cis isomer (thermal half-life t1/2 ꢀ 3.5 min under
these conditions). For MC, despite the extremely low photo-
conversion in the PSS (thermal half-life of a few seconds
under these conditions), the rate of condensation is enhanced
by a factor of 3.1 (see Figure S2 in the Supporting Informa-
tion). In stark contrast, DAE is truly bistable and can be
switched almost quantitatively and exclusively back and forth
by light irradiation.
A. F. Thꢂnemann
Federal Institute for Materials Research and Testing
Unter den Eichen 87, 12205 Berlin (Germany)
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[
] These authors contributed equally to this work.
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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