How Water in Aliphatic Solvents Directs the Interference of Chemical Reactivity in a Supramolecular System

Article abstract of DOI:10.1021/jacs.0c04962

Water is typically considered to be insoluble in alkanes. Recently, however, monomerically dissolved water in alkanes has been shown to dramatically impact the structure of hydrogen-bonded supramolecular polymers. Here, we report that water in methylcyclo

Full text of DOI:10.1021/jacs.0c04962

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Article
How Water in Aliphatic Solvents Directs the Interference
of Chemical Reactivity in a Supramolecular System
Mathijs F.J. Mabesoone, Gijs Ter Huurne, Anja R.A. Palmans, and Egbert Willem Meijer
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c04962 • Publication Date (Web): 16 Jun 2020
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How Water in Aliphatic Solvents Directs the Interference of Chemical Reactivity
in a Supramolecular System
Mathijs F.J. Mabesoone1, Gijs M. ter Huurne1, Anja R.A. Palmans1 and E.W. Meijer1*
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1Institute for Complex Molecular Systems and the Laboratory of Macromolecular and Organic Chemistry,
Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
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*Corresponding author: e.w.meijer@tue.nl
ABSTRACT: Water is typically considered to be insoluble in alkanes. Recently, however, monomerically dissolved
water in alkanes has been shown to dramatically impact the structure of hydrogen-bonded supramolecular
polymers. Here, we report that water in methylcyclohexane (MCH) also determines the outcome of combining a
Michael reaction with a porphyrin-based supramolecular system. In dry conditions, the components of the reaction
do not affect or destabilize the supramolecular polymer, whereas in ambient or wet conditions, the polymers are
rapidly destabilized. Although spectroscopic investigations show no effect of water on the molecular structure of the
supramolecular polymer, light scattering and atomic force microscopy experiments show that water increases the
flexibility of the supramolecular polymer and decreases the polymer length. Through a series of titrations, we show
that a cooperative interaction, involving the coordination of the amine catalyst to the porphyrin and complexation of
the substrates to the flexible polymers invokes the depolymerization of the aggregates. Water crucially stabilizes
these cooperative interactions to cause complete depolymerization in humid conditions. Additionally, we show that
the humidity-controlled interference in the polymer stability occurs with various substrates, indicating that water
may play a ubiquitous role in supramolecular polymerizations in oils. By controlling the amount of water, the
influence of a covalent chemical process on non-covalent aggregates can be mediated, which holds great potential to
forge a connection between chemical reactivity and supramolecular material structure. Moreover, our findings
highlight that understanding cooperative interactions in multicomponent non-covalent systems is crucial to design
complex molecular systems.
exciting systems have been reported that utilize
■ INTRODUCTION
kinetically trapped monomers or aggregation states
to install temporal behavior in supramolecular
polymers. In seminal work by Takeuchi, Sugiyasu and
co-workers, bisamidated porphyrin monomers were
shown to readily assemble into small isodesmically
formed J-aggregates, which gradually converted to
the thermodynamically stable, long H-aggregated
supramolecular polymers. Upon sonication of the
isodesmically formed J-aggregates, small H-
aggregated seeds that act as nuclei for seeded
polymerizations were formed.17 Similar behavior has
been shown by groups of Sugiyasu, Würthner, Ogi and
Sánchez for various other porphyrins18–20 as well as
Over the last two decades,
a
profound
understanding over the thermodynamic aspects of
supramolecular polymers has been achieved and as a
result a large variety of non-covalent structures can
be synthesized.1,2 Despite being dynamic, these
structures typically change their properties only as a
response to an external stimulus and do not show any
temporal change in structure or chemical
functionality without.3 In contrast, in natural systems,
spatiotemporal control over supramolecular
structure and reactivity is ubiquitous and leads to the
complex behavior observed in life.4,5 Recently,
chemists have endeavored to mimic these natural
reaction networks to obtain out-of-equilibrium
systems.6–12 Inspired by the chemistry of life, these
systems are often designed in aqueous environments.
Although aqueous environments provide great
opportunities to control chemical reactions, it is often
difficult to design and synthesize controlled
supramolecular structures in these polar media.13–15
for
a
wide range of other monomers.21–26 By
controlling the length of the flexible spacer between
the hydrogen-bonding groups at the monomer’s
periphery and its rigid core, control can be achieved
over the time-dependency of such living
supramolecular polymerizations.27 Aida and co-
workers used a similar approach to design an
initiated living supramolecular polymerization. In
this approach, a non-hydrogen bonded initiator can
activate an intramolecularly hydrogen-bonded
dormant monomer to form highly monodisperse
supramolecular polymers.28 These results show that
by controlling the flexibility of the monomers, great
temporal and structural control over supramolecular
polymerization can be obtained. However, these
In apolar media, the well-defined character of non-
covalent interactions such as charge-transfer
complexation and hydrogen-bonding interactions has
permitted the design of ordered and well-organized
supramolecular polymers. Such structures are in
thermodynamic equilibrium, giving materials that do
not evolve over time.1,16 In recent years, several
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systems do not incorporate in situ chemical
Scheme 1 a) Chemical structure of Zn-1, Zn-Me,
and H2-1. b) Base-catalyzed Michael addition of
thiophenol to N-propylmaleimide in MCH.
c) Various piperidine-based catalysts used in the
Michael reaction.
conversion that is required to obtain life-like
responses observed in chemical reaction networks
and, as a result, possible applications of these systems
remains limited.
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To extend temporal control in supramolecular
polymerizations from polymer structure to additional
chemical functionality, the polymerization process
has to be combined with chemical reactions.29 By
enabling the combination of supramolecular
polymerizations with control over chemical
functionality, many new material behaviors, including
feedback, adaptation and regulation can be obtained.
However, only a handful of these systems have been
reported. The first examples by the groups of Ulijn,
and Van Esch and Eelkema showed that the
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propensity
of
monomers
to
polymerize
supramolecularly can be controlled in situ by
reversible esterification and hydrolysis, giving rise to
transient supramolecular networks.6,7,30 The groups
of Hermans and George showed that also redox
reactions can be used to control supramolecular
aggregation.10–12,31–33 The fine temporal control over
the aggregation state that these aqueous systems
offer comes often at the cost of a decrease in control
over polymerization mechanism and polymer
morphology,7 which renders complete control over
the material properties more challenging.
the
supramolecular
polymer
is
observed.
Since
complex
material
responses
in
Surprisingly, when the system is studied in ambient
environments, we found that small amounts of water
in MCH can dramatically impact the polymer stability.
With several titrations, light scattering and atomic
force microscopy (AFM) experiments, we show that
the polymer length and rigidity are decreased when
water, the reaction substrates, and catalyst are
complexed to the supramolecular polymer. We
propose that this effect arises from the binding of
water and the reagents to the macrodipole at the
chain ends of the polymer. The chain ends are
subsequently stabilized by complexation of the
catalyst to metal center of the porphyrin. Several
other Michael substrates illustrate the generality of
this effect.
supramolecular systems can only be realized when
both structure and chemical conversion are precisely
controlled, there is a need to realize a connection
between controlled chemical reactivity and
supramolecular structure. In the past, we have shown
that the interaction between basic amines and Zn-1
(Scheme 1a) can induce counterintuitive behavior in
these multicomponent systems, such as dilution
induced
self-assembly,34
photo-induced
depolymerization35 and microfluidic separation of
various aggregate morphologies.36 Encouraged by our
previous results, where the interactions between
amines and zinc center were employed to switch the
aggregation of the porphyrin and the pyridines, we
here combine the supramolecular polymerization
with an amine-catalyzed Michael reaction. In our
attempts to realize such a system, we aim to combine
supramolecular polymers of Zn-1 with a chemically
relevant and biologically interesting base-catalyzed
Michael reaction between thiols and maleimides. In
this system, the zinc metallated porphyrin molecule is
particularly interesting, since the metal center offers
the ability to sequester amine-based organocatalysts,
thus forging a connection between supramolecular
structure and chemical reactivity.
■ RESULTS AND DISCUSSION
A base-catalyzed Michael reaction and water
depolymerize Zn-1 polymers. To combine
supramolecular polymers of Zn-1 with a chemical
reaction, we set out to identify chemical conversions
that occur at reasonable timescales in apolar media
like MCH. Ultimately, we identified the base-catalyzed
Michael addition of thiophenol, PhSH, to N-propyl
maleimide, NPrMal as a promising reaction for the
desired system (Scheme 1b). Because the apolar MCH
solution does not stabilize the charged reaction
Interestingly, we found that in the absence of
monomerically dissolved water in the MCH solvent,
no interference of the amine-catalyzed reaction with
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After identifying a suitable chemical reaction and
amine catalyst, we performed the Michael reaction in
the presence of Zn-1 polymers. When studying the
Zn-1 polymers in the reaction medium containing
PhSH, NPrMal and MePip, the initial results obtained
were irreproducible and inexplicable. Similar to an
earlier report,37 however, we serendipitously found
that ppm-levels of water, which amount to millimolar
concentrations, play a crucial role in controlling the
stability of the Zn-1 polymers in the reaction medium.
This observation, which hints at a ubiquitous, yet
poorly understood role of water in supramolecular
polymers, prompted us to investigate this role of
water in Zn-1 polymers in detail.
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The low solubility of Zn-1 polymers necessitated
the use of small amounts (<0.5 vol%) of CHCl3 in the
MCH solvent to retain the polymers in solution,
denoted here as MCH*. Consequently, all further
experiments were performed in the presence of a
small amount of CHCl3 co-solvent. When Zn-1, the
NPrMal and PhSH reagents together with MePip
catalyst are dissolved in dry MCH*, the Michael
reaction occurs and the MA product is gradually
formed, as indicated by the decrease in absorbance of
NPrMal at 300 nm (Figure 2a-b). During the reaction,
no change is observed in the CD-active absorption
peak at 392 nm, which originates from the helically
ordered, H-aggregated Zn-1. The absence of a change
in the absorption of the Zn-1 aggregates indicates that
the polymers are stable structures under these
conditions (see also Figure S6a).38 Hence, reaction
substrates and catalyst do not interfere with the
supramolecular polymer in dry MCH*.
Figure 1 Time-dependent absorbance of NPrMal at 350
nm of reaction mixtures containing 10 mM NPrMal and
10 mM PhSH with 0.5 mM of the various catalysts at 20 °C
in MCH.
intermediates, no reaction occurs without the use of
an organic base as catalyst. To study the effect of the
catalyst on the rate of the reaction, we studied the
Michael reaction with various catalysts by monitoring
the decrease in absorbance of the conjugated system
of NPrMal as the reaction proceeds (Scheme 1c and
Figure 1). Interestingly, pyridine is not sufficiently
basic to catalyze the Michael reaction and the non-
nucleophilic 2,2,6,6-tetramethylpiperidine, Me4Pip,
is too bulky to show high rates of conversion. By
decreasing the steric bulk around the aliphatic amine,
good catalytic activities were obtained, with
piperidine and 2-methylpiperidine, MePip, being the
most active catalysts.
Conversely, upon preparing a reaction mixture in
the presence of Zn-1 in ambient MCH* containing
To control the effect of the amine on the
(de)polymerization of Zn-1 by its binding to the zinc
core, the association constant of the catalyst should
not be too high or too low. Therefore, an ideal catalyst
has an intermediate association constant, between
104 and 105 M−1, similar to the functional ligands
reported earlier.34,35 The association constants of all
four investigated aliphatic amine catalysts to the non-
polymerizing Zn-Me model compound were
determined through UV-Vis titrations (See
Supplementary Information for details). We found
that the association constant of MePip (Ka=20.6⋅103
M−1) is in the range of the previously reported
functional ligands, while the association constant of
piperidine (Ka=949⋅104 M−1) is too high and those of
Me2Pip and Me4Pip (Ka=6.63⋅103 and 168 M−1,
respectively) are too low. Hence, MePip does not
depolymerize the polymers at the concentrations of
both Zn-1 and MePip used in this work while still
showing catalytic activity and is thereby an
12.5 ppm of monomerically dissolved water,39
a
strong influence of the reaction on the polymer is
observed. We note here that the exact concentration
of water is strongly determined by the environmental
humidity. Due to equilibration of the dissolved water
in MCH* with moisture from the air, precise control
over the dissolved water content is hard to achieve. In
the ambient environment, the Michael reaction
proceeds with a slightly higher rate, as indicated by
the slightly sharper decrease in absorbance of the
maleimide at 300 nm (Figure 2c-d). The absorption
band at 392 nm, however, shows a rapid decrease in
intensity, coinciding with a strong decrease in CD
intensity and an increase of the absorbance at 425 nm
(Figure 2c-d and S6b). These spectral changes
indicate
that
the
helical,
H-aggregated
supramolecular polymers of Zn-1 depolymerize to
free monomers, dimers or small aggregates in the
presence of reaction substrates and catalyst.34
Interestingly, control experiments in which one or
more of the components of the Michael reaction are
absent, no destabilization of the supramolecular
interesting
candidate
for
the
proposed
multicomponent system.
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Figure 3 a) AFM micrographs of samples drop casted from solutions of 10 μM Zn-1 in dry MCH*. [H2O] = 20.0 ppm. b) Count
rates at various scattering vectors, q, of10 μM solutions of Zn-1 solutions obtained from SLS experiments in ambient (circles)
and dry (squares symbols) MCH. Error bars indicate one standard deviation. In the case of the ambient samples, [H2O] = 28.8
ppm and in the case of the dry samples, the concentration of water was below the detection limit of the Karl-Fisher titrator.
polymers was observed (Figures S8-11). Combined,
these results show that unexpected additive effects
between water and one or more of the components of
the Michael reaction rapidly destabilize the Zn-1
supramolecular polymers. Since these cooperative
interactions crucially impact the stability of the
supramolecular polymer, they must be fully
understood before any additional complexity through
switching of the supramolecular system can be
introduced. Therefore, we set out to investigate the
effect of water and the additional components on the
supramolecular polymers.
with AFM. Similar observations have been made for
benzene-1,3,5-tricarboxamides, suggesting that
water-induced flexibility may be
a
general
phenomenon in supramolecular polymers.37 Thus,
water appears to considerably increase the flexibility
and solubility of the supramolecular polymers in the
MCH* solvent.
The increased rigidity and size of the Zn-1
polymers in the absence of water, as suggested by the
AFM results, is corroborated by static light scattering
(SLS) experiments on ambient and dry solutions of
Zn-1 (Figure 3b). The results show the samples in
ambient MCH* show a weaker angle dependency of
the scattering intensity, compared to the samples in
dry MCH*. Since no plateau could be observed in the
q-region measured, we unfortunately cannot
determine the absolute length of the polymers from
the SLS results. However, a comparison of the UV-Vis
spectra of unfiltered solutions of Zn-1 with solutions
that are filtered through various filters of various pore
sizes does suggest that a considerable fraction of the
polymers is longer than 1 μm (Figure S16), as has
previously been observed.38
Scattering and microscopy show that water and
substrates reduce bundling. Although in the
absence of one of the reaction components, no change
in the absorbance of the polymers is observed, this
merely indicates that the number of polymerized
monomers remains the same. The interactions of
water and the substrates may still lead to
destabilization of the polymers and a reduction in
polymer length.40,41 To probe changes in polymer
length and morphology, we first investigated the
effect of water on the polymers through AFM
experiments (Figures 3a and S12 and 13). Here, large,
bundled aggregates are observed for samples that are
drop casted from 10 μM solutions of Zn-1 in dry
MCH*. Conversely, only small and ill-defined particles
could be observed for samples drop casted from
ambient solutions, indicating that in ambient
solutions, the polymers are present as dynamic single
fibers or small bundles which cannot be measured
The change in slope observed in the scattering
results suggests that water induces flexibility and
dynamics in the supramolecular polymers. The
induction of flexibility is additionally supported by
the poor solubility of the Zn-1 supramolecular
polymers in dry, pure MCH. In the absence of water,
precipitation occurs within hours, necessitating the
use of a small amount of CHCl3 co-solvent in the stock
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Figure 2 Absorbance of 10 μM solutions of Zn-1 with 4 mM NPrMal, 4 mM PhSH and 0.5 mM MePip in dry MCH*(a)
and ambient MCH* (c) and the time dependency of the absorbance at 300 nm, 392 nm and 425 nm for solutions in dry
MCH* (b) and ambient MCH* (d).
solutions. In contrast, solutions of Zn-1 prepared in
ambient, pure MCH are stable for weeks, which can be
attributed to the better solubility of the more flexible
aggregates when water is present.
(Figure 4). In all four cases, the CD signal at 392 nm,
which indicates the presence of the polymeric H-
aggregates of Zn-1, is probed at varying substrate
concentration. The results show that in the absence of
the catalyst, the polymers are stable in the presence
of the reaction substrates (Figure 4, hollow symbols).
However, in the presence of 0.5 mM MePip, the
addition of NPrMal or PhSH leads to
depolymerization (Figure 4, solid symbols).
Moreover, the degree of depolymerization by the
additives is strongly influenced by water. In ambient
Combined, the SLS and AFM results strongly
suggest that water plays a pivotal role in breaking up
polymer bundles to form more flexible single polymer
chains. The breaking up of the polymer bundles
potentially allows the complexation of the reaction
components to lead to a destabilization of the
polymers and a reduction of the polymer length. To
further study the destabilization of the polymers by
the components of the reaction, titrations in which
these components are added to the polymers in
varying amounts were performed.
MCH*,
containing
13.9
ppm
water,
the
supramolecular polymers are completely destabilized
when more than 8 mM NPrMal or 100 mM PhSH is
present. In contrast, in dry MCH*, containing trace
amounts of water that are not detectable by Karl-
Fisher titration, the polymers are still stable in the
presence of more than 10 mM NPrMal and
destabilization by PhSH is considerably less effective.
Similar results are also obtained for titrations with
MA. In the combined presence of PhSH and NPrMal,
but absence of MePip, Zn-1 polymers only
Cooperative interactions between water and
reaction substrates decrease rigidity and length.
To identify which interactions are causing the
depolymerization of Zn-1 by the substrates of the
Michael reaction, we performed titrations of Zn-1
with the NPrMal and PhSH in the absence and
presence of MePip in both dry and ambient MCH*
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Figure 4 Titrations of 10 μM solutions of Zn-1 with NPrMal (a) and PhSH (b) in dry (squares) and ambient (circles)
MCH*. Hollow symbols indicate the absence of 0.5 mM MePip and solid symbols indicate the presence of 0.5 mM MePip
in the solutions. In the case of the ambient titrations, [H2O] = 13.8 ppm. In the case of the dry titrations, the concentration
of water was below the detection limit of the Karl-Fisher titrator.
depolymerize when water is present. (Figures S17
and S18). Together, the titrations show that water is a
prerequisite for the depolymerization of the
supramolecular polymers by the substrates of the
Michael reaction. However, the titrations do not
clearly indicate
a
mechanism via which
depolymerization occurs. To further probe the
mechanism, several control experiments are
required.
Nucleophilicity and bulkiness of catalyst and
reagents control depolymerization. To investigate
whether the basicity or nucleophilicity of the MePip
catalyst causes the destabilization, we performed
control titrations in dry MCH* of Zn-1 with NPrMal in
the presence of either non-nucleophilic, basic Me4Pip
or nucleophilic, weakly basic pyridine (Figure 5a).
These titrations show that when 0.5 mM pyridine is
present, a similar destabilization of the polymers by
NPrMal is observed as when 0.5 mM MePip is used
(see also Figures S25a-c).34 The results obtained for
PhSH and MA show similar trends (Figure S26). In
contrast to pyridine, the less nucleophilic Me4Pip
does not induce depolymerization by NPrMal as
measured by CD spectroscopy (see also Figures
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S25d-f). This difference indicates that nucleophilicity
rather than basicity of MePip causes the
depolymerization of the Zn-1 polymers.
porphyrin core of the supramolecular polymers. As
such, the decreased destabilization upon increasing
the steric bulk of the substrates corroborates that the
substrates, water and the catalyst act in concert
during the destabilization of the polymers.
The role of the nucleophilic amine in the
depolymerization of Zn-1 polymers is further
illustrated by a titration of H2-1, which does not
contain the Zn-center that can complex nucleophiles.
When H2-1 is titrated with NPrMal, PhSH or MA in
the presence of MePip, no decrease in CD intensity is
observed (Figure 5b and S27). The absence of any
change in CD intensity upon addition of the additives
in the presence of the amine catalyst indicates that all
monomers remain fully polymerized. Thus,
complexation of the nucleophilic amine of the MePip
catalyst to the zinc center of Zn-1 is an additional
crucial factor in the destabilization of Zn-1 polymers
upon addition of the substrates of the Michael
reaction. Together with the water-enhanced
interaction of the substrates with the polymers, the
nucleophilic complexation of MePip to the zinc core
of Zn-1 induces the depolymerization.
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Lastly, the influence of substrate structure on the
depolymerization is illustrated by titrations of Zn-1 in
ambient MCH with the bulkier N-tert-butylmaleimide,
tBuMal, and 4-tert-butylthiophenol, tBuPhSH,
substrates in the presence of 0.5 mM MePip. These
Figure 5 Titrations of (a) 10 μM solutions of Zn-1 with NPrMal with 0.5 mM of either Me4Pip or pyridine in the system
and (b) 10 μM solution of H2-1 with NPrMal with 0.5 mM MePip present in the system. In the titration of H2-1, the CD
maximum at 390 nm is probed. The titrations were performed in dry MCH* with [H2O] below the detection limit of the Karl-
Fisher titrator.
titrations show that the bulkier substrates destabilize
the supramolecular polymer to a lesser extent.
(Figure S30). Together with the decreasing angular
dependency of the scattering intensity in the presence
of the substrates (Figure 4), the decreased
destabilization of the bulky substrates hints at a
molecular interaction of the substrates close to the
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amine to the Zn-center of the porphyrin core (Figure
6). Consequently, the cooperative interactions
between water, amine catalyst, polar substrates and
the helix macrodipole lead to the depolymerization of
the Zn-1 polymers in the Michael reaction.
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Interference of the reaction in polymer stability
observed for various substrates. The results above
show that water plays a crucial role in controlling
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interference of
a
chemical reaction in the
supramolecular polymerization of Zn-1. Changing the
amount of water present in the system, which has
only a marginal effect on the covalent Michael
reaction, results in
a
strong effect on the
supramolecular polymers (Figure S32). Moreover, the
chemical reaction introduces a temporally controlled
destabilization of the supramolecular polymer as the
reagents are converted to the Michael product, which
has a subtly different interaction with the polymer.
Thus, by controlling the amount of water in the multi-
component supramolecular system, chemical
reactivity can be coupled to or decoupled from the
structural integrity of the polymer material.
Figure 6 Cartoon representation of a proposed structure
of the supramolecular complexation at the polymer chain
ends. The cooperative interactions between water,
NPrMal and the complexation of MePip to the Zn-center
in the porphyrin stabilize the free chain end of the
supramolecular polymer. This stabilization eventually
leads to the gradual breakdown of the supramolecular
polymer. Experimental evidence for the interactions is
indicated in the gray text.
Lastly, we show the generality of the influence of
polar substrates of covalent reactions on the stability
of non-covalent aggregates. For this, we performed
Michael reactions between PhSH and 2-cyclohexen-1-
The requirement of a nucleophilic site in the
catalyst and low steric hindrance in the substrates
allows us to speculate on the molecular picture of the
interaction that causes the destabilization of the
polymers. The SLS results observed indicate that the
polymers become more flexible and shorter in the
presence of water. This suggests that water can
induce defects in the hydrogen-bonded backbone of
the polymer. This proposed chain breakage leads to
the generation of dynamic and free chain ends, where
the supramolecular polymer presents polar amides to
the bulk solution. The polar substrates of the Michael
reaction can then bind at these chain ends, stabilizing
the shorter polymer chains. Interestingly, the
macrodipoles of the chain ends42 of the helical
peptides are well known to bind polar molecules,43–45
while water and the helix macrodipole are also known
to play a role in collagen bundling.46,47 We hypothesize
that in an analogous fashion, the polar and
monomerically dissolved water molecules39 greatly
enhance the complexation of the substrates to the
chain ends of the Zn-1 polymers. This interaction
stabilizes the free chain ends of short polymers and
oligomers, while in turn destabilizes the long
polymers. The affinity of water and the additives to
macrodipoles at the chain end also explains bundling
of the polymer bundles in the absence of water. The
bundling may shield the non-stabilized macrodipole
from the aliphatic solution in the absence of water or
reaction substrates or by antiparallel alignment of the
polymers in the bundles, the dipoles are canceled. In
the presence of the amine catalyst, the free chain ends
are additionally stabilized by the complexation of the
Figure 7 a) The MePip catalyzed Michael reaction
between PhSH and CycHex in MCH*. b) The time-
dependent absorption changes of 10 μM solutions of Zn-
1 containing 10 mM PhSH, 10 mM CycHex and 0.5 mM
MePip. In the ambient sample, [H2O] = 30.0 ppm and in
the dry sample [H2O] = 11.4 ppm.
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one, CycHex. Although the base-catalyzed Michael
various substrates highlights the general role of water
in these supramolecular systems.
reaction occurs with lower rates (Figure S33), a
strong effect of water on the destabilization of Zn-1
polymers is also observed in this reaction mixture
(Figure 7). In ambient MCH*, rapid depolymerization
occurs when 10 mM of the CycHex and PhSH reaction
substrates and 0.5 mM MePip catalyst are present,
while in dry solutions, no effect is observed. In
addition, the spectral changes during the
depolymerization are identical to the changes
observed with NPrMal (Figure S34), suggesting that
the molecular features of the interactions are also
identical. As such, water in alkane solvents seems to
play a pivotal role in the design of multicomponent
systems that not only display supramolecular
structure but are also coupled to covalent chemical
synthesis.
We anticipate that the incorporation of chemical
reactivity in supramolecular polymers will lead to a
new and exciting area of adaptive materials. To
accelerate the design of such materials, it is of
paramount importance that all interactions in these
systems are understood. As other results37 have
recently shown, water seems very likely to play a
crucial and ubiquitous, yet poorly understood role. By
taking the interactions of water with supramolecular
systems into account, it will be possible to combine
great control over supramolecular polymers in apolar
environments with the diversity of chemical
reactions. We hope this approach will enable the
design of increasingly adaptive and responsive
materials that may rival the complexity of natural
systems.
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■ CONCLUSIONS
■ ASSOCIATED CONTENT
Nature is an inexhaustible source of inspiration for
complex systems where assembly processes and
chemical reactions are operating in in great harmony.
It will take long before the stellar performance of
nature can be observed using non-natural structures.
However, the aim to unravel this complexity using
artificial molecular systems is an important next step
in chemistry.
Supporting information
The supporting information is available free of charge at
DOI 00…./xxxxxx
Synthetic procedures and characterization of
compounds, additional UV-Vis and CD spectra, AFM
images.
Here, we have shown that minute amounts of water
can play
a
crucial role in porphyrin-based
■ AUTHOR INFORMATION
multicomponent supramolecular systems in alkane
solutions. In apolar MCH* solutions, supramolecular
polymers of Zn-1 are highly stable against every
separate component of a base-catalyzed Michael
reaction of NPrMal with PhSH. However, in the
combined presence of all components, including the
product of the reaction, the polymers are readily
Corresponding author:
* e.w.meijer@tue.nl
ORCID
Mathijs F.J. Mabesoone: 0000-0003-1314-7215
Anja R.A. Palmans: 0000-0002-7201-1548
E.W. Meijer: 0000-0003-4126-7492
destabilized
and
form
small
aggregates.
Serendipitously, we found that the presence of water
in the apolar solvent crucially regulates this
destabilization of the polymers by the reaction
substrates.
Notes
The authors declare no competing financial interest.
A combination of spectroscopic titrations, light
scattering and AFM experiments indicates that water
induces flexibility in the supramolecular polymer and
breaks up bundled polymers. We propose that the
destabilization of the supramolecular polymers is due
to binding of water and the substrates at the chain
ends and subsequent cooperative binding of the
catalyst to the metal center. This destabilization
consequently depolymerizes the polymers into small
aggregates.
■ ACKNOWLEDGEMENTS
Ilja K. Voets is acknowledged for fruitful discussions. We
acknowledge financial support from NWO (TOP-PUNT
Grant 10018944) and the Dutch Ministry of Education,
Culture and Science (Gravitation program 024.001.035).
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