Switchable 3D networks by light controlled π-stacking of azobenzene macrocycles

Article abstract of DOI:10.1039/c2cc35974j

Azobenzene macrocycles were designed for switchable π-stacking interaction. After efficient preparation and characterization of azobenzene macrocycles containing electron rich as well as electron poor elements a dimeric analogue was synthesized, which formed reversibly 3D-networks. Gel formation was observed with aromatic solvents representing the first example of this type based on switchable azobenzene macrocycles. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2cc35974j

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Switchable 3D networks by light controlled p-stacking of azobenzene
macrocyclesw
Raphael Reuter and Hermann A. Wegner*
Received 17th August 2012, Accepted 1st November 2012
DOI: 10.1039/c2cc35974j
Azobenzene macrocycles were designed for switchable p-stacking
interaction. After efficient preparation and characterization of
azobenzene macrocycles containing electron rich as well as
electron poor elements a dimeric analogue was synthesized, which
formed reversibly 3D-networks. Gel formation was observed with
aromatic solvents representing the first example of this type based
on switchable azobenzene macrocycles.
Fig. 1 Schematic representation of 3D-networks built from dimeric
switchable azobenzene macrocycles. In the all-E conformation linked
p-stacks are formed (left). Irradiation induces isomerization to a
mixture of Z-isomers all deviating from planarity, which impedes
the supramolecular interaction (right).
Functionality in molecules is governed by their structure and
their spatial arrangement. In most cases the interactions of
multiple molecular components on a supramolecular level are
crucial for the successful fulfilment of a given task.1 The
reversible control of such supramolecular interactions on a
molecular basis will, therefore, give the power to control the
associated function.2 The azobenzene scaffold is one of the
most prominent molecular entities, which can be reversibly
altered in its spatial arrangement upon irradiation.3–5
Azobenzenes can be changed between the Z and the E con-
formation upon irradiation by light with appropriate
wavelength. This feature has been beautifully applied in a
number of functional systems.6,7 Azobenzenes have also been
used to control supramolecular interactions,8 such as hydrogen
bonds,9,10 metal complex formation11 or hydrophilic/hydrophobic
interactions. Additionally, the azobenzene moiety itself can
participate in the arrangement, which upon isomerization is
inhibited.12 In this case the type of intermolecular inter-
action is mainly limited to p–p interactions.13 In general, the
aggregation of p-stacks is a distinct property of large aromatics
and, in particular, of shape-persistent macrocycles.14–16 The
incorporation of azobenzenes into such disc-like shape-persistent
macrocycles offers the unique opportunity to change the
spatial arrangement from the two-dimensional flat structure to a
three-dimensional geometry allowing alteration of their supra-
molecular assembly.17 This principle has been beautifully applied
by Norikane and coworkers for a switchable liquid crystal
system.18 The detailed molecular interactions were described on
the basis of stacking in the solid-state structure,19 which we also
observed for cyclotrisazobenzenes.20,21 However, this phenomenon
has not manifested in solution so far.
In this report we add an additional dimension to the concept
of switchable p-stacking of azobenzene macrocycles in order
to control 3D networks (Fig. 1). If two macrocycles, capable
of switchable p-stacking, are connected, these building blocks
will not only form columnar assemblies, but also allow
connections between the stacks to build horizontal as well
as lateral networks. With this approach reversibly porous
structures can be created, which are attractive for storage of
small molecules with the advantage of optional release upon
light irradiation.
To realize such a light responsive three-dimensional network
based on switchable building blocks, first, an appropriate
monomeric azobenzene macrocycle was designed to investigate
the crucial switchable p-stacking. The incorporation of phenyl
rings with alternating electron density should enable the necessary
molecular interaction leading to the target macrocyclic structure 1
(Fig. 2).22 Phenyl moieties substituted with ester groups should be
less electron rich compared to the unsubstituted rings.
University of Basel, Department of Chemistry, St. Johanns-Ring 19,
4056 Basel, Switzerland. E-mail: hermann.wegner@unibas.ch;
Fax: +41-61-267-0976
w Electronic supplementary information (ESI) available: Experimental
procedures and analytical data; UV of isomerization of 1. See DOI:
10.1039/c2cc35974j
Fig. 2 p-Stacking interaction of two macrocycles with CO2Bu
residues as EWGs.
c
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Scheme 1 Synthesis of butyl ester substituted trisazobiphenyl macrocycles 1.
Fig. 3 1H-NMR spectra of 1 at different concentrations in CDCl3
before irradiation and after irradiation.
The synthesis relied on the established strategy developed in
our laboratory using Suzuki cross-coupling as well as Mills
reactions.23,24 Aniline 2 was accessed by known procedures
(Scheme 1).25–27 For the introduction of the boronic ester
required for the Suzuki cross-coupling Miyaura borylation
was employed.28 Other methods such as lithiation followed by
Li–B exchange were not suitable as the ester is probably not
stable to the harsh lithiation conditions with BuLi. After
Suzuki cross-coupling of the boronic ester 4 with bromide 2,
biphenyl 5 was obtained. The next step, the Mills reaction with
3-bromonitrosobenzene (6), proceeded with a good yield of
73%. The Suzuki reaction as well as the final oxidative
cyclization with lead tetraacetate furnished the desired macro-
cycle 1 in eight linear steps in an overall yield of 10%.
Scheme 2 Assembly of the macrocycle and connection via click
As anticipated, the NMR shifts showed a strong concentration
dependence suggesting significant p-stacking interaction (Fig. 3).
Upon irradiation (365 nm, room temperature) the UV as well as
the NMR changed significantly indicating the isomerization from
the all-E to a mixture of the Z-isomers. The NMR of the mixture
of Z-isomers did not show any shift at different concentrations
proving the effective inhibition of the aromatic p-interaction.
Interestingly, an attempt to inhibit the p-stacking by using
benzene as solvent led to an even stronger shift of the NMR-
signals at the same concentration.
reaction to bis-macrocycle 17.
acetylene (9) and phenol 8. This synthetic intermediate 10 was
connected with the benzylic alcohol 12 using the Mitsunobu
protocol. Reduction of the nitro functionality followed by
Suzuki cross-coupling yielded the desired biphenyl building
block 14.
The final transformation to build up the macrocycles began
with a Mills reaction. The following Suzuki cross-coupling
reaction delivered the precursor for the macrocyclization in
quantitative yield. Ring closure and deprotection led to the
macrocyclic alkyne 15, which was then converted to the
bis-macrocycle 17 by click reaction with the diazide 16.
The aromatic peaks of the dimeric macrocycle 17 showed a
significantly larger high field shift compared to the monomer
1 at similar concentrations indicating an even stronger
p–p-stacking for the dimer 17.
Already before the workup of the click reaction, it was
observed that product 17 formed a gel with the solvent
toluene, which could be attributed to the incorporation of
solvent molecules inside the 3D p-stacking network. The
process of gel formation was further explored with different
solvents (Table 1). The effect was only observed for aromatic
With a suitable molecular system for columnar p-stacking in
hand the preparation of a dimeric variation was commenced.
The general strategy was chosen according to the synthesis of
macrocycle 1. It was envisioned to combine two macrocycles
by a click-reaction, a Cu-catalysed [2 + 3] dipolar cyclo-
addition.29,30 Such a transformation required the incorporation
of a suitable coupling partner. For this purpose an alkyne
moiety was incorporated into one of the biphenyl moieties
(Scheme 2). Additionally, the design has been modified by
increasing the length of the alkyl chain in the ester moiety,
which should have beneficial effects on the stacking as well as
on the solubility. The aromatic alkyne part was prepared via
Sonogashira cross-coupling with tri-iso-propylsilyl (TIPS)
c
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Table 1 Gel formation of bis-macrocycle 17 in different solvents
with light. Further studies on the structure and formation of
the gel are ongoing.
Entry
Solvent
Concentrationa
Gel
The authors thank the Swiss National Science Foundation
and SystemsX.ch for financial support. H. A. W. is indebted to
the Fonds der Chemische Industrie for a Liebig fellowship.
1
2
3
4
5
Toluene
o-Xylene
CHCl3
CH2Cl2
THF
10
1.25
5
5
5
Yes
Yes
No
No
No
Notes and references
a
In % m/v.
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Fig. 4 Solution of 17 in ortho-xylene [c = 1.25% (m/v)]. Left: gel
before irradiation. Right: solution after irradiation at 365 nm for 3 h.
solvents. For ortho-xylene gel formation was detected at a
concentration of 1.25% (m/v) (Table 1, entry 2). In other
solvents like CHCl3 or CH2Cl2 this property was not observed
(Table 1, entries 3 and 4). Also in THF no gelification took
place (Table 1, entry 5). The preference for aromatic solvents
could originate from the cavities spanned from the aromatic
gelator, which support p-stacking with the solvent. The gel
was stable for at least one day at room temperature.
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Finally, the switching behaviour was investigated. When the
sample was irradiated with UV light at 365 nm the gel slowly
liquefied and after three hours a clear orange liquid was
obtained supporting the anticipated dissociation of the 3D
networks (Fig. 4). Although there have been numerous reports
on switchable gel systems based on azobenzenes,31,32 as well as
a recent non-switchable example based on shape-persistent
macrocyles,33 the scaffold displayed above is to the best of our
knowledge the first one, which combines these two principles.
Compared to the other examples of switchable azobenzene
gels where a group attached to the azobenzene is responsible
for the network formation required for gelation, it is in this
case the p-stacking of the azobenzene macrocycles, which
provides the necessary non-covalent interaction. Therefore,
the macrocyclic structure is essential for the network–gel
formation. The long alkyl chains might serve as spacers to
provide the necessary cavities for solvent incorporation. A
derivative of 17 with butyl instead of hexyl chains did not
show any gelation ability.
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´
In summary, a new molecular system based on dimeric
trisazobiphenyl macrocycles has been prepared, which
displays switchable p-stacking interaction upon UV irradiation.
This property has been utilized to build up light-controlled
three-dimensional networks, which form switchable gels with
aromatic solvents. The basic principle should also be applicable
to bind other small molecules and release them on demand just
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¨
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.