Photo- and thermoresponsive supramolecular assemblies: Reversible photorelease of K+ ions and constitutional dynamics

Article abstract of DOI:10.1002/anie.201210334

Freed at the flick of a photoswitch: An acyl hydrazone forms supramolecular assemblies upon the binding of specific salts, such as potassium thiocyanate. This process presents cation and anion selectivity as well as thermo- and photoreversibility (see scheme). In particular, the system displays photorelease of potassium cations and undergoes dynamic component exchange in the acyl hydrazone.

Full text of DOI:10.1002/anie.201210334

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Angewandte
Communications
DOI: 10.1002/anie.201210334
Photoresponsive Materials
Photo- and Thermoresponsive Supramolecular Assemblies: Reversible
+
Photorelease of K Ions and Constitutional Dynamics**
Ghislaine Vantomme and Jean-Marie Lehn*
The stimulus-controlled self-assembly of long-range organ-
ized supramolecular entities is a topic of active investigation,
in particular for its potential implementation in (photo-,
isomerization from (E)-1 to (Z)-1, which cannot bind the
cation (Scheme 1, Figure 1).
[1,2]
thermo-) responsive soft-matter devices and materials.
On
the other hand, materials containing reversible covalent
bonds may undergo constitutional change by the incorpora-
tion or exchange of components, and may thus behave as
[
3,4]
dynamic materials.
The combination of these three fea-
tures: supramolecular self-assembly, constitutional dynamics,
and stimulus responsiveness, is especially attractive. We
report herein such a case, in which supramolecular assemblies
generated from complexes of dynamic ligand molecules may
be dissociated by heating or irradiation with light, with the
photorelease of a metal ion.
Scheme 1. Photoisomerization of ligand 1 from its (E)-1 configuration
+
À
in the complex units [{((E)-1) K }SCN ] that form the supramolecular
2
crystalline assemblies to the (Z)-1 isomer, with release of the potas-
sium cation. Thermal back isomerization restores the metallosupramo-
lecular assemblies.
[
5]
Previous studies by our research group showed that acyl
hydrazones, such as 1–4, bind cations through the tridentate
[
6]
NNO binding site and have the very attractive feature of
Figure 1. Observation of the solid-state fiberlike crystalline material
+
À
[
{((E)-1) K }SCN ] by optical microscopy.
2
n
being double dynamic entities capable of 1) configurational
dynamics by E-to-Z photoisomerization stabilized by internal
[
7]
hydrogen bonding to the pyridine moiety and 2) constitu-
tional dynamics by component exchange involving the
reversible acyl hydrazone bond.
Acyl hydrazone 1 was found to form a complex with
À1
KSCN with a binding constant of 24.8 Æ 4.6m and 1:1
[3,4,8]
With the system de-
stoichiometry (from a Job plot; see Figure S1) in acetonitrile
1
scribed herein, we explored both of these features. Crystalline
supramolecular assemblies were generated reversibly by the
binding of a cation by ligand 1 and its release by photo-
solution at 258C, as determined from H NMR spectroscopic
data (see Figures S2 and S3 in the Supporting Information).
When the concentration of ligand 1 reached 70 mm in the
presence of KSCN (35 mm) at 258C (in a thermostatic bath),
a material composed of thin fiberlike objects (see Figure S4)
was generated. Crystals suitable for X-ray diffraction analysis
were obtained by the slow diffusion of diethyl ether into
a solution of 1–KSCN in acetonitrile. Determination of the
solid-state molecular structure showed that the crystalline
material is composed of assemblies of the complex [{((E)-
[*] G. Vantomme, Prof. J.-M. Lehn
Institut de Science et d’Ingꢀnierie Supramolꢀculaires (ISIS)
Universitꢀ de Strasbourg
Allꢀe Gaspard Monge, 67000 Strasbourg (France)
8
E-mail: lehn@unistra.fr
+
À
[
**] G.V. thanks le Ministꢁre de l’Enseignement Supꢀrieure et de la
Recherche for a doctoral fellowship. We thank Lydia Brelot and
Corinne Bailly for crystal-structure determination and Prof. Chris-
topher Hunter for the program used to determine binding
constants. We also thank the ANR (2010 BLAN-717-1 project) and
the ERC (Advanced Research Grant SUPRADAPT 290585) for
financial support.
1
) K }SCN ], in which ligand 1 in its (E)-1 form coordinates
2
+
to the K cation with 2:1 stoichiometry (Figure 2). These
complexes stack on top of each other at a distance of 4.696 ꢀ
between two successive flat ligand molecules (or K cations)
+
+
to form a long channel of K cations. The cations hold the
assembly together by also interacting with the oxygen sites of
+
the complexes above and below. The K ···O distances are
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201210334.
similar (2.90 ꢀ) within a complex and between stacked
3
940
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complexes: it may be large enough to accommodate perchlo-
rate but not triflate or methanesulfonate.
The combined selectivity towards both cations and anions
is illustrated by experiments involving the successive addition
of salts to a solution of 1. Separately, tetrabutylammonium
thiocyanate and potassium hexafluorophosphate did not yield
assemblies, but did so when mixed together. Various combi-
nations of salts were investigated for their ability to induce
assembly formation (at different concentrations in acetoni-
trile as well as in methanol in a few cases; see Table S1). The
results agree with the selectivities indicated above.
Heating of the solution containing the assemblies caused
their disappearance and gave a clear solution at 50 and 658C,
respectively, for 100 and 200 mm solutions of 1 in acetonitrile
with KSCN (0.5 equiv; see Figure S8). The process is rever-
sible: cooling of the solution regenerated the assemblies. The
melting temperature of the assemblies also depended on the
ligand and salt used (see the Supporting Information).
The crystalline assemblies described above were found to
have interesting photochemical properties. Acyl hydrazones
Figure 2. X-ray solid-state molecular structure of the crystalline
metallosupramolecular assemblies formed by the complex
+
À
[
{((E)-1)
2
K }SCN ]
n
: a) top view; b) side view showing the interaction
À
between adjacent units in each stack of the complex. The SCN anions
are disordered.
[
10]
[5]
complexes. The thiocyanate anions are disordered and occupy
the space between two stacks by forming hydrogen bonds
with the NÀH group of 1 (average N···N,S distance: 3.275 ꢀ).
such as 1 underwent photoinduced E-to-Z conversion. The
+
À
n
[{((E)-1) K }SCN ] assemblies obtained in acetonitrile were
2
found to disappear upon irradiation (1 h) with UV light (1
absorbs at 300 nm), which led to the formation of (Z)-1 in
Only a 1:1 complex was detected in solution, probably as the
result of an even weaker binding constant for the second
ligand than for the first. Whereas the free ligand 1 is expected
to adopt a W-type conformation, it undergoes conformational
switching to the U form by rotation about the ÀNHÀC(=O)
1
solution, as observed by H NMR spectroscopy. Similar
results were obtained in methanol (see the Supporting
Information). In both solvents, the photoinduced (E)-1-to-
(Z)-1 configurational transformation of the ligand in the
[
5,9]
+
amide bond on binding to the metal cation (see Scheme 1).
complex caused the concomitant release of K ions into the
Crystals of 3–KSCN assemblies were also obtained. The
assemblies of 3–KSCN had the same structure as that
observed for assemblies of 1–KSCN (see Figure S6).
solution (as (Z)-1 cannot bind the cations) and the disappear-
ance of the fiberlike assemblies (Figure 1). Thus, in acetoni-
trile, the total amount of cations in solution underwent
a significant change from 36% to the full amount of the added
salt; the cations were in part free and in part complexed by
nonisomerized ligand, according to the binding constant.
Kinetic studies showed a slightly lower rate of E-to-Z
isomerization in the solution containing the assemblies
To gain insight into the features that control this
remarkable self-assembly process, we carried out a number
of experiments. The yield for the formation of the assemblies
was determined to be 64% at 258C and 85% at À108C for
a solution of ligand 1 (100 mm) and KSCN (50 mm) in
acetonitrile; 36 and 15% of the ligand remained in solution,
respectively. The failure of acyl hydrazones 5 and 6 to
generate assemblies underlines the importance of both the
À2
À1
(5.33 ꢁ 10 min ) than in the solution of 1 alone (5.68 ꢁ
À2
À1
10 min ), probably owing to the stabilization of the E
+
state by the binding of K both in the assemblies and in
+
pyridine moiety, which binds to K , and the hydrogen bond
solution in acetonitrile (see Figures S14 and S15). Moreover,
in methanol, a solvent that disrupts hydrogen bonding, the
crystalline assemblies were regenerated by the return from
the metastable state (Z)-1 to the stable state (E)-1 after 2 days
at room temperature (see Figure S16). The rate of the thermal
back conversion from (Z)-1 to (E)-1 was faster in the
presence of KSCN or potassium triflate (conditions under
which no assembly occurs with triflate anions) than in the
absence of potassium salts, which shows that the return
transformation was somewhat accelerated by the formation of
the weak complex in solution (see Figures S17–S19). Heating
to 408C in the presence of KSCN (1 equiv) accelerated the Z-
to-E return transformation by a factor of 6 (see Figure S20).
Photoinduced dissociation of the assemblies also occurred in
acetonitrile, but they were not reformed in 1 month at 258C
owing to the higher stability of (Z)-1 in this solvent.
À
between the NÀH group and the SCN anion. The assemblies
formed selectively with potassium, rubidium, and ammonium
+
cations, but not with sodium or cesium cations, although Na
À1
also binds to 1 (K = 33.0 Æ 4.1m at 258C for NaSCN in
f
+
acetonitrile). The absence of assembly formation with Na
may be related to its smaller size (with a radius of about
+
0
.95 ꢀ, compared to 1.33 ꢀ for K ), which makes it unable to
bind to neighboring stacked ligands.
Assembly formation was found to depend on the geom-
etry of the anions (50–200 mm solution). It occurred with
linear triatomic anions (such as thiocyanate, azide, and
cyanate) as well as with anions of tetrahedral geometry
(
such as perchlorate), but not with octahedral (such as
hexafluorophosphate) or trigonal anions (such as acetate
and benzoate). Moreover, anion selection occurred in the
tetrahedral series: perchlorate gave assemblies, whereas
triflate and methanesulfonate did not. A possible explanation
rests in the space available between the two potassium
Irradiation of the fiberlike material in the solid state and
observation by optical microscopy revealed a change in their
morphology from well-defined crystalline sticks to amor-
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phous stains (see Figure S21). Thus, the complexes themselves
in the crystalline material are photolabile.
The rate of exchange decreased by 30% in the absence of
KSCN (Figure 3) and increased by a factor of 6 and 10 with
KSCN (5 equiv) and barium perchlorate (1 equiv; a stronger
binder of ligand 5), respectively. It remained unchanged in the
presence of potassium triflate (1 equiv; conditions under
which no crystalline assemblies are formed; see Figures S22–
S26). After 17 days, the amount of 1 was 7% larger in the
presence of KSCN than in its absence.
The presence of a reversible C=N bond in compounds 1–6
allows for the exchange of molecular subunits by a transimi-
[3,4,8]
nation reaction
(Scheme 2) and thus for a dynamic
constitutional modification in connection with the assembly
process. When acyl hydrazone 5 and KSCN were mixed in
Thus, although the formation of a complex
and its assembly induced rather weak
exchange catalysis and component selec-
tion (relative to that observed for other
[
3,4]
systems),
constitutional dynamics ena-
bled the generation of the entity able to
form the supramolecular assemblies.
In conclusion, acyl hydrazones such as
1
underwent self-assembly into long-
range organized fiberlike supramolecular
entities on binding specific metal salts.
The formation of the crystalline assem-
blies depended greatly on the charge and
size of the cation as well as on the
geometry of the anion. These assemblies
displayed a reversible response to the
physical stimuli heat and light. Heating of
Scheme 2. Dynamic covalent exchange reaction between acyl hydrazone 5 and benzhydrazide
to form (E)-1 and its assemblies by the complexation of KSCN in methanol.
methanol at 258C, no assemblies were formed. The addition
of benzhydrazide to the solution induced a slow hydrazide
exchange, which led to the formation of acyl hydrazone
the solution led to their gradual dissolution, and they were
regenerated upon simple cooling. Irradiation of the crystal-
line fiberlike assemblies induced configurational photoiso-
merization of the ligand from (E)-1 to (Z)-1 and caused their
disappearance. The process is thermoreversible: the crystal-
line assemblies were regenerated over time by thermal return
from the metastable state (Z)-1 to the stable state (E)-1.
The system displayed photoinduced cation release. On
photolysis of the crystalline metallosupramolecular assem-
blies, the release of the potassium cations, which are not
bound by the photogenerated form (Z)-1, led to a significant
increase in the cation concentration. This feature is of general
interest, as it may be used to trigger a process that depends on
1
(Figure 3). After 10 days, fiberlike assemblies appeared in
solution, at the same rate as the exchange. Thus, covalent
dynamics enabled the conversion of acyl hydrazone 5, which
cannot form assemblies, into 1, which can. On irradiation of
the mixture, the same behavior as described above was
observed, with the disappearance of the crystalline assemblies
as a result of (E)-1-to-(Z)-1 photoconversion.
[
11]
an ionic threshold. These effects hold for both the cations
and the anions, which are also released on photodestruction of
the assemblies. Acyl hydrazones such as 1 display double
shape switching under the action of different, orthogonal
effectors: a conformational W-to-U conversion on cation
binding and a photoinduced configurational change from the
E to the Z form. Finally, the metallosupramolecular crystal-
line assemblies behave as covalent dynamic materials, as they
incorporate reversible covalent bonds, which allow for con-
stitutional variation by component exchange under metal-ion
catalysis. Thus, a compound unable to undergo assembly may
be converted progressively into a compound capable of
generating a self-assembled fiberlike material.
Received: December 28, 2012
Revised: February 1, 2013
Figure 3. Ratio of the total amount of 1 formed (present as (E)-1+
Published online: February 28, 2013
(
Z)-1 in solution and later as (E)-1 in the assemblies) by dynamic
covalent exchange between 5 (300 mm) and benzhydrazide (1 equiv) in
the presence (&) and absence ( ) of KSCN (1 equiv) in methanol with
&
Keywords: crystalline assemblies · photolysis · photoswitches ·
potassium ions · self-assembly
respect to the amount of 5. After 15000 min (10 days), the slope
changes slightly as fiberlike crystalline objects appear.
.
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