A Photoswitchable Heteroditopic Ion-Pair Receptor

Article abstract of DOI:10.1021/jacs.8b08689

Designing light-switchable heteroditopic receptors is challenging because it necessitates simultaneous (de)activation of two separate binding sites. Herein, we present the first photoswitchable heteroditopic ion-pair receptor in which both cation and anion binding sites are simultaneously and reversibly switched OFF and ON by a single photoswitch. Our receptor is simple, low molecular weight, and readily synthesized from commercially available precursors. Single-crystal X-ray structures and NMR spectroscopic titrations support ion-pair binding to the receptor both in the solid state and in solution, with strong positive cooperativity between the cation and anion binding. The receptor can be completely switched OFF by UV light-triggered photoisomerization of an acylhydrazone C=N double bond and remains kinetically stable in the deactivated form due to an intramolecular hydrogen bond. Its re-activation could be achieved by light irradiation or, more effectively, by fast acid-catalyzed back-isomerization. Our simple photoswitchable ion-pair receptor may serve as a blueprint for the design of new generations of switchable receptors, transporters, soft materials, and self-assembled systems, where incorporation of a functional heteroditopic ON/OFF photoswitch has been challenging up to now.

Full text of DOI:10.1021/jacs.8b08689

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A Photoswitchable Heteroditopic Ion Pair Receptor
Zoran Kokan, and Micha# Jan Chmielewski
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A Photoswitchable Heteroditopic Ion Pair Receptor
,
†,‡
,†
Zoran Kokan* and Michał J. Chmielewski*
†
Faculty of Chemistry, Biological and Chemical Research Centre, University of Warsaw, Żwirki i Wigury 101,
0
‡
2-089 Warszawa, Poland
Division of Materials Chemistry, Ruđer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia.
Supporting Information Placeholder
cessfully implemented in ion pair receptors, including ion metath-
esis, pH, and redox control.
ABSTRACT: Designing light-switchable heteroditopic receptors
is challenging because it necessitates simultaneous (de)activation
of two separate binding sites. Herein, we present the first pho-
toswitchable heteroditopic ion pair receptor in which both cation
and anion binding sites are simultaneously and reversibly
switched OFF and ON by a single photoswitch. Our receptor is
simple, low molecular weight, and readily synthesized from
commercially available precursors. Single-crystal X-ray structures
and NMR spectroscopic titrations support ion pair binding to the
receptor both in the solid state and in solution, with strong posi-
tive cooperativity between the cation and anion binding. The
receptor can be completely switched OFF by UV light-triggered
photoisomerization of an acylhydrazone C=N double bond and
remains kinetically stable in the deactivated form due to an intra-
molecular hydrogen bond. Its re-activation could be achieved by
light irradiation or, more effectively, by fast acid-catalyzed back-
isomerization. Our simple photoswitchable ion pair receptor may
serve as a blueprint for the design of new generations of switcha-
ble receptors, transporters, soft materials, and self-assembled
systems, where incorporation of a functional heteroditopic
ON/OFF photoswitch has been challenging up to now.
4
2–44
To address the challenge of photoswitching in ion pair recep-
tors, we have considered a recently recognized, widely tunable,
4
5
yet underexploited class of photoswitches – acylhydrazones.
Acylhydrazones are versatile dynamic systems that operate under
45
46
47,48
49
light, thermal, pH
and even redox control. In particular,
acylhydrazones derived from pyridine 2-aldehyde possess poten-
tial anion and cation binding sites (i.e. hydrogen bond donors and
electron lone pairs) that form an intramolecular hydrogen bond
upon light-induced E/Z photoisomerisation of a C=N double
4
7,50,51
bond.
Thus, in the Z form the two recognition elements
effectively cancel out each other from interactions with other
species. This feature of 2-pyridyl acylhydrazones has already been
52
exploited in photoswitchable molecular shuttles, enantioselec-
7
46
tive catalysts and supramolecular assemblies. This led us to the
idea that incorporating a 2-pyridyl acylhydrazone switch into the
ion pair receptor could enable the simultaneous photo-induced
disruption of both cation and anion binding. Furthermore, acylhy-
drazones have often been used in cation and anion
47,50,53,54
recognition,
although their light triggered catch/release
46
functionality has rarely been exploited.
Herein, we present the first example of a photoswitchable het-
eroditopic ion pair receptor, E-1 (Scheme 1). E-1 is composed of
Irradiation by light is one of the most attractive ways to control
the properties of a chemical system. It has been utilized in a num-
the 2-pyridyl acylhydrazone photoswitch and 2,6-pyridine bisam-
18–23,55
ide – a privileged anion recognition motif.
majority of ion pair receptors,
In contrast to the
1
ber of photoresponsive host-guest systems, supramolecular as-
25,26
E-1 is a simple, low molecular
2,3
4,5
semblies and molecular devices, with applications in cataly-
weight receptor and could be prepared in three simple steps from
commercially available precursors (Scheme S1). Upon UV light
irradiation E-1 readily transforms into the Z-1 isomer, which is
remarkably stable and can be easily isolated by column chroma-
tography.
6–8
9,10
11,12
sis, materials chemistry
a few. Photoswitchable receptors
and cargo delivery,
just to name
1
3,14
are an important class of the
investigated systems because they facilitate the photo-controlled
catch and release of chemical species. Due to the biological,
environmental and industrial importance of charged species,
1
5–17
18–24
25–31
receptors for cations,
anions,
and ion pairs
have been
actively investigated. In particular, the ion pair receptors offer
distinct advantages over single ion receptors in terms of affinity
Scheme 1. The design and working principle of E-1
3
2–34
and selectivity due to cooperative effects.
However, while
photo-switching functions have been frequently utilized in cation
1
3,17
recognition,
studies on photoswitchable anion recognition are
1
4,35–41
far less common
and photoswitchable ion pair receptors
have not been reported yet. This may be due to the heteroditopic
nature of ion pair receptors, which makes simultaneous
(
de)activation of two different binding sites challenging. Never-
theless, various binding/release strategies have been already suc-
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Figure 1. Single crystal X-ray structures of E-1, Z-1, and complexes of E-1 with LiClO
4
4
, NaI, KI, and NH Br.
54
The X-ray crystal structure of E-1 shows a pre-organized anion
binding site with two amide groups adopting a syn-syn confor-
mation (Figure 1). On the contrary, the 2-pyridyl nitrogen atom
points away from the cation binding site, possibly due to lone
pairs repulsion. Five X-ray crystal structures of ion pair complex-
es of E-1 were also obtained (Figure 1, see also SI). All of them
support the proposed general binding mode and show cations and
anions in their respective binding sites depicted in Scheme 1.
alkali metal cations and, in a similar fashion, hydrogen bonding
to the ammonium cation. The ability of E-1 to bind anions and
cations of various geometries is of general interest because of
potential incorporation in a broader range of supramolecular
systems. Finally, the X-ray structure of Z-1 shows the receptor in
the ‘closed’ form, in which the 2-pyridine side arm occupies the
anion binding site.
Initial evidence of ion pair binding to E-1 in solution was ob-
–
Anions are bound to E-1 by two N–H···A hydrogen bonds and
1
tained from H NMR spectra in acetonitrile-d
3
(Figure 2). In the
–
exhibit also short C–H···A contacts. On the other hand, the cation
presence of TBABr (5 equiv.), significant downfield shifts were
observed for both NH and imine CH protons (Δδ ≈ +1 ppm) due
to anion binding, as could be anticipated from X-ray crystal struc-
binding site shows two binding modes: O,N,N coordination to
tures. On the contrary, with LiBPh
slightly, while significant change was observed for the 2-pyridyl
-H proton (Δδ ≈ –0.2 ppm). This was attributed to the rotation of
4
these protons moved only
3
the 2-pyridyl ring, as evident from the crystal structures and 2D
ROESY measurements (Figures S14, S15). In the presence of
LiBr, however, much more pronounced shifts were observed for
all signals sensitive to both anion and cation binding (Δδ ≤ 2.3
ppm and Δδ ≈ 0.3 ppm, respectively), as compared to TBABr and
4
LiBPh . This is highly indicative of LiBr ion pair binding to E-1.
In striking contrast, the Z-1 isomer exhibits only very small chem-
ical shifts changes in the presence of LiBr (Δδ ≤ 0.08 ppm), sug-
gesting very weak interactions with the salt.
To investigate the ion pair binding to E-1 in more detail, we
1
performed H NMR spectroscopic titrations with model alkali
metal halide salts (LiCl, LiBr, LiI, NaBr, NaI, KI), as permitted
by their solubility in acetonitrile-d
plexity of ion pair binding equilibria,
anion, and ion pair binding equilibria as well as the competing ion
3
. Due to the irreducible com-
2
2,23
all individual cation,
pairing equilibria were included in the ion pair binding model
56
(
Figure 2, see also SI).
+
Titrations of E-1 with individual anions and cations (as TBA
–
and BPh
4
salts, respectively) in acetonitrile-d
3
show excellent fits
Figure 2. Equilibria included in the ion pair binding model used
for fitting titration data and H NMR spectra of E-1 and Z-1 in the
to the simple 1:1 binding model (Figure S20). The equilibrium
constants obtained for anion binding follow the order of decreas-
1
–
–
–
absence and in the presence of various salts (2 mM, acetonitrile-
ing charge densities: Cl > Br > I (Table 1). Similarly, the ob-
+
+
+
d
3
, 5 equiv. of each salt). The chemical shift changes suggest
served trend of Li > Na > K comes from stronger coordination
strong cooperative binding of LiBr to E-1 and only very weak
interaction with Z-1 (see SI for details).
of E-1 to smaller cations with higher charge densities. This agrees
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(
a)
Table 1. The equilibrium constants (logK or logβ) and
1.5
(
a)
(b)
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cooperativity factors (α) for anion, cation and ion pair
binding to E-1 in acetonitrile-d
.
E-1, LiBr
3
1
0
0
.0
.5
.0
logK or logβ (α)
–
–
–
–
Ph
4
B
Cl
Br
I
−
+
E-1, Br
TBA
-
2.80
1.96
< 1
δ
+
Li
2.08
1.54
1.28
6.24 (23)
5.50 (29)
4.86 (26)
4.24 (15)
3.43 (8)
3.20 (9)
+
(c)
Na
/
+
+
(c)
(c)
E-1, Li
0
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K
/
/
Z-1, LiBr
(a)
average values from two experiments. Estimated errors ≤ 10 %.
×K ). For iodide, K was set as 10 M , even though
A C A
experiments suggest it to be lower. Therefore, the values of coop-
erativity factors α for iodide salts are lower estimates only.
solubility issues did not permit titration studies.
(b)
–1
α = βtot/(K
0
1
2
3
4
5
salt equivalents
(c)
(b)
Z-1, LiBr
with the solid state structures (Figure 1), where the bond lengths
0.0
+
+
+
with E-1 follow the order Li < Na < K .
−
E-1, Br
The titration experiments performed in the presence of both
halide anions and alkali metal cations revealed strong and highly
binding of ion pairs to E-1 (Table 1, Figure 3)
The observed cooperativity is unusually high for halide salts
binding to receptors in acetonitrile solution.
erativity factors were obtained in less competitive, chlorinated
Given the rigid, fairly planar structure of E-1,
the observed cooperativity is more likely due to electrostatic,
3
2–34
cooperative
δ
-0.2
+
2
7,57–60
E-1, Li
Larger coop-
33,34
solvent systems.
1
6,61
E-1, LiBr
rather than allosteric
interactions.
-
0.4
In contrast to E-1, the titration of Z-1 with LiBr, LiI and NaI
revealed extremely weak interactions, presumably with the cation,
which could not be properly fit to any model due to a very small
range of chemical shifts changes (Figures S94–99). These obser-
vations show that indeed the E-1 and Z-1 represent the binding
and non-binding forms, respectively, in accord with the working
principle proposed in Scheme 1.
0
1
2
3
4
5
salt equivalents
Figure 3. Chemical shift changes of (a) amide proton and (b) 3-H
proton of the 2-pyridyl moiety during H NMR spectroscopic
titrations of E-1 and Z-1 (2 mM, acetonitrile-d
ing their sensitivity towards anion and cation binding, respective-
ly. The data suggest cooperative binding of Li and Br to E-1 and
very weak interactions with Z-1. Data for Br and Li (as TBA
4
and Ph B salts, respectively) are shown for comparison.
1
3
) with LiBr, show-
+
–
UV-Vis spectra also indicated the ion pair interactions with E-1
Figure S105). The E-1 and Z-1 spectra in acetonitrile (1 mM)
feature absorptions with maxima at 304 and 318 nm, respectively,
in agreement with UV-Vis spectra for similar acylhydrazones. In
the presence of TBABr (20 equiv.), only a small blue shift from
–
+
+
(
–
4
5
The amplitude of switching cycles was greatly improved by al-
ternating UV irradiation with an acid treatment followed by neu-
tralization with a base
4
8,62
3
04 to 300 nm was observed for the E-1 due to anion binding, as
(Figure 4b). The back-isomerization of
supported by NMR spectroscopic titrations. In the presence of an
excess of LiBr (20 equiv.), a fully bound E-1·LiBr is expected to
dominate and, accordingly, the E-1 absorption red shifted from
04 to 315 nm. No spectral changes were observed for Z-1 in the
presence of LiBr (20 equiv.), indicating negligible interactions
(Figure S105).
Z-1 to E-1 was nearly quantitative within a few minutes of triflic
acid addition (1 equiv.). After neutralization with DIPEA, the
binding of LiBr to E-1 was restored (Scheme S21, Figure S103;
DIPEA triflate is not appreciably bound to E-1 in acetonitrile).
This orthogonality of the external stimuli (i.e. photo and chemi-
cal) can be very useful in the design of functional supramolecular
3
1
0,63–65
Reversible switching between the ion pair binding (E-1) and
non-binding (Z-1) forms was attempted both by irradiation with
light of two different wavelengths and by light-triggered E to Z
isomerization followed by acid catalyzed back-isomerization. An
systems.
Nevertheless, using the orthogonal photo and
chemical stimuli in flipping the configuration of hydrazone photo-
acetonitrile-d
LiBr (5 equiv.) provided photostationary states (PSS) of 58 % and
1 % Z-1 after a few minutes of irradiation with 365 and 315 nm
3
solution of E-1 (2 mM) containing an excess of
8
UV light, respectively (Figure S101). Once formed, the Z-1 is
very stable in this solution, as shown by the lack of any changes
in the H NMR spectra even after several days in the dark (Figure
1
S18).
Thus, thermal and ion-induced back-isomerization are negligi-
3
8
ble under these conditions. Switching cycles obtained by alter-
nating irradiation with the two wavelengths of UV light provided
only modest changes in the PSS (Figure 4a), supporting neverthe-
less the possibility of light-induced switching in more carefully
Figure 4. Switching between E-1 and Z-1 in acetonitrile-d
(2 mM) containing 5 equiv. of LiBr, using (a) different wave-
lengths of UV light; (b) UV irradiation and triflic acid/DIPEA
addition (1 equiv.).
3
4
5
optimized systems.
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switches is very rare.
Moreover, such approach to the itera-
10081–10206.
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tive catch and release of a guest molecule is, to the best of our
knowledge, unprecedented.
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In summary, this work presents the first ON/OFF photoswitch-
able heteroditopic ion pair receptor 1, with a simple and modular
design based on the acylhydrazone photoswitch. Despite its sim-
ple structure, E-1 effectively binds various alkali metal salts in
acetonitrile and shows strong cooperativity between the cation
and anion binding. The receptor exhibits two very stable configu-
rational isomers, E-1 and Z-1, which represent binding and non-
binding forms, respectively. The proposed modes of binding were
unambiguously characterized by X-ray structures and NMR spec-
troscopic investigations. The E-1 was found to bind anions, cati-
ons and ion pairs of different geometries. The photoswitching
between the ON and OFF modes was successfully achieved
through cycles of UV light irradiation with different wavelengths
and, more effectively, by alternating UV light irradiation with
acid/base treatment. Furthermore, the simple modular synthesis
and versatile ion binding properties of E-1 may allow its easy
incorporation in a broad range of supramolecular systems. Poten-
(
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ASSOCIATED CONTENT
Supporting Information
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002, No. 22, 2734–2735.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: xxxxxxxxxxxx
General information, synthetic procedures, spectroscopic charac-
terization, titration data, photoisomerization details (PDF)
X-ray structure of E-1 (CIF)
X-ray structure of E-1·LiClO (CIF)
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X-ray structure of E-1·NaI (CIF)
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X-ray structure of E-1·NH
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Br (CIF)
I (CIF)
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X-ray structure of Z-1 (CIF)
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AUTHOR INFORMATION
9
3.
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Corresponding Authors
Anion Receptors and Sensors. Chem. Soc. Rev. 2009, 38 (2), 506–
519.
E-mail: zkokan@irb.hr, mchmielewski@chem.uw.edu.pl
Notes
The authors declare no competing financial interests.
(25) Kim, S. K.; Sessler, J. L. Ion Pair Receptors. Chem. Soc. Rev. 2010,
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26) McConnell, A. J.; Beer, P. D. Heteroditopic Receptors for Ion-Pair
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27) Zakrzewski, M.; Kwietniewska, N.; Walczak, W.; Piątek, P. A
Non-Multimacrocyclic Heteroditopic Receptor That Cooperatively
Binds and Effectively Extracts KAcO Salt. Chem. Commun. 2018,
ACKNOWLEDGMENT
(
MCh gratefully acknowledges financial support from the Founda-
tion for Polish Science and European Economic Area through
Homing Programme.
5
4 (51), 7018–7021.
(
(
28) Tepper, R.; Schulze, B.; Bellstedt, P.; Heidler, J.; Görls, H.; Jäger,
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