Photocontrol of Polar Aromatic Interactions by a Bis-Hemithioindigo Based Helical Receptor

Article abstract of DOI:10.1002/chem.201604237

The first example of a bis-hemithioindigo (bis-HTI)-based molecular receptor was realized. Its folding and selective binding affinity for aromatic guest molecules can be precisely controlled by visible light and heat. The thermodynamically stable state of the bis-HTI is the s-shaped planar Z,Z-configuration. After irradiation with 420 nm light only the E,Z-configuration is formed in a highly selective photoisomerization. The E,Z-isomer adopts a helical conformation because of the implementation of repulsive steric interactions. The E,Z-configured helix is able to recognize electron-poor aromatic guests exclusively through polar aromatic interactions and also distinguishes between regioisomers. After heating, the Z,Z-configuration is completely restored and the aromatic guest molecule is efficiently released.

Full text of DOI:10.1002/chem.201604237

A Journal of
Accepted Article
Title: Photocontrol of Polar Aromatic Interactions by a bis-Hemithioindigo Based He-
lical Receptor
Authors: Manuel Guentner; Edgar Uhl; Peter Mayer; Henry Dube
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To be cited as: Chem. Eur. J. 10.1002/chem.201604237
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Chemistry - A European Journal
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Photocontrol of Polar Aromatic Interactions by a bis-Hemithioindigo Based Helical
Receptor
[
a]
[a]
[a]
[a]
Manuel Guentner, Edgar Uhl, Peter Mayer, and Henry Dube*
[
a] M. Sc. Manuel Guentner, M. Sc. Edgar Uhl, Dr. Peter Mayer, Dr. H. Dube
Department of Chemistry, Ludwig-Maximilians-Universität München
Butenandtstr. 5–13, 81377 Munich (Germany)
Fax: (+49) 89 2180-77756
E-mail: Henry.Dube@lmu.de
Abstract
The first example of a bis-Hemithioindigo (bis-HTI) based molecular receptor was realized.
Its folding and selective binding affinity for aromatic guest molecules can precisely be
controlled by visible light and heat. The thermodynamically stable state of the bis-HTI is the
s-shaped planar Z,Z-configuration. After irradiation with 420 nm light only the E,Z-
configuration is formed in a highly selective photoisomerization. The E,Z-isomer adopts a
helical conformation because of the conscious implementation of repulsive sterical
interactions. The E,Z-configured helix is able to recognize electron poor aromatic guests
exclusively via polar aromatic interactions and also distinguishes between regioisomers.
After heating the Z,Z-configuration is completely restored and the aromatic guest molecule is
efficiently released.
Main text
Spatial and temporal control of molecular interactions is a central topic for the generation of
complex molecular functions similar to the ones found in natural systems. At present great
[
1]
[2]
[2b, 3]
efforts are made in supramolecular, medicinal, and biological
chemistry as well as in
[
4]
the materials sciences to implement different control elements for the regulation of
molecular recognition processes. Using light for this purpose is of special high interest and
[
1c, 1e, 5]
numerous systems employ photoswitches as external control unit
or integral part of
A fundamental problem of most photoswitches is their
dependence on UV light, which has to be used for at least one switching step and leads to
[
1d, 3g, 6]
their molecular architecture.
[
7]
severe limitations for instance in biological or materials applications. Additionally, most
molecular interactions that can be altered by light irradiation at the moment are relatively
strong and directional in nature, e.g. hydrogen bonding,
metal-ligand bonds. Weaker and less directed interactions are much harder to control with
precision and have therefore not well been studied in the context of photoresponsive
recognition.
[
1c, 1e, 8]
[6c, 9]
ionic interactions,
or
[
10]
In the following we present the first example of a molecular receptor that undergoes
photoswitching in the visible region of the spectrum and binds guest molecules exclusively by
polar aromatic interactions. Release of the guest is conveniently achieved by a simple
heating step. To this end we have developed a completely new supramolecular system 1,
[
3b, 11]
which is based on Hemithioindigo (HTI) photoswitches.
Upon irradiation with visible light
receptor 1 changes its conformation from a well defined planar geometry into a well defined
folded helix. The helical form is able to bind electron-poor aromatic guest molecules
efficiently while the affinity of the planar form is extremely low. Therefore, a reversible
capture-release cycle can be realized by alternating irradiation and heating steps
interconverting the high and low affinity configurations of 1 in solution.
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Chemistry - A European Journal
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Bis-HTI 1 was synthesized in a short, convergent, and highly modular fashion (Scheme 1).
The central tricyclic bis-thiophenone ring 7 was obtained by twofold intramolecular Friedel-
[
12]
Crafts acylation starting from the already described bis-acid 6. The light sensitive
compound 7 was not isolated but directly subjected to the condensation reaction with two
equivalents of biphenyl aldehyde 5 yielding bis-HTI 1. Aldehyde 5 was synthesized in two
[
13]
[14]
steps via Suzuki cross-coupling from the known precursors 2 and 4 (see Supporting
Information for more details).
Scheme 1. Synthesis of bis-HTI 1. The thermodynamically stable Z,Z isomer of 1 is shown.
Bis-HTI 1 can exist in four different isomeric states depending on the particular configuration
of its two double bonds: Z,Z, E,Z, Z,E, or E,E. The Z,Z configuration is the thermodynamically
stable form, which is also obtained in >95% after heating of an isomeric mixture. After
irradiation of a toluene solution of Z,Z-1 with 420 nm light the E,Z isomer is obtained in 94%
yield (Figure 1). A quantum yield Z,Z/E,Z of 7.0% was determined for this process. Such an
efficient and highly selective photoisomerization in electronically coupled switching systems
[
15]
is usually impossible and a distinct property of bis-HTI 1.
The sole presence of the E,Z configuration after irradiation was directly proven by an
extensive NMR analysis in solution (see Supporting Information for the complete signal
assignment).
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Figure 1. (a) Bis-HTI Z,Z-1 undergoes highly selective (94%) photoisomerization to the E,Z
isomeric form after irradiation with 420 nm light. Heating restores the initial Z,Z isomer. E,Z-1
shows a highly enhanced binding affinity for electron poor aromatic substrates because of its
helical preorganization. The binding of 27DDF (8) via polar aromatic interactions can thus
1
effectively be controlled by irradiation and heat. (b) Aromatic region of the H NMR spectra
(
400 MHz, 300 K, [D ]toluene) of Z,Z-1 before irradiation (1), E,Z-1 after irradiation with 420
8
nm light (2), E,Z-1 after irradiation with 420 nm light and addition of 8 (3), and restored Z,Z-1
in the presence of 8 after heating (4). Signals marked with * are satellites of the solvent.
Especially unambiguous is the cross-peak signal between the proton of the remaining Z-
configured double bond and the closest carbonyl-carbon atom of the bis-thiophenone ring in
the HMBC NMR spectrum of E,Z-1 (Figure 2b and c). The backisomerization to the Z,Z
configuration was achieved by simple heating of the toluene solution. The thermal stability of
the metastable E,Z-1 was quantified by a kinetic analysis of its thermal conversion to the Z,Z
isomer at 60 °C (see Supporting Information, Figure S15). The free activation enthalpy of this
process is 28.2 kcal/mol, which leads to a favorably high bistability (a half life of 1.7 years at
25 °C) of receptor 1. After irradiation of E,Z-1 with 530 nm light only 32% of Z,Z-1 are
obtained in the photostationary state (pss). A quantum yield E,Z/Z,Z of 0.1% was determined
for this process. The reversible switching between the Z,Z and E,Z configuration is shown in
Figure 1.
Z,Z-1 and E,Z-1 can each adopt four different conformations in which the two biphenyl units
are rotated by 180° around the C-C single bonds connecting them to the isomerizable double
bonds. Introducing a n-pentyl substituent in one of the ortho-positions to this single bond
leads to a strong preference of only one conformation. In this conformation the n-pentyl
substituent is positioned at the less hindered side of the double bond to avoid repulsive steric
interactions with the bis-thioindigo fragment. The geometry of the preferred conformer could
directly be proven by strong cross-peak signals between the olefinic protons of the double
bonds and the benzylic protons of the n-pentyl substituents in the NOESY solution spectra of
Z,Z-1. NOE signals of the other rotameric conformers were barely visible (see Supporting
Information, Figure S5). Because of this negative preorganization the spatial relation of the
two terminal aniline groups is precisely defined and Z,Z-1 adopts a s-shaped geometry in
solution. The same geometry is also present in the crystalline state, as is revealed by the
crystal structural analysis of Z,Z-1 (Figure 2a).
The negative preorganization is also effective in E,Z-1, which was again proven by strong
cross-peak signals between the olefinic protons of the double bonds and the benzylic protons
of the n-pentyl substituents in the NOESY spectra of E,Z-1 (Figure 2d). Because of this
preorganization E,Z-1 must adopt a helically folded structure to avoid steric clashes between
the terminal aniline groups, which are now in close proximity of each other. Strong chemical
1
shifts of the aniline proton signals in the H NMR spectrum of the E,Z isomer are indicative
for this close proximity.
In the helical conformation of E,Z-1 the terminal electron rich aniline groups should be able to
bind electron deficient aromatic guest molecules via polar aromatic interactions in a
“
sandwich”-like fashion. Addition of 9-dicyanomethylene-2,7-dinitrofluorene (27DDF, 8) to a
solution of E,Z-1 in [D ]toluene did indeed cause pronounced shifts of the aniline-proton
8
1
-1
signals in the H NMR spectrum. A binding constant of 240±30 L·mol was determined by
titration for a E,Z-1 : 8 = 1 : 1 stoichiometry. To this end the NMR data were fitted using the
[
16]
program Bindfit, which is based on previously published non-linear regression binding
[
17]
programs. The established binding affinity is astonishingly high for a non-rigid receptor
system, which solely interacts via polar aromatic interactions with its substrate. Since
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addition of 27DDF affects only the proton signals of the terminal aniline-biphenyl groups of
E,Z-1 and all signals are shifted upfield the guest molecule must bind between these groups
in a planar fashion. This observation is a strong indication for the predicted chelate-like
binding mode resulting from multiple polar aromatic interactions in the host-guest complex.
1
A more detailed analysis of the H NMR spectra revealed that also the Z,Z isomeric form of 1
interacts weakly with 8. This weak binding apparently affects only one aniline moiety of Z,Z-1
(
the exo-aniline group, i.e. the left side of the molecule shown in Figure 1).
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Figure 2. (a) Crystal structure of Z,Z-1. (b) After irradiation of Z,Z-1 with 420 nm light the
helical isomer E,Z-1 is obtained in 94%. (c) Aromatic part of the HMBC-NMR spectrum (600
MHz, 300 K, toluene-d ) of E,Z-1. The selective photoisomerization resulting in formation of
8
only E,Z-1 was proven by HMBC cross-peak signals between protons 1 and 9 with the
carbonyl C atom 7; the chemical shift of proton 9 unambiguously shows a Z configured
double bond. The signal of the E configured double-bond proton 9‘ does not give a cross-
peak signal with the carbonyl C atom 7, therefore the Z,E configuration is excluded. (d)
Section of the NOESY-NMR spectrum (400 MHz, 300 K, [D ]toluene) of E,Z-1 proving one
8
single preferred conformation in which the n-pentyl groups and the protons of the double
bonds reside on the same side. Strong NOE cross-peak signals between the benzylic
protons (21 and 21‘) of the n-pentyl groups and protons 9 and 9‘ of the double bonds are
seen. The NOE cross-peak signal between proton 21‘ and the aromatic proton 15‘ is barely
visible. The corresponding cross-peak signal between protons 21 and 15 could not be
resolved.
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The corresponding binding constant was also determined via NMR titration and is very small
.
-1
(
K = 0.13±0.01 Lmol for a Z,Z-1 : 8 = 1 : 1 stoichiometry). Because of the very large
difference between the binding affinities of E,Z-1 and Z,Z-1 the aromatic guest molecule 8
can effectively be released after thermal backisomerization of E,Z-1 to Z,Z-1. Guest binding
and release is completely reversible and can be repeated multiple times (see Figure S21 in
the Supporting Information).
A binding study with 9-dicyanomethylene-2,5-dinitrofluorene (25DDF, 9) - a regioisomer of 8
-
revealed no signal shifts for E,Z-1 upon addition of this guest (see Figure S20 in the
Supporting Information). Apparently, bis-HTI receptor 1 is able to distinguish efficiently
between two regioisomeric electron poor aromatic guests, thus showing a remarkable
selectivity in its binding affinity.
In summary, we have developed a photoswitchable molecular receptor 1 whose affinity for
electron poor aromatic guests can be controlled with visible light and heat with extraordinary
high precision. Additionally, even regioisomeric guest molecules can efficiently be
distinguished with the new receptor. The core structure of this responsive system is a
completely new bis-HTI motive, which shows an unprecedented selectivity for its
photoisomerization reaction. After irradiation of 1 with 420 nm light only one of four possible
configurations - the E,Z isomer - is obtained in up to 94% yield. The conscious
implementation of negative preorganization further allowed us to precisely control the
conformations of receptor 1 and force it into only one of four possible geometries in each
isomeric state. In this way the reversible transition from a defined low-affinity planar structure
to a defined helical structure with high affinity could be realized. Bis-HTI 1 can therefore be
regarded as basic unit of a new type of photoswitchable foldamer, which possesses a
defined structure even in the unfolded state. The helicity based chiral properties of this new
receptor system will be explored in future studies especially in the context of enantioselective
recognition and chiroptical properties of responsive materials.
Acknowledgements
H. Dube thanks the “Fonds der Chemischen Industrie” for a Liebig fellowship and the
Deutsche Forschungsgemeinschaft (DFG) for an Emmy-Noether fellowship. We further thank
the Deutsche Forschungsgemeinschaft (SFB 749) and the Cluster of Excellence ’Center for
M
Integrated Protein Science Munich’ (CIPS ) for financial support.
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Text for Table of Contents
A novel bis-Hemithioindigo photoswitch is reported, whose conformation and helical folding
can precisely be controlled by visible light and heat. The helical form is obtained after a
highly selective photoisomerization and is able to bind electron deficient aromatic guests in a
regioselective manner. A simple heating step leads to effective release of the guest.
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Keywords
Host-guest systems • Molecular recognition • Photochemistry • Photochromism • Receptors
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