Halogen Bonding Tetraphenylethene Anion Receptors: Anion-Induced Emissive Aggregates and Photoswitchable Recognition

Article abstract of DOI:10.1002/anie.202107748

A series of tetraphenylethene (TPE) derivatives functionalized with highly potent electron-deficient perfluoroaryl iodo-triazole halogen bond (XB) donors for anion recognition are reported. ¹H NMR titration experiments, fluorescence spectroscopy, dynamic light scattering measurements, TEM imaging and X-ray crystal structure analysis reveal that the tetra-substituted halogen bonding receptor forms luminescent nanoscale aggregates, the formation of which is driven by XB-mediated anion coordination. This anion-coordination-induced aggregation effect serves as a powerful sensory mechanism, capable of luminescence chloride sensing at parts per billion concentration. Furthermore, the doubly substituted geometric isomers act as unprecedented photoswitchable XB donor anion receptors, where the composition of the photostationary state can be modulated by the presence of a coordinating halide anion.

Full text of DOI:10.1002/anie.202107748

Angewandte
Research Articles
Chemie
How to cite:
International Edition: doi.org/10.1002/anie.202107748
German Edition: doi.org/10.1002/ange.202107748
Supramolecular Chemistry
Halogen Bonding Tetraphenylethene Anion Receptors: Anion-
Induced Emissive Aggregates and Photoswitchable Recognition
Andrew Docker+, Xiaobo Shang+, Daohe Yuan, Heike Kuhn, Zongyao Zhang, Jason J. Davis,
Paul D. Beer,* and Matthew J. Langton*
Abstract: A series of tetraphenylethene (TPE) derivatives
functionalized with highly potent electron-deficient per-
fluoroaryl iodo-triazole halogen bond (XB) donors for anion
recognition are reported. 1H NMR titration experiments,
fluorescence spectroscopy, dynamic light scattering measure-
ments, TEM imaging and X-ray crystal structure analysis
reveal that the tetra-substituted halogen bonding receptor
forms luminescent nanoscale aggregates, the formation of
which is driven by XB-mediated anion coordination. This
anion-coordination-induced aggregation effect serves as a pow-
erful sensory mechanism, capable of luminescence chloride
sensing at parts per billion concentration. Furthermore, the
doubly substituted geometric isomers act as unprecedented
photoswitchable XB donor anion receptors, where the compo-
sition of the photostationary state can be modulated by the
presence of a coordinating halide anion.
revealed that the origin of this atypical fluorescent response
upon aggregation was, amongst other factors, attributable to
the restriction of intramolecular rotation which acts to
suppress non-radiative decay pathways of the excited state.
Indeed, the discovery of this behaviour stimulated intense
interest in the search for other AIE molecules, and recent
years have seen an ever-increasing library of AIE lumino-
genic (AIEgens) materials developed.[5] In this context,
tetraphenylethene (TPE) derivatives have risen to promi-
nence as synthetically accessible, modular, multivalent scaf-
folds for AIEgens in the construction of bio-imaging
probes,[6,7] optoelectronic devices[8] and responsive materi-
als.[9–12] In addition to their role as highly sensitive lumines-
cent reporter groups, TPE derivatives are also of interest by
virtue of their potential to serve as photoswitchable platforms
with large degrees of spatial control.[13] TPE derivatives have
also received significant interest in the development of
supramolecular sensors which couple a luminescent response
with molecular recognition events.[14–18] However, the consid-
ered employment of TPE-derivatives within supramolecular
host systems remains scarce,[19–22] with the overwhelming
majority of reported examples reliant on irreversible chem-
ical modification of the AIEgen core to sense the guest
species.[23]
Introduction
Organic luminescent materials frequently exhibit signifi-
cant concentration dependent photophysical properties. Tra-
ditional chromophores, such as extended planar aromatic
systems, are typically strongly fluorescent under dilute con-
ditions, but suffer considerable attenuation in emissive
properties with increasing concentration. This phenomenon
is known as aggregation-caused quenching (ACQ),[1–3] which
is attributable, in most cases, to excimer or exciplex forma-
tion. Whilst undesirable, ACQ was widely accepted as an
immutable characteristic of fluorescent material design.
However, Tangꢀs seminal report of aggregation-induced
emission (AIE) in a tetraphenylsilole derivative challenged
this precept.[4] Extensive experimental and theoretical studies
Within the rapidly expanding field of supramolecular
anion host-guest chemistry, halogen bonding and chalcogen
bonding intermolecular interactions have emerged as a val-
uable addition to the supramolecular toolbox.[24–27] These
sigma hole interactions frequently confer enhanced anion
affinity and unique selectivity profiles relative to hydrogen
bonding (HB) interactions within anion receptors,[28–37] sen-
sors[38–40] and transmembrane anion transporters.[41–44]
Herein, we report a series of multidentate TPE-based
anion receptors containing highly potent perfluoroaryl iodo-
triazole XB donor motifs. Detailed H NMR anion titration
[*] A. Docker,[+] Dr. X. Shang,[+] D. Yuan, H. Kuhn, Z. Zhang,
Prof. J. J. Davis, Prof. P. D. Beer, Prof. M. J. Langton
Department of Chemistry University of Oxford
Chemistry Research Laboratory
Mansfield Road, Oxford OX1 3TA (UK)
E-mail: paul.beer@chem.ox.ac.uk
matthew.langton@chem.ox.ac.uk
[+] These authors contributed equally to this work.
1
experiments reveal the significant influence of XB donor
geometry and multivalency on anion coordination mode and
binding affinity. Photophysical experiments, in combination
with dynamic light scattering (DLS) measurements, TEM
imaging and X-ray structure determination, reveal the
formation of XB-mediated anion-coordination-induced ag-
gregates, accompanied by a significant turn-on luminescence
response which serves as a powerful transduction mechanism
for chloride sensing. The bidentate iodo-triazole TPE deriv-
atives may be switched between strong and weak chloride
binding states by photoisomerisation, representing—to the
best of our knowledge—the first example of a photoswitch-
able XB anion receptor. Analysis of the photoswitching
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202107748.
ꢀ 2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
&&&&
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
process reveals a strong correlation between halide anion
respectively. In the case of 2·XBE and 2·XBZ, a mixture of
affinity and the position of the photostationary state.
both isomers was obtained with a combined yield of 58%,
because the parent bis-iodoalkyne was used as an inseparable
mixture of the geometric isomers. Pleasingly, however, the
introduction of the aryl-iodotriazole motifs enabled resolu-
tion of the stereoisomers by silica gel column chromatogra-
phy, facilitating the isolation of 2·XBE and 2·XBZ in 33% and
25% yield, respectively. Full synthetic procedures and
characterisation data are available in the Supporting Infor-
mation.
Results and Discussion
Synthesis of mono- and bi-dentate anion receptors
To determine the effect of varying the geometrical
arrangement of XB donor motifs around the TPE core on
the multi-dentate anion-recognition behaviour, synthetic
efforts were first directed towards preparing the mono- and
di-functionalised anion receptors 1·XB, 2·XB, 2·XBE and
2·XBZ (Scheme 1).
Anion-recognition experiments
Typically, the synthesis of the target receptors was
accomplished by treatment of the proto-alkyne precursors
with n-BuLi in anhydrous THF solution at ꢀ788C, followed
by reaction of the lithiated alkynes with iodine to afford the
respective iodo-alkynes (Scheme 1a). The target receptors
were subsequently synthesized via a copper(I)-catalysed
azide alkyne cycloaddition (CuAAC) reaction between the
appropriate iodo-alkyne derivative and perfluorophenyl
azide, in the presence of Cu(MeCN)4PF6 and the CuI
stabilising ligand tris(benzyltriazolylmethyl) amine (TBTA),
which afforded 1·XB and 2·XB in yields of 83% and 57%
With the series of mono- and di-functionalised TPE
receptors in hand, we examined their chloride anion-recog-
nition behaviour by 1H NMR anion titration experiments.
Aliquots of chloride, as the tetrabutylammonium salt, were
added to solutions of the respective receptor in [D8]THF,
monitoring the binding induced chemical shift perturbations.
In the case of 1·XB, 2·XB and 2·XBE, chloride binding
induced a downfield shift of the aryl signal directly adjacent to
the iodo-triazole moiety, consistent with XB mediated anion
complexation.
Scheme 1. a) Representative synthesis of 1·XB, via lithiation of proto-alkyne 1, subsequent iodination and CuAAC reaction with perfluorophenyl
azide. b) Chemical structures of halogen bond donor TPE derivatives 2·XB, 2·XBE, 2·XBZ, 4·XB and 4·XBPh, and the CH-hydrogen bond donor 4·HB.
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Table 1: Chloride association constants for mono- and disubstituted XB
receptors in [D8]THF.
Analysis of the titration isotherm for 1·XB using the
Bindfit programme[45,46] generated a 1:1 stoichiometric asso-
ciation constant, K1:1(1·XB) = 2380 Mꢀ1, representing the
binding affinity of chloride to one iodotriazole XB donor.
The integration of additional XB donors to the TPE core
enables higher host-guest complex stoichiometries to be
achieved, which in the case of 2·XB, 2·XBZ and 2·XBE
corresponds to a 1:2 host-anion stoichiometry. Depending
on the geometric arrangement of the two XB donors, anion
complexation occurs with either no cooperativity (binding of
the first anion does not affect binding of the second) or
negative cooperativity (binding of the first anion inhibits
binding of the second). In the former case, the 1:1 and 1:2
host-guest binding constants, K1:1 and K1:2 respectively, are
related by the expression K1:1 = 4K1:2, accounting for statis-
tical factors. Analysis of the chloride binding isotherms for the
di-iodotriazole derivatives 2·XB, 2·XBZ and 2·XBE (Fig-
ure 1c) revealed that the two XB donors in 2·XB and 2·XBE
bind the chloride anions in a non-cooperative manner, with an
interaction parameter a = 1, where a = 4K1:2/K1:1 (Table 1).
The chloride binding isotherm of 2·XBZ exhibits a more
complex profile, in which addition of up to 1 equivalent of
chloride induced an upfield shift in proton a (Figure 1b), after
which an inflection point is observed and subsequent addition
of chloride resulted in a downfield shift. This behaviour is
consistent with the initial formation of a 1:1 complex, and the
formation of a considerably weaker 1:2 complex at higher
concentrations of chloride. The reversal of chemical shift
Ka(Clꢀ)[a] [Mꢀ1
]
1·XB[b]
2·XB[c,d]
2·XBE[c,d]
2·XBZ[c]
K1:1
K1:2
a[e]
2380
n/a
n/a
5650[f]
1410[f]
1
4970
1240
1
23200
960
0.17
[a] Chloride added as its tetrabutylammonium salt, errors <10%.
[b] Fitted to a 1:1 host-guest binding isotherm. [c] Fitted to a 1:2 host-
guest binding isotherm. [d] Binding model assumes K1:1 =4K2:1. [e] In-
teraction parameter a=4K1:2/ K1:1, with a=1 indicating no cooperativity,
and a<1 indicating negative cooperativity. [f] Perturbation of the
equilibrium due to anion-induced aggregation effects not considered in
the data fitting. For full details of the binding models and raw data see
the SI, Table S1–3.
perturbation is interpreted to be a consequence of conforma-
tional rearrangement upon changing from 1:1 to 1:2 stoichi-
ometry to maximise the inter-anion separation (Figure 1a).
Analysis of the 1:1 stoichiometric binding constants for
the three di-iodotriazole geometric isomers 2·XB, 2·XBE and
2·XBZ provided evidence for the presence or absence of
bidentate anion binding with chelate cooperativity for each
isomer. The chloride binding constant obtained for the
reference mono-dentate compound 1·XB, K1:1(1·XB), quan-
tifies the binding of the anion to a single iodotriazole binding
site. The 1:1 binding constant for the di-iodotriazole deriva-
tives 2·XB, 2·XBE and 2·XBZ is given by 2K1:1(1·XB) in the
absence of chelate cooperativity and when statistical factors
Figure 1. a) Postulated anion-binding equilibrium for 2·XBZ. b) 1H NMR TBACl titration of 2·XBZ ([D8]THF, 500 MHz, 298 K). c) Chloride binding
isotherms for 1·XB, 2·XB, 2·XBE and 2·XBZ, where circles represent experimental data and solid lines represent fitted data.
&&&&
www.angewandte.org
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Figure 2. a) Postulated anion-binding equilibrium for 4·XB. b) 1H NMR TBACl titration of 4·XB ([D8]THF, 500 MHz, 298 K). c) Halide binding
isotherms for 4·XB, where circles represent experimental data and dashed lines are a visual aid.
associated with binding of one anion to a ditopic host are
considered. Inspection of the values of K1:1 for the di-
iodotriazole derivatives reveals that, in the case of 2·XB and
2·XBE, the two iodotriazole donors are too far apart to allow
for chelation of the anion in a bidentate fashion: K1:1(2·XB)
and K1:1(2·XBE) ꢁ 2K1:1(1·XB). In contrast, in 2·XBZ the two
iodotriazole derivatives are closer in space, which allows for
effective bidentate cooperative binding of the first chloride
anion, such that K1:1(2·XBZ) ꢁ 10K1:1(1·XB). The pronounced
negative cooperativity in binding of the second equivalent of
anion to 2·XBZ, reflected in the interaction parameter a =
0.17, presumably therefore arises due to the combination of
mutual anion-anion electrostatic repulsion and the require-
ment to break the bidentate XB-chloride interactions upon
formation of the ternary complex.
Synthesis and anion recognition of tetradentate receptors
Attention then turned towards the synthesis of the tetra-
substituted receptors 4·XB, 4·XBPh and 4·HB, which were
prepared from the corresponding tetra-iodo/proto alkyne
TPE derivatives using analogous procedures to those used for
1·XB (see the Supporting Information for further details).
1H NMR chloride titration experiments were conducted
on 4·XB, 4·XBPh and 4·HB, to elucidate the role and potency
of XB interactions on the anion-recognition process. Chloride
binding titration experiments with 4·XB revealed a chemical
shift perturbation profile with an inflection point similar to
that observed with 2·XBZ, in which up to two equivalents of
TBACl induced an almost linear upfield perturbation of the
aryl proton a, indicative of near quantitative formation of
a 1:2 receptor:anion complex. Addition of further equivalents
caused a progressive downfield shift, indicating the formation
of higher receptor:anion stoichiometries, presumably up to
Subsequent 1H NMR anion titration experiments of
2·XBZ with the heavier halides revealed that bromide and
iodide produced similar binding isotherms with somewhat
attenuated association constants, consistent with anion
basicity trends (Table 2).
[
]
*
1:4 (Figure 2b). Similar chemical shift perturbations were
observed for 4·XBPh, whilst binding to the hydrogen bonding
analogue 4·HB was weak (see Supplementary information for
further details). Furthermore, the preference for the lighter
halides observed for 2·XBZ appears to also translate to 4·XB,
Table 2: Halide association constants for 2·XBZ in [D8]THF.
Ka[a] [Mꢀ1
]
Clꢀ[b]
Brꢀ[b]
Iꢀ[b]
[*] The binding data could not be reliably fitted to a 1:4 host:guest
isotherm, presumably due to the presence of anion-induced
aggregates. However, visual inspection of the isotherm (noting the
inflection point at 2 equiv. of anion) indicates that the binding of the
first and second anion is strong (Kꢂ104 Mꢀ1, likely via the same
bidentate binding mode seen in 2·XBZ), with weaker binding of the
third and fourth equivalent.
[e]
K1:1
23200
960
8080
1330
1050
40
K1:2
[a] Anions added as their tetrabutylammonium salts, errors <10%.
[b] Fitted to a full 1:2 host-guest binding model. For full details of the
binding models and raw data see the ESI.
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
upon visual analysis of the bromide and iodide binding
[
]
*
isotherms (Figure 2c).
Solid-state structure determination
Further insight into the anion-binding mode of the
halogen bonding TPE systems was obtained by solid-state
characterisation of 4·XB (Figure 3) and in the presence of
a source of chloride anion (Figure 4).
It is noteworthy that repeated attempts to crystallize 4·XB
with a non-coordinating organic countercation (e.g. tetra-
ethylammonium,
tetrabutylammonium
or
benzyltri-
methylammonium) persistently yielded the NaCl complex
of 4·XB (Figure 4a). The propensity of 4·XB to crystalize with
an adventitious sodium cation may be rationalized by
inspection of the solid-state structure in which the Na+ forms
Figure 3. Solid state structure of 4·XB. Grey=carbon, blue=nitrogen,
white=hydrogen, light green=fluorine, purple=iodine.
a proximal ion-pair with a chloride anion simultaneously
4·XB molecules (Figure 4b). Close examination of the
structure reveals that the four-fold coordination of chloride,
ꢀ
ꢀ
exhibiting four C I···Cl XB interactions with four distinct
Figure 4. a) Solid state structure of 4·XB·NaCl, presented as a wire frame structure. b) Truncated view of chloride coordination in 4·XB·NaCl.
c) Space-filling representation of 4·XB·NaCl. Grey=carbon, blue=nitrogen, white=hydrogen, light green=fluorine, purple=iodine, cyan=so-
dium, green=chlorine.
in concert with other interactions (Figure S86), enforces close
proximity of the TPE aromatic cores, as clearly visualised in
the space filling representation (Figure 4c). To establish the
relative roles of the Na+ and Clꢀ in the formation of these
aggregated structures observed in the solid state, efforts were
then directed towards obtaining structural information in the
presence of other halides. Crystals suitable for X-ray diffrac-
tion were obtained by slow diffusion of pentane into a 9:1
CHCl3:acetone (v/v) solution of 4·XB in the presence of
excess NaI. Single-crystal X-ray analysis revealed the
[*] The chloride titration experiments were repeated at concentrations of
4·XB spanning 0.25–1.0 mM. The general form of the binding
isotherms were unchanged: stronger initial binding of the first two
equivalents of anion, an inflection point at ꢁ2 equivalents, and
weaker binding of the subsequent anions. This suggests that the
formation of aggregates, which are resolved by fluorescence, DLS
and TEM experiments but not by NMR, do not significantly affect the
overall form of the titration, but do hinder a quantitative analysis of
binding affinities.
&&&&
www.angewandte.org
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
NaI·4·XB complex displays an analogous four-fold anion
have demonstrated that the direction of lmax perturbation can
ꢀ
ꢀ
coordination via C I···I XB interactions in an approximately
square planar geometry around the iodide, however the
sodium cation in this case resides in a vacancy between the
central aromatic units constituting the tetraphenylethene core
(Figure S82). This is presumably due to the reduced electro-
static interaction between the larger iodide anion and the
sodium cation. The structural similarity of the obtained
sodium halide complexes reveals the crucial role of potent
XB-anion interactions in the assembly of the aggregates.
Encouraged by these findings, attention then turned to
investigating whether these anion-induced aggregates persist
in solution phase.
be closely correlated with the relative orientations of the
central aromatic units to the alkene core. Increasing planarity
between the constituent phenyl rings leads to increasing p-
conjugation and red-shifting of the emission, whilst adoption
of a perpendicular conformation decreases p-conjugation and
results in blue-shifted emission.[47,48] Close examination of the
solid state structures for 4·XB and its NaCl complex readily
rationalise the observed changes in the emission profile,
whereby the intermolecular coordination of the halide by
halogen bond interactions not only induces the formation of
closely intertwined complexes, restricting possible rotational
degrees of freedom, but also enforces a more perpendicular
geometry of the core aromatic scaffold (See Figure S85 and
Table S5). Addition of the non-coordinating anion hexa-
fluorophosphate did not affect the emission spectrum, further
demonstrating that XB-mediated coordination to chloride is
required to mediate aggregation (Figure S79). Dynamic light
scattering (DLS) measurements of freshly prepared THF
solutions (10ꢀ5 M) of 4·XB indicate that the free receptor is
non-aggregated in the absence of chloride, but the addition of
1 equivalent of TBACl induces the formation of aggregates on
the order of 100 nm in diameter (Figure 5b, bottom). Further
evidence for anion-induced aggregate formation of 4·XB was
provided by transmission electron microscopy (TEM) which,
consistent with results from DLS, revealed that the addition
of 1 equiv. of TBACl to a 1 mM solution of 4·XB in THF led
to the formation of ꢁ 100 nm sized particles, while no such
species were observed for the free receptor in the absence of
chloride (Figures S87 and S88).
Anion-coordination-induced aggregation and anion sensing
As anticipated from the well-established emission behav-
iour of TPE derivatives, measurement of the fluorescence
spectra in THF solution (10ꢀ5 M) revealed that 4·XB is
weakly emissive (lex = 350 nm). A significant increase in
emission intensity with increasing water fractions (fw) was
observed at fw > 60%, indicative of the anticipated aggrega-
tion-induced emission behaviour of TPE derivative 4·XB
(Figure 5a, top), and the formation of aggregates confirmed
by dynamic light scattering (DLS) measurements (Figure 5a,
bottom).
Addition of increasing equivalents of chloride to a THF
solution of 4·XB led to an increase in fluorescence intensity
(lmax = 520 nm), with a three-fold increase observed after the
addition of 10 equivalents (Figure 5b, top), accompanied by
a considerable hypsochromic shift ( ꢁ 20 nm). Recent studies
shedding light on the mechanism of AIE in TPE derivatives
To further explore the role of spatial orientation and
potency of the halogen bond donors in the formation of the
anion-mediated aggregates, the relative fluorescence re-
Figure 5. Fluorescence spectra (top) (THF, 10ꢀ5 M, lex =350 nm) and overlaid DLS measurements (bottom) of 4·XB in the presence of (a)
increasing water fraction (fw) and (b) Increasing equivalents of TBACl.
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
aggregate formation, in which the emission intensity of
4·XB in THF was measured in the presence of 10 equivalents
of a range of anions (Figure 7). In all cases except acetate, the
addition of an anion induced a fluorescence intensity increase.
In accordance with the anion basicity trend this increase
followed the order of Clꢀ > Brꢀ > Iꢀ. Notably, despite its di-
2ꢀ
anionic nature, SO4 elicited a diminished enhancement in
emission intensity in comparison to the strong chloride
response. The considerable fluorescence response for Clꢀ
corresponds to a limit of detection (LOD) of 5.14 mM
(183 ppb), constituting one of the most sensitive supramolec-
ular anion sensors to-date (Figure S75).[23,49]
Figure 6. Relative fluorescence responses of the series of receptors in
the presence of 10 equivalents of TBACl (THF, 10ꢀ5 M, lex =350 nm).
Photo-switchable anion recognition
In addition to the highly desirable luminescent properties
of TPE, unsymmetrical doubly substituted TPE derivatives
have the capability to function as molecular photo-switches in
which photoisomerisation switches between different spatial
arrangements of functional groups. Photo-switchable anion
receptors in particular have attracted interest for applications
including ion extraction and transport, enabling control over
when and where the hosts bind or release their targets.[50]
However, the utilization of TPE as a photoswitch has been
hindered by practical issues in their synthesis, due to
a combination of poor stereoselectivity and challenging
chromatographic separation of the closely related geometric
isomers. Consequently, the vast majority of reports to date
using these di-functionalised TPE derivatives in material or
polymer sciences have used mixtures of the isomers. Com-
pounds 2·XBE and 2·XBZ are prime candidates for developing
photo-switchable anion receptors due to the contrast in
chloride binding affinity of the two geometric isomers (K1:1 of
4970 and 23200 Mꢀ1, respectively), and fortuitously, could be
isolated by column chromatography.
sponse for all receptors was recorded in the presence of
10 equivalents of TBACl, and the data summarised in Fig-
ure 6. The largest emission intensity increases are observed
for 4·XB and 2·XB, and the formation of aggregates in the
presence of 1 equivalent of TBACl in both cases was
confirmed by DLS measurements. In contrast, 1·XB, 2·XBZ
and 2·XBE exhibited minor perturbations in fluorescence
intensity upon the addition of 10 equivalents of TBACl, and
DLS analysis revealed no evidence of aggregated species.
Together, these results confirm that the observed emission
enhancement arises from XB-mediated chloride induced
aggregation. The absence of anion-coordination-induced
aggregation with 2·XBZ, despite the receptor possessing
enhanced chloride affinity relative to all other mono- and
di-substituted TPE derivatives, demonstrates that spatial
orientation of the XB donors is crucial in the formation of
the chloride induced XB aggregates. Under analogous con-
ditions 4·XBPh and the hydrogen bonding analogue 4·HB also
demonstrated no significant emission intensity change, pro-
viding further strong evidence for the requirement for
multiple potent XB-anion interactions to drive aggregate
formation.
Irradiation of a solution of a pure sample of 2·XBE with
405 nm light triggered photo-isomerisation, generating an
approximately equal ratio of both isomers in the photo-
stationary state (52:48 2·XBZ:2·XBE), as determined by
integration of the NMR signals of the mixture. The effect of
405 nm irradiation on the UV vis spectrum of a solution of
2·XBE is shown in Figure 8a, resulting in a bathochromic shift
of the absorption at low wavelength (ca. 265 nm) and
a decrease in the absorption at 335 nm, with isosbestic points
at 270 nm and 310 nm confirming the unimolecular nature of
the isomerisation process. The photo-stationary state was
reached within 120 s, as deduced by the absence of further
changes in the absorption spectrum. No thermal isomeriza-
tion of 2·XBZ or 2·XBE was observed under ambient
conditions in THF solution over the course of the experiment
when excluded from light. Analogous irradiation experiments
starting with 2·XBZ generated a similar, but reversed set of
perturbations in the UV-vis profile, namely an increase in
absorption at 335 nm and a hypsochromic shift in the local
lmax absorption at approximately 265 nm.
Analogous experiments were also undertaken to inves-
tigate how the nature of the anionic guest influences
Given the contrasting anion-binding properties of the
isomers, we anticipated that the photostationary state may be
biased in favour of the Z isomer by coordination to an anion.
Figure 7. Relative fluorescence response for 4·XB in the presence of
10 equivalents of a range of anions (THF, 10ꢀ5 M, lex =350 nm).
&&&&
www.angewandte.org
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Figure 8. a) Photoisomerisation of 2·XBE. UV/Vis spectra of the photo-stationary state generated upon irradiation of 2·XBE with 405 nm light
(THF, 10ꢀ5 Mꢀ1). b) Photoisomerisation of 2·XBE in the presence of 10 equivalents of TBACl (1H NMR, [D8]THF, 500 MHz, 298 K).
To this end, analogous photoisomerisation experiments were Conclusion
conducted by irradiating the sample with 405 nm light in the
presence of 10 equivalents of TBACl. In the presence of the
halide, the PSS is biased in favour of the Z isomer, due to
strong XB coordination to the chloride (68:32 2·XBZ:2·XBE)
(Figure 8b). Similar experiments were undertaken with
10 equivalents of bromide and iodide (Table 3), revealing
that the bias of the PSS in favour of the Z isomer correlates
with the anion-binding affinity to 2·XBZ, namely Clꢀ > Brꢀ >
Iꢀ. In contrast, in the presence of non-coordinating anion,
PF6ꢀ, which demonstrated no binding to either receptor in
1H NMR titration experiments, no perturbation of the
position of the PSS was detected. Together, this data provides
evidence for photo-switchable XB-mediated anion recogni-
tion, in which strong binding to the 2·XBZ and weaker binding
to 2·XBE enables switching of binding using light, as well as
modulation of the obtained ratio of isomers in the photosta-
tionary state.
In conclusion, we report a series of halogen bonding TPE-
based anion receptors, in which the multivalency and relative
orientation of the strongly potent perfluoroaryl XB donors is
1
systematically investigated. H NMR anion titration experi-
ments elucidate the varied binding modes and stoichiometries
of the family of halogen bonding TPE derivatives. Fluores-
cence anion titration experiments demonstrate that the tetra-
substituted perfluorinated 4·XB receptor, experiences a dra-
matic increase in emission intensity upon the addition of
chloride, whilst the non-fluorinated 4·XBPh or hydrogen
bonding 4·HB analogues elicit no photophysical response.
Detailed analysis of the fluorescence spectra, dynamic light
scattering measurements, TEM imaging and X-ray crystal
structure determination demonstrate that the origin of this
sensory response is an unprecedented XB-anion coordina-
tion-induced aggregation mechanism. Furthermore, success-
ful isolation of difunctionalised geometric isomers 2·XBZ and
2·XBE facilitated the investigation of these TPE derivatives as
photoswitchable anion receptors, in which it was demonstrat-
ed that the position of the photostationary state could be
significantly biased towards 2·XBZ on the basis of enhanced
halide anion affinity. These results not only provide further
evidence for the advantages of XB interactions in the design
of potent anion receptors, but also point to new opportunities
in the development of XB anion-responsive materials,
switches and sensors.
Table 3: PSS ratios determined from irradiation of 1 mM of 2·XBZ.
Anion[a]
PSS Ratio (2·XBZ/2·XBE)[b]
None
Clꢀ
Brꢀ
Iꢀ
52:48
68:32
62:38
58:42
53:47
ꢀ
PF6
[a] 10 equivalents of anion added as their tetrabutylammonium salts,
10ꢀ3 M, [D8]THF. [b] PSS ratios determined from 1H NMR integration.
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
ˇ
´
[21] Z. Zhang, M. I. Hashim, C.-H. Wu, J. I. Wu, O. S. Miljanic,
Acknowledgements
Chem. Commun. 2018, 54, 11578 – 11581.
[22] Y.-X. Yuan, J.-H. Wang, Y.-S. Zheng, Chem. Asian J. 2019, 14,
760 – 764.
[23] M. H. Chua, K. W. Shah, H. Zhou, J. Xu, Molecules 2019, 24,
2711.
X.S. and M.J.L. thank the Royal Society for funding
(RGF\EA\181007). A.D. and H.K. thank the EPSRC for
studentships (Grant reference numbers EP/N509711/1 and
EP/R513295/1). Z.Z. thanks University of Oxford and China
Scholarship Council for a scholarship. M.J.L. acknowledges
the John Fell Oxford University Press Research Fund for
financial support. M.J.L. is a Royal Society University
Research Fellow. The authors thank Dr. Amber L. Thompson
and Dr. Kirsten E. Christensen for their helpful discussion
and advice with crystallographic data collection and refine-
ment.
[24] L. C. Gilday, S. W. Robinson, T. A. Barendt, M. J. Langton, B. R.
Mullaney, P. D. Beer, Chem. Rev. 2015, 115, 7118 – 7195.
[25] A. Brown, P. D. Beer, Chem. Commun. 2016, 52, 8645 – 8658.
[26] T. M. Beale, M. G. Chudzinski, M. G. Sarwar, M. S. Taylor,
Chem. Soc. Rev. 2013, 42, 1667 – 1680.
[27] G. Cavallo, P. Metrangolo, R. Milani, T. Pilati, A. Priimagi, G.
Resnati, G. Terraneo, Chem. Rev. 2016, 116, 2478 – 2601.
[28] M. G. Sarwar, B. Dragisic, S. Sagoo, M. S. Taylor, Angew. Chem.
Int. Ed. 2010, 49, 1674 – 1677; Angew. Chem. 2010, 122, 1718 –
1721.
[29] M. J. Langton, S. W. Robinson, I. Marques, V. Fꢃlix, P. D. Beer,
Nat. Chem. 2014, 6, 1039 – 1043.
[30] T. Bunchuay, A. Docker, A. J. Martinez-Martinez, P. D. Beer,
Angew. Chem. Int. Ed. 2019, 58, 13823 – 13827; Angew. Chem.
2019, 131, 13961 – 13965.
Conflict of Interest
The authors declare no conflict of interest.
[31] A. Docker, T. Bunchuay, M. Ahrens, A. J. Martinez-Martinez,
P. D. Beer, Chem. Eur. J. 2021, 27, 7837 – 7841.
Keywords: aggregation-induced emission · anion recognition ·
[32] Y. C. Tse, A. Docker, Z. Zhang, P. D. Beer, Chem. Commun.
2021, 57, 4950 – 4953.
fluorescence · halogen bonding · photoswitchable receptors
[33] T. Bunchuay, A. Docker, U. Eiamprasert, P. Surawatanawong, A.
Brown, P. D. Beer, Angew. Chem. Int. Ed. 2020, 59, 12007 –
12012; Angew. Chem. 2020, 132, 12105 – 12110.
[1] J. Mei, Y. Hong, J. W. Y. Lam, A. Qin, Y. Tang, B. Z. Tang, Adv.
Mater. 2014, 26, 5429 – 5479.
[2] J. Mei, N. L. C. Leung, R. T. K. Kwok, J. W. Y. Lam, B. Z. Tang,
Chem. Rev. 2015, 115, 11718 – 11940.
[34] A. Docker, P. D. Beer, Halogen Bonding in Solution, Wiley,
Hoboken, 2021, pp. 83 – 120.
[3] Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Soc. Rev. 2011, 40,
5361 – 5388.
[35] S. Chakraborty, R. Dutta, P. Ghosh, Chem. Commun. 2015, 51,
14793 – 14796.
[4] J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S.
Kwok, X. Zhan, Y. Liu, D. Zhu, B. Z. Tang, Chem. Commun.
2001, 1740 – 1741.
[5] S. Suzuki, S. Sasaki, A. S. Sairi, R. Iwai, B. Z. Tang, G. Konishi,
Angew. Chem. Int. Ed. 2020, 59, 9856 – 9867; Angew. Chem. 2020,
132, 9940 – 9951.
[36] F. Zapata, A. Caballero, P. Molina, I. Alkorta, J. Elguero, J. Org.
Chem. 2014, 79, 6959 – 6969.
[37] M. Cametti, K. Raatikainen, P. Metrangolo, T. Pilati, G.
Terraneo, G. Resnati, Org. Biomol. Chem. 2012, 10, 1329 – 1333.
[38] M. J. Langton, I. Marques, S. W. Robinson, V. Fꢃlix, P. D. Beer,
Chem. Eur. J. 2016, 22, 185 – 192.
[6] J. Qian, B. Z. Tang, Chem 2017, 3, 56 – 91.
[7] Q. Lu, C.-J. Wu, Z. Liu, G. Niu, X. Yu, Front. Chem. 2020, 8,
https://doi.org/10.3389/fchem.2020.617463.
[8] M. Yu, R. Huang, J. Guo, Z. Zhao, B. Z. Tang, PhotoniX 2020, 1,
11.
[9] R. Hu, A. Qin, B. Z. Tang, Prog. Polym. Sci. 2020, 100, 101176.
[10] N.-T. Chu, R. D. Chakravarthy, N.-C. Shih, Y.-H. Lin, Y.-C. Liu,
J.-H. Lin, H.-C. Lin, RSC Adv. 2018, 8, 20922 – 20927.
[11] H. Qu, X. Tang, X. Wang, Z. Li, Z. Huang, H. Zhang, Z. Tian, X.
Cao, Chem. Sci. 2018, 9, 8814 – 8818.
[39] R. Hein, A. Borissov, M. D. Smith, P. D. Beer, J. J. Davis, Chem.
Commun. 2019, 55, 4849 – 4852.
[40] S. C. Patrick, R. Hein, A. Docker, P. D. Beer, J. J. Davis, Chem.
Eur. J. 2021, 27, 10201 – 10209.
[41] A. V. Jentzsch, D. Emery, J. Mareda, S. K. Nayak, P. Metrangolo,
G. Resnati, N. Sakai, S. Matile, Nat. Commun. 2012, 3, 905.
[42] A. Vargas Jentzsch, S. Matile, J. Am. Chem. Soc. 2013, 135,
5302 – 5303.
[43] L. E. Bickerton, A. J. Sterling, P. D. Beer, F. Duarte, M. J.
Langton, Chem. Sci. 2020, 11, 4722 – 4729.
[12] X. Yan, P. Wei, Y. Liu, M. Wang, C. Chen, J. Zhao, G. Li, M. L.
Saha, Z. Zhou, Z. An, X. Li, P. J. Stang, J. Am. Chem. Soc. 2019,
141, 9673 – 9679.
[44] L. E. Bickerton, A. Docker, A. J. Sterling, H. Kuhn, F. Duarte,
P. D. Beer, M. Langton, Chem. Eur. J. 2021, https://doi.org/10.
1002/chem.202101681.
[13] Y. Xie, Z. Li, Chem. Asian J. 2019, 14, 2524 – 2541.
[14] H.-T. Feng, Y.-X. Yuan, J.-B. Xiong, Y.-S. Zheng, B. Z. Tang,
Chem. Soc. Rev. 2018, 47, 7452 – 7476.
[15] J. Feng, L. Yao, J. Zhang, Y. Mu, Z. Chi, C.-Y. Su, Dalton Trans.
2016, 45, 1668 – 1673.
[16] H. Fang, G. Cai, Y. Hu, J. Zhang, Chem. Commun. 2018, 54,
3045 – 3048.
[17] D. D. La, S. V. Bhosale, L. A. Jones, S. V. Bhosale, ACS Appl.
Mater. Interfaces 2018, 10, 12189 – 12216.
[45] “BindFit v0.5 j Supramolecular,” can be found under http://app.
supramolecular.org/bindfit/.
[46] P. Thordarson, Chem. Soc. Rev. 2011, 40, 1305 – 1323.
[47] E. P. J. Parrott, N. Y. Tan, R. Hu, J. A. Zeitler, B. Z. Tang, E.
Pickwell-MacPherson, Mater. Horiz. 2014, 1, 251 – 258.
[48] T. Zhang, G.-L. Zhang, Q.-Q. Yan, L.-P. Zhou, L.-X. Cai, X.-Q.
Guo, Q.-F. Sun, Inorg. Chem. 2018, 57, 3596 – 3601.
[49] N. Busschaert, C. Caltagirone, W. Van Rossom, P. A. Gale,
Chem. Rev. 2015, 115, 8038 – 8155.
[18] P. Sabater, F. Zapata, B. Lꢁpez, I. Fernꢂndez, A. Caballero, P.
Molina, Dalton Trans. 2018, 47, 15941 – 15947.
[50] S. Lee, A. H. Flood, J. Phys. Org. Chem. 2013, 26, 79 – 86.
[19] J. Zhao, D. Yang, Y. Zhao, X.-J. Yang, Y.-Y. Wang, B. Wu,
Angew. Chem. Int. Ed. 2014, 53, 6632 – 6636; Angew. Chem.
2014, 126, 6750 – 6754.
Manuscript received: June 10, 2021
[20] M. M. Watt, J. M. Engle, K. C. Fairley, T. E. Robitshek, M. M.
Haley, D. W. Johnson, Org. Biomol. Chem. 2015, 13, 4266 – 4270.
Accepted manuscript online: June 29, 2021
Version of record online: && &&
,
&&&&
&&&&
www.angewandte.org
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Research Articles
Supramolecular Chemistry
A. Docker, X. Shang, D. Yuan, H. Kuhn,
Z. Zhang, J. J. Davis, P. D. Beer,*
M. J. Langton*
&&&& — &&&&
Halogen Bonding Tetraphenylethene
Anion Receptors: Anion-Induced
Emissive Aggregates and
Photoswitchable Recognition
A series of halogen bonding (XB) tetra-
phenylethene (TPE) based anion recep-
tors are reported. The receptors exhibit
a remarkable anion-induced turn-on
luminescence response, in which halogen
bonding anion interactions drive the for-
mation of emissive nanoscale aggre-
gates. Disubstituted XB TPE derivatives
function as photoswitchable anion
receptors, in which modulation of pho-
tostationary states is intimately related to
anion affinity.
Angew. Chem. Int. Ed. 2021, 60, 2 – 11
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!