A Dynamic and Responsive Host in Action: Light-Controlled Molecular Encapsulation

Article abstract of DOI:10.1002/anie.201607693

The rational design of a flexible molecular box, oAzoBox⁴⁺, incoporating both photochromic and supramolecular recognition motifs is described. We exploit the E?Z photoisomerization properties of azobenzenes to alter the shape of the cavity of t

Full text of DOI:10.1002/anie.201607693

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International Edition: DOI: 10.1002/anie.201607693
German Edition: DOI: 10.1002/ange.201607693
Supramolecular Chemistry
Hot Paper
A Dynamic and Responsive Host in Action: Light-Controlled
Molecular Encapsulation
Seꢀn T. J. Ryan+, Jesffls del Barrio+,* Reynier Suardíaz, Daniel F. Ryan, Edina Rosta, and
Oren A. Scherman*
Abstract: The rational design of a flexible molecular box,
oAzoBox4+, incoporating both photochromic and supramolec-
ular recognition motifs is described. We exploit the E$Z
photoisomerization properties of azobenzenes to alter the
shape of the cavity of the macrocycle upon absorption of light.
Imidazolium motifs are used as hydrogen-bonding donor
components, allowing for sequestration of small molecule
guests in acetonitrile. Upon E!Z photoisomerization of
oAzoBox4+ the guest is expelled from the macrocyclic cavity.
ability to alter the interaction between a host and its guests is
usually limited to invasive actions, such as the addition of
strongly competing guest compounds or pH/redox switch-
ing.[10,11] Ideal triggering mechanisms should enable remote
control over guest uptake and release in a well-defined
spatiotemporal fashion by a practical and easily operated
stimulus, such as light. Indeed, the concept of photocontrolled
uptake and release of guest species has been achieved by
exploiting light-responsive guests and, less frequently, host
species.[12–14]
T
he encapsulation and active release of molecular species
A few groups have provided examples of both strategies
by controlling the dynamic encapsulation properties of cyclo-
dextrines, calixarenes, cucurbiturils, metal–organic cages, and
other systems with molecular switches, either appended to the
host or as a guest molecule.[15–26] Systems which rely on the
rearrangement or isomerization of a guest compound have
a relatively limited scope of applicability. In our view, light-
switchable molecular containers may impact a much wider
spectrum of technological applications. However, they also
suffer from undesirable drawbacks, such as cumbersome
synthesis and hindered isomerization properties by ring strain
and molecular crowding.[27]
comprises an area of research that attracts constant attention
and crosses both academic and industrial research interests, as
encapsulation processes are ubiquitous in product synthesis
and formulation.[1,2] One particular encapsulation strategy
consists of the selective inclusion of guest compounds within
the cavities of discrete, shape-persistent macrocycles and has
been applied to the solubilization and/or stabilization of
active ingredients and hazardous materials, sensing, separa-
tion, and purification technologies.[3–9] Owing to their
dynamic nature, binding events in host–guest complexes can
be controlled by a range of different stimuli. However, in spite
of the many examples of noncovalent complexation, our
We report, here, the synthesis of oAzoBox4+ (Figure 1),
a photoresponsive molecular box produced by a facile three-
step synthetic procedure. We have made use of two o-xylene-
bridged bis(imidazolium)-azobenzene motifs to impart both
[*] Prof. Dr. O. A. Scherman
Melville Laboratory for Polymer Synthesis, Department of Chemistry
University of Cambridge, Lensfield Road, Cambridge, CB2 1EW (UK)
E-mail: oas23@cam.ac.uk
Dr. J. del Barrio[+]
Schlumberger Gould Research
Madingley Road, Cambridge, CB3 0EL (UK)
and
Melville Laboratory for Polymer Synthesis
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge, CB2 1EW (UK)
E-mail: JBarrio2@slb.com
Dr. D. F. Ryan
NASA Goddard Space Flight Center, Greenbelt, MD (USA)
Dr. R. Suardꢀaz
Department of Chemistry, King’s College London
London SE1 1DB (UK)
Dr. E. Rosta
Department of Chemistry, King’s College London
London SE1 1DB (UK)
Dr. S. T. J. Ryan[+]
Melville Laboratory for Polymer Synthesis
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge, CB2 1EW (UK)
[+] These authors contributed equally.
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201607693.
Figure 1. Chemical structure of oAzoBox4+ (a), its synthesis (b), and
small molecule guests used in this study (c).
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light-responsiveness and receptor-like[28,29] properties to our
macrocycle. In contrast to the rigid structure of more conven-
tional azobenzene-containing macrocycles,[27,30] oAzoBox4+
exhibits a large and flexible architecture, which has two
consequences: Firstly, the photochromic properties of oAzo-
Box4+ are largely unaffected in comparison to model com-
pound, AzoBI2+ (Figure 1), by the embedment of the photo-
switches in a cyclic architecture and secondly, its high
flexibility is not detrimental to its recognition abilities.
Therefore, oAzoBox4+ is ideal for the realization of light-
controlled catch-and-release in solution.
oAzoBox4+ was synthesized according to Figure 1b.
Firstly, the reduction of commercially available 4-nitrobenzyl
alcohol and subsequent reaction with CDI yielded intermedi-
ate 2. Cyclization of 2 with an equimolar amount of a,a’-
dibromo-o-xylene, followed by salt metathesis, afforded
oAzoBox·4BF4 in approximately 25% yield after purification
by recrystallization. The solid-state structure (Figure 2)
reveals that oAzoBox4+ is substantially elongated, with
a length of 21.05 ꢀ, as measured by the distance between
the centroids of the o-xylene bridges. The breadth of the box,
measured as the average distance between the planes of the
two sets of parallel azobenzene phenyl units, was found as
4.3 ꢀ, affording an aspect ratio of approximately 5.
Figure 3. 1H NMR spectra (CD3CN, 500 MHz) of 4DPDO (a), E,E-
oAzoBoxꢀ4DPDO (b), E,E-oAzoBox4+ before (c) and after (d) UV light
irradiation and followed by addition of excess 4DPDO (e) (Ha
resonances are assigned to illustrate binding behavior; for full assign-
ments see the Supporting Information).
1
The H NMR spectrum of a freshly prepared solution of
oAzoBox4+ (Figure 3c) shows nine distinct resonances, which
are consistent with the all-trans E,E-oAzoBox4+ stereoisomer.
When a solution of E,E-oAzoBox4+ is irradiated with UV
light, a new set of signals arises, which evidences the
generation of an isomeric mixture of multiple components.
The new aliphatic signals in the 5.00–5.50 ppm region are
shifted upfield relative to the Hh proton resonance of E,E-
oAzoBox4+, which is consistent with the changes associated to
the E!Z AzoBI2+ photoisomerization. Furthermore, the
intensity of the Ha proton resonance of E,E-oAzoBox4+
decreases and three additional sharp and well-resolved
resonances, which are associated to the same type of Ha
resonance, appear at 7.95, 6.88, and 6.75 ppm. In combination,
these results suggest that the UV-light-promoted E!Z
isomerization generates a mixture of three distinct stereoiso-
mers which, at the photostationary state, can be identified as
E,E-oAzoBox4+ (18%), E,Z-oAzoBox4+ (38%), and Z,Z-
oAzoBox4+ (44%). The complete assignment of all proton
resonances of each individual stereoisomer was achieved
Figure 2. X-ray crystal structure of E,E-oAzoBox4+ (left) and geometry-
optimized molecular structures (B3LYP-D3(BJ)/TZVP) of E,E-oAzo-
Box4+ꢀ4DPDO (middle) and E,E-oAzoBox4+ꢀBPDC (right).
1
using two-dimensional COSY and ROESY H NMR spec-
troscopy (Figures S3–S10). Irradiation with visible light
largely restores the initial spectrum, which parallels our
electronic absorption spectroscopy results (E,E-oAzoBox4+:
64%, E,Z-oAzoBox4+: 28%, and and Z,Z-oAzoBox4+: 9%).
Smooth cycling between the two E- and Z-predominant states
was demonstrated without any noticeable degradation (Fig-
ure S11).
The photoisomerization properties of model azobenzene,
AzoBI2+, were first examined by electronic absorption
spectroscopy. AzoBI2+ (see Figure S1 in the Supporting
Information) exhibits a characteristic strong p–p* absorption
band at short wavelengths (lmax = 321 nm) and a weaker n–p*
absorption band at longer wavelengths (lmax = 445 nm). Upon
UV light irradiation (350 nm), the intensity of the band
corresponding to the p–p* transition strongly decreased,
whereas that of the n–p* transition slightly increased. These
spectroscopic changes can be directly ascribed to the E!Z
photoisomerization, which can be reverted using visible light
(420 nm). The electronic absorption spectrum of oAzoBox4+
matches that of AzoBI2+. The spectral changes associated to
the E!Z photoisomerization of oAzoBox4+ are analogous to
those of AzoBI2+. Therefore, it was estimated that the E$Z
photoisomerization behaviors of AzoBI2+ and oAzoBox4+
should closely resemble one another.
Thermal relaxation of the Z-predominant oAzoBox4+, via
the stepwise pathway outlined in Figure S15, was monitored
by thermal array 1H NMR spectroscopy at 313, 318, 323, and
328 K, as their stabilities are important considerations with
regard to the potential of oAzoBox4+ to act as a photo-
switchable molecular container. Fitting the data to the
appropriate kinetic models [Eqs. (S4)–(S7)] and the Eyring
equation [Eqs. (S8), (S9)] allowed extraction of the rate
constants k1 and k2 and the thermodynamic parameters DG°,
DH°, and DS° (Table 1, Figures S16–S34, Tables S1–S8).
The activation energy barrier (DG°) of Z,Z-oAzoBox4+!
E,Z-oAzoBox4+ is very similar to that of Z-AzoBI2+!E-
2
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Table 1: Thermodynamic data for the thermal Z!E isomerization of
oAzoBox4+ and AzoBI2+ at 293 K.
Switching
species
DG°
DH°
DS°
[kcalmolꢁ1
]
[kcalmolꢁ1
]
[calmolꢁ1 Kꢁ1
]
Z,Z!E,ZoAzoBox4+
E,Z!E,EoAzoBox4+
Z!EAzoBI2+
19.23ꢂ1.06
22.24ꢂ4.53
19.86ꢂ4.2
22.46ꢂ0.31
23.85ꢂ2.30
22.66ꢂ1.21
11.01ꢂ0.97
5.48ꢂ7.85
9.55ꢂ3.77
Figure 4. High-resolution mass spectrometry of E,E-oAzo-
Box4+ꢀ4DPDO (average values: x=0.2507, y=0.3338) (a). Global fit
by nonlinear regression of the 1H NMR shifts of the Ha, Hb, Hg and
Hh resonances to a 1:1 binding model[32] (b).
AzoBI2+ at 293 K (Figures S24, S34). DG° of E,Z-oAzo-
Box4+!E,E-oAzoBox4+ is slightly higher at 22.24 kcalmolꢁ1
(Figure S25), likely on account of the length disparity of E-
and Z-azobenzene. The magnitudes of the DG° values were
corroborated by computational studies (Tables S9, S10).
DFT calculations (B3LYP-D3(BJ)/TZVP, Figure S41)
revealed that the lowest energy molecular configuration of
Z,Z-oAzoBox4+ is 10.97 kcalmolꢁ1 greater than that of E,Z-
oAzoBox4+, which is in turn 14.43 kcalmolꢁ1 greater than that
of E,E-oAzoBox4+. Such a result, combined with the sim-
ilarity of the DG° values (Tables 1), indicates that ring
strain[31] does not play a significant role in affecting the
thermal isomerization mechanism for oAzoBox4+.
The inclusion geometry assigned to the bimolecular
complex was corroborated by quantum-mechanical calcula-
tions (Figure 2). The energy-minimized structure of E,E-
oAzoBox4+ꢀ4DPDO shows that the container adopts a cage-
like conformation with the phenyl rings of the azobenzene
moieties lying in parallel planes and the 4DPDO guest
included in the cavity of the macrocycle. Each of the oxygen
atoms of the guest are hydrogen-bonded to two of the four of
acidic Hd protons in an approximately symmetric fashion
(Figure S42). This interpretation of the binding is supported
by the significant downfield shift of the Hd proton resonance
and a series of 1H NMR titration experiments (see the
Supporting Information). Calculations also reveal that the
macrocycle adopts a significantly expanded conformation in
comparison to that of the solid-state structure upon guest
sequestration with an appreciably reduced aspect ratio of
about 3. Similar conclusions were also established by quan-
tum-mechanical calculations for E,E-oAzoBox4+ꢀBPDC
(Figure S46).
The large size of E,E-oAzoBox4+ led us to consider
whether small organic guest compounds could be accommo-
dated within the cavity of the macrocycle. The representative
compounds, 2,2’-dipyridyl N,N’-dioxide (2DPDO), 4,4’-dipyr-
idyl N,N’-dioxide (4DPDO), 4-phenylpyridine N-oxide
(PPO), and biphenyl-4,4’-dicarboxylate (BPDC) were
chosen to test the interaction of E,E-oAzoBox4+ with hydro-
1
gen bond acceptor aromatic structures (Figure 1c). H NMR
revealed a binding interaction between E,E-oAzoBox4+ with
4DPDO, PPO and BPDC (Figures 3b, S36, and S37).
However, no interaction was detected between E,E-oAzo-
Box4+ with 2DPDO, presumably on account of steric effects.
All cases of binding exhibited upfield shift perturbations of
the aromatic resonances, Ha and Hb, whereas proton
resonances Hg-i shifted downfield. The encapsulated guest
proton resonances shifted upfield on account of the shielding
by the azobenzene moieties at the long sides of E,E-oAzo-
Box4+.
Exposure of E,E-oAzoBox4+ꢀ4DPDO to UV light indu-
ces the E!Z isomerization of the host and the release of
4DPDO, evidenced by the downfield shift of the Hw and Hf
resonances (Figures 5 and S51). This result can be rational-
ized by assuming the 4DPDO affinity of E,Z-oAzoBox4+ and
Z,Z-oAzoBox4+ is negligible in comparison to that of E,E-
oAzoBox4+. Indeed, when excess 4DPDO was added into
a Z-predominant oAzoBox4+ isomeric mixture, no evidence
of interaction was detected between Z,Z-oAzoBox4+ and the
guest molecule (unperturbed Z,Z-Ha resonances) and only
extremely limited interaction was observed for E,Z- oAzo-
Box4+ (Figures 3d,e). An attempt to quantify the 4DPDO
affinity of the E,Z stereoisomers was unsuccessful on account
of the limited interaction between the guest and the host after
UV light irradiation. In any event, irradiating the mixture
with visible light reverts the system back to the E,E-
oAzoBox4+ꢀ4DPDO enriched state.
4DPDO was selected as a representative example to
illustrate the encapsulation potential of E,E-oAzoBox4+ in
acetonitrile. A 1H NMR titration of 4DPDO into E,E-oAzo-
Box4+ provided an association constant on the order of 103 mꢁ1
(Figure 4) corresponding to an association free energy of
about ꢁ4 kcalmolꢁ1, which is consistent with the computa-
tionally obtained value of ꢁ4.06 kcalmolꢁ1 (see the Support-
ing
Information).
The
formation
of
E,E-oAzo-
Box4+ꢀ4DPDO was also confirmed by mass spectrometry
1
(Figures 4a, S38). A control H NMR experiment, whereby
The thermal stability of Z-predominant oAzoBox4+ was
unaffected by the presence of 4DPDO (Figure S35). The low
4DPDO affinity of E,Z-oAzoBox4+ and Z,Z-oAzoBox4+ can
be attributed to a significant decrease in the size of the cavity
of the macrocycle. Additionally, favorable orientation of the
acidic Hd protons towards the interior of the cavity is lost
upon UV light irradiation, and consequently the possibility of
establishing concerted hydrogen-bonding interactions
between the host and guest. This was supported by calculated
4DPDO was mixed with an equimolar amount of E-AzoBI2+,
showed no evidence of interaction, suggesting that macro-
cyclic preorganization is a requirement for strong binding in
our system (Figure S13). Similar conclusions were obtained
from an analogous experiment with 4DPDO and a,a’-bis[3-
(1-methylimidazolium)]-o-xylene, a model subcomponent
analog of the o-xylene bridging unit of oAzoBox4+ (Fig-
ure S14).
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scholarship. J.D.B. thanks Marie Curie IEF (project number
273807). R.S. acknowledges the EC for a Marie Curie
fellowship (project number 622711). D.F.R is supported by
the NASA Postdoctoral Program administered by the Uni-
versities Space Research Association. This work was sup-
ported by the EPSRC (reference number EP/G060649/1), an
ERC Starting Investigator Grant (project number 240629),
and a Next Generation Fellowship from the Walters-Kundert
Foundation. The authors thank HECBioSim (EPSRC grant
number EP/L000253/1) via ARCHER, and the Ada Kings
HPC3 service. Felix Cosmin Mocanu is gratefully acknowl-
edged for assistance in the synthesis of compound 1.
Keywords: host–guest systems · hydrogen bonds ·
macrocycles · photochemistry · supramolecular chemistry
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structures of E,Z-oAzoBox4+ and Z,Z-oAzoBox4+ (Figur-
es S39, S40) and the high energy levels associated with the
putative
E,Z-oAzoBox4+ꢀ4DPDO
and
Z,Z-oAzo-
Box4+ꢀ4DPDO complexes (Figures S43–S45). The concept
of photocontrolled catch-and-release was also demonstrated
for BPDC (Figure S37).
In conclusion, we have demonstrated how the photo-
chromic macrocycle, oAzoBox4+, may be synthesized in three
simple steps and allows for the realization of the remote
controlled catch-and-release concept mediated by a photo-
switchable molecular container. Our work illustrates how the
incorporation of photochromic switching elements into a flex-
ible macrocyclic framework, which does not compromise
light-induced isomerization, can also exhibit relevant recog-
nition properties. These have been achieved by exploiting the
hereto unreported complementary hydrogen-bonding H-
donor imidizolium and H-acceptor N-oxide pairs, as well as
the H-acceptor carboxylate. The incorporation of molecular
switches into optimized organic container structures may be
regarded as a general approach to regulate encapsulation, in
a non-invasive fashion, of selected molecular species.
Such a strategy may also be used as a supramolecular
host–guest logic system, as distinct switching between stereo-
isomeric mixtures may be achieved, where the predominant
components posses association constants separated by orders
of magnitude.
Acknowledgements
S.T.J.R acknowledges the Cambridge Home and European
Scholarship Scheme and the Robert Gardiner memorial
4
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Received: September 2, 2016
Published online: && &&, &&&&
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Communications
Supramolecular Chemistry
S. T. J. Ryan, J. del Barrio,* R. Suardꢁaz,
D. F. Ryan, E. Rosta,
O. A. Scherman*
&&&& — &&&&
A Dynamic and Responsive Host in
Action: Light-Controlled Molecular
Encapsulation
One ring to rule them all … and in the
darkness bind them: A photoresponsive
tetracationic macrocycle features a facile
three-step synthesis and makes possible
the noncovalent binding of a set of guest
molecules, including N-oxide and car-
boxylate derivatives. The guest molecules
can be actively released by UV light
irradiation in a remote fashion.
6
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