Photocontrol over cucurbit[8]uril complexes: Stoichiometry and supramolecular polymers

Article abstract of DOI:10.1021/ja406556h

Herein we report the photocontrol of cucurbit[8]uril (CB[8])-mediated supramolecular polymerization of azobenzene-containing monomers. The CB[8] polymers were characterized both in solution and in the solid state. These host-guest complexes can be reversi

Full text of DOI:10.1021/ja406556h

Communication
pubs.acs.org/JACS
Photocontrol over Cucurbit[8]uril Complexes: Stoichiometry and
Supramolecular Polymers
Jesus del Barrio,† Peter N. Horton,‡ Didier Lairez,§ Gareth O. Lloyd,∥ Chris Toprakcioglu,⊥
́
and Oren A. Scherman*,†
†Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, U.K.
‡EPSRC National Crystallography Service, University of Southampton, Southampton SO17 1BJ, U.K.
§Laboratoire Leo
́
n Brillouin, CEA Saclay, Gif-sur-Yvette Cedex 91191, France
∥Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K.
⊥Department of Physics, University of Patras, Patras 26500, Greece
S
* Supporting Information
The photocontrolled molecular recognition of hosts with
ABSTRACT: Herein we report the photocontrol of
cucurbit[8]uril (CB[8])-mediated supramolecular poly-
merization of azobenzene-containing monomers. The
CB[8] polymers were characterized both in solution and
in the solid state. These host−guest complexes can be
reversibly switched between highly thermostable photo-
stationary states. Moreover, a remarkable stabilization of Z-
azobenzene was achieved by CB[8] complexation,
allowing for structural characterization in the solid state.
azobenzenes has been used to develop hydrogels, micelles,
molecular shuttles, ion channels, and drug-delivery vehicles.4
With regard to the combination of azobenzenes and CB[n],
Isaacs and co-workers demonstrated that CB[7] can promote the
E → Z isomerization of 4,4′-diaminoazobenzene in aqueous
solution at pH 3−6, as the high binding energy of the complex
overcompensates for the thermodynamic cost of populating the
Z form.5 This system was cleverly used to construct an indicator
displacement assay for a series of biologically relevant amines.
We recently described the stimuli-responsive properties of a non-
covalent complex based upon CB[8]-mediated heteroternary
complexation with 1,1′-dimethyl-4,4′-bipyridinium dication (1)
and an uncharged azobenzene derivative.3d In that case, the
extended E-azobenzene was able to act as a second guest for a
CB[8]·1 binary complex, leading to the formation of a 1:1:1
heteroternary complex. Conversely, the photogenerated bent-
shaped Z isomer was geometrically unfavorable for binding
inside the CB[8] cavity in the presence of 1.
Here we describe a new photoinduced complexation/
decomplexation process to control the binding stoichiometry
of a series of CB[8] complexes in a fully reversible manner. Kim
and co-workers demonstrated the growth of a CB[8] oligo-
(pseudorotaxane) from a metallic surface.6a More recently,
Zhang and co-workers reported a series of naphthalene- and
anthracene-containing monomers that polymerized in the
presence of CB[8].6b Herein we demonstrate for the first time
the formation of CB[8] supramolecular polymers from guest
monomers bearing azobenzene moieties both in solution and in
the solid state. We also report the photocontrol of CB[8]-
mediated supramolecular polymerization by means of photo-
induced E−Z isomerization of the azobenzene moieties.
xternal control of self-assembling systems has attracted
1
Econsiderable interest across many scientific disciplines. In
supramolecular chemistry, an important focus is the use of light
to harness reversible control over the state of host−guest
complexes, as it does not require additional components and
triggering can be easily achieved in a completely remote manner
where and when required. The photoinduced isomerization of
azobenzene compounds has been the molecular basis for a range
of such photosensitive materials and self-assembling systems.2
Thus, the inclusion of azobenzene compounds as guests in
different macrocyclic host molecules, including cyclodextrins,
calixarenes, pillararenes, and more recently cucurbit[n]urils
(CB[n]; the structures of CB[7] and CB[8] are shown in
Chart 1), can give rise to photosensitive host−guest complexes.3
Chart 1. Structures and Labeling of the Studied Compounds
We selected 1 and 1-[p-(phenylazo)benzyl)]pyridinium
chloride (E-2) (Chart 1) as model first and second guest
molecules for the investigation of the photoresponsive properties
of CB[8] complexes. Doubly charged guest 1 first binds to CB[8]
in a 1:1 ratio to form the CB[8]·1 complex with a binding
constant (K1) on the order of 105 M−1 (Scheme 1b).7 In general,
E isomers of azobenzene compounds are extended organic
Received: June 27, 2013
Published: July 23, 2013
© 2013 American Chemical Society
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time scale. The association strength of E-2 and CB[8]·1 was
obtained by isothermal titration calorimetry (ITC). Titration of
E-2 into an aqueous solution of CB[8]·1 resulted in an
exothermic isotherm; fitting to a 1:1 complexation model
(Figure S3) gave a K2 value of (2.6 0.4) × 104 M−1 after
correction for the heats of dilution.
Scheme 1. (a) Isomerization of 2; (b) Stepwise Formation of
CB[8]·1·E-2; (c) Isomerization of 2 in the Presence of CB[8]
and 1 and Equilibrium after UV Irradiation of CB[8]·1·E-2
In general, upon irradiation with UV light, the E isomer of an
azobenzene compound can be transformed into the Z isomer,
which can revert back to the E state through either a thermal or
photoinduced process (Scheme 1a). The photoisomerization
properties of 2 in the absence and presence of CB[8]·1 were
investigated by UV−vis spectroscopy. The main band associated
with E-2 decreased upon UV irradiation, and two new bands
appeared at ∼290 and ∼425 nm (Figure S12). These bands were
related to Z-2, and the changes in the UV−vis spectrum can be
ascribed to a decrease in the E:Z ratio after irradiation. The
lifetime of the Z isomer was found to be on the order of several
days, similar to those of other cationic azobenzene derivatives.4d
Figure 1a,b shows partial 1H NMR spectra of pure E-2 before and
after UV exposure. The set of upfield peaks in the NMR spectrum
of an irradiated sample of E-2 (Figure 1b) corresponds to the Z
isomer. The photostationary state of the Z isomer (PSSZ) of 2
was observed at an E:Z ratio of 20:80. Subsequent irradiation of
Z-2 with visible light (λ > 420 nm) resulted in a PSS with the E
form as the predominant isomer (∼90%).
When a solution of CB[8]·1·E-2 was exposed to UV light, the
band corresponding to the π−π* transition strongly decreased,
whereas the n−π* band became narrower and shifted to lower
wavelength (Figure S12). The changes in the UV−vis spectrum
indicated the efficient E → Z photoisomerization of 2, and the
presence of two isosbestic points at 273 and 397 nm established
that the system involves a clean two-state equilibrium. However,
the differences between 2 alone and CB[8]·1·E-2 after UV
irradiation certainly suggested different molecular environments
of Z-2 in the presence and absence of CB[8] and 1. In an effort to
elucidate the type of interactions between CB[8], 1, and Z-2, we
recorded the 1H NMR spectrum of an equimolar mixture of these
three molecules after irradiation with UV light (Figure 1d). The
spectrum is composed of a significant number of relatively sharp
resonances in comparison to that of CB[8]·1·E-2 (Figure 1c).
After UV irradiation, the signals corresponding to protons Hj and
Hk of the azobenzene guest remained almost unchanged,
whereas those corresponding to protons Hc−i were significantly
shifted upfield relative to those for Z-2 alone. Conversely, the
signals corresponding to protons Ha and Hb of 1 were shifted
downfield. These changes suggested that Z-2 was included inside
the CB[8] cavity, whereas 1 may have resided outside the host
cavity. The exclusion of 1 after UV irradiation of CB[8]·1·E-2
was clearly evidenced by a series of diffusion-ordered NMR
spectroscopy (DOSY) experiments. Before irradiation, the NMR
signals of CB[8], 1, and E-2 shared a single diffusion coefficient
(D = [2.72 0.07] × 10−10 m2 s−1) as they diffused together as
one entity (Figure S4). After UV irradiation, the D values of two
species were clearly separated, as 1 (D = [7.94 0.50] × 10−10
m2s−1) and the larger species CB[8]·Z-2 (D = [2.88 0.08] ×
10−10 m2 s−1) diffused independently (Figure S5). According to
the Stokes−Einstein equation (D = kBT/6πηr), the hydro-
dynamic radius of a molecular species (r) is inversely
proportional to D in a medium of viscosity η (kB is Boltzmann’s
constant and T is the absolute temperature).8 Thus, a
comparison of the sizes of two molecular species on the basis
of their D values is valid, assuming that they have a spherical
hydrodynamic shape and are at the same temperature.
structures that can form CB[8] ternary complexes with
complementary 4,4′-bipyridinium derivatives.3d The CB[8]·1
complex can accommodate one E-2 molecule in a second binding
event, which was initially investigated by UV−vis spectroscopy. A
relative decrease in intensity as well as a slight hypsochromic shift
of the main absorption band of the azobenzene chromophore
(π−π* transition at ∼320 nm) were detected when E-2 was
added to a solution of CB[8]·1 in a 1:1 ratio [Figure S1 in the
Supporting Information (SI)]. The band associated with the
n−π* transition at ∼430 nm was also decreased and broadened.
These changes are due to intermolecular interactions and/or
changes in the polarity of the chromophore environment and
indicate the formation of a CB[8]·1·E-2 ternary complex.
To further confirm the formation of the ternary complex, 1H
NMR titration experiments were performed in D2O by gradually
increasing the concentration of E-2 while the concentration of
CB[8]·1 was kept constant (Figure S2). The signals for the
protons in 1 and E-2 were shifted upfield and broadened, except
for those of Hi and Hj of E-2, which were slightly shifted
downfield (Figures 1 and S2). These changes indicated the
formation of a ternary complex with fast exchange on the NMR
Figure 1. Partial 1H NMR spectra (500 MHz, D2O, 298 K) of (a, b) 2
before (a) and after (b) UV irradiation with an E:Z ratio ∼0.40:0.60 and
(c, d) CB[8]·1·E-2 before (c) and after (d) UV irradiation with an E:Z
ratio ∼0.05:0.95. See Chart 1 for proton labeling. Signals corresponding
to protons of the Z-2 isomer are indicated with apostrophes. For clarity,
only the Z-2 signals are labeled in (b).
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We formulate the structures resulting after UV irradiation
primarily as a mixture of uncomplexed 1 and CB[8]·Z-2 (Scheme
1c). This result can be rationalized most simply by assuming that
Z-2 fully occupies the CB[8] cavity with a binding constant much
higher than K1. A series of ITC experiments allowed us to
confirm this hypothesis. When a solution of CB[8] was titrated
with Z-2, an exothermic isotherm was obtained, and fitting
suggested a binding constant of up to (1.0 0.4) × 107 M−1
(Figure S3). In comparison, the binding constant for preformed
CB[8]·Z-2 and 1 was too small to be measured by ITC.
The interactions between CB[8], 1, and 2 greatly influence the
photoswitchable behavior. First, the conversion of 2 after UV
irradiation of CB[8]·1·E-2 was determined to be >95% at PSSZ.
The higher conversion is likely related to the lack of spectral
overlap of the two isomers, thus allowing for selective excitation
at 360 nm. The photoinduced E → Z process is slower for
CB[8]·1·E-2 than for E-2 alone (Figure S13). The CB[8]·Z-2
complex also shows a slower thermal Z → E isomerization than
Z-2 alone, which could be attributed to a higher stability of the
binary complex. Indeed, the high stability of CB[8]·Z-2 allowed
for structural characterization in the solid state, which is quite
uncommon for both Z-azobenzene compounds and CB[8]
inclusion complexes. Pale-yellow single crystals of CB[8]·Z-2
were grown by slow evaporation of an aqueous solution of
CB[8]·1·E-2 after UV irradiation. Figure 2 shows the X-ray
angle neutron scattering (SANS) and static light scattering
(SLS). Surprisingly, large structures with a radius of gyration (Rg)
of ∼35 nm were detected by SANS (Figure S19). A similar Rg of
∼40 nm was obtained when the same mixture was analyzed by
SLS. These results give a more complete description of the
equilibrium between the 2:2 and n:n complexes that exists when
CB[8] and E-3 are mixed in a 1:1 ratio. The larger structures
detected by SANS and SLS are probably related to “short-lived”
solution-phase supramolecular polymers (n:n complexes), which
can be detected only by static techniques. We were also
successful in characterizing the structure of the CB[8]n·E-3n
supramolecular polymer in the solid state by synchrotron X-ray
diffraction measurements (Figure 3). The E-3 guest molecules
Figure 3. Side view of CB[8]n·E-3n. Colors as in Figure 2.
clearly form a polymeric structure through pseudorotaxane
interdigitation of CB[8] host molecules. To the best of our
knowledge, this crystal structure represents the first of its kind for
a linear CB[8]-mediated polymer.
The photoswitchable behavior of CB[8]·3 and its effect on the
host−guest complex stoichiometry was investigated by UV−vis
1
and H and DOSY NMR spectroscopies. A decrease in the
intensity of the π−π* absorption band as well as a large blue shift
of the n−π* band were detected after the photoinduced E → Z
isomerization of 3 in the presence of CB[8] (Figure S11). This
parallels the results observed for CB[8]·1·E-2. The changes in
1
the H NMR spectrum after UV exposure were also consistent
Figure 2. Side and top views of CB[8]·Z-2. Only the largest component
of Z-2 disorder is shown. C, gray; N, blue; O, red; H, white.
with those detected for the heteroternary complex and suggested
the formation of a 1:1 CB[8]·Z-3 complex with CB[8] tightly
bound to the azobenzene moiety of Z-3 (Figure S21). DOSY
experiments yielded D values for two different species after UV
irradiation: the 2:2 complex CB[8]2·E-32 (D = [2.16 0.03] ×
10−10 m2 s−1) and the 1:1 complex CB[8]·Z-3 (D = [2.82 0.05]
× 10−10 m2 s−1), which is roughly half the size of CB[8]2·E-32
(Figures S7 and S8). A drastic size reduction from ∼35 to ∼1 nm
was also observed by both SLS and SANS, indicating that the
host−guest complex after UV irradiation can be adequately
described as a 1:1 CB[8]·Z-3 complex. Thus, depolymerization
was indeed achieved after UV irradiation.
A 2:1 mixture of CB[8] and the guest molecule E,E-4 could
yield only linear supramolecular polymers, as the formation of
complexes with lower absolute stoichiometry would be
prevented by the design of 4.6b The 1H NMR spectrum of this
mixture contained a set of broad and ill-defined signals
presumably corresponding to polymeric species (Figure 4a).
The molecular size of the complexes could not be determined by
either DOSY (because of the absence of well-defined spectra) or
SANS (on account of the limited q range of the setup to which we
had access). Thus, the size determination relied only on SLS
experiments, which gave Rg ≈ 60 nm for a 2:1 mixture of CB[8]
and E,E-4. This larger Rg relative to that obtained by SLS for an
equimolar mixture of CB[8] and E-3 is likely due to both the
larger size of 4 and stronger association between the repeating
units of the supramolecular polymer. A control experiment was
performed wherein CB[7] (Chart 1) instead of CB[8] was mixed
crystal structure of CB[8]·Z-2. The Z-azobenzene moiety of the
guest is locked inside the CB[8] cavity, while the pyridinium
group remains at the carbonyl portal area. The complexed CB[8]
exhibits an ellipsoidal deformation characterized by the major
and minor axes at the equator (major, 14.1 Å; minor, 12.0 Å) and
the carbonyl portal (major, 10.8 Å; minor, 8.7 Å).
Guest molecule E-3 can be regarded as an AB-type monomer,
with A and B representing bipyridinium- and azobenzene-type
first and second CB[8]-binding motifs (Chart 1). A theoretical
equilibrium between a 2:2 complex and a linear supramolecular
polymer can be expected for a 1:1 mixture of CB[8] and E-3
(Scheme S2 in the SI). Comparison of the 1H NMR spectra for
E-3 in the absence and presence of CB[8] revealed that most of
the proton resonances of the guest were shifted upfield (Figure
S21), suggesting the formation of a host−guest inclusion
complex. Since the integration of the resonances was consistent
with either 2:2 or n:n (supramolecular polymer) stoichiometry,
the absolute stoichiometry was further investigated by DOSY,
which gave D = (2.16 0.03) × 10−10 m2 s−1 for a sample
prepared from equimolar amounts of CB[8] and E-3 (Figure S6).
This value is 79% of that for CB[8]·1·E-2, meaning that the
heteroternary complex is approximately half the size of the
CB[8] complex with E-3.9 Therefore, the absolute stoichiometry
of the latter is CB[8]2·E-32 on the basis of the DOSY results. The
molecular size of the complex was further investigated by small-
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AUTHOR INFORMATION
Corresponding Author
oas23@cam.ac.uk
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. K. Jackson (Wyatt Technology) and Dr. A.
Koutsioubas (Synchrotron SOLEIL) for helpful discussions.
J.d.B. is grateful for a Marie Curie Intra-European Fellowship
(Project 273807). G.O.L. thanks Heriot-Watt University for
financial support. This work was also supported by an ERC
Starting Investigator Grant (ASPiRe) and a Next Generation
Fellowship provided by the Walters-Kundert Foundation.
1
Figure 4. Partial H NMR spectra (500 MHz, D2O, 298 K) of a 1:2
mixture of 4 and CB[8] before (a) and after (b) UV irradiation. See
Chart 1 for proton labeling.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Procedures and additional data. This material is available free of
charge via the Internet at http://pubs.acs.org.
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