2-(4′-Pyridyl-N-oxide)-Substituted Hemithioindigos as Photoresponsive Guests for a Super Aryl-Extended Calix[4]pyrrole Receptor

Article abstract of DOI:10.1002/asia.201800463

We report the synthesis of two 2-(4′-pyridyl-N-oxide)-substituted hemithioindigos (HTIs). We probed their photoisomerization by using UV/Vis and ¹H NMR spectroscopy techniques. Light irradiation at λ=450 nm provoked the isomerization of the HTI Z isomer to the E counterpart to a large extent (≈80 % at the photostationary state). ¹H NMR titration experiments revealed the formation of thermodynamically and kinetically stable 1:1 inclusion complexes of the (Z)-HTI isomers with a super aryl-extended host (association constant>10⁴ m^?1). Photoirradiation at λ=450 nm of the inclusion complexes induced the isomerization of the bound HTI N-oxide to afford the (E)-HTI?calix[4]pyrrole complex. We determined accurate association constant values for the 1:1 inclusion complexes of the (Z)- and (E)-HTI isomers by using isothermal titration calorimetry experiments. The results showed that the stability constants of the (E)-HTI complexes were 2.2–2.8-fold lower than those of the (Z)-HTI counterparts, which explains the lack of light-induced release of the former to the bulk solution.

Full text of DOI:10.1002/asia.201800463

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: 2-(4'-pyridyl-N-oxide)-Substituted Hemithioindigos as
Photoresponsive Guests for a Super Aryl-Extended
Calix[4]pyrrole Receptor
Authors: Pablo Ballester, Giulia Moncelsi, Luis Escobar, and Henry
Dube
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Chemistry - An Asian Journal
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FULL PAPER
2-(4’-pyridyl-N-oxide)-Substituted Hemithioindigos as
Photoresponsive Guests for a Super Aryl-Extended
Calix[4]pyrrole Receptor
Giulia Moncelsi,[a] [c] Luis Escobar,[a] [c] Henry Dube* and Pablo Ballester*
[d]
[a] [b]
8
9
Abstract: We report the synthesis of two 2-(4’-pyridyl-N-oxide)-
component host assemblies i.e. capsules and cages. Rebek
and co-workers introduced the Z/E photoinduced isomerization
of hemithioindigo (HTI) derivatives to realize photocontrolled
guest exchanges in encapsulation complexes of dimeric
capsules derived from resorcinarene cavitands.10
substituted hemithioindigos. We probed their photoisomerization
1
using UV-Vis and
H NMR spectroscopic techniques. Light
irradiation at 450 nm provoked the isomerization of the HTI Z isomer
1
to the E counterpart in a large extent (~80 % at the PSS). H NMR
titration experiments revealed the formation of thermodynamically
Irie and co-workers exploited the differences in binding ability
exhibited by the Z/E isomers of 2-pyridyl-substituted HTIs with
two Zn-porphyrin receptors to control the shuttling of the former
between the latter using light irradiation. 11 The Z-to-E light-
induced isomerization of HTI derivatives requires longer
and kinetically stable 1:1 inclusion complexes of the Z-HTI isomers
4
-1
with a super aryl-extended host (K
a
> 10 M ). Photoirradiation at
4
50 nm of the inclusion complexes induced the isomerization of the
bound HTI N-oxide affording the E-HTI⊂calix[4]pyrrole complex. We
determined accurate association constant values for the 1:1
inclusion complexes of the Z and E-HTI isomers using isothermal
titration calorimetry experiments (ITC). The results showed a 2.2-
wavelengths
photoisomerization of azobenzenes (365 nm). Remarkably, the
E-HTI isomer can be reverted to the counterpart by
(410-430
nm)
than
the
trans-to-cis
1
2
Z
2
.8-fold decrease in the stability constants of the E-HTI complexes in
subsequent irradiation at wavelengths > 500 nm or completely
by thermal equilibration in the dark. The switching between the
two states shows high efficiency and proceeds with little
photofatigue. In recent years, HTI derivatives have been
comparison to the Z-HTI counterparts explaining the lack of light-
induced release of the former to the bulk solution.
13
exploited as peptide modulators, prototypes of molecular logic
gates,14 machines,15,16 or supramolecular receptors, and their
17
Introduction
18
19
switching properties studied and modulated for possible
applications at the bio-optical window (650-900 nm). The tetra-α
isomers of aryl- and super aryl-extended calix[4]pyrroles are
known to form thermodynamically and kinetically stable 1:1
inclusion complexes with pyridyl-N-oxide derivatives. The
association constant values determined for these inclusion
The photocontrolled uptake and release of guest molecules from
synthetic hosts relies on two strategies that share the use of
appended photoswitchable units and differ in the selection of the
binding partner to which they are covalently attached. 1 The
isomerization of the photoswitch, induced by light or heat,
modifies its shape, spectrum, size, polarity, hydrophobicity, etc.
In turn, these changes can be coupled to modulate the
selectivity and affinity of the host-guest complex. Examples of
unimolecular natural 2 and synthetic hosts 3 , 4 that bind
photoresponsive guests made extensive use of azobenzene
groups as photochromic switches.5,6,7 Likewise, photoresponsive
guests containing azobenzene units have been employed for the
photocontrolled binding affinity modulation using multi-
4
7
-1 20
complexes are in the range of 10 –10 M . The pyridyl-N-oxide
is deeply included in the aromatic cavity and forms four
convergent hydrogen bonds between the oxygen atom of the N-
oxide and the pyrrole NHs of the receptor. Additional - and
CH- interactions are established between the two binding
partners.
We undertook this work with the aim of evaluating the affinity
modulations experienced by the super aryl-extended
calix[4]pyrrole receptor 1 upon the binding of photoresponsive
pyridyl-N-oxides 2 and 3 containing an HTI molecular switch
(
Figure 1). We describe the synthesis of two 2-(4’-pyridyl-N-
[
a]
Prof. P. Ballester, MSc. G. Moncelsi and MSc. L. Escobar
Institute of Chemical Research of Catalonia (ICIQ), The Barcelona
institute of Science and Technology (BIST)
Av. Països Catalans, 16, 43007, Tarragona
E-mail: pballester@iciq.es
oxide)-substituted hemithioindigo derivatives. We study their
photoresponsive behaviour and probe the interaction of the Z
isomers with the super-aryl extended calix[4]pyrrole receptor 1
using
photoisomerization processes induced by light-irradiation of the
formed inclusion complexes. Finally, we use ITC experiments to
assess the binding constants of the inclusion complexes formed
by the Z and E isomers of the photoswitches with super aryl-
extended calix[4]pyrrole 1, and quantify the binding affinity
modulation exerted by the photoisomerization.
1
H
NMR spectroscopy. We investigate the
[
[
[
b]
c]
d]
Prof. P. Ballester
Catalan Institution for Research and Advanced Studies (ICREA)
Passeig Lluís Companys, 23, 08018 Barcelona, Spain
MSc. G. Moncelsi and MSc. L. Escobar
Universitat Rovira i Virgili, Departament de Química Analítica i
Química Orgànica, c/Marcelꞏlí Domingo, 1, 43007 Tarragona, Spain
Dr. H. Dube
Ludwig-Maximilians-Universität München, Department für Chemie
and Munich Center for Integrated Protein Science CIPSM, 81377,
Munich, Germany
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calix[4]pyrrole 1 was synthesized using a described procedure in
literature.20
Scheme 1. Synthetic schemes for the preparation of hemithioindigo N-oxides
Z-2 and Z-3. The synthesis of the aldehyde N-oxide 8 is also shown. PIP =
piperidine.
Figure 1. Molecular structures of: a) the super aryl-extended calix[4]pyrrole
receptor 1 and b) 2-(4’-pyridyl-N-oxide)substituted hemithioindigos 2 and 3, Z
(
left) and E (right) isomers. In the case of 3, the lower energy rotamers of the
e
two isomers are shown. The values of the angles defined by the O, CH and
C(CH atoms are indicated in the Z and E isomers. The E isomer displays a
3 3
)
pronounced bending of the pyridyl-N-oxide and the HTI unit, suggesting a less
favourable fit in the cylindrical-shaped cavity of 1 in cone conformation (vide
infra).
Photoisomerization Studies of HTIs Z‐2 and Z‐3. We first monitored
the photoisomerization process of the HTI N-oxides, Z-2 and Z-3,
at micromolar concentration in chloroform solutions using UV-
Vis absorption spectroscopy. We prepared a solution of 2 (30

M), that mainly contained the Z-2 isomer, and we acquired its
Results and Discussion
UV-Vis absorption spectrum. The absorption spectrum of Z-2
showed three bands of similar intensity with maxima centred at
Synthesis. The nucleophilic substitution reaction of 4-tert-
butylbenzenethiol with 2-bromo acetic acid afforded the aryl
thioglycolic acid 4 in 69% yield.21 The aryl acyl chloride formed
324, 369 and 457 nm, respectively. Then, the same solution was
photoirradiated at 450 nm and we recorded the UV-Vis spectra
at different irradiation times. The photoirradiation process
caused noticeable changes in the absorption bands of 2. After
“in situ” by reaction of 4 with thionyl chloride was cyclized under
Friedel-Crafts acylation conditions to provide the labile
thioindoxyl 5, which was used without further purification in the
condensation reactions with carboxaldehyde N-oxides 6 and 7.
Aldehyde 6 is commercially available. However, to the best of
our knowledge, N-oxide 7 has not been previously described in
literature. Aldehyde derivative 7 was prepared in two synthetic
steps starting from 3,4-lutidine. First, lutidine N-oxide 8 was
isolated as white needles in 63% using sodium perborate in
acetic acid as oxidant.22 Next, the para-methyl substituent of 8
3
0 s of irradiation the absorption spectrum remained unaltered
indicating that the photostationary state (PSS) was reached. At
the PSS, the high-energy band centred at 324 nm increased in
intensity. In contrast, the band appearing at 369 nm displayed a
decreased intensity and a bathochromic shift ( = +5 nm) was
observed. The lower energy band with maximum at 457 nm was
redshifted ( = +17 nm) and also diminished in intensity
(Figure 2). It is known that the absorption spectra of E-isomers
of most HTIs show moderate bathochromic shifts compared to
the Z-isomers.12 Consequently, the observed spectral changes
are in agreement with the increase in concentration of the E-2
isomer in solution and supported the Z-to-E photoisomerization
process. An analogous photoirradiation experiment was
performed with a freshly prepared solution (30 µM) of the HTI N-
oxide Z-3. The initial absorption spectrum of Z-3 is almost
identical to the one registered for Z-2 (three bands centered at
was selectively oxidized using SeO
2
to afford N-oxide 7 in 17%
yield after column chromatographic purification of the reaction
23
crude. The reactions of thioindoxyl 5 with N-oxides 6 and 7,
using piperidine (PIP) as base, produced the desired 2-(4’-
pyridyl-N-oxide)-substituted HTI derivatives, 2 and 3, as yellow
powders in 21% and 59% yield, respectively, after column
chromatographic purification and trituration with ethyl acetate
1
(
see Scheme 1). Based on H NMR analyses of CDCl
3
solutions,
308, 370 and 460 nm). The chloroform solution was irradiated
the isolated solids of the HTI derivatives contained the
thermodynamically stable Z-isomer in an extent larger than 95%
with 450 nm light and the PSS was reached within 30 s. The
resulting UV-Vis spectrum of the solution showed the same
earmarks for the Z-to-E photoisomerization of the HTI switch.
(see SI). Compounds 7, Z-2 and Z-3 were fully characterized by
a complete set of high resolution spectra (NMR and MS). We
obtained single-crystals of Z-3 suitable for X-ray diffraction
analysis (see SI). In the solid state, the angle defined by the O,
e
CH and C(CH
)
3 3
atoms of Z-3 was 211°. This value is in good
agreement with the one (208°) measured for its energy-
minimized structure (MM3). The super aryl extended
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Figure 2. UV-Vis absorption spectra of chloroform solutions of: (red line) the
Z-2 isomer (30 µM) and (blue line) the same solution after photoirradiation at
450 nm for 30 s.
Afterwards, the chloroform solutions of 2 and 3 at the PSS were
thermally equilibrated in the dark at 40 °C for 12 h. The UV-Vis
spectra of the equilibrated HTI N-oxide solutions were similar to
the ones acquired for the Z-isomers. All together, these results
established that: a) N-oxides 2 and 3 are clearly responsive to
light stimuli; b) light-irradiation at a wavelength of 450 nm
induced PSSs in which the E isomer is the major component
and c) the Z-to-E photoisomerization process can be thermally
reversed. The short times required to achieve the PSSs in the
Figure 3. Selected region of the 1H NMR spectra (400 MHz, CDCl
. 298 K)
3
recorded during the photoirradiation at 450 nm of a 6.5 mM solution of
hemithioindigo N-oxide Z-2 for a) 0; b) 3 min; c) 5 min; d) 7 min; and e) 10 min,
reaching the PSS after 7 min with a final Z:E ratio of 20:80. Protons assigned
to the E-2 isomer are marked with a ‡ symbol. * Residual solvent peaks.
Binding Studies of HTIs Z-2 and Z-3 Probed by NMR
Spectroscopy. We probed the interaction of HTI N-oxide Z-2
performed photoisomerization experiments are
consequence of the employed highly diluted conditions (30 µM)
vide infra).
a
direct
1
with super aryl-extended calix[4]pyrrole 1 using H NMR titration
1
(
experiments. The H NMR spectrum of super aryl-extended 1 in
We also studied the photoisomerization process of both HTI N-
3
CDCl solution (3.5 mM) is in agreement with a C4v symmetry
1
oxides in millimolar chloroform solutions using
H
NMR
(Figure 4a). However, the NH and -pyrrole proton signals are
somewhat broadened suggesting that 1 is involved in a dynamic
equilibrium between conformers. The addition of 0.4 equiv. of
HTI Z-2 to the millimolar solution of 1 produced the appearance
of a new set of signals that were assigned to the protons of
spectroscopy. Separate thermally equilibrated solutions of Z-2
and Z-3 (6-7 mM) were photoirradiated at 450 nm. The solutions
reached the PSSs after 7-10 min, as evidenced by the fact that
the 1H NMR spectra remained unchanged after this period of
time. The mixture of HTI isomers at the PSS showed an E:Z
1’
bound 1 (Figure 4b). Significantly, the pyrrole NH protons (H )
of bound 1 appeared highly downfield shifted (δ = 9.8 ppm, δ =
+ 2.1 ppm) suggesting their involvement in hydrogen bonding
interactions with the oxygen atom of the N-oxide. The -pyrrole
composition of 80:20 and 85:15 for 2 and 3, respectively (Figure
1
3
and S23). Most likely, the larger concentrations used in the H
NMR experiments are responsible for the longer irradiation times
required to achieve the PSSs in comparison to the analogous
UV-Vis experiments.
2’
protons (H ) in bound 1 also moved downfield (δ = + 0.3 ppm)
and the four signals of its aromatic protons shifted upfield. The
binding of the N-oxide Z-2 locked receptor 1 in the cone
conformation. As the amount of HTI Z-2 increased, the new set
of signals of bound 1 grew at the expenses of those of free 1. In
the presence of 1 equiv. of Z-2, only the set of signals assigned
to the protons of bound 1 was observed (Figure 4c). Taken
together, these results indicated that Z-2 and 1 formed a 1:1
inclusion complex, Z-2⊂1, that is kinetically stable on the
chemical shift timescale and for which a binding constant larger
4
-1
than 10 M can be estimated.
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of the E and Z isomer of 2 can be easily identified, in the region
of the NHs, as two separate singlets resonating at δ = 9.9 and
9
.8 ppm, respectively. Further irradiation (20 and 40 min)
induced the increase of the NH signal corresponding to the E-
⊂1 complex (δ = 9.9 ppm) at the expenses of the signals
2
assigned to the Z-2⊂1 counterpart (Figure 5c-d). After 60 min of
1
irradiation, the H NMR spectra of the solution remained
unchanged indicating that the PSS was reached24 (Figure 5e).
Remarkably, the PSS of the mixture of inclusion complexes 2⊂1
was reached after 1 h of photoirradiation. The times required for
the free HTI 2 and the 21 complex to reach the PSS are
significantly different, 7 min and 1 h, respectively. The UV-Vis
absorption spectrum of the 21 complex showed a drastic
reduction of intensity (absorbance) at 450 nm compared to free
2
(see SI). This observation supports the finding that
photoirradiation at this wavelength is less efficient in the case of
the inclusion complex and, thus, longer irradiation times are
required to achieve the PSS.
Figure 4. Selected region of the 1H NMR spectra (300 MHz, CDCl
, 298 K)
3
acquired during the titration of tetraester calix[4]pyrrole 1 with Z-2: a) 1; b) 1 +
.4 molar equiv. of Z-2; c) 1 + 1 molar equiv. of Z-2, and d) Z-2. Primed letters
and numbers correspond to the proton signals of bound components.
Residual solvents peaks.
0
*
All proton signals of Z-2 in the Z-2⊂1 complex were upfield
shifted in comparison to those of Z-2 free in solution. The largest
upfield shift was experienced by the N-oxide pyridyl protons α to
g’
the nitrogen atom, H (δ = -3.67 ppm). These observations
support the deep inclusion of Z-2 in the aromatic cavity defined
by the meso-aryl substituents of receptor 1 in cone conformation.
The inclusion is mainly driven by the formation of four
simultaneous hydrogen bonds between the pyrrole NHs of 1 and
the N-oxide oxygen atom of Z-2.
We also probed the interaction of HTI N-oxide Z-3 with
calix[4]pyrrole 1 using 1H NMR titration experiments in CDCl
3
solution. The obtained results were completely analogous to the
ones described above for the Z-2 counterpart (see SI). It is
worthy to note, that the two chemically non-equivalent protons
g’
to the nitrogen atom of the included pyridyl-N-oxide Z-3 (H and
h’
H ) experienced similar upfield shifts, δ = -3.6 and -3.1 ppm,
respectively. Moreover, the magnitude of the upfield shifts is in
agreement with the one experienced for the analogous protons
of Z-2. This result supports similar binding geometries for the Z-
Figure 5. Selected region of the 1H NMR spectra (400 MHz, CDCl
, 298 K)
3
acquired during the photoirradiation at 450 nm of the 1:1 Z-21 complex for:
a) 0; b) 10; c) 20; d) 40 and e) 60 min. Primed letters and numbers correspond
to proton signals of bound components of Z-21. Double primed letters and
numbers correspond to the proton signals of irradiated bound components of
E-21. * Residual solvent peaks.
2⊂1 and Z-3⊂1 inclusion complexes.
Light-irradiation Experiments of the Inclusion Complexes of
. As discussed above, the equimolar mixture of Z-2 and the
1
super aryl-extended 1, at mM concentration, afforded the
quantitative formation of the Z-2⊂1 complex. The resulting
At the PSS, the composition of the mixture of isomeric
complexes E-2⊂1:Z-2⊂1 was determined as ~ 75:25. This value
is in good agreement with the 80:20 mixture of HTI isomers
obtained at the PSS (450 nm) for free 2. We did not observe any
of the expected proton signals for either the free receptor 1 or
the free HTI N-oxide isomer E-2. All together, these results show
CDCl
3
solution was photoirradiated with 450 nm light and
1
analyzed at different time intervals using H NMR spectroscopy
1
(Figure 5). After 10 min of irradiation, the H NMR spectrum of
the solution showed the diagnostic signals of the initial Z-2⊂1
complex together with a new set of signals that were assigned to
the protons of the E-2⊂1 complex (Figure 5b). The complexes
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that the inclusion of the HTI N-oxide has a reduced effect in the
isomerization level of the molecular photoswitch and that its E-
complexes compared to those of the free guests. Simple
molecular modelling studies (MM3) of the inclusion complexes,
Z-2⊂1 and E-2⊂1, showed that the bound HTI 2 isomers are
somewhat shape, size and function complementary to the
aromatic polar cavity of 1 (Figure 6). The two inclusion
complexes are stabilized by four simultaneous hydrogen bonds
established between the N-oxide oxygen atom of the HTI and
the pyrrole NHs of the calix[4]pyrrole unit. This binding motif
induces receptor 1 to adopt the cone conformation.
4
form is also bound by 1 with a stability constant larger than 10
-
1
M . In short, at mM concentration, the Z-to-E photoisomerization
of the bound guest did not modulate the binding constant of the
resulting E-2⊂1 inclusion complex at a level suitable to induce
°
its dissociation. Thermal equilibration in the dark at 55 C for 12
h
of the mixture of isomeric complexes afforded a final
composition of ~ 5:95 E-2⊂1:Z-2⊂1, which is in agreement with
the composition of free HTI-2 after thermal equilibration.
Analogous results were obtained in the irradiation experiments
performed with the Z-3⊂1 complex. Photoisomerization at 450
nm produced a ~ 75:25 ratio of E-3⊂1:Z-3⊂1 complexes, a
value that is also in line with the photoisomerization level
obtained for free Z-3. As observed above for Z-2, the inclusion of
Z-3 in the calix[4]pyrrole receptor 1 does not affect the PSS
composition significantly.
In order to characterize the kinetic stability of the Z-3⊂1 complex,
we determined its rate dissociation constant value as k-1 = 1.8 s-1
using EXSY NMR experiments (see SI). This dissociation rate
constant translates into an energy dissociation barrier of ca. 18
kcalꞏmol-1 at 298 K, which is in complete agreement with the
observed slow chemical exchange between free and bound
components.
1
Using H NMR spectroscopy, we also monitored the thermal E-
to-Z isomerization processes at 25 °C. First, separate CDCl
3
solutions of the free guests, 2 and 3, and their inclusion
complexes, 2⊂1 and 3⊂1, at millimolar concentrations, were
irradiated at 450 nm in NMR tubes until the PSSs were reached
(
see SI). Immediately, the NMR tubes were introduced in the
1
spectrometer probe previously thermostated at 25°C. Multiple H
NMR spectra of the evolving mixtures of stereoisomers were
acquired every 5 min for the free guest and every 30 min for
their inclusion complexes. The E-to-Z composition of the
mixtures were determined from the integral values of selected
proton signals assigned to the respective isomers. The data
obtained by monitoring the increase in concentration for the Z
isomer, as well as the decrease of the E counterpart, were
plotted versus time. The data fit to a first-order kinetic equation
in all cases. Remarkably, the half-lives for the thermally induced
isomerization reactions of the free HTI guests were ca. 3 times
shorter than those of the bound counterparts (1.5 h versus 4.5 h).
These results indicate that the thermal E-to-Z isomerization of
the HTIs is also affected by the inclusion in 1. We calculated that
the free energy barriers for the isomerization processes are of
the order of ca. 23 kcalꞏmol-1. This magnitude corresponds to
the lower range of values measured for related processes. It is
known that the electron-donor or electron-acceptor nature of the
para-substituent at the stilbene fragment of HTIs has an effect
on the Z-to-E photoisomerization and E-to-Z thermal
Figure 6. Energy-minimized structures (MM3) of the inclusion complexes (top
and side view) of a) Z-2⊂1 and b) E-2⊂1. Included guest molecules are shown
as CPK models and receptor 1 is depicted in stick representation. Non-polar
hydrogens of 1 have been omitted for clarity.
In addition, multiple CH- and - interactions take place
between the aromatic rings of included HTI 2 and the four meso-
26,27
aromatic substituents of 1.
The linear shape of Z-2 provides
a better complementary to the deep and cylindrical aromatic
cavityꢀ defined by the meso-aryl substituent of 1 in cone
conformation. Upon complex formation, the bent shape of the E-
2
isomers provoked a significant flexing in the aryl-extended
aromatic walls of the receptor 1. This is mainly due to the
maximization of CH- and - interactions. Likewise, the shape
complementarity between the host and the guest in the E-3⊂2
complex is reduced compared to Z-3⊂2 analogue. In contrast, -
sulphur interactions seem to be favoured for the E-3⊂2 complex.
These type of interactions are observed in the crystal structures
of folded proteins. They are established between methionine or
12
isomerization rates.
The effect is more noticeable for the latter process, in which an
increase in the electron-donor nature of the substituent
promotes
a faster thermal E-to-Z isomerization reaction.
Surprisingly to us, the pyridyl N-oxide HTIs reported in this work
show thermal isomerization rates that are similar to those of
stilbene derivatives bearing a strong electron-donor dimethyl
amino para-substituent (σ+ = -1.7). 25 We surmise that the
involvement of the N-oxide unit in hydrogen bonding interactions
might be responsible for the observed increase in the half-life for
the thermal photoisomerization reactions of the inclusion
cysteine residues and aromatic side chains of neighbouring
amino acids.28,29,30 All in all, the calculated energy difference for
-1
the two isomeric complex is ~3 kcalmol , in favour of the Z-3⊂2
complex, which is the one including the thermodynamically
stable Z-3 isomer of the HTI derivative.
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(
2.2-2.8-fold), which corresponds to a free energy difference of
-
1
ITC Binding Studies. We performed a series of isothermal
titration calorimetry (ITC) experiments in order to assess the
modulation of thermodynamic stability experienced by the
inclusion complexes of
photoisomer of 2 and 3. The H NMR binding studies discussed
ca. 0.5-0.6 kcalmol . This value is significantly smaller than the
3 kcalmol-1 estimated form the molecular modelling studies
(MM3). We attribute the experimentally measured decrease in
binding affinity to a reduction in the number of CH- and -
interactions for the E-HTI1 complexes in comparison to the Z-
HTI1 counterparts and to the worst fit of shapes between host
and guest.
1 as a function of the included
1
above indicated that the four possible 1:1 isomeric inclusion
4
complexes featured association constant values larger than 10
-
1
M . Firstly, we determined the thermodynamic parameters for
the formation of the 1:1 complexes of with the
1
thermodynamically stable Z-form of the two photoswitches. The
injection of incremental aliquots of chloroform solutions of Z-2 or
Z-3, using a computer-controlled micro syringe, to a solution of
super aryl-extended 1 in the same solvent produced the release
of heat pulses. Next, we performed analogous ITC experiments
by placing in the syringe diluted chloroform solutions of mixtures
of HTIs isomers obtained by irradiating the photoswitches, 2 and
a
Table 1. Thermodynamic constant values (K , H, TS and G) determined
for the complexation of HTIs Z-2 and Z-3 and the photoisomerized mixtures
at the PSS (450 nm) with receptor 1 in chloroform solutions. All values
represent the average of at least two ITC titration experiments. Errors in K
a
and H are reported as standard deviations.
Complex[a]
Z-2⊂1
K
/106[a]
ΔH[b]
TΔS[c]
ΔG[c]
a
5.4 ± 0.5
2.6 ± 0.4
1.9 ± 0.5
5.2 ± 0.7
2.9 ± 1.0
2.3 ± 1.2
-8.1 ± 0.6
-6.8 ± 0.1
1.1 ± 0.6
1.9 ± 0.1
-9.2 ± 0.1
-8.7 ± 0.1
3
, with 450 nm light until their PSSs were achieved. The
1
photoisomerization levels were analyzed using
spectroscopy before and after performing the ITC experiments.
H NMR
Z‐2⊂1/E‐2⊂1
E‐2⊂1[d]
31
As discussed above, the E isomers were present in the
photoisomerized mixtures at approximately 80%. The integrated
and normalized data of all titrations provided sigmoidal binding
isotherms with inflection points centered at guest:host molar
ratio equal 1. The experimental binding isotherm showed very
good fits to the “one set of sites” binding model implemented in
the Microcal analysis software. The thermodynamic parameters
determined from the fits (K, H) are summarized in Table 1
together with those calculated from them (G and TS).
Because the thermodynamic values of the binding constants
derived from the ITC experiments performed with the mixtures of
photoisomers (entries 2 and 4 in Table 1) are weighted
averages of the two inclusion complexes that are formed, we
consider them as maximum estimates for the inclusion
complexes of the E isomers.
Z‐3⊂1
-11.6 ± 0.3
-10.1 ± 1.4
-2.4 ± 0.3
-1.3 ± 1.4
-9.2 ± 0.1
-8.8 ± 0.2
Z‐3⊂1/E‐3⊂1
E‐3⊂1[d]
-
1
-1
-1
[
a] M . [b] kcal∙mol . [c] kcal∙mol at 298 K. [d] Values calculated using
equation (1).
Conclusions
Using the equation (1), we determined more accurate stability
constant values for the E inclusion complexes (entries 3 and 6 in
Table 1):
In conclusion, we report the synthesis and characterization of
two photoswitchable hemithioindigo N-oxides, Z-2 and Z-3, that
are suitable for deep inclusion in the polar aromatic cavity
defined by the cone conformation of the super aryl-extended
calix[4]pyrrole 1. The photoirradiation of thermally stabilized,
milli- and micromolar, chloroform solutions of the HTI derivatives,
Z-2 and Z-3, with 450 nm light produced isomeric mixtures
enriched with the E-form to an extent of > 80% at the PSS. The
Z-to-E photoisomerization of the HTI derivatives can be reversed
by a thermal equilibration process producing mainly the Z-form.
1H NMR titration experiments evidenced the formation of
kinetically and thermodynamically stable 1:1 inclusion
complexes of the HTI derivatives Z-forms of 2 and 3 and the
super aryl-extended receptor 1. The photoirradiation of the Z-
HTI⊂1 complexes produced mixtures enriched with the E-HTI1
counterparts requiring extensive irradiation times to achieve the
PSSs compared to the analogous experiments performed with
the free Z-HTI. The photoirradiation of the inclusion complexes
produced identical levels of isomerization of the HTI derivatives
than those obtained for the free guest. At millimolar
concentration, the Z-to-E photoisomerization of the HTI guests
did not induce their release in detectable extents to the bulk
solution. The results from the ITC experiments allowed us to
determine that the binding constant values of all isomeric
(1) Ka(E⊂1) = [Ka(Z⊂1/E⊂1) - χZ⊂1 Ka(Z⊂1)]/χE⊂1
Where χZ⊂1 and χE⊂1 are the molar fractions of the two
complexes and Ka(Z⊂1/E⊂1) is the stability constant as derived
from the fits of the titrations with the mixtures of photoisomers.
a
Thus, the calculated stability constant values are K (E-2⊂1) =
6
6
-1
1
.9 ± 0.5 × 10 M-1 and K
a
(E-3⊂1) = 2.3 ± 1.2 × 10 M .
Several conclusions can be drawn from the tabulated and
calculated data. Firstly, the formation of all inclusion complexes
is mainly enthalpically driven. Secondly, entropy favours the
formation of the complexes with HTI 2 but opposes in the cases
of those of HTI 3. We assign this dissimilar behaviour mainly to
different solvation/desolvation processes of the HTI. Thirdly,
enthalpy-entropy compensation effects3 are also evident in the
studied binding processes.
2
The fact that the binding constant values for the pair of
complexes Z-2/Z-3 and E-2/E-3 with receptor 1 are very similar
indicates that the presence of the methyl group in the meta-
position of the pyridyl-N-oxide residue of 3 does not impact on
the thermodynamic stability of the inclusion complexes.
Remarkably, the Z-to-E photoisomerization of the HTI guests
produces a very small reduction in the binding constant value
6
-1
inclusion complexes were in the order of 10 M (G ~ -9.5 kcal
-
1
mol ). Likewise, we quantified that the modulation in binding
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affinity exerted by the isomerization of the guest is ca. 0.5
2
2
2
1
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The photoirradiation at 450 nm of a millimolar solution of super aryl-
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1
in chloroform produced its total
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absorption property of the included HTI 2 inhibited to a significant
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Experimental Section
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In order to assess the ratio of E:Z isomers after the completion of an ITC
The authors thank Gobierno de España MINECO (projects
CTQ2017-84319-P and Severo Ochoa Excellence Accreditation
3
experiment, we titrated a PSS-photoirradiated CDCl solution of HTI-2
(80:20) with receptor 1. An aliquot of the solution in the calorimeter cell
2
8
014-2018 SEV-2013-0319), FEDER funds (Project CTQ2017-
4319-P), the CERCA Programme/Generalitat de Catalunya,
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the composition of the E-21 and Z-21 complexes was close to the
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Keywords: Supramolecular chemistry • Molecular recognition •
Calix[4]pyrroles • Hemithioindigos • Photo-isomerization
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Entry for the Table of Contents (Please choose one layout)
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Two 2-(4’-pyridyl-N-oxide)-substituted
hemithioindigos are investigated as
photoresponsive guests to modulate
the binding stability of their inclusion
complexes with a super-aryl-extended
calix[4]pyrrole. Moderate changes in
the stability constant values of the
deep inclusion complexes produced
with the Z and E isomers are
Giulia Moncelsi, Luis Escobar, Henry
Dube,* Pablo Ballester*
Page No. – Page No.
2-(4’-pyridyl-N-oxide)-Substituted
Hemithioindigos as Photoresponsive
Guests for a Super Aryl-Extended
Calix[4]pyrrole Receptor
assessed using ITC experiments.
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