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Journal ArticleDOI

Quantitative Kinetic Modeling in Photoresponsive Supramolecular Chemistry: The Case of Water-Soluble Azobenzene/Cyclodextrin Complexes

17 Apr 2020-Journal of Organic Chemistry (American Chemical Society)-Vol. 85, Iss: 10, pp 6509-6518

TL;DR: The proposed procedure has been shown to be very robust and is applicable to any other photoresponsive host-guest system and to develop a relevant and reliable protocol applicable to all other types of complexes.
Abstract: Hydrophilic host-guest complexes, consisting of water-soluble azobenzene and α-, β-, or γ-cyclodextrins, have been proposed as a model to study supramolecular photoresponsive systems in aqueous environments through a full spectrometric approach combined with a simulation and data fitting methodology. Various essential and complementary spectroscopic techniques have been used: circular dichroism to determine whether the complex is formed or not, NMR for the stoichiometry elucidation, and UV-visible spectrophotometry to obtain the association equilibrium constant of each complex and the quantum yield for each photochemical process. A step-by-step fitting procedure is presented, which enables the determination of all thermodynamic and photokinetic parameters. A sequential methodology is applied to dissipate all uncertainties on the variability of the results and to develop a relevant and reliable protocol applicable to other types of complexes. The proposed procedure has thus been shown to be very robust and largely applicable to other photoresponsive host-guest systems.
Topics: Supramolecular chemistry (50%)

Summary (2 min read)

INTRODUCTION

  • Based on all recognition phenomena present in nature, host-guest interactions have been motivating the chemists’ research for a long time, starting back in the 60s, with the early work of Cram, Pedersen and Lehn and their Nobel Prize in 1987.
  • To determine these parameters, linearization and graphical determination have long been used, especially the continuous variation plots (so called Job’s plot) for the stoichiometry determination and BenesiHildebrand linearization for the association constant1.
  • Cycodextrins (CD) are among the most popular supramolecular hosts2.
  • From this time on, they have become a powerful building block to prepare advanced materials with photomodulable properties.
  • There are only a few quantitative studies about their photoisomerization mechanism.

RESULTS AND DISCUSSION

  • A monosubstituted azobenzene containing a tri moiety in 4-position (AZO) was synthesized following already described procedures7a (Chart 1).
  • The reported complexation between ethyleneoxide oligomers and -cyclodextrin was prevented by limiting the length of this Stoichiomerty Association constant Ka Molar absorption coefficients εAZO, εAZO@CD AZO quantum yields AZO@CD quantum yields • NMR: Job Plot • UV/Visible spectroscopy: titration • UV/Visible spectroscopy: photokinetics chain to three units13.
  • Nevertheless, even with this short hydrophilic unit, AZO could be solubilized in water at concentrations up to 10-4 M. NMR confirmed that the synthesized compound is the trans isomer, AZOt.
  • Chart 1. Molecular structure of synthesized AZO chromophore.

I - Inclusion complexes

  • Different spectroscopic techniques such as NMR, UV-visible absorption, or circular dichroism have traditionally been used to demonstrate supramolecular complexation.
  • This max appears at the same value as for free AZOt (346 nm) and AZOt@CD, while it is shifted to 337 and 354 nm for respectively and CD complexes.
  • The sign of Cotton effects indicates the relative orientation of the guest OH O O ON N into the CD axis11.
  • UV irradiation of AZOt@αCD and AZOt@γCD aqueous solutions led to circular dichroism silent samples .
  • These results suggest the formation of a new inclusion complex between AZOc chromophore and βCD.

Stoichiometry

  • Both circular dichroism and UV-visible spectrophotometry showed a signal to noise ratio incompatible with a precise determination of the stoichiometry of the inclusion complexes.
  • The concentration of a 2:1 complex, if any, would be negligible and the recorded data representative of the 1:1 complexation allowing a reliable determination of the corresponding association constant.
  • The authors main motivation was however the possibility to get the association constant and the spectra of the related complexes, both needed in the following for modeling the experiments under irradiation.
  • 1 stoichiometry, the calculated spectrum of AZOt2@CD was about twice the intensity of the AZOt@α- or βCD complexes, also known as Because of its 2.
  • This corresponded to AZOc n- π* band, and the isosbestic point that appears at 417 nm accounted for a simple trans-cis photoisomerization .

The model

  • Under irradiation, three processes had to be considered: direct and reverse photoisomerization and a thermal AZOc to AZOt transformation (eq (1)).
  • The spontaneous evolution from AZOc to AZOt was very slow leading to k-1 = 7x10 -6 s-1 at 23°C .
  • This value corresponds to a half-life of more than twenty-seven hours and its contribution is negligible when compared with the duration of the photochemical kinetics examined below (less than 15 min).
  • Nevertheless, this parameter was taken into account in the model and the value fixed in all following calculations.
  • The aim was then to reproduce the experimental kinetic curves in order to retrieve missing parameters.

Fitting procedure

  • Several parameters were already at hand: I0 was obtained by chemical actinometry (see experimental part), lr (= 1 cm) was determined by the geometry of the system (see experimental part), AZOt and AZOc had been obtained in previous section according to UV-visible spectroscopy titration.
  • The direct and reverse photochemical quantum yields at the two irradiation wavelengths, also known as Four parameters were left.
  • At that point, once the initial conditions of the third run were obtained, the model quantitatively reproduced the experimental curves, which validate the values of the extracted parameters.
  • The parameters related to complexation and photoisomerization of free AZO were fixed, the two unknown parameters being the photoisomerization quantum yields of AZO@CD at the two irradiation wavelengths.
  • The quantum yield of trans-cis photoisomerization increases with increasing solvent polarity, whereas the quantum yield of cis-trans photoisomerization is known to be independent of the polarity of the solvent27.

CONCLUSION

  • The authors have presented a new step-by-step method for quantitatively determine the photoreactivity of inclusion complexes, including stoichiometry, affinity constant, UV-visible spectra of all the involved compounds, and quantum yields at two irradiation wavelengths.
  • The authors have applied this method to study for the first time the photochemistry of different azobenzene-cyclodextrin inclusion complexes, widely used for preparing photoactive biomaterials, in water.
  • Indeed, the authors found that the affinity constants in water of AZOt@βCD and AZOc@βCD are similar, which can limit the potential applications of this supramolecular photoswitch.
  • The authors hope that the insights in the photoreactivity of AZO@CD inclusion complexes provided in this work will be useful for the design of new bioactive materials.
  • The procedure is secured since all the parameters are validated after each step.

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Quantitative kinetic modeling in photoresponsive
supramolecular chemistry: the case of water-soluble
azobenzene/cyclodextrin complexes
Jorge Royes, Camille Courtine, Corinne Lorenzo, Nancy Lauth de Viguerie,
Anne-Françoise Mingotaud, Véronique Pimienta
To cite this version:
Jorge Royes, Camille Courtine, Corinne Lorenzo, Nancy Lauth de Viguerie, Anne-Françoise Mingo-
taud, et al.. Quantitative kinetic modeling in photoresponsive supramolecular chemistry: the case of
water-soluble azobenzene/cyclodextrin complexes. Journal of Organic Chemistry, American Chemical
Society, 2020, �10.1021/acs.joc.0c00461�. �hal-02559449�

1
Quantitative kinetic modeling in photoresponsive
supramolecular chemistry: the case of water-soluble
azobenzene/cyclodextrin complexes
Jorge Royes
a,b ǂ
, Camille Courtine
a
, Corinne Lorenzo
b
, Nancy Lauth de Viguerie
a
, Anne-
Françoise Mingotaud
a,
*, Véronique Pimienta
a,
*
a
Laboratoire des IMRCP, Université de Toulouse, CNRS UMR 5623, Université Toulouse III -
Paul Sabatier, 118 Rte de Narbonne, F-31062 Toulouse cedex, France
b
ITAV, Université de Toulouse, CNRS, UPS, 1 place Pierre Potier 31106 Toulouse Cedex 1, France
ǂ
present address: PASTEUR, Dpartement de Chimie, cole Normale Supérieure, PSL University,
Sorbonne Universit, CNRS UMR 8640, 24 rue Lhomond, 75005 Paris France.
KEYWORDS (Word Style “BG_Keywords”). Host-guest, supramolecular, photochemistry,
azobenzene, cyclodextrin, modeling

2
TOC.
ABSTRACT (Word Style “BD_Abstract”). Hydrophilic host-guest complexes, consisting in a
water-soluble azobenzene and -, - or -cyclodextrins, have been proposed as a model to study
supramolecular photoresponsive systems in aqueous environments through a full spectrometric
approach combined to a simulation and data fitted methodology. Various essential and
complementary spectroscopic techniques were used: circular dichroism to determine whether the
complex was formed or not, NMR for the stoichiometry elucidation and UV-visible
spectrophotometry to obtain the association equilibrium constant of each complex and the quantum
yield for each photochemical process. A step by step fitting procedure is presented, which enables
the determination of all thermodynamic and photokinetic parameters. The sequential methodology
is applied in order to dissipate all uncertainties on the variability of the results and to develop a
relevant and reliable protocol applicable to other types of complexes. The proposed procedure has
thus been shown to be very robust and largely applicable to other photoresponsive host-guest
systems.

3
INTRODUCTION
Based on all recognition phenomena present in nature, host-guest interactions have been motivating
the chemists’ research for a long time, starting back in the 60s, with the early work of Cram,
Pedersen and Lehn and their Nobel Prize in 1987. Beside the synthesis of molecules with
programmed host-guest interactions, an essential part of supramolecular chemistry involves the
characterization of the produced complexes. For basic A/B complexes, two central parameters are
sought, namely the stoichiometry of the complex and its association constant. To determine these
parameters, linearization and graphical determination have long been used, especially the
continuous variation plots (so called Job’s plot) for the stoichiometry determination and Benesi-
Hildebrand linearization for the association constant
1
.
Cycodextrins (CD) are among the most popular supramolecular hosts
2
. Their high solubility in
water together with their central hydrophobic cavity make them interesting supramolecular hosts.
Various hydrophobic guests with different affinity to the CD inner pocket have thus been described,
ranging from adamantane to different aromatic molecules such as phenyl, naphthalene or ferrocenyl
moieties.
The interaction of photoreactive guests with CD may give rise to inclusion complexes, whose
formation and dissociation can be reversibly controlled through irradiation. For instance,
azobenzene derivatives-CD photoresponsive inclusion complexes have been known for more than
30 years
3
. From this time on, they have become a powerful building block to prepare advanced
materials with photomodulable properties. Indeed, the excellent biocompatibility of
azobenzenes@CD inclusion complexes makes them good photo-switches in the emerging field of

4
biointerfaces, or materials in contact with biological samples
4
. For example, self-healing materials
5
,
artificial muscles
6
, biomaterials with photomodulable mechanical properties
7
or drug controlled
release materials
8
have been described in the recent years.
Despite the numerous examples of azobenzene@CD complexes applications, there are only a few
quantitative studies about their photoisomerization mechanism. A couple of early works
characterized the photochemical
9
and thermal relaxation constants
10
of azobenzene@CD complexes
in different solvents. Later, Liu et al. studied the binding models and relative affinity of azobenzene
tethered cyclodextrins vs different aliphatic alcohol guests
11
. It is worth mentioning that all these
studies were carried out in organic solvents or aqueous mixtures of them due to the low solubility
of azobenzene compounds in water. This poor water solubility of azobenzenes@CD supramolecular
photoswitches is a strong limitation for biological applications. Different strategies to increase the
water solubility of the azobenzene@CD photoswitches have included grafting on hydrophilic
moieties, such as polyethyleneglycol
12
, polyacrylate
5, 13
, polysaccharides
7b
or peptides
14
, or
introducing charged moieties
15
such as ammonium
10, 16
or carboxylates
17
. All these studies are
however limited to the determination of the stoichiometry and affinity constant of the complex
formed by hydrophilic azobenzene derivatives as guests. Thus, there is a strong lack of thorough
characterization of the isomerization kinetics and related quantum yields for azobenzene@CD
complexes in water.
The purpose of this study was to fully characterize the complexation, kinetic and spectral properties
of a water soluble azobenzene compound (noted AZO) in presence of ,  and CD. A
quantitative approach of the photoisomerization processes in the presence of CD was obtained by
simulation and data fitting, following a step by step procedure described in scheme 1. We first
determined the stoichiometry and equilibrium complexation constants of the AZO compound in the

Figures (6)
Citations
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Journal ArticleDOI
Qiuhong Cheng1, Aiyou Hao1, Pengyao Xing1Institutions (1)
Abstract: Dynamically manipulating the handedness of supramolecular chirality and chiroptical properties is a challenging task, which is important in the fabrication of responsive and smart functional materials. Here we report the dynamic evolution of supramolecular chirality inversion and the inversion of corresponding circularly polarized luminescence via the multiple-constituent coassemblies driven by hydrogen bonds. Cyanostilbene-conjugated amino acids were employed as building units, self-assembling into nanoarchitectures with 21 column chirality. After being incorporated into a binary coassembly with melamine and its derivative, 21 column chirality was transformed into helical chirality with macroscopic nanohelices. Such structural evolution enabled chirality inversion accompanied by inversion of circularly polarized luminescence. Isomerization triggered by photo irradiation appended responsiveness to morphology, supramolecular chirality as well as chiroptical properties, and inversion of circularly polarized luminescence was further achieved specifically. This work illustrating the dynamic chirality evolution and responsiveness provides an alternative perspective for the design and fabrication of smart chiroptical materials.

Journal ArticleDOI
24 Aug 2021-Langmuir
Abstract: We synthesize and characterize a triblock polymer with asymmetric tetramethoxyazobenzene (TMAB) and β-cyclodextrin functionalization, taking advantage of the well-characterized azobenzene derivative-cyclodextrin inclusion complex to promote photoresponsive, self-contained folding of the polymer in an aqueous system. We use 1H NMR to show the reversibility of (E)-to-(Z) and (Z)-to-(E) TMAB photoisomerization, and evaluate the thermal stability of (Z)-TMAB and the comparatively rapid acid-catalyzed thermal (Z)-to-(E) isomerization. Important for its potential use as a functional material, we show the photoisomerization cyclability of the polymeric TMAB chromophore and calculate isomerization quantum yields by extinction spectroscopy. To verify self-inclusion of the polymeric TMAB and cyclodextrin, we use two-dimensional 1H NOESY NMR data to show proximity of TMAB and cyclodextrin in the (E)-state only; however, (Z)-TMAB is not locally correlated with cyclodextrin. Finally, the observed decrease in photoisomerization quantum yield for the dual-functionalized polymer compared to the isolated chromophore in an aqueous solution confirms TMAB and β-cyclodextrin not only are in proximity to one another, but also form the inclusion complex.

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TL;DR: This Account demonstrates some of the great advances in the development of supramolecular materials through host-guest interactions within the last 10 years, and uses the molecular recognition of CDs to achieve macroscopic self-assemblies, and this chemistry can direct these macroscopy objects into even larger aggregated structures.
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