Sa Yao

Clement Falaise

Anton A Ivanov

Nathalie Leclerc

Max Hohenschutz

Mohamed Haouas

David Landy

Michael Shestopalov

Pierre Bauduin

Emmanuel Cadot

Clément Falaise

Hofmeister effect in the Keggin-type polyoxotungstate series

ou non, émanant des établissements d'enseignement et de recherche

Introduction

Polyoxometalates (POMs) are a remarkable class of anionic polynuclear metal oxide clusters of transition metal ions like W VI or Mo VI . [START_REF] Falaise | Pope Heteropoly and Isopoly Oxometalates[END_REF] POMs are able to exchange reversibly and massively electrons without structural change, making them promising electro-active subcomponents of hybrid systems relevant for applications in the energy field. [START_REF] Gumerova | Synthesis, structures and applications of electron-rich polyoxometalates[END_REF] Recent studies highlighted that POMs are an appealing molecular 'electronreservoir' to construct lithium-, sodium-ion and redox flow batteries, relevant for sustainable processes. The development of straightforward preparation routes to hybrid materials including POM units through supramolecular contacts has led to unique molecular assemblies with novel properties and reactivity. [START_REF] Stuckart | Polyoxometalates as components of supramolecular assemblies[END_REF] In context, exploration of ionic and other non-covalent interactions (hydrogen bonding, van der Waals forces, etc.) between POMs and other types of molecular systems has attracted considerable interest, revealing the remarkable supramolecular behavior of POMs to self-assemble across multiple length scales ranging from discrete systems to gigantic spherical single-layered nanostructures known as "blackberries".

12,16-20

One of the most striking supramolecular properties of POMs arises from their ability to form in solution resilient nonbonding contacts with neutral surfaces such as organic macrocycles, micelles, proteins or polymers. [START_REF] Buchecker | Self-Assembly of Short Chain PoIy-N-isopropylacrylamid Induced by Superchaotropic Keggin Polyoxometalates: From Globules to Sheets[END_REF][START_REF] Buchecker | Polyoxometalates in the Hofmeister series[END_REF][START_REF] Kobayashi | Interplay of Hydrophobic and Electrostatic Interactions between Polyoxometalates and Lipid Molecules[END_REF][START_REF] Malinenko | Are Keggin's POMs Charged Nanocolloids or Multicharged Anions?[END_REF][START_REF] Moussawi | Nonconventional Three-Component Hierarchical Host-Guest Assembly Based on Mo-Blue Ring-Shaped Giant Anion, gamma-Cyclodextrin, and Dawson-type Polyoxometalate[END_REF][START_REF] Moussawi | Polyoxometalate, Cationic Cluster, and gamma-Cyclodextrin: From Primary Interactions to Supramolecular Hybrid Materials[END_REF][START_REF] Naskar | Nanometer-Size Polyoxometalate Anions Adsorb Strongly on Neutral Soft Surfaces[END_REF][START_REF] Paul | Investigating Polyoxometalate-Protein Interactions at Chemically Distinct Binding Sites[END_REF][START_REF] Solé-Daura | Structure-Activity Relationships for the Affinity of Chaotropic Polyoxometalate Anions towards Proteins[END_REF][START_REF] Wu | Complexation of Polyoxometalates with Cyclodextrins[END_REF] Actually, their solution behavior appears "counter-intuitive" because POMs are anionic inorganic discrete species considered generally as hydrophilic. Others and us established that association phenomena between POMs and organic moieties are driven by a solvent mediated effect arising from a water structure recovery upon release of the POM's and the moiety's hydration water. [START_REF] Buchecker | Polyoxometalates in the Hofmeister series[END_REF][START_REF] Moussawi | Polyoxometalate, Cationic Cluster, and gamma-Cyclodextrin: From Primary Interactions to Supramolecular Hybrid Materials[END_REF][START_REF] Assaf | The Chaotropic Effect as an Assembly Motif in Chemistry[END_REF] POMs as well as other polynuclear species (dodecaborates or electron-rich octahedral clusters) have been identified as water structure breakers, also named chaotropic entities after the Hofmeister classification.

22,32-34

Based on the current understanding of the Hofmeister series, the chaotropicity of an ion should increase when its charge density decreases, but regarding POMs more insight is needed into the influence of the ionic charge, especially the effect of redox change on their chaotropic character.

22, [START_REF] Assaf | The Chaotropic Effect as an Assembly Motif in Chemistry[END_REF] From a thermodynamic point of view, processes involving chaotropes are usually considered to be enthalpically driven accompanied by an entropic penalty, i.e. enthalpy-entropy compensation. n-, composed of twelve octahedral WO6 polyhedra that form an anionic tetrahedral cage {W12O40} 8-with a central cavity occupied by the heteroatom X (P 5+ , Si 4+ , B 3+ or H2 2+ ). The choice of X allows the control of the charge density of the nano-sized inorganic POM.

26,35

Herein, we study the formation of host-guest inclusion complexes of γ-CD and Keggin-type POM anions along the [XW 12 O 40 ] n-series with anionic charges from 3-to 6-(Fig. 1) while keeping constant the shape and size of the host and of the guest. Using a set of complementary techniques (NMR, ITC and electrochemistry), we show in this article that affinity between γ-CD and Keggin-type anions increases dramatically with decreasing ionic charge. Simultaneously, we evaluate the chaotropic behavior using a simple experimental procedure based on the propensity of the POM species to adsorb on the non-ionic ethylene glycol-based surfactant C 8 E 4 . Thus, we establish unambiguously in this report the direct correlation between the chaotropic nature and the formation of highly stable supramolecular complexes. The present study might open new opportunities for the development of smart supramolecular devices with responsive behavior based on redox changing of the POM unit.

Results and discussion

Review of Keggin/γ-CD interactions in the solid-state

The crystal structure of PW 12 @CD was found to be isostructural to the recently published compounds O 40 ] to obtain crystalline products. [START_REF] Zhang | Induced chirality and reversal of phosphomolybdate cluster via modulating its interaction with cyclodextrins[END_REF] In our case, only Na + and H + were present evidencing that the POM-CD packing in PW 12 @CD is highly flexible to accommodate various types of counter-cations without structural change. PW 12 @CD's structure consists of a supramolecular 1:1 adduct involving a [PW 12 O 40 ] 3-unit partially incorporated into the cavity of γ-CD through its secondary rim (Fig. 2a). This configuration is quite unique among the few known polyoxometalate/γ-CD composites that mostly feature interactions with the primary face. 3-reveals a different complexation mode with γ-CD involving the opposite face, namely, the primary rim (Fig. 2b). 5-. By keeping the charge constant, the basicity of Keggin molybdates is known to be stronger compared to their tungstate analogues as the charge density is higher on the oxygen atoms at the surface of the molybdates. 47, [START_REF] Kamata | Base Catalysis by Mono-and Polyoxometalates[END_REF] Furthermore, the ionic charge must also play a crucial role in the supramolecular association.

Stability in solution

To investigate the complexation of the Keggin-type anions by γ-CD, titration experiments were conducted in solution by means of a set of complementary techniques including NMR spectroscopy ( 1 H 1D and DOSY), electrochemistry, and isothermal titration calorimetry (ITC).

Isothermal titration calorimetry

ITC experiments were carried out to determine the thermodynamical fingerprint of the interaction of γ-CD respectively with the four Keggin-type heteropolytungstates. ITC thermograms and isotherms are shown in the Electronic Supplementary Information (see Figs. S4-6, ESI), and a summary of binding constants, enthalpy Δ r H* and entropy Δ r S* changes are given at 298 K in Table 1. Beside the 1:1 complexation, a second 1:2 step process is systematically considered, according to equations (1) and (2) shown below.

POM n-+ CD ! [POM@CD] n- (1)
[POM@CD] n-+ CD ! [POM@2CD] n- (2)
Nonetheless, only ITC data for silicotungstate were found consistent with the 2:1 binding model involving a sequential process. No significant heat exchange was observed in the case of [H 2 W 12 O 40 ]

6-indicating negligible interactions with γ-CD. For all other complexes, supramolecular binding revealed to be enthalpically driven. The enthalpic gain is accompanied by an entropic penalty according to the usual enthalpy-entropy compensation observed within CD-based host-guest series.

Electrochemical studies

Interactions of the Keggin-type ions with γ-CD have been studied further using cyclic voltammetry. As shown below, the presence of γ-CD alters significantly the redox properties of the POMs. Besides, the POM's redox state is expected to have high impact on the binding constants. As shown in Fig. 3, dependence of CVs with γ-CD appears strongly dependent on the nature of the Keggin anion.

For instance, as the amount of γ-CD increases up to 8 equivalents, the half-wave potential related to the two first monoelectronic transfers of the [PW 12 O 40 ]

3-ion decreases continuously (see Fig. 3). Actually, the electrochemical behavior of the [PW 12 O 40 ] 3-ion gives the most representative effect reflected by a rough dependency of about -60 mV/pCD where pCD = -Log[CD] (see Fig. S9,ESI). This variation indicates that γ-CD is involved in the predominant redox process written in equation ( 3), with two consecutive monoelectronic transfers, n = 3 or 4 for the first and the second redox processes, respectively. Furthermore, the complexation behavior of γ-CD should be rather consistent with x = 2 or 1 corresponding to a 2:1 and 1:1 supramolecular adduct, respectively.

[POM(CD Including the six redox components involved in each redox process, the Nernst equations are written as followed:

) x ] n-+ 1e-! POM(CD) x-1 ] (n+1)-+ CD (3)
𝐸 !"#$,! = 𝐸 ! ! + !" ! ln (!!! !:! !" !" !" !! !:! !" ! !:! !" !" !" ! ) (!!! !:! !"#! !" !" !! !:! !"#! ! !:! !"#! !" !" ! ) (4) 𝐸 !"#$,! = 𝐸 ! ! + !" ! ln (!!! !:! !"#! !" !" !! !:! !"#! ! !:! !"#! !" !" ! ) (!!! !:! !"#! !" !" !! !:! !"#! ! !:! !"#! !" !" ! ) (5)
where 𝐸 ! ! !

process and [CD] eq , the concentration in free γ-CD at the equilibrium. The values K 1:1 and K 1:2 correspond to the binding constants associated to equation ( 1) and ( 2), respectively, wherein the POM can be in its three redox states, oxidized (ox), one-electron reduced (red1) or two-electron reduced (red2) (see scheme 1). Depending on the Keggin-type POM, equations ( 4) or ( 5) were adapted and then used to extract the binding constants through the fitting of experimental shift of the apparent potential upon free γ-CD concentration (see ESI, . The overall binding constants values are reported in Table 2 and the comparison between experimental data and the simulated evolution of the observed apparent potential is reported in ESI (see Figs. S7-9). The binding constants determined by electrochemistry for [BW This electrochemical study enabled to restore a logical sequence within the Keggin-type series, corroborating the fact that the POM/γ-CD association is mainly related to the global charge of POMs. For instance, plotting LogK 1:1 versus the ionic charge of the Keggin-type anion results in a quite perfect linear relationship (see ESI, Fig. S10). Furthermore, analysis of the electrochemical data provides access to the binding constants of the lower oxidation states of the Keggin anions (see Table 2). The binding constants of the reduced POM (n+1)-exhibit lower values compared to those determined for the oxidized parent (POM n-), evidencing again the preponderant effect of the charge on the association process with the macrocyclic host. For instance, the binding constants of the one-electron reduced [PW 12 O 40 ] 4-anion (K 1:1 = 6000 M -1

) fall in similar range order to that found for the oxidized silicotungstate [SiW 12 O 40 ] 4- .

(K 1:1 = 9500 M -1 ) (see

34

Nevertheless, alteration of the redox properties through supramolecular interactions remains quite intriguing in the case of the Keggin-type polyoxometalates. γ-CD being a nonionic component, its interaction with POM does not change the ionic charge as observed generally with protons or lithium ions. In context, Proton-Coupled Electron Transfer (PCET) involving POM species is a well-known pH dependent process featured by an increase of the standard potential as pH decreases. Such an effect can even be extended to POMlithium aggregation process under reducing conditions in nonaqueous solvents. [START_REF] Konishi | Impact of the lithium cation on the voltammetry and spectroscopy of [XVM 11 O 40[END_REF] In the presence of γ-CD, the origin of the redox potential variation upon γ-CD interaction should arise mostly from a change in the solvation shell of the POM corresponding to a shed of surrounding water molecules upon binding to γ-CD resulting in a more hydrophobic environment around the POMs. The influence of the medium, i.e. solvent and electrolyte support, on the redox properties of the POM species is well documented experimentally as well as through theoretical calculations and related studies reinforce our interpretation of the γ-CD effect upon the redox properties of Keggin-type POMs. 4-ion undergoes a 600-700 mV potential shift toward the negative potential when the medium is changed from aqueous to organic such as acetonitrile or dimethyl sulfoxide. This effect has been attributed to the capacity of the solvent to stabilize the reduced derivatives and has been quantified by the acceptor number (AN) of the solvent showing that the reduction process becomes difficult as solvent AN decreases. Water exhibits a high AN value (58.4) while less hydrophilic or less polar media feature lower AN parameters. [START_REF] Gutmann | Emperical parameters for donor and acceptor properties of solvents[END_REF] Furthermore, the intensity of the peak current decreases significantly in the presence of γ-CD. This behavior accounts for the complexation process which lowers the diffusion coefficient of the parent oxidized POM and then consecutively affects the peak current intensity, according to the Randles-Sevcik equation. [START_REF] Bard | Faulkener Electrochemical Methods: Fundamentals and Applications[END_REF] This effect on the current peak n-. From this observation, the Keggin/γ-CD association appears as an appealing component for redox switches, and could be integrated into design stimuliresponsive supramolecular systems controlled by the redox state of the POMs.

NMR spectroscopy

The formation and stability of Keggin POM/γ-CD complexes in aqueous solution were further probed by Depending on the Keggin anion, the spectra underwent more or less strong alteration evidencing POM-specific recognition. Table 3 summarizes the extent of shift variations observed for H3, H5, and H6 after adding eight equivalents of each Keggin-type POM. As expected, the Furthermore, we note the line broadening of H3 signal for POM:γ-CD ratio lower than two which is the signature of strong POM/γ-CD interactions involved in a 1:2 stoichiometry supramolecular adduct. Further quantitative investigations were also carried out using DOSY NMR. This technique is a valuable tool to study dynamic equilibriums, and similarly to chemical shift, the observed diffusion coefficient D in the fast exchange limit can be described as a weighted average of a given species evolving between different states. [START_REF] Cohen | Analytical Methods in Supramolecular Chemistry[END_REF] Thus, the NMR observable of γ-CD in presence of POM corresponds to an average situation weighted on residency times between free (solvated) and bound states. The observed diffusion coefficients D as a function of POM:γ-CD molar ratio produce different profiles depending on the nature of the Keggin-type anions (see Fig. 5).

From the initial value for solvated γ-CD measured at 254 µm 3-(219 vs 226 µm 2 /s, see ESI for modeling, Fig. S20) highlights the remarkable solution behavior of the phosphotungstate ion as reported by Antonio et al. [START_REF] Antonio | pH-Dependent Interactions between Keggin Heteropolyanions in Dilute Solutions[END_REF] In this domain, it may result from some further aggregation which could be consistent with a γ-CD unit sandwiched by two [PW 12 O 40 ]

3-ions interacting with both primary and secondary faces of the γ-CD. This hypothesis is also quite consistent with the 1 H NMR titration which reveals interactions involving simultaneously both rims of the γ-CD.

Furthermore,

H NMR can also characterize the protoncontaining [H 2 W 12 O 40 ]

6-anion exhibiting a resonance at ca. 6.0 ppm. Very small changes in the diffusion coefficient D (245-260 µm 2 /s) were observed over the titration experiment, reflecting very weak interaction with γ-CD as previously identified.

Driving force of the aggregation processes

This comparative study evidences clearly that Keggin-type anions bind to γ-CD with different extents despite their isostructure. Although γ-CD is non-ionic, the charge of the POM plays a dramatic role in the molecular recognition process, both in its thermodynamic stability and in its supramolecular structure. ). It was proposed that the binding affinity is related to the charge density of the polyanion, which has an effect on the hydration strength, i.e. hydration free energy of the ions. The monotonous trend observed along the oxidized derivatives of the Keggin-type series could be understood by thermodynamic features of the hydration shell around the POM, which is directly related to its chaotropic nature. The weak charge density of the Keggin-type POM generates a loosely bound solvation shell of disordered water molecules with high structural entropy. The energy gain associated to the release of this hydration water into the bulk upon binding to non-ionic moieties (at micellar surfaces, water surfactant monolayer or macrocycles) was proposed as the main driving force of the superchaotropic effect of POMs and more generally of low charge density nano-ions. [START_REF] Buchecker | Polyoxometalates in the Hofmeister series[END_REF] The evolution of the cloud point of a non-ionic surfactant, tetra-ethylene glycol mono-octyl ether (C 8 E 4 ), upon addition of POMs was proposed as a tool to evaluate their chaotropic behavior

22

: The higher the observed cloud point increase, the stronger the interaction with the non-ionic surfactant and the stronger the chaotropic nature of the tested ion. This procedure was carried out for the Keggintype anions investigated here. Corresponding to the respective increase in the cloud point by each POM, Fig. 6 Overall, we showed herein that the Keggin-type POMs interact with the γ-CD through various recognition modes depending on the POM's chaotropic power. The different interaction types previously identified can be classified along the Keggin ion series related to the magnitude of the binding constants. As tentatively depicted in Fig. 7, lowering of the ionic charge gives rise to multimodal supramolecular processes consistent with our experimental observations. On the left side of the scale, the

Conclusions

A comparative study allowed for an establishment of the relationship between the ionic charge of Keggin-type polyoxotungstate anions and their affinity towards a non-ionic substrate, γ-cyclodextrin. As the charge is lowered systematically keeping molecular size and shape, the ability to form supramolecular complexes increases continuously as was also observed for the interaction of the POMs with non-ionic surfactant micelles. The effects governing such supramolecular assemblies are suggested to be directly related to the solvation properties of the Keggin ions and to their propensity to shed hydration water upon formation of POM-substrate contact. 3-ion is not restricted to the systems involving cyclodextrins and the present study gives a better understanding for other self-assembly processes involving non-ionic substrates in water. 21,59,61,62 Furthermore, POM/CD association has dramatic consequences on the POM's redox properties, i.e. a decrease in the standard redox potential. This effect can be understood as a partial removal of the hydration shell of the redox active ions arising from the presence of bulky γ-CD in the close vicinity of the POM. It is worth noting that opposite effects corresponding to a redox potential increase were commonly observed for interaction with cations such as protonation or ion-pairing with lithium cation. Finally, the propensity of the Keggin-type anions to bind to γ-CD can be dramatically tuned by reversible electron transfer as demonstrated herein. This opens opportunities for the design of smart molecular materials like sensors, artificial muscles, and actuators, responsive to redox or possibly light stimuli.

Experimental section

Chemicals

All reagents were purchased from commercial sources and used without further purification.

Materials characterization

Fourier Transform Infrared (FT-IR) Spectra were recorded on a 6700 FT-IR Nicolet spectrophotometer, using diamond ATR technique. The spectra were recorded on non-diluted compounds and ATR correction was applied.

Energy-dispersive X-ray spectroscopy EDS measurements were performed using a SEM-FEG (Scanning Electron Microscope enhanced by a Field Emission Gun) equipment (JSM 7001-F, Jeol). The measures were acquired with a SDD XMax 50 mm 2 detector and the Aztec (Oxford) system working at 15 kV and 10 mm working distance. The quantification is realized with the standard library provided by the constructor using Lα lines.

Thermal gravimetric analysis (TGA)

To determine water and organic contents, a Mettler Toledo TGA/DSC 1, STAR e System apparatus was used under oxygen or nitrogen flow (50 mL min -1

) at a heating rate of 5 °C min -1 up to 700 °C.

Single-Crystal X-ray diffraction analysis

Intensity data collections were carried out at T = 200(2) K with a Bruker D8 VENTURE diffractometer equipped with a PHOTON 100 CMOS bidimensional detector using a high brilliance IμS microfocus X-ray Mo Kα monochromatized radiation (λ = 0.71073 Å). Crystals were glued in paratone oil to prevent any loss of crystallization water. Data reduction was accomplished using SAINT V7.53a. The substantial redundancy in data allowed a semi-empirical absorption correction (SADABS V2.10) to be applied, on the basis of multiple measurements of equivalent reflections. Using Olex2, [START_REF] Dolomanov | OLEX2: a complete structure solution, refinement and analysis program[END_REF] the structure was solved with the ShelXT 69 structure solution program using Intrinsic Phasing and refined with the ShelXL 70 refinement package using Least Squares minimization. Heavier atoms (W) for each structure were initially located by direct methods. The remaining nonhydrogen atoms were located from Fourier differences and were refined with anisotropic thermal parameters. Positions of the hydrogen atoms belonging to the γ-CDs were calculated and refined isotropically using the gliding mode. In the compound PW 12 @CD, numerous free water molecules (H 2 O) located inside the voids are disordered. Thereby the contribution of solvent-electron density was removed using the SQUEEZE routine in PLATON, producing a set of solvent-free diffraction intensities. Crystallographic data for single-crystal X-ray diffraction studies are summarized in Table S1.

Isothermal Titration Calorimetry (ITC)

Formation constants and inclusion enthalpies were simultaneously determined for each γ-CD/POM system by the use of an isothermal calorimeter (ITC200, MicroCal Inc., USA). Degassed deionized water solutions were used in both cell (V 0 = 202.8 μL) and syringe (40 μL). After addition of an initial aliquot of 2 μL, 10 aliquots of 3.5 μL of the syringe solution were delivered over 7 s for each injection. The time interval between two consecutive injections was 70 s, which proved to be sufficient for a systematic and complete return to baseline. The agitation speed was set to 1000 rpm. The resulting heat flow was recorded as a function of time. ITC titrations were realized at three temperatures (288, 298 and 308 K), with 5 mM γ-CD solution in the syringe and 0.25 mM POM solution in the cell. Values and uncertainties of formation constants and inclusion enthalpies were determined by global analysis of the binding isotherms, by means of a dedicated homemade program, 71 implementing 1:1 and 1:1+1:2 binding polynomials.

Electrochemistry

Purified water was used throughout. It was obtained by passing water through a RiOs 8 unit followed by a Millipore-Q Academic purification set. All reagents were of high-purity grade and were used as purchased without further purification. Cyclic voltammetric (CV) experiments were carried out with an Methrohm Autolab PGSTAT12 potentiostat/galvanostat associated with a GPES electrochemical analysis system (EcoChemie). Measurements were performed at room temperature in a conventional single compartment cell. A glassy carbon (GC) electrode with a diameter of 3 mm was used as the working electrode. The auxiliary electrode was a Pt plate placed within a fritted-glass isolation chamber and potentials are quoted against a saturated calomel electrode (SCE). The solutions were deaerated thoroughly for at least 30 minutes with pure argon and kept under a positive pressure of this gas during the experiments.

Nuclear Magnetic Resonance (NMR)

All solution NMR spectra were measured in D 2 O at 26 °C. 1 H NMR spectra were obtained on a Bruker Avance 400 spectrometer at a Larmor frequency of 400.1 MHz, using 5 mm standard NMR tubes. The 1D 1 H spectra were recorded with one pulse sequence at 30° flip angle (pulse duration 2.7 µs), using 0.1 s recycle delay, 3 s acquisition time, and 8 number of scans. Translational diffusion measurements were performed using Bruker's "ledbpgs2s" stimulated echo DOSY pulse sequence including bipolar and spoil gradients. Apparent diffusion coefficients were obtained using an adapted algorithm based on the inverse Laplace transform stabilized by maximum entropy. [START_REF] Delsuc | Maximum entropy processing of DOSY NMR spectra[END_REF] Chemical shifts are reported relative to tetramethylsilane (TMS).

Cloud point measurements

Aqueous solutions containing 60 mM C 8 E 4 and a given amount of POM were prepared into 1 mL glass vials. The vials were placed in a temperature controlled water bath. The heating rate was constant at 1 K.min -1 and the CP was detected by visual inspection after occurrence of turbidity. The choice of the counter-ion has negligible effects on the cloud point evolution.

Please do not adjust margins Please do not adjust margins ) till pH ~ 3. The solution is kept boiling for 24 h and the pH remains at 3. After cooling to room temperature, the solution is then filtered, and rubidium chloride (5.5 g, 45 mmol) is added. The white solid product is collected, and finally dried in air. Yield: ~ 3.8 g (33%). IR (cm -1

): 956 (sh), 935 (s), 896 (s), 878 (s), 766 (vs), ) and KCl (0.1 mol.L -1

) was stirred until obtaining a clear solution. The solution was then allowed to stand for crystallization in air. Colorless crystals of BW 12 •2CD were obtained by slow ethanol vapor diffusion into water solution (10 mL vessel containing 2 mL of water solution was placed into a 250 mL vessel with a tight-fitting lid containing 50 mL of ethanol) after 3 days, and were collected, washed with mixture water/ethanol and dried on air. Yield 37 mg, 30%. FT-IR spectrum is given in Fig. S23. ICP-OES Anal. Calcd for BC 1.5 CsH 220 K 2 O 149 W 12 : B, 0.17; C, 18.64; K, 1.26; W, 35.66. Found: B, 0.2; C, 18.5; K, 1.2; W, 32.5. EDS showed Cs : K : W ratio = 1.7 : 2.1 : 12. TGA showed a weight loss of 8.4% in the 20-220 °C temperature range corresponding to the twentynine hydration water molecules (calculated 8.4%) and a weight loss of 42% in the 220-700 °C range assigned to the loss of 2 CD (calculated 41.1%). ) and γ-CD (0.138 g; 0.096 mmol) was added. The clear solution is stirred for 15 min. The solution was then allowed to stand for crystallization in air. Needle-like colorless crystals of PW 12 @CD suitable for single crystal X-ray diffraction appeared within 5 days. They were isolated by filtration and washed with cold water. Yield 0.35 g, 76%. FT-IR spectrum is given in Fig. S24. ICP-OES Anal. Calcd for C 48 H 100 Na 3 O 90 PW 12 : C, 13.03; Na, 1.56; P, 0.70; W, 49.87. Found: C, 12.8; Na, 1.6; P, 0.7; W, 48.9. EDS showed Na : P : W ratio = 2.7 : 1.1 : 12. TGA showed a weight loss of 3.9% in the 20-220 °C temperature range corresponding to the ten hydration water molecules (calculated 4.1%) and a weight loss of 26.8% in the 220-700 °C range assigned to the loss of 1.3 CD (calculated 27%). This journal is © The Royal Society of Chemistry 20xx

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Fig. 1

 1 Fig. 1 Structural representations of the molecular entities used as building blocks. (a) γcyclodextrin C48H80O40 (γ-CD) resulting from the condensation of eight glycopyranose units. The toroidal macrocycle delimits a hydrophobic cavity of 9 Å in diameter. (b) The Keggin-type POM, [XW12O40]n-, composed of twelve octahedral WO6 polyhedra that

  Attempts for isolation of the supramolecular adducts have been carried out systematically using different ratios of sodium salt of POMs Na m [XW 12 O 40 ]•nH 2 O with X = of crystallization such as slow evaporation of aqueous solution or ethanol vapor diffusion into aqueous solution containing cesium chloride. In all cases, crystalline products were obtained but only [BW 12 O 40 ] 5-and [PW 12 O 40 ] 3-led to single crystals suitable for XRD analysis. Their composition was established combining different complementary techniques (elemental analysis, EDS, XRD, and TGA). The compounds CsK 2 H 2 {[BW 12 O 40 ]•2(γ-CD)}•29H 2 O (BW 12 •2CD) and Na 3 {[PW 12 O 40 ]@(γ-CD)}•10H 2 O (PW 12 @CD) present different molecular organization as further discussed in the next section.

30

 30 Furthermore, the stoichiometry of the [PMo 12 O 40 ] 3-:CD adduct is 1:2, while a 1:1 stoichiometry was found with [PW 12 O 40 ] 3-although higher stoichiometry numbers should be expected as observed with Dawson-type POM with stoichiometry of 1:1 up to 1:3. 26 Indeed, while PW 12 @CD crystallized in a 1:1 solution mixture, other crystalline products were obtained with higher γ-CD contents (2 γ-CDs per POM Please do not adjust margins Please do not adjust margins unit), but we were unable, despite numerous repeated attempts, to solve the solid-state structures of these crystals featuring a tetragonal unit cell (a = b = 23.94 and c = 18.54 Å). The 1:1 adduct observed in PW 12 @CD exhibits several nonbonding contacts involving the terminal O t and bridging O b atoms of the POM and H3 and H5 protons (see Fig. 1a for H labeling) of the inner cavity of the γ-CD (see Fig. S1, in ESI). The short interatomic distances H3•••O t =W (from 2.69 to 3.31 Å), H3•••O b -W (from 2.41 to 2.88 Å), H5•••O t =W (from 2.29 to 3.38 Å), and H5•••O b -W (from 3.13 to 3.88 Å) reflect a strong and deep host-guest inclusion. As PW 12 @CD crystallizes in a cubic crystallographic system, the 1:1 host-guest adduct is surrounded by four host-guest units giving additional nonbonding contacts between the exposed face of the Keggin ion and the external surface of each of the neighboring γ-CDs (see Fig. S2, in ESI).

Fig. 2

 2 Fig. 2 Illustration of the different supramolecular assemblies observed in the solid-state involving γ-CD and the Keggin anions (a) [PW12O40] 3-in PW12@CD, (b) [PMo12O40] 3-from reference 30 , and (c) [BW12O40] 5-in BW12•2CD. In the solid-state, the organization of the [BW 12 O 40 ] 5-and γ-CD is significantly different since the crystalline structure of BW 12 •2CD reveals that the two components co-crystallize without forming an inclusion complex. Indeed, the [BW 12 O 40 ] 5is positioned near the γ-CD in close contact with its external wall (Fig. 2c). One [BW 12 O 40 ] 5-unit is surrounded by eight γ-CDs and each γ-CD is in contact with four [BW 12 O 40 ] 5-through the three outward protons H1, H2, and H4 as well as the H6 of the methoxy arm (see Fig. S3, ESI). This leads to a global stoichiometry of 2(γ-CD):1POM. Two neighboring γ-CDs are face-to-face connected through a hydrogen bonding network from their secondary rims. The [BW 12 O 40 ] 5-is therefore

21

 21 Both these aspects are expected to affect significantly solvent structuration around the POM, and should have important consequences in the formation and stability of the resulting POM-based supramolecular arrangement. Systematic studies in solution are thus needed to shed light on such effects.

Fig. 3 1 HClO4Scheme 1

 311 Fig. 3 Cyclic voltammetry of the [XW12O40] n-anions with (a) X = P 5+ , (b) Si 4+ , (c) B 3+ or (d) H2 2+ (0.5 mmol.L -1 , glassy carbon working electrode, scan rate 50 mV.s -1 ) in the presence of increasing amounts of γ-CD (from 0 to 8 equivalents). The experiments involving [PW12O40] 3-, [SiW12O40] 4-and [BW12O40] 5-have been performed in 25 mmol.L -1HClO4 aqueous solution, while acetate buffer solution (0.5 M CH3COONa; 0.5 M CH3COOH) has been used for the electrochemical measurements of [H2W12O40] 6-.

Figs. S7- 9 ) 1 . 1 .

 911 . The case of the metatungstate ion [H 2 W 12 O 40 ] 6-is trivial as neither ITC nor electrochemistry evidences any interaction with γ-CD. For the [BW 12 O 40 ] 5-ion, ITC experiments showed formation of a 1:1 adduct. Furthermore, the second wave is nearly independent of the γ-CD concentration meaning that the one-electron and the two-electron reduced forms do not interact with γ-CD. In such conditions, the expression of the apparent potential E peak,1 is adapted from equation (4) by using only the binding constant 𝐾 !:! !" . Decreasing further the ionic charge produces a richer and more complex behavior. The first wave of the [SiW 12 O 40 ] 4-anion undergoes a shift of about -60 mV per pCD unit (see ESI, Fig. S8) meaning the Nernst Equation can be derived from equation (3). The second wave exhibits only moderate variation, consistent with a weak complexation of the one-electron reduced form (red 1 ) and no complexation for the two-electron reduced form (red 2 ). Then the equation (5) is derived by using only the binding constant 𝐾 !:! !"# ! and matches the experimental data for 𝐾 !:! !"# ! = 130 M -Considering the first monoelectronic exchange, both the 1:1 and the 1:2 adduct must be considered for the oxidized form as shown by ITC. Then equation (4) should include 𝐾 !:! !" , 𝐾 !:! !! and 𝐾 !:! !"# ! = 130 M -Suitable fitting with experimental data was obtained with numerical values 9500 and 150 M -1 for 𝐾 !:! !" and 𝐾 !:! !" , respectively. Finally, the electrochemical behavior of the [PW 12 O 40 ] 3ion consists of two CD-dependent monoelectronic waves which move both by about -60 mV per pCD unit in accordance with equation (3). The second wave is satisfactory modeled by using 𝐾 !:! !"# ! = 6000 M -1 and 𝐾 !:! !"# ! = 30 M -1 while calculation of the first redox potentials requires to include 𝐾 !:! !" and 𝐾 !:! !" in addition to the binding constants related to the reduced form [PW 12 O 40 ] 4-, namely red 1 . From equation (4), good agreement was found between calculated and experimental data for 𝐾 !:! !" = 90000 M -1 and 𝐾 !:! !" = 1500 M -1

1 H

 1 intensity appears quite pronounced in the CVs of the [PW 12 O 40 ]3-ion, which exhibits the strongest affinity with the γ-CD and diminishes as the ionic charge of the POM increases. Diffusion coefficient change upon γ-CD complexation is also nicely supported by DOSY NMR study presented below.To sum up, electrochemical investigations demonstrate that the binding constants K 1:1 and K 1:2 increase by at least one order of magnitude as the global ionic charge decreases along the Keggin-type series [XW 12O 40 ]

1 H 39 , 45 1 H 1 H

 1394511 NMR spectroscopy in D 2 O. In previous separate studies, the interaction of γ-CD with both [PW 12 O 40 ] 3-and [SiW 12 O 40 ] Here, we conducted a comparative titration study of 2 mM aqueous γ-CD solution with the four Keggin-type anions. Fig. 4 shows the resulting NMR spectra in the range 3.7-4.5 ppm corresponding to the resonance domain of the most exposed protons to guest species, namely H3 and H5 are located inside the cavity and H6 on the top of the secondary rim (see Fig. 1 for protons numbering scheme and ESI Figs. S11-14 for the complete NMR spectra). These protons are the most sensitive to hostguest contact and are usually used as indicators to probe the inclusion phenomenon.Please do not adjust margins Please do not adjust margins

Fig. 4 1 H

 1 Fig. 4 1 H NMR spectra in the chemical shift range for H3, H5, and H6 resulting from the titration of 2 mmol.L -1 aqueous γ-CD solution with [XW12O40] n-POMs. (a) [PW12O40] 3-; (b) [SiW12O40] 4-; (c) [BW12O40] 5-and (d) [H2W12O40] 6-.

] 5 -

 5 . The second observation is the splitting of the initial singlet of the H6 protons into two unresolved doublets in [BW 12 O 40 ] 5-containing solution. This may be due to constrained dynamic rotation of the methoxy arms which narrows the signal of the two methylenic diastereotopic H6 protons. Such steric hindrance would result from the interaction of the POM specifically with the primary rim of the γ-CD since the H3 protons present in the secondary rim did not show any significant perturbation. The situation is totally different with [PW 12 O 40 ] 3-anion where besides the shift of H6, we can also appreciate strong effects on both H5 and H3 resonances with up to 0.53 and 0.36 ppm downfield shifts, respectively, indicating involvement of the secondary face of the γ-CD in the strong interaction with [PW 12 O 40 ] 3-.

  analyzing the chemical shifts variation arising from fast chemical exchange regime, quantitative treatment allows for a determination of the binding constants for 1:1 POM:γ-CD complexes (for further details, see Figs. S15-16, ESI). The values obtained for K 1:1 are 23 and 1100 M -1 for [H 2 W 12 O 40 ] 6-:CD and [BW 12 O 40 ] 5-:CD, respectively, that compare well with the ITC and electrochemistry results (see Tables1 and 2). In summary, NMR results are fully consistent with electrochemistry showing that the interaction strength with γ-CD increases following this order: [H 2 W 12 O 40 ] 6-< [BW 12O 40 ]

2 /

 2 s, the self-diffusion D drops more or less abruptly upon addition of POM and reaches a plateau at large excess of POM at 226 µm 2 /s in the case of [SiW 12 O 40 ] 4-and [BW 12 O 40 ] 5-, while for the [H 2 W 12 O 40 ] 6-ion, the decrease of the self-diffusion coefficient draws a smooth slope featured by larger selfdiffusion values as an indication of very weak POM-γ-CD interactions. For the [PW 12 O 40 ] 3-ions, the D profile is characteristic of strong interactions. The decrease of D is sharper, reaching even the lower limit value measured at 219 µm 2 /s. We note also a minimum value for the curves of both [SiW 12 O 40 ] 4-and [PW 12 O 40 ] 3-at ca. POM:γ-CD = 0.5 suggesting presence of bulkier intermediate species at these compositions. Both curve profiles have been modeled This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins satisfactorily using either a one-step complexation model for [H 2 W 12 O 40 ] 6-and [BW 12 O 40 ] 5-or a two-step complexation model for [SiW 12 O 40 ] 4-and [PW 12 O 40 ] 3-. Simulations of these curves considering binding constants consistent with those determined previously from ITC, electrochemistry and also 1D NMR studies allowed to estimate diffusion coefficients of γ-CD within 1:1 and 1:2 adducts. Fair agreements between calculated and experimental data were found using D values of 226 and 163 µm 2/s for POM@CD and POM@2CD adducts, respectively. Noteworthy, the same D values were used for the 1:1 or 1:2 adduct independently of the nature of the Keggintype anion (see ESI for details of modeling, Figs. S17-20), considering similar volume/size for the POM-CD adduct along the Keggin-type series. The observed lower limit value for D with [PW 12O 40 ]

Fig. 5

 5 Fig. 5 Diffusion coefficients of γ-CD as a function of POM:γ-CD molar ratio, measured from 1 H DOSY NMR of a solution of 2 mM of γ-CD with various POM content. Lines correspond to calculated curves with one-step 1:1 POM:CD complexation for [H2W12O40] 6-and [BW12O40] 5-, and with two-step 1:2 POM:CD complexation for [SiW12O40] 4-and [PW12O40] 3-.

1 )

 1 All experimental measurements (ITC, electrochemistry and NMR) have shown that the interaction with [H 2 W 12 O 40 ] 6-is very weak (K 1:1 ≈ 2.3 x 10 1 M -, while it is very strong with [PW 12 O 40 ] 3-(K 1:1 ≈ 1.6 x 10 5 M -1

  illustrates an increase in the chaotropic nature of the polyanions from [H 2 W 12 O 40 ] 6-to [PW 12 O 40 ] 3-, i.e. from high to low charge density. The superchaotropic P-, Si-and B-Keggin POMs produce much stronger increases in the cloud point compared to the classical chaotrope SCN -. The resulting ordering of POMs according to their chaotropic behavior is in full agreement with the series obtained by electrochemistry and NMR measurements.

Fig. 6

 6 Fig. 6 Cloud point evolutions of 60 mM C8E4 as a function of salt concentration for the four isostructural Keggin-ions PW12O40 3-, SiW12O40 4-, BW12O40 5-and H2W12O40 6-. The classical salts NaCl (neutral salt) and NaSCN (classical chaotropic salt) are included as references.

1 H

 1 [H 2 W 12 O 40 ] 6-ion shows nearly no ability to interact with γ-CD and the [BW 12 O 40 ] 5-species reveals only weak interactions that involve preferably the outer surface of the CD as observed in the crystal structure of BW 12 •2CD (see Fig. 2c). Lowering further the ionic charge down to [SiW 12 O 40 ] 4ion leads to the formation of 1:1 and 1:2 inclusion complexes involving mostly the primary face featuring higher binding constants. Finally, on the right side, the lowest ionic charge anion [PW 12 O 40 ] 3-gives rise to the highest binding constants reflected in the large diversity of supramolecular binding modes. As shown previously by NMR, both primary and secondary faces contribute to the POM's complexation and complementary DOSY NMR study suggested even the presence of an unusual supramolecular assembly corresponding to a γ-CD sandwiched by two [PW 12 O 40 ] 3-anion. Despite the lack of any structural model for such an arrangement, NMR showed clearly that in excess of POM, the primary face represented by H5 and H6 resonances and the secondary face detected by H3 resonances are both simultaneously involved in low selfdiffusion coefficient assembly compatible with a {POM⋅⋅⋅CD⋅⋅⋅POM} arrangement. The binding on the primary and secondary faces of the CD, which results in a large contact surface with the POM, is favored over the binding of POMs on the exterior side of the CD.

Fig. 7

 7 Fig. 7 Illustration of the most representative binding modes involved in the supramolecular assemblies of γ-CD (green torus) and the Keggin anions (spheres).

63 H 5 [ 64 K 4 [ 66 K 3 [

 635644663 K 4.3 Na 0.7 [BW 12 O 40 ]•17H 2 O, BW 12 O 40 ]•29H 2 O, SiW 12 O 40 ]•6H 2 O, 65 Na 0.6 H 3.4 [SiW 12 O 40 ]•16H 2 O, PW 12 O 40 ]•5H 2 O, Na 1.4 H 1.6 [PW 12 O 40 ]•12H 2 O, and H 3 [PW 12 O 40 ]•14H 2 O 65,67 were prepared according to literature procedures and checked by routine analyses prior using. The synthesis of Rb 6 [H 2 W 12 O 40 ]•11H 2 O is described below. Solutions were prepared in MilliQ-water. This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins

Rb 4 . 5 73 Na 2 WO 4 •

 457324 Na 1.5 [H 2 W 12 O 40 ]•11H 2 O The synthetic procedure is inspired from the syntheses of the tetraalkylammonium (TAA) salts, TAA 6 [H 2 W 12 O 40 ]•9H 2 O. 2H 2 O (10 g, 30 mmol) is dissolved in 30 mL of deionized water, and the resulted solution is acidified with HCl (6 mol.L -1

  ppm. ICP-OES Anal. Calcd for H 24 Na 1.5 O 51 Rb 4.5 W 12 : Na, 1.00; Rb, 11.10; W, 63.66. Found: Na, 0.9; Rb, 10.5; W, 58.0. TGA showed a weight loss of 5.5% in the 20-220 °C temperature range corresponding to the eleven hydration water molecules (calculated 5.7%).

CsK 2 H 2 {

 22 [BW 12 O 40 ]•2(C 48 H 80 O 40 )}•29H 2 O (BW 12 •2CD) A mixture of K 4.3 Na 0.7 [BW 12 O 40 ]•17H 2 O (68 mg; 0.02 mmol) and γ-CD (85 mg; 0.06 mmol) in 3.5 mL aqueous solution of CsCl (0.05 mol.L -1

Na 3 {

 3 [PW 12 O 40 ]•(C 48 H 80 O 40 )}•10H 2 O (PW 12 @CD) Na 1.4 H 1.6 [PW 12 O 40 ]•12H 2 O (0.3 g; 0.096 mmol) is dissolved in 10 mL aqueous solution of NaCl (0.2 mol.L -1

  -W distances in the 2.66-2.91 Å and 2.73-3.91 Å range, respectively. This arrangement with empty γ-CDs is unique in POM/γ-CD composites although it could occur with the smaller α-CD.

			44 Indeed, the [PW 12 O 40 ]	3-Keggin anion led to
	an extended framework with α-CD forming double layers of
	interconnected [PW 12 O 40 ]	3-and CD motifs through bridging
	alkali cations.	44 This highlights the importance of the size-
	matching effect upon the molecular recognition process,	46
	however this could not explain the difference of binding
	interactions observed in Keggin/γ-CD arrangement.
	Finally, it is surprising to see how γ-CD interacts so
	differently with isostructural nano-sized objects, namely
	[PW 12 O 40 ]	3-, [PMo 12 O 40 ]	3-, and [BW 12 O 40 ]

•••O t =W and H6•••O b

  behave as chaotropic species (water structure breakers).[START_REF] Assaf | The Chaotropic Effect as an Assembly Motif in Chemistry[END_REF] In such a case, the magnitude of the chaotropic character and the related binding constants are expected to increase with decreasing global charge. However, the highest enthalpic and entropic changes (and binding constants), were observed for [SiW 12 O 40 ] 4-and the lowest values were found for

	[PW 12 O 40 ]	3-. This result represents therefore an anomaly within
	the expected series, as confirmed by the electrochemical and
	NMR studies discussed below. The discrepancy here may have
	appeared for two reasons: i) the partial hydrolysis of the
	[PW 12 O 40 ]	3-anion in the ITC conditions (non-buffered and
	diluted solution) or ii) the simultaneous formation of
	numerous complexes with various stoichiometries, e.g. 1:1,
	1:2, 2:1, etc., (overparametrization).

49

These thermochemical fingerprints together with low values of both enthalpic and entropic changes suggests that Keggin This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins anions

Table 1

 1

	Binding constants (M	-1 ) of γ-CD with the Keggin-type POMs and associated
	thermodynamic parameters (kJ.mol	-1 ) at T = 298 K measured by ITC. Calculated
	uncertainties are inferior to 10%.
	POM					POM:CD	K	ΔrH*	ΤΔrS*	ΔrG*
	[H2W12O40]	6-	-	-	-	-	-
	[BW12O40]	5-	1:1	1032	-53.5	-36.3	-17.2
	[SiW12O40]	4-	1:1	17209	-56.4	-32.3	-24.2
						1:2	459	-55.0	-39.8	-15.2
	[PW12O40]	3-	1:1	2919	-35.1	-15.3	-19.8

50,51

  12 O 40 ]Moreover, the second binding constant K 1:2 is also quite large, around one order of magnitude higher than the K 1:2 value observed for [SiW 12O 40 ]

				5-	and
	[SiW 12 O 40 ]
	With a value of 9 x 10	4 M	-1 , the affinity of [PW 12 O 40 ]	3-for γ-CD
	is about one order of magnitude higher than the affinity of
	[SiW 12 O 40 ]

4-

ions are significantly lower compared to those found by ITC (see Table

1

). This difference is likely due to the different medium used in the ITC experiments (pure water) and in the electrochemical measurements (aqueous solution of 0.025 mol.L -1 HClO 4 ) and may arise from the presence of hydronium ions H 3 O + or perchlorate ion ClO 4 -involved in competition processes with the POM-CD aggregation. Importantly, analysis of the electrochemical data allows us to determine the K 1:1 binding constants of the [PW 12 O 40 ] 3-ion.

[START_REF] Huang | Application of polyoxometalate derivatives in rechargeable batteries[END_REF]

-. Such a high value corresponds to one of the largest affinity constants reported so far with γ-CD. 32,4-. This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins

Table 2

 2 Binding constants K1:1 and K1:2 involving γ-CD with the Keggin-type anions at different oxidation states as determined from electrochemistry. The accuracy is estimated to be within 10% of the corresponding value.

Table 2 )

 2 . A similar observation is made for the one electron-reduced silicotungstate [SiW 12O 40 ]

	5-and

57 Table 3

 573 Differences in chemical shifts (ppm) of protons H3, H5, and H6 of γ-CD before and after addition of eight equivalents of POM.

	POM				ΔδH3	ΔδH5	ΔδH6
	[H 2 W 12 O 40 ]	6-	0.02	0.06	0.07
	[BW 12 O 40 ]	5-	0.03	0.18	0.33
	[SiW 12 O 40 ]	4-	0.03	0.16	0.30
	[PW 12 O 40 ]

  While the most charged POMs, [H 2 W 12 O 40 ] 6-and [BW 12 O 40 ]

			5-
	showed weak to moderate affinity towards γ-CD, interacting
	mainly with its exterior wall, the lowest charge density species
	formed sandwich-type host-guest complexes involving either
	the primary face or the secondary face, featuring larger shared
	interfaces. Thus, in the Keggin-type series, the [PW 12 O 40 ]	3-ion
	previously identified as a superchaotrope	60 exhibits a
	remarkable capacity to bind γ-CD through its two faces, thus
	presenting unique aggregation behavior. The host-guest
	adaptability allows maximizing contact interfaces through
	weak dispersion forces, highly amplified through a solvent
	effect arising from the pronounced superchaotropic nature of
	the [PW 12 O 40 ]	3-ion. This remarkable solution behavior of the
	[PW 12 O 40 ]

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 9

Syntheses

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 11

Acknowledgements

This work is supported by a public grant overseen by the French National Research Agency as part of the "Investissements d'Avenir" program (Labex Charm3at, ANR-11-LABX-0039-grat). This research is also supported by the China Scholarship Council (CSC) to S. Y. (201904910419). Authors are grateful to Flavien Bourdreux and Cyrielle Rey for their help for elemental analyses.

Notes and references

Conflicts of interest

There are no conflicts of interests to declare.


Quentin Franquet

Xavier Matillon

Nicolas Terrier

Jean-Jacques Rambeaud

Sebastien Crouzet

Jean-Alexandre Long

Hakim Fassi-Fehri

Ricardo Codas-Duarte

Delphine Poncet

Thomas Jouve

Johan Noble

Paolo Malvezzi

Lionel Rostaing

Jean-Luc Descotes

Lionel Badet

Gaelle Fiard
email: gfiard@chu-grenoble.fr

The Mayo Adhesive Probability score can help predict intra-and postoperative complications in patients undergoing laparoscopic donor nephrectomy

Keywords: living donor, nephrectomy, laparoscopy, intraoperative complications, postoperative complications

Introduction

Renal transplantation is the treatment of choice for end-stage renal disease, offering the best results both in terms of quality of life and cost-effectiveness, even for older patients [START_REF] Axelrod | An economic assessment of contemporary kidney transplant practice[END_REF][START_REF] Heldal | Kidney transplantation: an attractive and cost-effective alternative for older patients? A cost-utility study[END_REF]. Kidneys obtained from living donors offer excellent results in terms of graft function and showed an improved overall graft survival compared to deceased donors, even with an increasing donor age [START_REF] Yohanna | A Propensity Score-weighted Comparison of Outcomes Between Living and Standard Criteria Deceased Donor Kidney Transplant Recipients[END_REF][START_REF] Ahmadpoor | Time-Varying Covariates and Risk Factors for Graft Loss in Kidney Transplantation[END_REF][START_REF] Englum | Outcomes in kidney transplant recipients from older living donors[END_REF].

The laparoscopic approach for donor nephrectomy has become the gold standard, offering reduced postoperative pain, accelerating the return to an active life, and improving the esthetic outcomes [START_REF] Andersen | Postoperative pain and convalescence in living kidney donors-laparoscopic versus open donor nephrectomy: a randomized study[END_REF]. These benefits have been proven at a time when donors were preferably chosen with limited excess weight and comorbidities. Still, organ shortage increasingly leads to the consideration of older, more frequently overweight, or even obese donors. These candidates, inherently bearing more comorbidities, can be at increased risk of intraoperative difficulties or intra-and postoperative complications. Although considerable effort has been deployed to estimate the risk for the donor of developing end-stage renal disease or long-term medical adverse outcomes after kidney donation, little is known about the predictive factors of complicated surgery or postoperative course [START_REF] Grams | Kidney-Failure Risk Projection for the Living Kidney-Donor Candidate[END_REF][START_REF] Janki | Long-term prognosis after kidney donation: a propensity score matched comparison of living donors and non-donors from two population cohorts[END_REF].

The Mayo Adhesive Probability score (MAP score) was first designed in 2014 to predict the risk of adherent perinephric fat and intraoperative difficulties in partial nephrectomy patients [START_REF] Davidiuk | Mayo adhesive probability score: an accurate image-based scoring system to predict adherent perinephric fat in partial nephrectomy[END_REF]. The MAP score was retrospectively determined to be an independent predictor of the risk of conversion in these patients, and associated with the risk of peri-and postoperative complications in another study suggesting using a cutoff of 3 [START_REF] Khene | Predicting morbidity after robotic partial nephrectomy: The effect of tumor, environment, and patient-related factors[END_REF][START_REF] Bier | Prediction of Postoperative Risks in Laparoscopic Partial Nephrectomy Using RENAL, Mayo Adhesive Probability and Renal Pelvic Score[END_REF]. The potential interest of the MAP score in predicting the difficulty of laparoscopic donor nephrectomy was recently suggested by Cockerill et al. who showed an association of the MAP score with the total operative time [START_REF] Cockerill | The Association of the Mayo Adhesive Probability (MAP) Score With Total Operative Time in Patients Undergoing Hand-assisted Laparoscopic Donor Nephrectomy[END_REF] .

The objectives of the present study were to evaluate whether the MAP score could help predict intra-and postoperative complications in patients undergoing laparoscopic living donor nephrectomy.

Population and methods

Population

A bicentric study was conducted in two academic departments. The charts of all kidney donors operated on between January 2004 and December 2017 were retrospectively reviewed. The study was granted institutional review board approval and registered with the INDS (French institute for health data) under the number MR 2910211218. Patients were excluded in case of missing follow-up data or if they refused the use of their medical data.

Imaging review and MAP score definition

Preoperative imaging of living donors included computed tomography with intravenous contrast and non-contrast, arterial, portal, and late excretory phases.

Laparoscopic living donor nephrectomy technique

All surgeries in each center were done or directly supervised by the same surgeons during the study period. Surgery was performed via a laparoscopic transperitoneal approach. A complete dissection of the renal pedicle was achieved (extra-gerotal dissection) before the kidney was dissected from its perinephric fat (intra-gerotal dissection) and placed in a laparoscopic retrieval bag. Extraction of the kidney was done through a short Pfannenstiel, lower midline or periumbilical incision, depending on past surgery history. One surgeon in center 2 developed the single-port approach after completing his learning curve in a highvolume institution.

Patients operated by an upfront open approach (sub-costal or lombotomy) were excluded.

Vascular control was obtained using a combination of nonabsorbable Hem-o-Lok clips, metallic clips, and surgical vessel staplers. In the case of Hem-o-Lok use, a second technique was used such as a metallic clip or manual suturing of the arterial stump using a nonabsorbable uncoated blue suture monofilament polypropylene suture size 4/0 (Prolene™).

Intra-and postoperative complications assessment

Intraoperative complications were obtained from the operating notes and, when possible, by reviewing the video recording of the surgery. All intraoperative events disrupting the surgical plan were noted. Conversion to open surgery was defined by the need to switch from laparoscopic to open approach via a sub-coastal or laparotomy incision.

Postoperative complications (until postoperative day 30) were obtained from the patient's medical charts, including daily medical and nurses' notes, vital signs flow sheets, discharge, and follow-up letters. The Clavien-Dindo classification was used for complication severity rating and reporting [START_REF] Clavien | The Clavien-Dindo classification of surgical complications: five-year experience[END_REF].

Data presentation and statistical analysis

Data are presented as numbers (percentages) or medians (IQR). Univariate analysis was performed using Fisher's exact test for nominal variables and Student's test for continuous variables. Risk factors assessed for complications were surgical approach, gender, anatomic variation, previous abdominal surgical history, MAP score, and obesity with a BMI ≥30 kg/m2. Multivariable analysis was performed using nominal logistic regression and Chisquare test or the least-squares method for nominal and continuous data, respectively. All analyses were performed using the SAS interface JMP (SAS version 14). Statistical significance was set at p<0.05. To account for the multiple tests performed in the comparison of the cohorts with or without a high MAP score, the Bonferroni correction was used. Seven tests were performed, therefore a value of p<0.007 was used to assess for statistical significance.

Results

Study flowchart and patients' characteristics:

The study flowchart is detailed in Figure 2. Overall, 307 donors constituted the study population, among whom 44 patients (14%) presented with a MAP score ³3. The concordance between readers was 93% for the overall score and no case was misclassified using a cutoff of 3.

Figure 2. Study flowchart

The median age was 52 years (43-60) and the median Body Mass Index (BMI) was 24.4 kg/m 2 (22-27). Donors were female in 198/307 (64%) of the cases. Nearly half of the patients (49%) had a previous history of abdominal surgery. Surgery was performed on the left side in 284/307 cases (93%), through a laparoscopic and single-port approach in 186 and 121 cases, respectively. An anatomical variation was described in 114 cases (37%). The detailed patients' characteristics are described in Patients with a high MAP score (≥3) were more frequently males (p<0.001), presented more frequently with obesity (p<0.001). As expected, there was a strong association between a high MAP score and the description of adhesive perinephric fat during surgery (p<0.001).

Intraoperative complications

Intraoperative difficulties were encountered in 50 patients (16%), including difficult dissection (n=35) and bleeding (n=17). Conversion to open surgery was required for 13 patients (4.2%) because of inability to safely achieve planes (n=6) or acute bleeding (n=7).

Bleeding control was performed by direct vessel control in all cases and splenectomy in one case.

On univariate analysis, a high MAP score (≥3) and the presence of an anatomical variation were significantly associated with the risk of intraoperative difficulty. A list of all anatomical variations encountered is available as Supplementary Table . There was no significant difference in the rate of intraoperative difficulties when comparing the various surgical approaches. However, the operative time was significantly increased with the single port approach (225 minutes (200-252) vs 155 minutes (125-175), p<0.0001). Obesity, male gender, and a past history of abdominal surgery were not associated with increased intraoperative difficulty. On multivariate analysis, a high MAP score (≥3) and the presence of an anatomical variation were still significantly associated with the of intraoperative difficulty (Table 2).

Regarding the risk of conversion to open surgery, a high MAP score (≥3) was the only significant risk factor in multivariate analysis (Table 3).

Risk factor

Postoperative complications

A total of 112 complications were noted in 99 donors (32%). Most complications (n=106) were classified as minor according to the Clavien-Dindo classification (Clavien I and II). Local wound complications accounted for half of all complications (n=49), including wound abscess (n=30), hematoma (n=9) and delayed wound healing (n=10). One thrombo-embolic complication occurred (pulmonary embolism).

Twelve donors (3.9%) presented with a severe complication requiring surgery (Clavien IIIa and IIIb) including one acute hemorrhage while the patient was in recovery, requiring emergency laparotomy (Clavien IV). No donor died.

Four donors underwent drain placement under local anesthesia for the drainage of chylous ascites. One small bowel obstruction due to internal hernia motivated iterative surgery without bowel resection. Two donors developed pleural complications (pneumothorax, pleural effusion) requiring drainage.

On multivariate analysis, a high MAP score (≥3) was the only significant factor associated with an increased risk of postoperative complications (Table 4).

Risk factor

Discussion

In a background of a worldwide organ shortage, the performance of laparoscopic donor nephrectomy was found to increase the number of kidney donations [START_REF] Kuo | Laparoscopic donor nephrectomy increases the supply of living donor kidneys: a center-specific microeconomic analysis[END_REF]. However, while donor selection is being less restrictive to include donors with more comorbidities and higher BMI, the safety of a procedure performed on healthy individuals should remain our main concern. Still, while a considerable effort has been deployed to assess the operative risk of patients undergoing partial nephrectomy for renal cancer, little has been done in the specific field of kidney donation [START_REF] Kutikov | nephrometry score: a comprehensive standardized system for quantitating renal tumor size, location and depth[END_REF][START_REF] Ficarra | Preoperative aspects and dimensions used for an anatomical (PADUA) classification of renal tumours in patients who are candidates for nephron-sparing surgery[END_REF]. We show here, using data gathered from two Other studies have recently reported that donors with a higher BMI or a male gender were at higher risk of postoperative complications [START_REF] Sterkenburg | Long-term Surgical Outcome and Risk Factors in Living Kidney Donors[END_REF]. Still, we are now in an era of anticipation, where preoperative imaging gives us insight into the precise patient-specific anatomy and a significant edge when dealing with a complicated case. It is therefore not surprising that imaging-derived features such as anatomical variations or the MAP score were found to be associated with intraoperative difficulties and the risk of conversion to open surgery. This highlights the importance of obtaining high-quality preoperative imaging and its interpretation by trained radiologists.

Nevertheless, it can be opposed that even in the presence of preoperative risk factors, it may be decided to pursue with the donation plan because the benefits expected outweigh the hypothetical risks. This is probably true. However, improving patient information and consent, and raising the level of awareness of the surgical team to a potential risk of conversion or postoperative complications may prove critical to improving donors' management, safety, and outcomes. Minor complications are not without socio-economic consequences when prolonging the length of stay or sick leave of the donor.

Several studies and meta-analysis focusing on intra-and postoperative complications after kidney donation reported variable minor complication rates, probably due to inconstant reporting [START_REF] Wilson | Laparoscopic versus open nephrectomy for live kidney donors[END_REF]. We describe a rate of major complications of 4%, comparable to previously published data [START_REF] Mjøen | Morbidity and mortality in 1022 consecutive living donor nephrectomies: benefits of a living donor registry[END_REF][START_REF] Burkhalter | Early complications after living donor nephrectomy: analysis of the Swiss Organ Living Donor Health Registry[END_REF]. The conversion rate in our study (4.2%) seems to be on the high side of usually reported conversion rates (around 2%) [START_REF] Fisher | 200 consecutive hand assisted laparoscopic donor nephrectomies: evolution of operative technique and outcomes[END_REF]. However, reporting bias is not unlikely as Rowley et al., based on a series of 1592 contemporary laparoscopic procedures, described a quite similar conversion rate of 4.6% [START_REF] Rowley | Risk factors for conversion to hand assisted laparoscopy or open surgery during laparoscopic renal surgery[END_REF].

We must acknowledge several limitations of the present study. Its retrospective design explains a significant risk of unreported events, especially for the post-operative period.

Complications and conversions were rare events, explaining the width of the confidence intervals around the estimated odds-ratios. The use of 2 techniques, conventional

  Anatomical variations were noted. The MAP score was determined by the same observer blinded to the patients' outcome. A random sample of 20 cases was also reviewed by 2 separate readers to assess interobserver reproducibility. A cutoff of 3 was used to define a high MAP score. Details of the MAP score definition are presented in Figure 1.

Figure 1 .

 1 Figure 1. MAP score definition. (A) Perinephric fat thickness (t) at the level of the renal vein. Inferior to 1cm: 0 point; 1-1.9cm: 1 point; superior to 2cm: 2 points. (B-D) Perinephric fat stranding (B) absent: 0 point; (C) mild: 2 points; (D) severe stranding: 3 points (RV=renal vein, Ao=aorta, VC=vena cava)

  academic centers, that the Mayo Adhesive Probability score (MAP score) with a cutoff of 3, can be used to estimate the risk of intraoperative difficulty, conversion to open surgery, and postoperative complications. Contrary to partial nephrectomy, the surgical technique for living donor nephrectomy can be adjusted to allow for a completely extra-gerotal dissection in the presence of adherent perinephric fat. Nevertheless, these results support an increased surgical difficulty, due to a more difficult mobilization of the kidney for safe vascular control, dissection of the upper pole and separation from the adrenal gland, and graft extraction.

Table 1 .

 1

		MAP Score < 3	MAP score ≥3	All patients	P-Value
		N=263	N=44	N=307
	Age (years) -median (IQR)	51 (43-59)	55 (48-63)	52 (43-60)	<0.001*
	Gender -N (%)				<0.001*
	Female	190 (72)	8 (18)	198 (64)
	Male	73 (28)	36 (82)	109 (36)
	ASA Score -N (%)				0.02
	1	194 (74)	24 (55)	218 (71)
	2	69 (26)	19 (43)	88 (28.7)
	3	0	1 (2)	1 (0.3)
	BMI (kg/m 2 ) -median (IQR)	24.1 (21-26.6)	26.7 (25-28.6)	24.4 (22-27)	<0.001*
	Past abdominal surgery (yes) -N	129 (49)	21 (48)	150 (49)	0.87
	(%)
	Laterality -N (%)				0.42
	Right	21 (8)	2 (5)	23 (7)
	Left	242 (92)	42 (95)	284 (93)
	Anatomical variation -N (%)	91 (35)	23 (52)	114 (37)	0.02

Table 1 .

 1 Patients' characteristics (MAP score=Mayo Adhesive Probability score; ASA

score=American Society of Anesthesiologists score; BMI=Body Mass Index; IQR=Interquartile Range; *statistically significant with Bonferroni correction)

Table 2 .

 2 Risk factors of intraoperative difficulty on multivariate analysis. (CI=Confidence

		Odds Ratio	95% CI	P-Value
	MAP Score ≥3	14.12	5.58-35.7	<0.001*
	Male gender	1.54	0.65-3.66	0.31
	Anatomical variation	2.25	1.11-4.53	0.02*
	Surgical approach	0.92	0.43-1.97	0.19
	Past abdominal surgery	1.01	0.49-2.04	0.98
	BMI≥30 kg/m 2	2.15	0.53-8.73	0.26
	Interval; MAP score=Mayo Adhesive Probability score; BMI=Body Mass Index; *statistically
	significant)
	Risk factor	Odds Ratio	95% CI	P-Value
	MAP Score ≥3	18.96	3.42-105.14	0.0042*
	Male gender	5.54		0.87-35.23	0.05
	Anatomical variation	2.86		0.73-11,14	0.12
	Surgical approach	0.31		0.05-1.67	0.25
	Past abdominal surgery	1.37		0.38-4.92	0.62
	BMI≥30 kg/m 2	3.18		0.28-35.26	0.29

Table 3 .

 3 Risk factors of conversion to open surgery on multivariate analysis. (CI=Confidence

	Interval; MAP score=Mayo Adhesive Probability score; BMI=Body Mass Index; *statistically
	significant)

Table 4 .

 4 Risk factors of postoperative complications on multivariate analysis. (CI=Confidence

		Odds Ratio	95% CI	P-Value
	MAP Score ≥3	2.55	1.20-5.40	0.01*
	Male gender	1.37	0.76-2.48	0.28
	Anatomical variation	1.31	0.79-2.18	0.29
	Surgical approach	1.96	1.31-3.95	0.07
	Past abdominal surgery	1.24	0.74-2.06	0.40
	BMI≥30 kg/m 2	1.32	0.52-3.30	0.55

Availability of data and material

De-identified data can be made readily available to reviewers upon request Code availability Statistical software can be made readily available to reviewers upon request

laparoscopic and single-port approaches, probably limits the reproducibility of our results.

Given both teams' limited experience, no patient was operated using a retroperitoneoscopic approach, although it could have been an interesting option for patients with previous abdominal surgeries. Similarly, most donors included in this study were of white Caucasian origin and the results may not be reproduced in a population with increased racial diversity.

Still, we believe that the interest of the findings obtained from "real-life" data, and the ease of implementation of the MAP score in daily practice will trigger future studies aiming at confirming these results and evaluating the impact of the score on donors' outcomes. The prospective KLIDE (Kidney Living Donor Evaluation) cohort is currently being recruited in our centers.

Conclusion

In this retrospective, bicentric study conducted among two academic departments, a high MAP score in multivariate analysis was associated with an increased risk of intraoperative difficulty, conversion to open surgery, and postoperative complications. Although the benefits of living donation may outweigh the risks and a high score does not mean that the donation project should be abandoned, these results can be used to improve donors' information and possibly safety by increasing the level of awareness of the surgical team. External validation and further prospective evaluation are mandated to confirm these results.

Declarations


S Geiskopf

Mathieu Stoffel

X Devaux

Erwan André

C Carteret

A Bouché

Michel Vergnat

Hervé Rinnert

Formation of SiP 2 Nanocrystals Embedded in SiO 2 from Phosphorus-Rich SiO 1.5 Thin Films

Si nanocrystals (Si-NCs) have been extensively studied during the last two decades due to their original electronic and optical properties. Additional functionalities can be expected if the nanocrystals can be effectively doped. This is, however, a challenging issue which is related with several physical and technological barriers. In particular, the increasing formation energy for substitutional dopants with the decreasing size can lead to a "selfpurification" mechanism for small nanocrystals [START_REF] Dalpian | Self purification in semiconductor nanocrystals[END_REF]. Nevertheless, both P and B doped Si nanocrystals embedded in a SiO 2 matrix have been successfully obtained by high temperature phase separation of Si rich SiO x thin films deposited by e-beam evaporation [START_REF] Perego | Phosphorus doping of ultra-small silicon nanocrystals[END_REF], plasma enhanced chemical vapor deposition [START_REF] Gutsch | Electronic properties of phosphorus doped silicon nanocrystals embedded in SiO 2[END_REF], ion implantation [START_REF] Khelifi | Efficient n-type doping of Si nanocrystals embedded in SiO 2 by ion beam synthesis[END_REF] or by using a combination of the two last techniques [START_REF] Ehrhardt | Incorporation of dopant impurities into a silicon oxynitride matrix containing silicon nanocrystals[END_REF]. Another important question concerned the localization of the dopants. König et al. [START_REF] König | Location and electronic nature of Phosphorus in the Si nanocrystal-SiO 2 system[END_REF] used atom probe tomography to demonstrate that in the case of P-doped Si nanocrystals embedded in SiO 2 prepared by plasma enhanced chemical vapor deposition, only 15 % of the P atoms are found within the nanocrystals while 30 % are localized at the interface with the SiO 2 matrix and 55% in the silica matrix. Alternatively, ion beam synthesis by silicon/dopant co-implantation appeared to be a viable route to obtain a high concentration of P atoms inside Si nanocrystals [START_REF] Khelifi | Efficient n-type doping of Si nanocrystals embedded in SiO 2 by ion beam synthesis[END_REF]. Very recently, it has been shown both theoretically [START_REF] Pi | Tight-binding calculations of the optical response of optimally P-doped Si nanocrystals: a model for localized surface plasmon resonance[END_REF] and experimentally [START_REF] Rowe | Phosphorus-doped Si nanocrystals exhibiting mid-infrared localized surface plasmon resonance[END_REF][START_REF] Kramer | Plasmonic properties of silicon nanocrystals doped with boron and phosphorus[END_REF][START_REF] Zhou | Comparative study on the localized surface plasmon resonance of boron-and phosphorus-doped silicon nanocrystals[END_REF][START_REF] Zhou | Boron-and phosphorus-hyperdoped silicon nanocrystals[END_REF] that either phosphorus or boron doped Si-NCS can be of interest for tunable localized surface plasmon resonances (LSPR). When the dopant concentration is very high, the solid solubility limit could be exceeded and alloying could become possible. Indeed, the Si-P equilibrium phase diagram established so far predicts that two stable compounds may form (SiP and SiP 2 ) depending on the relative amount of phosphorus [START_REF] Olesinski | The P-Si (Phophorus-Silicon) system Bulletin of Alloy Phase Diagrams[END_REF][START_REF] Safarian | Phase diagram study of the Si-P system in Si-rich region[END_REF][START_REF] Liang | Modeling of thermodynamic properties and phase equilibria of the Si-P system[END_REF]. Both orthorhombic and pyrite type bulk SiP 2 alloys were first synthesized by Wadsten in 1967 [START_REF] Wadsten | The crystal structures of SiP 2 , SiAs 2 and GeP[END_REF][START_REF] Wadsten | Synthesis of a pyrite type modification of SiP 2[END_REF]. Donohue et al. [START_REF] Donohue | Preparation and properties of pyrite-type SiP 2 and SiAs 2[END_REF] refined the structure of pyrite type SiP 2 by using X-ray powder diffraction. Based on galvanomagnetic and reflectance measurements, these authors claim that pyrite-type SiP 2 could be a semimetal. Farberovich et al. [START_REF] Farberovich | Band structure and density of states in SiP 2[END_REF] came to the same conclusion by calculating the band structure using the orthogonalized plane wave method.

Vogt et al. [START_REF] Vogt | Complete first order Raman spectra of the pyrite structure compounds FeS 2 , MnS 2 and SiP 2[END_REF] characterized the vibrational properties of pyrite type SiP 2 and obtained the frequencies of all Raman active phonons at room temperature. Meier et al. [START_REF] Meier | Ab initio simulation of the fundamental vibrational frequencies of selected pyrite-type pnictides[END_REF] performed ab initio calculations of the fundamental vibration frequencies of pyrite type SiP 2 . A good agreement was found with both Raman and infrared studies. Bachhuber at al. [START_REF] Bachhuber | First principles calculations on structure, bonding and vibrational frequencies of SiP 2[END_REF] further used first principle calculations to investigate the structure, the bonding and the vibrational properties of pyrite type SiP 2 . By using the vapor transport method, SpringThorpe [START_REF] Springthorpe | The preparation of single crystal orthorhombic SiP 2[END_REF] succeeded to prepare single crystals of orthorhombic SiP 2 . This compound was shown to be a p-type semiconductor with an optical bandgap of about 1.89 eV. By using the Sn flux method with Gd as a mineralizer in a fused silica tube, Zhang et al. [START_REF] Zhang | Structure and growth of single crystal SiP 2 using flux method[END_REF] identified by means of X-ray diffraction another orthorhombic phase of SiP 2 crystallizing in the Pnma group. Diffuse reflectance measurements as well as preliminary density functional theory (DFT) calculations suggest a semiconducting behavior with a bandgap close to 1.4 eV. Moreover, SiP 2 microparticles prepared by ball milling [START_REF] Kwon | Silicon diphosphide: a Si-based 3D crystalline framework as a high performance Li-ion battery anode[END_REF] but also nanocomposites consisting of SiP 2 and amorphous carbon as well as carbon nanotube wrapped SiP 2 [START_REF] Wang | Multi walled carbon nanotube-wrapped SiP 2 as a superior anode material for lithium-ion and sodium-ion batteries[END_REF] emerged as promising anode materials for high performance Li-and Na-ion batteries [START_REF] Reinhold | Surface and electrochemical studies on silicon diphosphide as easy-to-handle anode material for lithium-based batteries-the phosphorus path[END_REF][START_REF] Saddique | Enhanced silicon diphosphide-carbon composite anode for long cycle, high efficient sodium ion batteries[END_REF].

Very recently, Si x P y alloys (x,y =1,2) have gained a renewed interest since they exhibit a lamellar structure. This property might be of interest for engineering novel Si-based 2D materials, which could allow filling the gap between silicene and phosphorene. Density functional theory (DFT) calculations have shown that 2D-SiP x (x = 1, 2) are direct bandgap semiconductors [START_REF] Huang | Highly stable twodimensional silicon phosphides: different stoichiometries and exotic electronic properties[END_REF][START_REF] Zhang | Two dimensional SiP: an unexplored direct band-gap semiconductor[END_REF][START_REF] Malyi | Tailoring electronic properties of multilayer phosphorene by siliconization[END_REF][START_REF] Matta | Versatile two-dimensional silicon diphosphide (SiP 2 ) for photocatalytic water splitting[END_REF], which might be of particular interest for future optoelectronic applications. Moreover, according to its band structure, 2D-SiP 2 appears to be promising for photocatalytic water splitting [START_REF] Matta | Versatile two-dimensional silicon diphosphide (SiP 2 ) for photocatalytic water splitting[END_REF]. Till now, there are no reports available in the literature concerning the growth of orthorhombic SiP 2 either in thin films or nanocrystals. In this paper, we demonstrate the formation of orthorhombic SiP 2 nanocrystals embedded in silicon dioxide thin films obtained by annealing of phosphorus rich SiO 1.5 thin films at 1100°C. We provide a careful structural and vibrational study of SiP 2 nanocrystals. The experimental results are found to be in excellent agreement with density functional theory calculations of the vibration modes for the SiP 2 alloy. The optical properties of P-rich SiO 1.5 thin films are further discussed in relation with the obtained structural results.

Phosphorus-rich SiO 1.5 thin films were prepared by co-evaporation on Si(001) substrates held at room temperature of SiO and SiO 2 from e-beam guns and P from a GaP decomposition source [START_REF] Lippert | Heavy phosphorus doping in molecular beam epitaxial grown silicon with a GaP decomposition source[END_REF]. The SiO and SiO 2 fluxes were controlled by a quartz microbalance. The P content, which is defined by [P]/([Si]+[O]+[P]), was obtained by comparing the relative intensities of the P 2p, Si 2p and O 1s core levels measured using Xray photoelectron spectroscopy (XPS). The obtained values were cross-checked by secondary ion mass spectroscopy (SIMS) measurements. The total film thickness was about 200 nm for all samples considered in this study. The samples were post-grown annealed in a rapid thermal annealing furnace at 1100°C during 5 minutes under N 2 atmosphere. The structural properties of the samples were investigated by high resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) using a JEOL ARM 200 F probe corrected with cold field emission gun. EELS spectrum images were recorded with a Gatan GIF quantum ER filter. To avoid any phase precipitation due to ion or electron beam damages, the samples were mechanically thinned up to the electron transparency by the tripod method and the TEM was operated at 80 kV with a probe current lower than 70 pA for EDS and EELS spectrum images. For EELS investigations, zero-loss and core-loss spectrum images were recorded simultaneously to allow advanced data post processing (correction of energy drift, thickness correction). To separate overlapping Si edges and P edges and chemical states of Si and extract quantitative data, multiple linear least square fitting (MLS) was used using reference spectra recorded on pure Si and on pure thermal silica grown on a Si wafer. The reference spectra were recorded with the same STEM and spectrometer parameters than that of the sample investigated. Raman scattering measurements were performed at RT by using a 532 nm laser diode as excitation source. The scattered light is analyzed by a monochromator equipped with a 1800 lines/mm grating and a CCD detector. The steady state photoluminescence was excited with a 325 nm He-Cd laser and subsequently analyzed by a monochromator equipped with a 600 grooves/mm grating and measured by a photomultiplier tube operating in the visible-IR range. The spectra were corrected from the response of the detector. Density functional theory (DFT) calculations were performed with the CRYSTAL14 [START_REF] Dovesi | CRYSTAL 14 : a program for the ab initio investigation of crystalline solids[END_REF] periodic computation code using the B3LYP functional [START_REF] Dovesi | CRYSTAL 14 : a program for the ab initio investigation of crystalline solids[END_REF][START_REF] Becke | Density-functional thermochemistry.III.The role of exact exchange[END_REF] and a triple ζ localized (atom centered) basis set [START_REF] Peintinger | Consistent Gaussian basis sets of triplezeta valence with polarization quality for solid-state calculations[END_REF]. Both the atomic positions and the cell parameters of SiP 2 were optimized. The harmonic vibration frequencies were calculated at the Γ point and the corresponding intensities were evaluated through a coupled perturbed Kohn-Sham (CPKS) approach.

Figure 1 (a) shows a HRTEM image of a SiO 1.5 thin film containing 10 at. % of phosphorus after annealing for 5 minutes at 1100°C. Two kinds of features can be distinguished. Firstly, the interface with the substrate is not sharp but one can identify bumps which are in epitaxy with the substrate. A high resolution STEM-high angle annular dark field (HAADF) image of the interface between a typical bump and the Si substrate is shown in Fig. 1(b). The zone axis of the Si substrate is <110>. The interface is indicated by a red dash-dotted line. One can clearly identify the dumbbell of Si <110> both in the bump and in the substrate showing that the bumps are in epitaxy with the underlying Si substrate. Secondly, in the thin film, one can distinguish large, spherical shaped nanoparticles having typical sizes between 30 and 40 nm.

One can clearly distinguish the atomic planes demonstrating almost perfect crystallinity. We note, however, that these nanocrystals often contain a high density of twins and stacking faults. Similar particles can also be identified in samples containing 3 and 6 at. % of phosphorus (not shown here). Their size increases from about 12 nm in the film containing 3 at. % of phosphorus to 31 nm in the film containing about 10 at. % of phosphorus. Figure 1(c) displays a Fourier transform of the spherical nanocrystal identified in Figure 1(a). A simulation of the electron diffraction pattern of SiP 2 with Pbam orthorhombic structure (JCPDS file 00-044-1124) was compared to the Fourier transform. A good agreement is found for the zone axis [014]. This is the first experimental observation of SiP 2 nanocrystals crystallizing in an orthorhombic structure. In order to characterize the chemical composition of both types of nanostructures, we have performed EDS spectrum images (SI). Figure 2 shows a HAADF image (Fig. 2(a)) together with the chemical semi-quantitative maps for Si (Fig. 2 Three different peaks can be identified at 0.5, 1.7 and 2.0 keV corresponding to the Kα emission lines of oxygen, of silicon and of phosphorus, respectively. In the substrate, one can find about 1 at. % of phosphorus indicating that P diffusion has occurred during annealing at 1100°C. Moreover, the EDS spectrum extracted from the bump area shows that the bumps contain also about 1 at. % of phosphorus. The oxygen related peak at 0.5 keV originates from SiO 2 which is present both on top and below the bump. Finally, for the large nanocrystals, the Si-related peak at 1.7 keV originates from both Si atoms located in SiO 2 and in SiP 2 . Indeed, under the electron probe, X-rays are emitted from the whole thickness of the sample. Consequently, Si atoms in SiO 2 present both on top and below the nanocrystals will also emit. To obtain the real Si content in the nanoparticles, the contribution resulting from Si atoms present in SiO 2 have to be subtracted. This can be done easily by assuming that all oxygen atoms originate from the stoichiometric SiO 2 compound. A Si content of about 32.8 at. % and a phosphorus content of 67.2 at. % was then obtained, which is compatible with the presence of a SiP 2 compound. However EDS measurements do not allow to distinguish clearly between silicon atoms in SiP 2 nanoparticles, in silicon bumps and in the silicon dioxide matrix. Moreover, the EDS signal is known to be not fully localized. In order to get a more precise chemical mapping at the nanoscale and to be able to distinguish between Si (0) and Si (4+) , we have performed further STEM-EELS spectrum images. Figure 3(a) shows a STEM-HAADF image of an annealed film containing 10 at. % of phosphorus annealed at 1100°C. One can clearly distinguish both the nanoparticles and the bumps. Figure 3(b) shows an EELS spectrum integrated over the whole area shown in Fig. 3(a) (black solid line). To get a chemical map, we have performed a minimum least square fitting of this spectrum by using reference spectra for Si (0) and Si (4+) (from a lamella of thermal SiO 2 grown by oxidation of Si(100)) measured in the same experimental conditions (thickness, accelerating voltage, probe current, convergence and collection angle, etc.). Moreover, the fitting leads to a residual spectrum (orange solid line in figure 3(b) having an edge at 129 eV, which corresponds to phosphorus [START_REF] Wu | Atomic and electronic structure of exfoliated black phosphorus[END_REF]. By considering the reference spectra for both Si (0) and Si (4+) and the residual spectrum for phosphorus, we were able to reconstruct the chemical map for each element with a new MLS fitting of each pixel of the SI. Figure 3(c)-(d) show the chemical maps for both Si (0) and P. One can recognize that Si is present in the substrate, in the bumps and in the nanocrystals. P atoms are localized in the nanocrystals and at the interface between the substrate and the film, which originates from P diffusion during annealing. It is noteworthy that the maps for Si and P superimpose perfectly in the thin films indicating that both Si and P atoms are located in the nanocrystals. Full EELS chemical maps showing Si (0) , Si (4+) and P as well as the superposition of maps is presented in the Supplementary Information (Fig. S1).

Finally, one can discuss briefly the formation of the bumps at the interface with the substrate.

In case of pure SiO 1.5 films, they are not observed. Their size and height increases as the P content in the films increases. During annealing at high temperatures, both P atoms and Si atoms in excess which do not participate in either SiP 2 or SiO 2 will diffuse towards the substrate. The diffusion will be even enhanced for larger P contents thus leading to formation of larger bumps. The Si atoms eventually accumulate into holes created in the oxide at the interface thus leading to the formation of bumps in epitaxy with the Si(001) substrate.

All the previous characterizations were done at the nanometer scale. To obtain a spectroscopic signature of the SiP 2 nanocrystals, we have further performed Raman scattering measurements. Figure 4 (a) shows Raman spectra measured for SiO 1.5 thin films with various P contents ranging from 0.3 at.% to 10 at. % and annealed at 1100°C. The films were deposited onto silica substrates in order to get rid of the huge signal originating from the Si-TO line at 520.5 cm -1 . For P contents below 3 at. %, the spectra are essentially dominated by the signal originating from the silica substrate. However, for P contents of about 10 at. %, new and sharp peaks can be observed suggesting the formation of a new crystalline phase.

According to all structural results from this work, these peaks could possibly be a signature of the SiP 2 compound. To clarify the origin of these peaks, we have performed DFT calculations of the vibrational modes for the orthorhombic SiP 2 structure. Figure 4 (b) shows the experimental spectrum (blue solid line) as well as the calculated Raman spectra for the orthorhombic SiP 2 alloy (magenta solid line). It is obvious that a good agreement is obtained between the experimental and the calculated Raman spectrum thus further confirming that the observed nanocrystals consist indeed of SiP 2 . Table I summarizes the position of both calculated and measured vibrational wavenumbers. Some differences appear nevertheless between the theoretical and the experimental values, which could be due either to the approximations used for the DFT calculations or to the fact that a non-negligible strain may be exerted by the silica matrix onto the SiP 2 nanocrystals. The different vibrational modes are also identified next to the calculated vibrational wavenumbers. Finally, our structural investigations help to shed new light on the evolution of the photoluminescence of SiO 1.5 :P thin films with increasing P contents. Figure 5 displays the room temperature PL spectra measured for SiO 1.5 thin films with various P contents ranging from 0.3 at.% to 10 at. %. For the lowest P content (i.e. 0.3 at.%), the PL signal is characterized by a rather broad band extending from about 700 nm to 1000 nm. In this particular sample, SiP 2 nanocrystals could not be detected. The observed luminescence thus originates from radiative recombinations between charge carriers confined in pure Si nanocrystals. For the sample containing about 3 at. % of P, a strong decrease of the PL intensity is observed. Similar evolutions have been reported in the literature and are generally ascribed to the Auger effect [37] in Si-NCs containing P atoms which could then act as dopants. Spatially resolved EELS measurements performed on the same sample (see Fig. S1, Supplementary Informations) revealed the coexistence of both SiP 2 nanocrystals having typical sizes of about 15 nm and Si nanocrystals with sizes ranging between 4 and 6 nm. Only few of them have sizes below the Bohr radius of the exciton in bulk Si and can contribute to the observed photoluminescence thus explaining the strong intensity decrease when the P content increases from 0.3 at. % to 3 at. % of P.

Moreover, the PL peak shifts to larger wavelength thus suggesting an increase of the Si nanocrystal size, which is indeed observed. Finally, no PL could be detected for the sample having the highest P content (i.e. 10 at. %). In this case, all Si atoms are either involved in the SiP 2 nanocrystals or in the bumps at the interface with the substrate.

To summarize, we provided clear evidence of the formation of SiP 2 nanocrystals in Prich SiO 1.5 thin films annealed at 1100°C. The latter are found to crystallize in an orthorhombic structure. Elemental chemical mapping confirmed the SiP 2 stoichiometry and allowed us to determine the exact location of both P and Si atoms. Apart from the SiP 2 nanocrystals, P atoms are located in Si bumps, which are in epitaxy with the substrate, and at the substrate surface to a level of about 1 at. %. The vibrational properties are found to be in excellent agreement with DFT calculations of the vibrational modes for the SiP 2 alloy.

Finally, the photoluminescence properties of SiO 1.5 thin films with various P contents in the range 0.3-10 at. % can be understood by considering the structural properties. Our results shed new light into the complex evolution of ternary Si-O-P alloys during high temperature annealing.

  Figure 2(e). Three different peaks can be identified at 0.5, 1.7 and 2.0 keV corresponding to

Figure captions : Figure 1 :

 :1 Figure captions:

Figure 2 :

 2 Figure 2: (a) STEM-HAADF image of a SiO 1.5 thin film containing 10 at. % of P annealed at

Figure 3 :Figure 4 :

 34 Figure 3: (a) Spectrum image of a SiO 1.5 thin film containing 10 at. % of P annealed at

Figure 5 :

 5 Figure 5: Room temperature photoluminescence spectra of SiO 1.5 thin films with different P

Fig. 1

 1 Fig.1

Fig. 2 20Fig. 3 21Fig. 4

 234 Fig.2

Fig. 5

 5 Fig. 5

Acknowledgments

The authors would like to acknowledge the LUE (Lorraine Université d'Excellence) and the CPER MatDS projects for support.