Tucker J Mckenzie

Paul S Heaton

Kabir Rishi

Raj Kumar

Thomas Brunet

Gregory Beaucage

Olivier Mondain-Monval

Neil Ayres
email: neil.ayres@uc.edu

Storage moduli and porosity of soft PDMS polyMIPEs can be controlled independently using thiol-ene click chemistry

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Porous elastomeric polymers are used in a wide range of possible applications including biomaterials [START_REF] Eissa | Enhanced Differentiation Potential of Primary Human Endometrial Cells Cultured on 3D Scaffolds[END_REF][START_REF] Zhang | Highly Porous Hydrogels Templated within Emulsions Stabilized Using a Reactive, Crosslinking Triblock Copolymer[END_REF][START_REF] Whitely | Improved in Situ Seeding of 3D Printed Scaffolds Using Cell-Releasing Hydrogels[END_REF][START_REF] Akay | Microcellular PolyHIPE Polymer Supports Osteoblast Growth and Bone Formation in Vitro[END_REF][START_REF] Streifel | Hemostatic and Absorbent PolyHIPE-Kaolin Composites for 3D Printable Wound Dressing Materials[END_REF] and catalysis [START_REF] Yuan | Preparation and Recyclable Catalysis Performance of Functional Macroporous PolyHIPE Immobilized with Gold Nanoparticles on Its Surface[END_REF][START_REF] Yavuz | Acid and Base Catalysed Reactions in One Pot with Site-Isolated PolyHIPE Catalysts[END_REF] due to their elastomeric behavior, biocompatibility, gas permeability, thermal stability, and specific hydrophobic and dielectric properties. [START_REF] Noll | The Polymeric Organosiloxanes[END_REF][START_REF] Tonhi | High-Performance Liquid Chromatographic Stationary Phases Based on Poly(Dimethylsiloxane) Immobilized on Silica[END_REF] More recently, they were also shown to exhibit specific acoustic properties, including low sound velocities through the materials (from 40-120 m/s). [START_REF] Jin | Flat Acoustics with Soft Gradient-Index Metasurfaces[END_REF][START_REF] Brunet | Soft 3D Acoustic Metamaterial with Negative Index[END_REF][START_REF] Ba | Soft Porous Silicone Rubbers with Ultra-Low Sound Speeds in Acoustic Metamaterials[END_REF][START_REF] Kovalenko | Tailoring of the Porous Structure of Soft Emulsion-Templated Polymer Materials[END_REF][START_REF] Kovalenko | Tuning the Sound Speed in Macroporous Polymers with a Hard or Soft Matrix[END_REF] These soft acoustic metamaterials have been prepared from commercially available reagents such as poly(dimethyl siloxane) (PDMS) and lead to materials with negative acoustic indices when dispersed as small porous beads in a continuous matrix. [START_REF] Brunet | Soft 3D Acoustic Metamaterial with Negative Index[END_REF][START_REF] Ba | Soft Porous Silicone Rubbers with Ultra-Low Sound Speeds in Acoustic Metamaterials[END_REF][START_REF] Kovalenko | Tailoring of the Porous Structure of Soft Emulsion-Templated Polymer Materials[END_REF][START_REF] Kovalenko | Tuning the Sound Speed in Macroporous Polymers with a Hard or Soft Matrix[END_REF] However, the potential range of acoustic properties accessible using commercially available PDMS is limited.

One method to prepare highly porous materials is using emulsion templating with polymerized high internal phase emulsions (polyHIPEs). [START_REF] Brun | Hybrid Foams, Colloids and beyond: From Design to Applications[END_REF][START_REF] Kimmins | Functional Porous Polymers by Emulsion Templating: Recent Advances[END_REF][START_REF] Pulko | High Internal Phase Emulsion Templating -A Path To Hierarchically Porous Functional Polymers[END_REF][START_REF] Silverstein | PolyHIPEs: Recent Advances in Emulsion-Templated Porous Polymers[END_REF] This has been used to prepare porous PDMS materials. [START_REF] Kobayashi | Porous Membrane of Polydimethylsiloxane by Hydrosilylation Cure: Characteristics of Membranes Having Pores Formed by Hydrogen Foams[END_REF][START_REF] Grosse | Preparation of Microcellular Polysiloxane Monoliths[END_REF][START_REF] Martina | Siloxane Elastomer Foams[END_REF][START_REF] Hilborn | Emulsion-derived microporous siloxane foams[END_REF][START_REF] Tebboth | Inflatable Elastomeric Macroporous Polymers Synthesized from Medium Internal Phase Emulsion Templates[END_REF][START_REF] Kataruka | PDMS Polymerized High Internal Phase Emulsions (PolyHIPEs) with Closed-Cell, Aqueous-Filled Microcavities[END_REF] The term high is defined as when the volume fraction of the dispersed-phase of the emulsion exceeds 74% of the global volume. An emulsion with 24-74% dispersed phase by volume is called a medium internal phase emulsion (MIPE), and low internal phase emulsions (LIPEs) contain less than 24% of dispersed phase by volume. [START_REF] Wu | Macroporous Polymers Made from Medium Internal Phase Emulsion Templates: Effect of Emulsion Formulation on the Pore Structure of PolyMIPEs[END_REF] Polymerized emulsions are formed after polymerization of a monomer in the continuous phase, and a porous network is obtained after removal of the dispersed phase. This method is compatible with both hydrophilic and hydrophobic monomers depending if a water-in-oil (w/o) or oil-in-water (o/w) emulsion is used. [START_REF] Tebboth | Inflatable Elastomeric Macroporous Polymers Synthesized from Medium Internal Phase Emulsion Templates[END_REF][START_REF] Zhang | Porous Poly(High Internal Phase Emulsions): Prepared from Biomass-Derived Vanillin and Laurinol and Applied as an Oil Adsorbent[END_REF] For example, free radical polymerization of styrene and a divinylbenzene cross-linker is often used in synthesis of hydrophobic polyHIPE materials [START_REF] Woodward | Hypercrosslinked PolyHIPEs as Precursors to Designable, Hierarchically Porous Carbon Foams[END_REF] and hydrophilic monoliths have been prepared using acrylamide and N,N′-methylenebisacrylamide. [START_REF] Xie | Preparation of Porous Hydrophilic Monoliths: Effect of the Polymerization Conditions on the Porous Properties of Poly (Acrylamide-Co-N,N?-Methylenebisacrylamide) Monolithic Rods[END_REF] It has been suggested that a constraint with using free radical polymerization techniques is a lack of network homogeneity, leading to unpredictable materials properties including irreproducible Young's modulus. [START_REF] Zhang | Emulsion Templating: Porous Polymers and Beyond[END_REF][START_REF] Gibson | Cellular Solids Structure and Properties[END_REF] A way to circumvent this constraint may be to use macromolecular orthogonal coupling reactions (or "click chemistry"), [START_REF] Iha | Applications of orthogonal "click" chemistries in the synthesis of functional soft materials[END_REF][START_REF] Kolb | Click Chemistry: Diverse Chemical Function from a Few Good Reactions[END_REF] such as thiol-ene reactions. [START_REF] Kade | The power of thiol-ene chemistry[END_REF][START_REF] Bowman | Thiol-ene click chemistry[END_REF] This reaction can occur between functionalized polymers [START_REF] Nguyen | Ultrafast Diffusion-Controlled Thiol-Ene Based Crosslinking of Silicone Elastomers with Tailored Mechanical Properties for Biomedical Applications[END_REF] or small molecules, [START_REF] Sergent | Preparation of Thiol-Ene Porous Polymers by Emulsion Templating[END_REF][START_REF] Lovelady | Preparation of Emulsion-Templated Porous Polymers Using Thiol-Ene and Thiol-Yne Chemistry[END_REF][START_REF] Langford | Chemical Functionalization of Emulsion-Templated Porous Polymers by Thiol-Ene "Click[END_REF][START_REF] Ratcliffe | Optimized Peptide Functionalization of Thiol-Acrylate Emulsion-Templated Porous Polymers Leads to Expansion of Human Pluripotent Stem Cells in 3D Culture[END_REF][START_REF] Mezhoud | Novel Hypercrosslinking Approach toward High Surface Area Functional 2-Hydroxyethyl Methacrylate-Based PolyHIPEs[END_REF] and macromolecular thiol-ene reactions have been reported using cross-linked silicones. [START_REF] Nguyen | Ultrafast Diffusion-Controlled Thiol-Ene Based Crosslinking of Silicone Elastomers with Tailored Mechanical Properties for Biomedical Applications[END_REF] These reactions enable the properties of the final polyHIPE to be tuned based on stoichiometric ratios of the reactants and permit post-polymerization functionalization [START_REF] Lovelady | Preparation of Emulsion-Templated Porous Polymers Using Thiol-Ene and Thiol-Yne Chemistry[END_REF] or secondary click-reactions. [START_REF] Mezhoud | Novel Hypercrosslinking Approach toward High Surface Area Functional 2-Hydroxyethyl Methacrylate-Based PolyHIPEs[END_REF] In this study, we prepared emulsion-templated PDMS-based porous polymeric materials for potential acoustic applications using thiol-ene click-chemistry reactions to form polyMIPEs. We have focused on the synthesis of polyMIPEs, as porous-PDMS acoustic metamaterials are typically prepared in the MIPE regime rather than the HIPE regime. We have investigated the effects of the thiol to ene ratio, the composition and amount of the dispersed phase, and the concentration of surfactant. Significantly for future metamaterials applications, it was found that changing the chemical composition of the polyMIPE results in porous cross-linked silicones that have predictable and tunable emulsion morphology and mechanical properties. Finally, the acoustic properties of the porous materials have been tested with respect to the modulation of sound velocity through the PDMS polyMIPEs.

Experimental

Materials

The polymers [13-17% (mercaptopropyl) methylsiloxane]-dimethylsiloxane copolymer (thiolated-PDMS), vinyl terminated polydimethylsiloxane (vinyl-PDMS), and (30-35% dodecylmethylsiloxane-[7-10% hydroxy(propethyleneoxy (6-9) propyl) methylsiloxane] -(55-65% dimethylsiloxane) terpolymer (Silube J208-812) were purchased from Gelest (Morrisville, PA, USA) and used as received. The photoinitiator 2,2-Dimethoxy-2-phenylacetophenone (DMPA) and reagent grade dichloromethane (DCM) were purchased from Sigma-Aldrich (St.

Louis, MO, USA) and used as received.

Methods

Emulsion images were recorded using an Axio Vert.A1 inverted microscope (ZEISS), using ZEISS Efficient Navigation (ZEN) software. Samples were illuminated using bright field microscopy. Each emulsion was placed between a glass microscope slide and glass coverslip. The coverslip was gently pressed on by hand to limit vertical droplet-droplet stacking (Figure S1). The samples were observed before and after applying force and no difference in droplet shape or size was seen. Rheological analysis was performed using oscillatory frequency sweeps (0.1-100 Hz; 24 °C) with a Discovery Series Hybrid Rheometer (DHR) (Model HR-2, TA Instruments) using 20 mm diameter parallel plates and controlled temperature using an advanced Peltier system. A sample size of ~ 0.2 mL of each emulsion was used. Emulsions for rheological analysis were prepared without photoinitiator for ease of handling. PolyMIPEs were prepared using UV irradiation of the emulsions by pouring the emulsion into a well of a 6-well tissue culture polystyrene plate to a height of ~4mm. The sample was then irradiated (λmax =365 nm, 48 W, 6 min) from all sides in a mirrored enclosure. Total porosity measurements were obtained from dried polyMIPE samples using a home-built Archimedes balance. Pore morphology was obtained using a Scanning Electron Microscope (Low-Vac) (FEI XL-30) equipped with EDAX detector. Cross sections of the materials were cut from dried polyMIPEs and fixed onto aluminum stubs, sputter coated with gold/palladium, and imaged at an accelerating voltage of 15 kV. Mechanical properties of dried polyMIPEs were obtained using a Perkin Elmer Dynamic Mechanical Analyzer (DMA-8000) and processed using Pyris software. Sections of dried polyMIPEs for analysis were cut to ~ 3 mm thick, ~ 5 mm wide, and ~ 8 mm long. Rectangular tension frequency sweep experiments (0.1-70 Hz; 0.01 mm strain) were run on three separate samples for each polyMIPE formulation.

Ultra small-angle X-ray scattering (USAXS) studies were carried out at beamline 9 ID-C at the Advanced Photon Source, at the Argonne National Laboratory. [START_REF] Ilavsky | Development of Combined Microstructure and Structure Characterization Facility for in situ and operando Studies at the Advanced Photon Source[END_REF][START_REF] Ilavsky | Ultra-smallangle X-ray scattering instrument and the advanced photon source: History, recent development, and current status[END_REF] X-ray scattering data sets were reduced, de-smeared to account for the slit smearing and analyzed using Indra and Irena packages in IGOR pro. [START_REF] Ilavsky | Tool Suite for Modeling and Analysis of Small-Angle Scattering[END_REF] This type of USAXS analysis relies on considering the porous silicone matrix as an ideal two-phase system. [START_REF] Hu | Characterization of porous materials using combined small-angle X-ray and neutron scattering techniques[END_REF] The scattering contrast between the two phases (silicone matrix and pores) arise due to a difference in scattering length density of air and the silicone matrix. The average structural details of the pores were estimated by fitting the reduced scattering intensity, I(q) measured in cm -1 as a function of reciprocal space vector, q, measured in Å -1 to Porod's law [START_REF] Porod | Die Röntgenkleinwinkelstreuung Von Dichtgepackten Kolloiden Systemen[END_REF][START_REF] Debye | Scattering by an Inhomogeneous Solid. II. The Correlation Function and Its Application[END_REF] given in the following equation,

𝐼(𝑞) = 2𝜋∆𝜌 2 ( 𝑆 𝑉 ⁄ )𝑞 -4 (1).
where, ∆𝜌 2 (cm -4 ) is the squared difference of the scattering length densities of the silicone matrix and air and ( 𝑆 𝑉 ⁄ ) is the surface area to volume ratio of the pores. The scattering length densities of the two components were determined from the scattering contrast calculator available via the Nika package in IGOR pro. [START_REF] Ilavsky | Software for two-dimensional data reduction[END_REF] The term ( 𝑆 𝑉 ⁄ ) expressed in cm -1 can then be used to obtain the specific surface area expressed in m 2 /g from the measured densities of the porous materials. For spherical entities such as pores, the Sauter mean diameter is used to represent the pore diameter. [START_REF] Beaucage | Particle Size Distributions from Small-Angle Scattering Using Global Scattering Functions[END_REF] 𝑑 𝑝 = 6( 𝑆 𝑉 ⁄ )

-1

(2).

The acoustic characterization of samples was performed on the porous materials at ultrasonic frequencies. For each polyMIPE, two samples (32 mm in diameter) with different thicknesses d (1 and 2 mm) were used. Each sample was placed between two identical broadband ultrasonic (US) transducers (emitter and receiver, Olympus V301) with a diameter of 30 mm and a central frequency of 500 kHz. The US transducers were placed face to face and mounted on a linear manual stage, allowing the precise measurement of the sample thickness, i.e., the propagation distances d with an uncertainty of about 100 μm. The emitting transducer was excited with short (broadband) pulses generated by a pulser/receiver (Olympus, 5077PR) that was also used to amplify the electric signal recorded by the receiving transducer before its acquisition on a computer via an oscilloscope.

General MIPE preparation

Water-in-silicone inverse emulsions were prepared using a modified literature procedure. [START_REF] Kovalenko | Tailoring of the Porous Structure of Soft Emulsion-Templated Polymer Materials[END_REF] An emulsion containing equal thiol to alkene functional groups was prepared by adding thiolated-PDMS (2.5 g, 2.86 mmol thiol-functional groups) and vinyl-PDMS (8.57 g, 2.86 mmol alkenefunctional groups) in a glass vial and mixed on a vortex shaker. Silube (44 mg) (0.4 wt % compared to total weight of both PDMS components) was added and the mixture further vortexed to ensure a homogenous continuous phase. A dispersed phase consisting of either 1.5% wt/vol NaCl or CaCl2 solution in ultra-pure Mili-Q water was added to the vial containing the continuous phase to a volume of 40% and vortexed to form the emulsion. Each emulsion was characterized using optical microscopy and rheology.

General polyMIPE synthesis

PolyMIPEs were prepared using a modified literature procedure. [START_REF] Kovalenko | Tailoring of the Porous Structure of Soft Emulsion-Templated Polymer Materials[END_REF] The continuous phase was first prepared in an appropriately-sized glass vial. For a polyMIPE with a 1:1 (thiol to alkene) functional group, thiolated-PDMS (2.5 g, 2.86 mmol thiol-functional group) and vinyl-PDMS (8.57 g, 2.86 mmol alkene-functional group) were added and vortexed slightly to mix. In a separate glass vial DMPA (111 mg, 1.0 wt% with respect to weight of continuous phase) and Silube (44 mg, 0.4 wt% with respect to weight of continuous phase) were dissolved in approximately 0.3 mL of DCM.

This solution was added to the continuous phase and vortexed until homogenous. Nitrogen gas was bubbled through the reaction mixture to remove the DCM. The dispersed phase was then added as either a 1.5% wt/vol NaCl or CaCl2 solution in Mili-Q water. These two phases were vortexed until a viscous emulsion formed. The emulsion was poured into a 36 mm diameter well of a 6-well tissue-culture plate and irradiated with UV light (max = 365 nm) for 6 min and allowed to stand further for 5 min. The resulting polyMIPE was removed and weighed before being placed into the vacuum oven and dried for ~ 48 h at 24 C. The polyMIPE was weighed periodically to monitor water content loss. The final polyMIPEs were characterized using dynamic mechanical analysis and scanning electron microscopy.

Results and Discussion

We prepared polyMIPEs using thiol-ene click reactions within a water-in-silicone emulsion, where the continuous phase consisted of thiolated-PDMS and vinyl-terminated PDMS (Scheme 1). A commercially available surfactant, Silube, was used to stabilize the emulsions. We used the photoinitator 2,2-Dimethoxy-2-phenylacetophenone (DMPA) in UV light to initiate the thiol-ene reaction.

Scheme 1.

Crosslinking reaction between thiolated-PDMS and vinyl-PDMS.

We investigated four parameters to understand how the emulsion composition affected the properties of the resulting PDMS polyMIPEs. These parameters were the thiol to ene ratio, the type of salt solution used as the dispersed phase, the volume of dispersed phase added, and the concentration of surfactant used to stabilize the emulsion. The details of each emulsion formulations are given in Table 1. a The dispersed phase consisted of a 1.5 wt. % salt solution of either NaCl or CaCl2.

b The surfactant concentration was added as a weight percent with respect to the total weight of the continuous phase.

We first varied the stoichiometric ratio of thiol to ene functional groups while keeping the dispersed phase (40% by volume of a 1.5 wt. % sodium chloride solution and 0.4 wt. % concentration of surfactant with respect to the continuous phase) constant. This initial formulation was chosen based on previous work on PDMS-based polyMIPEs. [START_REF] Kovalenko | Tailoring of the Porous Structure of Soft Emulsion-Templated Polymer Materials[END_REF] The emulsions were characterized before polymerization using optical microscopy and rheology.

Optical microscopy images revealed a distribution of spherical droplets with the largest droplets being approximately 40-50 m in diameter. This is shown in Figure 1 for MIPE 1. Optical microscopy images were similar for all the formulations and can be found in the Supporting Information (Figure S1). The spherical droplets of aqueous dispersed phase packed together, with smaller droplets arranging themselves between larger droplets as shown in Figure 1a. These droplets could be isolated into smaller aggregates by gently pressing on the glass cover slide to better visualize the distribution of droplet sizes, as shown in Figure 1b When the emulsions were characterized using rheology there was little difference in either stress versus shear-rate (Figure 2a) or viscosity versus shear rate (Figure S3) plots upon changing the composition of the continuous phase (MIPEs 1-3, Table 1). This data suggests that changing the composition of the continuous phase with respect to the ratio of the two PDMS reagents does not alter properties of the final emulsion. In Figure 2 the data is an average of three separate emulsions for each formulation. 1.

We next explored if the type of salt dissolved in the aqueous phase had an effect on the emulsions.

The dissolved salt in the dispersed phase slightly increases the polarity of the solution, making it more insoluble with the oil phase and allowing for stable emulsions to be formed more readily. [START_REF] Akay | Microcellular PolyHIPE Polymer Supports Osteoblast Growth and Bone Formation in Vitro[END_REF][START_REF] Cameron | High Internal Phase Emulsions (HIPEs) -Structure, Properties and Use in Polymer Preparation[END_REF][START_REF] Wong | Hierarchical Polymerized High Internal Phase Emulsions Synthesized from Surfactant-Stabilized Emulsion Templates[END_REF] We repeated the formulations described in MIPEs 1-3 using a 1.5 wt. % calcium salt solution as the dispersed phase. In our case, the rheology behavior was similar for all formulations, as shown in Figure 2a. In Figure 2a the data for emulsions MIPE 1-6 are plotted to demonstrate that similar behavior is seen both within each series (i.e. 1-3 and 4-6) and between the series.

Therefore, there is no effect in the emulsion properties when using either NaCl or CaCl2 salt solutions.

Our initial emulsion formulations were based on reported syntheses of PDMS-based polyMIPEs for microfluidics applications that used a remarkably low level of surfactant. [START_REF] Kovalenko | Tailoring of the Porous Structure of Soft Emulsion-Templated Polymer Materials[END_REF] However, many reports of polyHIPE and polyMIPE syntheses use much higher amounts of surfactant, sometimes as high as 20 wt.% of the emulsion. [START_REF] Wong | Hierarchical Polymerized High Internal Phase Emulsions Synthesized from Surfactant-Stabilized Emulsion Templates[END_REF][START_REF] Cameron | The Influence of Porogen Type on the Porosity, Surface Area and Morphology of Poly(Divinylbenzene) PolyHIPE Foams[END_REF] Considering this, we used increasing concentrations of surfactant in our emulsions, specifically 0.4%, 1.0%, 3.0%, and 5.0% by weight with respect to the continuous phase, and characterized the resulting emulsions using rheology (Figure 2b). At these concentrations the stress versus shear-rate plot showed an inflection point in the data at low shear rates, implying a yield stress of ~1 Pa as the surfactant concentration in the MIPE increased.

experiments we maintained the stoichiometric ratio at 1:1 thiol to ene, while changing the sodium chloride solution dispersed phase from 40% by volume to 50%, 60%, and 70% by volume. The concentration of surfactant was held constant at 1.0% to ensure stable emulsions were formed.

Rheology data from these emulsions (Figure 2c) revealed a yield stress of ~2 Pa, and emulsions with the highest volume of dispersed phase possessed the highest yield stress. However, we were unable to form emulsions with higher than 70% volume of the dispersed phase using 1.0 wt. % surfactant with our current protocols.

We prepared polyMIPEs from the emulsions by adding a photoinitiator, DMPA, to the continuous phase to initiate thiol-ene reactions upon irradiating with UV light. All of the MIPE formulations produced porous monoliths after thiol-ene polymerization, and were characterized using SEM, total porosity, surface area analysis, and DMA. The SEM images of the polyMIPEs are presented in Figure 3. The formulations for each polyMIPE are provided in Table 1. The polyMIPEs appear to be similar with respect to pore size and interconnectivity of pores. The pore interconnectivity is particularly visible on the most porous samples. Interestingly, there appears to be sections of non-porous crosslinked PDMS present too, as highlighted by the arrow in Figure 3a and3b. The presence of non-porous PDMS is expected when considering the optical microscopy images of dilute emulsions where upon dilution with continuous phase, droplets maintained an aggregated state. Therefore, clusters of water droplets are formed before polymerization and appear as clustered porous sections in the polymerized materials after drying. This observation is again seen in Figure 3d-f where the dispersed phase salt is changed to a CaCl2 solution representing polyMIPEs 4-6. Similar to the rheology of the emulsions, these images do not show significant differences in polyMIPEs using NaCl or CaCl2 solutions dispersed phases. This gives further evidence that changing the salt in the dispersed phase does not impact the resulting polyMIPE. The polyMIPEs in Figure 3g-i were prepared from MIPEs 7-9. When the surfactant concentration was increased to 3% and 5% there appeared to be more interconnectivity between the pores with fewer regions of non-porous PDMS.

Figures 3j-l are polyMIPEs from MIPEs 10-12 where the volume of dispersed phase was increased to higher than 40%. These images show an increase in the number of pores in the polyMIPEs while maintaining the interconnected morphology. The areas of non-porous PDMS also appear to decrease on a qualitative analysis of the SEM images.

We calculated the surface area and average pore sizes using ultra-small angle X-ray scattering (USAXS) and total porosity calculations of the polyMIPEs were obtained using equation (3) where 𝜌 is the average density of the bulk PDMS (0.975 g/mL), 𝜌* is the density of individual polyMIPE samples, and Φ is total porosity. The results are also presented in Table 2.

-𝜌

* 𝜌 = Φ (3).
Using the Sauter mean size, dp, from USAXS for the pore diameter and the calculated porosity from equation ( 3), the specific surface area (SS) can be calculated with Equation 4,

𝑆 𝑆 = 6 Φ (1-Φ)𝜌𝑑 𝑝 ~ 10 -2 m 2 /g (4)
. 3PolyMIPEs 1-9 possess a porosity of approximately 40%, which corresponds to the volume of dispersed phase of 40% used to prepare these polyMIPEs. This similarity between the dispersed phase initial volume fraction and the final porosity indicates that no contraction of the materials occurred during drying. This lack of contraction in the polyMIPEs is due to interconnected pores, which allows for the removal of water throughout the materials upon drying. [START_REF] Kovalenko | Tailoring of the Porous Structure of Soft Emulsion-Templated Polymer Materials[END_REF][START_REF] Cameron | Study of the Formation of the Open-Cellular Morphology of Poly(Styrene/Divinylbenzene) PolyHIPE Materials by Cryo-SEM[END_REF] A systematic increase in total porosity was observed for polyMIPEs with increasing volume of dispersed phase up to 70%. The porosity of these PDMS polyMIPEs is unaffected by chemistry of the continuous phase, identity of the salt, or concentration of surfactant, and only by the volume of the dispersed phase.

The polyMIPEs were characterized using DMA to obtain the storage moduli (G'), and the results are shown in Figure 4. Tan delta () values were also obtained using DMA, and the values were consistently less than one (data not shown), which is representative of elastic materials. No variation in the storage moduli was observed over the frequency range used. The DMA results for polyMIPEs 2, 3, 5, and 6 are shown in Figure 4a. The storage moduli observed trend in the DMA results was comparable upon changing the stoichiometric ratio of the PDMS polymers in the continuous phase regardless if NaCl or CaCl2 was used in the dispersed phase (Figure 4a), suggesting that the salt used in the dispersed phase has no effect on the storage moduli of the final polyMIPEs. PolyMIPEs 1 and 4 were prepared using a 1:2 thiol to ene ratio and were the two materials with the lowest storage moduli (~ 90 and 35 kPa respectively), in fact, while the storage moduli of these polyMIPEs was consistently lower that the other materials prepared, these polyMIPEs did not produce materials with consistent behavior in the DMA analysis. Therefore, we have only reported results from the averages of the materials with higher thiol to ene ratios.

PolyMIPEs 2, 3, 5, and 6 with thiol to ene ratios of 1:1 or 2:1 resulted in G' values similar to each other (between ~200 and 320 kPa) and consistently higher than that for polyMIPEs 1 and 4. This is similar to literature reports of cross-linked silicone networks prepared using thiol-ene chemistry, where materials properties increased with higher ratios of thiol to alkene, from which it was concluded that excess thiol functional groups were necessary to overcome imperfections during formation of the network. [START_REF] Nguyen | Ultrafast Diffusion-Controlled Thiol-Ene Based Crosslinking of Silicone Elastomers with Tailored Mechanical Properties for Biomedical Applications[END_REF] In our work, we did not observe this exact trend. We found that our system had the highest storage moduli using 1:1 thiol to ene ratios. The DMA results in Figure 4a therefore show that the mechanical properties of the polyMIPEs can be controlled by changing the molar ratio of thiol-functionalized to ene-functionalized PDMS in a given formulation, while simultaneously maintaining the morphology and porosity of the material. The surfactant concentration had little effect on the storage moduli and only a slight decrease was observed for the highest surfactant concentration (Figure 4b). The surfactant was not purposefully removed after synthesis and therefore may act as a plasticizer reducing the modulus of the polyMIPE if it remains within the network. We did observe differences in G' with polyMIPEs using increasing amount of dispersed phase (PolyMIPEs 7, 10-12) due to the increasing porosity of the materials, as is expected theoretically. [START_REF] Peng | Controlling Morphology and Porosity of Porous Siloxane Membranes through Water Content of Precursor Microemulsion[END_REF][START_REF] Gibson | Cellular Solids Structure and Properties[END_REF] The DMA results from this series of polyMIPEs are shown in Figure 4c, PolyMIPEs prepared with 70% volume of dispersed phase (polyMIPE 12)

did not give consistent results when characterized by DMA (although the recorded storage moduli, was consistently lower than that from polyMIPEs 7, 10, and 11) and therefore the DMA results from this polyMIPE are omitted from plot 4c.

The acoustic properties of the polyMIPEs were characterized at ultrasonic frequencies to test the potential of these materials as acoustic metamaterials. Unfortunately, measurements could not be performed on the samples having the lowest shear moduli (polyMIPEs 1, 4 and 12) as they were too delicate to be manipulated between the transducers. An image of the instrumentation with a polyMIPE sample between transmitter and receiver is shown in Figure S4. The transmitted signals 22 through the samples were recorded as a function of time, as an example the data from polyMIPE 10 is presented in Figure 5. We deduced the sound speed in the sample from the difference of timeof-flight and found a value of the longitudinal phase velocity cL ≈ 40 (+/-20) m/s. This is a remarkably low value of the sound velocity and suggests that these polyMIPEs can be used as acoustic metamaterials. We found that the sound velocity does not vary as a function of the porosity or the shear storage (G') modulus of the other polyMIPEs within the accuracy of our experimental measurements, and values between 30 and 70 m/s were recorded. We performed calculations of the expected profiles of the velocity versus porosity using the Kuster and Toksöz 54 model to gain further insight into the experimental observations. It has been previously shown this model describes well samples with very low G0 values. [START_REF] Kovalenko | Tuning the Sound Speed in Macroporous Polymers with a Hard or Soft Matrix[END_REF] In the Kuster and Toksöz model, the acoustic longitudinal velocity is a function of only three parameters; the porosity Φ, the elastic bulk (K0) and shear storage moduli (G0) of the non-porous matrix. Therefore, we measured the shear storage moduli of non-porous materials prepared using the same PDMS formulations as the polyMIPEs. The storage moduli of the non-porous materials were found to range from 110 and 1170 kPa. The value of K0 is typically of 1 GPa for this type of matrix. In

Figure 1 .

 1 Figure 1. Optical microscopy images of MIPE 1. (a) As prepared emulsion. (b) Emulsion droplets

Figure 2 .

 2 Figure 2. (a) Stress versus shear rate of MIPEs 1-3 and MIPEs 4-6 comparing emulsions with

Figure 3 .

 3 Figure 3. SEM images of cross sections of dried polyMIPEs with varied components of the

  Figure 3a-c show polyMIPEs prepared from MIPEs 1-3. The polyMIPEs appear to be similar with

Figure 4 .

 4 Figure 4. (a) Storage modulus versus frequency plots of polyMIPEs 2, 3, 5, and 6 comparing

Figure 5 .

 5 Figure 5. Transmitted signals through polyMIPE 10 with a porosity of 49% for two different

Figure 6

 6 Figure 6 we plot the expected evolution of the velocity versus porosity profile for different values

Figure 6 .Figure 6 ,

 66 Figure 6. Evolution of the acoustic longitudinal velocity as a function of porosity for different

Table 1 .

 1 Formulations of the 12 MIPEs prepared in this study.

			Volume of
	MIPE	Thiol:Ene Ratio	Dispersed Phase	Surfactant Content b
			and Salt a
	1	1:2	40% (NaCl)	0.40%
	2	1:1	40% (NaCl)	0.40%
	3	2:1	40% (NaCl)	0.40%
	4	1:2	40% (CaCl2)	0.40%
	5	1:1	40% (CaCl2)	0.40%
	6	2:1	40% (CaCl2)	0.40%
	7	1:1	40% (NaCl)	1.00%
	8	1:1	40% (NaCl)	3.00%
	9	1:1	40% (NaCl)	5.00%
	10	1:1	50% (NaCl)	1.00%
	11	1:1	60% (NaCl)	1.00%
	12	1:1	70% (NaCl)	1.00%

Table 2 .

 2 Surface area and pore dimensions of the polyMIPEs.

	polyMIPE	Measured	Average Pore Size a	Total	Surface Area a
		Density (g/mL)	dp (m)	Porosity b	(m 2 /g)
				(+/-2%)
	1	0.6247	164	38%	0.0230
	2	0.6100	173	39%	0.0227
	3	0.6066	136	38%	0.0277
	4	0.6223	195	36%	0.0177
	5	0.6174	153	38%	0.0246
	6	0.6499	150	42%	0.0297
	7	0.6007	123	40%	0.0333
	8	0.5988	249	44%	0.0194
	9	0.6249	272	42%	0.0163
	10	0.5015	104	49%	0.0569
	11	0.4205	56	60%	0.1654
	12	0.3337	48	66%	0.2487

a Calculated from USAXS data b Calculated from Equation

Acknowledgements

TJM, PSH, and NA thank the National Science Foundation (DMR-1940518) for the resources to conduct this work, and thank Dr. Robert Lazenby, Hope Kumakli, and Prof. Ryan White for help with optical microscopy images.


Fabio N Güttler

Samuel Alleaume

Christina Corbane

Dino Ienco

J Nin

Pascal Poncelet

Maguelonne Teisseire

Exploring high repetitivity remote sensing time series for mapping and monitoring natural habitats -A new approach combining OBIA and k-partite graphs

Keywords: Natural habitats monitoring, remote sensing time series, OBIA, graph representation, SPOT4 Take-5

published or not. The documents may come

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INTRODUCTION

Enhancing the frequency of satellite acquisitions represents a key issue for Earth Observation community nowadays. The upcoming Sentinel-2 satellites, with a 5-day revisit time, will lead time series analysis of high resolution optical images into a new level of complexity. This kind of high revisit capacity will open new possibilities for several applications such as mapping and monitoring natural habitats. However, close temporal acquisition requires new methodologies for time series analyses as advertised by the scientific community [START_REF] Esa | Sentinel-2 Preparatory Symposium: session summaries[END_REF][START_REF] Blaschke | Geographic Object-Based Image Analysis -Towards a new paradigm[END_REF]. Recent reviews about the integration of remote sensing for conservation monitoring [START_REF] Nagendra | Remote sensing for conservation monitoring: Assessing protected areas, habitat extent, habitat condition, species diversity, and threats[END_REF] and Natura 2000 habitat monitoring [START_REF] Vanden Borre | Integrating remote sensing in Natura 2000 habitat monitoring: Prospects on the way forward[END_REF] pointed out remote sensing as a strong, but still underexploited, tool. In this paper we describe a new framework combining Object Based Image Analysis (OBIA) and k-partite graphs for studying spatiotemporal evolutions of natural and semi-natural habitats in a Mediterranean protected site. Our results were obtained from the analysis of a recent high repetitivity time series dataset provided by the SPOT4 Take-5 experience, carried out by the CNES during the first semester of 2013 with the purpose of simulating Sentinel-2 time series.

METHODS

Our study was performed within the Lower Aude Valley Natura 2000 site, located in south of France (43°14'22.85" N 3°11'52.24" E). Contiguous to the Mediterranean Sea, the site spreads over 4,842 ha and is mainly dominated (56.3 %) by Natural Habitat types of Community interest (NHCI). In total, 19 NHCI are part of the site, including 5 priority habitat types. The most widespread habitats are: Mediterranean salt meadows, Coastal lagoons and Mediterranean halophilous scrubs. From February to June 2013, the SPOT4 satellite acquired 29 images over the study site (one image every 5 days) in the context of the Take-5 experience. Only 8 images were completely cloudy free over the whole site. To avoid excessive irregular sampling we kept only the 5 dates presented in table 1. The images were provided at level-2A (ortho-rectified and corrected from atmospheric, environmental and slope effects) by the THEIA Land Data Centre. From our time series, we aimed to extract a set of k-partite graphs describing the evolution of the natural areas within the site. More in detail, each graph depicts a single area that evolves during the time. The general process we developed can be divided in 4 main steps as shown in figure 1.

After some preprocessing and data standardization (step 1) which includes spatial subset and spectral indices computation, we segment the images individually (step 2). Six raster layers are used for image segmentation, four of them correspond to the SPOT4 spectral bands and the other two are the Normalized Difference Vegetation Index (NDVI) [START_REF] Rouse | Monitoring vegetation systems in the Great Plains with ERTS[END_REF] and the Normalized Difference Water Index (NDWI) [START_REF] Gao | NDWI-A normalized difference water index for remote sensing of vegetation liquid water from space[END_REF]. Image segmentation was performed in eCognition Developer 8.8.1 (multiresolution segmentation algorithm) throughout a hierarchical rule-set resulting in a set of objects per timestamp (around 800-1000). The next main step (step 3) involves the selection of particular zones (Bounding Boxes) to monitor during the time. The set of Bounding Boxes is chosen among all the objects obtained over all the timestamps. We select the Bounding Boxes following three criteria: a) obtain a good coverage of all the study area, b) reduce the overlapping among Bounding Boxes and c) constrain the process to select big objects. The coverage requirement lies on the hypothesis that we would extract as much information as possible from the time series images and for this reason we will cover the entire study area. The second requirement helps the process to reduce redundancy and avoid following the same zone more than once. The last criterion specifies the following hypothesis: big objects are assumed to delimitate the maximum extent of a particular phenomenon. For this reason we need to detect such particular spatial configuration in order to follow a phenomenon throughout time.

Once the set of Bounding Boxes is filtered out, an evolution graph is created for each of them (step 4). Given a Bounding Box BB, we project BB over each timestamp of the time series and we select the objects overlapping with BB. The graph is built linking the objects of timestamp i with the objects of timestamp i+1. The weight of the link represents the degree of overlap between two objects. In this way we obtain graphs that have as many layers as the number of images in the time series. Another intrinsic characteristic of an evolution graph is that for a certain layer it will contain only one object (corresponding to the Bounding Box that constrains the graph spatial extent). Logically objects belonging to the same timestamp are not connected; this is the same for objects not belonging to two successive timestamps.

RESULTS

In total, 551 evolution graphs were generated for the entire Lower Aude Valley site. Each of the resulting graphs synthesizes the temporal behaviour of a specific area which can be guided by particular changes and evolutions, like phenological cycles or environmental events (i.e. flooding, fire, etc.). For the natural part of the site (non-agricultural), each graph represents a complex of natural habitats. Figure 2 presents two examples of evolution graphs obtained over areas characterized by distinct natural habitats and environmental factors.

The first example (A) presents a quite simple evolution graph as the boundaries of the objects remain very similar from one time stamp to the next one. This area is dominated mainly by halo-psammophile meadows, coastal-saltmarsh grass swards and Mediterranean saltmarsh scrubs. From T 0 up to T 2 the graph contains only a single object per layer, afterwards two small peripheral objects appear. By analyzing the objects spectral content (bands and vegetation indices), we notice gradual and smoothed changes (figure 3).

The NDVI in the end of February (T 0 ) is around 0.33 and grows gradually up to 0.45 (without considering the peripheral objects). This increase, from the winter up to end of the spring, corresponds to the expected changes with regard to the phenology of those habitats. The peripheral objects pointed to some interesting local variations. The first one appears at T 3 and corresponds to the area closer to the coastal dunes. The vegetation is sparser and the soil response contributes to keeping the values NDVI lower than 0.35. The second peripheral object appears in the last timestamp and is contiguous to a small coastal pond. The wetness of this area should contribute to a better vegetation development as the NDVI attains 0.52. NDWI values, which informs about vegetation liquid water content, are low (from 0.0 up to 0.18). Nevertheless, the temporal behavior is similar to those described for the NDVI.

The graph structure of the second example (B) is much more complex and encloses several objects in most of the layers. We can observe some drastic radiometric changes throughout the time series. Graph B represents the evolution of a temporary pond. Flooded during the winter (T 0 ) and the beginning of spring (T 1 ), the pond dries up progressively starting from T 2 . The water is gradually replaced by some pioneer terrestrial vegetation, i.e. Salicornia patula and annual Suaeda sp. Habitat spatial differentiation will depend mainly on the local variations of soil salinity and submersion time. Southern and Southeastern parts of the former pond will present more exuberant vegetation and denser covering of the soil. Such developments can be observed since T 2 and subsist up to last timestamp (objects presenting the higher NDVI values on figure 3 -graph B). Flooded areas as well as non-vegetated surfaces can follow during time and are also easily detectable in the NDVI profiles (negative and near zero values). NDWI evolutions are also informative to follow the annuals colonization or the emergence of bare soils but should not be used detect flooding events.

CONCLUSIONS

The approach described here allows connecting objects all along the time series even if they were originally obtained from independent segmentations. The combination of OBIA and k-partite graphs offers a new tool to analyze in-deep the spatiotemporal evolutions of a particular area. Natural and semi-natural habitats are characterized by phenological cycles and are sometimes associated to seasonal controlling factors, like flooding in our case. High repetitivity satellite time series provide relevant information to characterize such evolutions but the manual analysis of is a very hard task even for a skilled image analyst or an ecology field expert. In a more automatic way, the graphs generated by our approach synthesize complex spatiotemporal evolutions and can be useful for this kind of time-consuming task. Actually, graph structure associated to spectral content of the objects provides complementary and spatially pertinent information allowing prompt analysis of dense time series. As future improvements we plan to develop ranking and clustering algorithms for the evolution graphs. This higher level information should help the expert to post-process the results as the graphs will be already organized in groups with similar temporal behavior. Also, ranking methods can be useful to highlight the most dynamic areas and the timestamps when the most important changes arise. In our study case, only the most frequent and ecologically coherent associations (complex of habitats) were analyzed as the spatial resolution of SPOT-4 images (20m) is not enough to individualize most of the Lower Aude Valley natural habitats. Another data limitation is related the time length of the Take-5 experience, which concerned only the first semester of 2013 and avoided the observation of complete phenological cycles. To a certain extent, the upcoming Sentinel-2 time series should overcome these difficulties (spatial resolution of 10 m for visible-NIR bands and continuous acquisition expected for about 7 years). Lastly, the combination of punctual very high spatial resolution imagery (i.e. less than 2 m) should be considered as a potential option to improve the delimitation of natural habitats.

Figure 1 .

 1 Figure 1. Main steps of the time series processing for evolution graphs extraction.

Figure 2 .

 2 Figure 2. Examples of evolution graphs for (A) coastal meadows and scrubs and (B) temporary pond.

Figure 3 .

 3 Figure 3. NDVI temporal profiles of the evolution graphs A and B (showed in fig.2). Dots correspond to the mean NDVI value calculated for each object of the evolution graphs.

Table 1 .

 1 Acquisition date of the five selected SPOT4 Take-5 images over the Lower Aude Valley Natura 2000 site.

	Timestamp	Acquisition date
	T 0	27 February 2013
	T 1	03 April 2013
	T 2	18 April 2013
	T 3	13 May 2013
	T 4	02 June 213

ACKNOWLEDGEMENTS

The authors of this work would like to acknowledge the financial support of the French State managed by the National Research Agency in the framework of the program "Investissements d'Avenir" through the support of the GEOSUD project (ANR-10-EQPX-20).


Johnny Hu

Dr Jas S Ward

Dr Alain Chaumont

Kari Rissanen

Jean-Marc Vincent
email: jean-marc.vincent@u-bordeaux.fr

Valerie Heitz
email: v.heitz@unistra.fr

Henri-Pierre Jacquot De Rouville

Jean-Marc Vincent

Valérie Heitz

Prof ] J V Hu

Dr H.-P Heitz

Rouville Jacquot De

Lsamm

Prof K Rissanen

Dr J.-M Vincent

A Bis-Acridinium Macrocycle as Multi-Responsive Receptor and Selective Phase Transfer Agent of Perylene

published or not. The documents may come

Introduction

The past decades have seen the emergence of numerous covalent artificial receptors. [1] Supramolecular chemistry and selfassembly have also allowed the formation of sophisticated receptors incorporating several recognition units. [2] These receptors have gained in complexity with the integration of switching units allowing a control of the guest-binding properties. [3] However, multi-state systems responsive to different type of stimuli are still scarce in the literature and are based on the combination of molecular switches in either the host or the guests. Such systems were recently reported to control the association/dissociation process in host-guest complexes [4] and the motion in mechanically interlocked molecules. [5] In this context, 9-aryl-acridinium moieties are emerging building blocks [6] for the development of multi-responsive receptors of Polycyclic Aromatic Hydrocarbons (PAHs). Indeed, these building blocks are single components responsive to different stimuli (electron, light and nucleophile) thus avoiding the combination of several switches. Upon redox [7] or acid/nucleophile [8] stimulations, the acridinium fragments experience reversible and profound changes of their electronic properties and/or shape, two key parameters strongly affecting recognition processes (Scheme 1). Among the few studies reporting acridinium based switchable supramolecular systems, a family of [2]rotaxanes incorporating two acridinium stoppers was described by Abraham and coworkers. [9] The photochromic properties of the acridinium stoppers were used to trigger the shuttling of a tetracationic macrocycle. In a seminal work, Yoshizawa and coworkers recently reported the synthesis of the tetrakisacridinium receptor interacting with long hydrophilic molecules (e.g. coumarin and steroid derivatives). [10] Upon addition of nucleophiles, the formation of the corresponding tetrakisacridane macrocycle triggers the decomplexation of the guest molecules as the result of the modulation of the cavity size of the receptor. Scheme 1. Two possible mesomeric forms of the 9-aryl-N-methyl acridinium subunit highlighting the electronic and 2D to 3D structural changes occurring upon addition of redox and pH stimuli.

An additional interest of acridinium moieties resides in their ionic nature allowing their straightforward confinement in aqueous and organic phases according to the associated counterions. [10,11] This is of primary importance for applications in molecular separation/purification processes which mostly exploit aqueous/organic solvents biphasic systems. [12] Perfluorocarbons (PFCs) are the least polar liquids making them non-miscible at room temperature and ambient pressure with both water and organic solvents, hydrocarbons included. [13] In reason of their extremely poor solvation ability, PFCs are appealing phases for receptors confinement. [14] They do not compete with the hostguest interactions, thus increasing their strength, [15] but also potentially limit the number and amount of competing guests or hosts in the recognition phase. These unique properties have been exploited to develop potentiometric ion sensing assays exhibiting unequalled sensitivity and selectivity ranges, [15a-b, 16] as well as a few colorimetric/fluorogenic assays. [17] Rather surprisingly, application of PFCs and fluorous supramolecular receptors to selective separation/transportation processes have been much less explored. [18] It might be ascribed to the difficulty of getting receptors with enough fluorophilicity to impart high partitioning in the fluorous phase.

Herein, the four-step synthesis of the bis-acridinium cyclophane (1 2+ ) and its recognition properties towards PAHs is reported. The chemical and redox-controlled encapsulation/release properties of the cyclophane towards perylene (Per) is demonstrated. This dicationic host also showed its ability to switch between CH2Cl2 and perfluorocarbon phases using fluorophilic supramolecular anions. In addition, the confinement of the receptor in a fluorous phase was exploited to the selective extraction of Per from a mixture of PAHs.

Results and Discussion

Design and Synthesis. The multi-switchable bis-acridinium cyclophane 1•2PF6 was designed to target PAH guest molecules that are ubiquitous environmental pollutants, most of them being carcinogenic. [19] It is known that acridinium moieties can act as recognition units in -donor/-acceptor interactions with electron rich guests. [20] The targeted receptor 1•2PF6 was prepared in only four steps from commercially available 3,5-dibromoanisole (Scheme 2). First, 3,5-dibromoanisole (1 eq.) was borylated under Miyaura conditions using bis(pinacolato)diboron (2 eq.), Pd(dppf)Cl2 (10 mol%) and KOAc (6 eq.) in DMF. After purification, the corresponding bis(pinacolato)anisole 2 was isolated in 78% yield. Compound 2 was reacted with 2,6-dibromopyridine (4 eq.) under Suzuki conditions using Pd(PPh3)4 (20 mol%) and K2CO3 (2 N) in a toluene/ethanol/water mixture. After column chromatography, the semi-rigid spacer 3 was obtained in 60% yield. The dibromo derivative 3 underwent a bromo-lithium exchange using a solution of nBuLi (2 eq.) in hexanes (2.5 mol L -1 ) followed by reaction with dec-1-enyl-9(10H)-acridone (2 eq). [START_REF]-enyl-9(10H)-acridone was synthesized from commercially available materials, namely 9(10H)-acridone and 10-bromodecene, under basic conditions using NaH[END_REF] Aromatization of the bis-acridane intermediate by acidification of the reaction mixture using HCl (37 wt.%) led to the bis-acridinium tweezer 4•2Cl, then converted to the corresponding hexafluorophosphate salt (4•2PF6) by anion metathesis using KPF6. Tweezer 4•2PF6 was isolated as a yellowish solid in 33% yield. The key macrocyclization step was achieved in almost quantitative yield under olefin metathesis conditions using first-generation Grubbs catalyst (10 mol%) and Per (3 eq.) as template. [START_REF]formation of the receptor. Attempts using shorter hexenyl and octenyl chains led to the formation of a mixture of products, namely the desired macrocycle and a cyclic side product with a methylene missing[END_REF] Without template, the macrocyclization step did not proceed showing that Per effectively preorganizes both acridinium units in a face to face conformation necessary for the ring-closure step. [START_REF]During purification of 1•2PF6[END_REF] Scheme 2. Synthesis of the cyclophane 1•2PF6 from 3,5-dibromoanisole as commercially available material.

The receptor 1•2PF6 and all precursors were characterized by NMR, UV-vis spectroscopies and MS spectrometry (see SI). The 1 H NMR spectrum of 1•2PF6 (CD3CN, 298K) revealed characteristic triplets at 5.47 and 5.44 ppm ( 3 Jgem = 1.0 Hz) corresponding to the olefin protons of both expected diastereomers of the cyclophane, namely trans and cis in a 9:1 ratio (Fig. 2a). Crystals of the inclusion complex C6H6 ⊂ 1•2PF6 were grown by vapor diffusion of C6H6 into an CH3CN solution of 1•2PF6, and its structure was solved by X-ray diffraction analysis (Fig. 1b-c). [24] The preorganization of both acridinium moieties was clearly evidenced by the optimum distance for -stacking interactions (dC9-C9 = 7.198 Å and dN-N = 7.676 Å). Overall, the cavity size (7.2  15.6 Å) is well-adapted to PAHs such as Per. Recognition Properties. NMR and UV-vis studies were employed to probe the recognition properties of 1•2PF6 toward PAHs, namely Per, anthracene (Ant) and naphthalene (Naph). The 1 H NMR (CD3CN, 298 K) of the 1:1 mixture (c = 1.5  10 -3 mol L -1 ) of 1•2PF6 and Per revealed a chemical shift of all protons in comparison to the individual host and guest (Fig. 2). More especially, upfield shifts of the acridinium protons ((H1/8) = -0.42, (H3/6) = -0.41, and (H4/5) = -0.57 ppm) of 1•2PF6 for the trans isomer as well as of the Per protons ((H1/6/7/12) = -0.58, (H2/5/8/11) = -0.52 and (H3/4/9/10) = -0.44 ppm) suggested - interactions between the guest and the acridinium subunits. The inclusion complex Per ⊂ 1•2PF6 was confirmed by the downfield shifts of the inner proton Ho' of the 1,3-dipyridylanisyl spacer ( (Ho') = 0.42 ppm) and the olefin protons ((Htrans) = 0.22 and (Hcis) = 0.08 ppm). In addition, the existence of the inclusion complex was confirmed by control experiments using the 9pyridyl-N-acridinium moiety as host (see SI, Figure S4.1). Indeed, the 1 H NMR spectrum (CD3CN, 298 K) of the 1:1 mixture of 9pyridyl-N-acridinium and Per showed no chemical shifts of any proton signals suggesting a macrocyclic effect and the cooperative assistance between both acridinium moieties in 1•2PF6. Of particular interest, complex formation gave rise to a new electronic transition in the visible region (max = 606 nm; 606 ~ 670 L mol -1 cm -1 ) [START_REF]The molar extinction coefficient at 606 nm (606) was estimated from the determination of the binding constant (Ka) (Fig S4.3) showing 76% complex formation at the end of the titration[END_REF] attributed to a charge transfer band from Per to the acridinium units of the receptor, responsible for the color change from yellow to green (Fig. 23). This optical output was only observed for the complexation of Per and account of its higher -donor character in comparison to Ant and Naph. The poor solubility of Per did not allow NMR or UV-Vis titration experiments in CH3CN. In order to get higher solubility of both 1 2+ and Per in a common solvent, namely CH2Cl2, 1•2PF6 was converted to the corresponding tetrakis [3,5bis(trifluoromethyl)phenyl]borate (BArF) salt, 1•2BArF (see SI). Monitored by UV-Vis spectroscopy, titration experiments in CH2Cl2 allowed the estimation of an association constant of 1200 ± 38 L mol -1 between Per and 1•2BArF (fitted to a 1:1 binding model, see SI, Fig. S4.3). [START_REF]The 1:1 stoichiometry of the host-guest complex was determined by the minimum standard deviation after fitting to a 1:1, 2:1 and 1:2 binding model. Indeed, the interpretation of Job Plot titrations could be sometimes misleading in supramolecular chemistry[END_REF] Remarkably, this binding constant is 18 and 400 times higher than the binding constant determined between 1•2BArF and Ant (Ka = 66 ± 3 L mol -1 , see SI Fig. S4. 5-6) and Naph (Ka = 3 ± 0.08 L mol -1 , see SI Fig. S4. 7-4.8) respectively. [START_REF]Estimation of the binding constants between Ant and Naph were determined by 1 H NMR titration experiments in CD2Cl2[END_REF] Classical molecular dynamics simulation shed light on the dynamic behavior of the Per ⊂ 1 2+ complex in CH2Cl2. In the course of the calculated trajectory (10 µs), Per exchanges eleven times between the center of the cavity of host 1 2+ and bulk CH2Cl2 (see SI, Fig. S8.1-2) with an average residence time of about 650 ns (from 23 ns to 2151 ns). The calculated free energy difference of -26.8 kJ mol -1 between the associated and dissociated complex is in good agreement with the experimental value of -17.6 kJ mol -1 .

Multi-Switching Properties. To demonstrate the chemical switching from the bis-acridinium macrocycle, the macrocyclic bis-acridane 5 was prepared in quantitative yield by addition of a solution of KOH (1 mol L -1 ) to a solution of 1•2PF6 in CH3CN (see structure in Fig. 3). [START_REF]As expected, compound 5 was obtained as a mixture of two isomers, namely trans and cis[END_REF] Compound 5 exhibited no binding ability for Per in CD2Cl2 as demonstrated by 1 H NMR spectroscopy (see SI, Fig. S5.3-S5.4). This behavior can be rationalized by the drastic geometrical change of the acridane units and by the loss of aromaticity of the recognition units in 5 in comparison to its bisacridinium precursor 1•2PF6 (see SI). [START_REF]The dearomatization of the acridinium units in 1•2PF6 was observed by 1 H NMR experiments (400 MHz, DMSO-d6, 298 K) with the upfield shifts of the acridane protons (H1/8 = -0.80 ppm[END_REF] The reversible conversion between 5 and the receptor 1•2PF6 upon addition of protons (trifluoroacetic acid, TFA) or hydroxides (KOH) was evidenced by 1 H NMR and UV-vis studies (see SI, Fig. S5.1-2 and S5.6-7).

The chemical switching of macrocycle 5 to 1•2PF6 to efficiently release/catch Per was then probed by UV-vis spectroscopy following the variation in intensity of the Per to acridinium chargetransfer band (Fig. 3). Quantitative restoration of the Per ⊂ 1 2+ complex was achieved upon addition of 4.8 eq. of TFA. In other words, the dihydroxylated macrocycle 5 behaves as a protonactivated latent receptor for Per. In addition, Per was reversibly released upon addition of 9 eq. of tetrabutylammonium hydroxide (TBAOH, Fig. 3

inset).

Figure 3. UV-Vis spectra (CH2Cl2, l = 0.1 cm, 298 K) of a mixture of 5 (c = 1.5 × 10 -3 mol L -1 ) with 5 eq. of Per upon addition of TFA (0.8, 1.6, 2.4, 3.2, 4.0 and 4.8 eq.); Insert: UV-Vis spectra (CH2Cl2, l = 0.1 cm, 298 K) of a mixture of (c = 1.5 × 10 -3 mol L -1 ) with 5 eq. of Per upon addition of TBAOH (1.5, 3, 4.5, 6, 7.5 and 9 eq.).

The redox-switchable properties of 1•2PF6 were investigated by cyclic voltammetry experiments in C2H4Cl2 (Fig. 4). At 100 mV s - 1 , 1•2PF6 exhibits a reduction wave at a potential (Ec1) of -0.946 V vs Fc+/Fc and a reoxidation wave at -0.828 V vs Fc+/Fc (Ea1). This observation is in agreement with two quasi reversible oneelectron injection processes (Ep = 118 mV) leading to the formation of the diradical 1 0 at E1/2 = -0.887 V vs Fc+/Fc. [START_REF]Electrochemical experiments performed on 1•2PF6 in DMF showed a reversible behaviour (Ep = 66 mV, see SI, Fig. S6.8[END_REF] This observation is supported by the scan rate study performed from 50 to 800 mV s -1 (see SI, Fig. S6.1). Indeed, the plot of the cathodic and anodic intensity as a function of the square root of the scan rate (Ic and Ia = f( 1/2 )) shows a perfect linear relationship SI, Fig. S6.2 and S6.4). Noteworthy, the slight shift of the cathodic potential (Ec1) corroborates a process under diffusion control. In addition, spectro-electrochemical experiments were undertaken to provide evidences of the reduction of both acridinium units at the same potential. Upon reduction, the original spectrum of 1•2PF6 was converted to the spectrum of 1 0 (see SI, ) of a solution of 1•2PF6 (c = 1  10 -3 mol L -1 ) before (blue) and after addition of 10 eq. of Per (red) in the presence of TBAPF6 as supporting electrolyte (c = 0.1 mol L -1 ).

After addition of an excess of Per (10 eq.) corresponding to 74% complex formation, a cathodic shift of the reduction (Ec2 = -0.977 V vs Fc + /Fc) and re-oxidation processes (Ea2 = -0.875 V vs Fc + /Fc) were observed (E1/2 = -0.926 V vs Fc + /Fc). This observation corroborates the formation of the host-guest complex since the electron rich guest transfers some of its electronic density to the acridinium moieties making them more difficult to reduce. The half-wave potential difference (E1/2) was found to be -39 mV and allowed the estimation of a binding constant (Ka') of 54 L mol -1 between the reduced receptor 1 0 and Per. This 20 times lower association constant compared to the constant found between 1•2PF6 and Per (Ka = 1 200 ± 21 L mol -1 in C2H4Cl2 in the presence of 0.1 mol L -1 of TBAPF6; see SI, Fig. S4.9) clearly evidences the destabilization of the host-guest complex upon reduction. This binding constant corresponds to 34% complex formation between the 1 0 and Per showing that the host-guest association-dissociation can be controlled to a different extent using either a redox or a chemical stimulus. [START_REF]Interaction of Per with the reduced receptor 1 0 was also investigated by Classical Molecular Dynamics simulations (PMF and RDF calculations[END_REF] Phase transfer in perfluorocarbons. Interested in applying the receptor in selective transportation processes, perfluorcarbons (PFCs) were envisioned as potential liquid phases for the receptor confinement. [START_REF]Receptor 1 2+ proved to be insoluble in water as a chloride, sulfate and nitrate salt. Consequently, perfluorocarbons (PFCs) were considered as potential orthogonal phase to both water and organic solvents[END_REF] In 2011, some of us reported the transfer of a [Ru(bipy)3] 2+ dication from CH2Cl2 to a perfluorodecalin (PFD) phase by a markedly increase of the fluorine content coming from the associated counter-ions (Scheme 3). [START_REF] Corrêa Da Costa | [END_REF] These counter-ions are heteromeric supramolecular fluorous carboxylates-carboxylic acid anions easily formed in PFCs by an ionic H-bond interaction thus doubling the number of fluorous chains. [34] The back transfer from the PFD to CH2Cl2 was achieved by adding a tiny amount of a H-bond competitor, namely CH3OH, to the biphasic system thus disrupting the supramolecular anion. Scheme 3. Concept of fluorophilicity amplification applied to the phaseswitching of the [Ru(bipy)3] 2+ dication between CH2Cl2 and PFD phases. [START_REF]Interaction of Per with the reduced receptor 1 0 was also investigated by Classical Molecular Dynamics simulations (PMF and RDF calculations[END_REF] Based on these results, the easy confinement of 1 2+ in a fluorous phase was hypothesized using highly fluorophilic supramolecular anion (Fig. 5a-b). Accordingly, a stock solution of 1•2(RfCOO-H-OOCRf) (c = 0.55  10 -3 mol L -1 ) in perfluoromethylcyclohexane (PFMC, 6 mL) was conveniently prepared by stirring a solution of bis-acridane 5 (c = 1.88  10 -3 mol L -1 ) in CH2Cl2 (2 mL) with a solution of commercially available perfluoro-2, 5,8,11-tetramethyl-3,6,9,12-tetraoxopentadecanoic acid (RfCOOH, Rf = CF3CF2CF2O[(CF3)CFCF2O]3(CF3)CF) in PFMC (c = 7.5  10 -3 mol L -1 , 6 mL). The formation of 1•2(RfCOO-H-OOCRf) and its phase transfer were followed by UV-Vis spectroscopy and a transfer of 90% of 1•2(RfCOO-H-OOCRf) into the fluorous phase was determined (see SI, Fig. S7.1-2). [START_REF]Protonation of the pyridines of the receptor was not observed even in concentrated HCl and pure H2SO4[END_REF] The receptor 1•2(RfCOO-H-OOCRf) proved to be highly fluorophilic as revealed by a back extraction from the PFMC phase to a fresh CH2Cl2 solution of only 0.3 % (see SI, Fig. S7.3). Addition of a small amount of CH3OH (a volume of ~ 4% of CH3OH compared to that of PFMC) as a Hbond competitor led to the quantitative back transfer of the receptor from the PFMC to the CH2Cl2 phase, thus supporting the formation of supramolecular H-bonded anions in the fluorous phase. 5-6). In the fluorous phase, Per is almost exclusively present in the form of the inclusion complex. Under similar conditions but in the absence of 1•2(RfCOO-H-OOCRf), the Per concentration in the PFMC was as low as 1.3  10 -6 mol L -1 (see SI, section 7.2). From these data an association constant between Per and 1•2(RfCOO-H-OOCRf) of 4.66  10 5 L mol -1 was estimated in PFMC, c.a. 390 times higher than in CH2Cl2 (see SI, section 7.3). Release of the complexed Per was achieved by replacing the CH2Cl2 phase containing the excess Per by fresh CH2Cl2 (four times the volume of the PFMC phase). After manual shaking, fading of the green color of the fluorous phase was observed with concomitant appearance of the characteristic yellowish color of Per in the CH2Cl2 phase (see SI, Fig. S7.7-9). The recorded UV-Vis spectra of each phase confirmed the complete release of Per in CH2Cl2.

Finally, the ability of the fluorous system to selectively extract and release Per from a mixture of PAHs was evaluated (see SI, section 7.5). A biphasic system composed of a solution of 1•2(RfCOO-H-OOCRf) (c = 1.88  10 -3 mol L -1 ) in PFMC (1 mL) and an equimolar solution of Per, Ant and Naph (c = 2  10 -3 mol L -1 each) in CH2Cl2 (2 mL) was stirred for 60 min. After decantation, an aliquot of the greenish PFMC phase (0.5 mL) was transferred in a test tube and extracted with two aliquots of fresh CH2Cl2 (1 mL + 0.5 mL). Analysis of the combined CH2Cl2 phase by UV-vis spectroscopy and GC (see SI, Fig. S7.11-S7.13) revealed a Per concentration (c ~ 114  10 -6 mol L -1 ) far superior than Ant (c = 6.1  10 -6 mol L -1 ) and Naph (c = 9.4  10 -6 mol L - 1 ). These concentrations correspond to concentrations in the PFMC phase before release of ~ 342  10 -6 mol L -1 , 18.3  10 -6 mol L -1 and ~ 28.2  10 -6 mol L -1 for Per, Ant and Naph, respectively. Under similar conditions without 1•2(RfCOO-H-OOCRf), the extraction of Per, Ant and Naph in the PFMC phase led to concentrations of ~ 1.1  10 -6 mol L -1 , 5.1  10 -6 mol L -1 and ~ 27.1  10 -6 mol L -1 , respectively. This observation confirms that the amount of Naph recovered in the CH2Cl2 phase with and without 1•2(RfCOO-H-OOCRf) is essentially due to its innate partitioning. In addition, the partition coefficient (K = cPFMC/cCH2Cl2) of Naph between CH2Cl2 and PFMC is higher than that of Ant and much higher than that of Per. The recyclability of the fluorous phase was also assessed by conducting three consecutive extraction/release experiments from a CH2Cl2 phase containing a 1/1/1 molar ratio of Per, Ant and Naph (see SI for detailed procedure, Fig. S7. 19-22, Tables S7.4-5). Combined UV-Vis spectroscopy and GC analysis showed that the fluorous phase could be reused two times with the same efficiency, both in terms of yield of extracted/released Per (17.1%, 16.5% and 17.0% for the first, second and third extraction/release processes, respectively) and selectivity (Per/Ant/Naph molar ratios of 9.5/1/1.05, 8.7/1/1.1, 10.6/1/1.06 for the first, second and third extraction/release processes, respectively). Overall, these experiments demonstrate the ability of fluorous solvents combined with supramolecular receptor for selective transportation processes.

Conclusion

The efficient synthesis of a multi-responsive cyclophane incorporating two acridinium moieties as recognition units was described. Its ability to form host-guest complexes with perylene on account of -donor/-acceptor interactions was shown in solution. Upon addition of hydroxide anions, the chemical switching properties of this system lead to the formation of the corresponding bis-acridane derivative. This derivative is unable to interact with perylene thus demonstrating the ability of the cyclophane to work as an ON/OFF switch. The electrochemical switching properties of the bis-acridinium cyclophane clearly evidenced a decrease of the host-guest interactions upon reduction. Finally, the phase switching behavior of the macrocycle between a perfluorocarbon and an organic layer was explored and successfully applied to the straightforward enrichment of perylene from a mixture of polycyclic aromatic hydrocarbons. The multifunctional properties associated to the cyclophane receptor integrating multi-responsive acridiniums units are promising and will be further explored in the context of selective transportation processes and functional materials.

Figure 1 .

 1 Figure 1. a) Scheme of the receptor 1•2PF6, b) Side view and c) Top view of the crystal structure of C6H6 ⊂ 1 2+ showing the inclusion of a benzene molecule (thermal ellipsoids at 50% probability level; PF6 -anions and a benzene have been omitted for clarity).

Figure 2 .

 2 Figure 2. 1 H NMR (500 MHz, CD3CN, 298 K, c = 1.5  10 -3 mol L -1 ) spectra of (a) 1•2PF6, (b) the 1:1 mixture of Per and 1•2PF6, and (c) Per. The photographs of the solution of 1‧2PF6 and the 1:1 mixture of Per and 1•2PF6 are shown.

  Fig S6.6) revealing four maxima at 357 (20200), 367 (24100), 492 (8400) and 531 nm (8350). Upon re-oxidation, the spectrum of 1•2PF6 was substantially restored thus demonstrating the clean chemical reversibility of this electrochemical process (see SI, Fig S6.7).

Figure 4 .

 4 Figure 4. Cyclic voltammograms (C2H4Cl2, WE: Pt, CE: Pt, RE: Ag, 100 mV s - 1

Figure 5 .

 5 Figure 5. a) Conversion of the bis-acridane 5 to the highly fluorophilic bisacridinium 1•2(RfCOO-H-OOCRf); b) Photograph of a biphasic system taken after stirring and of a stock solution of 1•2(RfCOO-H-OOCRf) in PFMC (c = 0.55  10 -3 mol L -1 , 6 mL, stir bar in the bottom of the tube) and fresh CH2Cl2 (3 mL); c) Photographs and schematic representation of the extraction/release of Per using 1•2(RfCOO-H-OOCRf); i) magnetic stirring (30 min, 1000 rpm, stir bar present in the bottom of the tube) of an aliquot of the PFMC stock solution (1 mL) and a solution of Per (c = 4  10 -3 mol L -1 ) in CH2Cl2 (1 mL); ii) an aliquot (0.5 mL) of the PFMC solution containing Per ⊂ 1•2(RfCOO-H-OOCRf) was taken and fresh CH2Cl2 (2 mL) was added; iii) manual shaking for 60 s and decantation.

Acknowledgements

This work was supported by the CNRS, the University of Strasbourg, the University of Bordeaux and the University of Jyväskylä. We gratefully acknowledge the International Center for Frontier Research in Chemistry ic-FRC (www.icfrc.fr) for financial support. We are grateful to the Fédération de Chimie Le Bel FR 2010 of the University of Strasbourg. JH thanks the French Ministry of National Education for a PhD Fellowship. Prof. Jean-Pierre Launay is thanked for fruitful discussions. Damien Jardel is thanked for his help and discussions on GC experiments. Keywords: Multi-Responsive Receptor • Phase Transfer • Acridinium • Perfluorocarbons • Host-Guest Chemistry

Entry for the Table of Contents

A bis-acridinium cyclophane binds selectively and reversibly perylene upon chemical or redox stimuli in organic media. In addition, its straightforward phase-transfer into a perfluorocarbon was exploited for the enrichment of perylene from a mixture of PAHs.

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