Robbie Radjagobalou

Jean-François Blanco

Luca Petrizza

Mickael Le Béchec

Odile Dechy-Cabaret

Sylvie Lacombe-Lhoste

Maud Save

Karine Loubiere
email: karine.loubiere@toulouse-inp.fr.

Petrizza Luca

Mickael ‡+

Le Bechec

Sylvie Lacombe

Efficient Photooxygenation Process of Biosourced α-Terpinene by Combining Controlled LED-Driven Flow Photochemistry and Rose Bengal-Anchored Polymer Colloids

Keywords: Flow photochemistry, Singlet oxygen, Photoactive polymer colloids, Green conditions, Rose Bengal, Alpha-terpinene

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

INTRODUCTION.

Organic photochemistry is a key synthetic pathway for sustainable chemistry. [START_REF] Ciana | Clean and Easy Photochemistry[END_REF][START_REF] Hoffmann | Photochemical Reactions as Key Steps in Organic Synthesis[END_REF][START_REF] Bach | Photochemical Reactions as Key Steps in Natural Product Synthesis[END_REF] Multi-step syntheses of complex molecules are shortened and simplified, a portfolio of novel chemicals becomes accessible, and the photon acts as a "traceless reagent". Erreur ! Source du renvoi introuvable. [START_REF] Turro | Geometric and Topological Thinking in Organic Chemistry[END_REF][START_REF] Olivucci | Chemical Selectivity Through Control of Excited-State Dynamics[END_REF][START_REF] Schapiro | Using the Computer to Understand the Chemistry of Conical Intersections[END_REF][START_REF] Hoffmann | Photochemical Reactions of Aromatic Compounds and the Concept of the Photon as a Traceless Reagent[END_REF] Several fundamental principles of Green Chemistry are thus addressed by photochemistry. [START_REF] Anastas | Green Chemistry: Theory and Practice[END_REF] Among the photochemical processes, photooxygenation reactions are very attractive as they involve only singlet oxygen ( 1 O 2 ), a powerful selective oxidant, which is produced by safe photosensitization of oxygen in the visible range and at ambient temperature. [START_REF] Derosa | Photosensitized Singlet Oxygen and its Applications[END_REF] Singlet oxygen photosensitization corresponds to an energy transfer from catalytic amounts of an organic dye (photosensitizer) to molecular ground-state oxygen (triplet). [START_REF] Ghogare | Using Singlet Oxygen to Synthesize Natural Products and Drugs[END_REF] Photooxygenation reactions have proved to be very efficient for the preparation of endoperoxides from biosourced molecules such as terpene and furanone derivatives. [START_REF] Clennan | Advances in Singlet Oxygen Chemistry[END_REF][START_REF] Alberti | Stereochemistry of Singlet Oxygen Simple Alkenes: A Stereospecific Transformation[END_REF][START_REF] Iesce | 2+2] and [4+2[END_REF] Only a limited number of these transformations can be carried out thermally, as they require the use of complex oxidants containing either hazardous peroxides or hypervalent iodine compounds or metal catalysts. Thus, from an atom economy point of view, photooxygenations are of interest as their only requirements are catalytic amounts of a metal-free organic photosensitizer and molecular oxygen as an oxidant. It is also noteworthy that sensitized photooxygenations find applications not only in the synthesis of fine chemicals, [START_REF] Camussi | g-C 3 N 4 -Singlet Oxygen Made Easy for Organic Synthesis: Scope and Limitations[END_REF] but also in wastewater treatment or in the degradation of insecticides and pesticides. [START_REF] Díez-Mato | Phototransformation of Model Micropollutants in Water Samples by Photocatalytic Singlet Oxygen Production in Heterogeneous Medium[END_REF][START_REF] Manjon | On the Factors Influencing the Performance of Solar Reactors for Water Disinfection with Photosensitized Singlet Oxygen[END_REF][START_REF] Magaraggia | Treatment of Microbiologically Polluted Aquaculture Waters by a Novel Photochemical Technique of Potentially Low Environmental Impact[END_REF] The implementation of organic photooxygenations at industrial scale are limited to a few examples in the flavour, fragrance or pharmaceutical industries. [START_REF] Gollnick | Photooxygenation and its Application in Industry[END_REF][START_REF] Braun | Industrial Photochemistry. Handbook of Organic Photochemistry and Photobiology[END_REF][START_REF] Paddon | High-Level Semi-Synthetic Production of the Potent Antimalarial Artemisinin[END_REF][START_REF] Turconi | Semisynthetic Artemisinin, the Chemical Path to Industrial Production[END_REF] Batch reactors equipped with energy-demanding light sources are the main systems currently used for industrial photooxygenation set-ups. Non-eco-friendly solvents are generally used to maximize the singlet oxygen lifetime, and the photosensitizer (as solid powder) is regularly fed to minimize its degradation. Continuous-flow microstructured technologies, which involve channels or tubes from tens of micrometres to a few millimetres in diameter are now recognized as alternatives to batch processing. Their benefits for organic photochemistry have been highlighted recently. 22- 28Erreur ! Source du renvoi introuvable. The integration of LEDs (Light-Emitting Diodes) as light sources in these technologies provides new opportunities. They present high electrical yields, long lifetimes, and spatial compactness, with the possibility of selecting the emission spectral domain and modulating the emitted radiant power. [START_REF] Haas | Light as a Reaction Parameter -Systematic Wavelength Screening in Photochemical Synthesis[END_REF][START_REF] Sender | Light Sources for Photochemical Processes -Estimation of Technological Potentials[END_REF] Most of the literature related to LED-driven flow photochemistry considers a sensitizer solubilized in the reaction medium. [START_REF] Loponov | Optimization of a Scalable Photochemical Reactor for Reactions with Singlet Oxygen[END_REF][START_REF] Park | Continuous Flow Photooxygenation of Monoterpenes RSC Adv[END_REF][START_REF] Radjagobalou | Photooxygenation in an Advanced Led-Driven Flow Reactor Module: Experimental Investigations and Modelling[END_REF][START_REF] Horn | A method to determine the correct photocatalyst concentration for photooxidation reactions conducted in continuous flow reactors[END_REF][START_REF] Chaudhuri | Process intensification of a photochemical oxidation reaction using a Rotor-Stator Spinning Disk Reactor: A strategy for scale up[END_REF][START_REF] Lee | Scalable Continuous Vortex Reactor for Gram to Kilo Scale for UV and Visible Photochemistry[END_REF] In parallel, the concept of solid-supported sensitizers offers many advantages as it makes it easier to separate the organic sensitizer from the other reactants and products, avoiding expensive downstream separation. [START_REF] Lacombe | Solvent-Free Production of Singlet Oxygen at the Gas-Solid Interface: Visible Light Activated Organic-Inorganic Hybrid Microreactors Including New Cyanoaromatic Photosensitizers[END_REF][START_REF] Ronzani | Visible-Light Photosensitized Oxidation of Alpha-Terpinene using Novel Silica-Supported Sensitizers: Photooxygenation vs[END_REF][START_REF] Lapkin | Photo-Oxidation by Singlet Oxygen Generated on Nanoporous Silicon in a LED-Powered Reactor[END_REF][START_REF] Lacombe | Materials for Selective Photo-oxygenation vs. Photocatalysis: Preparation, Properties and Applications in Environmental and Health Fields[END_REF][START_REF] Mosinger | Fluorescent Polyurethane Nanofabrics: A Source of Singlet Oxygen and Oxygen Sensing[END_REF][START_REF] Zhang | Efficient Photooxygenation of Furans using Oxygen with Wool-Immobilizing Rose Bengal as Green Photosensitizer[END_REF][START_REF] Ronzani | Immobilized Organic Photosensitizers with Versatile Reactivity for various Visible-Light Applications[END_REF][START_REF] Dambruoso | TPPS Supported on Core-Shell PMMA Nanoparticles: the Development of Continuous-Flow Membrane-Mediated Electrocoagulation as a Photocatalyst Processing Method in Aqueous Media[END_REF][START_REF] Shen | Photoactive and metal-free polyamide-based polymers for water and wastewater treatment under visible light irradiation[END_REF][START_REF] Valverde | Rose Bengal Immobilized on Supported Ionic-Liquid-like Phases: An Efficient Photocatalyst for Batch and Flow Processes[END_REF] It is also a strategy for transposing poorly soluble photosensitizers into green solvents, while simultaneously enhancing their photostability and reusability. Solid-supported sensitizers in continuous-flow microstructured reactors were first implemented in the form of polymer/silica films cast onto the walls of the reactor. [START_REF] Carofiglio | Fullerene-Promoted Singlet-Oxygen Photochemical Oxygenations in Glass-Polymer Microstructured Reactors[END_REF][START_REF] Han | Immobilised Photosensitisers for Continuous Flow Reactions of Singlet Oxygen in Supercritical Carbon Dioxide[END_REF][START_REF] Lumley | Comparison of Photo-Oxidation Reactions in Batch and a New Photosensitizer-Immobilized Microfluidic Device[END_REF] However, the light that can be delivered per specific surface area of the reactor is significantly reduced in this configuration and the replacement of the substrate film is complex. Including porous monoliths or packed porous beads with finely tuned particle sizes in continuous-flow reactors has been reported as an alternative strategy. [START_REF] Amara | Applying Green Chemistry to the Photochemical Route to Artemisinin[END_REF][START_REF] Tobin | Polymer-Supported Photosensitizers for Oxidative Organic Transformations in Flow and under Visible Light Irradiation[END_REF][START_REF] Kong | High Throughput Photo-Oxidations in a Packed Bed Reactor System[END_REF][START_REF] Masuda | Robust Organic Photosensitizers Immobilized on a Vinylimidazolium Functionalized Support for Singlet Oxygen Generation under Continuous-Flow Conditions[END_REF][START_REF] Blanchard | Continuous Flow Photo-Oxidations Using Supported Photocatalysts on Silica[END_REF][START_REF] Thomson | Continuous-Flow Synthesis and Application of Polymer-Supported BODIPY Photosensitisers for the Generation of Singlet Oxygen; Process Optimised by In-Line NMR Spectroscopy[END_REF][START_REF] Tobin | BODIPY-Based Conjugated Microporous Polymers as Reusable Heterogeneous Photosensitisers in a Photochemical Flow Reactor[END_REF] Although it may be particularly suitable at lab-scale for material screening or reusability tests, transfer towards industrial production remains quite challenging: long reactors with narrow diameters are indispensable to avoid light limitations while reaching sufficient residence times for full conversions without resorting to a multi-pass mode. Such devices are also faced with issues related to high pressure drops and consumption of solvent. Consequently, the most straightforward suitable method in a perspective of a scale-up is to operate with a suspension of the small particles grafted with the sensitizer, and to transport this suspension by the flow from the inlet to the outlet of the microreactor. As a continuous flow of bubbles (air or pure oxygen) remains the most preferable option to supply oxygen to the reaction medium, this leads to operation under gas-liquid slurry flows (Figure 1). Such a concept has been successfully applied in microreactors for catalytic hydrogenation applications [START_REF] Enache | The Hydrogenation of Isophorone to Trimethyl Cyclohexanone Using the Downflow Single Capillary Reactor[END_REF][START_REF] Enache | Multiphase Hydrogenation of Resorcinol in Structured and Heat Exchange Reactor Systems: Influence of the Catalyst and the Reactor Configuration[END_REF][START_REF] Buisson | Slurry Hydrogenation in a Continuous Flow Reactor for Pharmaceutical Application[END_REF] but to date, only two studies have been reported for photooxygenation, and involved either conjugated microporous polymers (CMPs) [START_REF] Tobin | BODIPY-Based Conjugated Microporous Polymers as Reusable Heterogeneous Photosensitisers in a Photochemical Flow Reactor[END_REF] or functionalized mesoporous silica particles. [START_REF] Mendoza | Improving Continuous Flow Singlet Oxygen Photooxygenations with Functionalized Mesoporous Silica Nanoparticles[END_REF] The most relevant strategy for achieving good operability appears to be the implementation of submicronic materials covalently grafted to an efficient organic photosensitizer. Their smaller sizes mean that they are not prone to settle (in particular at the microreactor walls where the velocities are very low), unlike micrometric beads (such as the commercial Sensitox ® polystyrene particle). The challenge in the design of the polymer colloid materials as a photoactive heterogeneous substrate is to make an efficient contact possible between the grafted photosensitizer, oxygen, and the molecule to be oxidized. The average diffusion length of 1 O 2 is only tens of nanometres in a dense polymer, [START_REF] Ghogare | Using Singlet Oxygen to Synthesize Natural Products and Drugs[END_REF] while it is hundreds of nanometres, on average, in alcoholic media (the diffusion length being a combination of 1 O 2 lifetime in a surrounding medium and its diffusion rate). [START_REF] Ogilby | Singlet Oxygen: There is still something new under the Sun, and Ii is better than ever[END_REF][START_REF] Schweitzer | Physical Mechanisms of Generation and Deactivation of Singlet Oxygen[END_REF] As ethanol is one of the green solvents of reference for most photooxygenation of bio-based molecules, Rose Bengal photosensitizer-grafted polymer microgels exhibiting swelling ability in ethanol bring together most of the targeted relevant properties. [START_REF] Petrizza | Tuning Photosensitized Singlet Oxygen Production from Colloids Synthesized by Polymerization in Aqueous Dispersed Media[END_REF] These microgels were synthesized by an eco-friendly solvent-free polymerization process carried out in aqueous dispersed media. [START_REF] Petrizza | Tuning Photosensitized Singlet Oxygen Production from Colloids Synthesized by Polymerization in Aqueous Dispersed Media[END_REF] Rose Bengal (RB) photosensitizer offers interesting features as it is metalfree, non-toxic, commercially available, cheap, already used at industrial scale, and soluble in polar-protic solvents. It also exhibits intensive absorption bands in the visible range with a good singlet oxygen quantum yield in ethanol ( 2,ethanol = 0.75). [START_REF] Wilkinson | Quantum Yields for the Photosensitized Formation of the Lowest Electronically Excited Singlet State of Molecular Oxygen in Solution[END_REF][START_REF] Wilkinson | Rate Constants for the Decay and Reactions of the Lowest Electronically Excited Singlet State of Molecular Oxygen in Solution. An Expanded and Revised Compilation[END_REF] In the present work, the photoreactivity of poly(N-vinylcaprolactam-co-vinyl acetate-covinylbenzyl Rose Bengal) microgels (VBRB@MG) as heterogeneous photosensitizers will be investigated in a continuous-flow process for sustainable singlet oxygen sensitized photooxygenation of a bio-based molecule. Their efficiency will be investigated under accurate control of irradiation, light absorption and flow conditions. We will focus on the effects of both oxygen stoichiometry and medium absorbance on the photochemical kinetic law in such gasliquid slurry flows as it is crucial for further implementation at a larger scale. Here, the photooxygenation of -terpinene (Scheme 1) is carried out as a benchmark reaction in a dedicated LED-driven continuous microreactor (Figure 1). The performances, in terms of yield of photooxygenation, selectivity and photobleaching, of heterogeneous RB covalently anchored onto microgel are systematically compared to those of free soluble RB. Finally, the reusability of the photoactive colloids is assessed by monitoring the yield of -terpinene after several photooxygenation cycles.

EXPERIMENTAL SECTION

Photooxygenation reaction. Ascaridole was the main product of the photooxygenation of terpinene, with the minor product p-cymene (Scheme 1, top). [START_REF] Radjagobalou | Photooxygenation in an Advanced Led-Driven Flow Reactor Module: Experimental Investigations and Modelling[END_REF] Isoascaridole could be also formed from ascaridole by thermal isomerization (Scheme 1, bottom). [START_REF] Ronzani | Visible-Light Photosensitized Oxidation of Alpha-Terpinene using Novel Silica-Supported Sensitizers: Photooxygenation vs[END_REF] All the experimental details are described in Part S.1 of the Supporting Information.

Scheme 1. Photosensitized oxygenation of -terpinene (top). Thermally induced isomerization

of ascaridole to iso-ascaridole (bottom).

Photoactive polymer colloids. Photoactive colloids were synthetized by miniemulsion copolymerization of vinyl acetate (VAc), N-vinyl caprolactam (VCL), divinyl adipate (DVA) crosslinker and vinyl benzyl Rose Bengal (VBRB) monomer as previously reported. [START_REF] Petrizza | Tuning Photosensitized Singlet Oxygen Production from Colloids Synthesized by Polymerization in Aqueous Dispersed Media[END_REF] The continuous phase was a water phase and the polymerization took place in the monomer droplets created by ultrasonication. Two types of microgels were used in this work (see Table S.1):

VBRB@MG-8 and VBRB@MG-14 with respectively 8 mol-% or 14 mol-% of DVA crosslinker based on the overall amount of monomers and crosslinker. For both microgels, 0.70 mol % of VBRB was initially introduced. After their synthesis in water, the microgels were systematically redispersed in ethanol and immediately stored in the fridge at 4 °C. The so-called storage period corresponded to the duration between the synthesis of the microgels and the first use under continuous-flow photooxygenation conditions.

The UV-visible spectra of VBRB@MG-8 and VBRB@MG-14 in ethanol (Figure S.1) showed that the wavelength at which the absorption of grafted RB was maximal, noted  max (nm), was 569 nm and thus slightly longer than that of free RB in ethanol ( max = 557 nm), highlighting a bathochromic effect. TEM analysis revealed a quasi-spherical shape of colloids. [START_REF] Petrizza | Tuning Photosensitized Singlet Oxygen Production from Colloids Synthesized by Polymerization in Aqueous Dispersed Media[END_REF] The main colloidal features of microgel dispersed in water or ethanol are reported in Table S.1, together with values of singlet oxygen quantum yield of VBRB anchored in the microgels dispersed in ethanol and the photosensitizer loading. It should be highlighted that both microgels proved to be photoactive in ethanol phase as the values of singlet oxygen quantum yield, 2 (-), defined by the ratio of the number of moles of singlet oxygen produced to the number of moles of photon absorbed by the photosensitizer, ranged from 0.27 -0.35 (Table S.1). [START_REF] Petrizza | Tuning Photosensitized Singlet Oxygen Production from Colloids Synthesized by Polymerization in Aqueous Dispersed Media[END_REF] The efficiency in producing singlet oxygen under irradiation was mainly due to their ability to swell in ethanol, thus favouring the diffusion of oxygen and reactive species in the vicinity of the grafted photosensitizer. This is an interesting feature of these microgels. We previously demonstrated that the VBRB@MG microgels dispersed in water were not able to produce singlet oxygen as the polymer was in a collapsed state inside the microgels. [START_REF] Petrizza | Tuning Photosensitized Singlet Oxygen Production from Colloids Synthesized by Polymerization in Aqueous Dispersed Media[END_REF] Note that the linear P(VCL-co-VAc) chains forming the polymer network were soluble in ethanol but insoluble in water. [START_REF] Etchenausia | R Copolymerization of Vinyl Acetate and N-Vinylcaprolactam: Kinetics, Control, Copolymer Composition and Thermoresponsive Self-Assembly[END_REF] As generally observed for supported photosensitizers, values of 2 of VBRB@MG remained lower than that of free Rose Bengal in ethanol ( 2 = 0.75), [START_REF] Wilkinson | Rate Constants for the Decay and Reactions of the Lowest Electronically Excited Singlet State of Molecular Oxygen in Solution. An Expanded and Revised Compilation[END_REF] which could arise from the polymeric confinement of the photosensitizers and/or from structural modifications. However, the present singlet oxygen quantum yields were rather high when compared with other supported RB. [START_REF] Lacombe | Materials for Selective Photo-oxygenation vs. Photocatalysis: Preparation, Properties and Applications in Environmental and Health Fields[END_REF] The average loadings of VBRB@MG (Table S.1) were estimated at almost 215 ± 30 µmol RB .g driedMG -1 , and thus higher than the value for commercially available Sensitox ® micrometric particles (about 100 µmol RB .g polymer -1 ).

LED-driven microreactor. The homemade microreactor consisted of an FEP tube fixed in a channel carved in a flat plate aluminium substrate, and wound into an Archimedean spiral, as illustrated in Figure 1. [START_REF] Aillet | Microreactors as a Tool for Acquiring Kinetic Data on Photochemical Reactions[END_REF][START_REF] Mei | Hydrodynamics of Gas-Liquid Slug Flows in a Long In-Plane Spiral-Shaped Milli-Reactor[END_REF][START_REF] Mei | Gas-Liquid Mass Transfer around Taylor Bubbles Flowing in a Long In-Plane Spiral-Shaped Milli-Reactor[END_REF] Thanks to this in-plane spiral-shaped geometry, the device was compact (160 × 160 mm 2 ), and allowed the irradiation to be perfectly controlled over the whole tubing surface, in terms of both homogeneity and photon flux, while offering the possibility of long residence times.

Air bubbles

present study, the photon flux densities measured at the microreactor surface, noted r (µmol photon .m -2 .s -1 ), were equal either to 1440 or to 2134 µmol photon .m -2 .s -1 .

All experiments were carried out under Taylor flow conditions, i. e. stable segmented flows of air bubbles regularly spaced between liquid slugs were obtained in the tube (see Part S.5. of the Supporting Information). The spatial distribution of gas and liquid phases inside the tube (i. e.

the size of air bubbles and of the liquid slugs) was controlled by the volumetric ratio, noted q, defined by the ratio between the air volumetric flow rate, , and the liquid volumetric flow rate, , according to Eq. 1:

When the gas and liquid volumetric flow rates were changed, the stoichiometric ratio between oxygen and α-terpinene at the inlet of the microreactor was also modified. It was then possible to define the ratio between the molar flow rates of oxygen ( 2 ) that could be delivered by each bubble, and the molar flow rate of α-terpinene ( α ), noted f, according to Eq. 2

2 α
The definitions of these flow rates and the relationships between them, the residence time, (s), and the initial concentration of α-terpinene, C α ,0 (mol.L -1 ), are detailed in Part S.6. of the Supporting Information.

General procedure for the photooxygenation of α-terpinene under continuous flow using VBRB@MG photoactive colloids. For a given set of operating conditions, when steady-state was reached, the samples (of a few mL) were collected in an amber glass at the outlet of the microreactor. They were analysed immediately, thus preventing any evolution of the reaction medium between collection and analysis. The analytical methods have already been described by Radjagobalou et al. [START_REF] Radjagobalou | Photooxygenation in an Advanced Led-Driven Flow Reactor Module: Experimental Investigations and Modelling[END_REF] The RB-photobleaching level, R (-), was calculated as the ratio of the concentration of the photobleached photosensitizer with respect the initial concentration of RB (see Part S.7 of the Supplementary Information).

he conversion o α-terpinene, α , was de ined according the concentration o α-terpinene measured (Gas Chromatography measurements) at the inlet and outlet of the microreactor (see Part S.8 of the Supplementary Information). For all the photooxygenation experiments, it was shown that the conversion o α-terpinene could be considered equal to the yield of ascaridole, as no impurities were detected except the ones initially present in the starting material. A high selectivity was thus obtained.

Experiments were carried out in the spiral-shaped microreactor, using the synthetized photoactive colloids or free RB (i. e. RB directly solubilized in ethanol), for comparison. As far as possible, both types of experiments were compared at similar absorbances of the reaction medium measured at max , at the inlet of the microreactor (noted max , 0 ) in order to work under identical light attenuation conditions. Note that, for both cases (free RB or photoactive colloids), the values of the absorbance of the reaction medium inside the microreactor were kept smaller than unity in all the experiments. This thus corresponded to conditions where the characteristic optical path length, (m), is smaller than the characteristic light penetration distance, d pen (see Part S.7. of the Supporting Information). Consequently, under these conditions, the medium could not be considered " ully absorbing": the part o the incident lux not absorbed by the R molecules (typically 10 -20 %) was thus transmitted over the back optical wall of the microreactor.

A specific procedure was applied to the synthetized photoactive colloids before their implementation in the microreactor: the solvent was changed (from water to ethanol), and the related suspension was made to flow, without irradiation, in the microreactor for a certain time (see Part S.9. of the Supporting Information).

he initial concentration o α-terpinene, C α ,0 , was 0.035 mol.L -1 or 0.100 mol.L -1 . Based on the results obtained by Radjagobalou et al., [START_REF] Radjagobalou | Photooxygenation in an Advanced Led-Driven Flow Reactor Module: Experimental Investigations and Modelling[END_REF] the ratio between the gas and liquid volumetric flowrates, q, was set equal to 5, corresponding to long bubble lengths ( 1.8 cm) and short liquid slug lengths ( 0.4 cm). According to the values of C α ,0 and q, the stoichiometry ratio f was equal either to 1.5 (for C α ,0 = 0.035 mol.L -1 ) or to 0.5 (for C α ,0 = 0.100 mol.L -1 ), involving conditions respectively above and below stoichiometry in terms of oxygen compared to terpinene . More details about the calculations of these parameters are given in Part S.6 of the Supporting Information.

RESULTS AND DISCUSSION

A first series of photooxygenation of α-terpinene was carried out under continuous flow using VBRB@MG-8 and VBRB@MG-14 microgels as heterogeneous photosensitizers (Figure 2).

For both VBRB@MG microgels, the grafted photosensitizer efficiently produced singlet oxygen, quantitatively converting α-terpinene into ascaridole (conversion above 80 %, selectivity close to 98 %) in a short residence time (< 4 min). Several reasons could be provided to explain these good results. Firstly, this type of technology allows a short light penetration distance (~ 1 mm) while offering a high irradiated specific area (~ 1000 m 2 .m -3 ). The extensive flux of photons per unit of liquid volume (~ 1400 -2500 mol photon .m -3 .s -1 ) makes possible to reduce the irradiation times significantly in comparison to conventional batch photoreactors (typically varying from ten minutes to several hours) [START_REF] Loubière | Continuous-Flow Photochemistry: A Need for Chemical Engineering[END_REF] , even for concentrated chromophore solutions. In addition, gas-liquid Taylor flow allows very efficient mixing (i. e. frequency of contact) between reactive species as the recirculation times inside the liquid slugs between two consecutive bubbles vary typically between 10 and 100 milliseconds. [START_REF] Mei | Hydrodynamics of Gas-Liquid Slug Flows in a Long In-Plane Spiral-Shaped Milli-Reactor[END_REF] The gas-liquid Taylor flow also induces an intensification of the mass transfer of oxygen from gas bubbles to the reaction medium: the overall volumetric gasliquid mass transfer coefficients typically range from 0.1 to 2 s -1 , which is tens of times or even a hundred times higher than in batch photoreactors. [START_REF] Mei | Gas-Liquid Mass Transfer around Taylor Bubbles Flowing in a Long In-Plane Spiral-Shaped Milli-Reactor[END_REF] The high conversions observed would be also the result of the very good external liquid-solid mass transfer coefficients that can be achieved in C α ,0 = 0.035 mol.L -1 , q = 5, f ~ 1.5, r = 1440 µmol photon .m -2 .s -1 . The concentrations of the photoactive colloids were close to 0.025 g driedMG .L -1 and to 0.050 g driedMG .L -1 for max, 0 and for max,0 10, respectively.

In Figure 2, the photooxygenation kinetics are presented at different storage periods for both photoactive colloids. Over the eight months of storage, the variations of the conversion of α-terpinene with residence time remained similar for VBRB@MG-14 colloids. They also almost overlapped with the one of VBRB@MG-8 after 3 months of storage, despite the slight difference in singlet oxygen quantum yields ( 2 = 0.27 for VBRB@MG-8 and 2 = 0.35 for VBRB@MG-14, Table S.1). Moreover, longer residence times were required to achieve high conversions when the absorbance of the reaction medium was decreased (i. e. when the concentration of the photoactive colloids decreased). For instance, for VBRB@MG-14 after a storage period of 8 months, 62 % and 44 % of conversion were achieved at a residence time of 1.76 min for max, 0 = 0.1 and max,0 = 0.05, respectively. Such a trend is in agreement with the results observed using free RB. [START_REF] Radjagobalou | Photooxygenation in an Advanced Led-Driven Flow Reactor Module: Experimental Investigations and Modelling[END_REF] It is interesting to note that, for all the experiments reported in Figure 2, no photobleaching of the RB grafted onto the microgels was observed (see Part S.10 of the Supporting Information).

In conclusion of this part, it can be noted, first, that the slight differences of hydrodynamic diameters and singlet oxygen quantum yields (see Table S.1) between the two solid photosensitizer substrates (VBRB@MG-8 and VBRB@MG-14) had no influence on the kinetics of -terpinene conversion under the operating conditions tested. Second, their reactivity was stable over 8 months of storage with good reproducibility. After the synthesis, the colloids could thus be stored for 8 months and used without any change of properties and/or photooxygenation performances, an interesting criterion not often scrutinized in the literature.

The following series of experiments was intended to demonstrate the efficiency potentiality of flow photochemistry combined with photoactive colloids. For this purpose, the photooxygenation of -terpinene was deliberately investigated under less favourable conditions.

Ethanol was again chosen as a green solvent even though the lifetime of singlet oxygen is known to be shorter in ethanol than in less eco-friendly chlorinated solvents (13.5 µs in ethanol versus 160-265 µs in chlorinated solvents). 72 To implement a safe and easy process, the oxygen was obtained from the air instead of using a source of pure oxygen. The selected Rose Bengal photosensitizer has a strong ability to degrade by photobleaching, which could be explained by structural alterations under UV light exposure and/or a secondary electron transfer reaction when α-terpinene reacts with an excited Rose Bengal molecule to form p-cymene and a reduced form of Rose Bengal. [START_REF] Zakrzewski | Bleaching Products of Rose Bengal under Reducing Conditions[END_REF]74 Figure 3 compares the photooxygenation rates between the RB-grafted polymer colloids dispersed in ethanol and the free RB photosensitizer in ethanol. This comparison was made for different initial concentrations of RB photosensitizer, characterized by different values of absorbance, max, 0 , ranging from 0.05 to 0.5, at the inlet of the microreactor. The configuration was set with a molar excess of oxygen compared to -terpinene (f  1.5, see Eq. 2 and Part S.6.

of the Supporting Information). (▲ ree R ), max ,0 0. 5 ( VBRB@MG-8), max ,0 0.065 ( free RB), max ,0 0.05 (◊ R @M -14). Operating conditions: C α ,0 = 0.035 mol.L -1 , q = 5, r = 1440 µmol photon .m -2 .s -1 , f  1.5.

First, it is interesting to note that the photobleaching level of RB, R , remained below a few percent for the experiments carried out with free Rose Bengal (see Part S.10 of the Supporting Information). This result is in agreement with our previous results, [START_REF] Radjagobalou | Photooxygenation in an Advanced Led-Driven Flow Reactor Module: Experimental Investigations and Modelling[END_REF] demonstrating that an excessive amount of oxygen compared to -terpinene minimized the RB photobleaching.

Interestingly, the same trend was observed for the experiments with photoactive colloids (see Part S.10 of the Supporting Information).

Figure 3 shows that, for a given absorbance, the supported RB exhibited a reactivity similar to that of the free RB. High conversions of -terpinene (Figure 3) with a high selectivity in ascaridole (Figure S.7) were achieved in a few minutes of residence time. Increasing the initial grafted RB concentration (i. e. a higher initial absorbance) led to faster kinetics as for free RB.

For instance, with supported RB, for max,0 = 0.05, a residence time of 1.13 min led to a conversion of 30 %, while for max,0 = 0.5, a conversion of 99 % was obtained with the same residence time.

The ability of VBRB@MG to produce singlet oxygen clearly demonstrates the relevance of implementing submicronic colloidal photoactive microgels, able to swell in ethanol, as supported photosensitizer in a LED-driven microreactor. As the singlet oxygen quantum yield (

measured in batch was twice as high for free RB as for supported RB (see Table S.1), [START_REF] Petrizza | Tuning Photosensitized Singlet Oxygen Production from Colloids Synthesized by Polymerization in Aqueous Dispersed Media[END_REF] the very close correlation between the variation of the conversion of α-terpinene and the residence time observed between photoactive colloids and free RB (Figure 3) suggests that more complex physicochemical and photophysical phenomena occur in the vicinity of and/or inside the colloids. For instance, the local concentration of the various species including singlet oxygen, substrate and product, inside the swollen microgels or the lifetime of singlet oxygen could be different in the heterogeneous medium and in ethanol. Also, the presence of photoactive colloids could change the gas-liquid flow properties as these types of colloids are known for their propensity to adsorb at the bubble surface. [START_REF] Binks | Particles as Surfactants-Similarities and Differences[END_REF][START_REF] Valadbaigi | Generation of Ultra-Stable Pickering Microbubbles via Poly Alkylcyanoacrylates[END_REF][START_REF] Li | Egg White Protein Microgels as Aqueous Pickering Foam Stabilizers: Bubble Stability and Interfacial Properties[END_REF] This could induce some modifications in terms of gas-liquid hydrodynamics, such as bubble velocity and size, thickness of the liquid lubrication film at the walls, or liquid velocity field in the liquid slug. The interfacial gas-liquid mass transfer, and thus the amount of oxygen transferred from the bubbles per unit of time, could be modified in return. In the latter case, the fact that the observed variations of conversion with residence time are the same between colloids and free RB might reveal some more favourable operating conditions. Deeper investigations will be required to clarify these questions and thus to better understand these promising results.

The third series of experiments aimed at evaluating the resistance of the grafted-RB molecules to photobleaching, in particular in comparison with free RB. For this purpose, the operating conditions were modified. First, a stoichiometric excess o α-terpinene compared to oxygen was used at the inlet of the microreactor, in order to increase the probability o reaction between αterpinene and excited RB, giving rise to both p-cymene and photobleached RB. This is a way to better highlight a difference in RB photobleaching between the free photosensitizer and the supported one. [START_REF] Radjagobalou | Photooxygenation in an Advanced Led-Driven Flow Reactor Module: Experimental Investigations and Modelling[END_REF] In consequence, in the series of experiments reported in Figure 4, an initial αterpinene concentration of 0.105 mol.L -1 (instead of 0.035 mol.L -1 ) was imposed, leading to a stoichiometric ratio f equal to 0.5 (instead of 1.5). The photon flux density at the microreactor surface was also increased from 1440 to 2184 µmol photon .m -2 .s -1 , in order to increase the amount of photo-excited species produced per unit of time. The photooxygenation reaction was implemented at two different initial concentrations of photosensitizer by setting the initial absorbance of the medium at either 0.15 or 0.70. max , 0 0. 4 ( free RB), max , 0 0. 6 ( VBRB@MG-8). Operating conditions: C α ,0 = 0.105 mol.L -1 , q = 5, r = 2184 µmol photon .m -2 .s -1 , f  0.5. Empty symbols correspond to VBRB@MG microgels and full symbols to free Rose Bengal.

Figure 4 displays the evolution of the photobleaching levels of RB with the residence times for RB-grafted colloids and free RB. Note that, because the experiments were carried out with a stoichiometric excess o α-terpinene, the conversion of α-terpinene was incomplete (< 50 %) and reached a plateau (see Figure S.9) while the photobleaching level reached 75 % to 90 % at 1.7 minutes of residence time. For the smallest concentration of RB photosensitizer ( max , 0 0. ), no major difference was observed between grafted and free RB. Interestingly, for the highest concentration of RB photosensitizer ( max ,0 0. ), the VBRB@MG colloids allowed a decrease of 24 % in the photobleaching level between 0.5 and 1 min, and a decrease of 10 % between 1 and 2 min, compared to free RB. Thus, for a strong absorbing medium, the covalent anchoring of RB molecules inside submicronic polymer colloids could prevent their photodegradation. This finding is in agreement with the observations made by Petrizza et al. [START_REF] Petrizza | Tuning Photosensitized Singlet Oxygen Production from Colloids Synthesized by Polymerization in Aqueous Dispersed Media[END_REF] and is particularly interesting since the "optimal conditions" for operating a photoreactor correspond to an absorbance of photosensitizer close to one. 28Erreur ! Source du renvoi introuvable.

The objective of the last series of experiments was to evaluate the possibility of reusing the photoactive colloids for several photooxygenation cycles. Based on the previous results, two sets of experimental conditions (C α ,0 r ) were considered to cover two types of operating domains:  Case 1: (C α ,0 = 0.035 mol.L -1 ; r = 1440 µmol photon .m -2 .s -1 ), preventing RB photobleaching.

 Case 2: (C α ,0 = 0.100 mol.L -1 of -terpinene; r = 2184 µmol photon .m -2 .s -1 ), favourable to RB photobleaching. The procedure used is described in Part S.11 of the Supporting Information.

Figure 5 reports the conversion of α-terpinene, α , and the photobleaching level of RB, R , after each cycle, , of photooxygenation in the LED-driven spiral-shaped microreactor (all the related numerical values are reported in Part S.11 of the Supporting Information). For Case 1 (stoichiometric excess of oxygen), whatever the number of cycles (from 1 to 4), the α-terpinene conversions were almost identical (~ 33%) and no photobleaching occurred (~ 0.11 %). This is a key point showing that the photoactive colloids can be reused for several cycles. As expected, Case 2 (stoichiometric excess of α-terpinene) led to significantly higher photobleaching levels (~ 27 %) than Case 1 (~ 0.11%) for each cycle. Due to the photosensitizer photodegradation, lower conversions were observed, with a slight decrease from cycle 1 to cycle 4 (from 21 to 13 %).

Figure 1 .

 1 Figure 1. Picture illustrating the Archimedean spiral geometry of the LED-driven microreactor in which the slurry Taylor flow was generated.

slurry 71 Figure 2 .

 712 Figure 2. Conversion of α-terpinene, α , versus residence time, , for the photoactive colloids after various times of storage. Empty and full symbols correspond to

Figure 3 .

 3 Figure 3. Conversion of α-terpinene, α , versus residence time, . Empty symbols correspond to VBRB@MG microgels and full symbols to free Rose Bengal. For different values of absorbance measured at max at the inlet of the microreactor: max ,0 0.5 ( free RB,  VBRB@MG-8), max ,0 0.(▲ ree R ), max ,0 0. 5 ( VBRB@MG-8), max ,0 0.065 ( free RB),

Figure 4 .

 4 Figure 4. Photobleaching level of RB, R , versus residence time for different values of absorbance at the inlet of the microreactor: max , 0 0. 0 ( free RB,  VBRB@MG-14),

Figure 5 .

 5 Figure 5. Conversion of α-terpinene (full bars) and photobleaching level of RB (empty bars) after each cycle, , of photooxygenation in the LED-driven spiral-shaped microreactor. Black bars correspond to Case 1, and grey bars to Case 2. Operating conditions for Case 1:VBRB@MG-8; 56 = 1.46; C α ,0 = 0.035 mol.L -1 ; ; = 1.6; = 0.5 min; r = 1440 µmol photon .m - 2 .s -1 . Operating conditions for Case 2: VBRB@MG-14; C α ,0 = 0.100 mol.L -1 ; 56 = 1.80; = 0.5; = 0.72 min; r = 2184 µmol photon .m -2 .s -1 . Note that, for Case 1, as the related photobleaching level was too small, the related values are directly indicated in the figure and no corresponding black empty bar is plotted.

Krasnovsky, A. A. Jr.; Roumbal, Y. V.; Strizhakov, A. A. Rates of 1 O2 ( 1 Δg) Production upon Direct Excitation of Molecular Oxygen by 1270 nm Laser Radiation in Air-Saturated

medium and Photobleaching level (section S.7), Conversion o α-terpinene and yield of ascaridole (section S.8), Conditioning of the photoactive colloids synthesized for their implementation under reactive conditions (section S.9), Additional data related to Figures 2, 3 and 4 (section S.10), and Experiments to test the reusability of the photoactive colloids (section S.11). MG, Microgel colloid; PSS, photosensitizers supported; RB, Rose Bengal; SR, Swelling ratio; TEM, Transmission Electron Microscopy); VAc, vinyl acetate; VCL, N-vinyl caprolactam; VBRB, vinyl benzyl Rose Bengal; αT, α-terpinene.

Funding Sources

This work was sponsored by the French National Research Agency (ANR) under the Collaborative Research Project Program PICPOSS (ANR-15-CE07-0008-01).

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

The authors thank the French Research Agency for the funding under its Collaborative Research Project PICPOSS (ANR-15-CE07-0008-01). The authors also thank Jean-Claude Moschetti from CNRS for the picture illustrating the LED-driven spiral-shaped microreactor. ABBREVIATIONS A, absorbance; DLS, Dynamic Light Scattering; DVA, divinyl adipate; EtOH, ethanol; FEP, Fluorinated Ethylene Propylene tube; GC, gas chromatography; LED, Light-Emitting Diode;

CONCLUSION

This study has shown the photoreactivity efficiency of poly(N-vinylcaprolactam-co-vinyl acetate-co-vinylbenzyl Rose Bengal) microgels as heterogeneous photosensitizers in a continuous-flow process for sustainable singlet oxygen sensitized photooxygenation of a biobased molecule. The process allowed accurate control of irradiation (LED-driven spiral-shaped microreactor), light absorption (tunable absorbance) and flow conditions (slurry Taylor flow).

The efficiency of the swollen RB-grafted colloids in converting -terpinene into ascaridole with high conversion and high selectivity in short residence times has been clearly demonstrated. The efficiency was combined with sustainability of the process involving a green solvent (ethanol) and air as a less hazardous source of oxygen. The supported RB exhibited a reactivity similar to that of the free RB. Moreover, the photo-reactivity of RB-supported microgels was stable over 8 months of storage with good reproducibility. The photoactive colloids also proved to be reusable during several cycles. Finally, with a stoichiometric excess of -terpinene compared to oxygen, in presence of high photon flux and for the highest concentration of Rose Bengal photosensitizer, the VBRB@MG colloids allowed the RB photobleaching levels to be decreased compared to free RB. This important result suggests that the covalent anchoring of RB molecules inside submicronic polymer colloids can prevent their photodegradation.

ASSOCIATED CONTENT

The Supporting Information includes Chemicals (section S.1), Main colloidal features of microgels (section S.2), UV-VIS absorption spectra (section S.3), LED array: emission spectrum and radiant power (section S.4), Gas-liquid Taylor flows (section S.5), Monitoring of the operating parameters and relationships between them (Section S.6), Absorbance of the reaction

Author Contributions

The manuscript was written through contributions of all authors. All authors have approved the final version of the manuscript.

SYNOPSIS

Combining polymer swollen colloids and flow photochemistry for sustainable singlet oxygen sensitized photooxygenation of a bio-based molecule


Danylo Babich

Keiki Fukumoto

Benoit Corraze

Julien Tranchant

Maciej Lorenc

Hervé Cailleau

S-Y Koshihara

Laurent Cario

Etienne Janod
email: etienne.janod@cnrs-imn.fr

Unusually long carrier lifetime in a Mott insulator revealed by time-resolved Photoemission Electron Microscopy

Mott insulators display puzzling insulator to metal transitions under electric field. We study here the Mott insulator GaV4S8 using time-resolved Photoemission Electron Microcopy and reveal unusually long carrier lifetimes, supporting the mechanism of electronic avalanche.

The study in non-equilibrium conditions of an emblematic class of quantum materials, the Mott insulators, is currently developing into an active and fruitful branch of solid state physics. This research field opens up many perspectives by revealing unexpected physical properties based on new control parameters. Mott insulators excited either by ultrafast infrared or THz laser pulses [START_REF] Yamakawa | Mott transition by an impulsive dielectric breakdown[END_REF][START_REF] Giorgianni | Overcoming the thermal regime for the electric-field driven Mott transition in vanadium sesquioxide[END_REF], or by electric fields [START_REF] Cario | Electric-Field-Induced Resistive Switching in a Family of Mott Insulators: Towards a New Class of RRAM Memories[END_REF]display indeed non-trivial insulator to metal transitions (IMT's) which origin goes beyond the well-established bandwidth-and filling-controlled mechanisms [START_REF] Imada | Metal-insulator transitions[END_REF]. Interestingly such transitions could be used to build up a new electronics based on Mott insulators, the so-called Mottronics, including breakthrough devices such as Mott memories [START_REF] Cario | Electric-Field-Induced Resistive Switching in a Family of Mott Insulators: Towards a New Class of RRAM Memories[END_REF] and artificial neurons for artificial intelligence [START_REF] Adda | Mott insulators: A large class of materials for Leaky Integrate and Fire (LIF) artificial neuron[END_REF]. A common feature of out-of-equilibrium IMT's in Mott insulators is that they are based on the massive excitation of electrons from the Lower (valence) to the Upper (conduction) Hubbard Band. For example, recent studies proposed that the multiplication of carrier in a Mott insulator placed under electric field occurs thanks to impact ionization and electronic avalanche [START_REF] Guiot | Avalanche breakdown in GaTa4Se8-xTex narrow-gap Mott insulators[END_REF][START_REF] Diener | How a dc Electric Field Drives Mott Insulators Out of Equilibrium[END_REF]. However the efficiency of such mechanisms might vanish if carrier lifetime is too small, i.e. if electron-hole recombination due for example to defects is too fast. In classical semiconductors, the carrier lifetime varies from the millisecond in ultrapure silicon to the nanosecond in heavily doped silicon [START_REF] Tyagi | Minority carrier recombination in heavily-doped silicon[END_REF][START_REF] Del Alamo | Modelling of minority-carrier transport in heavily doped silicon emitters[END_REF]. As usual purity levels of Mott insulators are far from the one of ultrapure classical semiconductors, one might naively expect very small carrier lifetime in the nanosecond range. Experimentally, the carrier lifetime in Mott insulators has been scarcely explored, partly due to the lack of appropriate technique to measure it. This situation has recently evolved, with the advent of time-resolved photoemission electron microscopy (tr-PEEM), a technique well-suited to determine carrier lifetime [START_REF] Fukumoto | Femtosecond time-resolved photoemission electron microscopy for spatiotemporal imaging of photogenerated carrier dynamics in semiconductors[END_REF]. We report here a tr-PEEM study of the canonical Mott insulator GaV4S8. GaV4S8 is a Mott insulator exhibiting a lacunar spinel structure (see Figure 1a) built from a NaCl-like arrangement of GaS4 tetrahedra and V4S4 cubane-like units. In each V4S4 cluster, one out of the seven V 3d electrons is unpaired and lies on the threefold degenerate molecular t2 level. In the solid, the three narrow bands deriving from these t2 molecular levels are split by the on-site electron-electron repulsion, leading to an insulating state with a Mott-Hubbard gap of 0.3  0.1 eV [START_REF] Guiot | Avalanche breakdown in GaTa4Se8-xTex narrow-gap Mott insulators[END_REF]. The application of electric field pulses in GaV4S8 leads to an IMT, opening the possibility to use this compound in memory applications [START_REF] Cario | Electric-Field-Induced Resistive Switching in a Family of Mott Insulators: Towards a New Class of RRAM Memories[END_REF]. We performed tr-PEEM experiments at room temperature on the KEK experimental setup [START_REF] Fukumoto | Femtosecond time-resolved photoemission electron microscopy for spatiotemporal imaging of photogenerated carrier dynamics in semiconductors[END_REF] on a freshly cleaved GaV4S8 single crystal synthesized as described in Ref. [START_REF] Janod | Negative Colossal Magnetoresistance Driven by Carrier Type in the Ferromagnetic Mott Insulator GaV4S8[END_REF]. Figure 1b provides a schematic description of the electron pathway in a pump-probe tr-PEEM experiment. A first pump laser, either 1.2 eV / 180 fs / repetition rate 500 Hz or 1.1 eV / 10 ns / 200 Hz, promotes electrons in the conduction band. A second laser, the probe, with a tunable delay with respect to the pump expels the excited electrons from the conduction band toward the vacuum level, where electrons are accelerated and detected. A typical tr-PEEM experiment presents the ratio of photoelectrons number detected with and without pump laser versus the time delay between the pump and the probe. Figure 1c shows the results of a first experiment carried out without any pump aiming at establishing the work function  of GaV4S8 and performed by varying the probe energy between 4 and 4.8 eV. This curve displays a clear threshold effect consistent with a work function close to  = 4.6  0.1 eV. For the tr-PEEM experiments, we chose a probe energy of 4.47 eV slightly below the  value, in order to detect only the excited electrons in the Upper Hubbard Band without any pollution by those in Lower Hubbard Band. A first tr-PEEM experiment performed with the 1.2 eV / 180 fs pump laser reveals a striking feature: there is no detectable relaxation of the signal shown in Figure 1d in the 0-100 ps time windows after the pump laser. To confirm this result, a second series of experiment was carried out using the 1.1 eV / 10 ns pump laser. Figure 1e-f shows that the relaxation indeed occurs over a much longer timescale, with a fair description of the relaxation as the sum of three exponential decay terms with1 = 43 ns, 2 = 270 ns and 3 = 4 µs. By comparison with classical semiconductors [START_REF] Tyagi | Minority carrier recombination in heavily-doped silicon[END_REF][START_REF] Del Alamo | Modelling of minority-carrier transport in heavily doped silicon emitters[END_REF], the long carrier lifetime found here might look surprising in a Mott insulator possessing likely a high level of native defects. However, a comparison with other strongly correlated insulators might be more instructive to clarify its origin. Among the scarce studies of carrier lifetime in such compounds, extremely different values are observed, going from the sub-picosecond range in the Mott insulator 1T-TaS2 [START_REF] Perfetti | Time Evolution of the Electronic Structure of 1T-TaS2 through the Insulator-Metal Transition[END_REF] to 1-20 µs in the strongly correlated insulator VO2 [START_REF] Miller | Unusually long free carrier lifetime and metal-insulator band offset in vanadium dioxide[END_REF]. As proposed in Ref. [START_REF] Miller | Unusually long free carrier lifetime and metal-insulator band offset in vanadium dioxide[END_REF], a possible clue is the difference of nature and symmetry of the valence and conduction bands states. In a "oneband" Mott insulator like 1T-TaS2, the Lower and Upper Hubbard Bands derive from the same band and therefore display the same symmetries. In this case, the electron-hole recombination is permitted and hence very fast. Conversely, in a "three-band" Mott insulator like GaV4S8, the top of the LHB and the bottom of the UHB are most probably made of different and orthogonal V3d bands (see Suppl. Mater. of Ref. [START_REF] Camjayi | First-Order Insulator-to-Metal Mott Transition in the Paramagnetic 3D System GaTa4Se8[END_REF] and Figure 1a). This would result in a forbidden interband transition and hence in a slow electron-hole recombination. Even if further theoretical and experimental work is required to confirm this conjecture, the possibility to tune the carrier lifetime in Mott insulators by more than six orders of magnitude is of prime interest for artificial intelligence applications. The carrier lifetime is indeed controlling the operating frequency of artificial neurons based on Mott insulators [START_REF] Adda | Mott insulators: A large class of materials for Leaky Integrate and Fire (LIF) artificial neuron[END_REF]. In summary, we have detected unusually long carrier lifetime in the multiband Mott insulator GaV4S8 using the tr-PEEM technique. This result is important since it supports the proposed mechanism of electronic avalanche [START_REF] Guiot | Avalanche breakdown in GaTa4Se8-xTex narrow-gap Mott insulators[END_REF][START_REF] Diener | How a dc Electric Field Drives Mott Insulators Out of Equilibrium[END_REF] and will guide the future development of new hardware components in artificial intelligence.

© 2 0Figure 1 :

 21 Figure 1 : (a) crystallographic structure of GaV4S8 and molecular orbital diagram associated to the V4 tetrahedral cluster. (b) schematic description of the electron pathway in a tr-PEEM experiment. (c) PEEM energy scan of the probe laser without any pump on a GaV4S8 single crystal. (d-f) tr-PEEM signal vs various ranges of time delay between the pump and the probe in GaV4S8 at room temperature.

This work was carried out within the French-Japanese International Associated Laboratory IM-LED (CNRS).