Marta Feliz

Pedro Atienzar

Maria Amela-Cortés

Noee Dumait

Pierric Lemoine

Yann Molard

Stéphane ‡ Cordier

Noée Dumait

Supramolecular Anchoring of Octahedral Molybdenum Clusters onto Graphene and Their Synergies in Photocatalytic Water Reduction

Keywords: Metal cluster, graphene, photocatalysis, supramolecular chemistry

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INTRODUCTION

Transition metal clusters are molecular compounds that contain several metal atoms linked by metal-metal bonds. [START_REF] Cotton | Metal Atom Clusters in Oxide Systems[END_REF] With their exact and defined composition, they can be classified as intermediates between transition metal coordination complexes and metal nanoparticles. Their specificity, compared to coordination complexes, is the delocalization of electrons involved in the metal-metal bonds on the whole metallic architecture. As those are not confined on one metal center, transition metal clusters display intermediate behavior between atoms and bulk metals and, in particular show unique magnetic, optical or catalytic properties. [START_REF] Maverick | Luminescence and Redox Photochemistry of the Molybdenum(Ii) Cluster Mo6cl14(2-1)[END_REF][START_REF] Nocera | Electrochemical reduction of molybdenum(II) and tungsten(II) halide cluster ions. Electrogenerated chemiluminescence of tetradecachlorohexamolybdate(2-) ion[END_REF][START_REF] Larina | Electronic state of rhenium complexes with octahedral chalcocyanide cluster anions [Re(6)Q(8)(CN)(6)[END_REF][START_REF] Tulsky | Cluster-to-Metal Magnetic Coupling: Synthesis and Characterization of 25-Electron [Re6-nOsnSe8(CN)6](5-n)-(<i>n</i> = 1, 2) Clusters and {Re6-nOsnSe8[CNCu(Me6tren)]6[END_REF][START_REF] Barras | Fast photocatalytic degradation of rhodamine B over [Mo6Br8(N3)6]2-cluster units under sun light irradiation[END_REF][START_REF] Beltran | A photobleaching resistant polymer supported hexanuclear molybdenum iodide cluster for photocatalytic oxygenations and photodynamic inactivation of Staphylococcus aureus[END_REF][START_REF] Buzek | MoII Cluster Complex-Based Coordination Polymer as an Efficient Heterogeneous Catalyst in the Suzuki-Miyaura Coupling Reaction[END_REF][START_REF] Feliz | In situ generation of active molybdenum octahedral clusters for photocatalytic hydrogen production from water and their stabilization onto graphene oxide surfaces[END_REF][10] Yet, octahedral transition metal cluster compounds based on a Mo, W or Re scaffold (Scheme 1a) are particularly attractive to design functional hybrid nanomaterials with potential applications in optoelectronic, [11][12][13][14][15][START_REF] Bigeon | Efficient active waveguiding properties of Mo 6 nano-cluster-doped polymer nanotubes[END_REF] lighting, 17 theranostic [START_REF] Krasilnikova | Prospects of molybdenum and rhenium octahedral cluster complexes as X-ray contrast agents[END_REF][START_REF] Kirakci | X-ray Inducible Luminescence and Singlet Oxygen Sensitization by an Octahedral Molybdenum Cluster Compound: A New Class of Nanoscintillators[END_REF][START_REF] Solovieva | Singlet Oxygen Production and Biological Activity of Hexanuclear Chalcocyanide Rhenium Cluster Complexes [{Re6Q8}(CN)6]4[END_REF][START_REF] Brandhonneur | Molybdenum cluster loaded PLGA nanoparticles: An innovative theranostic approach for the treatment of ovarian cancer[END_REF] or photocatalysis 6-7, 9, 22 applications. Because molybdenum or tungsten elements are cheap and abundant, cluster-based compounds constitute promising alternatives to costly Ir(III)-, Pt(II)-or rare earth -containing luminophores, or non-environmentally friendly lead hybrid perovskites, or cadmium containing quantum dots. They are synthesized as R e v i s e d m a n u s c r i p t 3 Ax[M6X i 8X a 6] ternary alkali salts (A = alkali, x = 2 -4; X i = inner ligand, usually halogen or chalcogen atoms; X a = apical halogen ligand) using solid state chemistry techniques. Resulting powders or crystals behave like ceramics. That is probably the reason why, despite their outstanding functionalities, their use and studies were circumvented to a small community of scientists who, only around twenty years ago, opened the Pandora's box of their integration in functional materials and nanocomposites [START_REF] Golden | Monodisperse Metal-Clusters 10-Angstroms in Diameter in a Polymeric Host -the Monomer-as-Solvent Approach[END_REF][START_REF] Jackson | Efficient singlet oxygen generation from polymers derivatized with hexanuclear molybdenum clusters[END_REF] via solution chemistry. Since, octahedral transition metal clusters form an integral part of nanoscience and were integrated in functional hybrid materials by various approaches either by direct integration, or after apical ligand exchange, cationic metathesis or host-guest complexation. [START_REF] Cordier | Advances in the Engineering of Near Infrared Emitting Liquid Crystals and Copolymers, Extended Porous Frameworks, Theranostic Tools and Molecular Junctions Using Tailored Re6 Cluster Building Blocks[END_REF][START_REF] Cordier | Inorganic Molybdenum Octahedral Nanosized Cluster Units, Versatile Functional Building Block for Nanoarchitectonics[END_REF][START_REF] Molard | Clustomesogens: Liquid Crystalline Hybrid Nanomaterials Containing Functional Metal Nanoclusters[END_REF][START_REF] Guy | Lord of The Crowns: A New Precious in the Kingdom of Clustomesogens[END_REF] Among the metal cluster units, the [M6I i 8(perfluoroalkylate) a 6] 2-and [M6I i 8(perfluorobenzoate) a 6] 2-(M = Mo or W) cluster anions are the most emissive, with emission properties depending on the pKa of the carboxylic acids. [START_REF] Sokolov | Complexes of {W6I8}4+ Clusters with Carboxylates: Preparation, Electrochemistry, and Luminescence[END_REF][START_REF] Sokolov | Highly luminescent complexes [Mo6X8(n-C3F7COO)6]2-(X = Br, I)[END_REF][START_REF] Kirakci | A Highly Luminescent Hexanuclear Molybdenum Cluster -A Promising Candidate toward Photoactive Materials[END_REF][32][START_REF] Amela-Cortes | Versatility of the ionic assembling method to design highly luminescent PMMA nanocomposites containing [M6Qi8La6]n-octahedral nano-building blocks[END_REF][34][START_REF] Riehl | A ligand substituted tungsten iodide cluster: luminescence vs. singlet oxygen production[END_REF][START_REF] Akagi | ) Clusters[END_REF] Recently, they were used to develop theranostic tools, [START_REF] Kirakci | Octahedral molybdenum clusters as radiosensitizers for X-ray induced photodynamic therapy[END_REF] lighting devices [START_REF] Robin | Direct Integration of Red-NIR Emissive Ceramic-like AnM6Xi8Xa6 Metal Cluster Salts in Organic Copolymers Using Supramolecular Interactions[END_REF] or oxygen sensors, [START_REF] Amela-Cortes | Tuned Red NIR phosphorescence of polyurethane hybrid composites embedding metallic nanoclusters for oxygen sensing[END_REF] but surprisingly, their catalytic abilities have been scarcely explored so far. Yet, their excellent abilities to emit light combined to their isotropic architecture, i.e. the presence of six metal atoms arranged orthogonally, that could all play the role of catalytic centers, prefigure good opportunities in photocatalysis for this family of compounds. In this work, we investigate the photocatalytic abilities of [Mo6I i 8(OCOC2F5) a 6] 2-anion, [START_REF] Amela-Cortes | Versatility of the ionic assembling method to design highly luminescent PMMA nanocomposites containing [M6Qi8La6]n-octahedral nano-building blocks[END_REF] in one of the most emblematic reaction in the field of renewable energy: the production of hydrogen by water splitting using sunlight. [START_REF] Kudo | Heterogeneous photocatalyst materials for water splitting[END_REF][START_REF] Walter | Solar Water Splitting Cells[END_REF][START_REF] Maeda | Photocatalytic water splitting using semiconductor particles: History and recent developments[END_REF][START_REF] Zou | Noble metal-free hydrogen evolution catalysts for water splitting[END_REF] In this field, it is still very challenging to develop low-cost, but efficient enough, photocatalysts that can be further scaled up for industrial applications. [START_REF] Kudo | Heterogeneous photocatalyst materials for water splitting[END_REF][START_REF] Zou | Noble metal-free hydrogen evolution catalysts for water splitting[END_REF][START_REF] Watanabe | Dye-sensitized photocatalyst for effective water splitting catalyst[END_REF][START_REF] Zheng | Unconventional Route to Hairy Plasmonic/Semiconductor Core/Shell Nanoparticles with Precisely Controlled Dimensions and Their Use in Solar Energy Conversion[END_REF][START_REF] Wang | Nonepitaxial growth of uniform and precisely size-tunable core/shell nanoparticles and their enhanced plasmon-driven photocatalysis[END_REF][START_REF] Wang | Plasmon-Mediated Solar Energy Conversion via Photocatalysis in Noble Metal/Semiconductor Composites[END_REF][START_REF] Yang | ZnGa2-xInxS4 (0 ≤ x ≤ 0.4) and Zn1-2y(CuGa)yGa1.7In0.3S4 (0.1 ≤ y ≤ 0.2): Optimize Visible Light Photocatalytic H2 Evolution by Fine Modulation of Band Structures[END_REF][START_REF] Yu | Biomimetic Z-scheme photocatalyst with a tandem solid-state electron flow catalyzing H2 evolution[END_REF][START_REF] Hao | Surface N modified 2D g-C3N4 nanosheets derived from DMF for photocatalytic H2 evolution[END_REF][START_REF] Xia | Unraveling Photoexcited Charge Transfer Pathway and Process of CdS/Graphene Nanoribbon Composites toward Visible-Light Photocatalytic Hydrogen Evolution[END_REF][START_REF] Chang | Photocatalytic hydrogen production from glycerol solution at room temperature by ZnO-ZnS/graphene photocatalysts[END_REF][START_REF] Xu | Noble metal-free RGO/TiO2 composite nanofiber with enhanced photocatalytic H2-production performance[END_REF] Currently, most researches are directed toward the use of earth abundant transition metal materials and hybrid systems supported onto graphene. [START_REF] Zou | Noble metal-free hydrogen evolution catalysts for water splitting[END_REF][START_REF] Osterloh | Inorganic Materials as Catalysts for Photochemical Splitting of Water[END_REF][START_REF] Sun | Graphene based new energy materials[END_REF][START_REF] Wang | Recent Progress in Cobalt-Based Heterogeneous Catalysts for Electrochemical Water Splitting[END_REF][START_REF] Kamat | Graphene-Based Nanoassemblies for Energy Conversion[END_REF][START_REF] Han | Progress in graphene-based photoactive nanocomposites as a promising class of photocatalyst[END_REF][START_REF] Xiang | Graphene-Based Photocatalysts for Hydrogen Generation[END_REF] The potential of transition metal complex-based photocatalysts to replace conventional inorganic materials was recently reported. [START_REF] Huo | Mini-review on an engineering approach towards the selection of transition metal complex-based catalysts for photocatalytic H2 production[END_REF] Still, many challenges should be faced before R e v i s e d m a n u s c r i p t 4 graphene-metal clusters nanocomposites become photocatalysts to address environmental and energy-related issues, for instance in terms of photostability and activity. [START_REF] Attia | Metal clusters: New era of hydrogen production[END_REF][START_REF] Peiris | Metal nanoparticle photocatalysts: emerging processes for green organic synthesis[END_REF] In fact, it is well known that graphene, because of its zero band gap semiconducting properties due to its extended conjugated system, possesses a high conductivity and electron mobility that favor charge separation and electron transport. Therefore, supporting photocatalysts onto graphene usually prevents the direct recombination of the hole-electron pair formed during light excitation and, as a result, facilitates the photogenerated electron transfer leading to an enhanced H2 photocatalytic performance compared to neat photocatalysts. Hence, several photocatalysts, such as TiO2 nanoparticles, CdQ (Q = S, Se) quantum dots, MoS2 or more recently metal clusters, were supported onto graphene oxide (GO) or reduced graphene oxide (RGO). [START_REF] Zou | Noble metal-free hydrogen evolution catalysts for water splitting[END_REF][START_REF] Kim | Solar Photoconversion Using Graphene/TiO2 Composites: Nanographene Shell on TiO2 Core versus TiO2 Nanoparticles on Graphene Sheet[END_REF][START_REF] Chang | MoS2/Graphene Cocatalyst for Efficient Photocatalytic H2 Evolution under Visible Light Irradiation[END_REF][START_REF] Li | Charge-Transfer Induced High Efficient Hydrogen Evolution of MoS2/graphene Cocatalyst[END_REF][START_REF] Georgakilas | Noncovalent Functionalization of Graphene and Graphene Oxide for Energy Materials, Biosensing, Catalytic, and Biomedical Applications[END_REF][START_REF] Li | Graphene in Photocatalysis: A Review[END_REF] Here we report on the synthesis, characterization and photophysical properties of bright red NIR emitting octahedral molybdenum cluster complexes bearing functional organic counter cations able to form a supramolecular adduct with graphene. Properties of the most promising candidate in terms of surface coverage were further investigated, in particular the photocatalytic efficiency in the hydrogen evolution reaction (HER) from water.

RESULTS AND DISCUSSION

Designing the linker for the anchoring of transition metal cluster on the graphene sheet: the supramolecular way. It is now recognized that [M6X i 8X a 6] 2-emission properties are related to the pKa value of their apical ligands, and that the most efficient emitters possess apical ligands linked to metal atoms via an acetate function. [START_REF] Sokolov | Highly luminescent complexes [Mo6X8(n-C3F7COO)6]2-(X = Br, I)[END_REF]34,[START_REF] Akagi | ) Clusters[END_REF] Hence, the integrity of the [Mo6I i 8(OCOC2F5) a 6] 2-anion should be preserved within the photocatalytic system to preserve its outstanding ability to emit light. In earlier studies, we observed that the covalent grafting of metal clusters on the surface of GO sheets limited the accessibility on the cluster active sites for catalytic R e v i s e d m a n u s c r i p t 5 reactions. [START_REF] Feliz | In situ generation of active molybdenum octahedral clusters for photocatalytic hydrogen production from water and their stabilization onto graphene oxide surfaces[END_REF] Moreover, the graphene covalent functionalization alters the sp 2 structure of its lattice which results in defects and loss of electronic properties. Therefore, the apical ligand exchange approach that allows the covalent grafting of the polymetallic anion onto any type of surface [START_REF] Fabre | Electrochemical and Charge Transport Behavior of Molybdenum-Based Metallic Cluster Layers Immobilized on Modified n-and p-Type Si(111) Surfaces[END_REF][START_REF] Godet | Surface immobilization of Mo6I8 octahedral cluster cores on functionalized amorphous carbon using a pyridine complexation strategy[END_REF][START_REF] Cordier | Elaboration, Characterizations, and Energetics of Robust Mo6 Cluster-Terminated Silicon-Bound Molecular Junctions[END_REF] has to be discarded. The non-covalent functionalization of graphene preserves its outstanding structural and electronic properties along with the simultaneous deposition of new chemical groups on the surface, and leads to an enhanced dispersability and catalytic reactivity of the hybrid material. For these reasons, using non-covalent interactions to link both protagonists appears as the most profitable way to take advantage of their properties. In fact, non-covalent functionalization of graphene or graphene oxide has been extensively studied to bind fluorophores or functional organic or inorganic species onto their surface. [START_REF] Georgakilas | Noncovalent Functionalization of Graphene and Graphene Oxide for Energy Materials, Biosensing, Catalytic, and Biomedical Applications[END_REF][START_REF] Wang | Tunable Supramolecular Interactions of Aggregation-Induced Emission Probe and Graphene Oxide with Biomolecules: An Approach toward Ultrasensitive Label-Free and "Turn-On" DNA Sensing[END_REF][START_REF] Sabater | Immobilization of Pyrene-Tagged Palladium and Ruthenium Complexes onto Reduced Graphene Oxide: An Efficient and Highly Recyclable Catalyst for Hydrodefluorination[END_REF][START_REF] Zhang | Supramolecular Chemistry on Graphene Field-Effect Transistors[END_REF] This technic should lead to hybrid materials that combine the best of each individual component properties. Hence, using supramolecular interactions to maintain the cluster anion on the graphene surface seems the more appropriate method to maximize the cluster effect during catalytic reactions.

The adsorption of polycyclic aromatic hydrocarbons (PAHs) and ionic liquids based on imidazolium cations onto graphene nanosheets has been recently investigated. [START_REF] Zhou | Dispersion of graphene sheets in ionic liquid [bmim][PF6] stabilized by an ionic liquid polymer[END_REF][START_REF] Wang | Adsorption of Polycyclic Aromatic Hydrocarbons by Graphene and Graphene Oxide Nanosheets[END_REF] In the case of pyrene, its affinity to graphene is dominated by - interactions, whereas in the case of ionic liquids, these and other non-covalent interactions, such as C-H

•••, N••• or ion••• interactions,
take place in the adsorption of imidazolium cations and their associated counteranions on the graphene surface. [START_REF] Wang | Adsorption of Polycyclic Aromatic Hydrocarbons by Graphene and Graphene Oxide Nanosheets[END_REF][START_REF] Shakourian-Fard | Meta-Hybrid Density Functional Theory Study of Adsorption of Imidazolium-and Ammonium-Based Ionic Liquids on Graphene Sheet[END_REF] Recently, Coskun and co-workers have shown that bifunctional pyreneimidazolium molecules can interact with reduced graphene oxide through - and cation-interactions simultaneously with ionic liquid solvent molecules. [START_REF] Srinivasan | A bifunctional approach for the preparation of graphene and ionic liquid-based hybrid gels[END_REF] In this context, as depicted in Scheme 1a, we decided to synthesize an imidazolium cation bearing a pyrene group, noted as Py +

(1-methyl-3-(4-(pyren-1-yl)butyl)-1H-imidazol-3-ium), to counterpart the anionic charge of the funtionalized pyrene counterions should play a dual role: (i) to act as antenna and promote energy transfer to the luminescent {Mo6I i 8} 4+ cluster core, and (ii) to assist the cluster immobilization onto graphene surfaces by non-covalent interactions. To observe the benefit of the pyrene group on its functional ability to be adsorbed onto the graphene surface, we also functionalized the cluster anion with imidazolium cations bearing a long alkyl chain, noted as Im + (1-methyl-3-nonyl-1Himidazol-3-ium).

Synthesis and characterization of the Py2Mo, Im2Mo, and Py2Mo@Gene materials.

The cationic metathesis between the Cs2[Mo6I i 8(OCOC2F5) a 6] (Cs2Mo) compound and the PyBr or ImBr salts afford quantitatively the Py2Mo and Im2Mo complexes, respectively, as highlighted in Scheme 1b. The Py2Mo and Im2Mo cluster compounds were characterized by 1 H and [START_REF] Kirakci | X-ray Inducible Luminescence and Singlet Oxygen Sensitization by an Octahedral Molybdenum Cluster Compound: A New Class of Nanoscintillators[END_REF] F NMR spectroscopy, and electrospray ionization mass spectrometry (ESI-MS) in order to confirm the complexes identity and purity (see Experimental Section and Supporting Information for characterization details). The structure of the Py2Mo complex was further confirmed by singlecrystal X-ray diffraction on suitable crystals obtained by slow evaporation of a complex containing acetone:toluene (1:1) solution (see Supporting Information).

The adsorption capacities of Py2Mo and Im2Mo by non-covalent interactions onto the graphene surface, resulting in Py2Mo@Gene and Im2Mo@Gene hybrid nanomaterials respectively (Scheme 1c), were investigated. Corresponding adsorption densities (qe) and adsorption isotherms were also determined (Figure S3, Supporting Information). The maximum adsorption capacity observed for Py2Mo at equilibrium is 0.3827 mmol (Py2Mo)/g (graphene). Considering the surface area of graphene (474 m 2 /g), the calculated cluster density deposited onto graphene surface is 0.0008 mmol/m 2 . The maximum qe determined for Py2Mo is lesser than that reported for pyrene, [START_REF] Wang | Adsorption of Polycyclic Aromatic Hydrocarbons by Graphene and Graphene Oxide Nanosheets[END_REF] which is probably due to the high steric hindrance of the hybrid complex and to the fact that two R e v i s e d m a n u s c r i p t pyrene moieties are surrounding one metal cluster unit. The adsorption isotherms for both complexes shows a linear fitting, in agreement with Langmuir isotherms, with ca. 15% increment of the adsorption capacities for Py2Mo vs Im2Mo. These results support the relevant role of pyrene in reinforcing the non-covalent cluster immobilization onto graphene surfaces despite its bulkiness compared to the Im + cation. Hence, as the pyrene containing cation is the one that provides the best surface coverage, only the hybrid cluster compound Py2Mo and its adduct with graphene were further investigated. Py2Mo@Gene was thus obtained by mixing Py2Mo and graphene in dichloromethane at room temperature. The solid nanomaterial was isolated by filtration and washed with dichloromethane and diethyl ether, followed by vacuum drying.

The morphology of the Py2Mo@Gene material was characterized by high resolution transmission electron microscopy (HR-TEM). HR-TEM analyses of the Py2Mo@Gene material shows a heterogeneous distribution of 2-3 nm molybdenum cluster aggregates onto a few layer graphene support (Figure S4). The - and cation- interactions between the pyrene functionalized imidazolium cations and graphene allow the hybrid system to self-assemble strongly, and these interactions also lead to the entrapment of cluster molecules between graphene layers, resulting in the formation of a 3D network structure. In addition, the cluster anion- interactions are not discarded in the adsorptive interactions. [START_REF] Shakourian-Fard | Meta-Hybrid Density Functional Theory Study of Adsorption of Imidazolium-and Ammonium-Based Ionic Liquids on Graphene Sheet[END_REF] The energy-dispersive X-ray spectroscopy (EDS-STEM) analysis of the hybrid material confirms the presence of Mo, I and F atoms (Figure S5).

R e v i s e d m a n u s c r i p t 9

The Py2Mo@Gene nanomaterial was further characterized by FT-IR, Raman, and photophysical based techniques. The infrared spectra registered for the Py2Mo@Gene, Py2Mo and graphene materials (Figure 1a) shows that there is no shift of the Py2Mo and graphene peaks after the cluster immobilization. The infrared spectrum of the Py2Mo@Gene solid contains most of the characteristic peaks of the Py2Mo complex. The bands at 1580 and 1628 cm -1 of the graphenic materials are assigned to the C=C vibrations of the sp 2 carbon atoms and H-O-H vibrations of adsorbed water molecules, respectively. The Raman spectra of Py2Mo, graphene and the hybrid nanomaterial have been acquired in the solid state upon 325 nm laser irradiation (Figure ). The Raman bands between 150 and 500 cm -1 expected for the {Mo6X i 8} 4+ (X = halogen) cluster core complexes are not detectable, and only fluorescence has been obtained under 514 and 785 nm laser conditions. [START_REF] Preetz | Kristallstruktur und spektroskopische Eigenschaften der Clusteranionen [(Mo6Br 8i)X 6a]2-mit Xa = F, Cl, Br, I[END_REF][START_REF] Schoonover | Resonance Raman Spectra of [M6X8Y6]2-Cluster Complexes (M = Mo, W; X, Y = Cl, Br, I)[END_REF] The Py2Mo@Gene spectrum contains a very intense band at 1580 cm -1 (G band) characteristic of the graphene aromatic rings sp 2 carbon atoms vibration. [START_REF] Ferrari | Raman spectroscopy of graphene and graphite: Disorder, electron-phonon coupling, doping and nonadiabatic effects[END_REF] A wide band with a lower intensity appears at 1394 cm -1 and includes a pyrene group Raman shift and the D band of graphene support. Two additional low intense bands characteristic of the pyrene functionality appear at 1236 and 1621 cm -1 . [START_REF] Asher | Ultraviolet resonance Raman spectrometry for detection and speciation of trace polycyclic aromatic hydrocarbons[END_REF][START_REF] Rumelfanger | UV Resonance Raman Characterization of Polycyclic Aromatic Hydrocarbons in Coal Liquid Distillates[END_REF] The graphenic 2D band (2848 cm -1 ) does not show any displacement after the cluster immobilization.

Photophysical properties of the Py2Mo and Py2Mo@Gene materials.

Photophysical properties of Py2Mo were investigated in solution and compared to those of PyBr and Cs2Mo precursors. Py2Mo absorption spectrum shows the strongly pyrene absorbing bands centered at 325 nm (Figure 2) associated to the S0  S2 transition, by analogy with similar systems. [START_REF] Rumelfanger | UV Resonance Raman Characterization of Polycyclic Aromatic Hydrocarbons in Coal Liquid Distillates[END_REF][START_REF] Larsen | Extending the Light-Harvesting Properties of Transition-Metal Dendrimers[END_REF][START_REF] Amelia | A ratiometric luminescent oxygen sensor based on a chemically functionalized quantum dot[END_REF] The low absorption band intensity at 403 nm is associated to a metal to metal charge transfer ( 1 MMCT) transition within the {Mo6I i 8} 4+ cluster core. [START_REF] Ramirez-Tagle | Electronic structure and molecular properties of the [Mo6X8L6[END_REF][START_REF] Costuas | Relating the polarity-dependent fluorescence response of pyrene to vibronic coupling. Achieving a fundamental understanding of the py polarity scale[END_REF] This band is also observed in the Cs2Mo compound (Figure S6), which also shows absorption bands centered at 334 and 295 nm characteristic of the molybdenum to ligand cluster transitions. The appearance of these UV bands implies that there is a cluster contribution in the pyrene absorption region of the Py2Mo 87 Despite the presence of two pyrene units per Py2Mo, the excimer emission from pyrene at ca. 500 nm is not observed. This highlights the lack of intra and intermolecular pyrene-pyrene interactions, taking in consideration the high dilution conditions (10 -5 -10 -6 M). A low intensity band with a maximum localized at 675 nm, attributed to the molybdenum cluster core emission, is also detected. This emission band shows a bathochromic shift with respect to the emission wavelength (650 nm) measured in DMF, also reported for Cs2Mo in the solid state. [START_REF] Amela-Cortes | Versatility of the ionic assembling method to design highly luminescent PMMA nanocomposites containing [M6Qi8La6]n-octahedral nano-building blocks[END_REF] The disappearance of this band by bubbling O2 in the Py2Mo containing solution confirms the oxygen-quenching of the cluster units phosphorescence. Absolute quantum yield (AQY) of Py2Mo was measured exciting the sample every 5 nm from 355 up to 430 nm. The AQY decreases from 0.18 to 0.12 from 355 nm to 380 nm, i.e. when the pyrene emission is visible. This value stabilizes around 0.1 when the cluster is the only emitter. The {Mo6I i 8} 4+ cluster core bearing perfluoroalkylcarboxylates presents one of the highest quantum yield value among the octahedral {M6X i 8} 4+ (M = Mo, W or Re; L i = halogen or chalcogen) cluster complexes, and (nBu4N)2[Mo6I8(OOCC3F7)6], respectively, [START_REF] Sokolov | Highly luminescent complexes [Mo6X8(n-C3F7COO)6]2-(X = Br, I)[END_REF][START_REF] Kirakci | A Highly Luminescent Hexanuclear Molybdenum Cluster -A Promising Candidate toward Photoactive Materials[END_REF] while we recently reported an AQY value of 0.49 in deaerated acetone for Cs2Mo. [START_REF] Robin | Direct Integration of Red-NIR Emissive Ceramic-like AnM6Xi8Xa6 Metal Cluster Salts in Organic Copolymers Using Supramolecular Interactions[END_REF] Therefore, the Py2Mo AQY value is lower than the one calculated for its precursor Cs2Mo. In a first attempt, this observation can be explained as follow: the metal cluster red emission at 675 nm is subject to a partial energy transfer (ET) mechanism associated to intermolecular interactions between the pyrene and the molybdenum cluster. This ET is partly due to the spectral overlap between the emission band of the pyrene moiety and the absorption of the molybdenum cluster. A triplet-triplet state equilibrium between the triplet state of pyrene and the one of cluster could then be envisioned. This equilibrium would favor the nonradiative relaxation of the excited states (vide infra). Figure 3 presents the evolution of the emission intensity maximum value of both entities, versus the excitation wavelength from 310 nm up to 390 nm, and the excitation map for excitations ranging from 355 up to 500 nm. The optimal wavelength to observe the pyrene emission is, as expected, around its maximum of absorption. Meanwhile, a band followed by a slight intensity decrease can be observed between 340 and 360 nm for the molybdenum cluster, in agreement with the maximum absorption of the pyrene group. This disrupt in the emission evolution trend is followed by an increase when the cluster is the only species to absorb (see Figure S7 for the corresponding emission spectra). Then, the cluster emission slowly decreases due to its absorption lowering up to 500 nm. To assess in a first attempt that an effective transfer exists between the pyrene moieties and the anionic cluster unit, emission spectra of solutions containing Py2Mo or Cs2Mo at the same concentration, were recorded upon a 345 nm excitation. The increase of the cluster emission intensity by approximately 30%, is in good accordance with the pyrene-to-molybdenum cluster ET (Figure S8). The presence of pyrene [Re6Se8CN6] 4-anions. [START_REF] Camerel | Ionic Columnar Clustomesogens: Associations between Anionic Hexanuclear Rhenium Clusters and Liquid Crystalline Triphenylene Tethered Imidazoliums[END_REF] It shows that, a short and covalent link between the pyrene and the cluster unit is mandatory to observe a full ET between the two entities and thus an enhancement of its emission efficiency. [START_REF] Kirakci | Hexamolybdenum Cluster Complexes with Pyrene and Anthracene Carboxylates: Ultrabright Red Emitters with the Antenna Effect[END_REF] We also monitored the emission signal of a solution containing initially Cs2Mo to which a small aliquot of a PyBr solution was added (see Figure S9 for emission spectra).

We observed an increase of the molybdenum cluster complex red emission in the presence of was judged looking at the  2 value and the shape of the residuals distribution. The phosphorescence lifetime for Py2Mo (188 s, exc = 345 nm in DMF) corresponds to the lifetime of the emissive {Mo6I i 8}-localized triplet states, and is similar to those reported for other carboxylate [Mo6I i 8(OOC-R)6] 2-(R = CF3, 182 s; R = n-C3F7, 303 s) complexes in acetonitrile. [START_REF] Sokolov | Highly luminescent complexes [Mo6X8(n-C3F7COO)6]2-(X = Br, I)[END_REF][START_REF] Kirakci | A Highly Luminescent Hexanuclear Molybdenum Cluster -A Promising Candidate toward Photoactive Materials[END_REF] The higher emission lifetime of Cs2Mo (228 s, exc = 345 nm) suggests that the emission decay is faster for the Py2Mo complex, associated to any kind of counterion-cluster interaction. [START_REF] Vining | The influence of environmental effects on excited-state lifetimes. The effect of ion pairing on metal-to-ligand charge transfer excited states[END_REF] This decrease in the lifetime and the slight bathochromic shift of the cluster emission maximum when Cs + is replaced by Py + could be due to an electronic stabilization of the cluster excited state by the pyrene-imidazolium counterion. [START_REF] Divya | A fluorescent molecular probe for the identification of zinc and cadmium salts by excited state charge transfer modulation[END_REF][START_REF] Ma | Luminescence Color Tuning by Regulating Electrostatic Interaction in Light-Emitting Devices and Two-Photon Excited Information Decryption[END_REF][START_REF] Ma | Dynamic metal-ligand coordination for multicolour and water-jet rewritable paper[END_REF] The decrease of the pyrene fluorescence lifetime from PyBr to Py2Mo (200 and 164 ns, respectively upon exc = 340 nm, see Figure S10 for fluorescence decay profiles), confirms this partial interaction.

The transient absorption spectra of PyBr and Py2Mo compounds are presented Figure 4a. For PyBr, the spectrum shows the characteristic transient absorptions of the pyrene group triplet states of the functionalized imidazolium cation, which appear as two maxima centered at ca. 420 and 500 nm. [START_REF] Pankasem | Reflectance spectroscopic studies of the cation radical and the triplet of pyrene on alumina[END_REF] These triplet states are confirmed by quenching experiments with O2 (Figure S11). At low absorption times a negative band corresponding to the emission of pyrene group singlet state (Figures S12a andS12c) appears between 350-450 nm. The Py2Mo and Cs2Mo TA spectra acquired upon 355 nm excitation are very similar and show a broad absorption band with one maximum centered at ca. 520 and 550 nm, respectively (Figure S12 for Cs2Mo TA spectrum).

The temporal profiles acquired at 550 nm (Figure S13) show that the absorption lifetime of the triplet state for Py2Mo is faster than Cs2Mo, in agreement with the different lifetimes stated above. S11b andS11d).

The absorption temporal profiles measured for PyBr and Py2Mo indicate that the triplet pyrene group absorption at 420 nm involves a fast decay in the Py2Mo compound, in contrast to the profile measured for PyBr (Figure 4b). This result confirms the ET from the pyrene group of the imidazolium functionalized cation excited state to the molybdenum cluster anion. The deactivation of the pyrene group triplet state by the cluster core was confirmed by monitoring the decrease of the pyrene triplet state (420 nm) with sequential addition of small volumes of a stock solution of Cs2Mo to a solution of PyBr (see Figure S15 for temporal profiles).We propose two possible reaction paths in the energy transfer mechanism within the Py2Mo complex, by taking energy and the pyrene orbitals. 34, [START_REF] Pina | The effect of polyaromatic hydrocarbons on the spectral and photophysical properties of diaryl-pyrrole derivatives: an experimental and theoretical study[END_REF][START_REF] Takahashi | Donor-acceptor-structured 1,4-diazatriphenylene derivatives exhibiting thermally activated delayed fluorescence: design and synthesis, photophysical properties and OLED characteristics[END_REF][START_REF] Kurata | Isolation and Characterization of Persistent Radical Cation and Dication of 2,7-Bis(dianisylamino)pyrene[END_REF] In both paths, the increase of the cluster unit phosphorescence at 675 nm could be attributed to a combination of an ET and intersystem crossing (ISC) processes ensuring that the energy migrates from the pyrene group and the {Mo6I i 8} 4+ cluster core localized singlet excited states to the 3 MMCT state of the cluster unit. Upon excitation in the UV region, regarding to the first mechanism proposed (blue line, Scheme 2), the pyrene group S1 excited state transfers the absorbed energy to the molybdenum cluster S1 excited state, and the remaining energy is lost by fluorescence relaxation. The S1 cluster state goes forward to an intersystem crossing (ISC) to the T1 of the {Mo6I i 8} 4+ cluster core. In a second mechanism (red line, Scheme 2), the excited state of the pyrene group of the imidazolium functionalized counterion evolves to an ISC to its T1 triplet state, which transfers some energy to the emissive cluster-localized triplet state. We

R e v i s e d m a n u s c r i p t

17 previous studies realized on the [Mo6I i 8(OCO-pyrene) a 6] 2-complex for which, an intramolecular ET takes place quantitatively. [START_REF] Li | Graphene in Photocatalysis: A Review[END_REF] Additionally, taking into account the energy positions of T1(Py) (-3.1 eV) and T1(Mo6) (-3.4 eV) as well as the low lifetime detected by TAS, a triplet-triplet equilibrium can be discarded. Therefore, the reaction path referring to S1(Py) to T1(Py) ISC transition can be considered as less probable because of the low amount of pyrene triplet state formation.

Interactions between Py2Mo and graphene in Py2Mo@Gene.

The electronic interactions between Py2Mo and graphene, were evidenced by steady state and time resolved photoluminescence spectroscopies. In the steady state regime, adding aliquots of a single-and few-layer thickness graphene suspension in DMF to a DMF solution containing the Py2Mo complex (exc = 375 nm) leads to a progressive decrease of the pyrene and {Mo6I i 8} 4+ cluster core emission bands (see Figure S15a for emission spectra). This quenching of the Py2Mo photoluminescence characterizes a non-radiative ET from Py2Mo to the adjacent graphene surface, although the photoluminescence of graphene layers is negligible. [START_REF] Cao | Photoluminescence Properties of Graphene versus Other Carbon Nanomaterials[END_REF] The time resolved luminescence spectra (Figure S15b) confirm the ET from the pyrene localized excited states to the π system of graphene, as the pyrene group lifetime decreases upon increasing the graphene concentration. The emission decay profiles could be fitted with a double exponential function, which indicates a dynamic quenching of the pyrene excited states. Two lifetime components appear for each decay curve: one in the scale of pyrene lifetime and a faster one associated to the presence of graphene. The relative ponderation of both components evolves upon addition of graphene. In fact, the weight of the pyrene component changes from 32% after addition of 125 L of graphene to 14% with 850 L of graphene suspension. These observations are also consistent with an ET from Py2Mo to graphene. [START_REF] Economopoulos | Multichromophores Onto Graphene: Supramolecular Non-Covalent Approaches for Efficient Light Harvesting[END_REF] Moreover, steady-state and lifetime fluorescence R e v i s e d m a n u s c r i p t 18 quenching studies with graphene revealed deviation from linearity in the Stern-Volmer plots (Figure S16), indicating a collective effect of static and dynamic quenching. This could be explained considering the initial complex formation between the pyrene derivative compound and the graphene layers at the ground-state together with the collisional quenching of the remaining excited states of the pyrene units. The interaction between the Py2Mo complex and the graphene support in the Py2Mo@Gene nanomaterial was also studied by TAS. The absorption spectrum of the hybrid material (Figure S17) is dominated by a broad band absorption characteristic of graphene, which is attributed to the formation of delocalized electron and hole pairs along the nanostructure. [START_REF] De Miguel | Graphene as a Quencher of Electronic Excited States of Photochemical Probes[END_REF][100] The TAS of the Py2Mo@Gene hybrid exhibits the pyrene emission band with maximum at 400 nm and a small absorption band at 540 nm assigned to the triplet state of the molybdenum cluster. The low intensity of the molybdenum band and the short lifetimes (Figure 5) associated to the temporal profiles of the Py2Mo@Gene material confirm the quenching of the excited state of molybdenum cluster by the graphene support, and suggests that any energy transfer takes place also from the cluster complex to the graphene support. [START_REF] Economopoulos | Multichromophores Onto Graphene: Supramolecular Non-Covalent Approaches for Efficient Light Harvesting[END_REF] Therefore, this photoinduced charge transfer from the Py2Mo complex to the graphene layer produces a synergetic effect,

Photocatalytic HER studies.

A comparative photocatalytic activity study of the Py2Mo@Gene, Py2Mo and graphene materials for molecular hydrogen generation from water was studied in a water:acetone (50:45% v/v) mixture in the presence of triethylamine (TEA, 5% v/v) under UV-Vis irradiation. Irradiations were carried out, with continuous stirring, using a cylindrical Pyrex reactor and a fiber optic Xe light source (800-200 nm emission output, 1000 W/m 2 ). The evolution of photoreactions was monitored over time by means of gas chromatography. After 6 h of irradiation, dihydrogen was the only gas generated, and the performance of catalysts was evaluated in term of H2 yield. The H2 yields (mol of H2•gcat -1 ) with respect to time using the three photocatalysts are illustrated in Figure 6. The nanosized Py2Mo@Gene composite exhibits higher H2 production in the Figure 6. HER (µmol of H2•grcat -1 ) vs time plot by using Py2Mo@Gene (black line), Py2Mo (blue line), PyBr (green line) and PyBr-Gene (pink line) and graphene (red line) catalysts. Py2Mo, PyBr and PyBr-Gene used in equimolar amount as in Py2Mo@Gene. R e v i s e d m a n u s c r i p t 20 photocatalytic reaction than the Py2Mo and graphene counterparts, resulting in an increase of the photocatalytic activities by 280%. After 6 h of irradiation, the hydrogen formation rate by using Py2Mo@Gene, Py2Mo and graphene catalysts was 49, 18, and 5 mol of H2•gcat -1 •h -1 , respectively. The reaction rate of the hybrid nanomaterial is in the range of that reported for other MoS2/graphene catalytic systems in photochemical conditions (Table S2), although other complex systems are more efficient. The catalytic activity of Py2Mo@Gene is comparable to that reported for some TiO2-, CdS-and C3N4/graphene nanomaterials in the absence of Pt as cocatalyst.

The small amount of hydrogen generated by neat graphene can be ascribed to the production of active catalytic species by ageing the graphene materials under photochemical conditions. 105 The turnover number (TON) calculated with respect to the Py2Mo cluster for Py2Mo@Gene and Py2Mo samples were found to be 0.96 and 0.34, respectively. The turnover frequency (TOF) with respect to atomic molybdenum for Py2Mo@Gene corresponds to 7x10 -6 s -1 . This is of the same order than the TOF values we observed for the (TBA)2Mo6Br i 8@GO composite for photocatalytic HER in similar conditions, whereas the TOF for Py2Mo (3x10 -6 h -1 ) is two order of magnitude lower than that reported for the (TBA)2Mo6Br i 8F a 6 complex. [START_REF] Feliz | In situ generation of active molybdenum octahedral clusters for photocatalytic hydrogen production from water and their stabilization onto graphene oxide surfaces[END_REF] The reaction rates for the Py2Mo@Gene nanomaterial and for the Py2Mo complex are nearly constant, and no induction period appears, in contrast to the (TBA)2Mo6Br i 8F a 6 catalyst. This suggests that the integrity of the Py2Mo species is maintained under reaction conditions. After three reuse experiments of Py2Mo@Gene, the morphology of the nanomaterial remains intact (Figure S18), with 10% w/w molybdenum leaching after each experiment. Control experiments were carried out and show that Im2Mo/Gene, Cs2Mo/Gene and PyBr/Gene composites are less active than the Py2Mo@Gene hybrid, and that H2 production is attributed to the molybdenum clusters (see Figures 6 andS24). We detect a slight increase of the activity of Im2Mo/Gene with respect to Cs2Mo/Gene (see Figure S24), which could be attributed to a better contact of Im2Mo to the graphene surface promoted by the imidazolium counterions.

These results confirm that Py2Mo acts as both photosensitizer and catalyst for hydrogen production under UV-Vis irradiation, similarly to the (TBA)2Mo6Br i 8F a 6 photocatalyst. The enhancement of reaction yields provided by the graphene-supported cluster is due to the intimate contact between the Py2Mo catalyst and the graphene, and to the superior conductivity of graphene, since it acts as an efficient electron acceptor and transporter between the immobilized cluster active species (Figure 7). This is in agreement with reported works on dye-sensitized graphene-based photocatalyts. [START_REF] Huo | Mini-review on an engineering approach towards the selection of transition metal complex-based catalysts for photocatalytic H2 production[END_REF]106

CONCLUSIONS

We show in this work that red NIR emissive octahedral transition metal clusters can be easily associated to graphene nanosheets using non-covalent interactions. To do so, organic linkers made of an imidazolium head and either a long alkyl chain or a pyrene group were specifically designed to interact with the anionic cluster and the graphene surface simultaneously. Adsorption studies demonstrated that the pyrene containing cation was the most efficient one to bind the anionic cluster onto the graphene surface. The components association was demonstrated by several techniques such as HR-TEM, EDX analyses, FT-IR and Raman spectroscopies. Photophysical studies of the Py2Mo hybrid complex in solution evidenced a partial energy transfer between the pyrene containing cations and the anionic cluster unit. Taking into account the low amount of pyrene triplet state produced during light excitation, we suspect that this energy transfer is more likely to involve the S1(Py) to S1(Mo) transition followed by an ISC to the T1(Mo6) state than a transfer from the T1(Py) to the T1(Mo6) states.

Once supported onto graphene, Py2Mo transfers its energy to the graphene surface which in turns conducts electrons to the catalytic active sites for hydrogen generation. Photocatalytic HER production in water studies have demonstrated the potential of such hybrids in the generation of hydrogen. In fact, the H2 production efficiency of Py2Mo increases by a factor of 2.8 once grafted on graphene surface. The improvement of the H2 production activity is attributed to the synergetic effect between graphene and the hybrid cluster complex, since graphene facilitates the charge separation activity and enhances the energy and electron transfer of the cluster photocatalyst. In addition, the extended aromatic lattice and, in consequence, the excellent electronic conduction of graphene sheets is preserved during the grafting of Py2Mo, thanks to the use of non-covalent interactions to maintain the assembly. Hence, this supramolecular strategy opens wide perspectives

EXPERIMENTAL SECTION

Reagents and solvents. Graphite, triethylamine (TEA), diethyelether, methanol, acetone (ACS analytical grade), dichloromethane (Chromasolv for HPLC), and anhydrous DMF were obtained from commercial resources (Sigma-Aldrich and Scharlau). Graphene nanoplatelet aggregates (surface area 500 m 2 g - 1 ) were purchased from Strem. The Cs2Mo6I i 8(O2CC2F5) a 6 (Cs2Mo) precursor are prepared by following reported procedures. [START_REF] Amela-Cortes | Versatility of the ionic assembling method to design highly luminescent PMMA nanocomposites containing [M6Qi8La6]n-octahedral nano-building blocks[END_REF] The (1-methyl-3-(4-(pyren-1-yl)butyl)-1H-imidazol-3-ium bromide) PyBr salt has been prepared in a two-step reaction from 2-pyrenebutanol, 107 as described in the literature. 108 Dichloromethane and diethylether were dried and deoxygenated by passing these solvents through commercial columns of CuO followed by alumina under nitrogen atmosphere.

Synthesis of the materials.

Preparation of the Py2Mo complex: to an acetone solution of Cs2Mo, was added a solution of PyBr in methanol under argon. The mixture was magnetically stirred for 48 h in the dark and then was filtered through a Celite® pad. The orange solution was then evaporated to yield a red-orange powder. Preparation of the Py2Mo@Gene nanocomposite: an excess of Py2Mo (22 mg, 0.007 mmol) was added to a graphene suspension (15 mg in 60 mL of dried dichloromethane) in a Schlenk flask under argon. The mixture was sonicated with an ultrasound source (400 W, Branson ultrasonic bath) for 1 h, and magnetically stirred for 24 h. The solid product was separated from the solution by filtration under vacuum, was washed several times with dichloromethane and diethylether, and dried under vacuum to provide 25 mg of a black product identified as Py2Mo@Gene nanocomposite. This material was stored in a desiccator. The amount of molybdenum present in

1 .

 1 Scheme 1. Schematic representation of cluster compound precursor and organic imidazolium salts

Figure 1 .

 1 Figure 1. a) FT-IR (KBr) spectra of Py2Mo@Gene (red line), Py2Mo (blue line) and graphene (black

  cluster compound (see PyBr absorption spectrum in Figure S6 for comparison). The emission spectrum of Py2Mo in deaerated solution is presented in Figure 2. Upon excitation (exc = 310 -375 nm), two emission bands appear in the red and blue regions of the spectrum. The vibrational structure of the pyrene observed in the steady state fluorescence spectrum appears as a mirror image of the absorption spectrum. This fluorescence takes place from the S1 state, and is due to a R e v i s e d m a n u s c r i p t 11 rapid internal conversion (IC) from the S2 to the S1 states upon excitation into the most intense pyrene absorption band centered at 345 nm.

Figure 2 .

 2 Figure 2. Absorption (black line) and emission (red line) spectra of Py2Mo in deaerated solution

  that the ET from the pyrene containing cation to the cluster core is not quantitative. Such non-quantitative ET were also recently evidenced in solution for assemblies gathering blue emissive triphenylene containing imidazolium cations and Red NIR emissive

PyBr.

  The rise of the emission of Cs2Mo in the presence of PyBr is again in favor of a pyrene-tocluster ET. To further characterize this partial ET, time dependent emission and absorption properties were investigated by time correlated luminescence spectroscopy and transient absorption (TA) spectroscopy.

Figure 3 .

 3 Figure 3. a) Evolution of Py2Mo emission maxima at 375 nm and 675 nm upon excitation; b) Emission

  sensitive to O2 indicating their triplet state nature, as previously shown by steady state luminescence experiments (Figures

Figure 4 .

 4 Figure 4. a) TAS (5µs after pulse) and b) temporal profiles of Py2Mo (red) and PyBr (black)

  orbitals, calculated from the absorption and redox data of [Mo6I i 8(OCOC2F5) a 6] 2-(see Supporting Information, section IV for details about calculations),

2 .

 2 Scheme 2. Energy diagram of the possible reaction paths passed on the photophysical properties

Figure 5 .

 5 Figure 5. Absorption temporal profiles of Py2Mo@Gene (0.5 mg/mL, black line) and of Py2Mo

  separation process after irradiation and enhancing the photocatalytic conversion as we describe below.

Figure 7 .

 7 Figure 7. Representation of the photoexcited electron transfer and H2 evolution over Py2Mo@Gene

  research prospects to design most efficient anionic clusters/graphene hybrids for hydrogen production.

  1 H-NMR (400 MHz, acetone-d6): δ (ppm) = 1.96 (m, 2H, -CH2-), 1.77 (m, 2H, -CH2-), 3.81 (s, 3H, CH3-N), 4.25 (t, 2H, J=7,1Hz, -CH2-N), 7.67 (t, 1H, J=1,7Hz, -N-CH-CH-N-), 7.77 (t, 1H, J=1,7Hz, -N-CH-CH-N-), 8.36-7.95 (m, 9H, CAr), 9.08 (s, 1H, CH, -N-CH-N-); 19 F-NMR (376 MHz, acetone-d6): δ (ppm) = -83 (3F), -120 (2F); ESI-MS (CH3CN): m/z: 1284.3 [M(2-)], 339.5 [M+] R e v i s e d m a n u s c r i p t 24 Synthesis of 1-methyl-3-nonyl-imidazolium bromide (ImBr): 1-N-methylimidazol (1.03gr, 12.5mmol) and 1-Bromononane (3.33g, 15 mmol) were mixed with 50 ml of CHCl3. The mixture was stirred under reflux for 15 hours. After evaporation of solvent under vacuum, the obtained colorless oil is washed three times with 50 ml of a cyclohexane:ethyl actetate mixture (1:1). The oil was recovered via a separating funnel, dried under vacuum to yield the desired compound (yield: 67%). 1H-NMR (400 MHz, acetone-d6): δ (ppm) = 0.79 (t, 3H, CH3), 1.25-1.16 (m, 14H, (CH2)7), 1.84 (m, 2H, -CH2-CH2-N), 4.10 (s, 3H, CH3-N), 4.25 (t, 2H, -CH2-N), 7.45 (1H, s, -N-CH-CH-N-), 7.65 (1H, s, -N-CH-CH-N-), 10.24 (1H, s, -N-CH-N-). ESI-MS (CH3CN): 223.4 [M+]. Preparation of the Im2Mo complex: to an acetone solution of Cs2Mo was added a solution of ImBr in methanol under argon. The mixture was magnetically stirred for 48 h in the dark and then was filtered through a Celite® pad. The orange solution was then evaporated to yield a red-orange powder. 1 H-NMR (400 MHz, acetone-d6):  (ppm) = 0.88 (t, 3H, CH3), 1.37-1.28 (m, 14H, (CH2)7), 1.98 (m, 2H, -CH2-CH2-N), 4.11 (s, 3H, CH3-N), 4.41 (t, 2H, -CH2-N), 7.77 (1H, s, -N-CH-CH-N-), 7.83 (1H, s, -N-CH-CH-N-), 9.15 (1H, s, -N-CH-N-); 19 F-NMR (acetone-d6): δ (ppm) = -83 (3F), -120 (2F); ESI-MS (CH3CN): m/z: 1284.3 [M(2-)]. 223.4 [M+].

  1.74 % w/w) has been determined by ICP analysis of the liquid phase after treatment of the solid with aqua regia. This material has been characterized by XRD, HR-TEM, STEM/EDS, IR, and Raman techniques.

3 ( 9 )

 39 , G.; Costuas, K.; Tarasenko, M. S.; Ledneva, A. Y.; Naumov, N. G.; Lemoine, P.; Guizouarn, T.; Molard, Y.; Amela-Cortes, M.; Audebrand, N.; Cordier, S., Stabilization of Ni2+ dimers in hexacyano Mo6 cluster-based Prussian blue derivatives: experimental and theoretical investigations of magnetic properties. Dalton Trans. 2018, 47 (4), 1122-1130. 11. Garreau, A.; Massuyeau, F.; Cordier, S.; Molard, Y.; Gautron, E.; Bertoncini, P.; Faulques, E.; Wery, J.; Humbert, B.; Bulou, A.; Duvail, J.-L., Color Control in Coaxial Two-Luminophore Nanowires. ACS Nano 2013, 7 (4), 2977-2987. 12. Zhao, Y.; Lunt, R. R., Transparent Luminescent Solar Concentrators for Large-Area Solar Windows Enabled by Massive Stokes-Shift Nanocluster Phosphors. Adv. Energy Mater. 2013, , 1143-1148. 13. Amela-Cortes, M.; Garreau, A.; Cordier, S.; Faulques, E.; Duvail, J.-L.; Molard, Y., Deep red luminescent hybrid copolymer materials with high transition metal cluster content. J. Mater. Chem. C 2014, 2 (8), 1545-1552. 14. Prevot, M.; Amela-Cortes, M.; Manna, S. K.; Lefort, R.; Cordier, S.; Folliot, H.; Dupont, L.; Molard, Y., Design and Integration in Electro-Optic Devices of Highly Efficient and Robust OCOCnF2n+1)6]2-(n = 1, 2, 3) Nanoclusters. Adv. Funct. Mater. 2015, 25 (31), 4966-4975. 15. Robin, M.; Kuai, W.; Amela-Cortes, M.; Cordier, S.; Molard, Y.; Mohammed-Brahim, T.; Jacques, E.; Harnois, M., Epoxy Based Ink as Versatile Material for Inkjet-Printed Devices. ACS Appl. Mater. Interfaces 2015, 7 (39), 21975-21984.

  , K.; Tomokiyo, K.; Kuo, M. T.; Ishikawa, T.; Suzuki, M., Molecular design of glutathione-derived biochemical probes targeting the GS-X pump. Tetrahedron 1999, 55 (24), 7529-7540. 108. Wittmann, S.; Schätz, A.; Grass, R. N.; Stark, W. J.; Reiser, O., A Recyclable Nanoparticle-Supported Palladium Catalyst for the Hydroxycarbonylation of Aryl Halides in Water. Angew. Chem. Int. Ed. 2010, 49 (10), 1867-1870. Octahedral molybdenum clusters are red NIR phosphorescent emitters. [Mo6I8(OCOC2F5)6] 2-is one of the most emissive metal cluster, but demonstrates a low photocatalytic H2 production from water splitting. Its anchoring onto graphene using pyrene containing organic cations as supramolecular linkers, enhances the photocatalytic activity of the nanocomposite by 280 %.

Kumar, P.; Kumar, S.; Cordier, S.; Paofai, S.; Boukherroub, R.; Jain, S. L., Photoreduction of CO2 to methanol with hexanuclear molybdenum [Mo6Br14]2-cluster units under visible light irradiation. RSC Advances 2014, 4 (20), 10420-10423.

ACKNOWLEDGMENT

We would like to thank Dr. Marta Puche for her technical assistance with the preparation of dispersed graphene solutions at the ITQ.

Funding Sources

The Severo Ochoa Program (SEV-2016-0683), Ministerio de Ciencia, Innovación y Universidades (PGC2018-099744), Consejo Superior de Investigaciones Científicas (I-Link1063) and University of Rennes.

ASSOCIATED CONTENT

Supporting Information. Characterization techniques, detailed X-ray structural analysis of Py2Mo, ESI mass spectra, adsorption experiments, HR-TEM micrographs, STEM/EDS analysis of Py2Mo@Gene, photophysics, laser flash photolysis experiments, calculations of Py2Mo singlet and triplet excited state, and photocatalyic hydrogen production conditions. This material is available free of charge.

Accession

Notes

The authors declare no competing financial interests.


A Merdrignac

D Bergeat

F Robin

E Gaignard

K Turner

M Rayar

B Meunier

K Boudjema

L Sulpice

Laurent Pr

Sulpice

Impact of prior interventional treatment on the complications after Frey procedure for chronic pancreatitis

Keywords: chronic pancreatitis, pancreatic surgery, Frey procedure Study conception: AM and LS. Acquisition of data: AM, FR, EG and KT

Purpose: Chronic pancreatitis is an inflammatory disease responsible for pain partially explained by pancreatic duct dilatation. Early surgery has become the treatment of choice for hypertrophic pancreatic head with main pancreatic duct dilatation. Frey procedure (FP), combining both surgical resection and decompression, is one of the standard surgical procedures. However, a "step-up approach" with endoscopic or limited surgical procedures is still frequently proposed before referring to expert pancreatic centers. The aim of the study was to evaluate the impact of a prior treatment on postoperative complications of FP.

Methods: All 61 consecutive patients who underwent FP between 2006 and 2017 were included. Perioperative data and outcomes were analyzed and compared according to the presence of a prior treatment.

Results: Twenty-four patients did not receive any prior treatment and thirty-seven patients had a prior endoscopic or limited surgical treatment. Preoperative data and outcomes were similar between the 2 groups. The rate of biliary derivation during FP was significantly higher in the group without prior endoscopic procedure. A prior treatment was not a risk factor for major morbidity (Clavien grade ≥III).

Conclusions:

A first attempt of endoscopic or limited surgical procedures before FP may not influence postoperative complications. Even if not recommended, a "step-up approach" proposing a first less invasive treatment could still be proposed to the patients who want to delay a morbid surgical procedure.

Introduction

Chronic pancreatitis (CP) is characterized by pancreatic parenchymal lesions progressing from inflammation to fibrosis and calcifications. Pain is the main resulting symptom and its management is a therapeutic challenge [START_REF] Braganza | Chronic pancreatitis[END_REF]. Although the origin of pain is not clearly known and certainly multifactorial, an enlargement of main pancreatic duct due to obstruction by strictures or stones is generally observed. The aim of both endoscopic and surgical procedures is to decrease pressure in the main pancreatic duct. Endoscopic treatment involves sphincterotomy, dilatation of strictures by temporary stent, removal of stones and extracorporeal shock wave lithotripsy. Complications of endoscopic treatment are post endoscopic retrograde cholangiopancreatography (ERCP) pancreatitis (3.5%), infection (1.4%), hemorrhage (1.3%) and perforation (0.6%) [START_REF] Andriulli | Incidence Rates of Post-ERCP Complications: A Systematic Survey of Prospective Studies[END_REF]. Post ERCP complications lead to a death rate of less than 1% mostly due to severe pancreatitis. Surgical techniques consist in drainage of main pancreatic duct, resection or association of both. In cases of hypertrophic pancreatic head, surgical treatment mostly consists in hybrid procedures associating main pancreatic duct decompression and duodenum-preserving pancreatic head resection such as the Frey procedure (FP). Several previously published studies have demonstrated that FP offers an effective pain relief and is a safe surgical technique in the management of CP [START_REF] Zhou | Frey procedure for chronic pancreatitis: Evidencebased assessment of short-and long-term results in comparison to pancreatoduodenectomy and Beger procedure: A meta-analysis[END_REF]. Frey procedure is associated with 25% rate of postoperative morbidity and less than 1% rate of mortality [START_REF] Zhou | Frey procedure for chronic pancreatitis: Evidencebased assessment of short-and long-term results in comparison to pancreatoduodenectomy and Beger procedure: A meta-analysis[END_REF][START_REF] Suzumura | Short-and long-term outcomes of the Frey procedure for chronic pancreatitis: a single-center experience and summary of outcomes in Japan[END_REF]. The most frequent specific complication after FP are postoperative bleeding (10-20%) and pancreatic fistula (8-10%) [START_REF] Zhou | Frey procedure for chronic pancreatitis: Evidencebased assessment of short-and long-term results in comparison to pancreatoduodenectomy and Beger procedure: A meta-analysis[END_REF][START_REF] Ali | Endoscopic or surgical intervention for painful obstructive chronic pancreatitis[END_REF].

For patients with obstructive CP and dilated pancreatic duct, surgery is superior to endoscopy in terms of pain relief [START_REF] Strobel | Surgical therapy of chronic pancreatitis: Indications, techniques and results[END_REF][START_REF] Yang | Systematic Review of Early Surgery for Chronic Pancreatitis: Impact on Pain, Pancreatic Function, and Re-intervention[END_REF]. In a Cochrane review, morbidity and mortality seem not to differ between the two intervention modalities, but the power of the trials was not sufficient to detect the small differences expected in this outcome [START_REF] Strobel | Surgical therapy of chronic pancreatitis: Indications, techniques and results[END_REF]. Surgical intervention at an early stage of CP is a promising approach in terms of pain relief and pancreatic function.

Early surgery has become the strategy of choice for expert pancreatic centers [START_REF] Yang | Surgery for Chronic Pancreatitis: The Role of Early Surgery in Pain Management[END_REF][START_REF] Ke | Earlier surgery improves outcomes from painful chronic pancreatitis[END_REF][START_REF] Ali | Early surgery versus optimal current stepup practice for chronic pancreatitis (ESCAPE): design and rationale of a randomized trial[END_REF]. A A c c e p t e d M a n u s c r i p t randomized controlled trial has been conducted to compare early surgery vs endoscopy but results have not been published yet (ESCAPE trial ISRCTN45877994) [START_REF] Ali | Early surgery versus optimal current stepup practice for chronic pancreatitis (ESCAPE): design and rationale of a randomized trial[END_REF]. The development of central pain (sensitization) is a concern when surgical intervention is delayed [START_REF] Anderson | Mechanism, Assessment and Management of Pain in Chronic Pancreatitis: Recommendations of a Multidisciplinary Study Group[END_REF]. A delayed surgical intervention will not be beneficial in terms of pain-relief. Negi et al. conclude that patients should be referred for surgery before opiates are needed to relieve pain [START_REF] Negi | Pain relief after Frey's procedure for chronic pancreatitis[END_REF]. These results are confirmed by Ahmed Ali et al. who recommend surgery within 3 years of symptomatic CP [START_REF] Ali | Clinical Outcome in Relation to Timing of Surgery in Chronic Pancreatitis: A Nomogram to Predict Pain Relief[END_REF]. In patients with an enlarged pancreatic head, a recent international survey of pancreatologists has shown that 58% preferred initial surgery (pylorus preserving pancreaticoduodenectomy) versus 42% initial endoscopy [START_REF] Issa | Diagnosis and treatment in chronic pancreatitis: an international survey and case vignette study[END_REF].

However, currently, some patients are still referred to expert pancreatic centers after a first "step up approach". The impact of a prior endoscopic treatment on postoperative complications is not studied and known. Moreover, some patients do not present initial cephalic pancreatic complications of chronic pancreatitis and require first distal, decompressive or urgent pancreatic surgical procedures. The aim of the study was to evaluate the impact of a prior endoscopic or surgical treatment on the postoperative complications of Frey procedure for painful CP with hypertrophic pancreatic head.

A c c e p t e d M a n u s c r i p t

Patients and methods

Patients

A database of all consecutive patients who underwent the FP in a tertiary centre has been created and maintained prospectively since 2006 after institutional approval and informed consent. Data were collected regarding demographics, preoperative procedures (endoscopic, surgical, radiologic), surgical variables, length of hospital stay, postoperative morbidity, and follow-up. All patients operated between 2006 and 2017 were included in the analyses. No patient was excluded of the analysis. Diagnosis of CP was based on the clinical (clinical history, physical examination) and imaging findings (computed tomographic scan, magnetic resonance cholangiopancreatography, endoscopic ultrasound). Indication for surgery was systematically determined during a multidisciplinary meeting including surgeons, gastroenterologists and radiologists. Patients were divided into two groups according to the presence of a prior endoscopic procedure (attempts of pancreatic and/or biliary catheterism) or pancreatic surgery for CP.

Surgical procedure

FP was performed by one senior pancreatic surgeon (LS) according to the technique first described in 1987 by Frey [START_REF] Frey | Description and rationale of a new operation for chronic pancreatitis[END_REF]. The need for biliary diversion was determined preoperatively or perioperatively when common bile duct (CBD) stricture persisted as observed by perioperative cholangiography. The type of biliary diversion chosen in case of CBD stricture depended on the accessibility of the CBD in the resection cavity and was left to the discretion of the surgeon.

The technique of CBD reinsertion combined a 1-cm longitudinal cut of the distal CBD in the pancreatic head resection cavity and an anastomosis to pancreatic parenchyma using 5-0 polydioxanone interrupted sutures. A Roux-en-Y choledochojejunostomy was performed if bilioenteric anastomosis was chosen. The same Roux-en-Y jejunal loop was used for the bilioenteric anastomosis and the pancreaticojejunal anastomosis. The distal extremity of the jejunal loop was first anastomosed with the pancreas.

A c c e p t e d M a n u s c r i p t

Definitions

The postoperative morbidity was defined as any complication that occurred within 90 days after surgery. The complications were categorized according to the Dindo and Clavien classification [START_REF] Dindo | Classification of surgical complications: a new proposal with evaluation in a cohort of 6336 patients and results of a survey[END_REF]. Major complications were categorized as grades III to V in the Dindo and Clavien classification. Postoperative pancreatic fistula was classified into three groups according to the criteria given by the International Study Group of Pancreatic Fistula [START_REF] Bassi | Postoperative pancreatic fistula: an international study group (ISGPF) definition[END_REF].

Postoperative delayed gastric emptying was classified according to the criteria given by the International Study Group of Pancreatic Surgery (ISGPS) but only grade B and C were used for this study as our centre policy was to maintain the nasogastric tube until at least postoperative day 5 [START_REF] Wente | Delayed gastric emptying (DGE) after pancreatic surgery: a suggested definition by the International Study Group of Pancreatic Surgery (ISGPS)[END_REF]. Postpancreatectomy haemorrhage including intra-and extraluminal bleeding was classified according to the ISGPS definition [START_REF] Wente | Postpancreatectomy hemorrhage (PPH): an International Study Group of Pancreatic Surgery (ISGPS) definition[END_REF]. Biliary fistula was defined as the appearance of bile in peritoneal drains. Postoperative pain was defined as controlled if no analgesic was necessary or only level 1 analgesic according to the World Health Organization pain ladder (neither opioid nor weak opioid).

Statistical analysis

Qualitative variables are expressed as numbers with percentages and were compared using Fisher exact test. Quantitative variables are expressed as mean values ± the standard deviation (SD) and were compared using Mann-Whitney U test. p-values less than 0.05 were considered statistically significant. Analyses were performed with R statistical software (available: http://www.r-project.org/).

A c c e p t e d M a n u s c r i p t

Results

Sixty-one patients were operated for a FP between 2006 and 2017. Patients were divided in two groups: 37 patients with a prior endoscopic or pancreatic surgical procedure (60.7%) and 24 patients without prior interventional treatment for CP (39.3%). Nine patients underwent a prior pancreatic surgery: 3 cystogastric anastomosis, 3 distal pancreatectomies, 2 pancreaticojejunostomies, and 1 pancreatic necrosectomy. Twenty eight patients have had one or more prior endoscopic procedure: 5 pancreatic stenting, 2 biliary stenting, 3 cystogastric drainages and 21 attempts for endoscopic pancreatic drainage. The distribution of patients is presented in figure 1. Preoperative data were comparable between the two groups (table 1). The majority were male with CP due to alcohol abuse.

Postoperative data are presented in table 2. Operation time and hospitalization duration were similar. The rate of biliary derivation was significantly higher in the group without prior endoscopic procedure. The rate of major complication (Clavien ≥III) was not different including pancreatic fistula and hemorrhage.

Mean follow-up was 24.6±21.4 and 33±30.3 months in the group with and without prior endoscopic or surgical procedure respectively (p=0.3). Data on pain control at 2 years were available for 43 patients: 24 and 19 patients in the group with and without prior procedure respectively. Pain control was obtained for 95.8% and 84.2% patients (p=0.3) in the group with and without prior procedure respectively. Data on diabetes mellitus at 2 years were available for 46 patients: 30 and 16 patients in the group with and without prior procedure respectively. Diabetes mellitus was present in 46.6% and 50% (p=1) in the group with and without prior procedure respectively.

Univariate analysis for identification of risk factors for complications with Clavien grade ≥III is presented in table 3. Multivariate analysis was not realized because no risk factor was identified in univariate analysis.

A c c e p t e d M a n u s c r i p t

Discussion

The treatment of painful CP with hypertrophic pancreatic head could involve both endoscopic and surgical procedures. However, current recommendations indicate that the timing of surgical intervention is an important factor in clinical outcomes.

The "step-up approach" consists with a less invasive endoscopic treatment as first line therapy followed by surgical procedure in case of failure. Prolonged and repeated pancreatic stenting has been identified as a risk factor for failure after subsequent rescue surgery [START_REF] Ali | Clinical Outcome in Relation to Timing of Surgery in Chronic Pancreatitis: A Nomogram to Predict Pain Relief[END_REF].

Moreover, the Dutch pancreatitis Study Group has argued against the "step-up approach" on the basis that opioids, the mainstay of conservative treatment, do not alter disease progression and that reliance on them delays intervention [START_REF] Issa | Surgical and Endoscopic Treatment of Pain in Chronic Pancreatitis: A Multidisciplinary Update[END_REF].

Nevertheless, a first less invasive approach using endoscopic procedure is still frequently proposed to patients before referring to expert pancreatic centers or if patients refused surgery. The present study shows similar rate of postoperative complications after FP between the patients who had a prior endoscopic or surgical treatment and those who were free of prior interventional treatment. One can hypothesize that specific complications of endoscopic procedures could influence surgical outcomes. Post-ERCP pancreatitis leads to local inflammation that could increase morbidity of pancreatic surgery. Chronic pancreatitis may be a protector factor for post-ERCP pancreatitis [START_REF] Freeman | Pancreatic Stents for Prevention of Post-Endoscopic Retrograde Cholangiopancreatography Pancreatitis[END_REF]. Conversely, difficult or failed cannulation is a risk factor for post-ERCP pancreatitis. The majority of patients with a prior endoscopic procedure in this study underwent a failure of pancreatic drainage. The similar rates of surgical morbidity in the 2 groups of this study are in accordance with the protecting effect of CP on post-ERCP pancreatitis. Moreover, a prior endoscopic or surgical treatment is not a risk factor for complications with Clavien grade ≥III in the present cohort.

In the group of patients with a prior endoscopic or surgical procedure, the need for a biliary diversion during FP was significantly lower. Among these patients, two had a biliary stenting that could have avoided the need for a biliary derivation during surgery and explain the statistical difference between the 2 groups.

A c c e p t e d M a n u s c r i p t

The main limits of the present study are the small number of patients and the long study period in a monocentric cohort. However, the majority of published cohorts do not exceed fifty patients. Pancreatic surgery is more frequently indicated for pancreatic neoplasms than for CP in Western Europe. This fact explains the difficulty to identify risk factors for complications after FP. Follow up of patients with CP is often difficult and leads to biases due to missing data.

Conclusions

The management of painful CP with enlarged pancreatic head involves both endoscopic and surgical procedures after failure of medical treatment. Several studies have shown benefit for early surgery on pain relief. The present study shows that a first attempt of endoscopic procedure before surgery may not influence postoperative complications. A preoperative biliary stenting could avoid the need for a surgical biliary derivation combined to the FP. A "step-up approach" could not be recommended but, in cases of patients referred after prior interventional treatment, patients could underwent Frey procedure without excess risk of postoperative complications.

Compliance with Ethical Standards:

Disclosure of potential conflicts of interest: The authors declare that they have no conflict of interest.

Research involving Human

  Participants: All procedures performed in the study involving human participants were in accordance with the ethical standards of the institutional research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. Informed consent: Informed consent was obtained from all individual participants included in the study A c c e p t e d M a n u s c r i p t A c c e p t e d M a n u s c r i p t

A c c e p t e d M a n u s c r i p t

Figure 1 :

 1 Figure 1: Distribution of all patients according to prior treatment for chronic pancreatitis.

Table 1 :

 1 Preoperative data of all patients divided in two groups according to the presence of a prior endoscopic or surgical procedure for chronic pancreatitis (CP). The number of patients presenting with pancreatic fistula, delayed gastric emptying, or hemorrhage is equal to the sum of all grades to permit statistical analyses. SD: standard derivation.

		Prior surgical
	Total	and/or endoscopic	None prior procedure
		procedure
	n=61	n=37	n=24	p

Table 2 :

 2 Postoperative data of all patients divided in two groups according to the presence of a prior endoscopic or surgical procedure for chronic pancreatitis.

	M a n u s c r i p t
	A c c e p t e d

Table 3 :

 3 Univariate analysis for identification of risk factors for complications with Clavien grade ≥III

	14

A c c e p t e d M a n u s c r i p t

Legends Table 1: Preoperative data of all patients divided in two groups according to the presence of a prior endoscopic or surgical procedure for chronic pancreatitis.

Table 2: Postoperative data of all patients divided in two groups according to the presence of a prior endoscopic or surgical procedure for chronic pancreatitis.


Adrien Lemoine

Valeria Martinez

Francis Bonnet

Pain measurement and critical review of analgesic trials [pain scores, functional pain measurements, limits and bias of clinical trials]

Keywords: postoperative pain, pain scores, functional pain measurement

Randomized clinical trials designed to assess analgesic agents and/or techniques used for postoperative pain control, have several limitations, which are addressed in this article. Efficacy of analgesics cannot be limited to the evaluation of pain intensity or the amount of opioid rescue medication, but also means to evaluate parameters such as the delay and duration of the effect, the number of patients with satisfactory pain control, and side effects. Since combination of analgesics is the standard of care in clinical practice, its value also needs to be

documented. Eventually, analgesic treatments have to be considered in the settings of postoperative supportive care and enhanced recovery programmes after surgery.

Practice points

 A surgical procedure-specific, evidence-based, evaluation of postoperative pain treatments is better than a global approach.

 Besides the effect on pain intensity and opioid consumption, the efficacy of pain treatments should be assessed on other parameters such as the number of patients with satisfactory analgesia.

 A clinical approach of analgesic treatments should also consider the balance between efficacy and side effects  Postoperative analgesic protocols should be designed and applied, in the setting of enhanced recovery after surgery.

Research agenda

 New analgesic treatments should be compared to the standard of care for the surgical procedure considered.

 Combination of analgesics agents should be evaluated as well as single agents.

 The impact of postoperative pain treatment on the occurrence f chronic pain after surgery should be more commonly evaluated

Introduction

The European Medical Agency has made recommendation concerning the design of clinical studies aiming to evaluate new analgesic agents or techniques for the treatment of postoperative pain [START_REF]Guideline on the clinical development of medicinal products intended for the treatment of pain[END_REF]. Clinical trials must be prospective randomized, with parallel groups. Three arms at least need to be considered: the placebo group, the group of patient receiving the standard of care, and the group receiving the new treatment. In addition, it is necessary to plan separately trials on visceral surgery, on one hand, and orthopaedic surgery, on the other hand. Repeated administrations, corresponding to clinical practice, need to be evaluated and eventually the new treatments need to be assessed in specific cohort of patients (older patients, patients with co-morbidities, etc.). Evaluation of a new analgesic treatment also means not only quantifying the decrease in pain intensity but also the delay and the duration of its effect and the global satisfaction of the patient. Moreover to determine the use of a new analgesic treatment it is necessary to evaluate its efficacy but also its tolerance that depends on the occurrence of side effects more or less bothersome or even life threatening. In other words the clinical profile of an analgesic treatment is made off directs effects on pain intensity, rescue medication consumption, side effects and complications, and indirect effects such as accelerated recovery after surgery, decreased postoperative morbidity and shortening of hospital stay.

Randomized prospective controlled trials (RCTs)

RCTs' are, in fact, commonly designed with two arms: a control group and a treatment group. In both groups patients self-administer morphine or another opioid as a rescue, and pain intensity is measured at rest and on mobilisation with a visual analogue scale (VAS) or an equivalent method (simple numerical rating scale, verbal scale, etc.). The result of a study is considered positive when morphine consumption is significantly decreased in the treated group, and that is usually considered enough to draw conclusion provided satisfactory pain control is achieved (i.e. VAS scores < 30). Indeed, comparable levels of analgesia should be theoretically achieved in the two groups but with different opioid consumptions (lower in the treatment group). Actually, side effects of opioids prevent from using high doses in the control groups making, it possible a difference in pain scores between the control and the treated groups while the «true» difference in opioid consumption is underestimated. When a new analgesic treatment or technique is evaluated, small size RCTs are usually designed initially so that positive results need to be confirmed in large sized assays, and negative results are inconclusive because of the lack of statistical power.

The comparison of pain intensity scores is commonly based on comparison of mean values at several pre-set time-points. Other measurements have been developed to quantify the effect of analgesic agents, such as pain intensity difference. This parameter can be measured at each time point, and the sum of pain intensity differences (SPID) can also be calculated. [START_REF] Farrar | Defining the clinically important difference in pain outcome measures[END_REF] Although measurement of these parameters can be adapted to statistical evaluation, it is far from clinical relevance.

It's also true for calculation and comparison of area under curves of pain intensity over time.

Other issues need to be addressed such as the difference between statistical and clinical differences in pain intensity. A clinical difference means that the patient perceives that pain intensity has decreased. It has been evaluated that this feeling corresponds to at least a 10 to 18 mm change in the VAS score (0-100) corresponding to about 20% change in pain intensity. [START_REF] Todd | The minimum clinically important difference in physicianassigned visual analog pain scores[END_REF][START_REF] Myles | Measuring acute postoperative pain using the visual analog scale: the minimal clinically important difference and patient acceptable symptom state[END_REF] The threshold value that documents statistical significant difference, depends on the sample size, meaning that in large sized clinical trials there are more chances to achieve a threshold of significance, with a small absolute difference in pain intensity, below the threshold of clinical perception of the difference in pain scores. The rational is the same when one considers the decrease in opioid consumption. For example, paracetamol obtains most of the time a less than 10 mg decrease in morphine consumption over 24 hours and hundred of patients are required to make this difference significant [START_REF] Doleman | Preventive Acetaminophen Reduces Postoperative Opioid Consumption, Vomiting, and Pain Scores After Surgery: Systematic Review and Meta-Analysis[END_REF].

On the other hand due to a 30-50% decrease in morphine consumption, NSAIDs are easily demonstrated to be effective. ( 6) However, from a clinical point of view a decrease in morphine consumption makes sense only if it is associated with a decrease in the incidence of side effects such as nausea, vomiting, urinary retention, pruritus, or prolongation of postoperative ileus.

Another point is the absolute value of pain score corresponding to a pain free patient. It is commonly considered as close to 30mm (and not zero!). ( 4) However patients may experience satisfactory pain control at rest while remaining in pain with high VAS scores on mobilization.

The last point refers to the treatment applied in the control group. To make the difference in pain scores more dramatic, authors of clinical trials tend to minimize the analgesic treatment in the control group that is sometimes limited exclusively to rescue medication (morphine Patient Controlled Analagesia). However in real life patients are treated even if these treatments are considered less than ideal justifying other investigations to improve pain control. It is therefore important to consider that the patients need to benefit to the standard of care for a given surgical procedure to make a valuable comparison with a new treatment susceptible to be given to patient to improve their comfort.

When considering unmet needs in postoperative pain treatment, RCTs suffer from several flaws and limitations. Most of them are small size clinical trials as mentioned before. In addition, studies concerning the same agent or technique are heterogeneous because of different type of surgical procedures, different cohorts of patients, different doses, routes and timing of administration. This is especially true for academic research that presents a scattering approach of the evaluation of analgesic agents related to different clinical practices. This matter of facts commonly precludes from definite conclusion. There are consequently important difficulties to characterize the value and the clinical use of drugs as for example alpha-2 adrenergic agents, ketamine, gabapentinoids and dexamethasone. The problem is even more complex when combination of analgesic agents are considered because no one uses the same combination, at the same dose, and at the same time, for the same categories of patients. For example when considering the use of ketamine, some ones give a single dose after anaesthetic induction, others a continuous infusion or repeated doses and others use PCA devices. [START_REF] Brinck | Perioperative intravenous ketamine for acute postoperative pain in adults[END_REF] For gabapentin, doses range from 300mg given once to 1200-1600 mg during several days after surgery! (8)

Systematic reviews and meta-analysis: value and limitation

The multiplication of post-operative trials evaluating the same analgesic molecule has increased so much that an overview of the literature has become increasingly difficult to achieve, even for an expert in the field. Systematic reviews are based on a comprehensive, systematic search for scientific evidence at a given time. They make it possible to bring together a very large amount of information and give the most objective possible analysis of the literature. Once there is a critical mass of comparable studies designed to address a common research question, a meta-analysis can be done. Meta-analyses are based on two main principles. The first is that the data is aggregated to achieve a combined effect. The second is that the included studies are weighted, i.e. meta-analyses give more weight to the more accurate studies with a higher number of patients.

They are based on a rigorous, reproducible methodology. The value of a metaanalysis is that it quantifies treatment effects and their uncertainty, increases power, increases precision, explores differences between studies, settles controversies from conflicting studies, and generates new hypotheses.

Systematic review provides a quantitative summary through a meta-analysis.

Although meta-analyses are considered as providing the highest level of evidence, it is important wondering how confident we can be in the estimate of the observed effect. Confidence in a meta-analysis result is an important consideration that can be evaluated. [START_REF] Guyatt | GRADE guidelines: a new series of articles in the Journal of Clinical Epidemiology[END_REF] The GRADE methodology is a systematic approach rating the certainty of evidence in systematic reviews at four levels (high, moderate, low, very low). However five factors can reduce the confidence in the estimated effect.

The first is the risk of bias due to the inclusion of poor-quality studies in the meta-analysis, which includes the limitations of study design and execution of randomized control trials. For example, the difficulties of blinding a regional analgesic treatment, or a significant loss of follow-up, in studies evaluating the risk of chronic pain after surgery, are a common source of bias.

The second is inconsistency of results across studies, when they produce widely different estimates of the effect. It is important to look for the causes of the heterogeneity, which could be differences in populations, interventions or outcomes. For example, the surgical procedures, the doses of drugs or even the definition of pain, are frequent factors of heterogeneity in postoperative trials.

The examination of heterogeneity is an important question in the development of new hypotheses.

The third point is indirectness -that is, confidence that the results expressed relate to the heath care question that was posed at the beginning. This question was expressed in the PICO format (Population, Intervention, Control, Outcomes) present in the paragraph method of systematic review. The PICO format helps evaluating whether the evidence is sufficiently direct to provide reasonable confidence that it can be used to answer the health care question. Direct evidence comes from trials that measure the outcomes of interest. For example, in RCTs evaluating the benefit of strong opioids in chronic pain, it is common practice to exclude patients who have psychological vulnerability or addiction problems. This situation is far from real life and can downgrade the quality of evidence in such meta-analyses. [START_REF] Moisset | Use of strong opioids in chronic non-cancer pain in adults. Evidence-based recommendations from the French Society for the Study and Treatment of Pain[END_REF] The fourth factor is imprecision of the results. It depends from some criteria such as sample size: the larger the sample size, the greater the precision. The optimal information size depends on the incidence of events in the control group and on the level of difference that is considered clinically significant. It is important to address this point in a meta-analysis that combines rare events such as adverse events and/or when it includes outcomes considered as secondary outcomes in the primary study -such as chronic pain after surgery. [START_REF] Chaparro | Pharmacotherapy for the prevention of chronic pain after surgery in adults[END_REF] The fifth factor is publication bias. Publication bias leads to a false assessment of the effect due to selective publication of studies. It is known that positive trials are more likely to be published than negative trials in all fields. Studies funded by pharmaceutical companies are more likely to have positive outcomes. A funnel plot has to be used to check to document publication bias. However, a simple and effective way to check that the literature search is exhaustive is to ensure that in the method section of the systematic review at least three databases have been explored and that the search for unpublished trials is performed in the registration site such as ClinicalTrials.gov and WHO ICTRP. Unpublished data account for more than 70% of the evidence for some analgesics. [START_REF] Martinez | Perioperative pregabalin administration does not prevent chronic postoperative pain: systematic review with a metaanalysis of randomized trials[END_REF] While conventional meta-analyses can evaluate whether a particular analgesic is effective, they do not answer the clinician's question: what is the best treatment among the various existing treatments? Controlled analgesic trials rarely compare treatment with each other. The lack of direct comparison makes it difficult to determine a comparative efficacy or even tolerance of different analgesics treatments. Recently, a new form of overview has emerged to provide a solution to this issue: the network meta-analysis. ( 13) Network meta-analysis makes it possible to estimate the relative effectiveness of the different treatments available to treat the same condition. The "network" refers to available treatments and existing direct comparisons between these treatments (randomized controlled trials). A network meta-analysis, then, consists of a comprehensive quantitative overview using existing direct comparisons as well as credible indirect comparisons. In the indirect comparison, the efficacy of the two treatments A and B is compared via their respective efficacy against a common comparator. This method allows classifying treatments in terms of efficacy and tolerance. Furthermore, bringing together evidences of efficacy and/or tolerance of available treatments in the form of a network analysis, makes it possible to quickly visualize missing studies, and guides future research. In the field of pain management, several "datasets" in the scientific literature, could greatly benefit from such an approach. Not only do they make it possible to compare pharmacological treatments with each other [START_REF] Martinez | Non-opioid analgesics in adults after major surgery: systematic review with network meta-analysis of randomized trials[END_REF][START_REF] Terkawi | Pain Management Modalities after Total Knee Arthroplasty: A Network Metaanalysis of 170 Randomized Controlled Trials[END_REF][START_REF] Beloeil | Multicentre, prospective, double-blind, randomised controlled clinical trial comparing different non-opioid analgesic combinations with morphine for postoperative analgesia: the OCTOPUS study[END_REF] but also pharmacological treatments with non-pharmacological treatments. ( 12)

What are the unmet needs?

From a clinical point of view a few millimetres decrease in VAS score or a few milligrams decrease in morphine consumption does not make sense and is not demonstrative enough to induce significant changes in clinical practice except in passionate believers. When one wants to change its clinical practice for the better, a series of steps need to be followed. First of all it is necessary to determine whether patients are treated according to evidence-based medicine provided by systematic review of the clinical trials with reasonable methodological quality (see above). Once the analgesic protocol is determined and used in routine practice, it needs to be evaluated. There are several instances corresponding to an unsatisfactory pain control. Patients may remain permanently in pain during the postoperative period but this is usually not the case, otherwise a complete revision of the protocol needs to be performed. More frequently pain at rest is controlled but mobilization remains painful. It is as much worrisome that patients are included in programs requiring active participation. Persistent pain on movement may preclude the compliance of patients to rehabilitation process. Such an issue may require changing continuous administration for intermittent boluses or providing rescue medication to be taken before mobilisation. Last but not least, the analgesic protocol could be satisfactory in most of the patients while some of them remain in pain. The issue is consequently not to lower the mean level of pain intensity in the whole population but better to reduce the percentage of patients who are still in pain despite adequate compliance to the current treatment. Sometimes, these patients can be identified pre operatively and benefit from a special adaptation of the current treatment (for example patients with chronic pain already treated with analgesics) In most of the case it is not possible to clearly identify preoperatively these patients. Thus a clinical trial comparing the standard of care to a new therapeutic approach may consequently focus on reducing this percentage that could be considered as the main goal of the trial or the main criterion for comparison of the two groups of patients.

A comparison of the number-needed-to-treat (NNT) (how many patients do I have to treat before one of them benefit from the treatment) and the number needed to harm (NNH) (how many patients are exposed to a treatment before occurrence of a side effect) in the same population of patients may help to determine the value of introducing a new drug or treatment in the setting of postoperative pain. Thus the NNT to achieve 50% pain relief is commonly compared to the NNH to induce nausea or urinary retention…. However, because the occurrence of pain is common postoperatively, NNT values of analgesic agents are low whatever their efficacy (ranging between 3-5). Although comparisons of NNT may also account for efficacy, comparison of NNH is especially valuable in the setting of postoperative pain treatment in patients scheduled for rehabilitation process.

The pathway for a new drug to become a routine treatment

Beside development of new drugs by pharmaceutical companies that follow predetermined rules, academic research may promote the use of pre existing drugs or describe new (regional analgesic) technique to improve pain management. Such a development is usually based on preliminary cohort studies that describe the technique and suggest its efficacy. In case of regional analgesia, this approach is completed by anatomical studies that describe the diffusion of the solution and the nerve structures involved. Then, a few randomised controlled studies are published with encouraging results compared to the standard of care and key opinion leaders promote the technique. After a while, other studies are published documenting controversial results or including more important cohorts of patients confirming or not preliminary results and documenting the incidence of side effects. At this stage, results of all the studies published with adequate methodology are included in systematic review and meta-analysis eventually confirming the efficacy of the new promoted treatment. However, the conclusions of meta-analysis do not close the debate for two reasons: first of all systematic review commonly reveal the heterogeneity of studied protocols and surgical procedures. Secondly meta-analyses usually do not address the issue of doses, duration and mode of administration.

There is consequently a need for a confrontation of the literature and the clinical needs and to evaluate the balance between advantages and drawback of a new treatment. At least, the literature needs to be analysed considering two points : only articles referring specifically to the surgical procedure that is considered, need to be analysed to determine the value of a given analgesic treatment for controlling pain in this setting and eventually evidences must be found in the recent literature considering that the surgical techniques are continuously changing (laparoscopic instead of laparotomy, robotic surgery, etc.) as well as the global management of the patients (shortening of hospital stay, early mobilization, etc.). [START_REF] Tan | Optimizing pain management to facilitate Enhanced Recovery After Surgery pathways[END_REF][START_REF] Joshi | Guidelines for perioperative pain management: need for re-evaluation[END_REF]


Kris Bauchmuller

Alastair Glossop

Samir Audrey De Jong

come

Combining high-flow nasal cannula oxygen and non-invasive ventilation for pre-oxygenation in the critically ill: is a double-pronged approach warranted?