Patricia Bolle

Tarik Benali

Clotilde Menet

Marin Puget

Eric Faulques

Jérome Marrot

Pierre Mialane

Anne Dolbecq

Hélène Serier-Brault

Olivier Oms

Rémi Dessapt

Tailoring the Solid-state Fluorescence of BODIPY by Supramolecular Assembly with Polyoxometalates

A cationic boron dipyrromethene (BODIPY) derivative (1 + ) has been successfully combined with two polyoxometalates (POMs), the Lindqvist-type [W6O19] 2-and the β-[Mo8O26] 4-units, into three new supramolecular fluorescent materials

(1)2[W6O19]⋅2CH3CN, (1)2[W6O19] and (1)4[Mo8O26]⋅DMF⋅H2O. The resulting hybrid compounds have been fully characterized by a combination of single-crystal X-ray diffraction, IR and UV-Vis spectroscopies and photoluminescence analyses. This self-assembly approach prevents any π-π stacking interactions between the BODIPY units, responsible for aggregation-caused quenching (ACQ) effects, but also between the BODIPY and the POMs, avoiding intermolecular charge-transfer effects. Noticeably, the POM units do not only act as bulky spacers but their negative charge density drives the molecular arrangement of the 1 + luminophore, strongly modifying its fluorescence in the solid state. As a consequence, the 1 + cations are organized into dimers in (1)2[W6O19]⋅2CH3CN and (1)2[W6O19], which are weakly emissive at room temperature, and in a more compact layered assembly in (1)4[Mo8O26]⋅DMF⋅H2O which exhibits a redshifted and intense emission upon similar photoexcitation.

INTRODUCTION

The interest for organic solid-state luminescent materials has greatly increased in the last decade. Integrated in optoelectronic devices, [START_REF] Ostroverkhova | Organic Optoelectronic Materials: Mechanisms and Applications[END_REF] they found applications in various fields such as lasers, 2 organic light-emitting diodes (OLEDs), [START_REF] Bunzmann | Optically and Electrically Excited Intermediate Electronic States in Donor:Acceptor Based OLEDs[END_REF] sensors, [START_REF] Ni | Organic Thermally Activated Delayed Fluorescence Materials for Time-Resolved Luminescence Imaging and Sensing[END_REF] bioimaging [START_REF] Cheng | Assembly Strategies of Organic-Based Imaging Agents for Fluorescence and Photoacoustic Bioimaging Applications[END_REF] or anti-counterfeiting technologies. [START_REF] Yuan | Dynamic Anti-Counterfeiting Security Features Using Multicolor Dianthryl Sulfoxides[END_REF] Despite their high photostability and fluorescent quantum yields along with narrow emission bands, the well-known class of boron dipyrromethene (BODIPY) [START_REF] Loudet | BODIPY Dyes and Their Derivatives: Syntheses and Spectroscopic Properties[END_REF] fluorophores has been intensively investigated almost exclusively for their use in diluted solutions. Actually, in the condensed phase, the strong intermolecular π-stacking interactions between these aromatic planar dyes lead to aggregation-caused quenching (ACQ) effects, thus limiting their integration in efficient optical devices. To overcome this specific issue, several strategies have been reported up to now (Table S1). First, due to the great chemical versatility of this family of compounds, it is possible either to functionalize the BODIPY skeleton by bulky substituents that inhibit the aggregation of the chromophores, [START_REF] Zhang | Bulky 4-Tritylphenylethynyl Substituted Boradiazaindacene: Pure Red Emission, Relatively Large Stokes Shift and Inhibition of Self-Quenching[END_REF][START_REF] Vu | New Hindered BODIPY Derivatives: Solution and Amorphous State Fluorescence Properties[END_REF][START_REF] Lu | Tuning the Solid-State Luminescence of BODIPY Derivatives with Bulky Arylsilyl Groups: Synthesis and Spectroscopic Properties[END_REF][START_REF] Sirbu | One-Pot Synthesis of a Mono-O,B,N-Strapped BODIPY Derivative Displaying Bright Fluorescence in the Solid State[END_REF] or to synthesize specific dyes that are structurally arranged into emissive aggregates, exhibiting the wellknown aggregation-induced enhanced emission (AIE) behavior. [START_REF] Kim | Tailoring the Solid-State Fluorescence Emission of BODIPY Dyes by Meso Substitution[END_REF][START_REF] Zhang | Circularly Polarized Luminescence of AIE-Active Chiral O-BOD-IPYs Induced via Intramolecular Energy Transfer[END_REF][START_REF] Hu | Twisted Intramolecular Charge Transfer and Aggregation-Induced Emission of BODIPY Derivatives[END_REF][START_REF] Singh | An Unconventional Mechanistic Insight on Aggregation Induced Emission in Novel Boron Dipyrromethenes and Their Rational Biological Realizations[END_REF][START_REF] Mukherjee | Fine-Tuning Dual Emission and Aggregation-Induced Emission Switching in NPI-BODIPY Dyads[END_REF][START_REF] Liu | Recent Progress of BODIPY Dyes With Aggregation-Induced Emission[END_REF] A second strategy which avoids numerous time-consuming synthetic steps consists in the dilution of the BODIPY molecules into matrices. So, it has been demonstrated that the incorporation of the fluorophore into a polymer matrix diminishes the formation of non-emissive aggregates. [START_REF] Yeo | Effective Light-Harvesting Antennae Based on BODIPY-Tethered Cardo Polyfluorenes via Rapid Energy Transferring and Low Concentration Quenching[END_REF][START_REF] Zhu | Efficient Tuning Nonlinear Optical Properties: Synthesis and Characterization of a Series of Novel Poly(Aryleneethynylene)s Co-Containing BODIPY[END_REF][START_REF] Liras | Conjugated Microporous Polymers Incorporating BODIPY Moieties as Light-Emitting Materials and Recyclable Visible-Light Photocatalysts[END_REF] In the same way and following an elegant synthetic route, it has been recently shown that the entrapment of commercial BODIPY derivatives into a Metal-Organic Framework (MOF) strongly enhanced their solid-state luminescence. [START_REF] Glembockyte | Highly Photostable and Fluorescent Microporous Solids Prepared via Solid-State Entrapment of Boron Dipyrromethene Dyes in a Nascent Metal-Organic Framework[END_REF] However, in addition to the problem of long-term chemical stability, it remains difficult via this method to finely control the substrate organization in the MOF pores, and in absence of single-crystal X-ray diffraction characterizations, an accurate structural description of such chromophore@MOF composites remains difficult, hindering structure/property correlations.

In the last decade, we have shown that the combination of polyoxometalates (POMs), which can be described as anionic soluble metal oxides of early transition metals (usually W, Mo, V), [START_REF] Wang | Recent Advances in Polyoxometalate-Catalyzed Reactions[END_REF][START_REF] Du | Recent Advances in Porous Polyoxometalate-Based Metal-Organic Framework Materials[END_REF][START_REF] Proust | Functionalization and Post-Functionalization: A Step towards Polyoxometalate-Based Materials[END_REF] with photoactive organic or organometallic species strongly tunes their optical properties. As a striking example, while they exhibit poor solid-state photochromism properties at room temperature, pure organic spiropyran and spironaphthoxazine derivatives became highly photoactive when closely associated to POMs through ionic or covalent bonds. [START_REF] Dridi | Directing the Solid-State Photochromic and Luminescent Behaviors of Spiromolecules with Dawson and Anderson Polyoxometalate Units[END_REF][START_REF] Oms | Photo-and Electrochromic Properties of Covalently Connected Symmetrical and Unsymmetrical Spiropyran-Polyoxometalate Dyads[END_REF][START_REF] Saad | Design and Optical Investigations of a Spironaphthoxazine/Polyoxometalate/Spiropyran Triad[END_REF][START_REF] Hakouk | New Photoresponsive Charge-Transfer Spiropyran/Polyoxometalate Assemblies with Highly Tunable Optical Properties[END_REF][START_REF] Menet | Influence of Electronic vs. Steric Factors on the Solid-State Photochromic Performances of New Polyoxometalate/Spirooxazine and Spiropyran Hybrid Materials[END_REF] Recently, ionic supramolecular assemblies have been easily built by mixing different polyoxoanions and a luminescent cationic cyclometalated Ir(III) complex. [START_REF] Bolle | Drastic Solid-State Luminescence Color Tuning of an Archetypal Ir(Iii) Complex Using Polyoxometalates and Its Application as a Vapoluminescence Chemosensor[END_REF][START_REF] Bolle | Polymorphism and Structural Filiations in Five New Organic-Inorganic Hybrid Salts of the Heteroleptic Cationic Iridium(III) Complex and Polyoxometalates[END_REF] Remarkably, the solid-state emission of the resulting compounds shifts from green to red depending on the nature of the POM and the crystal packing. Moreover, some of us recently designed novel supramolecular fluorescent materials by coupling POMs with an AIE-active phospholium, [START_REF] Bolle | Strong Solid-State Luminescence Enhancement in Supramolecular Assemblies of Polyoxometalate and "Aggregation-Induced Emission"-Active Phospholium[END_REF] and efficient solid-state emitters have been obtained by directing favorably the balance between AIE and ACQ effects using both anion-π + and H-bonding interactions. In view of these significant results, we decided to enlarge our strategy to BODIPY entities with the hope to recover their outstanding physico-chemical properties in the solid state in a simple and controlled way. To date, very scarce examples of BODIPY/POM dyads have been mentioned in the literature, and in most of the cases, the organic and inorganic fragments have been linked through covalent bonds. 33-36' We can mention the synthesis in 2015 of a molecular fluorescent photoswitch obtained by combining in a single molecule an Anderson-type POM, a spiropyran group and a BODIPY moiety. [START_REF] Saad | A High Fatigue Resistant, Photoswitchable Fluorescent Spiropyran-Polyoxometalate-BODIPY Single-Molecule[END_REF] Very recently, two supramolecular assemblies of a cationic iodobodipy and polyoxovanadates have been elaborated for photochemotherapy in solution. [START_REF] Gu | Polyoxovanadate-Iodobodipy Supramolecular Assemblies: New Agents for High Efficiency Cancer Photochemotherapy[END_REF] However, to the best of our knowledge, the solid-state photophysical properties of such ionic or covalent hybrid systems remained uninvestigated. We report herein the straightforward synthesis of three crystallographically resolved POM/BODIPY supramolecular assemblies, obtained by combining octamolybdate or Lindqvist-type tungsten polyanions and a cationic BODIPY derivative [START_REF] Kim | A Gold Nanoparticle-Based Fluorescence Turn-On Probe for Highly Sensitive Detection of Polyamines[END_REF] (denoted as 1 + ), displayed in Figure 1. The solid-state photophysical properties at room temperature of the reported hybrids, namely (1)2[W6O19]⋅2CH3CN, (1)2[W6O19] and (1)4[Mo8O26]⋅DMF⋅H2O have been thoroughly investigated and pertinent structure-property relationships have been evidenced. As envisaged, the structural analysis well indicates the absence of undesired π-π stacking interactions between the BODIPY cations likely to induce non-emissive aggregates. Moreover, we emphasize the essential role of the POM onto the emission of the cationic fluorophore in the crystalline state.

2-EXPERIMENTAL SECTION

2.1. Synthetic procedures. All chemicals and reagents were purchased from major chemical suppliers and used as received except (NBu4)4[α-Mo8O26] 39 and (NBu4)2[W6O19] [START_REF] Sanchez | Electron Delocalization in Mixed-Valence Tungsten Polyanions[END_REF] which have been synthesized according to the reported procedures. ((CH3CH2)2NH2)2(NH4)2[Mo8O26], which was used as an optical reference of the β-[Mo8O26] 4-unit for solid-state UV-vis spectroscopy characterizations, was also synthesized as previously described. [START_REF] Hakouk | Sulfonium Polyoxometalates: A New Class of Solid-State Photochromic Hybrid Organic-Inorganic Materials[END_REF] 2.1.1. Synthesis of 4,4-Difluoro-8-(4-(4-(N,N,N-trimethyl)aminobutoxy))phenyl-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-sindacene bromide ((1)Br). (1)Br has been prepared by a slight modification of the previously reported procedure, [START_REF] Kim | A Gold Nanoparticle-Based Fluorescence Turn-On Probe for Highly Sensitive Detection of Polyamines[END_REF] according to the synthetic pathway displayed in the Supporting Information.

Synthesis of (1)2[W6O19]⋅2CH3CN and (1)2[W6O19].

(NBu4)2[W6O19] (0.0889 g, 4.70 10 -2 mmol) was dissolved in acetonitrile (3 mL). The colourless solution (solution 1) was stirred for a few minutes at room temperature. Meanwhile, a second solution (solution 2) was obtained by dissolving (1)Br (0.050 g, 9.36 10 -2 mmol) in acetonitrile (2 mL) at room temperature. Solution 2 was added dropwise to solution 1 under vigorous stirring and the resulting reddish-brown solution was heated at 40 °C for two hours, the precipitation of an orange powder being observed from the beginning of heating. The mixture was kept at room temperature and then filtered. Orange-red single crystals of (1)2[W6O19]⋅2CH3CN were obtained after a few days by slow diffusion of tert-butyl methyl ether into the filtrate. Yield in W: 38%. Anal. Calcd for C56H76B2F4N8O21W6: C, 28.03 ; H, 3.16 ; N, 4.67. Found: C, 28.69 ; H, 3.52 ; N, 4.67. FT-IR (KBr cm -1 ): 1 + cation 1609 (w), 1545 (m), 1508 (w), 1470 (m), 1408 (w), 1379 (w), 1363 (w), 1306 (m), 1286 (w), 1246 (m), 1196 (m), 1157 (m), 1118 (w), 1086 (w), 1070 (w), 706 (w), 478 (w); νW=O, νW-O-W 976 (vs), 814 (vs), 766 (w), 586 (m), 445 (s).

The obtained orange powder which had first precipitated was washed with ethanol, dried in air and dissolved in DMF (4 mL). The resulting solution was stirred at 50 °C for 1 h, and the system was set to standby at ambient temperature. Orange-red single crystals of

RESULTS AND DISCUSSION

Synthesis. (1)

Br was prepared in a three-step synthetic procedure via a slight modification of the previously reported protocol. [START_REF] Kim | A Gold Nanoparticle-Based Fluorescence Turn-On Probe for Highly Sensitive Detection of Polyamines[END_REF] 1 H (Figure S1) and [START_REF] Zhang | Circularly Polarized Luminescence of AIE-Active Chiral O-BOD-IPYs Induced via Intramolecular Energy Transfer[END_REF] C NMR spectroscopies as well as mass spectrometry data (Figure S2) confirmed the formation of the product, which was isolated as a reddish-brown powder. Unfortunately, attempts to obtain single crystals were unsuccessful. Orange-red single crystals of (1)2[W6O19] were isolated by recrystallization in DMF at room temperature of the orange powder obtained by mixing in acetonitrile (NBu4)2[W6O19] and two equivalents of (1)Br. In addition, orange-red single crystals of (1)2[W6O19]⋅2CH3CN were obtained by slow diffusion of tert-butyl methyl ether into the acetonitrile filtrate of the same (1)Br/[W6O19] 2-mixture. FT-IR spectroscopy (Figures S3-S4) well evidences that both compounds contain the [W6O19] 2-anion associated with 1 + cations. The presence of crystallized acetonitrile molecules in (1)2[W6O19]⋅2CH3CN has been also confirmed by elemental analyses and thermogravimetric analysis (Figure S5).

(1)4[Mo8O26]⋅DMF⋅H2O was obtained as red single crystals by mixing in acetonitrile (NBu4)4[Mo8O26] and four equivalents of (1)Br, and after recrystallizing the resulting reddishorange powder in DMF at room temperature. The presence of the β-[Mo8O26] 4-unit and a co-crystallized DMF molecule

N + B - N O N + CH 3 CH 3 C H 3 C H 3 F F CH 3 CH 3 CH 3
were confirmed by FT-IR spectroscopy (Figure S6). The presence of a co-crystallized water molecule, observed by X-ray diffraction data (see below), could be due to the presence of a small quantity of water in the organic solvents used or/and to moisture from air.

Structural Description. (1)2[W6O19]⋅2CH3CN.

The crystal structure of (1)2[W6O19]⋅2CH3CN contains two crystallographically equivalent 1 + cations assembled with one [W6O19] 2-unit into supramolecular {(1)2[W6O19]} ribbons running along the a-axis (Figure 2a). The polar alkylammonium groups of the 1 + molecules interact via electrostatic interactions including C-H•••O contacts with the oxygen POM surface, and the acetonitrile molecules are localized between 1 + cations. In these latter, the meso-phenyl ring is almost orthogonal to the indacene plane with a dihedral angle of 81.19°. This conformation should minimize unfavorable steric hindrances between the ortho-hydrogen atoms and the vicinal methyl groups. [START_REF] Liu | Bodipy Dyes Bearing Oligo(Ethylene Glycol) Groups on the Meso-Phenyl Ring: Tuneable Solid-State Photoluminescence and Highly Efficient OLEDs[END_REF] In addition, the butylammonium chain is in a folded conformation (Figure 2b), with a distance between the boron atom and the ammonium head group of 11.95 Å. In the {(1)2[W6O19]} ribbons, the boradiazaindacene groups form a columnar slipped stack along the a-axis. However, the POMs units keep the 1 + molecules away from each other, inducing a long centre-to-centre distance of 9.586 Å between the aromatic planes. In addition, the {(1)2[W6O19]} ribbons are stacked along the b-axis (Figure S7). The methyl substituents of the pyrrole groups interact with the POM unit via weak C-H⋅⋅⋅O contacts which prevent any π-π stacking interactions with the oxygen POM surface likely to induce intermolecular charge transfer processes. Between two adjacent ribbons, the 1 + molecules form isolated head-to-tail dimers with a distance between both indacene planes of 6.124 Å (Figure 2b). However, they are strongly offset from each other and interact through weak C-H⋅⋅⋅F and C-H⋅⋅⋅π hydrogen bonds, banishing any π-π stacking interactions. The dimers adopt a layered organization along the ab plane (Figure 2c), without any overlap of the aromatic planes of neighboring dimers. (1)2[W6O19]. The crystal structure of (1)2[W6O19] contains two crystallographically inequivalent 1 + conformers (hereafter labelled as 1 + _A and 1 + _B) which are assembled with the [W6O19] 2-units in a herringbone-like pattern (Figure 3a), to form supramolecular ribbons running along the a-axis. These latter are then aligned parallel to form {(1)2[W6O19]} layers that are stacked along the b-axis (Figure S8).

Strikingly, the lack of crystallized solvent molecules in (1)2[W6O19] induces significant changes in the molecular crystal packing compared to that of (1)2[W6O19]⋅2CH3CN. In particular, the 1 + volume concentration increases from 1.909 × 10 -3 mol.cm -3 in (1)2[W6O19]⋅2CH3CN to 1.978 × 10 -3 mol.cm - 3 in (1)2[W6O19]. The conformation of the 1 + molecules is also significantly modified. Indeed, while the angle between the meso-phenyl ring and the indacene plane (86.92° for 1 + _A and 75.3° for 1 + _B) is quite comparable with that observed in (1)2[W6O19]⋅2CH3CN (81.19°), the butylammonium chain is in a long unfolded conformation this time, which increases the length of the BODIPY molecules, the distance between the boron atom and the ammonium head group reaching 14.25 Å for 1 + _A and 14.12 Å for 1 + _B (11.95 Å in (1)2[W6O19]⋅2CH3CN). In the supramolecular {(1)2[W6O19]} layers, the 1 + molecules are organized into columnar slipped stacks along the a-axis. However, due to the presence of the POM units, the shortest distances between the indacene planes are all over 9 Å (Figure 3b), and they mainly interact through intermolecular C-H⋅⋅⋅F hydrogen bonds implying the methyl substituents of the pyrrole groups. Moreover, the indacene planes do not interact with the oxygen POM facets via π-π stacking interactions. Along the c-axis, each 1 + conformer is arranged into a headto-tail dimer (Figure 3c) with a long centre-to-centre distance between the aromatic indacene planes of 7.322 Å for 1 + _A and 6.012 Å for 1 + _B. As observed for (1)2[W6O19]⋅2CH3CN, the neighboring indacene groups are mainly stabilized by intermolecular C-H⋅⋅⋅π contacts between the meso-phenyl ring and the pyrroles groups rather than π-π stacking interactions. (1)4[Mo8O26]⋅DMF⋅H2O. The crystal structure of (1)4[Mo8O26]⋅DMF⋅H2O contains two crystallographically independent 1 + cations (hereafter labelled as 1 + _A and 1 + _B), one β-[Mo8O26] 4-unit, and DMF and water molecules. The ammonium head groups of the 1 + cations form a wavy honeycomb-like supramolecular 2D network, and the POM clusters are localized inside the cavities (Figure 4a). Each β-[Mo8O26] 4-unit is surrounded by twelve BODIPY molecules whom boradiazaindacene rings are oriented on both sides of the POM layer (Figure 4b and Figure S9). Four aromatic planes interact with the β-[Mo8O26] 4-unit via C-H⋅⋅⋅O and short O⋅⋅⋅C contacts implying the meso-phenyl ring. The 1 + volume concentration reaches 2.068 × 10 -3 mol.cm -3 i.e., an increase of 4.5% and 8% compared to those determined for (1)2[W6O19] and (1)2[W6O19]⋅2CH3CN, respectively. Consequently, the boradiazaindacene cores of the two 1 + conformers do not form supramolecular dimers as observed in (1)2[W6O19] and (1)2[W6O19]⋅2CH3CN, but they are alternately stacked in a layered arrangement along the (bc) plane, and mainly interact through a complex C-H⋅⋅⋅F and C-H⋅⋅⋅π hydrogen-bonding network (Figure 4c). The shortest centre-to-centre distance between the neighboring indacene planes is 6.523 Å, i.e. longer than the corresponding shortest distances in (1)2[W6O19] and (1)2[W6O19]⋅2CH3CN, and no π-π stacking interactions are observed.

Solid-state photophysical properties.

The absorption and photoluminescence (PL) properties of (1)Br in diluted solution have been already investigated, indicating that 1 + exhibits a green emission (λem = 511 nm) in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) aqueous buffer. However, its photophysical properties in the solid state remain still unexplored. The room-temperature solid-state UVvis absorption and steady-state PL spectra of (1)Br are depicted in Figure 5 and Figure S10. The intense absorption band at 460 nm could be attributed to vibrational bands of the strong S0S1 (π-π*) transition, while the less intense relatively broad band centered at 368 nm could correspond to the S0S2 (π-π*) transition. The weak absorption band at 538 nm could be assigned to intermolecular effects because it totally disappeared in diluted acetonitrile solution (Figure S11). 2D photoluminescence excitation-emission map (Figure S10) indicates that upon irradiation at λex = 365 nm, (1)Br exhibits a weak fluorescence at room temperature. Its PL spectrum shows a broad and multimodal emission band in the 550-900 nm range with a wavelength maximum at 750 nm with six distinguishable shoulders peaking at 518, 550, 580, 632, 679 and 815 nm (Figure S10). A mean fluorescence lifetime of τ = 259 ps has been determined by room temperature time-resolved spectroscopy (Figure S12). This orange emission is characterized by the Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of (0.58, 0.41) (Figure S13).

As expected, the solid-state emission of 1 + is significantly redshifted relative to the monomer in diluted solution [START_REF] Kim | A Gold Nanoparticle-Based Fluorescence Turn-On Probe for Highly Sensitive Detection of Polyamines[END_REF] due to strong intermolecular interactions in the condensed state. 42- 44 The weak emission of (1)Br suggests significant ACQ effects, as usually observed for BODIPY dyes free from bulky substituents grafted onto the boradiazaindacene group.

As shown in Figure 5, the solid-state UV-vis absorption spectra of

(1)2[W6O19]⋅2CH3CN, (1)2[
W6O19] and (1)4[Mo8O26]⋅DMF⋅H2O exhibit a weak overlap, limited to the 275-350 nm range, between the absorption bands of 1 + and the OM ligand-to metal charge-transfer (LMCT) transitions of the POM units (λmax = 280 nm and 300 nm for the [W6O19] 2- and β-[Mo8O26] 4-anions, respectively). This well justifies the choice of these POMs whose high energy LMCT absorptions do not compete with the 1 + photoexcitation upon 365-nm irradiation. In the 350-600 nm range, the spectra are indeed dominated by the intense absorption bands of 1 + .

The room-temperature steady-state PL excitation (PLE) spectra of the three hybrid POMs (Figure S14) show a broad band whom the maximum wavelength is located in the 350-400 nm range, in good agreement with the S0S2 (π-π*) transition of 1 + observed in the absorption spectra of the three salts (Figure 5). The room-temperature steady-state emission spectra of In marked contrast, (1)4[Mo8O26]⋅DMF⋅H2O exhibits a bright orange-red emission (Figure 6, inset), much more intense than those of (1)Br, (1)2[W6O19]⋅2CH3CN and (1)2[W6O19]. Its PL spectrum displays a broad band peaking at 682 nm with a shoulder at 720 nm, and the mean PL lifetime reaches 0.39 ns, i.e. higher than those of the two polyoxotungstate species reported herein.

These results clearly highlight that the solid-state PL response of 1 + can be tuned with the nature of the associated POM and the crystal packing. As envisaged, the bulky POM anions efficiently inhibit detrimental intermolecular π-π stacking between nearest neighboring BODIPY molecules which mainly interact via C-H⋅⋅⋅F and C-H⋅⋅⋅π hydrogenbonds. However, POMs do not act only as simple molecular spacers, and their negative charge density [START_REF] Misra | Beyond Charge Balance: Counter-Cations in Polyoxometalate Chemistry[END_REF] is also a key factor to drive the supramolecular arrangements of 1 + . Thus, when associated with [W6O19] 2-which has a low negative charge density, 1 + forms isolated head-to-tail dimers in both In contrast, β-[Mo8O26] 4-has a negative charge density higher than [W6O19] 2-and it interacts more strongly with 1 + cations, increasing their volume concentration in the crystal lattice. This significantly impacts the PL properties. Indeed, the emissions of (1)2[W6O19]⋅2CH3CN, (1)2[W6O19] and (1)4[Mo8O26]⋅DMF⋅H2O are characterized by the CIE chromaticity coordinates of (0.55, 0.43), (0.58, 0.41) and (0.62, 0.38), respectively (Figure S16) which results in a progressive red-shift of the emission color. However, comparing the three hybrid POMs of this series, it appears that the higher the 1 + volume concentration, the brighter the fluorescence. Considering that (1)2[W6O19]⋅2CH3CN and (1)2[W6O19] exhibit comparable weak fluorescence, we may assume that the molecular arrangement of 1 + into head-to-tail dimers should dramatically favor self-quenching processes. The improved PL performances of (1)4[Mo8O26]⋅DMF⋅H2O could be possibly attributed to the effect of the β-[Mo8O26] 4-unit on the cationic BODIPY arrangement which results in the absence of the undesirable dimeric stacking and in an increase of the shortest distance between the boradiazaindacene planes of nearest luminophores.

CONCLUSION (1)2[W6O19]⋅2CH3CN, (1)2[W6O19]

and (1)4[Mo8O26]⋅DMF⋅H2O represent very rare examples of crystallized hybrid compounds with a fluorescent cationic BODIPY (1 + ) derivative. The three structurally resolved supramolecular assemblies combine the 1 + molecule with two different POM anions. Importantly, in addition to their crucial structural role, the β-[Mo8O26] 4- and [W6O19] 2-polyanions were properly chosen as these poor electron acceptors exhibit high energy LMCT transitions in the 250-300 nm range, and they do not compete with 1 + upon photoexcitation at 365 nm. As anticipated, the in-depth structural investigation points out that the bulky POM anions prevent detrimental π-π stacking interactions between the BODIPY cations, which could have resulted in undesired ACQ effect. Moreover, the coupling of 1 + with POMs via non-covalent interactions significantly modifies its fluorescence in the crystalline state, and the three ionic assemblies exhibit contrasted colors and brightnesses which vary with the nature of the 4-which has a higher negative charge density induces a higher volume concentration in BODIPY cations while avoiding its dimeric arrangement, that leads to an intense emission at room temperature. This can be tentatively explained by the existence of weaker interactions between the cationic fluorophores in this compound compared to those found in the Lindqvist ones, these last exhibiting shorter centreto-centre distances between the neighboring indacene planes. This simple strategy of assembling then sounds promising to improve PL performances of BODIPY entities in the solid state, as long as limiting the choice of POMs to poor electron acceptors to avoid photo-induced electron transfer processes likely to cause the luminescence quenching of organic emitter. As such POMs offer a large diversity of size and charge, this work paves the way towards the design of new ionic assemblies with a high degree of organization, and will allow deeper investigating the precise impact of the POMs on the photophysical properties of cationic BODIPY derivatives in the crystalline state.

ASSOCIATED CONTENT Supporting Information

Physical measurements, experimental section, 1 H NMR spectra, ES/MS spectrum, TGA curve, crystallographic data, FT-IR and UV-vis spectra, 2D photoluminescence excitation-emission map, CIE chromaticity diagrams, luminescence decay curves and extracted lifetimes. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT

Aurélie Damond is gratefully acknowledged for performing mass spectroscopy analysis. This work was supported by the Ministère de l'Enseignement Supérieur et de la Recherche, the CNRS, the Université de Versailles Saint Quentin en Yvelines, the Université de Nantes, the LUMOMAT project supported by the Région des Pays de la Loire and a public grant overseen by the French National Research Agency (ANR) as part of the 400 500 600 700 800 900 Fluorescence Intensity (a. u.)

Figure 1 .

 1 Figure 1. Representations of (a) the cationic BODIPY (1 + ), (b) the [W6O19] 2-and (c) the β-[Mo8O26] 4-units considered in this study (dark blue octahedra = WO6, light-blue octahedra = MoO6, gold sphere: oxygen).

( 1 ) 2 [

 12 W6O19] were obtained after a few days. Yield in W: 34%. Anal. Calcd for C52H70B2F4N6O21W6: C, 26.97 ; H, 3.05 ; N, 3.63. Found: C, 27.41 ; H, 2.96 ; N, 3.49. FT-IR (KBr cm -1 ): 1 + cation 1610 (w), 1545 (m), 1508 (w), 1472 (m), 1406 (w), 1371 (w), 1306 (m), 1286 (w), 1250 (m), 1194 (m), 1155 (m), 1120 (w), 1084 (w), 704 (w), 476 (w); νW=O, νW-O-W 976 (vs), 812 (vs), 764 (w), 584 (m), 444 (s). 2.1.3. Synthesis of (1)4[Mo8O26]⋅DMF⋅H2O. (NBu4)4[α-Mo8O26] (0.050 g, 2.32 10 -2 mmol) was dissolved in acetonitrile (3 mL). The colourless solution (solution 1) was stirred for a few minutes at room temperature. Meanwhile, a second solution (solution 2) was obtained by dissolving (1)Br (0.050 g, 9.36 10 -2 mmol) in acetonitrile (3 mL) at room temperature. Solution 2 was added dropwise to solution 1 under vigorous stirring and the resulting reddish-brown solution was heated at 40 °C for two hours leading to the precipitation of a reddish-orange solid. The mixture was kept at room temperature and then filtered. The powder was washed with ethanol, and dried in air. Then the powder was dissolved in DMF (4 mL). The resulting solution was stirred at 50 °C for 1 h, and the system was set to standby at ambient temperature. Red single crystals were obtained after a few days. Yield in Mo: 32%. Anal. Calcd for C107H149B4F8Mo8N13O32: C, 41.54; H, 4.82 ; N, 5.89. Found: C, 42.23 ; H, 4.54 ; N, 6.69. FT-IR (KBr cm -1 ): 1 + cation 1610 (w), 1545 (m), 1508 (w), 1472 (m), 1406 (w), 1371 (w), 1306 (m), 1286 (w), 1250 (m), 1194 (m), 1155 (m), 1120 (w), 1084 (w), 1151 (w), 704 (w), 476 (w); νMo=O, νMo-O-Mo 976 (vs), 812 (vs), 764 (w), 584 (m), 444 (s).

Figure 2 .

 2 Figure 2. (a) Crystal packing diagram in (1)2[W6O19]⋅2CH3CN. (b) Supramolecular 1 + dimer. (c) Molecular packing of the 1 + dimers (ether groups are not displayed). Intermolecular C-H⋅⋅⋅π and C-H⋅⋅⋅F interactions are displayed as blue and pink dotted lines, respectively (dark blue octahedra = WO6, gold sphere: oxygen, pink sphere: fluoride, blue sphere: boron, green sphere: nitrogen, grey sphere: carbon. Hatoms are omitted for clarity).

Figure 3 .

 3 Figure 3. (a) Crystal packing diagram in (1)2[W6O19]. Molecular arrangements of the 1 + conformers (b) between two adjacent {(1)2[W6O19]} ribbons, and (c) into head-to-tail dimers (ether groups are not displayed). Intermolecular C-H⋅⋅⋅π interactions are displayed as blue dotted lines (dark blue octahedra = WO6, gold sphere: oxygen, pink sphere: fluoride, blue sphere: boron, green sphere: nitrogen, grey sphere: carbon. H-atoms are omitted for clarity).

Figure 4 .

 4 Figure 4. Crystal packing diagram in (1)4[Mo8O26]⋅DMF⋅H2O. (a) β-[Mo8O26] 4-units in the wavy honeycomb-like supramolecular 2D network of 1 + (only the N-atoms of the ammonium head groups are displayed). (b) Molecular packing of the 1 + cations around the POM units. (c) Structural arrangement of the 1 + conformers (ether groups are not displayed). Intermolecular C-H⋅⋅⋅π and C-H⋅⋅⋅F interactions are displayed as blue and pink dotted lines, respectively. (light blue octahedra = MoO6, gold sphere: oxygen, pink sphere: fluoride, blue sphere: boron, green sphere: nitrogen, grey sphere: carbon. H-atoms are omitted for clarity).

38

 38

4 [

 4 Mo8O26]⋅DMF⋅H2O monitored at λex = 365 nm are displayed in Figure6.As observed for (1)Br, (1)2[W6O19]⋅2CH3CN and (1)2[W6O19]exhibit weakly intense orange fluorescence upon similar photoexcitation. The PL spectrum of (1)2[W6O19]⋅2CH3CN shows a broad and multimodal band centered at 678 nm. The PL profile of (1)2[W6O19] is roughly comparable except the maximum wavelength which is shifted to 694 nm. These emissions are characterized by mean PL lifetimes of τ = 185 ps and 219 ps for (1)2[W6O19]⋅2CH3CN and (1)2[W6O19], respectively (FigureS15).

Figure 5 .

 5 Figure 5. Solid-state UV-vis absorption spectra of (a), (1)2[W6O19]⋅2CH3CN, (b) (1)2[W6O19] and (c) (1)4[Mo8O26]⋅DMF⋅H2O (red lines), compared with those of (1)Br (black lines) and the optical POM references ((NBu4)2[W6O19] or ((CH3CH2)2NH2)2(NH4)2[Mo8O26]) (blue lines).

( 1 ) 2 [

 12 W6O19]⋅2CH3CN and (1)2[W6O19].

Figure 6 .

 6 Figure 6. Room-temperature PL spectra of (1)2[W6O19]⋅2CH3CN (blue line), (1)2[W6O19] (black line) and (1)4[Mo8O26]⋅DMF⋅H2O (red line) monitored at λex = 365 nm. Inset: photograph of crystalline powders of (1)Br (left) and (1)4[Mo8O26]⋅DMF⋅H2O (right) under illumination at 365 nm.

  POMs and the design of the framework. The assembly of 1 + with [W6O19] 2-having a low negative charge density favors the organization of 1 + into supramolecular dimers in (1)2[W6O19]⋅2CH3CN and (1)2[W6O19], affording weak fluorescence. In strong contrast in (1)4[Mo8O26]⋅DMF⋅H2O, the association of 1 + with β-[Mo8O26]

(b) (c)(a)

Wavelength (nm)

"Investissements d'Avenir" program n°ANR-11-IDEX-0003-02 and CHARMMMAT ANR-11-LABX-0039.

AUTHOR INFORMATION

Corresponding Author *E-mail: remi.dessapt@cnrs-imn.fr *E-mail: olivier.oms@uvsq.fr ORCID Rémi Dessapt: 0000-0002-2862-7767 Olivier Oms: 0000-0002-0745-1651 Eric Faulques: 0000-0002-7761-8509 Hélène Serier-Brault: 0000-0002-3300-1347 Anne Dolbecq: 0000-0001-7940-4152 Jérome Marrot: 0000-0002-5351-4908 Pierre Mialane: 0000-0002-8519-7943

NOTES

The authors declare no competing financial interests.


Keywords:

Informalintroduction

Topic. This short paper summarises some of my research data on national cancer controlprogrammes.ItfocusesonthecaseofEngland,whichIcomparewithFrance in my ongoing doctoral dissertation. The theoretical period of observation stretches back to the early 1970s, but my main angle of analysis concentrates on a particular policy initiative, the NHS Cancer Plan, published in September 2000 by the Department of Health, and its most recent sequel, the Cancer Reform Strategy, announcedin2006andfinalisedasapolicydocumentin2007.Myresearchquestions pertaintotheemergenceofcanceronthepoliticalagenda,themethodologyofhealth planning,andtherelationshipbetweencancercarereformandhealthcarereform.

Introduction

The health policy case study presented in this paper concerns the successive policy initiatives that affected the English cancer sector from the mid-1990s onwards. The research problem that is stressed throughout the paper pertains to the relationship betweenthesereformsandthewider,structuralevolutionsoftheNHS,bothfeatured in Table 1. To some extent, the organisational and political dynamics of reform were indeeddifferentatthecancersector-levelandathealthsystem-level. Outline.Thefollowingpapercoverstwomajoraspectsofcancercarereform.Itfirst explores the organisational logic of recent reforms, as embodied in two major policy 1 Note on a methodological dilemma. It remains unclear to me how best to theorise the parallel yet distinct evolution of the cancer sector and the English general health services policy. Frameworks that address the differences between the global and the sectoral levels of publicpolicy(JobertandMuller),orbetweenpolicy'subsystems'(SabatierandJenkins-Smith, Howlett and Ramesh) might fit, although an approach based on medical specialization and fields might eventually reveal more adequate [START_REF] Pinell | Champmédicaletprocessusdespécialisation[END_REF], since the empirical backbone of suchdifferenceswas,andis,theprofessionalfieldofoncology.Somehistoricalinsightsonthat matterappearinPickstone (2007).

ESPANETdraft1 4/18
documents,knownasthe'Calman-Hinereport'(DepartmentofHealth1995)andthe NHS Cancer Plan (Department of Health 2000). It then turns to the political factors thatconstrainedthedraftingofanationalcancercontrolprogrammeinEngland.The publishedviewsofpolicyinsiderssuchasRobertHaward(2002)madeuptheprimary material for this preliminary work on the English cancer sector, along with policy documentation and pilot interviews within the voluntary sector and the specialised media.

Comparison.Aparticularlyproductiveapproachtotheseresearchquestionsconsists in asking them comparatively within similar yet differentiable national settings. The main limitation of this paper, on top of the rather tentative nature of most of its arguments,ishencethatsuchcomparisonisrestrictedtodatafromtheFrenchcancer sector, and that comparisons will be drawn asymmetrically, with no intention of presenting both cases in identical levels of detail. 2 Further research is needed to produceasystematiccomparisonfrombothnationalcases.

Organisationalchallengesforcancerservices

Theguidinghypothesisinthispaperisthatreformsandcurrentpolicytrendsinthe cancersectorarebestunderstoodagainsttwowidertrendsofhealthservicespolicy:

1. The internal market. First, one has to consider the historical context of the NHSReview3 thatwasledduringthepremiershipofMargaretThatcherunder the title Working for Patients (Department of Health 1989), and which then passedintolegislationwiththeNationalHealthServiceandCommunityCare Act, coming into effect on 1 April 1991. All accounts collected so far4 seem to

ESPANETdraft1 5/18
indicate that, at the very fundamental level of modern cancer care, the core principles perceived as required to articulate clinically efficient cancer care pathways contradicted the foundational logic of market models promoted by Conservativegovernmentsforthedeliveryofpublicservicessuchashealthand education. 5 Specifically,thenationalpolicyreportsthataddressedthedelivery of cancer treatment in England seemed to run counter the experimental substitutionofmarketforcestocentrallyplanneddecision-makingasthemain driver for health care decision-making, advocating instead for a state-led programmatic approach to cancer services. In that respect, it seems that the principalmechanismguidingthedevelopmentofservicesatthemesolevelof the cancer sector in the past decades was negative policy learning 6 from the macrolevelreformsofthehealthcaresystem.

To some extent, the explanation for this finding can be derived from the structuralcharacteristicsofthepathologyunderscrutiny:thesheercomplexity of cancer (due, among other factors, to the variety of sites on which multiple types of malignancies can develop) and the resulting complexity of cancer treatment (a range of therapeutic strategies scattered among medical specialties) cancelled the background assumptions behind a market vision of health care-such as perfect information and negligible transaction costs regarding the demand and supply of medical goods. 7 Two organisational SomeguidingprinciplesforthisresearchwereextractedfrompilotinterviewsinLondon,May 2007. 5 The successive Secretaries of State in office during the NHS Review were John Moore (June 1987-July1988)andKennethClarke(July1988-November1990),thenfollowedbyWilliam Waldegrave (November 1990-April 1992). As far as the personal characters of the three Ministers were concerned, the authority with which they (especially Kenneth Clarke, who accelerated the Review's implementation; Webster 2002) intended to lead their reform of the NHS clearly affected its further developments, insofar as the lack of local experimentation of internalmarketsystemsthroughpilotprojectswasanimportantlineofcriticismheldagainst theGovernment'saction(Klein2006,p.157). 6 Thishypothesisstartswiththeobservationthatofficialsmakedecisionsonthebasisofpast policyexperience,andthat,generallyspeaking,'politicalinteractionhasconstitutedaprocess of social learning expressed through policy' (Heclo 1974, p. 306); see [START_REF] Freeman | LearninginPublicPolicy[END_REF] for a reviewoftheliterature.

7 From the rather large literature on the organisation of cancer care written by medical professionals themselves (published in the form of short articles, letters and editorials in medical journals such as the British Medical Journalor Clinical Oncology), it appears that the centralisation of services is persistently perceived as a necessity induced by the scarcity of technicalequipment:meansofradiotherapytreatmentordiagnostictoolsbasedoncomputed tomographicimagerysimplycannotbecomeavailableinallhospitals.Theoppositionbetween system-specific and cancer-specific reforms did not start with the internal market: the

ESPANETdraft1 6/18
properties of cancer care will be detailed to provide some empirical justification to that general statement: the spread of cancer networks promoting collaboration over competition (Section 2.2), and the de facto cancellation of the purchaser-provider split within them (Section 2.3). Managerialaspectsofhealthcarereformweremoresuccessfulintopervading cancercare,eventhough,asdiscussedinSection2.4,theuseofauditingand guidelines can serve professional activities in parallel to exerting professional control.

Modernisation.

The second influence on policy-making for cancer services comes from New Labour's modernisation agenda, which has been a topic of general inquiry from the overall perspective of government, governance and publicmanagementreform(Newman2001;PollittandBouckaert2004)aswell as from the perspective of health services policy. 8 Sections 3.2 and 3.3 on the role of evidence and consultation in policy-making show that cancer was following,ifnotanticipating-aswasthecaseinFrance-changesthataffected all policies in European health care states. Finally, to further comparison between the cancer sector and European health care reform (Freeman and Moran2000),Section3.4brieflycoversissuesinfundingandbudgeting.

Competitionandcancernetworks

A distinctive feature of the economic vision carried by the NHS internal market was the disaggregation of medical supply into provider units, composed of hospitals and community services. Intended competition between these supposedly homogenous units, however, disrupted cancer clinical pathways which spread over the equipment andstaffofhospitals,duetothedistributionofspecialistsanddifferinginvestmentsin expensive technology. In that sense, 'the market was particularly detrimental to coordinatedcareacrossorganisationalboundaries,asitgeneratedcompetitionbetween hospitals,therebydiscouragingcollaboration'(Haward2002,pp.89-90).

AccordingtotherecommendationsoftheCalman-Hinereport,whichitselfdrewfrom a 1994 report by the Association of Cancer Physicians, 9 the dominant mode of organisation was instead to become the hub-and-spoke logic of cancer networks (Department ofHealth 1995, §4.1.1),with cancercentresandcancerunits connecting fundamental logic of cancer services organised around radium since the beginning of the century(Pickstone2007)alsocontradictedthedevelopmentofdistricthospitalsinthe1970s.

8 ForageneralreviewofmodernisationwithinBritishhealthpolicy,seeAlvarez-Rosete(2008);

for an illustration through mental health services, see 6 and Peck (2004). For a comparative assessment,seeNewmanandKuhlman(2007)andMartinPowell'spaper'ModernisingHealth CareinEurope,'presentedattheESPANETAnnualConferenceinVienna,2007.

9 Asforcomprehensivecancerpolicyframeworks,theideaofassemblingcancernetworkswas notnewtothe1990s:theRoyalSocietyofRadiographershadalreadysuggestedthemin1979.

ESPANETdraft1 7/18 theirrespectiveservicesandspecialistteams. 10 Whilesomepractitionerswereableto experiment delivery networks as soon as 1996 (16 transitional networks existed in 2000),thegeneralisationofcancernetworksoccurredonlywiththeNHSCancerPlan, amountingtotheformationof34networksencompassingallEnglishcancerservices byApril2001.

Cancellingoutthepurchaser-providersplit

Thepurchaser-provider split, asthecornerstoneof the'internal market'(atermthat did not trigger much enthusiasm even among the Secretaries of State in charge of implementingit;Ham2000,p.28-sq.),wasaparticularly.ThecreationofNHSTrusts wouldenablecompetitiononlyifachangeinpurchasingandprovidingpatternscould occur,whichwasfarfromthecaseinmanyinstances.Equipmentthatwascrucialto cancer treatment (radiotherapy) and diagnosis (imagery) required resource pooling amonghospitalsinsteadofprovidercompetition,asembeddedintheinternalmarket [START_REF] Haward | Commissioning Cancer Services[END_REF]. Similarly, the preference for a proliferation of district hospitals over larger units of care delivery ran counter the centralised model of care built around radiotherapy(Haward2002;Pickstone2007).

TheCalman-Hinereporttook,initsrecommendations,averydifferentdirectionthan the internal market by having 'both purchasers and providers re-examining their patternsofcancercare'(DepartmentofHealth1995, §2.5),whichtranslatedinpractice intoaformofcoordinationbetweenbothpartiesthatsupportedthelogicofcollective planningovercancertreatmentsandwentlargelycountertotheWalrasianprinciple ofmarketequilibriumthroughpricefixation.

Managerialism,professionalidentitiesandclinicalautonomy

A last dimension of organizational change related to the internal market pertains to thepossibilitythatthereformmighthavealteredprofessionalroleswithinthemedical sector. Data collected during the introduction of the reform suggested that clinical directors were effectively expressing a dual 'bureaucratized' identity, hybridizing managerial and clinical values into a single professional entity that did not keep administrativeandmedicalworkroutinesseparated,andeventradedthelatterforthe former ones at times [START_REF] Kitchener | The "Bureaucratization" of Professional Roles: The Case of ClinicalDirectorsinUKHospitals[END_REF]. Such modification in professional identities mightbeunderstoodprimafacieasavictoryfortheConservativeadministrationsthat insisted on introducing managerial notions such as performance and auditing within medicalpractice.However,thereareatleasttwowaystosuggestthattheintroduction ofmanagerialismwithinthemedicalfieldmightbeinterpreteddifferently:

1. First,eveninsituationswherepractitionersmighthavetoacceptnewformsof professionalscrutiny,thisdoesnotmeanthattheynecessarilylosetheability to enforce these controls themselves within their professional field. In that sense,'evidence-basedmedicinemay,inasimilarwaytoclinicalaudit,become appropriated and distorted by the still powerful medical professionals. This would involve hospital doctors protecting their autonomy by ensuring that practice developments continue to be largely internally driven rather than externallyimposed'(Kitchener2000,p.146;seealsoRosenandMays1998).

2. Asecondsuggestionpositsthatpractitionersarecapableofrecyclingexternal constraintsintointernalresources. 11 Usingthesamelineofreasoningusedby Patrick Castel about the effect of evidence-based clinical guidelines on professional autonomy within the cancer sector 12 may be used in the English context to hypothesize that, when doctors were able to take the managerial notionsintroducedbytheinternalmarketintotheirownhands,itthenformed a new strategic asset for them to increase their legitimacy among their peers and support their views while performing common medical tasks such as discussingtherapeuticstrategieswithcolleagues(CastelandMerle2002).

From this viewpoint, the 'alien set of values' (Klein 2006, p. 156) introduced by the internalmarketreformmighthavetranslatedintosomethingelsethanareductionin clinical autonomy and the downplaying of professional groups within the policy process.Theemergenceof'clinicalmanagers'(Llewellyn2001)identicallyguaranteed thatmanagementstayedinthehandsofpractitioners. 13 Anapplicationofthistheory toclinicalguidelinesisprovidedinalaterSectionon'evidencepolitics.' * It might hence be the case that the sub-sector dynamic of cancer was negating the macro-evolutionofthehealthsystemasawhole.Lateron,whentheNHSCancerPlan reiterated most of the policy recommendations of the Calman-Hine report only in a 11 'Internal'and'external'refertothesymbolicfrontiersofthemedicalprofessionalfield.

12 French oncologists working in comprehensive cancer centres protected their way of doing fromhospitalcompetitionbyresortingtoclinicalguidelinesinordertoclaimahigherquality ofcare,anticipatingalargermovetowardsclinicalstandardsintheFrenchhealthsystem.See [START_REF] Castel | Quand les normes de pratiques deviennent une ressourcepourlesmédecins[END_REF]Castel and Friedberg 2004; as well as Patrick Castel's doctoral dissertation (Paris, 2002). For a summary, see his paper 'Recent developments in cancercareinFrance. Competition or coordinatedcare?' at the CHSTM workshop 'Sites and Styles:ExploringtheComparativeHistoryofCancer,'March2007(pages2-3).

13 Somesub-professionaldynamicsarealsoidentifiablewithinthedifferentspecialtiesofcancer treatment.UsingdatafromFranceandfromtheinternationalscientificcommunity,Casteland Dalgalarrondo (2005) observed that paediatric oncologists have been spearheading the trend towardsstandardizationofpractices.

ESPANETdraft1 9/18 more operational fashion, 14 further demarcation from the policy style of the NHS internalmarketreformwasproduced.Lateron,theapproachofserviceplanningfora specificgroupofpathologieswasreplicatedtoawidegamutofdiseases,throughthe applicationofthehealthpolicyinstrumentofNationalServiceFrameworks. 15 Section3 covers some of the political adjustments made by the Labour government while writing.

Thepoliticsofcancercarereform

Issueemergenceandagenda-setting

While NHS reform is, like any other component of the nation's political economy, a systematicconcernforgovernmentsduetobudgetandperformanceissues,reformin thecancersectorrequireitsownspecificexplanatorynarrativetoexplainwhycancer waselevatedontothegovernmentalagendaasandistinctissue. Schematically,twotypesofissueemergencecansummarizetheexperienceofEngland andothercountriesinEurope.

England

1. Policy triggers. In the first case, pressure to act came from policy triggers [START_REF] Weaver | SettingandFiringPolicyTriggers[END_REF]). The publication of comparative cancer survival data by the EUROCAREepidemiologicalstudy 16 effectivelyworkedasaninvoluntarypolicy trigger,whichfiredawarningtogovernmentswithdisadvantageouspositions in survival league tables, namely Britain and Denmark. In the latter country,

Consultativepolitics

The NHS Review gave a particular profile to health policy-making that generated 'as muchresentmentbecauseof[its]styleasdisagreementoversubstance…thoseinvited tothemeetingswereselectednotbecausetheywererepresentativeoftheprofessional interests (the Royal Commission model) but precisely because they were unrepresentativeintheirsympathyforideasofradicalreform'(Klein2006,p.147).The resulting effect on the medical profession was 'outrage at its exclusion of the policy process' (Klein 2006, p. 154). The Calman-Hine report, being authored by an Expert Group commissioned by the Chief Medical Officers of England and Wales, already indicated a return to the more traditional routines of health policy-making in the hands of professionals. National Service Frameworks, as explained in research by ChristineHoggforthePatient'sForum(seealsoHogg2002),wentevenfurtherinthe institutionalisationoflegitimateviewpointsbyinvolvingpatientsinExpertReference Groups-whichdidnotapplytotheNHSCancerPlaninsofaras,althoughpresented as a National Service Framework, the consultation procedure was bypassed to accelerate its elaboration due to the sense of urgency derived from the EUROCARE data.

Evidencepolitics

From the analytical angle of objectivity and embedded authority, National Service Frameworksderivedtheirlegitimacyfromotherfactorsinadditiontothescopeofthe consultation process. Their extensive use of clinical evidence shaped the content of NSFs with the objectivity and legal-rational authority associated to evidence-based medicine.Inthatrespect,evidencehereplayedapoliticalroleintheacceptationand adoptionofNSFsasappropriateguidestowardshigh-qualityhealthcareservices.

To some extent, NSFs can also be understood as a product of policy learning. An appropriatedescriptorforpolicy-makingwithinthehealthsectorinthelate1980shad been:'experimental.'RudolfKlein'sdescriptionofthesituationaftertheNHSReview isparticularlytelling:

Therationalmodelofpolicy-making-firstexperiment,thenevaluate,finallydecidemaythusbebased,thecaseoftheNHSsuggests,onaseriesofirrationalassumptions aboutthefeasibilityofapplyingit.Incontrast,ministerialstrategyseemstohavebeen based -no doubt unconsciously -on a very different model of rationality in policymaking. This sees policy-making itself as an experiment -the testing of theory by puttingitintopractice.(Klein2006,p.158) Thepolicydocumentationfromthesubsequentdecadesuggeststhattherewasindeed asense,fromthe1990sonwards,thatthepastcontainedmanyoftheseexperimental policies. In the cancer sector, however, professional guidance remained the cornerstoneofservicedelivery.Earlieron,theCalman-Hinereporthadsuggestedthat 'Professional bodies should urgently develop guidance on the level of expertise and supportrequiredtomanagethecommonercancers'(Recommendation6).Attheend ofthe1990s,theCancerOutcomesGroupledbyRobertHawardhadhencedeveloped afull-fledgedseriesof'ImprovingOutcomes'standardizedprotocolsforseveraltypes ofcancer,providingthecancerfieldwithaclinical'goldstandard'(forananalysisof theroleofclinicalguidelines,TimmermansandBerg2003;seealsoDaly2005).

Expenditurepolitics

Defining an optimal level of health expenditure for the NHS had been deemed impossible by successive Royal Commissions, until the Wanless Inquiry proposed to usecomparativeexpenditurestatisticsfromotherWestEuropeancountries,National ServiceFrameworksandprospectivecalculationsofcostsinducedbydemographicand technologicalevolutions(Klein2006,p.220).

AcrucialaspectoftheNHSCancerPlanwasthatitvirtuallyring-fencedanadditional budgettobespentoncancerservicesoverthefirstthreeyearsofimplementationfor thePlan(DepartmentofHealth2000, §18).Theinitialpromiseofthegovernmentwas to spend £280 million in 2001-2002, £407 million in 2002[START_REF] Timmermans | The Gold Standard[END_REF], and £570 million in 2003-2004(Department of Health 2000, §11.3). From there on, political contention couldbeexpectedfromthreesides:

1. Adequate spending. First, whether the "cancer budget" was being spent accordinglytowhatthePlanhadrecommendedwasopentoquestion.Parallel reportsfromtheNationalAuditOfficeandtheCommitteeforPublicAccounts scrutinized public spending and concluded that investment targets had been effectively met (National Audit Office 2005b, §1.10), which is a separate issue fromothertargetsbeingmet. There will be no return to the old centralised command and control systems of the 1970s. That approach stifled innovationandputtheneedsofinstitutionsaheadoftheneedsofpatients.Butnorwill there be a continuation of the divisive internal market system of the 1990s. That approach which was intended to make the NHS more efficient ended up fragmenting decision-making and distorting incentives to such an extent that unfairness and bureaucracybecameitsdefiningfeatures.(DepartmentofHealth1997, §2.1)

ESPANETdraft1 14/18
ReformswithinthecancersectoratthetimeoftheCalman-HinereportandtheNHS CancerPlantendtoshowthatcentralised'commandandcontrol'wassuitableatleast during the programmatic phase of policy-making, in order to establish the organisationalstandardsbywhichcancerclinicalpathwaysshouldbeorganised.The market, then, was perceived as divisive among the large networks of collaboration required for the efficient treatment of cancer. Further to this change of style in organisationalprinciples,therewasagradualchangeofstyleinthepoliticalresources mobilized during the conception of national cancer control programmes. Both observationsmakeupthetwomainresultsofthispaper.

Coda

Two-levelgamesinhealthpolicy

Aslaterpolicytrendsshow,however,the'marketmodel'alternativeremainedexactly that: an alternative solution for policy-makers. An interesting development since the abolition of the internal market by the Labour government (Department of Health 1997) has indeed been its gradual exhumation from the policy graveyard in the past years,from2005onwards(Klein2006,ch.8).Akeyargumentintheinitiationofthe CancerReformStrategyindeedliedinthenecessityfortheorganisationofthecancer sector to take stock of the wider structural changes associated with the NHS. The Strategyitselfisaveryrecentpolicydevelopment,withitsmainframeworkdocument published in December 2007. Its content and focus on adapting the sub-sectoral dynamics of cancer services to 'patient choice' and other macro-level innovations showsthatthe'two-levelgame'betweenthehealthsectorandthecancersub-sector 30 continues to define the making of health policy for that specific pathology. Other medical conditions with equally strong defining social properties -such as a reasonably large epidemiological base, a body of specialised health professionals and voluntary sector organisations, idiosyncratic forms of treatment and a certain degree of institutionalised autonomy within clinical and research facilities -might express identical adjustments, which would then justify for the linguistic image of the 'twolevelgame'tobedevelopedintoamorecoherentanalyticalframework.

  both Left and Right governments), as several institutional arrangements incitedpolicy-makerstothinkofhealthcareandpublichealthmoreintermsofnationalplans. ThecorrespondingnotionsofplanningintheFrenchcontext,planificationanddirigisme,seem toapplycorrectlytotheevenmorestatistturntakenbyhealthpolicy-makersatthetime.In generalterms,governanceintheFrenchhealthsectorisfragmentedandlargelydysfunctional, and the French public health regime has been historically weak in comparison to the British one. From the viewpoint of ministerial units, national health programmes might have been conceived as an attempt to restore some coherence to the agenda of the General Health Directorate(DirectionGénéraledelaSanté,DGS),inaparallelefforttotheétatisationofhealth system regulatory policies through the National Union of Health Insurance Funds (Union Nationale des Caisses d'Assurance-Maladie, UNCAM) and the Hospitals and Health Services Directorate(DirectionGénéraledel'Hospitalisationetdel'OrganisationdesSoins,DHOS).

  1

	Timeline
	1988 Jan	MargaretThatcherannouncesministerialNHSReview.
	Jul	SocialServicesdetachedfromDHSS
		KennethClarkeappointedasSecretaryofStateforHealth.
	1989 Jan	WorkingforPatientspublishedalongwithseriesofworkingpapers
	1991 Jan	NationalHealthServiceandCommunityCareActcomesintoeffect.
	1995 Apr	'Calman-Hinereport'published.
	1997 May Labourgovernmentelected.
	1998 Dec	EUROCARE2comparativesurvivaldatapublished.
	1999 May Number10cancersummit.
		MikeRichardsappointedasNationalCancerDirector.
	2000 Sep	NHSCancerPlanpublished.
	2001	Labourgovernmentre-elected.
	2004 Mar	FirstNationalAuditOfficereportontacklingcancerinEngland.
	2005	Labourgovernmentre-elected.
		MimicmarketmechanismsreintroducedwithintheNHS.
	2006 Aug	King'sFundreportrecommendsupdatingtheNHSCancerPlan.
	Nov	CancerReformStrategylaunchedatBritainAgainstCancerConference.
	2007 Dec	CancerReformStrategypublished.
	Table1.ChronologyforNHSinternalmarketandcancerpolicies,1988-2007.

  Policy entrepreneurs. In the second case, the issue of cancer care emerged becauseaparticularsetofpolicyplayersexertedpoliticalpressureoverpolicymakers,advancingsolutionsalongtheirdemandforactionandcatchingmedia attention. Each country tends to follow its own dynamic of cancer patient mobilization.22 23 In England, the voluntary sector was less instrumental in producing the NHS Cancer Plan than it was at producing the Cancer Reform Strategy, announced by Secretary of State for Health Patricia Hewitt in November2006attheannualBritainAgainstCancerConferenceorganisedby theAll-PartyParliamentaryGrouponCancer.24

	2.	nationalcancersurvivalperformanceappearedtobe'markedlypoorer'thanin
		the rest of Western Europe; As a consequence, a Cancer Steering Committee
		wassetupin1998andtwoNationalCancerPlanswerepublishedin2000and
		2004(Appel2004;Haward2008;Pedersen2006).
		InBritain,theresultsoftheEUROCAREstudy(magnifiedbytheirmentionin
		an OECD economic survey of Britain published in 2000) 17 were surprising
		enough for an workshop of international experts to be convened at the
		DepartmentofHealthtoelucidatewhetheritsdatawereactuallyreliable.Asit
		occurred, experts agreed that the gap between Britain and the Continent was
		definitelyreal,andthatitshouldbedealtwithurgently.Asaresult,TonyBlair
		himself announced an 'action plan' for cancer at a top meeting with cancer
		expertsonThursday20May1999at10DowningStreet,appointinglaterinthe
		sameyearamedicaloncologist,PrMikeRichards,asNationalCancerDirector
		withresponsibilityfordraftingandimplementingthePlan. 18 Laterintheyear,
		more pressure on the executive came from the 'Costs of Cancer Care'
		conference 19 by the All-Party Parliamentary Group on Cancer. 20 Finally,
		accordingtooneinsider,itwastheparticularlydramatic'MavisSkeet'caseofa
		cancer patient who died while waiting for care during the winter 1999 that
		promptedgovernmentalactioninhealthservicespolicybyattractingextremely
		negativemediacoverage. 21
	17 OECD2000;mentionedinSmee(2008,p.92).Smeeobservesthat,interestingly,theprevious
	surveyfrom1994hadonlyunderlinedtheremarkablecost-efficiencyoftheNHS;toexplainthis
	changeinfocus,detailedhistoriesofBritishhealthpolicyshowthatattentiontoqualityofcare
	was driven by remarkable fiascos, such as the one that triggered the Bristol Royal Infirmary
	Inquiryof1995(see,e.g.,Baggott2004).
	14 ThegovernmentwashelpedinthistaskbythepublicationofareportonTheworkforceand
	training implications of the Calman-Hine report, jointly issued by the King's Fund, Macmillan 20 TheGroupmaintainsstronglinkswiththethirdsectororganisationsinvolvedincancercare
	CancerReliefandtheCancerResearchCampaignin1997. andcancerresearch.ThedominantplayersatthetimeweretheImperialCancerResearchFund
	15 Clearly,then,whatcouldbesensedfrom1995onwardsinthecancersectorbecameageneral andtheCancerResearchCampaignledbyGordonMcVie(bothorganisationslatermergedto
	principle of health service configuration in the early 2000s, indicating a further change in formCancerResearchUK).WithintheGroup,themainparliamentaryadvocateforcancercare
	governmentalrationalityregardingtheprinciplesofpolicy-making.SeeSection4. inthepastyearshasbeenDrIanGibson,LabourMPforNorwichNorthsince1997,whohasa
	16 EUROCARE is a research programme funded by the European Commission since 1989 that professionalbackgroundasaresearcherinmolecularbiologyandcancergenetics.Professional
	connects cancer registries throughout European Member States. Its role as an involuntary knowledgeofcancerandcancertreatmentarehencesharedattributesonallsidesoftheBritish
	policy trigger illustrates the indirect impact of European decisions onto national health cancerpolicycommunity.
	policies, even though the organisation of national health systems have remained a state 21 AccordingtoStevens(2008,p.108),theMavisSkeetcasedirectlytriggeredBlair's'Breakfast
	prerogative (for a similar view, see Trubek et al. 2008). Several observers commented at the withFrost'wishofaligningBritain'spublichealthexpenditurewiththeEUaverage(seeKlein
	timeontheuseofEUROCAREasameansofbenchmarkingbetweencountries,butnomajor 2006fordetailsontheannouncement).Inthewinter1999,theNHSwasthenfacingincreased
	controversyemergedonthatmatter.EUROCAREproduceditsfourthwaveofresultsin2007. delaysinsurgicaloperationsandanA&Ecrisisfollowinganoutbreakofflu.
	ESPANETdraft1 ESPANETdraft1	10/18 11/18

18

'PM'scancerdrive"dependsoncash",'BBCNews,20May1999.

19

'UKcancercare"failspatients",'BBCNews,9November1999.

  25 22 Patient organisations certainly played a major role in influencing the course and shape of cancer policies in France, as shown in postgraduate research by Marie Ravier and Sandrine Knobé as well as in personal research. For such dynamics in the United States (where communityoncologistsplayedanimportantroleinspeakingforcancerpatientsduringthelate 1970s), see the paper by Peter Keating and Alberto Cambrosio 'Patients and Protocols' at the CHSTM workshop 'Patients and Pathways: Cancer Therapies in Historical and Sociological Perspective,' October 2005 (pages 8-11), as well as the extensive literature on the politics of cancerand(generally)breastcancerintheUnitedStates.

	ESPANETdraft1	12/18

Note on comparative case studies. From background readings, it clearly appears that several other countries would make valid case studies, such as Denmark and Sweden in NorthernEurope,orCanadaandtheUnitedStatesinNorthAmerica.OnCanadaparticularly, seeHayter(2005)andhispaper'TheInaccessibleHighway:TheFailureofCentralizedCancer Care in Ontario, 1930-1990' presented at the CHSTM workshop 'Patients & Pathways: Cancer Therapies in Historical and Sociological Perspective,' October 2005. As for the second part of thispaper-thedraftingofnationalcancerplans-Swedenseemstobeaninterestingnegative case, where attempts to draft a national policy for cancer (encouraged by the national thirdsector organisation Cancerfonden) have been hampered by the decentralised nature of the healthcaresystemandoppositionbetweenregional.Regionalhealthsystemscannothowever fullyexplaintheabsenceofnationalstrategies,sincefederalregimessuchasSwitzerlandand Belgiumhaverecentlyengagedintomakingnationalcancerplans.

3 On that historical period, see[START_REF] Ham | ThePoliticsofNHSReform[END_REF],Klein (2006, ch. 6) and[START_REF] Webster | The National Health Service. A Political History[END_REF]; On the internalmarket,seeIsomandKandiah(2002)

.4 Asmentionedinseveralpartsofthispaper,accountsfromtheexistingscholarshiponcancer careandreadingsofpolicydocumentationwillultimatelybetriangulatedwithinterviewdata.

Comparative analysis of cancer patient mobilization is not developed, however patients themselves are actively learning from national experiences in cancer activism through conferencesorganisedbyumbrellagroupsliketheEuropeanCancerPatientCoalition(ECPC) orinternationalorganisationsliketheInternationalUnionAgainstCancer(UICC).

S.Boseley,'Charitieswinfightfornewcancerplan,'Guardian,30November2006.

Interviewdata,London,2007;NationalAuditOffice(2005b,p.7).

CancerResearchUKthroughtheCancer2020campaignandits250,000+petitionsignatures, andaconglomerateofvoluntaryorganisationswithintheCancerCampaigningGroup.

Interviewdata,London,2007.


Lucas Méthivier

Romane Braun

Francesca Chillà

Julien Salort

Turbulent transition in Rayleigh-Bénard convection with Fluorocarbon

We present measurements of the global heat-transfer and the velocity field in two Rayleigh-Bénard cells (aspect ratios 1 and 2). We use Fluorinert FC770 as the working fluid, up to a Rayleigh number 2 × 10 12 . The velocity field is inferred from sequences of shadowgraph pattern using a Correlation Image Velocimetry (CIV) algorithm. Indeed the large number of plumes, and their small characteristic scale, make it possible to use the shadowgraph pattern produced by the thermal plumes in the same manner as particles in Particle Image Velocimetry (PIV). The method is validated in water against PIV, and yield identical wind velocity estimates. The joint heat-transfer and velocity measurements allow to compute the scaling of the kinetic dissipation rate which features a transition from a laminar Re 5/2 scaling to a turbulent Re 3 scaling. We propose that the turbulent transition in Rayleigh-Bénard convection is controlled by a threshold Péclet number rather than Rayleigh number, which may explain the apparent discrepancy in the literature regarding the "ultimate" regime of convection.

Introduction. -Turbulent thermal convection is a modern and important problem involved in a lot of practical situations and linked to environmental challenge. Natural flows are indeed difficult to characterize and physicists use often some model systems, such as the Rayleigh-Bénard flows which can mimic a lot of situations and are easy to manage in laboratory. The definition of Rayleigh-Bénard (RB) flow is a fluid layer confined in a cell of characteristics height h on which a difference of temperature ∆T = T bottom -T top is applied to destabilize the flow, so that T bottom > T top . The non-dimensional parameters characterizing the forcing are the Rayleigh number

Ra = gα∆T h 3 νκ , (1)
and the Prandtl number

P r = ν κ , ( 2
)
where g is the gravity acceleration, α is the thermal expansion coefficient, ν is the kinematic viscosity and κ the thermal diffusivity, coupled with the aspect ratio Γ = D/h where D is the horizontal dimension of the cell. One of the response parameter is the non-dimensional heat transport, the Nusselt number,

N u = Qh λ∆T , (3)
where Q is the heat flux injected in the cell, and λ the thermal conductivity. The other important parameter is the Reynolds number

Re = U H ν , (4)
where U is a typical velocity and H a typical scale. As clearly shown in the literature [1,2], the RB flow is largely inhomogeneous and the velocity can change between one place or another of the cell, the definition of H can depend on the place in the cell as well. A point arouse about the scaling laws that can be built as function of the Rayleigh number. To have this kind of discussion, measurements of velocity must be realised in several part of the flow and at different Rayleigh numbers. Two techniques have essentially been used till now: optical measurements such as Laser-Doppler Velocimetry [3][4][5], Particle Image Velocimetry (PIV) [6][7][8][9][10][11] or Lagrangian tracking [12,13]; or the measurement of correlation time delay between two probes in a particular point [14][15][16][17], recently a measurement of tracking of plumes from shadowgraph pattern have been also introduced [18]. In this letter, we introduce a really cheap and easy to manage method, to get an estimate for the full velocity field. Once this technique is introduced, we will discuss the estimated Reynolds numbers, and their scaling. At the end we will discuss the friction coefficient, and how it relates to the turbulent transition.

Experimental set-up. -We are using in this study two parallelepiped Rayleigh-Bénard convection cells with different aspect ratios. For both cells, the width is 41.5 cm, and the depth is 10.5 cm. The height of the first cell, of aspect ratio Γ = 1, is 41.5 cm. It is the same cell as previously used [18]. The second is identical but was fitted with smaller glass walls of height 20 cm. Its aspect ratio is Γ = 2. For both cells, the depth is 4 time smaller than the length so the flow is quasi bi-dimensional.

The cells are made with glass to allow optical measurements with better quality than Polymethyl Methacrylate (PMMA) which yields spurious gradients of optical index. The plates are smooth and made of copper with a thin layer of nickel. The bottom plate is heated with a constant power. The top plate is cooled by a circulation of ethylene-glycol mixture, regulated by a Lauda Variocool chiller. The convection cell is placed inside a PMMA box, thermally regulated at the same temperature as the bulk of the flow. The aim is to reduce the heat losses while keeping the optical access. An additional heat insulation consisting in neoprene foam and a temperature regulated copper thermal screen may be installed around the convection cell. This additional insulation prevents measurements with optical means, but can be used to improve the accuracy of the Nusselt number estimates.

The working fluid is Fluorinert FC770 (produced by 3M), which allows to reach higher Rayleigh numbers than those we had in the same apparatus with deionized water [7]. To compute the non-dimensional parameters, accurate estimates of the physical properties of the fluids are required. While they are well known for deionized water, it is not the case for FC770. Previous studies using FC770, such as [19], used the physical properties at 25 °C provided by the manufacturer. However, a higher working temperature (40 °C) allows to increase the temperature difference, and span a wider range of Rayleigh number. Additionally, the temperature dependence of the physical parameters is useful to estimate the deviation from the Boussinesq approximation. It is therefore difficult to compare the Rayleigh and Nusselt numbers obtained in FC770 with other data from the literature without better estimates of the fluid properties.

Therefore, we got the physical parameters measured by Flucon GmbH, see table 1. The viscosity required to compute the Rayleigh number, is computed from this data as ν = η ρ . Special care must be taken regarding the thermal conductivity, λ, which is required both for computing the Nusselt number, and for the estimate of the heat diffusivity in the Rayleigh number, κ = λ ρcp . Indeed, most commercial apparatus have not been designed with a fluid such as FC770 which has both low thermal conductivity and low viscosity, and their design produce spurious natural convection. That is why we performed our own measurements of the thermal conductivity using a commercial thermal conductivity analyzer, model "TCi", from C-Therm Tech- nologies Ltd, inside a temperature regulated chamber. In this configuration, the measurement is performed on a thin film of liquid, and not with a heated wire. We find the following fit for λ(θ), which we use in the remainder of the paper,

λ(θ) = λ 0 + (∂ θ λ)θ, (5)
where λ 0 = 0.1114 W/m/K is the thermal conductivity at 0 °C, ∂ θ λ = 3 × 10 -4 W/m/K/°C, and θ is the temperature in °C. This value of λ is quite different from the estimate from the manufacturer, λ 3M = 0.063 W/m/K at 25 °C. It is unclear how spurious convection bias the measurement, as it depends on details of the apparatus which are not publicly available. As can be seen in Fig 1 , our Nusselt numbers are a bit below the values of the Grossmann-Lohse (GL) model [20]. They would however be much higher than the GL model if λ 3M was used instead. It is possible that our measurement with the TCi thermal conductivity analyser slightly overestimates λ.

The phenomenological value of λ that would offset our heat-transfer data so that it matches the prediction of the GL model is λ GL = 0.0945 W/m/K at 25 °C.

The heat transfer measurements are reported in table 2 for both aspect ratio cells without additional insulation, and in table 3 for Γ = 1 cell with the additional insulation. As can be seen in Figure 1, the scaling law for the Nusselt number obtained with FC770 is close to Ra 1/3 , up to Ra = 2 × 10 12 , in fair agreement with the results from He, et al [21], as well as the Grossmann-Lohse model [20]. At this stage, the puzzle remains why it seems to be in disagreement with the data from Chavanne, et al. [22], and from Niemela, et al. [23,24].

The interpretation proposed by Chavanne, et al. [22] to explain the heat-transfer enhancement that they report, is a laminar turbulent transition of the boundary layer, which yields the inertial regime predicted by Kraichnan [25]. This interpretation was backed by two observations: (i) a transition for the dissipated power from a laminar ǫ ∼ Re 5/2 regime to a turbulent ǫ ∼ Re 3 regime; (ii) a change in the statistics of the temperature fluctuations. The latter was later confirmed with a smaller thermometer [26]. In the following, we focus on velocity measurements, and estimates of the dissipation, to compare the dissipation scaling with the scaling in cells with and without heat-transfer enhancement.

Shadowgraph and velocity measurement. -In this range of Rayleigh numbers, most cells do not allow visualization, and the wind velocity is often estimated from the correlations of local thermometers [16,17,22,27,28]. In this work, we use optical methods which allow to resolve the velocity field. However, the cost of FC770 as a working fluid makes it unsuitable to particle seeding, as pollution of the working fluid would be very costly. Therefore, we based the measurements on shadowgraph, which is non-invasive and does not pollute the working fluid. The method investigated here may also be useful for other experiments where particle seeding would be impractical, such as fluids at extreme temperatures.

The set-up is similar to the one described in [18], and consists in direct shadowgraphy in diverging light. In this paper, the cell is illuminated by a punctual monochromatic light (M450LP1 LED from ThorLabs). The light source is far enough from the cell, that the light is almost parallel. The shadowgraph pattern is captured on a ground glass diffuser, and recorded with a PCO Pandas camera, with a resolution 2048 × 2048 pixels. An example of shadowgraph pattern is shown in figure 2. The frame rate of the image sequence varies between 15 Hz at low Ra and 40 Hz at high Ra, to accommodate for the faster plume velocities when Ra is higher. The duration of the sequence is a few turnover times, ranging from 3 minutes and 20 seconds to 8 minutes and 20 seconds.

There are several strategies to infer an estimate of the velocity fields from the shadowgraph images. In a previous work, we investigated a simple method based on space-time diagrams [18]. Because this method tracks the plumes, it allows to infer statistics on the plumes. However, the drawback is that the measurement is averaged over lines. In the present work, we investigate an alternative method which consists in applying to the shadowgraph pattern the same algorithm that we use for Particle Image Velocimetry (PIV), i.e. Correlation Image Velocimetry (CIV) from the CIVx toolbox implemented by Fincham and Delerce [29]. This is possible in this range of Rayleigh numbers, because plumes are relatively small and fill the entire volume, and therefore may act as local pattern for the correlation algorithm similar to the particles in PIV. One important difference between the velocity field obtained in this way compared to the velocity field in PIV is that the light pattern is integrated over the depth of the cell, while particles in PIV are imaged on a light sheet. In our case, however, the flow is quasi bi-dimensional, so the integra- tion across the depth does not blur excessively the flow field. The CIV box size has to be a bit larger than with PIV, typically 4 mm, for them to hold a usable plume pattern. This means that we cannot resolve the velocity field inside the boundary layers, which are typically one order of magnitude smaller. To validate the method, we applied it to the case of deionized water, for which we could compare against PIV measurement. As can be seen in Figure 3, the average flow pattern is well recovered, except in the corners where both upwelling and downwelling plumes are visible on the shadowgraph sequence, which results in small velocity magnitude in the CIV field. The vertical velocity component near the top and bottom plates also show a qualitative difference. This may be caused by wall flows, such as plume attachment along vertical walls. Reynolds number and friction coefficient. -There are different kinds of definitions for the Reynolds number in the literature: based on the average velocity, the average turnover time, or the velocity fluctuations. The position of the sensor also varies, which makes it difficult to directly compare the value of the Reynolds numbers. However, Musilovà, et al. have compared estimates based on the sloshing mode frequency, time of flight between thermometers, and average or fluctuating velocities from the Elliptic approximation, and found that all these estimates roughly scale identically [17].

In the following, we choose the height of the cell h, as the length scale H in the estimation of the Reynolds number, and the vertical component of the velocity inside the jet as the typical velocity U . From the mean velocity field, we compute velocity profiles at mid-height, by spatially averaging on a window of height 2 cm. Examples of velocity profiles are shown in Figure 4. We use the point closest to the walls on these velocity profiles to compute the Reynolds number. This is similar to measurements with a fixed sensor. The relative position in the present work is of order 1 % of the cell width, which is comparable to [17,28]. However, as noted previously [18], the nondimensional shape of the velocity profile does not seem to change significantly with the Rayleigh number. Therefore, the actual choice for the position of the velocity probe only produce a constant prefactor for the Reynolds number, which however could be as high as 40 % across the published data in the literature. The Reynolds number values are given in Table 2. As can be seen in Figure 5, they are in fair agreement with other Reynolds number data from the literature, with a scaling close to Ra 1/2 P r -0.7 .

We compute the friction coefficient, using the same approach used by [22]. Let us recall that the balance of 3: Global heat-transfer measurements in the Γ = 1 fluorocarbon cell, with the copper thermal screen for more accurate Nusselt number estimates. The Nusselt number is estimated using Q = Qraw -Q loss , with Q loss = -0.7 W for the 25 °C series, and Q loss = 2.5 W for the 40 °C series. kinetic energy yields the following exact relation [1,2],

Ra

N u P r Q raw ∆T T moy [W] [K] [°C]
ǫ = ν 3 h 4 (N u -1)RaP r -2 , (6)
where ǫ is the kinetic energy dissipation rate. In the developed turbulence limit, one may expect ǫ ∼ U 3 h ∝ Re 3 . The non dimensional ratio,

(N u -1)RaP r -2 Re 3 = ν(grad u) 2 u 3 /h , (7)
is similar to a friction coefficient, and scales like 1/ √ Re at low Reynolds number (viscous limit), and features a plateau in the high Reynolds number limit.

As can be seen in Figure 6, the friction coefficient in the FC770 experiment reaches a plateau at the highest forcing (at Rayleigh numbers larger than 10 12 ). In addition, the value of the plateau is in quantitative agreement with the results from Chavanne, et al. [22] and Niemela, et al. [16,23]. We may note also that the recent data from Musilovà, et al. [17] are also in quantitative agreement with those of Niemela, et al. [23,24]. The data from He, et al. [21,28] also features a plateau, but only at their highest forcing. They are slightly offset compared to the other measurements, but this is of no consequence. Indeed, it may be caused by their slightly lower Reynolds numbers, possibly resulting from the choice of the thermometer positions. In principle, and unlike intrinsic estimates such as the Taylor micro-scale Reynolds number R λ , all Reynolds number estimates which are based on large scale quantities are defined with a O(1) prefactor. The important point is that they reach the friction versus Re plateau, but only at their highest forcing, which is consistent with their transition observed at higher Rayleigh number compared to Chavanne, et al. [22], red 3-branch stars from Niemela, et al. [16] at aspect ratio 1, orange circles from He, et al. [21,28].

This seems to indicate that the transition to the "ultimate regime" may be controlled by the Reynolds number rather than the Rayleigh number, as previously suggested by Dubrulle to explain apparent contradictions between measurements in SF 6 (large Re) and in alcohol and water (lower Re) [30]. Although all experiments show a similar Ra versus Re scaling, one must not forget the P r dependency, as well as possible dependencies on the geometry and flow configurations. Therefore, a threshold on the Reynolds number does not translate into a threshold on the Rayleigh number. Additionally, while the plateau is clearly visible on the present FC770 data, there is no visible heat-transfer enhancement. The reason is that our data lies at the beginning of the friction plateau, and the transition on the Nusselt number is not very sharp.

However, as can be seen in Figure 7, the structure of the large scale circulation (LSC) is identical on the full range of Rayleigh numbers that can be obtained in the cell with FC770. It is also similar to the velocity field obtained in water (see Fig 3) at lower Rayleigh numbers, and in fair agreement with the LSC structure reported by Xia, Sun & Zhou [6] at Ra = 3.5 × 10 10 . They report however a change from an oval-shaped LSC to the same rectangular-shaped LSC that we observe, at a threshold Rayleigh number of order 10 10 . The present work indicates that this rectangular-shaped LSC is very robust and remains unchanged from Ra = 4 × 10 10 up to Ra = 2 × 10 12 , in a range where the scaling of the kinetic dissipation rate is compatible with the turbulent regime.

Additionally, as shown in Fig 8, in the high Reynolds number limit, it seems that all Nusselt number data can be fairly well collapsed with the phenomenological scaling, N u ∼ Re 0.8 P r 0.6 .

(8) More precisely, it seems that the threshold is controlled by the Péclet number, P e = ReP r, rather than the Reynolds number, and occurs for a Péclet number of order 10 5 . This is somewhat consistent with the analysis of Kraichnan [25].

Conclusion. -We showed that the flow in our cell undergoes a transition to a turbulent regime, where the kinetic energy dissipation scales like Re 3 , and the Nusselt number is a universal function of the Reynolds and Prandtl numbers. One possible reason for the apparent discrepancy in the literature is that the transition is controlled by the Péclet number. This might also explain why Roche finds that there is a Ra-dependant transition threshold for the Prandtl number [27]. Indeed, the measurements in SF 6 have a nearly constant Prandtl number of order 0.8, almost ten times lower than the Prandtl number in the high Rayleigh numbers data of Chavanne, et al. As can be seen in Fig 8, they reach similar Péclet number only at their higher forcing.

While the scaling of the Reynolds number with the Rayleigh number is quite robust, the effective Reynolds number may still differ, for a given Ra, in one experiment, compared to another experiment. This discrepancy may be due to the Prandtl number, effects of the geometry, or the actual flow configuration. In addition, it is not possible to define the Reynolds number in a universal way, in a flow which is neither isotropic nor homogeneous. That makes comparison of transition thresholds between experiments intrinsically challenging.

Yet, the intensity of the LSC does not seem to change in the turbulent regime, and the scaling of the Reynolds number is not significantly modified beyond the turbulent transition. This is consistent with the observations of Roche, et al. [27], which showed that the transition in Grenoble did not feature a dramatic change in the LSC either. It is particularly striking that the scaling of the Reynolds number versus Rayleigh number is incredibly consistent across all experiments, whether or not they feature a transition to an enhanced heat transfer regime. When the data gets to a plateau, the scaling is N u ∼ Re 0.8 P r 0.6 . The symbols are identical to previous figures, with the addition of the data from Wu Ph.D. thesis [31] (green crosses).

One possible mechanism is that the transition is triggered by the fluctuations, or boundary eddies, as suggested by Roche [32]. It is likely that the velocity fluctuations scale similarly to the average velocity, at least up to the turbulent transition, hence why Re based on the LSC intensity still acts as a good parameter. Remarkably, in the case of roughness-facilitated transitions Liot et al. observed at high Re that the intensity of the LSC was not changed above and below the transition to the enhanced heat transfer regime, but that the intensity of velocity fluctuations and their statistical features, were indeed significantly enhanced [7]. At smaller Re however, the roughness-triggered transition does not necessarily yield higher bulk velocity fluctuations [33], and might therefore be different in nature [34]. It would be useful to further study the velocity fluctuations, and the small scale structures below and above the turbulent transition.

Fig. 1 :

 1 Fig. 1: Heat transfer measurements in FC770 (present work) in the Γ = 2 cell (green diamonds at 25 °C) and in the Γ = 1 cell (open magenta diamonds at 25 °C, and cyan triangles at 40 °C). Several high-Rayleigh number heat-transfer measurements are shown for reference: the heat-transfer from Chavanne, et al. [22] (Γ = 1/2) in black stars, from He, et al. [21] (Γ = 1) in orange circles, from Niemela, et al. with Γ = 1 [23] in blue 3-branch stars and Γ = 4 [24] in red 3-branch stars. Solid lines: estimates from the Grossmann-Lohse model with the updated prefactors [20], for P r = 14.

Fig. 2 :

 2 Fig. 2: Shadowgraph images in the Rayleigh-Bénard cell with FC770. Left: Aspect ratio 1 cell at Ra = 1.3 × 10 12 (Mean temperature at 40 °C). Right: Aspect ratio 2 cell at Ra = 8.9 × 10 10 (Mean temperature at 25 °C).

Fig. 3 :

 3 Fig. 3: Right column: velocity field obtained by Liot, et al. [7] using Particle Image Velocimetry (PIV) at Ra = 6.9×10 10 and P r = 4.4 in water. Left column: velocity field obtained from Correlation Image Velocimetry from the shadowgraph pattern (present work) in the same cell and same Ra and P r. (a, b) mean horizontal component U , (c, d) mean vertical component V , (e, f) mean velocity magnitude √ U 2 + V 2 .

Fig. 4 :

 4 Fig. 4: Velocity profiles at mid-height in the Γ = 1 cell with FC770 as the working fluid. Solid green line: Ra = 3.6 × 10 11 , dashed purple line: Ra = 9.7 × 10 11 , dotted blue line: Ra = 2.1 × 10 12 .

1. 2 × 3 × 5 × 6 ReP r 0. 7 Fig. 5 :

 235675 Fig. 5: Reynolds number measurement with FC770 (present work) in the Γ = 2 cell (green diamonds at 25 °C) and in the Γ = 1 cell (open magenta diamonds at 25 °C, and cyan triangles at 40 °C).For reference: Reynolds number from Chavanne, et al.[22] (black stars) and from He, et al[28] (orange circles).

2 )Fig. 6 :

 26 Fig. 6: Friction coefficient versus the Reynolds number. Data with the FC770 (present work) in the Γ = 2 cell (green diamonds at 25 °C) and in the Γ = 1 cell (open magenta diamonds at 25 °C, and cyan triangles at 40 °C). Black star from Chavanne, et al.[22], red 3-branch stars from Niemela, et al.[16] at aspect ratio 1, orange circles from He, et al.[21,28].

Fig. 7 :

 7 Fig. 7: Velocity fields in the Γ = 1 cell with FC770. Left: Ra = 3.6 × 10 11 (Mean temperature 25 °C). Right: Ra = 2.1 × 10 12 (Mean temperature 40 °C).

8 P r 0. 6 )Fig. 8 :

 868 Fig.8: Compensated Nusselt number as a function of the Péclet number, P e = ReP r. When the data gets to a plateau, the scaling is N u ∼ Re 0.8 P r 0.6 . The symbols are identical to previous figures, with the addition of the data from Wu Ph.D. thesis[31] (green crosses).

Table 1 :

 1 Physical properties of FC770 measured by Flucon GmbH. Density ρ, heat capacity cp, and dynamic viscosity η, for temperatures (T ) ranging from -10 °C to 95 °C.

	T [°C] ρ [kg/m 3 ] c p [K/kg/K]	η [Pas]
	-10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.0 65.0 70.0 75.0 80.0 85.0 90.0 95.0	1890.3 1876.6 1863.5 1849.4 1836.7 1822.8 1809.8 1795.9 1782.1 1769.1 1754.9 1741.7 1728.6 1714.4 1700.8 1687.3 1673.8 1660.5 1647.2 1633.0 1619.5 1606.4	932.2 942.6 959.2 967.0 979.2 991.6 999.5 1007.4 1023.0 1032.7 1048.6 1055.2 1068.7 1071.8 1085.6 1093.8 1102.0 1115.9 1123.7 1135.4 1154.2 1160.8	3.0407 × 10 -3 2.6885 × 10 -3 2.3813 × 10 -3 2.1223 × 10 -3 1.9154 × 10 -3 1.7393 × 10 -3 1.5783 × 10 -3 1.4407 × 10 -3 1.3364 × 10 -3 1.2236 × 10 -3 1.1397 × 10 -3 1.0585 × 10 -3 9.9312 × 10 -4 9.2496 × 10 -4 8.7276 × 10 -4 8.2490 × 10 -4 7.7859 × 10 -4 7.3521 × 10 -4 6.9451 × 10 -4 6.6369 × 10 -4 6.3422 × 10 -4 6.0256 × 10 -4

Table 2 :

 2 Reynolds numbers obtained from shadowgraph, and estimated global heat-transfer, in both Γ = 1 and Γ = 2 cells, without the copper thermal screen (to allow visualization), using FC770 (P r between 11 and 14) as working fluids. × 10 11 364.7 14.1 12 073 1 3.6 × 10 11 382.5 14.1 13 909 1 4.0 × 10 11 396.3 14.1 14 391 1 4.4 × 10 11 406.6 14.1 14 818 1 4.8 × 10 11 415.1 14.0 15 904 1 6.6 × 10 11 459.8 14.1 18 469 1 9.7 × 10 11 530.4 14.1 22 038 1 1.1 × 10 12 557.2 14.1 23 338 1 1.3 × 10 12 577.8 12.0 29 597 1 1.7 × 10 12 635.3 11.9 32 186 1 1.8 × 10 12 627.4 12.5 31 321 1 2.0 × 10 12 682.6 11.9 34 454 1 2.1 × 10 12 712.3 11.9 36 082 1

	Ra	N u	P r	Re	Γ
	3.4 × 10 10 185.7 14.1 3.9 × 10 10 191.1 14.0 4.3 × 10 10 197.0 14.1 4.8 × 10 10 201.8 14.1 5.2 × 10 10 207.6 14.1 5.6 × 10 10 212.6 14.1 6.0 × 10 10 218.0 14.1 6.4 × 10 10 222.7 14.1 7.1 × 10 10 231.4 14.1 8.9 × 10 10 248.9 14.1 3.2	3487 3563 3829 4058 4283 4539 4641 4572 5124 5681	2 2 2 2 2 2 2 2 2 2

Table

We are very grateful to M. Boutaous, H. Pabiou and the staff at CETHIL laboratory for their help with the measurement of the fluid thermal properties. This work was funded by ANR-18-CE30-0007-01 JCJC "CryoGrad" project, and benefited from the resources of the PSMN computing centre in Lyon. We warmly thank M. Moulin and the mechanical workshop for the design and setup of the experimental apparatus. We thank B. Castaing and P. Roche for stimulating discussions.