Xiaoyan He

Lucie Norel
email: lucie.norel@univ-rennes1.fr.

Yves-Marie Hervault

Remi Métivier

 Anthony D ' Aléo

Olivier Maury

Stéphane Rigaut
email: stephane.rigaut@univ-rennes1.fr

Modulation of Eu(III) and Yb(III) luminescence using a DTE photochromic ligand

Keywords: Photochromism, ytterbium, europium, dithienylethene, luminescence, molecular switche

In this work, we show that a dithienylethene (DTE) modified dipicolinic amide ligand can be a versatile tool to modulate Eu(III) and Yb(III) luminescence using light as an external stimulus. The nature of the modulation depends on the lanthanide emitter: with the europium ion, the DTE ligand quench the red luminescence upon ring closure, whereas with the ytterbium ion, ring closure can be used to turn on the luminescence in the NIR range.

Introduction

Molecular-based switching devices in which key physical properties can be modulated with external stimuli are focusing much attention. [START_REF] Klajn | Nanoparticles functionalised with reversible molecular and supramolecular switches[END_REF][START_REF] Natali | Molecular switches as photocontrollable "smart" receptors[END_REF][START_REF] Zhang | Photochromic Materials: More Than Meets The Eye[END_REF][START_REF] Raymo | Photoactivatable synthetic fluorophores[END_REF][START_REF] Klajn | Spiropyran-based dynamic materials[END_REF][START_REF] Irie | Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators[END_REF][START_REF] Goestl | Remote-controlling chemical reactions by light: Towards chemistry with high spatio-temporal resolution[END_REF][8][START_REF] Erbas-Cakmak | Artificial Molecular Machines[END_REF] Among the chemical and physical properties that a molecule can switch, luminescence is particularly attractive to achieve smart materials as well as cell imaging. [START_REF] Irie | Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators[END_REF][START_REF] Tian | Photoswitching-Enabled Novel Optical Imaging: Innovative Solutions for Real-World Challenges in Fluorescence Detections[END_REF][START_REF] Fukaminato | Single-molecule fluorescence photoswitching: Design and synthesis of photoswitchable fluorescent molecules[END_REF] A typical example of such molecules is a dyad composed of photochromic and fluorescent dyes units in which the fluorescence emission is modulated by the photochromic reaction. [START_REF] Raymo | Photoactivatable synthetic fluorophores[END_REF] Such a modulation has been extensively studied, and two types of quenching mechanism are possible involving intramolecular energy transfer or electron transfer. [START_REF] Fukaminato | Single-Molecule Fluorescence Photoswitching of a Diarylethene-Perylenebisimide Dyad: Non-destructive Fluorescence Readout[END_REF][START_REF] Berberich | Toward fluorescent memories with nondestructive readout: Photoswitching of fluorescence by intramolecular electron transfer in a diaryl ethene-perylene bisimide photochromic system[END_REF] However, most systems involve a photochromic unit linked to an organic or a metal-organic fluorophore which luminescence is regulated by intramolecular energy transfer. This non-radiative process occurs from the excited state of the fluorophore (donor) to the photochromic unit (acceptor), generally in its conjugated state, and requires that the acceptor characteristic absorption band overlaps the emission of the donor (FRET, Förster Resonance Energy Transfer, or Dexter energy transfer). Dithienylethene (DTE) is one of the most promising photochromic systems due to its high fatigue resistance. [START_REF] Irie | Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators[END_REF] It can be efficiently switched from a non-conjugated form to a conjugated form upon UV irradiation, and switched-back using visible light. Therefore, this unit has been used for the photoregulation of various properties including sensing, 2, 14 NLO, [START_REF] Boixel | Second-Order NLO Switches from Molecules to Polymer Films Based on Photochromic Cyclometalated Platinum(II) Complexes[END_REF] magnetism, [START_REF] Pinkowicz | Control of the Single-Molecule Magnet Behavior of Lanthanide-Diarylethene Photochromic Assemblies by Irradiation with Light[END_REF] molecular conductance, [START_REF] Russew | Photoswitches: From Molecules to Materials[END_REF][START_REF] Orgiu | 25th Anniversary Article: Organic Electronics Marries Photochromism: Generation of Multifunctional Interfaces, Materials, and Devices[END_REF][START_REF] Meng | Orthogonally modulated molecular transport junctions for resettable electronic logic gates[END_REF][START_REF] Meng | Photo-modulable molecular transport junctions based on organometallic molecular wires[END_REF] redox behavior, [START_REF] Browne | Oxidative electrochemical switching in dithienylcyclopentenes, part 1: Effect of electronic perturbation on the efficiency and direction of molecular switching[END_REF][START_REF] Guirado | Understanding electrochromic processes initiated by dithienylcyclopentene cation-radicals[END_REF][START_REF] Liu | Diarylethene-Containing Carbon-Rich Ruthenium Organometallics: Tuning of Electrochromism[END_REF] NIR absorption, [START_REF] Poon | Synthesis, Photochromic, and Computational Studies of Dithienylethene-Containing beta-Diketonate Derivatives and Their Near-Infrared Photochromic Behavior Upon Coordination of a Boron(III) Center[END_REF] optical rotation, [START_REF] Okuyama | Chiral helicenoid diarylethene with large change in specific optical rotation by photochromism[END_REF] super resolution imaging, [START_REF] Raymo | Photoactivatable synthetic fluorophores[END_REF][START_REF] Li | A trident dithienyletheneperylenemonoimide dyad with super fluorescence switching speed and ratio[END_REF] triplet-triplet annihilation, [START_REF] Xu | Photoswitching of triplet-triplet annihilation upconversion showing large emission shifts using a photochromic fluorescent dithienylethene-Bodipy triad as a triplet acceptor/emitter[END_REF] catalysis, [START_REF] Blanco | Artificial switchable catalysts[END_REF] and so on. Although DTE can be a fluorescent switch itself, [START_REF] Yang | Aromaticity-Controlled Thermal Stability of Photochromic Systems Based on a Six-Membered Ring as Ethene Bridges: Photochemical and Kinetic Studies[END_REF][START_REF] Uno | In Situ Preparation of Highly Fluorescent Dyes upon Photoirradiation[END_REF] it serves as a non-fluorescent switch to modulate the fluorescence of a neighboring fluorophore through FRET as mentioned above, including in combinations of DTE and metal centers that have opened up new perspectives for the design of metal based photoswitchable fluorescent materials. [START_REF] Boyer | Photomodulation of Fluorescent Upconverting Nanoparticle Markers in Live Organisms by Using Molecular Switches[END_REF][START_REF] Tan | Near-Infrared Photochromic Diarylethene Iridium (III) Complex[END_REF][START_REF] Monaco | Multifunctional switching of a photo-and electro-chemiluminescent iridium-dithienylethene complex[END_REF][START_REF] Indelli | Triplet pathways in diarylethene photochromism: Photophysical and computational study of dyads containing ruthenium(II) polypyridine and 1,2-bis(2-methylbenzothiophene-3-yl)maleimide units[END_REF][START_REF] He | Ruthenium-Phenothiazine Electron Transfer Dyad with a Photoswitchable Dithienylethene Bridge: Flash-Quench Studies with Methylviologen[END_REF][START_REF] Jukes | Photochromic dithienylethene derivatives containing Ru(II) or Os(II) metal units. Sensitized photocyclization from a triplet state[END_REF][START_REF] Roberts | Charge Transfer and Intraligand Excited State Interactions in Platinum-Sensitized Dithienylethenes[END_REF][START_REF] Ko | Transition metal complexes with photochromic ligandsphotosensitization and photoswitchable properties[END_REF][START_REF] Moreno | Reversible Photomodulation of Electronic Communication in a pi-Conjugated Photoswitch-Fluorophore Molecular Dyad[END_REF][START_REF] Yam | Photochromic and luminescence switching properties of a versatile diarylethene-containing 1,10-phenanthroline ligand and its rhenium(I) complex[END_REF][START_REF] Lee | Metal coordination-assisted near-infrared photochromic behavior: A large perturbation on absorption wavelength properties of N,N-donor ligands containing diarylethene derivatives by coordination to the rhenium(I) metal center[END_REF][START_REF] Wong | Photochromic Thienylpyridine-Bis(alkynyl)borane Complexes: Toward Readily Tunable Fluorescence Dyes and Photoswitchable Materials[END_REF][START_REF] Chan | Tunable Photochromism in Air-Stable, Robust Dithienylethene-Containing Phospholes through Modifications at the Phosphorus Center[END_REF] A c c e p t e d m a n u s c r i p t Among all fluorophores, lanthanide occupy a very particular place due to their unique photophysical properties. [START_REF] D'aleo | Charge transfer excited states sensitization of lanthanide emitting from the visible to the near-infra-red[END_REF][START_REF] Moore | From Antenna to Assay: Lessons Learned in Lanthanide Luminescence[END_REF][START_REF] Eliseeva | Lanthanide luminescence for functional materials and bio-sciences[END_REF] Indeed, since the emission origins from the Laporte forbiddent f-f transitions, lanthanide complexes display long lived excited state and narrow bandwidth emission band ranging from the visible to the near infra-red spectral range (NIR), with high emission efficiency and a large (pseudo)-Stokes shift that attracts considerable interests for molecular materials and imaging applications. [START_REF] Moore | From Antenna to Assay: Lessons Learned in Lanthanide Luminescence[END_REF][START_REF] Armelao | Design of luminescent lanthanide complexes: From molecules to highly efficient photo-emitting materials[END_REF][START_REF] Dos Santos | Recent developments in the field of supramolecular lanthanide luminescent sensors and self-assemblies[END_REF][START_REF] Buenzli | On the design of highly luminescent lanthanide complexes[END_REF][START_REF] De Bettencourt-Diaz | Luminescence of Lanthanide Ions in Coordination Compounds and Nanomaterials[END_REF][START_REF] Carr | Lanthanide complexes as chiral probes exploiting circularly polarized luminescence[END_REF][START_REF] Montgomery | Cell-Penetrating Metal Complex Optical Probes: Targeted and Responsive Systems Based on Lanthanide Luminescence[END_REF][START_REF] Heffern | Lanthanide Probes for Bioresponsive Imaging[END_REF] Therefore, reversible modulation of this luminescence is an attractive target. Such modulation can be achieved chemically, i.e. with external stimulus like pH, ion or molecules, [START_REF] Pandya | Engineering emissive europium and terbium complexes for molecular imaging and sensing[END_REF][START_REF] Gunnlaugsson | Responsive lanthanide luminescent cyclen complexes: from switching/sensing to supramolecular architectures[END_REF][START_REF] Andrews | Luminescent probes based on water-soluble, dual-emissive lanthanide complexes: metal ion-induced modulation of near-IR emission[END_REF] or electrochemically. [START_REF] Lehr | Reversible redox modulation of a lanthanide emissive molecular film[END_REF][START_REF] Tropiano | Reversible Luminescence Switching of a Redox-Active Ferrocene-Europium Dyad[END_REF][START_REF] Sato | An alternately introduced heterometallo-supramolecular polymer: synthesis and solid-state emission switching by electrochemical redox[END_REF][START_REF] Yano | ON-OFF" switching of europium complex luminescence coupled with a ligand redox process[END_REF][START_REF] Kanazawa | Electrochemical luminescence modulation in a Eu(III) complex-modified TiO2 electrode[END_REF][START_REF] Piazza | d-f Heterobimetallic Association between Ytterbium and Ruthenium Carbon-Rich Complexes: Redox Commutation of Near-IR Luminescence[END_REF][START_REF] Norel | Lanthanide Sensitization with Ruthenium Carbon-Rich Complexes and Redox Commutation of Near-IR Luminescence[END_REF] In contrast, the modulation of europium luminescence triggered by light irradiation itself is quite rare. One can mention the elegant 30% modulation of europium luminescence promoted by the ligand field change due to a significant variation of the coordination sphere induced by photochromic ligands. [START_REF] Nakagawa | Nondestructive luminescence intensity readout of a photochromic lanthanide(III) complex[END_REF][START_REF] Nakagawa | Photoresponsive europium(III) complex based on photochromic reaction[END_REF][START_REF] Hasegawa | Recent progress of luminescent metal complexes with photochromic units[END_REF] In addition, few examples of intramolecular FRET were reported, also with europium complexes, containing an inorganic [START_REF] Wang | Reversible Luminescent Switching in a Eu(SiW10MoO39)(2) (13-)-Agarose Composite Film by Photosensitive Intramolecular Energy Transfer[END_REF] or organic [START_REF] Mei | A novel photo-responsive europium(III) complex for advanced anti-counterfeiting and encryption[END_REF] photochromic unit, or embedded in a pseudorotaxane supramolecular assembly controlled by a DTE unit. [START_REF] Cheng | Dual-Stimulus Luminescent Lanthanide Molecular Switch Based on an Unsymmetrical Diarylperfluorocyclopentene[END_REF] To the best of our knowledge, no example of modulation of the luminescence of other lanthanide emitters have been reported, especially those displaying emission in the NIR spectral range.

Herein, we describe the synthesis and the luminescence modulation of lanthanide complexes (Ln = Eu and Yb) by taking advantage of a DTE unit incorporated in a dipicolinic amide ligand. We demonstrate that depending on the lanthanide ion used, the DTE unit can be used in different ways to modulate the lanthanide luminescence intensity upon ring closure/opening reactions, including the unprecedented Yb(III) luminescence turn on. to the ligand 3o in equimolar quantities. [START_REF] Renaud | In search for mononuclear helical lanthanide building blocks with predetermined properties: Triple-stranded helical complexes with N,N,N',N'-tetraethylpyridine-2,6-dicarboxamide[END_REF][START_REF] Picot | Twophoton antenna effect induced in octupolar europium complexes[END_REF][START_REF] Pointillart | In Solution Sensitization of Er(III) Luminescence by the 4-Tetrathiafulvalene-2,6-pyridinedicarboxylic Acid Dimethyl Antenna Ligand[END_REF] The latter was readily achieved from precursor 1 [START_REF] Roberts | Linker-Dependent Metal-Sensitized Photoswitching of Dithienylethenes[END_REF] and 2 [START_REF] Picot | Synthesis, structures, optical properties, and TD-DFT studies of donor-pi-conjugated dipicolinic acid/ester/amide ligands[END_REF] via Sonogashira cross-coupling reaction in classical conditions (Scheme 1). These new compounds were fully characterized by NMR and IR spectroscopies, elemental analysis, and HR-MS. For the two lanthanide complexes 4o and 5o, paramagnetic shifts are observed due to the through space interactions between the observed nuclei and the 4f unpaired electrons (pseudocontact shifts) which depend on the magnetic anisotropy of the lanthanide ion (Figures 1 and2). 75- 78 We observed for 4o that the pyridine ring and the N-ethyl protons are deshielded compared to 3o, whereas the DTE protons (thiophene rings and methyl groups) are shielded. Coordination to the more anisotropic Yb(III) ion in 5o leads to similar changes but with a greater amplitude: the protons of the DTE unit are shielded to 6.33 and 4.62 ppm (thiophene rings) and 0.58 and 0.28 ppm (methyl groups), whereas the protons of the dipicolinic amide ligand appear as very broad peaks in the range 6-20 ppm. Absorption spectra and photochromism of 3o, 4o and 5o. The absorption spectrum of 3o shows several intense transitions in the UV range. Upon irradiation at 350 nm, a decrease of absorption is observed at 272 nm while two new bands appear at 383 and 610 nm (Figure 3). This is consistent with the assignment of the 272 nm transition to an open-DTE centered process in 3o and with the usual photochromic behavior of DTE units upon UV irradiation, i.e. ring-closure, giving rise to the lower energy band (610 nm) ascribed to an intra-ligand (IL) transition centered on the closed DTE moiety in 3c. [START_REF] Irie | Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators[END_REF] The isomerization was also studied by [START_REF] Klajn | Nanoparticles functionalised with reversible molecular and supramolecular switches[END_REF] H NMR that proved that a high photoisomerisation conversion (to 93% of 3c) can be reached in the photo-stationary state (PSS) (Figure S5). (OFCF = 0.320), 4o (OFCF = 0.177) and 5o (OFCF = 0.142) are much higher than those for cycloreversions (3c: CFOF = 0.008, 4c: CFOF = 0.008 and 5c: CFOF = 0.007) and consistent with the photostationnary state at ca. 95% of closed isomer (i.e. 93,0%,, 95.6% and 95.4%, respectively). Such values lie in the classical ranges for diarylperfluorocyclopentene compounds. [START_REF] Irie | Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators[END_REF] A c c e p t e d m a n u s c r i p t

If we look at the cyclization event, we notice that OFCF is two times higher for 3o. At this wavelength, its absorption coefficient is about half of that of the complexes that include the TTA contribution (see Fig 3). As the quantum yield is related to absorption coefficient, this explains that OFCF of the DTE unit appears two times lower for the two complexes (about half of the light absorbed by the TTA unit does not give rise to the photoreaction). For the cycloreversion, the situation is simpler, the visible absorption involving only the DTE unit in all compounds. Overall, we conclude that the DTE unit photoreactivity is similar in 3o and in the complexes. Then, in the light of the efficient photochromic behaviour for both complexes, we turned our interest toward the photophysical properties and their possible modulation. 6 using particular experimental conditions (small slit aperture) in order to prevent photoisomerization process during the luminescence measurement. For 4o, upon excitation at 350 nm, the emission features the 5 D0  7 FJ (J = 0-4) transitions characteristic of Eu(III) luminescence at 579 (J = 0), 592 (J = 1), 612 (J = 2), 652 (J = 3), and 700 nm (J = 4) (Figure 5). The luminescence decay time of this 5 D0 state was measured to be 260 s (using the 5 D0  7 F2 transition), and the single exponential decay curve indicates that only one emitting species is present in solution. With a 350 nm excitation, the emission spectrum of 4c is very similar but of significant lower intensity, 55-65% (vide infra), whereas excitation in the closed DTE centered transitions (at 550 nm) does not induce any luminescence from the Eu(III) center. These observations are confirmed with the excitation spectrum ( = 610 nm, 5 D0  7 F2 transition) that is composed of a single broad band centered at 345 nm corresponding to the -* transitions of the ligand 3o and to the TTA moieties (Figure S6). A luminescence lifetime of 250 s was measured for 4c. Overall the results are comparable to those observed in the pioneering example reported by Hasegawa and Kawai. [START_REF] Nakagawa | Photoresponsive europium(III) complex based on photochromic reaction[END_REF] The absence of Eu(III) sensitization mediated by the closed DTE antenna can be easily explained by its low lying excited state (550-580 nm, 17 000-18 000 cm -1 ) that is lower than the best accepting 5 D1

state and almost at the same energy than the 5 D0 state from the europium ion ( 5 D1 at 19 000 cm -1

and 5 D0 at 17 400 cm -1 ). The possible sensitization of Eu(III) upon excitation at 350 nm is more surprising and could be explained by the fact that the energy transfer from the TTA in its triplet state occurs faster to the Eu(III) 5 D0/1 state than to DTE triplet state, this later photophysical process acting as non-radiative deexcitation pathway. Yb(III) emission is generally less intense than the Eu(III) one and requires an excitation using more power and longer integration time to get an acceptable signal/noise ratio. Therefore, measuring the full luminescence spectrum of the open isomer 5o with excitation in the absorption band (ex = 350 nm) was not possible owing to the fast concomitant closing of the molecule forming 5c. In contrast, the emission spectrum of 5c could be measured upon excitation at 350 nm and displays the characteristic ytterbium transitions ( 2 F5/2 → 2 F7/2) in the NIR spectral range at 980 nm (Figure 6, black line). We also found out that 5c is also emissive upon excitation at ex = 580 nm, in the absorption band of the closed DTE unit, in marked contrast with the Eu(III) analogous 4c.

This visible excitation of 5c induced two competitive photophysical processes, (i) the opening of the DTE unit leading to 5o and (ii) the emission of 5c. Consequently, the emission is weaker but is characteristic of the Yb(III) ion (Figure 6, red line). This result can be easily explained by the position of the 2 F5/2 excited state of Yb(III) (10200 cm -1 ) lying at much lower energy than that of Eu(III). The sensitization mechanism may proceed either directly from the DTE ligand singlet excited state 79a or from its corresponding triplet one. 79b It is worth noting that the open form 5o does not absorb at this wavelength and does not present any emission under 580 nm excitation. nm, the luminescence decay of the europium complex was followed at  = 610 nm ( 5 D0  7 F2 transition). This luminescence decay occurs simultaneously with the gradual formation of the closed form 4c as observed by UV-visible absorption (vide supra). In a typical experiment displayed in Figure 7, this decay of the emission process follows a mono-exponential law with a characteristic lifetime of 10 s in these experimental conditions. In the PSS, the emission intensity reaches 65% of its initial value, whereas the optical density concomitantly measured at 600 nm is consistent with that of the above described absorption spectrum, i.e. with the conversion of 4o to 4c of ca. 95%. During the irradiation of 4c at  = 560 nm, the compound being not emissive, no emission signal is recorded. Concomitantly, compound 4o is fully regenerated on the basis of the absorption spectrum and the initial luminescence intensity is fully recovered as well. Note that the closing process (75 s in the present experimental conditions) is faster than the opening one (300 s).

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

Four opening closing processes are depicted on figure 7 showing that the process is fully reversible and occurs without notable alteration. In the case of the ytterbium complex, the photomodulation of the NIR emission can be achieved by using an alternative sensitization pathway through the closed DTE unit. Indeed, continuous irradiation in the large absorption band of the luminescent complex 5c in the visible range (ex = 580 nm) gradually leads to the formation of the open form 5o, and the decay of the luminescence emission at  = 1000 nm (Figure 8) could be observed and monitored. When the PSS is reached, (i) the optical density measured at 580 nm in this experiment is virtually zero, consistent with the total conversion of 5c to 5o, and (ii) the luminescence is turned off. Upon further irradiation of 5o at  = 350 nm, complex 5c is fully regenerated on the basis of the absorption spectrum, and the previous luminescence intensity is regenerated in spite of a rather low signal-to-noise ratio. Here again, the closing process (200 s in the present experimental conditions) is faster than the opening one (600 s). Three opening closing processes are depicted in Figure 8 showing that the process is fairly reversible in those experimental conditions. To the best of our knowledge, this is the very Igor-implemented (Wavemetrics) homemade software. [START_REF] Ordronneau | Tunable double photochromism of a family of bis-DTE bipyridine ligands and their dipolar Zn complexes[END_REF] The luminescence spectra were measured using a Horiba-Jobin Yvon Fluorolog-3® spectrofluorimeter, equipped with a three slit double grating excitation and emission monochromator with dispersions of 2. added. The reaction mixture was left under stirring at room temperature for 14 hours. Then, the solvents were evaporated to obtain a crude product that was purified by column chromatography using ethyl acetate and pentane (1/1) as eluent, to obtain 3o as a yellow powder (480 mg, 76 %). 3H, m-C6H5+H2'), 7.37-7.34 (m, 1H, p-C6H5), 7.28 (s, 1H, H2), 3.53 (q, 4H, 3 J = 7.0 Hz, H4 or H4'), 3.33 (q, 4H, 3 J = 7.0 Hz, H4 or H4'), 2.00 (s, 3H, H3 or H3'), 1.99 (s, 3H, H3 or H3'), 1.23 (t, 6H, 3 J = 7.0 Hz, H5 or H5'), 1.14 (t, 6H, 3 J = 7.0 Hz, H5 or H5'). [START_REF] Berberich | Toward fluorescent memories with nondestructive readout: Photoswitching of fluorescence by intramolecular electron transfer in a diaryl ethene-perylene bisimide photochromic system[END_REF]

  Synthesis of the new complexes 4o and 5o were achieved in CH2Cl2 with coordination of [Ln(TTA)3.2H2O] (4o: Ln = Eu, 5o:Ln = Yb, TTA = 2-thenoyltrifluoroacetonate)

A c c e p t e d m a n u s c r i p t Scheme 1 .

 1 Scheme 1. Synthetic pathways yielding complexes 4o and 5o and isomerization process.

Figure 1 .

 1 Figure 1. 1 H NMR spectrum of 4o (red line) and of PSS (97 % of 4c, blue line) obtained from irradiation of 4o in CD2Cl2 (= 350 nm, 45 h, ([c] ≈ 1 × 10 -3 mol.L -1 ).

Figure 2 . 1 H

 21 Figure 2. 1 H NMR spectrum of 5o (red line) and of PSS (93 % of 5c, blue line) obtained from irradiation of 5o in CD2Cl2 (= 350 nm, 71h, ([c] ≈ 1 × 10 -3 mol.L -1 ).

Figure 3 .

 3 Figure 3. Absorption spectra of 3o, 5o and the [Yb(TTA)3.2H2O] precursor in CH2Cl2 solutions (left). Absorption spectrum of 5c, obtained upon UV light irradiation to PSS (= 350 nm) of 5oand its comparison with 3c obtained under similar conditions (right). The Europium complex 4o show identical behavior (FigureS6). The initial spectra were recovered after bleaching at 580 nm.

Further 1 H

 1 NMR monitoring of the processes in CD2Cl2 ([c] ≈ 2 × 10 -3 mol.L -1 ) with integration of the CH3 and CH proton signals of the DTE unit unambiguously shows that the photochromic events upon UV irradiation are almost complete with the reaching of a photostationary state composed of

A c c e p t e d m a n u

  photokinetic differential equations for ring-closure and ring-opening photochromic reactions (see

Figure 4 .

 4 Figure 4. Continuous absorption time-evolution profile extracted at 610nm for 4o during a full cycle of photoisomerization under continuous 334 and 575 nm illumination

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

Figure 5 :

 5 Figure 5: Normalized emission spectra of 4o and 4c in dichloromethane at room temp (ex = 350 nm).

A c c e p t e d m a n u s c r i p t 1 Figure 6 :Modulation of the luminescence response under irradiation in the DTE absorption bands of 4 and 5 .

 165 Figure6: Emission spectra of 5c, in dichloromethane at room temperature, upon excitation at ex = 350 nm (black) and ex = 580 nm (red). The difference of emission profile depending on the excitation wavelength is due to experimental parameters. Under 580 nm excitation, the emission of 5c is low and it was necessary to use larger emission slit aperture to increase the signal/noise ratio. Consequently, resolution is lower.

Figure 7 :

 7 Figure 7: (left) Evolution of the emission at  = 610 nm upon irradiation at  = 350 nm observed at room temperature during conversion of 4o ([c] ≈ 1 × 10 -5 mol.L -1 ) to 4c, and the mono-exponential fit (red line) with = 10 s). (right) Representation of four opening-closing cycles: emission at  = 610 nm upon successive irradiation at  = 560 nm (blue) and at  = 350 nm (red).

A c c eFigure 8 :

 8 Figure 8: (left) Evolution of the emission at  = 1000 nm upon irradiation at  = 580 nm at room temperature leading to conversion of 5c ([c] ≈ 1 × 10 -5 mol.L -1 ) to 5o and the mono-exponential fit (red line) with = 10 s. (right) Representation of three opening-closing cycles: monitoring of emission at  = 1000 nm upon irradiation at  = 580 nm (opening), the signal being not measured during the closing process (irradiation at  = 350 nm). Integration time was 1s for each data points.

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

  Importantly, this work shows that it is possible to modulate Yb(III) luminescence using the ubiquitous DTE photochromic unit, and further chemical engineering with photochromic molecules should be possible to achieve improved switching processes. measured using a photodiode from Ophir (PD300-UV). The photochromic quantum yields were determined by probing the sample with a xenon lamp during the course of the light irradiation. Absorption changes were monitored by a CCD camera from Princeton Instruments mounted on a spectrometer (Roper Scientific). Kinetic profiles were analyzed by means of a Runge-Kutta-based

  photomultiplier tube. Spectra were reference corrected for both the excitation source light intensity variation (lamp and grating) and the emission spectral response (detector and grating). Near infrared spectra were recorded at an angle of 90° using a liquid nitrogen cooled, solid indium/gallium/arsenic detector (850-1600 nm) through a RG830 -2 mm-filter. Selective excitation at 350 nm requires the use of a CG400 filter in order to perfectly suppress the second harmonic excitation (700 nm) localized in the closed DTE absorption band.

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

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

  65,79a

  C NMR (125.76 MHz, CD2Cl2, 297 K): H3' or H3).[START_REF] Berberich | Toward fluorescent memories with nondestructive readout: Photoswitching of fluorescence by intramolecular electron transfer in a diaryl ethene-perylene bisimide photochromic system[END_REF] C NMR (125.77 MHz, CD2Cl2, 297 K): δ 167.9 (C6), 145.9 (C7), 142.9, 141.8, 135.1, 133.5, 129.4, 128.4, 127.3, 125.9, 125.6, 125.3, 124.4, 122.5, 119.9, 90.1, 46.4 (C4), 16.5 (C5), 14.6 (C3 or C3'), 14.4 (C3 or C3'). Most of the quaternary carbon could not be observed as well as C4' and C5'. IR (KBr) (ѵ, cm -1 ): 2212(s) (ѵC ≡ C), 1608(s) (ѵC=O). Elemental analysis for [10C62H47N3O8F15S5Eu]•6CH2Cl2: C 46.97, H 3.04 (Calcd: C 46.69, H 3.02). HR-MS ESI(m/z):1582.0837 ([M + Na] + , calcd: 1582.08374). , 5.66 (brs, 4H, H4 or H4'), 4.21 (brs, 6H, H5 or H5'), 3.66 (brs, 4H, H4 or H4'), 2.53 (brs, 6H, H5 or H5'), 1.98-1.91 (3H, HTTA-methine), 1.98(s, 3H, H3 or H3' ),1.91 (s, 3H, H3 or H3') . IR (KBr) (ѵ, cm -1 ): 2202(s) (ѵC ≡ C), 1610(s) (ѵC=O) . In a Schlenk tube, 3o (51.5 mg, 0.069 mmol) and Yb(TTA)3.(H2O)2 (60.4 mg, 0.069 mmol) were dried under vacuum for 30 min and dissolved in dichloromethane (5 mL). This solution was stirred at ambient temperature for 16 hours. The mixture was taken to dryness under vacuum to yield a light green powder (109 mg. 95 %).[START_REF] Klajn | Nanoparticles functionalised with reversible molecular and supramolecular switches[END_REF] H-NMR (400.16 MHz, CD2Cl2, ppm): δ 8.48 (br. s, 3H, HTTA), 7.12 (br. s, 3H, C6H5), 6.92 (br. s, 2H, C6H5), 6.79 (br. s, 3H, HTTA), 6.33 (br. s, 1H, H2), 4.92 (br. s, 3H, HTTA), 4.62 (br. s, 1H, H2'), 0.58 (s, 3H, H3' or H3), 0.28 (s, 3H, H3' or H3), -14.61 (br. s, 3H, HTTAmethine), additional broad features are observed for H1, H4/4' and H5/5'.[START_REF] Berberich | Toward fluorescent memories with nondestructive readout: Photoswitching of fluorescence by intramolecular electron transfer in a diaryl ethene-perylene bisimide photochromic system[END_REF] C NMR (125.77 MHz, CD2Cl2, 297 K): δ 149.4 , 143.3, 141.6, 140.3, 137.8, 135.80 (m), 134.3, 133.8, 132.4, 130.0, 128.6, 127.8, 127.7, 125.9, 124.9, 124.0, 122.8, 121.1, 116.3, 115.0, 114.4, 112.2, 110.0, 107.7, 90.7(m), 87.7, 83.6, 46.9 (br. s, C4 or C4'), 45.7 (br. s, C4 or C4'), 24.0 (br. s, C5 or C5'), 22.5 (br. s, C5 or C5'), 12.7 (C3 or C3'), 12.6 (C3 or C3'). IR (KBr) (ѵ cm -1 ): 2212(s) (ѵC

	δ 167.6 (s, C6), 154.7 (C7), 146.1 (s, C13), 143.0 (s), 142.1 (s, C16), 137.6 (m) and 135.6 (m), 133.7
	(s, C2'), 133.6 (s), 133.1 (s), 129.4 (s, C21 ), 128.4 (s, C22), 126.0 (s, C20 ), 125.7 (s, C12 or C18), (s, C4 or C4'), 40.4 (s, C4 or C4'), 14.8 (s, C3 or C3'), 14.7 (s, C3 or C3'), 14.5 (s, C5 or C5'), 12.92 (s, C5 or C5'). IR (KBr) (ѵ, cm -1 ): 2209 (s) (ѵC≡C), 1638(s) (ѵC=O). Elemental analysis for [C38H35N3O2F6S2]0.1CH2Cl2: C 60.93, H 4.66 (Calcd: C 60.83, H 4.72). HR-MS ESI(m/z): 766.1968 ([M+Na] + , calcd 766.19671), 744.2123 ([M+H] + , calcd 744.21477). Data for 3c: 1 H NMR (400 MHz, CD2Cl2, 297 K): δ 7.62-7.44 (m, 7H, C6H5 + H1), 6.72 (s, 1H, H2'),6.58 (s,1H, H2), 3.50 (q, 4H, 3 J = 7.1 Hz, H4 or H4'), 3.30 (q, 4H, 3 J = 7.1 Hz, H4 or H4'), 2.21 (s, 3H, H3 or H3'), 2.20 (s, 3H, H3 or H3'), 1.21 (q, 6H, 3 J = 7.1 Hz, H5 or H5'), 1.12 (q, 6H, 3 J = 7.1 Hz, H5 or H5'). compound 3o (51 mg, 0.069 mmol) and Eu(TTA)3.(H2O)2 (58.4 mg, 0.069 mmol) were dried under m a n u s c r i p t 124.9 (s, C1 ), 122.6 (s, C2), 120.5 (s, C11), 116.5 (m), 111.5 (m), 91.0 (s, C9), 87.7 (s, C10), 43.6 [C6H5─(C15S2F6H8)─C≡C─m,m-(CON(CH2CH3)2)2-C5H4N-Yb(TTA)3] (5o).
	vacuum for 30min and dissolved in dichloromethane (5 mL). This solution was stirred at ambient
	temperature for 16 hours. The mixture was taken to dryness under vacuum to yield to a green powder (106 mg, 100 %). 1 H-NMR (400.16 MHz, CD2Cl2, ppm): δ 9.58 (br. s, 2H, H1), 7.26-7.46 (m, 5H, C6H5), 7.14 (s, 1H, H2' or H2), 7.09 (br. s, 3H, HTTA), 6.82 (s, 1H, H2' or H2), 6.53 (br. s, 3H, HTTA), 6.21 (br. s, 3H, HTTA), 5.63 (br. s, 4H, H4 or H4'), 3.84 (br. s, 6H, H5 or H5'), 3.51 (br. s, 4H, H4 or H4'), 2.53 (br. s, 6H, H5 or H5'), 2.1 (s, 3H, HTTA-methine), 1.79 (s, 3H, H3' or H3), 1.69 (s, 3H, A c c e p t e d

[C6H5─(C15S2F6H8)─C≡C─m,m-(CON(CH2CH3)2)2-C5H4N-Eu(TTA)3] (4o). In a Schlenk tube, Data for 4c: 1 H NMR (400 MHz, CD2Cl2, 297 K): δ 9.85 (s, 2H, H1), 7.38-7.52 (m, 5H, C6H5), 7.11 (brs, 3H, HTTA ), 6.62 (s, 1H, H2'), 6.52 (brs, 3H, HTTA), 6.13 (brs, 3H, HTTA), 5.93 (s, 1H, H2 )≡ C), 1606(s) (ѵC=O). Elemental analysis for [C62H47N3O8F15S5Yb]•CH2Cl2: C 45.35, H 2.95 (Calcd: C 45.44, H 2.97). HR-MS ESI(m/z): 1359.1227 ([M-TTA] + , calcd: 1359.1226).

Acknowledgments. This work was supported by the Université de Rennes 1, the CNRS, the Agence Nationale de la Recherche (RuOxLux -ANR-12-BS07-0010-01)).

Data for 5c: [START_REF] Klajn | Nanoparticles functionalised with reversible molecular and supramolecular switches[END_REF] H NMR (400 MHz, CD2Cl2, 297 K): δ 8.53 (brs, 3H, HTTA), 7.01-7.22 (m, 5H, C6H5), 6.82 (brs, 3H, HTTA), 5.99 (brs, 1H, H2), 4.93 (brs, 3H, HTTA), 4.56 (brs, 4H), 2.96 (brs, 1H, H2'), 0.66 (s, 3H, H3' or H3), 0.47 (s, 3H, H3' or H3) -14.70 (s, 3H, HTTA-methine). IR (KBr) (ѵ, cm -1 ): 2201(s) (ѵC ≡ C), 1605(s) (ѵC=O).

Supporting Information Available: NMR spectra, isomerization studies. This material is available free of charge via the Internet at http://pub.acs.org.

Experimental Section.

General comments: The reactions were carried out under an inert atmosphere using Schlenk techniques. Solvents were dried and distilled under argon using standard procedures. 4-iodo-2,6bis(diethylcarbamoyl)pyridine, [START_REF] Picot | Synthesis, structures, optical properties, and TD-DFT studies of donor-pi-conjugated dipicolinic acid/ester/amide ligands[END_REF] and compound 1, [START_REF] Roberts | Linker-Dependent Metal-Sensitized Photoswitching of Dithienylethenes[END_REF] were prepared as previously reported.

YbCl3.6H2O (99.998%) and EuCl3 (99.99%) were purchased from ALDRICH and ALFA INORGANICS while Yb(TTA)3•2H2O and Eu(TTA)3•2H2O were prepared according to the literature. [START_REF] Voloshin | Singlet-singlet energy transfer from ketone to lanthanide ion beta-diketonates as studied by chemiluminescence quenching. First observation of infrared chemiluminescence of neodymium (III) and ytterbium (III) in solution[END_REF] All the reactions and handling involving DTE compounds were carried out in the dark.


Bowker Geoffrey

Fidelia Ibekwe-Sanjuan
email: fidelia.ibekwe-sanjuan@univ-amu.fr

Geoffrey C Bowker
email: gbowker@uci.edu

Implications of big data for knowledge organization

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

Introduction

The field of Knowledge Organisation (KO) is mainly concerned with constructing and maintaining knowledge artefacts, otherwise known as Knowledge Organisation Systems (KOSs) which are structured and controlled vocabularies such as bibliographic classification schemes, taxonomies and thesauri for libraries and organisations (Hodge 2000). Since the late 20 th century, mainstream KO research has focused largely on evolving bibliographic classification languages from the print and local computer media to the Internet and the World Wide Web and on bibliographic models and metadata for describing records. However, these systems have remained largely in the purview of libraries and librarians. The general public rarely has recourse to such systems to find things on the Internet, including books. The late 1980s witnessed the advent of the Internet, the Web and several advances in artificial intelligence (machine learning, natural language, text mining) which have since enabled the emergence of computer-supported techniques and tools for knowledge intensive tasks such as indexing, information retrieval, classification, ontologies and domain knowledge mapping. The KO community has been slow to integrate these automatic approaches into its methods. To the best of our knowledge, few KO publications have leveraged these automatic techniques, such as bibliometrics and data clustering to represent, organise or retrieve domain knowledge. [START_REF] Smiraglia | Modulation and specialization in North American knowledge organization: visualizing pioneers[END_REF]; Ibekwe-SanJuan and SanJuan 2010, 2004 and 2002; [START_REF] Chen | The Impact of the Sloan Digital Sky Survey on Astronomical Research The Role of Culture, Identity, and International Collaboration[END_REF].

Over the last decade, the world has become aware of the clarion call of Big Data. It has become one of the buzz phrases of our times, after Web 2.0 and social media, which pepper the utterances and writings of commentators, journalists, businesses, policymakers and scientists. While not a technology in itself, Big Data has huge technological, methodological, social and epistemological implications. It has put every sector of activity under enormous pressure as organisations grapple with scalability issues in their information ecosystem. Recent advances in data processing techniques have given rise to very robust Machine Learning algorithms that are capable of leveraging the huge amounts of data left by our daily use of digital devices to build predictive models. The list of applications where big data algorithms are now being used is constantly growing, from the more conventional search and retrieval to recommender systems such as the ones on Amazon and Netflix that suggest what individuals may want to read next or watch next based on their previous clicks and purchases; to targeted advertising; to stock exchange brokering; to trend detection, opinion mining, and information visualisation. While not infallible, these algorithms have attained a level of performance that is acceptable to humans. Moreover, they are programmed to work in the background in a non-intrusive manner, quietly gathering data and crunching them to provide users with suggestions and recommendations that can rival those of a human librarian or KO specialist. Thus big data algorithms raise the question of the relevance of humanly constructed KOSs and their capacity to keep up with the ever-increasing size of available data on specific topics and domains. A lot has been written on the relative merits and pitfalls of Big Data for science and society but mainly from the point of view of other fields and disciplines. Big Data is starting to become a topic of concern to KO scholars, as evidenced by recent discussions at the ASIST workshop of the 'Special Interest Group on Classification Research' in 2014 i . In that workshop, (Shiri 2014, 18) argued that the formal and "organized nature of linked data and its specific application" for building SKOSs, linked controlled vocabularies and knowledge organization systems, should "have the potential to provide a solid semantic foundation for the classification, representation, visualization and the organized presentation of big data". However, Shiri's article is focused on practical applications of linked data to KOSs and it is also more programmatic rather than based on actual empirical evidence of the use of SKOSs in applications requiring the processing of large amounts of data. Indeed, most of the fields he listed as areas where KO research should have relevance such as Natural Language Processing (NLP), Machine Learning, Text mining, Data mining, Information visualisation, semantic search engines, recommender systems and query expansion have made significant advances without input from KO research.

To the best of our knowledge, few attempts have been made to offer high-level analyses of the implications of big data for KO research and for KOSs. Our paper does not deal with specific empirical studies nor with specific applications of KOSs to Big Data. Instead, we focus on the conceptual, epistemological and methodological implications that Big Data could have for KO and KOSs. More specifically, the questions we try to bring answers to are as follows: i) how will the increasing dematerialisation of activities in every sector which leads to the huge increase in the amount of digital data available and in turn to the necessity to turn to algorithms to process and extract information from the data affect the design of KOSs? ii) In other words, will the era of "algorithmic governmentality ii " [START_REF] Rouvroy | Gouvernementalité algorithmique et perspectives d'émancipation[END_REF] ushered in by the big data phenomenon signal the demise of KOSs in their current form? iii)

How should KO scholars and practitioners position themselves with regard to the participatory paradigm of web 2.0 which functions conjointly with Big Data on the field?

We hope that this discussion will help bridge the current gap between two research communities (and their literatures) which have existed separately until now: the KO community on the one hand, and the data analysis and machine learning community on the other. In section §2, we will recall the ongoing debate within the KO community about the relevance of KOSs in the digital age. In section §3, we turn to the phenomenon of Big Data in order to highlight its characteristics and in section §4, underscore the conceptual, epistemological and methodological challenges it poses for KO research and practice. Finally, we will offer some concluding remarks in section §5 about the challenges facing KO in the near future.

On the relevance of bibliographic classification schemes and of the thesaurus in the digital and web era

Fears about the possible demise of an existing method or technique in the face of new ones are not new nor are they peculiar to the KO community. For instance, Almeida, Souza and Baracho (2015) issued a dire warning about the threats facing Information Science (IS) in the face of information explosion and other recent phenomena like Big Data, Cloud Computing and Social Networks. Instead of making IS stronger as a field, the pervasiveness of information, of digital data and of information technologies them have in fact weakened IS considerably as most of its original subject matters have now become the object of research of fields like engineering, computer science, linguistics, sociology, anthropology or economics. The consequence, according to the authors is a much deflated IS which risks becoming « a mere niche among other fields », with information professionals becoming like "the "remora", which feeds on the thematic leftovers of topics that other fields develop". The authors concluded that the initial research program for IS laid out in Borko's 1968 seminal paper "Information science: what is this?", "has become too broad for the IS field" and indeed, is "too broad for any research field". They suggested turning to interdisciplinarity as a way of negotiating IS's relations with other fields on the overlapping subject matters. Hjørland, a leading researcher of KO, has devoted several articles to analysing various issues in KO research and artefacts that may compromise the relevance of the field in the digital age. His criticisms revolve around three points which are of particular relevance to our discussion: (i) the possible obsolescence of universal bibliographic classification schemes, (ii) the neglect of subject knowledge by library classifiers, and (iii) the reluctance of the KO community to leverage data analysis techniques as an alternative to manually constructed KOSs. We will examine these criticisms in the light of the issues raised by Big Data for all sectors of activities dealing with digital artefacts of which KO is one.

Over-standardisation and over-normalisation of bibliographic classification schemes

In his 2012 article, Hjørland asked "Is classification necessary after Google?" While the title is provocative, it is also relevant. He made the observation that: "At the practical level, libraries are increasingly dispensing with classifying books" and "At the theoretical level, many researchers, managers, and users believe that the activity of "classification" is not worth the effort, as search engines can be improved without the heavy cost of providing metadata." (Hjørland 2012, 299).

Search engines now offer access to full text of digital contents to end users, thus alleviating the need for lengthy library borrowing procedures. Also, the Google Book indexing project aims to digitise most of the human production of books. When this project is completed, it will challenge even more strongly the traditional role of libraries as primary custodians of knowledge artefacts, especially in print and book formats, as more publications migrate to the digital media. Concerning universal bibliographic classification schemes, (Hjørland 2012, 299) observed that the Dewey Decimal Classification (DDC) and Universal Decimal Classification (UDC) were built from the point of view of "standardisation" rather than "tailored to different domains and purposes" and that sections of the UDC are obsolete; thus obsolete knowledge was being served in a flag-bearing product of KO. Aside from not being useful for online search and retrieval, their obsolescence has more profound implications: the most commonly used bibliographical classification schemes may not reflect the most current theories orienting research activities in some fields. Furthermore, (Hjørland 2015b) argued that KO should be concerned with theories of knowledge since theories are expressed on the linguistic level as concepts and concepts are the building blocks of KOSs. As observed by Hjørland, a classification is composed of statements of the sort that concept "A" is a kind of concept "B" or that concept "A" is related to concept "B". A classification can therefore be likened to a scientific theory, although we observe that it is of a much looser type with more limited implications and explanatory power than scientific theories. KOSs such as library classifications, thesauri and ontologies are therefore important auxiliaries of scientific theories because they reflect how concepts and objects in a domain are related to each other, from the point of view of a given scientific theory which guided the classification task.

Neglect of subject knowledge

As argued by Hjørland, a corollary of the slowness to update universal bibliographic classification schemes is the neglect of subject knowledge from KO scholars and practitioners: "My claim is that the neglect of the importance of subject knowledge has brought forward a crisis in KO, and that no real progress can be observed in the field. Of course, there is plenty of progress in the development of digital technologies which enable better kinds of knowledge representation and information retrieval. But such progress is brought to us from the outside; it is not something the field of KO has provided. It is important to realize that there is a need to make sure that the KOSs developed or studied within our field are sufficiently based on and related to current scientific theory (that is also the case with approaches based on numeric taxonomic methods). There is no short cut via user studies, common sense, or anything else." (Hjørland 2013, 179).

Hjørland cites the example of the field of astronomy where the evolution of theory led to Pluto being demoted in 2006 from the status of a planet to a dwarf planet. He argued that one would have expected library classification schemes, taxonomies, and thesauri to reflect this "discovery" without much delay to ensure that people seeking information about planets are not served outdated or incorrect knowledge. Unlike library classification schemes which may take years to update, Wikipedia pages dedicated to planets and to Pluto updated the state of knowledge in this field as soon as the discovery was validated by the community of scholars in astronomy.

Because scientific theories which are the result of scientific discoveries are not immutable facts that are true at all times but can be overturned by other competing theories, it is important for KO to be concerned with scientific theories and domain knowledge. This is a fundamental issue, which can be illustrated by the attempts to build a semantic web tied to a fixed ontology.

Ontologies are frequently changed by scientific disciplines as they grow, and categories have different ontological properties in related disciplines. The flexibility that is needed here cannot be generated within a universal subject classification; but this does not in the slightest obviate the need to classify -it just says something about the need for classifications to be flexible and adaptable. For example, in biology, a new fossil can uproot a classification system, which is not a problem if the change can propagate swiftly across multiple interrelated classifications (Bowker 2000). While we agree with the soundness of Hjørland's fundamental criticisms, it is important to underline that the role of universal bibliographic classifications is not only to represent the state of domain knowledge at every given moment in time but also to organise knowledge artefacts in physical spaces like libraries such that their relationship with one another can be perceived. Furthermore, given the dynamic and evolving nature of digital data and the uncertainties underlying the knowledge contained therein (see §3 hereafter for a discussion), universal bibliographic classifications cannot be expected to constantly change their classifications to follow every discovery made at each instant t. This will not only prove an impossible task to accomplish in real time for libraries but it can also be very disruptive for end users. There is, of necessity, a waiting period between a scientific discovery and its inclusion into universal bibliographic classifications that are known for portraying knowledge validated by the scientific community and which have acquired a certain degree of permanence. Also, the practical value of universal bibliographic classifications-that of enabling patrons collocate material artefacts in a physical space, is not entirely dependent on the theoretical "up-to-dateness" of their class structure. Finally, universal classification schemes like the DCC and UDC which are the focus of Hjørland's criticisms form only a subset of KOSs. The other types -thesauri, ontologies and specialised classification schemes are all domain-dependent knowledge artefacts that make no claim to universalism and should therefore be amenable to more frequent updates.

The reluctance to leverage automatic data analysis and knowledge representation techniques to build more up-to-date KOSs

The same concerns about the relevance of KOSs in the digital age were perceptible at the Fourth conference of the UK chapter of the International Society for Knowledge Organization (ISKO-UK) held on 14 July 2015 in London. The conference theme "Knowledge organization -Making a difference: the impact of knowledge organization on society, scholarship and progress" asked participants to "address the role that KO should have in the future, the opportunities that lie ahead for KO, and what difference it could really make for economic, scientific and/or cultural development".

In answer to that call, [START_REF] Soergel | Unleashing the Power of Data Through Organization: Structure and Connections for Meaning, Learning and Discovery[END_REF]) offered a somewhat mixed diagnosis. On the one hand, he asserted the pervasiveness of knowledge in all human endeavour which should logically ensure the necessity of KOSs in every domain and knowledge intensive applications. This is the optimistic viewpoint. On the other hand, he also acknowledged that many of the advances in automated techniques for knowledge extraction, representation, and dissemination were brought about by other scientific communities. Large-scale ontologies, knowledge and expert systems, information search and retrieval platforms, taxonomies, and semantic web technologies have been developed outside of the KO community. The lack of interoperability of many KOSs only aggravates the situation. Even within the KO community, silos are erected around KOSs which slow down their integration with other knowledge repositories and their reuse outside of the specific KO targeted applications. He called for "more communication between the largely separated KO, ontology, data modelling, and semantic web communities to address the many problems that need better solutions" (Soergel 2015, 401) and exhorted the KO community to recapture the terrain that it had abandoned to computer science by focusing not only on the classification of bibliographic metadata as it has done for centuries but to become involved in actually "structuring and representing the actual data or knowledge itself", issues that KO has "left to the ontology, artificial intelligence, and data modelling communities" (Soergel 2015, 403). This, he says, requires that the KO community should embrace computer applications such as information extraction, phrase sense disambiguation, and information retrieval which can benefit from insights from KO. For KO to continue to be useful to today and tomorrow's world, it must be prepared to work with data analysts, and computer scientists amongst others. [START_REF] Hjørland | Theories of knowledge organization -theories of knowledge[END_REF]) equally lamented the reluctance of the KO community to leverage automatic techniques to building KOSs. While both automatic data analysis techniques and manual approaches to designing of KOSs entail methodological and epistemological biases, automatic techniques represent a "bottom-up" approach to knowledge organisation. They can yield more descriptive domain knowledge organisation that reflects the current state of knowledge, rather than the prescriptive top-down approach to building KOSs. As such, they share some features with KOSs that rely on user-generated contents (UGC) such as folksonomies.

The debate initiated during the Fourth ISKO-UK conference generated a lively discourse on the relevance of the thesaurus for online information retrieval and other knowledge intensive applications. Through both a face-to-face meeting iii and scholarly publications gathered in a special issue of Knowledge Organization journal (volume 43, issue 3 published in 2016), many authors defended the continued relevance of bespoke thesauri for several online applications such as domain knowledge representation, multilingual search, and image retrieval. However, there was a consensus that the thesaurus has been displaced by general-purpose search engines, such as Google, as the standard tool for knowledge representation and search. The reasons given were similar to the ones that had dethroned the relevance of universal bibliographic classification schemes for document retrieval and knowledge representation: difficult-toimplement construction models, a tendency to over-standardisation and over-normalisation of semantic relations also known as "bundling" which reduces the diversity of possible semantic relations between domain concepts and objects to only "is-a" and "related-to", leading to tools that are inadequate for real-life situations. When confronted with building a knowledge representation scheme in enterprises and organisations, many information professionals admitted to not building "ISO standard compliant thesauri". According to their testimonies, "flexibility and pragmatism", rather than strict adherence to ISO guidelines, govern the endeavour. Several information professionals also stressed the necessity to seek ways to integrate user-generated contents (UGC) such as tags and looser synonyms into future KOSs. We will return to this point in section 4.4. In a blog post following the debate on the future of the thesaurus, (Dextre Clarke 2015) surmises that: "Given a discerning team of developers, curators, IT support staff and indexers, this sophisticated tool can and should function interoperably alongside statistical algorithms, NLP techniques, data mining, clustering, latent semantic indexing, linked data, etc. Networking and collaboration, not rivalry, are the future iv ." While such optimism is commendable, the operative words here are "can" and "should". Indeed, the expected interoperability and integration of KOSs into data mining and clustering techniques has not happened, despite all the common sense arguments advanced by KO practitioners and scholars. Perhaps one of the reasons lies in the fact that the two scientific communities have very little interaction with one another. More fundamentally, KOSs and indexing and clustering algorithms are designed from different epistemological and methodological assumptions. This makes their integration if not contradictory, at least difficult to achieve in practical terms. By their very nature, statistical and probabilistic models underlying indexing and clustering algorithms are designed to select data units based on their distribution, to model the behaviour of data units in a corpus and produce statistical tests and measures. Of course, there are cases of combined approaches integrating some humanly constructed knowledge bases into automatic systems but such architectures rarely scale up to industrial applications and would be intractable in the case of today's big data (Ibekwe-SanJuan and SanJuan 2002; 2010). Machine Learning models are precisely designed to "learn" from existing data in order to be able to classify unseen units in the future. Furthermore, decades of experimental studies in information retrieval, NLP, or semantic knowledge extraction and modelling tended to show that systems relying on consultation of external knowledge-rich databases during an online search slow down the retrieval speed without necessarily guaranteeing a significant increase in precision. As a participant in the ISKO-UK debate concluded, "in my experience findability is the big driver rather than interoperability v ". This is in line with [START_REF] Hjørland | Is classification necessary after Google?[END_REF], 301)'s assessment of the challenge facing KO on the practical level, which he framed as follows: "how can LIS professionals contribute to the findability of documents, given the availability of many competing services in the "information ecology?" It seems that researchers and practitioners agree on this point: if KO artefacts do not help people achieve the goal for which they were built, namely finding documents and information in our current web-centred information ecology, then they risk being relegated to the ash heap of history, replaced by technological solutions powered by NLP, statistical and machine learning algorithms. Having recalled the ongoing debates about the relevance of KO research and of KOSs in the digital age, we now turn to the concept of Big Data in order to determine how they may in turn affect KO research and artefacts.

What is Big Data?

The first task that awaits anyone who embarks on a discourse on Big Data is to define what it is. Apart from the fuzziness surrounding the nature and size of Big Data, there has been some debate about the origins of the term itself. The statistician Francis Diebold is generally credited with coining the term "Big Data" in a paper that he presented in 2000 entitled "Big Data Dynamic. Factor Models for Macroeconomic Measurement and Forecasting". Diebold himself noted that the term was already used earlier in a non-academic context in advertisements run by Silicon Graphics International (SGI) between late 1990 and 1998. A slide deck prepared by the former Chief Scientist at SGI, John Mashey was entitled "Big Data and the Next Wave of InfraStress vi ". Another occurrence of the term was found in a 1998 computer science paper by Weiss and Indurkhya. However, it was the data analyst Douglas Laney who in 2001 made a decisive contribution towards the current characterisation of the Big Data by coining the popular and catchy "three V's" of Big Data (Volume, Variety and Velocity) in an unpublished 2001 research note at META Group vii . Laney's 3Vs later expanded into 4Vs (3Vs + Validity) and now has a fifth V as well (4Vs + Veracity). Haven retraced the origins of the term, the question about what it is remains open. There is a consensus, at least from a physical standpoint, that Big Data represents volumes of data such that traditional database algorithms are unable to cope with it and that it requires more robust and distributed computer infrastructures and algorithms such as hadoop clusters, grid infrastructure and cloud clusters. This led [START_REF] Gray | eScience: A Transformed Scientific Method[END_REF] to consider that data-driven science will be the "fourth science paradigm". However, people rarely venture to indicate a minimum size after which data can undisputedly be said to become big. At what point is one truly justified of speaking of "Big Data"? (Boyd and Crawford 2012, 664) offered a characterisation of the different dimensions of big data: "Big Data is less about data that is big than it is about a capacity to search, aggregate, and cross-reference large data sets. We define Big Data as a cultural, technological, and scholarly phenomenon that rests on the interplay of:

(1) Technology: maximizing computation power and algorithmic accuracy to gather, analyze, link, and compare large data sets.

(2) Analysis: drawing on large data sets to identify patterns in order to make economic, social, technical, and legal claims.

(3) Mythology: the widespread belief that large data sets offer a higher form of intelligence and knowledge that can generate insights that were previously impossible, with the aura of truth, objectivity, and accuracy."

As wearable electronic devices become more pervasive especially in the health and fitness, home and car insurance sectors, more and more data will be collected such that the term "Big Data" will eventually lose its distinctive meaning since most digital data will be "big". If, as (Soergel 2015, 402) asserted (and we are in agreement), "Knowledge organization is needed everywhere, it is pervasive", then one will expect that Big Data which has also become a pervasive phenomenon that embodies knowledge, will have an impact on KO research and artefacts. The next section recasts the discussion of the epistemological assumptions underlying Big Datadriven inquiry in the light of the current concerns about the future of KO research and KOSs.

Possible implications of Big Data-driven inquiry for Knowledge Organisation research and artefacts

Broadly speaking, publications on Big Data seem to fall into three categories: (i) enthusiastic, (ii) critical, and (iii) nuanced. For the first category of Big data apostles, it represents the new El Dorado whose exploitation has the potential to accelerate the rhythm of scientific discoveries and innovations (Andersen 2008; Mayer-Schönberger & Cukier 2014; Gray 2009). Undeniably, the exploration of Big Data by sophisticated data exploration techniques has accelerated the rhythm of discoveries in some fields. The era of e-Science it has brought in its wake implies a culture of international collaboration. Although (Kitchin 2014, 10) is not a Big Data apologisthe has indeed offered more nuanced and somewhat critical appraisals of the phenomenon, he nevertheless acknowledged its opportunities for scientific inquiry: "There is little doubt that the development of Big Data and new data analytics offers the possibility of reframing the epistemology of science, social science and humanities, and such a reframing is already actively taking place across disciplines. Big Data and new data analytics enable new approaches to data generation and analyses to be implemented that make it possible to ask and answer questions in new ways."

For its critics, the power of Big Data is grossly overrated. The much mediatised errors of the Google Flu Trend algorithm in predicting the outbreaks and peaks of seasonal flu worldwide are often cited as a blatant case of big data algorithm failure. The refusal by Google analysts to release information on the exact datasets used to calculate such trends and the algorithmic processes involved only increased scholarly distrust (Auerbach 2014a, 2014b and 2014c; Marcus and Davis 2014; Thatcher 2014) as their study cannot be replicated by other scientists. Indeed, this is a general problem since big data algorithms used by private companies such as Facebook and Twitter tend to be proprietary and thus, are not directly amenable to academic analysis.

The third category of more nuanced publications, recognises that Big Data-driven inquiry has the potential to accelerate the rhythm of discoveries in some fields viii but at the same time that it has pitfalls of which we ought to be aware. (Boyd and Crawford 2012, 664) summarised the duality of the big data phenomenon thus: "Like other socio-technical phenomena, Big Data triggers both utopian and dystopian rhetoric. On one hand, Big Data is seen as a powerful tool to address various societal ills, offering the potential of new insights into areas as diverse as cancer research, terrorism, and climate change. On the other, Big Data is seen as a troubling manifestation of Big Brother, enabling invasions of privacy, decreased civil freedoms, and increased state and corporate control. As with all socio-technical phenomena, the currents of hope and fear often obscure the more nuanced and subtle shifts that are underway."

Boyd and Crawford were also amongst the first authors to frame high-level critical questions that we should be asking about the implications of big data driven inquiry for science. In the sections below, we will recall some of the ontological, epistemological and methodological implications of Big Data-driven inquiry which have been much debated in the Big Data literature and which may have implications for KO research and artefacts, given their current shortcomings discussed in section §2 above.

1. Data are social artefacts

Although data are often presented as a natural phenomenon just waiting to be collected, nothing could be farther from reality. Pushmann and Burgess (2014) discussed how the metaphors of "gold", "ocean", "torrent", "mineral and "oil" are attached to the term big data, thus giving the impression that it is a natural phenomenon. As all data analysts know, data gathering is not a neutral nor an objective endeavour. It is governed by pragmatism (the goals of the study) and bound by technical constraints imposed by the data providers. This limits possibilities in terms data sources and content. [START_REF] Ekbia | Big Data, Bigger Dilemmas: A Critical Review[END_REF](Ekbia et al. , 1531) also offered a timely reminder of the whole gamut of tamperings involved in the data processing stage: from the intent to collect governed by pragmatic goals and involving "multiple social agents with potentially diverse interests" to its generation which is often "opaque and under-documented", to "incomplete or skewed" data without even talking of "instrument calibration and standards that guide the installation, development, and alignment of infrastructures" nor of human practices involving filtering (deciding which variables to keep and which to discard, in the case of personal data, anonymisation which often leads to loss of context and distortion), cleaning and even intentional distortions which all annihilate the pretensions to "rawness" of data." This ensemble of tweaking makes data highly subjective and dependent on the aims of the project for which it is being collected. Data is therefore something that is constructed to suit a particular project and is by necessity always "incomplete". This point was aptly captured by [START_REF] Bowker | Data flakes: An afterword to "raw data" is an oxymoron[END_REF]) when he wrote that "raw data is an oxymoron" and that "data should be cooked with care", an opinion echoed by the French sociologist, (Bruno Latour 2014) who suggested that the French term for data "donnée" should be replaced by "obtenu" (obtained). Etymologically, "data" is the plural form of the latin word "datum" which means "that which is give" (i.e., the perfect passive participle of the verb "do, dare", "to give"). Hence the suggestions by Bowker and Latour reflect the etymology of the word. If all the data tweaking that take place during data processing were not sufficient reason to adopt a critical attitude towards data-driven discoveries, consider the fact that big data studies from social media such as Facebook and Twitter are notoriously hard to replicate due to the restrictions in data gathering imposed by the private companies that have appropriated the data. Hence, the timely reminder by (Boyd and Crawford 2012, 669) not to confuse data from social media with real people or that 'people' and 'Twitter users' are not synonymous", all the more so when a proportion of the data is generated by computer bots. A change in the dataset from which a study is conducted will also alter the "discoveries" made therein. [START_REF] Bowker | The Theory/Data Thing. Commentary[END_REF](Bowker , 1797) ) similarly underscored the theoretical incompleteness of data: "As Derrida (1998) argues in Archive Fever and Cory Knobel (2010) so beautifully develops with his concept of ontic occlusion, every act of admitting data into the archive is simultaneously an act of occluding other ways of being, other realities. The archive cannot in principle contain the world in small; its very finitude means that most slices of reality are not represented. The question for theory is what the forms of exclusion are and how we can generalize about them. Take the other Amazon as an illustration. If I am defined by my clicks and purchases and so forth, I get represented largely as a person with no qualities other than "consumer with tastes." However, creating a system that locks me into my tastes reduces me significantly. Individuals are not stable categories-things and people are not identical with themselves over time."

The foregoing observations underscore the transient and dynamic nature of big data which in turn render difficult if not impossible, the replicability of Big Data-driven studies. Yet, replicability is one of the canons of science. Thus many studies reference the state of a database at the time the study was done, but do not contain a copy of the database at that time. This holds a fortiori for studies of Twitter feeds where again one cannot access past states of the database. What are the implications of the aforegoing considerations for KO? Traditionally, many KO practitioners and researchers building classification and indexing systems have justified the inclusion of terms and their relations based on literary warrant. This involves gathering data about the usage of these terms in books and other knowledge artefacts in a given field. How will literary warrant be construed given that the available size of data from which such warrants can be drawn has grown exponentially and will continue to do so and also that the said data is constantly changing? If literary warrant is an important criterion for constructing KOS, it means that KO practitioners and researchers will have to better account for how the corpora guaranteeing this literary warrant are built. The KO community will need to confront data representativity issues by documenting precisely how the data were collected, who or what is represented, who or what is left out, the types of processing the data underwent from its collection to the knowledge acquisition and representation. This will better inform end users about how the knowledge artefacts were built and what they can be used for. Doing so will also lend more credibility to those KO artefacts that claim universal subject coverage like the encyclopedic bibliographic classification schemes. However, literary warrant is not the sole basis for warrants in building KOSs. Other types of warrants have been suggested such as "use warrant", "structural warrant", "educational warrant", "scientific/philosopical warrant", "semantic warrant" and, "cultural warrant". For a typology of warrants, see [START_REF] Howarth | Towards a typology of warrant for 21st century knowledge organization systems[END_REF].

If there exist other kinds of warrants that are not based on literary warrant, then the necessity for KOSs construction to scale up to Big Data becomes less crucial. This is not to say that data representativity is unimportant nor should it be neglected. It means however that the credibility of KOSs is not uniquely linked to their adherence to literary warrant. The essential argument here is that KO as a field needs to adapt to the changing nature of output in the social and natural sciences, to the extent that these in turn are being affected by the advent of Big Data. One model might be the high energy physics community (itself closely linked with the rise of the Web) where not only are data generated and shared in vast quantities in real time, but also literary warrant is created more rapidly than traditional publications through open archives like ArXiv.

Big data changes what it is to know

Epistemology is a philosophical account of what knowledge is and what knowing is. This is of particular import to the field of KO, a field which deals with the classification of existing knowledge accumulated over thousands of years of scientific inquiry. The sheer size of data and their dynamic and heterogeneous nature (e.g. image, text, sound) make it difficult to subject Big-Data driven inquiries to rigorous scientific verification. This could in turn result in being forced to abandon the principles of falsifiability and fallibilism of scientific theories laid down by Karl Popper and Charles S. Peirce which have guided scientific activity up till now -for some fields at least. If rapidly changing ontologies (characteristic of Big Data) are creating incommensurabilities on the fly, then we are moving into a more kuhnian (1957) world in which old theories just cannot be compared with new -rendering falsifiability somewhat obsoleteespecially until we create institutional mechanisms for preserving all states of a given database (a monumental and almost impossible task).

If as (Hjørland 2015b) convincingly argued, KO should be concerned with theories, then knowledge derived from big data-driven discoveries should impact the theories and epistemological positions from which future KOSs are constructed. Essentially, the field of KO needs to move from laying down the apodictic (that which we know for all time) to adapting to the new world of social and natural scientific knowledge by creating maximally flexible schemas -that, faceted rather than Aristotelean classifications. The great knowledge organisation schemata of the nineteenth and twentieth centuries drew on a period when the sciences were being 'invaded' by statistics (Hacking, Empire of Chance). This led social science (specifically Durkheim) to creating reified categories which in turn informed social policy. If the reified categories go away and 'big data' replaces statistics (it is surely invading all branches of human learning just as effectively), then we need to rethink the nature of our enterprise.

Yes, classification is still necessary after Google!

In answer to his provocative title "Is classification still necessary after Google?", [START_REF] Hjørland | Is classification necessary after Google?[END_REF]) concluded in the affirmative, despite advances in machine mediated information retrieval and Google because any classification is a choice between different viewpoints. We are in agreement with this analysis. Big data and Google have not removed the need for classification, quite the contrary! The ever-increasing amount of data means we need classification more than ever, but the nature of that classification needs rethinking. There is a simple logic to this: Big Data only works if it comes with good metadata. Each form of metadata in turn relies on fixed categories of one kind or another. It is not that classifications will go away, rather, they just become less visible. Thus, even in the world of Big Data, it may be very difficult to parse backwards a retronym such as 'biological mother' since the 'mother' category may be part of the data flowwhere 'surrogate mother' or 'adoptive mother' may not.

Big Data paradox: between automation and human labour

Whereas the Big Data phenomenon entails increased automation of tasks in many fields, datadriven algorithms also require constant human input to learn and improve their models and hence performances. We therefore have a paradoxical situation where Big Data leads ultimately to the replacement of humans by algorithms whilst at the same time requiring human labour (crowdsourcing) to improve the predictive powers of the said algorithms. [START_REF] Ekbia | Big Data, Bigger Dilemmas: A Critical Review[END_REF]) called this paradox "heteromation" whereby Big Data relies on the co-existence of two seemingly opposing modalities: human labour and automation.

Initially, there was a lot of scepticism about the quality and value of large-scale knowledge resources built through the crowdsourcing model but the success of the Wikipedia project in harnessing public participation to make it the most consulted and cited web site, as well as the proliferation of participatory science projects have silenced even the most vocal sceptics.

To cite only a few examples, in the field of astronomy, the SDSS project gave rise to Galaxy Zoo ix , a crowdsourcing project to identify and annotate 3D images of celestial objects taken by the SDSS telescope. Started in 2007, Galaxy Zoo has mobilised more than 150 000 amateur contributors who helped astronomers classify more than 230 million celestial objects. Likewise, the eBird x project led by the University of Cornell crowdsourced the immense task of inventorising of bird species. This enabled scientists to collect up to 160 millions observations from more than 1000 bird watchers all over the world, thus accounting for more than 95% of bird species. Lagoze (2014) acknowledged that it was "a highly successful citizen science project that for over a decade has collected observations from volunteer participants worldwide. Those data have subsequently been used for a large body of highly-regarded and influential scientific research." This would have been impossible for scientists alone given the scale of the task. Web 2.0 technologies have made harnessing the contributions of the masses possible at an unprecedented scale. As scientists and other professionals in giant tech companies rely more and more on machines and on volunteers, the existence of online communities of citizen scientists can lead to a blurring of frontiers between amateurs and specialists. By inviting large members of the public to partake in the scientific adventure, scientists ultimately relinquish some of their prerogatives and areas of past expertise. (Lagoze 2014) called this a "fracturing of the control zone". This makes many scholars and professionals understandably uneasy because it breaks down well-established barriers between experts and amateurs.

In the library, archives and museum realm, there are efforts to integrate the participatory model of knowledge production popularised by Web 2.0 and which the general public has come to expect. Concepts like "museum 2.0", "participatory museum" (Simon 2010) and "library 2.0" or "participatory libraries" have come to represent endeavours to leverage public participation and integrate UGC (e.g. folksonomies) into some of the KOSs artefacts xi -although this practice has found little traction in academic libraries (see Ibekwe-SanJuan and Ménard, 2015 for a review). Some information professionals at the thesaurus debate organised by ISKO-UK in 2015 have emphasised the need to liberate thesaurus construction from the shackles of top-down ISO normalisation rules and to integrate more UGC and more uncontrolled vocabulary terms. Making a similar argument, (Hjørland 2012, 308) writes: "While mainstream classification research is still based on the objectivist understanding (a document has a subject), the minority view (that document A is assigned subject X by somebody in order to support some specific activities) is gaining a footing. I believe this last view is decisive for making a future for classification in both theory and practice."

This will enable KOSs to reflect more up-to-date knowledge which will better serve the needs of specific applications and categories of users. It will also ensure that KOSs are integrating more diverse viewpoints through the implementation of recommender systems available on Web 2.0 platforms. Domain experts are more aware of term usages and of recent advances in their specific fields than cataloguers.

Conclusion

In this paper, we confronted the ongoing debates about the future of KO research with debates about the benefits and pitfalls of Big Data for scientific inquiry in order to determine how the latter might affect the former. In an era of Big Data, it appears even more unrealistic to hope that universal bibliographic classification schemes can be updated by a handful of "expert cataloguers or bibliographers" nor to ignore the participatory and collaborative paradigm which has made Web 2.0 platforms like Wikipedia, Facebook and Twitter successful. However, universal bibliographical classifications in libraries are typically not used alone but are integrated into broader systems (i.e. catalogs), which bring them together with subject heading systems, thesauri, and sometimes folksonomies. Public participation can be better leveraged to update subject headings and thus enhance the effectiveness of library classification schemes. Thus, it is not a matter of "either...or", i.e., either expert-built classification systems or participatory/collaborative systems, but "and ... and", i.e., determining how both approaches can be combined in designing KOSs for specific applications and categories of users.

The challenge for KO is therefore to reinvent itself in an information ecosystem filled with algorithms that are continuously crunching data and delivering digital content tailored to users' profiles rather than focusing on one-size-fits-all knowledge bases constructed a priori. This calls for a "rapprochement" between the KO and the computer and artificial intelligence communities as well as a significant opening up of Library and Information Science curricula to integrate subjects like epistemology, philosophy, statistics and data analysis techniques. Knowledge organisation will not go away as a field: it is central to the scientific endeavour. However, it needs to adapt to the new temporalities of theoretical development occasioned by the spread of Big Data across the social and natural sciences.

vi The company's overview affirms its chief scientist's claim to paternity of the term «

  In the late 90s, SGI's Chief Scientist at the time, John R. Mashey coined the term "Big Data." https://www.sgi.com/company_info/overview.html vii According to Diebold (2012), « META is now part of Gartner. » viii In particle physics, the discoveries of the Higgs Boson in 2012 and of the pentaquarks in 2015 are among some of the most significant recent scientific discoveries which would not have been possible without the Large Hadron Collider (http://home.cern/topics/large-hadron-collider) which generates massive data for physicists to analyse. ix http://www.galaxyzoo.org/x http://ebird.org/content/ebird/ xi See for instance some of the papers in the bibliography of Jennifer Trant : http://www.archimuse.com/consulting/trant_pub.html but also museomix initiatives in different countries