Frédéric Dumur
email: frederic.dumur@univ-amu.fr

Recent Advances on Visible Light Photoinitiators of Polymerization Based on Indane-1,3-dione and Related Derivatives

Keywords: Photoinitiator, push-pull dyes, photopolymerization, LED, visible light, low light intensity, indanedione, cyclization reaction, azafluorenone

Photoinitiators that can operate under visible light and low light intensity are actively researched as this last generation of photoinitiators can address different issues raised by the traditional UV photoinitiators. Notably, the safety concern related to the use of UV light, the high energy consumption resulting from the use of UV irradiation setups, the low light penetration of UV light within the photocurable resins are three major concerns that visible light photoinitiators successfully address. Besides, the photoinitiating ability and the polymerization kinetic should remain high in order these new generations of photoinitiators to be capable to replace the traditional UV photoinitiators. Among the different scaffolds examined for the design of efficient visible light photoinitiators, indane-1,3-dione has been identified as a promising scaffold for the design of various structures. By the presence of the activated methylene group standing between the two ketone groups, this electron acceptor is an ideal candidate for the design of push-pull dyes by one of the simplest reactions, namely the Knoevenagel reaction. This group is also a good electron acceptor as the two ketone groups can be advantageously used in the electronic delocalization by mesomerism. If this electron acceptor has been extensively studied for the design of push-pull dyes, its scope of application was not only limited to the design of push-pull dyes and products issued from cyclization reactions, the design of compounds with extended aromaticities, improvement of the electronwithdrawing ability by substitution of the ketone groups by appropriate groups were examined as possible chemical modifications enabling to improve the photoinitiating ability of photoinitiators based on indane-1,3-dione and its related derivatives. In this review, an overview of the different visible light photoinitiators based on indane-1,3-dione and derivatives is provided. To evidence the performance of the different photoinitiators, comparisons with reference compounds will be provided.

Introduction

In recent years, visible light photoinitiators have been the focus of numerous research efforts due to the numerous advantages these last generations of photoinitiators exhibit. [START_REF] Ganster | New photocleavable structures. Diacylgermane-based photoinitiators for visible light curing[END_REF][START_REF] Ganster | New Photocleavable Structures, 4: Acylgermane-Based Photoinitiator for Visible Light Curing[END_REF][START_REF] Cook | Enhanced visible radiation photopolymerization of dimethacrylates with the three component thioxanthone (CPTXO)-amine-iodonium salt system[END_REF][START_REF] Xiao | Synthesis and photopolymerization kinetics of benzophenone piperazine one-component initiator[END_REF][START_REF] Karaca | Mechanistic studies of thioxanthonecarbazole as a one-component type II photoinitiator[END_REF][START_REF] Esen | One-component thioxanthone acetic acid derivative photoinitiator for free radical polymerization[END_REF][START_REF] Vitale | Visible light curable restorative composites for dental applications based on epoxy monomer[END_REF][START_REF] Fors | Control of a living radical polymerization of methacrylates by light[END_REF][START_REF] Konkolewicz | Visible light and sunlight photoinduced ATRP with ppm of Cu catalyst[END_REF][START_REF] Haddleton | Polymer chemistry: Rooftop reactions[END_REF][START_REF] Treat | Controlled radical polymerization of acrylates regulated by visible light[END_REF][START_REF] Xu | Visible light photocatalytic thiol-ene reaction: An elegant approach for fast polymer post-functionalization and step-growth polymerization[END_REF] Contrarily to the traditional UV photoinitiators that are the subject of numerous controversies (possibility of eye and skin damages due to the use of UV light, use of irradiation setups involving high energy consumptions and inefficient energy-to-light conversions, heat release by the irradiation setups, low light penetration of the UV light within the photocurable resins), visible light photoinitiators clearly constitute a breakthrough in photopolymerization. Even if visible light photoinitiators exhibit several appealing features, visible light photoinitiators also exhibit limitations that can't be denied, namely the fact that these compounds and the formulations containing them must be handled with exclusion of visible light which also constitutes a hurdle for their uses with conventional working setups. Notably, visible photoinitiators such as titanocenes or borate dyestuff did not find widespread use in technical application except in niche applications. [START_REF] Kitano | Unexpected visible-lightinduced free radical photopolymerization at low light intensity and high viscosity using a titanocene photoinitiator[END_REF][START_REF] Davidenko | The efficiency of titanocene as photoinitiator in the polymerization of dental formulations[END_REF][START_REF] Degirmenci | Photoinitiation of cationic polymerization by visible light activated titanocene in the presence of onium salts[END_REF] Therefore, a great deal of effort has still to be done in order to render these photoinitiators appealing candidates for industrial applications.

Compared to the classical thermal initiators, a perfect spatial and temporal control can be obtained during the photopolymerization process. The kinetic of photopolymerization can also be fast and the possibility to polymerize without any solvents enabling a drastic reduction of the release of volatile organic compounds (VOCs) constitute the most attractive features of photopolymerization. [START_REF] Fouassier | Photopolymerization reactions under visible lights: principle, mechanisms and examples of applications[END_REF][START_REF] Lin | Dual-wavelength (UV and blue) controlled photopolymerization confinement for 3D-printing: Modeling and analysis of measurements[END_REF][START_REF] Lalevée | Recent developments of versatile photoinitiating systems for cationic ring opening polymerization operating at any wavelengths and und.er low light intensity sources[END_REF][START_REF] Tehfe | Photopolymerization reactions: On the way to a green and sustainable chemistry[END_REF][START_REF] Kostanski | Cationic polymerization using mixed cationic photoinitiator systems[END_REF] Compared to UV photoinitiators, a better light penetration can also be obtained with visible light, as shown in the graph presented in the Figure 1 and established for a polystyrene latex. [START_REF] Bonardi | High performance near infrared (NIR) photoinitiating systems operating under low light intensity and in presence of oxygen[END_REF] Indeed, if the light penetration is limited to 600 µm at 320 nm, this latter can reach 4 mm at 450 nm and 5 cm at 800 nm. As a result of this, the access to thick samples by photopolymerization is now possible. [START_REF] Garra | Photopolymerization of thick films and in shadow areas: A review for the access to composites[END_REF] In fact, scope of applications of photopolymerization has been totally revolutionized during the last decade, shifting from the elaboration of thin coatings to thick samples containing fillers. Parallel to this, emerging applications such as 3D and 4D printing have clearly provided a revival of interest for this polymerization technique. [START_REF] Crivello | Photopolymer materials and processes for advanced technologies[END_REF][START_REF] Genty | Fast polymerization at low temperature of an infrared radiation cured epoxy-amine adhesive[END_REF][START_REF] Zhang | A versatile 3D and 4D printing system through photocontrolled raft polymerization[END_REF][START_REF] Lee | Two-way 4D-printing: A review on the reversibility of 3dprinted shape memory materials[END_REF][START_REF] Bagheri | Photopolymerization in 3D Printing[END_REF][START_REF] Aduba | Vat photopolymerization 3D printing of acid-cleavable PEG-methacrylate networks for biomaterial applications[END_REF][START_REF] Dietlin | Photopolymerization upon LEDs: new photoinitiating systems and strategies[END_REF] Figure 1. Light penetration within a photocurable resin composed of a polystyrene latex with an average diameter of 112 nm. Reproduced with permission from Bonardi et al. [START_REF] Bonardi | High performance near infrared (NIR) photoinitiating systems operating under low light intensity and in presence of oxygen[END_REF].

Copyright 2018 American Chemical Society.

Over the years, several scaffolds have been identified as being promising structures for the design of efficient visible light photoinitiators, especially of Type II photoinitiators. Among these structures, phenothiazines 1, [START_REF] Abdallah | Phenothiazine derivatives as photoredox catalysts for cationic and radical photosensitive resins for 3D printing technology and photocomposites synthesis[END_REF] porphyrins 2, [START_REF] Bonardi | Different NIR dye scaffolds for polymerization reactions under NIR light[END_REF][START_REF] Al Mousawi | Zinc-tetraphenylporphyrin as high performance visible-light photoinitiator of cationic photosensitive resins for LED projector 3D printing applications[END_REF][START_REF] Noirbent | Metalated Porphyrins as versatile visible light and NIR photoinitiators of polymerization[END_REF] pyrenes 3, [START_REF] Tehfe | Design of new Type I and Type II photoinitiators possessing highly coupled pyrene-ketone moieties[END_REF][START_REF] Tehfe | New insights in radical and cationic polymerizations upon visible light exposure: role of novel photoinitiator systems based on the pyrene chromophore[END_REF][START_REF] Telitel | New core-pyrene π-structure organophotocatalysts usable as highly efficient photoinitiators[END_REF][START_REF] Dumur | Recent advances on pyrene-based photoinitiators of polymerization[END_REF][START_REF] Uchida | nonsalt 1-(arylmethyloxy)pyrene photoinitiators capable of initiating cationic polymerization[END_REF][START_REF] Mishra | 1-(Bromoacetyl)pyrene, a novel photoinitiator for the copolymerization of styrene and methylmethacrylate[END_REF] squaraines 4, [START_REF] Bonardi | Different NIR dye scaffolds for polymerization reactions under NIR light[END_REF] thioxanthones 5, [START_REF] Karaca | Thioxanthone-benzothiophenes as photoinitiator for free radical polymerization[END_REF][START_REF] Wu | Thioxanthone based one-component polymerizable visible light photoinitiator for free radical polymerization[END_REF][START_REF] Qiu | Thioxanthone photoinitiator containing polymerizable N-aromatic maleimide for photopolymerization[END_REF][START_REF] Dadashi-Silab | Shining a light on an adaptable photoinitiator: advances in photopolymerizations initiated by thioxanthones[END_REF] iodonium salts 6, [START_REF] Mokbel | Specific cationic photoinitiators for Near UV and visible LEDs: Iodonium vs. ferrocenium structures[END_REF][START_REF] Villotte | Design of Iodonium Salts for UV or Near-UV LEDs for Photoacid Generator and Polymerization Purposes[END_REF][START_REF] Zivic | A novel naphthalimide scaffold based iodonium salt as a one-component photoacid/photoinitiator for cationic and radical polymerization under LED exposure[END_REF] iridium complexes 7, [START_REF] Lalevée | Subtle ligand effects in oxidative photocatalysis with iridium complexes: application to photopolymerization[END_REF][START_REF] Lalevée | Iridium photocatalysts in free radical photopolymerization under visible lights[END_REF][START_REF] Lalevée | Photopolymerization of N-vinylcarbazole using visible-light harvesting iridium complexes as photoinitiators[END_REF][START_REF] Telitel | Photoredox catalysis using a new iridium complex as an efficient toolbox for radical, cationic and controlled polymerizations under soft blue to green lights[END_REF][START_REF] Telitel | Photoredox process induced polymerization reactions: iridium complexes for panchromatic photoinitiating systems[END_REF][START_REF] Tehfe | Structural effects in the iridium complex series: photoredox catalysis and photoinitiation of polymerization reactions under visible lights[END_REF] [START_REF] Bonardi | High performance near infrared (NIR) photoinitiating systems operating under low light intensity and in presence of oxygen[END_REF][START_REF] Bonardi | Different NIR dye scaffolds for polymerization reactions under NIR light[END_REF][START_REF] Mokbel | On demand NIR activated photopolyaddition reactions[END_REF] and zinc complexes 29 [START_REF] Tehfe | Zinc-based metal complexes as new photocatalysts in polymerization initiating systems[END_REF] can be cited as the most explored structures. However, the polymerization efficiency of these compounds is still insufficient to be used in industry so that a great deal of efforts has still to be done. Several issues of visible light photoinitiators have still to be addressed such as the stability of the resins upon storage and the photobleaching of the resins during photopolymerization. Indeed, contrarily to the traditional UV photoinitiators that are colorless compounds and that can furnish colorless coatings after polymerization, conversely, visible light photoinitiators are strongly colored compounds that can impose the color of the final coatings, except if photobleaching is obtained. Concerning photopolymerization, two distinct photoinitiation mechanisms have to be distinguished, differing by the way how the initiating radicals are formed.

The first approach relies in the homolytic photocleavage of photoinitiators (Type I photoinitiators) so that two radicals are simultaneously formed. As the main drawback of this approach, an irreversible consumption of photoinitiators is observed during polymerization so that the polymerization kinetic is highly dependent of the initial amount of photoinitiators introduced within the photocurable resins. Among Type I photoinitiators, 2,2-dimethoxy-2phenylacetophenone (DMPA), [START_REF] Tehfe | Design of new Type I and Type II photoinitiators possessing highly coupled pyrene-ketone moieties[END_REF] acyl phosphine oxides (phenylbis(2,4,6trimethylbenzoyl)phosphine oxide (BAPO), bis (2,4,6-trimethylbenzoyl)phenylphosphine oxide (TPO)) [START_REF] Xiao | Structural effects in the indanedione skeleton for the design of low intensity 300-500 nm light sensitive initiators[END_REF][START_REF] Bonardi | High performance near infrared (NIR) photoinitiating systems operating under low light intensity and in presence of oxygen[END_REF][START_REF] Bonardi | Different NIR dye scaffolds for polymerization reactions under NIR light[END_REF][START_REF] Mokbel | On demand NIR activated photopolyaddition reactions[END_REF][START_REF] Tehfe | Zinc-based metal complexes as new photocatalysts in polymerization initiating systems[END_REF][START_REF] Tehfe | Design of new Type I and Type II photoinitiators possessing highly coupled pyrene-ketone moieties[END_REF][146][START_REF] Meereis | BAPO as an alternative photoinitiator for the radical polymerization of dental resins[END_REF][START_REF] Rutsch | Recent developments in photoinitiators[END_REF][START_REF] Macarie | Photopolymerizable systems Containing Organophosphorus Compounds as Photoinitiators[END_REF][START_REF] Dietliker | III of Chemistry and Technology of UV & EB Formulation for Coatings, Inks and Paints[END_REF][START_REF] Baxter | acylphosphine oxides as photoinitiators for acrylate and unsatured polyester resins[END_REF][START_REF] Wang | Photopolymerization of Glycidyl Acrylate and Glycidyl Methacrylate Investigated by Differential Photocalorimetry and FT-I.R[END_REF][START_REF] Moore | Calorimetric Analysis of UV Curable Systems[END_REF][START_REF] Gatechair | Photoinitiators: an overview of mechanism and applications[END_REF] are among the most widely studied. Parallel to these cleavable photoinitiators, a second category of photoinitiators exists where the light-absorbing compounds can be consumed or not during the polymerization process, depending on the conditions. Notably, when an amine is introduced in the reaction media, the sacrificing agent can re-oxidize the reduced photosensitizer to its initial state, enabling to use it in catalytic amount. Conversely, in the absence of sacrificial amine, an irreversible consumption of the photosensitizer is also detected, as observed for Type I photoinitiators. These compounds are named Type II photoinitiators and these compounds can be combined with a co-initiator to undergo an intermolecular hydrogen abstraction or an electron/proton transfer reaction. [START_REF] Stephenson Kenning | Spatial and temporal evolution of the photoinitiation rate for thick polymer systems illuminated by polychromatic light: selection of efficient photoinitiators for LED or mercury lamps[END_REF][156][START_REF] Dietliker | Chemistry and Technology of UV and EB Formulation for Coatings, Inks & Paints[END_REF][158][START_REF] Hoyle | Thiol-click chemistry: a multifaceted toolbox for small molecule and polymer synthesis[END_REF][START_REF] Cramer | Kinetics of thiol-ene and thiol-acrylate photopolymerizations with real-time Fourier transform infrared[END_REF][START_REF] El-Roz | Mechanistic Investigation of the Silane, Germane, and Stannane Behavior When Incorporated in Type I and Type II Photoinitiators of Polymerization in Aerated Media[END_REF] Multicomponent systems comprising three and even more components can also be used. [START_REF] Ma | Multicomponent photoinitiating systems containing arylamino oxime ester for visible light photopolymerization[END_REF][START_REF] Fouassier | Three-component photoinitiating systems: towards innovative tailor made high performance combinations[END_REF][START_REF] Allonas | Excited state processes in a fourcomponent photosensitive system based on a bisimidazole derivative[END_REF] As interesting feature, when incorporated in three-component systems, the dye acts as a photosensitizer and can thus be regenerated during polymerization so that a catalytic amount of dye can be used, as shown in the Figure 2. In fact, visible dyes are commonly used to sensitize UV photoinitiators such as iodonium salts. Upon photoexcitation, an electron transfer from the photosensitizer towards the iodonium salt can occur in the excited state, inducing the decomposition of the iodonium salt and the formation of initiating radicals Ph • .

Therefore, in these systems, the iodonium salt is the photoinitiator and this compound is also consumed during the polymerization process, as in the case of Type I photoinitiators. As a result of this, the photosensitizer is oxidized, irreversibly consumed and cannot contribute anymore to the sensitization of the iodonium salt. To address this issue, addition of a sacrificial amine (N-vinylcarbazole (NVK) for example) as a third component in the resin enables the oxidized photosensitizer to be reduced and thus regenerated (See Figure 2). [START_REF] Bonardi | Organometallic vs organic photoredox catalysts for photocuring reactions in the visible region[END_REF] Therefore, the main difference between Type I and Type II photoinitiators relies in the use of a photosensitizer in the case of Type II photoinitiators that can be or not consumed, depending of the presence of a sacrificing agent. As shown in the Figure 2, two different types of polymerization can be induced by the use of three-component systems. Notably, by reaction of the phenyl radical Ph • with NVK, a highly reactive radical Ph-NVK • capable to initiate a free radical polymerization (FRP) is produced. Parallel to this, Ph • can also react with the iodonium salt Ph2I + , producing the cation Ph-NVK + capable to initiate a free radical-promoted cationic polymerization (FRPCP) of epoxides. Therefore, the concomitant polymerizations of acrylates and epoxides is thus possible, enabling to access to interpenetrated polymer networks (IPNs). [START_REF] Tomal | Multifunctional biphenyl derivatives as photosensitisers in various types of photopolymerization processes, including IPN formation, 3D printing of photocurable multiwalled carbon nanotubes (MWCNTs) fluorescent composites[END_REF][START_REF] Hola | New, highly versatile bimolecular photoinitiating systems for free-radical, cationic and thiolene photopolymerization processes under low light intensity UV and visible LEDs for 3D printing application[END_REF][START_REF] Andrzejewska | Recent advances in photo-induced free-radical polymerization[END_REF] Figure 2. Photoinitiating mechanisms involved by the Type I and the Type II photoinitiators.

Among dyes strongly absorbing in the visible range, push-pull dyes composed of an electron-donating group connected to an electron-accepting group by mean of a π-conjugated spacer have been examined in numerous research fields such as non-linear optics, [START_REF] Dumur | Electrochemical, linear optical, and nonlinear optical properties and interpretation by Density Functional Theory calculations of (4-N,N-dimethylamino-styryl)pyridinium pendant group associated with polypyridinic ligands and respective multifunctional metal complexes (RuII or ZnII)[END_REF][START_REF] Raimundo | Design and Synthesis of Push-Pull Chromophores for Second-Order Nonlinear Optics Derived from Rigidified Thiophene-Based π-Conjugating Spacers[END_REF][START_REF] Jędrzejewska | Synthesis and Linear and Nonlinear Optical Properties of Three Push-Pull Oxazol-5(4H)-one Compounds[END_REF][START_REF] Shi | Synthesis and second-order nonlinear optical properties of push-pull BODIPY derivatives[END_REF][START_REF] Mohammed | Synthesis and characterisation of push-pull flavin dyes with efficient second harmonic generation (SHG) properties[END_REF] organic photovoltaics [START_REF] Hains | Molecular semiconductors in organic photovoltaic cells[END_REF][START_REF] Wu | Organic sensitizers from D-π-A to D-A-π-A: effect of the internal electron-withdrawing units on molecular absorption, energy levels and photovoltaic performances[END_REF][START_REF] Clifford | Sensitizer molecular structure-device efficiency relationship in dye sensitized solar cells[END_REF][START_REF] Duan | Recent advances in water/alcoholsoluble π-conjugated materials: new materials and growing applications in solar cells[END_REF][START_REF] Liang | Arylamine organic dyes for dye-sensitized solar cells[END_REF][START_REF] Duan | Recent development of push-pull conjugated polymers for bulk-heterojunction photovoltaics: rational design and fine tailoring of molecular structures[END_REF] and organic field effect transistors. [190] pyridinium A10, [START_REF] Dumur | New chelating stilbazonium-like dyes from Michler's ketone[END_REF] methyl-containing tricyanofurans A11, [START_REF] Khattab | Synthesis and application of novel tricyanofuran hydrazone dyes as sensors for detection of microbes[END_REF] substituted tricyanopropenes A12, [START_REF] Davis | Preparation of cyan dyes from 6diethylaminobenzo[b]furan-2-carboxaldehyde[END_REF] pyran derivatives A13 and A14, [START_REF] Andreu | 4H-Pyran-4-ylidenes: Strong proaromatic donors for organic nonlinear optical chromophores[END_REF][START_REF] Andreu | Linear and v-shaped nonlinear optical chromophores with multiple 4Hpyran-4-ylidene moieties[END_REF] 1,1,3-tricyano-2substituted propenes A15, [START_REF] Cho | Two photon absorption properties of 1,3,5-tricyano-2,4,6-tris(styryl)benzene derivatives[END_REF] isoxazolones A16, [START_REF] Zhang | Merocyanine dyes containing an isoxazolone nucleus: Synthesis, X-ray crystal structures, spectroscopic properties and DFT studies[END_REF] hydantions and rhodanines A17, [START_REF] Marinado | Rhodanine dyes for dye-sensitized solar cells : spectroscopy, energy levels and photovoltaic performance[END_REF] indane-1,3-dione derivatives A18 remains the most efficient one (See Figure 3). Indeed, contrarily to numerous electron acceptors that cannot be modified subsequent to their syntheses such as A1-A17, on the opposite, numerous chemical modifications can be done on the indane-1,3-dione scaffold A18, subsequent to its synthesis, as shown in the Figure Notably, electron-withdrawing ability of indane-1,3-dione can be reinforced by performing Knoevenagel reactions with malononitrile on the ketone groups. [START_REF] Zhang | Panchromatic ternary photovoltaic cells using a nonfullerene acceptor synthesized using C-H functionalization[END_REF][START_REF] Tirelli | New terthiophene derivatives for ultrahigh molecular weight polyethylene-based absorption polarizers[END_REF][START_REF] Shang | A Triphenylamine-Containing Donor-Acceptor Molecule for Stable, Reversible, Ultrahigh Density Data Storage[END_REF][START_REF] Planells | Naphthyl Derivatives Functionalised with Electron Acceptor Units -Synthesis, Electronic Characterisation and DFT Calculations[END_REF][START_REF] Li | Energy-level modulation of small-molecule electron acceptors to achieve over 12% efficiency in polymer solar cells[END_REF] Presence of the activated methylene group make this molecule an excellent candidate for

Extension of aromaticity

Improvement of the electron-withdrawing ability

Synthesis of Push-pull dyes by Knoevenagel reactions

Cyclization reaction

elaborating push-pull dyes by Knoevenagel reactions while reacting the appropriate aldehyde. [START_REF] Feng | An A-D-A type small-molecule electron acceptor with end-extended conjugation for high performance organic solar cells[END_REF][START_REF] Li | Nonfullerene acceptors based on fused-ring oligomers for efficient polymer solar cells via complementary light-absorption[END_REF][209][START_REF] Matsui | Application of indoline dyes attached with strongly electronwithdrawing carboxylated indan-1,3-dione analogues linked with a hexylthiophene ring to dye-sensitized solar cells[END_REF][START_REF] Traskovskis | Stereoselective synthesis and properties of 1,3-bis(dicyanomethylidene)indane-5-carboxylic acid acceptor fragment containing nonlinear optical chromophores[END_REF] Electron-withdrawing ability of indane-1,3-dione can also be reinforced by using indane-1,3-dione-inspired acceptors of extended aromaticity compared to the parent structure. However, such compounds cannot be obtained from indane-1,3-dione. Finally, when nucleophilic bases are used during the synthesis of push-pull dyes, intramolecular cyclization reactions can occur by nucleophilic attack of a secondary amine onto the cyano groups, providing azafluorenones. [START_REF] Landmesser | A Novel Route to 1-Substituted 3-(Dialkylamino)-9-Oxo-9HIndeno[2,1-c]Pyridine-4-Carbonitriles[END_REF] When compared to the other photoinitiating systems, push-pull dye-based photoinitiating systems and especially indane-1,3-dione-based photoinitiating systems exhibit several advantages that are summarized in the Table 1. In this review, an overview of the different indane-1,3-dione derivatives used as photoinitiators of polymerization activable in the visible range and under low light intensity are reported. At present, several challenges remain concerning the use of indane-1,3-dionebased photoinitiating systems : design of dyes with high molar extinction coefficients enabling to reduce the photosensitizer content, control of the absorption maxima to perfectly fit the emission of the different irradiation sources, if possible designing panchromatic photoinitiators absorbing over the whole visible range, ability of the photoinitiating systems to bleach in order to provide colorless coatings. With regards to these different challenges, and to evidence the photoinitiating ability of the different indane-1,3-dione-based photoinitiating systems, comparisons with benchmark photoinitiators is also provided.

Push-pull dyes based on indane-1,3-dione

Push-pull dyes have been the focus of numerous research efforts for the design of chromophoric dyes due to their easiness of synthesis and the facile tunability of both the absorption maxima and the molar extinction coefficients. However, several studies also evidenced that all push-pull dyes could not be used as photosensitizers. Indeed, as demonstrated in 2013, only the dyes for which a linear and positive solvatochromism could be determined could also act as photoinitiators. [START_REF] Tehfe | Blue-to-red light sensitive push-pull structured photoinitiators: indanedione derivatives for radical and cationic photopolymerization reactions[END_REF] This point was notably demonstrated with D1 and D2 (See Figure 5) for which a positive solvatochromism could be evidenced with several solvent polarity scales such as the Kawski-Chamma-Viallet's, [START_REF] Kawski | Der wellenzahl von elecktronenbanden lumineszie-renden molecule[END_REF] McRae's, [START_REF] Mcrae | Theory of solvent effects on molecular electronic spectra. frequency shifts[END_REF] Bakhshiev's, [START_REF] Bakshiev | Universal Intermolecular Interactions and Their Effect on the Position of the Electronic Spectra of Molecules in Two-Component Solutions. VII. Theory (General Case of an Isotropic Solution)[END_REF] Lippert-Mataga's, [START_REF] Lippert | Dipolmoment und Elektronenstruktur von angeregten Molekülen[END_REF] Dimroth-Reichardt's, [START_REF] Reichardt | Solvatochromic dyes as solvent polarity indicators[END_REF] Kamlet-Taft's, [START_REF] Kamlet | Linear solvation energy relationships. 23. A comprehensive collection of the solvatochromic parameters[END_REF] Catalan's,[220] McRae's,[221] and Suppan's scales (See Figure 6). Reproduced with permission from Tefhe et al. [START_REF] Tehfe | Blue-to-red light sensitive push-pull structured photoinitiators: indanedione derivatives for radical and cationic photopolymerization reactions[END_REF]. Copyright 2013 American Chemical Society.

From the absorption viewpoint, an absorption extending between 350 and 750 nm for D1, 350 and 550 nm for D2 could be measured in acetonitrile solutions. As anticipated, pyrene being a weaker electron donor than dimethylaminobenzene, a blue-shift of the intramolecular charge transfer band could be determined for D2 (λmax = 464 nm, ε = 21 000 M -1 ) compared to that of D1 (λmax = 478 nm, ε = 38 000 M -1 ). Based on their absorption spectra, the two dyes proved to be suitable candidates for photopolymerization done with laser diodes emitting at 457, 473, 532, and 635 nm (100 mW.cm -2 ) or with a halogen lamp (370-800 nm range, 12 mW.cm -2 ). While examining the cationic polymerization (CP) of a difunctional epoxide i.e. 3,4- Dx (x = 1 or 2)/Iod system (0.3%/2% w/w) where Iod stands for diphenyliodonium hexafluorophosphate, a final monomer conversion of 35% was determined with D1 whereas no polymerization was detected with D2 (halogen lamp (370-800 nm range, 12 mW.cm -2 )). This trend was confirmed with the three-component Dx (x = 1 or 2)/NVK/Iod (0.3%/3%/2% w/w) system of higher reactivity for which no polymerization could be still detected with D2 upon irradiation with a laser diode at 473 nm (100 mW/cm²). Depending of the irradiation wavelengths, monomer conversions of 40% at 405 and 532 nm, 80% at 473 nm could be determined with the three-component D1/NVK/Iod (0.3%/3%/2% w/w) system. Differences of monomer conversions are directly related to the difference of absorption maxima of D1 at the irradiation wavelengths. Similarly, with a second monomer (triethylene glycol divinyl ether (DVE-3)), a final monomer as high as 95% could be determined for the three-component D1/NVK/Iod (0.3%/3%/2% w/w) system. A similar behavior was observed during the free radical polymerization (FRP) of trimethylolpropane triacrylate (TMPTA) (See Figure 7). Thus, a monomer conversion of 50% could be determined with the three-component system D1/NVK/Iod (0.3%/3%/2% w/w) upon irradiation at 473 nm (laser diode, 100 mW cm -2 ) in laminate for 200 s whereas no polymerization could be initiated with D2. Photolysis experiments done upon irradiation at 473 nm revealed for the two-component Dx (x = 1 or 2)/Iod systems, the decrease of the absorbance to be extremely fast with D1 whereas no modification of the absorption spectrum was detected for D2. It was thus concluded that the photoassisted electron transfer from D2 towards Iod was thus unfavorable, explaining the inability of D2 to initiate a polymerization. From a mechanistic viewpoint, the mechanism supporting the FRP and the CP was consistent with that depicted in the Figure 3.

Mismatch of the redox potentials between Iod and the excited dye is not the only reason supporting the inability of a dye to initiate a polymerization. Several other reasons can be mentioned. Thus, D3 proved to be a poor candidate for photoinitiation due to the lack of solubility in resins whereas presence of the weak electron donor in D4 could not provide a sufficient absorption in the visible range to this dye (λmax = 351 nm) in order to be usable (See Figure 8). [START_REF] Xiao | Structural effects in the indanedione skeleton for the design of low intensity 300-500 nm light sensitive initiators[END_REF] Conversely, use of strong electron donors such as carbazole in D5 (λmax = 448 nm) or anthracene (λmax = 467 nm) in D6 could furnish dyes strongly absorbing in the visible range. With the most reactive dye i.e. D5, a final monomer conversion of 59% was obtained with EPOX, upon irradiation at 457 nm with a laser diode (100 mW/cm²) and while using the three-component D5/Iod/NVK (0.5%/2%/3%, w/w/w) system. This value could be increased up to 64% while using a halogen lamp of broader emission spectrum (370-800 nm range, 12 mW.cm -2 ) than the laser diodes while nonetheless emitting a lower light intensity than the laser diodes. Conversely, EPOX conversion was reduced to 33% with D3 in the same conditions, due to its low solubility in resins. Copyright 2014 American Chemical Society. and r9b) and enabling the thiol-ene reaction to proceed.

,3 Dye + MDEA → Dye •-+ MDEA •+ → Dye-H • + MDEA (-H) • (r1) 1,3 Dye + R'-Cl (or R-Br) → Dye •+ + (R'-Cl) •-[or (R-Br) •-] (r2) Dye •-+ R'-Cl (or R-Br) → Dye + (R'-Cl) •-[or (R-Br) •-] (r3) Dye-H • + R'-Cl (or R-Br) → Dye + H + + (R'-Cl) •-[or (R-Br) •-] (r4) (R'-Cl) •-[or (R-Br) •-] → R' • + Cl -[or R • + Br -] (r5
D5 → 1 D5 (hν) and 1 D5 → 3 D5 (r6) 1,3 D5 + Ph2I + → D5 •+ + Ph2I • (r7a) D5 •+ + Ph2I • → D5 •+ + Ph • + Ph-I (r7b) Ph • + NVK → Ph-NVK • (r8a) Ph-NVK • + Ph2I + → Ph-NVK + + Ph • + Ph-I (r8b) Ph • + RS-H → Ph-H + RS • (r9a) RS • + R'-CH=CH2 → R'-CH • -CH2SR (r9b) R'-CH • -CH2SR + RSH → R'-CH2-CH2SR + RS • (r9c)

Scheme 2. Mechanism involved during the polymerization process of the thiol-ene reaction

Interest for D5 also relies in the fact that this dye was of equivalent efficiency in oxidative cycle (when combined with an iodonium salt) but also in reductive cycle (when combined with MDEA). Such a behavior is rarely observed and is directly related to its oxidation and reduction potentials perfectly fitting with those two additives. This dual photoinitiating ability of the carbazole-based photoinitiator D5 could not be evidenced with another strong electron donor of similar structure, namely phenothiazine. [START_REF] Abdallah | Phenothiazine derivatives as photoredox catalysts for cationic and radical photosensitive resins for 3D printing technology and photocomposites synthesis[END_REF] Indeed, in this last case, photolysis of the D7/Iod system revealed the absorption spectrum to remain unchanged upon irradiation. Conversely, a fast photolysis was observed for the D7/Nphenylglycine (NPG) system, revealing that D7 was easier to reduce than to oxidize (See Figure 10). With aim at red-shifting the ICT band of the push-pull dyes, introduction of electron donors with extended π-conjugated spacers was examined. A relevant example of this strategy was developed with D8, comprising Michler's aldehyde as the donor (See Figure 11). [START_REF] Tehfe | New push-pull dyes derived from Michler's ketone for polymerization reactions upon visible lights[END_REF] Figure 11. Chemical structure of D8 and additives.

Benefiting from the extended π-conjugated spacer, a red-shift of the ICT band from 478 nm for D1 to 532 nm for D8 was found. However, once again, D8 proved to be a poor photosensitizer. Especially, D8 could only work in oxidative conditions. Indeed, a final monomer conversion of 35% could be determined during the CP of EPOX with the threecomponent system D8/NVK/Iod2 (0.3%/3%/2% w/w) after 800 s of irradiation at 532 nm whereas no polymerization could even be initiated with the three-component D8/MDEA/R-Br (0.3%/4%/3% w/w) system (where MDEA stands for N-methyldiethanolamine and R-Br for phenacyl bromide (See Figure 11)). Here again, photolysis experiments in solution revealed the lack of reactivity for the D3/Iod2 combination. However, as interesting findings, presence of radicals could be still detected within the EPOX films even after several weeks of storage, evidencing their remarkable stabilities. Even if origin of this unexpected behavior could not be rationalized, such long-living radicals could be of crucial interest for applications in electronics such as data storage.

With aim at developing a structure-performance relationship, a series of ten indane-1,3-dione-based dyes D1, D8-D16 were examined in similar conditions (See Figure 12). [START_REF] Sun | Free radical photopolymerization and 3d printing using newly developed dyes: indane-1,3-dione and 1H-cyclopentanaphthalene-1,3-dione derivatives as photoinitiators in three-component systems[END_REF] First, a clear change of the absorption spectra of the dyes with the donor strength could be evidenced. Thus, absorption maxima ranging from 383 nm for D9 possessing the weakest electron donor to 567 nm for D8 exhibiting the strongest electron donor could be determined in dichloromethane (See Figure 13). Even if major differences in terms of absorption can be found between D8 and D9, choice was done by the authors to examine their photoinitiating abilities only at 405 nm, this wavelength being a reference for 3D printing applications. Reproduced with permission from Ref [START_REF] Sun | Free radical photopolymerization and 3d printing using newly developed dyes: indane-1,3-dione and 1H-cyclopentanaphthalene-1,3-dione derivatives as photoinitiators in three-component systems[END_REF].

While examining their photoinitiating abilities during the FRP of the tetrafunctional acrylate Ebecryl 40, the final monomer conversions obtained with the two-component Dx (x = 1,8-16)/Iod (0.1%/2% w/w) systems remained low, below 40%. While using a three-component Dx (x = 1,8-16)/Iod3/EDB (0.1%/2%/2% w/w) system enabling the dye to be regenerated during the polymerization process, all photoinitiating systems could outperform that of the reference system Iod3/ethyl dimethylaminobenzoate (EDB) except D13 for which a lower monomer conversion could be determined (60% conversion for ebecryl 40 after 400 s of irradiation at 405 nm). The best monomer conversion was determined for D12, reaching 93% after only 25 s.

Interestingly, an efficient photobleaching could be observed during the polymerization process with the three-component D12/Iod3/EDB (0.1%/2%/2% w/w) system so that a colorless coating could be obtained after polymerization. Such a property is highly researched in industries as it allows to produce colorless coatings. [START_REF] Li | Synthesis of furan derivative as LED light photoinitiator: One-pot, low usage, photobleaching for light color 3D printing[END_REF][START_REF] Oh | Photopolymerization and photobleaching of n-butyl acrylate/fumed silica composites monitored by real time FTIR-ATR spectroscopy[END_REF][START_REF] Nan | Different photoinitiating ability and photobleaching efficiency of erythrosine B derivatives in radical/cationic photopolymerization[END_REF][START_REF] Asmusen | Photobleaching of camphorquinone during polymerization of dimethacrylate-based resins[END_REF][START_REF] Ebner | Ultra-high through-cure of (meth)acrylate copolymers via photofrontal polymerization[END_REF][START_REF] He | Photoreaction and photopolymerization studies on squaraines dyes/iodonium salts combination[END_REF][START_REF] Pigot | New push-pull dyes based on 2-(3-oxo-2,3-dihydro-1Hcyclopenta[END_REF][START_REF] Pigot | Unprecedented nucleophilic attack of piperidine on the electron acceptor during the synthesis of push-pull dyes by a Knoevenagel reaction[END_REF][START_REF] Sun | Novel push-pull dyes derived from 1Hcyclopenta[b]naphthalene-1,3(2H)-dione as versatile photoinitiators for photopolymerization and their related applications: 3D printing and fabrication of photocomposites[END_REF][START_REF] Knoevenagel | Ueber eine Darstellungsweise des Benzylidenacetessigesters[END_REF][START_REF] Telitel | The 1,3-bis(dicyanomethylidene)-indane skeleton as a (photo) initiator in thermal ring opening polymerization at RT and radical or cationic photopolymerization[END_REF][START_REF] Crivello | Redox-initiated cationic polymerization: The diaryliodonium salt/benzoin redox couple[END_REF][START_REF] Crivello | Redox initiated cationic polymerization: Silane-N-aryl heteroaromatic onium salt redox couples[END_REF][START_REF] Crivello | Redox initiated cationic polymerization[END_REF][START_REF] Crivello | Redox initiated cationic polymerization: reduction of triarylsulfonium salts by silanes[END_REF][START_REF] Crivello | Redox-initiated cationic polymerization: reduction of dialkylphenacylsulfonium salts by silanes[END_REF] Photolysis experiments realized with the D12/EDB and D12/Iod systems proved the photobleaching to be extremely fast compared to that observed for the other dyes. It can be thus concluded that the superiority of D12 over the other dyes relies in its fast interaction with the two co-initiators, speeding up the polymerization process so that the oxidative and the reductive cycles could concomitantly contribute to the formation of initiating radicals (See Figure 14).

EDB •+ EDB • (-H) dye-H • FRP EDB EDB •+ Ph 2 I + Ph • Dye

Push-pull dyes based on 1H-cyclopentanaphthalene-1,3-dione and related derivatives

As mentioned in the introduction section, one efficient strategy to red-shift the absorption of push-pull dyes consists either to improve the electron-donating ability of the donor or to improve the electron-withdrawing ability of the acceptor. From a synthetic viewpoint, improvement of the electron-withdrawing ability of the electron-acceptor requires additional synthetic steps. Indeed, indane-1,3-dione is commercially available, facilitating the synthesis of numerous dyes. Conversely, when the electron acceptor is not commercially available, its synthetic access can constitute a major impediment for the development of a wide range of dyes. This approach was nevertheless examined by the group of Lalevée et al. who developed a series of dyes D17-D26 bearing 1H-cyclopentanaphthalene-1,3-dione as the electron acceptor (See Figure 16). [START_REF] Sun | Free radical photopolymerization and 3d printing using newly developed dyes: indane-1,3-dione and 1H-cyclopentanaphthalene-1,3-dione derivatives as photoinitiators in three-component systems[END_REF] To evidence the pertinence of the approach, a comparison with their indane-1,3-dione analogues D1, D8-D16 was established. Compared to the D1, D8-D16 series, a red-shift by ca. 20 nm was found for D17-D26 at identical electron donors. From a synthetic viewpoint, 1H-cyclopentanaphthalene-1,3-dione can be prepared in three steps starting from naphthalene-2,3-dicarboxylic acid according to the synthetic route depicted in the scheme 3. [START_REF] Pigot | New push-pull dyes based on 2-(3-oxo-2,3-dihydro-1Hcyclopenta[END_REF][START_REF] Pigot | Unprecedented nucleophilic attack of piperidine on the electron acceptor during the synthesis of push-pull dyes by a Knoevenagel reaction[END_REF] Scheme 3. Synthetic route to 1H-cyclopentanaphthalene-1,3-dione.

Here again, the best monomer conversion obtained during the FRP of Ebecryl 40 at 405 nm was obtained for D23 exhibiting the same electron donor than D12. Besides, contrarily to that obtained for D12, no photobleaching of the coating was observed with D23. Therefore, it can be concluded that such studies combining within a unique work the investigation of numerous dyes is essential to establish a structure-performance relationship. In 2020, two new dyes D27 and D28 were examined in a new study devoted to the FRP of TMPTA (See Figure 17). [START_REF] Sun | Novel push-pull dyes derived from 1Hcyclopenta[b]naphthalene-1,3(2H)-dione as versatile photoinitiators for photopolymerization and their related applications: 3D printing and fabrication of photocomposites[END_REF] As a typical feature nonetheless observed for push-pull dyes but also for chalcones, a severe reduction of the monomer conversion was observed for the ferrocene derivative compared to that of the carbazole analogue. Thus, upon irradiation at 405 nm under air, final monomer conversions of 76 and 51% were respectively determined after 400 s of irradiation using the three-component Dx (x = 27 or 28)/Iod3/EDB (0.1%/2%/2% w/w/w) systems (See Table 2). Reduction of the monomer conversion with the ferrocene derivative D28 is directly related to an inefficient electron transfer from the dye to the iodonium salt, what was demonstrated during the photolysis experiments. Recently, a series of dyes D29-D35 exhibiting redshifted absorptions compared to that of 1H-cyclopentanaphthalene-1,3-dione-based push-pull dyes D17-D26 were obtained by improving the electron-withdrawing ability of the electron acceptor (See Figure 18). [START_REF] Sun | Novel push-pull dyes derived from 1Hcyclopenta[b]naphthalene-1,3(2H)-dione as versatile photoinitiators for photopolymerization and their related applications: 3D printing and fabrication of photocomposites[END_REF] To achieve this goal, a procedure similar to that used for improving the electron-withdrawing ability of indane-1,3-dione was used. By substituting 1H-cyclopentanaphthalene-1,3-dione However, it has to be noticed that if the Knoevenagel reactions done to introduce the cyano group onto indane-1,3-dione are classically done in ethanol as the solvent, due to the remarkable stability of the EA1 anion in basic conditions resulting from the electronic delocalization, condensation of malononitrile on EA1 could only be realized in more drastic conditions than classically used and an elevation of the reaction temperature could be obtained by using 2-ethoxyethanol as the solvent. and D35 in 37 and 42% yields respectively. Conversely, D29-D33 could be prepared in standard conditions for a Knoevenagel reaction, namely in ethanol as the solvent and with diisopropylethylamine (DIPEA) as the base. Thus, the different dyes could be obtained with reaction yields ranging from 37% for D32 to 87% for D30 and 90% for D19. When tested as photoinitiators for the FRP of acrylates in three-component Dx (x = 29-35)/Iod3/EDB (0.1%, 2%/2%, w/w) systems, D34 and D33 proved to be less efficient than the reference twocomponent Iod/EdB system (2%/2%, w/w) (43 and 56% conversions vs 60% for Iod/EDB respectively). Conversely, for the other dyes, the three-component Dx (x = 29-35)/iodonium salt/amine (0.1%, 2%/2%, w/w) system could outperform the reference system (78% monomer conversion for D30, 80% for D31). Here again, the lower reactivity of the ferrocene-based dye D33 compared to that of the purely organic analogues was confirmed, the monomer conversion only reaching 56%.

EA1

Azafluorenones

Azafluorenones are polycyclic compounds whose synthesis has recently been identified as a side-reaction occurring during the synthesis of push-pull dyes when a nucleophilic base was used. [START_REF] Landmesser | A Novel Route to 1-Substituted 3-(Dialkylamino)-9-Oxo-9HIndeno[2,1-c]Pyridine-4-Carbonitriles[END_REF] Indeed, the first report mentioning the condensation of an activated methylene group to an aldehyde was reported as soon as 1896 by Knoevenagel and in this pioneering work, benzaldehyde was condensed to ethyl acetoacetate in the presence of piperidine. [START_REF] Knoevenagel | Ueber eine Darstellungsweise des Benzylidenacetessigesters[END_REF] However, when nucleophilic bases are used, an adverse nucleophilic attack onto the cyano groups of the electron acceptor can occur, as depicted in the Scheme 5. Using piperidine or morpholine as the nucleophilic base, compounds D36 and D37 could be prepared in 79 and 14% yields respectively. [START_REF] Pigot | Unprecedented nucleophilic attack of piperidine on the electron acceptor during the synthesis of push-pull dyes by a Knoevenagel reaction[END_REF] Scheme 5. Synthetic route to compounds 8 and 9.

In 2020, azafluorenones that are compounds rarely prepared were examined as potential photoinitiators for the FRP of TMPTA and a comparison with D30-D35 was established. [START_REF] Sun | Novel push-pull dyes derived from 1Hcyclopenta[b]naphthalene-1,3(2H)-dione as versatile photoinitiators for photopolymerization and their related applications: 3D printing and fabrication of photocomposites[END_REF] Surprisingly, dyes D36 and D37 could greatly outperform all the other dyes, the monomer conversion reaching 91 and 88% respectively (See Figure 19 and Table 3). In fact, a two-fold enhancement of the monomer conversion could be determined for dye D36 compared to dye D34. In light of the remarkable performance of dye D36, photopolymerization experiments using sunlight were carried out. As shown in the Figure 20 and despites the severe decrease of the light intensity (I0 < 5 mW/cm² in the 350-500 nm range for sunlight vs. 110 mW/cm² for the LED@405nm), the polymerization of TMPTA was determined as being ended after only 10 min. under sunlight exposure while using the threecomponent D36/Iod3/EDB (0.1%, 2%/2%, w/w) system. Finally, still to evidence the remarkable reactivity of dye D36, laser write experiments were carried out using the same resins. Here again, the high reactivity could be demonstrated by the precision of the letters written with the laser emitting at 405 nm (See Figure 21). a: iodonium salt / amine (2%/2%, w/w) without dye.

Nucleophilic adducts of indane-1,3-dione derivatives.

In 2020, the nucleophilic addition of piperidine onto 2-(3-oxo-2,3-dihydro-1Hcyclopenta [b]naphthalen-1-ylidene)malononitrile EA2 was reported for the first time. [START_REF] Pigot | Unprecedented nucleophilic attack of piperidine on the electron acceptor during the synthesis of push-pull dyes by a Knoevenagel reaction[END_REF] Surprisingly, this unexpected reaction could only occur while using 3-(

(dimethylamino)phenyl)acrylaldehyde as the aldehyde. If the corresponding push-pull dye D34 could not be obtained in these conditions, compound D38 could be prepared in 86% yield (See Scheme 6). To confirm its structure, crystals could be obtained, and the crystal structure solved.

Scheme 6. Synthetic route to dye D38, the mechanism of formation and the crystal structure of dye D38. Reproduced with permission from Ref [START_REF] Pigot | Unprecedented nucleophilic attack of piperidine on the electron acceptor during the synthesis of push-pull dyes by a Knoevenagel reaction[END_REF].

Examination of its photoinitiating ability during the FRP of TMPTA using the threecomponent D38/Iod3/EDB (0.1%, 2%/2%, w/w) system revealed D38 to be on par with dye D35

and to furnish only a slightly lower monomer conversion than dyes D36 and D37 (85% vs. 91 and 88% monomer conversion respectively). This new family of compound, even if difficult to prepare deserves to be more extensively studied in the future.

1,3-Bis(dicyanomethylidene)indane

As mentioned in the paragraph 3, anions of tetracyano derivatives of indane-1,3-dione are highly stable in solution so that the Knoevenagel reaction with these electron acceptors can only be carried in acidic conditions (i.e. acetic anhydride). Indeed, when the Knoevenagel reaction are performed in classical conditions, i.e. ethanol in the presence of piperidine, no reaction occurs. Considering that the deprotonation of these species is easy in solutions and that the anions are strongly colored, in 2013, 1,3-bis(dicyanomethylidene)indane D39 and 1-(dicyanomethylene)-3-indanone D40 were examined as strong acids capable to initiate the CP of various epoxides, namely (3,4-epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate (EPOX), (epoxycyclohexylethyl)methylsiloxane-dimethylsiloxane copolymer (EPOX-Si) and an epoxidized soybean oil (ESO) (See Figure 22). [START_REF] Telitel | The 1,3-bis(dicyanomethylidene)-indane skeleton as a (photo) initiator in thermal ring opening polymerization at RT and radical or cationic photopolymerization[END_REF] In fact, advantages of these molecules are threefold. First, remarkable stability of the anions enables D39 and D40 to act as acid generators. Second, due to the strong color of the anions, these latter can be used as photosensitizers for the decomposition of an iodonium salt (Iod). Indeed, as shown in the Figure 24, an absorption spectrum extending from 400 to 650 nm could be found for D39 in a protic acetonitrile/water solution. It has to be noticed that, in-situ generation of strong acids in resins is already reported in the literature, except that these strong acids have only been obtained by mean of redox reactions involving the use of transition metals. Thus, the generation of strong acids by reaction of a copper-catalyzed reduction of iodonium salts with benzoins, [START_REF] Crivello | Redox-initiated cationic polymerization: The diaryliodonium salt/benzoin redox couple[END_REF][START_REF] Crivello | Redox initiated cationic polymerization: Silane-N-aryl heteroaromatic onium salt redox couples[END_REF] or the platinum-catalyzed reduction of iodonium salts with boranes or silanes [START_REF] Crivello | Redox initiated cationic polymerization[END_REF][START_REF] Crivello | Redox initiated cationic polymerization: reduction of triarylsulfonium salts by silanes[END_REF][START_REF] Crivello | Redox-initiated cationic polymerization: reduction of dialkylphenacylsulfonium salts by silanes[END_REF][START_REF] Crivello | Redox initiated cationic polymerization: Reduction of diaryliodonium salts by 9-BBN[END_REF] have previously been reported in the literature. Besides, traces of residual metals can constitute a major impediment for further use of polymers.

Figure 23. UV-visible absorption spectra of D39 in tert-butylbenzene (a), in acetonitrile-water (b). Adapted from ref. [START_REF] Telitel | The 1,3-bis(dicyanomethylidene)-indane skeleton as a (photo) initiator in thermal ring opening polymerization at RT and radical or cationic photopolymerization[END_REF] with permission from The Royal Society of Chemistry

In the present work, by simply dissolving D39 in EPOX (2% w/w), a conversion of 45% could be determined at room temperature after 1200 s of monitoring (See Figure 24).

Conversely, D40 could not initiate any polymerization in the same conditions. In fact, these results were corroborated by NMR analyses experiments in deuterated DMSO. Indeed, if a complete deprotonation of D39 in DMSO was detected by NMR, coexistence of the protonated and the deprotonated form for D40 was also evidenced, supporting the lack of reactivity of D40 in resins.

Figure 24.

Monitoring of the EPOX conversion A) using D39 ((2% w/w) in EPOX; B) with (curve 1) and without (curve 2) irradiation with a laser diode at 532 nm. Adapted from ref.

[241] with permission from The Royal Society of Chemistry.

A B

A B

Possibility to photoactivate the CP of EPOX was also examined. By using the twocomponent D39/Iod (2%/2% w/w) system, as anticipated, an improvement of the monomer conversion was obtained, as shown in the Figure 25. Upon irradiation at 532 nm, the monomer conversion could be improved from 45% without light to 55% in the presence of light, indicative of a photoactivation of the polymerization process. To support the polymerization reaction, the following mechanisms were proposed in the absence (r10-r12) and in the presence of light (r13-r17).

A-H → A -+ H + (r10)

H + + M → H-M + (r11) H-M + + M →→→ polyether (r12) A -→ 1 A -+ H + (h) (r13) 1 A -+ Ph2I + → A • + Ph2I • (r14) Ph2I • → Ph • + Ph-I (r15) Ph • + EPOX → Ph-H + EPOX • (-H) (r16) EPOX • (-H) + Ph2I + → EPOX + (-H) + Ph2I • (r17)
Scheme 7. Mechanism of the cationic polymerization of epoxides.

Finally, considering that the iodonium salts are still accompanied by an inorganic counter-anion, the design of a soft salt with D39 and the iodonium salt was proposed (See Scheme 8). By use of a polar solvent and by using procedure previously reported in the literature for the design of soft salts, D39 could spontaneously deprotonate, enabling the formation of the soft salt D41.[248-250] By water treatment, HPF6 could be easily removed from the organic phase, enabling to get the organic salt in pure form.

Conclusion

To conclude, a series of 41 dyes based on indane-1,3-dione and related derivatives have been synthesized and tested as photoinitiators of polymerization under low light intensity and in the visible range. By finely tuning the substitution pattern, dyes absorbing between 300 and 850 nm were prepared (See Figure 27 and Table 4). By examining these different structures, several interesting features have been identified. Thus, elongation of the electron-acceptor greatly contributed to red-shift the absorption while maintaining a similar reactivity. In this series, a dye i.e. D12 was identified as being capable to bleach, even in three-component systems and this property is highly researched by industry. The most promising results in terms of photoinitiating abilities have been obtained with the less common dyes namely the azafluorenones (D36, D37) that clearly outperform the rest of the dyes. Parallel to this, D38 whose synthesis is unprecedented in the literature could furnish monomer conversions on par with that of D36 and D37. With aim of developing high-performance photoinitiators, future works will typically consist in developing new derivatives of these two families (azafluorenones and nucleophilic adducts).

It will also consist in developing crosslinkable dyes, with aim at addressing the extractability issue. Indeed, at present almost no crosslinkable push-pull dyes have been examined as photosensitizers, what deserves to be more systematically studied in the Future.

iron complexes 8 ,[ 53 - 57 ]

 85357 naphthalimides 9,[58-69] perylenes 10, [70-72] 2,3-diphenylquinoxaline derivatives 11,[73] diketopyrrolopyrrole 12,[74-76] flavones 13,[77-78] ferrocenes 14,[79-83] helicenes 15,[84-85] chromones 16,[86-88] coumarins 17,[89-94] copper complexes 18,[95-105] cyclohexanones 19,[106-109] dihydroanthraquinone 20,[110] acridones 21,[111-112] acridine-1,8-dione 22,[113-114] benzophenones 23,[115-117] camphorquinones 24,[119-120] carbazoles 25,[121-126] chalcones 26,[126-131] push-pull dyes 27,[132-140] cyanines 28

[ 181 ]

 181 If several electron acceptors have been examined for this purpose over the years such as pyrazines A1,[182] dicyanoimidazoles A2,[183] benzo[d]thiazoliums A3,[184] benzo[d]imidazoliums A4,[185] dicyanovinyl-thiophen-5-ylidenes A5[186] and 9-methylene-2,4,5,7-tetranitro-9H-fluorene A6[187], malononitrile A7,[188] (thio)barbituric derivatives A8,[189] Meldrum derivatives A9,

Figure 3 .

 3 Figure 3. Chemical structures of electron acceptors A1-A18 commonly used for the design of push-pull dyes.

Figure 4 .

 4 Figure 4. The possible chemical modification of the indane-1,3-dione scaffold.

[ 222 ]Figure 5 .

 2225 Figure 5. Chemical structures of D1 and D2.

Figure 6 .

 6 Figure 6. Linear plots obtained with different solvent polarity scales for D1. Bakhshiev's (a), Kawski-Chamma-Viallet's (b), Lippert-Mataga (c), McRae's (d) and Suppan's (e) scales.

  3,4-epoxy-cyclohexylcarboxylate (EPOX) with the two-component

Figure 7 .

 7 Figure 7. Chemical structures of the different monomers and additives used with D1 and D2 as photoinitiators.

Figure 8 .

 8 Figure 8. Chemical structure of D3-D6, the different monomers and additives

1

 1

) Scheme 1 .

 1 Scheme 1. Reactions involved during the polymerization process with the three-component Dye/MDEA/R-X systems

Figure 9 .

 9 Figure 9. Photopolymerization profiles of a Trithiol/DVE-3 blend in laminate with D5/Iod (0.5%/2%, w/w) upon irradiation (a) with a laser diode at 457 nm and (b) a LED at 462 nm. (1) DVE-3 conversion (2) trithiol conversion. Reproduced with permission from Xiao et al. [225].

1 2

 1 By combining several techniques such as cyclic voltammetry, laser flash photolysis experiments and electron spin resonance (ESR) spin trapping (ESR-ST), the following initiating mechanism could be deduced. Upon photoexcitation, an electron transfer can occur in the excited state between D5 and Ph2I + , enabling to reduce the iodonium salt and generate the reactive phenyl radicals Ph • (see r7a and r7b in Scheme 2). Upon reaction of Ph • with Nvinylcarbazole (NVK), the more reactive radicals Ph-NVK • can be formed (see r8a), reacting with Ph2I + and providing Ph-NVK + capable to initiate the CP of DVE-3 (see r8b). Parallel to this, Ph • can also react with the thiol derivative, producing the initiating thyil radicals (see r9a

Figure 10 .

 10 Figure 10. Chemical structure of D7 and NPG.

Figure 12 .Figure 13 .

 1213 Figure 12. Chemical structures of D9-D16.

Figure 14 .

 14 Figure 14. The concomitant mechanistic pathways enabling the polymerization of ebecryl 40.

Figure 15 .

 15 Figure 15. 3D patterns obtained by laser write experiments using the three-component system D12/Iod/amine (0.1%/2%/2% w/w/w) in Ebecryl 40. Reproduced with permission from Ref [227].

Figure 16 .

 16 Figure 16. Chemical structures of D17-D26.

Figure 17 .

 17 Figure 17. Chemical structures of D27 and D28.

  with one or two equivalents of malononitrile, two new electron-acceptors 2-(3-oxo-2,3dihydro-1H-cyclopenta[b]naphthalen-1-ylidene)malononitrile EA2 and 2,2'-(1Hcyclopenta[b]naphthalene -1,3(2H)-diylidene)dimalononitrile EA3 could be respectively obtained in 67 and 41% yields (See Scheme 4).

Figure 18 .

 18 Figure 18. Chemical structures of D29-D35.

Scheme 4 .

 4 Scheme 4. Synthetic routes to building blocks EA1-EA3 and push-pull dyes D34 and D35. Due to the strong electron-withdrawing ability of EA3, D34 and D35 should be prepared by refluxing EA3 in acetic anhydride with the appropriate aldehyde, providing D34

Figure 19 .

 19 Figure 19. Photopolymerization profiles of TMPTA using the three-component dyes/iodonium salt/amine (0.1%, 2%/2%, w/w) system. Irradiation with a LED@405 nm under air with dyes D27, D28, D30-D38. The curve 0 corresponds to the photoinitiating system (iodonium/amine) without dye. Reproduced with permission from Ref [239].

Figure 20 .Figure 21 .

 2021 Figure 20. TMPTA conversion vs. Time obtained upon irradiation with sunlight using the three-component D36/Iod3/EDB (0.1%, 2%/2%, w/w) system. Reproduced with permission from Ref [239].

Figure 22 .

 22 Figure 22. Chemical structures of D39 and D40 and the different epoxide monomers.

Scheme 8 .

 8 Scheme 8. Synthetic route to dye D41.

Figure 25 .Figure 26 .

 2526 Figure 25. Experimental (a) and theoretical (b) ESR spectra obtained upon irradiation of the soft salt D41 in tert-butylbenzene under nitrogen using phenyl-N-tert-butylnitrone as the spin trap agent. Adapted from ref. [241] with permission from The Royal Society of Chemistry

Figure 27 .

 27 Figure 27. Summary of the absorption properties of all dyes discussed in this review.

Table 1 .

 1 Comparisons between indane-1,3-dione-based dyes and the other synthetic dyes used as photoinitiators/photosensitizers.

	Parameters	Push-pull dyes	Other dyes
	Cost/synthesis	Compounds can be prepared in one step by	Other photoinitiators are often
		Knoevenagel reactions	prepared	by	multi-step
			synthesis,	rendering	these
			compounds more expensive
			than push-pull dyes.
	Environmental	Push-pull dyes are mostly prepared by	Most of synthetic dyes are often
	impact	refluxing in ethanol and often precipitate after	prepared in organic solvent. A
		reaction. Isolation of products in pure form can	purification	by		column
		be obtained by filtration.	chromatography	is	also
			required.
	Photochemical	Push-pull dyes are stable compounds often	Dyes can be highly stable,
	stability	used in solar cells. Therefore, their	especially	with	the
		photochemical stability is often good.	development of polyaromatic
			structures.
	Absorption	Push-pull dyes exhibit a broad absorption and	Other synthetic dyes can also
	range	their absorption spectra can be finely tuned.	exhibit a broad absorption.
	Photoinitiating	Push-pull dyes are ideal candidates for	Photoinitiating systems based
	ability	photoinitiation. Ability of dyes to release or	on dyes can compete or being
		acceptor electrons is the basis of their use in	on par with that of push-pull
		Organic Electronics (OFET, OLED, OPVs).	dyes-based systems
	Availability	Electron donors and acceptors used to design	Availability can be restricted,
		push-pull dyes can be both commercially	especially when multistep
		available. Especially, this is the case when	syntheses are used.
		indane-1,3-dione is used as electron acceptor.
		Compounds are generally obtained in high
		yield.

Table 2 .

 2 Final monomer conversions obtained upon irradiation at 405 nm of an Ebrecryl 40 resin using the three-

	component Dx (x = 17-26)/iodonium salt/amine (0.1%,2%/2%, w/w)
	Dye	0 (Blank) a	D9	D10	D11	D1	D12	D13
	FCs	60%	80%	86%	78%	66%	93%	60%
	Dye	D14	D15	D16	D8	D17	D18	D19
	FCs	80%	83%	83%	84%	50%	70%	78%
	Dye	D20	D21	D22	D23	D24	D25	D26
	FCs	92%	90%	92%	94%	56%	85%	92%

a: iodonium salt / amine (2%/2%, w/w) without dye.

Table 3 .

 3 Final monomer conversions obtained upon irradiation at 405 nm of a TMPTA resin

	using the three-component Dx (x = 27,28,30-38)/iodonium salt/amine (0.1%,2%/2%, w/w)
	Dye	0 (Blank) a	D27	D28	D30	D31	D32
	FCs	60%	76%	51%	92%	80%	78%
	Dye	D33	D34	D35	D36	D37	D38
	FCs	56%	43%	84%	91%	88%	85%

Table 4 .

 4 Summary of the photopolymerization characteristics of the different photoinitiating systems based on dyes D1-D41.

	dye	Absorption range	Molar extinction	Irradiation conditions	Additives	Monomers	Polymerization	Reference
		(nm)	coefficient				type
	D1	400-750	40 000	laser diodes at 405, 457, 473, 532,	Iod, NVK	TMPTA, EPOX,	FRPCP	[213]
				and 635 nm		DVE-3	FRP
	D2	350-550	22 000	laser diodes at 405, 457, 473, 532,	Iod, NVK	TMPTA, EPOX,	FRPCP	[213]
				and 635 nm		DVE-3	FRP
	D3	insoluble	-	halogen lamp, laser diode 457 nm	Iod, NVK,	EPOX, TMPTA,	FRPCP, FRP,	[225]
					MDEA, R-Br, R'-	DVE-3, trithiol	Thiol-ene
					Cl		reaction
	D4	300-450	29 900	halogen lamp, laser diode at 457	Iod, NVK,	EPOX, TMPTA,	FRPCP, FRP,	[225]
				nm	MDEA, R-Br, R'-	DVE-3, trithiol	Thiol-ene
					Cl		reaction
	D5	350-525	42 200	halogen lamp, laser diode at 457	Iod, NVK,	EPOX, TMPTA,	FRPCP, FRP,	[225]
				nm	MDEA, R-Br, R'-	DVE-3, trithiol	Thiol-ene
					Cl		reaction
	D6	350-550	7 700	halogen lamp, laser diode at 457	Iod, NVK,	EPOX, TMPTA,	FRPCP, FRP,	[225]
				nm	MDEA, R-Br, R'-	DVE-3, trithiol	Thiol-ene
					Cl		reaction
	D7	400-650	16 400	LED at 405 nm	Iod, NPG,	TMPTA, EPOX,	FRPCP, FRP	[226]
						TEGDMA
	D8	350-650	14 400	halogen lamp, laser diodes at	Iod, NVK,	TMPTA, EPOX	FRPCP	[134]
				457, 473, and 532 nm	MDEA, R-Br,		FRP
					(TMS)3Si-H
	D9	350-450	-	LED at 405 nm	Iod, EDB	Ebecryl 40	FRP	[227]
	D10	350-475	-	LED at 405 nm	Iod, EDB	Ebecryl 40	FRP	[227]
	D11	350-475	-	LED at 405 nm	Iod, EDB	Ebecryl 40	FRP	[227]
	D12	400-550	-	LED at 405 nm	Iod, EDB	Ebecryl 40	FRP	[227]
	D13	400-575	-	LED at 405 nm	Iod, EDB	Ebecryl 40	FRP	[227]
	D14	350-575	-	LED at 405 nm	Iod, EDB	Ebecryl 40	FRP	[227]
	D15	350-525	-	LED at 405 nm	Iod, EDB	Ebecryl 40	FRP	[227]
	D16	350-650	-	LED at 405 nm	Iod, EDB	Ebecryl 40	FRP	[227]

Acknowledgments

The author thanks Aix Marseille University and the Centre National de la Recherche Scientifique for financial supports.


Lorenzo Riparbelli
email: lorenzo.riparbelli@unifi.it

Fabrice Brémand

Paolo Dionisi-Vici

Jean-Christophe Dupré

Giacomo Goli

Frank Hesser

Delphine Jullien

Paola Mazzanti

Marco Togni

Elisabeth Ravaud

Riparbelli Lorenzo

Brémand Fabrice

Jean-Christophe Dupre

Goli Giacomo

Hesser Frank

Jullien Delphine

Mazzanti Paola

Togni Marco

Ravaud Elisabeth

Uzielli Luca

Gril Joseph

Studies for the Mona Lisa conservation: the implementation of its panel's Digital-Twin

Since 2004, the wooden panel of the "Mona Lisa" painting by Leonardo da Vinci has been studied by an international research group of scientists [START_REF] Lorenzo | Mona Lisa's digital twin: identifying the mechanical properties of the panel combining experimental data and advanced finite-element modelling[END_REF][START_REF] Gril | The Mona Lisa Project: An update on the progress of measurement and monitoring activities, Euromech conference[END_REF] and several experimental campaigns have been carried out to understand its characteristics and provide information for the Artwork's conservation [START_REF] Mohen | Au Coeur de La Joconde[END_REF]. Among these, the implementation of its "digital twin" is here presented as a fundamental step for its conservation. Indeed, this digital twin is providing a deeper understanding of the mechanical characteristics of the panel, and after it has been accurately calibrated, it will provide the means for evaluating the stress states which the Artwork undergoes when the surrounding climatic conditions vary. Moreover, it will allow to evaluate in a non-invasive way the effects produced on the Artwork by any changes of its framing conditions, including the internal stresses or the external forces that it can bear without damage. The implementation of the digital twin, through a Nelder-Mead optimization scheme, starts from the definition of the shape of the Artwork, through optical methods [START_REF] Brémand | Mechanical structure analysis by Digital Image Correlation and Fringe Pattern Profilometry[END_REF], and the identification of the boundary conditions, through an experimental campaign based on the use of a film sensitive to pressure [START_REF] Goli | Locating contact areas and estimating contact forces between the "Mona Lisa" wooden panel and its frame[END_REF]. During the whole year, while exhibited in the conditioned display case, the panel continuously tends to deform, due to the slight variations in the surrounding environment. Its mechanical behaviour is automatically monitored and recorded every 30 minutes by an ad hoc equipment placed close to its back face: four miniature load cells (located at the four corners) measure the forces pressing it against the frame, and three displacement transducers measure its deflection at mid height. Finally, a method is here described to compare two different framing conditions, through the numerical computation of point-by-point stress and deformation differences, in order to provide information for optimizing settings and constraints to Conservators.

Figure 1 :

 1 Figure 1: (left) the Mona Lisa panel inserted inside its frame; (centre) the contact areas between the panel and its frame, detected with pressure sensitive film; (right) the panel's numerical simulation to assess the stresses (σxx in MPa) induced in it by the contact forces.