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

Recent Advances on Water-Soluble Photoinitiators of Polymerization

Keywords: Photoinitiator, water-soluble, photopolymerization, LED, low light intensity, visible light

Visible light photopolymerization has extensively been studied during the last decades since alternatives to UV processes have to be found rapidly. Indeed, based on the Minamata Convention, Hg irradiation setups must be suppressed from industrial applications. Even if the use of visible light is beneficial in terms of depth of cure and safety for the manipulator, photopolymerization in water is still far aside to the other polymerization processes. At present, only few visible light photoinitiators are soluble in water, speeding down the development of greener polymerization processes. In this review, an overview of the recent advances on water-soluble photoinitiating systems is provided. Even if water-solubility is a key parameter to polymerize in water, the monomer conversion that can be achieved is also important. For these reasons, comparisons with reference photoinitiating systems soluble in water is provided.

Introduction

During the past decades, visible light photopolymerization has become the workhorse of photopolymerists due to the numerous disadvantages UV photopolymerization exhibits.

Nowadays, UV light is the focus of numerous safety concerns, UV light being responsible of skins cancers and eye damages. [START_REF] Armstrong | The epidemiology of UV induced skin cancer, Consequences Expo[END_REF][START_REF] De Gruijl | Skin cancer and solar UV radiation[END_REF][START_REF] Narayanan | Review: Ultraviolet radiation and skin cancer[END_REF][START_REF] Shao | Visible light initiating systems for photopolymerization: status, development and challenges[END_REF] Ozone is produced during irradiation, what constitutes another drawbacks. [START_REF] Shao | Visible light initiating systems for photopolymerization: status, development and challenges[END_REF] In 2013, the United Nations Environmental Program (UNEP) Minamata Convention on Mercury has resulted in a ban of Hg lamps, which was extensively used in industry to produce UV light. Since 2020, Hg lamps are no longer be allowed to be manufactured, imported and exported. Therefore, new light sources have to be used and, in this field, light-emitting diodes (LEDs) are promising irradiation sources due to their lowcosts, compactness, long operating lifetimes and low energy consumptions. Nowadays, photopolymerization is involved in numerous processes ranging from solvent-free paints, dentistry, adhesives, microelectronics, 3D and 4D printing, coatings and varnishes. [START_REF] Jasinski | Photopolymerization in dispersed systems[END_REF][START_REF] Noè | Cationic UV-Curing of Epoxidized Biobased Resins[END_REF][START_REF] Yuan | Low volume shrinkage photopolymerization system using hydrogen-bond-based monomers[END_REF][START_REF] Khudyakov | Kinetics of Photopolymerization of Acrylates with Functionality of 1-6[END_REF][START_REF] Dickens | Photopolymerization Kinetics of Methacrylate Dental Resins[END_REF][START_REF] Maffezzoli | Photopolymerization of dental composite matrices[END_REF][START_REF] Dikova | Optimization of Photopolymerization Process of Dental Composites[END_REF][START_REF] Andreu | 4D printing materials for vat photopolymerization[END_REF][START_REF] Chen | Photopolymerization and 3D/4D applications using newly developed dyes: Search around the natural chalcone scaffold in photoinitiating systems[END_REF][START_REF] Bagheri | Photopolymerization in 3D Printing[END_REF][START_REF] Fouassier | Three-component photoinitiating systems: towards innovative tailor made high performance combinations[END_REF][START_REF] Lalevée | Dyes and Chromophores in Polymer Science[END_REF][START_REF] Lalevée | Metal and metal-free photocatalysts: mechanistic approach and application as photoinitiators of photopolymerization[END_REF] Compared to thermal polymerization, photopolymerization exhibits several specificities such as the possibility to get a spatial and a temporal control of the polymerization process.

Photopolymerization can also be carried out without solvents, at room temperature limiting the release of volatile organic compounds (VOCs) but also the polymerization of temperature sensitive monomers. In order to improve the depth of cure, longer irradiation wavelengths have to be used. Indeed, as shown in the Figure 1, the depth of cure is dependent of the irradiation wavelengths used for the polymerization process. Thus, if the light penetration is limited to a few hundreds of micrometers at 350 nm, this value can increase up to 5 cm at 800 nm, enabling now to polymerize thick samples and samples containing fillers. [START_REF] Bonardi | High Performance Near-Infrared (NIR) Photoinitiating Systems Operating under Low Light Intensity and in the Presence of Oxygen[END_REF] Figure 1. Light penetration in a polystyrene latex with an average diameter of 112 nm.

Reprinted with permission from Bonardi et al. [START_REF] Bonardi | High Performance Near-Infrared (NIR) Photoinitiating Systems Operating under Low Light Intensity and in the Presence of Oxygen[END_REF] If the visible light approach seems to be appealing for photopolymerization, several drawbacks have however to be mentioned. Thus, visible light photoinitiators are coloured compounds often providing an undesired colour to the final coatings. This coloration can constitute a severe limitation for future applications of photopolymers. Visible light photons are also less energetic than UV photons so that more reactive photoinitiating systems have to be used to overcome this lower reactivity. 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5,12-dihydroindolo[3,2-a]carbazole scaffold : Fine tuning of the optical properties of visible light photoinitiators of polymerization[END_REF][START_REF] Xu | High-efficient carbazole-based photo-bleachable dyes as free radical initiators for visible light polymerization[END_REF][START_REF] Dumur | Recent advances on carbazole-based oxime esters as photoinitiators of polymerization[END_REF][START_REF] Hammoud | Dialkyl-5,12-dihydroindolo[3,2-a]carbazole-Based Oxime-Esters for LED Photoinitiating Systems and Application on 3D Printing[END_REF] iridium complexes, [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] Tehfe | Structural Effects in 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Structure/reactivity/efficiency relationship in photoinitiating systems upon visible lights[END_REF][START_REF] Xiao | Green light sensitive diketopyrrolopyrrole derivatives used in versatile photoinitiating systems for photopolymerizations[END_REF] truxene derivatives, [START_REF] Tehfe | New Cleavable Photoinitiator Architecture with Huge Molar Extinction Coefficients for Polymerization in the 340-450 nm Range[END_REF] triphenylamines, [START_REF] Dumur | Recent advances on visible light Triphenylamine-based photoinitiators of polymerization[END_REF][START_REF] Ladika | Synthesis and application of triphenylamine-based aldehydes as photoinitiators for multi-photon lithography[END_REF] coumarins, [START_REF] Abdallah | Coumarin derivatives as versatile photoinitiators for 3D printing, polymerization in water and photocomposite synthesis[END_REF][START_REF] Abdallah | Keto-coumarin scaffold for photoinitiators for 3D printing and photocomposites[END_REF][START_REF] Abdallah | Coumarins as Powerful Photosensitizers for the Cationic Polymerization of Epoxy-Silicones under Near-UV and Visible Light and Applications for 3D Printing Technology[END_REF][START_REF] Abdallah | Boron Compounds as Additives for the Cationic Polymerization Using Coumarin Derivatives in Epoxy Silicones[END_REF][START_REF] Chen | Photopolymerization of Coumarin-Containing Reversible Photoresponsive Materials Based on Wavelength Selectivity[END_REF][START_REF] Li | Coumarin-Based Oxime Esters: Photobleachable and Versatile Unimolecular Initiators for Acrylate and Thiol-Based Click Photopolymerization under Visible Light-Emitting Diode Light Irradiation[END_REF][START_REF] Rahal | Mono vs. Difunctional Coumarin as Photoinitiators in Photocomposite Synthesis and 3D Printing[END_REF][START_REF] Rajeshirke | Enhancement of NLO Properties in OBO Fluorophores Derived from Carbazole-Coumarin Chalcones Containing Carboxylic Acid at the N-Alykl Terminal End[END_REF][START_REF] Rahal | Design of ketocoumarin based photoinitiator for Free Radical Photopolymerization: Towards 3D printing and photocomposites applications[END_REF][START_REF] Rahal | 3-Carboxylic Acid and Formyl-Derived Coumarins as Photoinitiators in Photo-Oxidation or Photo-Reduction Processes for Photopolymerization upon Visible Light: Photocomposite Synthesis and 3D Printing Applications[END_REF][START_REF] Hammoud | Substituent effects on the photoinitiation ability of coumarin-based oxime-ester photoinitiators for free radical photopolymerization[END_REF][START_REF] Dumur | Recent advances on coumarin-based photoinitiators of polymerization[END_REF][START_REF] Liu | Recent advances on visible light Coumarin-based oxime esters as initiators of polymerization[END_REF] metal organic framework (MOFs), [START_REF] Brahmi | New hybrid MOF/polymer composites for the photodegradation of organic dyes[END_REF][START_REF] Zhang | Iron-Based Metal-Organic Frameworks (MOF) as Photocatalysts for Radical and Cationic Polymerizations under Near UV and Visible LEDs (385-405 nm)[END_REF][START_REF] Brahmi | New Hybrid Fe-based MOFs/Polymer Composites for the Photodegradation of Organic Dyes[END_REF] curcumin, [START_REF] Zhao | A new role of curcumin: as a multicolor photoinitiator for polymer fabrication under household UV to red LED bulbs[END_REF][START_REF] Crivello | Curcumin: A naturally occurring long-wavelength photosensitizer for diaryliodonium salts[END_REF][START_REF] Han | Facilely prepared blue-green light sensitive curcuminoids with excellent bleaching properties as high performance photosensitizers in cationic and free radical photopolymerization[END_REF][START_REF] Mishra | Curcumin, A Novel Natural Photoinitiator for the Copolymerization of Styrene and Methylmethacrylate[END_REF] perovskites, [START_REF] Brahmi | New hybrid perovskites/polymer composites for the photodegradation of organic dyes[END_REF][START_REF] Mokbel | Perovskites as new radical photoinitiators for radical and cationic polymerizations[END_REF] cyanines, [START_REF] Mokbel | On demand NIR activated photopolyaddition reactions[END_REF][START_REF] Mokbel | NIR Sensitizer Operating under Long Wavelength (1064 nm) for Free Radical Photopolymerization Processes[END_REF][START_REF] Launay | Near-infrared light for polymer re-shaping and re-processing applications[END_REF][START_REF] Caron | Near-Infrared PhotoInitiating Systems: Photothermal versus Triplet-Triplet Annihilation-Based Upconversion Polymerization[END_REF][START_REF] Bonardi | Photoinduced Thermal Polymerization Reactions[END_REF][START_REF] Launay | Safe near infrared light for fast polymers surface sterilization using organic heaters[END_REF][START_REF] Launay | Photothermal activation in the near infrared range for 4-dimensional printing using relevant organic dyes[END_REF] pyridinium salts, [START_REF] Ma | Charge Transfer Complexes (CTCs) with Pyridinium Salts: Toward Efficient Dual Photochemical/Thermal Initiators and 3D Printing Applications[END_REF][START_REF] Kminek | A water-soluble poly(methylphenylsilylene) derivative as a photoinitiator of radical polymerization of hydrophilic vinyl monomers[END_REF] Schiff bases, [START_REF] Lalevée | Dye photosensitized cationic ring-opening polymerization: Search for new dye skeletons[END_REF] silyl glyoximides, [START_REF] Kirschner | Silyl Glyoximides: Toward a New Class of Visible Light Photoinitiators[END_REF] chromones and flavones, [START_REF] Tehfe | New chromone based photoinitiators for polymerization reactions under visible light[END_REF][START_REF] You | Flavonol dyes with different substituents in photopolymerization[END_REF][START_REF] Al Mousawi | 3-Hydroxyflavone and N-Phenylglycine in High Performance Photoinitiating Systems for 3D Printing and Photocomposites Synthesis[END_REF] N-heterocyclic carbene boranes, [START_REF] Lalevée | N-Heterocyclic Carbene Boranes Accelerate Type I Radical Photopolymerizations and Overcome Oxygen Inhibition[END_REF] thiophene derivatives, [START_REF] Dumur | Recent Advances on Visible Light Thiophene-based Photoinitiators of Polymerization[END_REF] squaraines, [START_REF] Giacoletto | Recent advances on squaraine-based photoinitiators of polymerization[END_REF][START_REF] Xiao | Panchromatic Photopolymerizable Cationic Films Using Indoline and Squaraine Dye Based Photoinitiating Systems[END_REF][START_REF] Launay | NIR Organic Dyes as Innovative Tools for Reprocessing/Recycling of Plastics: Benefits of the Photothermal Activation in the Near-Infrared Range[END_REF][START_REF] Bonardi | Different NIR dye scaffolds for polymerization reactions under NIR light[END_REF][START_REF] Balcerak | Novel photoinitiators based on difluoroborate complexes of squaraine dyes for radical polymerization of acrylates upon visible light[END_REF] naphthalimides, [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][START_REF] Bonardi | New 1,8-Naphthalimide Derivatives as Photoinitiators for Free-Radical Polymerization Upon Visible Light[END_REF][START_REF] Zhang | Naphthalimide-Tertiary Amine Derivatives as Blue-Light-Sensitive Photoinitiators[END_REF][START_REF] Xiao | Naphthalimide Derivatives: Substituent Effects on the Photoinitiating Ability in Polymerizations under Near UV, Purple, White and Blue LEDs[END_REF][START_REF] Xiao | Naphthalimide-phthalimide derivative based photoinitiating systems for polymerization reactions under blue lights[END_REF][START_REF] Zhang | A benzophenone-naphthalimide derivative as versatile photoinitiator of polymerization under near UV and visible lights[END_REF][START_REF] Zhang | N-[2-(Dimethylamino)ethyl]-1,8-naphthalimide derivatives as photoinitiators under LEDs[END_REF][START_REF] Zhang | Structure Design of Naphthalimide Derivatives: Toward Versatile Photoinitiators for Near-UV/Visible LEDs, 3D Printing, and Water-Soluble Photoinitiating Systems[END_REF][START_REF] Zhang | UVviolet-blue LED induced polymerizations: Specific photoinitiating systems at 365, 385, 395 and 405 nm[END_REF][START_REF] Xiao | Blue Light Sensitive Dyes for Various Photopolymerization Reactions: Naphthalimide and Naphthalic Anhydride Derivatives[END_REF][START_REF] Xiao | Naphthalimide based methacrylated photoinitiators in radical and cationic photopolymerization under visible light[END_REF][START_REF] Noirbent | Recent advances on naphthalic anhydrides and 1,8naphthalimide-based photoinitiators of polymerization[END_REF][START_REF] Rahal | Naphthalimide-Based Dyes as Photoinitiators under Visible Light Irradiation and their Applications: Photocomposite Synthesis, 3D printing and Polymerization in Water[END_REF][START_REF] Rahal | Naphthyl-Naphthalimides as High-Performance Visible Light Photoinitiators for 3D Printing and Photocomposites Synthesis[END_REF][START_REF] Zivic | Novel naphthalimideamine based photoinitiators 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[START_REF] Ma | Imidazole based dual photo/thermal initiators for highly efficient radical polymerization under air with a metal-free approach[END_REF] porphyrins, [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] viologen derivatives, [START_REF] Dumur | New Chelating Stilbazonium-Like Dyes from Michler's Ketone[END_REF][START_REF] Cao | Photocatalysis of viologens for photoinitiated polymerization using carboxylic acid as electron donors[END_REF][START_REF] Schnabel | Cationic photopolymerization with the aid of pyridinium-type salts[END_REF] dihydroanthraquinones, [START_REF] Zhang | Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization[END_REF] anthraquinone, [START_REF] Xiao | Cationic and Thiol-Ene Photopolymerization upon Red Lights Using Anthraquinone Derivatives as Photoinitiators[END_REF] pyrrole derivatives, [START_REF] Dumur | Recent advances on visible light pyrrole-derived photoinitiators of polymerization[END_REF] iodonium salts, [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][START_REF] Mokbel | Specific cationic photoinitiators for near UV and visible LEDs: Iodonium versus 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] Tasdelen | Photoacid Generation by Stepwise Two-Photon Absorption: Photoinitiated Cationic Polymerization of Cyclohexene Oxide by Using Benzodioxinone in the Presence of Iodonium Salt[END_REF][START_REF] Crivello | Diaryliodonium Salts. A New Class of Photoinitiators for Cationic Polymerization[END_REF][START_REF] He | Photoreaction and photopolymerization studies on squaraine dyes/iodonium salts combination[END_REF][START_REF] Jun | Photopolymerization Initiated by Dimethylaminochalcone/Diphenyliodonium Salt Combination System Sensitive to Visible Light[END_REF][START_REF] Zivic | Recent Advances and Challenges in the Design of Organic Photoacid and Photobase Generators for Polymerizations[END_REF] 2,3-diphenylquinoxaline derivatives, [START_REF] Corakci | Long wavelength photosensitizers in photoinitiated cationic polymerization: The effect of quinoxaline derivatives on photopolymerization[END_REF][START_REF] Xiao | Multicolor Photoinitiators for Radical and Cationic Polymerization: Monofunctional vs Polyfunctional Thiophene Derivatives[END_REF] naphthoquinones, [START_REF] Hammoud | Naphthoquinone-based imidazolyl esters as blue-lightsensitive Type I photoinitiators[END_REF] benzylidene ketones, [START_REF] Bao | Double benzylidene ketones as photoinitiators for visible light photopolymerization[END_REF][START_REF] Fu | Photopolymerization of acrylate resin and ceramic suspensions with benzylidene ketones under blue/green LED[END_REF][START_REF] Dumur | Recent advances on benzylidene ketones as photoinitiators of polymerization[END_REF][START_REF] Zhao | Polyethylene glycol-functionalized benzylidene cyclopentanone dyes for two-photon excited photodynamic therapy[END_REF][START_REF] Fang | Cationic benzylidene cyclopentanone photosensitizers for selective photodynamic inactivation of bacteria over mammalian cells[END_REF][START_REF] Xue | Effect of Bridging Position on the Two-Photon Polymerization Initiating Efficiencies of Novel Coumarin/Benzylidene Cyclopentanone Dyes[END_REF][START_REF] Dumur | Recent advances on benzylidene cyclopentanones as visible light photoinitiators of polymerization[END_REF] Bodipy, [START_REF] Topa-Skwarczyńska | Development of the first panchromatic BODIPY-based onecomponent iodonium salts for initiating the photopolymerization processes[END_REF][START_REF] Xu | Utilizing Intramolecular Photoinduced Electron Transfer to Enhance Photothermal Tumor Treatment of Aza-BODIPY-Based Near-Infrared Nanoparticles[END_REF][START_REF] Skotnicka | New BODIPY Dyes Based on Benzoxazole as Photosensitizers in Radical Polymerization of Acrylate Monomers[END_REF][START_REF] Lu | Boron dipyrromethene (BODIPY) in polymer chemistry[END_REF][START_REF] Telitel | BODIPY derivatives and boranil as new photoinitiating systems of cationic polymerization exhibiting a tunable absorption in the 400-600 nm spectral range[END_REF][START_REF] Telitel | Photopolymerization of Cationic Monomers and Acrylate/Divinylether Blends under Visible Light Using Pyrromethene Dyes[END_REF] copper complexes, [START_REF] Xiao | Copper Complexes in Radical Photoinitiating Systems: Applications to Free Radical and Cationic Polymerization upon Visible LEDs[END_REF][START_REF] Xiao | Copper complexes: the effect of ligands on their photoinitiation efficiencies in radical polymerization reactions under visible light[END_REF][START_REF] Xiao | Copper and iron complexes as visible-light-sensitive photoinitiators of polymerization[END_REF][START_REF] Garra | Copper-Based (Photo)redox Initiating Systems as Highly Efficient Systems for Interpenetrating Polymer Network Preparation[END_REF][START_REF] Garra | Mechanosynthesis of a Copper complex for redox initiating systems with a unique near infrared light activation[END_REF][START_REF] Garra | Mechanosynthesized copper(I) complex based initiating systems for redox polymerization: towards upgraded oxidizing and reducing agents[END_REF][START_REF] Mokbel | Copper photoredox catalyst "G1": a new high performance photoinitiator for near-UV and visible LEDs[END_REF][START_REF] Mokbel | Simultaneous initiation of radical and cationic polymerization reactions using the "G1" copper complex as photoredox catalyst: Applications of free radical/cationic hybrid photopolymerization in the composites and 3D printing fields[END_REF][START_REF] Mousawi | Copper photoredox catalysts for polymerization upon near UV or visible light: structure/reactivity/efficiency relationships and use in LED projector 3D printing resins[END_REF][START_REF] Mau | Panchromatic Copper Complexes for Visible Light Photopolymerization[END_REF][START_REF] Mau | Concomitant initiation of radical and cationic polymerisations using new copper complexes as photoinitiators: Synthesis and characterisation of acrylate/epoxy interpenetrated polymer networks[END_REF][START_REF] Garra | Copper (Photo)redox Catalyst for Radical Photopolymerization in Shadowed Areas and Access to Thick and Filled Samples[END_REF][START_REF] Garra | A New Highly Efficient Amine-Free and Peroxide-Free Redox System for Free Radical Polymerization under Air with Possible Light Activation[END_REF][START_REF] Garra | New copper(I) complex based initiating systems in redox polymerization and comparison with the amine/benzoyl peroxide reference[END_REF][START_REF] Noirbent | Recent Advances on Copper Complexes as Visible Light Photoinitiators and (Photo) Redox Initiators of Polymerization[END_REF][START_REF] Borjigin | The new LED-Sensitive photoinitiators of Polymerization: Copper complexes in free radical and cationic photoinitiating systems and application in 3D printing[END_REF][START_REF] Rahal | Novel Copper Complexes as Visible Light Photoinitiators for the Synthesis of Interpenetrating Polymer Networks (IPNs)[END_REF][START_REF] Lin | Efficacy analysis of new copper complex for visible light (455, 530 nm) radical/cationic photopolymerization: The synergic effects and catalytic cycle[END_REF] conjugated polymers, [START_REF] Zhang | Polymeric Photoinitiators: A New Search toward High Performance Visible Light Photoinitiating Systems[END_REF] polyoxometalates, [START_REF] Ghali | LED and solar photodecomposition of erythrosine B and rose Bengal using H3PMo12O40/polymer photocatalyst[END_REF][START_REF] Brahmi | Performance improvement of the photocatalytic process for the degradation of pharmaceutical compounds using new POM/polymer photocatalysts[END_REF][START_REF] Brahmi | Polyoxometalates/polymer composites for the photodegradation of bisphenol-A[END_REF] dithienophospholes, [START_REF] Al Mousawi | π-Conjugated Dithienophosphole Derivatives as High Performance Photoinitiators for 3D Printing Resins[END_REF][START_REF] Dumur | Organic Electronics: An El Dorado in the Quest of New Photocatalysts for Polymerization Reactions[END_REF] perylenes, [START_REF] Xiao | Perylene derivatives as photoinitiators in blue light sensitive cationic or radical curable films and panchromatic thiol-ene polymerizable films[END_REF][START_REF] Xiao | Red-Light-Induced Cationic Photopolymerization: Perylene Derivatives as Efficient Photoinitiators[END_REF][START_REF] Dumur | Recent advances on perylene-based photoinitiators of polymerization[END_REF][START_REF] Tehfe | Green-Light-Induced Cationic Ring Opening Polymerization Reactions: Perylene-3,4:9,10bis(Dicarboximide) as Efficient Photosensitizers[END_REF] iron complexes, [START_REF] Zhang | Iron-Based Metal-Organic Frameworks (MOF) as Photocatalysts for Radical and Cationic Polymerizations under Near UV and Visible LEDs (385-405 nm)[END_REF][START_REF] Garra | Ferrocene-based (photo)redox polymerization under long wavelengths[END_REF][START_REF] Telitel | Iron complexes as potential photocatalysts for controlled radical photopolymerizations: A tool for modifications and patterning of surfaces[END_REF][START_REF] Zhang | Iron complexes as photoinitiators for radical and cationic polymerization through photoredox catalysis processes[END_REF][START_REF] Zhang | Visiblelight-sensitive photoredox catalysis by iron complexes: Applications in cationic and radical polymerization reactions[END_REF][START_REF] Zhang | Iron Complexes in Visible-Light-Sensitive Photoredox Catalysis: Effect of Ligands on Their Photoinitiation Efficiencies[END_REF][START_REF] Dumur | Recent advances on ferrocene-based photoinitiating systems[END_REF][START_REF] Dumur | Recent advances on iron-based photoinitiators of polymerization[END_REF] pyrenes, [START_REF] Tehfe | New insights into 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] 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][START_REF] Tehfe | Design of Type I and Type II photoinitiators possessing highly coupled pyreneketone moieties[END_REF][START_REF] Dumur | Recent advances on pyrene-based photoinitiators of polymerization[END_REF][START_REF] Tehfe | A Multicolor Photoinitiator for Cationic Polymerization and Interpenetrated Polymer Network Synthesis: 2,7-Di-tert-butyldimethyldihydropyrene[END_REF][START_REF] Telitel | New functionalized aromatic ketones as photoinitiating systems for near visible and visible light induced polymerizations[END_REF][START_REF] Tehfe | Polyaromatic Structures as Organo-Photoinitiator Catalysts for Efficient Visible Light Induced Dual Radical/Cationic Photopolymerization and Interpenetrated Polymer Networks Synthesis[END_REF] zinc complexes, [START_REF] Tehfe | Zinc-based metal complexes as new photocatalysts in polymerization initiating systems[END_REF] acridine-1,8-diones, [START_REF] Tehfe | Novel Highly Efficient Organophotocatalysts: Truxene-Acridine-1,8-diones as Photoinitiators of Polymerization[END_REF][START_REF] Xiao | Difunctional acridinediones as photoinitiators of polymerization under UV and visible lights: Structural effects[END_REF][START_REF] Xiao | Acridinediones: Effect of Substituents on Their Photoinitiating Abilities in Radical and Cationic Photopolymerization[END_REF] helicenes, [START_REF] Mousawi | Azahelicenes as visible light photoinitiators for cationic and radical polymerization: Preparation of photoluminescent polymers and use in high performance LED projector 3D printing resins[END_REF][START_REF] Al Mousawi | Visible Light Chiral Photoinitiator for Radical Polymerization and Synthesis of Polymeric Films with Strong Chiroptical Activity[END_REF] benzylidene ketones, [START_REF] Bao | Double benzylidene ketones as photoinitiators for visible light photopolymerization[END_REF][START_REF] Fu | Photopolymerization of acrylate resin and ceramic suspensions with benzylidene ketones under blue/green LED[END_REF] push-pull dyes, [START_REF] Garra | Ferrocene-based (photo)redox polymerization under long wavelengths[END_REF][START_REF] Tehfe | New pyridinium salts as versatile compounds for dye sensitized photopolymerization[END_REF][START_REF] Xiao | Julolidine or Fluorenone Based Push-Pull Dyes for Polymerization upon Soft Polychromatic Visible Light or Green Light[END_REF][START_REF] Mokbel | Michler's Ketone as an Interesting Scaffold for the Design of High-Performance Dyes in Photoinitiating Systems Upon Visible Light[END_REF][START_REF] Tehfe | New Push-Pull Dyes Derived from Michler's Ketone For Polymerization Reactions Upon Visible Lights[END_REF][START_REF] Mokbel | End capped polyenic structures as visible light sensitive photoinitiators for polymerization of vinylethers[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] Xiao | Structural Effects in the Indanedione Skeleton for the Design of Low Intensity 300-500 nm Light Sensitive Initiators[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] Sun | N-ethyl carbazole-1-allylidene-based push-pull dyes as efficient light harvesting photoinitiators for sunlight induced polymerization[END_REF][START_REF] Tehfe | Push-pull (thio)barbituric acid derivatives in dye photosensitized radical and cationic polymerization reactions under 457/473 nm laser beams or blue LEDs[END_REF][START_REF] Sun | High-performance sunlight induced polymerization using novel push-pull dyes with high light absorption properties[END_REF][START_REF] Mokbel | Photoinitiating systems of polymerization and in situ incorporation of metal nanoparticles into polymer matrices upon exposure to visible light: push-pull malonate and malononitrile based dyes[END_REF][START_REF] Helmy | Read de Alaniz, Design and Synthesis of Donor-Acceptor Stenhouse Adducts: A Visible Light Photoswitch Derived from Furfural[END_REF][START_REF] Sun | Sunlight Induced Polymerization Photoinitiated by Novel Push-Pull Dyes: Indane-1,3-Dione, 1H-Cyclopenta[b]Naphthalene-1,3(2H)-Dione and 4-Dimethoxyphenyl-1-Allylidene Derivatives[END_REF] chalcones, [START_REF] Chen | Photopolymerization and 3D/4D applications using newly developed dyes: Search around the natural chalcone scaffold in photoinitiating systems[END_REF][START_REF] Chen | Photoinitiators derived from natural product scaffolds: monochalcones in three-component photoinitiating systems and their applications in 3D printing[END_REF][START_REF] Tang | A high performance phenyl-free LED photoinitiator for cationic or hybrid photopolymerization and its application in LED cationic 3D printing[END_REF][START_REF] Xu | Allyloxy ketones as efficient photoinitiators with high migration stability in free radical polymerization and 3D printing[END_REF][START_REF] Xu | Design of ketone derivatives as highly efficient photoinitiators for free radical and cationic photopolymerizations and application in 3D printing of composites[END_REF][START_REF] Chen | Bis-chalcone derivatives derived from natural products as near-UV/visible light sensitive photoinitiators for 3D/4D printing[END_REF][START_REF] Liu | Design of photoinitiating systems based on the chalcone-anthracene scaffold for LED cationic photopolymerization and application in 3D printing[END_REF][START_REF] Giacoletto | Recent Advances in bis-Chalcone-Based Photoinitiators of Polymerization: From Mechanistic Investigations to Applications[END_REF][START_REF] Ibrahim-Ouali | Recent Advances on Chalcone-based Photoinitiators of Polymerization[END_REF][START_REF] Chen | In situ generation of Ag nanoparticles during photopolymerization by using newly developed dyes-based three-component photoinitiating systems and the related 3D printing applications and their shape change behavior[END_REF][START_REF] Chen | Water-Soluble Visible Light Sensitive Photoinitiating System Based on Charge Transfer Complexes for the 3D Printing of Hydrogels[END_REF][START_REF] Tehfe | Chalcone derivatives as highly versatile photoinitiators for radical, cationic, thiol-ene and IPN polymerization reactions upon exposure to visible light[END_REF][START_REF] Sun | In silico rational design by molecular modeling of new ketones as photoinitiators in three-component photoinitiating systems: application in 3D printing[END_REF][START_REF] Chen | Interpenetrating polymer network hydrogels using natural based dyes initiating systems: antibacterial activity and 3D/4D performance[END_REF] thioxanthones, [START_REF] Karaca | Thioxanthone-benzothiophenes as photoinitiator for free radical polymerization[END_REF][START_REF] Balta | An annelated thioxanthone as a new Type II initiator[END_REF][START_REF] Balta | Thioxanthone-Diphenyl Anthracene: Visible Light Photoinitiator[END_REF][START_REF] Dadashi-Silab | Shining a light on an adaptable photoinitiator: advances in photopolymerizations initiated by thioxanthones[END_REF][START_REF] Eren | Photophysical and Photochemical Studies of Novel Thioxanthone-Functionalized Methacrylates through LED Excitation[END_REF][START_REF] Qiu | Thioxanthone photoinitiator containing polymerizable N-aromatic maleimide for photopolymerization[END_REF][START_REF] Tar | Panchromatic Type II Photoinitiator for Free Radical Polymerization Based on Thioxanthone Derivative[END_REF][START_REF] Wu | Thioxanthone based one-component polymerizable visible light photoinitiator for free radical polymerization[END_REF][START_REF] Wu | High-Performance and Low Migration One-Component Thioxanthone Visible Light Photoinitiators[END_REF][START_REF] Wu | Visible light-emitting diode-sensitive thioxanthone derivatives used in versatile photoinitiating systems for photopolymerizations[END_REF][START_REF] Lalevée | Light-Harvesting Organic Photoinitiators of Polymerization[END_REF][START_REF] Esen | The investigation of photoinitiated polymerization of multifunctional acrylates with TX-BT by Photo-DSC and RT-FTIR[END_REF][START_REF] Lalevée | New thioxanthone and xanthone photoinitiators based on silyl radical chemistry[END_REF][START_REF] Gencoglu | A Water Soluble, Low Migration, and Visible Light Photoinitiator by Thioxanthone-Functionalization of Poly(ethylene glycol)-Containing Poly(β-amino ester)[END_REF] benzophenones, [START_REF] Liu | Monocomponent Photoinitiators based on Benzophenone-Carbazole Structure for LED Photoinitiating Systems and Application on 3D Printing[END_REF][START_REF] Xiao | Variations on the Benzophenone Skeleton: Novel High Performance Blue Light Sensitive Photoinitiating Systems[END_REF][START_REF] Zhang | Design of Novel Photoinitiators for Radical and Cationic Photopolymerizations under Near UV and Visible LEDs (385, 395, and 405 nm)[END_REF][START_REF] Liu | New multifunctional benzophenone-based photoinitiators with high migration stability and their applications in 3D printing[END_REF][START_REF] Liu | Novel Photoinitiators Based on Benzophenone-Triphenylamine Hybrid Structure for LED Photopolymerization[END_REF][START_REF] Liu | A monocomponent bifunctional benzophenone-carbazole type II photoinitiator for LED photoinitiating systems[END_REF][START_REF] Tehfe | Trifunctional Photoinitiators Based on a Triazine Skeleton for Visible Light Source and UV LED Induced Polymerizations[END_REF][START_REF] Lin | Efficacy Modeling of New Multi-Functional Benzophenone-Based System for Free-Radical/Cationic Hybrid Photopolymerization Using 405 nm LED[END_REF] anthracene, [START_REF] Dumur | Recent Advances on Anthracene-based Photoinitiators of Polymerization[END_REF] acridones, [START_REF] Abdallah | Acridone derivatives as high performance visible light photoinitiators for cationic and radical photosensitive resins for 3D printing technology and for low migration photopolymer property[END_REF][START_REF] Zhang | Novel panchromatic photopolymerizable matrices: N,N'-dibutylquinacridone as an efficient and versatile photoinitiator[END_REF] and phenazines, [START_REF] Tehfe | Photoinitiators based on a phenazine scaffold: High performance systems upon near-UV or visible LED (385, 395 and 405 nm) irradiations[END_REF]. By exploring these different structures, high performance photoinitiating systems could be identified. Noticeably, only few water-soluble photoinitiators were proposed in the course of these different investigations. Besides, water-soluble photoinitiators could pave the way towards polymerization done in greener conditions than that currently used. [START_REF] Tehfe | Photopolymerization Reactions: On the Way to a Green and Sustainable Chemistry[END_REF][START_REF] Pierau | Bio-sourced monomers and cationic photopolymerization-The green combination towards eco-friendly and non-toxic materials[END_REF] In this field, 2-hydroxy-1-[4-(2-hydroxyethoxy) phenyl]-2-methyl-1-propanone (Irgacure 2959) is among the most popular water-soluble photoinitiator (See Figure 2). [START_REF] Rouillard | Methods for Photocrosslinking Alginate Hydrogel Scaffolds with High Cell Viability[END_REF][START_REF] Ma | Synthesis of novel macrophotoinitiator for the photopolymerization of acrylate[END_REF][START_REF] Zhang | Regulating photochemical behavior and property of imidazolium-based water soluble polysiloxane macromolecular photoinitiators by anions[END_REF] Besides, its absorption maximum located in the UV range (λmax = 276 nm) and its water-solubility lower than 2 wt% are restrictive for numerous applications. issue could be addressed. [START_REF] Kamoun | Carboxylated camphorquinone as visible-light photoinitiator for biomedical application: Synthesis, characterization, and application[END_REF] A high degree of double bond conversion could be obtained while using an amine co-initiator. Polymeric benzil macrophotoinitiators were also designed and synthesized as water-soluble photoinitiators, benzil being less prone than the other photoinitiators to quenching reactions by radical termination. [START_REF] Catalina | Synthesis, photochemical and photoinitiation activity of water-soluble copolymers with anthraquinone chromophores as side-chain groups[END_REF][START_REF] Catalina | Photocalorimetric study on the photoinitiation activity of water soluble copolymers with pendent benzil moieties[END_REF] However, absorption of benzil is strongly UV-centered and cannot be used for visible light polymerization.

Extensive efforts were also done to design water-soluble photoinitiating systems with historical photoinitiators such as xanthene dyes. [START_REF] Bi | A Visible Light Initiating System for Free Radical Promoted Cationic Polymerization[END_REF] Latex nanoparticles comprising Eosin-Y, [START_REF] Arbeloa | Synthesis and characterization of latex nanoparticles using a visible-light photoinitiating system in reverse micelles[END_REF] inclusion complex of xanthene dyes with methylated-β-cyclodextrin, [START_REF] Li | A water-soluble supramolecular structured photosensitive initiation system: Me-β-CD complex of xanthene dye/aryliodonium salt[END_REF] or the formation of charge transfer complexes with the water-soluble triethanolamine [START_REF] Encinas | Xanthene dyes/amine as photoinitiators of radical polymerization: A comparative and photochemical study in aqueous medium[END_REF] were envisioned as possible strategies for water solubility. However, due to the complex structures of xanthene dyes, their chemical modification is a hard work. Considering that these water-soluble photoinitiators are difficult to modify in order to redshift their absorptions towards the visible range, photoinitiators of innovative structures have thus to be found. This observation is true for Type I but also for Type II photoinitiators. [START_REF] Tomal | Water-Soluble Photoinitiators in Biomedical Applications[END_REF] Several strategies were thus examined to convert organo-soluble dyes as water-soluble structures using complex approaches. Thus, water-dispersible diphenyl(2,4,6trimethylbenzoyl)phosphine oxide (TPO) nanoparticles obtained by oil-in-water microemulsions were reported in the literature. [START_REF] Pawar | High-performance 3D printing of hydrogels by water-dispersible photoinitiator nanoparticles[END_REF] However, drying of volatile microemulsions by spray drying is not easy to handle. Generation of nanoparticles of poly(9,9dioctylfluorene-alt-benzothiadiazole) and poly(9,9-di-n-octylfluorenyl-2,7-diyl) by aggregation of polymers in water was also proposed, hydrophobicity of the conjugated polymers facilitating the aggregation process in water. [START_REF] Gallastegui | Conjugated Polymer Nanoparticles as Unique Coinitiator-Free, Water-Soluble, Visible-Light Photoinitiators of Vinyl Polymerization[END_REF] However, these last approaches required nanoparticles preparation prior to polymerization processes.

In this review, an overview of the water-soluble photoinitiating systems developed during the last decade is provided. Over the years, several strategies were examined, consisting in introducing water-solubilizing chains such as poly(ethylene glycol) chains, to form charge-transfer complexes with water-soluble amines such as triethanolamine, to form salts or to introduce water water-soluble groups. Water-soluble Type I and Type II photoinitiators were also investigated, expanding the scope of applications. Comparison with reference photoinitiating systems is also provided to evidence the interest of these newly developed structures.

Water-soluble photoinitiators

Mono-acylophosphine oxides and bis-acylophosphine oxides derivatives

Mono-acylophosphine oxides and bis-acylophosphine oxides were extensively studied starting from the 90's as visible light photoinitiators of polymerization, the absorptions of these two families of dyes extending up to 450 nm. [START_REF] Müller | Recent Advances in Type I Photoinitiators for Visible Light Induced Photopolymerization[END_REF][START_REF] Moszner | Synthesis of bis(3-{[2-(allyloxy)ethoxy]methyl}-2,4,6trimethylbenzoyl)(phenyl)phosphine oxide -a tailor-made photoinitiator for dental adhesives[END_REF] In these two families, diphenyl(2,4,6trimethylbenzoyl)phosphine oxide (TPO) and phenyl bis (2,4,6-trimethylbenzoyl)phosphine oxide (BAPO) were undoubtedly the most widely studied. However, BAPO and TPO are insoluble in water. Notably, water solubility of TPO is only of 3.13 mg/L. [START_REF] Wang | General One-Pot Method for Preparing Highly Water-Soluble and Biocompatible Photoinitiators for Digital Light Processing-Based 3D Printing of Hydrogels[END_REF] In 1991, Schnabel and coworkers prepared two water-soluble derivatives of TPO, namely LiTPO and MgTPO (See Figure 3). [START_REF] Majima | Phenyl-2,4,6-trimethylbenzoylphosphinates as water-soluble photoinitiators. Generation and reactivity of O Ṗ(C6H5)(O-) radical anions[END_REF] LiTPO and Mg0.5TPO exhibited a good solubility in water since solubilities of 85 g/L and 13 g/L were respectively determined for the two photoinitiators. From the mechanistic viewpoint, LiTPO and Mg0.5TPO can undergo an α-scission at the carbonylphosphorus bond, producing a benzoyloxy radical and a radical anion which is highly reactive towards olefins such as acrylamide, methacrylamide, methyl methacrylate, acrylonitrile, styrene, [START_REF] Majima | Phenyl-2,4,6-trimethylbenzoylphosphinates as water-soluble photoinitiators. Generation and reactivity of O Ṗ(C6H5)(O-) radical anions[END_REF] or poly(ethylene glycol)diacrylate (PEGDA) (See Figure 3). [START_REF] Fairbanks | Photoinitiated polymerization of PEG-diacrylate with lithium phenyl-2,4,6trimethylbenzoylphosphinate: polymerization rate and cytocompatibility[END_REF][START_REF] Lin | Application of visible light-based projection stereolithography for live cell-scaffold fabrication with designed architecture[END_REF] But other monomers such as gelatin methacrylate, [START_REF] Calderon | Tubulogenesis of co-cultured human iPS-derived endothelial cells and human mesenchymal stem cells in fibrin and gelatin methacrylate gels[END_REF] norbornene-functionalized PEG macromers or pentaerythritol triacrylate (PETA) [START_REF] Erkoc | Photocurable pentaerythritol triacrylate/lithium phenyl-2,4,6trimethylbenzoylphosphinate-based ink for extrusion-based 3D printing of magnetoresponsive materials[END_REF] were also polymerized with LiTPO. [START_REF] Raza | The Influence of Matrix Degradation and Functionality on Cell Survival and Morphogenesis in PEG-Based Hydrogels[END_REF] In 2009, the synthesis of TPO salts was greatly improved by Huang and coworkers, and reduced to a simple and straightforward one-pot procedure, enabling to get LiTPO, NaTPO and Mg0.5TPO in 66, 77 and 53% yields respectively. [START_REF] Wang | General One-Pot Method for Preparing Highly Water-Soluble and Biocompatible Photoinitiators for Digital Light Processing-Based 3D Printing of Hydrogels[END_REF] 3D Printed structures containing up to 80% water could be produced with a high spatial resolution. Noticeably, a water solubility lower than that previously reported in the literature was determined for LiTPO, being only of 40.5 g/L (vs.

85 g/L for Schnabel and coworkers). [START_REF] Majima | Phenyl-2,4,6-trimethylbenzoylphosphinates as water-soluble photoinitiators. Generation and reactivity of O Ṗ(C6H5)(O-) radical anions[END_REF] Besides, this value is closed to that reported by Liska and coworkers (47 g/L). [START_REF] Benedikt | Highly efficient water-soluble visible light photoinitiators[END_REF] Solubilities of 49.5 and 15.5 g/L were also found for NaTPO and Mg0.5TPO. Here again, the solubility in water of NaTPO is different to that reported by Liska and coworkers (29 g/L). [START_REF] Benedikt | Highly efficient water-soluble visible light photoinitiators[END_REF] In the case of LiTPO, biocompatible hydrogels using a mixture of acrylamide and PEGDA could be obtained, paving the way towards nontoxic 3D-printed hydrogels. The development of simple synthetic procedures was not limited to TPO derivatives and a one-pot two-step synthesis of sodium bis(mesitoyl)phosphinate was proposed in 2015 by Grützmacher and coworkers. [START_REF] Müller | Simple One-Pot Syntheses of Water-Soluble Bis(acyl)phosphane Oxide Photoinitiators and Their Application in Surfactant-Free Emulsion Polymerization[END_REF] By the versatility of the synthetic approach, a series of ammonium salts and a sodium salt could be prepared in good yields (See Figure 4). In the case of BAPO-ONa, a solubility of 45 g/L could be determined, therefore comparable to that of LiTPO. However, this value is lower than that later reported by the same authors (60 g/L). [START_REF] Benedikt | Highly efficient water-soluble visible light photoinitiators[END_REF] BAPO-ONa could initiate the surfactant-free emulsion polymerization of styrene, enabling to get monodisperse and spherical nanoparticles. A comparative study carried out on NaTPO, LiTPO, BAPO-ONa and BAPO-OLi revealed the BAPO derivatives to exhibit a higher solubility in water that the TPO salts (See Figure 5). However, these water solubilities are comparable to that of 2-hydroxy-1-[3-(hydroxymethyl)phenyl]-2-methyl-1-propanone (API-180) (74 g/L) or 3-(4-benzoylphenoxy)-2-hydroxy-N,N,N-trimethyl-1-propanaminium chloride (Q-BPQ) (54 g/L), but greatly higher than that of Irgacure 2959 (5 g/L) (See structures in Figure 6). A remarkable stability in deionized water was also evidenced for the TPO and the BAPO salts since no decomposition was detected after one month of storage in the dark. Besides, a lower stability was determined in basic conditions (pH = 11) for all photoinitiators. Examination of their biocompatibilities with mouse fibroblast cells L929 revealed the cytotoxicity of BAPO and TPO salts to be in a similar range than the state-of-the-art photoinitiators (API-180, Q-BPQ or 2-hydroxy-1-[4-(2hydroxyethoxy) phenyl]-2-methyl-1-propanone (Irgacure 2959) (See Figure 7). More precisely, cytotoxicities of LC50 = 2.8 mmol/L for BAPO-ONa and LC50 = 2.8 mmol/L for BAPO-OLi were determined. Polymerization tests done on N-acryloylmorpholine upon irradiation in the 320-500 nm range and 400-500 nm range revealed the TPO salts to outperform the BAPO salts, irrespective of the irradiation conditions. Noticeably, if significant differences of monomer conversions were determined in the 320-500 nm range, almost similar monomer conversions were obtained with the four salts upon irradiation in the 400-500 nm range (See Figure 8). In 2021, Chemtob and coworkers used a different strategy to provide water-solubility by quaternization of a dimethylamino group on BAPO (See Figure 9). [START_REF] Le | Water-Soluble Photoinitiators from Dimethylamino-Substituted Monoacylphosphine Oxide for Hydrogel and Latex Preparation[END_REF] Examination of the absorption properties of MAPO-3 revealed the dye to absorb in the same range than LiTPO.

Besides, a higher absorption at 385 nm (wavelength used for the polymerization tests) could be determined for MAPO-3 than LiTPO (476 M -1 .cm -1 for MAPO-3 vs 188 M -1 .cm -1 for LiTPO respectively). Despites these differences of absorption, LiTPO and MAPO-3 could produce identical polymerization profiles during the FRP of PEGDA at 385 nm and 420 nm (See Figure 10). Besides, all structures exhibited a low solubility in water (See Figure 11). [START_REF] Jauk | Photoinitiators with Functional Groups 9: New Derivatives of Covalently Linked Benzophenone-amine Based Photoinitiators[END_REF][START_REF] Ullrich | Photoinitiators with functional groups. IX. Hydrophilic bisacylphosphine oxides for acidic aqueous formulations[END_REF] Indeed, solubilities of 0.2, 1.03, 2.11 and 1.50 mg/L were only determined for compounds 1-4.

An opposite situation was found for PEG-BAPO since an exceptional water solubility of 428 g/L was determined for this compound. Stability of the formulations is often not evoked by the authors in their respective works. In 2001, Norrby and coworkers developed a dimethylacrylamide polymer copolymerized by a TPO-based monomer (PPI-2) (See Figure 11). [START_REF] De Groot | Hydrophilic Polymeric Acylphospine Oxide Photoinitiators/Crosslinkers for in Vivo Blue-Light Photopolymerization[END_REF] This polymer proved to be soluble in water but also extremely stable in solution. Indeed, after 22 months of storage, the polymerization of 2-hydroxyethyl methacrylate (HEMA) remained as efficient as for the freshly prepared solution of polymeric photoinitiator, in terms of polymerization exotherm and polymerization rate.

Alpha-hydroxyalkylphenone

Quaternization of amine has been extensively used in the past for the design of watersoluble alpha-hydroxyalkylphenone. [START_REF] Knaus | Photoinitiators with Functional Groups. Part IV. Water-Soluble Photoinitiators Containing Quaternary Ammonium Groups[END_REF] However, most of these photoinitiators could only be activated in the UV range. [START_REF] Rouillard | Methods for Photocrosslinking Alginate Hydrogel Scaffolds with High Cell Viability[END_REF][START_REF] Xie | Preparation and characterization of an amphiphilic macrophotoinitiator based on 2-hydroxyl-2-methyl-1-phenylpropanone[END_REF][START_REF] Roth | Highly Water-Soluble Alpha-Hydroxyalkylphenone Based Photoinitiator for Low-Migration Applications[END_REF][START_REF] Liu | High-Performance Lignin-Based Water-Soluble Macromolecular Photoinitiator for the Fabrication of Hybrid Hydrogel[END_REF][START_REF] Hill | Strategies for Red-Shifting Type I Photoinitiators: Internal Electric Fields versus Lewis Acids versus Increasing Conjugation*[END_REF][START_REF] Wu | Synthesis and Characterization of a Novel Kind of Water-Soluble Macromolecular Photoinitiators and Their Application for the Preparation of Water-Soluble Branched Polymers[END_REF][START_REF] Wu | Water-Soluble Branched Polyacrylamides Prepared by UV-Initiated Polymerization Using a Novel Kind of Water-Soluble Macromolecular Photoinitiator[END_REF][START_REF] Liska | Photoinitiators with functional groups. V. New water-soluble photoinitiators containing carbohydrate residues and copolymerizable derivatives thereof[END_REF] Still based on this strategy, a water-soluble derivative of 2-hydroxy-2-methyl propiophenone (Irgacure 1173) was recently proposed in the literature (See Figure 12). [START_REF] Aubry | Development of Water-Soluble Type I Photoinitiators for Hydrogel Synthesis[END_REF] This photoinitiator could notably initiate the polymerization of a benchmark organosoluble methacrylate resin (bisphenol A-glycidyl methacrylate (BisGMA)/triethylene glycol dimethacrylate (TEGDMA) (70/30 w/w) upon irradiation at 385 nm and under air in the presence of N-heterocyclic carbene borane as an additive. Especially, a polymerization profile identical to that obtained with Irgacure 2959 could be obtained using 1 as the photoinitiator. residues such as sucrose, glucose, maltose, 1-amino-D-sorbitol. [START_REF] Knaus | Photoinitiators with functional groups. III. Water-soluble photoinitiators containing carbohydrate residues[END_REF] In 2018, Sun and coworkers developed polysiloxane macrophotoinitiators comprising imidazolium units in their scaffolds (See Figure 13). [START_REF] Zhang | Regulating photochemical behavior and property of imidazolium-based water soluble polysiloxane macromolecular photoinitiators by anions[END_REF] As interesting features, photoinitiating ability of macrophotoinitiators [Si-di2959Im][X] could be finely tune by the careful selection of the anion (X = Br -, Ph4B -, TsO -, NO3 -, CF3COO -) . Thus, shorter induction time than that obtained with Irgacure 2959 could be determined for macrophotoinitiators comprising Ph4B-, CF3COO -and NO3 -as the anions during the FRP of TPGDA. Anions also drastically influenced the water solubility and the following order of water solubility was determined for [Si-di2959Im][X] : i.e. L-PEG-2959. [START_REF] Liu | High-Performance Lignin-Based Water-Soluble Macromolecular Photoinitiator for the Fabrication of Hybrid Hydrogel[END_REF] Advantages of lignin are numerous : abundance in nature, intrinsic biosafety, presence of multiple reactive sites. [START_REF] Duval | A review on lignin-based polymeric, micro-and nano-structured materials[END_REF] Additionally, the hyperbranched structure of lignin also enables lignin to act as reinforcing fillers in composites or hydrogels. [START_REF] Gillet | Lignin transformations for high value applications: towards targeted modifications using green chemistry[END_REF][START_REF] Wang | UV Cross-Linkable Lignin Thermoplastic Graft Copolymers[END_REF][START_REF] Li | Thermally Stable and Tough Coatings and Films Using Vinyl Silylated Lignin[END_REF] Hydrogels prepared with L-PEG-2959 showed excellent swelling properties and a waterretention ability of 78% could be determined. Due to the hyperbranched structure of lignin, extractability of photoinitiators was considerably reduced compared to the parent Irgacure 2959.

Ph4B -< Br -< TsO -< NO3 -< CF3COO -. In the case of [Si-di2959Im][CF3COO -],

Figure 13. Chemical structures of [Si-di2959Im][X], [Si-En-2959Im-A]

[TsO] and L-PEG-2959.

Chalcones, benzylidene ketones and cyclanones

The design of water-soluble dyes can constitute a hard work, especially if numerous synthetic steps are necessary to convert a water-insoluble dye as a water-soluble dye. Recently, an interesting approach was developed, consisting in the formation of a charge transfer complex with a water-soluble amine such as triethanolamine (TEOA). As a result of this intermolecular charge transfer, water solubility is provided to a water-insoluble dye without extensive synthetic works. Jointly, a redshift of the absorption can be obtained, enabling to design visible light photoinitiating system. This approach was notably applied to a bischalcone, namely 2,6-bis(furan-2-ylmethylidene)cyclohexan-1-one (BFC) (See Figure 14). [START_REF] Li | Visible light and water-soluble photoinitiating system based on the charge transfer complex for free radical photopolymerization[END_REF] Starting from a bis-chalcone absorbing at 373 nm, the absorption maximum could be shifted at 400 nm in acetonitrile, together with a broadening of the ICT band. A good solubility was found for the BFC/TEOA CTC since clear solutions could be obtained up to 5 wt% in water.

Besides, polymerization tests of AM, PEGDA and HEA (50% in water) done at 405 nm revealed the optimum concentration in water to be 3 wt%, an inner filter effect occurring at higher CTC concentrations. Investigation of the photochemical mechanism revealed the initiating process to result from a photoassisted hydrogen abstraction on TEOA cation, producing the initiating α-aminoalkyl radicals (See Scheme 1). en-1-one Chal-PEG (See Figure 14). [START_REF] Chen | Water-Soluble Visible Light Sensitive Photoinitiating System Based on Charge Transfer Complexes for the 3D Printing of Hydrogels[END_REF] Due to the presence of a short PEG chain, watersolubility of the chalcone was low, and a saturation concentration around 0.1 wt% was determined. Upon formation of the chalcone/TEOA CTC, a good water solubility could be found, up to 5 wt%, therefore similar to the previous bis-chalcone BFC. Polymerization tests done at 405 nm for PEGDA (50% in water) revealed the optimum concentration to be lower, at 2 wt% as a decrease of the monomer conversion and the polymerization rate could be determined upon increase of the CTC concentration in PEGDA. Introduction of carboxylate groups constitutes another strategy to provide water solubility to benzylidene ketones. This strategy was applied as soon as 2009 for the synthesis of two-photon photoinitiators. [START_REF] Wan | Water-soluble benzylidene cyclopentanone dye for two-photon photopolymerization[END_REF] A Two-Photon Absorption (TPA) cross-section of 287 GM at 800 nm was notably determined for BSEA in water. This TPA cross-section is larger than that reported for water-soluble xanthene dyes such as Rose Bengal, Eosin and Erythrosine (< 10 GM at 800 nm) (See Figure 15). [START_REF] Campagnola | 3-Dimensional Submicron Polymerization of Acrylamide by Multiphoton Excitation of Xanthene Dyes[END_REF] However, compared to its water insoluble BDEA analogue, a decrease by ca 50% of the TPA cross-section was determined for BSEA in water, consistent with the trend previously observed by Bazan and coworkers. [START_REF] Woo | Water-Soluble [2.2]Paracyclophane Chromophores with Large Two-Photon Action Cross Sections[END_REF][START_REF] Woo | Solvent Effects on the Two-Photon Absorption of Distyrylbenzene Chromophores[END_REF] A maximum absorption located at 513 nm was found for BSEA in water. [START_REF] Yang | Carboxylate modified benzylidene cyclopentanone dyes for one-and two-photon excited photodynamic therapy[END_REF] A low threshold energy of 0.51 mW was determined during the FRP of SR610 (80% in water). In the same spirit, Liska and coworkers investigated a series of benzylidene cyclanones varying by the size of the central ring but also by the length of the chains used to connect the carboxylate groups. [START_REF] Li | Initiation efficiency and cytotoxicity of novel watersoluble two-photon photoinitiators for direct 3D microfabrication of hydrogels[END_REF] Notably, size of the central ring drastically impacted the TPA cross section of photoinitiators and replacement of cyclopentanone in P2CK by cyclohexanone in E2CK increased the TPA cross-section from 176 GM at 800 nm up to 201 GM for E2CK. [START_REF] Li | A Straightforward Synthesis and Structure-Activity Relationship of Highly Efficient Initiators for Two-Photon Polymerization[END_REF] Here again, a reduction of the TPA cross-section of water soluble dyes compared to their water insoluble analogues was detected, consistent with the results of Bazan and coworkers.

Noticeably, reduction of the length of the alkyl chain in G2CK slightly reduced the TPA crosssection compared to E2CK (163 GM for G2CK compared to 201 GM at 800 nm for E2CK).

Influence of the central ring size was also demonstrated for the T1, BSEA, T3 series (See Figure 16). [START_REF] Huang | Study on a series of water-soluble photoinitiators for fabrication of 3D hydrogels by two-photon polymerization[END_REF] Indeed, TPA cross-sections of 567 GM, 808 GM and 231 GM were respectively determined for the T1, BSEA and T3. The lowest TPA cross-section was measured for the most flexible structure, namely T3 comprising a central cyclohexanone. BSEA proved to be the most promising candidate for one photon and two-photon polymerizations. A low cytotoxicity was also determined for BSEA, 60% of HepG2 cells being still alive after 24 hours of incubation and for the same concentrations than used during the photopolymerization experiments.

Thioxanthones

Thioxanthone is a benchmark photoinitiator that was extensively studied in the past due to its low cost, easy availability and its high polymerization efficiency. [START_REF] Karaca | Thioxanthone-benzothiophenes as photoinitiator for free radical polymerization[END_REF][START_REF] Balta | Thioxanthone-Diphenyl Anthracene: Visible Light Photoinitiator[END_REF][START_REF] Dadashi-Silab | Shining a light on an adaptable photoinitiator: advances in photopolymerizations initiated by thioxanthones[END_REF][START_REF] Eren | Photophysical and Photochemical Studies of Novel Thioxanthone-Functionalized Methacrylates through LED Excitation[END_REF][START_REF] Wu | Thioxanthone based one-component polymerizable visible light photoinitiator for free radical polymerization[END_REF][START_REF] Wu | High-Performance and Low Migration One-Component Thioxanthone Visible Light Photoinitiators[END_REF][START_REF] Wu | Visible light-emitting diode-sensitive thioxanthone derivatives used in versatile photoinitiating systems for photopolymerizations[END_REF][START_REF] Lalevée | New thioxanthone and xanthone photoinitiators based on silyl radical chemistry[END_REF] Rapidly, derivatization of this high-performance structure was examined and by chemical engineering, water-soluble structures could be obtained at the beginning of the 2000s (See Figure 17). For instance, inclusion of thioxanthones in cyclodextrin (TX-β-CD) enabled to elaborate photoinitiators exhibiting similar spectral characteristics and photoactivity to that of the parent thioxanthone. [START_REF] Balta | Chemical incorporation of thioxanthone into β-cyclodextrin and its use in aqueous photopolymerization of methyl methacrylate[END_REF] However, polymerization in water was possible with this structure. By a Diels-Alder [4 + 2] click chemistry strategy, a poly(ethylene glycol) chain could be introduced, enabling to design a one-component polymeric photoinitiator PEG-TX. [START_REF] Akat | Poly(ethylene glycol)-thioxanthone prepared by Diels-Alder click chemistry as one-component polymeric photoinitiator for aqueous free-radical polymerization[END_REF] Copolymerization of a thioxanthone-based monomer with two water-soluble co-monomers, namely acrylamide (AAm) and 2-acryloxyethyl trimethylammonium iodide (MI) enabled to generate water-soluble copolymers (TX3-co-AAm, TX3-co-Cl, TX4-co-AAm, TX5-co-AAm and TX6-co-AAm) that could be advantageously used for the polymerization of acrylamide in water. [START_REF] Corrales | Novel water soluble copolymers based on thioxanthone: photochemistry and photoinitiation activity[END_REF] The dendritic strategy was also examined to design water-soluble thioxanthones, as exemplified with DAB-16-TX and DAB-64-TX. [START_REF] Jiang | Water-compatible dendritic macrophotoinitiator containing thioxanthone[END_REF] By a simple acetalization process of polyvinyl alcohol, PVA-TX could be obtained. [START_REF] Kork | Poly(vinyl alcohol)-Thioxanthone as One-Component Type II Photoinitiator for Free Radical Polymerization in Organic and Aqueous Media[END_REF] Interestingly, photopolymerization could proceed even in absence of co-initiators. Indeed, PVA-TX possesses both the light sensitive thioxanthone and the hydrogen donating sites in its structure. By nucleophilic substitution, polyethylene glycol monomethyl ether chains of different lengths could be introduced as peripheral groups in TX-MPEGs. [START_REF] Wu | Thioxanthone-Based Hydrophilic Visible Photoinitiators for Radical Polymerization[END_REF] However, photopolymerization of acrylamide in water could only be carried out in the presence of a hydrogen donor, namely Nmethyldiethanolamine (MDEA). In order to increase the molar extinction coefficient in the visible range, the design of thioxanthone-fluorene conjugates was examined by Yagci and coworkers. [START_REF] Yilmaz | Thioxanthone-Fluorenes as Visible Light Photoinitiators for Free Radical Polymerization[END_REF] By introducing a carboxylate group on TX-FLCOONa, a water-soluble dye could be obtained. Conversely, under its neutral form i.e. TX-FLCOOH, this dye was only soluble in organic solvents and monomers. Noticeably, no polymerization was detected with TX-FLCOONa when used as a monocomponent photoinitiator. However, TX-FLCOONa could act as an efficient initiator in the presence of hydrogen donors in water. In 2008, Yin and coworkers developed a polymeric photoinitiator containing thioxanthone and glucamine i.e. PTX-GA and its photoinitiating ability was compared to that of low-molecular-weight reference compounds namely TX-MGA and TX-PMAC [START_REF] Jiang | A novel amphipathic polymeric thioxanthone photoinitiator[END_REF] (See Figure 18). [START_REF] Jiang | Water-Soluble Polymeric Thioxanthone Photoinitiator Containing Glucamine as Coinitiator[END_REF] Figure 18. Chemical structures of TX-PMAC, TX-MGA, PTX-GA and APTX.

Considering that the three compounds comprise the same chromophore i.e.

thioxanthone, similar absorption spectra were found in water. Besides, photoreactivity was different. First, use of N-methyldiethanolamine (MDEA) as a co-initiator was required for TX-PMAC contrarily to TX-MGA and PTX-GA which comprise glucamine. In this case, glucamine could act doubly, first as a co-initiator and as a water-solubilizing agent. Photoluminescence experiments revealed PTX-GA to exhibit a weaker emission than TX-PMAC and TX-MGA, evidencing a more effective quenching of the excited state of thioxanthone by the amino groups of the polymer chains, attributable to a higher concentration of amino groups in the microenvironment of thioxanthone. [START_REF] Aydin | Mechanistic Study of Photoinitiated Free Radical Polymerization Using Thioxanthone Thioacetic Acid as One-Component Type II Photoinitiator[END_REF] Polymerization tests done on acrylamide and N,N'methylenebisacrylamide (MBAM) confirmed the trend determined during the photolysis experiments. Thus, PTX-GA furnished the highest monomer conversions upon irradiation with a UV light, consistent with the strong intramolecular interaction existing between the thioxanthone units and glucamine. Conversely, due to the less efficient intermolecular interactions between TX-PMAC and MDEA, less radicals could be produced, resulting in the lowest acrylamide conversion with this two-component system. A similar trend of reactivity was determined for another amphipathic polymeric thioxanthone photoinitiator i.e. APTX (See Figure 18). A few years later, the same authors developed a water-soluble thioxanthone (hPEA-TX) and a water-soluble benzophenone (hPEA-BP) based on hyperbranched poly(ether amine) chains (See Figure 20). [START_REF] Li | Hyperbranched poly(ether amine) (hPEA) as novel backbone for amphiphilic one-component type-II polymeric photoinitiators[END_REF] This strategy is not new since multistimuli responsive polymer nanoparticles based on azobenzene were prepared as soon as 2010 with poly(ether amine)

chains. [START_REF] Yu | Multistimuli Responsive Polymer Nanoparticles On the basis of the Amphiphilic Azobenzene-Contained Hyperbranched Poly(ether amine) (hPEA-AZO)[END_REF][START_REF] Yu | Multistimuli-responsive hyperbranched poly(ether amine)s[END_REF] Water solubility and photoinitiating abilities were compared to that of small molecules comprising glucamine side-chains (i.e. MGA-TX and MGA-BP). Interest of the hPEA-TX and hPEA-BP structures was the concomitant presence of a poly(ethylene glycol) chain for water solubility and a tertiary amine acting as a co-initiator. Noticeably, hPEA-TX and hPEA-BP showed a good solubility in organic solvents but also in water. A water solubility of 10 wt% was determined for the two dyes. Photolysis experiments done upon irradiation at 365 nm revealed the photolysis rate to be fast (around 240 s) for hPEA-TX and hPEA-BP whereas almost no photolysis could be detected for MGA-TX and MGA-BP in the same conditions. A different situation was found for the twocomponent MGA-TX/TEOA and MGA-BP/TEOA systems, consistent with a mechanism including the formation of a radical formed by the carbonyl compound (ketyl-type radical)

and another one on the amine. During the FRP of acrylamide in water, hPEA-TXs and hPEA-BPs exhibited higher heat flows and faster polymerization rates than the reference Irgacure 2959 at early stage. Noticeably, hPEA-TXs slightly outperformed hPEA-BPs in terms of final monomer conversion, attributable to the higher molar extinction coefficient of hPEA-TXs at 365 nm. Excellent conversions could also be obtained during the FRP of organosoluble hexanediol diacrylate (HDDA), trimethylolpropane triacrylate (TMPTA) and pentaerythritol tetraacrylate (PETTA), photopolymerization of these multifunctional monomers providing monomer conversions as high as 99, 80 and 60% respectively. In this case, monomer conversions obtained with hPEA-TXs and hPEA-BPs were greatly higher than that obtained with the low-molecular weight photoinitiators MGA-TX and MGA-BP. Overall, during the past decades, a significant effort has been devoted to develop water-soluble thioxanthones. By developing polymeric structures, a better solubility of thioxanthone in resins could be obtained. A reduction of the extractability of thioxanthone can also be achieved, polymerbased thioxanthones being intimately mixed with the polymer chains. This property is highly researched, especially for biological and food packaging applications. [START_REF] Matsushima | Non-extractable photoinitiators based on thiol-functionalized benzophenones and thioxanthones[END_REF][START_REF] Chen | Novel Multifunctional Polymeric Photoinitiators and Photo-Coinitiators Derived from Hyperbranched Polyglycerol[END_REF] Over the years, numerous strategies were developed to render thioxanthone a watersoluble photoinitiator. For instance, introduction of an imidazolium group on [AIPTX]Cl enabled to reduce the extractability of the photoinitiator while introducing a watersolubilizing imidazolium group (See Figure 21). [START_REF] Qiu | Water-soluble and polymerizable thioxanthone photoinitiator containing imidazole[END_REF] Interestingly, in these systems, the imidazole ring was used as a water-soluble hydrogen donor, thus acting as a co-initiator during photopolymerization. Indeed, thioxanthone is well-known to mostly participate to hydrogen atom abstraction events. [START_REF] Nikitas | Thioxanthone: a powerful photocatalyst for organic reactions[END_REF] By mean of a photoreduction reaction, ketyl radicals and imidazolyl free radicals could be produced, the latter being able to initiate a polymerization process. In the case of IPTX, the electron and proton transfer resulted from an intramolecular electron transfer whereas for [AIPTX]Cl, imidazole had to be introduced as an additive, enabling first to induce an intermolecular electron transfer followed by an intramolecular proton transfer (See Scheme 2). In 2012, Yagci and coworkers proposed a thioxanthone salt PEO-NH + (Me)2 TX-CH2COO -possessing a poly(ethylene oxide) dimethylammonium counter-anion that could act as a hydrogen donating group (See Figure 21 and Scheme 3). [START_REF] Yilmaz | Counteranion Sensitization Approach to Photoinitiated Free Radical Polymerization[END_REF] Interestingly, this polymeric photoinitiator could be prepared in one step, by a simple acid/base reaction between the dimethylamino-functionalized poly(ethylene oxide) and the acid-substituted thioxanthone, enabling to produce the salt in 94% yield. Introduction of carboxylic groups on the thioxanthone scaffold is not a novel approach since the first reports in this field were published by Fouassier and coworkers in 1988, [START_REF] Lougnot | Comparative reactivity of water soluble photoinitiators as viewed in terms of excited states processes[END_REF][START_REF] Lougnot | Water-soluble polymerization initiators based on the thioxanthone structure: a spectroscopic and laser photolysis study[END_REF] but other research groups later examined this approach. [START_REF] Aydin | Mechanistic Study of Photoinitiated Free Radical Polymerization Using Thioxanthone Thioacetic Acid as One-Component Type II Photoinitiator[END_REF][START_REF] Esen | One-Component Thioxanthone Acetic Acid Derivative Photoinitiator for Free Radical Polymerization[END_REF] Scheme 3. Mechanism involved with PEO-NH + (Me)2 TX-CH2COO - By introducing a thioacetic acid group or a carboxymethoxy group on thioxanthone, initiating radicals could be directly generated on photoinitiators (1A and 2A), as shown in the Scheme 4. [START_REF] Balta | Thioxanthone based water-soluble photoinitiators for acrylamide photopolymerization[END_REF] Due to the decarboxylation reaction occurring on 1A and 2A, the two photoinitiators could be used as monocomponent photoinitiators. Additionally, using this approach, thioxanthone could be connected to the polymer chains, drastically limiting its extractability.

Scheme 4. X = O, S 1A : X = O, 2A : X = S On her side, Avci and coworkers examined the introduction of bisphosphonic acidgroups as water-solubilizing agents for thioxanthone. This strategy was notably applied to TXBP [START_REF] Eren | Bisphosphonic Acid-Functionalized Water-Soluble Photoinitiators[END_REF] and TXBP2 [START_REF] Eren | A water soluble and highly reactive bisphosphonate functionalized thioxanthone-based photoinitiator[END_REF] both exhibiting an absorption maximum located at ca 400 nm. If the bisphosphonic acid-groups do not influence the absorption properties of the dyes, major differences in terms of water solubility could be evidenced. Thus, a remarkable water solubility could be determined for TXBP2, peaking at 86 g/L, far from that reported for Irgacure 2959 (5 g/L) and obtained for TXBP (0.05 g/L). Presence of multiple bisphosphonic acid-groups on TXBP2 was thus beneficial for greatly improving the water-solubility. For comparison, a benzophenone-based analogue BPBP was prepared and exhibited a water-solubility of 28 g/L (See Figure 22). Compared to the previous carboxylic acid derivatives for which a decarboxylation reaction could produce radicals, BPBP, TXBP and TXBP2 could not behave as monocomponent systems and these structures were used in combination with an iodonium salt or an amine (ethyl dimethylaminobenzoate (EDB)). As drawback of this approach, lower monomer conversions were obtained when combined with EDB than with the iodonium salt, attributable to an acid/base reaction occurring between the phosphonic acid groups and the amine. The development of watersoluble polymers containing thioxanthone was also examined, as exemplified with p(PEGDA575-TX) that could decompose without additives (See Figure 23). [START_REF] Gencoglu | A Water Soluble, Low Migration, and Visible Light Photoinitiator by Thioxanthone-Functionalization of Poly(ethylene glycol)-Containing Poly(β-amino ester)[END_REF]. Indeed, as shown in the Scheme 5, p(PEGDA575-TX) can undergo an intramolecular or an intermolecular photoreduction by the tertiary amines or the ether groups in its structure. A low extractability from the resulting polymers was determined, around 13%, greatly lower than that of thioxanthone (66%). A similar mechanism was proposed for another thioxanthone-based polymer i.e. PAATX. [START_REF] Eren | Water soluble polymeric photoinitiator for dual-curing of acrylates and methacrylates[END_REF] A low extractability was also evidenced for this photoinitiator, being three-times lower than that of thioxanthone. Finally, the same authors developed an original strategy to prepare a water-soluble macrophotoinitiator starting from poly(styrene). [START_REF] Temel | One-pot synthesis of water-soluble polymeric photoinitiator via thioxanthonation and sulfonation process[END_REF] By mean of a simultaneous thioxanthonation and sulfonation reaction, PSt-TX-WS could be prepared in one step according to the mechanism depicted in the Scheme 6. Scheme 6. One-pot synthesis of PSt-TX-WS.

Naphthalimides

Naphthalimides were extensively used as visible light photoinitiators due to their high photoluminescence quantum yields, the facile modification of the naphthalimide scaffold in order to provide the desired properties. In 2015, Lalevée and coworkers introduced an amino group onto NDP2 so that this dye could be encapsulated in 2,6-di-O-Me-β-cyclodextrin (See Figure 24). [START_REF] Zhang | Structure Design of Naphthalimide Derivatives: Toward Versatile Photoinitiators for Near-UV/Visible LEDs, 3D Printing, and Water-Soluble Photoinitiating Systems[END_REF] A slight blueshift of the absorption maximum was found after inclusion of NDP2 in cyclodextrin, peaking at 407 nm in water contrarily to 417 nm in acetonitrile. As drawback of this approach, a reduction of the polymerization efficiency during the cationic polymerization of EPOX at 405 nm was found for the two-component NDP2-CD/Iod system compared to its NDP2/Iod analogue, assigned to a confinement of the NDP2 +• radicals inside the cyclodextrin, therefore lowering its reactivity. This issue was addressed with Napht-4 using 4-((6-bromo-1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)methyl)cyclohexane-1-carboxylic acid as a water solubilizing agent. Precisely, the water-solubility could be obtained while converting Napht-4 as a sodium salt, the neutral form exhibiting only a weak watersolubility. [START_REF] Rahal | Naphthalimide-Based Dyes as Photoinitiators under Visible Light Irradiation and their Applications: Photocomposite Synthesis, 3D printing and Polymerization in Water[END_REF] Photopolymerization of a polyethylene glycol diacrylate/water blend could be carried out at 405 nm using the two-component Napht-4/triethanolamine (0.1%/1% w/w) system. Noticeably, by UV-visible absorption spectroscopy, a hyperchromic effect was detected in water for Napht-4 compared to acetonitrile (ε = 7000 M -1 .cm -1 in water vs. ε = 2570 M -1 .cm -1 in acetonitrile) and this trend is rarely observed between organo-soluble and watersoluble dyes. By varying the substituent onto the naphthalimide core and by introducing a silver salt inside the photocurable resins, antibacterial hydrogels could be prepared with dye 8 and dye 9, by generating in situ silver nanoparticles. [START_REF] Chen | Water-soluble/visible-light-sensitive naphthalimide derivative-based photoinitiating systems: 3D printing of antibacterial hydrogels[END_REF] Indeed, part of the radicals generated by the three-component dye/iodonium salt/amine system could be used to reduce the silver salt during photopolymerization, enabling to get a homogenous distribution of silver nanoparticles inside the polymer. However, a high polydispersity was also found between nanoparticles, ranging from small particles to aggregates. Noticeably, an optimal antibacterial activity was found for all resins initially containing a concentration of 4 wt% of silver nitrate.

Other structures

Naphthalimides were not the only hydrophobic dyes to be included by mean of a hostguest strategy into water-soluble cavities in order to produce water soluble initiators. Thus, the hydrophobic 2,7-bis(2-(4-pentaneoxyphenyl)vinyl)anthraquinone (N) could be immobilized into 2-hydroxypropyl-β-cyclodextrin, and furnished a two-photon polymerization initiator exhibiting a two-photon absorption cross-section of 200 GM at 780 nm. [START_REF] Xing | A water soluble initiator prepared through host-guest chemical interaction for microfabrication of 3D hydrogels via two-photon polymerization[END_REF] A drastic increase of the two-photon absorption cross-section could be obtained with bis [2-(1-methylpyridinium)vinyl]-9-methylcarbazole diiodide (BMVMC) upon complexation with cucurbit [START_REF] Yuan | Low volume shrinkage photopolymerization system using hydrogen-bond-based monomers[END_REF]uril (CB7). [START_REF] Gao | Ionic Carbazole-Based Water-Soluble Two-Photon Photoinitiator and the Fabrication of Biocompatible 3D Hydrogel Scaffold[END_REF] Thus, the two-photon absorption cross section of the complex increased up to 2500 GM whereas a value of only 750 GM was determined for the uncomplexed BMVMC at 830 nm. During the FRP of PEGDA in water, a resolution of 180 nm and a laser threshold of only 3.7 mW was determined. A slightly higher laser threshold of 4.5 mW was found for the BMVPC-CB7 inclusion complex differing from BMCMC-CB7 by the length of the alkyl chain introduced onto the carbazole moiety (See Figure 25). [START_REF] Zheng | Cucurbit[7]uril-Carbazole Two-Photon Photoinitiators for the Fabrication of Biocompatible Three-Dimensional Hydrogel Scaffolds by Laser Direct Writing in Aqueous Solutions[END_REF] Figure 25. Chemical structures of N, BMVMC and BMVPC.

A more common strategy to design water-soluble dyes consisted in introducing polyethylene glycol chains, poly(hydroxy) or polysiloxane chains. Some dyes are naturally substituted with water-soluble chains such as Riboflavin (vitamin B2) (See Figure 26). [START_REF] Nguyen | Two-photon polymerization of polyethylene glycol diacrylate scaffolds with riboflavin and triethanolamine used as a water-soluble photoinitiator[END_REF] In combination with triethanolamine, biocompatible polymers could be prepared. In 2013, Sun and coworkers modified a benzophenone with polysiloxane chains. [START_REF] Wang | Self-floating ability and initiating gradient photopolymerization of acrylamide aqueous solution of a water-soluble polysiloxane benzophenone photoinitiator[END_REF][START_REF] Zhang | Photoinitiability of a water-borne polysiloxane-modified benzophenone macromolecular photoinitiator[END_REF] However, it has to be noticed that introduction of ammonium substituents on benzophenone is not new since this strategy was used as soon as 1988 by Green and coworkers. [START_REF] Allen | Photochemistry of novel watersoluble para-substituted benzophenone photoinitiators: A polymerization, spectroscopic and flash photolysis study[END_REF] Various silicon contents were used in W-Si-HBP2 -A/B/C and a decrease of the water solubility upon elongation of the silicone chain length was detected, assigned to the hydrophobicity of polysiloxane. In water, longer triplet excited state lifetimes were found for all polysiloxane benzophenone photoinitiators compared to the parent 4-hydroxybenzophenone. Similarly, increase of the silicone chain length obviously influenced the triplet lifetimes of polymeric photoinitiators, resulting in a decrease of the excited state lifetimes upon elongation of the chain. It has to be noticed that water-soluble UV photoinitiators were previously reported in the literature, as exemplified with polymethylphenylsilane-co-poly(polyethyleneglycol acrylate) (compound 6). [START_REF] Liang | A facile method to prepare a polyethylene glycol modified polysilane as a waterborne photoinitiator[END_REF] Benzophenone derivatives were the focus of numerous studies for improving their water solubilities. Thus, a series of amphiphilic polymeric Michler's ketone photoinitiators (APMKs) were prepared by incorporating PEO short chains, the Michler's ketone units and the coinitiator amine (piperazine) within a unique polymeric chain. [START_REF] Wen | Amphiphilic polymeric Michler's ketone (MK) photoinitiators (APMKs) containing PEO chain and coinitiator amine[END_REF] Investigation of the Besides, the two-photon absorption cross-section determined at 800 nm was only of 120 GM for WSPI, far from that reported for inclusion complexes prepared with 2,7-bis(2-(4pentaneoxyphenyl)vinyl)anthraquinone (N), BMVMC and BMVPC. However, this value is comparable to that previously reported for the parent 4,4'-dialkylamino-bis(styryl)benzene derivatives. [START_REF] Woo | Solvent Effects on the Two-Photon Absorption of Distyrylbenzene Chromophores[END_REF][START_REF] Cumpston | Two-photon polymerization initiators for threedimensional optical data storage and microfabrication[END_REF] Quaternization of dimethylamino groups is not new since Allen and coworkers reported in 2001 the synthesis of a water-soluble anthraquinone, namely 2-(N,N,Ntrimethylammoniumpropoxy)anthraquinone chloride (AQW). [START_REF] Catalina | Synthesis, photochemical and photoinitiation activity of water-soluble copolymers with anthraquinone chromophores as side-chain groups[END_REF] Hydrogels containing up to 80% water could be obtained at the fast scanning speed of 10 mm/s and for a laser power of 140 mW. The conversion of dialkylamino group as ammonium salt is not still necessary.

Indeed, a diketopyrrolopyrrole group bearing dimethylamino groups exhibiting an excellent water solubility. [START_REF] Bouzrati-Zerelli | New violet to yellow light sensitive diketo pyrrolo-pyrrole photoinitiators: high performance systems with unusual bleaching properties and solubility in water[END_REF] The two-component DKPP1/N-methyldiethanolamine (MDEA) 0.15%/8% w/w system could led to an efficient polymerization of 2-hydroxyethyl acrylate (HEA) in water. A hydrogel containing 74% water while starting from a formulation of HEA only containing 40% water could be obtained upon irradiation at 470 nm. In 2020, Lalevée and coworkers reported a small molecule i.e. 2-oxo-2(tert-butyldimethylsilyl)acetic acid (DKSi-COOH) that showed an excellent water-solubility. [START_REF] Kirschner | Lalevée, 2-Oxo-2(tertbutyldimethylsilyl)Acetic Acid (DKSi-COOH) as a New Water-Soluble Visible Light Type I Photoinitiator for Free Radical Polymerization[END_REF] Interestingly, a good absorption in the visible range was found for this phenyl-free photoinitiator, and the design of phenyl-free photoinitiators remains scarce in the literature. [START_REF] Tang | A high performance phenyl-free LED photoinitiator for cationic or hybrid photopolymerization and its application in LED cationic 3D printing[END_REF] Besides, a broad absorption band extending between 400 and 500 nm and an absorption maximum at 442 nm could be determined for DKSI-COOH in acetonitrile. In fact, theoretical calculations revealed the Si-d orbitals to participate to the frontier orbitals of DKSI-COOH, enabling to lower the HOMO-LUMO gap and providing an absorption band in the visible range. A moderate solubility was however determined for DKSI-COOH in water since a solubility as low as 10 -2 M was measured. A good stability in acidic conditions was also determined since after 12 weeks in a methacrylic acid/water blend, no modification of the absorption spectrum was detected. As interesting feature, DKSI could be used as a monocomponent system and excellent photobleaching properties could also be observed during the polymerization of a BisGMA/TEGDMA blend. This ability is actively researched for numerous applications requiring colorless coatings. [START_REF] Dumur | Recent advances on photobleachable visible light photoinitiators of polymerization[END_REF] Figure 26. Chemical structures of various water-soluble photoinitiators.

Another group which seems to be promising for elaborating water-soluble dyes is the sulfonate group. An example of coumarin-disulfonate (CoumB) was recently reported in the literature. [START_REF] Abdallah | Coumarin derivatives as versatile photoinitiators for 3D printing, polymerization in water and photocomposite synthesis[END_REF] As drawback of this approach, sulfonation using the SO3.DMF complex is only applicable to dyes comprising a thiophene unit in their scaffolds, therefore drastically limiting the scope of application of this reaction. A more versatile approach was developed by Nie and coworkers that introduced a keto-acid group on a series of photoinitiators (BFA, TF-BFA and CC-BFA). [START_REF] He | Water-soluble benzoylformic acid photoinitiators for water-based LED-triggered deep-layer photopolymerization[END_REF] In this case, an oxidation of acetyl group using selenium oxide as the oxidizing agent in pyridine at reflux was used, enabling to get the different photoinitiators with reaction yields ranging between 62 and 80%. If this approach is appealing, it was only investigated for the design of UV-photoinitiators, the absorption of BFA, TF-BFA and CC-BFA not exceeding 350 nm.

Conclusion

The design of water-soluble dyes is an active research field since almost 30 years. Despites the intense research efforts, the design of water-soluble photoinitiators has mainly been successful for UV photoinitiators. Concerning water-soluble visible light photoinitiating systems, their properties and applications in aqueous media have not been fully explored yet. If numerous groups such as poly(ethylene glycol) chains, ammonium groups or carboxylate groups were extensively used, innovative strategies have been developed during the last few years. Thus, sulfonation of thiophene-based dyes, introduction of benzoylformic groups have recently been successfully used for designing visible light photoinitiating systems. In these different cases, if the water-solubility was obtained, this latter remained however limited when using benzoylformic groups.

Future research outlook

With aim at designing highly soluble dyes in water, the most promising strategy seems to consist in introducing multiple bisphosphonic acid-groups. Several groups also succeeded to introduced water-solubilizing chains also acting as co-initiators so that the resulting photoinitiators could be used as monocomponent photoinitiating systems. If water-solubility was examined for visible dyes, the combination of water-solubility and photobleaching has not been extensively investigated. Future works will consist in combining water-solubility and photobleaching ability in order to design photoinitiators perfectly fitting the industrial requirements. Recent works have notably evidenced that photobleaching could be efficiently obtained with chalcones or push-pull dyes, suppression of the π-conjugation between the donor and the acceptor in push-pull dyes enabling to suppress the color. [START_REF] Dumur | Recent advances on photobleachable visible light photoinitiators of polymerization[END_REF] Similarly, in chalcones, suppression of the electron acceptor proved to be an efficient strategy to get an efficient photobleaching. Introduction of water-soluble substituents on these structures could enable to combine water solubility and photobleaching.

Figure 2 .

 2 Figure 2. Chemical structures of water-soluble Type I photoinitiators.

Figure 3 .

 3 Figure 3. Chemical structures of LiTPO, NaTPO and Mg0.5TPO and mechanism of photocleavage.

Figure 4 .

 4 Figure 4. Chemical structures of water-soluble BAPO derivatives.

Figure 5 .Figure 6 .

 56 Figure 5. Comparison of the water solubility of BAPO and TPO salts with benchmark photoinitiators (API-180, Irgacure 2959 and Q-BPQ).Reproduced with permission of Ref.[START_REF] Benedikt | Highly efficient water-soluble visible light photoinitiators[END_REF]

Figure 7 .Figure 8 .

 78 Figure 7. Cytotoxicity tests done on mouse fibroblast cells L929 for BAPO and TPO salts and reference compounds. Reproduced with permission of Ref.[286]

Figure 9 .

 9 Figure 9. Chemical structures of MAPO-3.

Figure 10 .

 10 Figure 10. Monomer conversions determined during the FRP of PEGDA upon irradiation at 385 nm (left) and 420 nm (right). Reproduced with permission of Ref[288].Several groups also investigated the introduction of poly(ethylene glycol) chains as a tool to provide water-solubility to BAPO derivatives. In 2005, Liska and coworkers developed a series of BAPO derivatives with PEG chains introduced at different positions of the BAPO scaffold. Besides, all structures exhibited a low solubility in water (See Figure11).[START_REF] Jauk | Photoinitiators with Functional Groups 9: New Derivatives of Covalently Linked Benzophenone-amine Based Photoinitiators[END_REF][START_REF] Ullrich | Photoinitiators with functional groups. IX. Hydrophilic bisacylphosphine oxides for acidic aqueous formulations[END_REF]

[ 291 ]

 291 Improved solubility of PEG-BAPO compared to that of 1-4 can be assigned to the long PEG chains used in PEG-BAPO (Mn = 950) whereas an oligomeric 2-(2-methoxyethoxy)ethoxy chain was only used in 1-4, insufficient to provide water solubility. A good storage stability, a high photoreactivity during the FRP of hydrogels (50 wt% water of poly(ethylene glycol)diacrylate (PEGDA)) under visible-light irradiation (460 nm) was evidenced for PEG-BAPO. Especially, comparisons done during the FRP of PEGDA at 460 nm revealed PEG-BAPO to outperform BAPO-ONa and BAPO-OLi in terms of monomer conversions but also of polymerization rates. Notably, a severe reduction of the inhibition time was determined by using PEG-BAPO.

Figure 11 .

 11 Figure 11. BAPO derivatives bearing PEG chains.

Figure 12 .

 12 Figure 12. Chemical structures of 1.While examining the polymerization of a water-soluble mixture of monomers i.e. the ethoxylated trimethylolpropane triacrylate (ETPT)/PEGDA (1/2 w/w) blend, a different situation was found at 405 nm and the two-component system based on 1 could furnish a high monomer conversion contrarily to the two-component system based on Irgacure 2959 for which no polymerization could be detected in water. The superiority of 1 compared to Irgacure 2959 in combination with N-carbene borane was thus demonstrated. To the best of our knowledge, 1 is the only alpha-hydroxyalkylphenone ever reported in the literature capable to initiate a polymerization process at 405 nm.Attempts to provide water solubility to 2-hydroxy-1-[4-(2-hydroxy-ethoxy)phenyl]-2methylpropan-1-one (Irgacure 2959) is not new since pioneering works have been reported as soon as 1995 by Gruber and coworkers with the bonding of Irgacure 2959 to carbohydrates

  a three-fold improvement of the water solubility was measured compared to that of Irgacure 2959 (3.8 wt% vs. 1.2 wt% for Irgacure 2959). By elongating the poly(ethylene glycol) chain in the monofunctional macrophotoinitiator ([Si-En-2959Im-A][TsO], a significant improvement of the water solubility could be determined for the three macrophotoinitiators. Thus, the solubility increased from 2 wt% to 7 wt% and 12 wt% for [Si-En-2959Im-A][TsO] (n = 1, 3, 5) respectively.[303] Interestingly, due to the presence of imidazolium groups in the polymer backbone, polymer films prepared with [Si-E1-2959Im-A][TsO] exhibited an antibacterial activity higher than that of [Si-E3-2959Im-A][TsO] and [Si-E5-2959Im-A][TsO]. Conversely, no antibacterial activity was detected for coatings prepared with Irgacure 2959, evidencing the interest of introducing imidazolium groups on photoinitiators. Finally, in 2019, Zhao and coworkers reported for the first time a biosourced water-soluble photoinitiator based on lignin

Figure 14 .Scheme 1 .

 141 Figure 14. Chemical structures of BFC, TEOA and Chal-PEG.

Figure 15 .

 15 Figure 15. Chemical structure of BSEA and reference compound BDEA, P2CK, G2CK and E2CK.

Figure 16 .

 16 Figure 16. Chemical structures of T1, BSEA and T3.

Figure 17 .

 17 Figure 17. Various chemical structures of water-soluble thioxanthone derivatives.

[ 324 ]

 324 APTX proved to be very efficient for photoinitiation of acrylate monomers such as 2,2-bis[4-(acryloxypolyethoxy)phenyl] propane (ABPE-10), trimethylolpropane triacrylate (TMPTA) and acrylamide (AM) in water. In comparison to the low-molecular weight photoinitiators DETX and TX-PMAC, a higher monomer conversion could be obtained with APTX, irrespective of the monomers. In the same spirit, hyperbranched structures AHPTXs based on hyperbranched poly(ethylene imine) (HPEI) were reported in the literature (See Figure19).[START_REF] Wen | Amphipathic hyperbranched polymeric thioxanthone photoinitiators (AHPTXs): Synthesis, characterization and photoinitiated polymerization[END_REF] Here again, compared to the photoinitiating systems based on low-molecular weight structures such as ETX or TX-MGA, AHPTX1 could outperform the twocomponent systems 2-(2,3-epoxypropyloxy)thioxanthone/triethylamine (ETX/TEA) and TX-MGA/TEA systems, during the FRP of A-BPE-10 and acrylamide in aqueous solutions. The remarkable photoinitiating ability of AHPTX1 was ascribed to an efficient intramolecular hydrogen abstraction between the thioxanthone units and the amino groups of co-initiator, facilitating radical generation. On the opposite, intermolecular interactions between ETX, TX-MGA and triethylamine is less favorable, the viscosity of the resins and the distance between the two partners adversely affecting intermolecular interactions.

Figure 19 .

 19 Figure 19. Chemical structures of ETX and AHPTX1.

Figure 20 .

 20 Figure 20. Water-soluble thioxanthone based on hyperbranched poly(ether amine) chains.

Figure 21 .Scheme 2 .

 212 Figure 21. Chemical structures of IPTX, [AIPTX]Cl and PEO-NH + (Me)2 TX-CH2COO -

Figure 22 .

 22 Figure 22. Chemical structures of BPBP, TXBP and TXBP2.

Figure 23 .Scheme 5 .

 235 Figure 23. Chemical structures of PAATX and p(PEGDA575-TX).

Figure 24 .

 24 Figure 24. Chemical structures of NDP2, Napht-4, Dye 8 and Dye 9.

  composition of APMKs revealed the Michler's ketone/PEO/piperazine 2:3:1 ratio was required to generate water-soluble photoinitiators. By increasing the POE content, an improvement of the monomer conversion was obtained. An absorption maximum at 357 nm could be determined for all photoinitiators in water, irrespective of the composition. Results showed

	that APMKs could efficiently initiate the polymerization of hydrophilic and hydrophobic
	monomers. Notably, the polymerization of the water-soluble polyethylene glycol diacrylate
	(PEGDA), and the oil-soluble phenoxy ethyl acrylate (AMP-10G) and 2,2-bis[4-
	(acryloxypolyethoxy)phenyl]propane (A-BPE-10) monomers were examined in this work. By
	increasing the POE content in APMKs, an improvement of the PEGDA conversion was
	detected, assigned to an enhanced flexibility of the APMKs backbone and a higher content of
	coinitiator amino groups, facilitating radicals generation. Conversion higher than 90% could
	be determined by photoDSC. A good water solubility could also be provided by introducing
	ammonium groups onto dyes, as exemplified with 1,4-bis(4-(N,N-bis(6-(N,N,N-
	trimethylammonium)hexyl)amino)-styryl)-2,5-dimethoxybenzene tetraiodide (WSPI).[353]

Acknowledgments

Aix Marseille University and the Centre National de la Recherche Scientifique (CNRS) are acknowledged for financial supports.

Conflicts of Interest

The authors declare no conflict of interest.