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

Recent Advances on Visible Light Phenothiazine-based Photoinitiators of Polymerization

Keywords: Photoinitiator, phenothiazine, photopolymerization, LED, low light intensity

Visible light photoinitiators of polymerization are actively researched as these molecules can advantageously address the light penetration issue within the photocurable resins. With aim at developing dyes using a scaffold that can be easily modified, phenothiazine is one of those. In this review, an overview of the recent developments concerning phenothiazine-based photoinitiators of polymerization is provided. Over the years, structures of photoinitiators have greatly evolved and phenothiazine is now exclusively included in photoinitiators of more complex structures and not used alone as during the pioneering works.

Using this unprecedented approach, Type I and Type II photoinitiators have been designed and synthesized. To evidence the pertinence of the approach, comparisons with reference photoinitiating systems will be provided.

Introduction

During the past decade, photopolymerization under low light intensity and in the visible range has become an active research field. [START_REF] Lalevée | Recent Developments of Versatile Photoinitiating Systems for Cationic Ring Opening Polymerization Operating at Any Wavelengths and under Low Light Intensity Sources[END_REF][START_REF] Tehfe | Photopolymerization Reactions: On the Way to a Green and Sustainable Chemistry[END_REF][START_REF] Xiao | Visible light sensitive photoinitiating systems: Recent progress in cationic and radical photopolymerization reactions under soft conditions[END_REF][START_REF] Sun | Organic dye-based photoinitiating systems for visible-light-induced photopolymerization[END_REF][START_REF] Lalevée | Metal and metal-free photocatalysts: mechanistic approach and application as photoinitiators of photopolymerization[END_REF][START_REF] Xiao | Cationic and Thiol-Ene Photopolymerization upon Red Lights Using Anthraquinone Derivatives as Photoinitiators[END_REF][START_REF] Tehfe | Cationic photosensitive formulations based on silyl radical chemistry for green and red diode laser exposure[END_REF][START_REF] Garra | Redox two-component initiated free radical and cationic polymerizations: Concepts, reactions and applications[END_REF][START_REF] Zhang | Radical photoinitiation with LEDs and applications in the 3D printing of composites[END_REF][START_REF] Dumur | Recent Advances on Visible Light Metal-Based Photocatalysts for Polymerization under Low Light Intensity[END_REF][START_REF] Pigot | Recent advances on push-pull organic dyes as visible light photoinitiators of polymerization[END_REF][START_REF] Lalevée | Light-Harvesting Organic Photoinitiators of Polymerization[END_REF][START_REF] Shanmugam | Photocontrolled Living Polymerization Systems with Reversible Deactivations through Electron and Energy Transfer[END_REF] Search for a new generation of photoinitiators is supported by the fact that UV light is more and more contested in photopolymerization due to the adverse health effects associated to this range of wavelengths (blinding eye disease, skin cancer, …). In fact, disinterest for UV photopolymerization is the combination of three main parameters. UV irradiation setups are expensive and consume a lot of energy, what entail running costs. Light penetration within the photocurable resins is also extremely limited (below 600 µm), impeding the polymerization of thick or filled samples. [START_REF] Bonardi | High Performance Near-Infrared (NIR) Photoinitiating Systems Operating under Low Light Intensity and in the Presence of Oxygen[END_REF][START_REF] Garra | Photopolymerization processes of thick films and in shadow areas: a review for the access to composites[END_REF] To end, ozone is also produced during the polymerization process, what constitutes an additional drawback of UV photopolymerization. [START_REF] Shao | Visible light initiating systems for photopolymerization: status, development and challenges[END_REF] Besides, photopolymerization remains an appealing polymerization technique which exhibit several advantages compared to the traditional thermal polymerization. Notably, photopolymerization can be carried out in solvent-free conditions, avoiding the release of volatile organic compounds (VOCs). [START_REF] Tehfe | Photopolymerization Reactions: On the Way to a Green and Sustainable Chemistry[END_REF] A temporal and a spatial control can also be obtained, paving the way towards numerous practical applications. [START_REF] Zhao | Spatiotemporal control of polymer brush formation through photoinduced radical polymerization regulated by DMD light modulation[END_REF][START_REF] Xi | Spatial and Temporal Control of Thiol-Michael Addition via Photocaged Superbase in Photopatterning and Two-Stage Polymer Networks Formation[END_REF] Among applications, coatings, adhesives, dentistry, jewellery, microelectronics, spare parts and prototypes prepared by 3D printing can be cited as the most common ones. [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] Besides, in order to develop photoinitiating systems usable in these different applications, dyes absorbing in the visible range are required. To get this result, several options are available. Notably, redshift of the absorption spectra of traditional photoinitiators such as benzophenone or thioxanthone is a hard work as this shift can only be achieved by mean of an extensive synthetic work. Recently, investigations of dyes with structures totally disconnected from traditional photoinitiators have been carried out, providing a series of highly efficient photoinitiators activable in the visible range. 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as visible-light photoinitiator for biomedical application: Synthesis, characterization, and application[END_REF][START_REF] Santini | Photoinitiators in Dentistry: A Review[END_REF] chalcones, [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] Chen | Photopolymerization and 3D/4D applications using newly developed dyes: Search around the natural chalcone scaffold in photoinitiating systems[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] 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] 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] 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] 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] 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 operating under violet and blue LEDs and usable for various polymerization reactions and synthesis of hydrogels[END_REF][START_REF] Xiao | Design of High Performance Photoinitiators at 385-405 nm: Search around the Naphthalene Scaffold[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] 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] carbazoles, [START_REF] Zhang | The carbazole-bound ferrocenium salt as a specific cationic photoinitiator upon near-UV and visible LEDs (365-405 nm)[END_REF][START_REF] Al Mousawi | Carbazole Scaffold Based Photoinitiator/Photoredox Catalysts: Toward New High Performance Photoinitiating Systems and Application in LED Projector 3D Printing Resins[END_REF][START_REF] Al Mousawi | Carbazole Derivatives with Thermally Activated Delayed Fluorescence Property as Photoinitiators/Photoredox Catalysts for LED 3D Printing Technology[END_REF][START_REF] Al Mousawi | Novel Carbazole Skeleton-Based Photoinitiators for LED Polymerization and LED Projector 3D Printing[END_REF][START_REF] Mousawi | Carbazole-based compounds as photoinitiators for free radical and cationic polymerization upon near visible light illumination[END_REF][START_REF] Abdallah | Development of new high-performance visible light photoinitiators based on carbazole scaffold and their applications in 3d printing and photocomposite synthesis[END_REF][START_REF] Dumur | Recent advances on carbazole-based photoinitiators of polymerization[END_REF][START_REF] Liu | Nitro-Carbazole Based Oxime Esters as Dual Photo/Thermal Initiators for 3D Printing and Composite Preparation[END_REF][START_REF] Hammoud | 5,12-Dihydroindolo[3,2a]carbazole: A promising scaffold for the design of visible light photoinitiators of polymerization[END_REF] Schiff Bases [START_REF] Lalevée | Dye photosensitized cationic ring-opening polymerization: Search for new dye skeletons[END_REF] dihydroanthraquinones, [START_REF] Xiao | Cationic and Thiol-Ene Photopolymerization upon Red Lights Using Anthraquinone Derivatives as Photoinitiators[END_REF][START_REF] Zhang | Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization[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] 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] 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] 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] 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] Bonardi | Free-radical polymerization upon near-infrared light irradiation, merging photochemical and photothermal initiating methods[END_REF][START_REF] Launay | Near-infrared light for polymer reshaping and re-processing applications[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] 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 | Visible-light-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] diketopyrrolopyrroles, [START_REF] Zhang | Panchromatic photoinitiators for radical, cationic and thiol-ene polymerization reactions: A search in the diketopyrrolopyrrole or indigo dye series[END_REF][START_REF] Xiao | Diketopyrrolopyrrole dyes: 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] 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 new Type I and Type II photoinitiators possessing highly coupled pyrene-ketone 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] 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] 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] 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] 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 the Iridium Complex Series: Photoredox Catalysis and Photoinitiation of Polymerization Reactions under Visible Lights[END_REF][START_REF] Telitel | Photoredox catalysis using a new iridium complex as an efficient toolbox for radical, cationic and controlled polymerizations under soft blue to green lights[END_REF][START_REF] Telitel | Photoredox process induced polymerization reactions: Iridium complexes for panchromatic photoinitiating systems[END_REF][START_REF] Dumur | Iridium (III) complexes as promising emitters for solid-state Light-Emitting Electrochemical Cells (LECs)[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] 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] and cyclohexanones [START_REF] Li | The effect of polyethylene glycoldiacrylate complexation on type II photoinitiator and promotion for visible light initiation system[END_REF][START_REF] Li | In situ monitoring of photopolymerization by photoinitiator with luminescence characteristics[END_REF][START_REF] Li | Synthesis of furan derivative as LED light photoinitiator: One-pot, low usage, photobleaching for light color 3D printing[END_REF][START_REF] Xu | Novel ketone derivative-based photoinitiating systems for free radical polymerization under mild conditions and 3D printing[END_REF] could furnish dyes absorbing between 400 and 800 nm. By mean of these different works, photosensitizers activable under blue, green, orange or red lights have been proposed. Even if very efficient photoinitiating systems have been developed with these different structures, on major drawback for a generalized use : the cost. Indeed, easiness of synthesis, facile chemical modifications of the selected scaffold, cost of the starting materials are other parameters to consider, especially for industrial applications. In this field, phenothiazine is a low cost molecule that can be facilely modified. [START_REF] Al-Busaidi | Phenothiazine-based derivatives for optoelectronic applications: A review[END_REF][START_REF] Massie | The Chemistry of Phenothiazine[END_REF][START_REF] Buene | Phenothiazine and phenoxazine sensitizers for dyesensitized solar cells -an investigative review of two complete dye classes[END_REF][START_REF] Wu | Pharmacological exploitation of the phenothiazine antipsychotics to develop novel antitumor agents-A drug repurposing strategy[END_REF] Chemistry of this molecule was also extensively studied during the 70's due its incorporation in various drugs [START_REF] Cimbura | Review of Methods of Analysis for Phenothiazine Drugs[END_REF][START_REF] Peele | Phenothiazine Deaths: A Critical Review[END_REF][START_REF] Dastidar | Role of Phenothiazines and Structurally Similar Compounds of Plant Origin in the Fight against Infections by Drug Resistant Bacteria[END_REF] so that photopolymerists could take advantage of this detailed chemistry. Phenothiazine was also extensively studied for the design of chromophores specifically developed for dye-sensitized solar cells. [START_REF] Kim | Tuning of spacer groups in organic dyes for efficient inhibition of charge recombination in dye-sensitized solar cells[END_REF][START_REF] Yang | Optimization of Multiple Electron Donor and Acceptor in Carbazole-Triphenylamine-Based Molecules for Application of Dye-Sensitized Solar Cells[END_REF][START_REF] Lee | Novel Dπ-A system based on zinc-porphyrin derivatives for highly efficient dye-sensitised solar cells[END_REF] Concerning photopolymerization, the first report mentioning the use of phenothiazine as photoinitiators of polymerization was reported as soon as 1969 by Margerum and coworkers. [START_REF] Rust | Photopolymerization studies: I. Polymers from new photoredox catalyst system[END_REF] In this pioneering work, phenothiazine was used as a photooxidant capable to oxidize in its excited state methylene blue or thionine, thus generating initiating radicals for the free radical polymerization (FRP) of barium diacrylate. Following this work, phenothiazine, due to the lack of absorption in the visible range was typically used as a UV photoinitiator for photoinduced charge transfer polymerization of acrylonitrile by mean of exciplex formation between phenothiazine and acrylonitrile, [START_REF] Wang | Photoinduced charge-transfer polymerization of acrylonitrile initiated by N-substituted phenothiazines[END_REF] the polymerization of tetrahydrofuran using triphenylsulfonium hexafluoroarsenate and upon sensitization of the reaction by phenothiazine. [START_REF] Rodrigues | Cationic photopolymerization of tetrahydrofuran: A mechanistic study on the use of a sulfonium salt-phenothiazine initiation system[END_REF] More recently, different two-component phenothiazine/sulfonium salts initiating systems were used as photochemical initiators for the FRP of acrylates and the cationic polymerization (CP) of epoxides. [START_REF] Takahashi | Novel sulfonium salts as thermal and photoinitiators for epoxide and acrylate polymerizations[END_REF][START_REF] Takahashi | Photocationic and radical polymerizations of epoxides and acrylates by novel sulfonium salts[END_REF] More generally, phenothiazines are good candidates for the sensitization of onium salts ranging from iodonium salts to triarylsulfonium salts and dialkylphenacylsulfonium salts. [209,[START_REF] Gomurashvili | Phenothiazine Photosensitizers for Onium Salt Photoinitiated Cationic Polymerization[END_REF] With aim at reducing the photoinitiator content, different phenothiazine derivatives could advantageously be used as photoredox catalysts for the photoinduced metal-free atom transfer radical polymerization (ATRP) of methyl methacrylate. [START_REF] Pan | Mechanism of Photoinduced Metal-Free Atom Transfer Radical Polymerization: Experimental and Computational Studies[END_REF][START_REF] Treat | Metal-Free Atom Transfer Radical Polymerization[END_REF] When polymerizable units were connected to phenothiazine, phenothiazine-substituted monomers could act as efficient photosensitizers for the photolysis of onium salts. [START_REF] Gomurashvili | Monomeric and Polymeric Phenothiazine Photosensitizers for Photoinitiated Cationic Polymerization[END_REF] Polymers and copolymers bearing phenothiazine-pendant groups could thus be obtained. However, phenothiazine, if connected to the monomer was not still used as a photosensitizer for its own polymerization and an additional photoinitiator was commonly used. To illustrate this, the polymerization of different phenothiazine-substituted monomers was notably carried out with AIBN [START_REF] Liu | Vinyl monomers bearing chromophore moieties and their polymers. VII. Synthesis, photochemical, and initiation behavior of acrylic monomers bearing phenothiazine oxide moieties and their polymers[END_REF][START_REF] Qiu | Vinyl monomers bearing chromophore moieties and their polymers. III. Synthesis and photochemical behavior of acrylic monomers having phenothiazine moieties and their polymers[END_REF] or with an iron arene complex.

[216] Phenothiazine was also extensively used as a monomer for the design of conjugated polymers acting as active layers for inverted structure solar cells, [START_REF] Kim | Easily Attainable Phenothiazine-Based Polymers for Polymer Solar Cells: Advantage of Insertion of S,Sdioxides into its Polymer for Inverted Structure Solar Cells[END_REF] but also for diverse optoelectronic applications. [START_REF] Padhy | Synthesis and applications of low-bandgap conjugated polymers containing phenothiazine donor and various benzodiazole acceptors for polymer solar cells[END_REF][START_REF] Sang | Two Polythiophene Derivatives Containing Phenothiazine Units: Synthesis and Photovoltaic Properties[END_REF][START_REF] Liu | Synthesis and electroluminescent properties of a phenothiazine-based polymer for nondoped polymer light-emitting diodes with a stable orange-red emission[END_REF][START_REF] Cho | Saturated and Efficient Red Light-Emitting Fluorene-Based Alternating Polymers Containing Phenothiazine Derivatives[END_REF][START_REF] Kong | Phenothiazine-Based Conjugated Polymers: Synthesis, Electrochemistry, and Light-Emitting Properties[END_REF] With aim at shifting the absorption spectra of phenothiazine-based photoinitiators towards the visible range, various chemical modifications were carried out so that Type I or Type II photoinitiators could be prepared. In this review, an overview of the different visible light phenothiazine-based photoinitiators reported to date is provided. It has to be noticed that two-photon polymerization photoinitiators will not be discussed in this review, even if a few examples of phenothiazine-based structures have recently been reported in the literature. [START_REF] Hao | Four new twophoton polymerization initiators with varying donor and conjugated bridge: Synthesis and two-photon activity[END_REF][START_REF] Li | Enhancement of two-photon initiation efficiency based on conjugated phenothiazine and carbazole derivatives[END_REF][START_REF] Tian | Two novel two-photon polymerization initiators with extensive application prospects[END_REF] To evidence the interest of these structures, comparison with benchmark photoinitiators and/or reference photoinitiating systems is provided.

Phenothiazine-based Type II photoinitiators of polymerization

Acridone derivatives

Acridone are fluorescent compounds widely studied as fluorescent probes for chemosensors. [START_REF] Bahr | Highly photoresistant chemosensors using acridone as fluorescent label[END_REF][START_REF] Pál | An Acridone-Based Fluorescent Chemosensor for Cationic and Anionic Species, and Its Application for Molecular Logic Operations[END_REF][START_REF] Rothman | A new generation of fluorescent chemosensors demonstrate improved analyte detection sensitivity and photobleaching resistance[END_REF][START_REF] Isaac | Novel acridine-based thiosemicarbazones as 'turn-on' chemosensors for selective recognition of fluoride anion: a spectroscopic and theoretical study[END_REF] The first reports mentioning the use of acridone as photoinitiators were published in 1984 and 1985 by Banerjee and coworkers who combined chlorine or bromine with acridone for the polymerization of methyl methacrylate (MMA) in carbon tetrachloride. The polymerization was found to proceed via a free radical mechanism, but required the solutions to be heated at 40°C. More recently, acridone was incorporated in the two-component systems for the sensitization of an iodonium salt. However, due to the lack of absorption in the visible range, acridone-based photoinitiating systems had to be activated under UV light. [START_REF] Timpe | Excited states of ketones as electron donors-ketone-iodonium salt systems as photoinitiators for radical polymerization[END_REF] But the reaction could be done at room temperature. In 2018, an acridone substituted by two phenothiazine units was examined as a visible light photoinitiator for the cationic polymerization of epoxides and the free radical polymerization of acrylates at 405 nm (See Figure 1). [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] Compared to the previous works, several advantages can be noticed : the polymerization was carried out in solvent free conditions whereas the highly toxic carbon tetrachloride was previously used as the solvent by Banerjee and coworkers. The reaction could also be done at room temperature and no volatile gas (chlorine, bromine) had also to be used. Recently, a new family of fluorescent dyes, namely compounds with thermally activated delayed fluorescence (TDF) properties have been identified as molecules capable to exhibit excited state lifetimes comparable to that observed for phosphorescent materials. [START_REF] Dias | The Role of Local Triplet Excited States and D-A Relative Orientation in Thermally Activated Delayed Fluorescence: Photophysics and Devices[END_REF][START_REF] Uoyama | Highly efficient organic light-emitting diodes from delayed fluorescence[END_REF] As an interesting feature, long-living excited state lifetimes can be obtained with heavy-metalfree structures. [START_REF] Zhang | Organic nanostructures of thermally activated delayed fluorescent emitters with enhanced intersystem crossing as novel metal-free photosensitizers[END_REF] This point is of crucial importance, especially with aim at designing polymers for food packaging and biomedical applications. This point is also important for the design of photoinitiating systems, long-living excited state lifetimes letting more time for the excited dye to interact with the different additives. As specificity, TADF dyes are characterized by a small S1-T1 gap so that electrons from the triplet state (T1) can thermally repopulate the singlet state (S1) by mean of a reverse intersystem crossing (RISC). To get this result, the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) must be spatially separated, what can be achieved by introducing orthogonal groups in the structure. In 2017, the first TADF dyes used in photopolymerization were reported, based on carbazoles used as peripheral groups for a benzene ring. [START_REF] Al Mousawi | Carbazole Derivatives with Thermally Activated Delayed Fluorescence Property as Photoinitiators/Photoredox Catalysts for LED 3D Printing Technology[END_REF] In this structure, presence of the four carbazole units attached to the same benzene ring enforced the four carbazoles to stand perpendicular to the central aromatic ring. The same strategy was applied to A-2PTz, with two phenothiazine groups being orthogonal to the central acridone core. Theoretical calculations revealed the HOMO and LUMO orbitals to be well-separated, the HOMO being located onto the phenothiazine moieties and the LUMO on the acridone core (See Figure 2). Examination of the UV-visible absorption spectra of A-2PTz in toluene revealed the dye to absorb from 300 until 480 nm. A good overlap with the LED emitting at 405 nm was thus obtained. Examination of the photoinitiating ability of the acridone derivative in twocomponent A-2PTz/Iod (0.3%/1% w/w) system under air and upon irradiation at 405 nm for 800 s during the CP of (3,4-epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate (EPOX)

in thin films (25 μm) revealed the monomer conversion to peak at 62% whereas no polymerization was detected for Iod alone. It therefore demonstrates the crucial role of A-2PTz in radical generation. Decrease of the photosensitizer content from 0.3 wt% to 0.1% resulted in a significant decrease of the monomer conversion, this latter being reduced to 45%. These results are consistent with a reduction of the light absorption abilities of the photoinitiating system, reducing the radical concentration. A similar trend was evidenced during the FRP of trimethylolpropane triacrylate (TMPTA) in thin films in laminate. Thus, an improvement of the TMPTA conversion from 0 until 43% could be evidenced by increasing the dye concentration from 0.05 wt% until 0.5 wt% in the two-component dye/Iod (x%/1% w/w with x varying from 0.05 to 0.5%) system upon irradiation at 405 nm. If no polymerization was detected at 0.05% acridone concentration, introduction of N-phenylglycine (NPG) enabled to reach a TMPTA conversion of 45% (See Table 1). Photoinitiating ability of A-2TPz in reductive pathways was also examined with two different two-component dye/amine system. Noticeably, if no polymerization was detected with ethyl 4-(dimethylamino)benzoate (EDB), a final monomer conversion of 56% could be observed with NPG. In fact, A-2PTz proved to be as efficient in photooxidative processes (with Iod) than in photoreductive ones (with NPG). Finally, the high reactivity of the threecomponent A-2PTz/Iod/NPG (0.05%/1%/1% w/w/w) system at low dye concentration was confirmed during the FRP of TMPTA in thick films. Thus, a conversion as high as 84% could

HOMO LUMO A-2DPA

A-2PTz

HOMO LUMO

A-2DPA

A-2PTz

be obtained within 100 s. Finally, the A-2PTz/Iod (0.1%/1% w/w) system was used for the polymerization of a dental resin, namely a methacrylate blend composed of BisGMA/TEGDMA (70%/30% w/w). [START_REF] Chen | BisGMA/TEGDMA dental composite containing high aspect-ratio hydroxyapatite nanofibers[END_REF] The polymerization was examined under air, for thick samples (1.4 mm) upon irradiation with a LED at 405 nm i.e. in the conditions classically used in dentistry. Interestingly, a short inhibition time due to oxygen was detected, demonstrating the reactivity of the photoinitiating system. As anticipated, the performance of the dye/Iod system decreased when increasing the dye concentration. Thus, if a conversion of 66% was obtained at 0.05 wt%, it decreased to 19% at 0.3 wt% due to inner filter effects impeding light penetration within the photocurable resin. When replacing Iod by NPG, a severe reduction of the monomer conversion was observed, not exceeding 30% after 150 s of irradiation at 405 nm for the optimized conditions (See Table 2). The weak interaction of A-2PTz and amines was thus confirmed. Finally, use of the three-component A-2PTz/Iod/NPG (0.05%/1%/1% w/w/w) system enabled to greatly improve the monomer conversion, up to 81%. BisGMA/TEGDMA-based polymer was examined for samples prepared with the threecomponent A-2PTz/Iod/NPG (0.05%/1%/1% w/w) system using acetonitrile as the solvent to extract residues. Noticeably, after one weak in acetonitrile solutions, no characteristic peaks of A-2PTz were detected by UV-visible absorption spectroscopy, demonstrating that the migratability of A-2PTz was low. It was assigned to the high molecular weight of A-2PTz but also to the high crosslink density of the final polymer network, the monomer conversion reaching 81% with this photoinitiating system. Finally, to evidence the interest of this photoinitiating system, 3D printing experiments were carried out. 3D patterns with a high spatial resolution were obtained within a short writing time (around 1 min.), as shown in the Acridone → *Acridone (hν)

(r1) *Acridone + NPG → (Acridone-H) • + NPG(-H) • (r2) NPG(-H) • → NPG(-H;-CO2) • (r3) *Acridone + Ar2I + → Acridone •+ + Ar2I • → Acridone •+ + Ar • + ArI (r4) NPG + Iod → [NPG-Iod]CTC (r5) [NPG-Iod]CTC →→→ Ar • (r6) NPG(-H;-CO2) • + Ar2I + → NPG(-H;-CO2) + + Ar • + ArI (r7) (Acridone-H) • + Ar2I + → Acridone + H + + Ar • + ArI (r8) Scheme 1.
The different reactions involved in the FRP and CP with A-2PTz.

Interestingly, during the FRP of acrylates and methacrylates, the high efficiency of the three-component dye/Iod/NPG system was assigned to the ability of this amine (NPG) to decarboxylate. [START_REF] Al Mousawi | 3-Hydroxyflavone and N-Phenylglycine in High Performance Photoinitiating Systems for 3D Printing and Photocomposites Synthesis[END_REF][START_REF] Noirbent | Photoinitiators of polymerization with reduced environmental impact: Nature as an unlimited and renewable source of dyes[END_REF] As shown in the Scheme 1, upon photoexcitation of A-2PTz, an electron transfer can be promoted in the excited state between A-2PTz and NPG, generating NPG(-H)

•
that can subsequently decarboxylate and form NPG(-H;-CO2) • (See equations r2 and r3). Parallel to this, NPG is well-known to form a charge-transfer complex with Iod, also enabling to provide an additional source of radicals (see equation r6). [START_REF] Garra | Charge Transfer Complexes as Pan-Scaled Photoinitiating Systems: From 50 μm 3D Printed Polymers at 405 nm to Extremely Deep Photopolymerization (31 cm)[END_REF] Indeed, the [NPG-Iod]CTC charge transfer complex constitute an interesting approach to provide enhanced visible light absorption properties to the photoinitiating system, in addition to the photosensitizer. Overall, the high efficiency of the A-2PTz system can be assigned to the generation of initiating radicals by at least three different pathways. Hence, NPG(-H, -CO2)•, Ar• and NPG(-H, -CO2) + , acridone •+ are the different species responsible of the FRP and CP, respectively.

Benzophenone derivatives

Benzophenone is an historical photoinitiator of polymerization. [START_REF] Allushi | Conventional Type II photoinitiators as activators for photoinduced metal-free atom transfer radical polymerization[END_REF][START_REF] Wang | One-component photoinitiator based on benzophenone and sesamol[END_REF][START_REF] Temel | Photopolymerization and photophysical properties of amine linked benzophenone photoinitiator for free radical polymerization[END_REF][START_REF] Hou | Synthesis and Evaluation of 4-Benzophenone Methoxyl Methacrylate As a Polymerizable Photoinitiator[END_REF] Besides, its combination with phenothiazine was only examined in 2020. [START_REF] Rahal | Design of new phenothiazine derivatives as visible light photoinitiators[END_REF] In this work, two benzophenone derivatives were prepared by in silico design by molecular modelling, namely PT3 and PT4. For comparison, two other phenothiazine-based dyes were prepared i.e. PT1 and PT2 bearing formyl groups as lateral substituents and differing by the length of the alkyl chain (See Figure 4). Interestingly, comparison of the absorption properties of PT1-PT4 revealed the benzoyl groups in PT3 and PT4 to exhibit slightly higher electron-withdrawing abilities than the formyl groups. Indeed, if PT1 and PT2 absorb at 400 and 405 nm respectively, a redshifted absorption was determined for PT3 (408 nm) and PT4 (424 nm). Especially, a 3-fold enhancement of the molar extinction coefficient was determined for PT3 compared to that of PT4, consistent with a significant elongation of the π-conjugated system in this dye (See Table 3). Considering that all dyes absorb between 350 and 550 nm, a good overlap with the emission of the LED at 405 nm was found. The FRP of TMPTA and (oxybis(methylene))bis(2-ethylpropane-2,1,3-triyl)tetraacrylate (Ebecryl 40) was examined with different photoinitiating systems including two and threecomponent systems based on dyes, Iod, NPG or EDB, upon irradiation at 405 nm with a LED. If the dye or additives considered separately could not initiate the FRP of TMPTA or Ebecryl 40, interestingly, the Iod/NPG (1%/1% w/w) combination could initiate the FRP of Ebecryl 40 in thin and thick samples with a rather good efficiency. Indeed, final monomer conversions of 81% and 32% could be determined for thick and thin samples respectively during the FRP of Ebecryl 40 at 405 nm for 250 s. Similarly, a good monomer conversion was obtained during the FRP of TMPTA (48% conversion after 250 s). Conversely, EDB and Iod being not capable to form a charge transfer complex, no polymerization could be detected with this twocomponent system. In all cases, higher polymerization rates and final monomer conversions could be obtained in the presence of dyes. Concerning the two-component dye/Iod (0.1%/1% w/w) systems, PT1 proved to be the most efficient dye during the FRP of TMPTA in thick films (71% conversion) or Ebecryl 40 (87% conversion). Irrespective of the dye, increase of the photosensitizer concentration resulted in a decrease of the monomer conversion, as a result of inner filter effects. Conversely, no polymerization was detected for the two-component dye/EDB system, evidencing the lack of interaction between the two partners. Upon introduction of Iod, good TMPTA conversions could be obtained with the three-component dye/Iod/EDB (0.1%/1%/1% w/w/w) systems, ranging from 61% for PT3 to 76% for PT4. In the case of Ebecryl 40, the best conversion was obtained with PT2 (See Table 4). By replacing EDB by NPG, an enhancement of the monomer conversion was obtained for all three-component systems, attributable to the decarboxylation of NPG and the formation of additional initiating aryl radicals by the amine, improving the monomer conversion. Comparisons with the wellestablished benzophenone/EDB combination revealed all two-component PT2-based systems to outperform the reference system (64% conversion vs. 58% conversion with benzophenone) during the FRP of Ebecryl 40. The same holds true by comparing the different two-component dye/Iod systems with the two-component ITX/Iod (0.1/1% w/w) system (where ITX stands for 2-isopropylthioxanthone). Thus, if a conversion of 55% was obtained with ITX, conversions ranging from 62 to 71% could be obtained with PT1-PT4. In fact, photoinitiating ability of PT1-PT4 was determined as not being directly correlated to their absorption properties but governed by several parameters such as the free-energy change (ΔGet) for the electron transfer reaction, the electron transfer quantum yields or the photolysis rate. Due to the good reactivity of the different dyes, photocomposites were prepared using a LED conveyor, using a LED emitting at 395 nm. Indeed, photocomposites are now widely used in dental cements but also for different applications. [START_REF] Bayou | Simulation of conversion profiles inside a thick dental material photopolymerized in the presence of nanofillers[END_REF][START_REF] Kardar | Influence of temperature and light intensity on the photocuring process and kinetics parameters of a pigmented UV curable system[END_REF][START_REF] Kardar | Curing behaviour and mechanical properties of pigmented UV-curable epoxy acrylate coatings[END_REF][START_REF] Tauböck | Effect of modulated photo-activation on polymerization shrinkage behavior of dental restorative resin composites[END_REF] Advantages of photocomposites is that these structures are lightweight and robust. As drawback, their photopolymerization is often difficult due to a limited light penetration within the resin, severely reducing the depth of cure. In this work, glass fibers were impregnated within the resins (Ebecryl 40 or TMPTA) containing the three-component PT/Iod/NPG (0.1%/1%/1% w/w/w) system. Interestingly, to completely cure the composites, only one pass was needed for resins containing 50% glass fibers (See Figure 6). Concerning the photochemical mechanism supporting the efficient FRP and CP processes, similar equations to that proposed for acridone were proposed by the authors (See Scheme 1).

Benzophenothiazine derivatives

Phenothiazine phenylethyl)sulfonium hexafluoroantimonate (DPS-C1C12). In fact, triarylsulfonium salts are more difficult to reduce than diaryliodonium salts due to higher reduction potentials rendering their sensitizations difficult. [START_REF] Yağci | Externally stimulated initiator systems for cationic polymerization[END_REF] Considering that polynuclear aromatic hydrocarbons such as pyrenes or perylenes have been identified as good candidates for the sensitization of triarylsulfonium salts, [START_REF] Crivello | Dye-sensitized photoinitiated cationic polymerization. The system: Perylene-triarylsulfonium salts[END_REF] an extension of the aromaticity on the phenothiazine core was thus examined as a possible strategy to redshift its absorption towards the visible range compared to that of its phenothiazine counterpart. Interestingly, introduction

(3)

of an additional aromatic ring in benzophenothiazine enabled to design absorbing until 450 nm.

Chalcone derivatives

During the last two years, the design of chalcones as visible light photoinitiators of polymerization has become an active research field since around 150 structures have been reported in less than two years. [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] Interest for chalcones is supported by the easiness of synthesis, these structures being obtained by mean of a Claisen-Schmidt condensation between an acetophenone and an aldehyde. [START_REF] Yadav | Claisen-Schmidt Condensation using Green Catalytic Processes: A Critical Review[END_REF][START_REF] Qian | Synthesis of Chalcones via Claisen-Schmidt Reaction Catalyzed by Sulfonic Acid-Functional Ionic Liquids[END_REF][START_REF] Sun | Synthesis of four pyrene-containing chalcone derivatives: Achieving excellent third-order nonlinear optical properties by optimizing halopyridines[END_REF][START_REF] Yu | Tuning the solid-state fluorescence of chalcone crystals via molecular coplanarity and J-aggregate formation[END_REF] These syntheses can also be done in green conditions, the condensation reaction being performed in ethanol as the solvent and by using an aqueous solution of potassium hydroxide. In these conditions, the environmental impact of the synthesis is greatly minimized. Additionally, most of the chalcones can be recovered in pure form by a simple filtration of the reaction medium, by washing the final product with water to remove base traces. The work-up is thus minimized. Parallel to this, the design of bioinspired photoinitiators has been the focus of intense research efforts as a higher biocompatibility and a lower toxicity are both expected. [START_REF] Noirbent | Photoinitiators of polymerization with reduced environmental impact: Nature as an unlimited and renewable source of dyes[END_REF] In 2021, the first phenothiazinebased chalcone i.e. PTZ-PD was investigated as the photosensitizer for the FRP of tripropylene glycol diacrylate (TPGDA) (See Figure 10). From the absorption viewpoint, good light absorption properties were determined in the 350-550 nm range, resulting from an efficient intramolecular charge transfer from the peripheral phenothiazine towards the central and electro-deficient piperidone moiety. An absorption maximum at 447 nm combined with a molar extinction coefficient of 14 500 M -1 .cm -1 was determined. Considering that molar extinction coefficients of 13100 and 1900 M -1 .cm -1 could be still detected at 460 and 520 nm, polymerization tests were carried out at these two wavelengths. Investigation of the photoinitiating ability of TPGDA using two-component PTZ-PD/Iod2 (0.2%/2% w/w) systems at 460 and 520 nm revealed the monomer conversion to be higher at 460 nm (84% conversion after 150 s of irradiation) contrarily to 79% conversion at 560 nm. A good correlation between molar extinction coefficients at 460 and 520 nm and monomer conversions at the two wavelengths could be found. Investigation of the cure depth at 460 and 520 nm revealed the polymerization depths to be of 1.35 mm and 2.14 mm respectively. The higher polymer thickness obtained at 520 nm is consistent with a better light penetration at 520 nm than at 460 nm. [START_REF] Bonardi | High Performance Near-Infrared (NIR) Photoinitiating Systems Operating under Low Light Intensity and in the Presence of Oxygen[END_REF] Finally, the following mechanism was proposed by the authors to support the efficient FRP of TPGDA upon irradiation at 460 or 520 nm (See equations r9 and r10 in Scheme 2).

Notably, upon excitation of the dye, a photoinduced electron transfer towards the electrodeficient iodonium salt can occur, inducing its decomposition and generating aryl radicals capable to initiate the FRP of TPGDA.

*Dye + Ar2I + → Dye •+ + Ar2I • → Dye •+ + Ar • + ArI (r10)
Scheme 2. Photochemical reactions involved in the FRP of TPGDA using PTz-PD.

Cucurminoid derivatives

Natural dyes are extensively studied as photoinitiators of polymerization as these structures can greatly improve the biocompatibility of the resulting polymers. [START_REF] Wang | Anti-tumor bioactivities of curcumin on mice loaded with gastric carcinoma[END_REF][START_REF] Prasad | Curcumin and its analogues: a potential natural compound against HIV infection and AIDS[END_REF][START_REF] Khan | Synthesis and antiinflammatory properties of some aromatic and heterocyclic aromatic curcuminoids[END_REF] Natural dyes are generally easily available, at low cost, enabling to develop cheap photoinitiating systems. In this field, curcumin which can be found in an Indian spice plant can be cited as a relevant example. Curcumin was notably used as soon as 2005 by Crivello and coworkers for the sensitization of iodonium salts. [START_REF] Crivello | Indian Turmeric and its Use in Cationic Photopolymerizations[END_REF][START_REF] Crivello | Curcumin: A naturally occurring long-wavelength photosensitizer for diaryliodonium salts[END_REF] In fact, curcumin has become a popular photoinitiator of polymerization, [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] Mishra | Curcumin, a natural colorant as initiator for photopolymerization of styrene: kinetics and mechanism[END_REF][START_REF] Mishra | Curcumin, A Novel Natural Photoinitiator for the Copolymerization of Styrene and Methylmethacrylate[END_REF][START_REF] Lalevée | N-Vinylcarbazole: An Additive for Free Radical Promoted Cationic Polymerization upon Visible Light[END_REF][START_REF] Dietlin | Photopolymerization upon LEDs: new photoinitiating systems and strategies[END_REF] its absorption maximum ranging between 410 and 430 nm depending of the solvent polarity. [START_REF] Priyadarsini | Photophysics, photochemistry and photobiology of curcumin: Studies from organic solutions, bio-mimetics and living cells[END_REF] Curcumin is also soluble in numerous monomers and this molecule is also none toxic, making this molecule an appealing scaffold for the design of various derivatives. To end, curcumin exhibits promising antitumoral, anti-HIV, anti-inflammatory, and anti-oxidant properties. [START_REF] Wang | Anti-tumor bioactivities of curcumin on mice loaded with gastric carcinoma[END_REF][START_REF] Prasad | Curcumin and its analogues: a potential natural compound against HIV infection and AIDS[END_REF][START_REF] Khan | Synthesis and antiinflammatory properties of some aromatic and heterocyclic aromatic curcuminoids[END_REF] In 2018, the first phenothiazine-based curcuminoid (CMD4) was proposed by Wang and coworkers as a bluegreen light sensitive compound (See Figure 11). [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] Compared to curcumin that exhibits an absorption maximum at 423 nm, a redshift by ca. 50 nm could be determined for CMD4, peaking at 474 nm. Surprisingly, a reduction of the molar extinction coefficient could be determined for CMD4 compared to curcumin (45 100 M -1 .cm -1 vs. 55 300 M -1 .cm -1 for curcumin)

even if an extension of aromaticity can be observed in DCM4 compared to curcumin. A positive solvatochromism was determined by examining the position of the absorption maxima in solvent of different polarities. Interestingly, upon increase of the solvent polarity, the keto-enol equilibrium was shifted toward the enol form, complicating the analysis of the solvatochromism. [START_REF] Khopde | Effect of Solvent on the Excited-state Photophysical Properties of Curcumin ¶[END_REF][START_REF] Bi | A Visible Light Initiating System for Free Radical Promoted Cationic Polymerization[END_REF][START_REF] Bong | Spectral and Photophysical Behaviors of Curcumin and Curcuminoids[END_REF] Considering that molar extinction coefficients of 42 800 and 16 900 M -1 .cm -1 can be still observed at 460 and 520 nm, polymerization tests were thus carried out at these two wavelengths. Interestingly, CMD4 could act as a high-performance photosensitizer with Iod2 in twocomponent systems and high monomer conversions could be obtained during the CP of diglycidyl ether of bisphenol A (DGEBA) and during the FRP of TPGDA upon irradiation with blue-green lights. Thus, the conversion could reach up 47% for the CP of DEGBA after 150 s of irradiation at 460 nm and by using the two-component CMD4/Iod2 (0.2%/3% w/w) system.

This value is higher than that obtained with curcumin in the same conditions (42%). This trend was confirmed at 520 nm. If a conversion of 44% was still obtained at this wavelength with CMD4, it decreased to only 24% with the curcumin-based two-component system due to the lack of absorption of curcumin at this wavelength (ε (520 nm) = 3 M -1 .cm -1 ). Noticeably, initiating ability of the CMD4/Iod2 combination could be enhanced upon addition of Nvinylcarbazole (NVK). [START_REF] Lalevée | N-Vinylcarbazole: An Additive for Free Radical Promoted Cationic Polymerization upon Visible Light[END_REF] Thus, upon irradiation at 460 nm, the DEGBA conversion could increase up to 56% with the three-component CMD4/Iod2/NVK (0.2%/3%/0.2% w/w/w) system vs. 42% with the two-component system. A similar trend was found at 520 nm. When tested for the FRP of TPGDA, similar monomer conversions were found between curcumin and CMD4, around 88% upon irradiation at 460 nm and by using the two-component dye/Iod2 (0.2%/3% w/w) systems. Conversely, at 520 nm, a better photoinitiating efficiency was found for CMD4 compared to curcumin, due to a higher molar extinction coefficient. Thus, the TPGDA conversion decreased to only 19% whereas no polymerization was found for curcumin in the same conditions. Irrespective of the irradiation wavelengths, an improvement of the monomer conversions was obtained by reducing the photoinitiator content, as a result of inner filter effects at high dye concentration (See Figure 12). Similarly, introduction of triphenylphosphine (TPP) improved the monomer conversion, TPP being capable to overcome oxygen inhibition. [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] Belon | Overcoming the oxygen inhibition in the photopolymerization of acrylates: A study of the beneficial effect of triphenylphosphine[END_REF] This photobleaching ability detected for thick samples could be of high interest for 3D printing applications or dental restoration. Finally, considering the major improvement of the DEGBA conversion upon introduction of NVK, the following mechanism was proposed (See Scheme 3). Upon excitation of the dyes, a photoinduced electron transfer on the iodonium salt can occur in the excited state, producing aryl radicals. These latter can react with NVK, producing Ph-NVK • radicals. Finally, by reaction with iodonium salt, Ph-NVK + cations can be formed, inducing the CP of epoxides.

CMD4 → 1 CMD4 (hν) and 1 CMD4 → 3 CMD4 (r11)

1,3 CMD4 + Ph2I + → CMD4 •+ + Ph2I • (r12) Ph2I • → Ph • + Ph-I (r13) Ph • + NVK → Ph-NVK • (r14) Ph-NVK • + Ph2I + → Ph-NVK + + Ph • + Ph-I (r15)
Scheme 3. Photochemical mechanism involved with the three-component Dye/Iod2/NVK) system.

Phenothiazine-based push-pull dyes

Push-pull dyes constitute an interesting family of compounds as the electronic delocalization existing within these molecules can give rise to an intense absorption band located in the visible range and named intramolecular charge transfer (ICT) band. [START_REF] Bureš | Fundamental aspects of property tuning in push-pull molecules[END_REF] By carefully selecting both the electron-donating and electron-accepting groups, position of the ICT band can be finely tuned. In 2019, a series of phenothiazine-based push-pull dyes (B106, B107, B108 and B111) was examined as photoredox catalysts for the cationic and the free radical polymerization of monomers upon irradiation at 405 nm (See Figure 14). [START_REF] Abdallah | Phenothiazine derivatives as photoredox catalysts for cationic and radical photosensitive resins for 3D printing technology and photocomposite synthesis[END_REF] Figure 14. Chemical structures of B106, B107, B108 and B111.

From the absorption viewpoint, the different dyes showed a broad absorption band extending between 400 and 650 nm, assigned to the ICT bands. Absorption maxima ranging from 461 nm for B111 until 494 nm for B107 could be determined in acetonitrile (See Figure 15 and Table 5). The different dyes were thus suitable candidates for photoinitiation at 405 nm. Examination of their photoinitiating abilities in two-component dye/Iod (0.5%/1% w/w) systems revealed the EPOX conversions to range from 43% for B108 to 58% for B111 (See Table 6). Upon decrease of the photosensitizer content to 0.2 wt%, a severe reduction of the monomer conversion was observed for all dyes, except for B106 for which a slight increase of the EPOX conversion was detected (64% instead of 57%). While getting a deeper insight into the monomer conversion, no correlation with the light absorption properties of the different dyes could be found, B108 being the less efficient photosensitizer but exhibiting the highest molar extinction coefficient at 405 nm.

 (nm)

An opposite trend was found during the FRP of TMPTA (See Table 7). Indeed, by using the two-component dye/Iod (0.2%/1% w/w) system, B108 clearly outperformed the other dyes, the TMPTA conversion peaking at 54% after 100 s of irradiation at 405 nm. By using NPG, a clear enhancement of the monomer conversion was observed for all dyes. The highest conversion was obtained for B106 (59%) followed by B108 (56%). While examining the photoinitiating ability of dye/NPG combinations, only B107 and B108 could furnish a low monomer conversion, assigned to the low radical yield and the inability of the photoinitiating system to overcome the oxygen inhibition. These results were confirmed during the photolysis experiments of the dye/NPG combination in solution for which no or poor photolysis was detected upon irradiation at 405 nm. Once again, phenothiazine proved to be better candidate for photooxidative processes with Iod than in photoreductive processes with amines. To support the photoinitiating ability of these push-pull dyes, a mechanism comparable to that presented in the Scheme 1 was proposed by the authors. Photoredox catalyst behavior of dyes in three-component systems was demonstrated by examining the consumption of the dyes in three different conditions, namely, dye alone, dye/Iod combination and dye/Iod/NPG system.

A partial regeneration of B111 by using the three-component system was demonstrated, the consumption of B111 being only of 10% after 400 s of irradiation at 405 nm contrarily to 33% for the two-component system. The same photocatalytic behavior was demonstrated the same year with another phenothiazine-based push-pull dye, namely FcIn4 (See Figure 16). [START_REF] Han | Synthesis and electrochemical, linear and thirdorder nonlinear optical properties of ferrocene-based D-π-A dyes as novel photoredox catalysts in photopolymerization under visible LED irradiations[END_REF] Figure 16. Chemical structure of FcIn4.

Diphenyl sulfone and diphenyl sulfoxide-based dyes

The synthesis of polyaromatic structures is an efficiency strategy to elaborate dyes exhibiting high molar extinction coefficients, especially when long π-conjugated systems can be developed. Parallel to this, this approach is also used for designing dyes for which the migratability can be severely reduced compared to small molecules. [START_REF] Oesterreicher | Low migration type I photoinitiators for biocompatible thiol-ene formulations[END_REF] In 2015, two extended π-conjugated structures comprising two phenothiazine units as end groups were proposed by Wang and coworkers (See Figure 17). [START_REF] Wang | Aromatic amine-sulfone/sulfoxide conjugated D-π-A-π-D-type dyes in photopolymerization under 405 nm and 455 nm laser beams[END_REF] The two dyes showed a broad ICT band extending between 350 and 575 nm. More precisely, examination of their UV-visible absorption properties in dichloromethane revealed the absorption of PTZ-SO2 to be redshifted compared to that of PTZ-SO, their absorption maxima peaking at 370 and 388 nm respectively.

The higher electron-withdrawing ability of sulfone compared to sulfoxide is the main reason supporting the redshifted absorption of PTZ-SO2. Considering their respective absorptions, polymerization experiments were carried out at 405 and 455 nm. As shown in the Table 8, similar monomer conversions were obtained during the FRP of TPGDA. At the two different irradiation wavelengths, the final conversions are much higher than that obtained with the reference camphorquinone (CQ)/triethanolamine (TEOA) system.

In fact, at 405 nm, a 2-fold enhancement of the monomer conversion can be determined with the two dyes, compared to the reference system. Concerning the CP of the epoxy-based resin E51, high conversions could be also obtained with the two dyes whereas the reference system was totally inefficient at 455 nm due to the lack of absorption of this photoinitiating system at this wavelength. photopolymerization [START_REF] Xiao | Multicolor Photoinitiators for Radical and Cationic Polymerization: Monofunctional vs Polyfunctional Thiophene Derivatives[END_REF][START_REF] Kasapoglu | Photosensitized Cationic Polymerization of Cyclohexene Oxide Using a Phenacylanilinium Salt[END_REF][START_REF] Dadashi-Silab | Microporous Thioxanthone Polymers as Heterogeneous Photoinitiators for Visible Light Induced Free Radical and Cationic Polymerizations[END_REF][START_REF] Yilmaz | Thioxanthone-Fluorenes as Visible Light Photoinitiators for Free Radical Polymerization[END_REF] Besides, examination of thiophene was not limited to photosensitization and thiophene was also used for the design of π-conjugated polymers, light being used to initiate the polymerization of this monomer. [START_REF] Woods | Advances in the Synthesis of π-Conjugated Polymers by Photopolymerization[END_REF] In light of these results, the combination of thiophene and phenothiazine was logically examined for the design of visible light photoinitiators. In 2016, a series of six thiophene-based phenothiazine was examined for the FRP of TPGDA and the CP of an epoxy resin i.e. E51 upon irradiation at 405 and 455 nm (See Figure 18). [START_REF] Chao | Thiophene-substituted phenothiazine-based photosensitisers for radical and cationic photopolymerization reactions under visible laser beams (405 and 455 nm)[END_REF] Examination of the UV-visible absorption spectra in dichloromethane revealed a redshift of the absorption spectra from 363 nm for ThEPTZ to 375 nm for Th2EPTZ and 435 nm for ThFEPTZ (See Figure 19 and Table 9). Thus, the higher electron-withdrawing ability of the formyl group compared to the thiophene unit was thus demonstrated. Elongation of the alkyl chain on the phenothiazine moiety did not modify the absorption maxima but improved the solubility of the different dyes in monomers. Photopolymerization of E51 with laser diodes at 405 and 455 nm under air revealed the two-component ThPTZs/Iod2 (0.1%:

3.0%, w/w) systems to outperform the reference hexylphenothiazine (EPTZ)/Iod2 (0.1%: 3.0%, w/w) system (See Table 10). Overall, a 3-fold enhancement of the monomer conversion could be determined with the new derivatives compared to hexylphenothiazine. Thus, the best conversions were obtained with Th2EPTZ and Th2HPTZ at 405 nm (72 and 69% respectively after 500 s), ThFEPTZ and ThFHPTZ at 455 nm (73 and 75% conversion respectively).

Interestingly, a direct relationship could be established between molar extinction coefficients and monomer conversions, evidencing that the light absorption properties of dyes was the primary factor influencing their reactivities in two-component systems. While evaluating the photoinitiating ability of thiophene-based phenothiazine for the FRP of TPGDA under air, Th2EPTZ and Th2HPTZ, ThFEPTZ and ThFHPTZ clearly outperformed ThEPTZ and ThHPTZ in terms of final monomer conversions (See Table 11).

Notably, an improvement of the monomer conversion by ca. 20% was found between ThEPTZ/ ThHPTZ and Th2EPTZ/Th2HPTZ at 405 nm differing by the presence of an additional thiophene unit. At 455 nm, this difference of monomer conversion decreased to only 10%.

Finally, comparison with two reference EPTZ/Iod2 and CQ/TEOA systems revealed the new derivatives to induce a 4-fold enhancement of the TPGDA conversion, evidencing the pertinence of the approach.

Table 11. Monomer conversion determined during the FRP of TPGDA under air upon irradiation with different laser diodes for 100 and 200 s using dye/Iod2 (0.1%/1.0%, w/w) system. EPTZ/Iod2 (0.1%/1.0%, w/w) and CQ/TEOA (0.1%/10%, w/w) were used as reference systems.

Thioxanthone derivatives

Thioxanthone is an historical photoinitiator of polymerization strongly absorbing in the near-UV/visible range and in this family, 2-isopropylthioxanthone (ITX) is one of the most relevant examples. [START_REF] Dadashi-Silab | Shining a light on an adaptable photoinitiator: advances in photopolymerizations initiated by thioxanthones[END_REF][START_REF] Li | In situ monitoring of photopolymerization by photoinitiator with luminescence characteristics[END_REF][START_REF] Zhou | Theoretical and Experimental Study of Lightassisted Polymerization by Multimechanism Action[END_REF] Due to its remarkable efficiency, this promising scaffold was logically examined for the design of various polymeric or macromolecular photoinitiators [START_REF] Dadashi-Silab | Microporous Thioxanthone Polymers as Heterogeneous Photoinitiators for Visible Light Induced Free Radical and Cationic Polymerizations[END_REF][START_REF] Wu | Silicone-Thioxanthone: A Multifunctionalized Visible Light Photoinitiator with an Ability to Modify the Cured Polymers[END_REF][START_REF] Kreutzer | Poly(propylene oxide)-thioxanthone as one-component Type II polymeric photoinitiator for free radical polymerization with low migration behavior[END_REF][START_REF] Valandro | A novel biopolymeric photoinitiator based on chitosan and thioxanthone derivative: Synthesis, characterization and efficiency in photopolymerization[END_REF][START_REF] Eren | Onecomponent thioxanthone-based polymeric photoinitiators[END_REF][START_REF] Wu | Thioxanthone-Based Hydrophilic Visible Photoinitiators for Radical Polymerization[END_REF][START_REF] Jiang | A novel amphipathic polymeric thioxanthone photoinitiator[END_REF][START_REF] Gacal | Synthesis and Characterization of Polymeric Thioxanthone Photoinitatiors via Double Click Reactions[END_REF][START_REF] Jiang | Water-Soluble Polymeric Thioxanthone Photoinitiator Containing Glucamine as Coinitiator[END_REF], one-component photoinitiators [START_REF] Wu | High-Performance and Low Migration One-Component Thioxanthone Visible Light Photoinitiators[END_REF][START_REF] Yilmaz | Thioxanthone-Fluorenes as Visible Light Photoinitiators for Free Radical Polymerization[END_REF][START_REF] Wu | Thioxanthone based one-component polymerizable visible light photoinitiator for free radical polymerization[END_REF][START_REF] Karasu | Mechanistic Studies of Photoinitiated Free Radical Polymerization Using a Bifunctional Thioxanthone Acetic Acid Derivative as Photoinitiator[END_REF] or multi-component photoinitiating systems. [START_REF] Balta | Thioxanthone-Diphenyl Anthracene: Visible Light Photoinitiator[END_REF][START_REF] Wu | Visible light-emitting diode-sensitive thioxanthone derivatives used in versatile photoinitiating systems for photopolymerizations[END_REF][START_REF] Karaca | Thioxanthonation of fluorenone: Visible photoinitiator for radical polymerization[END_REF][START_REF] Tunc | Thioxanthone-ethylcarbazole as a soluble visible light photoinitiator for free radical and free radical promoted cationic polymerizations[END_REF] In 2020, the first example of phenothiazine/thioxanthone combination i.e. TX-2PTz was reported in the literature (See Figure 20). [START_REF] Mau | Donor-acceptor-donor structured thioxanthone derivatives as visible photoinitiators[END_REF] By substitution of thioxanthone by two phenothiazine units, a redshift of the absorption was determined for TX-2PTZ. Notably, an absorption maximum at 386 nm was determined for ITX, far from that of TX-2PTZ for which a shoulder at 415 nm could be found (See λmax (nm) εmax (M -1 .cm -1 ) ε@405nm (M -1 .cm -1 ) ITX 386 6500 1000 TX-2PTZ 305; 415 a 2400 2600 a shouldered band A similar trend was determined with another two-component system i.e. the TPX-2PTZ/Iod (0.18/2.0 w/w%) combination. Once again, a higher monomer conversion was obtained with the reference system (ITX/Iod (0.19/2.0 w/w%), 63% TMPTA conversion after 400 s of irradiation vs. 38% with the two-component TX-2PTZ/Iod (0.18/2.0 w/w%) system in thin films. Finally, the low reactivity of TX-2PTZ was confirmed during the CP of EPOX. If this unexpected behavior could be clearly evidenced for TX-2PTZ, no clear explanations could be provided to explain this counter-performance.

Phenothiazine-based Type I photoinitiators of polymerization

Simplification of the photocurable resins is an active research field. [START_REF] Liu | Monocomponent Photoinitiators based on Benzophenone-Carbazole Structure for LED Photoinitiating Systems and Application on 3D Printing[END_REF][START_REF] Liu | A monocomponent bifunctional benzophenone-carbazole type II photoinitiator for LED photoinitiating systems[END_REF][START_REF] Dumur | Recent advances on visible light photoinitiators of polymerization based on Indane-1,3-dione and related derivatives[END_REF] Indeed, if high monomer conversions can be obtained with type II photoinitiators, the dye acts as a photosensitizer for the sensitization of onium salts. In order to render the system photocatalytic, an amine is classically added so that efficient photoinitiating systems are composed of at least three different compounds. The multicomponent issue can be addressed with Type I photoinitiators and notably oxime esters. [START_REF] Qiu | Polymerizable Oxime Esters: An Efficient Photoinitiator with Low Migration Ability for 3D Printing to Fabricate Luminescent Devices[END_REF] Oxime esters can produce initiating species by cleavage of the N-O bond and by decarboxylation of the acyloxy radicals, generating radicals involved in the polymerization process. [START_REF] Liu | Nitro-Carbazole Based Oxime Esters as Dual Photo/Thermal Initiators for 3D Printing and Composite Preparation[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] Groenenboom | Photoinitiators and photoinitiation Part 11. The photodecomposition of some O-acyl 2oximinoketones[END_REF][START_REF] Muramatsu | Terminal Group Analyses of Photopolymerized Products Using a MALDI-TOFMS for the Study on the Oxime Ester type Photoinitiators[END_REF][START_REF] Miyake | Structure and Reactivity of Radicals Produced by Photocleavage of Oxime Ester Compounds Studied by Timeresolved Electron Paramagnetic Resonance Spectroscopy[END_REF][START_REF] Amat-Guerri | O-Acyl-α-Oxooximes and related compounds. Chemistry, Photochemistry, And Use As Photoinitiators For Radical Polymerizations[END_REF][START_REF] Fast | Wavelength-Dependent Photochemistry of Oxime Ester Photoinitiators[END_REF][START_REF] Xu | Synthesis and photopolymerization kinetics of oxime ester photoinitiators[END_REF][START_REF] Degirmenci | Synthesis of benzoin end-chain functional macrophotoinitiator of poly(d,l-lactide) homopolymer and poly(ε-caprolactone)-poly(d,llactide) diblock copolymer by ROP and their use in photoinduced free radical promoted cationic copolymerization[END_REF] In 2017, the first examples of phenothiazine-based oxime esters were reported by Wang and coworkers, differing by the end-groups connected by mean of a ethynyl spacer. [START_REF] Ma | Conjugated phenothiazine oxime esters as free radical photoinitiators[END_REF] Notably, a carbazole, a triphenylamine and a phenyl group were introduced in the structure of oxime esters (See Examination of the UV-visible absorption spectra of oxime esters in THF revealed the end-group to only slightly influence the absorption spectra of Ph-TPZ-OXE, PTA-TPZ-OXE and CZ-TPZ-OXE. In fact, all dyes showed a broad absorption band extending between 300 and 475 nm. Due to the electron-withdrawing ability of the oxime moiety, an intramolecular push-pull effect could occur so that absorption maxima located at 378, 383 and 382 nm could be determined respectively for the three dyes. Upon irradiation with a laser diode at 405 nm, all oxime esters showed remarkable efficiencies since conversions of Ph-PTZ-OXE, TPA-PTZ-OXE and CZ-PTZ-OXE could reach up to 66%, 86% and 83%, after only 20 s of irradiation.

Final conversions of 88%, 96% and 94% could be determined with Ph-PTZ-OXE, TPA-PTZ-OXE and CZ-PTZ-OXE respectively after 100 s of irradiation. At 455 nm, TPGDA conversions of 62, 71 and 68% were respectively determined with Ph-PTZ-OXE, TPA-PTZ-OXE and CZ-PTZ-OXE after 70 s of irradiation. For comparison, the reference CQ/TEOA system could only furnish a monomer conversion of 52% and CQ alone only 11% TPGDA conversion. Influence of the dye concentration was examined with TPA-PTZ-OXE at 405 and 455 nm. Interestingly, an optimum concentration of 0.3 wt% was determined at the two irradiation wavelengths (See Figure 22). Upon increase of the concentration, a reduction of the monomer conversion was logically observed, resulting from inner filter effects impeding light penetration within the photocurable resin. Even if counter-intuitive, oxime esters can also be used as photosensitizers for iodonium salts. This point was examined and two-component dye/Iod2 (0.2%/2% w/w) photoinitiating systems were tested with the different oxime esters. Markedly, Iod2 concentration had only little effects on the photopolymerization of TPGDA at 405 nm.

Conversely, at 455 nm, a high monomer conversion was only obtained at 2 wt% iodonium salt (around 90% after 200 s) whereas conversions not exceeding 40% were determined at the other concentrations (See Figure 23). Finally, thermal stability of the different oxime esters was examined by thermogravimetric (TGA) analyses under a nitrogen flow. A decomposition temperature between 185-190°C was found for all dyes, demonstrating that the synthesized oxime esters perfectly met the requirement for being usable in photopolymerization.

Conclusion

To conclude, phenothiazine is a versatile structure since more than ten different families of dyes were developed with this scaffold. If historically, phenothiazine was reduced to the development of UV photoinitiating systems, introduction of peripheral groups has enabled to design visible light photoinitiators. Among all combinations, the less promising one seems to be the thioxanthone/phenothiazine combination as the resulting photoinitiating systems only exhibited poor initiating abilities. With aim at simplifying the photocurable resins, Type I photoinitiators have been elaborated with phenothiazine in 2017. Despites promising results, no further works have been developed. At present, if monocomponent systems have been developed with Type I photoinitiators, the design of monocomponent Type II photoinitiators comprising phenothiazine has not been achieved yet. As already done for other structures such as 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] Villotte | Design of Iodonium Salts for UV or Near-UV LEDs for Photoacid Generator and Polymerization Purposes[END_REF], Bodipy, [START_REF] Topa-Skwarczyńska | Development of the first panchromatic BODIPY-based one-component iodonium salts for initiating the photopolymerization processes[END_REF] or coumarins [START_REF] Mokbel | Specific cationic photoinitiators for near UV and visible LEDs: Iodonium versus ferrocenium structures[END_REF][START_REF] Topa | One-component cationic photoinitiators based on coumarin scaffold iodonium salts as highly sensitive photoacid generators for 3D printing IPN photopolymers under visible LED sources[END_REF][START_REF] Topa | Moving Towards a Finer Way of Light-Cured Resin-Based Restorative Dental Materials: Recent Advances in Photoinitiating Systems Based on Iodonium Salts[END_REF], the direct connection of iodonium salt to phenothiazine could provide a unique opportunity to design mono-component systems. Future works on phenothiazine will also certainly consist in developing water-soluble photoinitiating systems. Indeed, if widely studied, no example of water-soluble phenothiazine has been reported to date. Design of such dyes would be of crucial interest for 4D printing applications.

Figure 1 .

 1 Figure 1. Chemical structure of A-2PTz, monomers and additives.

Figure 2 .

 2 Figure 2. Contour plots of the HOMO and LUMO orbitals of A-2PTz. Reprinted with permission of Abdallah et al.[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]

Figure 3 .

 3 Figure 3.

Figure 3 .

 3 Figure 3. FRP experiments for 3D printing: 1) Photo of the initial resin; 2) the 3D patterns obtained after 3D-printing experiments. 3) Characterization of the patterns by numerical optical microscopy. Reprinted with permission of Abdallah et al. [91]

Figure 4 .

 4 Figure 4. Chemical structures of PT1-PT4 and monomer.

Figure 5 .

 5 Figure 5. Absorption spectra of the investigated compounds in chloroform: (a) PT2, (b) PT4, (c) PT3 and (d) PT1. Reproduced with permission of Rahal et al.[START_REF] Rahal | Design of new phenothiazine derivatives as visible light photoinitiators[END_REF]

Figure 6 .

 6 Figure 6. Photocomposites obtained upon irradiation with a near-UV light (LED@395 nm), belt speed = 2 m/min, TMPTA resin in the presence of glass fibers for different photoinitiating systems: (1) PT2/Iod/NPG (0.1%/1%/1% w/w/w); (2) PT4/Iod/NPG (0.1%/1%/1% w/w/w); and(3) PT2/Iod/NPG (0.1%/1%/1% w/w/w) in TMPTA. Reproduced with permission of Rahal et al.[START_REF] Rahal | Design of new phenothiazine derivatives as visible light photoinitiators[END_REF]

  is well-known to lack of absorption in the visible range. In 2008, Crivello developed a series of benzophenothiazine in which an extension of aromaticity was introduced.[246] Interestingly, by varying the reaction conditions, isomers of the same structures could be obtained i.e. 12H-benzo[a]phenothiazine (12-PT-a), 12Hbenzo[b]phenothiazine (12-PT-b) and 7H-benzo[c]phenothiazine (7-PT-c) (See Figure 7). The different dyes were notably used for the sensitization of various triarylsulfonium salts i.e.

Figure 7 .

 7 Figure 7. Chemical structures of different benzophenothiazines and triarylsulfonium salts.Comparison of the photoinitiating ability of the three benzophenothiazine isomers during the cationic polymerization of limonene dioxide (4-(epoxyisopropenyl)-1-methyl-1cyclohexene-1,2-epoxide) revealed 7-PT-c to outperform the two other isomers. As shown in the Figure8, a faster polymerization process was observed for 7-PT-c, evidenced by monitoring the polymerization reaction by pyrometry. Interestingly, if differences of polymerization rates could be found between the three isomers, a similar exothermicity for the polymerization reaction was found, the maximum temperature reaching 170°C upon irradiation with a Hg-lamp (See Figure8).

Figure 8 .

 8 Figure 8. Monitoring of the polymerization of limonene dioxide using two-component dye/SOC-10 (0.1%/1% w/w) systems upon irradiation with a Hg-lamp. Reprinted with the permission of Crivello et al. [246]

Figure 9 .

 9 Figure 9. Photopolymerization of limonene dioxide monitored by pyrometry using twocomponent dye/sulfonium (0.1%/1% w/w) systems. sulfonium = SS, MeOTA and DPS-C1C12.Reprinted with the permission of Crivello et al.[START_REF] Crivello | Benzophenothiazine and benzophenoxazine photosensitizers for triarylsulfonium salt cationic photoinitiators[END_REF]

Figure 10 .

 10 Figure 10. Chemical structure of PTZ-PD, Iod2 and TPGDA

Figure 11 .

 11 Figure 11. Chemical structure of curcumin, curcuminoid derivative CMD4, monomer and additives.

Figure 12 .

 12 Figure 12. Photopolymerization profiles of TPGDA using two-component CMD4/Iod2 systems; (a) upon irradiation at 460 nm; (b) upon irradiation at 560 nm. Reprinted with permission from Han et al. [264]

Figure 13 .

 13 Figure 13. Modification of the sample colors during polymerization for TPGDA-based resins, using two-component CMD4/Iod (0.2%/3% w/w) systems. Thickness : 1 mm. Reprinted with permission from Han et al. [264]

Figure 15 .Table 5 .

 155 Figure 15. UV-visible absorption spectra (1) B106 (2) B107 (3) B108 and (4) B111 in acetonitrile. Reprinted with permission from Abdallah et al.[START_REF] Abdallah | Phenothiazine derivatives as photoredox catalysts for cationic and radical photosensitive resins for 3D printing technology and photocomposite synthesis[END_REF]

Figure 17 .

 17 Figure 17. Chemical structures of PYZ-SO and PTZ-SO2.

Figure 18 .

 18 Figure 18. Thiophene-based phenothiazine tested by Wang and coworkers.

Figure 19 .

 19 Figure 19. UV-visible absorption spectra of thiophene-based phenothiazine in dichloromethane. Reprinted with permission of Chao et al.[START_REF] Chao | Thiophene-substituted phenothiazine-based photosensitisers for radical and cationic photopolymerization reactions under visible laser beams (405 and 455 nm)[END_REF]

Figure 21

 21 Figure 21).

Figure 21 .

 21 Figure 21. Chemical structures of phenothiazine-based oxime esters.

Figure 22 .

 22 Figure 22. Photopolymerization profiles of TPGDA using TPA-PTZ-OXE at different concentrations upon irradiation at 405 nm (left) and 455 nm (right). Reprinted with permission of Ma et al.[START_REF] Ma | Conjugated phenothiazine oxime esters as free radical photoinitiators[END_REF]

Figure 23 .

 23 Figure 23. Photopolymerization profiles of TPGDA using the two-component TPA-PTZ-OXE/Iod2 (0.2%/x% w/w with x = 0.5-2wt%) at different concentrations upon irradiation at 405 nm (left) and 455 nm (right). Reprinted with permission of Ma et al.[START_REF] Ma | Conjugated phenothiazine oxime esters as free radical photoinitiators[END_REF]

Table 1 .

 1 Final monomer conversions determined during the FRP of MPTA using different photoinitiating systems. 100 s of Irradiation with a LED emitting at 405 nm.

TMPTA conversions at t = 100 s. thin sample (25 µm) in laminate TMPTA conversions at t = 100 s thick sample (1.4 mm) under air

	Two-component photoinitiating	Three-component	Two-component	Three-component
		system		photoinitiating	photoinitiating	photoinitiating
				system	system	system
	A-2PTz	A-2PTz A-2PTz	A-2PTz	A-2PTz	A-2PTz
	+Iod	+NPG	+EDB	+ Iod/NPG	+Iod	+ Iod/NPG
	n.p. a ; 30% b ;	56% d	n.p. d	45% e	55% a	84% e
	37% c ; 43% d
	n.p.: no polymerization
	a: acridone/additive: 0.05%/1% w/w

b: acridone/additive: 0.1%/1% w/w c: acridone/additive: 0.3%/1% w/w d: acridone/additive: 0.5%/1% w/w e: acridone/additives: 0.05%/1%/1% w/w/w

Table 2 .

 2 Final monomer conversions determined for a BisGMA/TEGDMA blend. 150 s of irradiation with LED emitting at 405 nm using different photoinitiating systems (under air; thickness = 1.4 mm).For dental applications (or food packaging), extractability of photoinitiators is a major issue as this molecule can be potentially toxic. Extractability of A-2PTz from the

	Two-component photoinitiating system		Three-component
				photoinitiating system
	Acridone/Iod or Acridone/amine (NPG or EDB)	Acridone/Iod/NPG
	A-2PTz	A-2PTz	A-	A-2PTz
			2PTz
	+Iod	+NPG	+EDB	+Iod/NPG

b ; n.p. c ; n.p. d 30% c ; n.p. d n.p. d

Table 3 .

 3 Light absorption properties of PT compounds at 405 nm and λmax.

		λmax (nm)	εmax (M -1 .cm -1 )	ε@405nm (M -1 .cm -1 )
	PT1	400	5910	5850
	PT2	405	5730	5730
	PT3	408	25280	25150
	PT4	424	9570	8290

Table 4 .

 4 Final monomer conversion determined during the FRP of TMPTA and Ebecryl 40 using different photoinitiating systems after 400 s, upon irradiation with a LED at 405 nm (Thickness = 1.4 mm).

	TMPTA
	Two-component photoinitiating	Three-component photoinitiating system
		system
	PT/Iod (0.1%/1% w/w) and PT/Iod	PT/Iod/EDB and PT/Iod/NPG (0.1%/1%/1% w/w/w)
		(0.2%/1% w/w)
	PT1/ Iod PT2/Iod PT3/Iod PT4/Iod PT1/Iod/amine PT2/Iod/amine PT3/Iod/amine PT4/Iod/amine
	71 % a	64 % a	62 % a	67 % a	74 % c	77 % c	72 % c	78 % c
	54 % b	68% b	32 % b	67% b	65 % d	75 % d	61 % d	76 % d
					Ebecryl 40
	PT1/ Iod PT2/Iod PT3/Iod PT4/Iod PT1/Iod/amine PT2/Iod/amine PT3/Iod/amine PT4/Iod/amine
	87 % a	85 % a	63 % a	81 % a	81% c	90 % c	88 % c	90 % c
	63 % b	68 % b	48 % b	72 % b	73 % d	87 % d	71 % d	81 % d
	a: PT/Iod (0.1%/ 1% w/w)		c: PT/Iod/NPG (0.1%/1%/1% w/w/w)
	b: PT/Iod (0.2%/1% w/w)		d: PT/Iod/EDB (0.1%/1%/1% w/w/w)

Table 6 .

 6 Final

	monomer conversions determined during the CP of EPOX using different
	photoinitiating systems (800 s of irradiation, LED emitting at 405 nm (thickness = 25 µm) under
	air.
	B106/Iod	B107/Iod	B108/Iod	B111/Iod
	57% (0.5%/1% w/w)	53% (0.5%/1% w/w)	43% (0.5%/1% w/w)	58% (0.5%/1% w/w)
	64% (0.2%/1% w/w)	44% (0.2%/1% w/w)	39% (0.2%/1% w/w)	47% (0.2%/1% w/w)

Table 7 .

 7 Final monomer conversions determined during the FRP of TMPTA using different photoinitiating systems (100 s of irradiation, LED emitting at 405 nm (thickness = 25 µm) in laminate.

	Two-component photoinitiating system	Three-component photoinitiating system
		dye/Iod (0.2%/1% w/w)		dye/Iod/NPG (0.2%/1%/1% w/w)
	B106	B107	B108	B111	B106	B107	B108	B111
	46%	26%	54%	33%	59%	55%	56%	48%

Table 8 .

 8 Final monomer conversions obtained using the two-component dye/Iod2 (0.1%/1% w/w) system for TPGDA and the two-component dye/Iod2 (0.1%/3% w/w) system for E51. Value determined after 100 s of irradiation.

				TPGDA			E51
			405 nm	455 nm		455 nm
		PTZ-SO/Iod2		75%	87%		68%
		PTZ-SO2/Iod2		76%	84%		73%
		CQ/TEOA		34%	52%		-
	1.8. Thiophene derivatives
	Thiophene is a structure widely used in Organic Electronics due to its remarkable
	electron-donating ability and its low oxidation potential.[274-278] At present, only few
	thiophene	derivatives	have	successfully	been	used	in	visible	light

Table 9 .

 9 Optical absorption properties of thiophene-based dyes

		λmax	εmax ×10 4	λmax	εmax ×10 4	εmax ×10 3	εmax ×10 3
	Dyes	(nm)	(M -1 •cm -1 )	(nm)	(M -1 •cm -1 )	(M -1 •cm -1 )	(M -1 •cm -1 )
						(405 nm)	(455 nm)
	ThEPTZ	307	3.18	363	0.87	3.37	0.33
	Th2EPTZ	317	4.69	375	1.26	8.87	1.86
	ThFEPTZ	327	9.93	435	1.02	6.93	9.19
	ThHPTZ	308	2.82	363	0.76	2.88	0.33
	Th2HPTZ	317	4.02	375	1.11	7.58	1.51
	ThFHPTZ	327	3.54	435	0.94	6.37	8.36

Table 10 .

 10 Monomer conversion determined for E51 under air upon irradiation with laser diodes for 300 s and final conversion (CF) obtained in the presence of ThPTZs/Iod2 (0.1%: 3.0%, w/w) and EPTZ/Iod2 (0.1%: 3.0%, w/w) as the reference.

		405 nm		455 nm
		300 s	CF	300 s	CF
	ThEPTZ/Iod2	58.2	62.3	62.8	64.1
	ThHPTZ/Iod2	56.5	60.8	64.4	65.6
	Th2EPTZ/Iod2	70.5	72.2	68.5	70.2
	Th2HPTZ/Iod2	67.5	69.4	69.2	70.9
	ThFEPTZ/Iod2	67.1	68.7	71.4	72.8
	ThFHPTZ/Iod2	65.8	67.3	72.7	74.7
	EPTZ/Iod2	14.7	20.3	15.9	21.4

Table 12

 12

	). Overall, an absorption

Table 12 .

 12 Absorption properties of the different thioxanthone derivatives.

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.