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

Recent Advances on Photoinitiating Systems Designed for Solar Photocrosslinking Polymerization Reactions

Keywords: Photoinitiator, sunlight, daylight, photopolymerization, LED, low light intensity, solar, sun

Photopolymerization is an active research field and recently, a great deal of efforts has been devoted to replace the energy-consuming UV irradiation setups by the popular and easily available energy-saving light-emitting diodes (LEDs). With aim at going a step further, sunlight can be used as an inexpensive and unlimited light source to perform polymerization processes. In this case, no energy is required anymore, and the use of photopolymerization for various outdoor applications becomes possible. Over the years, and with aim developing reliable photoinitiating systems, various dyes have been developed, enabling in the case of sunny days to efficiently promote polymerization processes. In this review, an overview on the different photoinitiating systems developed for polymerization processes done under sunlight or even daylight is provided. Polymerization efficiency is often compared to that obtained with artificial light sources, enabling to determine the efficacy of the photoinitiating systems under sunlight.

Introduction

During the past decades, photopolymerization under visible light has been extensively studied and identified as a promising alternative to the traditional UV photopolymerization. This intense research activity is supported by the different applications using photopolymerization. Among the most popular applications, adhesives, solvent-free paints, dentistry, coatings and varnishes, microelectronics, 3D and 4D printing can be cited as relevant examples. [START_REF] Jasinski | Photopolymerization in dispersed systems[END_REF][START_REF] Noè | Cationic UV-Curing of Epoxidized Biobased Resins[END_REF][START_REF] Yuan | Low volume shrinkage photopolymerization system using hydrogen-bond-based monomers[END_REF][START_REF] Khudyakov | Kinetics of Photopolymerization of Acrylates with Functionality of 1-6[END_REF][START_REF] Dickens | Photopolymerization Kinetics of Methacrylate Dental Resins[END_REF][START_REF] Maffezzoli | Photopolymerization of dental composite matrices[END_REF][START_REF] Dikova | Optimization of Photopolymerization Process of Dental Composites[END_REF][START_REF] Andreu | 4D printing materials for vat photopolymerization[END_REF][START_REF] Chen | Photopolymerization and 3D/4D applications using newly developed dyes: Search around the natural chalcone scaffold in photoinitiating systems[END_REF][START_REF] Bagheri | Photopolymerization in 3D Printing[END_REF][START_REF] Fouassier | Three-component photoinitiating systems: towards innovative tailor made high performance combinations[END_REF][START_REF] Lalevée | Dyes and Chromophores in Polymer Science[END_REF][START_REF] Lalevée | Metal and metal-free photocatalysts: mechanistic approach and application as photoinitiators of photopolymerization[END_REF][START_REF] Dumur | Recent advances on water-soluble photoinitiators of polymerization[END_REF][START_REF] Dumur | The future of visible light photoinitiators of polymerization for photocrosslinking applications[END_REF] But photopolymerization is also used in various fields since as the preparation of polymer electrolytes for dye-sensitized solar cells, [START_REF] Bella | Photoinduced polymerization: An innovative, powerful and environmentally friendly technique for the preparation of polymer electrolytes for dyesensitized solar cells[END_REF] nanoimprinting lithography for organic field-effect transistors, [START_REF] Yang | Insight into High-Performance Conjugated Polymers for Organic Field-Effect Transistors[END_REF] or the design of electroactive materials with charge transport properties. [START_REF] Sakaino | Charge transport in liquid crystal network of terthiophene-siloxane block molecules[END_REF] Photopolymerization also exhibits several specificities that set it apart from the conventional polymerization in solution. Notably, photopolymerization can be done in solvent-free conditions, enabling to polymerize without solvents. A temporal and a spatial control can also be achieved, what is beneficial for applications such as 3D-printing.

Photopolymerization processes can also be done at room temperature, enabling the polymerization of heat-sensitive monomers. Historically, photopolymerization was done under UV light. Besides, due to safety concerns, this approach is more and more discarded from industrial applications. Notably, UV light is responsible of skin cancers and eye damages. [START_REF] Armstrong | The epidemiology of UV induced skin cancer, Consequences Expo[END_REF][START_REF] De Gruijl | Skin cancer and solar UV radiation[END_REF][START_REF] Narayanan | Review: Ultraviolet radiation and skin cancer[END_REF][START_REF] Shao | Visible light initiating systems for photopolymerization: status, development and challenges[END_REF] Ozone is also produced during photopolymerization, requiring safety procedures. [START_REF] Shao | Visible light initiating systems for photopolymerization: status, development and challenges[END_REF] In 2013, the United Nations Environmental Program (UNEP) Minamata Convention on Mercury has banned Hg lamps from industrial applications and since 2020, mercury lamps can't be manufactured, imported and exported. Therefore, alternatives to these historical light sources have to be found. During the last decade, light-emitting diodes (LEDs) have emerged as unavoidable light sources due to their low-cost, compactness, long operating lifetime and energy efficiency. This last point is of crucial importance, in light of the "sobriety" energy savings drive launched during winter 2022 by all European countries to avoid winter power cuts. Use of energy consuming irradiation setups such as UV light sources is now totally disconnected from the reality. If LEDs are more power efficient than filament light sources, a step further can be achieved if sunlight is used as the light source. As specificities, emission spectrum of Sun is relatively broad, extending from the UV range until the infrared range (See Figure 1). [START_REF] Moreno-Sáez | Modelling the distribution of solar spectral irradiance using data mining techniques[END_REF][START_REF] Diffey | Solar Spectral Irradiance and Summary Outputs Using Excel[END_REF] However, light intensity is lower than that of artificial light sources (around 2 mW/cm²). [START_REF] Tehfe | Photopolymerization Reactions: On the Way to a Green and Sustainable Chemistry[END_REF] A strong dependency of the monomer conversion with the weather conditions (sunny days, cloudy days, polymerization experiments done during the summer or winter) is also evidenced. Ideally, an efficient polymerization during a rainy day of winter would be ideal, ensuring high monomer conversions during better weather conditions. However, if from an energetic viewpoint, photopolymerization with sunlight seems to be ideal, visible light photons are also less energetic than UV photons so that in order to overcome these two drawbacks, highly reactive photoinitiating systems have to be developed. Besides, polymerization with sunlight constitutes the most cost-effective approach since no irradiation setups are required and do not necessitate energy consumption. Ref. [START_REF] Diffey | Solar Spectral Irradiance and Summary Outputs Using Excel[END_REF] Over the years, photopolymerists have thus examined a wide range of structures with aim at identifying the most promising structures for sunlight polymerization among all photoinitiators that are activable under artificial light sources. Among the different families of dyes investigated, anthraquinone, [START_REF] Xiao | Cationic and Thiol-Ene Photopolymerization upon Red Lights Using Anthraquinone Derivatives as Photoinitiators[END_REF] chromones and flavones, [START_REF] Tehfe | New chromone based photoinitiators for polymerization reactions under visible light[END_REF][START_REF] You | Flavonol dyes with different substituents in photopolymerization[END_REF][START_REF] Al Mousawi | 3-Hydroxyflavone and N-Phenylglycine in High Performance Photoinitiating Systems for 3D Printing and Photocomposites Synthesis[END_REF] thiophene derivatives, [START_REF] Dumur | Recent Advances on Visible Light Thiophene-based Photoinitiators of Polymerization[END_REF] imidazole derivatives, [START_REF] Ma | Imidazole based dual photo/thermal initiators for highly efficient radical polymerization under air with a metal-free approach[END_REF] pyridinium salts, [START_REF] Ma | Charge Transfer Complexes (CTCs) with Pyridinium Salts: Toward 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processes[END_REF][START_REF] Xu | Utilizing Intramolecular Photoinduced Electron Transfer to Enhance Photothermal Tumor Treatment of Aza-BODIPY-Based Near-Infrared Nanoparticles[END_REF][START_REF] Skotnicka | New BODIPY Dyes Based on Benzoxazole as Photosensitizers in Radical Polymerization of Acrylate Monomers[END_REF][START_REF] Lu | Boron dipyrromethene (BODIPY) in polymer chemistry[END_REF][START_REF] Telitel | BODIPY derivatives and boranil as new photoinitiating systems of cationic polymerization exhibiting a tunable absorption in the 400-600 nm spectral range[END_REF][START_REF] Telitel | Photopolymerization of Cationic Monomers and Acrylate/Divinylether Blends under Visible Light Using Pyrromethene Dyes[END_REF] phenothiazines, [START_REF] Dumur | Recent advances on visible light Phenothiazine-based photoinitiators of polymerization[END_REF][START_REF] Deng | Novel chalcone-based phenothiazine derivative photoinitiators for visible light induced photopolymerization with 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phenothiazine-carbazole/triphenylamine as double donors in photopolymerization under 455 nm and 532 nm laser beams[END_REF][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][START_REF] Hao | Four new twophoton polymerization initiators with varying donor and conjugated bridge: Synthesis and two-photon activity[END_REF] curcumin, [START_REF] Zhao | A new role of curcumin: as a multicolor photoinitiator for polymer fabrication under household UV to red LED bulbs[END_REF][START_REF] Crivello | Curcumin: A naturally occurring long-wavelength photosensitizer for diaryliodonium salts[END_REF][START_REF] Han | Facilely prepared blue-green light sensitive curcuminoids with excellent bleaching properties as high performance photosensitizers in cationic and free radical photopolymerization[END_REF][START_REF] Mishra | Curcumin, A Novel Natural Photoinitiator for the Copolymerization of Styrene and Methylmethacrylate[END_REF] naphthoquinones, [START_REF] Hammoud | Naphthoquinone-based imidazolyl esters as blue-lightsensitive Type I photoinitiators[END_REF] 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] coumarins, [START_REF] Abdallah | Coumarin derivatives as versatile photoinitiators for 3D printing, polymerization in water and photocomposite synthesis[END_REF][START_REF] Abdallah | Keto-coumarin scaffold for photoinitiators for 3D printing and photocomposites[END_REF][START_REF] Abdallah | Coumarins as Powerful Photosensitizers for the Cationic Polymerization of Epoxy-Silicones under Near-UV and Visible Light and Applications for 3D Printing Technology[END_REF][START_REF] Abdallah | Boron Compounds as Additives for the Cationic Polymerization Using Coumarin Derivatives in Epoxy Silicones[END_REF][START_REF] Chen | Photopolymerization of Coumarin-Containing Reversible Photoresponsive Materials Based on Wavelength Selectivity[END_REF][START_REF] Li | Coumarin-Based Oxime Esters: Photobleachable and Versatile Unimolecular Initiators for Acrylate and Thiol-Based Click Photopolymerization under Visible Light-Emitting Diode Light Irradiation[END_REF][START_REF] Rahal | Mono vs. Difunctional Coumarin as Photoinitiators in Photocomposite Synthesis and 3D Printing[END_REF][START_REF] Rajeshirke | Enhancement of NLO Properties in OBO Fluorophores Derived from Carbazole-Coumarin Chalcones Containing Carboxylic Acid at the N-Alykl Terminal End[END_REF][START_REF] Rahal | Design of ketocoumarin based photoinitiator for Free Radical Photopolymerization: Towards 3D printing and photocomposites applications[END_REF][START_REF] Rahal | 3-Carboxylic Acid and Formyl-Derived Coumarins as Photoinitiators in Photo-Oxidation or Photo-Reduction Processes for Photopolymerization upon Visible Light: Photocomposite Synthesis and 3D Printing Applications[END_REF][START_REF] Hammoud | Substituent effects on the photoinitiation ability of coumarin-based oxime-ester photoinitiators for free radical photopolymerization[END_REF][START_REF] Dumur | Recent advances on coumarin-based photoinitiators of polymerization[END_REF][START_REF] Liu | Recent advances on visible light Coumarin-based oxime esters as initiators of polymerization[END_REF][START_REF] Zhu | Broad wavelength sensitive coumarin sulfonium salts as photoinitiators for cationic, free radical and hybrid photopolymerizations[END_REF] camphorquinones, [START_REF] Kamoun | Carboxylated camphorquinone as visible-light photoinitiator for biomedical application: Synthesis, characterization, and application[END_REF][START_REF] Santini | Photoinitiators in Dentistry: A Review[END_REF] iodonium salts, [START_REF] Zivic | A novel naphthalimide scaffold based iodonium salt as a one-component photoacid/photoinitiator for cationic and radical polymerization under LED exposure[END_REF][START_REF] Mokbel | Specific cationic photoinitiators for near UV and visible LEDs: Iodonium versus ferrocenium structures[END_REF][START_REF] Villotte | Design of Iodonium Salts for UV or Near-UV LEDs for Photoacid Generator and Polymerization Purposes[END_REF][START_REF] Tasdelen | Photoacid Generation by Stepwise Two-Photon Absorption: Photoinitiated Cationic Polymerization of Cyclohexene Oxide by Using Benzodioxinone in the Presence of Iodonium Salt[END_REF][START_REF] Crivello | Diaryliodonium Salts. A New Class of Photoinitiators for Cationic Polymerization[END_REF][START_REF] He | Photoreaction and photopolymerization studies on squaraine dyes/iodonium salts combination[END_REF][START_REF] Jun | Photopolymerization Initiated by Dimethylaminochalcone/Diphenyliodonium Salt Combination System Sensitive to Visible Light[END_REF][START_REF] Zivic | Recent Advances and Challenges in the Design of Organic Photoacid and Photobase Generators for Polymerizations[END_REF] 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] 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][START_REF] Dumur | Cationic iridium complex for the design of soft salt-based phosphorescent OLEDs and color-tunable light-emitting electrochemical cells[END_REF] truxene derivatives, [START_REF] Tehfe | New Cleavable Photoinitiator Architecture with Huge Molar Extinction Coefficients for Polymerization in the 340-450 nm Range[END_REF] 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 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[START_REF] Dumur | New Chelating Stilbazonium-Like Dyes from Michler's Ketone[END_REF][START_REF] Cao | Photocatalysis of viologens for photoinitiated polymerization using carboxylic acid as electron donors[END_REF][START_REF] Schnabel | Cationic photopolymerization with the aid of pyridinium-type salts[END_REF] 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][START_REF] Liu | Effect of Decarboxylation on the Photoinitiation Behavior of Nitrocarbazole-Based Oxime Esters[END_REF][START_REF] Hammoud | Chemical engineering around the 5,12-dihydroindolo[3,2-a]carbazole scaffold : Fine tuning of the optical properties of visible light photoinitiators of polymerization[END_REF][START_REF] Xu | High-efficient carbazole-based photo-bleachable dyes as free radical initiators for visible light polymerization[END_REF][START_REF] Dumur | Recent advances on carbazole-based oxime esters as photoinitiators of polymerization[END_REF][START_REF] Hammoud | Dialkyl-5,12-dihydroindolo[3,2-a]carbazole-Based Oxime-Esters for LED Photoinitiating Systems and Application on 3D Printing[END_REF][START_REF] Liao | Design, synthesis and properties of carbazole-indenedione based photobleachable photoinitiators for photopolymerization[END_REF][START_REF] Bin | Carbazolebased Anion Ionic Water-Soluble Two-Photon Initiator for Achieving 3D Hydrogel Structures[END_REF] cyanines, [START_REF] Mokbel | On demand NIR activated photopolyaddition reactions[END_REF][START_REF] Mokbel | NIR Sensitizer Operating under Long Wavelength (1064 nm) for Free Radical Photopolymerization Processes[END_REF][START_REF] Launay | Near-infrared light for polymer re-shaping and re-processing applications[END_REF][START_REF] Caron | Near-Infrared PhotoInitiating Systems: Photothermal versus Triplet-Triplet Annihilation-Based Upconversion Polymerization[END_REF][START_REF] Bonardi | Photoinduced Thermal Polymerization Reactions[END_REF][START_REF] Launay | Safe near infrared light for fast polymers surface sterilization using organic heaters[END_REF][START_REF] Launay | Photothermal activation in the near infrared range for 4-dimensional printing using relevant organic dyes[END_REF] benzylidene ketones, [START_REF] Bao | Double benzylidene ketones as photoinitiators for visible light photopolymerization[END_REF][START_REF] Fu | Photopolymerization of acrylate resin and ceramic suspensions with benzylidene ketones under blue/green LED[END_REF][START_REF] Dumur | Recent advances on benzylidene ketones as photoinitiators of polymerization[END_REF][START_REF] Zhao | Polyethylene glycol-functionalized benzylidene cyclopentanone dyes for two-photon excited photodynamic therapy[END_REF][START_REF] Fang | Cationic benzylidene cyclopentanone photosensitizers for selective photodynamic inactivation of bacteria over mammalian cells[END_REF][START_REF] Xue | Effect of Bridging Position on the Two-Photon Polymerization Initiating Efficiencies of Novel Coumarin/Benzylidene Cyclopentanone Dyes[END_REF][START_REF] Dumur | Recent advances on benzylidene cyclopentanones as visible light photoinitiators of polymerization[END_REF][START_REF] Egorov | Benzylidene Cyclopentanone Derivative Photoinitiator for Two-Photon Photopolymerization-Photochemistry and 3D Structures Fabrication for X-ray Application[END_REF] thioxanthones, [START_REF] Karaca | Thioxanthone-benzothiophenes as photoinitiator for free radical polymerization[END_REF][START_REF] Balta | An annelated thioxanthone as a new Type II initiator[END_REF][START_REF] Balta | Thioxanthone-Diphenyl Anthracene: Visible Light Photoinitiator[END_REF][START_REF] Dadashi-Silab | Shining a light on an adaptable photoinitiator: advances in photopolymerizations initiated by thioxanthones[END_REF][START_REF] Eren | Photophysical and Photochemical Studies of Novel Thioxanthone-Functionalized Methacrylates through LED Excitation[END_REF][START_REF] Qiu | Thioxanthone photoinitiator containing polymerizable N-aromatic maleimide for photopolymerization[END_REF][START_REF] Tar | Panchromatic Type II Photoinitiator for Free Radical Polymerization Based on Thioxanthone Derivative[END_REF][START_REF] Wu | Thioxanthone based one-component polymerizable visible light photoinitiator for free radical polymerization[END_REF][START_REF] Wu | High-Performance and Low Migration One-Component Thioxanthone Visible Light Photoinitiators[END_REF][START_REF] Wu | Visible light-emitting diode-sensitive thioxanthone derivatives used in versatile photoinitiating systems for photopolymerizations[END_REF][START_REF] Lalevée | Light-Harvesting Organic Photoinitiators of Polymerization[END_REF][START_REF] Esen | The investigation of photoinitiated polymerization of multifunctional acrylates with TX-BT by Photo-DSC and RT-FTIR[END_REF][START_REF] Lalevée | New thioxanthone and xanthone photoinitiators based on silyl radical chemistry[END_REF][START_REF] Gencoglu | A Water Soluble, Low Migration, and Visible Light Photoinitiator by Thioxanthone-Functionalization of Poly(ethylene glycol)-Containing Poly(β-amino ester)[END_REF][START_REF] Wang | New functionalized thioxanthone derivatives as type I photoinitiators for polymerization under UV-Vis LEDs[END_REF] anthracene, [START_REF] Dumur | Recent Advances on Anthracene-based Photoinitiators of Polymerization[END_REF] chalcones, [START_REF] Chen | Photopolymerization and 3D/4D applications using newly developed dyes: Search around the natural chalcone scaffold in photoinitiating systems[END_REF][START_REF] Chen | Photoinitiators derived from natural product scaffolds: monochalcones in three-component photoinitiating systems and their applications in 3D printing[END_REF][START_REF] Tang | A high performance phenyl-free LED photoinitiator for cationic or hybrid photopolymerization and its application in LED cationic 3D printing[END_REF][START_REF] Xu | Allyloxy ketones as efficient photoinitiators with high migration stability in free radical polymerization and 3D printing[END_REF][START_REF] Xu | Design of ketone derivatives as highly efficient photoinitiators for free radical and cationic photopolymerizations and application in 3D printing of composites[END_REF][START_REF] Chen | Bis-chalcone derivatives derived from natural products as near-UV/visible light sensitive photoinitiators for 3D/4D printing[END_REF][START_REF] Liu | Design of photoinitiating systems based on the chalcone-anthracene scaffold for LED cationic photopolymerization and application in 3D printing[END_REF][START_REF] Giacoletto | Recent Advances in bis-Chalcone-Based Photoinitiators of Polymerization: From Mechanistic Investigations to Applications[END_REF][START_REF] Ibrahim-Ouali | Recent Advances on Chalcone-based Photoinitiators of Polymerization[END_REF][START_REF] Chen | In situ generation of Ag nanoparticles during photopolymerization by using newly developed dyes-based three-component photoinitiating systems and the related 3D printing applications and their shape change behavior[END_REF][START_REF] Chen | Water-Soluble Visible Light Sensitive Photoinitiating System Based on Charge Transfer Complexes for the 3D Printing of Hydrogels[END_REF][START_REF] Tehfe | Chalcone derivatives as highly versatile photoinitiators for radical, cationic, thiol-ene and IPN polymerization reactions upon exposure to visible light[END_REF][START_REF] Sun | In silico rational design by molecular modeling of new ketones as photoinitiators in three-component photoinitiating systems: application in 3D printing[END_REF][START_REF] Chen | Interpenetrating polymer network hydrogels using natural based dyes initiating systems: antibacterial activity and 3D/4D performance[END_REF][START_REF] Yen | Triphenylamine-functionalized chalcones as onecomponent type II visible-light-absorbing photoinitiators for free radical photopolymerization[END_REF][START_REF] Gao | New long-wavelength D-π-A-π-D chalcone photoinitiator for visible light polymerization with photobleaching and biocompatibility properties[END_REF][START_REF] Deng | Synthesis and properties of novel bis-chalcone-based photoinitiators for LED polymerization with photobleaching and low migration[END_REF] dithienophospholes, [START_REF] Al Mousawi | π-Conjugated Dithienophosphole Derivatives as High Performance Photoinitiators for 3D Printing Resins[END_REF][START_REF] Dumur | Organic Electronics: An El Dorado in the Quest of New Photocatalysts for Polymerization Reactions[END_REF] iron complexes, [START_REF] Zhang | Iron-Based Metal-Organic Frameworks (MOF) as Photocatalysts for Radical and Cationic Polymerizations under Near UV and Visible LEDs (385-405 nm)[END_REF][START_REF] Garra | Ferrocene-based (photo)redox polymerization under long wavelengths[END_REF][START_REF] Telitel | Iron complexes as potential photocatalysts for controlled radical photopolymerizations: A tool for modifications and patterning of surfaces[END_REF][START_REF] Zhang | Iron complexes as photoinitiators for radical and cationic polymerization through photoredox catalysis processes[END_REF][START_REF] Zhang | Visiblelight-sensitive photoredox catalysis by iron complexes: Applications in cationic and radical polymerization reactions[END_REF][START_REF] Zhang | Iron Complexes in Visible-Light-Sensitive Photoredox Catalysis: Effect of Ligands on Their Photoinitiation Efficiencies[END_REF][START_REF] Dumur | Recent advances on ferrocene-based photoinitiating systems[END_REF][START_REF] Dumur | Recent advances on iron-based photoinitiators of polymerization[END_REF] benzophenones, [START_REF] Liu | Monocomponent Photoinitiators based on Benzophenone-Carbazole Structure for LED Photoinitiating Systems and Application on 3D Printing[END_REF][START_REF] Xiao | Variations on the Benzophenone Skeleton: Novel High Performance Blue Light Sensitive Photoinitiating Systems[END_REF][START_REF] Zhang | Design of Novel Photoinitiators for Radical and Cationic Photopolymerizations under Near UV and Visible LEDs (385, 395, and 405 nm)[END_REF][START_REF] Liu | New multifunctional benzophenone-based photoinitiators with high migration stability and their applications in 3D printing[END_REF][START_REF] Liu | Novel Photoinitiators Based on Benzophenone-Triphenylamine Hybrid Structure for LED Photopolymerization[END_REF][START_REF] Liu | A monocomponent bifunctional benzophenone-carbazole type II photoinitiator for LED photoinitiating systems[END_REF][START_REF] Tehfe | Trifunctional Photoinitiators Based on a Triazine Skeleton for Visible Light Source and UV LED Induced Polymerizations[END_REF][START_REF] Lin | Efficacy Modeling of New Multi-Functional Benzophenone-Based System for Free-Radical/Cationic Hybrid Photopolymerization Using 405 nm LED[END_REF] triphenylamines, [START_REF] Dumur | Recent advances on visible light Triphenylamine-based photoinitiators of polymerization[END_REF][START_REF] Ladika | Synthesis and application of triphenylamine-based aldehydes as photoinitiators for multi-photon lithography[END_REF] copper complexes, [START_REF] Xiao | Copper Complexes in Radical Photoinitiating Systems: Applications to Free Radical and Cationic Polymerization upon Visible LEDs[END_REF][START_REF] Xiao | Copper complexes: the effect of ligands on their photoinitiation efficiencies in radical polymerization reactions under visible light[END_REF][START_REF] Xiao | Copper and iron complexes as visible-light-sensitive photoinitiators of polymerization[END_REF][START_REF] Garra | Copper-Based (Photo)redox Initiating Systems as Highly Efficient Systems for Interpenetrating Polymer Network Preparation[END_REF][START_REF] Garra | Mechanosynthesis of a Copper complex for redox initiating systems with a unique near infrared light activation[END_REF][START_REF] Garra | Mechanosynthesized copper(I) complex based initiating systems for redox polymerization: towards upgraded oxidizing and reducing agents[END_REF][START_REF] Mokbel | Copper photoredox catalyst "G1": a new high performance photoinitiator for near-UV and visible LEDs[END_REF][START_REF] Mokbel | Simultaneous initiation of radical and cationic polymerization reactions using the "G1" copper complex as photoredox catalyst: Applications of free radical/cationic hybrid photopolymerization in the composites and 3D printing fields[END_REF][START_REF] Mousawi | Copper photoredox catalysts for polymerization upon near UV or visible light: structure/reactivity/efficiency relationships and use in LED projector 3D printing resins[END_REF][START_REF] Mau | Panchromatic Copper Complexes for Visible Light Photopolymerization[END_REF][START_REF] Mau | Concomitant initiation of radical and cationic polymerisations using new copper complexes as photoinitiators: Synthesis and characterisation of acrylate/epoxy interpenetrated polymer networks[END_REF][START_REF] Garra | Copper (Photo)redox Catalyst for Radical Photopolymerization in Shadowed Areas and Access to Thick and Filled Samples[END_REF][START_REF] Garra | A New Highly Efficient Amine-Free and Peroxide-Free Redox System for Free Radical Polymerization under Air with Possible Light Activation[END_REF][START_REF] Garra | New copper(I) complex based initiating systems in redox polymerization and comparison with the amine/benzoyl peroxide reference[END_REF][START_REF] Noirbent | Recent Advances on Copper Complexes as Visible Light Photoinitiators and (Photo) Redox Initiators of Polymerization[END_REF][START_REF] Borjigin | The new LED-Sensitive photoinitiators of Polymerization: Copper complexes in free radical and cationic photoinitiating systems and application in 3D printing[END_REF][START_REF] Rahal | Novel Copper Complexes as Visible Light Photoinitiators for the Synthesis of Interpenetrating Polymer Networks (IPNs)[END_REF][START_REF] Lin | Efficacy analysis of new copper complex for visible light (455, 530 nm) radical/cationic photopolymerization: The synergic effects and catalytic cycle[END_REF] 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] zinc complexes, [START_REF] Tehfe | Zinc-based metal complexes as new photocatalysts in polymerization initiating systems[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] silyl glyoximides, [START_REF] Kirschner | Silyl Glyoximides: Toward a New Class of Visible Light Photoinitiators[END_REF] squaraines, [START_REF] Giacoletto | Recent advances on squaraine-based photoinitiators of polymerization[END_REF][START_REF] Xiao | Panchromatic Photopolymerizable Cationic Films Using Indoline and Squaraine Dye Based Photoinitiating Systems[END_REF][START_REF] Launay | NIR Organic Dyes as Innovative Tools for Reprocessing/Recycling of Plastics: Benefits of the Photothermal Activation in the Near-Infrared Range[END_REF][START_REF] Bonardi | Different NIR dye scaffolds for polymerization reactions under NIR light[END_REF][START_REF] Balcerak | Novel photoinitiators based on difluoroborate complexes of squaraine dyes for radical polymerization of acrylates upon visible light[END_REF] pyrrole derivatives, [START_REF] Dumur | Recent advances on visible light pyrrole-derived photoinitiators of polymerization[END_REF][START_REF] Li | A pyrrole-carbazole photoinitiator for radical and cationic visible light LED photopolymerization[END_REF] conjugated polymers, [START_REF] Zhang | Polymeric Photoinitiators: A New Search toward High Performance Visible Light Photoinitiating Systems[END_REF] polyoxometalates, [START_REF] Ghali | LED and solar photodecomposition of erythrosine B and rose Bengal using H3PMo12O40/polymer photocatalyst[END_REF][START_REF] Brahmi | Performance improvement of the photocatalytic process for the degradation of pharmaceutical compounds using new POM/polymer photocatalysts[END_REF][START_REF] Brahmi | Polyoxometalates/polymer composites for the photodegradation of bisphenol-A[END_REF] 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-3,4:9,10-bis(Dicarboximide) as Efficient Photosensitizers[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 pyreneketone moieties[END_REF][START_REF] Dumur | Recent advances on pyrene-based photoinitiators of polymerization[END_REF][START_REF] Tehfe | A Multicolor Photoinitiator for Cationic Polymerization and Interpenetrated Polymer Network Synthesis: 2,7-Di-tert-butyldimethyldihydropyrene[END_REF][START_REF] Telitel | New functionalized aromatic ketones as photoinitiating systems for near visible and visible light induced polymerizations[END_REF][START_REF] Tehfe | Polyaromatic Structures as Organo-Photoinitiator Catalysts for Efficient Visible Light Induced Dual Radical/Cationic Photopolymerization and Interpenetrated Polymer Networks Synthesis[END_REF] 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] benzylidene ketones, [START_REF] Bao | Double benzylidene ketones as photoinitiators for visible light photopolymerization[END_REF][START_REF] Fu | Photopolymerization of acrylate resin and ceramic suspensions with benzylidene ketones under blue/green LED[END_REF] bisacylphenylphosphine oxide, [START_REF] Wu | Synthesis and characterization of low-migration bisacylphenylphosphine oxide photoinitiators[END_REF] quinoxalines, [START_REF] Corakci | Long wavelength photosensitizers in photoinitiated cationic polymerization: The effect of quinoxaline derivatives on photopolymerization[END_REF][START_REF] Pyszka | Study on New Dental Materials Containing Quinoxaline-Based Photoinitiators in Terms of Exothermicity of the Photopolymerization Process[END_REF][START_REF] Pyszka | Photoinitiation abilities of indeno-and indoloquinoxaline derivatives and mechanical properties of dental fillings based on multifunctional acrylic monomers and glass ionomer[END_REF][START_REF] Arsu | Photoinduced free radical polymerization initiated with quinoxalines[END_REF][START_REF] Aydın | Photoinitiated free radical polymerization of methylmethacrylate by using of quinoxalines in the presence of aldehydes[END_REF][START_REF] Bulut | A quinoxaline derivative as a long wavelength photosensitizer for diaryliodonium salts[END_REF][START_REF] Bulut | Quinoxaline derivatives as long wavelength photosensitizers in photoinitiated cationic polymerization of diaryliodonium salts[END_REF][START_REF] Cao | Triphenylamine-modified quinoxaline derivatives as two-photon photoinitiators[END_REF][START_REF] Balta | Quinoxaline derivatives as photoinitiators in UV-cured coatings[END_REF][START_REF] Kucybała | Development of new dyeing photoinitiators for free radical polymerization based on the 1H-pyrazolo[3,4-b]quinoxaline skeleton. Part 2[END_REF][START_REF] Podsiadły | The synthesis of novel, visible-wavelength, oxidizable polymerization sensitizers based on the 8-halogeno-5,12dihydroquinoxalino[2,3-b]quinoxaline skeleton[END_REF][START_REF] Yao | Combining photocleavable and hydrogen-abstracting groups in quinoxaline with thioether bond as hybrid photoinitiator[END_REF][START_REF] Sun | Study of methoxyphenylquinoxalines (MOPQs) as photoinitiators in the negative photo-resist[END_REF][START_REF] Ercan | Highly conjugated isoindigo and quinoxaline dyes as sunlight photosensitizers for onium salt-photoinitiated cationic polymerization of epoxy resins[END_REF] furan derivatives, [START_REF] Dumur | Recent Advances on Furan-Based Visible Light Photoinitiators of Polymerization[END_REF] push-pull dyes, [START_REF] Garra | Ferrocene-based (photo)redox polymerization under long wavelengths[END_REF][START_REF] Tehfe | New pyridinium salts as versatile compounds for dye sensitized photopolymerization[END_REF][START_REF] Xiao | Julolidine or Fluorenone Based Push-Pull Dyes for Polymerization upon Soft Polychromatic Visible Light or Green Light[END_REF][START_REF] Mokbel | Michler's Ketone as an Interesting Scaffold for the Design of High-Performance Dyes in Photoinitiating Systems Upon Visible Light[END_REF][START_REF] Tehfe | New Push-Pull Dyes Derived from Michler's Ketone For Polymerization Reactions Upon Visible Lights[END_REF][START_REF] Mokbel | End capped polyenic structures as visible light sensitive photoinitiators for polymerization of vinylethers[END_REF][START_REF] Telitel | The 1,3-bis(dicyanomethylidene)indane skeleton as a (photo) initiator in thermal ring opening polymerization at RT and radical or cationic photopolymerization[END_REF][START_REF] Xiao | Structural Effects in the Indanedione Skeleton for the Design of Low Intensity 300-500 nm Light Sensitive Initiators[END_REF][START_REF] Sun | Novel Push-Pull Dyes Derived from 1Hcyclopenta[b]naphthalene-1,3(2H)-dione as Versatile Photoinitiators for Photopolymerization and Their Related Applications: 3D Printing and Fabrication of Photocomposites[END_REF][START_REF] Sun | N-ethyl carbazole-1-allylidene-based push-pull dyes as efficient light harvesting photoinitiators for sunlight induced polymerization[END_REF][START_REF] Tehfe | Push-pull (thio)barbituric acid derivatives in dye photosensitized radical and cationic polymerization reactions under 457/473 nm laser beams or blue LEDs[END_REF][START_REF] Sun | High-performance sunlight induced polymerization using novel push-pull dyes with high light absorption properties[END_REF][START_REF] Mokbel | Photoinitiating systems of polymerization and in situ incorporation of metal nanoparticles into polymer matrices upon exposure to visible light: push-pull malonate and malononitrile based dyes[END_REF][START_REF] Helmy | Read de Alaniz, Design and Synthesis of Donor-Acceptor Stenhouse Adducts: A Visible Light Photoswitch Derived from Furfural[END_REF][START_REF] Sun | Sunlight Induced Polymerization Photoinitiated by Novel Push-Pull Dyes: Indane-1,3-Dione, 1H-Cyclopenta[b]Naphthalene-1,3(2H)-Dione and 4-Dimethoxyphenyl-1-Allylidene Derivatives[END_REF] acridones, [START_REF] Abdallah | Acridone derivatives as high performance visible light photoinitiators for cationic and radical photosensitive resins for 3D printing technology and for low migration photopolymer property[END_REF][START_REF] Zhang | Novel panchromatic photopolymerizable matrices: N,N'-dibutylquinacridone as an efficient and versatile photoinitiator[END_REF] and phenazines [START_REF] Tehfe | Photoinitiators based on a phenazine scaffold: High performance systems upon near-UV or visible LED (385, 395 and 405 nm) irradiations[END_REF] have been examined as potential candidates for the design of highly reactive solar photoinitiating systems. In fact, the search for new structures is also motivated by the difficulty to redshift the absorption of historical photoinitiators such as thioxanthone or benzophenone which exhibit UV-centered absorptions. Redshift of their absorptions is only possible by mean of multistep syntheses and difficult synthetic steps. this strategy is therefore not adapted for developing sunlight photoinitiators, and especially panchromatic dyes. Indeed, contrarily to LED sources that exhibit a narrow emission, emission is broad, extending from the UV range up to the near infrared range. To fit with the specificity of sunlight, photoinitiators of broad absorptions should be developed. [START_REF] Topa-Skwarczyńska | Development of the first panchromatic BODIPY-based onecomponent iodonium salts for initiating the photopolymerization processes[END_REF][START_REF] Telitel | Photoredox process induced polymerization reactions: Iridium complexes for panchromatic photoinitiating systems[END_REF][START_REF] Xiao | Amino and nitro substituted 2-amino-1H-benzo[de]isoquinoline-1,3(2H)-diones: as versatile photoinitiators of polymerization from violet-blue LED absorption to a panchromatic behavior[END_REF][START_REF] Tar | Panchromatic Type II Photoinitiator for Free Radical Polymerization Based on Thioxanthone Derivative[END_REF][START_REF] Mau | Panchromatic Copper Complexes for Visible Light Photopolymerization[END_REF][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 | Panchromatic Photopolymerizable Cationic Films Using Indoline and Squaraine Dye Based Photoinitiating Systems[END_REF][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] Zhang | Novel panchromatic photopolymerizable matrices: N,N'-dibutylquinacridone as an efficient and versatile photoinitiator[END_REF][START_REF] Zhang | Plasmon enhanced upconverting core@triple-shell nanoparticles as recyclable panchromatic initiators (blue to infrared) for radical polymerization[END_REF] Reactivity of photoinitiating systems is also strongly related to the light penetration that is possible inside the resins. Indeed, a light penetration ranging between 600 µm at 400 nm up to 5 cm at 800 nm can be obtained (See Figure 2). Benefiting from this remarkable light penetration in the visible range and especially in the near infrared range, the access to composites is thus possible, what is difficult to polymerize in the UV range. [START_REF] Garra | Photopolymerization processes of thick films and in shadow areas: a review for the access to composites[END_REF] Use of sunlight for polymer synthesis is not new since this approach was already used by Egyptians 4000 years ago to waterproof papyrus boats with an asphalt oil that was undergoing a polymerization reaction under sunlight. More recent than these historical Egyptians works, this strategy was notably used by Stoddart and coworkers for the preparation of single-crystalline ionic polymers by sunlight-induced single-crystal-tosingle-crystal topochemical photopolymerization of preorganized monomers perfectly arranged by crystallization, prior to the polymerization process. [START_REF] Guo | Single-Crystal Polycationic Polymers Obtained by Single-Crystal-to-Single-Crystal Photopolymerization[END_REF] An impressive proton conductivity of 3 × 10 -4 S.cm -1 could be determined after the light-triggered [2+2] cycloaddition reaction, enabling the polymer to be used as proton-conducting materials. In 2023, the photoinduced step-growth polymerization of 3,4-ethylenedioxythiophene (EDOT) under sunlight irradiation was reported by Kaya and coworkers, using phenacyl bromide as the photoinitiator. [START_REF] Kaya | A green and fast method for PEDOT: Photoinduced step-growth polymerization of EDOT[END_REF] By mean of this photooxidative coupling, the resulting conjugated polymer could be prepared in a green solvent and at room temperature under air atmosphere.

Interestingly, the final PEDOT polymer could be doped with the bromine anions during the polymerization reaction, improving its conductivity. PEDOT exhibiting a high conductivity of 1.6 S/cm and a high molecular weight (32.5 kg/mol) could be obtained with these innovative polymerization conditions. Reprinted with permission from Bonardi et al. [START_REF] Bonardi | High Performance Near-Infrared (NIR) Photoinitiating Systems Operating under Low Light Intensity and in the Presence of Oxygen[END_REF] The design of autocurable resins under sunlight is another important research topics, [START_REF] Chiang | Preparation and properties of UV-autocurable BTDA-based epoxy-multiacrylate resins. Effects of the degree of polymerization and the epoxy type[END_REF][START_REF] Chiang | Syntheses and cured films properties of UV-autocurable BTDA-based multiacrylate resins[END_REF][START_REF] Grassino | Novel UV-autocurable methacrylo-urethane polymeric films derived from m-digallic acid: Preparation and characterization[END_REF] as it paves the way for outdoor applications of photopolymerization such as paints, glues and others. In this field, Chiang and coworkers developed several benzophenones bearing various pendant (meth)acrylates groups that could efficiently polymerize under sunlight. [START_REF] Chiang | Syntheses and cured films properties of UV-autocurable BTDA-based multiacrylate resins[END_REF][START_REF] Chiang | Synthesis and properties of ultraviolet-curable resins via a thio-ene (thiol and allyl) addition reaction[END_REF] Presence of both crosslinkable groups and the chromophore on a unique molecule was beneficial for the elaboration of simple resins.

Recently, the design of adhesive polymers was reported by Steele and coworkers. [START_REF] Tan | Sunlight activated film forming adhesive polymers[END_REF] As interesting features, bioadhesives were formulated with carbene-based polymers comprising diazirine unit. If the formulations were stable under indoor lighting, covalent crosslinking could be easily activated under direct sunlight exposure. Use of sunlight for polymerization reaction was not limited to crosslinking applications. Sunlight photoinduced atom transfer radical polymerization (ATRP) [START_REF] Konkolewicz | Visible Light and Sunlight Photoinduced ATRP with ppm of Cu Catalyst[END_REF] and sunlight driven reversible addition-fragmentation chain transfer polymerization (RAFT) photopolymerization [START_REF] Allegrezza | Visible and sunlight driven RAFT photopolymerization accelerated by amines: kinetics and mechanism[END_REF] were also examined for the design of polymers of controlled sizes and architectures. As shown in these different examples, solar photopolymerization exhibits numerous advantages, justifying the efforts to develop highly reactive photoinitiating systems.

In this review, an overview of the different photoinitiating systems developed for sunlight induced photocrosslinking polymerization reactions is provided. Considering that Sun offers the possibility of outdoor applications, an important parameter remains the reactivity under ambient light of these photoinitiating systems. This is the reason why comparison with monomer conversions obtained with artificial light sources will be established. 3). [START_REF] Georglev | Photopolymerization of 2-(dimethylaminoethyl)methacrylate induced by diacetoxyiodobenzene and bis(trifluoroacetoxy) iodobenzene[END_REF] Reactivity of these photoinitiators was high since daylight was sufficient to initiate the polymerization process. 24 hours of exposure to daylight were however required to polymerize DAEM. With these two photoinitiators, the ability of the monomer to act as an electron donor for hypervalent iodine compounds was the keypoint to generate initiating free radicals. Mechanism of radical generation is presented in Scheme 1. Upon photoexcitation of the hypervalent iodine compound, a photoinduced electron transfer between the monomer and the chromophore can occur. Subsequent to this, a photoinduced decomposition of the iodine compound can occur, producing acetoxy radicals and that can decarboxylate, generating highly reactive alkyl radicals. In 1998, the elaboration of glass fiber reinforced composites prepared from epoxidized soybean and linseed oils was investigated by Crivello and coworkers in the presence of a sulfonium salt (See Figure 3). [START_REF] Mayana | Fabrication and Mechanical Characterization of Glass Fiber Reinforced Epoxy with CFA and SiC[END_REF] Using 4-(thiophenoxyphenyl)diphenylsulfonium hexafluoroantimonate (Ar3S + SbF6 -), and after extensive optimization of the reaction conditions, 2-mm-thick fiberglass epoxy composites could be polymerized after 25 min. of exposure to sunlight under air. It has to be noticed that the cationic polymerization (CP) of epoxides is well-known to be insensitive to oxygen inhibition, contrarily to the free radical polymerization (FRP) of acrylates. [START_REF] Lang | A Review on Modeling Cure Kinetics and Mechanisms of Photopolymerization[END_REF][START_REF] Pierau | Bio-sourced monomers and cationic photopolymerization-The green combination towards eco-friendly and non-toxic materials[END_REF][START_REF] Sangermano | Visible Light Induced Cationic Polymerization of Epoxides by Using Multiwalled Carbon Nanotubes[END_REF][START_REF] Lalevée | Free-radical-promoted cationic photopolymerization under visible light in aerated media: New and highly efficient silane-containing initiating systems[END_REF][START_REF] Lalevée | Oxygen-Mediated Reactions in Photopolymerizable Radical Thin Films: Application to Simultaneous Photocuring Under Air and Nanoparticle Formation[END_REF][START_REF] Malik | Review on UV-Induced Cationic Frontal Polymerization of Epoxy Monomers[END_REF][START_REF] Lin | A Critical Review for Synergic Kinetics and Strategies for Enhanced Photopolymerizations for 3D-Printing and Additive Manufacturing[END_REF] This second point was also examined in 1998 by Bendaikha and coworkers in Mulhouse (France) in parallel to the previous works done by

Sunlight

Crivello in New York (United States) on cationic processes. [START_REF] Decker | Interpenetrating polymer networks. II. Sunlight-induced polymerization of multifunctional acrylates[END_REF]314] Interpenetrating polymer networks could be obtained by polymerization of multifunctional acrylates. Considering that atmospheric oxygen has an inhibitory effect on free radical initiated reactions, polymerization experiments were carried out for solid and liquid samples. As anticipated, oxygen diffusion was lower in solid samples than in liquid samples, improving the acrylate conversion. Indeed, intensity of the solar radiation was not enough to generate sufficient free radicals capable to react with molecular oxygen dissolved in the resin and oxygen diffusing inside the resin during polymerization. When HDDA was dispersed in a poly(methyl methacrylate) (PMMA) matrix (50%), oxygen diffusion was greatly reduced so that 50% of monomer conversion could be obtained within 1 min. under sunlight by using 2 wt% TPO. By using 5 wt% TPO, a conversion of 75% could be obtained within 3 min. Fast polymerization processes could also be obtained with urethane-diacrylate (Ebecryl 284) or isobornyl-acrylate (IBOA) as the monomers. Following this work, the same authors demonstrated that solar polymerization could be initiated with photoinitiators other than TPO. Other structures such as Thus, under sunlight and within 1 min., a depth of cure of a few centimetres could be obtained with BAPO during the FRP of an acrylate resin composed of polyurethane-acrylate (Ebecryl 284), an aminopolyester tetraacrylate (Ebecryl 80) and HDDA. For comparison, a higher efficiency was determined for TPO. Thus, a conversion of 90% for the polyurethaneacrylate monomer and 98% for HDDA could be obtained with TPO (2 wt%) within one minute to sun exposure. Interestingly, these monomer conversions were comparable to that determined upon irradiation with a UV light (mercury lamp, I = 12 mW/cm²).

Highly reactive photoinitiating systems were also prepared for the cationic polymerization of Still done by Crivello and coworkers, a lower reactivity was evidenced with (1E,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione (curcumin), a yellow dye extracted from the spice turmeric (See Figure 7). [START_REF] Crivello | Curcumin: A naturally occurring long-wavelength photosensitizer for diaryliodonium salts[END_REF] Besides, it has to be noticed that another monomer was used for this study, namely an epoxidized Linseed oil. Photopolymerization of this resin could be obtained within 5 min., using the two-component curcumin/IOC-10.SbF6 (0.1%/1% w/w) system (where IOC-10.SbF6 stands for (4-n-decyloxyphenyl)phenyliodonium hexafluoro-antimonate.

The chemistry of silane derivatives

On his side, Fouassier and coworkers extensively worked on the development of photoinitiating systems based on Si-Si bond cleavage. [START_REF] Lalevée | Free-radical-promoted cationic photopolymerization under visible light in aerated media: New and highly efficient silane-containing initiating systems[END_REF][START_REF] Lalevée | New Highly Efficient Radical Photoinitiators Based on Si-Si Bond Cleavage[END_REF][START_REF] Lalevée | Acylgermanes: Excited state processes and reactivity[END_REF][START_REF] Lalevée | Germanes as efficient coinitiators in radical and cationic photopolymerizations[END_REF][START_REF] Lalevée | Tris(trimethylsilyl)silane (TTMSS)-Derived Radical Reactivity toward Alkenes: A Combined Quantum Mechanical and Laser Flash Photolysis Study[END_REF][START_REF] Lalevée | Tris(trimethylsilyl)silyl versus tris(trimethylsilyl)germyl: Radical reactivity and oxidation ability[END_REF] Tris(trimethylsilyl)silane(TTMSS) was notably identified as a promising additive for addressing the low-light intensity of sunlight and to overcome the oxygen inhibition. [START_REF] Belon | Overcoming the oxygen inhibition in the photopolymerization of acrylates: A study of the beneficial effect of triphenylphosphine[END_REF] Indeed, silyl radicals can react with molecular oxygen, generating peroxyl radicals that can subsequently rearrange, with the migration of two trimethylsilyl substituents, regenerating in turn initiating silyl radicals (See Scheme 2).

Scheme 2. Mechanism of oxygen consumption and formation of initiating silyl radicals.

Concerning solar polymerization, the first three-component Titanocene/TTMSS/onium salt system based on TTMSS and used for the polymerization of a cationic resins (i.e. EPOX) under air was proposed in 2009. [START_REF] Tehfe | Long Wavelength Cationic Photopolymerization in Aerated Media: A Remarkable Titanocene/Tris(trimethylsilyl)silane/Onium Salt Photoinitiating System[END_REF] Interestingly, upon irradiation with the polychromatic light (400-800 nm), higher monomer conversions could be obtained under air than in laminate during the free radical-promoted cationic polymerization (FRPCP) of EPOX (See Figure 8).

This behaviour is relatively unusual, considering that all free radical processes are adversely affected by oxygen inhibition. The crucial role of TTMSS in this photoinitiating system was evidenced by comparing the monomer conversion obtained with and without TTMSS. radicals. Subsequent to a hydrogen abstraction reaction occurring between the peroxyl radicals and TTMSS, silyl radicals could be formed. [START_REF] Lalevée | Tris(trimethylsilyl)silyl versus tris(trimethylsilyl)germyl: Radical reactivity and oxidation ability[END_REF] In the presence of Ph2I + i.e. in the case of three-component photoinitiating systems, two types of silyl cations can be formed i.e. R(RO)Si + OR and R3Si + according to the mechanism depicted in the Scheme 4.

R3Si• + O2 --› R3SiOO• R3SiOO• + Ph2I + --› R3SiOO + + Ph• + PhI R3SiOO• + R3SiH --› R3SiOOH + R3Si• R3Si-O2 --› R(RO)Si•-OR R(RO)Si•-OR + Ph2I + --› R(RO)Si + OR + Ph• + PhI R3Si• + Ph2I + --› R3Si + + Ph• + PhI Scheme 4.
Mechanism involved in the generation of aryl radicals in the case of the three-component titanocene/TTMSS/ Ph2I + system.

Overall, when three-component Titanocene/TTMSS/Ph2I + (0.1%/1%/1% w/w/w) systems are used, two types of cations are formed, namely R(RO)Si + OR and R3Si + that can both contribute to the FRPCP process. Following this work, other transition metals were investigated for the cationic polymerization of EPOX in three-component photoinitiating systems, as exemplified with dimanganese (0) decacarbonyl (Mn2CO10), dirhenium decacarbonyl (Re2CO10) or cyclopentadienyl iron(II) dicarbonyl dimer (Cp2Fe2CO4) (See Figure 9). [START_REF] Tehfe | Combination of transition metal carbonyls and silanes: New photoinitiating systems[END_REF] Figure 9. Chemical structures of different metal complexes used as photosensitizers by Fouassier and coworkers.

In the case of Mn2CO10, higher polymerization rates were obtained with the threecomponent Mn2CO10/TTMSS/Ph2I + (1%/3 %/1% w/w/w) system than with the Titanocene/TTMSS/Ph2I + (0.1%/3%/1% w/w/w) system upon irradiation with a Xenon lamp or a laser diode at 405 nm. Besides, similar final monomer conversions were obtained in the two cases after 400 s of irradiation. In the case of Re2CO10 and Cp2Fe2CO4, lower monomer conversions could be determined compared to that obtained with Titanocene and Mn2CO10.

Under sunlight, reactivity of the Mn2CO10/TTMSS/Ph2I + system was lower than the Titanocene/TTMSS/Ph2I + system since one hour of exposure under air was necessary to get a full curing of the EPOX resin (vs. 20 min. for the Titanocene-based system). Other metal complexes were also examined as photosensitizers such as tris(2,2'-bipyridine)ruthenium(II)

dichloride Ru(bpy)3.Cl2 (See Figure 10). [START_REF] Lalevée | Green Bulb Light Source Induced Epoxy Cationic Polymerization under Air Using Tris(2,2′bipyridine)ruthenium(II) and Silyl Radicals[END_REF] By using the three-component Ru(bpy)3.Cl2/TTMSS/Ph2I + (0.2%/3%/ 2% w/w/w) system during the CP of EPOX, and upon irradiation with laser diodes emitting at 405, 457, 473 and 532 nm (I = 100 mW/cm²), tack-free coatings could be obtained within 4 minutes. Under sunlight, 9 min. of irradiation were required to get a similar result. Still during the CP of EPOX, comparison with the threecomponent system based on tris(1,10-phenanthroline)ruthenium( II ) Ru(phen)3.Cl2 revealed this complex to furnish more reactive photoinitiating systems under sunlight than Ru(bpy)3.Cl2. [START_REF] Lalevée | Efficient dual radical/cationic photoinitiator under visible light: a new concept[END_REF] This is directly related to the better light absorption properties of the Ru(phen)3 2+ complex in the 350-600 nm range than the Ru(bpy)3 2+ complex, improving the monomer conversion. However, ruthenium was not the only rare and expensive metal to be used for the design of photosensitizers. Iridium is another metal scarcely available on

Earth. [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] In this field, the benchmark tris(2-phenylpyridine)iridium (III) Ir(ppy)3 (See Figure 10) was also incorporated in three-component Ir(ppy)3/TTMSS/Ph2I + (0.2%/3%/ 2% w/w/w) systems and tack-free coatings could be obtained in around 18 minutes during the CP of epoxidized soybean oil or limonene dioxide. [START_REF] Lalevée | A Novel Photopolymerization Initiating System Based on an Iridium Complex Photocatalyst[END_REF] For comparison, the same monomer conversion could be obtained within 1 200 s upon irradiation with a fluorescent bulb. with the three-component dye/TTMSS/Ph2I + (0.1%/3%/1% w/w/w) system were determined.

The three-component system based on ITX proved to be especially reactive under sunlight since tack-free coatings could be produced in 12 min. and 45 min. using limonene dioxide and epoxidized soybean oil as the monomers. A very fast curing was also evidenced for the biosourced epoxidized linseed oil (less than 9 min.) that could be used in combination with an electron donor i.e. 2-(dimethylamino)ethyl methacrylate and that required between 30 min. and one hour to fully cure poly(ethylene glycol) dimethacrylate (PEGDA). [START_REF] Pączkowska | Photochemical preparation of polymer-clay composites[END_REF] The lack of reactivity of DEBP, BPSK and benzil can be confidently assigned to the narrow absorption spectra of these dyes, that do not coincide with the emission spectrum of Sun. In fact, all highly reactive photoinitiators under sunlight often exhibit a panchromatic absorption, enabling to perfectly match the broad absorption of Sun.

Other photoinitiating systems developed with iodonium salts

In the chemistry of Fouassier and coworkers, the iodonium salt has been introduced as an additive capable to generate silyl cations according to the mechanism depicted in Scheme 13). [START_REF] Zhang | Zero VOC sunlight curable coatings[END_REF] Figure 13. Chemical structures of DEGBA, Basonyl black X-22 and HTOPPI.

Interestingly, if no polymerization of vinylcyclohexene dioxide or DEGBA could be detected upon irradiation for two months with a fluorescent light, conversely, the same resins could cure under sunlight within two hours, as a result of the broad emission of Sun. To support the photoinitiating ability of sun, the author suggested the 5% UV component of sunlight to be responsible to this efficient curing. In order to improve the light absorption properties of the photoinitiating systems, iodonium salts were combined with different photosensitizers. For instance, 2,3-bis(3,4-bis(decyloxy)phenyl)-5,8-bis(2,3-dihydrothieno [3,4-b] [START_REF] Jasinski | Photopolymerization in dispersed systems[END_REF][START_REF] Khudyakov | Kinetics of Photopolymerization of Acrylates with Functionality of 1-6[END_REF]-dioxin-5-yl)quinoxaline (DOPEQ) by its broad absorption extending between 300 and 550 nm could greatly facilitate the cationic polymerization of EPOX (See Figure 14). [START_REF] Bulut | A quinoxaline derivative as a long wavelength photosensitizer for diaryliodonium salts[END_REF] Figure 14. Chemical structures of DOPEQ, dopamine and cyclohexene oxide.

By using the two-component DOPEQ/Iod system (0.1%/1% w/w) (with Iod = diphenyliodonium hexafluorophosphate), polymerization of cyclohexene oxide could take place almost instantly, when exposed to a UV light (I = 10 mW/cm²) with an exothermicity of around 65°C. In the case of EPOX, a lower exothermicity (around 40°C) and a slower initiation process was evidenced since the maximal temperature was only obtained after 25 s, contrarily to a few second in the case of cyclohexene oxide. Under sunlight, 5 min. of exposure of the EPOX resin containing the two-component DOPEQ/Iod (0.1%/1% w/w) system were required to get a tack-free coating. In 2021, Yagci and coworkers examined the photopolymerization of dopamine using pyrene as the photosensitizer. [START_REF] Kaya | Mussel-Inspired Coatings by Photoinduced Electron-Transfer Reactions: Photopolymerization of Dopamine under UV, Visible, and Daylight under Oxygen-Free Conditions[END_REF] Comparison of the polymerization efficiency was carried out under visible and daylight irradiation. As interesting features, photochemical routes to access to polydopamine (PDA) are well-known to proceed much faster than the conventional synthetic methods while allowing a temporal control of the polymerization process. [START_REF] Du | UV-Triggered Dopamine Polymerization: Control of Polymerization, Surface Coating, and Photopatterning[END_REF][START_REF] Sheng | Brushing up from "anywhere" under sunlight: a universal surface-initiated polymerization from polydopamine-coated surfaces[END_REF][START_REF] Lemaster | Synthesis of Ultrasmall Synthetic Melanin Nanoparticles by UV Irradiation in Acidic and Neutral Conditions[END_REF] By using pyrene as the photosensitizer for diphenyliodonium hexafluorophosphate, a polymerization time of 2 hours was determined upon irradiation at 400 nm, enabling a dopamine conversion of 72% (See Figure 15). Under sunlight, 4 hours were required to get a monomer conversion approaching that obtained at 400 nm (65%). Finally, during the last four years, a family of dyes was examined for the first time for sunlight photopolymerization, namely push-pull dyes based on a D-π-A structure (where D and A respectively stand for donor and acceptor). These compounds typically consist in an electron-donating group connected to an electron accepting group by mean of a conjugated spacer. [START_REF] Bureš | Fundamental aspects of property tuning in push-pull molecules[END_REF] These dyes are of specific interest as their absorptions can be finely tuned by acting on the electron accepting and electron-withdrawing abilities of the two partners in conjugation. In 2020, Lalevée and coworkers proposed dye 8 for sunlight polymerization (See Figure 16). [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] It has to be noticed that this dye is of unusual structure as it results from an undesired nucleophilic attack of the amine used to catalyse the Knoevenagel reaction on the corresponding push-pull dye under synthesis. [START_REF] Pigot | Unprecedented Nucleophilic Attack of Piperidine on the Electron Acceptor during the Synthesis of Push-Pull Dyes by a Knoevenagel Reaction[END_REF][START_REF] Pigot | New push-pull dyes based on 2-(3-oxo-2,3-dihydro-1Hcyclopenta[END_REF] Figure 16. Chemical structure of dye 8, TMPTA and EDB.

As an undesired product of the reaction, this product could however be obtained in reasonable yield (79%), this product constituting in turn the main product of the reaction.

Despites the fact that the synthesis of this dye was unexpected, this structure was of interest for sunlight photopolymerization experiments. This dye was notably characterized by a broad absorption spectrum extending between 300 and 600 nm, with an absorption maximum located at 491 nm. Benefiting from this broad absorption and while using the three-component dye 8/Iod2/EDB combination (where Iod2 and EDB respectively stands for bis(4-tertbutylphenyl)iodonium hexafluorophosphate and ethyl dimethylaminobenzoate), this dye proved to be an interesting candidate for sunlight polymerization. Indeed, FRP experiments done on trimethylolpropane triacrylate (TMPTA) upon irradiation at 405 nm and under sunlight revealed dye 8 to be highly reactive. If a monomer conversion of 91% was obtained upon irradiation at 405 nm (I = 110 mW/cm²) for 400 s, this value only decreased to 88% under sunlight (I < 5 mW/cm² in the 350-500 nm range), while exposing the resin to sun for 30 min.

In the different study done by Lalevée and coworkers, the ability to design sunlight photoinitiator was determined as being strongly related to the ability to associate a strong donor and a strong acceptor. As a result of this combination, broadening of the absorption (design of panchromatic photoinitiators), high molar extinction coefficients and redshift of the absorption towards the visible range can be obtained. This point was evidenced with PP7 and PP8 which proved to be efficient candidates for sunlight photopolymerization (See Figure 17). [START_REF] Sun | N-ethyl carbazole-1-allylidene-based push-pull dyes as efficient light harvesting photoinitiators for sunlight induced polymerization[END_REF] Figure 17. Chemical structures of PP7, PP8 and TA.

Here again, comparison of the monomer conversions obtained at 405 nm during the FRP of (oxybis(methylene))bis(2-ethylpropane-2,1,3-triyl) tetraacrylate (TA) and under sunlight irradiation revealed the monomer conversion to be only slightly reduced under sunlight (92% in 1800 s vs. 95% conversion in 800 s at 405 nm for PP8) using the threecomponent PP8/Iod2/EDB (0.1%/2%/2% w/w/w) system. A similar situation was found for PP7 (95% conversion in 1800 s vs. 97% in 800 s at 405 nm). Interestingly, the two dyes could also act as monocomponent systems. Thus, if a monomer conversion of 85% was obtained with PP7 upon irradiation with a LED at 405 nm, this value decreased to 80% after 1800 s of exposure of the resin under sunlight (See Figure 18). Surprisingly, in the case of PP8, a higher conversion was obtained under sunlight than with the light source emitting at 405 nm (75% vs. 35%). This Ref. [START_REF] Sun | N-ethyl carbazole-1-allylidene-based push-pull dyes as efficient light harvesting photoinitiators for sunlight induced polymerization[END_REF].

c c

The previous trend concerning the similarity of monomer conversions obtained upon irradiation at 405 nm and under sunlight was confirmed with another series of push-pull dyes, still based on strong electron acceptors. [START_REF] Sun | High-performance sunlight induced polymerization using novel push-pull dyes with high light absorption properties[END_REF] In this series of dyes, presence of the pyrrole unit as electron donating group seemed to be a key-element for sunlight polymerization, certainly attributable to its low oxidation potential. Indeed, for all analogue dyes comprising thiophene could not initiate any polymerization process, despites similar absorption properties in terms of molar extinction coefficient or absorption maxima (See Figure 19). Here again, around 30 min. were required to get a monomer conversion of TA similar to that obtained at 405 nm upon irradiation for 400 s. Noticeably, monomer conversions reported in Table 1 were obtained during winter, a period where the solar illumination is the lowest. In this series of dyes, the highest decrease of monomer conversion was observed for bindone (CPD6) which exhibited the lowest molar extinction coefficient of the series (See Figure 20). In 2022, a series of push-pull dyes based on tetravinylindane as the electron acceptor and comprising an extended π-conjugated spacer was proposed (See Figure 21). [START_REF] Sun | Sunlight Induced Polymerization Photoinitiated by Novel Push-Pull Dyes: Indane-1,3-Dione, 1H-Cyclopenta[b]Naphthalene-1,3(2H)-Dione and 4-Dimethoxyphenyl-1-Allylidene Derivatives[END_REF] Benefiting from the elongated conjugated spacer, a red-shift of the absorption maximum combined with a broadening of the absorption could be determined for CP7, CP9 and CP11 compared to the previous one. Even if the polymerization tests have been carried out in the most favourable conditions, namely in July, all dyes furnished a TMPTA conversion higher than 90% within 30 minutes under sunlight. As other dyes based on the D-π-A scaffold are chalcones. Chalcones have been extensively studied for the design of visible light photoinitiating systems and in this field, the groups of Jacques Lalevée and Jun Nie have reported within the last three years not less than a hundred structures. [START_REF] Deng | Novel chalcone-based phenothiazine derivative photoinitiators for visible light induced photopolymerization with photobleaching and good biocompatibility[END_REF][START_REF] Chen | Photoinitiators derived from natural product scaffolds: monochalcones in three-component photoinitiating systems and their applications in 3D printing[END_REF][START_REF] Giacoletto | Recent Advances in bis-Chalcone-Based Photoinitiators of Polymerization: From Mechanistic Investigations to Applications[END_REF][START_REF] Ibrahim-Ouali | Recent Advances on Chalcone-based Photoinitiators of Polymerization[END_REF][START_REF] Wu | Photobleachable bischalcones-based oxime ester dyes for radical visible photopolymerization[END_REF] Prior to this work, a chalcone capable to initiate sunlight photopolymerization was reported as soon as 2014 namely Cal5 (See Figure 22). [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] A final conversion of DVE-3 of 65% could be obtained within 600 s under sunlight irradiation during the thiol-ene polymerization of trithiol/DVE-3 using the two-component system Cal5/Iod (0.5%/1% w/w). For comparison, a similar DVE conversion could be obtained in 200 s upon irradiation with a laser diode at 457 nm.

N-Vinylcarbazole as photosensitizer

N-Vinylcarbazole (NVK) is extensively used as an additive as this compound is capable to convert a free radical polymerization process as a cationic one. To do this, NVK is often used in three-component photoinitiating systems comprising a photosensitizer and an iodonium salt (See Figure 23). This mechanism is named free-radical promoted cationic polymerization (FRPCP). Using this strategy, the polymerization of interpenetrated polymer network is possible, by enabling the concomitant occurrence of a cationic and a free radical process. In 2016, Brovko and coworkers used NVK, not as an additive as classically used but as a photosensitizer for a mixture of triphenylsulfonium hexafluorophosphate salts (TPSHFPS) (See Figure 24). [START_REF] Samoilenko | Influence of N-vynilcarbazole on the photopolymerization process and properties of epoxy-acrylate interpenetrating polymer networks[END_REF] Cu complex

Oxidative cycle

Ph

Conclusion

Photopolymerization is used in a wide range of applications and the use of sunlight as the light source could enable to polymerize in more environmentally friendly and economic conditions. If solar photoinitiating systems are highly desired, only few systems have been developed during the last decade. If a series of transition metal complexes have been proposed in the past. However, their high cost and toxicity are not adapted for industrial applications.

Recently, photopolymerists focused on purely organic dyes that are easier to prepare and less expensive. Based on recent works, the most promising structure for designing sunlight photoinitiator/photosensitizers seems to be push-pull dyes, considering that eight structures have been proposed during the last three years. In all cases, efficiency of the photoinitiating systems was strongly related to the incorporation of carefully selected additives (silanes, sacrificial amines), iodonium salts being unavoidable for the design of efficient photoinitiating systems. If nanoparticles up-conversion seems to be a promising approach enabling to initiate polymerization processes in safe irradiation conditions, this approach still suffers from the low near-light-to-visible light conversion. However, use of nanoparticles in photopolymerization is recent and major advances will certainly be obtained in the future. Future works will certainly consist in investigating new D-π-A structures, this scaffold being certainly the most promising at present for the design of sunlight photosensitizers. As other possible alternative, the design of Type I photoinitiators such as oxime esters comprising push-pull dyes as the chromophore could greatly help to simplify the composition of the photocurable resins while maintaining a high reactivity. [START_REF] Hammoud | A review on recently proposed oxime ester photoinitiators[END_REF]

Figure 1 .

 1 Figure 1. Measured solar spectral irradiance distribution. Reproduced with permission of

Figure 2 .

 2 Figure 2. Light penetration in a polystyrene latex with an average diameter of 112 nm.

Figure 3 .Scheme 1 .

 31 Figure 3. Chemical structures of hypervalent iodine compounds used by Varvoglis and coworkers and DMAEM.

Figure 3 .

 3 Figure 3. Chemical structures of epoxidized soybean and linseed oils, and the sulfonium salt. The polymerization of hexanediol diacrylate (HDDA), isobornyl acrylate (IBOA), hydroxypropyl acrylate (HPA), tripropyleneglycol diacrylate (TPGDA) and urethane diacrylate (Ebecryl 284) was notably investigated using diphenyl(2,4,6trimethylbenzoyl)phosphine oxide (TPO) as the photoinitiator (See Figure 4).

Figure 4 .

 4 Figure 4. Chemical structure of TPO and different acrylate monomers used by Bendaikha and coworkers. Due to the strong oxygen inhibition, solar polymerization of acrylates only furnished polymers with tacky surfaces even after several days of exposure to sunlight, oxygen preventing an efficient polymerization of the surface layer of the coating in contact with air.

Figure 5 .

 5 photoinitiators (See Figure5).[START_REF] Decker | Performance analysis of acylphosphine oxides in photoinitiated polymerization[END_REF]

Figure 6 .

 6 Figure 6. Chemical structures of different photosensitizers, additives and monomers used by Crivello and co-workers.

Figure 7 .

 7 Figure 7. Chemical structure of curcumin and an iodonium salt.

  photoinitiating systems.[START_REF] Dumur | Recent advances on photobleachable visible light photoinitiators of polymerization[END_REF] Indeed, visible light photoinitiators are coloured compounds that can impose a colour to the final polymer if no photobleaching occur during the polymerization process.

Figure 8 .

 8 Figure 8. EPOX conversion obtained with different photoinitiating systems: (1)Titanocene/Ph2I + (0.1%/1% w/w) (2) Titanocene/TTMSS/Ph2I + (0.1%/1%/1% w/w/w) (3) Titanocene/TTMSS/Ph2I + (0.1%/3%/1% w/w/w) upon irradiation with a polychromatic light under air. Reproduced with permission of Ref.[START_REF] Tehfe | Long Wavelength Cationic Photopolymerization in Aerated Media: A Remarkable Titanocene/Tris(trimethylsilyl)silane/Onium Salt Photoinitiating System[END_REF] To support the photobleaching detected during the polymerization process, decomposition of the photosensitizer according to the mechanism depicted in the Scheme 3 was proposed. Thus, upon irradiation, cyclopentadienyl radicals (Cp•) and aryl radicals (Ar•) are formed, constituting initiating radicals and enabling a discoloration of the resins during photopolymerization (See Scheme 3).

Scheme 3 .

 3 Scheme 3. Mechanism proposed to support the photobleaching detected during photopolymerization with Irgacure 784. If a complete polymerization of EPOX could be obtained within 300 s upon exposure of the resin to a polychromatic light, under sunlight, the same conversion could be achieved after 20 min of sunlight exposure during a cloudy day of July. To support the higher monomer conversion obtained under air than in laminate, and on the basis of electron spin resonance spin-trapping (ESR-ST) experiments, the authors demonstrated that the carbon-centered radicals formed by photodecomposition of Titanocene could be easily converted into peroxyl

Figure 10 .

 10 Figure 10. Chemical structures of Ru and Ir complexes used in combination with an iodonium salt and TTMSS.Investigation of silane-based photoinitiating systems was not limited to metal-based photosensitizers and purely organic compounds were also examined as exemplified with acridinediones,[START_REF] Tehfe | Near UV-visible light induced cationic photopolymerization reactions: A three component photoinitiating system based on acridinedione/silane/iodonium salt[END_REF] phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO), 2,4,6trimethylbenzoyldiphenyl phosphine (TPO) or thioxanthone (ITX) (See Figure11).[START_REF] Lalevée | Recent advances in sunlight induced polymerization: role of new photoinitiating systems based on the silyl radical chemistry[END_REF] Under sunlight, irradiation time varying between 25 min. for the CP of EPOX using a threecomponent dye/TTMSS/Ph2I + system based on the acridinedione 3,4,6,7,9,10hexahydroacridine-1,8(2H,5H)-dione (AD) or 10 min. for the CP of limonene dioxide (LDO)

Figure 11 .

 11 Figure 11. Example of organic dyes used as photosensitizers for the design of threecomponent (dye/TTMSS/Iod).For comparison, use of the two-component ITX/Ph2I + (0.1%/1% w/w) system drastically elongated the irradiation times, ranging between 45 min. for limonene dioxide (LDO) up to 3 hours for the biosourced monomers (epoxidized linseed and soybean oils). The favorable effect of TTMSS on the polymerization efficiency under sunlight was clearly demonstrated, once again originating from the ability of silyl radicals to react with molecular oxygen and to regenerate initiating radicals. Finally, less efficient dyes were also used for the design of silanebased photoinitiating systems such as diethyl benzoylphosphonate (DEBP), 1-[(4benzoylphenylsulfanyl)phenyl]-2-methyl-2-(4-methyl-phenylsulfonyl)propan-1-one (BPSK), benzil (BZ) or Eosin-Y (Eo) that could only furnish tack-free coatings within three hours of sunlight exposure (See Figure12).[331] It has to be noticed that Eosin-Y was not the only xanthene dyes to be studied for the design of photoinitiators activable under sunlight. In 2004, Paczkowski reported a Rose bengal derivative namely (2'[6-(3-trimethylaminopropoxy)-2,4diiodo-3-oxo-3H-xanthen-9-yl]-benzyloxypropyltrimethyl-ammonium dibromide (DBEE)

Figure 12 .

 12 Figure 12. Chemical structures of different photosensitizers used for sunlight-induced photopolymerization in silane-based photoinitiating systems.

Figure 15 .

 15 Figure 15. Chemical structures of pyrene and dopamine.

  photopolymerization experiments have been carried out during a rainy day of november with clouds.

Figure 18 .

 18 Figure 18. Monomer conversions obtained during the FRP of TA using PP7 or PP8 alone (0.1 wt%) upon irradiation at 405 nm (I = 110 mW/cm²) or sunlight, or the three-component dye/Iod2/EDB (0.1%/2%/2% w/w/w) system under sunlight. Reproduced with permission of

Figure 19 .

 19 Figure 19. Chemical structures of CPD1-CPD6.

Figure 20 .

 20 Figure 20. UV-visible absorption spectra in acetonitrile of the push-pull dyes CPD1-CPD6 in acetonitrile.Reproduced with permission of Ref.[START_REF] Sun | High-performance sunlight induced polymerization using novel push-pull dyes with high light absorption properties[END_REF]

Figure 21 .Table 2 .

 212 Figure 21. Chemical structures of CP7, CP9 and CP11.

Figure 22 .

 22 Figure 22. Chemical structures of Cal5, DVE-3 and trithiol used for thiol-ene polymerization.

Figure 23 .

 23 Figure 23. mechanism involving NVK and enabling to convert a FRP process as a FRPCP.

Figure 24 .Scheme 5 .

 245 Figure 24. Chemical structures of epoxide, TEGDM, NVK and TPSHFPS.Upon photoexcitation, an exciplex can form between NVK and the sulfonium salt.Then, a photoinduced electron transfer between NVK and the sulfonium salts can occur, generating sulfanyl radicals and vinyl radicals. If vinyl radicals can rapidly recombine avoiding any back electron transfer, sulfanyl radicals can decompose, generating initiating aryl radicals according to the mechanism depicted in Scheme 5. Aryl radicals can also add on NVK, enabling after redox reaction with the sulfonium salt to generate NVK cations. These cations are capable to initiate the cationic polymerization of epoxides.

Figure 25 .

 25 Figure 25. Polymerization profiles of double bonds (blue); cycloaliphatic epoxy groups (red); aliphatic epoxy groups (black) with NVK (solid lines) and without NVK (dash lines).Reproduced with permission of Ref.[START_REF] Samoilenko | Influence of N-vynilcarbazole on the photopolymerization process and properties of epoxy-acrylate interpenetrating polymer networks[END_REF] 2.5. Nanoparticles up-conversionUV light is the focus of numerous safety concerns so that this technology is now progressively discarded from industrial applications. Besides, UV photons are energetic and can efficiently promote the formation of initiating species. Recently, generation of near-UV/visible light upon excitation in the near-infrared range was investigated as a possible alternative to UV irradiation. To get this result, up-converting nanoparticles have been recently developed for photopolymerization applications. As interesting features, these nanoparticles can up-convert low-energy photons into high-energy photons.[START_REF] Hermes | Challenges and limits of upconversion nanoparticles for cationic photopolymerization with UV initiators excited at 980 nm[END_REF][START_REF] Rocheva | Highresolution 3D photopolymerization assisted by upconversion nanoparticles for rapid prototyping applications[END_REF][START_REF] Liu | Extremely deep photopolymerization using upconversion particles as internal lamps[END_REF][START_REF] Wang | Upconversion Nanoparticle-Assisted Photopolymerization[END_REF][START_REF] Limberg | Triplet-Triplet Annihilation Photopolymerization for High-Resolution 3D Printing[END_REF][START_REF] Liu | Deep Thiol-ene Photopolymerization Using Upconversion Nanoparticle-assisted Photochemistry[END_REF] In 2022, this strategy was applied for the first time to sunlight photopolymerization.[START_REF] Liang | Near-infrared photon upconversion and solar synthesis using low-toxicity nanocrystals[END_REF] Indeed, limits the reaction rates and the scalability of such systems. Moreover, given the abundance of NIR photons in sunlight, near light-to-visible light conversion is thus possible. In the present case, Zn-doped CuInSe2 nanocrystals (abbreviated as ZCISe NCs) with the surface functionalized with 5-tetracene carboxylic acid (TCA) were used for triplet-triplet annihilation up-conversion (TTA-UC). Rubrene was used as a triplet annihilator. To initiate the FRP of TMPTA and the CP of EPOX, Iod and NVK were used as additives. Upon irradiation at 808 nm, a complete curing of TMPTA could be obtained after 12 min. of irradiation, using ZCISe-TCA NCs/rubrene couple. For comparison, direct excitation of rubrene upon irradiation at 532 nm enabled TMPTA to polymerize within 3 min. The higher monomer conversion obtained upon direct excitation of rubrene can be assigned to the low efficiency of the near light-to-visible light conversion. Under sunlight, an extremely fast polymerization process occurred since TMPTA polymerization could be obtained in only 30 s, using the ZCISe-TCA NCs/rubrene couple. Control experiments done with only rubrene revealed the polymerization process to occur in 2.5 min. Here again, higher polymerization efficiency under sunlight was assigned to the broad emission of sun, better fitting with the broad absorption spectrum of ZCISe-TCA NCs than the narrow emission of the light source emitting at 808 nm.

-induced polymerization 2.1. Pioneering works in 1987-2010

	Aside from Egyptians, photopolymerization under daylight has been investigated as
	soon as 1987 by Varvoglis and coworkers who developed two hypervalent iodine compounds,
	namely diacetoxyiodobenzene (DAI) and bis(trifluoroacetoxy)iodobenzene (BTI) which could
	efficiently promote the free radical polymerization of 2-(dimethylaminoethyl)methacrylate
	(See Figure

Table 1 .

 1 Final monomer conversions obtained during the FRP of TA at 405 nm (400 s) with a

	laser diode (I = 110 mW/cm²) and sunlight (30 min.) using three-component photoinitiating
	dye/Iod2/EDB (0.1%/2%/2% w/w/w) systems.
			Conversion at 405 nm
	No.	Push-pull dyes	with a laser diode	Conversion at sunlight
			(I = 110 mW/cm²)
	CPD1		~96%	~99%

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.