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

The Future of Visible Light Photoinitiators of Polymerization for Photocrosslinking Applications

Keywords: I and Type II photoinitiators, visible light, photopolymerization, LED, low light intensity, self-healing, charge transfer complex, photocrosslinking

Photopolymerization under visible light is an active research field whose intense research activity is supported by the recent advances in 3D and 4D printing. Within this perspective article, emerging trends in visible light photopolymerization for photocrosslinking applications will be presented. Notably, the performant multicomponent photoinitiating systems extensively used in the past are now progressively discarded in favor of mono-component systems easier to handle. The reduction of side-products formed during polymerization, the reduction of oxygen inhibition, the formation of dyes by intermolecular charge transfer interactions or the development of more ecological polymerization processes using sunlight as the irradiation source have recently been the focus of intense research efforts. Furthermore, emerging trends in processes favoring the elongation of the polymer lifetime such as self-healing will be presented and this perspective article aims at presenting the most recent breakthroughs in these attempts. To end, the future vision of visible light photopolymerization for photocrosslinking applications will be discussed.

Introduction

During the past decade, visible light photopolymerization has been the focus of intense research efforts supported by the numerous drawbacks the historical UV approach is currently facing. [START_REF] Ribas-Massonis | Free-Radical Photopolymerization for Curing Products for Refinish Coatings Market[END_REF][START_REF] Gallastegui | Fast Visible-Light Photopolymerization in the Presence of Multiwalled Carbon Nanotubes: Toward 3D Printing Conducting Nanocomposites[END_REF][START_REF] Shao | Visible light initiating systems for photopolymerization: status, development and challenges[END_REF][START_REF] Subervie | Visible-Light Emulsion Photopolymerization of Acrylates and Methacrylates: Mechanistic Insights and Introduction of a Simplified Sulfur-Based Photoinitiating System[END_REF][START_REF] Grotzinger | Visible light induced photopolymerization: speeding up the rate of polymerization by using co-initiators in dye/amine photoinitiating systems[END_REF] Notably, UV light is well-known to have adverse health effects depending on the duration of exposure and the wavelength used. [START_REF] Shao | Visible light initiating systems for photopolymerization: status, development and challenges[END_REF][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] In industry, appropriate safety precautions have thus to be used in order to perform UV photopolymerization. Ozone is also produced during photopolymerization, what constitutes a severe drawback. [START_REF] Shao | Visible light initiating systems for photopolymerization: status, development and challenges[END_REF][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] Parallel to this, UV irradiation setups are energy-consuming equipments. Considering that numerous countries have launched during winter 2022 "sobriety" energy savings drive to avoid winter power cuts, such energy consuming approaches are now totally inappropriate and disconnected from the reality. By displacing the irradiation wavelength from the UV range to the visible region, nonetheless the safety issue of operators can be efficiently addressed but a higher light penetration within the photocurable resin can also be achieved. [START_REF] Ma | How to overcome the light penetration issue in photopolymerization? An example for the preparation of high content iron-containing opaque composites and application in 3D printing[END_REF] Hence, if the light penetration is limited to around 600 µm in the UV range, this value can increase up to 2 cm at 800 nm, evidencing the interest of polymerizing under visible light. [START_REF] Bonardi | High Performance Near-Infrared (NIR) Photoinitiating Systems Operating under Low Light Intensity and in the Presence of Oxygen[END_REF] Interest for developing visible light photoinitiating systems is also supported by the scope of applications of photopolymerization. Notably, dentistry, 3D/4D printing and coatings can be cited as the most popular applications of photopolymerization. [START_REF] Santini | Photoinitiators in Dentistry: A Review[END_REF][START_REF] Tay | Effect of delayed activation of light-cured resin composites on bonding of all-in-one adhesives[END_REF][START_REF] Yoshida | Effect of photoinitiator on degree of conversion of unfilled light-cured resin[END_REF][START_REF] Bagheri | Photopolymerization in 3D Printing[END_REF][START_REF] Bao | Challenges and Opportunities in 3D Printing of Biodegradable Medical Devices by Emerging Photopolymerization Techniques[END_REF][START_REF] Geisler | 3D printing of optical materials by processes based on photopolymerization: materials, technologies, and recent advances[END_REF][START_REF] Ma | Charge Transfer Complexes (CTCs) with Pyridinium Salts: Toward Efficient Dual Photochemical/Thermal Initiators and 3D Printing Applications[END_REF] Photopolymerization also exhibits several interesting features that clearly distinguishes photoinduced polymerization from the thermal polymerization. Notably, a spatial and a temporal control of the polymerization process can be obtained for photopolymerization, what can be implemented as an accurate external control approach for polymer synthesis and composite elaboration. [START_REF] Skliutas | Polymerization mechanisms initiated by spatio-temporally confined light[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][START_REF] Legrand | Spatiotemporal Control of Supramolecular Polymerization and Gelation of Metal-Organic Polyhedra[END_REF][START_REF] Sun | Integrating temporal and spatial control of electronic transitions for bright multiphoton upconversion[END_REF] Photopolymerization can also be carried out in solvent-free conditions, limiting the release of volatile organic compounds (VOCs). [START_REF] Shao | Visible light initiating systems for photopolymerization: status, development and challenges[END_REF] Recently, major changes in the composition of the photocurable resins have been observed. Notably, the extensively used multicomponent systems are now replaced by mono-component systems, greatly simplifying the composition of the photocurable resins. [START_REF] Dumur | Recent advances on carbazole-based oxime esters as photoinitiators of polymerization[END_REF][START_REF] Ma | Conjugated phenothiazine oxime esters as free radical photoinitiators[END_REF][START_REF] Zhou | Bicarbazole-based oxime esters as novel efficient photoinitiators for photopolymerization under UV-Vis LEDs[END_REF][START_REF] Walton | Functionalised Oximes: Emergent Precursors for Carbon-, Nitrogen-and Oxygen-Centred Radicals[END_REF] Indeed, photoinitiators can be divided in two distinct categories. Notably, Type II photoinitiators can generate initiating species by hydrogen abstraction from aromatic ketones in the presence of hydrogen donors or by photoinduced electron transfer of a photosensitizer towards an onium salt. [START_REF] Allushi | Conventional Type II photoinitiators as activators for photoinduced metal-free atom transfer radical polymerization[END_REF][START_REF] Balta | An annelated thioxanthone as a new Type II initiator[END_REF][START_REF] Chen | Photocuring Kinetic Studies of TMPTMA Monomer by Type II Photoinitiators of Different Weight Ratios of 2-Chlorohexaaryl Biimidazole (o-Cl-HABI) and N-Phenylglycine (NPG)[END_REF][START_REF] Li | The effect of polyethylene glycoldiacrylate complexation on type II photoinitiator and promotion for visible light initiation system[END_REF][START_REF] Kirschner | 1-Aryl-2-(triisopropylsilyl)ethane-1,2-diones: Toward a New Class of Visible Type I Photoinitiators for Free Radical Polymerization of Methacrylates[END_REF][START_REF] Tar | Panchromatic Type II Photoinitiator for Free Radical Polymerization Based on Thioxanthone Derivative[END_REF][START_REF] Christmann | Triazine-Based Type-II Photoinitiating System for Free Radical Photopolymerization: Mechanism, Efficiency, and Modeling[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] However, drawback of this approach is that the efficiency of the intermolecular process is highly dependent of the viscosity of the monomer, the concentration of the two partners and the rate constant of interaction. Conversely, Type I photoinitiators can generate active radical species by homolytic α-cleavage of aromatic carbonyl compounds (See Scheme 1). [START_REF] Shanmugam | Photocontrolled Living Polymerization Systems with Reversible Deactivations through Electron and Energy Transfer[END_REF][START_REF] Dietlin | New Phosphine Oxides as High Performance Near-UV Type I Photoinitiators of Radical Polymerization[END_REF][START_REF] Kirschner | Lalevée, 2-Oxo-2(tertbutyldimethylsilyl)Acetic Acid (DKSi-COOH) as a New Water-Soluble Visible Light Type I Photoinitiator for Free Radical Polymerization[END_REF][START_REF] Hammoud | Naphthoquinone-based imidazolyl esters as blue-lightsensitive Type I photoinitiators[END_REF] As a result of this, Type I photoinitiators can act as mono-component systems, simplifying the composition of the photocurable resins as no additives are required to generate reactive species. Scheme 1. Radical generation with Type I and Type II photoinitiators. Some recent efforts have also attempted to design dyes by forming intermolecular charge transfer (ICT) complexes, still aiming at simplifying the elaboration of the photocurable resins. In this case, formation of dyes is reduced to the simple mixture of an electron donor and an electron acceptor, minimizing the synthetic efforts. Numerous reversible addition fragmentation chain transfer (RAFT) agents were also recently developed such as thiocarbonylthio-containing compounds [START_REF] Mckenzie | Investigation into the photolytic stability of RAFT agents and the implications for photopolymerization reactions[END_REF][START_REF] Carmean | Catalyst-Free Photoinduced End-Group Removal of Thiocarbonylthio Functionality[END_REF][START_REF] Mckenzie | Beyond Traditional RAFT: Alternative Activation of Thiocarbonylthio Compounds for Controlled Polymerization[END_REF] or organoiodide compounds. [START_REF] Shanmugam | Photocontrolled Living Polymerization Systems with Reversible Deactivations through Electron and Energy Transfer[END_REF][START_REF] Chen | Light-Controlled Radical Polymerization: Mechanisms, Methods, and Applications[END_REF][START_REF] Yamago | Recent progress in the use of photoirradiation in living radical polymerization[END_REF] These photoinitiators could not only initiate polymerizations but also serve as chain-transfer agents and could impart excellent control on polymer chain growth. Nitroxide-mediated photopolymerization has also recently emerged as an efficient polymerization technique enabling to get an excellent control over the monomer conversions as well as the polymer chain lengths. [START_REF] Guillaneuf | Toward Nitroxide-Mediated Photopolymerization[END_REF][START_REF] Telitel | Nitroxide Mediated Photopolymerization: A Versatile Tool for the Fabrication of Complex Multilayer Polyfunctional Copolymer Nanostructures[END_REF] These last polymerization techniques will not be further developed in this work, photopolymerization of multifunctional monomers being the focus of this article.

Within this perspective article, emerging trends in the design of photoinitiating systems photopolymerization for crosslinking applications will be first presented, mostly focusing on the recent advances in terms of Type I photoinitiators and water-soluble photoinitiators.

Secondly, the emerging trends for improving the light penetration in resins will be discussed.

To end, perspectives in photopolymerization for crosslinking applications will be proposed.

Emerging trends in the synthesis of photopolymers

Towards the simplification of the photocurable resins: The design of Type I photoinitiators

The design of multicomponent photoinitiating systems has long been an efficient strategy for the design of high-performance photoinitiating systems. [START_REF] Xiao | Visible light sensitive photoinitiating systems: Recent progress in cationic and radical photopolymerization reactions under soft conditions[END_REF][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] Dumur | Recent advances on carbazole-based photoinitiators of polymerization[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] Al Mousawi | 3-Hydroxyflavone and N-Phenylglycine in High Performance Photoinitiating Systems for 3D Printing and Photocomposites Synthesis[END_REF] Typically, the most efficient systems were three-component systems in which a dye was used as a photosensitizer for the sensitization of an onium salt (iodonium or sulfonium salts), generating radicals. [START_REF] Mokbel | Specific cationic photoinitiators for near UV and visible LEDs: Iodonium versus ferrocenium structures[END_REF][START_REF] Fouassier | Three-component photoinitiating systems: towards innovative tailor made high performance combinations[END_REF][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] Considering that the photosensitizer is oxidized in first step, presence of a sacrificial amine is thus required, enabling to regenerate the photosensitizer in its initial redox state. In these conditions, the photosensitizer can also be introduced in catalytic amount (See Figure 1). Using this three-component system, the free radical polymerization of acrylates or methacrylates, thiol-ene polymerizations are thus possible. [START_REF] Liu | High-Performance Photoinitiating Systems for LED-Induced Photopolymerization[END_REF] If this approach enabled to get high monomer conversions, the migration ability of the different components and the acute toxicity of amines have thus favored the development of simplified and less toxic

PI Ph • Ph 2 I + Reduction agent PI •+ Amine Amine •+

Oxidation agent

Photoinitiated polymerization

FRP of acrylates photoinitiating systems. Notably, in the past, efficient photoinitiating systems have been prepared with transition metal complexes based on iridium, platinum or ruthenium. [START_REF] Lalevée | Metal and metal-free photocatalysts: mechanistic approach and application as photoinitiators of photopolymerization[END_REF][START_REF] Bonardi | Organometallic vs organic photoredox catalysts for photocuring reactions in the visible region[END_REF][START_REF] Dumur | Recent Advances on Visible Light Metal-Based Photocatalysts for Polymerization under Low Light Intensity[END_REF] If the monomer conversions were exceptional, the scarcity of several metals on Earth, their elevated costs have enforced photopolymerists to find alternatives to these transition metal complexes. [START_REF] Dumur | Recent Advances on Visible Light Metal-Based Photocatalysts for Polymerization under Low Light Intensity[END_REF][START_REF] Meier | Photopolymerization with transition metal complexes[END_REF] Historically, dyes were used as photosensitizers for the sensitization of onium salts. [START_REF] Mokbel | Specific cationic photoinitiators for near UV and visible LEDs: Iodonium versus ferrocenium structures[END_REF] However, recently, a major effort has been devoted to develop Type I photoinitiators, namely molecules that can undergo an homolytic cleavage of a selected bond upon photoexcitation and that do not need any additives to generate initiating species. [START_REF] Müller | Recent Advances in Type I Photoinitiators for Visible Light Induced Photopolymerization[END_REF] In these conditions, a drastic simplification of the photocurable resins can be obtained. To benefit from a push-pull effect inducing a redshift of the absorption, a series of electron-donating groups were introduced on the acyl side of TPO so that an absorption extending up to 440 nm could be obtained for C1a-C1d (See Figure 3). [START_REF] Nazir | Push-Pull Acylo-Phosphine Oxides for Two-Photon-Induced Polymerization[END_REF][START_REF] Xie | A novel acyl phosphine compound as difunctional photoinitiator for free radical polymerization[END_REF] Using this strategy, a higher monomer conversion of trimethylolpropane triacrylate (TMPTA) could be obtained with C1a (64%) compared to that obtained with TPO (47%) at 420 nm. In an effort to redshift the absorption of TPO derivatives, Lalevée and coworkers succeeded to design a Type I photoinitiator with absorption maxima standing at λ > 400 nm. Thus, for C1e, an absorption maximum located at 405 nm was found in acetonitrile. [START_REF] Dietlin | Rational Design of Acyldiphenylphosphine Oxides as Photoinitiators of Radical Polymerization[END_REF] Benefiting from this absorption in the visible range, a threefold enhancement of the monomer conversion was obtained during the free radical polymerization (FRP) of trimethylolpropane triacrylate (TMPTA) in thick films upon irradiation at 395 nm under air (58% conversion for C1e (1 wt%) vs. 17% for TPO (1 wt%)). Besides, it has to be noticed that the molar extinction coefficient at maximum absorption was low, around 50 M -1 .cm -1 . Another strategy to increase the absorption in the visible range consisted in using polyaromatic structures, as exemplified with C1f. By introducing a carbazole unit, a molar extinction coefficient of 15 000 M -1 .cm -1 at absorption maximum could be obtained for C1f, higher than that of BAPO (1 000 M -1 .cm -1 ). [START_REF] Dietlin | New Phosphine Oxides as High Performance Near-UV Type I Photoinitiators of Radical Polymerization[END_REF] During the FRP of TMPTA at 395 nm, C1f greatly outperformed TPO-L, the monomer conversion increasing from 38%

for TPO-L (4 wt%) up to 90% for C1f (4 wt%). No polymerization with BAPO could be obtained in these conditions, BAPO being not soluble in TMPTA at 4 wt%. In the same spirit to the work done by Lalevée and coworkers on TPO, Grützmacher and coworkers introduced different aromatic groups on BAPO. However, considering that a nonconjugating methylene group was introduced between the phosphorus atom and the aromatic ring, only a minor influence on the absorption maximum was detected so that the absorption of C2a and C2b was almost similar to that of BAPO (See Figure 4). [START_REF] Eibel | Extending the Scope of Bis(acyl)phosphane Oxides: Additional Derivatives[END_REF] When tested as photoinitiators, monomer conversions similar to that obtained with BAPO were determined. A different situation was found while functionalizing γ-cyclodextrin with BAPO. [START_REF] Cosola | Synthesis of γ-cyclodextrin substituted bis(acyl)phosphane oxide derivative (BAPO-γ-CyD) serving as multiple photoinitiator and crosslinking agent[END_REF] Around ten BAPO units could be introduced as peripheral groups of γ-cyclodextrin in C2c

(See Figure 5). [START_REF] Cosola | Synthesis of γ-cyclodextrin substituted bis(acyl)phosphane oxide derivative (BAPO-γ-CyD) serving as multiple photoinitiator and crosslinking agent[END_REF] Using this approach, the photoinitiator concentration could be reduced from one order of magnitude compared to BAPO while maintaining an equivalent monomer conversion during the FRP of 1,6-hexanediol diacrylate (HDDA) upon irradiation at 400 nm.

Due to the presence of acrylate groups, crosslinkable groups could guarantee a low migration ability of the supramolecular structure in the final polymer network without impacting the photoactivity of BAPO.

Figure 5. Chemical structure of C2c and HDDA. [START_REF] Cosola | Synthesis of γ-cyclodextrin substituted bis(acyl)phosphane oxide derivative (BAPO-γ-CyD) serving as multiple photoinitiator and crosslinking agent[END_REF] The development of environmentally friendly photopolymerization processes is nowadays an important parameter and in 2020, Versace and coworkers reported the first TPO derivative to be prepared with the biosourced vanillin (See Figure 6). [START_REF] Breloy | Vanillin derivative as performing type I photoinitiator[END_REF] During the FRP of TMPTA or the polymerization of soybean oil epoxidized acrylate (SOA) upon a Hg-Xe lamp irradiation, C3 proved to be as effective as BAPO as photoinitiator. Recently, a novel photocleavable group i.e. oxime ester was introduced on different chromophores to generate visible light Type I photoinitiators. Thus, coumarins, [START_REF] Liu | Recent advances on visible light Coumarin-based oxime esters as initiators of polymerization[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] Qiu | Polymerizable Oxime Esters: An Efficient Photoinitiator with Low Migration Ability for 3D Printing to Fabricate Luminescent Devices[END_REF] coumarin-carbazole fused structures, [START_REF] Zhou | Wavelength-Dependent, Large-Amplitude Photoinitiating Reactivity within a Carbazole-Coumarin Fused Oxime Esters Series[END_REF] carbazoles, [START_REF] Hu | Conjugated Bifunctional Carbazole-Based Oxime Esters: Efficient and Versatile Photoinitiators for 3D Printing under One-and Two-Photon Excitation[END_REF] nitrocarbazoles [START_REF] Dumur | Recent advances on carbazole-based oxime esters as photoinitiators of polymerization[END_REF][START_REF] Liu | Effect of Decarboxylation on the Photoinitiation Behavior of Nitrocarbazole-Based Oxime Esters[END_REF][START_REF] Liu | Nitro-Carbazole Based Oxime Esters as Dual Photo/Thermal Initiators for 3D Printing and Composite Preparation[END_REF] benzocarbazoles, [START_REF] Sameshima | Improvement of the photopolymerization and bottom-curing performance of benzocarbazole oxime ester photoinitiators with red-shifted absorption[END_REF] tert-butylcarbazoles, [START_REF] Lu | Synthesis and Structure-Activity Relationship of N-Substituted Carbazole Oxime Ester Photoinitiators[END_REF] bicarbazoles [START_REF] Zhou | Bicarbazole-based oxime esters as novel efficient photoinitiators for photopolymerization under UV-Vis LEDs[END_REF] and fused bis-carbazoles, [START_REF] Hammoud | 5,12-Dihydroindolo[3,2a]carbazole: A promising scaffold for the design of visible light photoinitiators of polymerization[END_REF] triphenylamines, [START_REF] Hammoud | Novel phenylaminebased oxime ester photoinitiators for LED-induced free radical, cationic, and hybrid polymerization[END_REF][START_REF] Lee | Synthesis and free radical photopolymerization of triphenylamine-based oxime ester photoinitiators[END_REF] stilbenes, [START_REF] Chen | Substituted stilbene-based oxime esters used as highly reactive wavelength-dependent photoinitiators for LED photopolymerization[END_REF] phenothiazines, [START_REF] Ma | Conjugated phenothiazine oxime esters as free radical photoinitiators[END_REF][START_REF] Lee | Design and Synthesis of Novel Oxime Ester Photoinitiators Augmented by Automated Machine Learning[END_REF] phenyl thienyl thioethers, [START_REF] Wang | Remote effect of substituents on the properties of phenyl thienyl thioether-based oxime esters as LED-sensitive photoinitiators[END_REF][START_REF] Wang | Phenylthioether thiophene-based oxime esters as novel photoinitiators for free radical photopolymerization under LED irradiation wavelength exposure[END_REF] 1,2-bis(4-fluorophenyl) ethane-1,2-diones, [START_REF] Ding | Photochromic Polymers Based on Fluorophenyl Oxime Ester Photoinitiators as Photoswitchable Molecules[END_REF] pyrenes and anthracenes, [START_REF] Rahal | Investigation of pyrene vs Anthracene-based oxime esters: Role of the excited states on their polymerization initiating abilities[END_REF] or naphthalenes [START_REF] Lee | Naphthalene-Based Oxime Esters as Type I Photoinitiators for Free Radical Photopolymerization[END_REF] were substituted with varying oxime ester groups. By introducing nitro groups on carbazole, absorption of nitrocarbazole-based oxime esters C4a-C4d (See Figure 7)

could tail out into the visible range until 450 nm. [START_REF] Liu | Nitro-Carbazole Based Oxime Esters as Dual Photo/Thermal Initiators for 3D Printing and Composite Preparation[END_REF] An improvement of the stability of oxime esters upon storage due to the presence of the nitro group was also demonstrated. From a mechanistic viewpoint, the decarboxylation reaction of the acyloxy radical subsequent to the homolytic cleavage of the N-O bond upon irradiation is an important step to produce highly reactive radicals (See Figure 8). Indeed, alkyl and aryl radicals are more reactive than their alkyloxy or aryloxy analogues. Additionally, after decarboxylation reaction, radical recombination of alkyl or aryl radicals with the iminyl radicals is unfavorable, furnishing in turn alkyl or aryl radicals that can only contribute to polymerization. Using Real Time Fourier Transform Infrared spectroscopy (RT-FTIR), occurrence of a decarboxylation reaction could be monitored with the detection of a peak at 2337 cm -1 corresponding to CO2.

During the FRP of TMPTA, C4a could outperform TPO upon irradiation at 405 nm due to the high reactivity of the methyl radicals. A dual thermal and photochemical initiating ability could also be evidenced for C4a-C4c, what was never examined prior to this work. The high reactivity of the methyl radicals compared to the aromatic radicals was confirmed with all chromophores investigated as photoinitiators. The distance between the chromophore and the photocleavable group is another parameter to consider as it governs the efficiency of the photocleavage. In 2022, Lalevée and coworkers examined this point with two series of naphthalimide-based initiators (See Figure 9). [START_REF] Liu | N-naphthalimide ester derivatives as Type Ⅰ photoinitiators for LED photopolymerization[END_REF] If C5a could outperform TPO at 405 nm during the FRP of TMPTA, a twofold reduction of the monomer conversion was obtained with C5b, due to an inefficient energy transfer from the chromophore towards the photocleavable group. This compound being not capable to act as a Type I photoinitiator, a

Type II photoinitiating ability was evidenced for this compound. Recently, new photocleavable groups derived from the well-known oxime esters were proposed in 2022 such as the N-hydroxyphthalimide esters C6a [START_REF] Liu | N-naphthalimide ester derivatives as Type Ⅰ photoinitiators for LED photopolymerization[END_REF] or the imidazolyl esters C6b (See Figure 10). [START_REF] Hammoud | Naphthoquinone-based imidazolyl esters as blue-lightsensitive Type I photoinitiators[END_REF] Figure 9. Chemical structures of naphthalimide-based Type I photoinitiators.

In the case of the naphthoquinone-based imidazolyl esters, absorption tailing up to 500 nm could be determined, making these Type I photoinitiators, the dyes with the most redshifted absorption ever reported in the literature. [START_REF] Hammoud | Naphthoquinone-based imidazolyl esters as blue-lightsensitive Type I photoinitiators[END_REF] In thin films, numerous derivatives of C6b could outperform TPO at 455 nm during the FRP of TMPTA, evidencing the pertinence of these new structures. Besides, at 405 nm, monomer conversions obtained with C6b were similar to that determined with TPO, TPO absorbing more at 405 nm than at 455 nm and thus improving its photoinitiating ability at this wavelength. A good stability of the resins upon storage was also demonstrated. Indeed, after one month of storage in the dark, polymerization tests revealed the monomer conversions of the stored resins to be similar to that obtained for freshly prepared resins. In 2019, Morlet-Savary and coworkers evidenced the crucial impact of the substitution pattern of a series of dibenzothiophene-based phenyl glyoxylates on the polymerization performance. [START_REF] Pan | A substituent para-toortho positioning effect drives the photoreactivity of a dibenzothiophene-based oxalate series used as LED-excitable free radical photoinitiators[END_REF] By positioning the photocleavable group in ortho-position to the sulfur atom in C7a and C7b (See Figure 12), a redshift of the absorption maximum by ca. 30 nm could be obtained for the ortho-substituted compounds compared to the para-substituted ones, so that an absorption tailing up to 430 nm could be obtained. Jointly, a twofold enhancement of the monomer conversion could be obtained at 405 nm during the FRP of tripropylene glycol diacrylate (TPGDA) for C7a and C7b compared to C7c and C7d. Still based on the homolytic cleavage of an intercarbonyl C-C bond, Wang and coworkers developed in 2020 a series of carbazolyl α-diketones C8a-C8c (See Figure 12). [START_REF] Ma | Carbazolyl α-diketones as novel photoinitiators in photopolymerization under LEDs[END_REF] Benefiting from an absorption extending up to 430 nm, carbazolyl α-diketones proved to be better Type I photoinitiators than phenyl glyoxylates during the FRP of TPGDA at 405 nm, the monomer conversions ranging between 82 and 91% for C8a-C8c vs. 38-42% for C7a/C7b. Especially, C8a-C8c could outperform the well-establish camphorquinone/triethanolamine (TEOA) combination (70% conversion). The absorption of C9b and C9c extending up 500 nm, photopolymerization of TPGDA could be carried out at 405 nm but also at 460 nm. Due to the low bond dissociation energy of the O-SO2 bond and the photoacid generation ability of C9c, C9c could furnish a TPGDA conversion similar to that obtained with camphorquinone for a photoinitiator content as low as 0.125 wt% at 460 nm. Conversely, C9b proved to be ineffective as a monocomponent photoinitiating system and an acceptable monomer conversion could only be obtained when combined to additives (triethanolamine or diphenyliodonium salt).

In 2021, Yagci and coworkers developed a panchromatic cationic Type I photoinitiator based on phenacyl phenothiazinium salt with a remarkable absorption extending up to 900 nm. [START_REF] Zhu | Phenacyl Phenothiazinium Salt as a New Broad-Wavelength-Absorbing Photoinitiator for Cationic and Free Radical Polymerizations[END_REF] Upon irradiation, C10 could cleave homolytically, the cleavage leading to the formation of a phenacyl radical and a phenothiazine radical cation (See Figure 14). By hydrogen abstraction, Bronsted acid could be formed, enabling to initiate the cationic polymerization (CP) of epoxides. C10 demonstrated excellent photoinitiating properties for cationic, free radical and hybrid polymerization processes. Especially, the concomitant polymerization of epoxides and acrylates enabled to drastically reduce the oxygen inhibition occurring during FRP and to generate interpenetrated polymer networks (IPNs).

Photochemistry without dyes: The benefits of charge transfer complexes

The design of photoinitiators/photosensitizers with appropriate light absorption, redox properties and excited state lifetimes can constitute a real challenge from the synthetic viewpoint, especially if numerous derivatives should be synthesized to establish a structure/performance relationship. Recently, the formation of intermolecular charge transfer (ICT) complexes has been identified as a promising alternative strategy to multistep syntheses for the design of dyes. [START_REF] Garra | Visible light photoinitiating systems by charge transfer complexes: Photochemistry without dyes[END_REF] To get charge transfer complexes (CTCs), an electron donor should be mixed with an electron acceptor so that by intermolecular interaction in the ground state, a colored CTC can be obtained, even starting from colorless reagents. [START_REF] Wang | Multifunctional Features of Organic Charge-Transfer Complexes: Advances and Perspectives[END_REF][START_REF] Goetz | Chargetransfer complexes: new perspectives on an old class of compounds[END_REF] In this field, an interesting CTC was obtained between diphenyliodonium hexafluorophosphate C11a and 4-N,N-trimethylaniline C11b. [START_REF] Garra | On-Demand Visible Light Activated Amine/Benzoyl Peroxide Redox Initiating Systems: A Unique Tool To Overcome the Shadow Areas in Photopolymerization Processes[END_REF] Using this CTC, the photoactivated redox polymerization of a dental composite (bisphenol A diglycidyl ether dimethacrylate/triethylene glycol dimethacrylate (BisGMA/TEGDMA) (30/70 wt %) blend) could be obtained upon irradiation at 405 nm. In fact, charge transfer complexes were mainly developed for FRP processes, and the polymerization of acrylates or methacrylates or thiol-ene polymerizations could be successfully obtained with these systems. [START_REF] Xu | Phenothiazine-based charge-transfer complexes as visible-light photoinitiating systems for acrylate and thiol-ene photopolymerization[END_REF] Indeed, as shown in the Figure 15, a photoinduced electron transfer between the electron donor and the acceptor can occur within the CTC, inducing the decomposition of the iodonium salt and the generation of initiating aryl radicals. By replacing 4-N,N-trimethylaniline C11b by the Trost ligand C11c, a better reactivity could even be obtained. This positive effect was assigned to the dual behavior of phosphine, enabling first 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][START_REF] Ligon | Strategies to Reduce Oxygen Inhibition in Photoinduced Polymerization[END_REF][START_REF] Lin | Modeling the Kinetics, Curing Depth, and Efficacy of Radical-Mediated Photopolymerization: The Role of Oxygen Inhibition, Viscosity, and Dynamic Light Intensity[END_REF] and second to form an additional CTC with the iodonium salt (See Figure 16). Using the three-component system, polymerization of the BisGMA/TEGDMA blend was even possible at 470 nm. blend could be achieved at 850 nm with the three-component system C11g/C11e/triphenylphosphine C11h. A final monomer conversion of 84% upon irradiation at 850 nm was determined, outranking the reference camphorquinone/ ethyl dimethylaminobenzoate C11i/C11f system only furnishing a monomer conversion of 60% upon excitation at 470 nm (See Figure 16). The high conversion obtained with the threecomponent C11g/C11e/C11h system was assigned to a photothermic effect resulting from an efficient light-to-heat conversion at 850 nm. the FRP of the BisGMA/TEGDMA blend at temperature as low as 108°C but also upon irradiation at 405 nm. [START_REF] Wang | Charge Transfer Complexes as Dual Thermal and Photochemical Polymerization Initiators for 3D Printing and Composites Synthesis[END_REF] This dual curing ability (photochemical and thermal polymerization) could serve for the curing of glass fiber or carbon fiber composites using the 6-aminoindole C11k/C11e CTC. By using this CTC, a fast curing by the light of the surface limiting oxygen diffusion within the resin and an efficient dark thermal curing in depth of the BisGMA/TEGDMA blend could be obtained. [START_REF] Wang | Indole-based charge transfer complexes as versatile dual thermal and photochemical polymerization initiators for 3D printing and composites[END_REF] Recently, the ability to initiate a thiol-ene step-growth polymerization was examined with CTCs based on phenothiazines C11l or C11m (See Figure 16). At 405 nm, the two CTC C11l/C11e and C11m/C11e could outperform the reference and benchmark photoinitiator isopropylthioxanthone (ITX).

[100]

Water-soluble photoinitiators: From chemical modification to CTC approach.

Water-soluble photoinitiators are required in a wide range of applications ranging from drug delivery, 3D bioprinting, cell encapsulation, dentistry, in vitro polymerization and inkjet inks. [START_REF] Tomal | Water-Soluble Photoinitiators in Biomedical Applications[END_REF][START_REF] El-Molla | Synthesis of polyurethane acrylate oligomers as aqueous UV-curable binder for inks of ink jet in textile printing and pigment dyeing[END_REF][START_REF] Smith | Ordered Poly(p-phenylenevinylene) Matrix Nanocomposites via Lyotropic Liquid-Crystalline Monomers[END_REF][START_REF] Zhang | Micro-stereolithography of polymeric and ceramic microstructures[END_REF][START_REF] Du | Synthesis and Characterization of Photo-Cross-Linked Hydrogels Based on Biodegradable Polyphosphoesters and Poly(ethylene glycol) Copolymers[END_REF][START_REF] Delong | Covalently immobilized gradients of bFGF on hydrogel scaffolds for directed cell migration[END_REF] To access to these water-soluble structures, several strategies were developed in parallel. For instance, the formation of ionic photoinitiators by derivatization of benchmark structures such as TPO or TPO-L was proposed. Thus, lithium phenyl-2,4,6trimethyl-benzoylphosphinate C12a was proposed as a water soluble photoinitiator exhibiting an improved solubility compared to 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1propanone C12b (See Figure 17). [START_REF] Fairbanks | Photoinitiated polymerization of PEG-diacrylate with lithium phenyl-2,4,6trimethylbenzoylphosphinate: polymerization rate and cytocompatibility[END_REF] Thus, water solubilities of 40.5, 49.5 and 15.5 g.L -1 were determined for C12a, C12a' and C12a" respectively, higher than that determined for C12b

(2.15 g.L -1 ). [START_REF] Wang | General One-Pot Method for Preparing Highly Water-Soluble and Biocompatible Photoinitiators for Digital Light Processing-Based 3D Printing of Hydrogels[END_REF] At comparable intensities and initiator concentrations, the time required to reach the gelation of polyethylene glycol diacrylate (PEGDA) with C12a was one tenth the time required for C12b upon irradiation at 365 nm. Polymerization of PEGDA was also possible at 405 nm with C12a, what is precluded with C12b due to the lack of absorption at this wavelength. A remarkable cytocompatibility was also demonstrated, the survival rates of human neonatal fibroblasts exceeding 95% for hydrogels polymerized with C12a. To provide water-solubility, another strategy was also examined such as the introduction of ammonium salt in C12c. [START_REF] Le | Water-Soluble Photoinitiators from Dimethylamino-Substituted Monoacylphosphine Oxide for Hydrogel and Latex Preparation[END_REF] Figure 17. Chemical structures of C12a-C12e and PEGDA.

Numerous water-soluble BAPO derivatives were also proposed during the last threeyears, consisting in generating ionic structures such as C12d which exhibited in turn the same absorption maximum than C12a (383 nm for C12d vs. 380 nm for C12a). [START_REF] Benedikt | Highly efficient water-soluble visible light photoinitiators[END_REF] However, if the absorption of C12a extends up to 420 nm, an absorption tailing up to 430 nm was determined for C12d. Noticeably, C12a and C12d exhibited a water solubility 6-to 12 times higher than that of C12b. As an imidazolium salt, C12e could act as an initiating agent for the surfactantfree emulsion polymerization of styrene, yielding monodisperse and spherical nanoparticles. [START_REF] Müller | Simple One-Pot Syntheses of Water-Soluble Bis(acyl)phosphane Oxide Photoinitiators and Their Application in Surfactant-Free Emulsion Polymerization[END_REF] Introduction of poly(ethylene glycol) chains in C12f (See Figure 17) enabled to increase the water-solubility up to 300 g/L and to the best of our knowledge, this performance has never been equaled. [START_REF] Wang | A highly efficient waterborne photoinitiator for visible-light-induced three-dimensional printing of hydrogels[END_REF] Polymerization of hydrogels could be carried out at 460 nm with this dye.

Recently, several approaches were envisioned to design water-soluble dyes without requiring synthetic work. Notably, encapsulation of the naphthalimide C13a in 2,6-di-O-Meβ-cyclodextrin enabled to reach a conversion of 80% during the FRP of 2-hydroxyethyl acrylate (HEA) upon irradiation at 405 nm using methyldiethanolamine (MDEA) as a water-soluble amine to prepare the two-component system. [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] Hydrogels containing up to 70% water were obtained. Using sulfobutylether-β-cyclodextrin (SBE-β-CD), the anticancer agent i.e.

azonafide C13b could be converted as a water-soluble photoinitiator in combination with MDEA and hydrogels prepared with hydroxyethylmethacrylate (HEMA) as the monomer also contained 70% water (See Figure 18). [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] Figure 18. Chemical structures of C13a, C13b and HEA.

In 2022, a photocleavable group was proposed by Nie and coworkers for the first time as a water solubilizing agent for photoinitiators, namely 2-oxoacetic acid (See Figure 19). [START_REF] He | Water-soluble benzoylformic acid photoinitiators for water-based LED-triggered deep-layer photopolymerization[END_REF] Water solubilities of C14a and C14b were respectively of 10 and 5 wt%, greatly higher than that of C12b (1.2 wt%). [START_REF] Wei | Design of photoinitiator-functionalized hydrophilic nanogels with uniform size and excellent biocompatibility[END_REF][START_REF] Zhang | Design of green hydrophilic polysiloxanepolyether-modified macromolecular photoinitiators with ionic liquid character[END_REF] A monomer conversion as high as 80% could be obtained with the two dyes during the FRP of PEGDA in water, upon irradiation at 405 nm. Deep layer photopolymerization experiments revealed C14a and C14b to reach a polymerization depth of 6.3 and 6.7 cm, respectively, higher than that obtained with BAPO (less than 1 cm). [START_REF] He | Methyl Benzoylformate Derivative Norrish Type I Photoinitiators for Deep-Layer Photocuring under Near-UV or Visible LED[END_REF] Figure 19. Water-soluble dyes based on 2-oxoacetic acid as the water-solubilizing and photocleavable agent.

From the mechanistic viewpoint, C14a and C14b behaved as Type I photoinitiators and the mechanism of photodecomposition of 2-oxoacetic acid derivatives is comparable to that previously depicted in Figure 11 for phenyl glyoxylates.

The formation of CTC with triethanolamine was identified as another promising strategy for the design of water-soluble dyes. In this field, the water-soluble triethanolamine (TEOA) was widely used during the last two years as an agent providing water-solubility to water-insoluble dyes. In 2020, TEOA was thus used as an electron donor for C15a that could generate a CTC absorbing until 500 nm, the absorption maximum peaking at 400 nm. [START_REF] Li | Visible light and water-soluble photoinitiating system based on the charge transfer complex for free radical photopolymerization[END_REF] A solubility as high as 20 mg/mL was determined for the CTC. Based on its good solubility, 50% acrylamide in water could be polymerized using 3 wt% of C15a/TEOA CTC upon irradiation at 405 nm. In 2021, another water-soluble CTC was proposed with a chalcone, namely C15b possessing an ethylene glycol chain (See Figure 20). However, compared to the C15a/TEOA CTC, a severe reduction of the water solubility was found for the C15b/TEOA CTC, not exceeding 0.3 wt%. [START_REF] Chen | Water-Soluble Visible Light Sensitive Photoinitiating System Based on Charge Transfer Complexes for the 3D Printing of Hydrogels[END_REF] Figure 20. Example of chalcones used to form water-soluble CTCs.

Polymerization with NIR light: Towards safer polymerization conditions and high light penetration in resins

Photopolymerization in the near infrared (NIR) range is challenging owing to the low energy NIR photons exhibit. [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] Bonardi | Different NIR dye scaffolds for polymerization reactions under NIR light[END_REF][START_REF] Mokbel | On demand NIR activated photopolyaddition reactions[END_REF][START_REF] Mokbel | NIR Sensitizer Operating under Long Wavelength (1064 nm) for Free Radical Photopolymerization Processes[END_REF][START_REF] Noirbent | Metalated porphyrins as versatile visible light and NIR photoinitiators of polymerization[END_REF][START_REF] Mokbel | Towards new NIR dyes for free radical photopolymerization processes[END_REF][START_REF] Strehmel | Photochemistry with Cyanines in the Near Infrared: A Step to Chemistry 4.0 Technologies[END_REF][START_REF] Strehmel | NIR-Dyes for Photopolymers and Laser Drying in the Graphic Industry[END_REF][START_REF] Wang | NIR-sensitized hybrid radical and cationic photopolymerization of several cyanines in combination with diaryliodonium bis(trifluoromethyl)sulfonyl imide[END_REF][START_REF] Strehmel | Photophysics and photochemistry of NIR absorbers derived from cyanines: key to new technologies based on chemistry 4.0[END_REF] Besides, polymerization with NIR light allows a deeper light penetration within the photocurable resins than the UV light ((up to 5 cm vs 600 µm for UV light), enabling an improved curing of thick materials and the access to composites by allowing the polymerization of resins containing fillers. [START_REF] Bonardi | High Performance Near-Infrared (NIR) Photoinitiating Systems Operating under Low Light Intensity and in the Presence of Oxygen[END_REF] As an innovative approach, photopolymerization based on photon up-conversion was proposed as an efficient tool to polymerize in safe conditions. [START_REF] Chen | Near-Infrared Light Driven Photopolymerization Based On Photon Upconversion[END_REF][START_REF] Hermes | Challenges and limits of upconversion nanoparticles for cationic photopolymerization with UV initiators excited at 980 nm[END_REF] Various lanthanide-doped upconverting nanoparticles (UCNPs) were reported in the literature. However, NaYF4:Yb 3+ ,Tm 3+ or NaYF4:Yb 3+ ,Er 3+ remain the most widely used nanoparticles. [START_REF] Han | Upconversion Nanoparticles/Hyaluronate-Rose Bengal Conjugate Complex for Noninvasive Photochemical Tissue Bonding[END_REF][START_REF] Stepuk | Use of NIR light and upconversion phosphors in light-curable polymers[END_REF][START_REF] Ding | Platform of near-infrared light-induced reversible deactivation radical polymerization: upconversion nanoparticles as internal light sources[END_REF] Using UCNPs, photopolymerization with NIR light could be initiated with UV and visible light photoinitiators such as Eosin Y (λmax(abs) = 535 nm, λmax(exc) = 974 nm), [START_REF] Han | Upconversion Nanoparticles/Hyaluronate-Rose Bengal Conjugate Complex for Noninvasive Photochemical Tissue Bonding[END_REF][START_REF] Stepuk | Use of NIR light and upconversion phosphors in light-curable polymers[END_REF] Rose Bengal (λmax(abs) = 560 nm, λmax(exc) = 974 nm), [START_REF] Ding | Platform of near-infrared light-induced reversible deactivation radical polymerization: upconversion nanoparticles as internal light sources[END_REF] titanocene (λmax(abs)

= 400 nm, λmax(exc) = 980 nm), [START_REF] Liu | Extremely deep photopolymerization using upconversion particles as internal lamps[END_REF] BAPO (λmax(abs) = 370 nm, λmax(exc) = 980 nm), [START_REF] Méndez-Ramos | Infrared-light induced curing of photosensitive resins through photon up-conversion for novel cost-effective luminescent 3D-printing technology[END_REF] bis(4methoxybenzoyl)diethyl germanium (λmax(abs) = 400 nm, λmax(exc) = 974 nm), [START_REF] Chen | Upconversion-Nanoparticle-Assisted Radical Polymerization at λ=974 nm and the Generation of Acidic Cations[END_REF] fluorescein (λmax(abs) = 450 nm, λmax(exc) = 975 nm), [START_REF] Kocaarslan | Near-Infrared Free-Radical and Free-Radical-Promoted Cationic Photopolymerizations by In-Source Lighting Using Upconverting Glass[END_REF] benzaldehyde-substituted diacetylene (λmax(abs)

= 380 nm, λmax(exc) = 980 nm), [START_REF] Yang | Near-Infrared Circularly Polarized Light Triggered Enantioselective Photopolymerization by Using Upconversion Nanophosphors[END_REF] or camphorquinone (λmax(abs) = 470 nm, λmax(exc) = 974 nm). [START_REF] Stepuk | Use of NIR light and upconversion phosphors in light-curable polymers[END_REF][START_REF] Caron | Near-Infrared PhotoInitiating Systems: Photothermal versus Triplet-Triplet Annihilation-Based Upconversion Polymerization[END_REF] NIR dyes are also well-known to be capable to release heat upon photoexcitation, what was advantageously used to initiate the decomposition of thermal initiators. Using this approach, a photochemically induced thermal polymerization could be obtained with a thermal initiator such as 2-[N-tert-butyl-2,2-(dimethylpropyl)aminooxy]propionic acid i.e.

BlocBuilder® MA (See Figure 23). [START_REF] Bonardi | Different NIR dye scaffolds for polymerization reactions under NIR light[END_REF][START_REF] Noirbent | Metalated porphyrins as versatile visible light and NIR photoinitiators of polymerization[END_REF][START_REF] Bonardi | Thermal Initiators as Additives for Photopolymerization of Methacrylates upon Blue Light[END_REF][START_REF] Noirbent | New Donor-Acceptor Stenhouse Adducts as Visible and Near Infrared Light Polymerization Photoinitiators[END_REF][START_REF] Bonardi | Free-radical polymerization upon near-infrared light irradiation, merging photochemical and photothermal initiating methods[END_REF][START_REF] Bonardi | Fillers as Heaters for Photothermal Polymerization upon NIR Light[END_REF][START_REF] Bonardi | Photoinduced Thermal Polymerization Reactions[END_REF] charge transfer complexes could form between the ferrocene derivatives and the monomer and an absorption extending up to 850 nm could be detected for the CTCs (See Figure 24).

Remarkably, light-induced polymerization of C16c could take place upon irradiation at 780 nm. With aim at extending polymer lifetime, self-healing of polymers constitutes an appealing approach to repair macroscopic damages (cracks), to impart reprocessability to polymers and to provide longer lifetimes to crosslinked polymeric materials. [START_REF] Griebel | Polymerizations with elemental sulfur: A novel route to high sulfur content polymers for sustainability, energy and defense[END_REF][START_REF] Guo | Universally autonomous self-healing elastomer with high stretchability[END_REF] This point was notably examined upon incorporation of IR-813 in glycerol formal methacrylate (glyfoma)-based polymers (See Figure 25). [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] Upon irradiation at 785 nm (I = 2.55 W/cm²) for 10 min., cracks could be totally suppressed for polymers containing IR-813 at 0.1 wt%.

Polymerization with sunlight: Towards photopolymerization without artificial light sources.

At present, most of the photopolymerization processes are carried out with artificial light sources. However, with the development of highly reactive photoinitiating systems, Sun which is the cheapest light source on Earth can be envisioned as a plausible alternative to artificial light sources, especially for outdoor applications. [START_REF] Lalevée | Recent advances in sunlight induced polymerization: role of new photoinitiating systems based on the silyl radical chemistry[END_REF] In this field, push-pull dyes seem to be the most promising candidates for sunlight applications. Indeed, due to their remarkably high molar extinction coefficients, their broad absorption extending over the visible range making these dyes panchromatic photoinitiators, an interesting overlap between the emission spectrum of the Sun and the absorption of push-pull dyes can be found. [START_REF] Bureš | Fundamental aspects of property tuning in push-pull molecules[END_REF][START_REF] Pigot | Recent advances on push-pull organic dyes as visible light photoinitiators of polymerization[END_REF][START_REF] Dumur | Recent advances on visible light photoinitiators of polymerization based on Indane-1,3-dione and related derivatives[END_REF][START_REF] Sun | Organic dye-based photoinitiating systems for visible-light-induced photopolymerization[END_REF] In 2021, Lalevée and coworkers proposed of a series of push-pull dyes based on N-ethyl carbazole-1-allylidene as the electron donor (see C17a and C17b in Figure 26). [START_REF] Sun | N-ethyl carbazole-1-allylidene-based push-pull dyes as efficient light harvesting photoinitiators for sunlight induced polymerization[END_REF] Figure 26. Chemical structures of photoinitiators used for sunlight induced polymerization.

Using the three-component dye/C11e/ethyl dimethylaminobenzoate C17c (0.1%/2%/2% w/w) system, monomer conversions as high as 99% with C17a and 95% with C17b could be obtained during the FRP of a benchmark acrylate monomer after 30 min. of sunlight exposure, close to that determined upon irradiation at 405 nm with a light-emitting diode (LED) (99% with C17a and 98% with C17b). This trend was confirmed with two other series of push-pull dyes i.e. C17d-C17f [START_REF] Sun | High-performance sunlight induced polymerization using novel push-pull dyes with high light absorption properties[END_REF] and C17g-C17i, [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] for which the remarkable conversions of 99, 95, 92, 80, 88 and 88% could be determined with the three-component C17d-C17i/C11e/C17c (0.1%/2%/2% w/w) systems respectively during the polymerization of the same monomer at 405 nm.

Future vision

Photopolymerization under visible light has known during the last decade a revolution due to the emergence of new irradiation setups based on LEDs and the generalization of 3D

printing for various applications. Owing to its unique properties, photopolymerization is used nowadays in dentistry, coatings, adhesives as well as food packaging. Polymers are everywhere in our daily lives, even if an increasing awareness of the threat polymers poses to the environment is emerging in the population. Nowadays, an ongoing effort is focused on the synthesis of photoinitiators in more environmentally friendly conditions. In this field, chalcones constitute a relevant example of this and numerous structures have been proposed

by the groups of Lalevée and Nie. [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] 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] Li | A pyrrole-carbazole photoinitiator for radical and cationic visible light LED photopolymerization[END_REF][START_REF] Li | Improvement in the storage stability of free radical photocurable materials under sunlight based on the cis → trans photoisomerization of pyrrole chalcone photoinitiator[END_REF][START_REF] Li | Study on pyrrole chalcone derivatives used for blue LED free radical photopolymerization: Controllable initiating activity achieved through photoisomerization property[END_REF][START_REF] Li | Thermally activated pyrrole chalcone free radical photoinitiator with excellent stability to sunlight[END_REF] Notably, chalcones can be prepared in one step by a Claisen Schmidt condensation in ethanol using an aqueous solution of base (KOH or NaOH).

Benzylidene ketones constitute another family of compounds that can also be synthesized in one step, using similar reactions conditions. [START_REF] Bao | Double benzylidene ketones as photoinitiators for visible light photopolymerization[END_REF][START_REF] Dumur | Recent advances on benzylidene ketones as photoinitiators of polymerization[END_REF][START_REF] Xue | Novel benzylidene cyclopentanone dyes for twophoton photopolymerization[END_REF][START_REF] Wu | Multibranched benzylidene cyclopentanone dyes with large two-photon absorption cross-sections[END_REF][START_REF] Fang | Water-soluble benzylidene cyclopentanone based photosensitizers for in vitro and in vivo antimicrobial photodynamic therapy[END_REF][START_REF] Fang | Cationic benzylidene cyclopentanone photosensitizers for selective photodynamic inactivation of bacteria over mammalian cells[END_REF][START_REF] Wan | Water-soluble benzylidene cyclopentanone dye for two-photon photopolymerization[END_REF][START_REF] Wang | Initiation efficiency and cytotoxicity of a series of water-soluble benzylidene cyclopentanone for two-photon polymerization[END_REF][START_REF] Dumur | Recent advances on benzylidene cyclopentanones as visible light photoinitiators of polymerization[END_REF][START_REF] Hu | Benzylidene piperidones as photosensitizers for visible light photopolymerization[END_REF][START_REF] Xue | A bis-acrylate functionalized enone as photoinitiator and crosslinker in photopolymerization[END_REF][START_REF] Xue | Enone dyes as visible photoinitiator in radical polymerization: The influence of peripheral N-alkylated (hetero)aromatic amine group[END_REF][START_REF] Xue | Benzylidene ketones as visible light radical photoinitiator: The effects of electron-donating group and co-initiator[END_REF][START_REF] Xue | Pyrrole-based enone dyes as radical photointiator under 405/460 nm LED lamp: The effect of ketone structure[END_REF][START_REF] Xue | Color evolution of a pyrrole-based enone dye in radical photopolymerization formulations[END_REF] In the current search for reducing the environmental impact of polymers, the polymerization of biosourced monomers is now extensively studied. [START_REF] Pierau | Bio-sourced monomers and cationic photopolymerization-The green combination towards eco-friendly and nontoxic materials[END_REF] Besides, if an effort is given to develope environmentally friendly polymerization conditions such as the polymerization in water, end of life of polymers is not sufficiently considered at present. Introduction of monomers facilitating depolymerization reactions must now be anticipated. Development of photochemical processes enabling to depolymerize extensively used polymers such as polystyrene, polyacrylates and polyamides remains still unexplored today. Photochemistry could constitute an energy-saving strategy to the thermal depolymerization currently under use.

For the coming decade, it is believed that the development of monocomponent photoinitiating systems based on new photocleavable groups will discard the extensively and historically used Type II photoinitiating systems requiring a complex mixture of different compounds. An important challenge remains in the development of photocleavable groups that can be cleaved at lower energy than oxime esters and aryl glyoxylates. In this case, chromophores absorbing at longer wavelengths than that used presently could be used.

Development of photoinitiating systems that can be activated at long wavelength combine several advantages such as safety for the manipulator, energy efficiency and energy sobriety, as well as the possibility to polymerize thick samples. These different trends will provide a guidance for the design of the next generation of long-wavelength photoinitiating systems that will be used in the next decade.

Figure 1 .

 1 Figure 1. Example of three-component photoinitiating systems enabling to introduce the photosensitizer in catalytic amount.

Figure 2 .

 2 Figure 2. Chemical structures of phosphine oxides TPO, TPO-L and BAPO.

Figure 3 .

 3 Figure 3. Chemical structures of C3a-C3e and TMPTA.

Figure 4 .

 4 Figure 4. Chemical structures of different BAPO derivatives.

Figure 6 .

 6 Figure 6. Chemical structure of C3 prepared with the biosourced vanillin and epoxidized soybean oil.[START_REF] Breloy | Vanillin derivative as performing type I photoinitiator[END_REF]

Figure 7 .

 7 Figure 7. Chemical structures of C4a-C4c.[75]

Figure 8 .

 8 Figure 8. Mechanism of decomposition of oxime esters upon irradiation.

Figure 10 .

 10 Figure 10. Photocleavable groups derived from the well-known oxime ester function.Investigation for photocleavable structures was not limited to the homolytic cleavage of the N-O bond in oxime esters and related structures but other types of fragmentations were also examined. Notably, phenyl glyoxylates constitute another workhorse of photopolymerists.[START_REF] Kirschner | 1-Aryl-2-(triisopropylsilyl)ethane-1,2-diones: Toward a New Class of Visible Type I Photoinitiators for Free Radical Polymerization of Methacrylates[END_REF][START_REF] Kirschner | Lalevée, 2-Oxo-2(tertbutyldimethylsilyl)Acetic Acid (DKSi-COOH) as a New Water-Soluble Visible Light Type I Photoinitiator for Free Radical Polymerization[END_REF] These structures exhibit a Type I photoinitiator behavior by the homolytic cleavage of the intercarbonyl C-C bond, generating aryloyl and alkyl radicals after decarboxylation of the acyloxy radicals (See Figure11).

Figure 11 .

 11 Figure 11. Mechanism of fragmentation of phenyl glyoxylate 7a.

Figure 12 .Figure 13 .

 1213 Figure 12. Examples of phenyl glyoxylate derivatives C7a-C7d, carbazolyl α-diketones C8a-C8c and TPGDA.Recently, the same group investigated flavonoids as Type I photoinitiators.[START_REF] You | Novel Norrish type I flavonoid photoinitiator for safe LED light with high activity and low toxicity by inhibiting the ESIPT process[END_REF] By functionalizing the 3-hydroxy position of 3-hydroxy-4H-chromen-4-one in C9b-C9c, the excited state intramolecular proton transfer (ESIPT) existing in C9a could be efficiently suppressed (See Figure13).[START_REF] Sengupta | Excited state proton-transfer spectroscopy of 3-hydroxyflavone and quercetin[END_REF][START_REF] Mcmorrow | Intramolecular excited-state proton transfer in 3hydroxyflavone. Hydrogen-bonding solvent perturbations[END_REF][START_REF] Ameer-Beg | Measurements of Excited State Intramolecular Proton Transfer (ESIPT) in Room Temperature Solutions of 3-Hydroxyflavone and Derivatives[END_REF]

Figure 14 .

 14 Figure 14. Chemical structure of C10 and mechanism of photodecomposition.

Figure 15 .

 15 Figure 15. CTC interactions existing in the three-component iodonium salt/amine/Trost ligand combination and the cleavage occurring upon photoactivation. By modifying the composition of the three-component system and by using the 3-(dimethylamino)benzyl alcohol C11d/di-tert-butyldiphenyliodonium hexafluorophosphate C11e/4-(diphenylphosphino)styrene C11f combination, the same group succeeded to get a deep polymerization up to 9 cm upon irradiation at 405 nm of a BisGMA/TEGDMA blend.[104] Near infrared absorbing CTC could be even obtained with the electron donating tris[4-(diethylamino)phenyl]amine C11g.[105] Polymerization of the BisGMA/TEGDMA

Figure 16 .

 16 Figure 16. Chemical structures of C11a-C11m using to design CTCs and the mixture of monomer BisGMA/TEGDMA. The dual thermal and photochemical initiating ability of CTC was demonstrated with the C11e/4-(dimethylamino)phenyldiphenylphosphine C11j combination, enabling to initiate

  photoinitiators could be used without taking recourse to UV light for photoactivation.[START_REF] Demina | Polymerization Assisted by Upconversion Nanoparticles under NIR Light[END_REF] As shown in the Figure21, basic principle of triplet-triplet annihilation (TTA) up-conversion consists in exciting two photosensitizer molecules (Sen) with light that can then undergo an intersystem crossing (ISC) to their triplet excited states.[START_REF] Limberg | Triplet-Triplet Annihilation Photopolymerization for High-Resolution 3D Printing[END_REF] Energy of the excited photosensitizer can be transferred to nearby acceptors (Acc). Two molecules of acceptors can undergo a TTA, promoting one of the two molecules to its singlet excited state. A photon of higher energy than that of the incident light can be emitted, enabling to excite wide bandgap photoinitiators.

Figure 21 .

 21 Figure 21. Working principle of triplet-triplet annihilation (TTA) up-conversion. Reproducedwith permission of Ref.[START_REF] Limberg | Triplet-Triplet Annihilation Photopolymerization for High-Resolution 3D Printing[END_REF]

Figure 22 .

 22 Figure 22. Mechanism of up-conversion with UCNPs enabling to polymerize in the visible range while using NIR light. Reproduced with permission of Ref.[START_REF] Chen | Near-Infrared Light Driven Photopolymerization Based On Photon Upconversion[END_REF]

Figure 23 .

 23 Figure 23. Mechanism of thermal initiation upon NIR light irradiation with BlocBuilder® MA Recently, charge transfer complexes absorbing in the near infrared range were proposed as the light absorbing materials to initiate the FRP of ethyl 2-cyanoacrylate C16c.[154] Due to the low oxidation potentials of C16a and C16b, spontaneously, in C16c,

Figure 24 .

 24 Figure 24. Chemical structures of C16a and C16b used to generate CTC with C16c.As drawback of this approach, NIR dyes due to their intense colors induce the formation of deep colored coatings. The formation of colorless coatings with NIR photoinitiators constitute in fact one of the most difficult challenge to overcome for an industrial transfer of this technology.[START_REF] Liao | Design, synthesis and properties of carbazole-indenedione based photobleachable photoinitiators for photopolymerization[END_REF] Recently, Lalevée and coworkers incorporated NIR dyes into photopolymers for various applications. In these different works, polymerization of acrylates was triggered with BAPO i.e. upon UV light irradiation in order to maintain the chemical integrity of the NIR dyes and the NIR dyes were thus only prepositioned inside the photopolymers. Using this approach, surface sterilization could be successfully obtained using IR-940 and IR-1064 (See Figure25).[START_REF] Launay | Safe near infrared light for fast polymers surface sterilization using organic heaters[END_REF] Thus, upon irradiation of a TMPTA-based polymer comprising IR-1064 (0.1 wt%), a temperature as high as 200°C could be reached within 50 s of irradiation at 1064 nm (I = 2.55 W/cm²). This value was reduced to only 140°C with IR-940 (0.1 wt%) under the same illumination conditions. Bactericidal effect of the near infrared treatment was demonstrated with E. Coli bacterial cultures on polymer surface. After NIR light irradiation, a complete surface sterilization could be obtained, even for dye concentration as low as 0.06 wt%.

Figure 25 .

 25 Figure 25. Chemical structures of IR-940, IR-1064 and Glyfoma.

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.

List of abbreviations