Guillaume Noirbent
email: guillaume.noirbent@univ-amu.fr

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

Photoinitiators of polymerization with reduced environmental impact: Nature as an unlimited and renewable source of dyes

Keywords: photoinitiator, photopolymerization, visible light, natural product, Free radical polymerization, cationic polymerization

The development of new procedures aiming at reducing the environmental impact of polymerization processes is a major societal issue. In this field, light-assisted polymerization and especially visible light photopolymerization can address this issue by enabling in the future, Sun, to be used as the irradiation source. Presently, numerous visible light photoinitiators (xanthene dyes, porphyrins and phthalocyanines) are used in industry but their toxicities constitute a major issue for future uses of polymers. Therefore, photopolymerization is facing a short-term challenge and the development of visible light photoinitiators of polymerization has become a blooming field of research during the last decade. An ever-growing effort is thus done to develop new structures, effort which is also supported by the recent applications of photopolymerization in 3D-printing. Depending on the applications, use of synthetic photoinitiators can constitute a severe limitation for future applications of photopolymers so that the use of natural products has been identified as a promising alternative to address the toxicity or the biocompatibility issues. In this review, an overview of the different visible-light photoinitiators based on natural products is provided.

Introduction

Polymers are ubiquitous in our daily life and exist in endless varieties so that the development of new synthetic procedures that can reduce their production's environmental footprint are actively researched. [START_REF] Nkwachukwu | Focus on potential environmental issues on plastic world towards a sustainable plastic recycling in developing countries[END_REF][START_REF] Webb | Plastic degradation and its environmental implications with special reference to poly(ethylene terephthalate)[END_REF] Indeed, polymers are key parts of cars, furniture, packaging, textiles and electronics. [START_REF] Rhim | Bio-nanocomposites for food packaging applications[END_REF][START_REF] Hu | A review of actively moving polymers in textile applications[END_REF] From a synthetic viewpoint, the access to polymers is possible according to two distinct procedures, namely, by mean of heat-activated processes which constitute the main access to polymers but also by mean of light-activated polymerization processes. [START_REF] Khuong | The mechanism of the self-initiated thermal polymerization of styrene. theoretical solution of a classic problem[END_REF] Historically, photopolymerization was devoted to coatings and adhesives, [START_REF] Fouassier | Photopolymerization reactions under visible lights: principle, mechanisms and examples of applications[END_REF][START_REF] Mendes-Felipe | Stateof-the-art and future challenges of UV curable polymer-based smart materials for printing technologies[END_REF] but nowadays, importance of photopolymers is unquestionable, and photopolymers can find applications in innovative research fields such as micro and nanoelectronics, 3D and 4D printing. [START_REF] Shie | Review of polymeric materials in 4d printing biomedical applications[END_REF][START_REF] Dietlin | Photopolymerization upon LEDs: new photoinitiating systems and strategies[END_REF][START_REF] Rocheva | High-resolution 3D photopolymerization assisted by upconversion nanoparticles for rapid prototyping applications[END_REF][START_REF] Siddharth | 4D printing of materials for the future: Opportunities and challenges[END_REF][START_REF] Zhang | Developments in 4D-printing: a review on current smart materials, technologies, and applications[END_REF][START_REF] Pei | Technological considerations for 4D printing: An overview[END_REF][START_REF] Zhang | A versatile 3D and 4D printing a system through photocontrolled raft polymerization[END_REF][START_REF] Bagheri | Photopolymerization in 3D Printing[END_REF] Benefits of photopolymerization include: spatial and temporal control of the polymerization process, [START_REF] Zhao | Spatiotemporal control of polymer brush formation through photoinduced radical polymerization regulated by DMD light modulation[END_REF][START_REF] Jasinski | Photopolymerization in dispersed system[END_REF] reduced carbon footprint by polymerizing in solvent-free conditions, [START_REF] Niedner | Green engineered polymers: solvent free, room-temperature polymerization of monomer from a renewable resource, without utilizing initiator[END_REF] reduced manufacturing cost by enabling the use of low light intensity, cheap and compact light sources (light-emitting diodes (LEDs), laser diodes). [START_REF] Xiao | Visible light sensitive photoinitiating systems: Recent progress in cationic and radical photopolymerization reactions under soft conditions[END_REF] Another advantage of visible light photopolymerization also relies on the possibility to perfectly control the light penetration inside the photocurable resins which can range from a few hundreds of micrometers to centimeters, depending on the wavelength used. [START_REF] Bonardi | High performance near infrared (NIR) photoinitiating systems operating under low light intensity and in presence of oxygen[END_REF] To be an efficient photoinitiator, several prerequisites exist. Notably, the photoinitiator should exhibit a high molar extinction coefficient but also an absorption maximum perfectly fitting with the emission spectrum of the light source. By increasing the molar extinction coefficient, the photoinitiator concentration can be drastically reduced. [START_REF] Dumur | Recent advances on visible light metal-based photocatalysts for polymerization under low light intensity[END_REF] The excited state lifetime is another important parameter as its elongation can drastically improve the polymerization efficiency by providing more time for the photosensitizer to react with the different additives. As a result of this, reactive species can be formed more efficiently, speeding up the polymerization kinetic and improving the final monomer conversion. [START_REF] Bouzrati-Zerelli | A novel class of photoinitiators for polymerization reactions: Organometallic and organic photoredox catalysts with a thermally activated delayed fluorescence (TADF) property[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] Over the years, numerous structures have been proposed as visible light photoinitiators, all obtained by molecular engineering. [START_REF] Xiao | Visible light sensitive photoinitiating systems: Recent progress in cationic and radical photopolymerization reactions under soft conditions[END_REF][START_REF] Shao | Visible light initiating systems for photopolymerization: status, development and challenges[END_REF][START_REF] Steyrer | Visible light photoinitiator for 3D-printing of tough methacrylate resins[END_REF] Besides, the migratability of these small molecules within polymers can constitute a severe limitation for applications of photopolymers in food packaging [START_REF] Aparicio | Migration of photoinitiators in food packaging: A review[END_REF] or as biocompatible materials. [START_REF] Leonhardt | Biocompatibility of photopolymers for additive manufacturing[END_REF] Concerning photopolymerization, two distinct photoinitiating strategies can be distinguished, differing in the way how the radicals are formed. The first type of photoinitiators known as type I photoinitiators are structures that can cleave upon light excitation, producing initiating radicals. One of the major drawbacks of this approach is the irreversible consumption of photoinitiators during polymerization. In this field, 2,2-dimethoxy-2-phenylacetophenone (DMPA), [START_REF] Tehfe | Design of new Type I and Type II photoinitiators possessing highly coupled pyrene-ketone moieties[END_REF] acyl phosphine oxides, [START_REF] Dietlin | New phosphine oxides as high performance near-UV type I photoinitiators of radical polymerization[END_REF] phenylbis (2,4,6-trimethylbenzoyl)phosphine oxide (BAPO) [START_REF] Meereis | BAPO as an alternative photoinitiator for the radical polymerization of dental resins[END_REF] or bis (2,4,6-trimethyl-benzoyl)phenylphosphine oxide (TPO) [START_REF] Stephenson Kenning | Spatial and temporal evolution of the photoinitiation rate for thick polymer systems illuminated by polychromatic light: selection of efficient photoinitiators for LED or mercury lamps[END_REF] can be cited as relevant examples of cleavable photoinitiators. Parallel to this first category, dyes that will not cleave upon photoexcitation can also be used for photoinitiation but require these dyes to be associated with a hydrogen donor and/or an electron donor/acceptor. As a result of this, type II photoinitiators can undergo an intermolecular hydrogen abstraction or an electron/proton transfer reaction when opposed to the appropriate co-initiators. [START_REF] Dietliker | Chemistry and Technology of UV and EB Formulation for Coatings, Inks & Paints[END_REF][START_REF] Hoyle | Thiol-enes: Chemistry of the past with promise for the future[END_REF][START_REF] Hoyle | Thiol-click chemistry: a multifaceted toolbox for small molecule and polymer synthesis[END_REF][START_REF] Cramer | Kinetics of thiol-ene and thiol-acrylate photopolymerizations with real-time Fourier transform infrared[END_REF][START_REF] El-Roz | Mechanistic Investigation of the Silane, Germane, and Stannane Behavior When Incorporated in Type I and Type II Photoinitiators of Polymerization in Aerated Media[END_REF] Even if type I photoinitiators are extensively used in industry, a great deal of efforts is nowadays devoted to develop type II photoinitiators. Indeed, type II photoinitiators offer a unique opportunity to elaborate photocatalytic systems, notably by developing three-component photoinitiating systems. In this field, carbazoles, [START_REF] Dumur | Recent advances on carbazole-based photoinitiators of polymerization[END_REF] naphthalimides and naphthalic anhydrides, [START_REF] Noirbent | Recent advances on naphthalic anhydrides and 1,8naphthalimide-based photoinitiators of polymerization[END_REF] pushpull dyes, [START_REF] Pigot | Recent advances on push-pull organic dyes as visible light photoinitiators of polymerization[END_REF] pyrenes, [START_REF] Dumur | Recent advances on pyrene-based photoinitiators of polymerization[END_REF] iron [START_REF] Dumur | Recent advances on iron-based photoinitiators of polymerization[END_REF] and copper [START_REF] Noirbent | Recent advances on copper complexes as visible light photoinitiators and (photo)redox initiators of polymerization[END_REF] complexes, thioxanthones [START_REF] Dadashi-Silab | Shining a light on an adaptable photoinitiator: advances in photopolymerizations initiated by thioxanthones[END_REF] or benzophenones [START_REF] Tomal | Water-Soluble Photoinitiators in Biomedical Applications[END_REF] have been identified as remarkable photocatalysts in three-component photoinitiating systems. Lastly, natural products have emerged as photoinitiators addressing both the environmental and safety concerns. Indeed, photopolymerization is facing short-term and long-term challenge concerning the development of new monomers but also new photoinitiators issued from renewable resources. Especially, if the use of renewable resources for polymer production has received a substantial interest from industry by developing and incorporating biosourced monomers into polymers, [START_REF] Kristufek | Monomer design strategies to create natural product-based polymer materials[END_REF] photoinitiators based on natural products have somewhat been discarded from this interest. Indeed, for all natural products comprising unsaturated carbon chains, these latter can be easily epoxidized or chemically modified with acrylate groups by mean of simple and inexpensive processes. Notably, monomers derived from ricinoleic oil, [START_REF] Liu | Castor oil-based polyfunctional acrylate monomers: Synthesis and utilization in UV-curable materials[END_REF][START_REF] Phalak | Development of tri-functional biobased reactive diluent from ricinoleic acid for UV curable coating application[END_REF] cashew nutshell liquid, [START_REF] Kanehashi | Photopolymerization of bio-based epoxy prepolymers derived from cashew nut shell liquid (CNSL)[END_REF] or soybean oil [START_REF] Li | Itaconic acid as a green alternative to acrylic acid for producing a soybean oil-based thermoset: synthesis and properties[END_REF][START_REF] Liu | High biocontent natural plant oil based UV-curable branched oligomers[END_REF][START_REF] Wang | Facile one-step synthesis of bio-based AESO resins[END_REF][START_REF] Wu | High-performance soybean oil-based epoxy acrylate resins: "Green" synthesis and application in UV curable coatings[END_REF] can be easily prepared at the industrial scale. However, most of the natural compounds do not absorb in the visible range so that the development of monomers is facilitated compared to that of photoinitiators. Besides, natural photoinitiators provide a unique opportunity to find cheap, renewable and easily accessible photoinitiators. Interest for biosourced photoinitiators is not new since the first report mentioning the use of riboflavin as photoinitiator for the polymerization of acrylic monomers was reported as soon as 1967. [START_REF] Needles | Riboflavin-sensitized photopolymerizations of acrylic monomers in the presence of proteins or amino acids[END_REF][START_REF] Needles | Riboflavin-sensitized graft photopolymerizations of monomers onto fibers[END_REF] If natural dyes were discarded during 40 years in favor of synthetic dyes, in 2005, the first natural product to be revisited as photoinitiator of polymerization was curcumin which showed outstanding photoinitiating properties during the polymerization of styrene, so that the likehood that other natural dyes could do so was newly examined. [START_REF] Mishra | Curcumin, a natural colorant as initiator for photopolymerization of styrene: kinetics and mechanism[END_REF] However, several questions arise concerning natural dyes. If from a theoretical point of view, natural dyes appear as appealing candidates for photoinitiation, for an overall economic viability, the dye content in plants, the number of purification steps necessary for isolation and the overall yields are crucial parameters to consider. [START_REF] Yusuf | Handbook of Renewable Materials for Coloration and Finishing[END_REF][START_REF] Mirjalili | Extraction and Characterization of Natural Dye from Green Walnut Shells and Its Use in Dyeing Polyamide: Focus on Antibacterial Properties[END_REF] Natural dyes are promising candidates for photopolymerization done under low light intensity and under visible light. Notably, Natural dyes have the potential to cut down the photoinitiators costs by replacing expensive chemicals and synthetic processes by a simple extraction. In this field, several methods have been developed and extractions with aqueous, solvent, alkali or acidic extractions, microwave and ultrasonic-assisted extractions, enzymatic extraction or fermentation are popular methods. Indeed, dyes can be extracted from different parts of the plants (petals, leaves, roots) so that an adapted extraction procedure has to be developed for these different cases. Natural dyes are also abundant, and their production can be carried out without environmental threat. Synthetic and natural photoinitiators can be compared on the basis of their costs, photochemical stability, maximum absorbance, availability, costs, biocompatibility or environmental issues (See Table 1). On the basis of the extraction procedures, natural dyes are clearly less costly than organic dyes that requires solvents and purifications by chromatography to be isolated in pure form. Naturally derived dyes have not only been investigated for textile dyeing but also for research fields related to photochemistry. On the basis of their easy availability and low cost, natural dyes were notably used as sensitizers for dye-sensitized solar cells [START_REF] Andargie | Dye-sensitized solar cells using natural dye as light-harvesting materials extracted from Acanthus sennii chiovenda flower and Euphorbia cotinifolia leaf[END_REF][START_REF] Das | Performance of Natural Dyes in Dye-Sensitized Solar Cell as Photosensitizer[END_REF][START_REF] Richhariya | Natural dyes for dye sensitized solar cell: A review[END_REF][START_REF] Ghann | Fabrication, Optimization and Characterization of Natural Dye Sensitized Solar Cell[END_REF] or as photosensitizing agents for photodynamic therapy. [START_REF] Ormond | Dye Sensitizers for Photodynamic Therapy[END_REF][START_REF] Abrahamse | New photosensitizers for photodynamic therapy[END_REF][START_REF] Li | Photodynamic therapy based on organic small molecular fluorescent dyes[END_REF]. Over the years, several scaffolds have been identified as promising candidates for photochemistry and anthocyanins, [START_REF] Phan | The potential of anthocyanins from blueberries as a natural dye for cotton: A combined experimental and theoretical study[END_REF][START_REF] Wang | A method for dyeing cotton fabric with anthocyanin dyes extracted from mulberry (Morus rubra) fruits[END_REF] carotenoids, [START_REF] Vohra | Natural Dyes and Their Derivatives Integrated into Organic Solar Cells[END_REF][START_REF] Maoka | Carotenoids as natural functional pigments[END_REF] flavonoids [START_REF] Guinota | Serratula tinctoria, a source of natural dye: Flavonoid pattern and histolocalization[END_REF][START_REF] Park | Naturally Occurring Quinones and Flavonoid Dyes for Wool: Insect Feeding Deterrents[END_REF] and various derivatives of chlorophyll [START_REF] Zvezdina | Natural dyes based on chlorophyll and protoporphyrin derivatives[END_REF][START_REF] Syafinar | Chlorophyll Pigments as Nature Based Dye for Dye-Sensitized Solar Cell (DSSC)[END_REF][START_REF] Viera | Green Natural Colorants[END_REF] are among the most commonly studied scaffolds.

Since 2005, approximately twelve families of natural and biosourced dyes have been examined in the literature, as shown in the Figure 1. By selecting the appropriate family, polymerization could be initiated from the UV to the near-infrared range. Over the years, two specific strategies have been developed with natural products. The first one consists in directly using a natural dye, even if a purification step is required in order this molecule to be isolated in pure form and used a photoinitiator. In a second approach, a dye can also be prepared starting from a natural product but requires synthetic steps in order to be chemically modified and converted as a photoinitiator. As a result of this, a distinction can be established between photoinitiators based on natural dyes and those which are biosourced. In this review, an overview of the recent advances concerning the development of natural photoinitiators or co-initiator is provided. Especially, the two strategies consisting in directly using a natural dye as a photoinitiator or to prepare a photoinitiator starting from natural products will be presented.

Photoinitiators based on natural products

β-Carotene

Among dyes absorbing in the 400-500 nm region, β-Carotene is one of those. β-Carotene is the first carotenoid dye whose chemical structure could be determined as soon as 1831 by Wackenroder upon extraction from carrots. [START_REF] Hellmuth | Heinrich Wilhelm Ferdinand Wackenroder, 8 März 1798-14[END_REF] β-Carotene plays an important role in health and medicine as it constitutes a precursor of Vitamin A. [START_REF] Grune | β-Carotene is an important vitamin a source for humans[END_REF] β-Carotene is also characterized by a high molar extinction coefficient which can be advantageously used for the design of photoinitiators, around 140 × 10 3 M -1 .cm -1 in hexane. [START_REF] Doshia | Transporter-mediated biofuel secretion[END_REF] The first report mentioning the use of β-Carotene as photoinitiator was published in 2019 by Versace et al. [START_REF] Breloy | β-carotene/limonene derivatives/eugenol: green synthesis of antibacterial coatings under visible-light exposure[END_REF] In this work, monomers and photoinitiators are biosourced since mono and di-epoxy limonene monomers (originating from citrus) and eugenol (originating from clove oil, cinnamon, or pepper) are used as the monomers (See Figure 2). Interestingly, eugenol is nonetheless a monomer but also reported as a bactericide capable to disrupt cellular membranes. [START_REF] Pandima Devi | Eugenol (an essential oil of clove) acts as an antibacterial agent against Salmonella typhiby disrupting the cellular membrane[END_REF] Figure 2. Chemical structures of photosensitizers, the different monomers and the cationic photoinitiator.

While examining the photolysis of the two component β-Carotene/bis(4methylphenyl)iodonium hexafluorophosphate (Iod) system by UV-visible absorption spectroscopy in solution, a fast photobleaching occurring without 2-3 minutes could be observed upon irradiation at 405 nm, indicating that β-Carotene could efficiently sensitize the cationic photoinitiator Iod. Conversely, without Iod, almost no photobleaching could be observed, demonstrating that the modification of the absorption was originating from an efficient interaction between Iod and β-Carotene (See Figure 3) [START_REF] Breloy | β-carotene/limonene derivatives/eugenol: green synthesis of antibacterial coatings under visible-light exposure[END_REF]. system in deaerated chloroform upon irradiation with a LED@405 nm. Reproduced with permission from Breloy et al. [START_REF] Breloy | β-carotene/limonene derivatives/eugenol: green synthesis of antibacterial coatings under visible-light exposure[END_REF] Copyright 2014 American Chemical Society.

Laser-flash photolysis experiments revealed the excited state lifetime of β-Carotene to be of 8 µs in deaerated toluene and to fall to 1.7 µs under oxygen, evidencing the excited state to be a triplet state. These values are consistent with those reported in the literature. [START_REF] Nielsen | Singlet versus triplet reactivity in photodegradation of c40 carotenoids[END_REF] A rate constant of interaction evaluated at 10 8 Mol -1 .s -1 was determined, meaning that the process was under diffusion control. By cyclic voltammetry, a negative free energy change ΔG was determined according to the Rehm-Weller equation, evidencing the polymerization process to be highly favorable with this two-component system. [START_REF] Rehm | Kinetics of fluorescence quenching by electron and H-atom transfer[END_REF] Considering the broad absorption of β-Carotene, the cationic polymerization of epoxides and the thiol-ene polymerization could be initiated upon irradiation at 405, 455 and 470 nm, but also with a xenon lamp. Under optimized conditions for the three component β-carotene/Iod/trithiol (1/3/6% w/w/w) system, a final monomer conversion of 75% after 800 s of irradiation with a LED at 405 nm was determined for the epoxy groups of limonene 1,2-diepoxide (Lim). Slightly reduced values were determined at 455 and 470 nm (61 and 59% respectively). Conversely, a lower final monomer conversion (> 50%) was determined for the allyl groups of eugenol. This result was assigned to the dense polymer network formed by the cationic polymerization of limonene 1,2-diepoxide (Lim) and dipentene dioxide (DPDO), hindering the diffusion of the thiyl radicals towards the allylic group of eugenol, slowing down the thiol-ene polymerization. Comparison of the three-component β-carotene/Iod/trithiol system with that of the reference ITX/Iod/trithiol (1/3/6% w/w/w) system revealed the β-carotene-based system to outperform the reference system, irrespective of the irradiation wavelength (See Table 2). While examining the photoinitiating ability of the two-component β-Carotene/Iod (1/3% w/w) system, higher final monomer conversions were determined with DPDO compared to (3,4-epoxy-cyclohexane)methyl 3,4-epoxycyclohexylcarboxylate EPOX, despites higher polymerization rates. This was assigned to a faster formation of the polymer network in the case of EPOX, letting more unreacted functions isolated inside the polymer network. Such an unexpected behavior has already been reported in the literature for EPOX. [START_REF] Tehfe | Green chemistry: sunlight-induced cationic polymerization of renewable epoxy monomers under air[END_REF] Finally, to get antibacterial properties, the eugenol content within the monomer blend should be sufficient to have molecules of eugenol at the surface of the polymer film. Polymerization of the DPDO/Lim/Eug (50/25/25% w/w/w) monomer blend resulted in high monomer conversion for the epoxy group whereas lower allyl conversions were determined, whatever the conditions were. Comparison with the reference systems based on CQ and ITX revealed the eugenol polymerization to be improved with the CQ-based system. Indeed, in this case, a lower epoxide conversion was obtained, favoring the thiol-ene polymerization over the cationic one. On the opposite, high epoxide monomer conversions were obtained with βcarotene and ITX, favoring the CP of epoxides over the thiol-ene reaction (See Table 3 and Figure 4) [START_REF] Breloy | β-carotene/limonene derivatives/eugenol: green synthesis of antibacterial coatings under visible-light exposure[END_REF]. Finally, antibacterial properties of eugenol were evaluated for two bacteria with antibacterial assays against E. coli (Gram negative) and S. aureus (Gram positive). After three hours of incubation, a clear reduction of the total number of colonies of bacteria could be detected for the polymer films treated with eugenol. Interestingly, the antibacterial effect was more pronounced for E. coli than for S. aureus and this result was assigned to the difference of membrane structures. Indeed, E. coli is protected by an outer membrane of liposaccharides whereas E. aureus possess a thick peptidoglycans membrane.

Chlorophyll a

Chlorophyll is one of the most abundant natural compounds on Earth and this molecule is vital for photosynthesis, enabling plants to harvest photons from sunlight. As a result of this, carbon dioxide and water can be converted into oxygen and glucose. [START_REF] Eggink | The role of chlorophyll b in photosynthesis: Hypothesis[END_REF] From a mechanistic viewpoint, the first event consists in a photoinduced electron transfer from the excited chlorophyll towards an electron acceptor, inducing charge separation. [START_REF] Pinnola | Electron transfer between carotenoid and chlorophyll contributes to quenching in the LHCSR1 protein from Physcomitrella patens[END_REF] In 2015, Boyer and coworkers took advantage of the first step of photosynthesis to prepare photopolymers with controlled molecular weights, polydispersities and end group functionalities by mean of a photoinduced electron transfer reversible addition-fragmentation chain transfer (PET-RAFT). [START_REF] Shanmugam | Utilizing the electron transfer mechanism of chlorophyll a under light for controlled radical polymerization[END_REF] Chlorophyll a was extracted from spinach leaves with an appropriate solvent and purified by column chromatography before use. However, availability of chlorophyll a remained limited since only 24 mg could be isolated in pure form from 100 g of spinach leaves. Chlorophyll a is a strong reductant (Ered = -1.1 V vs. SCE in DMSO) that can oxidize molecules of lower reduction potential including thiocarbonylthio compounds such as RAFT agents. Interestingly, in the case of chlorophyll a and in contrast to what is classically observed for transition metal complexes, the oxidation process is not centered on the metal center but on the π-conjugated system of the porphyrin. Presence of a π-cation radical centered on the organic moiety could be demonstrated by EPR measurements. To evidence the ability of chlorophyll a to interact with RAFT agents, fluorescence quenching experiments were carried out upon irradiation at 461 nm and 635 nm, chlorophyll a possessing two absorption bands, the Soret and the Q-bands (See Figure 5) [START_REF] Shanmugam | Utilizing the electron transfer mechanism of chlorophyll a under light for controlled radical polymerization[END_REF]. At both wavelengths, a clear decrease of the fluorescence intensity with the amount of RAFT agent was clearly demonstrated, evidencing that initiating radicals could be formed. A good control of the narrow molecular weight distributions could be obtained with different monomers such as methyl acrylate (MA), methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA), pentafluorophenyl acrylate (PFPA), glycidyl methacrylate (GMA) or methacrylic acid 2-(dimethylamino)ethyl ester (DMAEMA). Among the six RAFT agents used to control the polymerization process, only 2-(n-butyltrithiocarbonate)propionic acid (BTPA), 4-cyanopentanoic acid dithiobenzoate (CPADB) and 3-benzylsulfanyl-thiocarbonylthiosulfanyl propionic acid (BSTP) could efficiently promote the polymerization of vinyl acetate, the three other being unsuccessful. Examination of the solvent effects revealed DMF and DMSO to furnish the lowest polydispersities whereas no control could be obtained while using acetonitrile or toluene. Comparison of the polymerization kinetics at 461 and 635 nm for the different monomers evidenced the propagation rate constant to be lower at 461 nm than at 635 nm. This result was assigned to competitive absorptions between chlorophyll a and the RAFT agents absorbing in this region. Surprisingly, increase of the concentration of chlorophyll a from 4 ppm to 10 ppm furnished higher propagation rate constants but negligible changes of the molecular weight or the molecular weight distributions could be evidenced, even at high monomer conversions (See Figure 6) [START_REF] Shanmugam | Utilizing the electron transfer mechanism of chlorophyll a under light for controlled radical polymerization[END_REF]. However, at 25 ppm, a propagation rate constant similar to that determined at 4 ppm was determined, resulting from self-quenching of chlorophyll a at high concentrations. Livingness of the polymers was also demonstrated by performing chain extension with various monomers. Finally, compared to transition metal photocatalysts such as Ru or Ir complexes previously used in similar conditions, [START_REF] Xu | Oxygen tolerance study of photoinduced electron transferreversible addition-fragmentation chain transfer (pet-raft) polymerization mediated by Ru(bpy)3Cl2[END_REF][START_REF] Shanmugam | Photoinduced electron transfer-reversible additionfragmentation chain transfer (pet-raft) polymerization of vinyl acetate and nvinylpyrrolidinone: kinetic and oxygen tolerance study[END_REF][START_REF] Xu | A robust and versatile photoinduced living polymerization of conjugated and unconjugated monomers and its oxygen tolerance[END_REF][START_REF] Seal | Unraveling photocatalytic mechanism and selectivity in pet-raft polymerization[END_REF][START_REF] Pan | Photomediated controlled radical polymerization[END_REF] the main advantages of chlorophyll a over these complexes was the possibility to activate the polymerization process at two distinct wavelengths, with a blue or a red light, while using a low photocatalyst content. From a mechanistic viewpoint, it could be established that the mechanism was similar to that previously reported in the literature for iridium or ruthenium photocatalysts (See Figure 7) [START_REF] Shanmugam | Utilizing the electron transfer mechanism of chlorophyll a under light for controlled radical polymerization[END_REF].

Bacteriochlorophyll a

Bacteriochlorophylls (BChl) are a class of photosynthetic dyes that can be found in numerous phototrophic bacteria and their chemical structures are comparable to that of the well-known chlorophyll a. These dyes were notably discovered in 1932 by C.B. van Niel, [START_REF] Van Niel | On the morphology and physiology of the purple and green sulphur bacteria[END_REF] and over the years, chemical structures of bacteriochlorophyll a to bacteriochlorophyll g were identified. [START_REF] Senge | Biosynthesis and Structures of the Bacteriochlorophylls[END_REF][START_REF] Harada | In vivo energy transfer from bacteriochlorophyll c, d, e, or f to bacteriochlorophyll a in wild-type and mutant cells of the green sulfur bacterium chlorobaculum limnaeum[END_REF] As specificities, organisms making use of bacteriochlorophylls for photosynthesis do not produce oxygen and make use of a spectral range different from that of those using chlorophyll a. Thus, if bacteriochlorophyll a and b absorb in the 805-890 nm, 835-1040 nm range respectively, other bacteriochlorophylls typically absorb between 650 and 750 nm. Therefore, bacteriochlorophylls absorb in the far-red and near-infrared range. The first report mentioning their uses as photoinitiators was published in 2016 by Boyer and coworkers with bacteriochlorophyll a which exhibits the most red-shifted absorption of the bacteriochlorophyll series. [START_REF] Shanmugam | Light-regulated polymerization under nearinfrared/far-red irradiation catalyzed by bacteriochlorophyll a[END_REF] Choice of bacteriochlorophylls for controlled/living radical polymerization (CLRP) reactions was notably motivated by the improved light penetration inside the photocurable resin while using a near-infrared light within the context of developing a sustainable and green polymerization process. [START_REF] Schröder | Towards sustainable polymer chemistry with homogeneous metal-based catalysts[END_REF] Bacteriochlorophyll a (BChl a) exhibits several appealing features such as a high intersystem crossing rate and a low reduction potential (-1.1 V vs. SCE). [START_REF] Musewald | Time-Resolved Spectral Investigation of Bacteriochlorophyll a and Its Transmetalated Derivatives [Zn]-Bacteriochlorophyll a and [Pd]-Bacteriochlorophyll a[END_REF][START_REF] Cotton | An electrochemical investigation of the redox properties of bacteriochlorophyll and bacteriopheophytin in aprotic solvents[END_REF] Interestingly, by the remarkable overlap between the absorption spectra of the RAFT agent cyanopentanoic acid dithiobenzoate (CPADB) and BChl a), an efficient photosensitization could be promoted (See Figure 8) [START_REF] Shanmugam | Light-regulated polymerization under nearinfrared/far-red irradiation catalyzed by bacteriochlorophyll a[END_REF], resulting in a methyl methacrylate (MMA) conversion of 54 and 74% upon irradiation for 20 hours in the near-infrared (850 nm) and the far-red (780 nm) region respectively. Control experiments also revealed the direct sensitization by BChl a to be ineffective, monomer conversions lower than 5% being obtained in the same conditions. To support the remarkable efficiency of BChl a as photosensitizer, a photochemical mechanism identical to that reported in the Figure 7 was proposed. Interestingly, higher polymerization rates were determined in the far-red region than in the near infrared region, what can be assigned to the higher molar extinction coefficient of BChl a in the far-red region. A linear monomer conversion with the irradiation time was also demonstrated, suggesting the concentration of initiating radicals to remain constant over time. Additionally, no inhibition period was observed upon irradiation at 780 nm whereas an inhibition period of 45 min. was detected while polymerizing at 780 nm. Here again, the better absorption of BChl a at 780 nm rather than at 850 nm was suggested as a plausible explanation supporting this inhibition period. Temporal control of the photopolymerization process was also evidenced when the light was switched off for 15 hours (See Figure 9) [START_REF] Shanmugam | Light-regulated polymerization under nearinfrared/far-red irradiation catalyzed by bacteriochlorophyll a[END_REF]. After this period, photopolymerization could be newly initiated by switching the light on. Finally, living character of the polymerization process was demonstrated by preparing PMMA-b-PtBuMA block copolymers (with tBuMA which stands for tert-butyl methacrylate). Versatility of the approach was also demonstrated by polymerizing various methacrylic monomers including glycidyl methacrylate (GMA), 2-dimethylaminoethyl methacrylate (DMAEMA) and oligo(ethylene glycol) methyl ether methacrylate (OEGMA). A good control of both the molecular weights as well as the molecular weight distributions could be obtained. High penetration of the near-infrared light in the polymerizable solutions was demonstrated by intercalating a paper barrier (a standard A4 white paper sheet) between the light source and the sample. In this case, a higher propagation rate constant was found at 850 nm rather than 780 nm, and this opposite trend was assigned to the higher transmittance of the nearinfrared light compared to the far-red light. Especially, a decrease of the monomer conversion with the paper sheet thickness was also shown (See Figure 10) [START_REF] Shanmugam | Light-regulated polymerization under nearinfrared/far-red irradiation catalyzed by bacteriochlorophyll a[END_REF].

Naphthoquinones

Among dyes absorbing in the 400-500 nm region, β-carotene is not the only compound absorbing in this region and 1,4-naphthoquinones can be cited as relevant examples. In 2020, two natural dyes, namely 5-hydroxy-1,4-naphthoquinone (5HNQ) that can be found in black walnut (Juglans nigra) and 2-hydroxy-1,4-naphthoquinone (2HNQ) which is a red-orange dye found in the leaves of the henna plant (Lawsonia inermis) were examined as photoinitiators of polymerization under visible light and low light intensity (See Figure 11). [START_REF] Peng | Naphthoquinone derivatives: Naturally derived molecules as blue-light sensitive photoinitiators of photopolymerization[END_REF] These two compounds are abundant in Nature since these two dyes can also be found in various plants, microbes or marine organisms. [START_REF] Widhalm | Biosynthesis and molecular actions of specialized 1,4naphthoquinone natural products produced by horticultural plants[END_REF][START_REF] Hook | Chapter 5 -Bioactive naphthoquinones from higher plants[END_REF] If the two molecules 2HNQ and 5HNQ are only isomers of position, a severe modification of their absorption properties could be determined in solution (See Figure 12) [START_REF] Peng | Naphthoquinone derivatives: Naturally derived molecules as blue-light sensitive photoinitiators of photopolymerization[END_REF]. However, compared to β-carotene (ε ~ 140 × 10 3 M -1 .cm -1 in hexane), lower molar extinction coefficients could be determined for the two dyes, with coefficient of 3227 M -1 .cm -1 at 330 nm for 2HNQ, 4014 M -1 .cm -1 at 420 nm for 5HNQ. On the basis of their respective absorptions, photopolymerization tests were carried out at 410 and 445 nm. Interestingly, while using 5HNQ in two-component systems with diphenyliodonium hexafluorophosphate (Iod1), N-phenyl glycine (NPG) and ethyl 4-dimethylaminobenzoate (EDB), higher final monomer conversions could be obtained with NPG and EDB during the free radical polymerization (FRP) of trimethylolpropane triacrylate (TMPTA). Thus, conversions of 25%, 30% and 36% could be determined after 300 s of irradiation at 410 nm in laminate with the two-component 5NHNQ/Iod1 (0.5%/2% w/w), 5HNQ/NPG (0.5%/2% w/w), 5HNQ/EDB (0.5%/2% w/w) systems. It could be thus concluded that naphthoquinones are easier to photoreduce than to photooxidize. Comparison between EDB and NPG revealed NPG to outperform EDB due to decarboxylation of NPG during photopolymerization, avoiding the deactivating back electron transfer to occur. [START_REF] Al Mousawi | 3-hydroxyflavone and N-phenylglycine in high performance photoinitiating systems for 3D printing and photocomposites synthesis[END_REF] Consistent with a reduction of the molar extinction coefficient at 445 nm, lower final monomer conversions could be determined during the FRP of TMPTA. Conversely, 2HNQ was unable to initiate a polymerization in two-component systems, irrespective of the irradiation wavelength or the co-initiator (See Figure 13) [START_REF] Peng | Naphthoquinone derivatives: Naturally derived molecules as blue-light sensitive photoinitiators of photopolymerization[END_REF]. While using the three-component 5HNQ/Iod1/EDB (0.5%/2%/2% w/w/w) system, a two-fold enhancement of the TMPTA conversion could be obtained, peaking at 47% after 300 s of irradiation at 410 nm. Besides, the monomer conversion remained lower than that determined for the reference ITX/EBD (0.5%/2% w/w) system which could provide a final monomer conversion of 49% in the same conditions. However, at 445 nm, this conversion falls to only 18% due to the weak absorption of ITX at this wavelength. This conversion was 2-fold reduced compared to that obtained with the two-component 5HNQ/Iod1/EDB (0.5%/2%/2% w/w/w) system (35% TMPTA conversion) (See Figure 13) [START_REF] Peng | Naphthoquinone derivatives: Naturally derived molecules as blue-light sensitive photoinitiators of photopolymerization[END_REF]. Therefore, it could be concluded that naphthoquinone was more adapted for photopolymerization done at 445 than 410 nm.

A surprising behavior could be evidenced for 2HNQ during the free-radical promoted cationic polymerization (FRPCP) of triethyleneglycol divinyl ether (DVE-3). Indeed, the two component 2HNQ/Iod1 (0.5%/2% w/w) system could outperform its 5HNQ analogues, both at 410 and 445 nm. Final monomer conversions as high as 92 and 87% could be respectively determined at 440 and 445 nm, higher than the values of 85 and 59% determined for the 5HNQ/Iod (0.5%/2% w/w) system.

Paprika

In 2018, paprika which is a well-known food dyes containing numerous carotenoids was investigated for the first time as a visible light photoinitiator for the FRPCP of a biosourced monomer derived from gallic acid. [START_REF] Sautrot-Ba | Paprika, gallic acid, and visible light: the green combination for the synthesis of biocide coatings[END_REF] From a composition viewpoint, this natural spice comprises several carotenoids including capsanthin, capsorubin, and cryptocapsin, [START_REF] Deli | Paprika carotenoids: analysis, isolation, structure elucidation[END_REF] but

A) B)

yellow xanthophylls which are also polyenic structures can also be found, as exemplified with β-cryptoxanthin, zeaxanthin or antheraxanthin. It has to be noticed that capsanthin, capsorubin, and cryptocapsin which are the three main carotenoids in paprika can be prepared by total synthesis. In 1983, their syntheses were investigated but required the collaboration of the research groups of Rüttimann and Weedon to be successful. [START_REF] Bowden | Carotenoids and related compounds. Part 37. Stereochemistry and synthesis of capsorubin[END_REF][START_REF] Rüttimann | ′r)-capsorubin, (3′s,5′r)-kryptocapsin und einigen verwandten verbindungen. ein neuer zugang zu optisch aktiven fünfringcarotinoidbausteinen durch hydroborierung[END_REF] Proof that paprika could react with the diphenyliodonium salt (Iod1) was demonstrated during the photolysis experiments of the papikra/4-(2-methylpropyl)phenyl iodonium hexafluorophosphate (Iod2) combination. A fast photobleaching could be observed within 50 s in toluene upon light irradiation with a Xe lamp. (See Figure 14) [START_REF] Sautrot-Ba | Paprika, gallic acid, and visible light: the green combination for the synthesis of biocide coatings[END_REF]. Conversely, no photobleaching was detected for paprika alone in the same conditions. Formation of photoacids was confirmed by introduction a pH indicator in the solution, making the papikra/Iod system a perfect candidate for the CP of epoxides. Based on the photochemical mechanism previously reported in the literature, the following mechanism could be proposed to support the polymerization of epoxides (See Figure 15). Briefly, upon photoexcitation of paprika, an electron transfer occurs from the excited photosensitizer towards the iodonium salt, generating phenyl radicals, Ph • . By intermolecular hydrogen abstraction with Ph • , aliphatic α-ether radicals (epoxy(-H) • ) can form. These radicals are capable to act as reducing agent for the iodonium salt, oxidizing epoxy(-H) • epoxy(-H) + . In the presence of water traces, epoxy(-H) + can react with water, producing H + as the initiating species. In turn, two sources of cationic initiating species are formed in situ, namely H + and epoxy(-H) + .

While polymerizing the epoxidized monomer derived from gallic acid, a final monomer conversion of 90% was reached after 1200 s of irradiation. A good resistance to scratch and nanoindentation could also be demonstrating, evidencing a visco-elastoplastic behavior for the paprika-derived films. As other interesting feature, the production of singlet oxygen by the paprika-containing polymer was clearly evidenced upon illumination of the polymer film, using 1,3-diphenylisobenzofuran (DPFB) as the singlet oxygen trap (See Figure 16). Therefore, due to singlet oxygen production, paprika coatings were thus likely to exhibit antibacterial properties, what was demonstrating by incubating E. coli and S. aureus during 6 h. Upon illumination, the proliferation was totally inhibited whereas a tremendous proliferation of bacteria was demonstrated for the paprika-based film maintained in the dark. Indeed, upon illumination, oxidation of all the fatty acids necessary for the growth of bacteria resulted in the death of the cells. Therefore, in this work, the dual role of photosensitizer and biocide could be pointed out for paprika.

Sesamin

Sesamin which is a natural product extracted from sesame seeds contains two cyclic acetals with activated methylene groups between the oxygen atom whose protons can be easily abstracted. Considering that type II photoinitiators undergo a bimolecular process where the excited state of the photoinitiator abstracts a hydrogen atom from a second molecule (i.e. the co-initiator) to generate the initiating free radicals, sesamin was thus determined as an appropriate molecule to act as a co-initiator for benzophenone (BP). In 2011, the twocomponent system BP/sesamin was used as the initiating system for the FRP of 1,6hexanedioldiacrylate (HDDA) (See Figure 17). [START_REF] Wang | Sesamin as a coinitiator for UV photopolymerization[END_REF] Figure 17. Chemical structure of sesamin, the different benzophenone-based photoinitiators used for comparison and the acrylate monomer (HDDA).

However, benzophenone is a UV photoinitiator so that the polymerization process could only be activated under UV light. Indeed, Sesamin doesn't absorb in the visible range and this molecule exhibit two absorption maxima located at 250 and 385 nm respectively. Examination of the polymerization tests revealed sesamin to greatly improve the monomer conversion. Indeed, the final monomer conversion could increase from 49% for BP alone to 85% for the two-component BP/sesamin (2%/3% w/w). Use of sesamin was not limited to BP and other aromatic ketones such as p-chlorobenzophenone (CBP) and methyl obenzoylbenzoate (OMBB) were also tested. In the same polymerization conditions, CBP showed the highest polymerization rate of the benzophenone series but furnish a final monomer conversion similar to that of BP (82% after 2 min. UV irradiation). Conversely, the OMBB/sesamin two-component system furnished slower polymerization kinetics and final monomer conversions (78% after 2 min. UV irradiation) (See Figure 18). If sesamin proved its photoinitiating ability, the main drawback of this dye remains its UV-centered absorption so that this dye remains of poor interest for photoinitiation, considering that, at present, all efforts are devoted to develop visible light photoinitiators.

Curcumin

Among natural products that were rapidly investigated as photoinitiators of polymerization, curcumin can be cited as a relevant example as the first report mentioning its use for the sensitization of onium salts was reported as soon as 2005. [START_REF] Crivello | Curcumin: A naturally occurring long-wavelength photosensitizer for diaryliodonium salts[END_REF] Curcumin is a phenolic substance extracted from the rhizomes of Curcuma longa, and curcumin is one of the ingredients composing the curry spice. [START_REF] Hewlings | Curcumin: A review of its' effects on human health[END_REF] Apart from photopolymerization, curcumin was also extensively studied for its biological activities, and curcumin was notably investigated for its anti-inflammatory, [START_REF] Ammon | Pharmacology of curcuma longa[END_REF] antioxidant [START_REF] Rao | Chemoprevention of colon carcinogenesis by dietary curcumin, a naturally occurring plant phenolic compound[END_REF] and antitumor [START_REF] Sharma | Antioxidant activity of curcumin and related compounds[END_REF] properties but also for cancer treatment [START_REF] Basnet | Curcumin: An anti-inflammatory molecule from a curry spice on the path to cancer treatment[END_REF] or as anti-Alzheimer agent. [START_REF] Mishra | The effect of curcumin (turmeric) on Alzheimer's disease: An overview[END_REF] Curcumin is also widely used to prevent sunburn, to treat various skin ailments so that this molecule is relatively nontoxic. [START_REF] Crivello | Design and synthesis of highly reactive photopolymerizable epoxy monomers[END_REF][START_REF] Chopra | Indigenous Drugs of India[END_REF][START_REF] Nadkarni | [END_REF][START_REF] Krishnamoorthy | The Wealth of India: A Dictionary of Indian Raw Materials and Industrial Products[END_REF] Due to its aromatic structure and its extended π-conjugated system, curcumin absorbs between 340 and 535 nm so that this natural dye is appropriate for photoinitiation under visible light. As the main interest of this dye, curcumin is relatively cheap and is soluble in most of the common monomers. Technically, curcumin can be extracted from turmeric powder which contains between 1.5 and 2% of curcumin. For a higher purity, Crivello et al. could get curcumin with a high purity after solvent extraction by recrystallizing the different extracts in isopropanol. Examination of the photopolymerization of cyclohexene oxide by optical pyrometry revealed the polymerization reaction to be relatively exothermic since a maximum temperature as high as 120°C could be measured while using 0.25 mol% of curcumin and 1 mol% of (4-n-decyloxyphenyl)phenyl iodonium hexafluoroantimonate (IOC-10.SbF6) and upon excitation with a lamp emitting between 355 and 460 nm. A fast polymerization was also demonstrated, a final monomer conversion of 70% being achieved within 20 s, without induction period (See Figure 19) [START_REF] Crivello | Curcumin: A naturally occurring long-wavelength photosensitizer for diaryliodonium salts[END_REF]. Interestingly, by replacing the iodonium salt by a diphenylsulfonium or a phenacylsulfonium salt, a lower exothermicity was demonstrated, these two salts being more difficult to reduce than the iodonium salt. [START_REF] Crivello | UV Curing Science and Technology[END_REF] While using a LED at 470 nm, the polymerization process could be initiated satisfactorily with the same two-component system since a reasonable exothermicity could be determined (83°C) with a relatively short induction time (30 s). Among the most interesting finding, the solar-radiation induced polymerizations of two biosourced monomers, namely, epoxidized soybean oil and epoxidized linseed oil could be successfully achieved but required 10 minutes of solar irradiation to produce a fully crosslinked polymer (See Figure 20). Noticeably, a dramatic color change occurred upon sunlight exposure, shifting from yellow for the liquid resin to orange for the crosslinked polymer (See Figure 21) [START_REF] Crivello | Curcumin: A naturally occurring long-wavelength photosensitizer for diaryliodonium salts[END_REF]. If highly colored coatings were obtained with curcumin, this issue was recently addressed with the development of curcuminoid derivatives, enabling to get an excellent photobleaching and colorless coatings. Following these pioneering works devoted to the cationic polymerization of epoxides, another group reported in 2007 the free radical polymerization of styrene with a catalytic concentration for curcumin as low as 10 -6 M. [START_REF] Mishra | Curcumin, a natural colorant as initiator for photopolymerization of styrene: kinetics and mechanism[END_REF] However, the polymerization tests were carried out at 257 nm, despites the strong absorption of curcumin in the visible range. Comparison of the photoinitiating ability of curcumin with that of benchmark photoinitiators such as azobisisobutyronitrile (AIBN) or benzoyl peroxide (BPO) revealed curcumin to outperform these photoinitiators. Thus, while using a concentration of 10 -6 M for curcumin, a final monomer conversion of 23% could be obtained after 20 min. of irradiation, contrarily to 6.7 and 3.8% for AIBN and BPO respectively, in the same conditions, and by using standard concentrations (5 × 10 -3 M).

In 2015, curcumin was revisited in the context of photopolymerization done under air and under low light intensity by using modern light sources i.e. household LED bulbs. [START_REF] Zhao | A new role of curcumin: as a multicolor photoinitiator for polymer fabrication under household UV to red LED bulbs[END_REF] By using the three-component curcumin/diphenyliodonium hexafluorophosphate (Iod1) /triphenylphosphine (0.5%/2%/2%, wt%) system, the free radical polymerization of a dental resin, namely a bisphenol A glycerolate dimethacrylate (Bis-GMA)/triethyleneglycol dimethacrylate (TEGDMA) (70 wt%/30 wt%) blend could be efficiently initiated at 455, 518, 594, 636 nm and even with a white LED with emission ranging from 410 to 750 nm. Final monomer conversions of 71, 54, 38, 34 and 56% could be respectively obtained for the FRP of the Bis-GMA/TEGDMA blend with the aforementioned light sources and upon irradiation for 300 s. Logically, a reduction of the monomer conversion was observed as a result of a reduction of the molar extinction coefficient of curcumin and thus the light absorption ability from 455 to 636 nm. Besides, irrespective of the irradiation wavelength, curcumin could greatly outperform the benchmark phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO) (28% monomer conversion, even when BAPO was excited at the appropriate wavelength of 455 nm (see Figure 22) [START_REF] Zhao | A new role of curcumin: as a multicolor photoinitiator for polymer fabrication under household UV to red LED bulbs[END_REF]. While examining the temperature effect on the monomer conversion, an increase of 22% was observed between 0 and 50°C for a polymerization done under UV light for 300 s, ascribed to an improved mobility of the generated radicals upon increase of the temperature.

Figure 22.

Final monomer conversions at different irradiation wavelengths for the FRP of Bis-GMA/TEGDMA using different three-component systems (curcumin/Iod1/triphenylphosphine (0.5%/2%/2%, wt%) and BAPO (0.5 wt%). Adapted from ref. [START_REF] Zhao | A new role of curcumin: as a multicolor photoinitiator for polymer fabrication under household UV to red LED bulbs[END_REF] with permission from The Royal Society of Chemistry Examination of the cytotoxicity of polymers on human fibroblast Hs-27 cells also revealed the curcumin-based polymer films to be almost no toxic since the cell viability was close to that determined for the control experiments (94.5%). These works pave the way towards the developments of biocompatible materials prepared with biosourced photoinitiators. Investigation of the cytotoxicity of curcumin is an active research field and the cytotoxicity of curcumin was nonetheless examined for human fibroblasts but also for mouse fibroblasts L929, furnishing similar results concerning cells viability. [START_REF] Gharakhloo | Cyclodextrin-based nanosponges for improving solubility and sustainable release of curcumin[END_REF] Still with aim at examining the biological properties of curcumin, other authors examine the antibacterial properties of coatings prepared with curcumin as the photosensitizer. By photopolymerization of epoxidized soybean oil with the two-component curcumin/iod1 (2.2%/6% wt%) system, antibacterial coatings against Escherichia coli and Staphylococcus aureus could be efficiently prepared. [START_REF] Condat | Photoinduced curcumin derivativecoatings with antibacterial properties[END_REF] To realize this, authors demonstrated the curcuminbased coatings to be capable to generate singlet oxygen under light irradiation so that the proliferation of Escherichia coli and Staphylococcus aureus could be inhibited by 95% and 99% respectively, even after 48 h of incubation.

With aim at reducing the toxicity of the coatings, substitution of a toxic co-initiator by the less toxic glycerol is an interesting approach that was examined in the context of the polymerization of urethane dimethacrylate. [START_REF] Bezerra Lisboa De Oliveira | Use of curcumin and glycerol as an effective photoinitiating system in the polymerization of urethane dimethacrylate[END_REF] By replacing ethyl p-dimethylaminobenzoate by glycerol, toxicity of the final coating could be greatly reduced, without impacting the thermal or the morphological characteristics of the coatings. From a mechanistic viewpoint, the initiating step is similar to that classically observed for amine-based co-initiators, consisting in a hydrogen abstraction from glycerol by curcumin in the excited state, generating an initiating species (See Scheme 1). Scheme 1. Chemical mechanism involved in the FRP of urethane dimethacrylate by the twocomponent curcumin/glycerol system.

Dihydroxyanthraquinones

Dihydroxyanthraquinone derivatives (DHAQs) constitutes a family of natural dyes that can nonetheless be extracted from various plants but that can also be found in numerous animal sources such as insects. [START_REF] Siva | Status of natural dyes and dye-yielding plants in India[END_REF] Typically, dihydroxyanthraquinones absorb between 350 and 550 nm and these inexpensive compounds were widely used in the past as dyes for textiles, paints or food but also in the medical field. [START_REF] Anouar | UV/Visible spectra of a series of natural and synthesised anthraquinones: experimental and quantum chemical approaches[END_REF][START_REF] Ghosh | Spectroscopic and in silico evaluation of interaction of DNA with six anthraquinone derivatives[END_REF][START_REF] Müller | Intramolecular hydrogen bonding in 1,8dihydroxyanthraquinone, 1-aminoanthraquinone, and 9-hydroxyphenalenone studied by picosecond time-resolved fluorescence spectroscopy in a supersonic jet[END_REF] Notably, in this family, alizarin (1,2-dihydroxyanthraquinone) is the most representative dye of this family and alizarin has long been used as staining agent for bone tissues due to its unique ability to complex free calcium. [START_REF] Lemlikchi | Hydroxyapatite and Alizarin sulfonate ARS modeling interactions for textile dyes removal from waste waters[END_REF][START_REF] Moriguchi | Elucidation of adsorption mechanism of bone-staining agent alizarin red S on hydroxyapatite by FT-IR microspectroscopy[END_REF] But quinizarin (1,4-dihydroxyanthraquinone) which can be extracted from the root of the common madder plant, Rubia tinctorum, is another relevant example of the dihydroxyanthraquinone family composed of ten isomers of positions. [START_REF] Derksen | Analysis of anthraquinones in Rubia tinctorum L. by liquid chromatography coupled with diode-array UV and mass spectrometric detection[END_REF] Quinizarin has notably been extensively used for textiles dyeing. Apart from these historical uses, scope of applications of dihydroxyanthraquinones has only scarcely evolved over time and only a few reports mentioning the use of these dyes in research fields such as Non-Linear Optics [START_REF] Sangsefedi | Fluorescence and nonlinear optical properties of alizarin red S in solvents and droplet[END_REF] or energy conversion [START_REF] Sun | Optical and electrical properties of purpurin and alizarin complexone as sensitizers for dye-sensitized solar cells[END_REF][START_REF] Taki | Novel Alizarin palladacyclic complexes as sensitizers in high durable dye-sensitized solar cells[END_REF] can be cited. Due to their strong absorption in the visible range, in 2016, the scope of applications of dihydroxyanthraquinone has been extended to photopolymerization, these dyes benefiting from remarkable molar extinction coefficients in the visible range. Notably, dihydroxyanthraquinones were examined for their ability to initiate the free radical polymerization (FRP) of methacrylates or the cationic polymerization (CP) of epoxides [START_REF] Zhang | Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization[END_REF]. It has to be noticed that prior to this work, attempts to use quinizarin as photoinitiator was unsuccessful for the cationic polymerization of epoxides, the free radical polymerization of acrylates, the thiol-ene photopolymerization or the generation of interpenetrated networks using laser diode at 635 nm or household red LED bulb at 630 nm. [START_REF] Xiao | Cationic and thiol-ene photopolymerization upon red lights using anthraquinone derivatives as photoinitiators[END_REF] In this work, the same authors examined four dihydroxyanthraquinones (12-DHAQ extracted from the roots of many families of plants Rubiaceae, Morinda, Gallium, Oldenlandia, 14-DHAQ extracted from the roots of Rubia tinctorum, 15-DHAQ and 18-DHAQ extracted from the plants of Hypericum genus) were used with different monomers and additives presented in the Figure 23. Especially, compared to the previous work in which photopolymerization experiments were carried out at 630 and 635 nm, [START_REF] Xiao | Cationic and thiol-ene photopolymerization upon red lights using anthraquinone derivatives as photoinitiators[END_REF] a shorter irradiation wavelength was used in these new investigations, namely 455 nm. Although the four molecules are isomers of position, modifications of the substitution pattern drastically alter their optical properties. Positions of the hydroxy groups onto the anthraquinone core confer significantly different absorption properties to dyes which are evidenced by the strong red-shift observed for 14-DHAQ having an absorption maximum at 477 nm compared to that of 15-DHAQ and 18-DHAQ exhibiting absorption maxima at 417 nm and 426 nm respectively. Interestingly, if a major modification of the absorption maxima can be found with the substitution pattern, similar molar extinction coefficients approaching 7000 M -1 .cm -1 could be detected for all dyes, enabling to compare the photoinitiating on the basis of the modification of their absorption maxima (see Figure 24) [START_REF] Zhang | Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization[END_REF]. Parallel to this, major differences could be evidenced concerning the solubility. Thus, due to its low solubility, absorption spectrum of 12-DHAQ could not be determined due to its insolubility in all solvents whereas a good solubility was found for the three other dyes. Red-shift of the absorption maximum of 14-DHAQ compared to that of the other dyes can be easily explained by theoretical calculations (See Figure 25) [START_REF] Zhang | Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization[END_REF]. Indeed, by molecular modeling, the lowest unoccupied molecular orbitals (LUMO) were determined as exhibiting a similar repartition for all dyes irrespective of the substitution pattern whereas the highest occupied molecular orbital (HOMO) of 14-DHAQ was determined to be more delocalized than that of the three other dyes. Therefore, it could be concluded that positions of the hydroxyl groups on the anthraquinone core could greatly modify the position of the HOMO orbital while letting the LUMO level invariant. Reproduced from ref. [START_REF] Zhang | Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization[END_REF] with permission from The Royal Society of Chemistry.

Face to their absorption spectra, DHAQs are therefore good candidates for photopolymerization done under blue light (455 nm). Upon photoexcitation, an electron from the ground state of DHAQs can pass to the singlet excited states and then interact with the additives present in the formulations (Iod1 or TEAOH) to produce radicals. Notably, radicals could be efficiently produced with two-component systems comprising 14-DHAQ (14-DHAQ/Iod1 (0.5%/2%, wt%) and 14-DHAQ/ TEAOH (0.5%/2%, wt%)). More generally, due to the low rate constants of interaction between DHAQ and the two additives NVK or R-Br, efficient photoinitiating systems could only be obtained with three-component systems DHAQ/Iod1/NVK (0.5%/2%/3%, wt%) and DHAQ/TEAOH/R-Br (0.5%/2%/3%, wt%) where the photosensitizer DHAQ is regenerated. In these conditions, the free radical polymerization of methacrylates (BisGMA/TEGDMA) could be initiated according to reactions r1-r7:

DHAQ → 1 DHAQ (hν) and 1 DHAQ → 3 DHAQ (r1) 1,3 DHAQ + Ph2I + → DHAQ •+ + Ph2I • (r2a) DHAQ •+ + Ph2I • → DHAQ + Ph2I + (r2b) Ph2I • → Ph • + Ph-I (r3) 1,3 DHAQ + TEAOH → DHAQ •-+ TEAOH •+ → DHAQ-H • + TEAOH • (-H) (r4a) DHAQ •-+ TEAOH •+ → DHAQ -+ TEAOH (r4b) Ph • + NVK → Ph-NVK • (r5) Ph-NVK • + Ph2I + → Ph-NVK + + Ph • + Ph-I (r6) DHAQ •-+ R-Br → DHAQ + (R-Br) •-→ DHAQ + R • + Br - (r7)
As anticipated, by regeneration of the photosensitizer, the three-component systems could greatly outperform the conversion rates obtained with the two-component systems. Thus, 36% and 28% final monomer conversions could be measured for the polymerization of the dental resin composed of a BisGMA/TEGDMA blend (70/30) with the two-component system 14-DHAQ/Iod and 14-DHAQ/TEAOH, respectively, after 300 seconds of irradiation at 455 nm vs. 50% and 48% conversions with the three-component systems 14-DHAQ/Iod1/NVK (0.5%/2%/3%, wt%) and 14-DHAQ/TEAOH/R-Br (0.5%/2%/3%, wt%), respectively. 15-DHAQ and 18-DHAQ were also able to initiate the FRP of methacrylates (42 and 55% monomer conversion after 300 s irradiation at 455 nm) in three-component systems whereas 12-DHAQ was ineffective due to its low solubility. As shown in the Figure 26a and 26b [START_REF] Zhang | Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization[END_REF], 18-DHAQ was the most efficient photoinitiator in three-component systems upon irradiation at 455 nm, far from the conversions obtained with 14-DHAQ and 15-DHAQ.

Due to its red-shifted absorption, 14-DHAQ could even initiate the FRP of methacrylates with the 14-DHAQ/Iod1/NVK (0.5%/2%/3%, wt%) three-component system at 518 nm, this dye absorbing until 550 nm. As anticipated, the three-component 18-DHAQ/Iod1/NVK (0.5%/2%/3%, wt%) system was the less efficient upon irradiation of the photocurable resin at 518 nm, this dye exhibiting the lowest absorption coefficient at this wavelength (see Figure 26c) [START_REF] Zhang | Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization[END_REF]. Consistent with the results obtained during the free radical photopolymerization of methacrylates, a similar order of reactivity could be established for the different dyes during the cationic polymerization of the EPOX-based resins. Precisely, the most efficient photoinitiator at 455 nm was once again 18-DHAQ followed by 14-DHAQ and 15-DHAQ with conversion rates of 79%, 67% and 15% (see Figure 26d) [START_REF] Zhang | Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization[END_REF], respectively after 800 seconds of irradiation using a three-component DHAQ/Iod1/NVK (0.5%/2%/3%, wt%) system, knowing that 12-DHAQ was ineffective due to its solubility. The two-component DHAQ/Iod1 (0.5%/2%, wt%) system does not allow to initiate the CP of EPOX due to the backelectron transfer reaction presented in reactions (r2a) and (r2b) Figure 26. Photopolymerization profiles of methacrylate functions of the Bis-GMA/TEGDMA blend (70%/30%, w/w) in laminate in the presence of (a) DHAQ/Iod1/NVK (0.5%/2%/3%, wt%) and CQ/Iod1 (0.5%/2%, wt%) as references, (b) DHAQ/TEAOH/R-Br (0.5%/2%/3%, wt%) and CQ/TEAOH (0.5%/2%, wt%) as references upon irradiation at 455nm, (c) DHAQ/Iod1/NVK (0.5%/2%/3%, wt%) upon irradiation at 518 nm and (d) EPOX under air in the presence of DHAQ/Iod1/NVK (0.5%/2%/3%, wt%) upon irradiation at 455nm. Reproduced from ref. [START_REF] Zhang | Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization[END_REF] with permission from The Royal Society of Chemistry.

To support the photoinitiating ability, photolysis experiments were carried out in acetonitrile with different two-component systems upon irradiation with a blue LED bulb at 455 nm in order to determine the rate constants of interaction between the dyes and the additives. Upon photolysis of the two-component DHAQ/Iod1 system at 455 nm for 10 min., no photobleaching was observed in solution due to the back-electron transfer reaction (see reactions (r2a) and (r2b)). Interestingly, addition of NVK to the acetonitrile solution of 18-DHAQ/Iod resulted in the precipitation of a polymer produced by polymerization of vinylcarbazole (poly(vinylcarbazole)) within 10 seconds. This result is in agreement with the results obtained during the photopolymerization tests where the three-component DHAQ/Iod1/NVK (0.5%/2%/3%, wt%) system was greatly faster than the two-component DHAQ/Iod1 (0.5%/2%, wt%) systems due to the reaction between NVK and the phenyl radicals produced (see reaction (r5)). Similarly, photolysis of the two-component DHAQ/TEAOH (0.5%/2%, wt%) systems in acetonitrile revealed a faster photobleaching with 18-DHAQ than with 15-DHAQ, consistent with the order of reactivity determined during the photopolymerization of the BisGMA/TEGDMA blend (see Figure 27) [START_REF] Zhang | Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization[END_REF]. Interestingly, examination of their absorption properties revealed these dyes to be suitable candidates for photopolymerization processes done at 518 nm. Indeed, a broad absorption extending from 350 to 500 nm for 127-THAQ, from 350 to 550 nm for 124-THAQ and 1258-THAQ could be determined by UV-visible absorption spectroscopy. Photoluminescence measurements also revealed the order of reactivity of the three dyes, photoluminescence quantum yields of 0.046, 0.00 and 0.013 being respectively determined for 124-THAQ, 127-THAQ, and 1258-THAQ. Due to the ability of 124-THAQ to promote more easily an electron in the excited state, it could be concluded that the most reactive dye would be 124-THAQ, then 1258-THAQ and 127-THAQ. While examining the rate constants of interaction of THAQ derivatives with Iod1 by fluorescence quenching experiments upon irradiation with a green LED (λexc = 518 nm), the quenching reaction was determined as being diffusion controlled for 124-THAQ (kq = 1.6 × 109 m -1 .s -1 ) whereas no quenching was found for the 1258-THAQ/Iod combination, indicating that 1258-THAQ could not react with Iod. Similarly, an efficient fluorescence quenching process could be evidenced for the 124-THAQ/R-Cl combination, demonstrating that an electron transfer from the singlet excited state of 124-THAQ towards R-Cl in the ground state was possible. Laser flash photolysis experiments confirmed the occurrence of the singlet route, no triplet state absorption being detected for 124-THAQ. As anticipated from the photophysical measurements, only 124-THAQ could promote the FRPCP of EPOX when incorporated in a three-component system 124-THAQ/Iod1/NVK (0.5%/2%/3%, wt%). In these conditions, a final monomer conversion of 50% could be determined after 2000 s of irradiation whereas no conversion was detected with the corresponding two-component 124-THAQ/Iod1 (0.5%/2%, wt%) system. Conversely, the monomer conversion remained limited with 1258-THAQ, peaking at 5% with the threecomponent system whereas no conversion was detected with 127-THAQ. While examining the FRP of the BisGMA/TEGDMA blend, conversion of 41 and 40% could be determined with the two-component 124-THAQ/Iod (0.5%/2%, wt%) and 124-THAQ/R-Cl (0.5%/2%, wt%) systems, consistent with the aforementioned photochemical reactivity of 124-THAQ with Iod and R-Cl. By using the three-component 124-THAQ/R-Cl/TEAOH (0.5%/2%/3%, wt%), the monomer conversion could be increasing up to 51% upon irradiation at 518 nm for 300 s. Here again, enhancement of the monomer conversion can be assigned to the regeneration of 124-THAQ by the sacrificial amine. Comparison of the reference system camphorquinone/TEAOH (0.5%/2%, wt%) system revealed even the two two-component 124-THAQ/Iod1 and 124-THAQ/R-Cl systems to outperform this reference system, the monomer conversion only reaching 35% in the same irradiation conditions.

Flavones

Photoinitiators are compounds capable to convert a liquid monomer as a solid material. Besides, beyond simply initiating the polymerization process, the photoinitiators or the photosensitizers can also provide additional properties to the final materials. Thus, photoluminescence, [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] Xiao | Red-lightinduced cationic photopolymerization: perylene derivatives as efficient photoinitiators[END_REF] modification of the mechanical properties, [START_REF] Xiao | Photochemical in situ elaboration of polyoxometalate (α-[SiMo12O40] 4-) / polymer hybrid materials[END_REF][START_REF] Mokbel | Iodonium-polyoxometalate and thianthreniumpolyoxometalate as new one-component UV photoinitiators for radical and cationic polymerization[END_REF] photochromism, [START_REF] Tehfe | A multicolor photoinitiator for cationic polymerization and interpenetrated polymer network synthesis: 2,7-di-tert-butyldimethyldihydropyrene[END_REF] photocatalysis for wastewater treatment, [START_REF] Ghali | Synthesis of polyoxometalate/ polymer composites under soft conditions for enhancement of dye photodegradation by photocatalysis[END_REF] or biological properties including antimicrobial, antifungal or bactericidal activities [START_REF] Lalevée | Decatungstate (W10O 4-32)/silane: A new and promising radical source under soft light irradiation[END_REF] can be cited as classical properties originating from the photoinitiators/photosensitizers. With regards to the bactericidal activities, natural dyes are candidates of choice and interest for this specific properties has gained an increased attention during the last decade. [START_REF] Singha | Antimicrobial activity of some natural dyes[END_REF] However, at present, only synthetic dyes have been examined for this purpose, as exemplified with porphyrins and phthalocyanines, [START_REF] Felgentrager | Singlet oxygen generation in porphyrin-doped polymeric surface coating enables antimicrobial effects on Staphylococcus aureus[END_REF][START_REF] Yano | Current states and future views in photodynamic therapy[END_REF] derivatives of Michler's ketone, [START_REF] Oh | A novel antimicrobial photosensitizer: hydroxyethyl Michler's ketone[END_REF] eosin Y and Rose Bengal [START_REF] Cohn | Photodynamic inactivation of yeast sensitized by eosin Y[END_REF][START_REF] Freire | Comparison of the effect of rose bengal-and eosin Y-mediated photodynamic inactivation on planktonic cells and biofilms of Candida albicans[END_REF][START_REF] Guo | Rose Bengal-decorated silica nanoparticles as photosensitizers for inactivation of gram-positive bacteria[END_REF] or methylene blue. [START_REF] Usacheva | Comparison of the methylene blue and toluidine blue photobactericidal efficacy against gram-positive and gram-negative microorganisms[END_REF][START_REF] Bovis | Photosensitisation studies of silicone polymer doped with methylene blue and nanogold for antimicrobial applications[END_REF][START_REF] Araujo | In vitro lethal photosensitization of S. mutans using methylene blue and toluidine blue O as photosensitizers[END_REF] Among natural dyes with potential biological applications, flavones are one of those. [START_REF] Wang | Bioactive flavonoids in medicinal plants: Structure, activity and biological fate[END_REF][START_REF] Wu | Flavones. 3. Synthesis, biological activities, and conformational analysis of isoflavone derivatives and related compounds[END_REF][START_REF] Vermaa | The biological potential of flavones[END_REF][START_REF] Zhang | Flavones and flavonols: Phytochemistry and biochemistry[END_REF] In 2015, quercitin (3,5,7,3',4'-pentahydroxyflavone) which is one of the most abundant flavone [START_REF] Hertog | Potential health effects of the dietary flavonol quercetin[END_REF][START_REF] Inal | Beneficial effects of quercetin on oxidative stress induced by ultraviolet A[END_REF][START_REF] Sestili | Quercetin prevents DNA single strand breakage and cytotoxicity caused by tert-butylhydroperoxide: free radical scavenging versus iron chelating mechanism[END_REF] was used for the first time as the photosensitizer for the cationic polymerization of antibacterial coatings. [START_REF] Condat | Development of photoactivable glycerolbased coatings containing quercetin for antibacterial applications[END_REF] Precisely, quercitin was used as the photosensitizer for a cationic photoinitiator i.e. 4-(2-methylpropyl)phenyliodonium hexafluorophosphate (Iod1). Choice of quercitin was also supported by previous works reported in the literature mentioning its use as anti-inflammatory and anti-allergic agent, [START_REF] Brown | A review of the genetic effects of naturally occurring flavonoids, anthraquinones and related compounds[END_REF][START_REF] Read | Flavonoids: naturally occurring anti-inflammatory agents[END_REF][START_REF] Ohnishi | Quercetin potentiates TNF-induced antiviral activity[END_REF] its antiviral and antioxidant activities. [START_REF] Russo | Bioflavonoids as antiradicals, antioxidants and DNA cleavage protectors[END_REF][START_REF] Casagrande | Protective effect of topical formulations containing quercetin against UVBinduced oxidative stress in hairless mice[END_REF][START_REF] Choquenet | Quercetin and Rutin as potential sunscreen agents: determination of efficacy by an in vitro method[END_REF] Based on its UV-centered absorption spectrum (λmax = 370 nm), photopolymerization experiments were carried out in the UV range using a Xenon lamp (200 W) (See Figure 29) [START_REF] Condat | Development of photoactivable glycerolbased coatings containing quercetin for antibacterial applications[END_REF]. Reproduced from ref. [START_REF] Condat | Development of photoactivable glycerolbased coatings containing quercetin for antibacterial applications[END_REF] with permission from The Royal Society of Chemistry Interestingly, photolysis experiments of the two-component quercitin/Iod1 system revealed the kinetic of interaction to be relatively fast since 97% of the initial absorbance intensity at 370 nm disappeared after 17 min. of irradiation. The concomitant photogeneration of H + was also demonstrated by introducing rhodamine B as an acid-base indicator. While examining the monomer conversion profile for the cationic polymerization of glycerol triglycidyl ether (GTE), a final monomer conversion of 75% could only be obtained after 1200 s of irradiation with the Xenon lamp under air with the two-component quercitin/iod1 (2.5%/4%, wt%) system (See Figure 30) [START_REF] Condat | Development of photoactivable glycerolbased coatings containing quercetin for antibacterial applications[END_REF]. Besides, the resulting coating exhibited a remarkable thermal stability until 375°C and mechanical analyses revealed the polymer to exhibit a rubber-like state at room temperature.

Figure 30.

Polymerization profile obtained while polymerization GTE with the twocomponent quercitin/iod1 (2.5%/4%, wt%) system under air with a xenon lamp. Chemical structures of quercitin, the iodonium salt and the monomer. Reproduced from ref. [START_REF] Condat | Development of photoactivable glycerolbased coatings containing quercetin for antibacterial applications[END_REF] with permission from The Royal Society of Chemistry.

Due to the relatively high amount of photosensitizer introduced into the photocurable resin, unreacted quercitin could be advantageously used to generate reactive species based on oxygen. Indeed, one of the most efficient strategy to disinfect a polymer surface consists in the production of reactive oxygen species upon irradiation of the surface with the appropriate light. [START_REF] Niedre | In vitro tests of the validity of singlet oxygen luminescence measurements as a dose metric in photodynamic therapy[END_REF][START_REF] Ragas | Quantification of photosensitized singlet oxygen production by a fluorescent protein[END_REF][START_REF] Ragas | Singlet oxygen in Escherichia coli: New insights for antimicrobial photodynamic therapy[END_REF][START_REF] Cahan | Light-activated antibacterial surfaces comprise photosensitizers[END_REF] Thus, quercitin-based coatings proved to be efficient antibacterial coatings since the proliferation of Escherichia coli and Staphylococcus aureus could be efficiently inhibited. Thus, an inhibition as high as 99% could be determined for the proliferation of Staphylococcus aureus, demonstrating that enough singlet oxygen was produced close to the bacteria to induce the bacterial cell death. Control experiments also revealed that quercitin could not inhibit bacteria proliferation in the absence of light, demonstrating that the antibacterial effect was originating from the generation of reactive oxygen species (See Figure 31) [START_REF] Condat | Development of photoactivable glycerolbased coatings containing quercetin for antibacterial applications[END_REF]. Interestingly, antibacterial effect could be kept, even 6 hours after the light was switched off. Notably, use of the synthetic 3-hydroxyflavone (3HF) enabled the free radical polymerization of the BisGMA/TEGDMA blend upon irradiation with LEDs emitting at 405 and 477 nm. [START_REF] Al Mousawi | 3-hydroxyflavone and N-phenylglycine in high performance photoinitiating systems for 3D printing and photocomposites synthesis[END_REF] However, scope of applications of 3HF was not limited to photosensitization and use of 3HF as a fluorescent probe to monitor the photopolymerization of an acrylate-based biocompatible monomer i.e. E-Shell 300 was also reported. [START_REF] Tomin | Use of 3-hydroxyflavone as a fluorescence probe for the controlled photopolymerization of the E-Shell 300 polymer[END_REF] Indeed, during photopolymerization, a 1.8fold enhancement of the photoluminescence intensity of 3HF could be demonstrated, enabling to correlate monomer conversion and photoluminescence intensity. More recently, a series of flavones bearing halogens was also developed and proved to be exceptional near-visible photoinitiators for the FRP of tripropylene glycol diacrylate (TPGDA). [START_REF] You | Flavonol dyes with different substituents in photopolymerization[END_REF] This year, two other natural flavones, namely myricetin (3,3′,4′,5,5′,7hexahydroxyflavone) which an be found in tomatoes, oranges, nut, berries and red wines, as well as chrysin (5,7-dihydroxyflavone) which can be found in honey, passion flowers (Passiflora caerulea and Passiflora incarnata), carrots or mushrooms (Pleurotus ostreatus) were examined as photosensitizers for photopolymerization experiments done at 385 and 405 nm (See Figure 32). [START_REF] Al Mousawi | Flavones as natural photoinitiators for light mediated free-radical polymerization via light emitting diodes[END_REF] Compared to quercitin, myricetin exhibits an excellent absorption in the 350-450 nm whereas the absorption spectrum of chrysin was found to be more limited, no absorption being detected after 400-420 nm. While examining the photoinitiating abilities of chrysin and myricetin for the FRP of a BisGMA/TEGDMA (70/30) blend using the twocomponent flavone/NPG (0.5%/1%, wt %) system, a final monomer conversion of 30% could be obtained with chrysin upon irradiation for 100 s with a LED emitting at 405 nm whereas no monomer conversion could be detected with myricetin. Inefficiency of myricetin as photoinitiators was assigned to its low solubility in monomer but also to its polyphenolic structure, phenols being classically used as stabilizing agents for monomers and polymerization inhibitors. [START_REF] Benov | The antioxidant activity of flavonoids isolated from corylus colurna[END_REF][START_REF] Lartigue-Peyrou | The use of phenolic compounds as free-radical polymerization inhibitors[END_REF][START_REF] Lochab | Naturally occurring phenolic sources: monomers and polymers[END_REF][START_REF] Kice | Inhibition of Polymerization. I. Methyl Methacrylate[END_REF] Figure 32. Chemical structures of myricetin and chrysin used as photosensitizers, the sacrificial amine (NPG) and the monomers (BisGMA/TEGDMA).

Photoinitiators based on renewable resources.

Vanillin-based type-I photoinitiator

Vanillin is a natural product extensively used in baking and this molecule can be extracted from a plant of the orchid family. However, due to the scarcity and the expense of the natural extracts, vanillin which is found on the market is often from synthetic origin. Indeed, two main sources of artificial vanillin exist, namely guaiacol which is a side product from the chemical industry and lignin which is a residue from the paper industry. [START_REF] Bensaid | Authentication of natural vanilla flavorings: isotopic characterization using degradation of vanillin into guaiacol[END_REF] In 2020, vanillin was used as the starting materials for the design of a type-I photoinitiator. [START_REF] Breloy | Vanillin derivative as performing type I photoinitiator[END_REF] It has to be noticed that prior to its use as raw materials for the design of photoinitiators, vanillin has also employed as starting materials for the design of different monomers. [START_REF] Navaruckiene | Vanillin-based thiol-ene systems as photoresins for optical 3D printing[END_REF][START_REF] Lebedevaite | Photoinitiator free resins composed of plant-derived monomers for the optical μ-3D printing of thermosets[END_REF] Type-I photoinitiators are a specific kind of photoinitiators capable to undergo a homolytic cleavage of a single bond upon photoexcitation, furnishing two radical fragments from the original photoinitiator. [START_REF] Eibel | Choosing the ideal photoinitiator for free radical photopolymerizations: predictions based on simulations using established data[END_REF] In the context of this study, an analogue of the well-known acyl benzoylphosphine oxide [START_REF] Kolczak | Reaction mechanism of monoacyl-and bisacylphosphine oxide photoinitiators studied by 31 P-, 13 C-, and 1 H-CIDNP and ESR[END_REF] was synthesized in a three-step procedure, furnishing ((4-(allyloxy)-3-methoxyphenyl)(hydroxy)methyl)diphenylphosphine oxide (PM) in 48% yield for the three steps (See Scheme 2). Especially, introduction of an allyl group enabled its immobilization inside the polymer network by crosslinkage. Due to the electron-donating ability of the methoxy and the allyloxy groups, a molar extinction coefficient higher than that of benchmark type-I photoinitiators such as 2,2-dimethoxy-2-phenylacetophenone (DMPA), 1-hydroxy-cyclohexyl phenyl ketone (HCPK) or phenyl bis (2,4,6-trimethylbenzoyl)phosphine oxide (BAPO) was determined. A molar extinction coefficient of 3800 M -1 .cm -1 was found at 345 nm. Scheme 2. Synthetic route to PM.

Considering that phosphinoyl and phenyl radicals are known to be highly reactive radicals towards acrylate monomers, TMPTA was selected as a relevant monomer to evaluate the photoinitiating ability of PM. However, no polymerization could be detected under air as a result of oxygen inhibition and the formation of unreactive peroxyl radicals. Conversely, photopolymerization of the more viscous soybean oil epoxidized acrylate (SOA) monomer (1200 cps) furnished the high conversion of 40% after 800 s of irradiation under air with a Hg-Xe lamp. In this case, the viscosity of the resin efficiently impeded oxygen to penetrate into the photocurable resin. By introducing trimethylolpropane tris(3-mercaptopropionate) (trithiol, TT) in the resin, this issue could be addressed, thyil radicals consuming dissolved oxygen, [START_REF] Breloy | β-carotene/limonene derivatives/eugenol: green synthesis of antibacterial coatings under visible-light exposure[END_REF][START_REF] Hoyle | Thiol-enes: Chemistry of the past with promise for the future[END_REF][START_REF] Hoyle | Thiol-click chemistry: a multifaceted toolbox for small molecule and polymer synthesis[END_REF][START_REF] Breloy | Well-defined titanium complex for free-radical and cationic photopolymerizations under visible light and photoinduction of ti-based nanoparticles[END_REF] according to the reactions r1-r3.

RS • + O2 → RSOO • (r1) RSOO • + RSH → RSOOH + RS • (r2) RS • + acrylate → poly(acrylate) (r3)
Comparison of the photoinitiating ability of PM with benchmark photoinitiator revealed PM to be on par with BAPO for the polymerization of TMPTA in laminate (72% conversion of PM vs. 70% for BAPO), also producing benzoyl and phosphinoyl radicals upon photodecomposition (See Figure 33) [START_REF] Breloy | Vanillin derivative as performing type I photoinitiator[END_REF]. This conversion is higher than that of another type I photoinitiator, namely DMPA (60% TMPTA conversion). Lower performance of DMPA can be assigned to the formation of less reactive radicals such as dimethoxybenzyl and carbonyl radicals.

Camphor-derived photoinitiators

Camphorquinone is a well-known photoinitiator of polymerization which is extensively used in dentistry in combination with hydrogen donors such as amines. [START_REF] Jakubiak | Camphorquinone-amines photoinitiating systems for the initiation of free radical polymerization[END_REF][START_REF] Bi | A visible light initiating system for free radical promoted cationic polymerization[END_REF][START_REF] Cook | Photopolymerization kinetics of dimethacrylates using the camphorquinone/amine initiator system[END_REF][START_REF] Nie | Photopolymerization of poly(melamine-coformaldehyde) acrylate for dental restorative resins[END_REF][START_REF] Jakubiak | Crosslinking photocopolymerization of acrylic acid (and N-vinylpyrrolidone) with triethyleneglycol dimethacrylate initiated by camphorquinone/ethyl-4-dimethylaminobenzoate[END_REF][START_REF] Yu | UV photopolymerization behavior of dimethacrylate oligomers with camphorquinone/amine initiator system[END_REF][START_REF] Jakubiak | Isothermal photo differential scanning calorimetry. crosslinking polymerization of multifunctional monomers in presence of visible light photoinitiators[END_REF] Interest for camphorquinone in dentistry is not new since the pioneering works in this field were published as soon as 1976 with the use of a UV light to activate the photopolymerization process. [START_REF] Ruyter | An infrared spectroscopic study of sealants[END_REF] Among amines commonly used in combination with camphorquinone, N,Ndimethyl-p-toluidine, 2-ethyl-dimethylbenzoate, N-phenylglycine can be cited as the most popular ones but many other amines were also examined. [START_REF] Cook | Photopolymerization kinetics of dimethacrylates using the camphorquinone/amine initiator system[END_REF][START_REF] Fujimori | Polymerization and photoinitiation behavior in the light-cured dental composite resins[END_REF][START_REF] Inomata | Visible light induced polymerization of methyl methacrylate sensitized by diketones with peroxides[END_REF][START_REF] Lindén | Applied photochemistry in dental science[END_REF][START_REF] Anseth | Polymeric dental composites: Properties and reaction behavior of multimethacrylate dental restorations, s[END_REF][198][START_REF] Nie | A reappraisal of the photopolymerization kinetics of triethyleneglycol dimethacrylate initiated by camphorquinone-N,N-dimethyl-ptoluidine for dental purposes[END_REF] From a synthetic viewpoint, access to camphorquinone is relatively easy since it be readily prepared from (+)-camphor in one step, by oxidation with selenium oxide. [START_REF] Młochowski | Developments in synthetic application of selenium (IV) oxide and organoselenium compounds as oxygen donors and oxygentransfer agents[END_REF][START_REF] Glišic | Selectivity of the complexation reactions of four regioisomeric methylcamphorquinoxaline ligands with gold(III): X-ray, NMR and DFT investigations[END_REF] Besides, over the years, development of new synthetic approach have been developed, not requiring the highly toxic selenium oxide to be used (See Scheme 3). [START_REF] Wang | efficient synthesis of camphorquinone from camphor[END_REF] Scheme 3. Synthetic route to camphorquinone.

By itself, camphorquinone is a poor photoinitiator but the addition of tertiary amines used as proton donors or as reducing agents in the photoinitiating system was extensively studied. Such two-component combinations proved to be effective systems for the polymerization of methacrylate-based resins. [START_REF] Jakubiak | Camphorquinone-amines photoinitiating systems for the initiation of free radical polymerization[END_REF][START_REF] Cook | Photopolymerization kinetics of dimethacrylates using the camphorquinone/amine initiator system[END_REF][START_REF] Jakubiak | Isothermal photo differential scanning calorimetry. crosslinking polymerization of multifunctional monomers in presence of visible light photoinitiators[END_REF][START_REF] Alvim | Analysis of camphorquinone in composite resins as a function of shade[END_REF][START_REF] Jakubiak | The role of amines in the camphorquinone photoinitiated polymerization of mutifunctional monomer[END_REF][START_REF] Kamoun | Crosslinking behavior of dextran modified with hydroxyethyl methacrylate upon irradiation with visible light-effect of concentration, co-initiator type, and solvent[END_REF][START_REF] Kamoun | HES-HEMA nanocomposite polymer hydrogel: swelling behavior and characterization[END_REF] A further improvement of the final monomer conversion can be obtained while introducing an iodonium salt. In this last case, a photocatalytic behavior can be observed for camphorquinone, as evidenced in the mechanism proposed in Figure 34. [START_REF] Cook | Enhanced photopolymerization of dimethacrylates with ketones, amines, and iodonium salts: the CQ system[END_REF][START_REF] Kim | The role of diphenyliodonium salt (DPI) in three component photoinitiator systems containing methylene blue (MB) and an electron donor[END_REF][START_REF] Wang | Effect of coinitiator and water on the photoreactivity and photopolymerization of HEMA/ camphorquinone-based reactant mixtures[END_REF] After excitation of camphorquinone with light, a photoinduced electron transfer between the excited camphorquinone and the ground state amine occurs, resulting in the generation of a α-aminoalkyl radical formed by hydrogen abstraction. [START_REF] Dadashi-Silab | Photoinduced electron transfer reactions for macromolecular syntheses[END_REF] Subsequently, reduction of the iodonium salt by the radical formed on camphorquinone allows the formation of phenyl radicals Ph• as the initiating species.

Mechanism involved in the generation of the initiating species Ph•

As the main drawback, camphorquinone exhibits a poor solubility in water which restrict its utilization for polymerization so that the synthesis of derivatives of this structure was examined. [START_REF] Magoshi | Formation of polymerized mixed heparin/albumin surface layer and cellular adhesional responses[END_REF][START_REF] Matsuda | Preparation of vinylated polysaccharides and photofabrication of tubular scaffolds as potential use in tissue engineering[END_REF][START_REF] Nakayama | Development of highperformance stent: gelatinous photogel-coated stent that permits drug delivery and gene transfer[END_REF][START_REF] Okino | In situ hydrogelation of photocurable gelatin and drug release[END_REF] Camphorquinone is undoubtedly a reference photoinitiator. Besides, over the years, several strategies were developed to improve its photoinitiating ability. Indeed, if camphorquinone in two-component CQ/amine systems can efficiently initiate a polymerization process, the polymerization kinetic remains slow. To improve this, synthesis of derivatives was envisioned. Interest for camphor derivatives is also supported by the fact that camphor is a cheap starting material and that chemistry of camphorquinone has been extensively studied by organic chemists so that polymerists could capitalized on this to develop new visible light photoinitiators. In this field, chemistry of camphorsulfonyl chloride C-1 has been largely studied. In 2010, a camphorquinone derivative possessing an acylphosphine oxide group C-5 was reported, in analogy to the chemical structure of the benchmark photoinitiator BAPO. [START_REF] Ikemura | Synthesis of a novel camphorquinone derivative having acylphosphine oxide group, characterization by UV-VIS spectroscopy and evaluation of photopolymerization performance[END_REF] This molecule could be obtained in four steps, starting from C-1, by two successive oxidations with potassium permanganate and selenium oxide, providing C-3. Then, treatment of C-3 with thionyl chloride followed by a Michaelis-Arbuzov reaction with methoxydiphenylphosphine furnished C-5 in 86% yield for the last step (See scheme 4).

Scheme 4. Synthesis of C-5.

Investigation of the photopolymerization reactivity of C-5 under different curing light sources revealed C-5 to furnish a faster polymerization kinetic than CQ (9.6 s vs 10.9 s) for the polymerization of dental resin bisphenol A-glycidyl methacrylate (BisGMA)-triethyleneglycol dimethacrylate (TEGDMA) with the two-component systems CQ or C-5/ ethyl pdimethylaminobenzoate (EDAB) (2%/2% w/w) upon irradiation with a visible light. Parallel to a faster polymerization process, the resulting polymers showed good mechanical properties, what is of crucial importance for dental photocomposites.

More recently, photoinitiating abilities of ketopinic acid (C-2) and the carboxylatedcamphorquinone C-3 were compared to that of the reference camphorquinone (CQ) and examined for the formation of polymer hydrogels. [START_REF] Kamoun | Carboxylated camphorquinone as visible-light photoinitiator for biomedical application: Synthesis, characterization, and application[END_REF] Due to the presence of carboxylic groups onto the two structures, polymerization of water soluble monomers such as 2hydroxyethyl methacrylate (HES-HEMA) could be carried out. As anticipated and due to a reduced solubility in water, all carboxylated photoinitiators could furnish faster polymerization kinetics than CQ in three-component systems CQ or C-1 or C-2/Iod/amine with non-toxic amines such as NPG and L-arginine. While examining the biocompatibility of the polymer films, a reduction of the cell viability was observed for all polymer films prepared with the three-component CQ or C-1 or C-2/Iod/amine systems. Conversely, a good biocompatibility was found for all polymers prepared with the two-component CQ or C-1 or C-2/amine systems.

1,4-Dihydroxyanthraquinone derivatives

As previously mentioned in the dihydroxyanthraquinone section, quinizarin is a natural dye that was used as a photosensitizer for the cationic polymerization of epoxides. With regards to the potential extractability of photoinitiators from the polymer films which can adversely impact the potential use of the polymer films, several strategies have been developed to immobilize the photosensitizers within the polymer networks by mean of covalent linkages. Typically, to reduce the migratability, polymerizable groups are introduced onto the photosensitizers/photoinitiators, as exemplified with several examples reported in the literature and comprising allylic [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] Sun | Free radical photopolymerization and 3D printing using newly developed dyes: Indane-1,3-dione and 1H-cyclopentanaphthalene-1,3-dione derivatives as photoinitiators in three-component systems[END_REF][START_REF] Pigot | Recent advances on push-pull organic dyes as visible light photoinitiators of polymerization[END_REF] or acrylate [START_REF] Xiao | Naphthalimide based methacrylated photoinitiators in radical and cationic photopolymerization under visible light[END_REF][START_REF] Yang | Naphthalimide dyes: Polymerizable one-component visible light initiators[END_REF][START_REF] Yang | A green and highly efficient naphthalimide visible photoinitiator with an ability initiating free radical polymerization under air[END_REF] groups in their backbones. This strategy was notably applied to quinizarin, and allyl quinizarin (QA) and epoxidized quinizarin (QE) were proposed as photosensitizers for free radical, cationic and thiol-ene polymerizations but also as antimicrobial agents capable to inhibit the bacterial proliferation upon treatment of the polymer surface with light (See Figure 35). [START_REF] Sautrot-Ba | Quinizarin derivatives as photoinitiators for freeradical and cationic photopolymerizations in the visible spectral range[END_REF] Figure 35. Chemical structures of the crosslinkable photoinitiators (epoxidized quinizarin) and the different acrylate and epoxy monomers, the cationic photoinitiator and the sacrificial amine.

As previously observed for quinizarin, photolysis experiments at 405 nm revealed QA and QE to efficiently interact in the excited state with Iod1, ensuring the formation of initiating radical species. Upon addition of an acid/base indicator (Rhodamine B), formation of H + in solution was clearly evidenced by UV-visible absorption spectroscopy, with the appearance of a new peak at ca. 550 nm corresponding to the protonated form of Rhodamine B. Due to the efficient production of Bronsted acids upon excitation of the two-component quinizarin/Iod1 (0.5%/2.0%, wt%) system, the cationic polymerization of EPOX could be promoted at 405, 455 and 470 nm, QA and QE strongly absorbing between 350 and 500 nm (See Figure 36 and Table 4). Markedly, no polymerization of EPOX could be initiated with quinizarin irrespective of the irradiation wavelengths, attributable to its phenolic structure. Indeed, polymerization of EPOX is a free radical promoted cationic polymerization (FRPCP) and phenols are well-known to act as radical inhibitors. The highest monomer conversions were obtained upon irradiation at 405 nm, corresponding to approximately the maximum absorption for the two dyes (QA and QE). Comparison with the reference camphorquinone revealed this photoinitiator to be unable to initiate the FRPCP of EPOX in the same conditions, demonstrating all the interest of QA and QE as photoinitiators. QA and QE could also promote the FRP of TMPTA while using the two-component quinizarin/N-methyldiethanolamine (MDEA) systems. Here again, the best performances were obtained at 405 nm due to the high molar extinction coefficients of QA and QE at this wavelength. Thus, final monomer conversions as high as 69 and 57% were respectively obtained with the two-component QA/MDEA and QE/MDEA systems respectively. Due to the poor absorption properties of CQ at 405 nm, no polymerization could be initiate even in laminate due to the poor initiating rate and oxygen inhibition. FRP of TMPTA could also be promoted with the two-component quinizarin/Iod1 (0.5%/2.0%, wt%) systems. Interestingly, the two-component quinizarin/Iod1 (0.5%/2.0%, wt%) system could outperform the twocomponent quinizarin/MDEA (0.5%/2.0%, wt%) system, irrespective of the irradiation wavelengths. Considering that radicals and cationic are concomitantly formed, interpenetrated polymer networks (IPN) could be obtained by simultaneously polymerizing TMPTA and EPOX under air and in laminate upon irradiation at 455 nm. As anticipated, due to oxygen inhibition, higher acrylate conversions were obtained in laminate than under air whereas a decrease of the EPOX conversion was found in laminate compared to that determined under air (See Table 5). These results are consistent with previous works reported in the literature. [START_REF] Breloy | Well-defined titanium complex for free-radical and cationic photopolymerizations under visible light and photoinduction of ti-based nanoparticles[END_REF] An enhancement of the TMPTA conversion in the TMPTA/EPOX (43%/57% w/w) blend compared to TMPTA alone was also demonstrated, resulting from the hydrogen-donating ability of EPOX, favoring the generation of additional radicals. [START_REF] Sangermano | visible light induced cationic polymerization of epoxides by using multiwalled carbon nanotubes[END_REF][START_REF] Li | Near-infrared-induced cationic polymerization initiated by using upconverting nanoparticles and titanocene[END_REF] Ability of QA and QE as photoinitiators for thiol-ene reaction was also examined. If high monomer conversions could be obtained while using QA or QE (0.5 wt%) as a monocomponent photoinitiator, addition of Iod1 could further increase the TMPTA conversion, peaking at 98 and 93% respectively for the two-component QA/Iod and QE/Iod1 (0.5%/2.0%, wt%) systems after 800 s of irradiation at 405 nm (See Table 6). Almost similar TMPTA conversions were obtained under air and in laminate during the thiol-ene reaction, consistent with the literature. [START_REF] Xiao | Cationic and thiol-ene photopolymerization upon red lights using anthraquinone derivatives as photoinitiators[END_REF] Indeed, in the presence of oxygen, thiyl radicals (RS•) are converted as peroxyl radicals (RSOO•) and reaction with trithiol can regenerate the initiating thiyl radicals. [START_REF] Foti | The surprisingly high reactivity of phenoxyl radicals[END_REF][START_REF] Hoyle | Thiol-ene click chemistry[END_REF] To support the enhancement of the monomer conversion upon addition of Iod1, contribution of the phenyl radicals Ph• issued from the decomposition of the iodonium salt to the FRP of TMPTA was suggested by hydrogen abstraction from trithiol, providing an additional source of thiyl radicals (See Figure 37).

Conclusion

Use of natural dyes as photoinitiators is a recent topic that deserve to be more largely investigated for the design of photoinitiating species for polymerization in the Future. At present, only nine families of natural dyes have been examined for this purpose, demonstrating that a lot of works has still to be done to exploit the full potential of Nature. Parallel to natural dyes, a few examples of biosourced photoinitiators have been designed and Future of this research topic will certainly consist in the valorization of renewable resources. From the viewpoint of absorption, a wide range of absorption is accessible. Indeed, as shown in the Figure 1, natural dyes have absorptions making possible to initiate polymerization reactions in the 300-550 nm region. Different types of polymerization are also accessible, ranging from the free radical polymerization of acrylates to the cationic polymerization of epoxides (See Table 7). Among dyes, sesamin is the only one that do not exhibit any absorption in the visible range. Conversely, the bacteriochlorophyll family allow to polymerize until 850 nm, enabling to get a remarkable light penetration within the resins.

Besides, as demonstrated in this review, a dye can act as a photoinitiator if its absorption maximum, its molar extinction coefficient, its excited state lifetime or its electrochemical properties are adapted with that of the different additives introduced into the photocurable resin. With regards to these different criteria, use of renewable resources for the design of photoinitiators is certainly the most straightforward route. Indeed, natural dyes often require extensive purification processes to be isolated in pure form, adversely affecting the interest of this approach. n.p. : not provided in the reference.

As a relevant example of this, paprika requires the spice to be chromatographied prior to photopolymerization experiments and can't be used directly. Natural dyes are certainly appealing candidates for photoinitiation but molecules that can be easily separated from their vegetables have still to be found.

Figure 1 .

 1 Figure 1. Representative absorption properties of natural and biosourced photoinitiators mentioned in this review. Dotted lines mean that dyes only weakly absorb at these wavelengths.

Figure 3 .

 3 Figure 3. Photolysis experiments for 1) β-carotene and 2) the two-component β-carotene/Iodsystem in deaerated chloroform upon irradiation with a LED@405 nm. Reproduced with permission from Breloy et al.[START_REF] Breloy | β-carotene/limonene derivatives/eugenol: green synthesis of antibacterial coatings under visible-light exposure[END_REF] Copyright 2014 American Chemical Society.

Figure 4 .

 4 Figure 4. Polymerization profiles for the polymerization of a DPDO/Lim/Eug blend (50/25/25%, w/w/w) (1) epoxy group conversion (2) conversion of the ene groups of lim (3) conversion of the ene groups of eugenol while using β-carotene/Iod/trithiol (1/3/6% w/w/w)under air upon irradiation at (a) 405 nm (60 mW/cm 2 ), (b) 405 nm (100 mW/cm 2 ), (c) 455 nm (d) 470 nm (e) xenon lamp. Reproduced with permission from Breloy et al. [77] Copyright 2014 American Chemical Society.

Figure 5 .

 5 Figure 5. UV-visible absorption spectrum of chlorophyll a in chloroform. Reproduced with permission from Shanmugam et al. [84] Copyright 2015 The Royal Society of Chemistry.

Figure 6 .

 6 Figure 6. Plot of the monomer conversion ln([M]0/[M]t) vs. irradiation time while using 4 and 10 ppm of chlorophyll a as the photoredox catalyst during the polymerization of MMA in DMSO at room temperature upon irradiation at 635 mn. CPADB was used as the RAFT agent with a molar ratio of [MMA]:[CPADB] 200:1. Reproduced with permission from Shanmugam et al. [84] Copyright 2015 The Royal Society of Chemistry.

Figure 7 .

 7 Figure 7. A) Photochemical mechanism supporting the PET-RAFT polymerization. B) Chemical structure of chlorophyll a. Reproduced with permission from Shanmugam et al. [84] Copyright 2015 The Royal Society of Chemistry.

Figure 8 .

 8 Figure 8. UV-visible absorption spectra of BChl a and CPADB. Reproduced with permission from Shanmugam et al. [93] Copyright 2016 John Wiley & Sons, Inc.

Figure 9 .

 9 Figure 9. a) MMA conversion vs. time upon irradiation at 780 or 850 nm; b) temporal control of the polymerization process at the two afore-mentioned irradiation wavelengths. Reproduced with permission from Shanmugam et al. [93] Copyright 2016 John Wiley & Sons, Inc

Figure 10 .

 10 Figure 10. Final monomer conversion of MMA vs. the paper sheet thickness upon irradiation at 850 nm. Reproduced with permission from Shanmugam et al. [93] Copyright 2016 John Wiley & Sons, Inc.

Figure 11 .

 11 Figure 11. Chemical structures of the two photosensitizers (2HNQ, 5HNQ), the monomers, additives and sacrificial amines.

Figure 12 .

 12 Figure 12. UV-visible absorption spectra of 2HNQ and 5HNQ in acetonitrile. Reproduced with permission from Peng et al. [97] Copyright 2020 Elsevier.

Figure 13 .

 13 Figure 13. Photopolymerization profiles of TMPTA in laminate upon irradiation at 410 or 445 nm for different two-component systems (A) and three-component systems (B) (5HNQ, ITX: 0.5 wt%, Iod1, EDB, NPG: 2 wt%) Reproduced with permission from Peng et al. [97] Copyright 2020 Elsevier.

Figure 14 .

 14 Figure 14. Photolysis experiments of (A) paprika alone (B) the two-component paprika/Iod2 in toluene upon irradiation with a Xe lamp. Reproduced with permission from Sautrot-Ba. [101] Copyright 2018 American Chemical Society

Figure 15 .

 15 Figure 15. Mechanism supporting the CP of epoxides.

Figure 16 .

 16 Figure 16. Chemical structure of paprika used as the photosensitizer, and cationic photoinitiator, the monomer, and the singlet oxygen trap agent.

Figure 18 .

 18 Figure 18. Photopolymerization experiments of HDDA using different benzophenone-based photoinitiators (2 wt%). [sesamin] = 2 wt%. Reprinted with permission from Wang et al. [105] Copyright © 2011 Springer.

Figure 19 .

 19 Figure 19. Optical pyrometry monitoring of the photopolymerization of cyclohexene oxide with and without curcumin (0.1 mol%) in the presence of the iodonium salt IOC-10.SbF6 (1 mol%) using a lamp with emission maximum at 407 nm. Reprinted with permission from Crivello et al. [106] Copyright © 2005 John Wiley & Sons, Inc.

Figure 20 .

 20 Figure 20. Chemical structures of curcumin, the cationic photoinitiator and the epoxy monomers.

[ 118 ]Figure 21 .

 11821 Figure 21. Color change of the photocurable resin based on epoxidized linseed oil before (A) and after (B) sunlight irradiation. Reprinted with permission from Crivello et al. [106] Copyright © 2005 John Wiley & Sons, Inc.

Figure 23 .

 23 Figure 23. Chemical structures of the four dihydroxyanthraquinones used as the photosensitizers, the different monomers and the different additives examined in this work.

Figure 24 .

 24 Figure 24. UV-visible absorption spectra of three dihydroxyanthraquinones 14-DHAQ, 15-DHAQ, 18-DHAQ recorded in acetonitrile. Reproduced from ref. [133] with permission from The Royal Society of Chemistry.

Figure 25 .

 25 Figure[START_REF] Steyrer | Visible light photoinitiator for 3D-printing of tough methacrylate resins[END_REF]. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of dihydroxyanthraquinone using DFT at the UB3LYP/6-31G* level. Reproduced from ref.[START_REF] Zhang | Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization[END_REF] with permission from The Royal Society of Chemistry.

Figure 27 .

 27 Figure 27. Photolysis experiments of a) 15-DHAQ/TEAOH and b) 18-DHAQ/TEAOH in acetonitrile upon irradiation with blue LED bulb at 455 nm. Reproduced from ref. [133] with permission from The Royal Society of Chemistry.

Figure 28 .

 28 Figure 28. Chemical structures of different polyhydroxy-anthraquinones-based photoinitiators.

Figure 29 .

 29 Figure 29. UV-visible absorption and photoluminescence spectra of quercitin in acetonitrile.Reproduced from ref.[START_REF] Condat | Development of photoactivable glycerolbased coatings containing quercetin for antibacterial applications[END_REF] with permission from The Royal Society of Chemistry

Figure 31 .

 31 Figure 31. Bacterial proliferation of Staphylococcus aureus for quercitin-based coatings with and without light irradiation. Reproduced from ref. [159] with permission from The Royal Society of Chemistry Photopolymerization with flavones was not limited to the UV range and several examples of visible light photopolymerization have been reported since this pioneering work. Notably, use of the synthetic 3-hydroxyflavone (3HF) enabled the free radical polymerization of the BisGMA/TEGDMA blend upon irradiation with LEDs emitting at 405 and 477 nm.[100]However, scope of applications of 3HF was not limited to photosensitization and use of 3HF as a fluorescent probe to monitor the photopolymerization of an acrylate-based biocompatible monomer i.e. E-Shell 300 was also reported.[START_REF] Tomin | Use of 3-hydroxyflavone as a fluorescence probe for the controlled photopolymerization of the E-Shell 300 polymer[END_REF] Indeed, during photopolymerization, a 1.8fold enhancement of the photoluminescence intensity of 3HF could be demonstrated, enabling to correlate monomer conversion and photoluminescence intensity. More recently, a series of flavones bearing halogens was also developed and proved to be exceptional near-visible photoinitiators for the FRP of tripropylene glycol diacrylate (TPGDA).[START_REF] You | Flavonol dyes with different substituents in photopolymerization[END_REF]

Figure 33 .

 33 Figure 33. Polymerization profiles obtained during the FRP of TMPTA in laminate a) PM (1 wt%) b) BAPO (1 wt%); during the FRP of SOA c) PM (1 wt%); d) BAPO (1 wt%) upon irradiation with a Hg-Xe lamp (60 mW/cm²). Reprinted from ref. [178] Copyright 2020, with permission from Elsevier.

Figure 36 .

 36 Figure 36. UV-visible absorption spectra of QA, QE and quinizarin (QZ) in acetonitrile. Reproduced with permission from Sautrot-Ba [223]. Copyright 2020 American Chemical Society

Figure 37 .

 37 Figure 37. Mechanism involved in the thiol-ene reaction for the two-component quinizarin/Iod1 system.

Table 1 .

 1 Comparisons between natural and synthetic dyes.

	Parameters	Natural dyes	Synthetic dyes
	Cost/synthesis	Compounds extracted from	Synthetic dyes can compete
		plants. Depending on the	with natural dyes in terms
		availability, natural dyes can	of cost. Notably, a great deal
		be low-cost.	of efforts is devoted to
			develop dyes accessible by
			mean of a one-step
			procedure from inexpensive
			reagents.
	Environmental impact	Common extraction
		procedures are mostly based
		on water-based extraction
		process, without impact for
		the environment.

Table 2 .

 2 Final monomer conversions (%) determined after 800 s of irradiation.

				405 nm	405 nm	455 nm	470 nm	Xe Lamp
			function	60	100	38	25	60
				mw/cm²	mw/cm²	mw/cm²	mw/cm²	mw/cm²
	β-carotene/Iod/	epoxy	84	84	70	74	75
	trithiol	(1/3/6%	ene	77	50	50	63	68
	w/w/w)
	ITX/Iod/	trithiol	epoxy	76	76	-	-	76
	(1/3/6% w/w/w)	ene	63	67	-	-	67

Table 3 .

 3 Final monomer conversions (%) determined after 800 s of irradiation.

				405 nm	405 nm	455 nm	470 nm	Xe Lamp
			function	60	100	38	25	60
				mw/cm²	mw/cm²	mw/cm²	mw/cm²	mw/cm²
	β-carotene/Iod/	epoxy	75	80	61	59	71
	trithiol	(1/3/6%	lim-ene	23	24	19	34	38
	w/w/w)		eug-ene	31	20	17	26	28
	ITX/Iod/	trithiol	epoxy	58	56	-	-	73
	(1/3/6% w/w/w)	lim-ene	24	33	-	-	35
			eug-ene	21	27	-	-	13
	CQ/Iod/	trithiol	epoxy	-	-	33	53	44
	(1/3/6% w/w/w)	lim-ene	-	-	59	24	54
			eug-ene	-	-	40	39	39

Table 4 .

 4 EPOX and TMPTA conversions obtained with the two-component quinizarin/Iod1 (0.5%/2.0%, wt%) or the two-component quinizarin/MDEA (0.5%/2%, wt%) systems upon irradiation at different wavelengths for 800 s.

		EPOX conversion under air
	Xenon lamp	LED@405 nm	LED@455 nm	LED@470 nm
	CQ/Iod1	n.p.	n.p.	n.p.	n.p.
	QZ/Iod1	n.p.	n.p.	n.p.	n.p.
	QA/Iod1	76%	56%	43%	52%
	QE/Iod1	57%	72%	58%	50%
		TMPTA conversion in laminate
	Xenon lamp	LED@405 nm	LED@455 nm	LED@470 nm
	CQ/MDEA	39	-	40	44
	QZ/MDEA	30	37	28	12
	QA/MDEA	55	69	54	44
	QE/MDEA	57	57	55	50
	CQ/Iod1	45	-	50	32
	QZ/Iod1	n.p.	15	n.p.	n.p.
	QA/Iod1	70	77	65	50
	QE/Iod1	75	80	66	36
	n.p. no polymerization.

Table 5 .

 5 Monomer conversion obtained during the photopolymerization of an EPOX/TMPTA (50/50 w/w) blend upon irradiation at 455 nm for 800 s.

	QA/Iod (0.5/2.0%, w/w)		QE/Iod (0.5/2.0%, w/w)
	under air	72	48	49	50
	in laminate	52	76	39	87

Table 6 .

 6 Monomer conversions obtained during the thiol-ene polymerization of a TMPTA/trithiol (47/57 w/w) blend upon irradiation under air and in laminate at different wavelengths.

		Xenon lamp	LED@405 nm	LED@455 nm	LED@470 nm
	CQ (0.5 wt%)	81(40)*	-	100 (89)	97 (68)
	CQ/Iod1	73 (n.p.)**	-	90 (57)	87 (76)
	(0.5%/2.0%, wt%)
	QZ (0.5 wt%)	75(42)	85 (90)	86 (90)	67 (n.p.)
	QZ/Iod1	91(62)	90 (85)	90 (73)	80 (15)
	(0.5%/2.0%, wt%)
	QA (0.5 wt%)	79 (70)	92 (91)	60 (67)	57 (25)
	QA/Iod1	98(97)	98 (95)	92 (95)	86 (84)
	(0.5%/2.0%, wt%)
	QE (0.5 wt%)	75 (80)	90 (96)	90 (45)	59 (n.p.)
	QE/Iod1	90 (98)	93 (100)	90 (98)	70 (40)
	(0.5%/2.0%, wt%)

* ( ) values obtained under air. ** n.p. no polymerization.

Table 7 .

 7 Summary of the photopolymerization characteristics and photophysical characteristics of the different dyes

		Ph 2 I +
		PI*
	RSH	Ph •
		PI +
	RS •	RSH
		RS •

Acknowledgment

This research was funded by Aix Marseille University and The Centre National de la Recherche (CNRS). This research was also funded by the Agence Nationale de la Recherche (ANR agency) through the PhD grant of Guillaume Noirbent (ANR-17-CE08-0054 VISICAT project).