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

Recent Advances on Ferrocene-based Photoinitiating Systems

Keywords: chalcone, push-pull dyes, photopolymerization, ferrocene, LED, ketone

Photoinitiators of polymerization with reversible electrochemical properties are actively researched as these molecules can advantageously be used in photocatalytic systems, enabling to reduce the photoinitiators content within the photocurable resins. In this field, ferrocene which is extensively used by electrochemists as a reference compound for cyclic voltammetry also exhibits a significant absorption in the visible range as well as a low oxidation potential so that this metallocene was used as a potential candidate for the design of photoinitiators of polymerization. Over the years, ferrocene has been examined in numerous polymerization processes going from anionic to cationic polymerizations, but also for the freeradical polymerization of acrylates or as a sacrificial electron donor in free radical polymerization processes. Parallel to this, the development of cheap, compact and energysaving irradiation setups based on light-emitting diodes (LEDs) have clearly favored the development of visible light photoinitiators and this technology has nowadays the potential to replace the well-established UV photopolymerization. In this review, an overview of the recent development of the ferrocene-based photoinitiating systems is provided. To evidence the interest of the ferrocene derivatives recently developed, comparisons with benchmark photoinitiators will be provided.

Introduction

Photopolymerization consists in converting a liquid monomer into a solid in the presence of light. [1][START_REF] Mendes-Felipe | Stateof-the-art and future challenges of UV curable polymer-based smart materials for printing technologies[END_REF][START_REF] Shukla | Review of basic chemistry of UVcuring technology[END_REF][START_REF] Chen | Light-controlled radical polymerization: mechanisms, methods, and applications[END_REF] Beyond this simple photoassisted physical transformation, numerous chemical mechanisms differing by the way how the initiating species are formed support this irreversible chemical transformation. [START_REF] Fouassier | Photopolymerization reactions under visible lights: principle, mechanisms and examples of applications[END_REF][START_REF] Garra | Photopolymerization of thick films and in shadow areas: A review for the access to composites[END_REF][START_REF] Sangermano | New Horizons in Cationic Photopolymerization[END_REF] Photopolymerization is a polymerization technique which is clearly less studied than the classical thermal polymerization and the difference of publications between the two polymerization modes attests of the higher interest for the classical thermal polymerization done in solution. Besides, photopolymerization exhibits several appealing features that can't be denied. Notably, the possibility to get a spatial and a temporal control of the polymerization process, the ability to polymerize without solvent enabling to drastically reduce the emission of volatile organic compounds (VOCs) can be cited as appealing features. [START_REF] Fouassier | Photoinitiators for polymer synthesis-scope, reactivity, and efficiency[END_REF][START_REF] Lin | Dual-wavelength (UV and blue) controlled photopolymerization confinement for 3D-printing: Modeling and analysis of measurements[END_REF] Recently, photopolymerization has known a revival of interest with the development of the 3D/4D printing technologies, and a great deal of efforts has thus been devoted to elaborate dyes strongly absorbing at 405 nm, which is the wavelength currently under use in 3D printers. [START_REF] Crivello | Photopolymer materials and processes for advanced technologies[END_REF][START_REF] Genty | Fast polymerization at low temperature of an infrared radiation cured epoxy-amine adhesive[END_REF][START_REF] Zhang | A versatile 3D and 4D printing system through photocontrolled raft polymerization[END_REF][START_REF] Lee | Two-way 4D-printing: A review on the reversibility of 3dprinted shape memory materials[END_REF][START_REF] Bagheri | Photopolymerization in 3D Printing[END_REF][START_REF] Aduba | Vat photopolymerization 3D printing of acid-cleavable PEG-methacrylate networks for biomaterial applications[END_REF][START_REF] Dietlin | Photopolymerization upon LEDs: new photoinitiating systems and strategies[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] Lee | Rapid High-Resolution 3D Printing and Surface Functionalization via Type I Photoinitiated RAFT Polymerization[END_REF] With aim at reducing the environmental impact of photopolymerization, greener processes are now widely explored and the use of biosourced photoinitiators and monomers, [START_REF] Noirbent | Photoinitiators of polymerization with reduced environmental impact: Nature as an unlimited and renewable source of dyes[END_REF][START_REF] Guit | Photopolymer Resins with Biobased Methacrylates Based on Soybean Oil for Stereolithography[END_REF][START_REF] Al Mousawi | Flavones as natural photoinitiators for light mediated free radical polymerization via light emitting diodes[END_REF][START_REF] Noè | Cationic photopolymerization of bio-renewable epoxidized monomers[END_REF][START_REF] Breloy | β-carotene/limonene derivatives/eugenol: green synthesis of antibacterial coatings under visible-light exposure[END_REF][START_REF] Sautrot-Ba | Paprika, gallic acid, and visible light: the green combination for the synthesis of biocide coatings[END_REF][START_REF] Condat | Photoinduced curcumin derivativecoatings with antibacterial properties[END_REF][START_REF] Condat | Development of photoactivable glycerolbased coatings containing quercetin for antibacterial applications[END_REF][START_REF] Breloy | Vanillin derivative as performing type I photoinitiator[END_REF][START_REF] Sautrot-Ba | Quinizarin derivatives as photoinitiators for freeradical and cationic photopolymerizations in the visible spectral range[END_REF][START_REF] Tehfe | Photopolymerization Reactions: On the Way to a Green and Sustainable Chemistry[END_REF] or the use of sunlight as the irradiation source to initiate polymerization processes are now extensively studied. [START_REF] Allegrezza | Visible and sunlight driven RAFT photopolymerization accelerated by amines: kinetics and mechanism[END_REF][START_REF] Lalevée | Recent advances in sunlight induced polymerization: role of new photoinitiating systems based on the silyl radical chemistry[END_REF][START_REF] Samoilenko | Influence of N-vinylcarbazole on the photopolymerization process and properties of epoxy-acrylate interpenetrating polymer networks[END_REF][START_REF] Cataldo | On cyanogen photopolymerization[END_REF][START_REF] Corrigan | Seeing the Light: Advancing Materials Chemistry through Photopolymerization[END_REF][START_REF] Sheng | Brushing up from "anywhere" under sunlight: a universal surface-initiated polymerization from polydopaminecoated surfaces[END_REF] The progress in these two fields is considerable. With aim at generating initiating radicals, two strategies can be developed. The first one consists in using Type I photoinitiators i.e. cleavable molecules that will generate the initiating radicals upon photoexcitation. As the main drawback of this approach, Type I photoinitiators are irreversibly consumed during the polymerization process so that these molecules cannot be used in catalytic amount (See Figure 1). [36][START_REF] Eibel | Choosing the ideal photoinitiator for free radical photopolymerizations: predictions based on simulations using established data[END_REF] Conversely, Type II photoinitiators can be used in catalytic quantity, the regeneration of the photosensitizer being possible in this last case. [START_REF] Allushi | Conventional Type II photoinitiators as activators for photoinduced metal-free atom transfer radical polymerization[END_REF][START_REF] Dumur | Recent advances on pyrene-based photoinitiators of polymerization[END_REF][START_REF] Noirbent | Recent advances on naphthalic anhydrides and 1,8naphthalimide-based photoinitiators of polymerization[END_REF][START_REF]Recent advances on visible light photoinitiators of polymerization based on indane-1,3dione and related derivatives, F. Dumur[END_REF][START_REF]Recent advances on copper complexes as visible light photoinitiators and (photo)redox initiators of polymerization, G. Noirbent[END_REF][START_REF] Pigot | Recent advances on push-pull organic dyes as visible light photoinitiators of polymerization[END_REF][START_REF]Recent advances on carbazole-based photoinitiators of polymerization, F. Dumur[END_REF][START_REF]Recent advances on visible light metal-based photocatalysts for polymerization under low light intensity[END_REF] To illustrate this, a sacrificial amine (such as N-vinylcarbazole (NVK)) can be typically used for this purpose (See Figure 1).

Type I photoinitiators

Type II photoinitiators

Indeed, upon photoexcitation of the photosensitizer, an electron transfer between the photosensitizer and the iodonium salt (Iod) can occur in the excited state. As a result of this, a decomposition of the photoinitiator takes place, generating the initiating phenyl radicals Ph • and allowing the free radical polymerization (FRP) of acrylates to proceed. Reactivity of the phenyl radicals can be further improved by reaction with NVK, promoting the formation of the highly reactive Ph-NVK • radicals. [START_REF] Bonardi | Organometallic vs organic photoredox catalysts for photocuring reactions in the visible region[END_REF][START_REF] Fouassier | Photochemical production of interpenetrating polymer networks; simultaneous initiation of radical and cationic polymerization reactions[END_REF][START_REF] Tomal | Multifunctional biphenyl derivatives as photosensitisers in various types of photopolymerization processes, including IPN formation, 3D printing of photocurable multiwalled carbon nanotubes (MWCNTs) fluorescent composites[END_REF][START_REF] Hola | New, highly versatile bimolecular photoinitiating systems for free-radical, cationic and thiolene photopolymerization processes under low light intensity UV and visible LEDs for 3D printing application[END_REF][START_REF] Andrzejewska | Recent advances in photo-induced free-radical polymerization[END_REF] These radicals Ph-NVK • can also react with the oxidized photosensitizer so that the cations Ph-NVK + can be generated, these cations being capable to initiate a free radical-promoted cationic polymerization (FRPCP) of epoxides.

Beyond the initiating mechanism, the monomer conversion and the reaction time are also strongly dependent of the light penetration within the photocurable resin. [START_REF] Bonardi | High performance near infrared (NIR) photoinitiating systems operating under low light intensity and in presence of oxygen[END_REF] Indeed, as shown in the Figure 2, if the light penetration is limited to a few hundreds of micrometers at 300 nm, conversely, at 800 nm, this latter can reach five centimeters, enabling to polymerize thick samples. In light of these considerations, it explains the ever-growing interest for developing visible light photoinitiators.

Figure 2.

Light penetration within a photocurable resin composed of a polystyrene latex with an average diameter of 112 nm. Reproduced with permission from Bonardi et al. [START_REF] Bonardi | High performance near infrared (NIR) photoinitiating systems operating under low light intensity and in presence of oxygen[END_REF].

Copyright 2018 American Chemical Society.

Over the years, a wide range of families have been designed and synthesized as efficient photoinitiators of polymerization, activable under visible light and even under low light intensity. Among these structures, naphthalimides, [START_REF] Noirbent | Recent advances on naphthalic anhydrides and 1,8naphthalimide-based photoinitiators of polymerization[END_REF][START_REF] Bonardi | New 1,8-naphthalimide derivatives as photoinitiators for free radical polymerization upon visible light[END_REF][START_REF] Zhang | Naphthalimide-tertiary amine derivatives as blue-light-sensitive photoinitiators[END_REF][START_REF] Xiao | Naphthalimide derivatives: substituent effects on the photoinitiating ability in polymerizations under near UV, purple, white and blue LEDs (385 nm, 395 nm, 405 nm, 455 nm or 470 nm)[END_REF][START_REF] Xiao | Naphthalimide-phthalimide derivative based photoinitiating systems for polymerization reactions under blue lights[END_REF][START_REF] Zhang | A benzophenone-naphthalimide derivative as versatile photoinitiator for near UV and visible lights[END_REF][START_REF] Zhang | N-[2-(dimethylamino)ethyl]-1,8-naphthalimide derivatives as photoinitiators under LEDs[END_REF][START_REF] Zivic | Novel naphthalimideamine based photoinitiators operating under violet and blue LEDs and usable for various polymerization reactions and synthesis of hydrogels[END_REF][START_REF] Zhang | Structure design of naphthalimide derivatives: towards versatile photoinitiators for near UV/Visible LEDs, 3D printing and water-soluble photoinitiating systems[END_REF][START_REF] Zhang | UV violet-blue LED induced polymerizations: specific photoinitiating systems at 365, 385, 395 and 405 nm[END_REF][START_REF] Xiao | Blue light sensitive dyes for various photopolymerization reactions: naphthalimide and naphthalic anhydride derivatives[END_REF][START_REF] Xiao | Naphthalimide based methacrylated photoinitiators in radical and cationic photopolymerization under visible light[END_REF] copper complexes, [START_REF] Xiao | Copper complexes in radical photoinitiating systems: applications to free radical and cationic polymerization under visible lights[END_REF][START_REF] Xiao | Copper complexes: the effect of ligands on their photoinitiation efficiencies in radical polymerization reactions under visible light[END_REF][START_REF] Xiao | Copper and iron complexes as visible-light-sensitive photoinitiators of polymerization[END_REF][START_REF] Al Mousawi | Copper photoredox catalysts for polymerization upon near UV or visible light: structure/reactivity/ efficiency relationships and use in LED projector 3D printing resins[END_REF][START_REF] Garra | New copper(I) complex based initiating systems in redox polymerization and comparison with the amine/benzoyl peroxide reference[END_REF][START_REF] Garra | Mechanosynthesis of a copper complex for redox initiating systems with a unique near infrared light activation[END_REF][START_REF] Mokbel | Copper PhotoRedox Catalyst "G1": A New High Performance Photoinitiator for Near-UV and Visible LEDs[END_REF][START_REF] Garra | Mechanosynthesized Copper (I) complex based initiating systems for redox polymerization: towards upgraded oxidizing and reducing agents[END_REF][START_REF] Garra | Copper-based (photo) redox initiating systems as highly efficient systems for interpenetrating polymer network preparation[END_REF][START_REF] Garra | Stable copper acetylacetonate-based oxidizing agents in redox (NIR photoactivated) polymerization: an opportunity for one pot grafting from approach and example on a 3D printed object[END_REF][START_REF] Mokbel | Simultaneous initiation of radical and cationic polymerization reactions using the "G1" copper complex as photoRedox catalyst: Applications of free radical/cationic hybrid photo-polymerization in the composites and 3D printing fields[END_REF] flavones, [START_REF] Guit | Photopolymer Resins with Biobased Methacrylates Based on Soybean Oil for Stereolithography[END_REF][START_REF] Al Mousawi | 3-Hydroxyflavone and N-phenyl-glycine in high performance photo-initiating systems for 3D printing and photocomposites synthesis[END_REF] iron complexes, [START_REF] Zhang | Iron complexes as photoinitiators for radical and cationic polymerization through photoredox catalysis processes[END_REF][START_REF] Telitel | Iron complexes as potential photocatalysts for controlled radical photopolymerizations: A tool for modifications and patterning of surfaces[END_REF][START_REF] Zhang | Visiblelight-sensitive photoredox catalysis by iron complexes: applications in cationic and radical polymerization reactions[END_REF][START_REF] Zhang | Novel iron complexes in visible-light-sensitive photoredox catalysis: effect of ligands on their photoinitiation efficiencies[END_REF][START_REF] Zhang | Iron-based metal-organic frameworks (MOF) as photocatalysts for radical and cationic polymerizations under near UV and visible LEDs (385 -405 nm)[END_REF] iodonium salts, [START_REF] Mokbel | Specific cationic photoinitiators for Near UV and visible LEDs: Iodonium vs. ferrocenium structures[END_REF][START_REF] Villotte | Design of Iodonium Salts for UV or Near-UV LEDs for Photoacid Generator and Polymerization Purposes[END_REF][START_REF] Zivic | A novel naphthalimide scaffold based iodonium salt as a one-component photoacid/photoinitiator for cationic and radical polymerization under LED exposure[END_REF] coumarins, [START_REF] Abdallah | Coumarin derivatives as high performance visible light photoinitiators/photoredox catalysts for photosensitive resins for 3D printing technology, photopolymerization in water and photocomposites synthesis[END_REF][START_REF] Li | Coumarin-Based Oxime Esters: Photobleachable and Versatile Unimolecular Initiators for Acrylate and Thiol-Based Click Photopolymerization under Visible Light-Emitting Diode Light Irradiation[END_REF][START_REF] Abdallah | Keto-coumarin scaffold for photo-initiators for 3D printing and photo-composites[END_REF][START_REF] Abdallah | Coumarins as powerful photosensitizers for the cationic polymerization of epoxy-silicones under near-UV and visible light and applications for 3D printing technology[END_REF][START_REF] Chen | Photopolymerization of coumarincontaining reversible photoresponsive materials based on wavelength selectivity[END_REF][START_REF] Rahal | Mono vs. difunctional coumarin as photoinitiators in photocomposite synthesis and 3D printing[END_REF][START_REF] Abdallah | Coumarin derivatives as versatile photoinitiators for 3D printing, polymerization in water and photocomposite synthesis[END_REF], acridine-1,8dione, [START_REF] Tehfe | Novel highly efficient organophotocatalysts: truxene-acridine-1,8-diones as photoinitiators of polymerization[END_REF][START_REF] Xiao | Acridinediones: effect of substituents on their photoinitiating abilities in radical and cationic photopolymerization[END_REF] iridium complexes, [START_REF] Lalevée | Subtle ligand effects in oxidative photocatalysis with iridium complexes: application to photopolymerization[END_REF][START_REF] Lalevée | Iridium photocatalysts in free radical photopolymerization under visible lights[END_REF][START_REF] Lalevée | Photopolymerization of N-vinylcarbazole using visible-light harvesting iridium complexes as photoinitiators[END_REF][START_REF] Telitel | Photoredox catalysis using a new iridium complex as an efficient toolbox for radical, cationic and controlled polymerizations under soft blue to green lights[END_REF][START_REF] Telitel | Photoredox process induced polymerization reactions: iridium complexes for panchromatic photoinitiating systems[END_REF][START_REF] Tehfe | Structural effects in the iridium complex series: photoredox catalysis and photoinitiation of polymerization reactions under visible lights[END_REF][START_REF] Dumur | Cationic iridium complex for the design of soft salt-based phosphorescent OLEDs and color-tunable Light-Emitting Electrochemical Cells[END_REF][START_REF] Nasr | Iridium (III) soft salts from dinuclear cationic and mononuclear anionic complexes for OLEDs devices[END_REF][START_REF] Dumur | Iridium (III) complexes as promising emitters for solidstate light-emitting electrochemical cells (LECs)[END_REF] dihydroanthraquinone, [START_REF] Zhang | Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization[END_REF] helicenes, [START_REF] Al Mousawi | Azahelicenes as visible light photoinitiators for cationic and radical polymerization: preparation of photoluminescent polymers and use in high performance LED projector 3D printing resins[END_REF][START_REF] Al Mousawi | Visible light chiral photoinitiator for radical polymerization and synthesis of polymeric films with strong chiroptical activity[END_REF] porphyrins, [START_REF] Bonardi | Different NIR dye scaffolds for polymerization reactions under NIR light[END_REF][START_REF] Al Mousawi | Zinc tetraphenylporphyrin as high performance visible-light photoinitiator of cationic photosensitive resins for LED projector 3D printing applications[END_REF][START_REF] Noirbent | Metalated Porphyrins as versatile visible light and NIR photoinitiators of polymerization[END_REF] cyclohexanones, [START_REF] Li | The effect of polyethylene glycoldiacrylate complexation on type II photoinitiator and promotion for visible light initiation system[END_REF][START_REF] Li | In situ monitoring of photopolymerization by photoinitiator with luminescence characteristics[END_REF][START_REF] Li | Synthesis of furan derivative as LED light photoinitiator: One-pot, low usage, photobleaching for light color 3D printing[END_REF][START_REF] Xu | Novel ketone derivatives based photoinitiating systems for free radical polymerization under mild conditions and 3D printing[END_REF] thioxanthones, [START_REF] Karaca | Thioxanthone-benzothiophenes as photoinitiator for free radical polymerization[END_REF][START_REF] Wu | Thioxanthone based one-component polymerizable visible light photoinitiator for free radical polymerization[END_REF][START_REF] Qiu | Thioxanthone photoinitiator containing polymerizable N-aromatic maleimide for photopolymerization[END_REF][START_REF] Dadashi-Silab | Shining a light on an adaptable photoinitiator: advances in photopolymerizations initiated by thioxanthones[END_REF] acridones, [START_REF] Abdallah | Acridone derivatives as high performance visible light photoinitiators for cationic and radical photosensitive resins for 3D printing technology and for low migration photopolymer property[END_REF][START_REF] Zhang | Novel panchromatic photopolymerizable matrices: N,N'-dibutyl-quinacridone as an efficient and versatile photoinitiator[END_REF] zinc complexes, [117] squaraines, [START_REF] Bonardi | Different NIR dye scaffolds for polymerization reactions under NIR light[END_REF][START_REF] Launay | NIR dyes as innovative tools for reprocessing/recycling of plastics: Benefits of the photothermal activation in the near-infrared range[END_REF] cyanines, [START_REF] Bonardi | New 1,8-naphthalimide derivatives as photoinitiators for free radical polymerization upon visible light[END_REF][START_REF] Bonardi | Different NIR dye scaffolds for polymerization reactions under NIR light[END_REF][START_REF] Mokbel | On demand NIR activated photopolyaddition reactions[END_REF] phenothiazines, [START_REF] Abdallah | Phenothiazine derivatives as photoredox catalysts for cationic and radical photosensitive resins for 3D printing technology and photocomposites synthesis[END_REF] camphorquinones, [START_REF] Kamoun | Carboxylated camphorquinone as visible-light photoinitiator for biomedical application: Synthesis, characterization, and application[END_REF][START_REF] Santini | Photoinitiators in dentistry: a review[END_REF] chromones, [START_REF] Al Mousawi | 3-Hydroxyflavone and N-phenyl-glycine in high performance photo-initiating systems for 3D printing and photocomposites synthesis[END_REF][START_REF] Tehfe | New chromone based photoinitiators for polymerization reactions upon visible lights[END_REF][START_REF] You | Flavonol dyes with different substituents in photopolymerization[END_REF] benzophenones, [START_REF] Tehfe | Design of new Type I & Type II photoinitiators possessing highly coupled pyrene-ketone moieties[END_REF][START_REF] Xiao | Variations on the benzophenone skeleton: novel high-performance blue light sensitive photoinitiating systems[END_REF][START_REF] Zhang | Design of novel photoinitiators for radical and cationic photopolymerizations under Near UV and Visible LEDs (385, 395 and 405 nm)[END_REF][START_REF] Liu | New multifunctional benzophenone-based photoinitiators with high migration stability and their application in 3D printing[END_REF][START_REF] Liu | Novel photoinitiators based on benzophenone-triphenylamine hybrid structure for led photopolymerization[END_REF] chalcones, [START_REF] Abdallah | Development of new high performance visible light photoinitiators based on carbazole scaffold and their applications in 3D printing and photocomposite synthesis[END_REF][START_REF] Tehfe | Chalcone derivatives as highly versatile photoinitiators for radical, cationic, thiol-ene and IPN polymerization reactions upon visible lights[END_REF][START_REF] Tang | A high performance phenyl-free LED photoinitiator for cationic or hybrid photopolymerization and its application in LED cationic 3D printing[END_REF][START_REF] Xu | Allyloxy ketones as efficient photoinitiators with high migration stability in free radical polymerization and 3D printing[END_REF][START_REF] Xu | Design of ketone derivatives as highly efficient photoinitiators for free radical and cationic photopolymerizations and application in 3D printing of composites[END_REF][START_REF] Chen | Novel D-π-A and A-π-D-π-A three-component photoinitiating systems based on carbazole/triphenylamino based chalcones and application in 3D and 4D Printing[END_REF][START_REF] Chen | Bis-chalcone derivatives derived from natural products as near-UV/visible light sensitive photoinitiators for 3D/4D printing[END_REF] perylenes, [START_REF] Xiao | Perylene derivatives as photoinitiators in blue light sensitive cationic or radical curable films and panchromatic thiol-ene polymerizable films[END_REF][START_REF] Tehfe | Green light induced cationic ring opening polymerization reactions: perylene-3,4:9,10bis(dicarboximide) as efficient photosensitizers[END_REF][START_REF] Xiao | Red-light-induced cationic photopolymerization: perylene derivatives as efficient photoinitiators[END_REF] pyrenes, [START_REF] Dumur | Recent advances on pyrene-based photoinitiators of polymerization[END_REF][START_REF] Tehfe | Design of new Type I & Type II photoinitiators possessing highly coupled pyrene-ketone moieties[END_REF][START_REF] Tehfe | New insights in radical and cationic polymerizations upon visible light exposure: role of novel photoinitiator systems based on the pyrene chromophore[END_REF][START_REF] Telitel | New core-pyrene π-structure organophotocatalysts usable as highly efficient photoinitiators[END_REF][START_REF] Uchida | nonsalt 1-(arylmethyloxy)pyrene photoinitiators capable of initiating cationic polymerization[END_REF][START_REF] Mishra | 1-(Bromoacetyl)pyrene, a novel photoinitiator for the copolymerization of styrene and methylmethacrylate[END_REF][START_REF] Tehfe | New insights into radical and cationic polymerizations upon visible light exposure: Role of novel photoinitiator systems based on the pyrene chromophore[END_REF] 2,3-diphenylquinoxaline derivatives, [START_REF] Xiao | Multicolor photoinitiators for radical and cationic polymerization: mono vs. poly functional thiophene derivatives[END_REF] carbazoles, [START_REF] Abdallah | Development of new high performance visible light photoinitiators based on carbazole scaffold and their applications in 3D printing and photocomposite synthesis[END_REF][START_REF] Xiao | Multicolor photoinitiators for radical and cationic polymerization: mono vs. poly functional thiophene derivatives[END_REF][START_REF] Zhang | The carbazole-bound ferrocenium salt as a specific cationic photoinitiator upon near-UV and visible LEDs (365-405 nm)[END_REF][START_REF] Al Mousawi | Carbazole scaffold based photoinitiators/photoredox catalysts for new LED projector 3D printing resins[END_REF][START_REF] Al Mousawi | Carbazole derivatives with thermally activated delayed fluorescence property as photoinitiators/photoredox catalysts for LED 3D printing technology[END_REF][START_REF] Al Mousawi | Novel Carbazole Skeleton-Based Photoinitiators for LED Polymerization and LED Projector 3D Printing[END_REF] diketopyrrolopyrrole, [START_REF] Al Mousawi | Carbazole-based compounds as photoinitiators for free radical and cationic polymerization upon near visible light illumination[END_REF][START_REF] Zhang | Panchromatic photoinitiators for radical, cationic and thiol-ene polymerization reactions: a search in the diketopyrrolopyrrole or indigo dye series[END_REF][START_REF] Xiao | Diketopyrrolopyrrole dyes: structure/reactivity/efficiency relationship in photoinitiating systems upon visible lights[END_REF] and push-pull dyes [START_REF] Xiao | Green light sensitive diketopyrrolopyrrole derivatives used in versatile photoinitiating systems for photopolymerizations[END_REF][154][START_REF] Xiao | Julolidine or fluorenone based push-pull dyes for polymerization upon soft polychromatic visible light or green light[END_REF][START_REF] Tehfe | New push-pull dyes derived from Michler's ketone for polymerization reactions upon visible lights[END_REF][START_REF] Mokbel | End capped polyenic structures as visible light sensitive photoinitiators for polymerization of vinylethers[END_REF][START_REF] Garra | Ferrocene-based (photo)redox polymerization under long wavelengths[END_REF][START_REF] Telitel | The 1,3-bis(dicyanomethylidene)-indane skeleton as a (photo) initiator in thermal ring opening polymerization at RT and radical or cationic photopolymerization[END_REF][START_REF] Mokbel | Michler's ketone as an interesting scaffold for the design of highperformance dyes in photoinitiating systems upon visible lights[END_REF][START_REF] Sun | Novel push-pull dyes derived from 1Hcyclopenta[b]naphthalene-1,3(2H)-dione as versatile photoinitiators for photopolymerization and their related applications: 3D-printing and fabrication of photocomposites[END_REF][START_REF] Tehfe | Push-pull (thio)barbituric acid derivatives in dye photosensitized radical and cationic polymerization reactions under 457/473 nm laser beams or blue LEDs[END_REF][START_REF] Dumur | Organic Electronics: an El Dorado in the quest of new photoCatalysts as photoinitiators of polymerization[END_REF] can be cited as the structures focusing the most intense research efforts at present. With aim at developing photocatalytic systems, reversibility of the redox processes occurring on the photosensitizer side is of crucial importance in order the regeneration of the photosensitizer to be possible. [START_REF] Corrigan | Photocatalysis in organic and polymer synthesis[END_REF] If the reversibility of the redox processes is researched, ferrocene is an ideal candidate (See Figure 3). Ferrocene which is a metallocene discovered in 1951 is characterized by a reversible one-electron oxidation process at potential lower than 0.5 V so that ferrocene can be combined with numerous photoinitiators. [START_REF] Kauffman | The discovery of ferrocene, the first sandwich compound[END_REF][START_REF] Kealy | A new type of organo-iron compound[END_REF][START_REF] Miller | Dicyclopentadienyliron[END_REF][START_REF] Werner | At least 60 years of ferrocene: the discovery and rediscovery of the sandwich complexes[END_REF][START_REF] Astruc | Why is ferrocene so exceptional?[END_REF][START_REF] Brunel | Ferrocene: An unrivaled electroactive building block for the design of push-pull dyes with near-infrared and infrared absorptions[END_REF][START_REF] Bao | Electrochemical oxidation of ferrocene: A strong dependence on the concentration of the supporting electrolyte for nonpolar solvents[END_REF][172] This metal complex is extensively used in electrochemistry as a reference compound for calibration. Considering its low oxidation potential, ferrocene is also an excellent electron-donating group so that numerous push-pull dyes were designed with this iron (II) complex. [START_REF] Ziessel | Boron Dipyrrin Dyes Exhibiting "Push-Pull-Pull" Electronic Signatures[END_REF][START_REF] Kaur | Second-order nonlinear polarizability of ferrocene-BODIPY donor-acceptor adducts. Quantifying charge redistribution in the excited state[END_REF][START_REF] Dhokale | Donor-acceptor perylenediimide-ferrocene conjugates: synthesis, photophysical, and electrochemical properties[END_REF][START_REF] Melomedov | Aminoferrocene and Ferrocene Amino Acid as Electron Donors in Modular Porphyrin-Ferrocene and Porphyrin-Ferrocene-Porphyrin Conjugates[END_REF][START_REF] Fabbrizzi | The ferrocenium/ferrocene couple: a versatile redox switch[END_REF][START_REF] Yokota | Computational investigations of electronic structure modifications of ferroceneterminated self-assembled monolayers: effects of electron donating/withdrawing functional groups attached on the ferrocene moiety[END_REF][START_REF] Manfredi | Ferrocene Derivatives Functionalized with Donor/Acceptor (Hetero)Aromatic Substituents: Tuning of Redox Properties[END_REF][START_REF] Nishikitani | Photo-and Electrochemical Properties of Linked Ferrocene and Viologen Donor-Acceptor-Type Molecules and Their Application to Electrochromic Devices[END_REF] This metallocene which is a cheap compound is also characterized by a low toxicity so that this complex is an ideal candidate for the design of photoinitiators. [START_REF] Patra | The Medicinal Chemistry of Ferrocene and its Derivatives[END_REF][START_REF] Babin | Ferrocenes as potential anticancer drugs, Facts and hypotheses[END_REF][START_REF] Lewkowski | Synthesis and ecotoxicological impact of ferrocene-derived aminophosphonates using a battery of bioassays[END_REF][START_REF] Singh | Ferrocene-appended pharmacophores: an exciting approach for modulating the biological potential of organic scaffolds[END_REF][START_REF] Peter | Ferrocene-Based Compounds with Antimalaria/Anticancer Activity[END_REF][START_REF] Emel'yanov | Ferrocene -a Nontoxic Antiknock Agent for Automotive Gasolines[END_REF][START_REF] Goermen | Synthesis, Cytotoxicity, and COMPARE Analysis of Ferrocene and [3]Ferrocenophane Tetrasubstituted Olefin Derivatives against Human Cancer Cells[END_REF] Compared to the aforementioned dyes, ferrocene-based photoinitiating systems exhibit advantages and disadvantages that are listed in the Table 1. Synthetic dyes are also photochemically stable.

Absorption range

Absorption spectra of ferrocene derivatives can be easily tuned so that dyes absorbing over the whole visible range can be designed

The same holds true for synthetic dyes.

Photoinitiating ability

Ferrocene derivatives can compete with benchmark photoinitiators. Besides, the reversibility of the redox process occurring on the ferrocene side can be a crucial advantage for the design of photocatalytic systems.

Synthetic dyes can compete with benchmark photoinitiators. Besides, reversibility of the redox process is not still ensured.

Availability

Ferrocene is available from numerous suppliers.

Most of the chemical used to elaborate dyes are also accessible. The limitation can be the cost for some of the starting materials.

Bioactivity

Ferrocene-based dyes can exhibit biological activities [START_REF] Peter | Atim Aderibigbe, Ferrocene-Based Compounds with Antimalaria/Anticancer Activity[END_REF][START_REF] Singh | Ferrocene-appended pharmacophores: an exciting approach for modulating the biological potential of organic scaffolds[END_REF][START_REF] Larik | Recent advances in the synthesis, biological activities and various applications of ferrocene derivatives[END_REF][START_REF] Narvaez-Pita | Ferrocene-steroid conjugates: Synthesis, structure and biological activity[END_REF][START_REF] Asghar | Bioactivity of new ferrocene incorporated N,N'-disubstituted ureas: Synthesis, structural elucidation and DFT study[END_REF][START_REF] Mbaba | Ferrocenyl and organic novobiocin derivatives: Synthesis and their in vitro biological activity[END_REF] Synthetic dyes can also exhibit biological activities.

In this review, an overview of the ferrocene-based photoinitiating systems reported to date in the literature is provided. Over the years, several polymerization techniques were investigated, ranging from cationic and anionic polymerizations to the free radical polymerization, the redox polymerization or the anionic ring-opening polymerization. To evidence the interest of the ferrocene-based photoinitiating systems, comparisons with benchmark photoinitiators will be provided.

Anionic polymerization of alkyl 2-cyanoacrylates

Ferrocene and alkyl-substituted ferrocenes

Photoassisted anionic polymerization has long been examined but remains less studied than the free radical polymerization or the cationic polymerization due to the longer reaction time this polymerization technique requires compared to the two others. [START_REF] Zivic | Recent advances and challenges in the design of organic photoacid and photobase generators for polymerizations[END_REF] Recently, alternatives to the conventional light-curing strategies based on photobase generations has been proposed in the literature, [START_REF] Kutal | A Novel Strategy for Photoinitiated Anionic Polymerization[END_REF][START_REF] Paul | Photoinduced Anionic Polymerization of Cyanoacrylates using Substituted Pyridine Pentacarbonyl Complexes of Tungsten or Chromium[END_REF][START_REF] Palmer | A New Photoinitiator Anionic Polymerization[END_REF][START_REF] Yamaguchi | Ferrocenes as Anionic Photoinitiators[END_REF][START_REF] Arsu | Photoinitiated Zwitterionic Polymerization of Alkyl Cyanoacrylates by Pyridinium Salts[END_REF][START_REF] Onen | Photoinitiated Polymerization of Ethyl Cyanoacrylate by Phosphonium Salts[END_REF] notably based on photoinduced electron transfer (PET) processes. [START_REF] Fink | Reactive Polymers Fundamentals and Applications[END_REF][START_REF] Burns | Polycyanoacrylates[END_REF][START_REF] Shantha | Developments and Applications of Cyanoacrylate Adhesives[END_REF] Typically, these strategies are based on the photooxidation of a metal complex when opposed to the appropriate electron-deficient partner. Using this strategy, a wide range of structures have been examined, including inorganic complexes (trans-Cr(NH3)2(NCS)4 -1 , Pt(acac)2 (with acac standing for acetylacetonate)), [START_REF] Palmer | A New Photoinitiator Anionic Polymerization[END_REF][START_REF] Lavallee | Efficient Substitutional Photochemistry of a Third-Row Transition Metal β-Diketonate Complex[END_REF] organometallic complexes (Cr(CO)5L and W(CO)5L with L standing for pyridine and related derivatives), [START_REF] Paul | Photoinduced Anionic Polymerization of Cyanoacrylates using Substituted Pyridine Pentacarbonyl Complexes of Tungsten or Chromium[END_REF] porphyrins, [START_REF] Aida | Metalloporphyrins as Initiators for Living and Immortal polymerizations[END_REF][START_REF] Sugimoto | Polymerization by Metalloporphyrin and Related Complexes[END_REF] pyridinium [START_REF] Arsu | Photoinitiated Zwitterionic Polymerization of Alkyl Cyanoacrylates by Pyridinium Salts[END_REF] and phosphonium [START_REF] Onen | Photoinitiated Polymerization of Ethyl Cyanoacrylate by Phosphonium Salts[END_REF] salts, violet leuconitrile and malachite green leucohydroxide. [START_REF] Jarikov | Anionic Photopolymerization of Methyl 2-Cyanoacrylate and Simultaneous Color Formation[END_REF] Even if the mechanism of generation of active initiating species differs with the different aforementioned photoinitiators, as common point, an anion is released during photoexcitation so that the chain growth can rapidly propagate by addition of repeating monomer units, according to the mechanism proposed in the Scheme 1. Scheme 1. Mechanism occurring while using the inorganic complex trans-Cr(NH3)2(NCS)4 -1 as photoinitiator.

Over the years, anionic polymerization of cyanoacrylates was not limited to inorganic complexes and ferrocene was also examined as a potential photoinitiator. As specificity, ferrocenes and ruthenocenes are well-known to be stable in numerous organic solvents. [START_REF] Geoffroy | Organometallic Photochemistry[END_REF] Besides, when dissolved in electron-accepting solvents, a charge transfer reaction can occur with the solvent, giving rise to a charge-transfer-to-solvent (CTTS) process with an absorption band located in the near-ultraviolet region. When excited in the UV region, a photoinduced electron transfer from ferrocene (Fc) to the solvent can occur, generating a radical anion (See Scheme 2). [START_REF] Traverso | Photooxidation of ferrocene in halocarbon solvents[END_REF][210][START_REF] Traverso | Photochemical substitution of ferrocene in halogenated hydrocarbon-ethanol solutions[END_REF][START_REF] Borrell | Photochemistry of the charge-transfer complex between ruthenocene and carbon tetrachloride[END_REF][START_REF] Granifo | Mechanistic organometallic photochemistry: observation of metastable ruthenocenium in photolysis of ruthenocene[END_REF][START_REF] Bergamini | Radical-forming electron-transfer; photoreactions involving iron group metallocenes[END_REF] This phenomenon was notably demonstrated with CCl4 as the solvent. Scheme 2. CCTS occurring between ferrocene (Fc) and CCl4.

Taking advantage of this unique ability of ferrocene to initiate electron transfers with electron accepting molecules upon photoexcitation, Kutal and coworkers transposed this reaction to electron deficient monomers i.e. ethyl 2-cyanoacrylate (CA). [START_REF] Sanderson | Classical Metallocenes as Photoinitiators for the Anionic Polymerization of an Alkyl 2-Cyanoacrylate[END_REF] Following the aforementioned mechanism, a poly(ethyl 2-cyanoacrylate) could be prepared upon irradiation with a polychromatic light emitting at wavelengths higher than 290 nm (See Scheme 3). Scheme 3. Mechanism of polymerization occurring with ferrocene upon irradiation at wavelengths > 290 nm.

Thus, by mixing ethyl 2-cyanoacrylate (CA) and ferrocene in THF, appearance of a new absorption band at 355 nm (See Inset in Figure 4) could be clearly evidenced. This band typically corresponds to a charge transfer CTTS interaction between ferrocene and CA in the ground state. Interestingly, the polymerization time could be efficiently controlled by the light source intensity. Thus, while polymerizing with a high-intensity mercury lamp (output >290 nm), the polymerization could be ended after approximately four minutes of irradiation whereas the use of a light source of low intensity emitting at 365 nm could only furnish the polymer in approximately thirty-five minutes. Evidence of the anionic character of the polymerization process was provided by introducing radical scavengers such as dioxygen or quinones in the formulations and no modification of the reaction time could be detected. Conversely, introduction of a strong acid, namely methanesulfonic acid (MSA) could exert a strong inhibition effect by protonating the growing polymer chains (See Figure 5). Control experiments also revealed that no cyclopentadienide anion was released in the reaction media, supporting the mechanism proposed in Scheme 3. Similarly, examination of the UV visible absorption spectra of the ferrocene/CA solution revealed upon irradiation the appearance of a characteristic band at 617 nm corresponding to the ferrocenium cation. If this strategy for initiating an anionic polymerization is appealing, rapidly, several limitations were evidenced. Notably, if radical anions of cyanoacrylates can initiate a polymerization, reactivity of these latter drastically reduces with elongation of the alkyl chains, affecting their potential use in practical applications such as instant adhesives. [216][START_REF] Brinkmann | Can the radical anion of alkyl-2-cyanoacrylates initiate anionic polymerization of these instant adhesive monomers?[END_REF] To overcome this drawback, in 2019, this topic was revisited with the development of electron-rich ferrocenes (See Figure 6). [START_REF] Faggi | Polymethylferrocene-induced photopolymerization of cyanoacrylates using visible and near-infrared light[END_REF] Interestingly, the authors demonstrated the possibility to apply this strategy to the polymerization of two cyanoacrylate monomers of interest, namely butyl 2-cyanoacrylate and 2-octyl 2-cyanoacrylate. Indeed, these two monomers can be found in numerous commercial formulations due to their low toxicities and their high viscosities [START_REF] Petrie | Cyanoacrylate Adhesives in Surgical Applications[END_REF]. Extrapolation of this approach to noncyanoacrylate monomers was also demonstrated. Notably, comparisons of the reduction potentials of ethyl 2-cyanoacrylate (ECA), butyl 2-cyanoacrylate (BCA), hexyl 2-cyanoacrylate (HCA) and 2-octyl 2-cyanoacrylate (OCA) revealed the reduction potentials of the four monomers to be almost similar, located at ca. -1.6 V vs. SCE in acetonitrile. Conversely, introduction of electron-donating methyl groups onto ferrocene resulted in a significant reduction of the oxidation potential, decreasing from 0.50 V vs. SCE for ferrocene (Fc) in acetonitrile to 0.24 V, -0.06 V and -0.15V for dimethyl, tetramethyl and pentamethylferrocene Me2-Fc, Me8-Fc and Me10-Fc respectively. By contrast, almost similar absorption spectra were found for the different ferrocene derivatives, the absorption maxima ranging from 441 nm for Fc to 424 nm for Me10-Fc. Based on the Rehm-Weller equation, negative free energy changes could be determined for all dyes, with values increasing going from Fc to Me10-Fc, demonstrating the feasibility of the electron transfer between the ferrocene derivatives and the different monomers [START_REF] Dadashi-Silab | Photoinduced Electron Transfer Reactions for Macromolecular Syntheses[END_REF] PET-driven curing experiments of cyanoacrylates done at 420 nm with a LED (46 mW/cm²) revealed that if no polymerization could be carried out with Fc, irrespective of the monomers, conversely an irradiation time as long as 1760 s were required to polymerize BCA with Me2-Fc, whereas the reaction times could be reduced to 93 and 85 s with Me8-Fc and Me10-Fc respectively. For the less reactive monomer of the series i.e. OCA, reasonable reaction times could be however determined since polymerizations ended after 164 and 140 s were respectively determined with Me8-Fc and Me10-Fc. Interestingly and resulting from the charge transfer reaction occurring between ferrocenes and the monomers, possibility to initiate a polymerization process in the near-infrared region was also demonstrated, by irradiating in the ferrocenium absorption band. Indeed, by dissolving Me10-Fc in ECA, appearance of an absorption peak at 790 nm corresponding to the ferrocenium cation could be clearly evidenced (See Figure 7). Thus, upon irradiation of the photocurable resin (photoinitiator concentration = 2.5 × 10 -3 M) at 780 nm with a diode laser (320 mW/cm²), a 6-fold elongation of the reaction time was determined (281 s), far from the reaction time determined upon excitation at 420 nm (50 s). Elongation of the reaction time was assigned to a light less energetic at 780 nm than at 420 nm. Finally, possibility to extend the PET-driven photopolymerization to monomers other than cyanoacrylates was demonstrated. Thus, the polymerization of the electron-deficient diethyl methylenemalonate (diEtMM) could be realized with the most reactive ferrocene derivative Me10-Fc, except that a 12-fold increase of the photoinitiator content was required (30 mM vs 2.5 mM in the high reactive monomer ECA). Using these conditions, the polymerization of diEtMM could be ended at 431 s, upon irradiation at 420 nm (I = 46 mW/cm²) (See Figure 8). Reproduced with permission from Faggi et al. [START_REF] Faggi | Polymethylferrocene-induced photopolymerization of cyanoacrylates using visible and near-infrared light[END_REF]. Copyright 2019 American Chemical Society.

Acetylferrocenes and benzoylferrocenes

Parallel to alkyl-substituted ferrocenes, acetyl and benzoyl ferrocenes were also investigated as anionic photoinitiators. The first report mentioning this approach was published in 1998 with a series of three ferrocenes I-III (See Figure 9). [START_REF] Yamaguchi | Ferrocenes as Anionic Photoinitiators[END_REF] In this initial work, anionic polymerizations were examined at three different wavelengths, namely 290, 436 and 536 nm. Indeed, as shown in the Figure 10, the three ferrocenes exhibit a strong absorption in the UV range, but an additional absorption band can also be detected in the visible range, extending until 600 nm for the three dyes. When tested as photoinitiators for the polymerization of ethyl 2-cyanoacrylate (ECA), a drastic reduction of the polymerization time was observed upon irradiation with a highpressure mercury lamp (200 W, λ > 290 nm). Thus, if a polymerization time of 552 s was determined with ferrocene I (C = 2 mM), this value reduced to 176 s and even 3 s for ferrocenes II and III respectively. Considering that ferrocenes II and III exhibit a lower absorption at 436 nm than at 290 nm, a severe elongation of the reaction times could be determined, increasing up to 900 s for ferrocene II (vs. 176 s at 290 nm) and 38 s for ferrocene III (vs. 3 s at 290 nm).

To support the anionic polymerization process, formation of carboxylate anions by photocleavage of the carbonyl bond accompanied by a decomposition of the ferrocene core was suggested as a plausible hypothesis, these hypotheses being based on previous works reporting the photoaquation of the carbonyl groups of ferrocene upon photoexcitation (See Scheme 4). [START_REF] Ali | Photochemistry of ferrocenyl ketones and acids in dimethyl sulfoxide and related solvents[END_REF] However, further mechanistic investigations revealed that these first hypotheses were not correct and that a photoinduced ring-metal cleavage producing the benzoyl-substituted cyclopentadienide anion was the exact mechanism producing the initiating species (See Scheme 4). [START_REF] Yamaguchi | Efficient photodissociation of anions from benzoylfunctionalized ferrocene complexes[END_REF][START_REF] Ding | Characterization of the Low-Energy Electronic Excited States of Benzoyl-Substituted Ferrocenes[END_REF] Figure 10. UV-visible absorption spectra of ferrocenes I-III in ethyl α-cyanopropionate. Reproduced with permission from Yamaguchi et al. [START_REF] Yamaguchi | Ferrocenes as Anionic Photoinitiators[END_REF]. Copyright 1998 American Chemical Society

Scheme 4. The two mechanisms proposed to support the anionic photopolymerization process.

Finally, stability of the photocurable ECA/ferrocene resins over time was examined by determining the influence of the substitution pattern on the benzoyl moiety (See Figure 11). [START_REF] Yamaguchi | Benzoyl-substituted ferrocenes: An attractive new class of anionic photoinitiators[END_REF] Indeed, one of the main challenges from the industrial viewpoint concerns the longterm storage of the photocurable resin. Towards this end, Kutal and coworkers demonstrated that by placing substituents in ortho-position of the carbonyl group on the phenyl ring of the benzoyl group, the resin stability could be greatly improved. Thus, if introduction of methyl groups in ferrocenes IV, VI and VIII could not significantly influence the resin stabilities (23-32 days) compared to that determined for the ferrocenes bearing unsubstituted benzoyl groups (ferrocene II and III, 16-32 days of stability), conversely, introduction of chlorine atoms in V and IX resulted in a 3-fold elongation of the resin stability, peaking up to 86 days in the dark and at room temperature for ferrocene IX. Finally, examination of the photoinitiating abilities revealed ferrocenes I-X could be separated in two groups of reactivity. Thus, a severe reduction of the photoinitiating ability was determined for the mono-substituted ferrocenes compared to the disubstituted ones, with a 3-to-10-fold elongation of the reaction times. It has to be noticed that, recently, benzoylferrocenes have been revisited in the context of air-drying paints, [START_REF] Honzícek | Modified Ferrocenes as Primary Driers for Formulations of Alkyd Paints[END_REF] in replacement of the well-established but contested cobalt, [START_REF] Honzícek | Curing of Air-Drying Paints: A Critical Review[END_REF][START_REF] Leyssens | Cobalt toxicity in humans -A review of the potential sources and systemic health effects[END_REF][START_REF] Simpson | The Evolution of Catalysis for Alkyd Coatings: Responding to Impending Cobalt Reclassification with Very Active Iron and Manganese Catalysts, Using Polydentate Nitrogen Donor Ligands[END_REF][START_REF] Stava | Properties of metallocene complexes during the oxidative crosslinking of air drying coatings[END_REF] manganese [START_REF] Matušková | Performance of Manganese(III) Acetylacetonate in Solvent-Borne and High-Solid Alkyd Formulations[END_REF][START_REF] Bouwman | A study of new manganese complexes as potential driers for alkyd paints[END_REF] and vanadium [START_REF] Preininger | A promising drying activity of environmentally friendly oxovanadium(IV) complexes in air-drying paints[END_REF][START_REF] Preininger | Drying activity of oxovanadium(IV) 2-ethylhexanoate in solvent-borne alkyd paints[END_REF][START_REF] Preininger | Oxovanadium(IV) complexes bearing substituted pentane-2,4-dionate ligands: Synthesis, structure and drying activity in solvent-borne alkyd paints[END_REF][START_REF] Charamzová | Honzícek, Benzyl substituted oxidovanadium(IV) pentane-2,4-dionates: Synthesis, structure and drying properties[END_REF][START_REF] Charamzová | Application of Oxovanadium Complex Stabilized by N,N,N,N-Chelating Ligand in Air-Drying Paints[END_REF][START_REF] Charamzová | Oxidovanadium(V) dithiocarbamates as driers for alkyd binders[END_REF]-based catalysts, classically used as primary driers for alkyd resins. However, if the ability of benzoylferrocenes to act as driers was demonstrated, no investigation of the chemical mechanism was carried out in this work.

Living anionic ring-opening photopolymerization of ferrocenophanes

Ferrocene is a robust structure since the dissociation energy of the cyclopentadienideiron(II) bond is around 380 kJ/mol. [START_REF] Lewis | Bond dissociation energies in ferrocene[END_REF] Besides, depending of the substitution on the ferrocene core, a cleavage of the cyclopentadienide-iron(II) bond upon irradiation can occur, as exemplified with acylferrocenes [239][START_REF] Bozak | Facile Photochemical Iron-ring Cleavage in Acylferrocenes[END_REF][START_REF] Clara | Ultrafast Dissociation Dynamics of Ferrocene and [3]-Ferrocenophan[END_REF] and boron-bridged [1]ferrocenophanes. [START_REF] Berenbaum | Ring-Opening Protonolysis of Strained Silicon-Containing Rings: A New Approach to Ions with Silylium Character[END_REF] However, to get this photoassisted reaction, the use of strained ring-tilted structures is required and two different mechanisms were proposed to support the polymerization of these ferrocenes. Thus, in the case of strained phosphorus-bridged [1]ferrocenophanes, photopolymerization under UV light was proposed as occurring by mean of a ring slippage, the polymer formation resulting from a photolytic ring-opening reaction and a haptotropic shift of a cyclopentadienide ligand. [START_REF] Mizuta | Photolytic Ring-Opening Polymerization of Phosphorus-Bridged [1]Ferrocenophane Coordinating to an Organometallic Fragment[END_REF] As shown in the Scheme 5, dissociation of the cyclopentadienide anion let vacant coordination sites on the metal center that can be temporarily occupied by the solvent molecules. By recombination of the freed and pendant cyclopentadienide anion with the uncoordinated metal center of another ferrocene unit, a fast polymerization process can thus occur. Proof of this mechanism was obtained by isolating the intermediate product resulting from the ring slippage reaction with trimethylphosphite. [START_REF] Mizuta | Ring-Opening Reaction of Phosphorus-Bridged [1]Ferrocenophane via Ring Slippage from η 5 -to η 1 -Cp[END_REF] In contrast, for thermal polymerizations, a complete different mechanism was evidenced for the B, Si, Ge, Sn and even P-bridged [1]ferrocenophanes, this second mechanism being based on the cleavage of the strained cyclopentadienide-heteroatom bond (See Scheme 5). Scheme 5. Chemical mechanism supporting the photopolymerization of strained, ring-tilted ferrocenophanes and the mechanism occurring during a thermal polymerization process.

Free radical polymerization via ferrocene-alkyl/aryl chloride charge transfer complexes

Ferrocene is an excellent electron donor that was notably used for the design of pushpull dyes, by covalent linkage of ferrocene to electron acceptors. Considering its low oxidation potential, ferrocene can also be advantageously used as an electron donor to form charge transfer complexes by intermolecular interaction with the appropriate electron acceptor. In the literature, numerous charge transfer complexes obtained by opposing ferrocene to tetracyanoethylene, [START_REF] Rosenblum | The Properties of a Ferrocene-Tetracyanoethylene Charge-Transfer Complex[END_REF] ozone, [START_REF] Pinelo | Charge-Transfer Complexes and Photochemistry of Ozone with Ferrocene and n-Butylferrocene: A UV-vis Matrix-Isolation Study[END_REF] polyiodomethanes, [START_REF] Velichko | Charge transfer complexes of ferrocene derivatives with polyiodomethanes[END_REF] tetranitromethane, [START_REF] Jaworska-Augustyniak | Physico-chemical properties of charge transfer complexes of ferrocene and 1,1′-dimethylferrocene in methanol and cyclohexane[END_REF] iodine, 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and p-chloranil [START_REF] Salman | Redox reactions via outer sphere charge transfer complexation: the interaction of ferrocenes with σ-and π-type acceptors[END_REF] have notably been reported. Considering the propensity of ferrocene to form intermolecular charge transfer complexes (CTCs), the possibility to initiate a free radical polymerization (FRP) of acrylates with ferrocene-alkyl/aryl halide CTCs was examined. [START_REF] Anderson | Photoinitiation of multifunctional acrylates via ferrocene-alkyl chloride charge transfer complexes[END_REF] Interestingly, upon photoexcitation of the CTC in the UV region, a decomposition of the CTC could occur, producing radicals according to the mechanism depicted in the Scheme 6. Notably, theoretical calculations carried out with numerous alkyl and aryl halides revealed that formation of the charge transfer complexes was inducing a geometrical deformation of ferrocene, due to the intercalation of the halogen between the two cyclopentadiene rings. Upon photoexcitation of the CTC in the UV range, an electron from the highest occupied molecular orbital (HOMO) i.e. the cyclopentadienide anions towards the lowest unoccupied molecular orbital (LUMO) i.e. the metal center can occur. As a result of this, an electron is transferred onto the halogen atom, inducing the dissociation of the radical anion from the ferrocenium cation, and subsequently decomposing into the initiating alkyl/aryl radical. Examination of the photoinitiating ability of the ferrocene/benzyl chloride CTC for the FRP of 1,6-hexanediol diacrylate (HDDA) upon excitation in the UV range with a high-pressure mercury lamp revealed the CTC to be a less efficient photoinitiating system than the well-established benzophenone/N-methyl-N,Ndiethanolamine (MDEA) two-component system. Introduction of substituents such as methoxy or cyano groups onto ferrocene did not significantly modify the polymerization rate. Besides, while irradiating at 313 nm, an anomalous behavior was detected for the cyanoferrocene/benzyl chloride CTC. Indeed, in this last case, the CTC could outperform the benzophenone/MDEA combination whereas the ferrocene/benzyl chloride or the methoxyferrocene/benzyl chloride CTCs were ineffective to induce the FRP of HDDA. To support this, a higher association constant for the cyanoferrocene/benzyl chloride CTC than that observed for the ferrocene/benzyl chloride or the methoxyferrocene/benzyl chloride CTCs was tentatively proposed to support the polymerization results. However, no study concerning the influence of the CTC concentration on the photoinitiating ability was carried out in this unique work devoted to demonstrate that the ferrocene-based CTCs could act as photoinitiating systems. Scheme 6. Mechanism of formation of radicals by photodecomposition of the ferrocene-alkyl halide CTC.

Free radical polymerization using ferrocene as a sacrificial electron donor

Efficiency of FRP processes is strongly related to the composition of the photocurable resins and the most reactive ones are undoubtedly the three-component systems. [START_REF] Ley | Tailoring a hybrid three-component photoinitiating system for 3D printing[END_REF][START_REF] Fouassier | Three-component photoinitiating systems: towards innovative tailor made high-performance combinations[END_REF][START_REF] Lalevée | Metal and metal-free photocatalysts: mechanistic approach and application as photoinitiators of photopolymerization[END_REF] Indeed, parallel to a photosensitizer and a radical generator that will react in the excited state and promote the formation of radicals, the photosensitizer can be oxidized or reduced in this initial step so that the photosensitizer is irreversibly consumed. Consequently, concentration of the photosensitizer decreases in the photocurable resin so that a decrease of the polymerization rate can be observed. To overcome this drawback, a third component can be introduced in the resin so that the oxidized or reduced photosensitizer can be regenerated in its initial state. In 2013, use of methyl ferrocenecarboxylate as a sacrificial electron donor was reported. [START_REF] Kawamura | Relevance of Linked Dye-Coinitiator in Visible Three-Component Photoinitiating Systems: Application to Red Light Photopolymerization[END_REF] In this work, two parallel strategies were examined, consisting in the covalent linkage of the photosensitizer to the radical generator or to mix the two partners within the photocurable resins (See Figure 12). As anticipated, photolysis experiments revealed the electron transfer between the radical generator and the photosensitizer to be faster upon linkage of the two partners. These observations are consistent with previous results reported in the literature concerning the covalent linkage of free radical generators to photosensitizers. [START_REF] Turro | Principles of Molecular Photochemistry[END_REF][START_REF] Ohta | External Electric Field Effects on Fluorescence in an Electron Donor and Acceptor System: Ethylcarbazole and Dimethyl Terephthalate in PMMA Polymer Films[END_REF][START_REF] Kawamura | Novel and efficient dye-linked radical generators for visible light photoinitiating polymerization[END_REF][START_REF] Kawamura | Photoinduced Intramolecular Electron Transfer between Carbazole and Bis(trichloromethyl)-s-triazine Generating Radicals[END_REF] Figure 12. Chemical structures of the photosensitizer, radical generator, monomer and electron donor.

As shown in the Figure 13, photopolymerization experiments of pentaerythritol tetraacrylate (PETA) upon irradiation at 635 nm with a laser diode (I = 10 mW/cm²) using threecomponent systems (S/T/Fc-CO2Me 0.03M/0.03M/0.09M) or (S-n-T/Fc-CO2Me 0.03M/0.09M with n = 2, 6) revealed the final monomer conversion to be drastically improved upon covalent linkage of the photoinitiator to the photosensitizer. Thus, if the PETA conversion was limited to 6% for the mixture of the three components, the monomer conversion could increase up to 23 and 33% while using the (S-6-T/Fc-CO2Me) and the (S-2-T/Fc-CO2Me) respectively. Therefore, distance between the two partners was determined as being a crucial parameter governing the electron transfer efficiency. Compared to the photoinitiating system based on S-6-T and S-2-T alone, an improvement of the monomer conversion was obtained upon addition of the sacrificial electron donor, confirming the regeneration of the photosensitizer by ferrocene. Thus, only final monomer conversions of 20 and 27% were obtained with S-6-T and S-2-T when used as photoinitiating systems without ferrocene. Photolysis experiments also revealed the electron transfer rate between S and Fc-CO2Me to be 20 times higher than that of determined for the S/T system so that a complete deactivation of the photoinitiating system was observed for the physical mixture of S, T and Fc-CO2Me, according to the mechanism depicted in the Figure 14. In this situation, the reductive path results in an efficient quenching of the excited state of the photosensitizer by ferrocene, without contributing to the formation of initiating radicals. Conversely, the oxidative path contributes to generate radicals

FRP h

A completely different situation exists when the radical generator is connected to the photosensitizer. Indeed, as shown in the Figure 15, both the oxidative and reductive paths can contribute to the formation of initiating radicals, therefore enabling to get high final monomer conversions. Thus, upon photoexcitation of the S-n-T photosensitizer, reaction in the excited state of ferrocene with S-n-T results in the rapid formation of the S •--n-T radical anion which by electron transfer onto the radical generator is rapidly converted as the S-n-T •-radical anion. By decomposition of the S-n-T •-radical anion and release of the chlorine anion, the initiating radical S-n-T • can be formed. Parallel to this, the oxidative path still produces initiating radicals so that when the photosensitizer is linked to the radical generator, a dual source of radicals can be obtained, improving the overall efficiency of the photoinitiating system.

Figure 15.

Mechanism occurring when the photosensitizer is linked to the radical generator in the photoinitiating system.

Free radical polymerization using ferrocene-based push-pull dyes.

The design of push-pull dyes comprising ferrocene as the electron donor is an active research field due to the reversibility of the oxidation process occurring on the ferrocene side. Introduction of a metal complex in a push-pull structure is also an insurance to produce dyes with high molar extinction coefficients. Parallel to this, interest for photopolymerization processes done at long wavelength and especially in the near infrared region is supported by the remarkable light penetration that can be achieved at these wavelengths so that the access to thick samples and even filled samples is possible. In 2019, the first report mentioning the polymerization of methacrylic resins with a laser diode emitting at 785 nm was published by Lalevée and coworkers. [START_REF] Garra | Ferrocene-based (photo)redox polymerization under long wavelengths[END_REF] In this work, nine different ferrocenes Fe1-Fe7, Fe7Me and ferrocene (Fc) were examined, including six push-pull dyes (See Figure 16). Interest for designing photoinitiators based on push-pull structures are the followings: 1) these dyes can be synthesized with high reaction yields (89-98% yields), by a Knoevenagel reaction enabling to develop a green approach during the synthesis. Notably, push-pull dyes can be synthesized

S-T

Fc-CO 2 Me S •--T Fc-CO 2 Me +• S +• -T •- Reductive cycle Cl - S +• -T • FRP S*-T S*-T S -T •- Cl - S -T • FRP Oxidative cycle
in ethanol and the purification can be limited to a simple filtration on a glass filter. 2) Dyes with high molar extinction coefficient can be obtained. 3) Reversibility of the oxidation process located onto the electron-donating part is ensured if ferrocene is used as the electron donor. 4) Absorption spectra of the push-pull dyes can be finely tuned by mean of the electron acceptors used to prepare the dyes. Indeed, as shown in the Figure 17, all push-pull dyes could exhibit a broad absorption extending between 350 and 800 nm except for Fe7Me for which an absorption extending between 350 and 1000 nm could be determined. Thus, if an absorption maximum located at 438 nm was determined for Fe2, an absorption redshifted by ca. 240 nm was found for Fe7Me, peaking at 675 nm in acetonitrile. Among the different dyes, Fe7Me proved to be the most solvatochromic dye of the series and a shift of the absorption maxima of 19 nm could be determined between hexane and chloroform as the solvents. [START_REF] Janowska | Ferrocenyl D-π-A chromophores containing 3-dicyanomethylidene-1-indanone and 1,3-bis(dicyanomethylidene)indane acceptor groups[END_REF] Considering the absorption spectra of the push-pull dyes, photopolymerization experiments could be carried out both at 405 nm and at 785 nm. While testing the polymerization efficiency of the two-component ferrocene/Iod1 (0.5%/1% w/w) systems at 405 nm (110 mW/cm²) with the resin 2, only moderate final conversions could be obtained using Fe3-Fe5 as the photosensitizers. Thus, final monomer conversion of 25%, 20% and 30% were respectively obtained with Fe3, Fe4 and Fe-5-based two component systems. Comparison with the three-component camphorquinone/ethyl-4-(dimethylamino)benzoate/4-diphenylphosphino styrene revealed this reference system to outperform the different aforementioned two-components systems (60% monomer conversion at 405 nm). While using 2-dppba as the phosphine, the three-component systems showed besides an enhancement of the monomer conversions so that a conversion of 72% could be determined with the three-component Fc/Iod1/2dppba (0.5%/1%/1.5% w/w/w) system. At 785 nm and based on the absorption spectra of the different dyes, only Fe7 and Fe7Me could initiate a polymerization process. Final monomer conversions of 48 and 50% could notably be obtained with the Fe7/Iod/2dppba (0.5%/2%/1.5% w/w/w) and Fe7/Iod/DMABA (0.5%/2%/1.5% w/w/w) three-component systems upon irradiation at 785 nm (I = 2.55 W/cm²) for 300 s. These results outperform those obtained with the three-component system camphorquinone/ethyl 4-(dimethylamino)benzoate/4-diphenylphosphino styrene obtained at 470 nm (60% monomer conversion). [START_REF] Bouzrati-Zerelli | A low migration phosphine to overcome the oxygen inhibition in new high-performance photoinitiating systems for photocurable dental type resins[END_REF] Considering that ferrocene is capable to initiate a redox decomposition of peroxides, [START_REF] Sarac | Redox polymerization[END_REF][START_REF] Kalenda | Ferrocene and some of its derivatives used as accelerators of curing reactions in unsaturated polyester resins[END_REF] the redox decomposition of ammonium persulfate (APS) was also examined. Optical pyrometric measurements revealed the polymerization of resin 1 to be exothermic and a maximum temperature of 60°C, 38°C and 37°C were determined with the three-component Fe4/APS/2dppba, Fe3/APS/2dppba and Fe7/APS/2dppba (0.1%/2%/1.5% w/w/w) systems. Parallel to a pure redox initiating system, possibility to photoactivate the polymerization process was demonstrated, enabling to develop a dual cure process. Thus, by using the four-component Fe7/APS/2dppba/Iod1 (0.1%/2%/1.5%/1% w/w/w/w) system and while irradiating at 785 nm (0.4 W/cm²), a three-fold reduction of the polymerization time could be obtained. Jointly, a more exothermic reaction .cm -1

) Wavelength (nm) could be initiated, speeding up the polymerization process (See Figure 18). This significant enhancement accounts from the dual cure process, where two different sources simultaneous contribute to generate initiating radicals. Indeed, parallel to the redox process that make use of the ferrocene derivative as a redox agent, jointly, the same ferrocene derivative can also be used as a photosensitizer that can sensitize the photodecomposition of the iodonium salt, producing phenyl radicals (See Figure 19). Recently, new ferrocene-based push-pull dyes (10 and 11) were examined as photoinitiators at 405 nm and their photoinitiating abilities were compared to a series of organic-based dyes 1-9. [START_REF] Sun | Novel push-pull dyes derived from 1Hcyclopenta[b]naphthalene-1,3(2H)-dione as versatile photoinitiators for photopolymerization and their related applications: 3D-printing and fabrication of photocomposites[END_REF] As specificity, the two ferrocene-based dyes were prepared with the same electron-acceptor, namely 1H-cyclopenta[b]naphthalene-1,3(2H)-dione (See Figure 20). Despites promising UV-visible absorption spectra in acetonitrile (See Figure 21), dyes 10 and 11 were less efficient than the organic dyes used for comparisons (except dye 4) during the FRP of TMPTA at 405 nm (110 mW/cm²). In the case of dye 4, low photoinitiating ability of this dye directly resulted from its low solubility in TMPTA, adversely affecting its photoinitiating ability. More surprisingly, the monomer conversions obtained with dyes 10 and 11 (51 and 56% respectively) were lower than that obtained for the reference twocomponent system Iod1/EDB (2%/2%, w/w) (60% monomer conversion). Parallel to the works done by Lalevée and coworkers on push-pull dyes prepared by Knoevenagel reactions, on his side, Wang and coworkers developed a series of ferrocenebased push-pull dyes FcIn1-FcIn4 by Sonogashira cross-coupling reactions (See Figure 22). [START_REF] Han | Synthesis and electrochemical, linear and third-order nonlinear optical properties of ferrocene-based D-π-A dyes as novel photoredox catalysts in photopolymerization under visible LED irradiations[END_REF] Influence of the spacer was examined by introducing a benzene, thiophene, carbazole, and phenothiazine group between indane-1,3-dione and ferrocene. In terms of absorption, influence of the conjugation length could be clearly evidenced since the dyes exhibiting the most red-shifted absorptions were FcIn3 and FcIn4 comprising a carbazole and a phenothiazine spacer. Thus, absorption maxima peaking at 463 and 528 nm were respectively found for these two dyes. When tested as photosensitizers in two-component (FcIns/ONI 0.1%/2% w/w) systems, monomer conversions of 56%, 74%, 86%, and 87% using FcIn1/Iod2, FcIn2/Iod2, FcIn3/Iod2, and FcIn4/Iod2 were respectively obtained during the FRP of tripropylene glycol diacrylate (TPGDA) upon irradiation with a blue LED at 450 nm. The high final conversions obtained with FcIn3 and FcIn4 directly account from the presence of the carbazole and the phenothiazine moieties in their backbones. [START_REF] Chao | Thiophene-substituted phenothiazine-based photosensitisers for radical and cationic photopolymerization reactions under visible laser beams (405 and 455 nm)[END_REF][START_REF] Zhou | D-D-A dyes with phenothiazine-carbazole/triphenylamine as double donors in photopolymerization under 455 nm and 532 nm laser beams[END_REF][START_REF] Ma | Conjugated phenothiazine oxime esters as free radical photoinitiators[END_REF] Indeed, introduction of carbazoles or phenothiazines in photoinitiators of low efficiency is a well-known strategy to improve the polymerization efficiency. As expected, a significant enhancement of the final monomer conversions was obtained with the three-component (FcIns/ONI/NMP 0.1%/2%/4% w/w/w) systems where the sacrificial amine i.e. N-methylpyrrolidone (NMP) could efficiently regenerate the photosensitizer. The most important improvements were determined for FcIn1 and FcIn2, the monomer conversions reaching 93 and 92% vs. 56 and 74% for the corresponding two-component systems (See Figure 24). Therefore, the efficient regeneration of the photosensitizers upon introduction of NMP clearly evidences that the ferrocene-based dyes can act as photoredox catalysts due to the reversible electrochemical properties of ferrocene.

Free radical polymerization using ferrocene-based chalcones

Nature of the photoinitiating systems drastically influence the future use of photopolymers.

Indeed, metal complexes are highly controversed photoinitiators/photosensitizers due to their potential toxicities and the resulting photopolymers can't be used for food packaging [START_REF] Lago | Photoinitiators: a food safety review, Food Addit[END_REF][START_REF] Liu | Photoinitiators in Breast Milk from United States Donors: Occurrence and Implications for Exposure in Infants[END_REF] or medical applications. [START_REF] Lebedevaite | Photoinitiator Free Resins Composed of Plant-Derived Monomers for the Optical µ-3D Printing of Thermosets[END_REF][START_REF] Tomal | Water-Soluble Photoinitiators in Biomedical Applications[END_REF][START_REF] Baroli | Photopolymerization of biomaterials: Issues and potentialities in drug delivery, tissue engineering, and cell encapsulation applications[END_REF][START_REF] Fisher | Photoinitiated Polymerization of Biomaterials[END_REF][START_REF] Stansbury | Curing dental resins and composites by photopolymerization[END_REF][START_REF] Khosroshahi | Photopolymerization of dental resin as restorative material using an argon laser[END_REF][START_REF] Maffezzoli | Photopolymerization of dental composite matrices[END_REF][START_REF] Dickens | Photopolymerization kinetics of methacrylate dental resins[END_REF][START_REF] Peppas | Hydrogels in Biology and Medicine: From Molecular Principles to Bionanotechnology[END_REF][START_REF] Ifkovits | Review: Photopolymerizable and degradable biomaterials for tissue engineering applications[END_REF] Recently, a great deal of efforts has been devoted to develop biosourced or bioinspired photoinitiating systems. The recent development of chalcones-based photosensitizers corresponds to this new interest, and ferrocene is also well-known to exhibit a low toxicity. Therefore, the combination of the two approaches was of interest. In 2020, the first ferrocenebased chalcones developed as photoinitiators of polymerization were reported for the first time (See chalcone 1-chalcone 6 in Figure 25). [START_REF] Chen | Photoinitiators derived from natural product scaffolds: Mono-chalcones in three-component photoinitiating systems and their applications in 3D printing[END_REF] It has to be noticed that if chalcones can be found in the Nature (fruits and flowers), [START_REF] Morita | Chalcone isomerase-like protein enhances flavonoid production and flower pigmentation[END_REF] evidently, no ferrocene-based chalcones can be naturally found so that these structures are bioinspired and not biosourced. However, these structures remain of interest. Thus, the FRP of PEG-diacrylate using a LED at 405 nm (110 mW/cm²) revealed the three-component chalcones/Iod1/EDB (1.5%/1.5%/1.5% w/w/w) systems to furnish monomer conversions ranging between 30% for chalcone 5 to 80% for chalcones 1 and 6 respectively. In fact, control experiments done with the Iod1/EDB combination revealed that only chalcones 3 and 5 furnished lower final monomer conversions than the reference Iod1/EDB system (49% conversion). Interestingly, if the ferrocene-based photoinitiating systems were efficient for the polymerization of thin samples, conversely, no polymerization could be detected for thick samples, in the same irradiation conditions. In 2021, two bis-chalcones (chalcones 7 and 8) were prepared with ferrocene and tested in the same conditions than previously used for chalcones 1-6 (See Figure 25). [START_REF] Chen | Bis-chalcone derivatives derived from natural products as near-UV/visible light sensitive photoinitiators for 3D/4D printing[END_REF] Figure 25. Chemical structures of chalcones1-8, the monomers, and the different additives.

When tested in three-component chalcones/Iod1/EDB (1.5%/1.5%/1.5% w/w/w) systems, chalcones 7 and 8 could provide final monomer conversion of 42 and 53%, therefore, monomer conversions lower or similar to that obtained with the reference Iod1/EDB system (49% conversion). Comparison with other bis-chalcones comprising purely organic electron donors revealed the two ferrocene-based chalcones to furnish lower final monomer conversions. These results were confirmed during the free radical promoted cationic polymerization (FRPCP) of 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (EPOX) for which monomer conversions of 66 and 50% were obtained with the three-component chalcone 7/Iod1/EDB (1.5%/1.5%/1.5% w/w/w) and chalcone 8/Iod1/EDB (1.5%/1.5%/1.5% w/w/w) systems respectively upon irradiation at 375 nm with a LED (I = 40 mW/cm²). Besides, these conversions are significantly higher than that obtained for the two component Iod1/EDB system (45% conversion), demonstrating the higher ability of the ferrocene derivatives to promote the FRPCP of epoxides than the FRP of acrylates.

Conclusion

To conclude, ferrocene has recently been examined as a potential candidate for the design of photoinitiating systems enabling to promote both the free radical polymerization of acrylates but also the cationic polymerization of epoxides. Over the years, a clear progress of the ferrocene-based structures has been observed. If the initial chemical modifications of ferrocene were limited to introduced alkyl groups to tune the redox potentials of the corresponding derivatives, recently, an acceleration of the modification has been observed during the last three years. Notably, the multiplication of push-pull dyes prepared by Knoevenagel or Sonogashira cross-coupling reactions, the development of mono and bischalcones clearly evidence ferrocene-based photoinitiators to become an active research field. Through these different works, the possibility to design photoinitiating systems activable in the near UV-visible region and now in the near-infrared region demonstrates the challenge, photopolymerization is now facing (See Figure 26). Ferrocene is a versatile building block for the design of initiating systems since, parallel to the cationic, anionic and free radical polymerizations, redox polymerizations were also reported with this metallocene. Notwithstanding this, examples of redox polymerizations with ferrocene remain scarce at present. With regards to the light penetration that can be obtained in the near infrared (NIR) and the infrared region, future dyes based on ferrocene will certainly be developed to operate in this specific part of the absorption spectrum. Indeed, in numerous visible light photoinitiators have been reported during the last decade, the number of NIR photoinitiators remains is still scarce and this topic deserves to be more widely studied in the Future.

Figure 1 .

 1 Figure 1. The two families of photoinitiators reported in the literature.

Figure 3 .

 3 Figure 3. Chemical structure of ferrocene.

Figure 4 .

 4 Figure 4. UV-visible absorption spectra of ferrocene in (a) THF and (b) in a 2:3 (V:V) mixture of ethyl 2-cyanoacrylate CA and THF. Inset: difference spectrum of (a) and (b). Reproduced with permission from Sanderson et al. [215]. Copyright 2002 American Chemical Society.

Figure 5 .

 5 Figure 5. Monitoring of the monomer conversion vs. irradiation time for solution containing 9.6 mM of ferrocene with and without methanesulfonic acid (MSA). Polychromatic light source (Mercury lamp, 110 mW/cm²) Reproduced with permission from Sanderson et al. [215]. Copyright 2002 American Chemical Society.

Figure 6 .

 6 Figure 6. Chemical structures of electron-rich ferrocenes.

Figure 7 .

 7 Figure 7. UV-visible absorption spectra of Me10-Fc in acetonitrile and ECA. Reproduced with permission from Faggi et al. [218]. Copyright 2019 American Chemical Society.

Figure 8 .

 8 Figure 8. Polymerization of diEtMM using Me10-Fc as the photoinitiator (C= 30 mM). Reproduced with permission from Faggi et al. [218]. Copyright 2019 American Chemical Society.

Figure 9 .

 9 Figure 9. Chemical structures of ferrocenes I-III.

Figure 11 .

 11 Figure 11. Chemical structures of ferrocenes IV-X.

Figure 13 .Figure 14 .

 1314 Figure 13. Final PETA conversion determined by FTIR measurements upon irradiation with a laser diode@635 nm using different photoinitiating systems.(3Fc stands for Fc-CO2Me) Reproduced with permission from Kawamura et al. [277]. Copyright 2013 Wiley

Figure 16 .

 16 Figure 16. Chemical structures of dyes, monomers and additives.

Figure 17 .

 17 Figure 17. UV-visible absorption spectra of (a): Fe7; (b):Fe4; (c):Fe6; (d):Fe3; (e):Fe5; (f): Fe1; (g): Ferrocene; (h): Fe2; (i): Fe7Me in acetonitrile. Reprinted with permission from Garra et al. [158]. Copyright © 2019 The Royal Society of Chemistry.

Figure 18 .

 18 Figure 18. Monitoring of the polymerization of resin 1 using Fe7/2dppba/APS (0.1%/1.5%/2% w/w/w) (a) and Fe7/2dppba/APS/Iod1 (0.1%/1.5%/2%/1% w/w/w/w) upon irradiation at 785 nm. Reprinted with permission from Garra et al. [158]. Copyright © 2019 The Royal Society of Chemistry.

Figure 19 .

 19 Figure 19. The dual cure process involved during the near-infrared activation of the redox polymerization process.

Fe

  Photoinitiated polymerization

Figure 20 .

 20 Figure 20. Chemical structures of ferrocene-based dyes 10-11, the organic dyes used for comparisons, the monomer, the free radical generator and the sacrificial amine.

Figure 21 .

 21 Figure 21. Left: UV-visible absorption spectra of 1-11 in acetonitrile. Right: Photopolymerization profiles of TMPTA using the three-component dye/Iod/EDB (0.1%/2%/2% w/w/w), irradiation at 405 nm with a LED under air. Reprinted with permission from Sun et al. [161]. Copyright © 2020 MDPI.

Figure 22 .

 22 Figure 22. Chemical structures of FcIn1-FcIn4.

Figure 23 .

 23 Figure 23. Absorption spectra of FcIn1-FcIn4 in dichloromethane. Reprinted from [286] Copyright (2019) with permission from Elsevier.

Figure 24 .

 24 Figure 24. TPGDA conversions determined with the two-component (FcIns/ONI 0.1%/2% w/w) systems (a) and three-component (FcIns/ONI/NMP 0.1%/2%/4% w/w/w) systems upon irradiation with a blue LED at 450 nm. Reprinted from [286] Copyright (2019) with permission from Elsevier.

Figure 26 .

 26 Figure 26. Absorption range of the different dyes mentioned in this review.

Fe1 300

 300

Table 1 .

 1 Comparisons between ferrocene-based photoinitiating systems and other photoinitiating systems.

	Parameters	ferrocene-based	other organic dyes
		photoinitiating systems
	Cost/synthesis	ferrocene is a cheap and easily	Depending of the structures,
		available compound.	starting compounds can be
		Ferrocene is also a versatile	expensive. Parallel to this,
		structure that be easily	solubility can be a major
		chemically modified. As an	issue, especially when
		interesting feature, ferrocene	polyaromatic structures are
		derivatives are highly soluble	targeted.
		in most of the common
		organic solvents
	Environmental impact	Ferrocene derivatives can be	The same purification
		purified by column	procedures can be
		chromatography or	developed than for
		recrystallization.	ferrocene derivatives
	Photochemical stability	Ferrocene derivatives are
		photochemically stable.

Acknowledgment

This research was funded by Aix Marseille University and The Centre National de la Recherche Scientifique (CNRS).