Keywords: Photoinitiating system, Ketone, Push-pull dye, Free radical polymerization, Three-component system, LED, Sunlight induced polymerization, 3D printing

Even though many organic dyes have been introduced to be photoinitiators for free radical polymerization in previous literature, the challenge for the design and development of novel photoinitiating systems based on dye scaffolds adapted to visible lights e.g. 405nm LED and sunlight remains open. Recently, many achievements in high-performance photoinitiating systems based on novel dye scaffolds as light-harvesting compounds and their applications under visible-light irradiation have been emerged and discussed in our former works. In the art-of-state section of this PhD thesis, novel synthesized chromophores with various chemical structures as reactive photoinitiators in the period of 2018-2021, are systematically summarized and classified into categories. Subsequently, various new chromophores with different chemical structures were designed by our group and their high visible light absorption and outstanding photoinitiating efficiency upon LED and UV light in free radical photopolymerization were confirmed by deep investigations using RT-FTIR, UV-vis spectroscopy, fluorescence approaches and ESR-spin trapping experiments. Remarkably, many investigated chromophores have been proved as efficient photoinitiors upon sunlight irradiation, which is widely introduced as a cheap, unlimited, and energy-saving natural light source. Moreover, these novel dye-based photoinitiating systems is also likely to induce photopolymerization in the presence of fillers. Hence, there is no doubt for new developed dye-based systems to expand new active researches and broader applications on development of many cross-sectional studies in modern sciences and technologies.

1.

Photopolymerization profiles of novel Indane-1,3-dione and 1Hcyclopentanaphthalene -1,3-dione derivatives.................................................................... 76 2. Proposed chemical mechanisms.......................................................................................81 2.1. UV-vis Absorption and Steady State Photolysis of the selected 10 dyes.................. 82 2.2. Consumption of Dyes in Photolysis Reactions...........................................................87 2.3. Chemical mechanisms in electron transfer reactions.............................................. 89 2.4. ESR spin-trapping experiments.................................................................................93 3. Direct laser write experiments based on dye 5 and 19................................................... 94 derivatives ....................................................................................................96 1. Photopolymerization profiles and light absorption properties of 3-(dialkylamino) -1,2dihydro-9-oxo-9H-indeno[2,1-c]pyridine -4-carbonitrile derivatives.................................98 1.1. Photopolymerization kinetics for newly push-pull dyes in three-component photoinitiating systems.................................................................................................... 98 1.2. Light absorption properties of the push-pull dyes..................................................101 2. Proposed chemical mechanisms.................................................................................... 101 2.1. Steady State Photolysis experiments of dye 8 and 9...............................................101 2.2. Chemical mechanisms in electron transfer reactions............................................ 106 2.3. ESR spin-trapping experiments...............................................................................108 3. Direct laser write experiments based on dye 8 and 9................................................... 110 3.1. Direct laser write experiments based on dye 8 and 9.............................................111 3.2. Direct laser write experiments for the photocomposites prepared by push-pull dyebased PISs and silica fillers.............................................................................................111 Nethylcarbazole-1-allylidene derivatives.............................................................................. 113 1.1. Photopolymerization kinetics with the newly proposed push-pull dyes in threecomponent photoinitiating systems...............................................................................115 1.2. Light absorption properties of the push-pull dyes..................................................118 2. Potential mechanisms of photopolymerization initiated by N-ethylcarbazole-1allylidene derivatives derivatives ......................................................................................120 2.1. Steady State Photolysis experiments of dye 3, 6, 7, and 8...................................... 120 2.2. Chemical mechanisms in electron transfer reactions for dyes.............................. 125 2.3. Proposed chemical mechanisms..............................................................................130 2.4. ESR spin-trapping experiments...............................................................................131 3. Direct laser write experiments based on dye 3, 6, 7 and 8............................................133 Part 1 Context and bibliography

Chapter 2: 3-(dialkylamino)-1,2-dihydro-9-oxo-9H-indeno[2,1-c]pyridine-4carbonitrile

General Introduction

Nowadays, several cross-sectional studies in modern sciences and technologies based on photo-induced polymerization, namely photopolymerization in the following contexts, have been successively established in academic laboratories and the production of the polymer industry. [START_REF] Zhu | 3D printing of multi-scalable structures via high penetration near-infrared photopolymerization[END_REF][START_REF] Layani | Novel Materials for 3D Printing by Photopolymerization[END_REF][START_REF] Zhang | 3D Printing of Photopolymers[END_REF][START_REF] Zakeri | A comprehensive review of the photopolymerization of ceramic resins used in stereolithography[END_REF][START_REF] Xu | Vat photopolymerization 3D printing for advanced drug delivery and medical device applications[END_REF] Compared to other methods, much attention is currently paid to photopolymerization due to the numerous advantages this polymerization technique exhibits, including low energy consumption, [START_REF] Xie | Preparation of poly (methyl methacrylate) microspheres via photopolymerization initiated by LED light source[END_REF] solvent-free processes, [START_REF] Yilmaz | Photoinduced metal-free atom transfer radical polymerizations: state-of-the-art, mechanistic aspects and applications[END_REF] spatial and temporal control, [START_REF] Corrigan | Seeing the light: advancing materials chemistry through photopolymerization[END_REF] environmentally friendly [START_REF] Bonneaud | Synthesis of α, β-unsaturated esters of perfluoropolyalkylethers (PFPAEs) based on hexafluoropropylene oxide units for photopolymerization[END_REF] and highly efficient process in mild conditions, e.g. ambient temperature. [START_REF] Tasdelen | Photoinduced free radical promoted cationic polymerization 40 years after its discovery[END_REF] To get high-performance photopolymerization reactions under visible light irradiations, different parameters which influence the quality of the photopolymerization process have to be considered, for instance, the selection of the appropriate photoinitiating systems (PISs) combining the suitable photoinitiators (PIs)/co-initiators, the monomer(s) of appropriate viscosity, the presence or not of inorganic fillers within the resins. Intensity of the light source and the wavelength of irradiation are also additional parameters governing the efficiency of the polymerization process. [START_REF] Xiao | Visible light sensitive photoinitiating systems: Recent progress in cationic and radical photopolymerization reactions under soft conditions[END_REF] In photochemistry, the term "photoinitiating systems (PISs)" is a generic name used to design the light-sensitive compound capable to react with the different co-initiators/additives introduced in the resin. [START_REF] Allen | Photochemistry and photophysics of polymermaterials[END_REF][START_REF] Belfied | Photoinitiated polymerization[END_REF][START_REF] Dietliker | compilation of photoinitiators commercially available for UV today[END_REF] Currently, the design of new photoinitiating systems PISs (or PIs) by elaborating dyes of innovative structures or by modifying the chemical structures of well-known dyes already reported as photoinitiators of polymerization is the focus of intense research efforts of photopolymerists. [START_REF] Kocaarslan | Near-IR and UV-LED Sensitized Photopolymerization with Onium Salts Comprising Anions of Different Nucleophilicities[END_REF][START_REF] Sari | Visible light induced free radical promoted cationic polymerization using acylsilanes[END_REF] The photochemical/chemical reactivity of dyes plays a crucial role in the design of high-performance PI systems. [START_REF] Fouassier | Photoinitiators for Polymer Synthesis-Scope, Reactivity, and Efficiency[END_REF] Except the case of one-component PISs (PIs) which combine the dual role of light absorption and generation of reactive species in the absence of co-initiator or additives, [START_REF] Fouassier | One-Component Photoinitiating Systems[END_REF][START_REF] Zhang | Cationic Photoinitiators for Near UV and Visible LEDs: A Particular Insight into One-Component Systems[END_REF][START_REF] Zhang | UV-violet-blue LED induced polymerizations: Specific photoinitiating systems at 365, 385, 395 and 405 nm[END_REF] the singlet (or triplet) excited state lifetime of the PISs (PI) generated upon absorption of light by two-component PISs comprising a PI + additive(s) or by three-component PISs comprising a PI + a co-initiator (s) + additive(s) should be sufficiently long in order the excited photosensitizer to interact with the different additives (e.g. iodonium salt, amine, N-vinylcarbazole (NVK), silane, etc.). In order to be more easily promoted in the excited state, photosensitizer should exhibit high molar extinction coefficients and long-living singlet (or triplet) excited state lifetimes. [START_REF] Grotzinger | Photopolymerization reactions initiated by a visible light photoinitiating system: dye/amine/bis(trichloromethyl)-substituted-1,3,5-triazine[END_REF][START_REF] Kabatc | Photopolymerization reactions initiated by a visible light photoinitiating system: cyaninedye/borate salt/1,3,5-triazine[END_REF][START_REF] Tehfe | New push-pull dyes derived from Michler's ketone for polymerization reactions upon visible lights[END_REF][START_REF] Ligon | Strategies to reduce oxygen inhibition in photoinduced polymerization[END_REF][START_REF] Timpe | Excited states of ketones as electron donors-ketone-iodonium salt systems as photoinitiators for radical polymerization[END_REF] Nowadays, iodonium salts are classically used in photoinitiating systems for photopolymerization due to their relatively good solubilities in most of the photocurable resins. [START_REF] Kabatc | New kinetic and mechanistic aspects of photosensitization of iodonium salts in photopolymerization of acrylates[END_REF][START_REF] Baralle | Polymeric Iodonium Salts to Trigger Free Radical Photopolymerization[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] Brömme | New iodonium salts in NIR sensitized radical photopolymerization of multifunctional monomers[END_REF][START_REF] He | Photoreaction and photopolymerization studies on squaraine dyes/iodonium salts combination[END_REF] On the other hand, amines also play an increasingly important role as a sacrificial electron donor in photopolymerization. Indeed, by their electron lone pair on the nitrogen atom, amines exhibit remarkable electron-donating abilities that can greatly improve the photoinitiation efficiency of PISs, notably by regenerating the photosensitizer in its initial redox state. [START_REF] Jiang | Polymeric photoinitiators containing in-chain benzophenone and coinitiators amine: effect of the structure of coinitiator amine on photopolymerization[END_REF][START_REF] Cook | Photopolymerization kinetics of dimethacrylates using the camphorquinone/amine initiator system[END_REF][START_REF] Wang | Highly efficient sulfur-containing polymeric photoinitiators bearing side-chain benzophenone and coinitiator amine for photopolymerization[END_REF][START_REF] Wei | Novel photosensitive thio-containing polyurethane as macrophotoinitiator comprising side-chain benzophenone and co-initiator amine for photopolymerization[END_REF][START_REF] Bai | Novel benzoxazine resins as photoinitiator comprising benzophenone and coinitiator amine for photopolymerization[END_REF] On the other hand, the rapid development of free radical polymerization (FRP) has resulted in the multiplication of PISs (PIs). However, efficiency of the photoinitiating step is strongly related to the adequation existing between the absorption spectra of the photosensitizers with the emission spectra of the light sources. [START_REF] Balta | Thioxanthone-ethyl anthracene[END_REF][START_REF] Corakci | Long wavelength photosensitizers in photoinitiated cationic polymerization: The effect of quinoxaline derivatives on photopolymerization[END_REF][START_REF] Doğruyol | Thioxanthone based 9-[2-(methyl-phenyl-amino)-acetyl]-thia-naphthacene-12-one as a visible photoinitiator[END_REF][START_REF] Podsiadły | N-substituted quinoxalinobenzothiazine /iodonium salt systems as visible photoinitiators for hybrid polymerization[END_REF][START_REF] Yang | Synthesis and characterization of polymerizable one-component photoinitiator based on sesamol[END_REF] A mismatch between the two spectra can result in an inefficient excitation of the Photoinitiator/photosensitizer. In free radical polymerization, initiating radicals are generated by interactions between the PI and the additives in PISs upon photoexcitation, and the chemical mechanism supporting the formation of initiating species is presented in the reactions (r1)-(r3): [START_REF] Hudson | Penetration of Laser Light at 808 and 980 Nm in Bovine Tissue Samples[END_REF] (r1) PI → 1 PI (h) and 1 PI→ 3 PI (r2)

1,3 PI + Ar2I + → PI •+ + Ar2I • (r3) PI •+ + EDB → PI + EDB •+
Here, the mechanism above supports the process that occurs in free radical polymerization processes initiated by three-component PISs. After promotion of the photosensitizer in its singlet or triplet excited state ( 1 PI or 3 PI) upon light irradiation, PI •+ radicals are generated by electron transfer from the excited 1 PI towards the electron acceptor e.g. the iodonium salt (r2) upon irradiation, while PI can be regenerated from PI •+ in presence of EDB (r3).

In recent years, organic dyes have been extensively studied as PIs. Organic dyes are characterized by appealing features such as high molar extinction coefficients, good light absorption properties over the visible range (mostly in the 400-700 nm range) and high photochemical stability. [START_REF] Liao | Photobleachable cinnamoyl dyes for radical visible photoinitiators[END_REF][START_REF] Ding | Conjugated dyes carrying N, N-dialkylamino and ketone groups: One-component visible light Norrish type II photoinitiators[END_REF][START_REF] Wang | Hierarchically porous π-conjugated polyHIPE as a heterogeneous photoinitiator for free radical polymerization under visible light[END_REF][START_REF] Fouassier | Dyes as photoinitiators or photosensitizers of polymerization reactions[END_REF][START_REF] Zhang | Dihydroxyanthraquinone derivatives: natural dyes as blue-light-sensitive versatile photoinitiators of photopolymerization[END_REF][START_REF] Sokołowska | Styryl dyes as new photoinitiators for free radical polymerization[END_REF][START_REF] Jędrzejewska | Studies on an argon laser-induced photopolymerization employing both mono-and bischromophoric hemicyanine dye-borate complex as a photoinitiator[END_REF][START_REF] Villegas | Aqueous photopolymerization with visible-light photoinitiators: Acrylamide polymerization photoinitiated with a phenoxazine dye/amine system[END_REF][START_REF] Han | Synthesis of novel photochromic spiropyran dyes containing quaternary ammonium salt or cinnamoyl moiety and their properties as photoinitiators[END_REF] Even if numerous dyes have already been investigated in photopolymerization, the development of highly reactive photoinitiating systems capable to efficiently initiate the free radical/cationic polymerization under mild irradiation conditions (low light intensity and under air) is still a challenge.

Concerning the monitoring of the polymerization reaction, different techniques can be used. One of the most popular ones consists in monitoring over time the intensity of the characteristic bands of the monomers by Real-Time Fourier

Transformed Infrared Spectroscopy (RT-FTIR), notably the C=C double bonds of TMPTA, TA, EPOX, etc. Indeed, during the course of polymerization, the monomer is converted as a polymer so that intensity of the C=C band decreases over time. [START_REF] Fouassier | Photoinitiator. Photopolymerization and photocuring: fundamentals and applications[END_REF][START_REF] Xiao | Photoinitiation mechanism of free radical photopolymerizationin the presence of cyclic acetals and related compounds[END_REF] Precisely, conversion of the monomer can be determined by mean of the modification of the peak area (A) in the FTIR spectra, enabling to evaluate the monomer conversion. For acrylates, monitoring of the polymerization reaction can be achieved while using the vibration peak at ~6130 cm -1 or 1630 cm -1 . By monitoring the peak area, conversions of benchmark monomers can be calculated by using the following relationship between conversion and irradiation time: [START_REF] Xu | 3D Printing of polydiacetylene photocomposite materials: two wavelengths for two orthogonal chemistries[END_REF] conversion (%) = (A0 -At)/A0 × 100 (eq 1)

Parallel to the artificial light sources that can be used to initiate polymerization processes, sunlight is still an unusual light source for photopolymerization but tends to be more and more widely used due to the numerous advantages. [START_REF] Allegrezza | Visible and sunlight driven RAFT photopolymerization accelerated by amines: kinetics and mechanism[END_REF][START_REF] Wang | Natural RAFT polymerization: recyclable-catalyst-aided, opened-to-Air, and sunlight-photolyzed RAFT polymerizations[END_REF][START_REF] Ciftci | Sunlight induced atom transfer radical polymerization by using dimanganese decacarbonyl[END_REF][START_REF] Decker | Interpenetrating polymer networks. II. Sunlight-induced polymerization of multifunctional acrylates[END_REF] Indeed, liquid resins can be solidified only in the irradiated area upon sunlight irradiation. It thus constitutes an innovative approach to design and produce three-dimensional patterns using a mild, cheap, unlimited, energy-saving approach. [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] Although sunlight constitutes a free and unlimited light source that partly avoids energy consumption, it is easy to understand that sunlight which possesses a broad emission spectrum is also capable to excite panchromatic photoinitiators. Conversely, artificial light sources often exhibit a more limited emission range than sunlight so that panchromatic photoinitiators can't be efficiently excited. Sunlight-induced polymerization reactions also can be performed at ambient temperature. [START_REF] Fouassier | Photopolymerization reactions under visible lights: principle, mechanisms and examples of applications[END_REF] Particularly, intense research efforts are currently done to discover photoinitiating systems enabling the preparation of polymer composites containing fillers. [START_REF] Xu | Effect of zeolite fillers on the photopolymerization kinetics for photocomposites and lithography[END_REF][START_REF] Liu | Photocurable modification of inorganic fillers and their application in photopolymers for 3D printing[END_REF][START_REF] Zhang | Photopolymerization of Zeolite/Polymer-Based Composites: toward 3D and 4D Printing Applications[END_REF][START_REF] Malas | Fabrication of high permittivity resin composite for vat photopolymerization 3D printing: Morphology, thermal, dynamic mechanical and dielectric properties[END_REF][START_REF] Zhang | Radical photoinitiation with LEDs and applications in the 3D printing of composites[END_REF] The combination of the two topics (sunlight irradiation, composite synthesis) could constitute a major breakthrough in photopolymerization in the coming years.

In conclusion, the unique features of organic dyes strongly absorbing in the visible range and the polymerization of resins containing fillers can greatly contribute to the contribution of photopolymerization in modern manufacturing processes such as the 3D-printing technology. [START_REF] Stansbury | 3D printing with polymers: Challenges among expanding options and opportunities[END_REF][START_REF] Wang | 3D printing of polymer matrix composites: A review and prospective[END_REF] Undoubtedly, it is possible to speed up the development of dye-initiated photopolymerizations, and their long-term impact of photopolymerization on 3D printing. Nowadays, 3D printing has already found an uncontested place in rapid prototype design in industry, and mass production of small-sized objects. [START_REF] Nadgorny | 2D and 3D-printing of self-healing gels: design and extrusion of self-rolling objects[END_REF][START_REF] Chen | 3D printing of ceramics: A review[END_REF][START_REF] Albar | Effective extrusion-based 3D printing system design for cementitious-based materials[END_REF][START_REF] Barnett | Large-scale 3D printing with a cable-suspended robot[END_REF][START_REF] Ben-Barak | Drop-on-demand 3D printing of lithium iron phosphate cathodes[END_REF][START_REF] Wang | Current status and prospects of polymer powder 3D printing technologies[END_REF][START_REF] Ventola | Medical applications for 3D printing: current and projected uses[END_REF][START_REF] Mu | Recent Advances in 3D Printing with Protein-Based Inks[END_REF][START_REF] Buswell | 3D printing using concrete extrusion: A roadmap for research[END_REF][START_REF] Deore | Direct printing of functional 3D objects using polymerization-induced phase separation[END_REF] Photopolymerization is also an interesting technique to produce integrated circuits in microelectronics, namely photolithography as well. [START_REF] Zhang | A versatile 3D and 4D printing system through photocontrolled RAFT polymerization[END_REF][START_REF] Bagheri | Photopolymerization in 3D printing[END_REF][START_REF] Zips | Direct stereolithographic 3D printing of microfluidic structures on polymer substrates for printed electronics[END_REF][START_REF] Lu | Additive manufacturing frontier: 3D printing electronics[END_REF][START_REF] Zarek | 3D printing of shape memory polymers for flexible electronic devices[END_REF][START_REF] Espera | 3D-printing and advanced manufacturing for electronics[END_REF] Finally, photopolymerization can be of interest for many other fields, e.g. i)

regenerative medicine and photocurable dental composites, [START_REF] Price | Effect of stepped light exposure on the volumetric polymerization shrinkage and bulk modulus of dental composites and an unfilled resin[END_REF][START_REF] Topa | Moving Towards a Finer Way of Light-Cured Resin-Based Restorative Dental Materials: Recent Advances in Photoinitiating Systems Based on Iodonium Salts[END_REF] ii) drug delivery, [START_REF] Xu | Vat photopolymerization 3D printing for advanced drug delivery and medical device applications[END_REF][START_REF] Baroli | Photopolymerization of biomaterials: issues and potentialities in drug delivery, tissue engineering, and cell encapsulation applications. Journal of salts-chlorinated solvents complexes[END_REF] iii) tissue engineering, [START_REF] Baroli | Photopolymerization of biomaterials: issues and potentialities in drug delivery, tissue engineering, and cell encapsulation applications. Journal of salts-chlorinated solvents complexes[END_REF]86 iv) 4D printing. 87,88 Concomitant development of these different research fields can also contributes to speed up the rapid expansion of photopolymerization in material chemistry.

State of the Art

Due to the development of visible light photoinitiating systems over the years, numerous dyes differing by their chemical structures have been designed and investigated as visible light photoinitiators to promote free radical polymerization. [89][90][91][92][93][94][95][96][97][98][99] Dyes depicted in this review are separated into three categories: (1) red/near IR light-sensitive dyes, e.g. cyanine, squaraine derivatives, etc.; (2) green light-sensitive dyes, e.g. ferrocene, curcuminoid derivatives, etc.; (3) purple, blue or UV light-sensitive dyes, e.g. cyclohexanone, chalcones derivatives, etc. These different chromophores proved to be as efficient as the traditional UV photoinitiators but adapted for visible light sources. [100][101][102][103] At present, numerous photoinitiating systems are now available for activating polymerization under purple, blue or UV light.

Photoinitiating systems activable in NIR range remains still scarce. Additionally, photopolymerization under purple/blue light or Ultra-Violet light are often applied in industry to perform free radical photopolymerizations. [START_REF] Tasdelen | Photoinduced free radical promoted cationic polymerization 40 years after its discovery[END_REF] Over the years, versatile purple /blue or UV light-sensitive dye-based PISs have been developed to efficiently initiate photopolymerization processes, especially free radical polymerization, under irradiation at 365 nm, 405 nm, 410 nm, or 445 nm. 104,105 2.1 Chemical mechanisms of three-component photoinitiating systems in free radical polymerization.

To systematically investigate the chemical mechanisms occurring during photopolymerization, the dye-based three-component PISs dye/PI/additive(s) can give rise to two specific interactions, namely dye/PI and dye/additive e.g. dye/Iod and dye/EDB (as observed in r2 and r3). Classically, interactions existing between additives can be investigated by observing the decreasing characteristic peaks (in the UV-visible spectra) of dyes upon irradiation, and this experiment is named steady-state photolysis. [START_REF] Xiao | Visible light sensitive photoinitiating systems: Recent progress in cationic and radical photopolymerization reactions under soft conditions[END_REF] To be more precise, since the ground state dye can be promoted in its excited state (dye*) upon visible light irradiation, the excited state of Interactions existing between the partner of the dye/Iod and dye/EDB combinations can be examined by the fluorescence quenching experiments, enabling to better understand the photoinitiation ability of dyes. According to the slope of the classical Stern-Volmer treatment, their Stern-Volmer coefficients (Ksv) can be extracted and the electron transfer quantum yields (et) can be calculated by the equation: 109,110  et = Ksv[additive]/(1+Ksv[additive]) (eq 2)

Figure 1. The mechanism involved in the generation of radicals. Reprinted from ref. [58], Copyright 2020, with permission from MDPI.

Red/Near IR light-sensitive dyes

Contrarily to UV light that can damage eyes, light sources emitting at long wavelengths offer safe irradiation conditions. Parallel to this, use of long wavelength for photopolymerization offers several advantages such as a reduced light diffusion and a deeper light penetration than that achieved with UV light. These irradiation conditions are also healthier and environmentally friendly for both human and environment. Face to these considerations, development of photoinitiating systems activable at long-wavelengths, and notably in the red/near-infrared range has attracted much attention of chemists in recent decades. 111,112 However, even if the use of long-wavelength light sources seems to be more advantageous than what can be obtained with other visible light sources for many aspects, free radical polymerization can be activated more efficiently with purple, blue, or UV light than with NIR light.

Consequently, light sources emitting a purple/blue light or a UV light are still preferred in industry. Indeed, at these wavelengths, photons are more energetic than in the near-infrared range, enabling to initiate more efficiently a polymerization process.

Whatever, the various systems able to initiate free radical polymerizations under NIR light have been reported in previous literature, 113,114 and classified in the next chapter of this PhD thesis.

Cyanine derivatives.

Cyanines are NIR photosensitive dyes that have been extensively studied for their photophysical properties. Cyanines are also characterized by a narrow absorption band whose position can be hardly tuned. Considering the position of their charge transfer band, cyanines can absorb a near-infrared (NIR) light emitted by laser diodes and generate initiating radicals with iodonium salts and various additives. [115][116][117][118] In A.

H. Bonardi's work,115 the authors studied five cyanine borate derivatives named dye 1-5 (see especially when combined with a thermal polymerization. In this case, higher final monomer conversions compared to that obtained with only the photochemical or the photothermal approach were obtained. Final conversions of acrylate monomers reached 70-90% within a short polymerization time upon irradiation at 785 nm with a LED. Interestingly, dye 10 showed no photothermal activity, so that its photoinitiation abilities could be clearly evidenced. Moreover, these PISs were also investigated upon excitation at 940 nm with a LED. Polymerizations showed high final conversions in these conditions (See Table 1). Differing from the other dyes that were totally inactive at 1064 nm, high final conversions of methacrylate resin in the presence of dyes 8 and 9 based PISs were observed upon irradiation at 1064 nm. This is notably due to the fact that these two dyes absorb at 1048 and 1061 nm, respectively. Surprisingly, dye 3

showed poor photoinitiation ability in FRP due to its low absorption at 940 nm.

Squaraine derivatives.

Commonly, the photoinitiation abilities of squaraine derivatives are affected by the different electron-donating properties and the π-conjugated bridge. According to the existing literature, squaraine dyes exhibit interesting absorption properties located in the visible and/or the infrared region. Absorption spectra can also be easily tuned. [117][118][119] In 2019, four squaraines were studied by A.-H. Bonardi et al.. 120 Their chemical structures and their absorption characteristics such as their molar extinction coefficients at the specific emission wavelengths of the light sources, at 785 or 940 nm, are gathered in the Table 2 (From dye 1-dye 4). Moreover, efficient polymerization processes upon exposure to 785 nm or 940 nm LEDs were obtained with four-component PISs comprising the aforementioned squaraine dyes. Final monomer conversions higher than 75% were obtained upon irradiation at 785 nm with the four dyes, and close to 80% at 940 nm LED with three dyes, excepted with dye 2

for the polymerization of a benchmarked acrylate resin, indicating that squaraines are promising NIR dyes for photoinitiation in the NIR range.

Squarilium derivatives.

As previously mentioned, optical properties of squaraines are strongly affected by the different electron-donating groups that can be attached at both ends of the π-conjugated bridge. Contrarily to squaraines for which the two carbonyl groups are attached on the same side of the cyclobutane group, conversely, squariliums possess a carbonyl group and an alcoolate group attached on opposite side of the cyclobutyl group. Interestingly, squariliums exhibited similar photoinitiation abilities than that found for squaraines, and these results are directly related to the similarity of structures between squaraines and squariliums. 121 Similarly, to study the light absorption properties and photoinitiation abilities of squariliums, the free radical polymerizations of acrylates were performed with four-component PISs (dye/Iod/4-dppba/BlocBuilder®MA) including four squarilium dyes (dyes 5-8), 120 and the molar extinction coefficients in the visible range or the infrared region (785 and 940 nm) are summarized in Table 2. Even if lower molar extinction coefficients were determined for these four squariliums compared to squaraines, photopolymerizations upon NIR irradiation (785 or 940 nm laser diode) could however efficiently initiated with these squarilium-based four-component PISs. For instance, high final function conversions of a methacrylate-based monomer Mix-MA in the presence of dyes 5 and 6 could be observed. Final monomer conversion of 84%

for dye 5 and 67% for dye 6 could be determined. Conversely, poor FCs were determined with dyes 7 and 8 (FC~31% for dye 7 and 2% for dye 8). Moreover, high FCs were detected for polymerizations done at 940 nm with a laser diode initiated for three squariliums, namely dyes 5, 7, 8, for which final conversions of monomer were greater than 60%. On the opposite, the FC obtained with dye 6 only reached 12%. To conclude, dye 5 was confirmed as the dye possessing the best photoinitiation ability at both irradiations of light sources 785 or 940 nm. So far, even if the highest final conversions were found for dye 8 at 785 nm laser diode, however, the dye was not able to initiate polymerization at the light source with the same wavelength.

Bodipy derivatives.

Many bodipy derivatives have already been used as NIR light-sensitive photoinitiators in previous publications. The light absorption and photoinitiation properties for this type of derivatives can be improved by substituting various moieties at different positions of the bodipy core. Using this strategy, bodipy found numerous applications in photochemistry. 122 Two bodipy derivatives were examined by A.-H. Bonardi et al. 120 as photoinitiators of polymerization upon irradiation with NIR light sources (940 and 1064 nm laser diodes). Interestingly, the two bodipy exhibited excellent photoinitiation abilities during the polymerization at the two considered wavelengths (FC of monomer~86% for dye 9 and 92% for dye 10 at 785 nm; 84% for dye 9 and 60% for dye 10 at 785 nm). The light absorption properties i.e.

the molar extinction coefficients at the wavelengths of the two light sources of the different bodipy derivatives are given in Table 2 (marked as dyes 9, 10). Interestingly, their strong absorption appeared around 500 nm. However, only low absorptions in the NIR red region were observed, especially at 940 and 1064 nm, which is in contradiction with their high initiation behaviors. To conclude, similar photoinitiation abilities were determined at 785 nm and 940 nm for dye 9. However, photoinitiation at 785 nm with dye 10 is much better than that determined at 940 nm.

Porphyrin derivatives.

Porphyrins are macrocyclic compounds which comprise 18 π-electrons and are characterized by two specific absorption bands, 1) the Soret band which is detected at around 400 nm, and a second absorption band i.e. the Q-band which is detected in the 500-700 nm range. Typically, the light absorption properties of porphyrins is highly dependent to the central metal cation and the substitution of the peripheral macroligands. 123 Thus, three porphyrins with the same ligands but differing by the central metal cation e.g. no central atom, copper, or palladium cation (as shown in However, even if many advantages such as a deeper light penetration in the monomers, were evidenced while using green light sources in the photopolymerization, surprisingly, few photoinitiating systems adapted for these wavelengths have been developed to date, compared to that done for purple or blue/UV light sources at a wavelength of 350-492 nm. Thus, ten novel PISs based on different types of green light-sensitive dyes have recently been reported in the literature, offering additionally new perspectives in pharmacology, etc. green light (520 nm). Four dyes (marked as dyes 1-4 in Table 3) exhibited high photoinitiation abilities (Final conversions ~90% for dyes 1-4 in dye/Iod/NMP three-component systems) upon green LED, consistent with their strong absorptions at 460 or 520 nm. In addition, the ferrocene substituted dye-based PISs were also efficient for the FRP under blue light irradiation (460 nm LED).

Ferrocene substituted derivatives

Curcuminoid substituted derivatives

Curcuminoids, abbrev. CMDs, are curcumin derivatives modified by different functional groups. As a type of linear diarylheptanoids, curcuminoid derivatives have commonly been used as long-wavelength absorption photoinitiators in photopolymerization for cationic and free radical photopolymerization under red or NIR light sources. From the existing publications, Han, W. et al. synthesized four curcuminoid derivatives by modifying the curcumin scaffold using various chemical groups such as carbazole, triphenylamine, dimethylaniline, and phenothiazine (namely dyes 5-8, respectively, see Table 3) to improve their electron-donating abilities as electron donors. 131 In this work, high molar extinction coefficients at a long-wavelength range of 250-700 nm were confirmed by UV-visible absorption spectroscopy. Their photoinitiation behaviors in free radical polymerization were studied upon irradiation with green and blue light sources which are adapted to their wavelengths at maximum molar extinction coefficients (Table 3). The final conversions of monomer, tripropylene glycol diacrylate (TPGDA) in the three-component PISs (CMDs/Iod/TPP), were ~85% for dye 5-8, which are higher than that (FC~80%) of CMD without an additional chemical group. Moreover, the final conversions of TPGDA using the CMDs/Iod/NVK combination also exhibited a similar photopolymerization process, indicating that the high photoinitiation abilities of curcuminoid derivatives in free radical polymerization upon green light sources.

Markedly, similar to the ferrocene derivatives, the curcuminoid derivatives also proved to be efficient photoinitiators in FRP upon irradiation with a blue light source (460 nm LED).

Benzylidene ketone derivatives.

Benzylidene ketone derivatives can act as type II photoinitiators to initiate the free radical polymerization with an electron donor such as an amine. 124 Fu,H. et al. reported that two benzylidene ketones with D-π-A-π-D conjugated structures (abbrev.

TPAK and CZK, marked as dyes 9 and 10 in Table 3, respectively) were used in two-component PISs with an amine (Triethanolamine, TTA) under both irradiations of blue (460 nm) / green (520 nm) light sources. Chemical structures and molar extinction coefficients determined in THF at both 460 nm and 520 nm by UV-visible absorption spectroscopy for dyes 9 and 10 are shown in Table 3. Considering that their wavelengths at highest extinction coefficients are greater than 400 nm, polymerizations could be initiated by the derivatives upon relatively long wavelength light sources e.g. green light. Moreover, the two dyes showed weak extinction coefficients in both 460 and 520 nm in their absorption spectra. To be more precise, dye 9 exhibited a higher extinction coefficient at 460 nm (ε 460nm ~18370 M -1 cm -1 ) compared to that of dye 10 (ε 460nm ~1000 M -1 cm -1 ). However, dye 10 showed a better extinction coefficient at 520 nm (ε 520nm ~410 M -1 cm -1 ) due to its obvious red shift reached above 500 nm compared to that of dye 7 (ε 520nm ~340 M -1 cm -1 ).

Photopolymerization experiments with the initiation of dye/TTA PISs at 520 nm LED were also performed by the authors. The final conversions for TPGDA reached 60%

for both dyes 9 and 10. However, the polymerization process in the presence of dye 10 is much faster than with dye 9, which is consistent with their extinction coefficient at 520 nm. These results indicate the efficient photoinitiation abilities of benzylidene ketone derivatives under green light irradiation. Remarkably, the photobleaching process was observed during the photopolymerization of TPGDA at 460 nm.

Benzylidene piperidones derivatives

Similar to the benzylidene ketone derivatives, there is also a type of chromophores with conjugated D-π-A-π-D structures that exists. These structures are named benzylidene piperidones and these dyes possess excellent visible light absorption and photoinitiation abilities in free radical photopolymerizations. Three benzylidene piperidones were selected and synthesized by authors Hu,T. et al.,126 their chemical structures and absorption properties are gathered in Table 3 and marked as dyes 11, 12, 13, respectively. To conduct the free radical polymerization, dye/Iod two-component PISs were designed using the dyes act as electron donors, which are different from benzylidene ketone derivatives used as electron acceptors. The final conversions of TPGDA in FRPs by the initiation of three dye-based PISs under green light sources (520 nm) were around 80%. However, their polymerization rates are quite different: polymerization process in the presence of dye 11 was the slowest one, while dye 13 took the fastest time compared to the others. These findings are fully consistent with the orders of their molar extinction coefficients: dye 11 < dye 12 < dye 13 at 520 nm (see Table 3), indicating that the light absorption properties could play an important factor capable to affect the polymerization process. Moreover, the FRP under blue light irradiation (460 nm) was also performed. Interestingly, on the investigation on the depth of cure (DoC) in the polymerization process, the polymerizations were performed at longer wavelengths (520 nm vs 460 nm) i.e.

longer wavelengths leading to better DoC.

Naphthoquinone derivatives

As reported in the literature by X. Peng,et. al.,132 two naphthoquinone chromophores, 5-hydroxy-1,4-naphthoquinone (5HNQ, marked as dye 1 in Table 4) and 2-hydroxy-1,4-naphthoquinone (2HNQ, marked as dye 2 in Table 4), exhibited excellent photoinitiation properties in free radical polymerization upon exposure to blue light with different wavelengths (410 nm and 445 nm LED). Since 5HNQ possesses an absorption maximum better fitting with the emission wavelength of the light sources (420 nm for 5HNQ; 330 nm for 2HNQ), it also exhibited a better photoinitiation ability (FC of TMPTA~42%) compared with 2HNQ (FC of TMPTA1 8%) in three-component systems (HNQ/Iod/EDB) at 410 nm LED, which results from the higher molar extinction coefficients at 410 nm of 5HNQ (3690 M -1 cm -1 ) compared to 2HNQ (228 M -1 cm -1 ). Additionally, the authors indicated that the polymerization in the presence of 5HNQ can also be induced at 445nm (ε 445nm ~2819 M -1 cm -1 ) LED. However, in the case of 2HNQ based system (ε 445nm ~88 M -1 cm -1 ), the polymerization was unable to be activated in the same conditions.

Flavonoid derivatives.

Flavonoid with N,N-diphenylamino group, 2-(4-(diphenylamino)phenyl)-3hydroxy-4H-chromen-4-one (abbreviated as 3HF-OH, dye 4 in Table 4), and its derivatives by substituting the hydroxyl group at the 3-position with benzoyl chloride and benzenesulfonyl chloride, flavonol carboxylate (abbreviated as 3HF-C, dye 5 in

Peripheral groups substituted cyclohexanone derivatives.

So far, cyclohexanone derivatives substituted with the peripheral groups, named as "ketones" in the following context, have occupied an important part in the free radical polymerization due to the substitution of central cyclohexanone by peripheral groups of different structures. By carefully selecting the peripheral groups, significant impacts on the absorption properties and thus the photoinitiating properties could be demonstrated. Thus, cyclohexanone derivatives could be functionalized with peripheral groups to achieve better photoinitiation abilities than that obtained with commonly used photoinitiators. Recently, this type of chromophores was widely studied as reactive photoinitiators for the free radical polymerization of acrylate monomer under visible light irradiation. As reliable results, their high light absorption properties at the near-UV and visible range made these dyes relatively sensitive to different emission wavelengths of LEDs, e.g. 365 nm, 385 nm and 405 nm. Besides, some other advantages such as photobleaching properties, high polymerization efficiency, and extinction coefficients were also reported densely in recent years. For instance, a furan substituted cyclohexanone, namely BFC, was systematically investigated as photoinitiator by Li,J. et. al.. 134 The results confirmed its high extinction coefficients in acetonitrile (ε max ~38200 M -1 cm -1 , ε 365nm ~35100 M -1 cm -1 ,

ε 385nm ~32600 M -1 cm -1 , ε 405nm ~11800 M -1 cm -1 )
, thus an excellent photoinitiation ability could be achieved in FRP under purple or blue light sources with various wavelengths, e.g. 365 nm and 405 nm. In addition, photobleaching was also observed after ~2min of irradiation on BFC solution, of which color turned from yellow to colorless. Inspired by this research, the newly developed ketones and their exact impact on free radical polymerization are systematically discussed in the following part.

Thiophene or furane substituted cyclohexanone derivatives.

In this work, six cyclohexanone derivatives substituted by thiophene and furane moieties were examined, namely dyes 6 to 11. 135 Here, their chemical structures, absorption properties, final conversions of acrylate monomer (TA) in the presence of ketone-based PISs upon irradiation with a 405 nm LED are gathered in Table 4. The ketones strongly absorb in the near-UV visible range from 365-372 nm, and their highest molar extinction coefficients are standing at high levels ranging from 25020~34920M -1 .cm -1 . Particularly, although all six ketones exhibited high molar extinction coefficients at 405nm (ε 405nm

) in the range between 7900~12000M -1 .cm -1 .

However To continue the works on the influence of the peripheral groups on cyclohexanone derivatives used as photoinitiators, cyclohexanones decorated with various groups were introduced in three-component systems to activate FRP upon 405 nm LED irradiation. Xu, Y. et al. investigated systematically twelve tertiary amines or anthracene substituted cyclohexanone derivatives (namely ketone 1-12 in Table 5) in combination with an amine and an iodonium salt, used for the initiation of FRP. 136 Their chemical structures and molar extinction coefficients in acetonitrile are summarized in Table 5. High molar extinction coefficients (greater than 40000M -1 •cm -1 )

were found in the visible region from 430 and 440 nm for all the tertiary amine substituted ketone derivatives (dye 1-6). On the other hand, the molar extinction coefficients of six anthracene substituted cyclohexanone derivatives (dye 7-12) are located at the same wavelengths ~250 nm and their εmax varied from 80,000 to 180,000 M -1 •cm -1 , which are much lower than that of the tertiary amine substituted ketones. The results of their remarkable light absorption abilities can be easily detected with their photoinitiation abilities in FRP: tertiary amine substituted cyclohexanone derivatives could induce the polymerization and high FCs varying from 80%-90% in the air were obtained, while the FCs in the presence of anthracene substituted cyclohexanone derivatives only stood at lower levels, ranging from 40% to 90%. Interestingly, tertiary amine substituted cyclohexanone derivatives furnished lower FCs of acrylate monomer for the polymerization experiments in laminate compared to that under air (as thick samples). This phenomenon could also be observed with dyes 7, 9, and 10, indicating the potential possibilities to use such dye-based PISs for the initiation of FRP in larger quantities. the initiation with four ketone-based systems (ketone 1 to 4). Here, both of their absorption properties and final conversions of TA during FRP were listed in Table 7.

Finally, silica fillers containing composites were successfully fabricated by 3D

printing under the initiation of ketone-based PISs (Figure 4).

The new composites open new potential application prospect for free radical polymerization. 1

 max ~416 nm ε max ~18800 M -1 cm -1 ε 405nm ~17900 M -1 cm -1 ~85% @405 LED 2  max ~405 nm ε max ~36200 M -1 cm -1 ε 405nm ~36200 M -1 cm -1 ~64% @405 LED 3  max ~405 nm ε max ~37700 M -1 cm -1 ε 405nm ~37700 M -1 cm -1 ~68% @405 LED 4  max ~434 nm ε max ~30800 M -1 cm -1 ε 405nm ~23000 M -1 cm -1 ~56% @405 LED  max ~366 nm ε max ~21700 M -1 cm -1 ε 405nm ~11100 M -1 cm -1 ~73% @405 LED  max ~375 nm ε max ~25200 M -1 cm -1 ε 405nm ~15500M -1 cm -1 ~64% @405 LED  max ~375 nm ε max ~22000 M -1 cm -1 ε 405nm ~14500 M -1 cm -1 ~63% @405 LED  max ~375 nm ε max ~23400 M -1 cm -1 ε 405nm ~15800 M -1 cm -1 ~56% @405 LED  max ~350 nm ε max ~16500 M -1 cm -1 ε 405nm ~3100 M -1 cm -1 ~69% @405 LED 10  max ~350 nm ε max ~2800 M -1 cm -1 ε 405nm ~900 M -1 cm -1 ~60% @405 LED 11  max ~355 nm ε max ~23600 M -1 cm -1 ε 405nm ~5300 M -1 cm -1 ~69% @405 LED 12  max ~355 nm ε max ~12900 M -1 cm -1 ε 405nm ~2700 M -1 cm -1 ~72% @405 LED 13  max ~360 nm ε max ~20200 M -1 cm -1 ε 405nm ~5700 M -1 cm -1
~66% @405 LED

In this context, strong maximum absorbances for the thirteen ketone derivatives were found in the near-UV or visible region and two groups of ketones could be identified: 1) ketone-1 to ketone 4, 2) ketone-5 to ketone-13, were separated on the basis of their different wavelengths at highest molar extinction coefficients ( max ) in a broad wavelength range. Precisely, the first group (ketone-1 to ketone-4) displayed absorption maxima in the visible region (from 405 nm to 435 nm). Conversely, the absorption maxima of the second group (ketone-5 to ketone-13) are located at the near-UV region (from 350 nm to 375 nm). Interestingly, similar to the ketone 9 and 10 mentioned in the part of 2.1.3, the wavelengths at the highest molar extinction coefficients ( max ) of ketone-1 to ketone-3 are also overlapped with the emission spectra of the 405 nm LED. Furthermore, among all the ketones, ketone-2 and ketone-3 exhibited the highest values for both the highest molar extinction coefficients (ε max ~36200 M -1 cm -1 for ketone-2; ε max ~37700 M -1 cm -1 for ketone-3) and molar extinction coefficients at 405 nm (ε 405nm ~36200 M -1 cm -1 for ketone-2; ε 405nm ~37700 M -1 cm -1 for ketone-3). However, the ketone-1/amine/Iod (a low molar extinction coefficient was determined for ketone-1: ε max ~18800 M -1 cm -1 ) furnished the highest final conversion of acrylates (~85%) during the free radical polymerization at 405 nm LED irradiation. FC values of acrylates upon initiation with ketone-2/amine/Iod and the ketone-3/amine/Iod three-component systems just reached at 64% and 68%, respectively, indicating that the high absorption properties of ketones are not directly linked to their excellent photoinitiation behaviors.

Chalcone derivatives.

Chalcones are a class of natural compounds that have attracted a widespread attention in various research fields due to their facile oxidation and good hole-transport abilities. Moreover, benefiting from their highly conjugated and planar structures, chalcones have also exhibited good optical properties and UV-photosensitivity so that these dyes were used for the design of photoinitiators in radical chemistry. 138 However, several rare three-component PISs (e.g. chalcone/Iod/amine) exhibited perfect performance upon LED irradiation below 365 nm. Fortunately, recently, major improvements concerning the photoinitiation ability of chalcones in the visible range have been achieved by developing chalcones exhibiting high molar extinction coefficients. Especially, this goal could be achieved by varying the substitution pattern of chalcones and efficient photoinitiators activable in the visible range and capable to initiate free radical polymerizations could be obtained.

Furane or ferrocene-based mono-chalcone derivatives.

Plenty of mono-chalcone derivatives based on furane and ferrocene moieties were used to induce the free radical polymerization of the epoxy compounds in previous literature. Extracted from the results published by Chen,H. et al.,139 five mono-chalcone derivatives with methoxy and iodine groups marked as chalcones 1-5 in this review, exhibited good photoinitiation behaviors in three-component systems (chalcone/Iod/EDB) and polymerization rates of FC~89.9%, 79.3%, 84.6%, 81.8% and 90.9% could be determined for chalcones 1-5, respectively during the FRP of PEG. These five dyes were the best candidates among the series of 23 investigated chalcones investigated in this work and presented in Table 8. Markedly, the authors indicated that formation of hydrogels was possible and could be prepared by laser write experiments. Excellent swelling ratios (55% for chalcone, 61% for chalcone 5) could be evidenced for the resulting polymers, by immersing the PISs/monomer products in deionized water for 24h.

Carbazole or triphenylamine-based mono-chalcone derivatives.

Inspired by the fact that good optical properties can be obtained with cyclohexanone-based chalcones, several photoinitiators based on chalcones have been examined by our group. In Chen,H et al.'s work,140 carbazoles and triphenylamines (TPA) were used to elaborate chalcones that proved to be excellent photoinitiators and 10 different carbazole or TPA-based chalcones (marked as chalcones 6-15 in Table 8) exhibiting remarkable visible light absorption properties were introduced into three-component PISs (Chalcones/Iod/EDB) for the free radical polymerization of PEG under 405 nm LED, leading to high polymerization rates (see light absorption properties and final conversions for chalcones 6-15 in Table 8) during the polymerizations. Interestingly, reversible deformation effects were observed by investigating the swelling ratio of 3D objects obtained from the chalcone-induced photopolymerizations (swelling ratios~60%), due to the thermal or hydrophilic responses of hydrogel PEG which evidence the possibilities to use chalcones for 4D activation in the future. Table 8. Chalcone derivatives applicable in PISs. 139,140 No.

Chalcone Absorption properties Conversion (PEG)

1 9, several chalcones with high light absorption properties e.g. chalcones 2, 5, 6, 9 (their maximum extinction coefficients are greater than 20000 M -1 cm -1 ) furnished high final conversions (FC) of PEG ranging from ~86% to ~95% at 375 nm LED in the FRP.

 max ~423 nm ε max ~21190 M -1 cm -1 ε 405nm ~18160 M -1 cm -1 ~90% @405 LED 2  ~79% @405 LED 3  max ~363 nm ε max ~15770 M -1 cm -1 ε 405nm ~5800 M -1 cm -1 ~85% @405 LED 4  max ~362 nm ε max ~18870 M -1 cm -1 ε 405nm ~4330 M -1 cm -1 ~82% @405 LED 5  max ~344 nm ε max ~16950 M -1 cm -1 ε 405nm ~790 M -1 cm -1 ~91% @405 LED 6  max ~425 nm ε max ~8930 M -1 cm -1 ε 405nm ~7450 M -1 cm -1 ~37% (thick) ~68% (thin) 7  max ~369 nm ε max ~20520 M -1 cm -1 ε 405nm ~4830 M -1 cm -1 ~15% (thick) ~86% (thin) 8  max ~369 nm ε max ~17740 M -1 cm -1 ε 405nm ~5960 M -1 cm -1 ~29% (thick) ~86% (thin) 9  max ~408 nm ε max ~23900 M -1 cm -1 ε 405nm ~23850 M -1 cm -1 ~82% (thick) ~82% (thin)  max ~370 nm ε max ~21100 M -1 cm -1 ε 405nm ~7020 M -1 cm -1 ~66% (thick) ~79% (thin)  max ~360 nm ε max ~24900 M -1 cm -1 ε 405nm ~3530 M -1 cm -1 ~52% (thick) ~77% (thin)  max ~405 nm ε max ~18740 M -1 cm -1 ε 405nm ~18740 M -1 cm -1 ~92% (thick) ~71% (thin)  max ~430 nm ε max ~7990 M -1 cm -1 ε 405nm ~6760 M -1 cm -1 ~53% (thick) ~58% (
Interestingly, FCs of chalcones 7 and 9 also reached ~90% at 405 nm during polymerization. These results confirmed that bis-chalcones can be used as efficient photoinitiators to activate free radical polymerization under blue or UV light irradiations e.g. 375 and 405 nm. However, in the cases of ferrocene-based bis-chalcones (bis-chalcones 3 and 8), the acrylate monomer could not be polymerized at all. Hence, the organic dyes with ferrocene groups were identified as being inefficient photoinitiators in free radical polymerization processes. No.

Bis-Chalcone

ε 405nm ~13670 M -1 cm -1 ε 375nm ~24000 M -1 cm -1 ~89% @375 LED ~91% @405 LED 10  max ~350 nm ε max ~49300 M -1 cm -1 ε 405nm ~2220 M -1 cm -1 ε 375nm ~24500 M -1 cm -1
~82% @375 LED ~62% @405 LED

Carbazole-based derivatives.

Owing to the outstanding electron-donating ability of the carbazole group, carbazole derivatives with a peripheral π -conjugated system were used in various photoinitiating systems for both the free radical and cationic polymerizations. 142 A high molar extinction coefficient ( ε max~18600 M -1 cm -1 ) at the maximum absorption wavelength (λmax~342nm) was observed for BPC (see Table 10, carbazole 1) and the photopolymerizations of TMPTA (or EPOX) were performed by initiating polymerizations with two-component BPC/Iod and three-component BPC/Iod/EDB with a 365nm LED. Remarkably, the three-component PIS could efficiently initiate the FRP of TMPTA and its FC could reach a high level (~70%, see Table 10). Interestingly, the FRP could also be initiated with BPC alone without adding any co-initiator upon irradiation at 365 nm. This photoinitiator also exhibited excellent efficiency which boosts a high FC at ~65% in photopolymerization.

Similarly, eight carbazole derivatives were subsequently reported by Liu, S. et al.

to investigate their photochemical properties more systematically. 143 Their chemical structures, light absorption properties and final conversions of TMPTA in the same polymerization conditions than previously used are also listed in Table 10. Among them, carbazoles 2-5 are benzophenone-carbazole compounds, which exhibited better absorption properties at 365 nm than acetyl or styryl-substituted carbazoles (carbazoles 6-9, in Table 10). Here, carbazoles 6-9 were used as reference compounds for the four investigated dyes. With the same methods and conditions to that used for carbazole 1, the photopolymerization initiated with carbazoles 2-9 based three-component PISs were conducted at 365 nm LED and the function conversions of TMPTA reached ~60% for carbazoles 2-5, which are better than that of carbazoles 6-9 for which lower monomer conversions ranging between 43%-60% could be obtained. Moreover, the FCs achieved with the three-component PISs proved to be better than that obtained with the two-component PISs. Markedly, carbazoles 2-5 could also initiate the FRP alone and the highest FCs were achieved ~64% by carbazole 2 which is of similar structure to that of carbazole 1. These results confirmed the certain possibility that carbazole derivatives can efficiently promote the FRP as excellent photoinitiators both with (or without) external co-initiators.

Table 10. Carbazole derivatives applicable in PISs. 142,143 No.

Carbazole Absorption properties

Conversion

(TMPTA)

1

 max ~342 nm ε max ~18600 M -1 cm -1 ε 365nm ~21800 M -1 cm -1 ~70% @365 LED 2  max ~334 nm ε max ~13910 M -1 cm -1 ε 365nm ~3270 M -1 cm -1 ~63% @365 LED 3  max ~325 nm ε max ~13900 M -1 cm -1 ε 365nm ~2210 M -1 cm -1 ~62% @365 LED 4  max ~334 nm ε max ~13350 M -1 cm -1 ε 365nm ~3460 M -1 cm -1 ~62% @365 LED 5  max ~325 nm ε max ~12400 M -1 cm -1 ε 365nm ~2170 M -1 cm -1 ~61% @365 LED 6  max ~332 nm ε max ~8590 M -1 cm -1 ε 365nm ~70 M -1 cm -1 ~43% @365 LED 7  max ~318 nm ε max ~7880 M -1 cm -1 ε 365nm ~130 M -1 cm -1 ~49% @365 LED 8  max ~339 nm ε max ~3320 M -1 cm -1 ε 365nm ~500 M -1 cm -1 ~55% @365 LED 9  max ~337 nm ε max ~3540 M -1 cm -1 ε 365nm ~540 M -1 cm -1
~50% @365 LED 3. Experimental section.

Preparation of investigated ketones & push-pull dyes.

All the investigated molecules including ketones and push-pull dyes were prepared by the partner laboratory of the Marseilles University (Dr. Frederic Dumur).

Other Materials.

The photopolymerizable monomers (TMPTA and TA), the co-initiator (iodonium salt; Speedcure 938) and electron donor (amine; Speedcure EDB, triethanolamine) were all purchased from Sigma-Aldrich, and their corresponding molecular structures are shown in Scheme 1. Particularly, no organic solvents were involved in this work. Iod: amine=0.1%:2%:2% in 1g monomer in weight percent) upon exposed to visible LED with wavelength at 405nm under air at room temperature, and all samples were dropped between 2 polypropylene films with 2 drops deposited.

Photopolymerization reactions monitored by Real Time

Fourier Transformed Infrared Spectroscopy (RT-FTIR).

For checking photopolymerization reactions, Light Emitting Diodes (LEDs) with 405nm was used for photopolymerization of TMPTA/TA, and at the sample surface the incident light intensity I0 was 110 mW.cm -2 . The photopolymerization reactions of photoinitiating systems were carried out between polypropylene (PP) film at room temperature in laminated. The conversion of the C=C double bond during free radical photopolymerization process of monomer (TMPTA/TA) was continuously monitored by Real Time Fourier Transformed Infrared Spectroscopy (RT-FTIR, JASCO FTIR 4100) and focused on the stretching vibration peak of acrylate C=C double bond between ∼1630 cm -1 or ∼6150 cm -1 versus irradiation time (for thick sample NIR was used to follow the polymerization process). 109,110,143 3.5.

UV-Visible absorption spectroscopy.

The UV-Visible absorbance properties of the ketone/push-pull dyes, different photoinitiating systems were characterized by JASCO V730 UV-visible spectrometer.

3.6.

Electron transfer reaction for ketones/push-pull dyes characterized by Cyclic Voltammetry and Fluorescence approaches.

Electron Spin Resonance (ESR) Spin Trapping (ESR-ST).

Electron spin resonance-spin trapping experiments were carried out using an X-band spectrometer (Bruker EMXplus). The radicals were observed under nitrogen saturated media at room temperature. N-tert-Butylphenylnitrone (PBN) was used as a spin trap agent in acetonitrile. 109,110 ESR spectra simulations were carried out using PEST WINSIM software.

3D printing experiments.

A computer programmed laser diode (Thorlabs) with spot size around 50 μm was

Molecular Modeling

Frontier Molecular Orbital calculations were performed by the Gaussian 03 suite of programs. 149,150 The simulation of UV absorption spectra and the triplet state energy levels for ten ketones were calculated with the time-dependent density functional theory at the MPW1PW91/6-31G* level of theory on the relaxed geometries calculated at the UB3LYP/6-31G* level of theory.

Photocomposites preparation

Photocomposites were prepared with three-component systems (dye/Iod/EDB, 0.1w%/2%/2%, w/w/w/) and a weight content of 20% was used for the silica fillers dispersed in the monomer (TA) at room temperature and under air.

Sunlight induced polymerization

The sunlight polymerization experiments were carried out around 12 am-14 pm on Nov., 3rd., 2020, in Mulhouse Area (+77°43′ E, -47°75′ N) of France, and the weather conditions were weather rainy, grey clouds, and the temperature is 10~11°C

(see Figure 5). The acrylate function conversion was followed by FTIR spectroscopy. The abilities of all the ketone-based three-component photoinitiating systems for the polymerization of acrylate monomers were investigated. Typical acrylate function conversion vs irradiation time profiles obtained using RT-FTIR are given in Figure 1.

For the TMPTA monomer, the highest final conversions (FCs) of the acrylate function groups among the presence of ketone (1 to 10)/Iod/amine was about 67% (ketone 5)

after LED irradiation for 400 seconds. In the case of TA, the FCs were higher than those of TMPTA except the one initiated by ketone 5-based PISs. The specific data of TMPTA or TA conversions were clearly illustrated in Table 1.

When using triethanolamine (TEA) as an electron donor in ketone/Iod/amine PISs, the highest FC (for ketone 8/Iod/TEA) was raised significantly, and beyond 70% can be attained after the LED irradiation for 400 seconds. Moreover, compared with TMPTA, TA led to more efficient processes. The use of TEA instead of EDB also improved the polymerization efficiency. Typical acrylate function conversion-time profiles for same photoinitiating systems are given in Figure 1a and the FCs are summarized in Table 1. As illustrated, the ketones 8, 9, and 10 proved to be excellent candidates to promote the photopolymerization efficiency and FCs of TA in the presence of TEA. For ketones 1-7, poorer polymerization profiles were observed.

Interestingly, we observed irreversible processes of photobleaching on ketones 9

and 10 upon irradiation with LED@405 nm (Figure 1b,1c). These changes might be caused by reaction of ketone with iodonium salt and/or amine (see the chemical mechanisms below, Scheme 2).

Proposed Chemical Mechanisms

For this part, the three best ketones (8, 9 and 10) were mainly selected for the mechanistic studies. respectively. However, after irradiation of LED@405 nm, obvious intensity decreases were observed for all the three ketones. Interestingly, the reaction process in the photoinitiating system with ketone 8 cost ~50 min, while others cost only less than 1 min. According to the proposed mechanism shown in Scheme 2, the reaction process can be divided to two parts: 1) the ketones in combination with iodonium salt, and 2) ketones in combination with amine. Two partial reactions are presented by UV visible absorption spectra both in Figure 3 and Figure 4, respectively. In the UV-visible absorption spectra presented above, all three ketone-based photoinitiating systems exhibited relatively high reactivity which was in agreement with the final conversions of the acrylate functions and also rates of polymerization initiated by these PISs. As strong evidences to the presence of ketone/Iod or ketone/amine interactions, the peaks of ketones (characterized by significant absorption of 425< <475 nm for ketone 8; 375< <425 nm of ketone 9; 375< <425 nm of ketone 10) are disappearing in any case. However, the photolysis process of ketone 8/Iod or ketone 8/amine in acetonitrile under irradiation of the LED@405 nm is very slow (about 50 min for disappearance of peaks, see Figure 3a, 4a), while the very high photosensitivity of the ketone 9, 10/Iod or ketone 9, 10/amine combinations can be observed which took less than 60s (Figure 3b, 3c, 4b, 4c).

Therefore, two-component PISs based on ketones 9 or 10 exhibited stronger activity than ketone 8. Overall, ketone 8 showed favorable photostability during photolysis process upon exposure to the 405 nm LED owing to its lower extinction coefficients (ε @405 =7530 M -1 cm -1 ; see Table 1) which could hinder efficiency of photolysis, compared to ketone 9 and 10 with high extinction coefficients (ε @405 = 52130 and 32070 M -1 cm -1 , respectively; see Table 1). Moreover, new absorption in 300 nm < <350 nm in UV spectra were observed in the presence of ketones 9 and 10.

Consumption of Ketones in Photolysis Reactions

The percentage of consumption of ketone vs. irradiation time is also summarized from changes in their respective UV-vis spectra by comparing three-component PISs (ketone/Iod/amine) with two-component PISs (ketone/Iod or ketone/amine), which is illustrated in Figure 5. According to Figure 5a, it is obvious that percentages of consumption of ketone 8 achieved by three-component PIS (ketone 8/Iod/amine) and two-component PIS (ketone 8/Iod) are much higher than that of ketone 8/amine combinations e.g. consumption of ketone 8 = 65% for ketone 8/Iod/amine or 55% for ketone 8/Iod vs. 35% for ketone 8/amine. For the three-component systems, we proposed that r1, r2 and r3 occurred in ketone 8, 9 and 10/Iod/amine systems. The generated radicals are given in Scheme 2. However, the ketone 8/amine two-component PIS interaction was very inefficient for which a weak interaction was observed compared to ketone 9 or 10/amine systems.

Interestingly, as shown in Figure 5b, for the same irradiation time, the ketone 9/Iod could achieve the highest consumption of ketone, while the ketone/Iod/amine led to the lowest consumption. Moreover, it is proposed that EDB •+ radical can be formed by electron transfer from EDB, as an N-aromatic electron donor to ketones (r4), as well as ketone can be regenerated from ketone-H • in the presence of iodonium salt (r5), which decelerated consumption of ketones.

In the case of ketone 10/Iod/amine system, the consumption profile of ketone 10 with three-component PIS (ketone 10/Iod/amine) was quite close to that reached when using two-component PISs based on ketone 10/Iod or ketone 10/amine combinations:

i.e. the consumption of ketone 10 for 3 PISs reached at 75%-80% (see Figure 5c).

Scheme 2.

Proposed photoinitiation step mechanisms of ketones/Iodonium/amine redox combination.

Steady State Photolysis of Other Ketones characterized by UV-vis Spectra

Steady-state photolysis experiments for the three component systems with ketones 1-7 were also carried out where the changes of UV-vis absorption in these PISs were clearly observed (Figure 6). Light absorption properties e.g. maximum absorption wavelengths λmax; extinction coefficients at λmax (ε max ) and extinction coefficients at the emission wavelength of the LED@405 nm(ε @405nm ), were determined and listed in Table 1. New light absorption with 400 nm < < 500 nm for ketone 1, 2, 3, 7 and 450 nm < < 550 nm for ketone 4, 5, and 6 were observed which was in full agreement with the proposed chemical mechanism of photolysis processes of ketones 8, 9, 10 as mentioned above. To conclude, photolysis is found favorable for all the ketones investigated.

However, the photolysis processes of ketones 1 to 6/Iod/amine under irradiation of the LED@405 nm were very fast (Figure 6, a-f) compared to that of ketone 7 (see Figure 6, g) which lasted ~300 min (Figure 6, g). The similar behavior was observed for the photolysis of ketone 8 mentioned above. Furthermore, photolysis of ketones 1, 2, 3 which possess larger maximum absorption wavelengths (locate at 371-372nm) also shown shorter reaction process, compared to ketones 4, 5, 6whose maximum absorption wavelengths located at 366-368nm (irradiation time less than 60s for ketones 1, 2, 3 vs. 180s for ketones 4, 5, 6, see Figure 6).

Cyclic Voltammetry and fluorescence experiments to study electron transfer reaction for ketones

The electron transfer was found to be favorable for ketones 8, 9, 10-based systems, which was confirmed by Cyclic Voltammetry experiments (Figure 7). In our work, each cyclic voltammogram was obtained by scanning between 0 and 2000 (or -2000) mV in acetonitrile using tetrabutylammonium hexafluorophosphate as the supporting electrolyte. All voltammograms showed an oxidation wave (no reduction peak could be observed in this range): the oxidation peaks of ketones 8, 9, 10, (located at 1.14 V, 1.24 V, 1.30 V versus the saturated calomel electrode (SCE), respectively) corresponds to the oxidation of ketone and the subsequent formation of ketone •+ . In contrast, the cyclic voltammetry with a potential sweep range from -2000 mV to 0V (against SCE) indicates that no current in reduction cyclic for ketone 8 (Figure 7a) can be observed. The reduction peaks of ketones 9 and 10 (located at -1.75 V, -1.69 V, respectively) correspond to the transition from the neutral ketone to its reduced state, ketone •-. This suggests the crucial role of ketones in the FRP process generating radicals to initiate monomer polymerization (see Scheme 2 above). By extracting Eox and Ered from the cyclic voltammograms (see Table 2), it is obvious that electron transfer processes in oxidation process of ketone 8 and whole cyclic processes of ketones 9, 10 are irreversible reactions. Fluorescence and UV-vis absorption associated experiments in acetonitrile for ketones 8, 9, 10 were demonstrated (see Figure 8) to seek the theoretical feasibility of interactions between ketone/Iod and ketone/amine. First, the crossing point of the UV-vis and fluorescence spectra allows the determination of the first singlet excited state energy (ES1) for ketones (ES1 = 2.58 eV for ketone 8, 2.66 eV for ketone 9 and 2.71 eV for ketone 10; Table 2). Second, the free energy changes (GIod or GEDB) for the electron transfer reactions between: (1) ketones as electron donors and Iod as electron acceptor; (2) amine (Speedcure EDB or TEA) as electron donors and ketones as electron acceptor were calculated from the equation 1 (or equation 2, respectively) using the oxidation potentials Eox (or Ered, respectively) and the excited state energies (ES1) of ketones (Table 2). Ketone/Iod interaction is proposed to hinder the transition from singlet state to the triplet state of the ketones. Fluorescence quenching on ketone 9/Iod was found i.e. a linear quenching on ketone 9 was indicated in fluorescence quenching and Stern-Volmer treatment data (see Figure 9). Iodonium salt (Speedcure 938) also acted as a relatively good quencher on ketone 10. However, there was no obvious fluorescence quenching observed on ketone 8/Iod. These phenomena are fully accordant with the results from laser flash photolysis and ESR-spin trapping experiments. Interaction constant (Ksv) and the electron transfer quantum yields ( et(S1) ) of ketones 9 and 10 from the slope of the Stern-Volmer were also calculated by Stern-Volmer equation (see Table 3). Fluorescence lifetime values of ketones 8, 9, 10 are also obtained in the present work and demonstrate very short time (less than 1.4 ns corresponding to the resolution time of our set-up; Table 3). No obvious fluorescence quenching of 1 ketones by amine was observed. a: from the equation presented in eq 5. 109,110 The presence of triplet states was also examined by laser flash photolysis.

Interestingly, triplet states were observed for ketones 9 and 10, but no triplet state for ketone 8 (Figure 10). There is a convincing explanation verified by the optimized geometries as well as the frontier orbitals.

ESR spin-trapping experiments based on ketones

For a better understanding of the ketone/Iod/amine interaction, ESR-spin trapping experiments were carried out on ketone/Iod solution under N2 in presence of PBN as the spin trap agent (Figure 11). The simulation gave the following hfc constants for the PBN spin adducts: aN = 14.8 G; aH = 2.8 G, and a radical is detected: and faster writing process than ketone 8 in the case for ketone 9, due to the highest photosensitivity of the ketone-based PISs as mentioned. Compared with ketones 8 and 9, an efficient photopolymerization processes also occurred in the irradiated area, and there was a difference for ketone 10 contained photoinitiating system, which was slower to approach a better 3D product with longer irradiation time.

Finally, the results of numerical optical microscopy obtained here clearly showed that the selection of higher light absorption properties for ketones (e.g. ketone 9 > ketone 10 > ketone 8) must be considered as important aspects to promote free radical photopolymerization process. Using well-established phosphine-oxides (TPO or TPO-L) as benchmark photoinitiators, the writing speeds are similar than those for the best developed systems proposed here. Recently, dyes with push-pull structures and excellent photoinitiation abilities as sensitive photoinitiators for visible-light-induced polymerizations have drawn a huge attention from the academic community. Considering that the absorption spectra of numerous dyes are difficult to tune, the push-pull effect existing in dyes comprising an electron donor connected to an electron acceptor by mean of a pi-conjugated spacer can be efficiently tune by elongating the length of the pi-conjugated spacer introduced between the electron donor and the electron acceptor. By improving the electronic delocalization, a significant increase of the molar extinction coefficients can also be achieved. [155][156][157][158][159][160][161][162][163][164][165][166] Thus, push-pull dyes have been widely used as novel PIs due to their interesting features, that were also valorized in applications ranging from Organic Electronics, e.g. organic photovoltaics (OPVs), 167, 168 organic light-emitting diodes (OLEDs), 169 organic-field effects transistors (OFETs) 170 to nonlinear optics (NLO) [171][172][173][174][175][176][177] and waste-water treatment. 178 In our works, numerous push-pull dyes have been investigated as PIs, as detailed below. ). 58

No. LED irradiation time (400s) at room temperature (Figure 1). The specific polymerization of Iod/Amine two-component PIS without dyes was also given and defined as the curve 0. As depicted in the Figure 1, typical acrylate function conversion vs irradiation time profiles indicate that the presence of the dyes is essential to promote the free radical photopolymerization process, based on the comparison established with the Iod/amine two-component systems. As exceptions, for dyes 6, 11 and 18, the three-component systems seem to be less efficient than the Iod/amine system (curve 0). In the specific case of dyes 5 and 17, the two dyes proved to efficiently promote the photopolymerization of TA and the highest conversions and fastest polymerization processes were obtained with these two dyes. Beyond 90% of final reactive function conversions were attained within 50 seconds. Interestingly, the two dyes have similar chemical structures except that the dye 17 possesses an additional aromatic ring compared to 5. Additionally, an irreversible photobleaching process was evidenced with the dye 5 during the photopolymerization process upon irradiation with the LED@405nm, what was not observed for the elongated version of dye 5, namely dye 17.

On the other hand, for all the other dyes, several excellent polymerization profiles were also obtained still upon irradiation with a LED@405nm. Among all dyes, ten of them including the two dyes 5 and 17 (gathered in Table 1) furnished high final conversions beyond 80% and short polymerization times (the polymerization being ended within 50 seconds). All data concerning the polymerization of TA are summarized in the Table 1. Compared to these 10 dyes, all the other dyes were less efficient to initiate photopolymerization processes.

Figure 1.

Photopolymerization profiles of TA (conversion rate of C=C bonds vs irradiation time) initiated by Iodonium (Iod) and amine (EDB) upon exposure to LED@405nm under air in the presence of dyes 1-21 at the same weight ratio: dye: Iod:amine=0.1%:2%:2% in TA. The irradiation starts for t = 10 s. Copyright 2020, Reprinted [58] with permission from MDPI.

Proposed Chemical Mechanisms

For the photopolymerization process initiated with the three-component PIS, the chemical mechanisms can be divided into two parts: (1) dyes acting as electron donors with Iod; (2) amine (EDB) acting as the electron donor and the dyes as the electron acceptors.

UV-vis Absorption and Steady State Photolysis of the selected 10 dyes

In this part, the ten best dyes reported were selected for the steady state photolysis experiments. Their molar extinction coefficients were determined by UV-visible absorption spectroscopy and the results are summarized in the Table 1.

These dyes exhibit excellent visible light absorption properties that can ensure their use in PISs upon visible LED light irradiation. For a better understanding of these absorptions, dyes 5 and 17 were selected for molecular modeling. The frontier orbitals involved in the lowest energy transition are depicted in the Figure 2. A charge transfer transition is found with the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) localized on the electron donor (amine) or electron acceptor (indane-1,3-dione) moieties, respectively. Photolysis of the ten best dyes (shown in the Table 1) in the presence of the iodonium salt and the amine were characterized by UV-visible spectroscopy. As evidenced by the UV-visible absorption spectra depicted in Figure 3, obvious photolysis and significant absorption decreases were observed for the photoinitiating systems comprising eight dyes among the ten examined. Among them, dyes 5, 19, and 20 displayed a relatively rapid photolysis process of decline, which only took five minutes, 25 min, 50 min in solution, respectively. Thus, we mainly focused on discussing the photolysis of dyes 5 and 19 to investigate their chemical mechanisms.

To evidence the contribution of the additional aromatic ring on the photopolymerization process, performances of the dyes 5 and 9 were compared to those of the dyes 17 and 19 bearing an additional aromatic ring. In addition, there is no obvious photolysis in the case of dyes 14, 15 and the photolysis in the photoinitiating system with dye 16 showed an increasing process (Figure 3d) i.e. the formation of a colored photoproduct.

In the case of the dye 5-based photoinitiating system, the dye 5/Iod/amine combination exhibited a rapid photolysis which was in agreement with the good initiating ability of this system. However, the photolysis process of the dye 17/Iod/amine system in acetonitrile under irradiation with the LED at 405 nm is very slow (Figure 3e), owing to its lower extinction coefficients (ε @405 =2180 M -1 cm -1 ; see Table 1), which could hinder the photolysis efficiency, compared to dye 5 which exhibit a higher extinction coefficient (ε @405 =6510 M -1 cm -1 ; see Table 1). In fact, no clear contribution of the additional aromatic ring on the photolysis process of dye 17 was evidenced.

The absorption peak of dye 19 (observed between 400 < λ < 650 nm) is disappearing within 25 minutes while the very high photostability of the dye 9/Iod/amine combination can be clearly demonstrated. Indeed, the photolysis experiment took more time, approaching 150 minutes for this system. Thus, it is in accordance with the fact that a higher molar extinction coefficient is found at the emission wavelength of the LED at 405 nm for dye 19 compared to dye 9 (ε @405 =350 M -1 cm -1 ; ε @405 =3790 M -1 cm -1 for dyes 9 and 19, respectively, see Table 1).

Interestingly, this trend is the opposite to that found for dyes 5,17, for which the In the presence of dye/amine interactions, no photolysis can be observed for the dye 17 (characterized by a significant absorption between 400 and 600 nm, Figure 4d) in any case, for dye 17/Iod combination, a slow photolysis is observed (Figure 4c).

This phenomenon can hardly be explained by clear and obvious photolysis experiments done on the dye 17/Iod/EDB three-component PIS. The main difference between dyes 5 and 17 can probably be ascribed to their different interaction behaviors with the iodonium salt and the amine. Therefore, such speculation can be verified by investigating their chemical mechanisms, which can be carried out by ESR-spin trapping experiments.

On the other hand, the photolysis process of dye 9/Iod or dye 9/amine in acetonitrile under irradiation of the 405nm LED is rather slow (Figure 5a,b). For the dye 19/EDB combination, a clear photolysis can be found (Figure 5d). Therefore, the dye 19/EDB based PIS exhibits a higher photosensitivity than the dye 9/EDB system.

For the interaction with Iod, a rather similar reactivity is found for dye 9 and dye 19

(Figure 5a vs. Figure 5c). Copyright 2020, Reprinted [58] with permission from MDPI. Copyright 2020, Reprinted [58] with permission from MDPI.

Consumption of Dyes in Photolysis Reactions

The consumption of dyes 5, 9, 17, 19 in percentage vs. irradiation time was illustrated in Figure 6. In details, the consumption of dye in percentage is calculated from changes of photolysis process in the UV-visible absorption spectra. Particularly, chemical mechanisms can also be investigated by comparing the three-component PISs (dye/Iod/amine) with the two-component PIS ones (dye/Iod or dye/amine).

Interestingly, it is obvious that the percentages of consumption profiles for all dyes (dye 5, 9, 17, 19) achieved by the three-component PIS (dye/Iod/amine) are higher than that reached when using the two-component PISs (dye/Iod or dye/amine), e.g. consumption of dye 5 = ~60% for dye 5/Iod/amine vs. ~50% for dye 5/Iod or ~30%

for dye 5/amine, (Figure 6a). Thus, we propose a plausible mechanism to support the formation of radicals according to the reactions depicted in the Scheme 1: r1, r2 and r3 are supposed to occur in the dye-based three-component PISs. As the generated radicals shown in Scheme 1, dye •+ radical is proposed to be generated by electron transfer from *dye to iodonium salt (r2) upon irradiation @405 nm, while dye-H • can be formed from *dye in the presence of EDB (r3). The consumption of dyes is accelerated by the simultaneity of r2 and r3 in three-component systems. Additionally, as shown in other systems, dye •+ can react with EDB (r4) or dye-H • with Iod (r5).

Similarly, the dye 9/Iod/amine combination seems to follow the same chemical mechanism than the dye 5/Iod/amine PIS. Indeed, the consumption changes for all PISs comprising the dye 9 (dye 9/Iod/amine, dye 9/Iod, or dye 9/amine) was quite similar to that observed for the dye 5/Iod/amine combinations (Figure 6a,b).

However, despite the mechanism mentioned above, there is a dissimilar process

Chemical mechanisms in electron transfer reactions

The theoretical feasibility of the interactions between dye/Iod (or dye/amine) was investigated by calculations of the free energy changes (ΔGIod or ΔGEDB, respectively) for the electron transfer reactions. According to the equations (eq. 1 or eq. 2), ΔGIod or ΔGEDB were determined from the oxidation potential Eox (or from the reduction potential or Ered, respectively) and the first singlet excited state energy (ES1).

The oxidation potentials (or reduction potentials) of dyes 5, 9, 17, 19 have been reported by literature 180 and their first singlet excited state energies (ES1) can be calculated from the crossing point of the UV-visible absorption and the fluorescence spectra (Figure 7). The results are gathered in the Table 2.

Fluorescence quenching experiments were carried out to evaluate the efficiency of the dye/Iod interaction. Interestingly, Iod was determined as acting as a relatively good quencher for dyes 5 and 16 (Figure 8a, b for dye 5/Iod, Figure 8c, d for dye 16/Iod). As shown in the Stern-Volmer treatment data, a linear quenching for dye 5 was found, whereas there was only a weak decrease for dyes 9 and 19. The Stern-Volmer constants (Ksv) and the electron transfer quantum yields (φ et(S1) ) for dyes 5, 9, 16 and 19 with Iod were also calculated (Table 2). Moreover, there was neither obvious fluorescence quenching observed for the dye 17/Iod combination, nor fluorescence quenching for the dyes/amine combination.

The calculated triplet state energies (density functional theory level) for a series of representative dyes are gathered in the Table 2. A triplet state pathway cannot be ruled out for Dye/Iod interaction (ΔG < 0; Table 2). In contrast, such a triplet state interaction is not favorable for the dye/amine combination (ΔG > 0; Table 2). Copyright 2020, Reprinted [58] with permission from MDPI.

Direct laser write experiments based on dye 5, 17 and 19

Some laser writing experiments using different PISs (dye/Iod/amine) in TA were performed and tridimensional patterns "TSY" were fabricated successfully (Figure 10). Dyes 5 and 19 were selected due to their good performances for the FRP of TA.

These two dyes were selected as being the best candidates of the 2 series of dyes (for irradiation at 405nm). Laser writing experiments with the Dye 17/Iod/amine PIS were also carried out. The 3D patterns were characterized by profilometric observations by using numerical optical microscopy. As expected, efficient photopolymerization processes occurred in the irradiated area, and the patterns clearly illustrated excellent 3D profiles, smooth surfaces and excellent spatial resolution (Figure 10b and10d).

Other experiments were performed with the dyes 5 or 19 but with a lower spatial resolution. in Table 1 (with their structures). To check their photoinitiation abilities, the eleven chromophores have also been introduced into three-component PISs (weight ratio: dye/Iod/EDB=0.1%/2%/2%, in 1g TMPTA), as photoinitiators to activate the polymerization upon irradiation with a LED at 405 nm within 400s at room temperature and the conversions were monitored by RT-FTIR. The profiles are reported in Figure 1a. Similar to the previous work, the specific curve identified as the blank reference (curve 0) in the absence of dyes initiated by Iod/EDB (weight ratio: 2%/2% in 1g TMPTA) was given for the photopolymerization of an acrylate monomer, namely TMPTA. In addition, the series of push-pull dyes were also investigated to activate polymerization processes under sunlight to replace the energy-consuming LEDs.

Table 1. Newly developed 3-(dialkylamino)-1,2-dihydro-9-oxo-9H-indeno [2,1-c] pyridine-4-carbonitrile based push-pull dyes applicable in PISs.

No. Iod/amine system (curve 0). Thus, it is convincing that push-pull chromophores with ferrocene groups can only hardly work as efficient photoinitiators in free radical polymerization process. In the specific case of dyes 5, 7, 8 and 9, the highest conversions and fastest polymerization processes were obtained with these four dyes, which proved to efficiently promote the photopolymerization of TMPTA. For instance, highest final reactive function conversions (FCs) were attained for dye 8 (beyond 90%), as well as dye 5, 7, 9 are also furnished high final conversions beyond 80%., and their short conversion times within 50s also evidenced their high photoinitiation abilities as well. Several excellent polymerization profiles were also obtained with the other dyes, e.g. dye 1, 2, 3, 6, which showed slightly lower performances (the FCs attained for the FRP of TMPTA were about 70%~80%) during free radical photopolymerization processes excepted for 4 dyes mentioned above.

Push-pull dyes Absorption properties Conversion (TMPTA)

To investigate the photopolymerization kinetics more precisely and specifically, the highest final reactive function conversions (FCs) concerning the polymerization of TMPTA in the presence of all eleven dyes are gathered in the Table 1. From the different experiments, a comparison of the FCs obtained with the eight most reactive dyes could be established and the following order of reactivity could be deduced: Dye 8 > dye 9 > dye 7 > dye 5> dye 2> dye 3 (or dye 1)> dye 6. Interestingly, dye 8, 9

have similar chemical structures but their light absorption properties are much lower than that of the other dyes, e.g. Dye 1 and dye 6 (See Figure 1). The light absorption properties of the push-pull dyes will be discussed in the following part.

Photoinitiation abilities of dye 8-based PIS were also investigated using Fourier

Light absorption properties of the push-pull dyes

Molar extinction coefficients of dyes were determined by UV-visible absorption spectroscopy in acetonitrile (see Figure 1c) and the light absorption parameters are gathered in Table 1. Dye 1, 2, 3, 5, 6 showed very high maximum molar extinction coefficients e.g. ε(Dye 1) = 38580 M -1 . cm -1 at λmax = 601nm, ε(Dye 2) = 24872 M -1 .

cm -1 at λmax= 582nm. ε(Dye 3) = 16330M -1 . cm -1 at λmax = 661nm, ε(Dye 5) = 22990M -1 . cm -1 at λmax = 727nm, ε(Dye 6) = 49650M -1 . cm -1 at λmax = 447nm in the UV-visible absorption spectra, which nicely fit to their relatively high photoinitiation abilities mentioned above. Interestingly, dyes 7, 8, 9 showed both poor maximum extinction coefficients at λmax and extinction coefficients @405 nm compared to dye 1, 2, 3, 5, 6, however, their performances on photopolymerization are much better. As expected, the lower maximum extinction coefficients of dyes 10 and 11 lead to their poor photoinitiation abilities as well.

Proposed chemical mechanisms of photopolymerization initiated by 3-(dialkylamino)-1,2-dihydro-9-oxo-9H-indeno[2,1-c]pyridine-4carbonitrile derivatives

Steady State Photolysis experiments of dye 8 and 9

To evidence the contribution of 3-(dialkylamino)-1,2-dihydro-9-oxo-9Hindeno [2,1-c] pyridine-4-carbonitrile group on the photopolymerization process, photolyses of dyes 8 and 9 were selected to be investigated in the presence of the iodonium salt and the amine which were characterized by UV-visible spectroscopy in acetonitrile under irradiation with the LED at 405 nm to investigate their chemical mechanisms. As shown in the UV-visible absorption spectra depicted in Figure 2a, obvious photolysis and significant absorption decreases were observed for the photoinitiating systems. On the other hand, the dye 8/EDB combination exhibited a slight photolysis (Figure 2c) which was in agreement with the extinction coefficient at the emission wavelength of the LED@405 nm (ε

@405nm =3340M -1 .cm -1
).

Moreover, in the case of the dye 8/Iod system, a strange photolysis process could be evidenced and showed an increase of the optical density in first step, indicating a colored photoproduct produced, and then a decrease after 8min (Figure 2c). Additionally, the dye 8/Iod/amine system exhibited a shoulder in the visible range (Figure 2a) which was assigned to the formation of the colored photoproduct mentioned above.

On the other hand, the photolysis of dye 8-based three-component PIS were also investigated upon irradiation of sunlight. As depicted in Figure 2d, an obvious decreasing tendency were observed for the range between 500-550nm, as well as the increasing tendency for the range between 350-500nm. Even if an excellent photolymerization on monmer (TMPTA) under sunlight were observed, as well the speed of photolysis under sunlight is faster than 405nm LED (compared to Figure 2a and 2d), however, the curve between the range from 375 to 500nm decreased after eight minutes, indicating the probable conversion of new photoproduct which generated before eight minutes due to multiple wavelengths in the sunlight. As evidenced by the UV-visible absorption spectra depicted in Figure 3, dye 9-based PISs also displayed a relatively rapid photolysis process of decline compared to the performances of dye 8, which only took five minutes in solution. As expected, dye 9 exhibited a similar photolysis process observed for three-component system dye 9/Iod/amine, resulting from their similarity in structures. However, the performance of the dye 9-based PISs in acetonitrile under irradiation with the LED at 405 nm during the photolysis process is more rapid and obvious compared to that in dye 8-based PISs (Figure 3, Figure 2, respectively), owing to its higher extinction coefficients at 405nm (ε @405 =4950 M -1 cm -1 ; see Table 1), which could govern the photolysis efficiency, as well as dye 8 exhibit a lower extinction coefficient at 405nm (ε @405 =3340 M -1 cm -1 ;

see Table 1).

The percentage of consumption of dye 9 vs. the irradiation time is also summarized from changes in their respective UV-visible spectra by comparing three-component PISs (dye/Iod/amine) with two-component PISs (dye/Iod or dye/amine), which is illustrated in Figure 3d. Obviously, percentage of consumption of dye 9 achieved by the three-component PIS (dye 9/Iod/amine) is much higher than that of two-component PISs (dye 9/Iod and dye 9/amine combinations e.g. consumption of dye 9 = 38% for dye 9/Iod/amine vs. 30% for dye 9/Iod or 28% for dye 9/amine. It is evidenced that the two-component PIS interactions (dye 9/Iod or dye 9/amine) were very inefficient for which a weak interaction was observed compared to dye 9/Iod/amine system.

Moreover, photolysis processes in the presence of dyes 5, 6 and 7-based PISs were also performed as shown in Figure 4. There is an obvious photolysis observed for dyes 5, 6 owing to their high extinction coefficient at 405nm (dye 5: ε @405 =8320M -1 cm -1 ; dye 6: ε @405 =22910M -1 cm -1 ; see Table 1). Furthermore, dye 7

shown a slow photolysis process in the concentration of dye: Iod: amine=1.26×10 -5 M:

1.46×10 -4 M: 4.07×10 -4 M, thus we increased the concentrations of Iod and amine in solution to accelerate the photolysis process, and the obvious tendency of decline was observed. We neglect the discussion on the photolysis of dyes 7 due to it possesses a low extinction coefficient at 405 nm (ε @405 =3880 M -1 cm -1 ; see Table 1).

Chemical mechanisms in electron transfer reactions

The singlet excited state energy (ES1) for the investigated dyes was calculated from the crossing point between the curve of normalized UV-vis absorption and the curve of normalized fluorescence spectra in acetonitrile (See Table 2, 2.28eV for dye 8 and 2.38eV for dye 9). The free energy changes (ΔGIod= -1.22eV, ΔGEDB=0.07eV for dye 8; ΔGIod= -1.36eV, ΔGEDB= -0.03eV for dye 9; see Table 2) for the electron transfer reactions were also carried out to investigate the theoretical feasibility of the interactions between dye/Iod (or dye/amine). For addition, the results of ΔGIod and ΔGEDB for dyes 8 and 9 were calculated from the oxidation potential (Eox) , the reduction potential (Ered) and the singlet excited state energy (ES1), and the oxidation potentials (or reduction potentials) of dyes 8, 9 have been reported by literature. 182 From the data, ΔGIod for dye 8, ΔGIod and ΔGEDB for dye 9 were less than 0, indicating the spontaneous interactions in dye 8/Iod, dye 9/Iod and dye 9/amine systems. Thus, the interactions will also be investigated by fluorescence quenching in the following context.

The triplet state energies (density functional theory level) for dyes 8, 9 are also calculated and shown in the Table 2. A triplet state pathway cannot be ruled out for Dye/amine interaction (ΔG < 0; Table 2). In contrast, such a triplet state interaction is not favorable for the dye/Iod combination (ΔG > 0; Table 2).

The fluorescence quenching experiments were performed in acetonitrile (Figure 5). For dye 8/EDB, there is no fluorescence quenching observed in acetonitrile. A linear quenching process was observed for dye 9 /EDB combination, its Stern-Volmer coefficients (Ksv=129 for dye 9/EDB; Table 2) and the electron transfer quantum yield (φet; Table 3) were also carried out. Interestingly, dye 8 (or dye 9)/Iod interaction showed an increasing tendency, which indicates that new photoproducts are generated (see Figure 5c,d). These results are in full agreement with the highly favorable free energy changes (∆Get(Dye/EDB)) for the electron transfer reaction which interact between dyes and EDB ( or Iod).

ESR spin-trapping experiments

To evidence the chemical mechanisms of push-pull dye-initiated photopolymerization, ESR-spin trapping experiments were carried out and some radicals were detected after mixing of dye 8/Iod and dye 8/amine in tert-butylbenzene under N2 in presence of PBN as the spin trap agent upon irradiation of 405nm LED:

PBN/Ar• was detected as hfc constants given by simulation aN = 14.38 G; aH = 2.15 G, in the dye 8/Iod PIS and PBN/ArNCH3CH2• detected as aN = 14.38 G; aH = 2.14 G in the dye 8/amine PIS(see Figure 6), which fully matching to literature data. 152 The dye 9/Iod and dye 9/amine solutions in tert-butylbenzene under N2 in presence of PBN as the spin trap agent were also characterized by ESR-spin trapping experiments upon irradiation of 405nm LED: As expected, radical adducts PBN/Ar• and PBN/ArNCH3CH2• were also detected and characterized by aN = 14.39 G; aH = 2.14 G and aN = 14.41 G; aH = 2.17 G (dye 9/Iod and dye 9/amine PISs, respectively, see Figure 7) in agreement with literature data. All these data are corresponding to the chemical mechanisms presented above in the Scheme 3. 153

Direct laser write experiments

Direct laser write experiments based on dye 8 and 9

Some laser writing experiments in the presence of different dye-based three-component systems in TMPTA acrylate as organic resins were performed to obtain tridimensional patterns. PISs dye 8/Iod/amine and dye 9/Iod/amine were selected as their best performances among the eleven dyes for the FRP of TMPTA in the mild condition (for irradiation at 405nm) and the 3D patterns "KES" were generated successfully (Figure 7). The 3D patterns were obtained by efficient photopolymerization processes and in a very short time and then characterized by profilometric observations by using numerical optical microscopy. The patterns of dye

Direct laser write experiments for the photocomposites prepared by push-pull dye-based PISs and silica fillers

Photocomposites were prepared with the dye 8 or 9/Iod/amine three-component systems (0.1%: 2%: 2% in monomer, w/w/w) and silica as filler (20% in monomer, w/w) where TMPTA was used as the monomer in this study. Laser writing experiments with the prepared photocomposites were also carried out. The 3D patterns after irradiation were characterized by profilometric observations by using numerical optical microscopy (Figure 9). As a result, laser writing experiments on these photocomposites comprising dyes 8 or 9 were performed rapidly but with a lower spatial resolution observed from attained patterns, compared to the dye 8 or 9

PISs without filler due to their transmittance were decreased by hiding of fillers.

However, dyes 8 or 9 also presented a relatively remarkable reactivity in the presence of filler, thus this kind of photocomposite is likely to be developed as a potential and useful photopolymerizable material in the future. Chapter 3

N-ethylcarbazole-1-allylidene derivatives

The chemical modification of an electron donor and its conversion as N-ethylcarbazole-1-allylidene was also considered as an efficient approach to enhance the electronic delocalization and improve the light absorption properties of traditional photoinitiators. So far, a series of 12 differed N-ethylcarbazole-1-allylidene derivatives were used in three-component PISs. These dyes noted as dyes 1-12 have the optical characteristics summarized in the Table 1. 183 Furthermore, enlightened by the fact that acrylate monomer can be well initiated by push-pull dyes based on

3-(dialkylamino)-1,2-dihydro-9-oxo-9H-indeno[2,1-c]pyridine-4-carbonitrile moieties
under sunlight irradiation, the photoinitiation behaviors of these novel dyes under sunlight were systematically investigated, including polymerization tests and stead state photolysis experiments.

Photopolymerization profiles and light absorption properties of novel N-ethylcarbazole-1-allylidene derivatives derivatives.

Table 1. Newly developed N-ethylcarbazole-1-allylidene derivatives based push-pull dyes applicable in PISs. As depicted in Figure 1a, photopolymerization kinetics of TA upon irradiation with a LED at 405 nm using three-component photoinitiating systems (PISs) composed of the different dyes (from dye 1 to 12, 0.1% in monomer, w/w), the iodonium salt (used as electron acceptor, 2% in monomer, w/w) and the amine (used as electron donor, 2% in monomer, w/w) were monitored by Real-Time Fourier

Transform Infrared spectroscopy (RT-FTIR) at room temperature. As observed for dyes 1-12, the final acrylate function conversions (FCs) determined at 400 s are gathered in the Table 1 as well as the monomer conversions obtained with the reference two-component PIS (2% Iod: 2% amine in TA) without dyes (cited as curve 0 in Figure 1). In fact, based on their photoinitiating abilities, the series of twelve dyes can be divided into two different families:

(1) those exhibiting a good photoinitiating ability such as dyes 3, 6, 7, 8 with high final conversions (FCs) reaching 98% or above. Polymerizations carried out with these dyes were also characterized by short induction period (T-T0 with T: Start time of polymerization; T0: Start time of light irradiation). Typically, polymerization processes are ended within 50s, evidencing their remarkable photoinitiation abilities.

In this latter series, the dye 7 is found the best one (combining high polymerization rate, low inhibition time and high FC).

(2) All the other dyes showed less efficiency and lower FCs comparing with the reference two-component PISs without dye. Their FCs obtained with these dyes only furnished 96% conversions or below. Interestingly, the lowest acrylate function conversion was obtained with dye 2, which only reached 83% after 400 s of irradiation at 405 nm. For the reference Iod/EDB system, the photoinitiating ability is ascribed to the presence of a photosensitive charge transfer complex as shown in ref.

192.

In light of the good photoinitiating abilities of dyes 3, 6, 7 and 8, the first family composed of these four push-pull dyes was thus selected for further investigations concerning the light absorption properties of photoinitiators and the chemical mechanisms examined in the following parts. Additionally, an order of induction time could be established with the four most reactive dyes examined in this work, namely dye 7 > dye 6 > dye 3 > dye 8.

To investigate the photoinitiation abilities more precisely and comprehensively, the photopolymerization profiles of TA initiated by dye 7 or dye 8-based PIS upon sunlight irradiation were also established and these latter are shown in Figure 1b (for dye 8) and Figure 1c (for dye 7). Remarkably, dye 7 and 8 acted as an excellent photoinitiator with high performance and furnished high final reactive function conversions (~99%) for the polymerization of TA under sunlight. Thus, photoinitiated processes done under mild conditions such as irradiation with sunlight provide novel solutions in the field of FRP, especially, the possibility to polymerize without using energy.

Furthermore, the polymerization of TA can also be observed under irradiation of LED or sunlight using one-component PIS (Figure 1), which only dye 7 (or dye 8) included without iodonium salts or amine. The photopolymerization kinetics are shown in Figure 1b or 1c (under 405nm LED for black curve, sunlight for red curve, respectively). Remarkably, all these data indicate that dye 7 (or 8) can be used as an efficient photoinitiator in photopolymerization, and sunlight was efficient as a reliable light source to initiate polymerization. The changes of FTIR spectra (ranging of 1600cm -1 to 1660cm -1 or 6120cm -1 to 6200cm -1 ) before and after sunlight irradiation using dye 8 alone were illustrated in Figure 2. This is ascribed to the panchromatic behavior of dyes 7 and 8 as light harvesting compounds (see in Figure 3 their broad absorption properties on the 350-700 nm range).

Light absorption properties of the push-pull dyes

Molar extinction coefficients of dyes were determined by UV-visible absorption spectroscopy in acetonitrile (see Figure 3) and the light absorption parameters are gathered in Table 1. As shown in Figure 3, the high maximum molar extinction coefficients at the maximum absorption wavelengths (λmax) are ranging between 6440 M -1 .cm -1 for dye 9 and 53200 M -1 .cm -1 for dye 12, from 454 nm for dye 11 to 579 nm for dye 10, respectively (see Table 1). For more details, dyes 3, 5, 11, 12 showed very high maximum molar extinction coefficients which were higher than 40000 M -1 .cm -1 at their maximum absorption wavelengths (λmax) e.g. ε(Dye 3) = 42460 M -1 .cm -1 at λmax = 531 nm, ε(Dye 5) = 43480 M -1 .cm -1 at λmax= 479 nm. ε(Dye 11) = 41120 M -1 .cm -1 at λmax = 454 nm, ε(Dye 12) = 53200 M -1 .cm -1 at λmax = 456 nm, in the UV-visible absorption range. Interestingly, compared to dyes 1, 2, 11, 12, dyes 3, 6, 7 showed relatively poor extinction coefficients at 405 nm: 11150 M -1 .cm -1 for dye 3, 8430 M -1 .cm -1 for dye 6 and 7000 M -1 .cm -1 for dye 7 which not fit to their photoinitiation abilities and which was discussed in the section of 3.2. Parallel to this, dyes 4, 9, 10 which showed lower molar extinction coefficients (less than 30000 M -1 .cm -1 ) at their absorption maxima also proved to be poorer photoinitiators, even if higher molar extinction coefficients were detected at 405 nm for dyes 4 and 10

(reaching 10000 M -1 .cm -1 ) compared to that of dyes 6, 7. In the case of dye 8, both good photoinitiation abilities and high molar extinction coefficients (ε max = 33500 M -1 .cm -1 at λmax = 447 nm,  @405nm = 19500 M -1 .cm -1 ) were found for this dye, and the results confirmed the high performances of dye 8 as a reactive photoinitiator in free radical polymerization under exposure to both 405 nm LED and sunlight. Furthermore, the weight concentration ratio of dyes/Iod/EDB in acetonitrile are fixed in all systems and set as: 0.0005%: 2%: 2% in dyes/Iod/EDB systems, 0.0005%: 2% in dyes/Iod systems and 0.0005%: 2% in dyes/EDB systems. Here, we proposed two partial interactions both existing in three-component systems: (1) dye/Iod and ( 2) dye/EDB. Remarkably, examination of these two reactions in solution was highly representative of the interactions existing in three-component systems. Conversely, dye 6 showed distinct photolysis processes compared to dye 3. Even if the decreasing tendencies were observed during the photolysis of the three systems: dye 6/Iod/EDB, dye 6/Iod and dye 6/EDB, however, photolysis of the dye 6/Iod system seemed to be less efficient than that of dye 6/EDB system: the consumption of dye 6 in dye/Iod only achieved ~10% while the consumption in dye/EDB reached ~13%. A similar process was found in the case of the dye 6/Iod/EDB three-component system (see Figure 5d), when compared to the dye 6/EDB interaction, indicating that the dye 6/EDB interaction drastically affects the efficiency of the integrated reaction in free radical polymerization process (See Figure 5).

Upon irradiation at 405 nm with a LED, just like for dye 3 and dye 6, photolysis processes were also examined in the case of dye 7-based PISs, comprising the dye consumption and 26% consumption for the dye/Iod/EDB system (see Figure 8), indicating that the interactions in three-component systems are comparable to that observed for the dye 7-base PISs.

Parallel to the LED-activated photolysis done with the three-component dye 8/Iod/EDB system, substitution of the 405 nm LED by sunlight to perform the different photolysis experiments was also investigated. As shown in Figure 7b, sunlight significantly activates the photolysis process. Surprisingly, the consumption of dye 8 is significantly accelerated in the presence of sunlight, and a higher dye consumption was obtained with sunlight than with a 405 nm LED, indicating the possible participation of UV sunlight (see Figure 8).

Chemical mechanisms in electron transfer reactions for dyes

The light absorption property is not the unique parameter to characterize the reactivity of a dye in photoinitiating systems. Indeed, redox processes, excited state lieftimes as well as quenching experiments are very important. In addition to the aforementioned conclusions, we also determined the theoretical parameters in the electron transfer reaction e.g. the singlet excited state energy (ES1), the Gibbs free energy changes (G), the triplet state energies (calculated at the density functional theory level), Stern-Volmer coefficients and the electron transfer quantum yields for the investigated dyes, which contributed to get a deeper insight into the chemical mechanisms of FRPs. The different paramaters are listed in the Table 2. The singlet excited state energies (ES1) of dyes were extracted by using the normalized UV-visible absorption and fluorescence spectra in acetonitrile (see Figure 9), and by using the crossing point of the two curves (See Table 2, 1.97 eV for dye 3;

2.00 eV for dye 6; 2.01 eV for dye 7). Conversely, no fluorescence spectrum could be recorded for dye 8 due to the presence of nitro group in this dye. The oxidation potential (Eox), the reduction potential (Ered) of for the electron transfer reaction of dyes 3, 6, 7, 8 have been extracted from their cyclic voltammograms (see Figure 10). The half peak oxidation potentials of dyes 3, 6, 7

(Eox) were calculated and gathered in Table 2 (0.29 V for dye 3, 0.18 V for dye 6, 0.78

V for dye 7). However, there is no current observed in oxidation cycle of dye 8 (see Figure 10d), indicating that its oxidation potential could be greater than 2.00V. In contrast, the reduction potential of four dyes are located at -0.62 V, -0.60 V, -0.65 V, -1.03 V (for dye 3, 6, 7, 8, respectively) and showed in Table 2. The free energy changes G S1 Iod or G S1 EDB for the electron transfer reaction between dyes and Iodonium salts or EDB were confirmed by equations (eq 3-6) and listed in Table 2 (GIod= -0.98eV, GEDB= -0.35eV for dye 3; GIod= -1.12eV, GEDB= -0.4eV for dye 6; GIod= -0.53eV, GEDB= -0.36eV for dye 7). G are < 0, which indicate the theoretical feasibility of these electron transfer reactions.

Calculations of the triplet energy levels ET1 (eV) of dyes 3, 6, 7, 8 were performed at DFT level, and gathered in Table 2. The triplet routes involved in mechanisms can be ruled out for the dye/EDB interactions due to all of their free energy changes at triplet state Get T1 are greater than 0, even if the dye/Iod interactions are less than 0 except dye 7/Iod combination.

The photoinitiating ability of dyes was also studied by examining the fluorescence quenching experiments on dye 3, dye 6 and dye 7 in acetonitrile as well as their Stern-Volmer coefficients (Ksv) and the electron transfer quantum yields (et)

were also carried out (see Table 2). Linear fluorescence quenching processes were observed (see Figure 11a, c for dye 3, Figure 12a 2) and high electron transfer quantum yields et were calculated by equation (eq 2) (Table 2). These data are in full agreement with the mechanisms discussed in photolysis part 2.2, and 2.3. Similarly, the reduced form Dye-H • can be oxidized by Iod to finish the catalytic cycle and the original dye was regenerated in the resin. Particularly, the byproducts Ar • , EDB • (-H) which are also created in these reaction pathways (see r4, r5 in Scheme 1), can be advantageously used as radicals confirming the chemical mechanisms.

3 rd , Nov. 2020, in Mulhouse Area (+77°43′ E, -47°75′ N) of France.

Hyperfine Couplings Constants (HFC) for the PBN radical adducts are given in Table 3 by simulation of the curves of ESR spectra, and corresponding to the literature data. 152,153 In some experiments, very minor signals ascribed to PBNox (the oxidized form of PBN) were observed and can be ascribed to side reactions. Thus, the feasibility of the chemical mechanisms presented above has clearly been proved. Following the examination of the photoinitiating abilities of the different dyes, the polymerization of TA acrylate in DLW experiments was examined and initiated with a selected set of three-component systems (dye 3 (or 6, 7, 8)/Iod/amine). These systems were selected due to their excellent behaviors in FRP. The formation of tridimensional patterns ("SY" for dye 3, "SK" for dye 6, "RZ" for dye 7, "ZH" for dye 8) was successfully observed in a very short time and specificities of these 3D

profiles, e.g. smooth surfaces, excellent spatial resolution, were demonstrated by profilometric observations via numerical optical microscopy (see Figure 15).

Direct laser write experiments for the photocomposites with silica fillers

The design of new photosensitive systems can be highly worthwhile for different applications (2D, 3D, 4D printing…). 184,185 To broaden the applications of the push-pull dyes, a possible application was proposed here and examined by producing photocomposites by FRP. Photocomposites were prepared by adding silica fillers into the monomer (TA) at a weight ratio of 20%. PISs used in this part are the same as used before, namely, the dyes 3, 7 and 8/Iod/EDB three-component systems (0.1%/2%/2% in monomer, w/w/w). Here, the laser acts as the irradiation source to print the 3D patterns of photocomposites: "SEP", "SRZ", "021" (for dye 3, 7 and 8, respectively) with a high spatial resolution. The different 3D patterns with high spatial resolution were characterized by profilometric observations as shown in Figure 16. properties of 12 push-pull dyes, as well as the final conversions of TA under the initiation of these dye-based PISs are gathered in Table 1. Despite these chromophores also possess excellent photoinitiation behaviors in free radical polymerization at 405nm light source irradiation, they exhibited poorer initiation abilities when compared with the family N-ethylcarbazole-1-allylidene derivatives discussed above, with an exception for dye 7 (FC~99%).

Table 1. Newly developed 4-dimethoxyphenyl-1-allylidene derivatives based push-pull dyes applicable in PISs. Remarkably, conditions of polymerization used in this work are the same than that used for the previous dyes, e.g. concentrations, light source, in the air, and in laminate.

Table 1 provided their chemical structures, absorption properties, and final monomer conversions in polymerization profiles of acrylate monomer (TA). Similarly, the weight ratios of three components involved in photopolymerization are 0.1%, 2%, and 2% for dyes, Iod, EDB in TA, respectively.

Light absorption properties and photopolymerization kinetics with

the newly proposed push-pull dyes.

. Light absorption properties of the push-pull dyes

Light absorption properties such as the molar extinction coefficients at the maximum absorption wavelengths (λmax) and the molar extinction coefficients at the 405nm (ε405nm) were determined for all dyes, and the relevant results are gathered in the Table 1. In addition, their UV-visible spectra are illustrated in Figure 1. Dyes can be divided into three families, based on their maximum molar extinction coefficients at the maximum absorption wavelengths (λmax): (1) Thus, the first group showed the highest maximum molar extinction coefficients (λmax) with values of 50990 M -1 •cm -1 at λmax = 391 nm for dye 1 and 55460 M -1 •cm -1 λmax = 404 nm for dye 3. These data support the high photoinitiation abilities evidenced for dyes 1 and 3 under exposure to both 405 nm LED and sunlight; (2) the second group identified as exhibiting a lower efficiency than that of the first group also showed lower extinction coefficients. Thus, molar extinction coefficients of 27740 M -1 •cm -1 at λmax = 368 nm for dye 2 and 28310 M -1 •cm -1 at λmax = 430 nm for dye 4 could be determined; (3) Interestingly, dyes 5 and 6 showed the smallest extinction coefficients at the maximum absorption wavelengths among all dyes: 10490 M -1 •cm -1 at λmax = 473 nm for dye 5, 7600 M -1 •cm -1 at λmax = 512 nm for dye 6 which was not in agreement with their high photoinitiation abilities as discussed in the polymerization profiles (see below). [START_REF] Wang | Hierarchically porous π-conjugated polyHIPE as a heterogeneous photoinitiator for free radical polymerization under visible light[END_REF] From molecular modeling data, a push-pull behavior is found with Highest Occupied Molecular Orbitals (HOMOs) and Lowest Unoccupied Molecular Orbitals (LUMOs) that are differently localized on the  system (Figure 2).

Parallel to this, all push-pull dyes were also investigated for their molar extinction coefficients (ε405nm) at 405 nm. For group (1), relatively high molar extinction coefficients were found at 405 nm for dyes 1 and 3 (standing at ε405nm = 17 680 M -1 •cm -1 for dye 1 and ε405nm = 55 350 M -1 •cm -1 for dye 3), which is consistent to their high performances in free radical polymerization at 405 nm. In the case of group

(3), both poor molar extinction coefficients at 405nm were found for dye 5 and 6

(ε405nm = 5990 M -1 •cm -1 , ε405nm = 2380 M -1 •cm -1 ), indicating that these two dyes acted as poor photoinitiators in free radical polymerization under exposure to both 405 nm LED and sunlight. Interestingly, dye 4 showed a high molar extinction coefficient (ε405nm=21820 M -1 •cm -1 ), even if a poor polymerization efficiency was found for dye 4-based PIS. Oppositely, a relatively low molar extinction coefficient at 405nm was found for dye 2 (ε405nm=4010 M -1 •cm -1 ), consistent with its poor photoinitiation ability observed during polymerization, even though dye 2 exhibited similar molar maximum extinction coefficient (εmax=27740 M -1 •cm -1 ) compared to that of dye 4. for dye 6 (blue curve), evidencing that these dyes can also act as efficient photoinitiators under natural light source and other mild conditions. Thus, an order of final conversions with these four dyes under sunlight exposure could be established: dye 1 > dye 3 > dye 5 > dye 6. Obviously, dyes 1 and 3 demonstrated better photoinitiation efficiencies compared to that of dyes 5 and 6. In conclusion, high photoinitiation abilities can be achieved by irradiation of sunlight in the presence of the investigated dyes as light harvesting compounds. Interestingly, the sunlight induced photopolymerizations were conducted in the winter of Mulhouse, which are supposed as a desirable season due to the high cloud density reduces the solar illumination with only diffuse sky radiation. Hence, better photoinitiation behaviors could be imagined in other seasons, e.g. summer, which possess stronger solar illumination to promote the photopolymerization.

Proposed chemical mechanism

Steady state photolysis experiments of dye 5, 6-based

Photoinitiating Systems

Interactions between dye, Iod and EDB in three-component systems were examined by steady state photolysis experiments upon irradiation with the LED@405 nm. In addition, the interactions of dye/Iod or dye/EDB were also investigated, and comparisons between the consumption of dyes both in three-component systems dye/Iod/EDB and two-component systems dye/Iod (or dye/EDB) were also established to confirm the chemical mechanisms occurring during polymerization.

Here, the solutions were prepared at 1.23×10 -5 M dye 5 or 1.44×10 -5 M dye 6, 2.93×10 -2 M Iod and/or 8.15×10 -2 M EDB in acetonitrile. Investigations concerning the photolysis of the two-component systems (dye/Iod and dye/EDB) in solution were highly consistent to the interactions existing in three-component systems, which will be discussed in the following part concerning the chemical mechanism. Firstly, the UV-visible absorption spectra of the dye 5-based three-component system increased when the 405 nm LED was used as irradiation source, even if decreasing tendencies were observed from dye 5/Iod and dye 5/EDB two-component systems (see Figure 4). The results indicated a new photoproduct was generated during the irradiation process for the three-component system (See Figure 4a). This phenomenon was also observed in the case of dyes 1 and 3, indicating the same chemical mechanisms occurred with these three dyes during photopolymerization.

However, dye 5 was consumed in dye 5/Iod and dye 5/EDB as shown in Figure 4b and c, proving the formation of new photoproducts existed under participation of the radicals obtained from the interactions between dye/ Iod and dye/EDB. Dye 6/Iod/EDB three-component system showed a decreasing photolysis process which is distinct from that observed for dye 5/Iod/EDB (see Figure 5). The two-component systems (dye/Iod and dye/EDB) showed similar behaviors than the three-component systems during the photolysis while the consumption of dye 6 three-component system and two-component systems were also illustrated to investigate the chemical mechanisms in detail (see Figure 5d). Even if the decreasing tendencies were also detected from the photolysis of dye 6/EDB, however, the consumption of dye 6 in dye 6/Iod system achieved higher level (~30%) than that of dye 6/EDB system (4%). Remarkably, both of their photolysis drastically contribute to the integrated interactions in three-component systems which the consumption of dye 6 reached higher percentage (~44%) at the end of irradiation (~100s), and higher efficiency of dye 6/Iod interaction gave more contributions when compared to the dye 6/EDB interaction in free radical polymerization process.

Cyclic Voltammetry and fluorescence experiments to study electron transfer reactions

As showed in Figure 6, the electron transfer of dyes 1, 3, 5, and 6 was investigated in Cyclic Voltammetry experiments. The oxidation potentials (Eox) and the reduction potentials (Ered) of dyes 1, 3, 5, and 6 were calculated from the half peak potentials in oxidation and reduction cyclic from their cyclic voltammograms (Table 2). No current was detected in the reduction cycle of dye 1 (see Figure 6a), indicating that its reduction potential is probably less than -2.00 V.

The singlet excited state energies (ES1) of dyes were calculated from the crossing point between the normalized UV-visible absorption and normalized fluorescence spectra of dye in acetonitrile solutions. The different values are gathered in Table 2.

After that, on the basis of chemical parameters such as the oxidation potential (Eox), the reduction potential (Ered) and the singlet excited state energy (ES1), it was possible to calculate the Gibbs free energy changes G S1 Iod or G S1 EDB for the electron transfer reaction in dye/Iod and dye/EDB systems according to equations (eq. 3-6). All the results were lower than 0, and the data are gathered in Table 2 (e.g. GIod= -2.09eV, GEDB> -0.07eV for dye 1 ; GIod= -2.02eV, GEDB= -0.64eV for dye 3; GIod= -1.22eV, GEDB= -0.82eV for dye 5; GIod= -1.05eV, GEDB= --0.31eV for dye 6),

indicating the theoretical feasibility of the electron transfer reactions.

Calculations of the triplet energy levels ET1 (eV) of dyes 1, 3, 5, and 6 were performed at the DFT level (Table 2). The triplet routes involved in the mechanisms can probably be ruled out for the dye/EDB interactions due to the positive values of their Gibbs free energy changes at the triplet state Get T1 (as exception of dye 6/EDB).

In contrast, the 3 dye/Iod interactions were all favorable. The involved chemical mechanisms during the polymerization processes were also investigated using fluorescence quenching experiments on dye 5 and dye 6 in acetonitrile (see Figure 7). Precisely, parameters such as the Stern-Volmer coefficients (Ksv) and the electron transfer quantum yields (et) were determined from the fluorescence quenching experiments. This is a useful method to evaluate the efficiency of the dye/Iod and dye/EDB interactions. These parameters are presented in Table 3. A decline process was found for the fluorescence quenching of dye 5/EDB and dye 6/Iod solutions (see Figure 7a for dye 5/EDB, Figure 7c for dye 6/Iod). Thus, linear quenching processes could be established using the Stern-Volmer treatment for fluorescence quenching as showed in Figure 7b

Direct laser write experiments

According to the polymerization behaviors of TA in the presence of the different dyes (see above), DLW experiments were carried out to evidence the potential applications of our three-component systems in practical applications with a selected set of push-pull dyes (dyes 5, 6) due to their excellent photoinitiation abilities. Indeed, the three-component systems comprising dyes 5, 6/Iod/EDB (0.1%/2%/2% in monomer, w/w/w) used in this part are the same than the compositions and concentrations used before. Moreover, the tridimensional patterns with smooth surfaces and excellent spatial resolution were also successfully fabricated via DLW experiments in a very short time, and profilometric observations on these 3D profiles were performed by numerical optical microscopy ("SRZ" for dyes 5, 6, see Figure 11a,b).

Photocomposites prepared with the selected PISs were also produced and examined by DLW experiments and numerical optical microscopy, respectively. Here, silica powers act as fillers added into the monomer (TA) with a weight ratio of 20%.

Markedly, 3D profiles of solidified photocomposites with an excellent spatial resolution were attained under initiation with dyes 5 and 6-based PISs ("SRZ" for dye 5/silica, dye 6/silica, see Figures 11c,d, respectively). As a result, the success of this kind of photocomposite proved the remarkable photoinitiation abilities of selected push-pull dyes even if in the presence of silica fillers and can be regarded as useful

Part 4 Photobase generator (PBG) as photoinitiators in radical polymerization

In 1990, the organic photobase generators (PBGs) with a photolabile carbamate group was utilized as a co-initiator to generate basic amines for the first time. 187 After that, many novel PBGs are widely reported as substitutes to replace the basic primary and secondary amines with relatively low electron-donating abilities which exhibit inefficient photoinitiation abilities in anionic polymerizations. Therefore researchers focused on searching new efficient PBGs in the last decade, and huge of developments have been achieved and described in previous reviews. [188][189][190][191][192][193] Many outstanding characteristics of PBGs are found, e.g. stability in air and the non-reactivity with metals, which is enable to be used for various applications from coatings in automotive to electronic industries, etc. Recently, two novel photoinitiators as photobase generators, have been supposed and reported in our group. Firstly, various concentrations of Irgacure 369 were dissolved in TMPTA at weight ratios: 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5% of TMPTA as the reference blanks, while a series of samples comprising Iragure 369 at the same weight ratios as reference blanks were prepared, and the BPO was added a specific concentration at 1% to build the PI/BPO systems. As illustrated in Fig. 1, higher conversion in the range of 80%-90% are observed for all PI/BPO systems with different concentrations after irradiation (~400s) at 405nm LED rather than their homologous reference blank without BPO (FC~60%-80%). However, the curve in polymerization profile of PI/BPO at the weight ratio 0.05%/1% is too noisy, thus we neglect this concentration combination in the following research. Interestingly, the combination of PI and BPO at the weight ratio, 0.4%/1% in TMPTA, furnished highest FC (~90%) among all investigated concentrations, thus we selected 0.4% as an ideal weight ratio for Irgacure 369 to further check the photoinitiation abilities of the combination PI/BPO in the different concentration of BPO. For addiation, the FCs and the final polymerization status (polymerized or not) are listed in Table 1.

On the other hand, the samples, including Irgacure 907 and BPO, were prepared in the same concentration and procedures, and the photopolymerzation of TMPTA were also successfully performed in the presence of this combination. Among them, a weight ratio of Irgacure 907/BPO at 0.2%/1.0% in TMPTA exhibited highest FC (~60%) among these samples. To evidence the contribution of BPO in photopolymerization, the samples comprising 0.4% (weight ratio in TMPTA) Irgacure 369 and different concentrations of BPO were prepared: 0.1%, 0.2%, 0.5%. After the irradiation of 405nm LED, as illustrated in Figure 2a, an increasing tendency of the FCs were observed as following the concentration increase of BPO. The highest FC was achieved in the polymerization of PI/BPO at the weight ratio 0.4%/0.5%, however, the mechanical strength of TMPTA in this weight ratio is too high, and the sample broke into several pieces under light irradiation during polymerization process. To avoid this circumstance, we tried to decrease the intensity of light and then discuss its effects on PI/BPO based systems.

As shown in Figure 2b, c, d, the photopolymerization of TMPTA initiated by PI/BPO systems were activated by three light intensity at 405nm LED: 50%, 40%, 30%, described by the percentages to highest light intensity (I0 = 110 mW•cm -2 ) of laser diode. As the results, a decrease of FCs was observed by declining the light irradiation intensity for the the PI/BPO combinations at the same weight ratio, however, the FCs curves became too noisy when the light intensity decrease to 40%.

Similarly, the photopolymerization status and FCs are listed in Table 2.

The expected chemical mechanisms can be given in reactions (r1)-( r3):

Chapter 3 :

 3 N-ethylcarbazole-1-allylidene derivatives................................................. 113 1. Photopolymerization profiles and light absorption properties of novel

  the dye was proposed to react with PI or additives in two different reactions. Thus, if the PI in reactions r1-r3 is replaced by dyes, the chemical mechanisms involved in the dye-based three-component photoinitiation systems can be established as being the followingAr2I + → Dye•+ + Ar2I • → Dye •+ + Ar • + ArI (r6) *Dye + EDB →Dye •-+ EDB •+ → Dye-H • + EDB • (-H) (r7) Dye •+ + EDB → Dye + EDB •+ (r8) Dye-H • + Ar2I + → Dye + Ar • + ArI + H +As shown in reactions r2 and r3, the reactive species, Dye •+ , Dye-H • , are generated by oxidation and reduction reactions of the dye/Iod and dye/EDB couples, respectively. During these two processes, Ar • , EDB • (-H) radicals are formed as byproducts and can be detected by ESR-spin trapping experiments, confirming the proposed chemical mechanisms. Subsequently, the initial state of dye can be regenerated in the resin according to the reactions, r4 and r5. Dye •+ which is the oxidized form of Dye can be reduced by EDB. Similarly, Dye-H • can be reduced by Iod to finalize the catalytic cycles and generate the reactive aryl radicals Ph • .

3 .

 3 photoinitiators for free radical polymerization (or cationic polymerization) under green light sources at 492-577 nm, especially at 520 nm.[125][126][127] Indeed, cationic polymerization upon green light irradiation is quite difficult, thus a majority of research is based on free radical polymerization.128, 129 Many newly developed green light-sensitive dyes based on different chemical structures exhibited outstanding photophysical and photochemical properties which are possible to enhance the photoinitiation abilities of PISs and the polymerization efficiency of FRPs.130

  photopolymerization in combination with Iodonium salts and N-methylpyrrolidone (NMP) to form the three-component PISs. In this work, the polymerization of monomer (TPGDA) initiated by the PISs was investigated with LEDs as the light sources and two different wavelengths were used, namely: blue light (460 nm) and

3 .

 3 Chalcone substituted cyclohexanone derivatives.The cyclohexanone derivatives based on the chalcone moiety (namely ketones) were also found as promising chromophores for photopolymerization due to strong light absorption in the near-UV/visible region (375-434 nm). Recently, Xu, Y.Y. et al.developed 13 ketones (marked as ketone-1 to ketone-13) as effective photoinitiators to initiate photopolymerization reactions via rational molecular structure design.137 Similarly, the synthesized ketone derivatives were examined in acetonitrile and their ground-state absorption spectra were also illustrated as shown in Figure3a, b. Free radical polymerizations of TA in laminate were induced by ketone//Iod/amine (0.1%/2%/2%, w/w/w) three-component PISs under 405 nm LEDs irradiation at room temperature (Figure3c, d). Particularly, it is believed that the ketone-based PISs could induce both free radical and cationic photopolymerizations, what was confirmed by

Figure 3 .

 3 Figure 3. UV-visible absorption properties of the thirteen ketone derivatives dissolved in acetonitrile: (a) for ketone 1 to ketone 6 and (b) for ketone 7 to ketone 13; Photopolymerization profiles of TA (C=C conversion vs. irradiation time) upon LED@405 nm irradiation in thin films condition (25 μm) in the presence of different ketone/amine/Iod (0.1%/2%/2%, w/w/w) photoinitiating systems: (c) for ketone 1 ketone 6 and (d) for ketone 7 to ketone 13, respectively. For the control experiment, only amine/Iod (2%/2%, w/w) two-component system was tested. LED, light-emitting diode. Copyright 2020, Reprinted [137] with permission from Wiley.

Figure 4 .

 4 Figure 4. Free radical photopolymerization of TA containing 20% (w%) silica filler for 3D printing experiments upon laser diode irradiation at 405 nm with ketone-1/amine/Iod system and characterization of the generated 3D patterns by numerical optical microscopy: (a) top surface morphology; 3-D overall appearance in (b) color and (c) black-and-white patterns, respectively. The scale bar is 4,000 μm. Copyright 2020, Reprinted [137] with permission from Wiley.

  photopolymerizations of acrylate monomer (PEG) were performed in the presence of bis-chalcone/Iod/EDB three-component PISs in laminate upon irradiation both at 375 and 405 nm LEDs at room temperature. As shown in the Table9, several chalcones

Scheme 1 .

 1 Scheme 1. Chemical structures of Iodonium salt (Speedcure 938), amine (Speedcure EDB) and monomers used3.3. Sample preparation.The ketones, iodonium salt (Iod or Speedcure 938), ethyldimethylamino benzoate (EDB or Speedcure EDB) or triethanolamine and monomer (TMPTA/TA) were successfully added into a small glass bottle and stored in the dark environment before photo irradiation. Here, the weight percents of the ketones were calculated from the monomer (TMPTA/TA) content and kept at 0.1% (w). To study the photolysis effect of three-component photoinitiating systems, both iodonium salt (Speedcure 938) and electron donor (Speedcure EDB/TEA) are elaborately selected under the same condition: co-initiated by Speedcure 938 and Speedcure EDB (ketone:

  17

  To investigate the chemical mechanisms induced by the three-component PISs during photopolymerization, several techniques, e.g. photolysis experiments and fluorescence approaches were performed by UV-visible absorption spectroscopy (JASCO V730 UV-visible spectrometer) and fluorescence spectroscopy (JASCO FP-6200 spectrofluorimeter), respectively. The singlet excited state energy level (Es1) was calculated from the crossing point of the UV-visible and fluorescence spectra. The Stern-Volmer coefficients (Ksv) were determined by the slopes of Stern-Volmer treatment for fluorescence quenching and the electron transfer quantum yields (et) were calculated using the eq. 2: 109, 110  et = Ksv[additive]/(1+Ksv[additive]) (eq 5)

  photomultiplier and a transient digitizer (Luzchem LFP 212) were used to perform nanosecond laser flash photolysis (LFP) experiments.110

Figure 5 .

 5 Figure 5. The weather and address of experiments performed for sunlight irradiation cut from websites 'google map' and 'www.time and date.com'.

Figure 1 (

 1 Figure 1 (a) Photopolymerization profiles of TA (conversion of C=C bonds vs irradiation time) initiated by Iodonium salt and TEA upon exposure to LED@405nm under air in the presence of ketone 1-10 at the same weight ratio: ketone: Iod: TEA=0.1%:2%:2% in 1g TA. (b) photos before (left) and after (right) photopolymerization for curve ketone 9; (c) photos before (left) and after (right) photopolymerization for curve ketone 10. Adapted from Ref. [151] with permission from The Royal Society of Chemistry.

2. 1 .Figure 2 .

 12 Photolysis of the ketones 8, 9, and 10 in the presence of iodonium salt & amine can be characterized by UV-visible spectrometer. The UV-vis spectra are depicted in Figure2. The photolysis quantum yields were not determined, the aim of these experiments was to highlight the different ketone/additive interactions. Obvious and significant decline of the UV-vis absorption was observed for photoinitiating systems comprising ketone 8, ketone 9, and ketone 10. Before irradiation, the maximum absorption of ketones 8, 9, and 10 appeared at 440 nm, 403 nm, and 405 nm,

Figure 2 .

 2 Figure 2. UV-vis absorption spectra of ketones 8, 9, 10 (2.08×10 -5 M; 1.08×10 -5 M; 1.23×10 -5 M, respectively.) with co-initiators: Iodonium (Speedcure 938, 1.46×10 -4 M) and amine (Speedcure EDB, 4.07×10 -4 M) upon exposure to LED@405nm under air in the solvent of acetonitrile: (a) ketone 8, (b) ketone 9 and (c) ketone 10. Adapted from Ref. [151] with permission from The Royal Society of Chemistry.

Figure 3 .

 3 Figure 3. UV-vis absorption spectra of ketones 8, 9, 10 (2.08×10 -5 M; 1.08×10 -5 M; 1.23×10 -5 M, respectively.) in the presence of Iodonium salt (Speedcure 938, 1.46×10 -4 ) upon exposure to LED@405nm under air in the solvent of acetonitrile: (a) ketone 8, (b) ketone 9 and (c) ketone 10. Adapted from Ref. [151] with permission from The Royal Society of Chemistry.

Figure 4 .

 4 Figure 4. UV-vis absorption spectra of ketones 8, 9, 10 (2.08×10 -5 M; 1.08×10 -5 M; 1.23×10 -5 M, respectively.) in the presence of amine (Speedcure EDB, 4.07×10 -4 M) upon exposure to LED@405nm under air in the solvent of acetonitrile: (a) ketone 8; (b) ketone 9 and (c) ketone 10. Adapted from Ref. [151] with permission from The Royal Society of Chemistry.

Figure 5 .

 5 Figure 5. Consumption of ketones vs. irradiation time @  = 405 nm (a) ketone 8/Iod/amine(); ketone 8/Iod(•); ketone 8/amine(). (b) ketone 9/Iod/amine(); ketone 9/Iod(•); ketone 9/amine(). (c) ketone 10/Iod/amine(); ketone 10/Iod(•); ketone 10/amine(). Adapted from Ref. [151] with permission from The Royal Society of Chemistry.

Figure 6 .

 6 Figure 6. UV-vis absorption spectra of ketone 1-7 (5×10 -6 g in 1g acetonitrile w/w) co-initiated with iodonium salt (1.46×10 -4 M, w/w) and EDB (4.07×10 -4 M, w/w) upon exposure to LED at 405nm under air in the solvent of acetonitrile: (a) ketone 1; (b) ketone 2; (c) ketone 3;(d) ketone 4; (e) ketone 5; (f) ketone 6; (g) ketone 7. Adapted from Ref. [151] with permission from The Royal Society of Chemistry.

Figure 7 .

 7 Figure 7. Cyclic voltammograms of electrochemical reactions of ketones 8, 9, 10 in acetonitrile solvent against saturated calomel electrode (SCE) under nitrogen saturated solution: (a) ketone 8; (b) ketone 9; (c) ketone 10. Adapted from Ref. [151] with permission from The Royal Society of Chemistry.

Figure 8 .

 8 Figure 8. Singlet state energy determination in acetonitrile for: (a) ketone 8; (b) ketone 9; (c): ketone 10. Adapted from Ref. [151] with permission from The Royal Society of Chemistry.

Figure 9 .

 9 Figure 9. (a) Fluorescence quenching of ketone 9 by Iodonium salt; (b) Stern-Volmer treatment for the ketone 9/Iodonium salt fluorescence quenching. Adapted from Ref. [151] with permission from The Royal Society of Chemistry.

Figure 10 .

 10 Figure 10. Laser flash photolysis for ketone 8, 9, 10: (a) ketone 8 (2.1×10 -5 M in acetonitrile); (b) ketone 9 (2.7×10 -6 M in acetonitrile w/w); (c) ketone 10 (4.1×10 -6 M in acetonitrile ). Laser excitation @ 355 nm. Adapted from Ref. [151] with permission from The Royal Society of Chemistry.

(

  CH3)3C6H4• which fully match to literature data.152 This suggests the redox reaction (r2) between ketone/Iod while Iod acted as an oxidizer leading to aryl radicals.Additional ESR-spin trapping spectra by mixing of ketone 8 (or 9) and amine were also performed. ArNCH3CH2• adduct is detected and characterized by aN = 14.1G; aH = 2.1G in agreement with literature data.153

Figure 11 . 3 .

 113 Figure 11. ESR spectra obtained from ESR-spin trapping experiment using PBN = 2 mg/mL (as spin trap agent); Iodonium salt (Speedcure 938) = 12.6 mg/mL and ketone 9, 10 = 0.8 mg/mL in acetonitrile under N2. left: (a), (c) ketone 9,10, Irradiation time =100s (red) and =0s (black) spectra; right:(b), (d) ketones 9,10, Irradiation time =100s (red) and simulated (black) spectra. Adapted from Ref. [151] with permission from The Royal Society of Chemistry.

Figure 12 .

 12 Figure 12. Free radical photopolymerization experiments for laser writing initiated by ketone based three-component photoinitiating systems in TA. Characterization of the 3D patterns by numerical optical microscopy: (left) top surface morphology (right) 3-D overall appearance of color pattern of ketone/Iod/TEA (0.1%/2%/2% w/w/w) in TA: (a) (b) for ketone 8/Iod/TEA; (c) (d) for ketone 9/Iod/TEA. (e) (f) for ketone 10/Iod/TEA.Adapted from Ref. [151] with permission from The Royal Society of

  electron-withdrawing ability of electron acceptors, an efficiency strategy consists in improving the electronic delocalization. Application of this strategy was notably applied to indane-1,3-dione for which an extended version of this well-known electron acceptor was developed. The first report mentioning the use of indane-1,3-dione-based push-pull dyes as photoinitiators was published in 2013.179 Subsequently, push-pull dyes based on 1H-cyclopentanaphthalene-1,3-dione, derived from indane-1,3-dione and obtained by extending the polyaromaticity of the electron acceptor, were proposed as photoinitiators of innovative structures and investigated in our previous work.[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] Moreover, the effects of the extension of aromaticity have also been proved by comparing push-pull dyes comprising indane-1,3-dione and 1H-cyclopentanaphthalene-1,3-dione as the electron acceptors. The two families of dyes exhibited a broad absorption extending over the visible range. Consequently, the different dues could be used as photoinitiators, especially dyes 1-21 (PPS 1

  of the acrylate monomer TA, the photoinitiation abilities of the two series of dye-based PISs (from dye 1 to 21) were investigated using Real-Time Fourier Transform Infrared spectroscopy (RT-FTIR) within the same

Figure 2 .

 2 Figure 2. Contour plots of HOMOs and LUMOs for Dye 5 and Dye 17 (isovalue=0.02). Copyright 2020, Reprinted [58] with permission from MDPI.

Figure 3 .

 3 Figure 3. UV-visible absorption spectra of different dyes with co-initiators: Iodonium salt (Speedcure 938, 1.46×10 -4 M) and amine (Speedcure EDB, 4.07×10 -4 M) upon exposure to LED@405nm under air in acetonitrile: (a) dye 5 (1.04×10 -5 M), (b) dye 9 (1.30×10 -5 M), (c) dye 10 (9.31×10 -6 M ), (d) dye 16 (6.46×10 -6 M), (e) dye 17 (9.20×10 -6 M), (f) dye 19 (1.11×10 -5 M), (g) dye 20 (8.33×10 -6 M ), (h) dye 21 (7.56×10 -6 M ). Copyright 2020, Reprinted [58] with permission from MDPI.

Figure 4 .

 4 Figure 4. UV-visible absorption spectra of dyes 5, 17 (1.04×10 -5 M; 9.20×10 -6 M, respectively.) (left) in the presence of the iodonium salt (Speedcure 938, 1.46×10 -4 M) upon exposure to LED@405nm under air in the solvent of acetonitrile: (a) dye 5, (c) dye 17 and (right) in the presence of the amine (Speedcure EDB, 4.07×10 -4 M) upon exposure to LED@405nm under air in the solvent of acetonitrile: (c) dye 5, (d) dye 17.

Figure 5 .

 5 Figure 5. UV-visible absorption spectra of dyes 9, 19 (1.30×10 -5 M; 1.11×10 -5 M, respectively.) (left) in the presence of Iodonium salt (1.46×10 -4 M) upon exposure to LED@405nm under air in the solvent of acetonitrile: (a) dye 9, (c) dye 19 and (right) in the presence of amine (4.07×10 -4 M) upon exposure to LED@405nm under air in the solvent of acetonitrile: (c) dye 9, (d) dye 19. Copyright 2020, Reprinted [58] with permission from MDPI.

  observed in the case of dye 19-based PIS. Compared to the dye 9/amine system, a slower consumption of dye 19 in the dye/Iod two-component PIS was observed during the same irradiation time e.g. the consumption of dye 19 = ~7.3% for the dye 19/Iod PIS vs. ~50% for the dye 9/amine PIS (Figure 6b,d). Therefore, for dye 19, r3 is probably favored over r2. Interestingly, as shown in the Figure 6c, for the same irradiation time, the dye 17/Iod/amine system led to a high consumption of the dye (~72%), whereas no obvious consumption of the dye in dye17/Iod or dye 17/amine can be observed. The chemical mechanism will be discussed in the part concerning the ESR spin-trapping experiments.

Figure 6 .Scheme 1 .

 61 Figure 6. Consumption of dyes vs. irradiation time upon irradiation with a LED@405 nm (a) dye 5/Iod/amine(); dye 5/Iod(•); dye 5/amine(). (b) dye 9/Iod/amine(); dye 9/Iod(•); dye 9/amine(). (c) dye 17/Iod/amine(); dye 17/Iod(•); dye 17/amine(). (d) dye 19/Iod/amine(); dye 19/Iod(•); dye 19/amine(). Copyright 2020, reprinted [58] with permission from MDPI.

Figure 7 .

 7 Figure 7. Singlet state energy determination in acetonitrile for: (a) dye 5; (b) dye 9; (c): dye 17; (d) dye 19. Copyright 2020, Reprinted [58] with permission from MDPI.

Figure 8 .

 8 Figure 8. (a) Fluorescence quenching of dye 5 by Iodonium salts; (b) Stern-Volmer treatment for the dye 5/Iodonium salts fluorescence quenching. (c) Fluorescence quenching of dye 16 by Iodonium salt; (d) Stern-Volmer treatment for the dye 16/Iodonium salt fluorescence quenching. Copyright 2020, Reprinted [58] with permission from MDPI.

Figure 9 .

 9 Figure 9. ESR spectra obtained from ESR-spin trapping experiment using PBN = 2 mg/mL (as spin trap agent); Iodonium salt (Iod) or amine (EDB) = 12.6 mg/mL and dye 5 = 0.8 mg/mL in tert-butylbenzene under N2.: (a) dye 5/Iod, Irradiation time =0s (black) and =10s (red) spectra; (b) dye 5/Iod, Irradiation time =10s (black) and simulated (red) spectra; (c) dye 5/amine, Irradiation time =0s (black) and =30s (red) spectra; (d) dye 5/amine, Irradiation time =30s (black) and simulated (red) spectra.

Figure 10 .

 10 Figure 10. Free radical photopolymerization experiments for laser write experiments for different dye-based three-component photoinitiating systems in TA. Characterization of the 3D patterns by numerical optical microscopy: (left) top surface morphology (right) 3-D overall appearance of the 3D pattern using dye/Iod/amine (0.1%/2%/2% w/w/w) in TA: (a) (b) for dye 5/Iod/EDB; (c) (d) for dye 17/Iod/EDB; (e) (f) for dye 19/Iod/EDB. Copyright 2020, Reprinted [58] with permission from MDPI.

  Photoinitiation abilities of the eleven dyes-based PISs (from dye 1 to 11) were investigated using Real-Time Fourier Transform Infrared spectroscopy (RT-FTIR) upon irradiation with a LED@405nm within 400s at room temperature. Typical acrylate function conversions vs irradiation time profiles are depicted in Figure1. In

Transform

  Infrared spectroscopy (FT-IR) under sunlight as a mild condition at room temperature. The monomer (TMPTA) with three-component PIS both upon irradiation with sunlight at 0min and 30min were gather in Infrared spectra as showed in Figure3a. The peaks in spectra obviously decreased, especially in the peak 6130 -1 cm, indicating the conversion of C=C bonds in TMPTA. Moveover, typical acrylate function conversions vs irradiation time (in 0,10, 20, 30min) profiles are also carried out. As shown in Figure1b, the profile indicate the conversion of monomer reached at ~88%, which evidence the dye-based three-component PISs can be photoinitiated by sunlight, and the free radical polymerization can be well promoted to a high level of conversion (~88%), compared with the photopolymerization upon irradiation with LED at 405nm. In conclude, by carefully selecting the push-pull dyes with excellent performance, novel and high-performance photoinitiating systems operating at household mild irradiations e.g. 405nm LED or sunlight are thus proposed.

Figure 1 .

 1 Figure 1. (a) Photopolymerization profiles of TMPTA (conversion rate of C=C bonds vs irradiation time) initiated by Speedcure 938 and Speedcure EDB upon exposure to LED@405nm under air in the presence of dye 1-11 at the same weight ratio: dye: Speedcure 938: Speedcure EDB=0.1%:2%:2% in 1g TMPTA. (b) Conversion of C=C bond initiated by push-pull dye 8 (0.1% in TMPTA w/w) with co-initiators: Iodonium

Figure 2 .

 2 Figure 2. UV-visible absorption spectra of dye 8 (8.51×10 -6 M in acetonitrile) with co-initiators: (a) Iod (Speedcure 938, 1.46×10 -4 M in acetonitrile) and amine (EDB, 4.07×10 -4 M in acetonitrile); (b) Iod (Speedcure 938,1.46×10 -4 M in acetonitrile); (c) amine (EDB, 4.07×10 -4 M in acetonitrile) upon exposure to a LED@405nm under air in acetonitrile as the solvent; (d) Iod (Speedcure 938, 1.46×10 -4 M in acetonitrile) and amine (EDB, 4.07×10 -4 M in acetonitrile) upon exposure to sunlight under air in acetonitrile as the solvent. Copyright 2020, Reprinted [181] with permission from MDPI.

Figure 3 .

 3 Figure 3. UV-visible absorption spectra of dye 9 (8.54×10 -6 M in acetonitrile) with co-initiators: (A) Iod (Speedcure 938, 1.46×10 -4 M in acetonitrile) and amine (Speedcure EDB, 4.07×10 -4 M in acetonitrile), (B) Iod (Speedcure 938,1.46×10 -4 M in acetonitrile) and (C) amine (Speedcure EDB, 4.07×10 -4 M in acetonitrile) upon exposure to LED@405nm under air in acetonitrile as the solvent. (D) Consumption of dye 9 vs. irradiation time @ λ = 405 nm: dye 9/Iod/amine(•); dye 9/Iod(); dye 9/ amine(). Copyright 2020, Reprinted [181] with permission from MDPI.

Figure 4 .

 4 Figure 4. UV-vis absorption spectra of (A) dye 5 (6.92×10 -6 M in acetonitrile), Iodonium salts (1.46×10 -4 M in acetonitrile) and EDB (4.07×10 -4 M in acetonitrile); (B) dye 6 (1.02×10 -5 M in acetonitrile), Iodonium salts (1.46×10 -4 M in acetonitrile) and EDB (4.07×10 -4 M in acetonitrile); (C) dye 7 (1.26×10 -5 M in acetonitrile w/w), Iodonium salts (2.92×10 -2 M in acetonitrile) and EDB (8.14×10 -2 M in acetonitrile) upon exposure to LED@405nm under air in the solvent of acetonitrile. Copyright 2020, Reprinted [181] with permission from MDPI.

Figure 5 .

 5 Figure 5. (a) Fluorescence quenching of dye 9 (8.54×10 -6 M in acetonitrile) by EDB); (b) Stern-Volmer treatment for the dye 9/EDB fluorescence quenching. Fluorescence quenching of (c) dye 8 (8.51×10 -6 M in acetonitrile); (d) dye 9 (8.54×10 -6 M in acetonitrile) by Iodonium salts. Copyright 2020, Reprinted [181] with permission from MDPI.

Figure 6 .

 6 Figure 6. ESR spectra obtained from ESR-spin trapping experiment using PBN = 2 mg/mL (as spin trap agent); Speedcure 938 = 12.6 mg/mL and dye 8= 0.8 mg/mL in tert-butylbenzene under N2. (a) dye 8/Iod PIS, Irradiation time =20s (green), =10s (red) and =0s (black) spectra, respectively; (b) dye 8/Iod PIS, Irradiation time =20s (black) and simulated (red) spectra; (c) dye 8/amine PIS, Irradiation time =210s (red) and =0s (black) spectra; (d)dye 8/amine PIS, Irradiation time =210s (black) and simulated (red) spectra. Copyright 2020, Reprinted [181] with permission from MDPI.

Figure 7 .

 7 Figure 7. ESR spectra obtained from ESR-spin trapping experiment using PBN = 2 mg/mL (as spin trap agent); Iodonium salts = 12.6 mg/mL and dye 9= 0.8 mg/mL in tert-butylbenzene under N2. (a) dye 9/Iod PIS, Irradiation time = 15 s (red) and = 0 s (black) spectra; (b) dye 9/Iod PIS, Irradiation time = 15 s (black) and simulated (red) spectra; (c) dye 9/EDB PIS, Irradiation time = 20 s (green), = 10 s (red) and = 0 s (black) spectra, respectively; (d) dye 9/EDB PIS, Irradiation time = 20 s (black) and simulated (red) spectra. Copyright 2020, Reprinted [181] with permission from MDPI.

8 -

 8 based PIS were clearly illustrated excellent 3D profiles, smooth surfaces and excellent spatial resolution (Figure 7a and 7a), however, a lower spatial resolution on surfaces was observed from attained patterns initiated by dye 9-based PIS, compared to the dye 8 PISs.

Figure 8 .

 8 Figure 8. Free radical photopolymerization experiments for laser writing initiated by dye-based three-component photoinitiating systems in TMPTA. Characterization of the 3D patterns by numerical optical microscopy: (left) top surface morphology (right) 3-D overall appearance of color pattern of dye/Iod/amine (0.1%/2%/2% w/w/w) in TMPTA: (a) (b) for dye 8/Iod/amine; (c) (d) for dye 9/Iod/amine. Copyright 2020, Reprinted [181] with permission from MDPI.

Figure 9 .

 9 Figure 9. Free radical photopolymerization experiments for laser writing initiated by dye-based three-component photoinitiating systems in TMPTA. Characterization of the 3D patterns by numerical optical microscopy: (left) top surface morphology (right) 3-D overall appearance of color pattern of dye/Iod/amine (0.1%/2%/2% w/w/w) in TMPTA: (a) (b) for dye 8/Iod/amine (0.1%/2%/2% w/w/w) in the presence of silica (20%, w/w) in TMPTA; (c) (d) for dye 9/Iod/amine (0.1%/2%/2% w/w/w) in the presence of silica (20%, w/w) in TMPTA. Copyright 2020, Reprinted [181] with permission from MDPI.

Figure 1 .

 1 Figure 1. (a) Photopolymerization profiles of TA (conversion of C=C bonds vs irradiation time) initiated by iodonium salts and EDB upon exposure to LED@405nm in laminate in the presence of dyes 1-12 at the same weight ratio: dye:Iod:EDB = 0.1%:2%:2% in TA. Curve 0: Iodonium salt: EDB=2%:2% in TA without dye; (b) Photopolymerization profiles of TA (conversion of C=C bonds vs irradiation time) initiated by: (green) the iodonium salt and EDB upon exposure to sunlight in laminate

Figure 2 .

 2 Figure2. FT-IR spectra of before and after the photopolymerization of monomer TA initiated by dye 8 alone (weight ratio: 0.1% in TA) in laminate: (a) upon exposure to LED@405nm LED for the acrylate functional peak ranging from 6120cm -1 to 6200cm -1 ; (b) upon exposure to LED@405nm for the acrylate functional peak ranging from 1600cm -1 to 1660cm -1 ; (c) upon exposure to sunlight for the acrylate functional peak ranging from 6120cm -1 to 6200cm -1 ; (d) upon exposure to sunlight for the acrylate functional peak ranging from 1600cm -1 to 1660cm -1 . Copyright 2021, Reprinted[183] with permission from Elsevier.

Figure 3 .

 3 Figure 3. UV-visible absorption spectra in acetonitrile of the push-pull dyes (dye 1-12) in acetonitrile. Copyright 2021, Reprinted [183] with permission from Elsevier.

Figure 4 .

 4 Figure 4. UV-visible absorption spectra of dye 3 (7.77×10 -6 M) with co-initiators: (a) Iod (2.93×10 -2 M) and EDB (8.15×10 -2 M), (b) Iod (2.93×10 -2 M) and (c) EDB (8.15×10 -2 M) upon exposure to LED@405nm under air, in acetonitrile. (d) Consumption of dye 3 vs. irradiation time @  = 405 nm: dye 3/Iod/EDB(•); dye 3/Iod(); dye 3/ EDB(). Copyright 2021, Reprinted [183] with permission from Elsevier.

Figure 5 .

 5 Figure 5. UV-visible absorption spectra of dye 6 (7.85×10 -6 M) with co-initiators: (a) Iod (2.93×10 -2 M) and EDB (8.15×10 -2 M), (b) Iod (2.93×10 -2 M) and (c) EDB (8.15×10 -2 M) upon exposure to LED@405nm under air in acetonitrile. (d) Consumption of dye 6 vs. irradiation time @  = 405 nm: dye 6/Iod/EDB(•); dye 6/Iod(); dye 6/EDB(). Copyright 2021, Reprinted [183] with permission from Elsevier.

7 /

 7 Iod/EDB three-component system and the dye 7/Iod or dye 7/EDB two-component systems (see Figure6a, b, c) upon exposure to the LED. The changes of consumption of dyes in PISs are shown in Figure6d. Interestingly, it has been demonstrated that dye 7/Iod and dye 7/EDB two-component PISs achieved similar dye-consumption percentages, more precisely, final dye-consumption of dye 7/Iod (~10.5%) was slightly higher than that of the dye 7/EDB system (~9.5%) during the photolysis process, which is different from the behaviors detected during the photolysis experiments of dyes 3 and 6. The consumption of dye 7 can be maximized in PISs composed of the three components (reached ~20%), proving that the interactions between dye 7/Iod/EDB are much efficient than that in the dye 7/Iod or dye 7/EDB two-component systems.

Figure 6 .

 6 Figure 6. UV-visible absorption spectra of dye 7 (7.16×10 -6 M in acetonitrile) with co-initiators: (a) Iod (2.93×10 -2 M in acetonitrile) and EDB (8.15×10 -2 M in acetonitrile), (b) Iod (2.93×10 -2 M in acetonitrile) and (c) EDB (8.15×10 -2 M in acetonitrile) upon exposure to LED@405nm under air in acetonitrile as the solvent. (d) Consumption of dye 9 vs. irradiation time @  = 405 nm: dye 7/Iod/EDB(•); dye 7/Iod(); dye 7/EDB(). Copyright 2021, Reprinted [183] with permission from Elsevier.

Figure 7 .

 7 Figure 7. UV-visible absorption spectra of dye 8 (6.02×10 -6 M in acetonitrile) with co-initiators: (a) Iod (2.93×10 -2 M in acetonitrile) and EDB (8.15×10 -2 M in acetonitrile) upon exposure to a 405 nm LED; (b) Iod (1.46×10 -4 M in acetonitrile) and EDB (4.07×10 -4 M in acetonitrile) upon exposure to sunlight; (c) Iod (1.46×10 -4 M in acetonitrile) and (d) EDB (4.07×10 -4 M in acetonitrile) upon exposure to 405nm LED under air. Copyright 2021, Reprinted [183] with permission from Elsevier.

Figure 8 .

 8 Figure 8. Consumption of dye 8 (at wavelength of maximum O.D: 564.5nm) vs. irradiation time in photolysis of: (a) dye 8/Iod/EDB(•) upon irradiation with a 405 nm LED; (b) dye 8/Iod/EDB() upon irradiation with sunlight; (c) dye 8/Iod() upon irradiation with a 405 nm LED; (d) dye 5/EDB( ) upon irradiation with a 405 nm LED. Copyright 2021, Reprinted [183] with permission from Elsevier.

Figure 9 .

 9 Figure 9. Singlet state energy determination in acetonitrile for: (a) dye 3; (b) dye 6; (c) dye 7. Copyright 2021, Reprinted [183] with permission from Elsevier.

Figure 10 .

 10 Figure 10. Cyclic voltammograms for the dyes 3, 6, 7, 8 in acetonitrile solvent against saturated calomel electrode (SCE) under nitrogen saturated solution: (a) dye 3; (b) dye 6; (c) dye 7; (d) dye 8. Copyright 2021, Reprinted [183] with permission from Elsevier.

  Figure 12b, d for dye 7; Table2) and high electron transfer quantum yields et were

Figure 11 .

 11 Figure 11. Fluorescence quenching of dye 3 (7.77×10 -6 M in acetonitrile) by: (a) iodonium salt (Iod); (c) amine (EDB). Stern-Volmer treatment for fluorescence quenching of the (b) dye 3/iodonium; (d) dye 3/EDB. Copyright 2021, Reprinted [183] with permission from Elsevier.

Figure 12 . 2 . 3 .

 1223 Figure 12. Fluorescence quenching of dye 7 (7.16×10 -6 M in acetonitrile) by: (a) Iodonium salts (Iod); (c) amine (EDB). Stern-Volmer treatment for fluorescence quenching of the (b) dye 7/Iod; (d) dye 7/EDB. Copyright 2021, Reprinted [183] with permission from Elsevier.

Figure 13 .

 13 Figure 13. ESR spectra obtained from ESR-spin trapping experiments under irradiation of 405nm LED using PBN = 2 mg/mL (as the spin trap agent); Iod = 2 mg/mL, EDB = 2 mg/mL and dye 8= 0.2 mg/mL in tert-butylbenzene under N2. (a)

Figure 14 .

 14 Figure 14. ESR spectra obtained from ESR-spin trapping experiments under irradiation of sunlight using PBN = 2 mg/mL (as the spin trap agent); Iod = 2 mg/mL, EDB = 2 mg/mL and dye 8= 0.2 mg/mL in tert-butylbenzene under N2. (a) dye 8/Iod PIS, Irradiation time =15min (red) and =0min (black) spectra, respectively; (b) dye 8/Iod PIS, Irradiation time =15min experimental (black) and simulated (red) spectra; (c) dye 8/EDB PIS, Irradiation time =15min (red) and =0min (black) spectra; (d) dye 8/EDB PIS, Irradiation time =15min experimental (black) and simulated (red) spectra. Copyright 2021, Reprinted [183] with permission from Elsevier.

Figure 15 .

 15 Figure 15. Free radical photopolymerization experiments in DLW experiments initiated by dye-based three-component photoinitiating systems in TA. Characterization of 3D patterns overall appearance: color patterns observed by numerical optical microscopy in the presence of: (a) dye 3/Iod/EDB (0.1%/2%/2% w/w/w); (c) dye 6/Iod/EDB (0.1%/2%/2% w/w/w). (b) dye 7/Iod/EDB (0.1%/2%/2% w/w/w); (d) dye 8/Iod/EDB (0.1%/2%/2% w/w/w). Copyright 2021, Reprinted [183] with permission from Elsevier.

Figure 16 .

 16 Figure 16. Free radical photopolymerization experiments in DLW experiments in TA. Characterization of 3D overall appearance of color patterns by numerical optical microscopy in the presence of: (a) dye 3/Iod/EDB/silica (0.1%/2%/2%/20% in TA, w/w/w); (b) dye 7/Iod/EDB/silica (0.1%/2%/2%/20% in TA, w/w/w/w); (c) dye 8/Iod/EDB/silica (0.1%/2%/2%/20% in TA, w/w/w/w). Copyright 2021, Reprinted [183] with permission from Elsevier.

  Unlike other light-harvesting compounds with dye scaffolds, numerous push-pull dyes without categorical structure have been synthesized and previously reported in the literature. Photochemistry of six push-pull dyes with excellent performance in free radical polymerization were investigated in Ref 184 , in combination with Iod and EDB as novel PISs to initiate the polymerization under 405nm LED and sunlight.

Figure 1 .

 1 Figure 1. UV-visible absorption spectra in acetonitrile of the push-pull dyes (dye 1-6) in acetonitrile. The curve number 1-6 corresponds to the dye 1-6. Copyright 2021, Reprinted [186] with permission from Elsevier.

Figure 2 . 1 . 2 .

 212 Figure 2. Contour plots of HOMOs and LUMOs for Dyes1-6; structures optimized at the B3LYP/6-31G* level of theory (Isovalue = 0.02). Copyright 2021, Reprinted [186] with permission from Elsevier.

Figure 3 .

 3 Figure 3. (a) Photopolymerization profiles of TA (conversion of C=C bonds vs irradiation time) initiated by iodonium salts and EDB upon exposure to LED@405nm in laminate in the presence of dyes 1-6 at the same weight ratio: dye:Iod:EDB = 0.1%:2%:2% in TA. Curve 0: Iodonium salt: EDB=2%:2% in TA without dye. The curve number 1-6 corresponds to the number of dye 1-6. The irradiation starts for t = 10 s. (b) Photopolymerization profiles of TA (conversion of C=C bonds vs irradiation time) initiated by the iodonium salt and EDB upon exposure to sunlight in laminate in the presence of dyes at the weight ratio: dye:Iod:EDB=0.1%:2%:2% in TA. (black) dye 1; (red) dye 3; (green) dye 5; (blue) dye 6. Copyright 2021, Reprinted [186] with permission from Elsevier.

Figure 4 .

 4 Figure 4. UV-visible absorption spectra of dye 5 (1.23×10 -5 M) with co-initiators in acetonitrile: (a) Iod (2.93×10 -2 M) and EDB (8.15×10 -2 M), (b) Iod (2.93×10 -2 M), (c) EDB (8.15×10 -2 M) upon exposure to LED@405nm under air for different time. Copyright 2021, Reprinted [186] with permission from Elsevier.

Figure 5 .

 5 Figure 5. UV-visible absorption spectra of dye 6 (1.44×10 -5 M) with co-initiators in acetonitrile: (a) Iod (2.93×10 -2 M) and EDB (8.15×10 -2 M), (b) Iod (2.93×10 -2 M) and (c) EDB (8.15×10 -2 M) upon exposure to LED@405nm under air for different time. (d) Consumption of dye 6 vs. irradiation time under the irradiation of LED@ 405 nm: dye 6/Iod/EDB(•); dye 6/Iod(); dye 6/EDB(). Copyright 2021, Reprinted [186] with permission from Elsevier.

Figure 6 .

 6 Figure 6. Cyclic voltammograms of dyes 1, 3, 5, 6 recorded in acetonitrile against saturated calomel electrode (SCE) under nitrogen saturated solution: (a) dye 1; (b) dye 3; (c) dye 5; (d) dye 6. Copyright 2021, Reprinted [186] with permission from Elsevier.

  for dye 5/EDB and Figure7dfor dye 6/Iod. Moreover, the slope of Stern-Volmer treatment also determine the Stern-Volmer coefficients Ksv and the electron transfer quantum yields (et), which were calculated from eq2. However, for the other combinations e.g. dye 5/Iod, dye 6/EDB, dye 1/Iod (or EDB) and dye 3/Iod (or EDB), no fluorescence quenching processes were observed, indicating poor interactions existing in these two-component systems. Remarkably, even if the Gibbs free energy changes of dye 5/Iod and dye 6/EDB combinations are less than 0 (GIod= -1.22eV for dye; GEDB= -0.314eV for dye 6) as mentioned above, the poor interactions within dye 5/Iod and dye 6/EDB confirmed by fluorescence quenching approaches are in full agreement with the results attained by steady state photolysis, for which only slight declines were observed during the photolysis processes.

Figure 7 .Table 3 .

 73 Figure 7. Fluorescence quenching of: (a) dye 5 (1.23×10 -5 M in acetonitrile) by the amine (EDB); (c) dye 6 (1.48×10 -5 M in acetonitrile) by iodonium salt (Iod). Stern-Volmer treatment for fluorescence quenching of the (b) dye 5/EDB; (d) dye 6/Iod. Copyright 2021, Reprinted [186] with permission from Elsevier.

Figure 8 .

 8 Figure 8. ESR spectra obtained from ESR-spin trapping experiments under irradiation of 405 nm LED using PBN = 2 mg/mL (as the spin trap agent); Iod = 2 mg/mL, EDB = 2 mg/mL and dye 6= 0.2 mg/mL in tert-butylbenzene under N2. (a) dye 6/Iod PIS, Irradiation time = 60 s (red) and = 0 s (black) spectra, respectively; (b) dye 6/Iod PIS, Irradiation time = 60 s experimental (black) and simulated (red)

Figure 9 .

 9 Figure 9. ESR spectra obtained from ESR-spin trapping experiments under irradiation of sunlight using PBN = 2 mg/mL (as the spin trap agent); Iod = 2 mg/mL, EDB = 2 mg/mL and dye 6 = 0.2 mg/mL in tert-butylbenzene under N2. (a) dye 6/Iod PIS, Irradiation time =15 min (red) and = 0 min (black) spectra, respectively; (b) dye 6/Iod PIS, Irradiation time = 15 min experimental (black) and simulated (red) spectra; (c) dye 6/EDB PIS, Irradiation time = 15 min (red) and =0min (black) spectra; (d) dye 6/EDB PIS, Irradiation time = 15 min experimental (black) and simulated (red) spectra. Copyright 2021, Reprinted [186] with permission from Elsevier.

Scheme 1 .

 1 photopolymerization (Scheme 1). Additionally, BPO was added in the acrlyate resins (TMPTA) to achieve higher mechanical properties. In this approach, the polymerization of TMPTA initiated by two-component systems comprising Irgacure 369 (or Irgacure 907) and BPO were investigated at 405nm LED in order to simplify the difficulty of three-component systems for the first step, then the highest conversion of TMPTA at the specific concentrations of photoinitiators and BPO were selected, and PBGs will added in the selected PI/BPO systems to continue the research in the future.

Figure 1 .

 1 Figure 1. Polymerization profiles of TMPTA initiated by 1% (w/w) BPO and Irgacure 369 at weight ratio: (a) 0.5%; (b) 0.4%; (c) 0.3%; (d) 0.2%; (e) 0.1%; (f) 0.05%.

  • in r1 act as primary initiating radicals. The presence of r2 and r3 in PI/BPO versus PI alone explains that the yield in initiating radicals is increased and therefore better polymerization performances reached.

Figure 2 .

 2 Figure 2. Polymerization profiles of TMPTA initiated by 0.4% (w/w) Irgacure 369 and BPO at irradiation of different intensity of 405nm LED: (a) 100%; (b) 50%; (c) 40%;(d)30%

For

  photoinitiator and BPO with excellent characteristics will be further investigated in the future.

Table

  The discoloration of the dyes could lead to a deeper light penetration into the acrylate-based resins and it could also improve the thickness of polymerization products. High final conversions of the methacrylate monomer were

	obtained (greater than 80% under air) in this work. The different borate-based
	cyanines dyes exhibited excellent photoinitiation abilities when a phosphine was
	added in the resin. Beyond simply regenerating the dye, the phosphine could also
	prevent the polymerization process from oxygen inhibition. Other commonly used
	NIR sensitive dyes e.g. porphyrin (chlorophyll B, chlorophyllin copper salt) or nickel
	dithiolene were also examined as photoinitiators and compared with the cyanine

1 for their chemical structures and light absorption abilities) combined with different iodonium salts and 4-diphenylphosphinobenzoic acid (4-dppba) in three-component PISs for the photopolymerization upon NIR lights emitting at 660 nm and 780 nm. Bleaching property could be demonstrated during photopolymerization.

borate derivatives. However, their photoinitiation behaviors were proved to be lower than that of the cyanine borate.

In another publication, 116 A. H. Bonardi et al. also reported several cyanines differing from the chromophores mentioned above by the substitution pattern (see dyes 7-12 in Table

1

Table 1 .

 1 Cyanine derivatives applicable in PISs.

	Dyes

Conversion of monomer/Mx-MA Applicable Light Source Ref. 1 ~75% in 660 nm ~55% in 785 nm (0.4 W/cm 2 ) ~85% in 785 nm (2.55 W/cm 2 ) 660 or 785 nm [115]

2 ~25% in 785 nm ~75% in 785 nm 785 nm [115]

3 ~70% in 785 nm ~60% in 940 nm 785 nm or 940 nm [115]

4 - 785 nm (2.55 W/cm 2 ) [115]

5 ~70% in 785 nm ~86% in 940 nm 785 nm (2.55 W/cm 2 ) or 940 nm [115]

6 ~55% in 660 nm 660 nm [115]

7 ~90% in 785 nm ~82% in 940 nm 785 nm or 940 nm [116]

8 ~75% in 1064 nm 1064 nm [116]

9 ~85% in 1064 nm 1064 nm [116]

10 ~80% in 785 nm ~90% in 940 nm 785 nm or 940 nm [116]

11 ~75% in 785 nm 785 nm [116]

12 ~90% in 785 nm ~75% in 940 nm 785 nm or 940 nm [116]

Table 2 ,

 2 Their molar extinction coefficients in acetonitrile at different wavelengths and their abilities to initiate free radical polymerizations were also discussed. Even if these porphyrins exhibited low light absorptions at 940 and 1064 nm (see Table2), however, the

	photopolymerizations initiated by dye-based four-component PISs furnished high
	final conversions for acrylate monomers upon irradiation of 940 or 1064 nm with
	laser diodes: 1) upon irradiation at 940 nm with a laser diode, final conversions
	reached 88%, 65%, 84% with dyes 11, 12, 13, respectively. Dye 11 without metal
	cation furnished the highest FC among three porphyrins. Furthermore, in the case of
	the other two metalated porphyrins, Pd-porphyrin exhibited a better photoinitiation

dyes 11-13, respectively)

, were investigated by A.-H

Bonardi et al.. 120

ability than that of the Cu-porphyrin. However, a shorter inhibition time could be evidenced with the second porphyrin 2) At 1064 nm, the same final conversions were detected for the three porphyrin dyes.

Table 2 .

 2 Squaraine

, Squarilium, Bodipy, Porphyrin derivatives used in PISs.

Table 3 .

 3 Green light-sensitive dyes applicable in PISs.

Table 4

 4

	) and flavonol sulfonate (abbreviated as 3HF-S, dye 3 in Table 4),
	respectively, combined with iodonium salt (Iod) or triethanolamine (TEOA) used as
	two-component photoinitiating systems exhibited the high photoinitiation activities
	and polymerization conversions on TPGDA under blue/near UV light sources e.g. 460
	nm and 405 nm LED. 133 From the results published by You, J. et al, strong
	photoinitiation abilities (e.g. FC~80%-90% for TPGDA under 3HFs/ONI) and good
	absorption properties/ high extinction coefficients can be observed for 3HF-S,
	3HF-OH, and 3HF-C from photopolymerization kinetics and the ultraviolet-visible
	(UV-vis) absorption spectra in the 250nm-470nm range as listed in Table 4.
	Remarkably, aggregation-induced emission (AIE) as additional characteristics on

2.5. Newly developed purple or blue light-sensitive dye-based three-component PISs for free radical polymerization with 405 nm LEDs.

	Considering that major developments of visible light-induced polymerization are
	focusing on photoinitiating systems adapted to purple or blue light sources, significant
	improvement of dye-based PISs have recently been achieved upon addition of an
	iodonium salt and a sacrificial amine (namely ethyl 4-(dimethylamino)benzoate, abbv.
	EDB) to activate FRP at 405 nm. Here, in this review, such newly developed
	dye-based PISs are classified and the relevant discussions are presented in the
	following context.

Table 4 .

 4 Naphthoquinone, pyrrole-based enone, flavonoid derivatives, thiophene, or furane substituted cyclohexanone derivatives applicable in PISs.

	, their poor photoinitiating abilities obtained with Iodonium salts and amine
	(ketone: Iod:EDB=0.1%:2%:2% in TA, weight ratio) in free radical polymerization
	were determined by RT-FTIR experiments, which only furnished low final
	conversions of acrylate monomer (TA) ranging from ~55% to ~81% in laminate.
	Interestingly, even if this ketone-activated polymerization in laminate exhibited
	poor performance during the irradiation, however, the polymerization of acrylate
	monomer involved by dye 9 and 11-based PISs enabled the polymerization of thick

Table 5 .

 5 Tertiary

	amine or anthracene substituted cyclohexanone-based dyes
	applicable in PISs. 136

Ketones Absorption properties (In acetonitrile) Conversion at 405nm

Table 7 .

 7 Chalcone substituted cyclohexanone-based dyes applicable in PISs.

137

Ketones Absorption properties

Conversion (TA, in laminate)

Table 9 .

 9 Chalcone derivatives applicable in PISs.141

  9H-carbazole-3,6-diyl) bis(phenylmethanone), abbreviated as BPC, was reported by Liu, S. et al. and used for the first time as photoinitiator in free radical polymerization.

									Such
	derivatives exhibited good optical properties and performances for
	photopolymerization under irradiation in the visible range, especially under purple,
	blue,	or	UV	light.	Previously,	a	novel	carbazole	derivative,
	(9-ethyl-

  the singlet state G S1 Iod or G S1 EDB was calculated from equation 1 or 2 (eq. 1 or 2)144, 145 where Eox, Ered, and E * (Es1), are the oxidation potential of the electron donor, the reduction potential of the electron acceptor, the excited state energy level (determined from UV-visible and luminescence experiments) for the initially formed ion pair, respectively. Similarly, the free energy change of triplet state Get was calculated from equation 3 and 4 (eq. 3, eq. 4)109, 110 where Eox, Ered, and E * (ET1), are the oxidation potential of the electron donor, the reduction potential of the electron acceptor and the triplet state energy level (determined from HOMO/LUMO frontier orbitals simulation), respectively. The reduction potential of Iodonium salt was -0.7V 146 and the oxidation potential of EDB was 1.0V according to literature data.

	Saturated Calomel Electrode SCE) in cyclic voltammetry (Voltalab PST006). In our
	work, each cyclic voltammogram was obtained by scanning between 0 and 2000 (or
	-2000) mV in acetonitrile using tetrabutylammonium hexafluorophosphate as the
	supporting electrolyte.

For measuring the redox potentials, ketones or push-pull dyes (oxidation potential Eox vs. SCE and reduction potential Ered vs. SCE) were dissolved in acetonitrile containing tetrabutylammonium hexafluorophosphate (Aldrich) as the supporting electrolyte. A platinum electrode was used as a working electrode and a saturated calomel electrode (SCE) was used as a reference electrode. (potential vs.

The fluorescence properties of the ketones were measured using a JASCO FP-6200 spectrofluorimeter. Moreover, for electron transfer reaction, the free energy change from

Table 1 .

 1 studied in our previous works.151 In this work, twelve ketones with violet/blue light absorptions were selected and combine with Iod and EDB/TEA as novel three-component PISs to promote the conversions of acrylate monomer in the photopolymerization process. The polymerization was performed upon exposure to 405 nm laser diode in laminate. The chemical structures, light absorptions, and conversions of C=C bond in acrylate monomer are gathered in Table1. Heterocycle substituted cyclohexanone-based dyes applicable in PISs.151

	Ketones	Absorption properties	Conversion at 405nm

Recently, photopolymerization initiated by visible light photoinitiators based on heterocycle functionalized cyclohexanone (namely ketone in the following context) has been extensively

Table 2 .

 2 Parameters characterizing the chemical mechanisms associated with ketones

	8, 9, 10 in acetonitrile. a
		Ketone 8	Ketone 9	Ketone 10
	Eox (eV)	1.01	1.10	1.11
	Ered (eV)	< -2.00	-1.59	-1.58
	E S1 (eV)	2.5 8	2.6 6	2.7 1
	E T1 (eV) b	2.0 3	1.8 1	1.8 2
	G S1 Iod (eV)	-0.8 7	-0.8 6	-0.9 0
	G S1 EDB (eV)	> 0.4 2	-0.0 7	-0.1 3

a: a re-evaluated value of reduction potential of -0.7 V is used according to 146 . b: calculated triplet state energy level at the DFT level.

Table 3 .

 3 Parameters characterizing the fluorescence properties of ketones 8, 9, 10 in acetonitrile: S1 lifetime (); Interaction constant (Ksv) between ketone and Iod systems calculated by Stern-Volmer equation; electron transfer quantum yield ( et(S1) ) of ketone/Iod interaction.

		τ(ns)	K sv	Iod (M	-1 )	 et(S1) Iod a
	Ketone 8	<1.4		-		-
	Ketone 9	<1.4		310		0.9 3
	Ketone 10	<1.4		144		0.8 6

Table 2 )

 2 Additionally, HOMO/LUMO (Highest Occupied Molecular Orbital HOMO and Lowest Unoccupied Molecular Orbital LUMO) of ketone 8 are mainly localized on the shorter carbon chain from the carbon ring to the pyrryl group with larger energy gap (ET1 = 2.03 eV, see Table2) whose triplet state is harder to be achieved in free radical polymerization, compared to ketones 9 and 10 with delocalized π bonds on longer carbon chain and lower energy gap (ET1=1.81eV, 1.82eV for ketone 9 and 10, respectively). Moreover, triplet state energy levels (ET1) of ketones were also simulated and depicted in Table1. By calculating the free energy change of triplet state Get from equation 3 and 4, the reaction of triplet state of ketone with iodonium salt is also favorable (G et Iod < 0 in but not for EDB. Therefore, a triplet state pathway cannot be ruled out for iodonium.

Table 1 .

 1 Summary of the FCs at 405 nm of acrylate monomers (TA) under

	three-component photopolymerization systems between dyes with indane-1,3-dione
	and 1H-cyclopenta naphthalene-1,3-dione group (0.1%, w/w), iodonium salt (Iod,

2%, w/w), and amine (EDB, 2%, w/w); Light absorption properties of ketones in acetonitrile: maximum absorption wavelengths λmax; extinction coefficients at λmax (ε max ) and extinction coefficients at the emission wavelength of the LED@405 nm(ε @405nm

Table 2 .

 2 Parameters characterizing the chemical mechanisms associated with dyes5, 9, 16, 17, 19 in acetonitrile a

				Dye 5	Dye 9	Dye 16	Dye 17	Dye 19
	E ox (eV)	0.5 4	0.4 9	0.7 9	0.7 9	0.4 9
	E red (eV)	-1.3 0	-1.3 0	-1.3 1	-1.3 1	-1.2 2
	E S1 (eV) b	2.3 2	2.0 2	2.2 9	2.2 9	2.1 2
	E T1 (eV) c	2.1	1.6	1.9	2.0	1.6
	G S1	Iod (eV)	-1.0 8	-0.8 3	-0.8 0	-0.8 0	-0.9 3
	G S1	EDB (eV)	-0.0 2	0.2 9	0.0 2	0.0 2	0.1 0

a: A re-evaluated value of reduction potential of -0.7 V is used according to ref. 146 b: From the values presented in ref. 180 c: calculated triplet state energy level at DFT level. d: From the equation presented in ref.

109, 110

Table 2 .

 2 Parameters characterizing the chemical mechanisms associated with dyes 8, 9 in acetonitrile a

E S1 (eV) E red (eV) E ox (eV) G S1 Iod (eV)

								G S1	EDB
								(eV)
	Dye 8	2.28	-1.35	0.36	-1.22		0.07
	Dye 9	2.38	-1.41	0.32	-1.36		-0.03
		ET1 (eV) b	Get T1 Iod (eV)	Get T1 EDB (eV)	K sv	EDB (M	-1 )	et(S1) EDB
	Dye 8	1.65	-0.59	0.7		-		-
	Dye 9	1.65	-0.63	0.76		129		0.937

a: A re-evaluated value of reduction potential of -0.7 V is used according to ref. 146 b: From the values presented in ref. 182 c: calculated triplet state energy level at DFT level.

Table 2 .

 2 Parameters characterizing the electron transfer reactions for the dyes 3, 6, 7, 8 in acetonitrile a

		E S1 (eV) ET1 (eV) b E red (eV)	E ox (eV)	G S1 Iod (eV)	G S1 EDB
											(eV)
	Dye 3	1.97	1.26	-0.62		0.29		-0.98		-0.35
	Dye 6	2.00	1.45	-0.60		0.18		-1.12		-0.4
	Dye 7	2.01	1.30	-0.65		0.78		-0.53		-0.36
	Dye 8		1.43	-1.03		>2.00
		Get T1 Iod	Get T1 EDB	K sv	Iod (M	-1 )	 et(S1) Iod	K sv	EDB (M	-1 )	 et(S1) EDB
		(eV)	(eV)
	Dye 3	-0.27	0.36	303.6		0.928		88.6		0.913
	Dye 6	-0.57	0.15		14.1		0.376		8.1		0.471
	Dye 7	0.18	0.35		14.3		0.379		0.47		0.053
	Dye 8	>1.27	0.6
	a: For Iod, the reduction potential of -0.7 V is used according to ref. 146
	b: calculated triplet state energy level at DFT level.

Table 3 .

 3 HFC constants determined by ESR-spin experiments for the four selected dyes.

					Dye 8 at	Dye 8 at
		Dye 3	Dye 6	Dye 7
					405nm LED	sunlight
	Iod	aN=14.4 aH=2.1	aN=14.4 aH=2.1	aN=14.4 aH=2.2	aN=14.4 aH=2.1	aN=14.4 aH=2.2
	EDB	aN=14.4 aH=2.1	aN=14.4 aH=2.1	aN=14.4 aH=2.1	aN=14.4 aH=2.1	aN=14.4 aH=2.3

Table 1 .

 1 Newly

developed 6 push-pull dyes with high light absorption properties applicable in PISs.

Table 2 .

 2 Parameters characterizing the chemical mechanisms associated with dyes 1, : For Iod, the reduction potential of -0.7 V is used according to ref.146 b: calculated triplet state energy level at DFT level.

	3, 5, 6 in acetonitrile. a, b
		Dye 1	Dye 3	Dye 5	Dye 6
	E S1 (eV)	3.07	2.96	2.39	2.17
	E T1 (eV) a	2.12	2.10	1.31	2.05
	G S1 Iod (eV)	-2.09	-2.02	-1.22	-1.05
	G S1 EDB (eV)	>-0.07	-0.64	-0.82	-0.31
	Get T1 Iod(eV)	-1.14	-1.16	-0.14	-0.93
	Get T1 EDB(eV)	>0.88	0.22	0.26	-0.19
	Eox (eV)	0.28	0.24	0.47	0.42

a

Table 4 .

 4 HFC constants determined by ESR-spin experiments for the dyes 1, 3, 6 (sunlight included).

		Dye 1	Dye 3	Dye 6 at	Dye 6 at
		405nm LED	405nm LED	405nm LED	sunlight
	Iod	aN=14.4 aH=2.1	aN=14.4 aH=2.2	aN=14.3 aH=2.2	aN=14.4 aH=2.2
	EDB	aN=14.4 aH=2.1	aN=14.4 aH=2.2	aN=14.4 aH=2.2	aN=14.4 aH=2.2

Table 1 .

 1 The final polymerization status and final acrylate function conversion (FCs) for TMPTA in the presence of different Irgacure 369/BPO syestems.

	Initiator (w/w in TMPTA)	Polymerization (Y/N)	Conversion (%) of TMPTA
	0.5% Irgacure 369+1% BPO	Y	85-90
	0.5w% Irgacure 369	Y	75-80
	0.4% Irgacure 369+1% BPO	Y	85-90
	0.4w% Irgacure 369	Y	80
	0.3% Irgacure 369+1% BPO	Y	80-90
	0.3w% Irgacure 369	Y	75-80
	0.2% Irgacure 369+1% BPO	Y	85-90
	0.2% Irgacure 369	Y	75-80
	0.1% Irgacure 369+1% BPO	Y	80-90
	0.1% Irgacure 369	Y	70-75
	0.05% Irgacure 369+1% BPO	Y	80
	0.05% Irgacure 369	Y	60

Table 2 .

 2 The final polymerization status and final acrylate function conversions (FCs) of TMPTA activated by different light intensities (Int. stands for light intensity).

	Initiator (w/w in TMPTA)/ Intensity of light	Polymerization (Y/N)	Conversion (%) of TMPTA
	0.4% Irgacure 369/100% Int.	Y	65
	0.4% Irgacure 369+0.1% BPO/100% Int.	Y	70
	0.4% Irgacure 369+0.2% BPO/100% Int.	Y	75
	0.4% Irgacure 369+0.5% BPO/100% Int.	Y	85
	0.4% Irgacure 369/40% Int.	Y	60
	0.4% Irgacure 369+0.1% BPO/40% Int.	Y	65
	0.4% Irgacure 369+0.2% BPO/40% Int.	Y	70
	0.4% Irgacure 369+0.5% BPO/40% Int.	Y	75
	0.4% Irgacure 369/30% Int.	Y	50-55
	0.4% Irgacure 369+0.1% BPO/30% Int.	Y	50-55
	0.4% Irgacure 369+0.2% BPO/30% Int.	Y	65
	0.4% Irgacure 369+0.5% BPO/30% Int.	Y	75-80

Chemistry.

Structure/reactivity/efficiency relationship

As all ketones exhibit good light absorption properties @405 nm, this parameter can hardly explain alone the difference in polymerization behavior. Therefore, the main difference between the ketone efficiency can probably be ascribed to the reactivity of their excited states with iodonium salt or amine. Ketones in comparison with other photosensitizers presented in previous works (see a review of possible sensitizers/photoinitiators for visible light in [START_REF] Xiao | Visible light sensitive photoinitiating systems: Recent progress in cationic and radical photopolymerization reactions under soft conditions[END_REF]154 ) are also able to initiate polymerization in mild conditions. Compared to photoredox catalysts, [START_REF] Xiao | Visible light sensitive photoinitiating systems: Recent progress in cationic and radical photopolymerization reactions under soft conditions[END_REF] the new proposed systems do not exhibit significant catalytic activity as the ketone is not fully regenerated (see the photolysis data above). The possibilities for bleaching (r2-r3) mainly govern the consumption of ketones during the photopolymerization process. additional aromatic ring in dye 17 was considered to provide a lower extinction coefficient at the emission wavelength of the LED at 405 nm. Thus, there is no clear evidence that the additional aromatic ring governs the molar extinction coefficient or the polymerization performance in FRP. In fact, the main factor governing the absorption at 405 nm is definitely the electron-releasing ability of the electron donor used for the design of the push-pull dyes, the electron acceptor being maintained constant in the two series.

Compared to the performances of dye 9, dye 10 exhibited a similar photolysis process to that observed for dye 9 in the presence of the iodonium salt and the amine, resulting from their similarity in structures. Their respective performances in photopolymerization can be found in supplementary information. Moreover, dyes 20 and 21 showed a similar photolysis process to that observed for dye 19 even though dye 20 possesses a lower extinction coefficient at 405 nm (ε @405 =1810 M -1 cm -1 ; see Table 1). Thus, we neglect the discussions on the photolyses of dyes 20, 21 as well.

ESR spin-trapping experiments

For a better understanding of the dye/Iod/amine interaction, ESR-spin trapping experiments were carried out on dye 5/Iod/amine (or dye 17/Iod/amine) and dye 5/Iod solution under N2 in presence of PBN as the spin trap agent (Figure 9a, b, c, d for dye 5, Figure 9e, f for dye 17). The simulation gave the following hfc constants for the PBN spin adducts: aN = 14.4 G; aH = 2.1 G for the dye 5/Iod PISs (aN = 14.7 G; aH = 2.8 G for the dye 17/Iod/amine PISs) which suggests the generation of the radical (CH3)3C6H4• fully matching to literature data. 152 It also suggests that the redox reaction (r2) between dye 5 and Iod takes place where Iod acts as an oxidizing agent leading to the formation of aryl radicals.

Additional ESR-spin trapping experiments carried out by irradiating the dye 5/amine solution were carried out and the results are presented in the Figure 9c and9d.

Interestingly, a radical adduct corresponding to PBN/ArNCH3CH2• adduct is detected and characterized by aN = 14.4 G; aH = 2.2 G in agreement with literature data. 153 As expected, there is no radical observed for dye 17/amine PISs. All these data are in agreement with the chemical mechanisms presented above in the Scheme 1.

Notably, these two species were detected by ESR-spin trapping experiments (see below). The results will be detailed in the next parts of this paper.

(r1)

Dye → *Dye (h)

Proposed photoinitiation step mechanisms of the dyes/Iod/EDB redox combination.

ESR spin-trapping experiments

Confirmation of the proposed chemical mechanisms could be obtained by characterizing the Ar 1. Here, a two-component PIS (2%

Iod: 2% amine in TA) without dyes (cited as curve 0 in Figure 1) was used as a reference system to make a comparison with the three-component systems. Indeed, dyes 1-6 showed different performances with the same content of co-initiators (Iod and EDB), indicating their different photoinitiation abilities. To clarify the results more clearly, we divided the investigated dyes into three different families:

(1) For dyes 5 and 6, high final acrylate function conversions (FCs) reaching 98% or above could be obtained, what is higher than the conversions obtained with the reference two-component Iod/EDB PISs without dye. In this last case, a final acrylate function conversion of 97% could be determined after 50 s of irradiation (set as curve 0 in Figure 2a). The initiation ability for this latter blank is ascribed to the formation to a photosensitive charge transfer complex between amine and iodonium salt. [START_REF] Fouassier | Dyes as photoinitiators or photosensitizers of polymerization reactions[END_REF] (2) For dye 1 and 3, even if these dyes showed lower efficiencies and lower FCs compared to dyes 5, 6 or blank, however, their polymerization processes were also characterized by shorter induction period (T-T0 with T: Start time of polymerization; T0: Start time of light irradiation). Precisely, their polymerizations were started within 10s, i.e. 7s for dye 1 and 4s for dye 3 while 40s for the blank. Thus, their remarkable photoinitiation abilities were also considered and investigated in the following context.

Interestingly, a short induction period (25s) was also obtained with dye 6, which furnished the highest acrylate function conversion (~99%). This is undoubtedly the most efficient photoinitiator of the series of dyes examined in this work.

(3) The other dyes (dyes 2 and 4) showed both poor efficiencies (lowest FCs) or longer induction periods compared to the other dyes: The acrylate function conversion obtained with dye 2 only furnished 73% and the induction period was 68 s; On the respectively. The oxidation reaction r2 could generate radical Dye

ESR spin-trapping experiments

In this part, a clear chemical interaction between the dyes and Iod or EDB upon excitation at 405 nm has also been observed during the ESR-spin trapping experiments. The Ar • and EDB • (-H) radicals formed from Iod and EDB in presence of dye during the irradiation with the LED could be detected in the different solutions, as a confirmation of the proposed chemical mechanism mentioned above. [START_REF] Sokołowska | Styryl dyes as new photoinitiators for free radical polymerization[END_REF] The solutions constituted by a mixture of the selected dyes (dyes 1, 3, 6) and the Iodonium salt (or EDB) in tert-butylbenzene at weight concentration ratio ([dye]=0.2 mg/mL; [Iod]= 2 mg/mL; [EDB]=2 mg/mL) under protection gas, N2, then the spin trap agent, PBN (2 mg/mL), was also dissolved into the solution. The experimental results and ESR spectra of dyes with Iod (or EDB) both before and after irradiation at 405 nm are presented in Figure 8 for dye 6. Particularly, the ESR spectra for the dye 6/Iod and dye 6/EDB combinations under sunlight were also photopolymerizable materials in the potential field of additive manufacturing/vat photopolymerization. (10), 1433-1440.

General conclusions

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