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The design of biosourced and/or bioinspired photoinitiators is an active research field as it offers a unique opportunity to develop photoinitiating systems exhibiting better biocompatibility as well as reduced toxicity. In this field, flavonoids can be found in numerous fruits and vegetables so these structures can be of interest for developing, in the future, polymerization processes, offering a reduced environmental impact but also better biocompatibility of the polymers. In this review, the different flavonoids reported to date as photoinitiators of polymerization are presented. Over the years, different modifications of the flavonoid scaffold have been examined including the grafting of well-known chromophores, the preparation of Type II photoinitiators or the introduction of photocleavable groups enabling the generation of Type I photoinitiators. Different families of flavonoids have also been investigated, enabling to design of high-performance photoinitiating systems. [ABSTRACT FROM AUTHOR]

During the last decades, multicomponent photoinitiating systems have been the focus of intense research efforts, especially for the design of visible light photoinitiating systems. Although highly reactive three-component and even four-component photoinitiating systems have been designed, the complexity to elaborate such mixtures has incited researchers to design monocomponent Type II photoinitiators. Using this approach, the photosensitizer and the radical/cation generator can be combined within a unique molecule, greatly simplifying the elaboration of the photocurable resins. In this field, sulfonium salts are remarkable photoinitiators but these structures lack absorption in the visible range. Over the years, various structural modifications have been carried out in order to redshift their absorptions in the visible region. In this work, an overview of the different sulfonium salts activable under visible light and reported to date is proposed. [ABSTRACT FROM AUTHOR]

This work is devoted to the study of two copper complexes (Cu) bearing pyridine ligands, which were synthesized, evaluated and tested as new visible light photoinitiators for the free radical photopolymerization (FRP) of acrylates functional groups in thick and thin samples upon light-emitting diodes (LED) at 405 and 455 nm irradiation. These latter wavelengths are considered to be safe to produce polymer materials. The photoinitiation abilities of these organometallic compounds were evaluated in combination with an iodonium (Iod) salt and/or amine (e.g., N-phenylglycine—NPG). Interestingly, high final conversions and high polymerization rates were obtained for both compounds using two and three-component photoinitiating systems (Cu1 (or Cu2)/Iodonium salt (Iod) (0.1%/1% w/w) and Cu1 (or Cu2)/Iod/amine (0.1%/1%/1% w/w/w)). The new proposed copper complexes were also used for direct laser write experiments involving a laser diode at 405 nm, and for the photocomposite synthesis with glass fibers using a UV-conveyor at 395 nm. To explain the obtained polymerization results, different methods and characterization techniques were used: steady-state photolysis, real-time Fourier transform infrared spectroscopy (RT-FTIR), emission spectroscopy and cyclic voltammetry. [ABSTRACT FROM AUTHOR]

[Display omitted] Iodonium salts are historical cationic photoinitiators extensively used for the polymerization of epoxides or vinyl ethers. As drawback, the absorption of iodonium salts is strongly UV-centered so that this UV photoinitiator can only operate in the visible range by mean of a photosensitization process. With aim at simplifying the composition of the photopolymerizable resins, numerous chromophores have been covalently linked to the iodonium salts, enabling to design monocomponent photoinitiating systems. In this review, an overview of the different dyes that have been connected to iodonium salts is provided. [ABSTRACT FROM AUTHOR]

[Display omitted] Photoinitiating systems activable under visible light are the focus of numerous studies, and this interest is directly related to the higher light penetration that can be obtained in this spectral range. Reactivity of the photoinitiating systems is another major concern of photopolymerists and this issue can be addressed by investigating new structures of dyes. In this field, naphthoquinones have only been scarcely examined as visible light photoinitiators of polymerization. Besides, since 2015, different strategies have been developed to design photoinitiators with naphthoquinone, and promising results have been obtained, positively paving the way for future developments. In this review, an overview of the different photoinitiators comprising a naphthoquinone unit is given and polymerization efficiencies of these new structures are compared to those of reference compounds. [ABSTRACT FROM AUTHOR]

A series of 18 iridium complexes ( Ir) have been prepared and studied. Their abilities to initiate both a ring-opening cationic polymerization (CP) using an Ir/iodonium salt couple and a free radical polymerization (FRP) in the presence of an Ir/amine/alkyl halide system under very soft irradiation (i.e., halogen lamp) or upon exposure to laser diodes @457 nm and @532 nm are investigated. Interestingly, these photoinitiating systems operate through an oxidative or a reductive photoredox catalytic cycle, respectively. Excellent polymerization profiles are obtained (reactive function conversions >70% for the CP of an epoxide and >60% for the FRP of low viscosity triacrylate). The photochemical mechanisms are studied by fluorescence, steady state photolysis, cyclic voltammetry, and electron spin resonance spin trapping experiments. [ABSTRACT FROM AUTHOR]

[Display omitted] The design of photoinitiating systems activable in the visible range and under low light intensity is an active research field supported by the vast applications of photopolymerization. In the search for new dyes that can exhibit panchromatic absorptions, anthraquinone has been identified as a promising scaffold. Over the years, numerous modifications have been carried out on this scaffold by molecular engineering, enabling to design water-soluble, panchromatic or photobleachable dyes. In this review, an overview of the recent advances concerning photopolymerization with anthraquinone-based photoinitiating systems is given. [ABSTRACT FROM AUTHOR]

[Display omitted] • Water-soluble photoinitiators have been examined as photoinitiators for one and two-photon polymerization. • Polymerization processes in the visible range have been initiated. • Photobleachable photoinitiators also exhibit interesting water-solubilities. • Polymerization of various monomers was possible under low light intensity. Photopolymerization is an active research field and recently, a great deal of efforts has been devoted to replace the energy-consuming UV irradiation setups by the popular and easily available energy-saving light-emitting diodes (LEDs). With aim at going a step further, sunlight can be used as an inexpensive and unlimited light source to perform polymerization processes. In this case, no energy is required anymore, and the use of photopolymerization for various outdoor applications becomes possible. Over the years, and with aim at developing reliable photoinitiating systems, various dyes have been developed, enabling in the case of sunny days to efficiently promote polymerization processes. In this review, an overview of the different photoinitiating systems developed for polymerization processes done under sunlight or even daylight is provided. Polymerization efficiency is often compared to that obtained with artificial light sources, enabling to determine the efficacy of the photoinitiating systems under sunlight. [ABSTRACT FROM AUTHOR]

[Display omitted] • Water-soluble photoinitiators have been examined as photoinitiators for one and two-photon polymerization. • Polymerization processes in the visible range have been initiated. • Photobleachable photoinitiators also exhibit interesting water-solubilities. • Polymerization of various monomers was possible under low light intensity. Visible light photopolymerization has been extensively studied during the last decades since alternatives to UV processes have to be found rapidly. Indeed, based on the Minamata Convention, Hg irradiation setups must be suppressed from industrial applications. Even if the use of visible light is beneficial in terms of depth of cure and safety for the manipulator, photopolymerization in water is still far aside to the other polymerization processes. At present, only few visible light photoinitiators are soluble in water, speeding down the development of greener polymerization processes. In this review, an overview of the recent advances on water-soluble photoinitiating systems is provided. Even if water-solubility is a key parameter to polymerize in water, the monomer conversion that can be achieved is also important. For these reasons, comparisons with reference photoinitiating systems soluble in water is provided. [ABSTRACT FROM AUTHOR]

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

[Display omitted] Photopolymerization under visible light has known major breakthroughs in recent years due to the development of new irradiation setups based on Light-Emitting Diodes (LEDs) enabling now to polymerize resins with low light intensities but also by the urgent need to develop efficient photoinitiating systems perfectly fitting the light sources currently under use in 3D printers. If the use of visible light is the key parameter to increase the thickness of the polymerized films, visible light photoinitiators are also strongly colored compounds that can adversely impact the future use of photopolymers for daily applications. Indeed, colorless coatings are highly desired by industry, what can not currently be achieved by use of visible light photoinitiators. To address this issue, the development of photobleachable photoinitiating systems has become an active research field. In this review, an overview of the recent advances on photobleachable photoinitiators is provided. Face to the difficulty to access to these highly desired structures, a wide range of structures have been explored and an overview of the different structures is given. [ABSTRACT FROM AUTHOR]

A series of conjugated polymers and oligomers previously used in organic photovoltaics and organic light-emitting diodes (OLEDs) have been examined for the first time as photoinitiators of polymerization. They address the present and future challenges facing photopolymerization: requirement of high extinction molar coefficients, absorption over the whole visible spectrum, ability to easily withdraw or release electrons. Interestingly, these compounds initiate the cationic polymerization of epoxides or the free radical polymerization of acrylates upon exposure of the photocurable formulations to a LED emitting at 405 nm. Some of them can potentially operate under blue, green, or red laser diodes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Benzylidene cyclopentanones have been examined as photoinitiators for one and two-photon polymerization. • Polymerization processes in the visible range have been initiated. • Benzylidene cyclopentanones exhibit interesting photophysical properties. • Polymerization of various monomers was possible under low light intensity. Photopolymerization is a rapidly evolving research field, supported by the growing demand for greener polymerization processes and the development of 3D printers used now for daily applications. With aim at developing dyes adapted for LEDs used in 3D printers, benzylidene cyclopentanones have been regarded as interesting structures due to their facile synthesis, low cost, high thermal and photochemical stability. Especially, benzylidene cyclopentanones proved to be better candidates than their benzylidene ketones analogue due to the rigidity provided by the central five-membered ring. As a result of this rigidification, higher monomer conversions could be obtained. Noticeably, benzylidene cyclopentanones could initiate one and two-photon polymerizations processes, what is relatively unusual for dyes to exhibit this dual photoinitiating ability. In this review, recent advances concerning benzylidene cyclopentanones are detailed. Compared to their acyclic analogues, a severe improvement of the photoinitiating ability has been demonstrated for benzylidene cyclopentanones, so that comparisons between these two families of photoinitiators is provided. [ABSTRACT FROM AUTHOR]

[Display omitted] • Benzylidene ketones have been examined as photoinitiators of polymerization. • Monocomponent photoinitiating systems have been prepared. • Benzylidene ketones exhibit interesting photobleaching properties during the polymerization process. • High depth of cure could be obtained with benzylidene ketones. Visible light photopolymerization is facing a revolution with the development of monocomponent photoinitiating systems greatly simplifying the composition of the photocurable resins. Among unimolecular photoinitiators, chalcones and related structures are extensively studied due to the easiness of synthesis, the versatility of the substitution pattern and the good thermal stability. Among chromophores that can be used for the design of photoinitiators, benzylidene ketones have been identified as promising scaffolds due to their facile syntheses reduced to a unique synthetic step, the possibility to access to these structures using environmentally friendly conditions and their good absorptions in the visible range. In this review, an overview of the recent advances on benzylidene ketones used as visible light photoinitiators is provided. Notably, these structures could be used for both one photon and two-photon polymerizations, evidencing the versatility of these structures. To evidence the interest of benzylidene ketones, comparisons with benchmark photoinitiators are provided. [ABSTRACT FROM AUTHOR]

[Display omitted] • Carbazole derivatives have been examined as photoinitiators of polymerization. • Polymerization processes in the visible range have been initiated. • Carbazole exhibit interesting photophysical properties. • Polymerization of various monomers was possible under low light intensity. Photopolymerization is currently facing a revolution with the development of new light sources that are light-emitting diodes. Considering the low light intensity of these light sources, new photoinitiators exhibiting a higher reactivity than those used in the past have to be developed. Parallel to this, a great deal of efforts is also devoted to simplify the photocurable resins. With aim at simplifying the composition of the resins, monocomponent systems are actively researched. Indeed, if Type II photoinitiators are efficient structures, these compounds need besides to be combined with additives, complexifying the composition of the resins. In this field, oxime esters that are monocomponent Type I photoinitiators are extensively studied since a few years. In this field, the most impressive developments have been obtained for carbazole-based oxime esters since not less than 100 structures have been proposed during the last three years, demonstrating the intense research activity. In this review, an overview of the recent development concerning carbazole-based oxime esters is presented. Comparisons with reference compounds will also be presented to evidence the interest of these new structures. [ABSTRACT FROM AUTHOR]

[Display omitted] • Pyrrole derivatives have been examined as photoinitiators of polymerization. • Polymerization processes in the visible range have been initiated. • Pyrrole exhibit interesting photophysical properties. • Polymerization of various monomers was possible under low light intensity. Photopolymerization is the focus of intense research efforts due to the recent need to develop visible light photoinitiating systems activable under low light intensity and in the visible range. This effort is notably supported by the recent development of 3D/4D printing and light cured dental composite resins requiring new polymerization approaches. With aim at developing new formulations combining fast polymerization kinetics, high final monomer conversions, low energy consumption and activation in the visible range, pyrrole has been identified as an interesting scaffold for the design of various dyes. Indeed, numerous derivatives such as porphyrins, diketopyrrolopyrroles, chalcones, push-pull dyes can be prepared with pyrrole and related derivatives. As the main advantages, pyrrole is a five-membered ring heterocycle characterized by an easiness of chemical modification, low oxidation potential, high thermal and chemical stability that make pyrrole an interesting scaffold for the design of photoinitiators. More precisely, pyrrole has been used for the design of numerous Type II photoinitiators. In this review, an overview of the different dyes based on pyrrole and related derivatives, and reported to date is presented. Comparisons with benchmark photoinitiators will be established to evidence the interest of these new structures. [ABSTRACT FROM AUTHOR]

[Display omitted] Photopolymerization is an active research field, and this intense research activity is supported by the numerous and emerging applications related to this polymerization technique. Especially, the historical UV approach is more and more discarded in favour of visible light photopolymerization making use of cheap, lightweight, compact, low intensity and energy-saving light sources that are light-emitting diodes (LEDs). With aim at developing photoinitiators fitting with these light sources, anthracene has been identified as a promising scaffold for the design of photoinitiators of polymerization. In this review, an overview of the recent advances concerning anthracene-based photoinitiators is provided. To evidence the interest of these structures, comparisons with benchmark photoinitiators will be provided. [ABSTRACT FROM AUTHOR]

[Display omitted] • Thiophene have been examined as photoinitiators of polymerization. • Polymerization processes in the visible range have been initiated. • Thiophene exhibit interesting photophysical properties. • Polymerization of various monomers was possible under low light intensity. Visible light photopolymerization has given a revival of interest to photopolymerization due to its recent use in 3D printing. With aim at developing visible light photoinitiating systems, structures disconnected from the traditional photoinitiators are more and more widely examined. Besides, several parameters such as the excited state lifetimes, the redox properties or the molar extinction coefficients of the newly prepared photoinitiators have to be considered in order photoinitiators to efficiently interact with the different additives introduced into the photocurable resins. In this field, thiophene is extensively used in Organic Electronics due to its low oxidation potential, its facile chemical modification and the possibility to design chromophores absorbing all over the visible range. Considering the numerous thiophene derivatives that have been prepared all over the years, thiophene was thus a candidate of choice for the design of visible light photoinitiators of polymerization. Over the years, structures of thiophene-based photoinitiators have greatly evolved and Type I or Type II photoinitiators have notably been developed. In this review, an overview of the recent advances concerning thiophene-based photoinitiators is provided. To evidence the interest of these structures, comparisons with reference photoinitiating systems are provided. [ABSTRACT FROM AUTHOR]

[Display omitted] During the past decades, a breakthrough has been obtained in the development of photoinitiators activable under low light intensity and in the visible range. In the search for new structures, biosourced or bioinspired molecules have been examined as potential candidates for photoinitiation. In this field, coumarins that can be found in numerous flowers, fruits, vegetables and plant species were determined as highly efficient photoinitiators of polymerization. Especially, these structures were determined as acting as efficiency photoinitiators in Type I and Type II initiating systems. In this review, an overview of the recent development on visible light coumarin-based photoinitiators of polymerization is provided. To evidence the interest of these structures, comparisons with reference photoinitiating systems will be provided. [ABSTRACT FROM AUTHOR]

In this article, five new organic dyes based on the naphthalimide scaffold (Napht-1–Napht-5) were synthesized and tested as high-performance photoinitiators for both the Free Radical Photopolymerization (FRP) of acrylates and the Cationic Polymerization (CP) of epoxides using blue Light-Emitting Diodes (LEDs) as a safe irradiation source (LED @405 nm and 455). In fact, very good photopolymerization profiles (high final conversions and high polymerization rates) were obtained once these photoinitiators were combined with an Iodonium salt (Iod) or Iod/amine NPG and NVK). Remarkably, these dyes were able to generate interpenetrating polymer networks (IPN) by polymerization of a blend of monomers. These experiments were carried out to improve the polymerization profiles as well as the mechanical properties of the obtained materials. Due to their high photoinitiation abilities, these compounds were used in some applications such as photocomposite synthesis, direct laser write, and 3D printing experiments. To determine the chemical mechanisms, the photochemical/photophysical properties of these compounds were studied using different characterization techniques such as UV–visible absorption spectroscopy, steady-state photolysis, Fluorescence quenching, time-resolved fluorescence spectroscopy, FTIR spectroscopy, and cyclic voltammetry [ABSTRACT FROM AUTHOR]

[Display omitted] Photopolymerization is an active research field requiring the design of new photosensitizers enabling to convert more efficiently liquid monomers as solids. Especially, a great deal of efforts is devoted to reduce the polymerization time, improve the monomer conversion while using safer irradiation conditions. To reach these goals, choice of the photosensitizers is crucial as it constitutes the unique element of the photocurable resins capable to interact with light. In this field, perylene which is a pigment of polycyclic aromatic structure has been studied as soon as 1978 for the design of photoinitiating systems. Since these pioneering works, a wide range of perylene-based photoinitiators have been developed, ranging from organic photocatalysts to water-soluble photoinitiators. In this review, an overview of the different perylene-based photoinitiators reported to date is provided. To evidence the efficiency of these structures, comparisons which benchmark photoinitiators is provided. [ABSTRACT FROM AUTHOR]

[Display omitted] • Chalcones have been examined as photoinitiators of polymerization. • Polymerization processes in the visible range have been initiated. • Chalcones can be prepared in one step. • Polymerization of various monomers was possible under low light intensity. Photopolymerization is an active research field facing a revolution due to the recent availability of cheap, lightweight, compact and energy-saving light sources that constitutes light-emitting diodes (LEDs). With regards to the low light intensity delivered by these new irradiation setups, the demand for photoinitiating systems activable under low light intensity, in the visible range, and exhibiting a high reactivity even under air are actively researched. With aim at developing dyes fitting the above requirements while remaining relatively uncoloured, chalcones are one of those. In this review, an overview of the recent development concerning chalcone-based photoinitiating systems is reported. With aim at evidencing the crucial interest of this family of dyes, comparisons with benchmark photoinitiators will be presented. [ABSTRACT FROM AUTHOR]

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

In this paper, nine organic compounds based on the coumarin scaffold and different substituents were synthesized and used as high-performance photoinitiators for free radical photopolymerization (FRP) of meth(acrylate) functions under visible light irradiation using LED at 405 nm. In fact, these compounds showed a very high initiation capacity and very good polymerization profiles (both high rate of polymerization (Rp) and final conversion (FC)) using two and three-component photoinitiating systems based on coum/iodonium salt (0.1%/1% w/w) and coum/iodonium salt/amine (0.1%/1%/1% w/w/w), respectively. To demonstrate the efficiency of the initiation of photopolymerization, several techniques were used to study the photophysical and photochemical properties of coumarins, such as: UV-visible absorption spectroscopy, steady-state photolysis, real-time FTIR, and cyclic voltammetry. On the other hand, these compounds were also tested in direct laser write experiments (3D printing). The synthesis of photocomposites based on glass fiber or carbon fiber using an LED conveyor at 385 nm (0.7 W/cm2) was also examined. [ABSTRACT FROM AUTHOR]

• Indane-1,3-dione derivatives have been examined as photoinitiators of polymerization. • Polymerization in the UV range and under visible light has been initiated. • Various strategies were developed to improve the light absorption properties. • Polymerization of various monomers was possible under low light intensity. Photoinitiators that can operate under visible light and low light intensity are actively researched as this last generation of photoinitiators can address different issues raised by the traditional UV photoinitiators. Notably, the safety concern related to the use of UV light, the high energy consumption resulting from the use of UV irradiation setups, the low light penetration of UV light within the photocurable resins are three major concerns that visible light photoinitiators successfully address. Besides, the photoinitiating ability and the polymerization kinetic should remain high in order these new generations of photoinitiators to be capable to replace the traditional UV photoinitiators. Among the different scaffolds examined for the design of efficient visible light photoinitiators, indane-1,3-dione has been identified as a promising scaffold for the design of various structures. By the presence of the activated methylene group standing between the two ketone groups, this electron acceptor is an ideal candidate for the design of push–pull dyes by one of the simplest reactions, namely the Knoevenagel reaction. This group is also a good electron acceptor as the two ketone groups can be advantageously used in the electronic delocalization by mesomerism. If this electron acceptor has been extensively studied for the design of push–pull dyes, its scope of application was not only limited to the design of push–pull dyes and products issued from cyclization reactions, the design of compounds with extended aromaticities, improvement of the electron-withdrawing ability by substitution of the ketone groups by appropriate groups were examined as possible chemical modifications enabling to improve the photoinitiating ability of photoinitiators based on indane-1,3-dione and its related derivatives. In this review, an overview of the different visible light photoinitiators based on indane-1,3-dione and derivatives is provided. To evidence the performance of the different photoinitiators, comparisons with reference compounds will be provided. [ABSTRACT FROM AUTHOR]

This work is devoted to investigate three coumarin derivatives (Coum1, Coum2, and Coum3), proposed as new photoinitiators of polymerization when combined with an additive, i.e., an iodonium salt, and used for the free radical polymerization (FRP) of acrylate monomers under mild irradiation conditions. The different coumarin derivatives can also be employed in three component photoinitiating systems with a Iod/amine (ethyl 4-dimethylaminobenzoate (EDB) or N-phenylglycine (NPG)) couple for FRP upon irradiation with an LED @ 405 nm. These compounds showed excellent photoinitiating abilities, and high polymerization rates and final conversions (FC) were obtained. The originality of this work relies on the comparison of the photoinitiating abilities of monofunctional (Coum1 and Coum2) vs. difunctional (Coum3) compounds. Coum3 is a combined structure of Coum1 and Coum2, leading to a sterically hindered chemical structure with a relatively high molecular weight. As a general rule, a high molecular weight should reduce the migration of initiating molecules and favor photochemical properties such as photobleaching of the final polymer. As attempted, from the efficiency point of view, Coum3 can initiate the FRP, but a low reactivity was observed compared to the monofunctional compound (Coum1 and Coum2). Indeed, to study the photochemical and photophysical properties of these compounds, different parameters were taken into account, e.g., the light absorption and emission properties, steady state photolysis, and fluorescence quenching. To examine these different points, several techniques were used including UV-visible spectroscopy, real-time Fourier Transform Infrared Spectroscopy (RT-FTIR), fluorescence spectroscopy, and cyclic voltammetry. The photochemical mechanism involved in the polymerization process is also detailed. The best coumarins investigated in this work were used for laser writing (3D printing) experiments and also for photocomposite synthesis containing glass fibers. [ABSTRACT FROM AUTHOR]

A series of eleven push–pull chromophores with specific structures have been designed for the free radical polymerization of acrylates, but also for the fabrication of photocomposites and 3D-printed structures. New photoinitiating systems comprising the different push–pull dyes showed excellent photochemical reactivities at 405 nm. Notably, polymerization reactions could be initiated with light-emitting diodes (LEDs) which constitute a unique opportunity to promote the free radical polymerization under mild conditions, i.e., low light intensity (e.g., sunlight) and under air. Photopolymerization is an active research field, and push–pull dyes have already been investigated for this purpose. Besides, it remains of crucial interest to investigate new reactive structures capable of efficiently initiating photopolymerization reactions. The plausible potential of these structures to act as efficient photoinitiators in vat photopolymerization (or 3D printing) and fabrication of photocomposites prompts us to select eleven new push–pull dyes to design multi-component photoinitiating systems activable with LEDs emitting at 405 nm. Precisely, a tertiary amine, i.e., ethyl dimethylaminobenzoate (EDB) used as an electron/hydrogen donor and an iodonium salt used as an electron acceptor were selected to behave as powerful co-initiators to construct three-component photoinitiating systems (PISs) with the different push–pull dyes. Among these new PISs, dye 8 and 9-based PISs could efficiently promote the free radical photopolymerization of acrylates upon exposure to a LED emitting at 405 nm also upon sunlight irradiation, highlighting their huge performance. Photoinitiating abilities could be explained on the basis of steady state photolysis experiments. Fluorescence measurements and electron spin resonance (ESR) spin-trapping experiments were also performed to obtain a deeper insight into the chemical mechanisms supporting the polymerization reaction and determine the way the initiating species, i.e., the radicals, are observed. Finally, two investigated dye-based PISs were applied to the fabrications of photocomposites. Three-dimensional patterns with excellent spatial resolutions were generated by the laser writing technique to identify the effects of photopolymerization of acrylates both in the absence and presence of fillers (silica). Interestingly, comparison between the 3D objects fabricated by the PISs/monomer systems and the PISs/monomer/filler photocomposites indicates that the newly designed photocomposites are suitable for practical applications. [ABSTRACT FROM AUTHOR]

The design of photoinitiators of polymerization is an active research field and the development of initiators activable in the visible range and under low light intensity have been the focus of numerous research efforts from both the academic and industrial communities. Recently, two families of polyaromatic compounds, i.e. 1,8-naphthalic anhydride and naphthalimides, have been extensively studied by different research groups due to their easiness of synthesis and the possibility to easily tune the photophysical properties. Especially, panchromatic, crosslinkable or water-soluble photoinitiators could be easily developed starting from the benchmark and cheap 1,8-naphthalic anhydride. In this review, an overview of the different photoinitiators based on naphthalic anhydride or the naphthalimide scaffold is presented. Based on the diversity of structures reported in the literature, a structure-performance relationship could be established in this review. [ABSTRACT FROM AUTHOR]

In this study, eight coumarins (coumarins 1–8) are proposed as near-UV and blue light sensitive photoinitiators/photosensitizers for the cationic polymerization (CP) of epoxysilicones when combined with 4-isopropyl-4'-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate (IOD). Among these coumarins, four of them (coumarins 1, 2, 6 and 8) have never been reported in the literature, i.e., these structures have been specifically designed to act as photoinitiators for silicones upon near UV and visible irradiation. Good final reactive epoxy function conversions (FCs) and also high rates of polymerization (Rp) were achieved in the presence of the newly proposed coumarin-based systems. The polymers generated from the photopolymerization of epoxysilicones can be considered as attractive candidates for several applications such as: elastomers, coatings, adhesives, and so on. The goal of this study focuses also on the comparison of the new proposed coumarins with well-established photosensitizers i.e., 1-chloro-4-propoxythioxanthone (CPTX), 9,10-dibutoxyanthracene (DBA) or some commercial coumarins (Com. Coum). As example of their high performance, the new proposed coumarins were also used for laser write experiments upon irradiation with a laser diode at 405 nm in order to develop new cationic 3D printing systems. [ABSTRACT FROM AUTHOR]

The design of photoinitiating systems with excellent photochemical reactivities at 405nm LED is one of the obstacles to efficiently promote free radical polymerization in mild conditions (e.g., low light intensity, under air). Here, our actual search for new multicomponent photoinitiating systems at 405nm LED prompts us to develop new dyes based on push–pull structures. In the present paper, we chose two series of new dyes which possess indane-1,3-dione and 1H-cyclopenta naphthalene-1,3-dione groups as the electron-withdrawing groups, since they have the great potential to behave as sensitive and remarkable photoinitiators in vat photopolymerization/3D printing. When incorporated with a tertiary amine (ethyl dimethylaminobenzoate EDB, used as electron/hydrogen donor) and an iodonium salt (used as electron acceptor) as the three-component photoinitiating systems (PISs), and among a series of 21 dyes, 10 of them could efficiently promote the free radical photopolymerization of acrylates. Interestingly, steady state photolysis experiments revealed different behaviors of the dyes. Fluorescence experiments and free energy change calculations for redox processes were also carried out to investigate the relevant chemical mechanisms. Additionally, the formation of radicals from the investigated PISs was clearly observed by electron spin resonance (ESR) spin-trapping experiments. Finally, stereoscopic 3D patterns were successfully fabricated by the laser writing technique. In this work, the use of push–pull dyes based on the naphthalene scaffold as photoinitiators of polymerization is reported for the first time in a systematic study aiming at investigating the structure–performance relationship for irradiation carried out at 405 nm. By carefully selecting the electron donors used in the two series of push–pull dyes, novel and high-performance photoinitiating systems operating at 405 nm are thus proposed. [ABSTRACT FROM AUTHOR]

Photoinitiators of polymerization activable under low light intensity and in the visible range are actively researched by both the academic and industrial communities. To efficiently harvest light and initiate a polymerization process, polyaromatic structures are ideal candidates. In this field, pyrene can be cited as a relevant example. Over the years, a wide range of Type I and Type II photoinitiators have been developed, ranging from push-pull dyes to polyaromatic structures, ferrocenium, ketones, chromones or acridinediones. In this review, an overview of the different pyrene-based photoinitiators reported to date is presented. Especially, a structure-performance relationship will be established with benchmark photoinitiators, evidencing all the interests of this polyaromatic structure. [ABSTRACT FROM AUTHOR]

Photoinitiators of polymerization that can be activated under visible light and low intensity are actively researched as these compounds constitute the next generation of photoinitiators that will replace the controverted UV-photoinitiators. Over the years, a family of compounds has been identified as being highly promising in light of its remarkable photoinitiating ability, photochemical stability, low cost and easiness to functionalize, namely carbazoles. In this review, an overview of the different carbazole-based photoinitiators reported to date is presented. Interestingly, carbazoles have mostly been used as electron donors in push-pull structures, as photosensitizers for iodonium salts, as ligands for ferrocenium salts or as chromophores for phenacyl onium salts. These different applications clearly demonstrate the versatility of this structure used as the elemental building block for the design of UV, near-UV or visible light photoinitiators. [ABSTRACT FROM AUTHOR]

Iodonium salts are well established photoacid generators, cationic photoinitiators, as well as additives commonly used in photoredox catalytic cycles. However, as a strong limitation, iodonium salts are characterized by low light absorption properties for λ > 300 nm so that these latter cannot be activated with cheap, safe, and eco-friendly near UV or even visible light emitting diodes (LEDs). To overcome this drawback, the covalent linkage of an iodonium salt to a chromophore absorbing at longer wavelength is actively researched. With aim at red-shifting the absorption spectrum of the iodonium salt, the synthesis of new compounds combining within a unique chemical structure both the chromophore (here the naphthalimide scaffold) and the iodonium salt is presented. By mean of this strategy, a polymerization could be initiated at 365 nm with the modified iodonium salts whereas no polymerization could be induced with the benchmark iodonium salt i.e., Speedcure 938 at this specific wavelength. To examine the effect of the counter-anion on the photoinitiating ability of these different salts, five different counter-anions were used. Comparison between the different anions revealed the bis(trifluoromethane)sulfonimide salt to exhibit the best photoinitiating ability in both the free radical polymerization of acrylates and the cationic polymerization of epoxides. To support the experimental results, molecular orbital calculations have been carried out. By theoretical calculations, the initiating species resulting from the photocleavage of the iodonium salts could be determined. The cleavage selectivity and the photochemical reactivity of the new iodoniums are also discussed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Two new copper complexes were investigated for both radical and cationic polymerisation under air. • The formations of acrylate/epoxy interpenetrated polymer networks were successfully performed. • Several characterisations of the interpenetrated polymer networks were done. • The advantages of IPNs are highlighted. Two new copper complexes were investigated in three-component visible light photoinitiating system for both radical and cationic polymerisation under air. Their reactivity and efficiency have been compared with that of a highly efficient photoredox catalyst. The formation of acrylate/epoxy interpenetrated polymer networks were successfully performed through concomitant initiation of radical and cationic polymerisation with low copper complex content upon either LED@405 nm or LED@455 nm irradiations. Several characterisations of the acrylate/epoxy IPNs final properties, such as the mechanical properties, adhesion properties, shrinkage and swelling, indicated the possibilities to adapt or tune their properties compared to radical polymerisation of TMPTA. [ABSTRACT FROM AUTHOR]

[Display omitted] • Squaraines have been examined as photoinitiators of polymerization. • Polymerization processes in the visible range and the near-infrared region have been initiated. • Squaraines can absorb in the UV range and in the near-infrared range. • Polymerization of various monomers was possible under low light intensity. The development of photoinitiating systems activable in the visible and the near-infrared region is an active research field. Compared to the traditional UV photopolymerization for which the light penetration into the photocurable resin remains limited, a significant enhancement of this latter can be obtained in the visible and the near infrared range so that the scope of applications of photopolymerization can be revolutionized. Among dyes whose absorption can be facilely tuned, squaraines can be cited as relevant examples. Depending on the peripheral groups introduced on both sides of the central four-membered squaric ring, absorptions ranging from 400 to 1000 nm can be obtained. In this review, an overview of the recent development concerning the squaraines-based photoinitiating systems is provided. To evidence the interest of these new photoinitiating systems, comparisons with reference systems will be established. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new oxime ester is investigated as a photobase generator. • A base-catalyzed thiol-acrylate Michael addition reaction is catalyzed. • This system accomplished a good performance for dual activation (light + heat). In this work, a previously designed and synthesized phenothiazine-based oxime ester was investigated as a photobase generator for the formation of thiol-acrylate networks through the polymerization of thiol-acrylate reactions and more appealingly the base-catalyzed thiol-acrylate Michael addition reactions. This system accomplished a complete disappearance and a full conversion of the characteristic acrylate peak within 10 min of irradiation with a light-emitting diode (λ = 405 nm) or within 1 or 2 min of the same irradiation source and 24 h of heat exposure (T = 60 °C). Photobase generation was evidenced using phenol red as a pH-indicator, enabling to confirm the production of methyl amine (CH 3 NH 2) as initiating species. Photoinitiation mechanism through which base reactive species are produced and the reactions by which the thiol-acrylate polymerization proceeds were investigated according to the base detection experiments, the obtained real-time Fourier transform infrared spectroscopy results, and according to the general photochemical base-generation reactions of oxime-esters. Finally, differential scanning calorimetry scans and Shore A and Shore D hardness experiments have been performed on the polymers obtained to examine their thermal properties and surface hardness features. [ABSTRACT FROM AUTHOR]

Twelve one-compound photo-fragmenting (type I) photoinitiators containing two oxime esters (OXEs) functionalities on the terminal moiety were synthesized. These esters have high light absorptivities in the visible range, and some of them showed better photoinitiation than the commercial photoinitiator diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), under the irradiation of a 405 nm LED. Photoinitiation mechanisms of these developed OXEs are proposed through theoretical calculations, the detection of CO 2 , and the ESR investigation of the free radical generation. 3D objects could be manufactured successfully by using the most reactive photoinitiators with a stereolithography 3D printer. In addition, these reactive OXEs also displayed a thermal initiation behavior, meaning that these compounds can served both as thermal or photochemical initiators. [Display omitted] • Twelve Type I photoinitiators containing two oxime esters (OXEs) are synthesized. • These esters showed better photoinitiation than benchmark photoinitiator. • Photoinitiation mechanisms of these OXEs are proposed. • 3D objects could be manufactured successfully using these photoinitiators. • A dual photo/thermal initiation behavior is found. [ABSTRACT FROM AUTHOR]

[Display omitted] In this article, ten organic dyes based on keto-coumarin (KC) derivatives (MeO-Coum1, MeO-Coum10) have been synthesized and characterized as high performance photoinitiators for the Free Radical Photopolymerization (FRP) of acrylates upon visible light exposure using a Light emitting diode (LED) @405 nm. The addition of Iodonium salt (Iod), amine [ethyl dimethylaminobenzoate (EDB) or N -phenylglycine (NPG)] and Iod/NPG couple in the photocurable resins have been carried out in order to prove their influences on the improvement on the photoinitiating abilities of keto-coumarins. The different dyes showed a very high ability to initiate the Free Radical Photopolymerization by introduction of these additives, using Two or Three-component photoinitiating systems based on MeO-Coum/Iod or amine (0.1% or 0.4%/1% w/w) or MeO-Coum/Iod/NPG (0.1% or 0.4%/1%/1% w/w/w) respectively. In fact, these photoinitiators have been tested in different applications. For example: in direct laser write to generate 3D patterns using a laser diode @405 nm, or for the photocomposite synthesis based on glass fibers. To characterize the initiation ability and to explain the reaction mechanisms in the photoinitiation step, several techniques have been used, such as UV–visible spectroscopy and steady state photolysis, fluorescence emission, RT-FTIR and cyclic voltammetry experiments. [ABSTRACT FROM AUTHOR]

[Display omitted] • 6 novel dyes based on push-pull structures are proposed. • Their abilities to initiate the polymerization process with sunlight were examined. • Photoinitiation abilities were remarkable upon sunlight irradiation. • The photochemical mechanisms are discussed and confirmed. • 3D patterns can be successfully prepared with the proposed systems. In this article, a series of three-component photoinitiating systems (PISs) based on push–pull dyes are proposed. The systems are composed of the newly synthesized push-pull dyes and different co-initiators, namely an iodonium salt and a tertiary amine (ethyl dimethylaminobenzoate EDB). Photoinitiation abilities and photochemical properties of the different investigated photoinitiating systems were studied in detail. Notably, the mechanisms related to the polymerization efficiencies were determined using different techniques such as UV–visible absorption and fluorescence spectroscopy. Moreover, the photoinitiation abilities of the three-component systems for the sunlight-induced polymerization processes were also examined, indicating that sunlight can act as an effective light source for photopolymerization. Finally, 3D patterns with a spatial resolution were successfully produced by the direct laser writing (DLW) approach, even with photosensitive resins containing silica fillers. [ABSTRACT FROM AUTHOR]

• 12 new push-pull dyes (never synthesized before) were synthesized and examined for their photoinitiation abilities. • The new dyes are efficient for direct laser write experiments. • A clear picture of the photochemical mechanisms is provided. • A specific dye (dye 8) proved to be the most reactive photoinitiator of the series upon irradiation with sunlight. In this article, the free radical polymerization of acrylates was successfully achieved with a series of twelve push-pull chromophores comprising the N -ethyl carbazole-1-allylidene group as the electron-donating group. Using this strong donor, the resulting dyes, never synthesized before, could efficiently initiate the free radical photopolymerization of acrylates upon mild irradiation conditions e.g. using sunlight. These novel multi-component systems comprising the above mentioned push-pull dyes, a tertiary amine (ethyl dimethylaminobenzoate EDB) and an iodonium salt could act as efficient photoinitiating systems (PISs) in free radical polymerization (FRP), leading to the formation of 3D patterns with precise shapes under the direct laser write (DLW) approach. Among these new PISs, dyes 3, 6, 7 and 8 were selected due to their efficient photoinitiation performances for chemical mechanism studies including steady state photolysis, fluorescence quenching measurements, electron spin resonance (ESR) spin-trapping experiments and cyclic voltammetry. Markedly, their excellent photochemical reactivities prompted us to investigate the performance of these multi-component photoinitiating systems upon sunlight as an ecofriendly approach. As a result, this research proved that sunlight could be used as a potential light source that can advantageously replace LEDs when highly reactive push-pull dyes are used as photosensitizers for the free radical photopolymerization of acrylate resins. Finally, 3D patterns could be prepared with the new photocomposites, and silica fillers could be even incorporated within the photosensitive resins so that a gradient of resolution could be successfully demonstrated. [ABSTRACT FROM AUTHOR]

Photoinitiators activable under visible light and low light intensity are actively researched as it can advantageously address the safety concerns raised by the traditional UV-light photopolymerization process as well as the light penetration issue. Among dyes strongly absorbing in the visible range, one of the most promising family of photoinitiators are push–pull dyes based on an electron-donating and an electron-accepting moiety connected at both ends of a polyenyl-type spacer. By acting on both the strength of the electron donor and the electron acceptor, position of the intramolecular charge transfer (ICT) band can be easily tuned from the near-UV to the near infrared region. In this review, an overview of the different D-π-A push–pull dyes used as visible photoinitiators of polymerization is presented. Over the years, a clear evolution in the chemical structure of the electron acceptors can be clearly observed, evidencing the strong activity existing around these structures. [ABSTRACT FROM AUTHOR]

• Six ketones were tested as visible light photoinitiators. • Free radical and cationic polymerization tests were carried out. • Polymerization tests were done under visible light. • Low intensity light sources were used. • Laser-write experiments were carried out. Six ketones (abbreviated as ketones 1–6) varying by the substitution pattern of the central cyclohexanones and the choice of the peripheral groups (thiophene or furane) were synthesized and proposed as unprecedented visible light sensitive photoinitiators, in combination with an amine and an iodonium salt, for the free radical polymerization of acrylates upon LED irradiation at 405 nm. For the photopolymerization of acrylates carried out as thin samples in laminate, all these ketones showed high photoinitiating abilities. Conversely, when tested as photoinitiators for thick samples, compared to the other 4 ketones, ketone 3 and ketone 5 both based onpiperidin-4-one as the central core and comprising furanes as peripheral groups proved to be the most efficient photoinitiators. Notably, the highest final polymerization conversion of Ebecryl 40, a tetrafunctional polyether acrylate, could be obtained using these two photoinitiators. The high photoreactivity of ketone 3 was highlighted by the steady state photolysis experiments. Meanwhile, ketone 3 could also promote the cationic polymerization of epoxides upon LED irradiation at 405 nm, in the presence of an iodonium salt. Interestingly, some 3D patterns could also be fabricated by free radical polymerization of Ebecryl 40 while using the ketone 3-based photoinitiating system. [ABSTRACT FROM AUTHOR]