Phenothiazine Derivative-Based Photoinitiators for Ultrafast Sunlight-Induced Free Radical Polymerization and Rapid Precision 3D Printing

In this work, we introduce twenty-six phenothiazine derivatives (PTZs) that were designed and synthesized as visible light photoinitiators. These compounds, in combination with an amine [ethyl 4-(dimethylamino)benzoate (EDB)] and an iodonium salt [di-tert-butylphenyl iodonium hexafluorophosphate (Iod)], could furnish high-performance three-component (PTZs/EDB/Iod) photoinitiating systems that were employed for the free radical polymerization of thick films of a low-viscosity model acrylate resin, namely, trimethylolpropane triacrylate (TMPTA) under visible light and sunlight exposure. A commercial thioxanthone, i.e., isopropylthioxanthone (ITX) was selected to design a reference ITX/EDB/Iod photoinitiating system. Double bond conversions of 87% and 76% were measured for the developed and synthesized photoinitiating systems under 405 and 450 nm light-emitting diode irradiation, respectively, and a conversion as high as 70% could be determined under sunlight irradiation─about 23 times higher than the conversion obtained with the comparable system prepared with the commercial photoinitiator. The relevant photoinitiation abilities and photochemical mechanisms are comprehensively investigated by a combination of techniques including real-time Fourier transform infrared spectroscopy, UV-visible absorption spectroscopy, fluorescence spectroscopy, steady-state photolysis, cyclic voltammetry, and electron paramagnetic resonance. Notably, the exceptional performance of the photoinitiators enabled the fabrication of 3D objects with precise morphology and superior resolution through 3D printing and direct laser write techniques. These findings not only provide opportunities for efficient polymerization under artificial and natural light conditions but also pave the way for scalable, cost-effective, environmentally sustainable, and green chemistry-driven curing applications.

Novel High-Performance Glyoxylate Derivative-Based Photoinitiators for Free Radical Photopolymerization and 3D Printing with Visible LED

Investigations concerning the glyoxylate moiety as a photocleavable functional group for visible light photoinitiators, particularly in the initiation of free radical photopolymerization remain limited. This study introduces nine innovative carbazole-based ethyl glyoxylate derivatives (CEGs), which are synthesized and found to exhibit excellent photoinitiation abilities as monocomponent photoinitiating systems. Notably, these structures demonstrate robust absorption in the near-UV/visible range, surpassing the commercial photoinitiators. Moreover, the newly developed glyoxylate derivatives show higher acrylate function conversions compared to a benchmark photoinitiator (MBF) in free radical photopolymerization. Elucidation of the photoinitiation mechanism of CEGs is achieved through a comprehensive analysis involving the decarboxylation reaction and electron spin resonance spin trapping. Furthermore, their practical utility is confirmed during direct laser writing and 3D printing processes, enabling the successful fabrication of 3D printed objects. This study introduces pioneering concepts and effective strategies in the molecular design of novel photoinitiators, showcasing their potential for highly advantageous applications in 3D printing.

Highly Efficient Room-Temperature Light-Induced Synthesis of Polymer Dots: A Programming Control Paradigm of Polymer Nanostructurization from Single-Component Precursor

Polymer dots (PDs) have raised considerable research interest due to their advantages of designable nanostructures, high biocompatibility, versatile photoluminescent properties, and recyclability as nanophase. However, there remains a lack of in situ, real-time, and noncontact methods for synthesizing PDs. Here we report a rational strategy to synthesize PDs through a well-designed single-component precursor (an asymmetrical donor-acceptor-donor' molecular structure) by photoirradiation at ambient temperature. In contrast to thermal processes that normally lack atomic economy, our method is mild and successive, based on an aggregation-promoted sulfonimidization triggered by photoinduced delocalized intrinsic radical cations for polymerization, followed by photooxidation for termination with structural shaping to form PDs. This synthetic approach excludes any external additives, rendering a conversion rate of the precursor exceeding 99%. The prepared PDs, as a single entity, can realize the integration of nanocore luminescence and precursor-transferred luminescence, showing 41.5% of the total absolute luminescence quantum efficiency, which is higher than most reported PD cases. Based on these photoluminescent properties, together with the superior biocompatibility, a unique membrane microenvironmental biodetection could be exemplified. This strategy with programming control of the single precursor can serve as a significant step toward polymer nanomanufacturing with remote control, high-efficiency, precision, and real-time operability.

Recent Advances in Monocomponent Visible Light Photoinitiating Systems Based on Sulfonium Salts

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.

Recent Advances and Challenges in Long Wavelength Sensitive Cationic Photoinitiating Systems

With the advantages offered by cationic photopolymerization (CP) such as broad wavelength activation, tolerance to oxygen, low shrinkage and the possibility of "dark cure", it has attracted extensive attention in photoresist, deep curing and other fields in recent years. The applied photoinitiating systems (PIS) play a crucial role as they can affect the speed and type of the polymerization and properties of the materials formed. In the past few decades, much effort has been invested into developing cationic photoinitiating systems (CPISs) that can be activated at long wavelengths and overcome technical problems and challenges faced. In this article, the latest developments in the long wavelength sensitive CPIS under ultraviolet (UV)/visible light-emitting diodes (LED) lights are reviewed. The objective is, furthermore, to show differences as well as parallels between different PIS and future perspectives.

High-Performance Photoinitiating Systems for LED-Induced Photopolymerization

Currently, increasing attention has been focused on light-emitting diodes (LEDs)-induced photopolymerization. The common LEDs (e.g., LED at 365 nm and LED at 405 nm) possess narrow emission bands. Due to their light absorption properties, most commercial photoinitiators are sensitive to UV light and cannot be optimally activated under visible LED irradiation. Although many photoinitiators have been designed for LED-induced free radical polymerization and cationic polymerization, there is still the issue of the mating between photoinitiators and LEDs. Therefore, the development of novel photoinitiators, which could be applied under LED irradiation, is significant. Many photoinitiating systems have been reported in the past decade. In this review, some recently developed photoinitiators used in LED-induced photopolymerization, mainly in the past 5 years, are summarized and categorized as Type Ⅰ photoinitiators, Type Ⅱ photoinitiators, and dye-based photoinitiating systems. In addition, their light absorption properties and photoinitiation efficiencies are discussed.

4,7-Diarylbenzo[c][1,2,5]thiadiazoles as fluorophores and visible light organophotocatalysts

A library of 26 electron donor-acceptor organophotocatalysts based on the benzo[c][1,2,5]thiadiazole (BTZ) group has been developed. These visible light organophotocatalysts were then used in a Minisci-type alkylation of heteroarenes under both batch and continuous flow conditions.

Fused Carbazole–Coumarin–Ketone Dyes: High Performance and Photobleachable Photoinitiators in Free Radical Photopolymerization for Deep Photocuring under Visible LED Light Irradiation

In this work, three dyes based on fused carbazole–coumarin–ketone structures were designed and synthesized. These dyes were named CCK–Me, CCK–Ph, and CCK–Tol in accordance with their different substituents. Their excellent...

Recent Advances in bis-Chalcone-Based Photoinitiators of Polymerization: From Mechanistic Investigations to Applications

Over the past several decades, photopolymerization has become an active research field, and the ongoing efforts to develop new photoinitiating systems are supported by the different applications in which this polymerization technique is involved-including dentistry, 3D and 4D printing, adhesives, and laser writing. In the search for new structures, bis-chalcones that combine two chalcones' moieties within a unique structure were determined as being promising photosensitizers to initiate both the free-radical polymerization of acrylates and the cationic polymerization of epoxides. In this review, an overview of the different bis-chalcones reported to date is provided. Parallel to the mechanistic investigations aiming at elucidating the polymerization mechanisms, bis-chalcones-based photoinitiating systems were used for different applications, which are detailed in this review.

Steric Manipulation as a Mechanism for Tuning the Reduction and Oxidation Potentials of Phenothiazines

Synthetic chemists customarily tune the redox characteristics of π-conjugated molecules by introducing electron-donating or electron-withdrawing substituents onto the molecular core, or by modifying the length of the π-conjugated pathway. Any steric effects of such efforts on molecular geometry typically affect both the neutral and charged (oxidized or reduced) states indiscriminately. However, in electroactive systems that undergo significant conformational changes upon oxidation or reduction, we can leverage the steric and inductive effects of substitution to attain considerable control over individual redox potentials. Here, we make use of density functional theory to elucidate the interplay between electronic and geometric effects of peripheral substitution on the model system of phenothiazine. For instance, we introduce substituents at positions ortho to the nitrogen atom (positions 1 and 9) to induce steric strain in the radical-cation state without significant effect on the neutral molecule, thereby augmenting the overall ionization potential. Notably, this steric effect persists for electron-donating substituents; the resulting ionization potentials therefore deviate from outcomes foretold by Hammett constants. Moreover, the same procedure has limited effect on electron affinities because of differences in phenothiazines' relaxation process upon reduction compared to oxidation. Our results promote molecular design guidelines for manipulating redox potentials in classes of electroactive compounds that experience dramatic changes in geometry upon ionization.

Novel Push–Pull Dyes Derived from 1H-cyclopenta[b]naphthalene-1,3(2H)-dione as Versatile Photoinitiators for Photopolymerization and Their Related Applications: 3D Printing and Fabrication of Photocomposites

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.

Colloidal properties of the metal-free semiconductor graphitic carbon nitride

The metal-free, polymeric semiconductor graphitic carbon nitride (g-CN) family is an emerging class of materials and has striking advantages compared to other semiconductors, i.e. ease of tunability, low cost and synthesis from abundant precursors in a chemical environment. Efforts have been done to improve the properties of g-CN, such as photocatalytic efficiency, designing novel composites, processability and scalability towards discovering novel applications as a remedy for the problems that we are facing today. Despite the fact that the main efforts to improve g-CN come from a catalysis perspective, many fundamental possibilities arise from the special colloidal properties of carbon nitride particles, from synthesis to applications. This review will display how typical colloid chemistry tools can be employed to make 'better g-CNs' and how up to now overseen properties can be levered by integrating a colloid and interface perspective into materials chemistry. Establishing a knowledge on the origins of colloidal behavior of g-CN will be the core of the review.

Monocomponent Photoinitiators based on Benzophenone-Carbazole Structure for LED Photoinitiating Systems and Application on 3D Printing

In this article, different substituents (benzoyl, acetyl, styryl) are introduced onto the carbazole scaffold to obtain 8 novel carbazole derivatives. Interestingly, a benzoyl substituent, connected to a carbazole group, could form a benzophenone moiety, which composes a monocomponent Type II benzophenone-carbazole photoinitiator (PI). The synergetic effect of the benzophenone moiety and the amine in the carbazole moiety is expected to produce high performance photoinitiating systems (PISs) for the free radical photopolymerization (FRP). For different substituents, clear effects on the light absorption properties are demonstrated using UV-Visible absorption spectroscopy. Benzophenone-carbazole PIs can initiate the FRP of acrylates alone (monocomponent Type II photoinitiator behavior). In addition, fast polymerization rates and high function conversions of acrylate are observed when an amine and/or an iodonium salt are added in systems. Benzophenone-carbazole PIs have good efficiencies in cationic photopolymerization (CP) upon LED @ 365 nm irradiation in the presence of iodonium salt. In contrast, other PIs without synergetic effect demonstrate unsatisfied photopolymerization profiles in the same conditions. The best PIS identified for the free radical photopolymerization were used in three-dimensional (3D) printing. Steady state photolysis and fluorescence quenching experiments were carried out to investigate the reactivity and the photochemistry and photophysical properties of PIs. The free radicals, generated from the studied PISs, are detected by the electron spin resonance - spin trapping technique. The proposed chemical mechanisms are provided and the structure/reactivity/efficiency relationships are also discussed. All the results showed that the benzophenone-carbazole PIs have a good application potential, and this work provides a rational design route for PI molecules. Remarkably, BPC2-BPC4, C6, C8 were never synthetized before; therefore, 5 of the 8 compounds are completely new.

Coumarins as Powerful Photosensitizers for the Cationic Polymerization of Epoxy-Silicones under Near-UV and Visible Light and Applications for 3D Printing Technology

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.

Photoinduced free radical promoted cationic polymerization 40 years after its discovery

Free radical promoted cationic photopolymerization has been described with its historical background, main principles and usage in polymer synthesis.

Design of new phenothiazine derivatives as visible light photoinitiators

In this article, four new phenothiazine derivatives (denoted as PT1, PT2, PT3 and PT4) are specifically in silico designed by molecular modelling for good light absorption properties @405 nm.

Photopolymerization with AIE dyes for solid-state luminophores

The photoinitiating activities of MPAs/ONI were evaluated. The AIE emission of MPAs occurred during photocuring. MPAs showed potential as fluorescent molecular probes to monitor the progress of photopolymerization.