This study focused on the design and synthesis of a series of novel acylphosphine oxide (APO) and acylphosphinate photoinitiators based either on diphenylphosphine oxide (DPO) or 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO), systematically investigating the structure-property relationships in terms of photochemical performance, polymerization efficiency and cytotoxicity. Polymerization experiments demonstrated that DPO-based photoinitiators exhibit excellent photopolymerization efficiency, while the structurally optimized DOPO-based Type I photoinitiators also exhibit good reactivity. Biocompatibility experiments indicate that acylphosphinates (e.g., the (6-oxidodibenzo[c,e][1,2]oxaphosphinin-6-yl)(2,4,6-trimethoxyphenyl)methanone (TMO-DOPO) derivative) exhibit no significant toxicity toward C3H10 T1/2 mouse mesenchymal stem cells, significantly outperforming the traditional 2,4,6-trimethylbenzoyldiphenylphosphine oxide (TPO) photoinitiator. These results highlight the DOPO role as a functional core in Type I initiation systems, providing an effective strategy for developing novel, efficient, and low-toxicity photoinitiators.

Photopolymerization-based additive manufacturing, particularly vat photopolymerization (VP), is undergoing a transformation driven by the synergistic evolution of light-emitting diode (LED) technology and innovative photoinitiating systems (PISs). The development of novel Type I (cleavage), Type II (hydrogen abstraction/ electron-proton transfer), and cationic photoinitiators has successfully extended the spectral sensitivity from the UV into the visible light regime. This red-shift is not merely an incremental improvement; it is a critical enabler for fabricating objects with greater depth, complexity, and speed while leveraging the energy efficiency, safety, and wavelength control of LEDs. These advances are accelerating innovation in fields from high-fidelity prototyping to biomedical engineering and dental materials. However, the full potential of these technologies is fundamentally constrained by a critical bottleneck: the (cyto)toxicity of many high-performance photoinitiators, which precludes their use in biocompatible and sustainable applications. This review provides a critical assessment of the state-of-the-art in several VP technologies and the chemistry of visible-light PISs that empower them. We critically analyze their initiation mechanisms and performance in 3D printing and, most importantly, provide a comprehensive evaluation of their cytotoxicity. By mapping the intricate relationship between chemical structure, photo-efficiency, and biological response, this review aims to establish a clear roadmap for the rational design of next-generation PISs that are not only highly efficient but also fundamentally safer and more sustainable.

In the pursuit of green polymer chemistry, natural sunlight represents the ideal energy source for photopolymerization due to its abundance and sustainability. While the transition from UV to LED light has improved energy efficiency, sunlight-driven photopolymerization offers a transformative path towards power-free and accessible material synthesis. The key challenge is the development of highly sensitive photoinitiating systems (PISs) capable of harnessing the broad solar spectrum. This review provides a comprehensive overview of the state-of-the-art in sunlight-induced photopolymerization. We explore the core photochemical mechanisms and survey the latest developed photoinitiators (PIs) and photocatalysts of versatile organic dyes. We highlight recent milestones where solar-driven systems have achieved polymerization rates comparable to their LED-activated counterparts, showcasing their practical viability. Furthermore, real-world applications in coatings, 3D printing, and biomaterials are discussed through specific case studies. By addressing current challenges and outlining future research directions, this review aims to promote further innovation in the rational design of solar-activated PISs, ultimately unlocking the full potential of sunlight as a cornerstone for sustainable manufacturing.

The development of high-performance visible-light photoinitiators (PIs) is critical for advancing 3D printing and thick-section manufacturing. In this study, two acylphosphinate photoinitiators (6-((9-benzyl-9H-carbazol-3-yl) (hydroxy)methyl)dibenzo[c,e] [1,2]oxaphosphinine 6-oxide (BCDOPO) and 6-((9-ethyl-9H-carbazol-3-yl)(hydroxy)methyl)dibenzo[c,e][1,2]oxaphosphinine 6-oxide (ECDOPO)) are designed, synthesized and evaluated in photopolymerizations and 3D printing. DFT calculations and UV-vis spectroscopy reveal a narrowed HOMO-LUMO energy gap of 3.9-4.3 eV and high molar extinction coefficients in the 385-405 nm range. Despite the low dissociation quantum yield of ECDOPO (approximate to 0.01), its superior light penetration properties enables it to exhibit significant light curing effects in thick samples (2 mm). To further amplify photoinitiation kinetics, ECDOPO is employed in a multicomponent system together with ethyl 4-(dimethylamino)benzoate (EDB) as a co-initiator and bis(4-tert-butylphenyl)iodonium hexafluorophosphate (IOD) as an electron acceptor. The three-component system can achieve the highest conversion in photopolymerization tests involving both thin (25 mu m) and thick (2 mm) samples in trimethylolpropane triacrylate (TMPTA) monomer. Leveraging this efficiency, clear and geometrically precise complex 3D objects are successfully fabricated via DLP printing technology. These findings demonstrate that carbazole-functionalized DOPO derivatives, when integrated into synergistic systems, provide a robust platform for high-resolution 3D printing and high-performance light-curable materials. Beyond technical performance, cytotoxicity evaluations confirm excellent biocompatibility for ECDOPO within relevant concentration ranges, opening avenues for safe application in biomedical devices and photopolymerizable materials.

The effect of structural tuning of porous polymers on the photopolymerization kinetics and properties of the final photopolymerization products either as macrophotoinitiators or nanofillers is presented in this work.

The development of efficient and earth-abundant alternatives to noble-metal photocatalysts remains an important research focus in controlled radical photopolymerization. In this work, two Schiff base Fe(III) complexes (Fe III Salen H and Fe III Salen EtO ) were synthesized and evaluated as photocatalysts for radical photopolymerizations. The FeIIISalenL complexes were characterized by FTIR, UV-vis, fluorescence spectroscopy, MALDI-TOF mass spectrometry, cyclic voltammetry, and molar conductance. Photoinduced polymerizations of methyl acrylate (MA) were conducted to investigate the controlling ability of the FeIIISalenL complexes under LED@365 nm. Phenacyl bromide (Ph-Br) served as the alkyl halide and electron acceptor, whereas ethyl 4-(dimethylamino)benzoate (EDB) acted as the electron donor in the system. Light-driven polymerizations yielded polymers with regulated molecular weight control and uniform distributions. Among the two complexes, Fe III Salen EtO exhibited superior performance, showing enhanced control over chain growth, as evidenced by the moderate polydispersity values. In addition, the water solubility of these complexes enables the photopolymerization of 2-hydroxyethyl methacrylate (HEMA) in aqueous media with high initial water fractions, resulting in hydrogels with high monomer conversion. Both complexes also mediated the synthesis of block copolymers (PHEMA-b-PBA), as confirmed by size exclusion chromatography (SEC), Raman, and 1H NMR analyses. The photoactivation mechanism was further investigated through spectroscopic analyses, providing insights into the redox and reactive processes involved in the photopolymerization reactions.

Archaeological and historical evidence indicates that natural dyes have been employed by human societies for millennia to achieve colouration in various materials. And light reveals this world of colours by painting everything it touches. However, there is much evidence that natural dyes photodegrade when exposed to light, fading cultural heritage work. Upon photon absorption, a molecule reaches an excited state, which promotes photochemical reactions leading to the formation of new compounds and thus the photodegradation. Understanding the underlying mechanisms of these processes helps elaborate effective strategies to mitigate them. These problematics have long been relevant across various fields. There have been many studies about the photodegradation of dyes, and this continues until today. Our study focuses on alizarin, a historical natural dye that can be extracted from madder roots. It has been used in many fields, textiles, paintings, since antiquity. A distinctive aspect of our study is the investigation of alizarin's less-studied basic (di-deprotonated) form, using a solution model under controlled conditions. By using techniques such as EPR and HPLC-UV-MS, we identified radical oxygen intermediates and degradation products while evaluating the influence of pH, solvent and light wavelength. These findings have important implications for preserving alizarin-based artworks, improving industrial dyeing practices, and informing environmental efforts like wastewater treatment and shedding light on computational modelling as a crucial research field in these topics. Overall, the study deepens our understanding of anthraquinone dye degradation and promotes the sustainable use of natural dyes.

Understanding radical processes in drying oils is essential for both heritage science and materials chemistry, as these reactions govern film formation and long-term stability in historical paintings. In this study, we investigate radical mechanisms in linseed oil, a traditional paint binder. Gold salts are used as probes to elucidate film-drying reactions. The introduction of gold salts into both raw and lead-treated linseed oils leads to the formation of gold nanoparticles, whose size, morphology, and distribution are monitored by spectroscopic methods and transmission electron microscopy (TEM). Under ambient conditions, spherical gold nanoparticles form during the drying of both oil types. Electron paramagnetic resonance (EPR) reveals oxygen-centered radicals (peroxyl (ROO●) or alkoxyl (RO●) radicals) involved in the oil-curing process. The addition of lead, following historical oil preparation recipes, increases radical activity and promotes the formation of additional species, suggesting more complex reaction pathways. In lead-treated oils, 3-4 nm lead-based nanoparticles are observed both in the oil bulk and in dried films, formed during the oil preparation.

AC:AC:Visible-light photopolymerization offers safer processing, lower energy input, and broader compatibility with advanced manufacturing than thermally initiated curing or short-wavelength UV processes. However, sustainable photoinitiating systems that operate efficiently under visible light remain limited. Here, a sustainable two-component strategy is reported by combining bio-derived oxime esters with natural or bio-inspired photosensitizers for 405 nm curing. Among the systems investigated, the synthesized oxime ester (OXE1) and menadione (MD, vitamin K3) pair show the best performance, enabling rapid photopolymerization of TMPTA with good formulation homogeneity, low coloration, and good storage stability. Electrochemical analysis and excited-state energetics indicate that direct single-electron transfer between MD and OXE1 is thermodynamically unfavorable. In contrast, steady-state photolysis, laser flash photolysis, CO2 evolution, and ESR spin trapping support a triplet-triplet energy transfer pathway, followed by N-O bond cleavage and decarboxylation, to generate initiating radicals. This work establishes a simple sensitization principle for converting UV-responsive bio-derived oxime esters into visible-light photoinitiators and provides a sustainable design strategy for photopolymerization and 3D microfabrication.

A short and versatile route to S‐arylthioxanthonium trifluoromethanesulfonates via a Cu‐catalyzed reaction of diaryliodonium trifluoromethanesulfonates and commercially available thioxanthones is described. Variously substituted S‐arylthioxanthonium salts were prepared in only one step from diaryliodonium salts in moderate to good yields. Then, we show one of the first applications of an S‐arylthioxanthonium trifluoromethanesulfonate, as an efficient aryl radical precursor by simple and mild direct 365‐nm LED irradiation. In preliminary experiments, this aryl radical was successfully trapped with furan without any metallic photocatalyst, thereby enabling the first formation of a new aryl–heteroaryl bond from a sulfonium salt derived from thioxanthone.

Copper(I) complexes have attracted considerable attention as efficient redox photocatalysts (PCs) in photopolymerization. In this study, two copper(I) complexes bearing a pyridine-benzothiazole ligand were thoroughly investigated as photocatalysts for the free radical photopolymerization of ethoxylated trimethylolpropane triacrylate under violet and green light irradiation. A three-component system comprising a copper complex, di-tert-butyl-diphenyl iodonium hexafluorophosphate, and ethyl dimethylaminobenzoate as additives was investigated under various conditions. The copper complexes exhibited remarkable photoinitiation capabilities, achieving high monomer conversion rates and highlighting significant structure-reactivity relationships. Based on free energy calculations and spectroscopic analyses, a mechanistic pathway was proposed. The optimal conditions were successfully applied in 3D printing, producing smooth and uniform 3D structures.

Photopolymerization continues to attract significant interest due to its environmental benefits, precise spatial-temporal control, and energy efficiency. In this study, we report a novel two-component photoinitiating system comprising a synthesized oxime ester derived from biobased carvone and a thioxanthone derivative photosensitizer (isopropylthioxanthone), which effectively addresses the inherent limitations of oxime esters under visible light. Mechanistic investigations, including steady-state photolysis, fluorescence quenching, electron spin resonance spectroscopy, laser flash photolysis, and computational analysis, demonstrate that Dexter triplet-triplet energy transfer is the dominant activation pathway, with a secondary contribution from photoinduced electron transfer. This synergistic mechanism facilitates rapid N-O bond cleavage and decarboxylation, leading to high photopolymerization efficiency under both visible-light and low-intensity solar irradiation. Importantly, the system exhibits excellent storage stability and broad applicability across various 3D printing technologies, including digital light processing, liquid crystal display, and direct laser writing. These findings advance the understanding of visible-light-driven radical generation via dual-energy and electron-transfer mechanisms and provide a promising platform for the development of next-generation photopolymerization-based manufacturing processes.

2,4,6-Trimethylbenzoyldiphenylphosphine oxide (TPO) is a highly efficient and widely used photoinitiator, but it is currently facing significant concerns regarding cytotoxicity. This work investigates a type-I photoinitiator designed as a safer alternative to TPO. In particular, a TPO analog was synthesized, namely 6-(2,4,6-trimethylbenzoyl)-(6H)-dibenz[c,e][1,2]oxaphosphorin 6-oxide (TDOPO), incorporating 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) motif. This compound presents promising characteristics as a type I photoinitiator. It can be synthesized efficiently in just two steps under mild conditions with a reduced environmental impact using commercially available chemicals, without need for extensive purification procedures. Notably, TDOPO exhibits absorption close to the visible spectrum, suggesting its potential for use in visible light curing applications. Comparative studies reveal that TDOPO exhibits strong photoinitiation ability for the free radical photopolymerization of acrylate in both thin and thick samples. The photochemical mechanism studies indicate its capability to generate three distinct types of free radicals: the 2,4,6-trimethylbenzoyl, an oxygen-centered and a phosphorus-centered free radical. In addition, TDOPO shows no cytotoxicity in a 20-hour assay, a result far superior to that of TPO. These findings underscore the potential of TDOPO as an effective and safer alternative to TPO in photopolymerization processes.

In this work, bisphenol A diglycidyl ether (BADGE) and four bio-based epoxy monomers were used to investigate the kinetics of cationic photopolymerization of epoxy monomers and anionic photopolymerization of thiol-epoxy reaction. The real-time Fourier transform infrared spectroscopy results showed that the cationic photopolymerization process was more efficient for the two bio-based epoxy monomers derived from sorbitol and glycerol compared to BADGE, which is considered a petro-based epoxy monomer. Considering the anionic photopolymerization process, in 25 mu m thin samples this process was more efficient in the presence of BADGE, whereas in 1.8 mm thick samples, the formulations accomplished complete epoxy function conversions and tack-free surface. On the other hand, the two bio-based epoxy monomers derived from vegetable oils exhibited very low reactivity in acid-catalyzed homopolymerization and using thiols as hardeners. The high efficiency of the photobase-generating system, capable of releasing strong base active species under the irradiation conditions used in this study was demonstrated using phenol red and UV-vis absorption spectra. Furthermore, the polymerization mechanism governing the thiol-epoxy reactions was outlined thoroughly. Finally, the thermo-mechanical properties of the polymers obtained from the thiol-epoxy reactions were analyzed using differential scanning calorimetry, dynamic mechanical analysis, and surface hardness tests, which revealed interesting properties for the polymers containing bio-based epoxy monomers derived from polyols.

Photopolymerization driven by long-wavelength visible LEDs offers a promising approach for biocompatible, low-energy 3D printing, minimizing light scattering and photodamage compared to conventional ultraviolet (UV) and near-UV systems. However, the inherently slow polymerization kinetics of long-wavelength photoinitiators (PIs) typically necessitate high light intensities, extended curing times, or excessive PI loadings, limiting their practical applications. To address these challenges, the design and synthesis of two novel donor-pi-acceptor (D-pi-A) PIs are reported, incorporating 2-phenylnaphtho[2,3-d]thiazole-4,9-dione as an electron acceptor and two naturally derived aldehydes (furfural and butyraldehyde) as electron donors. These compounds enable exceptionally rapid polymerization under blue and green light LEDs, as well as natural sunlight, even at low light intensities and minimal PI concentrations. The unprecedented curing speed and efficiency are further demonstrated through the fabrication of intricate 3D structures using three distinct printing techniques-direct laser writing (DLW), digital light processing (DLP), and liquid crystal display (LCD) printing. This work expands the frontiers of long-wavelength photopolymerization by combining high efficiency with eco-friendly, low-energy activation, paving the way for advanced applications in biocompatible additive manufacturing.

Photoredox catalytic systems are widely used in free radical polymerization as an important photoinitiating approach. However, many reported photoredox catalytic systems are limited by their low stabilities, high excitation powers, and low initiating efficiencies upon excitation in the visible region. Therefore, it is still a great challenge to develop efficient photoinitiating systems for photopolymerization under visible light. In this work, three new effective photosensitizers from N-(hexyl)benzothioxanthene-3,4-dicarboximide derivatives, namely 2-hexyl-1H-thioxantheno[2,1,9-def]isoquinoline-1,3(2H)-dione (BTXI), 5-bromo-2-hexyl-1H-thioxantheno[2,1,9-def]isoquinoline-1,3(2H)-dione (BTXI-Br) and 2-hexyl-1H-thioxantheno[2,1,9-def]isoquinoline-1,3(2H)-dione 6,6-dioxide (BTXIO), were designed by density functional theory calculation and synthesized as photoredox catalysts for visible light induced photopolymerization. When combined with initiators such as oxidants, that is, bis(4-tert-butylphenyl)iodonium hexafluorophosphate or sulfonium salts (i.e., thianthrenium salts, phenoxathiinium salt, phenothiazinium salt, dibenzothiophenium salt) and the reductant ethyl dimethylaminobenzoate to form three-component initiating systems, they showed good to high performance in visible light photo polymerizations with LED@405 nm and LED@450 nm. In addition, these photoinitiating systems enable the successful digital light processing and direct laser writing of 3D structures with high resolution, demonstrating a promising strategy for 3D printing applications.

This study presents the development and evaluation of five dyes with varying conjugated energy levels and donor-π-acceptor (D-π-A) structures as photoinitiators for free radical polymerization. Their photoinitiation efficiencies are systematically assessed under both visible-light LED and sunlight. Notably, the conversions reach up to 81% within just 30 s under sunlight, demonstrating the ultrafast and efficient polymerization capabilities of the dyes. The efficient electron transfer is facilitated by the D-π-A structure, where the conjugation is reduced or interrupted by the high distortion between the electron-withdrawing and the electron-releasing units. This distortion can prevent the overlap of frontier molecular orbitals, decreasing the energy difference between the ground state and the excited state of dyes, thereby enhancing the electron transfer reactivity with additives. Additionally, we propose a chemical mechanism for the electron transfer reaction in the three-component systems. The study also explores the application of naphtho[2,3-d]thiazole-4,9-dione-based dyes as donors in additive manufacturing demonstrating their effectiveness in three different 3D printing technologies, i.e., direct laser writing (DLW), digital light processing (DLP), and liquid crystal display (LCD). These three-component formulations achieve high-precision 3D printed objects, with detailed characterization and comparison of the resulting structures.

Three novel mono‐ and dimethacrylate‐based functional photoinitiators (PIs) are synthesized from aza‐Michael addition reaction of (i) amine derivative of 2‐hydroxy‐4′‐(2‐hydroxyethoxy)‐2‐methylpropiophenone (I2959) and 3‐(acryloyloxy)‐2‐hydroxypropyl methacrylate (AHM) by controlling their stoichiometry (I2959‐NH‐AHM, I2959‐N‐2xAHM) and (ii) allyl amine derivative of I2959 and AHM (I2959‐NA‐AHM). Density functional theory is used to predict bond dissociation energies and the absorption maxima of the PIs. These PIs have similar λ max, 272–280 nm, to I2959. The photocleavage mechanisms examined by electron spin resonance (ESR) indicate the formation of benzoyl and alkyl radicals. The homopolymerization reactivities of the PIs as monomers are found in the order of I2959‐N‐2xAHM > I2959‐NH‐AHM > I2959‐NA‐AHM. Their photoinitiating abilities of (meth)acrylate polymerizations studied using photo‐DSC are similar to I2959. The migration of I2959‐NA‐AHM and I2959‐N‐2xAHM from the cured material is alleviated significantly compared to I2959. As a result, these PIs are promising candidates for the synthesis of environmentally friendly coatings.

The development of photoinitiators (PIs) combining high initiation ability, low‐toxicity, and availability for high‐precision 3D printing is a key challenge in photopolymerization that has never been reported before. In this study, carbazole chalcone glyoxylate oxime ester derivatives (denoted as Cs, C1‐C5) containing both glyoxylate and oxime ester moieties with good light absorption properties in the visible range have been designed as type I PIs. Subsequent experimental results clearly show that the photoinitiation ability of C5 outperforms that of the benchmark commercial PIs (methyl benzoylformate (MBF) and diphenyl (2,4,6‐trimethylbenzoyl) phosphine oxide (TPO)) under the same conditions. In addition, C5 is successfully applied to 3D printing for the manufacture of large‐scale and high‐resolution object. The photochemical mechanism of C5 is systematically and comprehensively analyzed using a combination of steady state photolysis, decarboxylation reaction, fluorescence experiments, and electron spin resonance‐spin trapping (ESR‐ST) technology. Furthermore, the low‐toxicity of C5 is evidenced by cytotoxicity assays. The comprehensive molecular modeling and experimental approach adopted in this research has led to the development of novel PIs that are highly efficient and low‐toxic, and can be used for high‐precision 3D printing, which offers broad application prospects in the fields of environmental sustainability, visible light curing, and biomedical science.

In this study, we reported the design, synthesis, and comprehensive evaluation of a series of nitro carbazole‐based oxime photoinitiators (OPIs, OP1: oxime oxalate, OP2: oxime glyoxylate), as a series of efficient Type I photoinitiators (PIs) for the free radical photopolymerization (FRP) of trimethylolpropane triacrylate (TMPTA) and ethoxylated trimethylolpropane triacrylate (ETPTA) under blue light‐emitting diodes and sunlight irradiation. Computational molecular modeling was employed to predict the effect of OPIs structures on the photoinitiation properties. The predictions suggest that OP1 had a higher propensity for decarboxylation and therefore a better photoinitiation behavior. Compared to OP2, OP3, and commercial benchmark photoinitiator TPO, OP1 exhibits exceptional photoinitiation performance when exposed to LED@405 nm, LED@450 nm, and sunlight. OP1 undergoes decarboxylation to efficiently produce CO 2 and free radicals, thereby initiating the photopolymerization reaction. Remarkably, OP1 is successfully applied in 3D printing, producing complete morphology with high‐resolution structure, showcasing its potential for advanced manufacturing applications. The photochemical mechanism of OPIs is comprehensively elucidated using the monitoring of the CO 2 , steady state photolysis, UV–vis absorption spectroscopy, fluorescence spectroscopy, and electron spin resonance techniques. These experimental investigations are supported by data OP1 from the molecular modelling carried out. Additionally, thermal polymerization shows that OP1 had a high thermal initiation capability, and the composites are successfully prepared together with carbon fibers. The cytotoxicity of the synthesized oxime oxalate and TPO on human umbilical vein endothelial cells (HUVECs) results in a lower cytotoxicity for the oxalate than for TPO. Therefore, OP1, which has never been reported before, can be used as a highly efficient and low cytotoxic dual photo/thermal initiator. This research not only provides theoretical and practical insights into the design and development of new efficient Type I PIs, but also opens up new perspectives for curing applications that require scalability, cost‐effectiveness, environmental sustainability, and green chemistry.