Benzophenothiazine and benzophenoxazine photosensitizers for triarylsulfonium salt cationic photoinitiators

Novel D-π-A hemicyanine dye as photoinitiators for in situ hydrogel formation and DLP printing

The field of biofabrication imposes stringent requirements on the polymerization activity and biosafety of photopolymeric hydrogel systems. In this investigation, we designed and synthesized four hemicyanine dyes with a D-π-A structure specifically tailored for biofabrication purposes. These novel dyes, incorporating carbazole (CZ), triphenylamine (TPA), anthracene (AN), and benzodithiophene (BDT) as electron donors, along with heterocyclic salt (IN) as electron acceptors, were prepared using a straightforward synthesis method. The absorption maxima of ANIN, CZIN, and TPAIN exceeded 500 nm, rendering them suitable co-initiators for the free radical photopolymerization of acrylates under green-red light exposure facilitated by light-emitting diodes (LEDs) and the co-initiator iodonium salt (ION). Notably, CZIN and TPAIN, due to their robust dye absorption and efficient electron transfer to ION, functioned as high-performance photosensitizers. Meanwhile, BDTIN, with its strong and broad absorption range (400-600 nm), enhanced the accuracy of visible light photopolymerization. These dyes exhibit characteristics such as facile synthesis, heightened photo stability, and non-toxicity and also demonstrate the ability to discern the alkalinity of a solution to some extent. Furthermore, we explored the application of these hemicyanine dyes in 3D printing, showing potential to enhance printing resolution in DLP 3D printing (digital light process 3D printing).

Synthetic, biological and optoelectronic properties of phenoxazine and its derivatives: a state of the art review

Phenoxazines have sparked a lot of interest owing to their numerous applications in material science, organic light-emitting diodes, photoredox catalyst, dye-sensitized solar cells and chemotherapy. Among other things, they have antioxidant, antidiabetic, antimalarial, anti-alzheimer, antiviral, anti-inflammatory and antibiotic properties. Actinomycin D, which contains a phenoxazine moiety, functions both as an antibiotic and anticancer agent. Several research groups have worked on various structural modifications over the years in order to develop new phenoxazines with improved properties. Both phenothiazines and phenoxazines have gained prominence in medicine as pharmacological lead structures from their traditional uses as dyes and pigments. Organoelectronics and material sciences have recently found these compounds and their derivatives to be quite useful. Due to this, organic synthesis has been used in an unprecedented amount of exploratory alteration of the parent structures in an effort to create novel derivatives with enhanced biological and material capabilities. As a result, it is critical to conduct more frequent reviews of the work done in this area. Various stages of the synthetic transformation of phenoxazine scaffolds have been depicted in this article. This article aims to provide a state of the art review for the better understanding of the phenoxazine derivatives highlighting the progress and prospects of the same in medicinal and material applications.

Super-Reducing Behavior of Benzo[b]phenothiazine Anion Under Visible-Light Photoredox Condition

Herein we describe the anion of benzo[b]phenothiazine as a super reductant species upon excitation by visible light. In contrary to N-substituted phenothiazines or benzophenothiazines, this molecule holds extreme reducing power to promote single electron transfer-based reductive cleavage at a potential of -3.51 V vs SCE. As a proof, a plethora of aryl chloride substrates have been reductively cleaved to fabricate molecules of the class isoindolinone and oxindole. Moreover, an aryl-chloride bond has been homolytically cleaved to generate aryl radicals that have been utilized for C-C cross-coupling or C-P bond formation reactions. To prove its extreme reducing ability, some of the aryl fluoride bonds have been cleaved to generate aryl radicals. A detailed photophysical study including steady-state and time-resolved spectroscopic techniques explain the molecule's behavior upon light excitation, and that correlates with its reactivity pattern. Theoretical calculations disclose the benzophenothiazine anion to be slightly puckered at the ground state as the molecule is antiaromatic in nature. In contrast, the excited-state geometry is planar, which is also close to that of the intermediate after one electron transfer. Abating the antiaromaticity of the anionic species is partially responsible for its highly reducing behavior.

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.

Ceramic 3D Printing via Dye-Sensitized Photopolymerization Under Green LED

Photopolymerization-based ceramic 3D printing shows unmatched superiority in fabricating high-performance ceramic parts compared with the conventional preparation technology. Nevertheless, it remains challenging to achieve efficient 3D printing due to the light scattering in photosensitive ceramic slurries, increasing the width of solidification and reducing the curing depth during photocuring. Herein, we report an efficient ceramic 3D printing approach based on curcuminoid dye-sensitized photopolymerization under green light-emitting diode (LED). For deep penetration and minimal light scattering, ceramic bodies with good performance can be produced from a ceramic slurry with curcuminoid dye by using a green LED-digital light processing (DLP) 3D printer. Curcuminoid dye was found to provide the ability to transfer electrons to photoinitiator and play a role in improving the accuracy of the entire 3D printing process. The proposed approach here provides a viable solution toward efficient ceramic additive manufacturing by green LED-DLP-3D printing.

Benzophenothiazine and Its Cr(III)-Catalyzed Cross Dehydrogenative Couplings

In stark contrast to phenothiazines and their prevalence for cross-dehydrogenative amination reactions, benzophenothiazine has a pronounced preference for cross-dehydrogenative C-C bond-forming reactions. Moreover, the substrate is very versatile, leading to several new classes of C-C bond-forming reactions and many new oxidative coupling product architectures, including unprecedented indole fused paddlewheel-like structures.

Structural effect of oxazolone derivatives on the initiating abilities of dye-borate photoredox systems in radical polymerization under visible light

Three photoinitiating systems based on new oxazolone derivatives have been developed and their performance in initiation of radical polymerization of acrylate monomers has been tested by differential scanning calorimetry. The absorption characteristics of the oxazol-5(4H)-ones is compatible with the emission characteristics of different light sources like diode pulse solid state lasers. Thus, the dyes were used as sensitizers which are photoreduced during a photochemical reaction in the presence of phenyltriethylborate salt. Results showed that the increase in the dimensionality of the molecule extends the range of light absorption and increases the efficiency of the photoinitiation process. The photoreduction of the oxazolone–borate complex was studied using steady-state and nanosecond laser flash photolysis. The dye singlet and triplet were found to be quenched by the electron donor via an electron transfer process. Rate constants for the quenching of the excited states were high and were found to depend on the dye structure.

Three photoinitiating systems based on new oxazolone derivatives have been developed and their performance in initiation of radical polymerization of acrylate monomers has been tested by differential scanning calorimetry.

Mild synthesis of triarylsulfonium salts with arynes

Reactions between arynes and alkyl sulfides have been extensively studied over the past few decades. These reactions commonly end with a dealkylation process and thus deliver thioethers as final products. In contrast, the transformation described furnishes valuable triarylsulfonium salts, in lieu of thioethers, from arynes and diarylsulfides. The reaction features mild conditions and a broad substrate scope. A suite of functional groups such as ketones, esters, nitriles, aryl ethers and aryl halides is tolerated, which can be issues faced by traditional synthetic methods. The practicality of the reaction and its extension to the synthesis of triphenyl selenonium salt are also exhibited herein.

Facilely prepared blue-green light sensitive curcuminoids with excellent bleaching properties as high performance photosensitizers in cationic and free radical photopolymerization

Four blue-green light sensitive curcuminoids have been prepared using a facile method. High performances were found as photosensitizers in cationic and free radical photopolymerzation.

Synthesis of o-Aryloxy Triarylsulfonium Salts via Aryne Insertion into Diaryl Sulfoxides

The aryne insertion into "S═O" bond has been validated recently. This technology is elusively applied to the synthesis of thioethers. In contrast to the reported cases, the reaction described furnished o-aryloxy triarylsulfonium salts, in lieu of thioethers, in good to excellent yields. The reaction is also featured by its exquisite regioselectivity, broad substrate scope, and mild conditions (25 °C). Preliminary mechanistic studies suggest that the reaction probably proceeds in a sequential [2 + 2] cycloaddtion/O-arylation/protonation pathway.

Carbazole-based compounds containing aldehyde and cyanoacetic acid: optical properties and applications in photopolymerization

Carbazole-based aldehyde and cyanoacetic acid containing dyes were synthesized and applied as photosensitizers in photopolymerization with a LED light source at 455 nm.

Photoinduced Electron Transfer Reactions for Macromolecular Syntheses

Photochemical reactions, particularly those involving photoinduced electron transfer processes, establish a substantial contribution to the modern synthetic chemistry, and the polymer community has been increasingly interested in exploiting and developing novel photochemical strategies. These reactions are efficiently utilized in almost every aspect of macromolecular architecture synthesis, involving initiation, control of the reaction kinetics and molecular structures, functionalization, and decoration, etc. Merging with polymerization techniques, photochemistry has opened up new intriguing and powerful avenues for macromolecular synthesis. Construction of various polymers with incredibly complex structures and specific control over the chain topology, as well as providing the opportunity to manipulate the reaction course through spatiotemporal control, are one of the unique abilities of such photochemical reactions. This review paper provides a comprehensive account of the fundamentals and applications of photoinduced electron transfer reactions in polymer synthesis. Besides traditional photopolymerization methods, namely free radical and cationic polymerizations, step-growth polymerizations involving electron transfer processes are included. In addition, controlled radical polymerization and "Click Chemistry" methods have significantly evolved over the last few decades allowing access to narrow molecular weight distributions, efficient regulation of the molecular weight and the monomer sequence and incredibly complex architectures, and polymer modifications and surface patterning are covered. Potential applications including synthesis of block and graft copolymers, polymer-metal nanocomposites, various hybrid materials and bioconjugates, and sequence defined polymers through photoinduced electron transfer reactions are also investigated in detail.

D–D–A dyes with phenothiazine–carbazole/triphenylamine as double donors in photopolymerization under 455 nm and 532 nm laser beams

In order to enhance electron-donating ability, four novel organic dyes (CPC, TPC, CPN and TPN) with double electron donors have been designed and synthesized.

Analysis of association constant for ground state dye-electron acceptor complex of photoinitiator systems and the association constant effect on the kinetics of visible-light-induced polymerizations

We investigated the formation of ground state donor/acceptor complexes between xanthene dyes (rose Bengal (RB) and fluorescein (FL)) and a diphenyl iodonium salt (DPI) which is dissolved in 2-hydroxyethyl methacrylate (HEMA) monomer. To characterize the association constant of the complex, we have suggested a new analysis model based upon the Benesi-Hildebrand model. Because the assumption of the original Benesi-Hildebrand model is that the absorption bands are due only to the presence of the complex and that the absorption by the free component is negligible; the model cannot be applied to our systems, which is a dye-based initiator system. For each dye, the molar absorptivity of the ground state complex was evaluated as a function of wavelength and this analysis confirmed the validity of the modified Benesi-Hildebrand model. In addition, we observed the RB/DPI photoinitiator system failed to produce a perceptible polymerization rate but the FL/DPI photoinitiator system provided very high rates of polymerization. Based upon the association constant for these complexes, we concluded that the observed kinetic differences arise from the different association constant values of the ground state dye-acceptor complex, resulting in back electron transfer reaction.