Phenacyl Ethyl Carbazolium as a Long Wavelength Photoinitiator for Free Radical Polymerization

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.

Molecular Self-Assembly in Conductive Covalent Networks for Selective Nitrate Electroreduction to Ammonia

Electrochemical nitrate (NO3-) reduction in aqueous media provides a useful approach for ammonia (NH3) synthesis. While efforts are focused on developing catalysts, the local microenvironment surrounding the catalyst centers is of great importance for controlling electrocatalytic performance. Here, we demonstrate that a self-assembled molecular iron catalyst integrated in a free-standing conductive hydrogel is capable of selective production of NH3 from NO3- at efficiencies approaching unity. With the electrocatalytic hydrogel, the NH3 selectivity is consistently high under a range of negative biases, which results from the hydrophobicity increase of the polycarbazole-based electrode substrate. In mildly acidic media, proton reduction is suppressed by electro-dewetting of the hydrogel electrode, enhancing the selectivity of NO3- reduction. The electrocatalytic hydrogel is capable of continuous production of NH3 for at least 100 h with NH3 selectivity of ∼89 to 98% at high current densities. Our results highlight the role of constructing an internal hydrophobic surface for electrocatalysts in controlling selectivity in aqueous media.

Preparation of Xanthene-TEMPO Dyads: Synthesis and Study of the Radical Enhanced Intersystem Crossing

We prepared a rhodamine-TEMPO chromophore-radical dyad (RB-TEMPO) to study the radical enhanced intersystem crossing (REISC). The visible light-harvesting chromophore rhodamine is connected with the TEMPO (a nitroxide radical) via a C-N bond. The UV-vis absorption spectrum indicates negligible electron interaction between the two units at the ground state. Interestingly, the fluorescence of the rhodamine moiety is strongly quenched in RB-TEMPO, and the fluorescence lifetime of the rhodamine moiety is shortened to 0.29 ns, from the lifetime of 3.17 ns. We attribute this quenching effect to the intramolecular electron spin-spin interaction between the nitroxide radical and the photoexcited rhodamine chromophore. Nanosecond transient absorption spectra confirm the REISC in RB-TEMPO, indicated by the detection of the rhodamine chromophore triplet excited state; the lifetime was determined as 128 ns, which is shorter than the native rhodamine triplet state lifetime (0.58 μs). The zero-field splitting (ZFS) parameters of the triplet state of the chromophore were determined with the pulsed laser excited time-resolved electron paramagnetic resonance (TREPR) spectra. RB-TEMPO was used as a photoinitiator for the photopolymerization of pentaerythritol triacrylate (PETA). These studies are useful for the design of heavy atom-free triplet photosensitizers, the study of the ISC, and the electron spin dynamics of the radical-chromophore systems upon photoexcitation.

Coumarinacyl Anilinium Salt: A Versatile Visible and NIR Photoinitiator for Cationic and Step-Growth Polymerizations

A coumarinacyl anilinium (CAA) salt, facilely synthesized via a one-pot reaction, is shown to be a versatile visible and NIR photoinitiator for cationic and step-growth polymerizations. CAA salt exhibits superior photoinitiation performance as compared to commercial iodonium salt in cationic polymerization. Upon visible-light irradiation, this salt undergoes hemolytic and heterolytic cleavage and subsequent electron transfer and hydrogen abstraction reactions, forming reactive species capable of initiating cationic polymerization of epoxides and vinyl monomers. After a short irradiation period, polymerization also proceeds in the dark due to the non-nucleophilic nature of the counteranion. NIR-induced polymerizations were successfully conducted based on upconversion photochemistry. CAA salt can also initiate step-growth polymerization of N-ethyl carbazole (NEC) by oxidation of the monomer by the photochemically formed anilium radical cations. Subsequent proton release and radical coupling reactions essentially yield polycarbazole. CAA salt, featuring straightforward synthesis and long-wavelength sensitivity as well as versatile photoinitiating performance, has great potential in various applications.

Transient absorption spectroscopy of the electron transfer step in the photochemically activated polymerizations of N-ethylcarbazole and 9-phenylcarbazole

The polymerization of photoexcited N-ethylcarbazole (N-EC) in the presence of an electron acceptor begins with an electron transfer (ET) step to generate a radical cation of N-EC (N-EC˙+). Here, the production of N-EC˙+ is studied on picosecond to nanosecond timescales after N-EC photoexcitation at a wavelength λex = 345 nm using transient electronic and vibrational absorption spectroscopy. The kinetics and mechanisms of ET to diphenyliodonium hexafluorophosphate (Ph2I+PF6-) or para-alkylated variants are examined in dichloromethane (DCM) and acetonitrile (ACN) solutions. The generation of N-EC˙+ is well described by a diffusional kinetic model based on Smoluchowski theory: with Ph2I+PF6-, the derived bimolecular rate coefficient for ET is kET = (1.8 ± 0.5) × 1010 M-1 s-1 in DCM, which is consistent with diffusion-limited kinetics. This ET occurs from the first excited singlet (S1) state of N-EC, in competition with intersystem crossing to populate the triplet (T1) state, from which ET may also arise. A faster component of the ET reaction suggests pre-formation of a ground-state complex between N-EC and the electron acceptor. In ACN, the contribution from pre-reaction complexes is smaller, and the derived ET rate coefficient is kET = (1.0 ± 0.3) × 1010 M-1 s-1. Corresponding measurements for solutions of photoexcited 9-phenylcarbazole (9-PC) and Ph2I+PF6- give kET = (5 ± 1) × 109 M-1 s-1 in DCM. Structural modifications of the electron acceptor to increase its steric bulk reduce the magnitude of kET: methyl and t-butyl additions to the para positions of the phenyl rings (para Me2Ph2I+PF6- and t-butyl-Ph2I+PF6-) respectively give kET = (1.2 ± 0.3) × 1010 M-1 s-1 and kET = (5.4 ± 1.5) × 109 M-1 s-1 for reaction with photoexcited N-EC in DCM. These reductions in kET are attributed to slower rates of diffusion or to steric constraints in the ET reaction.

Phenacyl Phenothiazinium Salt as a New Broad-Wavelength-Absorbing Photoinitiator for Cationic and Free Radical Polymerizations

A novel broad-wavelength-absorbing photoinitiator based on phenacyl phenothiazinium hexafluroantimonate (P-PTh) possessing both phenacyl and phenothiazine chromophoric groups was reported. P-PTh absorbs light at UV, Visible and Near-IR region. Photophysical, photochemical, and computational investigations revealed that P-PTh in solution decomposes at all wavelengths by homolytic and heterolytic cleavages and generates cationic and radical species, which could efficiently initiate cationic and free radical polymerizations. It is anticipated that the photoinitiator with such wavelength flexibility may open up new pathways in curing applications of formulations of pigment systems.

Chemiluminescence-Induced Free Radical-Promoted Cationic Polymerization

Chemiluminescence (CL) has recently been featured as a new external light source for various photoinduced reactions with attractive features such as eliminating continuous energy supply and advanced light source setups. In the present study, the free-radical-promoted cationic polymerization of cyclohexene oxide, n-butyl vinyl ether, and N-vinyl carbazole under CL irradiation is described. The method is based on the visible-light-induced generation of electron donor radicals from bis-(4-methoxybenzoyl)diethyl germane (BAG), bis(2,4,6-trimethylbenzoyl) phenyl phosphinate, and camphorquinone by CL illumination followed by electron transfer to diphenyl iodonium hexafluorophosphate (Ph₂ I⁺ PF₆ ^- ) to form corresponding cations capable of initiating cationic polymerization. The applicability of the process to network formation is also demonstrated by using a bifunctional monomer, tri(ethylene glycol) divinyl ether.

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.

Photoinduced synthesis of poly(N-ethylcarbazole) from phenacylium salt without conventional catalyst and/or monomer

We report a new method for the photoinduced synthesis of poly(N-ethylcarbazole) (PEC).

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.