Tunable Organophotocatalysts for Polymerization Reactions Under Visible Lights

Electronic effects of conjugated aryl groups on the properties and reactivities of di(arylethynyl)tetracenes

The photochemical oxidations of acenes can cause challenges with their optoelectronic applications, such as singlet fission and organic transistors. At the same time, these reactions form the basis for many luminescent sensing schemes for 1O2. While diethynyl substitution is arguably the most widely adopted of the various substitution strategies to control oxidation and also improve solubility and processability of long acenes, the extent to which differences between the alkyne groups can influence key properties of long acenes remains largely unknown. This report therefore describes the effects of various arenes and heteroarenes on the electronic structures, optical properites, and reactivity with singlet oxygen for eight 5,12-di(arylethynyl)tetracenes. The fluorescence spectra of these tetracenes span approximately 100 nm, while their observed rate constants for reaction with singlet oxygen correlates strongly with the HOMO level, spanning one order of magnitude. They are also amenable to fluorescent materials that respond ratiometrically to singlet oxygen. Therefore, electronic effects of groups directly conjugated to ethynylacenes offer a useful chemical space for rational acene design.

Visible Light Induced Step-Growth Polymerization by Substitution Reactions

A new visible light induced step-growth polymerization of dibromoxylene, and diols using dimanganese decacarbonyl and diphenyliodonium salt is described. The polymerization is suggested to proceed by substitution reaction between dixylenium cations formed upon visible light irradiation in the presence of dimanganese decacarbonyl and diphenyl iodonium salt. For the described substitution reaction with diols as nucleophilic component, the scope of the process is studied. Furthermore, the presence of halide groups at chain ends of the resulting polymers provided the possibility of initiating subsequent free radical and free radical promoted cationic resulting in the formation of polyether-based block copolymers.

A dinuclear rhenium complex in the electrochemically driven homogeneous and heterogeneous H+/CO2-reduction

A dinculear Re(CO)3 complex with a proton responsive phenol unit and a pyrene anchor in the ligand backbone was investigated in the electrochemical CO2/H+ conversion in solution and adsorbed on multi walled carbon nanotubes (MWCNT) on an GC electrode surface. The pyrene group unit is introduced at the end of the ligand synthesis via a coupling reaction, which allows for a versatile ligand modification in order to tune the electronic properties or to introduce various anchor groups for heterogenisation at a late stage. The redox chemistry of the pyrene-α-diimine-Re(CO)3 complex, 1, was investigated in N,N-dimethylformamide (dmf), including IR-spectroelectrochemical (IR-SEC) characterisation of the short lived, reduced species. Subsequently, the electrochemical H+/CO2-reduction catalysis in dmf/water was investigated. The complex catalyses syngas formation yielding CO and H2 with similar rates, namely in Faraday yields of 45% and 35%, respectively. Since the similar complex without the pyrene anchor in the backbone, I, prefers CO2 over H+ reduction, the formation of syngas was rationalised by the small differences in the redox properties and pKa values of the phenol-pyrene unit in regard to phenol unit as in I. Subsequently, the complex was adsorbed on multi walled carbon nanotubes (MWCNT) on a GC electrode surface. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) confirmed coating of the electrode. The immobilised complex was utilised in the electrochemical CO2/H+ reduction in dmf/water, however, the complex quickly desorbed under reductive conditions, likely due to the good solubility of the reduced species. Water as a solvent prevents desorption as confirmed by XPS, however, then a preference for H2 formation over syngas formation was observed under electrocatalytic conditions. Thus, these experiments show, that the results obtained in aqueous organic solution are not easily transferable to the heterogeneous systems operating in water due to changes in the reaction rates for competing pathways.

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.

Photoinitiators derived from natural product scaffolds: monochalcones in three-component photoinitiating systems and their applications in 3D printing

Design of new photoinitiators based on natural Chalcone scaffold.

Novel D–π-A and A–π-D–π-A three-component photoinitiating systems based on carbazole/triphenylamino based chalcones and application in 3D and 4D printing

A series of carbazole or triphenylamine based mono-chalcones, displaying either D–π-A or A–π-D–π-A architecture have been designed as photoinitiators for 3D and 4D printing.

Design of Iodonium Salts for UV or Near-UV LEDs for Photoacid Generator and Polymerization Purposes

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.

Sulfinates and sulfonates as high performance co-initiators in CQ based systems: Towards aromatic amine-free systems for dental restorative materials

OBJECTIVE

The aim of our study is to develop amine-free photoinitiating systems (PISs) for the polymerization of representative dental methacrylate resins under blue light irradiation. PISs based on camphorquinone (CQ)/sulfinate and CQ/sulfonate, eventually in combination with an iodonium salt, are proposed and compared to the well-established CQ/amine system. The polymerization performances of thick (1.4 mm) samples of different methacrylate blends upon exposure to a commercial blue LED centered at 477 nm under air are described. Finally, the performances of the new developed PISs are evaluated for dental composites application.

METHODS

FTIR is used to monitor the photopolymerization profiles. ESR spectroscopy and electrochemical experiments are used to identify the radicals generated. Mechanical properties measurements and color stability measurements are carried out to determine the key properties of the dental composites prepared.

RESULTS AND SIGNIFICIANCE

The performances of the new proposed PISs for the photopolymerization of thick (1.4 mm) samples of methacrylate upon exposure to a blue dental LED under air are excellent. Similar or better performances and bleaching properties are obtained with the new proposed amine-free systems compared to those reached with the CQ/amine reference system. Dental composites with excellent mechanical properties and exceptional color stability are obtained. The involved chemical mechanisms for the initiation step were also established.

1-Aryl-2-(triisopropylsilyl)ethane-1,2-diones: Toward a New Class of Visible Type I Photoinitiators for Free Radical Polymerization of Methacrylates

1-Aryl-2-(triisopropylsilyl)ethane-1,2-diones (SEDs) are proposed here as a new class of visible Type I photoinitiators (PIs) for free radical polymerization under air upon exposure to blue (@455 nm) and green (@520 nm) LEDs. Remarkably, these new systems present good polymerization performances and excellent bleaching properties compared to camphorquinone-based systems, and transparent polymers are obtained upon visible light irradiation. Real-time Fourier transform infrared spectroscopy is used to monitor the polymerization profiles. Molecular orbital calculations are also carried out for a better understanding of the structure/reactivity relationship of the photoinitiators.

Curcuminoid-Based Difluoroboron Dyes as High-Performance Photosensitizers in Long-Wavelength (Yellow and Red) Cationic Photopolymerization

Difluoroboron β-diketonate dyes are reported to exhibit excellent photophysical properties (e.g., broad absorption and large extinction coefficients) and have the potential to act as high-performance photosensitizers in cationic photopolymerization (CP). In this study, four curcuminoid-based difluoroboron dyes (BF₂ Curs) are prepared. Their ability to initiate the CP of epoxides or vinyl ethers in combination with an iodonium salt under yellow and red LEDs is investigated. Some of the BF₂ Curs-based photoinitiating systems exhibit much higher efficiencies than the reported anthraquinone derivative Oil Blue N (OBN). The molecular structure of BF₂ Curs is found to play a critical role in the photoinitiating efficiencies of cationic polymerization.

Seeing the Light: Advancing Materials Chemistry through Photopolymerization

The application of photochemistry to polymer and material science has led to the development of complex yet efficient systems for polymerization, polymer post-functionalization, and advanced materials production. Using light to activate chemical reaction pathways in these systems not only leads to exquisite control over reaction dynamics, but also allows complex synthetic protocols to be easily achieved. Compared to polymerization systems mediated by thermal, chemical, or electrochemical means, photoinduced polymerization systems can potentially offer more versatile methods for macromolecular synthesis. We highlight the utility of light as an energy source for mediating photopolymerization, and present some promising examples of systems which are advancing materials production through their exploitation of photochemistry.

Organometallic vs organic photoredox catalysts for photocuring reactions in the visible region

Recent progresses achieved in terms of synthetic procedures allow now the access to polymers of well-defined composition, molecular weight and architecture. Thanks to these recent progresses in polymer engineering, the scope of applications of polymers is far wider than that of any other class of material, ranging from adhesives, coatings, packaging materials, inks, paints, optics, 3D printing, microelectronics or textiles. From a synthetic viewpoint, photoredox catalysis, originally developed for organic chemistry, has recently been applied to the polymer synthesis, constituting a major breakthrough in polymer chemistry. Thanks to the development of photoredox catalysts of polymerization, a drastic reduction of the amount of photoinitiators could be achieved, addressing the toxicity and the extractability issues; high performance initiating abilities are still obtained due to the catalytic approach which regenerates the catalyst. As it is a fast-growing field, this review will be mainly focused on an overview of the recent advances concerning the development of organic and organometallic photoredox catalysts for the photoreticulation of multifunctional monomers for a rapid and efficient access to 3D polymer networks.

Photoinduced metal-free atom transfer radical polymerizations: state-of-the-art, mechanistic aspects and applications

Photoinduced atom transfer radical polymerization has recently been the center of intensive research in synthetic polymer chemistry because of the unique possibility of topological and temporal control in addition to precise control of macromolecular structure offered by conventional ATRP.

N-[2-(Dimethylamino)ethyl]-1,8-naphthalimide derivatives as photoinitiators under LEDs

Four N-[2-(dimethylamino)ethyl]-1,8-naphthalimide derivatives (ANNs) with different substituents in the naphthalimide skeleton have been synthesized and can be used as versatile photoinitiators under various LEDs.

Organocatalyzed Atom Transfer Radical Polymerization: Perspectives on Catalyst Design and Performance

The recent development of organocatalyzed atom transfer radical polymerization (O-ATRP) represents a significant advancement in the field of controlled radical polymerizations. A number of classes of photoredox catalysts have been employed thus far in O-ATRP. Analysis of the proposed mechanism gives insight into the relevant photophysical and chemical properties that determine catalyst performance. Discussion of each of the classes of O-ATRP catalysts highlights their previous uses, their roles in the development of O-ATRP, and the distinctive properties that govern their polymerization behavior, leading to a set of design principles for O-ATRP catalysts. Remaining challenges for O-ATRP are presented, as well as prospects for further improvement in the application scope of O-ATRP.

New violet to yellow light sensitive diketo pyrrolo–pyrrole photoinitiators: high performance systems with unusual bleaching properties and solubility in water

New high performance photoinitiating systems for visible light exhibiting excellent bleaching properties and water solubility are proposed.

Synthesis of block copolymers by mechanistic transformation from photoinitiated cationic polymerization to a RAFT process

A novel synthetic strategy for the synthesis of block copolymers based on mechanistic transformation from photoinitiated cationic polymerization to radical addition fragmentation transfer polymerization is presented.

Photoinduced controlled radical polymerization of methacrylates with benzaldehyde derivatives as organic catalysts

Under 23 W CFL irradiation, block copolymers are obtained starting from a P_n-I macroinitiator in the presence of a benzaldehydic molecule-based catalytic system.

A novel photoinitiating system producing germyl radicals for the polymerization of representative methacrylate resins: Camphorquinone/R3GeH/iodonium salt

OBJECTIVES

The aim of our study is to find an amine free photoinitiating system (PIS) for the polymerization of representative dental methacrylate resins. A photoinitiating system (PIS) based on camphorquinone (CQ)/triphenylgermanium hydride/diphenyl iodonium hexafluorophosphate is proposed and compared to the conventional CQ/amine couple. The polymerization monitoring of thin (∼20μm) and thick (1.4mm) samples of a bisphenol A-glycidyl methacrylate (Bis-GMA)/triethyleneglycol dimethacrylate (TEGDMA) blend (70%/30% w/w) and of a urethane dimethacrylate (UDMA) upon exposure to a commercial blue LED centered at 477nm under air or in laminate is described. Finally, the impact of the photoinitiating system composition on the final polymer color is evaluated in detail.

METHODS

FTIR and DSC experiments are used to record the photopolymerization profiles. ESR spectrometry and steady state photolysis are used to detect the produced radicals. Color measurements are carried out to determine the key parameters in the bleaching of the different dental formulations.

RESULTS

The efficiency of the newly proposed PISs for the photopolymerization of BisGMA/TEGDMA and UDMA for thin (20μm) or for thick (1.4mm) samples upon exposure to a dental blue LED under air is excellent. It is noticeably higher than that of the CQ/amine reference couple. Excellent bleaching properties are also observed under irradiation in presence of the new PISs. A good correlation is found between the sample bleaching and the amount of Ph3GeH in the formulation. The excited state processes could be established. The overall chemical mechanisms for the initiation step were also clarified.

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.

Novel naphthalimide–amine based photoinitiators operating under violet and blue LEDs and usable for various polymerization reactions and synthesis of hydrogels

A series of naphthalimide derivatives containing tertiary amine groups (DNNDs) have been prepared.

New role of aminothiazonaphthalimide derivatives: outstanding photoinitiators for cationic and radical photopolymerizations under visible LEDs

Aminothiazonaphthalimide derivatives can act as photoinitiators (when combined with additives) to start the cationic polymerization or the radical polymerization upon exposure to visible LEDs or a polychromatic visible light.

Thiophene-substituted phenothiazine-based photosensitisers for radical and cationic photopolymerization reactions under visible laser beams (405 and 455 nm)

In the present work, we designed and prepared six new thiophene-substituted phenothiazine-based visible photosensitizers (ThPTZs) to solve the low absorptivity of phenothiazine in the visible light region.

A novel naphthalimide scaffold based iodonium salt as a one-component photoacid/photoinitiator for cationic and radical polymerization under LED exposure

A strong drawback to overcome for photoinitiators of cationic polymerization or photoacids is their low photosensitivity for long wavelengths.