Metal-supported and -assisted stereoselective cooperative photoredox catalysis

In this perspective, we review those stereoselective photocatalytic reactions that use synergy between photoredox catalysts and transition metal catalysts. In particular, we highlight the orchestrated interaction between two and more metals which not only enhance the turnover numbers, but also lead to increased selectivities. Aspects of green chemistry and sustainable developments are included. In this review, C-C, C-O, C-N and C-S forming reactions are discussed and a perspective on future developments is given.

Recent Advances on Visible Light Metal-Based Photocatalysts for Polymerization under Low Light Intensity

In recent years, polymerization processes activated by light have attracted a great deal of interest due to the wide range of applications in which this polymerization technique is involved. Parallel to the traditional industrial applications ranging from inks, adhesives, and coatings, the development of high-tech applications such as nanotechnology and 3D-printing have given a revival of interest to this polymerization technique known for decades. To initiate a photochemical polymerization, the key element is the molecule capable to interact with light, i.e., the photoinitiator and more generally the photoinitiating system, as a combination of several components is often required to create the reactive species responsible for the polymerization process. With the aim of reducing the photoinitiator content while optimizing the polymerization yield and/or the polymerization speed, photocatalytic systems have been developed, enabling the photosensitizer to be regenerated during the polymerization process. In this review, an overview of the photocatalytic systems developed for polymerizations carried out under a low light intensity and visible light is provided. Over the years, a wide range of organometallic photocatalysts has been proposed, addressing both the polymerization efficiency and/or the toxicity, as well as environmental issues.

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.

New N^C^N-coordinated Pd(ii) and Pt(ii) complexes of a tridentate N-heterocyclic carbene ligand featuring a 6-membered central ring: synthesis, structures and luminescence

Photoluminescent pincer-like Pd(ii) and Pt(ii) complexes with a rare tridentate N^C^N ligand with a six membered NHC central core are described. Solid-state packing exhibits M(ii)⋯M(ii) and π–π interactions.

Ionic Self-Assembly and Red-Phosphorescence Properties of a Charged Platinum(II) 8-Quinolinol Complex Associated with Ammonium-Based Amphiphiles

A series of ionic associates based on the platinum(II) chelate of 5-sulfo-8-quinolinol, [Pt(qS)2](2-), and ammonium-based amphiphiles is described. At variance with the prototypical neutral complex Pt(q)2 (q=8-quinolinol), these dianionic fluorophores, functionalized at the periphery with sulfonate groups, can be associated by the ionic self-assembly approach with various ammonium cations, such as (H2n+1Cn)2Me2N(+) (n=12, 16, 18) or complex ammonium cations carrying three Cn carbon chains (n=12, 14, 16) and an additional amide group. Investigations of their luminescence properties in solution, in the solid state, and, when possible, in thin films revealed that the phosphorescence properties in condensed phases are directly correlated to intermolecular interactions between the luminescent [Pt(qS)2](2-) centers. Of particular interest is also the formation of a columnar liquid-crystalline phase around room temperature (between -25 and +180 °C), as well as the very good film-forming ability of some of these fluorophores from organic solvents.

Mechanisms and applications of cyclometalated Pt(ii) complexes in photoredox catalytic trifluoromethylation

The incorporation of a trifluoromethyl group into an existing scaffold can provide an effective strategy for designing new drugs and agrochemicals. Among the numerous approaches to trifluoromethylation, radical trifluoromethylation mediated by visible light-driven photoredox catalysis has gathered significant interest as it offers unique opportunities for circumventing the drawbacks encountered in conventional methods. A limited understanding of the mechanism and molecular parameters that control the catalytic actions has hampered the full utilization of photoredox catalysis reactions. To address this challenge, we evaluated and investigated the photoredox catalytic trifluoromethylation reaction using a series of cyclometalated Pt(ii) complexes with systematically varied ligand structures. The Pt(ii) complexes were capable of catalyzing the trifluoromethylation of non-prefunctionalized alkenes and heteroarenes in the presence of CF3I under visible light irradiation. The high excited-state redox potentials of the complexes permitted oxidative quenching during the cycle, whereas reductive quenching was forbidden. Spectroscopic measurements, including time-resolved photoluminescence and laser flash photolysis, were performed to identify the catalytic intermediates and directly monitor their conversions. The mechanistic studies provide compelling evidence that the catalytic cycle selects the oxidative quenching pathway. We also found that electron transfer during each step of the cycle strictly adhered to the Marcus normal region behaviors. The results are fully supported by additional experiments, including photoinduced ESR spectroscopy, spectroelectrochemical measurements, and quantum chemical calculations based on time-dependent density functional theory. Finally, quantum yields exceeding 100% strongly suggest that radical propagation significantly contributes to the catalytic trifluoromethylation reaction. These findings establish molecular strategies for designing trifluoromethyl sources and catalysts in an effort to enhance catalysis performance.

Metal and metal-free photocatalysts: mechanistic approach and application as photoinitiators of photopolymerization

In the present paper, the photoredox catalysis is presented as a unique approach in the field of photoinitiators of polymerization. The principal photocatalysts already reported as well as the typical oxidation and reduction agents used in both reductive or oxidative cycles are gathered. The chemical mechanisms associated with various systems are also given. As compared to classical iridium-based photocatalysts which are mainly active upon blue light irradiation, a new photocatalyst Ir(piq)2(tmd) (also known as bis(1-phenylisoquinolinato-N,C (2'))iridium(2,2,6,6-tetramethyl-3,5-heptanedionate) is also proposed as an example of green light photocatalyst (toward the long wavelength irradiation). The chemical mechanisms associated with Ir(piq)2(tmd) are investigated by ESR spin-trapping, laser flash photolysis, steady state photolysis, cyclic voltammetry and luminescence experiments.

New core-pyrene π structure organophotocatalysts usable as highly efficient photoinitiators

Eleven di- and trifunctional compounds based on a core-pyrene π structure (Co_Py) were synthesized and investigated for the formation of free radicals. The application of two- and three-component photoinitiating systems (different Co_Pys with the addition of iodonium or sulfonium salts, alkyl halide or amine) was investigated in detail for cationic and radical photopolymerization reactions under near-UV-vis light. The proposed compounds can behave as new photocatalysts. Successful results in terms of rates of polymerization and final conversions were obtained. The strong MO coupling between the six different cores and the pyrene moiety was studied by DFT calculations. The different chemical intermediates are characterized by ESR and laser flash photolysis experiments. The mechanisms involved in the initiation step are discussed, and relationships between the core structure, the Co_Py absorption property, and the polymerization ability are tentatively proposed.