Fabrice Morlet-Savary

Subtle Ligand Effects in Oxidative Photocatalysis with Iridium Complexes: Application to Photopolymerization

Iridium(III) complexes were designed and evaluated as efficient photoinitiators of polymerization reactions in combination with iodonium salts and silanes. Mechanistically, these reactions were shown to proceed through oxidative photoredox catalysis, generating aryl and silyl radicals under very soft irradiation conditions (blue LED, xenon lamp, and even sunlight). These radicals can initiate the free radical polymerization of acrylates or can be oxidized during the catalytic cycle to promote the ring-opening polymerization of epoxy monomers. Remarkably, both the (photo)chemical reactivity and the practical efficiency are dramatically affected by the ligands. In addition, the central role played by the oxidation ability of the excited state of the photocatalyst is discussed.

Efficient dual radical/cationic photoinitiator under visible light: a new concept

A new concept based on the photoredox catalysis is proposed for the design of a photoinitiating system able to efficiently generate, in a single catalytic cycle, both a radical and a cation as initiating species. This is exemplified here by the tris(1,10-phenanthroline)ruthenium(II) (Ru(phen)32+) complex in combination with a diphenyl iodonium salt and a silane. Interesting and promising photopolymerization profiles in free radical photopolymerization (FRP) and free radical promoted cationic photopolymerization (FRPCP) are obtained under air and upon xenon lamp exposure and even soft irradiation conditions (fluorescence bulb, sunlight). An acrylate/epoxide blend is also easily polymerized under air using a fluorescent bulb. The mechanisms are investigated by ESR and Laser Flash Photolysis experiments.

Metal-Free, Visible Light-Photocatalyzed Synthesis of Benzo[b]phosphole Oxides: Synthetic and Mechanistic Investigations

Highly functionalized benzo[b]phosphole oxides were synthesized from reactions of arylphosphine oxides with alkynes under photocatalytic conditions by using eosin Y as the catalyst and N-ethoxy-2-methylpyridinium tetrafluoroborate as the oxidant. The reaction works under mild conditions and has a broad substrate scope. Mechanistic investigations have been undertaken and revealed the formation of a ground state electron donor-acceptor complex (EDA) between eosin (the photocatalyst) and the pyridinium salt (the oxidation agent). This complex, which has been fully characterized both in the solid state and in solution, turned out to exhibit a dual role, i.e., the oxidation of the photocatalyst and the formation of the initiating radicals, which undergoes an intramolecular reaction avoiding the classical diffusion between the two reactants. The involvement of ethoxy and phosphinoyl radicals in the photoreaction has unequivocally been evidenced by EPR spectroscopy.

Photopolymerization upon LEDs: new photoinitiating systems and strategies

This paper is focused on: (i) the recent developments in LED technology, allowing the design of novel and efficient light sources for the free radical and cationic photopolymerization of various monomers, the synthesis of interpenetrating polymer networks (IPNs) or thiol–ene photopolymerization; (ii) the existing and emerging applications in these areas; and (iii) the development of novel photoinitiators and photoinitiating systems specifically adapted for LED excitation.

A Novel Photopolymerization Initiating System Based on an Iridium Complex Photocatalyst

The use of a photocatalyst (tris(2-phenylpyridine)iridium [Ir(ppy)(3)]) being able to generate both radicals and cations to initiate free radical polymerization and ring opening polymerization is presented. Remarkably, under soft irradiations (fluorescence bulb, sunlight), excellent cationic polymerization profiles and final conversions are obtained. The involved mechanisms are investigated by ESR experiments.

Triplet-triplet absorption of 2-(2′-hydroxyphenyl) benzoxazole (HBO) in polar solvents

Abstract The transient TT absorption spectra of 2-(2′-hydroxyphenyl) benzoxazole (HBO) have been measured in liquid polar solvents and in polymethylthacrylate. In non-protonic polar solvents the main TT absorption is from the keto form of the compound ( 3 K ∗ ). With a triplet sensitizer the compound just gives a TT absorption of its enol from ( 3 E ∗ ) after triplet energy transfer from the sensitizer to HBO. The excited state intramolecular proton transfer (ESIPT) from the enol form to keto form occurs just in the singlet state. The protonic solvent can break the intramolecular hydrogen bond of HBO and decrease the ESIPT process. In solid solution both 3 E ∗ and 3 K ∗ can be formed and the TT absorptions were detected because of the inhibition of the rotational conformation transfer.

Spin—orbit-coupling-induced triplet formation of triphenylpyrylium ion: a flash photolysis study

Abstract The excited singlet of the 2,4,6-triphenylpyrylium ion (TPP + ) reacts with iodobenzene (PhI) by electron transfer leading to radicals and the triplet state of TPP + . The induced formation of 3 TPP + by spin—orbit coupling has been observed. An encounter complex intermediate is proposed for the quenching of 3 TPP +* by PhI. Homogeneous recombination of radicals is also observed in the millisecond time domain. Flash photolysis has been used to study the excited state reactions of TPP + .

Dyes as Photoinitiators or Photosensitizers of Polymerization Reactions

A short but up-to-date review on the role of dyes in the photoinitiation processes of polymerization reactions is presented. Radical and cationic reactions are largely encountered in the radiation curing and the imaging areas for use in traditional coating applications as well as in high tech sectors such as nanofabrication, computer-to-plate processing, laser direct imaging, manufacture of optical elements, etc. Recent promising developments concerned with the introduction of the silyl radical chemistry that enhances the polymerization efficiency are also discussed.

Efficiency and excited state processes in a three‐component system, based on thioxanthene derived dye/amine/additive, usable in photopolymer plates

The excited state interactions occurring when a three-component system of thioxanthene derived dye TXD/amine/additive (diphenyliodonium salt, CBr 4 , benzoyl peroxide, cumene hydroperoxide) is subjected to sensitization processes in the visible range, were investigated through time-resolved absorption spectroscopy, spectrofluorometry, and photolysis. The rate constants of the various operative processes were measured together with the values of the fluorescence quantum yields (e.g. ? f = 0.75 ± 0.07 in methanol) and the lifetimes of the singlet excited state of the dye (e.g. 6 ns in methanol). Singlet state quenching by methyldiethanolamine (MDEA) occurs with a rate constant k = 10 9 M -1 s -1 in methanol. The reactivity of the triplet excited state of the dye is very low (k = 5.6 X 10 4 M -1 s -1 for the TXD/MDEA interaction). The ketyl radicals that arise from the interaction of the singlet state of the dye with the amine, are quenched by such additives as CBr 4 (k = 6.7 X 10 5 M -1 s -1 ), or the onium salts (k = 5.7 X 10 5 M -1 s -1 ). The calculations of the yields of interaction of the singlet state of the dye with the two components of the system demonstrate that the process of quenching by the amine is the main one ( ? int = 0.5) compared to that by, e.g., an onium salt ( ? int = 0.07). Sensitivity of 0.3 mJ cm -2 obtained in a laser scanning system is also reported.

A dinuclear gold(I) complex as a novel photoredox catalyst for light-induced atom transfer radical polymerization

Controlled/living atom transfer radical polymerization of methacrylates and acrylates initiated by ethyl α-bromophenylacetate (EBPA) as the initiator in the presence of low loadings (1.25 mol% vs. initiator) of a dinuclear gold(I) complex based photocatalyst [Au2(μ-dppm)2]Cl2 has been accomplished in solution and in laminate under UVA and visible-light photoreductive conditions. In solution, the linear increase of molecular weights with methyl methacrylate (MMA) conversion and the low dispersity are consistent with a controlled/living process. In a film, trimethylolpropane triacrylate (TMPTA) was polymerized and the ethyl α-bromophenylacetate (EBPA)/[Au2(μ-dppm)2]Cl2 system resulted in a faster rate of polymerization compared to EBPA/Ir(ppy)3. Chain extensions of polymers were successfully conducted leading to block copolymers, which also confirms the living character of this new system. Photophysical experiments support a conventional photoredox-catalyzed ATRP mechanism. Finally, this approach utilizes a gold catalyst featuring better solubility and lower cost than the well-known Ir(ppy)3 complex.