Dominique Burget

Photopolymerization reactions under visible lights: principle, mechanisms and examples of applications

A general overview of visible light photoinduced polymerization reactions is presented. Reaction mechanisms as well as practical efficiency in industrial applications are discussed. Several points are investigated in detail: photochemical reactivity of photoinitiating system (PIS), short overview of available photoinitiators (PIs) and photosensitizers (PSs), mechanisms involved in selected examples of dye sensitized polymerization reactions, examples of applications in pigmented coatings usable as paints, textile printing, glass reinforced fibers, sunlight curing of waterborne latex paints, curing of inks, laser-induced polymerization reactions, high speed photopolymers for laser imaging, PISs for computer-to-plate systems.

Visible light induced photopolymerization : Speeding up the rate of polymerization by using co-initiators in dye/amine photoinitiating systems

The activity of six newly designed three-component systems (containing a dye, an amine and a triazine derivative) for the initiation of the photopolymerization of multifunctional acrylates under visible light has been evaluated. The selection of the dyes was based on thermodynamic considerations. A discussion of the photochemical reactivity of these systems reveals the role played by thermodynamics and outlines different aspects concerned with kinetics.

Photopolymerization Reactions Initiated by a Visible Light Photoinitiating System: Dye/Amine/Bis(trichloromethyl)-Substituted-1,3,5-Triazine

Addition of a bis(trichloromethyl)-substituted-1,3,5-triazine (Tz) to a dye/amine photoinitiating system leads clearly to an increased efficiency of polymerization under visible light irradiation. The polymerization rates obtained with three triazine derivatives with either phenosafranine, eosin or Rose Bengal as light absorbing species, were measured. Whatever the investigated dye/amine mixture, the addition of Tz led to a strong synergistic effect: the inhibition time decreased and the polymerization rate increased significantly when the three components were used jointly. The spectroscopic and redox properties of the Tz were measured. The system dye/amine/Tz was investigated through time-resolved laser spectroscopy and steady state fluorescence experiments. A detailed reaction mechanism was elaborated which shows that Tz acts mainly as an inhibitor scavenger. The involved inhibitor is the reduced dye (DH∗) arising from the first photochemical reaction between the excited states of the dye and the amine.

Determination of Fluorescence Yields, Singlet Lifetimes and Singlet Oxygen Yields of Water-Insoluble Porphyrins and Metalloporphyrins in Organic Solvents and in Aqueous Media

Abstract— Fluorescence quantum yields and singlet lifetimes for a wide range of hydrophilic to hydrophobic porphyrins and metalloporphyrins have been determined in toluene, methanol or acetone. Photosensitized singlet oxygen yields have been determined in the same solvents. For some porphyrins, the same quantities were determined in an aqueous medium, through use of an amphiphilic polymer to solubilize the porphyrin sensitizer and target molecule, 1, 3‐diphenylisobenzofuran. Because rate constants for the deactivation of singlet oxygen (kd) and for its reaction with a target molecule (ka) are unknown in such aqueous polymer systems, a new method was developed for evaluating yields of singlet oxygen formation that also provides a value for the ratio kd/ka. A variation observed in quantum yield of singlet oxygen production for the aqueous polymer system with variation in initial concentration of the target molecule is discussed.

On the necessity of distinguishing “n” from “π” quenchers in electron transfer studies

Abstract From the example of 1DCA* (where DCA is 9,10-dicyanoanthracene) fluorescence quenching by electron donors which exhibits different “π” and “n” Rehm-Weller plots for select compounds (pure “π” or “n” donors) it is shown that the quenching data of non-select compounds (donors exhibiting both “π” and “n” properties) need careful examination. Vertical ionization potentials rip for characterizing the donor ability reveal themselves to be more useful in the understanding of quenching rates than adiabatic potentials vIP or the redox potential E1/2.

On the interaction mechanism of radical photoinitiators with monomers

Rate constants of quenching of triplet excited ketones by several monomers were determined through time-resolved laser spectroscopy or culled from the literature. The semi-empirical calculation method PM3 allows the quenching mechanisms to be refined and can be used to predict the reactivity of aromatic ketones toward monomers. It is apparent from both experimental results and theoretical calculations that the rate constant (kq ) measured for the bimolecular quenching between the triplet state of a given aromatic ketone and both electron-rich as well as electron-poor monomers, depends linearly on the free enthalpy of formation of the regioselectively favored 1,4-biradical, which is the primary reaction step of the ketone/monomer interaction.

Structure/properties relationships in photoinitiators of polymerization—9. The behaviour of a new bis α-sulphonyl ketone derivative

Abstract The photochemistry of a new α-sulphonyl acetophenone characterized by high efficiency in the radical polymerization of acrylic coatings is presented. Its photochemical behaviour and the excited-state processes have been studied using time resolved laser spectroscopy and steady-state photolysis techniques; the photopolymerization activity has been evaluated. The photoinitiator works according to a β cleavage process in the triplet state (rate constant ⩽ 1.8 × 105 sec−1). Interactions with e.g. isopropanol, methylacrylate and methyldiethanolamine led to bimolecular quenching rate constants of 103, 7.4 × 105 and 3.2 × 108mol−1 sec−1, respectively. The main applications are UV curable inks and can be found in UV thin layer pigmented systems. One of its important peculiarities is the absence of odour developed after curing. Primary toxicological evaluations show its safety in handling.

Structures/properties relationships in photoinitiators of polymerization: XI. Behavior of new benzylbenzoin benzyl ethers

Abstract The photochemistry of five new substituted benzoin ethers has been investigated and their polymerization activities evaluated; the cleavage mechanisms have been explored using steady-state photolysis and mass spectroscopy and the excited state processes studied with time-resolved laser spectroscopy. The five compounds can be separated into two groups: three of them present a very interesting initiation efficiency for the polymerization of acrylates; photolysis leads to a cleavage reaction resulting from a very short triplet state and is followed by subsequent cleavage. The other two, which exhibit a better near UV–Vis light absorption, have very poor efficiencies as photoinitiators since they are very stable under irradiation. Full diagrams of excited state processes are discussed.

Photopolymerization induced materialization of the dipolar response from isolated metallic nanoparticles

We report on the characterization of the field diffracted by Au nanoparticles under optical excitation. The spatial distribution of the scattering diagram of the nanoparticles is materialized in real space through photopolymerization. These experiments find their motivations in optics where metallic nanoparticles are thought to find promising applications in plasmonics, but also in chemistry where nanometer scale polymerization mechanisms is a subject of current interest for both fundamental purposes or lithographic applications. For our experiments, the nanoparticles embedded in a photopolymerizable material are deposited on a glass substrate. The sample is then subjected to a global illumination and the field scattered by the particles enables for a local optical activation of the polymerization reaction. The nanometric sensitivity of the polymerization reaction determines the reactions transfer function and allows for a spatially controlled characterization of the field scattered by the particles. The shape of the resulting polymeric material, representing the particles spatial diffraction pattern, is subsequently characterized using Atomic Force Microscopy (AFM). For colloidal, randomly dispersed Au nanoparticles excited with linearly polarized light, the scattering induced topography is related to the dipolar response from the particle. More specifically, different components of the scattered field were identified that we assigned to the evanescent and progressive contributions of the dipoles field. Experiments in progress are aimed to study interacting particles with various shapes and sizes.

Benzylbenzoin benzyl ethers as new photoinitiators

The title benzoins are prepared and their photochemistry has been investigated. Product analysis studies indicate that the benzoin undergoes Norrish Type I photocleavage to generate benzoyl and benzyloxybenzyl radical, followed by thermal fragmentation to produce deoxybenzoin thereby generating benzyl radical. All three transient species are actually detected and characterized by laser flash photolysis of the benzoin in solution at room temperature. The efficiency of the benzoins as photoinitiator has been also investigated. Effects of ring substituents and polycyclic aromatic rings on the photoreactivity have been revealed.