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

Metal and metal-free photocatalysts: mechanistic approach and application as photoinitiators of photopolymerization Jacques Lalevée*1, Sofia Telitel1, Pu Xiao1, Marc Lepeltier2, Frédéric Dumur3,4,5, Fabrice Morlet-Savary1, Didier Gigmes3 and Jean-Pierre Fouassier6 Full Research Paper Open Access Address: 1Institut de Science des Matériaux de Mulhouse IS2M – UMR CNRS 7361 –UHA; 15 rue Jean Starcky, 68057 Mulhouse Cedex, France, 2Institut Lavoisier de Versailles, UMR 8180 CNRS, Université de Versailles Saint-Quentin en Yvelines, 45 avenue des Etats-Unis, 78035 Versailles Cedex, France, 3Aix-Marseille Université, CNRS, Institut de Chimie Radicalaire, UMR 7273, F-13397 Marseille, France, 4Univ. Bordeaux, IMS, UMR 5218, F-33400 Talence, France, 5CNRS, IMS, UMR 5218, F-33400 Talence, France and 6formerly: University of Haute Alsace/ENSCMu, 3 rue Alfred Werner, 68093 Mulhouse Cedex, France Beilstein J. Org. Chem. 2014, 10, 863–876. doi:10.3762/bjoc.10.83 Email: Jacques Lalevée* - © 2014 Lalevée et al; licensee Beilstein-Institut. License and terms: see end of document. Received: 17 January 2014 Accepted: 21 March 2014 Published: 15 April 2014 This article is part of the Thematic Series "Organic synthesis using photoredox catalysis". Guest Editor: A. G. Griesbeck * Corresponding author Keywords: LEDs; photoinitiators; photopolymerization; photoredox catalysis Abstract 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,C2’)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. Introduction Photoredox catalysis is now well-known and largely used in organic synthesis, especially in the development of sustainable radical-mediated chemical processes under very soft irradiation conditions (e.g., household fluorescence or LED bulbs, halogen lamps, sunlight, Xe lamp), e.g., enantioselective alkylation, cycloaddition, etc. [1-14]. Ruthenium- and iridium-based 863 Beilstein J. Org. Chem. 2014, 10, 863–876. organometallic complexes are mostly employed as radical sources: they exhibit an excellent visible-light absorption, long lived excited states and suitable redox potentials and they work through either an oxidation or a reduction cycle [1-12]. To avoid, however, the high cost, potential toxicity and limited availability of these structures, metal-free organic dye compounds (e.g., Eosin-Y, Nile Red, Alizarine Red S, perylene derivative or Rhodamine B etc.) were recently proposed for cooperative asymmetric organophotoredox catalysis [13,14]. This approach opened up [45-53] a new way for the design of a novel high performance class of PIs for FRP and FRPCP (where the photoinitiator is now referred as a photoinitiator catalyst PIC). It brings, among others, the following novel properties [45-55]: Photoredox catalysis was then introduced into the polymer photochemistry field (area) in the very past years (see a review in [15-22]). Indeed, in this area, free-radical photopolymerization (FRP, Scheme 1, reactions 1 and 2), cationic photopolymerization (CP, Scheme 1, reactions 3 and 4), free-radical promoted cationic photopolymerization (FRPCP, Scheme 1, reactions 5–7) or acid and base-catalyzed photocrosslinking (reactions 8 and 9) are initiated using photoinitiators (PI) which generate reactive species (radicals, cations, anions, radical cations, acids, bases). These PIs are often incorporated into multicomponent photoinitiating systems (PIS) where they primarily react with the other concomitant reagents or additives. • Almost no photoinitiator is consumed. • Since the spectral photosensitivity extends now from the UV to the visible, laser excitation in the purple, blue, yellow, green or red is feasible. • Low light intensities (as delivered, e.g., by household lamps and LED bulbs) can be used; this is a catalytic process without loss of efficiency with irradiation. • Photopolymerization under sunlight becomes reachable. • The production of the radical or cationic initiating species for the FRP of acrylates or the FRPCP of epoxides, respectively, is quite easy; polymerization of sustainable monomers can also be achieved (e.g., epoxidized soybean oil). • A possible dual behavior (simultaneous generation of radicals and cations that ensure the formation of, e.g., an epoxy/acrylate interpenetrated network IPN) is achieved. PIs and PISs have been extensively developed both in industrial R&D and academic laboratories [15-40]. PIs are usually organic molecules that are consumed during the light exposure [15-40]. The use of organometallic compounds as PIs was reported many years ago (see a review in [41]) and reintroduced in some recent papers dealing with, e.g., Cr, Ti, Fe, Rh, W, Pt, Ru, Ir, Zn, Zr-based derivatives [42-44]. On this occasion, it appeared that a photoredox catalysis behavior (allowing a PI regeneration while keeping a high reactivity/efficiency) can be introduced through a suitable selection of the PIs and PISs [45-55]. Examples of PICs proposed by us in the photopolymerization reactions are depicted in Figure 1 and Figure 2 (for metal based PICs and metal free PICs, respectively) [45-55]. Their reactivity parameters (redox potentials, excited state lifetimes) are given in the associated references. These PICs are typically used (see below) in combination with various additives (see Figure 3 below) in three-component photoinitiating systems, e.g., based PIC/iodonium salt (or sulfonium salt)/ tris(trimethylsilyl)silane (or N-vinylcarbazole) or PIC/amine/ alkyl halide. Also, relatively high intensity light sources (Hg, Xe or Hg–Xe lamps, laser diodes) can be obviously employed. Scheme 1: Examples of photoinitiating systems. 864 Beilstein J. Org. Chem. 2014, 10, 863–876. Figure 1: Previously reported PIC (based on metal complexes) [45-52]. Figure 2: Previously reported PIC (metal free organic molecules) [54,55]. However, and with more interest, very soft irradiation conditions (using, e.g., household fluorescence or LED bulbs, halogen lamps or even sunlight) are also suitable to polymerize radical and cationic monomers (see Figure 4 below) under polychromatic or monochromatic lights in the 350–700 nm and to get tack free coatings. The present paper will i) review the various possible mech (...truncated)