Charge Transfer Complexes as Pan-Scaled Photoinitiating Systems: From 50 μm 3D Printed Polymers at 405 nm to Extremely Deep Photopolymerization (31 cm) Patxi Garra, Bernadette Graff, Fabrice Morlet-Savary, Ce ́ line Dietlin, Jean-Michel Becht, Jean-Pierre Fouassier, and Jacques Laleve ́ e* Institut de Science des Mate ́ riaux de Mulhouse IS2M, UMR CNRS 7361, UHA, 15, rue Jean Starcky, Cedex 68057 Mulhouse, France * S Supporting Information ABSTRACT: Charge transfer complexes (CTC) between N- aromatic amines (donors) and iodonium salts (acceptors) are used here as photoinitiating systems (PIS) for the polymerization of clear methacrylate formulations under a 405 nm LED irradiation. Outstandingly, a complete spatial and temporal resolution is kept for 50 μm resolved 3D printed photopolymers at 405 nm (50 μm being the size of the printing laser used here). Photocuring of a high thickness (31 cm) is also possible. The photopolymerization propagation is rationalized and interpreted from both experimental (using thermal imaging experiments) and predicted data. An experimental/molecular modeling study also attempts to rationalize the CTC structure/reactivity/efficiency relationships. These systems are commercially available, stable, and metal-free and have a low toxicity. 1. INTRODUCTION Light-induced reactions are very interesting as they require small energy amounts for reactions at mild temperatures without significant emissions of VOC. Most of the time, they result in excellent spatial and temporal controls. Nevertheless, a huge limitation of them is the inner filter effect: light is mainly absorbed by layers close to the light source, and a deep light penetration is not possible (as due to the Beer-Lambert law 1 ). As a result, most applications of photochemistry are occurring close to the surface receiving the actinic light, for example in solar cells 2 or in continuous-flow photochemistry. 3 This is particularly true for free radical photopolymerization: 4-12 most of the current applications are often limited to thicknesses <100 μm in coatings, inks, paints, or composites even if polymers concern much wider sizes of samples (45% of the manufactured plastic materials are produced through free radical polymer- ization processes 13 ). There is therefore a need to (i) better understand the inner filter effect on in depth photopolymeriza- tion kinetics and (ii) to actually develop photoinitiating systems (PIS) capable of producing pan-scaled and well-resolved (e.g., for 3D printing) polymer materials, i.e., from the micrometric to the centimeter scale. In the literature, many authors are tackling the issues of light penetration, and this was summed up in a recent review. 14 Many years ago, mono- and bisacylphosphine oxides appeared as the only family of photoinitiators being able to polymerize clear, pigmented, filled, or glass reinforced fiber acrylate coatings. 15-20 Thanks to the particular bleaching properties of these compounds, the photocuring of 2.9 cm thick filled materials and ∼10-15 cm thick clear varnishes has been claimed. Nevertheless, long irradiation times and UV irradiation sources were necessary for the curing of very thick samples thanks to the photobleaching observed with acylphosphine oxides (i.e., 25 min to cure a 8.5 cm of clear acrylate formulation 21 ). Developing systems active under near-infrared irradiation sources 22-26 is a recent answer (to light penetration issues in photopolymerization) particularly relevant when fillers are causing some light diffusion enhancing inner filter effect, e.g., in composites. 27-30 Very recently, another advanced report stated the use of upconversion particles to produce the free radical photopolymerization of acrylate samples up to 13.7 cm. 31 These upconversion particles (0.3 wt % particles in the resin) re-emitted blue light (allowing photoinitiation by a titanocene photoinitiator) upon a 980 nm high-intensity (9.4 W/cm 2 ) laser beam. The evaluation of the spatial control of the method was not presented. Other recent different strategies involve the presence of latent species created under a primary irradiation that will diffuse through the entire polymerizable media, e.g., in controlled photopolymerization under air, 32-36 UV-induced photopolymerization triggered by photocaged amines, 37-39 a process based on a diffusion by a leuco form of the dye, 40 or in Received: October 11, 2017 Revised: November 22, 2017 Article Cite This: Macromolecules XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.macromol.7b02185 Macromolecules XXXX, XXX, XXX-XXX