Thioxanthone Derivatives as a New Class of Organic Photocatalysts for Photopolymerisation Processes and the 3D Printing of Photocurable Resins under Visible Light

Abstract

In the present paper, novel thioxanthone-based compounds were synthesised and evaluated as a component of photoredox catalysts/photoinitiating systems for the free-radical polymerisation (FRP) of acrylates and the ring-opening cationic polymerisation (CP) of epoxy monomers. The performance of the obtained thioxanthones in two- and three-component photoinitiating systems, in combination with amines, iodonium or sulphonium salt, as well as with alkyl halide, for photopolymerisation processes upon exposure to light emitting diodes (LEDs) with a maximum emission of 405 nm and 420 nm, was investigated. The studied compounds act also as one-component free-radical photoinitiators. Fourier transform real-time infrared spectroscopy was used to monitor the kinetics of disappearance of the functional groups of the monomers during photoinitiated polymerisation. Excellent photoinitiating efficiency and high final conversions of functional groups were observed. Moreover, the influence of thioxanthone skeleton substitution on photoinitiating efficiency was discussed. The photochemical mechanism was also investigated through cyclic voltammetry. It was discovered that thioxanthone derivatives can be used as a metal-free photoredox catalyst active for both oxidative and reductive cycles. Furthermore, a photopolymerizable system based on novel thioxanthone derivatives in a stereolithography three-dimensional (3D) printing technology under visible sources of light was used. The effects of photoinitiator type system and monomer type in photoresins during 3D printing processes were explored. The outcome of this research is the development of high-performance visible photosensitive resins with improved photosensitivity obtained thanks to the development of entirely novel photoinitiating systems specifically adapted for this application.

Visible-Light Amine Thioxanthone Derivatives as Photoredox Catalysts for Photopolymerization Processes

“…Fluorescence emission and excitation spectra were registered with a Quanta Master 40 (from Horiba) spectrofluorometer as described in our previous papers. 41,44 Fluorescence quenching studies of thioxanthen-9-one derivatives were carried out using the same spectrofluorometer and procedure described in detail elsewhere. 44 Method of Determining Oxidation and Reduction Potentials.…”

“…39 What is more, the thioxanthone molecule is a well-known photosensitizer 40 that is applied for photopolymerization processes. 41,42 A few years ago, Allushi et al investigated also thioxanthone and 2isopropylthioxanthone together with amine N,N,N′,N″,N′′pentamethyl-diethylenetriamine (PMDETA) and alkyl halides in atom transfer radical polymerization. 43 In this work, synthesis of structurally modified thioxanthonebased compounds was performed in order to obtain active components for two-and three-component photoinitiator systems for cationic and free-radical polymerization performed upon exposure to visible light.…”

New horizons for carbon dots: quantum nano-photoinitiating catalysts for cationic photopolymerization and three-dimensional (3D) printing under visible light

“…Scientists around the world are currently working on new molecules, the so-called iodine salt photosensitizers, to efficiently transfer electrons to the iodine salt and thus effectively trigger light-initiated polymerization. 19 Many types of compounds are available as effective photosensitizers of iodine salts, including biphenyl, 20,21 terphenyl, 1,22,28 thioxanthone, 23 pyridine [24][25][26] or 1,8-naphthalimide 27 derivatives. However, it is a constant challenge to develop more "greener" sensitizers derived from natural resources that are non-toxic and highly effective and can be successfully exploited in biomedical applications.…”

Recent progress in the development of highly active dyeing photoinitiators based on 1,3-bis(p-substituted phenylamino)squaraines for radical polymerization of acrylates

“…37,38 Over the years, the photosensitization abilities of many various organic dyes have been tested. [39][40][41][42] There is much information in the literature on photoinitiating systems containing different types of sensitizers, such as: BODIPY derivatives, [43][44][45][46] camphorquinones, [47][48][49][50] carbazoles, 51-53 coumarins, [54][55][56] cyanines, 40,57 ferrocenes, 58,59 flavones, 60 indoles, 61 squaraines, 40,43,62 thioxanthones [63][64][65][66][67] and many others. 68 The number of compounds that can act as a light absorber is really huge and impressive.…”