Shining a light on an adaptable photoinitiator: advances in photopolymerizations initiated by thioxanthones

Abstract

Photochemistry has been playing a central role in the synthetic polymer community. Aromatic ketones, examples of which include benzophenone, thioxanthone, camphorquinone, among others, are renowned for their excellent optical characteristics and have been extensively taken advantage of photochemically induction of polymerization processes. Of particular interest is thioxanthone due to its adaptability for bearing different functionalities and applications in various modes of photopolymerization which accomplishes photoinitiation in conjunction with other co-initiator compounds; a behavior that is referred to as bi-molecular photoinitiation. In this paper, we review the photochemistry of thioxanthonebased systems and their use in different modes of photoinitiated polymerizations. Citing examples from literature, the development of various photoinitiating systems based on thioxanthones along with an understanding of their mechanistic behavior has been elucidated in advance.Scheme 1 Typical representation of the photolysis of the radical photoinitiators based on Type I (top) and Type II (bottom) systems on the example of a benzoin derivative and thioxanthone, respectively.Scheme 3 Synthesis of thioxanthones by condensation of thiosalicylic acid and aromatic compounds in the presence of concentrated sulfuric acid.

Evaluating brominated thioxanthones as organo‐photocatalysts

“…[1,2] Thioxanthone and its derivatives are frequently used in organo-photocatalytic reactions [3,4] and as initiator in photopolymerization formulations. [5] Although 1 absorbs strongly in the near-UV spectral region (λ max = 381 nm), its absorption shows only a minor tailing in the visible region above 400 nm, which makes it only poorly suited as a sensitizer/catalyst for visible light illumination. We envisioned a simple structural modification of the thioxanthone scaffold that would result in a bathochromic shift without altering the triplet energy significantly.…”

2,2,2-trifluoroacetophenone-based D-π-A type photoinitiators for radical and cationic photopolymerizations under near-UV and visible LEDs

“…[8][9][10][11][12][13] However, most reported and commercial PIs exhibit less-than-perfect performance upon LED irradiation because they have a maximum absorbance below 365 nm where the most common and cost-effective UV/LED lamps irradiate. 14 In the reported PIs, substituted ketones (e.g., the well-known 2,2-dimethoxy-2-phenylacetophenone (DMPA), 15,16 benzophenone (BP), [17][18][19][20][21] or thioxanthone (TX) [22][23][24][25][26] ) can work as cleavable (Type I) or uncleavable (Type II) PIs, and their photogenerated radicals is used for universal application in free-radical polymerization (FRP), free-radical-promoted cationic photopolymerization, or in cationic polymerization (CP) (as photosensitizers). The excellent properties of ketone derivatives provide them with interesting potential applications for polymer synthesis.…”

Triazine‐Based Type‐II Photoinitiating System for Free Radical Photopolymerization: Mechanism, Efficiency, and Modeling

“…[12,13] However, recent studies have proved that triazine derivatives are actually efficient to act as co-initiators, via a photoinduced electron transfer from the singlet state of pyrromethene derivatives. [15][16][17] Interestingly, isopropylthioxanthone (ITX) which is one of the most versatile photoinitiator for free radical photopolymerization (FRP) [23] has never been combined with triazine derivatives in a two-component photoinitiating system for FRP, although that such a combination has been studied for photoacid generation. [24] In the present paper, we studied the ability of a Type-II photoinitiating system combining ITX as photoinitiator and 2,4,6-tris(chloromethyl)-1,3,5-triazine (triazine S, TS) as electron-accepting co-initiator to initiate acrylate photopolymerization.…”