Photoinitiation Mechanisms of Novel Phenothiazine‐Based Oxime and Oxime Esters Acting as Visible Light Sensitive Type I and Multicomponent Photoinitiators

Abstract

In this work, three new photoinitiators, based on the phenothiazine scaffold as a chromophore and potentially bearing the oxime ester functionality as an initiating group are designed and synthesized for the free radical polymerization of acrylates, the cationic polymerization of epoxides, and the formation of interpenetrated polymer networks upon irradiation with a light emitting diode emitting at 405 nm. These phenothiazine‐based oxime and oxime esters revealed impressive photoinitiation ability manifested by excellent polymerization rates and high final reactive function conversions. Significantly, they can be used as both; one‐component (Type I) and two‐component photoinitiating systems. Photoinitiation mechanisms through which reactive species are produced are investigated by means of different complementary techniques including real‐time Fourier transform infrared spectroscopy, UV–visible absorption spectroscopy, electron spin resonance spectroscopy, fluorescence (steady state and time resolved), cyclic voltammetry, and molecular modeling calculations. Thermal initiation behavior of the different oxime esters is also studied by using differential scanning calorimetry, highlighting their dual thermal/photochemical initiation ability. Finally, 3D printed objects are successfully fabricated by conducting both direct laser writing and 3D printing experiments.

An anthraquinone-based oxime ester as a visible-light photoinitiator for 3D photoprinting applications

“…The enthalpies of the cleavage process from the singlet state (Δ H cleavage S = BDE(N–O) − E S ) and from the triplet state (Δ H cleavage T = BDE(N–O) − E T ) are evaluated at −25.1 kcal mol −1 and −12.5 kcal mol −1 , respectively, suggesting an energetically favorable cleavage from both excited states. Interestingly, the calculated enthalpies of the cleavage process of the anthraquinone-based oxime ester are lower than those observed in the literature 51 favoring its use as a Type I photoinitiator.…”

Carbazole-fused coumarin based oxime esters (OXEs): efficient photoinitiators for sunlight driven free radical photopolymerization

“…16,20,23,[25][26][27] In the literature, many other OXEs exhibiting different chromophores have been reported by different research groups, including chalcones, coumarins, bicarbazoles, phenothiazines, stilbenes, phenyl thienyl thioethers, fluorophenyl and indoles. 19,22,[28][29][30][31][32][33][34][35][36] From the reactivity viewpoint, oxime esters based on carbazoles and coumarins both proved to be remarkable Type I photoinitiators, as evidenced in our previous works. In this context, we decided to design a hybrid structure to enhance the light absorption properties and the potential reactivity for sunlight polymerization, in which the two groups would be fused, generating a carbazole-fused coumarin structure.…”

Recent advances on phenothiazine-based oxime esters as visible light photoinitiators of polymerization

“…With aim at establishing the real initiating abilities of PTZ2 and PTZ3, comparisons were first established with PTZ1 which is only an oxime and not an oxime ester (See Figure 16). [147] But comparisons were also established with oxime esters differing by the group introduced on the oxime ester side (See Hex-1-Hex-10). The heavy atom effect was also examined, [148][149][150] a series of oxime esters being prepared with bromophenothiazine (See 1A-13A).…”

“…Reproduced with permission of Ref. [147] Interestingly, in this work, photoinitiating ability of a wide range of radicals (primary, secondary, tertiary radicals, aliphatic and aryl radicals) could be examined since oxime esters esterified with aromatic and aliphatic groups were designed and synthesized. Noticeably, in this work, the remarkable photoinitiating ability of PTZ3 capable to generate a methyl radical was undoubtedly the most reactive one (81% conversion).…”