In this study, a series of coumarin chromophore-based oxime-esters were designed and synthesized as visible light photoinitiators (PIs).

Four visible light triphenylamine-based oxime ester photoinitiators (TP-1–4) were synthesized and confirmed successfully through ¹H NMR, ¹³C NMR, and electron impact mass spectrometry (EI-MS) measurements as well as elementary analyses (EA). The design strategy for triphenylamine substituted photoinitiators involved the use of hydrogen atoms only for TP-1, one methoxy group for TP-2, two methoxy groups for TP-3 and two methyl groups for TP-4. The incorporation of triphenylamine derivatives in oxime esters gives near UV and visible light absorption characters with maximum absorption peaks at around 360 nm. Upon using a UV lamp as an irradiation source, all the oxime esters showed good double bond conversion efficiency in a range of 72–92% in the free radical polymerization of trimethylolpropane triacrylate (TMPTA) through photo-DSC experiments. Among all, TP-1 displayed the highest final conversion efficiency. Accordingly, the photochemical reaction behavior was studied using photolysis, cyclic voltammograms (CV), electron spin resonance (ESR) and theoretical calculations. Additionally, selected photoinitiating systems exhibited outstanding 3D patterns with high resolution and good edge definition through direct laser write (DLW) experiments @405 nm. This work thus demonstrates a judicious chemical design to fine-tune the photopolymerization character in triphenylamine oxime ester analogues.

In this work, a series of Type I photoinitiators (PIs), based on the naphthoquinone scaffold, were designed and synthesized for the first time in order to induce photopolymerization under visible light.

In order to discuss the polymerization effect from the substituted position and methoxy group of Type I photinitiators, a series of naphthalene-based oxime esters was designed and synthesized. Compared to the 2-naphthalene-substituted compound, the UV absorption region of the 1-naphthalene-based compound was greatly improved. In addition, the methoxy substitution exhibited longer absorption characteristics than did the methoxy-free one. The photochemical reaction behavior of these novel compounds was also studied by photolysis, cyclic voltammetry (CV), and electron spin resonance (ESR) experiments. Finally, the initiation abilities of naphthalene-based oxime esters toward trimethylolpropane triacrylate (TMPTA) monomer were conducted through the photo-DSC instrument under UV and a 405@nm LED lamp. Remarkedly, the naphthalene-based oxime ester (NA-3) that contains 1-naphthalene with o-methoxy substituent showed the rather red-shifted absorption region with the highest final conversion efficiency under UV (46%) and 405@nm LED (41%) lamp irradiation.

A series of oxime esters (TPA-DP, TPA-CZ and TPA-PT) have been developed and synthesized, in which three electron-donating groups (diphenylamine, carbazole, and phenothiazine) are connected to the 4,4′-positions of the triphenylamine to form three triarylamine oxime ester photoinitiators. All the compounds were prepared successfully; in addition, for reference the electron-donating free oxime ester (TP-1M) was also prepared for comparison. The influence of different electron-donating groups on the absorption region, theoretical calculations, and photochemical properties of oxime esters was studied. In addition, photo-DSC experiments under UV, LED@365 nm and LED@405 nm exposures were carried out to realize the corresponding photoreactivity. Although compound TPA-DP exhibits a rather red-shifted and broader absorption region, TPA-CZ and TPA-PT based formulations showed better photochemical and photoreactive properties. Furthermore, two-component photoinitiating systems of oxime ester/iodonium salt (Iod) showed an efficient free radical and cationic photopolymerization behavior under irradiation with a 405 nm LED. Finally, these oxime esters also displayed thermal initiation characteristics, indicating that these compounds are capable of acting as both thermal and photochemical initiators.