Abstract
The design of photoinitiating systems with excellent photochemical reactivities at 405nm LED is one of the obstacles to efficiently promote free radical polymerization in mild conditions (e.g., low light intensity, under air). Here, our actual search for new multicomponent photoinitiating systems at 405nm LED prompts us to develop new dyes based on push–pull structures. In the present paper, we chose two series of new dyes which possess indane-1,3-dione and 1H-cyclopenta naphthalene-1,3-dione groups as the electron-withdrawing groups, since they have the great potential to behave as sensitive and remarkable photoinitiators in vat photopolymerization/3D printing. When incorporated with a tertiary amine (ethyl dimethylaminobenzoate EDB, used as electron/hydrogen donor) and an iodonium salt (used as electron acceptor) as the three-component photoinitiating systems (PISs), and among a series of 21 dyes, 10 of them could efficiently promote the free radical photopolymerization of acrylates. Interestingly, steady state photolysis experiments revealed different behaviors of the dyes. Fluorescence experiments and free energy change calculations for redox processes were also carried out to investigate the relevant chemical mechanisms. Additionally, the formation of radicals from the investigated PISs was clearly observed by electron spin resonance (ESR) spin-trapping experiments. Finally, stereoscopic 3D patterns were successfully fabricated by the laser writing technique. In this work, the use of push–pull dyes based on the naphthalene scaffold as photoinitiators of polymerization is reported for the first time in a systematic study aiming at investigating the structure–performance relationship for irradiation carried out at 405 nm. By carefully selecting the electron donors used in the two series of push–pull dyes, novel and high-performance photoinitiating systems operating at 405 nm are thus proposed.
Highlights
Photopolymerization under visible light and low light intensity is an active research field still requiring the optimization of the chemical structures of photoinitiators [1,2,3,4,5,6,7,8,9,10,11,12]
For the photopolymerization process initiated with the three-component photoinitiating systems (PISs), the chemical mechanisms can be divided into two parts: (1) dyes acting as electron donors with iodonium salt (Iod); (2) amine (EDB)
Their molar extinction coefficients were determined by UV-visible absorption spectroscopy and the results are summarized in the Table 2
Summary
Photopolymerization under visible light and low light intensity is an active research field still requiring the optimization of the chemical structures of photoinitiators [1,2,3,4,5,6,7,8,9,10,11,12]. By increasing the molar extinction coefficients of photoinitiators, the quantity of the compound introduced into the photocurable resin can be drastically lowered while maintaining a constant absorption This point is of crucial interest, as the extractability and the migratability of photoinitiators within the polymer films can be a major issue for numerous applications such as food packaging [18,19] or the design of biocompatible materials or safety equipment [20,21,22,23]. By using long irradiation wavelengths, the polymerization of thick samples and the access to composites is rendered possible, contrarily to the traditional UV photopolymerization for which the polymerization of only thin samples is possible due to a limited light penetration at short wavelength [39] With regards to these different challenges (light penetration, excited state lifetimes, bleaching properties, molar extinction coefficients), the development of visible light photoinitiators is an active research field imposing new structures to be tested
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