Pyrromethene derivatives in three‐component photoinitiating systems for free radical photopolymerization

Abstract

1,3,5,7,8-pentamethyl pyrromethene difluoroborate complex (HMP) and 2,6-diethyl-8-phenyl-1,3,5,7-tetramethylpyrromethene difluoroborate complex (EPP) were used to initiate the polymerization of a diacrylate in a two-and a three-component photoinitiating system (PIS), together with an amine (ethyl-4-dimethylaminobenzoate, EDB) and triazine A (2-(4methoxyphenyl)-4,6-bis(trichloromethyl)-1,3,5-triazine, TA) as coinitiators. For both pyrromethene dyes, the highest conversion was achieved with the three-component PIS. As these dyes have high-fluorescence quantum yields, steady state and time-resolved techniques were used to study the possible fluorescence quenching by the amine and the triazine, as well as laser flash photolysis to investigate the electron transfer process that occurs in these PIS from either the singlet or triplet excited states. The electron transfer reaction is evidenced by using time-resolved photoconductivity. Experiments show that the main interaction between the dye and both coinitiators is through its excited singlet state and the process is more efficient when TA is present. The beneficial effect noted when both coinitiators are used in a three-component system is ascribed to secondary reactions between the coinitiators and intermediates that lead to the generation of higher amount of initiating species and the recovery of the initial dye.

Borate Salts as Coinitiators in Photoinitiating Systems for Laser Imaging.

“…Pyrromethene dyes HMP or EMP also lead to a good efficiency, although these dyes are known to react from their first excited singlet state [9]. Similar situation was found for the squarylium dye SQ, which exhibits a good reactivity.…”

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

“…Because TA highly absorbs around 355 nm, it was not possible to analyze its photosensitized mechanism with ITX by LFP. However, positive ΔG PEneT value of 0.19 eV (E T = 2.9 eV [12] ) compared to ΔG PEleT of −0.08 eV (E red = −1.12 V vs. SCE [16] ) tends to indicate that a photoinduced electron transfer is also operating for ITX/TA system. The Type-II PIS model was used with parameters specific to TA (initial concentrations, k et extracted with Equation (9) from the k q = 3.7 × 10 9 m −1 s −1 reported value [64] ) and a k bet value of 7.5 × 10 5 s −1 was found ( Figure 6) for a ΔG bet value of −2.63 eV.…”

“…It has three main advantages: (i) the fundamental state of the PI is regenerated and can participate to a new cycle, (ii) supplementary initiating radicals are produced, (iii) potential terminating agents are consumed. Among the numerous PCIS redox additives reported in the literature, triazine derivatives (Tz) have been extensively used as electronacceptors in combination with amines, [12][13][14][15][16][17] thiols, [18] or borate salts. [18][19][20][21] They have been found to enhance final conversion and rate of polymerization by reacting with the semi-reduced form of the PI.…”

“…[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.…”

Photocyclic Initiating System for Free Radical Photopolymerization Studied Through Holographic Recording

“…To improve and prevent the limitation of polymerization kinetics, three-component PIS were developed [3][4][5][6][7][8] and have been found to be by far more efficient than the corresponding two-component systems [9][10][11]. They generally include a photosensitizer which is typically a dye, an electron donor and as third component an electron acceptor [3][4][5][6][7][8].…”