Fundamental studies of three-component radical photoinitiators

Three-component systems containing a light-absorbing species (typically a dye), an electron donor (typically an amine), and a third component (usually an iodonium salt) have emerged as efficient visible-light sensitive photoinitiators. However, these systems are not well understood and several distinct mechanisms have been reported in the literature. In this work, photo-differential scanning calorimetry and in situ, time-resolved, laser-induced, steady-state fluorescence spectroscopy are used to study the initiation mechanism of several three-component systems.; Most studies were carried out on the simplified model system methylene blue, N-methyldiethanolamine and diphenyliodonium chloride because electrostatic repulsion prevents direct reaction between the dye and the iodonium salt. Although DSC studies reveal accelerated photopolymerization with increasing concentration of either amine or iodonium salt, laser-induced fluorescence experiments show that increasing the iodonium concentration actually slows consumption of the dye. We concluded that the primary photochemical reaction involves electron transfer from amine to dye to produce an active amine radical and a terminating dye-based radical. We suggest that the iodonium salt oxidizes the dye radical to regenerate the dye and simultaneously produce an initiating phenyl radical.; In the presence of oxygen, a retardation period precedes rapid decay of the dye fluorescence. Based on the impact of the amine and iodonium concentrations on the retardation period, our proposed mechanism includes an oxygen-scavenging pathway in which the tertiary amine radicals formed in the primary photochemical process consume the oxygen via a cyclic reaction mechanism.; The dyes Eosin Y and Eosin Y, spirit soluble were chosen as model anionic and neutral dyes. In contrast to the cationic methylene blue dye, the eosin dyes can react directly with both the amine and the iodonium components. In fact, electron transfer from eosin to iodonium salt is considerably faster than the eosin/amine interaction. Despite this, photo-DSC indicates that the eosin/iodonium interaction does not produce active radicals very efficiently. However, evidence from fluorescence spectroscopy suggests that the addition of amine increases the photopolymerization rate through amine-mediated regeneration of iodonium-bleached dye. As with methylene blue, the effect is twofold: an active amine radical replaces the presumably less active dye-based radical and the original eosin dye is regenerated.