New insights in photosensitizers-photoinitiators interaction

Abstract

Excited state processes of a large variety of polymerization photoinitiators (based on morpholino, hydroxyalkyl, amino ketones) in the presence of photosensitizers belonging to the thioxanthone series have been investigated through time-resolved laser-pumped dye-laser spectroscopy. Energy and electron transfer processes account for the experimental results. The high efficiency of the interaction reaction parallels the high reactivity of the couple photosensitizer-photoinitiator. The introduction of thioether substituents at the para position of the benzoyl moiety of various ketones, such as amino ketones or hydroxyalkyl phenyl ketones, leads to a change in the excited states of these compounds when exposed to UV. Interactions between a photosensitizer (such as a thioxanthone derivative) and some of these compounds are followed by using a dye laser which allows selective excitation of the photosensitizer. The efficiency of these interactions depends on the nature of the solvent by means of a balance between the two mechanisms of charge transfer or energy transfer. The latter is favoured in substituted derivatives, compared with the parent compounds, since the energy level of the lowest lying triplet state is reduced. Interactions between a thioxanthone derivative used as a photosensitizer and a range of polymerization photoinitiators have been investigated in low viscosity solvents and in photopolymerizable monomer/oligomer media. The data show the influence of the acceptor molecule (the photoinitiator) and the polarity and viscosity of the medium on the rate constant of interaction, the quenching by the monomer and the yield of the excitation transfer process. The high values (0.5–0.8) for the yield of excitation transfer that are found, match the well-known efficiency of these combinations in industrial UV curing applications.