Photochemistry and photocuring activities of novel substituted 4′-(4-methylphenylthio) benzophenones as photoinitiators

Abstract

The photoinduced polymerization activities of 19 novel substituted 4′-(4-methylphenylthio)benzophenones (or 4′-(4-tolylthio)-benzophenones) were determined and compared with that of the unsubstituted derivative itself in different monomers and prepolymers using real-time infrared (RTFTIR) and pencil hardness methods. Absorption, fluorescence and phosphorescence analyses, as well as photoreduction/photolysis studies, were undertaken on the compounds, and the data were related to the photopolymerization activities. Solvent shift studies on the absorption maxima of all the compounds indicate the presence of a long-wavelength single ππ ∗ state with some nπ ∗ character. The absorption maxima of the bistolythio, amino, chloro and nitro derivatives are more red shifted than those of the simpler alkyl, alkoxy, phenyl and phenoxy derivatives. Fluorescence and phosphorescence analyses indicate a high rate of intersystem crossing to the triplet state. The fluorescence emission maxima are markedly red shifted with increasing solvent polarity, indicating that the lowest excited singlet state exhibits a high degree of charge transfer character. The lowest excited triplet state of all the 4-substituted 4-tolythiobenzophenones is essentially a ππ ∗ configuration, with some nπ ∗ character, as indicated by the phosphorescence lifetime data. The lowest excited triplet state of the unsubstituted derivative, on the other hand, is essentially ππ ∗ in character with a longer emission lifetime and a lower quantum yield. The emission maxima are independent of the nature of the substitution, indicating the presence of a rigid molecular structure. The latter is confirmed by the observation of very high phosphorescence quantum yields. The bistolythio derivatives exhibit shorter phosphorescence emission lifetimes, indicating that their lowest excited triplet state is nπ ∗ in nature. The triplet ππ ∗ character of the amino derivatives is indicated by their increased emission lifetimes. The electron-withdrawing effect of the chloro and nitro groups markedly reduces the phosphorescence quantum yield. These effects are accounted for by the relative spacings of close-lying lowest excited singlet ππ ∗ and second excited triplet nπ ∗ states controlling the competitive processes of internal conversion and intersystem crossing. Compared with the unsubstituted 4-tolylthio derivative, all the alkyl-, alkoxy-, phenyl- and phenoxy-substituted derivatives exhibit higher activities as photoinitiators in the absence of an amine cosynergist. Lengthening or broadening of the alkoxy substituent also gives rise to an increased photoinitiation activity, whereas alkyl groups are less effective. The presence of an amine cosynergist enhances photocuring, giving rise to variable effects depending on the nature of the triplet exciplex interaction. The bistolythio derivatives exhibit the highest photoinitiation activity, whereas the 4-nitro group has a marked deactivating effect. The presence of the 4-amino group also enhances the photoinitiation activity, but only by RTFTIR measurements. The photoreduction/photolysis rates of all the 4-substituted derivatives are greater than that of the unsubstituted derivative and show some relation to the nature of the lowest excited triplet state. The photophysical and photochemical data are discussed in relation to the relative, photoactivities of the compounds as photoinitiators for curing acrylated multifunctional monomers.