Photochemistry of thioxanthones—IV. Spectroscopic and flash photolysis study on novel n-propoxy and methyl, n-propoxy derivatives

Abstract

The spectroscopic properties of seven oil soluble n-propoxy and methyl, n-propoxy substituted thioxanthone structures have been examined in various solvents and the data compared to their photopolymerization efficiencies, photochemical stabilities and flash photolysis behaviours in solution. Absorption maxima, extinction coefficients, fluorescence and phosphorescence spectra and quantum yields have been measured. Generally, all the compounds exhibit low fluorescence and high phosphorescence quantum yields but the ratio is solvent dependent. These observations are consistent with the high photoreactivity of the molecules operating in the lowest excited triplet state which is nπ* in character. Photopolymerization rates of n-butyl methacrylate with N-diethylmethylamine co-initiator correlate with the absorption maxima and extinction coefficients of the thioxanthones and also the degree of quenching of their fluorescence by the amine, indicating the importance of electron transfer. In the absence of a tertiary amine, transient formation on micro-second flash photolysis in 2-propanol is associated with the ketyl radical formed by the lowest excited triplet state abstracting a hydrogen atom from the solvent. In the presence of a tertiary amine, both ketyl radical and radical anion (at longer wavelengths) are observed, the latter being formed via abstraction of an electron from the amine. This is confirmed by a flash photolysis study using amines of various ionization potentials where a correlation is observed with radical anion absorbance. A pH study confirms the identity of the transient species. Activation of the thioxanthone molecule in the 3 and 4 positions with a methyl group enhances initiator activity whereas substitution in the t-position deactivates the molecule through intra-molecular hydrogen atom abstraction.