Photophysical properties and photoinduced polymerisation activity of novel 1-chloro-4-oxy/acyloxythioxanthone initiators

Abstract

The excited state characteristics of 13 novel 1-chloro-4-oxy/acyloxy derivatives of thioxanthone were determined using micro-, nano- and picosecond flash photolysis techniques. Triplet energy levels have also been determined using phosphorescence analysis while photoinitiated polymerisation activities were measured using photocalorimetry. All the initiators exhibit high photopolymerisation activity except the 4-hydroxy model and 2-methyl-4-n-propoxy derivatives. The triplet energies are found to be fairly insensitive to solvent polarity with an observed spectral broadening from non-polar to polar solvents. This is consistent with close lying mixed triplet states of 3 ππ* and 3n π* character. Both the least active 4-hydroxy and 2-methyl-4-propoxy derivatives exhibit lower triplet energies suggesting the presence of a less active lower triplet 3 ππ* state. Triplet–triplet absorption spectra are obtained with all the thioxanthones in the range 600–680 nm with a marked blue shift from non-polar to polar solvents owing to stabilisation of the lowest triplet state by solvent reorganisation. In photoreductive solvents such as methyl alcohol and 2-propanol a longer lived species is observed absorbing in the region 400–500 nm associated with the formation of the ketyl radical. This observation is highly solvent dependent and further supported by microsecond flash spectroscopy in 2-propanol. The 4-hydroxy derivative gave only weak transient absorption and is consistent with its much lower initiation activity. In the presence of a tertiary amine no ketyl radical is observed. Triplet lifetimes increase with solvent polarity confirming the presence of mixed 3 ππ* and 3n π* states where vibronic coupling influences the rate of intersystem crossing to the ground S 0 state. Bimolecular triplet quenching rate constants indicate all the thioxanthones, except the 2-methyl-4-n-propoxy and 4-hydroxy, interact strongly with a range of tertiary amines, DABCO, triethylamine and methyldiethanolamine. The lower triplet quenching constants for the 2-methyl-4-n-propoxy and 2-hydroxy derivatives (an order of magnitude) indicates weaker interaction by the amine and is consistent with their lower photoinitiation activities. Low triplet quenching rates are also observed in the presence of monomer (methyl methacrylate). Bimolecular triplet quenching rates are also measured with naphthalene and are similar to those for benzophenone except the 2-methyl-4-n-propoxy and 4-hydroxy, derivatives which are an order of magnitude less. From this data triplet molar extinction coefficients are determined and found to be higher than that for benzophenone owing to the presence of mixed states. Again, the 2-methyl-4-n-propoxy and 4-hydroxy derivatives exhibited lower values as did the 4-benzoyloxy derivative. Relatively high quantum yields of intersystem crossing are observed (<0.6) but are all lower than that of benzophenone with the side chain substituents having no significant effect on the rate. The growth rate of the triplet state could be measured and triplet maxima accurately determined via picosecond flash photolysis. Within the growth time of the triplet state (6–18 ps) there is a concurrent formation of the ketyl radical except for the 4-acetyloxy and 4-acryloxy derivatives. The importance of this technique in gaining valuable information on the relative rates of concurrent excited state reactions for thioxanthones is discussed.