Photophysical properties and photoinduced polymerisation activity of novel1-chloro-4-oxy/acyloxythioxanthone initiators

The excited state characteristics of 13 novel 1-chloro-4-oxy/acyloxy deriva tives of thioxanthone were determined using micro-, nano- and picosecond fl ash photolysis techniques, Triplet energy levels have also been determined using phosphorescence analysis while photoinitiated polymerisation activiti es were measured using photocalorimetry. All the initiators exhibit high ph otopolymerisation activity except the 4-hydroxy model and 2-methyl-4-n-prop oxy derivatives. The triplet energies are found to be fairly insensitive to solvent polarity with an observed spectral broadening from non-polar to po lar solvents. This is consistent with close lying mixed triplet states of ( 3)pi pi* and (3)n pi* character. Both the least active 4-hydroxy and 2-meth yl-4-propoxy derivatives exhibit lower triplet energies suggesting the pres ence of a less active lower triplet (3)pi pi* state. Triplet-triplet absorp tion spectra are obtained with all the thioxanthones in the range 600-680 n m with a marked blue shift from non-polar to polar solvents owing to stabil isation of the lowest triplet state by solvent reorganisation. In photo-red uctive solvents such as methyl alcohol and 2-propanol a longer lived specie s is observed absorbing in the region 400-500 nm associated with the format ion of the ketyl radical. This observation is highly solvent dependent and further supported by microsecond flash spectroscopy in 2-propanol, The 4-hy droxy 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 polarit y confirming the presence of mixed (3)pi pi* and (3)n pi* states where vibr onic coupling influences the rate of intersystem crossing to the ground So state. Bimolecular triplet quenching rate constants indicate all the thioxa nthones, except the 2-methyl-4-n-propoxy and 4-hydroxy, interact strongly w ith a range of tertiary amines, DABCO, triethylamine and methyldiethanolami ne. 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-me thyl-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 exh ibited lower values as did the 4-benzoyloxy derivative. Relatively high qua ntum yields of intersystem crossing are observed (<0.6) but are all lower t han that of benzophenone with the side chain substituents having no signifi cant effect on the rate. The growth rate of the triplet state could be meas ured and triplet maxima accurately determined via picosecond flash photolys is. Within the growth time of the triplet state (6-18 ps) there is a concur rent formation of the ketyl radical except for the 4-acetyloxy and 4-acrylo xy derivatives. The importance of this technique in gaining valuable inform ation on the relative rates of concurrent excited state reactions for thiox anthones is discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.