A Reappraisal of Some Properties of Thioxanthone as Photoinitiator by Thermal Lens and Picosecond Transient Absorption Spectroscopy

On the example of Thioxanthone (TX) it is shown that time resolved thermal lens (TRTL), spectroscopy and picosecond transient absorption spectroscopy (PTAS) provide valuable informations on the primary processes involved in the photoinitiation step. On one hand, TRTL is
used to determine the triplet quantum yield 'T in different solvents. It is found that is strongly solvent dependent and ranges from 0.56 in acetonitrile to 0.85 in n-hexane. On the other hand, the built up time scale of the 1 —* absorption recorded at 620 nm does not correspond to that deduced from the singlet state (recorded at 700nm). This effect is discussed on the basis of different excited states involved in the intersystem crossing process.