Solvent dependence of photophysical and photochemical behaviors of thioxanthen-9-one.

The photophysical and photochemical behaviors of thioxanthen-9-one (TX) in different solvents have been studied using nanosecond transient absorption spectroscopy. A unique absorption of the triplet state ³TX* is observed, which involves two components, ³nπ* and ³ππ* states. The ³ππ* component contributes more to the ³TX* when increasing the solvent polarity. The self-quenching rate constant k_sq of ³TX* is decreased in the order of CH₃CN, CH₃CN/CH₃OH (1:1), and CH₃CN/H₂O (1:1), which might be caused by the exciplex formed from hydrogen bond interaction. In the presence of diphenylamine (DPA), the quenching of ³TX* happens efficiently via electron transfer, producing the TX^·− anion and DPA^·+ cation radicals. Because of insignificant solvent effects on the electron transfer, the electron affinity of the ³nπ* state is proved to be approximately equal to that of the ³ππ* state. However, a solvent dependence is found in the dynamic decay of TX^·– anion radical. In the strongly acid aqueous acetonitrile (pH=3.0), a dynamic equilibrium between protonated and unprotonated TX is definitely observed. Once photolysis, ³TXH^+* is produced, which contributes to the new band at 520 nm. [ABSTRACT FROM AUTHOR]