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 (TX)-T-3* is observed, which involves two components, (3)n pi* and (3)pi pi* states. The (3)pi pi* component contributes more to the (TX)-T-3* when increasing the solvent polarity. The self-quenching rate constant k(sq) of (TX)-T-3* 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 (TX)-T-3* happens efficiently via electron transfer, producing the TX center dot- anion and DPA(center dot+) cation radicals. Because of insignificant solvent effects on the electron transfer, the electron affinity of the (3)n pi* state is proved to be approximately equal to that of the (3)pi pi* state. However, a solvent dependence is found in the dynamic decay of TX center dot- anion radical. In the strongly acid aqueous acetonitrile (pH=3.0), a dynamic equilibrium between protonated and unprotonated TX is definitely observed. Once photolysis, (TXH+*)-T-3 is produced, which contributes to the new band at 520 nm.