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 ^3 TX ^* is observed, which involves two components, ^3 n \pi ^* and ^3 \pi\pi^* states. The ^3 \pi\pi^* component contributes more to the ^3 TX ^* when increasing the solvent polarity. The self-quenching rate constant k_ \rmsq of ^3 TX ^* is decreased in the order of CH _3 CN, CH _3 CN/CH _3 OH (1:1), and CH _3 CN/H _2 O (1:1), which might be caused by the exciplex formed from hydrogen bond interaction. In the presence of diphenylamine (DPA), the quenching of ^3 TX ^* happens efficiently via electron transfer, producing the TX ^\cdot ^- anion and DPA ^\cdot ^+ 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^ \cdot ^ - anion radical. In the strongly acid aqueous acetonitrile (pH = 3.0), a dynamic equilibrium between protonated and unprotonated TX is definitely observed. Once photolysis, ^3 TXH ^+* is produced, which contributes to the new band at 520 nm.