Photoinduced Electron And Hydrogen Transfer Reactions Of Thioxanthone With Amines, Phenols And Alcohols

The quenching reactions of triplet thioxanthone (TX) by a series of amines, phenols, and alcohols were investigated by laser flash photolysis in deoxygenated acetonitrile. We obtained corresponding transient absorption spectra and quenching rate constants (k(q)). From changes in the transient absorption spectra, we determine that the electron transfer reactions occur between triplet TX and amines without an active hydrogen while electron/proton transfer reactions occur between triplet TX and amines with an active hydrogen. The appearance of hydrogenated radicals can be regarded as evidence for hydrogen transfer reactions in the TX/phenol and TX/alcohol systems. In the TX/amine systems, the quenching rate constants decreased with an increase in the free energy change (Delta G). This indicates that electron transfer reactions influence the quenching of triplet TX. In the TX/phenol systems, the quenching rate constants decreased with an increase in Delta G firstly, then increased with an increase in phenol cation acidity. This can be explained by considering that charge transfer and hydrogen transfer may play separate but important roles. In the TX/alcohol system, the quenching rate constants decreased with an increase in the alpha-C-H bonding energy of alcohols, and this indicates that the alpha-C-H bonding energy is a key factor during triplet TX quenching. By comparison with previous studies about the quenching reactions of triplet xanthone (XT) and fluorenone (FL) by a series of amines, phenols, and alcohols, it is established that because of a discrepancy in molecular configurations the quenching rate constants decrease according to the following order: XT, TX, and FL.