Photochemical reactions of triplet state of safranine-T studied by transient absorption spectroscopy in visible/near-IR regions

Properties and reactivities of the triplet state of safranine-T [(3)(STH+)*] have been studied by following the transient absorption bands measured with nanosecond laser flash photolysis. Solvent effect was found in the lifetime and T-T annihilation rate constant (k(TT)) of (3)(STH+)*; the lifetime in ethanol is quite long and k(TT) in water is too small to observe. For electron transfer of (3)(STH+)* with various electron donors, the reaction rate constants decrease with the oxidation potentials of the donors along with the Rehem-Weller relation. For the high electron-donors, the reaction rate constants were found to be solvent viscosity dependent rather than dielectric constants, suggesting the triplet exciplex formation prior to electron transfer. Different reactivities of (3)(STH+)* towards H-abstraction reactions have been observed depending on the nature of H-atom donors: with phenols, the reaction center of the triplet states has a highly electrophilic nature, while it has a nucleophilic character with respect to thiophenols, suggesting that the reactive position of (3)(STH+)* changes with the substrates.