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

Abstract

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.