Reductive fluorescence quenching of the photoexcited dihydroxy antimony(V) tetraphenylporphine cation in acetonitrile solution

At 298 K in acetonitrile solution, the dihydroxy antimony(V) tetraphenylporphine cation [SbV(TPP)(OH)2]+ shows an intraligand fluorescence at λmax=598 nm with τF=1.46 ns and a quantum yield of φF = 0.028. While the presence of dioxygen has no significant effect on the value of φF, the fluorescence is very efficiently quenched by Cl−, Br−, I− and SCN− ions. At quencher concentrations below 10−2 M the reaction follows Stern–Volmer kinetics with high rate constants close to the diffusion-controlled limit. Based on the observation of the reduced complex [SbIII(TPP)]+ as a permanent photoredox product, the quenching mechanism can be characterized as a bimolecular electron transfer process involving the lowest excited singlet (ππ*) state of the [SbV(TPP)(OH)2]+ cation.