Fluorescence quenching of water-soluble porphyrins. A novel fluorescence quenching of anionic porphyrin by anionic anthraquinone

The fluorescence quenching of 5-phenyl-10,15,20-tris(p-sulfonatophenyl)porphine (TPPS^3-) and 5,10,15,20-tetrakis(4-N-methylpyridyl)porphine (TMPyP⁴⁺) has been studied in water (pH 8.0) by using 9,10-anthraquinone-2,6-disulfonate (AQDS^2-) and methyl viologen (MV²⁺) as quenchers. While electrostatic repulsion is expected, AQDS^2- quenched the TPPS^3- fluorescence more efficiently than MV²⁺. The steady-state and time-resolved fluorescence measurements indicated that static quenching took place in the TPPS^3--AQDS^2- system. Studies on the absorption spectra and the effects of ionic strength on the fluorescence quenching indicated the formation of the ground-state complex of TPPS^3- and AQDS^2-. The thermodynamic parameters (ΔH and ΔS) suggested that the ground-state complex was formed via van der Waals interaction.