Photophysics and halide quenching of Soret-excited ZnTPPS4− in aqueous media

► Both the S1 and S2 states of water-soluble ZnTPPS4− are quenched by iodide ion. ► An intermolecular electron-transfer mechanism is involved. ► For S1 the quenching is diffusion limited at constant ionic strength. ► For S2, static quenching occurs within a tight sphere of action.