Fluorescence quenching of the S1 and S2 states of zinc meso-tetrakis(4-sulfonatophenyl)porphyrin by halide ions

Abstract

The excited-state quenching of zinc meso-tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS) by halide ions has been studied in water and various aqueous micellar solutions using both steady-state and time-resolved fluorescence techniques. The quenching efficiencies of the S₁ state of ZnTPPS (τ ≈ 1.93 ns) by halide ions in homogeneous aqueous solution increase in the order Cl⁻ < Br⁻ < I⁻; both dynamic and static quenching processes are involved. The mechanisms which may possibly be operating (i.e. electron transfer, heavy atom effect and Watkins mechanism) are discussed. Quenching of the short-lived S₂ state (τ ≈ 1.3 ps) of ZnTPPS is only possible using I⁻; it is suggested that the quenching mechanism proceeds via electron transfer between the fluorophore and I⁻ within the quenching sphere of action. The results are consistent with the Gibbs free energy change (ΔG°) involved in the charge-separation process. Finally, the fluorescence quenching of ZnTPPS by bromide ions in both cationic (DTAB and CTAC) and neutral (TX-100) micelles has been examined. The quenching observed in the cationic micelles is rationalised using a water-filled channel model, whereby Br⁻ ions diffuse down these channels to achieve close proximity to the fluorophore. No emission quenching was observed in the case of the neutral TX-100 micelles.