Excited-state Reactions of Coumarins in Aqueous Solutions. II. The Fluorescence Quenching of 7-Ethoxycoumarins by Halide Ions

Abstract

The fluorescence yield and the fluorescence lifetime of 7-ethoxycoumarins in aqueous solutions are reduced with the increase in the temperature and in the concentration of the quenching agents. The radiationless deactivation of the excited state in the absence of quenchers is temperature-dependent; its activation energy has been determined to be 32.5–34.8 kJ mol⁻¹. The quenching reaction due to halide ions proceeds via both a diffusional and a static process. The rate constant for the diffusional process has a value of (9.7–10.2)×10⁹ mol⁻¹ dm³s⁻¹ in the case of I⁻ as a quencher. The static quenching component can be approximately described by the V[Q] expression, with the static quenching constant, V, being 1.6–1.8 mol⁻¹ dm³. The quenching efficiencies that represent the probability leading to real quenching reactions by an encounter of the molecule and the halide ion are unity for I⁻, 0.39–0.42 for Br⁻, and almost zero for Cl⁻. The possible physical interpretation of the quenching mechanism is discussed in relation to the electron transfer between colliding species.