Excited-State Reactions of Coumarins in Aqueous Solutions. III. The Fluorescence Quenching of 7-Ethoxycoumarins by the Chloride Ion in Acidic Solutions

Publisher Website
Google Scholar
Abstract
The fluorescence yield of 7-ethoxycoumarins in acidic aqueous solutions becomes reduced with an increase in the chloride-ion concentration. In acidic solutions, two types of excited molecules, a neutral molecule and a protonated molecule with a positive charge, are generated by a UV light illumination. Only the excited state of a protonated molecule is quenched by a chloride ion; the quenching reaction proceeds via both a diffusional and a static process. The rate constant for the diffusional process is (3.7–5.8)×109 mol−1 dm3 s−1. The static quenching component can be approximately described by the V[Q] expression and the static quenching constant, V, is 1.2–1.7 mol−1 dm3. One possible physical interpretation of the quenching mechanism is discussed in relation to the charge of colliding species and the electron transfers between them.