Abstract
The fluorescence behavior of 7-hydroxycoumarin and 7-hydroxy-4-methylcoumarin has been studied in acidic aqueous solutions by changing the concentration of chloride ions as a fluorescence quencher. The intensity decrease and lifetime change in fluorescence from neutral and tautomeric forms of photo-excited states are explained by an interaction between protonated molecules generated in excited state and chloride ions. The magnitude of quenching-rate constant is estimated under steady-state conditions and the result is compared with the analysis using time-dependent response.

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