On the Fluorescence Quenching of Substituted Coumarins

In the present work the effect of different concentration of halide quenchers (Cl, Br, I) on 4-methyl-5-ethoxy-7-methoxy coumarin (1) and 4-methyl-5,7-diethoxy coumarin (2) in aqueous solutions have been carried out at 20 temperature. It was observed that the fluorescence intensity of these coumarin derivatives decrease with increase in the concentration of the quenchers except Cl quencher. Further, it was observed that the quenching due to halide ions proceeds via both a diffusional and static quenching process. The rate constants for diffusional as well as static component of quenching process have been calculated using modified Stern–Volmer relation. From the observed data it was found that I ion has very high quenching ability than Br ion and Cl ion behaves almost like a non-quencher. It was interpreted in terms of the sphere of action model by showing that the value of radius of sphere of action of the halide ion quencher is greater than the sum of the radii of the respective coumarin and quencher. Consequently, the order of the quenching ability of the halide quencher was found to be as I>Br>Cl and interestingly this is in the same order as of the ionizing energy of these halide ions. The present quenching process has been attributed to the electron transfer resulting between the colliding species.