The effect of substituents on the response of 3,4-dihydro-3-(2-oxo-2- phenylethylidene)-quinoxalin-2(1H)-one derivatives toward binding of Cu ²⁺

A new series of fluorogenic chelating reagents based on phenylethylidene-3,4-dihydro-1H-quinoxalin-2-one with different substituents (attached either to the quinoxaline-2-one (3) or phenyl ring (4)) have been investigated to examine the effect of the substituent (nature/position) on the spectral properties and response toward Cu ²⁺ in the presence of other metal cations, in ethanol. It was found that all of the examined ligands exhibit a pronounced response to Cu ²⁺ addition resulting in a red-shift in the UV-vis spectra and a strong quenching in the fluorescence spectra. Among the ligands examined, 3a exhibits the highest selectivity toward various metal cations. In general it appears that the best response selectivity of these ligands toward Cu ²⁺ ion is obtained by either EDG in 3 or EWG in 4. For example, the fluorescence intensity of 3a (with OMe substituent) increases to about three times that of the unsubstituted derivative.