Three colorimetric Cu(II) sensors based on a well-studied benzylideneaniline framework were successfully synthesized, their structural and electronic properties characterized, and their Cu(II)-sensing behaviour evaluated using a combination of spectroscopic and high-quality DFT techniques. Though all metal complexes were highly planar, the presence of the α-CH3 group led to considerable acoplanarity of the rings for all sensors, the extent of which was substituent-dependent. These structural features influence the electronic properties, and by extension, the spectroscopic properties of the compounds and their metal complexes, which, though highly similar, exhibited minor differences as a result of substituent effects. Of the various metal ions tested, all compounds responded uniquely to Cu(II) with which they bound in a 1:1 ratio, producing a distinct colour change from colourless to pink in response due to an MLCT process. Moreover, the binding process for all compounds was highly exergonic (ΔGbind = -20 kJ mol-1 – -26 kJ mol-1) and entropy-driven. The sensors produced detection limits as low as 0.16 ppm, offering greater sensitivity than methods based on FAAS, and gave linear ranges which had upper limits as high as 5 ppm. Whereas only zinc interferes with Cu(II) determination by the unsubstituted derivative, hard-soft interactions allow significant interference from various metals for the other two derivatives. Nonetheless, such competition can be reduced by controlling binding site hardness through substitution effects.

Exploratory, experimental, and DFT study of three small molecular benzylideneaniline-based Cu(II) sensors

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