Colorimetric sensing of Cu(II) by 2-methyl-3-[(pyridin-2-ylmethyl)-amino]-1,4-naphthoquinone: Cu(II) induced deprotonation of NH responsible for color changes

1,4-Naphthoquinone based chemosensors 1 and 2 detect only the presence of Cu2+ ions among other transition metal ions by changing color from orange to dark blue. Chemosensors 1 and 2 function as tridentate ligands, which bind Cu2+ ions through three functional groups: an amine nitrogen, a pyridine nitrogen and a quinone oxygen. On addition of Cu2+, both 1 and 2 exhibit a 168 nm red shift in absorption wavelength (pH 7.0). The effect on pH by the formation of these Cu2+ complexes was determined by UV-vis spectroscopic pH titration. In the pH range of 6-7.5, a maximum was observed at 634 nm and exhibited the formation of deprotonated 1-Cu2+ and 2-Cu2+ complexes. A reversible color change (orange-blue-orange) was observed when chemosensor 1 was mixed with Cu2+ in CH3CN. This represents a reversible process of deprotonation and re-protonation of the 1-Cu2+ complexes and also demonstrates the short half-life of thr deprotonated 1-Cu2+ complexes in CH3CN.