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

Abstract

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.