Colorimetric recognition of Cu(II) by (2-dimethylaminoethyl)amino appended anthracene-9,10-diones in aqueous solutions: deprotonation of aryl amine NH responsible for colour changes

Chromogenic receptors 2 and 3 undergo distinct colour changes from magenta to blue on gradual addition of Cu(II) and can be used as colorimetric probes for spectrophotometric and visual analysis of Cu(II) in the presence of biological metal ions Na(I), K(I), Mg(II), Ca(II), Fe(II), Zn(II) etc. in aqueous solution (methanol-water 1 : 1 v/v). On addition of Cu(II), both 2 and 3 exhibit a bathochromic shift of Delta lambda(max) = 114 nm for 2 and Delta lambda(max) = 150 and 265 nm for receptor 3. The protonation constants and formation constants of Cu(II) complexes of receptors 2 and 3 (at pH 7) and the effect of pH on formation of these complexes has been determined by the combination of UV-vis-pH titrations of receptors 2 and 3 and their Cu(II) complexes. These results and the emergence of only one peak at 610 nm for 2 and two distinct absorption peaks at 715 and 800 nm for 3 on addition of Cu(II) unambiguously point to mono- and di-deprotonation for 2 and 3, respectively.