Nature of 1-(2-aminoethylamino)-anthracene-9,10-diones Cu(II) interactions responsible for striking colour changes

Chromogenic sensors 2 on addition of Cu(II) undergo deprotonation at aryl NH to give remarkable colour change from red (lambda(max) 492 nm) to blue (lambda(max) 600 nm). This selective NH deprotonation enables 2 for both naked eye and spectrophotometric determination of Cu(II) even in the presence of other metal ions.