Colorimetric Sensing of Cu(II) in Aqueous Media with a Spiropyran Derivative via a Oxidative Dehydrogenation Mechanism

A spiropyran derivative containing a simple NO bidentate ligand (2) was synthesized. The ligand acts as a colorimetric chemodosimeter for selective Cu(2+) detection in aqueous media. The ligand, even when treated at elevated temperature (60 °C), exists as a spirocyclic form and shows almost no absorption in the visible region. This is because the electron donation by an amine substituent suppresses spirocycle-opening. Treatment of the solution with Cu(2+) at 60 °C, however, creates a strong absorption at 450-650 nm, whereas other metal cations do not promote spectral change. IR, NMR, and ab initio calculation revealed that Cu(2+) is coordinated with two 2 ligands and produces 1:2 Cu(2+)-amine complex intermediately. This is converted to the 1:2 Cu(+)-imine complex via oxidative dehydrogenation of amine moieties, along with the reduction of Cu(2+). The formation of the imine complex decreases the electron density of nitrogen atoms and weakens the electron donation ability. This thus promotes thermal spirocycle opening and facilitates coloration of the solution.