Coumarin-Based Dual Chemosensor for Colorimetric and Fluorescent Detection of Cu²⁺ in Water Media

A novel coumarin derivative (5) was synthesized and used as a colorimetric and fluorescent probe for selective detection of Cu²⁺ ions in the presence of other metal ions, with the detection limits of 5.7 and 4.0 ppb, respectively. Cu²⁺ ion reacts with probe 5 to form a 1:1 stoichiometry complex, resulting in a remarkable redshift of absorption maximum from 460 to 510 nm, as well as almost completely quenching fluorescence intensity of probe 5 at the wavelength of 536 nm. These changes can be distinctly observed by naked eyes. In addition, the working pH range of probe 5 is wide and suitable for physiological conditions, thus probe 5 may be used for detection of Cu²⁺ ions in living cells. The stable structures of probe 5 and its 1:1 complex with Cu²⁺ ion were optimized at the PBE0/6-31+G(d) level of theory. The presence and characteristics of bonds in compounds were studied through atoms in a molecule and natural bond orbital analysis. The formation of the complex led to a strong transfer of electron density from probe 5 as a ligand to Cu²⁺ ion, resulting in breaking the π-electron conjugated system, which is the cause of fluorescence quenching and color change of 5-Cu²⁺ complex.