New Sensitive and Selective Chemical Sensors for Ni 2+ and Cu 2+ Ions: Insights into the Sensing Mechanism through DFT Methods

We report the synthesis and theoretical study of two new colorimetric chemosensors with special selectivity and sensitivity to Ni 2+ and Cu 2+ ions over other metal cations in the CH 3 CN/H 2 O solution. Compounds ( E )-4-((2-nitrophenyl)diazenyl)- N, N -bis(pyridin-2-ylmethyl)aniline (A) and ( E )-4-((3-nitrophenyl)diazenyl)- N, N -bis(pyridin-2-ylmethyl)aniline (B) exhibited a drastic color change from yellow to colorless, which allows the detection of the mentioned metal cations through different techniques. The interaction of sensors with these metal ions induced a new absorption band with a hypsochromic shift to the characteristic signal of the free sensors. A theoretical study via time-dependent density functional theory (TD-DFT) was performed. This method has enabled us to reproduce the hypsochromic shift in the maximum UV–vis absorption band and explain the selective sensing of the ions. For all of the systems studied, the absorption band is characterized by a π → π* transition centered in the ligand. Instead of Ni 2+ and Cu 2+ ions, the transition is set toward the σ* molecular orbital with a strong contribution of the 3d x 2 - y 2 transition (π → 3d x 2 - y 2 ). These absorptions imply a ligand-to-metal charge transfer (LMCT) mechanism that results in the hypsochromic shift in the absorption band of these systems.