Herein, we have focused on a new colorimetric ligand synthesized from the reaction of 2-hydroxy-5-methylbenzene-1,3-dialdehyde with 2-amino-thiophenol, and investigated its activity as a sensor. In this regard, the sensory activity of the ligand towards different ions (Mn²⁺, Cu²⁺, Co²⁺, Fe²⁺, Fe³⁺, Zn²⁺, Ni²⁺, Cd²⁺, Ag⁺, Na⁺, Cs⁺, Mg²⁺, Al³⁺, Ba²⁺, K⁺, and Pb²⁺) was studied. The specificity of ion bindings is discussed through UV–Vis analysis. The ligand that was synthesized showed remarkable sensitivity, with a detection limit of 0.001 ppb. Additionally, the presence of Pb²⁺ ions can be visually detected through a color change from colorless to yellow. In the last part of this work, we seek to predict the available experimental measurements. Density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) are employed to examine the bonding between the ligand and the Pb²⁺ ion. The effect of water solvent was thoroughly examined for all the steps via the conductor-like Polarizable Continuum Model (CPCM). The theoretical findings revealed that electronic properties, including energy gap, adsorption energy, charge/energy transfer, and optical characteristics, undergo significant changes when Pb²⁺ cations are present. Hence, it can be inferred that the newly synthesized chemosensor (NC) is highly efficient in detecting Pb²⁺.

The selectivity of a novel chemosensor, based on a modified nitrobenzofurazan referred to as NBD-Morph, has been investigated for the detection of heavy metal cations (Co²⁺, Pb²⁺, Mg²⁺, Ag⁺, Cu²⁺, Hg²⁺, Ni2+, and Zn²⁺). The ligand, 4-morpholino-7-nitrobenzofurazan (NBD-Morph), was characterized using spectroscopic techniques including FT-IR and ¹H NMR. Vibrational frequencies obtained from FT-IR and proton NMR (¹H) chemical shifts were accurately predicted employing the density functional theory (DFT) at the B3LYP level of theory. Furthermore, an examination of the structural, electronic, and quantum chemical properties was conducted and discussed. DFT calculations were employed to explore the complex formation ability of the NBD-Morph ligand with Co²⁺, Pb²⁺, Mg²⁺, Ag⁺, Cu²⁺, Hg²⁺, Ni²⁺, and Zn²⁺ metal cations. The comparison of adsorption energies for all possible conformations reveals that NBD-Morph exhibits sensitivity and selectivity towards metal ions, including Pb²⁺, Cu²⁺, Ag⁺, and Ni²⁺. However, an assessment of their reactivity using QTAIM topological parameters demonstrated the ligand's greater complexation ability toward Cu²⁺ or Ni²⁺ than those formed by Pb²⁺ or Ag⁺. Additionally, molecular electrostatic potential (MEP), Hirshfeld surfaces, and their associated 2D-fingerprint plots were applied to a detailed study of the inter-molecular interactions in NBD-Morph-X (X = Pb²⁺, Cu²⁺, Ag⁺, Ni²⁺) complexes. The electron localization function (ELF) and the localized-orbital locator (LOL) were generated to investigate the charge transfer and donor-acceptor interactions within the complexes. Electrochemical analysis further corroborates the theoretical findings, supporting the prediction of NBD-Morph's sensory ability towards Ni²⁺ metal cations. In conclusion, NBD-Morph stands out as a promising sensor for Ni²⁺.