Synthesis and experimental/theoretical evaluations on redox potentials and electronic absorption spectra for copper symmetric bis(thiosemicarbazone) complexes

Electronic investigation of the effect of substituents on the SOD mimic activity of copper (II) complexes with 8-hydroxyquinoline-derived ligands

Density functional theory (DFT) calculations were used to study the superoxide dismutase (SOD) mimic activity of two Cu²⁺ complexes with ligands derived from 8-hydroxyquinoline (8-HQ). Electron-donating and -withdrawing substituent groups were inserted into the structures to verify changes in the reactivity. The theoretical parameters obtained were compared and validated with the experimental data available. The results showed that the reduction process occurs with greater participation of the 8-HQ ligand and the oxidation step occurs with participation of the copper atom in the complexes, where the electron received during the reduction step is used to reduce the Cu²⁺ to Cu⁺. The calculated electronic affinity showed good correlation with the experimental mimetic activity, and the analysis of this property, of total charge and of molecular orbitals indicated an increase in the mimetic activity with the insertion of electron-withdrawing substituent groups in the structures.

A coumarin derivative-Cu2+ complex-based fluorescent chemosensor for detection of biothiols

Herein, a novel fluorescent sensor has been developed for the detection of biothiols based on theoretical calculations of the stability constant of the complex between a Cu²⁺ ion and (E)-3-((2-(benzo[d]thiazol-2-yl)hydrazono)methyl)-7-(diethylamino) coumarin (BDC) as a fluorescent ligand. In this study, on the basis of density functional theory method, the Gibbs free energy of ligand-exchange reaction and the solvation model were carried out using thermodynamic cycles. The obtained results are in good agreement with the experimental data. The BDC–Cu²⁺ complex can be used as a fluorescent sensor for the detection of biothiols in the presence of non-thiol containing amino acids, with a detection limit for cysteine at 0.3 μM. Moreover, theoretical calculations of excited states were used to elucidate variations in the fluorescence properties. The computed results show that the excited doublet states D₂ and D₁ are dark doublet states, which quench the fluorescence of the complex.

This study opens a new research direction towards the utilization of complexes between metal ions and fluorescent ligands for the detection of biothiols based on the theoretical calculations of stability constants.