Regulating effect of the halogen atoms on the cofacial dinuclear Schiff base complexes: Synthesis, spectroscopy, electrochemistry, and DFT calculations

Abstract

The present study reports the synthesis and characterization of two new halogenated Schiff base copper (II) derivatives, namely [CuII2(3‐Cl‐Salmphen)2] (Cu‐Cl‐Salmphen) and [CuII2(3‐Br‐Salmphen)2] (Cu‐Br‐Salmphen), which were characterized by UV–Vis, FT‐IR, 1H‐NMR, and 13C‐NMR spectroscopy. Single crystal structural analysis revealed that each complex contains two crystallography independent, distorted copper centers, where each Cu center exhibits the same coordination environment with N2O2 donor atoms from two different ligands. To investigate the substitution effect of halogen atoms on the properties of the complexes, the non‐halogenated isomer [CuII2(H‐Salmphen)2] (Cu‐H‐Salmphen) was introduced for comparison. The differences between the halogenated and the non‐halogenated analogues were compared through spectroscopic, electrochemical, DFT, and TD‐DFT theoretical calculations. The optical experimental results indicated that the introduction of halogen atoms decreased the energy gap between the ground and excited states, leading to enhanced optical absorption capabilities of the compounds. Electrochemical tests showed that the halogenated analogues exhibited better electrochemical performance, and with the increase in an atomic number of the halogen atoms, the compounds displayed a gradual increase in cyclic voltammetry peak areas, diffusion coefficients, and conductivities. Theoretical calculations revealed that the introduction of halogen atoms simultaneously expanded the conjugated area of the molecules, decreased the HOMO‐LUMO energy levels of the complexes, and strengthened the intramolecular interactions. Furthermore, the potential mechanism of the tuning effect of halogen atoms on the charge transfer process was proposed.