Synthesis, characterization, and redox-responsive novel molecular switching behavior of copper (II)–Viologen adduct complexes

This study presents the synthesis and characterization of copper (II) complexes incorporating viologen units, designed to function as redox-responsive molecular switches. The tetradentate N₂O₂ ligand (AN) was first coordinated with copper (II) to produce the CuAN complex, which subsequently underwent axial coordination with pyridine, mono-methylviologen (C₁V⁺·PF₆⁻), and bis-viologen (V₂²⁺·2PF₆⁻), affording the corresponding adducts. A comprehensive set of analytical techniques, including FT-IR, UV-Vis absorption spectroscopy, XRD, TG analysis, mass spectrometry, and cyclic voltammetry, was employed to confirm their formation and investigate their structural and electronic properties. The spectroscopic results verified coordination through C=N and C–O groups, while XRD confirmed the crystalline nature of both the ligand and its complexes, with noticeable variations in d-spacing values due to axial ligation. UV-Vis spectra revealed distinct shifts in absorption bands, indicating enhanced electron delocalization and planarity upon coordination. Thermal analysis established differences in stability, with CuAN–V₂²⁺·2PF₆⁻ exhibiting the highest resistance to decomposition. Electrochemical studies further demonstrated the redox activity of the complexes, particularly the reversible π–dimerization of viologen radicals under chemical and electrochemical reduction. Overall, the obtained results highlight the potential of these copper–viologen adducts as promising candidates for molecular switches, responsive materials, and electrochemical devices.