Optical, Magnetic, Electrochemical, and Electrical Properties of 8-Hydroxyquinoline-Based Complexes with Al3+, Cr3+, Mn2+, Co2+, Ni2+, Cu2+, and Zn2

Metallic complexes containing the 8-hydroxyquinoline ligand were synthesized to investigate the effect of transition-metal ions on their electronic structure and electrical conductivity. The materials were characterized by UV–visible absorption spectroscopy, cyclic voltammetry, and electrical and magnetic measurements. Our results indicate that the π electrons localized on the ligands do not interact with the d electrons of the metal. The free ligand is electroactive only at very low and very high overpotentials. This trend is maintained when the ligands are part of the complexes. The 3d ions with an open-shell configuration, which are all in a high-spin state, show electron-transfer reactions lying in the middle of the potential window. Nevertheless, once in the solid state, these electroactive ions pin the Fermi level of the injecting electrode and act as charge traps. The mobility of electrons hopping among 3d levels is several orders of magnitude less than that of electrons hopping among the π* orbitals of the complexes with Al or Zn.