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

Abstract

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 UVÀvisible 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.

Exciton Coupling of Surface Complexes on a Nanocrystal Surface

“…The intramolecular and intermolecular interactions may also affect the photonic properties of ZnQ 2 and thus induce a redshift of the p!p* transition. [30,41] This is consistent with the exciton-coupling model. To interpret the 300-350 nm absorption/PLE band of ZnQ 2 , metal-to-ligand charge transfer (MLCT) was assumed.…”

“…The absorption band at 320-460 nm is assigned to the surface complex. [30] The increasing absorption of the surface complex in the sequence of ZnS-Q-1 < ZnS-Q-2 < ZnS-Q-3 again indicates its increasing surface density. The absorption band of the surface complex becomes broader and extends to longer wavelengths with increasing surface density (Figure 2 B).…”

“…To interpret the 300-350 nm absorption/PLE band of ZnQ 2 , metal-to-ligand charge transfer (MLCT) was assumed. [30] For the 300-350 nm absorption/PLE band of AlQ 3 , the localized p!p* transition was suggested. [45] However, neither the MLCT nor the localized p!p* transition can explain the evolution of the Q*'' peak in the PLE spectra of ZnS-Q-1/2/3: the intensity of the Q*'' band relative to that of Q* is rather low for identification at low surface concentration of the surface complex, but becomes stronger with increasing surface concentration of the surface complex.…”

“…It is generally accepted that the strongest optical absorption of ZnQ 2 at about 400 nm is the p!p* transition localized on the Q ligands and that the Zn atom contributes very little to the electron-density distribution of the frontier orbitals. [30,[42][43][44] Thus, Q*" is assigned as the uncoupled excitation state of the surface complex in ZnS-Q NCs. The intramolecular and intermolecular interactions may also affect the photonic properties of ZnQ 2 and thus induce a redshift of the p!p* transition.…”

“…400 nm) observed in the absorption/PLE of the ZnQ 2 complex. [30,31,46] Exciton coupling of uncoordinated 7-hydoxyquinoline or quinoline-derived chromophores in organic compounds [47,48] and of coordinated quinoline chromophores in Cu/Zn complexes has been observed. [49][50][51][52][53] Albrecht et al reported a lanthanum complex with three Q ligands showing exciton coupling of the coordinated Q at 280, 295, 350, and 480 nm in the absorption spectrum.…”