The effect of metal electron cloud on the luminescence characteristics of organic ligands: An experimental and theoretical investigation

The luminescence processes of metal complexes are complicated by intramolecular charge (energy) transfer from the metal to the ligand or from the ligand to the metal. The charge transfer strongly influences the excited state of the ligand and its luminescence characteristics. The luminescence characteristics of tris(8-hydroxyquinoline) aluminum (Alq(3)) and tris(8-hydroxyquinoline) gallium (Gaq(3)) are investigated to reveal the effect of the metal ion on the ligand. Emission from the complexes shows a significant red shift as the size of the metal ion increases from Al to Ga because of more efficient charge transfer from the metal to the ligand. Theoretical calculations on the structure and transition characteristics of the excited states of Alq(3) and Gaq(3) were performed. The calculated emission wavelength agrees with the experimental value and the effect of the metal electron cloud on the emission wavelength is clarified.