Optical and electrochemical properties and the calculated structure of pentacoordinate aluminum 8-hydroxyquinoline

In this study we report on the characterization of a series of pentacoordinate aluminum 8-hydroxyquinolines, AlQ₂X (X = F, Cl, Br), composed of two 8-hydroxyquinoline (8-HQ) groups and one halogen ligand. These were prepared by reacting 8-hydroxyquinoline and dialkylaluminum halide stoichiometrically. The λ_maxs of absorption and emission were in the range of 385-388 and 515-516 nm, respectively, which were similar to AlQ₃. The molar absorptivity of AlQ₂X is similar regardless of the X group but emission efficiency of AlQ₂X is 2-3 fold higher than that of AlQ₃ when X = F or Br, but not when X = Cl. This result can be attributed to decreased quenching of energy due to a less steric environment by reducing quinolinate content. The overall molecular orbital structures and the absorption spectra of AlQ₂X and AlQ₃ are very similar due to a single quinolinate unit. The value of the potential difference of AlQ₂Xs between anodic and cathodic waves (ΔE = 3.12 V) is close to the estimated HOMO-LUMO energy gap (the optical band gap, ΔE_optical ∼ 1240/λ_max), 3.19 eV. Detailed optical and electrochemical properties of AlQ₂X are discussed.