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 λmaxs 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, ΔEoptical ∼ 1240/λmax), 3.19 eV. Detailed optical and electrochemical properties of AlQ2X are discussed.