Fine-tuning the luminescent properties of metal-chelating 8-hydroxyquinolines through amido substituents in 5-position

New series of 5-substituted-8-hydroxyquinolines HLn (1–6) bearing aliphatic or aromatic amido groups were synthesised. The chelating ability of these ligands toward the zinc(II) ion was tested and the photophysical characterisation of the resulting complexes (ZnLn)2 · 2H2O (7–12) is reported and compared to those of the uncomplexed ligands. The photophysical data of 1–6 revealed interesting differences between aliphatic (1–3) and aromatic (4–6) amido-substituted species which, however, are no longer evident upon metal complexation. In fact while the ligands 1–3 showed a very high quantum yield (2, λem=470 nm; Φ=0.22) higher than that of the unsubstituted HQ compound, the ligands 4–6 displayed low quantum yield, similar to that of the complexes 7–12, which was in turn lower than that of ZnQ2·2H2O. The behaviour of these compounds is discussed with particular reference to the possibility of controlling the photophysical properties of such compounds through selective modification of the amido substituents.