8-Hydroxyquinoline (HQ)

8-Hydroxyquinoline is an organic compound with the formula C9H7NO. It is a derivative of the heterocycle quinoline by placement of an OH group on carbon number 8. This light yellow compound is widely used commercially, although under a variety of names. 8-Hydroxyquinoline is a monoprotic bidentate chelating agent. Related ligands are the Schiff bases derived from salicylaldehyde, such as salicylaldoxime, salen and salicylaldehyde isonicotinoylhydrazone (SIH).

The reaction of 8-hydroxyquinoline with aluminium(III) results in Alq3, a common component of organic light-emitting diodes (OLED's). Variations in the substituents on the quinoline rings affect its luminescence properties. The roots of the invasive plant Centaurea diffusa release 8-hydroxyquinoline, which has a negative effect on plants that have not co-evolved with it.