Knowledge Bank

“The expected phosphorescence of salicylaldehyde and other aromatic compounds that contain a hydroxy group ortho to a carbonyl group does not appear immediately in some solvents at $77\circ K$. Compounds that contain this arrangement of atoms, called the conjugated chelale system, have been found by others to have abnormal infrared and ultraviolet absorptions. The phosphorescence of salicylaldehyde is delayed in hydrocarbon solvents and is immediate in ethanol and some other solvents. The difference is ascribed to the existence of the intramolecular hydrogen bonded conjugated chelate system in the hydrocarbon solvent and the non-existence of the conjugated chelate system when the molecule is externally hydrogen bonded in ethanol; the phosphorescence is that of a normal aromatic carbonyl. The appearance and the nature of the spectra of o-methoxybenzoic acid, substituted salieylaldehydes, and salicylic acid and its derivatives in various solvents are consistent with this interpretation of the delay of phosphorescence.”