Ground and lowest excited triplet state dissociation constants of the 5-, 6- and 7-mononitro-8-quinolinols. Spectroscopic evidence for intramolecular hydrogen bonding in the lowest triplet state of 7-nitro-8-quinolinol

Abstract— The dissociation constants of 5-nitro-8-quinoIinol, 6-nitro-8-quinolinol and 7-nitro-8-quinolinol have been determined for the ground states of the molecules by potentio-metric titration, and for the lowest triplet states from the shifts of the O—O bands of phosphorescence occurring upon dissociation. The ground-state dissociation constants of the phenolic groups are found to be typical of ortho, meta, and para nitro phenols. The phosphorescences of all prototropic species derived from the 6-isomer appear to be anomalous and their origin is unknown. The anomalously low acidity of the neutral species of the 7-isomer in its triplet state, relative to that of the neutral species of the 5-isomer in its triplet state, is attributed to intramolecular hydrogen bonding in the lowest triplet sfate of the 7-isorner, between the phenolic proton and the 7-nitro group. The magnitudes of the two triplet-state disSociation constants of both the 5 and 7 isomers suggest that the predominant uncharged species in fluid solutions containing the triplet-state molecules are the excited zwitterions.