7‑Hydroxyquinoline-8-carbaldehydes. 2. Prototropic Equilibria

Prototropic equilibria were studied for a series of 7-hydroxyquinoline-8-carbaldehydes (7-HQCs) by 1H NMR spectroscopy, photostationary and time-resolved UV–vis spectroscopic methods, and quantum chemical computations. These molecules represent trifunctional proton-donating/accepting systems that in aqueous solutions may assume four main neutral and ionic structures: 7-quinolinol (OH), 7(1H)-quinolinone (NH), deprotonated anion (A), and protonated cation (C). Electronic absorption and fluorescence of 7-HQCs are rationalized in terms of the ground and excited-state long-range tautomerization (part 1) as well as protonation and deprotonation processes. The photophysical properties of neutral and ionic forms of 7-HQCs are compared with those of 7-hydroxyquinolines (7-HQs), synthetic precursors of the former. The experimental results are corroborated by ab initio computations.