Ground‐ and excited‐state prototropic tautomerism in anils of aromatic α‐hydroxy aldehydes studied by electronic absorption, fluorescence and ¹H and ¹³C NMR spectroscopies and semi‐empirical calculations

Proton transfer processes in both the ground and excited states in anils of aromatic α‐hydroxyaldehydes (salicylaldehyde, 2‐hydroxynaphthalene‐1‐carbaldehyde and the novel 10‐hydroxyphenanthrene‐9‐carbaldehyde) have been studied by a combination of spectroscopic techniques. Solution 1 H and 13 C NMR is used to establish the position of the tautomeric equilibria. UV‐visible absorption and fluorescence spectral data help to characterize the existence, in all cases, of excited‐state intramolecular proton transfer (ESIPT) pheonmena. Semi‐empirical calculations involving full geometry optimization and calculation of heats of formation for the ground state (AM1) and vertical excitation energies and oscillator strengths (INDO/S) are in agreement with the experimental observations.