Controlled shift in the tautomeric equilibrium of 4-((phenylimino)methyl)naphthalen-1-ol

4-((Phenylimino)methyl)naphthalen-1-ol and 4-((phenylimino)methyl)-2-(piperidin-1-ylmethyl)naphthalen-1-ol have been synthesized and their tautomeric properties were investigated using molecular spectroscopy (UV–Vis absorption/emission and NMR), X-ray crystallographic analysis and quantum-chemical calculations. The results show that with the implementation of a flexible piperidine ring a controlled shift in the tautomeric equilibrium will be achieved upon protonation/deprotonation in acetonitrile. The addition of a metal salt also shifts the tautomeric equilibrium, but at very high concentrations, which is caused not by a complex formation, but a special effect of the salt addition.