Fluorescence characteristics of Schiff bases derived from amino- and aminoalkylpyridines

The fluorescence characteristics of the Schiff bases 2-(3- pyridylmethylimi nomethyl)phenol (1), 2-(2-pyridyliminomethyl)phenol (2), N,N'-bis(salicylid ene)-2,3-pyridinediamine (3), N,N'-bis(salicylidene)-2:6-pyridinediamine (4 ) and 2-(2-amino-4-methoxymethyl-6-methyl-3 -pyridylmethyliminomethyl)pheno l (5) were studied in various solvents at different pH values. Correspondin g quantum efficiencies were determined. Compound 1, which showed a tendency towards tautomeric interconversion to ketoamine in polar protic solvents w as not fluorescent at pH < 8. The fluorescence of other compounds was very sensitive to solvent polarity and the pH of the medium. Compounds 2 - 4, pr eferably present as enolimines in all solvents, were not fluorescent in non -polar and moderately polar solvents, whereas weak emission was observed in polar solvents, like methanol, dimethylformamide and dioxane/water 1/1 (0. 001 < Q < 0.072). A significant increase in Stokes shifts and in quantum ef ficiencies was noted as a result of increasing polarity of dioxane/water mi xtures, indicating specific interactions with polar water molecules The emi ssion Has promoted at acidic pH values where a pyridinium cation was found (0.061 < Q < 0.519. in dioxane/water 1/1 at pH 3.4) Compound 5. which was a tautomeric mixture of enolimine and cyclic diamine in all solvents. was fl uorescent in polar as well as in non-polar media The quantum efficiency) va ried dependent on the: solvent and pH (0.023 < Q) < 0.435). The cyclic diam ine. i. e, the more rigid structure was supposed to be responsible for the fluorescence in non-polar and aprotic solvents as well as at neutral and we akly basic pH values.