Pyridylthiazoles: Highly Luminescent Heterocyclic Compounds

Absorption, fluorescence, and fluorescence excitation spectra of two substituted [(5-methyl-2-pyridine-2‘-yl-1,3-thiazole-4-yl)oxy]acetic acid and its methyl ester (2,2‘-pyridylthiazoles) are studied at various pH values
in aqueous solution. The acid exhibits pKa(1) = 2.10 ± 0.07 and pKa(2) = 3.45 ± 0.03, whereas the ester
pKa = 1.93 ± 0.03. The protonation site is the pyridyl-nitrogen. When protonated, the cisoid conformer is
the most stable; however, the transoid conformer is more stable in the deprotonated form. Fluorescence quantum
yields close to unity are found. Large Stokes shift values are explained by the shortening of the inter-ring
bond in the excited state. These compounds may be useful for metal sensing and as laser dyes.