Synthesis and Optical Properties of Substituted Derivatives of Oxazolo[5,4-b]Pyridine

Abstract

In continuation of our work on the synthesis and study of new properties of 4-substituted 3-aminopyridin-2- (1H)-ones, we carried out the synthesis and subsequent cyclization of the corresponding oxalyl amides. The aminolysis reaction of diethyl oxalate with 3 aminopyridin-2-(1H)-ones was carried out by boiling without solvent at a temperature of 150 °C. In this case, intermediate oxalic acid monoamides with the remainder of the ester group were also recorded and separately identified. It was shown that under the action of phosphorus oxychloride the synthesized oxalic acid diamides undergo fairly smooth intramolecular cyclization into symmetrical bis-derivatives of oxazolo[5,4-b]pyridine. The photoluminescent properties of our newly obtained oxazolo[5,4-b]pyridine derivatives 5-8a-c and 2,2'-bisoxazolo[5,4-b]pyridines 4a-c were studied, including such parameters as maximum absorption (λ), molar absorption coefficient (ε), Stokes shift, and quantum yield. All compounds were found to luminesce with a bluish-blue color and exhibit maximum absorption wavelengths in the range of 299–333 nm (in acetonitrile) and 281–317 nm (in toluene), which is associated ith the π–π* electron transition. A fairly large Stokes shift (83–128 nm) is observed for all compounds. It was also found that the presence of a carboxyl linker at the C-2 position of compounds 5-8a-c does not significantly affect the shift of the absorption band maxima and other spectral characteristics of the molecules. It should be noted that symmetrical conjugated 2,2'-bisoxazolo[5,4-b]pyridines 4a-c, featuring two oxazolo[5,4-b]pyridine rings, exhibit fairly high quantum yield values (φ ≈ 0.70–0.82) compared to the known standard quinine sulfate (φ ≈ 0.55), allowing their potential application as effective fluorophores.