In this work, we report on the synthesis, in-depth crystal structure studies as well as optical and magnetic properties of newly synthesized heterometallic quaternary selenides of the Eu⁺²Ln⁺³Cu⁺¹Se₃ composition. Crystal structures of the obtained compounds were refined by the derivative difference minimization (DDM) method from the powder X-ray diffraction data. The structures are found to belong to orthorhombic space groups Pnma (structure type Ba₂MnS₃ for EuLaCuSe₃ and structure type Eu₂CuS₃ for EuLnCuSe₃, where Ln = Sm, Gd, Tb, Dy, Ho and Y) and Cmcm (structure type KZrCuS₃ for EuLnCuSe₃, where Ln = Tm, Yb and Lu). Space groups Pnma and Cmcm were delimited based on the tolerance factor t’, and vibrational spectroscopy additionally confirmed the formation of three structural types. With a decrease in the ionic radius of Ln³⁺ in the reported structures, the distortion of the (LnCuSe₃) layers decreases, and a gradual formation of the more symmetric structure occurs in the sequence Ba₂MnS₃ → Eu₂CuS₃ → KZrCuS₃. According to magnetic studies, compounds EuLnCuSe₃ (Ln = Tb, Dy, Ho and Tm) each exhibit ferrimagnetic properties with transition temperatures ranging from 4.7 to 6.3 K. A negative magnetization effect is observed for compound EuHoCuSe₃ at temperatures below 4.8 K. The magnetic properties of the discussed selenides and isostructural sulfides were compared. The direct optical band gaps for EuLnCuSe₃, subtracted from the corresponding diffuse reflectance spectra, were found to be 1.87–2.09 eV. Deviation between experimental and calculated band gaps is ascribed to lower d states of Eu²⁺ in the crystal field of EuLnCuSe₃, while anomalous narrowing of the band gap of EuYbCuSe₃ is explained by the low-lying charge-transfer state. Ab initio calculations of the crystal structures, elastic properties and phonon spectra of the reported compounds were performed.

Terpyridines are well-known ligands in coordination chemistry due to their conforma-tional flexibility and strong metal-binding properties; therefore, they serve as attractive platforms for the design and synthesis of new functional derivatives. This study focused on the synthesis and comprehensive investigation of a new class of bis-oxazolo[5,4-b]pyridine derivatives, designed based on their structural similarity to terpyridines. Four novel compounds 4a–d were synthesized by cyclization of amide de-rivatives of 3-aminopyridin-2(1H)-ones using pyridine-2,6-dicarboxylic acid and its di-chloride as key acidic components. Their structures and purity were confirmed by melt-ing point analysis, high-resolution mass spectrometry, and detailed 1H NMR spectros-copy. Photophysical studies in chloroform, dichloromethane, and acetonitrile revealed that compounds 4a–c exhibit intense blue to deep blue fluorescence (л_max 323–347 nm) with high quantum yields (φi ≈ 0.32–0.84), attributed to р–π* transitions within the con-jugated ring system. These findings suggest their potential as air-stable phosphors for organic electronics. Computational modeling of 4a–c molecules provided insight into their electronic structures, conformational stability, and predicted optical behavior. The most stable conformers (4a–II, 4b–II, 4c–II′) showed decreasing HOMO–LUMO gaps and photoactivity from 4a to 4c, with 4a–II emerging as the most promising fluorophore due to its high symmetry and consistent emission. Overall, this study lays the foundation for future studies of bis(oxazolo[5,4-b]pyridine) derivatives in coordination chemistry and optoelectronic materials development.

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 with 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.

Terpyridines are well-known ligands in coordination chemistry, are valued for their conformational flexibility and strong metal-binding properties, and are also of interest as fluorophores. This study focused on the synthesis and comprehensive investigation of a new class of bis-oxazolo[5,4-b]pyridine derivatives, designed based on their structural similarity to terpyridines. Four novel compounds, 4a–d, were synthesized by cyclization of amide derivatives of 3-aminopyridin-2(1H)-ones using pyridine-2,6-dicarboxylic acid and its dichloride as key acidic components. Their structures and purity were confirmed by melting point analysis, high-resolution mass spectrometry, and ¹H, ¹³C NMR spectroscopy. Compounds 4a–c exhibit UV absorption at 323–357 nm and intense blue to deep-blue fluorescence (357–474 nm, цi ≈ 0.32–0.84) in chloroform, dichloromethane, and acetonitrile, attributed to p–p* transitions within the conjugated ring system. These findings suggest their potential as phosphors for organic electronics. Computational modeling of 4a–c molecules provided insight into their electronic structures, conformational stability, and predicted optical behavior. The most stable conformers (4a–II, 4b–II, 4c–II′) exhibited a progressive decrease in the HOMO–LUMO gap from 4a to 4c, correlated with the enhancement of photoactivity. Among them, compound 4a stands out as the most promising luminophore, displaying the most intense and narrow luminescence band, owing to its high molecular symmetry and stable emission characteristics. Overall, this study lays the foundation for future studies of bis(oxazolo[5,4-b]pyridine) derivatives in coordination chemistry and optoelectronic materials development.