A study on complexation of Zn(II) salts with 4-aryl-2-(pyridin-2-yl)quinolines: The influence of anions on the complex structures and their luminescent properties

A novel ligand, 6‑methoxy-4-(4-methoxyphenyl)-2-(pyridin-2-yl)quinoline (PQ) was synthesized and studied for its interaction with ZnX₂.nH₂O (X: Cl^-, OAc^-,${\text{NO}}_3^{-}$) to form three complexes of the type [Zn(X₂)(PQ)(H₂O)_n] (Zn1-Zn3) with high yields, ranging from 75 to 94 %. The structures of Zn1–Zn3 were fully characterized using ESI mass spectrometry, IR and ¹H NMR spectroscopy, and single-crystal X-ray diffraction (for Zn1 and Zn3). Spectroscopic analyses revealed that Spectroscopic analyses revealed that the central atom Zn(II) in Zn1, Zn2, and Zn3 has the coordination numbers of 4, 5, and 6, respectively. In these complexes, Zn(II) coordinates with the PQ ligand through its two nitrogen atoms, while the remaining coordination sites are occupied by anions such as Cl^-, OAc, ${\text{NO}}_3^{-}$, or by H₂O. The results also indicate that the optical properties of the Zn(II) complexes depend not only on the primary organic ligand QP but also on the secondary inorganic ligands. In THF, the complexes Zn1 and Zn3, which contain chlorido and nitrato ligands exhibit intense fluorescence blue emissions at 443 and 462 nm with quantum yields of 0.66 and 0.70, respectively. Although the complex Zn2 has the weakest emission in THF, it exhibited the strongest blue emission intensity in solid state. In addition, computational studies provided insights into the structural and electronic properties, showing a strong correlation with experimental data. The calculated absorption and emission spectra were promarily based on the transition between HOMO and LUMO (π-π*) with high oscillator strength.