Synthesis, Photophysical, and Electrochemical Characterization of Coumarin–Chalcone Derivatives and Their Al(III), Fe(III), Ni(II), and Zn(II) Complexes

Two new chalcones derived from coumarin and their corresponding Al(III), Fe(III), Ni(II), and Zn(II) complexes were synthesized. UV-visible absorbance spectra showed ligand-centered π → π* transitions, reducing optical band gaps from 3.20 eV for parent coumarin to 2.94-3.16 eV after conjugation. Solvent polarity also affected absorbance bands, whereas metal complexation preserved band gaps but increased absorbance intensity through ligand-to-metal charge transfer. Photoluminescence spectra exhibited blue-green emission within the range of 432-458 nm, influenced by solvent polarity and the degree of conjugation, with diamagnetic Al(III) and Zn(II) amplifying emission (CHEF effect) while paramagnetic Fe(III) induced significant quenching (CHEQ). Fluorescence lifetimes varied between 0.85 and 4.80 ns for the unbound ligands and 1.12 and 4.63 ns for the complexes; the prolonged yet significantly quenched Fe(III) complexes suggested static quenching, whereas the brief-lived, highly emissive Ni(II) complex demonstrated an increased radiative rate. Cyclic voltammetry showed irreversible anodic oxidation, with HOMO levels from oxidation onsets and LUMO from the optical band gap. Chalcone functionalization increased the HOMO level and cycling stability compared to the parent coumarin, whereas metal coordination, especially with Ni(II) and Zn(II), improved charge-transfer activity and stability without changing the band gap. These findings suggest optoelectronic and photonic applications for coumarin-chalcone derivatives and their metal complexes.