Heavy-Atom Driven Enhancement of Solid-State Luminescence in Cd(II)-Halide N-Heterocyclic Chalcogenones

A series of fluorescent cadmium halide complexes incorporating two N-heterocyclic chalcogenone (NHCh) ligands was synthesized and characterized. Coordination of two neutral NHCh ligands to CdX2 (X = Cl, Br, I) through S/Se donor atoms afforded mononuclear four-coordinate complexes with distorted tetrahedral geometry. Systematic variation in the halides or S/Se significantly influences the photophysical characteristics. Notably, the crystalline state emission quantum yield increases progressively from chlorine to iodine, while selone complexes exhibit consistently higher emission quantum yields than those of thiones. Among the series, CdI2(L2)2 displayed the highest photoluminescence quantum yield (PLQY) of 9.1% with an emission lifetime of 41 ns. The charge density difference (CDD) analysis revealed that the excited states are predominantly governed by anthracene-centered intraligand π→π* transitions. The fluorescent properties and their compatibility in the PMMA matrix were demonstrated through an LED coating experiment along with an artistic representation.