Near-infrared emissive mononuclear lanthanide(<scp>iii</scp>) complexes based on chiral Schiff base ligands: synthesis, crystal structure, luminescence, and magnetic properties

In this study, novel chiral mononuclear complexes (teaH)[Ln((<i>R</i>,<i>R</i>)-dnsalcd)₂] (Ln = Nd³⁺ (1), Ho³⁺ (2), Er³⁺ (3), Yb³⁺ (4), and Gd³⁺ (5)), where teaH = triethylammonium and (<i>R</i>,<i>R</i>)-H₂dnsalcd = (<i>R</i>,<i>R</i>)-<i>N</i>,<i>N'</i>-bis(3,5-dinitrosalicylidene)-1,2-cyclohexanediamine, were synthesized and characterized. Their structural features were analyzed <i>via</i> single-crystal diffraction, and their chiral and magnetic properties were examined through circular dichroism and magnetic susceptibility, respectively. Structural analysis revealed two distinct coordination modes: complexes 1 and 4 were isomorphous, each exhibiting a single intramolecular π-π stacking interaction, and complexes 3 and 5 were also isomorphous, each displaying two intramolecular π-π stacking interactions. Complexes 1 and 4 show strong near-infrared (NIR) emissions in both solid and solution states, attributed to efficient antenna effects. In contrast, complexes 2 and 3 exhibit weaker NIR emissions with Stark splitting features. Circular dichroism measurements confirmed the chiroptical activity of the complexes, and magnetic susceptibility data revealed typical lanthanide-type paramagnetic behavior. Density functional theory (DFT) calculations for complex 3 support a ligand-to-metal energy transfer mechanism, involving two sequential internal conversion processes.