The integration of slow magnetic relaxation and near-infrared (NIR) luminescence within a single molecular platform is an attractive strategy for developing multifunctional lanthanide materials.

Herein, we report three mononuclear Nd(iii) complexes featuring coumarin ligands and different auxiliary ligands, [Nd(coum)3(phen)(H2O)] (1), [Nd(coum)3(bbp)] (2) and [Nd(coum)2(NO3)(MeOH)3] (3) (coum = 3-acetyl-4-hydroxycoumarinate; phen = 1,10-phenanthroline; bbp = 2,6-bis(benzimidazol-2-yl)pyridine). Single-crystal X-ray diffraction reveals nine-coordinate Nd(iii) centres adopting distorted tricapped trigonal prismatic geometries. All complexes exhibit ligand-sensitized Nd3+ NIR emission, following the intensity trend 2 > 1 > 3 in the solid state. The superior NIR performance of the bbp derivative arises from the presence of three coumarin antenna ligands combined with the suppression of non-radiative quenching associated with the absence of coordinated O–H oscillators. Low-temperature magnetization measurements, supported by ab initio calculations, indicate rhombically anisotropic Nd3+ ground-states with predominantly axial character Alt-text is not available for this image., following the trend 1 > 2 > 3. Ac susceptibility measurements reveal field-induced slow relaxation of the magnetization in all three compounds, associated with two distinct relaxation pathways, with 1 exhibiting a thermally activated process (Ueff/kB ≈ 28 K under 0.4 T).

These results demonstrate that judicious modification of the ancillary ligand environment enables the tuning of both NIR luminescence and magnetic relaxation in coumarin-based Nd(iii) complexes, establishing them as promising bifunctional platforms for magneto-optical sensing, NIR photonics and emerging quantum technologies..