Tuning the optical and magnetic properties of lanthanide single-ion magnets using nitro-functionalized trispyrazolylborate ligands.

We report the synthesis, crystal structures, photophysical and magnetic properties of 11 novel lanthanide complexes with the asymmetrically functionalized trispyrazolylborate ligand 4-nitrotrispyrazolylborate, 4-NO 2 Tp - : [Ln(4-NO 2 Tp) 3 ] (Ln = La-Dy, except Pm). In-depth photophysical characterization of the ligands via luminescence, reflectance and absorption spectroscopic techniques, decay lifetimes, quantum yields supported by time-dependent density functional theory (TD-DFT) and natural bond order (NBO) analysis reveal that n -NO 2 Tp - ligands are dominated by intra-ligand charge transfer (ILCT) transitions and that second-sphere interactions are critical to the stabilization of the T 1 state of n -NO 2 Tp - ligands and hence their ability to sensitize Ln 3+ emission. The luminescence properties of the complexes indicate that 4-NO 2 Tp - is a poor sensitizer of Ln 3+ emission, unlike 3-NO 2 Tp - . Moreover, [Nd(4-NO 2 Tp) 3 ] (crystallized as a hexane solvate) displays single-molecule magnet (SMM) properties, with longer relaxation times and larger barrier than the non-functionalized [NdTp 3 ], attributed to the addition of the NO 2 -group and subsequent rigidification of the molecular structure.