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-NO2Tp–: [Ln(4-NO2Tp)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-NO2Tp– ligands are dominated by intra-ligand charge transfer (ILCT) transitions and that second-sphere interactions are critical to the stabilization of the T1 state of n-NO2Tp– ligands and hence their ability to sensitize Ln3+ emission. The luminescence properties of the complexes indicate that 4-NO2Tp– is a poor sensitizer of Ln3+ emission, unlike 3 NO2Tp–. Moreover, the [Nd(4-NO2Tp)3] (crystallized as a hexane solvate) displays single-molecule magnet (SMM) properties, with longer relaxation times and larger barrier than the non functionalized [NdTp3], attributed to the addition of the NO2-group and subsequent rigidification of the molecular structure.