Synthesis and Solid-State, Solution, and Luminescence Properties of Near-Infrared-Emitting Neodymium(3+) Complexes Formed with Ligands Derived from Salophen

A series of free ligands, H2L1, H2L2, H2L3, and H2L4, designed for the coordination and sensitization of near-infrared(NIR)-emitting Nd3+ were synthesized by modifying the salophen Schiff base with different numbers and locations of Br-substituents. The nature of the Nd3+ complexes in solution was etermined to be [ML2](-) by spectrophotometric titrations as an indication that the different substituents do not affect significantly the nature of the formed species. The structures were determined in the solid phase from X-ray diffraction experiments. The stoichiometries and structures in the solid state are different from those observed in solution. We established that the structures in the solid state can be partially controlled by the crystallization conditions. The ligands L-1 - L-4 have the ability to sensitize Nd3+ through intramolecular energy transfer from the ligand to the metal ion. We quantified that the numbers and locations of Br-substituents control the emitted luminescence intensity of the complex by the heavy-atom effect.