Synthesis of a Photoluminescent Ni(II) Complex With a Rigid Tetradentate Dicarbene Diphenolate Backbone

The development of photoluminescent Ni(II) complexes has recently attracted growing interest. However, it is highly challenging, and photoluminescent Ni(II) complexes are very rare. In this work, a square-planar Ni(II) complex bearing a rigid tetradentate naphthyridine-fused dicarbene diphenolate ligand, Ni(NpC₂O₂), was designed and synthesized. The complex exhibits blue ligand-originated fluorescence in DCM at room temperature. At 77 K, it shows weak phosphorescence at 515 nm with a lifetime of 51.8 μs in a frozen 2-MeTHF matrix. Based on the experimental studies and DFT/TD-DFT calculations, it is plausible that the phosphorescence originates from the T₇/T₈ state(s) via a high-lying intersystem crossing (HISC) process from the S₃ state. The assignment of anti-Kasha phosphorescence is in line with the excitation-dependent phosphorescence at 77 K. This work demonstrates that rigid and fused carbene ligands might be promising to light up Ni(II) complexes.