Luminescent N^C^N Pincer Ni(II), Pd(II), and Pt(II) Complexes with a Pendant Coumarin Group: The Role of Auxiliary Ligands and Environments

Square-planar <i>d</i>⁸-configured metal complexes and their excited states play a key role in photocatalysis, sensing, and optoelectronic devices. However, metal-to-ligand charge-transfer (MLCT) excited states involving transition metals, particularly those with a 3<i>d</i>⁸ electronic configuration, present challenges due to rapid nonradiative relaxation via low-lying metal-centered (MC) states. In this work, an isoelectronic and isostructural series of cyclometalated complexes [MX(dpb)] with M = Ni(II), Pd(II), and Pt(II), dpb = 1,3-di(2-pyridyl)phenide, and auxiliary ligand X = chlorido, azido, and triazolato were studied by UV/vis absorption, steady-state, and time-resolved photoluminescence spectroscopy in solution and frozen glassy matrix at 77 K, along with DFT calculations. Consistently, the Pd(II) and Pt(II) complexes exhibited a characteristic emission from their triplet ligand-centered (³LC) excited states. In contrast, Ni(II) complexes with auxiliary chlorido, azido, and triazolato^{COOCH3,COOCH3} ligands were nonemissive at low and room temperatures, due to the presence of low-lying MC-type ligand-field excited states. On the other hand, [Ni(triazolato^{Coumarin,COOCH3})(dpb)] showed phosphorescence from the T₅ state in a frozen glassy matrix at 77 K, since the restrictive environment limits structural relaxation, while at room temperature, the primary emission is due to singlet LC excited states from the coumarin moiety of the free ligand.