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 d8-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 3d8 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
triazolatoCOOCH3,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(triazolatoCoumarin,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.