Ligand-to-Ligand Charge Transfer (LL'CT) Transitions in Square-Planar Metal-Bipyridyl Complexes of Heteroatom-Substituted Maltol Derivatives.

A series of square-planar d8-metal complexes that exhibit ligand-to-ligand' charge-transfers (LL'CT) is reported, featuring bipyridines (L') as acceptors and α-hydroxy-thiopyrone (i.e., thiomaltol) derivatives as donors (L). Eleven complexes of the general formula [M(bpy)L]+ were synthesized, where M = Ni(II), Pd(II), or Pt(II); bpy = 2,2'-bipyridine or 4,4'-bis(tert-butyl)-2,2'-bipyridine; and L = 3-hydroxy-2-methyl-4H-pyran-4-thione, 3-hydroxy-2-methyl-4H-thiopyran-4-thione, 3-hydroxy-1,2-dimethylpyridine-4(1H)-thione, and 3-hydroxy-2-methyl-1-phenylpyridine-4(1H)-thione. Structural characterizations include nuclear magnetic resonance (NMR) spectroscopy, electron spray ionization-mass spectrometry (ESI-MS), and single-crystal X-ray diffraction. Photophysical properties were investigated via electronic absorption (UV/vis) and photoluminescence (PL) spectroscopies, and density-functional theory (DFT) computations compared the Frontier Molecular Orbitals (FMOs) for the ground states and Natural Transition Orbitals (NTOs) for CT excited-states. All complexes exhibit donor-to-acceptor LL'CT bands, admixed with metal-to-ligand (bpy) charge transfer (ML'CT). For Pt complexes, excitation into the LL'CT band results in emissions in the yellow-to-red region, attributed to ligand-centered (3LC or 3LL'CT) phosphorescence. In the hydroxypyridine-thione complexes, multiexponential decays suggest that these transitions are admixed with thermally activated delayed fluorescence (1TADF). Excited-state lifetimes fall within the nanosecond regime, with longer-lived components emerging at 77 K, consistent with temperature-suppressed TADF accompanying significant luminescence rigidochromism demonstrated by ∼1700 cm-1 blue-shifted λmax and ∼5-10-fold sensitized PL quantum yields in cryogen-cooled rigid-glassy media vs fluid solutions.