Peripheral-Carbazole-Group-Functionalized Tetradentate Carbene Platinum(II)/Palladium(II) Phosphorescent Complexes: Synthesis, Structures, and Optical Tuning

We designed and synthesized two novel tetradentate carbene ligands, IM-carme and IM-cardtb, both containing a peripheral carbazole group, with methyl and 3,5-di<i>tert</i>-butyl-phenyl groups on their carbene moieties, respectively. Thus, four new tetradentate carbene platinum(II) and palladium(II) complexes (Pt-carme, Pt-cardtb, Pd-carme, and Pd-cardtb) were synthesized. The X-ray structures reveal that the 3,5-di<i>tert</i>-butyl-phenyl group promotes a head-to-tail stacking arrangement and intermolecular metal-metal interactions (Pt-Pt or Pd-Pd), whereas the methyl group leads to a head-to-head stacking pattern and intermolecular π-π interactions. Notably, all complexes emit narrow-band (43-55 nm) phosphorescence ranging from sky blue to deep blue within the wavelength range of 439-502 nm, and the 3, 5-di<i>tert</i>-butyl-phenyl-substituted complexes (Pt-cardtb or Pd-cardtb) have longer emission wavelength, higher emission efficiency, and longer emission lifetime than the corresponding methyl-substituted complexes (Pt-carme or Pd-carme). Theoretical calculations demonstrate that the T₁ state of these complexes exhibits prominent ILCT from the peripheral carbazole group, skeletal carbazole-phenoxy moieties to the benzimidazole and pyridine moieties, as well as MLCT characteristics. Furthermore, the platinum complexes exhibit more pronounced MLCT effects than the corresponding palladium complexes, leading to shorter lifetime and higher efficiency. This research offers novel insights for designing tetradentate carbene platinum(II)/palladium(II) phosphorescent materials featuring peripheral functional groups.