Inter‐/Intramolecular <i>π</i> ‐Stacking in a Benzannulated NCN‐Coordinated Pd(II) Monomer and Dimer Pair: Synthesis, Structure, and Photophysics

The development of phosphorescent complexes of second‐row transition metals remains a goal of molecular design, driven by the generally lower cost, higher abundance and thus improved sustainability with respect to their applications. In this work, the synthesis, structures, and photophysical characterization of a mononuclear cyclometalallated palladium complex L PdOAc and its acetate‐bridged dinuclear analogue [( L Pd) 2 ( κ 2 ‐OAc)] + are presented, where L represents an NCN‐coordinated, cyclometallated 1,3‐bis(2‐quinolinyl)benzene. The benzannulated ligand stabilizes the dinuclear derivative through ligand–ligand interactions between π ‐systems, which is further facilitated by the acetate adopting a κ 2 ‐binding mode bridging the two Pd(II) centres. The mononuclear (but not the dinuclear) complex is weakly phosphorescent in the deep red region of the spectrum ( λ max = 676 nm) in solution at room temperature, while both compounds phosphoresce in frozen media at 77 K with essentially identical profiles, emission maxima, and lifetimes of 180 μs. Analysis of the photophysical properties and electronic structures of both monomer and dimer are supported by density functional theory (DFT) and time‐dependent DFT (TDDFT) simulations.