Effect of Self-Assembled Structure on the Luminescence of Platinum(II) and Palladium(II) Complexes with a C^N^C Tridentate N-Heterocyclic Carbene

Crystal structures and luminescence properties of square-planar Pt(II) and Pd(II) complexes, [M(bimpy)CN]PF6 (M = Pt, Pd; bimpy = 2,6-bis(3-methyl-1H-imidazol-2-ylidene)pyridine), were investigated by focusing on the effect of the C^N^C tridentate ligand with two N-heterocyclic carbenes on assembly induced luminescence. Both complexes formed similar self-assembled stacked structures with weak intermolecular interactions. The Pt(II) complex exhibited strong orange-red luminescence, which was assigned to carbene-assisted triplet metal–metal-to-ligand charge transfer emission. By contrast, the Pd(II) complex exhibited only weak luminescence at room temperature but strong 3ππ* emission at low temperatures owing to the phase transformation from a parallel to an oblique stacking structure. These findings indicate that even in loosely stacked systems, characteristic luminescence properties emerge from precise molecular arrangements.