Enhancement of Emission Efficiency and Color Tuning by Mixing Platinum(II) and Palladium(II) Complexes in a...

A series of mixed metal crystals comprising square-planar platinum(II) and palladium(II) complexes, K[M(CN)₂(dFppy)]·H₂O (M = Pt²⁺, Pd²⁺; dFppy = 2-(4,6-difluorophenyl)pyridinate), were prepared, and their unique luminescence properties-emerging upon mixing-were revealed. Seven mixed crystals with Pt/Pd ratios of 1:0.5, 1:1, 1:1.5, 1:2, 1:4.5, 1:10, and 1:13 adopt the same crystal structure (P2₁/c) as the pure metal complexes, in which the metal complexes are stacked with short M⋯M contacts (3.35-3.37 Å at 293 K). Reflecting the M⋯M interactions, the mixed crystals exhibited luminescence attributable to the emission from the triplet metal-metal-to-ligand charge transfer (³MMLCT) state. The emission color varied with the Pt/Pd mixing ratio, and notably, a significant enhancement in emission quantum yield was observed (e.g., Φ = 0.64 for the Pt/Pd = 1:2 crystal at room temperature). The emission efficiency further improved at temperatures above room temperature. These unprecedented emission properties are explained by energy transfer among multiple emission sites within the one-dimensional chain, involving excited oligomers containing various combinations of Pt and Pd complexes. Thus, metal mixing offers a new strategy for enhancing emission efficiency and enabling color tuning.