Enhancement of Emission Efficiency and Color Tuning by Mixing Platinum(II) and Palladium(II) Complexes in a One‐dimensional Chain

By mixing isomorphous Pt(II) and Pd(II) complexes, which have very low emission efficiencies at room temperature, in pure crystals, a series of mixed crystals with high emission efficiencies and luminescent colors depending on the mixing ratio were successfully constructed. The characteristic luminescence of the mixed crystals originates from excited oligomers with different Pt/Pd ratios.

Abstract

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.

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