Origin of Enhanced Efficiency and Reduced Roll-Off in Organic Light-Emitting Diodes Based on Pt(II) Complexes

Metal-metal-to-ligand charge-transfer (MMLCT) complexes are emerging as next-generation emitters that combine molecular precision with metallic coherence, achieving fast exciton decay and high photoluminescence quantum yields. Despite their promise, the lack of mechanistic insight linking molecular structure to exciton dynamics has hindered rational design. Here, we establish the microscopic origin of MMLCT emission in Pt(II) systems by integrating quantum-chemical modelling, ultrafast spectroscopy, and synchrotron-based x-ray probes to deliver an end-to-end mechanistic picture of MMLCT-state formation and decay. Direct measurements on Pt(fppz)₂ and its alkylated analogue Pt(8ppz)₂ quantify Pt-Pt spacings, revealing that few-ångström variations govern the emergence and coherence of the MMLCT state. Coherently stacked Pt(fppz)₂ aggregates (Pt-Pt ≈3.3 Å) exhibit sub-microsecond exciton decay with nearly complete exciton utilization, whereas Pt(8ppz)₂ (Pt-Pt >5 Å) yields long-lived ligand-centred emission. These contrasting exciton dynamics translate directly into different device performance, with Pt(fppz)₂-based organic light-emitting diodes (OLEDs) achieving ∼29% external quantum efficiency and negligible roll-off, enabling applications in visible-light communication, bioimaging, and transparent displays. Our findings establish a generalizable structure-property relationship for MMLCT emitters, extending beyond Pt(II) to aggregated d⁸-d⁸ and coinage-metal systems, unlocking new opportunities across optoelectronic and photonic technologies.