Accepted: 28 July 2026
A comprehensive investigation of the relationship between molecular structure, excited-state dynamics and organic light-emitting diode (OLED) performance is presented using square-planar Pt(II) complexes. The study focuses on the benchmark metal–metal-to-ligand charge-transfer (MMLCT) emitter Pt(fppz)₂ and its structurally modified analogue Pt(8ppz)₂, in which electron-withdrawing CF₃ groups are replaced by n-octyl chains. By combining quantum-chemical modelling, temperature-dependent extended X-ray absorption fine structure (EXAFS), grazing-incidence wide-angle X-ray scattering (GIWAXS), steady-state and time-resolved photoluminescence, and femtosecond transient absorption spectroscopy, the work establishes the critical role of short-range Pt–Pt interactions in controlling MMLCT-state formation and excited-state dynamics.
Structural analyses reveal that Pt(fppz)₂ forms a highly ordered and closely packed molecular arrangement with short Pt–Pt separations of approximately 3.32 Å, whereas Pt(8ppz)₂ exhibits a more disordered structure with Pt–Pt distances exceeding 5.5 Å. The short Pt–Pt separation in Pt(fppz)₂ enables strong orbital overlap and efficient MMLCT formation, resulting in fast exciton decay, near-unity photoluminescence quantum yield and coherent Pt–Pt vibrational motion. In contrast, the increased intermolecular separation in Pt(8ppz)₂ suppresses MMLCT formation and leads to slower ligand-centred and excimer-like emission.
The distinct photophysical behaviour of the two complexes translates directly into their OLED performance. Pt(fppz)₂-based OLEDs achieve a peak external quantum efficiency (EQE) of approximately 29% and maintain an EQE of about 23% at a luminance of 5000 cd m⁻² with minimal efficiency roll-off. The devices also retain an EQE above 10% at 35,000 cd m⁻² and above 5% at 80,000 cd m⁻², demonstrating excellent performance at high brightness. In comparison, Pt(8ppz)₂-based devices exhibit substantially lower efficiency and pronounced roll-off at increasing luminance.
The findings demonstrate that precise control of Pt–Pt molecular interactions provides a powerful strategy for optimizing exciton dynamics, charge transport and electroluminescence in Pt(II)-based OLED materials. The study establishes a clear structure–property–performance relationship and highlights Pt–Pt engineering as an effective approach for developing high-efficiency OLEDs with enhanced brightness, reduced efficiency roll-off and improved operational robustness.