Efficient Polaron Recombination and Fast Energy Transfer in a Deep Blue Phosphorescent Pt(II) Complex via Covalently...

Achieving high-efficiency deep-blue phosphorescent organic light-emitting diodes is fundamentally hindered by intermolecular kinetic bottlenecks in polaron recombination and exciton-transfer dynamics. We overcome these limits through a fused emitter architecture where a p-type host and a tetradentate Pt(II) dopant are covalently integrated into a single molecule, Pt-SP-tCz. Despite direct chemical integration, two units retain fully independent photophysical and electrochemical identities, establishing dual recombination sites within a single molecule, significantly enhancing radiative probability. The fused geometry collapses energy donor-acceptor separation to the molecular length scale, enabling exceptionally fast intramolecular energy-transfer pathway-zero-radius intramolecular energy transfer (ZRIET). When paired with an n-type host, the intramolecular p-type host facilitates exciplex formation, creating a second zero-radius pathway that transfers excitons directly to the Pt center-namely, the ZETPLEX mechanism. This dual short-range transfer framework produces a record-high Förster resonance energy transfer rate of 3.64 × 10⁸ s^{- 1}, and a recombination coefficient of 1.12 × 10^-7 cm³ s^-1 unattainable in conventional exciplex systems. A simplified two-component EML achieves an external quantum efficiency of 23.6% with deeper-blue emission and stability comparable to the optimized three-component devices. Our results demonstrate that molecular fusion provides a powerful strategy to simultaneously transcend intermolecular kinetic limits, establishing a new paradigm for high-performance deep-blue emission.