N-heterocyclic carbene-based tetradentate Pt(II) phosphors for blue organic light-emitting diodes: Molecular design, photophysics, and device performance

초록

N-heterocyclic carbene (NHC)-based tetradentate Pt(II) complexes have attracted significant attention as promising phosphorescent dopants for blue organic light-emitting diodes (OLEDs), because their rigid coordination frameworks enable narrow emission bandwidths and high color purity. However, square-planar Pt(II) complexes suffer from intrinsic limitations, including limited triplet metal-to-ligand charge transfer (3MLCT), long excited-state lifetimes, and susceptibility to intermolecular interactions in the solid state, which collectively lead to reduced efficiency and operational stability. From a structure-oriented perspective, this review systematically summarizes recent efforts aimed at overcoming these challenges, focusing on Pt(II) phosphorescent emitters based on the NHC-phenyl-O-carbazole-pyridine (NHCPhOCzPy) tetradentate ligand framework as a reference scaffold. The photophysical, electrochemical, and thermal properties are compiled together with key OLED device metrics, including emission wavelength, full width at half maximum (FWHM), Commission Internationale de L'& Eacute;clairage (CIE) coordinates, external quantum efficiency (EQE), efficiency roll-off, and operational lifetime. Furthermore, diverse molecular design strategies are discussed, such as suppression of dopant-dopant and host-dopant interactions through steric engineering, fused-ring extension, and enhanced molecular rigidity; modulation of charge injection and emitting dipole orientation by controlling the substituent and electronic-structure control; and mitigation of intrinsic 3MLCT limitations through incorporating nitrogen into the NHC framework to shorten decay lifetimes and enhance spin-orbit coupling. This comprehensive review also critically assesses the trade-offs among deep-blue color coordinates, high efficiency, and long device lifetime, and it proposes design guidelines for next-generation blue Pt(II) phosphorescent dopants.

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