Deep Blue Tetradentate Pt(II) Emitter Coordinated With Fused Fluorenyl N‐heterocyclic Carbene. High Efficiency, Narrow FWHM, and Superior Operational Lifetime LT₉₅ of 290 h at 1000 cd m⁻²

Blue tetradentate Pt(II) emitters and the corresponding organic light-emitting diodes (OLEDs) are promising for next-generation high-resolution displays. However, developing blue Pt(II) emitters that simultaneously achieve high efficiency, high color purity, and excellent operational stability remains challenging. In this study, we developed two new high-performance deep blue tetradentate Pt(II) emitters (Pt1 and Pt2) whose ligands contain 1-(3,5-di-tert-butylphenyl)-9,9-dimethyl-1,9-dihydrofluoreno[2,3-d]imidazolium or 3-(3,5-di-tert-butylphenyl)-9,9-dimethyl-3,9-dihydrofluoreno[2,3-d]imidazolium carbene moiety (DMFI). These Pt(II) emitters display excellent photophysical characteristics including high photoluminescence quantum yield (PLQY) of up to 0.90, narrow full width at half maximum (FWHM), short emission lifetime, and high horizontal transition dipole ratio of up to 0.80. As a result, deep blue phosphorescent OLEDs are fabricated with a narrow FWHM of 21 nm, a high maximum external quantum efficiency (EQE_max) of 28.6%, and Commission Internationale de l'Éclairage (CIE) coordinates of (0.13, 0.15). Even more impressively, the devices based on the deuterated co-host system and different device structures achieved an unprecedented excellent device lifetime LT₉₅ of 290 h at an initial luminance of 1000 cd m^-2, an EQE_max of 20.8%, and CIE coordinates of (0.14, 0.17).