Sterically Hindered Tetradentate [Pt(O^N^C^N)] Emitters with Radiative Decay Rates up to 5.3 × 10⁵ s⁻¹ for Phosphorescent Organic Light‐Emitting Diodes with LT₉₅ Lifetime over 9200 h at 1000 cd m⁻²

Abstract Described here are sterically hindered tetradentate [Pt(O^N^C^N)] emitters (Pt‐1, Pt‐2, and Pt‐3) developed for stable and high‐performance green phosphorescent organic light‐emitting diodes (OLEDs). These Pt(II) emitters exhibit strong saturated green phosphorescence (λ_max = 517–531 nm) in toluene and mCP thin films with emission quantum yields as high as 0.97, radiative rate constants (k_r) as high as 4.4–5.3 × 10⁵ s⁻¹ and reduced excimer emission, and with a preferential horizontally oriented transition dipole ratio of up to 84%. Theoretical calculations show that p‐(hetero)arene substituents at the periphery of the ligand scaffolds in Pt‐1, Pt‐2, and Pt‐3 can i) enhance the spin‐orbit coupling (SOC) between the lower singlet excited states and the T₁ state, and S₀→S_n (n = 1 or 2) transition dipole moment, and ii) introducing additional SOC activity and the bright ¹ILCT[π(carbazole)→π*(N^C^N)] excited state (Pt‐2 and Pt‐3), which are the main contributors to the increased k_r values. Utilizing these tetradentate Pt(II) emitters, green phosphorescent OLEDs are fabricated with narrow‐band electroluminescence (FWHM down to 36 nm), high external quantum efficiency, current efficiency up to 27.6% and 98.7 cd A⁻¹, and an unprecedented device lifetime (LT₉₅) of up to 9270 h at 1000 cd m⁻² under laboratory conditions.