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

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.