A novel orange-red thermally activated delayed fluorescence emitter with high molecular rigidity and planarity realizing 32.5% external quantum efficiency in organic light-emitting diodes

Abstract

Simultaneous optimization of photoluminescence quantum yield (Φ_PL) and horizontally oriented dipoles (Θ_‖) is considerably challenging for orange and red thermally activated delayed fluorescence (TADF) emitters, due to the conflicts between enhancing molecular rigidity and improving molecular planarity. Herein, a novel orange-red TADF emitter 10-(dipyrido[3,2-a:2′,3′-c]phenazin-11-yl)-10H-spiro[acridine-9,9′-fluorene] (SAF-2NP) was constructed with a donor–acceptor structure. The highly rigid donor and acceptor segments ensure the overall rigidity of the emitter. More importantly, the quasi-coplanar structure between the acceptor and the fluorene moiety in the donor unit enlarges the molecular plane without weakening rigidity. Consequently, SAF-2NP exhibited extremely high Φ_PL and Θ_‖ of 99% and 85%, respectively. The optimal organic light-emitting diode using SAF-2NP as the emitter and 4,4′-di(9H-carbazol-9-yl)-1,1′-biphenyl (CBP) as the host demonstrated an unparalleled external quantum efficiency of 32.5% and a power efficiency of 85.2 lm W⁻¹ without any extra light extraction structure. This work provides a feasible strategy to establish efficient orange and red TADF emitters with both high rigidity and planarity.