Thermally Controllable Tuning of Emission Properties of Phenoxazine‐Substituted Acridones as One Step Forward to Efficient Organic Light‐Emitting Diodes Based on Crystalline Emitters

Abstract The overwhelming majority of efficient organic light‐emitting diodes (OLEDs) are based on amorphous emitting layers. The devices with crystalline emitters usually show significantly worse performance. It is demonstrated that controlled thermal treatments of phenoxazine‐substituted acridones may lead to a spectacular improvement of photo‐ and electroluminescence of these luminophores. The three acridone derivatives studied, namely 2‐phenoxazine‐N‐hexylacridone (PHhA), 2,7‐bis‐(phenoxazine)‐N‐hexylacridone (bPHhA), and 2,7‐bis[3‐(phenoxazine)phenyl]‐N‐hexylacridone (PHPhhA), are characterized by relatively narrow photoluminescence spectra and photoluminescence quantum yields approaching 100%. These spectra can be reversibly tuned through appropriate thermal treatment of the deposited layers, undergoing shifts up to 92 nm upon transformation from amorphous to crystalline states. Strong thermally activated delayed fluorescence (TADF) observed for these compounds is manifested by a significantly higher reverse intersystem crossing rate of 1.92×10⁶ s⁻¹ as compared to the intersystem crossing rate of 5.68×10⁵ s⁻¹. These improvements are attributed to the annealing‐induced conformational equilibration due to crystallisation in combination with hosting. Adequacy of the proposed concept is demonstrated by the fabrication of devices exhibiting external quantum efficiencies reaching 20.7% – a record value for OLEDs with crystalline emitting layers. The presented approach of annealing‐induced conformational equilibration can lead to improved TADF properties for other fluorescent organic emitters presently considered inefficient.