Reversible Crystal-to-Crystal Phase Transitions with High-Contrast Luminescent Alterations for a Thermally Activated Delayed Fluorescence Emitter

Merging thermally activated delayed fluorescence (TADF) and mechanochromic luminescence (MCL) into one single molecule is a promising strategy for developing multifunctional organic materials. Herein, a unique multifunctional molecule TPA-DQP, comprising a large pi-conjugated diquinoxalino[2,3-a:2',3'-c]phenazine (DQP) as the acceptor and triphenylamine (TPA) as the donor, is designed and synthesized. TPA-DQP possesses polymorphism, efficient TADF emission as well as MCL property with high-contrast in emission colors from 576 to 706 nm. Reversible crystal-to-crystal phase transitions in response to external stimuli such as vapor fuming and heating are realized on the basis of the two polymorphs of TPA-DQP. The distinct crystal-to-crystal phase transition is attributed ultimately to the change of packing arrangements and intermolecular interactions of the two polymorphs under stimuli. Furthermore, TPA-DQP-based organic light emitting diode (OLED) device achieves external quantum efficiency as high as 18.3% at 676 nm, which represents the best performance for deep-red OLEDs based on MCL-active TADF emitters. This study reports a novel MCL-active TADF material that exhibits crystal-to-crystal phase transition and brings insight into the underlying relationship between molecular packing modes and the photoluminescent behavior.

Automated summary

The study introduces a novel multifunctional molecule TPA-DQP, which exhibits polymorphism, efficient TADF emission, and MCL property, with reversible crystal-to-crystal phase transitions in response to external stimuli. The TPA-DQP-based OLED device achieves high external quantum efficiency and represents the best performance for deep-red OLEDs based on MCL-active TADF emitters, providing insight into the relationship between molecular packing modes and photoluminescent behavior.