Room-temperature white and color-tunable afterglow by manipulating multi-mode triplet emissions

Abstract

Herein, we succeed in achieving room-temperature persistent white afterglow by manipulating multi-mode triplet emissions from a single purely organic phosphorescence molecule. Two D–A–A′–D type organic isomers pDCzPyCN and oDCzPyCN are designed and synthesized, with two carbazolyls as the donors and pyridine and a cyanogroup as the acceptors. Amazingly, oDCzPyCN and pDCzPyCN manifest white afterglow and green afterglow at room temperature, which lasts for over 3 s and 2 s, respectively. The white afterglow of oDCzPyCN is made up of thermally activated delayed fluorescence (TADF) (455 nm, ∼90 ms), distinguishable thermally activated delayed phosphorescence (TADP) (483 nm) and organic ultralong phosphorescence (OURTP) from the intermolecular interaction-stabilized triplet state (T₁*) (542 nm and 592 nm, ∼240 ms). The calculated CIEx,y chromaticity coordinates are (0.30, 0.35) in the white-light zone. The green afterglow of pDCzPyCN contains TADF (475 nm, ∼51 ms), TADP from the lowest molecular triplet state (T₁) released from T₁* (490 nm, ∼55 ms), and weak T₁* emission (542 nm and 592 nm, ∼46 ms and ∼49 ms). Fascinatingly, both isomers adopt the unique multi-mode triplet emission mechanism but different emission components play a leading role in the final afterglow for each isomer, leading to the different afterglow colors. Single crystal analyses and TD-DFT calculations evidence the T₁* phosphorescence. Temperature-dependent experiments validate the TADF and TADP of pDCzPyCN and oDCzPyCN. To the best of our knowledge, this is the first time that the afterglow color has been tuned and single component white afterglow has been finally realized by manipulating multi-mode triplet emissions. This work will help gain deep insight into the mechanism for organic afterglow and extend its application scope.