Achieving Visible‐Light‐Excitable Blue TADF‐Type Afterglow via Delicate Control of Excited States in Difluoroboron β‐Diketonate Systems

Abstract Blue afterglow constitutes of one of the primary afterglow colors and can convert into other afterglow colors through energy transfer. The reported studies show the fabrication of blue afterglow emitters, but most of them are formed by room‐temperature phosphorescence mechanism and require UVB lights as excitation source (these high‐energy lights may damage organic systems). Here we report visible‐light‐excitable blue thermally activated delayed fluorescence type (TADF‐type) afterglow materials via delicate control of excited states in difluoroboron β‐diketonate (BF 2 bdk) systems. Tiny change of the substituents in BF 2 bdk system has been found to pose significant influence on excited state energy levels and consequently narrow the singlet‐triplet splitting energy of the system. As a result, both forward and reverse intersystem crossing have been accelerated, leading to the emergence of BF 2 bdk's TADF‐type organic afterglow in rigid crystalline matrices. The resultant TADF‐type afterglow materials exhibit emission lifetimes of several hundred milliseconds, photoluminescence quantum yield (PLQY) of 24.7 % and display temperature responsive property.