Purely Organic Fluorescence Afterglow: Visible‐Light‐Excitation, Inherent Mechanism, Tunable Color, and Practical Applications with Very Low Cost

Abstract Purely organic materials with visible light excitable fluorescence afterglow are promising for applications. Herein, fluorescence afterglow with various intensity and duration was observed on fluorescent dyes once being dispersed in polymer matrix, thanks to the slow reverse intersystem crossing rate ( k RISC ) and long delayed fluorescence lifetime (τ DF ) derived from the coplanar and rigid chemical structure of the dyes. To verify the mechanism, different polymers were used to tune singlet‐triplet splitting energy based on solvent effect. And commercial acriflavine (Acf) film showed blue shifted fluorescence compared to purified one, with slower k RISC (≈10 0 s −1 ) and longer τ DF (0.6 s). Via energy transfer from Acf to rhodamine B, the afterglow color was further regulated, with the largest fluorescence quantum yield of 42.4 %. It was demonstrated that the materials worked on color tunable light sources, and low‐cost ($2 for 50 000 labels) anti‐counterfeit labels recognized by white light.