LAUSR

The mechanism of carbazole (Cz)‐based phosphors is still unclear since its isomer (1H‐benzo[f]indole, Bd) is discovered in 2020. Herein, the successful synthesis of four Cz/Bd derivatives is reported, named as 2CzBr, CzBdBr, 2BdBr, and 3Bd, and the general mechanism for their ultralong organic phosphorescence (UOP) is provided. Bd and its derivatives give double groups of phosphorescence, including the short‐wavelength phosphorescence with a short lifetime and the ultralong phosphorescence at long wavelengths, assigned to their neutral molecules and radical cations, respectively. Interestingly, the doped poly(methyl methacrylate) (PMMA) films of CzBdBr and 2BdBr show photo‐activated ultralong phosphorescence at room temperature. The activation of Bd derivatives ‘UOP involves three factors: 1) well dispersion in the matrix with limited amount, 2) generation of their radical cations and 3) the matrix‐mediated stabilization of radical cations. The function of Cz derivatives to activate the Bd derivatives’ UOP could be replaced by photo‐activation or using other matrixes. Significantly, the application of the doped PMMA films is practiced and gives an exciting result that the high‐resolution QR code could be reversibly printed and erased on the film. This research has expanded the understanding in the field of organic phosphorescence and it may pave a new way for its development.