Merging thermally activated delayed fluorescence and two-photon ionization mechanisms for highly efficient and ultralong-lived organic afterglow

It is challenging to simultaneously achieve high afterglow efficiency and long afterglow duration in organic systems. Here we report dopant-matrix materials with afterglow efficiency of 92.4 +/- 10.7 % and afterglow duration up to 1 h under ambient conditions. The materials exhibit thermally activated delayed fluorescence (TADF) mechanism at room temperature to harvest triplet energies and possess afterglow efficiency close to unity. At high excitation dose where two-photon ionization mechanism is switched on, the materials follow a power law decay of excited states due to retarded charge recombination and display hour-long afterglow duration. The TADF mechanism and two-photon ionization are respectively responsible for the high efficiency and hour-long duration. This two-mechanism strategy provides a new pathway to address efficiency-to-duration conflict in afterglow systems. Moreover, the present afterglow materials can serve as efficient donors of Fo center dot rster resonance energy transfer, exhibit excellent processability, and function as background-free bioimaging agents.