Continuous Condensed Triplet Accumulation for Irradiance‐Induced Anticounterfeit Afterglow

Abstract Afterglow room‐temperature emission that is independent of autofluorescence after ceasing excitation is a promising technology for state‐of‐the‐art bioimaging and security devices. However, the low brightness of the afterglow emission is a current limitation for using such materials in a variety of applications. Herein, the continuous formation of condensed triplet excitons for brighter afterglow room‐temperature phosphorescence is reported. (S)‐(‐)‐2,2′‐Bis(diphenylphosphino)‐1,1′‐binaphthyl ((S)‐BINAP) incorporated in a crystalline host lattice showed bright green afterglow room‐temperature phosphorescence under strong excitation. The small triplet–triplet absorption cross‐section of (S)‐BINAP in the whole range of visible wavelengths greatly suppressed the deactivation caused by Förster resonance energy transfer from excited states of (S)‐BINAP to the accumulated triplet excitons of (S)‐BINAP under strong continuous excitation. The steady–state concentration of the triplet excitons for (S)‐BINAP reached 2.3 × 10⁻² M, producing a bright afterglow. Owing to the brighter afterglow, afterglow detection using individual particles with sizes approaching the diffraction limit in aqueous conditions and irradiance‐dependent anticounterfeiting can be achieved.