Continuous Condensed Triplet Accumulation for Irradiance-Induced Anticounterfeit Afterglow

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 Forster 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 x 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. Continuous triplet accumulation over 1 wt.% in a host-guest crystalline material showing bright green afterglow emission is achieved due to suppressed Forster resonance energy transfer for accumulated triplet excitons under strong excitation. The medium, as well as a common green afterglow emitter, exhibits a promising irradiance-induced appearance for high-resolution afterglow imaging, providing novel anticounterfeiting capabilities.image