Exciton Dissociation and Recombination Afford Narrowband Organic Afterglow Through Efficient FRET

Abstract Organic afterglow with long‐persistent luminescence (LPL) after photoexcitation is highly attractive, but the realization of narrowband afterglow with small full‐width at half‐maximum (FWHM) is a huge challenge since it is intrinsically contradictory to the triplet‐ and solid‐state emission nature of organic afterglow. Here, narrow‐band, long‐lived, and full‐color organic LPL is realized by isolating multi‐resonant thermally activated delayed fluorescent (MR‐TADF) fluorophores in a glassy steroid‐type host through a facile melt‐cooling treatment. Such prepared host becomes capable of exciton dissociation and recombination (EDR) upon photoirradiation for both long‐lived fluorescence and phosphorescence; and, the efficient Förster resonance energy transfer (FRET) from the host to various MR‐TADF emitters leads to high‐performance LPL, exhibiting small FWHM of 33 nm, long persistent time over 10 s, and facile color‐tuning in a wide range from deep‐blue to orange (414–600 nm). Moreover, with the extraordinary narrowband LPL and easy processability of the material, centimeter‐scale flexible optical waveguide fibers and integrated FWHM/color/lifetime‐resolved multilevel encryption/decryption devices have been designed and fabricated. This novel EDR and singlet/triplet‐to‐singlet FRET strategy to achieve excellent LPL performances illustrates a promising way for constructing flexible organic afterglow with easy preparation methods, shedding valuable scientific insights into the design of narrow‐band emission in organic afterglow.