Transformation of organic afterglow mechanism from room-temperature phosphorescence to thermally-activated delayed fluorescence in intramolecular charge transfer systems

Organic afterglow materials have emerged as an important class of luminescent materials. These afterglow systems feature small rate constants of key photophysical processes, such as phosphorescence decay and nonradiative decay of triplet excited states. When coupled to other photophysical processes, these afterglow systems would give rise to intriguing photophysical behaviors. Here we report the transformation of afterglow mechanism from room-temperature phosphorescence (RTP) to thermally-activated delayed fluorescence (TADF) in intramolecular charge transfer (ICT) systems. Detailed studies reveal that the increase of the acceptor strength of ICT molecules can reduce singlet-triplet splitting energy and consequently open reverse intersystem crossing (RISC) pathway with moderate kRISC of 1-10 s-1 to harvest triplet energies, leading to the RTP-to-TADF afterglow transformation. Such observations provide deep understanding on the behaviors of organic triplets with small photophysical rate constants. Besides, these organic ICT molecules have been incorporated into aqueous emulsion systems to exhibit promising applications.