Experimentally Observed Reverse Intersystem Crossing-Boosted Lasing.

Thermally activated delayed-fluorescent (TADF) materials are anticipated to overcome triplet-related losses towards electrically driven organic lasers. Thus far, contributions from triplets to lasing have not yet been experimentally demonstrated owing to the limited knowledge about the excited-state processes. Herein, we experimentally achieve reverse intersystem crossing (RISC)-boosted lasing in organic microspheres with uniformly dispersed TADF emitters. In these materials, triplets are continuously converted to radiative singlets through RISC, giving rise to reduced losses in stimulated emission. The involvement of regenerated singlets in population inversion results in a thermally activated lasing; that is, the lasing intensity increases with increasing temperature, accompanied by accelerated depletion of the excited-state population. Benefiting from the suppression of triplet accumulations by RISC processes, a high-repetition-rate microlaser was achieved.