Organic RTP must overcome several challenges stemming from the absence of heavy-metal atoms. The most severe is the inherently small phosphorescence transition probability (i.e., rate constant kp), causing poor RTP quantum yields (p). kp can be 10 4 -10 5 s -1 for heavy-metal complexes, 3 and only ~10 s -1 or less for conventional organic compounds. Introducing heavy atoms, such as bromine, iodine, selenium, and tellurium, in conjunction with carbonyl functionalities, is a classical approach to enhancing kp. Despite its long history, however, an organic kp over ~100 s -1 and/or a p over 1% in solution are rarely observed (Figure , blue filled circles; Figure and Table ). The use of thiocarbonyls could provide a large kp in some cases (Fiugre 1a, blue open circles). However, they are associated with high (photo)chemical reactivities, thereby limiting their high p in perfluoroalkane solvents. The only practical way to improve p has been to suppress the nonradiative decay rate (knr). This approach has provided solid-state phosphors with RTP lasting subseconds after excitation cessation (persistent RTP or afterglow). However, it inevitably restricts the organic phosphors to rigid media, such as crystals, or tailored host-guest systems with strong intermolecular interactions. A game-changing strategy for environmentally-independent RTP could be the significant increase in kps. Complementary to the persistent RTP, fast RTP could have significant potential use in solutions or any non-rigid molecular environment, including biological conditions. Moreover, because fast RTP can avoid efficiency loss caused by the accumulation of the triplet excitons, it also could be used in optoelectronics such as OLEDs. Low color purity is another organic RTP challenge. Considering the luminescence of the same quantum efficiency, high-color-purity emission has a sharp and intense spectral peak that is visible, distinguishable, and vivid. These are essential feature for OLED displays and bioimaging. However, organic RTP is usually broad with 80-120-nm full-widths at half-maxima (FWHM)(Figure , Table ). Moreover, because of an inefficient intersystem crossing (ISC) from singlet to triplet states, organic phosphors often exhibit both RTP and fluorescence, further impairing color purity. As a long-known exception, 1,2-diketones, such as biacetyl and benzil, exhibit RTP with small FWHM values (i.e., narrowband RTP). 14 However, the RTP is feeble in solution. Thus, simultaneously achieving a high p and color purity in organic RTP remains a tremendous challenge.