Here, we disclose the inherent molecular RTP properties of the thienyl diketone derivatives 1: exhibiting high-efficiency narrowband RTP based on exceptionally large kps (Figure ). In particular, 1a exhibited 38.2% p in cyclohexane. To our knowledge, this is the highest efficiency for metal-free organic molecules in common solvents. Relative to benzil, the kp was increased by a factor of 100 from 39 s -1 to 5300 s -1 , enabling in the increase in p from 0.31% to 38% (Figure ). 14c Moreover, high p with narrow ~30-nm FWHM were observed in solution (Figure ). The high efficiency and color purity were also observed in various conventional polymer matrices and crystalline solids, indicating that the RTP was not dependent on specific environmental interactions and was an inherent molecular feature. The RTP mechanism and origin was revealed through single-crystal X-ray structure analysis, time-resolved photoluminescence (TRPL) spectroscopy, transient absorption spectroscopy (TAS), time-resolved infrared (TRIR) spectroscopy, and quantumchemical calculations. The shift from persistent to fast RTP highlighted a potential alternative to precious-metal-based RTP.