Efficient narrowband RTP from metal-free organic 3-bromo-2thienyl diketones was observed in solution, amorphous polymer matrices, and crystalline solids. Fast RTP, represented by a huge phosphorescence rate constant that was approximately 5,000 s -1 , was the key to outstanding RTP quantum yields in solution (38% under Ar) and in polymer films (up to 54% in air). Ultrafast spectroscopy and single-crystal X-ray structure analysis revealed that the RTP originated from the diketone planar conformation. Both experimental and theoretical analyses indicated that the planar conformer embodies ideal electronic structures for fast RTP: