1. the planar geometry with delocalized -system 2. -symmetric n-orbitals perpendicular to the -system 3. heavy atoms conjugated with both and n-electron systems 4. 3 (n,*) configuration of the T1 state Fullfilling these points would be a promising design principle for molecules with large Sn-S0 and <Sn|HSO|T1> in the same Sn states, and results in kp leap. In addition, the centrosymmetric nonbondingto-antibonding electronic transition of the 1,2-diketone skeleton was the key to the narrowband emission. The fast RTP (large kp) is potentially advantageous for optoelectronics, such as efficient triplet-harvesting OLEDs and scintillators, phosphorescence-based organic lasers, and triplet-to-singlet conversion via Förster resonant energy transfer. We believe that the fast RTP inherent in these systems could radically expand applications of metal-free organics.