To quantify the rapid ISC, we conducted femtosecond TAS (fsTAS) measurement with ~100-fs time resolution (Figure ). The broad transient absorption over 450-650 nm changed to sharp spectra with a peak at 580 nm in a ~10-ps timescale. We globally analyzed the TAS by a sequential model assuming two excited species, resulting in successful fitting with a timeconstant of the transition estimated as 7.9 ps (Figure ,e and Figure ). We also conducted TAS measurements over nanosecond timescales (Figure ), and observed that the TAS shape was unchanged up to the microsecond timescale. The long-lived spectral component was the phosphorescent T1 state, populated via ISC with a time constant of <10 ps for 1a in solution (Figure ). Because the ISC outcompeted other relaxation processes, we can reasonably assume that the ISC quantum yield (ISC) was unity. The total photoluminescence quantum yield can be expressed as PL = f + p, where f is the fluorescence quantum yield. Given that ISC ~1, phosphorescence dominates the total emission, and p ~PL = 0.38. To the best of our knowledge, this is the recordbreaking RTP efficiency for metal-free organic molecules in common solvents. In addition, with ISC ~1 confirmed, p = ISC kp p ~ kp p. For p = 0.38 and p = 72.7 s, kp was derived to be 5300 s -1 , which greatly exceeded those of previously reported organic phosphors, except for thiocarbonyl compounds (Figure and Table ).