The significance of the diketone skeleton for the huge kp was further evident when comparing 1b with the corresponding bromoaldehyde 2b and diketone without Br atoms 3b (Figure and Table ). All the compounds exhibited RTP in degassed solutions; however, p of 1b was much higher than those of 2b and 3b (24.6 % vs. 0.22% and 1.2%). The superior p of 1b was attributed to its large kp, which was more than one order of magnitude larger than the estimated kps of 2b and 3b. TDDFT calculations reasonably reproduced this trend; the calculated kp for 1a, 2a, and 3a were 5200, 8, and 110 s -1 , respectively (the silyl groups were replaced with H to reduce computation costs). The small kp of 2a was because of the mismatched <Sn|HSO|T1> and Sn-S0 (Table ). Thus, the T1 state of 2a had a 3 (,*) configuration, which coupled strongly to the Sn states with 1 (n,*) configurations (e.g., n = 5; <S5|HSO|T1> was as large as 247 cm -1 ). However, because the 1 (n,*) configuration diminishes Sn-S0 (e.g., |S5-S0| was only 0.22 D), intensity-borrowing was insufficient, resulting in a small kp. Hence, a large <Sn|HSO|T1> itself is insufficient and the combination with a large Sn-S0 in the same singlet states is essential for achieving huge kp. Indeed, the kp of 3a was larger than that of 2a, because the T1 state of 3a had a 3 (n,*) configuration. Thus, 3 (n,*) configurations prefer coupling with 1 (,*) configurations, whose Sn-S0 can be sizable (e.g., n = 3, |S3-S0| = 5.16 D; Table ). However, SOC between those states was not as effective as that for 1a because of the absence of the heavy atom, Br (e.g., <S3|HSO|T1> = 23 cm -1 ). Therefore, intensity borrowing was less effective, yielding a lower kp.