The natural transition orbitals (NTOs) of the S3 and T1 states provided an intuitive understanding of the large S3-S0 and <S3|HSO|T1> (Figure ). First, the S3 state (S3-S0 transition) had a pure 1 (,*) configuration, where the NTOs delocalized over the entire molecule in a planar geometry, which was favorable for large . On the other hand, the T1 state had a pure 3 (n,*) configuration, which enabled SOC with the pure 1 (,*) state, consistent with El Sayed's Rule. The pure (n,*)/(,*) characters are noteworthy, because excited states with mixed (n,*) and (,*) configurations diminish SOC. Most importantly, the highest-occupied NTOs (HONTOs) of the S3 and T1 states around the heavy atoms, especially Br, were mutually perpendicular. This is ideal because the spin-orbit Hamiltonian HSO involves the orbital angular-momentum operator that rotates the orbitals 90°. Thus, for the HONTO-constituting atomic orbital of Br, the in-plane -symmetric n orbital of T1 states became -symmetric, producting a significant spatial integral with the out-of-plane p orbital of the S3 state. The large spatial integral near the heavy nuclei boosts the heavy-atom effect, resulting in a large <S3|HSO|T1> = 167 cm -1 . Thus, 1 embodies an ideal electronic structure for intensity-borrowing that enhances kp: (1) a planar -system that produces large ; (2) carbonyl groups in the -plane that joint the pure 1 (,*) and pure 3 (n,*) states; and (3) heavy atoms with electrons conjugated to both the  and -/n-systems to boost <Sn|HSO|T1>.