To further analyze the ISC, the reference RO-DFT orbitals at the three different geometries of FC, CI 21,BLA and S 1min are presented in Fig. along with the contributions of each transitions in the parenthesis. It turned out that not only n, π and π * MOs but also n ′ , π ′ and π ′ * MOs are involved in the major transitions of S 1 , T 1 and T 3 states. At the FC structure, the characters of S 1 , S 2 , T 1 , T 2 and T 3 states may be denoted as nπ * , ππ * , ππ * , nπ * and ππ * , respectively. The orbital energy of n at FC is reduced from -9.10 to -9.25 (CI 21,BLA ) and -10.40 eV (S 1min ), which explains well the stability of S 1 and T 2 . Although the T 1 has an entirely ππ * character, however, the π orbital at FC and CI 21 is mostly composed of C=C π bonding, while that at S 1,min is located on oxygen. The character of T 3 is the combination of π ′ π * and ππ ′ * , utilizing the two different oxygen π orbitals. As the BLA increases such as S 1,min structure, the contribution of the former character increases. The lowest singlet excited state S 1 is mostly composed of nπ * with a minor n ′ π * at FC. However, the latter character dominates at S 1,min . Therefore, the significantly decreased SOC values of both S 1 /T 1 and S 1 /T 3 at S 1,min can be attributed to character changes. For example, the SOC between n ′ (S 1 ) and π ′ (T 3 ) can be small due to the small overlap between the corresponding orbitals.