According to the El-Sayed's rule, in order to compensate for the momentum change caused by electron spin reversal, it is necessary for an electron to jump in a mutually perpendicular orbit to balance the momentum change. Therefore, the initial and final states must have different transition properties to make ISC take place. But unfortunately, both S 1 and T 1 states of 7-PhQAD mainly have (π, π * ) character, indicating that the ISC and RISC between S 1 and T 1 state are nearly forbidden if only direct SOC is taken into account. The T 2 and T 3 are almost degenerate and close to S 1 in terms of energy (Table ), along with the large SOCs of S 1 -T 2 and S 1 -T 3 , suggesting that the population transition via S 1 -T n (n=2,3) may potentially play a crucial role in S-T crossings.