We explore possible intersystem crossing mechanisms from the S 1 minima. In TPA, the energy gaps between S/T pairs (∆E) at the S 1 geometry are 0.62 eV for the S 1 /T 1 and 0.22 eV for the S 1 /T 2 pair. Since these states are ππ * states, the spin-orbit couplings between S 1 /T 1 is small (∼4 cm -1 ) and ∼0 cm -1 for S 1 /T 2 pair. The estimated ISC rates, based on the Eq. 2, are negligible for the S 1 /T 1 transition and zero ISC rate for the S 1 /T 2 transition (Table , parameters used for the computations of all rates are given in the SI) at the S 1 geometry. The ISC S 1 (ππ * ) and T 1 /T 2 (ππ * ) states are negligible along the reaction coordinate for the C O lengths below 1.32 Å, as the gaps between states remain significant and spin-orbit couplings are close to zero. However, there is the possibility for the transition from the S 1 ππ * state to the higher-lying nπ * triplet state along the reaction pathway. The S 1 ππ * state crosses with the 1 3 nπ * triplet state at r C O = 1.32 Å. The minimum of the 1 3 nπ * state occurs at r C O ≈1.36 Å (Figure ), after the crossing between ππ * and nπ * states along the reaction coordinate (Figure ). At this geometry, the 1 3 nπ * state (T 3 ) is 0.2 eV above the S 1 vertical excitation. The first two triplet states are ππ * states, whereas the first two singlet states are nπ * and ππ * , respectively. The calculated ISC rate for the S 1 /1 3 nπ * transition at the minimum of the S 1 state is 1.6 × 10 5 s -1 . However, the spin-orbit couplings between these states increase along the reaction coordinate. The ISC rate computed applying the spin-orbit couplings in the vicinity of the crossing point between states (r C O ≈ 1.3 Å) is 9 × 10 5 s -1 , indicating that a part of the population can be transferred to the 1 3 nπ * state, and subsequently to the lower-lying T 1 and T 2 ππ * states.