In TPA, the lowest T 1 and T 2 states at their minima are ππ * states. The computed rISC rates from the T 1 and T 2 state minima (the T 2 state is the lowest triplet state at its minimum) to the S 1 state are negligible (10 -18 s -1 ) at the T 1 minimum and zero at the T 2 minimum), as a consequence of negligible/zero SOCs between (ππ * ) states. To explain the rISC mechanism, we take into account the higher-lying 1 3 nπ * state. At the minimum of this state (T 3 ), the lower-lying T 1 and T 2 states are ππ * states. The T 3 state can get populated due to excess vibrational energy obtained following the S 1 /T 3 ISC. At this geometry, there is a possibility for the rISC to the higher-lying S 2 ππ * state (corresponding to the S 1 state at the FC region) from the T 3 (1 3 nπ * ) state, due to a significant SOC between the states (10 cm -1 ). The computed rISC rate is ∼ 200 s -1 . The model used to compute the rISC rates (Eq. 2) does not take into account vibrational effects on spin-orbit interaction and vibronic coupling between triplet states, which could enhance the rISC rates. Another possible rISC mechanism at the T 3 geometry includes the transition from the ππ * T 1 /T 2 states to the close-lying S 1 (nπ * ) state. The T 2 /S 1 rISC is expected to be efficient, since the ∆E is only ∼ 0.13 eV, while the T 2 /S 1 SOC is 12.4 cm -1 . Moreover, the SOCs increase substantially along the reaction coordinate, surpassing 40 cm -1 for the r C O > 1.38 Å (Figure ), which could further increase the rISC rates.