A significant S 1 -T 1 gap at this geometry (0.71 eV), induces negligible rISC rate (section S6 in the SI). The T 2 state features C O stretching (r C O = 1.34 Å) and out-of-plane motion of -COOH group. Similar as at the S 1 geometry, S 1 , T 1 , and T 2 states are nπ * states at this geometry. We note that the T 2 state is the second triplet state at its minimum. A significant SOC between S 1 /T 1 pair (15.4 cm -1 ) and a relatively small energy gap (0.18 eV) induce more efficient rISC from T 2 to the S 1 state with the rate of 523 s -1 , in comparison with the T 1 /S 1 rISC. The T 2 /S 1 rISC rates can increase along the stretching coordinate, as a results of an increase of the SOCs between them. Similar as for TPA, the T 2 /S 1 SOCs significantly increase for r C O > 1.3 Å, reaching the values of 50 cm -1 (Figure ).