In IPA, two triplet states are in the vicinity of the S 1 state at its minimum (Figure ). The S 1 geometry features significant C O stretching (r C O = 1.44 Å) and out-of-plane motions of carboxylic group (Figure ). As a consequence, the S 1 , T for the S 1 /T 2 transition (Table ). Since the ISC rates to the T 1 state are larger compared to the fluorescence rates from the S 1 minimum (k f ≈1.7 × 10 8 s -1 ), a significant population transfer from the S 1 to T 1 state is expected. According to the reaction pathways (Figure ), the lowest-lying ππ * triplet states (T 1 and T 2 in the FC region) can be populated through internal conversion from the nπ * triplet state. This mechanism involves the planarisation of molecule and contraction of the C O bond.