In IPA, the T 1 state minium has a planar geometry with slightly elongated C O bond (r C O = 1.23 Å) compared to the ground state geometry (r C O = 1.21 Å) (Figure ). The S 1 state corresponds to the nπ * , whereas lower-lying T 1 and T 2 states correspond to the mixed nπ * /ππ * states at this geometry. The SOCs between the S 1 and T 1 states are 3 cm -1 .