The excitation to the S 1 state is followed by a relaxation to its minimum, where fluorescence and intersystem crossing can get activated. According to the PES and the spin-orbit couplings along the reaction coordinate (Figure and 5, the mechanisms involving triplet states are also possible. In the vicinity of the FC region, up to the (C=O) distance of ∼1.3 Å, where the T 1 and T 2 states are close-lying to the S 1 , the S 1 /T 1 and S 1 /T 2 couplings have small values (below 5 cm -1 ), indicating the possibility for slow intersystem crossing.