The excitation of a fluorescent molecule from the ground state (S0) to the second singlet excited state (S2) is followed by internal conversion to the first singlet excited state (S1). The molecule in S1 can either decay nonradiatively or emit light in the form of fluorescence to regenerate S0. Alternatively, it can undergo intersystem crossing to populate the first triplet excited state (T1) and then either decay nonradiatively or emit light in the form of phosphorescence to regenerate S0.
