The transition in phosphorescence, from a ground singlet state to a excited triplet state is forbidden, it has a much lower probability ,i.e., a slower rate. The transition in fluorescence is singlet to singlet (allowed). Solution The transition in phosphorescence, from a ground singlet state to a excited triplet state is forbidden, it has a much lower probability ,i.e., a slower rate. The transition in fluorescence is singlet to singlet (allowed)..