Chemiluminescence and photoluminescence:

The internal conversion refers to the transfer of the molecule from an excited singlet state to other while in case of intersystem crossing the molecule in the vibrational states of a singlet excited state crosses over to a vibrational level of a triplet state (rarely from a triplet to singlet). Thus, in both the cases the energies of the initial and final level must be the same.

 Spin multiplicity of an electronic state refers to the number of possible quantum states of the system. It is computed as (2S+1) where S is the total spin quantum number for the electronic state.

The spin multipliticities of the singlet and triplet states are 1 and 3, correspondingly.

 The difference among chemiluminescence and photoluminescence lies in the way the excited states are achieved. Within chemiluminescence, the energy needs for the excitation comes from the chemical reaction while in photoluminescence the energy is provided through a photon (light).The principle explains the phenomenon of activation procedure during the fluorescence and phosphorescence of a molecule.