Luminescence, the emission of light by a substance that has absorbed energy, is a process that involves the interaction of molecules with light. The energy-level diagram, or Jablonski diagram, is a graphical representation of these interactions, illustrating the various states and transitions a molecule can undergo. In a typical Jablonski diagram, the lowest horizontal line represents the ground-state energy of the molecule, which is usually a singlet state. This state represents the energies of most molecules in a solution at room temperature. The upper lines represent vibrational energy levels of three excited electronic states: the first and second electronic singlet states, and the first electronic triplet state.

Each of these four electronic states is associated with numerous vibrational energy levels. Absorption transitions can occur from the ground singlet electronic state to various vibrational levels of the excited singlet electronic states. The triplet transition involves a change in multiplicity, so it has a very low probability of occurrence. An excited molecule can return to its ground state through several mechanistic steps. Two of these steps, fluorescence, and phosphorescence, involve the emission of a photon, while others are radiationless processes. The favored route to the ground state is the one that minimizes the lifetime of the excited state. Therefore, if deactivation by fluorescence is rapid compared to the radiationless processes, such emission is observed. Conversely, fluorescence is either absent or less intense if a radiationless path has a more favorable rate constant.

Molecules excited to the first and second electronic singlet states rapidly lose any excess vibrational energy and relax to the ground vibrational level of that electronic state, through a nonradiational process called vibrational relaxation. The term internal conversion describes intermolecular processes that leave the molecule in a lower-energy electronic state without the emission of radiation. These processes are neither well-defined nor well-understood but are often highly efficient. Internal conversion can occur between two states of the same multiplicity (singlet-singlet or triplet-triplet), especially when two electronic energy levels are sufficiently close for there to be an overlap in vibrational energy levels.

Intersystem crossing, another deactivation process, is a crossover between electronic states of different multiplicity. Like internal conversion, the probability of intersystem crossing is enhanced if the vibrational levels of the two states overlap. Intersystem crossing is most common in molecules that contain heavy atoms, such as iodine or bromine, due to increased spin and orbital interactions.

Vibrational relaxation, internal conversion, external conversion, and intersystem crossing are all forms of radiationless deactivation where a molecule in an excited state loses energy without emitting a photon. On the other hand, fluorescence and phosphorescence involve the emission of a photon. In both cases, the molecule returns to a lower energy electronic state, but the difference lies in the multiplicity of the states involved and the lifetime of the excited state. In summary, the deactivation processes in luminescence are complex and involve a combination of radiative and non-radiative transitions. The efficiency of these processes can greatly affect the observed luminescent properties of a material.