Phosphorescence: Encyclopedia II - Phosphorescence - Details

Phosphorescence - Details

Electrons arranged in atomic configurations or more complex molecular orbitals group into pairs which follow the Pauli exclusion principle. Only singlet electrons can populate a single energy level, or orbital. The available orbitals and allowed (i.e. favored) transitions between orbitals are determined by the rules of quantum mechanics and modeled in the field of computational chemistry. Each possible orbital is associated with a set of quantum numbers, and allowed transitions involve only certain prescribed changes in quantum numbers between states. The allowed energy transitions of the material then determine the preferred absorbance and emission spectra, i.e. the wavelengths of light (or electromagnetic radiation) that easily interact with the system.

One measure of quantum numbers is the spin multiplicity (for a quick definition see term symbol). A singlet excited state results when an electron is promoted while conserving its spin (an "allowed" transition). Relaxation back to the ground state is very fast because the multiplicity doesn't change. Transition to the triplet state involves a "forbidden" spin flip (electrons cannot exist in between the two states in a molecule) to produce the triplet. This phenomenon is known as inter system crossing (ISC) and is kinetically slow, but thermodynamically favorable (the triplet is lower in energy). Once in the triplet state, relaxation back to the ground state necessarily involves another spin flip to avoid violating the Pauli exclusion principle, which is again kinetically slow, but thermodynamically favorable. In some cases this energy is dissipated by the emission of a photon corresponding to the energy difference between the triplet state and ground state, but often it is dissipated vibrationally (see phonon). The ratio between these two phenomena for a single molecule is known as the quantum yield of phosphorescence. Since the triplet state is lower in energy than the singlet excited state, the light is lower in energy (red-shifted) than if it had been emitted from a singlet excited state. Many compounds emit both from the singlet and triplet states and by measuring the difference in wavelength between the two the energy difference between the excited states can be calculated.

There are many facets to emission from triplet excited states and many people have spent entire careers studying the phenomenon. As one example, a process known as delayed fluorescence occurs when two triplets encounter each other (by delocalization in the solid state or encounter of two species in solution or the gas phase) and additively annihilate to produce one singlet excited state of higher energy. If this state then emits light it will be of the shorter wavelength associated with fluorescence emission, but on a time scale appropriate for phosphorescence.

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