Phosphorescence

Phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light. This is in some cases the mechanism used for “glow-in-the-dark” materials which are “charged” by exposure to light. Unlike the relatively swift reactions in fluorescence, such as those seen in a common fluorescent tube, phosphorescent materials “store” absorbed energy for a longer time, as the processes required to re-emit energy occur less often.

Jablonski diagram of an energy scheme used to explain the difference between fluorescence and phosphorescence. The excitation of molecule A to its singlet excited state (1A*)is followed by intersystem crossing to the triplet state (3A) that relaxes to the ground state by phosphorescence.

Quantum mechanical

After an electron absorbs a photon of high energy, it may undergo vibrational relaxations and intersystem crossing to another spin state. Again the system relaxes vibrationally in the new spin state and eventually emits light by phosphorescence.

Most photoluminescent events, in which a chemical substrate absorbs and then re-emits a photon of light, are fast, in the order of 10 nanoseconds. Light is absorbed and emitted at these fast time scales in cases where the energy of the photons involved matches the available energy states and allowed transitions of the substrate. In the special case of phosphorescence, the absorbed photon energy undergoes an unusual intersystem crossing into an energy state of higher spin multiplicity, usually a triplet state. As a result, the energy can become trapped in the triplet state with only classically “forbidden” transitions available to return to the lower energy state. These transitions, although “forbidden”, will still occur in quantum mechanics but are kinetically unfavored and thus progress at significantly slower time scales. Most phosphorescent compounds are still relatively fast emitters, with triplet lifetimes on the order of milliseconds. However, some compounds have triplet lifetimes up to minutes or even hours, allowing these substances to effectively store light energy in the form of very slowly degrading excited electron states. If the phosphorescent quantum yield is high, these substances will release significant amounts of light over long time scales, creating so-called “glow-in-the-dark” materials.

Equation

Phosphorescence

where S is a singlet and T a triplet whose subscripts denote states (0 is the ground state, and 1 the excited state). Transitions can also occur to higher energy levels, but the first excited state is denoted for simplicity.

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