Triplet state

(atomic physics) Electronic state of an atom or molecule whose total spin angular momentum quantum number is equal to 1.
(quantum mechanics) Any multiplet having three states.

An electronic state of a molecule that occurs when its total spin angular momentum quantum number S is equal to one. The triplet state is an important intermediate of organic chemistry. In addition to the wide range of triplet molecules available through photochemical excitation techniques, numerous molecules exist in stable triplet ground states, for example, oxygen molecules. See also Atomic structure and spectra; Molecular structure and spectra; Spin (quantum mechanics).

A triplet may result whenever a molecule possesses two electrons which are both orbitally unpaired and spin unpaired. Orbital unpairing of electrons results when a molecule absorbs a photon of visible or ultraviolet light. Direct formation of a triplet as a result of this photon absorption is a very improbable process since both the orbit and spin of the electron would have to change simultaneously. Thus, a singlet state is generally formed by absorption of light. However, quite often the lifetime of this singlet state is sufficiently long to allow the spin of one of the two electrons to invert, thereby producing a triplet. See also Molecular orbital theory.