Triplet state - triplet state

In a multi-electron atom or molecule, the sum of the quantum numbers of the spin angular momentum of all electrons (composite spin angular momentum quantum number S ) is 1. In this case, the multiplicity (2 S + 1) is 3, and it is called a triplet state. This state occurs only when the system has an even number of electrons and has two unpaired electrons. In other words, when the electrons in the system are divided into a pair of electrons whose spin directions are antiparallel to each other, two electrons with parallel spins remain in the end. These electrons with parallel spins cannot enter the same spatial orbital according to the Pauli principle, and are assigned to different spatial orbitals. The ground state of a molecule with an even number of electrons is generally a singlet state, but when two electrons are assigned to a degenerate highest occupied orbital, such as in oxygen molecules, the electrons are assigned to different orbitals with parallel spins according to Hund's rule, and some molecules have a triplet ground state. In general, the excited triplet state is lower than the excited singlet state, and is important in understanding the properties of the excited states of substances. In particular, phosphorescence of organic molecules is explained as radiation accompanying the transition from an excited triplet state to the ground singlet state, and is also used to explain the mechanisms of energy transfer and photochemical reactions.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

多電子原子や分子において，全電子のスピン角運動量の量子数の和(合成スピン角運動量量子数S)が1である状態．このとき，多重度(2S ＋ 1)は3となり，三重項状態といわれる．この状態は，系の電子が偶数個で不対電子を2個もつ場合にのみ生じる．つまり，系の電子を，スピンの方向が互いに反平行であるような2個の電子の組に分けるとき，最後に平行スピンをもつ2個の電子が残る場合である．この平行スピンをもつ電子は，パウリの原理によって同じ空間軌道に入ることはできず，異なった空間軌道に帰属される．偶数の電子を有する分子の基底状態は，一般に一重項状態であるが，酸素分子のように縮退した最高被占軌道に2個の電子が帰属されるときは，フントの規則に従い，電子は平行スピンをもって異なる軌道に帰属されることになり，基底状態が三重項状態になるものもある．一般には，励起三重項状態は励起一重項状態よりも低く，物質の励起状態の性質を理解するうえで重要な意味をもつ．とくに有機分子のりん光は，励起三重項状態から基底一重項状態への遷移に伴う放射として説明されるし，エネルギー移動や光化学反応の機構の説明にも用いられる．

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