系间窜越Intersystem crossing

自旋-轨道耦合常数大致随原子序数的四次方增大，因此含溴、碘或铂、铱、钌等重原子的分子系间窜越很快（“重原子效应”），量子产率可接近 1。埃尔-赛义德规则指出，轨道类型改变的跃迁（如 ¹(n,π*) → ³(π,π*)）比轨道类型不变的跃迁快约 1000 倍，所以芳香酮（如二苯甲酮）系间窜越效率很高。S₁ 与 T₁ 的能隙越小，系间窜越越快。

Spin–orbit coupling grows roughly with the fourth power of atomic number, so molecules containing bromine, iodine or metals such as platinum, iridium and ruthenium cross very quickly (the heavy-atom effect), with quantum yields approaching 1. El-Sayed's rule says that transitions changing orbital type, such as ¹(n,π*) → ³(π,π*), are about a thousand times faster than those that do not, which is why aromatic ketones such as benzophenone cross so efficiently. The smaller the S₁–T₁ gap, the faster the crossing.

1944 年刘易斯和卡沙通过研究刚性玻璃中有机分子的长寿命发光，证明其来自三重态，确立了系间窜越的概念。1960 年代埃尔-赛义德提出了系间窜越速率的规则。1990 年代末，福雷斯特和汤普森利用铱配合物的高效系间窜越实现了磷光 OLED，理论内量子效率达 100%。

In 1944 Lewis and Kasha, studying long-lived emission of organic molecules in rigid glasses, showed it came from the triplet state and established intersystem crossing. El-Sayed formulated his rules for crossing rates in the 1960s. In the late 1990s Forrest and Thompson exploited efficient crossing in iridium complexes to make phosphorescent OLEDs with a theoretical internal quantum efficiency of 100%.

系间窜越是磷光、三重态光敏化反应（如二苯甲酮光还原、[2+2] 环加成）、光动力疗法（光敏剂把能量传给 O₂ 产生单线态氧）和磷光 OLED 的关键步骤。热活化延迟荧光（TADF）材料利用很小的 S₁–T₁ 能隙实现反向系间窜越（T₁ → S₁），无需贵金属也能高效发光。

Intersystem crossing is the key step in phosphorescence, triplet-sensitised photochemistry (benzophenone photoreduction, [2+2] cycloadditions), photodynamic therapy (sensitisers passing energy to O₂ to make singlet oxygen) and phosphorescent OLEDs. Thermally activated delayed fluorescence (TADF) materials use a very small S₁–T₁ gap for reverse intersystem crossing (T₁ → S₁), emitting efficiently without precious metals.