El-Sayed rule

The El-Sayed rule states that radiationless transitions between electronic states with different orbital character are more probable because stronger spin–orbit coupling facilitates them.

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Intersystem crossing: A radiationless transition between electronic states of different spin multiplicity. The rule predicts when this transition gains probability from a change in orbital character.
Phosphorescence: Light emission from an excited state to a lower-energy state with different spin multiplicity. The rule helps explain how molecules populate the triplet states that emit phosphorescent light.
Mohamed El-Sayed: An Egyptian-American physical chemist known for research on excited-state dynamics and photochemistry. His name is attached to the empirical rule relating orbital character to intersystem crossing.
Laporte rule: A selection rule restricting electric-dipole transitions in centrosymmetric systems according to parity. It concerns spatial parity, whereas the El-Sayed rule links orbital character to spin–orbit coupling.
Spin–orbit coupling: An interaction between an electron's spin angular momentum and its orbital angular momentum. Different orbital character strengthens this interaction, enabling otherwise spin-forbidden transitions.
Fluorescence: Light emission from an excited singlet state to a lower singlet state. Competing intersystem crossing can divert excited molecules from fluorescence.
Jablonski diagram: A diagram showing electronic states and radiative and nonradiative transitions between them. It provides the standard visual framework for depicting transitions explained by the rule.
Fermi's golden rule: A formula estimating transition rates from coupling strength and the density of available final states. It gives a broader rate framework into which the rule's coupling effect can be understood.
Electronic state: A quantum state specifying the electronic configuration and energy of an atom or molecule. The rule compares the orbital character of the states connected by a transition.
Photochemistry: The study of chemical transformations initiated by light. Orbital-character-dependent state crossings shape which photochemical pathways molecules follow.