intersystem crossing

Intersystem crossing is a nonradiative process in physics where a molecule transitions between electronic states with different electron spins. This typically occurs when a molecule moves from a singlet state to a triplet state, or vice versa, involving a spin flip of one of the electrons. This process is generally slow compared to other molecular changes because altering an electron's intrinsic spin is improbable.123

Molecules absorb energy, often from light, which excites their electrons to higher energy levels. These excited states can be either singlet or triplet. Intersystem crossing allows a molecule to move between these states, influencing its subsequent behavior, such as phosphorescence or chemical reactions. This phenomenon is crucial in understanding various photochemical processes and luminescence.12