Intersystem crossing is a radiationless process involving a transition between two electronic states with different spin multiplicity.^[1]

[edit] Singlet and triplet states

When an electron in a molecule is excited (via absorption of radiation) to a higher energy level, either an excited singlet state or an excited triplet state will form. A singlet state is a molecular electronic state such that all electron spins are paired. That is, the spin of the excited electron is still paired with the ground state electron (a pair of electrons in the same energy level must have opposite spins, the Pauli exclusion principle). A triplet state is one in which the excited electron is no longer paired with the ground state electron; that is, they are parallel (same spin). Since excitation to a triplet state involves an additional spin transition, it is more probable that an excited singlet will form upon absorption of radiation.

Singlet and triplet energy levels.

When a singlet state nonradiatively transitions to a triplet state, that process is known as intersystem crossing. In essence, the spin of the excited electron is reversed. The probability of this process occurring is more favorable when the vibrational levels of the two excited states overlap, since little or no energy must be gained or lost in the transition. Intersystem crossing is most common in heavy-atom molecules (e.g. those containing iodine or bromine), as the spin/orbital interactions in such molecules become large and a change in spin is more favorable. In addition, the presence of paramagnetic species in solution enhances intersystem crossing.^[2]

The radiative decay from an excited triplet state back to a singlet state is known as phosphorescence, and is also considered another form of intersystem crossing since a transition in spin multiplicity is observed. The time scale of intersystem crossing is on the order of 10^-8 to 10^-3 s, one of the slowest forms of relaxation.^[3]

[edit] References