Intersystem Crossing

Intersystem crossing (ISC) is an isoenergetic radiationless process involving a transition between the two electronic states with different spin multiplicity.

Excited Singlet and Triplet States

When an electron in a molecule with a singlet ground state is excited (''via'' absorption of radiation) to a higher energy level, either an excited singlet state or an excited triplet state will form. 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, per the Pauli exclusion principle). In a triplet state 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 "forbidden" spin transition, it is less probable that a triplet state will form when the molecule absorbs radiation.

When a singlet state nonradiatively passes to a triplet state, or conversely a triplet transitions to a singlet, 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. As the spin/orbital interactions in such molecules are substantial and a change in spin is thus more favourable, intersystem crossing is most common in heavy-atom molecules (e.g. those containing iodine or bromine). This process is called " spin-orbit coupling". Simply-stated, it involves coupling of the electron spin with the orbital angular momentum of non-circular orbits. In addition, the presence of paramagnetic species in solution enhances intersystem crossing.

The radiative decay from an excited triplet state back to a singlet state is known as phosphorescence. Since a transition in spin multiplicity occurs, phosphorescence is a manifestation of intersystem crossing. The time scale of intersystem crossing is on the order of 10⁻⁸ to 10⁻³ s, one of the slowest forms of relaxation.

Metal Complexes

Once a metal complex undergoes metal-to-ligand charge transfer, the system can undergo intersystem crossing, which, in conjunction with the tunability of MLCT excitation energies, produces a long-lived intermediate whose energy can be adjusted by altering the ligands used in the complex. Another species can then react with the long-lived excited state via oxidation or reduction, thereby initiating a redox pathway via tunable photoexcitation. Complexes containing high atomic number ''d''⁶ metal centers, such as Ru(II) and Ir(III), are commonly used for such applications due to them favoring intersystem crossing as a result of their more intense spin-orbit coupling.'

Complexes that have access to ''d'' orbitals are able to access spin multiplicities besides the singlet and triplet states, as some complexes have orbitals of similar or degenerate energies so that it is energetically favorable for electrons to be unpaired. It is possible then for a single complex to undergo multiple intersystem crossings, which is the case in light-induced excited spin-state trapping (LIESST), where, at low temperatures, a low-spin complex can be irradiated and undergo two instances of intersystem crossing. For Fe(II) complexes, the first intersystem crossing occurs from the singlet to the triplet state, which is then followed by intersystem crossing between the triplet and the quintet state. At low temperatures, the low-spin state is favored, but the quintet state is unable to relax back to the low-spin ground state due to their differences in zero-point energy and metal-ligand bond length. The reverse process is also possible for cases such as ptz)₆.html"_;"title="Pentylenetetrazol.html"_;"title="e(Pentylenetetrazol">ptz)₆">Pentylenetetrazol.html"_;"title="e(Pentylenetetrazol">ptz)₆BF₄)₂,_but_the_singlet_state_is_not_fully_regenerated,_as_the_energy_needed_to_excite_the_quintet_ground_state_to_the_necessary_excited_state_to_undergo_intersystem_crossing_to_the_triplet_state_overlaps_with_multiple_bands_corresponding_to_excitations_of_the_singlet_state_that_lead_back_to_the_quintet_state.

__Applications_

__Fluorophores_

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Applications

Fluorophores

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