Spin-Orbit Coupling and Intersystem Crossing

In this section the effect of spin-orbit coupling on radiative and radiationless intercombinational transitions (transitions occurring between states of different multiplicity) will be investigated. We will be particularly concerned with the use of internal and external heavy atoms to induce spin-orbit coupling. The effect of heavy atoms on intercombinational processes occurring in aromatic hydrocarbons, carbonyl compounds, and heterocyclic compounds will be discussed. 

In the Dirac relativistic equation, spin is naturally included. It is possible to identify the energy terms in the Dirac equation and include them in the ordinary Hamiltonian as magnetic terms. In particular, the spin-orbit coupling term is important. This term is physically due to interactions between the electron spin and its motional spin around the atomic nucleus. Spin-orbit coupling increases in importance for heavy atoms. Transitions or curve crossings are no longer spin forbidden. 

FIGURE 12.7 Photo-processes in the excited states. Sq is the ground state PES Sj is the excited singlet state PES Tj is the excited triplet state PES. 

---

Klessinger M (1998) Triplet Photoreactions. Structural Dependence of Spin-orbit Coupling and Intersystem Crossing in Organic Biradicals. In Parkanyi C (ed) Theoretical Organic Chemistry-Theoretical and Computational Chemistry, Elsevier, Amsterdam, p 581. 

An RTF assay consists generally of four steps that are shown schematically in Figure 9. The purpose of the heavy-atom perturber is to encourage population of the triplet state via spin orbit coupling and intersystem crossing. The use of silver ion, as silver nitrate, is very eflFective in bringing this about and thereby enhancing the phosphorescence emission of many adsorbed PNA compounds. 

Triplet Photoreactions Structural Dependence of Spin-Orbit Coupling and Intersystem Crossing in Organic Biradicals... 

SPIN-ORBIT COUPLING AND INTERSYSTEM CROSSING IN BIRADICALS... 

C. M. Marian. Spin-orbit coupling and intersystem crossing in molecules. WTREs Comput Mol. Set, 2 (2012) 187-203. 

Let s summarize the key features of the process. A forbidden thermal pericyclic reaction leads to biradical character in the transition state. Spin-orbit coupling facilities intersystem crossing. The thermodynamics are such that both Ti and Sj of the product lie below the transition state for the thermal process. Put this all together and we have a remarkably efficient chemiluminescent process. 

In solution at room temperature, non-radiative de-excitation from the triplet state Ti, is predominant over radiative de-excitation called phosphorescence. In fact, the transition Ti —> S0 is forbidden (but it can be observed because of spin-orbit coupling), and the radiative rate constant is thus very low. During such a slow process, the numerous collisions with solvent molecules favor intersystem crossing and vibrational relaxation in So-... 

Fifth, the absence of significant phosphorescence is entirely consistent with the large a (Ti) — X (S0) spin-orbit coupling and the nonplanar geometry of the a state (vis a vis the planar X state), which leads to highly efficient a —> X intersystem crossing. 

---