Intersystem crossing in transition metal complexes

Intersystem crossing between zeroth-order states of different spin quantum numbers can be characterized as fast (≥1012 s−1) or slow. The bulk of the extant results on the fast unquenchable processes has been limited to the evaluation of efficiencies, the fraction of the initially excited molecules that undergo intersystem crossing. Cr(III) complexes have been the main systems studied. The increasing availability of sub-picosecond lasers has led to the determination of rates. Both prompt intersystem crossing which precedes thermalization and intersystem crossing from thermalized states contribute to the total efficiency, but the relative contributions of the two processes have not been determined. The magnitude of the efficiency reflects the relative rates of vibrational relaxation and prompt intersystem crossing in the Franck–Condon level, but both efficiencies and rates are available for few metal complexes.The published efficiency results are examined with a kinetic model. The relationship between the several different measures of intersystem crossing is considered. Identification of reactive states cannot be made from kinetic measurements alone. The reliability of the results is evaluated and the possible dependence on excitation wave length is judged.