Intersystem crossing through spin-orbit coupling in Os(II) complexes
The Os(fipz)(tfa)(CO)3 (tfa = trifluoroacetate, fipz=3-trifluoromethyl-5-(1-isoquinolinyl)-1,2-pyrazole) complex shows excitation-dependent emission quantum yield. The higher-energy excitation (< 340 nm) contributes more to phosphorescence than fluorescence, implying stronger intersystem crossing (ISC) for highly excited singlet states. Theoretical calculations confirm that the spin-orbit coupling (SOC) integrals between the lowest singlet excited state, S1, and the low-lying triplet excited states Tn (n=1-10), are negligibly small, whereas the higher singlet states show much stronger SOC with triplet excited states.
SOC is largest when both singlet (1dπ*) and triplet (3d’π*) states have strong MLCT character and when different d-orbitals are involved (d ≠ d’). This is analogous to the fast ISC between 1ππ* and 3ππ* states in organic systems (essentially the p ≠ p’ condition), and can be referred to as the ‘‘organometallics version of El-Sayed’s rule’’.
Spin-orbit coupling (SOC) constants between various low-lying singlet and triplet excited states of the Os(fipz)(tfa)(CO)3
