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 T_n (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

spin-orbit coupling TDDFT, intersystem crossing

Elise Yu-Tzu Li*, Tzung-Ying Jiang, Yun Chi and Pi-Tai Chou*. Semi-Quantitative Assessment of Intersystem Crossing Rate: An Extension of El-Sayed Rule to the Emissive Transition Metal Complexes. Phys. Chem. Chem. Phys. 16, 26184-26192 (2014)