Conservation laws, radiative decay rates, and excited state localization in organometallic complexes with strong spin-orbit coupling

摘要

There is longstanding fundamental interest in 6-fold coordinated $d^6$($t_{2g}^6$) transition metal complexes such as [Ru(bpy)$_3$]$^{2+}$ andIr(ppy)$_3$, particularly their phosphorescence. This interest has increasedwith the growing realisation that many of these complexes have potential usesin applications including photovoltaics, imaging, sensing, and light-emittingdiodes. In order to design new complexes with properties tailored for specificapplications a detailed understanding of the low-energy excited states,particularly the lowest energy triplet state, $T_1$, is required. Here wedescribe a model of pseudo-octahedral complexes based on a pseudo-angularmomentum representation and show that the predictions of this model are inexcellent agreement with experiment - even when the deviations from octahedralsymmetry are large. This model gives a natural explanation of zero-fieldsplitting of $T_1$ and of the relative radiative rates of the three sublevelsin terms of the conservation of time-reversal parity and total angular momentummodulo two. We show that the broad parameter regime consistent with theexperimental data implies significant localization of the excited state.