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

TL;DR

This paper presents a model for understanding the excited states of organometallic complexes with strong spin-orbit coupling, explaining their radiative decay and localization properties relevant for optoelectronic applications.

Contribution

The authors develop a pseudo-angular momentum model that accurately predicts excited state properties even with large symmetry deviations, providing insights into radiative rates and localization.

Findings

Model predictions align well with experimental data.

Excited states show significant localization.

Zero-field splitting explained by conservation laws.

Abstract

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