Optical spectroscopy of exchange-coupled ions: Insights into homonuclear, heteronuclear and mixed transition-metal/rare-earth systems

In 1988, McCarthy and Güdel published the seminal review Optical Spectroscopy of Exchange-Coupled Transition Metal Complexes, which not only compiled key spectroscopic results but also provided a deep insight into the theoretical foundations of the field, originally developed by Tanabe in the 1960s, which is undoubtedly the most insightful review in this topic to date. The work presented here aims to pay a modest tribute to Prof. Hans U. Güdel for his outstanding contributions and career -- a scientist who was a guiding figure for many of us entering the field of spectroscopy. This review provides an updated overview of key experimental findings where exchange-coupled systems play a fundamental role, many of which involved direct contributions from Prof. Güdel himself. We begin by discussing homonuclear transition-metal systems, such as Mn2+-Mn2+ and Mn3+-Mn3+ pairs, highlighting the unique features of their absorption and emission spectra, as well as their lifetimes-- phenomena that are not observed in isolated ions. The discussion then extends to heteronuclear mixed-metal systems, specifically transition-metal/rare-earth ion pairs, such as Mn2+-Yb3+. In these systems, a novel upconversion luminescence mechanism was proposed, based on exchange interaction via the Tanabe mechanism between Yb3+ and Mn2+. Significant enhancement of optical absorption and its temperature dependence was attributed to Mn2+-Cu2+ exchange interaction units. Finally, we compare results from two Ni2+-doped isostructural materials -- Rb2CdCl4 and Rb2MnCl4 -- emphasizing the potential of exchange coupling mechanism to enhance luminescence efficiency.