Composition Control in Molecular Cluster-Aggregates: A Toolbox for Optical Output Tunability via Energy Transfer Pathways.

Composition control is a powerful tool for obtaining high-performance lanthanide (Ln) luminescent materials with adjustable optical outputs. This strategy is well-established for hierarchically structured nanoparticles, but it is rarely applied to molecular compounds due to the limited number of metal centers within a single unit. In this work, we present a series of molecular cluster-aggregates (MCAs) with an icosanuclear core {Ln 2 Eu 2 Tb 16 } (Ln = Ce, Pr, Nd, Sm, Gd, Dy, Ho, Er, Tm, and Yb) in which we explore composition control, akin to nanoparticles, to modulate the optical output. More specifically, we target to understand how the presence of a third Ln III doping ion would impact the well-known Tb III → Eu III energy transfer and the ratiometric optical thermometry performance based on the Tb III /Eu III pair. Photophysical properties at room and at varying temperatures were investigated. Based on experimental data and well-established intrinsic features, such as spin-orbit coupling strength and Ln III 4 f energy levels' structure, we discuss the possible luminescent processes present in each MCA and provide insight into qualitative trends that can be rationally correlated throughout the series.