Inside-Out/Outside-In Tunability in Nanosized Lanthanide-Based Molecular Cluster-Aggregates: Modulating the Luminescence Thermometry Performance via Composition Control.

Modulating the optical property of a material via structural modification is a powerful tool for obtaining the desired optical output. If a material can be tuned inside (core) and outside (outer shell), then the degree of control is greater toward application. Herein, we present a lanthanide-based nanosized molecular cluster aggregate (MCA) that allows fine-tuning of the inner core via composition control akin to nanoparticles. At the same time, the tunable outer shell enables light-harvesting properties similar to molecular systems. As such {Eu4Tb16}, {Eu3Gd5Tb12}, {Eu2Gd10Tb8}, and {Eu1Gd15Tb4} compositions were synthesized, and their photophysical properties were investigated in solution and in the solid state. Controlling the composition and spacing of the emitter ions with the optically silent GdIII ions results in a decrease in the TbIII → EuIII energy-transfer process efficiency. Consequently, ratiometric luminescence thermometry performance is fine-tuned to reach a maximum relative sensitivity of 4.17% °C-1 at 36 °C for the {Eu4Tb16} MCA. This study demonstrates that the optical properties are intrinsic to individual MCA species rather than a collective intermolecular effect. The color change observed close to room temperature for {Eu2Gd10Tb8} suggests potential applications such as multistage anticounterfeiting technology.