Sensitivity-Range Integral (SRI): a novel approach to evaluating and optimizing luminescent thermometers. A case study of thermometric properties governed by intermolecular CT states in isomeric Eu/Tb coordination compounds

Abstract

The design of lanthanide-based molecular thermometers remains a major challenge, as their temperature sensitivity is strongly governed by subtle structural factors and the presence of competing relaxation pathways. A novel metric to compare the application potential was proposed: Sensitivity-Range Integral (SRI). The SRI accounts for both sensitivity and operating range, providing clearer performance insights. In this work, we investigate Eu³⁺, Tb³⁺, and mixed-metal Eu/Tb coordination compounds [LnL₃(bipy)] with two structurally isomeric ligands: 4,4,4-trifluoro-1-(1-methyl-1H-pyrazol-3-yl)butane-1,3-dione (HL¹) and 4,4,4-trifluoro-1-(1-methyl-1H-pyrazol-5-yl)butane-1,3-dione (HL²). Both ligand isomers form neutral mononuclear complexes with square-antiprismatic Ln³⁺ coordination environments, but distinct intermolecular interactions enable different intermolecular charge-transfer (CT) states, which modulate the temperature-dependent luminescence response. Mixed-metal Eu/Tb complexes show excellent relative thermal sensitivities 6.0% × K⁻¹ for Eu/TbL¹ and 8.5% × K⁻¹ for Eu/TbL². The achieved SRI values for the investigated compounds are the highest among any earlier reported thermometers: 734% for Eu/TbL¹ and 970% for Eu/TbL². These results highlight the crucial role of ligand design and CT states in tuning the performance of molecular luminescent thermometers and provide design guidelines for the next generation of lanthanide-based optical sensors.