Tunable Nd3+ Dimer Luminescent Molecular Thermometer via Stark Sublevels

Abstract

The near-infrared (NIR) luminescence thermometry performance of the Nd-based dimer {[Nd(acac)₃]₂(µ-bpm)}(Nd₂) (where acac⁻ denotes acetylacetonate and bpm is 2,2’-bipyrimidine) was investigated under both ligand- and metal-centered excitation. To resolve the Stark components associated with the ⁴F_3/2 → ⁴I_11/2 transition, ab initio electronic structure calculations were performed. The opposite temperature dependences of the lower- and higher-energy components of the ⁴F_3/2 level were used as a thermometric parameter, expressed through the luminescence intensity ratio (LIR). Under ligand-centered excitation at 370 nm, Nd₂ exhibited a maximum relative sensitivity (S_m) of 2.1 %•K^-1 at 85 K, across the 85-285 K range, demonstrating excellent thermal performance at low temperatures. Considering the relevance of NIR-to-NIR emitters for biomedical applications, the thermometric response was also evaluated under metal-centered excitation at 804 nm within the physiological range (294-332 K). In this case, an unprecedented S_m value of 2.5 %•K^-1 at 294 K was obtained, setting a new benchmark for Nd-based molecular thermometers and highlighting its potential for biological temperature sensing. Overall, Nd₂ operates as a versatile luminescent molecular thermometer whose thermal response can be tuned simply by selecting the excitation pathway (UV or NIR), enabling adaptable and application-specific thermometric behaviour.