β-Perhalogenated porphyrins enable sensitive cryogenic NIR lanthanide thermometry

Abstract

Optical thermometry based on luminescent lanthanide (Ln³⁺) complexes holds immense potential for non-invasive thermal sensing in biomedicine and optoelectronics, particularly when operating within the near-infrared (NIR) window (800–1700 nm). However, the practical deployment is often hindered by the inherently weak absorption of Ln³⁺ ions and a lack of rational design principles to optimize thermal sensitivity. While current strategies primarily focus on modulating vibrational relaxation or interionic interactions, they frequently overlook the critical influence of perturbations in the primary coordination environment. Such geometric perturbations are critical, as they govern the crystal-field splitting of 4f–4f transitions, thereby dictating the Stark sublevel populations essential for ratiometric sensing. To address this, we introduce β-perhalogenated porphyrins as a versatile platform. These ligands not only act as robust light-harvesting antennas to overcome the weak Ln³⁺ absorption but also allow precise tuning of the coordination geometry. We demonstrate that increasing the atomic radius of the halogen substituents systematically distorts the porphyrin framework; this structural distortion amplifies Stark splitting in Yb³⁺ and Er³⁺ complexes, achieving remarkable luminescence intensity ratio (LIR) temperature sensitivities of 17% K⁻¹ for Br-1-Yb and 4.5% K⁻¹ for Br-1-Er, respectively. Furthermore, through a comparative study of an Nd³⁺ analog (Br-1-Nd), we elucidate how thermally activated back energy transfer (BEnT), which is driven by a finely tuned ligand-to-metal energy gap (<200 cm⁻¹), dictates the complex decay kinetics and thermal quenching behavior. By correlating ligand-induced structural distortions with 4f-level splitting and thermal responses, this work establishes a rational framework for tailoring β-halogenated ligands to construct high-precision, NIR-emitting Ln³⁺ molecular thermometers.