β-Perhalogenated Porphyrins Enable Sensitive Cryogenic NIR Lanthanide Thermometry

Abstract

Optical thermometry utilizing luminescent lanthanide (Ln3+) complexes holds immense potential for non-invasive thermal sensing in biomedicine and optoelectronics, particularly within the near-infrared (NIR) window (800–1700 nm). However, the practical deployment of these probes is hindered by the inherently weak absorption of Ln3+ ions and the lack of rational design principles for optimizing the thermal sensitivity. While current strategies focus on modulating vibrational relaxation or interionic interactions, they often overlook the critical influence of pertubation of primary coordination enviroment. Perturbations in coordination geometry are pivotal, as they governs the crystal field splitting of 4f-4f transitions, thereby dictating the Stark sublevel populations required for ratiometric sensing. To address this, we introduce β-perhalogenated porphyrins as a versatile platform for engineering NIR-emitting Ln3+ thermometers. 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. Complementarily, we report a Nd3+probe (Br-1-Nd) that exploits temperature-dependent back energy transfer, yielding a sensitivity of 4%·K-1 driven by a finely tuned energy gap that is less than 200 cm-1. By correlating ligand-induced structural distortions with 4f-level splitting and thermal response, this work establishes a framework for tailoring -halogenated ligands to construct high-precision Ln3+ molecular thermometers.