A molecular dual-center emitter for ratiometric optical thermometry

Ratiometric optical thermometers have attracted significant interest due to their high accuracy, self-referencing capability and strong resistance to environmental fluctuations. However, most reported systems rely on doped materials, where random donor-acceptor distributions and batch-to-batch variability hinder reproducibility and restrict practical deployment for high-precision temperature sensing. Molecular ratiometric thermometers with precisely controlled donor-acceptor distances and fixed stoichiometry serve as promising candidates, yet remain largely unexplored. This work reports a dual-center emissive 3d-4f binuclear complex [ZnLSm(OAc)(NO₃)₂] (ZnSm), constructed using a Schiff base ligand (L). ZnSm exhibits two well-correlated emission bands at 485 nm (ZnL) and 644 nm (Sm³⁺), enabling quantitative temperature readout over a broad temperature range of 233-333 K with excellent reversibility and a high maximum relative sensitivity of 3.4% K^-1. Spectroscopic analyses and theoretical calculations reveal efficient ZnL-to-Sm³⁺ energy transfer mediated by the bridging Schiff base ligand, accounting for the temperature-dependent dual emission. Moreover, ZnSm can be readily processed into a transparent and flexible poly(methyl methacrylate) (PMMA) film (ZnSm@PMMA) while retaining its ratiometric thermometric performance, thus greatly enhancing its applicability for practical thermal mapping and device-integrated sensing. This work presents a robust molecular design strategy for developing high-performance, dual-center emissive ratiometric optical thermometers. Furthermore, the readily distinguishable color change in the visible range for both ZnSm and its film highlights their potential for advanced optical anti-counterfeiting and information encryption applications.