Anti-Thermal Quenching in Ndiii Molecular Near-Infrared Thermometers Operating at Physiological Temperatures

Examples of molecular complexes acting as thermometers operating at room temperature in near infrared region are scarce, therefore this work showcases the anti-thermal quenching effect on neodymium(III) molecular thermometers working in biological windows within the physiological temperature range. A mononuclear complex, [Nd(L)(NO<inf>3</inf>)<inf>3</inf>] (1Nd), where L is a macrocyclic ligand, was synthesized and used as a precursor to develop two novel species: a dinuclear, [(Nd(L)(NO<inf>3</inf>))<inf>2</inf>(µ-BDC)](NO<inf>3</inf>)<inf>2</inf>·H<inf>2</inf>O (2Nd), linked by 1,4-benzenedicarboxylate (BDC), and a hexameric, [(Nd(L))(µ-BTC)(H<inf>2</inf>O)]<inf>6</inf>·35H<inf>2</inf>O (6Nd), linked with 1,3,5-benzenetricarboxylate (BTC). Thermometric properties were studied in the physiological temperature range (292-332 K), utilizing 804 nm laser excitation (first biological window) and monitoring emissions in the second biological window (908, 1065, and 1340 nm) associated with the 4F<inf>3/2</inf> → 4I<inf>9/2</inf>, 4I<inf>11/2</inf>, 4I<inf>13/2</inf> transitions, respectively. Among the complexes, the hexamer 6Nd exhibited exceptional performance, with S<inf>r</inf> of 2.4%K−1 at 293 K, when luminescence intensity ratio (LIR) of two Stark components of the 4F<inf>3/2</inf> → 4I<inf>11/2</inf> emission was used, positioning it as a high-performance NdIII-based thermometer. All complexes displayed anti-thermal quenching behavior, surpassing the current molecular-based thermometers in the near-infrared region. Theoretical calculations using complete active space self consistent field (CASSCF) and Boltzmann population models between Kramers doublets of the 4F<inf>3/2</inf> level were performed to rationalize the anti-thermal behavior. (Figure presented.) © The Author(s) 2025.