Harnessing ligand design to develop primary and self-calibrated luminescent thermometers with field-induced single ion magnet behaviour in Dy3+ complexes

Abstract

Novel complexes {[Dy(L^N6en)(OAc)₂](NO₃)}·2H₂O (1·2H₂O) and {[Dy(L^N6prop)(OAc)₂](NO₃)}·CHCl₃ (2·CHCl₃), containing partially flexible symmetric N₆ macrocycles, are reported. We explore the influence of the spacer length between two symmetrical N₃ rigid moieties of the ligand on their structural, magnetic, and luminescence properties. Crystallographic analysis reveals the presence of Dy³⁺ ions in distorted tetradecahedral (1·2H₂O) or bicapped square antiprism (2·CHCl₃) environments. This underscores the increased flexibility of the L^N6prop ligand, resulting in greater distortion of the N₆ macrocycle plane. Both complexes exhibit single-molecule magnet behaviour under an optimal field of 2000 Oe, with 2·CHCl₃ displaying the highest U_eff value of 127 K, despite its less planar N₆ macrocycle. Luminescence measurements indicate that the ratio between the integrated intensity of the ligands and that of the the Dy^{3+ 4}F_9/2 → ⁶H_13/2 transition can define secondary luminescent thermometers. Maximum relative thermal sensitivity values of 2.3 (1·2H₂O) and 5.1% K⁻¹ (2·CHCl₃) are achieved. Furthermore, deconvolution of the ⁴F_9/2 → ⁶H_15/2 transition in 2·CHCl₃ supports the previous determination of the energy barrier for magnetic relaxation. This permits the demonstration of the first example of a Dy³⁺ primary luminescent thermometer based on two thermally coupled Kramer's doublets of the ⁴F_9/2 level. Remarkably, 2·CHCl₃ is also the first self-calibrated luminescent thermometer with magnetic relaxation operating within the 86–211 K range, showcasing its potential in precise temperature sensing applications.