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(LN6en)(OAc)2](NO3)}·2H2O (1·2H2O) and {[Dy(LN6prop)(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 LN6prop 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 Ueff 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 Dy3+ 4F9/2 → 6H13/2 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 4F9/2 → 6H15/2 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 4F9/2 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