High-Performance Luminescence Thermometer with Field-Induced Slow Magnetic Relaxation Based on a Heterometallic Cyanido-Bridged 3d–4f Complex

The 1:1:1 reaction of DyCl₃·6H₂O, K₃[Co­(CN)₆] and bpyO₂ in H₂O has provided access to a complex with formula [DyCo­(CN)₆(bpyO₂)₂(H₂O)₃]·4H₂O (1) in a very good yield, while [DyFe­(CN)₆(bpyO₂)₂ (H₂O)₃]·4H₂O (2) was also precipitated (also in a high yield) using K₃[Fe­(CN)₆] instead of K₃[Co­(CN)₆]. Their structures have been determined by single-crystal X-ray crystallography and characterized based on elemental analyses and IR spectra. Combined direct current (dc) and alternating current (ac) magnetic susceptibility revealed slow magnetic relaxation upon application of a dc field. μ-SQUID measurements and CASSCF calculations revealed high-temperature relaxation dynamics for both compounds. Low-temperature magnetic studies show the relaxation characteristics for 1, while for compound 2 the dynamics corresponds to an antiferromagnetically coupled Dy···Fe pair. High-resolution optical studies have been carried out to investigate the performance of compounds 1 and 2 as luminescence thermometers. For 1, a maximum thermal sensitivity of 1.84% K^–1 at 70 K has been calculated, which is higher than the acceptable sensitivity boundary of 1% K^–1 for high-performance luminescence thermometers in a broad range of temperature between 40 and 140 K. Further optical studies focused on the chromaticity diagram of compound 1 revealed a temperature shift from warm white (3200 K) at 10 K toward a more natural white color near 4000 K at room temperature.