Lanthanide Luminescence Thermometry and Slow Magnetic Relaxation in 3-D Polycyanidometallate-Based Materials

Two three-dimensional (3-D) polycyanidometallate-based luminescent thermometers with the general formula {Ln₄Co₄(CN)₂₄(4-benpyo)₁₇(H₂O)·7H₂O} Ln = (Dy(III)(1), Eu(III)(2)), based on the red-emissive diamagnetic linker [Co(CN)₆]^3- and the bulky pyridine derivative that possesses the N-oxide moiety, 4-benzyloxy-pyridine N-oxide (benpyo), were prepared for the first time. The structure of compound 1 has been determined by single-crystal X-ray crystallography while the purity and structure of 2 have been confirmed by CHN, Fourier transform infrared spectroscopy (FT-IR), and powder X-ray diffraction (PXRD) analysis. Magnetic AC susceptibility measurements at zero field show no single-molecule magnet (SMM) behavior indicating fast relaxation operating in 1. Upon application of an optimal field of 2 kOe, the SMM character of compound 1 is revealed while the τ(Τ) can be reproduced solely considering the Raman process τ^-1 = CT with C = 7.0901(3) s^-1 K^- and n = 3.58(1), indicating that a high density of low-lying states and optical as well as acoustic phonons play a major role in the relaxation mechanism. Micron-sized superconducting quantum interference device (μ-SQUID) loops show a very narrow opening in agreement with the AC susceptibility studies and complete active space self-consistent field (CASSCF) calculations. The interaction operating between the Dy(III) ions was quantified from CASSCF calculations. Good agreement is found by fitting the experimental DC χ_MΤ(Τ) and M(H), employing the Lines model, with J_Lines = -0.087 cm^-1 (-0.125 K). The excitation spectra of compound 2 are used for temperature sensing in the 25-325 nm range with a maximum relative thermal sensitivity, S_r = 0.6% K^-1 at 325 K, whereas compound 1 operates as a luminescent thermometer based on its emission features in the temperature range of 16-350 K with S_r ≈ 2.3% K^-1 at 240 K.