Multifunctional self-refrigerated multivariate {GdLn} (Ln = Dy, Tb, Tb/Eu) metal–organic frameworks

Abstract

Multivariate metal–organic frameworks (MOFs) containing multiple lanthanide ions present a compelling avenue for developing versatile materials with tailored properties. In this work, we synthesized “self-refrigerated” multifunctional carborane-based MOFs of formula unit {[(GdLn)₃(mCB-L)₄(NO₃)(DMF)_x]_n·Solv} strategically combining Gd³⁺ ions, known for their ability to achieve large magnetocaloric effects (MCE), with various lanthanides (Ln = Dy, Tb, Eu, Tb/Eu) intended to act as Single Molecule Magnet (SMM) or/and luminescent units. The intricate magnetic, thermal, and optical properties of these multivariate Ln-MOFs were unraveled through a comprehensive characterization employing dc and ac magnetometry, X-ray Absorption Spectroscopy (XAS), X-ray Magnetic Circular Dichroism (XMCD), and luminescence measurements. Element-selective XAS-XMCD technique proved instrumental in elucidating the magnetic properties of the individual lanthanides, and their contribution to the macroscopic properties of the MOFs. We demonstrate that Gd_1.5Ln_1.5 (Ln = Tb, Dy) MOFs exhibit multifunctionality, incorporating MCE, field-induced magnetic relaxation dominated by the anisotropic ion, and green emission for Ln = Tb. Conversely, Gd_1.5Ln_1.5 (Ln = Eu, Eu/Tb) MOFs display MCE, field-induced SMM behavior associated with Gd, and red/yellow luminescent emission for Ln = Eu and Eu/Tb substitutions, respectively. Our findings significantly contribute to our understanding of “complex magnetic molecular materials” and set a pathway for the design of multifunctional multi-lanthanide MOFs endowed with tailored properties for various technological applications.