Linker installation transformations in a 2-D rare earth MOF: increase of the dimensionality and turn on of the temperature sensing capability

Abstract

A new family of 2-D 8-connected rare earth (RE) MOFs based on a hexanuclear (RE³⁺)₆ secondary building unit (SBU), the increase of the dimensionality through single-crystal-to-single-crystal (SCSC) linker installation reactions and the turn-on of the temperature sensing capability upon these reactions are reported. The reaction of RE(NO₃)₃ with 4,4′-(hydroxymethylene)dibenzoic acid (H₂BCPM) in the presence of 2-fluorobenzoic acid (HFBA) in DMF/H₂O at 115 °C afforded compounds [RE₆(μ₃-OH⁻/F⁻)₈(BCPM)₄(NO₃)₂(H₂O)₄]_n (UCY-17(RE); RE: Y, Gd, Tb, Dy, Ho, Er) which represent rare examples of 2D 8-c MOFs based on a hexanuclear (RE³⁺)₆ SBU. The excellent quality of single crystals of UCY-17(Tb) and the distance between the terminal nitrate ions prompted us to investigate the SCSC exchange of NO₃⁻ anions by various dicarboxylate ligands aiming to bridge adjacent 2D nanosheets and form 3-D analogues. These SCSC linker installation reactions afforded a series of 3-D, 10-connected mixed linker MOFs with the general formulae [RE₆(μ₃-OH⁻/F⁻)₈(BCPM)₄(L)(H₂O)₄]_n (UCY-17(Tb)/L; H₂L = H₂BDC (1,4-benzenedicarboxylic acid), H₂ABDC (2-aminobenzene-1,4-dicarboxylic acid), H₂FBDC (2-fluorobenzene-1,4-dicarboxylic acid), and H₂NDC (1,4-naphthalenedicarboxylic acid)). The exchanged analogues of UCY-17(Gd) and UCY-17(Eu_0.05Tb_0.95) were synthesized and the thermometric properties of the latter were investigated. These studies revealed that there is no thermal evolution of the emission properties for the pristine MOF UCY-17(Eu_0.05Tb_0.95) whereas the exchanged analogues exhibit significant thermometric properties at higher temperatures (>270 K). The maximum thermal sensitivity of most exchanged derivatives appears at physiological temperatures and ranges between 300 and 355 K. Overall, this work proposes a promising strategy for increasing the dimensionality of 2-D MOFs and controlling the thermometric performances of mixed Eu_0.05Tb_0.95 MOFs.