Dramatic Structural Upheavals of 3D Lanthanide-Organic Frameworks Triggered by Moisture-Induced Single-Crystal-to-Single-Crystal Transformation

Due to the labile nature of coordinated bonds and variable coordination numbers and geometries, lanthanide-organic frameworks (LnOFs) often undergo single-crystal-to-single-crystal (SCSC) transformations upon external stimulation (light, heat, and solvent). However, SCSC transformations induced by moisture at room temperature (RT) remain exceedingly scarce in 3D LnOFs. Herein, we report a rare example of a moisture-induced SCSC transformation at RT in two 3D LnOFs. The needle-shaped SCs, (NH₄)[LnL₂]·4DMF (Ln = Eu, 1; Gd, 2) synthesized solvothermally from 3,3'-dihydroxybiphenyl-4,4'-dicarboxylic acid (H₂L) and Ln³⁺ cations, display an unprecedented anionic svq topology in LnOFs with the 1D rod-shaped [Ln(CO₂)₄]⁻ chains as secondary building units (SBUs). Remarkably, LnOFs 1 and 2 undergo irreversible moisture-induced SCSC transformations at RT to quantitatively produce two block-shaped SCs, [LnL1.5(H₂O)₃]·DMF (Ln = Eu, 3; Gd, 4) which cannot be directly prepared by a solvothermal reaction. LnOFs 3 and 4 represent the first 3-fold interpenetrated neutral pcu framework in LnOFs with the binuclear [Ln₂(CO₂)₆] cluster as the SBU. Notably, such a moisture-induced SCSC transformation simultaneously triggers significant structural changes involving space group, SBU, topology, framework charge, interpenetration, etc. Additionally, 1 exhibits strong red luminescence and lower water stability, while 3 shows complete luminescence quenching but higher water stability.