Lanthanide 5,7-Disulfonate-1,4-naphthalenedicarboxylate Frameworks Constructed from Trinuclear and Tetranuclear Lanthanide Carboxylate Clusters: Proton Conduction and Selective Fluorescent Sensing of Fe3+

The novel sulfonate-carboxylate ligand of 5,7-disulfonate-1,4-naphthalenedicarboxylic acid (H₄-DSNPDC) was synthesized, and its series of lanthanide compounds {[Ln₃(μ₂-OH)(DSNPDC)₂(H₂O)_x]·yH₂O}_n (JXNU-7; Ln = La³⁺, x = 10. y = 4; Ln = Nd³⁺, Sm³⁺ Eu³⁺, x = 9, y = 2) and {[Ln₄(μ₃-OH)₄(DSNPDC)₂(H₂O)₁₁]·28H₂O}_n (JXNU-8; Ln = Eu³⁺, Gd³⁺) are presented. JXNU-7 is a three-dimensional structure based on linear trinuclear Ln₃ building units, while JXNU-8 has a two-dimensional layer constructed from tetranuclear Ln₄(μ₃-OH)₄ building units. The representative Eu compounds of JXNU-7 and -8 show good proton conductive properties under high humidity. The hydrophilic sulfonate groups pointing to the pores and the water molecules included in the pores mainly contribute to the high proton conductivity for the materials. The presence of one-dimensional infinite hydrogen-bonded networks in channels of JXNU-7(Eu) facilitates a fast and efficient proton transfer, resulting in higher proton conductivity in comparison to that of JXNU-8(Eu). Additionally, JXNU-7(Eu) with a characteristic red emission exhibits a promising potential for selective sensing of Fe³⁺ ions in aqueous solution. Our work demonstrates the integration of functional organic components (sulfonate groups) and inorganic components (lanthanide centers) in MOFs for the successful preparation of multifunctional MOF materials.