3D Lanthanide Metal–Organic Frameworks Based on Mono‑, Tri‑, and Heterometallic Tetranuclear Clusters as Highly Selective and Sensitive Luminescent Sensor for Fe³⁺ and Cu²⁺ Ions

Five novel three-dimensional lanthanide metal–organic frameworks (Ln–MOFs) constructed by 4-(3′,5′-dicarboxylphenoxy) phthalic acid (H₄dcppa), namely, {[Eu­(Hdcppa)­(H₂O)₂]·H₂O}_<i>n</i> (<b>Eu-1</b>), [Eu_1.5(dcppa) (HCOO)_0.5(H₂O)₂]_<i>n</i> (<b>Eu-2</b>), {[Ln₂K₂(dcppa)₂(H₂O)₆]·<i>m</i>H₂O}_<i>n</i> (Ln = <b>Eu-3</b>, <b>Tb-4</b>, <b>Gd-5</b>, <i>m</i> = 5 for <b>Eu-3</b> and <b>Tb-4</b>; <i>m</i> = 4 for <b>Gd-5</b>) (HCOOH = formic acid) have been prepared by hydro­(solvo)­thermal method and fully characterized. Structural analyses indicated that H₄dcppa ligand took four different coordination fashions in <b>1</b>–<b>5</b> and, thus, resulted in diversity of the targeted Ln–MOFs: <b>Eu-1</b> displays the rare 3D (5,5)-connected <b>vbk</b> net, containing one-dimensional left- and right-handed helical chains; <b>Eu-2</b> possesses a 3D (3,8)-connected <b>tfz-d</b> topology constructed by trinuclear paddlewheel [Eu₃(CO₂)₆] SBUs; <b>3</b>–<b>5</b> are isostructural and show 3D (Ln-dcppa^4–) + 2D (K-dcppa^4–) → 3D (LnK-dcppa^4–) structure. <b>Eu-3</b>, with regular 1D channels and open Lewis basic oxygen atoms on the pore surface was utilized for specific sensing and binding of metal ions through Lewis base interactions, shows high selectivity, fast response time (8 min), and high sensitivity (<i>K</i>_SV = 4.3/5.2 × 10⁴ L/mol) for Fe³⁺ and Cu²⁺ ions with luminescent quenching. Furthermore, the detection limit of the sensor is in the 10^–6 M level. The triplet state (T₁ = 23364 cm^–1) of H₄dcppa studied by the Gd­(III)–MOF and <b>Gd-5</b> demonstrates that the ligand ideally populates Eu­(III)/Tb­(III) emission with luminescent quantum yields (Φ_overall) of 11.2% for <b>Eu-1</b>, 27.6% for <b>Eu-2</b>, 18.5% for <b>Eu-3</b>, and 62% for <b>Tb-4</b>, respectively