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}n (Eu-1), [Eu1.5(dcppa) (HCOO)0.5(H2O)2]n (Eu-2), {[Ln2K2(dcppa)2(H2O)6]·mH2O}n (Ln = Eu-3, Tb-4, Gd-5, m = 5 for Eu-3 and Tb-4; m = 4 for Gd-5) (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 1–5 and, thus, resulted in diversity of the targeted Ln–MOFs: Eu-1 displays the rare 3D (5,5)-connected vbk net, containing one-dimensional left- and right-handed helical chains; Eu-2 possesses a 3D (3,8)-connected tfz-d topology constructed by trinuclear paddlewheel [Eu3(CO2)6] SBUs; 3–5 are isostructural and show 3D (Ln-dcppa4–) + 2D (K-dcppa4–) → 3D (LnK-dcppa4–) structure. Eu-3, 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 (KSV = 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 Gd-5 demonstrates that the ligand ideally populates Eu­(III)/Tb­(III) emission with luminescent quantum yields (Φoverall) of 11.2% for Eu-1, 27.6% for Eu-2, 18.5% for Eu-3, and 62% for Tb-4, respectively.本研究通过水热（溶剂热）法合成了5种由4-(3′,5′-二羧基苯氧基)邻苯二甲酸（H4dcppa）构筑的新型三维镧系金属有机框架（Ln-MOFs），具体包括{[Eu(Hdcppa)(H₂O)₂]·H₂O}ₙ（Eu-1）、[Eu₁.₅(dcppa)(HCOO)₀.₅(H₂O)₂]ₙ（Eu-2）以及{[Ln₂K₂(dcppa)₂(H₂O)₆]·mH₂O}ₙ（其中Ln分别对应Eu-3、Tb-4、Gd-5，Eu-3与Tb-4的m值为5，Gd-5的m值为4，HCOOH为甲酸），并对所有产物进行了全面表征。结构解析结果表明，H4dcppa配体在1~5号配合物中存在4种不同的配位模式，由此构建得到结构多样的目标Ln-MOFs：Eu-1呈现罕见的三维(5,5)连接vbk拓扑网络，其结构中包含一维左手与右手螺旋链；Eu-2由三核桨轮状[Eu₃(CO₂)₆]次级结构单元（secondary building units, SBUs）构筑得到三维(3,8)连接tfz-d拓扑结构；3~5号配合物为同构体，呈现3D (Ln-dcppa⁴–) + 2D (K-dcppa⁴–) → 3D (LnK-dcppa⁴–)的复合结构。Eu-3具有规整的一维孔道，且孔道表面暴露路易斯碱性氧原子，可通过路易斯碱相互作用实现对金属离子的特异性识别与结合，对Fe³+和Cu²+离子表现出高选择性、快速响应（8 min）以及高灵敏度（针对Fe³+与Cu²+，斯特恩-沃尔默常数KSV=4.3/5.2×10⁴ L/mol），发光淬灭效果显著。此外，该传感器的检测限可达10⁻⁶ M量级。通过Gd(III)基MOF（即Gd-5）研究了H4dcppa的三重态能级（T₁=23364 cm⁻¹），结果表明该配体可有效敏化Eu(III)与Tb(III)的发光，各配合物的总发光量子产率（Φoverall）分别为：Eu-1 11.2%、Eu-2 27.6%、Eu-3 18.5%以及Tb-4 62%。