Hydrothermal synthesis, structure, and optical properties of two nanosized Ln₂₆@CO₃ (Ln = Dy and Tb) cluster-based lanthanide-transition-metal-organic frameworks (Ln MOFs)

Two Ln₂₆@CO₃ (Ln = Dy and Tb) cluster-based lanthanide-transition-metal-organic frameworks (Ln MOFs) formulated as [Dy₂₆Cu₃(Nic)₂₄(CH₃COO)₈(CO₃)₁₁(OH)₂₆ (H₂O)₁₄]Cl.3H₂O (1 ; HNic = nicotinic acid) and [Tb₂₆NaAg₃(Nic)₂₇- (CH₃COO)₆(CO₃ )₁₁(OH)₂₆Cl(H₂O)₁₅].7.5H₂O (2) have been successfully synthesized by hydrothermal methods and characterized by IR, thermogravimetric analysis (TGA), elemental analysis, and single X-ray diffraction. Compound 1 crystallizes in the monoclinic space group Cc with a = 35.775(12) Å, b= 33.346(11) Å, c = 24.424(8) Å, β = 93.993(5)°, V = 29065(16) ų, whereas 2 crystallizes in the triclinic space group P 1¯ with a = 20.4929(19) Å, b = 24.671(2) Å, c = 29.727(3) Å, α= 81.9990(10) °, β = 88.0830(10) °, γ = 89.9940(10) °, V= 14875(2) ų. Structural analysis indicates the framework of 1 is a 3D perovskite-like structure constructed out of CO₃@Dy₂₆ building units and Cu ⁺ centers by means of nicotinic acid ligand bridging. In 2, however, nanosized CO3@Tb26 units and [Ag3Cl]^{2 +} centers are connected by NicÅ bridges to give rise to a 2D structure. It is worth mentioning that this kind of 4d-4f cluster-based MOF is quite rare as most of the reported analogous compounds are 3d-4f ones. Additionally, the solid-state emission spectra of pure compound 2 at room temperature suggest an efficient energy transfer from the ligand Nic^- to Tb^{3 +} ions, which we called the "antenna effect". Compound 2 shows a good twophoton absorption (TPA) with a TPA coefficient of 0.06947 cmGM^-1 (1 GM = 10^-50 cm⁴ sphoton^-1), which indicates that compound 2 might be a good choice for thirdorder nonlinear optical materials.