Structure and Photoluminescence Tuning Features of Mn²⁺- and Ln³⁺-Activated Zn-Based Heterometal–Organic Frameworks (MOFs) with a Single 5-Methylisophthalic Acid Ligand

In attempts to investigate whether the photoluminescence properties of the Zn-based heterometal–organic frameworks (MOFs) could be tuned by doping different Ln³⁺ (Ln = Sm, Eu, Tb) and Mn²⁺ ions, seven novel 3D homo- and hetero-MOFs with a rich variety of network topologies, namely, [Zn(mip)]_n (Zn–Zn), [Zn₂Mn(OH)₂(mip)₂]_n (Zn–Mn), [Mn₂Mn(OH)₂(mip)₂]_n (Mn–Mn), [ZnSm(OH)(mip)₂]_n (Zn–Sm), [ZnEu(OH)(mip)₂]_n (Zn–Eu1), [Zn₅Eu(OH)(H₂O)₃(mip)₆·(H₂O)]_n (Zn–Eu2), and [Zn₅Tb(OH)(H₂O)₃(mip)₆]_n (Zn–Tb), (mip = 5-methylisophthalate dianion), have been synthesized hydrothermally based on a single 5-methylisophthalic acid ligand. All compounds are fully structurally characterized by elemental analysis, FT-IR spectroscopy, TG-DTA analysis, single-crystal X-ray diffraction, and X-ray powder diffraction (XRPD) techniques. The various connectivity modes of the mip linkers generate four types of different structures. Type I (Zn–Zn) is a 3D homo-MOF with helical channels composed of Zn₂(COO)₄ SBUs (second building units). Type II (Zn–Mn and Mn–Mn) displays a nest-like 3D homo- or hetero-MOF featuring window-shaped helical channels composed of Zn₄Mn₂(OH)₄(COO)₈ or Mn₄Mn₂(OH)₄(COO)₈ SBUs. Type III (Zn–Sm and Zn–Eu1) presents a complicated corbeil-like 3D hetero-MOF with irregular helical channels composed of (SmZnO)₂(COO)₈ or (EuZnO)₂(COO)₈ heterometallic SBUs. Type IV (Zn–Eu2 and Zn–Tb) contains a heterometallic SBU Zn₅Eu(OH)(COO)₁₂ or Zn₅Tb(OH)(COO)₁₂, which results in a 3D hetero-MOF featuring irregular channels impregnated by parts of the free and coordinated water molecules. Photoluminescence properties indicate that all of the compounds exhibit photoluminescence in the solid state at room temperature. Compared with a broad emission band at ca. 475 nm (λ_ex = 380 nm) for Zn–Zn, compound Zn–Mn exhibits a remarkably intense emission band centered at 737 nm (λ_ex = 320 nm) due to the characteristic emission of Mn²⁺. In addition, the fluorescence intensity of compound Zn–Mn is stronger than that of Mn–Mn as a result of Zn²⁺ behaving as an activator for the Mn²⁺ emission. Compound Zn–Sm displays a typical Sm³⁺ emission spectrum, and the peak at 596 nm is the strongest one (λ_ex = 310 nm). Both Zn–Eu1 and Zn–Eu2 give the characteristic emission transitions of the Eu³⁺ ions (λ_ex = 310 nm). Thanks to the ambient different crystal-field strengths, crystal field symmetries, and coordinated bonds of the Eu³⁺ ions in compounds Zn–Eu1 and Zn–Eu2, the spectrum of the former compound is dominated by the ⁵D₀ → ⁷F₂ transition (612 nm), while the emission of the ⁵D₀ → ⁷F₄ transition (699 nm) for the latter one is the most intense. Compound Zn–Tb emits the characteristic Tb³⁺ ion spectrum dominated by the ⁵D₄ → ⁷F₅ (544 nm) transition. Upon addition of the different activated ions, the luminescence lifetimes of the compounds are also changed from the nanosecond (Zn–Zn) to the microsecond (Zn–Mn, Mn–Mn, and Zn–Sm) and millisecond (Zn–Eu1, Zn–Eu2, and Zn–Tb) magnitude orders. The structure and photoluminescent property correlations suggest that the presence of Mn²⁺ and Ln³⁺ ions can activate the Zn-based hetero-MOFs to emit the tunable photoluminescence.