Structure and Photoluminescence Tuning Features of Mn<sup>2+</sup>- and Ln<sup>3+</sup>-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 Ln<sup>3+</sup> (Ln = Sm, Eu, Tb) and Mn<sup>2+</sup> ions, seven novel 3D homo- and hetero-MOFs with a rich variety of network topologies, namely, [Zn(mip)]<sub><i>n</i></sub> (<b>Zn–Zn</b>), [Zn<sub>2</sub>Mn(OH)<sub>2</sub>(mip)<sub>2</sub>]<sub><i>n</i></sub> (<b>Zn–Mn</b>), [Mn<sub>2</sub>Mn(OH)<sub>2</sub>(mip)<sub>2</sub>]<sub><i>n</i></sub> (<b>Mn–Mn</b>), [ZnSm(OH)(mip)<sub>2</sub>]<sub><i>n</i></sub> (<b>Zn–Sm</b>), [ZnEu(OH)(mip)<sub>2</sub>]<sub><i>n</i></sub> (<b>Zn–Eu1</b>), [Zn<sub>5</sub>Eu(OH)(H<sub>2</sub>O)<sub>3</sub>(mip)<sub>6</sub>·(H<sub>2</sub>O)]<sub><i>n</i></sub> (<b>Zn–Eu2</b>), and [Zn<sub>5</sub>Tb(OH)(H<sub>2</sub>O)<sub>3</sub>(mip)<sub>6</sub>]<sub><i>n</i></sub> (<b>Zn–Tb</b>), (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 (<b>Zn–Zn</b>) is a 3D homo-MOF with helical channels composed of Zn<sub>2</sub>(COO)<sub>4</sub> SBUs (second building units). Type II (<b>Zn–Mn</b> and <b>Mn–Mn</b>) displays a nest-like 3D homo- or hetero-MOF featuring window-shaped helical channels composed of Zn<sub>4</sub>Mn<sub>2</sub>(OH)<sub>4</sub>(COO)<sub>8</sub> or Mn<sub>4</sub>Mn<sub>2</sub>(OH)<sub>4</sub>(COO)<sub>8</sub> SBUs. Type III (<b>Zn–Sm</b> and <b>Zn–Eu1</b>) presents a complicated corbeil-like 3D hetero-MOF with irregular helical channels composed of (SmZnO)<sub>2</sub>(COO)<sub>8</sub> or (EuZnO)<sub>2</sub>(COO)<sub>8</sub> heterometallic SBUs. Type IV (<b>Zn–Eu2</b> and <b>Zn–Tb</b>) contains a heterometallic SBU Zn<sub>5</sub>Eu(OH)(COO)<sub>12</sub> or Zn<sub>5</sub>Tb(OH)(COO)<sub>12</sub>, 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<sub>ex</sub> = 380 nm) for <b>Zn–Zn</b>, compound <b>Zn–Mn</b> exhibits a remarkably intense emission band centered at 737 nm (λ<sub>ex</sub> = 320 nm) due to the characteristic emission of Mn<sup>2+</sup>. In addition, the fluorescence intensity of compound <b>Zn–Mn</b> is stronger than that of <b>Mn–Mn</b> as a result of Zn<sup>2+</sup> behaving as an activator for the Mn<sup>2+</sup> emission. Compound <b>Zn–Sm</b> displays a typical Sm<sup>3+</sup> emission spectrum, and the peak at 596 nm is the strongest one (λ<sub>ex</sub> = 310 nm). Both <b>Zn–Eu1</b> and <b>Zn–Eu2</b> give the characteristic emission transitions of the Eu<sup>3+</sup> ions (λ<sub>ex</sub> = 310 nm). Thanks to the ambient different crystal-field strengths, crystal field symmetries, and coordinated bonds of the Eu<sup>3+</sup> ions in compounds <b>Zn–Eu1</b> and <b>Zn–Eu2</b>, the spectrum of the former compound is dominated by the <sup>5</sup>D<sub>0</sub> → <sup>7</sup>F<sub>2</sub> transition (612 nm), while the emission of the <sup>5</sup>D<sub>0</sub> → <sup>7</sup>F<sub>4</sub> transition (699 nm) for the latter one is the most intense. Compound <b>Zn–Tb</b> emits the characteristic Tb<sup>3+</sup> ion spectrum dominated by the <sup>5</sup>D<sub>4</sub> → <sup>7</sup>F<sub>5</sub> (544 nm) transition. Upon addition of the different activated ions, the luminescence lifetimes of the compounds are also changed from the nanosecond (<b>Zn–Zn</b>) to the microsecond (<b>Zn–Mn</b>, <b>Mn–Mn</b>, and <b>Zn–Sm</b>) and millisecond (<b>Zn–Eu1</b>, <b>Zn–Eu2</b>, and <b>Zn–Tb</b>) magnitude orders. The structure and photoluminescent property correlations suggest that the presence of Mn<sup>2+</sup> and Ln<sup>3+</sup> ions can activate the Zn-based hetero-MOFs to emit the tunable photoluminescence.