Multidimensional (0D to 3D) Alkaline-Earth Metal Diphosphonates: Synthesis, Structural Diversity, and Luminescence Properties

A series of new alkaline-earth metal diphosphonate frameworks were successfully synthesized under solvothermal reaction condition (160 °C, 3 d) using 1-hydroxyethylidene-1,1-diphosphonic acid (CH₃C­(OH)­(H₂PO₃)₂, hedpH₄) as a diphosphonate building block and Mg­(II), Ca­(II), Sr­(II), or Ba­(II) ions as alkaline-earth metal ion centers in water, dimethylformamide, and/or EtOH media. These diphosphonate frameworks, (H₂NMe₂)₄­[Mg­(hedpH₂)₃]·3H₂O (<b>1</b>), (H₂NMe₂)₂­[Ca­(hedpH₂)₂] (<b>2</b>), (H₂NMe₂)₂­[Sr₃(hedpH₂)₄­(H₂O)₂] (<b>3</b>), and [Ba₃(hedpH₂)₃]·H₂O (<b>4</b>) exhibited interesting structural topologies (zero-, one-, two-, and three-dimensional (0D, 1D, 2D, and 3D, respectively)), which are mainly depending on the metal ions and the solvents used in the synthesis. The single-crystal analysis of these newly synthesized compounds revealed that <b>1</b> was a 0D molecule, <b>2</b> has 1D chains, <b>3</b> was a 3D molecule, and <b>4</b> has 2D layers. All compounds were further characterized using thermogravimetric analysis, solid-state ³¹P NMR, powder X-ray diffraction analysis, UV–vis spectra, and infrared spectroscopy. In addition, Eu­(III)- and Tb­(III)-doped compounds of <b>1</b>–<b>4</b>, namely, (H₂NMe₂)₄­[Ln_<i>x</i>Mg_1–<i>x</i>(hedpH₂)₂­(hedpH_2–<i>x</i>)]·3H₂O (<b>1Ln</b>), (H₂NMe₂)₂­[Ln_<i>x</i>Ca_1–<i>x</i>(hedpH₂)­(hedpH_2–<i>x</i>)] (<b>2Ln</b>), (H₂NMe₂)₂[Ln_<i>x</i>Sr_3–<i>x</i>(hedpH₂)₃(hedpH_2–<i>x</i>)­(H₂O)₂] (<b>3Ln</b>), and [Ln_<i>x</i>Ba_3–<i>x</i>(hedpH₂)₂­(hedpH_2–<i>x</i>)]·H₂O (<b>4Ln</b>) (where Ln = Eu, Tb), were synthesized, and their photoluminescence properties were studied. The quantum yield of <b>1Eu</b>–<b>4Eu</b> was measured with reference to commercial red phosphor, Y₂O₂S:Eu³⁺ (YE), and the quantum yield of terbium-doped compounds <b>1Tb</b>–<b>4Tb</b> was measured with reference to commercial green-emitting phosphor CeMgAl₁₀O₁₇:Tb³⁺. Interestingly, the compound <b>2Eu</b> showed very high quantum yield of 92.2%, which is better than that of the reference commercial red phosphor, YE (90.8%)