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 (1), (H2NMe2)2­[Ca­(hedpH2)2] (2), (H2NMe2)2­[Sr3(hedpH2)4­(H2O)2] (3), and [Ba3(hedpH2)3]·H2O (4) 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 1 was a 0D molecule, 2 has 1D chains, 3 was a 3D molecule, and 4 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 1–4, namely, (H2NMe2)4­[LnxMg1–x(hedpH2)2­(hedpH2–x)]·3H2O (1Ln), (H2NMe2)2­[LnxCa1–x(hedpH2)­(hedpH2–x)] (2Ln), (H2NMe2)2[LnxSr3–x(hedpH2)3(hedpH2–x)­(H2O)2] (3Ln), and [LnxBa3–x(hedpH2)2­(hedpH2–x)]·H2O (4Ln) (where
Ln = Eu, Tb), were synthesized, and their photoluminescence properties
were studied. The quantum yield of 1Eu–4Eu was measured with reference to commercial red phosphor, Y2O2S:Eu3+ (YE), and the quantum yield of terbium-doped
compounds 1Tb–4Tb was measured with
reference to commercial green-emitting phosphor CeMgAl10O17:Tb3+. Interestingly, the compound 2Eu showed very high quantum yield of 92.2%, which is better
than that of the reference commercial red phosphor, YE (90.8%).