New 2D zinc phosphonates with heptanuclear units, reversible dehydration–hydration and bright luminescence

Abstract

Two new zinc phosphonates with formulas [Zn₁₈(L)₁₂(H₂N(CH₂)₄NH₂)₂]·25.5H₂O (1) and [Zn₉(L)₆(HN(C₂H₄NH₂)₂)₂]·10H₂O (2) have been synthesized starting from zinc acetate, 1-C₁₀H₇-CH₂N(CH₂COOH)(CH₂PO₃H₂) (H₃L) and 1,4-butanediamine (for 1)/diethylenetriamine (for 2). Compounds 1 and 2 are characterized by single-crystal X-ray crystallography, powder XRD, elemental analysis, IR spectroscopy, and thermogravimetric (TG) analysis. In the crystal structures of compounds 1 and 2, Zn₇ heptanuclear clusters are bridged by [Zn₂(H₂N(CH₂)₄NH₂)] and [Zn(HN(C₂H₄NH₂)₂)] fragments, respectively, to form two-dimensional (2D) frameworks. Compound 1 is a dynamic framework upon the dehydration–hydration process and displays bright purple emission, while the framework of compound 2 is stable up to 250 °C under an air atmosphere. And the luminescence of compound 2 can be tuned by heating treatment.