Synthesis, structure and optical properties of two novel luminescent polar dysprosium metal–organic frameworks: [(CH3)2NH2][Dy(HCOO)4] and [N2H5][Dy(HCOO)4]

Abstract

We report the synthesis, crystal structures, Raman, infrared, electron absorption and emission spectra of two novel luminescent MOFs, [(CH₃)₂NH₂][Dy(HCOO)₄] and [N₂H₅][Dy(HCOO)₄]. These data show that [(CH₃)₂NH₂][Dy(HCOO)₄] and [N₂H₅][Dy(HCOO)₄] crystallize in polar space groups of Pna2₁ and Pca2₁ symmetry, respectively. The polar properties of [N₂H₅][Dy(HCOO)₄] come from the arrangement of template cations that possess an internal dipole moment as well as from the symmetry of the framework. In [(CH₃)₂NH₂][Dy(HCOO)₄], the Dy(III)–formate framework is centrosymmetric and the polar properties of this compound arise from the arrangement of the DMA⁺ cations. Vibrational studies reveal that the position of the ν(NN) band, which is very sensitive to coordination changes, is characteristic for hydrazine molecules forming onium ions. Optical studies show that both compounds exhibit emission of Dy(III) ions, in spite of full concentration of dysprosium ions in the studied compounds. Our data also show that radiative decay times, 57 μs for [(CH₃)₂NH₂][Dy(HCOO)₄] and 96 μs for [N₂H₅][Dy(HCOO)₄], are very long compared to lifetimes observed in inorganic compounds. We have also preformed Judd–Ofelt analysis of the optical absorption spectrum of the hydrazinium analogue that provides information on various optical parameters such as electric-dipole oscillator strengths, Judd–Ofelt parameters Ω_λ, branching ratios and spontaneous emission probabilities.