Turn-off luminescence sensing, white light emission and magnetic studies of two-dimensional lanthanide MOFs

Abstract

The lanthanide metal organic framework compounds [Ln(BPTA)_1.5(Bpy)]·0.5DMF (Ln = Y, Eu, Gd, Tb, Dy; 1a–5a) and [Ln(BPTA)_1.5(Phen)]·0.5DMF (Ln = Y, Eu, Gd, Tb, Dy; 1b–5b) were prepared by employing 2,5-bis(prop-2-yn-1-yloxy)terephthalic acid (2,5-BPTA) as the primary ligand and 2,2′-bipyridine (1a–5a) and 1,10-phenanthroline (1b–5b) as the secondary ligands. Single-crystal structural studies on [Gd(BPTA)_1.5(Bpy)]·0.5DMF (3a) and [Dy(BPTA)_1.5(Phen)]·0.5DMF (5b) indicated that the compounds have a two-dimensional structure. The Y compound exhibits blue emission, and the other compounds exhibit emission in the expected regions (λ_ex = 350 nm). White light emission was achieved by careful mixing of the red (Eu³⁺) and green (Tb³⁺) components in the blue emitting Y compound. Thus, Y_0.96Tb_0.02Eu_0.02 (bpy) and Y_0.939Tb_0.06Eu_0.001 (phen) were found to show white emission when excited using a wavelength of 350 nm. The introduction of N–N-containing ancillary ligands (i.e., bpy and phen) increased the overall quantum yield (QY) of white light emission to 31% and 43%, respectively. The high QY observed for the Tb and Eu compounds was found to be sensitive and selective for the fluorometric detection of azinphos-methyl pesticide and trinitrophenol (TNP) in an aqueous medium at the ppb level. The same behaviour was observed when utilising the compounds as onsite paper strip sensors. Their magnetic properties were also studied, revealing for the Tb and Dy derivatives slow relaxation of the magnetisation at low temperature. The present study highlights the usefulness of rigid π-conjugated molecules such as 2,2′-bipyridine and 1,10-phenanthroline in enhancing the many utilities of rare-earth-containing MOFs towards white light emission, the sensing of harmful and dangerous substances and magnetic properties.