Alkaline-earth and aminonicotinate based coordination polymers with combined fluorescence/long-lasting phosphorescence and metal ion sensing response

Abstract

Six novel coordination polymers (CPs) based on alkaline-earth (AE) metal ions and two sorts of aminonicotinate ligands, namely [M(μ-2ani)(μ₃-2ani)(μ-DMF)]_n [M^II = Sr (1) and Ba (2)], {[M(μ-6ani)₂(H₂O)₂]·3H₂O}_n [M^II = Ca (3) and Sr (4)], {[Ca(μ-6ani)₂(H₂O)₂]}_n (5) and {[Ba(6ani)₂(H₂O)₃]·7H₂O}_n (6) [where 2ani = 2-aminonicotinate and 6ani = 6-aminonicotinate] have been synthesized and well characterized. All compounds are firstly built up with AE-carboxylate infinite rods that provide compact 2D-layered structures in 1 and 2 but open architectures containing microchannels in an AE-6ani system. Interestingly, the architecture of 3 behaves as a flexible platform which undergoes spontaneous dehydration to yield compound 5. Structural transition occurring from 3 to 5 has been supported by periodic density functional theory (DFT) calculations and X-ray diffraction data. In this regard, compound 6 contains a high amount of solvent molecules which suggests the existence of pores in its structure. A deep study of the optical properties including a detailed experimental characterization and DFT based computational calculations of these compounds supports a fascinating photoluminescence performance which entails a bright blue emission accompanied by persistent green afterglows at low temperature, especially for compound 5 as reflected by its lifetime (τ ≈ 1.4 s). In addition to its strong room temperature emission properties, we took advantage of the open yet stable structure of compound 4, to check its viability as a new material sensitive to the presence of solvents and/or transition metal ions, via significant quenching of the luminescence of 4. Remarkably, 4 is shown to be a particularly efficient sensor for some water soluble pollutants.