A luminescent Eu coordination polymer with a near-visible excitation for the sensing and its homologues constructed from 1,4- benzenedicarboxylate and 1H-imidazo[4,5-f][1,10]-phenanthroline
The Eu-coordination polymer and its homologues, namely, five lanthanide coordination polymers (Ln-CPs), [Ln(BDC)1.5(IP)(H2O)]·nH2O (Ln = Sm 1, Eu 2, Gd 3, Tb 4, Dy 5) were constructed from 1,4-benzenedicarboxylate (BDC) and 1H-imidazo[4,5-f][1,10]-phenanthroline (IP). They feature 2D network structures with LnO6N2 units by bridging BDC ligands, while the IP molecules as terminal ligands are coordinated to the Ln(III) ions by bidentate chelating mode. The luminescence of these compounds was recorded. The Eu-CP has several distinct advantages including a broad excitation band (315 nm < λex < 410 nm) with the maximum excitation wavelength up to 375 nm, visible red emission, a good aqueous stability in a wide pH range of 4–10. Using the Eu-CP as a chemical sensor, highly luminescent sensing for emodin in aqueous medium was realized at excitation 375 nm. Remarkably, when excited at 400 nm visible light, the Eu-CP also exhibited similar luminescence sensing capacity. UV-Vis absorption spectral studies and density functional theory (DFT) calculations were performed to elucidate the possible mechanism of luminescence quenching of the Eu-CP by emodin. Similarly, the detection of common ions (Fe3+, Ag+, Cr2O72-/CrO42-, MnO4-) was also realized under the near-visible light of 375 or 400 nm. The experimental results prove that the Eu-CP is an excellent luminescence compound, as the excitation wavelength can be adjusted to the near-visible range (350 nm < λex < 410 nm), and strong luminescence and luminescence sensing for analytes can be achieved.