A luminescent Eu coordination polymer with near-visible excitation for sensing and its homologues constructed from 1,4-benzenedicarboxylate and 1H-imidazo[4,5-f][1,10]-phenanthroline

Abstract

A 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 in bidentate chelating mode. The luminescence of these compounds was recorded. Eu-CP has several distinct advantages including a broad excitation band (315 nm < λ_ex < 410 nm) with a maximum excitation wavelength of up to 375 nm, visible red emission, and a good aqueous stability in a wide pH range of 4–10. Using Eu-CP as a chemical sensor, high luminescence sensing for emodin in aqueous medium was realized at excitation of 375 nm. Remarkably, when excited with 400 nm visible light, Eu-CP also exhibited a 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 Eu-CP by emodin. Similarly, the detection of common ions (Fe³⁺, Ag⁺, Cr₂O₇²⁻/CrO₄²⁻, MnO₄⁻) was also realized under near-visible light of 375 or 400 nm. The experimental results prove that Eu-CP is an excellent luminescent compound, as its excitation wavelength can be adjusted to the near-visible range (350 nm < λ_ex < 410 nm), and strong luminescence and luminescence sensing for analytes can be achieved.