LnMOF@PVA nanofiber: energy transfer and multicolor light-emitting devices

Abstract

In this paper, we report the use of electrospinning methods for the preparation of polymeric nanofibers containing Lanthanide–Organic Frameworks (LnMOFs) and present a detailed investigation of their spectroscopic properties. The nanocomposites were based in polyvinyl alcohol (PVA) and two isotypical 2D LnMOFs ([Ln(DPA)(HDPA)], where H₂DPA is pyridine 2,6-dicarboxylic acid and Ln = Tb³⁺ and Eu³⁺ ions, which we have designated as EuMOF@PVA and TbMOF@PVA, respectively. Additionally, we prepared three tunable visible light-emitting devices based on nanofibers containing different [Tb(DPA)(HDPA)]/[Eu(DPA)(HDPA)] relative concentrations (95–5 wt%, 80–20 wt% and 50–50 wt%, designated as Tb_0.95Eu_0.05MOF@PVA, Tb_0.8Eu_0.2MOF@PVA, and Tb_0.5Eu_0.5MOF@PVA, respectively), and investigated the energy transfer among the optical centers. The emission spectrum of EuMOF@PVA presents typical narrow bands of the Eu^{3+ 5}D₀ → ⁷F_J transitions centered in a non-centrosymmetric site, and its profile indicates that in EuMOF@PVA the point group of the Eu³⁺ ion is distorted from the local C₁ symmetry observed in as-prepared material. Horrocks' rule applied to EuMOF@PVA leads to n_w = 0.40, in an indication that the encapsulation of LnMOF materials in PVA does not necessarily imply the coordination of water molecules in the inner-sphere of the lanthanide ion; thus, the distortion of the symmetry around of metal center can be solely associated to the steric effect of the PVA chains. The ⁵D₄ → ⁷F₅ transition centered at ca. 543 nm is the strongest in the emission spectrum of TbMOF@PVA, corresponding to ca. 52% of the integrated emission spectrum. The emission spectra of Tb_0.95Eu_0.05MOF@PVA, Tb_0.8Eu_0.2MOF@PVA, and Tb_0.5Eu_0.5MOF@PVA show lines typical of the Eu³⁺ and Tb³⁺ ions. The colors emitted by Tb_0.95Eu_0.05MOF@PVA, Tb_0.8Eu_0.2MOF@PVA and Tb_0.5Eu_0.5MOF@PVA are in the green-yellow (0.4083, 0.4803), yellow (0.4364, 0.4616) and orange (0.5042, 0.4212) portions of the chromaticity chart, respectively. The Tb³⁺ → Eu³⁺ 〈W_ET〉 and 〈n_ET〉 do not vary with the increasing of the acceptor (Eu³⁺) concentration, since the donor–acceptor interaction strength is a function of distance.