Assembly of one-, two-, and three-dimensional Ln(iii) complexes constructed from a novel asymmetric tricarboxylic acid: synthesis, structure, photoluminescence and tunable white-light emission

Abstract

By reacting an asymmetric semi-rigid V-shaped linker H₃dpob (H₃dpob = 3-(2′3′-dicarboxylphenoxy)benzoic acid) and Ln(NO₃)₃·6H₂O, nine novel Ln-based luminescent materials, from 1D to 3D, namely {[Eu(dpob)(phen)]·H₂O}_n (1), {[Ln(Hdpob)(ox)_0.5(H₂O)₂]}_n (Ln = Eu(2), Sm(3), Gd(4), Tb(5)), {[Eu(dpob)(H₂O)₂]·0.5H₂O}_n (6), and {[Ln(dpob)(H₂O)₂]·mH₂O}_n (Ln = Eu(7), Gd(8), Tb(9), m = 0.5 for 7 and 9; m = 1 for 8) (phen = 1,10-phenanthroline; H₂ox = oxalic acid) have been hydrothermally synthesized and characterized by single-crystal X-ray diffraction, infrared (IR) spectroscopy, elemental analysis, and PXRD. The crystal structures of 1–9 indicate that the coordination modes and coordination configuration of the H₃dpob ligand play critical roles in the formation of the lanthanide architectures. Complexes 1–5 display multiple structures from double-stranded 1D chains to 4-connected 2D layers, through [Eu₂(CO₂)₄] or [Eu₂(CO₂)₂] dinuclear units with different auxiliary ligands. Complexes 6 and 7 are genuine supramolecular isomers which are induced by the concentration effect. 6 possesses a 2D kgd network with a Schläfli symbol of (4³)₂(4⁶·6⁶·8³) built from 6-connected [Eu₂(CO₂)₂] units and 3-connected H₃dpob, which further connects to a (3,8)-connected tfz-d topology through O–H⋯O hydrogen bonds. 7 displays a 3D (3,6)-connected rtl network with the Schläfli symbol (4·6²)₂(4²·6¹⁰·8³). Eu complexes 1, 2, 6, and 7 as well as Tb complexes 5 and 9 could provide intense and bright characteristic ⁵D₀ → ⁷F_J/⁵D₄ → ⁷F_J red/green luminescence under UV excitation in the solid state at 298 K and 77 K. The calculated singlet and triplet energies of H₃dpob as well as phen and H₂ox ligands indicate that these ligands act as antenna chromophores that are able to efficiently absorb and transfer energy to Ln(III) ions. In complexes 1, 2 and 5, ligand-to-metal energy transfer processes could occur in mixed ligands. However, these processes occurred in a single H₃dpob ligand in complexes 6 and 7. With careful adjustment of the relative concentration of the lanthanide ions and by varying the excitation wavelengths of {[Gd_0.92Eu_0.04Tb_0.04(dpob)(H₂O)₂]·0.5H₂O}_n (10), tunable yellow (CIE coordinate: 0.51, 0.40) to white-light (CIE coordinate: 0.33, 0.34) emission has been obtained.