1-D helical chain, 2-D layered network and 3-D porous lanthanide–organic frameworks based on multiple coordination sites of benzimidazole-5,6-dicarboxylic acid: synthesis, crystal structure, photoluminescence and thermal stability

Abstract

One-dimensional to three-dimensional lanthanide coordination polymers 1–8 based on benzimidazole-5,6-dicarboxylic acid (H₃BIDC) have been synthesized under hydrothermal conditions at different pH values, generally formulated as {[Pr(HBIDC)(ox)_0.5(H₂O)]·H₂O}_n (1), [Yb(HBIDC)(ox)_0.5(H₂O)₂]_n (2), and [Ln(HBIDC)(ox)_0.5(H₂O)₃]_n [Ln = Ho (3), and Tb (4)] and {[Ln(H₂BIDC)(HBIDC)(H₂O)₃]·3H₂O}_n [Ln = Tb (5), Sm (6), Dy (7), and Gd (8), H₂ox = oxalic acid]. All coordination polymers have been characterized by elemental analysis, infrared spectra and single-crystal X-ray diffraction. The structural diversity, luminescence and thermal properties of all coordination polymers have been investigated. Coordination polymers 1–8 exhibit four different structural types: topological analysis has given the 3-D pcu network, with the point symbol of {4¹²·6³} in coordination polymer 1. Coordination polymer 2 exhibits a 4-connected 4⁴ topology, and coordination polymers 3–4 appear as 2-D (6,3)-connected hcb network topology. The 1-D helical infinite chain of coordination polymers 5–8 around the crystallographic 2₁ axis spread along the b axis direction, with different 1-D helical infinite chains forming 3-D supramolecular framework via hydrogen bonds and π–π stacking interactions. The coordination polymers 4 and 5 could be triggered to have intense characteristic lanthanide-centered green luminescence under UV excitation in the solid state at room and liquid nitrogen temperature, or dispersed in CH₂Cl₂ at 77 K. In coordination polymers 4 and 5, the oxalic acid introduced into coordination polymer 4 as a second ligand further sensitized the trivalent terbium ion, and resulted in longer fluorescence lifetimes of coordination polymer 4 (1058.58 μs at 298 K, 679.42 μs at 77 K in the solid-state, 867.82 μs in CH₂Cl₂ at 77 K) than coordination polymer 5 (595.06 μs at 298 K, 583.19 μs at 77 K in the solid-state, 584.38 μs in CH₂Cl₂ at 77 K). In coordination polymers 6 and 7, we not only measured emission spectra in the visible region, but also detected the infrequent NIR emission spectra in the near infrared region of samarium and dysprosium ions. The singlet excited state (30 303 cm⁻¹) and the lowest triplet state energy level (24 390 cm⁻¹) of H₃BIDC ligand were calculated based on the UV-vis absorbance edges of ligand and the phosphorescence spectrum of Gd(III) coordination polymer (8) at 77 K, showing that the effective extent of energy transfer from H₃BIDC ligand to lanthanide ions follows the sequence of Tb³⁺, Dy³⁺ > Sm³⁺. Finally, thermal behaviors of all coordination polymers were studied by thermogravimetric analysis, which exhibited high thermal stability.