Xinrong Liu

Two-/three-dimensional open lanthanide–organic frameworks containing rigid/flexible dicarboxylate ligands: synthesis, crystal structure and photoluminescent properties

Three series of two-dimensional and three-dimensional lanthanide coordination polymers {[Ln(ox)1.5(H2O)3]·mH2O}n [Ln = Ce (1), m = 2 and Pr (2), m = 3, H2ox = oxalic acid], {[Ln(ox)2(H3O)]·EtOH·3H2O}n [Ln = Nd (3), Sm (4), Eu (5), Gd (6), Tb (7), Dy (8), Ho (9), Er (10) and Yb (11), EtOH = ethanol] and {[Ln2(IMDC)2(H2O)3]·mH2O}n [Ln = Sm (12), Eu (13), Gd (14), Tb (15) and Dy (16), where m = 2.25 except for 15 where m = 1.75, H3IMDC = imidazole-4,5-dicarboxylic acid] have been synthesized under hydrothermal conditions. All the coordination polymers have been characterized by elemental analysis, IR spectroscopy and X-ray single-crystal diffraction. Coordination polymers 1–11 exhibit two different structural types: the 2-D structure of coordination polymers 1–2 are a (6,3)-connected hcb network, coordination polymers 3–11 appear as a 3-D diamond structure with 66-network topology. Coordination polymers 12–16 are 3-D structures constructed from the bridged carboxyl group of IMDC3−, in which IMDC3− bridged the DNA-like double-helix line and the triple-helix line, intertwined and connected into a 1-D chain and then conformed into a 3-D structure along the ab plane. The different structures of the two dicarboxylate ligands (oxalic acid as a flexible ligand and imidazole-4,5-dicarboxylic acid as a rigid planar ligand) were described and discussed. Furthermore, the luminescent properties in the visible region and the fluorescence lifetimes of the coordination polymers exhibited the characteristic transitions of the corresponding lanthanide ions. It was noted that coordination polymers 5 and 7 have long solid-state fluorescence lifetimes of 824.84 and 733.87 μs, respectively. The NIR emission spectra of the Nd (3), Sm (4 and 12), Dy (8 and 16) and Yb (3) coordination polymers in the solid-state were measured. The singlet excited state (30 842 cm−1 for oxalic acid, 32 258 cm−1 for H3IMDC) and the lowest triplet state energy level (23 753 cm−1 for oxalic acid and 22 371 cm−1 for H3IMDC) of the H2ox and H3IMDC ligand were calculated on the basis of the UV-Vis absorbance edges of the ligand and the phosphorescence spectrum of the Gd(III) coordination polymers 6 and 14 at 77 K, respectively. The relationship between the lowest triplet state energy level of the two dicarboxylate ligands and lowest resonance energy levels of the Sm(III), Eu(III), Tb(III) and Dy(III) ions were described and discussed. The results showed that the energy transfer from the oxalic acid ligands is much more effective than H3IMDC.

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

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.

Multiaxial Fatigue Crack Orientation and Early Growth Investigation Considering the Nonproportional Loading

The paper presents a comprehensive investigation of fatigue cracking behaviors under various nonproportional multiaxial cycle loading paths based on the critical plane approach. The maximum normal and shear stress/strain fields are presented to analyze the crack orientation and early growth directions in polar diagrams. The experimental observed crack paths and directions were compared with maximum strain loci of each angle to determine multiaxial fatigue failure mode. The results show that crack orientation and growth paths appear in the maximum shear and normal strain plane, respectively. Likelihood cracking regions of various loading paths are predicted according the determined failure mode. Besides, nonproportionality factor is introduced to characterize the degree of multiaxiality on these loading paths.