From Two-Dimensional Double Decker Architecture to Three-Dimensional<i>pcu</i>Framework with One-Dimensional Tube: Syntheses, Structures, Luminescence, and Magnetic Studies

Abstract

The hydrothermal reactions of lanthanide salts with substituted imidazole-4,5-dicarboxylic acids in the presence of aliphatic carboxylates afforded a new family of lanthanide metal–organic frameworks formulated as {Y3[(Heimda)4(μ2-HCOO)·3H2O]·H2O}n (1Y), {[Gd3(Heimda)4(μ2-HCOO)·4H2O]·2H2O}n (2Gd), {[Tb3(Heimda)4(μ2-HCOO)·4H2O]·2H2O}n (3Tb) and {[Nd3(Hpimda)2(μ2-HCOO) (μ2 -C2O4)2·6H2O]·4H2O}n (4Nd), (H3eimda = 2-ethyl-1H-imidazole-4,5-dicarboxylic acid, while H3pimda = 2-propyl-1H-imidazole-4,5-dicarboxylic acid). The structural diversity and photophysical and magnetic properties have been investigated. The polymer 3 triggers intense characteristic lanthanide-centered green luminescence under UV excitation, and it exhibits gradually increasing luminescence intensities when dispersed in water, ethanol, and DMSO as suspensions. After water molecules are liberated from the condensed frameworks of 4, the evacuated product (4a) exhibits nitrogen sorption properties at 77 K with a hysteresis, as well as strong characteristic emissions of the Nd(III) ion in the near-infrared (NIR) region. Polymer 2 displays very weak but significant ferromagnetic couplings between adjacent Gd(III) ions through the carboxylate bridging, whereas the depopulation of the Stark levels or possible antiferromagnetic interactions within both polymers 4 and 4a leads to a continuous decrease of χMT when the samples are cooled from 300 to 2 K.