ABSTRACT
Two new isostructural carboxylate-bridged lanthanide ribbons having the chemical formula [Ln2(4-ABA)6]n [4-ABA = 4-aminobenzoate, Ln: holmium (Ho) and erbium (Er)] were synthesized by a solvothermal method and fully characterized using multiple analytical, spectroscopic, and computational techniques. Single-crystal X-ray diffraction analysis reveals that both lanthanide coordination polymers (Ln-CPs) exhibit linear ribbon-like structures built up by dinuclear Ln2(4-ABA)6 units and bridged by carboxylate groups. Ln-CPs showed remarkably high thermal and chemical stabilities. Ho-CP and Er-CP exhibited similar band gaps of 3.21 eV and 3.22 eV, respectively, showing their photocatalytic ability under UV light. The photocatalytic activities of Ln-CPs were examined in the CO2 cycloaddition of epoxides to cyclic carbonates under solvent-free conditions, and full conversion (yields up to 99.9%) to the product was achieved. Ln-CP photocatalysts retained the same product yields over five consecutive cycles. Additionally, the experimental magnetic studies indicated that both Ln-CP crystals are antiferromagnetic at low T, which is confirmed by density functional theory calculations.
ABSTRACT
Two lanthanide metal-organic frameworks [Ln-MOFs, Ln = Eu(III), Tb(III)] composed of oxalic acid and Ln building units were hydrothermally synthesized and fully characterized by powder X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscope, and energy-dispersive X-ray spectroscopy. Furthermore, their magnetic susceptibility measurements were obtained using SQUID based vibrating sample magnetometer (MPMS 3, Quantum Design). Both Ln-MOFs exhibited highly efficient luminescent property. Solid-state photoluminescence (PL) measurements revealed phosphorescence emission bands of Eu-MOF and Tb-MOF centered at 618 nm (red emission) and 550 nm (green emission) upon excitation at 396 nm and 285 nm, respectively. Eu-MOF and Tb-MOF displayed a phosphorescence quantum yield of 53% and 40%, respectively. Time-resolved PL analyses showed very long lifetime values, at 600 and 1065 ± 1 µs for Eu-MOF and Tb-MOF, respectively. Calculations performed by density functional theory indicated a charge transfer form metal centres to the ligand which was in good agreement with the experimental studies. Therefore, this new mode of highly photoluminescent MOF materials is studied for the first time which paves the way for better understanding of these systems for potential applications.