Phase selectivity and tunable photophysical nature of rare earth metal–organic frameworks of EuxY1−x-PTC (H3PTC = 2,4,6-pyridine tricarboxylic acid; x = 0–1)

Abstract

Two rare earth metal–organic frameworks (MOFs), [Y₂(PTC)₂(H₂O)₂]·3H₂O (Y-PTC) and [Eu₂(PTC)₂(H₂O)₅]·H₂O (Eu-PTC) together with the solid solutions [Eu_2xY_2(1−x)(PTC)₂(H₂O)₅]·H₂O (Eu_xY_1−x-PTC, x = 0.013–0.82), were synthesized hydrothermally, and characterized by microanalysis, IR spectroscopy, TG, powder, and single crystal X-ray diffraction techniques. Eu-PTC and Y-PTC showed different crystal structures; however, all solid solutions were isomorphic to Eu-PTC even at x = 0.013, leading to the IR spectra and TG plots of the solid solutions to be similar to those of Eu-PTC but distinct from those of Y-PTC. DFT calculations for the crystal lattice energy demonstrated that the procedure for the crystallizing nucleation of Eu-PTC occurred prior to that of Y-PTC in the reaction solution, leading to the all solid solutions being isomorphic to Eu-PTC. The solid emission spectra at ambient condition showed that Y-PTC emitted ligand-based phosphorescence at 433 nm with a quantum yield (QY) of 27.02%, while Eu-PTC and Eu_xY_1−x-PTC (x = 0.013–0.82) emitted the characteristic luminescence of Eu³⁺ ions, and most solid solutions showed higher QYs than Eu-PTC; in particular, the QY of Eu_0.195Y_0.805-PTC was up to 29.48%, i.e., increased by 10% regarding Eu-PTC (19.86%). Interestingly, solid solutions with x = 0.013–0.395 showed excitation-wavelength-dependent luminescence, and such type of luminescence MOFs have promising applications including the areas of precise temperature, gas sensing and information encryption or anti-counterfeiting materials.