Heterometallic Europium(III)–Lutetium(III) Terephthalates as Bright Luminescent Antenna MOFs

Abstract

A new series of luminescent heterometallic europium(III)–lutetium(III) terephthalate metal–organic frameworks, namely (EuxLu1−x)2bdc3·nH2O, was synthesized using a direct reaction in a water solution. At the Eu3+ concentration of 1–40 at %, the MOFs were formed as a binary mixture of the (EuxLu1−x)2bdc3 and (EuxLu1−x)2bdc3·4H2O crystalline phases, where the Ln2bdc3·4H2O crystalline phase was enriched by europium(III) ions. At an Eu3+ concentration of more than 40 at %, only one crystalline phase was formed: (EuxLu1−x)2bdc3·4H2O. All MOFs containing Eu3+ exhibited sensitization of bright Eu3+-centered luminescence upon the 280 nm excitation into a 1ππ* excited state of the terephthalate ion. The fine structure of the emission spectra of Eu3+ 5D0-7FJ (J = 0–4) significantly depended on the Eu3+ concentration. The luminescence quantum yield of Eu3+ was significantly larger for Eu-Lu terephthalates containing a low concentration of Eu3+ due to the absence of Eu-Eu energy migration and the presence of the Ln2bdc3 crystalline phase with a significantly smaller nonradiative decay rate compared to the Ln2bdc3·4H2O.

Microcrystalline Luminescent (Eu1-xLnx)2bdc3·nH2O (Ln = La, Gd, Lu) Antenna MOFs: Effect of Dopant Content on Structure, Particle Morphology, and Luminescent Properties

“…The mass loss was observed at 60–150 °C. As previously reported [ 19 , 20 ], the mass loss in this temperature range can be attributed to the dehydration of the compounds, resulting in the formation of anhydrous terephthalate: Lu 2 bdc 3 ·nH 2 O → Lu 2 bdc 3 + nH 2 O. The average weight loss was found to be (5.31 ± 0.18)%, which corresponds to 2.62 ± 0.09 water molecules per unit formula of Lu 2 bdc 3 ·nH 2 O.…”

“…One of the useful approaches to overcome this problem is the so-called “antenna effect”, which suggests using light-harvesting ligands that transfer absorbed energy onto lanthanide ions [ 15 , 16 ]. Typical ligands used in such antenna complexes are aromatic and unsaturated molecules, like calixarenes, bipyridines, phenanthroline derivatives, and carboxylates, including terephthalates [ 12 , 17 , 18 , 19 , 20 ].…”

“…In addition, in the works devoted to the study of heterometallic REE-based MOFs containing both luminescent and non-luminescent ions, the mechanism of the mutual effect of REE on the photophysical properties is practically not disclosed. Recently, we reported on heterometallic europium–lutetium and terbium–lutetium terephthalate metal–organic frameworks (MOFs) that demonstrate a strong correlation between the luminescent properties of the complexes and the crystalline structure [ 19 , 20 ]. It has been shown that the luminescence of lanthanide (III) ions strictly depends on the local symmetry of emitting lanthanide ions [ 19 , 20 , 21 , 22 , 23 , 24 ].…”

“…Recently, we reported on heterometallic europium–lutetium and terbium–lutetium terephthalate metal–organic frameworks (MOFs) that demonstrate a strong correlation between the luminescent properties of the complexes and the crystalline structure [ 19 , 20 ]. It has been shown that the luminescence of lanthanide (III) ions strictly depends on the local symmetry of emitting lanthanide ions [ 19 , 20 , 21 , 22 , 23 , 24 ]. Therefore, in our studies, we have found that the fine structure of luminescence spectra, the lifetimes of excited states, and quantum yields change with crystalline phase change upon varying REE ion contents.…”

Europium(III)/Terbium(III) mixed metal-organic frameworks and their application as a ratiometric thermometer

“…We supposed that similarly to literature reports, energy transfer occurs via intersystem crossing from linker based triplet states to 5 D1 level, internal conversion into 4 D0 energy level occurred, followed by emission into 7 FJ levels. 34 The emission spectra of suspension of the pure MOFs in ethanol are very similar to the PL spectra in the solid state (Figure 2), with however a slight shift of the emission wavelength of the Eu 5 D0 7 F2 transition. Interestingly, when replacing ethanol with acetonitrile, the emission profile of TbBTC remains unaltered, while the emission profile of EuBTC is hugely affected.…”