The information is prepared on the basis of data from the information-analytical system RSF, informative part is represented in the author's edition. All rights belong to the authors, the use or reprinting of materials is permitted only with the prior consent of the authors.

Project titleHeterometallic terephthalates of rare-earth elements for luminescent sensors design

AffiliationFederal State Budgetary Educational Institution of Higher Education "Saint-Petersburg State University",

Research area 03 - CHEMISTRY AND MATERIAL SCIENCES, 03-203 - Chemistry of coordination compounds

KeywordsSolid solutions, coordination compounds, metal-organic frameworks, rare earth elements, sensors, luminescence, terephthalates

Annotation
In recent years, such area of chemical materials science as the design of functional nanomaterials is actively developed. Among this type of materials, one of the most promising are metal-organic frameworks (MOFs) containing rare-earth elements (REEs) due to their potential application as luminescent coatings for energy-saving lamps and LEDs, flexible screens, luminescent labels, and sensors for various analytes. To create luminescent sensors, one need to make nanocomposites with desired properties (specific surface area, luminescent properties, sensitivity to impurities, pH and temperature). It is known that the mutual presence of several rare-earth elements in MOFs significantly impact on such luminescent properties as the quantum yield and the fine structure of the luminescence spectra. In contrast with inorganic composites based on metal oxides and halides, this effect is poorly studied for metal-organic coordination compounds. Studies of the effect of the organic linkers structure on the photophysical properties of REE MOFs are non-systemic, and the theoretical approaches described in the literature cannot fully explain the correlations between quantum yield of luminescence and the structure of the organic linker. For the further development of this area of research, it is extremely important to reveal the relationship between the composition and properties of the synthesized materials. The effect of the addition of nonluminescent REE ions, the structure of the organic linker, and the morphology of particles on the luminescent and sensory properties of REE MOFs containing terbium and europium ions will be systematically studied for the first time in this project. The results of the project can significantly affect the current understanding of the relationship between the composition and structure of luminescent MOFs with rare-earth elements. One of expecting result of this project is disclosure of the mechanism of the effect of co-doping with nonluminescent REE ions on the quantum yield of luminescence and the dynamics of excited states. The systematic study of the dependence of photophysical properties on the structure of the organic linker will allow to understand the energy transfer mechanism in antenna complexes of europium and terbium. The results of the proposed project will uncover a more rational approach to the design of rare earth elements MOFs with strong luminescence, which can be used as components of smartphone screens and LEDs, and therefore it undoubtedly opens new prospects for the development of the Russian hi-tech technologies. In addition, the ultrasonic synthesis of luminescent micro- and nanocrystalline rare-earth elements-based MOFs with a high specific surface area will be worked out, and the factors affected on the size and the shape of particles will be analyzed. Luminescence quenching of synthesized materials (including micro- and nanocrystalline REE MOFs) with such toxic analytes as heavy metal ions and organic compounds will be studied. The combination of bright luminescence and high specific surface area of the small-sized particles of the REE MOFs will make it possible to determine these substances the concentrations of the abovementioned analytes of low concentrations. The sensitive express tests for some heavy metal ions and organic compounds, including dangerous phenols and explosive nitro compounds can be designed based on obtained materials, which will make a significant contribution to the ecology and safety of the Russian Federation. The aim of the project is to reveal the effect of the electronic structure of organic linkers, the nature of rare earth elements and their relative content on the structure, luminescent and sensory properties of heterometallic solid solutions of metal organic frameworks of various particle morphology. To achieve this goal, we the following problems need to be solved the project: 1) The synthesis of luminescent solid solutions of rare earth terephthalates containing two different REE ions; one is REE ions is luminescent: (LnxLn'1-x)2(1,4-bdc)3*nH2O (Ln=Eu, Tb; Ln' = Y, La, Gd, Yb, Lu). These materials will be synthesized from aqueous solutions of REE salts, terephthalic acid, and sodium hydroxide. 2) The analysis of the effect of the rare earth elements content on the phase composition and luminescent properties of crystallizing compounds (fine structure of the spectra and luminescence intensity, luminescence quantum yield, excited-state dynamics). 3) The synthesis of monosubstituted terephthalic acid 2-X-C6H3-1,4-(COOH)2, (X = OH, CN, Cl, Br, I) from 2-amino-terephthalic acid. 4) The synthesis of luminescent monosubstituted terephthalates of europium(III) and terbium(III): Ln2(2-X-C6H3-1,4-(COO)2)3*nH2O (Ln=Eu, Tb; X = H, NH2 , OH, CN, Cl, Br, I). These materials will be synthesized from aqueous solutions of REE salts, substituted terephthalic acids, and sodium hydroxide. 5) The analysis of the effect of a substituent on the composition and luminescent properties of crystallizing compounds (fine structure of the spectra and luminescence intensity, luminescence quantum yield, dynamics of relaxation of excited states). 6) The synthesis of some luminescent solid solutions of monosubstituted terephthalates of rare earth elements to find of the most pronounced luminescent compounds (which possesses the maximum quantum yield of luminescence). 7) The ultrasound-assisted synthesis of luminescent micro- and nanocrystalline solid solutions of monosubstituted rare earth terephthalates with high luminescence quantum yields from aqueous solutions of salts, monosubstituted terephthalic acids and sodium hydroxide. 8) The study of luminescence quenching of synthesized materials by heavy metal ions and organic compounds depending on the composition and morphology in order to design the most promising luminescent sensors. To achieve this goal, we the following problems need to be solved the project: 1) The synthesis of luminescent solid solutions of rare earth terephthalates containing two different REE ions; one is REE ions is luminescent: (LnxLn'1-x)2(1,4-bdc)3*nH2O (Ln=Eu, Tb; Ln' = Y, La, Gd, Yb, Lu). These materials will be synthesized from aqueous solutions of REE salts, terephthalic acid, and sodium hydroxide. 2) The analysis of the effect of the rare earth elements content on the phase composition and luminescent properties of crystallizing compounds (fine structure of the spectra and luminescence intensity, luminescence quantum yield, excited-state dynamics). 3) The synthesis of monosubstituted terephthalic acid 2-X-C6H3-1,4-(COOH)2, (X = OH, CN, Cl, Br, I) from 2-amino-terephthalic acid. 4) The synthesis of luminescent monosubstituted terephthalates of europium(III) and terbium(III): Ln2(2-X-C6H3-1,4-(COO)2)3*nH2O (Ln=Eu, Tb; X = H, NH2 , OH, CN, Cl, Br, I). These materials will be synthesized from aqueous solutions of REE salts, substituted terephthalic acids, and sodium hydroxide. 5) The analysis of the effect of a substituent on the composition and luminescent properties of crystallizing compounds (fine structure of the spectra and luminescence intensity, luminescence quantum yield, dynamics of relaxation of excited states). 6) The synthesis of some luminescent solid solutions of monosubstituted terephthalates of rare earth elements to find of the most pronounced luminescent compounds (which possesses the maximum quantum yield of luminescence). 7) The ultrasound-assisted synthesis of luminescent micro- and nanocrystalline solid solutions of monosubstituted rare earth terephthalates with high luminescence quantum yields from aqueous solutions of salts, monosubstituted terephthalic acids and sodium hydroxide. 8) The study of luminescence quenching of synthesized materials by heavy metal ions and organic compounds depending on the composition and morphology in order to design the most promising luminescent sensors.

Expected results
1) The synthesis of luminescent large-crystalline solid solutions of rare-earth terephthalates containing two different REE ions, one is REE ions is luminescent: (LnxLn'1-x)2(1,4-bdc)3*nH2O (Ln=Eu , Tb; Ln'= Y, La, Gd, Yb, Lu). 2) The determination of composition and morphology of synthesized solid solutions based on rare earth elements MOFs using modern physicochemical methods. 3) The analysis of influence of the content of rare earth elements on the phase composition and luminescent properties of crystallizing compounds (fine structure of the spectra and luminescence intensity, luminescence quantum yield, excited-state dynamics). 4) The synthesis of monosubstituted terephthalic acids 2-X-C6H3-1,4-(COOH)2, (X = OH, CN, Cl, Br, I) from 2-amino-terephthalic acid and determination of their composition. 5) The synthesis of luminescent monosubstituted) terephthalates of europium(III) and terbium(III): Ln2(2-X-C6H3-1,4-(COO)2)3*nH2O (Ln=Eu, Tb; X = H, NH2, OH, CN, Cl, Br, I). 6) The determination of composition of the synthesized monosubstituted terephthalates of europium(III) and terbium(III) and the particle morphology via modern physicochemical methods. 7) The analysis of the effect of a substituent on the composition and luminescent properties of crystallizing compounds (fine structure of the spectra and luminescence intensity, luminescence quantum yield, dynamics of excited states relaxation). 8) The synthesis of some luminescent solid solutions of monosubstituted terephthalates of rare earth elements in order to identify the most brightly luminescent compounds (having the maximum luminescence quantum yield), and determination of their composition and particle morphology. 9) The development and carrying out of ultrasonic synthesis of luminescent micro- and nanocrystalline solid solutions of monosubstituted terephthalates of rare earth elements with high luminescence quantum yields from aqueous solutions of salts, monosubstituted terephthalic acids and sodium hydroxide, and determination of morphology of particles. 10) The study of the luminescence quenching of the synthesized materials by heavy metal ions and organic compounds depending on the composition and morphology, and determination of the most promising materials for the creation of luminescent sensors. The results of the project can significantly affect the current understanding of the relationship between the composition and structure of luminescent MOFs with rare-earth elements. One of expecting result of this project is disclosure of the mechanism of the effect of co-doping with nonluminescent REE ions on the quantum yield of luminescence and the dynamics of excited states. The systematic study of the dependence of photophysical properties on the structure of the organic linker will allow to understand the energy transfer mechanism in antenna complexes of europium and terbium. The results of the proposed project will uncover a more rational approach to the design of rare earth elements MOFs with strong luminescence, which can be used as components of smartphone screens and LEDs, and therefore it undoubtedly opens new prospects for the development of the Russian hi-tech technologies. In addition, the ultrasonic synthesis of luminescent micro- and nanocrystalline rare-earth elements-based MOFs with a high specific surface area will be worked out, and the factors affected on the size and the shape of particles will be analyzed. Luminescence quenching of synthesized materials (including micro- and nanocrystalline REE MOFs) with such toxic analytes as heavy metal ions and organic compounds will be studied. The combination of bright luminescence and high specific surface area of the small-sized particles of the REE MOFs will make it possible to determine these substances the concentrations of the abovementioned analytes of low concentrations. The sensitive express tests for some heavy metal ions and organic compounds, including dangerous phenols and explosive nitro compounds can be designed based on obtained materials, which will make a significant contribution to the ecology and safety of the Russian Federation.

Annotation of the results obtained in 2022
A series of heterometallic metal-organic frameworks (MOFs) based on terephthalate ion and rare-earth element (REE) ions with the general formula (LnxLn'1-x)2(1,4-bdc)3‧nH2O (Ln = Eu, Tb; Ln' = Y, La, Gd, Yb, Lu) was studied at the first period of this project. The work consisted of three parts. In the first part, the synthesis of the abovementioned materials was performed and optimized. For each series of synthesized MOF, the morphology and particle size were found, the phase composition was determined, the unit cell parameters were refined, the luminescence spectra were recorded, the fine structure of the luminescence bands was analyzed, the lifetimes of the excited state and the luminescence quantum yields were obtained. On the basis of the experimental data, regularities of effect of the the content of rare-earth ions content on the composition, crystal structure, and luminescent properties (fine structure of the spectra and luminescence intensity, excited-states dynamic) of crystallizing compounds were analyzed. It was found that the synthesized MOFs (LnxLn'1-x)2(1,4-bdc)3‧nH2O (Ln=Eu, Tb; Ln'= Y, La, Gd, Yb, Lu) can be divided into three groups according to the effect of the nature of REE on the indicated properties: a group of compounds containing yttrium, lanthanum and gadolinium; a group of compounds containing ytterbium; a group of compounds containing lutetium For the series of MOFs (LnxLn’1-x)2(1,4-bdc)3‧nH2O (Ln = Eu, Tb; Ln’= Y, La, Gd) we have found the formation of terephthalates tetrahydrates (LnxLn'1-x)2(1,4-bdc)3‧4H2O corresponding to the crystalline phase Ln2(1,4-bdc)3‧4H2O (Ln = Ce-Yb). For a series of terephthalates simultaneously containing trivalent ions of lutetium and europium or terbium, the concentration of REE significantly affects the phase composition of crystallizing MOFs: mixed terephthalates can crystallize in the form of terephthalates tetrahydrates (LnxLn'1-x )2(1,4-bdc)3‧4H2O, terephthalates decahydrates (LnxLn'1-x)2(1,4-bdc)3‧10H2O, anhydrous terephthalates (LnxLn'1-x)2(1,4-bdc )3 or mixtures of different crystalline phases depending on the ratio of rare earth elements and the concentration of reagents. For compounds of the series containing ytterbium, the phase composition of crystallizing compounds depends on the temperature regime of synthesis. Europium-ytterbium and terbium-ytterbium terephthalates crystallize in the anhydrous phase when synthesized at room temperature at about 20°C, while with a slight increase in the temperature regime of synthesis (about 25°C), crystallization of the tetrahydrate occurs. The difference in the results obtained for different groups of compounds is explained by the difference in the sizes and characteristic coordination numbers of rare-earth ions. This leads to the observed dependence of the phase composition of MOFs on the concentration of REE ions in compounds containing lutetium, as well as to the sensitivity of the phase composition to synthesis conditions, in the case of compounds of lutetium and ytterbium. The luminescent properties of the compounds significantly depend on the local symmetry of the europium and terbium ions, as well as on the amount of water molecules coordinated to the ions, and thus on the phase composition of the compounds. It was found that the fine structure of the luminescence bands is determined by the crystalline phase of the compound and the central ion (europium or terbium) and does not depend on the concentration of the luminescent ion within one phase. This fact is clearly seen in series of compounds with lutetium, where a change in the phase composition is observed depending on the conditions of synthesis and the concentration of REE ions. A study of the luminescent properties showed that the lifetimes of the excited state depend on the concentration of the substituent ion in terephthalate, as well as on the type of crystalline phase. This dependence is explained by the presence of two luminescence quenching mechanisms that take place in the studied compounds. The first mechanism is associated with an increase in the probability of energy transfer between luminescent ions with an increase in their concentration and is pronounced to a greater extent for terbium compounds. For example, for mixed terephthalates, where no change in the crystalline phase is observed with a change in the REE ratio, the luminescence lifetimes are sharply reduced when the threshold ion concentration is reached, and a further increase in the content of the luminescent ion leads to quenching of the luminescence due to the interaction of these ions with each other. For europium-based mixed terephthalates, no such quenching is observed, which is probably due to the fact that the critical radius between luminescent REE ions, at which energy transfer is observed for europium, is larger than for terbium in these systems. The second quenching mechanism is associated with the transfer of energy to high-energy valence vibrational levels O-H of water molecules coordinated to europium or terbium ions in compounds. Higher values of quantum yields and the lifetime of the excited state of the compounds are associated with the formation of an anhydrous crystalline phase. The quantum yield for the anhydrous phase is much higher than for the tetrahydrate and decahydrate Ln2(1,4-bdc)3‧4H2O and Ln2(1,4-bdc)3‧10H2O due to the absence of water molecules coordinated to ions. The obtained results makes one possible to synthesize europium(III) and terbium(III) phosphors posessing high luminescence quantum yields. Thus, within the current the project, a sample of composition (Tb0.1Lu0.9)2(1.4-bdc)3‧1.4H2O with a high quantum yield of 95% was synthesized. The results are published in two articles related to the first quartile journals Q1, which can be found at the following links (open access): 1. Nosov, V.G.; Kupryakov, A.S.; Kolesnikov, I.E.; Vidyakina, A.A.; Tumkin, I.I.; Kolesnik, S.S.; Ryazantsev, M.N.; Bogachev, N.A.; Skripkin, M.Y.; Mereshchenko, A.S. Heterometallic Europium(III)–Lutetium(III) Terephthalates as Bright Luminescent Antenna MOFs. Molecules 2022, 27, 5763. https://doi.org/10.3390/molecules27185763 2. Nosov, V.G.; Toikka, Y.N.; Petrova, A.S.; Butorlin, O.S.; Kolesnikov, I.E.; Orlov, S.N.; Ryazantsev, M.N.; Kolesnik, S.S.; Bogachev, N.A.; Skripkin, M.Y.; Mereshchenko, A.S. Brightly Luminescent (TbxLu1−x)2bdc3·nH2O MOFs: Effect of Synthesis Conditions on Structure and Luminescent Properties. Molecules 2023, 28, 2378. https://doi.org/10.3390/molecules28052378

Publications

1. Nosov V.G, Kupryakov A.S., Kolesnikov I.E., Vidyakina A.A., Tumkin I.I., Kolesnik S.S., Ryazantsev M.N., Bogachev N.A., Skripkin M.Yu., Mereshchenko A.S. Heterometallic Europium(III)–Lutetium(III) Terephthalates as Bright Luminescent Antenna MOFs Molecules, Molecules 2022, 27, 5763 (year - 2022) https://doi.org/10.3390/molecules27185763

2. Nosov V.G., Toikka Y.N., Petrova A.S., Butorlin O.S., Kolesnikov I.E., Orlov S.N., Ryazantsev M.N., Kolesnik S.S., Bogachev N.A., Skripkin M.Y., Mereshchenko A.S. Brightly Luminescent (TbxLu1−x)2bdc3·nH2O MOFs: Effect of Synthesis Conditions on Structure and Luminescent Properties Molecules, Molecules 2023, 28, 2378. (year - 2023) https://doi.org/10.3390/molecules28052378

3. Mereshchenko A.S., Nosov V.G., Kolesnik S.S. Гетерометаллические металлорганические каркасные структуры лантаноидов – перспективные люминесцентные сенсоры Тезисы Международой научно–проктической конференции «Современные проблемы химии координационных соединений», Тезисы Международой научно–практической конференции «Современные проблемы химии координационных соединений», 22-23 декабря 2022, Бузара, Узбекистан, с. 546-548 (year - 2022)