St Petersburg University chemists synthesise hybrid luminescent polymers for sensors and gadget screens

Metal—organic frameworks (MOFs) represent a large class of crystalline materials defined as porous networks, consisting of metallic ions or clusters linked together by organic multidentate ligands. Using different combinations of metals and ligands allows obtaining materials with different specified structures and properties.

The research findings are published in a series of articles in the scientific journal Molecules.

Currently, metal-organic framework compounds are used as reaction catalysts in the production of sensors, rocket fuel additives, and phosphors, i.e. substances capable of emitting light under ultraviolet radiation, an electromagnetic field, or other external factors. 

Scientists from St Petersburg University are studying materials based on lanthanide compounds. They have studied patterns of changes in the shape and size of nanoparticles used in theranostics (that is an innovative field of medicine) by adding various lanthanides to the structure. Today, chemists from the University are continuing to study lanthanides.

Lanthanides refers to a family of 15 chemical elements of group III, period 6 of the periodic table, i.e. metals between atomic numbers 57 and 71 (from lanthanum to lutetium).

The emission spectrum of lanthanide ions contains very narrow lines, which makes their compounds promising for the development of new bright and contrasting luminescent paints and monitor screens, said the researchers. Although the lanthanide ions absorb light very poorly, we can synthesise antenna MOFs. The antenna refers to an organic compound that absorbs light efficiently and transfers its energy to the lanthanide. If all the conditions are selected correctly, such organic-inorganic hybrids will glow as brightly as organic phosphors, yet the glow colour will be more contrasting and saturated.

The research project "Heterometallic terephthalates of rare-earth elements for luminescent sensors design" is supported by the Russian Science Foundation. The project is carried out at the Department of Inorganic Chemistry at St Petersburg University using the research infrastructure and equipment at the resource centres at the St Petersburg University Research Park.

The luminescent antenna metal—organic frameworks can be used to design luminescent paints, document security features, elements of gadget screens, sensors and chemical compounds for detecting hazardous substances in food products and the environment. 

The chemists from St Petersburg University synthesised a series of such compounds that contain two ions: luminescent ions, i.e. europium or terbium ions, and nonluminescent ions, i.e. yttrium, lanthanum, gadolinium or lutetium ions. To this end, they used several synthesis methods, including ultrasound, to reduce the particle size and obtain MOFs with a large specific surface area, which is important in the development of luminescent sensors. 

"It turned out that yttrium, lanthanum and gadolinium ions in unlimited quantities replace europium and terbium ions in the overall structure, taking the same places in the crystalline structure. However, the introduction of a large amount of lutetium ion leads to a change in the structure, which depends on the synthesis method. As a result, anhydrous, tetrahydrate, decahydrate or 2.5-hydrate can be formed. At the same time, 2.5-hydrate of lutetium terephthalate and mixed terephthalates were obtained for the first time," explained Andrey Mereschenko, the head of the research team and Associate Professor in the Department of Laser Chemistry and Laser Materials Science at St Petersburg University.

The scientists from St Petersburg University found out how to increase more than twice the quantum yield of luminescence, which determines the brightness of the glow. To this end, it is necessary to partially replace europium and terbium ions with gadolinium and lutetium ions. For one of the compounds, simultaneously containing terbium and lutetium ions in a ratio of 1:9, the quantum yield was 95%. In other words, 95 particles out of 100 particles that absorbed ultraviolet light emitted green light. 

During the project, the University scientists not only synthesised and studied the properties of the new compounds, but also analysed the correlations between structural, optical, and photophysical properties at the level of the electronic structure. In particular, they proposed a new approach to assessing and describing the excitation energy transfer at the molecular level. The scientists showed that the quantum yield of luminescence of antenna complexes is determined by two parameters: how effectively energy is transferred from the antenna to the europium or terbium ion and how strongly the luminescence of lanthanide ions is quenched by other molecules, such as water molecules, present in the compounds. 

"We conducted an indirect assessment of the efficiency of energy transfer from the ligand-antenna, which is an acceptor of ultraviolet radiation energy, and determined the main parameters of the compound to calculate the efficiency of the ligand-antenna. This allowed us to take another look at the nature of energy transfer from a different angle and, as a result, determine the dominant factors influencing the behaviour of this mechanism and predict it," shared Oleg Butorlin, a co-author of the article and a student at St Petersburg University. 

The approach developed by the chemists from St Petersburg University is a major contribution to the development of photophysics of metal-organic frameworks with an antenna mechanism of energy transfer. In future, the scientists set tasks for applying the new approach to the analysis of metal-organic frameworks with other antenna ligands.