Scientists Of Ural Federal University Observe Strong Lanthanide Cation Luminescence in Fluorine-Containing Complexes

Researchers of Ural Federal University, together with colleagues from the Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences, created new complexes of lanthanides and studied their properties. The compounds are soluble in water and have a bright glow due to the inclusion of fluorine atoms in the system. Phosphors are used in the detection of various chemicals, imaging of biological objects, and more.

The results of the work were published in the journal Polyhedron.

Phosphors are used in a wide variety of fields: the detection of compounds (for example, reactive oxygen species and metals), and the study of biological objects, including in medicine, and in light sources, and in various electronic devices. Such compounds absorb energy and re-emit it in the form of light. Organic complexes of lanthanides are promising as a basis for the synthesis of phosphors. The positively charged ions (cations) of Group III metals of the 6th period of the periodic table are inside an organic “coat” called a ligand. It helps lanthanide to show its photophysical properties – this effect of their sensitization has been known since 1942. The ligand basically consists of two functional parts: the antenna absorbs energy and transfers it to the cation, and the chelate forms (saturates) bonds with the lanthanide, preventing its interaction with water molecules – because of them, significant suppression of luminescence is possible.

“We have synthesized complexes of europium (III) and terbium (III) in several stages. We had bipyridine as an antenna – these are two aromatic nitrogen-containing rings that form stable complexes with metals that can change their oxidation state. They have excellent optical performance and efficiently transfer electrons and energy. We used polyaminocarboxylic acid as a chelating ligand. Such systems are often modified by the addition of halogen atoms such as fluorine, bromine and chlorine. In this way, the efficiency of energy transfer from the ligand to the chelated lanthanide cation can be significantly increased and thereby the luminescence of the compound can be enhanced. We followed the same path: halogen (fluorine) atoms were included in the phenyl residue C6H5,” says Grigory Zyryanov, professor of the Department of Organic and Biomolecular Chemistry, Ural Federal University.

The authors confirmed the structure of the synthesized complexes by elemental analysis and mass spectrometry (based on ionization of samples). Most of all, scientists were interested in the photophysical properties and solubility of compounds. The absorption spectra were consistent with those previously described in the literature for similar compounds. The luminescence efficiency was estimated by such a parameter as the quantum yield of photoluminescence. It is expressed as a percentage and is equal to the ratio of the emitted number of light quanta (if excitation, as in this case, was with the help of light) to the absorbed. In the case of europium complexes, the maximum value (16.2%) was achieved by introducing a fluorophenyl residue into the system. This is about 1.5 times higher than the average for this class of compounds. The results with terbium were even better – 49.2% (in the case of the same fluorophenyl), which is almost 10 or more times more effective than the previously described similar complexes. Moreover, the latter complexes had poor solubility in water, which had negatively affected the use of such systems for bioimaging, while the systems developed by the authors do not have such drawbacks.

Thus, chemists were able to successfully combine excellent photophysical characteristics and photo-solubility. This is very important when developing new approaches to identifying various compounds in the environment, as well as bioimaging.