Scientists Discover Why Europium 'Misbehaves'

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Europium is a rare-earth metal responsible for the pure red glow in displays and other luminescent materials. For a long time, however, it refused to emit light when surrounded by certain organic molecules known as acylpyrazolone ligands. Chemists have now uncovered the reason: in europium complexes with these ligands, a 'black window' appears—a charge-transfer state in which the energy absorbed by the ligand is dissipated as heat rather than emitted as light. Understanding this mechanism opens the way to designing more efficient red-emitting materials for displays, fluorescent thermometers, and chemical sensors. The results have been published in Dalton Transactions.

Luminescent materials based on rare-earth metals, known as lanthanides, are used everywhere—from smartphone screens to medical sensors. Among them, europium holds a special place: it is responsible for the pure, vivid red glow in modern displays. To produce a full-colour image, displays rely on three primary colours: red, blue, and green. If the red component lacks purity or contains unwanted hues, overall colour reproduction suffers: the image loses saturation and appears less natural.

Europium has long been used in inorganic materials to produce bright red luminescence. However, chemists are seeking cheaper and more technologically versatile alternatives—organic compounds that are easier to process and apply to surfaces. To achieve this, special molecules called ligands are attached to the europium ion. These ligands act as 'antennas' by absorbing external energy, such as ultraviolet light, and transferring it to the metal ion, thus triggering its emission. However, when europium binds to such ligands, its luminescence intensity drops sharply. By contrast, the same antenna molecules effectively 'switch on' luminescence in other lanthanides, such as terbium and samarium.

An international team of researchers, including chemists from HSE University, the RAS Lebedev Physical Institute, Moscow State University, the RAS Kurnakov Institute of General and Inorganic Chemistry, Bauman Moscow State Technical University, and the Landau Institute for Theoretical Physics, investigated the reasons behind this behaviour of europium. To do so, they synthesised three series of compounds based on samarium, europium, and gadolinium with acylpyrazolone ligands.

Growing the crystals took ten years. After determining the crystals’ structure, the scientists began to investigate the luminescence of the resulting compounds: irradiating the samples with ultraviolet light and measuring how brightly they emitted, at which wavelengths, and how quickly the luminescence quenched. The key experiment was carried out at an ultra-low temperature (−196°C); cooling the samples suppresses many non-radiative processes, making it possible to reveal the hidden mechanisms of energy transfer.

As a result, the authors showed that by modifying the composition of the complex—for example, by replacing the counterion that balances its charge—it is possible to partially suppress quenching and 'switch on' europium luminescence in systems where it was previously absent. 

Normally, a ligand absorbs light energy and transfers it to the metal ion, which then releases it as visible emission. This process is known as luminescence. However, in europium complexes—unlike those of samarium—an additional charge-transfer state was found, when electron density—the 'cloud' of negative charge surrounding atoms—is transferred from the ligand to the metal ion, creating an alternative channel for energy loss without emitting light. This channel effectively intercepts the energy that would otherwise generate luminescence and dissipates it as heat. As a result, europium does not receive enough energy and fails to light up.

Yury Belousov

'We studied the luminescent properties of all the compounds in detail and finally identified the reason for europium’s "misbehaviour." Unlike samarium complexes, europium complexes activate an additional pathway for energy loss—a charge-transfer state from the ligand to the metal. This acts like a kind of black hole, absorbing the energy transferred to the europium ion from the ligand and preventing it from emitting light,' explains Yury Belousov, Associate Professor of the Joint Department of Inorganic Chemistry and Material Science with the RAS Kurnakov Institute of General and Inorganic Chemistry at the HSE Faculty of Chemistry and Senior Research Fellow at the RAS Lebedev Physical Institute.

The scientists also compared their results with data for a closely related ligand in which one of the phenyl groups was replaced by a cyclohexyl group—a six-membered carbon ring. It turned out that even such a minor modification dramatically alters the charge-transfer process for the better: the 'black window' disappears, and europium finally begins to glow. This finding confirmed the researchers’ hypothesis about the origin of europium luminescence quenching.

'Previously, chemists knew that europium was not compatible with acylpyrazolone ligands, but the reasons remained unclear. Now we understand the underlying mechanism. This knowledge makes it possible to deliberately tailor the environment around the europium ion—the choice of cation and the structure of the complex—in order to suppress unwanted charge-transfer states. If this process can be controlled, it will become possible to design not only bright red materials for displays but also highly sensitive fluorescent thermometers and chemical sensors based on europium, although before this study, europium complexes with these ligands were considered practically useless for the development of luminescent materials,' adds Prof. Belousov.

The study was conducted with support from the Russian Science Foundation and the Ministry of Science and Higher Education of the Russian Federation.