Researchers from HSE University and the Institute of Petrochemical Synthesis of the Russian Academy of Sciences have developed a method to precisely control both the color and intensity of the light emitted by rare earth elements. While these elements usually emit light in predictable ranges—cerium, for instance, typically glows in the ultraviolet—the team demonstrated that it is possible to significantly shift this emission by altering the elements’ chemical environment. Their findings, published in Optical Materials, could contribute to the development of innovative light sources, lasers, and display technologies.
Rare earth elements are widely used in microelectronics and luminescent devices due to their ability to emit light at specific wavelengths.
This emission results from electronic transitions, particularly between the 4f orbitals of lanthanide atoms, which are generally unaffected by their surroundings. In contrast, the 5d orbitals, though more sensitive to external influences, usually sit at energy levels too high to contribute meaningfully to luminescence.
The researchers challenged this assumption by creating metal-organic complexes of cerium, praseodymium, and terbium using specially designed organic ligands. These ligands formed a symmetrical environment of three cyclopentadienyl anions around each metal ion. The resulting electrostatic field altered the relative energy levels of the 5d orbitals, enabling previously inactive electronic transitions and shifting the luminescence spectrum.
In conventional compounds, cerium emits ultraviolet light with wavelengths between 300 and 400 nanometers. In the newly synthesized complexes, its emission shifted into the red region of the spectrum, with wavelengths reaching up to 655 nanometers. This shift indicates a reduction in the energy gap between the 4f and 5d orbitals. Similar changes were observed in praseodymium and terbium complexes.
To better understand this effect, the researchers explored the energy transfer process. In most known lanthanide compounds, energy is absorbed by a ligand, which then transfers it directly to the 4f electrons of the metal, causing luminescence. In the new materials, however, energy was transferred first to a 5d orbital before reaching the 4f electrons. This alternative route enabled emissions that had not previously been associated with these metals.
"Previously, shifts in luminescence were noted, but the mechanism behind them remained unclear. Working closely with physicists, we now understand how modifying a compound’s electronic structure can produce unexpected results," explained Daniil Bardonov, a master’s student at the HSE Faculty of Chemistry.
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Rather than focusing on isolated cases, the researchers synthesized a series of lanthanide complexes to identify trends and shared behaviors. The results reveal a clear connection between molecular structure and emission characteristics.
"This work shows how tuning the atomic environment allows us to influence electronic transitions and thus control the optical behavior of lanthanides," said Fyodor Chernenkiy, a bachelor's student involved in the study. "By designing compounds with specific structures, we can now produce materials with tailored luminescence.
"The team believes that being able to predict how a compound will emit light will streamline the development of custom luminescent materials, moving the field beyond trial-and-error approaches. Their research lays the groundwork for creating more efficient and versatile technologies in optics and photonics.
