AV¶ÌÊÓƵ

Geneva, April 23, 2025 - Dr Pierrick Berruyer.

The scientific journal Inorganic Chemistry highlights the research of the group of Claude Piguet, professor in the School of chemistry and biochemistry at the AV¶ÌÊÓƵ (UNIGE). On the cover, two Greek nymphs, Lampétie and her twin sister Phæthuse, perform a coordinated dance. The elegance of this ballet is the product of perfect synchronisation between the couple; the magic of the dance stops working if the partners get out of sync. In this image, the team of researchers at the AV¶ÌÊÓƵ presents a significant advance in the field of energy conversion thanks to the development of a molecule made up of two erbium centres solidly linked together. By imposing a relatively short distance, of the order of 1 nm, between the two charged erbium atoms (+3), the researchers orchestrate, as Lampétie and Phæthuse did, a choreography between the two centres, known as cross-relaxation. By allowing an exchange of energy between the two identical erbium centres, the new molecule they have synthesised is particularly effective in upconversion compared with the mono-erbium molecular complex. Upconversion is a phenomenon that enables low-energy light to be converted into higher-energy light. This phenomenon is used, for example, to convert infrared laser light into visible green light.

CREATIng HIGH-ENERGY PHOTONS BY ABSORBING LOW-ENERGY PHOTONS

Upconversion is a phenomenon in which low-energy photons are combined to produce a higher-energy photon. This process requires the successive absorption of two photons to emit a high-energy photon. As Prof. Piguet explains:

"the lifetime of the intermediate states is therefore a crucial parameter in upconversion. It is therefore crucial to design systems capable of storing energy after the absorption of a first photon to allow the absorption of a second photon before the deactivation of the intermediate".

Since the introduction of upconversion, efficient systems have been developed. These include solid materials doped with lanthanides, which have found applications in lasers, solar cells and display devices. The development of molecular solutions, particularly for applications in biological systems, remains a challenge marked by the short lifetimes of the intermediate states. The work of Prof. Piguet (UNIGE) and Prof. Charbonnière (AV¶ÌÊÓƵ of Strasbourg, former doctoral student at UNIGE) has proposed the first molecular systems to achieve upconversion in solution by combining different ions to optimise the optical properties of the complexes.

A Bi-metallic Erbium complexÌý

In this publication on the cover of Inorganic Chemistry, the proposed concept is based on the association of identical ions: two erbium(III) complexes are linked together by a relatively short molecular bond of around 1 nm. Dr Inès Taarit, corresponding co-author of the study, explains:

"the short distance between the two erbium atoms allows them to be coupled so that they exchange energy by a phenomenon known as cross-relaxation".

So when a photon is absorbed by one of the erbium ions, the energy can be transferred to the other ion before the intermediate state deactivates. This approach extends the lifetime of the intermediate states and increases the efficiency of energy conversion. The researchers show a factor of 100 improvement in upconversion efficiency for the bi-metallic complex compared with the mono-metallic complex.

These advances continue to advance knowledge in the field of upconversion and the resulting optical and technological applications. By proposing a new molecular approach, researchers are exploring a solution that seemed impossible at first sight. Just as the identical poles of magnets repel each other, the coexistence of the positive charges carried by erbium atoms seemed incompatible. However, the experiment finally proved the opposite and introduced cross-relaxation as a mechanism that could be exploited in the field of molecular upconversion.