The photo is sourced from uiying Ye / ETH Zurich
The lanthanide series includes 15 metals with atomic numbers 57–71 (from lanthanum through lutetium). As the emission spectrum of lanthanide ions exhibits very narrow lines, these compounds could be used to develop monitor screens, as well as bright and contrasting luminescent paints. Although the ions absorb light poorly, this shortcoming can be corrected by using metal-organic frameworks (MOFs) – lattice structures made of metal and organics that ensure the ions are synthesized with an antenna, i.e., an organic compound that absorbs light energy well and transfers it to the lanthanide. Under correctly chosen conditions, these hybrids will glow as brightly as fully organic luminophores, and their glow color will be more saturated and contrasting.
The scientists from SPbU synthesized a series of structures of this kind containing two ions at the same time, with one ion being luminescent (europium or terbium) and the other one being optically inert (yttrium, lanthanum, gadolinium or lutetium). The authors of the study used ultrasound to synthesize the compounds. This allowed them to reduce the particle size and obtain MOFs with a large specific surface area, which is important in the development of luminescent sensors.
“It emerged that ions of yttrium, lanthanum and gadolinium in unlimited quantities replace ions of europium and terbium in the overall structure, taking their places in the crystal lattice. However, the introduction of a large amount of lutetium ion changes the structure, which depends on the synthesis method. This results in the formation of anhydrous, tetrahydrate, decahydrate or 2.5-hydrate crystal hydrate. For the first time ever, we managed to obtain 2.5-hydrate crystal hydrate of lutetium terephthalate and mixed terephthalates,” Andrey Mereshchenko, associate professor at the Department of Laser Chemistry and Laser Material Science, is quoted as saying by SPbU.
The SPbU scientists also found out how to multiply the quantum yield of luminescence, which determines the brightness of the glow. Experiments showed that, in order to achieve that, europium and terbium ions need to be partially replaced by gadolinium and lutetium ions. In one of the compounds containing terbium and lutetium ions at a ratio of 1:9, the quantum yield was 95%: out of 100 particles that absorbed ultraviolet light, 95 particles emitted green light.
The authors also offered a new approach to describing the processes of transferring light excitation energy at the molecular level. Specifically, the scientists showed that the quantum yield of the luminescence of antenna complexes is determined by two parameters: the efficiency of energy transfer from the antenna to the europium or terbium ion and the extent to which the luminescence of lanthanide ions is quenched by other molecules, such as water molecules, which are present in the compounds.
“We performed an indirect assessment of the efficiency of energy transfer from the antenna ligand, which is an acceptor of ultraviolet radiation energy, and determined the main parameters of the compound, based on which we calculated the efficiency of the antenna ligand. This allowed us to take a look at the nature of energy transfer from another angle and subsequently determine the chief factors affecting the behavior of this mechanism, as well as to get a little closer to predicting it,” Oleg Butorlin, one of the authors of the article, is quoted as saying by SPbU.
