Editorial: Hot topic: luminescence in rare earth coordination compounds
Paula Gawryszewska1 
Lawrence W. Miller
Artur J. M. Valente- 1Faculty of Chemistry, University of Wroclaw, Wroclaw, Poland
- 2Department of Chemistry, University of Illinois Chicago, Chicago, IL, United States
- 3Department of Chemistry, CQC-IMS, University of Coimbra, Coimbra, Portugal
Editorial on the Research Topic
Hot topic: luminescence in rare earth coordination compounds
Rare-earth coordination compounds exhibit unique spectroscopic properties that include long emission decay times, high magnetic moments, narrow-bandwidth emissions, and large pseudo-Stokes shifts (Bünzli, 2015). These qualities underpin a wide array of photonic and biomedical technologies including light-emitting diodes [LEDs; (Kido and Okamoto, 2002)], anti-counterfeiting inks (Eliseeva and Bünzli, 2010), and chemical probes for diagnostics and bioimaging (Heffern et al., 2014). The ability of these metal ions to take part in luminescence resonance energy transfer (Qiu et al., 2022) and to exhibit photon upconversion (Wen et al., 2018) further expands their potential applications. Given their unique and fascinating properties, there is intense and ongoing interest in the fundamental and applied science of rare-earth coordination compounds and related molecular architectures, including metal-organic frameworks (Cui et al., 2014), polymer films, gels, hybrid materials (Binnemans, 2009), and nanomaterials (Wang et al., 2011).
The electronic configuration of rare-earth elements dictates their chemical and photophysical properties (Bünzli, 2015). Comprised of the lanthanide series, along with yttrium and scandium, the rare-earth elements are grouped together due to their similar geochemical properties. However, the only luminescent species are lanthanides with partially filled 4f orbitals (elements 58–70) from cerium to ytterbium. Lanthanides are predominantly trivalent (Ln3+) and form cations in solution. The larger radial expansion of the 5s25p6 subshells causes the 4f orbitals to be shielded from the influence of their surroundings. Consequently, lanthanide coordination is omnidirectional and metal-ligand interactions are electrostatic.
A rigorous understanding of the physicochemical properties of lanthanides informs the design of functional coordination compounds. The orbital shielding results in very weak perturbations of 4f electronic transitions by the ligand field that gives rise to lanthanides’ characteristically narrow, multi-line absorption and emission spectra. The 4f–4f electronic transitions are electric dipole (ED) forbidden to first order because the initial and final states belong to the same parity. The types of intraconfigurational 4f–4f electronic transitions that are allowed to first order are magnetic dipole (MD) or electric quadrupole (EQ) transitions (Tanner and Duan, 2010). The extinction coefficients (ε) of Ln3+ are typically small. This means that even if the compounds of these ions exhibit large values of intrinsic quantum yield (
This Research Topic highlights the science of luminescent coordination compound design with four different contributions. The first article included in this Research Topic highlights the relevance of the structure of lanthanide complexes on luminescent properties. Luminescent processes based on the absorption of visible light and emission in the visible/NIR region are relevant for the development of biosensors for biomedical applications. Ohmagari et al. describe the syntheses of a highly planar tetradentate ligand and its corresponding complexes by using a set of different lanthanide ions. The resulting compounds show thermosensitive, visible light-excited, dual-color luminescence that reflects simultaneous ligand phosphorescence and metal f-f emission. The emission characteristics are a consequence of π-electronic system coplanarity through H-bonding, and the color profile of these complexes’ luminescence varies depending on the coordinated lanthanide ion. These features are discussed in terms of structural and photophysical properties.
The dinuclear lanthanide compounds containing hexafluoroacetylacetonate and pyrene-based phosphine oxide, an organic-based luminophore, are the core of the research paper by Nakai et al. With this complex, lanthanide coordination sets the geometry of two pyrene moieties into a stacked configuration that exhibits excimer emission. The identity of the coordinated lanthanide, Eu(III), or Gd(III) affects pyrene stacking geometry and emission spectra. Moreover, the compounds with coordinated Eu(III) provide a thermosensitive ratiometric luminescence based on 4f*→4f and excimer emission. The use of lanthanide coordination to control aggregation structure opens up new avenues in the design of luminescent materials.
Dinga et al. describe the development and characterization of spherical polymer particles (polymer beads) that contain Eu(III) complexes. Polymer particles with Eu(III) or other luminescent lanthanides have found widespread use as highly detectable labels for a variety of bioanalytical assays and point-of-care diagnostics. As discussed in the study, polymer beads incorporate up to hundreds of individual coordinated Eu(III) compounds, and their luminescence and biocompatibility require careful selection of coordinating ligands and appropriate surface functionalization. The authors analyze the luminescence properties of pure Eu(III) complexes with trifluoro-substituted, aromatic β-diketones, and TOPO (trioctylphosphinoxide) co-ligands as well as the complexes incorporated into polymer beads. Emission quantum yields of the obtained polymer beads for all coordination compounds are higher than 80%. The surface of the polymer bead with the best properties was functionalized and model proteins were coupled (Avidine, Neutravidine). The core-shell beads were tested for ELISA-like analyses and for lateral flow immunoassays, and high-end commercial beads were compared as control. It has been shown that obtained beads, taking advantage of large amplification factors and ultimate brightnesses, enable determinations in the attomolar regime and that they are superior to commercial materials.
The last contribution to this topical issue is a review of the thermal and photophysical properties of Eu3+ and Tb3+ coordination compounds with phenyl-containing carbacylamidophosphates. Materials containing Eu3+ and Tb3+ are important for red and green emissions, and their coordination compounds with β-diketones are the most studied. However, β-diketones are not good Tb3+ emission sensitizers. Hence, there is a search for sensitizers among other groups of ligands, and one of them is carboxamidophosphates. Kariaka et al. provide an illuminating discussion of photophysics and energy transfer processes in lanthanide light-converting molecular devices. A variety of carbacylamidophosphate coordination compounds were evaluated for their photophysical properties and the results were compared with those of β-diketonates.
Author contributions
All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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References
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Bünzli, J.-C. G. (2015). On the design of highly luminescent lanthanide complexes. Coord. Chem. Rev. 293–294, 19–47. doi:10.1016/j.ccr.2014.10.013
Cui, Y., Chen, B., and Qian, G. (2014). Lanthanide metal-organic frameworks for luminescent sensing and light-emitting applications. Coord. Chem. Rev. 273–274, 76–86. doi:10.1016/j.ccr.2013.10.023
Eliseeva, S. V., and Bünzli, J.-C. G. (2010). Lanthanide luminescence for functional materials and bio-sciences. Chem. Soc. Rev. 39, 189–227. doi:10.1039/B905604C
Heffern, M. C., Matosziuk, L. M., and Meade, T. J. (2014). Lanthanide probes for bioresponsive imaging. Chem. Rev. 114, 4496–4539. doi:10.1021/cr400477t
Kido, J., and Okamoto, Y. (2002). Organo lanthanide metal complexes for electroluminescent materials. Chem. Rev. 102, 2357–2368. doi:10.1021/cr010448y
Qiu, X., Xu, J., Cardoso Dos Santos, M., and Hildebrandt, N. (2022). Multiplexed biosensing and bioimaging using lanthanide-based time-gated förster resonance energy transfer. Acc. Chem. Res. 55, 551–564. doi:10.1021/acs.accounts.1c00691
Tanner, P. A., and Duan, C.-K. (2010). Luminescent lanthanide complexes: Selection rules and design. Coord. Chem. Rev. 254, 3026–3029. doi:10.1016/j.ccr.2010.05.009
Wang, G., Peng, Q., and Li, Y. (2011). Lanthanide-Doped nanocrystals: Synthesis, optical-magnetic properties, and applications. Acc. Chem. Res. 44, 322–332. doi:10.1021/ar100129p
Keywords: rare earth elements, coordination compound, luminescence, biomedical, sensing
Citation: Gawryszewska P, Miller LW and Valente AJM (2023) Editorial: Hot topic: luminescence in rare earth coordination compounds. Front. Chem. 11:1258607. doi: 10.3389/fchem.2023.1258607
Received: 14 July 2023; Accepted: 14 July 2023;
Published: 08 August 2023.
Edited and reviewed by:
Luís D. Carlos, University of Aveiro, PortugalCopyright © 2023 Gawryszewska, Miller and Valente. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Artur J. M. Valente, avalente@ci.uc.pt