Jean-Claude G. Bünzli

Taking advantage of luminescent lanthanide ions

Lanthanide ions possess fascinating optical properties and their discovery, first industrial uses and present high technological applications are largely governed by their interaction with light. Lighting devices (economical luminescent lamps, light emitting diodes), television and computer displays, optical fibres, optical amplifiers, lasers, as well as responsive luminescent stains for biomedical analysis, medical diagnosis, and cell imaging rely heavily on lanthanide ions. This critical review has been tailored for a broad audience of chemists, biochemists and materials scientists; the basics of lanthanide photophysics are highlighted together with the synthetic strategies used to insert these ions into mono- and polymetallic molecular edifices. Recent advances in NIR-emitting materials, including liquid crystals, and in the control of luminescent properties in polymetallic assemblies are also presented. (210 references.).

Lanthanide Luminescence for Biomedical Analyses and Imaging

Keywords: Time-Resolved Fluorescence ; Resonance Energy-Transfer ; Near-Infrared Luminescence ; Double-Stranded Dna ; Prostate-Specific Antigen ; Photoinduced Electron-Transfer ; Europium-Tetracycline Complex ; Sybr-Green-I ; Terbium Complexes ; Optical Probes Reference EPFL-ARTICLE-149396doi:10.1021/cr900362eView record in Web of Science Record created on 2010-06-17, modified on 2017-05-12

Lanthanide NIR luminescence for telecommunications, bioanalyses and solar energy conversion

Present-day advanced technologies heavily rely on the exciting magnetic and spectroscopic properties of lanthanide ions. In particular, their ability to generate well-characterized and intense near-infrared (NIR) luminescence is exploited in any modern fiber-optic telecommunication network. In this feature article, we first summarize the whereabouts underlying the design of highly luminescent NIR molecular edifices and materials. We then focus on describing the main trends in three applications related to this spectral range: telecommunications, biosciences, and solar energy conversion. In telecommunications, efforts concentrate presently on getting easily processable polymer-based waveguide amplifiers. Upconversion nanophosphors emitting in the visible after NIR excitation are now ubiquitous in many bioanalyses while their application to bio-imaging is still in its early stages; however, highly sensitive NIR-NIR systems start to be at hand for both in vitro and in vivo imaging, as well as dual probes combining magnetic resonance and optical imaging. Finally, both silicon-based and dye-sensitized solar cells benefit from the downconversion and upconversion capabilities of lanthanide ions to harvest UV and NIR solar light and to boost the overall quantum efficiency of these next-generation devices.

New Opportunities for Lanthanide Luminescence

Trivalent lanthanide ions display fascinating optical properties. The discovery of the corresponding elements and their fist industrial uses were intimately linked to their optical properties. This relationship has been kept alive until today when many high-technology applications of lanthanide-containing materials such as energy -saving lighting devices, displays, optical fibers and amplifiers, lasers, responsive luminescent stains for biomedical analyses and in cellulo sensing and imaging heavily rely on the brilliant and pure-color emission of lanthanide ions. In this review we first outlined the basics of lanthanide luminescence with emphasis on f-f transitions, the sensitization mechanisms, and the assessment of the luminescence efficiency of lanthanide-containing emissive molecular edifices. Emphasis was then put on two fast developing aspects of lanthanide luminescence: materials for telecommunications and light emitting diodes, and biomedical imaging and sensing. Recent advances in NIR- emitting materials for plastic amplifiers and waveguides were described, together with the main solutions brought by researchers to minimize non-radiative deactivation of excited states. The demonstration in 1999 that erbium tris (8-hydroxyquinolinate) displayed a bright green emission suitable for orgenic light emitting diodes (OLEDs) was followed by realizing that in OLEDs, 25% of the excitation energy leads to singlet states and 75% to triplet states. Since lanthanide ions are good triplet quenchers, they now also play a key role in the development of these lighting devices. Luminescence analyses of biological molecules are among the most sensitive analytical techniques known. The long lifetime of the lanthanide excited states allows time- resolved spectroscopy to be used, suppressing the sample autofluorescence and reaching very low detection limits. Not only visible lanthanide sensors are now ubiquitously provided in medical diagnosis and in cell imaging, but the feasibility of using NIR emission of ions such as Ybm is now being tested because of deeper penetration in biological tissues.

Intriguing aspects of lanthanide luminescence

The enthralling properties of lanthanide luminescence have propelled luminescent probes, tags and materials based on these elements to the forefront of science and technology. In this minireview, attention is focused on the latest innovations and on less-known aspects of this field. Exciting new developments in bioimaging, therapy, drug delivery, security tags, luminescent sensors, and solar energy conversion are highlighted.

Rare earths: jewels for functional materials of the future

In recent decades, rare earths have become vital to a wealth of advanced materials and technologies including catalysts, alloys, magnets, optics and lasers, rechargeable hydride batteries, electronics, economical lighting, wind- and solar-energy conversion, bio-analyses and imaging. In this perspective article we give a broad overview of rare earth resources and uses first and then of selected applications in dedicated fields such as telecommunications, lasers, photovoltaics (solar-energy conversion), lighting (fluorescent lamps and OLEDs), luminescent probes for bio-analyses and bio-imaging, as well as magnetism and magnetic refrigeration.

Mono- and polymetallic lanthanide-containing functional assemblies: a field between tradition and novelty

The variable and versatile co-ordination behaviour of lanthanide metal ions, LnIII, limits their selective introduction into organised molecular or supramolecular architectures. The design of lanthanide-based devices is thus a special challenge since their specific electronic, magnetic or spectroscopic properties result from a precise control of the co-ordination sphere around the metal ions. The lock-and-key principle associated with the preorganisation of rigid macropolycylic multidentate ligands tailored for one particular LnIII only partially fulfils these structural requirements. The development of less constrained macrocyclic ligands or macrocycles bearing pendant arms allows a smooth transition toward flexible (predisposed) receptors leading to the application of the induced fit principle in lanthanide co-ordination chemistry. According to this concept, programmed secondary non-covalent interstrand interactions (π-stacking, hydrogen bonds, electrostatic repulsion) assist the complexation process leading to an ultra-fine tuning of the metallic co-ordination sites. These two complementary approaches are discussed and evaluated for the design of organised mono-, di- and polymetallic lanthanide complexes together with the consideration of new semi-rigid multidentate podands which combine both aspects.

Europium and Terbium tris(Dipicolinates) as Secondary Standards for Quantum Yield Determination

Abstract Aqueous solutions of europium(III) and terbium(III) tris(dipicolinates), around physiological pH are shown to be convenient secondary standards in the determination of quantum yields of lanthanide complexes containing these ions. Conditions for which a strict linearity is observed between the concentration of the solutions and the emission intensity are established. The speciation in these solutions, which contain a non‐negligible amount of bis species is presented and discussed on the basis of both stability constants and lifetime determinations of the Eu(5D0) level. The quantum yield Q L Eu displays a strong pH‐dependence: for a solution with an absorbance of 0.30, it increases sharply from about 2% at pH 2.5 to reach 11.5–12.5% in the pH range of 6–9. The proposed standard for EuIII is a solution of Cs3[Eu(dpa)3] 7.5 × 10−5 M in tris buffer 0.1 M (absorbance = 0.20) with a quantum yield of 13.5% ± 1.5% under excitation at 279 nm. For TbIII, we propose a standard solution of Cs3[Tb(dpa)3] 6.5 × 10−5 M in tris buffer 0.1 M (absorbance = 0.18) with a quantum yield of 26.5% ± 2% under excitation at 279 nm. Despite the speciation between bis and tris complexes, these two standard buffered solutions present a constant quantum yield within a reasonably large range of concentration and they are easy to handle, which makes them adequate for laboratory use.

Rare earth complexes with neutral macrocyclic ligands

Stoechiometrie, proprietes magnetiques et structurales. Etude en solution des complexes des ions Ln(III); constantes de stabilite

Intrinsic quantum yields and radiative lifetimes of lanthanide tris(dipicolinates)

The efficiency with which the surroundings of trivalent lanthanide ions sensitize their luminescence (eta(sens)) is a key parameter in the design of highly emitting molecular edifices and materials. Evaluation of eta(sens) requires the measurement of the overall and intrinsic quantum yields obtained upon ligand and metal excitation, respectively. We describe a modified integration sphere enabling absolute determination of these quantities on small amounts of solid samples or solutions (60 muL). The sphere is tested for linear response of emitted versus absorbed light intensities with increasing concentration of Cs(3)[Ln(dpa)(3)] solutions (Ln = Eu, Tb). The overall (Q = 29 +/- 2%) and intrinsic (Q = 41 +/- 2%) quantum yields obtained for Eu allow the direct calculation of eta(sens) (71 +/- 6%) while the radiative lifetime (tau(rad) = 4.1 +/- 0.3 ms) is calculated from Q and the observed lifetime. The intrinsic quantum yield matches the value extracted from emission parameters using the simplified equation proposed by Werts et al. but, on the other hand, the theoretical estimate using spontaneous transition probabilities calculated from Judd-Ofelt (JO) parameters is off by -25% (3.15 ms). In the case of Cs(3)[Tb(dpa)(3)], the molar absorption coefficient of the (5)D(4)<--(7)F(6) transition is too small to measure Q for the solution but this quantity could be determined for the microcrystalline sample (72 +/- 5%, tau(rad) = 1.9 +/- 0.1 ms). In this case, the JO theoretical estimate leads to a much too short tau(rad) value. The large difference in eta(sens) for microcrystalline samples of Eu (85%) and Tb (42%) tris(dipicolinates) is attributed to back energy transfer in the latter compound consecutive to a sizeable overlap between the (5)D(4)-->(7)F(6) emission and the absorption spectrum of the dipicolinate triplet, this overlap being smaller in the case of the solution. The overall quantum yield of Na(3)[Yb(dpa)(3)] in aqueous solution is very low (0.015 +/- 0.002%) due to both poor sensitization efficiency (8%) and small intrinsic quantum yield (Q = 0.178 +/- 0.003%; tau(rad) = 1.31 +/- 0.02 ms). For evaluating intrinsic quantum yields of Yb in aqueous solutions of coordination compounds from lifetimes, a value of 1.2-1.3 ms is recommended.