Recent startling interest for lanthanide luminescence is stimulated by the continuously expanding need for luminescent materials meeting the stringent requirements of telecommunication, lighting, electroluminescent devices, (bio-)analytical sensors and bio-imaging set-ups. This critical review describes the latest developments in (i) the sensitization of near-infrared luminescence, (ii) "soft" luminescent materials (liquid crystals, ionic liquids, ionogels), (iii) electroluminescent materials for organic light emitting diodes, with emphasis on white light generation, and (iv) applications in luminescent bio-sensing and bio-imaging based on time-resolved detection and multiphoton excitation (500 references).

The fascination for lanthanide optical spectroscopy dates back to the 1880s when renowned scientists such as Sir William Crookes, LeCoq de Boisbaudran, Eugène Demarçay or, later, Georges Urbain were using luminescence as an analytical tool to test the purity of their crystallizations and to identify potential new elements. The richness and complexity of lanthanide optical spectra are reflected in an article published in 1937 by J.H. van Vleck: The Puzzle of Rare Earth Spectra in Solids. After this analytical and exploratory period, lanthanide unique optical properties were taken advantage of in optical glasses, filters, and lasers. In the mid-1970s, E. Soini and I. Hemmilä proposed lanthanide luminescent probes for time-resolved immunoassays (Soini and Hemmilä in Clin Chem 25: [353][354][355][356][357][358][359][360][361] 1979) and this has been the starting point of the present numerous bio-applications based on optical properties of lanthanides. In this chapter, we first briefly outline the principles underlying the simplest models used for describing the electronic structure and spectroscopic properties of trivalent lanthanide ions Ln III (4f n ) with special emphasis on luminescence. Since the book is intended for a broad readership within the sciences, we start from scratch defining all quantities used, but we stay at a descriptive level, leaving out detailed mathematical developments. For the latter, the reader is referred to references Liu and Jacquier, Spectroscopic properties of rare earths in optical materials. Tsinghua University Press & Springer, Beijing & Heidelberg, 2005 and Görller-Walrand and Binnemans, Rationalization of crystal field parameters. In: Gschneidner, Eyring (eds) Handbook on the physics and chemistry of rare earths, vol 23. Elsevier BV, Amsterdam, Ch 155, 1996. The second part of the chapter is devoted to practical aspects of lanthanide luminescent probes, both from the point of view of their design and of their potential utility.

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. ScopeLanthanide luminescence has a special status in photonics in view of the unique spectroscopic properties of lanthanide ions with respect to light generation and amplication. Following early applications in lighting at the end of the 19 th century (Auer mantle, int stones), the discovery of the bright red emitting phosphor Y 2 O 3 :Eu III at the beginning of the 20 th century and the advent of the YAG:Nd III laser in 1964, the number of photonic applications of lanthanides has been burgeoning and they presently extend from laser physics to materials sciences, agriculture, and medical diagnostics. Selected examples are phosphors for lighting and displays, lasers, optical telecommunications, night vision, guiding systems, security inks, counterfeiting tags, luminescent coatings, probes for luminescent immunoassays, and bioconjugates for medical imaging and the photodynamic treatment of cancer. The liveliness of lanthanide photonics is demonstrated by the plentiful general 1 and specialised 2,3 reviews appearing at an accelerated pace.The aim of this minireview is to focus on some intriguing results obtained in areas such as biosciences where upconverting nanophosphors and long-persistence phosphors have opened whole new areas, as well as on selected unusual and/or not yet too explored aspects of lanthanide luminescence both at academic and practical levels, e.g. security and counterfeiting tags, luminescent probes, impact sensors, and solar energy conversion. For an executive summary on lanthanide spectroscopy, see the ESI. †

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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.

Heteronuclear lanthanide terephthalate coordination polymers with the general chemical formula [Ln2–2xLn′2x(bdc)3(H2O)4]∞, for which bdc2– symbolizes benzene‐1,4‐dicarboxylate (or terephthalate) and Ln and Ln′ represent trivalent rare earth ions, were synthesized and structurally characterized. Analysis of the Y/Lu compounds by 89Y and 13C solid‐state NMR spectroscopy was carried out, and the results support the hypothesis of randomly distributed lanthanide ions. The spectroscopic and colorimetric properties of this family of compounds were investigated in detail. The resulting data demonstrate that this series of compounds presents highly tunable luminescence properties and clearly indicate that intermetallic deactivation processes play an important role in the emission mechanism. Playing with intermetallic distances allows one to tune the color and the brightness of the lanthanide emission in these coordination polymers.

The development of efficient sensors for the determination of the water content in organic solvents is highly desirable for a number of chemical industries. Presented herein is a Mg(2+) metal-organic framework (MOF), which exhibits the remarkable capability to rapidly detect traces of water (0.05-5 % v/v) in various organic solvents through an unusual turn-on luminescence sensing mechanism. The extraordinary sensitivity and fast response of this MOF for water, and its reusability make it one of the most powerful water sensors known.

Luminescent lanthanide(III)-based molecular scaffolds hold great promises for materials science and for biological applications. Their fascinating photophysical properties enable spectral discrimination of emission bands that range from the visible to the near-infrared (NIR) regions. In addition, their strong resistance to photobleaching makes them suitable for long duration or repeated biological experiments using a broad range of sources of excitation including intense and focalized systems such as lasers (e.g., confocal microscopy). A main challenge in the creation of luminescent lanthanide(III) complexes lies in the design of a ligand framework that combines two main features: (i) it must include a chromophoric moiety that possesses a large molar absorptivity and is able to sensitize several different lanthanide(III) ions emitting in the visible and/or in the near-infrared, and (ii) it must protect the Ln(3+) cation by minimizing nonradiative deactivation pathways due to the presence of -OH, -NH and -CH vibrations. Herein, a new family of luminescent Ga(3+)/Ln(3+) metallacrown (MC) complexes is reported. The MCs with the general composition [LnGa4(shi)4(C6H5CO2)4(C5H5N) (CH3OH)] (Ln-1, Ln = Sm(3+)-Yb(3+)) were synthesized in a one pot reaction using salicylhydroxamic acid (H3shi) with Ga(3+) and Ln(3+) nitrates as reagents. The molecular structure of [DyGa4(shi)4(C6H5CO2)4(C5H5N) (CH3OH)] was obtained by X-ray analysis of single crystals and shows that the complex is formed as a [12-MCGa(III)shi-4] core with four benzoate molecules bridging the central Dy(3+) ion to the Ga(3+) ring metals. The powder X-ray diffraction analysis demonstrates that all other isolated complexes are isostructural. The extended analysis of the luminescence properties of these complexes, excited by the electronic states of the chromophoric ligands, showed the presence of characteristic, sharp f-f transitions that can be generated not only in the NIR (Sm, Dy, Ho, Er, Yb) but also in the visible (Sm, Eu, Tb, Dy, Tm). All Ln-1 complexes possess very high quantum yield values with respect to other literature compounds, indicating a good sensitization efficiency of the [12-MCGa(III)shi-4] scaffold. Especially, as of today, the Yb-1 complex exhibits the highest NIR quantum yield reported for a lanthanide(III) complex containing C-H bonds with a value of 5.88(2)% in the solid state. This work is a significant step forward toward versatile, easily prepared luminescent lanthanide(III) complexes suitable for a variety of applications including highly in demand biological imaging, especially in the NIR domain.