Upconversion and Anti-Stokes Processes with f and d Ions in Solids

Near-Infrared (NIR) to Red and Green Up-Conversion Emission from Silica Sol−Gel Thin Films Made with La0.45Yb0.50Er0.05F3 Nanoparticles, Hetero-Looping-Enhanced Energy Transfer (Hetero-LEET): A New Up-Conversion Process

Bright green and red luminescence has been generated with a 980 nm diode laser from silica sol-gel thin films made with La0.45Yb0.50Er0.05F3 nanoparticles through a newly described hetero-looping-enhanced energy-transfer (hetero-LEET) up-conversion process, which exhibits a power dependence similar to that of a photon avalanche (PA). The hetero-LEET mechanism is potentially more efficient than PA, ground-state absorption/excited-state absorption (GSA/ESA), and energy-transfer (ETU) mechanisms because it combines resonant ground-state absorption with a looping or feedback process.

Crystal Absorption Spectra in the Region of 4f−4f and 4f−5d Excitations in Tm2+-Doped CsCaCl3, CsCaBr3, and CsCaI3

Low-temperature absorption spectra of single crystals of Tm2+-doped CsCaCl3, CsCaBr3, and CsCaI3 in the spectral range from 8700 to 47000 cm-1 are presented. Weak sharp-line 4f-4f absorptions around 8800 cm-1 are essentially independent of the nature of the halide. More-intense broad absorptions cover the region between 12000 and 47000 cm-1. They are assigned to 4f-5d excitations and interpreted in terms of a simple qualitative picture taking into account the most important interactions. As a result of two counterbalancing effects, the onsets of the 4f-5d spectra are almost coincident in the three materials: The blue-shift of about 3000 cm-1 between chloride and iodide resulting from the decreasing crystal field splitting of 5d is roughly balanced by the red-shift resulting from the reduced energy gap between the average energy of the 4f13 and the 4f125d1 electron configurations. The absorption helps the understanding of the most unusual light emission properties of these materials.

Two-photon-excited fluorescence from silicate glass containing tantalum ions pumped by a near-infrared femtosecond pulsed laser

We report a novel phenomenon in sodium-calcium-silicate glass doped with Ta(5+). Under irradiation with a 780 nm femtosecond pulsed laser, strong blue emission centered at about 420 nm could be observed. The spectral characteristics are similar to those pumped by ultraviolet photons. The log-log correlation between integrated emission intensity and pump power reveals that a two-photon absorption process is involved in the phenomenon. It is suggested that the presence of localized Ta(5+)5d(0) energy levels is responsible for the appearance of the blue emission. The results indicate that transition metal ions without d electrons play an important role in fields of optics when embedded into glass hosts.

Preparation Process and Upconversion Luminescence of Er3+-Doped Glass Ceramics Containing Ba2LaF7 Nanocrystals

The preparation process and upconversion luminescence of the Er(3+)-doped glass ceramics containing Ba(2)LaF(7) nanocrystals were investigated. The formation of Ba(2)LaF(7) nanocrystals in the glass ceramics was confirmed by X-ray diffraction. Er(3+)-doped glass ceramics containing Ba(2)LaF(7) nanocrystals exhibited highly efficient upconversion luminescence in comparison with glasses. With the increase of heat treatment temperature the upconversion luminescence intensity increased gradually. The composition of glasses was also found to have significant influence on the crystallization process of glass ceramics. The mixture of Ba(2)LaF(7) and La(2)O(3) nanocrystals and the mixture of La(2)F(3) and La(2)O(3) nanocrystals in the glass ceramics could be obtained by controlling different compositions of glasses. The upconversion luminescence intensity also varied significantly with different nanocrystals in the glass ceramics.

Structures, Photoluminescence, Up-Conversion, and Magnetism of 2D and 3D Rare-Earth Coordination Polymers with Multicarboxylate Linkages

Four new rare-earth compounds, [Eu(NDC)1.5(DMF)2] (1), [Nd2(NDC)3(DMF)4].H2O (2), [La2(NDC)3(DMF)4].0.5H2O (3), and [Eu(BTC)(H2O)] (4), where NDC = 1,4-naphthalenedicarboxylate, BTC = 1,3,5-benzenetricarboxylate, and DMF = N,N-dimethylformamide, have been synthesized through preheating and cooling-down crystallization. Compounds 1-3 possess similar 2D structures, in which the NDC ligands link M(III) (M = La, Nd, and Eu) ions of two adjacent double chains constructed by NDC ligands and dinuclear M(III) building units. In compound 4, the Eu(III) ion is seven-coordinated by O atoms from six BTC ligands and one terminal water molecule in a distorted pentagonal-bipyramidal coordination environment. If the BTC ligand and the Eu(III) ion are regarded as six-connected nodes, respectively, the structure of compound 4 can be well described as a 3D six-connected net. Furthermore, compounds 1 and 4 exhibit strong red luminescence upon 355-nm excitation. Compound 2 displays interesting emissions in the near-IR region, and yellow (580 nm) pumping of this compound results in UV and intense blue emissions through an up-conversion process. The magnetic properties of compounds 1, 2, and 4 have been studied through measurement of their magnetic susceptibilities over the temperature range of 4-300 K.

New Type of Near-Infrared to Visible Photon Upconversion in Tm2+-Doped CsCaI3

Tm2+ doped in CsCaI3 displays unusual optical properties that are characterized by the existence of two metastable 4f-5d excited states in the near-infrared and visible spectral region, respectively. For the first time, a photon upconversion process based on sequential absorption of light by 4f-5d states is reported. The large absorption cross-section of the involved transitions allows highly efficient pumping in the NIR. An efficiency of 11% for the green upconversion luminescence is reached at 10 K, and the upconversion luminescence remains visible by eye up to room temperature. The energy positions of the relevant 4f-5d states and thus the photophysical and light emission properties can be tuned by chemical variation, such as placing the Tm2+ ion into the isostructural CsCaBr3 and CsCaCl3 lattices.

In vivo and scanning electron microscopy imaging of up-converting nanophosphors in Caenorhabditis elegans

We show here that upconversion phosphors can be imaged both by infrared excitation and in a scanning electron microscope. We have synthesized and characterized for this work up-converting phosphor nanoparticles nonaggregated nanocrystals of size range 50-200 nm. We have investigated the optical properties of 50-200 nm nanoparticles and found a square dependence of the emitted visible fluorescence on the infrared excitation and verified that under electron excitation similar narrow band emission spectra can be obtained as is seen with IR upconversion. The viability of the nanoparticles for biological imaging was confirmed by imaging the digestive system of the nematode worm Caenorhabditis elegans, and we have confirmed using energy-dispersive X-ray analysis that the up-conversion nanoparticles can be identified in a scanning electron microscope at high spatial resolution.

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.).

Photochemical Characterization of Up-Converting Inorganic Lanthanide Phosphors as Potential Labels

We have characterized commercially available up-converting inorganic lanthanide phosphors for their rare earth composition and photoluminescence properties under infrared laser diode excitation. These up-converting phosphors, in contrast to proprietary materials reported earlier, are readily available to be utilized as particulate reporters in various ligand binding assays after grinding to submicron particle size. The laser power density required at 980 nm to generate anti-Stokes photoluminescence from these particulate reporters is significantly lower than required for two-photon excitation. The narrow photoluminescence emission bands at 520-550 nm and at 650-670 nm are at shorter wavelengths and thus totally discriminated from autofluorescence and scattered excitation light even without temporal resolution. Transparent solution of colloidal bead-milled up-converting phosphor nanoparticles provides intense green emission visible to the human eye under illumination by an infrared laser pointer. In this article, we show that the unique photoluminescence properties of the up-converting phosphors and the inexpensive measurement configuration, which is adequate for their sensitive detection, render the up-conversion an attractive alternative to the ultraviolet-excited time-resolved fluorescence of down-converting lanthanide compounds widely employed in biomedical research and diagnostics.

Upconversion from aqueous phase lanthanide chelates

We have prepared and characterized several lanthanide ion complexes of multidentate ligands or chelates in an effort to develop new upconverting luminescent labels that can be immune to autofluorescence and photobleaching. This study has involved the characterization of various chelates of Nd, Er, and Tm with respect to relative luminescent efficiency and excited-state lifetimes and explored various two-photon stepwise excitation mechanisms. Using peak laser powers near 100 kW, the upconversion emissions of Nd in Nd(EDTA)2(5-) at 386 nm, Er in Er(DPA)3(3-) at 550 nm, and Tm in Tm(DPA)3(3-) at 480 nm, at levels of approximately 10(-12) moles can be detected.

Synthesis of Yb3+/Er3+ co-dopants sodium yttrium fluoride up-conversion fluorescence materials

Yb3+/Er3+ co-dopants sodium yttrium fluoride micro-powders were synthesized by a combustion method and the luminescence properties investigated. The results indicated that the up-conversion material synthesized by this method has high up-conversion efficiency and the fluorescence spectrum peak was sharp.

Making sense of lanthanide luminescence

The luminescence of trivalent lanthanide ions has found applications in lighting, lasers, optical telecommunications, medical diagnostics, and various other fields. This introductory review presents the basics of organic and inorganic luminescent materials containing lanthanide ions, their applications, and some recent developments. After a brief history of the discovery, purification and early spectroscopic studies of the lanthanides, the radiative and nonradiative transitions of the 4f electrons in lanthanide ions are discussed. Lanthanide-doped phosphors, glasses and crystals as well as luminescent lanthanide complexes with organic ligands receive attention with respect to their preparation and their applications. Finally, two recent developments in the field of luminescent materials are addressed: near-infrared luminescent lanthanide complexes and lanthanide-doped nano-particles.

A Simple Thermodynamic Model for Rationalizing the Formation of Self-Assembled Multimetallic Edifices: Application to Triple-Stranded Helicates

Reaction of the bis-tridentate ligand bis[1-ethyl-2-[6'-(N,N-diethylcarbamoyl)pyridin-2'-yl]benzimidazol-5-yl]methane (L2) with Ln(CF(3)SO(3))(3).xH(2)O in acetonitrile (Ln = La-Lu) demonstrates the successive formation of three stable complexes [Ln(L2)(3)](3+), [Ln(2)(L2)(3)](6+), and [Ln(2)(L2)(2)](6+). Crystal-field independent NMR methods establish that the crystal structure of [Tb(2)(L2)(3)](6+) is a satisfying model for the helical structure observed in solution. This allows the qualitative and quantitative beta23 (bi,Ln1,Ln2)characterization of the heterobimetallic helicates [(Ln(1))(Ln(2))(L2)(3)](6+). A simple free energy thermodynamic model based on (i) an absolute affinity for each nine-coordinate lanthanide occupying a terminal N(6)O(3) site and (ii) a single intermetallic interaction between two adjacent metal ions in the complexes (DeltaE) successfully models the experimental macroscopic constants and allows the rational molecular programming of the extended trimetallic homologues [Ln(3)(L5)(3)](9+).