Ulrich Kynast

Optically Functional Zeolites: Evaluation of UV and VUV Stimulated Photoluminescence Properties of Ce3+- and Tb3+-doped Zeolite X

The optical properties of Tb3+/Ce3+ doped zeolites are elucidated with emphasis on ultraviolet (UV) and vacuum ultraviolet (VUV) excitation and luminescence. Ce3+ sensitized Tb3+ emission with quantum yields of 85 % may be obtained at 330 nm excitation. Low absorptivity at 254 nm due to low Ce3+ concentrations or low Ce3+/Tb3+ ratios, which are required for the suppression of UV components, restricts their applicability as phosphors for Hg-based discharges, e.g., in conventional fluorescent lamps. Near band edge excitation at 172 nm revealed an immediate quantum yield of 50 % enabled by a zeolite Ce3+ (5d1) Tb3+ (4f75d1) energy transfer channel, which may be exploited for the down-conversion of the Xe2 excimer emission.

Laponite Blue: Dissolving the Insoluble

The neutral organic dye indigo forms an inorganic-organic hybrid material with nanoclays (see picture; blue circles on disks symbolizing indigo, spheres indicating liberated cations) and can thus be transferred into aqueous solution. Solids recovered from these solutions resemble the ancient Maya Blue pigment. The method can also be applied to other hydrophobic species and may open the gate for novel solution chemistry, including photonic and catalytic applications.

One dimensional energy transfer in lanthanoid picolinates. Correlation of structure and spectroscopy

Optical and structural properties of rare earth complexes with 2-pyridine carboxylic acid (‘Hpic’) are evaluated by luminescence spectroscopy, decay measurements, X-ray crystal structure determination, FTIR, DTA and metal content analysis. Corresponding Tb3+and Eu3+ complexes of this ligand are extraordinarily efficient with respect to their luminescence. In the crystalline state the series is isostructural and composed of M[Ln(pic)4]·nH2O (M = Na, NH4; Ln = Eu, Gd, Tb, Ho) with pic-linked [Ln(pic)4]− units forming a chain-like structure, which gives rise to a one-dimensional exchange communication between the rare earth ions; this energy transfer being confined to the chains. Energy transfer of the Coulomb type between the ligands appears to be of significance only, if suitable rare earth acceptor states are not accessible, as is shown for a series of [La(pic)4]− series, in which La3+ is gradually substituted by Tb3+ or Eu3+.

Nile-Red–Nanoclay Hybrids: Red Emissive Optical Probes for Use in Aqueous Dispersion

Water-dispersible and (bio)functionalizable nanoclays have a considerable potential as inexpensive carriers for organic molecules like drugs and fluorophores. Aiming at simple design strategies for red-emissive optical probes for the life sciences from commercial precursors with minimum synthetic effort, we systematically studied the dye loading behavior and stability of differently functionalized laponites. Here, we present a comprehensive study of the absorption and emission properties of the red emissive hydrophobic and neutral dye Nile Red, a well-known polarity probe, which is almost insoluble and nonemissive in water. Adsorption of this probe onto disk-shaped nanoclays was studied in aqueous dispersion as function of dye concentration, in the absence and presence of the cationic surfactant cetyltrimethylammonium bromide (CTAB) assisting dye loading, and as a function of pH. This laponite loading strategy yields strongly fluorescent nanoclay suspensions with a fluorescence quantum yield of 0.34 at low dye loading concentration. The dye concentration-, CTAB-, and pH-dependent absorption, fluorescence emission, and fluorescence excitation spectra of the Nile-Red-nanoclay suspensions suggest the formation of several Nile Red species including emissive Nile Red monomers facing a polar environment, nonemissive H-type dimers, and protonated Nile Red molecules that are also nonfluorescent. Formation of all nonemissive Nile Red species could be suppressed by modification of the laponite with CTAB. This underlines the great potential of properly modified and functionalized laponite nanodisks as platform for optical probes with drug delivery capacities, for example, for tumor and therapy imaging. Moreover, comparison of the Nile Red dimer absorption spectra with absorption spectra of previously studied Nile Red aggregates in dendrimer systems and micelles and other literature systems reveals a considerable dependence of the dimer absorption band on microenvironment polarity which has not yet been reported so far for H-type dye aggregates.

Semiconductor nanostructures via electrodeposition from ionic liquids

The fascinating properties of ionic liquids make it possible to synthesize semiconductor nanostructures via a simple and low-cost electrochemical pathway. The present paper summarizes our recent work on the synthesis of Si, Ge, and SixGe1–x nanostructures from ionic liquids: thin films, nanowires and photonic crystals. We also introduce our first results on the template-assisted electrodeposition of SixGe1–x photonic crystals from 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMIm]Tf2N) ionic liquid, and some optical measurements on the previously prepared Ge photonic crystals. Our results confirm that electrochemistry in ionic liquids is excellently suited to the synthesis of high-quality semiconductor nanostructures.

Efficient green emission from transparent Tb3+–silicone hybrid materials

Salicylate complexes of terbium have been modified by co-coordination with trioctylphosphine oxide, by which the complexes can be rendered soluble even in non-polar matrices. Highly transparent, green emitting composites based on rare ions can thus be obtained now by the incorporation of these complexes into silicone matrices, which has in the past been reserved for green emitting organic dyes. The optical properties of the novel hybrid systems have been characterised, showing efficient luminescence of the Tb3+ ions with emission quantum yields of up to 68%.

Multifold fluorescence enhancement in nanoscopic fluorophore-clay hybrids in transparent aqueous media.

Valuable emissive properties of organic fluorophores have become indispensable analytical tools in biophotonics, but frequently suffer from low solubilities and radiationless deactivation in aqueous media, that is, in biological ambience as well. In this report, nanoscaled dye-clay hybrids based on laponite, Na0.7 {(Li0.3 Mg5.5 )[Si8 O20 (OH)4 ]}, are taken advantage of to solubilize neutral dyes, which are natively not encountered in water. Previously reported efficiency and solubility bottlenecks of such hybrids can to a large extent be overcome by comparably simple chemical measures, as demonstrated here for two prominent examples, the fluorescent dyes Nile Red and Coumarin 153. On controlled co-adsorption of small bifunctional quaternary ammonium ions (Me3 N(+) C2 H5 OH and Me3 N(+) C2 H5 NH2 ) we observed an outright efficiency boost by an order of magnitude, and a 30-fold brightness gain. Even at higher concentrations, transparency and stability of the hybrid dispersions are retained, rendering them useful for employment as optically functional nanoparticles in bioassays and beyond.

Phthalocyanine blue in aqueous solutions

Using laponite nano-clay carriers, a facile method for the solubilisation of natively insoluble phthalocyanines into aqueous solution is described. Copper(II) phthalocyanine, technologically a most relevant pigment (C.I. Pigment Blue 15), thus yields hitherto unknown clear and stable aqueous dispersions of either colloidal α-CuPc or monomeric CuPc, depending on details of the preparation.

Potential of Nano-Sized Rare Earth Fluorides in Optical Applications

Rare earth fluorides are a class of materials with a high potential for optical applications. Fluoride lattices allow high coordination numbers for the hosted rare earth ions, but the high ionicity of the rare earth to fluorine bond leads to a wide band gap and very low vibrational energies. These two essential factors, in particular, contribute to their practicality for use in optical applications based on vacuum ultraviolet (VUV) and near infrared (NIR) excitation. The preparation and optical characteristics of rare earth fluoride nanoparticles and their embedding in polymeric, glassy or porous matrices are very promising for the eventual manufacture of transparent hybrid materials. Recent attempts to control the size of these particles down to the nano-scale and, at the same time, maintaining the performance of their macroscopic counterparts, indicate accessibility of hitherto unrealized optical properties and applications.

Rare-Earth Ions in Porous Matrices

Initially motivated by the commercial need for cheaper and environmentally friendly luminescent materials for application in fluorescent lamps and cathode ray tubes, the search for new matrices for optically active species has penetrated a scope far beyond “classical” solid-state materials. Porous matrices with voids ranging from the nano-to the microscale have become the subject of recent investigations. Crystalline, amorphous, organomorphous, nanosized matrices and matrices, which are amorphous on the atomic level but have a translational superstructure on the microscale (zeolites, sol-gel materials, polymers, nanoparticles and photonic crystals), are addressed. The optical technologies covered in this research range from mercury free discharge lamps, plasma displays, organic and polymeric light emitting diodes, and novel laser materials to biophotonics and the new generation of white emitting AlGaN solid-state light emitting diodes (LEDs). Due to their specific properties (e.g., high quantum yields, narrow line emission), rare-earth ions are indispensable components of these approaches, be it in the nanoscaling zeolites, sol-gel matrices, or as the active component in optically functional polymers. Optical properties of hybrid materials composed of either rare-earth ions as such, their complexes, or nanoparticles in these matrices, with potential application in the fields mentioned, will form the scope of the present report.