Mechanisms of Energy Transfer and Enhanced Stability of Carbidonitride Phosphors for Solid-State Lighting.

Phosphor-converted light emitting diodes (pcLEDs) produce white light through the use of phosphors that convert blue light emitted from the LED chip into green and red wavelengths. Understanding the mechanisms of degradation of the emission spectra and quantum yields of the phosphors used in pcLEDs is of critical importance to fully realize the potential of solid-state lighting as an energy efficient technology. Toward this end, time-resolved photoluminescence spectroscopy was used to identify the mechanistic origins of enhanced stability and luminescence efficiency that can be obtained from a series of carbidonitride red phosphors with varying degrees of substitutional carbon. The increasing substitution of carbon and oxygen in nitrogen positions of the carbidonitride phosphor (Sr2Si5N8-[(4x/3)+z]CxO3z/2:Eu2+) systematically changed the dimensions of the crystalline lattice. These structural changes caused a red shift and broadening of the emission spectra of the phosphors due to faster energy transfer from higher to lower energy emission sites. Surprisingly, in spite of broadening of the emission spectra, the quantum yield was maintained or increased with carbon substitution. Aging phosphors with lowered carbon content under conditions that accurately reflected thermal and optical stresses found in functioning pcLED packages led to spectral changes that were dependent on substitutional carbon content. Importantly, phosphors that contained optimal amounts of carbon and oxygen possessed luminescence spectra and quantum yields that did not undergo changes associated with aging and therefore provided a more stable color point for superior control of the emission properties of pcLED packages. These findings provide insights to guide continued development of phosphors for efficient and stable solid-state lighting materials and devices.

Raman spectroscopic investigation of tetraethylammonium polybromides.

A large number of polyhalides, especially polyiodides, have been discovered and studied, but definitive studies on polybromides remain scarce. Br(3)(-) is the only monovalent polybromide with a known structure. Higher-order monovalent polybromide anions have been proposed but not structurally confirmed as discrete species. In this study tetraalkylammonium polybromides with molecular formulas R(4)NBr(2x+1) (R = ethyl; x = 1-4) were prepared by reacting tetraalkylammonium monobromide or tribromide salts with gas-phase bromine. Distinct and characteristic Raman spectra were obtained from the solid polybromides in the spectral range between 100 and 400 cm(-1). Experimental Raman spectra were compared to ab initio calculations to propose the structure of these polybromide anions. A general agreement between the experimental and theoretical results was observed. This study demonstrates that Raman spectroscopy is a sensitive technique for probing the structure of discrete monovalent polybromides.

Inhibition by fibrates of plasminogen activator inhibitor type-1 expression in human adipocytes and preadipocytes.

Plasminogen activator inhibitor type-1 (PAI-1), an established marker and mediator of cardiovascular risk, is produced extensively in adipose tissue. Fibrates are hypolipidemic peroxisome proliferator activated receptor-alpha (PPARalpha) agonists. Recent laboratory and clinical observations indicate that they are also anti-atherosclerotic. Mechanisms responsible, however, remain to be fully understood. The present study was designed to elucidate modulation of PAI-1 expression in adipose cells by fibrates as a potential mechanism. Expression of PPARalpha was verified by PCR, immunohistochemistry, and Western blotting. In cultured preadipocytes and adipocytes gemfibrozil and fenofibrate significantly reduced PAI-1 protein expression by up to 55 +/- 5% and 34 +/- 4% under basal conditions and up to 56 +/- 6% and 31 +/- 6% under conditions of stimulation of the cells with 40 pM transforming growth factor (TGF)beta, respectively. Quantification of mRNA showed that the gemfibrozil-induced effect was at least in part regulated at the transcriptional level. Incubations with non-fibrate PPARalpha agonists showed similar reductions in PAI-1 expression. The decrease in PAI-1 expression induced by gemfibrozil was inhibited by MK886, a PPARalpha inhibitor. Furthermore, preadipocytes isolated from PPARalpha-deficient mice produced significantly more PAI-1 than those from wild-type mice upon stimulation with TGFbeta. Finally, fenofibrate reduced PAI-1 expression both in plasma and adipose tissue of hyperlipidemic mice. Our data support the view that PPARalpha activation down-regulates PAI-expression in adipose cells that may contribute in part to the reduction in cardiovascular mortality seen with fibrates in clinical trials.

Direct attenuation of plasminogen activator inhibitor type-1 expression in human adipose tissue by thiazolidinediones.

Adipose tissue produces substantial amounts of plasminogen activator inhibitor type-1 (PAI-1), an established cardiovascular risk factor. This study evaluated PAI-1 expression in human adipose tissue in response to thiazolidinediones, insulin sensitising drugs activating peroxisome proliferator-activated receptor-gamma (PPAR-gamma). Troglitazone, rosiglitazone, and ciglitazone significantly reduced PAI-1 protein expression in human preadipocytes under basal conditions and after stimulation of the cells with TGF-beta. Pioglitazone had no effect. In human adipocytes all four thiazolidinediones significantly attenuated PAI-1 expression. Signalling appeared to be mediated via PPAR-gamma and effects reflected, at least in part, changes in transcription. Accordingly, patients with insulin resistance may benefit from treatment with thiazolidinediones with respect to diminution of PAI-1 expression in adipose tissue and consequent potential reduction of cardiovascular risk.

High-throughput analysis in catalysis research using novel approaches to transmission infrared spectroscopy.

This study has demonstrated that high-throughput FTIR transmission measurements using a newly designed array-based support formed using silicon wells and a silicon wafer is a very useful and robust tool for the characterization of polymer composition for combinatorial materials research. The comonomer content in copolymers can be measured accurately with a fully automated throughput of >300 samples/day (8 h). The transmission measurement is more robust, reliable, and easier to automate than other spectroscopic methods. The support itself provides excellent resistance to aggressive organic solvents at elevated temperatures and allows the unattended deposition and preparation of polymer films for infrared analysis. Because of the excellent durability of the support with respect to the solvent, the support can be rinsed and reused many times. This high-throughput approach to infrared transmission spectroscopy can be used for measuring a wide array of polymer characteristics: vinyl content, geometrical isomers, crystallinity, and tacticity. As well, this IR approach can be used to predict the oxidative stability of the antioxidant packages. Because the support provides a means of containing hot polymer solutions while the solvent evaporates, the support is also suitable for high-throughput nanoindentation methods for the determination of modulus and other physical properties of the polymer.