4,021 research outputs found
Nonionic water-soluble polysilanes: Global conformation in solution and the occurrence of preferential solvation
No full textIn aqueous solution UV-absorption and fluorescence emission spectroscopy reveal that the nonionic water-soluble polysilanes poly(4,7,10-trioxaundecylmethylsilane) (1) and poly(4,7,10,13-tetraoxatetradecylmethylsilane) (2) are present in a highly folded random coil. In contrast, in organic solvents the backbone adopts a more extended conformation. These changes in backbone conformation are accompanied by changes in the average backbone silicon segmental length L from 15 to 21-24. Concomitantly, the exciton coherence length also exhibits a 2-fold increase. The results obtained for 1 and 2 are compared to those of poly(4,7,10-trioxahexadecylmethylsilane) (3) and poly(4,7,10,13-tetraoxanonadecylmethylsilane) (4), which contain instead of a CH3, a C6H13 moiety as the side chain end group. A detailed analysis of aqueous/organic binary solvent mixtures of 2 reveals that already a low mole fraction of the organic solvent induces large shifts in the photophysical properties; i.e., preferential solvation occurs at a mole fraction of ca. 0.15, Hence upon addition of small amounts of organic solvents, the solute-solvent interactions, which in H2O prevent the silicon backbone from adopting a more extended conformation, an reduced, causing the backbone to be less distorted. This is supported by the increase in the quantum yield Phi in going from polar aqueous to apolar organic solvents
Aggregation and solvatochromism of polysilanes and polysilynes in aqueous and binary solutions.
No full textWill biological agents supplant systemic glucocorticoids as the first-line treatment for thyroid-associated ophthalmopathy?
No full textIn this article, the two authors present their opposing points of view concerning the likelihood that glucocorticoids will be replaced by newly developed biological agents in the treatment of active, moderate-to-severe thyroid-associated ophthalmopathy (TAO). TAO is a vexing, disfiguring and potentially blinding autoimmune manifestation of thyroid autoimmunity. One author expresses the opinion that steroids are nonspecific, frequently fail to improve the disease and can cause sometimes serious side effects. He suggests that glucocorticoids should be replaced as soon as possible by more specific and safer drugs, once they become available. The most promising of these are biological agents. The other author argues that glucocorticoids are proven effective and are unlikely to be replaced by biologicals. He reasons that while they may not uniformly result in optimal benefit, they have been proven effective in many reports. He remains open minded about alternative therapies such as biologicals but remains skeptical that they will replace steroids as the first-line therapy for active, moderate-to-severe TAO without head-to-head comparative clinical trials demonstrating superiority. Despite these very different points of view, both authors are optimistic about the availability of improved medical therapies for TAO, either as single agents or in combination. Further, both agree that better treatment options are needed to improve the care of our patients with active moderate-to-severe TAO
The value of silence
No full textThis is an electronic version of the article published in Theatre Journal, 54(1):85-94, 2002 March. The published article is available at http://muse.jhu.edu/journals/theatre_journal/v054/54.1eng.pdfEng, David L.The Value of Silence.Theatre Journal, 54,(1):85-94, 2002.DOI: 10.1353/tj.2002.000
Occurrence of radical cation localization in chemically modified poly(methylphenylsilane): Poly(methylphenyl-co-4-dimethylaminophenylmethylsilane)s and poly (methylphenyl-co-4-bromophenylmethylsilane)s
No full textChemical modification of poly(methylphenylsilane) 1 using triflate chemistry gave two series of random copolymers with different composition ratios, viz., poly(methylphenyl-co-4-dimethylaminophenylmethylsilane)s 3-7 and poly(methylphenyl-co-4-bromophenylmethylsilane)s 8-9. The optical and electronic properties of the copolymers have been studied and are compared with those of the homopolymers 1 and poly(4-dimethylaminophenyl-methylsilane) 2. Electrochemical measurements in THF/LiClO4 give strong evidence that, in marked contrast to the results observed for 1 and 2, the onset of oxidation (V-i) of 3-7 and 8-9, respectively, does not represent the silicon backbone valence band. Instead, in these copolymers, a mixture of decoupled electronic domains exists of which those possessing the lowest V-i govern the electrochemical response. As a consequence, the radical cations become confined, i.e. localization occurs. This interpretation is further supported by fluorescence emission spectroscopy, the presence of a confined exciton is indicated by the appearance of an additional broad band for the copolymers. Remarkably, copolymers with optical and electronic properties comparable to those of 3-7 are also accessible by (reversible) protonation of the 4-dimethylamino substituents of 2
Water-soluble poly(4,7,10,13-tetraoxatetradecylmethylsilane): Enhanced yield and improved purity via polymerization using graphite-potassium (C8K) as reducing agent
No full textApplication of graphite-potassium (C8K, THF, 0 degrees C), instead of convential Wurtz-type coupling conditions (Na, toluene, 110 degrees C), as reducing agent for the preparation of non-ionic, watersoluble poly(4,7,10,13-tetraoxatetradecylmethylsilane) 1 gives a markedly enhanced yield as well as a high molecular weight polymer of high purity
Non-ionic polsyilanes and polysilynes in aqueous solution: Polymer structure vs. temperature.
No full textNonionic water-soluble polysilynes. Synthesis and properties of a novel class of functionalized materials
No full textThe nonionic water-soluble polysilynes poly(4,7,10-trioxaundecylsilyne) (1) and poly(4,7,10,13-tetraoxatetradecylsilyne) (2), which are inaccessible using the conventional Wurtz-type coupling with Na in refluxing toluene, have been prepared in reasonable yields using graphite potassium C8K as the reducing agent in THF at 0 degrees C. A mater-insoluble analogue of 1, viz. poly(4,7,10-trioxahexadecylsilyne) (3), is obtained in nearly quantitative yield under similar conditions. Despite the fact that 1 and 2 possess all the characteristic polysilyne-like (photo) physical properties, aqueous solutions of 1 and 2 unexpectedly exhibit thermoresponsive behavior; i.e., at 49 degrees C a lower critical solution temperature (LCST) is found. The presence of an LCST, which has to originate from folding/unfolding processes of the polysilyne backbone, suggests that polysilynes have a hybrid structure with a predominantly one-dimensional overall appearance consisting of linear fragments with small branches and/or incorporated (branched) cyclical instead of the previously proposed extended sheetlike and/or hyperbranched/dendritic structures. Additional support for a hybrid structure was given by semiempirical PM3 calculations on a variety of oligomeric model compounds. The PM3 results suggest that Si-Cl moieties incorporated in oligomers will be more reactive than monomeric Si-Cl groups. The calculations further indicate that linear chain extension is preferred over branching. Cyclic voltammetry in combination with absorption/excitation spectroscopy reveals that in going from the related polysilane to the polysilyne the valence band edge shifts ca. -0.7 V, while the conduction band edge remains virtually unchanged. Furthermore, it is demonstrated that polysilynes 1 and 2 are effective photoinitiators for radical polymerizations upon excitation at lambda 400 nm. This is exemplified for the conversion of methylacrylate into poly(methylacrylate)
Band gap modifications in functionalized poly(methylphenylsilanes)
No full textFor many applications of polysilanes in optoelectronic devices, it is desirable that polymer properties, such as their band gap energy levels, their (redox) stability, and their propensity to interact favorably with (semi)conducting inorganic substrates, can be tailored. It has been demonstrated that, by introduction of substituents in the aryl moiety of poly(methylphenylsilane) (1), i.e., poly(methyl-4-methylphenylsilane) (2), poly(4-methoxyphenylmethylsilane) (3), poly[4-(dimethylamino)phenylmethylsilane] (4), poly(3-methoxyphenylmethylsilane) (5), and poly[4-(2-methoxyethoxy)phenylmethylsilane] (6), these objectives can be achieved. For comparative purposes, poly(4,7,10,13-tetraoxatetradecylmethylsilane) (7) was also taken into consideration. Electrochemical measurements (cyclic voltammetry) in THF/LiClO4 of 1-7 show that the onset of oxidation V-i of each polysilane provides a reliable estimate of its valence band edge; within series 1-7 V-i shifts over ca. 0.7 V. Although it is impossible to obtain a reliable estimate of the conduction band edge due to the available potential window of THF/LiClO4, the position of the conduction band edge of the polysilanes is derived from their optical band gaps using fluorescence excitation and emission spectroscopy. The electrochemical and optical properties of the related polysilanes 1-5 correlate with the substituent Hammett constants (sigma(R)). The Hammett reaction constants (rho) indicate that the optical band gap (rho = 0.29) is less sensitive to electronic pertubations induced by the substituents than the valence band edge (rho = 0.85). From these results the response of the conduction band edge toward substituent induced electronic pertubations was estimated to be rho = 0.60. The experimental results are supported by semiempirical PM3 calculations on polysilane oligomers n = 1-10 of I and 4
Aggregation and solvatochromism of polysilanes and polysilynes in aqueous and binary solutions.
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