Photoinitiation rate profiles during polymerization of a dimethacrylate-based resin photoinitiated with camphorquinone/amine. Influence of initiator photobleaching rate

The photodecomposition of camphorquinone (CQ)/amine during the photo polymerization of a dimethacrylate-based resin under continuous irradiation was investigated in thick samples. The global CQ photoconsumption was measured by monitoring the decrease in light absorption as a function of irradiation time and the kinetics were satisfactory fitted to a first order expression where the rate constant of photobleaching was proportional to the irradiation intensity. In a thick sample, the photobleaching of the photoinitiator is accompanied by a deeper penetration of the light through the underlying layers. These gradients of photoinitiator concentration, light intensity and photoinitiation rate along the path of irradiation were calculated. The photodecomposition reaction was spatially inhomogeneous and the degree of nonuniformity increased with increased initial sample absorbance. The influence of the photobleaching process on the polymerization reaction was examined. The photobleaching rate of CQ was much slower than the polymerization rate and only 20% of the initial amount of CQ was consumed before the polymerization reaction had almost ceased. Results obtained in this research highlight the inherent interlinking of light attenuation and photobleaching rate in bulk polymerizing systems.     

Pathway for the stereocontrolled Z and E production of alpha,alpha-difluorine-substituted phenyl butenoates

An efficient preparation of pure ethyl Z- and E-alpha,alpha-difluoro-4-phenyl-3-butenoate 1a and 1b together with the corresponding acids 2a and 2b is described. The procedures involve stereocontrolled additions of *CF2CO2Et to phenylacetylene or beta-bromostyrene. Compound 1a is easily obtained by addition of *CF2CO2Et to phenylacetylene via a mechanism where the stereochemistry is controlled by an electron-transfer process to produce predominantly the Z vinyl anion. The product 1b is obtained by *CF2CO2Et addition-elimination to Z- or E-beta-bromostyrenes via a mechanism where the stereochemistry is controlled by steric factors in the conformational equilibration of the intermediates.     

Ergodic Directions for Billiards in a Strip with Periodically Located Obstacles

We study the size of the set of ergodic directions for the directional billiard flows on the infinite band ${\mathbb{R}\times [0,h]}$ with periodically placed linear barriers of length 0 < λ < h. We prove that the set of ergodic directions is always uncountable. Moreover, if λ/h ∈ (0, 1) is rational, the Hausdorff dimension of the set of ergodic directions is greater than 1/2. In both cases (rational and irrational) we construct explicitly some sets of ergodic directions.     

A holistic finite element and multi-body systems approach for the virtual product development

This article describes the creation of a holistic modeling of a machine tool using a multi-body-systems approach with an integrated flexible body. This lower order model is validated by a detailed finite element model and its eigenvalues are improved by a model-updating algorithm. The improved model is used for investigation of the machine vibration during the cutting process and the prediction of chatter. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electron density measurements of argon surface-wave discharges

The electron density of microwave-generated surface-wave discharges in argon have been measured using Stark broadening and calculated from the measured wavelengths of the standing surface wave. Results obtained from these two techniques compare well. The electron density varies from 10¹³ to 10¹⁴/cm³ for pressures ranging from 50 to 800 torr.     

Computational methods in risk theory: A matrix-algorithmic approach

This paper is concerned with the numerical computation of the probability ψ(u) of ruin with initial reserve u. The basic assumption states that the claim size distribution is phase-type in the sense of Neuts. The models considered are: the classical compound Poisson risk process, the Sparre Anderse process and varying environments which are either governed by a Markov process or exhibit periodic fluctuations. The computational steps involve the iterative solution of a non-linear matrix equation Q = Ψ (Q) as well as the evaluation of matrix-exponentials e^Qu. A number of worked-out numerical examples are presented.     

The Structure of [HSi9]3− in the Solid State and Its Unexpected Highly Dynamic Behavior in Solution

We report on the first unambiguous detection of the elusive [HSi₉]^3? anion in solutions of liquid ammonia by various ²⁹Si and ¹H NMR experiments including chemical exchange saturation transfer (CEST). The characteristic multiplicity patterns of both the ²⁹Si and ¹H resonances together with CEST and a partially reduced ¹H,²⁹Si coupling constant indicate the presence of a highly dynamic Si₈ entity and a Si?H moiety with slow proton hopping. Theoretical calculations corroborate both reorganization of Si₈ on the picosecond timescale via low vibrational modes and proton hopping. In addition, in a single‐crystal X‐ray study of (K(DB[18]crown‐6))(K([2.2.2]crypt))₂[ HSi₉ ]?8.5 NH₃, the H atom was unequivocally localized at one vertex of the basal square of the monocapped square‐antiprismatic cluster. Thus experimental studies and theoretical considerations provide unprecedented insight into both the structure and the dynamic behavior of these cluster anions, which hitherto had been considered to be rigid.     

Optimization of Aqueous Stability versus π-Conjugation in Tetracationic Bis(triarylborane) Chromophores: Applications in Live-Cell Fluorescence Imaging

The stability of tetracationic triarylboranes in dilute aqueous solution was investigated by tuning the steric demand of the linker in a (para-(N,N,N-trimethylammonio)xylyl)₂B–(linker)–B(para-(N,N,N-trimethylammonio)xylyl)₂ structure. With increasing steric bulk of the linker, namely 1,4-phenylene, 2,2′′′-(3,3′′′-dimethyl)-5,2′:5′,2′′:5′′,5′′′-quaterthiophene, 9,10-anthracenylene, and 4,4′′′-(5′-(3,5-dimethylphenyl))(5′′-(3′′′,5′′′-dimethylphenyl))-2′,2′′-bithiophene, the stability of the compounds increased. The anthracene-based chromophore, compound 3M is water-stable for at least 48 h, is nontoxic to cells and exhibits an exceedingly high fluorescence quantum yield of 0.86 in water making it an ideal candidate for confocal live-cell imaging of lysosomes.     

Improved Synthesis of Indirubin Derivatives by Sequential Build‐Up of the Indoxyl Unit: First Preparation of Fluorescent Indirubins

Indirubin, present in extracts of Isatis tinctoria and some other plant species, has promising cytotoxicity against a variety of cell lines by inhibition of cyclin‐dependent kinases. Chemical synthesis of its derivatives relies on the combination of isatins and 2,3‐dihydro‐1H‐indol‐3‐one (‘indoxyl’) derivatives and usually yields indigo as well as other by‐products. Inspection of the hydrolysis of the long‐known condensation products of 2‐thioxothiazolidin‐4‐one with isatins gave useful hints for an improved synthesis of indirubins: this reaction does not yield quinoline derivatives but 2‐(2,3‐dihydro‐2‐oxo‐1H‐indol‐3‐ylidene)‐2‐sulfanyl acetic acids. By substitution of the sulfanyl group in this oxindoles with anilines and straightforward cyclization under Nazarov conditions, a broad variety of indirubins substituted in the indoxyl ring system are thus available, usually in very good purity and yield. Use of naphthylamines in this reaction sequence yields various fluorescent substances with λ_fl at ca. 630 nm.