Fluorinated Azobenzenes for Shape‐Persistent Liquid Crystal Polymer Networks

Liquid crystal polymer networks respond with an anisotropic deformation to a range of external stimuli. When doped with molecular photoswitches, these materials undergo complex shape modifications under illumination. As the deformations are reversed when irradiation stops, applications where the activated shape is required to have thermal stability have been precluded. Previous attempts to incorporate molecular switches into thermally stable photoisomers were unsuccessful at photogenerating macroscopic shapes that are retained over time. Herein, we show that to preserve photoactivated molecular deformation on the macroscopic scale, it is important not only to engineer the thermal stability of the photoswitch but also to adjust the cross‐linking density in the polymer network and to optimize the molecular orientations in the material. Our strategy resulted in materials containing fluorinated azobenzenes that retain their photochemical shape for more than eight days, which constitutes the first demonstration of long‐lived photomechanical deformation in liquid‐crystal polymer networks.     

Ruthenium‐Grafted Vinylhelicenes: Chiroptical Properties and Redox Switching

The properties of mono‐ and bis‐Ru–vinyl[6]helicene complexes ( 2 a and 2 b , respectively), recently synthesized by using molecular engineering of helicenes based on the grafting of lateral organometallic substituents on the π‐helical backbone through a vinyl bridge, are presented. These helicene derivatives are thoroughly characterized, with special attention given to their chiroptical properties and redox switching activity. The UV/Vis and electronic circular dichroism (ECD) spectra of P and M enantiopure species, both in the neutral and oxidized states ([ 2 a ] ^{. +}, [ 2 b ] ^{. +}, and [ 2 b ]²⁺), are analyzed with the aid of quantum‐chemical calculations. The extended π‐conjugation facilitated by the vinyl moiety, clearly visible in the electronic structures of 2 a , b , introduces new active bands in the ECD spectra that consequently lead to a significant increase in optical rotation of Ru–vinylhelicenes compared with the organic precursors. The vibrational circular dichroism (VCD) spectra were measured and calculated for both the organic and organometallic species and constitute the first examples of VCD for metal‐based helicene derivatives. Finally, the redox‐triggered chiroptical switching activity of 2 a , b is examined in detail by using ECD spectroscopy. The modifications of the ECD spectra in the UV/Vis and NIR region are well reproduced and rationalized by calculations.     

Tandem post-synthetic modification for functionalized metal-organic frameworks via epoxidation and subsequent epoxide ring-opening

A sequence of two new post-synthetic modifications, epoxidation using dimethyldioxirane and subsequent epoxide ring-opening, was developed, maintaining the MOF network. A kinetic study shows that the functionalization of metal-organic frameworks with synthetically versatile and reactive oxirane groups is even possible for small pore diameters.     

Enhancement and quenching of single-molecule fluorescence

We present an experimental and theoretical study of the fluorescence rate of a single molecule as a function of its distance to a laser-irradiated gold nanoparticle. The local field enhancement leads to an increased excitation rate whereas nonradiative energy transfer to the particle leads to a decrease of the quantum yield (quenching). Because of these competing effects, previous experiments showed either fluorescence enhancement or fluorescence quenching. By varying the distance between molecule and particle we show the first experimental measurement demonstrating the continuous transition from fluorescence enhancement to fluorescence quenching. This transition cannot be explained by treating the particle as a polarizable sphere in the dipole approximation.     

Investigation of the quantitative properties of the quadrupole orthogonal acceleration time-of-flight mass spectrometer with electrospray ionisation using 3,4-methylenedioxymethamphetamine

This paper describes the investigation of the potential of a quadrupole orthogonal acceleration time-of-flight mass spectrometer (Q-TOF) equipped with an atmospheric pressure ionisation interface for quantitative measurements of small molecules separated by reversed phase liquid chromatography. To this end, the detection limits and linear dynamic range in particular were studied in an LC/MS/MS experiment using 3,4-methylenedioxymethamphetamine standards and 3,4-methylenedioxyethylamphetamine for internal standardisation. In a second phase, the experiment was repeated with real biological extracts (whole blood, serum, and vitreous humour). A calibration for 3,4-methylenedioxymethamphetamine and its metabolite 3,4-methylenedioxyamphetamine was prepared in each of these matrices again using 3,4-methylenedioxyethylamphetamine as internal standard. The resulting quantitative data were compared with those obtained by liquid chromatography with fluorescence detection for the same extracts. The Q-TOF results revealed excellent sensitivity and a linear dynamic range of nearly four decades (2-10 000 pg on-column, r(2) = 0.9998, 1/x weighting). Furthermore, all the calibration curves prepared in biological material were superimposable, LC/MS/MS and LC-fluorescence, and the quantitative results for actual samples compared very favourably. It was concluded that the Q-TOF achieves a linear dynamic range for quantitative LC/MS/MS work exceeding that of fluorescence detection and at much better absolute sensitivity. Copyright 1999 John Wiley & Sons, Ltd.     

Extremalpunktmethoden bei Koeffizientenproblemen

Ohne Zusammenfassung     

Nachweis primärer Halogenalkyle in der Tüpfelanalyse

Zusammenfassung Primäre Halogenverbindungen (CH₂Hal-Gruppen) können dadurch nachgewiesen werden, daß bei deren Erhitzen mit hydratischem oder anhydratischem Natriumthiosulfat auf 160 bis 180° Schwefeldioxyd abgespalten wird. Dieser Effekt beruht auf der Bildung und pyrolytischen Zersetzung sogenannter Buntescher Salze.Der Nachweis kann in der Arbeitsweise der Tüpfelanalyse ausgeführt werden und zeigt mikroanalytische Empfindlichkeit. Er ist nicht unmittelbar anwendbar in Gegenwart saurer oder durch Umlagerung potentiell saurer Verbindungen, sowie von Halogenverbindungen, die durch Hydrolyse Halogenwasserstoff abspalten.

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Summary Primary halogen compounds (CH₂Hal-compounds) can be detected through the finding that they yield sulfur dioxide if heated to 160–180° with hydrated or anhydrous sodium thiosulfate. This effect is due to the formation and pyrolytic decomposition of the so-called Bunte salts.The test can be made within the limits of spot test analysis and has microanalytical sensitivity. It cannot be applied directly in the presence of acids or of compounds which may become acidic through rearrangement, nor of halogen compounds, which yield halogen hydracids when hydrolyzed.

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Résumé II est possible d'identifier les combinaisons halogénées primaires (groupes CH₂Hal) en utilisant le fait que par chauffage entre 160 et 180° en présence de thiosulfate hydraté ou anhydre elles libèrent de l'anhydride sulfureux. Cet effet repose sur la formation et la décomposition pyrolytique du «Sel de Bunte». Cette identification peut être effectuée dans le cadre de l'analyse à la touche; sa sensibilité est microanalytique. Elle n'est pas directement applicable en présence d'acides ou de substances susceptibles de leur donner naissance ou encore en présence de combinaisons halogénées libérant des acides halohydriques par hydrolyse.