Weak exciton scattering in molecular nanotubes revealed by double-quantum two-dimensional electronic spectroscopy

The two-exciton manifold of a double-wall cylindrical molecular aggregate is studied using a coherent third order optical technique. Experiments reveal the anharmonic character of the exciton bands. Atomistic simulations of the exciton-exciton scattering show that the excitons can be treated as weakly coupled hard-core bosons. The weak coupling stems from the extended exciton delocalization made possible by the nanotube geometry.     

Energy-consuming micromechanisms in the fracture of glassy polymers. I. Chain scission in polystyrene

The number of chain scissions per unit area that occur during the fracture of partially annealed latex films from M_n ? 180,000 g/mol polystyrene particles of about 275 Å radius were measured and correlated to annealing times. A curve with four regimes was found. At short annealing times the curve is nearly flat, in what is called the chain pull-out regime. In the second regime, the number of chains broken per unit area increases with a 0.8 power of annealing time as entanglement of the diffusing polymer chains increases in neighboring host particles. This is in good agreement with Wool's theory which predicts a 0.75 power dependence. Then, after reaching a peak, the number of scissions decreases in the third regime, indicating a change in fracture mechanism. The number of chain scissions increases again in the fourth regime, as final healing of the film interface takes place. Fracture surface analysis reveals a rough surface for short annealing times and a smooth surface for longer annealing times. The number of polymer chain scissions per unit area of fracture surface showed no dependence on initial molecular weights for t ? τ_r where t and τ_r are annealing and relaxation times, respectively. The number of chain bridges crossing a unit area of interface was suggested as the basic molecular property. © 1992 John Wiley & Sons, Inc.     

Determination of heavy metals in seawater and in marine organisms by flameless atomic absorption spectrophotometry

Summary The appicability of flameless AAS to direct cadmium determinations in seawater samples is limited to a level of about 2g l^–1 [22]. Thus, for lower concentrations, a combination with an extraction method has been tested. It has been shown that the extraction can well be carried out with ammonium pyrrolidine dithiocarbamate (APDC). However, several modifications of the well known APDC/MIBK-Method were necessary. The key for the method presented was the choice of an appropriate solvent (CCl₄, C₂Cl₄) for the reagent as well as for its Cd-complex. Then a simple, sensitive and miniaturized method has been developed, which has then been applied to a few natural seawater samples.

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Bestimmung von Schwermetallen im Meerwasser und in marinen Organismen durch flammenlose AtomabsorptionsspektralphotometrieVII. Ein einfaches Extraktionsverfahren zur Bestimmung extrem niedriger Cadmiumgehalte in kleinen Seewasserproben

Zusammenfassung Die direkte Bestimmung von Cadmium im Meerwasser durch flammenlose AAS stößt unterhalb einer Konzentration von 2g l^–1 auf beträchtliche Schwierigkeiten [22]. Deshalb wurde eine kombinierte Methode erprobt, bei welcher das Metall zunächst extrahiert und dann mittels flammenloser AAS bestimmt wird. Es zeigte sich, daß die Extraktion gut mit Ammoniumpyrrolidindithiocarbamat (APDC) ausgeführt werden kann. Es wurden jedoch erhebliche Abwandlungen der wohlbekannten APDC-MIBK-Extraktionsmethode erforderlich. Erst als es gelang, ein geeignetes Solvens (CCl₄, C₂Cl₄) sowohl für das APDC als auch für seinen Cd-Komplex zu finden, konnte eine einfache, empfindliche und miniaturisierte Bestimmungsmethode entwickelt werden, mit deren Hilfe dann der Cd-Gehalt einiger Meerwasserproben bestimmt wurde.

     

The SI Unit Candela

The history of photometry, the measurement of light—and thus also the root of the basic unit for the luminous intensity, goes back to the seventeenth century. The theoretical foundations are provided by Johann H. Lambert in his treatise on photometry around 1760. With industrialization in the nineteenth century, there is an increasing demand for stable reference light sources for the measurement of light and for an reliable definition of the unit for light. But it is not until the middle of the twentieth century that the basic prerequisites had been created to use a common, internationally recognized unit, the Candela, as a measure of the visible light. The following article describes the basics and the boundary conditions that arise for the unit candela from the revision of the SI.     

Investigation of the interaction of Mercurochrome® constituents with proteins using liquid chromatography/mass spectrometry

The interaction of Mercurochrome®, a medical preparation based on the mercuryorganic compound merbromin, with free thiols in low molecular weight peptides and in proteins has been investigated by means of liquid chromatography (LC) and electrospray mass spectrometry (ESI-MS). β-Lactoglobulin A (β-LGA) from bovine milk (18.4 kDa) has been used as the model protein. It was found that, in contrast to assumptions in literature, the commercial product itself is a heterogeneous mixture of moderate chemical stability, which may contain precipitated Hg salts depending on storage time and conditions. Further variability results from different degrees of bromination of the fluorescein backbone of the compound. The formation of mercury compound–protein adducts was detected. The peptide sequence T13 containing a free thiol residue was identified as the binding site for mercury species after tryptic digestion of β-lactoglobulin A. While fresh Mercurochrome® tends to the formation of a Hg(II)-β-LGA adducts due to excess Hg²⁺ in solution, investigations after precipitation of Hg salts yield Hg(merbromin)(β-LGA) as the major product.     

Density matrix analysis, simulation, and measurements of electronic absorption and fluorescence spectra of spirobifluorenes

The linear absorption and fluorescence spectra as well as the oscillator strengths of 2,2',7,7'-tetraphenyl-9,9'-spirobifluorene (A), 2,2',7,7'-tetrakis(biphenyl-4-yl)-9,9'-spirobifluorene (B), and 2,2',7,7'-tetrakis(9,9'-spirobifluoren-2-yl)-9,9'-spirobifluorene (C) are calculated on the basis of the collective electronic oscillator (CEO) approach of Mukamel et al. (see, e.g., Chem. Rev. 2002, 102, 3171). The graphical visualization and quantitative characterization of CEO modes allows one to extract the real-space distribution of electronic excitations of the molecules under study. Effects of the lengthening and branching of the oligophenylene segments have been analyzed. The influence of the lowest excited (S1) vs ground-state (S0) geometry changes on the CEO modes is investigated and related to the geometry changes of the molecular parts. The obtained theoretical results are in good agreement with experimental trends observed in absorption and fluorescence data.     

POLARIZED UV-ABSORPTION SPECTRA OF RETINAL ISOMERS-I. MEASUREMENTS IN EXTREMELY THIN MONOCRYSTAL PLATELETS

Abstract Crystals of all- trans retinal and both different forms of 11- cis , 12-s- cis retinal were grown on quartz slides with faces (101), (001) and (101), respectively, forming thin platelets of less than 0.2 μm thickness. Polarized UV absorption spectra at room temperature were measured in the range from 20 to 43 × 10³ cm⁻¹ with a microscope-spectrophotometer. In this spectral range three diffuse absorption bands were observed for all crystal types at similar wave numbers. A main absorption band was found at 25–28 × 10³ cm⁻¹, and two further bands at 32–34 and 38–40 × 10³ cm⁻¹. In case of all- trans retinal the latter band is by far the weakest in this spectral range. Additionally, the crystal spectrum of all- trans retinal shows a shoulder at the low wavenumber side of the main band which cannot be resolved in the corresponding solution spectrum. In the crystal spectra of 11- cis , 12-s- cis retinal, however, only a strong dissymmetry is observed at this side of the main band.