Self‐Assembly of Tricuprous Double Helicates: Thermodynamics,Kinetics, and Mechanism

We report in this paper the coordination and kinetic properties of two oligobipyridine strands, which contain three 2,2′‐bipyridine subunits separated by oxydimethylene bridges, the 4,4′‐bis(CONET₂)‐substituted L and the 4,4′‐bis(CO₂Et)‐substituted L ′. Spectrophotometric measurements allowed the characterization of thermodynamic complexes and kinetic intermediates* which are involved in the self‐assembly process of L ₂Cu₃ and L Cu₃ helicates. The reaction presents positive cooperativity for the binding of two 2,2′‐bipyridine strands to the cuprous cations. While reactive kinetic intermediates* present distorted coordination geometries around Cu^I, the final rearrangement of the tricuprous bistranded helicates allows more closely tetrahedral coordination of each cation and reduces the interactions. Differences in the bulkiness and electronic properties of the L and L ′ substituents do not affect significantly the stability of the corresponding helicates, but greatly influence binding rates in the self‐assembly process.     

Electron- and energy-transfer properties of hydrophilic carotenoids

The antioxidant activities-expressed as the electron-donating properties-of five hydrophilic carotenoids (carotenoid surfactants) and three related hydrophobic carotenoids were investigated by flash photolysis. The electron-transfer rates of the carotenoids to the triplet state of the sensitizer 2-nitronaphthalene and the energy transfer rates of triplet 2-nitronaphthalene to the carotenoids were determined. The results demonstrate that the electron-donating effects of the hydrophilic and hydrophobic carotenoids were comparable when evaluated in acetonitrile. In the presence of water, however, electron transfer (i.e., antioxidant efficiency) was enhanced by a factor of four for the hydrophilic carotenoids. The increased hydrophilicity of carotenoids, therefore, could expand their antioxidant properties, thus facilitating their use as aqueous-phase radical scavengers. At the same time, it was shown that supramolecular assembly ("aggregation") of the amphiphilic carotenoids prevented electron transfer, thus deactivating the antioxidant function. Modulation of the biophysical properties of carotenoids through synthetic modification is capable of increasing the biological and medical utility of this natural class of predominantly hydrophobic antioxidant compound.     

Singlet oxygen quenching by thione analogues of canthaxanthin, echinenone and rhodoxanthin

Thione analogues of three naturally occurring carotenones (canthaxanthin, echinenone, and rhodoxanthin) were synthesized just over ten years ago, and it was reported that substitution of the oxygen atom by sulphur brings about a large red shift and some broadening in the optical absorption spectrum of the compound. Since the three carotenothiones are scarce, determination of their molar absorption coefficients presents a challenge. A method for relating the molar absorption coefficient of a carotenothione (Car-S) to that of its ketone analogue (Car-O) has been developed, which has revealed that the peak molar absorption coefficient of a Car-S is only about 60% of the corresponding value for Car-O. Using methylene blue as the sensitizer and acetonitrile as the solvent, we have also investigated the quenching (under photostationary conditions) of the 1270nm phosphorescence emission of singlet oxygen by each of the six carotenoids. The data conform to the Stern-Volmer relation, and show that substitution of a carbonyl oxygen atom by sulphur does not lead to an appreciable change in the value of the quenching constant, which is close to 1.5 x 10(10)M(-1)s(-1) for all six quenchers.     

chemie und ortografie

The German-speaking countries decided last year to reform their spelling in an extremely restrictive manner, resulting in nearly invisible changes in daily writing. German chemists have actively promoted new orthography rules. R. Pauli, in 1904, edited a journal with a progressive writing, far ahead of the recently adopted new spelling rules.     

Vibrational analysis of the Raman,infrared and electronic spectra of hexafluoromanganate(IV)

From the Raman, infrared and electronic spectra all of the fundamental vibrational frequencies were determined for both the ground state⁴ A _2g and the excited² E _g In addition some fundamental frequencies were established for the⁴ T _2g and⁴ T _1g excited states from low temperature absorption spectra.

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Zusammenfassung Aus den Raman-, Infrarot- und Elektronenspektren wurden alle Grundschwingungsfrequenzen für den Grundzustand⁴ A _2g und den angeregten² E _g-Zustand bestimmt. Ferner wurden noch einige der entsprechenden Größen für die angeregten Zustände⁴ T _2g und⁴ T _1g aus Tieftemperatur-Absorptionsspektren ermittelt.

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Résumé Toutes les fréquences fondamentales de vibration ont été déterminées pour l'état fondamental⁴ A _2g et l'état excité² E _g à partir des spectres Raman, infra-rouge et électronique. De plus, certaines fréquences fondamentales des états excités⁴ T _2g et⁴ T _1g ont été obtenues à partir des spectres d'absorption à basse température.

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Dedicated to the memory of Prof. Hans-Ludwig Schläfer.