Charge transfer in model bioinspired carotene-porphyrin dyads

We present a computational study based on accurate DFT and TD-DFT methods on model bioinspired donor-acceptor dyads, formed by a carotenoid covalently linked to a tetraphenylporphyrin (TPP) at the ortho position of one of the TPP phenyl rings. Dyadic systems can be used in the construction of organic solar cells and development of efficient photocatalytic systems for the solar energy conversion, due to the unique advantages they offer in terms of synthetic feasibility. This study aims to describe the influence of chemical modifications on the absorption spectra, in particular on the lowest energy charge transfer bands. Effects of different metals of biological interest, i.e., Mg, Fe, Ni, and Zn, and of H(2)O and histidine molecules coordinated to the metals in different axial positions are rationalized.     

Photoinduced electron transfer in Os(terpyridine)-biphenylene-(bi)pyridinium assemblies

A series of linearly arranged donor-spacer-acceptor (D-S-A) systems 1-3, has been prepared and characterized. These dyads combine an Os(II)bis(terpyridine) unit as the photoactivable electron donor (D), a biphenylene (2) or phenylene-xylylene (3) fragment as the spacer (S), and a N-aryl-2,6-diphenylpyridinium electrophore (with aryl = 4-pyridyl or 4-pyridylium in 1 or 2/3, respectively) as the acceptor (A). Their absorption spectra, redox behavior, and luminescence properties (both at 77 K in rigid matrix and at 298 K in fluid solution) have been studied. The electronic structure and spectroscopic properties of a representative compound of the series (i.e., 2) have also been investigated at the theoretical level, performing Density Functional Theory (DFT)-based calculations. Time-dependent transient absorption spectra of 1-3 have also been recorded at room temperature. The results indicate that efficient photoinduced oxidative electron transfer takes place in the D-S-A systems at room temperature in fluid solution, for which rate constants (in the range 4 × 10(8)-2 × 10(10) s(-1)) depend on the driving force of the process and the spacer nature. In all the D-S-A systems, charge recombination is faster than photoinduced charge separation, in spite of the relatively large energy of the D(+)-S-A(-) charge-separated states (between 1.47 and 1.78 eV for the various species), which would suggest that the charge recombination occurs in the Marcus inverted region. Considerations based on superexchange mechanism suggest that the reason for the fast charge recombination is the presence of a virtual D-S(+)-A(-) state at low energy--because of the involvement of the easily oxidizable biphenylene spacer--which is beneficial for charge recombination via superexchange but unsuitable for photoinduced charge separation. To further support the above statement, we prepared a fourth D-S-A species, 4, analogous to 2 but with a (hardly oxidizable) single phenylene fragment serving as the spacer. For such a species, charge recombination (about 3 × 10(10) s(-1)) is slower than photoinduced charge separation (about 1 × 10(11) s(-1)), thereby confirming our suggestions.     

Synthesis and properties of new poly(ether sulfone)amides

High molecular-weight aromatic polyamides were obtained from 1,5- and 2,6-bis-(4′-carboxy-4-phenylenoxy-sulfonyl)naphthalene by direct polycondensation reaction in N-methyl-2-pyrrolidone with various aromatic diamines, using triphenyl phosphite and pyridine as condensing agents. The polymers were characterized by elemental analysis, thermogravimetric analysis, differential scanning calorimetry, and infrared analysis. The polyamides, obtained in quantitative yield, possessed inherent viscosities in the range 0.42–1.70 dL/g, glass transition temperatures between 245–310°C, and 10% weight loss temperatures in nitrogen and air above 435 and 424°C, respectively. Most of the polymers were soluble in aprotic solvents. The effect of the structure on properties, such as solubility, T_g, and thermal behavior, were also studied. © 1996 John Wiley & Sons, Inc.     

Recent advances in the field of diolefin polymerization with transition metal catalysts

Recent advances in the field of catalysis for 1,3-diene polymerization and in the interpretation of the polymerization mechanism are examined. Catalysts prepared from methylaluminoxanes and soluble transition metal compounds are in general more active than the analogous systems prepared from AlR₃. With some catalysts, however, (e.g. lanthanide systems) a high activity is obtained only when transition metal compounds containing preformed metal-carbon bonds are used. Methylaluminoxanes affect also the stereospecificity of the polymerization. Active and stereospecific systems are obtained from monocyclopentadienyl derivatives of Ti and aluminoxanes. Recent views on the factors that determine stereospecificity are examined. Schemes are presented for the formation of iso- or syndiotactic polymers, with 1,2, cis-1,4 or trans-1,4 structure, from various dienes.     

Designing Cascades of Electron Transfer Processes in Multicomponent Graphene Conjugates

A novel family of nanocarbon-based materials was designed, synthesized, and probed within the context of charge-transfer cascades. We integrated electron-donating ferrocenes with light-harvesting/electron-donating (metallo)porphyrins and electron-accepting graphene nanoplates (GNP) into multicomponent conjugates. To control the rate of charge flow between the individual building blocks, we bridged them via oligo-p-phenyleneethynylenes of variable lengths by β-linkages and the Prato–Maggini reaction. With steady-state absorption, fluorescence, Raman, and XPS measurements we realized the basic physico-chemical characterization of the photo- and redox-active components and the multicomponent conjugates. Going beyond this, we performed transient absorption measurements and corroborated by single wavelength and target analyses that the selective (metallo)porphyrin photoexcitation triggers a cascade of charge transfer events, that is, charge separation, charge shift, and charge recombination, to enable the directed charge flow. The net result is a few nanosecond-lived charge-separated state featuring a GNP-delocalized electron and a one-electron oxidized ferrocenium.     

The asymptotic analyses of non-linear waves in magneto-thermoelastic solids with thermal relaxation

Summary We investigate the magneto-thermoelastic model with one-dimensional deformation proposed in [1] when Fourier's law is replaced by Cattaneo's Eq. [4]. By means of an asymptotic analysis, we point out the main features of the non-linear wave processes related to different hypotheses based on the order of magnitude of physical parameters connected to thermal relaxation, heat and electrical conduction. In the various cases we obtain the evolution equations which govern the wave amplitude.

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Sommario In questa Nota si prende in esame il modello della magneto-thermoelasticità con deformazioni undimensionali proposto in [1], sostituendo peró alla legge di Fourier l'equazione di Cattaneo [4]. Facendo uso di metodi perturbativi, si mette in evidenza il carattere dei processi ondosi non-lineari connessi con different ipotesi sugli ordini di grandezza dei parametri fisici legati alla conduzione del calore, al rilassamento termico e alla conducibilità elettrica finita. Nei vari casi si deducono le equazioni di evoluzione che governano l'ampiezza d'onda.

     

Divisible designs and groups

We study (s, k, ₁, ₂)-translation divisible designs with ₁0 in the singular and semi-regular case. Precisely, we describe singular (s, k, ₁, ₂)-TDD's by quasi-partitions of suitable quotient groups or subgroups of their translation groups. For semi-regular (s, k, ₁, ₂)-TDD's (and, more general, for the case ₂>₁) we prove that their translation groups are either Frobenius groups or p-groups of exponent p. Some examples are given for the singular, semi-regular and regular case.     

Pyrrolo[3,2-c][1,2,5]benzotriazocine: A new ring system

A nucleophilic substitution reaction in the pyrrole series, achieved by a neutral nucleophile, led to the key intermediate 7 which by reduction and successive diazotization afforded the new ring system pyrrolo[3,2-c][1,2,5]benzotriazocine 9 in good yield.     

Reactivity of asymmetric benzo-condensed diazines with nitrilimine dipoles in the 1,3-dipolar cycloaddition reactions

The reactivity of asymmetric benzo-condensed diazines in the 1,3-dipolar cycloaddition reactions with nitrilimines was investigated. The results demonstrated that, at variance with the symmetric quinoxaline, a certain grade of diastereoselectivity emerged. Moreover in the case of the 5-methylquinoxaline and quinazoline a mono-cycloadduct was obtained.     

New insights on testing the efficiency of methods of pricing and hedging American options

With reference to the evaluation of the speed–precision efficiency of pricing and hedging of American Put options, we present and discuss numerical results obtained on the basis of four different large enough random samples according to the relevance of the American quality (relative importance of the early exercise opportunity) of the options. Here we provide a comparison of the best methods (lattice based numerical methods and an approximation of the American Premium analytical procedure) known in literature along with some key methodological remarks.