Sonogashira alkynylation of unprotected 8-brominated adenosines and guanosines: fluorescence properties of compact conjugated acetylenes containing a purine ring

A practical Sonogashira alkynylation protocol for the preparation of 8-alkynylated adenosines and guanosines has been developed. Protection of the sugar hydroxyl substituents is not required; protection hinders the purification of these products. A preliminary fluorescent study is reported, which shows that the presence of a substituent on the phenylene ring influences the fluorescent properties considerably, an outcome that could be utilized in biological applications.     

A modified predictor–corrector scheme for the two-dimensional sine-Gordon equation

A three-time level finite-difference scheme based on a fourth order in time and second order in space approximation has been proposed for the numerical solution of the nonlinear two-dimensional sine-Gordon equation. The method, which is analysed for local truncation error and stability, leads to the solution of a nonlinear system. To avoid solving it, a predictor–corrector scheme using as predictor a second-order explicit scheme is proposed. The procedure of the corrector has been modified by considering as known the already evaluated corrected values instead of the predictor ones. This modified scheme has been tested on the line and circular ring soliton and the numerical experiments have proved that there is an improvement in the accuracy over the standard predictor–corrector implementation. This research was co-funded by E.U. (75%) and by the Greek Government (25%).     

On Monodromy of Generalized Analytic Functions

We discuss a number of topics concerned with certain boundary-value problems in the context of generalized analytic functions. Solution of the classical Riemann-Hilbert problem and the linear conjugation problem for analytic functions is described in appropriate function classes and the same scheme is applied to generalized analytic functions and vectors. In particular, we describe solution of the Riemann-Hilbert problem for generalized analytic functions and obtain an explicit analytic presentation of monodromy matrices in the case of generalized analytic vectors. __________ Translated from Sovremennaya Matematika i Ee Prilozheniya (Contemporary Mathematics and Its Applications), Vol. 15, Theory of Functions, 2004.     

Spatial coherence and coherent interactions of broadband dispersed laser beams

Analytical expressions are obtained that describe the changes in the degree of coherence and in the thickness of the coherence layers occurring upon propagation of a dispersed broadband laser beam. It is found that the greater the tilting of the coherence layers with respect to the phase fronts, the more rapidly the spatial coherence is violated with increasing distance. A comparison with the case of an undispersed beam is performed. It is shown that, as the beam propagates, the decrease in the degree of coherence is accompanied by the appearance of spatial fluctuations of this parameter. The degree of mutual coherence of intersecting dispersed beams with parallel correlated coherence layers, which determines the efficiency of their coherent interaction, is investigated. The existence of spatial fluctuations of the degree of mutual coherence is established.     

EPR-detected photoinduced electron transfer in three structurally related molecular triads

A comparative electron paramagnetic resonance (EPR) study has been performed on a series of structurally related molecular triads which undergo photoinduced electron transfer and differ one from the other in terms of the acceptor or donor moieties. The molecular triads, C-P-C₆₀, TTF-P-C₆₀ and C-P-P_F, share the same free-base, tetraarylporphyrin (P) as the primary electron donor, which after light excitation initiates the electron transfer process, but differ either in terms of the electron acceptor (fullerene derivative, C₆₀, versus fluorinated free-base porphyrin, P_F), or in terms of the final electron donor (carotenoid polyene, C, versus tetrathiafulvalene, TTF). All these molecular triads can be considered artificial photosynthetic reaction centers in their ability to mimic several key properties of the reaction center primary photochemistry. Photoinduced charge separation and recombination have been followed by time-resolved EPR in a glass of 2-methyltetrahydrofuran and in the nematic phase of the uniaxial liquid crystal E-7. All the triads undergo photoinduced electron transfer, with the generation of charge-separated states in both the low-dielectric environment of the 2-methyl-tetrahydrofuran glass and in anisotropic E-7 medium. Different photochemical pathways have been recognized depending on the specific donor and acceptor moieties constituting the molecular triads. In the presence of the tetrathiafulvalene electron donor singlet- and triplet-initiated electron transfer routes are concurrently active. Recombination to the low-lying carotenoid triplet state occurs in the carotene-based triads, while singlet recombination is the only active route for the TTF-P-C₆₀ triad, where a low-lying triplet state is lacking. Long-lived charge separation has been observed in the case of TTF-P-C₆₀: about 8 μs for the singlet-born radical pair in the glassy isotropic matrix and about 7 μs for the triplet-born radical pair in the nematic phase of E-7. For all the molecular triads, a weak exchange interaction (J?1 G) between the electrons in the final spin-correlated radical pair has been evaluated by simulation of the EPR spectra, providing evidence for superexchange electronic interactions mediated by the tetraarylporphyrin bridge.     

Finite-Band Potentials with Trigonal Curves

We construct Dubrovin equations and trace formulas for finite-band operators with trigonal curves and present some examples.     

Une formule de Landau–Zener pour un croisement non dégénéré et involutif de codimension 3Landau–Zener formula for non-degenerate involutive codimension 3 crossings

In this Note, we study a 2×2 system of evolution equations with some codimension 3 crossing. We derive two conditions of non-degeneracy. We focus on one of them and reduce our system to some Landau–Zener's type system. Using this reduction, we describe the energy transfer at the crossing by Landau–Zener formula for 2-scales semi-classical measures. To cite this article: C. Fermanian Kammerer, P. Gérard, C. R. Acad. Sci. Paris, Ser. I 335 (2002) 915–920.     

Structural evolution in amorphous silicon and germanium thin films.

The structural evolution in amorphous silicon and germanium thin films has been investigated by high-resolution transmission electron microscopy (HRTEM) in conjunction with autocorrelation function (ACF) analysis. The results established that the structure of as-deposited semiconductor films is of a high density of nanocrystallites embedded in the amorphous matrix. In addition, from ACF analysis, the structure of a-Ge is more ordered than that of a-Si. The density of embedded nanocrystallites in amorphous films was found to diminish with annealing temperature first, then to increase. The conclusions also corroborate well with the results of diminished medium-range order in annealed amorphous films determined previously by a variable coherence microscopy method.