Coherent effects in energy transport in model dendritic structures investigated by ultrafast fluorescence anisotropy spectroscopy

Measurements of ultrafast fluorescence anisotropy decay in model branched dendritic molecules of different symmetry are reported. These molecules contain the fundamental branching center units of larger dendrimer macromolecules with either three (C(3))- or four (T(d), tetrahedral)-fold symmetry. The anisotropy for a tetrahedral system is found to decay on a subpicosecond time scale (880 fs). This decay can be qualitatively explained by F?rster-type incoherent energy migration between chromophores. Alternatively, for a nitrogen-centered trimer system, the fluorescence anisotropy decay time (35 fs) is found to be much shorter than that of the tetramers, and the decay cannot be attributed to an incoherent hopping mechanism. In this case, a coherent interchromophore energy transport mechanism should be considered. The mechanism of the ultrafast energy migration process in the branched systems is interpreted by use of a phenomenological quantum mechanical model, which examines the two extreme cases of incoherent and coherent interactions.     

The convenient route to cd fragment for the synthesis of vitamin D3 relatives

The efficient degradation of $\text{1}$ to the α-methylene ketone $\text{4}$ is described. Compound $\text{4}$ was then converted to the allylic alcohol $\text{8a}$ - the precursor of vitamin D³ relatives.     

The IINS, IR and DFT studies of hydrogen bonds in 6-Furfuryl and 6-Benzylaminopurines

FTIR and IINS spectra of 6-Furfurylaminopurine (6-FAP) and 6-Benzylaminopurine (6-BAP) taken at different temperatures have been analysed and compared with the spectra calculated by the ab initio DFT/B3LYP method and the semiempirical PM3 method in the isolated molecule approximation, for the tautomers N3-H, N7-H and N9-H, and dimers with hydrogen bonds. For 6-FAP the best agreement between the calculated and experimental (at 20 K) spectra has been found for the N9-H tautomer, whose structure was established by X-ray diffraction. For 6-BAP the analogous agreement for the N9-H tautomer structure has been poor and much better for the N7-H tautomer. The vibrational spectra calculated for dimers of the molecules studied involved in hydrogen bonds, permitted also an interpretation of the bands whose positions and FWHM in the FTIR spectra changed with temperature.     

ResonanceRaman spectra of Iron(II) complexes with 4,4′-didodecyloxy-2,2′-bipyridine and 4,4′-dioctadecyloxy-2,2′-bipyridine

The resonanceRaman spectra of Fe(LC ₁₂)₃Cl₂ and Fe(LC ₁₈)₃Cl₂ (whereLC ₁₂ andLC ₁₈ denote 4,4′-didodecyloxy-2,2′-bipyridine and 4,4′-dioctadecyloxy-2,2′-bipyridine, respectively) have been measured along with their excitation profiles. The exciting lines of an Ar⁺ laser have been used. The bands appearing in theRR spectra within 1 200–1 600cm^?1 (expected to arise from thebipy moiety C-N and C-C vibrations) suffer the greatest resonance enhancements. Both depolarization ratios of theRaman bands and excitation profiles reveal the interaction of the resonant electronic states.     

X-ray diffraction and solid-state NMR studies of a germanium binuclear complex

A compound formulated as (C4H12N2)[Ge2(pmida)2(OH)2] x 4 H2O (where pmida(4-) = N-(phosphonomethyl)iminodiacetate and C4H12N2(2+) = piperazinedium cation), containing the anionic [Ge2(pmida)2(OH)2]2- complex, has been synthesised by the hydrothermal approach and its structure determined by single-crystal X-ray diffraction analysis. Several high-resolution solid-state magic-angle spinning (MAS) NMR techniques, in particular two-dimensional 1H-X(13C,31P) heteronuclear correlation (HETCOR) and 1H-1H homonuclear correlation (HOMCOR) experiments incorporating a frequency-switched Lee-Goldburg (FS-LG) decoupling scheme, have been employed for the first time in such a material. Using these tools in tandem affords an excellent general approach to study the structure of other inorganic-organic hybrids. We assigned the NMR resonances with the help of C...H and P...H internuclear distances obtained through systematic statistical analyses of the crystallographic data. The compound was further characterised by powder X-ray diffraction techniques, IR and Raman spectroscopy, and by elemental and thermal analyses (thermogravimetric analysis and differential scanning calorimetry).     

N-Vinyl as N-H protecting group. Synthesis of cyclic imines from N-vinyllactams and organolithium reagents

On reaction with organolithium reagents followed by hydrolysis and distillation, N-vinylpyrrolidone ( 1 ) gives five-membered ring imines 3-8 , whereas N-vinylcaprolactam ( 2 ) gives mixtures of seven-membered ring imines and the corresponding ∈-amino ketones.     

Conformational equilibria and photoinduced tautomerization in 2-(2′-pyridyl)pyrrole

Depending on the polarity and protic abilities of the solvent, 2-(2′-pyridyl)pyrrole can exist in either syn or anti rotameric forms. In nonpolar solvents, intramolecular excited state single proton transfer is observed, manifested by the appearance of low-energy tautomeric emission. The solvent-assisted excited state double proton transfer reaction is also detected. DFT calculations confirm low barriers for both single and double proton transfer processes in the lowest excited singlet state and show different character of the tautomerization in both cases: in the intramolecular reaction, mutual approach of two nitrogen atoms plays an important role.     

New derivatization method for determination of 3-chloro-4(dichloromethyl)-5-hydroxy-2(5H)-furanone in water

An extremely potent mutagen, 3-chloro-4(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) is commonly present in chlorinated drinking water. Due to its high mutagenic activity and according to World Health Organization guidelines its concentration should be controlled in drinking waters. Determination of MX is difficult due to ppt levels at which the compound usually exists in drinking waters. Derivatization of MX with 2-propanol is presented as a method which significantly lowers the GC–MS detection level compared to other alcohol derivatization agents.     

Direct asymmetric aldol-Tishchenko reaction of aliphatic ketones catalyzed by syn-aminoalcohol-Yb(III) complexes

The asymmetric direct aldol condensation of aldehydes with ethyl- and propylketones is catalyzed by syn-alpha-aminoalcohol-Yb(OTf)3 complexes, yielding the anti-1,3-diol monoesters with high diastereocontrol and good enantioselectivity. Three adjacent stereogenic centers are created in a simultaneous aldol condensation and Evans-Tishchenko reduction in an acyclic system.