Photophysical and electrochemical properties of meso-substituted thien-2-yl Zn(II) porphyrins

The influence of the thiophene ring on the ground and excited state properties of the porphyrin ring is investigated, when substituted at the meso-position. A series of mono-, di-, tri-, and tetra- meso-thien-2-yl porphyrins are studied and discussed with respect to the reference compounds zinc(II)-5,10,15,20-tetra(thien-2'-yl)porphyrin ( 1a) and zinc(II)-5,10,15,20-tetraphenylporphyrin (ZnTPP). The extended conjugated system zinc(II)-5-(5'-(5'-ethynyl-2'-thiophenecarboxaldehyde)thien-2'-yl)-10,15,20-triphenylporphyrin ( 4d) is also studied and shows enhanced charge transfer character due to the presence of the terminal aldehyde accepting group. A detailed analysis of ground and excited state UV-vis absorption, steady-state and time-resolved fluorescence, laser flash photolysis, and electrochemical data all point toward substantial electronic communication between the central Zn(II) porphyrin ring and the meso-thien-2-yl substituents, which is evident from excited state charge transfer character.     

Understanding the electronic structure, reactivity, and hydrogen bonding for a 1,2-diphosphonium dication

The experimental charge density for hexamethyldiphosphonium ditriflate has been determined from low-temperature high-resolution X-ray diffraction data. These results have been compared with theoretically calculated values for the isolated gas-phase compound. Analysis of the topological and atomic basin properties has provided insight into the exact nature of the P-P bond in both the crystalline and the gas-phase structures. The rho(b)(r) and nabla2rho(b)(r) values highlight the covalent nature of the P-P bond, while the atomic charges indicate a localization of the positive charges on the two phosphorus atoms. This seems to indicate that a covalent bond is formed despite a strong electrostatic repulsion between these two heteroatoms. The topological properties and electrostatic potentials have also been shown to provide significant insight into the chemical reactivity of the title compound. A topological analysis of P2Me4, P2Me5(+), and P2Me6(+2) species has provided information about the progression of the P-P bond in the synthesis of the title compound. An investigation of the different hydrogen-bonding networks present in the crystalline and gas-phase structures, along with their affect on the electronic structure of the title compound has also been investigated. This has all led to significant new insight into the electronic structure, reactivity, and weak hydrogen bonding in prototypical 1,2-diphosphonium dications.     

Role and substrate specificity of the Streptomyces coelicolor RedH enzyme in undecylprodiginine biosynthesis

The function of RedH from Streptomyces coelicolor as an enzyme that catalyses the condensation of 4-methoxy-2,2'-bipyrrole-5-carboxaldehyde (MBC) and 2-undecylpyrrole to form the natural product undecylprodiginine has been experimentally proven, and the substrate specificity of RedH has been probed in vivo by examining its ability to condense chemically-synthesised MBC analogues with 2-undecylpyrrole to afford undecylprodiginine analogues.     

Pyrazolo[1,5-a]pyridines as p38 kinase inhibitors

[reaction: see text] A convergent synthesis of substituted pyrazolo[1,5-a]pyridines has been achieved either via a regioselective [3 + 2] cycloaddition of N-aminopyridines with alkynes or by thermal cyclization of disubstituted azirines. Subsequent palladium-catalyzed introduction of pyridines or de novo synthesis of pyrimidines affords inhibitors of p38 kinase.     

SPINAL modulated decoupling in high field double- and triple-resonance solid-state NMR experiments on stationary samples

Continuous wave irradiation has limited bandwidth for heteronuclear 1H decoupling at high fields and for 13C decoupling in 1H/13C/15N triple-resonance experiments. SPINAL-16 modulation is shown to improve the efficiency of 1H and 13C heteronuclear decoupling on single crystals of peptides and on magnetically aligned samples of membrane proteins in bicelles, which is of particular importance because aqueous samples of biomolecules are lossy at high fields, which limits the strengths of the RF fields that can be applied.     

Pitch strength of regular-interval click trains with different length "runs" of regular intervals

Click trains were generated with first- and second-order statistics following Kaernbach and Demany [J. Acoust. Soc. Am. 104, 2298-2306 (1998)]. First-order intervals are between successive clicks, while second-order intervals are those between every other click. Click trains were generated with a repeating alternation of fixed and random intervals which produce a pitch at the reciprocal of the duration of the fixed interval. The intervals were then randomly shuffled and compared to the unshuffled, alternating click trains in pitch-strength comparison experiments. In almost all comparisons for the first-order interval stimuli, the shuffled-interval click trains had a stronger pitch strength than the unshuffled-interval click trains. The shuffled-interval click trains only produced stronger pitches for second-order interval stimuli when the click trains were unfiltered. Several experimental conditions and an analysis of runs of regular and random intervals in these click trains suggest that the auditory system is sensitive to runs of regular intervals in a stimulus that contains a mix of regular and random intervals. These results indicate that fine-structure regularity plays a more important role in pitch perception than randomness, and that the long-term autocorrelation function or spectra of these click trains are not good predictors of pitch strength.     

Computing multi-valued physical observables for the high frequency limit of symmetric hyperbolic systems

We develop a level set method for the computation of multi-valued physical observables (density, velocity, energy, etc.) for the high frequency limit of symmetric hyperbolic systems in any number of space dimensions. We take two approaches to derive the method.The first one starts with a weakly coupled system of an eikonal equation for phase S and a transport equation for density ρ:

The main idea is to evolve the density near the n-dimensional bi-characteristic manifold of the eikonal (Hamiltonian–Jacobi) equation, which is identified as the common zeros of n level set functions in phase space . These level set functions are generated from solving the Liouville equation with initial data chosen to embed the phase gradient. Simultaneously, we track a new quantity f = ρ(t,x,k)|det(_k)| by solving again the Liouville equation near the obtained zero level set = 0 but with initial density as initial data. The multi-valued density and higher moments are thus resolved by integrating f along the bi-characteristic manifold in the phase directions.The second one uses the high frequency limit of symmetric hyperbolic systems derived by the Wigner transform. This gives rise to Liouville equations in the phase space with measure-valued solution in its initial data. Due to the linearity of the Liouville equation we can decompose the density distribution into products of function, each of which solves the Liouville equation with L^∞ initial data on any bounded domain. It yields higher order moments such as energy and energy flux.The main advantages of these new approaches, in contrast to the standard kinetic equation approach using the Liouville equation with a Dirac measure initial data, include: (1) the Liouville equations are solved with L^∞ initial data, and a singular integral involving the Dirac-δ function is evaluated only in the post-processing step, thus avoiding oscillations and excessive numerical smearing; (2) a local level set method can be utilized to significantly reduce the computation in the phase space. These methods can be used to compute all physical observables for multi-dimensional problems.Our method applies to the wave fields corresponding to simple eigenvalues of the dispersion matrix. One such example is the wave equation, which will be studied numerically in this paper.     

Coherent population trapping of electron spins in a high-purity n-type GaAs semiconductor

In high-purity n-type GaAs under a strong magnetic field, we are able to isolate a lambda system composed of two Zeeman states of neutral-donor-bound electrons and the lowest Zeeman state of bound excitons. When the two-photon detuning of this system is zero, we observe a pronounced dip in the excited-state photoluminescence, indicating the creation of the coherent population-trapped state. Our data are consistent with a steady-state three-level density-matrix model. The observation of coherent population trapping in GaAs indicates that this and similar semiconductor systems could be used for various electromagnetically induced transparency type experiments.