Distance-Dependent Electron Transfer Kinetics in Axially Connected Silicon Phthalocyanine-Fullerene Conjugates

The effect of donor-acceptor distance in controlling the rate of electron transfer in axially linked silicon phthalocyanine-C₆₀ dyads has been investigated. For this, two C₆₀-SiPc-C₆₀ dyads, 1 and 2 , varying in their donor-acceptor distance, have been newly synthesized and characterized. In the case of C₆₀-SiPc-C₆₀ 1 where the SiPc and C₆₀ are separated by a phenyl spacer, faster electron transfer was observed with k_cs equal to 2.7×10⁹ s⁻¹ in benzonitrile. However, in the case of C₆₀-SiPc-C₆₀ 2 , where SiPc and C₆₀ are separated by a biphenyl spacer, a slower electron transfer rate constant, k_cs=9.1×10⁸ s⁻¹, was recorded. The addition of an extra phenyl spacer in 2 increased the donor-acceptor distance by ∼4.3 Å, and consequently, slowed down the electron transfer rate constant by a factor of ∼3.7. The charge separated state lasted over 3 ns, monitoring time window of our femtosecond transient spectrometer. Complimentary nanosecond transient absorption studies revealed formation of ³SiPc* as the end product and suggested the final lifetime of the charge separated state to be in the 3–20 ns range. Energy level diagrams established to comprehend these mechanistic details indicated that the comparatively high-energy SiPc^.+-C₆₀^.− charge separated states (1.57 eV) populated the low-lying ³SiPc* (1.26 eV) prior returning to the ground state.     

Radical reactions on pinene-oxide derivatives induced by Ti(III)

A practical, brief and selective synthesis of several pinene oxide derived terpenoids can be achieved from readily available starting materials. The key step is a radical reaction promoted by titanocene chloride.     

Chilling resistances of isolates of Pythium ultimum var. ultimum from the Arctic and Temperate Zones

Chilling resistances in moss pathogenic fungi, Pythium ultimum var. ultimum, from Longyearbyen, Svalbard (78 degree N, 15 degree E), located in the Arctic Zone and in the same isolates from Temperate Zone, were determined. Both strains had similar optimum growth temperatures. However, the strains from Svalbard could grow and survive at 0 - 5 degrees C. In addition, chilling treatment induced irregular mycelial morphology in the Arctic isolates. On the other hand, the isolates from Japan did not grow at temperatures below 5C and were destroyed after chilling stress (0 degree C for 3 days or at 4 degrees C for 1 week). The results suggested that isolates from Svalbard highly adapted to the severe spring condition in Polar environments.     

Characterization of the surface structure of gold nanoparticles and nanorods using structure sensitive reactions

The surface structure of gold nanorods has been determined by studying the behavior of electrochemical reactions sensitive to the structure and compared to that obtained by other structure characterization techniques. Lead underpotential deposition (UPD) reveals that the surface of the nanorods is composed by (111) and (110) domains, while (100) domains are practically absent from the surface. In the case of the oxygen reduction reaction, the formation of hydrogen peroxide as a final product of the reaction in the whole potential range also indicates that (100) domains are absent on the surface of the nanoparticles, corroborating the previous result. These results are compared with other surface structure information provided by other techniques.     

Absence of dc-conductivity in lambda-DNA

The electrical conductivity of biomaterials on a molecular scale is of fundamental interest in the life sciences. We perform first principles electronic structure calculations, which clearly indicate that lambda-DNA chains should present large resistance values. We also present two direct procedures to measure electrical currents through DNA molecules adsorbed on mica. The lower limit for the resistivity is 10(6) Omega . cm, in agreement with our calculations. We also show that low energy electron bombardment induces a rapid contamination and dramatically affects the measured conductivity, thus providing an explanation to recent reports of high DNA conductivity.     

Chebyshev collocation methods in thermoconvective problems

Chebyshev collocation methods are high-order methods. This means that high precision is obtained with low-order expansions. Then `small' matrices appear in the numerical implementation and reduced computing resources become necessary. Thermoconvective fluid dynamics problems are large ones, involving various partial differential equations for several fields in large dimensions. The models present a number of difficulties, such as the different orders of derivatives for the different fields, or lack of information on the boundary conditions for pressure. This paper presents a review of the specific characteristics of this method when it is applied to thermoconvective problems: the method, the types of problems, the different resulting models, the strategies to overcome the difficulties of the different derivative orders and the characteristics of the matrices, and the convergence properties. A comparison of its efficiency with other numerical methods like finite differences and finite elements is included.     

Interfacial differences in enhanced schottky barrier height Au/n-GaAs diodes deposited at 77 K

The use of cryogenic temperatures (∼77 K) during Au Schottky contact deposition onto n-GaAs produces an increase in barrier height from 0.73 eV for room temperature diodes to 0.82 eV. Not all Schottky metals show this enhancement—for example Pt and Ti do not show any significant change in barrier height whereas Au, Pd and Ni show increases between 7 and 18%. We used X-ray reflectivity to show that the main difference between Au deposited at 77 K and room temperature is a decreased metal roughness while the interfacial roughness between the Au and GaAs is basically the same. As the diodes are annealed to 300 °C both the difference in barrier height and interfacial roughness is lost. This is a simple method with potential for improving the performance of GaAs metal-semiconductor-field-effect-transistors (MESFETs).