Attachment of a Hydrogen‐Bonding Carboxylate Side Chain to an [FeFe]‐Hydrogenase Model Complex: Influence on the Catalytic Mechanism

Azapropanedithiolate (adt)‐bridged model complexes of [FeFe]‐hydrogenase bearing a carboxylic acid functionality have been designed with the aim of decreasing the potential for reduction of protons to hydrogen. Protonation of the bisphosphine complexes 4 – 6 has been studied by in situ IR and NMR spectroscopy, which revealed that protonation with triflic acid most likely takes place first at the N‐bridge for complex 4 but at the Fe? Fe bond for complexes 5 and 6 . Using an excess of acid, the diprotonated species could also be observed, but none of the protonated species was sufficiently stable to be isolated in a pure state. Electrochemical studies have provided an insight into the catalytic mechanisms under strongly acidic conditions, and have also shown that complexes 3 and 6 are electro‐active in aqueous solution even in the absence of acid, presumably due to hydrogen bonding. Hydrogen evolution, driven by visible light, has been observed for three‐component systems consisting of [Ru(bpy)₃]²⁺, complex 1 , 2 , or 3 , and ascorbic acid in CH₃CN/D₂O solution by on‐line mass spectrometry.     

Spectroscopic characterization of cobalt-containing mesoporous materials

Cobalt-containing mesoporous materials that have been prepared using different procedures have been comparatively characterized by transmission electron microscopy/energy-dispersive X-ray spectroscopy (TEM/EDS), extended X-ray absorption fine structure spectroscopy (EXAFS), X-ray absorption near edge spectroscopy (XANES), and ultraviolet-visible (UV-vis), near-infrared (NIR), and mid-infrared (mid-IR) spectroscopies, and the results provide new insights into the local environment and properties of cobalt in this type of material. TEM/EDS analyses have shown that tetraethyl orthosilicate (TEOS) may be less appropriate as a silicon source during the syntheses of cobalt-containing mesoporous materials, because the distribution of cobalt throughout the framework may become uneven. EXAFS has been determined to be the most suitable method for direct verification of framework incorporation, by identifying silicon as the backscatterer in the second shell. Such a direct verification may not be obtained using UV-vis spectroscopy. From EXAFS analyses, it is also possible to distinguish between surface-bound and framework-incorporated cobalt. There is a good agreement between the results obtained from XANES and UV-vis regarding the coordination symmetry of cobalt in the samples. The presence of cobalt in the silica framework has been determined to create Lewis acid sites, and these acid sites are suggested to be located at tetrahedral cobalt sites at the surface.     

Synthesis of an Orthogonal Topological Analogue of Helicene

The synthesis of an orthogonal topological pentamer analogue of helicene is presented. This analogue forms a tubular structure with its aromatic systems directed parallel to the axis of propagation, which creates a cavity with the potential to function as a host molecule. The synthetic strategy reported, based on a series of repeating Friedländer condensations that utilize pyrido[3,2‐d]pyrimidine moieties as protected amino aldehydes, allows for the facile access of higher generations of helical, tubular structures. As a result of the synthetic strategy, only a helical isomer of the pentamer is possible. The structure and absolute configuration of the pentamer were elucidated from a combination of NMR spectroscopic data, optical properties, X‐ray structures, and by comparison of an experimental electronic circular dichroism spectrum to a calculated spectrum.     

Photoinduced charge carrier dynamics of Zn-porphyrin-TiO2 electrodes: the key role of charge recombination for solar cell performance

Time resolved absorption spectroscopy has been used to study photoinduced electron injection and charge recombination in Zn-porphyrin sensitized nanostructured TiO(2) electrodes. The electron transfer dynamics is correlated to the performance of dye sensitized solar cells based on the same electrodes. We find that the dye/semiconductor binding can be described with a heterogeneous geometry where the Zn-porphyrin molecules are attached to the TiO(2) surface with a distribution of tilt angles. The binding angle determines the porphyrin-semiconductor electron transfer distance and charge transfer occurs through space, rather than through the bridge connecting the porphyrin to the surface. For short sensitization times (1 h), there is a direct correlation between solar cell efficiency and amplitude of the kinetic component due to long-lived conduction band electrons, once variations in light harvesting (surface coverage) have been taken into account. Long sensitization time (12 h) results in decreased solar cell efficiency because of decreased efficiency of electron injection.     

Material property estimates from ultrasound attenuation in fibre suspensions

An investigation of a new method for measuring fibre material properties from ultrasonic attenuation in a dilute suspension of synthetic fibres of uniform geometry is presented. The method is based on inversely solving an ultrasound scattering and absorption model of suspended fibres in water for the material properties of the fibres. Experimental results were obtained from three suspensions of nylon 66 fibres each with different fibre diameters. A forward solution to the model with reference material values is compared to experimental data to verify the model’s behaviour. Estimates of the shear and Young’s modulus, the compressional wave velocity, Poisson’s ratio and loss tangent from nylon 66 fibres are compared to data available from other sources. Experimental data confirms that the model successfully predicts that the resonance features in the frequency response of the attenuation are a function of diameter. Consistent estimated values for the compressional wave velocity and the Poisson’s ratio were found to be difficult to obtain but in combination gave values of shear modulus within previously reported values and with low sensitivity to noise. Young’s modulus was underestimated by 54% but was consistent and had low sensitivity to noise. The underestimation is believed to be caused by the assumption of isotropic material used in the model. Additional tests on isotropic fibre would confirm this. Further analysis of the model sensitivity and the reasons for the resonance features are required.     

On boundary layers

The concept of boundary layers, introduced by A. Volberg in [7], is generalized from subsets of the unit disk to subsets of general non-tangentially accessible (NTA) domains. Capacitary conditions of Wiener type series of both necessary and sufficient type for boundary layers are presented and the connection between boundary layers and minimally thin sets is studied.     

Flexible composites, strength, deformation, and fracture processes. 1. Reinforcement structures and tensile strength

Fiber-reinforced flexible composites are extensively used for different kinds of applications, for example, tubes, drive belts, tires, and coated fabrics. Typical for these materials are matrix materials allowing large strain deformation and reinforcement structures allowing bending. Apart from the tensile strength and limited bending stiffness, damage resistance and ductile-brittle transition characteristics are discussed. The tensile strength usually follows the rule of mixture. The mode of fracture and damage resistance, however, strongly depend on penetration of the matrix into the fiber bundles, textile structure, and internal friction. Models for the work of fracture and the ductile-to-brittle fracture transition are discussed.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Published in Mekhanika Kompozitnykh Materialov, Vol. 34, No. 6, pp. 747–760, November–December, 1998.     

Ultrafast excited-state isomerization dynamics of 1,1'-diethyl-2,2'-cyanine studied by four-wave mixing spectroscopy

Excited-state dynamics and solvent-solute interactions of 1,1'-diethyl-2,2'-cyanine iodine (1122C) in alcoholic solutions are investigated using time-integrated three-pulse photon-echo spectroscopy. 1122C serves as a model compound for ultrafast photoinduced isomerization-a key process in the light reception of plants, bacteria, and human vision. The photoreaction in 1122C is interrogated in dependence on solvent and excitation wavelength. The wavelength-dependent three-pulse photon-echo peak shift indicates strong alterations of the reaction pathways and points to the existence of a direct internal conversion channel in close proximity to the Franck-Condon point of absorption. The solvent-dependent S1-S0 internal conversion time does not follow conventional sheared viscosity dependence, suggesting that the solvent local friction has to be considered to account for the observed isomerization kinetics. The concerted discussion of transient grating and three-pulse photon-echo peak-shift data allows us to derive a complete picture of the solvent-solute interaction-controlled photoreaction. The results obtained are related to other work on reactive systems and are discussed in the framework of multilevel response functions.     

Appearance of intramolecular high-frequency vibrations in two-dimensional, time-integrated three-pulse photon echo data

An alternative experimental outline to measure homodyne detected three-pulse photon-echo data is presented. The novel experimental approach allowing for online monitoring and correction of experimental timing and stability is discussed in detail using the paradigm system of Nile blue in alcohol solution. It is shown that excellent signal-to-noise ratios together with high reproducibility of the data can be routinely achieved. We report in detail on the appearance of high-frequency intramolecular vibrations in the two-dimensional three-pulse photon-echo data and suggest that besides the conventionally discussed three-pulse photon-echo peak-shift the width of the integrated echo signal as a function of population time contains identical and easily accessible information on high-frequency intramolecular vibrations. A comparison of experimental data with theoretical modeling is performed showing that the observed echo-width oscillations are in line with predictions of the Brownian oscillator model.