Volume 220, Issue 4

The Scanning Electrochemical Microscope (SECM) can be used with different techniques of microstructured copper deposition. A first approach involves the electrolytic copper deposition on noble metals, whereby copper ions are released from a complex by a suitable tip reaction and then reduced on the polarised conducting surface to form a copper microstructure. The second approach is very similar to the first, but does not involve polarising the substrate. It generates a tip-induced microgalvanic cell, the positive electromotoric force of which is constituted by two electrochemical reactions at different areas of the substrate. Finally the electroless copper deposition is performed on nonconducting surfaces like glass or semiconducting surfaces like silicon. This involves locally reducing a suitable precursor film whose surface has been previously immobilised.

The phase diagram and the properties of the aqueous phases of the AB block copolymer poly-n-butylacrylate-block-polyacrylic acid (nBuA 100 -b-AA 150 ) has been studied as a function of the neutralisation degree α with NaOH. Aqueous solutions of the block copolymer with α < 0.1 are turbid and separate into two phases while solutions with α > 0.1 are transparent and single phase. The viscosities of such solutions increase with α and pass over a maximum at α = 0.5. Solutions with a concentration of Cp ≥ 1% and at α = 0.5 are highly viscoelastic and have a yield stress value. Cryo-TEM micrographs show that globular micelles are present in the viscoelastic solutions. The micelles have a polydisperse size distribution. The radius of the hydrophobic core varies from about 100 Å to 200 Å what is the length of the fully stretched hydrophobic block. The rheological properties of the solutions are explained on the basis of the globular core–shell micelles where the polyelectrolyte chains form a corona around the hydrophobic block. Most of the counter ions are in the condensed and free state inside the corona. Some of the counter ions are outside of the corona and form an electrical double layer whose thickness depends on α. The viscoelastic properties of the solutions are thus given by the dense packing of the charged globular micelles. The decrease of the viscosity for α > 0.5 comes about by the shrinkage of the thickness of the electrical double layer. With increase of excess salt the double layer and the corona also shrink and the viscoelastic properties break down. With adsorption of zwitterionic surfactants the viscoelastic properties also disappear. Cryo-TEM micrographs of the block copolymers at α = 0.5 with adsorbed zwitterionic surfactants show that the polyelectrolyte chains of the block copolymers are wrapped around small globular micelles of the zwitterionic surfactants. The supra-molecular structures look like raspberry-like micelles. The block copolymers show a very unusual amphiphilic behavior. For a neutralisation degree of α > 0.1 the compounds do not seem to adsorb at the water surface even though the compounds form micelles in the bulk solution. The micellar solutions have the same surface tension of water. The acidic form of the compound however is surface active.

The rate constants for the electrochemical reduction of Eu 3+ in water–acetone and water–dimethylformamide solvent mixtures with different parts of the organic component were measured in the temperature range 15°C–40°C. The activation parameters for this reaction were evaluated. While the activation enthalpy does not depend on the composition of the solvent, the activation entropy changes follows the changes of reaction rates. The entropy contribution (responsible for the form of dependence of rate constants on the composition of the solvent) is obviously related to structure changes of the resolvatated particle.

A novel empirical model for the diffuse double layer is found by generalizing the simple analytical equations of Gouy–Chapman theory. Two adjustable parameters are introduced into the Boltzmann equation for the exponential dependence of the ion–wall correlation functions on the diffuse layer potential. Optimal parameter values and model validation for 1:1 and 2:1 electrolytes are provided by Monte Carlo simulations. Simple relationships are obtained between these empirical parameters and those commonly associated with the mean-spherical approximation. The new empiricism accurately models diffuse layer potential profiles and ion–wall correlation functions for a restricted 1:1 electrolyte in a primitive solvent.

The UV/vis absorption spectra recorded during titration experiments (pH2 ↔ pH11, ΔpH ∼ 0.33) were subjected to a principal component analysis. The aim was to determine by means of the program SPECFIT the number of significant components and to reduce the noise in the spectra. The attempt to achieve a global fit of the “noise-reduced” data was in general successful if 5 or more independent species were considered. The necessity to include a larger number of species than the 4 occuring within the simple protonation-deprotonation scheme of a triprotic acid provides strong evidence for conformational heterogeneity at various levels of protonation. Only if this heterogeneity is taken into account, the analysis yields the correct p K a -values. For tetracycline, slow conformational changes are suggested to occur in the neutral state. Consequently, tetracycline adopts conformation 2 in alkaline solution if the pH is raised slowly up to pH11, whereas if it is dissolved directly in buffer solution at pH2 or pH11 it adopts conformation 1 (like in the crystal). In this conformation, tetracycline degrades rapidly in a methanol–water mixture around pH11 probably by methylation of the 11,12-β-carbonyl group. Sancycline and 10-propoxy-sancycline are stable at any pH, the conformational changes are proposed to occur preferentially in the anionic state. Because of these conformational changes, the p K a -values derived from forward and backward titration can differ up to 0.4 pH units. In addition, there is evidence that C10-OH does not dissociate at pH less than 11. In contrast to a widely accepted hypothesis, 4-epi-tetracycline also changes its conformation in the anionic form. This shows that CD spectra are not suited to draw conclusions on molecular geometry if the total net charge changes.

Nitrogen and carbon doped titania powders initiate acrylate polymerisation upon UV excitation. Time resolved photovoltage measurements afford surface charge carrier lifetimes of 7–8 ms for undoped and N-doped titania, whereas 12 ms are observed for the C-doped semiconductor. These values correlate with the observation that the polymerisation rate constants are similar for titania or N-doped titania but about two times larger for the C-doped material. Wavelength dependent polymerisation experiments indicate that visible light polymerisation is possible only with carbon-doped titania. This suggests that hole relaxation to dopant-centered surface states is much more efficient in nitrogen-doped titania. These experimental findings are in accord with the assumption that polymerisation is initiated by hole oxidation of the acrylate.

For the first time, intracomplex electron transfer reactions were tested in cytochrome c based nanohybrids ( i.e. , Fe(II)Cyt C and Fe(III)Cyt C ), to which polyanionic C 60 derivatives were electrostatically associated. Successful radical ion pair formation, namely, oxidized FeCyt C and reduced C 60 , was detected when divalent Fe(II)Cyt C is used. The so formed Fe(III)Cyt C · C 60 •− lives for 2.4 ± 0.5 μs.

The redox behaviour of N,N-dimethyl-p-phenylenediamine (PPD) and N,N-dimethyl-p-phenylenediamine · 2HCl (PPD · 2HCl) in acetonitrile was studied by means of cyclic voltammetry – both in the absence and in the presence of proton accepting pyridine. The oxidation occurs in two separate steps yielding first N,N-dimethyl-p-semiquinonediimine (SQDI + ) and subsequently N,N-dimethyl-p-quinonediimine (QDI 2+ or QDI + (H + )). In case of N,N-dimethyl-p-phenylenediamine the 1 st oxidation step is fully reversible, while the 2 nd oxidation step is slightly quasi-reversible. At 10°C the formal redox potentials of the PPD/SQDI + and SQDI + /QDI 2+ redox couples are −0.200 ± 0.005 V and 0.380 ± 0.010 V ( versus Fc/Fc + ), respectively. The synproportionation constant K syn = [SQDI + ] 2 /[PPD][QDI 2+ ] is 2.1 × 10 10 . Adding pyridine to the reaction mixture leads to a shift of the 2 nd anodic wave to less positive potentials. This shift is primarily due to proton transfer from QDI 2+ to pyridine to form QDI + , which overall renders the oxidation easier. The corresponding cathodic peak, on the other hand, disappears as the reduction of QDI + necessitates a proton. The formal oxidation potential of the redox couple SQDI + /QDI + (H + ) is about 0.190 V and, therefore, the synproportionation constant K syn = 1.7 × 10 6 . N,N-dimethyl-p-phenylenediamine · 2HCl lacks completely the 1 st anodic peak. The position of the anodic peak is given by the direct oxidation of PPDH + to QDI 2+ (H + ) and is similar to that of the SQDI + /QDI 2+ redox couple. Upon adding pyridine the 2 nd anodic peak shifts notably to less positive potentials and the redox process attributable to the PPD/SQDI + redox couple is seen. As pyridine influences the potential of the Ag pseudo-reference electrode a reference electrode based on the ferrocene/ferrocenium redox couple was used.

Structures of [M ⊂ 2.2.2] n+ obtained with MNDO, AM1, PM3, PM3/SPASS, and PM5, are evaluated against X-ray and DFT (B3LYP/LANL2DZp) geometries. DFT and X-ray structures agree well, followed by PM3/SPASS. The ion selectivity calculated with DFT and PM3/SPASS for a simple model reaction, agrees satisfactorily with the available complex-formation constants (log K S ). The applied theoretical treatment has a good predictive potential.

Simple synthetic procedure (reactions between iridium trichloride hydrate with a series of fluorine substituted 2-phenyl-N-methyl-benzimidazole derivatives L) leads to the cyclometallated Ir(III) complexes IrL 2 (acac) and IrL 2 (pic) where acac = acetylacetonate and pic = α-picolinate anions, respectively. All the investigated complexes exhibit strong absorption in the UV region, due to the spin allowed intra-ligand (IL) transitions, and moderately intense bands in the visible region, due to the metal-to-ligand-charge-transfer (MLCT) transitions. The complexes exhibit also strong luminescence in the deoxygenated 1:1 acetonitrile/dioxane solutions at 298 K as well as in the butyronitrile glasses at 77 K. In both experimental conditions the structured emissions (with the 0–0 transition within the 480–510 nm range of UV-Vis radiation) have been observed. Cyclic voltammetry investigations point out to reversible character of the electrochemical reactions leading to stable IrL 2 (acac) + or IrL 2 (pic) + cations and IrL 2 (pic) − anions. The corresponding IrL 2 (acac) − species have been found to be unstable under the applied experimental conditions (1:1 acetonitrile/dioxane solutions containing 0.1 M (C 4 H 9 ) 4 NPF 6 as the supporting electrolyte) most probably due to the irreversible electrochemical reduction of the acac − ligand. Appropriate combination of the electrochemical and luminescence properties found for the investigated complexes allows for the electron transfer generation of the excited 3 ∗IrL 2 (pic) and 3 ∗IrL 2 (acac) species. Triple-potential-step technique was used to create electrochemiluminescence (ECL) emission by annihilation of the electrochemically generated IrL 2 (acac) + or IrL 2 (pic) + cations and 4,4’-dicyano-biphenyl radical anion. Very high ECL emission efficiencies (close to the excited 3 ∗IrL 2 (acac) and 3 ∗IrL 2 (pic) luminescence yields) have been found.

The rates of degenerate electron exchange (electron self-exchange) of various cyanobenzenes have been measured by EPR line broadening technique in nine different solvents at room temperature. The molecules studied comprise besides benzene-1,2-dicarbonitrile, benzene-1,4-dicarbonitrile and benzene-1,2,4,5-tetracarbonitrile, the two isomeric tricyanobenzenes, benzene-1,2,3-tricarbonitrile and benzene-1,2,4-tricarbonitrile, the anion radicals of which have not been characterized before. The experimentally observed rates vary from 4.5 × 10 8 to 44.0 × 10 8 M −1 s −1 and show the pronounced dependence on the longitudinal relaxation times, τ L , of the solvents. The solvent dynamical effect so manifested is confirmed with remarkable clarity using solvents spanning a wide range of τ L -values, which comprise acetonitrile (0.2 ps) and o -dichlorobenzene (6.0 ps) at its extremes. The rate constants are compared with Marcus theory using the continuum model (CM) and the mean spherical approximation (MSA) for the outer sphere reorganization energies and Nelson’s method for the inner sphere reorganization energies. Furthermore, an estimation of the resonance splitting energies, V RP , is given based on the experimental rates.