Viktor Krasnikov

Molecular sieving properties of the cytoplasm of Escherichia coli and consequences of osmotic stress

We determined the diffusion coefficients (D) of (macro)molecules of different sizes (from ∼0.5 to 600 kDa) in the cytoplasm of live Escherichia coli cells under normal osmotic conditions and osmotic upshift. D values decreased with increasing molecular weight of the molecules. Upon osmotic upshift, the decrease in D of NBD‐glucose was much smaller than that of macromolecules. Barriers for diffusion were found in osmotically challenged cells only for GFP and larger proteins. These barriers are likely formed by the nucleoid and crowding of the cytoplasm. The cytoplasm of E. coli appears as a meshwork allowing the free passage of small molecules while restricting the diffusion of bigger ones.

Photovoltaic devices from fullerene oligophenyleneethynylene conjugates

Exploitation of the particular electronic properties of fullerenes for solar energy conversion has become a field of intensive investigation. The modification of semiconductor or metal electrodes with fullerene thin films has revealed promising photoelectrochemical properties. Good solar energy conversion efficiencies have also been obtained with sandwich-type conjugated polymer/fullerene heterojunctions. As part of this research, we have recently shown that fullerene derivatives, in which an oligophenylenevinylene (OPV) group is attached to C60, can be incorporated into photovoltaic cells. This molecular approach for solar energy conversion appears to be particularly interesting, since the bicontinuous network obtained by chemically linking the hole-conducting OPV moiety to the electron-conducting fullerene subunit prevents any problems arising from bad contacts at the junction, as observed for polymer/C60 blends. Furthermore, this new synthetic approach also offers great versatility for designed tuning of the photovoltaic system. We now report the preparation of plastic solar cells from fullerene ±oligophenyleneethynylene (OPE) derivatives 1 ±4. For compounds 1 and 2, the efficiencies of the resulting photovoltaic devices are similar to those prepared from corresponding fullerene ±OPV conjugates. Interestingly, by increasing the donating ability of the

Control of charge trapping in a photorefractive polymer

Modification of the trap density of the photorefractive polymer composite poly(N-vinyl carbazole) (PVK), 2,4,7-trinitro-9-fluorenone (TNF) and N,N-diethyl-para-nitroaniline (EPNA) was achieved with the addition of 4-(diethylamino)benzaldehyde diphenylhydrazone (DEH). Measurements of the response time, the phase shift and the amplitude of the photorefractive grating are presented.

Sexithiophene-C60 blends as model systems for photovoltaic devices

Large photovoltaic responses have been recently observed in devices based on donor-acceptor blends. Their enhanced performance, which relies on the formation of a bicontinuous network of internal heterojunctions, is very sensitive to the morphology of the blend. In this paper, we propose sexithiophene-C60 blends, prepared by coevaporation, as a model system for the study of these devices. Preliminary results suggest a microphase-separated morphology with a domain size comparable to the exciton diffusion range. Photovoltaic cells based on the blends are demonstrated.

Excimer luminescence from single crystals and films of a cyano-substituted phenylene-vinylene model compound

Abstract In the context of luminescence from the solid state of poly( para -phenylene vinylene)-type materials, we report a study of three octyloxy-substituted five-ring oligo( para -phenylene vinylene)s, two of which bear cyano substituents on the innermost vinylene bonds. For each compound, the molecular arrangement in the single crystal has been derived from a crystallographic analysis. For the compound which has the cyano group not directly adjacent to the central ring, synthesis and structure are described here in detail. This compound, in contrast to the other two, has molecular π -stacking in the crystal lattice. Only for this compound do we observe a strongly red-shifted, long-lived emission, which is therefore ascribed to an excimer-type intermolecular excitation. The observations of excimer-type emission from cyano-substituted alkoxy–PPVs and in related copolymers fit in very well with the molecular picture derived from this study of model compounds.

Energy level alignment at the conjugated phenylenevinylene oligomer/metal interface

In this letter we report an investigation of the interfacial electronic structure formed by metals and conjugated oligomers using ultraviolet photoelectron spectroscopy. Au and Ag were used as metal substrates for two five-ring phenylenevinylene oligomers: unsubstituted p-bis[(p-styryl)styryl]benzene (P5V4) and the analogous oligomer with 2-methoxy-5-(2’-ethyl-hexyloxy) substitution on the central ring (MEH-P5V4). We found for all interfaces a lowering of the energy levels of the organic overlayer by 0.4–1.2 eV. Remarkably, this energy lowering, presumably due to interface dipole layers, was always such as to keep the hole injection barrier nearly constant and therefore at most weakly sensitive to the work function of the metal or the ionization potential of the oligomer.

Transient behavior of photorefractive gratings in a polymer

The transient behavior of photorefractive gratings in the polymer composite poly(N‐vinyl carbazole) (PVK), 2,4,7‐trinitro‐9‐fluorenone (TNF), and N,N‐diethyl‐para‐nitroaniline (EPNA) doped with various amounts of 4‐(diethylamino)benzaldehyde diphenylhydrazone (DEH) is presented. The influence on the hole drift mobility due to the change in the trap density induced by DEH, was directly measured.

Analytical study of PPV-oligomer- and C60-based devices for optimising organic solar cells

A blend of a 5-ring n-octyloxy-substituted oligo(p-phenylene vinylene) and C60, sandwiched between two electrodes, has been used as the active layer for an organic solar cell. It delivered external quantum efficiencies up to 60% in the visible and 70% in the UV part of the spectrum. To unambiguously determine which parts of the bulk heterojunction structure are responsible for the rectifying behaviour and which can be considered as ohmic, the I/V characteristics of several other devices were investigated. It is found that the presence of C60 in a bulk heterojunction solar cell introduces fill factor reducing shunting paths.

Photorefractive polymer composite with net gain and subsecond response at 633 nm

By combining the well‐known photoconductor poly(N‐vinyl carbazole) sensitized with 2,4,7 trinitro‐9‐fluorenone and the electrooptic molecule N,N, diethyl‐substituted para‐nitroaniline, which is transparent at 633 nm, a photorefractive polymer composite suitable for applications with He‐Ne lasers was developed. Net gain of 18 cm−1 and 400 ms response time were measured on a 65‐μm‐thick sample.

Optical properties of single crystals and vacuum-deposited thin films of a substituted oligo(p-phenylene vinylene)

Abstract The optical properties and structural organization of a 5-ring n-octyloxy-substituted oligo(p-phenylene vinylene) have been investigated in the solid state. For this study solution-grown single crystals and vacuum-deposited thin films were used. The unit cell of the solution-grown single crystals was determined to be monoclinic, space group I2 a . The as-deposited oligomer films showed a polycrystalline structure. Annealing the film at a temperature of 120°C resulted in a morphological change, which manifested itself in an increase of crystalline domain size from approximately 4 μm to 20 μm. Both single crystals and thin films exhibit a high photoluminescence quantum yield of 0.5 and 0.7, respectively. In order to determine the relative importance of oxygen and water on photoluminescence quenching, we measured the luminescence decay of a vacuum-deposited film exposed to water and air. It was found that quenching is mainly due to contamination with oxygen from air rather than water and that this contamination is reversible to a great extent. We demonstrate that under intense laser excitation it is possible to obtain stimulated emission from both single crystals and thin films. Non-optimized single-layer light-emitting diodes prepared from the oligomer showed uniform green/yellow light emission with external electroluminescence efficiencies up to 0.15% using Ca as electron-injecting electrode. The electroluminescence efficiency is drastically increased by annealing.