Stéphane Boué

Electrical conductivity and infrared absorption of trans- polyacetylene in the presence of iodine

An electronic transition which can be assigned to either a charge-transfer band or to a positively charged soliton has been observed in the near-i.r. (shallow maximum at ca. 1800 nm) for thin films of iodine-doped trans-polyacetylene. From the analysis of this band intensity as a function of the iodine content it is deduced that the complex produced between the (CH)x polyene units (or the neutral solitons) and I2 has the average composition (CHI0.008)x and obeys a one-to-one stoichiometry with Keq≈ 64 dm3 mol–1. On the assumption that charge carriers originate from the production of the spectroscopically observed new species the relationship between the electric conductivity, σ, and the complex concentration has been qualitatively and quantitatively investigated. A logarithmic dependence of σ on the density of carriers has been shown and the experimental data are best accounted for by the three-dimensional variable range-hopping conduction mechanism.

Photovoltaic and rectification properties of In/trans-(CH)x/electrodag +502 schottky-barrier cells

Abstract The electrical and photovoltaic properties of In/trans-(CH)x/Electrodag +502 sandwich cells have been examined. The forward bias dark current obeys a modified Shockley equation. The junction reverse bias current fits the image-force lowering relationship. The capacitance-voltage characteristics were analysed. A photo-voltage and a photocurrent were observed under polychromatic or monochromatic illumination. Photovoltaic action spectra are similar to the adsorption spectra. The dependence of Isc and Voc on the light intensity was determined. The temperature effect on Io, C, Isc and Voc was also investigated.

Theoretical kinetic analysis of biphotonic processes

A simple kinetic scheme involving the successive absorption of two photons has been completely analysed and the solutions have been numerically computed within a wide range of parameter values. It is found that, aside from the classical monotonic behaviour described in many textbooks, a system can, under appropriate conditions, respond to photoexcitation in a number of unusual ways. In particular, for a given system and a single set of conditions more than one steady state can be achieved, not all of them being stable, and this results in the existence of critical light intensities around which the system behaves like a chemical amplifier or threshold discriminator. It is also shown by time-dependent analysis that the concentration in some species can be subject to overshoot or to damped oscillations before reaching its stationary value. Although a few experimental cases have been reported, such unusual phenomena have not been widely recognized in photochemistry, and it seems plausible that their possible occurrence in laboratories might have been overlooked or disregarded as being due to artefacts. The aim of this paper is to draw the attention of photochemists to the physical meaning of such unusual observations.

Novel approach to the study of electrical conduction in bromine-doped polyacetylene

Abstract Using X-ray analysis we showed that on doping with iodine or bromine trans -polyacetylene loses its crystalline character. The optical and electrical properties of bromine-doped films (approximately 1000 A thick) were investigated and the formation of charge transfer complexes was quantitatively correlated to the production of charge carriers and to the conductivity σ. Our results indicate that the most probable conduction mechanism which obtains at room temperature in halogen-doped polyacetylene is three-dimensional disordered hopping. The density of states N ( E F ), the hopping distance R and the carrier localized wavefunction α were estimated for this organic semiconductor.

Photovoltaic properties of In/trans-polyacetylene/Electrodag+502 Schottky barrier cells

The electrical properties of In/trans-polyacetylene/Electrodag+502 Schottky barrier cells were examined in the dark and under monochromatic and white light irradiation. The experiments were performed between −77 and +110 ° C. The In/trans-polyacetylene contact is strongly blocking. The photocurrent is proportional to the light intensity and the open-circuit photovoltage is an exponential function of light intensity. Owing to the low quantum yield for carrier production and collection, the fraction of light energy transformed into electrical energy is rather low (about 0.025%) under white light illumination.

Direct photolysis of the penta-1,3-dienes: Recognition of wavelength-dependent behaviour in solution

The quantum yields for geometrical isomerisation and 1,4-cyclomerisation of cis- and trans-penta-1,3-dienes differ for excitation at 228·8 and 253·7 nm; this is explained by a change in the vibrational modes through which the molecules relax on the S1 hypersurface.

Quenching of the excited singlet penta-1,3-dienes by Me3SnH: evidence against the allylmethylene configuration of the relaxed S1 state

The 1,4 concerted addition of Me3SnH to the singlet excited penta-1,3-dienes yields cis- and trans- adducts according to a stereochemistry which is incompatible with an allylmethylene relaxed configuration and suggests a doubly state.

Kinetics of solvated electrons in reaction with hexamethylphosphoric triamide

Solvated electrons, es–, have been generated in hexamethylphosphoric triamide (HMPA) by chemical and electrochemical means. When Na metal was used to release es–, two distinct bands, already reported in the literature, appeared in the absorption spectrum of the solution, which rapidly turned red–brown and decomposed according to irreproducible complex kinetics which could not be analysed. When Li was used instead of sodium, a single broad band extending into the IR region developed and later decayed obeying consistently first-order kinetics in absorbance. Kinetic rate constants are reported and half-lives at ambient temperature were about 1 h duration. A systematic study of the disappearance of solvated electrons in HMPA was carried out at several temperatures (from 7 to 27 °C) and an activation energy (85–89 kJ mol–1) is estimated both in the absence and in the presence of lithium perchlorate. However, the overall reaction rate is markedly increased in the presence of Li+; a mechanism is proposed to account for this lithium salt effect.

Electroreduction of fluorene, a five-membered ring-containing aromatic component of coal

Abstract Electrolysis of fluorene dissolved in THF-H2O-TBABF4 mixtures took place on a mercury cathode at −2.4 V vs. Ag/Ag+ to produce di-, tetra- and hexahydrofluorenes via a Birch-type reduction mechanism and without significant hydrogen evolution; the addition of six hydrogen atoms per molecule proved to be the upper limit attainable in this system, regardless of the charge transferred. Reduction of a coal sample in the same conditions, at the rate of 0.016 F g−1, noticeably increased the soluble fraction as well as the conversion yield in hydropyrolysis experiments.

Mechanisms of electrochemical hydrogenation of fluoranthene in HMPA with ethanol and aqueous HCL as proton donors

The electrochemical hydrogenation of the non-alternant aromatic hydrocarbon fluoranthene has been investigated in LiClO4-hexamethylphosphoric triamide; it has been found that hydrogenation cannot be achieved satisfactorily, not even to the dihydro derivative, when ethanol is used as the proton donor, whether present during or added after electrolysis. On the other hand, reaction of a stock of fluoranthene dianion with aqueous hydrochloric acid proves to be a clean, easy way to prepare 1,2,3,10b-tetrahydrofluoranthene, a product which requires donation of four electrons to fluoranthene. A mechanism is discussed to account for such a transfer of electrons.