Emile Vander Donckt

Sonochemistry: a systematic study of the maleate-fumarate isomerization in the presence of alkyl bromides

Abstract A systematic and quantitative study of diethyl maleate-diethyl fumarate induced isomerization has been performed in CCl4 using different ‘inert’ gases, different initiators and various experimental conditions. It was shown how changes in the experimental conditions, generally considered as minor in thermochemistry, can have dramatic effects in sonochemistry (for example, the ‘marble effect’).

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.

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.

Photochemical reactions of bifluorenylidene. Part 1.—Steady irradiation investigations

On irradiation with light of wavelengths < 310 nm, solutions of bifluorenylidene in isopropanol are phototransformed, the reaction products being bifluorenyl and 2-hydroxypropylbifluorenyl. The proportion of the two compounds and the quantum yield of disappearance of bifluorenylidene depend on its concentration. It is shown that auto-inhibition of the reaction is as efficient as quenching by anthracene, and that bifluorenylidene phosphorescence is observed only after excitation at wavelengths < 310 nm. These facts suggest that the primary step of the reaction takes place in the triplet manifold and that the reactive triplet state is not the T1 state. The quantum yield of disappearance of bifluorenylidene is an increasing linear function of the inverse of the square root of the intensity. A kinetic scheme which rationalizes these observations is proposed.

Photochemical reactions of bifluorenylidene. Part 2.—Flash photolytic investigations

A short lived species which absorbs between 360 and 530 nm is produced when a dilute solution of bifluorenylidene in a viscous paraffin solvent is flashed. The transient is assigned to the triplet phosphorescent state since the phosphorescence lifetime and relaxation time of the species at 77 K are identical within experimental uncertainties. The decay of the transient is strictly first order and the data at room temperature can be rationalized in terms of the equation τ–1transient=k3+k4[A(S0)] with k3(paraffin)=(440 ± 25) s–1 and k4(paraffin)=(3.7 ± 0.2)× 107 dm3 mol–1 s–1. The absence of a second order term in the kinetic equation is apparently due to a compensation of two opposing effects; a decrease of a triplet–triplet annihilation term and an increase of the self quenching term with the disappearance of A(TR).On flashing bifluorenylidene in isopropanol, a transient with a lifetime independent of the ground state concentration is monitored. It is assigned to the bifluorenyl radical AH since the amount of transient formed is always proportional to the amount of bifluorenylidene phototransformed by the flash. The rate constant for hydrogen abstraction from isopropanol by AH, k6, is determined to be 61 ± 7 dm3 mol–1 s–1.In viscous paraffin + isopropanol mixtures, A(TR) and AH are monitored in succession by kinetic spectroscopy. The rate constant for hydrogen abstraction from isopropanol by A(TR) in a long chain paraffin solvent is found to be k5= 550 ± 100 dm3 mol–1 s–1.Various attempts to find direct spectroscopic evidence for the elusive triplet state(s) of lower energy than TR were unsuccessful. Nevertheless, kinetic spectroscopy showed that the T1 state of anthracene is efficiently quenched by bifluorenylidene even though the T1 state of the donor is 60 kJ mol–1 below the phosphorescent level of bifluorenylidene.The photoreduction in isopropanol can be sensitized by triplet energy donors such as naphthalene and triphenylene. The quantum yield of the sensitized reaction is smaller than that of the reaction under direct irradiation. This is tentatively discussed in terms of a competition between energy transfer to the TR state and to the triplet state(s) of lower energy than TR.

External pressure effect on quenching of benz-(a)-anthracene and 9-cyano-anthracene fluorescence by dimethylmercury

The effect of external pressure on the interaction between 9-cyano-anthracene or benz-(a)-anthracene in the S1 state with dimethylmercury has been investigated using n-hexane and acetonitrile as solvents. It appears that 9-cyano-anthracene and dimethylmercury do not form an exciplex in acetonitrile, but that the fluorescence quenching involves a species of higher energy and smaller volume than that of the separated species (Eyring activated complex); in the non-polar solvent the slow fluorescence quenching takes place via a contact complex.On the other hand, exciplex formation between benz-(a)-anthracene and dimethylmercury, discussed previously on the basis of temperature effects, has been confirmed in the polar solvent. The volume contraction of the system, –9 ± 2 cm3 mol–1, is consistent with exciplex formation.

Unexpected electrochemical reduction of fluoranthene in the solvents DME and HMPA: new light onto the mechanism of hydrogenation to produce tetrahydrofluoranthene

The non-alternant aromatic hydrocarbon fluoranthene (Ar0) has been reduced, either chemically with Na or Li or by electrolysis, to the radical anion Ar˙– in the three solvents THF (tetrahydrofuran), DME (dimethoxyethane) and HMPA (hexamethylphosphoric triamide). The UV–VIS absorption spectrum of the orange–brown Ar˙– is quite similar in the three solvents and in all instances addition of H+–H2O has resulted in quantitative electron-back-donation along with H2 evolution and recovery of unchanged fluoranthene Ar0. Thus the usual Birch-type reduction to a dihydro derivative is totally inefficient in the cases under investigation. The two-electron reduction has also been achieved in these three solvents. The greenish-yellow dianion Ar2– produced in THF exhibits characteristic UV–VIS absorption patterns, disproportionates with Ar0 and reacts with H+–H2O only to evolve H2.With DME or HMPA a blood-red species is produced whose absorption spectrum is virtually the same and quite different from that observed in THF. In both solvents addition of H+–H2O leads to tetrahydrofluoranthene as a main reaction product but disproportionation is not observed at all in HMPA and this is not compatible with a regular dianion Ar2–.Reaction with D+–D2O instead of H+–H2O has shown that hydrogenation involves radical abstraction of H atoms from the solvent in both cases; this sheds new light onto the reaction mechanism. Furthermore, several other experiments indicate that the dianionic blood-red species is most likely a complex written as [Ar˙–⋯˙– Solvent], in which the Ar˙– moiety is bound to a solvated electron localized on a solvent molecule.

On the validity of fluorescence decay used as a technique to measure internal microviscosity in reverse micelles

Abstract Analysis of the fluorescence decay of a probe dissolved in the water core of reverse micelles has been reported as a means to measure the internal microviscosity of water under the assumption of a homogeneous distribution of the probe. We have carried out a computer analysis which shows that divergence from homogeneity associated with solvation enthalpy changes Δ G o as small as 0.5 kJ mole −1 is sufficient to affect significantly the fluorescence response of the system. There are known examples of non-homogeneous distribution of a solute within the water pool of a reverse micelle and we are led to conclude that if a homogeneous distribution of the excited probe is to be assumed, then the decay of its fluorescence is an unreliable technique for studying the microscopic internal properties of reverse micelles.

New fast and easy procedure for in n-doping polyacetylene with H4LiAl as the electron donor. A method for producing polyene anions

It has bee shown unambiguously that lithium aluminium hyride in tertrahydrofuran solution reduces polyacetylene to the poly-anionic n-semiconducting state with concomitant escape of gaseous hydrogen.