Eric Levillain

Polysulfides in dimethylformamide: only the redox couples S−n/S2−n are involved

Abstract The redox mechanism of polysulfides in nonaqueous solvents is examined. For that purpose, Li 2 S 6 + DMF solutions have been investigated by using cyclic voltammetry and time-resolved spectroelectrochemical experiments coupling spectrophotometry and cyclic voltammetry. It is shown that the only redox couples involved in these solutions are S − n /S 2− n with n = 3,4,6,8 and S 8c /S − 8c (c for cyclic). The experimental results are interpreted by the following mechanism: S 3 − + e − ⇔ S 3 2 − ( E 3 ) S 6 2 − ⇔ 2 S 3 2 − ( K 3 ) S 4 − + e − ⇔ S 4 2 ( E 4 ) S 8 2 − ⇔ 2 S 4 − ( K 4 ) S 6 − + e − ⇔ S 6 2 − ( E 6 ) S 8 e 2 ⇔ S 8 l − ( K 8 ) S 8 l − + e − ⇔ S 8 2 − ( E 8 ) 2 S 3 2 − ⇔ S 2 2 − + S 4 2 − ( K 3 D ) S 8 e + e − ⇔ S 8 e − ( E 0 ) 2 S 6 2 − ⇔ S 4 2 − ⇔ S 8 2 − ( K 6 D ) with E 3 E 4 E 0 E 6 E 8 S i 2 − + S j − ⇔ S i − + S j 2 − ( K i j r ) with i or j = { 3 , 4 , 6 , 8 } and i ≠ j This mechanism has been simulated and adjusted to the experimental voltammograms in broad temperature (233 to 313 K) and scan rate (50 to 2000 mV/s) ranges, recorded for an initial scan in the positive direction and in the negative direction. We have obtained a complete description of the redox properties of sulfur and polysulfides in this solvent. We have shown that only the radical anions S − n are reducible and that only the dianions S 2− n are oxidizable. It is also shown that this mechanism describes the two steps of the reduction mechanism of sulfur in DMF.

In situ time-resolved FTIR spectroelectrochemistry: study of the reduction of TCNQ

The efficiency and versatility of time-resolved FTIR spectroscopy has been used to follow concentration profiles of species produced during a cyclic voltammetric scan. It has been tested in situ and in resolved time, by probing the reduction of tetracyanoquinodimethane (TCNQ) on its first and second electrochemical wave. Besides the establishment of the method, the individual concentrations of TCNQ, of the monoanion and of the dianion were monitored at distinct infrared frequencies and the time derivatives of the concentration profiles were compared to the voltammograms.

First signals of electrochemically oxidized species of TTF and TTM-TTF: A study by in situ spectroelectrochemical FTIR and DFT calculations

A first study by in situ FTIR spectroelectrochemistry of TTF and TTM-TTF has been undertaken. The oxidation, in this case, is caused only by electrochemistry, which constitutes a clear advantage over chemical oxidation since no side products are present in the solution. In this context, we obtained the signals of neutral, radical cation, and dication species of TTF and TTM-TTF. The experimental conditions were chosen in order to avoid the possible formation of π-dimer species and to obtain a satisfactory signal-to-noise ratio. A weak signal was detected for TTF and a stronger one for TTM-TTF. The changes induced by the oxidation process in the IR spectra of TTF and TTM-TTF have been analyzed with the aid of B3P86/6-31G** density functional theory (DFT) calculations, which allows for a comprehensive assignment of the bands observed. DFT calculations show that the IR signal associated to the asymmetric stretching of the lateral CC bonds can be used as a structural signature to identify the different oxidation states of TTF and TTM-TTF. The frequency downshift and the intensity increase undergone by the νasym(CC) vibration upon oxidation are shown to be the key factors to understand the evolution of the IR spectra.

Synthesis of linear oligo-TTFs and their [2]rotaxanes

Two linear oligo-TTFs were synthesised employing a stepwise nstrategy involving two different thiolate protecting groups. These linear nTTFs were incorporated into donor–acceptor rotaxanes with the cyclic nacceptor, cyclobis(paraquat-p-phenylene). Moreover, a prototype rotaxane based on a bis(pyrrolo)-TTF nwas prepared and studied.

Estimation of π–π Electronic Couplings from Current Measurements

The π-π interactions between organic molecules are among the most important parameters for optimizing the transport and optical properties of organic transistors, light-emitting diodes, and (bio-) molecular devices. Despite substantial theoretical progress, direct experimental measurement of the π-π electronic coupling energy parameter t has remained an old challenge due to molecular structural variability and the large number of parameters that affect the charge transport. Here, we propose a study of π-π interactions from electrochemical and current measurements on a large array of ferrocene-thiolated gold nanocrystals. We confirm the theoretical prediction that t can be assessed from a statistical analysis of current histograms. The extracted value of t ≈35 meV is in the expected range based on our density functional theory analysis. Furthermore, the t distribution is not necessarily Gaussian and could be used as an ultrasensitive technique to assess intermolecular distance fluctuation at the subangström level. The present work establishes a direct bridge between quantum chemistry, electrochemistry, organic electronics, and mesoscopic physics, all of which were used to discuss results and perspectives in a quantitative manner.

Electrochemical study of polythionite solutions in HMPA

Abstract Polythionite solutions Li(SO 2 ) n , prepared by chemical reduction of sulfur dioxide by lithium, have been investigated by classical electrochemical techniques (cyclic voltammetry, electrochemical impedance spectroscopy). The study of Li(SO 2 )-hexamethylphosphoramide (HMPA) solutions shows that the dithionite ion S 2 O 4 2− is not electroactive, and confirms that the concentration of SO 2 − is very low in these solutions. The study of Li(SO 2 ) n -HMPA solutions with n > 1 allows to identify two redox couples: SO 2 /SO 2 − , and S 2 O 4 /S 2 O 4 − , and some of the homogeneous chemical reactions coupled to these electron transfers.

Identification of S2O4 in sulfur dioxide solutions in HMPA

The characteristics of the UV/VIS absorption spectra of SO2 solutions in HMPA have been accurately determined. The decomposition of the spectra by a nonlinear least-squares method shows the existence of two bands. The analysis of the absorbance variations of each band versus the total SO2 concentration shows that SO2 is in equilibrium with the dimer species S2O4. Quantitative analysis of these variations allows us to determine the equilibrium constant of the dissociation of S2O4 (into two SO2 units): K=(1.3±0.4)×10-2 mol dm-3. The influence of the addition of a supporting electrolyte has been examined: it shifts the equilibrium towards SO2, as expected for equilibria involving neutral species. The existence of S2O4 is also supported by the Raman spectra of SO2–HMPA solutions.These results have been correlated with an electrochemical study; the S2O4 reduction is observed as a prewave in the cyclic voltammograms at a potential about 150 mV higher than that of the reduction of SO2. This is also indicated by the difference of the electron affinity of these two species, which have been computed here. Moreover, the intensity of the prewave is related to the concentration of S2O4 in the solution. This confirms both the existence of this species and the attribution of the prewave to its reduction. Identification de S2O4 dans les solutions de dioxyde de soufre dans HMPA. Les caracte′ristiques des spectres dabsorption UV/VIS des solutions de SO2 dans HMPA ont e′te′ de′termine′es avec pre′cision. La de′composition des spectres par une me′thode de moindres carre′s non line′aires indique lexistence de deux bandes dabsorption. Le′tude des variations dabsorbance de chaque bande avec la concentration totale en SO2 montre que SO2 est en e′quilibre avec une espece dimere S2O4. Lanalyse quantitative de ces variations permet de de′terminer la constante de′quilibre de la dissociation de S2O4 (en 2 SO2): K=(1.3±0.4)×10-2 mol dm-3. Linfluence de lajout dun e′lectrolyte support a e′te′ examine′e: le′quilibre se de′place vers SO2, comme pre′vu pour les e′quilibres faisant intervenir des especes neutres. Lexistence de S2O4 a e′galement e′te′ mise en e′vidence par spectroscopie Raman des solutions SO2–HMPA.Ces re′sultats ont e′te′ corre′le′s avec le′tude e′lectrochimique: la re′duction de S2O4 est observe′e sous la forme dune pre′vague sur les voltammogrammes, a un potentiel supe′rieur denviron 150 mV par rapport a la re′duction de SO2. Le calcul de laffinite′ e′lectronique de ces deux especes indique e′galement une re′duction le′gerement plus facile pour S2O4. De plus, lintensite′ de la pre′vague est lie′e a la concentration de S2O4 dans la solution. Ceci confirme a la fois lexistence de cette espece et lattribution de la pre′vague a sa re′duction.

A spectroelectrochemical study of the reduction of a Schiff base cryptand

The electrochemical reduction of a bicyclic hexaimino Schiff base cryptand 1 (N[(CH2)2N-CH-meta-C6H4-CH=N(CH2)2]3N) and that of one of its strands 2 ((CH3)2CH-N=CH-meta-C6H4-CH=N-CH(CH3)2) has been studied by visible and near infrared in-situ spectroelectrochemical techniques. These results are in good agreement with those obtained using alkali metals, but in this case the effect of the formation of ion pairs is minimized through the use of tetrabutylammonium cations. It is confirmed that 1- and 1= have the same visible and near IR spectrum. The spectrum of the products of the electrochemical reduction of 2 is similar to those of 1- or 1=.

(TTF)2[TTF(CO2H)2(CO2)2]: a wholly TTF material containing TTF radical cations and TTF derived anions

Electrooxidation of tetrathiafulvalene (TTF) carried out in the presence of (Bu4N)2TTF(CO2H)2(CO2)2 as supporting electrolyte affords wholly TTF organic materials in which TTF cations are associated with TTF(CO2H)2(CO2−)2 as counteranions.

Aza-Analogues of Extended Tetrathiafulvalenes

Highly extended sulfur-rich tetrathiafulvalene derivatives are known to lead to original two-dimensional associations with noticeable electrochemical features. Different conjugated spacers have been introduced to separate two (or more) 1,3-dithiole units, polyenic chains being the most used. In this context, a literature search revealed only two examples of 1,3-dithiole derivatives containing azine spacers.1 In this context, we have prepared compounds 1 and 2a–d (Figure 1), by reaction of 1,3-dithiole-derived aldehydes or 1,3dithiolium salts with hitherto undescribed 1,3-dithiole(diethoxyphosphinyl)hydrazones.