M. Angélica del Valle

Redox chemistry and spectroscopy of 2-mercaptobenzoic acid and its manganese(II) and (III) complexes in dimethylsulphoxide

Abstract The redox behaviour of the ligand 2-mercaptobenzoic acid has been studied in dimethylsulphoxide. The combination of the dianion of the ligand with manganese(II) in a 1:2 mole ratio forms a colourless complex whose stability constant has been determined polarographically as 2.8 x 10 9 M −2 . Oxidation of this complex at −0.45 V vs SCE yields a red complex of manganese(III) whose stoichiometry is also 1:2. The complexes have been characterized by cyclic voltammetry, controlled-potential electrolysis, optical spectroscopy and magnetic susceptibility measurements. The formation constant of the [Mn III (2-MBA) 2 ] − complex has been evaluated spectrophotometrically as 9.3 x 10 7 M −2 . These results may be relevant to the development of an adequate model for the active centre of acid phosphatases.

Nucleation and growth mechanism of polycarbazole deposited by electrochemistry

Polycarbazole (PCz) thin films have been deposited by electro-oxidation of carbazole in LiClO4 + anhydrous acetonitrile onto SnO2 coated glass substrates, by potentiostatic method and the nucleation and growth mechanism (NGM) were studied. The obtained current time transients (j-t) were fitted using a mathematical equation with three contributions: instantaneous nucleation with two-dimensional (IN2D) or three-dimensional (IN3D) growth, and also a progressive nucleation with three dimensional (PN3D) growth. At the beginning, the IN2D contribution is predominant but, quickly the IN3D processes become more important. At a deposition time t > 17 s the IN3D corresponds to 80 % of the total current. The visualization by scanning electron microscopy of the surface morphology of the PCz films is in agreement with the NGM proposed by the mathematical method. The film covers very rapidly the SnO2 under layer related to the 2D process, even if some heterogeneities randomly distributed in the films, issued from the 3D processes, are also visible. After two minutes of deposition, the roughness of the whole surface of the films corroborates the 3D processes domination. Therefore, the information directly obtained from the (j-t) transients is a suitable and very useful tool to predict the working conditions in order to control the type of morphology of the film prepared by electropolymerization.

Redox chemistry of 3,4-dihydroxy-2-benzoic acids, its oxidation products and their interaction with manganese(II) and manganese(III)

Abstract The redox chemistry of the ligand 3,4-dihydroxybenzoic acid (3,4-DHBA) has been studied in dimethylsulphoxide and the conditions for the formation of the corresponding semiquinone and quinone have been determined. The manganese(II) and manganese(III) complexes with the different forms of this ligand have been characterized by cyclic voltammetry, UV-vis spectroscopy and magnetic susceptibility measurements. Neither the neutral nor the monoanionic form of the ligand show the formation of complexes with the metal ions. The dianion can be oxidized electrochemically to the corresponding semiquinone, at a more positive potential due to the presence of a deactivating group, forming a “peroxo-type” dimer. The subsequent oxidation of this species generates the corresponding quinone. The dianion and the semiquinone forms of the ligand produce manganese(II) and manganese(III) complexes with 1:2 stoichiometry, which is favoured by the ortho position of the hydroxide groups. These results may be relevant for the development of models for biological systems.

OLIGOMER CHAIN LENGTH EFFECT ON THE NUCLEATION AND GROWTH MECHANISMS (NGM) OF POLYTHIOPHENE

In this research a comparative study of the starting unit chain length effect on the electropolymerization of thiophene or its oligomers was carried out considering mono (1Th), bi (2Th), ter (3Th), tetra (4Th) and sexi (6Th)-thiophene as starting units. The deconvolved transient allows stating that the growth of polythiophene (PTh) employing each of the starting units have the same predominant contribution to the nucleation and growth mechanism (NGM). The others contributions disappear as the length of the starting unit chain increase. The results were validated by scanning electron microscopy (SEM) of PTh deposits obtained onto SnO2 coated glass, following a potential pulse program. Besides of corroborating the electropolymerization model, this study suggested the possibility of designing and performing suitable experiments leading to the attainment of electro-deposited conductive polymers bearing a desired morphology, appropriated for prospective applications.

Voltammetric and spectroscopic study of the manganese-quinizarine-quinaldic acid mixed- ligand complex in dimethylsulfoxide

Abstract The combination of manganese(II) with quinizarine and the monoanion of quinaldic acid in dimethylsulfoxide yields a soluble deep-blue complex whose Mn II : Qz 2− : AcQ − stoichiometry is 1 : 1 : 2. This complex is oxidized in two steps each involving one equivalent of charge per metal ion as indicated by controlled-potential electrolyses at +0.30 V vs S.C.E. and +0.45 V vs S.C.E. The association between the metal ion and the ligands prevails and the oxidations produce finally a dark-red complex of the same stoichiometry as the original deep-blue species. In the latter the metal is in the oxidation state +3 and the quinizarine dianion has been oxidized to the semiquinone form. The monoanion of quinaldic acid is unchanged. The combination of the semiquinone of quinizarine with Mn II and quinaldic acid yields a species in which the metal ion presents magnetic characteristic of manganese (III) and the semiquinone has been reduced to the dianion of quinizarine, indicating that intramolecular charge-transfer has occurred. The voltammetric behaviour of the Mn III -semiquinone-quinaldic acid complex and its electrochemical reduction indicate that it is binuclear which upon reduction at −0.20 V vs S.C.E. generates a Mn II Mn II mixed-valence species. The latter is in turn reduced at −0.95 V vs S.C.E. producing probably a mononuclear species of Mn II .

Redox chemistry and spectroscopy of the manganese-quinizarine-thiosalicylic acid mixed-ligand complex in aprotic medium

Abstract The addition of the monoanion of thiosalicylic acid to the insoluble compound formed between manganese(II) and the dianion of quinizarine forms a very intense blue soluble new complex. The stoichiometry of this complex is Mn:Qz:2-MBA = 1:1:2, in the presence of 4 equivalents of base in order to neutralize the two hydroxylic protons of quinizarine and the carboxylic proton of each equivalent of 2-MBA. This mixed-ligand complex is oxidized in several steps and a total of 4 equivalents of charge per metal ion are transferred in this process. The complete oxidation at +0.90 V vs S.C.E. produces the destruction of this mixed-ligand complex. However, it is possible to oxidize sequentially at +0.35 and +0.65 V vs S.C.E. and the association between the metal ion and the ligands prevails. These two oxidations involve 1 equivalent of charge each and lead to the formation of a dark red complex. The stoichiometry of this complex is maintained with respect to the original blue compound and it is proposed that the metal ion is in the +3 oxidation state, the quinizarine has been oxidized to its semi-quinonic form and the monoanion of thiosalicylic acid has not changed. The final process at +0.90 V vs S.C.E. corresponds to the oxidation of the monoanion of the latter which leads to the decomposition of the complex. It is concluded that the mixed-ligand complex formed after the two-electron oxidation process can be considered as a very good model for biological systems in which it is known that manganese plays an important role as a catalyst for redox processes and ligands such as those studied here are present.

Electrochemical synthesis of poly(3′-alkyl-terthiophenes). Characterization and applications

A new series of polymers obtained from 3′-alkyl-terthiophene monomers have been electropolymerized aimed at using them as raw materials for the development of electronic devices, e.g., solar cell and organic light-emitting diodes, among others. The polymers were characterized by infrared and UV–Vis spectroscopy and cyclic voltammetry. Cyclic voltammetry results revealed that during polymerization both terthiophene system and substituent groups are oxidized, but these processes are reversible. The products were tested in solar cells and the maximum yield obtained was 0.01%.

Poly(3,5-dichloroaniline) doped with different sulfonic acids

Poly(3,5‐dichloroaniline) was prepared by chemical oxidation in the presence of various sulfonic acids as doping agent, using potassium permanganate as oxidant. 1‐Naphtalene sulfonic acid, 2‐naphatalene sulphonic acid, 1,5‐naphtalene disulfonic acid, and p‐toluenesulfonic acid were the acids of choice. Infrared and UV‐Vis spectroscopy, utilized to characterize the polymers, revealed that the compounds exist in the emeraldine (conductive) oxidation state. The level of doping, conductivity, and morphology were determined as well. The presence of a sulfonic acid produces a morphological change, from granular to microtubule structures, which is responsible for the strong increase in the conductivity of the polymer.

PREPARATION OF PALLADIUM THIN FILMS AND THEIR USEFULNESS AS MODIFIED ELECTRODES

Se han obtenido, fotoquimicamente, peliculas delgadas nanoestructuradas de paladio, por irradiacion UV directa (254nm) de una pelicula amorfa de Pd[C8H17COCHCOC4H 9]2 depositada sobre Si(100) y sobre vidrio ITO, mediante la tecnica de spin-coating. La fotolisis de la pelicula de complejo b-dicetonato de paladio, por luz UV, conduce a la perdida de los ligandos de la esfera de coordinacion. Los derivados de las acetilacetonas fueron escogidos por su fotoquimica en estado solido, ya que la presencia de sustituyentes ramificados hidrocarbonados disminuye las interacciones moleculares y, mediante spin-coating, conduce a la formacion de peliculas de alta calidad. El producto de la fotolisis fue analizado por difraccion de rayos-X (XRD), microscoscopia electronica de barrido (SEM) y microanalisis. Posteriormente, la pelicula depositada sobre vidrio ITO se estudio eletroquimicamente, aplicando ciclos triangulares sucesivos a potenciales entre 0,00 y 1,30 V vs ECS por 15 minutos, en una disolucion electrolitica de H2SO4 0,50 M. El electrodo modificado presenta un perfil similar al de una lamina de paladio, sin embargo, es posible observar en el sistema modificado un desplazamiento de los potenciales de pico, de aproximadamente 0,04 V hacia valores menos positivos, lo cual es indicativo de un posible efecto catalitico. En base a ello, el sistema modificado ITO/Pd fue usado como sustrato electrodico para la polimerizacion de anilina en H2SO4 0,10 M, obteniendose un deposito polimerico, a potenciales menores que los requeridos sobre la lamina de paladio, indicando que el metodo descrito puede consituir una interesante herramienta para la preparacion de electrodos modificados, que podrian ser ventajosamente usados en la preparacion de materiales polimericos

Voltammetric study of the redox chemistry of 2,3-dihydroxy-quinoxaline and its zinc complexes in non-aqueous medium

Abstract The electrochemical behaviours of 2,3-dihydroxy-quinoxaline (2,3-DH 2 Qx) and the complex species formed with zinc(II) have been studied in non-aqueous media. Using dimethylsulphoxide or N,N -dimethylformamide as solvents the monoanion of the ligand is obtained quantitatively after reduction of one of the hydroxylic protons. The reduction potential for the hydroxylic proton is −1.93 V vs S.C.E. in DMSO and −1.68 V vs S.C.E. in DMF, reflecting the different donor number of the solvents. If acetonitrile is used, the simultaneous reduction of both hydroxylic protons is observed. The oxidation to the semiquinone is observed at −0.12 V vs S.C.E. in DMSO and DMF whereas it appears at −0.02 V vs S.C.E. in acetonitrile. Neither the protonated ligand nor its quinonic form exhibit the formation of stable complexes with zinc(II) whereas the dianionic species produces two coordination compounds having M : L stoichiometric ratios of 1:1 and 1:2. The semiquinone form of the ligand by itself shows the tendency to polymerize on the surface of the electrode and it is not stabilized in the presence of this metal ion. A neutral insoluble compound that passivates the electrode is formed when the semiquinone and this metal ion interact. It was not possible to study this complex in detail but the electrochemical evidence indicates that it has formed on the surface of the electrode.