Aniline-1,4-benzoquinone as a model system for the characterization of products from aniline oligomerization in low acidic media

Mediation of water-soluble oligoaniline by phenol in the aniline-persulfate system under alkaline conditions

Although synthesis of oligoaniline (OANI) by persulfate and aniline has been investigated in the recent years, the impact of phenol on the synthesized soluble OANI is still not clear. In this study, our results indicate that phenol and pH mediate the production of the blue water-soluble OANI (OANI_blue) in the reaction between sodium persulfate (SPS) and aniline under alkaline conditions, and the yields of OANI_blue increase with increasing concentrations of phenol and pH values. Quenching experiments rule out the contributions of SO₄˙^- and ˙OH to aniline oxidation and imply that the non-radical activation of SPS is an important pathway in the formation of OANI_blue. MALDI-TOF-MS analysis indicates that phenol apparently inhibits the polymerization degree of aniline in that the molecular weights of OANI_blue gradually decrease from 1586.4 to 684.6 when phenol is increased from 0 to 2.0 mM. FTIR and Raman analyses confirm the structure of aniline oligomers in OANI_blue and indicate that phenol inhibits the phenazine-like structure in OANI_blue and facilitates the transformation of benzenoid rings to quinoid rings in the oxidation products. However, simultaneous activation of SPS by phenol and aniline is likely to occur in the reaction system with the formation of PhNH˙, as indicated by DFT calculations. The high scavenging reactivity of phenol towards both PhNH₂˙⁺ and PhNH˙ implies that PhNH₂˙⁺ and PhNH˙ are not the intermediates in the formation of OANI_blue. DFT calculations also reveal that apart from the one-electron transfer pathway between aniline and SPS, the two-electron transfer pathway is also likely to occur in the presence of phenol, resulting in the formation of PhNH⁺/PhN˙˙ without producing PhNH₂˙⁺ and PhNH˙. The produced PhNH⁺/PhN˙˙ intermediates then couple with aniline, PhNH⁺, aminophenyl sulfate and its hydrolysate to form dimers, trimers, oligomers, and eventually OANI_blue. This study not only describes a novel method to prepare water-soluble OANI, but also gives new insight on the importance of phenol in the production of OANI_blue.

Peculiarities of Oxidative Polymerization of Diarylaminodichlorobenzoquinones

New oxidative polymerization monomers-diarylaminodichlorobenzoquinones were synthesised by alkylating aniline, m-phenylenediamine and methanilic acid with chloranil. Oxidative polymerization of diarylaminodichlorobenzoquinones was studied for the first time in relation to the concentration of the monomer, acid, and oxidant/monomer ratio. It was found that the synthesized monomers are highly active in the polymerization reaction, and the oxidation rate grows with the increase in the acid concentration. Only one arylamine group is involved in the polymerization reaction. The optimal oxidant/monomer ratio is stoichiometric for one arylamine group, despite the bifunctionality of the monomers. It was shown that the type of the substituent in the aniline ring (electron donor or electron acceptor) determines the growth of the polymer chain and the structure of the resulting conjugated polymers. A mechanism for the formation of active polymerization centers for diarylaminodichlorobenzoquinones was proposed. FTIR-, NMR-, X-ray photoelectron spectroscopy, and SEM were used to identify the structure of the synthesized monomers and polymers. The obtained polymers have an amorphous structure and a loose globular morphology. The frequency dependence of the electrical conductivity was studied.

Conducting polyaniline prepared in the solutions of formic acid: Does functionalization with carboxyl groups occur?

Polyaniline is a conducting polymer with an application potential in the field of biomedical engineering. By employing FTIR spectroscopy and conductivity measurements, it has been shown that the oxidation at stoichiometric peroxydisulfate-to-aniline mole ratio 1.25 in the solutions of formic acid in the range 0-10 M provides samples of a moderate conductivity of the orders 0.01-0.1 S cm^-1. They consist of polyaniline and aniline oligomers as typical of the aniline oxidation in weak acids. The detailed investigation of the infrared spectra indicates a partial ring-carboxylation of polyaniline at high acid concentrations. The extent of structural defects is higher for a series prepared at over-stoichiometric peroxydisulfate-to-aniline mole ratio 2.5, which provided only non-conducting samples. The reference sample series represented by poly(aniline-co-o-aminobenzoic acid) was also prepared and is used in the discussion of the infrared spectra.

DFT Studies of Distinct Anilines with p-Hydroxycinnamic Acids for Antioxidant Profile

BACKGROUND

Life style and jobs in current situations have generated increased free radicals such as hydroxyl (OH•) and superoxide (O2•) radicals, thereby increasing stress in humans. Interest in search of antioxidants that trap these free radicals has increased to relieve stress. β-carotene (provitamin A), ascorbic acid (vitamin C), tocopherol or vitamin E, Trolox; butyl hydroxy toluene and phenolic compounds are the well-known antioxidants. Several methods evaluate the antioxidant property existing in natural substances (medicinal plants and agri-food products) and synthetic compounds (2-methyl-3- (pyrrolidin-2-ylideneamino) quinazolin-4 (3H) -one and 3,3'- (1,4- phenylenebis (methanylylidene)) bis (azanylylidene) (2-methyl-quinazolin-4 (3H) -one).

OBJECTIVE

The objective of this study is to focus on complexes with p-hydroxycinnamic acids to trap free radicals in a greener way.

METHODS

Spectroscopic shifts and structural studies were employed to attribute electronic properties responsible for antioxidant profile. Spectroscopic shifts in wavenumbers were attributed with Fourier Transform Infrared Spectra (FTIR) and Fourier Transform Raman spectra (FT Raman Spectra). Structural studies were performed with Gaussian package, electron density method the B3LYP method, basis set 6-31(d) for attributing electronic properties responsible for antioxidant profile.

RESULTS

Interpretation of FTIR spectra revealed spectroscopic shifts in wavenumbers in all the complexes responsible for bonding. Further, studies confirmed the formation of complex with reduced intensities in Raman spectra. Computational studies revealed enhancement in molecular and electronic properties responsible for antioxidant power.

CONCLUSION

Studies revealed that complex with p-nitroaniline contribute to greater acceptor and donor power responsible for antioxidant power. These higher powers suggest the best antiradicals to trap free radicals.

The First Stages of Chemical and Electrochemical Aniline Oxidation—Spectroscopic Comparative Study

There are several types of aniline oligomers that can be formed in the early stages of aniline oxidation: linear oligomers with repeating units joined in para positions, and various branched and polycyclic oligomers, being the two most important groups. The fraction of these different oligomeric groups depends upon the reaction conditions of aniline oxidation. The aim of this study was to analyze the first products of the chemical and electrochemical oxidation of aniline at the (starting) pH 1 and 7, in order to specify the conditions of the formation of phenazine-like oligomers, and to test the theory that they have an important role in polyaniline film formation. We have confirmed that phenazine-like oligomers do not form at pH 1, neither in the chemical nor the electrochemical oxidation of aniline; however, they form in both chemical and electrochemical oxidation of aniline at pH 7. Phenazine-like oligomers are thus definitely not necessary intermediates for PANI film formation, not even in the chemical polymerization of aniline. Finally, the redox behavior of phenazine-like oligomers was demonstrated in a medium at pH 1.

Simultaneous Quantification of Electron Transfer by Carbon Matrices and Functional Groups in Pyrogenic Carbon

Pyrogenic carbon contains redox-active functional groups and polyaromatic carbon matrices that are both capable of transferring electrons. Several techniques have been explored to characterize the individual electron transfer process of either functional groups or carbon matrices individually. However, simultaneous analysis of both processes remains challenging. Using an approach that employs a four-electrode configuration and dual-interface electron transfer detection, we distinguished the electron transfer by functional groups from the electron transfer by carbon matrices and simultaneously quantified their relative contribution to the total electron transfer to and from pyrogenic carbon. Results show that at low to intermediate pyrolysis temperatures (400-500 °C), redox cycling of functional groups is the major mechanism with a contribution of 100-78% to the total electron transfer; whereas at high temperatures (650-800 °C), direct electron transfer of carbon matrices dominates electron transfer with a contribution of 87-100%. Spectroscopic and diffraction analyses of pyrogenic carbon support the electrochemical measurements by showing a molecular-level structural transition from an enrichment in functional groups to an enrichment in nanosized graphene domains with increasing pyrolysis temperatures. The method described in this study provides a new analytical approach to separately quantify the relative importance of different electron transfer pathways in natural pyrogenic carbon and has potential applications for engineered carbon materials such as graphene oxides.

Spectroscopic study of the highly homogeneous polyaniline film formation on gold support

The oxidation of aniline with ammonium peroxydisulfate in the aqueous solution of acetic acid has two subsequent phases: the oxidation of the neutral aniline molecules at low acidity, which was followed by the oxidation of the anilinium cations after the acidity became higher. The final polyaniline film deposited on immersed surfaces is usually contaminated with semi-crystalline oligomers which precipitated during induction period from the reaction medium. To obtain a homogeneous film, which is important in the fabrication of many molecular electronic devices, we have studied the course of aniline oxidation in a view of new experimental evidence. In the unique series of experiments, the silicon or gold supports have been immersed in the reaction mixture at crucial stages of oxidation reaction, and the deposits at the end of the reaction were analyzed. The growth of a highly homogenous film on the gold-coated glass substrate immersed in the reaction mixture at the end of the polymerization period has been observed. The molecular structure of the products was monitored with UV-visible, infrared, and Raman spectroscopies. The possible mechanism of the film formation and the molecular mechanism of the surface interaction of chemisorbed aniline oligomers with gold support are proposed.

In Situ Infrared Spectroscopy of Oligoaniline Intermediates Created under Alkaline Conditions

The progress of the oxidation of aniline with ammonium peroxydisulfate in an alkaline aqueous medium has been monitored in situ by attenuated total reflection (ATR) Fourier transform infrared spectroscopy. The growth of the microspheres and of the film at the ATR crystal surface, as well as the changes proceeding in the surrounding aqueous medium, are reflected in the spectra. The evolution of the spectra and the changes in the molecular structure occurring during aniline oxidation in alkaline medium are discussed with the help of differential spectra. Several processes connected with the various stages of aniline oxidation were distinguished. The progress of hydrolysis of the aniline in water and further an oxidation of aminophenol to benzoquinone imines in the presence of peroxydisulfate in alkaline medium have been detected in the spectra in real time. The precipitated solid oxidation product was analyzed by mass spectrometry. It is composed of oligomers, mainly trimers to octamers, of various molecular structures incorporating in addition to aniline constitutional units also p-benzoquinone or p-benzoquinoneimine moieties.

Detection of aniline oligomers on polyaniline-gold interface using resonance Raman scattering

In situ deposited conducting polyaniline films prepared by the oxidation of aniline with ammonium peroxydisulfate in aqueous media of various acidities on gold and silicon supports were characterized by Raman spectroscopy. Enhanced Raman bands were found in the spectra of polyaniline films produced in the solutions of weak acids or in water on gold surface. These bands were weak for the films prepared in solutions of a strong acid on a gold support. The same bands are present in the Raman spectra of the reaction intermediates deposited during aniline oxidation in water or aqueous solutions of weak or strong acids on silicon removed from the reaction mixture at the beginning of the reaction. Such films are formed by aniline oligomers adsorbed on the surface. They were detected on the polyaniline-gold interface using resonance Raman scattering on the final films deposited on gold. The surface resonance Raman spectroscopy of the monolayer of oligomers found in the bulk polyaniline film makes this method advantageous in surface science, with many applications in electrochemistry, catalysis, and biophysical, polymer, or analytical chemistry.