Pyrrole Oligomers Research Articles

Influence of ion-exchange on the electrochemical properties of polypyrrole films

Cyclic voltammetry, X-ray microanalysis (EDS), FTIR, and AFM were applied for the systematic examination of the influence of self-exchange and potential stimulated ion-exchange on the electrochemical properties of polypyrrole (PPy) films prepared in the presence of benzene sulfonate (BS), para-toluene sulfonate (pTS), and naphthalene-1, 5-disulfonate (NDS) anions. All electrochemical experiments were carried out in aqueous solutions. It was established that small inorganic anions, like NO3−, Cl−, ClO4− can relatively easily replace an essential part of the sulfonates in PPy/BS and PPy/pTS films simply during a 10min polarization at 0.4V. A single potentiodynamic cycle increases the completeness of the exchange of the anions remarkably. The EDS analysis carried out before and after the anion exchange showed that 5–15% of the sulfonate anions still remain in the PPy films. The ability of the double-charged SO42− anions to replace sulfonate anions is even more pronounced as the exchange process does not need additional electrochemical activation. The quantum chemical model calculations have shown that the interaction of the sulfate anions with charged pyrrole oligomers is essentially stronger compared to other anions under the study.The redox capacitance and the corresponding electrochemical doping level of the redoped PPy films are remarkably higher than those before ion-exchange, showing that the PPy matrix formed in the presence of sulfonate anions does not collapse after ion-exchange and it creates favorable conditions for increased mobility of inorganic anions.

Encapsulation of Nile Red in polypyrrole microvessels

Polymeric hollow structures are a popular topic in modern chemistry and materials science because of their potential importance in biomedical applications such as smart drug delivery systems. We report on the preparation of polypyrrole microvessels that are capable of encapsulating Nile Red, a solvatochromic dye that serves as a model of lipophilic drugs. Using a range of modern physicochemical methods including electron and optical microscopy, vibrational and fluorescence spectroscopy, and zeta potential measurements, we show that the dye can be encapsulated effectively through chemical polymerization of pyrrole onto p-xylene droplets that contain Nile Red. The fluorophore resides within the organic core of the microvessels but interacts with the polymer wall material (or pyrrole oligomers formed during polymerization) which is manifested as slower rotational dynamics and shorter fluorescence lifetime of the dye in comparison to bulk solvent. We also demonstrate release of the dye to the surrounding solution and show that this process is governed primarily by the miscibility of the organic core (p-xylene) with the external phase (solvent).

Kinetics and mechanism of pyrrole chemical polymerization

Proton NMR spectroscopy was used to study the kinetics of the chemical polymerization of pyrrole in water using ferric chloride as an oxidant. A kinetics model was proposed in which the consumption of pyrrole is due to a fast oxidation reaction, and a relatively faster reaction with the oxidized oligomers of pyrrole. This suggests the commonly used mechanism for polymerization of pyrrole to polypyrrole, which is an important conductive polymer, is not correct for polymerization in water. Instead the polymerization is based on a mechanism where the pyrrole monomer is attacked by a radical cation, leading directly to the pyrrole reacting with the oxidized oligomer. We conducted kinetics studies as a function of temperature and determined the activation energy, activation entropy and enthalpy for each step of the polymerization process for the first time. The activation enthalpy and activation entropy are respectively 77,091±17J and 24.25±0.06J/K for the initial step, and 70,971±14J and 24.25±0.06J/K for the propagation step.

Facile Synthesis of Alternating Benzene–Pyrrole Oligomers by Cyclization of Propargylic Dithioacetals and Imines and Their Fluorescent Properties

AbstractAlternating benzene–heteroaryl oligomers possess fascinating optoelectronic properties and a wide range of applications. This article presents the facile synthesis of benzene–pyrrole oligomers with diverse functional groups and the elongated alternating heterocycle–benzene–pyrrole oligoaryls. The syntheses are based on a one‐pot, three‐step reaction of propargylic dithioacetals and imines. The subtle influence of functional groups, such as ether, ester, hydroxy, and dithiacetal groups on the peripheral benzene rings, on the reaction, has been explored and the reaction conditions were finely changed accordingly to achieve reasonable yields. The elongation could be accomplished by using a similar procedure. The fluorescent properties of original and elongated alternating benzene–pyrrole oligomers were determined. Theoretical calculations preliminarily reveal that the fluorescent intensity is closely related to the molecular geometry and to the HOMO/LUMO energy difference in the ground state.

Electronic Structures of Oligomers of Pyrrole

The electronic structures of oligomers of pyrrole have been calculated using the extended Hückel tight-binding program. The energy diagrams are shown for oligomers as the number of the repeating unit in the oligomers increases from 1 to 16. The relationship between the energy gap of the oligomers and the number of the repeating unit in the oligomers is established. Blue-green photoluminescence and electroluminescence from one-dimensional polypyrrole are predicted.

Synthesis and characterization of poly[( R)-(−) and ( S)-(+)-3-(1′-pyrrolyl)propyl-N-(3″,5″-dinitrobenzoyl)-α-phenylglycinate]s as chiral oligomers of pyrrole

Chiral polypyrrole oligomers were prepared from ( R)-(−) ( 1) and ( S)-(+)-3-(1′-pyrrolyl)propyl-N-(3″,5″-dinitrobenzoyl)-α-phenylglycinate ( 2) by oxidative polymerization in the presence of FeCl 3 as an oxidant. The new polymers were characterized by 1H NMR, FTIR and UV–vis ( λ máx = 251 nm). TGA analyses of the corresponding polymers, poly1 and poly2, demonstrate an excellent thermal stability up to 204 °C, followed by two consecutive weight losses at higher temperatures. Relative number-average molecular weights of about 1.4 × 10 3 and 2.3 × 10 3, for poly1 and poly2, respectively, were measured by SEC, corresponding to oligomeric chains. Specific optical rotation measurements ( c 0.1, THF) for poly1 [α] D 20 = −44 and poly2 ([α] D 20 = +44) suggest that chirality is retained during the polymerization. Transversal section analysis of SEM images indicates the materials isolated right after the synthesis is highly porous.

Synthesis and Chain‐Length Dependence of the Electronic Properties of π‐Conjugated Dithieno[3,2‐b:2′,3′‐d]pyrrole (DTP) Oligomers

N-(2-ethylhexyl)dithieno[3,2-b:2',3'-d]pyrrole has been prepared and its dimer and trimer have been synthesized by Stille coupling. The electrochemical and optical properties of these compounds have been investigated by cyclic voltammetry, UV-Vis, and fluorescence emission spectroscopies. The obtained results show that these strongly luminescent compounds can be oxidized into stable cation radical and dication state. The analysis of the chain length dependence of the electronic properties indicates that the predicted bandgap of an ideal polymer chain should be considerably smaller than the experimental results reported until now. This difference is discussed in terms of reactivity of the dithieno[3,2-b:2',3'-d]pyrrole (DTP) unit.

Monte Carlo study of oligopyrroles in condensed phases

A classical model potential to simulate pyrrole oligomers in condensed phases is developed in this work. The new potential contains ten parameters that are optimized on a database of energy points calculated within the density functional theory approach. Based on this potential the condensed phase of systems composed of pyrrole oligomers with 4 and 12 monomers is studied as a function of system density. The binding energy, end-to-end distance, radius of gyration, vector and orientational order parameters, and pair correlation functions are reported at T=300 K. The mechanical equilibrium density is determined for both systems. The bulk modulus is reported at these densities, showing that systems composed of short oligomers are softer than systems containing longer oligomers. Analysis of pair correlation functions and order parameters indicates that at equilibrium the system of short oligomers has characteristics of a liquid while the system of longer oligomers shows a chain stacking trend.

Development of Self-Organizing, Self-Directing Molecular Nanowires: Synthesis and Characterization of Conjoined DNA−2,5-Bis(2-thienyl)pyrrole Oligomers

Specifically designed conducting polymers were prepared from monomers that are covalently linked to duplex DNA. These materials combine the self-assembly properties of DNA with those of conducting ...

Effects of Perfluorination on Thiophene and Pyrrole Oligomers

The effect of perfluorination on thiophene and pyrrole oligomers in neutral, cationic, and anionic states was investigated with density functional theory at the (TD)B3P86-30%/6-31G* level. For the title compounds fluorination leads to planarization. For pyrroles a band gap reduction of 0.58 eV results, as unsubstituted pyrroles are nonplanar and disordered in the solid state. For thiophene the band gap is slightly increased as long thiophene oligomers are almost planar. Ionization energies and electron affinities increase upon fluorination by 0.65 and 0.60 eV for polythiophene and by 0.45 and 0.90 eV for polypyrrole. Conduction band widths increase by 0.5 for polythiophene and by 0.7 eV for polypyrrole. Spectra of charged (doped) forms are almost identical to those of the parent systems. Like parent systems, fluorinated oligomers with chain lengths of more than six rings develop a third UV absorption that increases in strength and decreases in energy upon chain length increase.

Charge transport studies in fluorene – Dithieno[3,2- b:2′,3′- d]pyrrole oligomer using time-of-flight photoconductivity method

Charge mobility characteristics of a newly synthesised 2,6-bis[2-(9,9-dihexyl-9H-fluorene)]- N-(4-hexylphenyl)-dithieno[3,2- b:2′,3′- d]pyrrole oligomer ( DTP-FLU) was studied as a function of electric field and temperature using time-of-flight photoconductivity measurement. It is found that the DTP-FLU oligomer is a hole transporting material with a hole mobility of 7.7 × 10 −6 cm 2/Vs at an applied electric field of 2.9 × 10 5 V/cm at 298 K. The dependence of hole mobility with applied electric field and temperature is studied in detail by analyzing the experimental results using the Bassler’s Gaussian disorder model and Correlated disorder model. The energetic disorder parameter ( σ) = 100 meV, mobility pre-factor ( μ ∞) = 6.1 × 10 −4 cm 2/Vs and positional disorder parameter ( Σ) = 2.4 were extracted using Gaussian disorder model. The film morphology and photophysical properties of this new oligomer are also studied in detail.

Synthesis of ordered spiral and ring-like polypyrrole nanowires in cetyltrimethylammounium bromide crystalline suspension

Spiral dislocation morphology on the surface of cetyltrimethylammounium bromide (CTAB) crystallites has been discovered for the first time. By addition of ammonium peroxydisulfate into CTAB crystalline suspension, a drastic change in the morphology of result crystallites is observed from spiral to two dimensional (2D) islands. The spiral and 2D-island structures of these crystallites could be use as the templates for the synthesis of spiral and ring-like polypyrrole (PPy) nanowires, respectively, via direct chemical oxidative polymerization of pyrrole due to the oxidation reaction occurring preferentially at the steps of these crystallites. The mechanism of adsorption of pyrrole oligomers on the steps of these crystallites is proposed for the growth of PPy spiral and ring-like nanowires.

Synthesis of Conducting Polymer Spiral Nanostructures Using a Surfactant Crystallite Template

AbstractThe present work describes, for the first time, the synthesis of spiral nanostructures of conducting polymers by chemical oxidant polymerization using a hydrated surfactant (sodium dodecyl sulfonate (SDSn)) crystallite template. A spiral dislocation structure on the surface of a hydrated SDSn crystallite has been observed and is responsible for the growth of the spiral nanoarchitecture of conducting polymers (polypyrrole, polyaniline). Ammonium peroxydisulfate (APS) has a strong tendency to induce the formation of a spiral dislocation structure of hydrated SDSn crystallites. The mechanism of adsorption of pyrrole or aniline oligomers on the steps of dislocation is proposed for the growth of conducting polymer spirals.magnified image

Quantum-Chemical Calculations of the Electronic Properties of Pyrrole Oligomers

The electrochemical pyrrole polymerization mechanism at initial stages is theoretically studied through sequential monomer units coupling to obtain 2 to 9 oligomer formation. The relevance of these oligomers lies on their solubility in water as well as on the influence in uniformity of the final polymer film. The dependence of the growing chain electronic properties has been studied by the Polarizable Continuum Method (PCM) taking into account the influence of water as solvent effect. It was observed that the |HOMO-LUMO| energy difference, defined as Eg, decreases as the delocalization (resonance) of the π-electrons increases within the 5-member rings, which occur while the length of the oligomer molecule increases in line with literature reported experimental observation. The improvement in electron conduction of the stepwise oligomer product as a function of its chemical structure is also discussed.

Theoretical Modeling of the Doping Process in Polypyrrole by Calculating UV/Vis Absorption Spectra of Neutral and Charged Oligomers

Changes in absorption spectra during doping of oligopyrroles were investigated with time-dependent density functional theory on optimized structures of neutral, singly, and doubly charged pyrrole oligomers with up to 24 rings. In the absence of counterions, defects are delocalized. Counterions induce localization. For dications two polarons on the same chain are preferred over a bipolaron. Intragap absorptions arise in charged species, no matter whether defects are localized or delocalized. Cations and dications give rise to two sub-band transitions. The cation peaks have lower energies than those of dications. The first excitations of cations have lower oscillator strengths than the second; for dications the second peak is weaker than the first. For very long oligomers, the second sub-band absorption vanishes and a third one appears at higher energy. The behavior of pyrrole oligomers is analogous to that of thiophene oligomers. Theoretical UV spectra for cations and dications of short oligomers (six to eight rings) match experimental spectra of polypyrrole at low and at high doping levels, respectively. The error in the theoretical calculations is about 0.4 eV, slightly larger than for thiophene oligomers at the same level of theory.

Toward an Understanding of the Formation of Conducting Polymer Nanofibers

Introducing small amounts of additives into polymerization reactions to produce conducting polymers can have a profound impact on the resulting polymer morphology. When an oligomer such as aniline dimer is added to the polymerization of aniline, the nanofibers produced are longer and less entangled than those typically observed. The addition of aniline dimer can even induce nanofiber formation under synthetic conditions that generally do not favor a nanofibrillar morphology. This finding can be extended to both the synthesis of polythiophene and polypyrrole nanofibers. The traditional oxidative polymerization of thiophene or pyrrole only produces agglomerated particles. However, when minute amounts of thiophene or pyrrole oligomers are added to the reaction, the resulting polymers possess a nanofibrillar morphology. These results reveal important insights into a semirigid rod nucleation phenomenon that has hitherto been little explored. When polyaniline nucleates homogeneously, surface energy requirements necessitate the formation of ordered nuclei which leads to the directional polymerization of aniline. This ultimately leads to the one-dimensional nanofibrillar morphology observed in the final product. The synthetic procedures developed here are simple, scalable, and do not require any templates or other additives that are not inherent to the polymer.

Oxidative polymerization of pyrrole photocatalyzed by TiO2 nanoparticles and interactions in the composites

AbstractOxidative polymerization of pyrrole is initiated by photoexcited TiO2 nanoparticles. Pyrrole oligomers and polypyrrole (PPy) are continuously produced with reaction time. The conversion of pyrrole monomer changes as a function of the concentration of TiO2 nanoparticles. It is found that PPy in the composite has the conjugated structure and is partially oxidized with a formation of positively charged N+. Results from X‐ray photoelectron spectroscopy and Raman analysis consistently indicate that a strong interaction is established between the TiO2 and PPy. According to the results of UV–vis spectroscopy, a mechanism of photocatalytic oxidation is proposed for this polymerization. The interaction between TiO2 and PPy is found to arise from the photocatalytic reaction and discussed in terms of photoinduced Ti3+. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Pyrrole-Based Silane Primer for Corrosion Protection of Commercial AA 2024 T3

An alternative approach was developed for surface treatment of as-received commercial AA 2024 T3 by using a pyrrole-based silane (SiPy). For film deposition, just one immersion step is enough, followed by curing. SiPy layer structure contains both polysiloxane bonds and pyrrole oligomers, with some degree of doping, giving a highly coherent layer. The superior film quality with respect to simple polysiloxane, is probably the main reason for the better corrosion performance obtained for SiPy on 2024.

Study of the factors determining the mobility of ions in the polypyrrole films doped with aromatic sulfonate anions

The electrochemical redox behavior of the polypyrrole (PPy) films doped with benzenesulfonate (BS), p-toluenesulfonate ( pTS) and naphtalene-1,5-disulfonate (NDS) was investigated by cyclic voltammetry, the chemical composition of the films was determined by XDS, the surface morphology was characterized by AFM, and the interactions between pyrrole oligomers and the anions were modeled with quantum chemical methods. It is the first detailed comparative study of the properties of these interesting systems with two complementing interactions (electrostatic and aromatic stacking). It was shown than these relatively similar aromatic sulfonate anions have rather different behavior in PPy films. The results showed that the redoxactivity of polypyrrole films doped with sulfonate ions increases in the sequence pTS < BS < NDS. The comparison of voltammograms measured in the presence of different anions with PPy films synthesized in the same solution show that the PPy/ pTS films have the most densely packed and the PPy/NDS films the least densely packed structure of the three. The mobility of ions in these films is mainly determined by the packing of the films, which in turn partly depends on the interactions between the dopant-anion and polymer chains.

Induction of titanium reduction using pyrrole and polypyrrole in the ionic liquid ethyl-methyl-imidazolium bis(trifluoromethanesulphonyl)amide

The electrowinning of titanium currently involves high temperature processing and quite extreme condition. As part of a project investigating the use of ionic liquids to refine titanium, we have investigated the use of a polymeric nucleating agent to assist the electro-deposition of the metal. Initial attempts focussed on polypyrrole coatings on the working electrode. These were unsuccessful due to the low conductivity of the conducting polymer in the IL at the reductive potentials required to deposit the titanium. However, it was found that pyrrole was a very successful additive able to induce deposition of titanium species from a TiCl 4 containing ionic liquid electrolyte ([emim][Ntf 2]). Well characterised titanium containing polypyrrole co-deposits have been achieved, when pyrrole was introduced, however bulk metallic titanium was not observed. The presence of titanium species in these deposits was confirmed by XPS. It is thought that the growing pyrrole oligomers form nucleation sites either in situ at the electrode–liquid interface or as a thin film on the electrode allowing co-precipitation of reduced Ti species from solution.