Pyrrole Oligomers Research Articles

Theoretical study of pyrrole oligomers: Electronic excitations, relaxation energies, and nonlinear optical properties.

Theoretical study of pyrrole oligomers: Electronic excitations, relaxation energies, and nonlinear optical properties.

A comparative semi-empirical study of pyrrole and phosphole oligomers

The results of the semi-empirical MNDO and INDO/CI calculations of pyrrole and phosphole oligomers with a chain length of 2,4,6 and 10 monomer units are presented. The oligomers with the homolytically dissociated NH bond or PH bond (4 n+2 π-electron unit cell systems), and the oligomers with NLi and PLi bonds have also been studied. The electronic and conductive properties of the polymers are predicted by using the extrapolation of the respective heterocyclic oligomer data.

Products in solution during electrodeposition of polypyrrole

Oligomers of pyrrole accumulated in aqueous solution during the anodic oxidation of pyrrole at stationary and rotating electrodes have been identified and separated using UV-visible spectroscopy and chromatographic techniques. Oligomers of up to nine pyrrole units have been observed in stagnant solutions, concurrently with polypyrrole deposition onto the electrode surface. Stirring strongly inhibits accumulation of oligomers longer than three monomeric units and reduces the amount of polypyrrole film deposited, indicating that the establishment of nuclei for polymer growth occurs by precipitation of oligomeric species formed on the electrode surface and accumulated in its vicinity.

Observation of soluble pyrrole oligomers and the role of protons in the formation of polypyrrole and polybipyrrole

Combined rotating ring disk electrode (RRDE) and spectroelectrochemical evidence is presented that shows clearly that bipyrrole is formed in solution during the anodic polymerization of pyrrole in acetonitrile (ACN). Spectroelectrochemistry shows that a measurable solution concentration is built up, but the RRDE collection efficiency N r of 9×10 −3 (0.22 theoretical) shows that the yield is low. Other intermediates were observed and the presence of H + and protonated pyrrole is discussed. The half wave potential E 1 2 for the H + reduction wave ( N r = 0.22) at the ring shifts from −0.08 V vs. saturated sodium chloride calomel electrode (SSCE) in 0.1 M ClO 4 − to −0.90 V in 0.1 M (C 4H 9) 4NBF 4, and to −0.66 V in 0.1 M ClO 4 − salt+0.1 M pyridine or di- t-butylpyridine for 50 mM pyrrole solutions in ACN. An absorbance peak at 242 nm formed during electropolymerization is identified as being due to protonated pyrrole. Since this species goes on to form pyrrole black it may influence the conductivity of the electropolymerized product, and this suggests why the more basic BF 4 − is a preferred electrolyte. Authentic bipyrrole underwent two oxidations at an RRDE, with E 1 2 values of 0.55 and 1.15 V vs. SSCE in 0.1 M NaClO 4. The first E 1 2 was pH independent, while the second shifted to 0.55 V in 0.1 M NaClO 4 + 0.1 M pyridine to give a single two-electron wave. Polybipyrrole formed from oxidation of 0.03 mM bipyrrole in 0.1 M NaClO 4 + ACN has an oxidation peak at −0.35 V vs. SSCE in aqueous ClO 4 −, compared with more positive than −0.05 V for polypyrrole. The polybipyrrole oxidation peak is narrower and shows a better defined range of redox states than does polypyrrole, and coincides with a shoulder often observed for polypyrrole.

Well‐defined pyrrole oligomers: Electrochemical and UV/vis studies

The properties of oligo(pyrrole)s can give vital clues to the intrinsic properties and behavior of poly(pyrrole), which is important in the field of polyconjugated conducting polymers but difficult to obtain defect‐free. However, up to now little work has been done on oligo(pyrrole)s, perhaps because of their sensitivity to oxygen, which makes their handling and preparation difficult. The reported electrochemical analysis of well‐defined pyrrole oligomers has allowed both the determination of the limiting value for the potential of reversible charging of poly(pyrrole) and the production by electrochemical coupling of a virtually defect‐free poly(pyrrole).

Progress toward the development of a chemical synthesis for poly(2,5-pyrrole)

The synthesis of a number of N-Boc-protected pyrrole oligomers, as well as a method to isolate a narrow fraction of N-Boc-protected poly(2,5-pyrrole) (DP = 20; PDI = 1.2) are described. Preliminary studies certify that Boc groups can be removed from pyrrole rings. Single-crystal X-ray structures of protected and unprotected oligomers are presented.

A b i n i t i o studies on heterocyclic conjugated polymers: Structure and vibrational spectra of pyrrole, oligopyrroles, and polypyrrole

We report the results of a seiies of ab initio calculations on the structure and on the vibrational spectra of pyrrole and pyrrole oligomers performed with double zeta quality basis set.

The growth of polypyrrole films on electrodes

The kinetics of growth of polypyrrole films deposited onto electrode surfaces from aqueous solutions of pyrrole have been investigated. The rate-determining step for the growth of the film is the oxidation of pyrrole units or oligomers and their incorporation at the end of polymer chains. In spite of the positive potential imposed on the electrode during electrodeposition, polypyrrole remains essentially in its neutral state during growth and exhibits conductivities which are several orders of magnitude lower than those of already grown films. It is shown that the rate of film growth is limited by the low conductivity of the growing film, which was evaluated from potentiostatic current transients recorded during electrodeposition of polypyrrole.

Characterization of Polypyrrole

Abstract The renewed interest in the conducting polymer polypyrrole in 1968 was due to the efficient electrochemical synthesis of the polymer which made it possible to grow thin films with appropriate mechanical properties to facilitate the characterization of the polymer. Since then we have been able to grow thick films of polypyrrole with very good mechanical properties without compromising their electrical properties. In addition, good films can also be grown from aqueous media. Modifications can be achieved through copolymers with N-methylpyrrole and by alloying with other polymers. An insight into the structure of the polypyrrole polymers can be gained through the x-ray structural determination of some oligomers of pyrrole.

Electronic structure of polypyrrole and oligomers of pyrrole

The ultraviolet photoemission and optical absorption spectra of pyrrole and polypyrrole are analyzed using the spectroscopically parametrized CNDO/S3 model. Calculated charge densities, bond orders, and orbital eigenvalues for pyrrole are compared with the extensive literature on this molecule. The densities of valence states are computed for pyrrole, oligomers of pyrrole, and an infinite planar polypyrrole chain. They provide a good description of ultraviolet valence electron photoemission spectra measured for gas-phase pyrrole, condensed thin films of pyrrole, and thin-film polypyrrole. The photogenerated radical cation states in polypyrrole seem to extend over four or more pyrrole units for the higher binding energy σ-electron states but are localized on fewer units for the lowest binding energy π-electron states. The relationship between the density of valence states and the energy band structure of polypyrrole is established. The energies of the first few dipole allowed optical absorption transitions are calculated for pyrrole and its oligomers via a limited configuration interaction analysis. Examination of the trends in these transition-state energies as a function of the size of the oligomer suggests that the lowest-energy optically excited states in polypyrrole are excitons which extend over at least four pyrrole units. Comparison of the model predictions for ring and linear-chain molecular conformations indicates that both are compatible with existing spectroscopic measurements.