Energetics, structure, and charge distribution of reduced and oxidized n-pyrrole oligomers: A density functional approach

Abstract

Polypyrrole is a conjugated polymer prototype of conducting polymers. The energetically preferred spatial conformation of n-pyrrole oligomers (n=1-24) in both the reduced and oxidized phases is obtained and analyzed in this paper within the hybrid density functional theory. Binding energies, gap energies, radius of gyration, end-to-end distance, and vibrational frequencies are reported as functions of oligomer length. Reduced n-pyrrole are bent chains for all sizes showing a dramatic departure from planarity. Vibrational spectra of n-pyrrole oligomers indicate the presence of two fairly size-insensitive frequency regions, which increase in intensity with increasing oligomer size. Several oxidation levels were analyzed for n-pyrrole through the distribution of the carbon-carbon bond orders and single/double bond lengths. It is shown that the oxidation level is directly related to the way positive charge localizes along the n-pyrrole oligomer chain. If charge/n<13, the oligomers are bent and charge is delocalized; if charge/n>/=13, the oligomers are planar and charge notoriously localizes in n/charge regions along the backbone. Calculations with electronegative dopants show that charge localizes in the neighborhood of the dopant. It is demonstrated that one localized state in the gap between the highest occupied and lowest-unoccupied states appears for every +2e in the oxidation level. The band structure of infinite reduced polypyrrole gives a band gap energy in excellent agreement with experiment. The evolution of the band gap and the charge-localized band as a function of polypyrrole oxidation level is reported.

A combined molecular dynamics and experimental study of doped polypyrrole

“…As discussed in the introduction, several previous studies have examined the molecular and electronic structure of PPy oligomers using a range of ab initio theory [3, 8–10]. Earlier works were limited by computational cost and used less accurate basis sets.…”

“…Thus undoped oligo or polypyrrole have non-planar structures and are likely to be rather flexible. Most recently, a quantum mechanics (QM) study of the equilibrium structures of oligomers up to 24 rings in length showed that higher doping ratios produce more planar chains [10]. They also showed that explicitly including dopant ions in QM calculations with oxidized PPy chains increased the localization of positive charge near the doping sites.…”

Computational investigation of interaction of polypyrrole on the surface of single-walled carbon nanotube

“…Pyrrole and pyrrole oligomers have been subjected to several investigations determining various properties including electronic structure, vibrational frequencies, ionization potential, and electron affinity and effect of doping on these properties [12][13][14]. It has been found that aromatic compounds are adsorbed on CNT surfaces through stacking interactions [15][16][17][18][19][20].…”

Monte Carlo study of oligopyrroles in condensed phases

“…Oxidized PPy is a polycationic conductor that synthesizes forming thin or thick films. [17][18][19][20][21][22][23][24][25][26][27][28] However, the level of these calculations is computationally intractable for studying condensed phases that are not crystalline. By reduction, the electrical conduction property is lost and the structure of the conjugated polymer chain becomes benzenoid.…”

“…27 This potential treats the Py monomers as rigid molecules with a permanent dipole moment anchored at their centers and contains interactions pertaining to intra-and interoligomer chains. 27 This potential treats the Py monomers as rigid molecules with a permanent dipole moment anchored at their centers and contains interactions pertaining to intra-and interoligomer chains.…”