Organic polymers based on aromatic rings (polyparaphenylene, polypyrrole, polythiophene): Evolution of the electronic properties as a function of the torsion angle between adjacent rings

Abstract

We present ab initio Hartree–Fock and valence effective Hamiltonian (VEH) calculations on polyparaphenylene, polypyrrole, and polythiophene dimers and polymer chains. These polymeric materials are among the most studied compounds in the field of conducting polymers. We examine, as a function of the torsion angle between consecutive rings, the evolution of electronic properties such as ionization potential, bandgap and width of the highest occupied bands and of the carbon–carbon bond length between rings. This investigation is motivated by the fact that many derivatives of these compounds have substituents that lead to an increase of the torsion angle between adjacent rings, as a result of steric interactions. As expected, on going from a coplanar to a perpendicular conformation, the ionization potential and bandgap values increase and the width of the highest occupied bands decreases. This makes it more difficult to ionize or reduce the polymer chains and can result in achieving lower maximum conductivities on doping. However, since the evolution of the electronic properties is found to follow a cosine law (related to the decrease of the overlap between the π orbitals on adjacent rings), the modifications up to a ∼40° torsion angle are not very large. For instance, in all three polymers, the ionization potential value for a 40° torsion angle is about 0.4 eV larger than the coplanar conformation value. Therefore, substituents that lead to torsion angles between consecutive rings smaller than 40° are quite acceptable. Finally we discuss the importance, for the substituted compounds, of the possibility of achieving a coplanar conformation upon doping, in order to permit high intrachain mobilities of charge carriers such as bipolarons.

Ordering the amorphous – Structures in PBD LED materials

“…Additionally, in the solid state intermolecular interactions between the individual molecules packed into the three-dimensional crystal lattice contribute to a planarization of the molecule compared to the isolated state. In polyparaphenylene the steric interactions lead to torsion angles between adjacent rings of approximately 23°, where every second ring lies in the same plane [50]. Theoretical considerations give 27°f or chains that reduce to 17°for chains arranged in crystals [32] in contrast to investigations of p-quinquephenyl, p-sexiphenyl and p-septiphenyl that should exhibit a planar molecular structure in the crystal lattice [45].…”

Conformation dependence of molecular conductance: chemistry versus geometry

“…For this reason we have decided not to include the transport results of this molecule. HOMO to lower energies [19], which increases the charge on the molecule as it crosses the Fermi level. This behaviour is reminiscent of bonding and antibonding states with a coupling matrix element, which depends on the overlap between the π conjugated states of both rings 7 .…”

Electrochemical materials science: calculation vs. experiment as predictive tools in tailoring intrinsically conducting polythiophenes

“…This extension of overlap is associated with conformation of the chain; thus knowledge of the possible conformations and the energies involved in the conformational equilibrium can be used to understand the electric properties of these compounds. With this aim, theoretical methods have been applied to determine structures and electronic properties related to the electric conductivity [56][57][58][59][60][61]. On the other hand, polythiophenes substituted with aryl groups are interesting from several viewpoints.…”

“…Many applications of conjugated polymers, such as light emitting electrochemical cells [55,74], microactuators [75][76][77], energy storage [78], photovoltaic [79], electrochromic devices [80], and sensors [60,81] are based on electrochemical transitions between doped and neutral states or rely on the stability of a specific doping level.…”