D.A. dos Santos

Poly(p-phenylene-co-2,5-pyrazine): An AM1 investigation

Abstract We present an AM1 study of the geometry of some oligomers of a new electrically conductive polymer: the poly( p -phenylene-co-2,5-pyrazine) (PPPz). We examine the evolution of the total energy, the ionization potential and the inter-ring bond length as a function of the torsion angle between adjacent rings. We have also investigated the geometry of charged (ions +1 and −1) oligomers of PPPz. Based on the obtained results we can explain the observed increase in the conductivity for PPPz induced by p doping and we suggest a way of increasing its value.

Poly(alkylthiophenes): chain conformation and thermochromism

Abstract The role of conformational disorder in the electronic structure of alkyl-substituted polythiophene is investigated. Thermally-induced twisting of thiophene rings out of the main conjugation plane is assumed. AM1 geometry optimizations were carried out to obtain the molecular torsion potential curves for substituted bithiophenes. Total torsion potentials acting on inter-ring rotations are assumed to come from two contributions, the molecular potential and a phenomenological ‘solid state potential’ that accounts for inter-chain interactions. It is found that the molecular potential for rotations does not depend on the length of the alkyl group but only on the regiochemistry of substitution. A long, disordered thiophene chain is built to be representative of a classical probability distribution of torsion angles. The electronic structure associated with valence and conduction π bands is calculated within VEH pseudopotential theory and the NFC technique. The dependence of the optical gap on temperature is obtained and is in agreement with experiment.

Conformational study on alkyl-substituted thiophene oligomers

Abstract A series of Austin-method 1 geometry optimizations on alkyl-substituted thiophene small oligomers has been carried out. Torsion potential curves for dimers are obtained as a function of the length of the alkyl side group and the substitution position. Ground-state geometries are predicted to be quite independent of the radical length. On the other hand, different side-chain couplings give rise to distinct torsion potential curves. A planar ground-state geometry is obtained for 4,4′-dialkyl-bithiophene while in 3,3′ and 3,4′ coupled isomers, thiophenes are twisted by 90° but have distinct torsion barriers. The importance of the present results to the structural features of alkyl-substituted polythiophenes is discussed.

On the disordered distribution of defects in transpolyacetylene

Abstract We report a theoretical study on the effect of the presence of conformational defects (soliton type) on the electronic properties of large trans-polyacetylene chains. The negative factor counting method was used in a tight-binding parametrization with the geometrical data from Austin method 1 calculations. A comparative study of the ordered and disordered distribution of defects is presented. We discuss the implications of the presence of disorder on the electronic behavior of polyacetylene.

Protonic doping effects on the electronic behavior of poly(P-phenylene-Co-2,5-pyrazine)

Abstract We report a theoretical investigation of the effect of random protonation on the electronic behavior of a new conductive polymer (PPPz) using the NFC technique. The obtained results show that in absence of structural disorder protonation cannot by itself lead to a semiconductor-metal transition. A discussion of the protonic acid doping as a mechanism to increase the conductivity of organic polymers is presented.

Effects of geometrical disorder on the electronic structure of alkyl-substituted polythiophenes

Abstract In this work, the role of structure disorder on the alkyl-substituted polythiophene electronic spectrum is investigated. A series of AM1 calculations is carried out to estimate the total energy as a function of torsion angle of alkyl-thiophene dimers. The dependence of torsion energy on the radical length and substitution position are studied. The calculated torsion barriers are used to simulate the out-of-plane disordered twisting of thiophene rings within a simple statistical model. Electronic structure calculations on long disordered thiophene chains are performed by the combination of Valence Effective Hamiltonian method and the Negative Factor Counting technique. Thermally induced disorder on alkyl-substituted polythiophenes is shown to explain irreversible changes on infrared spectrum of the polymer upon heating and cooling cycles.

Clustering effects in polyaniline chains

Abstract Theoretical investigations on the effect of random protonation on the electronic behaviour of long finite chains of polyaniline have shown that the presence of disorder is an essential feature to explain the observed semiconductor-metal transition. In this work we have extended the study of disorder including a pair correlation (short range order) parameter in order to simulate the effects of segregation (clustering) in one-dimensional chains. Our results have shown that the inclusion of these aspects do not affect the general features earlier obtained, disorder is still in the origin of the semiconductor-metal transition.