Bernardo Laks

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.

Spectroscopic study of polyazopyrroles (a narrow band gap system)

Abstract Polyazopyrroles are systems that are derived from the dipyrroles by inserting an azo group among the pyrroles rings. This new polymer present a significant decrease in energy gap lowering from 2.5 eV to 1.0 eV. In this work we present a theoretical absorption spectrum obtained by the ZINDQ/S method using the geometries optimized from the AM1 and PM3 semiempirical methods. As there are experimental evidences that this system presents polaron defects, we also include the effect of the polaron type defects on the electronic structure. The results are in good agreement with the experimental ones.

Electronic structure of dithienopyrrole, dithienothiophene and thionaphtheneindole

Dithienopyrrole (DP), Dithienothiophene (DT) and Thionaphtheneindole (TNI) are electrochromic semiconductor polymers that possess very similar electronic structure. These materials were synthesized in the middle of the nineties with the intention to obtain an improvement in the processability of the electrochromic polymers and investigate the mechanism of charge transport in the polymeric net. In this work, we investigated the electronic, conformational and spectroscopic structure of oligomers formed by the mentioned systems. Density of states and wavefunction were calculated to study the conduction mechanism in the net of these polymers through polaron defect type. We used the semiempirical methods for calculations of electronic structure of the polymers and copolymers coupled with ZINDO/Spectroscopic method to simulate the behavior of absorption for neutral and charge systems. We adopted EHT description for the conformational defects and we investigated the electrical conduction as a function of defects concentration.

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.

Optical spectroscopy of heterocycles based on pyrrole, furan and thiophene

Abstract Recently Joshi et al synthesised a serie of nine triheterocycles based on pirrole, furan and thiophene. All the systems were chemically polymerised and oxidised with NOPF6. The experimental results indicated the presence of polarons in three of the systems that cause appreciable electrical conductivity values. In this work we present a theoretical spectroscopy study for oligomers builded with these materials. In our calculations we utilised the semi empirical AM1 method for the geometric simulation of the oligomers. Optical absorption spectra for neutral and charged systems were done by using ZINDO/S method.

Polycarbonitrile : A semiempirical, ab initio and density functional study of molecular stability

Abstract The theoretical literature data with respect to the electronic properties of this compound is quite scarce and makes use of the planar all-trans structure as the most stable for the calculations. In this work semiempirical (AM1 and PM3), ab initio (at the Hartree-Fock level) and density functional theory (using the correlation functional of Vosko, Wilk and Nussair) were used to analyse the conformational stability of the all-trans and all-cis dimers, trimers and tetramers of polycarbonitrile. The semiempirical and ab initio calculations at the Hartree-Fock level showed in general that the all-trans structure with respect to other conformers is the most unstable structure. The inclusion of electronic correlation energy through the MP2 calculations or the VWN functional method suggest that the trans structure is the most stable. The relative energies calculated at the correlated level presented differences around 2 kcal/mol among the different conformers. While the all-cis compounds presented a planar structure for any of the three methods, the all-trans polymer showed a strong deviation of planarity with a set of local minima in its energy surface. These results suggest that further calculations on the electronic properties of this polymer can be significantly different of those actually available in the literature.

An investigation of the electronic structure of the antitumor drug ellipticine and its derivatives: Part I. Geometrical AM1 study

Abstract Ellipticine and its derivatives are a class of molecules that show antitumor and cytotoxic activity with a multimodal mechanism of action. In this paper we report a preliminary Austin Method One (AM1) study of ellipticine and some molecules derived from it. We have observed a relationship between charge density distribution and biological selectivity. A mechanism that could improve cytotoxic activity is proposed.

A comparative study of ordered and disordered distribution of defects in polyazine derivatives

In this work we report the electronic structure of polyazine derivatives, in the presence of conformational defects like bipolaron. The Negative Factor Counting technique and the Inverse Iteraction Method were used to find the electronic density of states and the wavefunctions, respectively. Our results show a red shift at the Fermi energy for high concentration defect in disordered systems and in ordered systems we have localized wavefunctions. Together with the extention of the highest occupied molecular orbital (HOMO) state, this indicates the possibility to have the polymer in the metallic regime.

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.