Structure and electronic properties of alkylthiophenes coupled by Head-to-Tail and Head-to-Head regioselectivity

Marcos A De Oliveira,Wagner B De Almeida,Hélio F Dos Santos

doi:10.1590/s0103-50532004000600008

Abstract

In the present paper a theoretical analysis of alkylthiophenes with Head-Tail-Head-Tail (HT-HT) and Head-Head-Tail-Tail (HH-TT) regioselectivities up to 6 monomeric units was carried out. Structure and electronic properties for the syn and anti forms were calculated by ab initio Hartree-Fock (HF) and Density Functional Theory (DFT) approaches. The energy gap between the conduction and valence bands was obtained at HF and DFT levels and also calculated with the semiempirical ZINDO/S-CI method. The results showed that the isomer with HH-TT regioselectivity presents lower energy gap for longer oligomers and the substitution of thiophene by alkyl groups does not change significantly the energy gap. This is important in the sense that more processable polymers can be obtained through alkylation without decrease the energy gap that is a molecular properties related to the electronic conductivity. Regarding to the conformational equilibrium, the syn form was found to be stable only for the HT-HT isomer and in this case the energy gap was found to be lower than those calculated for the anti form.

Highlights

Conjugated organic polymers have been subject of intense experimental and theoretical researches since 1977 when this class of materials was fully tested.[1]
For the parent compound 2T, the inter-ring dihedral angle calculated for the anti form from the HF/6-31G(d) geometry (ω=147°) was found to be in good agreement with those obtained from higher levels of theory[12] and experimental data.[22]
The structure and energy gap were calculated for a series of alkylthiophenes using theoretical methodologies based on HF and Density Functional Theory (DFT) approaches

Summary

Introduction

Conjugated organic polymers have been subject of intense experimental and theoretical researches since 1977 when this class of materials was fully tested.[1] Several systems as poly(pyrrol), poly(aniline), poly(phenylthiophene) and poly(alkylthiophene) were prepared and their electronic and optical properties analyzed.[2] Doped organic molecules were widely tested for building electronic and photonic devices.[3]. The substituted α,α’-oligothiophenes have been considered adequate prototypes for intrinsic electro-active polythiophenes. Their structural and electronic properties can be modulated by group substitution. The polythiophenes are oxidized π-electron systems exhibiting conductivities of ~20 S.cm-1 and are stable at oxygen atmosphere.[3] The small oligothiophenes are usually not processable using common solvents, and one way to contour this obstacle is to substitute appropriated molecular positions by alkyl groups, that leads to processable (fusible and soluble) derivatives without losing conductivity

Methods

Results

Conclusion