Science Diliman, Vol 13, No 2 (2001)

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Structural Studies of Polythiophenes: An Ab Initio Evaluation of Coupling Through α and β Carbons

A. C. Alguno, R. V. Bantaculo, R. M. de Asis, R. M. Venquizo, A. A. Castillon, A. S. Dahili, H. Miyata, A. M. Bacala

Abstract


Extensive studies have been done both fundamentally and practically on conducting polymers like polythiophene (PT). Once positively doped, PT is relatively stable in air, due in part to the resonance effect of the sulfur which acts to stabilize a carbonium ion on the polymer chain (Yang & Chien, 1995). The drastic changes in electrical, optical and magnetic properties upon doping (Castillon, 2000) have attracted much attention from both theoretical and practical physicists.

In the undoped state, polythiophene is an insulator with a band gap of about 2.0 eV (Castillon, 2000) and can be used as heterojunction together with polypyrrole (PPy). Their combination exhibits excellent rectifying characteristics (Castillon, 2000). PT stable films can be prepared by either an electrochemical, or a chemical method (Roncalia, 1992).

Like polypyrrole, the neutral polythiophene as observed in the infrared (lR) and a Carbon13 NMR spectrum has shown that α-α' carbon linkages predominate (Roncalia, 1992; Yurtsener, 1999). Thus, the commonly accepted structure of PT is a linear chain of monomers bonded by α-α'' carbons.

This paper investigates the most probable carbon coupling during polymerization. The thermodynamical probabilities of linking α and β carbons in dimers through pentamers of polythiophenes were determined employing ab initio method.

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