STRUCTURAL STUDIES OF POLYTHIOPHENES

A theoretical investigation of the electronic structures of the oligom ers of thiophene (T) and their derivatives, namely, 2-methylthiophene (2MT), 3-methylthiophene (3MT), 2-cyanothiophene (2CT) and 3-cyanothio phene (3CT), are presented. The most stable forms of the monomer, dime r, trimer and tetramers are obtained by the semi-empirical quantum mec hanical methodology using AM1 parametrization. All possible binding si tes are investigated in order to understand the bonding in polythiophe nes. The 3-substituted molecules form dimers which lie nearly at trans -planar positions but 2CT and 2MT favor cis conformations. The activat ion energies between these structures and local minima change from 0.2 to 1.8 kcal/mol. Methyl- and cyano-substituted polythiophenes favor l inear growth mechanisms. The branching requires 3-6 kcal/mol per subst itution. Polythiophenes seem to form fairly flexible chains as underst ood from the relatively low rotational barriers.