A CONFORMATIONAL STUDY OF 2,5'-BITHIOPHENES SUBSTITUTED IN POSITION-3BY HALOGEN ATOMS CORRELATED WITH CONDUCTING AND ELECTROCHEMICAL PROPERTIES OF POLY(3-HALOGENOTHIOPHENE)S

Poly(3-halogenothiophene) s have different electrochemical and conduct ing properties depending on the halogen atom X involved (X: F, Cl, Br) . An electrochemical study (polymerization and cyclic voltammetry) giv es some information concerning the specific electronic effects of halo gen atoms. Nevertheless, conducting properties are not easy to underst and because not only has the electronic effect to be taken into accoun t but also steric control is probably of major importance. Molecular m echanics calculations on the simplest model of poly(3-halogenothiophen e)s, 2,5'bithiophene, permit us to evaluate the steric contributions o f the substituent on the energy and the conformation. First, we confir m that electronic (inductive electron-withdrawing) effects mainly expl ain the oxidation potentials of the monomers. For polymers we then sho w that not only electronic but also steric effects control the oxidati on potentials. In terms of conductivity we prove that steric effects m ainly justify the fact that poly(3-chlorothiophene) and poly(3-bromoth iophene) are fractured films exhibiting low conductivity. The sizes of the chlorine and bromine atoms induce a higher torsion angle between two adjacent thiophene rings, which may perturb the pi conjugated syst em along the helical or planar structures and also directly affect the conductivity. (C) 1998 Elsevier Science S.A. All rights reserved.