A conformational study of 2,5′-bithiophenes substituted in position 3 by halogen atoms correlated with conducting and electrochemical properties of poly(3-halogenothiophene)s

Poly(3-halogenothiophene)s have different electrochemical and conducting properties depending on the halogen atom X involved (X: F, Cl, Br). An electrochemical study (polymerization and cyclic voltammetry) gives some information concerning the specific electronic effects of halogen atoms. Nevertheless, conducting properties are not easy to understand because not only has the electronic effect to be taken into account but also steric control is probably of major importance. Molecular mechanics calculations on the simplest model of poly(3-halogenothiophene)s, 2,5′bithiophene, permit us to evaluate the steric contributions of the substituent on the energy and the conformation. First, we confirm that electronic (inductive electron-withdrawing) effects mainly explain the oxidation potentials of the monomers. For polymers we then show that not only electronic but also steric effects control the oxidation potentials. In terms of conductivity we prove that steric effects mainly justify the fact that poly(3-chlorothiophene) and poly(3-bromothiophene) 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 π conjugated system along the helical or planar structures and also directly affect the conductivity.