Steric and Electronic Effects in Methyl-Substituted 2,2'-Bipyrroles and Poly(2,2'-Bipyrroles)s: Part II. Theoretical Investigation on Monomers

The effects of N- and C(beta)-methyl substitution in pyrrole and 2,2'-bipyrrole were investigated through ab initio calculations and Atoms in Molecules analysis of the resulting wave functions. Replacement of a hydrogen atom with a methyl group in pyrroles lowers the ionization potential, with substitution at C3 being more efficient than N-substitution because of the larger release of pi population to the ring in the former case. Full geometry optimization at RHF/6-31G** level and as a function of the torsion angle tau between two adjacent rings demonstrates that the increasing loss of planarity in the 2,2'-bipyrrole, N,N'-dimethyl-2,2'-bipyrrole, and 3,3'-dimethyl-2,2'-bipyrrole series, adversely affects the positive contributions expected from methyl substitution. An intramolecular interaction energy model shows that the greater anti-planarization energy of N,N'-dimethyl-2,2'-bipyrrole, as compared to 3,3'-dimethyl-2,2'-bipyrrole, is due to the larger decrease in the stabilizing electrostatic term and to the larger increase in the destabilizing nonbonding contribution which occurs at tau=0° in the former. Calculations on the corresponding monocations and analysis of new conductivity measures on substituted poly(2,2'-bipyrrole)s suggest that the ease in achieving local chain planarity in doped polypyrroles should be more closely correlated to the antiplanarization energies of the charged monomers rather than to anti-planarization energies of the neutral monomers.