Conformational analysis on biphenyls with theoretical calculations: Modeling torsions in poly(para-phenylene)s with side chains

Conformational analysis was carried out on the methyl and methoxy substituted biphenyls (BP) with semi-empirical AM1 and PM3 quantum chemical calculations as well as Hartree-Fock (HF) ab initio calculations with 6-31G(d) basis set. The existence of methoxy groups mainly causes the increase in the energy barrier towards co-planar conformation but not much change in the torsion angles in the free molecules. Consequently, the flexibility of the corresponding polymers is reduced. The electronic donation effect of the alkoxy group plays an important role in the modification of related polymers. The existence of methyl groups not only largely increases the energy barrier towards co-planar conformation but also the equilibrium torsion angle. The related polymer will be greatly twisted and become more flexible. The steric interaction of the alkyl groups plays an important role in the modification of the electronic structures of related polymers. These different effects account for the large difference between the UV absorption maximum of alkyl- and alkoxy-substituted polymers with poly(para-phenylene)s (PPP) backbone in solutions.