The electrical behavior of pi -conjugated oligo(phenyleneethynylene) systems functioning as memory devices is studied using quantum chemistry methods, including density functional and Green function formalisms combined in a fully self-consistent manner. Electron charge alters a molecule impedance characteristic providing in some Gases distinguishable "impedance states" that can serve to determine experimentally the state of charge of the molecule. Conducting and nonconducting states can be strategically engineered by arranging substituents in a molecule. The NH2 group localizes the highest energy occupied electronic states whereas the NO2 group localizes the lowest energy unoccupied orbitals of the oligomer systems. These effects yield two complementary molecular memories, each occupying a volume smaller than 1 nm(3).