Comparison of Soliton Geometry and Charge-Density Wave Structure, and Band Gaps, Between Odd Polyene and Symmetrical Polymethine Cyanine (Brooker) Ions and T - Polyacetylene : Scf and Model Hamiltonian Approaches with Implications For Molecular Wires and Switches

The geometric and electronic structures of odd polyene anions and the isoelectronic Brooker ions are examined by SCF quantum chemical methods in both the small and large (infinite chain) limit. For both types of ion, a geometric and charge density wave soliton is established, with half widths 6.9–7.2 and 9.4 CH units respectively. The odd polyene anions retain C2v symmetry, but the Brooker ions are predicted to break symmetry for chain lengths larger than about 15 CH units. Where the finite-length ions are used as bridges (molecular wires) linking donor and acceptor groups, rates of charge transfer will be considerably enhanced over those of even-membered polyenes in symmetrical structures; where symmetry is broken, the ion may function as a charged soliton molecular switch. Relations between properties of finite-length ions and those of t-polyacetylene are discussed.