SOLITONS IN FINITE-LENGTH AND INFINITE-LENGTH NEGATIVE-DEFECT TRANSPOLYACETYLENE AND THE CORRESPONDING BROOKER (POLYMETHINECYANINE) CATIONS .2. CHARGE-DENSITY WAVE

The charge density wave structures of Brooker cations C2n-1H2n+3N2+ and their isoelectronic polyenes C2n+1H2n+3- are studied using ab initio SCF, AM1, and an empirical technique. The net charge on the soliton in the Brooker ions is + 1 with no net charge residing on the atoms at the chain ends. Hence two pi electrons are confined to the chain end regions: the properties and energy levels of the central region closely resemble those of the polyene cations rather than the isoelectronic polyene anions. In both the polyene and Brooker ions, the charge density wave is predicted to be slightly broader than that found previously (part I of this paper) for the corresponding geometric soliton; at least 20 CH units are required between the soliton centre and the chain ends in order that the infinite-chain limit of the soliton is established. When the chain length exceeds 17 CH units, it was shown in part I that AM1 calculations predict a lowering of the symmetry of the equilibrium geometry from C2v to C(s), with the geometric soliton localized at one end of the chain. Here, the charge distribution is shown to follow the geometric soliton, retaining its qualitative nature. Ions with asymmetric structures would be very useful as molecular switches as a small applied electric field could cause the ion to isomerize, with the soliton carrying its charge to the distant end of the ion.