HOLE, ELECTRON AND ENERGY-TRANSFER THROUGH BRIDGED SYSTEMS .8. SOLITON MOLECULAR SWITCHING IN SYMMETRY-BROKEN BROOKER (POLYMETHINECYANINE) CATIONS

Brooker ions NH2-(CH)2n+1-NH2+ and their derivatives are extremely imp ortant dye and photosensitizer materials, and, if the inner odd-polyen e chain length can be made sufficiently large, they will display the i nteresting conductivity properties of polyacetylene derivatives. They could be useful as molecular wires, with the conductivity actually exc eeding that of finite-length doped polyacetylene until Peierls distort ion sets in and solitons form. We explore the possibility, predicted b y AM 1 calculations, that, at sufficiently large chain lengths, solito ns form and are attracted to localized structures near the chain ends rather than forming a symmetric (C2v) Structure. While non-linear opti cal responses of the symmetrical Brooker ions are known to be already very large, calculations predict order of magnitude improvements in th ese properties upon localization. Also, the appearance of a new far-in frared intervalence-type transition is predicted. The isomerization re action which interconverts the localized soliton structures is a hole transfer process the passage of the soliton carrying with it the net i onic charge. Details of the kinetics are considered, including their m odulation by external electric fields and neighbouring charges, and es timates of the electronic coupling of the diabatic surfaces and of the reorganization energy are made. This suggests that, for Brooker ions with optimal properties, a molecular switch could be constructed with subpicosecond write times and at least microsecond data retention time s. The kinetics of this type of switch are fundamentally different fro m those of other systems which have been considered as possible molecu lar switches.