Hole, electron and energy transfer through bridged systems. VIII. Soliton molecular switching in symmetry-broken Brooker (polymethinecyanine) cations

Abstract

Brooker ions NH 2-(CH) 2 n+1 -NH + 2 and their derivatives are extremely important dye and photosensitizer materials, and, if the inner odd-polyene chain length can be made sufficiently large, they will display the interesting conductivity properties of polyacetylene derivatives. They could be useful as molecular wires, with the conductivity actually exceeding that of finite-length doped polyacetylene until Peierls distortion sets in and solitons form. We explore the possibility, predicted by AM1 calculations, that, at sufficiently large chain lengths, solitons form and are attracted to localized structures near the chain ends rather than forming a symmetric (C 2v) structure. While non-linear optical responses of the symmetrical Brooker ions are known to be already very large, calculations predict order of magnitude improvements in these properties upon localization. Also, the appearance of a new far-infrared intervalence-type transition is predicted. The isomerization reaction which interconverts the localized soliton structures is a hole transfer process, the passage of the soliton carrying with it the net ionic charge. Details of the kinetics are considered, including their modulation by external electric fields and neighbouring charges, and estimates 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 times. The kinetics of this type of switch are fundamentally different from those of other systems which have been considered as possible molecular switches.