Electronic coupling through saturated hydrocarbon bridges

Abstract

Through-bond (TB) coupling between the chromophores of a series of polynorbornyldienes 1( n ), α,ω-diethynyl[ n]staffanes 2( n ), α,ω-divinyl[ n]staffanes 3( n ), and a series of all-trans, n-alkyl compounds, X-(CH 2) l -X, where X=H 2CC 4( n ), HCC 5( n ), OH 6( n ), SH 7( n ), and planar NH 2 8( n ), have been investigated using ab initio MO theory, with the 3–21G basis set, in the Koopmans' theorem approximation. A natural bond orbital (NBO) analysis of the n-alkyl compounds, 4( n )– 8( n ), shows that much of the variation of the TB electronic coupling among the different series is governed by the self-energy of the chromophore orbitals. This conclusion was surmised from “tuning” experiments in which the chromophore self-energies in the NBO Fock matrix of one system are replaced by those of a different chromophore. Analysis of the TB pathways shows that the tight binding (nearest neighbor) model is grossly inadequate in explaining TB orbital interaction behavior, and that intrabridge interactions that skip over one or more bridge bonds also need to be included for a realistic analysis. In addition, model Hamiltonian matrices, constructed using NBO Fock matrix elements, demonstrate the importance of “retracing” pathways.