ELECTRONIC COUPLING THROUGH SATURATED-HYDROCARBON BRIDGES

Through-bond (TB) coupling between the chromophores of a series of pol ynorbomyldienes 1(n), alpha,omega-diethynyl[n]staffanes 2(n), alpha,om ega-divinyl[n]staffanes 3(n), and a series of all-trans, n-alkyl compo unds, X-(CH2)-X, where X=H2C=C 4(n), HC=C 5(n), OH 6(n), SH 7(n), and planar NH2 8(n), have been investigated using ab initio MO theory, wit h the 3-21 G basis set, in the Koopmans' theorem approximation. A natu ral bond orbital (NBO) analysis of the n-alkyl compounds, 4(n)-8(n), s hows that much of the variation of the TB electronic coupling among th e different series is governed by the self-energy of the chromophore o rbitals. This conclusion was surmised from ''tuning'' experiments in w hich the chromophore self-energies in the NBO Fock matrix of one syste m are replaced by those of a different chromophore. Analysis of the TB pathways shows that the tight binding (nearest neighbor) model is gro ssly inadequate in explaining TB orbital interaction behavior. and tha t intrabridge interactions that skip over one or more bridge bonds als o need to be included for a realistic analysis. In addition. model Ham iltonian matrices, constructed using NBO Fock matrix elements, demonst rate the importance of''retracing'' pathways.