Superexchange tunneling conductance in molecular wires

TL;DR

This paper develops a modified superexchange model to accurately describe nonresonant tunneling conductance in molecular wires, linking theoretical parameters to experimental data and clarifying conditions for model applicability.

Contribution

It introduces a modified superexchange model that accounts for delocalized molecular orbitals and connects it to the flat barrier model, enhancing understanding of charge tunneling in molecular wires.

Findings

The model accurately explains experimental tunneling conductance in specific molecular wires.

Parameters like energy gap and effective mass are linked to molecular orbital properties.

Conditions for the validity of the superexchange model are clearly formulated.

Abstract

The modified superexchange model is used to derive the expression for nonresonant tunneling conductance mediated by localized and delocalized molecular orbitals associated with the terminal and the interior molecular units respectively. The model is shown to work as long as delocalization of electron density in the chain's molecular orbitals is sustained during the tunneling. The criteria for reduction of the superexchange model of charge tunneling to the flat barrier model are formulated and the parameters of the barrier model (energy gap and effective electron mass) are specified in the terms of inter-site coupling and energy distance from the Fermi level to the delocalized wire's HOMO level. Application of the theory tothe experiment shows that the modified superexchange model is quite appropriate to explain the experimental results in case of the nonresonance tunneling conductance in --(CH$_2)$$_N$--NH$_2$ and HOOC--(CH$_2)$$_N$--COOH molecular wires.