Electronics and Chemistry: Varying Single Molecule Junction Conductance Using Chemical Substituents

TL;DR

This study investigates how chemical substituents on benzene diamine molecules influence their electrical conductance in single-molecule junctions, revealing the role of molecular orbital alignment and electron donating or withdrawing effects.

Contribution

It demonstrates that molecular conductance varies predictably with substituents, linking conductance to ionization potential and Fermi level alignment, advancing understanding of molecular electronics.

Findings

Conductance increases with electron-donating groups.

Conductance decreases with electron-withdrawing groups.

Conductance correlates inversely with ionization potential.

Abstract

We measure the low bias conductance of a series of substituted benzene diamine molecules while breaking a gold point contact in a solution of the molecules. Transport through these substituted benzenes is by means of nonresonant tunneling or superexchange, with the molecular junction conductance depending on the alignment of the metal Fermi level to the closest molecular level. Electron-donating substituents, which drive the occupied molecular orbitals up, increase the junction conductance, while electron-withdrawing substituents have the opposite effect. Thus for the measured series, conductance varies inversely with the calculated ionization potential of the molecules. These results reveal that the occupied states are closest to the gold Fermi energy, indicating that the tunneling transport through these molecules is analogous to hole tunneling through an insulating film.