Chemical Modulation of Charge Transport Perpendicular to the Molecular Plane

The electronic structure of molecules can have a considerable effect on charge transport through molecular junctions. However, this is known only where molecular anchoring groups define the transport direction to be along the molecular plane. It is not clear how different chemical substituents can affect conductance perpendicular to the molecular plane. In this study, the conductance measurement perpendicular to the plane of mesitylene substituted with electron-withdrawing groups (e.g., NO₂, Br) and electron-donating groups (e.g., CH₃) showed that the conductance increases by introducing electron-withdrawing groups and decreases as electron-donating groups are added to the mesitylene molecule. These results are supported by density functional theory and nonequilibrium Green’s function calculations. We demonstrated that the changes in the conductance perpendicular to the molecular plane correlate well with the Hammett constant of the corresponding functional groups, indicating the importance of the nature and strength of chemical substituents on the degree of conductance modulations, at least for mesitylene derivatives. The generalization of the Hammett parameters to the perpendicular charge transport through single molecules could serve as a predictive framework, motivating the design and fabrication of molecular devices with desired functionalities.