Tuning Charge Transport Properties of Asymmetric Molecular Junctions

Charge transport characteristics of asymmetric molecules containing a 9,9′-spirobifluorene platform coupled covalently to a phenylene ethynylene linker capped with either a thiol or a nitrile end group are investigated by break junction techniques. It is shown that the platform provides very good electronic coupling with metallic leads and the differences in the charge transport depend solely on the type of the anchoring group at the opposite end of the molecule. The SH-terminated molecule has 1 order of magnitude higher conductance compared to the CN-terminated one, and the charge transport path depends on the end group utilized. By a combined experimental break junction techniques and theoretical DFT calculations, it was demonstrated that in molecules containing SH-terminated phenylene ethynylene wire attached to the 9,9′-spirobifluorene platform the charge is transported through fluorene unit and covalently coupled phenylene ethynylene linker. For CN-terminated molecules the charge is transported through the thiolate termini of the 9,9′-spirobifluorene tripod. These studies demonstrate the potential of spirobifluorene platform for the bottom-up approach to molecular architectures by its immobilization with all three thiol groups to one of the electrodes without compromising charge transport via the conjugated backbone.