Conductance Trends in Single Molecule Junctions Formed Using Donor-Acceptor Links: Theoretical Analysis

The conductance of single molecule junctions using amine-gold links has been understood based on formation of a donor- acceptor bond involving the N lone pair and the s-orbital on an undercoordinated Au site on the electrode. Experiments probing junctions formed with alkanes terminated by dimethyl phosphines and methyl sulfides also show an unambiguous conductance signature. The structure and bonding in these junctions is analyzed using density functional theory based calculations. Like the amine link, the dimethyl phosphine and methyl sulfide bond to an under-coordinated Au site through a donor-acceptor motif. While the bond energy for the amine and methyl sulfide links are similar (0.6 eV), the dimethyl phosphine is significantly stronger (1.2 eV). Trends in measured junction conductance (amine $<$ sulfide $<$ phosphine) are analyzed in terms of available electronic channels.