Influence of Conformational Flexibility on Single-Molecule Conductance in Nano-Electrical Junctions

The temperature dependence of the single-molecule conductance of conformationally flexible alkanedithiol molecular bridges is compared to that of more rigid analogues which contain cyclohexane ring(s). Molecular conductance has been measured with a scanning tunneling microscope (STM) at fixed gap separation by observing the stochastic formation of molecule bridges between a gold STM tip and substrate (the so-called “I(t)” technique). Under these conditions, the junction can be populated by a wide distribution of conformers of alkanedithiol molecular bridges and a strong temperature dependence of the single-molecule conductance is observed. By contrast the rigid analogues that contain cyclohexane ring(s), which cannot form the thermally accessible gauche rich conformers open to the alkanedithiols, show no dependence of the single-molecule conductance on temperature. This comparison demonstrates that it is the conformational flexibility and access to thermally populated higher energy conformers of the linear polymethylene (alkane) bridges which leads to the temperature dependence. By removing this possibility in the cyclohexane ring-containing bridges, this conformational gating is excluded and the temperature dependence is then effectively suppressed.