Controlling the Electron Transfer Mechanism in Metal-Molecules-Metal Junctions

We show how the mechanism of electron transfer through molecules can be switched between different regimes by using a simple Hg-based metal-molecules-metal junction that allows for hosting of self-assembled monolayers (SAMs) of a large variety of molecular systems. We compare here the results obtained by using two different approaches in measuring electron transfer rate. Using a two-electrode assembly we have measured I-V curves through SAMs formed by different organic molecules (alkanethiols HS(CH2)n-1CH3 (n = 8, 10, 12, 14, 16), oligophenylene thiols HS(C6H4)kH (k = 1, 2, 3), or benzylic homologs of the oligophenylene thiols HSCH2(C6H4)mH (m = 1, 2, 3). The molecules incorporated have a very large HOMO-LUMO energy separation and their orbitals cannot align with the Fermi levels of the electrodes under an applied voltage. The molecules therefore behave as insulators, and the electron transport mechanism is characterized by a through-bond tunneling process. Using an electrochemical junction we have measured I-V curves through SAMs of molecules incorporating redox sites (ruthenium pentaamine pyridine-terminated thiol [HS(CH2)10CONHCH2pyRu(NH3)5](PF6)2). The incorporated redox sites have energetically low molecular orbitals which can align with the Fermi levels of the electrodes. A four-electrode configuration of the electrochemical junction allows control of the potentials of the electrodes with respect to the redox potential of the incorporated redox-active molecules. We show that under this control of potential the electron transport mechanism can be switched to different regimes and the current flowing through the junction can be modulated.