Electrochemical gate-controlled electron transport of redox-active single perylene bisimidemolecular junctions

Abstract

We report a scanning tunneling microscopy (STM) experiment in an electrochemicalenvironment which studies a prototype molecular switch. The target molecules were perylenetetracarboxylic acid bisimides modified with pyridine (P-PBI) and methylthiol (T-PBI)linker groups and with bulky tert-butyl-phenoxy substituents in the bay area. At a fixedbias voltage, we can control the transport current through a symmetric molecular wireAu|P-PBI(T-PBI)|Au by variation of the electrochemical ‘gate’ potential. The current increases by up to twoorders of magnitude. The conductances of the P-PBI junctions are typically a factor 3larger than those of T-PBI. A theoretical analysis explains this effect as a consequence ofshifting the lowest unoccupied perylene level (LUMO) in or out of the bias window whentuning the electrochemical gate potential VG. The difference in on/off ratios reflects thevariation of hybridization of the LUMO with the electrode states with the anchor groups.IT–ES(T) curves of asymmetric molecular junctions formed between a bare AuSTM tip and a T-PBI (P-PBI) modified Au(111) electrode in an aqueouselectrolyte exhibit a pronounced maximum in the tunneling current at−0.740, which is close to the formal potential of the surface-confined molecules. Theexperimental data were explained by a sequential two-step electron transfer process.