Redox activity and structural transition of heptyl viologen adlayers on Cu(100)

The redox behaviour and potential-dependent adsorption structure of heptyl viologen (1,1'-diheptyl-4,4'-bipyridinium dichloride, DHV(2+)) on a Cu(100) electrode was investigated in a chloride-containing electrolyte solution by cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy (EC-STM). The dicationic DHV molecules generate a few pairs of current waves in CV measurements which are ascribed to two typical one-electron transfer steps. STM images obtained in a KCl-containing electrolyte solution disclose a well-ordered c(2x2) chloride adlayer on a Cu(100) electrode surface. After injecting DHV(2+) molecules into the KCl electrolyte solution, a highly ordered 2D "dot-array" structure in STM images emerges on the c(2x2)-Cl modified Cu(100) electrode surface. DHV(2+) molecules spontaneously arrange themselves with their molecular planes facing the electrode surface and their long molecular axis parallel to the step edge. Such adsorption structure can be described by mirror domains and rotational domains which stably exist between 200 mV and -100 mV. One-electron reduction of the dications DHV(2+) around -150 mV causes a phase transition from a 'dot-array' assembly to a stripe pattern formed by DHV(*+) radical monocations in STM images which has a bilayer structure. With a further decrease of the applied electrode potential, the structure of the DHV(*+) adlayer undergoes a change from a loose stripe phase to a more compact stripe phase, a subsequent decay of the compact structure, and finally the formation of a new dimer phase. A further electron transfer reaction at -400 mV causes the formation of an amorphous phase on the chloride free electrode surface. In a reverse anodic sweep, the reproduction of the ordered DHV(*+) stacking phase occurs again on top of the chloride lattice.