Potential dependent structure transitions of heptyl viologen layers on Cu(1 0 0) studied by in situ STM and IRRAS

The redox behavior and potential dependent adsorption structure of heptyl viologen (1,1′-Diheptyl-4,4′-bipyridinium dichloride, DHV2+) on a Cu(1 0 0) electrode have been investigated by Cyclic Voltammetry (CV), in situ Electrochemical Scanning Tunneling Microscopy (EC-STM) and Infrared Reflection Absorption Spectroscopy (IRRAS). Current peaks in CV measurements of the redox-active DHV molecule are ascribed to two one-electron transfer steps and phase transformation processes. A highly ordered 2D dot-array structure emerges on the c(2 × 2)-Cl modified Cu(1 0 0) electrode surface between +200 and −100 mV in STM images. One-electron reduction of the dications DHV2+ around −150 mV causes a phase transition from a dot to a stripe pattern which has a bi-layer structure in the STM images. With a further decrease of the applied potential, the structure of the adlayer undergoes a further change from the bi-layer phase to a close-packed monolayer of stripes. An IRRAS analysis on the orientation of the bipyridine moieties reveals a slight tilt of the long molecular axis with respect to the surface in the dicationic phase, but a parallel orientation in the monocationic striped phases. Additionally a large fraction of the hydrocarbon chains are in gauche conformation with low order among them. Only in the bi-layer stripe phase evidence for a marginal order between the alkyl chains was found. Calculation of the average angle of the alkyl chains with respect to the surface normal reveals an orientation along the surface within the dot structure but an outward tilt within the closed packed monolayer due to space limitation between the stripes.