Self-assembly of viologen molecules at metal/electrolyte interfaces under <em>non-reactive </em>and<em> reactive</em> conditions

Abstract

Nowadays, organic chemistry can synthesize almost any kind of functional organic molecules, while surface physics on the other hand provides analytical techniques of high microscopic and spectroscopic resolution. Therefore, the symbiosis of organic chemistry and surface physics is a prerequisite to understand and design the physical and chemical properties of organic films. Particularly in solution, by applying electrochemical techniques, the electrochemical potential is an additional parameter to control the deposition and growth of organic layers. In supramolecular chemistry, the role of viologens as molecular building-blocks is particularly interesting thanks to the intra-molecular conformational changes between their different redox-states. Motivated by that approach, this work focuses on the surface redox-chemistry and the related structural phase behavior of electrodeposited viologens, specifically, Dibenzyl-Viologen (DBV) and Diphenyl-Viologen (DPV) adsorbed on a chloride modified Cu(100) or HOPG substrate, using a combination of the three methods: Cyclic Voltammetry (CV), in-situ Electrochemical Scanning Tunneling Microscopy (EC-STM) and ex-situ X-ray Photoelectron Spectroscopy (XPS).<br /> On the chloride precovered Cu(100) template, the DBV species adsorb under non-reactive conditions at high initial electrode potentials as DBV2+ dication and form a so-called cavitand phase. Electrostatic interaction between the negatively charged chloride layer and these dicationic viologens is taken to be the major driving force for their adsorption and subsequent lateral ordering on the chloride modified electrode surface. A new proposed structural model of the cavitand phase agrees with the obtained high resolution STM images and improves a previously published model considerably. Reducing the di-cationic DBV2+ species to the corresponding radical mono-cation DBV•+ causes a phase transition from the pre-existing DBV2+ cavitand phase to a stacking phase following a nucleation and growth mechanism. DBV•+ species are adsorbed with their main molecular axis parallel to the surface in a side-on adsorption geometry. Enhanced intermolecular π-π interactions are identified as the main driving force for the formation of 1D oligomer and polymer chains as the characteristic structural motif of the stacking phase. These structural motifs are generally independent of the electronic and structural substrate properties. Chloride desorption through the viologen film is discussed as the reason for an order–disorder transition within the viologen film at even more negative potentials.<br /> In contrast to the adsorption of the dication DBV2+ cavitand phase, DPV2+ species adsorbing from solution instantaneously react to the corresponding radical cation DPV•+ on the chloride-modified surface followed by the formation of a condensed (DPV•+)n stacking monolayer phase. The N 1s core level binding energy indicates only the presence of the corresponding mono-reduced DPV•+ species on the surface even at potentials more positive than the redox potential of the bulk solution species. Therefore, DPV can be regarded as prototype of a highly reactive viologen species. This stacking layer is highly hydrophobic which completely removes all water species from the chloride lattice. Upon adsorption and phase formation, the DPV releases also completely its solvation shell as a pre-requisite to undergo strong inter-viologen interactions, most likely in the form of π-π interactions. The structure of this stacking phase is characterized by the preferential adsorption at steps of [100] orientation, forming typical double rows along the step-edges on the chloride precovered template. <br /> By far, most of the EC-STM investigations have been performed under potentiostatic and equilibrium (or non-reactive) conditions. However, it is for the first time that a measured system was also prepared under reactive conditions, i.e. during an ongoing electron transfer reaction. Adsorbing DBV at the onset of the first reduction peak, the adsorption and reduction take place simultaneously, showing two new metastable DBV dimer phases. While dimer species are considered as thermodynamically stable in aqueous solution we find on surfaces, by contrast, metastable dimers that can irreversibly be converted either into the oxidized species (anodic potential sweep) or into the more compact and thermodynamically favorable “polymeric” stacking phase of radical mono-cations (cathodic sweep). The reactive DBV adsorption leads to considerably different potential-dependent surface compositions, coverages and 2D structures than known from the non-reactive DBV adsorption that is subsequently followed by the potential triggered electron transfer reaction, giving rise to the interesting observation of tip-induced effects.