Self-Assembly of Ordered Pyridinium Layers at Halide Precovered Copper(100)/Electrolyte Interfaces

Abstract

The present work aims at elaborating a systematic understanding of the monolayer formation of bi- and dipyridinium cations on a Cl anion covered or bare Cu(100) electrode surface as substrate, with particular focus on the role of the intra-molecular flexibility and extension of the molecular Pi-system. The molecular flexibility of organic cations is increased by introducing methylene bridges between adjacent aromatic rings, allowing them to rotate around the tetrahedral C-C bond. However, the extension of the molecular Pi-system is reduced by these methylene bridges because Pi-electrons cannot pass the tetrahedral methylene limiting their delocalization to one side of the bridge.<br /> In the first part, the two viologens (1,1’-disubstitued-4,4’-bipyridinium cations), DPV (DiPhenylViologen) and DBV (DiBenzylViologen), as well as a 1:1 mixture of both molecules are investigated by cyclic voltammetry and in situ scanning tunneling microscopy at the Cl-c(2x2)/Cu(100) electrode. Upon adsorption, even at the most positive electrode potentials, the highly redox active DPV dication is immediately reduced to its monocation radical state, which is likely related to the wide extension of the Pi-system over the whole DPV molecule. Adsorbed DPV monocation radicals form their characteristic Pi-stacked stripe phase, which prevents the adsorption of the more flexible DBV dication and hence, the formation of the DBV dication cavitand phase, as it occurs in pure DBV dication solution on the Cl/Cu(100) substrate. However, adsorbed DPV monocation radicals are displaced from the substrate by DBV monocation radicals, formed at electrode potentials lower than Ework = −270mV [vs. RHE]. The adsorbed DBV monocation radicals arrange in their own characteristic Pi-stacked stripe phase on the Cl-c(2x2) anion lattice. Further lowering the electrode potential induces the desorption of the Cl-c(2x2) anion lattice underneath the DBV stripe phase, leading to the conversion of the ordered stripe phase into an amorphous viologen layer on the bare Cu(100) electrode surface.<br /> In the second part, the 4,4’-bipyridinium core of DBV is modified in three different ways, resulting in three “non-viologen-type” molecules. These new molecules are studied to extend the understanding of the relation between molecular structure, redox activity and structure formation. The investigated molecules are DB-3,3’-BP (1,1´-Dibenzyl-3,3´-bipyridinium), C3-DBDP (1,1’-Dibenzyl-4,4’-(propane-1,3-diyl)-dipyridinium) and C7-DBDP (1,1’-Dibenzyl-4,4’-(heptane-1,7-diyl)-dipyridinium). Their dication state is less redox active than the initial DBV dication and thus, none of them is reduced to a lower redox state within the potential window accessible in aqueous solution.<br /> DB-3,3’-BP only forms an amorphous layer on Cl/Cu(100) or bare Cu(100) substrate, indicating that the position of the nitrogen atoms within the bipyridinium core affects the interaction of the organic cation to the substrate. C3-DBDP and C7-DBDP form ordered organic monolayers on Cl-c(2x2)/Cu(100), as well as on bare Cu(100), which might be a result of their higher intra-molecular flexibility compared to DPV and DBV. <br /> On Cl/Cu(100) and bare Cu(100), C3-DBDP dications arrange in an inter-linked stripe phase, which is suggested to be stabilized by the inter-molecular electrostatic attraction between directly facing pyridinium cations, whereas its missing commensurable relation to the substrate suggests that adsorbate-substrate attractions are of minor importance. <br /> The more flexible C7-DBDP dication forms two different ordered monolayers on the Cl/Cu(100) substrate: the C7-DBDP mesh phase and the C7-DBDP stripe phase. The former is energetically favored on the Cl/Cu(100) substrate and substitutes the latter after several hours. This changes when the potential is lowered into the Cl-c(2x2) desorption regime, where the C7-DBDP mesh phase decomposes into an amorphous layer. The C7-DBDP stripe phase domains on the other hand remain stable and even grow into the amorphous domains on the bare Cu(100) surface. However, the decomposition of the C7-DBDP stripe phase can be triggered at the lowest potentials accessible in the aqueous electrolyte. This decay triggered by lowering the potential, thus increasing the electrostatic attraction of C7-DBDP cations to the electrode, suggests that a second Cl anion species is adsorbed above or within the organic monolayer, essential for the stability the C7-DBDP stripes.