Cyclic voltammetry and XPS studies of monolayers deposited on gold and platinum electrodes displaced by mercaptopyridines

The synthesis and characterization of a side-by-side iron phthalocyanine dimer

The QCA paradigm is one of the approaches to decrease the size scale of computing devices. When molecules are used as QCA cells, they may be able to perform computing at room temperature. This paper describes a novel molecular QCA cell candidate which is a side-by-side iron phthalocyanine dimer, and an investigation of its optical and redox properties. The new dodeca(pentyloxy) substituted side-by-side iron phthalocyanine dimer, along with the octa(pentyloxy) iron phthalocyanine monomer, are soluble in non-polar organic solvents. These compounds were isolated by gel permeation chromatography (GPC) and high-performance liquid chromatography (HPLC) to final purities of 98% and 99%, respectively. The NMR spectra of both compounds in CDCl3 are broad due to aggregation, but become well resolved after the addition of the coordinating solvent pyridine-d5. Addition of pyridine also gives changes in the UV-vis spectra and electrochemical peaks of both monomer and dimer in dichloromethane indicative of axial iron coordination. The electrochemical data indicates the loss of pyridine ligands from the oxidized products of both monomer and dimer. The comproportionation constant of side-by-side phthalocyanine dimer shows that its oxidized and reduced mixed-valence complexes are fairly stable. The dimer is thus a candidate for molecular QCA systems.

Structure and electrochemistry of 4,4'-dithiodipyridine self-assembled monolayers in comparison with 4-mercaptopyridine self-assembled monolayers on Au(111)

4,4'-Dithiodipyridine (PySSPy) monolayers on Au(111) were investigated by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS) and in situ scanning tunneling microscopy (STM). The studies were performed in solutions of different anions and pHs (0.1 M H2SO4, 0.1 M HClO4, 0.1 and 0.01 M Na2SO4, 0.1 and 0.01 M NaOH). The cyclic current-potential curves in H2SO4 show current peaks at about 0.4 V, which are absent for all other electrolytes at this potential. The XPS data suggest that PySSPy adsorbs via the S endgroup on the gold surface and the S-S bond breaks during adsorption. From the chemical shift of the N(ls) peak, it is concluded that in acidic media the self-assembled monolayer (SAM) is fully protonated, whereas in basic solution it is not. The pKa is estimated to be 5.3. STM studies reveal the existence of highly ordered superstructures for the SAM. In Na2SO4 and H2SO4, a (7 x mean square root of 3) structure is proposed. However, whereas in Na2SO4 solutions the superstructure does not change with potential, in 0.1 M H2SO4 the superstructure is observed only negative of the current peak at +0.4 V. At more positive potentials, the film becomes disordered. The results are compared to those for 4-mercaptopyridine (PyS) SAMs. XPS experiments and current-potential curves indicate that both molecules adsorb in the same manner on Au(111), that is, even in the case of PySSPy the adspecies is PyS. The STM results, however, call for a more subtle interpretation. While in Na2SO4 solutions the observed superstructures are the same for both SAMs, markedly different structures are found for PySSPy and PyS SAMs in 0.1 M H2SO4.

Self-assembled monolayers of 4-mercaptopyridine on Au(111): a potential-induced phase transition in sulfuric acid solutions

In situ scanning tunneling microscopy images of self-assembled monolayers (SAMs) of 4-mercaptopyridine (4-MPy) on Au(111) recorded in neat 0.1 M H2SO4 solutions provided evidence for a potential-induced phase transition over the range 0.40-0.15 V versus saturated calomel electrode. Analysis of the data was consistent with the presence of a (5 x square root(3)) and (10 x square root(3)) superstructure (phase A) at the positive end, that is, 0.40 V, for which the local coverage, theta(loc), is about 0.2 (two 4-MPy molecules per unit cell), which compresses at the negative end, that is, 0.15 V, to yield a much denser superstructure (phase B, theta(loc) ca. 0.5). This behavior is unlike that reported for the 4-MPy-Au(111) SAM prepared by identical means, in 0.1 M HClO4 (or in sulfate solutions of a much higher pH) for which only the (5 x square root(3)) superstructure was observed over the same potential range. The compression associated with the phase A to phase B transition is attributed to the formation of a hydrogen-bonded network of bisulfate coordinated in turn to the 4-MPy layer via the acidic hydrogens of the pyridinium moieties. Such conditions promote better packing of adsorbed 4-MPy species, which are aided by intermolecular pi-pi ring interactions, resulting in higher local coverages.