Investigations on the monolayer structure of thiol SAMs and the influence of conjugated π-bonds on the electronic molecular conductivity

Electrically transmissive alkyne-anchored monolayers on gold

Well-ordered, tightly-packed (surface coverage 0.97 × 10-9 mol cm-2) monolayer films of 1,4-bis((4-ethynylphenyl)ethynyl)benzene (1) on gold are prepared via a simple self-assembly process, taking advantage of the ready formation of alkynyl C-Au σ-bonds. Electrochemical measurements using [Ru(NH3)6]3+, [Fe(CN)6]3-, and ferrocenylmethanol [Fe(η5-C5H4CH2OH)(η5-C5H5)] redox probes indicate that the alkynyl C-Au contacted monolayer of 1 presents a relatively low barrier for electron transfer. This contrasts with monolayer films on gold of other oligo(phenylene ethynylene) derivatives of comparable length and surface coverage, but with different contacting groups. Additionally, a low voltage transition (Vtrans = 0.51 V) from direct tunneling (rectangular barrier) to field emission (triangular barrier) is observed. This low transition voltage points to a low tunneling barrier, which is consistent with the facile electron transport observed through the C-Au contacted self-assembled monolayer of 1.

Oxygen attachment on alkanethiolate SAMs induced by low-energy electron irradiation

Reactions of (18)O2 with self-assembled monolayer (SAM) films of 1-dodecanethiol, 1-octadecanethiol, 1-butanethiol, and benzyl mercaptan chemisorbed on gold were studied by the electron stimulated desorption (ESD) of anionic fragments over the incident electron energy range 2-20 eV. Dosing the SAMs with (18)O2 at 50 K results in the ESD of (18)O(-) and (18)OH(-). Electron irradiation of samples prior to (18)O2 deposition demonstrates that intensity of subsequent (18)O(-) and (18)OH(-) desorption signals increase with electron fluence and that in the absence of electron preirradiation, no (18)O(-) and (18)OH(-) ESD signals are observed, since oxygen is unable to bind to the SAMs. A minimum incident electron energy of 6-7 eV is required to initiate the binding of (18)O2 to the SAMs. O2 binding is proposed to proceed by the formation of CHx-1(•) radicals via resonant dissociative electron attachment and nonresonant C-H dissociation processes. The weaker signals of (18)O(-) and (18)OH(-) from short-chain SAMs are related to the latter's resistance to electron-induced damage, due to the charge-image dipole quenching and electron delocalization. Comparison between the present results and those for DNA oligonucleotides self-assembled on Au (Mirsaleh-Kohan, N. et al. J. Chem. Phys. 2012, 136, 235104) indicates that the oxygen binding mechanism is common to both systems.

On the Interpretation of Multiple Waves in Cyclic Voltammograms of Self-Assembled Monolayers of n-Alkane Thiols on Gold

A homologue series of n-alkane thiols (CH3(CH2) m SH, MCm, m between 8 and 22) adsorbed on Au/mica substrates was studied in an alkaline environment using cyclic voltammetry (CV) and second harmonic generation (SHG). The known split of the reductive desorption signal in the cyclic voltammogram for thiols with chain length of m > 12 disappears again for m = 22. Simultaneous recording of SHG and CV data allowed correlation of thiol coverage with charge which was found to be linear, irrespective of the shape of the desorption signal. This evidences that the double peak structure of the desorption signal does not arise from the separation of faradaic and capacitive processes. The same is found for the double wave of the readsorption signal. The transient appearance of a split desorption signal for the one electron process of thiol desorption in the range of 12 < m < 22 is suggested to originate from intermolecular interactions.

Self-assembled monolayers of a bis(pyrazol-1-yl)pyridine-substituted thiol on Au(111)

Self-assembled monolayers (SAMs) of a bis(pyrazol-1-yl)pyridine-substituted thiol (bpp-SH) on Au (111)/mica were studied with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Using substrates precoated with perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA), preparation at elevated temperatures yields highly ordered layers whose structure is described by a rectangular (5 x radical3) unit cell containing one molecule. The bis(pyrazol-1-yl)pyridine (bpp) units exhibit pi-stacking along the 112 direction, and they are tilted significantly. We conclude the three imine nitrogen atoms in the bpp headgroup adopt a trans,trans arrangement.

Electron transfer studies on cholesterol LB films assembled on thiophenol and 2-naphthalenethiol self-assembled monolayers

We have formed the cholesterol monolayer and multilayer LB films on the self-assembled monolayers of 2-naphthalenethiol (2-NT) and thiophenol (TP) and studied the electrochemical barrier properties of these composite films using cyclic voltammetry and electrochemical impedance spectroscopy. We have also characterized the cholesterol monolayer film using grazing angle FTIR, scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Cholesterol has a long hydrophobic steroid chain, which makes it a suitable candidate to assemble on the hydrophobic surfaces. We find that the highly hydrophobic surface formed by the self-assembled monolayers (SAM) of 2-NT and TP act as effective platforms for the fabrication of cholesterol monolayer and multilayer films. The STM studies show that the cholesterol monolayer films on 2-NT form striped patterns with a separation of 1.0 nm between them. The area per cholesterol molecule is observed to be 0.64 nm2 with a tilt angle of about 28.96 degrees from the surface normal. The electrochemical studies show a large increase in charge transfer resistance and lowering of interfacial capacitance due to the formation of the LB film of cholesterol. We have compared the behavior of this system with that of cholesterol monolayer and multilayers formed on the self-assembled monolayer of thiophenol.

Photoswitching electron transport properties of an azobenzene containing thiol-SAM

The influence of conformational and electrical properties of azobenzene molecules on the electron transfer barrier properties of their SAMs was studied by SECM and ellipsometry.

Self-assembled monolayers of aromatic thiols stabilized by parallel-displaced pi-pi stacking interactions

Parallel-displaced pi-pi stacking interactions have been known to be the dominant force in stabilizing the double helical structure of DNA and the tertiary structure of proteins. However, little is known about their roles in self-assembled monolayers of other large pi molecules such as aromatic thiols. Here we report on a systematic study of the self-assembled monolayers of four kinds of anthracene-based thiols, 9-mercaptoanthracene (MA), (4-mercaptophenyl) (9-anthryl) acetylene (MPAA), (4-mercaptophenyl) (10-nitro-9-anthryl) acetylene (MPNAA), and (4-mercaptophenyl) (10-carboxyl-9-anthryl) acetylene (MPCAA) on Au(111), in which a spacer and different functional groups (NO2 and COOH) are intentionally designed to introduce and thus allow the investigation of various intermolecular interactions, in addition to pi-pi interactions in the base molecules. We find that all molecules form long-range-ordered monolayers and, more interestingly, that these assembled monolayers exhibit essentially the same fundamental packing structure. On the basis of high-resolution scanning tunneling microscopy observations, we propose the space-filling models for the observed superstructures and demonstrate that all superstructures can be understood in terms of the parallel-displaced pi-pi stacking interactions, despite the presence of competing dipole-dipole and H-bonding interactions associated with these specially designed functional groups.

Scanning Tunneling Microscopy, Fourier Transform Infrared Spectroscopy, and Electrochemical Characterization of 2-Naphthalenethiol Self-Assembled Monolayers on the Au Surface: A Study of Bridge-Mediated Electron Transfer in Ru(NH3)62+|Ru(NH3)63+ Redox Reactions

We have studied the structure, adsorption kinetics, and barrier properties of self-assembled monolayers of 2-naphthalenethiol on Au using electrochemical techniques, grazing-angle Fourier transform infrared (FTIR) spectroscopy, and scanning tunneling microscopy (STM). The results of cyclic voltammetric and impedance measurements using redox probes show that 2-naphthalenethiol on Au forms a stable and reproducible, but moderately blocking, monolayer. Annealing of the self-assembled monolayer (SAM)-modified surface at 72 +/- 2 degrees C remarkably improves the blocking property of the monolayer of 2-naphthalenethiol on Au. From the study of kinetics of SAM formation, we find that the self-assembly follows Langmuir adsorption isotherm. Our STM and FTIR results show that the molecules are adsorbed with the naphthalene ring tilted from the surface normal by forming a square root 3 x 3 R30 degrees overlayer structure. From our studies, we conclude that the electron-transfer reaction of ferro/ferricyanide in the freshly formed monolayer occurs predominantly through the pinholes and defects present in the monolayer. However, in the case of thermally annealed specimen, although the ferro/ferricyanide reaction is almost completely blocked, the electron-transfer reaction of hexaammineruthenium(III) chloride is not significantly inhibited. It is proposed that the electron-transfer reaction in the case of the ruthenium redox couple takes place by a tunneling mechanism through the high-electron-density aromatic naphthalene ring acting as a bridge between the monolayer-modified electrode and the ruthenium complex.