We compared the electrical properties of selfassembled monolayers (SAMs) formed on template-stripped Au from two homologous series of five different oligo-(phenylene)s bearing alkane thiol tails. The terminal phenyl ring is substituted by a 4-pyridyl ring in one series, thus the two differ only by the substitution of C−H for N. We formed tunneling junctions using the liquid metal eutectic Ga−In (EGaIn) as a nondamaging, conformal top contact that is insensitive to functional groups and measured the currentdensity, J, tunneling decay constants, β, and transition voltages, V trans . Conductance measurements alone did not sufficiently differentiate the two series of molecules. The length dependences of the two series of SAMs produced values of β of 0.44 and 0.42 Å −1 for pyridyl-and phenyl-terminated SAMs, respectively, which lie between the expected values for alkanethiolates and oligo(phenylene)s. The values of V trans were ∼0.3 V larger for the phenylterminated SAMs than for the pyridyl-terminated SAMs. A comparison of the values of J to highest occupied molecular orbital (HOMO) levels determined by density functional theory (DFT) calculations revealed an odd−even effect for the phenylterminated SAMs but not the pyridyl-terminated SAMs. Plots of V trans versus the measured shift in work function, measured with a Kelvin probe, reveal a roughly linear trend. Plots of the difference between HOMO and Fermi energies reveal a strong linear trend with two distinct series that clearly differentiate the two series of SAMs, even between SAMs with nearly identical HOMO levels, but only when the dipole-induced shift in vacuum level is considered. The influence of the electronic properties of the SAMs is clearly evident in the conductance data and highlights the importance of molecular dipoles in tunneling junctions comprising SAMs. Taken together, the data show that tunneling junctions incorporating EGaIn as a top contact are sensitive enough to differentiate SAMs that differ by the substitution of a single atom.

Vibrational sum-frequency-generation (SFG) spectroscopy experiments at electrified interfaces involve incident laser radiation at frequencies in the IR and near-IR/visible regions as well as a static electric field on the surface. Here we show that mixing the three fields present on the surface can result in third-order effects in resonant SFG signals. This was achieved for closed packed self-assembled monolayers (SAMs) with molecular groups of high optical nonlinearity and surface potentials similar to those typically applied in cyclic voltammograms. Broadband SFG spectroscopy was applied to study a hydrophobic well-ordered araliphatic SAM on a Au(111) surface using a thin-layer analysis cell for spectro-electrochemical investigations in a 100 mM NaOH electrolyte solution. Resonant contributions were experimentally separated from non-resonant contributions of the Au substrate and theoretically analyzed using a fitting function including third-order terms. The resulting ratio of third-order to second-order susceptibilities was estimated to be scriptO(10−10) m/V.

Based on electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), Au(111) electrodes modified by self-assembled monolayers (SAMs) of a homologous series of pyridine-terminated thiols with aromatic backbones have been investigated. An important correlation between the chain structure and film integrity in electrolytic media was found. Monolayers with odd numbers of methylene spacers in the molecular chain showed superior barrier properties compared to even numbered counterparts. A positive influence of an increase in the number of attached phenyl rings on the integrity of SAMs was observed. Furthermore, cathodic desorption of the investigated SAMs is characterized by multiwave desorption peaks and extraordinarily large cathodic charges indicating an unusual desorption process. Moreover, protonation behavior of the SAMs has been investigated by X-ray photoelectron spectroscopy (XPS) and electrochemical methods. Protonation has been found to be reversible and surface pK(a) values have been determined to be around 5 for all investigated monolayers.

Electroreductive desorption of a highly ordered self-assembled monolayer (SAM) formed by the araliphatic thiol (4-(4-(4-pyridyl)phenyl)phenyl)methanethiol leads to a concurrent rapid hydrogen evolution reaction (HER). The desorption process and resulting interfacial structure were investigated by voltammetric techniques, in situ spectroscopic ellipsometry, and in situ vibrational sum–frequency–generation (SFG) spectroscopy. Voltammetric experiments on SAM-modified electrodes exhibit extraordinarily high peak currents, which di er between Au(111) and polycrystalline Au substrates. Association of reductive desorption with HER is shown to be the origin of the observed excess cathodic charges. The studied SAM preserves its two–dimensional order near Au surface throughout a fast voltammetric scan even when the vertex potential is set several hundred millivolt beyond the desorption potential. A model is developed for the explanation of the observed rapid HER involving ordering and pre–orientation of water present in the nanometer–sized reaction volume between desorbed SAM and the Au electrode, by the structurally extremely stable monolayer, leading to the observed catalysis of the HER.