Tripodal Binding Units for Self-Assembled Monolayers on Gold: A Comparison of Thiol and Thioether Headgroups

Whereas thiols and thioethers are frequently used as binding units of oligodentate precursor molecules to fabricate self-assembled monolayers (SAMs) on coinage metal and semiconductor surfaces, their use for tridentate bonding configuration is still questionable. Against this background, novel tridentate thiol ligands, PhSi(CH(2)SH)(3) (PTT) and p-Ph-C(6)H(4)Si(CH(2)SH)(3) (BPTT), were synthesized and used as tripodal adsorbate molecules for the fabrication of SAMs on Au(111). These SAMs were characterized by X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The PTT and BPTT films were compared with the analogous systems comprised of same tripodal ligands with thioether instead of thiol binding units (anchors). XPS and NEXAFS data suggest that the binding uniformity, packing density, and molecular alignment of the thiol-based ligands in the respective SAMs is superior to their thioether counterparts. In addition, the thiol-based films showed significantly lower levels of contamination. Significantly, the quality of the PTT SAMs on Au(111) was found to be even higher than that of the films formed from the respective monodentate counterpart, benzenethiol. The results obtained allow for making some general conclusions on the specific character of molecular self-assembly in the case of tridentate ligands.     

Near-edge X-ray absorption fine structure spectroscopy of diamondoid thiol monolayers on gold

Diamondoids, hydrocarbon molecules with cubic-diamond-cage structures, have unique properties with potential value for nanotechnology. The availability and ability to selectively functionalize this special class of nanodiamond materials opens new possibilities for surface modification, for high-efficiency field emitters in molecular electronics, as seed crystals for diamond growth, or as robust mechanical coatings. The properties of self-assembled monolayers (SAMs) of diamondoids are thus of fundamental interest for a variety of emerging applications. This paper presents the effects of thiol substitution position and polymantane order on diamondoid SAMs on gold using near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and X-ray photoelectron spectroscopy (XPS). A framework to determine both molecular tilt and twist through NEXAFS is presented and reveals highly ordered diamondoid SAMs, with the molecular orientation controlled by the thiol location. C 1s and S 2p binding energies are lower in adamantane thiol than alkane thiols on gold by 0.67 +/- 0.05 and 0.16 +/- 0.04 eV, respectively. These binding energies vary with diamondoid monolayer structure and thiol substitution position, consistent with different degrees of steric strain and electronic interaction with the substrate. This work demonstrates control over the assembly, in particular the orientational and electronic structure, providing a flexible design of surface properties with this exciting new class of diamond nanoparticles.     

Self-assembled monolayers of cobalt(II)- (4-tert-butylphenyl)-porphyrins: the influence of the electronic dipole on scanning tunneling microscopy images

Self-assembled monolayers (SAMs) of cobalt(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin, a promising material for optical, photoelectrochemical, and chemical sensor applications, were prepared on Au(111) via axial ligation to 4-aminothiophenol, and studied by several surface science techniques. Scanning tunneling microscopy (STM) and spectroscopy (STS) measurements showed the apparent topology of the Au(111) herringbone structure reconstruction, but with bias-dependent contrast images and asymmetric I/V characteristics. Photoelectron spectroscopy confirmed the presence of metalloporphyrins on the surface, whereas near-edge X-ray absorption (NEXAFS) measurements revealed that the porphyrin ring was tilted by about 70 degrees with respect to the surface plane. The above effects are ascribed to the presence of oriented molecular dipole layers between the metal and the organic material as confirmed by a comparison with first-principles density-functional theory calculations. The measured bias-dependent STM profiles have been reproduced by a simple monodimensional tunneling model.     

Intermixed terpyridine-functionalized monolayers on gold: nonlinear relationship between terpyridyl density and metal ion coordination properties

Aiming at the functionalization of surfaces with terpyridine anchors for the coordinative deposition of additional layers, mixed self-assembled monolayers (SAMs) were prepared from binary solutions of 12-(2,2':6',2″-terpyridine-4'-yl)dodecane-1-thiol (TDT) and 1-decanethiol (DT). The SAMs and the order of the constituting molecules were analyzed by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and time-of-flight-secondary ion mass spectrometry (ToF-SIMS). The composition of the (TDT/DT)-SAMs and with it the surface density of terpyridyl groups correlates linearly with the relative concentrations of the two compounds in the solution used for depositing them. In marked contrast, the amount of terpyridine-coordinated Pd(II) ions significantly deviates from this trend with an optimum at a 1:3 ratio of TDT/DT. This indicates a major fraction of the terpyridines in TDT-rich SAMs not to be accessible for Pd(II) ion coordination. In agreement, NEXAFS spectroscopy reveals the alkyl backbones in TDT-rich SAMs not to be ordered, while they are preferentially upright oriented in the optimal 1:3-(TDT/DT)-SAMs. We interpret this in terms of terpyridine backfolding in TDT-rich SAMs, while they are located in accessible positions on top of the SAM in the 1:3-(TDT/DT)-SAM. While the alkyl backbones in the 1:3-(TDT/DT)-SAM are ordered, NEXAFS spectroscopy shows the terpyridyl groups not to have a preferential orientation in this SAM and thus retain enough flexibility to adjust to molecules that are deposited on top of the mixed SAM. In conclusion, the novel SAM does not undergo phase separation and consists predominantly of intermixed phases with adjustable surface density of quite flexible terpyridine anchor groups. The terpyridine-Pd(II) anchors are not only available for a future deposition of the next layer, but the metal ions also represent a sensitive probe for the accessibility of the terpyridyl groups.     

Molecular Suction Pads: Self‐Assembled Monolayers of Subphthalocyaninatoboron Complexes on Gold

Subphthalocyaninatoboron complexes with six long‐chain alkylthio substituents in their periphery are applicable for the formation of self‐assembled monolayers (SAMs) on gold. Such films are prepared from solution with the axially chlorido‐substituted derivatives and characterised by near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy, X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). The results are in accord with the formation of SAMs assembled by the chemisorption of both covalently bound thiolate‐type as well as coordinatively bound thioether units. The adsorbate molecules adopt an essentially flat adsorption geometry on the substrate, resembling a suction pad on a surface.     

Effect of ring substitution position on the structural conformation of mercaptobenzoic acid self-assembled monolayers on Au(111)

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, photoemission spectroscopy (PES), and contact angle measurements have been used to examine the structure and bonding of self-assembled monolayers (SAMs) prepared on Au(111) from the positional isomers of mercaptobenzoic acid (MBA). The isomer of MBA and solvent chosen in SAM preparation has considerable bearing upon film morphology. Carbon K-edge NEXAFS measurements indicate that the monomers of 2-, 3-, and 4-MBA have well-defined orientations within their respective SAMs. Monomers of 3- and 4-MBA assume an upright orientation on the Au substrates in monolayers prepared using an acetic acid in ethanol solvent. The aryl ring and carboxyl group of these molecules are tilted from the surface normal by a colatitudal angle of approximately 30 degrees . Preparation of 4-MBA SAMs using pure ethanol solvent, a more traditional means of synthesis, had no appreciable effect upon the monomer orientation. Nonetheless, S(2p) PES measurements illustrate that it results in extensive bilayer formation via carboxyl group hydrogen-bonding between 4-MBA monomers. In 2-MBA monolayers prepared using acetic acid/ethanol solvent, the monomers adopt a more prostrate orientation on the Au substrates, in which the aryl ring and carboxyl group of the molecules are tilted approximately 50 degrees from the surface normal. This configuration is consistent with an interaction between both the mercaptan sulfur and carboxyl group of 2-MBA with the underlying substrate. S(2p) and C(1s) PES experiments provide supporting evidence for a bidentate interaction between 2-MBA and Au(111).     

Dipole-dipole interactions and the structure of self-assembled monolayers

The structure of self-assembled monolayers (SAMs) on the gold (111) surface is still a matter of debate despite a considerable experimental and theoretical effort. We address the problem from a new perspective, studying the influence of electrostatic interactions on the degree of disorder in COOH-terminated SAMs. We show that the HS(CH2)(n-1)COOH molecules carry two dipole moments associated with their head- and tail-groups. Depending on the coupling of these dipole moments along the molecules, the structure of the COOH-SAMs either resembles the structure of the corresponding alkanethiol monolayers (short molecules, strong dipole coupling) or shows a more complex behavior (long molecules, weak dipole coupling). In the latter case, the monolayer exhibits a crystalline-like order with respect to the hydrocarbon chains and a high degree of disorder with respect to the carboxylic head-groups. These results resolve the controversy of experimental data on the degree of order in COOH-monolayers with near-edge X-ray absorption fine structure spectroscopy (Himmel, H.-J.; Weiss, K.; J?ger, B.; Dannenberger, O.; Grunze, M.; W?ll, Ch. Langmuir 1997, 13, 4943), probing the tail-groups, showing that the monolayer is largely disordered, and the infrared data (Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558) on the C-H stretching modes suggesting a crystalline-like order of the hydrocarbon chains.     

Thermal and mechanical aging of self-assembled monolayers as studied by near edge X-ray absorption fine structure

Self-assembled monolayers (SAMs) enable significant changes in the surface energy and/or specific interactions of surfaces, which are desirable for microelectromechanical systems (MEMS), superhydrophobic coatings, sensors, and other applications. However, SAMs often exhibit poor durability and rapid degradation upon mechanical, thermal, or moisture exposure. The chemical and orientational changes in SAMs due to mechanical and thermal degradation were investigated using near-edge X-ray absorption fine structure (NEXAFS) and the water contact angle. SAMs were based on unfluorinated or fluorinated linear hydrocarbons that form highly oriented and densely packed structures on silicon substrates. Complex chemical and orientational changes were observed via NEXAFS following degradation. Under heating in a dry, oxygen-rich environment, unfluorinated SAMs tended to cleave at C-C bonds on the main chain; below 250 °C, CH(3) groups were sequentially cleaved toward the surface, whereas above 250 °C, remaining hydrocarbon groups were converted to a graphitic coating dominated by C═C bonds. Under similar conditions, fluorinated SAMs began their chemical degradation at 350 °C and above, although the orientation decreased steadily from 150 to 300 °C; at and above 350 °C, the preferential removal of F occurred and the SAM was slowly converted to a graphitic layer. By contrast, under vacuum the fluorinated molecules were very thermally stable, showing good stability up to 550 °C; when degradation occurred, entire molecules were removed. Mechanical degradation followed two routes; both unfluorinated and fluorinated SAMs that were mechanically rubbed with smooth surfaces exhibited severe chemical degradation of the molecules, leading to an amorphous and poorly defined layer with C═C, C-C, C-H, and C-F bonds. Unfluorinated and fluorinated surfaces that were mechanically rubbed in the presence of free silicon particulates showed the rapid and complete destruction of both the molecular orientation and the protective SAM layer, even for short exposure periods. The resulting NEXAFS spectra were very similar to those produced by heating to 550 °C, suggesting that the friction created by granular particles may lead to extreme local heating.     

Self-assembled carbon nanotubes on gold: polarization-modulated infrared reflection-absorption spectroscopy, high-resolution X-ray photoemission spectroscopy, and near-edge X-ray absorption fine structure spectroscopy study

Recently we reported noncovalent functionalization of nanotubes in an aqueous medium with ionic liquid-based surfactants, 1-dodecyl-3-methylimidazolium bromide (1) and 1-(12-mercaptododecyl)-3-methylimidazolium bromide (2), resulting in positively charged single-wall carbon nanotube (SWNT)-1,2 composites. Thiolation of SWNTs with 2 provides their self-assembly on gold as well as templating gold nanoparticles on SWNT sidewalls via a covalent -S-Au bond. In this investigation, we studied the electronic structure, intermolecular interactions, and packing within noncovalently thiolated SWNTs and also nanotube alignment in the bulk of SWNT-2 dried droplets and self-assembled submonolayers (SAMs) on gold by high-resolution X-ray photoemission spectroscopy (HRXPS), C K-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS). HRXPS data confirmed the noncovalent nature of interactions within the nanocomposite of thiolated nanotubes. In PM-IRRAS spectra of SWNT SAMs on gold, the IR-active vibrational SWNT modes have been observed and identified. According to PM-IRRAS data, the hydrocarbon chains of 2 are oriented with less tilt angle to the bare gold normal in a SAM deposited from an SWNT-2 dispersion than those of 1 deposited from an SWNT-1 dispersion on the mercaptoethanesulfonic acid-primed gold. For both the dried SWNT-2 bulk and the SWNT-2 SAM on gold, the C K-edge NEXAFS spectra revealed the presence of CH-pi interactions between hydrocarbon chains of 2 and the pi electronic nanotube structure due to the highly resolved vibronic fine structure of carbon 1s --> R*/sigma*C-H series of states in the alkyl chain of 2. For the SWNT-2 bulk, the observed splitting and upshift of the SWNT pi* orbitals in the NEXAFS spectrum indicated the presence of pi-pi interactions. In the NEXAFS spectrum of the SWNT-2 SAM on gold, the upshifted values of the photon energy for R*/sigma*C-H transitions indicated close contact of 2 with nanotubes and with a gold surface. The angle-dependent NEXAFS for the SWNT-2 bulk showed that most of the molecules of 2 are aligned along the nanotubes, which are self-organized with orientation parallel to the substrate plane, whereas the NEXAFS for the SWNT-2 SAM revealed a more normal orientation of functionality 2 on gold compared with that in the SWNT-2 bulk.     

Effect of the chemical composition on the work function of gold substrates modified by binary self-assembled monolayers

This study demonstrated that the work function (Φ) of Au substrates can be fine-tuned by using series ratios of binary self-assembled monolayers (SAMs). By using pure amine- and carboxylic acid-bearing alkanethiol SAM on gold substrates, Φ of Au changed from 5.10 to 5.16 and 5.83, respectively, as determined by ultra-violet photoelectron spectrometry (UPS). The shift in Φ due to the use of different functional groups was rationalized by considering the dipole moments of the molecules anchored on the Au surface. A series of binary SAMs were fabricated by mixing carboxylic acid- and amine-terminated alkanethiols in the deposition solution. By mixing these functional groups in SAMs, a linear correlation between Φ with respect to chemical composition (hence the effective dipole moment on the Au surface) was observed. It was found that arbitrary Φ between extremes (5.16 and 5.83) controlled by respective functional groups can be obtained by changing the chemical composition of SAMs. The Scanning Kelvin Probe (SKP) was also used to measure the contact potential difference (CPD) between SAMs and referencing Au on a patterned substrate prepared by photo-lithography. It was found that the CPD of SAMs with different chemical compositions correlates to their Φ. However, the magnitude of the CPD was smaller than the difference in Φ measured by UPS that was possibly due to the adsorption of contaminants in air.     

Quantum theory of atoms in molecules charge-charge flux-dipole flux models for the infrared intensities of X(2)CY (X = H, F, Cl; Y = O, S) molecules

The molecular dipole moments, their derivatives, and the fundamental IR intensities of the X2CY (X = H, F, Cl; Y = O, S) molecules are determined from QTAIM atomic charges and dipoles and their fluxes at the MP2/6-311++G(3d,3p) level. Root-mean-square errors of +/-0.03 D and +/-1.4 km mol(-1) are found for the molecular dipole moments and fundamental IR intensities calculated using quantum theory of atoms in molecules (QTAIM) parameters when compared with those obtained directly from the MP2/6-311++G(3d,3p) calculations and +/-0.05 D and 51.2 km mol(-1) when compared with the experimental values. Charge (C), charge flux (CF), and dipole flux (DF) contributions are reported for all the normal vibrations of these molecules. A large negative correlation coefficient of -0.83 is calculated between the charge flux and dipole flux contributions and indicates that electronic charge transfer from one side of the molecule to the other during vibrations is accompanied by a relaxation effect with electron density polarization in the opposite direction. The characteristic substituent effect that has been observed for experimental infrared intensity parameters and core electron ionization energies has been applied to the CCFDF/QTAIM parameters of F2CO, Cl2CO, F2CS, and Cl2CS. The individual atomic charge, atomic charge flux, and atomic dipole flux contributions are seen to obey the characteristic substituent effect equation just as accurately as the total dipole moment derivative. The CH, CF, and CCl stretching normal modes of these molecules are shown to have characteristic sets of charge, charge flux, and dipole flux contributions.     

Molecular orientation in helical and all-trans oligo(ethylene glycol)-terminated assemblies on gold: results of ab initio modeling

The structural properties of self-assembled monolayers (SAMs) of oligo(ethylene glycol) (OEG)-terminated and amide-containing alkanethiols (HS(CH(2))(15)CONH(CH(2)CH(2)O)(6)H and related molecules with shorter alkyl or OEG portions) on gold are addressed. Optimized geometry of the molecular constituents, characteristic vibration frequencies, and transition dipole moments are obtained using density-functional theory methods with gradient corrections. These data are used to simulate IR reflection-absorption (RA) spectra associated with different OEG conformations. It is shown that the positions and relative intensities of all characteristic peaks in the fingerprint region are accurately reproduced by the model spectra within a narrow range of the tilt and rotation angles of the alkyl plane, which turns out to be nearly the same for the helical and all-trans OEG conformations. In contrast, the tilt of the OEG axis changes considerably under conformational transition from helical to all-trans OEG. By means of ab initio modeling, we also clarify other details of the molecular structure and orientation, including lateral hydrogen bonding, the latter of which is readily possessed by the SAMs in focus. These results are crucial for understanding phase and folding characteristics of OEG SAMs and other complex molecular assemblies. They are also expected to contribute to an improved understanding of the interaction with water, ions, and ultimately biological macromolecules.     

Atomic dipole moments calculated using analytical molecular second-moment gradients

We have implemented analytical second-moment gradients for Hartree-Fock and multiconfigurational self-consistent-field wave functions. The code is used to calculate atomic dipole moments based on the generalized atomic polar tensor (GAPT) formalism [Phys. Rev. Lett. 62, 1469 (1989)], and the proposal of Dinur and Hagler (DH) for the calculation of atomic multipoles [J. Chem. Phys. 91, 2949 (1989)]. Both approaches display smooth basis-set convergence toward a well-defined basis-set limit and give reasonable electron correlation effects on the calculated atomic properties. However, the atomic charges and atomic dipole moments obtained from the GAPT partitioning scheme are unable to provide even qualitatively meaningful molecular quadrupole moments for some molecules, and thus the atomic multipole moments calculated in this scheme cannot be considered well suited for analyzing the electron density in molecules and for calculating intermolecular interaction energies. In contrast, the DH approach gives atomic charges and dipole moments that by definition exactly reproduce the molecular quadrupole moments. The approach of DH is, however, restricted to planar molecules and thus suffers from not being applicable to molecules of arbitrary shape. Both the GAPT and DH approaches give rather poor results for octupole and hexadecapole moments, indicating that at least atomic quadrupole moments are required for an accurate representation of the molecular charge distribution in terms of atomic electric moments.     

Photocontrolled On‐Surface Pseudorotaxane Formation with Well‐Ordered Macrocycle Multilayers

The photoinduced pseudorotaxane formation between a photoresponsive axle and a tetralactam macrocycle was investigated in solution and on glass surfaces with immobilized multilayers of macrocycles. In the course of this reaction, a novel photoswitchable binding station with azobenzene as the photoswitchable unit and diketopiperazine as the binding station was synthesized and studied by NMR and UV/Vis spectroscopy. Glass surfaces have been functionalized with pyridine‐terminated SAMs and subsequently with multilayers of macrocycles through layer‐by‐layer self assembly. A preferred orientation of the macrocycles could be confirmed by NEXAFS spectroscopy. The photocontrolled deposition of the axle into the surface‐bound macrocycle‐multilayers was monitored by UV/Vis spectroscopy and led to an increase of the molecular order, as indicated by more substantial linear dichroism effects in angle‐resolved NEXAFS spectra.     

Characterization of self-assembled monolayers of porphyrins bearing multiple thiol-derivatized rigid-rod tethers

A series of multithiol-functionalized zinc porphyrins has been prepared and characterized as self-assembled monolayers (SAMs) on Au. The molecules, designated ZnPS(n) (n = 1-4), contain from one to four [(S-acetylthio)methyl]phenylethynylphenyl groups appended to the meso-position of the porphyrin; the other meso-substituents are phenyl groups. For the dithiol-functionalized molecules, both the cis- and the trans-appended structures were examined. The ZnPS(n) SAMs were investigated using X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and various electrochemical methods. The studies reveal the following characteristics of the ZnPS(n) SAMs. (1) The ZnPS(n) molecules bind to the Au surface via a single thiol regardless of the number of thiol appendages that are available per molecular unit. (2) The porphyrins in the ZnPS(3) and ZnPS(4) SAMs bind to the surface in a more upright orientation than the porphyrins in the ZnPS(1), cis-ZnPS(2), and trans-ZnPS(2) SAMs. The porphyrins in the ZnPS(3) and ZnPS(4) SAMs are also more densely packed than those in the cis-ZnPS(2) and trans-ZnPS(2) SAMs. The packing density of the ZnPS(3) and ZnPS(4) SAMs is similar to that of the ZnPS(1) SAMs, despite the larger size of the molecules in the former SAMs. (3) The thermodynamics and kinetics of electron transfer are generally similar for all of the ZnPS(n) SAMs. The general similarities in the electron-transfer characteristics for all of the SAMs are attributed to the similar binding motif.     

Molecular orientation of terephthalic acid assembly on epitaxial graphene: NEXAFS and XPS study

The adsorption of terephthalic acid molecules [C(6)H(4)(COOH)(2)), TPA] on a single layer of graphene grown epitaxially on Ni(111) has been investigated by means of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) at room temperature. The assignment of the NEXAFS resonances was aided by ab initio calculations for the free TPA molecule. For coverages up to a monolayer the molecular plane of TPA adopts a parallel orientation with regard to the epitaxial graphene (EG) layer. Deprotonation of TPA molecules at one monolayer coverage can be excluded. For TPA multilayers, the molecular plane is tilted on average by approximately 45° with respect to the sample surface.     

Self-assembled monolayers of peptide nucleic acids on gold surfaces: a spectroscopic study

We have characterized self-assembled monolayers (SAMs) of thiol-derivatized peptide nucleic acid (PNA) chains adsorbed on gold surfaces by using reflection absorption infrared spectroscopy (RAIRS) and X-ray photoemission spectroscopy (XPS) techniques. We have found that the molecular orientation of PNAs strongly depends on surface coverage. At low coverage, PNA chains lie flat on the surface, while at high coverage, PNA molecules realign their molecular axes with the surface normal and form SAMs without the need of co-immobilization of spacers or other adjuvant molecules. The change in the molecular orientation has been studied by infrared spectroscopy and it has been confirmed by atomic force microscopy (AFM). PNA immobilization has been followed by analyzing the N(1s) XPS core-level peak. We show that the fine line shape of the N(1s) core-level peak at optimal concentration for biosensing is due to a chemical shift. A combination of the above-mentioned techniques allow us to affirm that the structure of the SAMs is stabilized by molecule-molecule interactions through noncomplementary adjacent nucleic bases.     

Orientation of pentacene molecules on SiO2: from a monolayer to the bulk

Near edge x-ray absorption fine structure (NEXAFS) spectroscopy is used to study the orientation of pentacene molecules within thin films on SiO2 for thicknesses ranging from monolayers to the bulk (150 nm). The spectra exhibit a strong polarization dependence of the pi* orbitals for all films, which indicates that the pentacene molecules are highly oriented. At all film thicknesses the orientation varies with the rate at which pentacene molecules are deposited, with faster rates favoring a thin film phase with different tilt angles and slower rates leading to a more bulklike orientation. Our NEXAFS results extend previous structural observations to the monolayer regime and to lower deposition rates. The NEXAFS results match crystallographic data if a finite distribution of the molecular orientations is included. Damage to the molecules by hot electrons from soft x-ray irradiation eliminates the splitting between nonequivalent pi* orbitals, indicating a breakup of the pentacene molecule.     

Surface structure and chemical switching of thioctic acid adsorbed on Au(111) as observed using near-edge X-ray absorption fine structure

Thioctic acid (alpha-lipoic acid) is a molecule with a large disulfide-containing base, a short alkyl chain with four CH2 units, and a carboxyl termination. Self-assembled monolayer (SAM) films ofthioctic acid adsorbed on Au(111) have been investigated with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) to determine film quality, bonding, and morphology. Using standard preparation protocols for SAMs, that is, dissolving thioctic acid in ethanol and exposing gold to the solution, results in poor films. These films are highly disordered, contain a mixture of carboxyl and carboxylate terminations, have more than monolayer coverage, and exhibit unbound disulfide. Conversely, forming films by dissolving 1 mmol thioctic acid into 5% acetic acid in ethanol (as previously reported with carboxyl-terminated alkanethiols) forms ordered monolayers with small amounts of unbound sulfur. NEXAFS indicates tilted over endgroups with the carboxyl group normal on average 38 degrees from the surface normal. Slight angle-dependent intensity modulations in other features indicate alkyl chains statistically more upright than prostrate on the surface. Reflection-absorption Fourier transform infrared (RA-FTIR) spectra indicate hydrogen bonding between neighboring molecules. In such well-formed monolayers, a stark reorientation occurs upon deprotonation of the endgroup by rinsing in a KOH solution. The carboxylate plane normal is now about 66 degrees from sample normal, a much more upright orientation. Data indicate this reorientation may also cause a more upright orientation to the alkyl portion of the molecules.     

Stark effect,polarizabilities and the electric dipole moment of heteronuclear diatomic molecules in 1Σ states

The theory of second-order Stark effect in ¹Σ states of heteronuclear diatomic molecules is thoroughly reviewed. The rigorous treatment given demonstrates that by introducing rotational, vibrational and electronic branch polarizabilities, the intrinsic character of the second-order Stark effect in diatomic molecules can be shown to be related more closely to polarizabilities than to dipole moments. The well-known expression for the Stark shift in ¹Σ levels which is dominated by the square of the dipole moment is only a crude, though sufficient approximation whenever large dipole moments are involved. For small dipole moments, however, this approximation is likely to fail, leading to an erroneous determination of such dipole moments. In the limiting case of negligible influence of the molecular rotation on the vibronic matrix elements, the arithmetic mean of the electronic branch polarizabilities turns out to be equal to the well-known static electronic polarizabilities α_∥ and α_⊥. The results are applied to the interpretation of the Stark splitting in the A¹Σ⁺, υ′ = 5, J′ = 1 level of ⁷LiH, recently determined by Stark quantum-beat spectroscopy.