Thiolated monolayers

The fdr studies reveal that the alkyl chains in SAMs of thiolates on Au(lll) usually are tilted 26-28° from the surface normal, and display 52-55° rotation about the molecular axis. This tilt is a result of the chains reestabUshing VDW contact in an assembly with - 0.5 nm S—S distance, larger than the distance of - 0.46 nm, usually quoted for perpendicular alkyl chains in a close-packed layer. On the other hand, thiolate monolayers on Ag(lll) are more densely packed owing to the shorter S—S distance. There were a number of different reports on chain tilt in SAMs on Ag(lll), probably owing to different amounts of oxide, formed on the clean metallic surface (229,230,296,297). In carefully prepared SAMs of alkanethiolates on a clean Ag(lll) surface, the alkyl chains are practically perpendicular to the surface. 

For second-order NLO applications, the films need to be noncentrosymmetric. 4-Di(2-hydroxyethyl)amino-4 -a2oben2enephosphonate was used to form SAMs on 2irconium-treated phosphorylated surfaces. Further reaction with POCl and hydrolysis created a new phosphorylated surface that could be treated with 2irconium salt (341—343). The principal advantage of the phosphate systems is high thermal stabiUty, simple preparation, and the variety of substrates that can be used. The latter is especially important if transparent substrates are required. Thiolate monolayers are not transparent, and alkyltrichlorosilanes have a serious stabiUty disadvantage. 

Malicki M, Guan Z, Ha SD, Heimel G, Barlow S, Rumi M, Kahn A, Marder SR (2009) Preparation and characterization of 4 -donor substituted stilbene-4-thiolate monolayers and their influence on the work function of gold. Langmuir 25(14) 7967-7975... 

Balasubramanian R, Guo R, Mills AJ (2005) Reaction of Au55(PPh3)12Cl6 with thiols yields thiolate monolayer protected Au75 clusters. J Am Chem Soc 127 8126-8132... 

Fig. 3 Pentacene grown by supersonic molecular beam deposition to form near monolayer p-type FETs with thiolate monolayer modified Au source and drain contacts (a) visualized by atomic force microscopy and with well-behaved (b) /d-Eds and (c) -Eg characteristics...

Similar behavior of other aromatic disulfides and thiols on gold electrodes has been described based on the SERS experiments [167]. Adsorption of benzenethiol, benzenemethanethiol, p-cyanobenzenemethanethiol, diphenyl sulfide, and dibenzyl sulfide was studied on the roughened gold electrode. All these species adsorb dissociatively as the corresponding thiolates. Monolayers formed from symmetric disulfides were exactly like those formed from the corresponding thiols. These monolayers were stable in a wide potential window from -1-800 to —1000 mV (versus SCE), which was limited by the oxidation of the Au surface from the positive side and hydrogen evolution at —1000 to —1200 mV at the negative side. 

Thiol monolayers are not removed by solvents, but by sulfur-active chemicals which pass through the surface monolayers. Laser desorption mass spectrometry has shown that thiolate molecules are intact on the gold surface, but through air oxidation, some sulfonates develop. The relative stability of alkanethiol SAMs on gold to air oxidation is to be expected due to the covalent nature of the S—Au bond. Photooxidation via UV excitation of electrons in the metal surface is, however, possible and leads to sulfonate salts which have again been characterized by mass spectrometry as well as by XPS . Alkene-thiolate monolayers can best be desorbed from gold by a one-electron reductive path. Stable monolayers on gold were also obtained with benzenesulfinate. 

Motesharei, K., Myles, D. C. Multistep Synthesis on the Surface of Self-Assembled Thiolate Monolayers on Gold Probing the Mechanism of the Thiazolium-Promoted Acyloin Condensation. J. Am. Chem. Soc. 1997,119, 6674-6675. 

Thiolate monolayers can also be removed from the surface by anodic oxidation in 0.1 M KOH or in neutral electrolytes at 0.5-1.5 V. This process is much slower than the reductive desorption. It is not clear what products result from such oxidation. Formation of alkanesulphinic acids292, sulphonic acids237,296 and sulphate ions and CO2 were reported212. 

The closest analogues of thiols, alkaneselenols R—SeH, were found to form well-packed monolayers on the Au(lll) surface. The structure of these layers was studied by X-ray diffraction. An oblique unit cell was revealed, indicating a distorted hexagonal close-packed lattice of selenol molecules . Benzeneselenol and diphenyl diselenide form identical monolayers on gold, as shown with STM microscopy. Monolayers of benzeneselenol do not have vacancy pits typical for thiolate monolayers, but show the presence of small islands of gold (20-200 A) which were absent before deposition . 

Oxidation of monolayers is greatly increased under UV irradiation. Irradiation of alka-nethiol monolayers with a mercury lamp at ca 0.16 W cm for 25 min results in 60% oxidation of the thiolate group to the sulphonate. Irradiation at 3 W cm for 1 h leads to the complete oxidation of the monolayers . The sulphonate monolayer obtained can be removed by rinsing with neat solvent or by exchange with solution of another thiol. This method, along with electrochemical desorption (vide infra), is conveniently used to remove thiolate monolayers from the gold surface . The application of these techniques to generate patterned monolayers is discussed in Section X. 

Other useful methods for the quantitative destmction of thiolate monolayers on the gold surface include metastable atom °° or kiloelectronvolt ion bombardment and electron beam damage techniques . These methods are successfully used to prepare patterned monolayers. Quantitative removal of the thiol by an electron beam requires an e-beam dose of 10-100 mCcm . Recently, X-rays were shown to damage gold-thiol monolayers . ... 

Here X is the partial charge-transfer coefficient, namely, the fraction of electron charge transferred from a sulfur atom to the mercury upon adsorption, and a is the degree of dissociation of the sulfhydryl group. Since partial electron transfer from the sulfhydryl groups and their deprotonation are strictly correlated events, a and X are expected to assume close values. The Hg notation is merely used to denote chemisorption of the RSH thiol on Hg, and has no stoichiometric implications. As a matter of fact, under conditions of total charge transfer (a = 2. = 1), formation of mercurous [3, 69] or both mercurous and mercuric [77, 79] thiolate monolayers has been postulated. However, no definite... 

Our group has recently reported two novel examples of interfacial molecular complexation in thiolate monolayers containing well-defined host binding sites [26,27]. In both cases, the binding sites are provided by sulfur-containing monomeric receptors which can be co-assembled on Au surfaces to afford monolayers with molecular recognition properties. 

Gu T, Whitesell JK, Fox MA (2003) Energy transfer from a surface-bound arene to the gold core in ca-fluorenyl-alkane-l-thiolate monolayer-protected gold clusters. Chem Mater 15 1358-1366... 

Several precautions and limitations should be mentioned upon the application of chronopotentiometry as an electrochemical method to analyze interface properties of layered-modified-electrodes, and, specifically, monolayer-function-alized-electrodes (1) The chronopotentiometric pulse results in a potential shift on the electrode after the transition-time, x, resulting in the discharge of the electrolyte. This potential shift often ruins the chemically functionalized layer on the electrode (e.g., thiolated monolayers on Au-electrodes). Thus, it is essential to terminate the chronopotentiometric pulse at shorter time-intervals than the transition time, X, in order to eliminate the destruction of the functionalized electrode. (2) The electrode resistance, R, values derived from the chronopotentiometric exper-... 

Fig. 19 Molecular recognition between barbituric acid derivatives (25) and a diamino-pyridine-based receptor functionalized alkanethiol (26) on the surface of a self-assembled thiolate monolayer on a gold film...

Motesharei and Myles prepared thiazolium salts (e.g. 26, scheme 7) for the synthesis of self-assembled thiazolium thiolate monolayers on gold (fig-1) which promoted the acyloin condensation (Motesharei Myles 1997)... 

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