Trapping of molecules in alkanethiol self-assembled monolayer matrices

Abstract: Melting of crystalline solids (superlattices) of octadecanethiol and octanethiol protected silver clusters has been studied with x-ray powder diffraction (XRD), differential scanning calorimetry (DSC), and infrared (IR) spectroscopy. These solids have been compared with the silver thiolate layered compounds in view of their similarity in alkyl chain packing and x-ray diffraction patterns. Superlattice melting is manifested in XRD around 400 K as the complete disappearance of all the low angle reflections; only bulk silver reflections due to the cluster cores are seen at 423 K. The superlattice structure is regained upon cooling from a temperature close to its melting point. However, cooling from a higher temperature of 473 K does not regain the superlattice order, whereas thiolate melting is repeatedly reversible even at these temperatures. Transmission electron microscopy suggests aggregation of clusters during heating/cooling cycles. DSC shows two distinct transitions, first corresponding to alkyl chain melting and the second corresponding to superlattice melting. Only alkyl chain melting is observed in variable temperature IR and increased order is manifested upon repeated heating/cooling cycles. Alkyl chain assembly shows strong interchain coupling leading to factor group splitting in cluster superlattices upon annealing. In thiolates only one melting feature is seen in DSC and it produces gauche defects, whereas significant increase in defect structures is not seen in superlattices. Repeated heating/cooling cycles increase interchain interactions within a cluster and the superlattice order collapses.

Abstract: V(benzene)2 sandwich cluster cations produced in the gas phase were size-selectively deposited onto a self-assembled monolayer of n-hexadecanethiols (HDT-SAM) chemisorbed on a Au(111) surface as well as onto a bare Au(111) surface. The thermal chemistry of the neutralized clusters on each substrate was studied with temperature programmed desorption (TPD). From the analyses of the threshold in the TPD, the desorption activation energies of the clusters deposited were determined to be 64.4 ±12.8 kJ/mol for the Au(111) and 130 ±10 kJ/mol for the HDT-SAM. The remarkably large desorption activation energy from the SAM suggests that the deposited clusters are incorporated into the SAM matrix and firmly trapped inside the alkyl chains of the SAM.

Abstract: Reaction of a methyltriphenylphosphonium (TPP) ion with a 2-mercaptobenzothiazole (MBT) monolayer on polycrystalline gold under electrochemical conditions leads to the chemical modification of the monolayer. Surface-enhanced Raman spectroscopy (SERS) has been used to study the molecular nature of the transformation. MBT, which adsorbs in the thione form on Au, gets transformed to the thiolate form upon reaction. Thermal stability of the modified monolayer is substantially lower than the corresponding pure monolayers and complete desorption of the modified monolayer occurs below 473 K. Time, potential, and solvent dependence of the reaction has been investigated. Electron transfer from the monolayer to the approaching ion in solution is suggested as the cause of the reaction. XPS investigation shows the change in the valence states of the species concerned. While the parent MBT gets oxidized, the TPP moiety gets reduced. The results suggest that modified monolayers can be made by simple electrochemical procedures and the processes are similar to the corresponding gas-phase events (1994, R. G. Cooks, T. Ast, T. Pradeep, and V. H. Wysocki, Acc. Chem. Res. 27, 316).

Abstract: : Long-chain alkanethiols, HS(CH2)nX, adsorb from solution onto gold surfaces and form ordered, oriented monolayer films. The properties of the interfaces between the films and liquids are largely independent of chain length when n > 10; in particular, wetting is not directly influenced by the proximity of the underlying gold substrate. The specific interaction of gold with sulfur and other soft nucleophiles and its low reactivity toward most hard acids and bases make it possible to vary the structure of the terminal group, X, widely and thus permit the introduction of a great range of functional groups into a surface. Studies of wettability of these monolayers, and of their composition using X-ray photoelectron spectroscopy (XPS), indicate that the monolayers are oriented with the tail group, X, exposed at the monolayer-air or monolayer- liquid interface. The adsorption of simple n-alkanethiols generates hydrophobic surfaces whose free energy (19 mJ/sq. m) is the lowest of any hydrocarbon surface studied to date. Measurement of contact angles is a useful tool for studying the structure and chemistry of the outermost few angstroms of a surface. This work used contact angles and optical ellipsometry to study the kinetics of adsorption of monolayer films and to examine the experimental conditions necessary for the formation of high-quality films.

Abstract: Analytical Tools. Analysis of Film Properties. Analysis of Surface Properties. Langmuir-Blodgett Films. Self-Assembled Monolayers. Modeling of LB and SA Monolayers. Applications of LB and SA Films. Author's Final Note. Index.

Abstract: Self-assembled monolayers provide an ideal system for disentangling the fundamental events in interfacial electron transfer. Coadsorption of ferrocene-terminated alkanethiols with unsubstituted n-alkanethiols on evaporated gold films yields stable, electroactive self-assembled monolayers. Monolayers containing low concentrations of alkanethiols linked to ferrocene by a polar ester group (FcCO{sub 2}(CH{sub 2}){sub n}SH, Fc = ({eta}{sup 5}-C{sub 5}H{sub 5})Fe({eta}{sup 5}-C{sub 5}H{sub 4})) show thermodynamically ideal surface electrochemistry in 1 M HClO{sub 4}, indicating the ferrocene groups to be homogeneous and noninteracting. Higher surface concentrations or use of alkanethiols linked directly to the nonpolar ferrocene group (Fc(CH{sub 2}){sub n}SH) lead to broadened electrochemical features, indicating interactions among ferrocene groups or inhomogeneous sites. Longer chain lengths and lower ferrocene surface concentrations result in slower electron-transfer kinetics with the ferrocene groups. A fraction of the thiols in a monolayer exchange with thiols in an ethanol solution, but much of the monolayer remains unequilibrated after 10 days.