Charles J. M. Stirling

Reactions and Reactivity in Self-Assembled Monolayers**

Self-assembled monolayers (SAMs) are excellent models for studying interfacial reactions. Here monolayer chemistry is reviewed, focusing on the features that have no analogues in solution chemistry. The growth of surface-attached polymers, intrafilm reactions, chemistry, photochemistry and reactivity issues are all discussed.

Thin films of calix-4-resorcinarene deposited by spin coating and Langmuir-Blodgett techniques: determination of film parameters by surface plasmon resonance

Amphiphilic calix-4-resorcinarene (C-4-RA) has been deposited as thin films using both spin coating and Langmuir-Blodgett (LB) techniques. Both LB and spin-coated films were studied by surface plasmon resonance (SPR) using Kretschmanns prism configuration, and their optical parameters have been evaluated. LB film thickness d obtained from SPR curve fitting procedure was plotted as a function of the number of layers and from this, a bi-layer thickness of 2.34 nm was determined. The thickness of spun films produced at a spinning speed of 2000 rpm is found to be equivalent to that of six to eight monolayer LB films. Both types of films were found to have a similar refractive index value of about 1.47, Using SPR technique, LB and spun films of C-4-RA of comparable thickness were found to respond in a similar manner when exposed to 400 ppm toluene vapour, indicating a structural similarity between films produced by wet techniques, such as LB and spin coating

The electrochemistry of the ferri/ferrocyanide couple at a calix[4]resorcinarenetetrathiol-modified gold electrode as a study of novel electrode modifying coatings for use within electro-analytical sensors

Abstract The electrochemistry of the ferri/ferrocyanide redox couple has been studied at Au electrodes modified with calix[4]resorcinarenetetrathiol. Cyclic voltammetry in Fe(CN) 6 3− solutions yields three separate pairs of faradaic peaks. Evidence is given for these redox couples corresponding to the reduction of Fe(CN) 6 3− and the subsequent re-oxidation of Fe(CN) 6 4− in three differing steric arrangements. One pair of peaks suggest that when the Fe(CN) 6 3− ion resides within the calix[4]resorcinarene bowl, electron transport is facilitated by the calix[4]resorcinarene acting as a charge transfer mediator; in this arrangement the activation energy is found to be lowered by ∼24 kJ mol −1 . Another pair of peaks is thought to correspond to the reduction of Fe(CN) 6 3− as it approaches the Au electrode by packing itself in-between adjacent calix[4]resorcinarene molecules. The third pair of redox peaks is attributed to the reduction and subsequent re-oxidation of Fe(CN) 6 3− /Fe(CN) 6 4− when the ion resides above a saturated calix[4]resorcinarene coating; in this case the activation energy was raised by ∼45 kJ mol −1 . FTIR spectroscopy of calix[4]resorcinarene-coated Au electrodes and calix[4]resorcinarene-coated Au electrodes exposed to Fe(CN) 6 3− lends further support to this argument, by demonstrating that the Fe(CN) 6 3− ion resides within at least two and possibly three differing environments. Calix[4]resorcinarene modified electrodes previously exposed to ferricyanide lose the calix[4]resorcinarene coating together with a surface layer of gold when subsequently scanned in a phosphate buffer. It therefore appears that the calix[4]resorcinarene/Fe(CN) 6 3− association is stronger than the Au binding to the underlying glass material.

Size-quantization in extremely small CdS clusters formed in calixarene LB films

CdS nanoparticles have been formed within Y-type Langmuir-Blodgett (LB) films of cadmium salts of calix(8)- and calix(4)-arene by reaction with H2S. UV-vis absorption spectra of the LB films, measured at room temperature, show well-resolved transitions between size-quantization levels in CdS clusters. The size of the CdS particles, obtained by Gaussian fitting of the experimental spectra, is 1.5 +/- 0.3 nm, which is much less than those reported for fatty acid LB films. The particle size does net depend either on the type of calixarene or the number of LB layers. LB films were also characterised by X-ray diffraction and ellipsometry which show the film thickness do not change substantially after treatment with H2S. The mechanism of CdS nanoparticles formation is discussed

Complexation of metal ions with Langmuir-Blodgett films of novel calixarene azo-derivative

Abstract The complexation properties of Langmuir–Blodgett (LB) films of an amphiphilic calix(4)resorcinarene substituted by four arylazo groups with some heavy metal ions were studied using UV-vis spectral measurements. It was shown that soaking samples in metal salt solutions leads to irreversible decrease of absorbance at 460 nm. Efficiency of complexation is found to depend on both metal cations and anions. Effects of film thickness, solution concentration and time of exposure have also been studied. It was also shown that the addition of a small amount of aromatic compounds such as aniline, leads to a further decrease in the absorbance of the films. The presence of metal salts in the film was directly confirmed by XPS. The mechanism of ion complexation is discussed.

The temporal and thermal stability of pyroelectric calix[8]arene Langmuir–Blodgett films

Abstract Pyroelectric Langmuir–Blodgett (LB) films of calix[8]arene have been prepared using the alternate-layer technique. The technique allows the co-deposition of carboxyl and amino-substituted calix[8]arenes, and results in the alternating acid/amine/acid… layer structure. A pyroelectric coefficient of up to 15 μC m−2u2008K−1 has been obtained from a 13 alternate layer t-butyl calix[8]arene LB film. The pyroelectric response of this system has been shown to remain constant over ∼5000 continuous heating and cooling cycles. The introduction of metal ions into the LB matrix dramatically increases the temperature range over which the pyroelectric activity can be measured.

The effect of pendant chain structure on the pyroelectric behaviour of calix(8)arene Langmuir-Blodgett films

Abstract The variation of the pyroelectric response (temperature-dependent electric polarisation) of alternate-layer acid/amine films of calix(8)arenes differs according to the structure of their pendant hydrophobic chains. t-Butyl calix(8)arenes exhibit larger pyroelectric coefficients than the t-octyl analogues. A coefficient of 15 μC m −2 K −1 has been obtained for a 13-alternate-layer sample of acid- and amine-substituted t-butyl calix(8)arene.

Morphology and binding of spontaneously assembled multilayers of calix-4-resorcinarenes

Abstract Calix-4-resorcinarenes form multilayers (7–40 layers) by self-assembly from hydrophobic solvents such as hexane on various substrates. Hydrophilic solvents such as ethanol reverse this process. Infrared and X-ray photoelectron spectroscopy, together with X-ray and neutron reflectometry measurements are described. Comparison is made with similar measurements on Langmuir-Blodgett films of these materials. The aim of this work was to study the structure and morphology of the multilayers in the context of a model which has been proposed for their bilayer structure and which is based on the X-ray crystal structure of calix-4-resorcinarene 1. These multilayers have been found to bind glutaric acid selectively.

Methylenecyclopropanes in Elimination and Addition Reactions: Quantification of the Effects of Strain

The effect of strain in 1,2-elimination reactions that form methylenecyclopropanes has been evaluated for a series of leaving groups. The worse the leaving group, the greater is the inhibitory effect of strain build-up, which reaches 50% of the excess enthalpy differential for the poorest leaving group studied. In nucleophilic addition to an electrophilic methylenecyclopropane, comparison of strained and unstrained systems shows that about 60% of the excess enthalpy differential promotes the reactivity of the strained system.

Structures and binding of LB films of calix-8-arenes

Abstract A series of three calix-8-arenes have been successfully deposited as Langmuir-Blodgett films up to 100 layers thick and studied by means of a quartz crystal microbalance, X-ray diffraction and infra-red spectroscopy. These layers have been shown to gain weight when exposed to dilute aqueous solutions of caesium bromide over periods of up to several hours. From our results we deduce that the ions are adsorbed vertically through the film rather than in a transverse manner. The kinetics of adsorption are similar to those found for other LB systems.