Vibrational spectroscopy on intermolecular interactions in solutions and at interfaces

Abstract

In recent years, considerable progress has been made in the areas of molecular recognition and surface analysis. These fields meet in the field of sensor development, where the interaction between molecules and a suitably modified surface is of utmost importance. Vibrational spectroscopy is quite useful in these areas of research, as it may reveal the processes taking place at a molecular level. This thesis describes a number of applications of vibrational spectroscopy in the analysis of molecular recognition of molecules in solution, and in the characterisation of self-assembled interfaces. The study of molecular recognition of halide ions by specific hosts is reflected in the chapters 3, 4 and 5. In chapter 3, a new method is proposed for the determination of association constants applying infrared spectroscopy in combination with multivariate regression. This technique was developed to overcome the sensitivity problems arising when strongly associating complexes are investigated by NMR or UV/vis methods. A concentration profile for the complex is derived by correction of multivariate regression data. Subsequent iterative fitting of the corrected data yields the association constant. The regression part is not integrated in the process of the association constant determination itself. The separation of data treatment from the actual fitting procedure offers the means to evaluate the quality of the data set and the order of association before the actual calculation of the association constant. From simulated data, it is estimated that an association constant range of 10 2 -10 6 M -1 can be determined when measuring at millimolar concentration levels. In chapters 4 and 5 an infrared-spectroscopic study of the binding behaviour of urea and thiourea-substituted hosts is presented. Chapter 4 renders the results of the investigation of the complexation of chloride, bromide and iodide ion by a resorcinarene cavitand substituted on its upper rim with four N-(o-nitrophenoxyoctyl-ether) urea groups. Association takes place solely via hydrogen bonding by the urea moieties, and is well monitored by infrared spectroscopy. Association constants are high, about 10 4 M -1 , and a small preference for chloride over bromide and iodide is observed. Upon binding of the anion, the array of weak intramolecular bonds is disrupted and replaced by hydrogen bonds towards the halide lone pairs. No significant reorganisation of the urea o-nitrophenoxyoctyl-ether substituents is found, which suggests a re- ordering of the urea groups only upon binding. Co-operation of the bonding moieties is observed in the complexation of halide ions as a result of pre-organisation of the urea groups on the upper rim of the resorcinarene cavitand by weak hydrogen bonding interactions. Pre-organisation of the substituents was also observed in further investigations of the binding behaviour of thiourea-substituted cavitands (chapter 5). The ordering of the ligating side chains on the upper rim by intramolecular bonding enhances the capability of association by a factor 10 to 100, compared to a simple thiourea. Further analysis of the differences observed between association by the cavitand host and this model compound reveals that an analogy exists between complexation of a halide anion by the resorcinarene and complexation of an iodide anion by the model thiourea. The latter suggests that within the cavitand, chloride, bromide?130 and iodide are all complexed by at least two thioureido moieties, whereas the simple thiourea binds chloride and bromide in a 1 to 1 fashion, and only iodide in a 2 to 1 fashion. The chapters 6 to 10 specifically relate to the behaviour of molecules in monolayers and at interfaces. In chapter 2, a concise overview of the spectroscopic methods discussed in these chapters is given. The formation of monolayers on silicon by linking terminal alkenes to a Si-H modified surface is studied in chapter 6 and the feasibility to perform reactions on monolayers on silicon is evaluated. It is shown that these layers are exceptionally stable and that hydrolysis of an ester functionality is possible. In the course of this work, a baseline-correcting algorithm was developed (Appendix 1). The applicability of a rather uncommon method, Surface Electromagnetic Wave (SEW) spectroscopy, to the characterisation of monolayers on gold is discussed and checked in chapter 7. In theory, this technique is very well suited for the analysis of a surface, as the depth to which the interface is probed is only a few micrometers. The outcome is largely comparable to results obtained by reflection measurements, but a considerable amount of line broadening is observed. Considering the practical aspects, SEW spectroscopy is not recommended for routine measurements. The most versatile substrate for monolayer formation is gold. Chapter 8 describes the ordering processes as observed in chiral and racemic monolayers of three phenylalanyl- substituted w-thiol alkanoic acids on gold. Measurements were performed applying Infrared Reflection Absorption Spectroscopy (IRRAS). For these chiral molecules, packing is influenced by hydrogen bonding interactions between the end groups at the air-monolayer interface and the length of the spacer chain. For a racemic monolayer, a dense, yet unregular packing is observed. In chapter 9, not the monolayer itself is considered, but the influence of the substrate on the ordering of adlayers. Using Near-Infrared Surface-Enhanced Raman Spectroscopy, the orientation of selected cavitand molecules adsorbed to colloidal gold particles is estimated. The directing influence of planar gold substrates and surfaces covered by a self-assembled monolayer on the ordering of thick resorcinarene adlayers was investigated by Infrared Reflection- Absorption Spectroscopy. The orientation of the resorcinarene molecules positioned at the interface proves to be influenced by the underlying substrate, though the effect does not extend into the adjoining bulk of the sample layer. The final chapter deals with the dynamic process of adsorption of halide-complexing cavitands onto a chloride-containing monolayer as it was monitored by Surface Plasmon Resonance. It is shown that the resulting layers are stable in ethylacetate and acetone, and can be desorbed by hydrochloric acid in ethanol (pH~1). In the case of thiourea-containing compounds, adsorption is partially irreversible, possibly as a result of binding at defect sites in the monolayer. The results of the research described in this thesis demonstrate vibrational spectroscopy is a very powerful technique in the investigation of interactions at the molecular level, both in solution and at an interface.