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
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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.
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