Vibrational spectroscopy on intermolecular interactions in solutions and at interfaces (1999)

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\nbetween molecules and a suitably modified surface is of utmost importance. Vibrational\nspectroscopy is quite useful in these areas of research, as it may reveal the processes taking place\nat a molecular level. This thesis describes a number of applications of vibrational spectroscopy in\nthe analysis of molecular recognition of molecules in solution, and in the characterisation of\nself-assembled interfaces.\nThe study of molecular recognition of halide ions by specific hosts is reflected in the chapters\n3, 4 and 5. In chapter 3, a new method is proposed for the determination of association\nconstants applying infrared spectroscopy in combination with multivariate regression. This\ntechnique was developed to overcome the sensitivity problems arising when strongly associating\ncomplexes are investigated by NMR or UV/vis methods. A concentration profile for the\ncomplex is derived by correction of multivariate regression data. Subsequent iterative fitting of\nthe corrected data yields the association constant. The regression part is not integrated in the\nprocess of the association constant determination itself. The separation of data treatment from\nthe actual fitting procedure offers the means to evaluate the quality of the data set and the order\nof association before the actual calculation of the association constant. From simulated data, it is\nestimated that an association constant range of 10 2 -10 6 M -1 can be determined when measuring\nat millimolar concentration levels.\nIn chapters 4 and 5 an infrared-spectroscopic study of the binding behaviour of urea and\nthiourea-substituted hosts is presented. Chapter 4 renders the results of the investigation of the\ncomplexation of chloride, bromide and iodide ion by a resorcinarene cavitand substituted on its\nupper rim with four N-(o-nitrophenoxyoctyl-ether) urea groups. Association takes place solely\nvia hydrogen bonding by the urea moieties, and is well monitored by infrared spectroscopy.\nAssociation constants are high, about 10 4 M -1 , and a small preference for chloride over bromide\nand iodide is observed. Upon binding of the anion, the array of weak intramolecular bonds is\ndisrupted and replaced by hydrogen bonds towards the halide lone pairs. No significant\nreorganisation of the urea o-nitrophenoxyoctyl-ether substituents is found, which suggests a re-\nordering of the urea groups only upon binding. Co-operation of the bonding moieties is\nobserved in the complexation of halide ions as a result of pre-organisation of the urea groups on\nthe upper rim of the resorcinarene cavitand by weak hydrogen bonding interactions.\nPre-organisation of the substituents was also observed in further investigations of the binding\nbehaviour of thiourea-substituted cavitands (chapter 5). The ordering of the ligating side chains\non the upper rim by intramolecular bonding enhances the capability of association by a factor 10\nto 100, compared to a simple thiourea. Further analysis of the differences observed between\nassociation by the cavitand host and this model compound reveals that an analogy exists\nbetween complexation of a halide anion by the resorcinarene and complexation of an iodide\nanion by the model thiourea. The latter suggests that within the cavitand, chloride, bromide?130\nand iodide are all complexed by at least two thioureido moieties, whereas the simple thiourea\nbinds chloride and bromide in a 1 to 1 fashion, and only iodide in a 2 to 1 fashion.\nThe chapters 6 to 10 specifically relate to the behaviour of molecules in monolayers and at\ninterfaces. In chapter 2, a concise overview of the spectroscopic methods discussed in these\nchapters is given. The formation of monolayers on silicon by linking terminal alkenes to a Si-H\nmodified surface is studied in chapter 6 and the feasibility to perform reactions on monolayers\non silicon is evaluated. It is shown that these layers are exceptionally stable and that hydrolysis of\nan ester functionality is possible. In the course of this work, a baseline-correcting algorithm was\ndeveloped (Appendix 1).\nThe applicability of a rather uncommon method, Surface Electromagnetic Wave (SEW)\nspectroscopy, to the characterisation of monolayers on gold is discussed and checked in\nchapter 7. In theory, this technique is very well suited for the analysis of a surface, as the depth\nto which the interface is probed is only a few micrometers. The outcome is largely comparable\nto results obtained by reflection measurements, but a considerable amount of line broadening is\nobserved. Considering the practical aspects, SEW spectroscopy is not recommended for routine\nmeasurements.\nThe most versatile substrate for monolayer formation is gold. Chapter 8 describes the\nordering processes as observed in chiral and racemic monolayers of three phenylalanyl-\nsubstituted w-thiol alkanoic acids on gold. Measurements were performed applying Infrared\nReflection Absorption Spectroscopy (IRRAS). For these chiral molecules, packing is influenced\nby hydrogen bonding interactions between the end groups at the air-monolayer interface and\nthe length of the spacer chain. For a racemic monolayer, a dense, yet unregular packing is\nobserved. In chapter 9, not the monolayer itself is considered, but the influence of the substrate\non the ordering of adlayers. Using Near-Infrared Surface-Enhanced Raman Spectroscopy, the\norientation of selected cavitand molecules adsorbed to colloidal gold particles is estimated. The\ndirecting influence of planar gold substrates and surfaces covered by a self-assembled monolayer\non the ordering of thick resorcinarene adlayers was investigated by Infrared Reflection-\nAbsorption Spectroscopy. The orientation of the resorcinarene molecules positioned at the\ninterface proves to be influenced by the underlying substrate, though the effect does not extend\ninto the adjoining bulk of the sample layer.\nThe final chapter deals with the dynamic process of adsorption of halide-complexing\ncavitands onto a chloride-containing monolayer as it was monitored by Surface Plasmon\nResonance. It is shown that the resulting layers are stable in ethylacetate and acetone, and can\nbe desorbed by hydrochloric acid in ethanol (pH~1). In the case of thiourea-containing\ncompounds, adsorption is partially irreversible, possibly as a result of binding at defect sites in\nthe monolayer.\nThe results of the research described in this thesis demonstrate vibrational spectroscopy is a\nvery powerful technique in the investigation of interactions at the molecular level, both in\nsolution and at an interface.