The work described in this Thesis aimed at a better understanding of the structure-property relationships of supramolecular assemblies with a specific focus on hydrogen-bond dimers and polymers. The hydrogen-bond strength of (supra)molecular complexes in different solvents is usually determined by measuring the concentration dependent transition of dimers to monomers in the solvent of interest, e.g. with isothermal titration calorimetry (ITC) or nuclear magnetic resonance spectroscopy (NMR). However, these ensemble averaging methods may hide interesting effects, such as unresolved transition states or the influence of molecular aggregates in solution. They are also not applicable for complexes formed at low concentrations. Since supramolecular hydrogen-bonded systems are nowadays used and further developed as building blocks e.g. for self-healing polymers and molecular motors, single molecule measurements can provide new molecular level insight to support this development for future applications. Furthermore single molecule investigations are interesting from a fundamental point of view due to the disparity of theoretical calculations and bulk measurements of equilibrium dimerization constants Kdim observed in several of these systems. The experimental work described in this Thesis covers the synthesis and characterization of self-assembling supramolecular hydrogen-bonded arrays in the bulk as well as at surfaces. In particular atomic force microscopy-based single molecule force spectroscopy (AFM-SMFS) was used to investigate the energy landscape of the hydrogen-bonds in ureidopyrimidinone (UPy) and urea-aminotriazine (UAT) dimers and polymers in different solvents (2-propanol, 1-nonanol and hexadecane). Exploiting the Kramers-Bell-Evans approach, this technique directly yields equilibrium dimerization constants Kdim based on the measured bond strength values at different bond loading rates (stretching rates) of individual molecules, provided that an estimate for the on rate of the complex formation or the diffusive relaxation time is known. The formation and bond failure of supramolecular polymers of the two self-complementary quadruple hydrogen-bonded arrays is successfully studied as well. The rupture force observed as a function of the number of linkers N is in quantitative agreement with the theory on uncooperative bond rupture for supramolecular linkages switched in a series, which provides a new approach to determine the relevant kinetic parameters from SMFS measurements at one single loading rate.
|Award date||19 May 2011|
|Place of Publication||Enschede|
|Print ISBNs||978 905 335 400 1|
|Publication status||Published - 19 May 2011|