Design and synthesis of self-assembling systems with multiple hydrogen bonding interactions
Hydrogen bonding is one of the most useful interactions for the self-assembly of molecular subunits into well-defined supramolecular structures. Recent attention has been focused on systems capable of forming strong, directional, and reversible quadruple hydrogen bonds. Meijer et al. have made a significant impact in this area with the synthesis of stable DDAA dimers based on ureidopyrimidinones (Upy) with a 7 1 dimerisation constant greater than 10 M" in chloroform. This work is reviewed in Chapter I of this thesis. A less attractive feature of the Upy unit is the possibility of up to three tautomeric forms in solution. The nature of the substituents at C-6 has been shown to influence the tautomeric equilibrium. Chapter II of this thesis describes the synthesis of an ureidopyrimidinone compound incorporating an electron-donating group (/7-C6H4NH2) at the C-6 position, which has led to the formation of the dimeric form DADA in DMSO-6- It was the first time that the hydrogen bonded dimeric Upy was observed in such a polar solvent. Chapter III describes the use of quadruple hydrogen bonded Upy units in the chain extension of various energetic (PolyGlyn) and non-energetic (polyether and polycarbonate based) telechelic polymers, which has led to the formation of supramolecular polymers with increased elastomeric properties. In addition, the development of an alternative synthetic approach avoiding the use of isocyanate has been successfully achieved leading to supramolecular polymers of improved quality. In addition, the synthesis of bifunctional Upys incorporating small chiral spacers has been achieved and described in Chapter IV. Notably, it was found that the use of diethyl L-tartrate or butane diol led to the formation of extremely stable cyclic dimers in chloroform with a dimerisation constant greater than 108 M"1. The formation of new intramolecular hydrogen bonds within the cyclic species was observed in the crystal structure and was found to stabilise the dimeric form in solution. Finally, the design of a new quadruple hydrogen bonded DDAA array based on cytosine has been successfully achieved via a straightforward synthetic strategy (Chapter V). The structure of the linear DDAA/A ADD dimer was revealed in the solid state and in solution with a dimerization constant greater than 105 M"1 in chloroform. The synthesis of the first generation of polymers based on this new unit has been achieved. The obtained results show great potential for the future material applications.