Helical supramolecular aggregates based on ureidopyrimidinone quadruple hydrogen bonding.

A series of mono- and bifunctional compounds 2-7, based on the ureido pyrimidinone quadruple hydrogen bonding unit, was prepared to study the mode of aggregation of these compounds in the bulk and in solution. Compounds 2-7 exhibit thermotropic liquid crystalline properties, as evidenced by differential scanning calorimetry and optical polarization microscopy. The presence of an ordered hexagonal discotic (D(ho)) phase of 2 a was confirmed by X-ray diffraction on an aligned sample. In chloroform, the bifunctional compounds form cyclic dimers at millimolar concentrations, and these dimers exist in equilibrium with linear species above a critical concentration, which may be from 6 mM to greater than 260 mM, depending on the structure of the spacer. Circular dichroism measurements in chloroform did not show a Cotton effect. Dodecane solutions of compounds 3, 4 b, and 7 b display a Cotton effect at the absorption band of the phenyl-pyrimidinone unit. Amplification of chirality was observed in mixtures of 7 a and 7 b, but not in mixtures of 4 a and 4 b, indicating that 7 a and 7 b form mixed polymeric aggregates with a helical architecture in dodecane solution, whereas 4 a and 4 b do not. The Cotton effect is lost upon increasing the temperature. Half of the helicity is lost at 25 degrees C for 3 and at 60 degrees C for 4 b, suggesting that 3, bearing the shorter spacer, forms less stable columns than 4 b. Compound 7 b loses half of its helicity at 45 degrees C. Compounds 2 b, 5, and 6 do not exhibit helical organization, as evidenced by the absence of Cotton effects.

Hierarchical formation of helical supramolecular polymers via stacking of hydrogen-bonded pairs in water.

Bifunctional ureido-s-triazines provided with penta(ethylene oxide) side chains are able to self assemble in water, leading to helical columns via cooperative stacking of the hydrogen-bonded pairs (DADA array). Monofunctional ureido-s-triazines do not form such helical architectures. The presence of a linker, covalently connecting the two ureido-s-triazine units, is essential as it generates a high local concentration of aromatic units, favorable for stacking interactions. This hydrophobic stacking of the aromatic units occurs at concentrations as low as 5 x 10(-6) M and can be visualized by using fluorescence spectroscopy. The stacking generates a hydrophobic microenvironment that allows intermolecular hydrogen bonding to occur at higher concentrations because the hydrogen bonds are shielded from competitive hydrogen bonding with water. This hierarchical process results in the formation of a helical self-assembled polymer in water at concentrations above 10(-4) M. Chiral side chains attached to the ureido-s-triazine units bias the helicity of these columns as concluded from CD spectroscopy and "Sergeants and Soldiers" experiments.

Helical self-assembled polymers from cooperative stacking of hydrogen-bonded pairs.

The double helix of DNA epitomizes this molecule's ability to self-assemble in aqueous solutions into a complex chiral structure using hydrogen bonding and hydrophobic interactions. Non-covalently interacting molecules in organic solvents are used to design systems that similarly form controlled architectures. Peripheral chiral centres in assemblies and chiral side chains attached to a polymer backbone, have been shown to induce chirality at the supramolecular level, and highly ordered structures stable in water are also known. However, it remains difficult to rationally exploit non-covalent interactions for the formation of chiral assemblies that are stable in water, where solvent molecules can compete effectively for hydrogen bonds. Here we describe a general strategy for the design of functionalized monomer units and their association in either water or alkanes into non-covalently linked polymeric structures with controlled helicity and chain length. The monomers consist of bifunctionalized ureidotriazine units connected by a spacer and carrying solubilizing chains at the periphery. This design allows for dimerization through self-complementary quadruple hydrogen bonding between the units and solvophobically induced stacking of the dimers into columnar polymeric architectures, whose structure and helicity can be adjusted by tuning the nature of the solubilizing side chains.

Reversible polymers formed from self-complementary monomers using quadruple hydrogen bonding.

Units of 2-ureido-4-pyrimidone that dimerize strongly in a self-complementary array of four cooperative hydrogen bonds were used as the associating end group in reversible self-assembling polymer systems. The unidirectional design of the binding sites prevents uncontrolled multidirectional association or gelation. Linear polymers and reversible networks were formed from monomers with two and three binding sites, respectively. The thermal and environmental control over lifetime and bond strength makes many properties, such as viscosity, chain length, and composition, tunable in a way not accessible to traditional polymers. Hence, polymer networks with thermodynamically controlled architectures can be formed, for use in, for example, coatings and hot melts, where a reversible, strongly temperature-dependent rheology is highly advantageous.