625471-83-2Relevant academic research and scientific papers
Helical supramolecular aggregates based on ureidopyrimidinone quadruple hydrogen bonding
Hirschberg, J. H. K. Ky,Koevoets, Rolf A.,Sijbesma, Rint P.,Meijer
, p. 4222 - 4231 (2003)
A series of mono- and bi-functional 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 (Dho) phase of 2a was confirmed by X-ray diffraction on an aligned sample. In chloroform, the bi-functional 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, 4b, and 7b display a Cotton effect at the absorption band of the phenyl-pyrimidinone unit. Amplification of chirality was observed in mixtures of 7a and 7b, but not in mixtures of 4a and 4b, indicating that 7a and 7b form mixed polymeric aggregates with a helical architecture in dodecane solution, whereas 4a and 4b do not. The Cotton effect is lost upon increasing the temperature. Half of the helicity is lost at 25°C for 3 and at 60°C for 4b, suggesting that 3, bearing the shorter spacer, forms less stable columns than 4b. Compound 7b loses half of its helicity at 45°C. Compounds 2b, 5, and 6 do not exhibit helical organization, as evidenced by the absence of Cotton effects.
