Helical supramolecular aggregates based on ureidopyrimidinone quadruple hydrogen bonding

A series of mono- and bifunctional compds. 2-7, based on the ureido pyrimidinone quadruple hydrogen bonding unit, was prepd. to study the mode of aggregation of these compds. in the bulk and in soln. Compds. 2-7 exhibit thermotropic liq. cryst. 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 bifunctional compds. form cyclic dimers at millimolar concns., and these dimers exist in equil. with linear species above a crit. concn., which may be from 6 mM to greater than 260 mM, depending on the structure of the spacer. CD measurements in chloroform did not show a Cotton effect. Dodecane solns. of compds. 3, 4b, and 7b display a Cotton effect at the absorption band of the phenyl-pyrimidinone unit. Amplification of chirality was obsd. in mixts. of 7a and 7b, but not in mixts. of 4a and 4b, indicating that 7a and 7b form mixed polymeric aggregates with a helical architecture in dodecane soln., whereas 4a and 4b do not. The Cotton effect is lost upon increasing the temp. Half of the helicity is lost at 25°C for 3 and at 60°C for 4b, suggesting that 3, bearing the shorters spacer, forms less stable columns than 4b. Compd. 7b losses half of its helicity at 45°C. Compds. 2b, 5, and 6 do not exhibit helical organization, as evidenced by the absence of Cotton effects