The Effects of Microenvironment Polarity and Dendritic Branching of Aliphatic Hydrocarbon Dendrons on the Self-Assembly of 2-Ureido-4-pyrimidinones

Two series of aliphatic hydrocarbon-based G1-G3 dendritic 2-ureido-4-pyrimidinones (UPy) (S-Gn), and (L-Gn)(2), differing from one another by the distance between the branching juncture to the urea end, were prepared and characterized. These hydrocarbon dendrons were also appended to a p-aminonitrobenzene solvatochromic chromophore in order to probe their microenvironment polarity. While positive solvatochromism was observed which indicated the chromophore was solvent accessible, there was no significant difference between the microenvironment polarities on going from the G1 to the G3 dendrons. The self-as-sembling behavior and tautomeric preference of the dendritic UPy derviatives were examined by H-1 NMR spectroscopy. The dimerization constants (K-dim*) of the DDAA tautomers were unchanged at 10(7)M(-1) in CDCl3 at both 25 and 50 degrees C, which were comparable to those of UPy compounds bearing other nonpolar substitutents. Furthermore, the lower limits on the K'(dim*) of the DADA tautomeric forms of the (S-Gn)(2) and (L-Gn)(2) series were determined to be 10(6) and 10(5) M-1 in CDCl3, respectively. It was found that a closer proximity of the dendron branching juncture to the UPy unit could lead to a destabilization effect on the dimeric states. Hence, the (L-Gn)(2) dimers are more stable than those of (S-Gn)(2) in the DDAA form, but the latter are more stable than the former in the tautomeric DADA state. This study showed that both the highly nonpolar microenvironment and the proximity of the dendritic branching juncture to the UPy motif could alter the strength and profile of the hydrogen bond-mediated self-assembling process.