The Effects of Microenvironment Polarity and Dendritic Branching of Aliphatic Hydrocarbon Dendrons on the Self‐Assembly of 2‐Ureido‐4‐pyrimidinones |
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| Authors: | Chun‐Ho Wong Lai‐Sheung Choi Sui‐Lung Yim Dr Kwun‐Ngai Lau Prof Dr Hak‐Fun Chow Sin‐Kam Hui Prof Dr Kong‐Hung Sze |
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| Institution: | 1. Department of Chemistry and The Center of Novel Functional, Molecules The Chinese University of Hong Kong, Shatin, NT (Hong Kong SAR), Fax: (+852)?2603‐3057;2. Institute of Molecular Functional Materials, Areas of Excellence Scheme, University Grants Committee, Hong Kong SAR;3. Department of Chemistry, University of Hong Kong, Pokfulam Road (Hong Kong SAR) |
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| Abstract: | Two series of aliphatic hydrocarbon‐based G1–G3 dendritic 2‐ureido‐4‐pyrimidinones (UPy) ( S‐Gn )2 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‐assembling behavior and tautomeric preference of the dendritic UPy derviatives were examined by 1H NMR spectroscopy. The dimerization constants (Kdim*) of the DDAA tautomers were unchanged at 107 M ?1 in CDCl3 at both 25 and 50 °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 106 and 105 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. |
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| Keywords: | dendrimers dimerization self‐assembly solvatochromism tautomerism |
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