Department of Chemistry and The Center of Novel Functional Molecules, The Chinese University of Hong Kong, Shatin, NT, Hong Kong SAR.
Chem Asian J. 2010 Oct 4;5(10):2249-57. doi: 10.1002/asia.201000359.
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 derivatives were examined by ¹H NMR spectroscopy. The dimerization constants (K(dim*)) 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 substituents. 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.
我们制备并表征了两类脂肪族烃基 G1-G3 树枝状 2-脒基-4-嘧啶酮（UPy）（S-Gn）₂和（L-Gn）₂，它们在分支连接处到脲末端的距离上彼此不同。这些碳氢树枝还被连接到对氨基硝基苯溶剂致变色发色团上，以探测它们的微环境极性。虽然观察到正溶剂化变色，表明发色团可被溶剂接近，但从 G1 到 G3 树枝状，微环境极性没有显著差异。通过 ¹H NMR 光谱研究了树枝状 UPy 衍生物的自组装行为和互变异构体偏好。DDAA 互变异构体的二聚化常数（K(dim*）在 CDCl₃中于 25 和 50°C 下均保持不变，为 10⁷ M⁻¹，与具有其他非极性取代基的 UPy 化合物相当。此外，（S-Gn）₂和（L-Gn）₂系列中 DADA 互变异构形式的 K'(dim*)下限分别确定为 10⁶和 10⁵ M⁻¹在 CDCl₃中。结果发现，树枝状分支连接处更靠近 UPy 单元可能导致二聚态的不稳定性。因此，（L-Gn）₂二聚体在 DDAA 形式下比（S-Gn）₂更稳定，但后者在互变异构 DADA 状态下比前者更稳定。本研究表明，高度非极性的微环境和树枝状分支连接处与 UPy 基序的接近程度都可以改变氢键介导的自组装过程的强度和特征。
