Design and Function of Molecular Complex Sensors Based on Supramolecular Structure

本文报道了荧光探针/环糊精(CyD)络合物对水中离子和分子识别的超分子功能。首次研究了具有不同烷基间隔长度的苯并-15-冠-5荧光载体CN-15C5(n=1，3，5)，制备了用于水中碱金属离子识别的超分子CN-15C5/γ-CyD络合物传感器。在有机溶液中，C3-15C5以1：1的络合物形式表现出中等的Na~+选择性。然而，C3-15C5/γ-CyD络合物对水中的K~(++)离子有选择性的反应，并表现出芘二聚体的发射。γ-CyD包合物在水中的平衡分析表明，发射二聚体的主要成分是C3-15C5与K~(++)和γ-CyD形成的2：1：1的络合物。虽然C5-15C5/γ-CyD络合物的K~(++)敏感性与C3-15C5/γ-CyD络合物相当，但它也对Na~+有响应。在γ-CyD存在下，具有最短亚甲基间隔基的含氟离子载体C1-15C5对碱金属阳离子没有反应。因此，CN-15C5的烷基间距对响应函数的影响很大，C3-15C5/γ-CyD络合物在水中对K~+的选择性最高。硼酸荧光载体C4-PB/β-CyD络合物结合糖并在水中产生更强的荧光发射。C4-PB/β-CyD络合物的pH-荧光图谱表明，随着硼酸盐共轭碱的形成，荧光强度增加。加入果糖后，表观pKa降低到较低的pH，导致中性pH下的荧光增强。C4-PB/β-CyD络合物在糖结合时的荧光发射响应似乎是由于抑制了从芘供体到三角芳基硼酸受体的光致电子转移。

This study reports the supramolecular function of fluorescent probe/cyclodextrin (CyD) complexes for ion and molecule recognition in water. Benzo-15-crown-5 fluoroionophores, Cn-15C5 (n=1,3,5), with different alkyl spacer lengths were first examined to develop supramolecular Cn-15C5/γ-CyD complex sensors for alkali metal-ion recognition in water. In organic solutions, C3-15C5 shows moderate Na^+ selectivity based on 1:1 complex formation. However, the C3-15C5/γ-CyD complex is found to selectively respond to K^+ ion in water and to exhibit pyrene dimer emission. An equilibrium analysis of the γ-CyD inclusion complexes in water reveals that the major component for the dimer emission is a 2:1:1 complex of C3-15C5 with K^+ and γ-CyD. Although the K^+ sensitivity of the C5-15C5/γ-CyD complex is comparable to that of the C3-15C5/γ-CyD complex, it also responds to Na^+. The fluoroionophore C1-15C5, which has the shortest methylene spacer, exhibits no response to alkali metal cations in the presence of γ-CyD. Thus, the response function is strongly affected by the alkyl spacer length of Cn-15C5, and the highest K^+ selectivity in water is obtained for the C3-15C5/γ-CyD complex. The boronic acid fluoroionophore C4-PB/β-CyD complex binds sugars and produces increased fluorescence emission in water. A pH-fluorescence profile for the C4-PB/β-CyD complex reveals that the fluorescence intensity increases upon the formation of the boronate conjugate base. Upon the addition of fructose, the apparent pK_a decreases to a lower pH, resulting in increased fluorescence at neutral pH. The fluorescence emission response of the C4-PB/β-CyD complex upon sugar binding appears to be due to suppression of the photoinduced electron transfer (PET) from the pyrene donor to the trigonal arylboronic acid acceptor.