Inclusion Behaviors of Benzyl-Containing Asymmetric Viologen with Cucurbit[8]uril
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摘要: 研究芳环取代紫精分子与葫芦[8]脲(CB[8])的包结模式，对进一步构建相关超分子聚合物，甚至是刺激响应型材料具有重要意义。选取不对称的1-乙基-1’-苄基-4,4’-联吡啶溴化盐（EBV），通过核磁共振氢谱（1H-NMR）、等温滴定量热法（ITC）和高分辨质谱（HRMS）等手段，详细考察了EBV与CB[8]在水中的包结行为。研究结果表明，EBV的苄基单元会进入CB[8]内腔中，在经历1∶1（EBV与CB[8]的物质的量之比）包结后，最终形成1个CB[8]分子包结2个苄基的1∶2超分子体系。其中1∶1包结过程的结合常数为（1.65±1.22）×107 M−1（1 M=1 mol/L，下同），整个过程的表观包结常数为（1.34±0.193）×1013 M−2，对应的ΔH和−TΔS分别为(−64.4±3.19) kJ/mol和−9.43 kJ/mol，表明该主客体组装行为是由焓和熵共同驱动的。Abstract: Cucurbiturils (CB[n]s) is a hollow macrocyclic molecule formed by the condensation of glycoluril and formaldehyde under acidic conditions. The glycoluril units are linked together by methylene bridges, and the resulting cucurbiturils have hydrophobic cavity and polar carbonyl groups on both portals. Cucurbituril has strong inclusion ability for positively charged guest molecules such as protonated organic amines, pyridinium, and viologen. The study of the inclusion behaviors of aryl substituted viologen with cucurbit[8]uril urea (CB[8]) is of great significance for the further construction of related supramolecular polymers as well as stimulus responsive materials. In this work, asymmetric 1-ethyl-1'-benzyl-4,4'-bipyridine bromide (EBV) was used to investigate its inclusion behaviors with CB[8] in aqueous solution by means of 1H-NMR spectroscopy, isothermal titration calorimetry (ITC) and high resolution electrospray ionization mass spectrometry (ESI-HRMS). The results showed that the benzyl unit of EBV firstly occupied the cavity of CB[8] to form a 1∶1 inclusion complex, and a 1∶2 supramolecular system where one CB[8] molecule encircled two benzyl groups ultimately forms. The binding constant of the first 1∶1 inclusion process was (1.65±1.22)×107 M−1, and the corresponding ΔH1 and −TΔS1 are (−26.2±1.26) kJ/mol and 14.6 kJ/mol, respectively. The apparent inclusion constant of the whole process was (1.34±0.193)×1013 M−2, and the corresponding ΔH and −TΔS were (−64.4±3.19) kJ/mol and −9.43 kJ/mol, respectively, suggesting that the host-guest complexation was driven by both enthalpy and entropy.
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Key words:
- supramolecular chemistry /
- cucurbituril /
- viologen /
- host-guest interaction /
- self-assembling
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图 1 EBV的合成（a）及其与CB[8]组装成超分子的过程（b）
Figure 1. Synthesis of EBV(a) and the process of assembling it into supramolecules with CB[8]（b）
图 2 往EBV（c=2.86×10−2 mol/L）中逐渐加入CB[8]时生成物的1H-NMR（400 MHz，D2O）图谱（★为CB[8]峰）
Figure 2. 1H-NMR (400 MHz, D2O) spectra of products when CB[8] gradually added to EBV (c=2.86×10−2 mol/L) (★ is the peak of CB[8])
图 3 将EBV（1.30×10−3 mol/L）在298.15 K下滴加到CB[8]（1.00×10−4 mol/L）水溶液中的ITC图像
Figure 3. ITC pattern of the complexation of EBV(1.30×10−3 mol/L) with CB[8](1.00×10−4 mol/L) in aqueous solution at 298.15 K
图 4 EBV与CB[8]包结物的ESI-HRMS（(a)、(b)分别为1∶1、1∶2包结物的信号峰，其中上面为理论模拟图，下面为测试图）
Figure 4. ESI-HRMS of the EBV/CB[8] complex ( (a) and (b) are the signal peaks of 1∶1 and 1∶2 inclusions, respectively, the corresponding simulated spectra (up) and test spectra (down))
表 1 EBV和CB[8]包结行为的ITC测定值（298.15 K）
Table 1. ITC values of the complexation of EBV with CB[8] (298.15 K)
Item K1/M−1 K2/M−1 Ka/M−2 n1 n2 ΔH1/(kJ·mol−1) ΔH2/(kJ·mol−1) ΔS1/(J·mol·K−1) ΔS2/(J·mol·K−1) ΔH(kJ·mol−1)) −TΔS/(kJ·mol−1) Value 1.65×107 8.14×105 1.34×1013 0.975 1.034 −26.2 −38.25 48.94 −17.32 −64.4 −9.43 Error ±1.22×107 ±1.58×105 ±0.193×1013 ±0.062 ±0.068 ±1.26 ±1.936 − − ±3.19 − Ka= K1K2, ΔH=ΔH1+ΔH2, −TΔS=−T（ΔS1+ΔS2）; n1, n2—Stoichiometric ratio of the first and the second step inclusion process, respectively 
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