Abstract:
Cucurbiturils (CB
ns) 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 cucurbit8uril urea (CB8) 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 CB8 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 CB8 to form a 1∶1 inclusion complex, and a 1∶2 supramolecular system where one CB8 molecule encircled two benzyl groups ultimately forms. The binding constant of the first 1∶1 inclusion process was (1.65±1.22)×10
7 M
−1, and the corresponding Δ
H1 and −
TΔ
S1 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)×10
13 M
−2, and the corresponding Δ
H and −
TΔ
S 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.