2-(4'-Pyridyl)benzimidazole (4PBI) and 2-(2'-Pyridyl)benzimidazole (2PBI) acting as a guest molecule has been found to form inclusion complexes with cucurbit[n]uril in aqueous solution at pH = 4 as well as pH = 9 through its one out of two binding sites, benzimidazole and pyridyl moieties. Upon insertion in the cavity and binding at the portal of cucurbituril, absorption and fluorescence responses of guest molecule have been found to be changed dramatically. Therefore, with the help of steady state and time resolved fluorescence spectroscopy, it has been demonstrated that the both of the guest molecules interacts with cavity as well as portal. The efficiency of the formation of inclusion complex of cucurbituril with forms of 4PBI and 2PBI (C, T, N) is not equal and depend on the charge distribution on the species. It has been observed that the position of the nitrogen atom of the pyridyl part in both dyes is the main driving force to form the inclusion complexes. The cation of the dye gets inserted inside the hydrophobic cavity of cucurbituril from the benzimidazole side and from pyridyl side in case of 4PBI and 2PBI, respectively. The tautomer form also approaches to form inclusion complex with this macrocycle.   The formation of inclusion complexes is proved by quantum chemical calculations which have been performed for 4PBI-CB and 2PBI-CB type complexes with each form of the dye (C , T , N ). 1H NMR spectroscopy is found to prove the formation of inclusions complexes with 4PBI. The complexation occurring between host-guest molecule is effect the ESPT processes inside the cavity and responsible for the complexation with different degree of extent with different forms of the dye. The matching of the size of the host-guest system is also play important role. Therefore, the high energy water is removed from the cucurbituril cavity and renders it to formation of stable complex with 4PBI and 2PBI as well. The eradication of high energy water is main driving force for the complexation and it decreases with increasing size of the host molecules.

Interplay of hydrophobic and electrostatic interactions in the modulation of ground and excited state protonation-deprotonation equilibria of two positional isomers in their complexes with cucurbiturils

248th National Meeting (2014)

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