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Host-guest Complexation of Monoanionic and Dianionic Guests with Polycationic Pillararene Host: Same Two-step Mechanism but Striking Difference in Rate Upon Inclusion.

Supramolecular dynamic studies provide the most direct information to elucidate the binding mechanisms of the systems, and yet are underdeveloped in pillararene chemistry. Herein, we describe the first real-time study on the binding dynamics of a water-soluble per-substituted pillar[5]arene (H1) with pentanesulfonate (G1) and butane-1,4-disulfonate (G2). Both the host-guest complexes were formed via a two-step process. The first step, equilibrated within 1 millisecond for both guests, was associated with the formation of a 1:1 exclusion complex and the second step was the conversion of this exclusion complex to the inclusion complex. Threading and dethreading processes in the second step for G2 were at least million times slower than for G1. Kinetics results reveal that for H1, complexation with a charged guest may follow the same "two-step" mechanism regardless of the number of charged moieties in the guests and the rate of the complexation. This study may advance the mechanistic understanding necessary for further development of functional supramolecular systems.