Guest-shape-responsive fitting

As encapsulation of the guest depends on shape and size, tetrahedral lacton, which can better fit the cavity of the coordination cage, encapsulated and stabilized more than quinone. This can shift the equilibrium of two forms toward lactonization and disappearance of pink color. In this way, cavity-directed (and not pH-dependent) chromism can be obtained. Therefore, phthalein dyes remain colorless, even in a basic solution. Upon release of phthalein dyes by the addition of a competitive guest, the pink color will be recovered. The authors believed that this procedure could be used to control the responsiveness of target compounds to external stimuli without any chemical modification [46]. 

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