Cavity-Size-Dependent Recognition of Naphthalene Derivatives by 3,3′-Bipyridinium Tetracationic Cyclophanes

The recognition behavior of tetracationic cyclophanes is strongly influenced by the geometric match between the host cavity and aromatic guests. Herein, we compare two 3,3′-bipyridinium-based tetracationic cyclophanes, BPy-Box⁴⁺ and NBox⁴⁺, toward a series of naphthalene derivatives. ¹H NMR titration experiments reveal cavitydependent binding affinities and different recognition preferences. BPy-Box⁴⁺ binds naphthalene and hydroxylated naphthalene derivatives with comparable affinities in CD₃CN, whereas NBox⁴⁺ shows weak binding toward naphthalene but stronger association with selected hydroxylated guests. Chain-extended derivatives display lower affinities, suggesting that increased flexibility and steric bulk do not necessarily improve host-guest complexation. Computational analyses further support different preferred binding geometries in the two macrocyclic cavities. These results highlight cavity size and shape complementarity as key parameters for tuning the recognition behavior of tetracationic cyclophanes.