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Notes: ---The cooperative nature of non-covalent interactions which give rise to inclusion complexes involving cyclobis(paraquat-p-phenylene), 14+, and related cyclophane derivatives, 24+-44+, with substituted 1,4-phenyl and 4,4′-biphenyl guests has been studied by spectroscopic techniques and ab initio and semiempirical molecular orbital methods. Inclusion complex formation and stability are primarily determined by the combination of two main interaction modes involving aromatic stacking of the guest within the cyclophane cavity and external interactions between guest side arms and the exterior of the cyclophane. A balance between cavity and external forces results in supramolecular association and is shown to change depending upon the functionality and substitution of the guest. Cavity binding was probed using 1,4-phenyl and 4,4′-biphenyl guests, where for the 1,4-phenyl guests the primary basis for energy stabilization with 14+ is found to be short-range stabilizing electrostatic forces complemented by small amounts of polarizability and charge-transfer. In contrast, the cavity binding between substituted 4,4′-biphenyl guests and 14+ is determined by almost equal contributions of polarizability and electrostatics. The effect of solvent is shown to have only a small effect on the computed geometry of 14+ complexes, but its impact upon binding energies is substantial. The first solvation shell of the cyclophanes is computationally approximated by 12 acetonitriles and satisfies the requirements of the 16 relatively acidic protons on the bipyridinium groups. Good correlations between the computed (with solv ation) and experimental 14+ binding energies are found. The degree of linear correlation improves substantially when the comparison between computed and experimentally observed binding energies is restricted to structurally similar (number of aromatic rings, number of substituents and position of substitution) molecular guests. Furthermore, computed molecular properties, such as polarizability, maximum hardness, softness and electronegativity of the isolated guests, correlate well with 14+ binding energies based upon the same requirement of guest similarity. The non-covalent forces associated with the external cyclophane interactions were studied with guest molecules built from symmetrical 1,4-extensions of hydroquinone composed of aliphatic or ethyleneoxy side arms. In particular, side arm length and functionality, and the position and type of heteroatoms along the chain, were systematically varied to define the external interactions between the guest side arms and different host cyclophanes. Specifically, the ethyleneoxy linkages are shown to provide a large chelate and cooperative effect which direct the binding with 14+. In order to probe further the special geometric and electronic character of 14+, we have synthesized and tested a new supramolecular host, 24+, similar to 14+ but where a pentacycloundecane unit replaces one of the xylyl groups. Both experimental and computed data on the new host emphasize the ideal geometry and electronic nature of the 14+ molecular receptor for aromatic guests. The inclusion complexes discussed in this paper are important not only because they, or similar entities, are the main components of many rotaxanes, catenanes and other switchable molecules, but because the intermolecular interactions involved, such as electrostatics, polarizability and charge-transfer, are ubiquitous in supramolecular chemistry. The information reported on the specific interactions involving the 14+-44+ molecular receptors with substituted aromatic guests can also be extended by analogy to many systems of broad interest. © 1997 John Wiley & Sons, Ltd.