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Three neutral cyclophanes were synthesized, and their association with indole, an aromatic pi-donor, was studied. The cyclophanes were designed to contain a rigid, hydrophobic binding cavity with 1,4,5,8-naphthalenetetracarboxylic diimide or 1,5-dinitronaphthalene as the pi-acceptor. Two of the cyclophanes also contain a (S)-(valine-leucine-alanine) tripeptide unit to provide chiral hydrogen bonding interactions with guest molecules. Despite the fact that these cyclophanes contain a hydrophobic binding cavity of appropriate dimensions, their association with indole is very weak. In the case of cyclophanes derived from 1,5-dinitronaphthalene, steric interactions force the nitro groups out of the plane of the naphthalene ring, diminishing their effectiveness as pi-acceptors. A simple UV--visible titrimetric method, using N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) as a pi-donor, was used to rank the pi-acceptor strength of these and other aromatic units. These titrations show that 1,4,5,8-naphthalenetetracarboxylic diimide and 1,5-dinitronaphthalene derivatives are weaker pi-acceptors than viologens, which make good pi-acceptor cyclophanes. Methyl viologen is in turn a weaker pi-acceptor than anthaquinone disulfonate, suggesting that the latter may serve as a useful building block for pi-accepting cyclophane hosts.

Two π-accepting viologen substituents were coupled with the chiral barrier (1R,2R)-(+)-1,2-cyclohexanediamine to synthesize a rigid, C 2 -symmetric cyclophane host for aromatic guest molecules. UV-visible spectroscopic titrations show an association constant of 3.3 × 10 3 M -1 with indole, a hydrophobic guest, and constants in the range of 10 1 -10 2 M -1 for more hydrophilic indole derivatives in water. Tryptophan methyl ester complexes this host in aqueous base with an enantioselectivity ratio of 3.3, and an association constant of 2.0 × 10 2 M -1 for the preferred L-enantiomer. Protonation of tryptophan methyl ester causes both the L-and D-enantiomers to have the same affinity for the host, which is, within experimental error, the same as that of the D-enantiomer in base. The cyclophane host can be intercalated into R-zirconium phosphate, a lamellar solid acid, by first swelling the latter with tetranbutylammonium hydroxide. Because the intercalated cyclophane (layer spacing ) 14.7 Å) has a pre-organized binding pocket, indole intercalates from solution to give a 1:1 complex in the solid, with no change in interlamellar spacing. Adsorption isotherms corresponding to indole intercalation and complexation were Langmuirian, in contrast to the strongly cooperative binding found previously for complexation of π-donors with noncyclophane chiral hosts in R-zirconium phosphate.