Modular synthesis of pi-acceptor cyclophanes derived from 1,4,5,8-naphthalenetetracarboxylic diimide and 1,5-dinitronaphthalene

Three neutral cyclophanes were synthesized, and their association with indo le, an aromatic pi -donor, was studied. The cyclophanes were designed to co ntain a rigid, hydrophobic binding cavity with 1,4,5,8-naphthalenetetracarb oxylic diimide or 1,5-dinitronaphthalene as the pi -acceptor. Two of the cy clophanes also contain a (S)-(valine-leucine-alanine) tripeptide unit to pr ovide chiral hydrogen bonding interactions with guest molecules. Despite th e fact that these cyclophanes contain a hydrophobic binding cavity of appro priate dimensions, their association with indole is very weak. In the case of cyclophanes derived from 1,5-dinitronaphthalene, steric inter actions fo rce the nitro groups out of the plane of the naphthalene ring, diminishing their effectiveness as pi -acceptors. A simple UV-visible titrimetric metho d, 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 unit s. These titrations show that 1,4,5,8-naphthalenetetracarboxylic diimide an d 1,5-dinitronaphthalene derivatives are weaker pi -acceptors than viologen s, which make good;pi -acceptor cyclophanes. Methyl viologen is in turn a w eaker pi -acceptor than anthaquinone disulfonate. suggesting that the latte r may serve as a useful building block for pi -accepting cyclophane hosts.