Synthesis of Ex(n)Box cyclophanes.

A rapid and efficient synthesis of the extended bipyridinium-based class of cyclophanes--that is, Ex(n)Box(4+) (n = 0-3), where n is the number of p-phenylene rings inserted between the pyridinium rings--is demonstrated, resulting in much higher yields of products along with a reduced output of oligomeric byproducts. Although each cyclophane can be synthesized readily without the use of a precise stoichiometric amount of template, ExBox(4+) can be prepared in 66% yield (following crystallization) using six equivalents of pyrene in a template-directed protocol. This new methodology has been employed to synthesize, in modest yield, a nearly 2.5 nm long cyclophane consisting of 12 aromatic rings.