Cyclophanes as synthetic receptors
Strategies for improving guest recognition are discussed. The bis crown ethers (14), with well defined separation between the two macroc clic eceptors, form complexes with bis-alkyl ammonium cations H,$(CH,) %H3 and show a high selectivity for guest chain length. The big zinc porphyrin (18) is a selective host for 4 , 4 ' dipyridyl and forms complexes rather less readily with the diamines H,N(CH,) NH, without showing a high selectivity for chain length. The beha%our of synthetic ditopic receptors of the cyclophane type evidently depends upon the separation and rigidity of the bridges linking the two receptor systems and also upon the rigidity of difunctional guest molecules and the separation and orientation of the two functional groups.