Dimerization and Self-Sorting of Tetraurea Calix[4]arenes

Calix[4]arenes, substituted at their wide rim by four Urea functions, form hydrogen-bonded dimeric capsules. Covalent connection of adjacent urea functions within a calixarene molecule creates compounds with one to four loops. They show selective dimerization, since an overlap of loops is not possible within a dimer, and the urea substituent of the partner must be able to penetrate the loop. These selectivities can be developed into a sorting scheme for eleven tetraureas, where only six of potentially 35 dimeric combinations are realized. Together with the similar dimerization of triureas derived from triphenylmethane and the formation of tetramers from triurea monoacetamide calix[4]arenes, it is possible to design building blocks that form dendritic assemblies Uniform in size and structure.