Self-assembled dendrimers with uniform structure

Abstract

Calix[4]arenes substituted at their wide rim by four aryl urea residues (1) form hydrogen-bonded dimers in apolar solvents. Replacement of one urea residue by an acetamido moiety leads to calix[4]arene derivatives (5) which form hydrogen-bonded tetramers under the same conditions. Both self-assembly processes occur independently. Therefore, molecules have been prepared in which a tetra-urea calix[4]arene and a tri-urea mono acetamide derivative are covalently connected between their narrow rims by a long, mainly aliphatic chain [-O-(CH(2))(n)-C(O)-NH-(CH(2))(m)-O-] (7). In the presence of an equimolar amount of tetra-tosyl urea calix[4]arene () they form dendritic assemblies since the well known heterodimerization of tetra-tosyl and tetra-aryl urea calix[4]arenes prevents the formation of a cross-linked structure. Covalent connection of adjacent urea residues leads to tetra-loop derivatives (3) that cannot form homodimers, but instead form heterodimers with tetra-aryl or tetra-tosylureas. Therefore, similar dendrimers should be available using the selective dimerization observed for 3. The formation of a single, structurally uniform dendrimer from eight building blocks is confirmed by (1)H NMR spectra, showing only peaks that are also found for respective model assemblies. Translational diffusion coefficients of the assemblies have been determined using (1)H DOSY NMR.