Structural analysis of cylindrical and spherical supramolecular dendrimers quantifies the concept of monodendron shape control by generation number

In 1989, it was predicted that a change in dendritic shape to a nearly spherical one should occur upon increasing the generation number (Naylor, A. M.; Goddard, W. A., III; Kiefer, G. E.; Tomalia, D. A. J. Am. Chem. Soc. 1989, 111, 2339). The absence of long-range order required for X-ray analysis allowed only indirect evidence to be provided for this concept. This publication reports the synthesis of three generations of self-assembling monodendrons based on the AB₃ building block methyl 3,4,5- trihydroxybenzoate. The first 3,4,5-tris[p-(n-dodecan-1- yloxy)benzyloxy]benzoic acid and the second-generation methyl 3,4,5- tris{3',4',5'-tris[p-(n-dodecan-1-yloxy)benzyloxy]benzyloxy}benzoate monodendrons self-assemble into cylindrical supramolecular dendrimers that self-organize in a two-dimensional p6mm lattice. The third-generation monodendron 3,4,5-tris(3',4',5'-tris{3'',4'',5''-tris[p-(n-dodecan-1- yloxy)benzyloxy]benzyloxy}benzyloxy)benzoate self-assembles in a spherical dendrimer that self-organizes in a three-dimensional cubic Pm3̄n lattice. Structural analysis of these lattices by X-ray diffraction provided the first direct demonstration of the supramolecular dendrimer shape change from cylindrical to spherical and indirect determination of the average shape change of the monodendron from a quarter of a disk to a half of a disk and to a sixth of a sphere as a function of generation number. These results have demonstrated the concept of monodendron and supramolecular dendrimer shape control by generation number.