Rational Design of the First Spherical Supramolecular Dendrimers Self-Organized in a Novel Thermotropic Cubic Liquid-Crystalline Phase and the Determination of Their Shape by X-ray Analysis

Abstract

The rational design, the synthesis via a new convergent approach, and the characterization of four generations of monodendrons based on the AB3 building block methyl 3,4,5-trishydroxybenzoate are described. The first generation monodendrons are crystalline. The second generation methyl 3,4,5-tris-3‘,4‘,5‘-[tris(n-dodecyloxy)benzyloxy]benzoate (12Gn-AG-CH3, n = 2), the corresponding benzyl alcohol (12G2-AG-CH2OH) and benzoic acid (12G2-AG), and all higher generation monodendrons with n = 3 and 4 exhibit in addition to the crystal phase an isomorphous cubic liquid-crystalline (LC) phase of Pm3̄n space group. The cubic unit cell parameters of 12G2-AG, 12G3-AG, and 12G4-AG are 68.3, 79.2, and 84.0 Å, respectively. Computed electron density profiles demonstrated that in the cubic phase 12, 6, and 2 monodendrons of generation 2, 3 and 4 are self-assembled in supramolecular dendrimers resembling spherical micelles when they are centered at the corners and the body-center and respectively highly rounded tetrahedra when they are at 1/4 and 3/4 distance along one of the bisectors of each face of the cubic cell. This novel thermotropic LC phase is similar to that of the lyotropic Pm3̄n phase of biological lipids. These supramolecular dendrimers contain a poly(benzyl ether) core dispersed in an aliphatic matrix of nearly uniform density which is made up of the melted terminal long alkyl chains of the monodendrons. The size of the supramolecular dendrimer and of the entire unit cell increases with generation number as a result of a disproportionately large increase in the number of AB3 repeat units relative to the number of terminal alkyl chains in a monodendron which decreases its average cone angle. The present experiments provide the first examples of monodendrons which self-assemble into spherical supramolecular dendrimers forming a thermotropic cubic LC phase.