Columnar Liquid Crystals from Shape-Persistent Dendritic Molecules

摘要

Dendritic molecules of well-defined structure offer special opportunities in materials chemistry. In the area of self-organizing materials, correlations between molecular morphology and macroscopic symmetry are of fundamental as well as practical importance, yet this relationship is poorly understood at this time. The morphology of perfectly branched dendrimers evolves from an open, extended form to a more globular shape, as generation increases. Because of their globular shape, most higher generation dendrimers do not crystallize, although several reports have documented lyotropic and thermotropic liquid crystalline order. This intriguing behavior was first seen by Kim who observed lyotropic phases of hyperbranched polyaramides in N-methylpyrrolidone (NMP) solutions. Several groups have since discovered thermotropic liquid crystalline (LC) phases in dendritic materials. Percec et al. prepared a series of hyperbranched polymers based on flexible, branched meso-gens that exhibited nematic LC phasesresulting from the molecules ability to fold into anisotropic shapes. The corresponding regular dendrimers were also found to be liquid crystalline. Another type of dendritic architecture that displays liquid crystallinity involves functionalization of the periphery with mesogenic groups. The flexibility of the dendritic interior in these systems allows the peripheral mesogens to acquire nematic organization. Recently. Percec et al. used benzylic ether monodendrons to prepare tapered mesogenic architectures functionalized with endo receptors or polymerizable groups at their focal points. These molecules have been shown to organize into hexagonally packed columns in which a slice of the column (that is. one disc) is made of several of the wedge-shaped monodendrons. The monodendrons are suggested as being stacked in a face-to-face manner acting like the protein wedges found in the Tobacco Mosaic Virus. Another interesting recent report describes a "starlike" heptameric triphenylene derivative (a triphenylene mesogen substituted with six other triphenylene mesogens) which exhibited a discotic columnar mesophase. However, the hexagonal cell edge in these materials corresponded to individual triphenylene mesogens rather than the star molecule as a whole (that is registry of the entire molecule was not achieved).