Dendrimer, liquid crystalline

The use of ordered supramolecular assemblies, such as micelles, monolayers, vesicles, inverted micelles, and lyotropic liquid crystalline systems, allows for the controlled nucleation of inorganic materials on molecular templates with well-defined structure and surface chemistry. Poly(propyleneimine) dendrimers modified with long aliphatic chains are a new class of amphiphiles which display a variety of aggregation states due to their conformational flexibility [38]. In the presence of octadecylamine, poly(propyleneimine) dendrimers modified with long alkyl chains self-assemble to form remarkably rigid and well-defined aggregates. When the aggregate dispersion was injected into a supersaturated... 

Fig. 27. Percec s hyperbranched liquid crystalline dendrimer that forms isotropic and nematic phases based on monomeric gauche and anti conformations...

Mesomorphic dendrimers containing electroactive units have potential for construction of dendrimer based molecular switches. Deschenaux et al. reported [154] the synthesis and liquid-crystalline properties of a novel dendrimer containing six mesomorphic ferrocene units. Apart from exhibiting a broad enantiotropic smectic A phase as determined by polarized optical microscopy, DSC, and XRD studies, thermogravimetry revealed the excellent thermal stability of the macromolecule. 

Thermotropic liquid crystalline (LC) phases or mesophases are usually formed by rod-like (calamitic) or disk-like (discotic) molecules. Spheroidal dendrimers are therefore incapable of forming mesophases unless they are flexible, because this would allow them to deform and subsequently line up in a common orientation. However, poly(ethyleneimine) dendrimers were reported to exhibit lyotropic liquid crystalline properties as early as 1988 [123],... 

A key feature of our polyphenylene dendrimers is that they can be planarized and thus reduced in dimensionality by intramolecular dehydrogenation [29,35]. This results in large, fused polycyclic aromatic hydrocarbons (PAHs). PAHs serve as structurally distinct, two-dimensional subunits of graphite and show attractive properties such as high charge carrier mobility, liquid crystallinity, and a high thermal stability, which qualifies these materials as vectorial charge transport layers [81]. 

The Tfj-symmetrical hexaaddition pattern also represents an attractive core tecton for dendrimer chemistry [26, 31, 63-67]. Examples for such dendrimers, involving a core branching multiplicity of 12, are 38 and 39 [63, 64], Addition of six mesotropic cyanobiphenyl malonate addends produced the spherical thermotropic liquid crystal 40 [65], DSC and POM investigations revealed a smectic A phase between 80 and 133 °C. Interestingly, this spherical and highly symmetrical compound gives rise to liquid crystallinity despite the absence of molecular anisotropy. 

Various covalent and non-covalent approaches for the incorporation of fullerene building blocks into liquid crystalline mesomorphic materials have been developed [35]. Lyotropic mesophases (buckysomes) were obtained upon dissolving globular amphiphilic fullerene dendrimers in water [145]. 

G2-OH) and cholesteryl dendrimer 265 (G2-DLCP) (DLCP = dendritic liquid crystalline polymer) are shown in Scheme 27 as examples. 

The fascination of dendrimers is due essentially to the unique architecture of these molecules and the attendant possibilities of designing well-defined functional (macro)molecules [1], whose functional groups can participate in chemi-cal/physical processes [2], In discussions about functional dendrimers a distinction is made between dendrimers bearing more complex, function-bearing units (such as catalytically active, photoactive units, liquid-crystalline groups) and those whose functionalities serve to control simple chemical/physical molecular properties (such as solubility, viscosity). Since both fulfil a function, the word functional is used as a generic term in the following sections. 

Stilbenoid dendrimers are able to undergo aggregation. Depending upon the generation number, some of the pure substances form liquid-crystalline phases (Dha discotic hexagonal disordered phase Dra discotic rectangular disordered phase Dob discotic distorted phase). Differential scanning calorimetry (DSC) revealed phase transitions between 99°C and 0°C. 

Siloxanes, prepared in 1989 as representatives of silicon-based dendritic molecules ( silicodendrimers ), were the first dendrimers to contain heteroatoms other than the usual ones (N, O, S, halogens) [68]. As with the phosphodendri-mers (Section 4.1.10), their readily modifiable architecture and their pronounced thermostability hold promise of applications, for example, in the form of carbo-silanes as liquid-crystalline materials and catalyst supports. They can be subdivided into a number of basic types and their properties are presented below with the aid of characteristic representatives ... 

Fig. 8.6 Liquid-crystalline dendrimer (the central bonds have been lengthened for the sake of clarity)...

Percec et al.130bl have employed 1 — 2 C-branching and ether-type connectivity for the preparation of nonspherical, thermotropic liquid crystalline dendrimers. Observed thermotropic behaviors are predicated on mesogenic monomers that are capable of conformational isomerism. 

---