Maurice W. P. L. Baars

Host-guest chemistry of dendritic molecules

In this chapter we will discuss the contribution of dendritic macromolecules to the field of supramolecular host-guest chemistry. Since the first publications on dendrimers more than two decades ago, their properties as molecular recognition compounds have been discussed many times. A brief introduction to the common host-guest interactions in the traditional supramolecular field is accompanied by a short overview of specific properties of these highly branched, three-dimensional macromolecules. Emphasis will be placed on the existence of internal voids in the dendritic interior. Subsequently, an overview will be given of the reported host-guest systems based on dendritic molecules. The host-guest systems discussed are arranged by type of interactions: from topological encapsulation to electrostatic, hydrophobic or hydrogen-bonding interactions. This review will emphasize contributions in which the pre-organized three-dimensional dendritic structure and the high local concentrations of sites display cooperative effects and which could be of interest towards future applications.

The Localization of Guests in Water-Soluble Oligoethyleneoxy-Modified Poly(propylene imine) Dendrimers

Small-angle X-ray scattering measurements and UV/Vis titrations have been used to characterize a new water-soluble host?–?guest system based on oligoethyleneoxy-modified poly(propyleneimine) dendrimers. These experiments indicate that unique interactions between the host and guests lead to a specific location of the guests in the core of the dendrimers (see schematic representation).

LIQUID-CRYSTALLINE PROPERTIES OF POLY(PROPYLENE IMINE) DENDRIMERS FUNCTIONALIZED WITH CYANOBIPHENYL MESOGENS AT THE PERIPHERY

A family of smectic liquid-crystalline dendrimers has been synthesized by connecting rodlike cyanobiphenyl mesogens, with various spacer lengths, at the periphery of poly(propylene imine) dendrimers. A combination of differential scanning calorimetry, optical microscopy, and X-ray diffraction suggests that, under the influence of the mesogenic endgroups, the dendrimer has to adopt a highly distorted conformation.

Supramolecular Modification of the Periphery of Dendrimers Resulting in Rigidity and Functionality

The construction of 32 scaffolds at the periphery of a fifth generation poly(propyleneimine) dendrimer, in which glycinylurea-modified molecules can be selectively bound, represents a novel approach to functionalize dendrimers (see schematic representation). These supramolecular architectures have enhanced rigidity and show reversibility of the modification.

A scattering electro-optical switch based on dendrimers dispersed in liquid crystals

The dispersion of dendrimers in a liquid crystal (LC) is reported as a new approach to LC displays based on light scattering. It is shown that the performance of a fifth-generation palmitoyl-functionalized poly(propylene imine) dendrimer - with 64 legs, as shown in the Figure - as an electro-optical switch is superior to that of polymer-filled nematics and polymer-dispersed LC systems (see also cover).

Electro-Optical Switches Based on Polymer and Dendrimer Filled Nematics

Two-phase systems which can be switched electrically between a light scattering and a transparent state can be prepared based on either a dispersion of well-defined sub-micron sized crosslinked polymeric particles in a liquid crystal (LC) matrix (Polymer Filled Nematics, PFNs) or on a dispersion of palmitoyl-functionalised poly(propylene imine) dendrimers in an LC (Dendrimer Filled Nematics). The present paper describes the preparation of both systems and their properties. The PFNs can be electrically switched between a scattering and a transparent state by an appropriate choice of materials and refractive indices. The preparation of the disperse polymeric phase is separated from the preparation of the polymer/LC blend, which enables control over the morphology of the system. Rather surprisingly, it is found that, due to the rheological properties of the PFN blends, stable films of the blends can be simply produced by conventional coating processes. The dendrimer-stabilised nematics require extremely low switching voltages to switch from a scattering to a highly transparent state. Moreover, the switching process is totally reversible and hysteresis effects appear to be absent.

Synthesis and properties of redox-active dendrimers containing phenothiazines

A new family of redox-active dendrimers based on a poly(propylene imine) core with phenothiazines as the redox active unit have been prepared, and some of their properties have been investigated.