Lamellar to Columnar Mesophase Evolution in a Series of PAMAM Liquid-Crystalline Codendrimers

Abstract

The synthesis, chemical characterization and liquid crystalline behavior of a series of seven poly(amidoamine) (PAMAM) codendrimers are described. These compounds were obtained by grafting two types of terminal promesogenic units, that carry either one or two decyloxy chains, in various proportions onto the third generation of PAMAM dendrimer. The average number of promesogenic units was determined by using an original interpretation of the NMR spectra. X-ray diffraction studies show that these compounds exhibit lamellar and/or columnar mesophases. The type of mesophase is determined by the number of each kind of promesogenic units present in a given codendrimer and by the temperature. The evolution of the supramolecular organization of the different molecules is explained as a function of the number of alkoxy chains present around the dendritic core and compared to the models developed for the homodendrimers totally functionalized by promesogenic units with either one or two decyloxy chains.

Recent NMR Studies of Thermotropic Liquid Crystals

“…Liquid crystalline dendritic molecules, especially those of intermediate generations, may assume different shapes, as the inner dendritic cores are flexible and can adopt several configurations to give various mesophases. In the case of PAMAM's D r phase, the dendrimer assumed an average rectangular plate-like structure as shown by X-ray structural studies [213]. The T 1 frequency dispersion at high Larmor frequencies in the D r phase (80°C) was described by two BPP (ROT) contributions, particularly above 10 MHz, characterized by two correlation times associated with slow and fast local reorientations within the molecule, respectively.…”

Functional Carbazole Liquid‐Crystal Block Codendrimers with Optical and Electronic Properties

“…The experimentally measured parameters can be accounted for if we assume that the molecules adopt the usual cylinder model found for other dendritic mesogens in which the mesogenic units linked to the periphery of dendrimers lie statistically upward and downward of the dendritic matrix. [40][41][42] In the present case, by comparing the layer spacing and the length of the molecules of the dendrons and codendrimers in their most-extended conformation, calculated by molecular modelling, two molecules are necessary to fill the cylinder (14 a and 15 a'; Figure 2 a). The only exception is 17 a', for which the experimental data are consistent with one molecule filling the cylinder.…”

Fullerene‐Containing Liquid Crystals

“…A columnar slice contains 14 molecules of 31 and 10 molecules of the larger dendrimers 32 and 33. The postulated model of the molecular organization is derived from that of the lamellar organization, although the increasing ratio value destabilizes the layering by breaking the layers into ribbons as previously observed for lamellar-to-columnar phase transitions exhibited by polycatenar liquid crystals [33], fifth-generation carbosilane dendrimers [34], and statistical liquid-crystalline codendrimers [35]. Each columnar core is made up of mesogenic groups interacting through the cyanobiphenyl groups and is surrounded by the poly(benzyl ether) dendrons including the C 60 units.…”